Fourier transform microwave spectroscopy of the 13C- and 18O-substituted tropolone. Proton tunneling effect for the isotopic species with the asymmetric potential wells.

Fourier-transform microwave spectroscopy has been applied for the 13C/18O-substituted tropolone to observe tunneling-rotation transitions as well as pure rotational transitions. The tunneling-rotation transitions were observed between the 13C-4 and -6 forms as well as between 13C-3 and -7, between 13C-1 and -2, and between 18O-8 and -9 (we denote these tunneling pairs as 13C-46, etc., below) although they have an asymmetric tunneling potential due to the difference in the zero point energy (ZPE). From the observed tunneling splittings ΔEij (0.9800-1.6824 cm-1), the differences in ZPE Δij for the 13C-46, -37, -12, and 18O-89 species are derived to be -0.1104, 0.5652, -1.3682, and 1.3897 cm-1 to agree well with the DFT calculation. The state mixing ratio of the tunneling states decreases drastically from (44%:56%) to (8.7%:91.3%) for 13C-46 and 18O-89 with an increase in the asymmetry Δij of the tunneling potential function. The observed tunneling-rotation interaction constants Fij decrease from 16.001 to 9.224 cm-1 as the differences in ZPE Δij increase, while the diagonal tunneling-rotation interaction constants Fu increase from 1.767 to 13.70 cm-1, explained well by the mixing ratio of the tunneling states.

Fourier-transform microwave spectroscopy of the ClSO radical.

Pure rotational transitions of the ClSO radical have been observed by Fourier-transform microwave spectroscopy. a-type and b-type transitions, for both 35Cl and 37Cl isotopologues, were detected, and the observed very complicated fine and hyperfine components were assigned well. The intensities of the observed spectra of the two isotopologues correspond to the ratio of the isotope abundances of 35Cl and 37Cl. A total of 21 molecular constants were determined precisely for both 35ClSO and 37ClSO, including the rotational constants, centrifugal distortion constants, electronic spin-rotation constants, nuclear spin-rotation constants, magnetic hyperfine constants, and quadrupole coupling constants of chlorine. The molecular constants show ClSO to have the 2A'' electronic ground state with an out-of-plane unpaired electron. The spin density of the chlorine atom is about 10.6%, which is similar to that of the fluorine atom for FSO, about 8%. Results of the ClSO radical are compared with those of other triatomic radicals with similar structures, the XSS, XSO, and XOO radicals with X = H, F, and Cl, leading to a conclusion that the ClSO radical is more like FSO, but fairly different from the FOO and ClOO radicals.

Fourier-transform microwave spectroscopy of the s-trans-3-propenalyl (CH2CHCO) and 3-propenolyl (CH2CHCO) radicals.

Pure rotational transitions of two conformers of the CH2CHCO radical have been observed by Fourier-transform microwave spectroscopy, where one conformer is called the s-trans-3-propenalyl radical and the other the 3-propenolyl radical. The observed two conformers have different electronic states. The former, the s-trans-3-propenalyl radical, has the 2A' electronic state and can be written as CH2CHCO, where the unpaired electron resides mainly on the terminal CO carbon. On the other hand, the latter, 3-propenolyl radical has the 2A'' electronic state and can be written as CH2CHCO. We were able to observe pure rotational transitions of the two conformers. Since both of the species have an unpaired electron, there exist spin-rotation interactions due to the unpaired electron and the magnetic hyperfine interactions due to the three coupling protons. The observed very complicated spectra, caused by these interactions, were assigned, leading to detailed molecular constants including the fine and hyperfine coupling constants for both of the species. The determined molecular constants support the electronic structures of the two conformers. There exists a controversy as to which of the two conformers is the lowest energy one. Our present observation led to the conclusion that s-trans-3-propenalyl is the lowest energy conformer.

Detection of a C4 Criegee Intermediate: Fourier-Transform Microwave Spectroscopy of Methacrolein Oxide.

Pure rotational transitions of methacrolein oxide (MACRO) were observed by Fourier-transform microwave spectroscopy. Among the four low-lying conformers existing within an energy window of 3 kcal/mol, only the lowest-energy conformer, the anti-trans conformer, was detected in a discharged jet of a 1,3-diiode-2-methylprop-1-ene and O2 mixture diluted in Ar. Nineteen pure rotational transitions, in the frequency range from 10 to 25 GHz, most of them showing A/E splitting due to the methyl-top internal rotation, were observed and analyzed by the XIAM program, yielding the internal rotation barrier of 559 cm-1, which very well agrees with a theoretically calculated value, 558 cm-1, at the CCSD(T)/cc-pVTZ level of theory.

Gas-phase spectroscopic identification of the chlorovinyl radical.

