Product-specific reaction kinetics in continuous uniform supersonic flows probed by chirped-pulse microwave spectroscopy.

Experimental studies of the products of elementary gas-phase chemical reactions occurring at low temperatures (<50 K) are very scarce, but of importance for fundamental studies of reaction dynamics, comparisons with high-level quantum dynamical calculations, and, in particular, for providing data for the modeling of cold astrophysical environments, such as dense interstellar clouds, the atmospheres of the outer planets, and cometary comae. This study describes the construction and testing of a new apparatus designed to measure product branching fractions of elementary bimolecular gas-phase reactions at low temperatures. It combines chirped-pulse Fourier transform millimeter wave spectroscopy with continuous uniform supersonic flows and high repetition rate laser photolysis. After a comprehensive description of the apparatus, the experimental procedures and data processing protocols used for signal recovery, the capabilities of the instrument are explored by the study of the photodissociation of acrylonitrile and the detection of two of its photoproducts, HC3N and HCN. A description is then given of a study of the reactions of the CN radical with C2H2 at 30 K, detecting the HC3N product, and with C2H6 at 10 K, detecting the HCN product. A calibration of these two products is finally attempted using the photodissociation of acrylonitrile as a reference process. The limitations and possible improvements in the instrument are discussed in conclusion.

Pure Rotational Spectroscopy of the CH2CN Radical Extended to the Sub-Millimeter Wave Spectral Region.

We present a thorough pure rotational investigation of the CH2CN radical in its ground vibrational state. Our measurements cover the millimeter and sub-millimeter wave spectral regions (79-860 GHz) using a W-band chirped-pulse instrument and a frequency multiplication chain-based spectrometer. The radical was produced in a flow cell at room temperature by H abstraction from acetonitrile using atomic fluorine. The newly recorded transitions of CH2CN (involving N″ and Ka″ up to 42 and 8, respectively) were combined with the literature data, leading to a refinement of the spectroscopic parameters of the species using a Watson S-reduced Hamiltonian. In particular, the A rotational constant and K-dependent parameters are significantly better determined than in previous studies. The present model, which reproduces all experimental transitions to their experimental accuracy, allows for confident searches for the radical in cold to warm environments of the interstellar medium.

A novel Ka-band chirped-pulse spectrometer used in the determination of pressure broadening coefficients of astrochemical molecules.

A novel chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer has been constructed to cover the Ka-band (26.5 GHz-40 GHz) for use in the CRESUCHIRP project, which aims to study the branching ratios of reactions at low temperatures using the chirped-pulse in uniform flow technique. The design takes advantage of recent developments in radio-frequency components, notably, high-frequency, high-power solid-state amplifiers. The spectrometer had a flatness of 5.5 dB across the spectral range, produced harmonic signals below -20 dBc, and the recorded signal scaled well to 6 × 106 averages. The new spectrometer was used to determine pressure broadening coefficients with a helium collider at room temperature for three molecules relevant to astrochemistry, applying the Voigt function to fit the magnitude of the Fourier-transformed data in the frequency domain. The pressure broadening coefficient for carbonyl sulfide was determined to be (2.45 ± 0.02) MHz mbar-1 at room temperature, which agreed well with previous measurements. Pressure broadening coefficients were also determined for multiple transitions of vinyl cyanide and benzonitrile. Additionally, the spectrometer was coupled with a cold, uniform flow from a Laval nozzle. The spectrum of vinyl cyanide was recorded in the flow, and its rotational temperature was determined to be (24 ± 11) K. This temperature agreed with a prediction of the composite temperature of the system through simulations of the experimental environment coupled with calculations of the solution to the optical Bloch equations. These results pave the way for future quantitative studies in low-temperature and high-pressure environments using CP-FTMW spectroscopy.

Matrix Isolation ESR and Theoretical Study of ZnN.

The Zn14N, 67Zn14N, Zn15N, and 67Zn15N radicals have been formed by the reaction of a plume of zinc metal produced with laser ablation and either ammonia vapor or nitrogen atoms isolated in an inert neon matrix at 4.3 K. The ground electronic state of ZnN was determined to be 4∑- using electron spin resonance spectroscopy. The following magnetic parameters were determined experimentally for ZnN: g⊥ = 1.9998(3), g∥ = 2.0018(3), | D| = 7268(8) MHz, A⊥(14N) = -17.9(20) MHz, A∥(14N) = 1.5(20) MHz, A⊥(15N) = 25.1(20) MHz, A∥(15N)= -2.0(20) MHz, A⊥(67Zn) = 156(3) MHz, and A∥(67Zn) = 168(12) MHz. The low-lying electronic states of ZnN were also investigated using the complete active space self-consistent field technique. By plotting the potential energy surface, theoretical parameters for the ground state with a configuration of 8σ29σ210σ14π2 were determined, including re = 2.079 Å and De = 1.0 kcal/mol.

Low Temperature Kinetics of the Reaction Between Methanol and the CN Radical.

