用耦合簇理论及相关一致基研究NH2自由基的解析势能函数

Investigation on analytic potential energy function of NH_2 radical using coupled-cluster theory in combination with the correlation-consistent basis sets

运用CCSD(T)理论和Dunning等的系列相关一致基对NH2自由基的基态结构进行了优化,并使用优选出的cc-pV5Z基组对其进行了频率计算.得到的结果是:平衡核间距RNH=0.10247 nm,键角∠HNH=102.947°,离解能De=4.2845 eV,振动频率ν1(a1)=1546.0342 cm-1,ν2(a1)=3379.5543 cm-1和ν3(b2)=3474.4784 cm-1.对NH自由基及H2分子,使用优选出的cc-pV6Z基组对其基态的几何构型与谐振频率进行了计算并进行了单点能扫描,且将扫描结果拟合成了解析的Murrell-Sorbie函数.采用多体项展式理论导出了NH2自由基的解析势能函数,其等值势能图准确再现了它的离解能和结构特征.报导了NH2自由基对称伸缩振动等值势能图中存在的两个对称鞍点,对应于反应NH+H→NH2,势垒高度约为0.1378×4.184 kJ/mol.

The coupled-cluster singles-doubles-approximate-triples [ CCSD（T） ] theory in combination with the series of the correlation-consistent basis sets of Dunning and co-workers are used to calculate the equilibrium geometries of NH2 radical. By careful comparison between the available experiments and the present results, the cc-pV5Z basis set is found to be the most suitable one for further calculations. The values obtained at this basis set are of 0.10247 nm for the equilibrium bond length RN-H, 102.947° for the bond angle LHNH, 4.2845 eV for the dissociation energy De and 1546.0342 cm^-1,3379.5543 cm^-1 and 3474.4784 cm^-1 for the harmonic frequencies v 1 （a 1 ）, v2 （ a 1 ） and v3 （b2）, respectively. The equilibrium geometries, harmonic frequencies and potential energy curves of the ground-state NH radical and H2 molecules are calculated at the CCSD （T）/cc-pV6Z level of theory. The ab initio calculation points are fitted to the analytic Murrell-Sorbie function with the least-squares method. The spectroscopic parameters derived from the analytic potential energy functions are in good accord with the available experiments. The analytic potential energy function of the NH2 （C2v, X^2B1） radical is derived using the many-body expansion theory. This function correctly describes the configuration and dissociation energy of the ground-state NH2 radical.