J. Phys. Chem. 1996, 100, 4491-4495
4491
Effect of Pressure on the e- + Pyrimidine h Pyrimidine- Equilibrium in Nonpolar Solvents Pingyun Chen and Richard A. Holroyd* Department of Chemistry, BrookhaVen National Laboratory, Upton, New York 11973 ReceiVed: October 24, 1995; In Final Form: December 19, 1995X
The thermodynamics of the equilibrium reaction e- + pyrimidine h pyrimidine- was studied in tetramethylsilane (TMS) and 2,2,4-trimethylpentane (TMP) as solvents. In hexane, only the forward attachment reaction could be observed. The attachment rate constants (ka) increase with temperature in all solvents. The equilibrium shifts to the right with increasing pressure because of the large, negative reaction volumes (-200 to -330 cm3/mol). The volume changes are consistent with electrostriction by the pyrimidine anion when a glass shell of solvent molecules is included. The free energy of reaction at 298 K is -0.43 eV in tetramethylsilane at 1 bar and -0.70 eV in 2,2,4-trimethylpentane at 150 bar. The solution free energies indicate the gas-phase (adiabatic) electron affinity of pyrimidine is -0.24 eV.
Introduction The pulse conductivity technique employed here has previously been used to measure the thermodynamic and volume changes for electron attachment to CO2,1,2 toluene,3 and 1,3butadiene.4 These reactions are all energetically unfavorable in the gas phase. Electron attachment to DNA bases, generally, is also unfavorable in the gas phase. Calculations indicate that the gas-phase (vertical) electron affinities of the four DNA bases range from -1.23 to -0.32 eV.5 This study is concerned with attachment to the unsubstituted pyrimidine molecule: ka
e- + pyrimidine y\ z pyrimidinek
(1)
d
This reaction is unfavorable in the gas phase; the electron affinity of pyrimidine is estimated to be -0.33 eV, based on the position of an electron-scattering resonance.6 However, reaction 1 is quite favorable in solution. In polar solvents, pyrimidine readily attaches electrons; the rate constant for reaction with eaq- in water is 2 × 1010 M-1 s-1.7 The pyrimidine anion is stable and exhibits absorption bands in the ultraviolet in water7 and in MTHF.8 This study concerns the effect of pressure, temperature, and solvent on reaction 1 and leads to the equilibrium constant, volume change, and free energy of this reaction in solution. A value of the electron affinity of pyrimidine in the gas phase is inferred from the data. The results are compared to those for electron attachment to CO2 and 1,3-butadiene. Reaction volumes are large and negative for all three reactions and are attributed mainly to electrostriction by the anion. Electrostriction is to be expected around a neutral molecule with a dipole in a nonpolar solvent, and the measurements of volume changes in reaction 1 provide the opportunity to assess the magnitude of this effect. Pyrimidine has a dipole moment of 2.28 D.9 Several electron attachment reactions have been studied where both the attachment rate, ka, and the free energy of reaction, ∆Gr, were determined. These data for TMS as the solvent are examined here to ascertain the relationship of ka and ∆Gr. Experimental Section The TMS used was from Wacker, the TMP was “Omnisolve” grade (99.99%), and the n-hexane was from Wiley (99.8%). All solvents used were purified by washing several times with X
Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-4491$12.00/0
