J. Phys. Chem. 1984,88, 5513-5519 different symmetries; this DR in s-C6H3D3is hence primarily the result of different H M S matrices in the ground and excited states. The H M S matrices predict out-of-phase mixing of Q9 with QI8 but the mixing is more extensive in the excited state; also mixing of Qlaand Q19has led to reduced H-wagging of Q l 8 in the excited state. Direct inspection of the normal-mode displacements shows a larger decrease of the hydrogen character of Q t relative to @ in the excited state than in the ground state, in accordance with the experimental inference. VI. Concluding Remarks
-
The large number of vibronic bands observed in the lBpu
A’, two-photon spectra of deuterium-labeled benzenes and the
force field calculations allow construction of a more detailed potential surface than previously available for the lBZustate. Although experimental frequencies are necessary for establishing the force field, the reward is calculated mode forms and frequencies for all the modes, allowing further assignments and rationalizations. Use of the harmonic mode scrambling principles facilitates assignments of new vibronic features in the deuterated spectra while the Duschinsky rotation matrices illuminate the
5513
differences between ground- and excited-state mode forms. Extensions of these ideas to other molecules should prove useful for further understanding of molecular excited states in general. It will be interesting to see whether other molecules are just as susceptible to kinematic effects as benzene and if the phenomena are in general more important in two-photon spectra than in normal optical spectra.
Acknowledgment. We gratefully acknowledge financial support from the donors of the Petroleum Research Fund, administered by the American Chemical Society (K.K.-J.), and from the National Science Foundation (L.G.). We also acknowledge a generous grant of computer time from the Rutgers Center for Computer and Information Services. We thank Dr. Ronald Levy for penetrating discussions. Appendix
The normalized harmonic mode scrambling matrices and Duschinsky rotation matrices are given in Tables 111-X. Registry NO. C,&, 71-43-2; C&,, 1076-43-3; s-CsH$,, 1684-47-5; m-C&4D2, 14941-51-6.
Supersonic Beam Studies of Hydrogen-Bonded Indoles: Relative Interaction Strengths James Hager and Stephen C. Wallace* Department of Chemistry, University of Toronto, Toronto, Ontario, MSS 1 A1 Canada (Received: March 27, 1984)
The energetics of hydrogen-bonded indoles formed in a supersonic free jet expansion has been studied over a range of different complexing bases including alcohols, amines, and aromatic molecules. For these complexes a good correlation between the observed red spectral shift and the gas-phase proton affinity of the base has been found. This has permitted an analysis of the important interaction components in these hydrogen bonds. Time-resolved studies show a distinct fluorescence decay time shortening in these complexes relative to the bare molecule origin. This is explained in terms of an increased rate of intersystem crossing upon the formation of a hydrogen bond. Spectra of indole (HCC13)and indole (H2CC12)indicate that indole can also serve as a proton acceptor in a hydrogen bond.
Introduction
The study of the effect of vapor-phase hydrogen-bond formation on electronic transitions has been an area of significant interest and activity for many years.’ This work has been largely confined to fairly simple chromophores that are relatively free from problems associated with vibrational sequence congestion since electronic spectral shifts are usually only a few hundred wavenumbers at most. In addition, one is generally restricted to binding energies greater than kT. These difficulties have been overcome with supersonic molecular beam techniques that enable the study of larger and much more complicated molecules and weaker bonding energies.2 In this paper we wish to report our investigations on hydrogen-bonded indole and substituted indoles with a series of alcohols, amines, and aromatic complexing partners. Indole has long been used as a model compound for the amino acid tryptophan, which exhibits fluorescence and absorption spectra that are very sensitive to environmental effects3 By studying the isolated hydrogen-bonded complexes of the model indole systems one can begin to understand the varieties of interactions (1) Pimentel, G. C.; McClellan, A. L. “The Hydrogen Bond”; W. H. Freeman: San Francisco, 1960. (2) Levy, D. H. Annu. Rev. Phys. Chem. 1980, 31, 197. (3) Strickland, E.H.; Billups, C. Biopolymers 1973, 22, 1989. (4) Morokuma, K. J. Chem. Phys. 1971,55, 1236.
0022-3654/84/2088-5513$01.50/0
responsible for the tryptophanyl absorption bands of proteins. Hydrogen bonding is an interaction that is generally characterized by well-defined geometries and binding energies. The binding energy has been determined to arise largely from electrostatic forces; however, smaller contributions from longer range interactions can also be important in determining the hydrogenbond strengths. Upon,excitation there is a spectral shift of the hydrogen-bonded bands from the bare indole features that reflects the difference in binding energies between the ground and the first excited electronic states of the complex. For this T,T* transitionS we observe a spectral shift to lower energy when indole is the proton-donating half of the hydrogen-bonded complex, implying a greater “acidity” in the excited state. We have used the observed red electronic spectral shifts to order the strength of the gas-phase indole hydrogen bonds and shown that these can be correlated with the strength of the added base as measured by gas-phase proton affinities. This enables us to analyze the various components of the binding interaction that are the primary concern in these hydrogen-bonded series using Morokuma’s energy decomposition a n a l y ~ i s . ~ In addition to the above complexes where the indole moiety is the proton donor in the hydrogen bond, initial studies on indole-chloroform and indole-dichloromethane indicate that the (5) Hager, J.; Wallace, S. C. J. Phys. Chem. 1983, 87, 2121.
