1105
J . Phys. Chem. 1991, 95, 1105-1 112 Complexation would inhibit motion of the polymer chain and increase the T,. The breadth of the transition would reflect the range of environments in which the polymer is found (e.g., near a copper surface and far removed from the surface). Similar results are observed for block copolymers below the microphase separation t r a n ~ i t i o n . ~ ~ From the above discussion, a scenario for the formation of small copper particles is proposed. At 125 OC,copper formate initially decomposes in a rigid, glassy matrix creating nucleation sites. As the decomposition proceeds, the glass transition temperature in the uncomplexed regions is exceeded, and the mobility of the copper particles is increased. At this stage, growth would proceed as observed for solution systems.2'.22 Particles will grow until a critical size is achieved, further growth being limited by a protective polymer coating that complexes to the copper surface. Conclusions
Soluble copper formate-poly(2-vinylpyridine) complexes were prepared in a methanol solution where up to 50% of the pyridine moieties were complexed. The sterically hindered polymeric ligand precluded the complexation of more than one ligand about each (50) Bates, F. S.; Bair, H. E.; Hartney, M. A. Macromolecules 1984, 17, 1987.
copper site, thereby preventing cross-linking. The complexes were isolated either as thin films or as powders and were characterized by visible and infrared spectroscopy and thermal analysis techniques. Thermal decomposition of the precursor resulted in a redox reaction whereby Cu(I1) was reduced to Cu(0) and formate was oxidized to C 0 2 and H2. Incorporation of the reducing agent (formate) into the complex allowed the reaction to occur in the solid state. The reaction was studied by the previously mentioned techniques as well as by X-ray diffraction measurements and transmission electron microscopy. During the initial stages of the redox reaction, copper nucleation sites are formed within a glassy polymer matrix. As the reaction proceeds, the glass transition temperature of the matrix is exceeded and the copper mobility increases. Particles grow until a critical size (approximately 35 A) is achieved, further growth being limited by a protective polymer coating that complexes to the copper surface. Large copper particles are observed only above the decomposition temperature of the polymer matrix. Acknowledgment. We thank Mark Andrews for inspiration and helpful discussions. The kindness of Joe Abys for the longterm loan of the Inficon mass spectrometer is greatly appreciated. Initial measurements by A. M. Mujsce are gratefully acknowledged.
Solvent Control of Electronic Distribution in the MLCT Excited States of
[(bpy) (CO),Re1(4,4'-bPY )Re'(CO),(bpy)
12+
Gilles Tapolsky, Rich Duesing, and Thomas J. Meyer* Department of Chemistry, University of North Carolina, Chapel Hill,North Carolina 2751 4 (Received: October 19, 1989; In Final Form: August 13. 1990)
-
-
In the ligand-bridged complex [(bpy)(C0)3Re1(4,4'-bpy)Re'(CO),( bpy)]*+(bpy is 2,2'-bipyridine; 4,4'-bpy is 4,4'-bipyridine), either Re bpy or Re 4,4'-bpy excitation in acetonitrile or propylene carbonate leads to an equilibrium mixture of two metal to ligand charge transfer (MLCT) excited states. They are based on Re"(bpy') or ReI1(4,4-bpy'-). The excited states are formed rapidly (15 ns) upon excitation. They are interconverted by intramolecular electron transfer processes which, at room temperature, are rapid on the time scale for excited-state decay. As shown by transient absorbance measurements, the distribution between excited states is temperature and solvent dependent with polar solvents favoring Re11(4,4-bpy'-). Changes in solvent can be used to change the distribution between excited states and, thus, the electronic distribution within the molecule following MLCT excitation. In fluid solution, excited-state lifetimes for the equilibrium mixture between states were 870 20 ns in 1,2-dichloroethane,365 I O ns in acetonitrile, and 325 10 ns in propylene carbonate. As the temperature was decreased below 180 K in 4:l (v:v) ethanol/methanol, excited-state decay became nonexponential as the time scales for intramolecular electron transfer and excited-state decay became comparable. By 135 K, intramolecular electron transfer was frozen and the decay of the two excited states was kinetically uncoupled.
*
*
complexity arises because of relatively low molecular symmetries, the existence of multiple A* acceptor levels on the ligands, and the effects of spin-orbit coupling.'-3 The energies of the lowest
*
hV
I ( I ) (a) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Cwrd. Cfiem. Reo. 1988, 84, 85. (b) Meyer, T. J. Pure Appl. Chem. 1986, 58. 1193. (c) Balzani, V.; Sabbatini, N.; Scandola, F. Chem. Reu. 1986. 86. 319. (d) Fernuson. J.; Herren. F.: Krausz. E.: Vrbancich, J . Coord. Chem. Reu. 198c 64, 21. (e) Watts, R.J. J . Cfiem. Educ. 1983,60,834. (0 Kalyanasundaran, K. Coord. Chem. Rev. 1982,46,
159. (g) Balzani. V.; Moggi, L.; Manfrin, M. F.;Bolletta, F.;Lawrence, G. S. Coord. Cfiem. Reo. 1975, 15, 321.
0022-3654/91/2095-1 l05$02.50/0
\
/
I
lying states are affected by changes in substituents on the polypyridyl acceptor ligands and by changes in the remaining ligands 0 1991 American Chemical Society
1106 The Journal of Physical Chemistry. Vol. 95, No. 3, 1991
Tapolsky et al.
