J. Phys. Chem. 1992,96, 10591-10594
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Transient Resonance Raman Spectrum of meso-Tetraphenylporphlne: An Analysis of Chemical Factors That Influence the Dynamics of the Excited Trlplet States of Metalloporphyrlns Julio C. de Paula,*.+Valerie A. Walters,*.s Charles Nuhitis,$ Jeffrey Lind,+ and Kelly Haul Department of Chemistry, Haverford College, Haverford, Pennsylvania 19041, and Department of Chemistry, Lafayette College, Easton, Pennsylvania 18042 (Received: September I I, 1992; In Final Form: November 10, 1992)
We report the resonance Raman spectrum of the first excited singlet (SI) and triplet (TI) states of meso-tetraphenylporphine. The structure of the TI state is characterized by a weakening of the C,C, bonds and a strengthening of the C,N bonds relative to the ground electronic state. These changes are of considerably lower amplitude than those observed for zinc(I1) tetraphenylporphyrin, where Jahn-Teller instability of the TI state was observed (Walters, V. A.; de Paula, J. C.; Babcock, G. T.; Lcroi, G. E. J . Am. Chem. Soc. 19%9,111,8300).By comparing data on a number of compounds, we provide a preliminary analysis of the chemical factors that modulate the dynamics of the excited states of porphyrins.
Introduction The mo-tetraphenylporphine (TPP) ring system has been used extensively as a model in studies aimed at deciphering factors that control the biological function of porphyrins. For example, several complexes of TPP bearing substituents that act as electron or energy acceptors have been used as platforms for understanding the dynamics of the primary events of photosynthesis.'V2 Our own efforts toward a description of porphyrin photochemistry have been centered on the use of time-resolved vibrational spectroscopy as a tool for the elucidation of the dynamics of photoinduced electron-transfer reactions mediated by porphyrins. Complexes of TPP were chosen for these investigations because the structure and spectroscopy of its ground electronic state are understood in some detail, in both the metalated and freebase forms (designated as MTPP and H2TPP, respecti~ely).~ Hence, the electronic and vibrational properties of excited electronic states that participate directly in photochemistry may be inferred by comparison with properties of the ground state. Transient resonance Raman methods have been used to study excited states of a number of metalloporphyrinse7 and chlorophylls.* The time-resolved resonance Raman spectra of ZnTPP and its deuterated derivatives provided the first experimental evidence for a Jahn-Teller distortion of the first excited triplet state (T, state) of the m~lecule.~ The Jahn-Teller instability, which was predicted on theoretical grounds? results in an in-plane distortion of the porphyrin to a rectangular or kitelike shape. Hence, the symmetry point group is reduced from D4h(undistorted) to D2h(distorted). The change in geometry causes some modes to change in frequency as well as new modes to appear in the resonance Raman s p e c t r ~ m . ~ . ~ It is unlikely that the shifts in vibrational frequencies observed in ZnTPP are due solely to distortions brought about by JahnTeller instability. Even in the absence of Jahn-Teller distortions, some changes in vibrational frequencies are expected to occur because the electronic configurations of the So and TI states are different. In forming the TI state, one electron is promoted from an a2" bonding orbital to an e,* antibonding orbital; the shift in electron density results in weakening or strengthening of bonds relative to the ground electronic state, with attendant shifts in the vibrational spectrum. To gain further insight into the dynamics of porphyrin triplet states, we report an investigation of the transient resonance Raman spectrum of free base tetraphenylporphine, H2TPP. Comparison of the Raman spectra of the T I state of H2TPP and ZnTPP provides the first assessment of the chemical means by which excited-state dynamics may be tuned finely in porphyrins.
