Abstract
Long-lived parent negative ions formed via nuclear-excited Feshbach resonances at thermal electron energies have been observed for six NO2-containing benzene derivatives-namely, o-, m- and p-nitrophenol, and o-, m- and p-nitroaniline. The lifetimes, τ, of these ions at thermal electron energies, as determined by the slope method using a time-of-flight mass spectrometer, are 460±34, 31±1.3, 13.9±0.4, 46±2, 21±1, and 15±0.8 μs for o-, m- and p-nitrophenol, and o-, m- and p-nitroaniline, respectively. The measured lifetimes, as well as those reported previously for all known long-lived parent negative ions of benzene derivatives, are discussed and related to the respective molecular structures. The measured lifetimes depend on the electron donor-acceptor properties of the substituent groups and the interamolecular interaction between them, which is a function of their relative position around the benzene periphery. The results of this study suggest that, quite generally, NO2-containing benzene derivatives capture thermal electron very efficiently and form long-lived (τ > 10-6 s) parent negative ions in the absence of competing dissociative electron attachment processes which are usually very fast.
| Original language | English |
|---|---|
| Pages (from-to) | 1691-1703 |
| Number of pages | 13 |
| Journal | Journal of the Chemical Society, Faraday Transactions 2: Molecular and Chemical Physics |
| Volume | 69 |
| DOIs | |
| State | Published - 1973 |
Funding
A. Hadjiantoniou L. G. Christophorou J. G. Carter Long-lived parent negative ions formed via nuclear-excited Feshbach resonances at thermal electron energies have been observed for six NO 2 -containing benzene derivatives—namely, o -, m - and p -nitrophenol, and o -, m - and p -nitroaniline. The lifetimes, τ , of these ions at thermal electron energies, as determined by the slope method using a time-of-flight mass spectrometer, are 460 ± 34, 31 ± 1.3, 13.9 ± 0.4, 46 ± 2, 21 ± 1, and 15 ± 0.8 µ s for o -, m - and p -nitrophenol, and o -, m - and p -nitroaniline, respectively. The measured lifetimes, as well as those reported previously for all known long-lived parent negative ions of benzene derivatives, are discussed and related to the respective molecular structures. The measured lifetimes depend on the electron donor-acceptor properties of the substituent groups and the interamolecular interaction between them, which is a function of their relative position around the benzene periphery. The results of this study suggest that, quite generally, NO 2 - containing benzene derivatives capture thermal electron very efficiently and form long-lived ( τ > 10 –6 s) parent negative ions in the absence of competing dissociative electron attachment processes which are usually very fast. Long-lived Parent Negative Ions Formed via Nuclear-excited Feshbach Resonances Part 1 .-Benzene Derivatives t A. HADJIANTONIOU *§ AND J. G. CARTER,L. G. CHRISTOPHOROU Health Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 Received 30th March, 1973 Long-lived parent negative ions formed via nuclear-excited Feshbach resonances at thermal electron energies have been observed for six N02-contajcing benzene derivatives-namely, o-, rn-and p-nitrophenol, and o-, m-andp-nitroaniline. The lifetimes, T,of these ions at thermal electron energies, as determined by the slope method using a time-of-flight mass spectrometer, are 460+ 34, 31 k1.3, 13.9f0.4, 46i-2, 21 k 1, and 15f0.8 ,us for o-, rn-andp-nitrophenol, and o-, in-andp-nitroaniline, respectively. The measured lifetimes, as well as those reported previously for all known long-lived parent negative ions of benzene derivatives, are discussed and related to the respective molecular structures. The measured lifetimes depend on the electron donor-acceptor properties of the sub- stituent groups and the intramolecuIar interaction between them, which is a function of their relative position around the benzene periphery. The results of this study suggest that, quite generally, N02-containing benzene derivatives capture thermal eleurons very efficiently and form long-lived (T > 10-6s) parent negative ions in the absence of