|
· Spectroscopy and
Dynamics of Large-Amplitude Molecular Motions
Our principal focus is on fundamental
understanding of the interaction between the small-amplitude vibrations in a
molecule and large-amplitude motions such as internal rotation (torsion) or
inversion. These large-amplitude motions are active promoters of
intramolecular vibrational energy redistribution, as the coupling with the
vibrations creates channels for energy flow among the different modes.
Fascinating and complex effects are produced in the molecular infrared
spectra, which we are exploring experimentally and theoretically. Methanol
is a molecule of special interest, and we have extensive high-resolution Fourier
transform spectra of the vibrational fundamental bands for normal CH3OH
and several other isotopic species. We are engaged in the analysis of these
spectra, the development of high precision Hamiltonian models for the
spectral fitting, and ab initio theoretical exploratison of the coupling
between torsion and vibration for understanding of the interaction terms in
the Hamiltonian:
o
Analyses of Fourier transform
infrared spectra of methanol and its isotopomers
o
Global fitting of methanol ground
state far-infrared spectra
o
Ab-initio-assisted investigation of
torsion-vibration coupling
· Laboratory
Astrophysics - Database studies in Support of Cometary and Atmospheric
Spectroscopy of Methanol
Methanol is distributed widely
throughout the universe in interstellar clouds, and has also been firmly
identified in infrared spectra from comets, the terrestrial atmosphere, and
forest fires. We are collaborating with NASA astronomers in analyzing
spectra from recent comets. To provide the reliable data on line positions
and strengths that are essential for modeling and interpretation of
astronomical and remote sensing observations, we are critically evaluating
known spectra for inclusion of a comprehensive atlas in the HITRAN
international database of atmospherically important spectra.
·
Conformational
Studies of Biomolecules
As a route to insight into
proteomics, the important contemporary study of protein structure, dynamics
and function, we are investigating the conformations and torsional motions of
medium-sized biomimetic molecules that mimic the folding behaviour of
larger biomolecular parents. We are carrying out ab initio calculations on
supercomputers to determine the different low energy conformations
corresponding to internal rotations about the molecular backbone bonds, and
also have a major new experimental initiative underway on jet-cooled
infrared spectra of biomimetic species:
o
Ab Initio Calculation
of Biomimetic Conformations and Tunneling Paths
o
Pulsed-Jet
Molecular Beam Infrared Spectrometer
As
members of the Far-Infrared Beamline Team at the Canadian Light Source in Saskatoon, we are
also exploring the possibility of studying biomimetic torsional bands
utilizing the high brightness of the synchrotron FIR radiation.
· Laser-Based
Spectroscopy of Environmental and Atmospheric Molecules
We are investigating trace gas
sensing of molecules of environmental and atmospheric importance in
different spectral regions with laser spectrometers. This work is carried
out in the program of the Canadian Institute for Photonic Innovations
(CIPI), one of the national Networks of Centres of Excellence. Point
sampling and open path techniques are being explored for greenhouse and
pollutant gases. We are looking into detectability limits and the best
choices for observational spectral windows, and are examining line
broadening and pressure shifts to provide fundamental information for use
in atmospheric modeling and interpretation of airborne and satellite remote
sensing data. Our instruments are:
o
CO2-Laser/Microwave
Sideband Precision Tunable Infrared Source
o
Mid-Infrared Tunable Diode Laser Spectrometer
o
External Cavity Tunable Diode Laser
Spectrometer
Ab Initio Calculation of Biomimetic Conformations and
Tunneling Paths
With ab initio supercomputer quantum
calculations, we are mapping the potential energy surfaces of biomimetic
molecules in order to establish the stable low energy conformations and the
tunneling paths between them via torsion about the main backbone bonds. The
ab initio calculations are compared against experimental Fourier transform
microwave observations to identify the observed conformers and their
structures. In a major new initiative, we are building a pulsed-jet
molecular beam spectrometer to explore biomolecular infrared spectra in the
cold temperatures of biomolecules.
[back]
Pulsed-Jet Molecular Beam Infrared Spectrometer
In this apparatus, a large chamber is
mounted on a 20¡± diffusion pump to maintain a high vacuum. A multipass
optical White cell of 1-m base length is mounted in the beam chamber which
can be set for up to 40 passes. Tunable infrared radiation is sent through
an optical window into the cell and returns through a second window to be
detected outside. A pulsed slit 14 cm long is mounted inside the chamber
just above the two small mirrors at the far end of the cell, and is
connected to a high pressure mixture of the molecule being studied plus a
carrier gas such as argon. When the slit is briefly opened, a pulse of
molecules is sent across the radiation path, so that absorption of the
infrared radiation can be observed. The absorption bands give information
on the molecular vibrational frequencies and the rotational energies, so
provide insight into the forces within the molecule and the molecular
structure.
For the source of tunable infrared
radiation, we can use our CO2-laser/microwave-sideband source in
the 10 µm region where many vibrational fundamental bands occur.
Alternatively, we can use our
external cavity tunable diode laser in the 1.3 to 1.7 µm region of
vibrational overtones and combination bands. For our program of conformational
studies of biomolecules, we will employ the CO2-laser-based
sideband source to investigate bending and stretching fundamentals in the
10 µm region and their combinations with the torsional motions that we seek
to probe in connection with protein folding. Here, we have a collaboration
with the National Institute for Standards and Technology (NIST) in Gaithersburg, MD,
in which they are measuring Fourier transform microwave spectra of the
biomimetic species. Our infrared experiments will give complementary
information to the microwave studies, and we will also be able to
investigate symmetric species with no permanent dipole moment that have no
microwave spectrum.
The secret to the success of the
experiment is that in the supersonic expansion of the gas pulse through the
slit nozzle into high vacuum, remarkable cooling of the gas occurs, down to
rotational temperatures of a few K. Since these temperatures correspond to
very low energy, the molecules all fall down into the lowest few rotational
energy levels. Thus, the infrared spectrum of the gas is enormously
simplified, making it possible to study and resolve the absorption bands of
even large molecules. These would be extremely dense and weak at room
temperature, but with the spectral simplification, the increase in
intensity due to concentrating all the molecules in just the few lowest
levels, and the reduction in Doppler line width due to shooting the
molecular beam perpendicular to the radiation, well-resolved spectra can be
seen. This means that we can separate the vibrational bands due to
different conformers, resolve the rotational patterns of those bands to get
structural information, and see perturbations to the patterns due to
interaction with low-frequency torsional modes to get information about the
folding dynamics.
[back]
|
