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Possibilities of cyclization of side alkyl chains of n-alkylphenols and n-alkylbenzenes in the environment of a stationary phase.

1. INTRODUCTION

Gas chromatographic measurements of relative retention times ([R.sub.t,rel]) of n-alkylphenols and n-alkylbenzenes [C.sub.7]-[C.sub.12] on capillary columns with both polar and nonpolar stationary phases at temperatures of 125-140 [degrees]C showed that these retention times increase non-linearly with the number of carbon atoms in a molecule (z) (Fig. 1) (Buryan and Macak, 1982). Two linear areas in the consecutive intervals [C.sub.7]-[C.sub.9] and [C.sub.9]-[C.sub.12], with the slopes of each line being different, were proved in the relation log [R.sub.t,rel] = az + b (where a and b are constants), although only one line was expected to be found. The complete dependence was thus of a divergent character with break (Fig. 1). This dependence was under the mentioned conditions also found in the case of n-alkylphenol methylethers (Buryan and Macak, 1982).

In classic considerations on the dependence of retention times on the number of carbons in different homologous series of gas chromatographically separated substances, purely linear dependence without any divergence is always considered, be it for isotherm or non-isotherm separations (Purnell, 1962; Leibnitz and Struppe, 1966; Hartus and Habgood, 1966).

However, the divergence detected in the dependence log [R.sub.t,rel] = az + b suggests, that the behavior of molecules of the compounds in question during separations in stationary phases is more complicated than has been assumed by the classic considerations. The relation log [R.sub.t,rel] = a + bz should be expected to be purely linear as a result of the growing number of carbon atoms in the molecule (Simpson, 1970) and the corresponding increase of [R.sub.t,rel] or more precisely log [R.sub.t,rel] with boiling points ([T.sub.b]) of the given homologs (log [R.sub.t,rel] = a + b[T.sub.b]). The mentioned purely linear relation was frequently applied when identifying unknown substances using data acquired through gas chromatographic separations, but the validity of this usage is questionable.

[FIGURE 1 OMITTED]

In the study (Buryan and Macak, 1982), the relative retention times were expressed as a logarithm of the ratio of the retention time of the given n-alkylphenol substituted in the positions ortho, meta and para and phenol, and in the case of n-alkylbenzenes as a logarithm of the ratio of the retention time of n-alkylbenzene and benzene. The mentioned divergence (break) in the increase of retention characteristics was always observed at [C.sub.9].

In our work, the following possibilities were considered as an explanation of this phenomenon.

* Cyclization. In a high density stationary phase, the longer alkyl side chains of n-alkylphenols and n-alkylbenzenes are subject to cyclization as a result of the resistance force of this phase affecting molecules of the compounds during their thermal and diffusion motion. Consequently, common conventional aromatic-aliphatic molecules become new molecules with quasi-alicyclic parts. It is likely that in comparison with the conventionally conceived molecules, the resulting aromatic-quasi-alicyclic molecules are characterized by rather different, possibly even completely different non-covalent interactions between the molecules (chiefly van der Waals interactions), which then affect the retention characteristics.

* Association. The molecules of the compounds in question are of an aromatic-aliphatic character. Such molecules tend, in a dense environment (e.g. in organic gels), to form molecular aggregates, as was discovered during the research on the formation of coal structures (Straka et al., 2002; Straka, 2003). A similar phenomenon may occur in the environment of a dense stationary phase. The molecules in question may associate, which would lead to a change in retention characteristics. The association could also increase with the growing length of the side alkyl chain and might strongly manifest itself in the case of molecules containing more than 9 carbons.

* Non-covalent interactions with a stationary phase. With the increasing number of carbons in the molecule of n-alkylphenols and n-alkylbenzenes, intermolecular interactions between compounds in question and the stationary phase might change non-linearly with the growing number of carbons in the molecule. These interactions may manifest themselves differently in polar and nonpolar phases.

The mentioned phenomena can be evaluated and compared using methods of computational chemistry. In order to assess cyclization, association and different interactions, energies of covalent bonds and non-covalent interactions (mainly van der Waals) can be computed. For the purpose of this assessment, molecular conformations which are realistic and energetically advantageous or possible and are in correlation with the ascertained chromatographic data can also be estimated.

On the basis of experience with the evaluation of aromatic, aromatic-aliphatic, aromatic-alicyclic and phenolic structures in terms of energy using the methods of molecular and quantum mechanics (Carlson, 1991), two methods of molecular mechanics were selected for the calculations (see below).

The aim of the presented work is to explain the divergence (break) in the logarithmic dependence of relative retention time on the number of carbons in the molecule in the case of n-alkylphenols and n-alkylbenzenes by utilisation of computational methods of molecular mechanics and to clarify the behavior of these compounds in the environment of a stationary phase of a capillary column. Further, to describe a cyclization of side alkyl chains in a stationary phase as a new phenomenon.

2. CALCULATIONS

For considerations the retention data obtained on a nonpolar stationary phase of Apiezion K and a polar stationary phase of trixylenylphosphate-phosphoric acid (95:5) at temperature of 130 [degrees]C (Buryan and Macak, 1982) were taken into account.

