The acidity of polyhalogenated silanes and silyl radicals

The results of abinitio molecular orbital calculations at the MP4SDTQ/6-31++G(d,p)//HF/6-31++G(d,p) level have been used to calculate acidities of fluoro- and chloro-substituted silanes and silyl radicals. The radicals are more acidic than the silanes and substituent effects are also slightly larger in the radicals. For the gas phase deprotonation of fluorosilanes at 298 K, ΔH r (kcal/mol) values are SiH 4 , 378.5; SiH 3 F, 374.5; SiH 2 F, 366.7, and SiHF 3 , 351.0, i.e., interaction between fluorine atoms leads to increased enhancement of acidity. For chlorosilanes substituent effects are larger but strictly additive (13 kcal/mol for each Cl atom) with ΔH r values SiH 3 Cl, 365.4; SiH 2 Cl 2 352.5, and SiHCl 3 339.4. The electron affinities of silyl radicals calculated using isogyric reactions at the MP4SDTQ/6-31++G(d,p) level are too low by ~0.3 eV, but at the MP4SDTQ/6-311++G(2df,p) level the calculated electron affinity of SiH 3 is 1.39 eV, compared with an experimental value of 1.44 ± 0.03 eV. This higher level of theory gives calculated electron affinities of 1.53 eV for SiH 2 F and 1.92 eV for SiH 2 Cl. Heats of formation obtained by using isogyric reactions to calculate atomization energies at the MP4SDTQ/6-311++G(2df,p) level are within 3 kcal/mol of experimental values except for SiH 2 F (where the "experimental" value was obtained from linear interpolation between SiH 3 and SiF 3 ). [Formula: see text] (kcal/mol) calculated for the anions are SiH 3 − , 14.4; SiH 2 F − , −78.0; and SiH 2 Cl − , −37.6.