K. N .
Jha, G . L. Bolton, and 6 . R. Freeman
hifts in the Optical Spectra of Solvated thanol and Ethano11.2 . L. Bolton, and G. R . Freeman" Chemistry Department, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
(Received May 12. 7972)
PI:blications costs assisted by the University ot Alberta
At 295 K the esolv- absorption peak E m a x and the width of the band at half height W1,z were respectively 3.95 and 1.3 eV in methanol and 1.80 and 1.4 eV in ethanol. At temperatures between 170 and 350 K , dEm,,/ dT = -2.6 X 10-3 eV/deg in methanol and -3.2 X 10-3 eV/deg in ethanol. Increasing 7' increased Wllz slightly. The shifts in E,,, caused by changing the temperature or pressure (up to 6000 atm) in a given liquid (alcohol or water) correlate with the product of the dielectric constant and the density. td. Plots of E,,, L S cd overlap for methanol and ethanol, but the curve for water falls a t higher values o f ed for a given E,,,. Electron-solvent short range interactions appear to be important in determining E,,,. The esolv- spectra were not changed in shape by addition of up to 1.4 M NOH. The height of the absorption band increased with increasing base concentratidn due to scavenging of HR,,lv+in the spurs. The molar absorbancy (decadio) coefficient of esolv- was independent of temperature: (10.2 f 0.4) x lo3 M - l 6m-I in methanol, (9.4 =+ 0.4) x 103 in ethanol and (18.9 0.6) x lo3 in water. The oscillator strength of esoIv-, corrected for the refractive index of the solvent, is 0.4 in methanol and ethanol and 0.6 in water and ammonia.
~
~
t
~
~
The work reported herein was done in parallel with a study of the effects of high pressure on properties of solvated electrons in alcohols and water.3 Temperature and pressure effects provide complementary information about the properties of species in the liquid A number of temperatureG-I1 and p r e s s ~ r e ~ J ~studies -~3 of solvated electron optical absorption spectra in ammonia,7910911 ~ a t e r , 3 , ' 1 - ~and ~ alcohol^^,^^^ have been reported. The spectra shift to higher energies with d.ecreasing temperature and increasing pressure. The present article attempts to correlate the temperature and pressure effects. New information is also provided about the molar absorbancy (extinction) coefficient and oscillator strength of solvated electrons in alcohols. ~ x ~ e r i m e Section nt~~
1Materials. Absolute methanol, analyzed- reagent spectrophotometric grade from Baker Chemical Co., was refluxed for 12 hr after addition of 2 g of 2,4-dinitrophenylhydrazine and 0.5 g of concentrated sulfuric acid to 1 1. of the alcohol. The refluxing system was rinsed with the absolute methanol before filling and was flushed with ultrahigh purity argon (Matheson) before and during refluxing. The alcohol was then distilled and the center 50% was collected. For some experiments the purified methanol was further refluxed with 10 g/l. of sodium borohydride for 5 hr, while bubbling with argon, and distilled. Absolute reagent quality U.S.P. ethyl alcohol from U. S. Industrial Chemical Co. was usually used as received because of its exceptionally high purity.14 Some of it was treated with sodium borohydride as above. The borohydride treatment did not affect the measured optical absorption spectra, but it increased the solvated electron half-lives. For all alcohol distillations the apparatus was rinsed with the initial alcohol j m t prior to filling, and the receiver was rinsed with the first portions of distillate before collecting the center fraction. The main purpose of the rinsings was to The Journal of Physical Chemistry, Vol. 76, No. 25, 1972
~remove water ~ adsorbed on c the glass ~surfaces. The ~ Baker
methanol and U. S. I. ethanol reportedly contained 0.01 and 0.005 wt % water, respectively. Sodium borohydride (Fisher, 98%) and potassium hydroxide (Fisher certified ACS) were used as received. Translucent pellets of the latter, free of carbonate spots on the surface, were selected. "Singly distilled" water was obtained from a Barnstead still. Two further distillations, one from alkaline permanganate followed by a simple distillation in a Pyrex apparatus, provided "triply distilled" water. Sample Preparation. Cells and other glassware were cleaned with hot nitric acid, then rinsed several times with singly distilled water, once with dilute sodium bicarbonate solution, then several times with singly, then triply distilled water. The glassware was then dried at 388 K in a clean oven reserved for that purpose, Finally, it was rinsed with the appropriate alcohol or solution just prior to use. (1) Supported by the National Research Council of Canada, (2) This work was reported in part in a paper by M. G. Robinson, K. N. Jha, G . L. Bolton, and G. R. Freeman a? the C.I.C. Pulse Radiolysis Symposium, Pinawa, Manitoba, Can., Oct 26, 