J. Phys. Chem. 1981, 85, 2557-2562
saturation current density with the flame irradiated by the laser, A cm-2 Boltzmann constant, erg deg-l ionization rate constant for ground-stateNa atoms, S-1
ionization rate constant for excited-stateNa atoms, S-1
collisional quenching rate constant for excited-state Na atoms, s-l magnitude of the mobility of either positive or negative ions in the cold air spaces between the flame and the electrodes, cm2 V-' s-l magnitude of the mobility for positive ions (K+) or electrons (K-) in the flame where K- >> K+, cm2 v-1 s-l
separation of the parallel electrodes, cm number density of the positive (n+)or negative (n-) ions (electrons) in the flame, cm-3 number density of excess positive ions (n+')or excess electrons (n-') resulting from laser excitation, cm-3 number density of neutral metal atoms in the flame with no laser excitation, cm-3 number density of neutral metal atoms in the flame under the influence of laser excitation, transmitted laser power emerging from the flame, W laser power incident on the flame, W total volume ionization rate in the flame, cm-3s-l partial volume ionization rate due to natural flame species, cm-3 s-l partial volume ionization rate due to metal species aspirated into the flame, s-l
2557
fraction of neutral atoms maintained in the excited state by laser irradiation peak absorption cross section for the Na 3 s 3P3 transition at atmospheric pressure and 2300 cm2 time coordinate, s absolute temperature, K decay-time constant of the LEI signal, s rise-time constant for the LEI signal, s velocity of positive ions (U+)or electrons (U-)due to diffusion, cm s-l rise velocity of the burnt gases of the flame, cm s-l velocity of positive ions (u+) or electrons (v-) due to an externally applied electric field, cm s-l magnitude of the potential difference applied between the anode and the cathode, V minimum value of the applied potential (V,) for which the current density is maximized with electrodes contiguous to the flame; saturation voltage, V saturation voltage with electrodesremote from the flame, V saturation voltage with electrodes contiguous to the flame, and the flame irradiated by the laser, V saturation voltage with remote electrodes and laser irradiation of the flame, V Coulombic velocity imparted to positive ions (w+) or electrons (w-)by the electric field resulting from a unipolar charge excess, cm s-l spatial coordinate for one-dimensional model, ranging from 0 at the anode to L at the cathode, cm
- d,
Electron Spin Resonance Study of Silver Atom Solvation in Methyl Cyanide at 4.2 K A. S. W. LI and Larry Kevan" Departmnt of Chemistry. University of Houston, Houston, Texas 77004 (Received: December 16, 1980; In Final Form: April 24, 1981)
Silver atoms formed by X irradiation of frozen, methyl cyanide solutions of silver perchlorate are studied at 4.2 K by electron spin resonance. It is observed that there is no dramatic site change upon 77 K annealing but there is some increase in Ago intensity and sharpening of the hyperfine structure upon 77 K annealing. These results imply that there is some rearrangement of the atom toward equilibrium solvation upon thermal annealing. If the irradiated sample is subjected to visible