Shapes of Optical Spectra of Solvated Electrons
908
Shapes of Optical Spectra of Solvated Electrons. Effect of Pressure' Fang-Yuan Jou and Gordon R. Freeman' Chemistry Department, Universiw of Alberta, Edmonton, Alberta, Canada T6G 2G2 (Received December 20, 1976) Publication costs assisted by the University of Alberta
Absorption spectra were measured in water, methanol, ethanol, l-propanol, and l-butanol at 27 "C and pressures from 1bar to 2.2 kbar. The width of the band at half-height, Wllz, was divided into two portions, W , and Wb, representing the parts on the red and blue sides of the absorption maximum EA^^^. The average energy on the low-energy side of the band, represented by EA^^ - WJ, is nearly the same in water (1.36 eV at 1 bar) as in the alcohols (1.34-1.49 eV at 1bar). By contrast the average energy on the high-energy side of the band, represented by EA^^ + Wb),is -0.7 eV higher in alcohols (2.8-3.0 eV at 1 bar) than in water (2.2 eV at 1 bar). The results are consistent with the following model. The low-energy side of the band represents the transition between the ground state and the first excited state, both of which are determined mainly by the potential well created by the OH groups. In alcohols the higher excited states are greatly affected by the (more weakly scattering) alkyl groups. The model is qualitatively similar to that of Delahay, but we find that the overlapping Gaussian shaped lines are not all of the same width and that the high-energy tail does not have but varies from one type of liquid to another. The relative increase in EAmaa with the form (AIA,) 0: pressure is greater in the alcohols than in water; it correlates with the relative increase in liquid density rather than with that in dielectric constant. The smaller change in in water is balanced by a relatively larger increase in the width parameters. Oscillator strengths estimated from the present more complete spectra have similar values in the alcohols (0.69-0.75) and water (0.76).
Introduction Attention has recently been focussed on the different behavior of the two sides of the o tical absorption band of electrons solvated in ethers? The approximately Gaussian low-energy side of the band becomes wider with increasing temperature, while the approximately Lorentzian high-energy side becomes slightly narrower. In the theoretical interpretation of solvated electron spectra the two sides of the band should be treated separately. For example the low-energy side might be determined mainly by the distribution of ground state (trap) energies, while the high-energy side might contain an overlapping series of excited states and a continuum. The total width of the band at half-height, WllZ, is less informative than the separate widths W,and Wb of the two sides. In a continuation of the investigation of band shapes we have measured the pressure dependence of the spectra of electrons in several alcohols and in water. The results are reported below. Experimental Section Materials. From Fisher Co. were obtained methanol (Spectranalyzed grade), l-propanol (Certified grade), and potassium thiocyanate (Certified grade). The ethanol was Absolute Reagent grade from U.S.Industrial Chemical Co.; l-butanol was Reagent grade from BDH, nitrous oxide and argon (Ultra High Purity) were from Matheson Co. The ethanol from a newly opened bottle was used without further purification, because solvated electron half-lives in the deaerated alcohol were 5-6 ps. Methanol, l-propanol, and l-butanol were purified in the same way. To 1.5 L of alcohol was added 2 g of sodium borohydride. The solution was gently bubbled with argon and refluxed for 1day at 30 "C for methanol, or 50 "C for l-propanol and l-butanol, with a l-m condenser column (20 "C)packed with 3-mm 0.d. glass beads. Freshly cut 2 g of sodium, washed with the appropriate alcohol, was then added and the solution was refluxed for another day.
