ABSTRACT
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.
ABSTRACT
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.
ABSTRACT
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.
ABSTRACT
Pure rotational transitions of the dimethyl-substituted Criegee intermediate (dimethyl carbonyl oxide, acetone oxide), (CH3)2COO, were observed in the discharge plasma of a C(CH3)2I2/O2 gas mixture by Fourier-transform microwave spectroscopy. The observed spectra show small splittings due to the internal rotations of the two methyl groups. Precise rotational constants of the molecule and the barrier heights of the methyl internal rotations were experimentally determined.
ABSTRACT
Pure rotational transitions of hydroxymethyl hydroperoxide (HMHP) were observed in the discharged plasma of a CH2I2/O2/water gas mixture, where the water complex with the simplest Criegee intermediate CH2OO has been identified [M. Nakajima and Y. Endo, J. Chem. Phys. 140, 134302 (2014)]. Isotope experiments using heavy water support that the currently observed HMHP molecule was produced by the reaction of CH2OO with water vapor. The observed species was identified as the most stable conformer with the help of quantum chemical calculations. We also clarified that productions of formic acid and dioxirane are promoted by the existence of water vapor in the discharged reaction system.
ABSTRACT
The Ã(2)Σ(+)-XÌ(2)Πi band system of l-SiC3H in the region 14,700-16,300 cm(-1) was re-investigated by laser induced fluorescence (LIF) and fluorescence depletion spectroscopy. Rotational analyses were made for three intense bands 0(0)(0), 4(0)(1), and 6(0)(1)7(0)(1) by observing high-resolution LIF excitation spectra. The determined rotational constants demonstrate that SiC3H is linear in the à state, as is the case in the XÌ state, and the observed band types are consistent with the vibrational assignments. The ν3(â³) (C1-C2 stretch) level was identified in a newly observed dispersed fluorescence spectrum from the zero-vibrational level of the à state.
ABSTRACT
We observed microwave absorption spectra of some deuterated benzenes and accurately determined the rotational constants of all H/D isotopomers in the ground vibrational state. Using synthetic analysis assuming that all bond angles are 120°, the mean bond lengths were obtained to be r0(C-C) = 1.3971 Å and r0(C-H) = r0(C-D) = 1.0805 Å. It has been concluded that the effect of deuterium substitution on the molecular structure is negligibly small and that the mean bond lengths of C-H and C-D are identical unlike small aliphatic hydrocarbons, in which r0(C-D) is about 5 mÅ shorter than r0(C-H). It is considered that anharmonicity is very small in the C-H stretching vibration of aromatic hydrocarbons.
ABSTRACT
Two silicon-bearing carbon chain radicals, SiC2N and SiC3N, were detected in the laboratory by Fourier transform microwave spectroscopy. Molecular constants including the hyperfine coupling constants have been determined for the two radicals in the ground electronic states. The SiC2N and SiC3N radicals have linear structures in the (2)Π ground electronic states with inverted and regular fine structures, respectively, as are the cases for their isoelectronic radicals, SiC3H and SiC4H, indicating that the SiC(n)N radicals have similar electronic structures to the SiC(n +1)H radicals. The electronic structures of SiC2N and SiC3N in the ground states are discussed on the basis of the experimentally determined molecular constants.
ABSTRACT
The hydrogen-bonded complex between water and the simplest Criegee intermediate CH2OO was detected by Fourier-transform microwave spectroscopy under a jet-cooled condition. Both a-type and b-type rotational transitions were observed for H2O-CH2OO and D2O-CH2OO. The determined rotational constants enable us to conclude that the complex has an almost planar ring structure with the terminal oxygen atom of CH2OO being a strong proton acceptor.
ABSTRACT
Pure rotational transitions of the Ar-CS van der Waals complex have been observed by Fourier Transform Microwave (FTMW) and FTMW-millimeter wave double resonance spectroscopy. Rotational transitions of v(s) = 0, 1, and 2 were able to be observed for normal CS, together with those of C(34)S in v(s) = 0, where vs stands for the quantum number of the CS stretching vibration. The observed transition frequencies were analyzed by a free rotor model Hamiltonian, where rovibrational energies were calculated as dynamical motions of the three nuclei on a three-dimensional potential energy surface, expressed by analytical functions with 57 parameters. Initial values for the potential parameters were obtained by high-level ab initio calculations. Fifteen parameters were adjusted among the 57 parameters to reproduce all the observed transition frequencies with the standard deviation of the fit to be 0.028 MHz.
