Optimized geometry, vibration (IR and Raman) spectra and nonlinear optical activity of p-nitroanilinium perchlorate molecule: A theoretical study

The p-nitroanilinium perchlorate crystal synthesized by Marchewka et al. crystallizes in P2₁/c space group of monoclinic system with a = 5.003 (10) Å, b 10.685 (2) Å, c 17.914 (4) Å, β = 94.44 (3)° and Z = 4 [8]. The optimized structural parameters, bond length and bond angle calculated for p-nitroanilinium perchlorate molecule at B3LYP and HSEH1PBE levels using 6-311++G(d,p) basis set, compared with X-ray diffraction results are presented in Table 1. The experimental structure with the labeling of atoms and theoretical optimized structure are given Fig. 1a and Fig. 1b, respectively.

Fig. 1

The p-nitroanilinium perchlorate molecule has electron withdrawing nitro group and electron donating amino groups. Because of the electronic effects of push and pull type substituents the symmetry of the benzene ring is distorted. The electron donating substituents in the benzene ring, yield ring angles smaller than 120° at the point of substitution and slightly larger than 120° at the ortho and meta positions [18] and vice versa. The C–C bond lengths observed in the region of 1.3588 to 1.3812 Å [8] have been calculated in the ranges of 1.3890 to 1.3972 Å and 1.3851 to 1.3890 Å for B3LYP and HSEH1PBE levels, respectively. The Cl–O bond lengths were observed in the range of 1.4133 to 1.4420 Å [8], while these bond lengths have been calculated in the range of 1.4607 to 1.5329 Å for B3LYP and 1.4441 to 1.5138 Å for HSEH1PBE level. In our calculations, different Cl–O bond lengths have been found for ClO₄ group, since two O atoms (O12 and O13) resonance in the hydrogen bonding N–H···O, led to an increase in Cl–O bond lengths. Due to electron withdrawing nature of the nitro group, the bond angle at the point of C3-C4-C5 was found at 122.376° and 122.472°, respectively. Similarly, at the point of attachment of electron donating amino group, the bond angle of C5-C6-C1 was calculated at 122.124° and 122.160°, respectively. This demonstrates that the lone pair of electrons of the amino nitrogen is not delocalized into the ring, and is completely involved in the hydrogen bonding formation with ClO4 group. In order to compare results obtained at B3LYP and HSEH1PBE levels with X-ray results [8], the R² values have been presented in Table 1. According to these R² values, HSEH1PBE level gives closer results in bond lengths, while B3LYP gives closer results in bond angles.

Theoretical calculations display the presence of two different intermolecular hydrogen bonding interactions in PNAPC molecule. One of these is between donor N1-H12 and acceptor O2 moieties, while the second one is between donor N1-H13 and acceptor O3 moieties (Table 2). These intermolecular hydrogen bonding interactions play a vital role in constructing of PNAPC crystals. In addition to these hydrogen bonding interactions, short C–H···O contacts predicted for PNAPC crystal are presented in Table 2.

The IR and Raman spectra for PNAPC molecule have been simulated by using B3LYP and HSEH1PBE levels and the obtained results are given in Fig. 2 and Fig. 3 as compared with FT-IR and FT-Raman results [8]. The calculated vibration frequencies, the percentage of relative contribution of various internal co-ordinates and approximate descriptions of normal modes are listed in Table 3. Frequency calculations at the same levels of theory revealed no imaginary frequencies, indicating that an optimal geometry at these levels of approximation was found for PNAPC. It is well known that DFT levels overestimate the vibration wavenumbers due to the neglecting of anharmonicity, lack of electron correlation effects and basis set deficiencies [19]. So, the scaling factor 0.9614 [20] was used providing closer results to the experimental ones [8].

Fig. 2

Fig. 3

NBO analysis is used to provide a clear description of the stabilizing interactions between filled (donor) and empty (acceptor) orbitals and destabilizing interactions between filled orbitals [25], and it could enhance the analysis of intra-and intermolecular interactions. NBO theory also allows the assignment of the hybridization of atomic lone pairs and of the atoms involved in bond orbitals. The hyperconjugative interaction energy was deduced from the second-order perturbation approach [26, 27]. For each donor (i) and acceptor (j), the stabilization energy E⁽²⁾ associated with the delocalization i → j is estimated as:

