Quantum Chemical Rovibrational Analysis of the HOSO Radical

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Quantum Chemical Rovibrational Analysis of the HOSO Radical Ryan C. Fortenberry, Joseph S. Francisco, and Timothy J Lee J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b08121 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 9, 2017

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The Journal of Physical Chemistry

Quantum Chemical Rovibrational Analysis of the HOSO Radical Ryan C. Fortenberry,∗,† Joseph S. Francisco,‡ and Timothy J. Lee¶ †Georgia Southern University, Department of Chemistry & Biochemistry, Statesboro, GA 30460, U.S.A. ‡Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, U.S.A. ¶MS 245-3 NASA Ames Research Center, Moffett Field, California 94035-1000, U.S.A. E-mail: [email protected] Phone: 912-478-7694

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Abstract syn-HOSO is the most stable form of hydrogenated sulfur dioxide, a possible intermediate in the creation of sulfuric acid. Recent work has characterized the structure and rotational constants, but the present work provides new insights into the vibrational spectra of this molecule. The central S−O bond is weaker than the terminal S−O bond which allows for near-free rotation of the hydrogen atom. The stretches and the torsional modes are relatively bright infrared emitters/absorbers. The torsional mode is very low in frequency making it a potential target for THz observation. The other vibrational frequencies for HOSO are reduced relative to the corresponding frequencies in SO2 , H2 SO4 , and the HOS radical making the infrared features of cis-HOSO likely red-shifted in mixed spectral observation where oxygen, hydrogen, and sulfur are all found.

Introduction The chemistry of sulfur has a notable place on the periodic table due to its bond nature similar to that of carbon but is located in the chalcogen family with oxygen as the patriarch. The extra electrons and necessary additional basis functions for sulfur increase the cost per atom for quantum chemical computations involving this atom which has led to fewer studies of molecules containing element 16 as compared to the lighter, second-row atoms. However, the namesake of this festschrift produced many experimental studies on small molecules containing sulfur, and recent quantum chemical computations are filling the remaining voids. The highly electronically-complicated NS2 radical 1,2 is a prime example of how experiments on sulfurous molecules from a few decades ago are only now coming under quantum chemical scrutiny beyond initial, contemporary studies. 3 One of the most common and simplest sulfur-containing species is SO2 . Sulfur dioxide is prevalent in most natural environments where sulfur is present. This is true for atmospheric conditions like that of the Earth, Venus (most notably), and the sulfur-rich Jovian moon 2


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Io. 4 Furthermore, sulfur containing species and even sulfur-oxygen species are known in interstellar/circumstellar media. 5–7 SO2 can combine with water vapor to form sulfuric acid, a major component of acid rain on Earth and the clouds on Venus. Oxygen is so abundant in nearly all environments in the universe making associations of these family members a likely natural occurrence if the sulfur abundance is high enough. The simplest atomic addition to sulfur dioxide is the even more astronomically abundant hydrogen coming from various sources, not the least of which is water but could really come from any donor. Such an association is a likely beginning step in the formation of sulfuric acid from SO2 . The HOSO radical is the most stable of the [H,S,O2 ] family of molecules. 8 HOSO is isovalent to the patrician HO3 (HOOO) radical, a notoriously weakly bound structure. 9–11 While ozone is resonantly stable, the addition of the hydrogen atom creates a complex that easily dissociates into the oxygen molecule and the hydroxyl radical. 9 This molecule has been experimentally analyzed vibrationally with great care due to its fragility, 10 but HO3 may yet have astrochemical significance, especially as a source of hydroxyl radicals and oxygen molecules in cold environments or as an intdermediate. 11 Like its lighter analogue, the HOSO radical is also unstable, but this manifests itself in somewhat different ways. Most related, tetra-atomic, Cs radicals exhibit clear cis and trans confirmations as observed in the HOCO isomers and its sulfur analogues. 12–17 The cations exist only in the trans form, 13,18,19 but the HO3 radical has such a low barrier to isomerization at only 344 cm−1 , less than 1.0 kcal/mol or an ambient 500 K, 9,10 that classification as cis or trans on the HO3 radical is moot save for extremely cold environments, where it is trans. Interestingly, while trans-HO3 is the most stable conformer, HOSO is actually non-planar and syn with the trans isomer previously reported to be a transition state. 20,21 The trans-HOSO saddle point lies 2.28 kcal/mol above the syn (more than that in HO3 ). 20 Current estimates for the minimum value of the torsion range from cis planar to greater than 22.0◦ , 21 similar to the behavior of s-cis-1,3-butadiene. 22,23 Consequently, while HO3 and its

