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  • 1
    Electronic Resource
    Electronic Resource
    Structure and stability of the N-hydroxyurea dimer: Post-Hartree–Fock quantum mechanical study (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5784-5790 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The potential energy surface (PES) search of the N-hydroxyurea dimer was searched with second-order Møller–Plesset perturbation theory (MP2) and the 6-31G(d,p) basis set. Eight local minimum energy structures have been found. Four of them have relatively strong (ΔE∼−10 to −13 kcal/mol) intermolecular interactions and the others are moderately strongly interacting species (ΔE∼−3 to −7 kcal/mol). Final estimation of interaction energies was performed using the larger 6-311G(df,pd) and 6-311G(2df,2pd) basis sets. The predicted interaction energies are ΔE=−14.26 kcal/mol and −3.43 kcal/mol for the strongest and the weakest interacting forms of the studied complex, respectively, at the MP2/6-311G(2df,2pd)//MP2/6-31G(d,p) level of theory. The self-consistent field (SCF) interaction energy decomposition indicates the important influence of the deformation term magnitude on ΔE(SCF). The calculated electron correlation contribution to ΔE(MP2) depends on the geometry of the system and varies from −0.5 to −5 kcal/mol. The estimated influence of water on the stability (free energy of hydration) of N-hydroxyurea dimers using the self-consistent isodensity polarized continuum (SCI-PCM) model of solvation varies from ∼−11 kcal/mol to ∼−21 kcal/mol. The forms predicted to be more strongly interacting species in gas phase are less influenced by hydration than the more weakly interacting ones. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1288916
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  • 2
    Electronic Resource
    Electronic Resource
    Structures and energies of seleno derivatives of biuret.Ab Initio comparative studies of diselenobiuret, selenobiuret, and selenothiobiuret (1997)
    Springer
    Structural chemistry 8 (1997), S. 245-256 
    Details
    ISSN: 1572-9001
    Keywords: Ab initio ; potential energy surface ; MØller-Plesset ; intramolecular hydrogen bond
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Ab initio studies (LCAO-MO method) on conformers of three seleno derivatives of the biuret molecules diselenobiuret [I], selenobiuret [II], and selenothiobiuret [III] were carried out at the Hartree-Fock (HF) and MP2 levels. The molecular geometries of these species were fully optimized at the HF level and characterized by analysis of the harmonic vibrational frequencies using a split-valence triple-zeta basis set augmented by a set ofd polarization functions on heavy atoms andp polarization functions on hydrogen atoms [TZP(d, p)]. The total energies of the HF-optimized structures were calculated at the MP2 (frozen core) level using a larger TZP (2df, 2pd) basis set. The potential energy searches revealed a total of 11 minimum-energy conformers (assigned astrans-trans, trans-cis, cis-trans, andcis-cis) and seven transition-state species for the title molecules. The two predicted conformers for diselenobiuret (Ia=trans-trans andIc=cis-cis) are characterized byC 2 and the third byC s symmetry. For selenothiobiuret two forms (IIIa=trans-trans andIIId=cis-cis) possessC 1 and two (IIIb=trans-cis andIIIc=cis-trans) possessC s symmetries, respectively. For selenobiuret, four formsIIa=trans-trans (C1),IIb=trans-cis (C s),IIc=cis-trans (C 1), andIId=cis-cis (C1), were obtained as a result of gradient optimization. Comparison of the relative energies for the considered species indicated that thecis-trans forms are the most stable conformations for all three systems at both the HF and MP2 levels of theory.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02252968
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  • 3
    Electronic Resource
    Electronic Resource
    Biuret and its sulfur analogs: Structures and energies (1992)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 301-313 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The calculations for biuret (B) and its two sulfur analogs, thiobiuret (TB) and dithiobiuret (DTB), were carried out using the ab initio LCAO-MO method at the Hartree-Fock level. The molecular structure of B was fully optimized using standard 3-21G and 6-31G* basis sets, while geometry optimizations of the TB's and DTB's were performed at the 3-21G* and 6-31G* levels. Only trans-isomers were found to be minimum structures on the HF/3-21G* potential-energy surfaces, whereas cis-forms are the first-order transition structures. The relative energies of the cis-conformers at MP 2/6-31G**//HF/6-31G* + 0.9 zero point energy (ZPE) level are equal to 47.4, 51.7, and 59.7 kJ mol-1 for TB, B, and DTB, respectively. Because of relatively large dipole moments, cis-conformers are predicted to be stabilized in polar solvents.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560440216
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  • 4
    Electronic Resource
    Electronic Resource
    13C NMR studies of conformational isomerism in thioureas: Signal assignments via chemical shift and population trends (1983)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 21 (1983), S. 293-300 
    Details
    ISSN: 0030-4921
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 22.63 MHz 13C NMR spectra of a series of alkylated thioureas are reported. Characteristic Z and E spectral regions were found for the 13C = S resonances. The two regions were generally found to be non-overlapping for the series, with the region of the Z, Z resonances occurring more downfield than those of either the Z, E or E, Z conformers in the cases of 1,3-disubstitution. The Z, Z configuration became favored and the relative chemical shift difference (Rδ) increased linearly with increasing substituent size. At 217 K, hindered internal rotation caused a multiplicity of resonances which were normally single peaks in the broad band 1H decoupled 62.86 MHz 13C spectrum of CH3NHCSNH(CH2)2NHCSNHCH3 (2MTE) at room temperature. The trends in chemical shifts and populations were employed to assign tentatively the resonances of five of the six possible configurational isomers contributing to the 2MTE spectra at 217 K. The isomer populations are given. The 13C NMR spectra reported here led to signal assignments of Z and E isomers which supported prior 1H NMR results and contradicted more recent results of another 13C NMR study of N-methylthiourea. The major peak of the exchange doublet occurs at relatively high field strengths in both methanol-d5.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/omr.1270210502
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  • 5
    Electronic Resource
    Electronic Resource
    Hindered rotation in thioureas: Steric effects and conformationsThis research has been submitted by one of the authors (R. H. S.) as partial fulfillment of the requirements for the PhD degree in chemistry at Howard University, August 1972.Presented in part at the American Chemical Society South-eastern Regional Meeting, Charleston, South Carolina November 1973. (1975)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 7 (1975), S. 143-150 
    Details
    ISSN: 0030-4921
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The temperature dependent spectra of several mono-, di- and trialkylthioureas have been recorded. Free energy barriers to internal rotation about the C—N bonds have been calculated. In thioureas that were unsymmetrically substituted, free energy barriers were found to be different for each C—N bond with the more substituted amino group exhibiting the higher barrier. The monosubstituted thioureas showed different rotational barriers for the NH2 groups of the cis and trans isomers, respectively. The free energy barriers for the trans isomers were found to be substituent dependent and substantiate the reassignment of the high and low field substituent signals to the trans and cis isomers, respectively. The spectrum of 1-methyl-3-t-butylthiourea indicates restricted rotation of the t-butyl group at temperatures below 200 K.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1270070307
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