Microwave Detection of Sulfoxylic Acid (HOSOH) - The Journal of

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Microwave Detection of Sulfoxylic Acid (HOSOH) Kyle N. Crabtree, Oscar Martinez, Lou Barreau, Sven Thorwirth, and Michael C. McCarthy J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp400742q • Publication Date (Web): 28 Mar 2013 Downloaded from http://pubs.acs.org on April 6, 2013

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The Journal of Physical Chemistry

Microwave Detection of Sulfoxylic Acid (HOSOH) Kyle N. Crabtree,† Oscar Martinez Jr.,† Lou Barreau,†,¶ Sven Thorwirth,‡ and Michael C. McCarthy∗,† Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA, and I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany E-mail: [email protected]

∗

To whom correspondence should be addressed Harvard-Smithsonian Center for Astrophysics ‡ Universität zu Köln ¶ Current address: École Normale Supérieure de Cachan, 61 Avenue du Président Wilson, 94235 Cachan Cedex, France †

1 ACS Paragon Plus Environment


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Abstract Sulfoxylic acid (HOSOH), a chemical intermediate roughly midway along the path between highly-reduced (H2 S) and highly-oxidized sulfur (H2 SO4 ), has been detected using Fourier transform microwave spectroscopy and double resonance techniques, guided by new high-level CCSD(T) quantum-chemical calculations of its molecular structure. Rotational spectra of the two most stable isomers of HOSOH, the putative ground state with C2 symmetry and the low-lying Cs rotamer, have been measured to high precision up to 71 GHz, allowing accurate spectroscopic parameters to be derived for both isomers. HOSOH may play a role in atmospheric and interstellar chemistry, and the present work provides the essential data to enable remote sensing and/or radioastronomical searches for these species. Spectroscopic characterization of HOSOH suggests that other transient intermediates in the oxidation of SO2 to H2 SO4 may be amenable to laboratory detection as well.

Keywords: rotational spectroscopy, electronic structure, atmospheric chemistry, double resonance, coupled-cluster calculations

1

Introduction

Emissions of sulfur-containing gases from both natural and anthropogenic sources significantly impact the chemistry of the atmosphere. Sulfur emissions in the form of H2 S are eventually oxidized to SO2 ; subsequent chemical reactions yield H2 SO4 , an important nucleation agent with a profound influence on weather and climate. 1 While atmospheric oxidation of H2 S is generally thought to proceed via well-characterized reactions with OH radicals, 2 much less attention has been paid to possible reactions of H2 S with O2 , 3 which occur on the H2 SO2 potential energy surface and may involve formation of sulfoxylic acid and its isomers. H2 SO2 is also intrinsically interesting because it lies roughly midway between highly-reduced and highly-oxidized sulfur, and because it is one oxygen atom removed from sulfurous acid


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(H2 SO3 ), an elusive and unstable species similar to carbonic acid which has not been spectroscopically detected in the gas phase (this has been called “one of the greatest open challenges of inorganic chemistry” 4 ). In addition to its potential role in atmospheric chemistry, H2 SO2 may also play a role in interstellar chemistry. This species was recently detected via mass spectrometry as a photolysis product after UV irradiation and warm-up of an interstellar H2 S:H2 O ice analog. 5 Icy grain mantles, including H2 S ice, 6 have been invoked as a reservoir for sulfur, an element with a rich chemistry whose gas-phase abundance is depleted relative to its cosmic abundance in cold molecular clouds by about three orders of magnitude. 7 Because both H2 S and H2 O have been detected in comets, ice mixtures containing both species may plausibly exist in astronomical environments, as comet compositions reflect those of star-forming regions. 8 H2 SO2 might therefore be formed and liberated into the gas phase in sources that have UV flux and heating. Detection of H2 SO2 would aid in elucidating the poorly-understood chemistry of sulfur-bearing molecules in space. 9 H2 SO2 and its isomers have been extensively studied by ab initio theory. 3,10–14 The consensus that has emerged from these calculations is that the lowest-energy structure is sulfoxylic acid (HOSOH) with C2 symmetry, followed by another sulfoxylic acid rotamer with Cs symmetry, which lies only slightly higher in energy (∼1.3 kcal mol−1 ). Other structures are calculated to be minima on the potential energy surface, but are considerably higher in energy; several of these are illustrated in Figure 1. While the energy ordering of the isomers is generally the same from different calculations, there is still some uncertainty regarding their absolute energies relative to the global minimum. 14 Although it has been shown to be stable in the gas phase, 15,16 spectroscopic studies of isomers of H2 SO2 are scarce. The only spectroscopic data available to date on any isomer is a matrix-isolation infrared study by Fender et al., 17 who produced H2 SO2 via UV photolysis in a solid Ar matrix containing SO2 and H2 S. The authors assigned eight observed bands to vibrational modes of sulfinic acid (HS(−O)OH), but these assignments have been called into



