ARTICLE pubs.acs.org/JPCA
EPR and ENDOR Study of the Frozen Ammoniated Electron at Low Alkali-Metal Concentrations Andreas Meyer and Maurice van Gastel* Institut f€ur Physikalische und Theoretische Chemie, Rheinische Friedrich-Wilhelms-Universit€at Bonn, Wegelerstrasse 12, D-53115 Bonn, Germany ABSTRACT: Ammoniated electrons in dilute frozen solutions are examined using EPR spectroscopy under conditions where the formation of metallic nanoparticles is avoided. Two signals from two different species have been observed. One signal is metastable and decays irreversibly upon annealing. The metastable species saturates at a spin concentration of 10 nM. The annealing temperature for this species amounts to 60 K for frozen solutions of sodium in neat ammonia and is raised upon addition of metal iodide. The observed g value is smaller than the free electron g value and is compatible with a cluster-anion radical rather than with a cavity electron. The wave function of the unpaired electron contains about 6%-10% of 2p character at nitrogen. The observed g shift is fully compatible with previously reported theoretical calculations (Shkrob, I. A. J. Phys. Chem. A 2006, 110, 3967-3976). The second signal cannot be annealed in the frozen state. The line shape is homogeneous, and its width depends on the identity of the metal and at large metal concentrations on the metal concentration itself. Upon increasing alkali metal concentration above 0.15 MPM, the line shape changes from Lorentzian to Dysonian, indicating the presence of metal nanoparticles. A new ENDOR pulse sequence is introduced to investigate the presence of weakly coupled nuclear spins for homogeneous EPR lines. The observations are critically compared with available literature data.
1. INTRODUCTION Alkaline metals can be dissolved in liquid ammonia, resulting in deep blue colored solutions. This characteristic blue color was first observed by Sir Humphry Davy in 18081 after contacting potassium with ammonia gas and derives from an intense absorption in the infrared and an extended tail in the visible region. About 100 years later, the dissolution process was interpreted for the first time by Kraus,2,3 who investigated the properties of metal-ammonia solutions by physicochemical experiments. In particular, he inferred the existence of solvated electrons from conductivity measurements. While the macroscopic properties of metal ammonia solutions are well established owing to the work of Kraus and others,4-6 the investigation of the detailed electronic and geometric structure at the microscopic level is still a very active field of research. A widely spread model for the solvated electron was introduced by Ogg in 1946.7 In this model, which is able to explain the large solvation volume of sodium in liquid ammonia and similarities to F-centers in irradiated crystals of alkali halides, the solvated electron is located in a polarized, spherical solvent cavity. This “cavity model” was later refined by Jortner,8 who employed variational methods in combination with an empirical potential energy function to derive the distribution function of different bound states of the solvated electron in the cavity. Jortner’s model is able to account for the wavelength of maximum absorption in the infrared region and also provides an approximate cavity radius of about 3.2 Å.8 This number is compatible with later experiments by pump-probe techniques, r 2011 American Chemical Society
which demonstrate that the cavity-solvent border is made up of the first solvation shell of about six molecules.9 Despite the success in explaining many experimental observations, the cavity model is not able to predict all properties of the ammoniated electron. In particular, it predicts a symmetric line shape of the infrared absorption without a tail extending in the visible region. Tuttle et al. compared optical spectra of the solvated electrons with entities like F-centers and solvated anions. They observed marked differences in their spectroscopic signatures and suggested that the solvated electron is better described as a solvated anion complex, i.e., as a cluster of solvent molecules that shares the excess electron.10 NMR studies by Nakamura furthermore revealed a negative Knight shift—and thereby a negative spin density—in the proton resonance frequency,11 which supports the “cluster model”. Further support for the cluster model is derived from a recent study of molecular beams of Li(NH3)4 which display an absorption spectrum compatible with that of lithium-ammonia solutions.12 Moreover, DFT calculations, which describe the ammoniated electron as a cluster-anion radical, were able to semiquantitatively explain the observed 1H and 14N Knight shifts as well as the independence of the infrared absorption on the identity of the alkaline metal.13,14 Another experimental approach to obtain information about the microscopic solvation structure is EPR spectroscopy. Besides Received: January 20, 2011 Revised: February 2, 2011 Published: February 23, 2011 1939