in the coordination ~ p h e r e . ~Excited ,~ states of comparable en(X = NH2, CH3, H, C02Et), parallel sets of MLCT excited states ergies exist which are ligand centered 13(7r7r*)] or metal centered exist based on either 4,4'-(X)*-bpy or 4,4'-bpy as the acceptor ldd] in character. In some cases, these states are sufficiently close ligand. The relative energies of the states vary depending upon in energy that more than one contributes to excited-state propthe substituent -X.I2 erties. X X An example is the dianion [Fe(bpy)(CN),I2- (bpy is 2,2'-bipyridine). From transient absorbance measurements, it has been shown that changes in solvent can cause an inversion in the relative ordering of dd and MLCT states for this complex.6 4,4'-bpY 4,4'-owJPY In chromophoreq~encher~ or ligand-bridged complexes, MLCT In the parent complex in this series, [(bpy)(CO)3Re'(4,4'excitation followed by intramolecular electron or energy transfer bpy)Re'( CO) 3( bpy ) ] 2+, the a*(bpy)- and a*(4,4'-bpy)-based states can lead to new types of excited states.8-" For example, MLCT are nearly isoenergetic based on the results of electrochemical and excitation of [(4,4'-(NH2)2-bpy)(CO)3Re1(4,4'-bpy)Re1(CO)3spectral studies. As for [Fe(bp~)(cN),]~-, this raises the possibility (4,4'-(NH2)2-bpy)]2+leads to an excited state in which the excited of utilizing solvent effects to influence the electronic distribution electron occupies a 7r* level of the bridging ligand.I2 In the series within the molecule following MLCT excitation. [(4,4'-( X)2-bpy)(CO)3Re'(4,4'-bpy)Re1(CO)3(4,4'-( X),-bpy)] 2+ In forming the ReI1(4,4'-bpy'-)-based state, there is an electronic displacement of ca. 5.5 A from Re' to the center of the bridging (2) (a) Indelli, M. T.;Bignozzi, C. A.; Marconi, A.; Scandola, F. J . Am. Chem. SOC.1988, 110, 7381. (b) Juris, A.; Campagna, S.; Ibid, 1.; Lehn, ligand.i3.i4 In the Re"(bpy'-)-based state, the electronic disJ.-M.; Ziessel, R. Inorg. Chem. 1988, 27,4007. (c) Kalyanasundaran, K. J . placement is ca. 3 A.13Because of the greater displacement, the Chem. Soc. Faraday Trans. 1986,82,2401. (d) Kober, E. M.; Meyer, T. J. interaction with the solvent is greater for Re1I(4,4'-bpy'-) than Inorg. Chem. 1984,23,3877. (e) Kober, E. M.; Meyer, T. J. Inorg. Chem. for Re"(bpy'-). From classical dielectric continuum theory, the 1983,22, 1614. (0 Kober, E. M.; Meyer, T. J. Inorg. G e m . 1982,21, 3967. (g) Caspar, J. V.; Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1984, 23, 2104. difference in the energy of interaction with the solvent between (h) Wrighton, M. S.; Morse, D. L. Organomel. Chem. 1975, 97, 405. (i) two excited states in the same molecule is given by eq 1.i5qi6 In Morse, D. L.; Wrighton, M. S. J . Am. Chem. SOC.1974, 96, 261. the derivation of eq I , it was assumed that the change in electronic (3) Yersin, H.; Braun, D.; Hensler, G . ;Gallhuber, E. In Vibronic Prodistribution between states can be approximated by a dipole in cesses in Inorganic Chemistry; Ed.; Flint, C. D., Kluwer Academic: Dordrecht, Holland, 1989; p 195. (b) Braun, D.; Gallhuber, E.; Hensler, G . ; a sphere of radius a and that the process occurs in a medium of Yersin, H. Mol. Phys. 1989, 67, 417. (c) Ferguson, J.; Krausz, E. J . Lumin. static dielectric constant D,.The,quantities and jie,2are the 1987, 36, 129. (d) Ferguson, J.; Krausz, E. R. Chem. Phys. Lett. 1982, 93, dipole moments of excited states 1 and 2. Given the greater 21. (e) Ohsawa, Y.; Whangbo, M-H.; Hanck, K. W.; DeArmond, M. K. Inorg. Chem. 1984,23,3426. (f) Morris, D. E.; Hanck, K. W.; DeArmond, M. K. J. Am. Chem. SOC.1983, 105, 3032. (g) DeArmond, M. K.; Hanck, K. W.; Wertz, D. W. Coord. Chem. Rev. 1985, 64, 65. (4) (a) Denti, G.; Sabatino, L.; DeRosa, G.; Bartolotta, A.; DiMarco, G.; Ricevuto, V.; Campagna, S. Inorg. Chem. 1989,28,3309. (b) Wacholtz, W. F.; Auerbach, R. A.; Schmehl, R. H. Inorg. Chem. 1986, 25,227. (c) Indelli, M. T.; Bignozzi, C. A.; Marconi, A,; Scandola, F. J . Am. Chem. Soc. 1988, 110, 7381. (5) (a) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1982, 21, 3967. (b) Caspar, J. V.; Meyer, T. J. Inorg. Chem. 1983, 22, 2444. (c) Barqawi, K. R.; Llobet, A,; Meyer, T. J. J . Am. Chem. SOC.1988,110, 7751. (d) Kober, E. M.; Marshall, J. L.; Dressick, W. J.; Sullivan, B. P.; Caspar, J. V.; Meyer, T. J. Inorg. Chem. 1985,24, 2755. (e) Caspar, J. V.; Kober, E. M.; Sullivan, 8. P.; Meyer, T. J. J . Am. Chem. Soc. 1982, 104. 630. (6) Winkler, J. R.; Creutz, C.; Sutin, N . J . Am. Chem. SOC.1987, 109, 3470. (7) (a) Chen, P.; Danielson, E.; Meyer, T. J. J . Phys. Chem. 1988, 92, 3708. (b) Chen, P.; Westmoreland, T. D.; Danielson, E.; Schanze, K. S . ; Anthon, D.; Neveux, Jr., P. E.; Meyer, T. J. Inorg. Chem. 1987, 26, 11 16. (c) Danielson, E.; Elliot, C. M.; Merkert, J. W.; Meyer, T. J. J . Am. Chem. SOC.1987, 109, 2519. (d) Schanze, K.S.; Neyhart, G . A.; Meyer, T. J. J . Phys. Chem. 1986, 90, 2182. (e) Chen, P.; Curry, M.; Meyer, T. J. Inorg. Chem. 1989, 28, 227 I .
dipole length for Re"(4,4'-bpy'-), it is predicted by eq 1 that this state should be stabilized relative to Re"(bpy*-) as the dielectric constant of the solvent is increased. We have attempted to utilize this effect to influence the electronic distribution within [ (bpy)(CO)3Re'(4,4'-bpy)Re'(CO)3(bpy)] 2+ following MLCT excitation. Experimental Section Materials. The solvents acetonitrile, propylene carbonate, 1,2-dichloroethane (DCE) (Burdick and Jackson), and dimethyl sulfoxide (DMSO) (Aldrich) were spectrophotometric grade and used without further purification. The preparations of the salts [(bPY)(Co)3Re'(4,4'-bPY)Re'(CO)3(bPY)I(PF6)2, [(bPY)(Co)3Re'(b~a)Re'(Co),(b~~)l(PF~)2, and [ ( b p ~ ) R e ' ( C 0 ) ~ ( 4 Etpy)](PF,) are described elsewhere.Izb
(8) Fuchs, Y.; Lofters. S.; Dieter, T.; Shi, W.; Morgan, R.; Strikes, T. C.; Gafney, H. D.; Baker, A. D. J . Am. Chem. SOC.1987, 109, 2691. (b) Braunstein, H. C.; Baker, D. A.; Strekas, T. C.; Gafney, D. H. Inorg. Chem. 1984,23, 857. (c) Furue, M.; Kinoshita, S . ; Kushida, T. Chem. Phys. Lett. 1987, 2355. (9) (a) Kambara, T.; Hendrickson, D. N.; Sorai, M.; Oh, S. M. J. Chem. Phys. 1986, 85, 2895. (b) Oh, S. M.; Hendrickson, D. N.; Hassett, K. L.; Davis, R. E. J . Am. Chem. Soc. 1985, 107,8009. (c) Dong, T-Y.; Kambara, T.; Hendrickson, D. N. J . Am. Chem. SOC.1986,108,4423, (d) Dong, T-Y.;
Hendrickson, D. N.; Iwai, K.; Cohn, M. J.; Geib, S. J.; Rheingold, A. L.; Sano, H.; Motoyama, 1.; Nakashima, S . J . Am. Chem. SOC.1985, 107, 7996. (e) Woehler, S. E.; Wittebort, R. J.; Oh, S . M.; Hendrickson, D. N.; h i s s , D.; Strouse, C. E. J . Am. Chem. SOC.1986, 108, 2938. (f) Dong, T-Y.; Hendrickson, D. N.; Pierpont. C. G . ;Moore, M. F. J. Am. Chem. SOC.1986, 108, 963.
