Structure and potential energy function of cyclopent-3-enone. Part 2.—Out-of-plane ring modes from far infra-red, Raman and microwave spectra: ring bending potential function
Abstract
A Q branch progression identified with the fundamental and hot bands of the ring bending vibration νB(B1) of the C2v molecule cyclopent-3-enone has been observed in the far infra-red spectrum of the gas. The vibrational separations determined are: [graphic omitted]
These values are supported by relative intensity measurements of νB satellites in the rotational spectrum, which give [graphic omitted] after accounting for nuclear spin statistics appropriate to a B1 vibration of a C2ν molecule in which 3 pairs of equivalent protons are exchanged by C2.
The infra-red inactive ring twisting mode, νT(A2), is measured in the Raman spectrum of the gas at 379.0 cm–1, in good agreement with the value 390 ± 10 cm–1 from microwave relative intensity measurements of the νT satellites in the rotational spectrum.
Vibrational separations from the far infra-red spectrum lead to the simple, one dimensional, single minimum potential function for the ring bending vibration V(Z)= 80.78 (0.5 Z2+ 0.0102 Z4) cm–1 in terms of the dimensionless coordinate Z. This function is shown to be similar to those determined from the variation of rotational constants with VB.