Lone-Pair Lone - Pair Interactions

The lone-pair ionisation potentials of the halogens, disulphides and hydrazines are analysed by a simple MO procedure including orbital overlap. Calculations of Slater overlap integrals for adjacent sp2 and sp3 orbitals show that the energy of the cis isomer is lower than the energy of the trans in the former case but the reverse in true for sp3 hybridised orbitals. Gaussian overlap integrals are greater for the trans form than for the cis, both for sp3 and sp2 nitrogen orbitals. Experimental evidence in support of these conclusions for sp2 systems is to be found in the greater stability of the Z isomers of sulphonylamines and sulphur diimides.