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On electron solvation in polar liquids
ON ELECTRON
1 July 1977
CHEMICAL PHYSICS LETTERS
Volume 49, number 1
SOLVATION
IN POLAR LIQUIDS
Larry KEVAN* and Kenji FUEKI Department of Synthetic Chernistry. CSzikusa-ku. Nagoya. Japan
Faculty
of Engïtleering.
Nago_va University,
Reccived 25 February 1977
Experimental and theoretical evidente for the domïnance served dwing
electron
solvation
of fikt
solvation
shell orientation
on the spectra1
chages
ob-
is evaluated.
When an electron is suddenly injected into a polar fluid such as water, alcohols or ethers it becomes solvated as indicated by optica1 and electron spin resonance (ESR) spectra [ 1] _ Diverse experimental studies suggest that solvation occurs by orientation of polak molecules about the electron [2-61. It has been implicitly assumed that the spectra1 changes observed are primarily due to orientation of first solvation shell molecules. Recently, Kenney-Wallace has argued that the observable spectral changes are due to orientation of second or third solvation shell molecules which can be treated as a dielectric continuum [7] _We would lïke to point out both experimental and theoretical results that seem incompatible with this recent suggestion and which support the concept that orientation of first solvation shell molecules is responsible for the spectra1 changes cbserveä during electron solvation. Electron solvation is observed by two quite different techniques, optica1 absorption and ESR. Irreversible optica1 blue-shifts and ESR linewidth changes both indicate electron solvation and correlate with each other [ll. Thus, any interpretation of the optical data should also be consistent with the ESR data. Furthermore, the ESR data are more directly relatable to quantitative information about the geometry of the electron solvation shell. Second moment analyses of ESR linewidths of presolvated and solvated electrons * On sabbitical leave from the Department oí Chemistry , Wayne State University, Detroit, Michïgan 48202, ÜSA.
in alkanes and ethers yield electron-proton distance ïnformation and show that the major geometrical changes during electron solvation involve distances corresponding to protons in the fìrst solvation shell [SI. This is independentty confirmed by lineshape analysis of the matrix line in electron nuclear double resonance experiments on presolvated and solvated electrons in ethers and alcohols [Y] . Since these linewidth changes correlate with optica1 blue-shifts, also characteristic of presolvated and solvated electrons, we conclude that the experimental evidente is streng for these optica1 shifts to be associated with orientation of firsr solvation shell molecules. The magnitude of the optica1 spectra1 shift in ethanol has been successiully calculated by the semicontinuum model [lO] on the basis of fïrst solvation shell dipole orientation in a dielectric continuurn [ 111 _ If only the orientational polarization of the dielectric continuum, corresponding to second and more distant solvation shell orientation, were considered the optica1 spectral shift could not be explained theoretically. Since the semicontinuum model has proved quite successful in explaining and correlatïng many features of solvated electrons in polar systems [ 1,121, it appears that there is considerable theoretical support for the identiflcation of the spectra1 shifts in electron solvation with orientation
ofjìrst
soivation
shell molecules.
Kemey-Wallace has shown an interesting
linear correlation between the rate constant for electron solvation in alcohols (methanol through decanol) and (D;: - D;~)?J-~ where Dop and Ds are the optica1 101
Volume 4S, number 1 and static dielectric
CHEMICAL PHYSICS LETTERS
constants and q the viscosity. [7]. is proportional to the long range poIarization potential between the electron and the dielectric continuurn, this correlation was interpreted to indicate thar the electron soIvation rate from the spectral shift klnetics corresponds to this polarization potential acting on the second and further solvation shells to cause their orientation. It appears to US that the electron-dipole distance must be included to properly represent the polarization potential. More importantly we fee1 that this interpretarion ís neitber unique nor required for the following reason. We have previously shown that electron solvation times (inverse rate constants) for different alcohols can be successfuhy calculated from the rotational alignment time of fust solvation shell alcohol dipoles using a short range charge-dipole interaction [13]_ The essential point is that the electron solvation rate is driven by the elecrric field of tbe electron acting on tbe molecular dipoles and impeded by the viscosity which opposes molecular rotation. However, a correlation wiii exist whether the molecules are eitber in the fust or second solvation shell and whether the electrostatic force acting on the dipoles is written as either Clse/r2 or (Dg. --DS’ )efr2 where CIs is the fractional electron charge within distaxe I and r is the electron-dipole distance [ 131. Thus we do not fee1 that Renney-Wallace’s interesting correlation adrhits of a definítive interpretation to distinguish the relative importante of short range versus long range electrostatic interactions or fìrst versus síícond solvation shell orientation. On the otber hand, the ESR evidente and the theoretical calculation on the
Since (Do;’ -DS’)
102
1 July 1977
spectral shifts appear to US to unambiguously support the domlnance of fust solvation shell orientation on the spectral changes observed during electron solvation. This work was supported by the Jipan Society for of Science and the U.S. Energy Research and Development Administration under contract AT(l1 -l)-2086.
