J. Phys. Chem. 1996, 100, 15027-15032
15027
Experimental Evidence for Metastable Hydrosulfonium Radical H3S• Martin Sadı´lek and Frantisˇek Turecˇ ek* Department of Chemistry, Box 351700, UniVersity of Washington, Seattle, Washington 98195-1700 ReceiVed: March 21, 1996; In Final Form: May 28, 1996X
Variable-time neutralization-reionization mass spectrometry is used to generate isotopomers of metastable hydrosulfonium radicals, H3S•, of lifetimes ranging between 0.2 and 2.8 µs. The fractions of metastable H332S•, H334S•, D332S•, D334S•, and D2H32S• increase with the increasing internal energy of the precursor cations. Intramolecular isotope effects favor less of H from D2HS•. Ab initio calculations at the MP2/6-311++G(3df,2p) level indicate existence of symmetry-restricted 2A′ and 2A′′ electronic states of H3S•. Formation of metastable radicals from vibrationally excited precursor ions is discussed.
Introduction Hypervalent radicals are transient intermediates produced by one-electron reduction of organic or inorganic cations of the “onium” type.1 Hypervalent radicals derived from first-row hydrides, e.g., NH4• and H3O•, have been studied in detail by experiment1,2 and theory3 and found to exist in shallow potential energy minima in their ground electronic states. In contrast, experiment and theory have disagreed on the stability of hypervalent radicals derived from second-row hydrides, e.g., H2Cl• or CH2ClH4,5 and H3S•.6 In particular, the latter species has been observed by Griffiths et al. as a product of collisional neutralization of H3S+ in the gas phase and claimed to have a lifetime exceeding 0.56 µs.6 The latter authors used neutralization-reionization mass spectrometry (NRMS)7 to generate the radical and analyze it following collisional reionization to cations. In NRMS, fast ions of kiloelectronvolt kinetic energies are allowed to collide with thermal atomic or molecular targets, which serve as electron donors. Due to the short donor-acceptor interaction time, which is typically in the 10-15 s range, the electron transfer is considered a vertical process.7c Both exothermic and endothermic electron transfer is possible in high-energy collisions, since, in the latter case, the energy balance is maintained by converting a fraction of the fast ion kinetic energy.7,8 Excitation in the fast neutral species occurs upon collisional neutralization due to Franck-Condon effects in the vertical electron transfer9 and/or formation of excited electronic states.10,11 In contrast to the experimental results, a recent detailed ab initio study by Smart and Schiesser mostly identified H3S• as a saddle point corresponding to a transition state for H• radical attack at H2S.12 However, the latter results strongly depended on the basis set used, and a shallow energy minimum was found with one set of calculations that employed the Hay-Wadt pseudopotential.12,13 This apparent disagreement between experiment and theory has led us to reexamine the metastability of H3S• formed by femtosecond collisional reduction of its cation. Sulfur hydrides represent an experimentally challenging system due to the presence of the stable 33S isotope (0.75%), which causes isobaric overlaps of transient H332S• with the stable H233S that may affect the mass spectra.7b We use a combination of 2H and 34S isotopomers to separate hydrosulfonium radicals from hydrogen * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, August 15, 1996.
