Spectra of Aromatic Radicals in Stretched Polymer Film
The Journal of Physical Chemistry, Vol. 83, No. 19, 1979 2501
Polarized Absorption Spectra of Aromatic Radicals in Stretched Polymer Film. 2.' Radical Ions of Anthracene and Pyrene Hlroshl Hiratsuka" and Yoshie Tankakit Department of Chemistry. Tokyo Institute of Technology, Ohokayama, Meguro-ku, Tokyo 152, Japan (Received October 3 1, 1978)
Radical anions and cations of anthracene and pyrene are prepared in polymer film upon y irradiation at 77 K by choosing a suitable polymer film-solvent system in which anthracene or pyrene is incorporated. Polarized absorption spectra of these radical ions are determined in stretched polymer film. It is shown that qualitatively reliable information is obtained on the polarization of electronic transition by dichroism analysis. The dichroic behavior of polarized absorption spectra of the radical anion is very similar to that of the cation and the pairing property of the ions of alternant hydrocarbons is demonstrated from the viewpoint of polarization. preparation of radical ions poly(viny1 alcohol), poly(viny1 Introduction chloride), and poly(ethy1ene) films were employed in A large number of investigations on electronic absorption spectra of aromatic radical ions have been r e p ~ r t e d . ~ , ~ combination with various solvents which were available for the y-ray irradiation technique of organic r n a t r i ~ e s . ~ Polarization measurements, however, have scarcely been The poly(viny1 chloride) and poly(ethy1ene) films used reported on unstable aromatic radical ions, except for the were commercially available and the poly(viny1 alcohol) study of aromatic anions produced in 2-methyltetrahydrofuran glass by means of alkali metal r e d u c t i ~ n . ~ ? ~film was prepared by pouring its hot aqueous solution on glass plate. Degree of polymerization of poly(viny1 alcohol) The stretched film technique is one of the useful was 1400. The thicknesses of these films were 0.2-0.3 mm. methods for the determination of electronic transition Hydrocarbon molecules were incorporated into the film polarization and has been employed in studying the by soaking it in a solution of the compounds in a suitable electronic absorptionG6and emission spectragfor a number solvent (see Results and Discussion, and also ref 3). The of stable molecules. This method has not yet been applied sample concentration of the film was about 10 mM. 6oCo to unstable radicals which may appear as important iny-ray irradiation was carried out at 77 K for 2 h with a dose termediates in chemical reactions. rate of 1.08 X lo6 rd/h. The absorption spectrum was In a previous paper1 the stretched film technique was measured in the range from 350 to 850 nm with a Jasco applied to polarization measurement of the absorption SS-50 type spectrophotometer which was modified to a spectrum of acridine semiquinone radical, the so-called double beam system and equipped with a Rochon poacridine C radical, produced by y irradiation of acridine larizer. Base line was recorded for the sample film before in poly(viny1 alcohol) film. In this paper we extend this y irradiation and then the absorption spectrum of the film technique to the radical ions of aromatic hydrocarbon irradiated film was recorded. The difference between the molecules. Our concern is on the following points: (1)how absorbances before and after irradiation was ascribed to can we produce radical ions in polymer film, (2) do the the absorption of the species produced. products show any dichroism, and (3) can we obtain useful Polarization measurement was carried out with stretched information on transition moment from polarization sample film. Film was stretched in a thermal chamber at measurement? First, for the preparation of radical ions, about 70 "C. Two marks, 1 cm apart in the stretching we intend to apply y irradiation of low temperature rigid direction, were made with ink on the film before stretching glassy matrices, which contain aromatic molecule^,^ to our and the film was stretched until the distance between these polymer film systems. We examine the efficiency in marks was 4 cm. Commercial poly(ethy1ene) and polyproducing radical ions for various combinations of polymer (vinyl chloride) films used were rolled during manufacture film and solvent which is used to incorporate the parent and the stretching was carried out parallel to the rolling molecule into the film. The third point can be studied by direction. The electronic absorption spectrum with pocomparing the result of polarization measurement in the larized light was obtained for two different directions of present work with that obtained by other methods. the electric vector, i.e., parallel (A ) and perpendicular (A,) In order to investigate these points