competing dissociative electron attachment processes which are usually very fast. A large number of polyatomic molecules capture thermal and epithermal electrons in the gas phase via nuclear-excited Feshbach resonances and form long-lived (z > s) parent negative ions.3 In this type of resonance the electron is trappedby losing energy solely to the nuclear motion of the molecule. No electronic excita- tion of the molecule is involved. A number of workers have measured cross sections 3-8 and autodetachment 5-79lifetimes 39 9-12 (z > s) for such resonant states. In almost all cases studied the cross sections are large and sharply increasing with decreasing electron energy at *thermal and epithermal energie~.~. On the other hand, although the long (> s)autodetachment lifetimes have been found for many systems to be independent of the incident electron energy, in the present study (Part 3) l3 and in a previous one,' z has been found to decrease with increasing incident electron energy for a number of long-lived parent polyatomic negative ions. We undertook a systematic study whereby we investigated and identified structures which capture thermal and epithermal electrons forming long-lived nuclear excited Feshbach resonant states. In the present paper (Part 1) the parent negative ions, their yields as a function of incident electron energy, the parent negative ion lifetimes, and their dependence on electron energy are reported, discussed, and related to the respective molecular structures for six NO,-containing benzene derivatives-namely, t Research sponsored by the U.S. Atomic Energy Commission under contract with Union Carbide Corporation. '+ University of Tennessee, Knoxville, Tennessee ; on leave of absence from Democritos Nuclear Research Center, Athens, Greece. S Also Department of Physics, University of Tennessee, Knoxville, Tennessee. 11-55 1691 LONG-LIVED NEGATIVE IONS u-, m-,and p-nitrophenol, and u-, m-,and p-nitroaniline. Also, all available informa- tion on long-lived parent negative ions of benzene derivatives is summarized and dis- cussed. In Part 2 l4 these studies are extended to other aromatic and nonaronzatic struc- tures in an effort to identify structural functional groups responsible for the formation of long-lived nuclear-excited Feshbach resonant states. In Part 3 l3 the variation of the autoionization lifetime with incident electron energy observed and measured for four organic molecules is presented, discussed, and theoretically treated. EXPERIMENTAL A. TIME-OF-FLIGHT MASS SPECTROMETER In this study a time-of-flight (TOF) mass spectrometer has been used which was coil- structed at our Laboratory and is similar to the Bendix (model 14-206) massspectro meter. It consists of four basic parts : (i) the "source "which is made up of the electron gun and the collision chamber, (ii) the field-free flight tube in which mass separation takes place, (iii) the detection system which consists of the electron multiplier and the associated electronics for the detection and measurement of the negative-ion current and the presentation of the negative-ion spectra, and (iv) the vacuum system. (i) THE SOURCE Fig. 1is a schematic diagram of the electron source which consists of a retarding potential difference (RPD) eIectron gun and the collision chamber. A Lambda power supply (model LH 119 FM) was employed to maintain a constant d.c. current of 1.5 to 2.5 A through a I /ELECTRON COLLECTOR I 1.h50-200 voLTs COLLISION , CHAMBER i I ;-TO ION ACCELERATGR , AND FLIGHT TUBEI BACK1 NG- PLATE I I ANODE RETARDING OFFERENCE ELECTRON BIAS VOLTAGE RETARDING ENERGY VOLTAGE FIG.1.