High Accuracy ab Initio Calculations of Rotational–Vibrational Levels of the HCN/HNC System

Highly accurate ab initio calculations of vibrational and rotational–vibrational energy levels of the HCN/HNC (hydrogen cyanide/hydrogen isocyanide) isomerising system are presented for several isotopologues. All-electron multireference configuration interaction (MRCI) electronic structure calculations were performed using basis sets up to aug-cc-pCV6Z on a grid of 1541 geometries. The ab initio energies were used to produce an analytical potential energy surface (PES) describing the two minima simultaneously. An adiabatic Born–Oppenheimer diagonal correction (BODC) correction surface as well as a relativistic correction surface were also calculated. These surfaces were used to compute vibrational and rotational–vibrational energy levels up to 25 000 cm–1 which reproduce the extensive set of experimentally known HCN/HNC levels with a root-mean-square deviation σ = 1.5 cm–1. We studied the effect of nonadiabatic effects by introducing opportune radial and angular corrections to the nuclear kinetic energy operator. Empirical determination of two nonadiabatic parameters results in observed energies up to 7000 cm–1 for four HCN isotopologues (HCN, DCN, H13CN, and HC15N) being reproduced with σ = 0.37 cm–1. The height of the isomerization barrier, the isomerization energy and the dissociation energy were computed using a number of models; our best results are 16 809.4, 5312.8, and 43 729 cm–1, respectively.本文针对HCN/HNC（氰化氢/异氰化氢）异构化体系的多种同位素取代物，报道了高精度从头算（ab initio）振动与振转能级的计算结果。研究采用全电子多参考组态相互作用（MRCI）电子结构计算方法，在包含1541个几何构型的计算网格上，使用最高至aug-cc-pCV6Z的基组完成了全部计算。基于所得ab initio能量，我们构建了可同时描述两个势能极小点的解析型势能面（PES），并额外计算了绝热玻恩-奥本海默对角修正（BODC）面与相对论修正面。依托上述势能面，我们计算了能量最高至25000 cm⁻¹的振动与振转能级，该组能级与已发表的大量HCN/HNC实验能级的均方根偏差σ仅为1.5 cm⁻¹。为探究非绝热效应的影响，我们对核动能算符引入了合适的径向与角向修正；通过经验拟合确定两个非绝热参数后，四种HCN同位素取代物（HCN、DCN、H¹³CN与HC¹⁵N）在7000 cm⁻¹以下的观测能级均被精准重现，均方根偏差降至0.37 cm⁻¹。本研究采用多种模型计算了异构化势垒、异构化能与解离能，所得最优结果分别为16809.4 cm⁻¹、5312.8 cm⁻¹与43729 cm⁻¹。