3886
J . Phys. Chem. 1985, 89, 3886-3890
Dichromium Revisited: A Resonance Raman Study of Cr, Isolated in Ar, Kr, and Xe Matrices M. Moskovits,* W. Limm, Department of Chemistry and Erindale College, University of Toronto, M5S 1Al Canada
and T. Mejean Laboratoire de la Spectroscopie Infrarouge, UiriversitP de Bordeaux I , Talence, France (Received: February 19, 1985)
Laser-induced resonance Raman progressions were observed for Cr2 isolated in solid Ar, Kr, and Xe. In Xe the vibrational constants were W / = 438 cm-' and W / X / = 14.5 cm-l. Another progression with vibrational constants we = 417 cm-' and o,x, = 12 cm-I but built upon a transition at 154 cm-' is assigned to one ending on a low-lying excited state of Cr2, possibly The discrepancy between the value of AGIl2 observed for Cr, in the gas phase as compared to its value of symmetry for the matrix isolated molecule is considered. One concludes that it is due to an unusual matrix effect due possibly to the fact that the ground-state potential for Cr, has a double minimum.
3xg+.
Interest in the spectral properties of metal clusters, stimulated to a large measure by the hope that what is learned may be used to understand heterogeneous catalysis better, has found substance in two groups of studies: matrix isolation spectroscopy and spectroscopy in isentropically cooled molecular beams. The two techniques normally give consistent results, and often complementary results in that the fluorescence excited in beams is normally too faint to disperse and therefore, it is often difficult to obtain ground-state information about clusters in beams, apart from rotational constants. Although it is easy to generate clusters containing many tens of atoms by both routes, the complexity of the spectra which are being obtained with transition-metal diatomics has caused workers in the cluster field to stay at this first stop in the cluster journey somewhat longer than anticipated. Among the diatomics, the one which has attracted the most attention has been Cr,. This is in part because a number of sophisticated molecular orbital calculations, often at an inordinate level of configuration interaction, have been unable to predict a strongly bound ground state for Cr2 while all experimental evidence points to a molecule with a short bond (1.68 A), a high frequency cm-I), and therefore high bond order. Cr2 is also unique in that its value of W / determined from a resonance Raman spectrum of the argon-matrix isolated' molecule, 427.5 cm-', and that estimated from the value of AG1/2of a hot band in a gas phase2 spectrum, 470 cm-', possess the largest difference between the gas-phase and matrix frequency hitherto reported: so large, in fact, that it has been suggested that the matrix spectrum was not that of Cr2 at all but perhaps of Cr3 or of another chromium-containing species. For comparison one should note that for V, and Cu2, the other two diatomics for which both gas-phase and matrix vibrational frequencies are known, the differences do not exceed 3 cm-l. In this article we report further studies with Cr in these matrix environments, solid Ar, Kr, and Xe, carried out in order to attempt to resolve this discrepancy. Experimental Section
The apparatus and experimental procedure have been described elsewhere.' Briefly, chromium vapor, produced by heating the metal in a tungsten basket to the sublimation point, was cocondensed with Ar, Kr, or Xe vapor onto a polished aluminum surface cooled to 12 K by means of a Displex (Air Products) refrigerator. (1) D. P. DiLella, W. Limm, R. H. Lipson, M. Moskovits, and K. V. Taylor, J. Chem. Phys., 77, 5263 (1982). (2)V. E.Bondybey and J. H. English, Chem. Phys. Lett., 94,443(1983).