For calculations the molecular mechanics methods were chosen and two force fields, MM+ and AMBER (Becker and Allinger, 1982; Allinger and Yuh, 1982; Howard et al., 1994), were used. The reason is that these methods use an analytical and relatively simple potential energy functions for describing the interactions between a set of atoms, further, they are empirical and accurate and very suitable for small organic molecules. Important is that atom types, not atoms, are the fundamental basis for calculating interactions. In these methods the interaction potential describes both bonding and non-bonding interactions. In the potentials the following energetic terms were calculated:

* bond stretching ([E.sub.bond]), which is associated with deformation of a bond from its standard equilibrium length,

* bond angle bending ([E.sub.angle]), which is associated with the deformation of an angle from its normal value,

* stretch-bend ([E.sub.stretch-bend]); bond stretch and angle bending cross term, which includes coupling between bond stretching and angle bending,

* dihedrals ([E.sub.dihedral]); torsional energy, which is associated with the tendency of dihedral angles to have a certain n-fold symmetry and to have minimum energy,

* van der Waals ([E.sub.vdWaals]), which describes the repulsive forces keeping two non-bonded atoms apart at close range and attractive forces drawing them together at long range,

* electrostatic ([E.sub.elst]), which describes the classical non-bonded electrostatic interactions, particularly dipole-dipole interactions.

These energetic terms were calculated both by MM+ and AMBER methods, except [E.sub.stretch-bend], which was calculated only by means of the MM+.

The mentioned potential energies of covalent bonds and non-covalent interactions were calculated for common (conventional) n-alkylphenols and n-alkylbenzenes and also for models of cyclized forms of these compounds. The conceptions of the cyclized forms were formulated on the basis of the study of distribution of electron densities (atomic charges in a molecule) in common and cyclized molecules. As expected, in n-alkylphenol molecules, a high electron density was detected on the oxygen of the hydroxyl group and a very low electron density on the hydrogens of the terminal methyl group. The closing of the cyclanic ring was thus easily implemented by a hydrogen bridge as shown in Fig. 2 a. The conception of the cyclized form of alkylbenzenes was more complicated. The electron density on the alkyl carbon in the [alpha] position with respect to the benzene ring was discovered to be considerably higher in the case of the cyclized form than on the other carbons and, especially, on hydrogen atoms of the methyl group in an alkyl chain. The closing of the cyclanic ring was thus implemented by an interaction of methyl hydrogen with the ascribed charge +[delta] and the benzene ring with the ascribed charge -[delta] (Fig. 2 b). The basic conceptions of cyclized forms and models of these forms, for the sake of calculations demonstrated on n-propylphenol and n-propylbenzene, are also shown in Figs. 2 c, d, namely for the case of ortho n-propylphenol and alicyclic n-propylbenzene. In the latter case, the cyclization occurs until it reaches the position neighboring the position of a propyl substituent (i.e. n-propylbenzene is cyclized to the position ortho with respect to the propyl). The cyclization here is realized on the terminal methyl group (-C[H.sub.3] cyclization).

[FIGURE 2 OMITTED]

Besides the-C[H.sub.3] case, cyclization was also considered and calculated for the group -C[H.sub.2]-neighboring C[H.sub.3]-(i.e. in the position a with respect to C[H.sub.3]-, Fig. 3, -C[H.sub.2]-[alpha] cyclization) and for the group -C[H.sub.2]-in the position [beta] with respect to the group C[H.sub.3]-(Fig.4,-C[H.sub.2]-[beta] cyclization). These cyclizations were considered for molecules from methyl- up to hexylphenol and from methyl- up to hexylbenzene. With n-alkylphenols and n-alkylbenzenes the cyclization was considered and calculated for the positions ortho, meta and para; the mentioned energy terms were calculated for 252 cases altogether. The calculated energies of covalent bonds and non-covalent interactions for common and model cyclic forms were compared, and it was ascertained that energy changes are related to the determined retention times.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The program HyperChem also enables to perform measurements of the distances between atoms of defined molecules. This possibility was utilized for the measurement and comparison of the maximum sizes of common and cyclized molecules of the examined compounds. The part of the program, Atomic Charges, was also used for the study of distribution of electron densities in the molecules of the observed compounds.

3. RESULTS AND DISCUSSION

With regard to the temperature of the chromatographic column (125-140 [degrees]C), a possibility of side-chain cyclization was considered first. This possibility was based on the conception that in high-density stationary phase, the longer side alkyl chains of n-alkylphenols and n-alkylbenzenes are subject to deformation as a result of resistance of this phase affecting molecules of these compounds in motion. Conventional aromatic-aliphatic molecules are thus transformed into aromatic-quasi-alicyclic molecules. Cyclization is then accompanied by decrease in the effective size of molecules, which is significant for [C.sub.9] and larger molecules. Aromatic-quasi-alicyclic molecules of a smaller size are more easily mixed with the dense stationary phase, and the formed system is, in comparison with a system with common aromatic-aliphatic molecules, more homogenous and thus thermodynamically more stable. The reduction of the maximum size ([D.sub.max]) of molecules during the cyclization of the side chain is shown in Table 1. The change in size of molecules moving through a chromatographic column is then accompanied by changes in intermolecular interactions and subsequently in change in relative retention time.

Calculation results for the energies of covalent bonds and non-covalent interactions for ortho n-alkylphenols (common, cyclized through C[H.sub.3]-, cyclized as -C[H.sub.2]-[alpha] and -C[H.sub.2]-[beta]) and n-alkylbenzenes both common and cyclized into the position ortho with respect to the substituent (again cyclization through C[H.sub.3]-, as -C[H.sub.2]-[alpha] and -C[H.sub.2]-[beta] are summarized in Tables 2, 3 and 4. From these tables it is evident that cyclization hardly brings any important changes in covalent bonds (only small changes in electrostatic interactions and some expected changes in the [E.sub.angle] term). However, substantial changes in van der Waals interactions between nonbonded atoms inside molecules took place. Changes in van der Waals forces inside the cyclized molecules must also be reflected in changes of these forces between molecules. The dependence of van der Waals forces on the number of carbons (z), demonstrated in Figs. 5 and 6, is of the same (divergent) character as the detected dependence 1 of logarithms of retention times on z (Fig. 1). This finding is in accordance with the fact that the same dependences were detected both in cases of polar and nonpolar phases and also in the cases of alkylphenols and alkylbenzenes, because retention data and their changes are in the given case related with intermolecular forces rather than with the structure of the considered compounds.

