J. Phys. Chem. 1992,96, 2385-2390 surfactant molecules in the micelle and displaces solvent molecules from the polar core of the aggregates. Once most of the cyclohexane has been squeezed out, further addition of water reduces n. Generally, wet TX-100 reverse micelles are larger than the dry ones, and the saline aggregates are in between in size. Dh generally increases with water content, but not monotonically. However,
2385
the size of all aggregates is reduced by increasing temperature. Acknowledgment. This material is based in part upon work supported by the National Science Foundation (Grant CHE8706345), the R. A. Welch Foundation, the Texas Advanced Research Program under Grant 1766, and Alcon Laboratories, Inc. Their support is gratefully acknowledged.
Experimental Determination of the Electron Affinities of Nitrobenzene, Nitrotoluenes, Pentafluoronitrobenzene,and Isotopic Nitrobenzenes and Azulenes E. C.M. Chen, E. S. Chen, M. S. Milligan, School of Natural and Applied Science, University of Houston, Clear Lake, Houston, Texas 77058
W. E,Wentworth,* and J. R.Wiley? Department of Chemistry, University of Houston, Houston, Texas 77204 (Received: June 10, 1991) The absolute electron affinities of nitrobenzene, m-, 0-,and p-nitrotoluene, pentafluoronitrobenzene, deuterated nitrobenzene, ISN-substitutednitrobenzene, azulene, and deuterated azulene have been determined by measuring the temperature dependence of the response of the electron capture detector (ECD). These are the first ECD determinations of the absolute electron affinities of these compounds and the only values of the electron affinities of the isotopically substituted compounds. The values for the substituted compounds do not differ from the unsubstituted compounds by more than the error. The best reproducibility for a single ECD determination is *0.02 eV. The ECD electron affinities differ from the literature values by less than the errors in the determinations. The electron affinities are as follows (eV): C6H5N02= 1.0 0.02, C6D5NO2 = 1.O f 0.03, C6H5l5NO2 = 0.99 f 0.03, C ~ F S N = O ~1.5 f 0.25, C,oHg = 0.69 f 0.04, C,oDg = 0.70 f 0.03, m-C7H,NO, = 0.98 0.03, o-C7H7N02= 0.89 + 0.03, and p-C7H7N02= 0.94 0.03.
*
*
*
Introduction The adiabatic electron affinity (EA) of organic molecules is an important fundamental property. Prior to 1960, there were no routine experimental methods for the determination of molecular electron affinities in the gas phase. This was emphasized by Mulliken in 1953 as follows: “Quantitative methods for determining molecular electron affinities are not very well developed, but there seem to be possibilities for the future.”’ It has taken over three decades for that prediction to come true. As recently as the mid-1980s there were fewer than a dozen values of molecular electron affinities which had passed the crucial test of being determined by more than one independent experimental procedure in the gas phase.* Presently, more than 200 molecular electron affinities have been reported, the majority of which have been determined by either the electron capture detector (ECD) method or the thermal charge transfer (TCT) methode3 However, only about seven molecules have been studied by both techniques (see Table I). In this paper, a distinction is made between organic molecules and organic radicals. Many electron affinities of organic radicals and small molecules such as O2 and SOz have been determined quite accurately by photodetachment and photoelectron ~pectroscopy.~ In addition the purpose of this paper is to present new ECD data for the subject compounds and to establish the experimental reproducibility of the ECD electron affinities determined using an internal standard. This will better establish the relationship between the ECD and TCT results. Another objective is to investigate the effect of isotopic substitution on the electron affinity. In the late 197Os, relative electron affinities of organic molecules were determined by using ICR mass spectrometry to measure the equilibrium constant for thermal charge-transfer reactions (TCT-ICR):4 AB-
+ CD
-
AB
+ CD-
‘Present address: University of Texas, Permian Basin, Odessa, TX 79762.
