J. Phys. Chem. 1990, 94. 1141-1144
1141
catalyze the reduction and reoxidation of diffusing 2,6-AQDS, but in larger amounts, it may block the reaction, perhaps because of limited electronic conductivity. It is interesting, however, that the reduction of Cd2+ is not blocked, even though the reduction potential for CdZ+is still in the adsorption region and the electrode is already sluggish toward the reduction of 2,6-AQDS. Concluding Remarks. The ease with which 2,6-AQDS and 1,5-AQDS develop very strongly bound, stable, and reversibly active adsorbate layers renders them very attractive for further electrochemical applications. We have, in fact, already begun to use them in studies of fast surface processes at ultramicroel e c t r ~ d e s . ~Additional ~,~~ clarification of fundamental behavior of these substances a t interfaces would yield useful scientific insights. High on the agenda should be a fuller exploration of the behavior of 1,5-AQDS and a more secure definition of the chemical basis for the dramatic reorganization in adlayers of 2,6-AQDS.
BRS-100
1
EIUOLTI
Figure 13. Cyclic voltammograms of 5 X lo4 M 2,6-AQDS in 0.1 M HNOl at 10 mV/s. Successive cycles are numbered in sequence.
being needed at high scan rates or low concentrations. Simply holding the potential somewhere in the adsorption region will cause growth in the catalysis peak in a subsequent cyclic voltammogram. The origin of the catalysis and inhibition is not clear. It is probably due to the deposition of some kind of extended film (e.g., by precipitation or polymerization) of 2,6-AQDS on the surface of the electrode.28 In a small quantity, the deposited material may
Acknowledgment. We are grateful to the National Science Foundation for supporting this work through Grants CHE-8 106026 and CHE-86-07984. Ann Zielinski’s assistance in the preparation of the final manuscript is greatly appreciated. Professor David Y. Curtin made valuable comments on hydrogenbonded structures that might exist in our systems. (31) Faulkner, L. R.; Walsh, M. R.; Xu, C. In Proceedings of ElecrroFinn Analysis, An International Conference on Electroanalytical Chemistry; Ivaska, A,, Ed.; Plenum: New York, in press. (32) Walsh, M. R.; Xu,C.; Faulkner, L. R. Manuscript in preparation.
Pressure-Induced Coordination Change in the Linear-Chain Metal Halide Complex Trimethylsulfonium Trichloromercurate(II) A. Brillante,tB* P. Biscarini,t and K. Syassen*,l Dipartimento di Chimica Fisica e Inorganica, Universitli di Bologna, viale Risorgimento 4, I-401 36 Bologna, Italy, and Max- Planck-Institut f u r Festkorperforschung, Heisenbergstrasse 1, 0-7000Stuttgart 80, Federal Republic of Germany (Received: June 26, 1989)
We have measured Raman spectra of the quasi-one-dimensionalionic complex trimethylsulfonium trichloromercurate(II), [(CH,),S]+[HgCI,]-, at pressures up to 12 GPa. A major discontinuityin the pressure dependenceof intramolecular vibrational frequencies is observed near 1.2 GPa, which is accompanied by drastic changes of band profiles in the Hg-Cl stretching region. An interpretation is given in terms of a presure-induced stereochemical change, involving an increase of the coordination number of chlorine ligands around the mercury atoms and a change of the anion (HgCI,) local symmetry from planar trigonal to tetrahedral, or pseudotetrahedral. The related stoichiometry involves the presence of either HgC1, units sharing two apexes along the linear chain or of the dimerized Hg2C1, species, where one edge is shared between tetrahedral units.
Introduction The chemistry and spectroscopy of mercury and its coordination compounds have been reviewed in ref 1-3. In this paper we report a high-pressure Raman investigation of trimethylsulfonium trichloromercurate(II), [(CH,),S]+[HgCl,]-. This compound (hereafter denoted as [Me+] [HgCl,]) crystallizes in the monoclinic system, space group P2,,,, with 2 = 4.4 The crystal structure consists of a trigonal planar arrangement of the HgCl, moieties. Two additional Hg-Cl contacts contribute to an ”effective” coordination number2 of 5, giving rise to a bridged structure forming quasi-one-dimensional infinite chains, as is illustrated in Figure 1. Each chain is surrounded by sulfonium cations with pyramidal configuration. Universiti di Bologna. * Max-Planck-lnstitut fur Festkorperforschung.