The same results have been obtained in the case of ortho, meta and para n-alkylphenols cyclized on the group -C[H.sub.3] and even in the case of n-alkylbenzenes, cyclized on the group -C[H.sub.3] as well to ortho, meta and para positions with respect to the substituent. Another example is shown in Fig. 7. Therefore, attention was focused on systematic calculations of the energies of van der Waals interactions. Numerical values of these energies show strength of van der Waals forces and results are summarized in Tables 5 and 6. From the data in these tables it is evident that in all the cases the divergence in the size of these interactions occurs at z = 9 (to be more specific when phenol or benzene are substituted by n-propyl). However, this result was ascertained only for alkylphenols and alkylbenzenes cyclized on the terminal methyl group. In the case of -C[H.sub.2]-[alpha] and -C[H.sub.2]-[beta] cyclizations, the detected [E.sub.vdWaals] values were similar to common n-alkylphenols and n-alkylbenzenes or no relevant dependences on z were found.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

An increase in van der Waals interactions inside the cyclized molecules (i.e. intramolecularly) must also be reflected in an increase of forces between molecules (i.e. intermolecularly). More intense effect of attraction forces between molecules should then manifest itself through a corresponding increase of boiling temperatures of the [C.sub.9]-compounds and higher in both types of molecules in question. The increase of intermolecular forces caused by cyclization was confirmed by comparison of boiling points of normal and cyclic hydrocarbons [C.sub.3]-[C.sub.8]. The results are shown in Table 7 (the values of boiling points were taken from the compendium (Spravotschnik Chimika, 1951) and verified against the compendium (Vecera et al., 1975)). From the values shown in Table 7 it is evident that boiling points of cyclic hydrocarbons increase with the number of carbons z in the molecule non-linearly, divergently, similarly to van der Waals forces and, consequently, relative retention times. Due to obvious importance of the dependence of the [T.sub.b] values on z in the case of normal and cyclic hydrocarbons, this dependence is graphically depicted in Fig. 8 as a difference of boiling points ([DELTA][T.sub.b]) of these hydrocarbons in dependence on z.

If a change in the relative retention time as a result of cyclization is caused by a change in the size of the molecule moving in a chromatographic column and a change in both intramolecular and intermolecular van der Waals forces, then the related cyclization of alkyl chain could really be, in the case of n-alkylphenols, accompanied by formation of a hydrogen bridge between the oxygen in the phenolic group -OH and a hydrogen of the terminal group -C[H.sub.3] of the alkyl; in other words, in the case of n-alkylphenols, the tendency to form hydrogen bridges resulting in a heterocyclic ring with six or more members being formed including also one C-C bond of the aromatic ring of phenol as shown in Fig. 2 could actually be considered. Similarly, in the case of alkylbenzenes, a tendency to integrate could exist between the benzene ring (-[delta]) and a hydrogen of the terminal group of the -C[H.sup.3] alkyl (+[delta]), resulting in a formation of a five-membered or more-membered ring, including also one C-C bond of the aromatic ring of benzene (Fig. 2).

[FIGURE 8 OMITTED]

The question of association. The molecules of the compounds in question are of an aromatic-aliphatic character. As already mentioned above, in the environment of a dense stationary phase, molecular aggregates may be formed. The molecules in question can thus associate in the separation zone, which would lead to a change in retention characteristics. It is likely to be that the association would increase with the increasing length of the side n-alkyl chain increasing and would strongly be manifested in the case of molecules with a number of carbons higher than 9. Therefore, the close association was investigated both in the case of n-alkylphenols and n-alkylbenzenyes for the system of two molecules. This was done by calculating the energies of van der Waals interactions ([E.sub.vdWaals]) using the above-mentioned methods after the two common molecules (i.e. without the side chains being cyclized) were conformed until the state of the energy minimum was reached. The calculated energies were then compared with the energies of van der Waals interactions in two non-associated (isolated) molecules. The data summary is in Table 8.

From the data in Table 8 it can be seen that intensive nonbonding physical interactions occur in the association of two molecules of n-alkylphenol or n-alkylbenzene. These interactions can determine the elution/retention behavior of molecules when they are moving through the chromatographic column. However, unlike in the case of cyclized forms no dependence of the [E.sub.vdWaals] values on the length of the n-alkyl chain was detected. Moreover, if we take into consideration that the measurements were taken at temperatures 125-140 [degrees]C, the close association of the molecules is unlikely.

The question of non-covalent interaction with the stationary phase. As has been already suggested, in the case of both the examined n-alkylphenols and n-alkylbenzenes, an increase in the number of carbons in the molecule may be accompanied by a change in the intermolecular interaction between a non-conventional compound and the stationary phase. A consequence of the non-linear changes in these interactions would also be a non-linear progression of retention times. However, these interactions would have to change differently in the case of polar and nonpolar stationary phases, with the dependence of the elution/retention times on the number of carbons consequently being of a completely different character for each type of the phase. Nevertheless, this was not observed. As shown in Fig. 9, the character of the dependences is the same, measured in the case of alkylbenzenes for both the polar and nonpolar phases. It seems that not even non-covalent interactions with the stationary phase are the cause of the observed progression of retention times.

Therefore, it is the cyclized form of the considered compounds that is preferred. The preference for the cyclized form arises also from the fact that cyclanic-aromatic ethers with five- and six-membered cyclanic rings with oxygen and an interconnected aromatic ring (Fig. 2) (which are thermodynamically stable compounds) are formed. (In the case of para methylphenol, higher values of some covalent energies were calculated due to higher strain of bonds, however, no values on principle eliminating a hypothetical cyclization were found).

