Spectroscopy of free radicals and radical containing entrance-channel complexes in superfluid helium nano-droplets

The spectroscopy of free radicals and radical containing entrance-channel complexes embedded in superfluid helium nano-droplets is reviewed. The collection of dopants inside individual droplets in the beam represents a micro-canonical ensemble, and as such each droplet may be considered an isolated cryo-reactor. The unique properties of the droplets, namely their low temperature (0.4 K) and fast cooling rates ($\sim10^{16}$ K s$^{-1}$) provides novel opportunities for the formation and high-resolution studies of molecular complexes containing one or more free radicals. The production methods of radicals are discussed in light of their applicability for embedding the radicals in helium droplets. The spectroscopic studies performed to date on molecular radicals and on entrance / exit-channel complexes of radicals with stable molecules are detailed. The observed complexes provide new information on the potential energy surfaces of several fundamental chemical reactions and on the intermolecular interactions present in open-shell systems. Prospects of further experiments of radicals embedded in helium droplets are discussed, especially the possibilities to prepare and study high-energy structures and their controlled manipulation, as well as the possibility of fundamental physics experiments.Comment: 25 pages, 12 figures, 4 tables (RevTeX

Structures of the linear silicon carbides SiC 4 and SiC 6 : Isotopic substitution and Ab Initio theory

V. D. Gordon,a) E. S. Nathan, A. J. Apponi, M. C. McCarthy, and P. Thaddeus are with Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138 and Division of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, Massachusetts 02138 -- P. Botschwina is with the Institut fu¨r Physikalische Chemie der Universita¨t Go¨ttingen, Tammannstr, 6. D-37077 Go¨ttingen, GermanyThe structures of two linear silicon carbides, SiC4 and SiC6, have been determined by a combination of isotopic substitution and large-scale coupled-cluster ab initio calculations, following detection of all of the singly substituted isotopic species in a supersonic molecular beam with a Fourier transform microwave spectrometer. Rotational constants obtained by least-squares fitting transition frequencies were used to derive experimental structures; except for those nearest the center of mass, individual bond lengths for both chains have an error of less than 0.008 Å. Accurate equilibrium structures were derived by converting the experimental rotational constants to equilibrium constants using the vibration–rotation coupling constants from coupled-cluster calculations, including connected triple substitutions. Equilibrium dipole moments and harmonic vibrational frequencies were also calculated for both chains. On the basis of the calculated vibration–rotation and l-type doubling constants, weak rotational satellites from a low-lying vibrational state of SiC4 were assigned to v6 , a bending mode calculated to lie about 205 cm -1 above the ground state. A recommended ab initio equilibrium structure for SiC8 has also been established. © 2000 American Institute of Physics. @S0021-9606~00!01537-3#Chemistr

Astronomical identification of CN-, the smallest observed molecular anion

We present the first astronomical detection of a diatomic negative ion, the cyanide anion CN-, as well as quantum mechanical calculations of the excitation of this anion through collisions with para-H2. CN- is identified through the observation of the J = 2-1 and J = 3-2 rotational transitions in the C-star envelope IRC +10216 with the IRAM 30-m telescope. The U-shaped line profiles indicate that CN-, like the large anion C6H-, is formed in the outer regions of the envelope. Chemical and excitation model calculations suggest that this species forms from the reaction of large carbon anions with N atoms, rather than from the radiative attachment of an electron to CN, as is the case for large molecular anions. The unexpectedly large abundance derived for CN-, 0.25 % relative to CN, makes likely its detection in other astronomical sources. A parallel search for the small anion C2H- remains so far unconclusive, despite the previous tentative identification of the J = 1-0 rotational transition. The abundance of C2H- in IRC +10216 is found to be vanishingly small, < 0.0014 % relative to C2H.Comment: 5 pages, 4 figures; accepted for publication in A&A Letter

THE EQUILIBRIUM GEOMETRIES OF C3C_{3} AND C3C_{3}

$^{1}$Coupled Cluster method with single and double excitation operators and a quasiperturbative treatment of the effects of connected triple excitations; K. Ragavachari, G.W. Trucks, J.A. Pople and M. Head-Gordon, Chem. Phys. Lett. 157, 479 (1989). $^{2}$N. Moazzon-Ahmadi, S.D. Flatt and A.R.W. McKrllar, Chem. Phys. Lett, 186, 201 (1991). $^{3}$K.P Huber and G. Hersberg, Molecular Spectra and Molecular Structure, IV Constats of Diatomie Molecules, Van Nostrand, New York, 1979.Author Institution: Instutut f\""{u}r Physikalische Chemie, der Universit\""{a}tMaking use of large-scale CCSD(T)-$calcualtions^{1}$ the equilibrium geometry of $C_{5}$ was calculated to be linear with $R_{1e}$ (outer CC) = 1.28959 {\AA} and $R_{2e}$ (inner CC) = 1.28190 {\AA}, with uncertainties of ca 0.0005 {\AA}. The corresponding equilibrium rotational constant is $B_{e}$ = 25550.6 MHz in good agreement with an approximate experimental value of 2548.7 $MHz.^{2}$ Analogous calculations of $C_{3}$ yield a linear equilibrium structure with $R_{e}$ = 1.29431 {\AA} for $C_{2}$ a value of 1.24209 {\AA} is obtained. The latter differs from $experiment^{3}$ by 0.0004 {\AA}