A study of solvent polarity and hydrogen bonding effects on the nitrogen NMR shieldings of N-nitramines and ab initio calculations of the nitrogen shieldings of C-nitro, N-nitro and O-nitro systems

摘要

High-precision nitrogen NMR shieldings, bulk susceptibility corrected, are reported for dimethylnitramine (1) and diethylnitramine (2) in a variety of solvents which represent a wide range of solvent properties from the point of view of polarity as well as hydrogen bond donor and acceptor strength. The observed range of solvent-induced nitrogen shielding variations of I and 2 is significant for the amino-type nitrogens, up to about 10 ppm, and originates essentially from the deshielding effect of the increasing polarity of solvent. On the other side, the nitro nitrogens reveal a very weak and rather chaotic response to solvent effects, within about 2 ppm. This is in a sharp contrast with the behavior of the nitrogen shieldings of C-nitro and O-nitro groups where solvent effects induce variations within at least 6 ppm which follow a regular pattern of enhanced magnetic deshielding with increasing polarity involved. This apparent insensitivity of the N-nitro group shielding to solvent effects seems to stem from the cancellation of opposite effects, the shielding effect of the electron charge migration to the nitro group upon increasing solvent polarity, and the deshielding effect produced by the flattening of the pyramidal arrangement of bonds at the amino nitrogen with the increasing polarity of the medium. Ab initio quantum-mechanical calculations using the GIAO/B3PW91/6-311 + +G** approach and geometry optimizations employing the same basis set and density functionals show an excellent linear correlation with the experimental data and reproduce not only major changes but also most of the subtle variations in the experimental nitrogen shieldings of the nitro systems as a whole. (C) 2002 Elsevier Science B.V. All rights reserved. [References: 20]