-Schematic diagram of the RPD electron source. A. HADJIANTONIOU, L. G. CHRISTOPHOROU, J. G. CARTER 1693 5-mil tungsten filament. The electron beam is pulsed every 100 p into the collision chamber by a + 7 V amplitude and a 1 ,us wide pulse applied at the first electrode (electrode 1 in fig. 1) which is normally biased by a negative potential. The ions formed in the collision chamber are pushed out into the flight tube by a negative pulse applied on the backing plate about 0.5 p after the anode pulse. This negative pulse has an adjustable amplitude of 50 to 250 V and a width of 4 p. A bias voltage is also applied to the backing plate to compensate for the overshoot of the backing plate pulse. A quasi-monoenergetic electron beam was obtained by application of the RPD tech-nique.” The energy resolution was -0.15 eV. (ii) FIELD-FREE FLIGHT TUBE Negative ions formed in the collision chamber are pulsed through the ion grids into the flight tube (fig. 2) by the backing plate pulse. The ion acceleration is achieved by a high d.c. voltage applied to the acceleration grid which is in contact with the tube liner. The d.c. voltage can be varied from 1 to 4.5 kV in 0.5 kV intervals. I 2 FIG.2.--Schematic diagram of the time-of-flight mass spectrometer. DISTANCE FROM TO ACCELERATION GRID 3 ion focus grid 2 0.7 cm ion focus grid 1 1.4cm horizontal deflection plates 61.5 cm electron multiplier 150 cm The ion beam can be focussed by adjusting the voltage applied to two sets of deflection plates, horizontal and vertical. The horizontal deflection plates are also used to completely deflect all ions out of the beam during the negative-ion lifetime measurements. When the plates are off, they are at the same potential as the liner, thus not affecting the ion flight paths. The ion lens, a concentric cylinder in the flight tube, acts as a converging lens for the ion beam. (iii) DETECTION SYSTEM The ions, and the neutral molecules which are formed in the flight tube from the ions via autodetachment, are detected at the end of the flight tube by an electron multiplier (Bendix M306). The electron pulse from the electron multiplier is fed to the timing filter amplifiers through a high impedance unity gain preamplifier (see fig. 3). The negative pulse can be amplified from 2 to 200 times in the amplitude range from +0.5 to -6.0 V. It was observed that for the study of a signal corresponding to a certain mass, the optimum setting of the amplifiers should be such that the amplitude of the amplified pulse was approximately 4 V. From the amplifiers the signal is fed to the fast discriminator which is used as a pulse shaper. The discriminator, triggered by an input signal, gives only one output pulse which is reshaped to a standard 5 V amplitude and 500 ns wide pulse. The discriminator level can be adjusted LONG-LIVED NEGATIVE IONS from 0.15 to 1V. In the present experiment the discriminator level was usually adjusted to at least 1V less than the amplitude of the weakest pulse coming from the amplifiers (which is that from the neutrals) to ensure no discrimination in the detection of ions and neutrals. The pulse from the discriminator is fed to the fast coincidence unit and is recognized only when it is in coincidence with the gate pulse from the gate and delay generator. The gate and delay generator are triggered by the master pulse. The gate pulse, variable in time and of 0.4 ps width, is synchronized with the signal from the mass of interest displayed on the screen of the oscilloscope, which is also triggered by the master pulse. The two input pulses are reshaped, and when they overlap within the set resolving time, the circuit recognizes a co-incidence event and gives a standard output pulse. The resolving time may vary from 10 to 100ns. TlMt TO PULSt GATE ~OELAY HEIGHT ELECiWY AWLTIER AMPJF!ER DISCRIMINATOR COINCIDENCE GENERATOR CONVERTER RATEMETER MULTIPLIER RECORDER STRETCHER PRE-AMP 0Y lr OUT UUL-FIG.3.-Block diagram of the time-of-flight mass spectrometer detection system. The positive pulse from the fast coincidence system is changed to a negative pulse by the inverting transformer (IT 100; see fig. 3) and is fed to the time-to-pulse height converter (TPHC) as the stop pulse. The start pulse is furnished from the master pulser and then delayed by the Rutherford generator. The TPHC measures the time difference from the start to the stop pulse and furnishes an output amplitude proportional to the time difference. One of the two outputs of the TPHC is connected to the ratemeter and then to a Brown recorder, and the other output to a 512 multichannel analyzer. (iv) VACUUM SYSTEM The vacuum system consists of a 15 CFM forepump, a six-inch oil diffusion