2-DNMR

Fig. 2. DNMR spectrum of the isopropyl methyl groups of Al(iPr, iPr-/3-dik)3 in chlorobenzene solution. From Ref.22)...

Although many pulse sequences have been investigated for 2-Dnmr. only a very limited number are of practical interest. One deterrent to their more widespread use is the time required and hence the cost of collecting the data. The time factor arises because the pulse sequences have to be repeated hundreds of times whilst varying the time interval during the evolution period, t, in order to provide sufficient data for the final 2-D contour-plot. Three of the more valuable 2-D techniques are described below. 

It must be emphasized that all time-dependent chemical phenomena, such as tautomerism, are sensitive to temperature changes. In this section, we treat classic DNMR (dynamic NMR) studies and cases of changes in the NMR spectra with temperature together. In Table XI we have classified these studies according to the physical state of the sample and to the nuclei... 

The imbalance between and NMR studies in the solid state (Section VI,F) partly reflects the fact that it is easier to introduce N than into heterocyclic compounds, particularly azoles (DNMR in the solid state usually requires isotopic enrichment). Compared to solution studies, solid-state intermolecular proton transfer between tautomers has the enormous advantage that the structure of the species involved is precisely defined. 

The largest series of papers (third column) is related to the study of annular tautomerism in azoles. This class of tautomerism is especially well suited for DNMR because in many cases Kt = 1 (autotrope) and because the activation barriers are in the range easy to be measured by NMR. To... 

AH, AS, AGt2i3 = 11-7 kcal mol (CD2CI2), AHt and d5. Larina et al. (98MRC110) used DNMR to determine the activation barrier of 4-trimethylsilylpyrazole (65) (dGtc = H-9 kcal mol ) and DNMR to determine the activation barrier of 3(5)-methylpyrazole (66) (54% 66a -46% 66b, AGtc = 10 kcal mol ) [similar barriers have been reported for other pyrazoles (93CJC1443)]. In the case of 3(5)-trimethylsilylpyrazole only the 3-substituted tautomer is present, preventing the determination of the barrier. 

In the same paper, Limbach et al. (87AGE934) reported the study of the compound related to porphyrins, porphycene 79. Porphycene behaves very differently from FI2P in CPMAS DNMR experiments. Its behavior was explained assuming that the four porphycene tautomers (two cis and two trails) are present in the crystal and interconverting very rapidly due to short N-H...N distances in the seven-membered pseudorings. 

The dynamic NMR (DNMR) spectroscopy has been used in studies of stereomutations of non-symmetrical di-Schiff bases [18].39 It was shown that the hindered Schiff bases exist in DMSO in two chiral conformations. The presence of a pair of conformers being in equilibrium was explained by the existence of two stereogenic axes a g (aligned to Cl—N8 bond) and a 7 (aligned to C6—N7 bond) due to restricted rotation around two Ar—N bonds. The trans to cis interconversion as well as enantio- or diastereoisomerisation barriers for the compounds studied have been established using line shape analysis. 

SCHEME 1. Conformational map of the RcRn and RcSn diastereomers of /V-ethyl-iV-methyl-2-aminobutane (EMAB). Interconversions among conformers within dashed boxes are fast on the NMR time scale at 104 K. Those between dashed boxes occur via rotations about the methine carbon-nitrogen bond with barriers which are DNMR-visible. The interconversion between the solid boxes occurs via nitrogen inversion (disstereomeric interconversion). The values in parentheses are MM2-80 results. Reprinted with permission from Reference 71. Copyright (1988) American Chemical Society... 

SCHEME 2. Conformational map of A-ethyl-A-methyl-2-aminopropane (EMAP). Reproduced with permission from reference 73. MMP2-87 relative energies in parenthesis. 6MMP2-87 Avalues in parenthesis. The rotational barrier for the direct GG GG exchange was calculated as 8.5 kcal mol-1. "lower limit for DNMR detection in this study... 

For Af-ethyl-Af-tert-butylneopentylamine (ETNA, 40)74, three successive processes are sequentially revealed during the cooling procedure in the DNMR spectra, with barriers of 8.1, 7.3 and 6.0 kcalmol-1. As in the diisopropylamine series, the highest barrier is attributed to an inversion/rotation process (40a -o- 40b), and the lower ones to rotations... 

Tertiary amines of class (iii) were also studied by Lunazzi and coworkers75. A series of beta-substituted-alpha-amino naphthalenes 41a-d-44a-d were investigated using DNMR and NOE experiments and molecular mechanics. Such systems allow for separation between rotational processes, whose barriers around Ar—N bonds are quite high (15-23 kcal mol 1). and inversion, known to have particularly low barriers... 

TABLE 24. Experimental (DNMR) and calculated (MM3) inversion barriers of several azabicyclo and azatricyclo compounds... 

In /V,/V-dialkylated members of this group (1, A = NR, R2, B = H or alkyl), several authors have observed fast thermal isomerizations at the double bond, using the DNMR technique. Most compounds of this type also display hindered rotation about the C—N bond with substantial barriers (see, e.g., ref. 30). Compounds of the general type 9 and 10 (R = H or Me) with a variety of... 

Shvo and Belsky (42) have also studied a methylthio analog of 23, R = Me (24). Here the C=C barrier is above the DNMR region (>27.5), that is, at least... 

Ketene mercaptals (27) have rather high C=C barriers (Table 5), and only those with quite strongly electron-accepting groups fall within the region accessible to the DNMR method (45-49). As an example, the C=C barrier for 27... 

In the first DNMR studies of push-pull ethylenes, a strong effect of solvent polarity on the C=C barriers was noted. Thus Kende et al. (64) found AG = 18.0 kcal/mol for 46a in N,/V-dimethylformamide (dielectric constant e = 38) and 19.4 kcal/mol in Ph20 ( = 4). Similar observations have been made by many other workers, and they have been seen as a strong support for a zwitterionic transition state. Kessler et al. (140) observed reasonably linear correlations between AG for two ketene aminals and the solvent polarity parameter T (141) with variations in AG of ca. 2.5 kcal/mol over T values between 25 and 46. Similarly, Shvo et al. (78) found linear correlations between log km and the polarity parameter Z (141) for three compounds from Table 12. 

One important relationship which has survived from the earliest days of DNMR [1] and which is still used to estimate exchange rates is the relationship between the exchange rate which is the rate that just results in coalescence of the A and B signals into a broad, single, flat-topped absorption. From differentiation of the lineshape equation, it is possible to obtain the simple expression [1]... 

A double-bond configuration can have a strong impact on the conformational behavior of a molecule, and this may be monitored by dynamic NMR (DNMR) spectroscopy. 

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