0 1984 American Chemical Society
Hager and Wallace
5514 The Journal of Physical Chemistry, Vol. 88, No. 23, 1984
li
E V -100
-300
U
400
& - 50C
-20 R E L ~ T I V E ENERGY t d 1
II
I
0
I
,
-100
,
I
,
I
-200
-300
-200
-300
, -400
-500
-600
-400
-500
-600
I/
-*,I 0
-100
Figure 1. Fluorescence excitation spectra of (a) indole-methanol and (b)
indole-ethanol hydrogen-bonded complexes. indole molecule itself can act as a proton acceptor in a weak hydrogen bond. These results can be discussed in terms of the molecular interactions responsible for the anesthetic action of these halocarbons. First, we present the qualitative features of the fluorescence excitation spectra and the obvious trends. Following this is an analysis of these trends discussed in terms of the various interaction components important in these hydrogen-bonded systems.
1 0
1
l -100
l
l -200
I
RELATIVE
I
-300
I
I -400
ENERGY
I
I
-500 (cm-')
I
l -600
I
Figure 2. Fluorescence excitation spectra of (a) indole-2-propanol, (b) indole-1-propanol,and (c) indole-1-butanol hydrogen-bondedcomplexes.
Results A. Indole-Alcohol Complexes. Hydrogen-bonded spectra of indole and water, methanol, ethanol, and 1,4-dioxanehave recently been reported6,'. These fluorescence excitation spectra show red
spectral shifts that range from 135 cm-' for water to 218 cm-' for ethanol. We have extended these measurements to 1-propanol, 2-propanol, and 1-butanol and have also identified previously unreported features in the methanol and ethanol spectra. Figure 1 (parts a and b) shows the fluorescence excitation spectra of the indole-methanol and indole-ethanol hydrogenbonded systems in the vicinity of the bare molecule origin. The large features in these spectra are in good agreement with those previously published and show spectral shifts of -160 cm-' for indole-methanol and -214 cm-I with indole-ethanol. In addition to these major bands, there is substantial far-red-shifted structure that arises under conditions of greater alcohol concentration in the expansion mixture. The lowest energy feature in the case of the methanol complex is located a t -472 cm-' and that for the ethanol complex is at -518 cm-' from the bare molecule origin. Based on the alcohol concentration dependence of these bands we assign them to complexes of indole-n(methano1) and indolen(ethano1) where n 1 2. The complexity of these features appears to preclude assignment to a particular cluster size based on fluorescence studies alone; consequently, we have recently undertaken laser ionization mass spectroscopic studies to resolve this question. Figure 2 shows similar spectra with alcohols 2-propanol, 1propanol, and 1-butanol. When the alkyl side chain of the alcohol is lengthened, new low frequency modes are introduced into the hydrogen-bonded molecule. This is especially noticeable for indole-1-propanol (Figure 2b), showing a prominent 12-cm-' progression. The spectral shifts are similar to that of the ethanol complex with -225 cm-' for 1-propanol, -232 cm-' for 2-propanol, and -215 and -258 cm-' for 1-butanol and are all obtained from spectra taken at lower alcohol concentration, where the far-redshifted spectral features are absent. At lower energies, we again see the appearance of a broad concentration-dependent band. There is still some discernible structure in this region for the 2-propanol complex, but as the alcohol chain length is increased, as in 1-butanol, this is seen to quickly diminish. In fact, the 1-butanol spectrum shows only a very slight rise in the base line in the 525-cm-' spectral region and none of the discrete structure so prominent in the lower alcohols.
(6) Montoro, J.; Jouvet, J.; Lopez-Campillo, A.; Soep, B. I.Phys. Chem. 1983,87,3582.
(7) Nibu, Y.; Abe, H.; Mikami, N.; Ito, M. J. Phys. Chem. 1983,87,3898. (8) Abe, H.; Mikani, N.; Ito, M. I. Phys. Chem. 1982, 86, 1768.
