J. Phys. Chem. 1990, 94, 1357-1366 it is impossible to observe intensity changes in the spectrum of the base, which might suggest that higher rotational states of the bases are permitted within the complex. The shifts of the Q band that have been observed for Lewis bases in these studies are greater than what has been observed for van der Waals molecules such as Xe...H2I1 and even H2-.HF’o (see Table 111). We suspect that the binding energies between H, and these bases is at least as large as that found for H2-Xe. These associations should therefore be observable in the absence of a matrix. If there is a significant binding energy holding hydrogen to the base, then it must also follow that the nature of the matrix is not a major factor in the observed spectrum. Expect for the expected shift of all vibrational features in switching to krypton from argon, the spectra of the base complexes with hydrogen are the same. One wonders if the trends that are observed in these studies will continue for much stronger bases that by necessity must bear an ionic charge. In that context, it is surprising has yet to be observed29even though the proton affinity that H3’ of the shifted Q band that of H- is 1674 k J / m 0 1 . ~ ~ ~In~ light results from the adsorption of H, in zeolites that are ascribed to interactions of Na+ with H2,32it would be interesting to observe the result of adsorbing H2 in a substance with free anions such as sodalite or noselite. Finally, significant interactions must be present between hydrogen and aqueous, charged bases because hydrogen isotope exchange is observed.” We note one example of a hydrogen complex in which the higher rotational states of the hydrogen are more strongly bound than the J = 0 state. For the T-shaped association H2-.HF, the state that is correlated with the J = 1 state of dihydrogen is more (29) Chalasifiski, G.; Kendall, R. A.; Simons, J. J . Phys. Chem. 1987, 91, 6151. Kendall, R. A.; Simons, J.; Gutowski, M.; Chakasifiski, G. J . Phys. Chem. 1989, 93, 621. Chalasifiski, G.; Gutowski, M. Chem. Rev. 1988,88, 943. (30) Huheey, J. E. Inorganic Chemistry, 3rd ed.; Harper & Row: New York, 1983; p 301. (3 I ) There have been several reports of coordinated H3, in all cases with an open arrangement of the hydrogen atoms rather than an arrangement as found in H3+. Chaudret, B.; Commenges, G.; Jalon, F.; Otero, A. J . Chem. Soc., Chem. Commun. 1989, 210 and references cited therein. (32) Cohen de Lara, E. R. Mol. Phys. 1989, 66, 479. (33) Halpern, J. Adu. Catal. 1959, I I , 301.
1357
strongly bound than is the state originating from the J = 0 state.34 This distinction arises from a very anisotropic potential and from the fact that the state that is correlated to the J = 0 state permits the H-H bond to be oriented in an unfavorable orientation with respect to HF.35 In this light, it is proper to question whether p-H2forms a van der Waals complex at all with a base. We have unsuccessfully attempted to observe the Raman spectrum of perturbed p-H2 in argon matrices of trimethylamine and p-H2, which had been formed by 02-catalyzed relaxation. The signal-to-noise ratio was such that we should have been able to observe a signal of perhaps half of the expected intensity. Thus, it is difficult to claim that p-H2 does not form a van der Waals complex with bases. A definitive conclusion must await better Raman measurements. We note that a perturbed H D was detected by Raman methods in a mixture of H D and trimethylamine. Taken by itself, this experiment would suggest that p-HZshould also associate with base in an analogous fashion. In conclusion, we have been able to observe interactions of hydrogen with a series of Lewis bases. These associations must probably be classed as van der Waals complexes that are not so rigid that the rotation of the hydrogen molecule is quenched. In fact, the H-H stretching vibration becomes infrared-active as a consequence of this internal motion. Thus, the zero-phonon Q bands of H2 and D2 are readily observable while that of H D is not so easily observed. The magnitude of the frequency shift of the Q band correlates with the base strength of the base, suggesting that the interaction is based on hydrogen acting as an acid.
Acknowledgment. Support for this work was provided by the donors of the Petroleum Research Fund, administered by the American Chemical Society (R.L.S.), and by the Louisiana Education Trust Fund, administered by the Louisiana Board of Regents (R.L.S. and S.L.W.). Registry No. H2, 1333-74-0; NH3, 7664-41-7; C5H5N, 110-86-1; (CH3)3N, 75-50-3; H20, 7732-18-5; D, 7782-39-0. (34) Lovejoy, C. M.; Nelson, D. D., Jr.; Nesbitt, D. J. J . Chem. Phys. 1987,87, 5621. (35) Lovejoy, C. M.; Nelson, D. D., Jr.; Nesbitt, D. J. J . Chem. Phys. 1988, 89, 7 180.
Normal-Coordinate Analyses of the Ground and %LCT Excited States of Tris(bipyridlne)ruthenium( I I ) Dennis P. Strommen,’ Prabal K. Mallick? Gerald D. Danzer,2 Richard S. Lumpkin,* and James R. Kincaid*i2 Chemistry Department, Marquette University, Milwaukee, Wisconsin 53233, and Chemistry Department, Carthage College, Kenosha. Wisconsin 53140 (Received: June 15* 1989) The resonance Raman and time-resolved resonance Raman (TR’) spectra of the title compound and nine of its deuteriated derivatives as well as the I5Nand 15N2-3,3’-2H2analogues are reported. These data are used to refine the previously reported ground-state force field and to derive a corresponding force field for the anion-radical fragment of the ’MLCT excited state. These force fields reproduce the observed frequencies with average errors of 1%. In addition, the derived fields adequately reproduce the observed shifts upon ’MLCT-state formation, in general giving shifts of appropriate sign (some shifts are to higher frequencies) and approximately correct magnitudes. Finally, the excited-state normal-mode formulations (potential energy distributions) are compared with those derived for the ground-state species, and structural implications for the ’MLCT state are discussed.
-
Introduction During the past decade, intense activity has been directed toward understanding the structure and reactivity of metal-toligand charge-transfer (’MLCT) excited states of tris(bipyridine)ruthenium(II), [Ru(b ~ y ) , ~ *, + ]and related c~mplexes.~” ( I ) Chemistry Department, Carthage College, Kenosha, WI 53140. (2) Chemistry Department, Marquette University, Milwaukee, WI 53233.
