- Email: [email protected]
Low-energy electron interaction with nitrobenzene: C6H5NO2
ARTICLE IN PRESS
Vacuum 81 (2007) 1180–1183 www.elsevier.com/locate/vacuum
Low-energy electron interaction with nitrobenzene: C6H5NO2 A. Pelca,, P. Scheierb, T.D. Ma¨rkb a
Institute of Physics, Maria Curie-Sklodowska University, 20-031 Lublin, Poland Institut fu¨r Ionenphysik und Angewandte Physik, Leopold Franzens Universita¨t, Technikerstr. 25, A-6020 Innsbruck, Austria
b
Abstract Low-energy electron attachment to C6H5NO2 (nitrobenzene) in the gas phase is reported in the electron energy range from about 0 up to 10 eV with an energy resolution of 120 meV. Dissociative and nondissociative electron attachment to nitrobenzene were observed. From the numerous ions observed, the two most abundant were NO 2 and C6H5NO2 . Based on comparison of the abundance of studied ions with Cl in the dissociative electron attachment to CCl4 at 0.8 eV, estimates of cross sections for the all observed ions were obtained 20 2 21 2 for the first time (e.g. s(NO m and s(C6H5NO m ). 2 ) ¼ 4.6 10 2 ) ¼ 3.8 10 r 2007 Elsevier Ltd. All rights reserved. Keywords: Electron attachment; Nitrobenzene
1. Introduction Most of the nitrobenzene produced is used to manufacture a chemical called aniline used for production of dyes. Nitrobenzene is also used to produce lubricating oils such as those used in motors and machinery. A small amount of nitrobenzene is used in the manufacture of pharmaceuticals, pesticides, synthetic resins and as an intermediate in the production of several products such as photographic chemicals and artificial sweeteners [1,2]. Nitrobenzene as the smallest of the nitroaromatics, is a model compound for studies of various characteristics as e.g. kinetics of nitroaromatic explosives [3], binding and aromaticity of substituted benzene rings [4,5]. Formation of NO 2 ions in dissociative electron attachment (DEA) to explosive nitro compounds has been used to distinguish these substances from others [6]. In previous studies [7–10], several discrepancies exist between the results concerning the observed negative ions and the electron energy positions of the observed resonances. In order to clarify these discrepancies, we have investigated in the present work, electron attachment to nitrobenzene with an increased electron energy resolution
Corresponding author.
E-mail address: [email protected] (A. Pelc). 0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.01.011
and increased ion detection efficiency in the electron energy range from about 0 to 10 eV. Nitrobenzene is a third nitro compound studied in our laboratory. Previously, we have investigated the DEA to the two simplest nitro molecules—nitromethane and nitroethane [11,12]. 2. Experimental The electron attachment spectrometer used in present studies consists of a molecular beam system, a highresolution trochoidal electron monochromator (TEM) and a quadrupole mass filter with a pulse counting system for analysing and detecting the ionic products. The apparatus has been described previously in detail [13]. Briefly, electrons emitted from a heated hairpin filament are extracted by an electric field of several V/mm and guided by an axial magnetic field of 50 Gauss produced by Helmholtz coils outside the vacuum chamber. After passing two electrostatic lenses, the electron beam enters the dispersive element where a weak electrostatic field of about 2 V/cm perpendicular to the magnetic field is applied. These two crossed fields act on electrons causing their deflection inversely proportional to the electron axial velocity (parallel to the magnetic field). Subsequently, at the exit of the dispersive element a fraction of the electrons distribution is allowed to pass through the orifice in the exit
ARTICLE IN PRESS A. Pelc et al. / Vacuum 81 (2007) 1180–1183