1971,. (3) M. G. Robinson. K. N. Jha, and G. R. Freeman, J. Chem. Phys., 55, 4933 (1971). (4) 0 . E. Weigang, Jr., and W. W. Robertson i n "High Pressure Physics and Chemistry" Voi. 1, R. S. Bradley, Ed., Academic Press. London, 1963, Chapter4. (5) J . Jortner, Ber. Bunsenges Phys. Chem., 75,696 (19711. (6) (a) M. C. Sauer, Jr., S. Arai, and L. M. Dorfman. J , Chem. Phys., 42, 708 (1965); (b) S. Arai and M. C. Sauer, Jr.. ibid,, 44, 2297 (1966). (7) R. K. Quinn and J. J. Lagowski, J. Phys. Chem., 73,2326 (1969). (8) J. H.Baxendale and P. Wardman, Chem. Commun., 429 (1971). (9) R. Vogelsgesang and U. Schindewolf, Ber. Bunsenges. Phys. Chem., 75,651 (1971). (IO) R . Olinger, U . Schindewolf, A. Gaathon, and J. Jortner. Ber. Bunsenges. Phys. Chem., 75,690 (1971) (11) B. D. Michael, E. J. Hart, and K. H. Schmid?, J . Phys. Chem.. 75, 2798 (1971). (12) U. Schindewolf, H. Kohrmann, and G. Lang, Angew. Chem., lnt. Ed. Engl., 8 , 512 (1969). (13) R. R. Hentz, Farhataziz, and E. M. Hansen, J. Chem. Phys., 55, 4974 (1971). (14) S. M. S. Akhtar and G. R . Freeman, J . Phys. Chem.. 75, 2756 (1971).
~
3877
Temperature Shifts in Optical Spectra
c
Cl
Figure 1. Technique for deaerating and sealing samples (see
text).
Samples containing a solute were usually prepared using a microsyringe to transfer the required volume of a stock solution to a cell containing solvent. Stock solutions were made by weighing. Most samples in the Suprasil optical cells (1 x 1 x 4.5 cm) were deoxygenated by bubbling for 220 min with 25 cm3/min of ultrahigh-purity argon through a stainless steel needle (Figure 1A). The needle was then withdrawn until it was just out of the solution and the argon flow rate was increased somewhat (Figure 1B). The solution was then cooled to 195 K. The tubing was then heated gently with a flame a t the place where it was to be sealed, before the needle was withdrawn further. This flushed volatile substances from the heated glass wall. The needle was then withdrawn to just above the point to be sealed (Figure 1C) and the seal was made as quickly as possible. For some experiments at room and elevated temperatures the argon-flushed sample was simply sealed with a Teflon stopcock. The stopcock leaked a t low temperatures. Some samples were degassed on a vacuum line, using freezepump-thaw cycles, prior to sealing with a flame. All three techniques resulted in the same electron lifetime for a given type of sample at r o a n temperature. Temperature Apparatus. The optical cell was held snugly on three sides by a one-piece blackened-brass holder. A steel spring formed the fourth side of the holder. An adjust. able slit, usually set at 0.3 x 2.5 cm, was attached to the side of the holder. Two copper-constantan thermocouples were attached to different points of the holder and another was glued to the side of the optical cell using G.E. RTV silicone rubber adhesive. The cell holder was fixed in a Styrofoam box of 12 x 12 x 27 cm outside and 7 x 7 X 17 cm inside dimensions. At the place where the electron beam entered the box the Styrofoam was thinned from the outside to l cm to minimize spreading of the beam. The analyzing light beam entered and left the box through windows that were evacuated Suprasil cylinders ( 2 x 2 em) press-fitted into the sides of the box. At very low temperatures a stream of dry air was used to keep the windows frost free. Cold nitrogen or hot air entered the box through the bot. tom and left through an insulated 30 cm high chimney in the lid. The rate of flow of cold nitrogen was controlled by adjusting the voltage across a 1-kW capacity nichrome coil immersed in a 50.1. dewar of liquid nitrogen. The flow
rate and temperature of hot air were controlled by attaching Variacs to the fan motor and heating coil of a heat gun (Master Applicance Corp., Model HG-SOIL). Temperatures from 130 to 360 K could he maintained within f l K. Temperatures above 370 K caused distortion of the Styrofoam under the cell holder. The sample was assumed to he at the desired temperature when the thermocouple glued to the cell indicated the same temperature as those attached to the holder. Spectrophotometry. Light from a 450-W xenon arc lamp passed through a quartz lens and an iris and was further focused and transported using spherical and plane frontsurfaced (aluminum, coated with silicon monoxide) mirrors. T h e beam passed through 1.0 cm of sample. The entrance and exit slits of a Bausch and Lomb Model 33-86-25 monochromator containing a no. 33-86-02 grating were set to give a bandpass of 10 nm a t 500 nm. The monochromator wavelength scale was calibrated using a mercury lamp and a helium-neon gas laser ( A 632.8 nm). The light detector was a SGD-444-2 photodiode from EG & G Inc. (response 20% at 400 nm and 80% a t loo0 nm; sensitivity 0.5 pA/rW a t 900 nm; response linear over seven decades of incident power). The