0022-3654190/2094-1357$02.50/0
One of the most important contributions was made by Dallinger, Woodruff, and co-~orkers,~.’ who demonstrated the utility of (3) Kalyanasundaram, K. Coord. Chem. Rev. 1982, 46, 159. (4) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 8 5 . ( 5 ) DeArmond, M. K.; Hanck, K. W.; Wertz, D. W. Coord. Chem. Rev. 1985, 64, 65.
0 1990 American Chemical Society
1358 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990
time-resolved resonance Raman (TR3) spectroscopy for interrogation of 3MLCT state vibrational modes. Our group,s as well as Mabrouk and Wrighton? subsequently demonstrated that such measurements are ideally suited for documenting selective population of the various ligand-localized excited states in bis-heteroleptic complexes. Recently, several similar studies have been reported. I Given this demonstrated utility and the potential relevance of excited-state vibrational frequencies for obtaining detailed structural parameters of 3MCLT states,I2 we have undertaken a systematic study of excited-state vibrational modes for the parent compound [ R ~ ( b p y ) ~*~and + ] several related complexes. Utilization of vibrational data to obtain insight into the structure and dynamics of these excited states will require a reasonably accurate evaluation of the normal-mode formulations, a generally elusive goal for polyatomic systems of this size.13 The most effective approach to this general problem involves acquisition of vibrational data for a large number of isotopically labeled analogues of the compound of interest, although this is often a synthetically demanding task. Nevertheless, in the present work, we have utilized this approach to investigate the excited-state vibrational modes of the parent compound of this class, Le., R ~ ( b p y ) , ~ +We . report here the TR3 spectra of R ~ ( b p y ) , ~ +its, lSN-substituted analogue, nine selectively deuteriated species, and the complex of the “doubly labeled” ligand, [ ‘SNz]-3,3’-dideuterio-2,2’-bipyridine. These data provide the basis for a normal-coordinate calculation (NCA) of the anion-radical fragment of the 3MLCT excited state and further refinement of our earlier NCA of the ground state.I4 As in our previous report of the NCA of the ground-state species, the derivation of the final (modified valence) force field employed a systematic procedure that restricts perturbations in diagonal and off-diagonal elements by maintaining fixed relationships among these during the minimization procedure. The resulting force field, whose elements are realistically interrelated, yields calculated frequencies that are in quite satisfactory agreement with those observed.
Experimental and Calculational Procedures Materials. The preparation and purification of most of the compounds studied have been described p r e v i o u ~ l y . ’ ~The ~~~ tris( 4,4’-dideuterio-2,2’-bipyridine) complex was prepared from the corresponding 4,4’-dibromo-2,2’-bipyridine analogue9 via zinc dust reduction in 2 H 2 S 0 4 / 2 H 2 0s01ution.l~ The 3,3‘,4,4’- and 3,3’,6,6’-tetradeuteriatedanalogues (from the corresponding dideuteriated analogues) were generated via exchange of the 3,3’-protons with a 1/10 (v/v) solution of 0.10 M N a 0 2 H in 2 H 2 0 and hexadeuteriated dimethyl s ~ l f o x i d e . ~The ~ ~ ’4,4’,5,5’,6,6’~ hexadeuterio derivative was prepared from the perdeuteriated complex by the same procedure with natural-abundance reagents. Spectral Measurements. Ground-state Raman spectra were obtained as previously described.14 The excited-state Raman spectra were obtained from aqueous solutions that were (1-3) X (6) Dallinger, R. F.; Woodruff, W. H. J. Am. Chem. Soc. 1979,101,4391. (7) Bradley, P. G.; Kress, N.; Hornberger, B. A.; Dallinger, R. F.; Woodruff, W. H. J. Am. Chem. Soc. 1981, 103, 7441. (8) McClanahan, S. F.; Dallinger, R. F.; Holler, F. J.; Kincaid, J. R. J. Am. Chem. Soc. 1985, 107.4853. (9) Mabrouk, P. A,; Wrighton, M. S. Inorg. Chem. 1986, 25, 526.
(IO) Chung, Y. C.; Leventis, N.; Wagner, P. J.; Leroi, G. E. J. Am. Chem. SOC.1985, 107, 1416. ( 1 1 ) Kumar, C. V.; Barton, J. K.; Gould, I. R.; Turro, N. J.; Van Houten, J. Inorg. Chem. 1988, 27, 648. (12) Casper, J. V.; Westmoreland, T. D.; Allen, C. H.; Bradley, P. G.; Meyer, T. J.; Woodruff, W. H. J. Am. Chem. SOC.1984, 106, 3492. ( 1 3) Woodward, L. A. Infroduction to the Theory of Molecular Vibratiom and Vibrational Spectroscopy; Oxford University Press: London, 1972. (14) Mallick, P. K.;Danzer, G. D.; Strommen, D. P.; Kincaid, J. R. J . Phys. Chem. 1988, 92, 5628. (15) McClanahan, S . ; Kincaid, J. J. Ramon Specfrosc. 1984, 15, 173. (16) Danzer, G. D.; Golus, J. A,; Strommen, D. P.;Kincaid, J. R. J. Raman Specirosc., in press. (17) Bak, B. J. Org. Chem. 1956, 2/, 797. (18) (a) Constable, E. C.; Seddon, K . R. J. Chem. Soc., Chem. Commun. 1982. 34. (b) Nord, G. Department of Inorganic Chemistry, University of Copenhagen, Denmark, private communication.
