J. Phys. Chem. 1983, 87,5083-5090
on the statics and dynamics of dilute aqueous solutions of weakly hydrated, spherical ions. The information obtained is of a type which is useful for the interpretation of experimental data, and the comparison with experiment shows that currently available potentials provide a satisfactory description on the ion-water interactions in solution. There are some statistical problems connected with the fact that the ions are present only in low concentration, but these are confined to studies of the ion dynamics, and there are few difficulties associated with the calculation of either the structure around the ion or the dynamics of the water molecules in the region close to the ion. Secondly, the calculated structural properties are in very good agreement with the experimentally determined radial distribution functions. The results may therefore be used with some confidence in predicting coordination numbers and ion-water geometries for those systems for which experimental measurements are so far lacking. Thirdly, the ionic self-diffusion coefficients, though not measurable with high accuracy, reproduce the tendency to increase with ion size which is a special feature of ionic solutions. However, the mobilities of the ions are more profitably analyzed by an indirect approach based on the behavior of the ions in the first coordination shell. Fourthly, the behavior of the velocity autocorrelation functions of the ions and of their coordination shell suggests that the solventberg model provides a satisfactory picture of the dynamics only for ions such as Li+. Fifthly, a definition
5083
of hydration number in terms of a time scale defined by the relative rates of breakup of the local order in the solution and in pure water leads to results which are in reasonable accord with estimates based on electrochemical measurements. Finally, we have incorporated the results of the simulations into a theoretical model which relates the ionic mobility to the drag experienced by a sphere moving in a fluid of nonuniform viscosity. The fluid viscosity in the vicinity of the ion may be characterized either by the reorientation correlation times of water molecules in the region or by their residence times in the first coordination shell. In both cases, good agreement is found with ionic mobility data. An attractive feature of the model is the fact that it provides a natural link between the structure (coordination number) and dynamics (ionic mobility) of the solution, the concept of residence time (and hence of hydration number) playing the key, intermediary role. Acknowledgment. We are grateful for the financial support provided by the SERC through the award of a Research Studentship (to R.W.I.) and an Advanced Fellowship (to P.A.M.). We thank Dr. G. W. Neilson and Dr. J. R. Newsome for supplying us with their neutron diffraction data in tabular form, and Dr. J. N. Agar for a critical reading of the manuscript. Registry No. HzO, 7732-18-5; Li, 7439-93-2; Na, 7440-23-5; K, 7440-09-7; F-,16984-48-8; C1-, 16887-00-6.
ARTICLES Solvated Phenol Studied by Supersonic Jet Spectroscopy Akira Oikawa, Haruo Abe, Naohiko Mikami, and Mitsuo Ito” Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan (Received:May 10, 1983)
Fluorescence excitation spectra, dispersed fluorescence spectra, and mass-selected multiphoton ionization (MPI) spectra have been observed for the complexes formed between phenol and various solvents prepared in a supersonic free jet. The spectra of the 1:l (phenol, solvent) and 1:2 complexes have been identified from dependence of the fluorescence excitation spectra upon the pressure of solvent or He and from the mass-selected MPI spectra. It was found that all the solvents studied (water, methanol, ethanol, dioxane, and benzene) induce red shifts of the phenol absorption by formation of the 1:l hydrogen-bonded complex. The second solvent molecule which interacts with the 1:l complex results in red or blue shift of the absorption of the 1:1 complex by the 1:2 complex formation depending on the nature of the interaction. Intermolecular vibrations of the 1:l and 1:2 complexes were also obtained for both the ground and excited states.
Introduction “Solvation” is probably the most important concept which is widely used in all aspects of chemistry. However, solvation is the most difficult subject to explore on the molecular level. The answers to naive questions such as how is a solute molecule surrounded by solvent molecules, how many of the solvent molecules are called solvating molecules, what are the structure and dynamics of the solvated molecule, and so on are still vague. These fundamental questions would be solved if we could pursue
experimentally the processes by which an isolated solute molecule is subsequently surrounded by individual solvent molecules. Recently, the supersonic expansion technique was proved to be very useful to prepare isolated ultracold molecules in the gas phase1 and it can be applied to explore solvation processes on the molecular level. From the above viewpoint, weakly bound intermolecular compounds such (1) See, for example, D. (1980).
