Signatures of isomerization in photodissociation of trans- crotonaldehyde probed by multiphoton ionization mass spectrometry

Abstract

We report the observation of new isomerization effects in the UV-photodissociation of trans-crotonaldehyde upon multiphoton excitation by the third harmonic (355 nm) pulses of a Nd:YAG laser. A time-of-flight mass spectrometric analysis reveals formation of acetaldehyde, acetyl, and methoxy radical cations as signatures of isomerization processes. A small segment of the multiphoton ionization spectrum of jet-cooled crotonaldehyde is recorded by tuning the laser frequency around 355 nm. An oxetene type transient intermediate in the ground state has been considered for acetaldehyde formation following a photochemical model suggested earlier (Reguero; et al. J. Am. Chem. Soc. 1994, 116, 2101−2114) for such compounds. Likewise, for methoxy radical formation, a trans–cis isomerization about the C═C double bond has been considered in a triplet surface. Electron ionization mass spectra of the compound are also recorded by varying the electron kinetic energy in the range 11–70 eV. Ionic fragments in the mass spectra of the two ionization processes are dramatically different. Our suggested mechanisms for isomerization and fragmentation channels are substantiated by density functional theory calculations. Combined experimental and calculated data lead us conclude that isomerization occurs in neutral potential energy surfaces prior to dissociation and photoionization.