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J. Phys. Chem. 1982, 86, 2567-2569
Dispersed Fluorescence Spectra of Hydrogen-Bonded Phenols in a Supersonic Free Jet Haruo A b , Naohlko Mlkaml, Mlteuo Ito,' Lbpartment of Ctmmlshy, Faculty of Sclence, Tohoku Universlty, Sendel 980, Japan
and Yasuo Udagawa Institute for Molecular Science, Okazakl444, Japan (Received: Mer& 16, 1982)
Dispersed fluorescence spectra are reported for hydrogen-bonded complexes of phenol with various protonaccepting molecules prepared in a supersonic free jet. The spectra show a well-resolved structure characteristic of intermolecular vibrations from hydrogen bonding in the ground state. The possibility of excitation energy redistribution in the excited state leading to different conformational isomers is also suggested.
Introduction In our previous paper,' the fluorescence excitation spectra of hydrogen-bonded complexes of phenol with various proton acceptors prepared in a supersonic free jet were reported. In the spectra, low-frequency intermolecular vibrations from hydrogen bonding in the electronic excited state gave well-resolved sharp structures and were readily assigned to intermolecular stretching and bending vibrations. In this paper, we report the fluorescence spectra of the hydrogen-bonded complexes of phenol with dioxane, methanol, ethanol, diethyl ether, and benzene in a supersonic free jet by exciting various vibronic levels involving the intermolecular stretching and bending vibrations. The observed spectra exhibit well-resolved structures due to intermolecular vibrations from hydrogen bonding in the electronic ground state. Since intermolecular vibrations reflect directly the nature of the hydrogen bonding, their finding has great significance in the study of hydrogen bonding. Vibrational relaxation of the hydrogen-bonded complex in the electronic excited state into different conformational isomers through the intermolecular vibrations is also suggested. Experimental Section Fluorescence spectra were observed for the hydrogenbonded phenols with dioxane, methanol, ethanol, diethyl ether, and benzene. The preparation procedure of the hydrogen-bonded complexes and the pulsed supersonic jet apparatus used were essentially the same as described previously.'V2 The second harmonic of a dye laser (Quanta-Ray PDL) pumped by the third harmonic of a NdYAG laser (Quanta-Ray DCR-1A) was used as exciting light. The fluorescence spectra were obtained with a l-m Spex monochromator used in the second order of grating blazed at 500 nm. A HTV R562 photomultiplier with a Brookdeal 9415/9425 boxcar integrator was used for detection. Results and Discussion Intermolecular Vibrations in the Ground State. It was shown in our previous paper that the fluorescence excitation spectra of the hydrogen-bonded complexes show two main structures due to intermolecular stretching (v,) and bending (up) vibrations of the hydrogen bond. In the present study we have obtained dispersed fluorescence (1)H. Abe, N.Mikami, and M. Ito, J. Phys. Chem., 86,1768(1982). ( 2 ) N.Mikami, A. Hiraya, I. Fujiwara, and M. Ito, Chem. Phys. Lett., 74,531 (1980). 0022-365418212086-2567801.2510
TABLE I: Intermolecular Vibrational Frequencies of Hydrogen-Bonded Phenols in the Ground State and the First Excited Electronic State (in cm-') ground state excited statea bend- stretch- bend- stretchproton
ing
ing
ing
acceptor
VP
VU
VP
VU
22 22 21, 38 19 26
162 153
27 24 31 23 20b
175 150
methanol
ethanol
ing
diethyl ether dioxane 128 137 benzene 50 a Taken from ref 1. From the present study of fluorescence spectra the value in ref 1was revised.