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The present study aims to investigate whether the above structure could be affected by external pressure via a stereochemical interconversion, going along with a change of the characteristic coordination of mercury. Due to the sensitivity of intramolecular vibrational modes to the metal-halogen coordination, a Raman investigation under pressure of [Me,S] [HgCl,] can probe structural changes around the metal atom. From the variation of intramolecular Raman-active fundamentals with (1) Levason, W.; McAuliffe, C. A. In The Chemistry of Mercury; McAuliffe, C. A., Ed.; MacMillan Press: London, 1977; Chapter 2, p 67. (2) Grdenic, D. Q. Reu. 1965, 19, 303. (3) A d a m , D. M.; Hills, D. J. J . Chem. Soc., Dalton Trans. 1978. 776, and references therein. (4) Biscarini, P.; Fusina, L.; Nivellini, G.D.; G.Pelizzi, G.J . Chem. Soc., Dalton Trans. 1977, 664.
0 1990 American Chemical Society
1142 The Journal of Physical Chemistry, Vol. 94, No. 3, 1990
Brillante et al.
Figure 1. Schematic view of the molecular packing of [Me3S][HgCI,] chains.
pressure, we find evidence of a reversible phase change at around 1.2 GPa. The results are interpreted in terms of a discontinuous change in the coordination number of mercury from a trigonal to a tetrahedral (or pseudotetrahedral) structure.
0
200
400
600
800
RAMAN SHIFT (cm-1)
Experimental Details
I
(bl
I
The compound [Me$] [HgC13] has been prepared and characterized as reported el~ewhere.~ Single crystals were obtained by crystallization from methanol. Raman spectra under pressure were measured by using a gasketed diamond window cell in combination with the ruby luminescence method for pressure mea~urement.~As pressure media we have used either paraffin, isopentane, or a 3: 1 mixture of isopentane/methylcyclohexane. Spectra were recorded in near backscattering configuration using 488-nm excitation from an argon ion laser and a scanning Raman spectrometer equipped with a photon-counting system. The incoming power, measured at the diamond window, was kept low, ranging from 20 to 50 mW, to avoid sample damage. All spectra were taken at room temperature. Results and Discussion Raman spectra of single crystals of [Me3S][HgCI3] at ambient pressure compare well with the previously reported data on powder ~ a m p l e s .Figure ~ 2 shows Raman profiles at various pressures up to 10.5 GPa. The spectra do not substantially change on using different pressure media. Strong variations of the signal to background ratio are observed depending on the orientation of the crystal axes with respect to the polarization of the exciting laser line, a common feature for materials formed of quasi-onedimensional chains. The spectra in Figure 2 refer to a single crystal mounted in the cell with the direction of the chain axis perpendicular to the polarization direction of the laser beam (low-background direction) and in paraffin as pressure medium. An inspection of the Raman spectra in Fig. 2a reveals that a drastic change of band profiles occurs at fairly low pressures between 1.05 and 1.4 GPa. The structure in the Hg-CI stretching region near 280 cm-' is particularly affected. Figure 3 shows the observed Raman frequencies as a function of pressure. For clarity, the mode frequencies characteristic of the [(CH,),S]+ ion are shown separately in Fig. 3b. The curves in Figure 3 represent results from least-squares fits of second-order polynomials to the experimental data. The mode assignments, zero-pressure frequencies, and pressure coefficients are given in Table I. The experimental results clearly indicate a major change of the Raman spectra at about 1.2 GPa. At this pressure the ul(Hg-C1) symmetric stretching frequency near 273 cm-' splits into a doublet. At the same time a discontinuous frequency change occurs in the phonon region below 150 cm-'. In the C-S frequency region discontinuities are observed at 1.2 GPa for the C-S-C deformation mode at 305 cm-I and the symmetric C-S stretching at 650 cm-I, (5) Piermarini, G.J.; Bloch, S.; Barnett, J. P.; Forman, R. A. J. Appl. Phys. 1975, 46, 2774.