H.Levy, Annu. ReL. Ph>s. Chem., 31,
0022-3654/83/2087-5083$01.50/00 1983 American Chemical Society
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The Journal of Physical Chemistry, Vol. 87, No. 25,
1983
as van der Waals molecule^,^-'^ hydrogen-bonded comp l e ~ e s , ' ~and - ~ ~charge-transfer c ~ m p l e x e have s ~ ~been ~~~ studied in a supersonic free jet, and fairly detailed information has been obtained for the solvation processes especially for rare gas atoms.15 However, studies dealing with the usual solvents which are most frequently used in chemistry are very f e ~ . ~ ~ $ ~ ~ i ~ ~ In the last few years, we studied the electronic spectra of phenols hydrogen-bonded with various solvent molecules having proton-accepting ability in a supersonic free jet and obtained information about intermolecular vibrations of the hydrogen bond in the electronic ground and excited states, the existence of isomeric complexes, and vibrational relaxation.18-20In these studies, the partial pressure of the solvent was kept as low as possible in order to see the first step of the complexation. Therefore, the complexes that we studied there were the 1:l complexes. In the present study, we are mainly interested in further steps of solvation, that is, the interaction between the 1:l complex and additional solvent molecules. In the present paper, we report the fluorescence excitation spectrum of the mixture of phenol and solvent (water, methanol, ethanol, 1,4-dioxane, or benzene) in a supersonic free jet and its dependence on the pressure of solvent or He. From the observed pressure dependence, the spectra of the complexes of different compositions were differentiated and they were identified from the time-offlight mass-selected MPI spectra to be 1:l and 1:2 complexes. The dispersed fluorescence spectra of the 1:2 complexes were also observed. From the observed results, the second step of solvation, that is, the interaction of the second solvent molecule and the 1:l hydrogen-bonded complex, is discussed. Experimental Section The fluorescence excitation spectra of jet-cooled com(2) R. E. Smalley, L. Wharton, D. H. Levy, and D. W. Chandler, J. Chem. Phys., 68, 2487 (1978). (3) A. Amirav, U. Even, and J. Jortner, Chem. Phys. Lett., 67,9 (1979). (4) J. M. Steed, T. A. Dixon, and W. Klemperer, J. Chem. Phys., 70, 4940 (1979). ~. ~. (5)'s. M. Beck, M. G. Liverman, D. L. Monts, and R. E. Smalley, J . Chem. Phys., 70, 232 (1979). (6) J. E. Kenny, K. E. Johnson, W. Sharfin, and D. H. Levy, J. Chem. P ~ Y S .72, , 1109 (i98o). (7) T. R. Hays, W. Henke, H. L. Selzle, and E. W. Schlag, Chem. Phys. Lett., 77, 19 (1981). (8) A. M. Griffiths and P. A. Freedman, Chem. Phys., 63,469 (1981). (9) D. E. Powers, J. B. Hopkins, and R. E. Smalley, J . Phys. Chem., 85, 2711 (1981). (10) A. Amirav, U. Even, and J. Jortner, J . Phys. Chem., 85, 309 (1981). (11)K. H. Fung, W. E. Henke, T. R. Hays, H. L. Selzle, and E. W. Schlag, J. Phys. Chem., 85, 3560 (1981). (12) J. B. Hopkins, D. E. Powers, and R. E. Smalley, J . Phys. Chem., 8.5. 3739 ._ - - - 11981). ~----, (13) P. R. R. Langridge-Smith, D. V. Brumbaugh, C. C. Haynam, and D. H. Levy, J . Phys. Chem., 85, 3742 (1981). (14) K. H. Fung, H. L. Selzle, and E. W. Schlag, Z. Naturforsh. A, 36, 1338 (1981). (15) A. Amirav, U. Even, and J. Jortner, J . Chem. Phys., 75, 2489 (1981). (16) S. Leutwyler, U. Even, and J. Jortner, Chem. Phys. Lett., 86 439 (1982). (17) U. Even and J. Jortner, J . Phys. Chem., 87, 28 (1983). (18) H. Abe, N. Mikami, and M. Ito, J. Phys. Chem., 86,1768 (1982). (19) H. Abe, N. Mikami, M. Ito, and Y. Udagawa, J. Phys. Chem., 86, 2567 (1982). (20) H. Abe, N. Mikami, M. Ito, and Y. Udagawa, Chem. Phys. Lett., 93, 217 (1982). (21) P. M. Felker and A. H. Zewail, Chem. Phys. Lett., 94,448 (1983). (22) Y. Nibu, H. Abe, N. Mikami, and M. I b , J. Phys. Chem.,in press. (23) K. Fuke and K. Kaya, Chem. Phys. Lett., 91, 311 (1982). (24) T. D. Russell and D. H. Levy, J . Phys. Chem., 86, 2718 (1982). (25) N. Gonohe, N. Suzuki, H. Abe, N. Mikami, and M. Ito, Chem. Phys. Lett., 94, 549 (1983), (26) N. Gonohe, H. Abe, N. Mikami, and M. Ito, J . Phys. Chem., in press.
Oikawa et al.
J,
/A /
36200 /
/
36100 WAVENUMBER / cm-I
36coO
'
3660
'
8 'o*$
Figure 1. Fluorescence excitation spectra of phenol-water mixtures in a supersonic free jet. Pressures of phenol and helium are fixed at 0.8 and 2280 torr, respectively, and the pressure of water is varied.
,
/
36100
36200
,
36300
WAVENUMBER Icrn-1
Flgure 2. Fluorescence excitation spectra of phenol-benzene mixtures in a supersonic free jet. Pressures of phenol and helium are fixed at 0.8 and 2280 torr, respectively, and the pressure of benzene is varied.
plexes of phenol with various solvent molecules were observed with the same apparatus as that reported elsehere.'^,^^ The partial pressure of phenol in the nozzle was about 0.8 torr (vapor pressure of phenol at 310 K), and that of the solvent was controlled in the range up to its vapor pressure a t room temperature by the use of a thermoregulator (Toyo LHC-ll), in which a sample holder containing the solvent was immersed. Helium was used as a diluent gas and its pressure was controlled in the range from 760 to 4560 torr. The gaseous mixture of phenolsolvent-He was expanded continuously into a vacuum chamber through a 50-pm nozzle. The partial pressure of the solvent in the nozzle Po(S)was inverted to a reduced pressure PI by the use of the relation given by28
PI =
(m/mHe)1'2(pCLHe+PhOH/I*S+PhOH)1'zp~(s)
I
where m and 1.1 denote the average molecular mass of helium and the solvent, and the reduced mass, respectively. The second harmonic of a dye laser (Molectron DL-14) pumped by a nitrogen laser (Molectron UV22 or UV24) with a spectral width of 1.5 cm-l was used as the exciting light. The fluorescence excitation spectra were obtained 7 mm downstream from the nozzle, and the laser intensity was kept constant over the spectral region studied. Phenol (Wako, >99%) was purified by repeated sublimation under vacuum. Water, methanol, ethanol, benzene, and 1,4-dioxane were used as the solvent molecules. They were of spectral or special grade and were used without (27) N. Mikami, A. Hiraya, I. Fujiwara, and M. Ito, Chem. Phys. Lett., 74, 531 (1980). (28) G. M. McClelland, K. L. Saenger, J. J. Valentini, and D. R. Herschbach, J . Phys. Chem., 83, 947 (1979).
