J. Phys. Chem. 1982, 86, 1768-1771
1768
Fluorescence Excitation Spectra of Hydrogen-Bonded Phenols in a Supersonic Free Jet Haruo A b , Naohlko Mlkaml, and MHruo Ito' Depaflment of Chemlstty, Faculty of Science, Tohoku Unlverslty, Sendal980, Japan (Received: November 10, 1981)
The fluorescence excitation spectra of hydrogen-bonded complexes of phenol with various proton-accepting molecules prepared in a supersonic jet have been observed. The hydrogen-bonded complexes exhibit spectra having a well-resolved structure characteristic of intermolecular vibrations of the hydrogen bonding.
Introduction The effect of hydrogen bonding on the S1 So absorption spectrum of phenol at 275 nm has been extensively studied in the solution state. The hydrogen-bonded complexes of phenol with proton-accepting molecules such as ethers and alcohols are known to shift the spectra to longer wavelengths from that of free phenol by 200-400 cm-l, depending on the proton-accepting power of the bases.'" The corresponding gaseous-state absorption spectra have never been observed because of spectral congestion due to many hot bands. Recently it has been demonstrated that the use of a supersonic free jet is an excellent method to prepare an ultracold molecule in the gas phase and to significantly reduce the spectral cong e ~ t i o n . ~We report here the fluorescence excitation spectra of various hydrogen-bonded phenols prepared in a supersonic free expansion. The observed spectrum of each hydrogen-bonded complex exhibits a well-resolved structure characteristic of the intermolecular vibrations of the hydrogen bonding. Existence of more than one hydrogen-bonded species was found for each proton-accepting molecule except dioxane and methanol. +
Experimental Section The fluorescence excitation spectrum of the pulsed supersonic jet composed of a gaseous mixture of phenol and proton acceptor seeded in 2.5 atm of He gas was measured with an apparatus described elsewhere.s The second harmonic of a dye laser (Molectron DL-24) pumped by a pulsed nitrogen laser (Molectron UV-22) was used as the exciting light. The laser frequency covered was from 280 to 260 nm with fwhm of 1cm-'. The spectra were observed 14 mm downstream from an orifice of 400-pm diameter. Phenol (Wako, >99%) was dehydrated and purified by sublimation under vacuum 3 times. Dioxane, water, dimethyl ether, diethyl ether, tetrahydrofuran, methanol, ethanol, 1-propanol, cyclohexene, and benzene were used as the proton-accepting molecules; they were of spectral grade or special grade and were used without further purification. The partial pressure of phenol in the gas mixture was about 400 mtorr (vapor pressure of phenol at room tem-
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(1)D.L.Gerrard and W. F. Maddams, Spectrochim. Acta, Part A , 34, 1205,1213 (1978). (2)G. Nemethy and A. Ray, J. Phys. Chem., 77,64 (1973). (3)H.Baba and S.Suzuki, J. Chem. Phys., 35, 1118 (1961). (4)S. Nagakura, J. Chem. SOC.Jpn., 74,1154 (1953). (5)S. Nagakura and H. Baba, J. Am. Chem. Soc., 74,5693(1952);J. Chem. SOC.Jpn., 72,3 (1951). (6)M. Ito, J. Mol. Spectrosc., 4, 106 (1960). (7)D.H.Levy, Annu. Rev. Phys. Chem., 31, 197 (1980). (8)N. Mikami, A. Hiraya, I. Fujiwara, and M. Ito, Chem. Phys. Lett., 74,531 (1980). 0022-3654/82/2086-1768$01.25/0