0022-3654/85/2089-3886$01 SO10
Raman and fluorescence spectra were Ar+ laser excited and detected by means of a SPEX double monochromator equipped with photon counting and interfaced to a Tektronix 4052 computer. Results Figures 1 and 2 and show portions of the resonance Raman spectrum obtained from two Cr-containing Ar matrices, the first dilute, the second more concentrated in metal. The spectra are dominated by two progressions beginning at 396 and 306 cm-' previously assigned to Cr2 and Cr3. The relative intensities of the two progressions vary with metal concentration in the expected manner, Le., the more concentrated matrix exhibits a relatively more intense Cr, spectrum. The ratio of intensities of the two progressions also varies with exciting line. With high-frequency (458 or 476 nm) Ar+ laser excitation the intensity of the trimer progression is weak compared to that of the dimer while with 488and 514-nm excitation the relative intensities of the trimer bands are greater. This is not unexpected since Cr2 is known to have a strong absorption3 near 460 nm. The results suggest, however, that there is a weak trimer band at or near 488 nm since it is with excitation in that region that one sees the most intense (relative) Cr3 emissions. The spectrum excited with 458 nm is marred by a broad fluorescence whose origin has not yet been determined. With 488-nm and to a lesser extent with 476-nm excitation, one sees another progression consisting of three lines at 154.5, 548.5, and 919 cm-'. The vibrational constants calculated for this progression, we = 417, use= 15.75 cm-', are close to those found for the ground state, Le., WP= 427.5 and w / x c = 15.75 cm-'. The intensities of these emissions were determined to be proportional to the laser power within experimental error. Upon matrix warmup the two progressions beginning at 154.5 and 396 cm-' did not change their relative intensities nor were their relative strengths affected by increasing the concentration of the metal, implying that they belong to the same carrier. Not unexpectedly, the intensity of the progression ascribed to Cr3 did increase upon matrix warmup. Since chromium possesses four abundant isotopes a high-resolution spectrum of the 396-cm-' line was recorded. This is shown in Figure 3 along with a calculated spectrum of equivalent resolution, assuming the carrier to be Cr2 and intensities of various peaks to be proportional to the natural isotopic abundance of the particular species. With 476-nm laser excitation the high-resolution spectrum measured immediately after deposition does not (3) (a) E.P.Khdig, M. Moskovits, and G. A. Ozin, Notum (tondon)254, 503 (1975); (b) M. J. Pellin and D. M. Gruen, J. Chem. Phys., 79, 5887 (1983).
0 1985 American Chemical Society
Raman Study of Cr, Isolated in Inert Gas Matrices
The Journal of Physical Chemistry, Vol. 89, No. 18, 1985 3881
C r / Ar
.... ... ... . .
.................
u 33K
488
I
100
I
m
I I 1 I I I I ' 303 400 500 600 700 800 900 WAVE NUMBER ( c d
Figure 1. Portions of the emission spectrum obtained from chromium containing, solid argon matrices excited with the Ar' laser lines indicated.
1.0
J I I I I I I I I I I 388 392 396 400 404 408 WAVENUMBER (cm-') Figure 3. A higher resolution spectrum of one of the fundamentals of the resonance Raman spectrum of Cr2 isolated in solid argon: (A) as deposited (B) and (C) annealed to the temperatures indicated (D) computed based on the natural abundance of the four isotopes of chromium.
4 514.5
> 0.15-
c_
-.......... w 0.102
I-
2 -
0 ' I ' 388 392
1
396
'
1
400
'
l
404
'
I
408
WAVENUMBER (cm-')
Figure 4. Measured spectrum (bottom) and (top) calculated spectra of the isotopic structure that one expects if Cr2 molecules in three different matrix sites contribute to the observed spectrum. Arrows indicate the assumed positions of the emissive maxima of the three contributing species.
Figure 2. As in Figure 1 but with a matrix more concentrated in chromium. The numbers "2" and "3" in the top spectrum refer to two progressions assigned to Cr2 and Cr3, respectively.
correspond well to the calculated spectrum. Upon warmup the profile of the high-resolution spectrum changes (Figure 3) until it resembles, after the matrix has been warmed to 33 K, the calculated spectrum rather closely. This suggests that the 396-cm-' progression consists of a n overlap of several progressions each with almost the same vibrational constants. Each of these progressions almost certainly belongs to Cr, isolated in a slightly different
matrix site. Upon warmup species in less stable sites disappear leaving behind Cr, isolated in a single site. The results of an attempt to determine how many Cr2 species were necessary to reproduce the high-resolution spectrum obtained immediately after deposition is shown in Figure 4 which was constructed by using three Cr2species whose major features (those belonging to SZCr2)were assumed to come at the frequencies indicated by the arrows. The fit, although acceptable, is still not perfect, implying at least four matrix sites. It is interesting to note that a high-resolution absorption spectrum of Cr2 shows the presence of four matrix sites4 The three most highly populated (4) M. Moskovits, W. Limm, and J. Mejean, J. Chem. Phys., 82, 4875 (1985).