Fourier transform microwave spectra for two isomers of the chlorine substituted vinyl radical have been observed in the 4-52 GHz frequency region. The observed radicals (2A') have been generated using electric discharges of diluted dichloro derivatives of ethylene as molecular precursors. Fine and hyperfine components observed for each rotational transition are fully assigned in the present study to two isotopologues (35Cl and 37Cl), and precise molecular constants are determined for both radicals.

Fine and hyperfine coupling constants of the cis-ß-cyanovinyl radical, HCCHCN.

A Fourier-transform microwave spectrum of the cis-ß-cyanovinyl radical is re-measured for the Ka = 0 ladder of the a-type transitions up to 30 GHz and the 212-111 transition at 19.85 GHz. Four b-type transitions are also observed using a MW-MW double-resonance technique. Fine and hyperfine components observed for each rotational transition are fully assigned in the present study, and the precise molecular constants are determined for the radical. From the comparisons of the hyperfine coupling constants with those of the vinyl radicals, it is concluded that the substitution of one of the ß-hydrogens by the cyano group has little effect on the electronic structure of the vinyl radical.

Spectroscopic detection of gas-phase HOSO2.

The HOSO2 radical was detected by microwave spectroscopy in a discharge plasma of a SO2/H2O gas mixture. The observed spectrum shows tunneling splittings due to the OH torsional motion. A least-squares analysis considering interactions between the two torsional sublevels of the ground vibronic state, 0+ and 0-, reproduces the observed transition frequencies with a standard deviation of ca. 3 kHz. The splitting between the two torsional sublevels is accurately determined to be 24.3 MHz for HOSO2 and 0.08 MHz for DOSO2. The potential barrier for the OH torsional motion is estimated to be 1150 cm-1 from a one-dimensional hindered rotor model.

Laboratory microwave spectroscopy of the doubly deuterated cyanomethyl radical, D2CCN.

The microwave spectrum of the doubly deuterated cyanomethyl radical (D2CCN) in its ground electronic state (2 B 1) has been observed for the lowest four rotational transitions (NKa,Kc = 10,1-00,0, 20,2-10,1, 21,2-10,1 and 21,1-11,0) using a Fourier transform microwave spectrometer in combination with a pulsed discharge nozzle. A total of 394 hyperfine components were measured and subjected to a least squares analysis which allowed determining twelve hyperfine constants for nitrogen and deuterium nuclei. With this new set of molecular constants we obtained accurate predictions for low N rotational transitions with hyperfine structure, and searched for this species in TMC-1.

Reactivity and internal dynamics in the Criegee intermediate CH2OOCO2 system: A rotational study.

The reaction system between the simplest Criegee intermediate, CH2OO, and the greenhouse gas CO2 has been investigated by Fourier transform microwave spectroscopy. The CH2OO-CO2 weakly bound complex was identified in the rotational spectrum, where inversion doublets due to the tunnelling motion between two equivalent configurations of the complex, with CO2 located at one side or the other side of the CH2OO plane, were observed. Using a two-state torsion-rotation Hamiltonian, a complete set of rotational and centrifugal distortion constants for both tunneling states were derived. In addition, the torsional energy difference between both states could be accurately determined, being 23.9687 MHz. The non-observation of the cycloaddition reaction product is in agreement with our ab initio calculations and with previous results that concluded that the reactivity of CIs toward CO2 is measured to be quite limited.

Detection of a Criegee Intermediate with an Unsaturated Hydrocarbon Substituent: Fourier-Transform Microwave Spectroscopy of Methyl Vinyl Ketone Oxide.

Pure rotational transitions of methyl vinyl ketone oxide, or the so called methyl-vinyl Criegee intermediate, were observed by Fourier-transform microwave spectroscopy. Among the four possible conformers of this species, predicted by theory within an energy window of 3 kcal/mol, only the lowest-energy conformer, the syn-trans form, was detected in a discharged jet of a 1,3-diiode-but-2-ene (either in Z- or E-conformation) and O2 mixture diluted in Ar. Thirty pure rotational transitions with internal rotation splitting of the methyl top were observed. The observed frequencies were analyzed by the XIAM program, yielding an internal rotation barrier of 702.8(28) cm-1, which reasonably agrees with a theoretical value of 680 cm-1 determined with CCSD(T)/cc-pVTZ.

Probing Criegee intermediate reactions with methanol by FTMW spectroscopy.

