Reports: UR651272-UR6: Understanding Diradical Behavior in Asphaltene Model Compounds

Carol Parish, University of Richmond

A Mechanistic Study of the 2-Thienylmethyl + HO2 Radical Recombination Reaction.  Radical recombination reactions are important in the combustion of fuel oils. Shale oil contains alkylated heteroaromatic species - the simplest example of which is the 2-thienylmethyl radical. The ab initio potential energy surface for the reaction of the 2-thienylmethyl radical with the HO2 radical has been examined. Seventeen product channels corresponding to either addition/elimination or direct hydrogen abstraction have been characterized for the first time. Direct hydrogen abstract from HO2 proceeds via a weakly bound van der Waals complex which leads to 2-methyl thiophene, 2-methylene-2,3-dihydrothiophene or 2-methylene-2,5-dihydrothiophene depending upon the 2-thienylmethyl radical reaction site. The addition pathway for the two radical reactants is barrierless with the formation of three adducts, as distinguished by HO2 reaction at 3 different sites on the 2-thienylmethyl radical. The addition is exothermic by 37 ~ 55 kcal mol-1 relative to the entrance channel and these excess energies are available to promote further decomposition or rearrangement of the adducts, leading to nascent products such as H, OH, H2O and CH2O. The reaction surfaces are characterized by relatively low barriers (most lower than 10 kcal mol-1). Based upon a careful analysis of the overall barrier heights and reaction exothermicities, the formation of O2, OH and H2O are likely to be important pathways in the radical recombination reactions of 2-thienylmethyl + HO2. This work was published in the Journal of Physical Chemistry A.

Mechanisms for the Reaction of Thiophene and Methylthiophene with Singlet and Triplet Molecular Oxygen. Mechanisms for the reaction of thiophene and 2-methylthiophene with molecular oxygen on both the triplet and singlet potential energy surfaces (PESs) were investigated using ab initio methods. Geometries of various stationary points involved in the complex reaction routes were optimized at the MP2/6-311++G(d, p) level. The barriers and energies of reaction for all product channels were refined using single-point calculations at the G4MP2 level of theory.  For thiophene, CCSD(T) single point energies were also determined for comparison with the G4MP2 energies. Thiophene and 2-methylthiophene where shown to react with O2 via two types of mechanisms, namely, direct hydrogen abstraction and addition/elimination. The barriers for reaction with triplet oxygen are all significantly large (i.e., > 30 kcal mol-1), indicating that the direct oxidation of thiophene by ground state oxygen might be important only in high temperature processes. Reaction of thiophene with singlet oxygen via a 2+4-cycloaddition leading to endoperoxides is the most favorable channel. Moreover, it was found that alkylation of the thiophene ring (i.e., methyl-substituted thiophenes) is capable of lowering the barrier height for the addition pathway. The implication of the current theoretical results may shed new light on the initiation mechanisms for combustion of asphaltenes. This work was published in the Journal of Physical Chemistry A with undergraduate coauthors Matthew Fanelli, Justin Cook and Furong Bai. 

Pyrolysis Mechanisms of Thiophene and Methylthiophene in Asphaltenes. The pyrolysis mechanisms of thiophene in asphaltenes were investigated theoretically using density functional and ab initio quantum chemical techniques. All of the possible reaction pathways were explored using B3LYP, MP2 and CBS-QB3 models. A comparison of the calculated heats of reaction with the available experimental values indicates that the CBS-QB3 level of theory is quantitatively reliable for calculating the energetic reaction paths of the title reactions. The pyrolysis process is initiated via four different types of hydrogen migrations. According to the reaction barrier heights, the dominant 1,2-H shift mechanism involves two competitive product channels, namely, C2H2 + CH2CS and CS + CH3CCH. The minor channels include the formation of CS + CH2CCH2, H2S + C4H2, HCS + CH2CCH, CS + CH2CHCH, H + C4H3S and HS + C4H3. The methyl substitution effect was investigated with the pyrolysis of 2-methylthiophene and 3-methylthiophene. The energetics of such systems were very similar to that for unsubstituted thiophene, suggesting that thiophene alkylation may not play a significant role in the pyrolysis of asphaltene compounds. This work was published in the Journal of Physical Chemistry A.

Thirteen undergraduate students and 5 high school student worked on these and other projects full-time this past summer. Two undergraduates and 1 high school student were supported directly on the ACS-PRG grant  (Nelson, Zeldin and Jaini).

This past year, my students were responsible for 22 presentations at regional, national and international meetings.  (7 student poster presentations at the 2007 Southeastern Regional meeting of the American Chemical Society (SETCA), 1 invited student poster at the Beckman symposium at the national ACS meeting in New Orleans, 3 student poster presentations at the 2008 Schrodinger User Group meeting, 2 student talks at the first annual Virginia Tech – Univ of Virginia Theory Summit, 1 student poster at the annual Beckman symposium, 6 student poster presentations at the 2008 MERCURY conference in computational chemistry and 2 invited student talks at the NSF-PIRE symposium at the University of Vienna.)  Six of my students will also be presenting their work at the 2008 Southeastern Regional meeting of the American Chemical Society (SETCA) this coming November.  Five of my seniors have graduated; 1 student is pursuing the D.O. degree at the Philadelphia College of Osteopathic Medicine, 1 student is working as a software engineer at Microsoft and a third student is in a biotechnology graduate program at Georgetown University.  Two other students are working during 08-09 and planning to apply to law school and medical school.