88
J . Phys. Chem. 1994, 98, 88-94
Determination of the Electron Affinities of Molecules Using Negative Ion Mass Spectrometry E. C. M. Chen School of Natural and Applied Sciences, University of Houston Clear Lake, Houston, Texas 77058
J. R. Wiley Division of Science and Engineering, University of Texas, Permian Basin, Odessa, Texas 79762
C. F. Batten and W. E. Wentworth' Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received: June 1 , 1993; In Final Form: October 18, 1993'
The electron affinities of sulfur hexafluoride, nitrobenzene, two chloroethylenes, and hexafluorobenzene have been obtained by analyzing the temperature dependence of thermal electron attachment negative ion mass spectra. The electron affinities are as follows: SF6, 1.07 f 0.07 eV; CaHsN02, 1.0 f 0.06 eV; C6F6, 0.83 f 0.2 eV; C2C14,0.64 f 0.03 eV; CzHC4,0.40 f 0.22 eV. The values support the literature values for SF6 and C6HsN02 and the chloroethylenes. On the basis of these results, an experimental procedure for measuring molecular electron affinities is presented.
Introduction The electron affinity (EA) is the energy released when a negative ion is formed by reaction with thermal electrons in the gas phase at 0 K. It is one of the fundamental properties of significanceto all electron-transfer reactions. An absolute value for this quantity can be obtained by measuring the temperature dependence of the equilibrium constant, K,, for the reaction
+
M e-- Mwhere M is any reactant: atoms, radicals, molecules, or clusters. If the electron is considered the same as any other reactant and has a thermal distribution, then this is analogous to the data required to obtain the energetics of any reaction. Typically, an appropriate log Kqvs reciprocal temperature plot is used to obtain theenthalpy of thereactionat the temperatureof the experiment. The enthalpy is then corrected to a standard temperature using the appropriate heat capacities. In the case of the reactions with an electron, the equation for the equilibrium constant can be simplified and the energy change at 0 K determined directly. If the partition functions of the negative ion and the neutral are the same except for the spin degeneracy, and the spin degeneracy of the negative ion is canceled by that of the electron, then the temperature dependence is In Kq = 12.43- 3 1 2 ln(T) - E A / R T (see kinetic model), and the electron affinity is obtained directly from the experimental data. This is equivalent to assuming that the temperature dependence of the heat capacities of the negative ion and the neutral are the same and that the heat capacity of the electron is equal to '/2R. If the partition functions do not cancel, then an additional temperature correction would be necessary. In order to apply this technique, four things are necessary: (1) a source of thermal electrons, (2) a source of a constant or measurable concentration of the species under investigation, and (3) a way of measuring the concentrations of the ions and/or electrons, and (4)a way to measure temperature in the reaction zone. In the 1930s the electron affinities of the halogen atoms were determined by equilibrium measurements. In 1953 these were ~
*Abstract published in Advance ACS Absrraas, December 15, 1993.
0022-3654/94/2098-0088~04.50/0
the only reliable values reported in a review of electron affinities by Pritchard.' In the 1960s two thermodynamic procedures, the magnetron method2 and the ECD method? were applied to the determination of the electron affinities of complex organic molecules. Other procedures for the determination of electron affinities have been summarized in a collection of experimental values of electron affinitie~.~ Spectroscopicmethods based upon photodetachment and photoelectron spectroscopy have produced the most precise values of electron affinities of atoms, small molecules, and radicals. Relative molecular electron affinities have been determined by using high-pressure mass spectrometric measurement of equilibrium constants for thermal electrontransfer reaction^.^ The major advantage of the mass spectrometric measurements is the identificationof the electron-transfer product. However, the electron affinity of the reference compound must be known in order to obtain absolute values. The purpose of this article is to describe a new thermodynamic procedure for obtaining absolute molecular electron affinities. This method uses negative ion mass spectrometry to measure the equilibrium constant for the reaction with thermal electrons as a function of temperature. The procedure described in this paper has the advantage of ion identification while giving an absolute value for the electron affinity. The abundance of a parent negative ion under the conditions of our experimentscan be related to the appropriate rate constants through a kinetic model. When the concentrations of electrons and neutral are maintained constant, the abundance of the parent negative ion is proportional to the equilibrium constant at temperatures where the detachment rate constant predominates over the rate constant for other losses. This condition normally occurs at the higher temperatures of the experiment and is of primary importance to the present work. The first part of the discussion deals with the kinetic model and the verification of the above assumption. Next we show how our data agree with the kinetic model by using a nonlinear least-squares procedure to fit the data to an equation involving activation energies, E I and E-,, and preexponential terms, A1 and&, for rate constants of thermal electron attachment and detachment. The activation energy for thermal 0 1994 American Chemical Society