Methanol (CH3OH) is considered by astronomers to be the simplest complex organic molecule (COM) and has been detected in various astrophysical environments, including protoplanetary disks, comets, and the interstellar medium (ISM). Studying the reactivity of methanol at low temperatures will aid our understanding of the formation of other complex and potentially prebiotic molecules. A major destruction route for many neutral COMs, including methanol, is via their reactions with radicals such as CN, which is ubiquitous in space. Here, we study the kinetics of the reaction between methanol and the CN radical using the well-established CRESU technique (a French acronym standing for Reaction Kinetics in Uniform Supersonic Flow) combined with Pulsed-Laser Photolysis-Laser-Induced Fluorescence (PLP-LIF). Electronic structure calculations were also performed to identify the exothermic channels through which this reaction can proceed. Our results for the rate coefficient are represented by the modified Arrhenius equation, k(T) = 1.26 × 10-11(T/300 K)-0.7 exp(-5.4 K/T), and display a negative temperature dependence over the temperature range 16.7-296 K, which is typical of what has been seen previously for other radical-neutral reactions that do not possess potential energy barriers. The rate coefficients obtained at room temperature strongly disagree with a previous kinetics study, which is currently available in the Kinetics Database for Astrochemistry (KIDA) and therefore used in some astrochemical models.

A matrix isolation ESR investigation of Mg+-N2.

The adducts formed between 25Mg+ with 14N2 and 25Mg+ with 15N2 have been trapped in a solid neon matrix and studied with electron spin resonance (ESR) spectroscopy. These radical species were formed through the interaction of laser ablated magnesium and nitrogen gas. The Mg+-N2 radical species was found to have a ground electronic state of 2Σ+ in a linear configuration with discrete coupling to the proximate nitrogen resolved in the spectra. Fitting the ESR spectra allowed magnetic parameters to be determined as follows: g⊥ = 2.0012(5), g∥ = 2.0015(8), A⊥(1-14N) = 32(3) MHz, A∥(1-14N) = 34(5) MHz, A⊥(1-15N) = 45(4) MHz, A∥(1-15N) = 47(6) MHz, A⊥(25Mg) = -581(5) MHz, and A∥(25Mg) = -582(5) MHz, and estimates derived for A⊥(2-14N) = 1(2) MHz, A∥(2-14N) = 2(5) MHz, A⊥(2-15N) = 2(2) MHz, and A∥(2-15N) = 4(6) MHz. Ab initio calculations using the coupled-cluster single double triple methodology showed that the linear form was 59.7 kcal mol-1 more stable than the T-shaped form. The potential energy curve around the equilibrium geometry was explored using the complete active space self-consistent field approach, and Hartree-Fock singles and double configuration interaction and multireference singles and double configuration interaction calculations of the hyperfine coupling constants were undertaken, and reasonable agreement with the experiment was observed.

A matrix isolation ESR investigation of the MgCH radical.

The MgCH radical and its magnesium-25, carbon-13, and deuterated isotopologs have been isolated in low temperature neon matrices and examined by the matrix isolation electron spin resonance technique for the first time. The radicals were formed through the reactions of laser ablated natural abundance magnesium metal and magnesium-25 enriched magnesium metal with carbon-13 and deuterated isotopologs of acetone. The MgCH radical was shown to have a X4Σ- ground electronic state, and the magnetic parameters determined for this state were g⊥ = 2.001 81(45), g∥ = 2.0018(10), D = 4970(5) MHz, A⊥(13C) = 115(6) MHz, A∥(13C) = 65(15) MHz, A⊥(H) = 34(6) MHz, A∥(H) = 5(10) MHz, A⊥(D) = 5(3) MHz, A⊥(25Mg) = 82(5) MHz, and A∥(25Mg) = 85(10). Comparisons are made between the electronic structure of this radical and the MgCH3 and MgN radicals. Theoretical hyperfine parameters were also evaluated for the MgCH radical, and a potential energy surface for the low-lying electronic states was constructed using complete active space multiconfigurational self-consistent field theory. The leading configuration (96.6%) for the X4Σ- ground electronic state was shown to be 1σ22σ23σ21π44σ25σ26σ27σ12π12π1 with an Mg-C bond length of 2.041 Å for a fixed C-H bond length of 1.090 Å. The Mg-C bond dissociation energy (De) was 48.26 kcal/mol. The optimized geometry from a density functional theory calculation using the B3LYP functional gave a Mg-C bond length of 2.061 Å and a C-H bond length of 1.090 Å.

A matrix isolation ESR and theoretical study of MgN.

Matrix isolation experiments have been conducted on the Mg14N, 25Mg14N, Mg15N, and 25Mg15N radicals which were formed by the reaction of a plume of magnesium metal produced with laser ablation and either acetonitrile vapour or nitrogen atoms. The radicals were isolated in an inert neon matrix at 4.3 K and studied with electron spin resonance spectroscopy. The ground electronic state of MgN was determined to be 4Σ-. The following magnetic parameters were determined experimentally for MgN: g⊥ = 2.004 78 (2), g∥ = 2.001 72 (4), |D| = 9797 (6) MHz, A⊥(14N) = 19.7 (2) MHz, A∥ (14N) = -4.0 (3) MHz, A⊥(15N) = 27.5 (3) MHz, A∥ (15N) = -5.7 (3) MHz, A⊥ (25Mg) = -60.7 (5) MHz, and A∥(25Mg) = -65 (3) MHz. The low-lying electronic states of MgN were also investigated using the complete active space multiconfigurational self-consistent field technique. By plotting the potential energy surface, theoretical parameters for the ground state with a configuration of 5σ26σ27σ12π12π1 were able to be determined, including re = 2.090 Å and De = 11.28 kcal/mol.