High-resolution infrared measurements on HSOH: Analysis of the OH fundamental vibrational mode

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1 Available online at Journal of Molecular Spectroscopy 247 (2008) High-resolution infrared measurements on HSOH: Analysis of the OH fundamental vibrational mode Oliver Baum *, Thomas F. Giesen, Stephan Schlemmer I. Physikalisches Institut, Universität zu Köln, Germany Received 16 August 2007; in revised form 8 October 2007 Available online 22 October 2007 This paper is dedicated to Prof. Josef Hahn on the occasion of his 65th birthday. Abstract High-resolution infrared measurements of the OH-stretching mode of oxadisulfane, HSOH, at 3625 cm 1 have been recorded using a Bruker IFS 120 HR Fourier transform spectrometer. More than 1300 lines have been assigned to the m(oh) fundamental vibration mode, which is a hybrid band showing a c-type perpendicular band and an a-type parallel band spectrum of an asymmetric rotor molecule. The splitting due to the torsional-tunneling has not been observed in this band. The band center position at (20) cm 1 as well as rotational and centrifugal distortion constants for the m(oh) vibrational excited state have been obtained from a least-squares fit analysis of a semirigid rotor. In addition the a OH experimental vibration rotation correction terms of the OH-stretching mode have been derived and compared to values used in an earlier semi-empirical calculation of the HSOH structure. All data are in very good agreement with high level ab initio calculations and confirm the assignment of an earlier matrix isolation spectrum at 3608 cm 1 to the m(oh) fundamental mode. Ó 2007 Elsevier Inc. All rights reserved. Keywords: HSOH; Oxadisufane; OH-stretching mode; High-resolution infrared spectroscopy; FT spectroscopy 1. Introduction * Corresponding author. address: baum@ph1.uni-koeln.de (O. Baum). Gas-phase studies on HSOH were long time impeded due to the problem of synthesizing this reactive and unstable molecule. First experimental evidence of a skew chain structure, very similar to the cases of HOOH and HSSH, came from high-resolution millimeter-wave spectroscopy on HSOH and H 34 SOH [1]. The authors used flash vacuum pyrolysis of di-tert-butyl sulfoxide to produce HSOH, a novel pathway which had been introduced by Hahn et al. and Behnke in 2001 [1,2]. Pure rotational spectra of the perdeuterated isotopologue D 32 SOD, produced in a rf-discharge of D 2 O and D 2 S were reported by Behnke et al. [3] and later by Brünken et al. [4]. Baum and co-workers presented first gas phase spectra of a singly deuterated oxadisulfane, HSOD, in its vibrational ground state [5]. The authors also derived a semi-empirical equilibrium structure of HSOH from the ground state rotational constants A 0, B 0, and C 0 of HSOH, H 34 SOH, DSOD, HSOD and from calculated vibration rotation interaction constants a A,B,C. It is the purpose of this paper to present the first ro-vibrational spectra of HSOH and to derive an experimental value for a of the first excited OH-stretching mode. Infrared data on HSOH vibrational modes are rare. In 1977 Smardzewski et al. [6] trapped ozone (O 3 ) and hydrogen sulfide (H 2 S) in an argon matrix and recorded infrared spectra of products formed via photolysis. Based on ab initio calculations they assigned four spectral features to fundamental modes of HSOH. Recently Beckers et al. [7] identified five of six fundamental vibrational modes of matrix isolated HSOH formed by pyrolysis of di-tert-butyl sulfoxide. Furthermore, they used characteristic /$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi: /j.jms

2 26 O. Baum et al. / Journal of Molecular Spectroscopy 247 (2008) spectral features of the present gas-phase measurements on HSOH to confirm their assignment of two matrix peaks at (43) and (8) cm 1 to the OH- and SH-stretching fundamental modes. Here, we present the complete high-resolution infrared spectrum of the OHstretching mode of HSOH and derive gas phase molecular constants. The analysis of the SH-stretching mode is complicated by a strong interaction with at least one energetically close lying combination band and will be presented in a forthcoming paper. 