Study of hydrogen bonding and solvent polarity effects on the nitrogen NMR shieldings ofN,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.

Synthesis and NMR studies of 2- and 3-fluorosubstitued five-membered heterocycles

“…Let us now consider the trends observed in the shielding or deshielding of the fluorine nuclei in particular compounds studied. By analogy to the presentation of the nitrogen data applied by Witanowski in his numerous papers [28,29], we will present our results in terms of Ds, which represents the differences in the fluorine nuclear shielding constants of the compounds studied and that of fluorotrichloromethane used as reference. Thus, we use the expression ''fluorine shielding'' for Ds, since the positive sign corresponds to an increase in magnetic shielding.…”

Shielding in and around Oxazole, Imidazole, and Thiazole: How Does the Second Heteroatom Affect Aromaticity and Bonding?

“…The difference between the N1 and and N3 shieldings in 1-methylimidazole, measured in cyclohexane by Witanowski et al,33 amounts to 113.52 ppm. Surprisingly, the gas-phase HF-GIAO/6-31++G(d,p)//HF/6-31++G(d,p) estimate of this difference, 122.5 ppm, 34 is better than our HF-GIAO difference of 137.4 ppm, obtained using a larger basis, 6-311++G(d,p), and for a geometry, optimized at a higher level of theory, MP2(FC)/aug-cc-pVTZ.…”

“…Isotropic shieldings for all nuclei in oxazole, imidazole and thiazole, selected nuclei in 1-methylimidazole, and NICS(0), NICS(0.5) and NICS(1) values (in ppm), calculated at the HF-GIAO/6-311++G(d,p) and MP2-GIAO/6-311++G(d,p) levels of theory.data [33][34][35]. The difference between the N1 and and N3 shieldings in 1-methylimidazole, measured in cyclohexane by Witanowski et al,33 amounts to 113.52 ppm.…”

A Study of the Nitrogen NMR Spectra of Azoles and their Solvent Dependence

“…These investigations involve the study of the same molecule in a variety of solvents under similar conditions and this allows the investigators to isolate various aspects of the solvent effects on the molecular property studied. [2][3][4][5] In the present work, in order to understand the effect of the solvent on NMR parameters, we analyze the shielding constants and NQCC's of the nitrogen atoms in a complete set of parent structures of N-methyl-substituted azoles (see Figure 1). Within this group of molecules we have at our disposal a total of 23 chemically nonequivalent nitrogen atoms and the analysis of their NMR properties, when solvated in a number of solvents, provides a fairly large set of data.…”