Strommen et al. M in the metal complex. The sample solution was spun in an N M R tube, and the scattered light was collected with a 135O backscattering geometry and a conventional two-lens (quartz) collection system. Dispersion was achieved by using a Spex Model 1403 double monochromator equipped with a Hamamatsu Model R928 photomultiplier tube. The photomultiplier tube output was typically amplified by a factor of 25 via a Stanford Research Systems (SRS) Model SR240 fast preamplifier and then averaged by a S R S Model SR250 gated integrator and boxcar averager. The boxcar was triggered by the pretrigger output on the Nd:YAG laser (the excitation source), and the internal delay on the boxcar was adjusted so that the signal from the photomultiplier tube coincided with the 12-11s gate width. Typically, 50 events were averaged per point (0.50-cm-’ increments). The averaged output was passed through a locally constructed voltage-to-frequency converter (analogous to the Spex Model DM103) to allow the use of a Spex Model D M l B computer controller for data acquisition. The preamplifier, boxcar, and voltage-to-frequency converter were powered by a S R S Model SR280 power supply and display module, all of which were contained in a standard NIM-bin. A Quanta-Ray (Spectra Physics) Model DCR-3A Nd:YAG laser (operated at 20 Hz) was used as the excitation source. This laser has a near-Gaussian beam profile unlike the doughnut-shaped beam profile on earlier Nd:YAG lasers. The 1064-nm fundamental was frequency-tripled via a Spectra Physics Model HG-2 harmonic generator equipped with KD*P crystals. The collinear fundamental and harmonics were separated via a Spectra Physics Model PHS-1 prism harmonic separator. The 354.7-nm third harmonic was directed to the sample via several 90° SI-UV quartz prisms. Typically, the Q-switch delay on the laser was adjusted to produce an average power (at the laser) of 125-175 mW of the third harmonic. Calculation. Normal-coordinate calculations employed the well-documented Schactschneider programs.19 Derivation of the force field for the ground state was accomplished in a manner similar to, but slightly different from, that previously described.14 Briefly, an initial set of diagonal stretching force constants (Le., f(C-C) andflC-N)) were estimated from logfvs log r and other well-known relationships (see ref 14 for details). Initial values for the 1-2 stretchstretch interaction constants (i.e.,h,z) were estimated according to the f ~ r m u l a f i = , ~1/20cfl,l + &). The other stretch-stretch interaction constants cfi,3 and fi,4)were linked to thefl,2 constants through fixed relationships +f2,3)]. incorporated into the Z matrix,19 e.g:,fIs = -1/2[,1/2(fl,2 Note that 1-3 interactions are negative and 1-4 interactions are positive. lnitial values for all other constants (i.e., diagonal bending, C-H stretching, C C H bending, and stretch-bend interactions) were transferred from the previously reported force field.14 The systematic refinement procedure was controlled in the following manner. The general strategy in the initial cycles is to allow a certain subset of constants to vary while all others are held at fixed values. Perturbations are controlled, both in magnitude and number, in order to prevent the development of physically unacceptable values for a particular force constant. Force constants that exhibit a tendency to diverge at any stage of the refinement procedure are temporarily removed from the perturbation. Owing to the relatively large effects of 6(CCH) wagging (and stretch-wag interaction constants) on the absolute values of the calculated frequencies and given the large number of specifically deuteriated analogues, these constants were those initially entered into the perturbation routine. Inasmuch as initial values for angle bending constants are difficult to estimate, this subset (as well as v(Ru-N)) was next entered into the perturbation procedure, all others being held constant. The purpose of these initial perturbation cycles is to obtain a set of angle bending and stretch-wag interaction constants that yield an approximate fit, when taken in combination with diagonal stretching and stretch-stretch interaction constants that have been (19) Schachtschneider, J. H. Technical Reports No. 231-64 and 57-65, Shell Development Co., Emoryville, CA.
Ground and )MLCT Excited States of R ~ ( b p y ) ~ ~ + fixed at values that are typical of aromatic systems. Following these initial iterative routines, small manual adjustments were made on individual elements with the aid of a locally generated program that provides calculated frequency changes for each mode for a fixed (small) force constant change. Primarily, this was done in an attempt to adjust the calculated Ai5N shifts to those observed. Obviously, it is unreasonable to expect that an accurate fit can be obtained from fixed, estimated stretching and stretch-stretch interaction constants. In fact, at this stage of the adjustment procedure, the overall error in the calculated frequencies was -2%. Thus, following these initial adjustments, the iterative procedure was further employed as follows. The stretchstretch interaction constants, the G(CCH) wagging, and the stretch-wag interaction constants were allowed to fluctuate simultaneously. Furthermore, no assumptions were made regarding relative values within each of these subsets. Thus, all stretch-stretch constants were set to an initial value of 0.60, the three G(CCH) wagging constants to 0.51, and the stretch-wag interaction constants to 0.1 1. These constants were permitted to vary within a range of acceptable values; Le., stretchstretch constants were allowed to vary between 0.25 and 0.80, G(CCH) between 0.49 and 0.54, and stretchwagging constants between 0.08 and 0.14. During these iterative cycles, if a constant approached a limiting value, it was fixed at this value and the iteration continued. After several cycles, the average error was reduced to -1.4%. Finally, the diagonal stretching constants were allowed to enter the perturbation routine while holding all other constants at the values derived in the previous series of perturbations. The iterative cycles involving diagonal stretching constants were continued while carefully monitoring the relative values of the constants. During this set of iterations, the relative values of the diagonals remained as initially estimated from the available structural information. Finally, the adjusted diagonal stretching constants were held constant and the stretch-stretch, wagging, and stretch-wag interaction constants were allowed to fluctuate. The final error in the calculated frequencies was 1.2%. When the NCA of the 3MLCT state is conducted, it is necessary to employ a slightly different approach inasmuch as there is obviously no structural data available from which estimates of initial values for stretching and stretch-stretch interaction constants can be derived. In fact, as will be discussed later, it is possible to utilize vibrational spectroscopic data, along with results of normal-mode calculations, to obtain excited-state structural information. In this regard, it is important that no assumptions are made concerning 3MLCT-state structure. Given this condition, the following procedure was used to derive the 3MLCT-state force field. The final ground-state force field was used as the initial force field for the 3MLCT-state calculation, with the following exceptions. All C-C stretching force constants were assigned an initial value of 5.8 mdyn/A and both C-N stretching force constants were set equal to 5.4 mdyn/A. That is, a general lowering of the force constants is expected based on the general lowering of frequencies in the 3MLCT state (vide infra), but no assumptions are made regarding relative bond strengths (Le., all u(C-C) constants are set to the same initial value). Similarly, all stretch-stretch interaction constants were set equal to 0.50 mdyn/A, a value that is slightly lower than the average value for the ground-state force field. The G(CCH) wagging and stretch-wag interaction constants were set equal to 0.51 mdyn/(A rad2) and 0.110 mdyn/rad, respectively, but the other angle bending and stretch-band interaction constants were fixed at the values derived for the ground-state force field and were not permitted to vary during the 3MLCT-stateforce field refinement. This restriction seems appropriate inasmuch as bending force constants are not expected to change substantially in the present case. This expectation has been previously supported by studies of the vibrational spectra of systems undergoing similar changes (e.g., neutral parent to anion radical) wherein pure bending modes exhibited insignificant frequency shifts.20
-
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1359 Thus, the initial point for the 3MLCT-state force field refinement employs a “ground-state-like” set of bending and stretch-bend interaction constants and a set of stretching an stretchstretch interaction constants that do not incorporate any assumptions regarding relative bond strengths. From this point, no manual adjustments are made and only the iterative fitting routine is used to derive the final force field. During the iterative cycles, the stretching, stretch-stretch, wagging, and stretch-wag interaction constants were allowed to fluctuate simultaneously. The iterative procedure was allowed to continue unless any force constant assumed an unreasonable value; Le., the values for stretching constants are expected to fall between 5.0 and 7.0 mdyn/A, the stretch-stretch between 0.25 and 0.70 mdyn/A, wagging between 0.48 and 0.55 mdyn/(A rad2), and stretch-wag constants between 0.08 and 0.14 mdyn/rad. At the end of the final iterative adjustment procedure, the average error of the calculated frequencies was -0.8%.
