Potential formation of three pyrimidine bases in interstellar regions

TL;DR

This study explores the potential formation pathways of pyrimidine bases in interstellar environments, predicts a new observable precursor molecule, and provides quantum chemical data to aid future detection efforts.

Contribution

It introduces a comprehensive chemical network model for pyrimidine base formation in space and proposes a new precursor molecule, C4N3OH5, for observational verification.

Findings

Cytosine likely forms in cold, dense interstellar regions.

Uracil formation from cytosine and water is improbable under interstellar conditions.

C4N3OH5 is a more abundant precursor to cytosine and potentially observable.

Abstract

Work on the chemical evolution of pre-biotic molecules remains incomplete since the major obstacle is the lack of adequate knowledge of rate coefficients of various reactions which take place in interstellar conditions. In this work, we study the possibility of forming three pyrimidine bases, namely, cytosine, uracil and thymine in interstellar regions. Our study reveals that the synthesis of uracil from cytosine and water is quite impossible under interstellar circumstances. For the synthesis of thymine, reaction between uracil and :CH2 is investigated. Since no other relevant pathways for the formation of uracil and thymine were available in the literature, we consider a large gas-grain chemical network to study the chemical evolution of cytosine in gas and ice phases. Our modeling result shows that cytosine would be produced in cold, dense interstellar conditions. However, presence of cytosine is yet to be established. We propose that a new molecule, namely, C4N3OH5 could be observable in the interstellar region. C4N3OH5 is a precursor (Z isomer of cytosine) of cytosine and far more abundant than cytosine. We hope that observation of this precursor molecule would enable us to estimate the abundance of cytosine in interstellar regions. We also carry out quantum chemical calculations to find out the vibrational as well as rotational transitions of this precursor molecule along with three pyrimidine bases.