Reaction Pathway and Rovibrational Analysis of Aluminum Nitride Species as Potential Dust Grain Nucleation Agents

Abstract

A dust nucleating agent may be present in interstellar or circumstellar media that has gone seemingly undetected and unstudied for decades. Some analyses of the Murchison CM2 meteorite suggest that at least some of the aluminum present within condensed as aluminum nitrides instead of the long studied, but heretofore undetected suite of aluminum oxides. The present theoretical study utilizes explicitly correlated coupled cluster theory and density functional theory to provide a pathway of formation from alane (AlH$_3$) and ammonia to the cyclic structure, Al$_2$N$_2$H$_4$ which has the proper Al/N ratio expected of bulk aluminum nitrides. Novel rovibrational spectroscopic constants are computed for alane and the first two formed structures, AlNH$_6$ and AlNH$_4$, along the reaction pathway for use as reference in possible laboratory or observational studies. The $\nu_8$ bending frequency for AlNH$_6$ at 755.7 cm$^{-1}$ (13.23 $\mu$m) presents a vibrational transition intensity of 515 km mol$^{-1}$, slightly more intense than the anti-symmetric C$-$O stretch of carbon dioxide, and contains a dipole moment of 5.40 D, which is $\sim 3 \times$ larger than that of water. Thus, the present reaction pathway and rovibrational spectroscopic analysis may potentially assist in the astrophysical detection of novel, inorganic species which may be indicative of larger dust grain nucleation.