Experimental Section The fluorescence excitation spectra were obtained by crossing a continuous helium-free jet expansion seeded with indole and the appropriate complexing partner with the frequency-doubled light of a Nd:YAG pumped dye laser system. A nominal 100-pm pinhole was used as the expansion orifice with a backing pressure of approximately 3.5 to 4.0 atm. Under these conditions background chamber pressure was always below 10 mtorr. The laser light intersected the free jet expansion 5 mm (50 nozzle diameters) downstream of the pinhole. The laser system consisted of a Quanta Ray NdYAG (DCR-1) pumped dye (PDL-1) laser using various red dyes which was then frequency doubled in KD*P to cover the 2800- to 2900-A spectral range. The line width of the UV light was approximately 0.2 cm-' with pulse energies 1 5 mJ. For the time-resolved studies the laser linewidth was reduced to 0.01 cm-l (as measured by a 0.25-cm-' spectral free energy Fabry-Perot) by introducing an intracavity etalon. The relative concentrations of all species in the expansion mixture was controlled by a flow system described in a previous p~blication.~ For this study the liquid complexing partner was placed in a stainless steel vessel attached to a separate He flow line monitored by one of two Hastings linear mass flowmeters. The other mass flow meter monitored the flow of the expansion mixture. Indole (99+%) and the substituted indoles (99%) were obtained from Aldrich and used without further purification. Methanol, ethanol, 2-propanol, 1-propanol, 1-butanol, benzene, toluene, p-xylene, diisopropylamine,dichloromethane, and chloroform were used as complexing partners and were of spectral grade or distilled-in-glass HPLC grade and used without further purification. Ammonia and trimethylamine were obtained from Matheson and introduced into the expansion mixture as previously d e ~ c r i b e d . ~
The Journal of Physical Chemistry, Vol. 88, No. 23, 1984 5515
Hydrogen-Bonded Indoles
-" I
I
0
-100
I
I -200
- 500
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-300
RELATIVE ENERGY (cm-') c
I 0
1 0
1
I -100
I
-200 l -300RELATIVE E N E R G Y (err-')
5
- 400
0
I
I -100
I
l -200
l
I
-300
I
I
-400
0
R E L A T I V E E N E R G Y (ern-') Figure 4. Fluorescence excitation spectra of indole-benzene hydrogen-
bonded complex at two different benzene concentrations in the expansion mixture: (a) -0.1% benzene and (b) -0.2% benzene.
1 0
I -100
I
I
-200 -300 RELATIVE ENERGY (cm-')
I -400
I -500
Figure 3. Fluorescence excitation spectra of (a) indole-ammonia, (b)
indole-trimethylamine, and (c) indole-diisopropylamine hydrogenbonded complexes. B. Indole-Amine Complexes. The spectra of indole-ammonia, indole-trimethylamine, and indole-diisopropylamine are shown in Figure 3. The resulting spectral features are quite similar to those of the alcohol complexes; however, the red shifts are slightly greater, -227 cm-' for NH,, -325 cm-' for NMe3, and -350 20 cm-' for diisopropylamine. The uncertainty of k 2 0 cm-' for the indole-diisopropylamine origin reflects the lack of a single dominant transition in this energy region. From these spectra, and considering the low-temperature environment in which they were obtained, one can tentatively identify several of the less intense transitions in Figure 3a,b with excited state stretching and bending frequencies associated with the hydrogen bond. For ammonia one obtains v&nd = 36 cm-' and v,trd& = 163 cm-', and for trimethylamine these values are 36 and 108 cm-' respectively. These are roughly the values expected based on the results of other hydrogen-bonded systems.'* Further spectroscopic studies with the appropriate isotopically substituted molecules are necessary to confirm these assignments. Finally, one should note the number of low-frequency modes present in the diisopropylamine spectrum.
*
(b)
R E L A T I V E ENERGY (crn-ll
Figure 5. Fluorescence excitation spectra of (a) indole-toluene and (b)
indole-p-xylene hydrogen-bonded complexes.
C . fndole-Aromatic Complexes. Figures 4 and 5 show the fluorescence excitation spectra of indole complexed to the aro-
Hager and Wallace
5516 The Journal of Physical Chemistry, Vol. 88, No. 23, 1984
I
2'
18
RELATIVE E N E R G Y (crn-l)
Figure 6. Fluorescence excitation spectrum of the indole-HCC13 hydrogen-bonded complex.
matics benzene, toluene, and p-xylene. In this case we are forming 7r hydrogen bonds. That indole forms this type of hydrogen bond in the ground state has been shown by several authorsgJOusing IR spectroscopy and constitutes the mechanism of indole autoassociation. The example of indole-benzene (Figure 4a) is the simplest of the three and will be considered first. The most intense red-shifted feature is located at -160 cm-' from the bare molecule origin and consists of a doublet separated by -9 cm-'. This doublet may arise from a very low frequency mode involving rocking or rotation of the benzene molecule about the hydrogen bond. A similar splitting of 4 cm-' has been noted in the phenol-benzene spectrumus In addition, there are other low frequency modes that are too low in energy to involve the constituent molecules and therefore must arise from the relative motion of one with respect to the other. At higher benzene concentrations (Figure 4b) a broad continuum grows in beneath the sharp structure and is undoubtedly due to the presence of multiple benzene molecules in the complex. Such behavior is common for organic molecules with complexing partners such as argon5~"and has been attributed to the spectral manifestation of the building up of a distribution of complex sizes with a large number or argon atoms. The same explanation appears to be valid in the case of indole-n(benzene) as well. Parts a and b of Figure 5 show the broad features associated with the indole-toluene and indole-p-xylene, respectively. Even at very low concentrations of the aromatic complexing partner bands characterized by a fwhm of 140 cm-' for toluene and -80 cm-l for xylene rapidly grow into the spectrum. The toluene spectrum is also characterized by a series of sharp peaks separated by 12-15 cm-I lying atop the bell-shaped broad feature. The structure on the red side of the broad band is always seen to be better resolved than that on the blue regardless of the toluene concentration in the beam. In contrast, one can see that there is little structure associated with the p-xylene complexes. We have been unable to identify a single sharp spectral origin in these systems, which may be the result of the combination of many low frequency vibrational modes and overlapping transitions of higher oligimers. D. Indole as a Proton Acceptor. When CHC13 is used as a complexing partner a rather broad highly structured band is observed with a maximum at approximately 120 cm-' to the blue of the indole origin (Figure 6). We have interpreted this behavior in terms of an indole-HCC1, hydrogen bond. In this case the indole molecule is acting as a proton acceptor, and thus the spectral shift is to higher energies. Gonohe et al.lz have proposed a similar explanation for the fluorbenzene-HCC1, complex which shows a blue spectral shift of 117 crn-'.