we chose anthracene to the stretching direction. Tke dichroic ratio, Rd, is and pyrene as a typical cata- and peri-condensed aromatic defined as AII/A, and was used for dichroism analysis. hydrocarbon molecules. Preparation and identification of radical ions are investigated by comparing the resulting MO calculation absorption s ectra of sample films with those previously reported.2ai2b)J~ We failed to obtain essential information MO calculations were carried out for radical ions by use of Roothaan's restricted Hartree-Fock open shell method to identify radical ions in polymer film from ESR spectra. in the R electron approximation. Configuration interaction Since no polarization measurement has been carried out calculation was limited to all the singly excited and ground on these radical cations, our results of the study of poconfigurations. One- and two-center electron repulsion larization for both of radical ions are compared only for integrals were estimated by the Pariser-Parr approxithe radical anions with those reported p r e v i ~ u s l y . ~ mation. Valence state ionization potential, electron afExperimental Section finity, and resonance integral were set equal to 11.22,0.62, Anthracene, pyrene (Ultra Pure Grade, Tokyo Kasei Co., and -2.318 eV, respectively. All bond lengths were set Ltd.), and the solvents employed (Guaranteed Reagent equal to 1.395 A. Grade) were used without further purification. For the Results and Discussion Preparation and Identification of Radical Ions. In The Department of Materials Science and Technology, Technological University of Nagaoka, Nagaoka, Niigata 949-54, Japan. order to examine the preparation of radical ions of an0022-3654/79/2083-250 1$01 .OO/O
@ 1979 American Chemical Society
2502
The Journal of Physical Chemistry, Vol. 83, No. 19, 1979
H. Hiratsuka and Y. Tanizaki
1
I
I
I -I
Wave number ( x l o 3
25
20
15
10
Wave number ( x l o 3 crn-')
ern-')
Figure 2. Absorption spectra of pyrene in y-irradiated PVA (a) and PVC (b) films.
Flgure 1. Absorption spectra of anthracene in y-irradiated PVA (a) and PVC (b) films.
thracene and pyrene in polymer films, optical absorption spectra of the irradiated sample films have been compared with those of corresponding radical ions prepared in rigid glassy matrices by other m e t h o d ~ . ~As* a~result ~ ~ ~it~has ~ been found that the solvent in the film plays an important role in producing the radical ions efficiently. For preparation of radical anions, the combination of poly(viny1 alcohol) film and sec-butylamine solvent (PVA-BuA system) is available. The poly(viny1 chloride) film and sec-butyl chloride (PVC-BuC1) or carbon tetrachloride (PVC-CClJ sys,tem is suitable for preparation of radical cations. Figure l a shows the absorption spectrum of the species produced upon y irradiation of anthracene in a PVA-BuA system at 77 K. It has been compared with the spectra of anthracene radical anions which were prepared in glassy matrices of tetrahydrofuran by sodium metal reduction,2a and of 2-methyltetrahydrofuran (MTHF) by y irradiat i ~ n The . ~ ~spectrum in Figure l a is very similar to those of the radical anion with respect to the band positions and the relative intensities. It is ascribed to the anthracene radical anion. Figure l b shows the spectrum determined for the species produced upon irradiation of anthracene in a PVC-BuC1 system. It has been compared with the spectra of the corresponding radical cation prepared in oxidizing reage n k Z d However, the latter spectra are considered to be disturbed partly by some absorptions which are not intrinsic to the radical cation, and definite identification has not been possible. Thus the spectrum in Figure l b has been compared with that of the anthracene radical cation prepared in sec-butyl chloride (BuC1) glass by y irradia t i ~ n Since . ~ ~ there is close resemblance between them, the spectrum in Figure 1b is ascribed to the anthracene radical cation. The resulting absorption spectra upon irradiation of PVA-BuA and PVC-BuC1 systems containing pyrene are shown in Figure 2a and ab, respectively. The spectrum of pyrene in irradiated PVA film corresponds well to that of the pyrene radical anion prepared by sodium metal reductionzb and by y irradiation of MTHF glass.3c The spectrum of pyrene in irradiated PVC film also resembles
j
0
#
f
P
2
1.0
f
Figure 3. (a) Polarized absorption spectra of the anthracene radical anion in y-irradiated PVA film. The solid and broken curves indicate the absorbances when the electric vectors of the incident polarized light are parallel (All) and perpendicular ( A to the stretching direction of the film, respectively. Rd is defined as A I I I A l . (b) Calculated spectrum of the anthracene radical anion. Open circles (f) represent forbidden transitions.
