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J. Phys. Chem. 1996, 100, 19292-19294
Solvent Effects on Molecular and Ionic Spectra IX: The Change in Dipole Moment Accompanying Metal to Ligand Charge Transfer Absorption in Pentaaminopyridylruthenium(II) J. Zeng,† N. S. Hush,†,‡ and J. R. Reimers*,† School of Chemistry F11 and Department of Biochemistry, UniVersity of Sydney, NSW 2006, Australia ReceiVed: June 20, 1996; In Final Form: September 4, 1996X
We have previously modeled solvent effects on the metal to ligand charge transfer (MLCT) spectra of Ru2+(NH3)5-pyrazine, Ru2+(NH3)5-pyrazine-H+, and Ru2+(NH3)5-pyridine and predicted the change ∆µ in dipole moment on excitation of Ru2+(NH3)5-pyrazine and Ru2+(NH3)5-pyrazine-H+. Prompted by a recent observation of ∆µ for Ru2+(NH3)5-pyridine by electroabsorption spectroscopy, we review the results of our previous simulations and evaluate ∆µ. The agreement found between the observed and a priori calculated values is not as close as found for the other complexes but, given the difficulties involved in both the theory and experiment, is very encouraging.
Inspired by the observation of electroabsorption spectra of inorganic complexes by Boxer et al.,1,2 we have developed a method for modeling specific solvation effects on electronic spectra. This method involves two steps, generation using molecular simulation techniques of an ensemble of configurations representing explicitly the structure of the solvent around the chromophore and evaluation of the solvent shift from the solvent structure. Earlier work3-7 concentrated on the second aspect and involved studies of azines in water, systems for which detailed experimental information is available. More recently, we have examined issues concerned with the reliable generation of liquid structures around the charged inorganic complexes, considering initially Fe2+(H2O)6 8 and Ru2+(NH3)5-pyridine.9 Lastly, this work culminated in the modeling10 of Boxer’s electroabsorption spectra of Ru2+(NH3)5-pyrazine and Ru2+(NH3)5-pyrazine-H+. Since then, Shin et al.11 have obtained the electroabsorption spectrum of Ru2+(NH3)5-pyridine; here, we reconsider our previous simulations of this complex and extract from them relevant results. The general nature of chargetransfer electronic transitions in inorganic complexes is of considerable current theoretical interest.11-14 In general,15-18 the molecular parameter that can be most reliably extracted from electroabsorption spectroscopy is the change in dipole moment on excitation, ∆µ. None of the properties are straightforward to calculate a priori, but ∆µ and the absorption band frequency ν are perhaps the simplest. Here, we consider these properties only. We evaluate these properties for a complex isolated in the gas phase and calculate from the liquid structure the solvent shift ∆ν and change in dipole moment induced by solvation. Originally,9 we performed a variety of liquid simulations based on three different solvent-solute intermolecular pair potentials. Of these, only the one named ΦESP (obtained using Kollman’s19-22 potential function parametrized with ab initio electrostatic-potential-derived charges) was deemed to provide a realistic liquid structure. The other functions were used to examine the dependence of the solvent shift on the liquid structure and are not considered here. During the solvent shift evaluation, charge distributions and polarizabilities are required describing the isolated complex in both its ground and excited †
School of Chemistry F11. Department of Biochemistry. X Abstract published in AdVance ACS Abstracts, November 15, 1996. ‡
S0022-3654(96)01860-6 CCC: $12.00
TABLE 1: Results of Calculations Using the Ground and dπ f π* MLCT Excited State SDCI, MCSCF, INDO/S-CI, and INDO/MRCI Wave Functions for Ru2+(NH3)5-Pyridinea solution electronic structure method geometry SDCI MCSCF INDO/S-CI INDO/S-CI INDO-MRCI obsd30
SCF SCF SCF mod mod
gas phase ν
solvent shift
80 38 ( 4 39 34 31
-5.0 -8 ( 1 -10.4 -6.8 -6.3
MLCT band-center 75 30 ( 4 29 27 25 24.5
a The gas-phase absorption energies, in 1000 cm-1, are evaluated at the SCF-optimized geometry and at a modified geometry10 in which the Ru-N bond lengths are set to the values observed for Ru2+(NH3)5pyrazine. The solvent shifts and hence the solution absorption energies, all in 1000 cm-1, are deduced by configurational averaging over the previously determined9 liquid structure ΦESP. The MCSCF results and error bars are obtained by consideration of three different MCSCF calculations and are not necessarily conservative (these extensive calculations are actually the simplest ab initio calculations that produce qualitatively reasonable results).