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Figure 1. CW EPR spectra of (a) a liquid sodium-ammonia solution at 195 K, νMW = 9.3619 GHz; (b) a frozen sodium-ammonia solution at 20 K, 0.05 M in Naþ, νMW = 9.3889 GHz; (c) a rapidly cooled sodium-ammonia solution (0.05 M in Naþ) at 20 K, νMW = 9.3910 GHz; (d) a frozen sodium-ammonia solution, 0.13 M in NaI, 0.18 M in Naþ, νMW = 9.3766 GHz. A few spectra have been recorded with minimum possible modulation amplitude and frequency in order to more accurately estimate the line widths of signals 4 and 6. The low signal-to-noise ratio is due to the low spin concentration of typically2 10-8 M and the reduced modulation amplitude (0.003 mT) and microwave power (0.1 mW). Gaussian derivative: dG(B)/dB 2 2 = A(d/dB)e-2((B-B0)/C) with peak-to-peak linep width ffiffiffi in Table 1 equal to C. Lorentzian derivative: dL(B)/dB = D(d/dB)(1/(1 þ 4(B - B0) /E )) with peak-to-peak line width in Table 1 equal to E/ 3.
the standard continuous wave (CW) experiment, two techniques are widely employed. The first of these two is called electron spin echo envelope modulation (ESEEM) spectroscopy, which makes use of the simultaneous occurrence of allowed and forbidden spin transitions that are both excited owing to the finite bandwidth of the microwave pulses. The set of excited transitions gives rise to spin coherences, which cause the echo amplitude to be modulated in time. Fourier transformation of these modulations yields a spectrum in frequency domain from which the nuclear hyperfine coupling constants can be deduced. The second technique is called electron nuclear double resonance (ENDOR) spectroscopy, which can be considered as an EPRdetected NMR experiment. In the ENDOR measurements, a radio frequency (RF) pulse is applied in between the microwave pulses. The RF radiation induces nuclear spin transitions. If the electron spin is coupled to the corresponding nuclear spin, the EPR signal will change owing to polarization transfer between electronic and nuclear spin states. For the ammoniated electron, the methods are only scarcely applied. Kevan employed ESEEM spectroscopy to gain knowledge about the geometric structure of solvated electrons in various solvents.15 He excluded ammonia from his studies because of stability issues in the frozen state. Concerning the stability, the results of Feher on alkaline metal conduction electron paramagnetic resonance (CEPR)16 are often quoted, which state that metal nanoparticles precipitate upon freezing and that the observed signals arise from conduction electrons in the nanoparticles. In a recent study by Chiesa et al, ammoniated electrons localized on a MgO surface have been studied by EPR spectroscopy.17 Catterall demonstrated that it is possible to stabilize solvated electrons in frozen
ammonia,18 and Kaplan et al. observed essentially the same signal characteristics as those seen by Feher et al., however, for nonconducting paramagnetic systems, such as NO2 absorbed on MgO.19 In this contribution we re-examine metal-ammonia solutions by EPR spectroscopy and focus on the solid (frozen) state under dilute conditions where no precipitation occurs. We examine the processes that occur upon freezing and demonstrate that it is possible to study ammoniated electrons by ESEEM and ENDOR spectroscopies; the latter has even been applied to a homogeneous line shape.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Liquid ammonia is obtained by distillation of a 25% aqueous solution of ammonia under reduced pressure and collecting the solid raw ammonia at 77 K. The raw ammonia is then distilled again under atmospheric pressure, yielding highly pure liquid ammonia. Pieces of sodium are cut from the inside of a large rod of sodium in an anaerobic atmosphere. The metal pieces are molten under reduced pressure and subsequently moved to another part of the glass container. The resulting metal pieces are cut, weighed, and added to a measured volume of liquid ammonia under Schlenk conditions. The resulting blue solution is transferred to an EPR tube. A few experiments have been carried out with potassium and lithium. Potassium is treated according to the procedure described for sodium, and lithium is cleaned mechanically. Sodium iodide (purity >99%) is bought from Applichem and heated to 350 C under reduced pressure for about 3 min prior to use to 1940