0022-365419212096-2385$03.00/0
In the mid-1 980s, the “high-pressure” (TCT-HPMS) pulsed electron beam technique was applied to this reaction and in addition the temperature dependence was determined so that enthalpy and entropy values were obtained. These relative values were referenced to the ECD value for benzophenone. Later, the TCT values were referenced to the electron affinity of SO2so that the TCT method and the ECD method can be independently ~ompared.~,~ The previously reported ECD values range from 0.15 f 0.05 eV for naphthalene2 to 0.88 f 0.04 eV for tetracene.’ The ECD procedure has been improved to allow measurement of higher electron affinities. We report the electron affinities for the nitrotoluenes C6F5N02,C6H5N02,C6D5NO2,and C6H5l5NO2and the azulenes CloHgand CLODs.Multiple determinations have been carried out for many compounds. The ECD results differ from the TCT results by less than the experimental error. The effect of isotopic substitution on the electron affinity is small. The best reproducibility that was realized in the multiple determinations is f0.02 eV. The ECD values are compared with recently determined half-wave reduction potentials* and photodetachment threshold^.^ The photodetachment thresholds give upper limits to the electron affinity while the half-wave reduction potential data can be used to estimate solvation energies for the anions. The photodetachment thresholds are all greater than the EAs obtained from the ECD. The combination of the ECD data with the (1) Mulliken, R. S.In Molecular Complexes; Mulliken, R. S., Person, W. B., Eds.; Wiley: New York, 1981; p 373. (Original reference: Proceedings of Int. Conference on Theoretical Physics, Kyoto and Tokyo, Sept 1953). (2) Chen, E. C. M.; Wentworth, W. E. Mol. Crysr. Liq. Crysr. 1989, 171, 271. (3) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G . J. Phys. Chem. Re$ Data 1988, 17. (4) Rains, L. J.; Moore, H. W.; McIver, R. J. J. Chem. Phys. 1978, 68, 3309. ( 5 ) Caldwell, G.;Kebarle, P. J. Chem. Phys. 1984, 80, 1. (6) Kebarle, P.; Chowdhury, S. Chem. Rev. 1987, 87, 513. (7) Lyons, L. E.; Morris, G. C.; Warren, L. J. J. Phys. Chem. 1968, 72, 3677. (8) Shalev, H.; Evans, D.H. J. Am. Chem. SOC.1989, 1 1 1 , 2667. (9) Mock, R. S.;Grimsrud, E. P. J. Am. Chem. SOC.1989, 111, 2861.
0 1992 American Chemical Society
Chen et al.
2386 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 TABLE I: Electron Affinities (eV) EA
molecule hexafluorobenzene
formula C6F6
carbon disulfide
oxygen nitric oxide sulfur hexafluoride nitromethane
ECD
TCT 0.52 f 0.1 1.0 > EA < 0.65 (ICR) 0.51 f 0.1
0.60 f 0.1
1.05 f 0.1 0.48 f 0.1
0.46 f 0.05 0.1 f 0.1 1.15 f 0.156 0.45 f 0.1
0.86 f 0.03
anthracene azulene nitrobenzene
1.01 i 0.1
>0.90 f 0.1
m-nitrotoluene
0.99 f 0.1
0.80 i 0.2
4-F-BP 4-CI-BP 1-naphthaldchyde
2-naphthaldeh yde
f 0.1 f 0.1
f 0.05 (ICR) f 0.1
0.66 f 0.05
f 0.05 (ICR) t 0.05 (ICR) f 0.1 f 0.1 f 0.1 f 0.1
0.80 f 0.1 0.68 f 0.03 0.62 f 0.03 0.56 f 0.05 0.66 f 0.01
“Original sources and data can be found in refs 2 and 3. ”etermined
half-wave reduction potentials demonstrate that the solvation energies of the anions of nitrobenzene and the nitrotoluenes are constant to fO.O1 eV for a given solvent.
Kinetic Model In the ECD, thermal electrons are generated at a constant rate, k,& and the electrons recombine with the positive ions, as follows:2.1+’5 Ar/CH4
+
-+ -
+B
k
~
5
e-
P+
.+
using the ECD/MS.
