Gas and matrix infrared spectra of n-methyl-ethylidenimine and valence force field

Infrared spectra of gaseous and matrix isolated trans-CH₃N:CHCH₃, CD₃N:CHCH₃ and CH_N:CDCD₃ have been recorded from 4000 to 200 cm^?1 under medium resolution. Nearly complete assignments have been established based upon rotational contours, isotope shifts and group frequency correlations. A number of Fermi resonances have been detected mainly related to CH(D) stretching fundamentals. ln particular, the light N-methyl group gives rise to a complicated Fermi multiplet in the CH-stretching region. An examination of Q sub-band structures resulted in the identification of a number of Coriolis resonances, which in most cases involve pairs of nearly coinciding ‘E-type“ methyl deformation fundamentals. The rotational fine structure of three bands has been analysed by means of combination differences providing estimates for effective rotational constants A(u)-$\text{1}\text{2}$ B(u)+C(u)]. From a normal coordinate analy a 30 parameter valence force field has been obtained, which allows an approximation of 85 experimental frequencies with an RMS deviation of 7.4 cm^?1.