Natural-abundance nitrogen-15 nuclear magnetic resonance spectroscopy. Nitrogen chemical shifts of alkylpyridines, picolinium, and lutidinium ions, and picoline N-oxides

Abstract

the nitrogen resonance positions of the methylpyridines are small except for the para-substituted compounds. This is rationalized qualitatively in terms of competitive inductive vs. hyperconjugative interactions between the substituent and the nitrogen, tert-Butyl substitution at the ortho position induces upfield shifts characteristic of -gauche interactions. Protonation or /V-oxide formation has little effect on the manner in which methyl groups influence the shielding, suggesting that the lone-pair electrons in the free base play only a minor role in the response of the nitrogen nucleus to alkyl substitution. The pyridinium ion resonance positions are markedly influenced by solvent and concentration. Picoline and lutidine chemical shifts do not correlate with electron densities calculated by the INDO method, but chemical shifts calculated by the Karplus-Pople method or the Witanowski modification thereof qualitatively reproduce trends. The validity of these types of calculations is assessed.

Hydroacridines: part 29.15N NMR chemical shifts of 9‐substituted 1,2,3,4,5,6,7,8‐octahydroacridines and theirN‐oxides—Taft, Swain–Lupton, and other types of linear correlations

“…As may be seen, in CH 3 OD, the only solvent capable of hydrogen bonding, the resonance of 1a appears shifted upfield by 13.6-22.3 ppm, and that of 1b by 9.9-16.7 ppm, as compared with the other five solvents incapable of hydrogen bonding. A similar shielding effect of hydrogen bonds upon 15 N chemical shifts has been observed also for pyridines and pyridine Noxides in H 2 O versus DMSO, [2] and in CF 3 CH 2 OH versus DMSO. [7] Since Taft substituent constants obviously cannot account for shielding effects due to hydrogen bonds, the 9-amino compounds 6a,b were ruled out from the regression analyses and so an additional improvement of the correlation was achieved for both octahydroacridines and their N-oxides, according to Eqns (1) and (2), respectively.…”

Experimental and quantum‐chemical studies of 1H, 13C and 15N NMR coordination shifts in Au(III), Pd(II) and Pt(II) chloride complexes with picolines

“…The observed metal-induced 15 184.3, 173.4, 170.3 ppm (and 183.2 ppm); [40] we have recalculated them using the 358.1 ppm relative shift of both references [41] [42] Such behaviour, i.e. the general increase of 15 N shielding constants upon the engagement of lone-pair electrons of picoline nitrogens into the N-H, N-Li or N-M (M = Au, Pd, Pt) bondings, confirms our previous suggestions [13,17,22,23] that the main reason for this effect is the replacement of the n N → π * electronic transition by the σ N−H → π * , σ N−Li → π * or σ N−M → π * ones.…”

“…All [45] (original values 310.1 ppm, 305.9 ppm, 297.6 ppm, referenced to liquid NH 3 , have been recalculated with respect to neat CH 3 NO 2 using the 381.7 ppm relative shift of both references [41]); in DMSO-d 6 -2pic −66.0 ppm, 3pic −70.1 ppm, and 4pic −78.3 ppm [45] (original values were referenced to liquid NH 3 : 315.7, 311.6, 303.4 ppm); as neat liquids-2pic −62.6 ppm, 3pic −61.7 ppm, and 4pic −70.2 ppm. [40] was observed for the py complexes (84.8 → 99.5, 99.3 → 113.6 ppm [13,17] [17] ). The same effect was noted for Pd(II) and Pt(II) chloride coordination compounds with 2,2 -bipyridine, 1,10-phenanthroline and their methyl or phenyl derivatives, as well as 2,2 -biquinoline.…”

“…for the relevant picoline N-oxides, detected in DMSO-d 6 at −89.6 ppm (2picO), −87.5 ppm (3picO), −96.3 ppm (4picO) and −87.5 ppm (for pyO), the δ N(1) decrease, noted upon the formation of N-O bonds, was much smaller (by 18.0 ppm, 11.7 ppm, 12.2 ppm and 18.8 ppm, respectively). [40] Some other factors increasing the 15 Tables S3 and S4). …”

2,6‐Bis(diazaboryl)pyridine: A Superbasic Sterically Demanding Pyridine Ligand.

“…Alkyl substituents at the 2‐ and 6‐positions are reported to induce a slight downfield shift (e.g. 2,6‐di‐tertbutylpyridine: −70.4 ppm) . Synthesis of 2,6‐bis(trimethylsilyl)pyridine and characterization by 15 N NMR revealed a resonance at −23.9 ppm.…”