(IO) (a) Schmehl, R. H.; Auerbach, R. A,; Wacholtz. W. F. J . Phys. Chem. 1988, 92,6202. (b) Wacholtz, W. F.; Auerbach, R. A,; Schmehl, R. S. Inorg. Chem. 1986. 25, 227. (c) Murphy, Jr., W. R.; Brewer, K. T.; Petersen, J. D. Inorg. Chem. 1987, 26, 3376. (d) Moore, K. J.; Lee, L.; Figard, J. E.; Gelroth, A.; Wohlers, D. H.; Petersen, J. D. J . Am. Chem. SOC. 1983, 105, 2274. Gelroth, J. A,; Figard, J. E.; Petersen, J. D. J . Am. Chem. SOC.1979. 101, 3649. (k) Petersen, J. D.; Murphy, Jr., W. R.; Sahai, R.; Brewer, K. T.;Ruminski, R. R. Coord. Chem. Rev. 1985, 64, 261. ( I I ) (a) Nishizawa, M.; Ford, P. C. Inorg. Chem. 1981, 20, 2016. (b) Rumininski, R. R.: Cockroft, T.: Shoup, M. Inorg. Chem. 1988.27, 4026. (c) Rumininski, R. R:; Kiplinger, J.; Cockroft, T.; Chase, C. Inorg. Chem. 1989, 28, 370. (d) Curtis, J. C.; Bernstein, J . S.; Meyer, T. J. Inorg. Chem. 1985, 24. 385. (12) (a) Tapolsky, G . ; Duesing, R.; Meyer, T. J. J . Phys. Chem. 1989.93, 3885. (b) Tapolsky, C.;Duesing, R.; Meyer, T. J. Inorg. Chem. 1990, 29, 2285.
u)
bpa
Measurements. UV-visible spectra were recorded on a Hewlett-Packard 845 1A diode-array spectrophotometer in 1 cm path length cuvettes. Emission spectra were recorded on a Spex Fluorolog-2 emission spectrophotometer equipped with a 450-W Xenon lamp and a cooled Hamamatsu R928 photomultiplier tube and were corrected for instrument response. Cyclic voltammograms were recorded by using a Princeton Applied Research Model 173 potentiostat, a Princeton Applied Research Model 175 sweep generator, and a Soltec XY Model 64148 recorder. (13) (a) Chen, P.; Curry, M.; Meyer, T. J. Inorg. Chem. 1989.28, 2271. (b) Sullivan, B. P. J . Phys. Chem. 1989,93, 24. (c) Kober, E. M.; Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1984, 23, 2098. (14) (a) Imamura, A.; Hoffman, R. J . Am. Chem. SOC.1968, 90, 5379. (b) Yamaguchi, S.;Yashimizu, N.; Maedas, S . J . Phys. Chem. 1978,82, 1078. (15) (a) Marcus, R. A. J. Chem. Phys. 1956,24,979. (b) Cannon, R. P. Ado. Inorg. Chem. Radiochem. 1978, 21, 179. ( I 6) (a) Brunschwig, B, S.; Ehrenson, S.; Sutin, N. J. J. Phys. Chem. 1987, 91, 4717. (b) Brunschwig, B. S.; Ehrenson, S.; Sutin, N. J. J . Phys. Chem. 1986, 90, 3657. (17) Luong. J . C.; Nadjo, L.; Wrighton, M. S. J . Am. Chem. Soc. 1978, 100, 5790.
Electronic Distribution in Polypyridyl Complexes
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1107
TABLE I: Electrochemicala and Spectral Datab in CHJCN at 293 f 2 K oxidation reduction E V v complex 6&1 4,4'-bpy0/- (bpyo/-) II(~PY)(C~),R~I~(~P~))~+ +1.87 -1.17 4 / 2 3
I[(b~y)(CO),Re1~(4,4'-bp~)l~~+ 1.90
-1.06
-1.20
~m,,,
nm 352 320 308 268
x 10-31, mol-l.L.cm-l
252
33.5
340 3 20 310 275 (sh)
17.6 33.4 29.4 28.4 40.8
-- ----
da(Re)
6.7 21.5 22.2 33.9
a
a a A
A
r*(bpy)
a* A*
a* a*
da(Re) da(Re) a
assignment
-
r*(bpy,4,4'-bpy) a*(4,4'-bpy); A
a*
A*
A*
246 A A* In 0.1 M [N(n-Bu),](PF,) vs. SSCE. The El values were obtained by cyclic voltammetry by using a platinum bead working electrode at a scan rate of 0.1 V/s. The potentials cited for the Re"/' couples are oxidative peak potentials, Ep,a.Values of ,Ell2 for the Re"/' couples can be calculated from the relationship E l I 2(V) = E , , - 0.030. The El12values for the ligand-based reduction potentials were calculated as the midpoint between peak potentials for the oxidative and reductive waves. The differences in peak potentials for the two one-electron oxidations at Re and the two reductions at bpy are irresolvable. bsh is shoulder; A,, f 2 nm, c f 5%. Voltammograms were acquired in argon deaerated 0.1 M [N(nC4H9)4](PF6)-CH3CN solutions vs the sodium chloride calomel electrode (SSCE) by using a platinum disk as the working electrode and a scan rate of 0.1 V/s. The supporting electrolyte was recrystallized twice from CH3CN/H,0. Emission decay profiles were acquired by using a PRA LN 1000/LN 102 nitrogen laser/dye laser combination for sample excitation. Emission was monitored at a right angle to the excitation by using a PRA B204-3 monochromator and a cooled, IO-stage, Hamamatsu R928 PMT coupled to either a LeCroy 9400 or a LeCroy 8013 digital oscilloscope, both of which were interfaced to an IBM PC. The absorbance (in I-cm cuvettes) of the sample solutions was -0.1 at the excitation wavelength. Solutions were deoxygenated by Ar bubbling. Transient absorbance measurements were performed by using the third harmonic of a Quanta Ray DCR-2A Nd:YAG laser. The excitation beam was at a right angle to an Applied Photophysics laser kinetic spectrometer. The spectrometer consisted of a 250-W pulsed Xe arc probe source, a f/3.4 grating monochromator, and a five-stage PMT. The output was coupled to a Tektronix 791 2 digital oscilloscope interfaced to an IBM PC. Electronic control and synchronization of laser, probe, and digital oscilloscope was achieved by electronics of our own design. Solutions were freeze-pump-thaw degassed (five cycles) and the ground-state absorbance at the excitation wavelength was typically -0.7. In the attcmpts to quantify the excited-state distribution, the transient absorbance difference spectra for the complexes were acquired sequentially by using the same ground-state absorbance, and the same excitation energy and wavelength (4 mJ/pulse at 355 nm). The absorbances were taken immediately after the laser pulse (5 ns). The relative fractional populations were estimated by using eqs 2a and 2b. In these equations, AANH2and AA4.Etpy
constant was calculated from the fractional populations. Temperature-dependent studies were conducted by using an Oxford DN 1704 cryostat with an Oxford 3120 temperature controller for steady-state emission and transient absorbance experiments and a Janis 6NDT cryostat with a Lake Shore Cryotronics DRC84C temperature controller for emission lifetime experiments. Transient emission and transient absorbance decay profiles were fit to either eq 4a or 4b by using a Levenberg-Marquardt fitting procedure. The preexponential terms in eq 4b, a , and a2,were I(t) = a, exp(-kt) I(t) = a, exp(-rlt)
(44
+ a2 exp(-r,t)
(4b)
normalized by dividing the entire transient waveform by the maximum changes in absorbance or emission intensity.
