Journal
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Volume 13, Issue 15, Pages 7154-7161Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c0cp02710c
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Funding
- National Science Council of Taiwan, Republic of China [NSC 99-2113-M-001-025-MY3]
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Ion imaging coupled with (2 + 1) resonance-enhanced multiphoton ionization (REMPI) technique is employed to probe CO(v '' = 0) fragments at different rotational levels following photodissociation of methyl formate (HCOOCH3) at 234 nm. When the rotational level, J(CO)'', is larger than 24, only a broad translational energy distribution extending beyond 70 kcal mol(-1) with an average energy of about 23 kcal mol(-1) appears. The dissociation process is initiated on the energetic ground state HCOOCH3 that surpasses a tight transition state along the reaction coordinate prior to breaking into CO + CH3OH. This molecular dissociation pathway accounts for the CO fragment with larger rotational energy and large translational energy. As J(CO)'' decreases, a bimodal distribution arises with one broad component and the other sharp component carrying the average energy of only 1-2 kcal mol(-1). The branching ratio of the sharp component increases with a decrease of J(CO)''; (7.3 +/- 0.6)% is reached as the image is probed at J(CO)'' = 10. The production of a sharp component is ascribed to a roaming mechanism that has the following features: a small total translational energy, a low rotational energy partitioning in CO, but a large internal energy in the CH3OH co-product. The internal energy deposition in the fragments shows distinct difference from those via the conventional transition state.
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