2026 J. zyxwvuts Phys. Chem. 1983, zyxwvut 87, 2026-2032 Effect of Substituting Silicon for Carbon on Molecular Proton Affinities Monlca L. Hendewerklt Reglna Frey,$ and Davld A. Dlxon" Depaflment of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 (Received: August 26, 1982) The absolute proton affinities for the small molecules RNH2,ROH, RPH2,and RSH for R zyx = H, CH3, and SiH3 have been calculated at the SCF level by using double {basis sets with polarization functions on the heavy atoms. Zero-point energy differences have been calculated at the STO-3G level. Substitution of SiH3 for CH3 leads, in general, to a decrease in the proton affinity. The proton affinity of (CH3)3SiN(CH3)2 zyx was determined by ion cyclotron resonance spectroscopy to be 5.6 kcal/mol less than that of (CH3)3CN(CH3)2 in good agreement with the calculated difference of 6.9 kcal/mol for PA(H3SiNH2) and PA(H3CNH2).Methyl and silyl cation affinities for NH3, H20,PH3, and H2S have also been calculated. The silyl cation affinities are found to be much smaller than the methyl cation affinities. The dependence of zero-point energy differences on substituent and on the chemical nature of the central atom is also discussed. Introduction There are a number of recent experimental studies of the gas-phase ion-molecule reactions of silicon-containing Much of this work has focussed on conden- sation reactions of the trimethylsilyl cation while other work has examined the effect of SiR3 as a substituent on gas-phase proton affinities and ion-molecule chemistry. As part of our combined approach using molecular orbital (MO) theory5 and ion cyclotron resonance (ICR) spec- troscopy6 for the study of gas-phase ion-molecule chem- istry, we have examined the substituent effect of SiH3 as compared to CH, on the proton affinity of some small molecules using MO theory. The calculations also give silyl and methyl cation affinities for this set of molecules. We have previously demonstrated the importance of using adequate basis sets and including zero-point effects in the calculation of proton affinitie~.~ For the calculations reported here, we have employed a double f basis set with heavy atom polarization functions (DZ+D) and optimized geometries from minimum basis set calculations. The DZ+D basis was chosen based on our previous experience in calculating relative proton affinities for methyl-sub- stituted amines.% The substituent effects in our present calculations are found to be strongly basis set dependent. The approximate molecular orbital method PRDD07 has recently been extended to include second-row and transition-metal atoms.8 For the first row, good agreement between PRDDO and ab initio minimum basis set calcu- lations is usually We present a comparison of our PRDDO results with the ab initio STO-3G results as an aid in calibrating the PRDDO method for second-row species. Calculations All ab initio calculations were done with the program HOW: 4.1° All PRDDO calculations were performed with the new version of the PRDDO program. The STO-3G basis set'l was employed for the minimum basis set ab initio calculations. Exponents for the PRDDO calculations (an MBS of STO's) were the same as those employed in the STO-3G work. The DZ+D basis set was taken from Dunning and Hay12 with the following contraction schemes: (4)/[2] for hydrogen, (9,5,1)/[3,2,1]for first-row atoms, and (11,6,1)/[6,4,1] for second-rowatoms. All geometries were optimized at the MBS level in the appropriate molecular Present address: Chemistry Department, University of California-Berkeley, Berkeley, California. * Present address: Chemistry Department, University