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HOSO analogue are members of the same periodic family, they exhibit properties unique to themselves. This work will focus on the analysis of the rovibrational properties of cis-HOSO. While the Cs structure is quite likely not the actual minimum, the low barriers combined with the ease of computation for the planar conformer justify the use of the Cs , cis arrangement for computation herein. The expected errors due to a large amplitude motion will be within the accuracy of the methodology employed. 24,25 Standard errors for the employed methodology, described in the next section, in the computation of fundamental vibrational frequencies have been as good as 5.0 cm−1 for typical radical species 15,17 and even better for closedshell species. 19,26–29 While noble gas matrix data exist for three of the fundamentals, 30 the complete spectrum has yet to be produced in the gas phass. A previous combined theoretical and experimental study 21 has provided exceptionally accurate descriptions of the rotational constants and associated rotational spectra for HOSO allowing for benchmarking of this molecule. Additionally, the differences in the computed geometrical and spectroscopic values for the cis-planar and syn/skew structures are so slight that our current approach should be valid in the provision of the as-of-yet undetermined fundamental vibrational frequencies.

Computational Details Coupled cluster theory 31,32 at the singles, doubles, and perturbative triples level [CCSD(T)] 33 and the restricted open-shell Hartree-Fock reference 34–36 within the MOLPRO 2015.1 quantum chemistry program 37,38 are utilized for all computations. Geometry optimizations with the aug-cc-pV5Z basis set as well as the aug-cc-pV(5+d)Z basis set 39,40 for sulfur are appended with differences between the Martin-Taylor core correlating basis set 41 including and excluding core electrons. From this reference geometry, 743 symmetry-unique (805 total) points produce the quartic force field (QFF) potential energy surface (PES). Displacements of 0.005 ˚ A for bond lengths and 0.005 radians for angles and the torsion are produced from

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the following coordinate system:

I1 (a′ ) = O1 − S

(1)

I2 (a′ ) = S − O2

(2)

I3 (a′ ) = O2 − H

(3)

I4 (a′ ) = 6 O1 − S − O2

(4)

I5 (a′ ) = 6 S − O2 − H

(5)

I6 (a′′ ) = τ (O − S − O − H)

(6)

with the atoms labeled from Figure 1. Again, the τ is held to be 0.0◦ in the optimization of this structure. This portion of the PES is so very flat that a QFF would struggle to define it. 24,25 As a result, I6 will only be utilized for the harmonic portion of the computation, but the quality of the other fundamental frequencies will not be greatly affected since I6 is the only non-totally-symmetric mode. 42 Figure 1: Depiction of the equilibrium CcCR geometry of cis-HOSO.

