Infrared photodissociation of weakly bound "mass tags" is widely used to determine the structures of ions by analyzing their vibrational spectra. Molecular hydrogen is a common choice for tagging in cryogenic radio-frequency ion traps. Although the H2 molecules can introduce distortions in the target species, we demonstrate an advantage of H2 tagging in the analysis of positional isomers adopted by the molecular anions derived from decarboxylation of formylbenzoates. Attachment of H2 to the carbanion centers of three such isomers yields distinct shifts in the H2 stretch, which can be used to determine the distribution of isomers in an unknown sample. Electronic structure calculations indicate that the position-dependent shifts are due to different reactivities of the carbanion sites with respect to an intracluster proton-transfer reaction with the H2 molecule. We exploit this spectroscopic method to quantify the surprisingly facile migrations of the anionic center that have been previously reported for phenide rearrangements.

Decarboxylation of carboxylate ions in the gas phase provides a useful window into the chemistry displayed by these reactive carbanion intermediates. Here, we explore the species generated by decarboxylation of two benzoate derivatives: 2-formylbenzoate (2FBA) and 2-benzoylbenzoate (2BBA). The nascent product anions are transferred to a cryogenic ion trap where they are cooled to ∼15 K and analyzed by their pattern of vibrational bands obtained with IR photodissociation spectroscopy of weakly bound H2 molecules. The structures of the quenched species are then determined by comparison of these spectra with those predicted by electronic structure calculations for local minima on the potential energy surface. The 2-phenide carbanion generated by decarboxylation of 2FBA occurs in two isomeric forms that differ in the orientation of the formyl group, both of which yield a very large (∼110 cm-1) redshift in the stretching frequency of the H2 molecule attached to the anionic carbon center. Although calculated to be a local minimum, the analogous 2-phenide species could not be isolated upon decarboxylation of 2BBA. Rather, the anionic product adopts a ring-closed structure, indicating efficient nucleophilic attack on the pendant phenyl group by the nascent phenide. The barrier for ring closing is evaluated with electronic structure calculations.

In the presence of a halide ion, hypohalous acids can adopt two binding motifs upon formation of the ion-molecule complexes [XHOY]- (X, Y = Cl, Br, I): a hydrogen (HB) bond to the acid OH group and a halogen (XB) bond between the anion and the acid halogen. Here we isolate the X-bonded Cl-·IOH ion-molecule complex by collisions of I-·(H2O)n clusters with HOCl vapor and measure its vibrational spectrum by IR photodissociation of the H2-tagged complex. Anharmonic analysis of its vibrational band pattern reveals that formation of the XB complex results in dramatic lowering of the HOI bending fundamental frequency and elongation of the O-I bond (by 168 cm-1 and 0.13 Å, respectively, relative to isolated HOI). The frequency of the O-I stretch (estimated 436 cm-1) is also encoded in the spectrum by the weak v = 0 → 2 overtone transition at 872 cm-1.

It remains an open question as to whether polycyclic aromatic hydrocarbons (PAHs) can be efficiently formed in the ISM by bottom-up mechanisms involving growth from small aromatic precursors like benzene.However, the lack of dipole moment renders benzene invisible in the radio regime making estimation of the abundance of benzene in the ISM difficult.The recent detection of benzonitrile in the Taurus Molecular Cloud (TMC)-1 has caused excitement in the astrochemical community as it is the first aromatic molecule detected in the interstellar medium (ISM) using radio astronomy.Benzonitrile is thought to form via the neutral-neutral reaction between the CN radical and benzene, and therefore may serve as a chemical proxy to determine the abundance of benzene.The abundances of aromatic species in ISM environments are not well understood, in part due to a lack of experimental kinetics data.Both rate constants and product-branching ratios for the reactions of aromatic molecules must be measured at low temperature in order to input these reactions into astrochemical models and accurately predict abundances.Benzene and toluene are two of the aromatic species detected in the atmosphere of Titan and their reactions with the CN radical have been studied down to 105 K by Trevitt et al.Here, we have extended this study down to 15 K to approach dense cloud conditions and have measured the rate constants of the reactions of benzene and toluene with the CN radical using the well-established CRESU technique (Cinétique de Réaction en Ecoulement Supersonique Uniforme, or Reaction Kinetics in Uniform Supersonic Flow) combined with the Pulsed Laser Photolysis-Laser-Induced Fluorescence method.I will also discuss our recent progress in combining chirped-pulse micro/mm-wave spectroscopy with the CRESU method and how we plan to employ this technique to measure product branching ratios for reactions of the CN radical with aromatics at low temperatures.