S0022-3654(96)00864-7 CCC: $12.00
sulfide isotopomers and avoid interferences. We also employ the recently introduced variable-time neutralization-reionization method10,14 to distinguish neutral and ion dissociations. Ab initio calculations are utilized to analyze the stability of excited states in the H3S• system pertinent to vertical transitions between the ionic and neutral potential energy surfaces. We show that the vertical nature of fast electron transfer can give a clue to the observed metastability of H3S•. Experimental Section Methods. Neutralization-reionization (NR) measurements were carried out on a tandem quadrupole accelerationdeceleration mass spectrometer described previously.15 Briefly, the precursor cations were prepared in a chemical ionization (CI) source by protonation of H2S or deuteronation of D2S. The ions were extracted and focused by the source ion optics and by the first quadrupole analyzer operated in the radio-frequencyonly mode and accelerated to 8250 eV kinetic energy. Stable cations of 15-20 µs lifetime entered the neutralization cell, which was floated at the acceleration potential, and a fraction of ions were neutralized by glancing collisions with dimethyl disulfide. The neutralization reagent was introduced from a heated glass manifold at a pressure to achieve 70% transmittance of the precursor ion beam. The remaining ions were separated electrostatically by a cylindrical lens floated at +250 V15 and located 5 mm down-beam from the neutralization cell. This creates a field gradient of 16.5 kV/cm that the neutral intermediates pass through before reionization. The fast neutral intermediates were reionized by collisions with oxygen at pressures achieving 70% transmittance of the precursor ion beam. Reionization in a floated conduit attached to the neutralization cell14,15 sampled neutral intermediates of lifetimes in the 0.21.8 µs range. Reionization in a down-beam collision cell sampled neutral intermediates of 2.8-2.9 µs lifetimes. A detailed description of the variable time experiments has been given elsewhere.10,14,16 The spectra were obtained by linked scanning of the deceleration lens potential and the ac and dc potentials at the mass-analyzing quadrupole to obtain unit mass resolution in the neutralization-reionization spectra. This linked scan mode also resolves precursor ions at adjacent m/z.17 For NR fragments, interferences may occur at low m/z values due to kinetic energy release in the fragmentation.16,18 For example, the dissociation of 8250 eV H332S to 32S forms the products with 7543 eV average kinetic energy, which is spread by 101.7xT about the mean value, where T is the kinetic energy release in the fragmentation. For large T, which are typical for © 1996 American Chemical Society
15028 J. Phys. Chem., Vol. 100, No. 37, 1996
Sadı´lek and Turecˇek
TABLE 1: Total ab Initio Energiesa species H3S+ H3S+ (2A′ f ion)c H3S+ (2A′′ f ion)d H3S• (2A′, TS) H3S• (2A′) H3S• (2A′′) H3S• (2A′ f 2A′′)f
MP2/ MP2/ 6-31++G(2d,p) 6-311++G(3df,2p) 〈S2〉 ZPVEb -399.109 284 -399.307 133 -399.311 882e -399.301 320 -399.305 136e -399.268 547 -399.271 032e -399.123 864 -399.124 112e
-399.169 310 -399.083 561 -399.159 729 -399.375 928 -399.381 003e -399.370 561 -399.374 700e -399.337 650 -399.340 457e -399.189 424 -399.189 699e
0 0 0 0.79 0.75 0.78 0.75 0.77 0.76
TABLE 2: Thermochemical Data for the H2S and H3S+ Systems ∆Hr,298a
reaction H2S + H3O+ f H3S+ + H2O H2S + CH5+ f H3S+ + CH4 H2S + (2B1)H2S•+ f H3S+ + H2S + (2A1)H2S•+ f H3S+ + H2S + H• f H3S• (TS) H3S+ f H2S•+ + H• H3S+ f HS+ + H2
65.9 43.2 47.6 a
HS• HS•
-15.5 -161 -31 -300 50 410 343
Based on data from ref 33 in units of kJ mol-1.
a In units of hartrees, 1 hartree ) 2625.5 kJ mol-1. b Zero-point vibrational energies, corrected by 0.93, in units of kJ mol-1. c By vertical ionization of (2A′)H3S•. d By vertical ionization of (2A′′)H3S•. e After annihilation of higher spin states, ref 40. f By vertical excitation of (2A′)H3S•.