that of pyrene radical cation prepared in y-irradiated BuCl glass3cand in oxidizing reagentseZbHence it is concluded that the pyrene radical anion and cation are produced in PVA and PVC films by y irradiation, respectively. Polarized Absorption Spectra in Stretched Polymer Films. Anthracene. Figure 3a shows the polarized absorption spectra of the anthracene radical anion determined in &etched PVA film at 77 K. It is noted that the first absorption band in the region below 12 X lo3 cm-l is missed in the Figure owing to the limitation of the spectrophotometer employed. Dichroic ratio, Rd (All/AL), has been determined in the region 12-25 X IO3 cm". It has been found that the variation of the Rd curve for the radical anion is generally smaller than that for the parent molecule.6 This fact may be attributed to the solvent contained in the film and/or to the high concentration of the sample molecule. These large amounts of residual
The Journal of Physical Chemistry, Vol. 83, No. 19, 1979 2503
Spectra of Aromatic Radicals in Stretched Polymer Film
TABLE I : Calculated and Experimental Results for Anthracene Radical Ionsa transition energy (X
io3 c m - ' )
polarizationb
obsd calcd 1 2 3 4 5 6
13.70 14.70 17.13 23.51 24.84 26.66
obsd
anionC
cationd
calcd
anion
cation
13.7e -15.5
13.6 -16.0
f Y Z f
Z Y 2
Z Y Z
24.8
22.6
Z
Z
z
oscillator str (calcd)
0.0000 0.1308 0.0225 0.0000 0.1144 0.0000
f
Y and Z are the molecular long and short axes, rea The calculated result is available for the radical anion and cation. In irradiated poly(viny1 alcohol) film. d In irradiated poly(viny1 spectively. f means that the transition is forbidden. chloride) film. e Absorption maximum,
solvent (- 10% by weight) and sample molecule ( 10 mM) are required to produce a measurable amount of radical ion in the thin film by y irradiation.1° These conditions are different from those in the usual measurement in the stretched film technique.6 Therefore we only analyze the Rd curve qualitatively. In dichroism analysis with stretched film it has been usually considered that the molecular long axis inclines preferentially along the stretching direction of film. This has been already ascertained experimentally for the acridine semiquinone radical (acridine C radical) produced in stretched PVA film upon y irradiation (the solvent was methanol).' In the case of anthracene and pyrene radical ions we may assume the same situation. As these ions have Da symmetry, the polarization direction of T-T* transition should be along the long or short molecular axes. Hence .the absorption band with a large Rd value should be polarized parallel to the long axis, while that with a small Rd value should be perpendicular to it. The Rd curve in Figure 3a is analyzed as follows. The intense second absorption band at about 14 X lo3 cm-l shows the largest Rd value and is polarized parallel to the long axis ( Y axis polarization). The weak first band located in the region below 13 X lo3 cm-l, in which a lowering of the Rd curve is observed, should be polarized parallel to the short axis (2 axis polarization). Rather intense absorption at about 25 X lo3 cm-l is considered to be polarized along Z axis because of its small Rd value. In addition to these bands we can detect another transition band in the region 15-20 X lo3 cm-' from the Rd curve behavior. In this region there appears well-defined structure and it seems to be attributed to some progression of the intense second absorption. However, the Rd curve is depressed at about 15 X lo3cm-l, and it is easily pointed out that the main part of absorption in the region 15-20 X lo3 cm-l should be assigned to a Z-axis polarized tranition different from the second transition. Its 0-0 peak should be a t about 15.5 X lo3 cm-l. The presence of this Z-axis polarized transition band has been suggested by Hinchliffe et al.ll and by Shida and I ~ a t a but , ~ direct ~ experimental detection has not been made. Our result for the polarization determination is in good agreement with the theoretical result represented in Figure 3b by the stick spectrum. The assignment of these bands is tabulated in Table I. Our result also agrees well with that obtained by Hoijtink and Z a n d ~ t r athough ,~ the third absorption band (Z-axis polarized weak absorption in the region 15.5-20 X lo3 cm-l) was not assigned by them. This assures us that the stretched film technique combined with y irradiation gives sufficiently reliable information on the polarization directions of the radical ions produced. Figure 4 shows the polarized absorption spectra of the anthracene radical cation in stretched PVC film at 77 K. The Rd value is also determined in the region 12-25 X lo3 N
a l '
U C
m
ft VI
n 4
I
I
25
20 Wave number
15 (x
10
lo3 crn-')
Figure 4. Polarized absorption spectra of the anthracene radical cation in y-irradiated PVC film.
