Skip to main content

High Resolution Laser Spectroscopy of Radical Containing Complexes and Radical-Radical Reaction Products in Helium Nanodroplets

 

High Resolution Laser Spectroscopy of Radical Containing Complexes and Radical-Radical Reaction Products in Helium Nanodroplets

Dr. Gary Douberly

Department of Chemistry, University of Georgia, Athens, Georgia 30602 USA

Helium nanodroplet isolation (HENDI) is a versatile technique for many forms of molecular spectroscopy.  Helium nanodroplets provide a dissipative medium for studying at 0.4 Kelvin, the structure and dynamics of novel systems such as free-radicals, metal clusters, and molecular clusters.  In this talk, I will discuss our recent use of HENDI and infrared laser spectroscopy to investigate the CH3 + O2 and C3H3 + O2 reactions.  HENDI is also used to investigate the effect of the superfluid helium environment on the spectroscopy and dynamics of the OH radical and its complexes with O2 and C2H2

Hydrocarbon radicals generated in an effusive pyrolysis source are picked-up by helium droplets and cooled to 0.4 K prior to the addition of single O2 molecules.  In this experimental configuration, a reaction may occur between sequentially picked-up and cold reactants.  The resulting products of this low temperature reaction are probed spectroscopically downstream from the pick-up zones.  The CH3 + O2 reaction leads barrierlessly to the methyl-peroxy radical, and although the droplets must dissipate an energy of ~30 kcal/mol, the infrared spectra reveal a large abundance of droplets containing the cold CH3O2 radical.  Theoretical studies have predicted an approximately 2-4 kcal/mol barrier in the entrance channel of the C3H3 + O2 reaction.  Because of this prediction, a weakly bound “entrance channel” C3H3--O2 van-der-Waals complex was expected, given the rapid cooling provided by the dissipative helium environment.  In addition to the trans-acetylenic isomer of the propargyl-peroxy (C3H3O2) radical, we now have experimental evidence for the metastable C3H3--O2 complex.

The fundamental vibrational band of the helium solvated hydroxyl radical (OH) has two sharp Q(3/2) lines.  The splitting is consistent with a fivefold increase in the parity (lambda type) doubling of the 2P3/2 state in helium droplets relative to the gas phase.  This parity splitting increase is rationalized in terms of the differences in the potential energy surfaces for He-OH(A') and He-OH(A").  Rotationally resolved OH and CH stretching bands of the T-shaped OH-C2H2 complex reveal that the electronic angular momentum of OH in the complex is only partially quenched, and to a similar degree as observed in the gas phase.1,2  This indicates that the helium droplet environment does not significantly affect the electronic intermolecular interactions in OH-C2H2.

HO3 and HOOO-(O2)n clusters have also been assembled in helium nanodroplets.  The trans-HOOO isomer is observed and has vibrational band origins within 1 cm-1 of the gas phase values.3,4  This supports recent theoretical calculations, which show that the HO + O2 reaction is barrierless.  Neither of the two other predicted stable isomers, namely cis-HOOO and the hydrogen-bound OH-O2 species,5 were found within a broad survey scan.  HOOO-(O2)n clusters grow in to the red of the n1 band of trans-HOOO as the O2 pick-up pressure is increased.  HOOO-(O2)n cluster bands are resolved up to n=4.  High level ab initio calculations are being carried out to help understand the geometries of these multiple O2 clusters.

(1)        J. B. Davey, M. E. Greenslade, M. D. Marshall, M. I. Lester, and M. D. Wheeler, J. Chem. Phys. 121, 3009 (2004)

(2)        M. D. Marshall, J. B. Davey, M. E. Greenslade, and M. I. Lester, J. Chem. Phys. 121, 5845 (2004)

(3)        Derro, E. L.; Murray, C.; Sechler, T. D.; Lester, M. I. J. Phys. Chem. A 2007, 111, 11592

(4)        Derro, E. L.; Sechler, T. D.; Murray, C.; Lester, M. I. J. Chem. Phys. 2008, 128.

(5)        Braams, B. J.; Yu, H. G. PCCP 2008, 10, 3150.

For more information on Dr. Douberly and his research, click here.

Faculty Host: Dr. Dong-Sheng Yang

Date:
-
Location:
CP-137

The Origin and Evolution of Photosynthesis

Dr. Hyman Hartman of MIT will be presenting a seminar entitled The Origin and Evolution of Photosynthesis.

Abstract:

The origin and evolution of photosynthesis is considered to be the key to the origin of life. This eliminates the need for a soup as the synthesis of the bioorganics are to come from the fixation of carbon dioxide and nitrogen. No soup then no RNA world or Protein world.  Cyanobacteria have been formed by the horizontal transfer of green sulfur bacterial photoreaction center genes by means of a plasmid into a purple photosynthetic bacterium.

The fixation of carbon dioxide is considered to have evolved from a reductive dicarboxylic acid cycle  (Chloroflexus) which was then followed by a reductive tricarboxylic acid cycle (Chlorobium) and  finally by the reductive pentose phosphate cycle (Calvin cycle).

The origin of life is considered to have occurred in a hot spring on the outgassing early earth. The first organisms were self-replicating iron-rich clays which fixed carbon  dioxide into oxalic and other dicarboxylic acids. This system of replicating clays and their metabolic phenotype then evolved into the sulfide rich region of the hotspring acquiring the ability to fix nitrogen. Finally phosphate was incorporated into the evolving systemwhich allowed the synthesis of nucleotides and phospholipids.

If biosynthesis recapitulates biopoesis, then the synthesis of amino acids preceded  the synthesis of the purine and pyrimidine bases. Furthermore the polymerization of the amino acid thioesters into polypeptides preceded the directed polymerization of amino acid esters by polynucleotides. Thus the origin and evolution of the genetic code is a late development and records the takeover of the clay by RNA.

Faculty Host: Dr. Marcelo Guzman

Date:
-
Location:
CP-137

The Methoxy Radical: An Incredible Playground for Hi-resolution Molecular Spectroscopists

Dr. Jinjun Liu of the University of Louisville will be presenting a seminar entitled The Methoxy Radical: An Incredible Playground for Hi-resolution Molecular Spectroscopists.

Faculty Host: Dr. Dennis Clouthier

Date:
-
Location:
CP-137

Advancing our Understanding of Blood and its Functions with Biotechnology

Dr. Dana Spence of Michigan State University will be presenting a seminar entitled Advancing our Understanding of Blood and its Functions with Biotechnology.

For more information about Dr. Spence and his research, click here.

Faculty Host: Dr. Susan Odom

Date:
-
Location:
CP-137
Subscribe to