the formation of S+ in NRMS,19 a fraction of 32S+ ions may be formed with 7135-7150 eV kinetic energy corresponding to dissociation m/z 37 f m/z 32, transmitted, and appear as an interference peak in the spectrum of H334S. For the precursor ion preparation, H2S (Matheson, 99.5%) was introduced from a heated manifold at 60 °C into a tight ion source at 1.8 × 10-6 Torr, as measured on the diffusion pump intake and corrected for the ionization gauge response.20 The typical ion source conditions were as follows: temperature 230 °C, emission current 1 mA, electron energy 100 eV. The reagent gas, CH4 (Matheson, 99.97%), H2O, or D2O (Cambridge Isotope Laboratories, 99.9% D), was introduced at a pressure measured as (2-3) × 10-5 or (5-10) × 10-5 Torr, which corresponded to ion source pressures of 0.2-0.3 and 0.5-1 Torr, respectively. The same corrected pressure was maintained when using H2S or D2S under self-CI conditions21 to maximize the H3S+ or D3S+ ion current. The extent of protonation or deuteronation was checked by recording standard mass spectra. At the higher pressures used, the intensity ratios [H3S+]/[H2S•+] were always >10. D2S was synthesized by adding D3PO4 in D2O (99.9% D) to dry ZnS and condensing the gas at -196 °C on a vacuum line. D2S was further purified by vacuum distillation at -20 °C. Calculations. Standard ab initio calculations were carried out using the Gaussian 92 suite of programs.22 Geometries were optimized with Møller-Plesset,23 frozen-core calculations truncated at second order (MP2), using the 6-31++G(2d,p) and, in some instances, 6-311++G(3df,2p) basis sets24 to obtain stationary points that were characterized as minima (all frequencies real) or saddle points (one imaginary frequency). Spinunrestricted formalisms (UHF, UMP2) were used for the openshell species. MP2 frequencies were corrected by 0.93 and used to calculate zero-point vibrational energies.25 The 2A′′ excited state of H3S• was constructed from the wave function of the 2A′ state by promoting the electron from the 8a′ SOMO to the 3a′′ vacant orbital and reoptimizing the molecular geometry under Cs symmetry constraints. Baczkay’s quadratic convergence method26 was used to preserve the symmetry of the Hartree-Fock wave functions. Single-point calculations were also carried out for H3S+ using the optimized neutral geometries to obtain vertical ionization energies. The total ab initio energies and zero-point corrections are summarized in Table 1. Neutralization-Reionization Results Precursor H3S+ ions were generated by exothermic protonation with gas-phase acids whose acidities increased in the series H3O+, H2S•+, and CH5+ (Table 2). The protonation reagents
Figure 1. Neutralization-reionization spectra of self-protonated H332S+ obtained with CH3SSCH3 (70% transmittance) and O2 (70% transmittance) at (a) 0.2 Torr and g2.8 µs, (b) 0.6 Torr and g2.8 µs, (c) 0.2 Torr and 0.2-1.8 µs, and (d) 0.6 Torr and 0.2-1.8 µs.
were chosen such as to vary the amount of energy available to the H3S+ ions being formed. The internal energy of H3S+ was further varied roughly by adjusting the reagent gas pressure. Within the pressure regimes used, the low-pressure conditions of 0.2-0.3 Torr allow the ions to undergo on average 50-70 collisions with the reagent gas during their residence time in the ion source, whereas 120-240 collisions are possible on average under the high-pressure conditions of 0.5-1 Torr. Neutralization-reionization (NR) spectra7 were obtained for H3S+ ions prepared under both pressure regimes and at two lifetimes of the neutral intermediates. Figure 1a,b shows the NR spectra of H332S+ prepared by selfCI at 0.2 and 0.6 Torr of H2S and at long neutral lifetimes of g2.8 µs. Figure 1c,d likewise shows the NR spectra at short neutral lifetimes of 0.2-1.8 µs. Survivor H3S+ ions appear in all the NR spectra, but effects of both the ion source pressure and the neutral lifetime are observed. The relative abundances of surviving H3S+ increase at lower ion source pressures. This effect is in part due to the contribution of stable H233S at m/z 35, whose fraction increases at low pressure due to incomplete protonation and competing electron ionization. These factors limit the pressure range over which H3S+ can be reliably generated in a standard chemical ionization source. Based on the relative neutralization-reionization efficiencies of H3S+ and H2S•+,27 the isobaric contributions were estimated to account for ca. 30% of the survivor ion intensity for ions prepared by
Metastable Hydrosulfonium Radical H3S•
J. Phys. Chem., Vol. 100, No. 37, 1996 15029
Figure 2. Neutralization-reionization spectra of self-protonated H334S+ obtained with CH3SSCH3 (70% transmittance) and O2 (70% transmittance) at (a) 0.2 Torr and g2.8 µs, (b) 0.6 Torr and g2.8 µs, (c) 0.2 Torr and 0.2-1.8 µs, and (d) 0.6 Torr and 0.2-1.8 µs.
