Molecular Structures of 10-Valence Electron Species. 4. Potential

Potential energy surface (PES) searches of HSiX+ (X = O, S, Se) ions were performed using HF and MP2 computational methods. Split-valence triple-ζ is...
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J. Phys. Chem. 1996, 100, 7361-7366

7361

Molecular Structures of 10-Valence Electron Species. 4. Potential Energy Surfaces and Properties of HSiX+ (X ) O, S, Se) Andrzej Nowek and Jerzy Leszczyn´ ski* Department of Chemistry, Jackson State UniVersity, 1400 Lynch Street, Jackson, Mississippi 39217 ReceiVed: June 20, 1995; In Final Form: December 21, 1995X

Potential energy surface (PES) searches of HSiX+ (X ) O, S, Se) ions were performed using HF and MP2 computational methods. Split-valence triple-ζ isotropic basis sets augmented by sets of p and d polarization functions on hydrogen atoms and sets of d and f polarization functions on heavy atoms (TZP(2d,2p) and TZP(2df,2pd)) were used. Unlike those of the linear form of HOSi+, the strongly bent HSSi+ and HSeSi+ global minimum-energy structures are predicted at both HF and correlated levels of theory, in contradiction to Mulliken-Walsh’s rules. Isomeric HSiS+ and HSiSe+ forms are higher-energy local minima on the studied PESs.

Introduction Small molecules and molecular ions are often used to develop and test simple rules relating molecular structure to a specific number of valence electrons. As an example, the class of 10valence-electron HAB molecular systems has attracted a lot of attention in the last decade. Some of the species belonging to this class, e.g., HCO+,1,2 HOC+,3 HNC,4,5 and HCS+,6 have been detected in interstellar clouds. Such detection accelerates experimental studies on these interesting ions. These species have also been extensively studied by theoretical methods, because they have unconventional geometrical structures. The relatively small size of these systems allows for the use of an accurate, state-of-the-art computational approach. According to the classical Walsh7-Mulliken8 rules, these systems should be linear in their ground states. However, rigorous quantummechanical calculations predict for many of the 10-valenceelectron species that the global minima correspond to strongly bent structures (for example, HOGa, HSGa and HSeGa,9 HSGe+ and HSGe+ 10). In addition, the existence of quasilinear systems (HOAl11 and HOGe+ 10) has been predicted by theoretical methods. It has been concluded that, even for such simple systems, the applied level of theory and size of the basis set determine the number of minimum-energy points on the potential energy surface (PES).10,11 Evidently, the oxygen-containing members of this class represent particularly troublesome computational problems. The most difficult case seems to be represented by the linear HOC+ ion. An extensive investigation by Yamaguchi et al.12 revealed that this ion is a transition-state structure at the HF level, independent of the quality of the basis set used, from DZP to QZ3P(f,d) saturated by one set of s and p diffuse functions for C and O and s for H. However, HOC+ is a minimum-energy species at two correlated levels, CISD and CCSD, when the DZP basis set is used. Interestingly, extension of this basis set to TZP2P+diff and QZ3P+diff again revealed a transition-state structure. The minimum-energy structure was finally confirmed using triple-ζ and quadruple-ζ basis sets when a set of f (C and O atoms) and d (hydrogen) polarization functions was added. Molecular ions of formula XYH+ (X ) C, Si, Ge, Ga; Y ) O, S, Se) are among the best-known 10-valence electron systems. There is a number of experimental and theoretical * Author to whom correspondence is to be addressed. X Abstract published in AdVance ACS Abstracts, April 1, 1996.