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Table 1. Line Shape, Line Width (mT), and g Value of the Signals 1 - 6 Shown in Figure 1a c(Naþ)
line shape
line width (mT)
g value
1
Lorentzian
0.0170 (0.0001)
2.0014 (5)
2
Lorentzian
0.1230 (0.0002)
2.0018 (5)
0.1
3
Lorentzian
0.123 (0.001)
2.0018 (5)
0.1
4
Gaussian
0.0066 (0.0006)
2.0000 (5)
0.1
5
Lorentzian
0.070 (0.002)
2.0019 (5)
0.36
6
Gaussian
0.0050 (0.0004)
2.0000 (5)
0.36
c(e-)
16.1 1.4 · 4.2 10-4 · 3.4 10-2 · 7.0 10-3
Also included are the concentrations of sodium ion, c(Naþ) [MPM] and the spin concentration, c(e-) [10-6 M] at 20 K. For signals 5 and 6, sodium iodide is present in the frozen solution and the amount of added metallic sodium amounts to 0.1 MPM. a
remove traces of water. All glass and quartz containers are heated to 350 C under reduced pressure prior to contact with the solution of solvated electrons. Solutions in the concentration range of 0.025-0.5 mol % metal (MPM), corresponding to 0.01-0.2 M at -70 C, have been prepared. To prepare quenched solutions, a solid sample is thawed and then rapidly frozen inside the helium cryostat to temperatures below 60 K. During the time that the sample is liquid, the spin concentration of the sample diminishes owing to the formation of hydrogen gas and metal amide. Although the utmost care is taken to prepare clean samples, occasionally, signals with differing line widths have been observed, which may stem from unremoved contamination in the quartz containers. 2.2. EPR Spectroscopy. Continuous wave (CW) EPR measurements are conducted on a Bruker ESP 300E EPR spectrometer with an ER 4102ST-3731 resonator and on a Bruker ELEXSYS 580 EPR spectrometer equipped with a 4122SQHEW resonator, which allows determination of the absolute number of spins present in the sample by double integration (accuracy (20%). ESE-detected EPR, ESEEM, and ENDOR experiments are conducted on a Bruker ELEXSYS 580 EPR spectrometer equipped with either an MD4 (ENDOR) or an MD5 (EPR, ESEEM) resonator and an Oxford Instruments CF935 helium flow cryostat. The minimum modulation amplitude of the EPR spectrometer amounts to 0.003 mT. In quenched solution, signals with an exceptionally narrow line width on the order of the minimum modulation amplitude have been observed when the modulation frequency is reduced from 100 to 30 kHz. The line width of these signals is likely even smaller than this value, and 0.003 mT has to be considered as an upper limit. The accuracy of the g value determination is limited by the accuracy of the field controller of 0.08 mT. The field has been calibrated by use of a TEMPO sample, for which the g value amounts to 2.0056.20
3. RESULTS A typical CW-EPR spectrum of a sodium-ammonia solution at 195 K is shown in Figure 1a. The spectrum is characterized by an exceptionally narrow and structureless signal, which agrees with previous observations.21 A fit of this signal with a Lorentzian line shape gives an upper bound for the peak-to-peak line width of 0.017 mT. The g value amounts to 2.0014. The line width, line shape, and g value of this signal, denoted 1, are included in Table 1. Upon freezing a dilute solution in liquid nitrogen, this signal is converted to a broader signal, denoted 2, as shown in Figure 1b. The spin concentration (in 10-6 M) and alkali metal concentration (in MPM) are included in Table 1. The line shape can be
Table 2. Metal (M), Metal-Iodide (MI), and Metal (Mþ) Concentration in MPM and Spin Concentration Obtained for Signals 3 and 5 in 10-6 M and 4 and 6 in 10-8 M Frozen Metal-Ammonia Solutions
a
M
c(MI)
c(Mþ)
c(3, 5)
c(4, 6)
Na
0.291
0.020
0.5
1.8b
Na
0
0.093
18.8
0.28
Na
0.558
0.140
56.5a
1.4
Li
0
0.020
0.04
0.04b
Li
0
0.023
2.3
0.06
Li
0
0.023
3.7
Li
0
0.163
24.9a
0
K
0.291
0.580
232.5a
0
1.2
b
Dysonian line shape. Sample significantly reduced color upon the thaw-quench cycle.