The alcohol was then distilled through the column, which was allowed to warm up. The middle 0.8 L was collected in a bulb that contained a gas bubbler and a joint that could be fitted to'the pressure system, so contact with air was avoided. The solvated electron half-life after a dose of 1.4 X 1017 eV/g at 1bar and 27 "C was 2.5 ps in methanol, 5.0 ps in ethanol, 3.4 ps in l-propanol, and 4.0 ps in l-butanol. Treatment with acidic 2,4-dinitrophenylhydrazine shortened the electron half-life in an alcohol that was purified in the above manner. The reason is not known. Water was distilled from acidic dichromate, then from alkaline permanganate, then from no additive, in a triple reflux-distillation unit. The final collection flask was protected from the lab atmosphere by a U tube and bubbler containing distilled water. The electron half-life in the water was 6 ps. The nitrous oxide was purified by bubbling it through 30 wt. % potassium hydroxide solution and collecting it in a trap at -100 "C, while pumping on the trap to remove oxygen and nitrogen. Subsequent warming of the trap provided the pressure required to bubble the nitrous oxide through the KSCN solution. Techniques. The pressure3 and optical2 systems are described in the references indicated. In addition to equipment used earlier for infrared and visible light measurements,2 we used the following for the ultraviolet range. The UV grating was a Bausch and Lomb 33-86-07 (1200 grooves/mm, 200-700 nm). The bandpass was 7 nm. The UV detector was an RCA 1P28 photomultiplier; we used only the first six dinodes, followed by an amplifier with 1504 gain. The power supply for the 1P28 was gated on-off with the lamp shutter, which opened for an internal of 1s. The power supply was set to 650-800 V, depending on the light intensity. The amplifier was connected by a 93-0 coaxial cable to the Tektronix 7623 oscilloscope. Optical slits were placed just before and just after the cell such that the light beam passed close to the steel window but did not reflect from it. The slit width was 1.34 The Journal of Physical Chemlstry, Vol. 87, No. 9 , 1977
F.-Y. Jou and 0. R. Freeman
910 I
I
1"": 12
8
;1
E
2 06-
3
U
Q
I
10 08
06
\ 13000
1
0.4
04 02
021 400
450
500 A , nrn
550
11500
Figure 1. The optical absorption spectrum of aqueous (SCN)2-: 0, 1 bar and 27 O C ; 0,2 kbar and 27 OC;A, 1 bar and 59 O C (ref 5). ,A,, = 478 f 4 nm (2.59 eV), W1,2= 0.77 eV.
mm for dosimetry and the determination of Gt,, values; it was 2.0 mm for the measurement of spectrum shapes. Corning filters were placed in front of the monochromator to eliminate higher order light interference: CS 2-64 for X between 700 and 1100 nm, CS 3-66 for 600-700 nm, CS 3-72 for 470-600 nm, CS 4-97 for 380-470 nm, CS 7-51 for 330-380 nm, and CS 7-54 for 270-330 nm. The radiation was a 1.0-pus pulse of 2.1-MeV electrons. The electrons were severely scattered by the steel window in the pressure vessel and the dose inside the cell decreased with increasing distance from the window. A new window became permanently bulged by the pressure. This increased the distance between the window and the light beam and, therefore, decreased the dose in the slice of liquid being observed. A new window was given a preliminary distortion treatment by pressurizing the cell to 2.4 kbar and holding it there for 2 h. Dosimetry. The dosimeter was oxygen saturated 5 mM KSCN, using Gt(478 nm) = 21 000 (100 eV M cm)-' for (SCN);.4 The absorption spectrum of (SCN)L was independent of pressure, as shown in Figure 1. The temperature independence was confirmed by inclusion of the points obtained earlier at 59 'C.' In reasonable agreement with earlier measurernent~,~ Gt(478 nm) was found to be 2.0 X lo4for 0.1 mM and 2.3 X lo4 for 100 mM KSCN, corresponding to G(OH scavenged) = 2.7 and 3.0, respectively. Saturating the solutions with nitrous oxide (-25 mM) increased G((SCN);) by a factor of 2.02 f 0.02 in 0.1 mM KSCN, 2.08 f 0.02 in 5 mM, and 2.00 f 0.02 in 100 mM. By moving the optical slits it was found that the radiation dose decreased approximately linearly with mass depth (g/cm2) beyond the steel window in the cell. The apparent dose measured with fixed slits decreased with increasing pressure, due to an increase of the space between the light beam and the window caused by elastic distortion of the window, and due to the increasing fluid density. The dose decrease was attributed to scattering of the electron beam. Dosimetry was done at each pressure. The pressure independence of the absorption spectrum (Figure 1)and of G(OH)6 indicate that G((SCN);) is also independent of pressure. To convert dosimetry from the aqueous solution to the alcohols at a given pressure, it was assumed that the decrease in dose due to scattering of the electron beam was proportional to the density ratio (alcohol/water) The Journal of Physical Chemistry, Vol. 81, No. 9, 1977
0
2
1
4
3
E, eV Figure 2. The normalized spectra of solvated electrons in water at 27 O C and different pressures: 0, experimental values. The full curves were calculated from the best fit of eq 1 and 2, using the parameter values in Table I. Successive curves are displaced vertically by 0.2 units to avoid overlap. The arrows indicate EAmxThe dashed curves represent the decomposition of the band at 1 bar into two Gaussian shaped lines and an E-"' tail. X represents the experimental value of (AIA,) minus the contribution(s) from the Gaussian(s). The values of ,EAmax, gfor the two lines are E, = 1.72, g, = 0.36 and E2 = 2.30, g2 = 0.25 eV.
