878
J . Phys. Chem. 1988, 92, 878-880
Comparisons between different methods, as we have reported in this paper, can be helpful in designing future developments.
Acknowledgment. H.M. is grateful for an exchange program between the Academy of Sciences (GDR) and the Consiglio Nazionale delle Ricerche (Italy) that made possible his stay in Milano during the spring of 1985. A.G. acknowledges partial financial help from Minister0 della Pubblica Istruzione (Fondo Ricerca 40%). Thanks are due to Mrs. A. Trunschke for assistance in performing the extended Hiickel calculations. The work reported in this paper was inspired by M. Simonetta and his passionate interest in theoretical chemistry for chemisorption and catalysis. He died on January 6, 1986. The paper does not carry his signature only because the authors want to take full responsibility for their own mistakes.
Appendix
Extended Hiickel calculations were performed using R. Hoffmann's ICON program. The VOIP's were derived by selfconsistent charge iteration on PtCO with a quadratic dependence for the Pt VOIP's and a linear dependence for the C and 0 ones. The final parameters used were the following: C 2s, 1.625 (21.44); C 2p, 1.625 (-1 1.64); 0 2s, 2.275 (-29.40); 0 2p, 2.275 (-12.80); Pt 6s, 2.554 (-9.203); F't 6p, 2.535 (-4.656); for the Pt 5d double-f functions, the exponents were 6.013, 2.696 and the coefficients of double-f expansion 0.633,0.551, respectively. The Pt 5d VOIP was -1 1.67. Regism NO.CO, 630-08-0; Ptj, 110743-43-6; Pt4, 110743-44-7; Pt,, 110743-45-8; Pt,, 110743-46-9.
The Tetrahydrfdosulfonlum Dfcation, Has2+. Hydrogen-Deuterium Exchange of DH,S+ in FSO,D:SbF, and D,HS+ in FS03H:SbF, and Theoretical Calculationstla George A. Olah,**C. K. Surya Prakasb,* Michael MarceUi,*and Koop Lammertsma*s Donald P. and Katherine B. Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, Los Angeles, California 90089- 1661, and Department of Chemistry, University of Alabama, Birmingham, Alabama 35294 (Received: February 12, 1987)
Isotopomerichydridosulfoniumions undergo proton/deuterium exchange in (Magic Acid) FS03DSbFSand FS03H-SbFs-SOI solutions as monitored by 'H NMR spectroscopy. The rate of exchange increases with the increase in acidity of the superacidic medium, indicating the involvement of isotopomeric tetrahydridosulfonium dications in the exchange process. The structure of tetrahydridosulfonium ion, H4SZ+,2, was also probed by ab initio theory. The tetrahedral symmetry ( Td)structure of H4SZ+,2, was found to be the minimum energy structure at the HF/6-31G* level. Although structure 2 is thermodynamically unstable (dissociationto H3S++ H+ is preferred by 25.2 kcal/mol) it appears to have significant kinetic stability (deprotonation barrier 59.2 kcal/mol).
Introduction The parent of sulfonium ions i s protonated HIS, Le., the hydridmulfonium ion H3S+, 1. Olah and co-workers2observed H3S+ for the first time in FS03H:SbF5:S02media at low temperature by 'H N M R spectroscopy. H3S+appeared as a singlet a t 6 'H 6.6 from tetramethylsilane. H3S+has also been observed in the gas phase by mass spectrometric techniques.* Christe3 has subsequently isolated H3S+SbF6- salt by treating H2Swith HFSbF, at very low temperature. H
H
2+
1
2
2*
I
I
3
4
4. The ab initio theoretical calculations performed a t the HF/
6-31G* level showed that although the tetrahydridooxonium dication, H402+(4), is thermodynamically unstable (dissociation is exothermic by 59.2 kcal/mol) it is indicated to have significant kinetic stability (deprotonation barrier 39.4 kcal/mol). In continuation of our work we have found now that isotopic hydridosulfonium ions also undergo proton-deuterium exchange at high acidities, indicating the involvement of the isotopomeric tetrahydridosulfonium dication, H4S2+( 4 ) , in the exchange process. The structure of H4S+ (4) has also been probed by a b initio theoretical calculations at the HF/6-3 l G * level. Results and Discussion The isotopomeric hydridosulfonium ions were prepared by protonating DIS or H# in tenfold excess of 4 1 FSO3H:SbF5/SO2 and 4:l FSO3D:SbF5/SO2(Magic Acids) solutions, respectively, at -78 "C. The 'H N M R spectra of the isotopomeric ions were recorded at -50 O C . The 'H N M R chemical shifts of three isotopomeric sulfonium ions H3S+,H2DS+,and HD2S+are 6 'H 6.600, 6.639, and 6.674, respectively. For convenience the exchange rate of the ions with the superacidic solutions was monitored by the integration of isotopomeric hydridosulfonium vs the acid peak intensities. The exchange studies were carried out at -50 O C , since at higher temperatures the isotopomeric hydridosulfonium ions decompose oxidatively giving elemental sulfur.
