Chemical exchange by nuclear magnetic resonance. IV. Concentration dependence of the nuclear magnetic resonance spectral properties of some N,N-dimethylamides and -thioamides

Hindered Rotation in Some Organic Dithiocarbamates

“…38 Steric effects of the morpholinyl group in the planar Downloaded by [University of Bristol] at 00:28 04 November 2014 Table 3 Accurate spectral parameters of 5 at −60 • C ABXY spin system(trans/C = S) LMUV spin system(cis/C = S) δ A = 3.92 ppm δ X = 3.74 ppm δ L = 4.34 ppm δ U = 3.78 ppm δ B = 3.96 ppm δ Y = 3.78 ppm δ M = 4.39 ppm δ V = 3.82 ppm state could also raise its energy and thus lower the barrier. Such an effect has also been observed for G = when Me is replaced by iPr or iBu with slight difference for formamides (Me 2 NCHO: 87.5, iPr 2 NCHO: 86.1) 23-29 and more strongly for acetamides (Me 2 N-CO-CH 3 : 72.7, iPr 2 N-CO-CH 3 : 67.7) 23,26,29,30 or dithiocarbamates (Me 2 N-CS-SMe: 66.5 ± 2, iBu 2 N-CS-SMe: 63.7 ± 0.02). 4a, 8,39,40 Interestingly, Table 2 shows also that the free energies of activation in the range 58-70 kJmol −1 obtained for the dithiocarbamates 1-5 are nearly similar to or slightly higher than those reported for carbamates, 41 but lower than that in structurally related thioamides, which is in fair agreement with the results already reported by Kalikhman et al for dithiocarbamate 1 and its derivatives.…”

“…The equilibrium distribution of Z and E isomers appears to be independent of solvent for 2 (K • 295 ≈ 1.5, Z ≈ 60%), whereas it strongly depends upon solvent polarity for N-monosubstituted dithiocarbamates 3 and 4, in good agreement with solvent dependence observed for primary carbamates. 22 This suggests that 2 can essentially exist in the monomeric form or partially in a dimeric form (D) held together by relatively weak dipolar forces (Figure 3), as observed for N,N-dimethylthioacetamide (DMTA) for which molecular weight studies in carbon tetrachloride have shown average molecular aggregates of 1:1 molecules/unit at concentration of 0.05 M), 1,23 whereas N-monosubstituted dithiocarbamates can exist as a strongly hydrogen bonded cis dimer (with trans N-R ) or as a trans linear chain (with cis N-R, Figure 4) as previously reported for related systems. 1,24 It is worth to note that the NH chemical shifts of monosubstituted dithiocarbamates show a strong dependence on solvent polarity.…”

“…23,26,[35][36][37] Changing Me to O(CH 2 CH 2 ) 2 on the nitrogen in Me 2 N-CS-SEt however leads to a significant lowering of G = from 67.1 to 58.9 kJmol −1 . This is likely due to the electronic withdrawing nature of oxygen in the morpholine ring, which would lead to a decreased rotational barrier, consistent with trends observed for carbamates.…”

The proton relaxation and Overhauser effect in the nuclear magnetic resonance spectra of some amides

“…An exception is the CCI, solution where TI are longer than in the pure liquid although the viscosity of CC1, (0.97 cP) is higher than N,N-dimethylformamide (0.80 cP). Probably this is an effect arising from some degree of selfassociation in the pure liquid because of electric dipole-dipole interaction between the molecules (32). The extrapolated differences of the relaxation rates for cis-and trans-methyl protons (l/Tl), -(IIT,), are also roughly proportional to the viscosity with the same exception (in CCl, this difference is smaller than in pure liquid).…”

Notion of viscosity Arrhenius temperature forN,N-dimethylacetamide withN,N-dimethylformamide binary mixtures and its pure components

“…[23] On the other hand, DMF also has high solvating power with respect to a lot of organic and inorganic substances and is miscible with almost all common polar and non-polar solvents. Raman and infrared spectra of liquid DMF have been recorded by Fini and Mirone, [24] X-ray diffraction [25] and NMR [26][27][28] study of liquid DMF indicates that there are no significant interactions; only a weak hydrogen bond interaction existed between DMF molecules. We also add a supplementary parameter such as the Arrhenius temperature (T A ) for each pure component which has been calculated by using the equation: Table 3 summarises some Arrhenius parameters of the pure liquids studied in previous works.…”