HEATS OF FORMATION OF SIHMCLN CALCULATED BY AB-INITIO MOLECULAR-ORBITAL METHODS

Optimized geometries and vibrational frequencies for SiH(m)Cl(n) were calculated at the MP2/6-31G(d,p) level. Energy differences were computed at the MP4/6-31+G(2df,p) level (all structures) and the G-2 level (structures containing no more than two chlorines). The heats of formation of SiH(m)Cl(n) were estimated using the following isodesmic and isogyric reactions: [(4-n)/4]SiH4 + (n/4)SiCl4 --> SiH4-nCln, SiHm + SiH4-nCln-->SiHCln + SiH4, 3/4Si + 1/4H-2 + SiH3Cl --> SiCl + 1/2H + 3/4SiH4,1/2Si + 1/2H-2 + SiH4-nCln - SiH2-nCl + H + 1/2SiH4, and 1/4Si + 1/2H + SiH4-nCln --> SiH3-nCln + 1/4H-2 + 1/4SiH4. The calculated heats of formation (kcal/mol; 298 K, 1 atm) are as follows: SiCl, 36.5 +/- 1.5; SiHCl, 15.0 +/- 1.5; SiCl2, -38.6 +/- 1.5; SiH2Cl, 8.0 +/- 1.5; SiHCl2, -34.0 +/- 1.5; SiCl3, -75.8 +/- 1.5; SiH3Cl, -32.0 +/- 1.5; SiH2Cl2, -74.2 +/- 1.5; SiHCl3, -116.8 +/- 1.5 (based on experimental values for SiH(n) and SiCl4). The present study uses a significantly higher level of theory than that used in earlier work and confirms the heats of formation predicted by the best previous theoretical studies to within 4-0.8 kcal/mol. The theoretical heats of formation are within the error bars of the most recent experimental values except for SiCl (46 +/- 5 kcal/mol), suggesting that a new experimental value would be desirable for SiCl.