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Molecular Properties and H-Bonding in N-8-Quinolinyl-2-hydroxynaphthaldimine and its Azo-Analogue.

Byline: Tareq Irshaidat

Summary. Schiff bases are important class of molecular materials. In

this computational organic chemistry study the title compounds were exploited by DFT/B3LYP based on the experimental studies recommendations by Fita (Chem. Phys. Lett. 2005 416 305) and Fabian (J. Phys. Chem. A 2004 108 7603). The MP2 method was compared and showed some differences but with general qualitative agreement. In addition to the O/H-N1 H-bond the study provides evidence on the involvement of the H-N1 proton in a second H-bond (N2/H-N1) and that it leads to substantial diverse effects on the structures the NMR chemical shifts the atomic charges the orbitals interaction the tautomerism energies and the band gaps. Examining the NBO results reveals that the N2/H-N1 interaction in the more stable keto-tautomer is critical and can enhance the electron delocalization through the p-system toward both the quinoline and the naphthalene segments. Thus the details are new addition to the H-bond theory.

These compounds have several interesting features: 1. they are better conductors at the molecular level than a known Schiff base molecular conductor 2. both frontier orbitals distribute over the extended conjugated p-system which permits efficient intermolecular interaction 3. their fluorescence is enhanced significantly after complexation with several metal ions and 4. their synthesis is not complicated therefore they are interesting models and building blocks for a variety of molecular technology applications specially switching and electroluminescence devices.

Keywords: H-bond tautomerism Azo-dyes Schiff bases DFT NBO.

Introduction The ortho-hydroxy Schiff bases and the azo- analogues are interesting families of organic compounds. The biological the chemical the theoretical and the technological aspects are the subject of several reviews [1-4]. The important factors that contributed in studying them extensively are the possibility of preparing a wide variety of derivatives through an uncomplicated synthesis routes and their stability under normal atmospheric conditions. They have been used as ligands that can form stable complexes with nearly all the transition metals. They are building blocks for several molecular technology applications for example: thermochromic and photochromic molecular materials polymers liquid crystals data storage information processing anticorrosive materials optical switching and molecular conductors.

Many of these applications are associated directly with the fact that both the ortho-hydroxy Schiff bases and the analogous azo-compounds can change their structure by a reversible tautomerism process. The proton-electron delocalization process is possible under the effect of temperature change or upon illumination with electromagnetic radiation of a specific wave length. The proton-electron delocalization process is a 15-proton shift that is essentially possible only due to the unique structures of these compounds that include a strong neutral H-bonding between oxygen and nitrogen. In general the Schiff bases that are derived from salicylaldehyde exist mainly as the enol-tautomer with a variable contribution from the keto-tautomer (depending on the substituents) while those derived from 2- hydroxynaphthaldehyde are dominated by the keto- tautomer structure [5]. This observation is attributed to the lower aromatic character of naphthalene [6].

The previous studies indicate that the keto- tautomer is the more stable structure in both HNQ [7- 25] and HNQ-azo [26-31] (Fig. 1). Due to the technological applications of these families of compounds a better understanding of the electronic structure is vital for designing new useful compounds. Therefore the purpose of this study is to understand the consequences of the N2/H-N1 dipole- dipole interaction on the electronic structures. To gain a deeper insight HNQ and HNQ-azo are compared with the isomeric structures and with other derivatives generated by replacing the hydrogen atom next to N2 with an electron donating and an electron withdrawing group (OMe and CN respectively) (Fig. 1). Computational Details

All the calculations were executed using Gaussian 03W [32]. All the structures were optimized using the B3LYP hybrid functional [33 34]. In studying malonaldehyde we found [35] that the 6-31G(dp) basis set [36] can produce geometries better than that produced by the DZP++ basis set [37] and closer to the CCSD(T)/cc-PVQZ results therefore we adopted the 6-31G(dp) basis set in the geometry optimization of all the structures. The calculated frequencies indicated that the number of the imaginary frequencies of each of the optimized minimum structures is equal to zero while each optimized transition state has one imaginary frequency. At the same theory level the natural bond orbital analysis was performed using the implemented NBO 3.1 program [38]. This theory can evaluate the second-order interaction energy that is associated with an electron density delocalization from a bonding pair or a

lone pair to an empty orbital as in hydrogen bonding (more information can be found elsewhere [39]). Single point energy calculations were performed using B3LYP along with the 6-311+G(2dp) basis set [40 41] and also using the second-order MAllerPlesset (MP2) [42] perturbation theory and the 6-311+G(dp) basis set. At the B3LYP/6-311+G(2dp)//6-31G(dp) level of theory the GIAO [43] method was used to calculate the NMR chemical shifts. Tetramethylsilane (TMS) was used as a reference for hydrogen while ammonia was used as a reference for nitrogen. The ChemCraft graphic inter-phase was used to generate the input files and to read the output files.