perature), and that of the proton acceptor was controlled in the range up to ita vapor pressure at room temperature by adjusting a needle valve of a reservoir containing the proton acceptor. The total pressure of the gas mixture and He carrier gas was kept at 2.5 atm. At these conditions the background pressure in the expansion chamber was about 2 X lo4 torr at 8 Hz repetition rate of the nozzlevalve opening. Results and Discussion Figure 1 shows the longer-wavelength region of the fluorescence excitation spectra of phenol vapor at room temperature (A) and the supersonic free jet of phenol seeded in 2.5 atm of He gas (B). It is apparent that none of the vibrational hot bands observed in the room-temperature spectrumg are present in the supersonic-jet spectrum and that the 0,Oband in the jet becomes extremely sharp. From the disappearance of the vibrational hot bands and the band sharpening, the vibrational and rotational temperatures of the free jet are estimated to be less than 50 K and about 5 K, respectively. The supersonic-jet spectrum shows no sign of formation of phenol dimer or phenol-He complex. Figure 1C shows the supersonic-jet spectrum of the gaseous mixture of phenol and dioxane. Beside the 0,Oband of free phenol, several new bands appear in the longer-wavelength region displaced by 200-400 cm-' from the 0,Oband of free phenol. Since their intensities vary with the concentration of dioxane, they are readily assigned to hydrogen-bonded complex between phenol and dioxane. The spectrum of the complex exhibits a simple structure consisting of three progressions of 23 cm-' (a-c,) as shown in Figure 1C. The a and b progressions have similar relative intensity distributions and are separated by 137 cm-'. Since the frequencies of 23 and 137 cm-' are too small for the intramolecular vibrations of the constituent molecules, they are assigned to the excited-state vibrations characteristic of the hydrogen bond. We assign 23 and 137 cm-' to the bending and stretching vibrations, respectively, of the hydrogen bond formed between phenol and dioxane. This assignment is reasonable in comparison with the known frequencies of the hydrogen bond for other complexes.1° Progression c in Figure 1C starts from the shorter-wavelength position shifted by 4 cm-' from the first member of the a progression and the intensity distribution of the c progression is different from that of the a or b progression. Moreover, the intensity of the c progression increases with an increase in temperature of the jet. We therefore interpret the c progression as due to transitions from u" (9)H. D.Bist, J. C. D. Brand, and D. R. Williams, J. Mol. Spectrosc., 24,413 (1967). (10)G. C.Pimentel and A. L. McClellan, "The Hydrogen Bond", W. H. Freeman, San Francisco, CA, 1960,p 68; S. G. W.Ginn and J. L. Wood, Spectrochim. Acta, Part A, 23,611 (1967).
0 1982 American Chemical Society
The Journal of phvsical Chemistry, Vol. 86, No. 10, 1982 1769
Hydrogen-Bonded Phenols in a Supersonic Free Jet
36000
35900
36100
36200
36300
36400
excitation frequency (cm-1)
0
Flgure 3. Fluorescence excitation spectrum of phenol-water in a supersonic free jet. 22 23 23
ma
1
.
,
.
,
I
36600
I
.
36iOO
'
36200
excitation frequency ( cm-1 )
Flgm 1. Fluoregcence excitatbn spectra of (A) phenol vapor at room temperature, (B) phenol in a supersonic free jet, and (C) phenol-dk oxane mlxture in a supersonic free jet. 35800
36000
36200
36400
exutation frequency (cm-1)
Flgum 4. Fluorescenceexcitation spectra of (a) phenocdknethylether, (b) phenol-diethyl ether, and (c) phenol-tetrahybofwan in a supersonic free jet.
00
I
I 12;
6ab
1;
18;
I
36000
36200
36400
36600
36800
37000
37200
37400
exClfo1lon 'requcncy (cm-1)
Flgure 2. Supersonic-jet spectrum of phenol-dloxane in the entire spectral reglon studied. The main vlbronlc bands of free phenol and those of the hydrogen-bonded phenol are indicated separately by two horizontal lines above the figure. Assignments for the vlbronlc bands of free phenol are taken from ref 9.