well fitted by a Lorentzian, albeit with small deviations in the outer region of the spectrum. At 20 K, the g value and the line width of this signal amount to 2.0018 and 0.123 mT. The g value, which is slightly smaller than the free electron g value (ge = 2.0023), is found to be independent of the identity of the metal within the experimental accuracy. The line width does depend on the identity of the metal; the line width measured in a frozen potassium solution is on the order of 1 mT, while lithium gives rise to line widths similar to sodium. At 20 K, the line shape changes to Dysonian for metal concentrations larger than 0.15 MPM and becomes similar to that observed by Feher.16 It is possible to retain a narrow signal similar to the one in the liquid solution in the solid phase by rapidly freezing the sample in the helium cryostat of the EPR spectrometer to temperatures below 60 K (signal 4; cf. Figure 1c).18 The g value of the quenched signal amounts to 2.0000, which is significantly smaller than the free electron g value. The line width amounts to 0.0066 mT. The signal now has a Gaussian line shape, indicative of inhomogeneous broadening. By double integration, the spin concentration of signal 4 amounts to 4.2 10-10 M, which is only 0.01%-0.1% of the spin concentration of signal 3. Signal 4 irreversibly decays upon annealing the sample at a temperature of 60 K. Spin counting of the signals leads to another remarkable observation. The double integral of signal 3 increases with increasing amounts of alkaline metal, as indicated in Table 2 and Figure 2. Absolute spin concentrations ranging from about 1010 to 1014 spins/mm3, corresponding to 1.7 10-8 to 1.7 10-4 M solutions, have been measured for this signal. The lowest concentrations have been obtained for solutions which appeared nearly colorless. Unlike signal 3, the double integral of signal 4 does not increase with the amount of alkaline metal; it saturates 1941
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Figure 2. Correlation between metal concentration c(Mþ) and spin concentration obtained for the signals 3, 5 (b) and 4, 6 (9). A linear fit of the former signal gives a correlation factor R = 0.96. Figure 4. Two-dimensional, three-pulse ESEEM spectrum of signal 4 obtained at 10 K with the π/2-τ-π/2-T-π/2-τ echo sequence. Pulse lengths π/2 = 32 ns; initial values of pulse separations τ =120 ns, T = 200 ns. B0 = 348.4 mT.
Figure 3. ENDOR spectrum of signal 3 at 10 K showing sodium resonances, detected via manipulation of the T2 decay (see inset). B0 = 349.99 mT.