products of recombination are neutrals. The appropriate rate constants are indicated above the arrows. The electrons have a thermal distribution and hence react just as any other chemical species. The system of differential equations resulting from these reactions can be solved at “steady state” or at “low’ reaction times. The two approaches lead to the same expression for the electron capture coefficient, K4
(1)
(7) At steady state, the capture coefficient is calculated from the molar response according to the equation K = k d m O l awhile r at “low” reaction times, the equation is K = 2K,.,,Jtp The quantity Kmolar is the integrated molar response while the quantity kN is the pseudo-first-order rate constant for ion recombination; kN = kN’ [P’] . Two types of electron attachment have been observed in the ECD; nondissociative and dissociative. In the temperature range used in these studies, the test molecules all undergo nondissociative capture and k2 and k l z are zero, giving
kl
+ e- k-1 AB-
AB+eTk12-A+B-
-% A + Bk ‘N AB- or B- + P+ neutrals AB-
0.68 f 0.04 0.75 f 0.1 1 >0.7 0.4
kd
e- P+ neutrals (2) The reaction time, t , is defined by a periodic pulse which collects the electrons an8 allows the positive ions to build up. The rate equations for these reactions can be solved to give b = k&kD at steady state ( t , about 500 ps) and at “low” reaction times ( t , about 50 ps), to give b = k&, where b is the electron concentration and kD = kD’[P+].When the electron concentration is measured as a function of t , both kpRe and kD can be estimated. In the presence of a capturing species the following additional reactions take place: AB
0.49 f 0.1 1 0.4 f 0.2
0.63 i 0.05 0.64 i 0.05
0.69 0.62 0.69 0.64 0.74 0.85 0.70 0.65 0.60 0.69
biacetyl benzophenone (BP)
otheP 1.2 f 0.07 >1.8 0.895 f 0.02 0.90 f 0.3 0.50 f 0.2 1.06 f 0.06 1.0 f 0.2 0.440 f 0.005 0.024 f 0.01
-
(3) (4) (5)
(6)
The sample is designated as AB, the electrons e-, the positive ions P+,the radical A, and the dissociated anion B-, while the (10) Wentworth, W. E.; Chen, E. C. M. In Elecrron Capture; Zlatkis, A., Poole, C. F. Eds.; Elsevier: Amsterdam, 1981; Chapter 3, pp 27-68. ( 1 1 ) Chen, E. C. M.; Wentworth, W. E.; Lovelock, J. E. J . Phys. Chem. 1966, 70, 445. (12) Chen, E. C. M.; Wentworth, W. E. J . Phys. Chem. 1985,89,4099. (13) Chen, E. C. M.; Shuie, L. R.; Chen, E. S. D.; Wentworth, W. E. Manuscript in preparation. (14) Chen. E. C. M.: George, - R. D.; Wentworth, W. E. J . Chem. Phys. 19&, 49, 1973. (15) Chen. E. C. M.: Chen. E. S. D.: Wentworth. W. E. J . Phvs. Chem. 1991, 95, 520.
If k-, >> kN,then K = klkN/k-land the molecular electron affinity may be obtained since K = (AI/A-,)kNT3/*exp(EA/RT). If there are insufficient data in the “high-temperature” region where this expression applies, then a nonlinear least-squares procedure can be used to obtain the electron affinity from all of the available data. With the following assumptions and definitions: kN = AN is a constant, -EA = El* - E-,*, k, = exp(-E1*/RT), and k-, = A-IT exp(-E-,*/RT), the expression for K then becomes K=
A-lP/’{l
AIANexp(EA/RT) + & / ( L I T ) exp(E-,*/RT))
(9)
The experimental variables are K and T, and the parameters which are obtained from the data are AIAN/A-,,EA, ANIAI, and E-I*. Similar reactions occur for another capturing species, S,which can be used as an internal standard. The corresponding rate
Determination of the Electron Affinities constants are designated by k2s, kls, k I s ,and k N d . The corresponding rate constants for the test compound are designated as klAB, k-IAB, and k N A d . If ( k N s k2S) >> Lis, then the molar response Ks = kls = AlST1I2is the ECD response for S at all temperatures, The ratio of the response for two compounds is given by A ~ A B A N A Bexp@A/R TI R= (10) A-1 A d i s Z l 1 + ANAB/(A-~ABT)(~xP(E*-IAB/RT)) The value of R is simply the ratio of the molar responses. Using this expression for R, the experimental data are adjusted in a weighted nonlinear least-squares procedure to establish the electron affinity and the other three parameters. Alternatively, R can be converted to K by simply multiplying R by the known value of AIST1I2,which is simply the rate constant for thermal electron attachment for the reference molecule, kls.
