Structural and computational studies of 2,5-diarylpenta-2,4-dienenitriles

2,5-Diarylpenta-2,4-dienenitriles 1–9 were synthesized and characterized by the high resolution 1H, 13C, 1H–1H COSY and 1H–13C COSY spectra. Spectral data indicate the trans arrangement of the side chain protons H(3), H(4) and H(5) and trans orientation of H(3) proton with respect to cyano group in 1–9. Computational calculations were carried out for some possible structures and they support the conformation in which all side chain protons are trans to each other and H(3) proton is trans to CN group. Moreover, syn orientation of NO2 group with respect to H(5) proton in 4–6 is revealed by computational calculations and chemical shift data. From the favored conformations, geometrical parameters, HOMO–LUMO energies, dipole moment, polarizabilities and first order hyperpolarizabilities were determined theoretically. The highest βtot is observed for the nitrile 9 where electron releasing substituent (OCH3) and electron withdrawing substituent (NO2) are present at the opposite ends of the conjugated system and hence it is the best NLO candidate. The NLO character decreases according to the order 9 > 3 > 5 > 4 > 7 > 8 > 6 > 2 > 1. The 1H and 13C chemical shifts in gaseous state as well as in solution were also determined theoretically by DFT method and they are in agreement with the experimental values. NBO analyses were further carried out for the minimum energy conformer. From 1H and 13C chemical shifts, the effect of introduction of substituents (NO2, OCH3, Cl) in the phenyl ring on the chemical shifts of the side chain protons [H(3), H(4) and H(5)] and carbons [C(3), C(4) and C(5)] were analyzed in detail.

► Conformations of nine arylacrylonitriles were predicted by NMR. ► DFT calculations done to predict favoured conformation, geometrical parameters, 1H, 13C spectral data, NBO analysis and HOMO-LUMO energies. ► NLO behavior investigated by polarizability calculations. ► Comparisons of experimental and theoretical 1H and 13C chemical shifts.