Experimental Section Materials. meso-Tetraphenylporphine (5,10,15,20-tetraphenyl-21H,23H-porphine)was synthesized and p d i e d according to published protocols.I0 The final product was free of chlorin impurity, as judged by absorption and fluorescence spectroscopies. Tetrahydrofuran (THF, Aldrich) was either of spectral quality or freshly distilled. CS2 (Aldrich, spectrometric grade) and CHJ (Aldrich) were used without further purification. Instrumentation for Raman Spectroscopy. The resonance Raman spectrum of the ground electronic state of H2TPP was obtained with excitation by about 25 mW of 441.6-nm radiation from a HeCd laser (Liconix 2040B). Transient Raman spectra were obtained with a pumpprobe configuration, where one energetic laser pulse at 436 nm populates the excited state and simultaneously probes it by resonant Raman s~attering.4~~~ This design takes advantage of the fact that both the SIand the T I states of H2TPP absorb strongly at 436 nm and are long-lived relative to the pulse width of a Q-switched Nd:YAG l a ~ e r . ~ J I J ~ The 532-nm output of a Continuum Surelite Q-switched Nd:YAG laser (10 Hz) was Raman-shifted in 85 psi of H2(g) and dispersed by a Pellin-Broca prism. The 436-nm component of the beam was focused by a cylindrical lens (25" diameter, 76-mm focal length) to a slitlike image at the sample position. For the experiments described in this paper, the energy density at the sample varied from 1.1 to 31 mJ/cm2. The sample consisted of a 0.5 mM solution of H2TPP held in a 4m"mdiameter quartz tube (Wilmad 708-SJH) and degassed by three or four freezepumpthaw cycles to a vacuum of 6 mTorr. The tube was placed in a nitrogen-flow cryostat (Wilmad WG821) kept at -40 OC. In order to prevent local heating of the sample by the laser beam, the quartz tube was spun at about 10-15 Hz with a stream of compressed dry air. Raman scattering was analyzed by a homebuilt spectrometer. Backxattered radiation was collected by a pair of 38-"diameter quartz lenses (with f/l and f / 8 ) , a polarization scrambler, and an Omega Filters (Brattleboro, VT) sharp cutoff filter with a design wavelength of 442 nm (75% transmission at 449 nm). The radiation was dispersed in a ISA THR-1000 1.0-mf / 8 spectrograph equipped with a 1800 groove/" holographic grating. A Princeton Instruments IRY-l024/B microchannel plate intensified photodiode array cooled thermoelectrically at -23 OC was the detector. All transient Raman spectra were obtained at a spectral resolution of about 8 cm-l. Sample integrity was ascertained by UV-vis absorption spectra of the solutions before and after irradiation at 436 nm. In all cases, no evidence of photodacomposition was found.
Results
'* Lafayette Haverford College. College.
spectroscow d the E~dtd E k t ~ &strtes of HZTPP. Figure 1 shows the effect of pulsed laser excitation on the resonance
0022-3654192 12096-10591S03.00/0 , 0 1992 American Chemical Society I
,
10592 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992
r\
Letters
a.
