ISOMERS OF HYDROGEN FLUORIDE CLUSTERS (HF)N(HF)_{N} (N=4-6) FORMED IN HELIUM NANODROPLETS

Author Institution: Department of Chemistry, The University of North Carolina at Chapel HillThe infrared spectra (in the spectral region $3100 cm^{-1}$ to $3900 cm^{-1}$) are reported for hydrogen fluoride clusters formed in helium nanodroplets. Comparisons with previous gas phase spectra reveal new isomeric forms of $(HF)_{n} (n=4-6)$, corresponding to tailed ring structures (i.e. trimer to pentamer rings to which an HF monomer is attached). Pendular state spectroscopy (in the presence of a large DC electric field) is used to establish that these isomers are polar. In addition, comparisons between the experimental and calculated transistion dipole moment directions provide further support for the assignments of the various vibrational bands. The ab initio calculations (at the MP2 / 6-311++G(3df,3pd) level of theory) support the assignment of various spectral features to the $(3+1)$ and the $(4+1)$ HF complexes. Data of this type provides insights into the barriers associated with the insertion of an HF molecule into a preexisting cyclic complex

THE HCN-X (X=Na, K, Rb, Cs) COMPLEXES FORMED ON THE SURFACE OF HELIUM NANODROPLETS

Author Institution: Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599Complexes of hydrogen cyanide with alkali atoms are formed on the surface of helium nanodroplets. The surface bound complexes are probed with infrared laser spectroscopy, and the B" rotational constants for HCN-X (X=Na, K, Rb, Cs) are found to be strongly droplet size dependent. Varying the droplet size, the moment of inertia of each complex scales as the square of the helium droplet radius. In addition, for a fixed droplet size, the moment of inertia scales linearly with the total mass of the complex. These results are consistent with complex rotation on the surface of the droplet with the center of mass at the droplet center

EXPERIMENTAL THERMOCHEMISTRY OF GAS PHASE CYTOSINE TAUTOMERS

Author Institution: Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556Enthalpies of interconversion are measured for the three lowest energy tautomers of isolated cytosine. The equilibrium distribution of tautomers near 600 K is frozen upon the capture of the gas phase species by low temperature helium nanodroplets. The temperature dependence of the gas phase cytosine tautomer populations is determined with infrared laser spectroscopy of the helium solvated species. The interconverison enthalpies obtained from the van't Hoff relation are $1.14 \pm 0.21$ and $1.63 \pm 0.12$ for the \textbf{C31} $\rightleftharpoons$ \textbf{C32} and \textbf{C31} $\rightleftharpoons$ \textbf{C1} equilibria, respectively. \textbf{C31} and \textbf{C32} are rotamers of an enol tautomer, and \textbf{C1} is a keto tautomer. The interconversion enthalpies are compared to recent CCSD(T) thermochemistry calculations of cytosine tautomers

ON THE OUTCOME OF THE REACTIONS BETWEEN HYDROCARBON RADICALS AND O2_{2} IN HELIUM NANODROPLETS

Author Institution: Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556Helium nanodroplet isolation and infrared laser spectroscopy are used to investigate the CH$_{3}$ + O$_{2}$ and C$_{3}$H$_{3}$ (propargyl) + O$_{2}$ reactions. The hydrocarbon radicals are generated in an effusive pyrolysis source located upstream from a differentially pumped O$_{2}$ gas pick-up cell. In this experimental configuration, the reaction occurs between sequentially picked-up and presumably cold fragments. The CH$_{3}$ + O$_{2}$ reaction leads barrierlessly to the methyl-peroxy radical, and despite having to dissipate an energy of approximately 30 kcal/mol, the infrared spectra reveal a large abundance of droplets containing the cold CH$_{3}$O$_{2}$ radical. Theoretical studies have predicted an approximately 2-4 kcal/mol barrier in the entrance channel of the C$_{3}$H$_{3}$ + O$_{2}$ reaction. Therefore, we initially expected to see a weakly bound "entrance channel" C$_{3}$H$_{3}$--O$_{2}$ van-der-Waals complex, given the rapid cooling provided by the dissipative helium environment. However, only the trans-acetylenic isomer of the propargyl-peroxy radical is observed. The dipole moment of this species is measured with infrared laser Stark spectroscopy