Structures of Al+(C2H4)n clusters: Mass-selected photodissociation and ab initio calculations

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Abstract

Al+(C2H4)n clusters were produced by reaction of laser ablated Al+ ions with ethene molecules seeded in a pulsed molecular beam, and detected with a reflectron time-of-flight mass spectrometer. The mass spectrum shows that an Al+ ion can combine with at least eight ethene molecules to form Al+(C2H4)n clusters. Based on the photodissociation experiments and ab initio calculations, we suggest that an Al+ ion can strongly interact with one or two C atoms to form Al–C σ-bonds and trigger addition reaction of ethene molecules to develop chain or ring structures. The interaction of Al+ ion with ethene is different from the interaction between V+ ion and ethene molecules reported previously (Int. J. Mass. Spectrom. 295 (2010) 36), wherein a V+ ion can only combine with no more than four ethene molecules at similar experimental conditions and causes little change in the structures of the ethene molecules.

Highlights

► We propose that Al–C σ-bonds can be formed in Al+(C2H4)n (n > 1) clusters. ► The strong interaction between Al+ and ethene molecules weakens the Cdouble bondC bond significantly. ► Al+ can trigger addition reaction of ethene molecules to develop chain or ring structures.

Introduction

Investigations of metal ion-hydrocarbon molecular interactions have received extensive interests because such interactions play important roles in various chemical fields, such as catalysis, lubrication, hydrogen storage, oxidation and reduction reactions [1], [2], [3]. In catalysis processes, bond-formation and bond-breaking usually occur at unsaturated metal centers. Electron-deficient aluminum complexes are important for the study of homogeneous and heterogeneous catalysis [4], [5], for example, the zeolites doped by Al have been widely used as heterogeneous catalysts due to its high activity, good selectivity and chemical stability [6], [7], [8]. Since aluminum and its compounds are important in heterogeneous catalysts, the interaction between aluminum ion and organic molecules received special attention. Kasai and McLeod examined the electron spin resonance (ESR) spectra of aluminum–ethene complex in rare-gas (neon and argon) matrices. They found that the Al(C2H4) complex had a π-coordinated structure with a dative bond of donation from a half-full p orbital of Al into the antibonding π orbital of ethene [9], [10]. Mitchell et al. determined the binding energy between Al atom and ethene to be greater than 0.69 eV by observation of the temperature dependence of the equilibrium constant of the Al–ethene reaction in the range of 288–333 K [11]. Manceron and Andrews investigated the infrared spectrum of Al(C2H4) in solid argon and ethene. They showed that Al atom formed a symmetric π-complex with ethene by interacting with equivalent CH2 groups [12]. Chenier et al. [13] detected the formation of alumiocyclopentane in the reaction of Al atom with ethene molecules at low temperature by ESR method. Schwarz and co-workers determined the structure and bond dissociation energy of Al–ethene using FTICR mass spectroscopy and ab initio calculations [14]. Kleiber and co-workers studied photodissociation spectroscopy of Mg+(C2H4) and Al+(C2H4) [1], [15]. They also reported photodissociation spectroscopy studies of weakly bound Al+–alkene bimolecular complexes (ethene, propene, and 1-butene) in the 216–320 nm range [16]. The geometric structure and electronic property of Al(C2H4) were also investigated using different theoretical methods by several theoreticians [17], [18], [19], [20].

In the past, the investigations of aluminum with ethene molecules were mainly focused on Al–C2H4 complex with a single ethene molecule except that Al(C2H4)2 was studied by Chenier et al. [13]. To our knowledge, there is no investigation on the cationic Al+(C2H4)2–6 clusters. In this work, we studied the interaction between Al+ ion and ethene molecules using mass-selected photodissociation of Al+(C2H4)n clusters combining with ab initio calculations.

Section snippets

Experimental method

The experiments were conducted on a home-built reflectron time-of-flight mass spectrometer, which has been described in detail elsewhere [21]. Aluminum–ethene clusters were generated in a laser vaporization source, where a rotating and translating aluminum disc target was ablated by the second harmonic of a Nd:YAG laser (532 nm, 2.331 eV/pulse, Continuum Surelite II-10) to produce Al+ ions, then the Al+ ions reacted with the ethene molecules seeded in argon carrier gas (∼4% ethene) expanding into

Mass spectrum

A typical mass spectrum of Al+(C2H4)n clusters is presented in Fig. 1. The prominent mass peaks are Al+(C2H4)n (n = 0–8) clusters. The mass signals of Al+(C2H4)9 and Al+(C2H4)10 clusters can also be detected although they are not shown in the spectrum. From Fig. 1, we can see that the ion intensities decrease gradually from Al+(C2H4) to Al+(C2H4)3, then increase from Al+(C2H4)3 to Al+(C2H4)4, after that the ion intensities decrease from Al+(C2H4)4 to Al+(C2H4)8 again. In addition to the Al+(C2H4)n

Conclusions

Al+(C2H4)n clusters were produced in a supersonic molecular beam by laser vaporization and studied with laser photodissociation and ab initio calculations. For the dissociation of Al+(C2H4)2–5, the largest fragment ions were generated by loss of two ethene molecules. For the dissociation of Al+(C2H4)6, the largest fragment ion was generated by loss of three ethene molecules. The dissociation of Al+(C2H4)n (n = 1–6) clusters can eliminate all ethene molecules to generate Al+ fragment ion. Based on

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC, Grant No. 20933008). The theoretical calculations were conducted on the ScGrid and Deepcomp7000 of the Supercomputing Center, Computer Network Information Center of Chinese Academy of Sciences.

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