12-
100
2E,, where E, is the transition energy to the bottom of the continuum from the most probable configuration in the ground state." Plots of (AIA,) against E-3 are shown in Figure 11. The dashed curve in Figure 10 also displays this relation. The tail of the spectrum in water has a different shape, which is approximately ET'.' for 2.8 < E < 3.7 eV; the power may be smaller at higher energies.I3 Perhaps the difference between the tails in alcohols and water is explained by the alkyl group contributing the "room" needed for the electron in the higher levels of excitation. Electron mobilities are much higher in alkanes" than in water,lgwhich indicates that the electron scattering cross section of the alkyl group is much smaller than that of the OH group. Pressure Dependence of Optical Spectra. To compare the relative magnitudes of the pressure effects on different parameters B, values of B-l (dB/dP) are listed in Table 11. The relative increase in Ehm is greater in the alcohols than in water. It is interesting that the relative magnitude of the pressure induced change in correlates with that in the liquid d e n ~ i t y ~ "rather ~ ' than with that in the This is a further indielectric constant (Table 11).21~22 dication that the energy levels are affected primarily by short-range interactions, rather than by long-range polarizations. The smaller change in E A m a x in water is balanced by a relatively larger change in the width parameters (Table 11). The band width in the alcohols is independent of pressure up to 2 kbar, with the possible exception of 1butanol. Oscillator Strengths. The oscillator strength f of an optical absorption band may be estimated from f = 3.49X 10-5.f~E dE (3) where EE is the molar absorbancy at photon energy E?3 Although there is general agreement about the values of , ~ ~ is~ still ~ ~ con~~~~ the product GfiemaXin a l ~ o h o l s there troversy about those of Gfi24-26and hence of emu. For the
-
The Journal of Physlcal Chemistry, Vo/. 81, No. 9 , 1977
E - ~ e,
~
-
~
Flgure 11. Plots of A/&
against E3for the high-energy side of the solvated electron spectra in alcohols and water at 1 bar and 27 O C . Successive curves are displaced vertically by 0.2 units.
present estimates we take e, = 1.0 X lo4M-' cm-I in all four alcohols and 1.9 X lo4My cm-'in water? The spectra were extrapolated beyond the highest ex erimental value of E to (e~/e,,) = 0 using ( E ~ / E , , ) 0: ,??. The oscillator strengths so obtained (Table IV) are lgrger than those reported earlier: mainly because the latter spectra were inadequately extrapolated to high energies. Furthermore, a correction had previously been applied for the effect of the internal field due to polarization of the solvent molecules surrounding the absorber:' but the proper magnitude of the correction is not know@ and is now not used. Although the error in f estimated by eq 3 may be 20-30%,28129the errors in the present systems might be expected to be in the same direction. The oscillator strength of the solvated electron band in alcohols is therefore similar to that in other liquids (Table IV), contrary to earlier report^.^^^ The shape of the high-energy tail differs somewhat from one type of liquid to another. We have reanalyzed recent results for electrons in tetrahydrofuran and diethyl ether at low temperatures2and found that 0: for (EE/c,) < 0.4. Using this function to extrapolate the bands to E = m increased the areas under the bands by about 10% over those obtained by the arbitrary linear extrapolation to eE = 0 at E = 4 eV. However, the uncertainties in the values of emax are -15% in the ethers, so we simply state that f = 0.7 f 0.1 in the ethers.