0
200
400
600
800
RAMAN SHIFT (cm-'1 Figure 2. Raman spectra of [Me3S][HgCI3] a t different pressures (a) below 2 GPa and (b) above 2 GPa. Vertical bars indicate cation internal vibrations.
whereas the asymmetric one at 728 cm-' also splits into a doublet. The latter effect becomes more evident at higher pressures. We attribute the major changes in the Raman spectra near 1.2 GPa to a phase transition. As noted already, the most significant spectral region affected in this phase change is that of the mercury-halogen bond vibration around 280 cm-l. By further increasing the pressure, a new band builds up at the low-energy side of the Hg-CI stretching frequency, its intensity grows continuously at the expense of the latter, and it eventually becomes predominant at about 10 GPa. All pressure effects on mode frequencies are fully reversible on returning to ambient pressure. The samples remain colorless in the pressure range investigated here. The frequency of the vI mode is very sensitive to environmental changes around the mercury atom. Its value is most indicative in probing the structural change of the mercury-chlorine anion induced by an increase in coordination number, as determined by the number of chlorine ligands. A summary of normal-pressure data is given in Table 11. (6) Adams, D. M.; Appleby, R. J. Chem. SOC.,Dalton Trans. 1977, 1530. (7) Barnes, W.; Sundaralingam, M. Acta Crystallogr. B 1973,29, 1868. (8) Klemperer, W.; Lindeman, L. J. Chem. Phys. 1956, 25, 397. (9) Janz, G. J.; Baddiel, C.; Kozlowski, T. R. J. Chem. Phys. 1964, 40, 2055
(10) Kecki, Z . Spectrochim. Acta 1962, 18, 1165. ( 1 1) Canty, J.; Raston, C. L.; Skelton, B. W.; White, A. H. J . Chem. Soc., Dalton Trans. 1982, 15. (12) Rolfe, J. A.; Sheppard, D. E.; Woodward, L. A. Trans. Faraday SOC. 1959, 50, 1275.
The Journal of Physical Chemistry, Vol. 94, No. 3, 1990 1143
Coordination Change in [Me3S][HgC13]
TABLE Ii: Structural and Spectral Properties of Selected Chloromercury(I1) Coordination Compounds (m Denotes Coordination Number) chem structure bond lengths ul(soln). . . . v,(solid). .. . cm-l species HgCI,, pm cm-l m sym 325b 315' D,h (223; 227)" [HgC12] linear . . . 2 305' 312' [HOC&]- trigonal D,h (245; 240; 242)' 279' 273' 3 2920 282d 280h [HgCI4l2- tetrahedral Td 250' 261/ 267* 4 2688 269 [Hg,CI6l2- dimeric D,h see ref 13 266' (pseudotet) 286' 4
-
1
2401 ! T
I
1401 I
I
I
" Reference 6. In water, ref 7. In organic solvents, ref 8. 'Reference 9. 'Reference 4 and this work. /In dimethyl sulfoxide, ref 4. 8In methanol, ref 10. hIn water, ref 4. 'Reference 1 1 . 'In water, ref 12. 'Reference 13. PRESSURE (GPa)
760 -
700
6 80
660
300 0
2
6
4
8
1 0 1 2 1 4
PRESSURE ( G P d Figure 3. Pressure dependence of Raman-active fundamentals (a) in the lattice region and of the [HgC13]- anion and (b) of the [(CH,),S]+ cation. Different symbols refer to different sample loadings. Full and open symbols correspond to increasing and decreasing pressure, respectively. TABLE I: Vibrational Freawncies and Their Assignments for phase I mode assignt and svm lattice HgCI, - - den- (e')
u0, b, m i 1 / GPa cm-I 74.7 1 1 . 3 (15) 100.5 8.6 (11)
HgC13 sym stretch (al') C-S-C def (a,) C-S sym stretch (a,) C-S asym stretch (e)
271.9 305.1 650.3 728.5
3.1 (13) 3.9 (7) 6.9 (6) 5.4 (7)
phase I1 uo, b, cm-l/ c, cm-'/ GPa GPa2 cm-' 96.1 5.6 (5) -0.17 (2) 248.9 270.5 284.6 654.4 733.4 733.2
1.4 (3) 7.8 (4) 13.9 (5) 6.7 (4) 2.3 (5) 5.5 (6)
0.49 (2) -0.10 (2)
-0.44 (2) -0.05 (2) 0.13 (2) -0.02 (2)
" Zero-pressure frequencies and first- and second-order pressure coefficients are from a least-squares fit of the equation u(P) = u0 + bP + CP2 to the experimental data. Error bars for c,, are less than 0.5 cm-'. The effect of increasing pressure can be related to an increase in coordination number of mercury. Figure 1 shows that interaction between the planar trigonal HgCl, units along the chain is by two intermolecular bridged bonds, thus yielding a quasi(13) Goggin, P. L.; King, P.; McEwan, D. M.; Taylor, G . E.; Woodward, P.; Sandstrom, M . J . Chem. Soc., Dalton Trans. 1982, 875.
