Structure and hydrogen bonding of solid N1-alkyl-N2-arylthioureas. 13C CP/MAS, IR and semi-empirical AM1 studies

Article abstract of DOI:10.1002/(SICI)1097-458X(199603)34:3<207::AID-OMR858>3.0.CO;2-0

Seven crystalline N¹-alkyl-N²-arylthioureas were studied by ¹³C CP/MAS NMR and by IR spectroscopy. The double set of signals in ¹³C CP/MAS spectra indicates that molecules of N¹-ethyl-N²-(3-methylphenyl)thiourea and N¹-ethyl-N²-(4-methylphenyl)thiourea are crystallographically non-equivalent. N¹-Propene-N¹-phenylthiourea forms cyclic dimers with two N²-H...S hydrogen bonds, as confirmed recently by x-ray diffraction. The broad v_NH maxima at 3175-3295 cm^-1 in the IR spectra indicate that in other thioureas both N¹-H and N²-H protons are involved in hydrogen bonding and that the non-bonded N¹-H proton in cyclic dimers of N¹-propenethioureas is probably an exception. Semi-empirical AM1 calculations showed that the N²-H proton is more positively charged than the N¹-H proton and therefore should be preferentially involved in N²-H...S hydrogen bonds.