On the estimation of second moment for organic compounds in wide-line PMR: analysis of group geometry

Abstract

Neutron diffraction data available in the literature have been analyzed statistically to arrive at the ideal geometry for the commonly occurring groups, NH₄⁺, NH₃⁺, CH₃, CH₂, and CH; this has yielded optimum experimental values for the bond length and bond angle parameters. They are: N-H distance of 1.030 Å in both NH₄⁺ and NH₃⁺ groups, H-N-H angle of 109.47° and 108.1° respectively in NH₄⁺ and NH₃⁺ and 110.8° for the corresponding X-N-H angle in -NH₃⁺. For CH₃, CH₂, and CH groups the bond length C-H is 1.090 Å, while the H-C-H and X-C-H angles are respectively 108.2° and 110.5° in CH₃ and 107.2° and 109.3° in CH₂. The importance of using precise values of both bond lengths and bond angles for the ideal geometry when estimating theoretical second moments is emphasized.