concentrated sulfuric acid and then with Millipore water. Subsequently, the solvents were dried over molecular sieves, degassed on a vacuum line, passed through an evacuated column containing silica gel and molecular sieves, and finally stored over NaK. The rate of attachment of electrons to residual impurities was checked prior to addition of pyrimidine in each solvent. The pyrimidine used was from Aldrich (99%) and was degassed prior to use. GC/MS analysis revealed traces of decalin and tetramethoxypropane in the pyrimidine, neither of which would be expected to interfere by attaching electrons. The method of solution preparation and the pulse conductivity technique and data analysis are described elsewhere.1,4 Densities (F), compressibilities (χ), and thermal coefficients of expansion (RT) were determined by fitting known density data vs pressure to a Tait equation: do/d ) 1 - A ln(1 + P/B). The density data for TMP was from Bridgman10 and A ) 0.086 96, B ) 314 bar at 89 °C and dB/dT ) -4.8 in this range. For TMS, the data and form of the Tait equation suggested by Parkhurst11 were used. Results Attachment Rates. Electrons attach to pyrimidine in nhexane as well as in TMS and TMP. In n-hexane, however, only the forward attachment rate is observed; that is, the equilibrium constant for reaction 1 is large. From the decay of the current following a pulse, the observed rate constant, kobs ) ka[Py], is obtained, and the results for n-hexane are shown in Figure 1. The pyrimidine concentration [Py] was varied over a 10-fold range, and linear dependence of kobs on [Py] is obtained only at high concentrations. At lower concentrations, the values of kobs fall below the line through the higher concentration points. Very similar results were obtained for TMS. The reason that the points at low concentration are low is not clear. We suspect there is partial adsorption of pyrimidine on the stainless steel walls and electrodes of the pressure cell. Values of the rate constant for attachment, ka, reported in Table 1, are derived from the points at high concentration (>0.2 µm). The rate constants for TMP are based on data for a 0.19 µm solution. The large rate constant in TMP precluded studies at higher concentrations because of time response limitations. Although the rate constants for attachment are large, they are comparable to rate constants reported for attachment to C2HCl312 and CCl413 in these solvents. © 1996 American Chemical Society
4492 J. Phys. Chem., Vol. 100, No. 11, 1996
Chen and Holroyd
Figure 1. Observed attachment rate to pyrimidine in n-hexane vs pyrimidine concentration at temperatures in °C as indicated. Dashed lines correspond to rate constants given in Table 1 for 500 bar.
Figure 2. Arrhenius plot of attachment rate. (O) in TMS at 100 bar; (3) in TMP at 50 bar; (0) in hexane at 1 bar.
TABLE 1: Electron Attachment to Pyrimidine ka, ×10-12 m-1 s-1
µD, cm2/ (V s)
ka/µDFT, ×10-11 V/ (M K cm2)
temp, °C
pres, bar
21 21 21 21 40 40 40 40 60 60 60
1 500 1000 2000 1 500 1000 2000 500 1000 2000
In n-Hexane 2.12 0.072 1.67 0.052 1.45 0.045 1.01 0.036 3.34 0.108 2.68 0.078 2.34 0.064 1.68 0.051 4.2 0.11 3.6 0.094 2.85 0.073
1.5 1.6 1.5 1.25 1.6 1.6 1.6 1.4 1.7 1.6 1.6
20 20 20 20
50 500 1000 2000
In 2,2,4-Trimethylpentane 35.7 6.53 33 7.38 30.2 7.95 19.2 8.8
0.27 0.21 0.17 0.093
21 21 21 21
1 500 1000 2000
In Tetramethylsilane 33.7 103 34.8 93 30.6 90 25 90
0.017 0.018 0.016 0.012
Figure 3. Equilibrium constant for e- + pyrimidine ) pyrimidinein TMS as a function of pressure and temperature (in °C): (O) 21, (1) 29.5, (0) 39.3, (9) 46.8.