-
(9) Bernard-Houplain, M.; Sandorfy, C. Can.J. Chem. 1973, 51, 1075. LautiE, A.; LautiE, M. F.; Gruger, A.; Fakhri, S . F.Spectrochfm.Acta Part A 1979, 36, 85. (11) Amirav, A.; Even, U.; Jortner, J. J . Phys. Chem. 1982, 86, 3345. (12) Gonohe, N.; Abe, H.; Mikami, N.; Ito, M. J. Phys. Chem. 1983,87, (10)
4406.
I
I
19
20
21
21
Figure 7. Fluorescence excitation spectrum of the indole-H2CC12com-
plex showing the prdgession intervals. The structure associated with the indole-HCC1, spectrum is quite regular on the red side of the band maximum, with spacings of approximately 8 cm-'. Past this point, however, the spectrum becomes quite complex, and we have not analyzed it further. This structure may be due to a distribution of cluster sizes similar to that mentioned in the toluene and xylene spectra or alternatively to a drastic change in equilibrium geometry upon electronic excitation. We have also investigated the indole-CH2C12 complex (Figure 7) and have found some unusual behavior. A well-developed vibrational progression begins at -+12 cm-' and finally dies out around +160 crn-'. In all we have identified three progressions with spacings of 21 crn-', one of which is made up of seven members. The question of what type of bonding interactions are operative in this complex is difficult to answer unambiguously at the present time. The main possibilities include (a) hydrogen bonding or (b) van der Waals interactions. van der Waals interactions usually give rise to red spectral shifts with a complexing partner as polarizable as dichloromethane unless there is a substantial change in dipole moment upon electronic e~citati0n.l~In indole the dipole moment changes in the S1 state, but by only a relatively small amount (10.14 f 0.051 D).14 In addition we have found that indole-CF,Cl, gives rise to a red electronic spectral shift.5 On the other hand, it has been shown that CH2C12does form hydrogen bonds with some proton acceptors, though not as strongly as CHC13.15 We favor the view that ?r hydrogen bonding is the appropriate explanation for the blue-shifted spectral features we have observed. The type of interactions between species such as indole and halocarbons are important in determining the nature of anesthetic action which involves changes in weak molecular associations. Trudeau et al.15 have demonstrated that fluorocarbon type anesthetics hinder the formation of hydrogen bonds in solution and that most potent anesthetics, and "hydrogen-bond breakers", are usually those containing an acidic hydrogen such as HCC13. In our indole-HCC1, and -H2CC12 spectra it appears that ?r hydrogen bonding is the relevant interaction. This is also the mechanism responsible for indole autoassociation, and thus, in solution, indole-HCC13 formation would be expected to compete directly with indole dimerization, reducing the concentration of the latter species. Consequently, it is of great interest to be able to observe directly the formation of indole-halocarbon species in an isolated environment where direct measurements of interaction energies can be made. Work toward this objective is in progress at the present time. E . Hydrogen Bonds Involving Substituted Indoles. We have extended our studies to several substituted indoles including 1methylindole, 5-methylindole, 5-methoxyindole, and 7-methylindole as well as the hydrocarbon analogue indene. As expected, (13) Even, U.;Jortner, J. J . Chem. Phys. 1983, 78, 3445. (14) Chang, C.; Wu, C.; Muirhead, A. R.; Lombardi, J. R. Photochem. Photobiol. 1974, 19, 347. (15) Trudeau, G.; Dumas,J.; Dupuis, P.; Gutrin, M.; Sandorfy, C. Top. Curr. Chem. 1980, 93, 91.