Results The absorption spectrum of [(bpy)(CO),Re1(4,4'-bpy)Re1(C0),(bpy)l2+ above 300 nm is dominated by a series of u r*(bpy) and du(Re) u*(bpy,4,4'-bpy) transitions. In Figure 1 A are shown absorption spectra for [(bpy)(C0),Re1(4,4'bPY)Re'(CO)3(bPY)12+ and [(bPY)(Co),Rel(bPa)Re1(co)3(bpy)12+ in CH3CN. In Figure 1B are shown spectra for [(bpy)(C0)3Re'(4,4'-bpy)Re'(C0)3(bpy)]2+in three solvents. In order to deconvolute the contributions from du(Re) u*(bpy) u*(4,4'-bpy) in [(bpy)(CO),Rel(4,4'-bpy)Re1and du(Re) (CO),(bpy)lZ+, the spectra of the two were subtracted. The resulting difference spectra are shown as insets in Figure 1B. Absorption maxima and molar extinction coefficients are listed in Table I. Electrochemical data in 0.1 M [N(n-C4H9).,](PF6) in CH3CN are also listed in Table 1. The splittings between the two Re"/' and bpyO/-couples were too small to resolve. Emission spectra for [(bpy)(CO)3Rel(4,4'-bpy)Re'(CO)3at 370 nm: fraction Re"(bpy'-) = (2a) (bpy)l2+and [ (bpy)(CO),Rel(bpa)Re'(CO),(bpy)j2+in CH3CN are shown in Figure 2. The spectral profiles are independent of excitation wavelength from 350 to 420 nm. The emission quantum AA - u4-Etpy at 580 nm: fraction Re"(4,4'-bpy'-) = yield is excitation dependent for the 4,4'-bpy-bridged complex. AANH2 It decreases by ca. 15% between A,, = 420 and A,, = 350 nm (2b) where the absorbance is becoming increasingly u T * in character. This effect was noted earlier and attributed to possible are the maximum transient absorbance changes for [(4,4'less than unit conversion efficiencies bitween RA* and MLCT (N H2)2-b~y)(CO)~Re'(4,4'-bpy)Re'(CO)~(4,4'-(NH~)~-bpy)l 2+ excited states.12bEmission maxima, quantum yields, and lifetimes and [(bpy)(CO)3Re'(4-Etpy)]+, respectively. The MLCT excited in CH3CN, 1,2-dichloioethane, and propylene carbonate are listed states in these complexes are known to be 4,4'-bpy based or bpy in Table 11. At room temperature, the emission decays followed based, respectively.I2 The quantity AA is the experimentally single-exponential kinetics in all three solvents. observed transient absorbance change for [(bpy)(CO),Re1(4,4'In Figure 3 are shown transient absorbance difference spectra bpy)Re'(CO)3(bpy)]2+. The total concentration of excited states for [(bpy)( CO)3Re'(4,4'-bpy) Re'(CO)3( bpy)] 2+ acquired 50 ns was [Re"*] = [Re"(bpy'-] + [ReI1(4,4'-bpy'-)]. The equilibrium after laser excitation at 355 nm in CH,CN, 1,2-dichloroethane, relating the two excited states is defined in eq 3. The equilibrium or propylene carbonate. The decay kinetics were first order and K = [Re1I(4,4'-bpy'-)] /[Re"(bpy'-)] (3) lifetimes were the same within experimental error as those obtained
-
-
-
-
(
(
)
-
I108 The Journal of Physical Chemistry. Vol. 95, No. 3, 1991
Tapolsky et al.
TABLE II: Ouantum Yields. Emission Maxima. and Emission Lifetimes at 293 f 2 K'
propylene carbonate
acetonitrile complex
be,
Xmas, nm
ns 200 365
A,,
f 10% 7 f
2%.