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At each point, CCSD(T)/aug-cc-pVTZ, aug-cc-pVQZ, and aug-cc-pV5Z energies are extrapolated on the QFF PES to the one-particle complete basis set (CBS) limit via a threepoint formula. 43 The energy differences in the CCSD(T)/MT energies with and without core electrons as well as the CCSD(T)/cc-pVTZ differences with the scalar relativistic DouglasKroll 44 (DK) correction included and excluded are added to the CBS energy. This produces the “CcCR” QFF 15,45–47 from inclusion the CBS energy (“C”), core correlation (“cC”), and relativistic (“R”) inclusions. All computations involving the sulfur atom include the additional d functions even if not explicitly stated. MP2/6-31+G∗ results utilizing the Gaussian09 program 48–50 generate the double-harmonic intensity predictions. The least-squares fitting is tight (on the order of 10−15 a.u.2 ) and produces the equilibrium geometry. A refitting provides the force constants resulting from the refit zero gradients. The above-defined internal force constants are transformed into cartesian coordinates using the INTDER 51 program in order to provide for easier translation into the SPECTRO 52 program. The latter utilizes second-order perturbation theory for vibrations (VPT2) 53,54 and rotations 55 to produce the vibrational frequencies along with the spectroscopic constants. Standard HOSO and the 18 O/34 S isotopologues possess a ν5 +ν4 = ν3 type-2 Fermi resonance as well as ν3 /ν2 Darling-Denisonn resonance. DOSO possesses a ν5 + ν3 = ν2 type-2 Fermi resonance, a ν4 /ν3 Darling-Denisonn resonance, and a ν4 /ν3 C-type Coriolis resonance.

Results and Discussion Structure and Rotational Constants The ground electronic state for cis-HOSO is 2 A′′ with the highest-occupied or singly-occupied orbital (HOMO and SOMO, respectively) lying in the π antibonding orbital comprised of three out-of-phase p orbitals from each of the oxygen, sulfur, and oxygen atoms, respectively. Similar behavior has been noted for HSS, HSO, and HOS. 56 The aug-cc-pV5Z T1 diagnostic is 0.019 indicating that single-reference methods are adequate to characterize HOSO. 57 6


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One of the reasons for this molecule’s flat potential lies in the electron density and partial charges. Each atom has a significant (MP2/6-31+G∗ ) Mullikan partial charge. O1 is -0.623, S is 0.895, O2 is -0.797, and H is 0.525. A side product of this has the terminal, negative O1 interacting with the positive hydrogen stabilizing the structure in a the observed cyclic form. The weakness of the bonding is further born out in the CcCR force constants provided in Table 1. The F11 O1 −S force constant (that on the terminal end of the molecule) at 8.925 mdyne/˚ A2 is in the range expected for S−O bonds, and the F33 7.900 mdyne/˚ A2 is also expected for O−H bonds. 56 However, the F22 force constant, and subsequent bond strength, for the internal S−O2 bond is less than half of that of the other S−O bond. Explicitly, the CCSD(T)/aug-cc-pV5Z dissociation energy of cis-HOSO into SO and OH is 70.2 kcal/mol, markedly less than the 125 kcal/mol standard S−O bond dissociation energy. Additionally, the S−O2 bond length at 1.630 ˚ A (Table 2) is 0.163 ˚ A longer than the O1 −S. Hence, the central S−O2 bond is reduced in strength compared to the O1 −S bond. These five items (HOMO, partial charges, force constants, bond energy, and bond lengths) highlight the strong electron localization in the OH and terminal SO portions as well as the weakened strength present in the central bond of this molecule. As a result, the central S−O2 bond is more free to rotate reducing the barrier and flattening the potential. This flat rotation is also highlighted in the F66 force constant of 0.0004 mdyne/˚ A2 , a very small value. Comparison of the Rα vibrationally-averaged CcCR rotational constants built around the zero-point-corrected geometrical constraints, given in Table 2, and the experimental values from Ref. 21 is excellent for the B and C constants in this near-prolate molecule with differences of less than 0.002 cm−1 after converting the experimental values from the reported MHz. The A constant is nearly as good with a difference of 0.003 cm−1 , where A values are known to differ from experiment in similar molecules more than B and C. 15 As a result, the subsequent vibrational frequencies, except for the torsional mode, should be of similar accuracies as those computed with CcCR QFFs and VPT2 previously.

