Aromaticity in X₃Y₃H₆ (X = B, Al, Ga; Y = N, P, As), X₃Z₃H₃ (Z = O, S, Se), and phosphazenes. Theoretical study of the structures, energetics, and magnetic properties

Abstract

A systematic estimation of aromaticity in X₃Y₃H₆, 1, (X = B, Al, Ga; Y = N, P, As), P₃N₃H₆, 2, and X₃Z₃H₃, 3 (Z = O, S, Se), has been conducted using structural, energetic, and magnetic criteria. Estimates based on aromatic stabilization energy (ASE) calculations predict that 1BN (1; X = B, Y = N) and 1BP are equally aromatic. Contrary to this, we have found, from magnetic susceptibility exaltation (MSE) and from the nucleus independent chemical shift (NICS) data at the B3LYP/6-31G^∗ level, that 1BN is not aromatic while 1BP is. This emphasizes the fact that energetic and magnetic criteria need not be parallel. On the basis of MSE and NICS values, all 1XP compounds show strong aromatic character; 1XAs are borderline aromatic while 1XN compounds are nonaromatic. Despite being aromatic, all 1XP and 1XAs compounds are found to prefer nonplanar geometries. MSE and NICS criteria can also diverge quite strongly; this has been observed in the X₃Z₃H₃ family. MSE values for 3BS, 3BSe, 3AlO, 3AlS, and 3GaS are more than half of the MSE value for benzene, indicating substantial aromatic character. However, NICS estimates point to the contrary; none of the type 3 compounds are aromatic. The problem with the ASE and MSE is that both depend on the choice of the reference systems while NICS, which avoids the need for reference molecules, is impossible to vary experimentally. In spite of this epistemological deficiency of NICS, we find it complementary to the ASE and MSE criteria. Despite the existence of a large number of well-established structures and substantial aromatic stabilization energy, phosphazenes, 2, are not aromatic according to NICS data.