The hexafluoroantimonate salt of H3S+is a stable white solid a t low temperatures and reacts with water to give HIS gas. This reaction can be conveniently used to generate H2S. The ion 1 has been thoroughly characterized by vibrational spectroscopy and normal-coordinate a n a l y ~ i s . ~ In our preceding worP we have found that the hydronium ion, H30+,3, undergoes hydrogen-deuterium exchange with very high acidity systems (i.e., increase in exchange rate with increase in acidity) indicating that the nonbonded electron pair of oxygen in the onium ion 3 is capable of interacting with a second proton (1) (a) Onium Ions. 36 at the University of Southern California. For part through the involvement of protonated hydronium dication, H402+, 35 see: Laali, K.; Olah, G. A. Rev. Chem. Intermed. 1985, 6, 237. t In memoriam of Massimo Simonetta.
*University of Southern California. University of Alabama.
f
(2) Olah, G. A,; OBrien, D. H.; Pittman, C. U.,Jr. J. Am. Chem. Soc. 1961, 89, 2996. ( 3 ) ChristL, K. 0. Inorg. Chem. 1975, 14, 2230. (4) Olah, G. A.; Prakash, G. K. S.; Barzaghi, M.; Lammertsma, K.; Schleyer, P. v. R.; Pople, J. A. J. Am. Chem. Soc. 1986, 108, 1032.
0022-3654/88/2092-0878$01 .50/0 0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 4, 1988 879
Tetrahydridosulfonium Dication
TABLE I: Total (in au) and Relative (in kcal/mol) Energies of SH4*+and SH3+ 3-21G(*)// 6-3 lG*//
geometry SHd2+
Td
SH3-H"
C3"
SH2+
D4h
3-21G(*)
6-31G*
-397.03973 (0.00) -396.95629 (5 2.36) -396.65641 (240.53)
-398.89852 (0.00) -398.80726 (57.26) -398.50689 (245.74) -398.941 84 (-27.18)
SH3+"
on 6-31G* geometries
HF/6-31G** -398.91 44 (0.00) -398.8 1844 (60.21) -398.52265 (245.82) -398.9525 (-23.91)
MP2/6-31GS* -399.05 127 (0.00) -398.95822 (58.39) -398.65575 (248.18) -399.09133 (-25.14)
MP3/6-31GS* -399.07221 (0.00) -398.97763 (59.35) -398.68041 (245.85) -399.1 1226 (-25.13)
MP4SDTQ/ 6-31G** -399.07774 (0.00) -398.98347 (59.15) -398.688 (244.56) -399.1 1784 (-25.16)
From ref 13. TABLE 11: 631C* Harmonic Frequencies for SH4*+Structures
structures
frequencies,cm-I
SH?'
