Spectral data are presented for three acceptor complexes of trimethylamine and for two of their perdeuteroisotopes over the range 33-4000 cm-1. Data were taken both at 300° and 83°K over the near-infrared region. This has allowed a complete assignment of all intramolecular motions and the discovery of several of the weaker lattice intercombination frequencies. Data are reported in all three cases for low-lying degenerate bending motions in the frequency range 33-80 cm"1. The revised assignments reported here have permitted a complete description of the skeletal intramolecular potential which predicts properly the observed isotope effects. The model proposed suggests a revision of previous considerations regarding force fields in charge-transfer complexes. In particular, it is necessary to account for skeletal stretch-bend interactions.