Figure 3. Neutralization-reionization spectra of self-deuteronated D3S+ obtained with CH3SSCH3 (70% transmittance) and O2 (70% transmittance): (a) D332S+, 0.2 Torr and g2.8 µs; (b) D332S+, 0.2 Torr and 0.2-1.8 µs; (c) D334S+, 0.2 Torr and g2.8 µs; (d) D334S+, 0.2 Torr and 0.2-1.8 µs.
low-pressure ionization but were within one standard deviation of the reproduced survivor ion intensities under high-pressure conditions. In order to eliminate this interference, NR spectra of H334S+ at m/z 37, occurring at natural 34S abundance (4.2%), were recorded for two pressure regimes and neutral lifetimes (Figure 2a-d). These spectra show interference-free survivor ions whose relative abundances depend on the ion source pressure and neutral lifetime in a similar fashion as observed for the 32S isotopomers. The ion relative intensities in the NR spectra, obtained as integrated peak areas, are summarized in Table 3 for all combinations of ionization conditions and neutral lifetimes. The small peaks at m/z 32 and 33 in Figure 2a-d are due to penetration of slow 32S and H32S ions from dissociations of the abundant H332S (see Experimental Section). A salient feature of the relative abundances of surviving H3S+ is their dependence on the neutral lifetimes. For the interference-free spectra of H334S•, a decrease of H334S+ relative abundance is observed at longer neutral lifetimes for precursors prepared by high-pressure ionization with H2S•+ and CH5+. The least exothermic protonation with H3O+ gives smaller fractions of surviving H334S+ as compared to the previous two reagents. The results indicate that there is a substantial fraction of metastable H3S• formed by vertical neutralization that dissociate within the 1.8-2.8 µs interval, corresponding to unimolecular rate constants in the (3-4) × 105 s-1 range. The existence of long-lived H3S• is thus clearly established. The NR spectra of deuterium-labeled ions D332S+ and D334S+ are shown in Figure 3a-d for neutral lifetimes of 0.2-1.8 and 2.8 µs. The spectra of D332S+, which were obtained by lowpressure self-CI of D2S, are given in Figure 3a,b. Abundant survivor D3S+ ions are observed whose relative abundances increase at short neutral lifetimes. Because of an isobaric interference of D234S•+ at m/z 38, NR spectra of D334S+ at m/z 40 were also obtained at natural 34S abundance, which excluded further overlaps. The spectra (Figure 3c,d) show substantial survivor ions for D334S+ whose relative abundances slightly increase at short neutral lifetimes. The integrated relative abundance of D334S+ (13-15%) is slightly higher than that for
TABLE 3: Neutralization-Reionization Spectra of H3S+ relative abundancea ionization H2
S•+
>2.8 µsh
0.16-1.8 µsh
pressureb)
(low H332S H334S H2S•+ (high pressurec) H332S H334S H3O+ (low pressured) H332S H334S H3O+ (high pressuree) H332S H334S CH5+ (low pressuref) H332S H334S CH5+ (high pressureg) H332S H334S
14 9 1.7 1.5 14 4 2.1 1.3 20 11 3.1 3.2
15 12 2.8 2 18 4 0.8 0.6 15 9 1.9 4.5
a % relative to the sum of reionized intensities. b 1.8 × 10-5 Torr corrected pressure of H2S as measured outside the ion source. c 5 × 10-5 Torr of H2S. d 1.9 × 10-5 Torr of H2O. e 6 × 10-5 Torr of H2O. f 3 × 10-5 Torr of CH . g 1.1 × 10-4 Torr of CH . h Neutral lifetime. 4 4
H334S+ obtained by low-pressure ionization (Table 3). In contrast, no survivors were produced from D332S+ prepared by the less exothermic deuteronation with D3O+. In this case the intermediate D3S• dissociated completely within 0.2 µs. Overall, the NR spectra of D3S+ show substantially smaller isotope effects on the hypervalent radical stability that observed previously for the first-row radicals H3O• and NH4•.1,2,7c Mixed isotopomers D2H32S+ were prepared by exothermic protonation with CH5+ of D232S at low and high methane pressure. The NR spectra showed survivor ions at m/z 37 at both pressure regimes (Figure 4a-d). The survivor ion relative abundances showed a weak dependence on the neutral lifetime, similar to those for CH5+-protonated H3S+ (Table 3). In these experiments, the presence in methane plasma of C3H3+ at m/z 39 precluded measurements for the isobaric D2H34S. C3H3+
15030 J. Phys. Chem., Vol. 100, No. 37, 1996
Sadı´lek and Turecˇek TABLE 4: Corrected Harmonic Frequencies in H3S+ and H3S• sym
frequency (cm-1)a
H3S+
C3V
H3S•(TS)
C1
H3S•(2A′)
Cs
H3S•(2A′′)b
Cs
2534 (e, νsym); 2521 (a1, νasym); 1196 (e, δasym); 1037 (a1, δsym umbrella)b i571 (νasym(HfSfH)); 2674 (νsym); 1804 (νsym(HfSrH)); 1244 (δasym); 931 (δsym); 575 (δsym) 2081 (a′, νsym); 1983 (a′, νasym); 1389 (a′′, δasym); 1164 (a′′, νasym + δasym); 926 (a′, δsym); 419 (a′, δsym) 2093 (a′, νasym); 2062 (a′, νsym); 1359 (a′, νasym); 993 (a′, δsym)
species
a MP2/6-31++G(2d,p) harmonic frequencies corrected by 0.93. Reference 30 gives experimental frequencies for H3S+ as 2526, 2521, and 1033 cm-1. b
Figure 4. Neutralization-reionization spectra of D2H32S+ from CH5+ protonation of D2S obtained with CH3SSCH3 (70% transmittance) and O2 (70% transmittance): (a) 0.2 Torr and g2.8 µs; (b) 0.6 Torr and g2.8 µs; (c) 0.2 Torr and 0.2-1.6 µs; (d) 0.6 Torr and 0.2-1.6 µs.