ABSTRACT
An alkyl-substituted Criegee intermediate syn-CH3CHOO was detected in the gas phase through Fourier-transform microwave spectroscopy. Observed pure rotational transitions show a small splitting corresponding to the A∕E components due to the threefold methyl internal rotation. The rotational constants and the barrier height of the hindered methyl rotation were determined to be A = 17 586.5295(15) MHz, B = 7133.4799(41) MHz, C = 5229.1704(40) MHz, and V3 = 837.1(17) cm(-1). High-level ab initio calculations which reproduce the experimentally determined values well indicate that the in-plane C-H bond in the methyl moiety is trans to the C-O bond, and other two protons are directed to the terminal oxygen atom for the most stable structure of syn-CH3CHOO. The torsional barrier of the methyl top is fairly large in syn-CH3CHOO, implying a significant interaction between the terminal oxygen and the protons of the methyl moiety, which may be responsible for the high production yields of the OH radical from energized alkyl-substituted Criegee intermediates.
ABSTRACT
Pure rotational spectra of the allenyloxy radical (CH2=CCHO) were observed by Fourier transform microwave (FTMW) and FTMW-millimeter wave double-resonance spectroscopy. Molecular constants including the hyperfine interaction constants of CH2=CCHO in the (2)A(â³) ground electronic state were precisely determined. Ab initio calculations indicate that CH2=CCHO has a linear C-C-C backbone with Cs symmetry, where the formyl group is in the Cs plane and perpendicular to the methylene group. The determined rotational constants and the inertial defect agree well with those derived from the calculations, implying that the calculated molecular structure is reasonable. The fine and hyperfine constants also agree with those derived from the calculated spin density, where the unpaired electron is located mainly on the central carbon atom. The ground state CH2=CCHO can, thus, be described as taking the formylvinyl (CH2=C-CH=O) form rather than as the allenyloxy (CH2=C=CH-È®) form.
ABSTRACT
The simplest Criegee intermediate CH2OO was detected in a discharged supersonic jet of a CH2Br2 and O2 gas mixture by Fourier-transform microwave spectroscopy. The experimentally determined rotational constants of CH2OO and its isotopologues enabled us to derive the geometrical structure. The determined OO and CO bond lengths, which are relevant to a discussion on its electronic structure, are 1.345(3) and 1.272(3) Å, respectively. The CO bond length is close to that of a typical double bond and is shorter than that of the OO bond by 0.07 Å, indicating that CH2OO has a more zwitterionic character H2C = O(â)-O(â) than biradical H2C-O-O.
ABSTRACT
Pure rotational spectra of the H2O-trans-HOCO complex have been observed by Fourier transform microwave (FTMW) spectroscopy and millimeter-wave FTMW double resonance spectroscopy. The complex was produced in a supersonic jet by discharging a mixture gas of CO and H2O diluted in Ar. The observed rotational lines consist of two groups of transitions with different hyperfine patterns. This is explained by considering the internal rotation of the H2O monomer in the complex. The molecular constants including the fine and hyperfine coupling constants have been determined for the two groups of lines. The hydrogen bond distance between H2O and the trans-HOCO monomer has also been determined with other structural parameters fixed to ab initio values. The hydrogen bond distance, 1.794 Å, is much shorter than that of the water dimer, and similar to those of water-acid complexes. The Fermi coupling constant of the proton of HOCO is compared with that of the trans-HOCO monomer, leading to the conclusion that there is an induced effect on the spin density on the proton of HOCO by the complex formation.
ABSTRACT
The Ã(3)A"-XÌ(3)Σ(-) electronic transition of the HCCN∕DCCN radical was observed by laser-induced fluorescence spectroscopy. Rotationally resolved excitation spectra were observed for HCCN and DCCN, and effective molecular constants of the upper state were determined. The observed intensities of the rotational lines were anomalous, probably due to a level-dependent non-radiative decay process in the excited state. Fluorescence depletion spectroscopy was applied in order to observe non-fluorescent vibronic levels. A dispersed fluorescence spectrum was also observed to determine the vibrational level structure in the ground electronic state. The observed vibrational structures in the fluorescence depletion and dispersed fluorescence spectra were tentatively assigned based on the results of ab initio calculations.