In hexane, the attachment rate constants increase with temperature and decrease with pressure. In this solvent, the electron mobility, µD, also increases with temperature and decreases with pressure. This parallelism suggests that the rate is diffusion controlled, in which case ka and µD should be related by
ka ) 4πReffD ) 4πReffµDkbT
(2)
that is, the quantity ka/µDFT should be a constant. As can be seen in Table 1, this quantity is fairly constant for hexane. The density, F, is included to convert the rate constant to molar units. The average value of ka/µDFT corresponds to an effective radius of reaction of 2.4 nm. The temperature dependence of the attachment rate in n-hexane, shown in Figure 2, indicates an activation energy of 4.4 kcal/mol, the same as the activation energy associated with the mobility of the electron.12 In TMP and TMS, the rate constants are larger than in hexane. Nevertheless, in TMP, the ratio ka/µDFT is an order of magnitude less than in n-hexane. In TMS, this ratio is 2 orders of magnitude less. Thus, the rates are far from diffusion controlled in these solvents. The temperature dependence of the attachment rate constants at 50 bar are fit to an equation of the form k ) A exp(-∆E*/RT) and ∆E* ) 1.4, and 3.0 kcal/mol in TMS and TMP, respectively. As shown in Table 1, ka is nearly constant at pressures between 1 and 500 bar, but a slight
Figure 4. Equilibrium constant for e- + pyrimidine ) pyrimidinein TMP as a function of pressure and temperature (in °C): (0) 89.5, (]) 100, (4) 108, (O) 116.
decrease in rate is observed at higher pressures. This decrease is unusual, as most electron attachment reactions accelerate with increasing pressure as a result of the negative volume change associated with anion formation. Equilibrium. Figures 3 and 4 show how the equilibrium constant for reaction 1 shifts with pressure and temperature in TMS and TMP. In both solvents, Keq increases with pressure and decreases with increasing temperature. The equilibrium was observed near room temperature in TMS and near 100 °C in TMP. The reaction volumes, determined from the slopes in Figures 3 and 4, are given in Table 2. For TMS, ∆Vr is close to -200 cm3/mol at the lower pressures; for TMP, ∆Vr is between -190 and -330 cm3/mol. Values of electrostriction by the pyrimidine ion were calculated for comparison and are also shown in Table 2. The values based on the classical equation1
Vel ) -(e2/2Ra)(χ/32)( - 1)( + 2)
(3)
Pressure Effect on e- + Pyrimidine h Pyrimidine-
J. Phys. Chem., Vol. 100, No. 11, 1996 4493
TABLE 2: Reaction Volumes Vel(Pyr-) temp, °C
pres, bar
∆Vd*, cm3/mol
∆Vr, cm3/mol
21.0 29.5 39.3 46.8
125 225 175 250
210 201 215 194
-220 -200 -233 -205
89.3 100 108 116
125 125 150 150
185 286 277 329
-188 -284 -274 -333
a
F, g/cm3
cont.
glass
RT, 103 K-1
∆Scal, cal/(K mol)
Solvent: Tetramethylsilane 0.663 1.874 1.835 0.666 1.879 1.66 0.651 1.854 1.93 0.652 1.856 1.79
-160 -145 -159 -145
-243 -226 -240 -226
1.36 1.30 1.36 1.31
-38a -36 -40 -37
Solvent: 2,2,4-Trimethylpentane 0.651 1.864 2.057 0.643 1.852 2.344 0.643 1.851 2.46 0.640 1.846 2.804
-173 -196 -206 -234
-248 -254 -252 -257
1.41 1.46 1.49 1.52
-30 -42 -40 -43
χ, 104 bar-1
∆Scal from (R/χ)∆V.
TABLE 3: Thermodynamic and Rate Parameters temp, pres, ∆Gr, ∆Hr, ∆Sr, ∆Hd*, ∆Sd*, cal/ °C bar kcal/mol kcal/mol cal/(K mol) kcal/mol (K mol) Solvent: Tetramethylsilane -20.0 -32.4 -19.9 -30.1 -21.1 -32.4 -20.0 -27.8
21 21 21 21
100 200 300 400
-10.5 -10.9 -11.6 -11.9
100 100 100 100
50 100 150 200
Solvent: 2,2,4-Trimethylpentane 16.1a -26.4 -34 25.4 -15.8a -23.0 -24 22.0 -15.6a -21.3 -19 20.3 -15.5a -19.7 -14 18.7
a
21.5 21.5 22.4 22.2
41 39 40.5 39 37 27 21 16
∆Gr° at 298 K.