Results and Discussion A . Ground-State Normal Modes. The ground-state resonance Raman (RR) data for the additional deuteriated analogues and the two ISN-labeled complexes were used to further refine our previously derived force field.I4 The calculated frequencies for all compounds are in excellent agreement (- 1% average error) with those observed (Table I). It is especially satisfying to note that the observed AI5N shifts for both the ]H8 and 3,3’-’H2 complexes are well reproduced. The newly derived ground-state force field, based on a much more extensive set of vibrational data, yields force constants and PED’S that differ somewhat from those published p r e v i ~ u s l y .For ~ ~ this reason, final ground-state normal-mode formulations are illustrated in Figure 1. B . Excited-State Normal Modes. Before proceeding to a summary of the 3MLCT-state NCA, it is necessary to summarize the spectral data and assignments of fundamentals. A rather detailed discussion is warranted not only by the fact that the original7 interpretation of the TR3 spectra has been challenged recently21abut also because it is inappropriate to select 3MLCTstate fundamental modes from the rather complicated TR3 spectra without clearly justifying the choices made. I . General Spectral Patterns. The resonance Raman spectra of the ground- and 3MLCT-excited-state species, for the parent compound, are shown in Figure 2. The ground-state spectrum in trace B was obtained with 350.7-nm excitation that corresponds closely to the 354.7-nm excitation used for the 3MLCT spectrum shown in trace C. As was originally pointed out by Dallinger, Woodruff, and c o - ~ o r k e r sthe , ~ TR3 spectrum (trace C) is most reasonably interpreted in terms of a ligand-localized 3MLCT state, Le., [ R ~ ~ + ( b p y ) ~ ( b p y - )Thus, ] ~ + . the observed spectrum is comprised of two sets of bands. The set of more intense features (labeled vi, in trace C) is associated with the anion-radical fragment, while the set of weaker lines, whose frequencies essentially match those of the ground-state species, is considered to arise from the two remaining coordinated (“neutral-ligand”) bipyridines. The bpy- modes are strongly enhanced as a consequence of the nearly direct resonance of the 354.74111 laser excitation with the bpya-a* transition located at -360 nm.22 The a-a* transitions associated with the nonradical bipyridine fragments of the 3MLCT state are located near 320 n m z 2 The unfavorable resonance condition results in relatively weak enhancement of the neutral-ligand modes with excitation at 354.7 nm. This interpretation has been challenged in a recent report by Hopkins and co-workers,21who suggest that the features previously’ assigned to the neutral-ligand fragments actually arise from a residual population of ground-state molecules, arguing that pure 3MLCT-state spectra are not available without resorting to spectral (20) Jeanmaire, D. L.; Van Duyne, R. P. J . Am. Chem. SOC.1976, 98, 1029. (21) (a) Orman, L. K.; Hopkins, J. B. Chem. Phys. Lett. 1988, 149, 375.
(b) Orman, L. K.; Chang, Y. J.; Anderson, D. R.; Yabe, T.; Xu, Xiaobiny; Yu,Sou-Chang; Hopkins, J. B. J . Chem. Phys. 1989, 90, 1469. (22) Braterman, P. S.; Harriman, A,; Heath, G. A,; Yellowlees, L. J. J . Chem. SOC.Dalton Trans. 1983, 1801.
1360
The Journal of Physical Chemistry, Vol. 94, No. 4 , 1990
Strommen et a]. 1491
1563
L I320
1276
Ill0
1067
I I76
1045
w Figure 1. Normal-mode descriptions for ground-state Ru(bpy)3Z+-Al symmetry class.
subtraction techniques. Further experiments, currently in progress , ~ ~needed to resolve this issue. in our l a b ~ r a t o r yare Regardless of the origin of the neutral-ligand bands in the TR3 spectrum, it is generally agreed that the bpy- fragment may be treated separately in terms of normal mode composition (Le., there is no mixing of the bpy- fragment modes with those of the remaining coordinated bipyridines). Thus, the present discussion is focused on the normal modes of the isolated Ru(bpy-) fragment and the frequencies of interest are those labeled vi, in Figure 2 (trace C). 2. Specific Spectral Assignments for the Anion-Radical Fragment. Adherence to the Dallinger/Woodruff interpretation of the TR3 spectrum leads to the expectation of a set of "bpy--like" modes superimposed on an essentially ground-state-like spectrum, the latter associated with modes of the two remaining coordinated (23) Danzer. G. D.; Kincaid, J. R. J . Phys. Chem., submitted for publication.