[FIGURE 9 OMITTED]

From the thermodynamical aspect, it is the affinity of the low-molecular compounds that is important in the given context in question with the stationary high-molecular phase when they are being mixed with this phase. The degree of affinity is the change in Gibbs energy of mixing when the molecules of alkylphenol or alkylbenzene are blended with high-molecular chains of the stationary phase ([DELTA][G.sub.mix]) at constant temperature and pressure:

[DELTA][G.sub.mix] = [DELTA][H.sub.mix] - T [DELTA][S.sub.mix] < 0 (1)

where [DELTA][H.sub.mix] is the enthalpy of mixing, [DELTA][S.sub.mix] the entropy of mixing and T the temperature of the GC separation (K). The more negative [DELTA][G.sub.mix] is, the better the mixing of the compounds will be (in question with the stationary phase). Since the entropy of the system always increases when the components are mixed, and the entropic term of the equation (1) is thus always negative (-T [DELTA][S.sub.mix] < 0), the mixing/solubility depends mainly on the value [DELTA][H.sub.mix]. The thermodynamic condition of the solubility of a low-molecular element in the stationary phase, or the mixing of the two substances is then:

[DELTA][H.sub.mix] < T [DELTA][S.sub.mix]. (2)

The blending will thus be the best in the case [DELTA][H.sub.mix] = 0, when also the solubility of one component in the other will be maximal as well. However, this is an ideal case. Parameters of solubility [delta] were introduced for practical purposes, numerically characterizing solubility of low-molecular and high-molecular substances (Hildebrand and Scott, 1959). A low-molecular substance a solubility parameter of which will be identical to the solubility parameter of the high-molecular substance will achieve maximal dissolution during the mixing, because [DELTA][H.sub.mix] = 0 in this case. Since cyclization increases the [delta] in the case of hydrocarbons, e.g. in the case of hexane-cyclohexane from 15.1 (hexan) to 16.8 (cyklohexan) (Brandrup and Immergut, 1975), the mixing is improved, because the parameter for high-molecular stationary phases (Mleziva, 1993) is approximately 16-18. Cyclization thus facilitates the mixing of alkylphenols or alkylbenzenes with high-molecular stationary phases, because the thermodynamic condition for mixing is better fulfilled.

On the whole, the data obtained can serve both an analytical methodology for the analysis of aromatics and phenolics, which is still topical (Naczk and Shahidi, 2004), and a deeper insight into the problem of van der Waals forces/non-covalent interactions which is also actual (Hobza et al., 2006).

4. CONCLUSION

Side alkyl chains of n-alkylphenols become cyclized in both polar and nonpolar stationary phases of capillary columns, with a possible formation of hydrogen bridges between the oxygen of the phenolic -OH group and a hydrogen of the methyl group of the side alkyl chain. In the case of n-alkylbenzenes, cyclization is made possible due to the interaction between the benzene ring and a hydrogen of the terminal methyl group of the alkyl. In the case of the formed aromatic-quasi-alicyclic molecules, the effect of van der Waals forces thus increases not only intramolecularly but also intermolecularly, with a consequent increase in boiling points, mainly in the case of n-alkylphenols and n-alkylbenzenes with the number of carbons in a molecule higher than 9. This results in a divergence in the retention characteristics of the mentioned compounds observed in the dependence of the logarithm of the relative retention time on the total number of carbons in the molecule. Cyclization of side alkyl chains in a dense stationary phase is a quite new phenomenon.

ACKNOWLEDGEMENT

Grant Agency of the Academy of Sciences of the Czech Republic supported this work as the project No. IAA300460702 and Institute Research Plan Ident. Code AVOZ30460519; further, support from Ministry of Education, Youth and Sports (MSM 604 613 7304) is acknowledged.

REFERENCES

Allinger, N. L. and Yuh, Y.H.: 1982, Quantum Chemistry Program Exchange. Indiana University, Bloomington.

Becker, U. and Allinger, N.L.: 1982, Molecular Mechanics. American Chemical Society, Monograph 177, Washington D.C.

Brandrup, J. and Immergut, E.H.: 1975, Polymer Handbook. Willey, New York.

Buryan, P. and Macak, J.: 1982, Partial explanation of the anomaly in the relationship between the logarithm of retention and the carbon number of monohydric phenols, J. Chromatogr. 237, 381-388.

Carlson, G.A.: 1991, Computer studies of coal molecular structure. In: Proceedings-1991 International Conference on Coal Science, September 16-20, Ed. the IEA Agency, Newcastle upon Tyne, 24-27.

Hartus, W.E. and Habgood, H.W.: 1966, Programmed temperature gas chromatography. J. Wiley & Sons, New York-London-Sydney.

Hildebrand, J. H. and Scott, R.L.: 1959, Solubility of Non-Electolytes. Reinhold Publ. Co., New York.

Hobza, P., Zahradnik, R. and Muller-Dethlefs, K.: 2006, The world of non-covalent interactions: 2006, Collect. Czech. Chem. Commun. 71, 443-531.

Howard, A., McIver, J. and Collins, J.: 1994, HyperChem Computational Chemistry. Publ. No. 40-00-03-00, Hypercube Inc., Waterloo (Ontario-Canada).

Leibnitz, E. and Struppe, H.G.: 1966, Handbuch der Gas-Chromatographie. AVG & Portik K.-G., Lepzig.

Mleziva, J.: 1993, Polymers. Sobotales, Praha, (in Czech).

Naczk, M. and Shahidi, F.: 2004, Extraction and analysis of phenolics in food, J. Chromatogr. A 1054, 95-111.

Purnell, H.: 1962, Gas Chromatography. J. Wiley & Sons, New York-London-Sydney.