Hydrogen-Bonded Indoles
The Journal of Physical Chemistry, Vol. 88, No. 23, 1984 5517
TABLE I: Spectral Shifts of Hydrogen-Bonded Indoles and Proton Affinities of the Associated Bases
1 2
3 4 5
6 7 8
9 10
11
complexing base H,O M~OH benzene 1,4-dioxane EtOH 1-PrOH 2-PrOH 1-BuOH 3"
N(W3 (i-Pr),NH
indole
Av. cm-I 5-methylindole 5-methoxyindole
7-methylindole
PA," kcal mol-' 173.0 184.9 183.7d 194.1 190.3 191.4 192.4" 192.6 205.0 224.3 228.9
-135'
-160 -166 -188C -214 -225 -232
-178
-1 15
-122
-233
-166
-252
-176 -188 -1 92
-171
-343
-270
-227
-258 (-215) -227
-323 -350 f 20
"Proton affinity values ( f l kcal mol-') from ref 21a unless otherwise noted. *Reference 6. CReference7. dReference 21b. eReference 27. /Reference 2112. no bands characteristic of hydrogen-bond formation could be detected in the cases of 1-methylindole or indene where there is no N-H moiety available. This is further confirmation that we are indeed observing hydrogen-bonding phenomena in the case of the unsubstituted indole complexes. The spectral shifts observed for indole and other substituted indole complexes are given Table I and are all roughly the same as those previously discussed for indole. One should note that there is the same trend among the alcohols, of increasing red spectral shift with increasing alkyl substitution. F. Time-Resolved Studies. We have previously reportedS the fluorescence decay time of the bare indole origin as 17.5 f 0.5 ns, which is substantially different from the value of 30 ns recently reported by Bersohn et a1.16 The 17.5-ns lifetime was found to be independent of the interrogation distance downstream from the expansion orifice and the carrier gas. A possible explanation for this difference can be found in our earlier publication.s The excitation of any hot band structure in the vicinity of the origin results in a persistent long 7 tail in the fluorescence decay. If the excitation pulse frequency width includes some hot band features that overlap the origin, this long T component will be evident, leading to a longer observed decay time. The dye laser frequency width in our studies was reduced to 0.01 cm-' using an intracavity etalon compared to the 0.3 cm-' of Bersohn et al. This may be the source of the discrepancy. When an alcohol or amine is introduced into the expansion we have found a distinct shortening of the complex fluorescence decay times. In fact these lifetimes were unresolvable from our 5 4 s laser pulse. These results are in good agreement with the fluorescence studies in solution on indoles in various hydrogenbonded solvents where the decay times are found to be in the 3-6 ns range for a variety of alcohols and water." A possible mechanism for these reduced fluorescence decay times is an increased rate of intersystem crossing induced by the prsence of the hydrogen-bonding partner. Additional related data come from the behavior of indole in pyridine solution and complexed with pyridine in our supersonic expansion. Pyridine has been found to quench all indole fluorescence in solution and has been attributed to either a very high rate of internal conversion to the ground state or intersystem crossing to the triplet manifold.18 In our studies we find that as pyridine is introduced into the expansion the bare indole fluorescence decreases rapidly but without the addition of any spectral features. At high pyridine concentrations all of the isolated indole bands completely disappear without any trace of a complex feature. Given the behavior of the alcohols, we favor the view that intersystem crossing and not internal conversion is responsible for the decreased fluorescence lifetimes and the absence of pyridine features since it has been shown to be a major deactivation pathway for indole excited in the S1state.lga Calculations1gbput (16) Bersohn, R.; Even, U.; Jortner, J. J . Chem. Phys. 1984, 80, 1050. (17) Meech, S. R.; Phillips, D.; Lee, A. G. Chem. Phys. 1983, 80, 317. (18) Mataga, N.; Keufu, T. Mol. Phys. 1963, 7, 137.
Proton A f f i n i t y (cm-' x 1.50 1.70
1.S
-~004-
" t
-
-300
v
-loot
t
01 I60
I
I
I
I
1
I
180 200 220 Proton A f f i n i t y (kcal mol-')
I
0
Figure 8. Correlation of electronic spectral shifts with gas-phase proton affinities. The numbers of the associated bases are listed in Table I. The two triangles indicate the two possible origins in the indole-1-butanol spectrum.
T1 at 1.3 eV below S,, T2 and T3 at lower energies than S1, and T4 and T5 approximately isoenergetic with SI. Thus, the perturbation of the indole radiative rate by hydrogen-bond formation probably results in better coupling with this dense triplet manifold.
Discussion From the above results one can see that there is a wide range of behavior for hydrogen-bonded indole complexes. Indole can act as a proton donor or proton acceptor in a hydrogen bond depending on the nature of the complexing partner. In addition, there is the obvious trend that complexing with amines leads to spectral shifts that are generally larger than those of alcohols, which are larger than that of water. This suggested to us the possible correlation between spectral shift, which is a measure of the relative H-bond strength,20and the basicities of the proton acceptors as measured by gas-phase proton affinities.21 Ault et a1.22have previously used proton affinities as a method of un(19) (a) Klein. R.: Tatischeff. I.: Bazin. M.: Santus. R. J . Phvs. Chem.
1981, 85, 670. (b) Lerner, D. A.'; Howowitz, P. M.; Eyleth, E. M. J . Phys. Chem. 1977. 81. 12. (20) Pimentei, G. C. J. Chem. Phys. 1957, 79, 3323.
