Reaction of CN^– with F, Cl, O, and S Atoms: Attachment or Associative Detachment?

Abstract

Highly correlated ab initio wave functions within the UCCSD(T)-F12 approach have been used to map the potential energy surfaces (PESs) describing the reactivity of the CN(-) (X(1)Σ(+)) anion with neutral atoms present in interstellar media (F, Cl, O, and S). With the H atom, for comparison, the reaction [CN(-)((1)Σ(+)) + H((2)S)] evolves along the PES of the X(2)Σ(+) electronic ground state of HCN(-) (or HNC(-)) until the crossing with the X(1)Σ(+) electronic ground state of HCN (or HNC), where electron detachment occurs. The process is rather similar to the two halogen atoms F and Cl, with some differences due to the larger electron affinity of the halogens, making possible the existence of ClCN(-) in a (2)Σ(+) state. The reaction of CN(-) with O and S atoms proceeds via a multistep mechanism. The lowest electronic state at long distance, the (3)Π state arising from the [CN(-)((1)Σ(+)) + O/S((3)P)] reaction channel, does not correlate with the X(1)Σ(+) ground state of the XCN(-) anion (X = O or S). This (3)Π state and its bent components cross at medium RXC (RXN) distances the X(1)Σ(+) ground state of XCN(-) or XNC(-), and at shorter distances the X(2)Π state of the neutral XCN or XNC where the extra electron can detach. With both O and S atoms, it is shown that the spin-orbit couplings can efficiently lead the [CN(-)((1)Σ(+)) + O/S((3)P)] reaction toward the stable X(1)Σ(+) ground state of XCN(-) and XNC(-).

Probing the Potential Energy Surfaces of BrCN– by Dissociative Electron Attachment

“…Difficulties in both processes lead to frustrations or controversies in terms of the understanding of the chemical reactions and molecular properties. Recently, the PESs of anionic cyanogen halides (XCN – , where X = Cl, Br, or I) have attracted a great deal of attention, aiming to reveal the photodissociation dynamics of the isolated and solvated anion XCN – (refs − ), dissociative electron attachment (DEA) to the neutral XCN, , and the collisional reactions between CN – and atomic targets . With respect to the neutral, XCN – can be further stabilized in its ground state X 2 Σ + by elongating the X–CN bond and maintaining the linear structure (e.g., BrCN – shown in Figure ).…”

Cold reactive and nonreactive collisions of Li and Rb with C2− : Implications for hybrid-trap experiments

“…For example, trap losses have been experimentally observed for the Rb + OH − system 23 . We have previously shown that choosing another alkali or alkaline earth atomic species would not lead to detachment 24 since the vertical detachment energy of MOH − depends on M. An other example is the CN − anion for which the AED reaction can occur for some collision partners despite having a large electron affinity (3.862 eV) 25,26 . In conclusion, it is difficult to rely on general molecular properties to predict whether or not to expect AED reactions at low temperatures without explicitly calculating both the anion and neutral PES.…”

Associative detachment (AD) paths for H and CN− in the gas-phase: astrophysical implications

“…The above reaction has been extensively studied computationally because of the challenge provided by the instability of the hydrogen cyanide to electron attachment processes [11,16]. It is essentially a recombination reaction that is exothermic to forming the neutral system and therefore markedly favoured at the low temperatures of many of the ISM environments.…”