Combined Electron-Diffraction, Spectroscopic, and Theoretical Determination of the Structure of N‑Deuterio-trans-Methyldiazene, CH3NND. Conformational Effects of the NN Double Bond

Gas phase electron-diffraction (GED) data obtained at a nozzle-tip temperature of 273 K have been combined with spectroscopic vibrational–rotational constants to determine the structure of trans-methyldiazene, an important prototype for the NN double bond. The N-deuterio form CH3NND was used in the study since it is appreciably more stable than CH3NNH. Both the theoretical and experimental results are consistent with a planar C s trans-CNND framework. The experimental results (r α 0/r g 273) are 1.465(2)/1.467(2) Å for the CN bond, 1.248(1)/1.251(1) Å for the NN double bond, and 1.037(17)/1.048(17) Å for the ND bond. The NND angle is 105.9(20)/105.6(20)° and the CNN angle is 112.4(5)/112.2(5)°, where the uncertainties in parentheses are twice the standard deviation from a simultaneous least-squares fit of the GED and microwave data. For the methyl group, both theory and experiment indicate that two CH bonds are symmetrically arranged out of the molecular plane while the third CH′ lies in the plane in an eclipsed (not staggered) cis-H′CNN arrangement. Theoretical calculations (B3LYP/cc-PVnZ and CCSD­(T)/cc-PVnZ) suggest a slight distortion of the methyl group, with a tilt of the methyl top axis about 5° away from the NN bond. The experimental data are consistent with this picture but are equally consistent with an undistorted methyl group. Inclusion of distortions predicted by theory in a complete basis set limit (CBS) lead to a preferred analysis with average values of 1.086(5)/1.106(5) Å for the CH bond length and an average HCH angle of 108.3(8)/107.8(8)°. Features of the structure of methyldiazene and related compounds are discussed. It is found that the short NN bond length in the diazenes produces much greater steric repulsion than in analogous ethylene compounds and this effect leads to some interesting conformational and distortion differences for attached CH3 groups.