Formation of N3O3− anion in (NO)n−: photoelectron spectroscopy and ab initio calculations
Introduction
Anions of dimeric molecules often form tightly bound species where the excess electron is delocalized to some extent over the dimer framework. In that case, the formula (M)2−, where M denotes a molecule, no longer appropriately represents their chemical identities. For example, the dimer anion of carbon dioxide (CO2)2− is characterized as a C2O4− molecular anion having D2d symmetry [1]. Recent photodetachment [2]and photodissociation [3]experiments provide evidence for a covalent, cyclic (CS2)2− anion 4, 5. Because of their intrinsic stabilities, the addition of another molecule to the dimer anion results in the formation of an ion–neutral complex, such as C2O4−·CO2 and C2S4−·CS2, where the third constituent monomer is reluctant to share the excess charge. In the case of nitric oxide, however, the situation seems to be somewhat different. Posey and Johnson have measured the photoelectron spectrum of (NO)2− and have shown that the anion can be represented as a resonance-stabilized NO−·NO dimer [6]. Subsequent ab initio calculations suggest that the NO−·NO dimer has a trans-ONNO− structure (Scheme. 1) 7, 8.
Whereas these experimental and theoretical investigations indicate the formation of a strongly bound (NO)2− anion, other evidence for a stable trimer anion has been presented by a mass spectrometric study of (NO)n−[9]. Carman has found a distinct odd–even intensity alternation in the mass spectrum; the (NO)n− anions with odd sizes predominate over those with even sizes, indicating that odd-sized clusters are more stable than even-sized ones [9]. Based on these experimental findings combined with a simple MO consideration, he proposed that a trimer core (NO)3− having a closed-shell electronic configuration is formed in larger clusters [9]. As neutral NO can readily dimerize to complete spin pairing 10, 11, the preferential stabilization of the odd-sized clusters could be attributed to the formation of neutral NO pairs around the (NO)3− core. Thus, the odd-sized clusters are formulated as (NO)3−·(NO)2k, where k=(n−3)/2 [9]. More recently, Kondow and co-workers have studied the dissociation processes of (NO)n− (3≤n≤40) in collision with a silicon surface [12]. They have observed that (NO)3− is the dominant product in the region of collision energy below 1 eV per molecule, also implying that the (NO)n− cluster contains an (NO)3− core.
In this Letter, we have measured the photoelectron spectra of (NO)n− with 1≤n≤7 to probe their electronic structures. The size dependence of the spectral features provides us with information as to how the electronic structures evolve as the cluster size increases. In particular, our attention is focused on the spectral change, such as that in the band profiles and the spectral shift, from n=2 to n=3 to test the (NO)3−-core hypothesis. We have also done ab initio calculations to study the stabilities and structures of the (NO)3− anion to complement the experimental findings.
Section snippets
Experimental
Details of the experimental apparatus have been described elsewhere [13]. The apparatus consisted of a cluster-ion source, a tandem time-of-flight (TOF) mass spectrometer and a photoelectron spectrometer. The (NO)n− clusters were produced by electron attachment to neutral NO clusters by using an electron-impact ionized free jet [14]. The neutral NO clusters were prepared by pulsed supersonic expansion of a 1:4 mixture of NO and Ar gas at a stagnation pressure of 1–2 atm. Impurities in the
Photoelectron spectra
Fig. 1 shows the photoelectron spectra of (NO)n− with 2≤n≤7 measured at 4.66 eV, together with the NO− spectrum taken at 2.33 eV. An overview of the photoelectron spectra in Fig. 1 reveals a drastic change in the spectral features with cluster size. The monomer spectrum is composed of a vibrational progression with a spacing of ∼0.23 eV. The observed spectral features reproduce well those measured by Ellison [17]except for a difference in the spectral resolution. The vibrational progression has
Acknowledgements
The authors wish to thank Professor Tamotsu Kondow, Dr. Hisato Yasumatsu and Mr. Hitoshi Yamaguchi for providing us with their results of surface-induced dissociations of (NO)n− clusters prior to publication. The theoretical part of this work has been performed on the SP2 computer at the Computer Center of the Institute for Molecular Science. This work has been supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture and by the Joint Studies
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Present address: Data Storage Institute, 10 Kent Ridge Crescent, Singapore 119260.