Simpson, C.: 1970, Gas Chromatography. Kogan Page, London.

Spravotschnik Chimika: 1951, Basic properties inorganic and organic compounds. GNT Izd. Chim. Lit., Moskva. (in Russian).

Straka, P.: 2003, Chemical structure of coal substance, Acta Montana, Series AB, No. 12(132), 7-47.

Straka, P., Brus, J. and Endrysova, J.: 2002, Solid-state NMR spectroscopy of Ostrava-Karvina coals, Chemical Papers 56, 182-187.

Vecera, M., Gasparic, J., Churacek, J. and Borecky, J.: 1975, Chemical tables of organic compounds. SNTL, Praha, (in Czech).

Pavel STRAKA (1) * Petr BURYAN (2) and Jana NAHUNKOVA (1)

(1) Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, v.v.i. V Holesovickach 41, 182 09 Prague 8, Czech Republic

(2) Institute of Chemical Technology, Technicka 5, Prague 6, 166 28 Prague 6, Czech Republic

* Corresponding author's e-mail: straka@irsm.cas.cz

(Received August 2007, accepted October 2007)
Table 1 Reduction of the maximum molecular
size (Dmax) during the cyclization of the
side chain (comparison of Dmax values for
common and cyclized forms of n-alkylphenols).

Molecule                      [D.sub.max] (nm)
                           Position of substituent

                           ortho    meta   para

Common:
Methylphenol                0.56    0.59   0.67
Ethylphenol                 0.72    0.72   0.80
n-Propylphenol              0.82    0.83   0.91
n-Butylphenol               0.93    0.95   1.04
n-Pentylphenol              1.06    1.08   1.15
n-Hexylphenol               1.19    1.21   1.28

Cyclized on -C[H.sub.3]:
Methylphenol                0.58    0.49   0.47
Ethylphenol                 0.65    0.55   0.48
Propylphenol                0.72    0.61   0.57
Butylphenol                 0.72    0.66   0.54
Pentylphenol                0.80    0.70   0.61
Hexylphenol                 0.78    0.71   0.73

Table 2 Potential energies of covalent bonds and
non-covalent interactions in ortho n-alkylphenols
(common, cyclized on C[H.sub.3]-, cyclized as
-C[H.sub.2]-[alpha] and -C[H.sub.2]-[beta]).
Calculated by the MM+ program. Energetic terms:
[E.sub.bond]-bond stretching, [E.sub.angle]-bond
angle bending, [E.sub.stretch-bend]-stretch-bend
(bond stretch and angle bending cross term),
[E.sub.dihedral]-torsional energy, [E.sub.vdWaals]-van
der Waals  interactions, [E.sub.els]-the classical
non-bonded electrostatic interactions (particularly
dipole-dipole interactions). For [D.sub.max] see
Table 1.

n-Alkylphenols   [E.sub.bond]   [E.sub.angle] [E.sub.stretch-bend]
                                                    (kJ/mol)

Phenol               4.57           9.43             -0.443

Common:

Methylphenol         6.90           9.68             -0.589
Ethylphenol          8.80           9.68             -0.577
n-Propylphenol      10.70           9.68             -0.577
n-Butylphenol       12.60           9.68             -0.573
n-Pentylphenol      14.50           9.68             -0.568
n-Hexylphenol       16.40           9.69             -0.568

Cyclized on C[H.sub.3]:

Methylphenol        37.06         232.19             -1.986
Ethylphenol         12.93          24.32              0.180
Propylphenol        11.09           4.56             -0.042
Butylphenol         12.81           4.82             -0.004
Pentylphenol        14.53           4.22             -0.004
Hexylphenol         16.67           4.18             -0.230

Cyclized as
  -C[H.sub.2]-[alpha]:

Ethylphenol         12.93          24.32              0.180
Propylphenol        14.83          24.14              0.155
Butylphenol         16.73          24.15              0.159
Pentylphenol        18.63          24.15              0.159
Hexylphenol         20.53          24.15              0.163

Cyclized as
  -C[H.sub.2]-[beta]:

Propylphenol        14.83          24.14              0.155
Butylphenol         12.83           4.37             -0.050
Pentylphenol        14.71           4.36             -0.046
Hexylphenol         16.64           4.37             -0.046

n-Alkylphenols   [E.sub.dihedral]   [E.sub.vdWaals]   [E.sub.elst]

Phenol                -23.32             12.11          0

Common:

Methylphenol          -31.35             17.12           0.205
Ethylphenol           -31.94             41.25           0.920
n-Propylphenol        -31.94             44.24           0.920
n-Butylphenol         -31.94             47.23           0.920
n-Pentylphenol        -31.94             50.40           0.920
n-Hexylphenol         -31.94             53.58           0.920

Cyclized on C[H.sub.3]:

Methylphenol          -16.79             11.39           0.004
Ethylphenol           -16.75             12.09           0.786
Propylphenol          -23.86             27.61           0.334
Butylphenol             2.26             49.25           0.146
Pentylphenol            8.44            210.20           0.247
Hexylphenol             4.29                -            0.096

Cyclized as
  -C[H.sub.2]-[alpha]:

Ethylphenol           -16.75             12.09           0.786
Propylphenol          -17.14             14.22           0.786
Butylphenol           -15.67             33.18           0.786
Pentylphenol          -15.68             38.08           0.786
Hexylphenol           -15.68             40.93           0.786

Cyclized as
  -C[H.sub.2]-[beta]:

Propylphenol          -17.14             14.22           0.786
Butylphenol           -23.86             29.67           0.334
Pentylphenol          -21.73             41.34           0.334
Hexylphenol           -21.76             44.21           0.334

n-Alkylphenols   [D.sub.max]
                    (nm)

Phenol              0.56

Common:

