Synchrotron infrared spectroscopy of the ν4, ν8, ν10, ν11 and ν14 fundamental bands of thiirane

Journal of Molecular Spectroscopy 316 (2015) 32–37

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Synchrotron infrared spectroscopy of the m4, m8, m10, m11 and m14 fundamental bands of thiirane Corey J. Evans a,⇑, Jason P. Carter a, Dominique R.T. Appadoo b, Andy Wong c, Don McNaughton c a

Department of Chemistry, University of Leicester, Leicester LE1 7RH, UK Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia c School of Chemistry, Monash University, Wellington Road, Clayton, Victoria 3800, Australia b

a r t i c l e

i n f o

Article history: Received 16 June 2015 In revised form 21 July 2015 Available online 5 August 2015 Keywords: Thiirane Synchrotron High-resolution spectroscopy Coriolis coupling Fundamental bands

a b s t r a c t The high-resolution spectrum of thiirane has been recorded using the far-infrared beamline at the Australian synchrotron facility. Spectra have been recorded between 700 cm1 and 1200 cm1 and ro-vibrational transitions associated with four fundamental bands of thiirane have been observed and assigned. The effects of Coriolis coupling were observed in the upper energy levels associated with the m4 (1024 cm1) and the m14 (1050 cm1) fundamental bands as well as in the m11 (825 cm1) and the m8 (895 cm1) fundamental bands. The m10 (945 cm1) fundamental band was also observed and was found to have no significant perturbations associated with it. For each of the observed bands rotational and centrifugal distortion constants have been evaluated, while for all but the m10 fundamental band, Coriolis interaction parameters have been determined for the upper states. The ground state constants have also been further refined. Crown Copyright Ó 2015 Published by Elsevier Inc. All rights reserved.

1. Introduction Thiirane is a cyclic molecule with the formula C2H4S. It is the sulfur analogue of oxirane (ethylene oxide, C2H4O), a molecule that has been extensively studied since its discovery in the interstellar medium [1]. Oxirane has been found to be of significant astrophysical importance, while thiirane can also be formed under similar interstellar conditions. It is suggested that oxirane and thiirane are formed in the presence of high-energy radiation, which upon interaction with oxygen or sulfur atoms, excites them from their atomic ground electronic state to a higher electronic state that then interacts with ethene to create a three-membered ring across the double bond [2,3]. Thiirane may find a use in astronomy as a diagnostic tool for comets; more specifically, it could be used as an indicator of comet temperature through the quantitative study of thiirane abundances in comet comae [3]. It has also been implicated as a precursor for prebiotic organosulfur compounds, meaning it may play an important role in the development of life elsewhere in the universe [4]. Due to its interstellar importance the high-resolution infrared spectrum of oxirane has been extensively studied, while thiirane has been less studied using high-resolution techniques. Thiirane has been under the spectroscopic looking glass for over 70 years. The first mid-infrared study was carried out by Thompson ⇑ Corresponding author. http://dx.doi.org/10.1016/j.jms.2015.07.010 0022-2852/Crown Copyright Ó 2015 Published by Elsevier Inc. All rights reserved.

and Dupré in 1940 [5]. In 1951, Thompson and Cave published further work on thiirane with an improved spectrometer resulting in re-assignment of observed bands and better determination of band positions [6]. In 1952, Guthrie et al. reported the observation of the infrared spectra of liquid thiirane and of the solid polymer [7]. Venkateswarlu and Joseph published work on the structure, the force constants and the Coriolis coupling coefficients associated with thiirane [8,9]. In 1986, Allen et al. carried out a comprehensive study of the low resolution (1 cm1) infrared and Raman spectra of thiirane and evaluated the force constants. A computational chemistry study on thiirane was also carried out by Allen et al. using different levels of theory to help in the assignment of the infrared and Raman spectra and to determine the structure of thiirane [10]. More recently Bane et al. published a high-resolution infrared study (0.0019 cm1) on the m5 and m15 fundamental vibrational modes of thiirane [11]. In this study they also re-evaluated the ground state spectroscopic constants of thiirane and carried out a computational chemistry investigation at the B3LYP/cc-pVTZ level of theory. A number of microwave spectroscopic studies have also been carried out in order to characterize the ground vibrational state of thiirane and its different isotopologues [12–16]. The two most pertinent studies for our investigation being those of Hirose et al. and Hirao et al. [17,18] Hirose et al. carried out a study on the microwave spectra of the ground vibrational state as well as the five lowest excited vibrational states of 12C2H32 4 S in addition to