3888 The Journal of Physical Chemistry, Vol. 89, No. 18, 1985
Moskovits et al.
Cr/Ar
5
.. ..
I
J
.
::
I
405
l
! l
I
1
l
410 W A V E N U M B E R Icm-' )
1
41 5
1
1
I
,
Figure 7. Observed (points) and calculated (line) isotopic fine structure of the fundamental of the resonance Raman spectrum of Cr2 isolated in
solid Xe.
465.9
I
1
-'3
392
1
I
I
400 404 W A V E NUMBER (cm-1)
408
396
Figure 5. High-resolutionspectra obtained at three matrix locations of progressively lower chromium concentration (A) to ( C ) compared to computed spectrum (D).
:E
5
2'5Jt 2.0 514.5 I .o
WAVENUMBER (cm-')
465.9
3
Figure 8. As in Figure 6 but with two other wavelengths of laser excitation which excite a progression based on 154 cm-I.
2
458
h XX, 400 500 600 700 800 900 WAVENUMBER (cm-')
Figure 6. Emission spectra obtained from chromium containing solid Xe
matrices by using the indicated wavelengths of laser excitation. of which show abundances not unlike the proportions used to produce Figure 4. Upon warmup the Cr2 population in all but one site is substantially reduced. Another aspect of these various sites is shown in Figure 5 , which shows the high-resolution spectrum of the 396-cm-' line excited a t different points in the matrix corresponding to different metal loading. (The cold receiving surface on which the matrix is formed is so placed that a gradient in the metal concentration is created below a region of almost constant metal concentration.) The high-resolution spectrum observed in the region of the matrix in which the metal concentration was low corresponds best to the calculated spectrum. This may mean that some of the subordinate sites correspond to Cr2 with other chromium species, perhaps atoms in the vicinity. Upon warming these disappear perhaps as a result of C r diffusion and reaction. Alternatively, the high concentration region is perhaps more disordered as a result
of the high level of doping and warmup serves mainly to heal the crystalline faults thereby reducing the concentration of Crz in the less stable sites. Examples of spectra obtained with Xe matrices are shown in Figure 6. The major progression now begins with a fundamental at 409.4 cm-' and has vibrational constants u c = 438.0 and WX:/ = 14.5 cm-'. Unlike the spectra obtained in Ar the spectral feature at 409 cm-' shows a broad phonon wing in addition to its zero phonon band. A high-resolution spectrum (Figure 7) of the fundamental agrees well with the calculated (isotopic) spectrum implying either one major site or, if several, sites for which Cr, has nearly identical vibrational constants. In addition to the progression based on 409 cm-] the spectra shown in Figure 6 contain other features. With 465.9-nm excitation one sees two features at 348 and 380.8 cm-l which are assigned to sequence components of the 409-cm-' transition. This phenomenon, seen before for Ti2, and V26 (as well as for numerous other diatomics not involving transition metals), implies that the u" = 1 and u" = 2 levels of Cr, are radiatively populated and that the vibrational lifetime is sufficiently long to allow sufficient population to accumulate in those two states. That these truly are due to multiphoton transitions rather than to new spectral features which coincidentally fall at the frequencies expected for sequences was demonstrated by means of a laser fluence study in which it was found that transitions originating in u" = 0 were more or less proportional to laser power while those proposed to begin in the u" = 1 or 2 state were found to depend on a power of the laser fluence greater than one. The features at 670 and 733 cm-' may likewise be sequence components of the first ( 5 ) M. Moskovits and D. P. DiLella, J . Chem. Phys., 73, 4917 (1980). (6) C.CossS., M. Fouassier, T. Mejean, M. Tranquille, D. P. DiLella, and
M. Moskovits, J . Chem. Phys., 73, 6076 (1980).
Raman Study of Cr2 Isolated in Inert Gas Matrices
The Journal of Physical Chemistry, Vol. 89, No. 18, 1985 3889
5L\
Cr/ Xe
4
W A V E N U M B E R (cm-')
Figure 10. As in Figure 6 but for Cr in solid Kr.