light at 4.2 K, a new Ago site, site CNI, is observed which is similar to A$ site 2020 in the EtOH-H20 solvent system. The site CNI disappears upon 77 K annealing and can be recovered upon visible light illumination. Introduction Atomic and ionic silver =e important chemical species not only because of their well-known role in the photographic process but also because of their catalytic properties and their sensitivity as probes for local condensed phase environments. Their catalytic efficacy has been demonstrated with regard to the partial oxidation of ethylene on aluminum oxide' and the formation of saturated nitrogen compounds from ammonia on A Silver has also been implicated in the cleavage of water on silver-exchanged Y eol lite.^^^ The large isotropic hy(1) Kilty, P. A.; Sachtler, W. M. H. Catal. Rev. 1974, 10, 1. (2) Kim, Y.; Seff, K. J. Am. Chem. SOC.1977,99, 7055. (3) Kim, Y.; Gille, J.; Seff, K. J. Am. Chem. SOC.1977, 99, 7057. (4) Jacobs, P. A.; Utterhoven, J. B.; Beyer, H. K. J. Chem. Soc., Chem. Commun. 1977,128. 0022-3654/81/2085-2557$01.25/0
perfine coupling of atomic silver has been effectively used 8s a probe Of the extent Of 10Cd environmental interaction in aqueous media,&ll al~ohols,'~J~ and even (5) Leutwyler, S.; Schumacher, E. Chimia 1977, 31, 475. (6) Narayana, M.; Li, A. S. W.; Kevan, L. J.Phys. Chem. 1981,85,132. (7) Abou-Kais, A,; Vedrine, J. C.; Naccache, C. J . Chem. Soc., Faraday Trans. 2 1978,74,959. (8) Zhitnikov, R. A,; Kolesnikov, N. V.; Kosyakov, V. I. Zh. Eksp. nor. 1963, 4'4 1204; Sou. phYs.JETp (End. 2'm.d.) 1963,17,815. (9) Shields, L. J. Chem. Phys. 1966,44, 1685. (10) Bales, B. L.; Kevan, L. Chem. Phys. Lett. 1969,3,484. (11) Brown, D. R.; Symons, M.C.R.J.Chem. Soc., Faraday Tram. I 1977, 73, 1490. (12) Zhitnikov, R. A.; Peregud, D. P. Fiz. Tuerd. Tela 1970,12,1993; Sou. Phys.-Solid State (Engl. Transl.) 1971,12, 1583. (13) Li, A. S. W.; Kevan, L. J. Phys. Chem. 1980, 84, 2862. (14) Zhitnikov, R. A.; Kolesnikov, N. V.; Kosyakov, V. I. Zh. Eksp. Teor. Fiz. 1962,43,1186; Sou. Phys.-JETP (Engl. Traml.)1963,16,839.
0 1981 American Chemical Society
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The Journal of Physlcal Chemistry, Vol. 85,No. 17, 1987
Li and Kevan
TABLE I: Hyperfine Splittings, g Values, and High-Field Line Widths for lWAgAtoms Produced at 4 K and Observed at 4 K When 0.6 M AgCl,/CD,CN Was X-Irradiatedg ~
radicals Ago/free atom Ago/D,O (site I)b Ago/D,O (site II)c Ag0/8EtOH + 2H,O (site 2020)c Ago/CD,CN (site II)c AgO/CD.CN (site IId 14N(AgoiCD,CN),site I1 “N( Ag” /CD,CN) 11 I4N(Ag2+ /CD,CN)lf
g value“
1.9993 f 1.9964 * 2.0005 * 1.9965 f 2.0024 * 1.9966 f 2.3212 * e
Aiso, MHz
2 2 3 6 5 6 8
Aupp, MHz
1977 1763 f 1 1466 f 1 2020 f 2 1600f 5 2006 t 5 16.0 i 0.3 70.1 f 0.7 74
9.0 f 0.5 15.6 r 0.6 10.4 f 0.4 41.9 f 0.3 38.5 ~t1.0
-
ref 26 16 15 13 this work this work this work this work this work
a The uncertainties refer to the last digit in the g value. Site initially formed at 4 K. Site formed after thermal excitation at 77 K. Site initially formed at 4 K after optical bleaching with visible light. e Unavailable because of interference with central features. f Measurements made at 77 K. The uncertainties given are average deviations.
