Spectra and structure of small ring molecules. XLVII. Rotational and vibrational spectra and conformational stability of cyclobutylcarboxylic acid fluoride

The microwave spectrum of cyclobutylcarboxylic acid fluoride, c‐C4H7CFO, has been recorded from 18.0 to 40.0 GHz. The a‐type R‐branch transitions have been observed and assigned for the ground and two vibrationally excited states of the asymmetric torsional mode as well as for the first vibrationally excited state of the ring puckering fundamental. The rotational constants were determined from the frequency fit to a rigid rotor model for the ground vibrational state to be: A=5467.35±0.03, B=1884.76±0.01, and C=1558.64±0.01 MHz. These constants are shown to be consistent with an equatorial‐gauche conformation (i.e., the CFO group is in the equatorial position relative to the ring and the C=O bond is eclipsing, or nearly so, one of the C–C bonds of the ring). From relative intensity measurements the frequency for the asymmetric torsion for this conformer is estimated to be 72±10 cm−1. From the Stark effect the dipole moment components were determined to be: ‖μa‖ =2.97±0.02, ‖μb‖ =0.84±0.01, ‖μc‖ =0.35±0.01,...