Vicinal NMR proton-proton coupling constants. An NBO analysis

A method of natural bond orbital (NBO) interactions between bonds and antib onds, sigma (m) --> sigma (n)*, has been developed for analyzing vicinal pr oton-proton coupling constants, (3)J(HH). The contribution to (3)J(HH) from such an interaction sigma (m) --> sigma (*)(n) is defined as J(mn*) and is obtained following these three steps: (i) the SOS scheme is used to calcul ate 3JHH; (ii) the same calculation is repeated after deleting the relevant off-diagonal elements in the NBO Fock matrix; (iii) J(mn*) is obtained as the difference between those calculated in steps i and ii. Application of t his method to the ethane and fluoroethane molecules shows that the main con tribution to (3)J(HH) comes from the through space term while the substitut ion of a hydrogen in ethane by a fluorine changes this through space term a nd, in addition, gives a direct contribution to (3)J(HH).