The most detailed picture of local environmental interaction has been worked out for silver atoms in aqueous media by using electron spin resonance (ESR)15 and electron spin-echo (ESE)ls spectroscopy. It is found that silver atoms generated by electron reduction of the ion at 4.2 K are produced in a nonequilibriumstate for the atom. Thermal excitation at 77 K is sufficient to reorient the first solvation shell waters so as to achieve a near equilibrium solvation state for the atom. In contrast to polycrystalline aqueous matrixes, silver atoms generated at 4.2 K in glassy alcohol matrixes17do not show geometrical changes in the first solvation shell interpretable as a solvation process upon thermal excitation. This may be related to the constraints on atomic and molecular motions in glassy vs. polycrystalline media.18 To compare with silver atom solvation in polar, polycrystalline aqueous media, we here report results in polycrystalline methyl cyanide. Silver atoms have recently been generated in methyl cyanide at 77 K.19 Here we extend such studies to the generation of silver species at 4.2 K and explore the effects of thermal and photoactivation on local environmental changes around the silver atoms. Experimental Section Ca. 0.15 mL of freshly prepared 0.5 M AgC104was added to 3.0-mm i.d. Spectrosil quartz tubes which were sealed in air. The tubes were quick-frozen by plunging into liquid nitrogen to form white polycrystalline samples. The samples were irradiated in liquid helium in an ESR Dewar with a Siemens X-ray tube (Model AGW61) connected to a Kristalloflex-2 power supply. All irradiations were carried out a t 60 kV and 50 mA; doses were typically 0.9 Mrd at a dose rate of 0.8 Mrd h-l. Spectra were obtained with a Varian E-102 ESR spectrometer. Annealing at 77 K was done by removing the samples from liquid helium, plunging them into liquid nitrogen for a specified time, and reinserting them into liquid helium. The frequency was measured directly with a microwave frequency counter. The calibration on the field sweep and the field was checked against the hyperfine splitting and g factor of hydrogen atoms trapped in the irradiated Spectrosil tubesm in which the samples were measured. The ESR parameters were derived from the Breit-Rabi equation.21 Optical (15) Kevan, L.; Hase, H.; Kawabata, K. J. Chern. Phys. 1977,66,3834. (16) Ichikawa,T.;Kevan, L.; Narayana,P. A. J.Chern. Phys. 1979,71, 3792. (17) Ichikawa,T.;Kevan, L.; Narayana,P. A. J.Phys. Chern. 1979,83, 3378. (18) Ichikawa, T.; Li, A. S. W.; Kevan, L. J. Chern. Phys., in press. (19)Alesburv. C. K.:. Svmons. M. C. R. J. Chern. SOC.,Faraday Trans. _ 1 1978, 76, 244: (20) Ogawa, S.; Fessenden, R. W. J. Chern. Phys. 1964,41,1616. (21) Briet, G.; Rabi, I. Phys. Rev. 1931, 38,208.
I09 2’ Agll
107 2+ Agll
I I Ill
I
H H
Flgure 1. ESR of 0.8 M AgCIO,/CD&N X-Irradlated and observed at 77 K.
bleaching experiments with visible light were done at “low intensity” with a 40-W tungsten lamp with a high-energy cutoff at 400 nm or at “high intensity” with a 500-W slide projector with a high-energy cutoff at 370 nm. The relative intensity of these two sources at the sample position was measured to be 200. Results Naturally occurring silver consists of 48% ‘@Agand 52% lo7Ageach of which has a nuclear spin I = 1/2. One hyperfine doublet arises from each isotope, resulting in a four-line spectrum. The ratio of the nuclear moments (g(109)/g(107) = 1.15) of the two silver isotopes is often large enough to permit resolution of all four lines of Ago, with the bigger splitting assigned to 10sAg. 77 K Experiments. Figure 1 shows the spectrum of 0.6 M AgC104/CD&N X-irradiated and observed at 77 K. Deuterated compounds were used throughout this study because the hyperfine components are narrower. The major radical obtained for 77 K X irradiation is .CDS, which exhibits a septet near the center of the spectrum. Deuterium has a nuclear spin of 1, and interaction of three deuterons with an unpaired electron gives a septet with relative intensities 1:3:67:63:1. With higher gain, A$ and Ag2+ species were also observed. In Figure 1 the peaks marked H are the hydrogen atoms trapped in the irradiated quartz sample tube which also serve for field calibration.20 The broad peaks of the Ago species have nine hyperfine components indicating interaction with four equivalent nitrogens since I = 1 for 14N. The Ag2+species also shows nine hyperfine lines with a larger 14Nsplitting. Because of the small isotropic silver hyperfine splitting, the 10sAg2+and lo7Ag2+peaks are not resolved. The ESR parameters of the Ago species were calculated by the
The Journal of Physical Chemistty, Vol. 85, No. 17, 198 1
Silver Atom Solvation in Methyl Cyanide at 4.2 K
2559
W
5
3-
_I
[r W
>k
2 2t-
z pl
a
DOSE, MRAD
Flgure 3. Agoyleld vs. dose for 0.6 M AgCI04/CD3CNX-irradiated and observed at 77 K.