spectra by exciting the electronic origin (O,O), bending vibrational levels (@, n = 1,2), and stretching vibrational level (ai) of the S1(mr*) state for each hydrogen-bonded complex. In Figure 1are shown the fluorescence spectra obtained by exciting (a) the electronic origin (0,O)of free phenol (at 36 348 cm-') and (b) that of the phenol-dioxane complex (at 35937 cm-l) in a supersonic free jet. In spectrum b, several low-frequency bands can be seen on the lowerfrequency side of the 0,Oband (exciting level) and a similar spectral pattern appears repeatedly for the 6ay band, while spectrum a has no such bands except for those due to intramolecular vibrations of free phen01.~ Therefore, spectrum b is definitely due to the hydrogen-bonded complex of phenol-dioxane. Figure 2 shows dispersed fluorescence spectra (b-e) near the exciting line of phenol-dioxane complex in a supersonic free jet together with the fluorescence excitation spectrum (a) indicating each exciting position. The first interval of 19 cm-' seen commonly in spectra b-e of Figure 2 is readily interpreted to be the fundamental frequency of the intermolecular bending vibration (uB) in the electronic ground state. This is consistent with the ground-state frequency of 18 cm-' obtained from hot-band analysis of the fluorescence excitation spectra in the previous paper. Moreover, the intermolecular stretching vibration (v,) is easily identified by the interval of 128 cm-' as shown in spectra b, c, and especially e in Figure 2, which corresponds to 137 cm-' in the electronic excited state. However, irregular structure beyond the vB fundamental is not easily interpreted. Such an irregularity was observed (3)H.D.Bist, J. C. D. Brand, and D. R. Williams,J. Mol. Spectrosc., 24, 402,413 (1967).
0 1982 American Chemical Soclety
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The Journal of Physical Chemistry, Vol. 86,No. 14, 1982
Letters
I!
(a)
/I
36100
36200 36300 excitation frequency(cm-1)
202221 22192:
n-rl-rn
coo
600
800
reicttve
200
,
0
frequency ( c m - ' )
Figure 1. Dispersed fluorescence spectra obtained by exciting the electronic origin (a) 0,O of free phenol (at 36348 cm-'), (b) 0,O of the phenol-dioxane complex (at 35 937 cm-I), and (c) of Sa: of the p h a noi-dioxane complex (at 38 423 cm-') in a supersonic free jet.
35900
300
200
36000 36100 excitation frequency icm-1 )
100 0 relative
300 200 100 f r e q u e n c y cm-1 )
b
Flgure 2. Dispersed fluorescence spectra of the phenol-dioxane complex in a supersonic free jet (b-e). Each excitiig level is indicated in the fluorescence excitation spectrum (a).
more or less for all complexes studied and will be discussed later. The fluorescence spectra of phenol-methanol (0,O and P i excitation) and phenol-ethanol (0,O excitation) show
1
400 300 200 100 relative frequency (cm-1)
.
1
0
Flgure 3. Dispersed fluorescence spectra of the phenol-benzene complex in a supersonic free jet (b and c). Each exciting level is indicated in the fluorescence excitation spectrum (a).
similar spectral patterns to that of phenol-dioxane, and the v, and uB ground-state frequencies were obtained for these complexes. They are listed in Table I together with the frequencies of the other complexes and the excitedstate frequencies obtained in our previous study. Figure 3 shows the fluorescence spectra of the phenolbenzene complex which is a typical r-type hydrogenbonded complex. In contrast to the other spectra described above, long progressions of almost the same interval of -20 cm-' are seen except for the first interval, which is clearly different from the former. Furthermore, an anomalous intensity distribution is found for the progression in Figure 3c. We have already suggested that the fluorescence excitation spectra indicate the existence of more than one hydrogen-bonded species such as conformational isomers around the hydrogen bonding. In the case of phenol-benzene, we have not assigned the first interval of 4 cm-' in the excitation spectrum. Now we have obtained the fluorescence spectra by exciting just the two levels forming this interval. We took therefore the first intervals of 25 and 27 cm-' in spectra b and c of Figure 3 as the vB fundamentals of two different but nearly the same conformational isomers. The bands forming the 20-cm-l progression are possibly due to fluorescence from the level which is not excited directly. In other words they suggest the possibility of energy redistribution in the excited state leading to the different isomers. In the following subsection we discuss this possibility in some detail. Energy Redistribution in the Excited State of Hydrogen-Bonded Complexes. Figure 4 shows the fluorescence spectra of phenol-diethyl ether in a supersonic free jet. Since the first intervals of spectra b and d of Figure 4 are the same (20 cm-l), it is confirmed that the two bands b and d in the excitation spectrum shown in Figure 4a are due to the same hydrogen-bonded species and may be assigned to its 0,O and PA. However, the strongest band
J. Phys. Chem. 1982, 86, 2569-2571 C
30 21
m
I
35900 36000 excitation frequency (cm-1)
I
I
35600
35700
35600
"
31 20
35900
frequency (cm-1)
Flgure 4. Dispersed fluorescence spectra of the phenol-diethyl ether complex in a supersonic free jet (b-d). Each exciting level Is Indicated in the fluorescence excitation spectrum (a).