S0022-3654(95)01739-4 CCC: $12.00

data on HCO+/HOC+ (see ref 12 and references cited therein). Also, HCS+/HSC+ (e.g., ref 13) and HCSe+/HSeC+ 14 were extensively studied theoretically. Tao15 studied HCS+/HSC+, HSiO+/HOSi+, and HSiS+/HSSi+ ion pairs at lower levels of theory. Srinivas et al.16a studied HSiO+/HOSi+ and HSiO•/ HOSi• systems using mass spectroscopic techniques and provided evidence that these species do exist in the gas phase. Their high-level calculations, carried out in addition to experimental studies, indicate that linear HOSi+ and HOSi• are the global minimum-energy structures. Recently, Yamaguchi and Schaefer16b presented a detailed study on the HSiO+/HOSi+ system at the correlated level of theory (coupled-cluster CCSD(T)) using large basis sets of triple-ζ quality augmented with polarization and diffuse functions. Fox et al.17 experimentally determined the gas-phase proton affinities (PA) of SiO and SiS at room temperature and supplemented these studies with theoretical predictions on HSiO+/HOSi+ and HSiS+/HSSi+ systems. They concluded that the measured PA values correspond to protonation of the heteroatom of SiO and SiS. Berthier et al.18 investigated stabilities of HSiO+/HOSi+ and HSiS+/HSSi+ isomers at the correlated (CIPSI) level. Botschwina and Rosmus19 calculated the spectroscopic properties of HOSi+ at the correlated (CEPA) level of theory. In the present paper, we report the results of systematic Hartree-Fock (HF) and post-Hartree-Fock studies on the PES, stability, and properties of HSiX+/HXSi+ systems (X ) O, S, Se). The reported results (including transition-state structures for all systems) will enrich existing data on the still-intriguing class of 10-valence-electron HAB species, verify applicability of simple Mulliken-Walsh rules for determination of their ground-state structures, and, in addition, guide experimental investigation of their molecular structure and vibrational frequencies. Methods Calculations were performed on the title compounds using ab initio LCAO-MO methods.20 Two basis sets were used in the reported calculations. The first was standard Pople’s21a and McLean-Chandler’s valence triple-ζ21b augmented by two sets of p polarization functions on hydrogen (ζ1 ) 0.375, ζ2 ) 1.5), and two five-component d polarization functions on oxygen (ζ1 ) 0.646, ζ2 ) 2.584), silicon (ζ1 ) 0.9, ζ2 ) 0.225), and sulfur (ζ1 ) 0.325, ζ2 ) 1.3),21c while the partially uncontracted [433111/43111/4*] Huzinaga’s basis set supplemented with two © 1996 American Chemical Society

