We experimentally study the dynamics of chemical reactions relevant
to catalysis, combustion, and atmospheric chemistry. Our research
employs photolytic and pyrolytic techniques to generate molecular
beams containing neutral transition metal atoms, complexes,
free radicals, and aerosol particles. Reactions are initiated
by crossing such beams with a second molecular beam under single
collision conditions. The angular and velocity distributions of
the reaction products from single bimolecular collisions are measured
to learn about: 1) the branching ratios for competing reactions;
2) the reaction mechanism(s); and 3) the disposal of excess energy
into the products' degrees of freedom.
Further details about our recent work as well as preprints
and reprints of journal articles may be found at our group
website.
Transition metal chemistry
We study the bimolecular reaction dynamics of ground
state and electronically excited transition metal atoms, clusters,
and complexes with small molecules using a rotating source crossed
molecular beams apparatus. (See Figure) A transition metal beam
is generated by laser vaporization from a solid metal rod and
entrained in a stream of inert carrier gas from a piezoelectrically
actuated pulsed valve. The resulting beam is collimated
by a skimmer and then crosses a second molecular beam containing
the hydrocarbon reactant of interest. Each beam has
a narrow velocity distribution which may be controlled by selecting
different inert carrier gases. It is therefore possible to study
bimolecular reactions of neutral species over a relatively wide
range of collision energies (3-50 kcal/mole). The neutral products
from the reactions are ionized using vacuum ultraviolet radiation,
mass analyzed, and counted as a function of arrival time at a
detector located approximately 30 cm from the reaction zone.
We are particularly interested in C-H and C-C bond activation...
i.e., the insertion of d-electron species into the normally
unreactive C-H and C-C bonds of alkanes, alkenes, alkynes, and
aldehydes. An underlying theme of our work is to understand
the electronic and dynamic factors controlling this process. For
example, which electronic and orbital configurations of the transition
metal favor insertion? How effective is initial translational
and vibrational energy in enhancing reaction? What can we learn
about the reaction mechanism, potential energy barriers for insertion,
and dynamics of molecular elimination by studying the competing
C-H and C-C activation channels as a function of collision
energy?
Polyatomic free radical chemistry
We produce polyatomic free
radicals such as OH by laser photolysis of stable precursor molecules
at the orifice of a pulsed nozzle. After characterizing the radicals
using spectroscopic techniques, their bimolecular reactions are
studied under well-defined single collision conditions.
Recently, we initiated the first vibrationally- resolved study
of the simplest 4-atom reaction OH + D2 ->
HOD + D. In these experiments, the velocity and angular
distributions of D atom products are measured at very high angular
and velocity resolution using the high-n Rydberg time-of-flight
method. In this case, the reactant beams are fixed and a
detector may be rotated in the plane of the beams in order to
measure product angular and velocity distributions. (See Figure).
From energy conservation, measurements of the kinetic energy released
to the recoiling D atoms provide insight into the internal state
distributions of the molecular counterfragment, in this case HOD.
Our study demonstrated for the first time that vibrational energy
is deposited into HOD in a highly mode-specific manner, with preferental
formation of HOD with two quanta of local mode OD stretching excitation.
Comparison of our experimental results with predictions from recent
quantum scattering calculations provides a very stringent test
of calculated potential energy surfaces for 4-atom reactions,
particularly in the region of the transition state and exit channel
for reaction.
Atmospheric aerosol chemistry
Much of the chemistry relevant to ozone destruction and air
pollution involves heterogeneous processes on aerosols, dust particles,
and ice surfaces. We have recently initiated a program to
study such chemical reactions under precisely-defined conditions
using particle and molecular beam techniques combined with novel
detection methods.
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Hinrichs, R.Z.; Schroden, J.J.; and Davis, H.F. C-C versus C-H Bond Activation of Alkynes
by Early Second Row Transition Metal Atoms. submitted to J. Chem. Phys. Abstract
Schroden, J.J.; Wang, C.C.; and Davis, H.F. Competition between C-C versus C-H Activation
in Reactions of Neutral Yttrium Atoms with
four Butenes.
J. Phys. Chem. in press.
PDF*
Hinrichs, R.Z.; Schroden, J.J.; and Davis, H.F. Competition between C-C versus C-H Activation
in Reactions of Neutral Yttrium Atoms with Cyclopropane
and Propene. J. Phys.
Chem., in press. PDF*
Lin, C.; Witinski, M.F.; and Davis, H.F. Oxygen Atom Rydberg Time-of-Flight spectroscopy-
ORTOF. J. Chem. Phys. 2003,
119, 251. PDF*.
Hinrichs, R.Z.; Schroden, J.J.; and Davis, H.F. Competition between C-C and C-H Insertion
in Prototype Transition Metal-Hydrocarbon Reactions.
J. Am. Chem. Soc. 2003, 125, 861.
PDF*.
A full list of publications can be found here http://www.chem.cornell.edu/hfd1/publist.htm
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