A theoretical and empirical approach to solvent effects on nitrogen nuclear shielding

Abstract

Solvent effects on nitrogen shieldings are shown to vary from almost zero to about 26 ppm. These shielding variations can be analysed in terms of a variety of hydrogen bonding interactions and non-specific solute-solvent interactions. Steric effects are also taken into account. The non-specific interactions are found to be well accounted for by INDO/S-SOS shielding calculations incorporating the solvaton model. Nitrogen NMR is shown to be very suitable for distinguishing between a variety of solute-solvent interactions.

Solvent Effects on Nitrogen Chemical Shifts

“…The magnitude of the δ 14N of nitromethane (1, Fig. 1) was reported by Webb et al to show an up to 10 ppm solvent dependency, and its higher alkane homologues up to $5-6 ppm dependency [23,24]. The δ 14N of the nitro group predominantly depends on solvent polarity, as described by the dielectric constant ε.…”

“…5) is in its trans form, and experiences an up to 20 ppm variation of its δ 14N [24]. Methanol induces a considerable shielding of its nitrogen due to hydrogen bond donation to its lone pair (Table 3) as Figure 5 The structures of methyl nitrate (11), tert-butyl nitrite (12), phenyl-Nmethylnitrone (13), acetone oxime methyl ether (14), and [1,4]benzoquinone mono-(O-methyl-oxime) (15).…”

“…Upon changing the solvent from cyclohexane to trifluoroacetic acid, the δ 14N of pyridine (44) [5,24] and of pyridine N-oxide (45) [49] (Fig. 10) alter by À38 and À30 ppm, respectively ( Table 3).…”

“…22 ppm per unit scale), and it is capable of C-H hydrogen bond donation to Lewis basic solvents (b ¼ À3.30 ppm per unit scale). Strong hydrogen bond donors typically have a term a of $20 ppm per unit scale [24], and thus the hydrogen bonds of 1 are comparably weak. The solvent dependence of aromatic nitro functionalities, for example of nitrobenzene (2), is comparable to that of the aliphatic ones, i.e.…”

Medium effects on nitrogen nuclear shielding in methyl nitrate and methyl isothiocyanate assessed by the solvaton model

“…In cases where steric effects prevent the close approach of CHC13 to hydrogen bond acceptor sites, a value of zero for a provides a better agreement with the experimental data. 7 We conclude that the solvaton model is very attractive for studying medium polarity effects on nitrogen shielding when specific interactions, such as hydrogen bonding, are absent. The agreement between the nitrogen shielding variation, as a function of solvent, predicted by INDO/S-SOS calculations incorporating the solvaton model, and that observed experimentally for methyl nitrate and methyl isothiocyanate, is quantitatively acceptable.…”