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Figure 1: Structures of several isomeric forms of H2 SO2 . The isomers are ordered vertically in rough order of increasing energy. Structural parameters for the two rotamers of sulfoxylic acid are listed in Table 1.


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question on the basis of quantum-chemical calculations. 11,12 A recent examination 14 of the theoretical and experimental data suggests that only four of the observed bands of Fender et al. can be reasonably assigned to sulfinic acid, though matrix effects and anharmonicity have not been thoroughly investigated. In this paper, we present the detection of the rotational spectrum of the C2 and Cs rotamers of sulfoxylic acid (HOSOH) using a combination of Fourier transform microwave (FTM) spectroscopy and microwave-microwave double resonance (DR) techniques, which were guided by quantum-chemical calculations. The paper consists of (i) new coupled cluster calculations of the geometrical structures and dipole moments of the most stable isomers of H2 SO2 (Section 2); (ii) the experimental techniques employed for the detection of the newly found HOSOH rotational spectra (Section 3); and (iii) discussion of the spectroscopic data and related experiments that might be fruitfully undertaken now that ground state rotamers of HOSOH have been spectroscopically characterized for the first time (Section 4).

2

Quantum-chemical calculations

High-level quantum-chemical calculations were performed at the coupled-cluster (CC) level using the CC singles and doubles level augmented by a perturbative treatment of triple excitations, CCSD(T). 18 Using the molecular structures reported by Napolion et al. 14 as starting geometries, all calculations were performed using the CFOUR program package 19,20 in combination with Dunning’s hierarchies of correlation consistent polarized valence and polarized core-valence sets: in the frozen-core approach, the d-augmented correlation consistent basis set cc-pV(T+d)Z was used for the sulfur atom along with standard basis sets cc-pVTZ for hydrogen and oxygen (denoted CCSD(T)/cc-pV(T+d)Z in the following). When considering all electrons in the correlation treatment, the cc-pwCVXZ (X = T and Q) basis sets 21 were used. The best equilibrium structures of the Cs and C2 forms of HOSOH have been calculated at



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the CCSD(T)/cc-pwCVQZ level of theory (Table 1), which has been shown on several occasions to yield equilibrium structures of very high quality for molecules harboring second-row elements. 22–25 Harmonic and anharmonic force fields were calculated in the frozen-core approximation at the CCSD(T)/cc-pV(T+d)Z level of theory using analytic second-derivative techniques 26,27 followed by additional numerical differentiation to calculate the third and fourth derivatives needed for the anharmonic force fields. 27,28 Theoretical ground state rotational constants were then obtained from the relation B0 = Be − ∆B0 with similar equations for the A0 and C0 rotational constants. Here, the equilibrium rotational constants Be were obtained from the CCSD(T)/cc-pwCVQZ equilibrium structures and corrected for the effects of zero-point vibration calculated at the CCSD(T)/cc-pV(T+d)Z level of theory. In addition, the CCSD(T)/cc-pV(T+d)Z force field calculation yields the quartic centrifugal distortion parameters. The calculated spectroscopic parameters are summarized in Table 2. Table 1: Structural Parameters of Sulfoxylic Acid Quantity

C2 HOSOH

r(H−O) 0.9622 r(O−S) 1.6364 ̸ (HOS) 108.14 ̸ (OSO) 103.28 τ (HOSO) 84.34 µb µc

0.6 0.0

Cs HOSOH units 0.9616 1.6367 108.59 103.64 ±90.56

Å Å deg. deg. deg.

