摘要
应用UHF/6—311G法确定C_2H的平衡几何,能量、偶极矩、力常数与振动频率,并用MP3/6—31G法计算了离解能,由此所导出的分析势能函数成功地表迷了星际分子C_2H的结构与动力学特征。
The ethynyl radical C_2H has been the subject of both experimental and the oretical investiga- tions for its astrophsical interest. It is wall established that it is linear and its ground state is ~2∑^+. However, beyond these many questions about the strcture and spectra of this molecule are still open. The present work devotes to provide the molecular potential function of C_2H in the whole region as accurate as possible, and examine the dynamic feature which has been not yet dealt with. Using UHF method the optimum geometry of C_2H has been found to be 1.226A, 1.054A and 180° with 6-311G basis. and to be 1.209A, 1.057A, and 180°with 6-311G basis for R_(CC), R_(CH) and CCH respectively. As indicated earlier made by Z. H. zhu, the addiction of polarization functions to the basis set leads to significant improvement in bond angles but not in bond lengths. Based on the geometry of 6-311G basis, we have calculated the force field given in Table 2, the re- sults of the force constants f_(CC) and f_(CH), and the stretching vibration frequencies v_1(CH) and v_3(CC) are in good agrement with all the available experimental data. It is impossible to find the accurate dissocciation energy using basis 6-311G or 6-311G without configuration interaction. Therefore, the third-order perturbation with smaller basis set of 6-31G was used to the calcula- tion of dissociation energy. The calculated De of C_2H is 12.06eV, which is remarkable satisfaction in comparison with thermodynamic determination of 12.1 leV. The potential energ function would play the key role in examining the features of molecular dy- namics. The ab-initio approach is likely to be valuable for some stationary regions, but it is extremely arduous and time-consuming task for giving an acceptable potential surface in the whole reactive region. Instead of that, we have used the so-called many-body expansion theory developed by J. N. Murrell and his coworkers, whivh is successful now in use for atom-diatom reactive collisions. The molecular potential function of many-body expansion for C_2H has been derived based on the energy, geometry and fome field caculated above for its equilibrium configuration, in addition, the experimental data for diatomics. It is obvious from Fig.1 that there will be no activation energy for the motrcular formation processes of stable C_2H starting from either C_2 + H or CH + C, which is significant for the interstallar molecules. From Fig.2 and 3, it seems to us that hydrogen at- om is much liable to have the intramolecular migration along bond C-C, but the carbon atom is not the case.