electrode, which is slightly displaced (2 mm) with respect to the entrance electrode. The resulting beam of monochromatized electrons is then accelerated to the desired energy and introduced into a collision chamber, where it is crossed with the molecular beam of molecules (formed via gas flow through an orifice with 20 mm diameter). The anions generated by the electron attachment process are extracted by a weak electrostatic filed (0.5 V/cm) into the quadrupole mass filter where they are analysed and detected by the channeltron. After crossing the collision region the remaining electrons are collected in a Faraday cup and the electron current can be monitored during the experiments using the picoammeter. With the TEM, we are able to achieve energy distributions of about 30 meV at full-width at half-maximum (FWHM) independent of electron energy. To determine this energy spread and to calibrate the energy scale wellknown cross sections for the formation of Cl or SF 6 by electron attachment to CCl4 or SF6, respectively is used. In the present study, a 10 to 1 mixture of nitrobenzene and CCl4 were used. Generation of Cl/CCl4 is characterised by two main resonances at 0 and 0.8 eV. The first one was used for calibration of the electron energy scale and to determine of the electron energy spread (the apparent width of this peak (FWHM) is a measure for the electron energy resolution of the electron beam). Production of Cl ion at 0.8 eV with well-known cross-section value of 5 1020 m2 [14,15] was used to obtain estimates of the cross section of the electron attachment to nitrobenzene. For this calibration procedure, we have neglected discrimination effects for ions in the reaction chamber due to their different kinetic energy and assume constant transmission efficiency through the QMS and identical detection efficiencies for Cl and for the ions produced via electron attachment to nitrobenzene. In the present experiments, the electron energy spread (FWHM) and the electron current intensity were 120 meV and 50 nA, respectively. This relatively low resolution used was a reasonable compromise between product ion intensity and incident electron energy distribution. Sample of the nitrobenzene of 99% purity was purchased from Sigma Aldrich, Vienna, Austria. The experiments have been performed at a pressure ranging from 4.7 107 up to 6.6 107 mbar. 3. Results and discussion The lowest unoccupied molecular orbital (LUMO) of nitrobenzene has mainly b1 (p*) NO2 character, but it is strongly mixed with the component of b1 symmetry of the degenerate benzene p* (e2u) molecular orbital [16,17]. Moreover, due to the rather low LUMO energy nitrobenzene has a very pronounced electron acceptor properties. The electron affinity of nitrobenzene is 1.00070.06 eV [18]. Electron attachment (in energy range of about 0–10 eV) to nitrobenzene in the gas phase proceeds dissociatively and nondissociatively and results in formation of 33
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negative ions (see Table 1). The observed anions comprise five distinct groups around the most intense q/m ratios of 39, 51, 65, 76, 93, (corresponding to C3H 3 , C4H3 , C5H5 , C6H4 , C6H5O ). In addition C6H5NO2 , C6H4NO2 and anions detected in case of previous studies [11,12] of DEA to nitro compound (O, OH, C2H, CN, NO, CNO, NO 2 ) were measured. From many possible channels leading to formation of negative anions in electron attachment to nitrobenzene, we will discuss in details only these in which anions are produced with the highest abundance (see Fig. 1). 3.1. Formation of C6H5NO 2 All studied previously NO2 containing benzene derivatives [19,20] are known to form parent negative ions. Nitrobenzene reaction (1) and benzonitrile are the only known monosubstituted benzenes forming long-lived anions through nondissociative, thermal electron attachment [21], e.g., e þ C6 H5 NO2 ! ðC6 H5 NO2 Þ] :
(1)
] (C6H5NO 2 ) ion is formed via the nuclear excited Fesbach resonance—mechanism in which the kinetic energy of incident electron goes solely into nuclear motion of target molecule. The lifetime obtained for this ion is t ¼ 17.5 ms [22]. Formation of the parent anion was observed in all previous investigations of electron attachment to nitrobenzene [7–10].
