J. Phys. Chem. 1983, 87, 3582-3584
3582
Comparison of the Hydrogen Bond Formation of Indole in Solution and in a Supersonic Expansion 1. Montoro, C. Jouvet, A. Lopez-Camplllo, and 6. Soep' Laboratolre de Photophysique Mol6culaire du CNRS, + Bitlment 2 13, Universl Paris-Sud, 9 1405 Orsay Cedex, France (Received: April 4, 1983; I n Final Form: July 25, 1983)
Hydrogen bonding in indole has been deduced from the comparison of the spectra obtained between room temperature and 77 K in polar and nonpolar solutions. The fluorescence excitation spectra of hydrogen-bonded complexes of indole with various proton-acceptingmolecules such as alcohol, dioxane, and water have also been observed in supersonic expansion with helium. Introduction There exists an important body of information on hydrogen-bonded molecules, investigated through ultraviolet spectroscopy of solutions at low temperatures. The data show that intermolecular hydrogen bonding produces shifts and band broadenings on the absorption and emission spectra of free molecules in solution. Here, we propose to bridge this information with the direct observation of hydrogen-bonded complexes in a supersonic jet expansion. The specific absorption of the hydrogen-bonded species can be measured with great detail and accuracy in the low-temperature jet where perturbations due to solvent effects are absent. Still the advantage of the solution method lies in its simplicity so as to provide guide lines for spectroscopic studies in supersonic expansion. We have investigated indole and N-methylindole (NMI) which present great biological interest, since indole is the basic chromophore of the tryptophan residue found in many protein systems. Here, we report the observation of hydrogen-bonded complexes both in the condensed phase and the low-temperature supersonic expansion. Experimental Section The species under investigation (indole-water, indolealcohol, and indole-dioxane complexes) were formed in a supersonic free jet expansion. Indole and NMI maintained at temperatures between 20 and 50 " C in an oven and mixed to the helium carrier gas at pressures varying from 1.5 to 40 bar. Under these conditions the fraction of solute molecules varied from to The complexing molecules (alcohol, dioxane, water) mixed to helium were then added to the indole-He mixture, through a separate tubing just preceding the expansion. The relative concentration of foreign dioxane and alcohol complexing molecules to indole could be adjusted by its reservoir temperature (-40 " C ca 0.5 torr) or by changing the flow rate of its mixture with helium. Mixtures were expanded through a 100-pm nozzle into vacuum. Pumping was achieved by a 1000 m3/h roots pump backed by a rotatory pump. Total fluorescence excitation spectra were obtained with a N2 pump frequency doubled dye laser with a 2-cm-' resolution in the UV. Spectral scans were recorded together with the fringes of a Fabry Perot etalon (reference frequency 6.78 cm-I). The emission spectra of solutions were analyzed with a Perkin-Elmer Model MPF-3 fluorescence spectrophotometer. Fluorimetric studies of solutions between room temperature and 77 K were performed in a home-built cryostat using nitrogen gas as the coolant.
'Associ6 I l'Universit6 Paris-Sud. 0022-3654/83/2087-3582$O 1.5OlO
Indole (Aldrich) and NMI (Fluka A.G) were purified by vacuum sublimation and distillation, respectively. Results and Discussion UV Spectroscopy of Indole and NMI in Solution. By using UV absorption spectroscopy, Cazeau-Dubrocaet al.l have shown the formation of hydrogen-bonded complexes of indole in the ground state with polar solvent molecules at room temperature. These results are supported by those obtained here from UV emission spectroscopy of indole and NMI in solution, the emission being far more sensitive to the presence of a polar solvent. In polar solutions, the fluorescence spectrum of indole displays a strong Stokes shift as compared to the fluorescence from a nonpolar solution, while the corresponding absorption spectrum is only weakly displaced. In order to study correctly the emission of hydrogenbonded complexes in polar solutions, one finds it necessary to separate the dipolar interaction between the solute and the polar solvent molecules from the effect of the complexation. This dipolar interaction can be determined on the NMI molecule where the presence of a methyl group prevents the formation of the N-H complex. In addition the dipole moment of NMI is similar to that of indole. For this purpose, we have compared the emission spectra of NMI in polar (alcohol) and in nonpolar (paraffins) solutions obtained at different temperatures. At room temperature, the emission spectrum of NMI in butanol, presented in curve B in Figure la, is shifted by 14 nm ( N 1400 cm-') to the red as compared to the emission of NMI in 3-methylpentane (3MP) displayed in curve A in Figure la. We have interpreted this shift as due to the increased excited-state stabilization energy of NMI by its polar solvent cage. This increased stabilization energy results from the greater dipole moment of NMI in the excited state with respect to the ground state. This interpretation is based on data obtained from the temperature-dependent2and the time-resolved emission spectra of NMI in polar solvent^.^ At short times or high viscosity (low temperatures) the speectrum of NMI in polar solutions is unshifted (see Curve C, figure la) with respect to the nonpolar solutions. We explain these results by the time- or viscosity-limited reorientation of the solvent polar molecules surrounding the solute, thus hindering the stabilization of the excited NMI. We shall use this temperature dependence of emission spectra to eliminate the similar solvation effect in indole (1) C. Cazeau-Dubroca, F. Dupuy, M. Martineaud, and A. LopezCampillo, Chem. Phys. Lett. 23, 397 (1973). (2) T. Montoro, Tesina, Facultad de Ciencias fisicas, Universidad Complutense, Madrid. (3) 0. Benoist D'Azy, A. Lopez-Campillo, and T. Montoro, to be submitted for publication.
