57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

Overview

The funding cycle from 2011-2012 represents the first year of funding for this project from the American Chemical Society Petroleum Research Fund.  Already this funding has made a significant impact on the research progress in my group.  The ACS PRF was used to support one full-time graduate research assistant, one month of the PI's summer salary, graduate student conference travel, and the purchase of consumables required for assembling our two-dimensional infrared (2D IR) spectrometer and preparing samples of model asphaltene compounds.

The funding provided to this project has made a significant impact on my student's career, as well as my own.  Ms. Jenée Cyran is now beginning her third year of graduate school.  Ms. Cyran has been fully supported on this ACS PRF project and thus has benefited from having time in the lab in order to gain traction in her research.  She has made significant progress in her research and as a result attended the 6^th International Conference on Coherent Multidimensional Spectroscopy in Berlin, Germany this past summer.  During this conference we were able to participate in the exchange of ideas that is sure to push our work forward in the near future.

Scientific Progress

A two-pronged approach was taken in year 1 (2011-2012) of this project in order to be in a position to accomplish the scientific tasks outlined in this research project.  To this end, Ms. Cyran worked on developing the appropriate molecular system for our investigations into asphaltene nanoaggregation and worked with a second student in my research group, Jacob Nite, to build our 2D IR spectrometer. 

Two-dimensional infrared spectroscopy is a third order, nonlinear optical spectroscopy, which requires three electric fields to be overlapped in spatially and temporally at the sample.  Subsequently, the sample emits the third order signal that depends on the wave vector direction, the relative time delays between the excitation pulses, the relative polarization and phases of the excitation pulses.  Recently, dramatic improvements have been made in acquisition times and the relative ease with which an absorptive 2D IR spectrum can be collected through utilizing mid-IR pulse shaping technology.  During year 1 of this project we have built our 2D IR spectrometer and during this process discovered a path to make Bragg-regime pulse shaping technology compatible with broad bandwidth, mid-IR pulses.  Based on our results, we filed a provisional patent application and our manuscript titled, "Active Bragg angle compensation for shaping ultrafast mid-infrared pulses," has been accepted for publication in Optics Express.  We are now working towards collecting 2D IR spectra of a chemical system that will serve to model asphaltenes.

We have identified two model compounds that will allow us to answer to critical questions regarding asphaltene nanoaggregation.  Based on previous efforts in the literature, two compounds routinely used to model asphaltene behavior are violanthrone-79 and N,N'-dioctyl-3,4,9,10-perylenedicarboxide; the chemical structures are shown in Figure 1.  Ms. Cyran has developed a protocol for sample preparation of the model compounds in deuterated toluene and carbon tetrachloride.  These solvents are chosen in order to consider the solubility definitions of asphaltenes and to take care to remove solvent contributions to the linear and 2D IR spectra of the samples.  Currently, our efforts are focused on extracting an accurate local mode description of the carbonyl oscillators in each of the model compounds and using 2D IR spectroscopy to determine the effect the molecular symmetry has on the molecular stacking configurations in nanoaggregates.  Representative linear IR spectra corresponding to nanoaggregates of violanthrone-79 and N,N'-dioctyl-3,4,9,10-perylenedicarboxide are provided in Figure 1.  In addition to preliminary experiments, Ms. Cyran has performed a suite of electronic structure calculations that provide a local mode description of the carbonyl stretching vibrations in violanthrone-79 and N,N'-dioctyl-3,4,9,10-perylenedicarboxide.  These calculations suggest that the vibrational transition dipoles associated with the carbonyl groups directly in the ring structures lie along the C=O bond and are degenerate in energy.  During year 2 of this project, we will develop a vibrational coupling model that describes the vibrational interactions between these oscillators using electronic structure calculations in conjunction with our 2D IR spectra.  We will then be able to determine a detailed picture of the molecular stacking configurations in asphaltene nanoaggregates.  In Figure 2, two possible stacking configurations for violanthrone-79 are shown.  We will use 2D IR spectroscopy to discriminate between these two scenarios.

Figure 1. Chemical structures of violanthrone-79 and N,N'-Dioctyl-3,4,9,10-perylenedicarboxide.  Representative linear IR spectra of the carbonyl stretching region are shown for each model compound in the lower panels; the violanthrone-79 is in deuterated toluene and the perylene derivative is in carbon tetrachloride.

Figure 2. Two hypothetical stacking configurations of violanthrone-79 molecules.  The vibrational signatures in the 2D IR spectrum will be dependent upon the distances and relative orientations of the carbonyl groups in the nanoaggregates that form.  It is expected that five to eight molecules make up a nanoaggregate.