3.2. Formation of NO 2 The most abundant negative ion observed in DEA to the nitrobenzene is NO 2 . This ion could be formed via reaction: e þ C6 H5 NO2 ! ðC6 H5 NO2 Þ] ! NO2 þ C6 H5 :
(2)
Using known standard entalphies of the formation for all species involved in this reaction (Df H0g (C6H5NO2) ¼ 68.537 0.07 kJ/mol, Df H 0g ;(NO2) ¼ 33.10 kJ/mol, Df H0g ðC6 H5 Þ ¼ 339 8 kJ=mol [23]), the bond dissociation energy was obtained as D(C6H5–NO2) ¼ 3.1770.1 eV. Considering the bond dissociation energy and the electron affinity of NO2 molecule (EA(NO2) ¼ 2.27370.005 eV) we derived, that channel described via reaction (2) is endothermic by 0.8570.20 eV. The appearance energy estimated from our experiment is 0.370.2 eV. This disagreement shows that the first resonance (peak at 0.7 eV) proceeds via other mechanism. Modelli et al. [10] attributed 0.7 eV resonance to dissociation of the molecular anions formed on the high-energy side of the p* (a2) resonance. The transfer of the extra electron from the antisymmetric ring p* (a2) MO to the NO2 group requires a vibrational motion which reduces the molecular symmetry, and thus needs additional energy. Second resonance (at 1.4 eV) has appearance energy of 0.870.2 eV which fits correctly with the derived value.
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Table 1 Peak positions and absolute partial cross-sections for the fragment ions obtained in the previous [7–10] and present experiments m/z (possible fragment)
Peak position (eV) Jaeger et al. [9]
16(O) 17(OH) 25(C2H) 26(CN) 30(NO) 39(C3H 3) 40(C3H 4) 41(C2HO) 42(CNO) 46(NO 2) 49(C4H) 50(C4H 2) 51(C4H 3) 52(C4H 4) 63(C5H 3) 64(C5H 4) 65(C5H 5) 66(C4H2O) 67(C4H3O) 68(C4H4O) 75(C6H 3) 76(C6H 4) 77(C6H 5) 78(C5H2O) 79(C5H3O) 91(C6H3O) 92(C6H4O) 93(C6H5O) 94(C5H4NO) 104 105(C6H3NO) 106(C6H4NO) 108 122 (C6H4NO 2) 123 (C6H5NO 2)
Obtained EA crosssections (m2) Compton et al. [7,8]
Modelli et al. [10]
5, 6.5, 7.5 4.2, 6.5 4.5, 5, 7 4.2 3.8
4.2 4.2 1.5, 4
1.3, 3.64
0.7, 1.25, 2.9, 3.7
3.8 4.2
3.6
4.2
3.6
1.8, 4.2, 6.5 3.8 4 4.5 3.8
3.6
3.8 4.2 0.3, 4.8
Present
4.8, 0.2, 5, 4.2 3.8 5, 4.2 4 3.8 0.7, 7 4.2 4.2, 4.5 3.1, 4.2 3.8 3.8 4.2 4.5 4.2, 4.2 4.2, 3.8, 3.4 3.8 4.2 3.8 3.8
7 4.2, 7 7
7
1.4, 3.1, 3.8, 6.6
6.5 4.2
6.4 6.5 6.5
1.6 1021 2.1 1022 1.9 1022 1.4 1021 9.0 1023 8.0 1023 4.5 1023 1.0 1022 7.9 1022 4.6 1020 5.9 1023 3.7 1023 4.3 1023 3.6 1023 2.4 1023 1.6 1023 1.9 1022 4.0 1023 1.5 1022 5.3 1024 1.4 1021 1.4 1021 2.3 1022 2.0 1022 7.4 1023 1.7 1022 1.6 1022 2.9 1022 4.1 1023
3.8 4
7.9 1023 3.2 1023
3.5, 7 0
2.2 1023 3.8 1021
3.6 3.8 0.15
0
Peaks at 1.4 and 3.8 are associated with electron attachment into the second p* MO of b1 symmetry. Instead of three resonances mentioned, NO 2 is also formed by capture of the electron with the energy of 3.1 and 6.6 eV. The origin of these two channels is not evident. The formation of NO 2 at 3.1 eV ions could be complementary to the production the C5H 3 ion. The channel at 6.6 eV (observed in present studies for the first time) could be coupled with the production of neutral fragments: C4H3, C6H3 or C6H5 (their negative ions were measured at 6.6 eV). Due to a lack of the thermodynamical data for C5H3, C4H3, C6H3 and C6H5, it is impossible to deduce which channel is responsible for the formation of the nitrogen dioxide anion at these two energies. 3.3. Formation of O Ejection of an oxygen anion from nitrobenzene occurs at two electron energies: 4.8 and 7 eV. Possible reactions