1,
0.0325 I[ ( ~ P Y ) ( C ~ ) I R ~ I , ( ~ P ~ ) I ~ +590 0.0332 I[(~PY)(CO)~R~I~(~,~'-~PY)~~+ 585
'The excitation wavelength was 380 nm. A,,
f
2 nm,
nm 590 590
bem
0.0264 0.0267
1,2-dichloroethane
ns 175 325
T,
,,,X,
nm 575 570
be,,, 0.135 0.170
ns 510 870
T,
TABLE 111: Temperature-Dependent Emission and Absorption Transient Decay Data in 4 1 (v:v) Ethanol/Methanol
emission T, K 295 260 250 230 200 190 I80 I70 I65 160 I55
rl(aAas-' 3.12 X IO6 2.75 X IO6 2.67 X IO6 2.51 x 106 2.18 X IO6 2.07 X 4 x 107 1.93 X IO6 2.17 X IO7 (0.45)* 1.33 X IO7 (0.57)b 9.71 X IO6 (0.69) 7.69 X IO6 (0.77) 6.37X IO6 (0.55)
13s
6.8 X IO6 (0.37) 6.33X IO6 (0.17) 2.11 x 1066 5.52 X IO6 (0.15)b 1.72 X IO6
I30 12s I20 I I6 IO0 90 77
1.46 X IO6 1.01 x 106 6.9 x 105 4.67x 105 2.70 x 105 2.56 x 105 2.15 x 105
I50 145 140
absorption r2(a2),"s-I
2.02 X IO6 (0.75) 1.88 X 1.78 X 1.89 X 1.95 X 2.24 X
IO6 IO6 IO6 IO6 IO6
(0.60) (0.43) (0.31) (0.30) (0.45)
rl(al):s-I
r2(a2):
s-I
monitoring A, nm
3.08 X IO6
350-650
2.16 X IO6 2.14 X IO6
580 390
7.19 X 8.06 X 1.16 X 2.56 X 2.24 X
IO6 (0.11) IO6 (0.1I) IO'(0.31) IO6 (0.85)b
1.03 X 1.02 X 9.11 X 6.21x
IO6 (0.89) IO6 (0.89) IO' (0.69) 105 (0.15)
61OC 59OC 58OC 370 370
2.40 X IO6 (0.64) 2.34 X IO6 (0.87) 2.32 X IO6 (0.88) 9.52x 105 -1.33 X I O 6
590 370
'The estimatcd crror in the lifetimes by transient emission and absorption is fS% for the simple first-order decays and &IO% for the biexponential fits. I n the emission experiments the monitoring wavelength was 580 nm. The data were fit to the relationships in eqs 4 and 5 by using a Levenberg-Marquadt fitting procedure. The biexponential fits were normalized by dividing the entire waveform by the maximum change in absorbance or emission intensity. The preexponential terms sum to 1 f 0.1. The 10% error is a reflection of the noise in the data. bResults are given for both singlc- and double-exponential fits. The error in the lifetimes that are less than 100 ns was 130% due to the small amount of data that were fit. In these decay profiles, there was an initial increase in the absorbance-time profile followed by a decay. by emission decay, Table 111. The relative absorbances at the maxima of 368-372 or 570-610 nm were solvent dependent. The ratio ASEO/A370 was 0.2 in 1,2-dichloroethane, 0.5 in CH3CN, and 0.67 in propylene carbonate. The range of solvents that could be used was limited. With DMSO or N-methylformamide, photoreduction of the excited state occurred. With acetone, ASEO/A370 was critically dependent on the water content of the solvent and the spectra were difficult to reproduce. Emission spectra and transient absorbance difference spectra were acquired in 4:l (v:v) ethanol/methanol as a function of temperature. Some unusual effects were observed at low temperatures. The integrated emission intensity increased slightly from 295 to 180 K , decreased from 180 to -I60 K , and then increased continuously through the glass to fluid transition at 140-120 K, Figure 4. Temperature-dependent emission decay curves were exponential in behavior above 180 K, became nonexponential from 180 to 140 K, and were nearly exponential below the glass to fluid transition, Table 111. Reasonable fits of the data from 180 to 140 K were obtained by using the biexponential function in eq 4b. The results are listed in Table 111. Transient absorbance difference spectra were acquired in the range 295-220 K in propylene carbonate and in the range 295-1 35 K in 4:l (v:v) ethanol/methanol. The absorbance changes in propylene carbonate are listed in Table IV. Data were acquired at 155 K at a variety of monitoring wavelengths in 4:l (v:v) ethanol/mcthanol following excitation at 355 nm, Table 111. The
-
TABLE I V Temperature-DependentTransient Absorbance Changes in Propylene Carbonateo temp, K lifetime, ns AA,,,,b mA AAcan.bmA 295 325 47 31 258 370 51 3s 239 420 51 38 220 466 51 39
Uncertainties are f2 K in T; f3% in r; 1 2 milliabsorbance units (mA) in An. bThe AA values were maximum absorbance changes following laser flash photolysis. data could be satisfactorily fit to the biexponential function in eq 4b, although the parameters obtained in the fits were wavelength dependent. Inithe region 570-610 nm, where the absorbance change is dominated by a ligand localized 7r 7r*(4,4'bpy'-) transition in Re"(4,4'-bpy'), an initial growth was observed in the absorbance-time decay curve, Figure 5. There was no rounding of the decay curve at 368-372 nm. This is a region where 7r 7r*(bpy'-;4,4'-bpy'-) transitions occur for Re"(bpy*-) and Re"(4,4'-bpy*-). The transition based on Re"(bpy'-) is expected to be the stronger absorber of the two based on comparisons with and the Re"(bpy'-)-based state in [Re1(bpy)(C0)3(4-Etpy)]+78.'8 the Re1[(4,4'-bpy'-)-based state i n [(4,4'-(NHJ2-bpy)-
-
-
( 1 8 ) Mahon, C.; Reynolds,
W.L. Inorg. Chem. 1967,6,1927
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1109
Electronic Distribution in Polypyridyl Complexes
10000 5000
0
t
\
H
i
s
B
WAVELENGTH (nn)
Absorption spectra of [(bpy)(C0),Rei(4,4'-bpy)Rei(CO),(bpy)]*+(-) and [(bpy)(CO),Re'(bpa)Re1(CO),(bpy)12+ (- -) in CH,CN. (B)Absorption spectra of [(bpy)(CO)3Rel(4,4'-bpy)Rei(CO)p(bpy)]z+ in CH3CN (-), propylene carbonate (--), and 1,2-dichloroethane The diffcrencc spcctra between [(bpy)(CO),Rel(4,4'-bpy)Rei(CO)3(bpy)]2+ and [(bpy)(CO),Re'(bpa)Re'(CO),(bpy)]z+are shown in the inset for the three solvents. Figure 1. ( A )
(-e-).