Criegee intermediates (CIs) are carbonyl oxides generated from ozonolysis of unsaturated hydrocarbons in the atmosphere. The relatively long lifetime of CIs makes possible the bimolecular reactions with other atmospheric agents. These reactions can potentially be fast enough to contribute significantly to the tropospheric budgets of those species or to alter the rate of generation of secondary organic aerosols (SOAs). In particular, the new adducts formed in these reactions contribute SOA formation because they have larger molecular weights and lower vapor pressures than the reactants and are more condensable. α-alkoxyalkyl hydroperoxides are the nascent products derived from the insertion reactions of CIs with alcohols. In this work we report the direct detection of methoxymethyl hydroperoxide (HOOCH2OCH3, MMHP) and methoxyethyl hydroperoxide (HOOC(CH3)HOCH3, MEHP) as the reaction products between the CIs, CH2OO and CH3CHOO and methanol. High resolution Fourier transform microwave spectroscopy has been used to identify one and two conformers of MMHP and MEHP, respectively. The pre-reactive complex CH2OO-CH3OH was not observed in this experiment, which indicates that CH2OO shows a different reactivity toward methanol than that toward water. Our results for CH3CHOO + CH3OH show that MEHP is produced in similar ratios when syn- and anti-CH3CHOO react with methanol vapor.

Observation of hydroperoxyethyl formate from the reaction between the methyl Criegee intermediate and formic acid.

Rapid reactions of carboxylic acids with Criegee intermediates provide efficient gas phase removal processes and are proposed to trigger the formation of new molecular compounds, which are implicated in the growth of atmospheric aerosols. The new adducts formed in these reactions are known as hydroperoxide esters and are predicted to be condensable, with lower vapor pressures than the reactants. We report here the direct detection of hydroperoxyethyl formate (HOOCH(CH3)OCHO) formed in the gas phase reaction between the methyl substituted Criegee intermediate, CH3CHOO, and formic acid. Two different isomers of this hydroperoxide ester have been observed using the combination of pulsed Fourier-transform microwave spectroscopy with isotopic substitution experiments and high-level quantum chemical calculations. The identification of hydroperoxyethyl formate as the main nascent product of the CH3CHOO and HCOOH reaction differs from previous results that reported the formation of vinyl hydroperoxide as the reaction product of this reaction.

The Criegee intermediate-formic acid reaction explored by rotational spectroscopy.

The atmospheric reaction of the simplest Criegee intermediate, CH2OO, with formic acid has been investigated in the gas phase by pulsed Fourier-transform microwave spectroscopy. The dominant nascent product from this reaction was identified as hydroperoxymethyl formate (HOOCH2OCHO), for which two different conformations, formed through independent insertion mechanisms, were observed in the discharged plasma of a CH2I2/O2/formic acid gas mixture. The conformational identifications are supported by the observation of 13C species in natural abundance together with the chemically mono substituted deuterium isotopologues. These isotopic observations further suggest that hydroperoxymethyl formate slightly decomposes, producing formic anhydride (OHCOCHO) in a dehydration reaction.

Fourier transform microwave spectroscopy of Criegee intermediates: The conformational behaviour of butyraldehyde oxide.

Four conformers of the n-propyl-substituted Criegee intermediate, CH3CH2CH2CHOO, also named n-butyraldehyde oxide, have been observed by Fourier transform microwave spectroscopy. The transient species was produced using a pulsed electric discharge of a gas mixture of 1,1-diiodobutane/O2 diluted in Ar or Ne. The observation of only syn species is in contrast to the results of other previous studies of alkyl-substituted Criegee intermediates, in particular, those for the structural isomers, isobutyraldehyde oxide ((CH3)2CHCHOO), and methyl-ethyl-ketone oxide (C2H5C(CH3)OO), for which syn and anti species have been observed coexisting in the gas phase.

High-resolution vibration-rotational spectra and rotational perturbation of the OO-stretching (ν6) band of CH2OO between 879.5 and 932.0 cm-1.

We report the observation of a rotationally resolved ν6 band associated with the OO-stretching mode of the simplest Criegee intermediate, CH2OO, in the range of 879.5-932.0 cm-1 (11.37-10.73 µm) at an optical resolution of 0.0015 cm-1. The spectra were recorded with a tunable cw external-cavity quantum-cascade laser (EC-QCL) system coupled with a Herriott multipass absorption cell. Over one thousand lines were assigned to determine the rovibrational parameters of the ν6 band. Rotational parameters A', B', and C' and higher-order parameters ΔJ, ΔJK, ΔK, δJ, and δK were determined precisely. Rotational perturbations for Ka' = 3, Ka' = 6, and Ka' ≥ 11 on high-J levels were observed. In particular, for Ka' = 6, frequency shifts as a function of J' showed an avoided crossing; the shifts were analyzed in terms of a Coriolis interaction between modes ν6 and ν8 of CH2OO. According to the distinct temporal profiles, lines of a hot band near 899.5 cm-1 were also identified.

Conformational preferences of Criegee intermediates: Isopropyl substituted carbonyl oxide.