+
O' I d 0
& &
460 6 b 760 860 WAVENUMBER (cm-' )
&
Figure 9. The effect of annealing on the spectrum of Cr in solid Xe: bottom, as deposited;others, annealed to temperatures indicated. 458-nm Ar+ excitation.
-
harmonic. Their high intensity compared to the 0 2 feature at 789 cm-' argues against this, however, as does the fact that with 458-nm excitation these two high-frequency features are visible while the two sequence components associated with the fundamental are missing. There is, in addition, another spectral feature a t 340.7 cm-I. This is visible most clearly with 458-nm excitation, although it was seen with every exciting line attempted. With 514-nm excitation (Figure 8), in fact, it is the major feature in the spectrum, although this is more due to the weakness with which the 409-cm-l progression is excited than to the strength of the 340-cm-' feature itself. Upon warmup the 340- and 670-cm-' features disappeared while at the same time the one at 409 cm-' diminished in intensity while features previously ascribed to Cr3 grow in (Figure 9). The progression based on 409 cm-' is assigned to Cr,. Its unusually large matrix shift of 11 cm-' on going from Ar to Xe may, in fact, provide a hint to the resolution of the problem alluded to in the Introduction. The assignment of the 340-cm-' line is more difficult. Good-quality high-resolution spectra could not be obtained and we prefer not to speculate upon its origin. The spectrum obtained with Kr matrices resembles closely that obtained in Ar (Figure 10). The progression ascribed to Cr2 begins with a feature at 395.1 cm-I.
Discussion Cr2 has recently been the subject of intense spectroscopic study. In addition to the work cited in the Introduction, Riley et al.7 propose, on the basis of the observed modulation of the intensity of the rotational branches of the A X transition of Cr2, that Cr2 possesses at least two states intermediate in energy between those two states. They argue convincingly that the modulation is due to a perturbing state with a vibrational constant around 270 cm-' which crosses the A state and a predissociating state which crosses the perturbing state near u = 0. Pellin and Gruedb report two fluorescence bands which they attribute to Cr,, one with a vibrational spacing of approximately 240 cm-'which they state which correlates with Cr(7S) claim originates from a 3Cg+
-
(7) S.J. Riley, E.K. Parks, L.G. Pobo,and 79, 2577 (1983).
S.Wexler, J . Chem. Phys.,
Cr(z7po)and terminates in a 3Eu+ state some 8000 cm-' above the ground state. In addition, Pellin and Gruen postulate a 'nu state about 3.8 eV above the ground state which they excite and which crosses to the fluorescing 3C,+state. Hence seven states of Cr2 are currently "known", two of which, the X'C,+ and the 3Cu+, correlate with ground-state chromium atoms i.e. Cr(7S). The actual situation is, of course, considerably more complex than this. Not counting spin degeneracies, Cr('S) can give rise 5Cg+, 'E,,+,9Cg+,IICu+, I3C,+. to seven states: ICe+,3Cu+, The exact placement of these states is unknown. Recently Goodgame and Goddard* published the results of a modified generalized valence-bond calculation that (in contrast to an earlier calculationg) predicts the bond energy, bond length, and force constant of Cr2 satisfactorily. The ground state is predicted to be characterized by a double well potential in which the outer well a t around 3 8, comes about as a result of s electron interaction which become repulsive at the short bond length. The 3Custate is also a double well state which lies near the dissociation limit of the ground state, according to this calculation. States of higher multiplicity (within the ground state manifold) possess only the outer minimum. A somewhat different picture is presented by Baykara et al.1° who produce a ground-state potential for Cr2 which, though not as good as that of Goodgame and Goddard so far as the dissociation energy is concerned, predicts the bond length and force constant satisfactorily. In the view of Baykara et al. the ground-state potential has a single well, albeit a rather