Electron attachment to some naphthoquinone derivatives: long- lived molecular anion formation N. L. Asfandiarov 1,2 *, S. A. Pshenichnyuk 1,3 , A. S. Vorob’ev 1 , E. P. Nafikova 1 , Y. N. Elkin 4 , D. N. Pelageev 4 , E. A. Koltsova 4 and A. Modelli 5,6 1 Institute of Molecule and Crystal Physics, Ufa Research Centre, Russian Academy of Sciences, Prospect Oktyabrya 151, 450075 Ufa, Russia 2 Bashkir State Pedagogical University, Oktyabrskoy Revolutsii st. 3a, 450000 Ufa, Russia 3 St. Petersburg State University, Uljanovskaja 1, 198504 St. Petersburg, Russia 4 Pacific Institute of Bioorganic Chemistry, Far Eastern Branch of Russian Academy of Sciences, Prospect 100 Let Vladivostoku 159, Vladivostok 690022, Russia 5 Dipartimento di Chimica ’G. Ciamician’, Universitá di Bologna, via Selmi 2, 40126 Bologna, Italy 6 Centro Interdipartimentale di Ricerca in Scienze Ambientali (CIRSA), Universitá di Bologna, via S. Alberto 163, 48123 Ravenna, Italy RATIONALE: Electron Affinity (EA) is one of the fundamental properties of a molecule. EA values can be measured with various experimental methods, although their availability is still relatively limited. We make an attempt to use Dissociative Electron Attachment Spectroscopy (DEAS) data for evaluation of the EAs of twelve naphthoquinone (NQ) derivatives. METHODS: Naphthoquinone (NQ) and eleven of its hydroxyl derivatives were investigated by means of DEAS. A combined investigation of NQ and juglone by means of the Electron Transmission Spectroscopy (ETS) and DEAS techniques, with the support of density functional theory (DFT) calculations, allowed us to elucidate the empty-level structures of NQ and its hydroxyl derivatives. RESULTS: All molecules under investigation form extremely long-lived molecular anions associated with three resonant states (except for NQ, where only two long-lived resonances were observed). The hydroxyl substituents of NQ cause an increase in EA and number of internal degrees of freedom (N), and, as a result, an increase in the mean electron autodetachment lifetimes of the molecular negative ions (NIs). Evaluation of the EAs from the measured lifetimes of the molecular NIs through a simple Arrhenius approximation gives results in reasonable agreement with those obtained with DFT calculations. CONCLUSIONS: NI lifetime measurements by means of a modified DEAS instrumentation can provide quantitative data of EA. A simple Arrhenius approximation seems to be adequate to describe the process of electron detachment from molecular anions. Copyright © 2014 John Wiley & Sons, Ltd. Quinones are a class of compounds widely distributed in plants, fungi and some animals, being products of their secondary metabolism. In particular, naphthoquinones are naphthalene derivatives which possess two keto groups and, generally, one or more hydroxyl substituents. Many quinones exhibit an interesting range of pharmacological properties, such as antibacterial, antifungal, antiviral, anti- inflammatory, enzyme-inhibitive, antioxidant or radical scavenging properties. Ubiquinone and vitamin K (a derivative of 1,4-naphthoquinone) play a crucial role in the electron transfer process of the cell respiratory system of mammals. [1,2] Theoretical calculations have been used only recently to characterize their electronic structures and the energetics of proton transfer and hydrogen abstraction processes, with the aim of correlating calculated quantities with their biological activities. [3,4] NQ, a number of its derivatives and some related compounds were investigated by means of mass spectrometry and spectroscopic techniques. [5–8] Theoretical investigations of these compounds are also available. [9,10] The energies of the frontier molecular orbitals (MOs), namely, the lowest unoccupied MO (LUMO) and the highest occupied MO (HOMO), define the so-called energy gap which is often used as a descriptor to correlate the molecular electronic structure with chemical reactivity [11] and biological properties [12,13] in quantitative structure–activity relationship (QSAR) studies. The energy of the LUMO and the electron affinity (EA) of a neutral molecule play an important role in the properties of the corresponding molecular negative ion (NI), defining its stability relative to electron detachment (mean autodetachment lifetime, τ a ). [14,15] Therefore, quantitative evaluations of the EAs of quinone derivatives and the characterization of their empty-level structures constitute a highly topical question. * Correspondence to: N. L. Asfandiarov, Institute of Molecule and Crystal Physics, Ufa Research Centre, Russian Academy of Sciences, Prospect Oktyabrya 151, 450075 Ufa, Russia. E-mail: nail@anrb.ru Copyright © 2014 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2014, 28, 1580–1590 Research Article Received: 27 January 2014 Revised: 23 April 2014 Accepted: 27 April 2014 Published online in Wiley Online Library Rapid Commun. Mass Spectrom. 2014, 28, 1580–1590 (wileyonlinelibrary.com) DOI: 10.1002/rcm.6934 1580