Experimental and Computational Evidence for “Double Pancake Bonds”: The Role of Dispersion-Corrected DFT Methods in Strongly Dimerized 5-Aryl-1λ2,3λ2-dithia-2,4,6-triazines

Crystal structures are reported for bicyclic 3-CF 3 C 6 H 4 CN 5 S 3 and monocyclic 3-CF 3 C 6 H 4 CN 3 S 2 , the latter of which is strongly dimerized in a cis -cofacial geometry [3-CF 3 C 6 H 4 CN 3 S 2 ] 2 . The title compounds have previously been characterized in solution by NMR, displaying spectra that are consistent with the structure of [3-CF 3 C 6 H 4 CN 3 S 2 ] 2 in the crystal with anti -oriented CF 3 substituents. The interannular binding was investigated using density functional theory (DFT) methods. However, the DFT-optimized geometry spreads the aryl rings too far apart (centroid-centroid distances of ≥4.353 Å versus experimental distance of 3.850 Å). Significant improvements are obtained with dispersion-corrected DFT functionals B3LYP-D3, B3LYP-D3BJ, M062X, and APFD using the 6-311+G(2d,p) basis set. However, all of these overbind the aryl rings with centroid-centroid distances of 3.612, 3.570, 3.526, and 3.511 Å, respectively. After selecting B3LYP-D3BJ/6-311+G(2d,p) as the best method, five alternative dimer geometries were tested, and all were found to be binding; however, anti cofacial-4 (matching the structure in the solid state) is the most stable. Computed energies of the remainder are as follows: +7.0 kJ mol -1 ( syn -cofacial-5), +26.7 kJ mol -1 ( anti -cofacial-64), +27.0 kJ mol -1 ( syn -cofacial-150), +102.0 kJ mol -1 (S,S-antarafacial), and +103.7 kJ mol -1 (S,N-antarafacial), where the suffixes are torsional angles around the CN 3 S 2 thiazyl ring centroids. The binding in the four most stable cofacial dimers may be described by "double pancake bonding".