Assessment of Computational Methods for Calculating Accurate Non-covalent Interaction Energies in 1,2,3,5-Dithiadiazolyl Radicals

Abstract

The strength of non-covalent intermolecular interactions between dithiadiazolyl (DTDA) radicals XCNSSN (X = F, Cl, Br, CN) were benchmarked using a high-level post-Hartree−Fock ab initio CCSD(T) theory with a complete basis set and corrected with a counterpoise (CP) correction for basis-set superposition error (BSSE). A range of density functional theory (DFT) functionals and basis sets were then screened to identify appropriate DFT methods which would reflect the benchmark data. These identified that minimal basis sets tended to overestimate the interaction energies, whereas inclusion of additional diffuse and polarization functions led to underestimation of these energies. The application of a CP correction for BSSE generally led to inferior performances in relation to uncorrected data for DFT calculations. The relative strengths of the intermolecular interactions were implemented to rationalize the trends in packing within the series XCNSSN (X = F, Cl, Br, CN). The nature of these interactions was also probed through an Atoms in Molecules approach which reveals intermolecular bond critical points for the different types of interactions, which are all diagnostic of non-covalent interactions between DTDA radicals. These are compared with previous experimental charge-density studies on related DTDA radicals. The best-performing DFT methods were then applied to the αand β-polymorphs of the larger radical p-NCC 6 F 4 CNSSN which was too large for benchmark studies. These confirmed the small energetic differences between α and β phases with the top five best-performing methods and functionals correctly reflecting the relative phase stabilities.

Robust S4⋅⋅⋅O Supramolecular Synthons: Structures of Radical‐Radical Cocrystals [p‐XC6F4CNSSN]2[TEMPO] (X=F, Cl, Br, I, CN)

“…−35 kJ mol −1 ) [33–35] >CN⋅⋅⋅S−S (ca. −26 kJ mol −1 ) [27] ∼C−I⋅⋅⋅N (−26 kJ mol −1 ) [27] >C−Br⋅⋅⋅N (ca. −16 kJ mol −1 ) [27] .…”

“…À 16 kJ mol À 1 ). [27] While the S 4 •••O interaction is antagonistic with respect to the CN•••SÀ S interaction (i. e., the DTDA radical cannot simultaneously form both these interactions for 5), there is potential for the halo derivatives 3 and 4 (X=Br, I) to form N•••XÀ C halogen bonds alongside the S 4 •••O interaction. We now review the packing of this series of radical cocrystals.…”

“…These contacts are shorter than the sum of the van der Waals radii (ΣvdW = 3.55 Å). Additional electrostatic δ + S•••N δÀ interactions generate SN-II type Interactions [18,27] (Figure 2) between these chains, running parallel to the crystallographic a-axis.…”

“…À 35 kJ mol À 1 ) [33][34][35] > CN•••SÀ S (ca. À 26 kJ mol À 1 ) [27] ~CÀ I•••N (À 26 kJ mol À 1 ) [27] > CÀ Br•••N (ca. À 16 kJ mol À 1 ).…”

“…In all cases, the large twist angle between C 6 F 4 and DTDA rings associated with the presence of the two ortho-F groups, potentially destabilizes the thermodynamically favored cisoid dimer. [41] When this is coupled with the presence of strong structure-directing CN•••SÀ S interactions or N•••IÀ C halogen bonds, [27] then this appears to suppress dimerization for both X=I or CN. For the weaker N•••BrÀ C halogen bond then there is an evident fine balance between the dimeric phase (where there is no N•••BrÀ C halogen bond) and the monomeric phase where N•••BrÀ C halogen bonds generate supramolecular chains.…”