Comment on “Insights into the Nature of the Chemical Bonding in

Comment Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

pubs.acs.org/JPCA

Comment on “Insights into the Nature of the Chemical Bonding in Thiophene-2-thiol from X‑ray Absorption Spectroscopy” M. Jake Pushie,† Julien J. H. Cotelesage,‡ Linda Vogt,‡,¶ Monica Barney,∥ Ingrid J. Pickering,‡,¶ and Graham N. George*,‡,¶ †

Department of Surgery, University of Saskatchewan, Royal University Hospital, 103 Hospital Drive, Saskatoon, Saskatchewan S7N 0W8, Canada ‡ Molecular and Environmental Sciences Group, Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada ¶ Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5C9, Canada ∥ Chevron Energy Technology Company, Richmond, California 94802, United States

J. Phys. Chem. A 2016, 120 (35), 6929−6933. DOI: 10.1021/acs.jpca.6b05874

R

ecently, we reported on the sulfur K-edge spectroscopy of toluene solutions of thiophene-2-thiol (1), which showed the substantial presence of intense spectral features arising from 1s → (CS)π* transitions, attributable to a thione moiety.1 This was interpreted as indicating the presence of one or more thione tautomers in solution (2 or 3).

In our paper we neglected to discuss the suggestions of dimerization reactions of thiophene-2-thiol. Ponticello et al.2 have reported that liquid thiophene-2-thiol slowly solidifies on storage under nitrogen, by reacting with the thermodynamically least favorable1 thione tautomer 3 to give the thione-containing dimer 4, a yellow solid that could be readily converted back to the liquid monomer.

Figure 1. Schematic structures of thiophene-2-thiol (1), its possible tautomers 5H-thiophene-2-thione (2) and 3H-thiophene-2-thione (3), and the dimeric product of the reaction of 1 and 3 4-(2thienylsulfanyl)tetrahydrothiophene-2-thione (4). DFT-computed equilibrium relative energies and geometries are shown adjacent to the schematic structures. The computed energy for 4 is expressed per mole of monomer.

To investigate the possible formation of dimers, we performed density functional theory (DFT) geometry optimizations using DMol3 and Biovia Materials Studio Version 20163,4 using the Perdew−Burke−Ernzerhof functional both for the potential during the self-consistent field procedure and for the energy. Dmol3 double numerical basis sets included polarization functions for all atoms with all-electron core treatments. Solvation effects were modeled using the Conductor-like Screening Model (COSMO)5 in Dmol3 with a dielectric value representing toluene (ε = 2.38). Transitionstate searches additionally used 13 kJ/mol orbital smearing to assist with the convergence of the self-consistent field calculations. The geometry-optimized energy-minimized DFT structures and relative energies of 1−4 are shown in Figure 1 (1−3 are as previously reported1). We find that the dimer 4 is predicted to be thermodynamically more stable than all of the © XXXX American Chemical Society

monomeric forms calculated, which agrees with the finding of Ponticello et al. of near-complete conversion on long-term storage. Transition-state search calculations were performed for the tautomerization of 1 to 3 and for the reaction of 1 and 3 to make the dimer 4. The tautomerization shows a substantial barrier energy of 230.0 kJ/mol, with a transition-state structure in which the thiol proton of 1 is midway between two carbons Received: January 13, 2018 Revised: March 15, 2018 Published: March 23, 2018 A

DOI: 10.1021/acs.jpca.8b00420 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Comment

The Journal of Physical Chemistry A

Bonding in Thiophene-2-thiol from X-ray Absorption Spectroscopy. J. Phys. Chem. A 2016, 120, 6929−6933. (2) Ponticello, G. S.; Habecker, C. N.; Varga, S. L.; Pitzenberger, S. M. An Unusual Dimer of 2-Mercaptothiophene. J. Org. Chem. 1989, 54, 3223−3224. (3) Delley, B. An All-Electron Numerical Method for Solving the Local Density Functional for Polyatomic Molecules. J. Chem. Phys. 1990, 92, 508−517. (4) Delley, B. From Molecules to Solids with the DMol3 Approach. J. Chem. Phys. 2000, 113, 7756−7764. (5) Klamt, A.; Schüürmann, G. COSMO: A New Approach to Dielectric Screening in Solvents with Explicit Expressions for the Screening Energy and its Gradient. J. Chem. Soc., Perkin Trans. 2 1993, 2, 799−805.

(Figure 2A). The dimerization also gives a substantial barrier energy of 95.0 kJ/mol (of monomer) so that the dimerization

Figure 2. Computed transition-state geometries for the tautomerization of 1 to 3 (A) and for the dimerization reaction of 1 and 3 to yield 4 (B).

reaction, although thermodynamically favorable, is expected to proceed only slowly, with the thiol proton transfer to carbon being the transition-state step (Figure 2B). On the basis of the sulfur K-edge X-ray absorption spectroscopy (XAS), our previous estimated contribution of the fraction of total sulfur present as thione in toluene solutions of thiophene-2-thiol was 16%.1 This ratio is consistent with the predicted proportion of 1 and 2 based on simple thermodynamic arguments.1 Pure dimer would correspond to 25% total sulfur as thione, and computed spectra (not illustrated) suggest that the thione peak of pure dimer 4 would be too intense relative to the reported experimental spectrum.1 However, we neglected to consider the possible presence of the dimer in our original analysis. Given the above arguments and the fact that the pure dimer is a yellow crystalline solid, rather than the clear liquid that we used to prepare the solutions for our work, we believe that our observations still constitute evidence of observation of the tautomeric forms in solution, but more work is warranted to definitively show that the observed 1s → (CS)π* transition of the sulfur K-edge XAS is due to monomeric tautomers and that significant quantities of the dimeric form are not present. As noted by Ponticello et al.2 NMR can also be used to detect the presence of dimer and possibly tautomers if the rate of interconvsion is sufficiently slow, and future work will include such measurements.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1 306 966 5722. ORCID

M. Jake Pushie: 0000-0001-7494-5427 Ingrid J. Pickering: 0000-0002-0936-2994 Graham N. George: 0000-0002-0420-7493 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Research at the Univ. of Saskatchewan is supported by a grant from the Chevron Energy Technology Company, the Natural Sciences and Engineering Research Council (G.N.G., I.J.P.), the Univ. of Saskatchewan, and by Canada Research Chairs (G.N.G., I.J.P.).

■

REFERENCES

(1) Cotelesage, J. J.; Pushie, M. J.; Vogt, L.; Barney, M.; Nissan, A.; Pickering, I. J.; George, G. N. Insights into the Nature of the Chemical B

DOI: 10.1021/acs.jpca.8b00420 J. Phys. Chem. A XXXX, XXX, XXX−XXX