Results and Discussion

NH The effect of the N2/H-N1 Interaction on the Structures

Selected structural data are presented in Table-1. In the enol-tautomers the smallest N1/H and N2/H distances are observed in the two methoxy N2 derivatives which indicates that despite of the large N2/H distance (larger than 2.5 angstroms) the substituent still have effect on the N2/H-N1 interaction. These results support that the N2/H-O H- bond strength has the OMegreater than Hgreater than CN order. The same trend is concluded based on the N2/H distance of the keto-tautomer.

Table-1: Selected structural data (A ).

Structure N1/H N2/H N2/H

HNQ-azo

(H)###1.616 2.697###0.0###119.0###1.034 1.775 2.315###0.0

(OMe)###1.593 2.644###-0.053###118.6###1.035 1.750 2.281###-0.034

(CN)###1.618 2.725###0.028###118.7###1.034 1.759 2.339###0.024

Isomer-

###1.621###-###-###115.1###1.037 1.661###-###-

HNQ

(H)###1.637 2.869###0.0###114.7 1.035 1.782 2.271###0.0

(OMe)###1.613 2.747 -0.122 114.1 1.037 1.755 2.233###-0.038

(CN)###1.647 2.929 0.060 114.0 1.037 1.754 2.286###0.015

Isomer 1.638###-###-###110.5 1.046 1.628###-###-

In the Keto-tautomer the Y-N1-H N1-H and O/H values do not show a specific correlation with the nature of the substituent (H OMe CN). On the other hand upon switching to the isomeric structures both the O/H distance and the Y-N1-H bond angle decrease while the N1-H bond length increases which indicates that the absence of the N2/H-N1 H-bond in the isomeric structures causes strengthening of the O/H-N1 H-bond. Therefore the observed changes confirm that the N2/H-N1 interaction is an interfering factor with respect to the O/H-N1 H-bond and not a negligible dipole-dipole interaction and has noticeable consequences on the structures.

The effect of the N2/H-N1 interaction on the NMR chemical shifts

In general a chemical shift gives an idea about the magnitude of the electron density that is surrounding a nucleus which might be the electron density of the atom itself and also the electron densities of the neighboring atoms (bonded or through-space). Selected NMR chemical shift data are presented in Table-2. In the title structures the electron densities that affect the hydrogen atom nucleus are the electron density of the hydrogen atom itself and the electron densities of oxygen N1 and N2 atoms. In general it is noticed that the hydrogen atom in all the studied structures exhibits large chemical shift values and they indicate that the hydrogen atom nucleus exists in electron deficient environment specially in the HNQ-azo derivatives.

Table-2: Selected NMR chemical shifts (ppm).

Structure###N2###N2###H###H

###HNQ-azo

###(H)###335.6###-###17.3###-

###(OMe)###267.1###-68.5###17.3###0.0

###(CN)###344.6###9.0###17.3###0.0

###Isomer-azo###351.0###15.4###18.2###0.9

###HNQ###

###(H)###338.4###-###15.9###-

###(OMe)###270.2###-68.2###15.9###0.0

###(CN)###348.2###9.8###16.1###0.2

###Isomer###354.0###15.6###17.1###1.2

From the chemical shift values of the N2 atom and as expected the methoxy group increases the electron density on N2 while the cyano group decreases it (by resonance) with respect to the hydrogen substituent. This is deduced from the large variation in the chemical shift of N2 (N2). Despite that the chemical shift of the hydrogen atom does not change significantly among the three substituents (OMe H and CN). This observation illustrates that the N2/H-N1 interaction is controlled mainly by the electron density of the N2 lone pair and not by the total electron density of N2.