Figure 5. MP2/6-31++G(2d,p) optimized structures. Bond lengths are in angstroms; bond and dihedral angles are in degrees.
gives an abundant survivor ion on neutralization-reionization,29 which obliterates the contribution of D2H34S•. The relative abundances of D2S•+ (m/z 36) and HDS•+ (m/z 35) in the NR spectra allow one to evaluate the kinetic isotope effect for loss of H and D, respectively. Comparing the experimental [m/z 36]/[m/z 35] ratios with the expected statistical value of 0.5 gives kH/kD ) 2.1-2.6, favoring H loss. Since the HD2S• radical is substantially less stable than its cation (vide infra), it can be expected that the isotope effect refers mainly to the radical fragmentation. A conspicuous feature in the NR spectra is the low relative abundance of HS+, compared to DS+ (Figure 4a-d). This is a manifestation of a primary isotope effect on the sequential loss of two deuterium atoms or D2 from D2HS•. Ab Initio Results Optimization of H3S• geometry without any symmetry constraints led to a slightly distorted Cs-like structure (Figure 5), which corresponded to the transition state for the H2S + H• radical substitution, in keeping with the previous calculations.12 The potential energy surface along the reaction path from the
saddle point was examined for H-S bond lengths in the 1.51.8 range, and it showed a smooth energy decrease with increasing H-S separation. In order to characterize the structure of H3S• formed by vertical neutralization, we considered symmetry constraints pertinent to its H3S+ precursor. Due to the C3V symmetry of H3S+, vertically neutralized H3S• must belong to the same symmetry group and have three equivalent S-H bonds, if formed from the ground, all V ) 0, vibrational state of the ion. Excitation of the e stretching and bending modes in H3S+ (Table 4) leads to ions with Cs symmetry, which give Cs radicals upon vertical neutralization. Optimization with UMP2/6-31++G(2d,p) of H3S• under the constraint of equal S-H bond lengths resulted in a bound Cs structure of the 2A′ state (Figure 5) that showed all real vibrational frequencies (Table 4). Two pure H-S stretching modes were obtained that exhibited somewhat lower wavenumbers than those in H3S+ (Table 4).30 A third, softer, mode was obtained as a combination of asymmetric stretch and H-S-H bend at 1164 cm-1 (Table 4). This low wavenumber indicates a rather shallow potential energy minimum along the corresponding internal coordinate. The 2A′ state is 21 kJ mol-1 above the transition state for the H2S + H• reaction and is therefore metastable toward dissociation to these products. A 2A′′ excited state was found by UMP2/6-31++G(2d,p) and UMP2/6-311++G(3df,2p) optimizations (Figure 5, Table 1); the latter calculations make the 2A′′ state only 1.09 eV less stable than the 2A′ state. Frequency analysis of the 2A′′ state gave large negative curvatures of the potential energy surface along two bending modes, while the stretching modes had positive frequencies (Table 4). We have examined this result by several calculations in which the bond and dihedral angles were varied stepwise, while the molecular symmetry was constrained and the remaining bond parameters were reoptimized. However, these calculations showed potential energy increases along the bending modes, indicating again a local energy minimum for the fully optimized structure of the 2A′′ state. We conclude that the latter state is probably bound, although the failure of the analytical harmonic frequency calculations indicates difficulties with the second-derivative matrix. Both the 2A′ and the 2A′′ state show substantial adiabatic ionization energies of 5.7 and 4.6 eV, respectively. Vertical transitions between H3S+ and the 2A′ and 2A′′ states of H3S• were examined by single-point UMP2/6-311++G(3df,2p) calculations (Table 1). The corresponding Franck-Condon energies in H3S+ amount to 351 and 25 kJ mol-1 for vertical ionization of the 2A′ and 2A′′ states, respectively. By microscopic reversibility, substantial Franck-Condon effects can be expected for the formation of the 2A′ state, but not the 2A′′ state.
