Keto-enol tautomerization and intermolecular proton transfer in photoionized cyclopentanone dimer in the gas phase

Abstract

Time-of-flight mass spectra of cyclopentanone and its clusters cooled in a supersonic jet expansion have been measured following 4-, 3-, and 2-photon ionizations by the 2nd, 3rd, and 4th harmonic wavelengths, respectively, of a Q-switched Nd:YAG laser. The mass spectra reveal signatures of energetically favored keto to enol tautomerization of the molecular ion leading to intermolecular proton transfer, and this observation is found sharply dependent on the ionization wavelengths used. Electronic structure calculation predicts that in spite of the energetic preference, keto-enol conversion barrier of isolated molecular ion is high. However, the barrier is significantly reduced in a CH⋯O hydrogen-bonded dimer of the molecule. The transition states associated with tautomeric conversion of both cyclopentanone monomer and dimer cations have been identified by means of intrinsic reaction co-ordinate calculation. In a supersonic jet expansion, although a weakly bound dimer is readily generated, the corresponding cation and also the protonated counterpart are observed only for ionization by 532 nm. For other two ionization wavelengths, these species do not register in the mass spectra, where the competing reaction channels via α-cleavage of the ring become dominant. In contrast to the report of a recent study, we notice that the intact molecular ion largely survives fragmentations when ionized from the 2-photon resonant 3p Rydberg state as intermediate using nanosecond laser pulses, and the corresponding resonant 3-photon ionization spectrum has been recorded probing the intact molecular ion.

Thermal desorption effects on fragment ion production from multi-photon ionized uridine and selected analogues

“…There are numerous examples in the literature of isomeric effects on electronic excited state dynamics 9 and on the relative production of fragment ions in mass spectrometry experiments. 46 Therefore, the plausibility of this attribution depends primarily on the idea that the relative populations of different uridine isomers can change significantly as a function of the desorption laser power (and hence foil temperature) in the present experiments. The calculations in Section 3 show that conformational isomeric transitions of uridine can occur at much lower temperatures than decomposition, and a range of higher barriers have been reported for different isomeric transitions in the literature.…”

Barrierless Proton Transfer in the Weak C–H···O Hydrogen Bonded Methacrolein Dimer upon Nonresonant Multiphoton Ionization in the Gas Phase

“…47 In one of our previous studies we have shown the occurrence of IMPT in the photoionized jetcooled dimeric complex of a small cyclic ketone, cyclopentanone. 50 In that case, the enol form was found to be more stable in the cationic ground state, and the energy barrier for keto to enol conversion was shown to reduce drastically in the hydrogen bonded dimeric adduct. Recently, an almost barrierless IMPT was claimed to be occurring in the dimeric complex of acetaldehyde, upon 1-photon VUV ionization, and the same was corroborated also by means of electronic structure theory methods.…”

Tautomers of homophthalic anhydride in the ground and excited electronic states: analysis through energy, hardness and vibrational signatures

“…There are also some important developments regarding the characterization of tautomerisation processes and particularly transition states [12][13][14]. Apart from single molecules, the size of the clusters and their respective structures play an important role in determining the tautomerisation process [15] in those cases.…”