Theoretical analysis of solvent effects on nitrogen NMR chemical shifts in oxazoles and oxadiazoles.

Using quantum chemistry methods we have evaluated the solvent effects on the (14)N NMR chemical shifts in five oxa- and oxadiazoles dissolved in twelve solvents. These solvents differ in their polarity with the dielectric constants varying from 2 to 80. Moreover, three of them have a hydrogen-bond donor character. All possible hydrogen-bonding in the water solution with the oxygen and nitrogen (hydrogen-acceptor) centers in oxazoles (2) and oxadiazoles (3) have been considered in our studies. It has been shown that both the pure solvent and hydrogen-bonding effects are significant and result in (14)N magnetic shielding increase. In water solutions the pure solvent effect is larger than the hydrogen-bonding effect. In addition, the solvent effect has been analyzed in terms of its direct and indirect contributions. It should be emphasized that our theoretical results for (14)N chemical shifts in oxa- and oxadiazoles remain in a very good agreement with the accurate experimental data.