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.
