Charge transfer in systems of conjugated bonds in cyanobiphenyl molecules: Quantum-chemical calculations of the structure and vibrational spectra

Abstract

Quantum-chemical calculations of the IR absorption spectra and geometric and electronic structure of cyanobiphenyl molecules Open image in new window (I) were performed for various angles between benzene ring planes by the B3LYP/6-31+G** method. It was shown that the stablest conformation of I (X=OCH3, OC3H7) should be the twist conformation with α= 37°, which was in agreement with the gas-phase experimental data. Rotation of benzene rings with respect to each other changed the relative orientation of the interacting π orbitals of the bridge ring carbon atoms and caused charge redistribution over molecule atoms, in particular, over terminal X and CN group atoms. The calculated period of charge oscillations on the alkyl and nitro groups coincided with the period of reversible charge transfer (∼5–10 fs) between the conjugated subsystems (benzene ring + substituent) observed as the α angle changed. The rate of charge transfer between the electron donor and electron acceptor groups was calculated to be (3–6)×105 m/s. Charge oscillations on benzene ring carbon atoms and donor and acceptor groups did not cause similar dipole moment oscillations and vibrations in the IR spectrum. The dipole moment of the molecule decreases as the angle between benzene ring planes increases, and the passage to the “perpendicular” conformation should increase the C≡N stretching vibration frequency by ∼5 cm−1 and decrease the intensity of the IR band by ∼2 times. The elongation of the aliphatic chain in the X group did not cause noticeable changes in the geometric and electronic structure of the molecule.