Thermochemistry of Hydrochlorofluorosilanes:  A Gaussian-3 Study

Abstract

A high-level theoretical study on the thermochemistry of hydrochlorofluorosilanes has been carried out using the Gaussian-3 (G3), and its computationally less expensive variant G3(MP2), methods. In this paper, we report the heats of formation of 15 silanes, their adiabatic ionization energies (IEs), electron affinities (EAs), and proton affinities (PAs), as well as the acidities of 10 hydrosilanes. Good to excellent agreement with the available experimental data is found for essentially all calculated quantities. The only exceptions are those involving the SiF 4 + cation, such as the IE and PA of SF 4 . The origin of this failure is not immediately clear, even though this failure is not confined to the G3 methods. Because many of the thermochemical data for the industrially important silanes are not available in the literature, the results reported here may be taken as reliable estimates.

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

“…These values are slightly higher than our G3(CC) adiabatic IE a s of 11.55 and 10.52 eV for SiCl 4 and SiBr 4 , respectively. The G3(CC) IE a (SiCl 4 ) is slightly lower than previous G3 value of 11.66 eV [55] and G2(MP2) of 10.67 eV (with Cs symmetry for SiCl 4 + ) [51]. At B3LYP/6-31+G(2df,p) level, both classical and non-classical structures are identified for cations of SiH 3 X, SiH 2 X 2 , and SiHX 3 (Figs.…”

“…Experimental adiabatic IE a s were only available for SiH 4 [31] and SiF 4 [42], while values for other species listed on NIST website [66] were exclusively obtained from electron impact ionization and photoelectron spectroscopy, which measure vertical IEs. Theoretical predictions are available to fluorinated and chlorinated silanes [29,51,54,55], especially the IE a values for fluoroand chlorosilanes obtained from previous G3 study [55] differ from current G3(CC) ones only slightly, within 0.03 eV except for SiH 2 Cl 2 , for which G3(CC) value is 0.08 eV lower.…”

“…On the other hand, the reported IE a (SiF 4 ) were rather diverse. Kickel et al [42] found that early studies obtained IE a (SiF 4 [31]; c Adiabatic IE from isogyric reactions at MP4/6-31G(d,p) level [29]; d Adiabatic IE from G3 calculation [55]; e Vertical IEs from photoelectron spectroscopy [86]; f Vertical IEs from photoelectron spectroscopy [88]; g Vertical IE from photoelectron spectroscopy [90]; h Vertical IE from photoelectron spectroscopy [80][81][82][83]; i Adiabatic IE from ion chemistry [42]; j Adiabatic IE from CCSD(T)/AVTZ + MP2/CBS [54]; k Vertical IE from photoelectron spectroscopy [89]; l From photoelectron spectroscopy [81]; m From electron impact ionization [84]; n From photoelectron spectroscopy [87]; o From photoionization mass spectrometry [75].…”

“…With limited experimental studies, quantum chemistry calculations have been used to predict the enthalpies of formation of fluorinated and chlorinated silanes and free radicals [17,29,[47][48][49][50][51][52][53][54][55][56][57][58][59] and of cations SiH x F y + , SiF x + , and SiCl x + [29,51,[53][54][55]60,61], while none on brominated species. Theoretical studies have found large discrepancies to as well as occasional agreement with the experimental results.…”

Quantum-chemical calculation of the thermodynamics of multistep hydrolysis of MX4 molecules (M = C, Si, Ge; X = H, F, Cl) in the gas phase

“…Therefore, estimation of these parameters by quantum-chemistry methods is of interest. Previously, quantum-chemical calculations were used for estimating the reactions of silicon fluorides and chlorides (see, e.g., [11,12]); however, the series (M = C, Si, Ge; X = H, F, Cl) was not systematically studied in these works. In addition, earlier quantum-chemical calculations rank below the state-of-theart methods in accuracy.…”