Masakazu Nakajima

Communication: Determination of the molecular structure of the simplest Criegee intermediate CH2OO

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(⊕)-O(⊖) than biradical H2Ċ-O-O.

Communication: Spectroscopic characterization of an alkyl substituted Criegee intermediate syn-CH3CHOO through pure rotational transitions

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.

Development of microwave-optical double-resonance spectroscopy using a Fourier-transform microwave spectrometer and a pulsed laser

A new type of microwave-detected microwave-optical double-resonance (MODR) spectroscopy has been developed using a Fourier-transform microwave spectrometer and a tunable pulsed dye laser. In this method, a free-induction decay (FID) signal was detected instead of the microwave (MW) absorption. To demonstrate the performance, we measured the MODR spectra of the CCS and C 4 H radicals in supersonic jets generated by a pulsed-discharge nozzle. Since the pulsed sources are employed for both the optical and microwave radiations, it is possible to control the relative timing of irradiations of the MW and optical pulses. We were able to obtain two different types of spectra; one is the ordinary population labeling spectrum, and the other is a spectrum obtained by breaking the coherence of molecules. In the latter case, more than 50% of depletion of the FID signal was observed, which is unable to be attained when noncoherent phenomena are used to detect the double-resonance signal.

The dΠg3-cΣu+3 band system of C2

A two-dimensional fluorescence (excitation/emission) spectrum of C2 produced in an acetylene discharge was used to identify and separate emission bands from the dΠg3←cΣu+3 and dΠg3←aΠu3 excitations. Rotationally resolved excitation spectra of the (4←1), (5←1), (5←2), and (7←3) bands in the dΠg3←cΣu+3 system of C2 were observed by laser-induced fluorescence spectroscopy. The molecular constants of each vibrational level, determined from rotational analysis, were used to calculate the spectroscopic constants of the cΣu+3 state. The principal molecular constants for the cΣu+3 state are Be=1.9319(19)cm−1, αe=0.01855(69)cm−1, ωe=2061.9cm−1, ωexe=14.84cm−1, and T0(c−a)=8662.925(3)cm−1. We report also the first experimental observations of dispersed fluorescence from the dΠg3 state to the cΣu+3 state, namely, dΠg3(v=3)→cΣu+3(v=0,1).A two-dimensional fluorescence (excitation/emission) spectrum of C2 produced in an acetylene discharge was used to identify and separate emission bands from the d (3)Pi(g)<--c (3)Sigma(u) (+) and d (3)Pi(g)<--a (3)Pi(u) excitations. Rotationally resolved excitation spectra of the (4<--1), (5<--1), (5<--2), and (7<--3) bands in the d (3)Pi(g)<--c (3)Sigma(u) (+) system of C2 were observed by laser-induced fluorescence spectroscopy. The molecular constants of each vibrational level, determined from rotational analysis, were used to calculate the spectroscopic constants of the c (3)Sigma(u) (+) state. The principal molecular constants for the c (3)Sigma(u) (+) state are B(e)=1.9319(19) cm(-1), alpha(e)=0.018 55(69) cm(-1), omega(e)=2061.9 cm(-1), omega(e)x(e)=14.84 cm(-1), and T(0)(c-a)=8662.925(3) cm(-1). We report also the first experimental observations of dispersed fluorescence from the d (3)Pi(g) state to the c (3)Sigma(u) (+) state, namely, d (3)Pi(g)(v=3)-->c (3)Sigma(u) (+)(v=0,1).

Laser induced fluorescence spectroscopy of the HC4S and DC4S radicals

Abstract Vibronic band systems for the HC 4 S and DC 4 S radicals were observed in supersonic jets for the first time by laser induced fluorescence (LIF) spectroscopy. The rotational structures of the bands were resolved by high-resolution scans. It was found that all the observed bands are 2 Π 3/2 – 2 Π 3/2 transitions. Band origins and effective rotational constants of the origin bands are determined to be T 0 =19 980.687(1) and B ′ eff =0.046580(4) cm −1 for HC 4 S, T 0 =20 034.8748(9) and B eff ′ =0.044294(3) cm −1 for DC 4 S. Ab initio calculations were carried out to interpret the vibronic transitions.

Observation of the dΠg3←cΣu+3 band system of C2

A new band system of C2, dΠg3←cΣu+3 is observed by laser induced fluorescence spectroscopy, constituting the first direct detection of the cΣu+3 state of C2. Observations were made by laser excitation of cΣu+3(v″=0) C2, produced in an acetylene discharge, to the dΠg3(v′=3) level, followed by detection of Swan band fluorescence. Rotational analysis of this band yielded rotational constants for the cΣu+3(v″=0) state: B0=1.9218(2)cm−1, λ0=−0.335(4)cm−1 and γ0=0.011(2)cm−1. The vibrational band origin was determined to be ν3−0=15861.28cm−1.

Spectroscopic characterization of the complex between water and the simplest Criegee intermediate CH2OO.

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.

Pure rotational spectrum of the NCCS radical studied by Fourier-transform microwave spectroscopy

Pure rotational transitions of the NCCS radical, showing resolved fine and hyperfine splittings, have been observed by Fourier-transform microwave spectroscopy in a discharged supersonic jet of acetonitrile and carbon disulfide. Since the transitions have been observed at frequencies corresponding to the even multiples of the rotational constant, it is concluded that the NCCS radical has a bent structure in the ground electronic state, X 2A′, and the Ka=0 ladder of the radical has been observed under the jet-cooled condition. Precise molecular constants, including the hyperfine constants of the nitrogen nucleus, are determined by a least-squares fit for the observed transition frequencies using a standard asymmetric top Hamiltonian. The determined rotational constant is compared with results of high-level ab initio calculations in order to confirm the spectral carrier to be the bent NCCS radical.

Laser induced fluorescence spectroscopy of the HC6S radical

Abstract New vibronic bands were observed in the 17 000–17 600 cm −1 region by laser induced fluorescence (LIF) spectroscopy in the discharged products of a mixture of C2H2 and CS2 in a supersonic jet. The effective rotational constants and the position of the vibronic band origin have been determined to be B ″ eff =0.01910(2) cm −1 , B ′ eff =0.01891(2) cm −1 , and T 0 =16961.5161(5) cm −1 , respectively. The spectral carrier of the bands has been confirmed to be the HC6S radical from the chemical behavior of the products and the determined rotational constant.

Spectroscopic observation of higher vibrational levels of C2 through visible band systems.

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.