Recently, noncovalent interactions in complexes and crystals have attracted considerable interest. The current study was thus designed to gain a better understanding of three seminal types of noncovalent interactions, namely: hydrogen, halogen and tetrel interactions with p-systems. This study was performed on three models of Lewis acids: X 3 -C-H, F 3 -C-X and F-T-F 3 (where X ¼ F, Cl, Br and I; andT ¼ C, Si, Ge and Sn) and three p-systems as Lewis bases: benzene (BZN), 1,3,5-trifluorobenzene (TFB) and hexafluorobenzene (HFB). Quantum mechanical calculations, including geometrical optimization, molecular electrostatic potential (MEP), maximum positive electrostatic potential (V s,max ), Point-of-Charge (PoC), potential energy surface (PES), quantum theory of atoms in molecules (QTAIM) and noncovalent interaction (NCI) calculations, were carried out at the MP2/aug cc-pVDZ level of theory.The binding energies were additionally benchmarked at the CCSD(T)/CBS level. The results showed that:(i) the binding energies of the X 3 -C-H/p-system complexes were unexpectedly inversely correlated with the V s,max values on the hydrogen atom but directly correlated with the X atomic sizes; (ii) the binding energies for the F 3 -C-X/p-system and F-T-F 3 /p-system complexes were correlated with the s-hole magnitudes of the X and T atoms, respectively; and (iii) for the F 3 -C-F/p-system complexes, the binding energy was as strong as the p-system was electron-deficient, indicating the dominating nucleophilic character of the fluorine atom. NCI analysis showed that the unexpected trend of X 3 -C-H/p-system binding energies could be attributed to additional attractive interactions between the X atoms in the X 3 -C-H molecule and the carbon atoms of the p-system. Furthermore, the I 3 -Sn-H molecule was employed as a case study of hydrogen, halogen and tetrel interactions with p-systems. It was found that hydrogen and halogen interactions of the I 3 -Sn-H molecule correlated with the electron-richness of the p-system. In contrast, tetrel interactions correlated with the electron deficiency of the p-system.

ABSTRACT:Ab initio calculations at the unrestricted Hartree-Fock (UHF) level have been performed to investigate the hydrogen abstraction reactions of · OH radicals with methane and nine halogen-substituted methanes (F, Cl). Geometry optimization and vibrational frequency calculations have been performed on all reactants, adducts, products, and transition states at the UHF/6-31G * level. Single-point energy calculations at the MP2/6-31++G * level using the UHF/6-31G * optimized geometries have also been carried out on all species. Pre-and postreaction adducts have been detected on the UHF/6-31G * potential energy surfaces of the studied reactions. Energy barriers, E ‡ , reaction energies, E r , reaction enthalpies, H r , and activation energies, E a , have been determined for all reactions and corrected for zero-point energy effects. Both E a and H r come into reasonable agreement with the experiment when correlation energy is taken into account and when more polarized and diffuse basis sets are used. The E a values, estimated at the PMP2/6-31++G * level, are found to be in good agreement with the experimental ones and correctly reproduce the experimentally observed trends in fluorine and chlorine substitution effects. A linear correlation between E a and H r is obtained, suggesting the presence of an Evans-Polanyi type of relationship.

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Ab initio molecular orbital calculations are performed on the planar ground states (S 0 ), the 90 • -twisted triplet (T 1 ), and pyramidalized singlet (S 1 ) excited states of ethylene, methaniminium cation (MC), monocyano-(MCE), 1,1-dicyano-(DCE), 1,1-dihydroxy-(DHE), and 1,1-dicyano-2,2-dihydroxy (DCHE) ethylenes. Equilibrium geometries are optimized at the Hartree-Fock (HF) level with the 6-31G * basis set. Electron correlation corrections are estimated by optimizing the HF/6-31G * geometries at the (U)MP2/6-31G * level and then by carrying out single-point calculations at the fourth-order Møller-Plesset perturbation theory ((U)MP4/6-311G * * //MP2/6-31G * ). The effects of various types of perturbations on the structures, energetics, dipole moments, and state ordering of S 0 , S 1 , and T 1 are carefully investigated. "Positive" S 1 -T 1 splittings are estimated at the HF level for all studied molecules, while "negative" S 1 -T 1 splittings are obtained at the MP2 level for MC, DHE, and DCHE.

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