J . Phys. Chem. 1986, 90, 250-255 superimposition of the C-C antisymmetrical stretching mode with a SO, symmetrical stretching mode. We notice that we have no difference between the spectra for 65%, 70%, and 80% SDS samples. We cannot say if the so-called complex hexagonal phase by Luzzati and H u ~ s o is n ~a deformed ~ hexagonal phase as argued by Leigh et al.34but it seems to us that a complex hexagonal phase would have to give a spectrum rather different from that of the neat phase. Stated differently, a deformed hexagonal phase would have to give a result rather similar to those for the hexagonal phase. As the spectra are the same for all these concentrations, if we have a complex hexagonal phase, it must be like a rolled bilayer in order to have some interactions between adjacent chains.
Conclusion Raman scattering can give us informations about chain ordering in surfactant aqueous solution. We believe now that these results are quite well understood. We have shown that the apparent unusual behavior of the C-C antisymmetrical mode is due to a superimposition of the symmetrical mode of SO,. This mode is only observed in the high-temperature phase of the solutions and is probably due to a change in the symmetry of the polar head(34) I . D. Leigh, M. P. McDonald, R. M. Wood, G. J. Tiddy, and M. A. Trevethan, J. Chem. SOC.,Faraday Trans. I , 77, 2867 (1981).
group. This change might be due to a rotation of the SO3group caused by electrostatic repulsion of the polar heads. We think that it would be interesting to study the behavior of water in these solutions. The fact that the melting temperature of the ice is 5 K lower than that of pure water seems to indicate that a great proportion of water inside the solution is linked to the surfactant. These results might be dependent on the concentration. Experiments are in progress to verify these facts. For concentrations that gives spherical or rodlike micelles we have shown that a gel phase can be observed and studied over a wide range of temperature following different cooling procedures. This does not seem to be the case for CTAB solutions where we do not observe any difference in the spectra following the different cooling procedures. There is still an unresolved question concerning the complex hexagonal phase: the behavior of the CH2 vibrations looks like that of the CH2 vibrations of the lamellar phase (neat phase). However, the spectra are also not so different from those of the hexagonal phase. The main difference is that the gel phase is not observed. We think that these systems, at high concentrations, require further investigations.
Acknowledgment. Thanks are due to Professor M. Jaffrain for helpful discussions.
Resonance Raman and Infrared Studies on Axial Coordination to Chlorophylls a and b in Vitro Masao Fujiwara and Mitsuo Tasumi* Department of Chemistry, Faculty of Science, The University of Tokyo, Bunkyo- ku, Tokyo 1 1 3, Japan (Received: June 17, 1985; In Final Form: September 23, 1985)
The resonance Raman and infrared spectra of chlorophyll a were observed in various solvents. The Raman spectra in the 1620-1510-cm-' region were classified into two groups, depending on the solvent used. Three Raman bands at 1612-1606 (weak), 1554-1551 (strong), and 1529-1527 (medium) cm-' (group I) observed in n-hexane, CCI4, CS2, diethyl ether, acetone, ethyl acetate, and ethanol solutions shift, respectively, to 1599-1 596, 1548-1 545, and 1521-1 5 18 cm-' (group 11) in tetrahydrofuran, dioxane, pyridine, and methanol solutions. In the solutions giving rise to the spectrum of group I the Mg atom of chlorophyll a is mostly five-coordinated (with one axial ligand), whereas the six-coordinated species (with two axial ligands) is the major fraction in the solutions giving rise to the spectrum of group 11. The infrared spectra obtained from n-hexane, diethyl ether, acetone, and tetrahydrofuran solutions supported the findings in the Raman. The Raman and infrared spectra of chlorophyll b show similar solvent dependency with some minor features differing from the spectra of chlorophyll a, indicating that chlorophyll b behaves in the same manner as chlorophyll a with respect to the axial coordination by solvent molecules.
