High-Level Ab Initio Calculations for (Potential) Interstellar Anions: Structures, Spectroscopic Properties and Energetics

摘要

During the past three years, the first four molecular anions could be identified in the interstellar or circumstellar medium. In 2006, McCarthy et al. [1] were able to demonstrate that the carbon chain anion C_6H~- is the carrier of harmonically related radio lines, which were detected with the Nobeyama 45-m radio telescope already more than a decade earlier [2]. The first astronomical detection of C_4H~-, based on laboratory microwave and millimeter wave spectroscopy [3], was reported by Cernicharo et al. [4]. With the aid of laboratory data [3], the largest anion found so far outside of the solar system, C_8H~-, was detected in two well-known astronomical sources [5-7]. These three linear anions appear to be closely related to the isoelectronic series of well-known neutral interstellar molecules of type HC_(2n-1)N, the cyanopolyynes. The fourth molecular anion detected so far is linear C_3N~- [8], which in chemical terms may be regarded as the conjugate base of the ubiquituous interstellar molecule HC_3N. As far as the spectroscopic properties of the noted four anions is concerned, experimental information is still scarce and a major part of our current knowledge derives from high-level ab initio calculations carried out at Gottingen [9-12]. Among theoretical predictions for properties of molecules of astronomical interest, ground-state rotational constants (A_0, B_0, and C_0) play a very important role. The ab initio calculation of a B_0 value (the only non-vanishing constant for a linear species) is commonly divided into two parts, the calculation of the equilibrium value B_e and the ground-state vibrational contribution ΔB_0, such that B_0 = B_e - ΔB_0. The reliable ab initio calculation of B_e values for molecules of the type of the known interstellar anions to an accuracy of better than 0.1 % is hardly feasible at present and so empirical corrections to calculated equilibrium structures are often the recommended procedure [13]. Recommended equilibrium structures for the four known interstellar anions [9, 10] are shown in Fig. 1. Characteristic is the significant elongation of the terminal CC bond by 0.04 - 0.05 A compared with the corresponding protonated species HC_(2n)H (n = 2-4) and HC_3N. ΔB_0 values may be calculated easily from cubic force fields by means of traditional second-order perturbation theory in normal coordinate space, now often abbreviated VPT2. For the four anions, the calculated ΔB_0 values (from CCSD(T) calculations with basis set of at least cc-pVQZ quality) are rather small, typically less than 0.1 % of the B_e values [9, 10]. Nevertheless, explicit calculation is important since this information is not known apriori.