7362 J. Phys. Chem., Vol. 100, No. 18, 1996

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five-component d polarization functions was used for selenium (ζ1 ) 0.144, ζ2 ) 0.489).22 This modified basis set is used throughout the paper and, for convenience, will be referred to as TZP(2d,2p). The second TZP(2df,2dp) basis was derived from the previous one by the inclusion of seven-component f polarization functions on oxygen (ζ ) 1.4), silicon (ζ ) 0.32), and sulfur (ζ ) 0.55) and five-component d polarization functions on hydrogen (ζ ) 1.0).21c The f orbital exponent for selenium (ζ ) 0.216) was calculated from the geometric sequence of corresponding exponent values of O and S atoms. The effects of electron correlation were accounted for using Møller-Plesset perturbation theory within a frozen core (fc) approximation (for Se, 1s, 2s, 2p, 3s, 3p, and 3d; for S and Si, 1s, 2s, and 2p electrons were excluded), up to full fourth order. MP4(SDTQ) and CI energies correct to the fourth order (CISD,4) are also given. The stationary-point structures were fully optimized using an analytical gradient method at the HF/ TZP(2d,2p), MP2(fc)/TZP(2d,2p), and MP2(fc)/TZP(2df,2pd) levels of theory. For each of the equilibrium structures, harmonic vibrational frequencies and absolute IR intensities were calculated at some levels. All calculations were performed using the GAUSSIAN92 set of programs.23 Results and Discussion Available experimental data for the SiX diatomics allow one to test the performance of the applied methods in prediction of bond lengths. The optimized molecular geometric parameters for HSiX+/HXSi+ ions and their SiX subunits are depicted in Figure 1. For SiO, SiS, and SiSe, the experimental gas-phase bond lengths are known.24 The Si-X bond distances of 1.5097, 1.9293, and 2.0583 Å increase through the O-S-Se series. The corresponding values calculated at the HF/TZP(2d,2p) level are systematically underestimated, whereas predicted bond lengths for SiO, SiS, and SiSe at MP2(fc)/TZP(2d,2p) overestimate experimental distances by 0.019, 0.024, and 0.028 Å, respectively. A similar tendency was observed for GeX (X ) O, S, Se),10 CSe,14 and CS.13 In contrast, bond lengths predicted at HF/TZP(2d,2p) for GaS and GaSe are larger than those optimized at the MP2(fc)/TZP(2d,2p) level. The Si-X bond lengths in HSiO+, HSiS+, and HSiSe+ ions are shorter than in the corresponding SiX diatomic species. In addition, the predicted interatomic distances for the minimum-energy species are shorter at HF than at the correlated level of theory. The opposite trend is observed for HOSi+, HSSi+, and HSeSi+. This regularity has also been noticed in the carbon12-14 and germanium10 analogs. For polar SiO (µ ) 3.09 D24), SiS (µ ) 1.73 D24), and SiSe diatomics with Si+X- polarity, Si-X bond polarity upon the X-H bond formation in the HXSi+ ions decreases (the Si and H atoms cancel the electronegativity of the central X atom). As a side effect, lengthening of the SX distance is observed. For HSiX+ species, the bonded H and Si atoms enhance the Si-H bond polarity, and the Si-X bond length becomes shorter. It seems, however, that due to the delicate nature of bonding of HCO+/HOC+ systems, the above explanation is not applicable. According to Hammond’s postulate,25 very exoergic reactions have transition-state geometries resembling the reactants, with the transition states becoming more like the products with decreasing exoergicity. For the predicted transition-state structures (Figure 1), r(HSi) < r(HX) (X ) O, S, Se). This observation supports Hammond’s postulate; however, a decrease in exothermic effects in HSiX+/HXSi+ is not followed by an increase of H-Si and a decrease of H-X distances (productlike structure). The addition of higher polarization functions of f (heavy atoms) and d (hydrogen) symmetry improves the agreement of

Figure 1. Optimized geometries of the studied systems. Bond lengths are in angstroms and angles in degrees.

predicted and experimental Si-X bond distances (by as much as 0.009 Å). For the SiSH+ ion, the SiS bond length predicted at the MP2(fc)/TZP(2df,2pd) level (1.9885 Å) is shorter by 0.0125 Å than that obtained using the TZP(2d,2p) basis set. Generally, for bond lengths optimized at the MP2(fc) level of

10-Valence Electron Species

J. Phys. Chem., Vol. 100, No. 18, 1996 7363

TABLE 1: Calculated Energies for HSiX+, HXSi+, and SiX (X ) O, S, Se) Speciesa TZP(2d,2p) MP2

TZP(2df,2pd) HF

TZP(2d,2p)

MP2

MP2

+

MP2 ZPE THERM MP4 CISD,4 HF ZPE THERM

HSSi Bent -686.983887 0.008504 0.009923 -687.022898 -686.996639 -686.772519 0.009032 0.011224

MP2 ZPE THERM MP4 CISD,4 HF ZPE THERM

HOSi+ Linear -364.443891 0.012647 0.001589 -364.468464 -364.436436 -364.160562 0.013339 0.016646

