Optical Absorption Spectris of Solvated Electron
References arid Notes8 (1) Cf.. for example, 0 . Dryssen, D.Jagner, and F. Wengelin, "Computer Calculation of Ionic Equilibria and Titration Procedures," Almqvist and Wiksell, Stockholm, 1918. (2) T. Anfalt and 13. Jagner, Anal. Chim. Acta, 57, 165 (1971). (3) F. Ingman, A Johanssori, S.Johansson, and R. Karlsson, Anal. Chim. Acta . .- .-, 64. .., 113 11!373\. (4) D. M. Barry and I.. Meites, Anal. Chim. Acta, 68, 435 (1974). (5) D. M. Barry, I-. Maltes, aind B. H. Campbell, Anal. Chim. Acta. 69, 143 (1974). (6) M. J. D. Brand and 6.A. Rechnitz, Anal. Chem., 42, 1172 (1970). (7) A. F. Isbell, ,Jr., R. L.. Pecsok, R . H. Davies, and J. H. Purnell, Anal. Chen?., 45, 2363 (1973). (8) S.H. Pipor, Trans. faradi?ySoc.. 25, 348 (1929). (9) F. V. Ryer, OilSctap, 25, 310 (1946). (10) J. Luoassen, J. Phys. Chejm., 70, 1824 (1966). (11) H. L. Rosano, K. Rreindel, J. H. Schulman, and A. J. Eydt, J. Colloid lnterface Sci., 22, 58 (19618).
2631 (12) H. L. Rosano and M. E. Feinstein, Rev. Franc. Corps Gras, 13, 661 (1966). (13) M. E. Feinstein and H. L. Rosano, J. Collold Interface Sci., 24, 73 (1967). (14) M. E. Feinstein and H. L. Rosano, J. Phys. Chem., '83, 601 (1969). (15) L. Meites, "The General Muitiparametric Curve-Fitting Program CFT3," 2nd ed, Computing Laboratory, Department of Chemistry, Clarkson College of Technology, Potsdam, N.Y., 1974. (16) Primes denote assumed values, which are at first equal to the initial estimates and are then adjusted by the operation of the basic curve-fitting program. (17) Listings, in both Basic and Fortran IV, of the program SOAP, with brief directions for use, may be obtained by remitting $25.00, to defray the costs of duplication and postage, to the Computing Laboratory of the Department of Chemistry, Clarkson College of Technology, Potsdam, N.Y. 13676. A copy of the parent curve-fitting program CFT3, accompanied by a 150-page manual of explanation and instruction, and with hard-copy listings in Basic, Polybasic, and Fortran IV, may be obtained at the same time for a total remittance of $45.00. (18) R. C. Merrill and R. Getty, J. Phys. Colloid Chem., 52, 774 (1948).
Optical Absorptian Spectrum of the Solvated Electron in Some Liquid Amides and Amines1 James F. Gavlas, F. Y. Jou, and Leon M. Dorfman' I ) t ? p a W " of Chemistry, The Ohio State University, Columbus, Ohio 43210 (Received Ju/y I , 1974) PLrhficationcosts assisted by the U. S. Atomic Energy Commission
The optical absorption spectrum of the solvated electron has been determined, at room temperature, in several amides and amines. The absorption maxima are found to be as follows: N,N- dimethylformamide, 1680 nm; N,N- dimethylacetamide, 1800 nm; N,N- diethylformamide, 1775 nm; dimethylamine, 1950 nm; 1 ,%propanediamine, 1500 nm. The results for these amides are not in accord with an empirical correlation of such data recently proposed by Freeman. The general spectral grouping of the absorption of es-- in a variety of liquids with type of compound is discussed. Rate constants have been determined for reactions of es - in ethanol with formamide, N - methylformamide, and N,N- dimethylformamide. The values are respecLlvel57 4.8 X lo8, 2.2 X lo8, and 1.1 X lo9 M-l sec-l.
Introduction Experimental siiwveys of the spectrum of the solvated electron in various types of liquid have i n d i ~ a t e d ~a , ~ marked spectral grouping of the absorption maxima with type of compounci. Although there is considerable overlap of these spectral groupings, the maxima for the aliphatic alc o h o l ~are ~ found mostly in the visible to near-infrared;s the maxima for the ethers6 are in the region from 1800 to 2300 nm; several amines which have been reported3x7 are in the region from about 1300 to 2000 nm. These observations seem to indicate that molecular structure, in terms of functional groups in the molecule, is indeed an important factora (along with the dielectric behavior of the liquid) contributing to the optical properties. This paper contains results for the spectrum in three amides, two of which have been the subject of a recent reportY with which our results are sharply a t variance. These reported spectre in the amides have a special bearing on the matter of spectral grouping. Spectra have also been determined in two armies and in a hydroxy ether. Kinetics of reactions of the solvated electron in ethanol with several of
these compounds have been investigated and rate constants determined.
