Theoretical insights into nature of π-hole interactions between triel centers (B and Al) and radical methyl as a potential electron donor: Do single-electron triel bonds exist?

Abstract

UMP2/aug-cc-pVTZ calculations are carried out to investigate the geometry, interaction energy and bonding properties of single-electron triel bond (SETB) interactions in binary X3Z···CH3 complexes, where Z = B, Al and X = H, F, Cl, Br, CN, NC, OH and CH3. The estimated binding distances are found to be in the range of 2.129–3.321 and 2.358–2.555 A for X3B···CH3 and X3Al···CH3 complexes, respectively, which are much smaller than the sum of the van der Waals radii of corresponding interacting atoms. The strength of SETBs strongly depends upon the nature of the Z and X substituents. For a given Z atom, the presence of electron-withdrawing groups such as F or CN in ZX3 molecule tends to increase the absolute value of interaction energy, while a reverse trend is seen for the electron-donating groups (OH and CH3). According to quantum theory of atoms in molecule, all strong SETB interactions indicate a partially covalent character. The analysis of the intermolecular orbital interactions in the title complexes also indicates that the amount of charge transfer from the single-electron occupied p orbital of radical methyl to the empty p orbital of Z atom increases in the order CN > NC > H > CH3 > F > Cl > Br > OH.