are from 50 to 100 cm3/mol less in magnitude than the observed reaction volumes. Here the value of Ra, the effective radius of the anion, is assumed to be 0.264 nm, the same as that reported for benzene.14 Electrostriction volumes were also calculated as in ref 15 by assuming that a glass shell of solvent molecules surrounds the ion out to a radius of 0.71 nm. The shell encompasses 7.8 molecules with TMS as solvent and 5.8 molecules with TMP. The volumes predicted for the glass shell model are in reasonable agreement with the observed reaction volumes; the predicted volumes are in much better agreement with experiment than those from the classical calculation. The enthalpy and entropy of reaction were calculated from the dependence of ln Keq on 1/T. The enthalpy and entropy changes (see Table 3) are comparable to those observed for electron attachment to CO21,2 and butadiene.4 Values of ∆Gr, calculated from -RT ln Keq for TMS at 21 °C and various pressures, are given in Table 3. The values of ∆Gr for TMP were obtained by extrapolation of the ln Keq vs 1/T plot to 298 K. Detachment Rates. The rate constants for autodetachment from pyrimidine- in TMS, kd, are shown in Figure 5. The decrease in rate constant with increasing pressure corresponds to a positive activation volume. Similar results were obtained for TMP. The derived values of ∆Vd* shown in Table 2 are in excellent agreement with the reaction volumes in magnitude. Values of the activation energy, ∆Hd*, and activation entropy, ∆Sd*, for the detachment reaction are shown in Table 3. These were calculated by fitting the temperature dependence of the detachment rate at different pressures to the equation
kd ) (kBT/h) exp(-∆Hd*/RT + ∆Sd*/R)
(4)
The detachment activation entropies are large and quite comparable to what was observed for detachment from CO2anion2 and butadiene anion.4
Figure 5. Rate constant for detachment from pyrimidine anion in TMS as a function of pressure and temperature in °C: symbols as in Figure 3.
Discussion Activation Energies. The equilibrium, reaction 1, has a small activation energy in the forward (attachment) direction and a much larger activation energy of around 22 kcal/mol in the reverse direction (detachment). Generally, we have found that activation energies for attachment to solutes in TMS are small.12 For electron attachment to CO2, the activation energy is zero.1 Consequently, the activation energy for detachment of an electron from CO2- is equal in magnitude but opposite in sign to the enthalpy of the attachment reaction, which is about -20 kcal/mol.1 Detachment from butadiene- in hexane has an activation energy around 20 kcal/mol,4 similar to the CO2 case. Pyrimidine behaves similarly; the activation energy for detachment at 100 bar is 21.5 kcal/mol, and the enthalpy of reaction is -20 kcal/mol. The small activation energy for attachment to pyrimidine is not without precedent. The rate constant for attachment to CCl4 is 5.4 × 1013 M-1 s-1 in TMS, and the activation energy is 2.3 kcal/mol. It has been suggested16 that electron attachment in nonpolar liquids is related to the gas-phase attachment cross-section dependence on electron energy. The corresponding energy in the liquid phase is shifted by the polarization energy of the product anion. For a vertical transition, the rate will depend on how close this energy matches the conduction band energy, which for TMS is -0.56 eV.17 There is a peak in the attachment cross section for pyrimidine at 0.33 eV in the gas phase.6 An increase in rate with temperature can be attributed to a better match of the energy to this maximum. This concept typically leads to different temperature effects in different solvents because the conduction band energies of the electron in each solvent differ. For pyrimidine, the activation energy is positive, suggesting that in TMS the electron energy state is above the peak in the attachment cross section. Apropos this discussion is the recent suggestion of a stable dipole-bound anion of uracil.18 In this state, the electron is
4494 J. Phys. Chem., Vol. 100, No. 11, 1996
Chen and Holroyd
TABLE 4: Calculated Electrostriction around Pyrimidine from Equation 5a temp, °C
pres, bar
21 21
200 800
22 59 59 89
50 250 850 175
a
χ, 104 bar-1
Vel(dipole), cm3/mol)
TMS 1.886 1.968
1.64 0.89
-4.2 -2.4
TMP 1.943 1.925 1.997 1.878
1.34 1.28 0.73 1.99
-3.5 -3.4 -2.0 -5.1
R ) 0.264 nm and µ ) 2.28 D.
