Photoionization mass spectroscopy of clusters of alkali metal atoms with methyl vinyl ketone and acrolein: intracluster oligomerization initiated by electron transfer from a metal atom

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Abstract

We have investigated the photoionization mass spectrometry of clusters of an alkali metal atom (M: Na and K) and methyl vinyl ketone (MVK) or acrolein (AC) to discuss the intracluster oligomerization. In the mass spectra of M(MVK)n, strong ion signals at n=3 are observed. This magic number is explained by the cyclohexane derivative formation in the intracluster oligomerization of MVK initiated by electron transfer from an alkali metal atom. The results of the calculation for Na(MVK) based on density functional theory also show the presence of intracluster electron transfer. On the other hand, in M(AC)n, intensity enhancement is observed at n=4. These intensity enhancements may be due to intracluster reaction in M(AC)n, which is different from the case of M(MVK)n.

Introduction

In recent years, the anionic polymerization of vinyl compounds in the condensed phase received much attention as a method to obtain various useful materials [1], [2]. It has been established that strong bases, such as alkali metals and alkyllithium compounds, initiate anionic polymerization of vinyl compounds that bear electron-withdrawing groups. An electron transfer from these strong bases causes cleavage of the CC double bond of the vinyl monomer to yield a carbanion. This carbanion reacts with another monomer to produce a propagated oligomer chain, which sequentially reacts with other monomer. Thus, the formation of the contact ion pair between the vinyl molecular anion and its counter-ion is important in the initial step of the polymerization. The anionic polymerization of α,β-unsaturated ketone and aldehyde, such as methyl vinyl ketone (MVK: CH2CHCOCH3) and acrolein (AC: CH2CHCHO) is well studied in various reaction conditions (initiator, solvent and temperature) to identify the structures of polymerized products [2]. In the review article for works of polymerization of AC [3], it was pointed out that acrolein is one of the few monomers which contain two polymerizable groups (CC double bond and carbonyl group) of different reactivities.

In recent years, ionic oligomerization in the gas phase has also been studied to some extent in order to elucidate the elementary reaction processes [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. For this purpose, gas phase studies should have some advantages with following features: (1) it is possible to discuss reactivity which is free from the solvent effect, and (2) assignments of the polymerized products are directly obtained by mass spectrometric method. Especially, studies on gas-phase clusters were performed as a microscopic model for the initial step of anionic polymerization. Anionic clusters of vinyl compounds were extensively investigated by Kondow and coworkers [6], [7], [8], [9], [10], [11], [12]. In their studies, trimeric terminations were observed for cluster anions of acrylonitrile and its derivatives, and they concluded that the intracluster oligomerization is initiated by electron attachment to acrylonitrile trimer and forms a stable anion radical that has a ring structure.

The contact ion pair consisting of a vinyl molecular anion and its counter-ion is also expected to be another model for the initial step of anionic oligomerization. Clusters of an initiator and vinyl molecules (VM) are the models of such contact ion pairs to discuss the mechanism of the propagation reaction of the ion pair with another monomer. In the authors’ group, intracluster reactions have been investigated for clusters containing VM and an alkali metal atom (M) by means of a time-of-flight mass spectrometer (TOF-MS) with a cluster beam source [20], [21], [22], [23]. In these studies, the magic numbers at M(VM)3 (M=Li, Na and K) were observed in their photoionization mass spectra, and it was concluded that these magic numbers are due to the trimeric unit with a cyclohexane derivative which is produced by intracluster anionic oligomerization. Thus, the evidence of anionic oligomerization was found for the neutral clusters and the electron transfer from metal to VM initiates the intracluster oligomerization. In addition, for alkali metal–methyl acrylate clusters, the mechanism of anionic oligomerization is found to be different from the methyl acrylate cluster anions [21]. From these studies, it has been found that the existence of the alkali metal atom in these clusters has a critical effect on the reactions in the initial step of the anionic oligomerization.

In this study, we have investigated clusters containing the alkali metal atom (Na, K) and MVK or AC by photoionization mass spectrometry in order to discuss the initial steps of the oligomerization of these molecules. The possibilities of the intracluster reactions are discussed from the size distributions observed in the photoionization mass spectra. We have compared the present results with those in the previous studies of M(VM)n and have discussed the intracluster anionic oligomerization initiated by electron transfer from the alkali metal atom. The quantum chemical calculations have also been performed to obtain the information about intracluster electron transfer in Na(MVK) and Na(AC).

Section snippets

Experimental section

Present experiments were performed by using an apparatus reported previously [20], [21], [22], [23]. We used two-stage differentially pumped chambers consisting of a cluster source and a Wiley—McLaren type TOF-MS [24]. The clusters of metal atoms with molecules were produced by a pickup source [25], [26], [27] consisting of a combination of laser vaporization [28] and pulsed supersonic expansion. A mixture gas of helium and sample (MVK, AC) was expanded from a pulsed valve with a stagnation

Calculation

The quantum chemical calculations for free molecules (MVK and AC) and 1:1 complexes (Na(MVK) and Na(AC)) were performed to examine the possibility of intracluster electron transfer. These calculations were carried out by using a density functional theory program of the Gaussian 94 package [29]. The 6-31+G∗ basis set and B3LYP functional [30] were utilized in these calculations. There are two conformers, s-trans and s-cis, for both MVK and AC. We calculated the optimized structures of these

Photoionization mass spectra of M(MVK)n and M(AC)n (M = Na and K)

Fig. 1a shows a typical mass spectrum obtained by one-photon ionization of Na(MVK)n by irradiation with a laser beam of 5.99 eV. A series of cluster ions of Na+(MVK)n is mainly observed up to n=16. The intensity of Na+(MVK)3 is observed intensely with respect to adjacent n ions. This intensity-anomaly (magic number) behavior at n=3 is independent of the energy of the ionization laser in the region between 4.15 and 5.99 eV. We also measured the photoionization mass spectrum of clusters containing

Conclusion

We have measured the photoionization mass spectra of clusters of alkali metal atom (M) with methyl vinyl ketone (MVK) or acrolein (AC). Intense ion signals assignable to M+(MVK)3 ions are observed for M=Na, K. In comparison with the result of size distribution of Na+(MVK)n ions formed by ion–molecule (cluster) reactions in the cluster source, the obtained magic numbers at M+(MVK)3 are found to be due to the stability of neutral clusters, M(MVK)3. From the previous results of the studies of

Acknowledgements

This work was partly supported by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Science, Sports and Culture. K. Ohshimo is supported by a Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists. F.M. also acknowledges financial support from the Kurata Foundation and Mitsubishi Foundation.

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