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

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.Contribution No. 564 from the Department of Crystallography and Biophysics, University of Madras, Guindy Campus, Madras-600025, India.