NMR spectra, GIAO and charge density calculations of five‐membered aromatic heterocycles

The B3LYP/6‐31 + G(d) molecular geometry optimized structures of 17 five‐membered heterocycles were employed together with the gauge including atomic orbitals (GIAO) density functional theory (DFT) method at the B3LYP/6‐31 + G(d,p), B3LYP/6‐311 + + G(d,p) and B3LYP/6‐311 + G(2d,p) levels of theory for the calculation of proton and carbon chemicals shifts and coupling constants. The method of geometry optimization for pyrrole (1), N ‐methylpyrrole (2) and thiophene (7) using the larger 6‐311 + + G(d,p) basis sets at the B3LYP/6‐31 + G(d,p), B3LYP/6‐311 + + G(d,p), B3LYP/6‐31 + G(2d,p) and B3LYP/cc‐pVTZ levels of theory gave little difference between calculated and experimental values of coupling constants. In general, the 1 H and 13 C chemical shifts for all compounds are in good agreement with theoretical calculations using the smaller 6‐31 basis set. The values of n J HH ( n = 3, 4, 5) and rm n J CH ( n = 1, 2, 3, 4) were predicted well using the larger 6‐31 + G(d,p) and 6‐311 + + G(d,p) basis sets and at the B3LYP/6‐31 + G(d,p), B3LYP/6‐311 + + G(d,p), B3LYP/6‐31 + G(2d,2p) levels of theory. The computed atomic charges [Mülliken; Natural Bond Orbital Analysis (NBO); Merz‐Kollman (MK); CHELP and CHELPG] for the B3LYP/6‐311 + + G(d,p) geometry optimized structures of 1–17 were used to explore correlations with the experimental proton and carbon chemical shifts. Copyright © 2007 John Wiley & Sons, Ltd.