On the other hand the absence of the N2 lone pair effect in the isomeric structures causes an increase in the chemical shift of the hydrogen atom (Isomer-azo= 0.9 and Isomer= 1.2ppm) which indicates that the total electron density surrounding the nucleus of the hydrogen atom decreased. This result is directly related and consistent with the structural changes that are observed in the isomeric structures and is clarified as the following. The absence of the N2 lone pair effect in the isomeric structures allows the H-N1 proton to move further toward the oxygen atom thus the H-N1 bond length increases while the O/H-N1 distance decreases (Tabl1 1). This in turn lowers shielding the H-N1 proton by the N1 electron density.

The Effect of the N2/H-N1 Interaction on the Natural Population Analysis (NPA) Charges

Selected NPA charges are presented in

Table-3: Selected (a) natural population analysis (NPA) charges.

###HNQ-Azo###HNQ

###H###OMe###CN###Isomer###H###OMe###CN###Isomer

###Oxygen

###Ot###8.600###8.619###8.591###8.622###8.625###8.646###8.617###8.651

###C=O###1.995###1.995###1.995###1.995###1.995###1.995###1.995###1.995

###C=O###1.954###1.955###1.953###1.956###1.963###1.964###1.962###1.965

###OLP1###1.968###1.967###1.967###1.965###1.966###1.965###1.966###1.964

###OLP2###1.879###1.879###1.878###1.867###1.877###1.876###1.874###1.857

###N2

###N2t###7.458###7.546###7.415###7.447###7.453###7.543###7.410###7.445

###N2LP###1.916###1.893###1.914###1.925###1.914###1.891###1.912###1.925

###H(-)###0.534###0.538###0.533###0.555###0.512###0.521###0.512###0.529

s-bond pair the p-bond pair and the two lone pairs. The electron density of the s-bond pair is constant in all the derivatives which is consistent with the fact that this pair is at low energy level in the molecular orbital diagram and can not be delocalized. The p- bond electron density is higher in energy but still lay at low energy and varies only slightly. The magnitude of the electron density of the s- and p-bond electron pairs is nearly equal to 2-electrons which indicates that the C=O group does not gain any significant electron density from the conjugated system. On the other hand it is observed that there is a difference in the magnitude of the electron densities of the first and the second lone pairs (LP1 and LP2) of oxygen. The LP2 is the lone pair that is involved directly in H- bond with H-N1 and has a lower value which is attributed to delocalization of electron density toward the anti-bonding orbital of the H-N1 bond.

Qualitatively the magnitude of the delocalized electron density from LP2 to the anti-bonding orbital of the H-N1 bond increases in the isomeric structures which is explained as the following. In absence of N2 (in the isomeric structures) the hydrogen atom becomes closer to the oxygen atom (Table-1) therefore the donation from the closer lone pair of the oxygen atom; LP2 becomes more efficient. This result is consistent with the observed increase in the electron density of the hydrogen atom upon switching to the isomeric structures (HNQ-azo= 0.555e and HNQ= 0.529e) and consistent with the observed NMR chemical shift changes (Table-2).

The Effect of the N2/H-N1 Interaction on the Charge Delocalization Energies

Table-4 presents the E(2) energies that are associated with the selected charge delocalization processes. The N1 atom donates a significant amount of electron density to both the quinolinyl and the naphthyl groups (as indicated by the values of N1/C=Y" and N1/C=C"). On the other hand due to the absence of the N2/H-N1 H-bond in the HNQ- isomer" and the HNQ-azo-isomer" the total amount of the delocalization energy that is associated with the N1/C=Y" and N1/C=C" interactions decreases (the N1pt decreases by 7.93kcal/mol with respect to HNQ-azo-H and 13.27kcal/mol with respect to HNQ- H). This change indicates that the second H-bond (N2/H-N1) can enhance the charge delocalization in the p-system. Despite that the change in the delocalization energy is not large for a single molecule but a cumulative effect may have a significant enhancement of the conductivity (in molecular and macromolecular materials).

