STRAIN ENERGIES IN HOMOATOMIC NITROGEN CLUSTERS N-4, N-6, AND N-8

Strain energies and resonance energies can be obtained as the energy c hanges for appropriate homodesmotic reactions using ab initio calculat ed total energies as the energies of the reactants and products involv ed. Homodesmotic reactions conserve bond types and preserve valence en vironments at all atoms, requirements that favor the cancellation of b asis set and electron correlation errors in the ab initio energies. In this paper we calculate strain energies and resonance energies for N- 4, N-6, and N-8 clusters in a number of chemically significant but, fo r nitrogen, hypothetical structural forms. The nitrogen cluster strain energies are generally of the same order of magnitude as those of iso structural hydrocarbon clusters, and individual differences can be exp lained by using the ring strain additivity rule and recognizing the ef fect of the presence of lone pairs of electrons on nitrogen clusters b ut not on the hydrocarbons. Resonance energies of the nitrogen cluster s are much smaller than those of the comparable aromatic hydrocarbons. The differences can be rationalized by considering the relative stren gths of CC and NN single and double bonds. Strain and resonance energi es of nitrogen clusters are compared with those previously reported fo r homoatomic clusters of phosphorus and arsenic. Trends through the se ries are remarkably similar, but strain energies for clusters from low er periods are progressively smaller. Strain and resonance have been i mportant organizing concepts in organic chemistry for many years. Esti mates of corresponding parameters for inorganic analogs are only now b ecoming available.