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 ν_B(B₁) of the C_2v 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 ν_B satellites in the rotational spectrum, which give [graphic omitted] after accounting for nuclear spin statistics appropriate to a B₁ vibration of a C_2ν molecule in which 3 pairs of equivalent protons are exchanged by C₂.

The infra-red inactive ring twisting mode, ν_T(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 ν_T 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 V_B.