Microwave spectrum, structure, dipole moment and centrifugal distortion of nitrosomethane. Dipole moment of acetaldehyde

Abstract

The microwave spectra of ten isotopic species of nitrosomethane have been measured enabling structures for CH₃NO and CD₃NO to be determined independently. The mean of these structures is found to be : NO = 1.211, C—N = 1.480, C—H_i(in plane)= 1.094, C—H₀(out-of-plane)= 1.094 Å, ∠CNO = 113.2, ∠H_iCN = 111.0, ∠H₀CN = 107.2 and ∠H₀CH₀= 109.2°. The methyl group is tilted by 2.5° away from the oxygen atom. The orientation of the dipole moment in nitrosomethane has been determined from isotopic measurements. For CH₃¹⁵NO, higher order Stark perturbation terms, arising through an accidental near degeneracy, had to be included to obtain an accurate dipole moment. The components of the dipole moment are; µ_a= 2.262, µ_b= 0.516 and µ= 2.320 ± 0.004 D. The orientation of the dipole moment in the related molecule acetaldehyde has been determined from isotopic work and previous Stark effect measurements have been refined to give µ_a= 2.537, µ_b= 1.062 and µ= 2.750 ± 0.006 D.

A, E splittings in the microwave spectra of CH₃NO and CD₃NO have been analysed by both the principal axis method (PAM) and the internal axis method (IAM) for a rigid top–rigid frame model. An attempt to fit the spectrum of CD₃NO was made using Woods' IAM treatment in conjunction with Watson's centrifugal distortion theory. More precise distortion constants were then obtained from the spectra of sym-CH₂DNO and sym-CHD₂NO and used to determine a preliminary harmonic force field for nitrosomethane. A few perturbations in the spectra of the symmetric species have been observed due to sym–asym interactions. The structure, bonding and vibrational properties of nitrosomethane are discussed.