pump, a freon baffle, a liquid nitrogen trap, and a number of valves. One automatic high vacuum valve is located between the flight tube and the freon baffle and is connected to the vacuum protection system. A second valve is located between the diffusion pump and the forepump, and a third valve isolates the flight tube from the forepump. The pressure is monitored with a Bayard-Alpert type ionization gauge. B. MASS IDENTIFICATION Ions of the same charge but different masses have different times of Aight given by the equation t =L(MJ2E)3, (1) where t is the time of flight, L is the length of the flight tube, M is the mass of the ion, and E A. HADJIANTONIOU, L. G. CHRISTOPHOROU, J. G. CARTER 1695 is the ion kinetic energy. By measuring the times of flight of two negative ions present in the spectrometer, the mass of one can be determined from Mx = M(tx/t)2 (2) if the mass of the other is known. Sulphur hexafluoride (SF,) was used as the calibrating mass in this study, and the times of flight were accurately determined by the delayed sweep-time measurement using the 454 Tektronix oscilloscope. The parent negative ions were clearly identified (within 0.2a.m.u.). For weak signals the error in determining the mass of the negative ion was the largest (about 1 a.m.u.) because the signal was spread out, and the start of the negative-ion pulse was not sharp. The sample gas pressures employed were usually between 1 and 4x Torr. The background pressures were between 1 and 2x lo-' Torr. c. MEASUREMENT OF AUTOIONIZATION LIFETIMES Let NTbe the number of negative ions that enter the flight tube at t = 0. Some of these will autoionize during their flight and will become neutral molecules. Both neutral molecules and surviving negative ions are detected at the end of the flight tube, hence NTis measured. Subsequently, the ions are deflected by applying a "flat top " potential on the horizontal deflection plates, thus allowing the neutral molecules No to pass unaffected and be detected. The number of ions, N-, that have not decayed in travelling the distance between the accelera- tion grid and the deflection plates is equal to NT- NO. If we assume that autodetachment obeys an exponential decay law, the negative-ion lifetime can be determined from the relation Nr-No N-z = -?/In ___ = --/In (3)~NT NT where the time of flight, t, is the time needed for the ions to travel the distance between the acceleration grid to the deflection plates. Edelson and co-workers first used the TOF mass spectrometer to measure autodetach- ment lifetimes of long-lived negative ions. Their method was employed later by0thers.5-7, 11, 12 These latter workers used the so-called " slope method " to determine the autodetachment lifetime, i.e., they obtained z from a plot of (-In (N-/NT))against the time of flight, t. The variation of t is obtained by varying the ion kinetic energy. Such a plot must be a straight line passing through the origin ; the autodetachment lifetime is then given as the inverse of the slope. For molecules with a very weak negative ion signal and/or a weak neutral-molecule signal (small capture cross section 01'a very long-lived negative ion), it was not possible to use the slope method because of the scattering of the data. In this case the autodetachment lifetime was determined as follows. First, SF6 was admitted to the TOF mass spectrometer. When the established l6 SF;* lifetime was obtained by the slope method (proper operation of the instrument), the molecule of interest was admitted to the TOF mass spectrometer, and a large number of measurements of NT and No were made for different acceleration energies. The autodetachment lifetime was, then, taken to be the average of all lifetimes calculated from these measurements, using eqn (3). A discussion of the possible sources of error in lifetime measurements has been given by Collins, Christophorou and Carter.l1 The low pressures employed (N 1 to 4x lo-, Torr) ensured that processes such as collisional detachment and collision-induced dissociation can be neglected. D. NEGATIVE-ION CURRENT AS A FUNCTION OF ELECTRON ENERGY The current for parent and fragment negative ions as a function of the incident electron energy was measured using the RPD method. The energy scale was calibrated by com-parison with well-established negative ion peaks (e.g., Cl-/HCI, 0-/N20,SF; /SF,) when both the sample molecule and the calibrating gas were present in the TOF mass spectrometer. The SF, *was not used for the energy scale calibration because the energy scale may be under-estimated, as pointed out by Schulz l7 and Christophorou and co-workers.18 LONG-LIVED NEGATIVE IONS RESULTS A. 0-, m-AND p-NITROPHENOLS 1. 