= 1 of the bending vibration in the ground state to u' = 1 , 2 , and 3 of the corresponding bending vibration in the excited state. This shows that the frequency of the ground-state bending vibration must be 18 cm-l, which is smaller than 23 cm-' of the excited-state bending vibration. The larger bending frequency in the excited state agrees with the general observation that the hydrogen bonding of phenol becomes stronger in going from So to S1(T,T*). Figure 2 shows the supersonic-jet spectrum of the gaseous mixture of phenol and dioxane in the entire spectral region observed. It is seen that the spectral feature shown in Figure 1C repeatedly appears in combination with the main vibronic bands of free phenol." The simple structure of the phenoldoxane spectrum suggests the existence of only one hydrogen-bonded species. In this respect, the phenoldoxane complex is the most simple one among the (11) Such a spectral feature as shown in Figure 2 was observed for all complexea between phenol and the other proton acceptors studied and could be interpreted a~ vibronic bands of phenol in the complexes.
complexes studied here since, as shown later, several hydrogen-bonded species exist in the case of other protonaccepting molecules. Figure 3 shows the supersonic-jet spectrum of a mixture of phenol and water. Several sharp bands appear in the wavelength region longer than the 0,Oband of free phenol, and they are also assigned to the hydrogen-bonded complexes between phenol and water. A spectral feature of the phenol-water mixture that is different from the phenol-dioxane spectrum is that no progression of the bending vibration is seen in the former. However, bands related by the frequency interval of 120 cm-' are found in the phenol-water spectrum; 120 cm-' corresponds to 137 cm-' of the phenol-dioxane complex and is assigned to the stretching vibration of the hydrogen bond formed between phenol and water. Several band origins from which the 120-cm-' progressions start indicate the existence of more than one hydrogen-bonded species. At least four kinds of complexes are evident from Figure 3. Since water is capable of self-association,the hydrogen-bonded complexes of phenol-(H20), with n = 1, 2, 3, ... are expected to be formed. Moreover, for (H20), with n # 1,various kinds of conformational isomers are possible. Therefore, it is natural that several different hydrogen-bonded species are detected in the spectrum. Although identification of each hydrogen-bonded species is not possible from the observed spectrum, the strongest band at the longest wavelength is probably assigned to the complex with monomeric water. The spectra of the hydrogen-bonded complexes with other proton-accepting molecules possess more or less both features represented by the spectra of phenol-dioxane and
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The Journal of physlcal Chemistry, Vol. 86,No. 10, 1982 27 27
Abe et al.
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(a)
23 23
m
3 5900 I
35900
.
I
,
,
36100
36100 excitation frequency (cm-1)
36300
36100
excitation frequency ( cm-1) Figure 5. Fluoresence excitation spectra of (a) phenol-methanol, (b) phenol-ethanol, and (c) phenol-1-propanol in a supersonic free jet.
phenol-water. Figure 4 shows the spectra of the complexes with dimethyl ether, diethyl ether, and tetrahydrofuran. The spectral features of the complexes with these ethers are similar to those of the phenol-dioxane spectrum with respect to appearance of the excited-state bending vibration. However, each spectrum is hard to analyze in terms of only one hydrogen-bonded species, suggesting the existence of more than one species for each case. The existence of more than one complex is similar to the case of phenol-water. However, the bands associated with the stretching vibration of the hydrogen bond are not found in the case of these ethers. Since ether is not a self-associating molecule, the origin of different hydrogen-bonded species should be sought elsewhere. In the case of diethyl ether, several intramolecular rotational isomerd2will give different hydrogenbonded species. This could be part of the reason for the complexity of the spectrum. In the case of dimethyl ether, two prominent band origins from which the progression of the bending vibration of -30 cm-' starts are observed, and they are separated by 107 cm-', as seen in Figure 4a. This 107-cm-' frequency is not a stretching vibration of the hydrogen bond. If it were a stretching vibration, we could expect the appearance of a longer progression of 107 cm-', judging from the observed intensities of the two bands. I t is therefore considered that the two bands are due to different hydrogen-bonded species. The two species are not ascribed to the intramolecular rotational isomers of the proton-accepting molecule because dimethyl ether itself has no rotational isomers. Although identification of these two species is not possible at the present time, we feel that they might be intermolecular rotational isomers around the hydrogen bond. The complexes with ethers give rise to spectra at the longest-wavelength region compared with the other complexes studied. As a result, the vibronic bands of the Complexes involving the excited-state 6a vibration of phenol appear near the 0,Oband of free (12) W.Wieser, W.G. Laidlaw, P. J. Krueger, and H. Fuhrer, Spectrochim. Actu, Purt A , 24, 1055 (1968).
