Formation of prebiotic hydrogen cyanide dimers in an astrophysical environment

Abstract

The past half-century has seen much advancement in the fields of astronomy and astrochemistry, but an emerging and highly interdisciplinary field, known as astrobiology, now seeks to answer one of the oldest and most fundamental questions in science: What is the origin of life? Prebiotic molecules — those that are proposed to be part of the processes leading to the origin of life — and the formation of nucleobases in different astrophysical environments have thus far been the primary area of focus. This study was spurred on by the detection of an HCN dimer (H2C2N2) by the Green Bank Telescope, along with possible formation routes leading to adenine (H5C5N5), one of the nucleobases. The relatively low temperatures ( 10 K) and densities of the relevant environments naturally leads one to suspect the involvement of nontrivial quantum mechanical effects in the chemical processes. The possibility of using an open quantum systems approach to the problem of HCN dimer formation or even of spontaneous dimerisation on the surfaces of interstellar ice grains was investigated. This required gaining an understanding of current methods of investigation regarding surface reactions, seeing if there are any gaps in the theory that can be filled by quantum mechanics, attempting to remedy this if they are, indeed, found. We expected to be able to model the ice-surface and accompanying molecules as a two-level system (or, at the very least, not something incredibly complex) in which the important quantum-mechanical effects are accounted for, and to subsequently model the reaction process. The ice-surface had to first be modelled using current, conventional methods. The complexity and thoroughness involved in the modelling by means of computational quantum chemistry (CQC) was unclear on the outset of the study, which has, admittedly, turned into more of a literature review regarding the problem as described in the title. This project thus attempts to provide the reader with background on the problem, current and previous methods with which prebiotic chemical problems have been investigated, a familiarity with many of the different concepts, and finally how such a problem can be solved given a larger time-investment. To this end we look specifically at the problem of HCN dimerisation both on the surface of interstellar ice-grains, as well as in the gas-phase. A chemical reaction pathway for gas-phase dimerisation of HCN and a model for the ice-grain surface are both developed.