Electron Affinities of Molecules
The Journal of Physical Chemistry, Vol. 98, No. 1, 1994 89
electron attachment and the electron affinity of the molecules are then compared to values from the literature. We use literature values for the rate constant for thermal electron attachment to SF69 to calculate the absolute response and consequently obtain of the ratio AlIA-1. This ratio can be calculated from fundamental constants if certain assumptions are made concerning the partition functions of the neutral molecule and the negative ion. The agreement of the experimental values with that calculated from fundamental constants further supports the kinetic model and the assumptions concerning the temperature dependence of the equilibrium constant. Data for the chloroethylenes and one set of data for SF6 were collected as a part of a series of experiments designed to study dissociative electron attachment using a commercial chemical ionization source operated at a pressure of about 1 Torr.6 We reported these data previously as the ratio of the parent negative ion to the dissociated species and demonstrated that an excited state of the parent negative ion was involved in the thermal electron attachment reaction. This article was prompted by a reviewer who suggested that we examine the temperature dependence of the parent negative ions rather than the ratio. Theother SF6data were taken with a63Ni atmospheric pressure ionization source and were also published as ion ratio^.^ The C6F.5 and C6HSN02 data were taken to characterize the atmospheric pressure ionization source, about 10 years ago, but have not been previously reported. The agreement of the values for electron affinities obtained from these data obtained at a different time using a different apparatus with the most recent literature values for SF649s37and C&,N024-s’8 convinced us of the utility of the experimental method which we postulate in this paper.
peak divided by the amount injected. This assumes that the instantaneous negative ion concentration is proportional to the concentration of the neutral. The ionic reactions are fast relative to the elution time of the chromatographic peak since the slowest kinetic process is on theorder of 100p s while the chromatographic peaks elute over a period of tens of seconds. The integration over the peak is justified if the ion abundance [AB-] is proportional to the concentration of the analyte, a ([AB-] = ka; see eq 7), being eluted from the chromatographic column. This will be true when the extent of electron capture is low. The relationship between the extent of capture and concentration of analyte has been thoroughly investigated in studies using the electron capture detector.8J4 The ion concentration is equal to the concentration of electrons captured. The extent or fraction of capture is linear with concentration with a 5% deviation at a 5% capture of electrons. The integration of the ion response over the chromatographic peak is actually an integration over the volume of gas flowing during the time of elution. J[AB-] d V = kJa d V = knAs where the integration over the analyte concentration gives the moles of analyte injected in the gas chromatographic column which is measured. For this reason, the ratio of the integrated ion intensity is proportional to the constant, k, which we show is related to kinetic rate constants in the next section. The advantage of integration over the peak is that variations in the peak width are taken into account. An alternative method of data reduction would utilize the ion abundance at the peak rather than the integrated abundance. This would assume that the peak width remains constant. The presence of SF6 in the sample gives a check on the assumptions in the data reduction since the response is temperature independent a t temperatures less than 500 K, as has been observed in many different experiments.9 Details of the experimental apparatus used to obtain the atmospheric pressure ionization spectra have been gi~en.~JO Briefly, the mass spectrometer is an Extranuclear Spectra EL quadrupole mass spectrometer fitted with a custom atmospheric pressure ionization source. Mass spectra were taken after injection of fixed amounts of SF6 into a gas chromatograph or were taken a t a constant concentration of CaF6 and C ~ H S N established O~ by bleeding through a Teflon permeator held at constant temperature. The temperature of the detector was then changed, and mass spectra were taken again. The temperature was measured with a thermocouple in the ionization source block. The comments regarding the potential errors in temperature also apply to these data. The carrier gas was a mixture of argon and methane. Data for SF6were obtained using computer control as described previ~usly.~ The data for C6F6 and C ~ H S N were O ~ obtained with a manual scan and were recorded on an X-Y recorder. A full scan over a mass range of 4-400 was carried out initially at selected temperatures. The data were taken by scanning a 2 amu mass range about the molecular ions 123 or 186. The ion abundancies were obtained from peak heights.