0, 0.4
describing the DEA to oxygen atom can be: e þ C6 H5 NO2 ! ðC6 H5 NO2 Þ] ! O þ C6 H5 NO;
(3a)
e þ C6 H5 NO2 ! ðC6 H5 NO2 Þ] ! O þ C6 H5 þ NO; (3b) e þ C6 H5 NO2 ! ðC6 H5 NO2 Þ] ! O þ C5 H4 þ CH þ NO; (3c) e þ C6 H5 NO2 ! ðC6 H5 NO2 Þ] ! O þ C5 H4 þ CNO þ H; (3d) e þ C6 H5 NO2 ! ðC6 H5 NO2 Þ] ! O þ C5 H4 þ CN þ OH: (3e)
The thermodynamical data for reaction (3a) are not accessible, however these data are available for reactions (3b)–(3e). From the energetic point of view, the reaction
ARTICLE IN PRESS A. Pelc et al. / Vacuum 81 (2007) 1180–1183
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40 30 C6H5NO2-
20 Cross section [*10-22 m2]
10 0 450 300
NO2-
150 0 16 12
O-
8 4 0 0
2
4
6
8
10
Electron energy [eV] Fig. 1. Ion yields for C6H5NO 2 , NO2 , O fragment anions formed via electron attachment to nitrobenzene.
(3b) is the most favourable—this reaction is endothermic by 4.8170.2 eV (Df H0g ðOÞ ¼ 249:18 0:10 kJ=mol, (Df H0g ðC6 H5 Þ ¼ 339 8 kJ=mol, Df H0g ðNOÞ ¼ 90:26 kJ=mol and EA(O) ¼ 1.46 eV [23]). Appearance energy for resonances at 4.8 and 7 eV (obtained in our experiment) are 3.070.2 and 5.070.4 eV, respectively. This indicates that reaction (3b) can be responsible for DEA process at 7 eV. The originating of the resonance at 4.8 eV is not obvious, especially if the thermodynamical data for possible neutral fragments (e.g. C6H5NO, C5H5, etc.) are not known. For the rest of anions, we provide the measured partial cross sections and the energetic position of resonances leading to their generation (Table 1). 4. Conclusion The nondissociative and dissociative electron attachment (with energy of 0–10 eV) to nitrobenzene has been studied. We observed 33 anions from which 13 (m/z: 39, 40, 49, 50, 51, 52, 64, 66, 67, 68, 76, 79, 94 amu) were measured for the first time. Simultaneously, two of previously measured ions with q/m ratio 104 and 108 were not detected. Intensities of ions generated have been converted to absolute cross sections by comparison with cross section of Cl/CCl4. Acknowledgement Work partially supported by the FWF, Wien, Austria and by the Polish Ministry of Science and High Education from scientific budget in 2006–2007, as a research project (Grant no.1 P03B 021 30).
References [1] Cheremisinoff PF, Morresi AC. Benzene: basic and hazardous properties. Park Ridge, NJ: Marcel Dekker; 1979. [2] Swinfin ST, Jackman FA. Benzene and its industrial derivatives. New York: Halstead Press; 1975. [3] Brill TB, James KJ. Chem Rev 1993;93:2667. [4] Borisenko KB, Bock CW, Hargittai I. J Phys Chem 1994;98:1442. [5] Urbanowicz P, Kupka T, Wrzalik R, Pasterny K. J Mol Struct 1999;482–483:409. [6] Laramee JA, Mazurkiewicz PH, Berkout V, Deinzel ML. Mass Spectrom Rev 1996;15:15. [7] Compton RN, Huebner RH, Reinhardt PW, Christophorou LG. J Chem Phys 1968;48:901. [8] Christophorou LG, Compton RN, Hurst GS, Reinhardt PW. J Chem Phys 1966;45:536. [9] Ja¨ger K, Henglein A. Z Naturforsch 1967;22a:700. [10] Modelli A, Venuti M. Int J Mass Spectrom 2001;205:7. [11] Sailer W, Pelc