electronic states, and we performed9 ab initio singles and doubles CI (SDCI) and various multiconfigurational selfconsistent-field (MCSCF) calculations, as well as some closedshell INDO/S-CI23-25 calculations. The calculated gas-phase transition energies, solvent shifts, and predicted solution absorption frequencies (averaged over 500 liquid configurations) are shown in Table 1 (in the original,9 the columns in the corresponding table were unfortunately printed somewhat misaligned). Two different geometries were used to evaluate the gas-phase transition energies and electronic structures. Most calculations were performed at the ab initio-SCF-optimized structure, this being chosen, since no experimental structural information is available. We subsequently realized that in this the Ru-N bond lengths are long compared to those observed26 for Ru2+(NH3)5-pyrazine; a second structure was obtained by modifying the first to set the Ru-N (pyridine) distance to 2.006 Å and the Ru-N (NH3) distances to 2.16 Å. As is clear from Table 1, the transition energy, electronic structure, and solvent shift are sensitive to these geometry changes, and results obtained at the modified geometry are more consistent with the experimentally observed transition energy. In studies10 of Ru2+(NH3)5-pyrazine and Ru2+(NH3)5© 1996 American Chemical Society
Solvent Effects on Spectra
J. Phys. Chem., Vol. 100, No. 50, 1996 19293
TABLE 2: Revised Coordinates, in Å, for Ru2+(NH3)5-Pyridine in Terms of the Pyridyl Long (L), Short (S), and Normal (N) Axes, and the INDO/MCSCF Atomic Point Charges, in e, and Dipoles, in debye, Used in the Solvent-Shift Evaluation (A ≡ Ammonia) Coordinates
ground state
MLCT excited state
atom
L
N
S
q
µL
µN
µS
q
µL
µN
µS
Ru1 N2 C8, C9 C10, C11 C12 H13, H14 H15, H16 H17 NA3 HA3 HA3 NA4, NA5 HA4, HA5 HA4, HA5 HA4,HA5 NA6,NA7 HA6,HA7 HA6,HA7 HA6,HA7
1.6056 -.4004 -1.0905 -2.4639 -3.1695 -.5296 -2.9679 -4.2432 3.7556 4.1168 4.1321 1.6117 1.6189 .7845 2.4117 1.6216 1.6240 .8003 2.4292
-.0032 -.0004 .0010 .0026 .0031 .0007 .0033 .0041 .0061 .9493 -.4613 -1.5428 -2.4583 -1.4981 -1.4912 1.5363 2.4527 1.4903 1.4809
.0000 0 (1.1435 (1.1867 0 (2.0561 (2.1336 0 0 0 (.8124 (1.5007 (1.0745 (2.0765 (2.1151 (1.5008 (1.0771 (2.0849 (2.1050
-.5240 -.0491 .1169 .0294 .0921 .0175 .0445 .0483 -.0789 .1599 .1596 -.0840 .1602 .1698 .1578 -.0837 .1601 .1696 .1580
.0125 -.1550 .0384 .0333 .0464 0 0 0 .1782 0 0 .0029 0 0 0 .0046 0 0 0
.0007 .0001 -.0003 -.0001 .0001 0 0 0 .0023 0 0 -.1305 0 0 0 .1295 0 0 0
0 0 -.0540 -.0270 0 0 0 0 0 0 0 (.1177 0 0 0 (.1180 0 0 0
.0476 -.3482 .0287 .0197 -.0204 .0128 .0452 .0471 -.0976 .1634 .1606 -.0769 .1618 .1730 .1606 -.0765 .1616 .1729 .1607
.0225 -.1279 .0414 .0339 .0442 0 0 0 .1839 0 0 .0031 0 0 0 .0047 0 0 0
.0003 .0006 -.0001 -.0002 0 0 0 0 .0023 0 0 -.1270 0 0 0 .1261 0 0 0
0 0 -.0546 -.0265 0 0 0 0 0 0 0 (.1150 0 0 0 (.1153 0 0 0