the Promotion
References [ 11 L. Kevan, Advan. Radiat. Chem. 4 (1974) 181. [2] L. Kevan, J. Chem. Phys. 56 (1972) 838. [3 ] J.H. Baxendale and P. Wardman. J. Chem. Sec. Faraday Trans. 169 (1973) 584. [4] W.J. Chase and J.W. Hunt, 1. Phys. Chem. 79 (1975) 2835. [5] G.A. Kenney-Wahace and C.D. Jonah, Chem. Phys. Letters 39 (1976) 596. [6j H. Hase, M. Noda and T. Higashïmura, J. Chcm. Phys. 54 (1971) 2975. [7] G.A. Kenney-Wallace, Chem. Phys. Letters 43 (1976) 529. [8] D.P. Lim and L. Kevan, Chem. Phys Letters 40 (1976) 517 191 H. Hase, F.Q.H. Ngo and L. Kevan, J. Chem. Phys. 62 (1975) 985. 1101 K. Fueki, D.-F. Feng and L. Kevan, J. Am. Chem. Sec. 95 (1973) 1398. [ 111 K. Fueki, D.-F. Feng and L. Kevan, J. Chem. Phys 56 (1972) 5351. [J2] N.R. Kestuer, in: Electron-soIvent and anion-solvent interactions, eds. L. Kevan and B. Webster (Elsevier, Amsterdam, 1976). [13] K. Fueki, D.-F. Feng and L. Kevan, J. Phys. Chcm. 78 (1974) 393; 80 (1976) 1381.
1 July 1977
CHEMICAL PHYSICS LETTERS
Volume 49, number 1
SOLVATION
IN POLAR LIQUIDS
Larry KEVAN* and Kenji FUEKI Department of Synthetic Chernistry. CSzikusa-ku. Nagoya. Japan
Faculty
of Engïtleering.
Nago_va University,
Reccived 25 February 1977
Experimental and theoretical evidente for the domïnance served dwing
electron
solvation
of fikt
solvation
shell orientation
on the spectra1
chages
ob-
is evaluated.
When an electron is suddenly injected into a polar fluid such as water, alcohols or ethers it becomes solvated as indicated by optica1 and electron spin resonance (ESR) spectra [ 1] _ Diverse experimental studies suggest that solvation occurs by orientation of polak molecules about the electron [2-61. It has been implicitly assumed that the spectra1 changes observed are primarily due to orientation of first solvation shell molecules. Recently, Kenney-Wallace has argued that the observable spectral changes are due to orientation of second or third solvation shell molecules which can be treated as a dielectric continuum [7] _We would lïke to point out both experimental and theoretical results that seem incompatible with this recent suggestion and which support the concept that orientation of first solvation shell molecules is responsible for the spectra1 changes cbserveä during electron solvation. Electron solvation is observed by two quite different techniques, optica1 absorption and ESR. Irreversible optica1 blue-shifts and ESR linewidth changes both indicate electron solvation and correlate with each other [ll. Thus, any interpretation of the optical data should also be consistent with the ESR data. Furthermore, the ESR data are more directly relatable to quantitative information about the geometry of the electron solvation shell. Second moment analyses of ESR linewidths of presolvated and solvated electrons * On sabbitical leave from the Department oí Chemistry , Wayne State University, Detroit, Michïgan 48202, ÜSA.