Three conformers of the isopropyl-substituted Criegee intermediate, (CH3)2CHCHOO, have been observed by Fourier transform microwave spectroscopy. The transient species was produced using a pulsed electric discharge of a gas mixture of 1,1-diiodo-2-methylpropane/O2 diluted in Ar or Ne. The use of different carrier gases in the supersonic expansion reveals the difference of the collisional relaxation process between anti-conformers. The conformational relaxation pathways have been investigated theoretically and are presented as well. In light of these results, the previous study on the ethyl-substituted Criegee intermediate, where the absence of one of the four possible conformers was associated with collisional relaxation processes, has been re-examined. Here we report the detection of a new conformer of the ethyl-substituted Criegee intermediate observed using Ne as the seeding gas.

The reaction between the methyl Criegee intermediate and hydrogen chloride: an FTMW spectroscopic study.

Experiments using pulsed Fourier-transform microwave spectroscopy have been performed to probe the reaction between the methyl substituted Criegee intermediate, CH3CHOO, and hydrogen chloride. Two conformers of the product derived from the insertion of HCl within CH3CHOO, chloroethyl hydroperoxide, formed in the discharged plasma of a CH3CHI2/O2/HCl gas mixture, were identified through their experimentally determined rotational and quadrupole coupling constants. The relative abundance of the detected species indicates that the reaction of the anti-CH3CHOO conformer is faster than that of the syn-CH3CHOO conformer. The results presented in this work constitute the first experimental information about the reaction between CH3CHOO and HCl.

Identification and Self-Reaction Kinetics of Criegee Intermediates syn-CH3CHOO and CH2OO via High-Resolution Infrared Spectra with a Quantum-Cascade Laser.

The Criegee intermediates, carbonyl oxides produced in ozonolysis of unsaturated hydrocarbons, play important roles in atmospheric chemistry. The two conformers of CH3CHOO exhibit distinct reactivity toward several atmospheric species, but a distinct conformer-specific probe is challenging because ultraviolet and infrared absorption bands of syn- and anti-CH3CHOO overlap at low-resolution. Employing a quantum-cascade laser and a Herriott cell, we recorded the O-O stretching bands of CH2OO and syn-CH3CHOO in region 880-932 cm-1 at resolution 0.0015 cm-1. In addition to completely resolved vibration-rotational lines of CH2OO extending over 50 cm-1, some spectral lines associated with hot bands were identified. Spectral lines solely due to syn-CH3CHOO were also identified. Probing these lines, we determined the rate coefficient for the self-reaction of syn-CH3CHOO to be kself = (1.6 ± 0.60.5) × 10-10 cm3 molecule-1 s-1, about twice that of CH2OO.

The reactivity of the Criegee intermediate CH3CHOO with water probed by FTMW spectroscopy.

The reaction of Criegee intermediates with water is one of the dominant removal mechanisms for these species in the atmosphere. The reactivity of alkyl substituted Criegee intermediates has been shown to be affected by the nature and location of the substituents. CH3CHOO, acetaldehyde oxide, can be considered as a prototypical Criegee intermediate with a single alkyl substituent to examine the conformer specific reactivity for Criegee intermediates. Pulsed Fourier-transform microwave spectroscopy has been used to probe the products resulting from the reaction between CH3CHOO and water. The hydrogen-bonded complex between CH3CHOO and water together with the reaction product, hydroxyethyl hydroperoxide, were observed in the discharged plasma of a CH3CHI2/O2/water gas mixture. The experimentally determined rotational parameters support the identification of the complex between water and the syn-CH3CHOO conformer and two conformers of hydroxyethyl hydroperoxide, produced from the anti-CH3CHOO conformer and water.

Detection of Microwave Transitions between Ortho and Para States in a Free Isolated Molecule.

Microwave transitions between the ortho and para states of the S_{2}Cl_{2} molecule in a free isolated condition are observed for the first time. In the theoretical treatment, we derive eigenfunctions of an effective Hamiltonian including the ortho-para interaction to calculate the intensities and frequencies of forbidden ortho-para transitions in the cm-wave region and pick up some promising candidates for the spectroscopic detection. In the experiment, transitions of the S_{2}Cl_{2} molecule under a supersonic jet condition are observed with a Fourier transform microwave spectrometer. Seven hyperfine resolved rotational transitions including the lowest rotational level are detected as the ortho-para transitions at the predicted frequencies within the experimental errors. The observed intensities are about 10^{-3} times the allowed transitions, which are consistent with the predictions based on the intensity-borrowing model. This result suggests that the ortho-para conversion of this molecule occurs in a few thousand years through spontaneous emission even in a circumstance where molecular collisions occur rarely like in interstellar space.