Identification of Two Stable Side-Chain Orientations of Valine Methyl Ester by Microwave Spectroscopy.

The rotational spectra of two valine methyl ester (ValOMe) conformers have been measured using a cavity-based Fourier-transform microwave spectrometer in the range of 9-18 GHz. Ten conformers of ValOMe were modeled using the ωB97XD/6-311++G(d,p) level of theory, and separate spectra arising from two lowest-energy conformations were observed and assigned. 44 rotational transitions were assigned to conformer I, the lowest-energy configuration, and were fit to Watson's A-reduced Hamiltonian: A = 2552.0145(5) MHz, B = 1041.8216(3) MHz, and C = 938.54890(22) MHz. 14N nuclear quadrupole hyperfine splittings were resolved, and 231 hyperfine components were fit to χaa = -4.187(7) MHz, and χbb-χcc = 1.269(5) MHz. The spectrum of conformer I also reveals tunneling splittings arising from the methyl rotor. XIAM was used to fit the barrier to the internal rotation of the methyl rotor, and the best-fit V3 barrier was found to be 401.64(19) cm-1. 47 rotational transitions were assigned for conformer II (ΔE = 2.08 kJ mol-1), and the fitted rotational constants are A = 2544.2837(3) MHz, B = 1092.3654(15) MHz, and C = 896.3131(12) MHz. 264 hyperfine components were fit to χaa = -4.187(7) MHz and χbb-χcc = 1.518(6) MHz, and the best-fit V3 barrier was found to be 409.74(16) cm-1.

Low temperature reaction kinetics inside an extended Laval nozzle: REMPI characterization and detection by broadband rotational spectroscopy.

Chirped-Pulse Fourier-Transform millimeter wave (CP-FTmmW) spectroscopy is a powerful method that enables detection of quantum state specific reactants and products in mixtures. We have successfully coupled this technique with a pulsed uniform Laval flow system to study photodissociation and reactions at low temperature, which we refer to as CPUF ("Chirped-Pulse/Uniform flow"). Detection by CPUF requires monitoring the free induction decay (FID) of the rotational coherence. However, the high collision frequency in high-density uniform supersonic flows can interfere with the FID and attenuate the signal. One way to overcome this is to sample the flow, but this can cause interference from shocks in the sampling region. This led us to develop an extended Laval nozzle that creates a uniform flow within the nozzle itself, after which the gas undergoes a shock-free secondary expansion to cold, low pressure conditions ideal for CP-FTmmW detection. Impact pressure measurements, commonly used to characterize Laval flows, cannot be used to monitor the flow within the nozzle. Therefore, we implemented a REMPI (resonance-enhanced multiphoton ionization) detection scheme that allows the interrogation of the conditions of the flow directly inside the extended nozzle, confirming the fluid dynamics simulations of the flow environment. We describe the development of the new 20 K extended flow, along with its characterization using REMPI and computational fluid dynamics. Finally, we demonstrate its application to the first low temperature measurement of the reaction kinetics of HCO with O2 and obtain a rate coefficient at 20 K of 6.66 ± 0.47 × 10-11 cm3 molec-1 s-1.

Multichannel Radical-Radical Reaction Dynamics of NO + Propargyl Probed by Broadband Rotational Spectroscopy.

Chirped-pulse rotational spectroscopy in a quasi-uniform flow has been used to investigate the reaction dynamics of a multichannel radical-radical reaction of relevance to planetary atmospheres and combustion. In this work, the NO + propargyl (C3H3) reaction was found to yield six product channels containing eight detected species. These products and their branching fractions (%), are as follows: HCN (50), HCNO (18), CH2CN (12), CH3CN (7.4), HC3N (6.2), HNC (2.3), CH2CO (1.3), HCO (1.8). The results are discussed in light of previous unimolecular photodissociation studies of isoxazole and prior potential energy surface calculations of the NO + C3H3 system. The results also show that the product branching is strongly influenced by the excess energy of the reactant radicals. The implications of the title reaction to the planetary atmospheres, particularly to Titan, are discussed.

Uniform supersonic flow sampling for detection by chirped-pulse rotational spectroscopy.

Chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy is a powerful near-universal detection method finding application in many areas. We have previously coupled it with supersonic flows (CPUF) to obtain product branching in reaction and photodissociation. Because chirped-pulse microwave detection requires monitoring the free induction decay on the timescale of microseconds, it cannot be employed with good sensitivity at the high densities achieved in some uniform supersonic flows. For application to low-temperature kinetics studies, a truly uniform flow is required to obtain reliable rate measurements and enjoy all the advantages that CP-FTMW has to offer. To this end, we present a new setup that combines sampling of uniform supersonic flows using an airfoil-shaped sampling device with chirped-pulse mmW detection. Density and temperature variations in the airfoil-sampled uniform flow were revealed using time-dependent rotational spectroscopy of pyridine and vinyl cyanide photoproducts, highlighting the use of UV photodissociation as a sensitive diagnostic tool for uniform flows. The performance of the new airfoil-equipped CPUF rotational spectrometer was validated using kinetics measurements of the CN + C2H6 reaction at 50 K with detection of the HCN product. Issues relating to product detection by rotational spectroscopy and airfoil sampling are discussed. We show that airfoil sampling enables direct measurements of low temperature reaction kinetics on a microsecond timescale, while rotational spectroscopic detection enables highly specific simultaneous detection of reactants and products.

Radical-Radical Reaction Dynamics Probed Using Millimeterwave Spectroscopy: Propargyl + NH2/ND2.

We apply chirped-pulse uniform flow millimeterwave (CPUF-mmW) spectroscopy to study the complex multichannel reaction dynamics in the reaction between the propargyl and amino radicals (C3H3 + NH2/ND2), a radical-radical reaction of importance in the gas-phase chemistry of astrochemical environments and combustion systems. The photolytically generated radicals are allowed to react in a well-characterized quasi-uniform supersonic flow, and mmW rotational spectroscopy (70-93 GHz) is used for simultaneous detection of the reaction products: HCN, HNC, HC3N, DCN, DNC, and DC3N, while spectral intensities of the measured pure-rotational lines allow product branching to be quantified. High-level electronic structure calculations were used for theoretical prediction of the reaction pathways and branching. Experimentally deduced product branching fractions were compared with the results from statistical simulations based on the RRKM theory. Product branching was found to be strongly dependent on the excess internal energy of the C3H3 and NH2/ND2 reactants.

Fourier Transform Microwave Spectrum of Propene-3-d1 (CH2═CHCH2D), Quadrupole Coupling Constants of Deuterium, and a Semiexperimental Equilibrium Structure of Propene.

The ground-state rotational spectrum of propene-3-d1, CH2═CHCH2D, was measured by Fourier transform microwave spectroscopy. Transitions were assigned for the two conformers, one with the D atom in the symmetry plane (S) and the other with the D atom out of the plane (A). The energy difference between the two conformers was calculated to be 6.5 cm-1, the S conformer having lower energy. The quadrupole hyperfine structure due to deuterium was resolved and analyzed for both conformers. The experimental quadrupole coupling and the centrifugal distortion constants compared favorably to their ab initio counterparts. Ground-state rotational constants for the S conformer are 40582.157(9), 9067.024(1), and 7766.0165(12) MHz. Ground-state rotational constants for the A conformer are 43403.75(3), 8658.961(2), and 7718.247(2) MHz. For the A conformer, a small tunneling splitting (19 MHz) due to internal rotation was observed and analyzed. Using the new rotational constants of this work as well as those previously determined for the 13C species and for some deuterium-substituted species from the literature, a new semiexperimental equilibrium structure was determined and its high accuracy was confirmed. The difficulty in obtaining accurate coordinates for the out-of-plane hydrogen atom is discussed.

Probing the Electronic Environment of Methylindoles using Internal Rotation and (14)N Nuclear Quadrupole Coupling.