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C.J. Evans et al. / Journal of Molecular Spectroscopy 316 (2015) 32–37

measuring lines from the ground vibrational state of the 13 12 12 C CH32 C2H34 4 S and 4 S isotopologues. They found that energy levels associated with four of the five excited vibrational states observed for 12C2H32 4 S were perturbed via Coriolis coupling [17]. Finally, in 2001, Hirao et al. carried out a study to determine the r0 structure of thiirane by recording the millimeter-wave spec12 12 trum, 340–360 GHz, of the 12C2H32 C2H33 C2H34 4 S, 4 S, 4 S and 13 12 C CH32 S isotopologues [18]. 4 In this investigation we have expanded on the far-infrared work of Bane et al. [11] to now include the mid-infrared region (1200– 700 cm1). The spectroscopic parameters associated with the m4, m8, m10, m11 and m14 fundamental vibrational modes of thiirane have been evaluated for the first time, while the ground state spectroscopic constants have been re-evaluated using a new set of ground state combination differences (GSCD) in conjunction with microwave transitions from the studies of Hirose et al. and Hirao et al. and the GSCD set from the study by Bane et al. [11,17,18]. 2. Experiment The experiments were performed on the THz/far-infrared beamline at the Australian synchrotron facility, with two regions of the spectrum recorded: 1200–880 cm1 (region one) and 880–700 cm1 (region two). Sample vapor of thiirane (98%, Sigma–Aldrich, Australia) was used without further purification and released in to a multi-pass White cell equipped with KBr windows and coupled to a Bruker IFS 125HR spectrometer capable of achieving a nominal resolution of 0.00096 cm1. The spectra were recorded with a KBr beamsplitter, a scan rate of 80 kHz and a liquid nitrogen cooled MCT detector. Spectra were taken with the sample pressure within the cell maintained at 150 and 500 mTorr for region one, while for region two a pressure of 200 mTorr was used. The multi-pass cell had an optical pathlength of 24 m and was at ambient temperature throughout the experiment (assumed to be 300 K for spectral simulations). For region one, 73 interferograms were recorded and then averaged, while for region two, 95 interferograms were recorded and averaged. Each interferogram was recorded with a nominal resolution of 0.00096 cm1 with each set averaged then Fourier transformed using a 4P apodization function and a post-zero fill factor of 8. All spectra were calibrated using the ethene line positions from the HITRAN database [19]. Geometry optimizations and subsequent vibrational analysis were performed at the B3LYP/aug-cc-pVTZ level of theory using Gaussian 09 [20]. These calculations determined structural parameters, rotational constants, centrifugal distortion constants and Coriolis coupling coefficients (f) as well as infrared intensities and the fundamental harmonic vibrational wavenumber values. Assigned lines were first fitted using PGopher [21] and then re-fitted using Pickett’s fitting program SPFIT [22], in order to compare results against the previous study by Bane et al. In all fits, Watson’s S-reduced Ir Hamiltonian was used [23]. Simulations were carried out using PGopher. The FWHM linewidth chosen was 0.0020 cm1, which is close to the predicted Doppler width; however, for the spectrum of m11 a linewidth of 0.0025 cm1 was used due to the poor signal-to-noise associated with the weakness of the band. Uncertainties in line positions were taken as 10% the FWHM values.

Fig. 1. Optimized structure (at the B3LYP/aug-cc-pVTZ level of theory) of thiirane with bond lengths in picometers.

3.1. Analysis of the m11 and m8 fundamental bands The only band located in region two is m11 which has B1 symmetry and results in a C-type band which is centered around 825.5 cm1. The m11 vibration can be described as a CH2 twisting mode and has a predicted intensity of only 0.4 km mole1. C-type bands have the selection rules of DKa = ±1; DKc = 0, ±2 and DJ = 0, ±1. The spectrum was overlaid on to the simulation generated by PGopher and this allowed a total of 2549 transitions to be assigned ranging between Ka = 0–31 and J = 3–53. Nuclear spin statistics as described previously were observed as clear intensity alternations throughout the band. All transitions were assigned an uncertainty of 0.00025 cm1 in the fitting procedure due to the poor signal-to-noise of the spectrum caused by the weakness of the band. These assignments were confirmed by GSCD generated from the known ground state constants. Intense features from ethene and possibly the m10 band of thiirane are seen throughout the R-branch side of the m11 band [19,24–26]. In the initial fits of the observed transitions of m11 it was apparent that the upper state energy levels associated with this band were perturbed due to the fitted centrifugal distortion terms being significantly different from those associated with the ground vibrational state. The mostly likely perturbing state being the m8 fundamental, assigned as a CH2 rocking mode, which has A2 symmetry and is infrared inactive. The band center for m8 has been estimated to be 895 ± 1 cm1 [10]. For molecules with C2v symmetry there can be the following Coriolis interactions: Interaction type

