Ethyl + O2 in Helium Nanodroplets: Infrared Spectroscopy of the Ethylperoxy Radical.

Helium-solvated ethylperoxy radicals (CH3CH2OO•) are formed via the in situ reaction between 2A' ethyl radical and 3Σg- dioxygen. 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. An infrared spectrum of ethylperoxy, in the CH stretching region, is recorded with species-selective droplet beam depletion spectroscopy. Spectral assignments are made via comparisons to second-order vibrational perturbation theory with resonances (VPT2 + K) based on coupled-cluster full quartic force fields. Cubic and quartic force constants, evaluated using a small basis set, are transformed into the normal coordinate system of the higher level quadratic force constants. This transformation procedure eliminates the mismatch between normal modes, which is a source of error whenever normal coordinate force constants from different levels of theory are combined. The spectrum shows signatures of both the C1 gauche and C s trans rotamers in an approximate 2:1 ratio; this is despite the prediction that the gauche rotamer lies 44 cm-1 lower on the zero-Kelvin enthalpic potential surface for torsional interconversion. Helium droplets are 0.4 K at equilibrium; therefore, in situ ethylperoxy production is highly nonthermal.

Infrared Spectrum of Fulvenallene and Fulvenallenyl in Helium Droplets.

Fulvenallene is the global minimum on the C7H6 potential energy surface. Rearrangement of fulvenallene to other C7H6 species and dissociation to produce fulvenallenyl radical (C7H5) is carried out in a continuous-wave SiC pyrolysis furnace at 1500 K. Prompt pick-up and solvation by helium droplets allows for the acquisition of vibrational spectra of these species in the CH stretching region. Anharmonic frequencies for fulvenallene, fulvenallenyl, and three isomers of ethynylcyclopentadiene are computed ab initio; VPT2+K spectral simulations are based on hybrid CCSD(T) force fields with quadratic (cubic and quartic) force constants computed using the ANO1 (ANO0) basis set. The acetylenic CH stretch of the fulvenallenyl radical is a sensitive marker of the extent by which the unpaired electron is delocalized throughout the conjugated propargyl and cyclopentadienyl subunits. The nature of this electron delocalization is explored with spin density calculations at the ROHF-CCSD(T)/ANO1 level of theory. Atomic partitioning of the spin density allows for a description of the fulvenallenyl radical in terms of two resonance structures: fulvenallenyl is approximately 24% allenic and 76% acetylenic.

Sequential Capture of O(3P) and HCN by Helium Nanodroplets: Infrared Spectroscopy and ab Initio Computations of the 3Σ O-HCN Complex.

Catalytic thermal cracking of O2 is employed to dope helium droplets with O(3P) atoms. Mass spectrometry of the doped droplet beam reveals an O2 dissociation efficiency larger than 60%; approximately 26% of the droplet ensemble is doped with single oxygen atoms. Sequential capture of O(3P) and HCN leads to the production of a hydrogen-bound O-HCN complex in a 3Σ electronic state, as determined via comparisons of experimental and theoretical rovibrational Stark spectroscopy. Ab initio computations of the three lowest lying intermolecular potential energy surfaces reveal two isomers, the hydrogen-bound (3Σ) O-HCN complex and a nitrogen-bound (3Π) HCN-O complex, lying 323 cm-1 higher in energy. The HCN-O to O-HCN interconversion barrier is predicted to be 42 cm-1. Consistent with this relatively small interconversion barrier, there is no experimental evidence for the production of the nitrogen-bound species upon sequential capture of O(3P) and HCN.

Infrared Stark and Zeeman spectroscopy of OH-CO: The entrance channel complex along the OH + CO → trans-HOCO reaction pathway.

Sequential capture of OH and CO by superfluid helium droplets leads exclusively to the formation of the linear, entrance-channel complex, OH-CO. This species is characterized by infrared laser Stark and Zeeman spectroscopy via measurements of the fundamental OH stretching vibration. Experimental dipole moments are in disagreement with ab initio calculations at the equilibrium geometry, indicating large-amplitude motion on the ground state potential energy surface. Vibrational averaging along the hydroxyl bending coordinate recovers 80% of the observed deviation from the equilibrium dipole moment. Inhomogeneous line broadening in the zero-field spectrum is modeled with an effective Hamiltonian approach that aims to account for the anisotropic molecule-helium interaction potential that arises as the OH-CO complex is displaced from the center of the droplet.

Mid-infrared signatures of hydroxyl containing water clusters: Infrared laser Stark spectroscopy of OH-H2O and OH(D2O)n (n = 1-3).

Small water clusters containing a single hydroxyl radical are synthesized in liquid helium droplets. The OH-H2O and OH(D2O)n clusters (n = 1-3) are probed with infrared laser spectroscopy in the vicinity of the hydroxyl radical OH stretch vibration. Experimental band origins are qualitatively consistent with ab initio calculations of the global minimum structures; however, frequency shifts from isolated OH are significantly over-predicted by both B3LYP and MP2 methods. An effective Hamiltonian that accounts for partial quenching of electronic angular momentum is used to analyze Stark spectra of the OH-H2O and OH-D2O binary complexes, revealing a 3.70(5) D permanent electric dipole moment. Computations of the dipole moment are in good agreement with experiment when large-amplitude vibrational averaging is taken into account. Polarization spectroscopy is employed to characterize two vibrational bands assigned to OH(D2O)2, revealing two nearly isoenergetic cyclic isomers that differ in the orientation of the non-hydrogen-bonded deuterium atoms relative to the plane of the three oxygen atoms. The dipole moments for these clusters are determined to be approximately 2.5 and 1.8 D for "up-up" and "up-down" structures, respectively. Hydroxyl stretching bands of larger clusters containing three or more D2O molecules are observed shifted approximately 300 cm(-1) to the red of the isolated OH radical. Pressure dependence studies and ab initio calculations imply the presence of multiple cyclic isomers of OH(D2O)3.

Infrared Spectroscopy of OH··CH3OH: Hydrogen-Bonded Intermediate Along the Hydrogen Abstraction Reaction Path.

Substantial non-Arrhenius behavior has been previously observed in the low temperature reaction between the hydroxyl radical and methanol. This behavior can be rationalized assuming the stabilization of an association adduct in the entrance channel of the reaction, from which barrier penetration via quantum mechanical tunneling produces the CH3O radical and H2O. Helium nanodroplet isolation and a serial pick-up technique are used to stabilize the hydrogen bonded prereactive OH··CH3OH complex. Mass spectrometry and infrared spectroscopy are used to confirm its production and probe the OH stretch vibrations. Stark spectroscopy reveals the magnitude of the permanent electric dipole moment, which is compared to ab initio calculations that account for wide-amplitude motion in the complex. The vibrationally averaged structure has Cs symmetry with the OH moiety hydrogen bonded to the hydroxyl group of methanol. Nevertheless, the zero-point level of the complex exhibits a wave function significantly delocalized over a bending coordinate leading to the transition state of the CH3O producing reaction.