MP2 ZPE THERM MP4 CISD,4 HF ZPE THERM MP2 ZPE THERM MP4 CISD,4 HF ZPE THERM

HSeSi+ Linear -2687.126760 0.006594

-364.475814 0.013195 0.016224 -364.501534 -364.466146

-364.288289 0.007214 -364.320038

MP2 ZPE MP4 CISD,4 HF ZPE

-2687.097828 0.005642 -2687.130634 -2687.110634

MP2 ZPE THERM MP4 CISD,4 HF ZPE THERM

-364.134684 0.002721 0.005098 -364.160684 -364.111185

MP2 ZPE THERM MP4 CISD,4 HF ZPE THERM

-2686.890291 0.001284 0.003826 -2686.924407 -2686.892460

SiOH+ TS

MP2 ZPE THERM MP4 CISD,4 HF ZPE THERM

-2687.158821 0.007393 0.010754 -2687.196635 -2687.172503

Bent

MP2 -2687.185870 0.007467 0.010811 -2687.225290 -2087.196486

-2686.965830 0.007950 0.011224 HSSi+ Linear -686.955699 0.007430

-686.993763 0.007553

-686.994447 -686.962900

-687.036107 -686.997881 -686.733112 0.007845

-687.001994 0.009099 0.012106 -687.042082 -686.999871

MP2 ZPE THERM MP4 CISD,4 HF ZPE THERM

HSiSe+ Linear -2687.152712 0.008490 0.011642 -2687.187516 -2687.154438 -2686.946631 0.009189 0.012166

-364.317591 0.007273 -364.350873 -364.291541

MP2 ZPE MP4 CISD,4 HF ZPE

-686.907277 0.006070 -686.951427 -686.916822

MP2 ZPE THERM MP4 CISD,4 HF ZPE THERM

-686.713128 0.001662 0.004124 -686.748134 -686.713951

-363.964701 0.007392 SiSeH+ TS

MP2 ZPE THERM MP4 CISD4 HF ZPE THERM

HSiO+ Linear -364.349448 0.010088 0.013010 -364.377075 -364.321652 -364.037902 0.011299 0.014061

-2687.922017 0.007011 HSiS+ Linear -686.964747 0.009007 0.012037 -687.000737 -686.965341 -686.741542 0.009805 0.012672

HSeSi+

MP2 ZPE THERM MP4 CISD,4 HF ZPE THERM

-2687.158175 0.006673 -2687.193325 -2687.158175

MP2 ZPE MP4 CISD,4 HF ZPE

a

-687.020479 0.008606 0.011781 -687.062857 -687.029976

TZP(2df,2pd) HF

SiSH+ TS

-364.379667 0.010120 0.013010 -364.409058 -364.349678

-2687.180259 0.008662 0.011763 -2687.217793 -2687.179775

-686.943368 0.006142 -686.991172 -686.678202

-686.678202 0.006160

-2687.123372 0.005671 -2687.167431 -2687.133447

-2686.893045 0.005739 SiO

-364.163396 0.002749 0.005126 -364.190865 -364.136955

-363.829507 0.003219 0.005098 SiSe

SiS

-686.749414 0.001705 0.004160 -686.788956 -686.747224

-686.491885 0.001841 0.005587

-2686.917449 0.001312 0.003846 -2686.954518 -2686.916756

-2686.687916 0.001418 0.003926

Energy values in au. THERM stands for the sum of vibrational, rotational, and translational temperature contributions at 298 K.

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TABLE 2: Calculated Relative Energies for 1A′ HXSi+ vs 1Σ+ HSiX+ (X ) O, S, Se) Systemsa system +

HSiO HSiO+ b TS HSiS+ HSiS+ TS HSiSe+ HSeSi+ TS

HF

CISD,4

MP2(fc)

MP4(fc)(SDTQ)

TOT

0 -76.9

0 -7.0 (-73.1) 35.2 (36.5)

0 -59.3 (-60.3) 38.4 (40.0)

0 -57.3 (-58.0) 35.8 (36.5)

0 -55.7 (-56.1) 30.4 (34.7)

0 -19.6 (-7.6) 30.4 (58.4)

0 -12.0 (-11.6) 36.1 (36.8)