Experimental Section Low-pressurechemical ionization mass spectra were taken with a Hewlett Packard 5988A GC-MS equipped for negativechemical ionization as described previously.6 The cooling gas was C02, dried and maintained at a pressure of 0.85 f 0.05 Torr in the ion source. The pressure was adjusted at each temperature and read using the thermocouple gauge on the ionization gauge controller. The instrument was tuned using m / z values of 35, 137, and 452 and their isotopic masses. The tuning was optimized for the low mass range at 200 OC and was kept constant throughout the temperature variations. However, tuning response profiles (response versus tuning parameters) were checked to verify that the shapes did not change with temperature. A gas chromatograph operated in the split injection mode with a fused silica capillary column was used to provide in line purification of the samples. Two microliter injections containing about 0.5 pmol of C2HCl3, 0.2 pmol of C2C14, and SF6 as a reference were made from a single mixture containing these compounds diluted in hexane. Three well resolved peaks at about 0.7, 2, and 4 min were observed in the chromatogram for this mixture with no extraneous peaks or interferences. Although the source temperature was set by a computer workstation, temperature measurement is a potential source of error because the thermocouple is mounted away from the ion source. The measured temperature could be proportional to the temperature in the gas phase, and this proportionality constant could be a function of temperature, This variation was minimized by independently maintaining a constant total pressure in the ion source. The lowest temperature was obtained by allowing the system toequilibrate overnight. Temperatures were thenchanged and allowed to equilibrate for 45-60 min. Mass spectra were obtained via the computer workstation. The parent ion abundancies were calculated using integrated sums of all isotopic masses of the ions and subtracting the background intensities. The actual experimental quantity which is reported is the ion intensity integrated over the chromatographic
Kinetic Model The reactions which can take place subsequent to reaction of a molecule AB with a thermal electron are given below. In some cases, more than one bound excited state of the negative ion may be involved, but for simplicity only one bound negative ion state, AB-*, is indicated.
90 The Journal of Physical Chemistry, Vol. 98, No. I, 1994 AB- or Band/or
-
+ P+
kN'
neutrals
(3a)
.4"
AB - o r Blosses (3b) The steady-state treatment for the various anions leads to the expression
Thequantity a is the instantaneousconcentrationof thecompound k, = k,'[~+]
+ kdiRwion= a constant
and k-l*
(7) For these compounds, k2 is shown by experiment to be small relative to k-1 in the temperature region of the experiment6f.11 and (8) Kex = k,/(k-l + kN) Since kN is temperature independent and k-1 increases exponentially with temperature, at low temperatures, k-l > kN so that
preexponential term and an electron affinity of 0.8 eV has a value of 1.0 s-1 at 300 K and a value of lo7 s-1 at 600 K. By substituting the relationships kN = AN, kl = A 1 T 1 J 2e x p (-El/Rl"). and k-1 = A-1T exp(-E-I/RT), eq 8 becomes [AB-]
A , T ' I Z exp(-El/RT)
--I
a[e-]
A_, T exp(-E-,/RT)
+ AN
(11)
If the negative ion concentration is measured at different temperatures at a measurable concentration of analyte and a constant concentration of [e-], then eq 12 has four parameters, (A-~IAN), (AI/&), El, and E-I, and twovariables, [AB-]/a[e-] and T. A nonlinear least-squares procedure can be used to obtain the parameters, which give the electron affinity of the molecule (EA = -E1 E-I). The other important quantity which can be obtained from these parameters is the ratio AI/&, which can be related to the equilibrium constant for reaction 1 as follows:
+
+ k2* + k, >> kN
The second assumption implies that any concentration of excitedstate negative ions is small relative to that of the dissociated species and the ground-state parent negative ion. It would be easier not to include the intermediate excited state of the parent negative ion for the purposes of this paper. However, the temperature dependence of the dissociated species clearly demonstrates the existence of an excited state of the parent negative ion for the chloroethylenes and for SF6.6.7 Therefore, we include an excited state as a general case and neglect its contribution to the measured parent negative ion abundance for the compounds at the higher temperatures of major concern to this work. At low temperatures, there may be a finite concentration of the excited species for SF6 and the chloroethylenes. This is reasonable since under the conditions of the experiment, the intermediate negative ion is stabilized to the ground state, dissociates, or undergoes detachment at high temperatures. At 1 Torr and 300 K, the collision frequency is about lo7 s-I and a factor of 1000 higher at 1 atm. Under these conditions, eq 4 becomes
Kc,
Chen et al.