cm-l. The Rd curve behavior is nearly the same as that of the radical anion (though the absolute value is small) and the analysis of the Rd curve can be carried out in a similar manner as that for the radical anion. The intense second absorption at 13.5 X lo3 cm-l and the first band below 13 X lo3 cm-l are polarized along the long (Y)and short (2)axes, respectively. The third band in the region 16-20 X lo3 cm-l shows Z-axis polarization and its 0-0 peak appears a t about 16 X lo3 cm-l. This band is located a t slightly higher energies than that of the radical anion and its overlap with the second intense band is smaller than that of the anion. The intense band a t 22.6 X lo3 cm-l is polarized parallel to Z axis and is assigned to the fifth transition. This band corresponds to the 25 X lo3 cm-l band of the anion. The assignment of the spectrum of the cation is also included in Table I. (In this table the calculated results are available for the radical anion and the cation since the same results are obtained for both radicals.) From the above assignments it can be seen that the pairing property of the radical ions of alternant hydrocarbon is satisfied in the case of anthracene with respect to transition directions.12 Pyrene. Figure 5 shows the polarized absorption spectra of the pyrene radical anion. In this figure the first two peaks observed in the region 9-12 X lo3 cm-l are missed owing to the limitation of the spectrophotometer. According to their large Rd value, the absorption bands a t about 14 and 20.5 X lo3 cm-l are polarized parallel to the long axis (Z-axis polarization). The absorption bands with small Rd values at 12,16-20, and 26 X lo3 cm-l should be polarized along the short axis (Y-axis polarization). This determination of the polarization of visible bands is identical with that of Hoijtink and Z a n d ~ t r aexcept ,~ for
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The Journal of Physical Chemistry, Vol. 83, No. 79, 1979
H. Hiratsuka and Y. Tanizaki
TABLE 11: Calculated and Experimental Results for Pyrene Radical Ionsa transition energy ( X IO3 cm-')
polarizationb
obsd calcd 1
anionC
10.56 14.93 16.50 20.11 24.14 29.40 29.61 30.03 31.74
2
3 4 5 6 7
8 9
obsd cationd
calcd
anion
cation
oscillator str (calcd) 0.0000
f 13.8
12.6 15.0
16-20e 20.3
20.0 22.0
26.1
Y
Y
Z
Z
Z
0.0643 0.0001
Y
Y
Y
0.0181
Z f f
Z
Z
0.3174 0.0000
Y
Y
0.0000 0.0014 0.0000
f
*
a The calculated result is available for the radical anion and cation. Y and Z are the molecular short and long axes, respectively. f means that the transition is forbidden. C In irradiated poly(viny1 alcohol) film. d In irradiated poly( vinyl chloride) film. e Shoulder.
!:
Rd
25 25
20
Wave number
15
10
( x 103cm-l )
Flgure 5. Polarized absorption spectra of the pyrene radical anion in y-irradiated PVA film.
the band in the region 16-20 X lo3 cm-l, which was assigned to the absorption of its proton adduct. Shida and Iwata pointed out that this band is inherent to the pyrene radical anion and assigned it to the fourth (Y-axis polarized) transition by reference to the calculated result.3c Our analysis agrees with their conclusion. The polarized absorption spectra of the pyrene radical cation are shown in Figure 6. The first two peaks which are not seen in Figure 5 appear in the region 12-14 X lo3 cm-l. The bands at 12 and 22 X lo3cm-l show the smallest and largest Rd value in the observed region and are considered to be polarized along the shorter and longer axes, respectively. Thus the first two peaks are assigned to the second (Y-axispolarized) transition and the intense 22 X lo3cm-l band to the fifth (Z-axis polarized) transition. The 15 X lo3 cm-l band in Figure 6 should correspond to the 14 X lo3 cm-l band in Figure 5, the third transition of the radical anion. These tentative assignments for the pyrene radical ions are shown in Table 11. As is the case for the anthracene radical ions, the pairing property is also satisfied for pyrene radical ions with respect to transition directions.
