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The ethyl radical has been isolated and spectroscopically characterized in 4He nanodroplets. The band origins of the five CH stretch fundamentals are shifted by < 2 cm-1 from those reported for the gas phase species [S. Davis, D. Uy, and D. J. Nesbitt, J. Chem. Phys. 112, 1823 (2000); T. Häber, A. C. Blair, D. J. Nesbitt, and M. D. Schuder, J. Chem. Phys. 124, 054316 (2006)]. The symmetric CH2 stretching band (v1) is rotationally resolved, revealing nuclear spin statistical weights predicted by G12 permutation-inversion group theory. A permanent electric dipole moment of 0.28 (2) D is obtained via the Stark spectrum of the v1 band. The four other CH stretch fundamental bands are significantly broadened in He droplets and lack rotational fine structure. This broadening is attributed to symmetry dependent vibration-to-vibration relaxation facilitated by the He droplet environment. In addition to the five fundamentals, three a1′ overtone/combination bands are observed, and each of these have resolved rotational substructure. These are assigned to the 2v12, v4 + v6, and 2v6 bands through comparisons to anharmonic frequency computations at the CCSD(T)/cc-pVTZ level of theory. [ABSTRACT FROM AUTHOR]

The HOOO hydridotrioxygen radical and its deuterated analog (DOOO) have been isolated in helium nanodroplets following the in situ association reaction between OH and O2. The infrared spectrum in the 3500-3700 cm-1 region reveals bands that are assigned to the ν1 (OH stretch) fundamental and ν1 + ν6 (OH stretch plus torsion) combination band of the trans-HOOO isomer. The helium droplet spectrum is assigned on the basis of a detailed comparison to the infrared spectrum of HOOO produced in the gas phase [E. L. Derro, T. D. Sechler, C. Murray, and M. I. Lester, J. Chem. Phys. 128, 244313 (2008)]. Despite the characteristic low temperature and rapid cooling of helium nanodroplets, there is no evidence for the formation of a weakly bound OH-O2 van der Waals complex, which implies the absence of a kinetically significant barrier in the entrance channel of the reaction. There is also no spectroscopic evidence for the formation of cis-HOOO, which is predicted by theory to be nearly isoenergetic to the trans isomer. Under conditions that favor the introduction of multiple O2 molecules to the droplets, bands associated with larger H/DOOO-(O2)n clusters are observed shifted ∼1-10 cm-1 to the red of the trans-H/DOOO ν1 bands. [ABSTRACT FROM AUTHOR]

C3H3+ ions produced with a pulsed discharge source and cooled in a supersonic beam are studied with infrared laser photodissociation spectroscopy in the 800–4000 cm-1 region using the rare gas tagging method. Vibrational bands in the C–H stretching and fingerprint regions confirm the presence of both the cyclopropenyl and propargyl cations. Because there is a high barrier separating these two structures, they are presumed to be produced by different routes in the plasma chemistry; their relative abundance can be adjusted by varying the ion source conditions. Prominent features for the cyclopropenyl species include the asymmetric carbon stretch (ν5) at 1293 cm-1 and the asymmetric C–H stretch (ν4) at 3182 cm-1, whereas propargyl has the CH2 scissors (ν4) at 1445, the C–C triple bond stretch (ν3) at 2077 and three C–H stretches (ν2, ν9, and ν1) at 3004, 3093, and 3238 cm-1. Density functional theory computations of vibrational spectra for the two isomeric ions with and without the argon tag reproduce the experimental features qualitatively; according to theory the tag atom only perturbs the spectra slightly. Although these data confirm the accepted structural pictures of the cyclopropenyl and propargyl cations, close agreement between theoretical predictions and the measured vibrational band positions and intensities cannot be obtained. Band intensities are influenced by the energy dependence and dynamics of photodissociation, but there appear to be fundamental problems in computed band positions independent of the level of theory employed. These new data provide infrared signatures in the fingerprint region for these prototypical carbocations that may aid in their astrophysical detection. [ABSTRACT FROM AUTHOR]

The proton-bridged dimers of nitrogen, e.g., N2–H+–N2 and N2–D+–N2, are produced in a pulsed-discharge supersonic nozzle source, mass selected in a reflectron time-of-flight spectrometer, and studied with infrared photodissociation spectroscopy using the method of messenger atom tagging with argon. Both complexes are studied from 700–4000 cm-1. These spectra reproduce the high frequency vibrations seen previously but discover many new vibrational bands, particularly those in the region of the shared proton modes. Because of the linear structure of the core ions, simple vibrational spectra are expected containing only the antisymmetric N–N stretch and two lower frequency modes corresponding to proton stretching and bending motions. However, many additional bands are detected corresponding to various combination bands in this system activated by anharmonic couplings of the proton motions. The anharmonic coupling is stronger for the H+ system than it is for the D+ system. Using anharmonic proton vibrations computed previously and combinations of computed harmonic frequencies, reasonable assignments can be made for the spectra of both isotopomers. However, advanced anharmonic computational treatments are needed for this system to confirm these assignments. [ABSTRACT FROM AUTHOR]

High-resolution infrared (IR) spectroscopy is used to investigate the Mg–HF and Mg–(HF)2 van der Waals complexes. Both complexes are formed and probed within helium nanodroplets. Rotationally resolved zero-field and Stark spectra are assigned to a linear binary complex composed of a Mg atom bound to the hydrogen end of the HF molecule. Although high level ab initio calculations predict a fluorine bonded complex, none of the observed IR bands can be assigned to this complex. The collocation method is employed to determine the bound states on the two-dimensional intermolecular Mg–HF potential energy surface. The ground and first excited state wave functions for this potential surface have zero amplitude in the well corresponding to the fluorine bonded complex, consistent with experiment. The two HF stretching bands of the Mg–(HF)2 complex are observed and assigned using a combination of the spectral symmetry, ab initio calculations, pick-up cell pressure dependencies, and dipole moment measurements. Comparisons with the helium solvated HF dimer show large changes to the HF stretching frequencies upon the addition of a single Mg atom to the hydrogen side of (HF)2. [ABSTRACT FROM AUTHOR]

