The unexpectedly low sensitivity of the nitrogen NMR shieldings of covalent azides to solvent effects

Abstract

The nitrogen shieldings of covalent azides are shown to be essentially independent of molecular interactions in solutions of a large variety of solvents. They present a rare case where substituent effects, exerted by an aryl or alkyl group attached to the azide, clearly swpass the range of solvent induced effects on the nitrogen shieldings. This occurs in spite of the large net change splitting in the electronic structure of the azide group and the presence of lone pair electrons on this group. The observed induced range of solvent effects does not exceed 2 ppm in most cases which is similar to that previously observed for dinitrogen (N2) and an azo bridged structure.

Solvent Effects on Nitrogen Chemical Shifts

“…Terms a and s are large and negative for both, reflecting the solute to solvent hydrogen bond formation and the influence of solvent polarity. The δ 14N of tetramethylthiourea (39) and thiourea (40) exhibit comparable solvent dependence, $11 ppm, to the corresponding ureas 37 and 38 (Table 3) [46].…”

“…7) and phenyl azide 23 are almost solvent independent (Table 3) [39]. Some nitriles are capable of undergoing solvent-induced azido-tetrazoloazino valence tautomerism, as depicted in Figure 6 The structures of methyl cyanide (16), ethyl cyanide (17), isopropylcyanide (18), tert-butylcyanide (19), cyanamide (20), and N,N-dimethylcyanamide (21).…”

“…9) show a low, <3.6 ppm, solvent dependence [43]. Hydrogen bond donor solvents, such as trifluoroethanol cause a slight increase of δ 15N of Figure 9 The structures of N,N-dimethylmethane-sulphonamide (33), N,Ndimethylbenzene-sulphonamide (34), benzene-sulfonamide (35), N,Ndimethylmethanesulphinamide (36), tetramethylurea (37), methylurea (38), tetramethylthiourea (39), thiourea (40), N,N-dimethylacetamidine (41), N-methylacetimidate (42), and methylacetimidate (43). 34 (Table 3), implying a decrease in the double-bond character of its sulfurnitrogen bond as a result of hydrogen bond donation to the oxygen of the sulphonyl group, rather than to its nitrogen.…”

“…The solvent effects on the δ 14N of 1,2-diazine (46) 1,3-diazine (47), 1,4-diazine (48), 1,3,5-triazine (49), and 1,2,4,5-tetrazine (50, Fig. 10) depend on the number and on the position of the nitrogens of their azine core, yet the direction of the alteration is identical [39,50]. The largest solvent-induced δ 14N alteration, À48 ppm, is seen for 46, whereas the smallest, À10 ppm, for 50.…”

Correlations among experimental and theoretical NMR data to determine the absolute stereochemistry of darcyribeirine, a pentacyclic indole alkaloid isolated from Rauvolfia grandiflora

“…The increase of dipole moment for systems alkaloid-DMSO may change the charge density on the chemical structure and some carbon and hydrogen nuclei can be either shielded or deshielded. Polarization of the medium by a polar solvent (DMSO) causes the shielding of the nucleus [40,41]. As result, calculated carbon chemical shift values must be better considered for systems alkaloid-DMSO than the corresponding values for isomer treated alone.…”