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Synthesis and conformational analysis of sterically congested (4r)-(-)-1-(2,4,6-trimethylbenzenesulfonyl)-3-n-butyryl-4-tert-butyl-2-imidazolidinone: X-ray crystallography and semiempirical calculations.

1. Introduction

The energy based conformational searching technique is a considerable computational request and is still an active area of research. When the property of interest is energy, the following methodology is indicated: full conformational search using molecular mechanics, followed by geometry optimization using semiempirical model for selected conformers, and finally single-point calculation using ab initio models for selected conformers [1]. On the other hand, steric bulkiness of chiral 2-imidazolidinones [2] plays an effective role in greatly enhancing stereoselectivity, and so sterically congested chiral 2-imidazolidinones [3-5] represent promising auxiliaries for providing excellent diastereocontrol. We reported the synthesis and chiral application of 4-tertbutyl-2-imidazolidinone which were greatly enhanced by the occurrence of N-arylsulfonyl fragments [4]. Moreover, several 2-imidazolidinone derivatives containing diarylsulfonylurea pharmacophore have been synthesized and screened for antitumor activity against various human solid tumors [6-17]. More interestingly the structure of the arylsulfonyl-2-imidazolidinone such as 4-benzamido-3-methyl-1-tosyl-2-imidazolidinone and (S)-(+)-1-[1-(4-aminobenzoyl) indoline-5sulfonyl]-4-phenyl-4,5-dihydroimidazol-2-one has elucidated using X-ray analysis [18,19]. Recently, we reported about the X-ray analysis and computational studies of trans-1acetyl-4,5-di-tert-butyl-2-imidazolidinone in which the crystal unit cell showed two independent molecules connected together by two intermolecular hydrogen bonds [20].


In such a way and in continuation of our previous report [20] we studied single-crystal X-ray and the theoretical conformational analysis of (4R)-(-)-1-(2,4,6-trimethylbenzenesulfonyl)-3-butyryl-4-tert-butyl-2-imidazolidinone (3), as a cyclic arylsulfonylurea, focusing on the configuration of substituents around the 2-imidazolidinone core and to establish the factors that influence this configuration and if this configuration can be predicted for new substituted 2-imidazolidinone.

2. Experimental

2.1. Procedure for Synthesis of (4R)-(-)-1-(2,4,6-Trimethylbenzenesulfonyl)-3-n-butyryl-4-tert-butyl-2-imidazolidinone (3). n-Butyl lithium (1.5 M in hexane, 0.5 mmoL) was added to a stirred solution of compound 2 (0.5mmoL) in THF (10 mL) at -78[degrees]C under nitrogen atmosphere for 10 min and n-butyryl chloride (1.0 mmoL) was added dropwise at -78[degrees]C. The reaction mixture was stirred at room temperature for 1 h and then was quenched by passing through silica gel (EtOAc, 100 mL), evaporated under vacuum, followed by column chromatography on silica gel (EtOAc: hexane) to afford compound 3 in quantitative yield.

Compound (4R)-3 (97%): white crystals, mp 125-127[degrees]C (Hexane), IR (KBr) [v.sub.max]/[cm.sup.-1], 1736, 1701 (CO), 1320, 1175 (S[O.sub.2]); [[[alpha]].sub.D.sup.26] = -12.0[degrees] (c 1.00, CH[Cl.sub.3]); [sup.1]H-NMR (CD[Cl.sub.3], 500 MHz): [delta] 6.99 (s, 2H), 4.42-4.40 (d, 1H, 7 = 8.5 Hz), 3.97-3.96 (d, 1H, J = 8.5 Hz), 3.85-3.81 (t, 1H, 7 = 8.5 Hz), 2.85-2.81 (m, 1H), 2.74-2.67 (m, 1H), 2.65 (s, 6H), 2.31 (s, 3H), 1.66-1.58 (m, 2H), 0.93 (s, 9H), and 0.92-0.88 (t, 3H, 7 = 7.3 Hz).

2.2. X-Ray Data Collection, Structure Solution, and Refinement for (4R)-(-)-1-(2,4,6-Trimethylbenzenesulfonyl)-3 n-butyryl-4-tert-butyl-2-imidazolidinone (3)

2.2.1. Data Collection. (4R)-(-)-1-(2,4,6-Trimethylbenzenesulfonyl)-3-butyryl-4-tert-butyl-2-imidazolidinone (3) (Scheme 1) was prepared according to our previous report [4, 21]. A colorless plate crystal of [C.sub.20][H.sub.30][N.sub.2][O.sub.4]S having approximate dimensions of 0.25 x 0.10 x 0.25 mm was mounted on a glass fiber. All measurements were made on a Rigaku AFC7R diffractometer with graphite monochromated Cu-K[alpha] radiation and a rotating anode generator. Cell constants and orientation matrix for data collection obtained from a least-squares refinement using the setting angles of 25[degrees] carefully centered reflections in the range 59.17[degrees] < 20 < 59.87[degrees] corresponded to a triclinic cell (Table 1).

The data were collected at a temperature of 20 [+ or -] 1[degrees]C using the [omega] - 2[theta] scan technique to a maximum 2[theta] value of 120.1[degrees]. Omega scans of several intense reflections, made prior to data collection, had an average width at half-height of 0.28[degrees] with a take-off angle of 6.0[degrees]. Scans of (1.78 + 0.30 tan [theta])[degrees] were made at a speed of 16.0[degrees]/min (in omega). The weak reflections (I < 10.0[sigma] (7)) were rescanned (maximum of 5 scans) and the counts were accumulated to ensure good counting statistics. Stationary background counts were recorded on each side of the reflection. The ratio of peak counting time to background counting time was 2 : 1. The diameter of the incident beam collimator was 0.5 mm and the crystal to detector distance was 235 mm. The computer-controlled slits were set to 3.0 mm (horizontal) and 3.0 mm (vertical).

2.2.2. Data Reduction. Of the 4942 reflections which were collected, 4649 were unique ([] = 0.059); equivalent reflections were merged. The intensities of three representative reflections were measured after every 150 reflections. The linear absorption coefficient, [mu], for Cu-K[alpha] radiation is 16.0 [cm.sup.-1] and an empirical absorption correction based on azimuthal scans of several reflections was applied which resulted in transmission factors ranging from 0.81 to 1.00. The data were corrected for Lorentz and polarization effects and a correction for secondary extinction were applied (coefficient = 6.35905[e.sup.-06]).