C3H5+ cations are probed with infrared photodissociation spectroscopy in the 800–3500 cm-1 region using the method of rare gas tagging. The ions and their complexes with Ar or N2 are produced in a pulsed electric discharge supersonic expansion cluster source. Two structural isomers are characterized, namely, the allyl (CH2CHCH2+) and 2-propenyl (CH3CCH2+) cations. The infrared spectrum of the allyl cation confirms previous theoretical and condensed phase studies of the C2v charge delocalized, resonance-stabilized structure. The 2-propenyl cation spectrum is consistent with a Cs symmetry structure having a nearly linear CCC backbone and a hyperconjugatively stabilizing methyl group. [ABSTRACT FROM AUTHOR]

Helium nanodroplets are applied to stabilization and the highly reactive metal atom-HCN complexes are studied. Theoretical calculations are used for describing the reaction mechanisms and energetics of these metal atom reactions with HCN.

The first gas phase infrared spectrum of the tert-butyl cation is described. The photodissociation methods are described by using new infrared lasers and ion sources with supersonic cooling in order to detect the structures of carbocations in the gas phase.

Three constitutional isomers of both Ca2H2 and Ca2H4 have been characterized with molecular electronic structure theory. Correlation methods as complete as CCSDT(Q) and basis sets as large as cc-pwCV5Z have been used to converge the relative energies within chemical accuracy (≤1 kcal mol−1). Anharmonic vibrational frequencies were computed using second-order vibrational perturbation theory employing CCSD(T)/cc-pwCVTZ cubic and quartic force-fields and a CCSD(T)/cc-pwCVQZ quadratic force field. The monobridged [Ca(μ2-H)CaH] and dibridged [Ca(μ2-H)2Ca] isomers of Ca2H2 were predicted to lie 6.5 and 12.9 kcal mol−1 below the energy of the classical HCaCaH linear isomer, respectively. Despite the energetic favorability of the bridged Ca2H2 isomers, we conclude (surprisingly) that only the higher energy linear structure has been observed in the laboratory. At 0 K, the tribridged [Ca(μ2-H)3CaH] isomer of Ca2H4 is predicted to be enthalpically favored by 0.9 kcal mol−1 in comparison to the enthalpy of the dibridged [HCa(μ2-H)2CaH] structure. Comparison of experiment with our computed frequencies suggests that the observed vibrational features arise from both the dibridged and the tribridged Ca2H4 structures. [ABSTRACT FROM AUTHOR]

Avidin coated the peptide tubules without denaturing confirmed by fluorescein-tagged D-biotin attachment onto the avidin-coated peptide tubules is presented. The result indicates that the protein tubules may be attached at desired surfaces using biological interactions such as antibody-antigen recognition and protein tubules can be used for fabrication such as microelectronics and protein array biosensors.

Isoprene (C5H8) is an abundant, reactive tropospheric hydrocarbon, derived from biogenic emissions. A detailed understanding of the spectroscopy of isoprene is therefore desirable. Isoprene monomer is isolated in helium droplets and its infrared spectrum is measured in the CH stretching region. Anharmonic frequencies are predicted by VPT2+K simulations employing CCSD(T) force fields with quadratic (cubic and quartic) force constants computed using the ANO1 (ANO0) basis set. The vast majority of the spectral features can be assigned to trans-isoprene on the basis of these computations. Some features of the higher energy gauche conformer are also assignable, by comparison to experiments using heated isoprene. Convergent ab initio thermochemistry is presented for the isomerization pathway, for which the partition function explicitly accounts for the eigenstates associated with separate, uncoupled one-dimensional potential surfaces for methyl torsion and internal rotation between rotamers. The respective 0 and 298.15 K trans/gauche energy differences are 2.82 and 2.52 kcal/mol, which implies a room temperature gauche population of 2.8%. [ABSTRACT FROM AUTHOR]

Chlorine atoms, generated through the thermal decomposition of Cl2, are solvated in superfluid helium nanodroplets and clustered with HCl molecules. The H-Cl stretching modes of these clusters are probed via infrared laser spectroscopy. A band centered at ∼2880.8 cm-1 is assigned to the binary Cl-HCl complex on the basis of HCl pressure dependence and difference mass spectra. The band lies in the "free" HCl stretching region, implying that the complex is not hydrogen bound. Furthermore, the breadth of the band (∼2 cm-1 fwhm) is consistent with an assignment to a predominantly b-type component of the H-Cl stretch, as the dominant b-type selection rules and A rotational constant allow for high energy rotational excitations that efficiently couple to droplet excitations, resulting in fast rotational deactivation. Despite the lack of rotational fine structure, which would verify the assignment, the observed band is consistent with the stabilization of a weakly bound complex having an approximately L-shaped geometry. Frequency computations for a rigid, L-shaped complex reveal that the transition dipole moment vector points almost entirely along the b inertial axis; indeed, the signal-to-noise ratio in our experiment precluded the observation of an a-type component of the HCl stretching band for the complex. No bands were observed that could be assigned to a linear H-bonded Cl-HCl complex. Additionally, we located bands that are consistent with the formation of Cl2-HCl, Cl2-(HCl)2, and Cl-(HCl)2. Two vibrations of the Cl-(HCl)2 complex were found, and harmonic frequencies and intensities computed for a cyclic structure are consistent with the observations. [ABSTRACT FROM AUTHOR]