Structure###N1/H###N2/H###O/H###N2/H###N1/C=Y###N1/C=C###N1t(a)

###HNQ-azo

###(H)###62.95###1.58###32.56###4.30###49.80###42.67###92.47

###(OMe)###69.56###1.92###36.09###4.97###52.16###41.92###94.08

###(CN)###62.38###1.33###34.40###3.71###47.19###44.36###91.55

###Isomer###63.41###-###33.39###-###50.07###34.47###84.54

###HNQ

###(H)###61.19###0.70###33.40###5.09###56.50###48.05###104.55

###(OMe)###67.52###1.18###37.77###6.05###59.36###46.95###106.31

###(CN)###58.81###0.49###37.07###4.58###54.11###50.56###104.67

###Isomer###61.46###-###38.56###-###58.12###33.16###91.28

A value of delocalization energy is not a direct measure of H-bond energy but can be used to establish a qualitative comparison. The delocalization energies that are associated with N1/H-O (N1/H enol) and O/H-N1 (O/H keto) indicate qualitatively that the H-bond in the enol-tautomer (N1/H-O) is stronger. On the other hand and relatively the N2/H- N1 interactions may be classified as weak H-bonds.

The Effect of the N2/H-N1 Interaction on the Tautomerism Energies

Fabian and co-workers illustrated that DFT/B3LYP can calculate energy parameters that are in good agreement with the experimental tautomerism energies [44] for anils and azo- analogues derived from hydroxynaphthalenes. On the other hand Hargis and co-workers illustrated that B3LYP underestimates the activation energy barriers (Ea) but they were able to derive a linear equation (Ea(corrected)=1.73Ea(B3LYP)+0.38) that can be used to correct the B3LYP results (of Ea) to become equivalent to those that may be produced using very accurate coupled cluster methods at the complete basis set limit [45]. This equation was used to correct the B3LYP activation energies in Table-5 (Ea between parentheses).

The E results of HNQ-azo-H and HNQ-H from the three computational procedures are consistent with the experimental results and indicate that the two tautomers are not of similar stability and the keto-tautomer is the structure that represents them. As known from previous results [44] a keto- tautomer is more stable in an azo-compound than in the analogous Schiff base the same E divergences are observed in Table-5. Therefore consistent with Fabian recommendation [44] these observations indicate that these are adequate computational procedures.

The systematic effect of the three substituents (H OMe and CN) on the activation energy (Ea Table-5) and on the energy difference (E) values is observed only in case of the Schiff bases. On the other hand the three (H OMe and CN) azo-derivatives have similar keto-tautomer relative stabilities which imply that this azo- molecular system exhibits the maximum possible relative stabilization for the keto-tautomer. The observed decrease in the energy difference (E) of the isomeric structures (HNQ-isomer-azo and HNQ- isomer) indicates that the N2/H-N1 hydrogen bond participates noticeably in stabilizing the keto- tautomer.

It is obvious (Table-5) that the stability of the keto-tautomer in the Schiff base derivatives of the three substituents has the following order: CNless than Hless than MeO. This sequence illustrates that the N2/H-N1 interaction is stronger in case of an electron donating substituent and weaker in case of an electron withdrawing substituent. This is an acceptable result knowing that an electron donating group can increase the electron density on N2 by resonance and therefore strengthens the N2/H-N1 interaction. On the other hand the small variations among the three substituents signify that the total charge of N2 has only a small influence on the strength of the N2/H-N1 interaction while the major participation is attributed to the N2-lone pair (which is constant electron density regardless of the substituent).

The activation energies (Table-5) of the isomeric structures (of HNQ-azo and HNQ) are less than those of the other derivatives. This result illustrates that the N2/H-N1 hydrogen bond disfavors converting the keto-tautomer to the enol-tautomer. The Ea values are large and indicate that the proton- electron delocalization can be a temperature- controlled process. However under the standard conditions the title compounds are pure keto- tautomers which is consistent with the experimental results. Inabe-Schiff base is a known molecular conductor [46] (Fig. 2). The calculations show that the enol-tautomer is more stable by 6.12kcal/mol B3LYP/6-31G(dp)) 5.80kcal/mol (B3LYP/6 311+G(2dp)//6-31G(dp)) and 10.50kcal/mol MP2/6-311+G(dp)//6-31G(dp)). The band gap (BG) of the enol-tautomer of this compound has the following values: 3.42 eV (B3LYP/6-31G(dp)) 3.40 eV (B3LYP/6-311+G(2dp)//6-31G(dp)) and 8.59 eV (MP2/6-311+G(dp)//6-31G(dp))

which are higher than the values of the Schiff bases and the azo-derivatives (Table-5). This indicates that the title compounds are better conductors at the molecular level. Fig. 3 shows that the frontier orbitals of the title compounds distribute over the entire molecular skeletons which allows efficient intermolecular p-p interaction and electron mobility. As a result the title compounds will demonstrate a better overall performance.

