Infrared Laser Magnetic Resonance Research Articles

Spectroscopic study of the 14NH radical in vibrationally excited levels of the X3Σ− state by far infrared laser magnetic resonance

Rotational transitions of vibrationally excited NH (ν = 1 and 2) in the ground X3Σ− electronic state have been detected by far-infrared laser magnetic resonance. The first four rotational intervals were investigated (N = 1 − 0, 2 − 1, 3 − 2, and 4 − 3) with a total of 17 far-infrared laser lines ranging in wavelength from 81.1 to 373.6 µm. For some rotational intervals, transitions between all components of the Zeeman manifolds were observed. An effective Hamiltonian was used to model the experimental measurements. The magnetic hyperfine parameters have been determined for the first time for the ν = 2 level and have been significantly improved for the ν = 1 level. In addition, the rotational, centrifugal distortion, spin-spin, spin-rotation, and Zeeman parameters have all been experimentally determined for the ν = 1 and 2 levels of NH.

Infrared laser magnetic resonance spectroscopy of the ν3 fundamental and associated hot bands of the NCO free radical

The v 3 = 1 ← 0 (out-of-phase stretching vibration) transition wavenumbers of gas-phase X ˜ 2 Π NCO have been measured, in many cases with sub-Doppler resolution, by mid-infrared laser magnetic resonance (LMR) spectroscopy. In addition to the fundamental transition from the 2Π ground vibronic state, hot bands from the v 2 = 1 2Σ and 2Δ and from the v 2 = 2 2Φ vibronic states were detected and analyzed. The 2Σ vibronic level, with a spin–orbit coupling only partly quenched by the Renner–Teller effect, is characterised by complex and rapid tuning of energies in the magnetic field. This is the first successful analysis of magnetic resonance spectra for non-unique Renner–Teller vibronic states. The new LMR transitions were combined with data from previous studies and analysed, using an effective Hamiltonian in a 40-parameter fit to 660 transitions and combination differences. Several new coupling parameters are required, having magnitudes generally consistent with predictions from standard vibration–rotation theory.

Global analysis of spectra of the PO 2 radical in the gas phase

A global analysis of all available and identified spectra of the PO2 radical has been performed. In this analysis, the infrared spectra of the nu(3) band, the microwave and far infrared laser magnetic resonance transitions in the ground state are fitted simultaneously to the effective Hamiltonian for asymmetric top molecules. Since the analysis absorbs reliable experimental data extensively, it bears the most precise and most complete set of molecular constants for PO2. These molecular constants can, be used to reproduce the spectra of PO2 accurately. Attention is also paid to the similarity and difference between PO2 and NO2.

Laser magnetic resonance spectrum of the OD radical ( [formula omitted], v=0) at 118.6 μm

Far infrared laser magnetic resonance (FIR LMR) spectrum of OD ( 2Π 3/2 , v=0) has been observed. Data are presented for the Zeeman components of the rotational transition J=3.5→4.5 observed at 118.6 μm using a discharge water vapour laser spectrometer. Theoretical values of the transition magnetic field strengths have been calculated using the best available molecular constants. The agreement between theory and experiment confirms the spectroscopic assignments.

Kinetics of the reactions of HCO with H and D atoms

AbstractThe rate constants for the reactions HCO+H → products (1) and HCO+D → products (2) were determined at room temperature in an isothermal discharge flow reactor. Atom and radical concentrations in the reactions were monitored using Electron Paramagnetic Resonance (EPR) for H and D atoms and Far Infrared Laser Magnetic Resonance (FIR‐LMR) and Laser Induced Fluorescence (LIF) for HCO. The overall rate constants describing the decay of the HCO concentrations with H and D, respectively, were found to be k1=(6.8±2.0)×1013 cm3 mol−1 s−1 and k2=(8.3±2.5)×1013 cm3 mol−1 s−1. The results are discussed assuming a mechanism that allows for parallel reaction pathways via a direct H atom abstraction reaction or via an addition‐elimination reaction through a short‐lived highly vibrationally excited formaldehyde complex.

Rate constant for the reaction CH3+CH2(X̃3B1) at 298 K

AbstractThe rate constant for the reaction between CH3 and CH2 (X̃3B1 = 3CH2) radicals, was determined at T=298 K and p=1.33 mbar in an isothermal discharge flow reactor. Far Infrared Laser Magnetic Resonance (FIR‐LMR) was employed for monitoring 3CH2 and CH3 radical concentration‐versus‐time profiles. CH3 was determined in the form of NO via the conversion reaction CH3+NO2 → CH3O+NO. The CH3 and 3CH2 radicals were generated by the reactions F+CH4 → CH3+HF and F+CH3 → 3CH2+HF. The rate constant for the recombination reaction was determined together with the rate constant for reaction (1). The measurements gave a rate constant value for reaction (1) of k1 = (6.5±1.2)·1013 cm3/mol s.