Concluding Remarks Irradiation with y rays can produce the radical anions and cations of anthracene and pyrene in polymer films by choosing a suitable combination of polymer film and the solvent which is employed to incorporate the parent molecule into the film. The following systems are available for the preparation of the radical ions: for the anion, poly(viny1 alcohol) film and sec-butylamine; and for the cation, poly(viny1 chloride) film and sec-butyl chloride or
20 Wave number ( x
15
I
10
l o 3 cm-l)
Flgure 6. Polarized absorption spectra of the pyrene radical cation in y-irradiated PVC film.
carbon tetrachloride. Using these systems, one can determine the polarized absorption spectra for the radical ions in stretched polymer films and reliable information is obtained on the electronic transition moment. Owing to its relative simplicity, the stretched film technique combined with y irradiation is attractive for determining the direction of the transition moment of aromatic radicals and radical ions. Our method has been examined for a number of aromatic compounds, such as naphthalene, phenanthrene, biphenyl, and their derivatives. These results will be published soon. Acknowledgment. We are much indebted to Dr. Y. Hatano of the Tokyo Institute of Technology for many helpful discussions and critical reading of the manuscript.
References and Notes (1) Pari 1: H. Hiratsuka, K. Sekiguchi, Y. Hatano, and Y. Tanizaki, Chem. Phys. Lett., 55, 358 (1978). (2) (a) P. Balk, G. J. Hoijtink, and J. W. H. Schreurs, Red. Trav. Chim. Pays-Bas, 76, 813 (1957); (b) P. Balk, S.de BruiJn,and G. J. Hoijtink, Recl. Trav. Chim. Pays-Bas. 76, 907 (1957); (c) G.J. Hoijtink, N. H. Velthorst, and P. J. Zandstra, Mol. Phys., 3,533 (1960); (d) W. I. Aalbersberg, G. J. Hoijtink, E. L. Mackor, and W. P. Weljland, J. Chem. Soc., 3049, 3055 (1959); (e) P. Bennema, G. J. Hoijtink, J. H. Lupinski, L. J. Oosterhoff, and J. D. W. van Vwrst, Mol. Phys., 2, 431 (1959). (3) (a) W. H. Hamill, "Radical Ions", E. T. Kaiser and L. Kevan, Ed., Interscience, New York, N. Y., 1968, p 321; (b) T. Shida and W. H. Hamill. J. Chem. Phvs., 44, 2375, 4372 (1966); (c) T. Shida and S.Iwata, J. Am. Chem-. SOC.,95,3473 (1973), and references cited therein. (4) G. J. Hoijtink and P. J. Zandstra, Mol. Phys., 3, 371 (1960). (5) K. H. J. Buschow and G. J. Holjtink, J. Chem. Phys., 40, 2501 (1964); K. H. J. Buschow, J. Dieleman, and G. J. Hoijtink, Mol. Phys., 7, 1 (1964). (6) H. Hiratsuka, Y. Tanizaki, and T. Hoshi, Spectrochlm. Acta, Pari A, 28, 2375 (1972), and references cited therein. T. Yoshinaga, H.
Copper-Ammonia Complexes Adsorbed on Silica Gels Hiratsuka, and Y. Tanizaki, Bull. Chem. SOC.Jpn., 50, 3096 (1977). (7) E. W. Thulstrup, J. Michl, and J. H. Eggers, J. Phys. Chem., 74, 3868 (1970); J. Michl, E. W. Thulstrup, and J. H. Eggers, bid., 74, 3878 (1970). (8) A. Davidsson and 6 . Norden, Chem. Scr., 9, 49 (1976). (9) J. J. Dekkers, G. Ph. Hoornweg, C. Maclean, and N. H. Velthorst, Chem. Phys., 5, 393 (1974). (10) For a general description of the preparation of radical ions in rigid
The Journal of Physical Chemistry, Vol. 83,
No. 19, 1979 2505
matrices by y irradiation, see ref 3a. (11) A. Hinchliffe, J. N. Murrell, and N. Trinajstic, Trans. Faraday Soc., 62, 1362 (1960). (12) A. D. Mclachlan, Mol. Phys., 2, 271 (1959); J. Michl, J. C b m . Phys., 61, 4270 (1974). The pairing property of mono- and divalent ions of some aromatic molecules was studied also by MCD spectrum. See, for example, R. J. Van der Wal and P. J. Zandstra, Chem. Phys. Lett., 36,500 (1975).
