Spectroscopic characteristics of the cyanomethyl anion and its deuterated derivatives ^⋆

¹ Indian Centre for Space Physics, Chalantika 43, Garia Station Rd., 700084 Kolkata India
e-mail: ankan@csp.res.in; liton@csp.res.in
² S. N. Bose National Centre for Basic Sciences, Salt Lake, 700098 Kolkata, India
e-mail: chakraba@bose.res.in

Received: 12 August 2013
Accepted: 17 November 2013

Abstract

Context. It has long been suggested that CH₂CN⁻ (cyanomethyl anion) might be a carrier of one of the many poorly characterized diffuse interstellar bands. In this paper, our aim is to study various forms (ionic, neutral, and deuterated isotopomer) of CH₂CN (cyanomethyl radical) in the interstellar medium.

Aims. The aim of this paper is to predict spectroscopic characteristics of various forms of CH₂CN and its deuterated derivatives. Moreover, we would like to model the interstellar chemistry for predicting the column densities of such species around dark cloud conditions.

Methods. We performed detailed quantum chemical simulations to present the spectral properties (infrared, electronic, and rotational) of various forms of the cyanomethyl radical. Moller-Plesset perturbation theory along with the triple-zeta, correlation-consistent basis set is used to obtain different spectroscopic constants of CH₂CN⁻, CHDCN⁻, and CD₂CN⁻ in the gas phase. They are essential for predicting rotational spectra of these species. Depending on the total number of electrons, there are several allowed spin states for various forms of the cyanomethyl radical. We performed quantum chemical calculations to find out, energetically, the stablest spin states for these species. We computed IR and electronic absorption spectra for different forms of CH₂CN. Moreover, we have also implemented a large gas-grain chemical network to predict the column densities of various forms of the cyanomethyl radical and its related species. To mimic physical conditions around a dense cloud region, the variation in the visual extinction parameters were considered with respect to the hydrogen number density of the simulated cloud.

Results. Our quantum chemical calculation reveals that the singlet spin state is the stablest form of cyanomethyl anion and its deuterated forms. For confirming the detection of the cyanomethyl anion and its two deuterated forms, namely CHDCN⁻ and CD₂CN⁻, we present the rotational spectral information of these species in the Appendix. Our chemical model predicts that the deuterated forms of cyanomethyl radicals (especially the anions) are also reasonably abundant around the dense region of the molecular cloud.