1
2
3
DOSE, MRAD
Figure 2. CD3 yield vs. dose for 0.6 M AgCI0,/CD3CN X-Irradiated and observed at 77 K.
Briet-Rabi equation.21 The ESR parameters of the Ag2+ ions were evaluated directly from the spectra taking the midpoint between the fourth and fifth peaks as the resonance field. The results are listed in Table I. They agree well with those of previous workers.l9 The silver species at 77 K only decrease in intensity when bleached by visible light. The Agospecies decreases faster than the Ag2+species. The CD3 intensity increases as much as 2-fold after extended bleaching. There is very little change in the silver species on storing in the dark at 77 K for 30 days. This behavior is quite different from that of the Ago/D20systemaZ2 The frozen sample is initially a white polycrystalline solid, and it becomes very colorful after X irradiation. The portion of the sample that directly faces the window of the X-ray gun receives the most X irradiation and turns an intense pink with black or deep purple portions in the center. The outer portion that receives less X irradiation turns a bright yellow that is characteristic of monoatomic silver species. Visible light bleaches the darker color before the yellow color. These color observations qualitatively indicate that species other than monoatomic silver species are formed at high dose. Quantitative dose-yield curves support this. Figures 2-4 show the yield of CD3, A$, and Ag2+with respect to dose for 77 K X irradiation. Each figure shows the relative yield of that radical only. Absolute yields were not measured so the relative yields in Figures 2-4 are not intercomparable. The same sample was used to obtain these curves with periodic interruption of the X irradiation. Note that the Ago and Ag2+species only increase linearly with dose to -0.8 Mrd before plateauing while .CD3 increases beyond 3.3 Mrd. The general shape of the spectrum does not change much with dose except that the hyperfine structure of the low-field peak of Ago becomes less defined with higher dose. At 3.3 Mrd, this hyperfine structure becomes unresolved. (22) Bales, B. L.;Kevan, L.J. Chem. Phys. 1971,55, 1327.
li 2
I
3
1
DOSE, MRAD
Flgure 4. Ag2+ yield vs. dose for 0.6 M AgCIO4/CD3CNX-irradiated and observed at 77 K. 2927 G 2927G I
i~
I07 Ag
3433 G l00G
,
i i
IH
H II "
109Ag0
Gain X Z
Flgure 5. ESR of 0.6 M AgC104/CD3CNX-irradiated and observed at 4 K. The high- and low-field peaks of trapped Ago are shown.
4 K Experiments. General Considerations. Figure 5 shows the ESR spectrum of 0.6 M AgC104/CD3CNirradiated and observed at 4 K. The Ago yield is low and is severely masked by the hydrogen signal of the X-irradiated quartz tube. By comparison with the 77 K spectra, the g values and the isotropic hyperfine constants can be estimated, and it is found that there is no dramatic change in the ESR parameters between the 4 and 77 K irradiations. The 14Nhyperfine structure is barely observable. Ag2+ ions are also observed under very high gain and modulation amplitude. Trapped D atoms are also observed, and their yield is also very low. Their concentration is roughly equal to that of Ago.
