N NMR spectra

A detailed study of spectra of compounds 1, 2, and 3 has been published as part of a general study of azolopyridines (84OMR209). The shifts are shown in Table III. The N shifts have been used to determine the structure of 7-amino-triazolopyridines 128 and 129 (89T7041). The shifts recorded were 56.8, 56.2 (Nl), 245.4, 246.3 (N2), 320.6, 316.8 (N7a), all from nitromethane as standard at 380 ppm the absorption for the amine was at 345.5, 350 ppm in accordance with the amino structure shown, rather than the imino forms 128a and 129a. 

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C-nmr data have been recorded and assigned for a great number of hydantoin derivatives (24). As in the case of H-nmr, useful correlations between chemical shifts and electronic parameters have been found. For example, Hammett constants of substituents in the aromatic portion of the molecule correlate weU to chemical shifts of C-5 and C-a in 5-arylmethylenehydantoins (23). Comparison between C-nmr spectra of hydantoins and those of their conjugate bases has been used for the calculation of their piC values (12,25). N-nmr spectra of hydantoins and their thio analogues have been studied (26). The N -nmr chemical shifts show a linear correlation with the frequencies of the N—H stretching vibrations in the infrared spectra. 

The structure of cytosine is certainly the aminooxo form (60) which has been confirmed by X-ray analyses <73AX(B)1234), UV and NMR data <63JCS3046), Raman spectra <67SA(A)255l) and even by N NMR spectra (72JPC5087). 

The possibility offered by new instruments to obtain N NMR spectra using natural abundance samples has made " N NMR spectroscopy a method which holds no interest for the organic chemist, since the chemical shifts are identical and the signal resolution incomparably better with the N nucleus (/ = ) than with " N (/ = 1). H- N coupling constants could be obtained from natural abundance samples by N NMR and more accurately from N-labelled compounds by H NMR. Labelled compounds are necessary to measure the and N- N coupling constants. 

With improvements in Instrument sensitivity and the use of techniques such as enhancement by polarization transfer (INEPT), it can be expected that natural abundance N NMR spectra will become increasingly Important in heterocyclic chemistry. The chemical shifts given in Table 10 illustrate the large dispersion available in N NMR, without the line broadening associated with N NMR spectra. 

The result of a study of 15 N NMR spectra of cyclic nitrones of 3-imidazoline-3-oxide and of the corresponding nitroxyl radicals has been reported (422). 

Fig. 7 Solid state N-NMR spectra of GS-3/3 at 1 40 in oriented DMPC bilayers. The arrow indicates the new resonance emerging at high peptide concentration, from which the upright alignment of gramicidin S in the membrane was calculated. The dashed line corresponds to the respective powder pattern of the lyophilized peptide...

Figure 1.49 Left H NMR spectra of 125 in d -THF (298 K, left) and in scC02/d -THF (180 bar, 323 K, right). Right N NMR spectra of 129 in d -acetone (298 K, left) and scC02/d -acetone (110 bar, 313 K, right a smaller signal of a second species is visible under these conditions).

N NMR spectroscopy has been described in some detail in CHEC-I <84CHEC-I(5)682> however, no N NMR spectra of 1,2,3-triazoles were reported before 1982 <84CHEC-i(5)684>. Since that time, N NMR spectroscopy has been widely applied to triazole and benzotriazole compounds. 

A number of 5,5-dialkyldihydro-4.ff-l,2,3-triazole-4-ones and 5-amino-4,4-diphenyl-4//-1,2,3-triazoles have been studied by N NMR spectroscopy. Their N chemical shifts (corrected to the external standard of ammonia) are listed in Figure 11 <93CB103>. The N NMR spectra of triazole and benzotriazole boron derivatives have been reported <89IC4022>. 4-(l-Azido-l-methylethyl)-lH-1,2,3-triazole shows N chemical shifts (D20/pyridine, MeN02 external standard) 3 — 284.6 (Na), - 161.5 (Ny), d - 135.8 (N/9), d-9lA,d- 79.2, and d - 60.4 ppm <89CB9l 1>. 

There are very few references to the N NMR of 1,2,3-thiadiazoles. The N NMR spectra of 15 monosubstituted 1,2,3-thiadiazoles have been published <93JHC301> and selected data are given in Table 7. 

Solvent and concentration effects in nitrogen NMR studies can be very significant. The N NMR chemical shifts for 1,2,4-thiadiazole (1) in ether solution (1 3 v/v) are - -106 for N-2, and -1-70 for N-4. These values are shielded with respect to nitromethane. A similar degree of shielding is observed in the N NMR spectra of 1,2,4-oxadiazole and in 1,2,5-thiadiazole <840MR(22)215>. 

A detailed comparative analysis of the H, C, and N NMR spectra of 1,2,5-thiadiazoles and the shifts relative to analogous heterocycles has been carried out <84CHEC-I(6)513>. 

This has been further confirmed by N NMR spectra which indicate >95 /o of the 5-one form with the tautomeric proton bonded to the N-4 atom (85MRC166,86BCJ3263). The C NMR shift of C-5 in structure (4 R = H) also confirms the tetrazolinone form, being about 148-149 ppm for X = O and about 163-165 ppm for X = S in agreement with 1,4-disubstituted model compounds... 

The cyclic structure of the mesoionic 3-substituted anhydro-5-hydroxy-l,2,3,4-oxatriazolium hydroxides (4) has been established through their dipole moments, mass spectra, x-ray spectra, and C and N NMR spectra. 

It has been stated that, when specific hydrogen-bonding effects are excluded, and differential polarizability effects are similar or minimized, the solvent polarity scales derived from UV/Vis absorption spectra Z,S,Ei 2Qi),n, Xk E- ), fluorescence speetra Py), infrared spectra (G), ESR spectra [a( " N)], NMR spectra (P), and NMR spectra AN) are linear with each other for a set of select solvents, i.e. non-HBD aliphatic solvents with a single dominant group dipole [263]. This result can be taken as confirmation that all these solvent scales do in fact describe intrinsic solvent properties and that they are to a great extent independent of the experimental methods and indicators used in their measurement [263], That these empirical solvent parameters correlate linearly with solvent dipole moments and functions of the relative permittivities (either alone or in combination with refractive index functions) indicates that they are a measure of the solvent dipolarity and polarizability, provided that specific solute/ solvent interactions are excluded. 

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