located in a diffuse orbital next to the positive end of the dipole. It can be thought of as a complex of a localized electron and a dipole. Such states are stable in the gas phase for molecules with dipoles greater than 1.6 D.19 Such a state may be involved as an intermediate in electron attachment to pyrimidine, but we have no evidence for or against its role. Clearly, in the ground state of the pyrimidine anion, the electron is not in such a diffuse orbital. The observed large volume changes associated with electrostriction imply the electron is in a compact molecular orbital. Volume Changes. The activation volume for attachment to pyrimidine is zero at low pressure. This differs from the results for other solutes. For attachment to CO2, C5F12, and N2O in TMS, the activation volumes are -68, -28, and -54 cm3/mol, respectively.20 As a consequence, the magnitude of the activation volume for detachment from CO2- is about two-thirds of the magnitude of the reaction volume.1,2 Here we find the activation volumes (see Table 2) for detachment from pyrimidine- are equal in magnitude to the reaction volumes. The volume change in reaction 1 is the difference in molar volume of pyrimidine- and the sum of the molar volumes of the electron and the neutral pyrimidine. As shown in the Results section, the overall reaction volumes are in agreement with the molar volume of the pyrimidine- anion, calculated with the glass shell electrostriction model.15 In this respect, the results for pyrimidine are consistent with the large negative volume changes observed for electron attachment to CO2,1,2 toluene,3 and butadiene.4 The role of the dipole moment of pyrimidine (2.28 D) needs to be considered. Electrostriction of the solvent occurs around a dipole in a nonpolar liquid.21,22 The volume of electrostriction is given by
3 Veldipole ) -(3Nµ2/R3) ∂/∂P (2 + 1)2
(5)
Veldipole ) -(Nµ2/R3)χ( - 1)( + 2)/(2 + 1)2
(6)
or
where the differential, ∂/∂P, has been evaluated as in ref 1. Putting values of χ and into eq 6 leads to the values of Veldipole given in Table 4. These values are negligible relative to the observed reaction volumes. Entropy Changes. The attachment reactions we have previously studied are characterized by a large negative entropy of reaction which is comparable (but opposite in sign) to the entropy of activation for detachment. For attachment to CO2 in TMS, ∆Sr ranges from -28 to -46 cal/(K mol).1 For butadiene in hexane, ∆Sr values are in the same range.4 For pyrimidine, we find the overall reaction entropy change is between -14 and -32 cal/(K mol).
As found previously,4 the entropy change should be proportional to the volume change; that is
Sel ) (RT/χ)∆V
(7)
where RT is the coefficient of thermal expansion. The observed volume changes were used in eq 7 along with values of RT and χ to calculate the entropy changes. The calculated values for the experimental conditions are given in Table 2 in the last column; the average (for TMS) is -38 cal/(K mol). For these conditions, that is, below 300 bar, the average observed entropy change is -32 cal/(K mol), in reasonable agreement. For TMP, the average calculated value is -39 cal/(K mol) for pressures below 150 bar, and the average observed entropy change is -26 cal/(K mol). There is a negative entropy term associated with the electrostriction around the pyrimidine dipole which is given by