1
1 1200
1350
1500
1650
Raman shift (wavenumbers)
R a m a n s h i f t ( w a v e n u m bers)
Figure 1. Rmnance Raman spectra of H2TPPin THF. Conditions are as described in the Experimental Section: (a) continuous excitation with 25 mW at 442 nm; (b) pulsed excitation at 436 nm, 1.1 mJ/cm2;(c)
Figure 2. Difference resonance Raman spectra of H2TPP. The spectrum obtained at low flux was subtracted from that obtained at high flux, as described in the Rcsults section. All other conditions are as described in the Experimental Section: (a) solvent, THF; low flux, 1.1 ml/cm2; high flux, 3 1 mJ/cm2; scaling factor, 1.672. (b) solvent, CS,; low flux, 1.1 mJ/cm2; high flux, 31 mJ/cm2;scaling factor, 1.797. (c) solvent, 4 1 THFCHJ; low flux, 1.1 mJ/cm2;high flux, 29 mJ/cm2;scaling factor,
pulsed excitation at 436 nm, 31 mJ/cm2. Raman spectrum of HzTPP in THF. Under excitation with a continuous source at 442 nm (Figure la), spectral features are observed that match closely those observed previously by Stein et al." The same features are reproduced under pulsed excitation at 436 nm, provided that the energy density is about 1,l mJ/cm2 (Figure lb). At higher energy densities, new features are apparent in the spectrum. These new features are labeled as ES in Figure IC (obtained at 31 mJ/cm2) and include (a) a shoulder to the low-frequency side of the 1551-cm-I band and (b) an increase in intensity of the 1355- and 1600-cm-' bands relative to other features. Spectra taken at low powers after exposure to high powers did not show these features. In order to describe these spectral changes more fully, we carried out a subtraction of the spectrum obtained at 1.1 mJ/cm2 from that obtained at 31 mJ/cm2. The procedure assumed that the band at 1551 cm-I was due solely to scattering from the ground electronic state of H2TPP. The difference spectrum was then scaled in such a manner as to give zero intensity at 1551 cm-I. No features with negative intensities were observed. The result was a spectrum of species generated during high excitation powers, cormkd for bleaching of the ground state. This scaled difference spectrum (Figure 2a) shows that five new bands appear as a result of an increase in power; these are located at 1224, 1355, 1500, 1540, and 1598 cm-l. We ascribe these features to excited electronic stat& of H2TPP. A sixth broad feature at about 1450 cm-l is assigned to nonremnant Raman modes from the THF solvent that beoome slightly more intense under high powers. It is possible that the band at 1224 cm-' may also have a contribution from the THF solvent. In order to distinguish between THF and porphyrin features, we acquired Raman spcctra of H2TPP in CS2at low and high powers of 436-nm radiation (Figure 2b). The use of CS2 as solvent is advantageous as it possesses no vibrational bands in the 12001700-cm-l region. The spectrum shows that there are no excited state features between 1360 and 1490 cm-I and that the band at 1224 cm-I is due to porphyrin excited state. The difference spectrum is not of high quality, however, possibly due to a solvent effect on the lifetimes of the excited states. Therefore, most of the experiments reported herein were performed in THF. The data in Figure 2a,b show that excited electronic states of HzTPP are populated by the 6-ns laser pulse and that these states may be probed by resonant Raman scattering at 436 nm. As both the SIand TI states of H2TPP absorb strongly at 436 nm and are
1.055.
TABLE I: Assignment of the Transient Rwmnce Spectra of H,TPP excited-state freq (cm-I) state assignt 1598 1540 1500 1367 1355 1225
TABLE 11: Assignments of Vibrational Bands of the TI States of H2TPP and ZnTPP vibrational freauencies (in cm-') for HZTPP ZnTPP mode descriptions" So TI shift So T1536 shift phenyl v2,
(CpHCPH)
c,c,
C,N phenyl (CMCPH) ~ 4 ,
1603 1558 1355 1230
1598 1540 1367 1225
-5 -18 +12 -5
1599 1551 1357 1236
1594 1517 1389 1236
-5 -34 +32 0
long-lived relative to the laser pulse, we expect that vibrational bands from both the S, and T I states are observed under our experimental conditions. In order to distinguish between vibrational modes of excited singlet and triplet states, we obtained a difference spectrum of H2TPP in a mixed solvent system containing 80% THF and 20% CH31 by volume. The presence of CH31introduces a heavy atom effect which results in an increase in the quantum yield of triplet formation and shortenssinglet-state lifetimes. The resulting difference spectrum (Figure 2c) shows the following effects: (a) the 1500-cm-' band is very weak relative to the features at 1540and 1598 cm-l, and (b) the l367-cnr1 band is strong relative to the features at 1540 and 1598 cm-I. The band at 1224 cm-I appears to be broad in comparison to its analog in the spectrum of Figure 2a. We believe that this effect is due to convolution of a CHJ mode with the 1224-cm-' band of the porphyrin. h i g " t of Vibrational Bands to &e St and TIStates. The data described above allow us to assign the observed vibrational bands in the difference spectra to the SIor TI state of H2TPP. We based our assignments of Table I1 on the fact that the T I vibrational bands should appear in both THF and THF/CH,I
