Structure, stability, and optical absorption spectra of small TinCx clusters: a first-principles approach

Sergio Gámez-Valenzuela,José I Martínez,Julio A Alonso,Gonzalo Santoro

doi:10.1093/mnras/stab2735

Abstract

ABSTRACT Titanium carbide molecular clusters are thought to form in the circumstellar envelopes of carbon-rich asymptotic giant branch (AGB) stars but, to date, their detection has remained elusive. To facilitate the astrophysical identification of those clusters in AGB and post-AGB environments, the molecular structures and optical absorption spectra of small TinCx clusters, with n = 1–4 and x = 1–4, and some selected larger clusters, Ti3C8, Ti4C8, Ti6C13, Ti7C13, Ti8C12, Ti9C15, and Ti13C22, have been calculated. The density functional formalism, within the B3LYP approximation for electronic exchange and correlation, was used to find the lowest energy structures. Except the clusters having a single Ti atom, the rest exhibit three-dimensional structures. Those are formed by a Ti fragment surrounded in general by carbon dimers. The optical spectra of TinCx, computed by time-dependent density functional theory, using the corrected CAM-B3LYP functional, show absorption features in the visible and near-infrared regions which may help in the identification of these clusters in space. In addition, most of the clusters have sizable electric dipole moments, allowing their detection by radioastronomical observations.

Highlights

Titanium carbide (TiC) dust is predicted to form in the inner regions of the circumstellar envelopes (CSEs) of carbon rich (C-rich) Asymptotic Giant Branch stars (AGBs) (Agúndez, M. et al 2020)
The identification of TiC crystals embedded in graphite spherules extracted from the Murchinson meteorite supports this prediction (Bernatowicz et al 1991; Bernatowicz et al 1996) but, to date, the detection of molecules comprised of titanium and carbon that may serve as precursors for the formation of TiC dust has remained elusive
The reaction of Ti with acetylene (C2H2) has been suggested as a possible pathway for the formation of stoichiometric TiC molecules (Chigai et al 1999; Gail & Sedlmayr 2013), and chemical equilibrium calculations of AGBs atmospheres show that the depletion of atomic Ti starts at stellar radii in which most carbon is locked into C2H2

Summary

INTRODUCTION

Titanium carbide (TiC) dust is predicted to form in the inner regions of the circumstellar envelopes (CSEs) of carbon rich (C-rich) Asymptotic Giant Branch stars (AGBs) (Agúndez, M. et al 2020). The mass spectrometric studies of anionic TinCx− clusters performed by Wang et al (1997, 1998) revealed new stable anionic clusters and evidenced the importance of C2 dimers in stabilizing the structure of these clusters These authors measured the photoelectron spectra of the most stable anionic clusters, namely:. Apart from the spectrum of the highly stable Ti8C12 cluster, the optical absorption of other Ti-C clusters of relevance in astrochemistry (Agúndez, M. et al 2020) has not been previously addressed. With this in mind, we have used TD-DFT to calculate the visible and near infrared photoabsorption spectrum of small TinCx clusters, with n = 1–4 and x = 1–4, and some selected larger clusters: Ti3C8, Ti4C8, Ti6C13, Ti7C13, Ti8C12, Ti9C15, and Ti13C22.

COMPUTATIONAL APPROACH

STRUCTURE AND STABILITY OF THE CLUSTERS

ABSORPTION SPECTRA

Findings

CONCLUSIONS