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2387
+
Experimental Section The experimental procedure for obtaining thermal electron attachment data has been described in detail in previous artic l e ~ . The ~ *basic ~ ~experimental ~ ~ device is a gas chromatograph with an electron capture detector operated in the pulsed mode. The electron capture detector is an ionization cell where electrons are generated from ionization of the carrier gas by beta particles from a radioactive foil. The electrons are thermalized by collisions with methane. The electron concentration is measured by the application of a brief pulsed voltage. This creates a reaction zone in which thermal electrons and positive ions reach a steady value. The sample is introduced into the reaction zone via a gas chromatograph so that a highly purified sample is obtained. The electron concentration is measured alternatively in the presence of the sample, [e-], and with only the carrier gas, b. The decrease in the electron current in the presence of the capturing species, b - [e-], is the typical response of the electron capture detector. Finally, the temperature is changed and the procedure repeated. The integrated ECD response at each temperature is combined with the experimental flow rate and the moles of material injected to obtain a molar response. The electron capture coefficient, K , is then obtained by multiplying by kD at steady state or by multiplying by 2 / t , in the linear region. The detector used for the nitro compounds was a concentric electrode detector with a scandium tritide source. The data for the azulenes were obtained with a concentric electrode detector with a 63Nisource on an absolute basis using the temperature equilibration method. The specific column used is not critical except that it must be able to separate tne components and the bleed must not be detrimental to the operation of the ECD. A DB-5 bonded phase capillary column was used for the nitrocompounds. The azulene data were obtained on a 75 ft by 0.02 in. poly(pheny1 ether) column. The flow rate through the column was 1.5 mL/min, and the total flow through the detector was 75 mL/min of Ar/lO% methane. The carrier gas was passed through two traps to remove water (Big Three MT-200-24) and oxygen (Big Three, 0-T3-2). The solvent was heptane and was checked for electron-capturing impurities. All samples and standards were reagent grade, but this is not critical since the samples were purified as the measurements were being made. The sample was split 60/l at the inlet. The time between pulses was 500 ps and a 40-V pulse of I-ps duration was applied. The recent modifications in the experimental procedure are a more precise measurement of the temperature, the use of an internal standard, and the use of the four-parameter nonlinear least-squares data reduction procedure. Previously, the temperature in a metal block was allowed to come to equilibrium and the temperature of the detector was taken as the temperature of the block. In the current experiments, a small thermocouple is placed directly in the gas stream within the detector and the temperature measured after each injection. This eliminates the necessity of waiting for equilibrium so that more data can be obtained in a shorter time period. The use of an internal standard should improve precision. There are chromatographic conditions, such as the split ratio, flow rate,
t
t L
c
1.75
2.00
2.25
1OOO1T
Figure 1. In KT’I2vs 1000/T for CCI,, C7FI4,and C6H4Br2,
column temperature, and electrometer sensitivities, which vary. By using an internal standard, these variations will cancel. Since the relative response to the internal standard is used, the precise amount of sample injected can be adjusted so that the response can be measured accurately while remaining in the linear region of response. If a single solution is used for all of the temperatures, the exact molar ratio is not required to obtain the electron affinity. Using an internal standard also eliminates the need to measure instrumental parameters of the ECD since they cancel in the expression for the ratio. This is especially important in the steady state mode. The use of the nonlinear least-squares procedure for data reduction eliminates the subjective estimation of the linear region in the data. In addition, it gives quantitative estimates for the errors in the electron affinity and allows the determination of the electron affinity in cases where there are few data points in the linear region. Of course, the errors for these determinations will be larger than those where there are many points in the linear region. Results and Discussion The data for C7F14, p-C6H4Br2,and CC14 are given in Figure 1 as a plot of In (KT1l2)vs lOOO/T. The capture coefficient for the internal standards is equal to the rate constant for thermal electron attachment, kl, expressed as K = kl .= A 1 T 1 / 2 .The as can be seen in Figure 1. We response is proportional to studied CC14because its kl value is near the theoretical maximum value and dissociative electron attachment is sufficiently exothermic so that there is no activation energy. C7FI4was used as the internal standard for C6FSNO2,and C6H4BrZwas used for the other compounds. The capture coefficient is also equal to kl when (k2 + k ~ >> ) kl. For C6H4BrZand C7FI4molecular negative ions are formed at low temperatures but the molecular negative ion dissociates at high temperatures. Because the threshold for dissociative electron attachment is low, (k2 kN) >> kl a t all temperatures and the response is equal to kl. We have determined that C7FI4 forms a molecular negative ion at low temperatures but dissociates at temperatures above 500 K.I3 The activation energy for the rate constant for thermal electron attachment of C7FI4was found to be 0.07 eV in an early ECD study.I4 Studies of C7FI4in a flowing afterglow give an activation energy of 0.04 eVI6 for kl. The ECD behavior of C7Fl, and C6H4Br2is not unique since a similar behavior has been observed for SF6.I7
+
(16) Alge,
E.;Adams, N. G . ;Smith, D.J . Phys. B 1984, 17, 3827.