Solvated Phenol In a Supersonlc Jet
further purification except that water contamination in methanol and ethanol was extensively removed by a desiccant (Nishio, molecular sieve 3A). The dispersed fluorescence spectra of the phenol complexes were observed with the pulsed supersonic jet apparatus already described e l s e ~ h e r e . ' ~The , ~ ~spectra were dispersed by a Nalumi 0.75-m grating spectrograph with a spectral resolution of about 5 cm-l and were observed 10 mm downstream from the nozzle. The mass-selected MPI spectra of the phenol complexes were observed by time-of-flight technique with a resolution of about 10 mass number per 94 mass number (free phenol).
The Journal of Physical Chemistry, Vol. 87, No. 25, 1983 5085
, 35900 36033 36100 362co 36303 Results and Discussion WAVENUMBER /cm ' Pressure Dependence of Fluorescence Excitation Figure 3. Fluorescence excitation spectra of phenol-methanol mixSpectra. Figure 1shows the fluorescenceexcitation spectra tures in a supersonic free jet. Pressures of phenol and helium are fixed of phenol-water mixtures in a supersonic free jet. The at 0.8 and 2280 torr, respectively. The pressure of methanol is 3.2 spectral region corresponds to the S1-So transition of (A) and 8.3 (B) torr. phenol. The pressures of phenol and helium are fixed, and that of water is varied. Main bands of the spectrum are already assigned by our groupls and by Fuke and K a ~ a , ~ ~ 1.3 torr who used the mass-selected MPI method: the band a a t 1I I C 36 352 cm-l is the electronic origin (0,O) of free phenol; b-d at 35997,36 118, and 36 153 cm-', respectively, are assigned to phenol-H20 complex; and e a t 36 259 cm-' is assigned to phen~l-(H,O)~ complex. It is seen from the figure that, at low pressure of water, only the bands of the phenol-H,O complex appear, while as the concentration of water increases the band of the phenol-(H20), complex begins to appear and grows more rapidly than those of the phenol-H20 complex. In this way, the 1:2 complex exhibits 35900 36033 36100 36200 36300 a pressure dependence different from that of the 1:l complex, providing a useful means for classification of the WAVENUMBER / cm-' complexes having different coordination number. Flgure 4. Fluorescence excitation spectra of phenolilioxane mixtures In Figure 2 are shown the fluorescence excitation spectra in a supersonlc free jet. Pressures of phenol and helium are fixed at of phenol-benzene mixtures in a supersonic free jet ob0.8 and 2280 torr, respectively. The pressure of dioxane is 1.3 (A) and 4.5 (B) torr. tained a t various pressures of benzene. The band a a t 36 352 cm-l in the figure is the 0,O band of free phenol. The from the longer wavelength bands e-g (at 35 830,35 845, bands b-d (at 36 205,36 210, and 36 225 cm-l, respectively) and 35 860 cm-l, respectively). were assigned by Abe et al.19920to the 1:l complex formed Analysis of the Pressure Dependence. Amirav, Even, between phenol and benzene. The assignment is supported and Jortner15have derived the theoretical formula for the from their parallel pressure dependences. On the other formation process of van der Waals molecules prepared hand, the bands e-g (at 36 036, 36 070, and 36 101 cm-', in a supersonic free jet, and applied them for binary system respectively) exhibit a pressure dependence different from consisting of tetracene and rare gas. By the use of their that of the b-d bands. The different pressure dependence method, we derived a similar formula for the phenol comis clearly seen, for example, by comparison of the two plexes where the system includes three components, spectra obtained a t the benzene pressures of 5.6 and 11 phenol, solvent, and He. First, we shall deal with the torr: the intensities of the b-d bands decrease in going intensity decrease of the 0,O band of free phenol accomfrom 5.6 to 11torr, while the e-g bands show an intensity panied by the complex formation. We assume that the 1:l increase. Such pressure dependences suggest that the b-'d complex is formed via the following three-body recombiand e-g bands belong to complexes of different componation process: sition. It will be shown later that the b-d and e-g bands are assigned to phenol-C6H6 and phenol-(C9H6)2 comki P S HeP S He (1) plexes, respectively. It may be worthwhile to point out that a broad background absorption grows with an increase of where P and S denote phenol and solvent molecules, rethe benzene pressure and the spectrum approaches more spectively, and Iz, is the three-body recombination rate or less the solution spectrum a t room temperature. constant. Since under our experimental conditions the The solvent pressure dependence was also observed for concentration of helium is much larger than those of mixtures of phenol-methanol and phenol-dioxane, and phenol and solvent, the following processes can be netheir results are shown only for two different pressures of glected: the solvent in Figures 3 and 4. It is seen from the pressure P + s + P-PS + P dependence that, in the case of phenol-methanol, the bands b-d (at 35935,35963, and 36 112 cm-l, respectively) P +s+s-PS +s can be classified into one group and the band e (at 36 166 cm-') into another group. Similarly, in the case of pheThen, a procedure quite similar t o that developed by Amirav et al.I5 can be applied for our system and we obtain nol-dioxane, the shorter wavelength bands b-d (at 35938, 35 962, and 36 079 cm-', respectively) can be differentiated (2) [PI = [PI0 exp(-K$HepS) (29) K. Fuke and K. Kaya, Chem. Phys. Lett., 94, 97 (1983). where [PI is the concentration of free phenol and is reI
1
I
1
1
1
+ +
,
I
I
+
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The Journal of Physical Chemistry, Vol. 87, No. 25, 1983
Oikawa et al.
