Penny M. Rowea* , Lea Fortmannb, Timothy L. Guascoc , Aedin Wrightd, Amy Rykene , Emma Sevierf, Grace Stokesg, Amanda Mifflind , Rachel Wadeh, Haiyan Chengi, William Pfalzgraffj , Justin Beaudoink, Isha Rajbhandarib, Kena Fox-Dobbsf & Steven Neshybad a NorthWest Research Associates, Bellevue, Washington 98052b Economics, University of Puget Sound, Tacoma, Washington 98416c Chemistry, Millikin University, Decatur, Illinois 62522d Chemistry, University of Puget Sound, Tacoma, Washington 98416e Education, University of Puget Sound, Tacoma, Washington 98416f Geology, University of Puget Sound, Tacoma, Washington 98416g Chemistry & Biochemistry, Santa Clara University, Santa Clara, California 95053h Physics, Edmonds Community College, Lynnwood, Washington 98036i Computer Science, Willamette University, Salem, Oregon 97301j Chemistry, Chatham University, Pittsburgh, Pennsylvania 15232k Interdisciplinary Arts and Sciences, University of Washington Tacoma, Tacoma, Washington 98402

Carbon-hydrogen (C-H) vibration modes serve as key probes in the chemical identification of hydrocarbons and in vibrational sum-frequency generation spectroscopy of hydrocarbons at the liquid/gas interface. Their assignments pose a challenge from a theoretical viewpoint. In this work, we present a detailed study of the C-H stretching region of dimethyl sulfoxide using a new ab initio molecular dynamics (AIMD) module that we have implemented in NWChem. Through a combination of AIMD simulations and static normal mode analysis, we interpret experimental infrared and Raman spectra and explore the role of anharmonic effects in this system. Comprehensive anharmonic normal mode analysis of the C-H stretching region casts doubt upon previous experimental assignments of the shoulder on the symmetric C-H stretching peak. In addition, our AIMD simulations also show significant broadening of the in-phase symmetric C-H stretching resonance, which suggests that the experimentally observed shoulder is due to thermal broadening of the symmetric stretching resonance.

Non-resonant tip-enhanced Raman images of dimercaptostilbene on silver reveal that different vibrational resonances of the reporter are selectively enhanced at different sites on the metal substrate. Sequentially recorded images track molecular diffusion within the diffraction-limited laser spot which illuminates the substrate. In effect, the recorded time resolved (dt = 10 s) pixelated images (25 nm x 8 cm-1) broadcast molecule-local field interactions which take place on much finer scales.