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P4), while pH 0.2 and 7.0 favor of H4(TMpy-P4)2+and H2(TMpy-P4), respectively. Raman bands at 333, 808, 900, 967, 1003, 1297, and 1556 cm-I are only observed at pH 2 1.2 and are therefore assigned to H2(TMpy-P4), since those due to H3(TMpy-P4)+ would have been expected to appear at pH 0.2 and reach maximum intensity at pH 1.2. A band that occurs at 1556 cm-I is the counterpart of a band observed a t 1544 cm-' at pH 51.2, which is assigned to H4(TMpy-P4)2+or H3(TMpy-P4)+. It is noteworthy that the band a t 1556 cm-' is polarized (as are all of the other bands) for 406.7-nm excitation, while it is anomalously polarized ( p = 1.1) for 488.0-nm excitaton! Three other bands that occur at 188,240, and 940 cm-' disappear below pH 1.2 and are therefore assigned to the doubly protonated species. A band near 1373 cm-' at pH 0.2 loses intensity and shifts to 1364 cm-I above pH 1.2. Since this is one of the metal-sensitive bands, it is tempting to explain the observed shift in terms of a change in core size upon deprotonation. A few additional changes occur in the weaker bands, but they will not be discussed here. Finally there is no evidence for bands unique to H3(TMpy-P4)+. D. interaction of Ni(TMpy-P4) with DNA and CpG. As pointed out previously, Ni(TMpy-P4) exists in 4-and 6-coordinate forms which can easily be detected via their characteristic R R spectra. When the 6-coordinate form intercalates between base pairs of DNA, it must lose its coordinated water molecules and change from 6- to 4-coordination. Therefore it can be used as a probe to monitor intercalation. As the concentration of the intercalated species increases, we expect to see concomitant in-
Divanadium, the 6
-+
creases in the intensities of the 1580-, 1375-, and 392-cm-' bands which are characteristic of 4-coordination. Indeed this is exactly what was observed when we recorded the spectrum of a mixture of DNA and the porphyrin (see Figure 7A,B). The dinucleotide CpG has been reported to exist as stacked monomers35in aqueous solutions, while X-ray studies of crystals formed after its interaction with drugs such as proflavin and acridine orange show that CpG forms hydrogen-bonded dimers which have the particular drug intercalated between base pairs.36 When we recorded the R R spectra of mixtures of CpG and the nickel porphyrin, we saw significant increases in the intensities of the bands which are characteristic of 4-coordination (see Figure 7C). However, the changes are not nearly as pronounced as in the case of DNA. Although it is tempting to consider the porphyrin as acting as a kind of template for the formation of a hydrogen-bonded dimer of CpG, one must also consider structures in which the porphyrin intercalates into a monomeric CpG or in which two nonbonded CpGs stack above and below the porphyrin. Clearly, more research will be required to answer this question.
Acknowledgment. This research was partly supported by Marquette Univesity through a biomedical research program from the National Institute of Health (RR07196). (35) Prescott, B.; Gamache, R.; Livrarnento, J.; Thomas, G. J., Jr. Eiopolymers 1974, 13, 1821. (36) Waring, M. J. Annu. Reo. Eiochem. 1981,50, 159.
6" Transition: Experiment and Xa Theory
Mark P. Andrewst and Geoffrey A. Ozin* Lash Miller Chemical Laboratories, University of Toronto, Toronto, Ontario, Canada M5S 1AI (Received: December 3, 1985)
A new structured absorption belonging to matrix-isolated V2 has been discovered in the region 750-850 nm. Given the short
1.76-A internuclear distance established elsewhere for this molecule, spin-restricted and spin-polarized SCF-Xa-SW MO calculations suggest three strong ( u + 2s) bonds in this formally pentuply bonded molecule. The theoretically predicted 32,-ground state agrees with that established by experiment. Some assignments of the optical transitions belonging to V, are made on the basis of the X a calculations. In particular, the lowest energy structured transition is assigned to the 6 6* excitation. Similar transitions have been found for Cr2 and Nbz and some attempt is made herein to explain them in the context of current issues in &bonding in metal-metal bonded dimers.
-
Introduction Ligand-free metal atoms and clusters are currently the focus of considerable multidisciplinary interest. They have been the subject of a recent comprehensive review1 and a number of monographs deal specifically with the topic.2 Much effort has been devoted to theoretical and experimental attempts to elucidate the structural and electronic properties of transition-metal atoms and low nuclearity clusters in weakly and strongly interacting supports. In the ideal limit we define a "weakly interacting" support as one having a multidimensional potential describing the guest-host interaction which remains relatively unchanged during an electronic or vibronic transition, so that the processes conserving the number of phonons are f a ~ o r e d . ~ In this limit, the spectrum of the guest species will resemble that of the relevant gas-phase atom or nonrotating molecule. This particular study is a continuation of our earlier work with matrix-entrapped d i ~ a n a d i u m . ~ We have now extended our matrix optical spectroscopy to the red where additional low-energy transitions are observed in the region 'Current address: AT&T Bell Laboratories, Murray Hill, NJ.
0022-3654/86/2090-2852$01.50/0
of 740-840 nm. These new observations, together with the recently reported gas-phase bond length (1.76 A) and electronic ground state (32;) of divanadium,5 provide a basis for an SCF-Xa scattered wave examination of the electronic structure and bonding in the molecule. The following is an account of our experimental and theoretical studies of V2. (1) G. A. Ozin and S . A. Mitchell, Angew. Chem., In?.Ed. Engl., 22,674 (1983). (2) (a) D. M. Gruen in Cryochemistry, M. Moskovits and G. A. Ozin, Ed., Wiley, New York, 1976,chapter 10. (3) (a) V. E. Bondybey and L. E. Brus, Advances in Chemical Physics, Vol. 41, I. Prigogine and S.A. Rice, Ed., Wiley, New York, 1980,p 269;(b) Matrix Isolation Spectroscopy, NATO Advanced Study Series. Series C, Mathematical and Physical Sciences, Vol. 76,A. J. Barnes, W. J. OrvilleThomas, A. Muller, and R. Gauffres, Ed., Reidel, Dordrecht, Holland, 1981, chapter 13-17. (4) T. A. Ford, H.Huber, W. Klotzbucher, E. P. Kundig, M. Moskovits, and G. A. Ozin, J . Chem. Phys., 66, 524 (1977); W. E. Klotzbucher, S. A. Mitchell, and G. A. Ozin, fnorg. Chem., 16, 3063 (1977). (5) P.R.R. Langridge-Smith, M. D. Morse, G. P. Hansen, R. E. Smalley, and A. J. Merer, J . Chem. Phys., 80, 593 (1984).
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 13, 1986 2853
Divanadium
t P s
w
m