anharmonic one to which s and d electrons contribute. The 6 orbitals, however, have a low degree of overlap and their contribution to the bonding is small. We will adopt the Goodgamffioddard picture for the bonding in Cr,, because it served better in explaining a long-lived metastable state of Cr, which we recently reported4 in terms of the "isomerization" of Cr, from the outer minimum to the inner one. One should state that our results may also be explained, in terms of the Baykara potential, with certain changes in interpretation. In addition to the ground-state manifold, Cr, is expected to possess a large number of excited states within 1 or 2 eV of the ground state. In particular, promoting an su electron from the ground us level to the u, level will result in two new states of symmetry ICu+ and 3C,+.These states may not differ greatly in energy from the ground state and the system which we see built upon 154 cm-' may well be due to transitions from the 3Z:U+ state state is shown by those authors to cross proposed above. The 3Cg+ the A'C,+ excited state to which the initial (allowed) absorption step is made. The emissions in question are only seen after one has achieved resonance with the A state (i.e., with 467-nm laser excitation or shorter wavelengths), and even then they are rather weak suggesting that only a small fraction of the molecules have state. crossed to the upper 3Cg+ A similar argument may be made in terms of the potential of Baykara et al. if instead of promoting an su, electron we promote a do electron to the du, level, since according to their picture those (8) M. M.Goodgame and W. A. Goddard 111, preprint. (9) M. M. Goodgame and W. A. Gcddard 111, J . Phys. Chem., 85, 215( 198 1). (10) N. A. Baykara, B. N. McMaster, and D. R. Salahub, Mol. Phys., 52, 891 (1984).
3890
J. Phys. Chem. 1985,89, 3890-3894
two states are separated by a small energy interval. Let us now consider the discrepancy between the reported gas-phase and matrix results. First let us review what is known vibrationally about gas-phase Cr,. Since only the A X transition has been studied either by fluorescence excitation or twophoton photoionization spectroscopy in cooled beams, only a few vibrational transitions have been observed. Precise values for the vibrational constants of the two states are not known although Riley et al.' report a value of 396.82 cm-' for AGl/, of the A state and 0,' - w," = -20.9 cm-I and wlx,'- w,"x," = 17.3 cm-' for the two states. Bondybey2reports a value of 452.3 cm-' for ACllz of the X state based on a single, weak transition which he ascribes to a vibrational hot band. (The possibility that the transition was from an electronic hot band was not considered.) The value reported for AGIlZin the A state is remarkably close to the value we find for Cr2 in argon matrices. We therefore must consider the possibility that the resonance Raman progression that we are observing is not taking place in the ground state of Cr2 but in the A state, radiatively populated. The linear dependence upon laser power must then be blamed on saturation even at the very low laser fluences we employ. The reason we do not see resonance Raman in the ground state would then simply be due to the fact that relaxed fluorescence competes effectively with RR. Another possibility is that the ground state of Cr, in the matrix is not the same as that in the gas phase. That is, that under the influence of the matrix the true ground state has switched places with a low-lying electronically excited state. Such an event has been previously suggested to occur for Ni atoms isolated in solid Ar, Kr, and Xe," for which the true ground state, based on a d8s2 configuration, has exchanged places with another state, formed from the d9sl configuration, which in the gas phase lies approximately 200 cm-' above the ground state. The last possibility is that Cr,, unlike most other strongly bonded diatomics, shows an unusually large matrix shift.
-
(11) B. Breithaupt, J. E. Hulse, D. M. Kolb, H. H. Rotermund, W. Schroeder, and W. Schrittenlacher, Chem. Phys. Left., 95, 513 (1983).