(9)
(10) Kcx= kl/k-l = Kq A plot of ln[AB-] vs 1 / T should be relatively constant at low temperatures and should decrease at high temperatures with a slope determined by the electron affinity of the molecule (see below, EA = -El + E-1). All of the data for AB- do indeed have this general structure. The above kinetic model is exactly the same as that used for the determination of electron affinities of molecules from the electron capture detector. From ECD data,14 it has been determined that kN = 103-104 s-I and is relatively temperature independent. It has also been determined that the rate constant for detachment for a compound with a nominal
The partition function of the negative ion (g[AB-1) is assumed equal to that of theneutral (g[AB]), except for thespindegeneracy term. That term is canceled by the degeneracy term of the electron. Substituting for the translational partition function of the electron, the equilibrium constant becomes
where h and k are the Planck and Boltzman constants, reapectively, and me is the mass of the electron. By comparison with eq 12 at high temperatures,
and ln(AI/A-1) = 12.43 if the concentrations are in units of mol/ L.
Results and Discussion The experimental data are given as ln([AB-]so vs 1 / T where [AB-] is the measured abundance of the parent negative ion and sf is a scale factor described below. The low-pressure data for SF6 and the chloroethylenes are plotted in Figure 1, and the atmospheric pressure data for SF6,C~HSNOZ, and C6F6 are plotted in Figure 2. Two independent sets of API data for SF6 have been presented. For the chloroethylenes, C6F6, and SF6, no extraneous peaks were observed. Above 350 OC, peaks at 107 (M - 16) and 46 (NOz-) and, below 110 OC, dimer (246) and trimer (369) peaks were observed for CsHsN02. The extreme temperatures were not used in the data reduction. The lines that are shown have been obtained using the parameters established by the nonlinear least-squares method using eq 12. The good fit is apparent. The least-squares parameters are given in Table 1 with their associated errors. The absolute values of Ke, were obtained by reference to the rate constant for thermal electron attachment to SF6 at 298 K which is well established as 2 X lOI4 L/(mol s ) and ~ values of AN = lo3 s-1 for the API data and 104 s-1 for the low-pressure data. Values of ANof this order of magnitude have been obtained in ECD ~ t u d i e s .The ~ higher value for the low-pressure data is reasonableconsidering the influence of diffusion losses. The scale factor isdefined by sf = kI/(AN[SF6-])where [SF6-] isinarbitrary
The Journal of Physical Chemistry, Vol. 98, No.
Electron Affinities of Molecules In{[AB-]sf)
L
/
2.0
1.5
2.5
looo/r [ OK -11 Figure 1. Plots of In( [AB-]sf) versus 1000/Tfor the parent negative ions of SF6, CzHCI,, and CzCI,. See text for the definition of sf. The curves have been drawn using the least-squares parameters in Table 1. The measurements were taken at about 1 Torr with C02 as a cooling gas. Ln{ [AB-]sf}
I
25-0
i t
/
i: / / 15.0
1.5
2.0
2.5
lOOO/T’ [ OK -11
Figure 2. Plots of In{[AB-]sf)versus 1000/Tfor the parent negative ions Of SF6, C&N02, and C.5F.5. Sce text for the definition of sf. The curves have been drawn using the least-squares parameters in Table 1 . The measurements were taken at about 1 atm with CH4 as a cooling gas.
TABLE 1: Lerwt-Squares Parameters Used To Calculate the Curves in Figures 1 and 2 compound EI,eV In(A,/AN) SFs 0.04 f 0.02 29.9 f 0.3 C&N02 CsFs CzClr C2CI3H
4.01t0.02 0.01 f 0.01 4 . 0 2 f 0.01 0.02 0.03 4 - 1 0f 0.02 0.23 0.30
* *
28.2f0.3 27.0 f 0.3 27.4 f 0.4 29.3 f 1.1 21.3 f 0.7 27.2 f 9.1
E-I, eV
ln(A-I/AN)
1.02 f 0.10 1.09f0.11 1.10 f 0.21 0.98 f 0.06 0.85 f 0.21 0.54 f 0.03 0.62 f 0.15
17.2 f 2.0 15.7f2.3 14.7 f 4.0 14.7 f 1.0 18.8 f 5.9 7.8 f 0.4 13.4 f 4.0
units and kl and AN are as given above. The same scale factor is then used for all the data. The scale factor does not affect the magnitude of the electron affinities or the errors in the parameters
I, 1994 91