at spin concentrations above 10-8 M. Moreover, it becomes increasingly difficult to observe signal 4 at metal concentrations above 0.15 MPM; cf. Table 2. Above this alkali metal concentration, the line shape of signal 3 becomes considerably distorted and attains a Dysonian line shape. When sodium iodide is added to the ammonia solution, the annealing temperature is raised above 100 K. It thus becomes easier to retain signal 4, since it is sufficient to freeze the solution with liquid nitrogen (signals 5 and 6; see Figure 1d). The g values remain unchanged upon addition of sodium iodide; the widths of signals 4 and 6 are also almost equal; cf. Table 1. At an alkali metal concentration below 0.03 MPM, the spin concentration for signal 6 amounts to about 10% of the spin concentration for signal 5. With sodium iodide, the spin concentration for signal 6 lies between 1.7 10-9 and 1.7 10-8 M. As was the case with signals 4 and 3, the spin concentration of signal 6 does not correlate with that of signal 5. The annealing temperature is also increased in dilute lithium solutions ( νZ) in the ( quadrant. The spectrum is characterized by a signal at the diagonal at the free proton Zeeman frequency (νZ(1H) = 14.9 MHz). In the ( quadrant, two signals are present. First is a signal on the antidiagonal from (0, 0) MHz to (8, -8) MHz. Second, a low-intense signal is visible from (0, 0) MHz up to (5, -10) MHz.
4. DISCUSSION At concentrations of alkali metal smaller than 0.15 MPM, the EPR spectra of frozen ammoniated electrons are characterized by structureless signals of either Gaussian or Lorentzian line shape. Although hyperfine structure is absent in frozen solution, important information can be extracted, first, from the number of observed signals, second, from the presence of additional signals upon quenching, third, from their irreversible decay upon annealing, and most strikingly from the observed g value, which for signals 4 and 6 is significantly smaller than the free electron g value of 2.0023. Moreover, by using hyperfine resolving techniques such as ENDOR and ESEEM spectroscopy, the hyperfine interaction with surrounding nuclei has been detected. At this point it is stressed that the spin concentration used for these measurements is exceptionally small (about 10 nM), which is a consequence of the small concentration of alkali metal necessary to avoid precipitation of nanoparticles and which also somewhat 1942
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The Journal of Physical Chemistry A limits the signal-to-noise ratio. The information obtained from the spectra is discussed and compared to literature data in the remainder of this section. To the best of our knowledge, frozen ammoniated electrons at alkali-metal concentrations smaller than 0.15 MPM have not yet been investigated by EPR spectroscopy. EPR spectra of liquid solutions at a concentration of 0.1 M have been reported22,23 and gave rise to a g value and line width of 2.0012 and 0.013 mT, in line with our observations (cf. Figure 1a, Table 1). CEPR spectra for electrons in sodium metal are available from the Feher group.16 These spectra are unusual in the respect that the line shape of the EPR signal is not Lorentzian or Gaussian, but Dysonian. According to Dyson’s theory, the absorptive and dispersive components of the response may mix under conditions that the microwave field is unable to fully penetrate the sample and the signal arises from the layer of sodium atoms within the skin depth of the (polarizable) metal nanoparticle.24 Levy has performed measurements on frozen ammoniated electrons at larger concentrations and found a line width at 20 K of 0.5 oersted, which under the assumption that the relative permeability of frozen ammonia is about 1 corresponds to 0.05 mT.22 This width is compatible with the one observed for signals 3 and 5. He has not observed signal 4, which is in line with our observations that this signal vanishes at increasing alkalimetal concentration. Rather, he, as well as Feher et al., observed precipitation of metal nanoparticles,16,22 which agrees with our observations at metal concentrations above 0.15 MPM. In experiments on colloid nanoparticles by Edwards et al., a Dysonian line shape was found at low temperature.25 Their spectra include a broad, Lorentzian signal and a much narrower signal, similar to our observations, though the line width and the g value of the signals differ, which is likely related to the alkali-metal concentration or the solvent. Edwards et al. also found that their observations for sodium nanoparticles do not match well with those expected from the theory of conducting metal nanoparticles.25 Catterall et al. have investigated relaxation processes of a narrow signal in frozen metal-ammonia solutions,24 which may be similar to our signal 4, albeit that they observed a homogeneous signal. In our