Letters solvents and should decrease in the presence of CH31 due to a heavy atom effect. We observe that the intensity of the 1367-cm-l band increases relative to the intensity of the 1355-cm-I band in the presence of CH31. Hence, we assign the 1367-cm-I band to the T I state and the 1355-cm-I band to the SIstate. Likewise, the relative intensity of the 1500-cm-l band decreases in CH31, and we assign it to the SIstate. This assignment is supported by the observation of bands at 15 15 and 15 11 cm-I in the fluorescence excitation spectrum of H2TPP in a supersonic jet.I4J5
Discumion Adgmmt of TIState Baods to Porphydn Modes. We amve at assignments of the modes observed in the transient resonance Raman spectrum of H2TPP by a method similar to that of Sat0 et al.7 Instead of using data on isotopically substituted compounds, they used results from normal coordinate analyses of the ground state in conjunction with molecular orbital arguments to predict the shift of vibrational bands in excited states relative to the ground state. To assign the observed TI state peaks to vibrational modes of the porphyrin, we examined the electron density patterns of the porphyrin molecular orbitals, which are discussed in detail by Gouterman3 and Maggiora.16 The arguments below hold strictly only for the D4hpoint group, but we will assume that they may be applied qualitatively to H2TPP,which belongs to the D2hpoint group. In DG symmetry, the highest occupied molecular orbitals have aZuand a I usymmetries and are nearly degenerate. In mesosubstituted porphyrins, the aZuorbital is slightly higher in energy. The a2, orbital has most of its electron density at the bridging (meso) carbons and at the pyrrole nitrogens, while the aluorbital has considerable electron density at the carbons a and fl to the pyrrole nitrogens. In the azuorbital, the C,N bonds have strong antibonding character, whereas the C,C, bonds have strong bonding character. The lowest unoccupied molecular orbitals are doubly degenerate and have eBsymmetry. These antibonding orbitals possess electron density at the meso, a,8, and N positions. The C a bonds have slight bonding character, and the C,C, bonds have slight antibonding character. The excited singlet states of aluegand azueBconfigurations are nearly degenerate and mix. In contrast, the alueBand a2ueBtriplet states cannot mix for symmetry r e a ~ o n sand, ~ * ~therefore, exist as pure configurations. The T I states of H2TPP and four-coordinate metalated analogs, such as ZnTPP, have aZueB configurations. It is possible, therefore, to predict the direction of vibrational band shifts for the T I state of H2TPP, relative to ground-state vibrational frequencies, by considering the bonding/antibonding nature of the bonds in the occupied molecular orbitals of a given configuration. For example, promotion of an electron from an a?, orbital to an eB+orbital leads to an a2ueBexcited-state configuration, with an attendant weakening of the C,C, bonds (downward frequency shifts) and strengthening of the C,N bonds (upward frequency shifts). Similar arguments may be used to predict that formation of an alueBconfiguration leads to strengtheningof the C,C, and C,N bonds, as has been observed in H20EP.7 The amsiderations above may be used in conjunction with mode assignments for the ground state of H2TPP,given by Stein et al.,I3 to arrive at an assignment of the modes active in the resonance Raman spectrum of the T I state (Table 11). For example, the v2 mode of the ground state of H2TPPappears at 1558 cm-l and is expected to shift to lower frequencies in the T1 state because it has considerable C,C, stretching character. Thus, we assign the 1540-cm-I band of the resonance Raman spectrum of the T I state to v2. Similar arguments were used to arrive at the assignment of v4. These assignments will be confirmed by similar studies on isotopically substituted tetraphenylporphyrin. Nonetheless, our approach is sound, as it is based on knowledge of the electronic structure of the excited state and of the vibrational pattern of the ground state. Our assignments are also consistent with data on ZnTPP and deuterated derivativePb and on a porphyrin triplet