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Vibrational Frequencies Simple addition of the hydrogen atom to SO2 to produce HOSO shifts the vibrational frequencies notably. Benchmarked values for sulfur dioxide has the two stretches taking place in the 1150 cm−1 and 1360 cm−1 regions for the symmetric and antisymmetric stretches, respectively. 58 While some mixing of the stretches is present between the two HOSO heavy atom stretches, the higher frequency ν2 fundamental mode can be thought of as the S−O1 stretch on the terminal end of the molecule. The lower frequency ν4 is more of the stretching between the SO and OH groups. Consequently, the ν2 HOSO frequency at 1172.2 cm−1 (shown in Table 2) is slightly higher in frequency than the symmetric stretch in SO2 . However, the ν4 at 781.0 cm−1 is less than 60% that of the antisymmetric stretch and less than 70% that of the symmetric stretch. Hence, one of the stretches is largely coincident when hydrogenating sulfur dioxide, but the other changes the spectrum drastically. The reduction in the O−S−O bend between HOSO and SO2 is not as drastic but is still significant. The bend reduces from close to 520 cm−1 to the HOSO ν5 of 395.6 cm−1 , or 76% lower. These frequencies, as well as the others discussed below, are in very good agreement with the anharmonic values computed by Wheeler and Schaefer 20 also given in Table 2. The double-harmonic intensities for HOSO are similarly comparable as the frequencies to the known intensities for SO2 . The HOSO ω2 intensity of 780 km/mol is quite bright, right in line with the antisymmetric stretch intensity of SO2 . 58 The ω4 250 km/mol intensity is 2/3 this value but also bright. The symmetric stretch of SO2 is an order of magnitude less intense than its antisymmetric counterpart. Hence, one of the intensity magnitudes is conserved when moving from SO2 to HOSO, while the other other is increased. The S−O1 stretch is actually the most intense absorber/emitter with the other S−O2 stretch the nextmost. The torsion is computed to be third with the hydride stretch fourth and still fairly bright at 139 km/mol. The bends are relatively dim. These intensities are likely the reason why ν1 , ν2 , and ν4 were observed in the matrix isolation studies with ν6 falling below the observing window. 30 8


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Even though the ν1 hydride stretch is dimmer than the heavy-atom stretches, its CcCR anharmonic vibrational frequency is still the highest frequency at 3567.3 cm−1 , nearly the same as all of the other frequencies combined making it almost coincident with the anharmonic zero-point vibrational energy of 3582.3 cm−1 . Comparison to sulfuric acid 59 indicates that the O−H stretching behavior is similar between the two species: H2 SO4 is 3609.2 cm−1 , and HOSO is, again, 3567.3 cm−1 . The CcCR O−H VPT2 anharmonic stretch in HOS is 3592 cm−1 , 56 in between these two values and within comparable range. As a result, the HOSO hydride stretch appears to be reduced compared to the simpler HOS and more complicated H2 SO4 structures owing to its bonding environment. The experimental H−O−S sulfuric acid symmetric and antisymmetric bends are nearly coincident with one another at around 1138 cm−1 and conclusively 1157.1 cm−1 , respectively. 59 The CcCR ν3 hydride bend is 1034.1 cm−1 , a reduction of 123 cm−1 or 89.4 %. The HOS bend is 1157.7 cm−1 , significantly closer to sulfuric acid than HOSO. The torsional anharmonicity cannot be treated explicitly with a QFF. Hence, the 31.0 cm−1 CcCR harmonic frequency is really of the greater value here than the anharmonic where none of the anharmonic terms involving I6 are actually included. This is a first estimate of the torsional frequency, but it is likely not as accurate as the predictions of the other vibrational modes.