but does affect the absolute magnitude of the ratios Al/ANand A-IIAN. The slope of the data in the high-temperature region determines the electron affinity and is independent of the scale factor since ln(sf) is added to each value of ln[AB-1. The electron affinities and values of ln(A1/A-l) obtained from the parameters are given in Table 2. The errors have been calculated using the variances and covariances obtained from the least-squares analysis and represent one standard deviation. The larger errors in the values for one set of SF6 data and the C2HC13 and C6F6 data are due to the limited amount of data in the two regions. All of the intercept values are consistent with the value of 12.43 calculated from the fundamental constants within the experimental error. The electron affinities are compared with other literature values and, except for C6F6, do not differ from the most recent literature values by more than one standard deviation. In 1961,12theelectron affinityof nitrobenzenewas determined to be greater than that for SO2 (1.1 eV)? In 1973, a lower limit of 0.7 eV was obtained from endothermic charge-transfer measurement^.^' A lower limit of 0.8 eV was alsoobtained from ECD data in 1966.14 The electron affinity was measured using thermal charge-transfer experiments in both high-pressurepulsed experiments15 and ICR experiments.16 The most precise value of the electron affinity was obtained from multiple determinations of the temperature dependence of the ECD.8 The value for nitrobenzeneobtainedin this study agrees with all of the literature estimates. The value for nitrobenzene is especially important because of the precision in the absolute value and the agreement between the determinations which have been reported. It can be used as a general test compound for the current method. The activation energy for the rate constant for thermal electron attachment to nitrobenzene is small. This is in agreement with the results obtained using the ECD.8 An extensive list of the experimental values of the electron affinities of SF6 that have been reported are given in refs 4 and 7. The values range from 0.3 to 1.4 eV. The first value that was reported was 1.4 f 0.4 eV obtained from magnetron data in 1964. In the mid- 1970s a number of collisional ionization studies bracketed theelectronaffinity between 0.3 and 0.75 eV.4*7J7These are now considered to be low and possibly to refer to an excited state. Drazic and Braumanla noted the inconsistency of the alkalimetal beam (AMB) value with the experimental lifetimes. In 1983, Heneghan and Bensonlg reported a value of 1.39 f 0.13 eV based on the calculation of the preexponential term for the rate constant for electron detachment and ion lifetimes. Also in 1983, Lifshitz reported a value of 0.8 eV for the electron affinity based on ion lifetimes.20 These values require a model for the negative ions and an estimation of the preexponential term for detachment and are not strictly experimental values. The first recent experimental value of the electron affinity of SF6, 1.Of 0.2 eV, was reported by Streit from bracketing chargetransfer reactions.21 In 1985, a more precisevalue of 1.05 0.1 eV was obtained from bracketing charge-transfer reactions.’ In 1988,wereportedavalueof 1.15f0.15eVfromthe temperature dependence of the ion ratios measured in the API source.7 In order to obtain this value, an independent measure of the threshold for the formation of SFs- from SF6 was required. The present value uses that same data for the parent negative ion but does not require the auxiliary data. All three of the values reported in Table 2 overlap the experimental values reported since 1984 at the one standard deviation level. The values of El for SF6 determined in this study are consistent with the value of 0.028 eV determined in ECD studies.7 There is disagreement in the values for the electron affinity of C6F6 determined using the TCT method (0.53 h 0.1 C V ) and ~ the value obtained using the ECD method (0.86 0.03 eV).3 We have attributed this difference in terms of an excited state for C6F6-. The ECD value corresponds to the ground state of the
*
*
92
Chen et al.
The Journal of Physical Chemistry, Vol. 98, No. 1 , 1994
TABLE 2 Kinetic and Thermodynamic Properties for Thermal Electron Attachment Reactions' compound EA(this work), eV M A I lA-1) EA(lit.), eV EA(EIIz),eV 1.06 0.10 12.7 i 2.0 1.4 i 0.4 (MGT-2) Sh 12.5 i 2.3 1.05 0.1 (TCT-5) 1.07+0.11 1.10 0.22 12.3 i 4.4 1.0 i 0.2 (KIN-21) 0.54 i 0.2 (AMB-17) 0.8 (lifetimes-20) 1.4 0.1 (lifetimes-19) 12.8 1.2 1.OO O.OZ(ECD-8) 1.O 0.06 1.0 i 0.1 1.01 i O.lO(TCT-4) 0.96 O.ll(TCT-15) 0.97 i O.OS(TCT-16) >0.70 (EnCT-13) >1.1 (KIN-12) >0.8 (ECD-14) c6F6 0.83 t 0.22 10.5 5.9 0.86 i 0.03(ECD-l1) 0.52 i 0.1 (TCT-5) 1.2 i 0.1 (MGT-2) 1.8 i 0.3 (EnCT-13) c2c4 0.64 0.03 13.5 i 0.7 0.65 i 0.1 0.39 0.1 CzCllH 0.40 i 0.22 13.8 i 11.5 Abbreviations: magnetron (MGT), thermal charge transfer (TCT), electron capture detector (ECD), half-wave reduction potential ( E l / 2 ) ,kinetic bracketing (KIN), endothermic charge transfer (EnCT).