Introduction Elucidation of the coordination state of the central Mg atom in chlorophylls and closely related molecules has been one of the important subjects in physicochemical studies of photo~ynthesis.'-~ The results obtained in the previous studies may be most briefly summarized as follows. X-ray analyses have been performed for the crystals of ethyl ,~ chlorophyllide a dihydrate: ethyl chlorophyllide b d i h ~ d r a t e and (1) Katz, J. J.; Oettmeier, W.; Norris, J. R. Philos. Trans. R . SOC.London, Ser. B 1976, B273, 227-253. (2) Katz, J. J.; Norris, J. R.; Shipman, L. L.; Thurnauer, M. C.; Wasielewski. M. R. Annu. Reo. Biophys. Bioeng. 1978, 7 , 393-434. (3) Lutz, M. In "Advances in Infrared and Raman Spectroscopy"; Clark, R. J . H., Hester, R. E., Eds.; Wiley Heyden: London, 1984; Vol. 1 1 , pp 21 1-300. (4) Chow, H.-C.; Serlin. R.; Strouse, C. E. J. Am. Chem. SOC.1975, 97, 7 230-7237. (5) Serlin, R.; Chow, H.-C.; Strouse, C. E. J. Am. Chem. SOC.1975, 97, 7237-1242.
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a water-soluble complex of bacteriochlorophyll a (Bchl a ) and a protein.6 In the former two, the Mg atom is coordinated by the four pyrrole nitrogens and a water molecule (axial ligand); Le., the Mg atom is five-coordinated (see Figure 1). The water-soluble Bchl a-protein complex has seven Bchl a molecules, and the seven Mg atoms all appear to be five-coordinated. In five cases the possible ligand is a histidine side chain, in one case a carbonyl oxygen of the polypeptide chain, and in the other case a water molecule. Early infrared7q8and IH NMR9 studies of chlorophylls dissolved in nonpolar solvents indicated that the Mg atom of one chlorophyll molecule was coordinated either by the carbonyl group of another (6) Matthews, B. W.; Fenna, R. E.; Bolognesi, M. C.; Schmid, M. F.; Olson, J. M. J . Mol. Bioi. 1979, 131, 259-285. (7) Boucher, L. J.; Strain, H. H.; Katz, J. .IJ. . A m . Chem. SOC.1966,88, 1341-1346. (8) Ballschmiter, K.; Katz, J. J. J. Am. Chem. SOC.1969, 91, 2661-2677. (9) Katz, J. J.; Strain, H. H.; Leussing, D. L.; Dougherty, R. C. J. Am. Chem. SOC.1968, 90, 784-791.
0 1986 American Chemical Society
Studies on Axial Coordination to Chlorophylls a and b
The Journal of Physical Chemistry, Vol. 90, No. 2, 1986 251
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1 L Coordination to the Mg atom of chlorophyll. L, axial ligand; N, pyrrole nitrogens; Ct, center of the hole in the chlorin ring. Figure 1.
to form molecular aggregates, or by a nucleophile such as water which was added to the solvent. This conclusion was later confirmed by infrared,'O resonance Raman," and electronic abs ~ r p t i o n ' ~ Jstudies, ~ - ' ~ and it is now established that the Mg atom is usually five-coordinated in nonpolar solvents. In polar solvents, on the other hand, electronic absorption ~ p e c t r a ' ~have . ' ~ provided evidence of the six-coordination of the Mg atom with two axial ligands (solvent molecules) above and below the chlorin ring. In these studies the contribution of Katz and his collaborators was particularly notable. More recently Cotton and Van Duynel5 studied the coordination interactions between Bchl a and some molecules such as pyridine by resonance Raman spectroscopy. They pointed out that a few Raman bands in the region of 1620-1510 cm-' could be used to determine the coordination number (five or six) of the Mg atom. We have examined the 1620-1510-cm-' region in the resonance Raman and infrared spectra of chlorophyll a (Chl a ) in various solvents and have found similarities and differences between the coordination properties of Chl a and Bchl a. The coordination properties of chlorophyll b (Chl b) have been found to be almost the same as those of Chl a. These results are reported in this paper.
