Structural and Theoretical Studies of 2-amino-3-nitropyridine

Geometrical optimization, spectroscopic analysis, electronic structure and nuclear magnetic resonance of 2-amino-3-nitropyridine (ANP) were investigated by utilizing ab-initio (MP2) and DFT(B3LYP) using 6311++G(d,p) basis set. Geometrical parameters (bond lengths, bond angles and torsion angles) were computed and compared with the experimental values obtained using X-ray single crystal measurements of the title compound. IR spectra were obtained and assigned by vibrational analysis. Comparing the theoretically calculated values (bond lengths, bond and dihedral angles) using both B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) methods of calculations with the experimentally determined data by X-ray single crystal measurements, all the data obtained in this investigation were considered to be reliable. The theoretical infrared spectra have been successfully simulated by means of DFT and MP2 levels of calculations. The H and C nuclear magnetic resonance (NMR) chemical shifts of 2-amino-3-nitropyridine were calculated using the GIAO method in DMSO solution using IEF-PCM model and compared with the experimental data. Intramolecular hydrogen bonding interaction in this compound was investigated by means of the NBO analysis. The calculated HOMO and LUMO energies show that charge transfer occurs within the molecule.


Introduction
2][3] A common and often productive route to such materials has been to utilize molecules with electron donating and accepting moieties attached to a conjugated system.[6][7] Crystal structure of 2-amino-3-nitropyridine has been reported. 8The vibrational spectra of substituted pyridine have been extensively investigated. 9Recently ab-initio Hartree-Fock and DFT study on vibrational spectra and molecular structures of 2-amino-3,4-, and 5nitropyridine have been published. 10 the present investigation detailed quantum chemical calculations including geometrical optimization, vibrational spectra, NMR chemical shifts and electronic transition were performed using density functional theory calculations.The theoretically calculated values were evaluated and compared with the available experimental data of this compound.

Computational Details
The molecular geometry optimization, energy and vibrational frequency calculations were performed with the GAUSSIAN 03W software package 11 , using DFT( B3LYP) and MP2 levels combined with the standard 6-311++G(d,p) basis set.Using triple zeta basis set for valence electrons with polarization and diffusion function for both heavy atoms and hydrogen is reliable.The Cartesian representation of the theoretical force constants were computed at the optimized geometry by assuming C s point group symmetry.Scaling of the force field was performed using 0.9613 and 0.942 scale factors for B3LYP and MP2 levels of calculations, respectively.The geometry was fully optimized without any constraint with the help of analytical gradient procedure.The optimized structure is a minimum because all the frequencies are positive.By the use of GAUSSVIEW molecular visualization program 12 with symmetry considerations along with available related molecules, vibrational wavenumber assignments were made with a high degree of accuracy.
The natural bonding orbital (NBO) calculations were performed at the B3LYP/6-31G++(d,p) method in order to investigate the electronic structures of the optimized geometry corresponding to the formation of C-H---O and C-H---N hydrogen bonding.The hyperconjugative interaction energy was deduced from the second-order perturbation approach. 13 where q i , is the donor orbitals occupancy, ε j, ε i the diagonal elements (orbital energies) and ) , ( j i F is the off diagonal NBO Fock matrix elements.These calculations are valuable to gain insight into the vibrational spectroscopy, molecular parameters, NMR chemical shifts and electronic transitions of the title structure.All the calculations are performed by using the Gaussian 03W program package on a personal computer. 11

Geometrical parameters
The labeling of atoms in 2-amino-3-nitropyridine is given in Figure 1.The calculated structural parameters, total energy and dipole moment using B3LYP and MP2 methods are collected in Table 1.The bond lengths and angles determined at the DFT level of theory are in good agreement with the X-ray data. 8From the theoretical values we can find that most of the optimized bond lengths are slightly longer or shorter than the experimental values at the MP2 and DFT levels, due to the fact that the theoretical calculations belong to isolated molecules in gaseous phase and the experimental results belong to molecule in solid state.
The results of dihedral calculations indicate that the NH 2 and NO 2 groups are coplanar with pyridine ring as was demonstrated by X-ray diffraction measurements.Comparing bond angles and lengths of B3LYP with those of MP2, as a whole the formers in majority case are bigger than the latter and the B3LYP calculated values correlates well compared with the experimental results.

Natural Population Analysis
Table 2 listed the natural atomic charges of the title compound together with Mulliken charges calculated by B3LYP/6-311++G(d, p) level of theory and basis set.The results showed redistribution of electron density among the atoms due to the hydrogen bonding interaction.The N 10 acquires highly negative charge compared with the other N atoms in the compound and the H 12 and H 13 hydrogen atoms become more acidic.

