Experimental and Theoretical Studies of the Basicity and Proton Affinity of SiF 4 and the Structure of SiF 4 H 1

A combined experimental and theoretical approach has been employed to establish the basicity and proton affinity of SiF4 and the structure of SiF4H . The kinetics and energetics for the transfer of a proton between SiF4, N2, and Xe have been explored experimentally in helium at 0.35 6 0.02 torr and 297 6 3 K with a selected-ion flow tube apparatus. The results of equilibrium constant measurements are reported that provide a basicity and proton affinity for SiF4 at 297 6 3 K of 111.4 6 1.0 and 117.7 6 1.2 kcal mol , respectively. These values are more than 2.5 kcal mol lower than currently recommended values. The basicity order was determined to be GB(Xe) . GB(SiF4) . GB(N2), while the proton-affinity order was shown to be PA(Xe) . PA(N2) . PA (SiF4). Ab initio molecular orbital computations at MP4SDTQ(fc)/ 6-31111G(3df,3pd) using geometries from B3LYP/6-311G(d,p) indicate a value for PA(SiF4) 5 118.7 kcal mol 21 that is in good agreement with experiment. Also, the most stable structure of SiF4H 1 is shown to correspond to a core SiF3 1 cation solvated by HF with a binding energy of 43.9 kcal mol. Support for this structure is found in separate SIFT collision induced dissociation (CID) measurements that indicate exclusive loss of HF. (J Am Soc Mass Spectrom 1999, 10, 848–855) © 1999 American Society for Mass Spectrometry