J. Phys. Chem. 1987, 91, 517-526
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FEATURE ARTICLE Spectroscopy and Dynamics of Aromatic Molecules Having Large-Amplitude Motions Mitsuo Ito Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan (Received: July 21, 1986; In Final Form: October 8, 1986)
The spectroscopy and dynamics of aromatic molecules having large-amplitude motions are described with examples mainly obtained in our laboratory. A qualitative explanation is made of various laser electronic spectroscopies applied to jet-cooled molecules which are useful for the study of the largeamplitude motion of a molecule. Newly developed sensitized phosphorescence excitation spectroscopy and stimulated emission spectroscopy have proved to be powerful tools in elucidating roles of the large-amplitude motions in intramolecular relaxation processes. Three types of large-amplitude motions are discussed. One is the internal rotation of the CH3 group in several conjugated molecules. It is shown that the internal rotation of the CH, group of substituted toluenes dramatically changes upon electronic excitation. The barrier to the internal rotation is found to arise from hyperconjugation in the absence of steric hindrance. Experimental findings are also given for the role of the internal rotation of the CH3 group in intersystem crossing, vibronic coupling, and intramolecular vibrational redistribution (IVR). The second type of large-amplitude motion dealt with is the large-amplitude torsion of the C-C bonds connecting the ethylene group and the phenyl groups of trans-stilbene in its ground state. The potential for the torsional mode is very anharmonic and easily deforms by coupling with other vibrations. It is shown from the stimulated emission spectrum that the C-C torsion greatly accelerates IVR. A brief description is also given of the torsional potentials of tolan and biphenyl. The last type of large-amplitude motion is the ‘butterfly tunneling” which was found for the excited state of 1,2,4,5tetrafluorobenzene. This is a new type of tunneling which makes the molecule flexible in the excited state. Finally, implications of the study of large-amplitude motion and possible extension of the study to van der Waals molecules are mentioned.
Introduction Flexibility of a molecule is one of the important factors in chemical reaction and attracts a great deal of interest from both theory and experiment. The flexibility arises from the largeamplitude oscillational motion of a molecule which usually has a low frequency and a large anharmonicity. Well-known examples of large-amplitude motions are the internal rotation of a CH3 group, the umbrella motion in NH3,the torsion of the C-C bond in ethane, and so on. Vibrational potentials for these types of motion are one of the main subjects in rotational and vibrational spectroscopies such as microwave, Raman, and infrared. Much information is now available for the potentials of small polyatomic molecules in their electronically ground states. For large polyatomic molecules like benzene derivatives, however, the information is rather limited even for the ground states and virtually nonexistent for the electronically excited states. The large-amplitude motion of a molecule is greatly affected by the environment of a group or a part in the molecule which is undergoing the large-amplitude motion. The motion is easily subject to a change by intramolecular and intermolecular interactions. The potential of a large-amplitude motion therefore reflects such interactions and gives a good measure of the molecular structure and dynamics. Upon excitation of a molecule to an electronically excited state, a great change generally occurs in electronic structure. The change will be sensitively reflected by a change in the potential of the large-amplitude motion, which provides a good characterization of the electronic state and also reveals the electronic origin of the large-amplitude motion. One of the aims of the present paper is to show how the potential of the large-amplitude motion of a molecule changes upon electronic excitation and what molecular properties are responsible for the potential change. Another aim is to see the dynamical aspect of a large-amplitude motion. The large-amplitude motion of a molecule will play a major role in various relaxation processes in the molecule. Following Fermi’s golden rule, the nonradiative relaxation rate
from an initially prepared state a to interacting isoenergetic states b is given by
where Velis the electronic part of the interaction matrix element, F the Franck-Condon factor, and p the density of states in b. The great anharmonicity inherent in a large-amplitude motion contributes to an increase in F and the low frequency of the largeamplitude motion leads to a great increase in p . Therefore, these dual roles make the large-amplitude motion primarily important in relaxation processes. Several examples illustrating the roles of large-amplitude motions in intersystem crossing and intramolecular vibrational redistribution (IVR) will be given. Microwave, infrared, and Raman spectroscopies have traditionally been used for the study of the large-amplitude motions of molecules in electronically ground states. These spectroscopies, however, are difficult to apply to molecules in electronically excited states. For the studies of the potentials and dynamics of largeamplitude motion in both electronically