2. Experiment High-resolution infrared measurements on HSOH were performed using a Fourier transform (FT) spectrometer (Bruker IFS 120) at the Bergische Universität, Wuppertal/Germany. The spectral range from 3500 to 3800 cm 1 was recorded with a spectral resolution of cm 1. HSOH is unstable under laboratory conditions and thus has been produced under constant flow conditions via flash vacuum pyrolysis of di-tert-butyl sulfoxide. The precursor was synthesized by selective oxidation of di-tert-butyl sulfide with hydrogen peroxide/ selenium dioxide (see Drabowicz and Nikolajczyk [8]). During the measurements a total pressure of 1 mbar was kept in the absorption cell. The FT-signal was detected with a fast liquid nitrogen cooled InSb bolometer and the signal to noise ratio was significantly enhanced by using multipass optics with an effective absorption pathlength of 40 m. To achieve high frequency accuracy of the HSOH line positions the spectrum of the by-product H 2 O and precise H 2 O line positions published by Toth [9] were used as reference for absolute frequency calibration. 3. Data and analysis According to the geometry of HSOH, with the OHbond lying in the a-c-plane of the principal axis-system, an a-type parallel and a c-type perpendicular spectrum with almost same band intensities are expected when the OH-stretching mode is excited. In Fig. 1 the band center region of the recorded spectrum is plotted, which clearly shows the characteristic pattern of a parallel type spectrum with strong unresolved q Q branches at the band center and a serious of overlapping q P- and q R- branches. Furthermore two resolved strong Q branches peaked towards the band center at cm 1 are most prominent and were consequently assigned to the p Q 1 - and r Q 0 -band of a perpendicular type band. For clarity we show in Fig. 2 the simulated spectra of the parallel and perpendicular bands in the band center region based on the molecular constants derived in the final data analysis and in the lower trace the recorded HSOH spectrum for comparison. It turned out, that the strongest lines in the recorded spectra are due to impurities of the sample (see transitions marked with an asterisk). The in situ production of HSOH from its precursor is accompanied by several by products, such as water, iso-butene, and sulfur dioxide, as discussed by Beckers et al. [7]. Strongest transitions in the recorded range between 3500 and 3800 cm 1 belong to the H 2 O molecule and had to be identified before further data treatment The c-type perpendicular band The HSOH ground state rotational constants A 0 = (45) cm 1, B 0 = (41) cm 1, and C 0 = (40) cm 1 [1] give evidence for an asymmetric rotor molecule close to the case of an accidental prolate top with Ray s asymmetry parameter j = The spacing of adjacent Q-branches of a perpendicular band is of order (2A 00 B 00 C 00 ) which is cm 1, taking the highly accurate HSOH ground state rotational constants into account. Furthermore, except for the p Q 1 -branch and the r Q 0 -branch, each p,r Q Ka -branch splits into two sub-bands due to asymmetry splitting, which is most prominent in case of low K a quantum numbers. The orientation of the p,r Q Ka -branches is depicted in the Fortrat diagram in Fig. 3. Assigned transitions are marked with circles, and the line intensities are indicated by the size of the crosses. The branches are only slightly affected by centrifugal distortion, indicating HSOH to be a rather rigid molecule in its vibrational excited OH-stretching mode The a-type parallel band For the parallel band of the OH stretching fundamental vibrational mode only transitions with DK a = 0 and DJ = 0, ± 1 occur leading to a set of q P Ka, q Q Ka, and q R Ka -branches for each K a. In the rigid rotor approximation the spacing of adjacent lines in the P and R branches are approximately (B + C) which is of order 1 cm 1 for HSOH. The complete parallel band is a superposition of K a sub-bands which show for K a > 0 a splitting of branches due to molecular asymmetry. Most pronounced is the asymmetry splitting of lines in the branches with low K a values. To assign P- and R-branch transitions a Loomis- Wood plot has been used (see Fig. 4). The plot uses a prolate symmetric top rigid rotor molecule as reference, where the deviation Dm ¼ m exp ðm 0 þðeb 0 þ eb 00 Þm þðeb 0 eb 00 Þm 2 Þ with eb ¼ 1 ðb þ CÞ, is plotted versus quantum numbers 2 m = J + 1 for R-branch transitions, and m = J for P-branch transitions. The more than 1300 identified a-type and c-type transitions were simultaneously fitted in a least-squares analysis to molecular parameters of a Watson type Hamiltonian in S-reduction. The pure rotational part of the Hamiltonian has the expression:

