RAPID COMMUNICATIONS IN MASS SPECTROMETRY Rapid Commun. Mass Spectrom. 2003; 17: 2327–2336 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/rcm.1197 Phenol, chlorobenzene and chlorophenol isomers: resonant states and dissociative electron attachment { Roustem V. Khatymov*, Mars V. Muftakhov and Victor A. Mazunov Institute of Molecule and Crystal Physics, Ufa Research Centre of the Russian Academy of Science, Prospekt Oktyabrya 151, 450075 Ufa, Russia Received 2 June 2003; Revised 20 August 2003; Accepted 20 August 2003 This paper reports a study of resonant dissociative electron attachment (DEA) to the phenol, chlor- obenzene, p-, m-, and o-chlorophenol molecules. On the basis of spectroscopic and thermochemical approaches the resonant states of the molecular negative ions (NIs) and the structures of some dis- sociative decay products are assigned. In the electron energy range up to 3 eV, DEA processes are determined by the two 2 [p*]-shape resonances resulting mainly in formation of [M–H]  and/or Cl  ions. At higher electron energies the energy correlation between peaks in the negative ion effective yield curves and bands of UV spectra allowed identification of the core-excited resonances. The peculiarities of Cl  ion formation and the vibrational fine structure on the effective yield curves of the [M–H]  ions are discussed. The mass spectrometric procedures for measurement of relative cross sections for NI formation are described. Copyright # 2003 John Wiley & Sons, Ltd. Many natural and synthetic chemical compounds contain a benzene ring as a part of their molecular structure. Physical and chemical properties of simple benzene derivatives are often determined by the properties of benzene itself (aroma- ticity, light absorption features. etc.), as has been thoroughly investigated and widely described in the literature. For science and practical applications, however, the distinctive features of substituted benzenes are scarcely less important, and they await detailed and complete exploration by various methods. Chlorine-containing aromatic compounds, for instance, are often known for their enhanced toxicity, which has not yet been rationalized entirely. The enhanced toxicity leads to many other problems connected both with detection, identification and quantitative analysis of these compounds in environmental samples, and with their cleanup. Studies of the behavior of such toxins under electron interaction condi- tions, including electrons of moderate energy (0–15 eV), could give invaluable information for solutions to these pro- blems. Elucidation of the fragmentation pathways of such molecular systems, and of electronic and spatial structures of the molecules and their fragments, is of critical importance for the development of mass spectrometric ecological moni- toring methods, techniques for cleanup of these toxins, and for clarification of the origins of their toxicity. In the present contribution we study dissociative electron attachment (DEA) to ortho-, meta- and para-chlorophenol (OCP, MCP, and PCP), chlorobenzene (CB), and phenol (PH) molecules, under isolated conditions. These chlorinated derivatives of benzene are considered to be potential toxins as precursors of another dangerous class of compounds, the chlorinated dibenzodioxins (dioxins). 2,3 Our interest was attracted by the peculiarities of DEA to chlorophenol (CP) isomers, which implies the need for clarification of formation mechanisms of negative ions (NIs), identification of NI spectroscopic states, and determination of the structures of dissociative decay products. 4 Phenol and chlorobenzene molecules represent the building blocks of chlorophenols and therefore were also studied as model systems. EXPERIMENTAL The experiments were performed using a magnetic sector mass spectrometer (model MI-1201B; Sumy, Ukraine), rebuilt for generation and detection of negative ions 5 (the possibility to work with positive ions is retained). A block diagram of the mass spectrometer is shown in Fig. 1. Electrons emitted by a tungsten filament (cathode) pass through a trochoidal elec- tron monochromator (TEM) to form a beam with narrow energy distribution. 6 The transmitted electrons are then accelerated to the desired energy by varying the ionization chamber (IC) potential through a computer-controlled digi- tal-to-analog converter (DAC). The ions produced in the IC by the interaction of a molecular beam with the electrons are immediately extracted by an electric field imposed by an extracting electrode, and accelerated toward the analyzing magnet. The signals due to transmitted mass-analyzed ions, detected by a secondary electron multiplier, are sent to a Copyright # 2003 John Wiley & Sons, Ltd. *Correspondence to: R. V. Khatymov, Institute of Molecule and Crystal Physics, Ufa Research Centre of the Russian Academy of Science, Prospekt Oktyabrya 151, 450075 Ufa, Russia. E-mail: LMSNI@soros.bashedu.ru { This publication is part of a series on the investigation of chlor- oaromatic compounds. 1 Contract/grant sponsor: Russian Foundation for Basic Research; contract/grant numbers: 02-03-97908, 02-03-33196, 02-02-97903.