ground and excited states, electronic spectroscopy dealing with electronic transition is most useful. Laser electronic spectroscopies which are applied to the molecule in a supersonic jet are particularly powerful for this purpose. Several laser spectroscopies useful for the study of a large-amplitude motion will be briefly described. Laser Spectroscopies Applied to Molecules in Supersonic Jets Before we discuss large-amplitude motions of molecules, several laser spectroscopies which are useful for the study of the motions will be briefly mentioned. Since the large-amplitude vibration of a molecule usually has a small frequency and a great anharmonicity, it often causes a spectral congestion resulting from the appearance of many hot bands in any spectrum of a gaseous sample at room temperature. Therefore, the preparation of molecules in a well-defined initial state is essential in the spectroscopy of large-amplitude vibrations. The molecules prepared
0022-3654/87/2091-0517$01.50/00 1987 American Chemical Society
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by supersonic expansion] are most suitable for this purpose. The expansion of a gaseous mixture of sample molecules and rare gas atoms (He or Ar) at several atmospheres into vacuum through a small nozzle orifice produces a supersonic jet, in which the molecule is cooled down by the collision-induced transfer of thermal energy of the molecule to translational energy of the rare gas atoms. The molecules in a supersonic jet are in a nonequilibrium state and the temperature of the molecules is generally different for the translational, rotational, and vibrational degrees of freedom. In most cases, the translational and rotational temperatures are less than a few degrees kelvin and 10 K, respectively. The vibrational temperature, however, is greatly different from molecule to molecule. In benzene, for example, T, is as high as 50 K, but it is less than 30 K in aniline under the same expansion condition. In general, the molecule having many low-frequency modes can be efficiently cooled down and its vibrational temperature becomes low. The aromatic molecules having low-frequency large-amplitude modes which will be discussed here have vibrational temperatures less than 10 K in supersonic jets. Therefore, the molecules are virtually in the zero-point energy levels in the ground state and they are also in a collision-free condition. Although a jet-cooled molecule is collision free and is in the well-defined lowest vibrational state, the concentration of the molecules is so low that usual spectroscopic means like absorption are difficult to apply. Vibrational spectroscopy such as Raman and infrared is also not easy to apply to jet-cooled molecules because of the relatively low sensitivity. Electronic spectroscopy, on the other hand, has a sensitivity much higher than that of the vibrational spectroscopy and is suitable for the study of molecules at low concentrations such as those in a supersonic jet. Various laser spectroscopies utilizing electronic transitions of jet-cooled molecules will be briefly described below. One-Color Laser Spectroscopies. (a) Fluorescence Excitation Spectrum. Laser-induced fluorescence is widely used for the observation of electronic transitions of a jet-cooled molecule. When tunable dye laser light irradiates a jet-cooled molecule and the total fluorescence emitted from the molecule is detected by scanning the laser frequency, the fluorescence excitation spectrum is obtained which practically coincides with the electronic absorption spectrum of the molecule. Since the electronic transition occurs exclusively from the zero-point level in the electronic ground state, all the vibronic bands in the excitation spectrum represent the vibrational levels in the electronically excited state. Because of the absence of hot bands, the low-frequency vibrational levels can be detected with a high resolution. ( b ) Dispersed Fluorescence Spectrum. When a jet-cooled molecule is excited by laser light of a frequency which coincides with that of a vibronic band in the fluorescence excitation spectrum, the molecule is excited to a specific vibronic level and emits fluorescence due to the transitions from this level to various ground-state vibrational levels. The spectrum obtained by dispersing the fluorescence with a monochromator is the dispersed fluorescence spectrum, from which the ground-state vibrational levels can be obtained. The information obtained from the dispersed fluorescence spectrum is, therefore, the same as that obtained from infrared and Raman spectroscopies. However, by selecting the vibronic level excited, one can obtain many dispersed fluorescence spectra, each of which shows a characteristic intensity distribution of the vibronic bands. The intensity distribution is determined by the Franck-Condon factors between the vibrational level in the excited-state and the ground-state vibrational levels reached by the electronic transition. The correspondence between the ground- and excited-state vibrational modes as well as the geometrical changes of a molecule between the ground and electronically excited states can be obtained from analyses of the intensity distributions. The dispersed fluorescence spectrum is particularly useful in determining the potentials of a large-amplitude mode in the ground and excited states and their relative position along the coordinate of the mode. (1) Levy, D.
