中性和阳离子丁酮团簇的结构及稳定性的理论研究

使用密度泛函B3LYP方法,在6-31G*和6-311+G**基组水平上计算中性和阳离子丁酮团簇(CH_3COC_2H_5)_n和(CH_3COC_2H_5)_n~+(n 7)的稳定结构,并比较不同尺寸团簇之间的相对稳定性.中性和阳离子丁酮团簇的结构具有相似性:n=3—7时,组成团簇的丁酮的平均几何参数基本相同,单环结构最稳定;随着团簇尺寸的增加,双环结构的稳定性逐渐上升.通过平均结合能、一阶差分能、HOMO-LUMO能隙等计算分析可知:在所研究的各种尺寸团簇中,(CH_3COC_2H_5)_3是最稳定的中性团簇,与实验中的最强峰对应;(CH_3COC_2H_5)_4~+是最稳定的阳离子团簇.通过电离能计算得到丁酮分子的垂直电离能为9.535 eV与实验值相符,同时证明中性和阳离子丁酮二元团簇的结构变化较大.研究结果为实验中丁酮团簇碎片离子的形成机理提供一定的理论依据,并且为进一步研究酮类分子团簇的生长规律提供有价值的信息.

Theoretical study on the structure and stability of neutral and cationic butanone clusters

The molecular clusters have attracted increasing attention in recent years due to their applications in areas of laser, synchrotron radiation, molecular beam and time-of-flight mass spectrometry. The cluster structures can be speculated by the mass spectrum measurement and predicted by the quantum chemical methods. It is very important for understanding the solvation kinetics and nucleation to explore the formation and growth of clusters. Meanwhile, it is also beneficial to understanding the atomic or intermolecular interactions in the clusters.The molecular clusters have been studied in our previous work. The acetone clusters (CH₃COCH₃)_n (n ≤ 12) were observed by 355 nm pumping laser. The structures of (CH₃COCH₃)_n with n=2-7 were calculated by density functional theory, and some structures of clusters with low energy were given. Subsequently, several butanone cluster fragment ions and protonated butanone (CH₃COC₂H₅, which is formed by taking a methyl change into ethyl from acetone CH₃COCH₃) clusters were observed by measuring the mass spectra under the irradiations of 355 nm and 118 nm laser lights, respectively.It is important to determine the stable cluster structures and explain the dynamics of the clusters by theoretical calculation. The stable geometric structures of neutral and cationic butanone clusters are optimized at B3LYP/6-31G^* and B3LYP/6-311+G^** levels based on the density functional theory. The structural characteristics and stabilities of butanone clusters with various sizes are also analyzed. The average binding energy, first-order difference energy, HOMO-LUMO gap and ionized energy are further discussed systematically in the present work. The results show that the structures of (CH₃COC₂H₅)_n and (CH₃COC₂H₅)_n⁺ have similar characteristics, single-ring structure is the most stable for them when n=3-7, and the structures also occur in some hydrogen bonded clusters, such as (H₂O)_n (n=3-6), (NH₃)_n (n=3-6), (CH₃OH)_n (n=3-6), and (HCHO)_n (n=3-8). Moreover, the stability of double ring structure rises with cluster size increasing. The (CH₃COC₂H₅)₃ has the best stability in neutral clusters (CH₃COC₂H₅)_n with n=2-7, and it corresponds to the strongest peak in experiment. In addition, the (CH₃COC₂H₅)₄⁺ is the most stable in the cationic clusters with corresponding sizes. Furthermore, the vertical ionization energy of butanone molecule is 9.535 eV via theoretical calculation, which is in agreement with the experimental data. At the same time, the structures of (CH₃COC₂H₅)₂⁺ and (CH₃COC₂H₅)₂ are proved to be different by the ionization energy. The results provide a theoretical basis for the formation mechanism of butanone cluster fragment ions in experiment, and it is beneficial to the further study of growing the ketone clusters.