0-NITROPHENOL The parent negative-ion current as a function of the incident electron energy for o-C6H40HNO; *, SF, *, and C1-/HCl is given in fig. 4(a). This figure is representa- tive of 15 independent measurements using the RPD method. The measurements were repeated after a year's time using a new electron gun, and were found to be in excellent agreement with the initial data. The o-C6H40HN0;* current peaks at the same energy as SF;* and both appear at lower energy than the SF; from SF6 resonance which peaks at -0.37 eV.* The ion current peak for all parent negative (a> I (b) (4electron energy/eV FIG.4.-The negative ion current against electron energy for (a) o-nitrophenol (O), SFG (O),and C1-(A) from HCl ; (6) m-nitrophenol (a)and SF6 (0); and (c) p-nitrophenol(0) and SFs (0).The shapes of SF;*, 0-, rn-and p-C6H40E",* are instrumental. A. HADJIANTONIOU, L. G. CHRISTOPHOROU, J. G. CARTER 1697 ions reported in this paper coincided in energy with that for SF;* and appeared at lower energies than the SF, from SF6. The energy scale for o-nitrophenol was calibrated also against the C1-from HCl which peaks at 0.8 eV.19 The full width at half height for the 0-C6H40HN02* resonance w2.s always 2 0.2 eV and similar to that for the SF;* resonance when both molecules were simultaneously in the system. The width of the SF;* resonance is instrumental (see discussion in ref. (3), (8), and (20)) and so is that of o-C6H40HNO;*. Using the same procedure as for the negative ion current measurements, the mean autodetachment lifetime of o-C6H40HNO;* was measured and found to vary with the incident electron energy. These results are presented and discussed in Part 3.13 The mean autoionization lifetime for o-C6H40HN0; * was determined by the slope method to be 460 ps at -0.0 eV incident electron energies. In fig. 5 a plot is given of the negative logarithm of N-divided by NT(NT= N-+NO)against the times of flight corresponding to 2.0, 2.5, 3.0, 3.5, 4.0, and 4.5 kV acceleration voltages. The points plotted for o-C6H40HN0;* are the averages of three different measurements. The straight line drawn through the data points in this and all similar plots is the least- squares fit of the data points including the origin. The lifetime obtained by the slope method at -0.0 eV is rslope= 460+ 34 ps. time of flight, t/ps ~l ~FIG.5.-The -In (N-/NT) against time of flight, t,for o-nitrophenol( ~ = 460k 34 ps) ~; m-nitro-~ phenol (Tslope = 31 k 1.3 ps) ; and p-nitrophenol (Tslope = 13.9k0.4 ps). The right-hand-side ordin- ate is only for o-nitrophenol. 2. YY1-NITROPHENOL AND p-NITROPHENOL The parent negative-ion currents as a function of incident electron energy for m-C6H40HN0;* and SF;* and p-C6H40HNO;* and SF;* are presented in fig. 4(b)and 4(c), respectively. Fig. 4(b) is representative of six and fig. 4(c) is representa- tive of thirteen independent measurements using the RPD method. The auto- detachment lifetime for m-C6H40HNO;*, as determined from the data points in fig. 5-which are the averages of four independent measurements- is z~~~~~= 31 1.3 ps. Similarly, the autodetachment lifetime for p-C6H40HN0,*, as determined from the (-In (N-/NT),t) data points in fig. 5 (the points plotted are the averages of nine independent measurements), is z~~~~~= 13.950.4 ps. Under our experimental conditions the lifetimes for m-C6H40HNO;*, p-C6H40HNO;* and SF, * were constant with incident electron energy across the respective resonances (see, however, Part 3).13 LONG-LIVED NEGATIVE IONS B. 0-,m-AND p-NlTROANILlNES The parent negative-ion currents as a function of the incident electron energy for o-C6H4NH2N0;* and SF;*, rn-C6H4NH2N0;* and SF;*, andp-C,H,NH,NO;* and SF;* are presented in fig. 6(a), 6(b),and 6(c),respectively. Fig. 6(a),6(b),and 6(c) are, respectively, representative of 6, 10, and 7 independent measurements using the RPD method. The energy scale was calibrated against the 0-from N20 resonance which has a maximum at 2.25 eV.21 (4 -12 -II -10 9-8-7-6-5-4-8 c.