Infrared Laser Spectroscopy of the CH3OO Radical Formed from the Reaction of CH3 and O2 within a Helium Nanodroplet

Helium Droplet Infrared Spectroscopy of the Butyl Radicals

Butyl radicals ( n-, s-, i-,} and tert-butyl) are formed from the pyrolysis of stable precursors (1-pentyl nitrite, 2-methyl-1-butyl nitrite, isopentyl nitrite, and azo- tert-butane, respectively). The radicals are doped into a beam of liquid helium droplets and probed with infrared action spectroscopy from 2700-3125 cm-1, allowing for a low temperature measurement of the CH stretching region. The presence of anharmonic resonance polyads in the 2800-3000 cm-1 region complicates its interpretation. To facilitate spectral assignment, the anharmonic resonances are modeled with two model Hamiltonian approaches that explicitly couple CH stretch fundamentals to HCH bend overtones and combinations: a VPT2+K normal mode model based on CCSD(T) quartic force fields and a semi-empirical local mode model. Both of these computational methods provide generally good agreement with the experimental spectra.

Chemistry and physics of dopants embedded in helium droplets

Helium droplets represent a cold inert matrix, free of walls with outstanding properties to grow complexes and clusters at conditions that are perfect to simulate cold and dense regions of the interstellar medium. At sub-Kelvin temperatures, barrierless reactions triggered by radicals or ions have been observed and studied by optical spectroscopy and mass spectrometry. The present review summarizes developments of experimental techniques and methods and recent results they enabled.

Can a gas phase contact ion pair containing a hydrocarbon carbocation be formed in the ground state?

So far, no conclusive evidence of a ground-state contact ion-pair containing a hydrocarbon carbocation has been given in the gas phase.

An Intramolecular Hydrogen-Shift in a Peroxy Radical at Cryogenic Temperatures: The Reaction of 2-Hydroxyphenyl Radical with O2

Peroxy radical hydrogen-shifts are pivotal elementary reaction steps in the oxidation of small hydrocarbons in autoignition and the lower atmosphere. Although these reactions are typically associated with a substantial barrier, we demonstrate that the [1,5]H-shift in the peroxy species derived from the 2-hydroxyphenyl radical 1 is so facile that it even proceeds rapidly in an argon matrix at 35 K through a proton-coupled electron transfer mechanism. Hydrogen-bound complexes of o-benzoquinone are identified as the main reaction products by infrared spectroscopy although their formation through O-O bond scission is hampered by a barrier of 11.9 kcal mol^-1 at the ROCCSD(T)/cc-pVTZ/UB3LYP/6-311G(d,p) level of theory.

tert-Butyl peroxy radical: ground and first excited state energetics and fundamental frequencies

The lowest adiabatic electronic transition origin and fundamental vibrational frequencies are computed, with high accuracy, for the tert-butyl peroxy radical.

Matrix Isolation Studies of Novel Intermediates in the Reaction of Trimethylaluminum with Ozone

The thermal reaction of ozone with trimethyl aluminum was explored using twin jet, concentric jet, and merged jet deposition into cryogenic matrixes. Infrared spectroscopy and density functional theory calculations were employed to identify and characterize the products formed in each case. Together, these deposition techniques provide information over the essentially full course of the gas-phase reaction. At short times with twin jet deposition, the primary product is the O atom insertion product (CH₃)₂AlOCH₃. With merged jet deposition and longer gas-phase mixing times, the methyl peroxy radical H₃COO· was seen in good yield along with final stable products H₂CO, H₃COH, and C₂H₆. Production of Al₂O₃ and its deposition onto the walls of the reaction tube as a powdery film was noted as well. All of these outcomes were combined to propose a reaction mechanism for this system. Of particular note, the observation of H₃COO· provides clear evidence for a free radical component to the overall mechanism.

Ethylperoxy radical: approaching spectroscopic accuracy via coupled-cluster theory

Highly reliable ground and excited state properties of the conformers of ethylperoxy radical are predicted using coupled-cluster theory. This research has implications for future characterization of intermediates in tropospheric and low-temperature combustion processes.

The methylsulfinyl radical CH3SO examined

Our computational investigations broaden the scope of currently available experimental results on the methylsulfinyl radical, a key atmospheric species.

Photoinduced molecular dissociation and photoinduced recombination mediated by superfluid helium nanodroplets

Photoinduced predissociation of Cr₂ in helium nanodroplets causes stable, quantum state specific spatial separation followed by geminate recombination upon photoionization.

Monocarbon cationic cluster yields from N2/CH4 mixtures embedded in He nanodroplets and their calculated binding energies

The formation of monocarbon cluster ions has been investigated by electron ionization mass spectrometry of cold helium nanodroplets doped with nitrogen/methane mixtures. Ion yields for two groups of clusters, CHmN2(+) or CHmN4(+), were determined for mixtures with different molecular ratios of CH4. The possible geometrical structures of these clusters were analyzed using electronic structure computations. Little correlation between the ion yields and the associated binding energies has been observed indicating that in most cases kinetic control is more important than thermodynamic control for forming the clusters.

Automation of an "Aculight" continuous-wave optical parametric oscillator

We report the automation of a continuous-wave, singly resonant, optical parametric oscillator (Lockheed-Martin Aculight ARGOS 2400-SF-15). This commercially available optical parametric oscillator (OPO) is capable of producing >1 W of continuously tunable idler output between 2.2 and 4.6 μm. An algorithm based on the feedback from a high accuracy wavemeter is implemented to synchronize three separate OPO tuning elements; the translation of a fan-out type periodically poled lithium niobate crystal, the rotation of an intracavity etalon, and the continuous tuning of the pump and idler wavelengths via piezoelectric strain of the tunable fiber pump laser. This allows for several hundred wavenumbers of efficient, automatic, continuous tuning of the idler wave. Continuous feedback from the wavemeter limits the absolute frequency accuracy to ±20 MHz. The broad, automatic tuning of the OPO is demonstrated via its implementation as a probe laser for the infrared action spectroscopy of methanol solvated in helium nanodroplets. LabVIEW virtual instruments for the automation of this OPO laser system are reported, along with detailed schematics of the associated hardware developed at the University of Georgia.