In this study, Rushworth et al used a combination of IR, Raman, and INS spectroscopies to comprehensively characterise the vibrational spectroscopy of the non-planar thianthrene. It is reported in the literature that the C–S stretch modes are assigned in the 600 – 800 cm-1 range however, they found that while there are modes that involve sulphur motion in this region, there is a consequence of motion in the ortho-phenylene rings. The modes that are driven by the C–S stretches are found in the ~400 – 500 cm-1 range. The authors claimed that C–S–C bending modes occur 200 – 300 cm-1 range, which has not been previously characterised. The results presented by the authors are of interest to the community and based on the results and discussion I recommend its publication. However, the authors should address the following points to further improve the clarity of the paper.
1. In the introductory section, the authors discussed very briefly about the structures of thianthrene, anthracene, phenazine and dibenzo-1,4-dioxin. Expanding on the importance and geometrical, optical and electronic properties of these molecules will widen the scope of the paper and make it more accessible to general readers.
2. In the results and discussion section, in Figure 4, the calculated and observed INS spectrum has been presented in which the authors discussed various assignments. The spectra in Figure 4 look mostly similar however at ~1600 cm-1 there are two pronounced peaks in the theoretical spectrum. The authors should comment on the origin of these two peaks.
3. In Figure 6, the authors presented a comparison of the out-of-plane C–H bending mode for an isolated thianthrene molecule and unit cell of thianthrene. They found that in the isolated molecule, the two rings move out of phase with equal amplitudes, which is different in the case of the unit cell of thianthrene. The origin of this difference may be related to the intramolecular interactions in the solid-state model. Did the authors test their results by putting two isolated molecules closer to each other? How would the presence of more than one molecule in close proximity change the bending modes? A systematic set of calculations (with parallel and lateral orientations) may provide some insights into the fact that similar differences are not seen for every mode. The authors should at least comment on this.
4. The authors should briefly mention how they performed calculations on the isolated molecules in the computational details.
5. Following on from comment 3, as seen in Figure 7, the vibrational modes in isolated and solid-state models are similar. Could the authors comment if this similarity is related to the weaker lateral interaction as compared to parallel interaction between the thianthrene molecules in the solid-state model?