0 -13.9 (-13.0) 30.9 (31.9)

0 -14.2 (-13.3) 25.1 (30.0)

0 -11.3 (-10.5) 27.5 (29.1)

0 -3.8 (-3.5) 34.4 (35.7)

0 -5.7 (-4.7) 30.0 (31.6)

0 -6.4 (-5.5) 28.2 (29.7)

45.9 0 -19.4 39.7 0 -12.0 33.6

a TZP(2d,2p) and TZP(2df,2pd) basis sets (in parentheses). MP4(fc)(SDTQ) and CISD,4 values calculated with MP2(fc)-optimized geometry. TOT stands for MP4(fc)(SDTQ) energy values corrected for (not scaled) ZPE(MP2). b Linear.

theory using TZP(2d,2p) and TZP(2df,2pd) basis sets, bond lengths differ by a few thousandths of an angstrom while bond angles change by less than 2°. For HSiO+/HOSi+ species, two minimum-energy linear isomers have been found at all studied levels of theory using both basis sets (Tables 1 and 2). The more stable HOSi+ form lies 76.9, 59.3, 60.3, 55.7, and 56.1 kcal/mol below the HSiO+ form at the HF/TZP (2d,2p). MP2(fc)/TZP(2d,2p), MP2(fc)/ TZP(2df,2pd), MP4(fc)(SDTQ)/TZP(2d,2p)//MP2(fc)/TZP(2d,2p)+ZPE, and MP4(fc)(SDTQ)/TZP(2d,2p)//MP2(fc)/TZP(2df,2pd)+ZPE levels of theory, respectively. These results agree well with those reported by Srinivas et al.16a (62.6 and 56.2 kcal/mol), which were obtained using MP2(full)/TZP(3df,3pd) and MP4(full)(SDTQ)/6-311G(3df,3pd)//MP2(full)/ 6-311G(3df,3pd)+ZPE methods. The energy barrier for HSiO+ f HOSi+ isomerization amounts to 40.0 kcal/mol at MP2(fc)/ TZP(2df,2pd) and 36.5 kcal/mol at MP4(fc)(SDTQ)/TZP(2df,2pd)//MP2(fc)/TZP(2df,2pd) levels (Table 2) and differs insignificantly from the MP2(full)/6-311G(3df,3pd) (37.5 kcal/ mol) and MP4(full)(SDTQ)/6-311G(3df,3pd)//MP2(full)/6311G(3df,3pd) (36.9 kcal/mol)16a results. Moreover, for the HSiO+/HOSi+ system, values of the isomerization energy and barrier height obtained using the substantially smaller TZP(2d,2p) basis set, the MP2(fc) and MP4(fc) levels differ only slightly (less than 3 kcal/mol) from those predicted with the larger TZP(3df,3pd) basis set in the full MP2 and MP4 schemes. It follows that the amount of the correlation energy recovered at MP2(fc)/TZP(2d,2p) and at MP4(fc)/TZP(2d,2p)//MP2(fc)/ TZP(2d,2p) is quite sufficient. However, Yamaguchi and Schaefer16b used a TZP(2df,2pd) basis set of similar quality at the CCSD(T) level of theory and obtained values of 64.1 and 29.4 kcal/mol for the SiOH+ f HSiO+ isomerization energy and the barrier height, respectively. Interestingly, for the HSiS+ and HSiSe+ species the PES search reveals three stationary points (Tables 1 and 3). The linear HSSi+ and HSeSi+ are second-order transition structures, with the large imaginary frequencies of 564i and 361i cm-1 for HSeSi+ and 609i and 403i cm-1 for HSSi+ at HF/TZP(2d,2p) and MP2(fc)/TZP(2df,2pd) levels, respectively. The global minimum-energy conformations correspond to strongly bent structures (Figure 1). The HSiS+ and HSiSe+ species form higher-energy linear isomers. A possible explanation for the substantially different predicted geometries for HXSi+ systems when X ) O (linear) and X ) S, Se (bent) can be derived from different electronic