case, signal 4 has a Gaussian line shape, and it turned out to be possible to observe a Hahn echo signal with pulsed EPR methodology, both of which are clear indications for inhomogeneous broadening. Though care has to be taken to compare our observations with previously reported experiments performed with larger alkalimetal concentration, especially when metal nanoparticles precipitate out, the presence of two signals seems to be in general agreement with the number of signals observed previously. Since signals 4 and 6 have only been observed upon rapid quenching of the metal-ammonia solution, since these signals irreversibly disappear upon annealing above 60 K (or above 100 K if metal iodide is present), and since the signal is inhomogeneously broadened, we conclude that signals 4 and 6 concern a species that is heterogeneous and energetically less favored than the species that corresponds to signals 3 and 5. According to a classification of concentration regions reviewed by Cohen and Thompson and based on experimental observations,3,26,27 a concentration of about 0.1 MPM corresponds to a dilute solution in which the solvated electrons and cations are associated, but at which no nanoparticles are formed.5 The absence of a Dysonian line shape in our experiments is completely in line with this classification and with the absence of nanoparticles. The most straightforward tentative explanation for the presence of two
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signals at these concentrations is that signals 3 and 5 correspond to a solvated electron-cation complex and signals 4 and 6 may involve quenched electrons which only upon annealing are able to become associated to a cation. We now turn our attention to the g values. The deviation of the g value from the free electron g value, called the g shift, is determined by spin-orbit coupling and contains information about the orbitals in which the unpaired electron resides. An extended theory for g values for conduction electrons in metals has been developed and has been successfully applied in combination with CEPR experiments to explain the observed negative g shift. Given the lack of metal nanoparticles, we have investigated whether the solvated electron also attains a negative g shift when described as a molecular entity. For the solvated electron as a molecular species, the significant negative g shift of signals 4 and 6 can only be attributed to a second-order effect of spin orbit coupling and orbital Zeeman interaction;28 i.e., the wave function of the solvated electron gives rise to second-order matrix elements of the orbital angular momentum operator (see eq 1). This would be in direct support of the cluster model, since the electron in a cavity does not carry orbital angular momentum. In order to gain insight as to whether a negative shift of the g value is also characteristic for a solvated electron in an ammonia cluster, we have performed a DFT calculation (PBE0 functional, TZVP basis)29 of the smallest conceivable model cluster, (NH3)2-. In the geometry optimized structure, the two molecules form a linear N-H 3 3 3 H-N substructure. The calculation gives rise to a wave function of the unpaired electron, which is mainly localized at the six protons but also contains a small amount of 2p character at nitrogen (2% on each N). The isotropic g value amounts to 2.0020, which is indeed negative, although nowhere near the observed g value. The g shift is comprised of a relativistic mass correction (-0.000 08), an even smaller gauge correction, and a dominated by spin orbit coupling and orbital Zeeman interaction (-0.0002) owing to the p character at nitrogen according to Δgi ¼ -
e jLi jψ0 æ ∑e 2λÆψ0jLεi jψ-e æÆψ ε e
ð1Þ
0
where λ is the spin-orbit coupling parameter of nitrogen, the index e runs over all excited states of energy εe, i = x, y, z, and L represents the orbital angular momentum operator. Since the g shift is a second-order property for which the wave function is required four times, the experimentally observed g shift will be attained with this formula for two nitrogen atoms which each carry 6%-7% spin density. Though the diammonia radical anion is too small and not a representative model for the solvated electron, it is very gratifying to note that the recent and more advanced calculations of Shkrob et al. for monoanionic ammonia clusters were able to qualitatively explain the observed Knight shifts by NMR spectroscopy using exactly the same ansatz.13 In these calculations the spin density at nitrogen was found to be on the order of 10%, with only a small variation with the size and particular geometry of the cluster used in the calculation. Larger spin densities at two protons of about 30% were found, which seem to be relatively constant with the cluster size of 8-12 ammonia molecules. This spin density distribution seems to be fully compatible with the observed g value of signals 4 and 6 and implies that it may not necessarily mean that electrons in metallic nanoparticles are responsible for this signal. Concerning the size 1943