0 1983 American Chemical
Society
The Journal of Physical Chemistry, Vol. 87, No. 19, 1983 3583
a
'
1
0.0 34560 cm-'
35286 cm-'
-w.
1
i'YdJL
.-
I
Flgure 2. Low-pressure fluorescence excitation spectra of indole (A) and N-methylindole (6) in a supersonic free jet.
b
b
Flgure 1. (a) Emission spectra of N-methylindoie in (A) 3-methylpentane at 300 K and 1.5 X lo-" M, (6)butanol at 300 K and 1.5 X M, and (C) butanol at 113 K and 1.5 X lo-' M. (b) Emission spectra of Indole In (A) 3-methylpentane at 300 K and 1 X lo4 M, (6) butanol at 300 K and 1.5 X lo4 M, and (C) butanol at 113 K and 1.5 x 10-4 M.
polar solutions. At room temperature, the emissions of indole in butanol (curve B, Figure lb) and in 3MP (curve A, Figure lb) are shifted by 22 nm (2250 mc-l) with respect to each other. By cooling the sample, the emission of indole in butanol is shifted to the blue by 18 nm (1800 cm-') because of the limited reorientation of the solvent polar molecules around the solute, as in the case of NMI. We have interpreted the remaining 4-nm (450 cm-') shift as due to the emission of the complex between indole in the ground state and butanol, in agreement with data obtained by absorption spectroscopy.' Let us now connect this information with the direct observation of the hydrogen-bonded complex in a supersonic jet expansion. Hydrogen-Bonded Complexes in the Jet Expansion. The assignments to the hydrogen-bonded complexes of the spectral shifts observed for indole in solution have been confirmed by supersonic expansion experiments on indole in helium. ( a )Indole Complexes. Indole easily forms complexes with alcohol, dioxane, and water. All these complexes show the same red shift of about 150 cm-' with respect to the indole transition. The fluorescence excitation spectrum of indole expanded in pure He is displayed in Figure 2A. The origin band is located at 35 240 cm-' and is always accompanied on the red side by weak hot bands which can be identified by temperature effeds. The intensity of these (4) J. M.Hollas, Spectrochim. Acta, 198 753-67 (19763).
C
+V Figure 3. Fluorescence excitation spectra of indole-ethanol (a), indole-dioxane (b), and indole-water (c) in a supersonic free jet.
hot bands decreases with the jet temperature, Le., the increase of the backing pressure Pa. However, these bands cannot be totally eliminated in our experimental conditions with a vibrational temperature of 50 K. A t 720 cm-' to the blue from the origin, there appears an intense band which can be assigned to a benzene ring vibration, according to the spectra obtained in the vapor phase.5 When a small partial vapor pressure of alcohol (0.5 torr) is added to the helium carrier gas through the separate line, new satellite bands shifted by 218 cm-' appear on the red side of each indole transition, as shown in Figure 3a for the 0-0 transition. The intensity of these satellites is proportional to the alcohol concentration and hence it can be attributed to an indole-alcohol complex. Similar satellite bands can be observed in the excitation spectra of indole mixed with dioxane (Figure 3b) at a partial pressure of 0.5 torr. These bands are located on the red side at 176 and 191 cm-' from the origin. Here the (5) Nitish, K. Sanyal, S. L. Srivastava, and Sita Ram Tripathi, Spectrochim. Acta, Part A, 38, 933-5 (1982).