Isotopologues In order to augment the literature, the D, herein. Table 2 contains the D and

34

34

S, and

18

O isotopologue data are also included

S data. The deuteration, naturally, shifts the spectral

features the most due to a larger relative mass increase for the atom. The hydride stretch and bend frequencies are notably decreased, as expected. Most interestingly, the ν4 O2 − S stretch is also reduced, by 13.2 cm−1 , relative to the standard isotopologue, to 767.8 cm−1 . This behavior implies that the internal S−O stretch has many properties of a dissociation pathway for the creation of sulfur monoxide and the hydroxyl radical. The heavy-atom bend, 9



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ν5 is also reduced as a result of a greater mass present on one of the oxygen atoms due to the strong O−H bond. 34

S substitution behaves expectedly with little change to the B and C constants. The

vibrational frequencies involving this atom decrease by a few cm−1 . The two S−O stretches are reduced more than the bend and torsion. Table 3 contains the

18

O data. Each successive

18

O addition reduces the rotational

constants and vibrational frequencies, again, as expected. These additional data can assist in laboratory experiments probing the [H,S,O2 ] potential energy surface utilizing isotopic substitution.

Conclusions The detailed spectroscopic data provided in this work will enable more refined experimental analysis and extraterrestrial observation of the HOSO system. It exists only as a cis conformer and has a relatively weak central S−O bond. Intramolecular hydrogen bonding stabilizes the cis conformer similar to that predicted for the cis-HOCO− anion. 16 The computed rotational constants agree well with previous theory and experiment 21 making further characterization of this molecule herein likely of similar accuracy. The vibrational frequencies of HOSO are all reduced compared to similar molecular species including the HOS/HSO radicals and sulfuric acid. Hence, environments where various [Hx ,Sy ,Oz ] compounds may be found will have the HOSO frequencies produced at the lower ends of the spectral windows. As a result of these positions and the notable intensities of the vibrational modes, chemical mechanisms that involve this tetra-atomic structure should allow for relatively straightforward detection. Finally, the notable intensity and low frequency of the torsional mode make it and its overtones of interest to THz observation of planetary atmospheres and beyond. While the CcCR harmonic predictions imply that the fundamental will occur in the 915 GHz range and

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the first overtone, assumedly, close to 1.83 THz, subsequent computations or experimental analysis must be undertaken in order to isolate these frequencies conclusively. However, if they are so low in frequency, excitations of this mode may serve as markers of sulfur chemistry observed from remote sensing of planetary atmospheres or even circumstellar envelopes or even atmospheric spectroscopic characterization of the Earth.

Acknowledgement RCF wishes to acknowledge support from NASA grant NNX17AH15G issued through the Science Mission Directorate. This work is also further supported by the National Aeronautics and Space Administration through the NASA Astrobiology Institute under Cooperative Agreement Notice NNH13ZDA017C issued through the Science Mission Directorate.