0.5 -
r m
4
5
a2-
I
z
4 O't Ln z
- 005I
,z 002$0 001I
I
v
0
10 20 PRESSURE CF SOLVENT / torr
00051
Flgure 5. Pressure dependence of the intensity of the 0,O band of free phenol: (a) solvent pressure dependence, (b) He pressure dependence. The phenol pressure is 0.8 torr. I n b, (If,w)"ps is plotted against p H e because, even if the pressure of solvent is fixed, its reduced pressure slightly changes by the change of He pressure.
TABLE I: Three-Body Recombination Coefficients between Phenol and Various Solvents in a Supersonic Free J e t solvent benzene methanol ethanol 1,4-dioxane water a
105K~(rz)a/ torr-2
3.6 5.5 8.8 15.5 2.6
(0.98) (0.99) (0.95) (0.98) (0.98)
105K,b/
Avc/
torre2
cm-'
1.9 (k0.5) 2.8 (tO.9) 7.2 ( r 3 . 5 ) 7.5 ( t 1 . 9 ) 8.5 ( i 6 . 5 )
147 417 409 414 355
0002[ 1
5
Figure 6. Dependences of the intensities of the bands of phenol(dioxane), complex normalized to the 0,O band intensity of free phenol upon pressure of dioxane (A) and pressure of He (B). In A, p b is fixed at 2280 torr and the intensities are estimated from the peak height. In B, p s is slightly varied (1.3-1.4 torr) and the intensities are estimated from the band area because of the different rotational temperatures for different He pressures. Notations of the bands should be referred to Figure 4.
Under the same assumption as that given by Amirav et al.15 that the three-body recombination rate constant a t the nozzle k , is independent of n, we have the relation
[psnl / [PI = (KpH$S)"
represents the quality of the linear fit t o the data points. Spectral K , value obtained by change of He pressure. red shift f r o m t h e 0, 0 band of free phenol induced by the 1:l complex formation.
placed by the intensity of the 0,O band, and [PI, is a numerical constant. pHeand p s are the pressure of helium and the reduced pressure of solvent in the nozzle, respectively. K , represents a three-body recombination coefficient which is roughly proportional to kl. On the basis of eq 2, the intensity of the 0,O band of free phenol was plotted against p s and pHe in Figure 5. Figure 5a shows the results obtained when the pressure of helium is fixed and that of solvent is varied, while the results for fixed solvent pressure and varied He pressure are shown in Figure 5b. As seen from the figures, the plots for each case roughly give a straight line. The slope of the line gives the K1 value in eq 2. In Table I the K1 values obtained are summarized. The value obtained from Figure 5a must be identical with that obtained from Figure 5b. They agree within a factor of 2 except for water, giving qualitative support for eq 2. Except for water, the increasing order of K1 is consistent between the results obtained by the pressure variances of solvent and He, and it is the order of benzene, methanol, ethanol, and 1,4-dioxane. It is interesting that this order roughly agrees with the order of the spectral red shifts from the 0,O bands of free phenol induced by the complex formation, which are given in the last column of Table I. The exceptional features of water are not well understood. Appreciable concentrations of water dimer, trimer, and so on even in the gas phase might be responsible for the observed deviation. Next, we shall consider the pressure dependence of the band intensity of the phenol-(solvent), complex. We assume that the l:n complex is prepared from the 1:n-1 complex by the following three-body recombination process:
+ He 2 PS, + He
2
PWPS/x103 torr2
K , value obtained by change of solvent pressure, r 2
S + PS,-l
g n.31
!