Vibrational Assignments
The IR spectrum of the investigated molecule was recorded as KBr disc in the range 4000 -400 cm -1 (see Figure 2).The detailed vibrational assignment of fundamental modes of 2amino-3-nitropyridine along with calculated IR wavenumbers at B3LYP and MP2 level using 6-311++G(d, p) basis sets have been collected in Table 3.
According to the theoretical calculations, 2-amino-3-nitropyridine (ANP) has structure of C s point group.The molecule has 15 atoms and 39 normal modes of vibrations.All the fundamental vibrations are active in both IR and Raman.The normal vibrations are distributed as 27 A' (in plane) and 12 A" (out-of plane) coordinates.
The results showed that the DFT (B3LYP) and MP2 methods applied in this work leads to vibrational wavenumbers which are in good agreements with the experimental data.The small difference between the experimental and calculated vibrational modes could be attributed to the formation of intermolecular hydrogen bonding and to the fact that the experimental results belong to solid phase while the theoretical belong to isolated gaseous phase.

C-NH 2 Vibrations
The -NH 2 symmetric and asymmetric stretches in the range of 3717-3575 cm -1 are in agreement with experimental value in the range of 3630-3488 cm -1 .The calculated -NH 2 scissoring vibration at 1656 cm -1 is in excellent agreement consistent with the expected characteristic value at 1593 cm -1 .The NH 2 rocking mode has been identified at 1095 cm -1 and also in agreement with the computed value at 980 cm -1 .The NH 2 wagging computed at 425 cm -1 show excellent agreement with FT-IR experimental data at 410 cm −1 .

-NO 2 Vibrations
The characteristic group wavenumbers of the nitro group are relatively independent of the rest of the molecule, which make this group convenient for identifications.Aromatic nitro compounds have strong absorption due to the asymmetric and symmetric stretching vibrations of the NO 2 group at 1647 and 1595 cm -1 respectively.The hydrogen bonding has little effects on the NO 2 asymmetric vibrations. 15In the present investigation, values of asymmetric and symmetric modes of vibrations are computed at 1656 and 1378 cm -1 respectively in agreement with the experimentally recorded values at 1593 cm -1 and 1310 cm -1 .The bending vibrations of NO 2 group (scissoring, rocking, wagging and twisting) contributed to several normal modes in the low frequency region.It flows from Table 3 that the bands computed at 843 cm -1 and 716 cm -1 are assignable to NO 2 scissoring and wagging respectively in good agreement with the corresponding experimental data.

C-H Vibrations
The heteroaromatic structure shows the presence of C-H stretching vibrations in the range of 3100-3000 cm -1 15 which is the characteristic region for the ready identification of C-H stretching vibrations, and the bands are not affected by the nature of substitutents.Hence the band at 3100 cm -1 has been designated to C-H stretching in agreement with the theoretically calculated value at 3221 cm -1 .The computed out of plane bending vibration of C -H at 992 cm -1 is comparable with the experimentally observed value at 950 cm -1 .

C-H---O hydrogen Bonding
In addition to the strong intramolecular hydrogen bond, H 11 ---O 14 , DFT predicts the weak hydrogen bonding interaction H 7 ---O 15 .Recently it has been established that a C-H group can be a hydrogen bond order.Although the C-H---O interactions are considered weak they form 20-25% of the total number of hydrogen bonds constituting the second most important group. 16This interaction could be of greater importance in NLO-systems.Ab-initio calculations have been particularly useful in the identification of C-H---O hydrogen bonds in which the C-H donor group is strengthened, shortened and blue-shifted (U-shifted in the stretching vibrational wavenumber. 17The intramolecular contacts of H 7 ---O 15 occurs with H---O distance of 2.67Å which is significantly shorter than the Van der Waals separation between the oxygen atom and the H-atom (2.72 Å) indicating the possibility of the intramolecular C-H---O interaction in 2-amino-3-nitropyridine.The calculated C-H---O angle for this interaction is well within the angle limit as the interaction path is not necessarily linear. 18

Natural Bond Orbital Analysis
Natural bond orbital (NBO) analysis provides an efficient method for studying intra-and intermolecular bonding and charge transfer or conjugated interaction in molecular systems. 19means energy of hyperconjugative interactions (stabilization energy).b Energy difference between donor and acceptor i and j NBO orbitals.c F(i,j) is the Fock matrix element between i and j NBO orbitals.
The intramolecular C-H---O hydrogen bonds are exposed in Table 4 by the interaction between the oxygen lone-pairs LP 2 O 15 and the antibonding σ* C 3 -H 7 not withstanding that the energetic contribution (0.65) of hyperconjugative interaction are weak, these E (2) values are chemically significant and can be used as a measure of the intramolecular delocalization.The strengthening and contraction of C 3 -H 7 bond is due to rehyperdization 20 , which is revealed by the low value of ED (0.76) in the  * C 3 -H 7 orbitals.