0.6 2.8

D D

Equilibrium structures are calculated at the ae-CCSD(T)/cc-pwCVQZ level of theory.

3

Experimental details

Rotational spectra were measured by use of a Balle-Flygare type 29 FTM spectrometer, which has been described previously. 30,31 HOSOH was produced in an electrical discharge in the throat of a pulsed supersonic expansion source, which was fed by a gas mixture of Ne, H2 , and SO2 in a 100:20:1 ratio at a stagnation pressure of 2.5 kTorr. The gas pulse duration 6 ACS Paragon Plus Environment

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Table 2: Spectroscopic constants for HOSOH (in MHz). C2 HOSOH — Calculated Constant A0 B0 C0 103 DJ 102 DJK DK 103 d1 104 d2 Constant A0 B0 C0 103 DJ 102 DJK DK 103 d1 104 d2

Experiment

Equilibrium

26 164.3069(35) 8 590.5274(24) 6 713.1594(12) 8.79(12) −5.13(13) 0.336a −2.884a −1.77a

26 362.0 8 617.0 6 761.7 8.65 −5.12 0.336 −2.884 −1.77

Experiment

Equilibrium

26 517.9847(12) 26 720.6 8 508.7351(8) 8 534.1 6 683.8531(6) 6 731.5 8.549(33) 8.244 −4.844(14) −4.833 0.34709(23) 0.3384 −2.768(25) −2.693 −2.6(6) −1.7

Vib. Contribution Ground vib. state +174.9 26 187.1 +26.2 8 590.8 +46.1 6 715.6 ··· ··· ··· ··· ··· ··· ··· ··· ··· ··· Cs HOSOH — Calculated Vib. Contribution Ground vib. state +165.6 +29.0 +46.7 ··· ··· ··· ··· ···

26 555.0 8 505.1 6 684.8 ··· ··· ··· ··· ···

Experimental values are given with 1σ uncertainties in units of the last decimal place. Equilibrium rotational constants are from CCSD(T)/cc-pwCVQZ equilibrium structures, centrifugal distortion constants and effects of zero-point vibration calculated at the CCSD(T)/cc-pV(T+d)Z level of theory. a Constrained to calculated value.



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(280-320 µs, equivalent to a flow of ∼20 cm−3 min−1 at standard temperature and pressure) and discharge voltage (1.0-1.4 kV) of the source were set on any given day by optimizing the intensity of one or more of the following: initially, the 10,1 − 00,0 transition of HOSO at 17137 MHz, 32 and subsequently the 20,2 − 11,0 transition of Cs sulfoxylic acid at 10419 MHz, or the 11,0 − 10,1 transition of C2 sulfoxylic acid at 19451 MHz. The FTM spectrometer has an operating range of roughly 5-43 GHz; transitions at higher frequencies were measured by means of microwave-microwave DR spectroscopy. 24,33,34 In our DR experiments, the frequency of the FTM spectrometer is tuned to observe a specific transition of the species of interest; this transition frequency acts as the probe. Then, during the free induction decay of the FTM signal, the molecular beam is irradiated by a second radiation source oriented perpendicular to both the molecular beam and cavity axis, which are collinear in our instrument; this transition frequency acts as the pump. Pump radiation is produced by a synthesizer in the 0.5-26 GHz frequency range; higher frequencies are generated by doubling (26-40 GHz), quadrupling (40-65 GHz), or octupling (60-90 GHz) the synthesizer output, followed by additional amplification. If the transitions resonant with the pump and probe radiation share a common state, the FTM signal intensity is either enhanced or depleted, depending on the exact transitions chosen and the properties of the beam. 35 In a typical experiment, the FTM free induction decay is averaged for a predetermined number of discharge pulses, and its Fourier transform is integrated over the frequency range containing the probe signal. Then, the pump radiation source is tuned in frequency, and the process repeated until the entire line profile is observed. Once a line is detected by this method, the power of the pump radiation is adjusted and the line remeasured to ensure that the lineshape is not flattened due to oversaturation. A Gaussian profile is then fit to the resultant spectrum; a typical DR measurement is shown in Figure 2. In addition to measurements above the frequency range of the FTM instrument, we also used DR to identify lines sharing a common state, which was crucial in the initial stages of