*
*
*
*
* *
*
*
negative ion while the TCT value corresponds to an excited state. In addition, there are two values of the electron affinity which are higher than the ECD value. The magnetron value (1.2 f 0.07 eV)2 has an error which is smaller than many encountered in magnetron values. The value of 1.8 f 0.3 eVI3obtained from the endothermiccharge-transfer studies is probably too high since it is larger than the measured electron affinity of pentafluoronitrobenzene.538 The present value (0.83 f 0.22 eV) is in agreement with the ECD value and does overlap the TCT value if the mutual errors are considered. The value of the activation energy for thermal electron attachment for C6F6 is approximately zero as determined in this study and in ECD studies." The literature values for the electron affinities of the chloroethylenes were obtained from half-wave reduction potentials.22 The present values are the only absolute gas-phase determinations ofthesequantitiesandagreewith theliteraturevalues. Ingeneral, rate constants for primary processes may have small negative activation energies but do not have large negative activation energies. The activation energy for C2Cl4 is negative by about 0.1 eV, which could indicate that an excited state may be involved in the low-temperature data. In the case of the chloroethylenes, the existence of an excited negative ion state has also been demonstrated from the ion ratios.6 The values of A I / A Nand A - I / A Nare lower for C2Cl4 than for the other compounds. This could be due to a steric term for electron attachment as evidenced by the negative activation energy for electron attachment or the fact 'that an excited state contributes to the low-temperature process. It must be emphasized that the electron affinity is determined from the data in the high-temperature region and is independent of the specific values of the intercept and the values of El and E-1 that are obtained from the least-squares data analysis. The agreement of the values of the experimental electron affinities with literature values as described above is taken as evidence supporting the potential of a mass spectrometric method of measuring electron affinitiesfrom the temperature dependence of theparent negativeions. The proposed methodcan becompared with the accepted methods which have produced molecular electron affinities. Presently, more than 200 molecular electron affinities have been reported, the majority of which have been determined by the ECD or the TCT method. The majority of the others have been determined using the alkali-metal beam (AMB) method or the magnetron method. In this paper a distinction is made between organic molecules and organic radicals. Many electron affinities of organic radicals and small molecules such as 02 and SO2 have been determined quite accurately by photodetachment and photoelectron spectroscopy.4
About 50 values have been determined by more than one technique.3v4J The magnetron,2 electron capture detector,' and thermal electron-transfers methods are thermodynamic procedures. The TCT method is based on the measurement of the equilibrium constant for thermal electron transfer. The magnetron method and the ECD method are based upon the measurement of the temperature dependence of the equilibrium constant for the reaction of thermal electrons with a molecule as is the proposed method. In the magnetron method, thermal electrons are generated at a heated filament and react with a known concentration of the neutral molecule. The electron and ion concentrations are measured by the application of a magnetic field. The temperature is measured with an optical pyrometer. The apparent electron affinity is obtained by plotting ln([AB-]/([e-] a) versus 1/T. The direct capture process is identified by the fact that the ion current decreases with increasing temperature. The apparent electron affinity is corrected to absolute zero by subtracting 2RT. Alternatively, the absolute entropy can be calculated and is generally consistent with the experimental value. The values of the electron affinities which have been determined with the magnetron range from 0.8 to 3.24 eV.2 The precision of the method is determined from the data reduction procedure and has been quoted as 0.05-0.4 eV.2.3 Systematic errors in the electron affinitiesof variousquinones have been attributed to the influence of excited states.' In the ECD method, thermal electrons are generated in an Ar/methane mixture by beta particles. The electron concentration monitored with and without a test sample is measured by the brief application of a pulsed voltage. A small, highly purified amount of the test sample is introduced via a gas chromatograph. The temperature is then changed and the process repeated. For compounds which do not dissociate, the molar response of the detector is related to the rate constants