Experimental Section Chlorophylls were extracted from spinach leaves with acetone and precipitated by adding dioxane and water.I6 Purification of Chl a and Chl b was performed by column chromatography with DEAE-Sepharose CL-6B and Sepharose CL-6B.I7 Solid chlorophylls were obtained by evaporating the solvents (n-hexane and 2-propanol) in vacuo. The sample obtained in this way was not completely dry as mentioned later. All the solvents used for spectral measurements were of reagent grade, and they were dried before use with Wako molecular sieves 3A. Raman spectra were observed for 10-3-10-4 M solutions in a rotating cell at room temperature. An N E C GLG-2018 He-Cd laser, a microcomputer-controlled Spex 1401 double monochromator, and a Hamamatsu R649 photomultiplier were used for Raman measurements. The average power of the 441.6-nm line was 15 mW at the sample point, and the spectral slit width was set at 8 cm-I. Infrared measurements were made for M solutions in a KBr cell of path length 0.1 mm. A Perkin-Elmer 180 infrared spectrophotometer was used. Observed spectrometer readings (wavenumbers) were calibrated with the bands of indene (for Raman) and polystyrene (for infrared). Results and Discussion Raman Spectra of Chl a. The resonance Raman spectra of Chl a in the region of 1750-1470 cm-' observed in various solvents (lO),Cotton, T. M.; Loach, P. A.; Katz, J. J.; Ballschmiter, K. Photochem. Photoblol. 1978, 27, 735-749. (1 1) Lutz, M. J . Raman Spectrosc. 1974, 2, 497-516. (12) Cotton, T. M.; Trifunac, A. D.; Ballschmiter, K.; Katz, J. J. Biochim. Biophys. Acta 1974, 368, 181-198. (13) Evans, T. A.; Katz, J. J. Biochim. Biophys. Acta 1975,396,414-426. (14) Shipman, L. L.; Cotton, T. M.; Norris, J. R.; Katz, J. J. J . Am. Chem. SOC.1976, 98, 8222-8230. ( I S ) Cotton, T. M.; Van Duyne, R. P. J . A m . Chem. SOC.1981, 103, 6020-6026. (16) Iriyama, K.; Ogura, N.; Takamiya, A. J . Biochem. (Tokyo) 1974,76,
I
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Raman spectra of Chi a (10-3-10-4 M) in (a), n-hexane; (b) CCI,; (c) CS,; (d) diethyl ether; (e) acetone; (f) ethyl acetate; (g) ethanol; (h) tetrahydrofuran; (i) dioxane; 6 ) pyridine; (k) methanol. Asterisks indicate solvent bands. Figure 2.
