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

Abstract

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”.

Pancake Bonding Seen through the Eyes of Spectroscopy

“…Table 1 summarizes the calculated bond distances (R calc ), experimental bond distances (R exp ), calculated bond dissociation energies (BDE calc ), experimental bond dissociation energies (BDE exp ), local stretching force constants (k a ), local mode vibrational frequencies (ω a ), bond strength orders (BSO n), electron densities (ρ b ), and energy densities (H b ) for the targeted CC bonds of of targeted contacts bonds of dimers 1-6 and rings of 1-3. Table 2 summarizes symmetry, singlet and triplet C⋯C contact distance (R(CC)), energy values of SOMOs (E SOMO ), and triplet/singlet (ΔE ST ) for all species investigated in this work (1)(2)(3)(4)(5)(6). Figure 2 shows the equilibrium geometries for the HCNTeTeN 3 dimer (C 2 )in singlet and triplet states.…”

“…[2] In regard to the phenalenyl-based dimers, as the substituent size increased from 4 to 6 the stability of the system steadily declined as the steric repulsion between the substituent groups hindered the monomers of these dimers from changing into a orientation of lower energy. [3] As the radius of the chalcogen atoms di-chalcodiazoyl dimers 1-3 increase (Te < Se < S) the strength of the C⋯C contacts decreases. As the strength of the chalcogen⋯chalcogen interactions (i.e.…”

“…The initial concept of "pancake bonding" was constructed by Mulliken and Person as to characterize the overall shape and bonding mechanisms of donoracceptor π systems [1]. More recently the term "pancake bonding" has primarily been used to describe the formation of stabilizing parallel π-π interactions between two or more open-shell free radicals, those of which are typically planar and/or consist of light-atoms [2][3][4]. Such interactions have received a considerable amount of interest as they allow one to synthesize novel radical-based materials, via electron or hole through-space delocalization, that exhibit unique magnetic [5], optical [6], and electronic properties (i.e.…”

“…singlet electronic state) [9]. The overlapping of antibonding (π * ) SOMOs is the basis of pancake bonds as this interaction leads to the following distinctive features [4]: i) contact bond distances that are beyond the usual C(sp 3 )-C(sp 3 ) bond length (1.54 Å) but are also much shorter than the bonds of closed shell dimers that are held together by vdW forces (sum of vdW radii = 3.40 Å) (ii) due to direct atom-to-atom overlap, SOMO-SOMO overlapping strongly favors configurations that yield maximum overlap orientations which lower the energy of the two radical SOMOs iii) low lying singlet (singlet-singlet) and triplet (singlet-triplet) electronic excited states, iv) negative singlet-triplet splitting energies (i.e., ΔE ST = E(singlet) -E(triplet)) for stable open shell singlet pancake bonded complexes [10] and v) interaction energies larger than those of vdW interactions. Bond dissociation energies (BDE) of pancake bonded system have been estimated to be smaller than those of a normal covalent system but larger than dimers subject to typical π-stacking where this type of π-stacking is observed for DNA base pairs [11] (vdW π stacking interactions and pancake bonds are different).…”

Chalcogen controlled redox behaviour in peri-substituted S, Se and Te naphthalene derivatives

“…75 Pancake bonding represents a challenging case for intermolecular bonding, with typical features of interaction distances considerably shorter than van der Waals contact distances, multireference singlet ground states, and concurrently important dispersion interactions. 73,[78][79][80][81][82][83][84] This is further complicated in [N23]BF 4 by the large coulombic charges amongst the cation radicals and BF 4 À counterions. The only charged radicals for which the energy of pancake bonding has been quantified, to our knowledge, is for dimers of anion radicals of tetracyanoethene (TCNE), which have been extensively investigated.…”