766
------I
668
I
71947
L
"neutral bipyridines".' Thus, it is instructive to compare the TR3 spectrum of Ru(bpy),*+ with the R R spectrum of the lithium bipyridine anion radical (Li(bpy)). In fact, Dallinger, Woodruff, and co-workers presented such a comparison in their work' and (mainly on this basis) argued that the optical electron was highly localized on a single bipyridine, Le., the vi, frequencies are quite similar to those of Li(bpy). Our results and interpretation are in essential agreement with this conclusion. However, the availability of the large data set for all of the isotopically labeled complexes, as well as the corresponding R R spectra of lithium anion radicals for several isotopically labeled bipyridines, reveals interesting and previously unrecognized correlations of groundand 3MLCT-state frequencies (Le., correlations of vi with vi,). The TR3 spectrum of R ~ ( b p y ) , ~(trace + C) is compared in Figure 2 with the R R spectrum of Li(bpy) in T H F (trace D). In order to clarify the discussion, the features observed in trace D are labeled as vi,,. The details of the preparation and spectral acquisition conditions for Li(bpy) and several isotopically labeled analogues will be given in a separate report.I6 At this time, it
The Journal of Physical Chemistry, Vol. 94, No. 4 , 1990 1361
Ground and 3MLCT Excited States of R ~ ( b p y ) , ~ +
l
r
t 500 00
"17'
y
7
I1 i i
w
110000
~
I700 00
Raman shift (cm-')
Figure 2. Resonance Raman and time-resolved resonance Raman spectra: (A) 983-cm-l band of the sulfate internal standard; (B) 1043-~m-~ band, polarized while the 1028-cm-l band is depolarized (this latter band was inadvertently assigned to u I S in our earlier workI4);(C) region between arrows obtained with higher resolution conditions; (D) bands due to T H F denoted by an asterisk.
is necessary only to note the following points. The TR3 spectrum of the 'MLCT state (trace C) is an approximate composite of the spectra shown in traces B and D. Thus, the neutral-ligand modes are readily identified at frequencies observed in the ground-state spectrum (trace B). The set of features labeled vi, exhibit frequencies and intensities that are comparable to the bands observed in trace D. It is useful to compare the spectra shown in traces B and D in order to establish the behavior of the individual modes upon anion-radical formation. The four highest frequency modes of coordinated neutral bipyridine are observed at 1608 ( u s ) , 1563 (&), 1491 (u,), and 1450 cm-I (us), the last being enhanced with 350.7-nm excitation but not IMLCT excitation. In the case of Li(bpy) (trace D), the four highest frequency modes occur at 1554 (us,,), 1502 (Vg"), 1485 (u7") , and 141 2 cm-l (Vg"). Thus, while large downshifts are observed for u s , Vg, and us, only a small downshift (6 cm-I) is observed for u7. We wish to point out that the weak feature observed at 1590 em-I in trace D is not assigned to us,, but, owing to its large ISN shift,16 is ascribed to an overtone or combination band. In the TR3 spectrum (trace C), features are observed at 1548 and 1427 cm-' that are readily ascribable to us, and us?. The cluttered region near 1500 cm-' requires close inspection. Figure 3 gives an expanded view of the spectra of several complexes obtained with higher resolution than that shown in Figure 2C. As can be seen in Figure 3A, this region actually contains at least three features located at 1506 (Vg'), 1495 (u7,), and 1482 em-'. The 1482-cm-l feature is assigned to the overtone of the relatively strong 742 cm-' ( u I 6 , ) mode. As can be seen in Figure 3B, this assignment is supported by the large I5N shift; Le., VI,' exhibits a 4- or 5-cm-' I5N shift. We note that a feature ascribable to 2uI6,is observed in the TR3 spectra of most of the deuteriated analogues. This region also contains some contribution from u7 (Le., the neutral-ligand mode). It is apparently not shifted from its ground-state frequency and is obscured by overlap with u7, at 1495 cm-I. This interpretation is also supported by the fact that, in the TR3 spectra of several of the deuteriated species, an isolated u7 mode is observed within 1 cm-' of its ground-state frequency
u
m
I :
1362 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990
Strommen et al.
e,
f
0 *
1;61
I
I ’ I
E M
. I
-
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0 + P
?
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1522
1475
1550
Raman shift (cm-’) Figure 3. TR3 spectra of R~(bpy),~+ and several of its isotopically sub-
stituted analogues.
(e.g., see traces C and D in Figure 3). Of most interest is the behavior of v9 upon anion-radical formation. The frequency for coordinated bpy is 1320 cm-’ (Figure 2A,B). The Li(bpy) derivative (Figure 2D) exhibits a band at 1350 cm-’ (v9”), while the TR3 spectrum of R ~ ( b p y ) , ~(Figure + 2C) contains a similar band at 1365 cm-l, which is assigned to u9,. The nature of this mode (Le., the PED) and the structural implications of the large shift to higher frequency warrant further discussion (vide infra). Other (uit) modes in the TR3 spectra of the various isotopomers are readily correlated with corresponding (vi) modes of the coordinated bipyridine and exhibit frequencies that are comparable to the corresponding modes of Li(bpy). It does not seem necessary to provide a detailed discussion of all of these specific assignments. Rather, it is sufficient to point out that all of the observed bands that are not assigned to bpy- (see Table 11) correspond to features that are ascribable to the neutral-ligand fragments. In addition, it is emphasized that these latter features are observed within a few cm-’ of their corresponding ground-state frequencies. Finally, in a few cases, modes are not observed (see Tables I and 11). This is especially true in the low-frequency region (C700 an-’) of the TR3 spectra, where no bands assignable to uI7,,vI8,, ui9,,and uZV were observed, although the corresponding modes (uI7-ul9)are observed for most of the ground-state species. This difference is related to the fact that these low-frequency modes are more effectively enhanced by an MLCT resonant transition than by the a-a* resonant transition for the bpy- fragment of the 3MLCT excited state.
e 2
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3.c
e,
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e
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n
5
D
58
f
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The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1363
Ground and 3MLCT Excited States of R ~ ( b p y ) ~ ~ +
-
0
4
5
7
9
11
13
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formation.
