J. Phys. Chem. 1981, 85,3364-3366
3304
Absolute Proton Affinities of PH, and H,S Dennis S. Marynick, Department of Chemistty, Universl?yof Texas-Arlington, Arlington, Texas 760 10
Kerin Scanion, Robert A. Eades, and David A. Dixon*-+ Department of Chemistty, University of Minnesota, Minneapolis, Mlnnesota 55455 (Received: August 9, 198 1)
The absolute proton affinities (PA) of PH3 and H2S have been determined from ab initio molecular orbital theory. Calculations were made at the SCF/CI level including all single and double excitations and a correction for quadruple excitations using a near Hartree-Fock STO basis. Zero-point energy corrections were made based on scaled SCF calculations by using a polarized double zeta GTO basis. The calculated values are PA(PH3) = 188.1 and PA(H2S) = 171.7 kcal/mol. A comparison with experimental values is presented.
Introduction Absolute proton affinities (PA) are extremely difficult to determine from experiment due to the scarcity of accurate heats of formation of i0ns.l Absolute proton affinities are important for converting relative proton affinities which are available from experiment, e.g., ion cyclotron resonance spectroscopy2 or high-pressure mass spe~trometry,~ to an absolute scale for use in a range of thermodynamic cycles. We are interested in determining proton affinities as part of our combined experimental/ theoretical studies of gas-phase ions.44 It has previously been demonstrated that accurate proton affinities can be calculated from ab initio molecular orbital theory. The two molecules for which accurate proton affinities (PA) have been calculated are NH? and H20a7 The PA's of these two compounds span a range of almost 40 kcal/mol. In order to help further establish the absolute scale of proton affinities, we have calculated from ab initio molecular orbital theory the absolute proton affinities of PH3 and H2S, the second row analogs of NH, and H20. These calculations provide further information about the effects of basis set size, zero-point energy corrections, and correlation energy corrections on the values of the absolute proton affinities. The proton affinity of a base B (PA(B)) is usually defined from experiment as PA(B) = -AH298 of reaction l.
+
B H++BH+ (1) We have previously shown4 that the value for PA(B) is given by (2) to a very good approximation. The term PA(B) = -AH298 = -AEoelec - AZPE + (5/2)RT (2) AZPE is the difference in zero-point energies of B and BH" and the term (5/2)RT is a thermodynamic temperature is the energy difference correction term. The term hEodec between B and BH+ at 0 K and is determined from our ab initio calculations. This term includes both the dominant Hatree-Fock energy difference and any correlation corrections. For H20 the correlation correction is only 1.5 kcal/moP but for NH,, it is double this value (3 kcal/mol).4
Calculations The calculations were performed by using large basis sets of both Slater and Gaussian type orbitals (STO's and GTO's). The SCF-CI (all single and double excitations from the valence space to the virtual space) calculations 'Alfred P. Sloan Foundation Fellow (1977-81). Henry Dreyfus Teacher-Scholar (1978-83).
Camille and
0022-3654/81/2085-3364$01.25/0
TABLE I : G e o m e t r y Parameters f o r the Calculation of A E ~ ~ ~ C molecule
symmetry
r(H-X)"
O(H-X-H)b
PHaC
CW
PH,+ HzSe H3S+
Td
1.413 1.392 1.328 1.350
93.1 109.5 92.2 94.5
CZLJ c3u
a Bond length in A . ti Bond angle in degrees, SCF-CI optimized. Reference 11. SCF-CI optimized. This work. The C I included all single and d o u b l e excitations f r o m the valence orbitals i n t o the virtual orbitals w i t h eigenvalues less t h a n 9.0 au. e Experimental geometry. Reference 14. f SCF-CI optimized. Reference 13.