2.2.3. Structure Solution and Refinement. The structure was solved by direct methods [22] and expanded using Fourier techniques [23]. The nonhydrogen atoms were refined anisotropically and hydrogen atoms were included but not refined. The final cycle of full-matrix least-squares refinement (Least-squares: function minimized: [SIGMA][omega]([absolute value of Fo] - [[absolute value of Fc]].sup.2]), where [omega] = 1/[[sigma].sup.2](Fo) = [[[[sigma].sup.2.sub.c](Fo) + ([p.sup.2]/4)[Fo.sup.2]].sup.1] and [[sigma].sup.2](Fo) = e.s.d. based on counting, p = p-factor) was based on 4409 observed reflections (I > 3.00a (I)) and 729 variable parameters and converged (largest parameter shift was 0.09 times its esd) with unweighted and weighted agreement factors of


The standard deviation of an observation of unit weight (standard deviation of an observation of unit weight: [square root of [SIGMA][omega]([absolute value of Fo] - [[absolute value of Fc]]).sup.2]/(No - Nv)], where: no. = number of observations and nv = number of variables) was 1.26 and the weighting scheme was based on counting statistics and included a factor (P = 0.080) to down-weight the intense reflections. Plots of [SIGMA][omega][([absolute value of Fo] - [[absolute value of Fc]].sup.2]) versus [absolute value of Fo], reflection order in data collection, sin [theta]/[lambda], and various classes of indices showed no unusual trends. The maximum and minimum peaks on the final difference Fourier map corresponded to 0.15 and -0.20 [e.sup.-]/[[Angstrom].sup.3], respectively.

Neutral atom scattering factors were taken from Cromer and Waber [24] and anomalous dispersion effects were included in Fcalc [25]; the values for [DELTA]F' and [DELTA]F" were those of Creagh and McAuley [26]. The values for the mass attenuation coefficients are those of Creagh and McAuley [26]. All calculations were performed using the teXsan [27] crystallographic software package of Molecular Structure Corporation and crystal data summary is given in Table 1. The selected bond lengths, angles, and torsion angles are given in Tables 2-4 and the molecular structure with the atom-numbering scheme and the packing within the cell lattice are shown in Figures 1 and 3, respectively

2.3. Computational Calculations. All molecular modeling calculations were performed using HyperChem version 8.0.6 [28], running on "Windows Vista" operating system installed on an Intel core 2 duo PC with a 2.66 GHz processor and 2000 Mb RAM.

2.3.1. Conformational Search. Conformational analyses of isolated molecule 3 (3A, 3B, and 3C) were done in the same way using the procedure which is suggested for conformational flexible compounds when the property of interest is energy [1]. Initial X-ray structures for the molecules 3A, 3B and 3C were used for conformational analysis with HyperChem 8.0 [28]. The MM+ [29] (calculations in vacuum, bond dipole option for electrostatics, and RMS gradient of 0.01kcal/mol) conformational searching in torsional space was performed using the multiconformer method [30, 31]. Each molecule 3A, 3B, and 3C was subjected to a separate conformational search and the most stable conformer was energy minimized using semiempirical MO methods AM1 [32] and PM3 [33] included in MoPAC version 2009 [34] using HyperChem as GUI. Vibration frequencies calculation for each conformer was characterized to be the stable structure (no imaginary frequencies).

3. Results and Discussion

As a result of the potential conformational flexibility of the substituent groups of compound 3, we have used solid state molecular structures (as obtained from single crystal X-ray diffraction analysis) to obtain realistic structure as starting geometries for the quantum chemical calculations. Additionally, the data obtained by X-ray diffraction analysis shed light on some interesting features of its molecular structures. Some structural characteristics of compound 3 are the geometrical parameters around the ring nitrogen atoms such as the relative orientation of the n-butyryl and the 2,4,6-trimethylbenzenesulfonyl groups at N1 and N2 positions. Although, the preferred conformation in the solid phase can be different from the solution structure and in gas phase, the X-ray diffraction data are useful for comparative purposes. Overall, the combination of experimental and computational results can help in understanding the physical and chemical properties of this molecule.

3.1. X-Ray Crystal Structure of (4R)-(-)-1-(2,4,6-Trimethylbenzenesulfonyl)-3-n-butyryl-4-tert-butyl-2-imidazolidinone (3). The molecular solid state structure of 3 and numbering system are indicated in Figure 1. Geometrical parameters for compound 3 are collected in Tables 2-4. In the crystal structure, compound 3 crystallizes in the P1 space group (Table 1) and exists in three independent conformationally different molecules in the unit cell (3A, 3B, and 3C; see Figures 1, 2, and 3). Recently, crystal structures having more than one molecule in the unit cell have aroused interest, since these compounds can help in understanding the interactions responsible for packing as well as to guide the design of technologically useful materials [35]. The three molecules present, certain disorder in the core 2imidazolidinone and the substituents linked to such ring. Thus, there is some ambiguity in the atomic positions of the 2-imidazolidinone skeleton, tert-butyl, n-butyryl, and 2,4,6-trimethylbenzenesulfonyl groups of three molecules: 3A, 3B, and 3C. These disorders are expected due to the conformational flexibility of the tert-butyl, n-butyryl, and 2,4,6-trimethylbenzenesulfonyl moieties. The five-membered imidazolidinone ring assumes distorted envelope conformation (half-chair; LISLOO) which may be related to angle strain (angle strain is calculated as the difference between the internal angle and the ideal [sp.sup.3] angle of 109.5[degrees]) [36]. Atoms system ZN2-C3-C2 of 3A, 3B, and 3C are deviated from ideal [sp.sup.3] angle by 6.2[degrees], 6.8[degrees], and 7.3[degrees], respectively. Similarly <C3-C2-N1 system of 3A, 3B, and 3C deviates by 8.5[degrees], 7.9[degrees], and 7.8[degrees], respectively, from the ideal value [17, 18, 20, 36-41]. The same pattern was observed with angle system <N2-C2-N2. The structures of 3A, 3B, and 3C depicted in Figures 1-3 are those more likely on the basis of standard bond distances and angles [17, 18, 20, 41]. In the three molecules (3A, 3B and 3C), the geometrical parameters of the 2-imidazolidinone ring are quite similar with few distortions (Tables 2-4). The geometries of <C4-N1-C2 and <C4-N1-C1 atoms are almost planar rather than the most stable pyramidal form with bond angles of ca. 120[degrees]-126[degrees] for the three molecules 3A, 3B, and 3C. Similarly the geometries of <S1-N2-C1 and <S1-N2-C3 are also planar with bond angles ca. as 122.1[degrees] (3A), 122.6[degrees] (3B), 121.7[degrees] (3C), 124.5[degrees] (3A and 3B), and 125.4[degrees] (3C), respectively [17, 18, 20, 38-41]. This geometry makes the two nitrogen atoms in each molecule distinguishable from a geometrical point of view. Moreover, the planarity angle <N2-C1-N1-C4 of molecules 3A, 3B, and 3C was deviated from the planar urea form by 23 [degrees]-26[degrees] with ca-154.1[degrees], -150.9[degrees], and -157.1[degrees], respectively [20, 41-44]. It must be indicated that the two nitrogen atoms in each molecules occupy anti-positions relative to the mean plane of the ring system. This anti-position of both nitrogen atoms in each molecule is one of the reasons which make the central ring nonplanar [20]. This distortion leads to trans-geometry of n-butyryl fragment around one nitrogen atom and the sulfonyl moiety of the other nitrogen atom. As expected from the previous results [20], as a result of electron distribution, flexibility, and steric congestion of the system, the 2-imidazolidinone rings in 3A, 3B, and 3C were nonplanar and adopted distorted envelope conformation (half-chair; LISLOO). A common characteristic molecules 3A, 3B, and 3C is the dihedral angle between ferf-butyl and the 2-imidazolidinone rings, with values of -74.0[degrees] (3A), -67.5[degrees] (3B), and -73.5[degrees] (3C) [20, 4346]. Similarly the dihedral angle between n-butyryl group and the 2-imidazolidinone rings is -9.6.0[degrees] (3A), -6.1[degrees] (3B), and -7.8[degrees] (3C) as expected in order to minimize unfavourable steric interactions between ferf-butyl and n-butyryl moiety. Another common feature of the three molecules is the relative orientation of the n-butyryl. They adopt a fransoid conformation and each n-butyryl group is nearly out of the plane of the corresponding 2-imidazolidinone (dihedral angles 6[degrees]-9[degrees]). A remarkable structural feature of the solid-state structures of molecules 3A, 3B, and 3C is the bond angles, whereas the geometrical distortion is manifested in the smaller <C2-C3-N2 (102.2[degrees]) and <N1C2-C3 (101.0[degrees]) bond angles (Figure 1, Table 3) [20, 38-41]. In addition, the structures of molecules 3A, 3B, and 3C were superimposed in order to reveal the conformational differences of the three molecules (Figure 2). The strategy of overlay fit to match 2-imidazolidinone rings and examines any spatial differences between the atoms of the peripheral fragments. The results show that atoms of the n-butyryl, and 2,4,6-trimethylphenylsulfonyl groups occupy different spatial positions relative to the plane of 2-imidazolidinone ring which may explain the existence of such three molecules in one unit cell.