(CO)3Re1(4,4'-bpy)Re1(CO)3(4,4'-(N H2)2-bpy)]2+.7d-'2a Discussion From the data in Table I, bpy and 4,4-bpy are comparable as acceptor ligands. Because of this, and as shown in Figure I , the lowest energy Re bpy and Re 4,4'-bpy transitions are badly overlapped in [(bpy)(CO)3Re1(4,4'-bpy)Re1(CO)3(bpy)]2+. The MLCT states that result from these transitions are also nearly equal in energy and both appear following MLCT excitation. The basis for this conclusion is the appearance of characteristic oxidation-state markers in the transient absorbance difference spectra in Figure 3. The sharp, intense feature at 368-372 nm appears It in the diffcrcnce spectrum of [Re1(bpy)(CO)3(4-Etpy)]+.7aJ8 arises from a A A* transition at bpy'-. The strongly absorbing feature at 570-610 nm, and the weaker feature at 390 nm, appear in the difference spectrum of [(4,4'-(NH2),bpy)(C0),Re1(4,4'bpy)Re'(C0),(4,4'-(NH,),-bpy)l2+. They arise from a x T* transitions at 4,4'-bpy'-.7dJ2a
-
-
-
-
The relative absorbances at the marker wavelengths are solvent dependent. In propylgne carbonate, the bridge-based absorption at 570-610 nm and the accompanying shoulder at 390 nm are favored compared to acetonitrile. In 1,2-dichloroethane the bpy'--based absorption at 368-372 nm dominates the spectrum. From these observations, we conclude that excitation of [(bpy)(CO)3Re1(4,4'-bpy)Re'(C0)3(bpy)]2+ at 355 nm leads to a solvent-dependent, excited-state mixture of Re"(bpy'-) and Re1'(4,4'-bpy'-). The two states are oxidation-state isomers of each other. Both are present at our earliest observation times ( 1 5 ns), and the distribution between them is independent of excitation wavelength. The two states decay simultaneously, by simple first-order kinetics, with the same lifetime. These experimental observations are consistent with the kinetic model in Scheme I. In the model, Re bpy or Re 4,4'-bpy excitation is followed by intramolecular electron transfer which equilibrates the excited states, and is rapid (k > 2 X IO8 s-'), on the time scale for excited-state decay. In this kinetic limit, the experimental lifetimes
-
-
1110 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 I
Tapolsky et al. 0.08 [
I
I
1
0.06
0.04
0.02
0.00 1 350
I
I
I
450
550
650
Wavelength (nm) 450.00
625.00 WAVELENGTH (nm)
800.00
Figure 2. Emission spectra at constant absorbance (OD = 0.1) in CH$N at 295 K following 380-nm excitation of [(bpy)(CO)3Re'(4,4'-bpy) Re'(CO),( bpy)12+ (-) and [ (bpy)(CO)3Re'(bpa)Re'(CO)3-
B
(bpy)I2+ (--h
in Table I I are related to the constants in Scheme I by eq 5 , with K = k1fk-i. f-1
+
k2 k3K = -I+K
+ 73-IK 1+K
~2-l
(5) 0.01
The solvent effects demonstrate that it is possible to utilize changes in the solvent to influence intramolecular electronic distribution following MLCT excitation. The effect of solvent dielectric constant on the distribution between isomers is predicted by eq 1. We were unable to acquire sufficient data to test this prediction quantitatively but our observations are consistent qualitatively. In the relatively nonpolar solvent 1,2-dichloroethane (D,= 10.4), the Re"(bpy'-)-based excited state is dominant. As = 36.2) the polarity of the solvent is increased from acetonitrile (0, to propylene carbonate (D,= 65.1), the Re"(4,4'-bpyo-)-based state is increasingly favored, consistent with eq 1. The distribution between oxidation-state isomers is also slightly temperature dependent. From the data in propylene carbonate in Table IV, the bridge-based isomer, which is the dominant light absorber at 580 nm, is favored as the temperature is decreased. This shows that AH is negative for the equilibrium in Scheme 1. We have attempted to estimate the fractional distribution between Re"(bpy'-) and Re"(4,4'-bpy'-) in [(b~y)(CO)~Rel(4,4'-bpy)Re'(C0),(bpy)J2+* by making quantitative transient absorbance comparisons with [Re1(bpy)(C0),(4-Etpy]+* and
1
0.00 350
(19) The assumption that the efficiency of reaching the emitting state is I is commonly made, e.&: (a) Creutz, C.; Chou, M.; Thomas, L. W.;Kumura, 0.;Mitchoi, M.; Sutin, N. J . Am. Chem. Soc. 1980, /02, 1309. (b) Laschisch, U.; Infelta, P.; Gratzcl, M . Chem. f h y s . Lett. 1979, 62, 317.
,
I
I
450
550
650
Wavelength (nm)
0.06 0.05 0.04 0.03
0.02 0.01
[(4,4'-(NH2)zbpy)(CO)3Re'(4,4'-bpy)Re'(CO)3(4,4'-(NH2)2bpy)l2+*,and using eq 2. At 370 nm, the Re"(bpy') excited state is the major light absorber. At 580 nm, the Re1I(4,4'-bpy') excited state is the only light absorber. Based on the maximum absorbance changes at these wavelengths, the calculated fractions of the bridge-based state at 293 f 2 K were 1,2-dichloroethane (10.10 f 0.02),acetonitrile (0.45 f 0.03), propylene carbonate (0.55 f 0.04). The calculations were based on the following assumptions: ( I ) The emitting MLCT states are reached with equal efficiency for the three complexes.19 (2) The molar extinction coefficients for the bpy-based states in [(bpy)(CO),Re1(4,4'-bpy)Re1(CO)3(bpy)I2+*and [Re1(bpy)(CO),(4-Etpy]+*are the same. (3) The molar extinction coefficients for the 4,4'-bpy bridge-based states in [ (bpy)(C0),Re1(4,4'-bpy) Re'( CO),( bpy)12+* and [ (4,4'( N H2)2bpy)(C0)3Re1(4,4'-bpy)Re1(C0)3(4,4'-(N H2)2-bpy)12+* are the same. Emission. In polar solvents at room temperature, both the Re"(bpy'-)- and Re"(4,4'-bpy0-)-based states may contribute to the emission from [ (bpy)(CO),Rel(4,4'-bpy)Rel(C0)3(bpy)]*+*.
-
0.00 350
450
550
650
Wavelength (nm)
Figure 3. Transient absorbance difference spectra obtained 50 ns after the laser pulse at 295 k,2 K for [(bpy)(CO),Rel(4,4'-bpy)Re'(CO)3(bpy)]*+in freeze-pump-thaw deoxygenated solutions: (A) 1,2-di-
chloroethane, (e) acetonitrile, and (C) propylene carbonate. The ground-state absorbance was -0.7 at the excitation wavelength, 355 nm. The laser pulse energy was 4 mJ/pulse.