PRESWRE OF HELIUM /x@torr
(3)
(4)
where n is the coordination number, and K is the unified three-body recombination coefficient. In Figure 6 are shown, for example, the plots of the intensities of several bands of the phenol-dioxane complex against the partial pressures of dioxane (Figure 6A) and of He (Figure 6B). In the figures, the ordinate represents the relative intensity of the complex normalized to that of the 0,O band of free phenol. It is seen from Figure 6A that the plots for each band are approximately on a straight line except for the plots a t the highest pressure studied. According to eq 4, the slope of the line gives the coordination number n, whose values are also given in the figure. n values obtained for the bands b-d (indicated in Figure 4, and assigned by Abe et al.18Jsto the O,O, 0,0+22 (bending mode of hydrogen bond), and 0,0+137 cm-' (stretching mode of hydrogen bond), respectively, of the phenol-dioxane hydrogen-bonded complex) are about 1, while n 2 for the e-g bands (indicated in Figure 4). Similar results are also obtained from Figure 6B, where the pressure of helium is varied, although somewhat larger n values were obtained for the e-g bands compared with n 2 obtained from Figure 6A. These results indicate that the b-d bands belong to the phenol-dioxane complex and the e-g bands to the phenol-(dioxane)2 complex. This conclusion will be confirmed later from the time-of-flight mass-selected MPI spectra. Plots similar to Figure 6, A and B, were obtained also for the bands of the complexes with water, methanol, and benzene. In all cases, the plots for each band give nearly a straight line over the pressure ranges studied (1-5 atm for He, 0.5-10 torr for methanol, 0.3-15 torr for benzene, and 0.1-6 torr for water). n values obtained from the straight lines are summarized in Table I1 together with the results obtained from a mass ionization experiment described later. In general, n values obtained from the pressure variations of solvent and of He are quite different. However, in all cases, the bands can be classified into two groups having small and large n values. It will be con-
-
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The Journal of Physical Chemistry, Vol. 87, No. 25, 1983
Solvated Phenol in a Supersonic Jet
5087
TABLE 11: S u m m a r y of t h e Experimental Data for t h e Main Bands of Various Phenol-Solvent Complexes with Their Assignmentsa band notationb
-
i;/cm-l
assignmentC
n ps
pHe
36205 36210 36225 36036 36070 36101
Phenol-Benzene PS,O,O 0.61 PS,’O,Od 0 . 6 1 PSIPA 0.60 PS,Ot 1.2 PS,Pi 1.2 PSzP,2 0.86
1.1 0.99 0.80 2.2 1.6 2.4
35935 35963 36112 36166
Phenol-Methanol PS,O,O 0.26 PSIPA 0.31 PS,oi 0.56 PS,O; 1.5
1.2 1.0 1.2 1.6
35938 35962 36079 35830 35845 35 860
Phenol-Dioxane PS,O; 1.1 PSIPA 1.2 PS,ot 1.2 PS,O: 2.2 PS,PA 2.5 PS,Pi 2.2
1.1 1.5 1.0 3.3 2.4 3.1
35997 36118 36153 3 6 259
Phenol-Water PS,O; 0.49 PSIPA 0.45 PS,ui 0.53 PS,O: 1.5
mass
1 0 20
Ew
2
1
1 2
1.5
le 1.4 2.8
lcoxo
MASSNUMBER
2e
a The coordination number n obtained b y the pressure dependences of solvent and He and by mass-selected MPI spectra are given in t h e last three columns. Band notaPSI a n d PS, tions are the same as those in Figures 1-4. represent phenol-(solvent) a n d phenol-(solver t ) , c o m plexes, respectively. p and u designate intermolecular bending a n d stretching vibrational modes, respectively. P S I ’ shows a conformational isomer of PS,. e F r o m ref 29.
‘
firmed later from the mass-selected MPI experiment that the bands of small n value are due to the 1:l complex and the bands of large n value to the 1:2 complex. In this way, the pressure dependence was found to be very useful for classification of the complexes. However, the facts that the n value obtained differs in general from the coordination number and that there is no consistency between the results obtained from the pressure variations of solvent and helium are probably ascribed to breakdown of the “democratic” assumption adopted here that the three-body recombination rate constant k, is independent of n. In all the cases studied, the bands due to the 1:2 complex appear after achieving sufficient growth of the bands of the 1:l complex and in some cases we found growth of the 1:2 complex at the expense of the 1:l complex. These facts seem to support the stepwise formation of the l:n complex from the 1:n-1 complex, which was assumed here and by Amirav e t al.15 Mass-Selected MPI Spectra. Mass-selected MPI spectra have been observed for the phenol-benzene and phenol-dioxane complexes by means of the time-of-flight technique to confirm the classification of the bands described in a preceding section. As an example, the timeof-flight mass spectra of the phenol-benzene mixture are shown in Figure 7A. The mass spectra a, b, and e in Figure 7A are those obtained by excitation of the bands a, b, and e in Figure 2 (a is the 0,O band of free phenol; b and e are the bands of the complex a t 36 205 and 36 036 cm-l, respectively). In each spectrum, only one sharp and strong peak appears and its mass number corresponds to free phenol (94), phenol-(C6H6) (94 -t 78), and phenol-
36100 36203 36300 WAVENUMBER / cm-’
Figure 7. (A) Timesf-flight mass spectra of phenol-benzene mixture in a supersonic free jet obtained by exciting the bands a, b, and e shown in Figure 2. (B) Mass-selected MPI spectra (b and e) obtained by fixing the mass numbers to the strongest peaks of mass spectra (b and e) of A. a in B is the fluorescence excitation spectrum of phenol-benzene mixture.
