Gas phase cyclobutyl radical (C₄H₇) is produced via pyrolysis of cyclobutyl methyl nitrite (C₄H₇(CH₂)ONO). The nascent radicals are promptly solvated in liquid He droplets, allowing for the acquisition of the infrared spectrum in the CH stretching region. Anharmonic frequencies are predicted by VPT2+K simulations based upon a CCSD(T)/ANO0 force field. Several resonance polyads emerge in the 2800-3000 cm⁻¹region as a result of anharmonic coupling between the CH stretching fundamentals and CH₂ bend overtones and combinations. Evidence of rotational fine structure is observed for two bands. The vibrationally averaged cyclobutyl radical geometry and the C₄H₇ potential energy surface will be discussed. In agreement with the findings by Schultz¹ and coworkers, 1,3-butadiene is formed from cyclobutyl ring opening and H atom loss, given a sufficiently high pyrolysis temperature. However, signatures of 1-methylallyl and allylcarbinyl radicals, proposed¹ as intermediates along the above mentioned reaction path, are yet to be seen. 1. Schultz, J.C., Houle, F.A., Beauchamp, J.L. J. Am. Chem. Soc. 1984, 106, 7336-7347.

Atomic oxygen (³ P) is generated via thermolysis in a commerical thermal gas cracker (Mantis Ltd. MGC-75). Complexes with HCN were investigated to qualitatively assess the doping efficiency of O(³ P) into a helium droplet. Theoretical calculations of a linear O ··· HCN (³ Σ) complex at the CCSD(T)/aug-cc-pVTZ level are consistent with the rotational constants extracted from the rotational substructure in the experimental spectra, and with dipole moments approximated from Stark spectra. The thermal source will be used to study reactions between O(³ P) and hydrocarbons in helium droplets, and preliminary data on this topic will be presented.

Helium-solvated ethylperoxy radicals (CH₃CH₂OO•) are formed via the in situ reaction between ethyl radicals and ~X³Σ_g⁻ O₂. The reactants are captured sequentially through the droplet pick-up technique. Helium droplets are doped with ethyl radical via pyrolysis of di-tert-amyl peroxide or n-propylnitrite in an effusive, low-pressure source. A mid-infrared spectrum of ethylperoxy is recorded with species-selective droplet beam depletion spectroscopy. Spectral assignments in the CH stretching region are made via comparisons to second-order vibrational perturbation theory with resonances (VPT2+K) based on coupled-cluster quartic force fields. Gauche and trans conformers are predicted to be nearly isoenergetic; however, the spectrum indicates that one dominant conformer is present. Indeed, in several previous studies in our group, where chemical reactions were conducted inside droplets, only a single conformer of the product was observed. Exploration of the ethylperoxy potential energy surface, particularly along the CCOO torsional and CO stretching coordinates, motivates an explanation that is based upon an adiabatic funneling mechanism that leads to the exclusive production of one conformer. The slower torsional degree of freedom is cooled more rapidly than the higher frequency stretching and bending coordinates owing to the stronger coupling between the torsional modes and the collective modes of the helium droplet. The reactants are cooled into the torsional well that stabilizes first during their approach on the PES.

The reaction of chlorine atoms (Cl) with isoprene (C₅H₈) in solid para-hydrogen (p-H₂) matrices at 3.2 K has been studied using infrared spectroscopy. Mixtures of C₅H₈ and Cl₂ were co-deposited in p-H₂ at 3.2 K, followed by irradiation at 365 nm to cause the photodissociation of Cl₂ and the subsequent reaction of Cl atoms with C₅H₈. Upon 365 nm photolysis, a series of new lines appeared in the infrared spectrum, with the strongest appearing at 807.8 and 796.7 cm⁻¹. To determine the grouping of lines to distinct chemical species, secondary photolysis was performed using a low-pressure Hg lamp in combination with various filters. Based on the secondary photolysis behavior, it was determined that the majority of the new lines belong to two distinct chemical species, designated as set A (3047.2, 1482.2, 1459.5, 1396.6, 1349.6, 1268.2, 1237.9, 1170.3, 1108.8, 807.8, 754.1, 605.6, 526.9, 472.7 cm⁻¹) and set B (3112.7, 1487.6, 1382.6, 1257.7, 1229.1, 1034.8, 975.8, 942.4, 796.7, 667.9, 569.7 cm⁻¹). The most likely reactions to occur between Cl and C₅H₈ under the low temperature conditions in solid p-H₂ are the addition of the Cl atom to the four distinct alkene carbon atoms to produce the corresponding chlorine atom addition radicals (ClC₅H₈). Quantum-chemical calculations were performed at the B3PW91/6-311++G(2d,2p) level of theory for the four possible ClC₅H₈ radicals in order to determine the relative energetics and the predicted harmonic vibrational spectra for each radical. The calculations predict that the addition of Cl to each of the four carbons is exothermic, with relative energies of 0.0, 74.5, 67.4, and 7.9 kJ/mol for the addition to carbons 1 – 4, respectively. When the lines of set A and B are compared to the scaled harmonic vibrational spectra for all four of the possible Cl addition radicals, it is found that the best agreement for set A is with the radical produced by the addition to carbon 4 (1-chloromethyl-2-methylallyl radical) and the best agreement for set B is with the radical produced by addition to carbon 1 (1-chloromethyl-1-methylallyl radical). Therefore, the lines of set A and B are assigned to these radicals, respectively.