A, Matejcik S, Illenberger E, Scheier P, Ma¨rk TD. J Chem Phys 2002;117:7989. [12] Pelc A, Sailer W, Matejcik S, Scheier P, Ma¨rk TD. J Chem Phys 2003;119:7887. [13] Grill V, Drexel H, Sailer W, Lezius M, Ma¨rk TD. Int J Mass Spec Ion Proc 2001;205:209. [14] Klar D, Ruf M-W, Hotop H. Int J Mass Spec Ion Proc 2001;205:93. [15] Chu SC, Burrow PD. Chem Phys Lett 1990;172:17. [16] Smyth KC, Schiavone JA, Freund RS. J Chem Phys 1974;61:1789. [17] Wilden DG, Comer J. J Phys B Atom Molec Phys 1980;13:627. [18] Chen ECM, Wiley JR, Batten CF, Wentworth WE. J Phys Chem 1994;98:88. [19] Johnson JP, McCorkle DL, Christophorou LG, Carter JG. J Chem Soc Faraday Trans II 1975;71:1743. [20] Christophorou LG, Hadjiantoniou A, Carter JG. J Chem Soc Faraday Trans II 1973;69:1713. [21] Naff WT, Compton RN, Cooper CD. J Chem Phys 1971;54:212. [22] Hadjiantoniou A, Christophorou LG, Carter JG. J Chem Soc Faraday Trans II 1973;69:1691. [23] NIST Chemistry WebBook, /http://webbook.nist.govS.
Vacuum 81 (2007) 1180–1183 www.elsevier.com/locate/vacuum
Low-energy electron interaction with nitrobenzene: C6H5NO2 A. Pelca,, P. Scheierb, T.D. Ma¨rkb a
Institute of Physics, Maria Curie-Sklodowska University, 20-031 Lublin, Poland Institut fu¨r Ionenphysik und Angewandte Physik, Leopold Franzens Universita¨t, Technikerstr. 25, A-6020 Innsbruck, Austria
b
Abstract Low-energy electron attachment to C6H5NO2 (nitrobenzene) in the gas phase is reported in the electron energy range from about 0 up to 10 eV with an energy resolution of 120 meV. Dissociative and nondissociative electron attachment to nitrobenzene were observed. From the numerous ions observed, the two most abundant were NO 2 and C6H5NO2 . Based on comparison of the abundance of studied ions with Cl in the dissociative electron attachment to CCl4 at 0.8 eV, estimates of cross sections for the all observed ions were obtained 20 2 21 2 for the first time (e.g. s(NO m and s(C6H5NO m ). 2 ) ¼ 4.6 10 2 ) ¼ 3.8 10 r 2007 Elsevier Ltd. All rights reserved. Keywords: Electron attachment; Nitrobenzene
1. Introduction Most of the nitrobenzene produced is used to manufacture a chemical called aniline used for production of dyes. Nitrobenzene is also used to produce lubricating oils such as those used in motors and machinery. A small amount of nitrobenzene is used in the manufacture of pharmaceuticals, pesticides, synthetic resins and as an intermediate in the production of several products such as photographic chemicals and artificial sweeteners [1,2]. Nitrobenzene as the smallest of the nitroaromatics, is a model compound for studies of various characteristics as e.g. kinetics of nitroaromatic explosives [3], binding and aromaticity of substituted benzene rings [4,5]. Formation of NO 2 ions in dissociative electron attachment (DEA) to explosive nitro compounds has been used to distinguish these substances from others [6]. In previous studies [7–10], several discrepancies exist between the results concerning the observed negative ions and the electron energy positions of the observed resonances. In order to clarify these discrepancies, we have investigated in the present work, electron attachment to nitrobenzene with an increased electron energy resolution
Corresponding author.