configurations and energies of the highest occupied molecular energies of SiO

...(2π)4(7σ)2

e(π) ) -0.465, e(σ) ) -0.435 au

and SiS

...(9σ)2(3π)4

e(σ) ) -0.392, e(π) ) -0.386 au

(MO energies calculated at the HF/TZP(2d) level). It should be noted that the energy difference between the lone pair σ and π MOs is noticeably larger for SiO (0.03 au) than for SiS (0.006, 0.009 au for SiSe, respectively). As a result, creation of the hybridized σ-π angular (nonlinear) bonding molecular orbitals upon HSSi+ and HSeSi+ formation is enforced. The correlation energy contributes to the relative stability of the isomeric pairs more significantly for selenium-containing species [12.0 (HF/ TZP(2d,2p)), 3.5 (MP2(fc)/TZP(2df,2pd)), 4.7 (MP4(fc)(SDTQ)/ TZP(2df,2pd), and 5.5 kcal/mol (MP4(fc)(SDTQ)/TZP(2df,2pd)+ZPE(MP2(fc))] than for their sulfur analogs (19.4, 11.6, 13.0, and 13.3 kcal/mol, respectively). Interestingly, for the HSiSe+/HSeSi+ pair, the energy difference between the linear and bent forms predicted at the MP2(fc)/TZP(2d,2p) level of theory amounts to only 67% for the MP4(fc)(SDTQ) value, whereas for the corresponding heavier germanium analog this ratio equals 92%. HOSi+ and HSSi+ dissociation enthalpies to SiO + H+ and SiS + H+ closely approximate gas-phase proton affinities (PAs). Experimentally determined heteroatom PA values of -189.3 ( 2.6 for SiO and -170.3 ( 2.0 kcal/mol for SiS17 coincide with the values of -182.0 and -163.4 kcal/mol, respectively, obtained at the MP4(fc)(SDTQ)/TZP(2df,2pd) level (Table 4). Also, the decreasing PA magnitude in the series SiO > SiS > SiSe is predicted at all levels of theory used. The reverse tendency for the silicon-end PA17 is also supported (Table 4). The two minimum-energy structures are well separated energetically for all isomeric pairs. Barriers to the isomerization reaction predicted at the HF level are generally larger than corresponding barriers obtained at correlated levels, except for HSiSe+ f HSeSi+. For this system, the barrier height calculated at the MP2(fc) level is slightly higher (34.4 kcal/mol) than the HF estimate (33.6 kcal/mol, Table 2). At the highest applied level of theory (MP4(fc)(SDTQ)/TZP(2df,2pd)), activation energies for linear-bent isomerization for the sulfur and selenium analogs are almost equal (31.9 and 31.6 kcal/mol, respectively), whereas the relative energies of these two isomeric pairs differ significantly, 13.0 (13.3 kcal/mol corrected for ZPE) for HSiS+/ HSSi+ and 4.7 (5.5 kcal/mol corrected for ZPE) for the HSiSe+/ HSeSi+ pair. The predicted harmonic vibrational frequencies and IR intensities for HSiX+/HXSi+ (X ) O, S, Se) are reported in Table 3. For SiOH+ our MP2(fc)/TZP(2df,2pd) results of 364 (π), 1154 (σ), and 3877 (σ) cm-1 are in good agreement with the recorded anharmonic experimental frequencies of 1227 and 3662 cm-1. Interestingly, values obtained using the substantially smaller TZP(2d,2p) basis set (290, 1128, and 3841 cm-1, respectively) are closer to experimental ones. A similar tendency occurs at the CCSD(T) level of theory.16b It should be noted that the anharmonicity of the HX stretching mode is usually large.26 For all minimum-energy structures, the calculated harmonic vibrational frequencies are larger at the HF than at the correlated level (Table 3). All considered transition-state structures are characterized by a similar bending mode imaginary frequency (MP2(fc)/TZP(2df,2pd) level) of 1110i, 1120i, or 1065i cm-1 (HOSi+, HSSi+, and HSeSi+, respectively). The

