3-014A Center for Science and Technology
Physical chemistry; biophysical chemistry; quantum chemistry and inelastic neutron scattering applied to molecular solids; fluorescence; solid state preparation of polyacetylene
- B.S., 1967, California Institute of Technology
- Ph.D., 1972, Harvard University
- NSF Postgraduate Fellow, 1967-1972, Caltech & Harvard University
- Camille & Henry Dreyfus Teacher Scholar, 1975-77
- Alfred P. Sloan Foundation Fellow, 1975-1977
- NIH Research Career Development Awardee, 1975-1980
- Fellow of the American Physical Society, 1994-
- CHE 347: Physical-Analytical Chemistry Laboratory
- CHE 357: Physical Chemistry Laboratory
- CHE 436/636: Advanced Physical Chemistry
- CHE 546: Molecular Spectroscopy and Structure
Our current research involves two aspects of the importance of the vibrational zero point level of molecules. The observed properties of a molecule are an average of that property over the range of geometry spanned by the zero point probability distribution. In the case of molecules with potential energy surfaces that have two (or more) minima this averaging can have a major effect on properties. This is particularly the case when the potential energy rises steeply at large deformations. The change in a property due to replacement of one isotope by another such as deuterium for hydrogen is the difference in the zero point average for these isotopes.
Replacement of H by D in rigid saturated hydrocarbon species results in changes in the NMR spectra of all the 13C atoms in the molecule. This is easily observed with natural abundance 13C. The carbon to which the D is attached has the largest change in its chemical shift on the order of 400 parts per billion. The effect for neighboring carbons decreases with the number of intervening bonds being ca. 100 ppb for next neighbor carbons and 50 to -20 ppb for more distant carbon atoms. We have recently shown that these effects can easily be computed with precision comparable to that of the experiment. This computation involves calculations of the NMR chemical shift and the energy using a standard program at the optimized geometry and with one H (or D) atom displaced to a new position along the stretch or bend of the C-H bond. From the energy calculations one can compute the zero point level wave function for H and for D. The square of the wave function is the probability distribution. This can be used to average the change of the 13C chemical shifts from their value at the equilibrium geometry. The difference in the average of this variation for H and for D is the isotope shift. Direct comparison with experiment is possible.
We are currently extending this computational method to the case of peptides for applications to NMR studies of proteins. Several research groups are using deuterium substitution of amino acids as probes of protein conformation. The current level of analysis of the results is rather rudimentary and depends on knowing the molecular structure. Our plan is to perform computations of the type described above on small peptides that have limited flexibility. We will then compare the results with the experimental changes in the 13C and 15N chemical shifts due to isotopic substitution at either CH or NH positions. Experiments will be carried out in several solvents in order to determine the effect of solvent change. We expect this to be significant for the NH(D) case.
-annulene, a ring of 18-CH units, has 4n+2 π-electrons with n = 4. This is analogous to benzene with n = 1. Like benzene there are two equivalent patterns of double and single bonds around the ring. These are found to be minima in the potential energy surface by most computational methods that treat electron correlation. When a molecule has a double minimum potential it is usually assumed that the minima represent “the structure” of the molecule. Ammonia is usually thought of as being pyramidal despite the fact that it tunnels from one minimum to the other in a few picoseconds. In the case of -annulene the crystal structure shows a D6h symmetric structure but the observed NMR spectrum is closer to that computed for a structure corresponding to either minimum in a Hartree-Fock calculation. We have shown in a 2012 publication that the zero-point level in this case is so high that both minima are populated and that the computed zero-point averaged NMR spectrum is in reasonable agreement with experiment.
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 1450 cm-1 means that the barrier height is insufficient for the zero point level to be below the barrier. This means that polyacetylene will not exhibit bond-alternation and should be a conductor. This has been described in a 2013 publication. The methods in use for the preparation of “polyacetylene” all result in mixtures of finite chain length materials. We are developing methods for the synthesis of polyacetylene that produce a high molecular weight material in which the reactive conjugated chains are constrained to be in their all-trans conformation insulated from chain-chain interactions. In a 2013 publication we have shown that the urea inclusion co-crystal of (E,E)-1,4-diiodo-1,3-butadiene shown in the diagram. When exposed to light this material ejects its iodine atoms and produces conjugated polyene chains as shown by Raman spectroscopy. Current efforts are directed at producing high molecular weight samples of this type.
In some of our studies we use inelastic neutron scattering (INS) spectroscopy. INS 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 have been performed at the ISIS facility of the Rutherford Appleton Laboratory (www.isis.rl.ac.uk) using TOSCA (www.isis.rl.ac.uk/molecularspectroscopy/tosca/). The resulting publications are given in the attached full publication list. 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.
- Hudson, Bruce S.; Allis, Damian G., “The structure of -annulene: Computed Raman spectra, zero-point level and proton NMR chemical shifts” J. Mol. Struct. (2012), 1023, 212-215. DOI:10.1016/j.molstruc.2012.05.016
- Hudson, Bruce S.; Allis, Damian G, “Bond alternation in infinite periodic polyacetylene: Dynamical treatment of the anharmonic potential”, J. Mol. Struct. (2013), 1032, 78-82. DOI:10.1016/j.molstruc.2012.07.051
- Lashua, Amanda F.; Smith, Tiffany M.; Hu, Hegui; Wei, Lihui; Allis, Damian G.; Sponsler, Michael B.; Hudson, Bruce S., “Commensurate Urea Inclusion Crystals with the Guest (E,E)-1,4-Diiodo-1,3-Butadiene”, Crystal Growth & Design (2013), 13(9), 3852-3855. DOI:10.1021/cg400980b
- Dinca, Steluta A.; Lashua, Amanda F.; Sponsler, Michael B.; Hudson, Bruce S., “Oriented, insulated polyacetylene chains in an inclusion complex by photopolymerization”, MRS Online Proceedings Library (2015) 1799, 1-6. DOI:10.1557/opl.2015.486
- Marshall, Madalynn G.; Lopez-Diaz, Valerie; Hudson, Bruce S., “Single-Crystal X-ray Diffraction Structure of the Stable Enol Tautomer Polymorph of Barbituric Acid at 224 and 95 K” Angewandte Chemie, International Edition (2016), 55(4), 1309-1312. DOI:10.1002/anie.201508078