(C6H6), (94 + 2 X 78) for a, b, and e, respectively, within an error of 3%. We observed also the mass-selected MPI spectrum by fixing the gate time corresponding to each complex. The results obtained are shown in b and e in Figure 7B for the 1:l and 1:2 complexes. It is seen that the bands b-d appear strongly in the spectrum b, while the bands e-g are enhanced in intensity in the spectrum e. Therefore, we conclude that the b-d bands are due to the phenol-C6H6 complex and the e-g bands to the phenOl-(C&& complex. This conclusion is consistent with that derived from the pressure dependence and also from the vibrational analysis of the fluorescence excitation spectrum.18 Similar measurements for the phenol-dioxane complex showed conclusively that the bands b-d (see Figure 4) are assigned to the 1:l complex and the bands e-g to the 1:2 complex. The results obtained by the mass ionization experiments are listed in the last column of Table 11. It may be worthwhile to mention that the mass spectrum obtained by exciting the spectral region of the phenol(dioxane)2 complex exhibits a weak peak ascribed to phenol-(dioxane), complex in addition to the strong peak due to the 1:2 complex. This implies the existence of bands due to the 1:3 complex near the spectral region of the 1:2 complex. Several weak bands are found a t 10-30 cm-l displaced on the longer wavelength side from the e band and also in the spectral region of the 1:2 complex. They might be ascribed to the 1:3 complex. However, because of their weakness, the mass-selected MPI spectrum of the 1:3 complex could not be obtained. Solvent-Induced Spectral Shift. In Table I1 are summarized the main bands of the various complexes together with their tentative assignments. Most of the low-frequency fundamentals given there are assigned to intermolecular vibrations in the excited state. In each complex, the strong band at the longest wavelength was taken as the 0,O band, which is underlined in the table. By reference to the positions of the 0,O bands, we notice an interesting propensity for the spectral shift of the
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The Journal of Physical Chemistty, Vol. 87,No. 25, 1983 IpJ3Jl
On the other hand, benzene or dioxane can act only as a proton-accepting molecule. Therefore, there seems no reason that the second solvent molecule is preferentially bound on the oxygen atom of phenol. The observed red shifts for benzene and dioxane suggest that dispersive interaction is responsible for binding of the second solvent molecule with the 1:l hydrogen-bonded complex. Red shift induced by dispersive interaction is known as polarization red shift and the magnitude of the red shift is nearly proportional to the polarizability of the solvent molecule.30 The larger red shift for benzene (169 cm-l relative to the 0,O band of the 1:l complex) than that of dioxane (108 cm-l) is qualitatively explained by larger polarizability of benzene. In the case of a van der Waals molecule consisting of a substituted benzene and a nonpolar solvent molecule like CC14,26the polarization red shift amounts to 70-150 cm-l. Therefore, we feel that the red shifts for benzene and dioxane are due to the dispersive interaction between the 1:l complex and the second solvent molecule. The 1:2 complex in which the second molecule is bound by dispersive interaction presumably has a structure like
t’i
/
1
35900
1
,
36000
Oikawa et ai.
1
,
36100
WAVENUMBER / cm-’
Figure 8. Dispersed fluorescence spectra of phenoi-(C6H6), complex in a supersonic free jet obtained by exciting the bands e, f, and g in Figure 2.
2 1
complex relative to the 0,O band of free phenol (at 36 352 cm-’). In the formation of the 1:l complex, the 0,O band shifts toward the red from the 0,O band of free phenol by in which the second solvent molecule is located above the 147, 355,414, and 417 cm-l for benzene, water, dioxane, benzene plane of the pheno1.12J3 and methanol, respectively. It was shown in our previous As mentioned in a previous section, we have an indicapaperla that the spectral red shifts of the 1:l complexes tion for dioxane that there exist the bands due to the 1:3 have good relationships with the heat of formation of the complex near or on the slightly longer wavelength side of hydrogen bond and also with the intermolecular vibrational the 0,O band of the 1:2 complex. This suggests that the frequency of the hydrogen bond. It is concluded from the third solvent molecule is weakly bound to the 1:2 complex observed relationships that the 1:l complex is the hydroand it does not induce an appreciable effect on the elecgen-bonded complex in which phenol and solvent serve as tronic state of the 1:2 complex. In this sense, the number a proton donor and a proton acceptor, respectively. of strongly solvating dioxane molecules is 2 and more Therefore, the first solvent molecule which interacts with dioxane molecules may be regarded as forming a medium. bare phenol is bound by the hydrogen atom of OH of Vibrational Analyses of Fluorescence Excitation phenol. Spectra and Dispersed Fluorescence Spectra of 1:2 ComIt is interesting to know how and where the second ~ ~ fluorescence J~ excitation plexes. In previous p a p e r ~ , the solvent molecule is bound. It is seen from Table I1 that spectra and dispersed fluorescence spectra of the 1:l hythe 0,O band of the 1:2 complex appears on the red side drogen-bonded complexes in a supersonic free jet were of the corresponding 1:l complex for benzene and dioxane, reported and we obtained the intermolecular stretching but on the blue side for methanol and water. The blue and bending vibrations of the hydrogen bond in the shift of the 1:2 complex relative to the 1:l complex was electronic ground and excited states from the analyses of already discussed for phenol-water by Fuke and K a ~ a . ~ ~the spectra. In this section, we shall describe briefly the They concluded that the second water molecule is acting vibrational structures of the fluorescence excitation and as a proton donor and is bound a t the oxygen atom of OH dispersed fluorescence spectra of the 1:2 complexes. of phenol as shown by The fluorescence excitation spectrum of the phenol(C6H6)2 complex shown in Figure 2 has a rather simple 2 vibrational structure consisting of the e-g bands (see Figure 2) and their frequency intervals are about 30 cm-l. O /H , The longest wavelength band e a t 36 036 cm-l is taken as the 0,O band. The dispersed fluorescence spectra obtained by exciting the e-g bands are shown in Figure 8. Two kinds of low-frequency vibrations in the ground state are found and they are of -12 and -35 cm-’. From the 0” observed Franck-Condon pattern, the ground-state vibration of -35 cm-’ corresponds to the excited-state frequency of -30 cm-l. However, the excited-state counterpart of the ground-state vibration of 12 cm-l could not Because of similar blue shift, the 1:2 complex of methanol be identified from the excitation spectrum. We tentatively will assume a similar structure. Water and methanol have both proton-accepting and -donating powers, and phenol also has the same properties. The structure shown above (30) H. C. Longuet-Higgins and J. A. Pople, J. Chem. Phys., 278 192 (1957). is one in which these characters are fully employed.