2560
Li and Kevan
The Journal of Physical Chemlstty, Vol. 85, No. 17, 7981 3200 G
3200G
(CD3CN)Z
Figure 8. ESR of 0.6 M AgCiOJCD,CN X-irradiated and observed at 4 K. The central portion of the spectrum is shown, an admixture of .CD3 and (CD3CN)d. 2933 G
2933 G
GAIN X2
Flgure 8. 77 K observation of 4 K X-irradiated 0.0 M AgCIO,/CD,CN after being stored in the dark at 77 K for 50 days (a) before and (b) after bleaching with low intensity visible light.
2933 G
li h 3200 G
ilil(1
(0)
GAIN X2 5
(b) GAIN X I
I.nnl
(C)
GAIN X1.5
Figure 7. ESR of 0.6 M AgCi04/CD3CNX-irradiated and observed at 4 K. The low-field spectrum of Ago is shown (a) before and (b) after 5-s annealing at 50 K and (c) after 2-min annealing at 77 K. Note the growth of Ago in b and the better resolved peaks in c.
-
TABLE 11: ESR Parameters of Central Spectrum
Radicals Observed at 4 K When 0.6 M AgCIO,/CD,CN Was X-Irradiated at 4 K radical g value Aiso, MHz AuPp, MHz (CD,CN),-
*CD, a
2.0015 rt 8 2.0019 f 4
47.6 f 0.6 10.6 f 0.3
6.4 f 0.4 * 0.4
4.4
The uncertainties refer to the last digit in the g value.
The central portion of the 4 K spectra is different from the 77 K spectra. It is a quintet superimposed on a septet as shown in Figure 6. The g values and the isotropic hyperfine splittings are listed in Table 11. The septet arises from .CD3 radicals. The quintet is identified= from previous studies as an electron interacting with two CD&N molecules, (CD,CN),. The quintet dominates at low dose while the septet dominates at high dose up to 1.8 Mrd. At 2.4 Mrd only the wings of the quintet are observable. Samples with added fluoride (0.2 M NaF) were also studied for both 4 and 77 K X irradiation. It is found that added P does not enhance the Agoyield in contrast to the case of Ago/D20.24 4 K Experiments. Thermal Annealing. When the sample is annealed for 5 s at -50 K above the liquid level in the helium Dewar and reexamined at 4 K, the Ago and Ag2+intensities increase by 4-6 times. This is shown in Figure 7. When the sample is annealed for 2 min a t 77 K followed by 4 K observation, the hyperfine structure becomes much better resolved. After 77 K annealing, the (23) Egland, R. J.; Symons, M. C. R. J. Chern. SOC.A 1970, 1326. (24) Bales, B. L.; Kevan, L. J. Phys. Chern. 1970, 71, 1098.
11“
ill
(a’
II. I
Figure 9. ESR of 0.6 M AgCi04/CD3CNX-irradiated and observed at 4 K after optical bleaching of sample with low intensity visible light and (a) before and (b) after 2-min annealing at 77 K. Note the recovery of the quintet after 77 K annealing.
Ago and Ag2+spectra are similar to those after 77 K irradiation. The center part of the spectrum is not affected much by annealing at -50 or 77 K except for some improvement in the hyperfine resolution. Also the relative intensities of the septet approached more closely to the theoretical values after annealing. If the 4 K X-irradiated sample is stored a t 77 K in the dark for 50 days, the central features change to a broad peak with barely resolvable hyperfine, as shown in Figure 8a. However, the septet hyperfine becomes resolved with a 2-fold intensity increase after bleaching with “low intensity” visible light, as shown in Figure 8b. The bleached samples recovered a purplish dark color after being stored at 77 K for several days. Trapped D atoms observed at 4 K disappear completely upon brief 77 K annealing and reexamination at 4 K. 4 K Experiments. Optical Bleaching. When the 4 K sample is irradiated with “low intensity’’ visible light a t 4 K for 10 min (40-W lamp), there is no observable change in the Ago and Ag2+spectra. This is the same for samples annealed to -50 K. However, in the center of the spectrum the quintet disappears and leaves the septet. It is not possible to determine whether the disappearance of the quintet increases the intensities of the septet because the septet is severely masked by the quintet before bleaching. When this bleached sample is annealed at 77
The Journal of Physical Chemistry, Voi. 85, No. 17, 1981 2561
Silver Atom Solvation in Methyl Cyanide at 4.2 K 2933 1
G
3439
I
G
Flgure 10. ESR of 0.8 M AgCIO,/CD&N X-Irradiated and observed at 4 K after bleaching with 1 min of projector light. A new site of splitting 2006 MHz, termed site CNI, is formed.