absorption signal was amplified and recorded as a voltage on either a Tektronix Type 549 storage oscilloscope or a Tektronix 7704 oscilloscope. Traces were photographed with a Polaroid Model C12 camera using type 47 or 410 Polaroid Land film. Incident light intensity was recorded on a digital voltmeter 20-40 rsec before the electron pulse. The amplifier had 136-ohm input impedance, a 35-nsec response time and had negative feedback to give linearity and stability. A light shutter was used to protect the sample from photolysis and warming. The rise time of the total measuring system was 70 nsec. The light shutter and monochromator (350-800 nm) were operated by remote control. Irradiation and Dosimetry. The 1.0- or O.lO-@ec pulses of 1.7-MeV electrons from a van de Graaff generator gave doses of 9 x 10'6 or 3 x 10'6 eV/g, respectively. The dose delivered in each pulse was monitored by a secondary emission monitor (SEM) or by current collected from the cell holder. These monitors were calibrated against the optical absorption produced in oxygen-saturated 2.0 m M KSCN aqueous solutions. To clarify earlier information'JJ6 the (SCN)Z- absorption spectrum was measured a t several timperatures in the range 293 to 332 K (Figure 2 ) : both Amax 478 + 4 nm and the absorbance per unit dose were independent of temperature. The extinction coefficient16 ((SCNh-) 76W M-l em-' was used. Assuming t h a t in the bulk solution G(OH)bu,k = G(e.,-)ci = 2.7, G(OH) scavenged by 2 m M KSCN is 2.9. This will be shown in a KSCN concentration study to be published. By comparison with the KSCN dosimeter, oxygen saturated aqueous solutions containing 5, 10, and 50 m M Fez+ gave G(Fe3+) = 15.5 f 1.1 under the present conditions. The total response time of the 10 m M ferrous dosimeter was 3 sec compared to. 360 K when necessary. The resulting values of t(Am,x) are given in Table 111. Within the experimental uncertainty the values of e(Xmax) are independent of temperature and of the presence or absence of 1 mM base: t(hmax) = (10.2 f 0.4)103 M - l cm-l in methanol and (9.4 f 0.4)103 A4-l cm-l in ethanol. The Journal of Physical Chemistry, Vol 76,
No 25, 7972
35
Figure 10. Plots of esolv- spectra to determine oscillator methanol; --.-.-, ethanol. T h e dashed strengths: -, portions were estimated by making bell-shaped extrapolations of the appropriate curves in Figures 3 and 4.
10.0 9.5 10.0
2.0
30
25
20
~ 1 1 c0m -~l )
CI-130H,neutral 1.9
IS
The values of t(hmax) derived from the other basic solutions are listed in Table 11; they are independent of KOH concentration, within experimental error. Because of the uncertain kinetics in the concentrated solutions the values in Table I11 are preferred to those in Table II. The early estimates of t(Amax), 17,000 M-1 cm-1 in methanol and 15,000 M - l c m - l in ethanol,6a were too high because low values of Gf, were used. Measurements were also made with neutral water (Table 111): €(Amax) = (18.9 f 0.6) 103 M-1 cm-1, in agreement with the previously reported 18.4 x lo3 M-1 cm-1,11 and 18.5 X 103M-l cm-1.61 Oscillator Strength f of esolv- " The oscillator strength of an optical absorption band is given by62 f = 4.32 X 10-'[9no/(no2 "r
2)2]J63
dc
(12)
where is the refractive index of the medium in which the absorber is dissolved and c y is the molar absorbancy a t wave number F. The term in square brackets i s an internal field correction; it equals unity in a low-pressure gas and 0.80 in ethanol a t 173 K (Table IV). Values of no were obtained from ref 63. The es0lv- spectra plotted on a wavelength scale are approximately bell-shaped (Figures 3 and 4 and ref 111, so the three most complete curves in Figures 3 and 4 were extrapolated to zero absorbance, then transferred onto a wave number scale (Figure 10). The oscillator strengths estimated by numerical integration under these curves are given in Table IV, along with those for electrons in water11 and ammonia.64 The values reported earlier6a,64,65were not corrected for the influence of the internal field of the solvent. When this is done and adjustment is made for the change in the values of e( Am,,), reasonable agreement is obtained between the new and old sets of data. The oscillator strengths appear to be J. C. Russell and G. R . Freeman, J. Phys. Chem., 72,816 (1968). K. N. Jha and G. R. Freeman, J. Chem. Phys,, 51,2846 (1969). G. V . Buxton, F. S. Dainton, and M. Hammerli, Trans. Faraday Soc., 63,1191 (1967). E. M. Fielden and E.J. Hart, Radiaf, Res., 32, 564 (f967). W. Kauzmann, "Quantum Chemistry," Academic Press, New York, N. Y., 1957, p 581. "International Critical Tables," Voi. 1 and 7, McGraw-Hill, New York, N. Y., 1930. D. F. Burow and J. J. Lagowski. Advan. Chem. Ser., No. 50, 125 (1965). See summary of f values, uncorrected for no. in Tabie XVI of J Jortner, Actions Chink Biol. Radiaf., 14, 7 (1970), p 64.