The essential points of the above discussion are that the R R bands of Li(bpy) can be directly correlated with bands of similar frequency in the TR3 spectra of Ru(bpy),*+ for all of the isotopomers studied (see Table I1 and ref 16) and that while there is a general lowering of frequencies upon anion-radical formation, certain modes exhibit substantial shifts to higher frequencies. 3 . 3MLCT-State Normal-Coordinate Calcularion. As mentioned earlier, the 3MLCT-state calculation is plagued by a somewhat larger uncertainty (relative to the ground state) in selecting fundamentals and by a lack of corresponding IR data. Furthermore, whereas reliable structural data is available for the ground-state species, none is directly available for the 3MLCT state. This represents an additional handicap in that initial values of diagonal stretching constants (and therefore the linked stretch-stretch interaction constants) cannot be estimated from the log f vs log r relationship. It may be argued that it is valid to employ structural parameters of "3MLCT-state models", such as low-oxidation-state metal complexes of bip~ridine:~to estimate
)MLCT structure. However, inasmuch as one of our ultimate objectives is to utilize the present NCA to extract 3MLCT structure from vibrational data, it is obviously inappropriate to bias the calculation with initial estimates of force constants that are based on an assumed structure. In this regard, it is probably worth pointing out that the utilization of the ground-state structure to generate a G matrix does not bias the calculation. Thus, we have confirmed the fact that small (C5%) alterations in bond lengths and bond angles do not lead to significant variations of the calculated frequencies (provided the correct symmetry is maintained). The frequencies and the potential distributions are essentially determined by the set of force constants employed. The observed and calculated anion-radical frequencies of all of the compounds studied are given in Table 11. As can be determined by inspection of the table, agreement between observed and calculated frequencies is quite satisfactory, given the large number of isotopically labeled analogues included and the relatively
(24) Chisholm, M. H.; Hoffman, J. C.; Rothwell, I . P.; Bradley, P. G.; Kress, N.; Woodruff, W. H. J . Am. Chem. SOC.1981, 103, 4945.
( 2 5 ) Rillema, D. P.; Jones, D. S.; Levy, H . A. J . Chem. SOC.,Chem. Commun. 1979, 849.
1364 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 TABLE 111: Ground and ’MLCT Potential Energy Distributions (PED) for Ru(bpy),*+
% PEDa
7‘
1
11
8 8‘ 9 9‘
12 13 14 15 16 17 18
1Ob
IO’
19
11 1 I‘
15 15‘ 16 16‘ 17 17’ 18 18’ 19 19‘ 20 20‘
symbolb
2 3 4 5 6 7 8 9 10
6 6’ 7
14‘
TABLE I V Force Constants“
no.
mode no. 5 5‘
12 12’ 13 13’ 14
Strommen et al.
.
e
,
,
6 ( c c H j 30; v(c3-c4) 14, u(C6-N) 11, u(cg-cf,) 10 6(CCH) 26; V(c3-c~) 9, U(C6-N) 13, U ( C & , ) 13 y(C4-C5) 18, u(C2-N) 13; 6(CCC) 12, G(C2CjH) 11, G(CsC6H) 10 4C4-CS) 1 I , v(C2-N) 12; B(CCC) 24, G(C2C3H) 7, G(C&H) 7 ) G(CCC) 12 ~ ( C s - c , ) 18, p(C4-CS) 11, v ( C ~ - C ~11; u ( C ~ - C ~13. ) u(C4-CS) 28, v ( C 4 4 ) 9; S(CCC) 12 6(CCC) 41; u(C2-N) 13, u ( C ~ - C ~12; ) B(CCH) 9 S(CCC) 35; u(C2-N) 13, Y ( C ~ - C ~26; ) G(CCH) 8 6(CCC) 44: u ( C ~ - N ) 11, ~ ( R u - N ) I O 6(CCC) 43; u(C2-N) 12, v(R-N) 8 u(Ru-N) 24, ~(C2-Czt) 20, U(c6-N) 12; G(CCC) I I ~ ( R U - N )20, u(C2-C2t) 14, v(C,-N) 13; v(CCC) 11 G(C6NRu) 50, G(NC2Ci) 10; ~ ( R U - N )14 ~ ( C ~ N R 47; U ) B(NC2C2,) IO; ~ ( R u - N ) 16 ~ ( R u - N )29; d(CjC2C2,) 18, G(C6NRU) 11, G(NC2C3) I O u(Ru-N) 30: G(C3C2C2r) 19, d(C6NRU) 1 I , h(NC2C3) I O
“The sum over diagonal force constants for a given mode is, in general, greater than 100%owing to negative contribution from off-diagonal elements (i.e., interaction force constants). bWe note that this mode contains two relatively large (negative) contributions from interaction constants (numbers 22 and 27) with the result that the percentage contributions from u(C2-N), u(C6-N), and u(C2-C3) are probably slightly overestimated. small number of force constants employed. The average error of the calculated frequencies is -I%, and in only a few cases is the error for an individual mode greater than 2% (vide infra). Obviously, employing a larger number of constants would improve the agreement, but this was considered to be inappropriate, given our intention to eventually utilize this force field as a reference field for structurally related molecules. The potential energy distributions (PED’S) (natural-abundance complex) for both the ground- and 3MLCT-excited-states are given in Table 111. Before comparing the mode compositions of the two states, it is important to identify the essential goal of the present calculations and to clarify the rationale employed in refining the force fields. As always, the primary objective of the NCA is to obtain calculated frequencies that are in reasonable agreement with those observed. However, in the present situation, where a comparison of two electronic states is the essential goal, it is considered more important to faithfully reproduce the shifts observed in the 3MLCT state relative to the ground-state frequencies. It is not likely that the 35 constant force fields are adequate to reproduce the groundand )MLCT-state frequencies to the level of accuracy required to yield calculated frequency shifts (GS-)MLCT) that are in precise agreement with those observed. However, it is reasonable to expect that the signs of the calculated shifts should be in agreement with those observed and that the calculated magnitudes
20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
ground state
3MLCT
2.192 5.072 5.116 5.431 6.752 6.459 7.214 6.658 5.697 0.280 0.641 0.763 0.871 0.5376 0.5117 0.5432 1.246 1.043 0.788
2.192 6.159 5.116 5.328 5.067 6.539 5.799 6.417 5.436 0.280 0.641 0.763 0.87 1 0.5321 0.5025 0.5534 2.246 1.043 0.788
1-2 Stretch-Stretch Interaction Constantsd 0.534 0.530 0.794 f [5-61 0.748 0.447 f [6-71 f [7-81 0.774 fw 9 1 0.605 f 14-91 0.599
f 12-41 f [2-51 f [4-51
Stretch-Bend Interaction Constants” f [5-141,f 16-14] 0.119 f[6-151, f[7-151,f[8-151 f i g - 161,f [9- 161 f [2-121, f[2-131 f[4-121, f[5-131 f[4-171,f[5-171 f [4-191,/[9-191 f[5-18],f[6-18], f[7-18], etc.