It is not possible to deconvolute their contributions. In comparing the emission spectrum of [(bpy)(CO)3Rel(L)Rel(CO)3(bpy)]2+, L = 4,4'-bpy, with that for L = bpa, Figure 2, there is no obvious evidence for a Re1l(4,4'-bpy'-)-based emission. The emission that is observed is probably dominated by Re"(bpy'-). In related complexes, emission quantum yields and radiative rate constants ( k , ) for 4,4'-bpy-based states are lower than for bpy-based states of comparable energy. For example, the quantum yield for emission from Re"(bpy'-) in [(bpy'-)(CO),Re"(bpa)Re'(CO),(bpy)12+*is 0.033. It is only 0.009 for the Re1I(4,4'-bpy'-)-based emission in [ (4,4'-( CH3)2bpy)(CO)3Re'(4,4'- bpy'-) Re"( C0)3(4,4'-(CH3)2-bpy)]2+*. The emission energies are 590 and 585 nm, respectively, in CH3CN.12 The radiative decay rate constant, k4, is ca. 8 times greater for the Re"(bpy'-) emitter. Even though k , is less for Re1I(4,4'-bpy'-), its lifetime appears to be comparable to the lifetime of the Re"(bpy'-)-based state.
1*4rl
Electronic Distribution in Polypyridyl Complexes
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991
1.8
1 .o 120
160
200
T
. - 75
175
Tomp.ntun (K)
15. I m A Y
H f
g.gamA
-a
3
-
I
P
2
decay kinetics became decidedly nonexponential and the profiles of the intensity-time curves became wavelength dependent. An example of an absorbance-time trace at 155 K is shown in Figure 5. Excited-state decay kinetics were exponential for the model complexes, [(bpy)(CO)3Re1(bpa)Re1(CO)3(bpy)]2+ and [Re'(bpy)(C0),(4-Etpy)]+, over this temperature range. The nonexponential decays for the 4,4'-bpy-bridged complex appear to arise from a coupling in time scale between intramolecular electron transfer and excited-state decay. The decay data could be fit satisfactorily to the biexponential function in eq 4b (Table 111) which is qualitatively consistent with this The clearest evidence for slow electron transfer comes from the initial growth in absorbance at 580-620 nm in Figure 5. From this observation, it can be inferred that, at 155 K, Re {bpy, 4'4'-bpyJ excitation at 355 nm leads to an excited-state distribution that is not at equilibrium. Initially, there is an excess of Re"(bpy'-). The initial growth in absorbance at 580-620 nm following the excitation arises because bpy'4,4'-bpy electron transfer occurs to give Re"(4,4'-bpy'-) as the system attempts to equilibrate. Electron transfer occurs on the same time scale as decay of Re"(bpy'-) or of ReI1(4,4'-bpy'-). The correspondence in time scales couples the processes kinetically and explains the nonexponential behavior. Slow intramolecular electron transfer and a nonequilibrium excess of Re1'(4,4'-bpy'-) following excitation at 380 nm also provides an explanation for the decrease in that occurs from 200 to 150 K, Figure 4. As the temperature is decreased and the rate of intramolecular electron transfer slows, a greater fraction of excited-state decay must occur via the ReI1(4,4'-bpy*-) state which is a weak emitter. The increase in +'em at the fluid to glass transition (140-120 K) is caused, in part, by the increase in energy gap between the excited and ground states that occurs in this region.20 This interpretation of the temperature dependence demands that intramolecular electron transfer be more highly dependent on temperature than is excited-state decay. This is an expected result. In Scheme I, k , and k-, are rate constants for the ligand to ligand electron transfers, bpy'4,4'-bpy or 4,4'-bpy'bpy, for which AG -0. A thermal barrier exists for these reactions. The barrier arises from the intramolecular and solvent reorganizational requirements which are posed by the changes in electron content at the donor and acceptor sites. On the other hand, the excited-state decay processes are known to be, at best, only slightly temperature dependent in fluid s ~ l u t i o n . ~In* the ~ ~ nonradiative process, which dominates excited-state decay for these complexes, Table 11, the important feature microscopically is the loss of energy into the surrounding molecular vibrations. There is no requirement for thermal activation to cross a barrier or to improve vibrational overlap. By 135 K electron transfer appears to be frozen on the time scale for excited-state decay. The evidence for this conclusion is as follows: (1) Transient absorbance decay kinetics at 580 nm, where Rel1(4,4'-bpy'-) is the sole absorber, were single exponential with T = 1050 ns following excitation at 355 nm. (2) At 370 nm, where both Re"(bpy'-) and Re"(4,4'-bpy'-) absorb, the decay kinetics were nonexponential. (3) Emission decay at 135 K, monitored at 550-600 nm, was nearly single exponential with T = 590 ns following excitation at 390 nm. These observations are consistent with excited states in which (1) decay is kinetically
-
275
Figure 4. Integrated emission intensities following 380-nm excitation of [(bpy)(CO)3Re1(4.4'-bpy)Re'(CO)3(bpy)]z+ in a 4:l (v:v) EtOH/MeOH solution as a function of temperature. The inset shows an expanded plot of the region from 120 to 220 K. 20.3mA
1111
4. B4mA
-309. uA
554. ns
1.27~0
1.99uc
2.70~5
3. 42uc
Time
I
I
I
Figure 5. (A, top) Absorbance-time decay curve and kinetic fit for [(bpy)(CO)3Rei(4,4'-bpy)Re1(CO),( bpy)]*+ in a freeze-pump-thaw deoxygcnatcd 4:l (v:v) EtOH/MeOH solution at 155 f 3 K. The excitation wavelength was 355 nm and the transient absorbance signal was monitored at 610 nm. (B,middle) Residual and (C, bottom) autocorrelation plots, obtained for a fit of the data to the biexponentional function in eq 5 with the parameters u, = 0.1 I , r, = 7.19 X IO6, u2 = 0.89, rz = I.03 X IO6, are also shown.
In [(bpy)(CO)3Re'(bpa)Re'(CO)3( bpy)12+,T for Re"( bpy')-based emission is 190 ns in CH3CN. By using 190 ns for rz in eq 6 and ~ r 3 ) / 2 and assuming that K = 1 in CH3CN, T = 370 ns = ( T + r 3 = 550 ns. Temperature Dependence. Single-exponential absorbance and emission decay kinetics were observed in 4: 1 (v:v) EtOH/MeOH in the temperature range 295-180 K. From 180 to 145 K, the
-
-
(20) (a) Lumpkin, R. S.; Meyer, T. J. J . Phys. Chem. 1986,90,5307. (b) Heitele, H.; Flinckh, P.; Weeren, S.; Michel-Beyerle, M. E. J . Phys. Chem. 1989, 93, 5173. (c) Flinckh, P.; Volk, M.; Michel-Beyerle, M. E. J . Phys. Chem. 1988, 92,6584. (d) Barigeletti, F.; Juris, A,; Balzani, V.; von Zelewsky, A. J . Phys. Chem. 1987, 91, 1095. (21) (a) Alberty, R. A.; Miller, W. G.J . Chem. Phys. 1957,26, 1231. (b) Danielson, E. Unpublished results. (22) (a) Giordano, P. J.; Wrighton, M. S. J . A m . Chem. SOC.1979, 101. 2888. (b) Barigelletti, F.; Belser, P.; von Zelewsky, A,; Juris, A,; Balzani, V. J . Phys. Chem. 1985, 89, 3680. (c) Giordano, P. J.; Fredericks, S. M.; Wrighton, M. S.; Morse, D. L. J . A m . Chem. SOC.1978, 100, 2257. (23) (a) Barquwi, K. R.;Llobet, A,; Meyer, T. J. J . Am. Chem. Soc. 1988, 110, 7751. (b) Rillema, D. P.; Taghdiri, D. G.;Jones, D. S.; Keller, C. D.; Worl, L. A.; Meyer, T. J.; Levy, H. A. Inorg. Chem. 1987, 26, 578.
