My research involves the vibrations of molecules and molecular complexes, often in the crystalline solid state, using both experimental and theoretical methods. The experimental methods include IR and Raman spectroscopy and inelastic neutron scattering. The theoretical methods include conventional Gaussian calculations of isolated molecules or clusters and periodic methods that take the crystal lattice into account. Much of this work involves the study of the motions (including the zero point level) of H atoms and often the replacement of H by D. Replacement of H by D in alkanes results in changes in the NMR chemical shift of the remaining H atoms and in 13C resonances. This is due to the change in the distribution of positions sampled by D vs. H in the ground vibrational state. Exchange of D for H in hydrogen bonded crystals results in changes in different changes in the zero point and thermal energy for distinct polymorphic forms of the material. In some cases a different form is obtained with D than H.
Inelastic neutron scattering (INS) spectroscopy is a technique of vibrational spectroscopy that differs from IR and Raman spectroscopy in several respects. One of these is that there are no selection rules in INS so that many vibrations that are not seen by the optical methods can be observed. The unique feature of INS is that the intensity of vibrational transitions is dominated by motions of the hydrogen atoms in the material. If hydrogen is present it dominates the scattering. For example, methyl rotations, which are very weak in other kinds of spectra, are very strong in INS. All other atoms, including deuterium, do not scatter appreciably by comparison. This permits selective deuteration experiments in which parts of a sample are "removed" by substitution of D for H. Most of our inelastic neutron scattering experiments are performed at the ISIS facility of the Rutherford Appleton Laboratory (www.isis.rl.ac.uk) near Oxford, England using the spectrometer called TOSCA (www.isis.rl.ac.uk/molecularspectroscopy/tosca/). A new Spallation Neutron Source (SNS) has just been put into operation at Oak Ridge National Laboratory (http://neutrons.ornl.gov/aboutsns/aboutsns.shtml) which we will use soon.
INS spectra can be very easily computed from known atomic scattering cross sections. This can be used to establish the specific tautomeric form present in a crystal (as shown in the figure) or to determine the conformation present (which we have done for cycloalkanes and cycloalkenes). We have recently used Raman spectroscopy to establish the conformation of the strained hydrocarbon trans-cyclooctene. In the complex 4-methylpyridine:pentachlorophenol the hydrogen bond proton moves as the temperature is lowered. This has been explained on the basis of periodic calculations. Proton motion also happens in L-alanine alaninium nitrate, an analogue of a well-known ferroelectric material, digylcine nitrate. In cases like the Zundel cation ([H2O-H-OH2]+) the H (or D) atom may have two equivalent positions at large OO separation or a flat "quartic oscillator" at observed OO distances. This results in peculiar vibrational spectra.
A double minimum potential has been proposed to explain the properties of the infinite conjugated polymer polyacetylene. Specifically, it has been thought that this results in stable bond alternation. However, given that the vibration that interchanges the bonds is near 1500 cm-1 means that the barrier height is insufficient for the zero point level to be below the barrier. This means that polyacetylene should be a conductor.
Another new project of the Hudson Lab involves studies of polymorphic pharmaceutricals and new methods for obtaining them. This includes the use of deuterium exchange and separation of polymorphic forms using density gradient methods. This work is carried out in collaboration with Jack Melton, Research Professor, who has extensive experience in the pharmaceutical industry.
Kulterer, M. R.; Hudson, B. R., Raman spectra and simulation of cis- and trans-cyclooctene for conformational analysis compared to inelastic neutron scattering. J. Raman Spectroscopy, in press.
Hudson, M. R.; Allis, D. G; Ouellette, W.; Hudson, B.S. L-Alanine Alaninium Nitrate, a Homologue of a Ferroelectric Material. Phys. Chem. Chem. Phys. 2009, 11, 9474.
Hudson, M. R.; Allis, D. G.; Hudson, B. S. The inelastic neutron scattering spectrum of nicotinic acid and its assignment by solid-state density functional theory. Chem. Phys. Lett. 2009, 473(1-3), 81-87.
O'Leary, D. J.; Allis, D. G.; Hudson, B. S.; James, S.; Morgera, K. B.; Baldwin, J. E. Vicinal Deuterium Perturbations on Hydrogen NMR Chemical Shifts in Cyclohexanes. J. Am. Chem. Soc. 2008, 130, 13659-13663.
Rivera, S. A.; Allis, D. G.; Hudson, B. S. Importance of Vibrational Zero-Point Energy Contribution to the Relative Polymorph Energies of Hydrogen-Bonded Species. Cryst. Growth Des. 2008, 8(11), 3905-3907.