K for 2 min and reexamined at 4 K, the quintet has partially recovered, as shown in Figure 9. Both 77 K unannealed and annealed samples behaved like this. In contrast to “low intensity” optical bleaching, when the 4 K sample is irradiated with “high intensity” visible light for 4 min (500-W slide projector), a new Ago site, which we will designate CNI, appears. This is shown in Figure 10. In the subsequent discussion we will designate the Ago site observed at 77 K as site CNII. The ESR parameters of site CNI are calculated by employing the Breit-Rabi equation, and the results are listed in Table I. The low-field peak of this new site is a broad single peak, whereas its high-field peak is a poorly resolved triplet. This new site disappears completely after 15-s annealing at 77 K so that only site CNII is present at 77 K. After first generating site CNI by optical bleaching, and destroying it by thermal annealing, we find that site CNI can be regenerated at 4 K by optical bleaching. This cycle can be continued several times with a decrease in the intensity of sites CNI and CNII after each cycle. With high intensity optical bleaching (1rnin with 500-W slide projector) the center of the spectrum also changes. The quintet disappears and the septet remains. However, the quintet can no longer be recovered upon 77 K annealing. High intensity bleaching at either 4 or 77 K destroys the quintet permanently. Discussion Silver atoms are generated in these experiments by the reaction of radiation-produced electrons with silver ions. Likewise the Ag2+species are formed by reaction of radiation-produced holes or solvent cations with Ag+. At 77 K both Ago and Ag2+ species are formed in reasonable yield. Previous work on the 77 K radiolysis of the pure solvent, CH3CN, has shown that the radiation-produced electrons are trapped as dimeric anions, (CH,CN),-, which can dissociate into methyl radicals coupled to CN-.,, We also observe methyl radicals in irradiated silver salt solutions of CH3CN and presume that they arise by the same mechanisms as in the pure solvent. The dose-yield plots at 77 K (Figures 2-4) show that the silver species undergo secondary reactions at higher dose. This has been observed previously for silver species in aqueous systems.” However, the methyl radicals show a more nearly linear dose-yield plot, which suggests that secondary reactions are not so important for them. The A$ species produced at 77 K has nitrogen hyperfine structure indicative of equivalent interaction with four solvent molecules. Its g factor and silver hyperfine coupling are very similar to the amorphous values for Ago generated in an aqueous matrix at 77 K (see Table I: Ag/D20 (site 11)). It is also known from electron spin-echo studies that A$ in water matrices has four water molecules in its first solvation shell.lB Three of these waters are oriented with the negative end of their molecular dipoles toward AgO while the fourth has reoriented so that one OH bond is toward the Ago suggestive of a H-bonding inter-
action; this geometry seems to be the optimum for a solvated silver atom in aqueous matrices. When Ago is generated at 4 K in aqueous matrices, it has a different solvation shell structure (see Table I: Ago/D20 (site I)); all four waters are oriented equivalently with the negative ends of the water dipoles toward A$. This structure seems to be expected for Ag+ solvation. One objective of the present investigation was to search for other ion/atom solvation shell changes in the different polar solvent CDSCN. When Ago is generated at 4 K in CD3CN,only a very low yield of Ago is found. Furthermore, it has the same magnetic parameters as the Ago species generated at 77 K in CD3CNin contrast to what is found in water matrices. The origin of the low yield of Ago at 4 K is partly due to trapping of some of the radiation-produced electrons as (CD,CN),- as indicated by observation of the quintet in the center of the 4 K spectrum. Thermal annealing at 50 K or above increases the Agoyield by 4-6 times while the (CD3CN),- yield is little changed. Since the (CD3CN),yield is 100 times greater than that of the silver species, it is hard to detect a small decrease in (CD,CN),- equivalent to the increase in the silver species. We suggest that the increase in silver species is due to detrapping a few electrons from (CD3CN),- species which are formed at 4 K in a nonequilibrium configuration. It is