0.115 0.142 0.311 0.145 0.144 0.256 0.056
state
0.534 0.578 0.367 0.364 0.512 0.594 0.730 0.224
0.1 10 0.126 0.105 0.311 0.145 0.144 0.256 0.056
“All stretching force constants are in units of mdyn/A, bending constants in (mdyn/A)/rad2, and stretch-bend interactions in mdyn/ rad. b u and 6 refer to stretching and bending diagonal constants. c6(CCH) refers to both G(C3C4H) and G(C4C5H) force constants. dInteraction force constant symbols refer to the two numbers of the interacting diagonal constants.
not be grossly different. As can be seen by inspection of the plots given in Figure 4, generally this goal has been accomplished. In most cases, the pairs of calculated and observed GSJMLCT shifts are of the same sign and approximately equal magnitude. Considering all of the isotopomers studied and the large number of comparisons of individual shifts, it is satisfying that only a few pairs show poor agreement (Le., a shift of opposite sign to that observed). It is important to emphasize that the agreement between observed and calculated GS-3MLCT shifts is obtained while maintaining good agreement between observed and calculated frequencies and physically realistic force fields. As in the case of the ground-state calculation, it is especially satisfying to note that the derived )MLCT-state force field yields calculated 15N shifts that are in general agreement with those observed for both and 3,3’-2H, analogues, even though no manual adjustthe ‘H8 ments were made in arriving a t the 3MLCT force field. The final force fields for both the ground-state and 3MLCTstate are given in Table IV. The final 3MLCT state normal modes are plotted in Figure 5. C. Comparison of Ground- and 3MLCT-State Normal Modes. The present analyses reveal several important features concerning the relationship of ground- and 3MLCT-state normal modes. The conclusion of primary importance is that the modes of the anion-radical fragment of the 3MLCT state exhibit frequencies and relative intensities that are quite similar to those of the corresponding lithium derivatives. Thus, the interpretation originally
Ground and 'MLCT Excited States of R ~ ( b p y ) ' ~ +
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1365 14%
w 1212
I285
I365
I I64
w w 323 I100
1028
1014
742
664
355
270
I89
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Figure 5. Normal-mode descriptions for excited-state [R~(bpy),~+]--A~ symmetry class.
proposed by Dallinger, Woodruff, and co-workers' is strongly supported. However, inspection of the PEDS in Table I11 indicates that there is not a direct correspondence of modes between the two states. The disparity of mode compositions is especially severe in the four highest frequency modes (uS/u5, through V 8 / V s ? ) . While the contribution of u(C2-Cz,) diminishes in u9, (relative to u g ) , it remains the only stretching coordinate that contributes significantly to this mode. Thus, the substantial shift to higher frequency in the 'MLCT is directly related to the increase in the u(C2-C2,)force constant (number 2 in Table IV). While the other shifts observed upon 3MLCT-state formation are not predictable simply from inspection of the PED'S (because of the disparity in mode compositions), they are reproduced very well by the derived force fields, as Figure 4 clearly demonstrates. Comparison of the derived force constants for the two states can provide qualitative structural and bonding information. Thus, the values given in Table IV indicate that the C2-Cz. bond strengthens considerably, whereas the C4-Cs and C2-C3 bonds are substantially weaker in the 3MLCT state. Both C-N bonds
,
weaken slightly while the C3-C4 and c5-c6 bonds change very slightly. These qualitative estimates are in general agreement with expectations based on M O calculation^^^^^^ and "3MLCT-state model compound" structural data.24 On the other hand, a more satisfying approach is to obtain quantitative estimates of bond length changes. In a separate report, we provide a thorough discussion of the development, justification and testing of a procedure which can be used to extract such information from the
PED'S.^^ Acknowledgment. This work was supported by a grant from the Department of Energy, Basic Energy Science Division (Grant ER13619). Such support does not constitute an endorsement by (26) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1985, 24, 106. (27) McPherson, A. M.; Fieselmann, B. F.; Lichtenberger, D. L.; McPherson, G. L.; Stucky, G . D. J . Am. Chem. SOC.1979, 101, 3425. (28) Mallick, P. K.; Strommen, D. P.; Kincaid, J . R. J . Am. Chem. SOC., in press.
1366
J . Phys. Chem. 1990, 94, 1366-1372
D.O.E. of the views expressed in this work. We acknowledge use of a graphics program for plotting vibrational eigenvectors that was developed in the laboratories of Professor R. Mathies at the University of California-Berkeley. We also thank Dr. Janusz Golus for help with the preparation of several compounds used
in this study and Mark Bartelt for his expert assistance in constructing several electronic components. Registry No. ISN, 14390-96-6; D2. 7782-39-0; Ru(bpy)32+, 151 5862-0.
An Investigation of Tautomerism in Adenine and Guanine Michael Sabio,? Sid Topiol,*qt and William C. Lumma, Jr. Department of Medicinal Chemistry, Berlex Laboratories, Inc., I IO East Hanover Avenue, Cedar Knolls, New Jersey 07927 (Received: August 21, 1989)
The geometries of an extensive set of adenine and guanine tautomers have been optimized by using the semiempirical AM1 method and ab initio Hartree-Fock method with the 3-21G basis set. Tautomeric energies were evaluated at the AM1 and the Hartree-Fock STO-3G and 3-21G levels. For selected (low-energy) tautomers of both adenine and guanine, correlation effects were evaluated at the MP2 (Maller-Plesset perturbation theory to second order) level with the 6-31G and 6-31G*(5D) basis sets. For the same selected tautomers of guanine, geometry optimizations were performed at the RHF/6-3 1G*(5D) level. The results suggest that the most stable form of these purines at the AMI, STO-3G, and 3-21G levels is 1A and ZA, Le., the one usually assumed. At the RMP2/6-31G*(sD)//RHF/6-31G*(5D) level (with the zero-point energy correction estimated at the RHF/3-21G//RHF/3-21G level) there is an enol tautomer of guanine which is less than 2 kcal/mol higher in energy than the most stable (keto) tautomer. All other tautomers are unlikely to be experimentally observed in the gas phase. In these systems, the effects due to polarization functions and correlation do not seem to be additive.