for electron attachment, detachment, and ion loss, exactly as in the proposed method. The reported values of the electron affinities which have been measured using the ECD range from 0.15 to 1.5 eVa8 The precision of the ECD values is determined in the data reduction procedure and ranges from 0.02 to 0.2 eV depending on the amount of data in the high-temperature In the thermal electron-transfer methods, the free energy for thermal electron transfer from the anion of one molecule to the neutral of another is measured by determining theconcentrations of the ions and the neutrals. Two experimental procedures have been used, the pulsed high-pressure mass spectrometers and the ICRl5 mass spectrometer. If the temperature dependence of the
Electron Affinities of Molecules
The Journal of Physical Chemistry, Vol. 98, No. 1, 1994 93
equilibrium constant is measured, then both the entropy and enthalpy of the reaction are determined. Alternatively, it can be assumed that the entropy terms cancel and the electron affinity differences are equal to the change in free energy. In either case, the electron affinity of the reference compound must be known toobtain absolutevalues of theelectron affinities. Absolutevalues of the electron affinities ranging from 0.5 to 3.14 eV have been reported using the electron affinity of SO2 as the reference points5 The nominal precision of the TCT values determined using the high-pressure pulsed mass spectrometer is given as 0.1 and 0.05 eV for the ICR measurements.15 The threshold for ion pair formation from the reaction of a molecule with an alkali-metal beam has been combined with the ionization potential of the alkali metal to give the electron affinity of a molecule. These measurements have given accurate values for the electron affinities of the diatomic halogens but give low values for SFa. The electron affinities of several quinones have been determined and indicate that the values determined using the magnetron method are low. Electron affinities in the range 0.5-2.80 eV have been reported with a nominal error of 0.1-0.2 eV.334 The proposed method combines the advantages of the ECD method and the TCT method. The negative ions are identified by mass, and absolute electron affinities are obtained. However, the method is presently limited to the determination of electron affinities of about 1.5 eV. If the recombination and ion losses can be decreased or the upper temperature limit increased, then higher electron affinities can be measured. The experimental procedure meets the four requirements set up earlier in the following manner. The thermal distribution of electrons is achieved by using COZor CH4 to moderate or cool the electron energy by collisions. The electron concentration was maintained constant by maintaining the same voltages in the ion source and by keeping the analyte concentration low so that the ion concentration is small relative to the total number of electrons. The constant values of the ion intensities at low temperatures indicate that this assumption is valid. The relative concentration of the test species was determined by injection of a known amount of material or by a constant permeation rate. The concentrations were not specifically measured but were calibrated by using the known magnitude of the rate constant for thermal electron attachment to SF6. The ion abundance was measured with the mass spectrometer and the temperature measured with thermocouples. The next step in establishing the validity of this procedure is to measure electron affinities of other compounds using this technique. We plan to do this in the future, but we have written this paper to encourage those with negative ionization mass spectrometers to utilize this method to measure molecular electron affinities. To achieve this objective, we suggest a number of factors which are important to the technique. An inert cooling gas such as COz rather than methane should be used to minimize ion molecule reactions. The electron energy should be examined by measuring the ratio of SFJ-/SF~a t low temperatures? If the value is significantly greater than 0.005 at 373 K, hyperthermal electrons may be present. The ratio can vary with electron energy resolution. The maximum number of ions that can be produced in the ion source should be determined experimentally by injecting a large amount of a material such as SF6. The concentrations of the negative ions of the test species should be maintained at a small fraction of this maximum value. The temperature dependence of a standard compound such as nitrobenzene should be measured. If a gas chromatographic method of sample introduction is used, this could be achieved by adding the standard to the mixture. A number of compounds may be investigated in a single chromatographic run by preparing a solution with known compositions of the test compounds. A single solution should be used for all measurements. The