Experimental Section As in earlier work,1° a Varian V-7715A electron linear accelerator was the source of the 4-MeV electrons. This accelerator is capable of generating either pulses of from 10- to 80-nsec duration, with a beam current of 600 mA, or pulses of from 100- to 1400-nsec duration, with a beam current of 325 mA. In general, the shorter, higher intensity pulses were used when the species under observation had lifetimes of less than about 1psec. The 200-nsec 325 .mA pulse delivers a dose of approximately 1.2 X IOl7 eV/g in water. All experiments were conducted at ambient room temperature (23 f 2'). The radiolysis cells were made of fused quartz, with optical windows of high-purity silica. Cells of' three different sizes were employed in this work. Liquids which absorbed the analyzing light only weakly in the wavelength region of interest were irradiated in cells having optical paths of 20.0 mm. When stronger optical absorption by the liquid was The Journal of Physical Chemistry, Vol. 7R, No. 25, 1974
32
encountered~cells having optical paths of either 5.0 or 3.0 mm were needed, These cells were connected to Pyrex storage bulbs, which were equipped with Teflon stopcocks and Teflon-seal joints to allow connection to a vacuum system. A 500-W flashed xenon arc lamp was the source of the a n a ~ y ~ ~light n g beam. Appropriate Corning glass filters were placed in the optical path before the cell to prevent photolysis of the sample, and in front of the monochromator to e ~ ~ m i ~ ~~econd-order ate diffraction and scattered light effects. Fast optical absorption spectrophotometry was used both to monitor the kinetics and to map the spectra of the transient species. Transient absorptions could be monitored in the wavelength region 230-2300 nm with pub~ ~ c r o s e c o ntime d resolution. Five-nanosecond time resoluat ~ ~ a v e ~ e n gbelow t h s 1000 nm. The optitems employed in this laboratory have detai~elsewhere.6Ji Formamide and NJV- dini$thylformamide were both reagent grade from Fisher Scientific Go. N - Methylformamide, N,N- d ~ $ t h y ~ f ~ ) r m a m ~Nd emethylacetamide, 7 N,Nd ~ ~ e t h y l ~ c e ~ a m iNd @methy~propionamide, ,7 dimethylamine and 1~2-~ropanedianiine were obtained from Eastalcohol, USP, was from ; the 2-methoxyethanol was with Einde (Union Carbide) m o ~ e ~ skve ~ ~ ~ and a r distilled at reduced pressure under argon. The ~ - ~ e t h o x y e t h a nwas o l distilled at 1 atm first from an acidic 2,4-dinitrophenylhydrazine solution and then from sodium, The ethanol was distilled from a sodium athoxide solution under an argon atmosphere. The 3 , 2 - ~ r o ~ ~ nwas e drefluxed ~ ~ ~ for ~ ~several ~ i hours with potassium under B nitrogen atmosphere and then distilled. The ~ i m e t h y ~ a m iwas n ~ ~purified by means of two vacuum distillations. 411 of these liquids, except for dimethylamine, were d e ~ ~ s ~s e~ ~~ ~ e ~after i a distillation t e ~ y by subjection to at least two f r e ~ ~ e - p u m ~ - t h acycles w and were stored under vacu ' the dark until needed. The dimethylamine was ed before the vacuum distillations were carried out. the ~ ~ ~ e ~ ~ y and ~ a the m i1,2-propann e e ~ ~ were a vacwim ~ ~ distilled ~ ~ einto Pyrex storage flasks c o n ~ ~ i nm~rrors ~ n ~ of triply istilled potassium a t least 24 hr, but not more than 48 hr, before they were to undergo irradiation. Ail samples to be irradiated were introduced under vacuum into the rex reservoirs to which the radiolysis cells were connect For a typical ~ x ~ i ~with r ~ethanol, ~ e ~2-methoxyethan~ t 01, or the amines, a proximately 35-45 ml of liquid were used. After each electron pulse, the irradiated liquid was mixed with that in the reservoir to dilute any stable radiolysis products that may have been formed. The dilutions were always at leiasit 15-fold or greater, since the approximate voiuineb of I b t ? cells having 20.0, 5.0, and 3.0-mm optical paths were 2 , 1, and 0.6 ml, respectively. No reservoir sample was e ~ e sut,jected r to a total of more than about 68 pukes, and over the course of any experiment no change was ~ ~ e r ~ine either ~ t ~ ~ ~or the decay kinetics of thebyield the ~ p t absorbing ~ ~ ~transient ~ ~ species y in these liquids. Experiments with the amides were more difficult to conduct because of the relative ease with which the amides decompose upon ~ ~ ~ a ~ i aThe t i Qdecomposition ~. products were found to affect the yields of the transient species, as well as their rates of decay, both in formarnide and in N,N- ~ ~ ~ e t h y ~Therefore, ~ ~ r ~each ~ sample ~ ~ dof eamide . was irradiated only ome, after which it was discarded. The The Journa! of Physicel C,hemislry, Val, 78. No. 25, 1974
J . F. Gavlas, F. Y.Jou, and
L. M. Dorfman
cell in which a liquid amide was irradiated was always connected to two 100-ml Pyrex bulbs. One of these served as a reservoir for liquid which had not yet been irradiated; the other was used to contain liquid after it had been irradiated. Following each electron pulse, the contents of the radiolysis cell were emptied into the waste bulb, and the cell was refilled from the reservoir bulb. This procedure was followed until all of the liquid initially contained in the reservoir had been irradiated.