E-mail address: [email protected] (A. Pelc). 0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.01.011
and increased ion detection efficiency in the electron energy range from about 0 to 10 eV. Nitrobenzene is a third nitro compound studied in our laboratory. Previously, we have investigated the DEA to the two simplest nitro molecules—nitromethane and nitroethane [11,12]. 2. Experimental The electron attachment spectrometer used in present studies consists of a molecular beam system, a highresolution trochoidal electron monochromator (TEM) and a quadrupole mass filter with a pulse counting system for analysing and detecting the ionic products. The apparatus has been described previously in detail [13]. Briefly, electrons emitted from a heated hairpin filament are extracted by an electric field of several V/mm and guided by an axial magnetic field of 50 Gauss produced by Helmholtz coils outside the vacuum chamber. After passing two electrostatic lenses, the electron beam enters the dispersive element where a weak electrostatic field of about 2 V/cm perpendicular to the magnetic field is applied. These two crossed fields act on electrons causing their deflection inversely proportional to the electron axial velocity (parallel to the magnetic field). Subsequently, at the exit of the dispersive element a fraction of the electrons distribution is allowed to pass through the orifice in the exit
ARTICLE IN PRESS A. Pelc et al. / Vacuum 81 (2007) 1180–1183
electrode, which is slightly displaced (2 mm) with respect to the entrance electrode. The resulting beam of monochromatized electrons is then accelerated to the desired energy and introduced into a collision chamber, where it is crossed with the molecular beam of molecules (formed via gas flow through an orifice with 20 mm diameter). The anions generated by the electron attachment process are extracted by a weak electrostatic filed (0.5 V/cm) into the quadrupole mass filter where they are analysed and detected by the channeltron. After crossing the collision region the remaining electrons are collected in a Faraday cup and the electron current can be monitored during the experiments using the picoammeter. With the TEM, we are able to achieve energy distributions of about 30 meV at full-width at half-maximum (FWHM) independent of electron energy. To determine this energy spread and to calibrate the energy scale wellknown cross sections for the formation of Cl or SF 6 by electron attachment to CCl4 or SF6, respectively is used. In the present study, a 10 to 1 mixture of nitrobenzene and CCl4 were used. Generation of Cl/CCl4 is characterised by two main resonances at 0 and 0.8 eV. The first one was used for calibration of the electron energy scale and to determine of the electron energy spread (the apparent width of this peak (FWHM) is a measure for the electron energy resolution of the electron beam). Production of Cl ion at 0.8 eV with well-known cross-section value of 5 1020 m2 [14,15] was used to obtain estimates of the cross section of the electron attachment to nitrobenzene. For this calibration procedure, we have neglected discrimination effects for ions in the reaction chamber due to their different kinetic energy and assume constant transmission efficiency through the QMS and identical detection efficiencies for Cl and for the ions produced via electron attachment to nitrobenzene. In the present experiments, the electron energy spread (FWHM) and the electron current intensity were 120 meV and 50 nA, respectively. This relatively low resolution used was a reasonable compromise between product ion intensity and incident electron energy distribution. Sample of the nitrobenzene of 99% purity was purchased from Sigma Aldrich, Vienna, Austria. The experiments have been performed at a pressure ranging from 4.7 107 up to 6.6 107 mbar. 3. Results and discussion The lowest unoccupied molecular orbital (LUMO) of nitrobenzene has mainly b1 (p*) NO2 character, but it is strongly mixed with the component of b1 symmetry of the degenerate benzene p* (e2u) molecular orbital [16,17]. Moreover, due to the rather low LUMO energy nitrobenzene has a very pronounced electron acceptor properties. The electron affinity of nitrobenzene is 1.00070.06 eV [18]. Electron attachment (in energy range of about 0–10 eV) to nitrobenzene in the gas phase proceeds dissociatively and nondissociatively and results in formation of 33
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negative ions (see Table 1). The observed anions comprise five distinct groups around the most intense q/m ratios of 39, 51, 65, 76, 93, (corresponding to C3H 3 , C4H3 , C5H5 , C6H4 , C6H5O ). In addition C6H5NO2 , C6H4NO2 and anions detected in case of previous studies [11,12] of DEA to nitro compound (O, OH, C2H, CN, NO, CNO, NO 2 ) were measured. From many possible channels leading to formation of negative anions in electron attachment to nitrobenzene, we will discuss in details only these in which anions are produced with the highest abundance (see Fig. 1). 3.1. Formation of C6H5NO 2 All studied previously NO2 containing benzene derivatives [19,20] are known to form parent negative ions. Nitrobenzene reaction (1) and benzonitrile are the only known monosubstituted benzenes forming long-lived anions through nondissociative, thermal electron attachment [21], e.g., e þ C6 H5 NO2 ! ðC6 H5 NO2 Þ] :
(1)
] (C6H5NO 2 ) ion is formed via the nuclear excited Fesbach resonance—mechanism in which the kinetic energy of incident electron goes solely into nuclear motion of target molecule. The lifetime obtained for this ion is t ¼ 17.5 ms [22]. Formation of the parent anion was observed in all previous investigations of electron attachment to nitrobenzene [7–10].