To conclude, it is found that the solid state conformations of the three molecules of 3 (3A, 3B,and 3C)inthe unit cell are quite similar, showing minor differences in some bond length and bond angle and major differences in some torsion angles at the peripheral substitution. However, and despite the high congestion in the molecular structures of these compounds, they form quiet molecular packing that likely reflects the subtle influence of the diverse intermolecular interactions.

The crystal packing of 3 is indicated in Figure 3. The molecules are arranged in a layer constituted by three molecules of 3A, 3B, and 3C and that are maintained by numbers of CH-O, CH-[pi], and [pi]-[pi] interactions [45-47]. The main structural feature of the packing of 3 is that two molecules are quite parallel to each other and connected by [pi]-[pi] interactions of the two aryl fragments (5.03 [Angstrom]) with coordinates -4.250, -2.462, and -0.875, while the third molecule was arranged in a lateral arrangement and approximately in opposite direction to the other two molecules. Additionally, the intramolecular interactions within each molecules of 3 involve O atom of sulfonyl fragment and hydrogen atoms from the C[H.sub.3] of 2,4,6-trimethylphenyl group (1.902-2.081 [Angstrom]). The main putative interactions CH-O, as inferred by relatively short distances and suitable orientations, are indicated in Figure 3. On the basis of the short distances and the wide angle, the CH-O intermolecular interactions are likely to be quite strong and an important factor to determine the crystal packing; these bonds can be considered as nonclassical hydrogen bonds [45-47] involving CH as H-bond donors. Moreover, the third opposite molecule was showing CH-[pi] interaction of the alkyl part of the butyryl moiety of this molecule and the aromatic fragment of the middle molecule (3.86 [Angstrom]). Similarly the C[H.sub.3] group of the 2,4,6-trimethylbenzenesulfonyl group of the middle molecule interacted with aromatic moiety of the third opposite lateral molecule through CH-[pi] interaction (3.60 [Angstrom]). Finally, it must be indicated that the relative orientation between two parallel molecules of 3 in the crystal packing and the third opposite lateral molecule might indicate, besides steric suitability, a tendency to minimize the polarity of the crystal (compensating the dipole moments of the molecules) [48].

3.2. Computational Studies. Despite the interesting properties of the 1-arenesulfonyl-2-imidazolidinones, these compounds have been scarcely studied from a computational point of view [ 20,49,50]. Our goal was to compute quantumchemical derived properties that would be useful as starting points for understanding the properties of this type of ring system. Moreover, the other main task of conformational analyses of isolated molecules 3A, 3B, and 3C was to examine the stable conformations and a global energy minimum for each molecule. If there was considerable energy difference between the lowest energy of 3A, 3B, and 3C type of conformer, then we concluded that theoretical calculations predicted one type of geometric molecule. Since, the size and the variety of heteroatoms in the 2-imidazolidinones are considerable, a full semiempirical geometrical optimization is computationally very demanding. This work is simplified, if we use a realistic structure as starting geometry for the MM conformational search and quantum-chemical calculation. Therefore, the structures obtained by X-ray diffraction analysis are suitable to this end. Since compound 3 appears as three independent molecules in the asymmetric unit, the three structures (molecules 3A, 3B, and 3C) were separately submitted to the conformational search using molecular mechanic MM+ and the energy minima conformer together with the highest energy conformer were subjected to full semiempirical AM1 and PM3 geometry optimization (Figures 4 and 5). Each conformer was confirmed as minimum or transition state on the basis of frequency calculation using AM1 results.

3.2.1. Theoretical Calculations. Taking into account our interest in the structural study of 2-imidazolidinone, the choice of computational methods which could reproduce the experimental data with reasonable agreement was relevant. Thus, we analyzed the conformational behaviour of compound 3 using semiempirical AM1 and PM3 quantumchemical calculations. Conformational performance of the 2imidazolidinone 3 was examined by the rotation and orientation in the space of the flexible tert-butyl, n-butyryl and 2,4,6-trimethylbenzenesulfonyl groups. Heat of formation, relative energies and dipole moment are collected in Table 5 and characteristic torsional angles, bond angles, and bond distance are also tabulated to illustrate the final geometries obtained. For such compound 3 the MM and semiempirical calculations led to six minimum energy conformations (Figures 4 and 5) within energy differences less than 8kcal/mol (Table 5). Additionally, the molecular structure of compound 3 was determined byMM+ and semiempirical AM1 and PM3 calculations to assess the accuracy of the theoretical methods used for compound 3. Conformations of the single molecules predicted by AM1, more than MM+ and PM3 methods, were approximately similar to that in the crystal (Figure 4).

