HCnH− Anion Chains with n ≤ 8 Are Nonlinear and Their Permanent Dipole Makes Them Potential Candidates for Astronomical Observation

Abstract

To be detectable in space via radio astronomy, molecules should have a permanent dipole moment. This is the plausible reason why HCnH chains are underproportionally represented in the interstellar medium in comparison with the isoelectronically equivalent HCnN chain family, which is the most numerous homologous series astronomically observed so far. In this communication, we present results of quantum chemical calculations for the HCnH family at several levels of theory: density functional theory (DFT/B3LYP), coupled-cluster expansions (ROCCSD(T)), and G4 composite model. Contradicting previous studies, we report here that linear HCnH− anion chains with sizes of astrochemical interest are unstable (i.e., not all calculated frequencies are real). Nonlinear cis and trans HCnH− anion chains turn out to be stable both against molecular vibrations (i.e., all vibrational frequencies are real) and against electron detachment (i.e., positive electroaffinity). The fact that the cis anion conformers possess permanent dipole is the main encouraging message that this study is aiming at conveying to the astrochemical community, as this makes them observable by means of radio astronomy.

Critical analysis of radical scavenging properties of atorvastatin in methanol recently estimated via density functional theory

“…Tables 1, S2, and S3) we performed unrestricted (UB3LYP) calculations. Therefore, it is worth mentioning that, similar to other similar cases [19, 20, 21, 22], we can safely rule out spin contamination artifacts (cf. Table 1).…”

Comprehensive Quantum Chemical Characterization of the Astrochemically Relevant HCnH Chain Family: An Attempt to Aid Astronomical Observations

“…Most importantly from astrochemical perspective (see our preliminary report [ 88 ] and Section 2.5 for further details), we found that, irrespective of their parity, linear HC n H − anion conformers with $$n \le 8$$ are unstable against molecular vibrations, that is, not all computed vibrational frequencies are real. Our calculations showed that cis and trans (C 2 v and nearly C 2 h symmetry, respectively) anion HC n H − nonlinear conformers with $$n \le 8$$ (like those depicted in Figure ) correspond to structures wherein all vibrational frequencies are real.…”

“…To understand why the results presented in this paper for HC n H − anions —partially reported recently [ 88 ] —are important from an astrochemical perspective, one should first note that both neutral HC n H 0 and cation HC n H + are linear chains possessing inversion symmetry. Their D$$_{\infty h}$$ symmetry is incompatible with a permanent dipole moment ($$\mu \equiv 0$$).…”

“…The astrochemical relevance of the HC n H − anions was emphasized in a recent report. [ 88 ] Reporting that nonlinear anion HC n H − conformers with $$n\le 8$$ are stable is a finding of paramount importance for space sciences. In contrast with the linear HC n H 0 and HC n H + chains, anion HC n H − cis conformers possess permanent dipole.…”

Two Theorems and Important Insight on How the Preferred Mechanism of Free Radical Scavenging Cannot Be Settled

“…The solvent (water) was treated within the polarized continuum model (PCM) [31] using the integral equation formalism (IEF) [32]. Similar to other cases studied recently [33][34][35], spin contamination did not appear to be an issue for the unrestricted spin (UB3LYP) approaches, as witnessed by the values of the total spin values S 2 before and after annihilation of the first spin contaminant presented in the following. For reasons explained below, we also performed restricted open shell (ROB3LYP) calculations, which are more demanding computationally for larger molecules.…”