Biology (HMC), Chemistry (HMC)
The interaction between urea and tetrabutylammonium acetate was investigated in dimethylformamide/ dimethyl sulfoxide solutions using ¹H 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 ¹H NMR spectra show that ∆G^‡ 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^‡ 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^‡ for rotation about both C(O)-N bonds of 1,1-dimethylurea.
© 1996 American Chemical Society
K. A. Haushalter, J. Lau, and J. D. Roberts. 1996. An NMR Investigation of the Effect of Hydrogen Bonding on the Rates of Rotation about the C-N Bonds in Urea and Thiourea. J. Amer. Chem. Soc. 118: 8891-8896. DOI: 10.1021/ja961380k