Adenine and guanine have been observed in meteor material. Because of this, they are thought to have been formed in the interstellar medium. However, while there exist several theoretical works exploring the mechanisms of formation for adenine, guanine has received only minor attention. In this work, a retrosynthetic analysis strategy was employed to suggest a new mechanism of guanine formation from species previously observed in the interstellar medium: HCN, CCO, HNCNH, and its isomer H₂NCN. MP2/6-311++G(2d,2p) calculations were used to evaluate the Gibbs free energy of the minimum and transition states involved in a simple reaction mechanism, involving only five steps. The intermediate species are highly polar (dipole moment range of 4.35–7.72 D), which make them candidates for observation in the rotational spectra. In terms of energy, this new mechanism is very exergonic, and the energy barriers are monotonically decreasing. The value of the energy barrier for the first step in this mechanism was substantially reduced when water molecules were explicitly introduced as a way to represent water-ice-coated grain surfaces.

中文翻译：

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星际介质中先前观察到的物种形成鸟嘌呤-异构体的新机制

在流星物质中已观察到腺嘌呤和鸟嘌呤。因此，它们被认为是在星际介质中形成的。然而，尽管有一些理论研究探索腺嘌呤的形成机理，但是鸟嘌呤仅受到了很小的关注。在这项工作中，采用逆合成分析策略提出了一种由先前在星际介质中观察到的物种：HCN，CCO，HNCNH及其异构体H ₂形成鸟嘌呤的新机制。NCN。MP2 / 6-311 ++ G（2d，2p）计算用于评估简单反应机制（仅涉及五个步骤）中涉及的最小和过渡态的吉布斯自由能。中间物种是高极性的（偶极矩范围为4.35-7.72 D），这使它们成为旋转光谱中观察的候选对象。在能量方面，这种新机制非常容易发挥作用，并且能量壁垒正在单调减少。当明确引入水分子作为代表水冰覆盖的谷物表面的一种方式时，该机理第一步的能垒值就大大降低了。