H.Annu. Reo. Phys. Chem. 1980, 31, 197.
It0
(c) Sensitized Phosphorescence Excitation Spectrum. There are many molecules which are nonfluorescent. For these molecules, the spectroscopy utilizing laser-induced fluorescence is of no use. However, when the molecule in a jet is nonfluorescent but phosphorescent, a spectrum corresponding to the absorption spectrum can be obtained by monitoring the total phosphorescence emitted from the molecule and it is called a phosphorescence excitation spectrum. This method is very popular for molecules in the condensed phase. However, when applying the method to a molecule in a jet, one encounters a great difficulty in detecting the phosphorescence signal by a space-fixed detector because of the long life time of the phosphorescence and the high velocity of the phosphorescent (triplet-state) molecule in the jet. These difficulties can be overcome by placing an appropriate solid phosphor downstream from the jet.2 A jet-cooled molecule is electronically excited by laser light to an excited singlet state, from which the long-lived triplet state is produced by intersystem crossing. The triplet-state molecule travels downstream from the jet at a high speed and collides against the solid phosphor. Collision-induced energy transfer to the phosphor then results in sensitized phosphorescence. When the sensitized phosphorescence is detected by a detector placed near the solid phosphor as the laser frequency is scanned, the sensitized phosphorescence excitation spectrum of the jet-cooled molecule is obtained.* When a jet-cooled molecule is fluorescent as well as phosphorescent, both the fluorescence excitation spectrum and the sensitized phosphorescence excitation spectrum can be simultaneously measured with two detectors, one for the fluorescence and another for the phosphorescence, the former being placed near the crossing point of the laser beam and the jet, and the latter near the solid phosphor. Both spectra represent the same electronic transition from the ground state to the excited singlet state and give the same vibronic band positions. The intensity distribution of the vibronic bands, however, is in general different between the two spectra. This difference reflects the difference in intersystem crossing rates of the different vibronic levels of the excited singlet-state molecules. The simultaneous measurements of the fluorescence excitation and sensitized phosphorescent excitation spectra are particularly useful in elucidating the role of largeamplitude motions in the intersystem crossing process. (6)Multiphoton Ionization Spectrum. The multiphoton ionization technique3 is also very useful in the measurement of electronic transitions of a jet-cooled molecule. This method can be applied to nonemissive molecules. When the laser power is strong, the excited-state molecule produced by one-photon absorption of the ground-state molecule in a jet can absorb another one or two photons (in total, two or three photons) to reach the ionization continuum. There, the molecular ion is generated. By detecting the molecular ion while scanning the laser frequency, the one-photon resonant two or three-photon ionization spectrum is obtained which practically coincides with the absorption spectrum due to the transition from the ground state to the one-photon resonant excited state. Since all the ions generated can be efficiently collected by applying an appropriate electric field, multiphoton ionization (MPI) spectroscopy is a highly sensitive spectroscopic means. Because of the high sensitivity, the MPI method is often used for the measurement of the twophoton absorption spectrum of a jet-cooled molecule in spite of its very small cross section. The two-photon resonant MPI spectrum is very useful in detecting vibronic levels which are electric dipole forbidden since there is a difference in selection rules. This is especially true for the molecules having inversion symmetry for which irregular vibronic levels associated with an anharmonic large-amplitude mode can be identified only by the use of the one-photon absorption spectrum together with the two-photon absorption spectrum. Fluorescence excitation, sensitized phosphorescence excitation, and one-photon resonant MPI spectra all give the same vibronic (2) Abe, H.; Kamei, S . ; Mikami, N.; Ito, M . Chem. Phys. Leu. 1984, 109, 217. ( 3 ) Johnson, P. M. J. ChemPhys. 1975.62, 4562; 1976, 64,4143, 4638.