2388
Chen et al.
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992
40.0 Ln K T 3 / 2
m-C.H,(CH,)NO,
30.0
20.0
1
C I
1
~
I
I
I
1 I 1
1
1
1
1
l
l
l
l
l
~
t
1 l 1
2.0 2.5 IOOO/T Figure 2. In R vs 1000/Tfor C6H5NO2:R is the relative molar response. The different symbols are for different determinations.
.
p-C.H,FNO,
0
-C,H,FNO,
40.0 Ln KT3’2
tC
Ln
/
/
//
, ,
i i , , ,
1.00
,
~
r
2.00
#
j
t
i
3.00
,
,
t
,
~
,
,
,
lOOO/l
Figure 3. In KT312 vs 1000/T for C6F5NO2,C6D5NO2, C6H5NO2, C6H5I5NO2,and 0-, m-,and p-C6FH4NO2.
To illustrate the reproducibility of the experimental data for multiple determinations of C6HSNO2are plotted as In R vs 1000/T in Figure 2. The good fit is apparent from the curves calculated from the least-squares parameters. In Figure 3, we plot all of the absolute data for C6F5NO2,C6H5N02,C6D5NO2,and C6H5’5N02 and show the calculated response curves using the weighted average values of the least-squares parameters. Representative absolute values of the ECD response for c6H4(CH3)N02are shown as In (KT3/2)vs 1000/T in Figure 4. The absolute values were obtained by multiplying the relative values by the response of the internal standard. Shown in Figure 5 are ECD data for CIOH8and Cl,,Ds and C6H5N02which were obtained in 1969 on an absolute basis but have not been published. The average values of the electron affinities obtained from these data are given in Table I1 and are compared to half-wave reduction potential data and photodetachment thresholds. For convenience (17) Chen, E. C. M.; Shuie, L. R.;D’sa,E. D.; Batten, C. F.; Wentworth, W. E. J . Chem. Phys. 1988,88, 4711.
1.00 2.00 3.00 lOOO/T Figure 5. In KP12 vs 1000/T for azulene, deuterated azulene, and nitrobenzene.
in comparison, the recently determined electron affinities of the fluoronitrobenzenes are included in Table II.I5 The individual values of the electron affinities and the standard deviations, s ( l u ) , for the nitro compounds and the weighted averages and the standard deviation of the average are given in Table 111. The standard deviation of the average must be smaller than the smallest individual value of the standard deviation. If the u values for each of the determinations were the same, the combined standard deviation would decrease as the reciprocal of the square root of the number of determinations. The equations used to obtain these quantities are E 4 a v ) = CEAi/[siI2/Z1/[sil2 [s(av)12 = l / X 1 / [ s i l z
In 196 1, the electron affinity of nitrobenzene was found to be less than that of S02.18 In 1966, a lower limit of 0.8 f 0.2 eV (18) Henglein, A,; Muccini, G. A. J . Chem. Phys. 1959, 31, 1426.