3 q Q 8 q Q 7 q Q 6 q Q 5 q Q 4 q Q 3 q Q 2 q Q 7 q Q 6 q Q 5 q Q 4 q Q 3 q Q 2 O. Baum et al. / Journal of Molecular Spectroscopy 247 (2008) calculated spectrum T=300K intensity (a.u.) p Q p 4 Q p 3 Q p 2 Q r r 1 Q r 0 Q 2 Q 3 r Q 1 experimental spectrum Fig. 1. Measured and calculated spectrum of HSOH at the m(oh) band center region. The calculated spectrum at 300 K is based on molecular constants from the final data analysis. The p Q 4 to r Q 3 branches of the c-type band and the q Q 2 to q Q 7 branches of the a-type band are indicated. The strong absorption lines in the experimental spectrum are mainly due to the H 2 O by-product. (a) calculated parallel band (b) calculated perpendicular band p Q r 1 Q 0 intensity (a.u.) J= (c) calculated spectrum =J (d) experimental * * * spectrum * ** * * * * ** ** * byproducts Fig. 2. Band center region of the m(oh) stretching mode: (a) Calculated spectrum of the parallel band, (b) perpendicular band, and (c) superposition of both spectra using molecular constants from the final data analysis. The parallel band spectrum shows the q Q 2 to q Q 8 branches. The p Q 1 and r Q 0 branches are the dominant structure in the perpendicular band. (d) Experimental spectrum, spectral features of by-products are indicated with an asterisk. bh ¼ 1 2 ðb þ CÞ bj 2 þ A 1 ðb þ CÞ bj 2 z 2 þ 1 4 ðb CÞð bj 2 þ þ bj 2 Þ D J bj 4 D JK bj 2 bj 2 z D b K J 4 z þ d b 1J 2 ðbj 2 þ þ bj 2 Þ þ d 2 ðbj 4 þ þ bj 4 ÞþH b J J 6 þ H JK bj 4 bj 2 z þ H b KJ J 2 bj 4 z þ H b K J 6 z þ h 1 bj 4 ðbj 2 þ þ bj 2 Þþh b 2J 2 ðbj 4 þ þ bj 4 Þþh 3ðbJ 6 þ þ bj 6 Þ Transitions up to J 00 = 35 and K 00 a ¼ 7 have been included in the final fit using Pickett s program spfit [10]. The ground state molecular constants were fixed to the highly accurate values from recent microwave measurements [1]. The analysis yields a precise value for the band center frequency of the m(oh) stretching mode, its rotational constants A, B, and C as well as all quartic centrifugal distortion constants D J, D K, and D JK. Except H J, the sextic terms and higher order constants were not significant and thus have been fixed to their corresponding ground state values. In addition the asymmetry parameters d 1 and d 2 have been derived. All molecular constants as well as the ground state values from [1] and for comparison high level ab initio values [7,11] are given in Table 1. The dimensionless weighted root-meansquare deviation value of 1.13 shows the excellent

4 p Q 5 p Q 4 p Q 3 p Q 2 p Q r r r Q r Q r Q r 1 Q 0 Q Q 5 28 O. Baum et al. / Journal of Molecular Spectroscopy 247 (2008) J' Fig. 3. Fortrat diagram of the m(oh) perpendicular band up to K 00 a ¼ 5. Assigned transitions are marked by circles, where the size of the crosses indicates their intensities. Fig. 4. Loomis-Wood diagram of parallel band P- and R-branch transitions up to K a =7.ForR-branch transitions the quantum numbers m = J + 1 and for P-branch transitions m = J have been used. Assigned transitions are marked by circles, where the size of the crosses indicates their intensities. The asymmetry splitting of transitions is most prominent in the q R 1 - and q P 1 -branches. agreement of the experimental data with the semirigid rotor model. 4. Discussion The analysis of high-resolution infrared data proves HSOH to be a fairly rigid molecule. The B and C rotational constants in the fundamental OH-stretching mode are less than 0.15% smaller than the ground state values. The same is true for the centrifugal constants. The A rotational constants in the excited state changes by 1% and contributes most significantly to the vibration-rotation correction term P i a i=2. The experimental value a A OH ¼ A00 A 0 ¼ 0:084607ð25Þ cm 1 is in excellent agreement with the theoretical value of cm 1, obtained from high level CCSD(T)/cc-pVTZ calculations of the harmonic and anharmonic force fields [11]. The values of a B OH and a C OH are two orders of magnitude smaller than aa OH and close to the ab initio values (see Table 1). In our previous analysis of the HSOH structure [5] we used ab initio values for a which prove satisfactory by the here presented results. HSOH has two rotameric forms, which are separated by tunneling barriers with V cis 