3.2. Formation of NO 2 The most abundant negative ion observed in DEA to the nitrobenzene is NO 2 . This ion could be formed via reaction: e þ C6 H5 NO2 ! ðC6 H5 NO2 Þ] ! NO2 þ C6 H5 :
(2)
Using known standard entalphies of the formation for all species involved in this reaction (Df H0g (C6H5NO2) ¼ 68.537 0.07 kJ/mol, Df H 0g ;(NO2) ¼ 33.10 kJ/mol, Df H0g ðC6 H5 Þ ¼ 339 8 kJ=mol [23]), the bond dissociation energy was obtained as D(C6H5–NO2) ¼ 3.1770.1 eV. Considering the bond dissociation energy and the electron affinity of NO2 molecule (EA(NO2) ¼ 2.27370.005 eV) we derived, that channel described via reaction (2) is endothermic by 0.8570.20 eV. The appearance energy estimated from our experiment is 0.370.2 eV. This disagreement shows that the first resonance (peak at 0.7 eV) proceeds via other mechanism. Modelli et al. [10] attributed 0.7 eV resonance to dissociation of the molecular anions formed on the high-energy side of the p* (a2) resonance. The transfer of the extra electron from the antisymmetric ring p* (a2) MO to the NO2 group requires a vibrational motion which reduces the molecular symmetry, and thus needs additional energy. Second resonance (at 1.4 eV) has appearance energy of 0.870.2 eV which fits correctly with the derived value.
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Table 1 Peak positions and absolute partial cross-sections for the fragment ions obtained in the previous [7–10] and present experiments m/z (possible fragment)
Peak position (eV) Jaeger et al. [9]
16(O) 17(OH) 25(C2H) 26(CN) 30(NO) 39(C3H 3) 40(C3H 4) 41(C2HO) 42(CNO) 46(NO 2) 49(C4H) 50(C4H 2) 51(C4H 3) 52(C4H 4) 63(C5H 3) 64(C5H 4) 65(C5H 5) 66(C4H2O) 67(C4H3O) 68(C4H4O) 75(C6H 3) 76(C6H 4) 77(C6H 5) 78(C5H2O) 79(C5H3O) 91(C6H3O) 92(C6H4O) 93(C6H5O) 94(C5H4NO) 104 105(C6H3NO) 106(C6H4NO) 108 122 (C6H4NO 2) 123 (C6H5NO 2)
Obtained EA crosssections (m2) Compton et al. [7,8]
Modelli et al. [10]
5, 6.5, 7.5 4.2, 6.5 4.5, 5, 7 4.2 3.8
4.2 4.2 1.5, 4
1.3, 3.64
0.7, 1.25, 2.9, 3.7
3.8 4.2
3.6
4.2
3.6
1.8, 4.2, 6.5 3.8 4 4.5 3.8
3.6
3.8 4.2 0.3, 4.8
Present
4.8, 0.2, 5, 4.2 3.8 5, 4.2 4 3.8 0.7, 7 4.2 4.2, 4.5 3.1, 4.2 3.8 3.8 4.2 4.5 4.2, 4.2 4.2, 3.8, 3.4 3.8 4.2 3.8 3.8
7 4.2, 7 7
7
1.4, 3.1, 3.8, 6.6
6.5 4.2
6.4 6.5 6.5
1.6 1021 2.1 1022 1.9 1022 1.4 1021 9.0 1023 8.0 1023 4.5 1023 1.0 1022 7.9 1022 4.6 1020 5.9 1023 3.7 1023 4.3 1023 3.6 1023 2.4 1023 1.6 1023 1.9 1022 4.0 1023 1.5 1022 5.3 1024 1.4 1021 1.4 1021 2.3 1022 2.0 1022 7.4 1023 1.7 1022 1.6 1022 2.9 1022 4.1 1023
3.8 4
7.9 1023 3.2 1023
3.5, 7 0
2.2 1023 3.8 1021
3.6 3.8 0.15
0
