Electron Impact Catalytic Dissociation: Two‐Bond Breaking by a Low‐Energy Catalytic Electron

Abstract

In low-energy electron impact on neutral molecules, the free electron and the molecule often form an intermediate metastable electron-molecule compound. [1][2][3] This compound may either release the excess electron by autodetachment (AD) [1] or fragment by a reaction called dissociative electron attachment [DEA, Eq. (1)]: [1][2][3] e À þ AB ! ðABÞA wide variety of chemical transformations initiated by electron impact can be attributed to DEA. [1][2][3] In particular, it is well known that in organic molecules DEA efficiently leads to bond dissociation, producing radical and radical-anion molecular fragments. In general, in all examples studied and understood theoretically, DEA leads to an anionic fragment and a neutral fragment. [1][2][3] Recently, mass spectroscopic measurements monitoring DEA of fructose have shown that several neutral fragments may appear in addition to the anionic fragment.[4] The underlying mechanisms have not been clarified yet, but it may be suspected from the detected products that the two fragments initially formed by DEA as in Equation (1) have further fragmented by several stepwise reactions. In any case, electron-molecule reactions are seen to be chemically rich and are of chemical interest.Herein we report on a new elementary reaction (i.e., single-step reaction) mechanism of an electron and a molecule in a metastable compound which we call (two or more) bond breaking by a catalytic electron (BBCE). Unlike in DEA, the formed compound anion dissociates into nonradical neutral molecular subunits and a free electron, which plays the role of a catalyst [Eq. (2)]:where we stress that at least two bonds (not a double bond) break in the elementary reaction path. Notice that the electron is freed in the course of the elementary reaction, that is, the electron is not attached to any of the chemical products of the elementary reaction and, thus, we refer to this mechanism as an electron-catalyzed mechanism. This elementary reaction involves both bond breaking and detachment of the electron. The key differences between BBCE and DEA are as follows: 1. More than one s bond (two-center bond) is broken in the elementary BBCE reaction. 2. The products formed in the BBCE are neutral. 3. The electron is released in the course of the elementary BBCE reaction.Below, we illustrate this mechanism more precisely by investigating electron impact on the quadricyclanone (QDCO) molecule. As will become clearer, the low-energy electron impact on QDCO can be viewed as proceeding via a compound negative ion metastable state. We have acquired considerable experience in the ab initio computation of energy and lifetime of metastable anions using non-Hermitian quantum chemical methods. [5][6][7] Recently, the introduction of a so-called continuum remover complex absorbing potential [8] and its implementation in Greens function methods [9] have made non-Hermitian quantum chemical methods applicable to larger systems. The efficient identification of the metastable states and the correct scaling of the electronic ener...

Communication: Low-energy free-electron driven molecular engineering: In situ preparation of intrinsically short-lived carbon-carbon covalent dimer of CO

“…6,7,10 The catalytic electron reaction channels have been identified and reported very recently. 6,7,10 In this reaction channel, the electron-molecule compound, i.e., the NIRS, formed by the resonant capture of the LEE undergoes nuclear relaxation such that it yields the neutral molecular products by setting the electron free during the course of the nuclear-relaxation. 6,7,10 One may also note that the overall process (see Eq.…”

“…6,7,10 In this reaction channel, the electron-molecule compound, i.e., the NIRS, formed by the resonant capture of the LEE undergoes nuclear relaxation such that it yields the neutral molecular products by setting the electron free during the course of the nuclear-relaxation. 6,7,10 One may also note that the overall process (see Eq. (6)) is also an example of a barrierless four-body fragmentation reaction.…”

“…The ability to selectively initiate sitespecific and bond-specific chemical transformations in target molecules by tuning the kinetic energy of a LEE has opened up unprecedented opportunities for single-molecular engineering. [1][2][3][4][5][6][7][8][9][10] The creation of an electronically metastable long-lived electron-molecule compound state, also known as a negative ion resonance state (NIRS), by the resonant attachment of a LEE to the molecular target is fundamental to these transformations. By tuning the kinetic energy of the LEE to be the resonant capture energy of the target molecule, the electronmolecule compound states of different energies can be created.…”

“…While the resonant capture of the LEE into a two-center localized orbital is responsible for the LEE induced site-selective reactions, single-step reactions involving more than one bond are induced by the resonant capture of the LEE into a multi-center orbital. 6,7,12 The possibilities of bond-strength manipulation are multiplied and become even more fascinating when twoparticle one-hole (2p-1h)-type NIRSs are created, where the resonance capture also involves an electronic excitation in the target molecule. 8 Besides the high degree of site-and bond-selectivities, one of the most important consequences of the LEE induced bond strength manipulation is the possibility that, when induced by the LEE, an apparent uni-molecular multi-step or a highbarrier single-step thermal reaction of the target molecule can be transformed into a barrierless or a low-barrier reaction.…”

“…8 Besides the high degree of site-and bond-selectivities, one of the most important consequences of the LEE induced bond strength manipulation is the possibility that, when induced by the LEE, an apparent uni-molecular multi-step or a highbarrier single-step thermal reaction of the target molecule can be transformed into a barrierless or a low-barrier reaction. 6,7 Barrierless or low-barrier single-step reactions are attractive as they are characterized with high quantum yield efficiencies and fast reaction rates. The LEE induced low-barrier reactions are the manifestations of high orbital energy splitting between the anti-bonding orbital, where the electron is captured, and the corresponding bonding orbital of the target molecule.…”