Department of Chemistry

Physical and Theoretical Chemistry

Research ImageThe Department of Chemistry at the University of North Carolina at Chapel Hill, offers a wide range of research opportunities in theoretical and experimental physical chemistry. Our program has broadened from its traditional areas of excellence in molecular chemical physics to include research activities in biophysical and surface chemistry, and materials and environmental sciences. Experimental efforts within these areas utilize state-of-the-art instrumentation, such as high-resolution and ultra-fast laser systems, molecular beam techniques, mass spectrometry, ion-scattering, scanning probe microscopy, and magnetic resonance spectrometry.

Research in theoretical chemistry involves developing computational models of chromatin, the structure of complex fluids, and polymer dynamics. Students at UNC have access to high-performance computer workstations, as well as RENCI/UNC Research Computing, which is home to one of the best computing facilities in the world, including a 4160-processor Dell Linux cluster.

 

Recent Research Highlights

Ultrafast Dynamics in DNA Components

Researchers in the Moran Group, as published in the Journal of Physical Chemistry A, are using femtosecond laser spectroscopies to examine a thymine family of systems chosen to expose the interplay between excited state deactivation and two distinct vibrational energy transfer (VET) pathways. One from the base to the deoxyribose ring, the second between neighboring units in a dinucleotide. They report that relaxation in the ground electronic state accelerates markedly as the molecular sizes increase from the nucleobase to the dinucleotide.

Research Image

Overall, the researchers conclude that the transfer of vibrational quanta from thymine to the deoxyribose ring couples significantly to the internal conversion rate, whereas the neighboring unit in the dinucleotide serves as a secondary heat bath. In natural DNA, it follows that (local) thermal fluctuations in the geometries of subunits involving the base and deoxyribose ring are most important to this subpicosecond relaxation process.

 

Molecular Relaxation Processes

Nonlinear laser spectroscopies in the deep UV spectral range are motivated by studies of biological systems and elementary processes in small molecules. In an invited perspective article published in Chemical Physics, the Moran Group discusses recent technical advances in this area with a particular emphasis on diffractive optic based approaches to four-wave mixing spectroscopies.

Research Image

Applications to two classes of systems illustrate present experimental capabilities. First, experiments on DNA components at cryogenic temperatures are used to uncover features of excited state potential energy surfaces and vibrational cooling mechanisms. Second, sub-200 fs internal conversion processes and coherent wavepacket motions are investigated in cyclohexadiene and α-terpinene. Finally, the group members propose new experimental directions that combine methods for producing few-cycle UV laser pulses in noble gases with incoherent detection methods, for example photoionization, in experiments with time resolution near a single femtosecond. These measurements are motivated by knowledge of extremely fast non-adiabatic dynamics and the resolution of electronic wavepacket motions in molecules.

 

Representative Publications

Imaging Charge Separation and Carrier Recombination in Nanowire p-i-n Junctions Using Ultrafast Microscopy. Michelle M Gabriel , Erik Grumstrup , Justin R. Kirschbrown , Christopher W. Pinion , Joseph D Christesen , David F. Zigler , Emma M Cating , James F. Cahoon , and John Michael Papanikolas. Nano Lett., Just Accepted Manuscript, DOI: 10.1021/nl5012118, Publication Date (Web): May 27, 2014.

Ultrafast Carrier Dynamics of Silicon Nanowire Ensembles: The Impact of Geometrical Heterogeneity on Charge Carrier Lifetime. Erik M. Grumstrup , Emma M. Cating , Michelle M. Gabriel , Christopher W. Pinion , Joseph D. Christesen , Justin R. Kirschbrown , Ernest L. Vallorz III, James F. Cahoon, and John M. Papanikolas. J. Phys. Chem. C, 2014, 118 (16), pp 8626–8633.

Ultrafast Carrier Dynamics in Individual Silicon Nanowires: Characterization of Diameter-Dependent Carrier Lifetime and Surface Recombination with Pump-Probe Microscopy. Erik M. Grumstrup , Michelle M. Gabriel , Emma M. Cating , Christopher W. Pinion , Joseph D. Christesen , Justin R. Kirschbrown , Ernest L. Vallorz III, James F. Cahoon, and John M. Papanikolas. J. Phys. Chem. C, 2014, 118 (16), pp 8634–8640.

Free Energy Barrier for Melittin Reorientation from a Membrane-Bound State to a Transmembrane State. Sheeba J. Irudayam, Tobias Pobandt, and Max L. Berkowitz. J. Phys. Chem. B, 2013, 117 (43), pp 13457–13463.

Interplay between Vibrational Energy Transfer and Excited State Deactivation in DNA Components. Brantley A. West, Jordan M. Womick, and Andrew M. Moran. J. Phys. Chem. A, 2013, 117 (29), pp 5865–5874.

Uncovering Molecular Relaxation Processes with Nonlinear Spectroscopies in the Deep UV. Brantley A. West, Brian P. Molesky, Paul G. Giokas, Andrew M. Moran. Chemical Physics, Volume 423, 23 September 2013, Pages 92-104.

Gas Phase Acidity Measurement of Local Acidic Groups in Multifunctional Species: Controlling the Binding Sites in Hydroxycinnamic Acids. Andres Guerrero, Tomas Baer, Antonio Chana, Javier González, and Juan Z. Dávalos. J. Am. Chem. Soc., 2013, 135 (26), pp 9681–9690.

Melittin Creates Transient Pores in a Lipid Bilayer: Results from Computer Simulations. Kolattukudy P. Santo , Sheeba J. Irudayam , and Max L. Berkowitz. J. Phys. Chem. B, 2013, 117 (17), pp 5031–5042.

Pump–Probe Microscopy: Spatially Resolved Carrier Dynamics in ZnO Rods and the Influence of Optical Cavity Resonator Modes. Brian P. Mehl, Justin R. Kirschbrown, Michelle M. Gabriel, Ralph L. House, and John M. Papanikolas. J. Phys. Chem. B, 2013, 117 (16), pp 4390–4398.

Interfacial Energy Conversion in RuII Polypyridyl-Derivatized Oligoproline Assemblies on TiO2. Da Ma, Stephanie E. Bettis, Kenneth Hanson, Maria Minakova, Leila Alibabaei, William Fondrie, Derek M. Ryan, Garegin A. Papoian, Thomas J. Meyer, Marcey L. Waters, and John M. Papanikolas. J. Am. Chem. Soc., 2013, 135 (14), pp 5250–5253.