The Journal of Physical Chemistry, Vol. 96,No. 5, 1992 2389
Determination of the Electron Affinities
TABLE II: Electron Affinities, Pbotodetachment Thresholds, and Half-Wave Reduction Potentials
EA, eV molecule
ECD"
V vs cobaltocenium/cobaltocene
TCTb
+
1.50 0.25 1.00 f 0.02 0.99 f 0.03 1.00 f 0.03 0.89 f 0.03 0.98 f 0.03 0.94f 0.03 >1.3 f 0.15 1.1 f 0.15
1.16 f 0.05 0.69 f 0.04 0.73 f 0.08 0.69 f 0.15 0.70 f 0.03
+
1.45 0.1 1.01 f 0.10 0.97 f 0.05 1.00f 0.05' 0.99 f 0.05' 0.92 f 0.10 0.89 f 0.05 0.99 f 0.10 0.93 f 0.05 0.95 + 0.10 0.91 f 0.05 1.23 f 0.1 1.18 f 0.1 1.15 f 0.05 1.07 f 0.1 1.02 f 0.1 1.04f 0.1 1.12 f 0.1 1.05 f 0.1 1.04 f 0.05 0.69 f 0.1 0.68 f 0.048
E~D'
E A ( E ~ I ~ ) ~ THF
DMF
AN
DMSO
0.250
0.211
0.163
0.276
1.18
(1 .OO)
1.24
1 .oo 0.99 0.89
0.483
0.360
0.212f 0.224 0.322
1.18
0.98
0.401
0.271
0.232
0.183
1.10
0.95
0.434
0.304
0.270
0.216
1.24
1.20
0.256
0.125
0.100
0.046
1.20
1.11
0.345
0.216
0.176
0.125
1.15
1.09
0.362
0.233
0.202
0.152
0.380
+
"This work see text. The fluoronitrobenzene results are taken from ref 19. bReferences 3 and 6. 'Reference 9. dEA(X) = EA(NB) E1,2(X) - EIl2(NB). Reference 8; THF, tetrahydrofuran; AN, acetonitrile; DMF, dimethylformamide; DMSO, dimethyl sulfoxide. /Reference 21. 8
Determined from temperature dependence of kl.22
TABLE III: Electron Affinities and Standard Deviations
EA, eV compound
ECD
0.94 f 0.03 1.01 f 0.05 1.03f 0.02 C6H5N02 0.99 f 0.03 1.00f 0.06 1.01 f 0.23 C~HS"NO~ 0.99 f 0.05 0.98 f 0.05 0.95 f 0.04 m-CH3c6H4No2 0.97 f 0.03 P-CHjC6H4NO2 0.96 f 0.03 0.95 0.04 0.88 f 0.03 o-CH&HdNO2 0.84 f 0.95 C6DSN02
+
measd
av, eV
0.96 f 0.05 1.02 f 0.05 1.00f 0.02 1.04f 0.04 0.97 f 0.04 1.00f 0.03 1.01 f 0.05 1.00 f 0.21 1.01 f 0.03 0.99 f 0.05 0.91 f 0.04
0.99 f 0.03 0.98 f 0.03 0.94 f 0.03
0.91 f 0.03 0.89 f 0.03
was obtained for the EA of nitrobenzene from ECD data.I9 This was later revised to 0.98 f 0.18 eV by applying the fcur-parameter nonlinear least-squares procedure to the ECD data shown in Figure 4. This illustrates the value of the data reduction procedure. The errors in the TCT-ICR values are quoted as f0.05 eV, and the TCT-HPMS values have an error of fO.10eV. Within these l i i t s , there is complete agreement between the values determined with the ECD and those obtained with the TCT as seen in Table 11. Indeed, the average deviation between the ECD and the TCT values is only f0.03 eV and is much better than the quoted error. The ECD electron affinities are all less than the photodetachment thresholds as anti~ipated.~ The individual results are shown in Table 111. The solvation energy of the anions as determined from the EA and the El12data are constant for the nitrobenzenes and the nitrotoluenes. This does not imply that the solvation energies are constant for all nitrobenzenes. Assuming constant solvation energies the electron affinities can be determined from the half-wave reduction potentials by using the equation EA(* = EA(nitrobenzene) + El 2(X)- E1,2(nitrobenzene). The electron affinities calculated in tfiis manner are given in Table 11. The values are independent of the solvent and are all consistent with the ECD and TCT values.8 (19) Chen, E. C. M.; Wentworth, W. E. J . Chem. Phys. 1975, 63, 3183.