We favor the last explanation. To begin, the shift (1 3 cm-I) observed on going from Kr to Xe is already condderably larger than what one normally encounters. Second, although one may blame the linear dependence of the intensity of the RR progression observed with laser fluence on saturation when one is pumping resonantly, one is much less likely to see saturation with nonresonant pumping (as, for example, with 514.5-nm excitation). Yet even in that case one sees a proportional dependence. Hence the near coincidence between the AGl/, interval in the A state of the gas-phase molecule and its value for the X state of the solid-argon-isolated diatomic is just that. (In fact, there is no coincidence in the case of solid-Xe-isolated Cr,.) The possibility of state swapping is much harder to reject. Experiments in solid Ne might shed some light on this but our current cryostats place that medium outside our reach. Moreover, the "swapping" state would also have to be of ICgsymmetry in order to explain the close correspondence between the gas phase and the matrix absorptions of the molecule. In total, we feel the most likely explanation to be the one based on an unusually large matrix effect. The double minimum ground state of this molecule may provide a rationale for such an unusual matrix shift. The outer minimum and the potential barrier may be affected quite appreciably by the matrix since they lie in the region of internuclear separations where substantial repulsive reactions with the matrix should set in. This may, in turn, result in a larger than usual effect upon the curvature of the inner well resulting in a reduced harmonic frequency and increased anharmonicit y. We speculate, therefore, that the ground state of Cr, in the exactly as in the gas phase, but that its ground matrix is 'Eg+, state potential has been modified by the matrix to a slightly larger degree than for most other strongly bonded diatomics.
Acknowledgment. We thank Drs. Dieter Gruen and Mike Pellin for valuable comments and discussions, NSERC for financial support of this work, and NATO for a travel grant. Registry No. Crz, 12184-82-6;Ar, 7440-37-1; Kr, 7439-90-9; Xe, 7440-63-3.
Redox Reactivity of Transition-Metal Phthalocyanines: Ligand Radical Formation vs. Metal Center Oxidation D. K. Geiger, G. Ferraudi,* K. Madden, J. Granifo, and D. P. Rillema Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: March 11, 1985)
The oneelectron oxidations of cobalt(II), cobalt(III), copper(II), and nickel(I1) tetrasulfophthalocyaninehave been investigated by using stop-flow, pulse radiolysis, and photolysis techniques. The optical and ESR spectral data obtained in all cases are consistent with the primary formation of the ligand-centered radical cation. The rate constants for the oxidation of the Cu(II), Ni(II), and Co(II1) complexes with Ce(IV) have been determined to be around lo6 M-' s-I. In the case of the Co(I1) complex, spectral changes detected in stop flow and pulse radiolysis experiments are consistent with the formation of the cobalt(I1) phthalocyanine radical and its rapid conversion to the stable Co(II1) species. The cobalt(III), copper(II), and nickel(I1) phthalocyanine radicals disproportionate, resulting in the regeneration of the parent phthalocyanine, M"+(aq), and ligand degradation products.
Introduction Oneelectron oxidized metallophthalocyanine radicals have been observed as intermediates of thermal and photochemical reactions.'-12 For example, electrolysis and chemical oxidants, e.g., (1) Cahill, A. E.; Taube, H. J . Am. Chem. SOC.1951, 73, 2487. (2) Rollman, L. D.; Iwamoto, R. T. J . Am. Chem. SOC.1968, 90, 1435. (3) Lever, A. B. P.; Minor, P. C.; Wilshire, J. P. Inorg. Chem. 1981, 20, 1950. (4) Dolphin, D.; James, B. R.; Murray, A.; Thornback, E. Can. J . Chem. 1980, 58, 1125.
Ce(IV) or chlorine,'-'J2 were used for the thermal generation of these radicals. In photolysis, the same species were generated as (5) Prasad, D. R.; Ferraudi, G. Znorg. Chem. 1982, 21, 4241. (6) Clarck, D. W.; Yandle, J. R. Inorg. Chem. 1972, 11, 1738. (7) Ferraudi, G.; Srisankar, E. V. Inorg. Chem. 1978, 17, 3164. (8) Ferraudi, G. Inorg. Chem. 1979, 18, 1005. (9) Muralidharan, S.;Ferraudi, G. J . Phys. Chem. 1983.87, 4877. (10) Muralidharan, S.; Ferraudi, G.; Schmatz, K. Inorg. Chem. 1982,21, 2961. (1 1) Sorek, Y.; Cohen, H.; Mulac, A.; Schmidt, H. K.; Meyerstein, D. Inorg. Chem. 1983, 22, 3040.
0022-3654/85/2089-3890$01.50/00 1985 American Chemical Society
