VIBRATIONAL-SPECTRA OF CYCLOPROPENES .1. IR-SPECTRA OF 3,3-DISUBSTITUTED CYCLOPROPENES

The IR spectra were measured and the vibrational problem was solved for a series of 3,3-disubstituted cyclopropenes [R1: CH3; R2: C(CH3)3 (I), C6H5 (II), C=C-C(CH3)3 (III), C=CH (IV)]. It was shown that a number of vibrations of the cyclopropene ring [symmetrical vibration of the ring, stretching and deformational (planar and out-of-plane) vibrations of the vinyl C-H bonds] are characteristic in frequency and form. On the basis of an analysis of the influence of a substituent at the C3 atom on the force field and the frequencies of the characteristic vibrations, it was concluded that the effect of conjugation of the substituent with sigma(Walsh) of the MO of the ring on the distribution of electron density in the ring plays a determining role.