Study of hydrogen bonding and solvent polarity effects on the nitrogen NMR shieldings of N,N‐dimethylacetamidine

Abstract

Solvent effects on the nitrogen shieldings of N,N-dimethylacetamidine (1) were found to be extremely large for the imino group (about 120 ppm) and for the amino moiety (about 50 ppm). A detailed analysis of the solvent-induced variations revealed contributions from three large interactions. These are due to solvent polarity, hydrogen bonding from solute to solvent, via the NH moiety of the solute, and from solvent to solute, involving the nitrogen lone pairs of the solute. For the imino moiety, large shielding effects are observed due to solvent polarity and solvent-to-solute hydrogen bonding, whereas solute-to solvent hydrogen bonding leads to a large decrease in nitrogen shielding. For the NMe2 group, the changes are of opposite sign and smaller in magnitude. Ab initio CHF-GIAO magnetic shielding calculations employing a 6–31++G** basis set and geometries optimized using the same basis set are reported for 1 and some related molecules. The experimental range of nitrogen shielding considered is about 270 ppm and shows an excellent linear correlation with the calculated results. The least-squares standard deviation amounts to only 1.7% of the observed shielding range. The effects on the nitrogen shielding of 1 caused by full protonation are accurately reproduced within this correlation. Copyright © 2000 John Wiley & Sons, Ltd.