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Table 1: The cis-HOSO CcCR QFF Quadratic, Cubic, and Quartic Force Constantsa (in mdyn/˚ An ·radm ). F11 8.925 288 F333 -55.0558 F554 0.0534 F4331 0.58 F5421 -1.41 F21 0.321 298 F411 -0.3388 F555 -0.9203 F4332 -0.29 F5422 0.29 F22 4.178 035 F421 -0.5889 F1111 311.22 F4333 0.17 F5431 0.12 F31 -0.065 346 F422 -0.0502 F2111 1.68 F4411 -0.61 F5432 0.38 F32 0.112 479 F431 0.1966 F2211 2.23 F4421 3.86 F5433 -0.93 F33 7.899 995 F432 0.1219 F2221 -1.53 F4422 0.03 F5441 -0.17 F41 0.282 904 F433 0.1208 F2222 125.96 F4431 -0.35 F5442 0.21 F42 -0.157 312 F441 -2.0410 F3111 1.26 F4432 -0.31 F5443 0.10 F43 -0.071 417 F442 -1.5987 F3211 -0.21 F4433 -1.73 F5444 0.68 F44 1.465 891 F443 0.0310 F3221 -0.25 F4441 4.89 F5511 -0.98 F51 0.035 111 F444 -2.8744 F3222 3.61 F4442 5.01 F5521 0.59 F52 0.337 315 F511 -0.1776 F3311 -1.59 F4443 -0.72 F5522 -1.02 F53 0.215 171 F521 0.0989 F3321 0.82 F4444 8.76 F5531 -0.31 F54 -0.250 985 F522 -0.7026 F3322 -2.93 F5111 -0.45 F5532 0.72 F55 0.583 468 F531 0.1251 F3331 0.65 F5211 0.91 F5533 -2.11 F66 0.000 405 F532 -0.5898 F3332 -1.45 F5221 -0.61 F5541 0.15 F111 -57.9685 F533 0.0966 F3333 340.54 F5222 0.19 F5542 -0.21 F211 -0.6999 F541 -0.0157 F4111 -0.67 F5311 -0.99 F5543 -0.57 F221 -0.3032 F542 0.0250 F4211 2.08 F5321 0.01 F5544 -1.15 F222 -24.8640 F543 -0.0035 F4221 1.66 F5322 0.68 F5551 0.18 F311 0.0683 F544 0.1449 F4222 0.99 F5331 -0.53 F5552 1.35 F321 0.1230 F551 0.0313 F4311 -0.27 F5332 -0.18 F5553 -0.30 F322 -0.8489 F552 -0.4589 F4321 -0.47 F5333 -2.34 F5554 0.26 F331 -0.0076 F553 -0.3705 F4322 0.37 F5411 0.81 F5555 -1.70 F332 0.1856 a The numbers correspond to the respective internal coordinate defined in the text with coordinate 6, the torsion, removed due to a flat potential. See text for details.

