title: 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.
authors.affiliation: Department of Physical Chemistry, Indian Association for the Cultivation of Science, 2A Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India.
authors.forename: Arup K
authors.initials: AK
authors.lastname: Ghosh
authors.affiliation: Department of Physical Chemistry, Indian Association for the Cultivation of Science, 2A Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India.
authors.forename: Piyali
authors.initials: P
authors.lastname: Chatterjee
authors.affiliation: Department of Physical Chemistry, Indian Association for the Cultivation of Science, 2A Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India.
authors.forename: Tapas
authors.initials: T
authors.lastname: Chakraborty
mesh_terms: D003517:Cyclopentanes / Q000737:chemistry* / Q000528:radiation effects*; D019281:Dimerization; D006860:Hydrogen Bonding; D007477:Ions / Q000737:chemistry* / Q000528:radiation effects*; D007834:Lasers; D008027:Light; D013058:Mass Spectrometry; D008956:Models, Chemical; D055668:Photochemical Processes; D011522:Protons; D011789:Quantum Theory
chemical_list: D003517:Cyclopentanes; D007477:Ions; D011522:Protons; C007201:cyclopentanone