-c 3-2-wl (b) (c) electron energy /eV FIG.6.-The negative ion current against electron energy for (a)a-nitroaniline(O), SF6 (0),and 0-from N20(A);(b) m-nitroaniline (0)and SF6 (0);and (c) p-nitroaniline(0)and SF6 (0).The shapes of SF:*, 0-,m- and p-C6H4NH2N0,* are instrumental. The autodetachment lifetimes for 0-,rn-, and p-C,H4NH2NO; * as determined from the -In (WINT)against t plots in fig. 7 (the data points plotted are the averages of three independent measurements) are, respectively, 46 &2, 21 5 1, and 1520.8 ps. A. HADJIANTONIOU, L. G. CHRISTOPHOROU, J. G. CARTER 1699 Under our experimental conditions the lifetimes for 0-,m-, andp-C6H4NH2N0i* and SF, * were constant with electron energy across the respective resonances (see, however, Part 3).13 time of flight, t/ps FIG.7.-The -In (N-/NT) against time of flight, t,for o-nitroaniline(wope= 46 f2 ps) ; m-nitro-~ = 15 k0.8 ps). The right-hand-side ordinate aniline (T~=I21~f~1 ps) ; and p-nitroaniline (TSI~~~ is only for o-nitroaniline. c. SUMMARY OF LIFETIME DATA FOR BENZENE DERIVATIVES Table 1 summarizes all the experimental results on the lifetimes of long-lived parent negative ions of benzene derivatives formed via nuclear-excited Feshbach resonances. TABLE1 .-LONG-LIVEDPARENT NEGATIVE ION LIFETIMES OF BENZENE DERIVATIVES FORMED VIA A NUCLEAR-EXCITED FESHBACHRESONANCE AT THERMAL ENERGIES molecule formula /PS ref. nitro benzene 17.5 12 benzonitrile -5 12 o-nitrotoluene 13 5 m-nitrotoluene 18.8 5 p-ni trot oluene o-nitrophenol rn-nitrophenol p-nitrophenol o-nitroaniline 14 460 31 13.9 46 5 PWa PW PW PW m-ni troaniline 21 PW p-nitroaniline cinnamaldehyde m-chloronitrobenzene 15 12 47 PW 5 12 [2HJnitrobenzene 22 12 hexafluorobenzene 12 22 chloropentafluorobenzene bromopentafluorobenzene pentafluorobenzonitrile pentafluorobenzaldehyde octafluorotoluene 17.6 21 17 36 12.2 12 12 12 12 22 0 present work; seealso ref. (13). LONG-LIVED NEGATIVE IONS D. DISSOCIATIVE ELECTRON ATTACHMENT No effort was made to study in detail the yields of fragment negative ions from these molecules. However, all six molecules studied were observed to dissociate by capturing electrons with energies greater than thermal. The predominant frag- ment negative ions observed for o-C6H40HNO2 below -5 eV were C6H4NO,, C6H40H-, and NO,. The negative ion currents for these ions peaked, respectively, at 1.00 and 3.50, 4.40, and -3.50 eV. For o-C6H4NH2NO2, the predominant ions observed below -5 eV were C6H4NO2, C6H4NH;, and NO;. The negative ion currents for C6H4N0, and NO; peaked at 3.15 eV. DISCUSSION AND CONCLUSIONS From this and previous studies on some NO,-containing benzene derivatives (see table l), it is suggested that these molecules capture thermal electrons very efficiently and form long-lived parent negative ions, unless a dissociative electron attachment process-which is usually very much faster-competes with parent negative-ion formation. This latter case seems to apply for M-C~H~INO~.~~In these long-lived aromatic parent negative ions the electron is maintained by the whole molecule which acts as an electron acceptor and hence may appropriately be called an “ electro-phore ”. The NO2group lowers the energy of the benzene ring due to charge migra- tion from the ring to the nitro group (“ inductive effect ”). In non-cyclic compounds the electrophore may be a functional group comprised of only a part of the poly- atomic molecule. In these aromatic structures the view that the whole molecule and not only the NO, group participates in the capture of the electron is supported by the observed lowering of the peak energies of the compound negative-ion resonances (CNIR) upon multiple halogen substitution (see table 2) and possibly, also, by the observed changes (table 1) in the lifetimes with the positions and the natures of the