2216 cm 1 and V trans 1579 cm 1 [1]. The tunneling splitting of ground state rotational levels is (21) cm 1 as has been reported by Winnewisser et al. [1]. Quack et al. [12] calculated in their quasiharmonic quasiadiabatic channel RPH approximation with aug-cc-pvtz basis set a torsional splitting for the ground state of cm 1 which agrees very well with the experimental value. For non of the vibrational excited states an experimental value of the splitting has been determined, but by Quack et al. a value of cm 1 for the m(oh) state has been calculated. The instrumental resolution of the presented data is cm 1 and the calculated Doppler line width in the vibrational excited m(oh)

5 O. Baum et al. / Journal of Molecular Spectroscopy 247 (2008) Table 1 Spectroscopic parameters for the vibrational ground state and the m(oh) first exited state of HSOH (cm 1 ) Parameter Theoretical [7,11] v =1 state a Experimental [this work] v = 1 state b Experimental[1] v = 0 state A (25) (45) B (16) (41) C (17) (40) D J (22) (46) D JK (32) (16) D K (51) (67) d (52) (65) d (52) (35) H J (100) (13) H JK c (13) H KJ c 0.996(11) H K c (38) h c (26) h c (27) h c (70) a A OH (25) a B OH (16) a C OH (17) m (20) a CCSD(T)/cc-pVQZ + CCSD(T)/cc-pVTZ vibrational correction. b In the analysis values for the v = 0 state were fixed to data taken from Ref. [1]. c Values are fixed to constants of the vibrational ground state. state is m D (FWHM) = cm 1. Accordingly, the tunneling splitting is not resolved in the recorded spectra. Recently, Beckers et al. [7] published vibrational spectra of HSOH trapped in a cryogenic Argon matrix. The authors used the here presented gas phase measurements to confirm their assignment of a matrix peak at cm 1 to the fundamental band of the OH-stretching mode. The matrix value is 17.2 cm 1 lower in frequency compared to the gas phase value. In most cases matrix effects shift the band centers to lower frequencies which also seems to be the case for the m(oh) mode of HSOH. The same authors also presented coupled cluster calculations and derived vibrational frequencies and band intensities for all six fundamental modes. Their m(oh) = cm 1 value is 31.7 cm 1 higher than the experimental gas phase value, which corresponds to 1% of accuracy. The band center frequency of the m(sh) stretching mode is calculated to be cm 1 with a band intensity 4-5 times weaker than the m(oh) band [7]. The gas phase spectrum shows a rotationally resolved band centered at 2538 cm 1, which can clearly be assigned to a vibrational mode of HSOH. The band intensity is by a factor of five weaker compared to the m(oh) band, which is in good agreement with the predicted ratio. The analysis of the SH fundamental stretching mode is complicated by strong interaction with energetically close lying levels. Other than the OH-stretching vibration, the ro-vibrational transitions of the m(sh) show clear line splittings, which are of order 0.04 cm 1 and a factor of 20 larger than the value calculated by Quack et al. [12]. A detailed analysis of the band at 2538 cm 1 will be given in a forthcoming paper. Acknowledgments The authors thank H. Beckers for assistance during the gas-phase experiments and S. Esser and J. Hahn for synthesizing the precursor. This work has been supported by the Deutsche Forschungsgemeinschaft via research Grant SFB 494, and by the European Community within the FP6 Marie Curie Research Training Network, QUASAAR. References [1] G. Winnewisser, F. Lewen, S. Thorwirth, M. Behnke, J. Hahn, J. Gauss, E. Herbst, J. Chem. Phys. 9 (2003) [2] M. Behnke, Ph.D. thesis, University of Cologne, [3] M. Behnke, J. Suhr, S. Thorwirth, F. Lewen, H. Lichau, J. Hahn, J. Gauss, K. Yamada, G. Winnewisser, J. Mol. Spectrosc. 221 (2003) [4] S. Brünken, M. Behnke, S. Thorwirth, K.M.T. Yamada, T. Giesen, F. Lewen, J. Hahn, G. Winnewisser, J. Mol. Struct. 742 (2005) [5] O. Baum, S. Esser, N. Gierse, S. Brünken, F. Lewen, J. Hahn, J. Gauss, S. Schlemmer, T.F. Giesen, J. Mol. Struct. 795 (2006) [6] R. Smardzewski, M. Lin, J. Phys. Chem. 66 (1977) [7] H. Beckers, S. Esser, T. Metzroth, M. Behnke, H. Willner, J. Gauss, J. Hahn Chem. Eur. J. 12 (2006) [8] J. Dabrowicz, M. Mikolajczyk, Synthesis (1978) [9] R. Toth, J. Opt. Soc. Am. B 10 (1993) [10] H. Pickett, J. Mol. Spectrosc. 148 (1991) [11] J. Gauss, private communication. [12] M. Quack, M. Willeke, Helv. Chem. Acta 86 (2003)