The arrangement around the N2-S1 and N1-C4 bonds mainly determines the geometry of the N-substituent groups. Based on the geometrical comparison, these forms can be classified into two groups characterised by the torsion angle <C1-N1-C4-C5 denoted as conformer-A, -B, -C and -D for transoid butyryl fragment and conformer-E and -F for cisoid butyryl fragment (around -23.8[degrees] and 137.9[degrees], resp.). For each group there are two or more possible orientations of the 2,4,6- trimethylbenzenesulfonyl moiety (torsion angle <C1-N2-S1-C12) with a similar energy content. However, the relative energy content of these conformers indicates a strong preference for conformations A-B, while the C-F forms are strongly destabilised. Therefore, it seems that the spatial orientations of the O=S=O of 2,4,6-trimethylbenzenesulfonyl and C=O of n-butyryl group relative to C=O of 2-imidazolidinone ring should be affecting the dipole moment as trans-orientation leads to a decrease of this force and vice versa. The high dipole moment represented by cis orientation probably due to the destabilising through-space interactions of the lone pairs of oxygen atoms yields much greater energy differences such as C-F conformations. In light of these findings, A-B conformations were more preferred compared with C-F conformations which are unfavourable and their participation may be negligible. Therefore, it seems that the orientation of n-butyryl group with 2,4,6-trimethylbenzenesulfonyl moiety exerts a significant effect on the conformational preferences of the compound 3 and this behaviour may be attributed to a combination of steric and electronic factors.

The AM1 method shows that the relative orientation of the aryl group of the most stable conformer-A is practically fixed in an anticonformation relative to position of tertbutyl fragment. These features are in concordance with the behaviour of reported molecules [17, 18]. Moreover, the aryl group can adopt two symmetric and isoenergetic conformations in which the tert-butyl and the aryl groups are syn and anti-positions (torsion angle <C1-N2-S1-C12 about -61.7, 61.7[degrees]). Moreover, the energy content of the three similar conformation of the conformer-A is very close with a slight predominance of the orientation of the 2,4,6-trimethylbenzenesulfonyl group as their interconversion requires a low cost (0.05 kcal/mol).

3.2.2. Comparison of the X-Ray and Calculated Structures. The crystal structure of 3 confirms the approximate behaviour of such compound in the gas phase (theoretical calculations). The good agreement between experimental torsion angles determined for 3 and those calculated for the conformer-A (Table 5) supports the correctness of the calculations. Because the barrier of energy of rotation of the three forms of conformer-A is very low so conformerA was predicted representing all the three molecules of X-ray data. The low Gibbs energies of rotation between all possible transoid rotamers indicate the easy conversion between the three molecules in solution, and the solid state crystal structure was obtained in which it showed the important role of intermolecular interaction in stabilizing the molecules in solid states. The different disposition of the n-butyryl group (torsion angle ZC1-N1-C4-C5 = -6.1- -9.6[degrees] in the solid state and -23.8[degrees] for the more favourable orientation computed using AM1 method) may be ascribed to the packing in the crystal structure. Similarity the different orientation of 2,4,6-trimethylbenzenesulfonyl (torsion angle <C1-N2-S1-C12 = -65.9[degrees] - -75.1[degrees] in the solid state and -61.7[degrees] for the most stable conformer-A may be attributed to CH-O, CH-[pi], and [pi]-[pi] interactions in the crystal packing. Owing to theoretical calculations account for very low energy differences between the three dispositions around the N2-S1 bond, their interconversion can take place easily. It was suggested that the change in the spatial orientation of the 2,4,6-trimethylbenzenesulfonyl group could be facilitated by the intermolecular interaction in the crystal structure. The anti-conformation of 2,4,6-trimethylbenzenesulfonyl adopted in the solid state relative to tert-butyl group would be more favourable for their formation due to the CH-O being sterically more accessible with lower dipole moment. These results confirm the flexibility of the 2,4,6-trimethylbenzenesulfonyl group in these 2-imidazolidinone derivatives and the strong dependence on intermolecular interactions as was previously suggested. Moreover, the great similarity between these conformers is the bond length with only 0.05 [Angstrom] deviation among them. Hence, calculations at the semiempirical levels of the conformational energies of compound 3 indicate that the ideal gas-phase global energy minimum conformation is partially observed in the solid state. Rather, the effects of intermolecular interactions in the crystal structure cause the molecules to adopt higher-energy conformations, which correspond to local minima in the molecular potential energy surface. Finally to probe similarity and differences between the three-dimensional structures of the conformer-A and molecules 3A, 3B, and 3C, molecular superposition has been performed (Figure 6). The strategy of overlay fit to match 2imidazolidinone rings and examines any spatial differences between the atoms of the 4-tert-butyl, n-butyryl, and 2,4,6trimethylbenzenesulfonyl. The results show that atoms of the 4-tert-butyl, butyryl, and arenesulfonyl groups occupy different spatial positions relative to each other as described above.

4. Conclusion

The crystal structures of (4R)-(-)-1-(2,4,6-trimethylbenzenesulfonyl)-3-n-butyryl-4-tert-butyl-2-imidazolidinone (3) were reported. This compound 3 crystallized in layers formed by crystallographic independent molecules. These crystallographic motifs are the consequence of the interplay of the diverse intermolecular interactions in the crystal packing. The crystal packing showed three molecules of compound 3 were stacked as a result of intermolecular interaction. [ANGSTROM] computational analysis of compound 3 was performed using the MM+ force field and fully optimized with semiempirical [ANGSTROM]M1 and PM3 MO methods. The comparison of experimental versus calculated values for the selected bond lengths and angles of 3 is presented and the relative errors in calculated values are less than 3%. Both the experimental and calculated values agree that compound 3 is a sterically congested molecule. Theoretical conformational analyses have pointed out two factors that determine the conformation of the system under investigation. The first one is intermolecular interaction of the crystal packing, such as CH-O, CH-[pi], and [pi]-[pi] interactions, which stabilises and favours the occurrence of three independent molecules and the second factor is steric hindrance between substituents. The generally reasonable agreement between theoretical and experimental results have confirmed that the method which was applied for the theoretical conformational analysis of 2-imidazolidinone is good and useful for related organic molecules. Therefore, these results must be regarded as approximated and only with qualitative and comparative purposes. Moreover, the small differences between X-ray and calculated structures are consequence of different states of matter. During the theoretical calculation single isolated molecule is considered in vacuum, while many molecules are treated in solid state during X-ray diffraction. However, all the calculated geometric parameters, obtained by three used models (MM+, AM1, and PM3), represent good approximations and they can be applied as groundwork for prediction and exploring the other properties of the conformers.

5. Supporting Information Available

Crystallographic data for the structure in this paper have been deposited with the Cambridge Crystallographic Data Centre as the Supplementary Publication (no. CCDC 734938). Copies of the data can be obtained, free of charge, through application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +441223 336033 or e-mail:

Conflict of Interests

The author(s) declare(s) that there is no conflict of interests regarding the publication of this paper.


The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding work through the research group Project no. RGP-VPP-163.


