The 3‐21G(N*) basis set: An economical polarized basis set for ab initio studies on nitrogen‐containing molecules

The 3‐21G basis set shares with its older cousin, the 4‐31G basis set, a tendency to overestimate valence angles at nitrogen atoms and to underestimate seriously barriers to inversion at such atoms. The 6‐31G* basis set generally yields greatly improved results in these respects. It is here shown that, for a variety of molecules, supplementation of the 3‐21G basis set at three‐ or two‐coordinate nitrogen atoms with a set of six d ‐functions having exponent 1.0 leads to optimized geometries and inversion barriers at such nitrogen centers in good agreement with results obtained with the 6‐31G* basis set. This supplemented basis set, designated as 3‐21G(N*), also leads to calculated vibrational frequencies in good agreement with those calculated with the 6‐31G* basis set. The 3‐21G(N*) basis set offers an economical alternative to the 6‐31G* basis set, particularly for molecules containing several first‐row atoms other than nitrogen.