(21) (a) Aue, D. H.; Bowers, M. T. In "Gas Phase Ion Chemistry"; Bowers, M. T. Ed.; Academic Press: New York, 1979; Vol. 2, pp 1-51. (b) Lau, Y. K.; Kebarle, P. J . Am. Chem. SOC.1976, 98, 7452. (c) Wolf, J. F.; Staley, R. H.; Koppel, I.; Toogepera, M.; McIver, R.T., Jr.; Beauchamp, J. L.; Taff, R. W. J . Am. Chem. SOC.1977, 99, 5417.
5518 The Journal of Physical Chemistry, Vol. 88, No. 23, 1984
derstanding the strength of the interaction due to hydrogen bonding as related to vibrational spectral shifts. This has been extended by Barnes23 to a wide range of base strengths and a smooth, but curved correlation was found for weak and medium strength hydrogen bonds. In the present study we are measuring electronic spectral shifts and using proton affinities to gain insight into the type and strengths of the interactions responsible for these spectral shifts. These values are given in Table I and plotted in Figure 8 where one can see that there is a reasonably linear correlation between spectral shift and proton affinity. This behavior helps gain some understanding into the type of interactions responsible for our observations as discussed below. Noncovalent complexes which involve hydrogen bonds have been widely investigated by the energy decompositiqn approach of Morokuma and co-workersZ4 in an attempt to determine structures and bond energies. These studiesZShave led to a greater understanding of hydrogen bonds, donor-acceptor complexes, and proton affinities. In the ab initio S C F molecule orbital calculations of M o r o k ~ m the a ~ total ~ ~ interaction energy is divided into the following physically meaningful components: electrostatic (AEB), polarization (AE~L),exchange (AEEX),and charge transfer (AE,,). Thus, the S C F interaction energy can be written as
We are particularly interested in the electrostatic and polarization components of the hydrogen-bonding interaction. The electrostatic component is the interaction energy between the charge distributions of the isolated molecules. Briefly, the individual contributions to the overall interaction energy are the differences
where EAoand EBoare the energies of the isolated molecules. The approximations used to describe the entire system are ql = A+Ao.d+Bo
where and are the SCF wave functions for the monomer whose energies are EAoand EBo,respectively. The antisymmetrizer .d indicates that electron exchange is allowed within each monomer unit. This gives the electrostatic interaction energy
+
AEEs = El - (EAo EB’)
The polarization components of the total interaction energy is defined as the charge redistribution energy within the monomer fragments. The relevant wave function is +ZA
=
A$A’A+Bo
where is the S C F wave function for A in the presence of B and is different from The energy differences (AEPL)A = EZA - El and (AEpL)B = EZB - E, are the polarization energies of molecule A and molecule B, respectively. This contribution is the sum of polarization and dispersion energy. The remaining (22) Ault, B. S.; Sternback, E.; Pimentel, G. C. J . Phys. Chem. 1975, 79, 615. (23) Barnes, A. J. In “Molecular Interactions”; Tarajczak, H., OrvilleThomas, W. J., Eds.; Wiley: New York, 1980; Vol. I, pp 273-99. (24) (a) Morokuma, K. J . Chem. Phys. 1971,55, 1236. (b) Umeyama, H.; Morokuma, K. J . Am. Chem. Soc. 1976, 98, 4400. (c) Umeyama, h.; Morokuma, K. J . Am. Chem. SOC.1977, 99, 1316. (d) Morokuma, K.;
Kitaura, K. In “Molecular Interactions”;Ratajzak, H., Eds.; Orville-Thomas, W. J., Wiley: New York, 1980; Vol. I, pp 21-66. (25) (a) Kollman, P. J . Am. Chem. SOC.1977, 99,4875. (b) Kollman, P.; Rothenberg, S. J. Am. Chem. SOC.1977, 99, 1333. (c) Kollman, P.; McKelvey, J.; Johansson, A.; Rothenberg, S. J. Am. Chem. SOC.1975, 97, 955. (d) DelBene, J. E. J. Am. Chem. SOC.1973.95, 5460. (e) DelBene, J. E. J . Chem. Phys. 1973, 58, 3139. (26) Aue, D. H.; Webb, H. M.; Bowers, M. T. J . Am. Chem. SOC.1976, 98, 311. (27) Dr. R. Houriet, unpublished results. Personal communication from
Prof. A. G. Harrison.
