LETTER pubs.acs.org/JPCL
Silicon Oxysulfide, OSiS: Rotational Spectrum, Quantum-Chemical Calculations, and Equilibrium Structure Sven Thorwirth,*,† Leonie Anna M€uck,‡ J€urgen Gauss,*,‡ Filippo Tamassia,§ Valerio Lattanzi,||,^ and Michael C. McCarthy*,||,^ †
I. Physikalisches Institut, Universit€at zu K€oln, Z€ulpicher Str. 77, 50937 K€oln, Germany Institut f€ur Physikalische Chemie, Universit€at Mainz, 55099 Mainz, Germany § Dipartimento di Chimica Fisica e Inorganica, Universita di Bologna, Viale del Risorgimento 4, I-40136 Bologna, Italy Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, United States ^ School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States
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‡
ABSTRACT: Silicon oxysulfide, OSiS, and seven of its minor isotopic species have been characterized for the first time in the gas phase at high spectral resolution by means of Fourier transform microwave spectroscopy. The equilibrium structure of OSiS has been determined from the experimental data using calculated vibrationrotation interaction constants. The structural parameters (rOSi = 1.5064 Å and rSiS = 1.9133 Å) are in very good agreement with values from high-level quantum chemical calculations using coupled-cluster techniques together with sophisticated additivity and extrapolation schemes. The bond distances in OSiS are very short in comparison with those in SiO and SiS. This unexpected finding is explained by the partial charges calculated for OSiS via a natural population analysis. The results suggest that electrostatic effects rather than multiple bonding are the key factors in determining bonding in this triatomic molecule. The data presented provide the spectroscopic information needed for radio astronomical searches for OSiS. SECTION: Molecular Structure, Quantum Chemistry, General Theory
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arbonyl sulfide (carbon oxysulfide), OCS, is a molecule wellinvestigated spectroscopically in the laboratory, and its pure rotational and rotation-vibration spectra have been studied at high spectral resolution by many groups. (See, for example, refs 1 and 2 and references therein.) It also is an abundant and widespread molecule in the interstellar medium and thus of astronomical relevance (ref 3 and references therein). In contrast, little is known about the isovalent silicon analog OSiS, silicon oxysulfide. In the laboratory, OSiS was first characterized through matrix isolation spectroscopy by Schn€ockel 30 years ago.4 More recently, some evidence has been obtained for the production of OSiS through flash-vacuum thermolysis.5 OSiS has also received comparably little attention from quantum chemistry6,7 despite its potential siliconsulfur double bond, which should render OSiS an interesting target for bond analysis studies. The present investigation is aiming at a characterization of OSiS in the gas phase with a focus on the following issues: (i) a determination of its structural parameters, (ii) a detailed discussion of its chemical bonding, and (iii) providing of spectroscopic information so that a radio astronomical search can be conducted with confidence. For these reasons, a joint experimental and theoretical investigation is reported here. In the experimental side, the rotational spectrum of OSiS has been obtained by Fourier transform microwave (FTMW) spectroscopy, whereas on the theoretical side, results from high-level quantum chemical calculations are reported. From a combination of the experimental r 2011 American Chemical Society
ground-state rotational constants of a total of eight isotopic species and calculated zero-point vibrational corrections ΔB0 to the rotational constants, an accurate empirical (semiexperimental) equilibrium structure has been determined. The spectroscopic search for OSiS in the radio band was guided by high-level quantum chemical calculations performed at the coupled-cluster (CC) level of theory, primarily using the CC singles and doubles level augmented by a perturbative treatment of triple excitations, CCSD(T).8 All calculations were performed using the program suite CFOUR9 in combination with basis sets from Dunning’s hierarchies of correlation consistent polarized valence and polarized corevalence sets (cc-pVXZ, cc-pV(Xþd)Z, cc-pCVXZ, and cc-pwCVXZ with X = D, T, Q, 5, and 6).1012 A theoretical best estimate of the molecular structure (Figure 1) and the equilibrium rotational constant Be were obtained using additivity and extrapolation techniques13,14 in which the various contributions to the energies obtained with different basis sets were summed up and the leading contributions, that is, SCF and CCSD(T), were furthermore extrapolated to the basis-set limit. The best estimate is then given as fc-CCSD(T)/cc-pV¥Z þ ΔT/cc-pVTZ þ ΔQ/ cc-pVDZ þ Δcore/cc-pCV5Z with the frozen-core (fc) Received: March 18, 2011 Accepted: April 29, 2011 Published: May 05, 2011 1228
dx.doi.org/10.1021/jz200368x | J. Phys. Chem. Lett. 2011, 2, 1228–1231
The Journal of Physical Chemistry Letters
LETTER
Figure 1. Semiexperimental equilibrium structure of OSiS (bond lengths in angstroms) as determined in the present study and the best theoretical estimate (in italics).
Table 1. Experimental Transition Frequencies (in MHz) of OSiS and Its Isotopologuesa transition J0 J00
O29SiS
O30SiS
OSi34S
10
7460.9428
7448.0006
7435.4678
7429.4221
21
14921.8705
14895.9908
14870.9209
14498.8303
32 43
22382.7757 29843.6416
22343.9568 29791.8831
22306.3490 29741.7379
21748.2151 28997.5637
54
37304.4574 0
00
transition J J
a
OSiS
18
OSiS
10
7055.9870
21
14111.9632
32
21167.9159
43
28223.8345
54
35279.7092
18
O29SiS
14093.4148
18
O30SiS
14075.4178
18
OSi34S
13705.7963
Estimated experimental uncertainties (1σ) are 2 kHz.
CCSD(T) contribution extrapolated to the basis-set limit (indicated by cc-pV¥Z) and augmented by corrections for a full CC singles, doubles, triples (CCSDT)15 treatment (using the ccpVTZ basis), for the effect of quadruple excitations at the CC singles, doubles, triples, quadruples (CCSDTQ)16 level (using the cc-pVDZ basis), and for core-correlation effects treated at the CCSD(T)/cc-pCV5Z level. These best estimates have been shown to provide bond lengths and angles with an accuracy of better than 0.001 Å and 0.1.13 The zero-point vibrational correction ΔB0 to the rotational constant was computed at the fc-CCSD(T)/cc-pV(Qþd)Z level using a perturbative scheme as outlined in ref 17. The best estimate for the groundstate rotational constant, obtained using atomic masses, was then obtained from the relation B0 = Be þ ΔB0. A similar approach has been employed successfully in recent studies of H2SiS18 and HPSi.19 Electronic contributions ΔBel to the ground-state rotational constant20 calculated at the fc-MP2/cc-pVQZ level of theory were found to be on the order of 50 kHz only and hence negligible. OSiS exhibits the simple rotational spectrum of a linear closedshell molecule in its ground vibrational state with rotational lines separated by two times the rotational constant B. With a B value of ∼3.7 GHz, five rotational transitions are accessible in the 5 to 42 GHz frequency range of our FTMW spectrometer, and all five were observed for the parent isotopic species 16O28Si32S (Table 1), produced through a discharge of a gas mixture consisting of water, hydrogen sulfide, and silane highly diluted (0.2%) in neon. A sample spectrum showing a survey scan around the J = 21 transition at 14.9 GHz is shown in Figure 2. From this data set, the rotational constant and the quartic centrifugal distortion constant D were determined (Table 2) to high precision. The calculated B0 = 3727.9 MHz differs by
