Photoelectron spectroscopy of acetone cluster anions, [(CH3)2CO]n-(n=2,515)

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Abstract

Photoelectron images of [(CH3)2CO]n-(n=2,515) were recorded at 3.49 eV. Analysis of the images provided the vertical detachment energies (VDEs) and photoelectron angular distributions (PADs) of [(CH3)2CO]n-. The n-dependence of these quantities starts with VDE = 0.83 ± 0.03 eV and β  −0.3 at n = 2, and it ends up with 2.83 ± 0.03 eV and ≈0.1 at n = 15. These findings, in conjunction with ab initio results, indicate: (1) the formation of a specific anion structure at n = 2; and (2) the presence of a solvent-stabilized (CH3)2CO valence anion in the larger analogues.

Graphical abstract

Geometrical structures and electronic properties of acetone cluster anions, [(CH3)2CO]n-(n=2,515), are probed by photoelectron imaging spectroscopy combined with ab initio calculations. The dimer anion exhibits a distinctive stability due to the formation of a specific anion structure with twin pyramidally-deformed monomers.

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Introduction

The anion of acetone ((CH3)2CO = Acn) has long been a target of many experimental and theoretical investigations as a typical example of aliphatic ketyl radical anions in condensed phases. They are formed in the reaction of hydrated electrons with acetone, which acts as an efficient electron scavenger [1]. Electron spin resonance (ESR) spectroscopy in solution [2] and in rare gas matrices [3] has revealed that (Acn) is pyramidal in shape at the α carbon. A theoretical calculation has also predicted a pyramidal geometry as the most stable structure of (Acn), where the excess electron is accommodated in the πCO antibonding orbital [4]. On the other hand, the valence state anion of acetone in the gas phase is unstable relative to autodetachment [5]; (Acn) is formed only as a ‘dipole-bound’ anion, where the excess electron is captured in the electrostatic field induced by the dipole moment of acetone (2.88 D) [6]. Thus, the electron trapping mechanism of acetone is definitely phase-dependent.

This invokes a naive question regarding the electronic properties of acetone cluster anions, (Acn)n-: whether a valence anion is formed in (Acn)n- or whether (Acn)n- contains a solvated electron surrounded by multi-dipoles, and whether or not these situations depend on the cluster size. There have been several experiments conducted in the same context for cluster systems other than (Acn)n-; coexistence of two forms of cluster anions, one involving valence anions and the other solvated electrons, is observed at certain sizes in pyridine [7], acetonitrile [8], and formamide [9] cluster anions.

In the present study, the electronic properties of (Acn)n- with n = 2, 5–15 are probed by photoelectron imaging spectroscopy. From the observed photoelectron images, we have determined the VDEs and the PADs as a function of the cluster size. The discussion on these experimental findings with the aid of ab initio calculations has provided information on the geometrical structures and electronic properties of (Acn)n- prepared in our cluster beam.

Section snippets

Experimental

The experimental apparatus consists of a negative ion source, a collinear tandem time-of-flight (TOF) mass spectrometer and a photoelectron spectrometer. An electron-impact ionized free jet [10] was used to generate (Acn)n- clusters. The sample gas was prepared by passing neat Ar through a reservoir containing acetone at 20 °C and expanded through a pulsed nozzle (General Valve 9-279-900) with a stagnation pressure of ≈1 atm. The free jet was then crossed with a 250-eV electron beam at the

Mass spectra

Fig. 1 shows a typical mass spectrum of anions formed in the electron-impact ionized free jet. The smallest (Acn)n- detected is the pentamer anion, (Acn)5-. Smaller members, n < 5, were not observed via ionization of acetone/Ar gas mixture under various beam conditions. The onset of the size distribution was always observed at n = 5, although the overall mass distribution was found to depend on the stagnation pressure, the duration of the pulsed beam and the distance between the nozzle and the

Acknowledgements

We wish to thank Professor Jean-Pierre Schermann for valuable discussions regarding the structure of the dimer anion. We are also grateful to Professor Hanna Reisler and Mr. Boris Karpichev for providing us with the BASEX program. Professor Kazuo Takatsuka is gratefully acknowledged for the loan of high performance computers. This work is partly supported by a Grant-in-Aids for Scientific Research (Grant Nos. 15350006 and 18550007) from the Ministry of Education, Culture, Sports, Science and

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