J . Phys. Chem. 1990, 94, 4712-4114
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C H J O fragment ion. However, we observe no signal in this channel. Instead, we observe AI+ fragments from AI+-acetone, presumably by a mechanism like that described for AI+-benzene. Ag+-acetone dissociates to acetone and acetyl ions. This suggests that charge transfer occurs first, followed by bond breaking in the acetone ion. Crude energetic information can also be obtained from our experiments. I n aluminum complexes, where no charge transfer is observed, it can only be concluded that the complex binding energies are less than the dissociation photon energy. The charge-transfer systems, however, are more informative. From the known IPS and photon energies, we can place an upper limit on the ion-organic binding energy: Dd’(M+-R) 5 hv - [IP(R) - IP(M)] This procedure gives D Z 5 1.55 eV for Ag+-benzene (observed at X 5 385.7 nm). For Ag+-acetone (only studied at 302.5 nm), we subtract off the additional energy to form the acetyl ion from the acetone ion (0.64 eV), yielding Dol’ 5 1.31 eV.
The mechanisms suggested here have several implications for further experiments. The simplest is that dissociative charge transfer should also occur in aluminum complexes at higher energies. I t may also be possible to distinguish between the two mechanisms proposed for silver complexes by tunable laser experiments. If there is any sharp structure in these spectra, it should be a characteristic of the state carrying the oscillator strength. Thus, excitation of the solvated a-a* transition in Ag+-benzene should produce excited benzene vibronic resonances, whereas excitation into the charge-transfer state above its dissociation limit would produce an unstructured continuum. Spectra at lower energy, however, may be able to access bound resonances in the Ag-benzene’ state, with vibronic activity in the Ag-benzene coordinates. These and other experiments on different metalorganic complexes are currently under way in our Ihboratory.
Acknowledgment. We acknowledge the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work.
ARTICLES Fluctuation of Be2+ between Four Acceptors and Be2+ Polarizability of Cation Bonds: A FTIR Study Bogumil Brzezinski Department of Chemistry, A . Mickiewicz University, 60- 780 Poznafi, Poland
and Ceorg Zundel* Institute of Physical Chemistry, University of Munich, 0-8000 Miinchen 2, F.R.G (Received: January 13, 1989: In Final Form: November 15, 1989)
The tetrakis(tetrabuty1ammonium) salt of phenyl- 1,2-bis(2,2-ethanedicarbxylic acid) (1) and of 1,1,6,6-hex-3-enetetracarboxylic acid (2) and the Bezt 1:l complexes of these salts were synthesized and studied by FTIR spectroscopy. An intense IR continuum shows that the Be2+ions fluctuate between the four -COY groups. The systems show large polarizabilities, the so-called Be2+polarizability, since the Bezt ions can easily be shifted within these complexes. A four-minima cation potential is present.
Introduction If the proton fluctuates in the AH-B F= A--.H+B hydrogen bonds with double-minimum proton potential, these hydrogen bonds show the so-called proton polarizability which is about 2 orders of magnitude larger than the usual polarizabilities due to distortion of electron systems.’ These large polarizabilities occur since with symmetrical double-minimum potentials the wave function of the lowest state is symmetrical and that of the first excited state antisymmetrical. If this antisymmetrical state is admixed to the ground state an asymmetrical change distribution arises. With these types of potentials the two lowest states are very neighboring. Thus, already, relatively weak electrical fields can admix the first excited state to the ground state and induce in this way an asymmetrical change distribution, i.e., polarization.’s2 Such hydrogen bonds cause continua in the infrared spectra. Thus, the presence of such hydrogen bonds is indicated by such IR ~ o n t i n u a . ~ M.; Zundel, G. J . Phys. Chem. 1987 91. 5170. (2) Weidemann, E. G.; Zundel, G. Z . Narurforsch 1970, 25a, 627.
(1) Eckert.