The thermal decomposition of methyl acetate (CH₃COOCH₃) has been studied in a set of flash pyrolysis micro-reactors. Samples were diluted to (0.06 – 0.13%) in carrier gases (He, Ar) and subjected to temperatures of 300 - 1600 K at roughly 20 Torr. After residence times of approximately 25 – 150 μseconds, the unimolecular pyrolysis products were detected by vacuum ultraviolet photoionization mass spectrometry at 10.487 eV (118.2 nm). Complementary product identification was provided by matrix isolation infrared spectroscopy. Decomposition began at 1000 K with the observation of (CH₂=C=O, CH₃OH), products of a four centered rearrangement with a ∆_rxnH₂₉₈ = 39.1 ± 0.2 kcal mol⁻¹. As the micro-reactor was heated to 1300 K, a mixture of (CH₂=C=O, CH₃OH, CH₃, CH₂=O, H, CO, CO₂) appeared. A new novel pathway is calculated in which both methyl groups leave behind CO₂ simultaneously, ∆_rxnH₂₉₈ = 74.5 ± 0.4 kcal mol⁻¹. This pathway is in contrast to step-wise loss of methyl radical, which can go in two ways: ∆_rxnH₂₉₈ (CH₃COOCH₃ → CH₃ + COOCH₃) = 95.4 ± 0.4 kcal mol⁻¹, ∆_rxnH₂₉₈ (CH₃COOCH₃ → CH₃COO + CH₃) = 88.0 ± 0.3 kcal mol⁻¹.

The rapidly growing list of potential plant-derived biofuels creates a challenge for the scientific community to provide a molecular-scale understanding of their combustion. Development of accurate combustion models rests on a foundation of experimental data on the kinetics and product branching ratios of their individual reaction steps. Therefore, new spectroscopic tools are necessary to selectively detect and characterize fuel components and reactive intermediates generated by pyrolysis and combustion. Substituted furans, including furanic ethers, are considered second-generation biofuel candidates. Following the work of the Ellison group, an 8-18 GHz microwave study was carried out on the unimolecular and bimolecular decomposition of the smallest furanic ether, 2-methoxy furan, and it`s pyrolysis intermediate, the 2-furanyloxy radical, formed in a high-temperature pyrolysis source coupled to a supersonic expansion. Details of the experimental setup and analysis of the spectrum of the radical will be discussed.

Building upon the foundation provided by previous work, the X₁²Π_3/2 and X₂²Π_1/2 states of the transient radical, BrO, were interrogated in previously unprobed spectral regions (0.5 to 1.7 THz) by employing JPL developed high-resolution cascaded frequency multiplier sources. Like other members of the halogen monoxides (XO), this species has been the target of several recent atmospheric remote sensing studies and is a known participant in a catalytic ozone degradation cycle. For the current work, BrO is generated in an inductively coupled plasma under dynamic flow conditions and rotational lines are observed directly at their Doppler-limited resolution. New spectral transitions including those owing to both the ground (ν=0) and excited (ν=1 and 2) vibrational states of isotopologues composed of permutations of natural abundance ¹⁶O, ¹⁸O, ⁷⁹Br, and ⁸¹Br are fit to a global Hamiltonian containing both fine and hyperfine terms. In addition to further refining existing spectroscopic parameters, new observations will be made available to remote detection communities through addition to the JPL catalog. New findings will be discussed along with future plans to extend these studies to other halogen monoxides (X=Cl and I) and the more massive halogen dioxides (OXO & XOO).

The electronic spectrum of the jet-cooled SnH₂ radical has been detected by LIF spectroscopy. The radical was produced in a pulsed electric discharge through a precursor mixture of SnH₄ in argon. Each band in the LIF spectrum consists of a small number of rovibronic transitions to the lowest energy (K_a = 0, J = 0,1,2,3) rotational levels in the excited state. High resolution spectra of the ^pP₁(1) line of the 2²₀ band show 7 components whose relative intensities are characteristic of the tin major isotopic abundances. The emission spectra are also consistent with assigning the spectrum as due to SnH₂. The fluorescence lifetimes of the upper state rotational levels decrease with increasing J', indicative of a rotationally dependent predissociation process in the excited state, similar to that previously observed in SiH₂ and GeH₂. Fluorescence hole burning experiments have located the upper state K_a = 2 levels which allow a determination of the molecular structure.

