Structure and Vibrational Force Field of Methyldifluoroamine, (CH_3NF_2. An Electron-Diffraction Investigation Augmented by Microwave and Infrared Spectroscopic Data and by Ab Initio Molecular Orbital Calculations

摘要

The structure of methyldifluoroamine, CH_3NF_2, was determined by gas-phase electron diffraction augmented by rotational constants from microwave spectroscopy taken from the literature and by results from molecular orbital calculations. The structural results are consistent with C_s symmetry for a molecule with staggered bonds. The experimental bond distances and bond angles (r_α~0/r_g) and ＜α), with estimated 2σ uncertainties are C-H = 1.104/1.124(5) A (average value), N-F = 1.406/1.408(2) A, C-N = 1.467/1.469(6) A, C-N-F = 104.1(2)°, F-N-F = 101.7(2)°, N-C-H_(anti) = 109.9(11)°, N-C-H_(gauche) = 106.5(10)°, H_(gauche)-C-H_(gauche) = 110.6(28)°; the subscripts indicate orientation with respect to the nitrogen lone pair. A scaled quantum-mechanical (SQM) quadratic vibrational force field was evaluated by symmetrizing the quantum-mechanical (MP2/6-311++G(d, p)) Cartesian force constants and scaling the results to fit observed infrared wavenumbers from the literature. The N-F stretching force constants for the other fluoromaines NF_3 and CH_3)_2NF were also determined in a similar fashion. Contrary to an earlier report, the values were fond to increase with decreasing bond length consistent with Badger's rule.