The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 10593 state with alueBconfiguration (vide infra). Comparison with ZnTPP and H20EP. Previous assignments of the resonance Raman spectrum of the TI state of ZnTPP are summarized in Table 11. The TI statts of both H2TPPand ZnTPP exhibit two bands attributed to phenyl nodes, the CPHCPHand the CMCPHstretches, which appear at essentially the same frequencies as in the ground state. This implies that the electronic excitation is localized onto the porphyrin ring alone and that phenyl-porphyrin conjugation in the TI state of these porphyrins does not OCCUT. The relatively strong intensity of the phenyl modes in the transient Raman spectrum, however, suggests the possible delocalization of electrons onto the phenyl rings in the higher energy triplet state which is in resonance with the probe beams5 Comparison between our data on H2TPPand those on ZnTPPS6 allows for an assessment of the effect of symmetry on the dynamics of the excited triplet states of porphyrins. ZnTPP belongs to the D4hpoint group, and its TI state is subject to Jahn-Teller distortions, which reduce the effective symmetry of the molecule to DU and cause large shifts in the frequencies of v2 and v4. In H2TPP,the modes shift in the same direction, but the magnitudes of the shifts are substantially smaller than those of ZnTPP. The TI states of both molecules have a2,,eg configurations, so that the direction of the shifts are explained readily. The relative magnitudes of the shifts may be attributed to a combination of effects. If the shifts are governed by the extent to which the excited state of the porphyrin is prone to Jahn-Teller distortions, then the shifts in ZnTPP (D4hpoint group) are expected to be sizable, while those in H2TPP are expected to be small. This is because H2TPP belongs to the D2h point group and is not subject to Jahn-Teller instability. In this case,the frequency shifts in H2TPP may be due solely to changes in the bonding character of the bonds upon formation of the TI state. Alternatively, the shifts may be due to differences in the electronegativity of the atoms occupying the core of the porphyrin. This model, first proposed by Perng and BocianI7 in an attempt to explain the resonance Raman spectra of anion radicals of H20EP and ZnOEP, suggests that metal ions at the porphyrin core perturb the eB*orbital by increasing electron density at the pyrrole nitrogen. As a result, the model predicts that the C,N bonds are stronger in aheg excited states of ZnTPP than of H2TPP. This is because the electrondeficient Zn2+ion draws more electron density to the pyrrole nitrogens than H+ions. This is, in fact, observed. However, this model fails to predict the observed pattern of shifting observed for C&, vibrations in the excited triplet state of ZnTPP and H2TPP. Therefore, we believe that Jahn-Teller distortions explain our data more accurately than inductive effects. Further comparison between our data on H2TPP and those of Sat0 et ala7for H20EP allows for a determination of the effect of electronic configuration on the resonance Raman spectra of T I states of porphyrins. Octaethylporphyrins contain only hydrogens at the meso positions. In D4hsymmetry, this makes the aluorbital higher than the a2uorbital, so that the TI and SIstates of OEP complexes have BlueBconfig~rations.~J~ Sat0 et al.7 observed a +14-cm-I shift in the frequency of ut upon formation of the T I state of H20EP; the position of v4 was not clear from their spectra. We note that the 18-cm-' downshift in v2 of an aZuegtriplet (H,TPP) is similar in magnitude to the 14-cm-I upshift in the same mode of an alueBtriplet (H20EP). This indicates that both porphyrins undergo similar changes in bond order for the C,C, bonds, in a fashion that seems to be unrelated to the configuration of the excited state. Acknowledgment. J.d.P. acknowledges funding by the donors of the Petroleum Research Fund, administerd by the American
Chemical Society, and the Olin Charitable Trust of Research Corporation (Grant C-3007). J.d.P., V.A.W., and C.N. received support from the Pew Science Program in Undergraduate Education (Mid-Atlantic Cluster). References and Notes (1) Wasielewski, M. R. Chem. Reu. 1992, 92, 435. (2) Morgan, B.; Dolphin, D. In Merol Complexes with Tefropyrrole figonds, I; Buchler, J. W., Ed.;Springer-Verlag: Berlin, 1987; p 115.