Seldipole ) -∂∆G/∂T ) -[3Nµ2Rd/(R3(2 + 1)2)] ∂/∂d (8) For pyrimidine in TMP, the calculated entropy change amounts to only -1 cal/(K mol) at 89 °C and 100 bar. This is not very significant compared to the observed entropy change. Free Energy of Reaction. The free energy of reaction 1 in TMS at 21 °C and 1 bar is -0.43 eV, from the value of the equilibrium constant at these conditions. This can be related to the corresponding gas phase value by
∆G°(1)soln ) ∆G(1)gas + ∆Gel - ∆Gsoln(e-)
(9)
where ∆Gel is the free energy of polarization of the TMS by the pyrimidine anion. From the Born equation and a value of the radius of the anion of 0.264 nm, ∆Gel is -1.24 eV in TMS. The term ∆Gsoln(e-) can be approximated for TMS by Vo, the conduction band energy, which is -0.56 eV.17 From these values and eq 9, one obtains the free energy of reaction 1 in the gas as 0.25 eV. A similar calculation may be made for the TMP results. However, since the equilibrium was studied at higher temperatures, an extrapolation to 25 °C is necessary. The value given in Table 3 for 25 °C and 50 bar is ∆Gr ) -0.70 eV. The value of ∆Gsoln(e-) is reported to be -0.40 eV under these conditions,20 and ∆Gel for the anion is -1.33 eV; thus, ∆G(1)gas ) 0.23 eV from the TMP data. These values, obtained in TMS and TMP, indicate the anion is unstable (autodetaches) in the gas phase (a positive ∆G implies a negative electron affinity). Our value thus agrees quite well with energies of resonances in electron transmission experiments of -0.335 and -0.25 eV.23 Note that entropy changes for electron attachment reactions are generally small in the gas phase.24 In addition to the data for pyrimidine reported here, there exist data on attachment rate constants (ka) for several similar aromatic compounds (A) for which the free energies of the reaction
e- + A ) Aor thresholds for the reverse detachment reaction are also known. Thus, it is of interest to examine how ka varies with ∆Gr, the energy gap. A compilation for TMS is shown in Table 5. The data for reactions with ∆Gr near -0.5 eV are from studies similar to this where the equilibrium constants were determined. The two compounds for which the driving force is greater than 2.0 eV are for quinones; in this case, the threshold energy, Eth, for photodetachment from A- was measured and we are taking ∆Gr as approximately equal to Eth. For the quinones, the rate
Pressure Effect on e- + Pyrimidine h Pyrimidine-
J. Phys. Chem., Vol. 100, No. 11, 1996 4495
TABLE 5: Electron Attachment Rate Constants and Free Energies of the Reaction e- + A ) A- in TMS compds
temp
ka (M-1 s-1 ×10-12)a
∆Gr, eV
ref
biphenyl naphthalene phenanthrene pyrimidine triphenylene coronene pyrene benzperylene trans-stilbene perylene C6F6 benzoquinone duroquinone
297 299 308 294 295 296 296 296 296 296 RT 293 293
7.2 9.8 14.0 52 17.0 7.2 14 5.9 3.9 4.3 120 0.044 0.062
-0.30 -0.33 -0.436 -0.43 -0.50 -0.73 -0.87 -0.94 -1.04 -1.11 -1.61 -2.32 -2.26
25 26 26 this study 26 27 27 27 27 27 28, 29 30 31
a
These rate constants are for 1 bar and are expressed in molar units.