Td
S H : +
D4h
SH3-H"
C3"
H2DS+in 1:4 FS03D:SbF5at -50 OC slowly undergoes protondeuterium exchange. The 'H N M R peak intensity at S 'H 6.62 (broad) decreases with corresponding increase in the intensity in acid region. The half-life for the exchange was approximately 300 min. Similarly the D2HS+ ion underwent deuterium-proton exchange with a half-life of approximately 240 min. This indicates a deuterium isotope effect of k H / k D 1 ~ 2 . When the same experiments were carried out in either 1:l FS03D:SbF5 or 1:l FS03H:SbF5 the exchange rate increased and occurred with half-lifes of less than 210 and 170 min, respectively, indicating faster exchange rates with increase in acid strength.5a Two mechanisms are possible for the observed proton-deuterium exchange in the isotopomeric hydridosulfonium ion in the = -18 to -21.5). One of highly acidic Magic Acid system (Ho these might involve a deprotonation/dedeuteriation equilibrium of the isotopic hydridosulfonium ion with hydrogen sulfide followed by reprotonation/redeuteriation by the acid system (Scheme I, shown for HDzS+). The other involves protonation/deuteriation of the nonbonded electron pair of the isotopomeric sulfonium ion (Scheme 11, shown for HD,S+). SCHEME I
-
HD2S+
* 22 HDS
DH2.S'
* -
SCHEME I1 H+
-D+
ZPE, kcal/mol
1109 (Tz), 1332 (E), 2687 (AI), 2703 (T2) 434 (E), 1 1 16 (Al), 1340 (E), 2809 (Al), 2822 (E), 708i (A,) 1407 (A*,,). 1677 (&), 2010 (Bzg), 2377 (Aig), 2385 (E,,), 2440
HD2S+ G [H2D2SI2+eHzDS+
H2S
H3S+ 1
& [H3DSl2+,* -H3S+ 1
The observed isotopic exchange per se does not unequivocally permit differentiation between the two mechanisms in Schemes I and 11. However, the increase in exchange rate observed with an increase in the acid strength of Magic Acid (1:l) seems to support the mechanism in Scheme I1 strongly. 1:l Magic Acid has an Hovalue of - 2 1 ~ 1:4 , ~ of~ about -18. Scheme I would be expected to show the opposite effect as dissociation of H3S+ would be facilitated by decreasing acidity. To further ascertain the structure and stability of H4S2+,4, we carried out ab initio theoretical calculations using the GAUSSIAN 82 series of programs.6 The geometries were optimized within each assumed symmetry by using the 3-21G(*) and 6-31G* basis sets.7 Single-point calculations at the 6-31G** level, which has polarization (p type) functions on hydrogen in addition to the (5) (a) The exchange rates measured are only approximate, since there is some oxidative decomposition of the hydridosulfonium ion into elemental sulfur even at -50 OC. This problem compounded when we attempted to use much stronger fluoroantimonic acid system. (b) Olah, G. A,; Prakash, G. K. S.;Sommer, J. Superacids; Wiley-Interscience: New York, 1985. (6) The GAUSSIAN82 package of programs was employed. Binckley, J. S.; Frisch, M.; Raghavachari, K.; DeFrees, D.; Schlegel, B.; Whiteside, R. A,; Fluder, E.; Seeger, R.; Pople, J. A. Carnegie-Mellon University, Pittsburgh, PA.
(Big),
2490 ( € 3 ~ ~ )
24 26.9 18.8
d-type functions on sulfur, were carried out on the 6-3 l G * optimized structures and are denoted as HF/6-3 1G**//6-3 lG*.7 Valence-shell electron correlation corrections were calculated by Mdler-Plesset perturbation theory at the second (MP2), third (MP3), and fourth order, including all single, double, triple, and quadruple substitutions (MP4SDTQ).* The final energies corThe harmonic respond to MP4SDTQ/6-3lG**//HF/6-3lG*. vibrational frequencies and associated zero-point energies were determined from the analytical second derivatives at the HF/63 l G * level using the corresponding optimized geometries. The total and relative energies of SH42+and SH3+are indicated in Table I. Along with the isoelectronic first-row species, BH,, CH4, and BH4,9the recently reported OH42+dication? and the second-row species AlH4-, SiH4, and PH4+,9the isoelectronic SH+ : dicationI0 also is expected to be tetrahedral. Indeed, at the HF/6-31G* level of theory, diprotonated hydrogen sulfide (SH:)' with Td symmetry is a minimum; all the eigenvalues of the Hessian matrix are positive (the harmonic frequencies listed in Table 11). Although SHd2+is thermodynamically unstable toward deprotonation by 25.2 kcal/mol (MP4SDTQ/6-3 lG**//HF/632G*), it has significant kinetic stability as the dissociation barrier is calculated to be 59.2 kcal/mol (same level). Table I shows that these values are nearly the same whether or not valence electron correlation interactions are included. The heat of formation of SH:+ is estimated to be 584 kcal/mol from AHf0(SH3+= 193.8 and the calculated kcal/mol," AHfo(H+) = 365.2 kcal/m01,'~~'~ heat of deprotonation. The S-H bond distance in the tetrahedral structure is 1.396 A. The deprotonation transition structure exhibits an S-H bond length of 2.750 A; the other bonds shorten to 1.340 A and the HSH bond angle becomes 117.3O. U