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(3) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 3, p 1. (4) (a) Findsen, E.; Shelnutt, J. A.; Ondrias, M. J. Phys. Chem. 1988, 92, 307. (b) Chikishev, A. Y.;Kamalov, V. F.; Koroteev, N. I.; Kvach, V. V.; Shkurinov, A. P.; Toleutaev, B. N. Chem. Phys. Lett. 1988, 144, 90. ( 5 ) (a) Walters, V. A.; de Paula, J. C.; Babcock, G. T.; Leroi, G. E. J. Am. Chem. Soc. 1989, 111,8300. (b) Nam, H. H.; Walters, V. A.; de Paula, J. C.; Babcock, G.T.; Leroi, G.E. In Proceedings ofthe XIIth International Conference on Raman Spectroscopy; Dung, J. R., Sullivan, J. F., Eds.;Wiley and Sons: New York, 1990; p 618. (6) Reed, R. A.; Purrello, R.; Prendergast, K.; Spiro, T. G.J . Phys. Chem. 1991, 95, 9720. (7) Sato, S.;Asano-Someda, M.; Kitagawa, T. Chem. Phys. Letr. 1992, 189, 443. (8) (a) Nishizawa, E.; Hashimoto, H.; Koyama, Y. Chem. Phys. Lett. 1989,164,155. (b) Nishizawa, E.; Koyama, Y. Chem. Phys. Lett. 1990,172, 317. (9) Gouterman, M. Ann. N .
Y.Acad. Sci. 1973, 206, 70.
(10) Fuhrhop, J.-H.; Smith, K.M. Laboratory Methods in Porphyrin u d Metalloporphyrin Research; Elsevier Scientific: Amsterdam, 1915; p 13. (1 1) Rodriguez, J.; Kirmaier, C.; Holten, D. J. Am. Chcm.Soc. 1989,111, 6500. (12) Darwent, J.; Douglas, P.; Harriman, A.; Porter, G.; Richoux, M.-C. Coord. Chem. Rev. 1982, 44, 83. (13) Stein, P.; Ulman, A,; Spiro, T. G. J. Phys. Chem. 1984, 88, 369. (14) Even, U.; Magen, J.; Jortner, J.; Friedman, J.; Levanon, H. J. Chem. Phys. 1982, 77, 4374. (1 5) One might note that the spectrum in ref 14 consisted of series of bands
which differed in the number of quanta in a phenyl torsional mode with a frequency of 18 cm-I. Assuming that their choice of the origin of the series was off by one quantum, their values for the fundamental bands could be off by 18 cm-I, bringing them very close to the frequencies we assign to the S, state in our spectrum. (16) Maggiora, G. M. J . Am. Chem. Soc. 1973,95,6655. (17) Perng, J.-H.; Bocian, D. F. J . Phys. Chem. 1992, 96,4804.
Multifrequency ESEEM Spectroscopy of Ammonia Adsorbed on Silica-Supported Reduced Molybdenum Oxide Sarah Cosgrove Larsent and David J. Singel* Department of Chemistry, Harvard University, Cambridge, Massachusetts 021 38 (Received: September 8, 1992; In Final Form: November 3, 1992)
The structural environment of a MoS+center on silica-supported molybdenum oxide, with adsorbed ammonia, was probed by ESEEM spectroscopy. Features attributable to IH and I4Nare observed in the spectra and indicate the direct coordination of ammonia to the MoS+. Nuclear hyperfine and quadrupole coupling constants,elicited by multifrequency ESEEM techniques, are compared with ones recently determined by us for the analogous vanadium oxide system ( J . Phys. Chem. 1992,%, 9007). While the I4N quadrupole coupling is slightly smaller in the molybdenum system, the hyperfine coupling constant, 4.68 MHz, is essentially identical to the average of the principal coupling constants found in the vanadium system.
