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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.

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.

Rotational and rovibrational transitions with parity doublings and hyperfine structures of the Ne-NO complex in the electronic ground state have been observed by Fourier-transform microwave spectroscopy, and they are analyzed by employing a free-rotor model with a standard deviation of the least-squares fit to be 12 kHz. A 2-dimensional intermolecular potential energy surface for the Ne-NO complex has been determined from the present high-resolution spectroscopic data with the aid of high-level ab initio calculations.

The equilibrium constants for the formation of ClOO from Cl and O2 are experimentally measured at 212−245 K using cavity ring-down spectroscopy. A van't Hoff plot analysis yields ΔHr = 4.8 ± 1.5 kcal mol-1. The Cl−OO bond dissociation energy is determined to be 4.69 ± 0.10 kcal mol-1 from the present and previously reported temperature dependences of the equilibrium constant by the third-law analysis utilizing our recent accurate rotational spectroscopic data. High level ab initio calculations using MRSDCI+Q with the complete basis set extrapolation are also performed. The present ab initio calculations yield the Cl−OO bond dissociation energy to be 4.56 kcal mol-1.

Pure rotational spectra and their Stark effects of X-CO 2 type complexes (X:Ne, Kr, and Xe) have been studied by pulsed-nozzle Fourier-transform microwave spectroscopy. Structures, intermolecular force fields, and electric dipole moments in the ground state have been determined, which have been discussed in connection with the previously reported results of Ar-CO 2 and Hg-CO 2

O({sup 3}P) atom reactions with ethylene, propylene, and 1-butene were studied by use of kinetic microwave absorption spectroscopy where the atomic oxygen was supplied via ArF excimer laser photolysis of SO{sub 2}. Time evolutions of vinoxy, HCO, and H{sub 2}CO were pursued. The fraction of vinoxy production in the propylene and I-butene reactions at 30 mTorr was found to be 0.29 {plus minus} 0.15 and 0.37 {plus minus} 0.15, respectively. The fractions seemed to increase with the decrease in pressure, by comparing the present results with those of several previous researchers. Therefore, at least some part of the vinoxy production fraction is pressure dependent. The reaction scheme explaining the pressure-dependent part is suggested as follows. Initially, a triplet biradical is produced through (O{sup 3}P) attack to the terminal carbon atoms of the C{double bond}C double bond, which is then converted to a singlet biradical. Subsequently, a hydrogen atom migrates and then the C-C bond adjacent to the original C{double bond}C double bond dissociates to yield the vinoxy and corresponding alkyl radicals. Thus, some part of the vinoxy radical is produced via a quite different mechanism from the case of ethylene reaction, where it is produced via a direct substitution channelmore » on a triplet surface.« less

Dihydrogen trioxide, HOOOH, which is a species with fundamental importance for understanding the chain formation ability of the oxygen atom, was detected in a supersonic jet by a Fourier transform microwave spectrometer with a pulsed discharge nozzle, together with double resonance and triple resonance techniques. Its precise molecular structure was determined from the experimentally determined rotational constants of HOOOH and its isotopomer, DOOOD. Many of the microwave and millimeter wave transitions can now be accurately predicted, which could be facilitated for remote sensing of the molecule to elucidate its roles in various chemical processes.

Hydration of ion pairs is an essential process in various physicochemical phenomena occurring in solutions. Isolated clusters of an ion pair solvated with finite number of waters have been considered as a model system for the critical evaluation of microscopic interactions involved in the process, and theoretical studies have contributed exclusively to the subject up to now. Here we report the first experimental characterization of structure and internal dynamics of hydrated ion pairs, NaCl-(H2O)n (n = 1-3). The measurements of their rotational spectra have proven that the clusters have cyclic forms, in which Na+ and Cl- ions are strongly interacted with the O and H atoms of the solvent molecules, respectively. The Na-Cl distance shows a pronounced increase with the successive addition of water molecules. The separation for n = 3 approaches the value predicted for the contact ion-pair state in aqueous solution by recent molecular dynamics simulations.

The branching ratio of the two channels of O(/sup 3/P) + ethylene reaction, one yielding vinoxy + atomic hydrogen (a) and the other methyl + formyl (b), was determined for C/sub 2/H/sub 4/ and C/sub 2/D/sub 4/ by means of a microwave kinetic spectroscopic method. The ratio of (b)/(a) was found to be 6 +/- 1.5 in the C/sub 2/D/sub 4/ reaction compared with 1.2 +/- 0.2 in the C/sub 2/H/sub 4/ reaction. This fact could be interpreted by a reaction mechanism in which the unimolecular dissociation on the triplet surface (channel a) competes with the intersystem crossing to a singlet surface followed by 1,2-hydrogen migration and C-C cleavage (channel b).