Interaction band symmetries

a-type b-type c-type

A1/A2 and B1/B2 A1/B1 and A2/B2 A1/B2 and A2/B1

An interaction between m8 and m11 would be a c-type Coriolis interaction. For a first-order c-type Coriolis interaction between the upper energy levels associated with the m11 and m8 fundamental bands the

3. Result and discussion

can be approximated using Eq. (1). Coriolis coupling term n11;8 c

The optimized structure of thiirane at the B3LYP/aug-cc-pVTZ level of theory is shown in Fig. 1. The point group of thiirane is C2v. Assuming an Ir representation, the nuclear spin statistics for the equivalent hydrogens are (in the form KaKc = weight): ee = 5, eo = 5, oe = 3, oo = 3. The axis system used was the same as that by Allen et al. [10].

n11;8 c

¼C

fc11;8

"

x8 x11

1=2



x11 þ x8

1=2 #

 2C  fc11;8

ð1Þ

At the B3LYP/aug-cc-pVTZ level of theory the Coriolis coupling coefficient (fc11;8 ) equals 0.851 which results in a Coriolis coupling term of n11;8  13600 MHz. It has been found from the previous c

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C.J. Evans et al. / Journal of Molecular Spectroscopy 316 (2015) 32–37

Table 1 Fitted spectroscopic parameters of the ground state, and the m11 and m8 fundamental bands of thiirane. Parameter 1

m0 (cm )

a b c d

Ground state

A (MHz) B (MHz) C (MHz) DJ (kHz) DJK (kHz) DK (kHz) d1 (kHz) d2 (kHz) HJ (Hz) HJK (Hz) HKJ (Hz) HK (Hz) h1 (mHz) h2 (mHz) h3 (mHz)

– 21973.63495(62) 10824.89248(25) 8026.24744(20) 7.05229(43) 13.7818(15) 16.4264(81) 2.192568(70) 0.428857(53) 0.00237(16) 0.19909(90) 0.6967(84) 0.516(13) 1.238(56) 0.406(47) 0.627(11)

nC11;8 (MHz) No. of lines r (RMS)

4917(IR)+239(mw) 1.051

Ground state (Calc)a

m11

– 21997.85 10566.26 7881.37 7.49 14.50 17.49 2.15 0.41 0.013 0.24 0.82 0.78 6.95 1.50 0.71

m8

825.470735(21) 21907.2065(49) 10824.4621(22) 8016.6401(30) 6.8562(23) 12.475(11) 17.5983(92) 2.30659(67) 0.43494(36) 0.00237c 0.19909c 0.6967c 0.516c 1.238c 0.406c 0.627c 13600d

b

893.9d 21990.869(52) 10790.079(22) 8020.802(22) 7.424(28) 14.40(30) 16.4264c 1.9662(88) 0.4279(60) 0.00237c 0.19909c 0.6967c 0.516c 1.238c 0.406c 0.627c

2549(IR)+42(mw) 1.059

40(mw)

Calculated at the B3LYP/aug-cc-pVTZ level of theory. Figures in brackets are one standard deviation according to the least squares fit in units of the least significant figure quoted. Fixed to the ground state value. Fixed during fit.

work on thiirane by Bane et al. that DFT functionals such as B3LYP with a suitable basis set can predict the zeta (f) values associated with interacting states of thiirane quite accurately [11]. Since m8 is infrared inactive re-analysis of the work by Hirose et al. was carried out to try and obtain spectral information on m8. Hirose et al. measured microwave transitions from a number of excited vibrational states of thiirane [17]. In this work, by using the fitted constants of the upper state, we could assign two set of transitions observed by Hirose et al. (me and md) as belonging to m11 and m10. The remaining set of unassigned transitions (mc) were assigned to m8. Inclusion of the microwave lines from m8 = 1 and m11 = 1 in addition to the first-order c-type Coriolis coupling term ) improved the fit significantly allowing the sextic centrifugal (n11;8 c distortion terms associated with upper energy levels of both bands to be fixed to the ground state values. The microwave lines were given a nominal uncertainty of 0.15 MHz. Eq. (2) shows the matrix elements associated with the c-type Coriolis interaction.