If Li’ or Na’ ions fluctuate in a double-minimum or a fourminima cation potential these cation bonds show so-called cation poiarizabilitie~.~-~ I n t r a m o l e ~ u l a ras~ ~well as intermolecular7 cation bonds with this property may form if the affinity of the acceptors for the cations is large.4 If double-minimum potentials or potentials with four minima are present, the Li+ bonds cause continua in the far-IR below 400 cm-’ and, in the case of broad In the case of fluctuating flat single minima, below 300 NaC ions these continua occur only below 200 cm-‘ due to the large molecular mass of these cations.6 The affinity of Bezt ions to acceptors is usually particularly large; therefore, one should expect that if Bezt ions are present (3) Zundel, G.; Fritsch. J . In Chemical Physics of Soluarion; Dogonadze, R. R.. KBlmBn, E.. Kornyshev, A. A., Ulstrup, J., Eds.; Elsevier: Amsterdam, 1986; Vol. 11, pp 21-96. (4) Brzezinski, B.; Zundel. Ci. J . Chem. Phys. 1984. 81, 1600. (5) Brzezinski, B.; Zundel, G . J . Chem. Sor., Faraday Trans. I 1985,81, 2375. ( 6 ) Brzezinski, 8.; Zundel, G.; Kramer, R. J . Phys. Chem. 1988.96, 7012. (7) Brzezinski, B.: Zundel, G.: Kramer. R. Chem. Phys. Let?. 1988, 146, 138.
0022-3654/90/2094-4712$.02.50/00 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 4773
Fluctuation of Be2+ between Four Acceptors
0
a
b
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WAVENUMBER [ l/CM] Figure 2. Carboxylate region of the FTIR spectra of acetonitrile solutions: (---) tetrakis(tetrabuty1ammonium)salts of the acids, and (-) Be2+complexes of these salts. (a, top) Compound 1. (b, bottom) Compound 2. 01
500
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WA%'NUMBER [1/CM]
Figure 1. FTIR spectra of acetonitrile solutions: pure Be (C104)2, (---) tetrakis(tetrabuty1ammonium) salts of the acids, and (-) BeZ+ complexes of these salts. (a, top) Compound 1. (b, bottom) Compound 2. (e-)
between acceptors, cation bonds are formed with double- or multiminima potentials. This problem is studied in the following. Results and Discussion The following two acids, 1 and 2, were synthesized. ,COOH
.COOH 1
2
The tetrakis(tetrabuty1ammonium) salts and the mono Be2+ complexes of these salts were studied by FTIR spectroscopy. These spectra are shown in Figures 1 and 2. The dotted spectra in Figure 1 are the spectra of the pure Be(C104)2 solutions in acetonitrile. An intense, relatively narrow ion motion band is found with maximum a t 953 cm-I. Furthermore, an intense v,(C104) vibration is found at 935 cm-'. The fact that this symmetrical vibration is found in the IR spectrum shows that the Tdsymmetry of the Clod- ions is strongly disturbed by the Be2+ ions. These results show that the Be2+ ions still strongly interact with the C10, ions in the solutions. The same is shown by the strong splitting of va,(ClO4-). Due to removal of degeneracy instead of one band two intense bands are found
with maxima a t 1190 and at 11 10 cm-'.* The appearance of the totally symmetrical vibration v, of the C104- ion as well as the removal of the degeneracy of the antisymmetrical stretching vibration Y , proves that the Td symmetry of the C104- ion is strongly disturbed. Such a strong disturbance of the Td symmetry is impossible if the c104-ions were present in a pure solvent environment. Thus, this strong disturbance of the symmetry demonstrates that also in the highly polar acetonitrile solutions the Bez+ ions still interact strongly with the C104ions since there is no other reason for such a disturbance of the symmetry of the C104- ions. Hence, the ion motion band at 953 cm-l is not caused as usual in polar by the motion of the cation in a cage of solvent molecules but by the interionic vibration of the Be2+ and C104- ions. Usually in such a polar solvent as acetonitrile one would expect that the ion motion band is caused by the ion fluctuation in a cage of solvent molecules (ref 13, pp 53-55). To show this fact, we give the following literature data: Evans and Lo9 observed in the far-IR with alkylammonium salts in benzene a band which they (8) Hathaway, B. J.; Underhill, A. E. J . Chem. Soc. (London) 1961 3091. (9) Evans, J. C.; Lo, G . Y . 