4’
-
Solvated Phenol in a Supersonic Jet
The Journal of Physical Chemistty, Vol. 87, No. 25, 7983 5089
1312 13 13
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rrm
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.
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,
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,
36100
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I
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,
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Flgure 9. Dispersed fluorescence spectra of phenol-(dioxane), complex in a supersonic free jet obtained by exciting the bands e, f, and g in Figure 4.
assign -12 and -35 cm-' to the modes involving parallel and perpendicular motions, respectively, of the second benzene molecule relative to the molecular plane of phenol of the 1:l complex (hereafter they are referred to as bending p and stretching u, respectively). The assignments seem reasonable compared with the intermolecular vibrations (of 20 and 50 cm-' for stretching and bending, respe~tive1y)'~J~ of the a-hydrogen bond of the 1:l complex, because the bonding of the second molecule is supposed to be weaker than the a-hydrogen bonding of the 1:l complex. The fluorescence excitation spectrum of the phenol( d i ~ x a n ecomplex )~ consists of three main bands e-g (see Figure 4) which are members of a progression of 15 cm-'. The longest wavelength band e a t 35830 cm-I is the 0,O band. In Figure 9 are shown the dispersed fluorescence spectra obtained by exciting the e-g bands. Although the spectra are rather complicated, we can find two groundstate vibrations of 13 and -45 cm-l in all the spectra. The vibration of 13 cm-I corresponds to the excited-state vibration of 15 cm-'. The observed two vibrations may be assigned to the bending (p) and stretching ( 0 ) vibrations of the second dioxane molecule from reasoning similar to that given for the benzene case. The vibrational frequencies are comparable with the corresponding frequencies of the phenol-(C6H6), complex, implying similar intermolecular interactions between the second solvent molecule and the 1:l complex for benzene and dioxane. The very low vibrational frequencies support the conclusion given in a previous section that the second solvent molecule is bound by dispersive interaction. The frequencies of the stretching modes of -45 cm-' for dioxane and -35 cm-I for benzene are comparable with the frequency of 40 cm-l observed for fluorobenzene-CC1, complex by Gonohe e t a1.26and also with the calculated frequency of 40-50 cm-l for tetracene-rare gas complex reported by Ondrechen et al.31 The frequency of the stretching vibration of the hydrogen bond formed with the (31) M. J. Ondrechen, 2.Berkovitch-Yellin, and J. Jortner, J. Am. Chern. Soc., 103,6586 (1981).
Figure 10. (A) Dispersed fluorescence spectrum of phenol-(H,O), complex in a supersonic free jet obtained by exciting the band e in Figure 1. (B) Dispersed fluorescence spectrum of phenol-(methanol), complex in a supersonic free jet obtained by exciting the band e in Figure 3.
first dioxane molecule in the 1:l complex is 128 cm-' in the ground state,Ig which is much larger than -45 cm-' for the second dioxane molecule. Therefore, there is no doubt that the second solvent molcule is much more weakly bound than the first solvent molecule. As mentioned in a previous section, the bands of the fluorescence excitation spectra of the phen~l-(H.,O)~ and phenol-(methanol), complexes appear on the blue side of the corresponding 1:l complex. The e bands (at 36259 cm-' for water in Figure 1and at 36 166 cm-' for methanol in Figure 3) are the strongest bands of the 1:2 complex and they are assigned to the 0,O bands. The other bands are very weak and have no regular vibrational structure. The dispersed fluorescence spectra obtained by exciting the e bands are shown in Figure 10, A and B. In the spectra, the vibrational frequency of 187 cm-' and the two vibrational frequencies of 41 and 192 cm-' are found for water and methanol, respectively. Because of their rather high frequencies, they are probably the vibrations associated with the hydrogen bond involving the second methanol or water molecule which is acting as a proton donor. We tentatively assigned the 41 cm-' (methanol) to the ground-state bending vibration of the hydrogen bond, and 187 (water) and 192 cm-' (methanol) to the stretching vibrations of the hydrogen bonds. It is interesting that the stretching frequency is larger than that of the hydrogen bond of the 1:l complex (153 cm-' for water, 162 cm-l for m e t h a n ~ l ) , in ~ ~which J ~ the first solvent molecule is acting as a proton acceptor. The above results obtained clearly indicate that the second solvent molecule in the 1:2 complex is weakly bound by dispersive force for nonpolar solvent molecules having no proton-donating power like benzene an dioxane, while the second molecule is rather strongly bound by hydrogen bonding for solvents possessing both proton-accepting and -donating abilities.
Conclusion The foregoing results have shown that phenol is solvated by a t least two solvent molecules. The first solvent molecule is bound by the hydrogen bond, and the second molecule by hydrogen bond or by dispersive force depending on the proton-donating ability of the solvent. In one case, an indication has been obtained for the existence
5090
J. Phys. Chem. 1903, 87,5090-5098
of the 1:3 complex, in which the third solvent molecule is considered to be very weakly bound and has no appreciable effect on the electronic state of phenol. It may be inferred from these results that the phenol solution is composed mainly of the free phenol, the 1:l complex and the 1:2 complex with the proportions determined by temperature and concentration of the solution. Since we know now the spectra and the intermolecular vibrations in the ground and excited states of the various complexes, it will be possible to simulate the solution spectra of phenol in various solvents which are usually broad.32 Such a sim-
ulation will be expected to contribute a great deal to the understanding of the solution state on the molecular level and it is now in progress in our laboratory. Acknowledgment. We thank Mr. Yasushi Tomioka and Mr. Minoru Ichijo for their experimental assistance. Registry No. HzO, 7732-18-5;phenol, 108-95-2;methanol, 67-56-1;ethanol, 64-17-5;1,4-dioxane,123-91-1;benzene, 71-43-2. (32) D. L. Gerrard and W. F. Maddams, Spectrochim. Acta, Part A , 34, 1205 (1978), and references therein.