where q_i is the donor orbital occupancy, ϵj and ϵ_i the diagonal elements and F(i, j) is the off diagonal NBO Fock matrix element. NBO analysis was performed to elucidate the intramolecular, hybridization and delocalization of electron density within PNAPC by using B3LYP and HSEH1PBE levels with the 6-311++G(d,p) basis set. The second order perturbation theory analysis of Fock matrix in NBO shows strong intermolecular hyper conjugative interactions presented in Table 4. In NBO analysis, the larger energy values of hyper conjugative interactions (E(2)), the more intensive is the interaction between electron donors and electron acceptors, i.e. the more donating tendency from electron donors to electron acceptors, the greater the extent of conjugation of the whole system. NBO analysis indicates the strong intramolecular hyperconjugative interactions formed by the orbital overlap between bonding and antibonding orbitals, leading to inter and intramolecular charge transfer (Table 4). There occurs a strong intramolecular hyperconjugative interaction of π electrons from (C4-C5) to π* (N2-O5) with the energy values of 24.48 and 21.51 kcal/mol at B3LYP and HSEH1PBE levels, respectively. The intramolecular hyperconjugative interaction of σ (C1-C6) distributes to σ* (C1-C2) and (C5-C6), leading to stabilization of 4.34 and 2.88 kcal/mol, respectively. This enhances further conjugation with antibonding orbital of π* (C2-C3) and π* (C4-C5) which leads to strong delocalization of 17.30 and 19.99 kcal/mol, respectively. Interactions of the same kind were also calculated for PNAPC molecule (Table 4). These interactions resulted in molecular charge transfer causing stabilization of PNAPC, and the large hyperconjugative interaction energies values are the indicators of weakening of the respective bonds. NBO analysis clearly explains the formation of H-bonded interaction between the LP(O) and σ*(N-H) antibonding orbitals. The stabilization energies E(2) associated with hyperconjugative interactions LP(3) O2 → σ*(N1-H12) and LP(3) O3 → σ*(N1-H13), were equal to 19.61 and 19.65 kcal/mol, which quantify the extent of intermolecular hydrogen bonding (Table 4).

Density functional theory calculations have been proven to be a useful tool in the description of the relation among the electronic structure and its NLO response of a molecular system. The DFT approach allows the definition of molecular NLO properties as an inexpensive way to design molecules by analyzing their potential and determine the hyperpolarizability tensors of molecules before synthesis of the molecules [28, 29]. The significance of the polarizability and the first hyperpolarizability of a molecular system depends on the efficient electronic communication between donating and withdrawing groups, due to the fact that it is the basis to intramolecular charge transfer [30]. The intramolecular charge transfer from the donating to the withdrawing group through a conjugated path can induce large variations in both the molecular dipole moment and molecular polarizability, making IR and Raman activity strong at the same time [31]. Furthermore, the molecular hyperpolarizability can be enhanced in organic molecules by substitutions with NH3 and NO2 groups, which are involved in hydrogen bond interactions. The N=O, C=C and N–H stretching vibrations were calculated as strong and simultaneously active in IR and Raman spectra. So, the calculated stretching vibrations clearly demonstrate the presence of charge transfer interactions in PNAPC, making highly polarizable molecule. The first hyperpolarizability (β), dipole moment (µ) and polarizability (α) were calculated using B3LYP and HSEH1PBE 6-311++G(d,p) basis set on the basis of the finite-field approach [32-34]. Dipole moment of a molecule was used to study the intermolecular interactions. Additionally, dipole moment as a reflector of the molecular charge distribution can be used as descriptor to depict the charge movement across the molecule depending upon the centers of positive and negative charges. The dipole moment for PNAPC molecule were calculated as 7.244 and 7.216 Debye by using B3LYP and HSEH1PBE levels, respectively. Theoretical values indicate that dipole moments are dominant in the x and y directions but negligible in z direction. The magnitude of the molecular polarizability (α) and hyperpolarizability (β) are the important key factors in a NLO system. The computed β values of PNAPC were found as 11.272 × 10⁻³⁰ and 10.220 × 10⁻³⁰ esu by using B3LYP and HSEH1PBE levels, respectively. Urea and PNA were the prototypical molecules used in the study of the NLO properties of molecular systems and are frequently used as a threshold value for comparative purposes [35-37]. Obtained hyperpolarizability values are given in Table 5, in comparison with urea and PNA. From Table 5, the PNAPC molecule exhibits significant NLO character as compared with urea and PNA molecules [35]. Therefore, the PNAPC complex might be a potential excellent nonlinear optical material. The NLO results also indicate the electrons of the studied molecule can be easily polarized with changing environments such as lights and fields.