Flgure 6. Fluorescence excltatlon spectra of (a) phenol-cyclohexene and (b) phenol-benzene in a supersonic free jet.
phenol with the same structure as the origin bands of the complexes seen in Figure 4. The spectra of complexes formed with several alcohols are shown in Figure 5. In the case of methanol, the spectrum is surprisingly simple in spite of the possible existence of various kinds of polymeric methanols. The observed spectrum is analyzed simply by the excited-state bending and stretching vibrations of one hydrogen-bonded species. The hydrogen-bonded species giving rise to the observed spectrum is probably the complex with monomeric methanol. The absence or weakness of the bands which are assigned to complexes formed with polymeric methanols is due to their small concentrations and/or low fluorescence quantum yields. In the case of ethanol and 1-propanol, the spectra become complicated by the appearance of several band origins as seen from Figure 5, b and c. The complexity may be ascribed to the intramolecular rotational isomers of ethanol and 1-propanol and/or the intermolecular rotational isomers around the hydrogen bond. Judging from the result of methanol, the possibility of hydrogen-bonded complexes with polymeric alcohols seems to be small. Figure 6 shows the spectra of phenol-cyclohexene and phenol-benzene. The bands appearing on the longerwavelength side of the 0,O band of free phenol may be assigned to the complexes. Now the binding partner of phenol is a molecule involving a *-electron system. The most probable complexes to be formed will be ones involving ?r hydrogen bonding. Appearance of the low-frequency bending vibrations and spectral features similar to those of the other hydrogen-bonded complexes studied support the hypothesis that OH of phenol directly participates in the complex formation. In the case of phenol-cyclohexene, hydrogen bonding between OH of phenol and C=C of cyclohexene is most probable. However, as seen from Figure 6a, there exist several complexes for phenol-cyclohexene. They are broadly classified into two groups: the complexes giving the spectra at 36 100-36 200 cm-' and those at 35 950-36 000 cm-'. A similar situation is also seen in the case of phenol-benzene, where one group gives the bands at 36200-36300 cm-' and another at 36 000-36 100 cm-'. The classification into two groups is
J. Phys. Chem. 1982, 86, 1771-1775
TABLE I: Intermolecular Vibrational Frequencies of Hydrogen-Bonded Phenols in the First Excited Electronic State and the Spectral Red Shifts from the 0,O Band of Free Phenol (in cm-') proton acceptor
v,ja
water methanol ethanol 1-propanol
27 24 23 29 26 3o 31 23 21 20 16
dimethyl ether diethyl ether dioxane
THF
cy clohexene benzene a
Bending.
Stretching.
VU b
A uC
121 175 150 131
355 41 5 409 395 447 455 56 2 431 411 477 235 147
137 101 50
Spectral shift.