4 . J . Phys. Chem. 1965, 69,3223. (10) (a) Edgell, W. F.; Watts, A. T.; Lyford, J.; Risen, Jr., W. M. J . Am. Chem. SOC.1966,88, 1815. (b) Edgell, W. F.; Lyford, J.; Wright, R.; Risen, Jr., W. M.; Watts, A. J . Am. Chem. SOC.1970, 92, 2240. (11) Maxey, B. W.; Popov, A. I. J . A m . Chem. SOC.1967, 89, 2230. (12) Maxey, B. W.; Popov, A. I . J . Am. Chem. SOC.1969, 91,20. (13) Buxton, T. L.; Caruso, J. A. J . Phys. Chem. 1973, 77, 1882. (14) McKenney, W. J.; Popov, A. I. J . Phys. Chem. 1980,84, 535. (15) Wuepper, J. L.; Popov, A. I. J . A m . Chem. SOC.1969, 91, 4352. (16) Popov, A. I. Pure Appl. Chem. 1975, 41 275. (17) Hourdakis, A,; Popov, A. I . J . Solurion Chem. 1977, 6,299.
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the spectrum of the Be2+complex no nonshifted u,,C02- band is assigned to an interionic vibration in contact ion pairs (ion motion observed. These results taken together demonstrate that all C O T band). Edge11 and co-workersloa,bhave studied some salts in groups strongly interact with the Be2+ ion. Hence, the Be2+ ions tetrahydrofuran; the position of this far-IR band depends on both interact strongly with the four carboxylate ions of the compounds. cations and anions and hence they assigned this vibration also to Thus, they fluctuate between four acceptors. an interionic vibration in ion pairs. Hence, in these cases the ion Compared with the continua caused by Li+ bonds with large motion band is also an interionic vibration. Completely different Li+ polarizability, in the case of these Be2+complexes the continua results have, however, been obtained with all other solutions of occur in a region of relatively high wave numbers. This demsalts in polar aprotic solvents. In these solvents the position of onstrates that the cation potential is relatively steep at the -COT this band only depends on the type of cations present. This is true for all salt solutions in dimethyl sulfoxide,11.'2,15,16 ~ u l f o l a n e , ' ~ . ~ ~ ions. This potential is a four-minima potential. The splitting of va,(CO,-) is probably caused by electromagnetic and acetonitrile." From these pyridine,l4-I7 1-methylpyr~lidone,'~ coupling of Y,,(CO~-)of the four - C 0 2 - groups in the complex. results all these authors concluded that the ion motion band, In the Be2+complex of 2 an additional intense band is observed observed with more polar solvents, is caused by the vibration of at about 1655 cm-'. This band is the u(C=C) stretching vibration, the cations between two solvent molecules or in a cage of solvent which is intensified and shifted from about 1620 cm-' toward molecules. The result that in acetonitrile solutions the Be2+ ions higher wavenumbers due to the strong interactions in the cominteract strongly with the Clod- ions shows that the ion motion plex.2' band in the acetonitrile solution of Be(C104)2 is no vibration of the BeZ+ion in a cage of acetonitrile molecules but an interionic Conclusions vibration. This may be due to the strong interaction of the Be2+ The Be2+ ions form complexes with tetracarboxylates. They with the CIO; ions. This conclusion is confirmed by the following interact strongly with the four -C02- groups. An intense IR result. In the case of acetonitrile solutions