J. Phys. Chem. 1983, 87, 3584-3586
3584
40 34560 cm-1
\=43
Bars
F+25
Bars
F+15
Bars
I
3
-V
5,49
OC
Flgure 4. Fluorescence excitation spectra of pure N-methylindole in a supersonic free jet at various N-methylindole pressures.
spectra display more transitions than for alcohol. These dioxane-complex transitions are separated by spacings comparable to those observed for phenol-dioxane complexes6 and are therefore assigned to the hydrogen bond bending vibration. When water is added, a new band of the indole-water complex appears in the lower wavelength region displaced by 135 cm-' from the 0-0 free indole band (Figure 3c). The similar displacement of all the complex bands from the indole origin for molecules as different as dioxane, ~~
alcohol, and water pleads in favor of a similar kind of bonding. Similar displacements were observed for the 0-H- - -0bond of phenol Complexes? Here we suggest that the observed shift reveals the hydrogen bonding of the NH group of indole. Indole can only be a hydrogen donor to dioxane, thus, all complexes involve indole as a donor. In contrast, for the same ta* transitions, when the fluorescent molecule is an hydrogen acceptor as is the case for isoquinolein,' the hydrogen-bonded complexes are characterized by a much smaller shift. There still lies the possibility of hydrogen bonding with the a electron cloud of the benzene ring. To ascertain NH hydrogen bond formation, we have substituted NH with a N-methyl group. ( b )NMI Complexes. The excitation spectrum (Figure 2b) of pure N-methylindole expanded in helium shows the same characteristic vibrations as indole. The origin, located at 34 560 cm-l, id displaced by 680 cm-l from that of pure indole. When the helium pressure is increased, we observe, as is shown in Figure 4, the (He),, (He)2,... complexes of N-methylindole attributed through their helium pressure dependence. Moreover in the presence of dioxane or alcohol, no additional band appears except for those of the helium complexes in the same concentration range as we used for indole (0,5 torr partial pressure). Hence polar systems are not easily condensed on the indole ring and the bands on the complexes observed for indole are characteristic of the N-H hydrogen bond complexes. We have found evidence for hydrogen bond formation in supersonic jet expansion of indole. The bands assigned to the complex are characterized by shifts of ca. 150 cm-' to the red of indole transitions. These shifts are similar to those deduced by Kadiri from UV absorption spectroscopy in solution.8 Thus solution spectroscopy can provide useful information on the hydrogen-bonded complexes of various molecules.
Acknowledgment. We thank the referee for indicating to us the possible presence of water complexes. Registry No. Indole, 120-72-9;N-methylindole, 603-76-9; ethanol, 64-17-5; dioxane, 123-91-1;water, 7732-18-5.
~
(6) Hauro Abe, Nachiko Mikami, and Mitsuo Ito, J.Phys. Chem., 83, 1768-71 (1982).
(7) P. M. Felker and A. H. Zewail, Chem. Phys. Lett., 94, 448 (1983). (8) A. M. Kadiri, These docteur d'dtat, Universite de Bordeaux, 1978.
Variation of Counterion Binding in Micelles of Cetyltrimethylammonium Hydroxide Hernan Chalmovlch,' Iolanda M. Cuccovla, Deparmento de Bloquimica, Instituto de Qulmica, UniversMade de Sa0 Paulo, Sa0 Paulo, Brazil
Clifford A. Bunton,' and John R. Moffatt Department of Chemistty, University of California, Santa Barbara, California 93 106 (Received: M y 16, 1983; I n Final form: June 28, 1983)
Reaction of N-methyl-4-cyanopyridiniumfluoroborate (1) with OH- in aqueous micelles of cetyltrimethylammonium hydroxide (CTAOH) occurs wholly in the aqueous pseudophase. Comparison of the rate constants in CTAOH and NaOH allows estimation of the concentrations of free and micellar bound OH-. The values of a,the fractional ionization of the micelles, decrease with increasing concentration of OH-, in agreement with evidence from reactions of substrates bound to micelles of CTAOH. The effects of normal, nonfunctional, micelles upon reaction rates and equilibria are generally treated on the
assumption that reactants are distributed between micelles and water, treating each as a pseudophase, and with no
0022-3654/83/2087-3584$0 1.50/0 0 1983 American Chemical
Society