of Utah, Salt Lake City, Utah. symmetry. Initially pointwise geometry optimization was done with the computationally efficient PRDDO method. The PRDDO-optimized geometries were used as the starting points for the STO-3G calculations and the ge- ometries were gradient 0ptimi~ed.l~ The geometries of the fragment molecules and ions CH3+, CH4,NH,, NH4+, HzO, H30+, SiH,, SiH3+, PH,, PH4+, H,S, and H3S+ were gradient optimized with the DZ+D basis set. The STO-3G optimum geometries were employed in the DZ+D calcu- lations for molecules with two non-hydrogenic atoms ex- cept that the C-H, Si-H and A (heteroatom)-H bond distances were scaled. The A-H bond distances in the neutral molecules were set equal to the A-H distance in the appropriate hydride obtained from the optimum ge- ometry a t the DZ+D level, e.g., the 0-H bond distance in CH30H is set equal to the bond distance in H,O(DZ+D). The C-H and Si-H distances for these neutral molecules (1) (a) I. A. Blair, J. H. Bowie, and V. C. Trenerry, zyxw J. zyx Chem. SOC., Chem. Commun., 230 (1979); (b) I. A. Blair, G. Phillipou, and J. H. Bowie, Aut. J. Chem., 32,59(1979); (c) I. A. Blair and J. H. Bowie, Aut. J. Chem., 32,1389(1979); (d) V. C. Trenerry, J. H. Bowie, and I. A. Blair, J. Chem. Soc., Perkins Trans. 2, 1640 (1979); (e) V. C. Trenerry, I. A. Blair, and J. H. Bowie, Aut. J. Chem., 33,1143 (1980); (0 V. C. Tren- erry, G. Klaes, J. H. Bowie, and I. A. Blair, J. Chem. Res., 5, 386 (1980); (g) J. H. Bowie, Acc. Chem. Res., 13,76 (1980). (2) K. J. Shea, R. Gobeille, J. Bramblett, and E. Thompson, zy J. Am. Chem. SOC., 100,1611 (1978). (3) W. J. Pietro, S. K. Pollack, and W. J. Hehre, J. Am. Chem. SOC., 101,7126 (1979). (4) (a) M. L. Hendewerk, D. A. Weil, T. L. Stone, M. R. Ellenberger, W. E. Farneth, and D. A. Dixon, J. Am. Chem. Soc., 104,1794(1982); (b) M. R. Ellenberger, M. L. Hendewerk, D. A. Weil, W. E. Farneth, and D. A. Dixon, Anal. Chem., 54,1309(1982); (c) D. A. Weil, M. L. Hendewerk, and D. A. Dixon, unpublished work. (5) (a) R. A. Eades, K. Scanlon, M. R.. Ellenberger, D. A. Dixon, and D. S. Marynick, J. Phys. Chem., 84,2840 (1980); (b) D. S. Marynick, K. Scanlon, R. A. Eades, and D. A. Dixon, ibid., 85, 3364 (1981); (c) R. A. Eades, D. A. Weil, D. A. Dixon, and C. H. Douglas, Jr., ibid., 85,981 (1981); (d) R. A. Eades, D. A. Weil, M. R. Ellenberger, W. E. Farneth, D. A. Dixon, and C. H. Douglaea,Jr., J. Am. Chem. SOC., 103,5372 (1981). (6) (a) M. R. Ellenberger, R. A. Eades, M. W. Thomsen, W. E. Far- neth, and D. A. Dixon, J. Am. Chem. SOC., 101,7151(1979); (b) M. R. (7) T. A. Halgren and W. N. Lipscomb, J. Chem. Phys., 85, 1569 (8) D. S. Marynick, private communication. (9) T. A. Halgren, D. A. Kleier, J. H. Hall, Jr., L. D. Brown, and W. N. Lipscomb, J. Am. Chem. SOC., 100,6595(1978). (10) M. Dupuis, J. Rys, and H. F. King, Jr., J. Chem. Phys., 65, 111 (1976); (b) H. King, M. Dupuis, and J. Rys, "National Resource for Computational Chemistry Software Catalog", Vol. 1, Program No. (11) W. J. Hehre, R. F. Stewart, and J. A. Pople, J. Chem. Phys., 51, 2657 (1969). (12) T. H. Dunning, Jr., and P. J. Hay in "Methods of Electronic Structure Theory", Vol. 3, H. F. Schaefer, III., Ed., Plenum Press, New York, 1977, Chapter 1. (b) f(d) = 0.4, 0.5, and 0.6 for Si, P, and S, respectively, T. H. Dunning, Jr., private communication. (13) P. Pulay in "Applications of Electronic Structure Theory", Vol. 4, H. F. Schaefer, 111, Ed., Plenum Press, New York, 1977, Chapter 4. Ellenberger, D. A. Dixon, and W. E. Farneth, ibid., 103, 5377 (1981). (1973). QHOZ-(HONDO), 1980. 0022-3654/83/2087-2026$0 1.50/0 0 1983 American Chemical Society