NMR Spectra
The molecular structure of the title compound was optimized, then, gauge-including orbital (GIMO) 13 C-NMR and 1 H-NMR chemical shifts calculations of the title compound were carried out using B3LYP/6-31++G(d, p) method.The calculations were performed in DMSO solution using IEF-PCM model, rather than in the gas phase and the values obtained were compared with experimental data and the data are listed in Table 5.The results showed excellent agreement between experimental and computed chemicals shifts.Figures (3&4) show experimental NMR spectra of ANP.

Electronic Properties and UV Spectra
The UV-Vis absorption spectrum of the sample in ethanol is shown in Figure (5) together with the experimental spectrum in the same solvent.On the basic fully optimized groundstate structure, TD-DFT/B3LYP/6-311++G(d,p) calculations have been used to determine the low-lying excited states of ANP.The calculated results involve vertical excitation energies, oscillator strength (f) and wavelengths.Typically according to Frank-Condon principle, the maximum absorption peak (λ max ) corresponds in an UV-Vis spectrum to vertical excitation.TD-DFT/B3LYP/6-311++G(d,p) predicts one intense electronic transition at (260.85 nm) with an oscillator strength f=0.3685, that shows excellent agreement with measured experimental data (λ max =260 nm) as shown in Table 6.Both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are the main orbitals taking part in chemical reactions.The HOMO energy characterizes the ability of electron donating; LUMO characterizes the ability of electron accepting.The energy gap between the highest occupied and the lowest unoccupied molecular oribtals characterizes the molecular electrical transport properties because it is a measure of electron conductivity.The energy gap is largely responsible for the chemical and spectroscopic properties of the molecules. 21This also used by the frontier electron density for predicting the most reactive position in π-electron systems and also explains several types of reactions in conjugated systems. 22e HOMO is located over the NO 2 and NH 2 groups, the HOMO-LUMO transition implies an electron density transfer to pyridine ring from N=O and N-H bonds.The HOMO and LUMO orbitals significantly overlap in the position for ANP.The molecular orbitals are sketched in Figure (6).
The calculated self-consistent field (SCF) energy of ANP is (−508.189a.u).The HOMO-LUMO energy gap explains the eventual charge transfer interactions taking place within the molecule.The small value of band gap reflects the chemical activity of the molecule and encourages the application of ANP as non-linear optical materials.

Thermodynamic Properties
Several calculated thermodynamic parameters are presented in Table 7.

Dipole Moment
Asymmetric molecules generally have non-zero dipole moments in the respective ground electronic states.Based on the stable geometry of ANP in the ground state, the dipole moment in vacuum and in different solvent environments was calculated using TD-DFT.The dipole moment is found to increase with an increase in solvent polarity and this facilitates the charge transfer probability.The high values of dipole moment of ANP signifies high delocalization of charges, resulting in the formation of relatively loose structured, charge separated species.

Conclusion
Geometric optimization, FT-IR, NMR and UV-Vis spectra have been computed by DFT using B3LYP/6-311G++(d,p) level of theory and basis set combination.
The results showed good agreement between the experimental and theoretical data indicating the validity of the DFT level of theory and basis applied to ANP molecule for prediction of both structural and spectroscopic data of the title compound.The investigation of C-H … O intramolecular interaction in 2-amino-3-nitro pyridine by DFT showed that the calculated C-H … O angle for this interaction is well within the angle limit as the interaction is not necessary linear.The small HOMO-LUMO energy gap value computed using DFT indicates that the title compound could be of potential use as non-linear optical material (NLO).

Figure 6 .
Figure 6.HOMO and LUMO orbitals and energy plot of ANP.

Table 2 .
Natural atomic charges (e) of ANP calculated at B3LYP/6-31++G(d, p) level of the theory and basis set.

Table 4 .
Second-order perturbation theory analysis of Fock-matrix in NBO analysis corresponding to the intermolecular C-H--O hydrogen bonds of ANP.