Infrared spectra of nitrostyrene derivatives

The IR spectra of a series of 6 phenylnitroethenes (PNE) and 6 phenylnitropropenes (PNP) were assigned, based on a normal coordinates calculation performed on styrene and styrene-d₈. Some frequencies were sensitive to the electronic properties of the substituents, and others to the substitution on Cβ. AM1 minimum energy conformations and rotational barriers around the ClCα bond were calculated. According to these calculations, PNE are planar and PNP display an angle of 45° between the ethylenic and aromatic planes. AM1 underestimates the height of the electronic barriers, which is however modulated by the electron donor properties of the substituent on the ring. A correlation between the calculated electronic barrier and the CC ethylenic stretching frequency was observed.