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1600 1400 20,2 - 11,0 Signal

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1200 1000 800 600

Cs HOSOH 31,2 - 20,2

400 200 70018.0 70018.5 70019.0 70019.5 70020.0 70020.5 Frequency (MHz)

Figure 2: The 31,2 − 20,2 transition of Cs HOSOH at 70019.2507 MHz as measured by DR spectroscopy. The probe frequency for this measurement was that of the 20,2 − 11,0 transition at 10419.2650 MHz. The red dots are the integrated intensity of the FTM signal as the pump frequency is tuned (see text), and the black line is a fit of the profile using a Gaussian lineshape. the spectroscopic investigation to facilitate assignment. Discharge sources generally produce a rich chemistry, which results in a large number of species with many rotational lines; correctly identifying the molecular carrier of an unidentified line requires special care. For each of the two isomers detected here, we began by conducting spectral surveys for a pair of transitions sharing a common state; each search covered roughly ±0.5% around the predicted transition frequency from the quantum-chemical calculations. As an example, for C2 sulfoxylic acid, surveys were conducted for the 11,1 − 00,0 transition calculated near 32902 MHz and the 20,2 − 11,1 transition near 12874 MHz. In each survey, ∼10-15 lines were found; these were screened according to whether they persisted after turning off the discharge, and also after removal of H2 from the source gas mixture. Roughly 50% of the lines in each survey were eliminated from further consideration by these tests. The remaining lines from the two surveys were then cross-screened by DR to establish if any shared a common state; an example of such a match is shown in Figure 3. A third transition was then identified using the same procedure, and the rotational constants A0 , B0 , and C0 were fit by least-squares to reproduce the measured transition frequencies. The remainder of



the spectroscopy proceeded by iteratively measuring transitions and refining spectroscopic parameters, occasionally linking lines by DR to avoid misassignment/misidentification.

C2 HOSOH 20,2 - 11,1

No DR DR (11,1 - 00,0)

FTM Signal

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-0.2

-0.1 0.0 0.1 Frequency offset (MHz)

0.2

Figure 3: Microwave-microwave DR spectroscopy of C2 HOSOH. Shown here is the FTM spectrum of the 20,2 − 11,1 transition at 12891.0254 MHz with (red) and without (black) pump radiation at the frequency of the 11,1 − 00,0 transition (32877.2050 MHz).

4

Results and discussion

The quantum-chemical calculations find both the C2 and Cs forms of sulfoxylic acid to be near-prolate asymmetric rotors (κ = −0.807 and −0.817, respectively). Although both forms have a b-type spectrum, the Cs isomer possesses a much stronger c-type spectrum (see Table 1). The observed intensities of the b-type spectra of both isomers were similar, suggesting they are nearly equally abundant in the supersonic beam. Transition frequencies were fit by a Watson S-reduced effective Hamiltonian 36 using the SPFIT program. 37 Derived spectroscopic constants are shown in Table 2, and their values agree remarkably well in all cases with the values from quantum-chemical calculations (most within 1%). Each isomer is discussed in more detail in the following sections.


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4.1

C2 HOSOH

Once the initial three transitions (11,1 − 00,0 , 20,2 − 11,1 , and 21,1 − 20,2 ) were assigned and preliminary rotational constants derived, the spectrum was simulated again, yielding more refined predictions for unmeasured transitions. As additional transitions were measured and assigned, distortion terms– initially constrained to their quantum-chemical values– were allowed to vary. A total of seven transitions of C2 HOSOH were measured and assigned; these are listed in Table 3 and range up to J = 4 and Ka = 1. In all, five parameters were allowed to float (A0 , B0 , C0 , DJ , and DJK ), while the remaining three (DK , d1 , and d2 ) were held fixed. The energy levels inferred from the effective Hamiltonian and the measured transitions are shown in Figure 4. Table 3: Measured rotational transitions of C2 HOSOH

line no.