substituents. From the data in tables 1 and 2 it is observed that only for multiple fluorine substitu- tion are long-lived parent negative ions formed, although multiple halogenation of any kind lowers the energy of the n-electron state (increases the molecules’ electron affinity) in which the electron is captured. The strong C-F bond allows multiply- fluorinated benzene derivatives to form long-lived parent negative ions by making the competing process of dissociative electron attachment energetically not possible at thermal energies. The weaker C-I bond (and possibly the C-Cl and C-Br bonds in multiply-substituted halogenated compounds) allows dissociative electron attach- ment and hence no long-lived, parent-negative-ion formation. The molecules in table 2 are expected to be, in the gas phase, electron donors while those in table 1 are electron acceptors. The measured parent negative ion lifetimes are seen to depend on the electron donor-acceptor properties of the substituent groups and on the intramolecular inter- action between the substituent groups which varies with their relative position on the benzene periphery. The substituents (NO,, OH ; NOz, NH, ; NO,, CH3) affect the charge distribution in the benzene ring. Substituents such as NH2, OH, and CH3 lead to an increase in electron density at the ortho and para positions, while substitu- ents such as NO2 lead to a decrease in electron density at the meta position although the decrease is less than at the ortho- and para-positions. Thus the ionization potentials, IP, of aniline (7.7 eV),23 phenol (8.5 ev) 23 and toluene (8.8 eV) 23 are smaller and that of nitrobenzene (10.15 ev) 24 is larger than the benzene ionization potential (9.25 eV).23 Hence the experimental finding that T0-c6H40HN0,*(460 ,Us) > t,-C6H4NHzN0,*(46 PSI ’‘o-C6H&H3NO$ (13 PSI A. HADJIANTONIOU, L. G. CHRISTOPHOROU, J. G. CARTER 1701 is consistent with the fact that IPOH(13.4 eV) > IPNH,(11.4 ev) > IPc,,(9.8 eV).23 This suggests that the lifetimes are longer when the electron withdrawing ability of NO, is the least affected by the electron donating capacity of the second substituent, a property that may be expected to decrease with increasing IP of the second substituent. 2.-BENZENE DERIVATIVES (INCLUDING N-HETEROCYCLIC)TABLE FOR WHICH NO LONG-LIVED PARENT NEGATIVE IONS HAVE BEEN OBSERVED IN THE GAS PHASE (PRESSURES < Torr). maximum of compoundnegative-ion resonance/ molecule eV benzene 1.30 a 1.40b toluene 1.50 fluorobenzene 1.4Ob chlorobenzene 0.86 bromobenzene 0.84 o-chlorotoluene 1.4Ob 1,3-difluorobenzene 0.60 1,Zdichlorobenzene 0.36 b lY3,5-trifluorobenzene 0.30 1,2,3,4-tetrafluorobenzene -OC pyridine d 0.84 1.30 pyridazine (1 ,2-C4H4N2) d 0.70 1.44 pyrimidine (1,3-C4H4N2) 0.80 1.90 a? pyrazine (1 ,4-C4H4N2) 1.10 1.53 H. H. Brongersma, J. A. v.d. Hart and L. J. Oosterhoff, in Proceedings of Nobel Symposium 5 (Interscience Publishers, New York, 1963, p. 211. b R. N. Compton, L. G. Christophorou and R. H. Huebner, Phys. Letters, 1966,23,656. c W.T. Naff, C. D. Cooper and R. N. Compton,J. Chem.Phys., 1968,49,2784. d See a discussion of these double resonances and their relative positions in ref. (e) below. e M. N. Pisanias, L. G. Christophorou, J. G. Carter and D. L. McCorkle, J. Chem. Phys., 1973, 58, 2110. The decrease in the parent negative ion lifetime for the three nitrophenol and nitroaniline isomers in the sequence 7,-> 7,-> z, may be due to the change of the charge density in the benzene ring, which is a function of the nature and relative positions of the two substituents, but also possibly due to a change in the effective number of degrees of freedom as a result of an intramolecular bonding involving the two substituents mainly at the ortho position. Consistent with the decrease of z with increasing distance between the substituents is a large body of information (see, for example, ref. (24)-(27)) which indicates a strong intramolecular interaction between the two substituents at the orrho position. The intramolecular hydrogen bonding is stronger (and more local) for o-nitrophenol than for o-nitroaniline, the shift in the electron density in the OH group