Ammonia, nitrogen chemical shift

Frequently the chemical shifts (Table 2.3) of molecular fragments and functional groups containing nitrogen complement their H and C shifts. The ammonia scale of N shifts used in... 

IUPAC recommends CD3N02 (90% in CDC13) as the chemical shift standard for both 14N and 15N). However, some spectroscopists reference nitrogen spectra to liquid NH3 (not a very convenient standard ) and IUPAC recommend this as an alternative for 15N. The chemical shift of CH3N02 with respect to liquid ammonia is 380.5 ppm. 

Nitrogen NMR data have been obtained using both the low-abundance, spin = 1/2 N and the predominant spin = 1 N nuclei. Several different references have been used for nitrogen NMR including aqueous ammonia, ammonium salts, acetonitrile, nitric acid, and nitrates. Current opinion favors neat nitromethane, and the compilation of the known data for 1,4-oxazines (Table 6) is expressed with respect to this reference and arranged in order of the observed chemical shift. For the C-labeled compound 87, carbamate rotamers lead to two separate signals and the value of 7c n can be determined -283.98 (d, J 9) and -284.46 (d, 7 11) <2001JOC8010>. 

NMR Spectrometry. Liquid phase and NMR spectra were recorded on a Varian XL300 NMR spectrometer at carbon and nitrogen resonant fi equencies of 75.4 and 30.4 MHz, respectively, using a 10 mm broadband probe. Quantitative NMR spectra of the unreacted fulvic and humic acid samples were recorded in DMSO-d6, 99.9 atom % as previously described (23). INEPT (24) and ACOUSTIC (25) N NMR spectra were recorded on the aniline-reacted fulvic and humic acids. Refocussed INEPT (proton decoupled) spectra were recorded as previously described (9). ACOUSTIC spectra, with the exception of the bimessite catalyzed sample, were recorded with the use of paramagnetic relaxation reagent (100-200 mg chromium (III) acetylacetonate). Acquisition parameters included an 18,656.7 Hz spectral window (613.7 ppm), 0.5-s acquisition time, 45° pulse angle, 2.0-s pulse delay, and t delay of 0.1 ms. Neat formamide in a 5 mm NMR tube, assumed to be 112.4 ppm, was used as an external reference standard for all spectra. N NMR chemical shifts are reported in ppm downfield of ammonia, taken as 0.0 ppm. 

Frequently the chemical shifts (Table 2.3) of molecular fragments and functional groups containing nitrogen complement their 77and C shifts. The ammonia scale of N shifts used in Table 2.3 shows very obvious parallels with the TMS scale of C shifts. Thus, the N shifts (Table 2.3) decrease in size in the sequence nitroso, nitro, imino, amino, following the corresponding behaviour of the C shifts of carbonyl, carboxy, alkenyl and alkyl carbon atoms (Table 2.2). 

There is still no unique standard for the calibration of15N NMR spectra. Six solid standards as well as liquid ammonia are frequently used as external references for the calibration of nitrogen shieldings in the solid state. Although chemical shifts in solids do not depend on the temperature and concentration (as the chemical shifts of liquid standards) there are discrepancies in the literature data on the exact chemical shifts for particular standards. In order to avoid errors created by conversion of literature data from different chemical shift scales, precise resonance positions of chemical shift standards are required. Since the difference in the resonance frequency between proton decoupled spectra... 

Curiously in the case of 59, the N spectrum was referenced to nitro-methane, while for 60 the spectrum was referenced to ammonia. Expressing both N chemical shifts on the same scale (ammonia = 0 ppm), the chemical shifts of the nitrogen resonances were 209.1 and 152.0 ppm, respectively, which is a relatively large difference in chemical shift for the introduction of a hydroxyl substituent in 60, which is fairly far removed from N1. 

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