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The Journal of Physical Chemistry A of the cluster, measurements by femtosecond spectroscopy of V€ ohringer et al. give rise to a cluster size of six molecules.9 The g value of signals 2, 3, and 5 amounts to the free electron g value within the accuracy of the experiment (cf. Table 1). Whether or not the wave function of the electron is more delocalized, contains less p character at nitrogen, is subject to rapid spectral diffusion, or is located in a cavity instead of on a cluster remains unclear for these species. In either case, given the homogeneous line shape, the species that contributes to this signal must be a unique one with essentially no heterogeneity. Additional information about these species has been obtained from ENDOR measurements. The one-pulse ENDOR sequence given in Figure 3 was the only ENDOR pulse “sequence” by which signals have been obtained for a homogeneous EPR line. The ENDOR spectrum clearly displays a signal at the free sodium frequency, indicating that the electron is weakly coupled to sodium and in agreement with the qualification by Cohen and Thompson of an associated electron-sodium pair.5 The hyperfine interaction with sodium is negligible, indicating that 3s spin density at sodium is absent. Signals of nitrogen atoms and protons have not been observed. A possible reason is that the hyperfine coupling constants are anisotropic and given the low spin concentration may be beyond the present detection limit or that the dynamics and spectral diffusion at 20 K are still much faster than the nanosecond time scale of the EPR experiment and do not allow observation of the nitrogen and proton hyperfine coupling constants. In contrast to signals 2, 3, and 5, a Hahn echo signal is observable for signals 4 and 6, which also opens the possibility to perform ESEEM experiments. The signal of weakly coupled protons at 14.9 MHz and a broad signal near the sodium Zeeman frequency are observable. The hyperfine coupling constants are essentially zero, and the resonances occur at the Zeeman frequency. Still, the presence of a proton resonance is yet another confirmation that the electrons are not located in metal nanoparticles. Invariably, the maximum spin concentration measured for signals 4 and 6 is found to be about 10 nM. Since the solvent is the only common parameter in these samples, it might be that the frozen ammonia can only harbor a maximum amount of conformations that give rise to signals 4 and 6. This interpretation is corroborated by the fact that, even in quenched frozen solutions of pure sodium or potassium (without added salts), spin concentrations of the same order have been obtained. Below 60 K, the signals show no sign of decay, in accordance with results obtained by Bosch using quenching condensation techniques in combination with optical spectroscopy.30 Her experiments show that it is possible to prepare a metastable species of solvated electrons at 20 K which can be converted to more stable species by annealing. For sodium, she observed the onset of lattice dynamics at 58 K. Annealing at 90 K leads to the complete decay of the signals observed at 20 K and the appearance of a new signal. The discussed influence of iodide ions may be interpreted in the following way. Iodide ions are known to have a structure breaking effect in water.31 The iodide ions are expected to give rise to an increased concentration of lattice defects in quenched solutions. The observed spin concentration for signal 6 would then correlate with the concentration of suitable lattice defects. Bosch proposes a structure of microcrystalline domains where the metastable electrons are localized between the borders of the domains, stabilized by molecular dipoles of ammonia molecules pointing at each other.30
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Another proposition for electron localization in the liquid state was made by Catterall and Mott.32 In their model, the solvated electron is stabilized by molecular dipoles in a vacancy in the bulk liquid made up of a few ammonia molecules. Instead of forming a network of hydrogen bonds, the N-H bonds of the ammonia molecules which build up the vacancy for the solvated electron are directed at each other. The importance of the polar N-H bond is in line with our observations and the calculations of Shkrob.13 As noted above, the spin density in the dimeric ammonia radical anion resides mainly on the protons, preferring the protons which are part of the N-H 3 3 3 H-N substructure, which is the structure of a Bjerrum defect.33 Since this structure is energetically favored in the radical anion, the term defect may actually