are shown in Figure 2. The bands in the carbonyl stretching region (1750-1650 cm-I) are relatively weak in intensity and do not give much information on the coordination state of the Mg atom. Only n-hexane solution gives the "aggregation peak" at 1654 cm-I, which is assignable to the stretching vibration of the C-9 keto carbonyl group coordinating to the Mg atom of another Chl a molecule.8 (As shown later, a corresponding infrared band is strongly observed at 1657 cm-I.) CC14 and CS2 solutions show extremely weak aggregation peaks, probably because of the disaggregation of Chl a by a small amount of coexisting water.* The band at 1700-1684 cm-' is assignable to the free C-9 keto carbonyl group, whereas the band at 1669-1668 cm-l in ethanol and methanol solutions is due to the hydrogen-bonded carbonyl group. According to Katz and his co-workers,13J4the electronic absorption bands, the Q,(O,O) band in particular, of six-coordinated species are red-shifted in comparison with those of five-coordinated. Chl a dissolved in tetrahydrofuran (THF),12 dioxane,18 and pyridineI3 is known to show the red-shifted Q,(O,O) absorption which suggests the presence of the six-coordinated species in these solvents. In this context, it is interesting to note that the resonance Raman spectra in Figure 2 can be classified into two groups: group I consisting of the spectra obtained from n-hexane, CCl,, CS2, diethyl ether, acetone, ethyl acetate, and ethanol solutions and group I1 from THF, dioxane, pyridine, and methanol solutions. The Raman spectra in group I have three bands at 1612-1 606 (weak), 1554-1551 (strong), and 1529-1527 (medium) cm-I. These bands are found, respectively, at 1599-1596 (weak), 1548-1545 (strong), and 1521-1518 (medium) cm-I in group 11. The Raman spectra in groups I and I1 may be correlated, respectively, with Chl a molecules having the five-coordinated and six-coordinated Mg atoms. We observed the visible absorption spectra of Chl a in all the solvents used for the Raman measurements. The results showed that the classification employed for the Raman spectra could be applied also to the visible absorption spectra. However, only pyridine solution showed a distinct absorption peak at about 639 nm (assignable to the red-shifted Q,(O,O) transition of the sixcoordinated specie^'^), whereas the other group I1 solutions (THF, dioxane, and methanol) showed some increases in absorption intensity (without distinct peaks) at 620-630 nm and decreases
901-904.
(17) Omata, T.; Murata, N. Photochem. Photobiol. 1980, 31, 183-18s.
(18) Seely, G. R.; Jensen, R. G. Spectrochim. Acta 1965, 21, 1835-1845.
252
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WAVENUMBER Figure 3. Raman spectra of Chl a (1.0 X M) in mixed solvents of n-hexane and tetrahydrofuran (THF). THF concentrations: (a) 0.012 M; (b) 0.025 M; (c) 0.037 M; (d) 0.062 M; (e) 0.12 M; (f) 0.24 M; (g) 0.59 M; ( h ) 1.2 M; (i) 6.3 M; (j)12.4 M (neat THF).
in absorption intensity at 570-580 nm, in comparison with the spectra of the group I solutions. These rather delicate differences in the visible absorptions among the group I1 solutions seem to indicate that the five-coordinated species coexists with the sixcoordinated in the group I1 solutions also, although the six-coordination is almost complete in pyridine solution. Then, we face a problem of how to interpret the Raman spectra of THF, dioxane, and methanol solutions which, seemingly, do not show the bands assignable to the five-coordinated species. To get an insight into this problem, we performed the following measurements. We observed the Raman spectra of Chl a in mixed solvents of n-hexane and T H F at various mixing ratios. As shown in Figure 3, the Raman spectrum at a small T H F concentration (0.012 M, THF/n-hexane molar ratio = 0.0016) has the three bands at 1608, 1554, and 1529 cm-l, which clearly belong to group I. At this T H F concentration, the aggregation peak at 1654 cm-' is very weak in intensity, indicating that the addition of T H F to n-hexane effectively breaks down Chl a self-aggregates with the formation of the THF-Chl a (1:l) complex. In other words, the five-coordinated species is dominant at low T H F concentrations. With increasing T H F concentration all the three Raman bands continuously shift to lower frequencies and reach the group I1 frequencies at the T H F concentration about 1.2 M (THFln-hexane molar ratio = 0.17). The intensity of the 1529-cm-* band at first increases relatively to that of the 1554-cm-' band and then decreases. As a result, the relative intensity of the 1554- and 1529-cm-' bands at 0.012 M T H F concentration is almost equal to that of the 1545- and 1521-cm-' bands in neat THF. The continuous shifts of the bands can occur for systems in which the five- and six-coordinated species coexist, if we take into account the bandwidths (20 cm-') and the relatively wide spectral slit width (8 cm-I). Simple spectral synthesis shows that it is not possible