The rate constants for thermal electron attachment for c6H5NO2, C6D5NO2,and C6Hs15N02differ by less than the experimental error.20 The electron affinities of these compounds determined with the ECD also differ from the TCT values by less than the experimental error. Using TCT-ICR?I the equilibrium constant for the reaction of C6HsN02- with C6DsNO2 k 0.7 while that for the reaction with C6H51SN02 is 1. This implies that the EA of C6D5NO2is 0.01 eV less than that of C6H5NO2and that the EA of C6Hs15N02 is the same as that for C6HSNo2.The TCT measurements can be used to obtain accurate relative electron affinities, but absolute values can be obtained only by reference to a known EA. Using the ECD value of 1.00 f 0.02 eV for C6H5N02,we obtain the TCT values for C6DSNOzand C6H5I5NO2given in Table 11. The ECD values differ from the TCT-ICR values by less than the errors. Half-wave reduction potentials have been measured for C6DSNo2and C6H51sN02.22 Assuming constant solvation energy differences, the EAs can be calculated and are included in Table 11. The values are consistent with the values of the electron affinities of the isotopically substituted nitrobenzene determined from ECD and TCT data. The parent negative ions of the nitro compounds have been observed in API mass spectrometry experiments at 4739 and at 523 K in chemical ionization experiment^.^^ The lowest energy dissociative process for the nitro compounds is the formation of R' + NOT, which is endothermic by about 0.9 eV. There are no reactions other than detachment, dissociation, and recombination which are fast enough to be competitive in an electron capture detector. Of these reactions, only the detachment reaction will release electrons and hence lower the molar response of the ECD as temperature increases. The good agreement between the electron affinities obtained by the different techniques is evidence that detachment is the predominant process and is responsible for the decrease in the response of the ECD with increasing temperature. The electron affinity of C6F5N02 is now the maximum value which has been determined from the temperature dependence of (20) Zlatkis, A,; Wentworth, W. E.; Ranatunga, R.; Chen, E. C. M.; Milligan, M. S. Chromatographia 1990. (21) Stevenson, G. R.; Reiter, R. C.; Espe, M. E.; Bartmess, J. E. J . Am. Chem. SOC.1987, 109, 3847. ( 2 2 ) Goodnow, T. T.; Kaifer, A. E. J. Phys. Chem. 1990. 94, 7682. (23) Stemmler, E. A,; Hites, R. A. Electron Capture Negative Ion; VCH Publishers: New York, 1988.
2390 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 ECD data. The curve can be used as an upper bound as shown in Figures 3-5. The standard deviation in this value, f0.25 eV, is larger because of the limited data in the linear region. The ECD value is fortuitously only 0.05 eV greater than the TCT value.24 The dissociative reaction involving R and NO2- is observed for the fluoronitrobenzenes because of the large value of the Arrhenius preexponential term AAlhfor the rate constant for dissociation. The ECD response for the para and ortho isomers increases at higher temperatures due to dissociation. This illustrated graphically in Figure 3 where the calculated ECD response for m-,0-, and p-C6H4FN02are plotted. At temperatures around 475 K the capture coefficient decreases. At about 550 K, the response for the ortho isomer increases. Finally at the highest temperature, the K value for the para isomer increases perceptively. The capture coefficients of the nitrobenzenes and the nitrotoluenes do not increase at higher temperatures. Assuming that the bond dissociation energy of the negative ions are approximately the same, the value of the Arrhenius preexponential term of the rate constant for dissociation is less than that determined for p-C6H4FNO2, which is 2 X 10l8 s-l. Two sets of ECD data have been published for azulene. In 1966, we reported a value of 0.59 f 0.07 eV for the EA for azulene using a linear least-squares line through the “high-temperature” data. We also used a “fured” intercept and the “high-temperature” data to obtain a value of 0.66 eV for the electron affinity. Using the four-parameter least-squares adjustment to all of the data, a value of 0.73 f 0.08 eV is obtained. In 1981, another laboratory reported a value of 0.52 eV using the linear least-squares fit to the “high-temperature” data and a value of 0.6 eV using the “fixed” i n t e r ~ e p t . ~We ~ have again used the nonlinear leastsquares procedure to obtain the EA and find a value of 