The use of MM/QM calculations of 13 C and 15 N chemical shifts in the conformational analysis of alkyl substituted anilines.

Abstract

The calculation of the 13 C and 15 N NMR chemical shifts by a combined molecular mechanics (Pcmodel 9.1/MMFF94) and ab initio (GIAO (B3LYP/DFT, 6-31+G(d)) procedure is used to investigate the conformations of a variety of alkyl substituted anilines. The 13 C shifts are obtained from the GIAO isotropic shielding (Ciso) with separate references for sp3 and sp2 carbons (δc=δref-Ciso). The 15 N shifts are obtained similarly from the GIAO isotropic shielding (Niso) with reference to the 15 N chemical shift of aniline. Comparison of the observed and calculated shifts provides information on the molecular conformations. Aniline and the 2,6-dialkylanilines exist with a rapidly inverting symmetric pyramidal nitrogen atom. The 2-alkylanilines have similar conformations with the NH2 group tilted away from the 2-alkyl substituent. The N,N-dialkylanilines show more varied conformations. N,N-dimethylaniline has a similar structure to aniline, but N-ethyl, N-methylaniline, N,N-diethylaniline, and N,N-diisopropylaniline are conformationally mobile with two rapidly interconverting conformers. In contrast, the anilines substituted at C2 and the nitrogen atom exist as one conformer where the steric interaction between the C2 substituent and the N substituent determines the conformation. In 2-methyl-N-methylaniline, the nitrogen atom is pyramidal as usual with the N-methyl opposite to the 2-methyl, but in 2-methyl-N,N-dimethyl aniline, the NMe2 group is now almost orthogonal to the phenyl plane. This is also the case with 2-methyl-N,N-diethylaniline and 2,6-diisopropyl-N,N-dimethylaniline. The comparison of the observed and calculated 15 N chemical shifts confirms the above findings, in particular the pyramidal conformation of aniline and the above observations with respect to the conformations of the N,N-dialkylanilines.