using the STO basis were performed with the program POLYCAL~on an IBM 370/155 computer. The term hEoe'ec was determined with the STO basis set. The SCF calculations using a GTO basis set were performed with the program HONDO (Version 5)9on a VAX 11-780 computer.1° The term AZPE was determined by using the GTO basis. The geometries for calculating the hEodec term for PH3,11 PH4+,12and SH3+l3 were obtained from geometry optimizations at the SCF-CI level by using polarized doublezeta (DZP)STO basis sets. The geometry for H2S was taken from experiment.14 The geometry parameters are (1)S. G. Lias, D. M. Shold, and P. Ausloos, J. Am. Chem. SOC. 102, 2540 (1980). (2) (a) J. F. Wolf, R. H. Staley, I. Koppel, M. Taagepera, R. J. McIver, Jr., J. L. Beauchamp, and R. W. Taft, J. Am. Chem. SOC., 99,5417 (1977); (b) D. H. Aue and M. T. Bowers, "Gas Phase Ion Chemistry", Vol. 2, M. T. Bowers, Ed., Academic Press, New York, 1979, Chapter 2. (3) Y.K. Lau. Ph.D. Thesis. Universitv of Alberta. 1979. (4) R.A. Eades, K. Scanlon,' M. R. Ellenberger, D.'A. Dixon, and D. S. Marynick, J. Phys. Chem., 84,2840 (1980). (5) (a) R. A. Eades, D. A. Weil, D. A. Dixon, and C.H. Douglass, Jr., J. Phys. Chem., 85, 981 (1981); (b) R. A. Eades, D. A. Weil, M. R. Ellenbereer, W. E. Farneth, D. A. Dixon. and C. H. Douelass, Jr., J. Am. Chem. S&.; 103,5372 (1981). ( 6 ) (a) M. R. Ellenberger, R. A. Eades, M. W. Thomsen, W. E. Farneth. and D. A. Dixon. J . Am. Chem. SOC..101.7151 (1979): (b) M.R. Ellenberger, W. E. F G e t h , and D. A. Dixon, J. Phys. Chem.,'85,'4(1981); ( c ) M. R. Ellenberger,D. A. Dixon, and W. E. Farneth, J. Am. Chern. SOC., 103,5377 (1981). (7) G. H. F. Diercksen, W. R. Kraemer, and B. 0. Roos, Theor. Chem. Acta (EerZ.),36, 249 (1975). (8) R. M. Stevens, J. Chem. Phys., 61, 2086 (1974). (9) H. F. King, M. Dupuis, and J. Rys, National Resource for Computer Chemistry Software Catalog, Vol. 1,Program No. QH02 (HONDO), 1AV"". ann
(10) This computer is located in the Department of Chemistry, University of Minnesota. (11) D. S. Marynick and D. A. Dixon, J. Phys. Chern., submitted for publication. (12) This geometry was optimized at the DZP-CI level for this work. (13) D. A. Dixon and D. S. Marynick, J. Chem. Phys., 71,2860 (1979).
0 1981 American Chemical Society
The Journal of Physical Chemistty, Vol. 85, No. 23, 198 1 3305
Letters
TABLE 11: Theoretical Harmonic Frequencies in c m - ' f o r Neutral Molecules and I o n s mode
PH,a
u,
v2
2469 (A,) 1 2 0 8 (A,)
ug uq
2482 (E) 1266(E)
PH,' 2669 (A,) 1242 ( E ) 2715 (T,) 1108(T,)
H2Sb
H,S+
2830 (A,) 1348 (A,) 2 8 3 6 (B,)
2 7 7 9 (A,)
1196 (A,) 2805 ( E ) 1357 ( E )
For comparison t h e experimental a n h a r m o n i c frequencies (see c ) a r e U , = 2323, u 2 = 9 9 2 , u , = 2 3 2 8 , and v 4 7 1122. For comparison the experimental a n h a r m o n i c frequencies (see c ) are v , = 2 6 1 5 , u2 = 1183, and v 3 = 2628. Experimental frequencies f r o m G. Herzberg, "Electronic S t r u c t u r e o f Polyatomic Molecules", Van Nostrand-Reinhold, N e w Y o r k , 1 9 6 6 , p p 586 a n d 610.
given in Table I. The final energy calculations for AE
