Collisional Quenching Dynamics and Reactivity of Highly Vibrationally Excited Molecules

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Date
2008-11-13
Authors
Liu, Qingnan
Advisor
Mullin, Amy S.
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Abstract
Highly excited molecules are of great importance in many areas of chemistry including photochemistry. The dynamics of highly excited molecules are affected by the intermolecular and intramolecular energy flow between many different kinds of motions. This thesis reports investigations of the collisional quenching and reactivity of highly excited molecules aimed at understanding the dynamics of highly excited molecules. How do molecules behave in collisions with a bath gas? How do the energy distributions evolve in time? How is the energy partitioned for both the donor and bath molecules after collisions? How do molecule structure, molecule state density and intermolecular potential play the role during collisional energy transfer? To answer these questions, collisional quenching dynamics and reactivity of highly vibrationally excited azabenzene molecules have been studied using high-resolution transient IR absorption spectroscopy. The first study shows that the highly vibrationally excited alkylated pyridine molecules impart less rotational and translational energy to CO<sub>2</sub> than pyridine does. The strong collisions are reduced for donors with longer alkyl chains by lowering the average energy per mode but longer alkyl chain have increased flexibility and higher state densities that enhance energy loss via strong collisions. In the second study, the role of hydrogen bonding interactions is explored in collision of vibrationally excited pyridines with H2O. A torque-inducing mechanism is proposed that involves directed movement of H2O between sigma- and pi-hydrogen bonding interactions with the pyridine donors. In the third study the dynamics of strong and weak collisions for highly vibrationally excited methylated pyridine molecules with HOD are reported. Lower limits to the overall collision rate are directly determined from experimental measurements and compared to Lennard-Jones models which underestimate the collision rate. The fourth study explores reactive collisions of highly vibrationally excited pyridine molecules. D-atom abstraction reactions of pyridine-d<sub>5</sub> and Cl show a rate enhancement of ~90 with vibrational energy. A single quantum of C-D stretching vibration is observed to be used for the vibrational driven reaction. Reactions of 2-picoline-d<sub>3</sub> with Cl do not show a similar enhancement. For this case, the fast rotation of -CD<sub>3</sub> group in highly vibrationally excited 2-picoline-d<sub>3</sub> inhibits the D-atom abstraction.
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