Study of solvent effects on the nitrogen NMR shieldings of some indolizines

Abstract

Solvent effects on the nitrogen NMR shieldings of indolizine and three azaindolizines are presented for a range of thirteen solvents. The results are discussed in terms of hydrogen-bonding an solvent polarity effects. 'Pyridine-type' nitrogen atoms show a much larger range of solvent effects than do 'pyrrole-type' nitrogens; solvent to solute hydrogen bonding is the dominant contribution to the changes in shielding for the 'pyridine-type' nitrogen atoms as the solvent is varied. Solvent polarity effects are important for both types of nitrogen atom in azaindolizines and can be attributed to a solvent-induced electronic charge separation between the nitrogen atoms concerned.Quantum chemical calculations involving the solvaton model are used to provide supporting evidence for the importance of solvent polarity effects on nitrogen shieldings in azaindolizines.

Solvent Effects on the Nitrogen NMR Shieldings in Oxazole and Oxadiazole Systems

“…As shown in Table 1 the effects of solvents on the nitrogen shieldings of means of Eq. [1] . As shown in Table 3, the s term, which represents the response of the nitrogen shielding to the oxazole and oxadiazole systems are significant and range changes in solvent polarity, reveals that more-polar sol-tems currently studied, which in principle is more electron attracting than the pyrrole-type nitrogen atoms contained vents result in a significant increase in the solute nitrogen shielding, by up to about 9 ppm on the shielding scale.…”

“…The resulting geometry is found to be planar with a reasonable approximately correspond to changes produced by the s term in Eq. [1].…”

“…Solvent effects on nitrogen NMR shieldings (chemical shifts) of nitrogenous five-membered ring heteroaromatic compounds have proven to be a valuable means of obtaining insight into solute-solvent molecular interactions (1)(2)(3). We have previously shown that a study of solvent effects can disclose information on solvent polarity/polarizability effects on the solute, as well as hydrogen-bonding effects between solvent and solute.…”

Study of the characterization and curing of a phenyl benzoxazine using15N solid-state nuclear magnetic resonance spectroscopy

“…16 However, it has been shown that 15 N-NMR chemical shifts are also sensitive to hydrogen-bonding effects in solution. 17,18 Further, it has been shown that there is significant hydrogen bonding in these kinds of benzoxazine compounds. 19 However, in solid-state NMR, it is expected that hydrogen bonding may shift resonances only slightly, but, more importantly, may lead to an increased line width.…”

Solvent Effects on Nitrogen Chemical Shifts

“…11) [52] experience rather limited solvent effects, up to 2 ppm, as compared to their pyridine-type nitrogen (N1 to N3) that alters its chemical shift up to 30 ppm upon changing the solvent (Table 3), similar to pyridine (44). Accordingly, N1, N2 and N3 of 55-57 may interact with hydrogen bond donor solvents, as reflected by their large and positive term a ( Table 2).…”