- A H (Kcal/mol)
't 0
100
200 red shift
300 dV
400
500
(cm-1)
Flgun, 7. Plot of the spectral red shift vs. the heat of formation of the hydrogen bond In the ground state taken from ref 13.
also supported by the difference of the frequency of the
bending vibration between the two spectral regions. It is concluded from these observations that there exist two types of 7~ hydrogen-bonded complexes with quite different
1771
bond strengths. As seen from Figure 6, each group also has several complexes, suggesting the existence of complexes with slightly different structures. Table I summarizes the excited-state bending and stretching vibrational frequencies and the spectral red shifts of the complexes from the 0,Oband of free phenol. In the cases where several complexes are involved, those giving the strongest bands are included in the table. It can be seen from the table that there is a qualitative correlation among the bending and stretching frequencies and the spectral shifts. Therefore, the observed spectral shift gives a good measure of the strength of hydrogen bonding. In Figure 7, the spectral shift is plotted against the heat of formation of the hydrogen bond in the ground state.13 A good correlation between the two quantities is seen from the figure. In all of the cases except phenol-water, the progressions of the excited-state bending vibration were observed in the spectra. The appearance of the progression seems to indicate appreciable change of the angle 0-H ...A (A is a proton-accepting molecule) in the electronic transition. The change might be ascribed to the steric hindrance arising from the bulkiness of the accepting molecule. Finally, it is concluded that the supersonic free jet technique is a very powerful way to investigate hydrogen-bonded complexes and gives detailed information about these complexes. Identification and structure of the hydrogen-bonded complexes are now under study in our laboratory by means of fluorescence spectra and multiphoton ionization-mass spectra. Acknowledgment. We thank Dr. Jun-ichi Murakami for his experimental assistance and useful suggestions. (13) Data taken from: (a) A. S. N. Murthy and C. N. R. b o , Appl. Spectrosc. Reu., 2, 69 (1968), Table 2; (b) G. C. Pimentel and A. L. McClellan, Annu. Reu. Phys. Chem., 22,347 (1971); (c) G. C. Pimentel and A. L. McClellan, 'The Hydrogen Bond", W. H. Freeman, San Francisco, CA, 1960.
Test of Laplace Transform Inversion of Unlmoiecular Rate Constant Wendell Forst Department of Chem&fty and Centre de Recherches sur les Atomes et les M&cules (CRAM), Universit6 Lava/, Q&ec, Canade G1K 7P4 (Received: June 17, 1081; I n Final Form: November 16, 1081)
"Exact" quantum-mechanical rate constants k(E) of specified energy E for the nonadiabatic decomposition of N20are used to calculate, by Boltzmann averaging, the exact limiting high-pressureunimolecular rate constant k,. The temperature dependence of this k , is non-Arrhenius. Inversion leads to the recovery of a good smooth-function approximation to the exact k(E) if the activation energy E,, is expressed at least as a fourth-degree polynomial in temperature. This is because k(E)in the example chosen has an unusual energy dependence, and so provides a severe test. The pressure dependence of E,, (but not of kUi) is shown to reflect sensitively the temperature dependence of k,, and through it the energy dependence of k(E).
Introduction It is well-hown that at some temperature the high-pressure unimolecular rate constant k, is by definition the Boltzmann average of k(E), the microcanonical rate constant for decomposition of molecules having specified energy E. This average can be considered operationally' as the Laplace transform of k(E)N ( E ) / Q ,i.e. km
@'L{k(E) N ( E ) J
(1)
0022-3654/82/2086-177 1$01.25/0
where N ( E ) is the density of states of the decomposing molecule at E , and Q is its partition function at T. It is now also well-known2-6that one can recover k(E) from k , by inversion (Le., by taking the inverse Laplace (1) Slater, N. B. h o c . Leeds Philoe. Lit. SOC.,Sci. Sect. 1955, 6, 259. (2) Forat, W. J . Phys. Chem. 1972, 76, 342. (3) Forat, W. In "Reaction Transition States"; Dubois, J. E., Ed.; Gordon and Breach New York, 1972; p 75. (4) Forat, W. J . Phys. Chem. 1979, 83, 100. (5) Forat, W.; Turrell, S. Znt. J. Chem. Kinet. 1981, 13, 283.
0 1982 Amerlcan Chemical Society