Reactive and Elastic Scattering of ions on Molecules
3883
TABLE IV: Oscillator Strengths for the Visible Absorption Band of esolv- in ROH
CzM50H
173 294
18.0 14.5
9.4 9.4
CH30H
183
18.0
10.2
294 292 203
15.8
H,Q
13.9 7.25
10.2 18.9 49.5
N0364
0.80 0.82 0.82
0.39 0.36 0.41 0.61
0.84 0.84
0.80d
Acknowledgment We wish to express our appreciation to
0.44
0.?86a 0.6565
0.39
0 65O 0 62
0.77
aValue from the literature adjusted to the new e(o,,) and for the internal field of the solvent internal field correction (0 84) cancels the revision of c(Xma,) from 15 800 to 18,900 M-' c m - ' 1 33 at 289 K 63
lower than previously thought, especially in methanol and et h a n d .
0.876a
Estimated from the 298 K spectrum in ref 1 1 CThe Assuming no = 1 4 at 203 K by ext+'apolation from
M. G. Robinson for many useful discussions and for his contributions toward construction of the optical system. We wish to thank the staff of the Radiation Research Center for their aid with the electronic equipment.
Elastic Scattering of Ions on Molecules rnim Henglein Hahn-A/lei~tier-institufftir Kernforschung, Sektor Strahienchemie, Berlin- Wannsee, Germany (Received May 75. 1972)
Publication cos:s assisted by Hahn-Meitner-lnstitut fur Kernforschung
Angular and energy distributions of product ions from simple ion-molecule reactions X+ + E)z XD" + D, where X+ = Ar+, Kr+, or o z + , were measured. The results were discussed in terms of three collision models; i e., spectator stripping, inipulsive isotropic scattering, and intermediate complex formation. The properties of protonated molecules XH+ and of possible intermediates XH2' of the ion-molecule reactions were deduced from experiments on the elastic scattering of H+ and of H2' on X. The potential curves of molecules XH+ were obtained with high accuracy if rainbow ondulations with fine structure could be observed in the differential scattering cross sections. The data from elastic scattering experiments were used to obtain potential energy level diagrams showing the important parts of the potential energy of ion-molecule reactions along the reaction coordinate. In the case of the reaction Ar+ -t l-1~-* AsH+ + H, 80 to 90% of the heat of reaction is liberated as internal energy of the product ion iupon the approach of the reactants, the rest appearing as translational energy upon the separation of the products. Nearly all of the heat of reaction is liberated as translational energy upon the separation of the products in the ease of the reaction Kr+ H2 KrHt H. This reaction has an activation energy of about 0.7 eV if it is initiated by the 2P3/2 ground state of Kr+. In the case of the reaction Hz+ + Hz H3 + H, an impulsive scattering mechanism is proposed at low collision energies, since no indication of a potential well for a long-lived H4+ intermediate could be obtained from the elastic scattering experiments. +
+
-
I. Introduction Studies on reactive and elastic scattering of ions on molecules contribute to our knowledge of the kinematics and dynamics of elementary chemical processes. This research is carried out today under the following two main aspects: (i) the description of ion-molecule reactions in terms of simple collision models which explain the observed angular and velocity distributions and total cross sections, such collision models often are similar to those developed in nuclear physics; and (ii) the a b initio and
+
-
+
semiemDirical calculation of Dotentials and of trajectories of the colliding particles. It is the purpose of the present reactive paper to review Some Of the more recent and elastic scattering and t o show how a combination of both types of studies may be used in order to obtain a deeper insight into the collision mechanism of ion-molecule reactions' 11. Reactive Scattering (A) Apparatus and Presentation of Data. In the first exThe Journal of Physical Chemisfry, Vol 76 No 25, 1972
