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Variable temperature (−105 to −145°C) studies of the infrared spectra (3500–400 cm−1) of methylamine, CH3NH2, dissolved in liquid krypton have been recorded. From these data, the hydrogen bonding enthalpy has been determined to be 530 ± 29 cm−1 (6.34 ± 0.35 kJ/mol). The elusive ν13 and ν14 fundamentals, which are strongly mixed CH3 rock and NH2 twist, have been observed at 1244 and 876 cm−1, respectively. These assignments are supported by frequency predictions from ab initio MP2/6-31G(d) calculations where the predicted infrared intensities for these two vibrations are 0.054 and 0.002 km/mol. The ab initio predicted infrared spectrum compares very favorably with that observed in the krypton solution. Normal coordinate calculations have also been carried out for four other isotopomers of methylamine, CH3NHD, CH3ND2, CD3NH2, and CD3ND2 and vibrational assignments given from previously reported infrared spectra of matrix isolated samples. The Raman spectrum of these latter three isotopes, along with the normal species, have been predicted from MP2/6-31G(d) calculations and the results compared to the experimental spectra. The equilibrium structural parameters have been obtained from ab initio calculations utilizing several different basis sets with full electron correlation by the perturbation method to second order. These predicted values are compared to the previously reported experimental structural parameters.