Methylphenol        0.56
Ethylphenol         0.72
n-Propylphenol      0.82
n-Butylphenol       0.93
n-Pentylphenol      1.62
n-Hexylphenol       1.93

Cyclized on C[H.sub.3]:

Methylphenol        0.58
Ethylphenol         0.65
Propylphenol        0.72
Butylphenol         0.72
Pentylphenol        0.79
Hexylphenol         0.78

Cyclized as
  -C[H.sub.2]-[alpha]:

Ethylphenol         0.65
Propylphenol        0.72
Butylphenol         0.82
Pentylphenol        0.88
Hexylphenol         0.94

Cyclized as
  -C[H.sub.2]-[beta]:

Propylphenol        0.72
Butylphenol         0.81
Pentylphenol        0.89
Hexylphenol         0.99

Table 3 Potential energies of covalent bonds and
non-covalent interactions in meta n-alkylphenols
(common, cyclized on [CH.sub.3]-, cyclized as
-[CH.sub.2]-[alpha] and -[CH.sub.2]-[beta]).
Calculated by the MM+ program. For symbols see
Table 2.

n-Alkylphenols    [E.sub.bond]   [E.sub.angle]   [E.sub.stretch-bend]
                                                       (kJ/mol)

Phenol                4.57           9.43               -0.443

Common:
Methylphenol          6.90           9.68               -0.589
Ethylphenol           8.80           9.68               -0.577
n-Propylphenol       10.70           9.68               -0.577
n-Butylphenol        12.60           9.68               -0.573
n-Pentylphenol       14.50           9.68               -0.568
n-Hexylphenol        16.40           9.69               -0.568

Cyclized on C[H.sub.3]:

Methylphenol         10.26           38.69              -0.961
Ethylphenol          13.81           36.63              -3.064
Propylphenol         29.75           64.02               3.703
Butylphenol          31.13           59.18               1.726
Pentylphenol         19.09           29.00               0.581
Hexylphenol          16.64            4.66              -0.038

Cyclized as
  -C[H.sub.2]-[alpha]:

Ethylphenol          13.81           36.63              -3.064
Propylphenol         15.72           36.38              -0.380
Butylphenol          17.62           36.38              -0.376
Pentylphenol         19.52           36.38              -0.376
Hexylphenol          21.42           36.38              -0.372

Cyclized as
  -C[H.sub.2]-/[beta]:

Propylphenol         29.75           64.02               3.703
Butylphenol          31.66           63.70               3.478
Pentylphenol         33.56           63.71               3.478
Hexylphenol          35.45           63.71               3.482

n-Alkylphenols    [E.sub.dihedral]   [E.sub.vdWaals]   [E.sub.elst]

Phenol                 -23.32              12.11            0

Common:
Methylphenol           -26.33              14.57          0.113
Ethylphenol            -26.92              39.44          0.113
n-Propylphenol         -26.92              42.49          0.113
n-Butylphenol          -26.92              45.48          0.113
n-Pentylphenol         -26.92              48.66          0.113
n-Hexylphenol          -26.92              51.84          0.113

Cyclized on C[H.sub.3]:

Methylphenol           389.88              32.45          0.002
Ethylphenol            369.97              95.81          0.493
Propylphenol           243.10             385.13          0.159
Butylphenol             -1.34            3173.33          0.054
Pentylphenol           -13.30            2941.51          0.042
Hexylphenol            -10.80            8006.79          0.054

Cyclized as
  -C[H.sub.2]-[alpha]:

Ethylphenol            369.97              95.81          0.493
Propylphenol           371.00             140.79          0.493
Butylphenol            373.11            1252.08          0.493
Pentylphenol           373.11            1250.91          0.493
Hexylphenol            373.11            1252.79          0.493

Cyclized as
  -C[H.sub.2]-/[beta]:

Propylphenol           243.10             385.13          0.159
Butylphenol            243.36             388.27          0.159
Pentylphenol           245.48             397.56          0.159
Hexylphenol            245.48             400.36          0.159

n-Alkylphenols    [D.sub.max]
                     (nm)

Phenol               0.56

Common:
Methylphenol         0.59
Ethylphenol          0.72
n-Propylphenol       0.83
n-Butylphenol        0.95
n-Pentylphenol       1.08
n-Hexylphenol        1.21

Cyclized on C[H.sub.3]:

Methylphenol         0.49
Ethylphenol          0.55
Propylphenol         0.61
Butylphenol          0.66
Pentylphenol         0.69
Hexylphenol          0.71

Cyclized as
  -C[H.sub.2]-[alpha]:

Ethylphenol          0.55
Propylphenol         0.66
Butylphenol          0.66
Pentylphenol         0.77
Hexylphenol          0.81

Cyclized as
  -C[H.sub.2]-/[beta]:

Propylphenol         0.61
Butylphenol          0.71
Pentylphenol         0.75
Hexylphenol          0.85

Table 4 Potential energies of covalent bonds
and non-covalent interactions in n-alkylbenzenes
cyclized into the position meta with respect
to the substituent (common, cyclized on
C[H.sub.3]-, cyclized as -C[H.sub.2]-[alpha]
and -C[H.sub.2]-[beta]). Calculated by the MM+
program. For symbols see Table 2.

n-Alkylbenzenes   [E.sub.bond]   [E.sub.angle]   [E.sub.stretch-bend]
                                                       (kJ/mol)

Benzene              4.573           9.434              -0.443

Common:

Methybenzene          7.060           0.247             -0.142
Ethylbenzene          8.958           0.247             -0.130
n-Propylbenzene      10.860           0.247             -0.130
n-Butylbenzene       12.757           0.247             -0.125
n-Pentylbenzene      14.659           0.247             -0.125
n-Hexylbenzene       16.561           0.247             -0.125