P
References and Notes Asslsted financially by the National Research Council of Canada. F.-Y. Jou and G. R. Freeman, Can. J . Chem., 54, 3693 (1976). 0. L. Bolton, M. G. Robinson, and G. R. Freeman, Can. J. Chem., 54, 1177 (1976). G. L. Bolton, K. N. Jha, and 0. R. Freeman, Can. J. Chem., 54, 1497 (1976)K. N. Jha, G. L. Bolton, and G. R. Freeman, J. Phys. Chem., 76, 3876 (1972). R. R. Hentz, Farhataziz, D. J. Milner, and M. Burton, J. Chem. Phys., 48, 2995, 4154 (1967). P. W. Bridgeman, "The Physics of Hlgh Pressure". Bell 8. Sons, London, 1931: (a) pp 128-130; (b) 341. M. C. Sauer, Jr., S. Aral, and L. M. Dorfman, J . Chem. Phys., 42, 708 (1965). Farhataziz and L. M. Perkey, J. Phys. Chem., 78, 1651 (1975). E. A. Shaede, L. M. Dorfman, G. J. Flynn, and D. C. Walker, C8n. J. Chem., 51, 3905 (1973). M. G. Roblnson, K. N. Jha, and G. R. Freeman, J. Chem. Phys., 55, 4933 (1971). R. R. Hentz, Fattxitazlz, and E. M. Hansen, J. Chem. Phys., 5S, 4974 (1971). R. Lug0 and P. Delahay, J . Chem. Phys., 57, 2122 (1972).
EPR from 0-Alkyl Thioesters and 0-Alkyl Selenoesters
915
(14) R. A. Ogg, Jr., J . Am. Chem. SOC.,68, 155 (1946). (15) M. S. Matheson and L. M. Dorfman in “Pulse Radiolysis”, M.I.T. Press, Cambridge, Mass., 1969, p 170. (16) See “optical line broadenlng” and “phonon” in the Glossary to Can. J . Chem., 55 (1977), Electrons in Fluids issue: J. Jortner, private communication. (17) P. Delahay, J. Chem. Phys., 55, 4188 (1971). (18) J.-P. Dodelet, K. Shinsaka, and G. R. Freeman, Can. J. Chem., 54, 744 (1976). (19) K. Schmidt and M. Anbar, J . Phys. Chem., 73, 2846 (1969). (20) R. W. Gallant, “Physical Properties of Hydrocarbons”, Vol. 1, Gulf Publishing Co., Houston, Tex., 1968.
(21) (22) (23) (24) (25) (26) (27) (28) (29)
S. Kyropoulos, Z . Phys., 40, 507 (1926). F. Buckley and A. A. Maryott, &ti. Bur. Stand. Circ., No. 589 (1958). A. Maccoll, 0 . Rev. Chem. SOC., London, 1, 16 (1947). G. R. Freeman, Natl. Stand. Ref. Data Ser., Nati. Bur. Stand., No. 48 (1974). J. H. Baxendale and P. Wardman, Nati. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 54 (1975). D. W. Johnson and 0. A. Salmon, Can. J. Chem., In press. W. Kauzmann, “Quantum Chemistry”, Academic Press, New York, N.Y., 1957, p 581. N. Q. Chako, J. Chem. Phys., 2, 644 (1934). R. S. Mulliken and C. A. Rieke, Rep. Prog. Phys., 8, 231 (1941).
Electron Paramagnetic Resonance Spectra of Some Radicals from 0-Alkyl Thioesters and 0-Alkyl Selenoesters’ D. Forrest,2a.bK. U. Ingold,*2’and
D. H. R. Barton2’
Division of Chemistry, National Research Council of Canada, Ottawa, Canada KIA OR9 and chemistry Department, Imperial College, London SW7 2AY, England (Received December 6, 1976) Publication costs assisted by the National Research Council of Canada
EPR spectra are reported for the radicals formed by addition of CF3., Me3Snq,and (EtO)zPOradicals to the C=S double bond of some thioesters and to the C=Se double bond of some selenoesters. The radicals formed by addition to thioacetate, selenoacetate, and related selenoesters adopt, for steric reasons, a conformation in which the added radical is in the eclipsed position with respect to the C, 2p, orbital. Most radicals formed by addition to thioformates and selenoformates adopt a partly staggered conformation in which the added radical lies between the eclipsed position and the C, 2p, nodal plane.