Introduction Supported molybdenum oxide catalysts are important for the oxidation of organic substrates.'" Mo5+ centers that actively coordinate molecular adsorbates have been identified on reduced molybdenum oxide catalysts by EPR (electron paramagnetic resonance) spectroscopy.'-I4 Depending on the type of support and the method of preparation, the Mo5+ EPR spectra show variations that have been interpreted as reflecting various MoS+ coordination numbers and geometries.'" In addition, structural details of the interaction of Mo5+with adsorbed W O have been elucidated through the analysis of ligand hyperfine structure in Mo5+EPR spectra.I0J' The utility of this approach is limited, however, as ligand hyperfine structure is rarely resolved in Mo*+ EPR powder spectra. A pulsed EPR technique, ESEEM (electron spin echo envelope modulation) spectroscopy, has proven useful in measuring hyperfine interactions not resolved in EPR spectra. ESEEM has been employed, in particular, to characterize the interactions between Mo5+on dispersed metal oxides and various adsorbates, including DzO, CD30H, and CH30D.13J4Analysis of 2Hmodulation effects in these systems revealed the number of adsorbate molecules directly coordinated to the Mo5+ center. ESEEM signals from adsorbed ammonia were consistent with an intimate nitrogen-molybdenum interaction but were not quantitatively ana1y~ed.I~ On the basis of infrared studies, Belokopytov et al. suggested that the ammonia on a molybdenum oxide surface is directly coordinated to the m01ybdenum.l~ We have recently used multifrequency ESEEM to characterize the interaction of V4+ centers on silica-supported vanadium oxide with adsorbed ammonia.I6 We demonstrated that quantitative interpretation of the 14Nmodulation effects is possible and that 'Present address: Department of Chemical Engineering, University of California, Berkeley, CA 94720.
it leads to a detailed structural picture. The vanadium and molybdenum systems should have similar electronic and structural properties: V4+ and MoS+both have d1valence electron configurations with the unpaired electron, typically, in the d orbital. Therefore, the analogous molybdenum system shoulg also be amenable to detailed ESEEM studies. In this Letter, we report results from a multifrequency ESEEM study of silica-supported, reduced molybdenum oxide with adsorbed ammonia. Aside from the intrinsic intmst in the structure of Mo5+ centers on catalytic oxides, these results also assume importance in providing a point of comparison between ligand hyperfme coupling constants in analogous Mo4+and V4+systems; the latter have been widely studied by ESEEM, but the former have not, despite the importance of molybdenum chemistry in chemical and biochemical catalysis. The ESEEM spectra observed display a great similarity to those of the previously studied vanadium system; both reflect direct coordination of ammonia to the metal center. The I4N quadrupole and hyperfine coupling constants found in both systems are very similar, although the spectra reveal that the 14Nhyperfine interaction is mort anisotropic in the Mo5+system. ESEEM from protons bound to an atom directly coordinated to the metal center, presumably the nitrogen atom of the ammonia ligand, is observed in both systems. The measured electron-proton dipolar interaction indicates that the metal-proton distance is slightly larger in the molybdenum versus the vanadium complex. Experimental Metbods Samples of silica-supported molybdenum oxide, with a molybdenum weight loading of approximately 296, were prepared by standard impregnation prooedures.SJ3J4 In order to remove Fe3+ impurities, the silica gel (Fisher Scientific, 100-200 mesh) was washed with 1 N HC1, r i d with distilled water, and dried at 100 OC for several hours. An aqueous solution of paramolybdate,
0022-36S4/92/2096-10594$03.00/00 1992 American Chemical Society
