Infrared Emission from Photodissociation of Methyl Formate [HC(O)OCH 3 ] at 248 and 193 nm: Absence of Roaming Signature

LANFRI, LUCIA; WANG, YEN-LIN; PHAM, TIEN V.; NGUYEN, NGHIA TRONG; PACI, MAXI BURGOS; LIN, M. C.; LEE, YUAN-PERN

JOURNAL OF PHYSICAL CHEMISTRY A

AMER CHEMICAL SOC

Año: 2019

1089-5639

Following photodissociation at 248 nm of gaseousmethyl formate (HC(O)OCH 3 , 0.73 Torr) and Ar (0.14 Torr),temporally resolved vibration−rotational emission spectra ofhighly internally excited CO (ν ≤ 11, J ≤ 27) in the 1850−2250cm −1 region were recorded with a step-scan Fourier-transformspectrometer. The vibration−rotational distribution of CO isalmost Boltzmann, with a nascent average rotational energy (E 0 R )of 3 ± 1 kJ mol −1 and a vibrational energy (E 0 V ) of 76 ± 9 kJmol −1 . With 3 Torr of Ar added to the system, the averagevibrational energy was decreased to E 0 V = 61 ± 7 kJ mol −1 . Weobserved no distinct evidence of a bimodal rotational distributionfor ν = 1 and 2, as reported previously [Lombardi et al., J. Phys.Chem. A 2016, 129, 5155], as evidence of a roaming mechanism. The vibrational distribution with a temperature of ∼13000 ±1000 K, however, agrees satisfactorily with trajectory calculations of these authors, who took into account conical intersectionsfrom the S 1 state. Highly internally excited CH 3 OH that is expected to be produced from a roaming mechanism wasunobserved. Following photodissociation at 193 nm of gaseous HC(O)OCH 3 (0.42 Torr) and Ar (0.09 Torr), vibration−rotational emission spectra of CO (ν ≤ 4, J ≤ 38) and CO 2 (with two components of varied internal distributions) wereobserved, indicating that new channels are open. Quantum-chemical calculations, computed at varied levels of theory, on theground electronic potential-energy schemes provide a possible explanation for some of our observations. At 193 nm, the CO 2was produced from secondary dissociation of the products HC(O)O and CH 3 OCO, and CO was produced primarily fromsecondary dissociation of the product HCO produced on the S 1 surface or the decomposition to CH 3 OH + CO on the S 0surface.