Halogenated silanes, radicals, and cations: Theoretical predictions on ionization energies, structures and potential energy surfaces of cations, proton affinities, and enthalpies of formation

Abstract

Quantum chemistry study has been carried out on the structure and energetics of halogenated silanes, radicals, and cations (SiH x X y 0,+1, X = F, Cl, Br; x + y = 1–4). The geometries are optimized at B3LYP/6-31+G(2df,p) level. The adiabatic ionization energiess (IE as), relative energetics of cations, proton affinities (PAs) of silanes, and the enthalpies of formation are predicted using G3(CC) model chemistry. Non-classical ion complex structures are found for hydrogenated cations and transition states connecting classical and non-classical structures are also located. The most stable cations for silylene and silyl radicals have their classical divalent and trivalent structures, and those for silanes have non-classical structures except for SiH 3Br + and SiH 2Br 2 +. The non-classical structures for halosilane cations imply difficulty in experimentally measurement of the adiabatic ionization energies using photoionization or photoelectron studies. For SiH 3X, SiH 2X 2, and SiHX 3, the G3(CC) adiabatic IE as to classical ionic structures closest to their neutrals agree better with the photoelectron spectroscopic measurements. The transition states between classical and non-classical structures also hamper the photoionization determination of the appearance energies for silylene cations from silanes. The G3(CC) results for SiH x 0,+1 agree excellently with the photoionization mass spectrometric study, and the results for fluorinated and chlorinated species also agree with the previous theoretical predictions at correlation levels from BAC-MP4 to CCSD(T)/CBS. The predicted enthalpy differences between SiH 2Cl +, SiHCl 2 +, and SiCl 3 + are also in accordance with previous kinetics study. The G3(CC) results show large discrepancies to the collision-induced charge transfer and/or dissociation reactions involving SiF x + and SiCl x + ions, for which the G3(CC) enthalpies of formation are also significantly differed from the previous theoretical predictions, especially on SiF x + ( x = 2–4). The G3(CC) IE a and PA of SiF 4 are significantly different from previous experimental and theoretical studies; however, they are supported by current benchmark calculations at level of CCSD(T)/CBS + core-valence correction.