also possible that other matrix-trapped electrons are formed at 4 K, perhaps as CD,CN-, and that these are thermally detrapped to form additional silver species. The optical bleaching results, to be discussed below, tend to support the former interpretation. Note that also only a small Ag2+yield is generated by radiolysis at 4 K and that its yield is also increased by thermal annealing to 50 K or above. By analogy, this implies that matrix-trapped holes or solvent cations are trapped at 4 K which are only detrapped to generate Ag2+ by thermal annealing. To generate more Ago in CD3CN at 4 K, we attempted to detrap matrix-trapped electrons by optical bleaching. When the 40-W tungsten lamp was used, there was no change in the Ago concentration. However, the quintet in the center of the spectrum assignable to (CD3CN),- was converted to a septet characteristic of CDS. Annealing at 77 K reforms some of the quintet. This is the same cyclic behavior seen at 77 K in earlier work2, which was interpreted as follows:
-
(CD,CN),-
%CD3. C N - + CD3CN
(1)
where CD3..CN- is a caged pair that recombines on thermal excitation. In this case the electron is never freed to react with Ag+ to form additional silver species. However, when the high intensity, shorter wavelength projector light is used for bleaching, a new silver species i s formed. In addition, the .(CD,CN),- quintet is bleached and only a CD3septet is observed, but most significantly the quintet cannot now be regenerated by thermal excitation. Thus, we suggest that the detrapped electron does not remain in the solvent cage as CN- coupled to the methyl radical. Instead, the shorter wavelength light detraps the electron from (CD3CN),- with sufficient energy that the electron moves several molecular diameters away giving it an opportunity to react with Ag+ or the much higher intensity light causes multiple excitations which also moves the electron out of the solvent cage. The most significant point is that the additional silver species produced by high intensity optical bleaching at 4 K have different magnetic parameters characteristic of less strongly solvated species than those found at 77 K. These
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J. Phys. Chem. 1981, 85, 2562-2567
are shown in Table I as Ago/CD3CN(site I). The g factor of Ago in site I is closer to the free-electron and the silver isotropic hyperfine coupling in site I is closer to the free-atom value than in site 11. Furthermore Ago in site I seems to be a precursor to Ago in site 11 since CNI converts to CNII on thermal annealing at 77 K. Also CNII can be converted back to CNI at 4 K by optical excitation with visible light. Since the signals are weak, the interconversion is only semiquantitative. This thermal/photo cyclic behavior is analogous to that found for Ago in water matrices.26 By analogy with the fairly complete ion/atom solvation picture obtained for Ago in water matrices, we interpret Ag+/Ago solvation in Cd3CN matrices as follows. We suggest that Ag+ is only weakly solvated by CD3CN,which is expected from evidence that methyl cyanide generally solvates cations weakly.27 Thus, we interpret that Ago in CNI is characteristic of the Ag+ solvation shell in CD3CN. Indeed, since Ago in CNI has the magnetic parameters of a nearly free silver atom, the solvation is quite weak. This is not the equilibrium solvation geometry for the atom however. Upon thermal excitation by warming above 50 K, A$ converts from site CNI to CNII, which we interpret as an atom solvation process leading to a near equilibrium solvent environment for the atom. Geometrically we surmise that this atom solvation process has brought the four first solvation shell CD3CN molecules closer to Ago. This leads to some delocalization of the unpaired electron spin density from the Ago to the solvent molecules. This decreases the Ago isotropic coupling from 2006 to 1500 MHz and increases the N coupling on the solvent from unresolvable to a resolvable 16 MHz. In addition, the decreases in the g factor of Ago indicate some admixture of other orbitals such as 5p into the unpaired electron wave function. We suggest that the delocalization causes the solvation shell of the atom to be more compact than that (25) Ramsey, N. F. In ‘‘Molecular Beams”; Clarendon Press: Oxford, 1965. (26) Kevan, L. J. Chem. Phys. 1978,69,3444. (27) Kuntz, I. D.; Cheng, C. J. J. Am. Chem. SOC.1975, 97, 4852.
