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

The Raman (50-3500cm-1) and infrared (200-3500cm-1) 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 C2 (gauche). For comparison purposes, M05-2X, M06-2X method were used in addition to B3LYP, MP2=full and MP4=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 (C2) being a local minimum with an energy barrier of 2.82kcal/mol. The energy difference ranged from 4.87 to 13.70kcal/mol favors the C2h conformer in good agreement with Raman and infrared spectral analysis. These results were also supported by the observed J value (H9-H10) at 10.8Hz which is consistent to 10.19Hz estimated for centrosymmetric conformer (C2h) rather than 4.81Hz predicted for gauche (C2). 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.