not be appropriate. This structure competes with the hydrogen bond network structure of neat ammonia, which is disrupted by structure breaking anions like iodide. This argument is further supported by observations on solvated electrons by Kevan who noticed that crystalline samples are not well suited to take up excess electrons produced by photoinjection.34 The maximum obtainable spin concentration for signal 6 may thus be increased by further disrupting the solvent structure. However, if the metal concentration becomes larger than 0.15 MPM, metallic nanoparticles are formed and the electrons will be be taken up by the nanoparticles. In a general perspective, our observation of a quenched solvated electron with angular momentum reveals important information about the initial stages of electron trapping and solvation in liquids and clusters, a very active field of research.35-40 Since the species can be annealed, it is likely quenched during a stage where the electrons are located in a nonrelaxed local environment. In this respect, the quenched electron is closely related to the much investigated weakly bound electrons that can be observed upon photolysis of the solvent. The weakly bound electrons are usually short-lived (picosecond time scale or faster); they can be stabilized at liquid helium temperatures in D2O.41 Our observation of angular momentum for the quenched solvated electrons would, if carried over one-toone to weakly bound electrons, imply that the initial processes of electron solvation after photolysis would involve an excitonic species. This exciton is best described as a nonrelaxed solvent cluster radical anion in which the unpaired electron occupies an orbital which includes p character and which likely consists of a linear combination of σ* antibonding orbitals of the individual solvent molecules. It is conceivable that this exciton is similar to what one would call a nonbonding orbital in molecules, i.e., an exciton which after relaxation results in dissociation of the cluster and liberation of the solvated electron.
5. CONCLUSIONS AND OUTLOOK By EPR spectroscopy of dilute frozen solutions of metalammonia, two signals from two different solvated electrons have been observed. One signal is metastable and can be annealed. The annealing temperature amounts to 60 K for frozen solutions of sodium in neat ammonia and to temperatures above 100 K upon addition of metal iodide. The EPR signal of the metastable species is reminiscent of the signal recorded in liquid metalammonia solutions and provides important insight into the electronic structure of the solvated electron in liquid ammonia. Mainly based on the significant g shift, the second-order matrix elements of the angular momentum operator are nonzero for the solvated electron. The signal is thus compatible with a cluster1944
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The Journal of Physical Chemistry A anion radical in which the unpaired electron occupies the otherwise empty molecular orbitals of the cluster. The wave function attains about 6%-10% of 2p character at nitrogen. The observed g shift supports the cluster model and is fully compatible with previously reported theoretical calculations.13 The metastable species saturates at a concentration of 10 nM. The second signal cannot be annealed in the frozen state, and its line width depends on the identity of the metal and at large metal concentrations on the metal concentration itself. Particularly noteworthy is the change in line shape at metal concentrations above 0.15 MPM, where its line shape changes from Lorentzian to Dysonian, indicating the formation of metal nanoparticles. Both line shapes are compatible with homogeneous broadening. We applied a new ENDOR pulse sequence and were able to detect weakly coupled sodium. The g value, which is almost equal to the g value of the free electron, is compatible with a delocalized state of the species responsible for the broad signal. Based on the present observations at dilute concentrations, it will be informative to investigate solutions of alkali metal with concentrations larger than 0.15 MPM at different temperatures, to obtain more information about the transition to the metallic state by the accompanying change of the g value and line width. Another interesting aspect is the comparison of the alkali metals to the earth alkali metals. Additionally, ENDOR and ESEEM experiments with isotope-substituted (15NH3, ND3) solvents are planned. Hopefully, it will become possible to disentangle the contributions of the metal ions, nitrogen, and hydrogen atoms and obtain a more detailed picture of dilute solvated ammoniated electrons at metal concentrations below 0.15 MPM.
’ AUTHOR INFORMATION Corresponding Author
*Tel. þ49 228 73 2919; fax þ49 228 73 2551; e-mail vgastel@pc. uni-bonn.de.
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