Hager and Wallace two components are AEa, which is the energy gained by allowing charge transfer between the fragments, and AEEX;which is the Pauli exchange repulsion between the fragments. From the perspective of this treatment, hydrogen bonding is an interaction that is mainly electrostatic in nature but is also influenced by other contributions24bsuch as polarization. It is this polarization contribution that is responsible for the so-called “alkyl substituent effect” in proton affinity studies; i.e., alkyl groups are found to stabilize the resulting positive charge upon p r o t o n a t i ~ n . ~So, ~~,~ for example, methylamine is found to be a stronger gas-phase base than ammonia even though the electron density localized on the nitrogen atom is greater for ammonia.2s Thus the electrostatic contribution to stabilization is reduced but the polarization is increased, resulting in an overall stabilization of the complex.24d In overly simplistic terms, the alkyl group removes electron density from the nitrogen atom in the neutral base but provides electron density upon protonation. Morokuma and K i t a ~ r have a ~ ~performed ~ variational calculations for the molecular interaction components of a wide variety of complexes and have compared the effects of methyl substitution on these complexes. The trial complexes of interest to us here are the ammonia/water (H,N-HOH) and the ammonia/proton (H3N-Hf) dimers. The effect of methyl substitution on the protonated complex is, as described above, an overall stabilizing factor. Whereas the change in the electrostatic interaction upon methyl substitution is destabilizing, the stabilizing effect of the polarization interaction leads to a net increase in proton affinity in agreement with experiment.21a For the hydrogen-bonded system, however, the stabilizing effects of the polarization and charge-transfer interactions were found to be insufficient to overcome the destabilization due to the decreasing electrostatic and exchange interactions. The general trend of these calculations for methyl substitution of the proton acceptor is A(AEpL), A(AEcT)< 0 (stabilizing) and A(AEES), A(AEBx)> 0 (destabilizing), with an overall destabilization of the hydrogen bond. The sign of the individual components seems to be universal. These results are contrary to what we have observed for the hydrogen-bonded indoles. The reason appears to be that these calculations of Morokuma and Kitaura deal with a relatively strong hydrogen-bonded complex, while we are investigating substantially weaker interactions because of the “less acidic” nature of the N-H moiety. Thus, in the present case, the total electrostatic interaction energy will be considerably less than for the ammonia-water complex and the contribution from the polarization energy will play a more important role. We have also examined the published data of Abe et a1.* on the more strongly hydrogen-bonded phenol complexes in light of our present results. The correlation between spectral shift and base proton affinity is not as good as it is for the indole systems, showing substantial deviation over a smaller range of proton affinity values, especially in the case of alkyl substituents. We interpret this behavior in terms of a stronger electrostatic contribution in these complexes which results in a stronger hydrogen bond more on the level with that in the Morokuma and Kitaura calculations. In the case of the indole complexes the electrostatic interaction will be smaller than in the phenol systems, as indicated by the relative acidities of the proton-donating functionalities. Thus, one would expect a greater relative contribution from the polarization and charge transfer interactions leading to the observed alkyl substituent effects. The correlation of spectral shift with the proton affinities of the respective bases suggests that the polarization interaction component is an important contribution for the stabilization of hydrogen-bonded indoles, but any additional effects from charge transfer interactions cannot be discounted. These results point out the importance of including factors other than purely electrostatic interactions when investigating the relatively weak hydrogen bonds such as those observed in this study. Conclusions We have reported the spectra of indole hydrogen bonded with a series of alcohols, amines, and aromatic partners. For these
J. Phys. Chem. 1984, 88, 5519-5526
complexes the indole chromophore was found to be the protondonating substituent. The spectral shifts of these complexes, and thus the relative strength of the hydrogen bond, are shown to give a reasonable linear correlation with the basicities of the partners as measured by the respective proton affinities. This correlation was also found to hold in general for substituted indoles. The use of the gas-phase proton affinity, which has previously been found to be sensitive to the polarizability of the attached alkyl groups, has made it possible to compare the important interaction components in terms of Morokuma's energy decomposition analysis. We have interpreted the ordering of indole hydrogen-bonded spectral shifts in terms of the interplay of both electrostatic interaction and polarization interaction components. It appears that for weakly bonded systems such as those containing indole as the proton donor, polarization forces can become important in determining electronic spectral shifts. We have also shown that indole and species such as halocarbons that are important in anesthetic activity can participate in hy-
5519
drogen bonds where the indole moiety is the proton acceptor. This observation provides a unique opportunity to investigate the dynamics of such biological processes in an isolated environment. Further investigations on these hydrogen-bonded indoles are currently in progress in our laboratory.
Acknowledgment. The financial support of the Natural Sciences and Engineering Research Council of Canada and the Petroleum Research Fund, addnistered by the American Chemical Society, is gratefully acknowledged. We wish to thank Dr. R. Houriet for providing us with unpublished values of proton affinities and Prof. A. G. Harrison for helpful discussions. Registry No. H20, 7732-18-5;MeOH, 67-56-1; EtOH, 64-17-5; 1PrOH, 71-23-8;2-PrOH, 67-63-0; 1-BuOH, 71-36-3; NH,, 7664-41-7; N(Me)3, 75-50-3; (i-Pr)2NH, 108-18-9; HCC13,67-66-3;H2CC12,7509-2; benzene, 71-43-2; 1,4-dioxane, 123-91-1; indole, 120-72-9; 5methylindole, 614-96-0; 5-methoxyindole, 1006-94-6; 7-methylindole, 933-67-5.