Results and Discussion Spectrum of e s- in Liquid Amides. The optical absorption spectrum of the solvated electron has been determined in each of three amides: N,N- dimethylformamide (DMF), N,N- dimethylacetamide (DMAd), and N,Ndiethylformamide (DEF). Shown in Figure P are the room temperature spectra in these amides over the range 800-2100 nm. The wavelengths at which the absorption maxima occur are DMF, 1680 f 30 nm; DMAd, 1800 f 50 nm; 150 nm. Data from which our optical absor tion spectrum of e,in DMF have been determined are presented in Figure 2, extending from 550 to 2100 nm. As indicated in Figure 2, this spectrum is a composite of results obtained in three separate experiments. This absorption band, having ZI maximum a t 1680 nm, can be eliminated by saturation of the DMF with N20 at a pressure of 1 atm. In the presence of anthracene (10-2 M ) the 1680-nm band is also eliminated and the spectrum of the anthracene radical anion i s observed instead. Both high reactivity toward NzO and reaction with aromatic hydrocarbons such as anthracene Lo form aromatic radical anions are, of course, typical of the solvated electron. Low signal-to-noise ratios in DEF, primarily the result of low optical densities due to decay of the short-lived e,during the electron pulse, were responsible for the large uncertainty in the location of the optical absorption maximum for the spectrum of e,- in that amide. In an earlier publication, Hayashi, et a1 the observation of e,- in both DMF and identified bands with maxima at 650 and and DMAd, respectively, with the s o ~ v ~ electron. ~ed As the spectrum in Figure 2 shows, t,here is DO indication, in our work, of any optical absorption maxiraium at or near 650 nm in DMF, the location of ,A, reported by Mayashi, et al. 9 Our determination of these absorption maxima differs from the values which theyg reported by about 1000 nm. Our new spectral data suggest that in the amides, as in the alcohols, ethers, and amines, grouping of es-. optical absorption bands by class of compound may QCCUT. The range of ,A, values in the amides, including the estimated uncertainties, is from 1650 to 1925 nm. The locations of the absorption maxima for e,- in numerous polar liquids, including the amides, are illustrated in Figure 3. The e,- spectra in the amides give some ~ n d ~ c a t ~ofothe n relative importance of the structural class of the molecules of the solvating liquid, compared to its bulk dielectric properties, in determining the location of A., From Figure 3 the absorption maxima of the e,- spectra in the amides can be seen to be located entirely within the spectral region characteristic of e,- in the amines. T h e optical properties of e,- in the amides thus appear. to be si those in the amines, despite the much higher static dielectric constants of the amides. The hydroxy compounds, which have dielectric constants closer to those of the am-
Optical Absorption Spectra of Solvated Electron
2633
TABLE I: Solvated Electron Transition Energies and Solvent Properties at Room Temperature DMF 0.738
D,
36.7U
0.689 40.2*
0.698 29.6b
d, g/cm3
0.9445a 4.13 7.844 1.73
0.936GC
4.60 9.691 1.84
0.9030c*d 4.30
Db
lOZ4a,cm36 &!K
L
1200 1600 WAVELENGTH(nm)
d
2000
DEF
E,,,, eV P,
0 ..-.”i-8OC
DMAd
11.538 1.84
a J. W. Vaughn in “The Chemistry of Non-Aqueous Solvents,” Vol. 2, J. J. Lagowski, Ed., Academic Press, New York, N.Y., 1967, Chapter 5. b Reference 12. e R. C. Weast, Ed., “Handbook of Chemistry and Physics,” 52nd ed, The Chemical Rubber Company, Cleveland, Ohio, 1971. d R . Gopal and S. A. Rizvi, J. Indian Chem. Soc., 439 179 (1966). e Calculated from bond refractions given in N. E. Hill, W. E. Vaughan, A. H. Price, and M. Davies, “Dielectric Properties and Molecular Behaviour,” Van NostrandReinhold, London, 1969, p 238.
Figure 1. Optical absorption spectra at room temperature of the solvated electron in liquid amides: DMF (. * DMAd (- - -), DEF (-).
--
s),
I
I
I
I
l
i
I
0 20T j-r--\
0 0
160
320 3
Dsdg,
Qp (x
480
640
IOz3g
Figure 4. Correlation of Freeman for transition energy for spectrum e,- with liquid properties. ,E is plotted against Q,dgl