bipyramidal trigonal structure. One would expect that the bond lengths of the weaker intermolecular van der Waals interactions of the farther atoms are more strongly affected by increasing pressure compared to the intramolecular bond. Reducing intermolecular spacings would create extra covalent bonds and increase the coordination number of the metal atom. The expected consequences is a decrease of the related stretching frequency in the new structural symmetry. We advance the hypothesis that such a molecular rearrangement is related to the phase transition which occurs at 1.2 GPa. We can attribute the rising of the new band above 1.2 GPa at the lower energy side of the symmetric Hg-Cl stretching of the trigonal form (see Figure 2) to the corresponding band in the tetrahedral (or a pseudotetrahedral) form. The related stoichiometry is then referred to quasi-one-dimensional chains of HgC14 units, sharing two apexes in the chain. A second possibility is the formation of dimeric [Hg2Cl6]*-anions where one edge is shared between tetrahedra along the chain. The pressure dependence of the new band is anomalous in the sense that its first-order pressure coefficient is quite small and that the mode frequency exhibits a pronounced superlinear shift with increasing pressure (see Table I). This behavior is similar to the pressure dependence of vibrations of bridging halogens in some linear-chain mixed-valence Pt and Au complexes, where even a negative first-order pressure coefficient is observed indicating a reduction of a Peierls-like d i s t o r t i ~ n . ' ~ In the case of [Me$] [HgC13] the anomalous frequency shift under pressure is taken as evidence for a continuous strengthening of the Hg-CI interaction in the newly formed bond with increasing pressure. Raman spectra above 10 GPa possibly indicate a second phase transition. In particular, at that pressure the v(P) relationship of the Hg-Cl stretching mode changes (see Figure 3a) and the new band, attributed to the tetrahedral form, becomes predominant. As to the other Raman fundamentals the strong band at 75 cm-' moves up by some 15 cm-' at the transition pressure of 1.2 GPa. This band has been assigned to a lattice vibration: and its average slope du/dP = 11 cm-I before the phase change is compatible with an external mode.** Above 1.2 GPa the slope of the curve is reduced. By supposing no large change in the mode Griineisen parameter, one can assume, in first approximation, that the new phase with larger coordination on mercury is less compressible, as expected on the basis of a model structure of chains where van der Waals contacts H g C I are reduced on passing from sp2 to sp3 hybridization on mercury. The pressure dependence of Raman modes of (CH3)$ units does not indicate major changes in the local symmetry of the cation, though (see Figure 3b) it clearly confirms the transition pressure at 1.2 GPa. (14) Tanino, H.; Holtz, M.; Hanfland, M.; Syassen, K.; Takahashi, K. Phys. Reu. E 1989, 39, 9992 and references therein. ( 1 5) Ferraro, J. R. Vibrational Spectroscopy at High External Pressure; Academic Press: New York, 1984.
1144
J. Phys. Chem. 1990, 94, 1144-1 146
In conclusion, high-pressure Raman spectra of [Me3S][HgCI,] provide evidence for a pressure-induced transition at 1.2 GPa, with major involvements on vibrational modes. The best evidence lies in the Hg-CI stretching region, which is the most significant for the identification of a local structural change around mercury. We suggest a transition from planar trigonal HgCI, to tetrahedral, or pseudotetrahedral, HgCI4 units sharing two apexes or one edge along the chain. This phase change is reversible, confirming the larger stability, at ambient pressure, of the trigonal planar with respect to the tetrahedral form of coordinated mercury. Although
phase transitions of tri- and tetrachloromercurate compounds have been known for some time to occur by changing temperature,I6-'* to our knowledge this is the first example of a pressure-induced coordination change in this class of materials. (16) Barr, R. M.; Goldstein, M. J . Chem. Soc., Dalton Tram. 1974, 1180; Ibid. 1976, 1593. (17) Prasad, P. S. R.; Sathaiah, S.; Bist, H. D. Chem. Phys. Lett. 1987, 142, 341. (18) Poulet, H.; Mathieu, J. P. J . Phys. 1979, 40, 1079.
