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Conformational Preferences and Internal Rotation in Alkyl- and Phenyl-Substituted Thiourea Derivatives
journal contribution
posted on 2006-04-13, 00:00 authored by Vyacheslav S. Bryantsev, Benjamin P. HayPotential energy surfaces (PES) for rotation about the N−C(sp3) or N−C(aryl) bond and energies of stationary
points on PES for rotation about the C(sp2)−N bond are reported for methylthiourea, ethylthiourea, isopropylthiourea, tert-butylthiourea, and phenylurea, using the MP2/aug-cc-pVDZ method. Analysis of alkylthioureas
shows that conformations, with alkyl groups cis to the sulfur atom, are more stable (by 0.4−1.5 kcal/mol)
than the trans forms. All minima adopt anti configurations with respect to nitrogen pyramidalization, whereas
syn configurations are not stationary points on the MP2 potential surface. In contrast, analysis of phenylthiourea
reveals that a trans isomer in a syn geometry is the global minimum, whereas a cis isomer in an anti geometry
is a local minimum with a relative energy of 2.7 kcal/mol. Rotation about the C(sp2)−N bond in alkyl and
phenyl thioureas is slightly more hindered (9.1−10.2 kcal/mol) than the analogous motion in the unsubstituted
molecule (8.6 kcal/mol). The maximum barriers to rotation for the methyl, ethyl, isopropyl, tert-butyl, and
phenyl substituents are predicted to be 1.2, 8.9, 8.6, 5.3, and 0.9 kcal/mol, respectively. Corresponding PESs
are consistent with the experimental dihedral angle distribution observed in crystal structures. The results of
the electronic structure calculations are used to benchmark the performance of the MMFF94 force field.
Systematic discrepancies between MMFF94 and MP2 results were improved by modification of selected
torsion parameters and one of the van der Waals parameters for sulfur.
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bondtorsion parametersrotationalkyl groups cissulfur atomphenyl thioureasstructure calculationssyn geometryterttrans isomerdihedral angle distributionMP 2 resultsSystematic discrepanciessyn configurationsInternal Rotationcis isomerMMFF 94MP 2energy surfaceskcalvan der Waals parametersPESnitrogen pyramidalizationMMFF 94 force fieldConformational Preferencescrystal structuresphenyl substituentstrans formsanalysis
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