0.69 f 0.15 eV. In 1969, we measured ECD responses and obtained the values 0.69 f 0.04 eV for CloHs and 0.70 f 0.03 eV for CIoD8 using the nonlinear least-squares data analysis. Our data and the curves calculated from the least-squares parameters are shown in Figure 5. Azulene is important because electron affinities have been obtained from HPMS-TCT data in two different manners, one based on the determination of the equilibrium constant for charge transfer as a function of temperature and the other the determination of the temperature dependence of the rate constant for detachment. The ECD values agree with the TCT values as seen in Table 11. In addition, the temperature dependence of the detachment rate constant has been determined in an electron capture detector photodetachment, ECD-PD, device. The activation energy for detachment is consistent with the electron affinity.26 The electron affinities for azulene and the deuterated azulene differ by less than the experimental error. In addition the different ECD values of the electron affinities differ from the TCT values by less than the experimental error. Finally, the other values differ (24) Dillow, G. W.; Kebarle, P. J . Am. Chem. Soc. 1989, 111, 5592. (25) Wojnarovits, L.; Foldiak, G.J . Chromatogr. 1981, 206, 511. (26) Mock,R. S.; Grimsrud, E. P. Int. J . Mass. Spectrom. Ion Phys. 1989, 94, 293.
Chen et al. from the ECD values by less than the experimental error. Thus there are six independent determinations of the electron affinity, four from ECD measurements and two by HPMS which do not differ by more than the experimental error. As seen in Table I, there are two TCT values which do differ from literature values, by more than the experimental error. The ECD value for the electron affinity of C6F6 is 0.33 eV higher than the TCT value. The other molecule is CS2 where the ECD and the TCT values are 0.3 eV lower than the photoelectron spectroscopy value of 0.895 f 0.02.3 We have postulated that these differences can be explained by excited states of the negative ions2 The best reproducibility of the ECD values is f0.02 eV. On this basis, the differences in the EAs of the isotopically labeled compounds must be less than this in order to be measured in the ECD. In general, the largest difference in EAs of isotopically substituted compound3 is less than 0.02 eV, so that the results of this study are consistent with the literature values.
Conclusions This paper shows that the absolute electron affinities of the subject compounds determined with the ECD and the absolute EAs obtained from the relative TCT values calibrated to the electron affinity of SO2agree with an average deviation of f0.03 eV. The good agreement between the values of the other electron affinities determined in the ECD and in the TCT experiments is a confirmation of the validity of both methods. There are now about 20 molecular electron affinities determined with the ECD which have been confirmed by an independent experimental procedure (Tables I and 11). In about half of these cases the ECD values were published first. However, there is one compound, C6F6, where the ECD value and the TCT value differ by more than the quoted error. The reproducibility of the ECD measurements is approximately f0.03 eV for the compounds reported in this study. The electron affinities of C6D5NO2,C6HSNO2,and C6H515N02 do not differ by more than the precision of the ECD method. The electron affinities of azulene and deuterated azulene do not differ by more than the ECD reproducibility. If the ECD electron affinities are combined with literature values of half-wave reduction potential, the solvation energies of nitrobenzene and the nitrotoluenes in a given solvent are constant. The differences in half-wave reduction potentials in five different solvents are equal to the electron affinity differences with an average deviation of less than fO.O1 eV. By comparison with the data for the fluoronitrobenzenes, it can be stated that the Arrhenius preexponential terms for the dissociation of the anion of the nitrobenzenes and nitrotoluenes are less than 2 X lo’* s-I. Acknowledgment. This work was supported by the Robert A. Welch Foundation, Grant E095, and a University of Houston-Clear Lake Chemistry Departmental grant. Registry No. ISN, 14390-96-6; D1, 7782-39-0 PhN02, 98-95-3; C,D5N02, 4165-60-0; PhlSN02, 368 1-79-6; C6F6.880-78-4; azulene, 275-51-4; perdeuterioazulene, 727-60-6; m-nitrotoluene, 99-08-1; onitrotoluene, 88-72-2; p-nitrotoluene, 99-99-0.
