Molecular Structure of tert-Butylazide: A Gas-Phase Electron Diffraction and Quantum Chemical Study

The molecular structure of tert-butylazide has been determined by gas-phase electron diffraction and quantum chemical calculations. The HF/6-31G* and B3LYP/6-31G** calculations yielded near Cs symmetry for the tert-butyl group, anti conformation of the (C)N—N bond with respect to one of the \(({\text{N}}){\text{C}}{\kern 1pt} - {\kern 1pt} {\text{C}}({\text{H}}_3 )\) bonds, and an essentially free rotation around the \(({\text{C}}){\text{N}}{\kern 1pt} - {\kern 1pt} {\text{N}}({\text{N}})\) bond with a 0.34 kcal/mol energy difference between syn and anti conformations of the CNNN moiety, the anti being the more stable form. The electron diffraction analysis was carried out by modeling a mixture of conformational isomers, generated by rotating the terminal nitrogen of the azide group, using a computed rotational potential. The data are consistent with Cs symmetry for the tert-butyl group. The \(({\text{C}}){\text{N}}{\kern 1pt} - {\kern 1pt} {\text{N}}\) bond, however, was found to be rotated out of the anti position, with respect to one of the \(({\text{N}}){\text{C}}{\kern 1pt} - {\kern 1pt} {\text{CH}}_3 \) bonds, by 12.5(12)°. The electron diffraction analysis yielded the following bond lengths (rg), bond angles, and torsional angles: \({\text{C}}{\kern 1pt} - {\kern 1pt} {\text{H 1}}{\text{.111(5) }}{\AA}\), \(({\text{N}}{\kern 1pt} - {\kern 1pt} {\text{N}})_{{\text{terminal}}} {\text{ 1}}{\text{.143(4) }}{\AA}, (N{\kern 1pt} - {\kern 1pt} {\text{N}})_{central} {\text{ 1}}{\text{.240(5) }}{\AA}{\text{, (C}}{\kern 1pt} - {\kern 1pt} {\text{C)}}_{{\text{mean}}} {\text{ 1}}{\text{.534(3) }}{\AA}{\text{, N}}{\kern 1pt} - {\kern 1pt} {\text{N}}{\kern 1pt} - {\kern 1pt} {\text{N 171}}{\text{.8(20)}}^\circ {\text{, N}}{\kern 1pt} - {\kern 1pt} {\text{N}}{\kern 1pt} - {\kern 1pt} {\text{C 118}}{\text{.6()10}}^\circ {\text{, N}}{\kern 1pt} - {\kern 1pt} {\text{C}}{\kern 1pt} - {\kern 1pt} {\text{C 102}}{\text{.5(6)}}^\circ {\text{ and 110}}{\text{.5(4)}}^\circ {\text{, C}}{\kern 1pt} - {\kern 1pt} {\text{C}}{\kern 1pt} - {\kern 1pt} {\text{H 111}}{\text{.2(4)}}^\circ {\text{, C}}{\kern 1pt} - {\kern 1pt} {\text{C}}{\kern 1pt} - {\kern 1pt} {\text{C 110}}{\text{.2(7)}}^\circ {\text{ and 112}}{\text{.6(7)}}^\circ {\text{, N}}{\kern 1pt} - {\kern 1pt} {\text{N}}{\kern 1pt} - {\kern 1pt} {\text{C}}{\kern 1pt} - {\kern 1pt} {\text{C 167}}{\text{.5(12)}}^\circ {\text{, 50}}{\text{.1(6)}}^\circ {\text{, and - 75}}{\text{.2(10)}}^\circ \).