c in Figure 4a giving the fluorescence spectrum of the first interval of 38 cm-l in Figure 4c must be due to another hydrogen-bonded species. Irregular bands beyond the first interval coincide in their spectral position among spectra b, c, and d in Figure 4, although their intensity distributions are quite different. The fact that the same spectral pattern appears regardless of excitation of different species and/or different levels gives evidence for relaxation of a part of the excitation energy into the common emitting levels of some different
2569
conformational isomers. Similar relaxation is seen for all the complexes studied here. Recently Smalley and his co-workers have shown that phenylalkanes display rapid intramolecular vibrational relaxation from well-localized ring modes to the bends and torsions of the alkyl chain.4 Similar phenomena were also observed for phen~xyalkanes~ and l-phenyl-l-alkynes.6 For hydrogen-bonded phenols the situation is similar to that of phenoxyalkanes. When compared with the chemical bonds of the alkyl chain, however, intermolecular hydrogen bonding is very loose and, as a result, hydrogen-bonded complexes have many low-frequency quanta of the intermolecular bending and torsion around the hydrogen bonding. These low-frequency quanta should increase the state density even at small excess energy. The broad spectral feature shown in Figure 2e (EexCeBB = 137 cm-'; g;) may therefore be explained as due to such an intermolecular vibrational relaxation. Because us is a intermolecular vibration and not a well-localized intramolecular vibration, there arises another question of whether or not hydrogen bonding is effective in accelerating intramolecular vibrational relaxation. Figure ICshows the fluorescence spectrum obtained by exciting the 6a; band of the phenol-dioxane complex in a supersonic free jet. The striking broadness of the 6a: band in contrast to the corresponding sharp band in the fluorescence spectrum of free phenol clearly shows that hydrogen bonding is an effective source to increase the state density and accelerate the intramolecular relaxation. Observation of the fluorescence spectra of hydrogenbonded phenols by exciting many other ring-localized modes is now under study and will be presented elsewhere. In conclusion, it became clear that fluorescence spectra give significant information on the low-frequency intermolecular vibrations in the ground state and the excitation energy redistribution in the excited state of hydrogenbonded complexes. (4) D. E. Powers, J. B. Hopkins, and R. E.Smalley, J. Chem. Phys., 72. 5721 (19801. ' ( 5 ) J. B. Hopkins, D. E.Powers, and R. E. Smalley, J. Chem. Phys., 74, 6986 (1981). (6) D. E.Powers, J. B. Hopkins, and R. E. Smalley, J . Chem. Phys., 74, 5971 (1981).
Photodissociation and Reactive Scattering Resonances D. C. Clary Department of ChemiStfY, University of Manchester Institute of Science and Technology, Manchester M60 100, United Kingdom (Received: April 5, 1982)
It is proposed that reactive scattering resonances might be the origin of oscillating structures in the UV photodissociation spectra of polyatomic molecules containing light atoms.
Introduction A simple interpretation of oscillating structures in the uv photodissociation spectra of polyabmic molecules has been presented by Pack.' He considered symmetric triatomic molecules XY2 and noted that the upper, dissociative electronic state should be treated as a reactive
scattering problem. He also assumed that the motion on this upper State could be separated into a b u n d symmetric stretch mode ( 2 ) and a dissociative asymmetric stretch mode ( X ) . The photodissociation cross section was then approximated by the ~ n - " - C h d o n formula fJ
(1) R. T Pack, J . Chem. Phys., 65, 4765 (1976). 0022-3654/82/2086-2569$01.25/0
=
F C J ( ~ ( . 2 : ~ E , f ) l ~ ( X , n i ) ) ( ~ ( ~ , U f ) l ~ ' ( Z , U i ) ) 1(1) 2 Vf
0 1982 American Chemical Society