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TABLE 3: Predicted Harmonic Vibrational Frequencies and IR Absolute Intensities for HSiX+, HXSi+, and SiX (X ) O, S, Se)a TZP(2d,2p) quantityb symm

a

MP2 freq int

A′ A′ A′

527 624 2581

π σ σ

TZP(2df,2pd) HF

freq

HSSi+ Bent 26 578 98 670 72 2717

MP2 int

freq

int

TZP(2d,2p) quantityb symm

MP2 freq int

TZP(2df,2pd) HF

freq

MP2 int

freq

int

65 67 23

423 505 2349

31 57 39

HSSi+ Linear 133 609i 184 110 695 216 285 2744 354

413i 674 641

129 111 275

HSeSi+ Bent 31 468 59 545 36 2477

40 138 56

527 643 2608

26 97 77

A′ A′ A′

414 490 2341

290 1128 3841

HOSi+ Linear 56 265 275 115 1204 181 618 4120 645

365 1145 3877

250 114 618

π σ σ

441i 654 2607

π σ σ

388i 504 2390

HSeSi+ Linear 98 564i 155 81 545 267 156 2532 212

361i 521 2337

104 82 156

π σ σ

439 1241 2309

HSiO+ Linear 6 537 0 1474 82 2411

27 58 78

439 1251 2313

7 0 82

π σ σ

416 791 2331

1 41 35

426 807 2336

0 3 44

π σ σ

389 617 2332

HSiSe+ Linear 0 471 5 672 35 2418

0 33 27

413 626 2350

0 5 35

A′ A′ A′

1098i 1347 1820

499 3 332

1110i 1355 1837

347 127 527

A′ A′ A′

1095i 771 1893

466 40 183

393 0 65

1120i 786 1910

493 786 190

A′ A′ A′

1070i 583 1893

293 1 15

1065i 596 1180

485 27 118 730

30

102

748

30

HSiS+ Linear 0 500 3 881 44 2423 HOSi+ TS 1127i 1249 1991 HSeSi+ TS 467 879i 26 462 109 2050

354 129 528

σ

730

30

SiO 808

102

748

30

σ

564

17

SiSe 622

60

576

17

σ

HSSi+ TS 1006i 659 2039

SiS 808

Frequencies in cm-1, IR absolute intensities in km/mol.

TABLE 4: Calculated Proton Affinities for SiX (X ) O, S, Se)a SiO + H+ f HSiO+ SiO + H+ f SiOH+ SiS + H+ f HSiS+ SiS + H+ f SiSH+ SiSe + H+ f HSiSe+ SiSe + H+ f SiSeH+

HF

MP2(fc)

MP4(fc)(SDTQ)

-120.0 -194.7 -147.8 -168.6 -152.9 -166.3

-125.8 (-126.7) -190.6 (-183.1) -148.9 (-149.5) -162.6 (-161.6) -155.8 (-155.3) -160.9 (-160.8)

-126.7 (-127.9) -189.7 (-182.0) -149.5 (-149.8) -165.1 (-163.4) -156.3 (-156.2) -163.2 (-162.3)

a In kcal/mol. TZP(2d,2p) and TZP(2df,2pd) (in parentheses) basis sets. Values include zero-point vibrational (not scaled) energy, sum of thermal energies, and ∆(PV) work terms at 298 K.