Far Infrared Laser Magnetic Resonance Detection of CF3, CHF2, and CH2F

AbstractFar infrared laser magnetic resonance (FIR‐LMR) spectra of CF3, CHF2, and CH2F radicals were observed in the 300–800 μm wavelength region in the reactions of F atoms with a series of different hydrocarbon and partially fluorinated hydrocarbon molecules. To identify the carriers of the observed resonance patterns and to exclude or identify contamination of the spectra by additional resonances from other paramagnetic species, numerous chemical tests were developed and performed. With optimized sources and the chemical tests in hand, many spectra could be unambiguously attributed to CF3, CHF2, and CH2F, respectively. The present study reports the first gas phase detection of CHF2 radicals in their ground electronic state.

The NCN 211310Hot Band Measured by Infrared Laser Magnetic Resonance Spectroscopy

The NCN 211310Hot Band Measured by Infrared Laser Magnetic Resonance Spectroscopy

New rotational transitions in 14N 32S [formula omitted] detected by laser magnetic resonance at 561.3 and 618.4 μm

New far infrared laser magnetic resonance (FIR LMR) spectra of 14N 32S ( 2Π 3 2 ) have been observed at 561.3 and 618.4 μm using DCOOD and C 2H 3Br optically pumped lasers, respectively. In the first case, the spectrum can be assigned to the rotational transition J = 10.5 → 11.5 of the ground vibrational state. In the second case, the spectrum can be attributed to the transition J = 9.5 → 10.5 in the first excited vibrational state. This is the first detection by LMR of 14N 32S vibrationally excited. The agreement between the theory and experiment confirms the spectroscopic assignments.

Far infrared laser magnetic resonance detection of CHD (X̃ 3A″)

Far infrared laser magnetic resonance (FIR-LMR) spectra of the CHD radical in its X̃3A″ electronic ground state were observed and assigned. The radicals were generated in the reaction of Na atoms with CHDBr2. LMR spectra were observed using seven laser lines at wavelengths around 100 to 200 μm. The spectra were assigned to six different rotational transitions and the molecular parameters of CHD were determined by a least squares fit. A number of additional transitions, observed using a laser line at λ=184.3 μm but not yet assigned in detail, were attributed to a coupling between the accidentally almost degenerate NKaKc=505 and 413 rotational levels induced by the εab term in the spin–rotation Hamiltonian and by the off-diagonal components of the hyperfine coupling tensor.

Primary products of the elementary reactions of CH2CO with F, Cl, and OH in the gas phase

AbstractThe absolute yields of the primary products of the reactions of CH2OH with F and Cl atoms and OH radicals were measured in the gas phase at room temperature using a discharge flow apparatus connected to a Far Infrared Laser Magnetic Resonance (FIR‐LMR) spectrometer. Quantitative determinations of the reaction pathways were carried out by employing reference reactions with known product yields. The measurements yielded the branching ratios for the following channelsmagnified imageThus, the main reaction channels were found to be addition of the F, Cl, or OH, respectively, to the CH2 moiety followed by elimination of CO.

Detection of OH+ in its a 1Δ state by far infrared laser magnetic resonance

The spectrum associated with the J=3←2 transition of OH+ in the a 1Δ(v=0) state has been observed by far infrared laser magnetic resonance spectroscopy. A new microwave discharge source enabled the detection of this spectrum, which is the first observation of the rotational spectrum of an ion in a metastable state. Assignment and least-squares fitting of the observed transitions have determined the following molecular constants: B0=494.420 388 (22) GHz, the proton hyperfine parameter a=74.84 (32) MHz, gL′ = 1.000 915 (15), and gr=−0.001 815 (18), with the 1σ uncertainties of the last digits in parentheses. The relationship of these parameters to the geometric and electronic structure of OH+ is discussed.

Far infrared LMR spectra of monobromomethyl radicals

Far infrared laser magnetic resonance spectra of the monobromomethyl radicals, CH 2Br and CD 2Br, in the gas phase have been observed at twelve wavelengths between 336 and 1224 μm. The assignment of the spectra to CH 2Br and CD 2Br is based on chemical tests and identification of the rotational transitions involved. Identical spectra have been obtained from two different sources, the reactions of F atoms with CH 3Br (CD 3Br) and of Na atoms with CH 2Br 2 (CD 2Br 2). Although most of the spectra are congested and irregular and do not permit a complete assignment to be made, an analysis based on a planar C 2v structure and symmetric top model yields B+ C=0.744(8) and 0.621(15) cm −1 for CH 2Br and CD 2Br, respectively.