The frontier molecular orbitals play an important role in the electric and optical properties for of molecular systems. The HOMO represents the ability to donate an electron, while LUMO represents the ability to accept an electron. The small energy gap between the HOMO and LUMO leads to efficient electronic charge transfer from donating groups to withdrawing groups, making the molecule highly polarizable.

According to Fig. 4, HOMO is mainly localized over the perchlorate group, however LUMO is characterized by a charge distribution on nitroanilinium group. So, the HOMO→ LUMO transition implies an electron density transfer from perchlorate to the remaining parts of PNAPC complex. The HOMO and LUMO were simulated by using B3LYP and HSEH1PBE levels, and illustrations obtained at B3LYP level are presented in Fig. 4. The HOMO and LUMO energies were calculated as −8.2281 and −3.6830 eV at B3LYP level, −8.1285 and −3.7505 eV at HSEH1PBE level, respectively (Table 6). The energy gap between the HOMO and LUMO were found as 4.5451 and 4.3780 eV. These energies indicate the lowering of energy gap and also reflect the NLO activity of PNAPC. Furthermore, the chemical hardness (η) is a good indicator of the chemical stability. The soft molecules are more polarizable than the hard ones since they need small energy to excite one electron from a ground state to excited state. Electronegativity (χ) and chemical hardness (η) can be calculated using the frontier molecular orbital energies [38, 39]. The χ and η values were calculated as 5.9555 and 2.2775 eV at B3LYP level (Table 6). Consequently, PNAPC molecule is a polarizable complex and also can be used as an efficient NLO material.

Fig. 4

The molecular electrostatic potential (MEP) is a very useful descriptor for determining reactive sites for electrophilic and nucleophilic reactions as well as hydrogen-bonding interactions [40]. It is well known that MEP simultaneously displays molecular size, shape as well as positive, negative and neutral electrostatic potential region. The color scheme for the MEP surface is red (electronrich or partially negative charge), blue (electrondeficient or partially positive charge), light blue (slightly electron-deficient region), yellow (slightly electron-rich region), respectively. Areas of low potential, red, are characterized by an abundance of electrons. Areas of high potential, blue, are characterized by a relative absence of electrons. The MEP surface for PNAPC was simulated at B3LYP/6311++G(d,p) basis set, and depicted in Fig. 5.

Fig. 5

As can be seen from the MEP surface of PNAPC, while the regions having the most negative potential are over the perchlorate group, oxygen and chloride atoms with a maximum value of −0.122 a.u., the regions having the most positive potential are over the hydrogen atoms of ammonia group with a maximum value of +0.122 a.u. So, the MEP clearly indicates the formation of the intermolecular hydrogen bonding interactions. In Fig. 5, the negative MEP is also localized over nitrogen and oxygen atoms of the nitro group and is reflected as a yellowish color, while the positive MEP is localized over the ring of hydrogen atoms and is reflected as bluish. This is an expected behavior, since electrostatic potential correlates with electronegativity and partial charges. The electrostatic surface potential (ESP) and the contour plot of ESP were simulated by using B3LYP/6311++G(d,p) level. The polarization effect is also visible in these illustrations as can be seen in Fig. 6.

Fig. 6

The Mulliken atomic charge is directly related to the vibrational properties of the molecular systems, and explains how the electronic structure changes under atomic displacement; it is therefore related directly to the chemical bonds present in the molecule [41]. It affects dipole moment, polarizability, electronic structure and much more properties of molecular systems.

Mulliken atomic charges calculated at the B3LYP and HSEH1PBE levels with the 6-311++G (d, p) are collected in Fig. 7. The maximum negative charge belongs to O2 and O3 atoms and its value is −0.541. The maximum positive charge belonging to Cl atom is 1.128. The atomic charges of all hydrogen atoms in the NH3 group are positive. The atomic charges of two hydrogen atoms involved in the hydrogen bonding interaction are 0.323, while the third is 0.266. So, the H atoms involved in the hydrogen bonding interactions have higher positive charge than the third one. These charge variations clearly reveal the existence of N–H···O intermolecular hydrogen bonding. The electron withdrawing character of NO2 group in PNAPC is demonstrated by the decrease of electron density on ring carbon atom. The N1 atom has negative charge, while the N2 atom has positive charge due to the electronegativity of oxygen atoms. From Fig. 7, the carbon atoms have either positive or negative charges between −0.054 and 0.128, while all of the ring hydrogen atoms have positive charge.

Fig. 7