Cyclized on C[H.sub.3]:

Methylbenzene         7.829         123.134             -3.582
Ethylbenzene         10.266          29.465             -2.274
Propylbenzene        12.858          21.611             -1.058
Butylbenzene         27.337          50.051              2.479
Pentylbenzene        30.443          49.667              0.982
Hexylbenzene         17.217           5.304             -0.117

Cyclized as
  -C[H.sub.2]-[alpha]:

Ethylbenzene         10.266          29.465             -2.274
Propylbenzene        12.344          29.565             -2.182
Butylbenzene         14.241          29.565             -2.182
Pentylbenzene        16.143          29.565             -2.178
Hexylbenzene         18.045          29.565             -2.174

Cyclized as
  -C[H.sub.2]-[beta]:

Propylbenzene        12.858          21.611             -1.058
Butylbenzene         14.755          21.657             -1.058
Pentylbenzene        16.657          21.657             -1.053
Hexylbenzene         18.555          21.657             -1.049

n-Alkylbenzenes   [E.sub.dihedral]   [E.sub.vdWaals]   [E.sub.elst]

Benzene               -23.324            12.114           0

Common:

Methybenzene          -26.334             14.852          0
Ethylbenzene          -26.919             39.727          0
n-Propylbenzene       -26.919             42.778          0
n-Butylbenzene        -26.919             45.771          0
n-Pentylbenzene       -26.919             48.956          0
n-Hexylbenzene        -26.919             52.137          0

Cyclized on C[H.sub.3]:

Methylbenzene         194.742             41.503          0
Ethylbenzene          292.847             34.230          0.560
Propylbenzene         305.633             54.081          0.242
Butylbenzene          185.379            257.722          0.159
Pentylbenzene          -2.504           3102.939          0.134
Hexylbenzene           -0.573               -             0.125

Cyclized as
  -C[H.sub.2]-[alpha]:

Ethylbenzene          292.847             34.230          0.560
Propylbenzene         294.372             51.493          0.560
Butylbenzene          297.114            456.540          0.560
Pentylbenzene         297.114            457.585          0.560
Hexylbenzene          297.114            458.588          0.560

Cyclized as
  -C[H.sub.2]-[beta]:

Propylbenzene         305.633             54.081          0.242
Butylbenzene          308.701             55.765          0.242
Pentylbenzene         311.523             78.927          0.242
Hexylbenzene          311.523             81.619          0.242

n-Alkylbenzenes   [D.sub.max]
                     (nm)

Benzene                -

Common:

Methybenzene         0.59
Ethylbenzene         0.72
n-Propylbenzene      0.82
n-Butylbenzene       0.95
n-Pentylbenzene      1.06
n-Hexylbenzene       1.19

Cyclized on C[H.sub.3]:

Methylbenzene        0.47
Ethylbenzene         0.51
Propylbenzene        0.57
Butylbenzene         0.62
Pentylbenzene        0.66
Hexylbenzene         0.69

Cyclized as
  -C[H.sub.2]-[alpha]:

Ethylbenzene         0.51
Propylbenzene        0.62
Butylbenzene         0.72
Pentylbenzene        0.81
Hexylbenzene         0.95

Cyclized as
  -C[H.sub.2]-[beta]:

Propylbenzene        0.57
Butylbenzene         0.65
Pentylbenzene        0.75
Hexylbenzene         0.84

Table 5 Energies of van der Waals interactions
([E.sub.vdWaals], kJ/mol)in the molecules of
n-alkylphenols and n-alkylbenzenes. Calculated
by the MM+ program. Positions "ortho", "meta",
"para" at n-alkylbenzenes mean cyclization
into the positions with respect to the substituent.

n-Alkylphenols    ortho     meta      para

Common:

Methylphenol      17.117     14.57     14.6
Ethylphenol       41.248     39.44     39.47
n-Propylphenol    44.237     42.49     42.51
n-Butylphenol     47.226     45.48     45.51
n-Pentylphenol    50.402     48.66     48.69
n-Hexylphenol     53.583     51.84     51.87

Cyclized on C[H.sub.3]:

Methylphenol      11.395     32.45     44.96
Ethylphenol       12.089     95.81    109.4
Propylphenol      27.609    385.1     411.3
Butylphenol       49.249   3173.0    1037
Pentylphenol     210.2     2942
Hexylphenol      -         8007      1132

                                     -

Cyclized as
  -C[H.sub.2]-[alpha]:

Ethylphenol       12.089     95.81    109.4
Propylphenol      14.22     140.8     735.8
Butylphenol       33.177   1252       129.3
Pentylphenol      38.076   1251       132.1
Hexylphenol       40.931   1251       135

Cyclized as
  -C[H.sub.2]-[beta]:

Propylphenol      14.22     385.1     411.3
Butylphenol       29.665    388.3     479.1
Pentylphenol      41.34     397.6    3399
Hexylphenol       44.208    400.4    3395

n-Alkylbenzenes  "ortho"   "meta"    "para"

Common:

Methylbenzene     14.852     14.85    14.85
Ethylbenzene      39.727     39.73    39.73
n-Propylbenzene   42.778     42.78    42.78
n-Butylbenzene    45.771     45.77    45.77
n-Pentylbenzene   48.956     48.96    48.96
n-Hexylbenzene    52.137     52.14    52.14

Cyclized on C[H.sub.3]:

Methylbenzene     10.396     41.5     23.99
Ethylbenzene      11.227     34.23    46.87
Propylbenzene     14.254     54.08    67.42
Butylbenzene      27.642    257.7    339.3
Pentylbenzene     64.514   3103      832.2
Hexylbenzene     476.77    -         905.6

Cyclized as
  -C[H.sub.2]-[alpha]:

