An NMR Investigation of the Effect of Hydrogen Bonding on the Rates of Rotation about the C−N Bonds in Urea and Thiourea

Abstract

Abstract:The interaction between urea and tetrabutylammonium acetate was investigated in dimethylformamide/ dimethyl sulfoxide solutions using 1 H and 15 N NMR. The chemical-shift behavior of the urea protons is consistent with a urea-acetate hydrogen-bonded complex involving both carboxylate oxygens and the urea hydrogens trans to the carbonyl oxygen with K assoc ) 120 ( 10. Line shape analysis of the temperature-dependent 1 H NMR spectra show that ∆G q for rotation about the C-N bond of urea changes only slightly from 11.0 ( 0.1 to 11.2 ( 0.1 kcal/mol on 1:1 molar addition of tetrabutylammonium acetate to a dilute solution of urea. A parallel investigation of the interaction of thiourea with tetrabutylammonium acetate gave a binding constant, K assoc ) 90 ( 10. The ∆G q for rotation about the C-N bond of thiourea was found to increase from 13.5 ( 0.1 to 14.0 ( 0.1 kcal/mol on 1:1 addition of tetrabutylammonium acetate to a dilute solution of thiourea in dimethylformamide/dimethyl sulfoxide. Measurements were also made of the self-association of several ureas and of ∆G q for rotation about both C(O)-N bonds of 1,1-dimethylurea.

Novel dimeric cholesteryl-based A(LS)2 low-molecular-mass gelators with a benzene ring in the linker

“…6b and Table 3. With reference to the figure and the table, it is seen that the signals of the amide protons shifted gradually to a lower field with increasing the concentration of 1, 2, or 3 from 20 to 40 mg/ml, in support of the tentative conclusion revealed by temperature-dependent 1 H NMR measurements [36]. The shifts are 0.04, 0.17, and 0.14 ppm for compounds 1, 2, and 3, respectively, again, following the order of 2 > 3 > 1.…”

Synthesis and structural characterization of novel 2-benzimidazolylthioureas: adducts of natural isothiocyanates and 2-amino-1-methylbenzimidazole

“…Both calculated energies are up to 7-9 kcal/mole higher than those determined experimentally for 1,3-dialkyl thioureas [24]. Since it is well known that the respective theoretical values for thioureas are usually 3-4 kcal/mole smaller [25], actual energy barriers in DMSO should be even higher than predicted theoretically for isolated molecules. The calculations also showed that mutual interconversion T1A¡T1B requires about 2 kcal/mole more energy in the case of 2c than 2b (Fig.…”

Calculations on the Electronic Excited States of Ureas and Oligoureas

“…39 Another area of potential further study relates to the barrier to the rotation around the C-N bonds, which is substantially lower in ureas than in amides. 40 It is possible that some of the urea groups in oligoureas are in cis/trans or cis/cis conformations instead of the trans/trans conformation used in our calculations and that this could have a significant effect on the resulting CD spectra. We will investigate this in future studies.…”