′ ′′ JK ′ ′ − JK ′′ ,K ′′ a ,Kc a c

1 2 3 4 5 6 7

20,2 − 11,1 11,0 − 10,1 21,1 − 20,2 31,2 − 30,3 30,3 − 21,2 41,3 − 40,4 11,1 − 00,0

DR linksa (line no.) 3, 7 1 5 4 1

Frequency (MHz) 12 19 21 24 29 29 32

891.0254 450.9030 470.9035 758.8445 506.5050 584.8250 877.2050 rms

obs. - calc.b (kHz) 3.9 0.0 7.1 5.8 −1.0 −5.3 −4.8 4.6

All transitions in this table were measured with FTM spectroscopy, and are estimated to have an uncertainty of 2 kHz. a Other transitions to which the indicated line has been linked with DR. b Calculated frequencies are from a fit of Watson’s S-reduced effective Hamiltonian to the experimental data. The spectroscopic constants are listed in Table 2.

The rms deviation of our best fit is 4.6 kHz, slightly more than twice the estimated uncertainty of the measurements. To improve the predictive power of the present effective Hamiltonian model, it would be desirable to measure one or more transitions involving a state with Ka = 2. However, our initial attempts were unsuccessful, either by DR or FTM spectroscopy. A number of lines were detected in these searches, but we were unable to



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Figure 4: Measured rotational transitions of C2 sulfoxylic acid and energy levels inferred from the effective Hamiltonian using the spectroscopic constants listed in Table 2. All transitions were measured with FTM spectroscopy. definitively assign any of them to C2 HOSOH.

4.2

Cs HOSOH

Owing to the strong c-type spectrum for the Cs isomer, many more lines have been detected compared to the C2 isomer. In total, 23 rotational transitions were measured (Table 4); of the observed transitions, 13 were measured with FTM spectroscopy, and the other 10 with DR. The total rms of the deviations between fitted frequencies and measured frequencies is 6.9 kHz, but this number cannot be directly compared with the FTM uncertainty as it contains DR measurements, which have greater uncertainty (discussed further below). Breaking the data into subsets, the rms of the FTM set is ∼1 kHz, and that of the DR set ∼10 kHz. The energy levels inferred from the fit are shown in Figure 5. To critically assess the accuracy of the present DR measurements, we measured the same transition (the 11,0 − 00,0 transition of Cs sulfoxylic acid) with both FTM and DR spectroscopy, and compared the frequencies. Because the molecular beam travels with a velocity ∼850 m/s as measured from the Doppler splitting in the FTM spectra, a systematic Doppler shift, possibly as large as 10 kHz, could arise if the DR radiation is not strictly 12 ACS Paragon Plus Environment

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Table 4: Measured rotational transitions of Cs HOSOH

line no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

′ JK ′ ′ a ,Kc

−

′′ JK ′′ ′′ a ,Kc

20,2 − 11,0 20,2 − 11,1 31,3 − 30,3 21,2 − 20,2 22,1 − 31,3 11,1 − 10,1 11,0 − 10,1 21,1 − 20,2 30,3 − 21,1 31,2 − 30,3 30,3 − 21,2 11,1 − 00,0 11,0 − 00,0 21,2 − 10,1 32,2 − 31,2 21,1 − 10,1 22,1 − 21,1 22,0 − 21,1 22,1 − 21,2 22,0 − 21,2 32,1 − 31,3 31,2 − 20,2 41,4 − 30,3

type c b c c c c b b c b b b c b c c c b b c c c b

DR linksa (line no.) 13, 22 12 21, 23 14, 19, 20 12 16, 17, 18 15 2, 6 1 4 10 8 8 8 4 4 3 1 3

10 12 14 16 16 18 19 21 23 24 28 33 35 46 51 52 54 54 59 59 62 70 70

Frequency (MHz)

obs. - calc.b (kHz)

419.2650 244.1259 022.3698 316.1779 737.4408 009.0127 833.8696 790.6259 263.7126 964.8986 738.1594 201.5631 026.4240 569.3136(72) 372.9308(104) 043.7577(60) 023.5012(181) 155.2862(97) 497.9741(131) 629.7053(86) 969.6753(99) 019.2507(144) 895.3361(96)

0.0 1.2 1.6 −0.7 0.9 2.3 −0.6 0.9 0.1 −0.3 0.5 −1.3 −0.2 0.1 −8.3 −2.4 −19.4 10.8 7.1 −16.5 −1.4 14.3 −1.9

Total rms FTM rms DR rms

7.0 1.0 10.3

All transitions with frequencies below 40 GHz were measured with FTM spectroscopy, and are estimated to have an uncertainty of 2 kHz. The remaining transitions were measured with DR, and their 1σ uncertainties are listed in units of the last digit. a Other transitions to which the indicated line has been linked with DR. b Calculated frequencies are from a fit of Watson’s S-reduced effective Hamiltonian to the experimental data. The spectroscopic constants are listed in Table 2.