pyrazine-H+, as well as in studies of the bacterial photosynthetic reaction center27 and for generalized electron-transfer problems,28 we have found that for the spectra of processes involving long-range electron transfer (eg., MLCT transitions), it is essential in an INDO framework to perform MCSCF-type calculations. It is, of course, well-known that this is required for ab initio calculations, but the same principle also applies to INDO calculations: the configuration interaction must be evaluated in a manner that treats the ground and excited states equivalently. Within the INDO framework, such MCSCF calculations, using fixed-weight configurations, are easily performed using established25 restricted-open-shell technology. In this approach for Ru2+(NH3)5-pyridine, all possible lowspin configurations involving two electrons in the metal dπ and ligand lowest-unoccupied molecular orbitals are considered. Results obtained by applying this method (named “INDO/ MRCI”) using Krogh-Jesperson’s parameters29 via our own multireference configuration-interaction INDO program are also shown in Table 1, together with the solvent shift estimated using the ΦESP liquid structure. The geometry used and the calculated ground-state and excited-state atomic point charges and dipoles are given in Table 2; ground- and excited-state polarizability differences evaluated as before9,10 are (115, 70, 30) and (80, 45, 22) au, respectively, in the (L, S, N) axis system (see Table 2). Assuming that the geometric effect provides the same frequency lowering (5000 cm-1) to the ab initio MCSCF results as found for INDO/S-CI, the combined ab initio and INDO/ MRCI results indicate a solvent shift in the range of -6000 to -8000 cm-1 and a solution transition energy near 25 000 ( 4000 cm-1; the band is observed30 at 24 500 cm-1. Similar agreement is also found10 between the calculated and observed transition energies in Ru2+(NH3)5-pyrazine and Ru2+(NH3)5pyrazine-H+. These calculations are a priori and contain no arbitrarily adjustable parameters. As is found also for the spectra of azines in solution (see, for example, ref 4), the most difficult step in the computational proceedure appears to be the reliable determination of the chromophore’s solvated geometry and unperturbed gas-phase electronic structure. Calculated and observed11 values for the change in dipole moment on excitation are shown in Table 3. This includes both results previously obtained9 and new results obtained using the INDO/MRCI charge distributions. Results previously obtained2,10,16 for Ru2+(NH3)5-pyrazine and Ru2+(NH3)5-pyrazine-H+ are also included for comparison. Note that for Ru2+(NH3)5-pyridine the differences found between the stan-
TABLE 3: Calculated and Observed Dipole Moment Changes ∆µ, in debye, Accompanying the MLCT Absorption of Some Ru2+(NH3)5 Complexes condensed phase complex pyridine
pyrazine pyrazine-H+
electronic structure method geometry SDCI MCSCF INDO/S-CI INDO/S-CI INDO-MRCI MCSCF INDO-MRCI MCSCF INDO-MRCI
SCF SCF SCF modd modd obsd26 obsd26 obsd26 obsd26
gas phase
calcd soln
obsd glassa
7.0 11 ( 2 13.5 8.7 8.1 8.8 8.6 1.0 -1.2
7.1 5.5 5.5 0.3 0.8
3.4b 3.5b, 4.8 ( 1.3c 3.5b, 4.8 ( 1.3c
