Discovery of long-lived excited electronic states of vinylchloride, vinylbromide, vinyliodide, and acrylonitrile cations

Abstract

Charge exchange technique has been used to detect the presence of long-lived excited electronic states of some monosubstituted ethene cations. The technique is based on the criterion that charge exchange between polyatomic species is efficient only when the energy of reaction is close to zero or negative (⌬E Յ 0), or the exoergicity rule. The à 2 A' states of vinylchloride, vinylbromide, vinyliodide, and acrylonitrile cations have been found to have long lifetimes (tens of microsecond or longer) while all the excited electronic states of vinylfluoride cation have been found to undergo rapid dissociation or nonradiative relaxation to the ground state. The long-lived states found are those displaying well-resolved vibrational structures in the photoelectron spectra. In particular, these are the states generated by removal of an electron from the in-plane nonbonding p orbitals of halogens or the in-plane orbital of the triple bond C §N. The present findings are very similar to those for the monosubstituted benzene cations reported previously. T wo assumptions made in the theory of mass spectra, or quasi-equilibrium theory (QET), are rapid conversion from excited to ground electronic states for molecular ions generated by the ionization method used and also rapid intramolecular vibrational redistribution in the ground electronic state. Then statistical treatment to evaluate dissociation rate constants becomes equivalent to the Rice-Ramsperger-Kassel-Marcus (RRKM) theory for unimolecular reaction and is often called RRKM-QET [1]. These two hypotheses have been actively investigated over the years, emphasis having been placed more on the latter than the former.There are some cases where the hypothesis of rapid conversion from an excited to ground electronic states prior to dissociation is not valid. The most frequently observed are the cases of direct dissociation in a repulsive excited electronic state [2][3][4][5][6][7][8][9][10][11][12][13][14]. Predissociation via bound-to-repulsive transition has been observed for dissociation of simple molecular cations also [15][16][17][18]. Dissociation in a bound excited state, or an "isolated" electronic state, has been hardly observed in ionic cases [19 -23]. Hence, searching for isolated electronic states of polyatomic cations remains a mission yet to be fulfilled.In our recent investigation on photodissociation of benzene cation [24], we found evidences for a very long lifetime (30 s or longer) of its à 2 E 2g state. In a subsequent work [25], a method based on charge exchange in collision cells of a modified double focusing mass spectrometer was developed for routine search for long-lived excited states with conventional mass spectrometry. Observations made were summarized as a hypothesis termed the exoergicity rule, which states that an energy resonant (⌬E ϭ 0) or exoergic (⌬E Ͻ 0) charge exchange occurs efficiently at low collision energy while endoergic one (⌬E Ͼ 0) is hardly observable.Here RE(A ϩ ) is the recombination energy of A ϩ . RE is equivalent to the ionizat...

Low-Lying Electronic States and Cl-Loss Photodissociation of the C2H3Cl+ Ion Studied Using Multiconfiguration Second-Order Perturbation Theory

“…We have carried out high-level ab initio calculations for the low-lying electronic states of the C 2 H 3 Cl + ion, and in the present article we will report our calculation results concerning properties and Cl-loss dissociation processes of the low-lying states and discuss these results in comparison with the experimental results reported recently by Kim et al [3][4][5] and previously by other groups (see below).…”

“…We also calculated the f value for the X 2 A′′ r A 2 A′ transition, and the calculated f value is very small (5 × 10 -6 ), which is compatible to the experimental fact that the A state has a long lifetime. 3 CASPT2//CASSCF Cl-Loss Dissociation Potential Energy Curves. In Figure 2 shown are the CASPT2//CASSCF Cl-loss dissociation PECs for the 1 2 A′′ (X 2 A′′), 1 2 A′ (A 2 A′), 2 2 A′′ (B 2 A′′), and 2 2 A′ (C 2 A′) states of the C 2 H 3 Cl + ion, and the CASPT2//CASSCF relative energies for the reactants and asymptote products of the four states to the ground-state reactant are given in parentheses.…”

Dissociation of the Phenylarsane Molecular Ion: A Theoretical Study

“…Recently, the existence of the long-lived electronic excited states of some polyatomic molecular ions was reported. [20][21][22][23] Some of the metastable 1a ions in the electronic excited state possibly preferred the dissociation to 2a + H• without the H scrambling, which was intensively detected in the MID experiment. Further experimental investigations are needed in order to examine this possibility.…”

Isomerization and Dissociation of the Acrylonitrile Radical Cation: A Theoretical Study

“…The ion-molecule reactions related with acrylonitrile have been investigated by several groups. [3][4][5][6][7][8][9] For the characterization of the acrylonitrile ion, the following experimental techniques were used: electron attachment, 10 electron impact ionization, 11 charge exchange ionization, 12 and tandem mass spectrometry.…”