1112
J. Phys. Chem. 1991, 95, I 1 12-1 119
uncoupled, (2) emission is dominated by Re"(bpy'-), and (3) the lifetime of Rel1(4,4'-bpy*-) is longer by a factor of -2. In the fluid to glass transition, emission decay curves are nonexponential and the emission intensity increases. Similar observations have been made for related complexes.20.22 The nonexponential decays arise from a coupling in time scale between excited-state decay and the reorientation of solvent dipoles near the excited state. Once the glass is reached, the decay kinetics become nearly single exponential and the integrated emission intensity nearly constant, Figure 4. We presume that intramo-
lecular electron transfer is frozen in the glass and does not occur. Although emission may occur from both states, it is dominated by Re"(bpy'-). We have been unable to obtain evidence for emission from Re"( 4,4/-bpy'-) by time resolving the emission spectrum at 77 K . Acknowledgment. Acknowledgementsare made to the National Science Foundation under grant CHE-8806664 for support of this research and to RHONE-POULENC GROUP for support for G.T.
Isotropy in Ionic Interactions. 2.' How Spherical I s the Ammonium Ion? Comparison of the Gas-Phase Clustering Energies and Condensed-Phase Thermochemistry of K+ and NH,+ Joel F. Liebman,* Mitchell J. Romm, Department of Chemistry and Biochemistry, University of Maryland, Baltimore County Campus, Baltimore, Maryland 21 228
Michael Meot-Ner (Mautner)," Center for Chemical Technology, National Institute for Standards and T e ~ h n o l o g y , ~ Gaithersburg, Maryland 20899
S . M. Cybulski,2band Steve Scheiner* Department of Chemistry and Biochemistry, Southern Illinois University at Carbondale, Carbondale, Illinois 62901 (Received: February 6. 1990; In Final Form: May 21, 1990)
High-pressure mass spectrometry is used to obtain the following clustering enthalpies and entropies of NH4+: With acetonitrile, AHo(n-I,n) (kcal mol-') n = 1, 27.6; n = 2, 21.2; n = 3, 14.2; n = 4, 11.7; and the corresponding ASo(n-l,n) (cal mol-! K-I) 24.2, 25.4, 17.5, and 22.2. With benzene, AHo(n-I,n) (kcal mol-!) n = 2, 17.0; n = 3 , 14.2; and the corresponding ASo(n-l ,n) (cal mol-' K-I) 30.5 and 32.9. Comparison of cluster iod energies is made with the values for the corresponding species containing K+. Classical electrostatic analyses, based on "numerical experiments" and on a formal multipolar analysis, are used to probe the nature of the interactions. Morokuma and electrostatic analyses of quantum chemical calculations on K+.N H3and NH,+.NH, provide additional insight. Finally, documentation is given by using examples from condensed-phase media: lattice and solvation energies, crystal dynamics, conductancesof electrolytesolutions, magnetic resonance measurements on NH4+ solutions, and the interactions of some key biochemical species.
Introduction The interaction of gas-phase ions with neutral "solvent" or "ligand" molecules and the resultant formation of "cluster ions" is a seminal topic in chemistry and related di~ciplines.~*~ Quoting from the major data compendium of the energetics of these species:b "Work in this area spans a broad range of fields including geophysics, electrochemistry, organic chemistry and chemical physics ... [and is] relevant to phenomena such as nucleation, the development of surfaces, catalysis, solvation, acid-base chemistry, and atmospheric properties." Much of this interest in cluster ions has arisen because these species interpolate atomic and simple dmolecular ions in the gas phase with condensed-phase ionic solutions in the bulk in a systematic way. Studies such as the current investigation provide values for the thermodynamic
properties of enthalpy and entropy of complexation. By uniquely defining the species of interest, these studies allow for both qualitative and quantitative understanding of structure/energy relations instead of comparatively vague concepts such as "solvent shell". More precisely, both the "central" ion and the nature and number of clustering neutrals can be unambiguously assigned for the cluster ion of interest, and the equilibrium constant, enthalpy, and entropy of gain or loss of a neutral molecule directly determined. In this paper we continue our comparison6 of the cluster chemistry of K+ and NH4+, two of the simplest monoatomic and polyatomic ions, respectively.' The complexing neutral species we will discuss include H20, the most classic but most complicated of solvents; NH,, another polar inorganic solvent; and CH3CN and CsH6, two important aprotic organic solvents that are polar
(1) In homage of Professor Vojtech Frie on the occasion of his 70th birthday. Part I: Meot-Ner (Mautner), M.; Cybulski, S.M.;Scheiner, S.; Liebman, J. F. J. Phys. Chcm. 1988, 92, 2738. (2) (a) Current address: Department of Chemistry, University of Canterbury, Christchurch I , New Zeland. (b) Current address: Department of Chemistry, University of Ottawa, Ottawa, K I S 586, Canada. (3) Formerly known as the Center for Chemical Physics, National Bureau of Standards. (4) Taft, R. W. Prog. Phys. Org. Chem. 1983, 14, 247. ( 5 ) Mark, T. D.; Castleman, A . W.,Jr. Adu. AI. Mol. Phys. 1985.20, 65. Keesee, R. G.; Castleman, A. W., Jr. J. Phys. Chem. ReJ Data 1986, 16, 1017.
(6) Deakyne, C. A.; Meot-Ner (Mautner) M. J. Am. Chem. Soc. 1985, 107,475. As is the case in most studies on ion energetics, both this cited paper and the current one discuss entropies as experimental measurements of interest and importance but ignore them insofar as comparison with calculational theory. (7) We consider K+ to be particularly simple because it has a Is electron configuration and, in the absence of associating neutrals, is spherically symmetric as well as closed shell. The low electronegativity of potassium also "guarantees" the absence of covalent binding of the ion to the associating neutrals. Likewise, we consider NH4+to be simple because of its high symmetry and may safely assume the absence of covalent bonding, save hydrogen bonding, with any added neutral.
0022-3654/9 I /2095-11 I2$02.50/0
0 199 I American Chemical Society