Peaks at 1.4 and 3.8 are associated with electron attachment into the second p* MO of b1 symmetry. Instead of three resonances mentioned, NO 2 is also formed by capture of the electron with the energy of 3.1 and 6.6 eV. The origin of these two channels is not evident. The formation of NO 2 at 3.1 eV ions could be complementary to the production the C5H 3 ion. The channel at 6.6 eV (observed in present studies for the first time) could be coupled with the production of neutral fragments: C4H3, C6H3 or C6H5 (their negative ions were measured at 6.6 eV). Due to a lack of the thermodynamical data for C5H3, C4H3, C6H3 and C6H5, it is impossible to deduce which channel is responsible for the formation of the nitrogen dioxide anion at these two energies. 3.3. Formation of O Ejection of an oxygen anion from nitrobenzene occurs at two electron energies: 4.8 and 7 eV. Possible reactions
0, 0.4
describing the DEA to oxygen atom can be: e þ C6 H5 NO2 ! ðC6 H5 NO2 Þ] ! O þ C6 H5 NO;
(3a)
e þ C6 H5 NO2 ! ðC6 H5 NO2 Þ] ! O þ C6 H5 þ NO; (3b) e þ C6 H5 NO2 ! ðC6 H5 NO2 Þ] ! O þ C5 H4 þ CH þ NO; (3c) e þ C6 H5 NO2 ! ðC6 H5 NO2 Þ] ! O þ C5 H4 þ CNO þ H; (3d) e þ C6 H5 NO2 ! ðC6 H5 NO2 Þ] ! O þ C5 H4 þ CN þ OH: (3e)
The thermodynamical data for reaction (3a) are not accessible, however these data are available for reactions (3b)–(3e). From the energetic point of view, the reaction
ARTICLE IN PRESS A. Pelc et al. / Vacuum 81 (2007) 1180–1183
1183
40 30 C6H5NO2-
20 Cross section [*10-22 m2]
10 0 450 300
NO2-
150 0 16 12
O-
8 4 0 0
2
4
6
8
10
Electron energy [eV] Fig. 1. Ion yields for C6H5NO 2 , NO2 , O fragment anions formed via electron attachment to nitrobenzene.
(3b) is the most favourable—this reaction is endothermic by 4.8170.2 eV (Df H0g ðOÞ ¼ 249:18 0:10 kJ=mol, (Df H0g ðC6 H5 Þ ¼ 339 8 kJ=mol, Df H0g ðNOÞ ¼ 90:26 kJ=mol and EA(O) ¼ 1.46 eV [23]). Appearance energy for resonances at 4.8 and 7 eV (obtained in our experiment) are 3.070.2 and 5.070.4 eV, respectively. This indicates that reaction (3b) can be responsible for DEA process at 7 eV. The originating of the resonance at 4.8 eV is not obvious, especially if the thermodynamical data for possible neutral fragments (e.g. C6H5NO, C5H5, etc.) are not known. For the rest of anions, we provide the measured partial cross sections and the energetic position of resonances leading to their generation (Table 1). 4. Conclusion The nondissociative and dissociative electron attachment (with energy of 0–10 eV) to nitrobenzene has been studied. We observed 33 anions from which 13 (m/z: 39, 40, 49, 50, 51, 52, 64, 66, 67, 68, 76, 79, 94 amu) were measured for the first time. Simultaneously, two of previously measured ions with q/m ratio 104 and 108 were not detected. Intensities of ions generated have been converted to absolute cross sections by comparison with cross section of Cl/CCl4. Acknowledgement Work partially supported by the FWF, Wien, Austria and by the Polish Ministry of Science and High Education from scientific budget in 2006–2007, as a research project (Grant no.1 P03B 021 30).
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