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Ibrahim A. Al-Swaidan, (1) Adel S. El-Azab, (1,2) Amer M. Alanazi, (1) and Alaa A.-M. Abdel-Aziz (1,3)

(1) Department of Pharmaceutical Chemistry, College of Pharmacy, KingSaud University, Riyadh 11451, Saudi Arabia

(2) Department of Organic Chemistry, Faculty of Pharmacy, Al-Azhar University, Cairo 11884, Egypt

(3) Department of Medicinal Chemistry, Faculty of Pharmacy, University of Mansoura, Mansoura 35516, Egypt

Correspondence should be addressed to Alaa A.-M. Abdel-Aziz;

Received 2 October 2013; Revised 4 November 2013; Accepted 7 November 2013; Published 5 January 2014

Academic Editor: Fernanda Carvalho

TABLE 1: Summary of crystal data, intensity data collection, and
structure refinement for compound 3 at 20.0[degrees]C.

(a) Crystal data

Empirical formula                     [C.sub.20][H.sub.30][N.sub.2]
Formula weight                                   394.53
Crystal color, Habit                        Colorless, plate
Crystal dimensions                        0.25 x 0.10 x 0.25 mm
Crystal system                                  Triclinic
Lattice type                                    Primitive
No. of reflections used                  25 (59.2-59.9[degrees])
  for unit cell
Determination (2[theta]
Omega scan peak width                         0.28[degrees]
  at half-height
                                        a = 10.6216(5) [Angstrom]
                                        b = 16.532(1) [Angstrom]
                                        c = 8.9572(9) [Angstrom]
Lattice parameters                    [alpha] = 91.193(6)[degrees]
                                       [beta] = 93.849(6)[degrees]
                                      [gamma] = 88.097(4)[degrees]
                                    V = 1568.2(2) [[Angstrom].sup.3]
Space group                                      P1 (#1)
Z value                                             3
[D.sub.calc]                               1.253 g/[cm.sup.3]
[F.sub.000]                                      636.00
[mu] (CuK[alpha])                           15.98 [cm.sup.-1]

(b) Intensity measurements

Diffractometer                                Rigaku AFC7R
Radiation                            CuK[alpha] ([lambda] = 1.54178
                                         Graphite monochromated
Attenuator                               Ni foil (factor = 8.82)
Take-off angle                                6.0[degrees]
Detector aperture                           3.0 mm horizontal
                                             3.0 mm vertical
Crystal to detector distance                     235 mm
Voltage, current                              35 kV, 150 mA
Temperature                                  20.0[degrees]C
Scan type                                   [omega]-2[theta]
Scan rate                               16.0[degrees]/min (in w)
                                              (up to 5 scans)
Scan Width                         (1.78 + 0.30 tan [theta])[degrees]
2[[theta].sub.max]                           120.1[degrees]
                                               Total: 4942
No. of reflections measured        Unique: 4649 ([] = 0.059)

(c) Structure solution and refinement

Structure solution                       Direct methods (SIR92)
Refinement                              Full-matrix least-squares
P factor                                         0.0800
Anomalous dispersion                      All nonhydrogen atoms
No. of observations                               4409
  (7 > 3.00[sigma](I))
No. of variables                                   729
Reflection/parameter ratio                        6.05
Residuals: R; Rw                              0.037; 0.056
Residuals R1                                      0.037
No. of reflections to calc R1                     4409
Goodness of fit indicator                         1.26
Max shift/error in final cycle                    0.09
Maximum peak in final diff. map      0.15 [e.sup.-]/[[Angstrom].sup.3]
Minimum peak in final diff. map     -0.20 [e.sup.-]/[[Angstrom].sup.3]

TABLE 2: Experimental bond lengths ([Angstrom]).

Atom   Atom   Distance   Atom   Atom   Distance

S1A    O3A    1.416(3)   S1A    O4A    1.428(3)
S1A    N2A    1.671(3)   S1A    C12A   1.777(4)
S1B    O3B    1.430(4)   S1B    O4B    1.421(4)
S1B    N2B    1.664(3)   S1B    C12B   1.773(4)
S1C    O3C    1.416(3)   S1C    O4C    1.425(3)
S1C    N2C    1.669(3)   S1C    C12C   1.768(4)
O1A    C4A    1.215(5)   O1B    C4B    1.219(5)
O1C    C4C    1.210(5)   O2A    C1A    1.200(4)
O2B    C1B    1.209(5)   O2C    C1C    1.203(5)
N1A    C1A    1.376(5)   N1A    C2A    1.485(5)
N1A    C4A    1.408(5)   N1B    C1B    1.380(5)
N1B    C2B    1.485(5)   N1B    C4B    1.401(5)
N1C    C1C    1.386(5)   N1C    C2C    1.476(5)
N1C    C4C    1.413(5)   N2A    C1A    1.395(5)
N2A    C3A    1.469(5)   N2B    C1B    1.386(5)
N2B    C3B    1.473(5)   N2C    C1C    1.386(5)
N2C    C3C    1.486(5)   C2A    C3A    1.523(5)
C2A    C8A    1.556(6)   C2B    C3B    1.531(5)
C2B    C8B    1.549(5)   C2C    C3C    1.520(5)
C2C    C8C    1.550(5)   C4A    C5A    1.507(6)
C4B    C5B    1.503(6)   C4C    C5C    1.486(6)
C5A    C6A    1.531(6)   C5B    C6B    1.507(7)
C5C    C6C    1.516(6)   C6A    C7A    1.490(8)
C6B    C7B    1.476(9)   C6C    C7C    1.492(9)
C8A    C9A    1.533(7)   C8A    C10A   1.533(6)
C8A    C11A   1.502(7)   C8B    C9B    1.515(7)
C8B    C10B   1.530(6)   C8B    C11B   1.508(7)
C8C    C9C    1.515(8)   C8C    C10C   1.523(6)
C8C    C11C   1.532(7)   C12A   C13A   1.401(5)
C12A   C17A   1.411(6)   C12B   C13B   1.412(6)
C12B   C17B   1.407(6)   C12C   C13C   1.410(6)
C12C   C17C   1.403(6)   C13A   C14A   1.383(6)
C13A   C18A   1.502(6)   C13B   C14B   1.408(7)
C13B   C18B   1.487(7)   C13C   C14C   1.404(6)
C13C   C18C   1.502(6)   C14A   C15A   1.377(6)
C14B   C15B   1.372(7)   C14C   C15C   1.355(7)
C15A   C16A   1.363(6)   C15A   C19A   1.500(6)
C15B   C16B   1.376(7)   C15B   C19B   1.507(8)
C15C   C16C   1.368(7)   C15C   C19C   1.503(7)
C16A   C17A   1.396(6)   C16B   C17B   1.392(7)
C16C   C17C   1.404(6)   C17A   C20A   1.500(6)
C17B   C20B   1.506(7)   C17C   C20C   1.506(6)

TABLE 3: Experimental bond angles (deg).