of the ion. Thus, we have another example of a striking silver atom solvation effect. One might also interpret these changes as due to redistribution among different types of trapping sites. However, on the basis of our previous detailed studies in aqueous matrices, we believe the solvation description is best. The reversal of atom solvation, which we will term desolvation, by optical excitation of site CNII at 4 K can only be understood qualitatively. We offer the same explanation advanced for the optically driven desolvation of Ago in water matrices. Namely, the excited state has significant charge-transfer-to-solvent character such that the excited state looks more like a Ag+ core to the nearest CN groups so they move toward a geometry consistent with this. And at 4 K they become locked into this resolvated atom geometry. It remains to comment on the low yield of Ago in site CNII initially formed in the 4 K radiolysis. It seems probable that this is due to the most energetic secondary electrons produced by the radiation. These energetic electrons initially form Ago in CNI, but the additional energy available converts CNI to CNII. The majority of the secondary electrons are trapped by the matrix. When these are thermally detrapped, they initially react with Ag+ to produce Ago in CNI, but site CNI is not observable because the thermal energy is sufficient to cause immediate conversion to site CNII. It is also interesting to note that the magnetic parameters of Agoin site CNI are similar to those of a site “2020” observed at 77 K in alcohol-water mixed matrices13 (see Table I). Likewise, a similar site has been seen in methyl cyanide-water mixed matrices at 77 K and termed species B.19 These sites in mixed matrices have been assigned to a mixed first solvation shell and to some sort of an interstitial site created by local disorder in the mixed matrix. To the extent that site CNI is indeed analogous, this interpretation of the 2020 site and species B may need to be reassessed. Acknowledgment. This work was partially supported by the Department of Energy.
Infrared Spectra of the Isobutyl and Neopentyl Radicals. Characteristic Spectra of Primary, Secondary, and Tertiary Alkyl Radicals J. Pacansky,” D. W. Brown, and J. S. Chang IBM Research Laboratory, San Jose, California 95 193 (Received: December 3 1, 1980; In Final Form: May 14, 108 1)
Infrared spectra are presented for the isobutyl and neopentyl radicals for the first time. Significant differences are observed in the P-CH stretching and pyramidal bending regions between primary alkyl radicals with straight chains and those with branched chains like the isobutyl and neopentyl radicals. These are compared with spectra of other alkyl radicals to establish characteristic infrared spectra of primary, secondary, and tertiary alkyl radicals.
Introduction Characteristic infrared spectra of organic materials are very important for identifying functional Doups, and investigating structural changes a particular group exerts on the rest of a molecular system.l,2 In order to determine
characteristic spectra, we investigated a series of structurallY related systems sPectroscoPicallY to establish a trend. Although this is not a trivial task, it is nevertheless somewhat easier for molecular systems with closed shells. The task is nontrivial and challenging for systems with
(1) L. J. Bellamy, “Advances in Infrared Group Frequencies”, Methuen, London, 1968.
(2) C. N. Rao, “Chemical Applications of Infrared Spectroscopy”, Academic Press, New York, 1963.
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0 1981 American Chemical Society