Fujiwara and Tasumi to resolve distinctly the group I and I1 bands under these experimental conditions. This means that the spectral pattern of a system in which the five- and six-coordinated species coexist may be classified into group 11, if the six-coordinated species is the major fraction. Therefore, the Raman results for THF, dioxane, and methanol solutions do not contradict what are derived from their visible absorption spectra. The above-mentioned variation of the relative intensity of the 1529-cm-' band with increasing T H F concentration is an interesting phenomenon but is difficult to explain at present. Similar shifts of the Raman bands in going from five- to sixcoordination were found for the Bchl a-pyridine system by Cotton and Van Duyne." An intense band near 1609 cm-I (designated band A by Cotton and Van Duyne) and a less intense band near 1530 cm-' (designated band B) are characteristic of the five-coordinated species. In the six-coordinated species, band A shifts to 1594 cm-' and band B splits into a doublet at 1531 and 1519 cm-I. In addition to bands A and B, a weak shoulder at about 1589 cm-' of the five-coordinated species also shifts to 1577 cm-I in the six-coordinated species. Band B of the five-coordinated species seems to have two components, one of which is not sensitive to the number of axial ligands and always appears at about 1530 cm-'. If this is the case, both Chl a and Bchl a have three axial-ligand-sensitive bands in the 1620-1 5 10-cm-' region of their Raman spectra, although the intensity patterns are quite different. The downshifts of the axial-ligand-sensitive Raman bands of Chl a upon six-coordination may be explained in the same manner as in the cases of metallop~rphyrinsl~-~~ and Bchl a.I5 As mentioned earlier,4 the Mg atom in ethyl chlorophyllide a dihydrate is five-coordinated with a water molecule being the axial ligand. As shown in Figure 1, the Mg atom is not located in the center of the hole in the chlorin ring but displaced 0.39 A from the plane of the chlorin ring (more precisely, from a plane formed by three pyrrole nitrogens). As far as we know, no X-ray analysis has been reported for Chl a (or its very closely related analogues) containing the six-coordinated Mg atom. However, it has been shown for di(pyridine)magnesium(II) octaethylporphyrinate22 that the Mg atom lies in the center of the porphyrin ring and the bonds between Mg and the two axial ligands are equal in length. Therefore, it is reasonable to consider that in Chl a in the group I1 solutions the six-coordinated Mg atom occupies the center of the hole in the chlorin ring. This movement of the Mg atom into the hole center would induce an expansion of the chlorin ring, which is made possible with elongation of the bond lengths in the conjugated macrocycle. This bond elongation, in turn, decreases the frequencies in the 1620-1510-cm-' region which are assignable to the C=C stretching modes3 However, we have no other support for the view that the six-coordinated Mg atom in solution occupies the ring center. This requires a potential for the Mg atom with a single minimum in the ring plane. Another explanation of the observed results may well be derived by assuming a potential minimum for the Mg atom on either side of the ring plane, i.e., a double-minimum potential. If the barrier height between the two minima is sufficiently low, loosening the ring vibrations might still be expected. Accordingly, the possibility of this doubleminimum potential may not be excluded at present. A difference is found between the coordination properties of Chl a and Bchl a . According to Cotton and Van Duyne,I5 Bchl a in absolute ethanol is predominantly six-coordinated. On the contrary, the Raman spectrum of Chl a in ethanol (Figure 2) definitely belongs to group I, indicating that Chl a is five-coordinated in this solvent. It should be noted that chemically cognate solvents can form different coordination species. As already discussed, methanol solution gives a Raman spectrum belonging (19) Spaulding, L. D.; Chang, C. C.; Yu, N.-T.; Felton, R. H . J . A m . Chem. SOC.1975, 97, 2517-2525. (20) Spiro, T. G.; Stong, J. D.; Stein, P. J . A m . Chem. SOC.1979. 101. 2648-2655. (21) Kincaid, J. R.; Urban, M . W.; Watanabe, T.: Nakamoto, K. J . Phys. Chem. 1983, 87, 3096-3101. (22) Bonnett, R.; Hursthouse, M. B.; Abdul Malik, K M.; Mateen, B. J . Chem. SOC.,Perkin Trans. 2 1977, 2072-2076.