experiments should be carried out in as short a period of time as possible, but adequate time must be allowed for temperature stabilization. The source pressure must be kept constant. The procedure for tuning the mass spectrometer and data collection given in the Experimental Section should be followed. Stemmler and Hite have measured negative ion mass spectra using a Hewlett Packard instrument a t two temperatures. The data are pertinent to the proposed methoda23The observation of the parent negative ion as the base peak at 523 K implies an electron affinity greater than 0.7 eV. The electron affinities of many of the compounds reported in the compilation have not been measured previously. For example, the electron affinities of nitrophenols have not been reported, but nitrophenols have a base peak of the parent negative ion at 523 K. Based on simple substituent effects, the electron affinity of these compounds should be about 1.2 eV, in agreement with the above limit. Finally, a new negative ion mass spectrometric source recently described by Laramee, Kocher, and Deinzer24 may be invaluable in characterizing negative ion states if combined with temperature dependence measurements. This source is the combination of an electron monochromator with a mass spectrometer system to generate negative ions. If the intensity of the parent negative ion is measured as a function of temperature, the data might be used for the determination of molecular electron affinities. One specific case where this source is an improvement over others is for aromatic hydrocarbons. With this source the parent negative ions of pyrene and fluoranthene are observed whereas with others no parent negative ions were observed for anthracene or pyrene.25 Conclusions
On the basis of the temperature dependence of the parent negative ions generated by reaction of thermal electrons with a molecule, the electron affinity of the molecule can be determined. There are some restrictions to the ion source, but the data which have been examined were taken from a chemical ionization source using COZas a cooling gas and an atmospheric pressure ionization source using CHI as a cooling gas. The current results form the basis for a hypothesis which can now be tested by others with similar equipment. In addition to meeting this objective, the good agreement of the present values with recent electron affinities reported for SF6 and C ~ H ~ N support OZ the present values and the values in the literature. Gas-phase electron affinities of C2C 4 and C2HC13 have been obtained which agree with values obtained from half-wave reduction potentials. Acknowledgment. We are grateful to the Robert A. Welch Foundation Grant E095 and Chemistry Department Grants to the University of Texas Permian Basin and University of Houston Clear Lake. References and Notes (1) Pritchard, H. 0. Chem. Rev. 1953, 52, 529. (2) Page, F. M.; G d e , G.C. Negative Ions and the Magnerron; Wiley: New York, 1969. (3) Chen,E.C. M.; Wentworth, W. E.MoI. Crysr.Li9.Crysr. 1989,171, 271. (4) Lias, S.G.;Bartmess, J. E.; Liebman, J. F.; Holmes,J. L.; Levin, R. D.; Mallard, W. G.J. Phys. Chem. ReJ Dara 1988, 17. ( 5 ) Kebarle, P.; Chowdhury, S. Chem. Rev. 1987, 87, 513.
(6) Wiley, J. R.; Chen, E. C. M.; Wentworth, W. E. J. Phys. Chem. 1993, 97, 1256. (7) Chen,E.C. M.;Shuie,L.R.;DesaiD’sa,E.;Batten,C. F.; Wentworth, W. E. J . Chem. Phys. 1988,88, 4711. (8) Chen, E. C. M.; Chen, E.S.; Milligan, M. S.;Wentworth, W.E.; Wiley, W. R. J. Phys. Chem. 1992, 96,2385. (9) Adams, N. G.; Smith, D.; Alge, E. J. Phys. E 1984, 15, 461. (10) Wentworth, W. E.; Desai D’sa, E.;Batten, C. F.; Chen, E.C. M.J. Chromarogr. 1987. 390, 249. ( 1 1) Wentworth, W. E.;Limero, T.; Chen, E.C. M. J. Phys. Chem. 1987, 91, 245. (12) Henglein, A.; Muccini, G.A. J. Chem. Phys. 1959, 31, 1426. ‘(13) Lifshitz, C.;Tiernan,T. 0.;Hughes, B. M. J. Chem. Phys. 1973,59. 3182.
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(14) Chen, E.C. M.Doctoral Dwrtation, Universityof Houston, 1966. (15) Grimsrud, E.P.;Caldwell,G.;Kebarle, P.J. Am. Chem. Soc. 1985, 107,4627. (16) Fukuda, E. K.;McIver, R. T. J. Am. Chem. Soc. 1985,107,2291. (17) Compton, R. N.;Cooper, C. D. J. Chem. Phys. 1973,59,4140. (18) Dradc, P. S.; Brauman, J. I. J . Am. Chem. Soc. 1982,104, 13. (19) Hencghan, S.P.;&nm, S. W. Ini. J. Chem. Kfnet. 1983,15,109. (20) Lifshitz, C. J. Phys. Chem. 1983,87,3474.
(21) Streit, G. J. Chem. Phys. 1982,77,826. (22) Wiley, J. R.;Chcn, E. C. M.; Chen, E. S. D.; Richardson,P.; Red, W.R.;Wenworth, W.E.J. Electmaaal. Chem. 1991,307,1961. (23) Stemmier, E.A.;Hit- R.A.Electron Captun NegafitwIons;VCH Publishen: New York, 1988. (24) Laramet, J. A.;Kwher, C. A.; Dei-, M.L. Anal. Chem. 1992, 64, 2317. (25) Buchanan, M. V.; Olerich, G. Org. Mars. Spectrom. 1 W , 19, 486.