Atom   Atom   Atom   Angle      Atom   Atom   Atom   Angle

O3A    S1A    O4A    118.5(2)   O3A    S1A    N2A    108.4(2)
O3A    S1A    C12A   110.8(2)   O4A    S1A    N2A    102.9(2)
O4A    S1A    C12A   110.6(2)   N2A    S1A    C12A   104.3(2)
O3B    S1B    O4B    117.6(2)   O3B    S1B    N2B    103.7(2)
O3B    S1B    C12B   110.8(2)   O4B    S1B    N2B    108.8(2)
O4B    S1B    C12B   110.5(2)   N2B    S1B    C12B   104.3(2)
O3C    S1C    O4C    118.2(2)   O3C    S1C    N2C    107.7(2)
O3C    S1C    C12C   110.1(2)   O4C    S1C    N2C    103.9(2)
O4C    S1C    C12C   111.2(2)   N2C    S1C    C12C   104.7(2)
C1A    N1A    C2A    111.9(3)   C1A    N1A    C4A    126.2(3)
C2A    N1A    C4A    120.4(3)   C1B    N1B    C2B    111.0(3)
C1A    N1B    C4B    126.4(3)   C2B    N1B    C4B    120.5(3)
C1C    N1C    C2C    111.7(3)   C1C    N1C    C4C    126.4(3)
C2C    N1C    C4C    120.5(3)   S1A    N2A    C1A    122.1(2)
S1A    N2A    C3A    124.5(2)   C1A    N2A    C3A    111.1(3)
S1B    N2B    C1B    122.6(3)   S1B    N2B    C3B    124.5(3)
C1B    N2B    C3B    111.3(3)   S1C    N2C    C1C    121.7(3)
S1C    N2C    C3C    125.4(3)   C1C    N2C    C3C    111.0(3)
O2A    C1A    N1A    128.9(3)   O2A    C1A    N2A    124.4(3)
N1A    C1A    N2A    106.7(3)   O2B    C1B    N1B    127.9(4)
O2B    C1B    N2B    124.7(4)   N1B    C1B    N2B    107.4(3)
O2C    C1C    N1C    128.0(4)   O2C    C1C    N2C    125.4(4)
N1C    C1C    N2C    106.6(3)   N1A    C2A    C3A    101.0(3)
N1A    C2A    C8A    113.5(3)   C3A    C2A    C8A    113.2(3)
N1B    C2B    C3B    101.6(3)   N1B    C2B    C8B    113.8(3)
C3B    C2B    C8B    114.6(3)   N1C    C2C    C3C    101.7(3)
N1C    C2C    C8C    114.6(3)   C3C    C2C    C8C    113.4(3)
N2A    C3A    C2A    103.3(3)   N2B    C3B    C2B    102.7(3)
N2C    C3C    C2C    102.2(3)   O1A    C4A    N1A    119.0(4)
O1A    C4A    C5A    122.7(4)   N1A    C4A    C5A    118.2(3)
O1B    C4B    N1B    119.1(4)   O1B    C4B    C5B    122.4(4)
N1B    C4B    C5B    118.4(3)   O1C    C4C    N1C    118.1(4)
O1C    C4C    C5C    123.5(4)   N1C    C4C    C5C    118.4(3)
C4A    C5A    C6A    111.4(4)   C4B    C5B    C6B    112.3(4)
C4C    C5C    C6C    112.5(4)   C5A    C6A    C7A    111.6(4)
C5B    C6B    C7B    112.3(5)   C5C    C6C    C7C    112.9(4)
C2A    C8A    C9A    110.3(4)   C2A    C8A    C10A   106.3(4)
C2A    C8A    C11A   113.9(3)   C9A    C8A    C10A   108.2(4)
C9A    C8A    C11A   108.0(5)   C10A   C8A    C11A   110.1(4)
C2B    C8B    C9B    110.9(3)   C2B    C8B    C10B   106.3(4)
C2B    C8B    C11B   112.0(3)   C9B    C8B    C10B   109.9(4)
C9B    C8B    C11B   107.9(4)   C10B   C8B    C11B   109.8(4)
C2C    C8C    C9C    110.8(4)   C2C    C8C    C10C   107.4(3)
C2C    C8C    C11C   111.8(3)   C9C    C8C    C10C   108.5(4)
C9C    C8C    C11C   108.6(5)   C10C   C8C    C11C   109.7(4)
S1A    C12A   C13A   118.1(3)   S1A    C12A   C17A   120.4(3)
C13A   C12A   C17A   121.4(4)   S1B    C12B   C13B   120.3(3)
S1B    C12B   C17B   118.3(3)   C13B   C128   C17B   121.3(4)
S1C    C12C   C13C   118.4(3)   S1C    C12C   C17C   120.7(3)
C13C   C12C   C17C   120.8(4)   C12A   C13A   C14A   117.5(4)
C12A   C13A   C18A   125.7(4)   C14A   C13A   C18A   116.8(4)
C12B   C13B   C14B   117.2(4)   C12B   C13B   C18B   127.2(4)
C14B   C13B   C18B   115.6(4)   C12C   C13C   C14C   117.8(4)
C12C   C13C   C18C   125.2(4)   C14C   C13C   C18C   117.0(4)
C13A   C14A   C15A   122.9(4)   C13B   C14B   C15B   122.4(4)
C13C   C14C   C15C   122.3(4)   C14A   C15A   C16A   118.2(4)
C14A   C15A   C19A   121.1(4)   C16A   C15A   C19A   120.6(4)
C14B   C15B   C16B   118.6(4)   C14B   C15B   C19B   120.2(5)
C16B   C15B   C19B   121.2(5)   C14C   C15C   C16C   118.9(4)
C14C   C15C   C19C   121.5(5)   C16C   C15C   C19C   119.6(4)
C15A   C16A   C17A   123.1(4)   C15B   C16B   C17B   122.8(4)
C15C   C16C   C17C   122.8(4)   C12A   C17A   C16A   116.8(4)
C12A   C17A   C20A   126.3(4)   C16A   C17A   C20A   116.9(4)
C12B   C17B   C16B   117.6(4)   C12B   C17B   C20B   125.4(4)
C16B   C17B   C20B   117.0(4)   C12C   C17C   C16C   117.2(4)
C12C   C17C   C20C   126.7(4)   C16C   C17C   C20C   116.1(4)

TABLE 4: Experimental dihedral angles (deg).