Excited-State Polarizabilities and Dipole Moments of Diphenylpolyenes and Retinal Morgan Pondert and Richard Mathies” Deparfment of Chemistry, University of California, Berkeley, California 94720 (Received: May 18, 1983)
Electric-field-induced changes in the optical absorption spectra of linear polyenes have been used to derive values for excited-state dipole moments and polarizabilities. The electric field spectra of diphenylbutadiene (DPB), diphenylhexatriene (DPH), diphenyloctatetraene (DPO), and diphenyldecapentaene (DPD) were measured in dioxane solution. The increases in long-axis polarizability upon excitation to the first dipole-allowed (lBJ excited state are 170 f 30 A3 for DPB, 270 f 20 A3 for DPH, 350 f 10 A3 for DPO, and 420 f 40 A3 for DPD. The solution-phase, long-axis, excited-state polarizabilities of these linear polyenes are 3-4 times larger than the ground-state polarizabilities. The dipole moment and polarizability changes upon excitation to the “lB,” excited state of all-trans-retinal (ATR) were measured in poly(methy1methacrylate) (PMM) films. The long-axis polarizability increases by 600 f 100 A3 and the dipole moment increases by 13.2 f 0.6 D. These solution-phase polarizabilities and dipole moments were converted to gas-phase values by using an ellipsoidal cavity reaction field model. The increases in long-axis polarizability upon electronic excitation in the gas phase are 80 f 10 A3 for DPB, 127 f 6 A3 for DPH, 170 f 4 A3 for DPO, 210 f 20 A3 for DPD, and 200 f 20 A3 for ATR. Because the excited-state polarizability of retinal is so large, the conversion of its solution-phase, excited-state dipole moment (19.8 f 0.7 D) to a gas-phase value (7 f 1 D) involves an unusually large reaction field factor. The relation of these results to proposed mechanisms for regulation of the absorption maximum in visual pigments and bacteriorhodopsin is discussed.
Introduction Conjugated polyenes play an important photochemical role in visual pigments and in the energy-converting protein bacteriorhodopsin.l Because the primary event in the photochemistry of these systems is the excited-state isomerization of the polyene retinal (Figure l ) , detailed information about the excited-state structure of retinal and related polyenes is needed to understand the functioning of these pigments. The two lowest singlet excited states of polyenes possess “Ag”and “B,” symmetry2and are very close in energy, with the exact order dependent on chain length.3 Other important excited-state properties of polyenes, such as their charge distribution and polarizability, are not as well understood. In this paper we use the electric-field-induced broadening of optical absorption spectra4to directly measure excited-state dipole moments and polarizabilities for all-trans-retinal and a series of diphenylpolyenes. These experiments will help to characterize the electrostatic interactions between retinal’s excited state and its protein environment, which are thought to play a central role in several aspects of the photochemistry of ‘Present address: Southern Research Institute, Birmingham, AL 35255.
visual pigments and bacteriorhodopsin. For example, the protonated Schiff base of 11-cis-retinal absorbs maximally at -440 nm in solution, while the ,,,A of this same chromophore in visual pigments ranges from 430 to 575 nm.5 Similarly, the absorption of the all-trans-retinal protonated Schiff base shifts from -450 nm in solution to 568 nm in bacteriorhodopsin. It has been proposed that these red shifts are caused by the electrostatic interaction of retinal’s excited state with charged amino acid residues? Furthermore, the photoisomerization quantum yield of the 11-cis-retinal protonated Schiff base rises dramatically upon going from solution to the visual protein environment,’ suggesting some specific catalytic interaction be(1) R. R. Birge, Annu. Reu. Biophys. Bioeng., 10, 315 (1981). (2) Assuming an effective CZhpoint group. (3) B. S. Hudson, B. E. Kohler, and K. Schulten in “Excited States”, Vol. 6, E. C. Lim, Ed., Academic Press, New York, 1982. (4) W. Liptay in “Excited States”, Vol. 1, E. C. Lim, Ed., Academic Press, New York, 1974, pp 129-229. ( 5 ) J. N. Lythgoe in “Handbook of Sensory Physiology”,Vol. 7/1, H. J. A. Dartnall, Ed., Springer-Verlag, New York, 1972, pp 604-24. (6) (a) A. Kropf and R. Hubbard, Ann. N . Y. Acad. Sci., 74,266 (1958); (b) M. Arnaboldi, M. G. Motto, K. Tsujimoto, V. Balogh-Nair, and K. Nakanishi, J. A m . Chem. SOC.,101, 7082 (1979); (c) K. Nakanishi, V. Balogh-Nair, M. Arnaboldi, K. Tsujimoto, and B. Honig, ibid., 102, 7945 (1980); (d) F. Derguini, C. G. Caldwell, M. G. Motto, V. Balogh-Nair, and K. Nakanishi, ibid., 105, 646 (1983).
0022-3654/83/2087-5090$01.50/00 1983 American Chemical Society