ChemInform Abstract: b‐Dipole Transitions in trans‐HOCO Observed by Far Infrared Laser Magnetic Resonance.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

B-dipole transitions in trans-HOCO observed by far infrared laser magnetic resonance

Far infrared laser magnetic resonance spectroscopy is used to measure components of 12 rotational transitions in the ground state of the HOCO radical. The transitions are all b-dipole in character in contrast to the a-dipole rotational spectrum previously reported [Radford, Wei, and Sears, J. Chem. Phys. 97, 3989 (1992)]. The new data determine the A rotational constant to high precision and allow the determination of several centrifugal distortion constants for the first time. The hyperfine coupling in the radical leads to observable splittings in several of the observed transitions and these are used to estimate two of the four expected nonzero hyperfine parameters in the radical.

Far-Infrared Laser Magnetic Resonance Detection of CH2Cl

Abstract New Far Infrared Laser Magnetic Resonance (LMR) spectra have been detected in the reaction of Cl atoms with CH2CO. Based on chemical and kinetic evidence they were assigned to CH2Cl radicals. The assignment was substantiated by subsequent experiments which employed the reactions of F atoms with CH3Cl and Na atoms with CH2Cl2, respectively. The three different sources yielded identical spectra

Kinetics of the Reaction of OH Radicals with CH2CO

AbstractThe kinetics of the homogeneous gas phase reaction OH + CH2CO → products (1) has been investigated at room temperature and pressures between 1.4 mbar ≤ p ≤ 2.5 mbar using the discharge flow method. Reactors with Teflon wall coatings and with different surface‐to‐volume ratios were employed to exclude effects of heterogeneous reactions. OH radicals were generated via the reaction F + H2O. Their concentration‐versus‐time decay profiles were followed under pseudo‐first‐order conditions, [CH2CO] → [OH]0 with a Far Infrared Laser Magnetic Resonance (FIR‐LMR) spectrometer. The rate constant of the title reaction was found to be k1, (296 K) = (7.2 ± 2.0)·1012 cm3/mol·s.

Far infrared L.M.R. of[Xtilde]2Π NCO

Pure rotational transitions in the [Xtilde] 2Π3/2 state of the NCO radical have been measured by far infrared laser magnetic resonance (L.M.R.) spectroscopy. Analysis of the Zeeman patterns leads to new determinations of the centrifugal distortion parameters, D and H, the lambda doubling parameter q and the g-factors g r and g′L.

Collision‐Induced Intersystem Crossing in CH2 (ã 1A1): A Quantitative Analysis Using the Mixed‐State Model

AbstractThe mechanism of collision‐induced intersystem crossing (ISC) in CH2 from the ã 1A1 first excited singlet to the X̃ 3B1, triplet ground electronic state, CH2 (ã) + M → CH2 (X̃) + M, is investigated. Collisional ISC rate constants are calculated for four selected collider gases, M = He, Ar, N2, and CH4, using the “mixed‐state” model of Freed and Gelbart. The most important triplet‐perturbed rotational levels in the ã‐state are identified from the spectroscopic data reported in the literature. The rates of rotational relaxation processes, which according to the model effectively govern the collision‐induced ISC, are determined by studying the collision broadening of CH2 (X̃) Far Infrared Laser Magnetic Resonance (FIR‐LMR) transitions. The collisional ISC rate constants for CH2 in the ortho nuclear spin modification are calculated to be kISC = 1.4 · 1012, 4.0 · 1012, 7.8 · 1012, and 6.6 · 1012 cm3/mol · s for M = He, Ar, N2, and CH4, respectively. The results are in good agreement with published experimental rate constants.

Measurement of the 2D5/2–2D3/2 fine structure interval in metastable nitrogen atoms at 1.15 mm by laser magnetic resonance

Far infrared laser magnetic resonance (LMR) spectra of metastable (2p3)2D nitrogen atoms have been measured using laser lines in carbonyl fluoride (COF2) at 1135 μm and methanol (CH3OH) at 1224 μm. The concentration of N 2D was optimized by electron paramagnetic resonance (EPR) using a hybrid EPR/LMR spectrometer. The far infrared cavity of this spectrometer was designed to oscillate at wavelengths longer than 1 mm. The LMR spectra were analyzed using g factors derived from the EPR spectrum [Radford and Evenson, Phys. Rev. 168, 70 (1968)] and yield a precise value of the 2D5/2–2D3/2 fine structure separation of −261 447.7(2) MHz, −8.720 957(7) cm−1(2σ).