A Comparative Theoretical and Spectroscopic Study of Aminomethylbenzoic Acid Derivatives as Potential NLO Candidates

: Three aminomethylbenzoic acid derivatives were theoretically studied at M062X/6-311++G(d,p) level in a vacuum, namely 2-ammonio-5-methylcarboxybenzene perchlorate ( 1 ), 4-(ammoniomethyl) carboxybenzene nitrate ( 2 ) and 4-(ammoniomethyl)carboxybenze perchlorate ( 3 ). The compounds’ structures were fully optimized and compared with the single-crystal X-ray diffraction results, showing a very close agreement with the experimental structural parameters. Their IR, 1H- and 13C-NMR spectra were calculated and examined in detail. Furthermore, the molecular electrostatic potential (MEP) maps of the studied compounds were investigated and the strength of the non-covalent interactions evaluated. In addition to these results, the NLO properties of the three compounds were predicted.

Quantum chemical calculations have been considered an attractive research area in the last few years [14][15][16][17]. Therefore, modelling studies of the salts (1)-(3) were carried out by using the M06-2X with a 6-311++G(d,p) basis set in the gas phase and the struc-tures of the three compounds were fully optimized at the same level and compared with the experimental single-crystal X-ray diffraction results previously reported [11]. The IR and the NMR spectra of the mentioned compounds were calculated and examined in detail. Furthermore, the molecular electrostatic potential (MEP) maps of the related compounds were investigated. In addition to these results, the interaction strength between the anions and the cations were calculated and discussed in detail. The NLO behavior of the studied compounds was furthermore theoretically evaluated.

Materials and Methods
The computational processes are performed by GaussView 5.0.9 [18] and Gaussian 09 AS64L-G09RevD.01 [19] programs. The M06-2X function of the hybrid density functional theory (DFT) has been derived by Zhao et al. in 2008 [20]. It was adopted in the calculations together with the 6-311++G(d,p) basis set. In the IR computations, no imaginary frequency was observed. As for the NMR spectra of the related compounds, they were calculated with a gauge-independent atomic orbital (GIAO) method. In the calculation of chemical shift values, tetramethylsilane (TMS) was used as a reference substance. Additionally, in the calculations of the MEP maps, the electrostatic potential (ESP) charges were taken into consideration.

Comparative Theoretical and Experimental Structural Study
The optimized structures of compounds (1)-(3) are represented in Figure 1. Selected experimental and calculated geometric parameters are given in Table 1. The accuracy of the calculated results towards the experimental ones was investigated by plotting the scatter graphs. Hence, the regression coefficients (R 2 ) were calculated from these graphs for each compound and given in the related table.  According to Table 1, the experimental bond distances vary from 1.203 to 1.507  for compound (1), from 1.213 to 1.505  for compound (2), and in the range 1.241-1.516  for compound (3). These distances are related to the C─CO2H and C─CH3/C─CH2─ bonds. Whereas, the associated calculated values are found to be (1.212-1.505 ), (1.200-1.507 ) and (1.241-1.516 ), respectively. Consequently, the calculated and experimental geometric parameters (bond lengths and angles) are very close to each other and it can be easily seen that there is a good agreement in terms of regression coefficients, with R 2 worth 0.9998, 0.9998 and 0.9993 for compounds (1)-(3), respectively.
The cationic and anionic moieties in compounds (1)-(3) were optimized separately at the same level of theory as follow: 2-ammonio-5-methylcarboxybenzene (+1) (A), 4-(ammoniomethyl) carboxybenzene (+1) (B), perchlorate (-1) (C) and nitrate (-1) (D) and the total energy of the whole structures are given in Table 2. There is an intramolecular hydrogen bond (labeled "c" in Figure 1) in the cationic moiety (A) and it is therefore more stable than the cation (B). Additionally, there are two other intermolecular hydrogen bonds as labeled in Figure 1 as "a" and "b" in compound (1). As for compounds (2) and (3), they exhibit two intermolecular non-covalent interactions ("a" and "b"). The corresponding interaction energies are calculated as −407.916, −477.384 and −434.142 kJ mol −1 for compounds (1)-(3), respectively. These results show that the interaction in compound (2) is stronger than those of compounds (1) and (3). It implies that the associated hydrogen bond is stronger due to the electronegative nitrogen atom. The single-crystal X-ray diffraction results [11] show that compound (2)