Metastable Hydrosulfonium Radical H3S•
J. Phys. Chem., Vol. 100, No. 37, 1996 15031
Figure 6. Distributions of vibrational states in H3S+ for the e mode (1196 cm-1).
Discussion The variable-time NR spectra of H3S+, D3S+, and D2HS+ show unequivocally that metastable sulfonium radicals of 0.22.8 µs lifetimes are formed upon vertical reduction of these cations. The present results thus support the earlier observation of Griffiths et al.6 In view of the extensive calculations reported previously,12 it appears unlikely that a bound H3S• structure could exist on the potential energy surface of the lowest-energy doublet state, which most likely corresponds to a saddle point. Even if a weakly bound state existed, vibrational excitation and tunneling could be expected to cause fast dissociation by hydrogen loss within 10-9 s, analogous to that observed for H3O•.1,2 One special feature of the formation of metastable H3S• in these experiments is that it occurs on an extremely short time scale. For 8250 eV H3S+ interacting with the electron donor over the distance of 5 Å, the electron transfer must take place within 2.3 fs. This is more than 5 times shorter than the shortest vibrational period (13.2 fs) of the e mode in H3S+ (νH-S ) 2533 cm-1, Table 4). The radicals are thus formed with the geometry and symmetry of the given Vibrational state of the ion precursor, which, however, is sampled at random for neutralization.17 The NR spectra show that the fraction of metastable H3S• depends on the ionization conditions. Since H3S+ is the most stable structure produced by protonation of H2S, the observed effects of ionization conditions must be due to different internal energies of the precursor ions. We use the results of ab initio calculations (Tables 1 and 4) to analyze the vibrational excitation in the precursor ions and discuss how it affects the formation of metastable H3S•. Protonation of H2S with thermal H3O+ is ∼15 kJ mol-1 exothermic (Table 2), which sets a limit to the internal energy deposited in the H3S+ ion. At the low-pressure limit, H3S+ ions will have internal energies composed of the thermal vibrational energy in H2S (1.5 kJ mol-1) and a portion of the protonation exothermicity. At the high-pressure limit, a fully thermalized H3S+ ion will have only 1.7 kJ mol-1 of internal energy at 500 K, due to its low vibrational heat capacity. Protonation of H2S with CH5+ is 161 kJ mol-1 exothermic, which sets an upper limit of 162.5 kJ mol-1 for the average
vibrational energy of H3S+ formed under low-pressure conditions. Protonation under self-CI conditions is interesting, as the gas phase acid, H2S•+, can be formed in its 2B1 ground electronic state31 or in a long-lived 2A1 excited state, which lies 2.77 eV higher in energy.32 It is noteworthy that the 2A1 state has a lifetime of several microseconds and accounts for a significant fraction of H2S•+ formed by electron ionization at 150 eV.32 Proton transfer onto H2S from the (2B1) ground state of H2S•+ is 31 kJ mol-1 exothermic, which sets the limit for the average vibrational energy in the H3S+ formed at 32.5 kJ mol-1. Protonation with the metastable (2A1) state of H2S•+ is 300 kJ mol-1 exothermic and should form a highly excited, but stable, H3S+. The pertinent dissociation energies for H3S+ are given in Table 2.33 In order to estimate vibrational excitation in H3S+, the vibrational energies were converted to the corresponding vibrational temperatures and used to evaluate the fractions of V ) 0-9 states.34 In particular, excitation of the e bending modes at ν ) 1196 cm-1 results in HSH angles which bring the H3S+ structure closer to those in the 2A′ and 2A′′ states of H3S•. Vertical neutralization of such vibrational states could enhance formation of bound radicals through favorable Franck-Condon factors. Figure 6 shows the distribution of vibrational states of the e bending mode corresponding to the maximum vibrational temperatures achieved by exothermic protonations. The fraction of V > 0 states is negligible in thermalized H3S+. Vertical neutralization of thermalized H3S+ thus must form H3S• radicals with highly nonrelaxed geometry and promote dissociation. For exothermic protonations, the fractions of V g 0 states rapidly increase with the ion vibrational temperature, and although some collisional deexcitation undoubtedly occurs, the probability of stable H3S+ ions leaving the ion source in a vibrationally excited state should also increase. Since the vertical transition (2A′′)H3S• f H3S+ requires only 25 kJ mol-1 vibrational excitation in the ion, protonation with CH5+ and self-CI with H2S•+ should produce fractions of H3S+ in vibrational states favoring vertical reduction to metastable H3S•. These conclusions are consistent with the trend in the experimental data, which show that greater protonation exothermicities and lower collisional deexcitation favor the formation of metastable hydrosulfonium radicals.
