Nitrogen-15 magnetic resonance spectroscopy. XIV. Natural-abundance nitrogen-15 chemical shifts of amines

Abstract

It has been found possible to measure the shifts of a variety of amines to an accuracy of 0.2 ppm with the isotope at its natural-abundance level. Successful observation of these resonances with our existing equipment is dependent on a favorable nuclear Overhauser effect arising from irradiation of the protons. Chemical exchange of protons directly bound to the amine nitrogen can reduce the signal intensity sufficiently to make detection of the 15N resonance difficult or impossible. The 15N shifts generally correlate linearly with the 13C shifts of alkanes derived by replacing the amine nitrogen with an appropriately substituted carbon. Detailed examination leads to deshielding a and ß substituent shift parameters, a shielding y effect, and possible deshielding associated with hydrogen bonding. Effects of constraints in geometrically rigid systems as well as incorporation of the nitrogen into six-, five-, and three-membered rings are discussed and compared with corresponding 13C shifts. Parallel trends were found for all cases. Natural-abundance 15N spectroscopy is thus a practicable, if far from routine, means of structural analysis. Recentdevelopments in nmr instrumentation have . greatly facilitated the application of 13C spectroscopy to problems in organic chemistry.2 Characteristic functional group chemical shifts, well-defined substituent and geometrical parameters, the virtual absence of 13C-13C spin-spin coupling in natural abundance, and the favorable Overhauser effect and spectral simplicity associated with proton noise-decoupling to eliminate 13€spin-spin interactions have all contributed to the success which 13C nmr spectroscopy now enjoys. Fourier-transform spectroscopy can be expected to further expand areas of cmr investigations, especially in the biochemical realm. The relatively small use of nitrogen nmr spectroscopy stands in sharp contrast. Even though involved in one of the first applications of nmr to structure elucidation,3 456the spectroscopy of 14N and 15N has nevertheless been relatively little exploited. The reasons for this are straightforward: nitrogen-14, while the dominant isotope (natural abundance = 99.64%), has a large nuclear quadrupole moment associated with its spin quantum number (/ = 1) which may provide an efficient relaxation mechanism and, when operative, usually results in both large line widths and substantial errors in chemical-shift measurements. Line widths of over 1000 Hz and uncertainties of up to 60 ppm have been reported for 14N resonances.4,5 As a result, subtle differences in chemical shifts of structurally related compounds are generally undetectable, and only gross correlations between structure and chemical shift have been possible.4-12 As a further consequence of quad-(1) (a)

A 13C‐ and 15N‐NMR. Study of Some 1‐Aryl‐3, 3‐dimethyl Triazene Derivatives

“…The position of thr. amine nitrogen atom N(3) at considerably lower field than that of either (CH&NH (-25 ppm [5]) or 1,l-dimethyl liydrazine (+ 27 ppm 161) more strongly suggests Ihat this nitrogen is involved in the adjacent n-system2). Similar 1% chemical shifls have been found for the aniline nitrogen of some phenylhydrazones (120-130 ppm [Sl) where some overlap of the aniline lone pair to form an extended n-system is to be expected.…”

n‐π‐Interaction in Enamines and Enaminoketones. A 15N‐NMR. Study

“…However, the enamine nitrogen is always less shielded than the nitrogen of a closely related amine. l5N shifts of aliphatic amines have been shown to lie, generally, between -360 and -315 ppm [9] [lo]. Thus, enamine chemical shifts extend beyond this range towards lower shielding constants.…”

Charge distributions and chemical effects. XXXVIII. Correlations between nuclear magnetic resonance shifts and electronic charges of nitrogen atoms

“…Supporting evidence is offered by linear correlations involving the resonance shifts of "N nuclei and those of the corresponding carbon nuclei in analogous hydrocarbons, showing that for a large variety of substituents (i.e., over a wide chemical shift range) shielding of the ISN and I3C nuclei in aromatic (42) and nonaromatic (17,24,25,43) systems is governed by the same electronic factors. This holds true for 170 as well (44).…”

“…The large solvent effects on propylamines have been rationalized on the basis of changes in the populations of the gauche conformers (17,24). The importance of these interactions is revealed by the correlations observed for the "N chemical shifts of the saturated amines with the I3C shifts of the corresponding carbons in analogous hydrocarbons (17,(24)(25)(26), namely, by the very existence of separate, solvent-sensitive, linear correlations for primary, secondary, and closely related groups of tertiary amines. Results of this sort, obtained with great care for a large variety of compounds and interpreted in detail by Roberts and co-workers (17,(24)(25)(26), are instrumental in a more balanced assessment of the charge-shift correlations shown in Fig.…”