The Journal of Physical Chemistry, Vol. 90, No. 2, 1986 253 1
.
1
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1500 WAVENUMBER Figure 5. Infrared spectra of Chl a (1.0 X M) in mixed solvents of n-hexane and tetrahydrofuran (THF). THF concentrations: (a) 0.24 M; (b) 0.59 M; (c) 1.1 M; (d) 12.4 M (neat THF). Asterisk indicates a solvent absorption.
1
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WAVENUMBER Figure 4. Infrared spectra of Chl a (1.0 X lo-* M) in (a) n-hexane; (b)
diethyl ether; (c) acetone; (d) tetrahydrofuran. Asterisk indicates a solvent absorption. to group I1 in contrast to ethanol solution. The Raman spectra of diethyl ether and THF solutions are classified into groups I and 11, respectively. Infrared Spectra of ChZ a. The infrared spectra of Chl a observed in n-hexane, diethyl ether, acetone, and T H F are shown in Figure 4. The other solvents used for the Raman measurements, namely, CC14,CS2,ethyl acetate, ethanol, dioxane, pyridine, and methanol, have their own absorption bands which overlap the Chl a bands in the 1620-1510-~m-~ region. The infrared spectrum of n-hexane solution has four bands in the carbonyl stretching region at 1739 (weak), 1724 (shoulder), 1701 (weak), and 1657 (intense) cm-'. The 1739- and 1724-cm-' bands are assigned to the ester carbonyl group, the 1701-cm-' band to the free keto carbonyl, and the 1657-cm-' band to the aggregation peak.
Previously Ballschmiter and Katzs made an extensive study of the effects caused by coexisting water on the infrared spectra of chlorophylls in nonpolar solvents. Comparison of the spectrum of n-hexane solution in Figure 4 with the results obtained by them suggests that the Chl a sample used in the present study contains water and that the water/Chl a molar ratio in this sample is less than 1 (presumably around 0.5). Presence of this amount of water would hardly give significant effects on the infrared and Raman spectra obtained in polar solvents, because polar solvents and water act more or less similarly on Chl a and the solvent concentrations are larger than the concentration of water by a factor of 103-105. In the infrared spectra of n-hexane, diethyl ether, and acetone solutions (called the three solutions), three bands are commonly observed at 1609-1607, 1552-1549, and 1534-1533 cm-I. The infrared band at 1609-1607 cm-' in the three solutions clearly shifts to 1597 cm-' in THF solution. Since this downshift corresponds well to that of the Raman band at 1612-1606 cm-] in group I, the infrared band at 1609-1607 cm-I and the Raman band at 1612-1606 cm-I must be due to either the same vibrational mode or normal modes made of common vibrational components (coordinates). The infrared band at 1552-1549 cm-' in the three solutions remains a t about the same frequency (1 548 cm-I) in THF solution. Therefore, it is unlikely that this band corresponds to the Raman band at 1554-1551 cm-l of group I. The infrared band at 1534-1533 cm-I in the three solutions appears to be associated with the aggregation, as mentioned below. Infrared spectra were also observed in mixed solvents of nhexane and T H F at various concentrations, as shown in Figure 5. At the THF concentration of 0.24 M (THFln-hexane molar ratio = 0.032) the aggregation peak at 1657 cm-I and the band a t 1534 cm-' almost disappear, suggesting that the latter may also be associated with the aggregation. (Note that the Chl a con-
254
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Figure 7. Infrared spectra of pheophytin a (1 .O X lo-* M) in (a) diethyl ether; (b) tetrahydrofuran.