Introduction The purines adenine and guanine serve as the base portions of the cyclic nucleotide second messengers adenosine 3'5'-cyclic monophosphate (CAMP) and guanosine 3'5'-cyclic monophosphate (cGMP), respectively, in addition to their presence in the ribonucleosides adenosine and guanosine (and the corresponding deoxyribonucleosides). Knowledge of the physical chemical properties of these purines is therefore clearly essential in understanding many important biochemical processes. The nature of the purine base is likely to play a direct role in the selectivity of recognition of cAMP and cGMP at their sites of action as second messengers, as well as at the active sites of enzymes at which they are hydrolyzed (e.g., their corresponding phosphodiesterases).] Moreover, their properties could also be indirectly involved in the recognition of their corresponding cyclic nucleotides. For instance, it has been suggested2 that the relative conformation about the sugar-to-base bonds of cAMP and cGMP, which varies with the base portion, is a primary component in the recognition of ligands at cGMP phosphodiesterase 11. One of the more fundamental properties to consider in understanding these purines is the relative tautomeric states. Tautomerism can affect both direct and indirect features, such as those mentioned above, of the parent systems. For example, models in which ligand tautomerism is involved in activation of histamine-H2 receptor^^-^ and serotonin receptors6,' have been proposed and may have implications for other receptor systems.* Similarly, active site tautomerism plays a role in the mechanism of action of enzymes such as the serine protease^^^'^ or DNAse." Indeed, it has already been suggested that guanine tautomerism plays a role in biochemical processes.12.'3 Computational chemical studies permit a direct analysis of such properties. In studies appearing in the literature, the structures of the most likely forms of adenine and guanine, as well as other selected tautomers, have been determined.14-24 These studies include examinations of the most likely tautomers by semiempirical approaches as well as ab initio calculations with the STO-3G and 3-21G basis sets. In this work, we study an extensive series of tautomers of adenine and guanine by using the semiempirical AMI method as well as ab 'Present address: Sandoz Research Institute, East Hanover, New Jersey 07936.
0022-3654/90/2094-1366$02.50/0
initio Hartree-Fock and Maller-Plesset perturbation theory calculations to second order with the STO-3G, 3-21G, 6-31G, and 6-31G*(5D) basis sets. The present results should facilitate the understanding of the role of tautomers of these moieties in biological systems and processes.
(1) Charbonneau, H.; Beier, N.; Walsh, K.A,; Beavo, J. A. Proc. Natl. Acad. Sci. USA 1986,83, 9308. (2) Wells, J. N.; Garst, J. E.; Kramer, G. L. J . Med. Chem. 1981, 24, 954. (3) Topiol, S.; Weinstein, H.; Osman, R. J . Med. Chem. 1984, 27, 1531. (4) Weinstein, H.; Mazurek, A. P.; Osman, R.; Topiol, S . Mol. Pharmacol. 1986, 29, 28. (5) Weinstein, H.; Chou, D.; Johnson, C. L.; Kang, S.; Green, J. P. Mol. Pharmacol. 1976, 12, 738. (6) Osman, R.; Weinstein, H.; Topiol, S.; Rubenstein, L. Clin. Physiol. Biochem. 1985, 3, 80. (7) Oman, R.; Topiol, S.; Rubenstein, L.; Weinstein. H. Mol. Pharmacol. 1987, 32, 699. (8) Topiol, S . Trends Biochem. Sci. 1987, 12, 419, (9) Blow, D. M.; Steitz, T. A. Annu. Reu. Biochem. 1970, 39, 63. (IO) Hartley, B. S.; Shotton, D. M. In The Enzymes; Boyer, P. D., Ed.; Academic Press: New York, 1971; Vol. 3, pp 323-373. (11) Suck, D.; Oefner, C. Nature 1986, 321, 620. (12) Topal, M. D.; Fresco, J. R. Nature 1976, 263, 285. (13) Rhoads, R. E.; Hellman, G. M.; Remy, P.; Ebel, J. P. Biochemistry 1983, 22, 6048. (14) Davis, A.; Warrington, B. H.; Vinter, J. G. J. Comput.-Aided Mol. Design 1987, 1 , 97. (15) Norinder, U . J. Mol. Srruct. (THEOCHEM) 1987, 151, 259. (16) Walker, G. A.; Bhatia, S. C.; Hall, J. H., Jr. J . Am. Chem. Soc. 1987, 109, 7629. (17) Del Bene, J. E. J. Cfiem. Phys. 1983,87, 367. (18) Les, A,; Kukawska-Tarnawska, B. J. Mol. Struct. (THEOCHEM) 1986. 148. 45. (19) Bartzsch, C.; Weiss, C.; Hofmann, H.-J. J. Prakr. Chem. 1984,326, 407. (20) Sygula, A.; Buda, A. J. Mol. Struct. (THEOCHEM) 1983, 92,267. (21) Sygula, A.; Buda, A. J. Mol. Struct. (THEOCHEM) 1985,121, 133. (22) Zielinski, T. J.; Breen, D. L.; Haydok, K.; Rein, R.; MacElroy, R. D. Int. J . Ouant. Chem. 1978, 3 5 5 . (23)Latajka, Z.; Person, W. B.; Morokuma, K. J. Mol. Srruct. (THEOC H E W 1986, 135, 253. (24) Thewalt, U.; Bugg, C. E.; Marsh, R. E. Acta Crystallogr. 1971,827, 2358.
0 1990 American Chemical Society