150

DR FTM

32,1 32,2

41,4

22,1 22,0

31,2 E/h (GHz)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

100

31,3 30,3

50 20,2 0

21,1 21,2 11,0 11,1

10,1 00,0

Cs HOSOH

0

1 Ka

2

Figure 5: Measured rotational transitions of Cs sulfoxylic acid and energy levels inferred from the effective Hamiltonian, using the spectroscopic constants listed in Table 2. Transitions measured with FTM spectroscopy are indicated by red solid lines, and those measured with DR by blue dashed lines. perpendicular to the molecular beam. The DR measurement was repeated a total of 20 times; after every fifth measurement, the pump radiation angle was adjusted to test for any systematic Doppler shifts with beam direction (none were observed). An average frequency of 35026.4212 ± 0.0061 MHz was derived, which differs from the FTM measurement by only 2.8 kHz. The maximum absolute deviation for any given measurement was 12 kHz, and the uncertainty of the line center frequency from the Gaussian fit ranged from 10-20 kHz. Thus, any systematic Doppler shift arising from the precise orientation of the pump radiation relative to the molecuar beam is well below the limiting uncertainty for DR measurements, which arises from the Gaussian fit to the DR line profile.

4.3

Discussion

Using the spectroscopic data presented here, remote sensing of sulfoxylic acid is now possible in the radio band. A particularly intriguing prospect is a radioastronomical search for these molecules in star-forming regions where H2 S and H2 O ices are expected to be present. A very recent chemical model proposes that polysulfanes (H2 Sn , n = 2 − 8) may be refractory reser-


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voirs of sulfur on interstellar grains, and these would be formed from UV or other processing of H2 S ice. 38 These models do not include HOSOH, but based on the experimental results of Jimenez-Escobar et al., 5 it would likely be expected to form under the same conditions in the presence of H2 O ice. Moreover, sulfoxylic acid is isoelectronic with H2 S3 (HSSSH vs. HOSOH) and if formed, may be liberated into the gas phase. The rotational spectra of HSSSH 39 and the structurally-similar HSSOH 40,41 have also been measured, and detection of any of these species would provide strong support for the validity of the aforementioned chemical model. Another avenue for future studies is an investigation of the formation mechanism of HOSOH in the discharge source. By forming isotopologs of HOSOH using mixtures of H2 and D2 , it may be possible to determine whether the formation involves sequential hydrogenation of SO2 or if H2 is added directly through a three-body process. In the latter case, HOSOH and DOSOD would be greatly favored relative to HOSOD, whereas in the former case a statistical distribution of H/D isomers would be expected. Such experiments could shed light onto possible formation pathways of HOSOH in the atmosphere, which may have an influence on models of sulfuric acid formation. Further isotopic studies involving D2 , S18 O2 , 34

SO2 , etc., would allow for exact structure determination and provide benchmarks for ab

initio theory. Detection of still more isomers of H2 SO2 might also be fruitfully undertaken. At least one isomer of sulfinic acid (isomer 1 in Figure 1) is also produced in our discharge; we have observed the 10,1 − 00,0 , 20,2 − 10,1 , and 30,3 − 20,2 transitions of this species under the same conditions used to produce sulfoxylic acid. 42 What role, if any, these higher-energy isomers play in atmospheric and interstellar chemistry remains to be studied.

Acknowledgement The work in Cambridge is supported by NSF Grant No CHE-1058063. K.N.C. has been supported by a CfA Postdoctoral Fellowship from the Smithsonian Astrophysical Observa15 ACS Paragon Plus Environment


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tory. S.T. gratefully acknowledges funding by the Deutsche Forschungsgemeinschaft (DFG) through grants TH 1301/3-1 and TH 1301/3-2.

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Microwave Detection of Sulfoxylic Acid (HOSOH) - The Journal of

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