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M) in (a) diethyl Figure 6. Raman spectra of pheophytin a (2.0 X ether; (b) acetone; (c) tetrahydrofuran: (d) dioxane.
centration for infrared measurements is 100 times higher than that for Raman measurements and that the THF concentration for disaggregating Chl a is accordingly higher.) At the same T H F concentration the 1607-cm-I band is broadened, having a shoulder on the low-frequency side. With increasing THF concentration this shoulder grows until it becomes the 1597-cm-' band in neat THF solution. Unlike the Raman case, the two bands a t 1607 and 1597 cm-I could be partially resolved in the infrared due to a higher instrumental resolution (2 cm-I). The above infrared results that the band at 1609-1607 cm-' in n-hexane, diethyl ether, and acetone solutions shifts to 1597 cm-' in T H F solution are in agreement with the findings in the Raman. Raman and Infrared Spectra of Pheophytin a. As described above, the Raman and infrared bands of Chl a in groups I and II have been correlated with the difference in axial coordination. However, the downshifts of the bands from group I to I1 might be associated with bulk solvent effects. This possibility may be eliminated by performing the same measurements for pheophytin a (Mg-free compound of Chi a ) . If bulk solvent effects are
I ; I u 700 d%"MBER
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Figure 8. Raman spectra of Chl b (1 .O X lo4 M) in (a) CS2; (b) diethyl ether; (c) acetone: (d) methyl acetate: (e) ethanol; (0tetrahydrofuran; (g) dioxane; (h) methanol. Asterisks indicate solvent bands.
responsible for the downshifts, similar downshifts should be observed also for pheophytin a by changing the solvent. The Raman and infrared results are shown, respectively, in Figures 6 and 7. In the 1620-1510-cm-' region of the Raman spectra (Figure 6), only one distinct band is always observed at 1583-1582 cm-I, regardless of the solvent type. In the infrared spectra (Figure 7), no downshifts are observed for the bands in the 1630-1540-cm-'
J. Phys. Chem. 1986, 90; 255-260 region in going from diethyl ether to T H F solution. (Probably the weak infrared band at 1584-1581 cm-' corresponds to the intense Raman band at 1583-1582 cm-'.) It may be concluded from these observations that bulk solvent effects do not contribute to the downshifts from group I to 11. Raman and Infrared Spectra of Chl b. Like Chl a, Chl b also showed shifts of the Raman and infrared bands, depending on the solvent used. In Figure 8 are shown thc; Raman spectra of Chl b in the region of 1800-1470 cm-' observed in eight kinds of solvents, which were also used to observe the Raman spectra of Chl a. Chl b was more fluorescent than Chl a when excited with the He-Cd 441.6-nm line. The fluorescence was so strong from CCI, and pyridine solutions that no Raman spectra could be obtained from these solutions. The solubility of Chl b in n-hexane was too low to observe the Raman spectrum in this solvent. As shown in Figure 8 the C-9 keto carbonyl and the C-3 formyl carbonyl stretchings are observed, respectively, at about 1700 and 1666 cm-' in non-hydrogen-bonding solvents. These bands shift, respectively, to about 1675 and 1648 cm-l in ethanol and methanol, which form hydrogen bonds with the above carbonyl groups. Under the present experimental conditions, Chl b does not seem to form self-aggregates in all these solvents including nonpolar
cs2.
The patterns of the Raman spectra of Chl b in the 162015 10-cm-' region can be classified in the same way as for Chl a. The spectra from CS2, diethyl ether, acetone, ethyl acetate, and ethanol solutions belong to group I and those from THF, dioxane, and methanol solutions to group 11. The 1607-cm-' band in group I shifts to 1596-1594 cm-' in group 11, although these bands are extremely weak in some solvents. A difference between Chl a and Chl b is found in the 1570-1 540-cm-' region; the group I band
255
at 1566-1564 cm-' of Chl b is replaced by a doublet at 1559 and 1551-1549 cm-' in group 11. The origin of this doublet is not clear at present. It is conceivable that the group I band at 1566-1 564 cm-' consists of two overlapping components which give rise to the doublet in group 11. The band at 1523-1520 cm-' in group I shifts to 1519-1516 cm-' in group 11. The infrared spectra of Chl b were also observed in diethyl ether and THF solutions. A medium-intensity band observed at 1610 cm-' in diethyl ether solution clearly shifts to 1597 cm-' in T H F solution. This downshift is very similar to the results obtained for Chl a (Figure 4). The above results indicate that Chl b behaves in the same manner as Chl a with respect to the axial coordination by solvent molecules.