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Table 2: The CcCR cis-HOSO Equilibrium and Zero-Point (Rα ) Geometries, Vibrational Frequencies (with Intensitiesa ), and Spectroscopic Constants. HOSO DOSO HO34 SO This Work Previousb This Work Previousb This Work Previousb r0 (O1 −S) ˚ A 1.467 093 1.443 1.465 932 1.467 098 r0 (S−O2 ) ˚ A 1.630 475 1.657 1.627 996 1.630 479 r0 (O−H2 ) ˚ A 0.985 582 0.957 0.980 638 0.985 581 6 (O−S−O) 108.609 108.8 108.440 108.605 6 (S−O−H) 107.856 108.3 107.723 107.858 1.165 307 1.1683621 1.056 679 1.059 486 1.133 119 1.135 786 A0 cm−1 B0 cm−1 0.318 354 0.3202227 0.308 592 0.311 768 0.318 239 0.320 110 0.253 371 0.2515336 0.243 060 0.241 209 0.251 739 0.249 922 C0 cm−1 A1 cm−1 1.159 378 1.050 769 1.137 402 0.318 890 0.309.184 0.318 775 B1 cm−1 C1 cm−1 0.253 418 0.243 098 0.251 781 1.160 518 1.050 323 1.128 695 A2 cm−1 B2 cm−1 0.317 658 0.308 099 0.317 554 0.252 832 0.245 530 0.251 211 C2 cm−1 A3 cm−1 1.170 920 1.061 496 1.138 424 0.317 604 0.307 721 0.317 494 B3 cm−1 −1 0.252 699 0.242 315 0.251 084 C3 cm −1 A4 cm 1.163 482 1.057 165 1.131 247 −1 0.316 268 0.306 627 0.316 169 B4 cm −1 C4 cm 0.251 811 0.241 619 0.250 215 −1 1.180 483 1.068 479 1.147 884 A5 cm −1 B5 cm 0.317 987 0.308 461 0.317 871 −1 0.252 582 0.242 387 0.250 953 C5 cm 10.321 10.439 20.234 DJ kHz DJK kHz -73.131 -64.548 -70.320 0.554 0.393 0.526 DK MHz d1 kHz -3.403 -3.575 -3.433 d2 kHz -0.232 -0.264 -0.237 31.233 -73.084 27.668 HJ mHz -0.370 -0.695 -0.345 HJK Hz HKJ Hz -4.846 -2.869 -4.562 32.885 20.929 30.544 HK kHz h1 mHz -18.963 -60.735 -19.954 -38.064 -27.189 -37.096 h2 mHz h3 mHz -1.573 -3.379 -1.281 re (O1 −S) ˚ A 1.464 073 – – ˚ re (S−O2 ) A 1.626 501 – – re (O−H2 ) ˚ A 0.967 372 – – 6 (O−S−O) 108.300 – – 6 (S−O−H) 107.631 – – − Ae cm 1 1.148 787 1.038 511 1.116 814 − 0.326 135 0.317 997 0.325 884 Be cm 1 Ce cm− 1 0.253 948 0.243 454 0.252 274 1.12 1.17 – – µ D −1 ω1 cm H−O2 3757.4 (139) 3757 2733.8 3757.4 −1 1203.7 (780) 1202 1192.0 1192.1 ω2 cm S−O1 ω3 cm−1 H−O−S 1084.3 (25) 1085 842.5 1082.2 −1 799.5 (250) 798 790.2 791.5 ω4 cm O2 −S −1 ω5 cm O−S−O 394.0 (34) 392 369.5 391.8 −1 30.8 (180) 23.4 30.7 ω6 cm torsion −1 Zero-Point cm 3582.3 2946.0 3570.6 −1 3567.3 3576 2632.7 3567.3 ν1 cm H−O2 1172.2 1184 1169.6 1160.5 ν2 cm−1 S−O1 −1 ν3 cm H−O−S 1034.1 1055 810.1 1032.6 −1 781.0 784 767.8 773.6 ν4 cm O2 −S −1 ν5 cm O−S−O 395.6 389 376.0 393.4 −1 c 31.0 25.8 31.0 ν6 cm torsion a ∗ The MP2/6-31+G double harmonic intensities for the standard isotopologue are in parentheses given in km/mol. b Rotational and structural experimental results with insights from theoretical computations, UHF-CCSD(T)/aug-cc-pwCV(T+d)Z, are given in Ref. 21. The vibrational frequencies are ROCCSD(T)/cc-pV5Z with cc-pVTZ anharmonic corrections from Ref 20. c Estimated due to the flat the potential along this coorinate.

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Table 3: The quencies.

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O CcCR cis-HOSO Rotational Constants and Vibrational Fre-

H18 OS18 O H OSO HOS O This Work Previousa A0 cm−1 1.149 052 1.126 630 1.109 189 1.117 273 −1 0.299 248 0.302 439 0.284 484 0.285 658 B0 cm 0.240 421 0.241 448 0.229 136 0.227 651 C0 cm−1 −1 ω1 cm H−O2 3745.3 3757.4 3745.3 −1 1203.0 1168.8 1167.3 ω2 cm S−O1 1079.6 1073.8 1070.0 ω3 cm−1 H−O−S −1 ω4 cm O2 −S 770.4 798.9 769.8 −1 387.6 384.6 378.1 ω5 cm O−S−O ω6 cm−1 torsion 30.6 30.7 30.6 −1 Zero-Point cm 3556.2 3554.9 3528.7 ν1 cm−1 H−O2 3556.5 3567.3 3556.5 −1 1171.6 1127.3 1128.1 ν2 cm S−O1 −1 ν3 cm H−O−S 1029.7 1033.5 1029.1 752.2 780.3 752.4 ν4 cm−1 O2 −S −1 388.2 386.6 379.0 ν5 cm O−S−O −1 c 30.5 31.1 30.5 ν6 cm torsion b Experimental rotational constants from Ref. 21. 18

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Quantum Chemical Rovibrational Analysis of the HOSO Radical

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