Experimental and theoretical studies of the basicity and proton affinity of SiF4 and the structure of SiF4H+

A combined experimental and theoretical approach has been employed to estab lish the basicity and proton affinity of SiF4 and the structure of SiF4H+. The kinetics and energetics for the transfer of a proton between SiF4, N-2, and Xe have been explored experimentally in helium at 0.35 +/- 0.02 torr a nd 297 +/- 3 K with a selected-ion flow tube apparatus. The results of equi librium constant measurements are reported that provide a basicity and prot on affinity for SiF4 at 297 +/- 3 K of 111.4 +/- 1.0 and 117.7 +/- 1.2 kcal mol(-1), respectively. These values are more than 2.5 kcal mol(-1) lower t han currently recommended values. The basicity order was determined to be G B(Xe) > GB(SiF4) > GB(N-2), while the proton-affinity order was shown to be PA(Xe) > PA(N-2) > PA (SiF4). Ab initio molecular orbital computations at MP4SDTQ(fc)/ 6-311++G(3df,3pd) using, geometries from B3LYP/6-31+G(d,p) ind icate a value for PA(SiF4) = 118.7 kcal mol(-1) that is in good agreement w ith experiment. Also, the most stable structure of SiF4H+ is shown to corre spond to a core SIFT cation solvated by HF with a binding energy of 43.9 kc al mol(-1). 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) (C) 1999 American Society for M ass Spectrometry.