Microwave Spectrum, Dipole Moment, and Barrier to Internal Rotation of Fluoral

Abstract

About 700 transitions have been measured and assigned in the microwave spectrum of the ground vibrational state and first excited torsional state of natural and deuterated trifluoroacetaldehyde. Most of these lines occurred as doublets due to tunneling through the barrier to internal rotation of the trifluoromethyl group. The splittings of these doublets were used in conjunction with a recently published computer program to compute independent values of the barrier in the ground state of the normal isotopic species and the excited states of both isotopes. The final averaged barrier estimate is 885±75 cal/mole, the first such value of a three‐fold barrier of a CF3 group to be obtained from microwave splittings. This barrier is considerably lower than that (1150 cal/mole) for acetaldehyde. In general the fit with a rigid—rigid model is good but not perfect. The dipole components are estimated to be μ = 0.15 D and μ b = 1.64 D. Two novel techniques employed in the assignment of the very intense and rich fluoral spectrum were the assignment, by least‐squares polynomial curve fitting, of a Q‐branch series of quite high J values as the initial step in solving the spectrum, and the application of the double resonance technique to the location of the weak R‐branch spectrum of the excited state.