Photochemistry of Tris(2,2'-bipyrldyl)ruthenlum( I I ) in Colloidal Clay Suspensions Pushpito K. Ghosh and Allen J. Bard* Department of Chemistry, The University of Texas, Austin, Texas 78712 (Received: May 31, 1984)
The spectroscopy (absorption, emission, and resonance Raman), photophysics, and photochemistry of Ru(bpy),2+ (bpy is 2,2'-bipyridine) in colloidal smectites have been studied. The structure of Ru(bpy)>+ is shown to remain relatively unperturbed in the adsorbed state. The clay interlayers segregate R ~ ( b p y ) ~from ~ + exchangeable Na+ ions, resulting in high local concentrations of the complex cations in the interlayer. This in turn leads to very efficient excited-stateself-quenchingprocesses (60% of the initially formed Ru(bpy)32+*decays with an average lifetime of 5 5 ns when as little as 7% of the molecules are excited) under normal pulsed laser conditions. Ru(bpy)32+also segregates from MV2+(methylviologen) in the interlayer, leading to poor efficienciesfor the quenching of the excited state by adsorbed MVZ+compared to that in homogeneous solution. The excited state is readily quenched, however, by the neutral viologen, PVS (propylviologen sulfonate). In contrast to the segregation from Na+ and MV2+,R ~ ( b p y ) randomly ~~+ mixes with Zn(bpy)?+ in the interlayer, as evidenced by the progressive decrease in excited-state self-quenching rate for increasing ratios of [Zn(bpy)>+]: [ R ~ ( b p y ) ~ ~ ' ] .
Introduction Clays constitute a unique class of inorganic colloids with special structural features that make them versatile as catalysts' and catalyst supports.2 Of special interest are the swelling phyllosilicate minerals known as smectite clays. The smectites (e.g., hectorite, montmorillonite) can be readily dispersed in water, possess a large surface area (-750 m2/g), have a very high cation-exchange capacity (- 100 mequiv/100 g), and exhibit unusual intercalation and swelling properties. Recently we described the electrochemistry of several different ions confined to electrode surfaces modified with such a clay.3 It is of interest in connection with such clay-modified electrodes to investigate how ions from solution are incorporated into such clays. In the work described here we report our observations on the spectroscopy and photochemistry of R ~ ( b p y ) ~in~ colloidal + suspensions of sodium hectorite and sodium montmorillonite. Due to its unique ground- and excited-state properties4-" the photochemistry of (1) (a) Weiss, A. Angew. Chem., Znt. Ed. Engl. 1981, 20, 850. (b) Shimoyama, A,; Johns, W. D. Nature (London),Phys. Sci. 1971, 232, 140. (c) Durand, B.; Fripiat, J. J.; Pelet, R. Clays Clay Miner. 1972, 20, 21. (2) Van Olphen, H. "An Introduction to Clay Colloid Chemistry"; Wiley: New York, 1977. (3) Ghosh, P. K.; Bard, A. J. J . Am. Chem. SOC.1983, 105, 5691. (4) Demas, J. N.; Crosby, G. A. J . Mol. Spectrosc. 1968, 26, 7 2 . ( 5 ) Lyte, F. E.; Hercules, D. M. J . Am. Chem. SOC.1969, 91, 72. ( 6 ) Demas, J. N.; Adamson, A. W. J . Am. Chem. SOC.1973, 95, 5159.
0022-3654/84/2088-5519$01.50/0
Ru(bpy),2+ in clay suspensions is of considerable interest.I2 In addition, its possible use as a pillaring agent (molecular prop) in smectites has also been di~cussed.'~We have obtained absorption and emission data for absorbed R ~ ( b p y ) and ~ ~ +have probed its structure by resonance Raman spectroscopy. We have also used the techniques of laser flash photolysis and time-correlated single-photon counting to obtain emission decay profiles of Ru( b p ~ ) ~in ~the + adsorbed state. We assign the efficient selfquenching to triplet-triplet annihilation under normal (high-in(7) (a) Navon, G.; Sutin, N. Znorg. Chem. 1974,13,2159. (b) Sutin, N.; Creutz, C. Adu. Chem. Ser. No. 1978, 168, 1. (c) This single-exponential decay in good agreement with previous work indicates that the more complex behavior found with clays cannot be attributed to impurities in the Ru(bpy),CI, sample. (8) (a) Laurence, G. S.; Balzani, V. Znorg. Chem. 1974, 13, 3663. (b) Balzani, V.; Bolletta, F.; Gandolfi, M. R.; Maestri, M. Top. Curr. Chem. 1978, 75, 1. (9) Young, R. C.; Keene, F. R.; Meyer, T. J. J. Am. Chem. SOC.1977,99, 2468. (10) Bock, C. R.; Meyer, T. J.; Whitten, D. G. J . Am. Chem. SOC.1974, 96, 4710. (1 1) (a) Harrigan, R. W.; Hager, G. D.; Crosby, G. A. Chem. Phys. Lett. 1973,21,487. (b) Hager, G. D.; Crosby, G. A. J. Am. Chem. SOC.9175,97, 703 1. (12) (a) Nijs, H.; Fripiat, J. J.; Van Damme, H. J. Phys. Chem. 1983,87, 1279. (b) Nijs, H.; Cruz, M. I.; Fripiat, J. J.; Van Damme, H., J . Chem. SOC. Chem. Commun. 1981, 1026. (13) Pinnavaia, T. J. Science (Washington, D.C.)1983, 220, 365.
0 1984 American Chemical Society
