Reinvestigation of benzothiazoline-2-thione and 2-mercaptobenzothiazole tautomers: Conformational stability, barriers to internal rotation and DFT calculations

Tarek A. Mohamed, Ahmed M. Mustafa, Wajdi M. Zoghaib, Mahmoud S. Afifi, Rabei S. Farag, Yehia Badr

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21 Citations (Scopus)

Abstract

The Raman (3600-100 cm-1), infrared (4000-400 cm-1), far-infrared (400-200 cm-1), 1H and 13C NMR spectra of solid benzothiazoline-2-thione (BTT) are recorded. Additionally, the 1H NMR spectra were obtained at a temperature range from -90 to +90 °C. In addition, 2-mercaptobenzothiazole (MBT) can either be cis (S-H is eclipsing C{double bond, long}N bond) and/or trans (S-H is eclipsing C{double bond, long}S bond). Therefore, we carried out geometry and frequency calculations for thio-keto and thio-enol conformations using Density Functional Theory (DFT) methods of B3LYP, B3PW91 and SVWN utilizing 6-31 G(d) basis set. Theoretical predictions are consistent with vibrational measurements, which are in favor of the thio-keto (thione) tautomer at ambient temperature (25 °C). From the variable temperature 1H NMR spectra, it is evident that the thione BTT is the only existing tautomer at ambient (25 °C), low- (down to -90 °C) and high- (up to +90 °C) temperatures. Therefore, the thio-keto (BTT) and thio-enol (MBT) equilibrium is unquestionably excluded. B3LYP and B3PW91 calculations are in agreement with vibrational spectra as well as X-ray structural parameters of BTT rather than SVWN at 6-31 G(d) basis set. In addition, we calculated the S-H barriers to internal rotation for cis/trans chalcogen analogues of MBT utilizing B3LYP/MP2 potential surface scans with the 6-31 G(d) basis set. The cis/trans S-H barriers were 4.15/3.61, 3.39/2.90 and 3.71/3.36 kcal/mol for 2-mecaptobenzoxazole (MBO), 2-mercaptobezothiazole (MBT) and 2-mercaptobenzoselenazole (MBSe) analogues, respectively.

Original languageEnglish
Pages (from-to)27-36
Number of pages10
JournalJournal of Molecular Structure: THEOCHEM
Volume868
Issue number1-3
DOIs
Publication statusPublished - Nov 15 2008

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Keywords

  • H and C NMR
  • Barriers to internal rotation
  • Benzothiazoline-2-thione
  • DFT calculations
  • Infrared
  • Raman
  • Thio-keto and thio-enol tautomerism

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Biochemistry
  • Condensed Matter Physics

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