Atom   Atom   Atom   Atom      Angle

S1A    N2A    C1A    O2A      20.7(5)
S1A    N2A    C3A    C2A     144.3(3)
S1A    C12A   C13A   C18A     -4.1(5)
S1A    C12A   C17A   C20A     5.4(6)
S1B    N2B    C1B    N1B     -162.1(3)
S1B    C12B   C13B   C14B    -178.1(3)
S1B    C12B   C17B   C16B    177.0(3)
S1C    N2C    C1C    O2C      23.1(6)
S1C    N2C    C3C    C2C     142.4(3)
S1C    C12C   C13C   C18C     -5.3(6)
S1C    C12C   C17C   C20C     5.8(6)
O1A    C4A    N1A    C2A      2.6(6)
O1B    C4B    N1B    C1B     170.5(4)
O1B    C4B    C5B    C6B     -13.6(6)
O1C    C4C    N1C    C2C      4.7(6)
O2A    C1A    N1A    C2A     -167.5(4)
O2A    C1A    N2A    C3A     -175.7(4)
O2B    C1B    N1B    C4B      29.6(6)
O2C    C1C    N1C    C2C     -170.1(4)
O2C    C1C    N2C    C3C     -171.9(4)
O3A    S1A    N2A    C3A     -112.6(3)
O3A    S1A    C12A   C17A    -10.7(4)
O3B    S1B    N2B    C3B      13.6(4)
O3B    S1B    C12B   C17B     38.3(3)
O3C    S1C    N2C    C3C     -120.7(3)
O3C    S1C    C12C   C17C    -12.7(4)
O4A    S1A    N2A    C3A      13.6(4)
O4A    S1A    C12A   C17A    -144.1(3)
O4B    S1B    N2B    C3B     -112.4(3)
O4B    S1B    C12B   C17B    170.5(3)
O4C    S1C    N2C    C3C      5.4(4)
O4C    S1C    C12C   C17C    -145.6(3)
N1A    C2A    C3A    N2A      23.3(4)
N1A    C2A    C8A    C10A    171.6(4)
N1A    C4A    C5A    C6A     176.7(4)
N1B    C2B    C3B    N2B      23.2(4)
N1B    C2B    C8B    C10B    176.3(4)
N1B    C4B    C5B    C6B     162.9(4)
N1C    C2C    C3C    N2C      25.1(4)
N1C    C2C    C8C    C10C    170.3(4)
N1C    C4C    C5C    C6C     -178.1(4)
N2A    S1A    C12A   C17A    105.8(3)
N2A    C1A    N1A    C4A     -154.1(4)
N2B    S1B    C12B   C13B    106.4(3)
N2B    C1B    N1B    C2B      12.4(4)
N2B    C3B    C2B    C8B     -99.9(4)
N2C    S1C    C12C   C17C    102.9(3)
N2C    C1C    N1C    C4C     -1571(3)
C1A    N1A    C2A    C3A     -22.4(4)
C1A    N1A    C4A    C5A      -9.6(6)
C1A    N2A    C3A    C2A     -18.8(4)
C1B    N1B    C2B    C8B     100.9(4)
C1B    N2B    S1B    C12B    -65.9(3)
C1C    N1C    C2C    C3C     -22.5(4)
C1C    N1C    C4C    C5C      -78(6)
C1C    N2C    C3C    C2C     -21.9(4)
C2B    N1B    C4B    C5B     -168.1(3)
C3A    N2A    S1A    C12A    129.3(3)
C3A    C2A    C8A    C9A     168.9(4)
C3A    C2A    C8A    C11A     47.3(5)
C3B    C2B    N1B    C4B     141.7(3)
C3B    C2B    C8B    C10B    -67.5(5)
C3C    N2C    S1C    C12C    122.1(3)
C3C    C2C    C8C    C9C     168.1(4)
C3C    C2C    C8C    C11C     46.9(5)
C4A    C5A    C6A    C7A     -173.9(4)
C4B    C5B    C6B    C7B     -173.8(5)
C4C    C5C    C6C    C7C     -173.4(5)
C12A   C17A   C16A   C15A     -0.2(6)
C12B   C17B   C16B   C15B     1.2(6)
C12C   C17C   C16C   C15C     -0.9(6)
C13A   C12A   C17A   C20A    -176.8(4)
C13A   C14A   C15A   C19A    -175.9(4)
C13B   C12B   C17B   C20B    177.4(4)
C13B   C14B   C15B   C19B    177.4(5)
C13C   C12C   C17C   C20C    -177.4(4)
C13C   C14C   C15C   C19C    -175.2(4)
C14A   C15A   C16A   C17A     -2.0(6)
C14B   C15B   C16B   C17B     1.0(7)
C14C   C15C   C16C   C17C     -2.4(7)
C15A   C16A   C17A   C20A    179.4(4)
C15B   C16B   C17B   C20B    -178.5(4)
C15C   C16C   C17C   C20C    179.5(4)
C17A   C16A   C15A   C19A    175.6(4)
C17B   C16B   C15B   C19B    -178.7(4)
C17C   C16C   C15C   C19C    176.5(4)