constants are upper limits because the attachment is believed to involve an excited state A-*.30 The data for reactions with ∆Gr around -1.0 eV are from a study of photodetachment spectra, but the reported thresholds were not actually observed and are only estimated from electron affinities. Some generalizations are clear. For attachment to occur, ∆Gr must be negative. The rate constants for attachment to aromatic compounds are generally considerally less than the maximum observed rate in TMS, which is close to 2 × 1014 M-1 s-1 for CH3I and SF6.12 The rate constant for attachment to C6F6 is an exception to this. The rate constant for pyrimidine, for which ∆Gr is -0.43 eV, is larger than that for any of the compounds studies, if we exclude C6F6. For very large energy gaps, the rate constants are small. Summary. Overall, electron attachment to pyrimidine bears many similarities to attachment to other solutes like CO2 and butadiene. The reaction volumes are large and negative, consistent with a compact product ion. Detachment of the electron from pyrimidine- is driven by a large entropy of activation. The free energy of electron attachment to pyrimidine is -0.43 eV in TMS and -0.70 eV in TMP. These values lead to a gas-phase electron affinity of -0.24 eV. Acknowledgment. We thank H. Schwarz for assistance and M. Sevilla for helpful suggestions. This research was carried
out at Brookhaven National Laboratory and supported under Contract DE-AC02-76CH00016 with the U.S. Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences. References and Notes (1) Nishikawa, M.; Itoh, K.; Holroyd, R. J. Phys. Chem. 1988, 92, 5262. (2) Ninomiya, S.; Itoh, K.; Nishikawa, M. J. Phys. Chem. 1993, 97, 9488. (3) Itoh, K.; Nishikawa, M.; Holroyd, R. J. Phys. Chem. 1993, 97, 503. (4) Holroyd, R. A.; Schwarz, H. A.; Stradowska, E.; Ninomiya, S.; Itoh, K.; Nishikawa, M. J. Phys. Chem. 1994, 98, 7142. (5) Sevilla, M. D.; Besler, B.; Colson, A.-O. J. Phys. Chem. 1995, 99, 1060. (6) Mathur, D.; Hasted, J. B. Chem. Phys. 1977, 16, 347. (7) Moorthy, P. N.; Hayon, E. J. Phys. Chem. 1974, 78, 2615. (8) Kato, T.; Shida, T. J. Am. Chem. Soc. 1979, 101, 6869. (9) Vaughan, W. E., Ed. Dig. Lit. Dielectr. 1975, 37, 42. (10) Bridgman, P. W. Proc. Am. Acad. Arts Sci. 1932, 67, 19. (11) Parkhurst, H. J., Jr.; Jonas, J. J. Chem. Phys. 1975, 63, 2698. (12) Allen, A. O.; Gangwer, T. E.; Holroyd, R. A. J. Phys. Chem. 1975, 79, 25. (13) Allen, A. O.; Holroyd, R. A. J. Phys. Chem. 1974, 78, 796. (14) Ben-Amotz, D.; Willis, K. G. J. Phys. Chem. 1993, 97, 7736. (15) Schwarz, H. A. J. Phys. Chem. 1993, 97, 12954. (16) Henglein, A. Can. J. Chem. 1977, 55, 2112. (17) Holroyd, R. A.; Cipollini, N. E. In Proceedings of the Sixth International Conference on Radiation Research; Okada, S., et al., Eds.; Toppan: Tokyo, 1979; p 228. (18) Oyler, N. A.; Adamowicz, L. J. Phys. Chem. 1993, 97, 11122. (19) Simons, J.; Jordan, K. D. Chem. ReV. 1987, 87, 535. (20) Holroyd, R. A. In Linking the Gaseous and Condensed Phases of Mattersthe BehaViour of Slow Electrons; Christophorou, L. G., Illenberger, E., Schmidt, W. F., Eds.; NATO-ASI Series B, Plenum: New York, 1994; Vol. 326, p 443. (21) Whalley, E. J. Chem. Phys. 1963, 38, 1400. (22) Issacs, N. S. Liquid Phase High Pressure Chemistry; Wiley: New York, 1981; p 191. (23) Nenner, I.; Schulz, G. J. J. Chem. Phys. 1975, 62, 1747. (24) Kebarle, P.; Chowdhury, S. Chem. ReV. 1987, 87, 513. (25) Warman, J. M.; De Haas, M. P.; Zador, E; Hummel, A. Chem Phys. Lett. 1975, 35, 383. (26) Holroyd, R. A. Ber. Bunsen-Gesell. Phys. Chem. 1977, 81, 298. (27) Sowada, U.; Holroyd, R. A. J. Chem. Phys. 1979, 70, 3586. (28) van den Ende, C. A. M.; Nyikos, L.; Warman, J. M.; Hummel, A.; Radiat. Phys. Chem. 1982, 19, 297. (29) Sowada, U.; Holroyd, R. A. J. Phys. Chem. 1980, 84, 1150. (30) Holroyd, R. A. J. Phys. Chem. 1982, 86, 3541. (31) Holroyd, R. A. Unpublished results.
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