CN is known for its fast reactions with hydrocarbons at low temperatures, but relatively few studies have focused on the reactions between CN and aromatic molecules. The recent detection of benzonitrile in the interstellar medium, believed to be produced by the reaction of CN and benzene, has ignited interest in studying these reactions. Here, we report rate constants of the CN + toluene (C7H8) reaction between 15 and 294 K using a CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme; reaction kinetics in uniform supersonic flow) apparatus coupled with the pulsed laser photolysis-laser-induced fluorescence (PLP-LIF) technique. We also present the stationary points on the potential energy surface of this reaction to study the available reaction pathways. We find the rate constant does not change over this temperature range, with an average value of (4.1 ± 0.2) × 10-10 cm3 s-1, which is notably faster than the only previous measurement at 105 K. While the reason for this disagreement is unknown, we discuss the possibility that it is related to enhanced multiphoton effects in the previous work.

The low-temperature reaction between CN and benzene (C 6 H 6 ) is of significant interest in the astrochemical community due to the recent detection of benzonitrile, the first aromatic molecule identified in the interstellar medium (ISM) using radio astronomy. Benzonitrile is suggested to be a low-temperature proxy for benzene, one of the simplest aromatic molecules, which may be a precursor to polycyclic aromatic hydrocarbons. In order to assess the robustness of benzonitrile as a proxy for benzene, low-temperature kinetics measurements are required to confirm whether the reaction remains rapid at the low gas temperatures found in cold dense clouds. Here, we study the C 6 H 6 + CN reaction in the temperature range 15–295 K, 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. We obtain rate coefficients, k ( T ), in the range (3.6–5.4) × 10 −10 cm 3 s −1 with no obvious temperature dependence between 15 and 295 K, confirming that the CN + C 6 H 6 reaction remains rapid at temperatures relevant to the cold ISM.

Arom. compds., such as toluene and benzene, are important anthropogenically emitted volatile org. compds. in urban areas, and lead to the prodn. of secondary org. aerosols. However, there are many questions that remain about the mechanism of their atm. oxidn. and very few direct observations of the radial intermediates have been made. Therefore, we have used mid-IR pulsed-laser photolysis cavity ringdown spectroscopy to directly detect the hydroxy-cyclohexadienyl radicals formed from the addn. of OH to toluene and benzene in lab. expts. Vibrational spectra and kinetic models of the reaction chem. will be presented.

The microwave spectrum of the vinyl chloride-hydrogen chloride complex, presented at last year's symposium, is greatly complicated by the presence of two chlorine nuclei as well as an observed, but not fully explained tunneling motion.Indeed, although it was possible at that time to demonstrate conclusively that the complex is nonplanar, the chlorine nuclear quadrupole hyperfine splitting in the rotational spectrum resisted analysis.With higher resolution, Balle-Flygare Fourier transform microwave spectra, the hyperfine structure has been more fully resolved, but appears to be perturbed for some rotational transitions.It appears that knowledge of the quadrupole coupling constants will provide essential information regarding the structure of the complex, specifically the location of the hydrogen atom in HCl.Our progress towards obtaining values for these constants will be presented.

H. O. Leung and M. D. Marshall, J. Chem. Phys. 126, 114310 (2007).H. O. Leung, M. D. Marshall, W. T. Cashion, and V. L. Chen, J. Chem. Phys. 128, 064315 (2008).

The microwave spectra of six isotopologues each of 2-chloro-1,1-difluoroethylene and its argon complex are obtained in the 5.6-20.5 GHz region using a combination of broadband chirped pulse and Balle-Flygare cavity Fourier transform microwave techniques. The structure of the halogen substituted ethylene has been determined using a combination of theory and experiment. Argon is found to form a nonplanar complex with this haloethylene, locating in the CICCF cavity, which maximizes its interactions with the heavy atoms in the ethylene subunit. A combined analysis of the chlorine quadrupole coupling constants of the monomer and of the complex allows the sign of the only nonzero off-diagonal component of the quadrupole coupling tensor for the monomer to be determined; thus, the complete tensor for the monomer has been derived. (C) 2014 Elsevier Inc. All rights reserved.