Computational studies, NMR, Raman and infrared spectral analysis of centrosymmetric (2Z,4Z)-Hexa-2,4-dienedinitrile

Abstract

The Raman (50–3500[Formula: see text]cm[Formula: see text]) and infrared (200–3500[Formula: see text]cm[Formula: see text]) spectra of (2Z,4Z)-Hexa-2,4-dienedinitrile (C6H4N2; cis,cis-HDDN) have been recorded. Initially, three conformers were proposed based on following point groups: C2h, C2v and [Formula: see text] (gauche). For comparison purposes, M05-2X, M06-2X method were used in addition to B3LYP, MP2[Formula: see text]full and MP4[Formula: see text]full quantum mechanical calculations employing 6-31G(d) and 6-311+G(d,p) basis sets using Gaussian 09 program. Aided by potential energy surface scan, the C2h conformer is verified as a global minimum and the gauche ([Formula: see text]) being a local minimum with an energy barrier of 2.82[Formula: see text]kcal/mol. The energy difference ranged from 4.87 to 13.70[Formula: see text]kcal/mol favors the C2h conformer in good agreement with Raman and infrared spectral analysis. These results were also supported by the observed [Formula: see text] value (H9-H[Formula: see text]) at 10.8[Formula: see text]Hz which is consistent to 10.19[Formula: see text]Hz estimated for centrosymmetric conformer (C2h) rather than 4.81[Formula: see text]Hz predicted for gauche ([Formula: see text]). Complete vibrational assignments are proposed herein based on normal coordinate analysis (NCA) and potential energy distributions (PEDs) combined with theoretical vibrational frequencies and force constants in internal coordinates. It is crucial that NCA using VEDA 4 program with mixing shows large deviations from ours due to neglecting symmetry elements while mixing molecular vibrational motions. The results of these spectroscopic and theoretical studies are reported herein and compared with similar molecules whenever appropriate.