Table-5: The activation energies(a) (kcal/mol) the energy difference (E= E(enol)-E(keto)) (in kcal/mol) and the band gaps(b) (eV) for the keto-tautomer based on the three methods (M)(c).

Structure###M###Ea###E###BG###Structure###M###Ea###E###BG

###HNQ-azo###HNQ

###5.46###3.94

###(H)###1###-3.63###2.93###(H)###1###-1.07###3.15

###(9.83)###(7.20)

###6.07###4.51

###2###-3.96###2.87###2###-1.18###3.09

###(10.88)###(8.18)

###3###6.76###-3.64###7.86###3###6.46###-1.08###8.19

###5.12###4.08

###(OMe)###1###-3.58###2.91###(OMe)###1###-1.84###3.19

###(9.24)###(7.44)

###5.54###4.75

###2###-3.64###2.85###2###-1.98###3.13

###(9.96)###(8.60)

###3###6.31###-3.59###7.82###3###7.10###-1.83###8.22

###5.19###-3.31###3.27###+0.18###2.96

###(CN)###1###2.90###(CN)###1

###(9.36)###(6.04)###(3.24)

###5.51###3.94###2.91

###2###-3.28###2.84###2###-0.19

###(9.91)###(7.20)###(3.20)

###8.03

###3###7.44###-4.13###7.87###3###6.01###+0.36

###(8.34)

###3.53###-1.26###2.85###3.40###3.25

###Isomer-azo###1###Isomer###1###+0.22

###(6.49)###(2.97)###(6.26)###(3.53)

###4.10###2.80###3.87###3.19

###2###-1.56###2###+1.41

###(7.47)###(2.921)###(7.08)###(3.48)

###7.71###8.32

###3###4.00###+2.80###3###6.14###+6.06

###(7.93)###(8.62)

Conclusion

Several points are revealed and they are summarized as the following:

1. This computational organic chemistry study investigated a molecular case were a hydrogen atom is involved in two hydrogen bonds. Ortho- hydroxy Schiff bases and the analogous azo- dyes are very well known examples for the O/H-N and N/H-O hydrogen bonding. Incorporating a second hydrogen bond into their structures increases the level of the complexity of the electronic structures and makes the analysis a challenging task. However we found that the relative analysis with respect to the proposed derivatives was necessary and enlightening for recognizing the consequences of the second hydrogen bonding on the electronic structures.

2. The changes in the total atomic charge of N2 indicate that the effect of the N2/H-N1 H-bond on the structural parameters the NMR chemical shifts the delocalization energies E Ea is attributed essentially to the electron density of the N2 lone pair while the two substituents (OMe CN) have only small effects by resonance. 3. Qualitatively and in general the B3LYP and the MP2 calculations produced homogeneous results consistent with the experimental data. They indicate that the absence of the N2/H-N1 H-bond decreases the stability of the keto- tautomer significantly which confirms that this five-membered H-bond interaction (despite it is being weak) is a critical factor.

4. The activation energy decreases in absence of this H-bond (N2/H-N1) therefore it participates appreciably in preventing the spontaneous thermal conversion of the keto-tautomer to the enol-tautomer at room temperature. However the values indicate that this conversion is kinetically possible in presence of the necessary energy (thermal or electromagnetic energy or in presence of a chemical effectors). This makes the title compounds and other similar possible compounds candidates for molecular switching applications [47 48].

5. The studies proved that HNQ can fluoresce in the visible region. On the other hand replacing the H-N1 proton with aluminum tin chromium and molybdenum [49-53] boosts the fluorescence intensity significantly in the visible region which is attributed to the enhanced rigidity that is gained upon complexation. In this study HNQ and HNQ-azo are characterized as new possible molecular conductors. Therefore coupling fluorescence and molecular conductivity in such complexes can produce materials suitable for switching and electroluminescence applications like photodiodes.

Acknowledgments

We thank Al-Hussein Bin Talal University for supporting this research through the research grant 78/2008.

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