High-resolution rotational spectra were recorded in the 10.5-21.0 GHz frequency range for seven singly methylated indoles. (14)N nuclear quadrupole hyperfine structure and spectral splittings arising from tunneling along the internal rotation of the methyl group were resolved for all indole species. The nuclear quadrupole coupling constants were used to characterize the electronic environment of the nitrogen atom, and the program XIAM was used to fit the barrier to internal rotation to the measured transition frequencies. The best fit barriers were found to be 277.1(2), 374.32(4), 414.(5), 331.6(2), 126.8675(15), 121.413(4), and 426(3) cm(-1) for 1-methylindole through 7-methylindole, respectively. The fitted barriers were found to be in good agreement with barriers calculated at the ωB97XD/6-311++G(d,p) level. The complete set of experimental barriers is compared to theoretical investigations of the origins of methyl torsional barriers and confirms that the magnitude of these barriers is an overall effect of individual hyperconjugative and structural interactions of many bonding/antibonding orbitals.

Microwave, structural, conformational, vibrational studies and ab initio calculations of isocyanocyclopentane.

The infrared and Raman spectra (3200-50 cm(-1)) of the gas, liquid or solution, and solid have been recorded of isocyanocyclopentane, c-C5H9NC. FT-microwave studies have also been carried out and 23 transitions were recorded for the envelope-axial (Ax) conformer. Variable temperature (-65 to -100 °C) studies of the infrared spectra (3200-400 cm(-1)) dissolved in liquid xenon have been carried out. From these data, both the Ax and envelope-equatorial (Eq) conformers have been identified and their relative stabilities obtained. The enthalpy difference has been determined to be 102±10 cm(-1) (1.21±0.11 kJ mol(-1)) with the Ax conformer the more stable form. The percentage of the Eq conformer is estimated to be 38±1% at ambient temperature. The conformational stabilities have been predicted from ab initio calculations by utilizing several different basis sets up to aug-cc-pVTZ from both MP2(full) and density functional theory calculations by the B3LYP method. Vibrational assignments have been made for the observed bands for both conformers with initial predictions by MP2(full)/6-31G(d) ab initio calculations to obtain harmonic force constants, wavenumbers, infrared intensities, Raman activities and depolarization ratios for both conformers. The structural parameter values for the Ax form are; for the heavy atom distances (Å): C≡N = 1.176 (3); Cα-N=1.432 (3); Cα-Cß,Cß'=1.534 (3); Cß-Cγ,Cγ'=1.542 (3); Cγ-Cγ'=1.554 (3) and angles (°):∠Cα-N≡C=177.8 (5); ∠CßCα-N=110.4 (5);

Microwave and infrared spectra, adjusted r0 structural parameters, conformational stabilities, vibrational assignments, and theoretical calculations of cyclobutylcarboxylic acid chloride.

The FT-microwave spectrum of cyclobutylcarboxylic acid chloride, c-C4H7C(O)Cl, has been recorded and 153 transitions for the (35)Cl and (37)Cl isotopologues have been assigned for the gauche-equatorial (g-Eq) conformation. The ground state rotational constants were determined for (35)Cl [(37)Cl]: A = 4349.8429(25) [4322.0555(56)] MHz, B = 1414.8032(25) [1384.5058(25)] MHz, and C = 1148.2411(25) [1126.3546(25)] MHz. From these rotational constants and ab initio predicted parameters, adjusted r0 parameters are reported with distances (Å) rCα-C = 1.491(4), rC═O = 1.193(3), rCα-Cß = 1.553(4), rCα-Cß' = 1.540(4), rCγ-Cß = 1.547(4), rCγ-Cß' = 1.546(4), rC-Cl = 1.801(3) and angles (deg) τCγCßCß'Cα = 30.9(5). Variable temperature (-70 to -100 °C) infrared spectra (4000 to 400 cm(-1)) were recorded in liquid xenon and the g-Eq conformer was determined the most stable form, with enthalpy differences of 91 ± 9 cm(-1) (1.09 ± 0.11 kJ/mol) for the gauche-axial (g-Ax) form and 173 ± 17 cm(-1) (2.07 ± 0.20 kJ/mol) for the trans-equatorial (t-Eq) conformer. The relative amounts at ambient temperature are 54% g-Eq, 35 ± 1% g-Ax, and 12 ± 1% t-Eq forms. Vibrational assignments have been provided for the three conformers and theoretical calculations were carried out. The results are discussed and compared to corresponding properties of related molecules.