Experimental Thermochemistry of the SiCl and SiBr Radicals; Enthalpies of Formation of Species in the Si−Cl and Si−Br Systems

Abstract

Gaseous reaction equilibria involving the radical species SiCl and SiBr were studied by mass spectrometric monitoring of the molecular beam issuing from a heated effusion cell source, and the results were used to derive the enthalpies of formation ∆ f H°2 98 (SiCl,g) ) 36.8 kcal mol -1 and ∆ f H°2 98 (SiBr,g) ) 48.7 kcal mol -1 , along with the dissociation energies D°0(SiCl) ) 98.9 kcal mol -1 and D°0(SiBr) ) 84.7 kcal mol -1 , all (2 kcal mol -1 . Thermochemical data were also obtained for the species SiCl 2 and SiBr 2 , confirming earlier data in the literature. Nonobservance of the species SiCl 3 and SiBr 3 was used to set lower limits of -84 and -40 kcal mol -1 to the enthalpies of formation of the gaseous trichloride and tribromide, respectively. The results are compared with experimental and theoretical values in the literature, clarifying previous discrepancies regarding the species SiCl 3 and SiBr 3 . From a full evaluation of the literature and this work, a recommended set of enthalpies of formation of the gaseous silicon mono-, di-, tri-, and tetrahalide species is given.

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

“…Another ion-chemistry study by Murthy and Beauchamp measured the enthalpy difference between SiH 2 Cl + , SiHCl 2 + , and SiCl 3 + [45,46]. Large discrepancy exists between experiments for these radicals and cations, e.g., f H • 298 K (SiCl 3 ) of −334.7 ± 8.4, −390.4 ± 16.7, and ≥−351.5 ± 8.4 kJ/mol have been reported by Walsh [27], JANAF-1985 [26], and Hilderbrand et al [28].…”

“…The corrected values for SiH 4 and Si 2 H 6 were adopted in photoionization study on SiH x [31] and in a large number of theoretical studies on the thermochemistry of silicon hydrides [98][99][100][101][102] where they were taken as reference for calibrating the theory and setting up the correction parameters. However, Feller and Dixon [59] [27]; e From G3 atomization energies [55]; f BAC-MP4 calculation, using 34.3 ± 2.1 kJ/mol for the enthalpy of formation of SiH4, for silicon hydrides and chlorinated compounds [48], and fluorinated compounds [47]; g Fluorinated silanes at MP4/6-31G(d,p) level using isodesmic reactions [49]; h MP4/6-31++G(2d,2p) with isodesmic reactions based on experimental SiHn and SiF4 [17]; i Chlorinated compounds at MP4/6-31+G(2df,p) level using isodesmic reactions [50]; j SiFx (x = 1-3), from the calculated CCSD(T)/CBS bond dissociation energies [54]; k SiClx (x = 1-3), from the calculated G2(MP2) bond dissociation energies [51]; l From heat of decomposition [96,97]; m Chemical equilibrium; n From threshold energies in reaction of Si + + SiF4 [40]; p From photoionization of SiHx [31]; q From collision-induced dissociation and charge transfer reactions [41]; r From chemical equilibrium study [28]. 79.9 kJ/mol(G3(CC)) where halosilanes were quantified using electron impact ionization mass spectrometry.…”

“…The early results were evaluated and collected in data compilations such as CATCH [25], JANAF-1985 [26], and a review by Walsh [27]. Recently, Hilderbrand et al [28] reported new enthalpies of formation for SiCl x and SiBr x (x = 1-3) from the gas-phase equilibrium study.…”

Thermochemical Properties of Halides and Halohydrides of Silicon and Carbon

“…Modeling accuracy and correctness depend crucially on the quality of the kinetics and thermochemistry data input. Thermochemistry data of some halides and halohydrides of Si and C are provided in databases, 6-8 review, 9 as well as experimental [10][11][12][13][14][15][16] and theoretical studies.…”

“…Modeling accuracy and correctness depend crucially on the quality of the kinetics and thermochemistry data input. Thermochemistry data of some halides and halohydrides of Si and C are provided in databases, 6-8 review, 9 as well as experimental [10][11][12][13][14][15][16] and theoretical studies.17-32 Nevertheless, inconsistency derived from various means of achieving data along with many data mismatches have rendered it far from complete. Here we present thermochemical properties from quantum chemical calculations of a complete set of halides and halohydrides of Si and C, namely SiX n H m and CX n H m with X = F, Cl and Br, and n+m ≤4, covering the temperature range of 298-2500 K. The study range accommodates applications such as CVD process, surface etching of semiconductors, combustion, as well as fundamental studies.…”