occurringmainly in the oxygen atom of that group while the H atom is slightly drawn to the NO2 group.28 Also consistent with the above is the fact that -C6H,0HN0,*(460 PSI 9 TC6H5N%*(17-5PSI, although IPo,(13.4 eV) 2: IPH(13.01 eV).23 1 702 LONG-LIVED NEGATIVE IONS That iiitr:iinolecular bontling kt ween the substit ucnts affects the parent negative- ion lifetime is supported also froin thc ohservsd differences in the lifetimes between the 0-and m-isomers and the nz-and p-isomers. Thesc differences decrease in going from nitrophenols to nitroanilines, to nitrotoluenes, and in going from 0-and m-to rn-and p-isomers. Thus, A(T,--z,-)OH = 429 PAS> A(T,--~~,,-)Ntl~= 25 C~S> A(T,--T”,-),-~I~ = -6 PS and A(T,,--T~-)OI~= 17 PLS> A(T~),--T~-)NE~~= G ,US 2 A(TJt,--T~-)CH~ = 5 /ts. The intramolecular bond between the two substituents weakens in the order OH > NH2 > CH, aiid 0-> in-> p-. Finally, the finding that z,-C~~~CH~~~~~(~~J P>PUS)fi zp-C611,~~s~~,*(14 may be attributed to the low IPcH3(compared to TPNH2and IPoH)which may result in a much larger electron migration to the benzene ring. It seems that the effect of this migration overshadows any opposite effect due to intramolecular complexing at the ortho position for o-C6H4CH3N02. J. N. Bardsley and F. Mandl, Rep. Progr. Phys., 1968,31,471. L. G. Christophorou,Atomic and Molmdar Radicitimr Physics (Wilcy-in tcrscicnce, Ncw York, 1971), Chap. 5, p. 331. L. G. Christophorou, Atomic arid Molecrrlar Radiation Physics (Wiley-Interscience, New York, 1971), Chap. 6. L. G. Christophorou and R. P. Blaunstein, Rad. Res., 1969, 37, 229. L. G. Christophorou,3. G. Carter, E. L. Chaney and P. M. Collins, Paper presented at the IVth International Congress of Radiation Research, Evian, France, June 1970 (Gordon and Breach in press). E. L. Chaney, L. G. Cluistophorou, P. M. Collins and J. G. Carter, J. Cltein.Phys., 1970, 52, 4413. P. M. Collins, L. G. Christophorou, E. L. Chaney and J. G. Carter, Chem. Phys. Letters, 1970, 4, 646. L. G. Christophorou, D. L. McCorkle and J. G. Carter, J. Chem. Phys., 1971,54,253. D. Edelson, J. E. Griffiths and K. B. McAffee, Jr., J. Chem. Phys., 1962,37,917. lo R.N. Compton, L. G. Christophorou, G. S. Hurst and P.W. Reinhardt, J. Chem.Phys., 1966, 45,4634. *l P. M. Collins, L. G. Christophorou and J. G. Carter, Oak Ridge National Laboratory Rept. NO.ORNL-TM-2614 (1970). l2 W. T. Naff, R, N. Compton and C.D. Cooper, J. Chent.Phys., 1971,54,212. l3 L. G. Christophorou,A.Hadjiantoniou and J. G. Carter, Part 3, 1973, 69, 1713. l4 A. Hadjiantoniou, L. G. Christophorou and J. G. Carter, Part 2, 1973, 69, 1704. lS R. E. Fox, W.H. Hickam, D. J. Grove and T.Kjeldaas, Jr., Reu. Sci. insrr., 1955,26, 1101. l6 This statement applies to our experimental arrangement. Under different experimental condi- tions the autoionization lifetime of SF;* may be different. Thus, the lifetime of SFi* was found to be much longer (-500 ps) in an Ion Cyclotron Resonance experiment (J. M. S. Henis and C. A.Mobie, J. Chem. Phys., 1970, 53, 2999). See a discussion of this and of the dep- endence of the autoionization lifetime on incident electron energy in ref. (13).*’G. 3. Schulz, J. Appl. Phys., 1960, 31, 1134. l8 L. G. Christophorou, R. N. Cornpton, G. S. Hurst and P. W. Reinhardt, J. Chem. Phys., 1966,45, 536. l9 L. G. Christophorou, R. N. Compton and H. W. Dickson, J. Chem.Phys., 1968, 48, 1949. 2oA. Stamatovich and G. J. Schulz, Rev. Sci. hstr., 1968, 39, 1752. 21 E. L. Chaney and L. G. Christophorou,J. Chenr. Phys., 1969,51, 883. 22 W. T. Naff, C. D. Cooper and R. N. Conipton, J. Chent. Phys., 1968,49,2784. 23 Photoionization value. L. G. Christophorou,Atontic and Molecular Radiation Physics (Wiley-tnterscience, New York, 1971), Appendix IT. A. HADJIANTONIOU, L. G. CHRISTOPHOKOU, J. G. CARTER 1703 24 S. Nagakura and J. Tanaka, J. Cheni. Phys., 1954, 22, 236. 25 I. T. Millar and H. D. Springall, The Organic Chemistry ofNitrogeiz (Clarendon Press, Oxford, 3rd edn., 1966). 26 J. Tanaka and S. Nagakura, Chetn. SOC.Japan, Pure Clienz. See., 1956, 24, 1274. 27 K. Semba, Bull. Chem. SOC.Japan, 1960,33, 1640. 28 L. B. Zubkova and V. I. Danilova, Zzvest. Fiz., 1967, lQ, 152.