corresponding frequency for HOC+ amounts to ca. 1500i cm-1 at various levels of theory,12 whereas corresponding frequencies for HSC+ [785.8i (MP2/6-31G(d,p)27] and for HSeC+ [733i cm-1 (HF/DZ(d,p))14] are significantly smaller. Conclusions The presented results of uniform calculations at the HF/TZP(2d,2p) and the correlated MP2(fc) and MP4(fc)(SDTQ) levels of theory with TZP(2d,2p) and TZP(2df,2pd) basis sets for HSiX+/HXSi+ (X ) O, S, Se) ions reveal the similarities of these compounds to their carbon and germanium analogs. The minimum-energy oxygen-containing structures are linear, as is the case for their carbon analogs. For the HGeO+/HOGe+ pair, the global minimum-energy bent HOGe+ structure is predicted; however, the linear form has virtually the same energy.10 Sulfur- and selenium-containing strongly bent HXSi+ and HXGe+ are the global minimum-energy conformers, whereas the linear forms are the second-order transition structures.

However, all the most stable carbon analogs are predicted to be linear. Our results support the suggestion by Buenker et al.27 that, for HAB systems, structures with the heavier element at the terminal position of the molecule are strongly preferred. For heavier-element-containing species, i.e., HXGe+, HXGa (X ) O, S, Se), and HSSi+, the bent conformers correspond to the minimum-energy forms, in contradiction of the Muliken-Walsh rules; this trend has exceptions. For example, in HSeSi+ and HSeGe+, selenium is a central atom. Despite this fact, linear HSiSe+ and HGeSe+ conformers are minimum-energy species. The calculated geometries and molecular parameters obtained at correlated levels with TZP(2d,2p) and TZP(2df,2pd) basis sets are almost identical. This observation is important from a practical point of view because inclusion of higher-symmetry polarization functions in the basis sets significantly increases the time of calculation and computer storage requirements. All title species are thermodynamically stable at both the HF and correlated levels of theory and should be experimentally observable. We have furnished calculated geometrical parameters, harmonic vibrational frequencies, and IR intensities to aid future spectroscopic studies. Acknowledgment. This work was supported in part by ARPA-Naval Regional contracting NOO174-93-RC-00004 and the National Science Foundation Grant RII-8902064. The authors thank the Mississippi Center for Supercomputing Research for allotment of computer time to perform some of the calculations which are presented here. References and Notes (1) Buhl, D.; Snyder, L. E. Nature (London) 1970, 228, 267. (2) Klemperer, D. Nature (London) 1970, 227, 1230.

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Nowek and Leszczyn´ski (18) Botschwina, P.; Rosmus, R. J. Chem. Phys. 1985, 82, 1420. (19) Berthier, G.; Pauzat, F.; Yuanqi, Tao J. Mol. Struct. (THEOCHEM) 1984, 107, 39. (20) See, for example: Hehre, W. H.; Radom, L.; v. Schleyer, P.; Pople, J. A. Ab Initio Molecular Orbital theory; John Wiley and Son: New York, 1986. (21) (a) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. (b) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639. (c) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80, 3265. (22) Huzinaga, S.; Andzelm, J.; Klobukowski, M.; Radzio-Andzelm, E.; Sakai, Y.; Tatewaki, H. Gaussian Basis Sets for Molecular Orbital Calculations; Elsevier: New York, 1984. (23) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schelgel, H. B.; Robb, M.; Replogle, E. S.; Gomperts, R.; Anders, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzales, C.; Martin, R. I.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A.; GAUSSIAN92, ReVision A; Gaussian, Inc.: Pittsburgh, PA, 1992. (24) CRC Handbook of Chemistry and Physics, 73rd ed.; CRC Press: Cleveland, OH, 1992-93. (25) Hammond, G. S. J. Am. Chem. Soc. 1995, 55, 334. (26) (a) LeClercq, J. M.; Allavena, M.; Bouteiller, Y. J. Chem. Phys. 1983, 78, 4606. (b) Hannachi, Y.; Silvi, B.; Bouteiller, Y. J. Chem. Phys. 1991, 94, 2915. (27) Buenker, R. J.; Bruna, P. J.; Peyerimhoff, S. D. Isr. J. Chem. 1980, 19, 309.

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