Methylnitrene free radical. Ground state vibrational fundamentals and harmonic force field from jet-cooled emission spectra of the A 3E–X 3A2 band system

The à 3E–X̃ 3A2 UV emission spectra of the jet-cooled methylnitrene radical CH3N and the isotopomers 13CH3N and CD3N have been examined with moderate resolution. Analysis of the vibrational band progressions gives a complete set of all six ground state vibrational fundamentals for CH3N: The symmetric hydrogen stretch, ν″1=2943(04) cm−1; the symmetric hydrogen bend, ν″2=1349(04) cm−1; the C–N stretch, ν″3=1040(04) cm−1; the asymmetric hydrogen stretch, ν″4=2989(04) cm−1; the scissors, ν″5=1490(04) cm−1; and the rock, ν″6=903(08) cm−1. A Jahn–Teller distortion in the 3E excited state activates ground state progressions of the e-type modes. These vibrational frequencies, along with those from 13CH3N and CD3N, determine the 12 coefficients of a ground state harmonic force field. The resulting force constants indicate a strong C–N bond and weak C–H bonds, accompanying considerable departure from a usual methyl structure. The structure and force field are discussed in the context of a model involving sigma bond optimization of the C–N bond along with hyperconjugation.