hJ; K 1jn11;8 jJ; Ki ¼ c

1 11;8 n ððJðJ þ 1Þ  KðK  1ÞÞ1=2 Þ 2 c

ð2Þ

Since no lines emanating from the m8 fundamental band were observed in the infrared spectrum there were some correlation and the m8 band origin problems associated with the fit and so n11;8 c were fixed in the fit. Firstly the Coriolis coupling term (n11;8 ) was c fixed to the calculated value of 13,600 MHz. Next the m8 band origin was manually changed and subsequent fits carried out until the RMS error of the fit was minimized. The final constants are given in Table 1. The error associated with n11;8 is estimated to be ±500 MHz c and the error associated with the m8 band origin is estimated to be ±1 cm1. Two microwave lines (24213.98 MHz and 16348.04 MHz) associated with m8 = 1 were weighted out of the initial fits as they gave obs-calc values of >2 MHz. Subsequent analysis showed the transition at 24213.98 MHz belongs to the ground state and should be assigned as 5919,40–5919,41. Based on predictions the transition at 16348.04 MHz most likely belongs to m5 = 1 or m15 = 1. Fig. 2 shows the observed band with an expanded section showing how well the simulation based on the fitted parameters models the band. This fit and the results from the B3LYP/aug-cc-pVTZ calculation are available as Supplementary material.

3.2. Analysis of the m10 fundamental band The first band studied in region one was the m10 fundamental band, which is centered at 945 cm1. This band is a C-type band with B1 symmetry and the vibration is assigned as a CH2 rocking mode. A total of 2897 infrared transitions ranging between Ka = 0–31 and J = 0–45 were assigned to this band. Initial fits showed the centrifugal distortion constants are in very good agreement with the ground state values, indicating that this band is not significantly perturbed. As this spectrum had significantly better signal-to-noise than that of the m11 fundamental band, each observed transition was assigned an uncertainty of 0.0002 cm1, which is more in line with the experimental uncertainty. As previously mentioned it was found the microwave transitions associated with m10 = 1 match those assigned to md in the study by Hirose et al. [17]. A final fit was made including the 41 microwave transitions and the fitted parameters can be found in Table 2 Fitted spectroscopic parameters of the m10, m4 and m14 fundamental bands of thiirane. Parameter

m10

m4

m14

m0 (cm1)

945.769563(15)a 21993.0979(62) 10779.0778(16) 7989.6486(20) 6.8640(11) 14.457(11) 16.094(18) 2.14073(56) 0.40367(37) 0.002371b 0.0497(78) 0.563(19) 0.517(18) 1.238b 0.406b 0.6267b – 2897(IR)+41(mw) 0.998

1024.227651(25) 21975.583(15) 10843.7006(49) 8010.9095(28) 7.2082(20) 12.488(38) 19.911(87) 2.3148(13) 0.46768(50) 0.002371b 0.299(14) 0.6967b 0.427(56) 1.238b 0.406b 0.6267b 35.6250(18) 899(IR) 0.950

1051.2177960(90) 21994.9438(38) 10809.6245(15) 8005.2572(13) 7.02807(40) 13.8693(55) 16.9911(95) 2.21040(58) 0.43979(23) 0.002371b 0.1657(21) 0.5849(73) 0.4274(71) 1.76(13) 0.555(85) 0.6267b

A (MHz) B (MHz) C (MHz) DJ (kHz) DJK (kHz) DK (kHz) d1 (kHz) d2 (kHz) HJ (Hz) HJK (Hz) HKJ (Hz) HK (Hz) h1 (mHz) h2 (mHz) h3 (mHz) Fab (MHz) No. of Lines r (RMS)

7197(IR)

a Figures in brackets are one standard deviation according to the least squares fit in units of the least significant figure quoted. b Fixed to the ground state value.

