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, NH 4 + , NH 3 + , 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 A in both NH 4 + and NH 3 + groups, H-N-H angle of 109.47° and 108.1° respectively in NH 4 + and NH 3 + and 110.8° for the corresponding X-N-H angle in −NH 3 + . For CH3, CH2, and CH groups the bond length C-H is 1.090 A, 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.