Atom   Atom   Atom   Atom   Atom      Angle

S1A    S1A    N2A    C1A    N1A     -158.4(2)
S1A    S1A    C12A   C13A   C14A    174.9(3)
S1A    S1A    C12A   C17A   C16A    -175.1(3)
S1A    S1B    N2B    C1B    O2B      17.4(5)
S1B    S1B    N2B    C3B    C2B     148.0(3)
S1B    S1B    C12B   C13B   C18B     2.7(6)
S1B    S1B    C12B   C17B   C20B     -3.4(5)
S1C    S1C    N2C    C1C    N1C     -156.5(3)
S1C    S1C    C12C   C13C   C14C    175.0(3)
S1C    S1C    C12C   C17C   C16C    -173.8(3)
S1C    O1A    C4A    N1A    C1A     167.1(4)
O1A    O1A    C4A    C5A    C6A      0.1(6)
O1B    O1B    C4B    N1B    C2B      8.5(5)
O1B    O1C    C4C    N1C    C1C     170.2(4)
O1C    O1C    C4C    C5C    C6C      4.0(7)
O2A    O2A    C1A    N1A    C4A      26.8(6)
O2A    O2B    C1B    N1B    C2B     -167.0(4)
O2B    O2B    C1B    N2B    C3B     -176.3(4)
O2C    O2C    C1C    N1C    C4C      23.3(6)
O2C    O3A    S1A    N2A    C1A      48.6(3)
O3A    O3A    S1A    C12A   C13A    171.4(3)
O3A    O3B    S1B    N2B    C1B     178.1(3)
O3B    O3B    S1B    C12B   C13B    -142.5(3)
O3B    O3C    S1C    N2C    C1C      42.0(3)
O3C    O3C    S1C    C12C   C13C    170.4(3)
O3C    O4A    S1A    N2A    C1A     174.9(3)
O4A    O4A    S1A    C12A   C13A     38.0(3)
O4A    O4B    S1B    N2B    C1B      52.1(4)
O4B    O4B    S1B    C12B   C13B    -10.3(4)
O4B    O4C    S1C    N2C    C1C     168.2(3)
O4C    O4C    S1C    C12C   C13C     37.5(4)
O4C    N1A    C1A    N2A    C3A      5.2(4)
N1A    N1A    C2A    C8A    C9A      54.5(5)
N1A    N1A    C2A    C8A    C11A    -67.1(5)
N1A    N1B    C1B    N2B    C3B      4.2(4)
N1B    N1B    C2B    C8B    C9B      56.9(5)
N1B    N1B    C2B    C8B    C11B    -63.7(5)
N1B    N1C    C1C    N2C    C3C      8.5(4)
N1C    N1C    C2C    C8C    C9C      52.0(5)
N1C    N1C    C2C    C8C    C11C    -69.3(5)
N1C    N2A    S1A    C12A   C13A    -72.1(3)
N2A    N2A    C1A    N1A    C2A      11.6(4)
N2A    N2A    C3A    C2A    C8A     -98.4(4)
N2B    N2B    S1B    C12B   C17B    -72.7(3)
N2B    N2B    C1B    N1B    C4B     -150.9(3)
N2B    N2C    S1C    C12C   C13C    -74.1(3)
N2C    N2C    C1C    N1C    C2C      9.5(4)
N2C    N2C    C3C    C2C    C8C     -98.5(3)
C1A    C1A    N1A    C2A    C8A      99.1(4)
C1A    C1A    N2A    S1A    C12A    -69.5(3)
C1A    C1B    N1B    C2B    C3B     -22.8(4)
C1B    C1B    N1B    C4B    C5B      -6.1(6)
C1B    C1B    N2B    C3B    C2B     -18.0(4)
C1C    C1C    N1C    C2C    C8C     100.3(4)
C1C    C1C    N2C    S1C    C12C    -75.1(3)
C1C    C2A    N1A    C4A    C5A     -174.2(3)
C2B    C2C    N1C    C4C    C5C     -173.4(4)
C3A    C3A    C2A    N1A    C4A     144.2(3)
C3A    C3A    C2A    C8A    C10A    -74.0(4)
C3A    C3B    N2B    S1B    C12B    129.6(6)
C3B    C3B    C2B    C8B    C19B    173.2(4)
C3B    C3B    C2B    C8B    C11B     52.5(5)
C3C    C3C    C2C    N1C    C4C     145.0(3)
C3C    C3C    C2C    C8C    C10C    -73.5(4)
C3C    C4A    N1A    C2A    C8A     -94.3(4)
C4A    C4B    N1B    C2B    C8B     -94.7(4)
C4B    C4C    N1C    C2C    C8C     -92.3(4)
C4C    C12A   C13A   C14A   C15A     0.6(6)
C12A   C12B   C13B   C14B   C15B     1.3(6)
C12B   C12C   C13C   C14C   C15C     -1.5(6)
C12C   C13A   C12A   C17A   C16A     2.7(5)
C13A   C13A   C14A   C15A   C16A     1.8(6)
C13A   C13B   C12B   C17B   C16B     -2.2(5)
C13B   C13B   C14B   C15B   C16B     -2.3(6)
C13B   C13C   C12C   C17C   C16C     3.0(6)
C13C   C13C   C14C   C15C   C16C     3.7(7)
C13C   C14A   C13A   C12A   C17A     -3.0(5)
C14A   C14B   C13B   C12B   C17B     1.0(5)
C14B   C14C   C13C   C12C   C17C     1.9(6)
C14C   C15A   C14A   C13A   C18A    179.8(4)
C15A   C15B   C14B   C13B   C18B    -179.4(4)
C15B   C15C   C14C   C13C   C18C    178.8(4)
C15C   C17A   C12A   C13A   C18A    178.0(4)
C17A   C17B   C12B   C13B   C18B    -178.2(4)
C17B   C17C   C12C   C13C   C18C    177.7(4)

TABLE 5: Heat of formations, relative energies, dipole moments, and
selected geometric parameters for the significant conformations of 3
computed using semiempirical AM1 MO level of theory. Comparative
analysis with crystal structure 3 (a).

Property (a)      Conformer          Conformer     Conformer
                  A (b,c)            B             C

<C1-N1-C4-C5      -23.8              -24.6         -28.0
                  (-9.9, -40.8)
<C1-N2-S1-C12     -61.7              61.7          -109.1
                  (-46.2, -68.1)
<N2-S1-C12-C13    -81.5              -98.3         -97.7
                  (-71.8, -75.2)
<C3-C2-C8-C10     -65.9              -62.7         -63.9
                  (-64.9, -55.1)
<C1-N1-C2         110.0              109.9         110.1
<C1-N2-C3         108.8              108.7         108.8
N1-C4             1.410              1.410         1.41
N2-S1             1.67               1.67          1.67
S1-C12            1.68               1.68          1.69
Hf (kcal/mol)     -135.596           -135.069      -134.115
Er (kcal/mol)     0.000              0.527         1.481
Dipole (Debye)    3.57               4.45          4.90

Property (a)      Conformer     Conformer     Conformer     3A
                  D             E             F

<C1-N1-C4-C5      -24.7         156.4         137.9         -9.6

<C1-N2-S1-C12     136.3         -45.7         -107.7        -69.5

<N2-S1-C12-C13    -81.2         -80.3         83.7          -72.1

<C3-C2-C8-C10     -62.5         -66.8         -59.9         -74.0

<C1-N1-C2         109.9         109.0         110.7         111.9

<C1-N2-C3         108.8         108.7         108.8         111.1

N1-C4             1.41          1.41          1.41          1.408

N2-S1             1.67          1.67          1.66          1.777

S1-C12            1.69          1.67          1.69          1.671

Hf (kcal/mol)     -132.739      128.334       127.122       --
Er (kcal/mol)     2.857         7.262         8.474         --
Dipole (Debye)    6.35          5.06          9.01          --

Property (a)      X-ray      3C

<C1-N1-C4-C5      -6.1       -7.8

<C1-N2-S1-C12     -65.9      -75.1

<N2-S1-C12-C13    -72.7      -73.5

<C3-C2-C8-C10     -67.5      -73.5

<C1-N1-C2         111.0      111.0

<C1-N2-C3         111.3      111.0

N1-C4             1.401      1.413

N2-S1             1.664      1.669

S1-C12            1.773      1.768

Hf (kcal/mol)     --         --
Er (kcal/mol)     --         --
Dipole (Debye)    --         --

(a) All values correspond to fully optimized geometries.

(b) Relative energies for the three similar conformations resulted from
the separate conformational analysis of 3A, 3B, and 3C around the N1-C4
and N2-S1 bonds: 0.000, 0.110, and 0.05 kcal/mol, respectively.

(c) Values in bold, plain, and italic text corresponding to MM+, AM1,
and PM3 geometry optimization, respectively.
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Title Annotation:Research Article
Author:Swaidan, Ibrahim A. Al-; Azab, Adel S. El-; Alanazi, Amer M.; Abdel-Aziz, Alaa A.-M.
Publication:Journal of Chemistry
Article Type:Report
Date:Jan 1, 2014
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