A new method for the deoxygenation of secondary alcohols has recently been de~eloped.~ The alcohol, R’OH, is first converted to an 0-alkyl thiobenzoate, R’OC(= S)C6H5,or 0-alkyl-S-methyl dithiocarbonate, R’OC(= S)SCH3,or 0-alkyl selenobenzoate, R’OC(=Se)C6H5, and the ester is then refluxed with tri-n-butylstannane in toluene. With the thiobenzoate or selenobenzoate the following free-radical chain yields the deoxygenated alcohol, R’H: n-Bu,Sn.
+ X=C
/
\
C6H5
./
-+
C6H5
n-Bu,SnXC
\
OR’
OR‘
1 1
I
--+
C6H5
R’. + n-Bu,SnXC
\\
0
R e + n-Bu,SnH -+ R’H + n-Bu,Sn. X = S, Se; R’ = secondary alkyl
The ease of this reaction implies that tri-n-butyltin adds readily to thiobenzoates and to selenobenzoates and this, in turn, suggests that the intermediate adduct radical, 1, might be examined by EPR spectroscopy. Such radicals are of interest in themselves and would be of even greater interest if a family of related radicals, 2, could be produced by the addition of transients other than n-Bu3Sn.to esters other than the benzoates, i.e. R,M*
/
+ X=C\
R’‘
/
-+
R“
hMXC
\
OR‘
OR’
2
X = S, Se; R’ = primary, secondary, tertiary alkyl; R” = H, alkyl, aryl
There have been very few EPR spectroscopic studies of
radical additions to the C=O double bond in esters4v5 or ketones4-’ or to the C=S or C=Se double bonds in thioketones” and selenoketones11s12and there have been no studies of additions to thioesters or selenoesters. There has also been only one report on the EPR spectroscopy of carbon-centered radicals which have two different atoms from group 6 of the Periodic Table attached directly to the a carbon.13 We were also intrigued by the possibility that some 1 or 2 might undergo /3 scission sufficiently rapidly that 0-alkyl thioesters would provide a convenient route for generating R’* from R’OH at ambient temperatures. Experimental Section Materials. 0-Ethyl thioformate was prepared by the method of Mayer and Berthold:14 yellow liquid with an ozone-like smell; bp 86-87.5 “C at 750 Torr (lit.1486.5-87.5 “C); proton NMR in CDC13, 6 9.50 (1H, s), 4.54 (2 H,.q, J = 7 Hz), 1.43 (3 H, t, J = 7 Hz). 0-tert-Butyl thioformate was prepared in low yield by the procedure of Barton and M~Combie:~ yellow liquid with a vile smell; NMR in CDC13, 6 9.63 (1H, s), 1.52 (9 H, s). 0-Ethyl thioacetate was prepared by the method of Ohno et al.:15 yellow liquid; bp 109-110 “C at 760 Torr (lit.15 105-109 “C); proton NMR in CDC13,6 4.41 (2 H, q, J = 7 Hz), 2.52 (3 H, s), 1.38 (3 H, t, J = 7 Hz). 0-Ethyl thiobenzoate and 0-2-methylbutyl thiobenzoate were prepared as yellow liquids by the general procedure of Barton et a1.16 The former has a bp 74-75 “C at 0.4 Torr, and proton NMR in CDC13, 6 8.03 (2 H, m), 7.26 (3 H, m), 4.62 (2 H, q, J = 8 Hz), 1.50 (3 H, t, J = 8 Hz), and the latter has proton NMR in CDC13, 6 8.00 (2 H, m), 7.30 (3 H, m), 4.40 (2 H, d, J = 7 Hz), 1.50-0.80 (9 H, m). 0-Cholesteryl selenoformate (mp 89-91 “C) and 0-ethyl selenobenzoate were prepared as previously de~cribed.~ 0-Ethyl selenoformate was obtained as a ca. 15% solution in a mixture of dimethylformamide, methylene chloride, and tetrahydrofuran: NMR in CDC13,6 11.94 (1H, s), 4.62 The Journal of Physlcal Chemistry, Vol. 8 1 No. 9 , 1977 ~