ESR Study of Copper-Ammonia Complexes in Solution Adsorbed on Silica Gels. 1. Wide-Bore Silica Gels Giacomo Martini" and Leo Burlamacchi Istituto di Chimica Fisica, UniversU di Firenze, 50 12 1 Firenze, Italy (Received February 5, 1979)
The behavior of water-ammonia solution filling the pores of silica gels with pore diameters 520 nm was studied by differential scanning calorimetry and electron spin resonance of Cu(I1) paramagnetic probes. The copper-ammonia complexes are only in part chemisorbed on surface sites by bonding with deprotonated silanol groups. The remainder are dissolved into the intracrystalline liquid as a pentaammine complex. Upon freezing, this part segregates out as solid crystals dispersed in the frozen water-ammonia mixture. The freezing points of water-ammonia mixtures of different concentration do not correspond to those predicted by the phase diagram for bulk unadsorbed mixtures. This fact was discussed in terms of long-range solid-to-liquid interactions and of preferential adsorption of ammonia molecules on the solid surface.
Introduction Electron spin resonance (ESR) has been used in the past for the study of the coordination and bonding of copper(I1) complexes with nitrogenous ligands adsorbed on silica gels and on other porous supports.l-ll The major aim of those works was to give further information on the behavior of metal ions bonded on surface sites for its implications in catalytic processes. Thus, almost all reported results were obtained from dried or vacuum-heated samples. However, it is also possible to study the behavior of Cu(I1) complexes, such as C U ( H ~ O )or~ ~ copper-ammine + complexes, still dissolved in intracrystalline liquid of fully solvated samples. The ion itself can be used in this case as a paramagnetic probe to obtain information on the nature of the intracrystalline l i q ~ i d . ~ ~ J ~ This paper reports a study on Cu(1I) water-ammonia solution adsorbed on wide-pore silica gels (pore diameter 1 2 0 nm). The ESR results from copper-ammonia complexes adsorbed on narrow-pore silica gels (pore diameter 110 nm) will be analyzed in the following paper, with particular emphasis on the stoichiometry and coordination of surface adsorbed c0mp1exes.l~ Experimental Section Sample Preparation. Solutions of copper-ammonia complexes ([Cu(II)] from 0.025 to 0.1 hl) at different copper concentration were prepared by dissolving the appropriate amounts of Cu(C104)2.4H20solid salt (Ventron) in aqueous solutions of ammonia a t different concentrations ([",] = 4.7, 8.4, and 16.6 M corresponding to pH 11.8, 12.4, and 13.8, respectively). Three samples of silica gels (Merck adsorbent for chromatography) were used with monodispersed pores of diameters of 20,50, and 100 nm, respectively, called S20, S50, and S100. The physical properties of these supports are reported in Table I. Impregnation with copper-ammonia solution was carried out by putting 3.0 g of the porous support into 10 cm3 of the appropriate solution, storing for 24 h in closed 0022-3654/79/2083-2505$0 1.OO/O
TABLE I: Silica Gels Used in T h i s Work type s20
S50
SlOO
pore diam, nm 20 50 100
surface pore area, m2/g vol, cm3/g 150 50 25
0.65 0.65 0.65
flasks at room temperature, and then filtering. After filtration, the solid was quickly dried by gentle smearing on a filter paper immediately stopping the operation as soon as an apparently, freely running, dry powder was obtained, in order to minimize ammonia evaporation. Freshly prepared samples were used in each experiment. Electron Spin Resonance Measurements. ESR spectra were registered with a Bruker 200tt spectrometer operating in the X band. The samples were sealed into a quartz capillary with an inner diameter 1 mm. Temperature variations were obtained with a Bruker B-ST 100/700 variable temperature assembly. Differential Scanning Calorimetry Measurements. DSC diagrams were registered with a Perkin-Elmer DSC 1B instrument with a constant rate of heating. The usual volatile-sample pans were used for the measurements below room temperature. Optical Spectra. Optical measurements were carried out with a Perkin-Elmer Model 200 UV-VIS spectrophotometer.
-
Results The copper-ammonia species in the solutions used for adsorption were first checked by ESR and optical spectra. At room temperature, all ESR spectra showed four well-resolved isotropic hyperfine lines due to the interaction of the unpaired electron with the I = 312 nuclear spin of 63Cuand 65Cuisotopes. At 77 K, the same solutions, after addition of 5% glycerol in order to prevent crystallization, gave rise to typical glass ESR spectra whose magnetic parameters together with the absorption maxima observed in the optical spectra at room temperature are 0 1979 American Chemical Society
