Fragment Ion Formation in Resonance Enhanced Multiphoton Ionization (REMPI) of n-Propyl Phenyl Ether in a Supersonic Jet

Abstract

Resonance enhanced multiphoton ionization (REMPI) mass spectra of different conformational isomers of n-propyl phenyl ether at 30 K give the same fragmentation patterns. Laser REMPI excitation spectra exhibit three principal conformers in a supersonic free jet expansion. The most abundant species in the jet happens also to contribute the longest wavelength 0,O band. SCF calculations suggest that the lowest energy structure corresponds to a gauche geometry in which all the carbons except that of the methyl group are essentially coplanar with the oxygen. This is confirmed by experimental observation of a predicted blue shift for a prominent vibrational overtone when the propyl group is partially deuterated (0,/3-d2 or a,a,T,T,T-d5). Time-of-flight mass spectra of deuterated analogues of each of three conformers exhibits propene expulsion to yield PhOH'+ and PhOD'+ (the principal fragment ions) in which all seven alkyl hydrogens have become randomized within the chain. REMPI of individual conformers (via intermediacy of the lowest vibrational levels of their excited singlet electronic states) therefore gives the same outcome as does field ionization or electron impact source mass spectra. The predominant decomposition mechanism of the radical cation involves an ion-neutral complex, nPrOPh*+ -. (iPr+ PhO'), in which the hydrogens of the iPr+ undergo rapid internal transpositions prior to the ultimate decomposition step. Ab initio computations on a model system concur with the experimental inference that this mechanism operates regardless of the conformation of the precursor neutral.