Infrared spectra and configuration of N,N'-diaryl-thioureas

Galabov, Boris; Vassilev, G. N.; Neyková, Neda; Galabov, Angel S.

doi:10.1016/0022-2860(78)85002-9

Cited by 30 publications

(17 citation statements)

References 11 publications

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“…This observation is supported by the C a −N rotational barriers, N-inversion barriers, charge distributions, and electron occupancies of the lone pair on nitrogen, etc. Analogous effects are observed on going from amides to thioamides. , At the MP2 level of theory employed in this study, comparison of results for thiourea, 1 , with those obtained for urea 10 are fully consistent with this observation. On going from urea to 1 , the C a −N bonds become shorter (from 1.395 to 1.375 Å), the nitrogen becomes more planar (γ from 343.2 to 349.5°), and the barrier of rotation increases (from 8.16 to 8.63 kcal/mol).…”

Section: Resultssupporting

confidence: 83%

Conformational Preferences and Internal Rotation in Alkyl- and Phenyl-Substituted Thiourea Derivatives

Bryantsev

Hay

2006

J. Phys. Chem. A

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Potential energy surfaces (PES) for rotation about the N-C(sp(3)) or N-C(aryl) bond and energies of stationary points on PES for rotation about the C(sp(2))-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(sp(2))-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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Section: Resultssupporting

confidence: 83%

Conformational Preferences and Internal Rotation in Alkyl- and Phenyl-Substituted Thiourea Derivatives

Bryantsev

Hay

2006

J. Phys. Chem. A

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“…The thiourea displayed a similar solvent dependence where the syn–anti and syn–syn conformers are predominant in the gas phase and low-dielectric solvents such as chloroform, but the preference for the anti–anti conformation increases in more polar solvents such as DMSO . This is consistent with infrared measurements of Schreiner’s thiourea catalyst that revealed the presence of several conformers in chlorinated solvents, while in THF only the anti–anti conformer is observed …”

Section: Introductionsupporting

confidence: 82%

Anion Binding Affinity: Acidity versus Conformational Effects

et al. 2020

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High-level quantum chemical calculations were used to elucidate the gas-and solution-phase conformational equilibria for a series of symmetrically substituted (thio)ureas, (thio)squaramides, and croconamides. Gas-phase calculations predict that the thermodynamic conformer of many of these anion receptors is not the dual-hydrogenbond-facilitating anti−anti conformer as is commonly assumed. For N,N′-diaryl thiosquaramides and croconamides, the syn−syn conformer is typically the predominant conformer. Solution-phase calculations show that the anti−anti conformer is increasingly stabilized as the polarity of the solvent increases. However, the syn− syn conformer remains the lowest energy conformation for croconamides. These predictions are used to explain the acidity versus chloride binding affinity correlations recently reported for some of these compounds. The chloride binding constants for thioureas and croconamides are significantly lower than expected on the basis of their pK a values, and this may be due in part to the need for these receptors to reorganize into the anti−anti conformer. Experimental NMR nuclear Overhauser effect (NOE) measurements of an asymmetrically substituted squaramide and its thio analogue are consistent with the syn−syn conformation being predominant at 298 K. The conformational equilibria should therefore be an important consideration for the design and development of future anion receptors and organocatalysts.

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“…Evidence for these equilibria is seen in the IR spectra of 2 in different chlorinated solvents. 18 NMR studies of diarylthioureas show the existence of a single species (the aa conformer) at room temperature in THF and toluene, with the as-form appearing at lower temperatures. 19 In the knowledge that the aa-conformer has been detected experimentally in a number of solvents, we found that COSMO-RS solvation performs rather better than other implicit solvation treatments, which predict a much smaller population of the aa-conformer.…”

Section: Resultsmentioning

confidence: 99%