Relationship between /sup 15/N/sup 13/C coupling constants and hybridization

The relationship for carbon-nitrogen coupling, parallel /sup 1/J/sub 13C 15N parallel = (80)/sup -1/ (percent S/sub C/) (percent S/sub N/), was explored to determine why the relationship holds for some CN constants while failing badly for others. To analyze the problem (percent S/sub C/)(percent S/sub N/) was computed from INDO localized orbitals, using the formula (percent S/sub C/)(percent S/sub N/) approximately (100 P/sub S/sub C/sub S/sub N/)/sup 2/; where the P/sub S//sub C/sub S//sub N/ is defined as 2 ..sigma../sub j/ C/sub jS/sub C/ C/sub jS/sub N//, the sum running over products of 2S coefficients in the localized orbital(s) of the CN bond(s). Two simple rules were formulated for the validity of the relationship: the CN bond cannot be a nitrile, isocyanide, nitrile oxide or isocyanate; and the single or double CN bond must have no s-containing lone pairs on C or N; pure p lone pairs are permitted. (DDA)