Conclusion The resonance Raman and infrared spectra of Chl a and Chl b in various solvents are usually classified into either group I or 11. Group I is correlated with a system in which the five-coordinated species (one axial ligand to Mg) is dominant, whereas group I1 is correlated with that in which the six-coordinated species (two axial ligands) is the major fraction. However, spectra intermediate between the two groups may be obtained, as exemplified by the cases shown in Figure 3, for systems in which the five- and six-coordinated species coexist with comparable concentrations. Acknowledgment. We are grateful to Dr. Hidenori Hayashi for his technical guidance in sample preparations. This work was supported by a Grant-in-Aid for Special Distinguished Research (No. 56222005) from the Ministry of Education, Science, and Culture.
Electron Correlation Effects in Ligand Field Parameters and Other Properties of CuF, Sergei Yu. Shashkint and William A. Goddard III* Arthur Amos Noyes Laboratory of Chemical Physics.1 California Institute of Technology, Pasadena, California 91 125 (Received: July 24, 1985)
The effect of electron correlation on ligand field splittings in CuF2 is examined. We find that charge-transfer (CT) effects play an important role in stabilizing the z2g+and states (but not 2Ag) and that electron correlation enhances the CT, raising ligand field splittings by about 20%. Electron correlation has an important effect on the g tensor but not on geometry and vibrational frequencies. Simplified configuration interaction schemes that should be practical for much larger clusters are suggested and tested.
1. Introduction Cluster models have proved useful in theoretical studies of the electronic structure of crystals containing transition-metal ions: for example, the use of the cluster [NiF,]" to model KNiF3. These studies have generally been at the Hartree-Fock (HF) level, and the discrepancies with experiment are generally attributed to electron correlation (many-body) effects. For instance, the is calculated crystal-field splitting parameter 1ODq for about 80% of the experimental value.'S2 As the initial step in a program for investigating the role of electron correlation in electronic properties of such systems, we examined the electronic structure of the linear molecule CuF2. The geometry and energy spectrum of CuF2 have previously been Permanent address: Physics Department, A. M. Gorkii Ural State University, Sverdlovsk, 620083, U.S.S.R. *Contribution No. 7256.
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studied within the framework of molecular orbitals (MO) using ab initio HF, multiple scattering X,, and semiempirical INDO method^.^,^ Limited configuration interaction (CI) studies5,, for a fixed geometry have also been performed recently. In order to determine the influence of electronic correlation on ligand field splitting, we carried out various C I calculations on the ground and excited electronic states of CuF2. We find that charge-transfer (CT) configurations play a particularly important (1) Moskowitz, J. W.; Hollister, C.; Hornback, C. J.; Basch, H. J . Chem. Phys. 1970, 53, 2570. (2) Wachters, A. J. H.; Nieuwpoort, W. C. Phys. Reu. B 1972, 5, 4291. (3) Basch, H.; Hollister, C.; Moskowitz, J. W. Chem. Phys. Let?. 1969,
., .,.
d IO
(4) de Mello, P. C.; Hehenberger, M.; Larsson, S.; Zerner, M. J . Am. Chem. SOC.1980, 102, 1278. ( 5 ) Larsson, S.; Roos, B. 0.;Siegbahn, P. E. M. Chem. Phys. Let?. 1983,
96,436. (6) Ha, T.-K.; Nguyen, M. T. Z. Naturforsch. A 1984, 39, 175.
0 1986 American Chemical Society