the ion motion band continuum shows that the Be2+ ion fluctuates between the four is found at about 400 cm-'.4,17 The relatively high position of the acceptors. The systems show large polarizabilities, so-called Be2+ ion motion band in acetonitrile solutions of Be(C104)2shows (Li+ polarizabilities since the Be2+ions can easily be shifted within these and Be2+ have comparable molecular mass) that the affinity of complexes. The Be2+ ions are present in a relatively narrow the Be2+ ions to the CIO; ions is so large that the Be2+ ions vibrate four-minima cation potential. between the C104- ions in a relatively narrow single-minimum potential in O--.Be2+--0 bonds. Experimental Section The spectra drawn with solid lines in Figure 1 show the 1 :1 Be2+ The preparations of the acids 1 and 2 as well as the analytical complexes of the tetrakis(tetrabuty1ammonium) salts of the data are given in ref 22. Tetrakis(tetrabuty1ammonium) salts of compounds 1 and 2. For comparison the spectra of the tetrathe investigated acids were prepared as follows: 1 M solutions kis(tetrabuty1ammonium) salts are given as dashed lines. were prepared of acid 1 or acid 2, respectively, in carefully dried In the spectra drawn with solid line the ion motion band has methanol. T o these solutions a 1 M solution of tetrabutylcompletely vanished. This result shows that the Be2+ ions form ammonium hydroxide was added (acid:hydroxide ratio 1:4). The complexes with the tetrakis(tetrabuty1ammonium) salts of comsolvent was removed under reduced pressure and the residues were pounds 1 and 2. This result is confirmed by the bands of the C104dissolved in acetonitrile. ions. us at 935 cm-l has completely vanished, and the same is true The Be2+ complexes were prepared by mixing equal volumes with the splitting of uaS(CIO4-). Both results show that the C104of a 0.2 M acetonitrile solution of the tetrakis(tetrabuty1ions are now in symmetrical solvent environments. Thus, the CIO, ammonium) salts of acid 1 or acid 2, respectively, and a 0.2 M ions are no longer bound to the Be2+ ions. acetonitrile solution of beryllium perchlorate. Figure 1 shows that instead of the ion motion band an intense All manipulations with the substances were performed in a continuous absorption is observed. This continuum begins at about carefully dried C0,-free glovebox. 1100 cm-l and extends with decreasing intensity down to about The I R spectra of the tetrabutylammonium salts and of the 300 cm-I. These continua demonstrate that the Be2+ions fluctuate beryllium complexes were taken in 0.1 M acetonitrile solutions in these systems and that the Be2+bonds show large so-called Be2+ (0.1 mol/dm3) with the FTIR spectrometer IFS 113 V (Bruker, polarizability. Karlsruhe, FRG) using a cell with Si windows (sample thickness, In Figure 2 a comparison of the spectra drawn with solid line 0.4 mm; detector, DTGS; resolution 2 cm-'; and NSS = 250). with those drawn with dashed line shows that v,(CO;) is shifted in the Be2+complex about 40-50 cm-' toward higher wavenumbers Acknowledgment. Our thanks are due to the Deutsche Forcompared with the tetrakis(tetrabuty1ammonium) salt and shows schungsgemeinschaft and to the Fonds der Chemischen Industrie doublet structure. Of particular importance is the fact that in for providing the facilities for this work. (18) Ault, B. S.; Pimentel, G. C. J. Phys. Chem. 1975, 79, 621. (19) Tsatsas, A . T.; Stearns. R. W.; Risen, Jr., W . M. J. Am. Chem. Soc. 1972. 94, 5241. (20) Cahen. Y. M.: Popov, A . I . J . Solurion Chem. 1975, 4 , 599.
(21) Duddell, A . A. In Spectroscopy and Structure of Molecular Complexes; Yarwood, J . , Ed.: Plenum Press: London, 1973; p 428. ( 2 2 ) Brzezinski, B. Pol. J . Chem., in press.