Spectral Analyses
The IR spectrum of the studied compounds are calculated and represented in Figure  2a. The most significant bands are labeled in the same figure and the vibration mode and corresponding frequencies are given in Table 3. In Figure 2b, we illustrated the reported experimental FTIR spectra [11].  Furthermore, the 1 H-and 13 C-NMR spectra of the studied compounds were calculated at the same level of theory. The chemical shift values of the related atom are calculated by using TMS. The chemical shift values of the hydrogen and carbon atoms are given in Table 4. According to Table 4, the chemical shift values of the aromatic carbon atoms are in the range of 135-186 ppm. As for the aliphatic carbons, the chemical shift value is calculated to be 24.1 ppm for compound (1) and nearly 50 ppm for compounds (2) and (3), since the amine group is connected to this carbon in both compounds. The chemical shift values of hydrogen atoms in the aromatic ring are in the range 8-10 ppm vs. 2-5 ppm for the hydrogen atoms connected to the aliphatic carbon. The chemical shift values of some hydrogens are high due to the presence of the hydrogen-bonding networks and the electronegative atoms. These results show that there are three hydrogen bonds in compound (1) and two hydrogen bonds in compound (2) and (3).

Molecular Electrostatic Potential (MEP) Maps and Contour Diagrams
The molecular electrostatic potential (MEP) maps are important because they allow investigating the active regions, the reaction mechanisms or the interaction regions. The MEP maps of the studied structures are calculated and represented in Figure 3. There are red, yellow, green and blue regions in the figure, which are determined in terms of the electron density. The electronically rich regions are mainly represented in red or yellow whereas the electronically poor ones are displayed in blue. The blue regions are dominant around the hydrogen atoms in the studied compounds, while the red ones are dominant nearby the oxygen atoms which are linked to the chlorine or the nitrogen atoms. The red regions are appropriate to nucleophilic attacks. Particularly, the environment of the carboxyl group and the benzene ring are in the green and yellow. The electron densities are decreasing from red to blue and therefore, the red and yellow regions are more active than the green and blue ones.

Non-Linear Optical Properties
The non-linear optical (NLO) properties can be affected by the molecular structure, the electromagnetic field, the frequency. It is important in providing the key functions of frequency shifting, the optical modulation, the optical switching, the optical logic and the optical memory for the technologies in areas such as telecommunications, signal processing and optical interactions. The NLO properties of the related compounds are predicted by calculating some quantum chemical parameters, which are the total static dipole moment (μ), the average linear polarizability (α), the anisotropy of polarizability (Δα), and the first hyper polarizability (β). These parameters are calculated by using Equations (1)-(4) and given in Table 5.  The NLO properties increase with the increasing of the total static dipole moment, the average linear polarizability, the anisotropy of polarizability and the first hyper polarizability. The related quantum chemical parameters of compounds (1)-(3) are higher than the urea's values. Therefore, the three compounds are better than urea and could be good candidates for NLO applications. According to the mentioned descriptors and the following ranking: Compound (1)  It is worth noted that compound (1) is the best candidate for NLO applications.

Conclusions
Computational investigations of three aminomethylbenzoic acid derivatives were performed by using the M062X method with the 6-311++G(d,p) basis sets in the gas phase. The structural and spectral analyses were carried out in detail, presenting a very good agreement between the theoretical results and the experimental values. The interaction energies were furthermore calculated and the hydrogen bond strengths of the mentioned compounds were evaluated, showing the presence of intramolecular and intermolecular O-H…O, O-H…N and N-H…O hydrogen bonds. Additionally, the MEP maps of the related compounds were examined and the NLO properties estimated, concluding that the three compounds are good candidates for NLO applications.