Combined Effect of Isotopic Substitution, Temperature, and Magnetic Fieid on the Lifetimes of Triplet Biradicals Jin-Feng Wang, V. Pushkara Rao, Charles Doubleday, Jr.,* and Nicholas J. Turro* Department of Chemistry, Columbia University, New York, New York 10027 (Received: July 5, 1989)
Lifetimes T for decay of triplet biradicals derived from 2-phenylcycloalkanoneswere measured by nanosecond transient absorption under conditions designed to probe the contribution of intersystem crossing and chain dynamics to the observed decay. The temperature dependence of T was measured at both 0 and 2 kG magnetic field, and also in the presence of 0.004 M MnCI2. Under a variety of conditions of temperature, solvent viscosity, and magnetic field, the lifetime of the biradical derived from 2-phenylcyclododecanonewas compared with its perdeuterated analogue and with a 1:l mixture of 1,2J3C2 and 1,l 2J3C2 isotopomers. The magnetic isotope effect on T and the temperature dependence of Mn2+quenching support chain dynamics as the rate-limiting step at -85 "C and intersystem crossing as the rate-limiting step at room temperature. However, the magnetic field effect on T reaches a maximum around -50 OC and persists even at -99 OC,in contrast to the magnetic isotope effect and Mn2+quenching, which are absent at low temperature. The significance of this observation is discussed.
Flexible triplet biradicals offer an excellent opportunity to study the effect of chain motions and chain conformations on the electron spin dynamics. In the general description shown in Scheme I, triplet biradical decay is due to a competition between intersystem crossing (ISC) and chain dynamics.] The other major process in Scheme I, product formation from the singlet biradical, is assumed to be much faster than ISC or chain dynamics.' To understand the dynamics of triplet biradicals, our procedure is to change ISC or chain dynamics in a well-defined way and observe the effect on the triplet biradical lifetime T. An important parameter is the relative contribution of spin dynamics vs chain dynamics to T under given conditions. The goal is a comprehensive model of the total dynamics, which would, for example, allow one to predict the optimum conditions for isotopic enrichment. Two limiting cases of Scheme I are clear. If chain motions are much faster than ISC, e.g., at high temperature and low viscosity, then T is determined only by ISC and 7 - I is the conformationally averaged ISC rate constant. If chain motions are much slower than ISC, e.g., at low temperature or very high viscosity, then T is essentially a measure of the rate at which the biradical can adopt a conformation poised for product formation. The transition from rate-limiting ISC to rate-limiting chain dynamics is not expected to be sharp, and under a broad range of conditions both are expected to contribute to T . Because of this inherent complexity, we have examined five different methods of probing the interplay of spin and chain dynamics: the effect on T of variations in temperature, solvent viscosity, external magnetic field, paramagnetic nuclei, and bimolecular encounters with paramagnetic ions. We have reported extensively on the temperature dependence of T in the earth's magnetic field2 and the magnetic field effect (1) Doubleday, C.; Turro, N. J.; Wang, J.-F. Acc. Chem. Res. 1989, 22, 199. (2) (a) Wang, J.-F.; Doubleday, C.; Turro, N . J. J . Am. Chem. SOC.1989, 111,3962. (b) Zimmt, M. B.; Doubleday, C.; Turro,N. J. J . Am. Chem. SOC. 1986, 108, 3618.
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SCHEME I Triplet precursor
*...*
chain dynamlcs
e...*
lscll .=
chain dynamics
== * . Products
SCHEME I1
+ disproportionation
at rmm temperat~re.~In this paper we report unexpected results obtained from varying both temperature and magnetic field together, as well as results on isotopic substitution, increase in solvent viscosity, and paramagnetic ion quenching. Lifetimes of biradicals 2, were measured, derived from type I photolysis of ketones 1, (Scheme 11), where n is the number of carbons in the biradical chain. The products were previously r e p ~ r t e d . ~ (3) (a) Wang, J.-F.; Doubleday, C.; Turro, N. J. J . Phys. Chem. 1989, 93, 4780. (b) Zimmt, M. B.; Doubleday, C.; Turro, N. J. J . Am. Chem. SOC. 1985, 107, 6726. (4) Lei, X.;Doubleday, C.; Turro, N. J. TefrahedronLeu. 1986, 27, 4675.
0 1990 American Chemical Society