Ethylbenzene      11.227     34.23    46.87
Propylbenzene     11.608     51.49    54.72
Butylbenzene      21.916    456.5    216
Pentylbenzene     24.658    457.6    218.1
Hexylbenzene      27.362    458.6    220

Cyclized as
  -C[H.sub.2]-[beta]:

Propylbenzene     11.608     54.08    67.42
Butylbenzene      16.386     55.77    70.17
Pentylbenzene     27.312     78.93   132.4
Hexylbenzene      29.783     81.62   134.9

Table 6 Energies of van der Waals
interactions ([E.sub.vdWaals],
kJ/mol) in the molecules of
n-alkylphenols and n-alkylbenzenes.
Calculated by the AMBER program.
Positions "ortho", "meta", "para"
at n-alkylbenzenes mean cyclization
into the positions with respect to
the substituent.

n-Alkylphenols   ortho         meta              para

Common:

Methylphenol     17.54         13.53            13.58
Ethylphenol      36.17         35.04            35.09
n-Propylphenol   36.54         35.48            35.53
n-Butylphenol    37.13         36.07            36.13
n-Pentylphenol   37.90         36.86            36.91
n-Hexylphenol    38.71         37.67            37.72

Cyclized on C[H.sub.3]:

Methylphenol      14.93        43.83             65.28
Ethylphenol       11.05       141.61            175.12
Propylphenol      19.08      2975.8            6246.6
Butylphenol       38.38      4 x [10.sup.6]    9 x [10.sup.4]
Pentylphenol     703.1       4 x [10.sup.6]       -
Hexylphenol         -        4 x [10.sup.10]      -

Cyclized as -C[H.sub.2]-[alpha]:

Ethylphenol      11.05        141.61            175.12
Propylphenol     11.42        249.91          17755
Butylphenol      37.26      31143               195.47
Pentylphenol     41.70      31142               195.82
Hexylphenol      42.22      31142               196.36

Cyclized as -C[H.sub.2]-[beta]:

Propylphenol     19.08      2976               6247
Butylphenol      19.35      2977               6387
Pentylphenol     28.03      2982               8 x [10.sup.6]
Hexylphenol      28.44      2983               8 x [10.sup.6]

n-Alkylbenzenes   "ortho"       "meta"       "para"

Common:

Methylbenzene      13.90        13.90        13.90
Ethylbenzene       35.43        35.43        35.43
n-Propylbenzene    35.87        35.87        35.87
n-Butylbenzene     36.48        36.48        36.48
n-Pentylbenzene    37.26        37.26        37.26
n-Hexylbenzene     38.07        38.07        38.07

Cyclized on C[H.sub.3]:

Methylbenzene      12.92        55.41        24.34
Ethylbenzene       14.35        40.43        53.46
Propylbenzene      11.52        61.62        74.65
Butylbenzene       16.90      1082.8       4182.5
Pentylbenzene      61.29      6 x         35913
                                [10.sup.6]
Hexylbenzene         -            -            -

Cyclized as -C[H.sub.2]-[alpha]:

Ethylbenzene       14.35        40.43        53.46
Propylbenzene      13.74        65.87        60.95
Butylbenzene       21.82      5888.8        777.48
Pentylbenzene      22.05      5887.9        777.15
Hexylbenzene       22.44      5887.1        777.06

Cyclized as -C[H.sub.2]-[beta]:

Propylbenzene      11.52        61.62        74.65
Butylbenzene       11.85        61.63        75.59
Pentylbenzene      20.19        94.76       194.70
Hexylbenzene       20.24        95.01       194.76

Table 7 Boiling points of normal
and cyclic hydrocarbons
[C.sub.3]-[C.sub.8] and energies
of intramolecular van der Waals
interactions.

Alkane          [T.sub.b]     [E.sub.vdWaals]   [E.sub.vdWaals]
               ([degrees]C)    (MM+)(kJ/mol)    (AMBER)(kJ/mol)

n-Propane         -44.5            8.469             1.835
n-Butane           -0.5           11.867             2.796
n-Pentane          36.15          15.132             3.666
n-Hexane           68             18.363             4.514
n-Heptane          98.34          21.577             5.355
n-Octane          125.5           24.787             6.191
Cyclopropane      -33.5            0.134             0.067
Cyclobutane        13             11.265             1.233
Cyclopentane       49.262         14.768             2.399
Cyclohexane        81             22.121             5.338
Cycloheptane      118.48         204.757           721.522
Cyclooctane       150            501.123          7839.966

Table 8 Energies of van der Waals
interactions in the association of
two molecules of a common n-alkylphenols
and n-alkylbenzenes in comparison
with those of two non-associated
(isolated) molecules. Calculated by
the AMBER method.

Compound                     [E.sub.vdWaals
                             (associate)] (kJ/mol)

Phenol                       2.189 x [10.sup.10]
ortho n-Methylphenol-ortho   2.427 x [10.sup.10]-8.945
  n-Hexylphenol                x [10.sup.13]
para n-Methylphenol-para     2.943 x [10.sup.9]-4.193
  n-Hexylphenol                x [10.sup.12]
Benzene                      2.291 x [10.sup.15]

Compound                     [E.sub.vdWaals(2 isolated
                             molecules)] (kJ/mol)

Phenol                       24.512
ortho n-Methylphenol-ortho   35.087-77.414
  n-Hexylphenol
para n-Methylphenol-para     27.162-75.441
  n-Hexylphenol
Benzene                      25.076
COPYRIGHT 2008 Akademie Ved Ceske Republiky, Ustav Struktury a Mechaniky Hornin
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Author:Straka, Pavel; Buryan, Petr; Nahunkova, Jana
Publication:Acta Geodynamica et Geromaterialia
Article Type:Report
Date:Jan 1, 2008
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