Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Molecular structure, Mulliken charge, frontier molecular orbital and first hyperpolarizability analysis on 2-nitroaniline and 4-methoxy-2-nitroaniline using density functional theory
Graphical abstract
Introduction
Research in the field of designing nonlinear optical materials has gained momentum in the recent past on account of its role in optical parametric amplifiers, optical parametric oscillators, etc. [1], [2], [3]. Organic crystals with delocalized π electrons usually display large nonlinear optical (NLO) response and they are the potential candidates for applications in the emerging areas of photonics and laser technology [4]. It is noticed that the molecules that are substituted with donors and acceptors which exhibit intramolecular charge transfer. The π electron cloud movement from donor to acceptor makes the molecule highly polarized. The important parameters for optimizing the NLO properties are the strength of the donor and acceptor groups of the conjugated system. The peculiar physical properties of this interesting class of compounds are governed by the low band gaps. Due to their potential applications in photonic devices, the nonlinear optical properties of molecules, and their hyperpolarizabilities have become an area of extensive research and lot of experimental [5], [6] and theoretical efforts [7], [8], [9] are focused on the first hyperpolarizabilities of molecules. Aniline based compounds play a very important role in designing organic materials for molecular electronics. The high first hyperpolarizabilities of many nitro aniline molecules have led to describe them as prototypes for second harmonic generation. Ab initio and density functional theory (DFT) calculations are excellent alternative methods in the design of NLO molecules and help to predict some properties of the new materials, such as molecular dipole moments, polarizabilities and hyperpolarizabilities [10], [11], [12], [13], [14]. In the present study, density functional theory (DFT) was employed with the aim of calculating the energy gap, dipole moment, polarizability and first hyperpolarizability of 2-nitroaniline and 4-methoxy-2-nitroaniline molecules.
Section snippets
Computational details
Theoretical calculations were carried out using Gaussian 03 W [15] package of programs. Gaussian 03 W has already proved to be an important tool predicting molecular structures, spectroscopic properties and molecular origins of NLO properties [16], [17], [18], [19], [20]. The single 2NA molecule consists of an aniline molecule and a nitrate group, whereas 4M2N molecule additionally has a methoxy group (Fig. 1). Its atomic positions were obtained from ChemOffice ultra software program [21]. The
Geometry optimization
The optimized molecular geometry (Fig. 1) represents an isolated single molecule under ideal conditions with a stationary point at the potential energy surface; the convergence was confirmed by observing no imaginary vibrational wavenumbers obtained in the calculation. Table 1 shows the bond lengths and angles for optimized 2NA and 4M2N molecules from B3LYP method. The calculated bond lengths (C–C) of the ring vary from 1.380 Ǻ to 1.425 Ǻ. The phenyl ring appears to be a little distorted from its
Conclusions
In the present study, molecular geometry, frontier molecular orbital and hyperpolarizability analysis of 2NA and 4M2N molecule was done using density functional theory method for the first time. The optimized molecular geometry parameters were calculated at density functional theory method. The frontier molecular orbital analysis explaining the eventual charge transfer interactions taking place within the molecules. The calculated reactivity descriptors, hardness and electrophilicity index lead
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