Td
0 4h
c3v
Comparison of SH4+with the OH42+dication? which has a barrier of 43.9 kcal/mol (MP4SDTQ/6-3 lG**//HF/6-3 1G*) (7) (a) 3-21G(*) basis: Pietro, W. J.; Francl, M. M.; Hehre, W. J.; DeFrees, D. J.; Pople, J. A.; Binckley, J. S. J . Am. Chem. SOC.1982, 104, 5039. (b) 6-31G* and 6-31G** basis: Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (8) Binckley, J. S.; Pople, J. A. Int. J . Quantum Chem., Symp. 1975, 9, 229. Pople, J. A.; Binckley, J. S.; Seeger, R. Ibid. 1976, 10, 1 . Krishnan, R.; Pople, J. A. Int. J . Quantum Chem. 1976, 14,91. (9) Krogh-Jespersen,M. B.; Chandrasekhar,J.; Wurthwein, E. U.; Collins, J. B.; Schleyer, P.v.R. J . Am. Chem. SOC.1980, 102, 2263. (10) Adams, D. B. J . Chem. SOC.,Faraday Trans. 2 1977, 73, 991. (11) Prest, H. F.; Tzeng, W.-B.; Brom, J. M.; Ng, C. Y. J . Am. Chem. SOC.1983, 105, 7531. (12) Rosenstock, H. M.; Draxl, K.; Steiner, B. M.; Herron, J. T. J . Phys. Chem. R e j Data, Suppl. 1977, I, 6. (13) Whiteside, R. A.; Frisch, M. J.; Pople, J. A. The Carnegie-Mellon Quantum Chemistry Archive, 3rd ed., 1983.
880 The Journal of Physical Chemistry, Vol. 92, No. 4, 1988
for the exothermic deprotonation (59.3 kcal/mol, same level), suggests that S H t + may be obtained more readily by protonation by protonation of OH3+. The thermodyof SH3+than namics of the process is illustrated in the proton exchange reaction (eq 1). It must be remembered that these calculational data refer
+
-
SH3+ OH42+ SH42++ OH3+ M0= -27.7 kcal/mol
(1)
to isolated-state conditions in which the protonation of SH3+is thermodynamically unfavorable. However, solvation effects play an important role in the condensed phase and could give a very structured superacid solution of SH3+containing dication-like species with C3, symmetry (see 5). H
H
H'
+..y
31 s
5
We have also calculated the planar SH42+dication. This D4h structure is less stable than its Td isomer by 245 kcal/mol (MP4SDTQ/6-3 lG**). This energy difference is surprisingly large in reference to the OHd2+isomers, where it is 94.6 kcal/mol (same level)., Despite its high thermodynamical instability, planar SH42+represents a local minimum on the potential energy surface.17 The unscaled 6-31G* harmonic frequencies are listed in Table 11. Surprisingly, the S-H bond distances in planar SHq2+ (1.379 A) are 0.017 A shorter than in the tetrahedral isomer. Whereas tetrahedral SHt+ may be viewed as diprotonated SH2, planar S H P represents dioxidized sulfurane (SH,).The neutral hypervalent SH, has been the subject of several theoretical studies;14the perfluoro compound SF4is experimentally k110wn.l~ The C2, (trigonal bipyramid) and C,, (square pyramidal) symmetry structures have been considered for sulfurane SH,, the latter structure being preferred.'4a It is not surprising that double oxidation from the lone pair of the hypervalent sulfurane leads to a planar S H t + structure. Indeed, the LUMO (azu)of planar SH42+is a sulfur 3p-orbital perpendicular to the plane of the dication. The HOMO of this kinetically stable 8 valence electron AX4 species is of bl, symmetry, with nonbonding hydrogen interactions. This is evident from 3-21G calculations,'* although (14) (a) Yoshioka, Y.; Goddard, J. D.; Schaeffer, H. F.; 111. J. Chem. Phys. 1981, 74, 1855. (b) Smolyar, A. E.; Zuybin, A. S.; Khaikina, E. A.; Charkin, 0. P. Zh. Neorg. Khim. 1980, 25, 307. (c) Chen, M. M. L.; Hoffmann, R. J . Am. Chem. Sor. 1976, 98, 1647. (d) Gleiter, R.; Veillard, A. Chem. Phys. Lett. 1976,37,33. (e) Schwenzer, G. M.; Schaeffer, H. F., 111. J. Am. Chem. SOC.197S, 97, 1393. (15) Tolles, W. M.; Gwinn, W. D. J . Chem. Phys. 1962, 36, 11 19.