The C₃ molecule has been detected in a variety of astrophysical objects thanks to the well-known 4050 Å (A¹Π_u-X¹Σ⁺ _g) electronic transition as well as the two IR active modes of the electronic ground state: ν₂ ( ∼  63.42 cm⁻¹) and ν₃ ( ∼  2040.02 cm⁻¹)L. Gausset, G. Herzberg, A. Lagerqvist, B. Rosen, Astrophysical Journal, 45-81 (1965); T. F. Giesen et al., The Astrophysical Journal, 551, L181-L184 (2001); K. W. Hinkle, J. J. Keady, P. F. Bernath, Science, 241, 1319-1322 (1988) Previous laboratory data in the ν₃ region, obtained using diode laser spectroscopy and the photolysis of allene to produce C₃, permitted measurement of the fundamental (0,0,1)Σ-(0,0,0)Σ as well as the hot bands: (0,1,1)Π-(0,1,0)Π; (0,2,1)Σ-(0,2,0)Σ; (0,2,1)∆-(0,2,0)∆ and provided insights on the anharmonicity of the (0,nν₂,1) vibrational patternK. Kawaguchi et al., J. Chem. Phys., 91, 1953-1957 (1989) We have recorded the absorption spectrum of C₃ in the 1800-2100 cm⁻¹ region (at a resolution of 0.003 cm⁻¹) using the Bruker IFS 125 Fourier Transform spectrometer at the AILES beamline of Synchrotron SOLEIL. C₃ was produced in a DC discharge of methane heavily diluted in helium. The rovibrational temperature of C₃ produced in our discharge is noticeably higher than in Ref. [4], which allowed us to extend measurements to higher J values. More interestingly, we assigned new hot bands involving higher quanta of the ν₂ bending states: (0,nν₂,1) with n ranging from 0 to 5. Despite the absence of Q branches for these bands, which results in a possible ambiguous J-assignment of P and R lines, the large variety of data considered in this work, in addition to our experimental data and including observations of comet spectra, allows confident assignments. L. Gausset, G. Herzberg, A. Lagerqvist, B. Rosen, Astrophysical Journal, 45-81 (1965); T. F. Giesen et al., The Astrophysical Journal, 551, L181-L184 (2001); K. W. Hinkle, J. J. Keady, P. F. Bernath, Science, 241, 1319-1322 (1988). K. Kawaguchi et al., J. Chem. Phys., 91, 1953-1957 (1989).

A very strong LIF spectrum was observed in the 655 - 600 nm region from the products of an electric discharge through a dilute mixture of silicon tetrachloride and oxygen in argon. The same spectrum was obtained from a Cl₃Si-O-SiCl₃ precursor in argon. The LIF bands do not have resolved rotational structure, suggesting that the carrier of the spectrum is a heavy molecule. Emission spectra show substantial differences depending on which upper state vibronic level is probed, and these differences are readily understood if we assume that the spectrum is due to the Cl₃SiO free radical with a significant Jahn-Teller effect in the ground ²E state. This conclusion is reinforced by our own ab intio calculations of the ground and excited state vibronic energy levels, band contours, chlorine isotope effects, and electronic excitation energies. Cl₃SiO is the first siloxy radical to be detected in the gas phase.

Targeting an anion with the formula CH₃O₃ for exploration with photoelectron imaging spectroscopy, we determine its identity to be dihydroxymethanolate, an anion largely absent in the literature, and the conjugate base of the hypothetical species orthoformic acid. Comparing the observed photoelectron spectrum to CCSD-EOM-IP and CCSD-EOM-SF calculations completed in QChem and Franck-Condon overlap simulations in PESCAL, we are able to determine with confidence the connectivity of the atoms in this molecule.

The SiCF free radical has been detected through the A²Σ⁺-X²Π band system in the 605 - 550 nm region. The radical was produced in an electric discharge through a dilute mixture of CF₃SiH₃ in high pressure argon and studied by laser induced fluorescence. The vibronic levels of the ground and excited states have been measured through LIF and emission spectroscopy and a Renner-Teller analysis has been undertaken for the ground ²Π levels. The observed vibrational frequencies, partially resolved rotational band contours, Renner-Teller parameter, and electronic excitation energy are in accord with our predictions from high level ab initio (CCSD(T)/aug-cc-pVTZ) calculations. Theory shows that the radical has a silicon-carbon double bond in the ground state and a much shorter triple bond in the excited state. This is the third in the series of SiCX (X = H, Cl, and F) free radicals we have produced and studied in the gas phase.