Subject(s)
Nitriles/chemistry , Free Radicals/chemistry , Quantum Theory , Spectrum AnalysisABSTRACT
In order to examine significant roles of conformations played in various research fields, a molecule with two internal-rotation axes of high symmetry, 1,4-pentadiene, was studied in detail through the observation of its rotational spectra by using various types of microwave spectroscopy, Stark modulation and Fourier transform in the centimeter-wave region, direct absorption in the millimeter-wave region, and centimeter-/millimeter-wave combinations for double resonance, along with ab initio molecular orbital calculations. The molecule was confirmed to exist in three rotameric forms: skew-skew, cis-skew, and skew-skew'. For the cis-skew form, rotational spectra not only in the ground vibrational state, but also in three excited C-C torsional states were detected. Rotational and centrifugal distortion constants were precisely determined by the analysis of all the observed spectra, in addition to the relative energies of the three isomers and the torsional frequencies for the cis-skew form, as estimated from the observed spectral line intensities. The skew-skew form was found to be the most stable among the three isomers, the cis-skew form higher in energy than the skew-skew by 172 ± 66 cm(-1), and the skew-skew' form higher in energy than the cis-skew by 44 ± 26 cm(-1). These experimental results were compared with those derived from a two-dimensional potential energy surface calculated by ab initio molecular orbital methods, in order to obtain a global view of molecular dynamics taking place on the surface, while paying attention to unique features of internal rotation characteristic of two dimensions.
ABSTRACT
Higher vibrational levels of the C2 molecule than those observed so far were investigated for the X(1)Σ(g)(+), A(1)Π(u), a(3)Π(u), c(3)Σ(u)(+), and d(3)Π(g) states through the Phillips, Swan, and d(3)Π(g)-c(3)Σ(u)(+) band systems under a jet-cooled condition. The term values and the molecular constants for 21 new vibronic levels were determined from rotationally resolved excitation spectra. The determined term values and rotational constants were compared to those derived from high-level ab initio potential curves. Perturbations identified in low J levels of the d(3)Π(g) (v = 8) state are most likely to be caused by the 1(5)Π(g) (v = 3) state.
ABSTRACT
We investigated the reactivity of O((1)D) towards two types of hydrogen atoms in CH(3)OH. The reaction was initiated on irradiation of a flowing mixture of O(3) and CD(3)OH or CH(3)OD at 248 nm. Relative vibration-rotational populations of OH and OD (1 ≤ v ≤ 4) states were determined from their infrared emission recorded with a step-scan time-resolved Fourier-transform spectrometer. In O((1)D) + CD(3)OH, the rotational distribution of OD is nearly Boltzmann, whereas that of OH is bimodal; the product ratio [OH]/[OD] is 1.56 ± 0.36. In O((1)D) + CH(3)OD, the rotational distribution of OH is nearly Boltzmann, whereas that of OD is bimodal; the product ratio [OH]/[OD] is 0.59 ± 0.14. Quantum-chemical calculations of the potential energy and microcanonical rate coefficients of various channels indicate that the abstraction channels are unimportant and O((1)D) inserts into the C-H and O-H bonds of CH(3)OH to form HOCH(2)OH and CH(3)OOH, respectively. The observed three channels of OH are consistent with those produced via decomposition of the newly formed OH or the original OH moiety in HOCH(2)OH or decomposition of CH(3)OOH. The former decomposition channel of HOCH(2)OH produces vibrationally more excited OH because of incomplete intramolecular vibrational relaxation, and decomposition of CH(3)COOH produces OH with greater rotational excitation, likely due to a large torque angle during dissociation. The predicted [OH]/[OD] ratios are 1.31 and 0.61 for O((1)D) + CD(3)OH and CH(3)OD, respectively, at collision energy of 26 kJ mol(-1), in satisfactory agreement with the experimental results. These predicted product ratios vary weakly with collision energy.
ABSTRACT
The spectra of 1-indanyl-based resonance-stabilized radicals containing a hydroxyl group are identified in an electrical discharge containing indene and its alkylated derivatives. It is argued that such species form by addition of a discharge-nascent hydroxyl radical, formed from trace water, to the π bond on the five-membered ring of the parent molecule. The spectral carriers are identified by analysis of their excitation and emission spectra guided by the results from quantum chemical calculations. All three hydroxylated radicals are found to exhibit origin bands in the 21300 cm(-1) region: the 2-hydroxy-indan-1-yl radical at 21364 cm(-1), the 2-hydroxy-2-methyl-indan-1-yl radical at 21337 cm(-1), and the 2-ethyl-2-hydroxy-indan-1-yl radical exhibiting two origins of similar intensity at 21287 and 21335 cm(-1).
ABSTRACT
Electronic spectra of the BÌ-XÌ transition of the 1-methylvinylthio radical were observed in a discharged jet of propylene sulfide by laser-induced fluorescence spectroscopy. Identification of the spectral carrier was made by comparing the observed spectra with results of molecular orbital calculations, in particular, for vibrational frequencies, rotational contour simulations, and the Franck-Condon simulations. Vibrational structures observed in the electronic spectra indicate that the 1-methylvinylthio radical can be regarded as a molecule with C(s) symmetry at the zero-point levels of both the excited and ground states.
