Identification of Isomers of Nitrotoluene via Free Electron Attachment Philipp Sulzer, Andreas Mauracher, Stephan Denifl, Fabio Zappa, Sylwia Ptasinska, Manuel Beikircher, Arntraud Bacher, Nina Wendt, Abid Aleem, Flaminia Rondino, † Stefan Matejcik, ‡ Michael Probst, Tilmann D. Ma 1 rk, § and Paul Scheier* Institut fu ¨r Ionenphysik und Angewandte Physik and Center for Molecular Biosciences Innsbruck, Universita ¨ t Innsbruck, Technikerstrasse 25, A-6020 Innsbruck, Austria Free electron attachment to the three different isomers of mononitrotoluene molecules in the gas phase is studied using a crossed electron-molecule beams technique. In contrast to previous studies for a large number of negative ions, the presently measured relative cross section curves are recorded with an electron energy resolution of better than 100 meV. For several product anions including the nitro anion NO 2 - , remarkable differences for the three isomers are observed. In almost all fragment anion efficiency curves, the 2-nitrotoluene exhibits pronounced differences from the two other isomers. In contrast, 3- and 4-nitrotoluene disagree only slightly in a few fragment anions from each other. Nitroorganic compounds are molecules with a significant potential for industrial use, particularly as explosives or propellant. The interaction of electrons with nitroorganic compounds, in particular electron attachment, plays an important role in under- standing the reactivity of these compounds. Therefore, we have recently carried out studies concerning dissociative electron attachment (DEA) to the simple nitroorganic molecules nitrome- thane and nitroethane. 1,2 Among the nitro compounds, explosives such as trinitrotoluene form a group of chemicals of considerable interest for environmental and analytical chemistry. 3 Moreover, the detection of explosives is a topic of increasing interest. 4 A wide variety of very sensitive methods has been developed for envi- ronmental analysis of explosives or their degradation products (see the review 5 ). Nitro compounds contain one or more nitro (NO 2 ) functional groups; thus, they possess very pronounced electron-acceptor properties due to the low energy of the lowest unoccupied (π*) orbital of the NO 2 group. Therefore, the interaction between low- energy electrons with nitro derivatives was a subject of many studies. 1,2,6,7 Particularly, nitrobenzene and nitrotoluene were studied rather early by using the swarm technique reported by Christophorou et al. 8 These investigators observed nondissociative electron attachment to nitro compounds, which form long-lived molecular anions and dissociative electron attachment that pro- duce intense NO 2 (m/z 46) fragment anions. Numerous studies described that this nitro anion can serve as a fingerprint for the identification of the neutral compound 9-11 and thus its great potential as a marker for the detection of explosives. 9,12 Further- more, Havey et al. 9 demonstrated by measuring the NO 2 - resonance energies for 25 different nitro aromatic compounds including several isomeric species that it is possible to distinguish structural isomers of nitro compounds, including the three isomers of mononitrotoluene. Chen and Wu 13 and later Chen and Chen 14 performed computational studies about the stability and internal rotational barriers of the three nitrotoluene (NT) isomers. Ac- cording to their calculations, 13 the 4-NT is the most stable isomer and for 2-NT the steric effect between the nitro group and the methyl group leads to a torsional angle of the nitro group up to 22°. Both, experiment 9 and theory 13 find that the proximity of the nitro and the methyl group of the 2-NT leads to substantial differences between this isomer and the two other forms, i.e., 3-NT and 4-NT, and only very small deviations between the latter two. Although, besides the dominant NO 2 - , many other fragment anions upon DEA to nitroorganic compounds have been de- tected, 1,2,10,11 these product anions have not been investigated concerning the identification of the precursor substance or possible isomeric effects. * To whom correspondence should be addressed. E-mail: paul.scheier@ uibk.ac.at. † Current address: Chemistry Department (N.E.C.-“La Sapienza”), P.le Aldo Moro 5, I-00185 Roma, Italy. ‡ Current address: Department of Experimental Physics, Comenius University, Mlynska dolina F2, 84248 Bratislava, Slovakia. § Also adjunct professor: Department of Experimental Physics, Comenius University, Mlynska dolina F2, 84248 Bratislava, Slovakia. (1) Sailer, W.; Pelc, A.; Matejcik, S.; Illenberger, E.; Scheier, P.; Ma ¨rk, T. D. J. Chem. Phys. 2002, 117, 7989-7994. (2) Pelc, A.; Sailer, W.; Matejcik, S.; Scheier, P.; Ma ¨rk, T. D. J. Chem. Phys. 2003, 119, 7887-7892. (3) Yinon, J.; Zitrin, S. In Modern Methods and Applications in Analysis of Explosives; Wiley: New York, 1996. (4) Yinon, J. Anal. Chem. 2003, 75, 99A-105A. (5) Moore, D. S. Rev. Sci. Instrum. 2004, 75, 2499-2512. (6) Modelli, A.; Venuti, M. Int. J. Mass Spectrom. 2001, 205,7-16. (7) Laramee, J. A.; Kocher, C. A.; Deinzer, M. L. Anal. Chem. 1992, 64, 2316- 2322. (8) Christophorou, L. G.; Compton, R. N.; Hurst, G. S.; Reinhardt, P. W. J. Chem. Phys. 1966, 45, 536-547. (9) Havey, C. D.; Eberhart, M.; Jones, T.; Voorhees, K. J.; Laramee, J. A.; Cody, R. B.; Clougherty, D. P. J. Phys. Chem. A 2006, 110, 4413-4418. (10) Boumsellek, S.; Alajajian, S. H.; Chutjian, A. J. Am. Soc. Mass Spectrom. 1992, 3, 243-247. (11) Yinon, J.; Boettger, H. G.; Weber, W. P. Anal. Chem. 1972, 44, 2235- 2237. (12) Laramee, J. A.; Mazurkievicz, P.; Berkout, V.; Deinzer, M. L. Mass Spectrom. Rev. 1996, 15, 15-42. (13) Chen, P. C.; Wu, C. W. J. Mol. Struct. (Theochem) 1995, 357, 87-95. (14) Chen, P. C.; Chen, S. C. Comput. Chem. 2002, 26, 171-178. Anal. Chem. 2007, 79, 6585-6591 10.1021/ac070656b CCC: $37.00 © 2007 American Chemical Society Analytical Chemistry, Vol. 79, No. 17, September 1, 2007 6585 Published on Web 08/08/2007