Olah et al. a t higher levels there is some sulfur 4d,2+ orbital participation. In contrast, planar which has the same number of valence electrons but lacks d-orbital stabilization, is a transition structure for intramolecular H ~ c r a m b l i n g .Because ~ of the ubonding nature of the H O M O (with d-orbital participation), planar SH42+shows resemblance with the well-studied planar CH3+,l6which has two valence electrons less. In comparison with the isoelectronic second-row species AlH4-, SiH,, and PH4+,9 SH42+shows the largest energy difference between the tetrahedral and planar structures.
Conclusion Similar to the oxygen atom of the hydronium ion 3," the sulfur atom of the hydridosulfonium, 1, possesses an unpaired electron pair which is capable of interacting with an additional proton forming H4S2+,2. Our experimental studies as well as a b initio theoretical calculations indicate the involvement of tetrahydridosulfonium dication, 2 in the proton exchange of hydridosulfonium ion 1 at increasingly higher superacidities. Experimental Section H2S gas (99.5%) was purchased from Matheson. D2S was prepared by reacting Na2S with aqueous D2SO4 (10% in D20). The superacids were freshly prepared from doubly distilled FS03H/FS03D and SbF5. FS03D was obtained from Columbia Organic Co. SO2 (Air Products) was used without further purification. Preparation of Hydridosulfonium Ions. The isotopomeric hydridosulfonium ions were prepared by introducing H2Sor D2S into a tenfold excess of the appropriate Magic Aid system in SO2 solution at -78 OC. The 'H N M R spectra were obtained and the exchange study carried out on a Varian Associates Model XL-200 spectrometer equipped with a 5-mm variable-temperature lH/19F dual frequency probe. Theoretical Calculations. Calculations were performed with Pople's GAUSSIAN 82 package of programs.6 Acknowledgment. Support of the work at USC by the National Science Foundation is gratefully acknowledged. We thank one of the referees for his constructive comments. Registry NO. H3S*, 18155-21-0; FSOSH, 7789-21-1; SbFS, 7783-70-2; H4SZ+,64291-72-1. (16) (a) Rabrenovic, M.; Proctor, C. J.; Ast, T.; Herbert, C. G.; Brenton, A. G.; Beynon, J. H. J . Phys. Chem. 1983,87, 3305. (b) Ast, T.; Porter, C. J.; Proctor, C. J.; Beynon, J. H. Chem. Phys. Lett. 1981, 78, 439. (c) Pople, J. A.; Tidor, B.; Schleyer, P. v. R. Chem. Phys. Lett. 1982, 88, 533. (d) Siegbann, P. E. M. Chem. Phys. 1982.66, 443. (17) The valence electronic configuration is 3a1,22e,41b, 2. Although the Dlh isomer with the 3a1l2e,44a2; configuration gives smalfer energy differe n m with the Td isomer, is., 153.41 (3-21G') and 157.76 (6-31G') kcal/mol, the number of imaginary frequencies for this state is 4 at both HF/3-21G* and HF/6-31GZ. We have not investigated other electronic states nor incorporated electron correlation in the optimizations. (18) At 3-21G, without d-orbital participation, the energy difference between the Td and D4*isomers is slightly higher and amounts 297.9 kcal/mol, with both species representing minima.