in alkanes and ethers yield electron-proton distance ïnformation and show that the major geometrical changes during electron solvation involve distances corresponding to protons in the fìrst solvation shell [SI. This is independentty confirmed by lineshape analysis of the matrix line in electron nuclear double resonance experiments on presolvated and solvated electrons in ethers and alcohols [Y] . Since these linewidth changes correlate with optica1 blue-shifts, also characteristic of presolvated and solvated electrons, we conclude that the experimental evidente is streng for these optica1 shifts to be associated with orientation of firsr solvation shell molecules. The magnitude of the optica1 spectra1 shift in ethanol has been successiully calculated by the semicontinuum model [lO] on the basis of fïrst solvation shell dipole orientation in a dielectric continuurn [ 111 _ If only the orientational polarization of the dielectric continuum, corresponding to second and more distant solvation shell orientation, were considered the optica1 spectral shift could not be explained theoretically. Since the semicontinuum model has proved quite successful in explaining and correlatïng many features of solvated electrons in polar systems [ 1,121, it appears that there is considerable theoretical support for the identiflcation of the spectra1 shifts in electron solvation with orientation
ofjìrst
soivation
shell molecules.
Kemey-Wallace has shown an interesting
linear correlation between the rate constant for electron solvation in alcohols (methanol through decanol) and (D;: - D;~)?J-~ where Dop and Ds are the optica1 101
Volume 4S, number 1 and static dielectric
CHEMICAL PHYSICS LETTERS
constants and q the viscosity. [7]. is proportional to the long range poIarization potential between the electron and the dielectric continuurn, this correlation was interpreted to indicate thar the electron soIvation rate from the spectral shift klnetics corresponds to this polarization potential acting on the second and further solvation shells to cause their orientation. It appears to US that the electron-dipole distance must be included to properly represent the polarization potential. More importantly we fee1 that this interpretarion ís neitber unique nor required for the following reason. We have previously shown that electron solvation times (inverse rate constants) for different alcohols can be successfuhy calculated from the rotational alignment time of fust solvation shell alcohol dipoles using a short range charge-dipole interaction [13]_ The essential point is that the electron solvation rate is driven by the elecrric field of tbe electron acting on tbe molecular dipoles and impeded by the viscosity which opposes molecular rotation. However, a correlation wiii exist whether the molecules are eitber in the fust or second solvation shell and whether the electrostatic force acting on the dipoles is written as either Clse/r2 or (Dg. --DS’ )efr2 where CIs is the fractional electron charge within distaxe I and r is the electron-dipole distance [ 131. Thus we do not fee1 that Renney-Wallace’s interesting correlation adrhits of a definítive interpretation to distinguish the relative importante of short range versus long range electrostatic interactions or fìrst versus síícond solvation shell orientation. On the otber hand, the ESR evidente and the theoretical calculation on the
Since (Do;’ -DS’)
102
1 July 1977
spectral shifts appear to US to unambiguously support the domlnance of fust solvation shell orientation on the spectral changes observed during electron solvation. This work was supported by the Jipan Society for of Science and the U.S. Energy Research and Development Administration under contract AT(l1 -l)-2086.
the Promotion
References [ 11 L. Kevan, Advan. Radiat. Chem. 4 (1974) 181. [2] L. Kevan, J. Chem. Phys. 56 (1972) 838. [3 ] J.H. Baxendale and P. Wardman. J. Chem. Sec. Faraday Trans. 169 (1973) 584. [4] W.J. Chase and J.W. Hunt, 1. Phys. Chem. 79 (1975) 2835. [5] G.A. Kenney-Wahace and C.D. Jonah, Chem. Phys. Letters 39 (1976) 596. [6j H. Hase, M. Noda and T. Higashïmura, J. Chcm. Phys. 54 (1971) 2975. [7] G.A. Kenney-Wallace, Chem. Phys. Letters 43 (1976) 529. [8] D.P. Lim and L. Kevan, Chem. Phys Letters 40 (1976) 517 191 H. Hase, F.Q.H. Ngo and L. Kevan, J. Chem. Phys. 62 (1975) 985. 1101 K. Fueki, D.-F. Feng and L. Kevan, J. Am. Chem. Sec. 95 (1973) 1398. [ 111 K. Fueki, D.-F. Feng and L. Kevan, J. Chem. Phys 56 (1972) 5351. [J2] N.R. Kestuer, in: Electron-soIvent and anion-solvent interactions, eds. L. Kevan and B. Webster (Elsevier, Amsterdam, 1976). [13] K. Fueki, D.-F. Feng and L. Kevan, J. Phys. Chcm. 78 (1974) 393; 80 (1976) 1381.