15032 J. Phys. Chem., Vol. 100, No. 37, 1996 Long-lived, high Rydberg states have also been suggested to explain metastability of hypervalent onium radicals.35 Highly excited Rydberg molecules consist of an ionic core and a weakly bound electron, such that the molecular geometry of such electronic states closely resembles that of the ion.36 Formation of a high Rydberg state by vertical electron capture thus could be expected, to the first order, to be independent of the ion vibrational energy, contrary to the present observations. In addition, vibronic coupling may result in autoionization of Rydberg molecules.37,38 Hence, increasing the vibrational energy in the H3S+ precursors is predicted to diminish the fraction of metastable H3S•, which is not corroborated by the experimental data. Furthermore, Rydberg states are susceptible to field ionization in electrostatic field gradients, which limit the highest principal quantum number (n) of the nonionizing radical state. For hydrogen-like states, Gellene and Porter reported that electronic states of n g 24(E)-1/4 should be ionizable in a field gradient E given in kV/cm.37 In the present experiments, the neutralized beam passes through a field gradient of 16.5 kV/cm, which is capable of ionizing Rydberg states of n g 12. Duncan and co-workers39 observed red shifts in ionization energies (IE) due to electrostatic field gradients (E) according to the empirical formula ∆IE ≈ 0.023xE, where E is in the units of kV/cm and IE is in electronvolts. Hence, for the 16.5 kV/cm gradient, Rydberg states within 0.1 eV of the ionization threshold should be unbound. Due to the combination of vibrational energy in H3S• and the external electrostatic field, high Rydberg states are likely to be ionized and hence removed from the population of metastable radicals. One can therefore conclude that high Rydbergs do not constitute a significant fraction of the observed metastable H3S•. Conclusions Metastable H3S• and D3S• of lifetimes up to 2.8 µs are formed by vertical neutralization of vibrationally excited cations. The behavior of these metastable radicals shows that they likely do not correspond to high Rydberg states of n g 12. Symmetryrestricted 2A′′ and 2A′ states are identified by ab initio calculations and predicted to be bound. The observed metastability of H3S• is attributed to the formation of symmetry bound states by vertical neutralization of H3S+. Acknowledgment. Financial support of this work by the National Science Foundation (Grant CHE-9412774) is gratefully acknowledged. The ab initio computations were conducted by using the resources of the Cornell Theory Center, which receives major funding from the National Science Foundation and New York State with additional support from the Advanced Research Projects Agency, the National Center for Research Resources at the National Institutes of Health, IBM Corporation, and members of the Corporate Research Institute. References and Notes (1) (a) Williams, B. W.; Porter, R. F. J. Chem. Phys. 1980, 73, 5598. (b) Gellene, G. I.; Porter, R. F. Acc. Chem. Res. 1983, 16, 200. (2) (a) Selgren, S. F.; Gellene, G. I. J. Phys. Chem. 1987, 87, 5804. (b) Volatron, F. J. Mol. Struct. 1989, 186, 167. (3) (a) Cardy, H.; Liotard, D.; Dargelos, A.; Poquet, E. Chem. Phys. 1983, 77, 287. (b) Kaspar, J.; Smith, V. H., Jr.; McMaster, B. N. Chem. Phys. 1985, 96, 81. (c) Kassab, E.; Evleth, E. M. J. Am. Chem. Soc. 1987, 109, 1653. (d) Boldyrev, A. I.; Simons, J. J. Chem. Phys. 1992, 97, 6621.
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