C.J. Evans et al. / Journal of Molecular Spectroscopy 316 (2015) 32–37

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Fig. 2. (a) The m11 region. The intense features between 860 to 830 cm1 belong to ethene. (b) Subsection of m11 showing the observed spectrum (black) with the simulation based on fitted parameters (red). The simulation was calculated at 300 K with a Gaussian linewidth of 0.0025 cm1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. (a) The m10 region. The intense feature at 950 cm1 belong to ethene. (b) Subsection of m10 showing the observed spectrum (black) with the simulation based on fitted parameters (red). The simulation was calculated at 300 K with a Gaussian linewidth of 0.0020 cm1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 2, again the microwave lines were given a nominal uncertainty of 0.15 MHz. A portion of the fitted spectrum can be seen in Fig. 3, along with the simulated spectrum and it shows there is good agreement between the two. As seen in the m11 fundamental band, intense features from ethene (emanating from the m7 band) can be observed throughout the band, in particular on the R-branch side [19,24–26]. The presence of ethene in the recorded spectra is not unusual and it has been previously observed in experiments involving photolysis and thermolysis of thiirane [27–30]. If not a contaminant within the sample (which is unlikely), then the most likely generation of ethene would be via an infrared multiphoton dissociation (IRMPD) process; however, further work is needed to investigate this phenomenon. The fortuitous presence of ethene in the sample meant that it could be used as a calibrant. The detailed fit of m10 is available as Supplementary material.

3.3. Analysis of the m14 and m4 fundamental bands The next bands studied in region one were the m4 and m14 fundamental bands that are situated between 1000 and 1100 cm1. The m14 vibration centered at 1051 cm1 is assigned as a CH2 wagging mode, and has B2 symmetry and so has a B-type appearance. It dominates the region having a predicted intensity of 22.5 km mole1, which is significantly more intense than any other band in the region and so the assignment of m14 was undertaken first. B-type bands have the selection rules of DKa = ±1, DKc = ±1 and DJ = 0, ±1. It was found early on in the fitting process that some type of perturbation was affecting the upper energy levels associated with this band. Since the m4 fundamental band is centered only 25 cm1 to the red (1024 cm1) this was thought to be the most likely source of the perturbation. The m4 vibration is assigned as a CH2 wagging mode and has A1 symmetry

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Fig. 4. (a) The m14 and m4 region. The arrow indicates position of the m4 Q-branch. (b) Subsection of m14/m4 showing the observed spectrum (black) with the simulation based on fitted parameters (m14 – red and m4 – blue). The simulation was calculated at 300 K and with a Gaussian linewidth of 0.0020 cm1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 3 Vibration assignment of thiirane (C2H4S).

hJ; K 1jFab jJ; Ki ¼

Mode (symmetry)

Wavenumber values (cm1) Observed

Calculatedd

Intensity (km mole1)

m1 (A1) m2 (A1) m3 (A1) m4 (A1) m5 (A1) m6 (A2) m7 (A2) m8 (A2) m9 (B1) m10 (B1) m11 (B1) m12 (B2) m13 (B2) m14 (B2) m15 (B2)

3013.5a 1456.8a 1109.9a 1024.228c 628.140b   895 ± 1a 3088.0a 945.769c 825.471c 3013.0a 1435.9a 1051.218c 669.740b

3023.9 1448.5 1104.3 1011.4 600.0 3097.7 1161.4 871.3 3111.8 923.4 809.2 3023.2 1427.9 1044.3 638.3

16.0 2.6 1.6 0.7 25.5 0 0 0 3.4 3.3 0.4 11.7 0.9 22.5 0.4

Mode description

C–H stretch CH2 scissor C–C stretch CH2 wag C–S stretch C–H stretch CH2 twist CH2 rock C–H stretch CH2 rock CH2 twist C–H stretch CH2 scissor CH2 wag C–S stretch

1 F ab ðJðJ þ 1Þ  KðK  1ÞÞ1=2 ð2K  1Þ 2

ð3Þ

Overall a total of 7197 lines (Ka = 0–35 and J = 0–61) were assigned to m14 and 899 lines (Ka = 0–34 and J = 3–45) were assigned to m4 with most lines from m4 coming from the P-branch side of the band. The wavenumber value of each observed transition was assigned an uncertainty of 0.0002 cm1. The fitted parameters can be seen in Table 2. The only issue with the fit is that some of sextic centrifugal distortion parameters have changed sign compared to their ground state values. This could be a result of only having a small spectral data set associated with m4. A portion of the fitted spectrum can be seen in Fig. 4 and it clearly shows that lines from both the m14 and m4 fundamental bands have been modeled well. The final fit is available as Supplementary material. Table 3 lists the updated band centers for the fundamental vibrational modes of thiirane along with the results from the B3LYP/aug-cc-pVTZ calculation.

a

Allen et al. Bane et al. c This work. d This work. B3LYP/aug-cc-pVTZ level of theory. Harmonic wavenumber values are scaled by 0.968. b

thus an A-type appearance. The m4 vibration has a predicted intensity of only 0.7 km mole1 and the selection rules for an A-type band are DKa = 0, ±2; DKc = ±1 and DJ = 0, ±1. Calculations at the B3LYP/aug-cc-pVTZ level of theory show that for a first-order c-type Coriolis interaction (where fc14;4 = 0.028) between the upper energy levels of m4 and m14 is approximately equal to 441 MHz. Inclusion of the first-order Coriolis term using ¼ 441 MHz) did not help lower the overall standard devia(n14;4 c tion of the initial fits; however, the small value of the Coriolis term makes sense given the nature of vibrations involved. A second-order Coriolis term, Fab or gab, was then included and with careful increasing of its value and subsequent fits it was found to be the dominant interaction with some obvious intensity stealing/borrowing affecting the m14 and m4 bands which can be simulated accurately using PGopher. Eq. (3) shows the matrix elements used for the second-order Coriolis term.

4. Ground state A number of previous studies have evaluated the ground state constants of thiirane. The first comprehensive analysis was carried out by Hirao et al. in 2001 in which they fitted 226 transitions to Watson’s A-reduced Hamiltonian and evaluated parameters up to and including the sextic centrifugal distortion constants except for HJ which was fixed to zero [18]. In the study by Bane et al., the ground state constants were revaluated using a combination of current microwave measurements (from Hirose et al. and Hirao et al.) and the GSCD obtained from the observed transitions associated with the m5 and m15 bands [11,17,18]. The analysis by Bane et al. used Watson’s S-reduced Hamiltonian and they also evaluated up to and including the sextic centrifugal distortion constants except for HK which they fixed to zero. In this study we have taken the infrared transitions from this work and the study by Bane et al. and have produced a new set of GSCD. This new set of 4917 far-infrared transitions along with the 239 microwave transitions from the work of Hirose et al. and Hirao et al. have resulted in a refined set of ground state parameters which includes all the sextic centrifugal distortion constants. The far-infrared transitions obtained from the GSCD were given a nominally uncertainty of 0.00028 cm1 and include A, B and

C.J. Evans et al. / Journal of Molecular Spectroscopy 316 (2015) 32–37

C-type transitions. The fitted parameters can be seen in Table 1. The resulting fit has a RMS deviation of 1.05 and the parameters are in close agreement with work by Bane et al.; however, both HJ and HK have been evaluated for the first time in the same fit. As found by Bane et al. four microwave transitions were omitted from the initial fit and they corresponded to the transitions 12728.23 MHz (144,11–152,14), 13888.85 MHz (95,5–103,8), 17364.65 MHz (141,13–133,10) and 18403.57 MHz (173,15–165,12) which were all recorded by Hirose et al. [17]. Using the fitted data as a guide the transition at 12728.23 MHz is most likely wrongly assigned and should be assigned as 6120,41–6120,42, the transition at 13888.85 MHz can be assigned as 52,3–52,4 for m5 = 1, while the transition at 17364.65 MHz is actually 114,7–114,8 for m10 = 1. The transition at 18403.57 MHz could not be assigned to any particular state belonging to thiirane and so was not included in the final fit of the ground state. These new ground state parameters were then used for all subsequent fits. The determined ground state parameters compare extremely well with the constants calculated at the B3LYP/aug-cc-pVTZ level of theory which are also list in Table 1. The agreement with the quartic and sextic centrifugal distortion constants is particularly good and shows that the calculation is modeling the system well. Detailed results from this re-analysis can be found in Supplementary material. 5. Conclusions Ro-vibrational transition from four fundamental bands of thiirane have been observed, assigned and fitted to determine spectroscopic parameters, including rotational constants and centrifugal distortion constants. Second-order c-type Coriolis coupling (Fab) was observed between the upper energy levels of the m4 and the m14 fundamental bands, while a strong first-order ) was observed between the upper energy Coriolis interaction (n11;8 c levels of m11 and the infrared inactive band m8. The results from the analysis of these perturbations were in line with the f values calculated at the B3LYP/aug-cc-pVTZ level of theory. The m10 band was found to be free of perturbations. Microwave transitions from the excited states of the m8, m11 and m10 vibrational modes of thiirane were also included in the analysis. The ground state parameters have been further refined from the previous work by Bane et al. and Hirao et al., which involved increasing the number of GSCD transitions in the fit and the evaluation of both HJ and HK for first time in the same fit. Acknowledgment We thank the Australian Synchrotron for the allocation of beamtime for this project. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jms.2015.07.010.

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