The Journal of Physical Chemistry, Vol. 91, No. 3, 1987 519
Feature Article bands as far as their spectral positions are concerned. The intensity distribution of the vibronic bands, however, is usually different among the three spectra. The intensity distribution is determined by the fluorescence quantum yields, phosphorescence quantum yields, and ionization cross sections, respectively, of the individual vibronic levels excited. Comparisons among these three types of spectra provide us with information on the competition between the three dissipation processes from the vibronic level. One-color laser spectroscopies, except for dispersed fluorescence spectroscopy, are high-resolution spectroscopies since the spectral resolution is determined only by the resolution of the laser used. The energies of excited-state vibrational levels obtained from these spectroscopies can be determined quite accurately. Therefore, an accurate potential can be obtained for the excited state. The spectral resolution of the dispersed fluorescence spectrum, on the other hand, is limited by the resolution of a dispersing monochromator which is usually much lower than the laser resolution. As a result, the ground-state potential determined by the dispersed fluorescence spectrum is less accurate than that of the excited-state potential. However, this situation can be improved by the use of two-color laser spectroscopies which will be described below. Spectroscopies with Two Color Lasers. ( a ) Stimulated Emission Spectroscopy (Ionization Dip and Fluorescence Dip Spectra). As was mentioned above, the resolution of the dispersed fluorescence spectrum is rather poor. However, the resolution can be greatly improved by utilizing stimulated emission. A molecule in a jet is excited by the first laser light of frequency v 1 to a selected vibrational level in an excited electronic state. Introducing the second tunable laser light of frequency v2, the excited molecule experiences stimulated emission when v 2 is resonant with a ground-state vibrational level. The stimulated emission can be probed by the depopulation of the excited-state molecules. The depopulation can be detected by two ways. One is to observe the intensity of the fluorescence from the excited l e ~ e l . ~In, ~the presence of v l , the excited-state molecule emits fluorescence of a constant intensity. When stimulated emission occurs at the frequency of v2, the fluorescence intensity decreases because of the depopulation of the excited molecules. Therefore, observing the total fluorescence while scanning the frequency of v2, one obtains the so called (two-color) fluorescence dip spectrum. The dip spectrum is essentially the same as the dispersed fluorescence spectrum, but the spectral resolution is much higher in the former than in the latter because the resolution is determined by the laser resolution of v2. Another method to probe the depopulation of the excited molecules is the detection of ions produced by the absorption of v2 photons by the excited m ~ l e c u l e . ~The * ~ molecule in a jet is excited to a selected vibronic level with vl. The laser intensity of v I is kept as low as possible so that multiphoton ionization does not occur with v 1 alone. By introducing tunable dye laser light of frequency v2, the molecular ion is generated by the absorption of a v2 photon if v1 + v2 is larger than the adiabatic ionization potential of the molecule. The laser light of v2 can also induce stimulated emission from the excited vibronic level to a groundstate vibrational level. When stimulated emission occurs, the ion signal is reduced by the depopulation of the excited molecules. Detecting the ion signal in scanning the laser frequency v2, one obtains the (two-color) ionization dip spectrum, which is identical with the two-color fluorescence dip spectrum. In the case of the ion dip spectrum, v2 plays the dual roles of the upward ionization and downward stimulated emission. Since the ions can be efficiently detected, ionization dip spectroscopy is more sensitive than fluorescence dip spectroscopy. However, the ground-state vibrational energy region (vl - v 2 ) covered by the ion dip spectrum is restricted to the range 0 2hvl - IP, while there is no restriction in the fluorescence dip spectrum.'
As will be shown in a later section, stimulated emission spectroscopy is very useful in determining the ground-state potential of a large-amplitude motion and also in elucidating the role of the large-amplitude motion in intramolecular vibrational redistribution (IVR). ( b ) Two-Color Ionization Threshold Spectrum. This is a spectroscopy by which the vibrational levels of a cationic ion are obtained.* The experimental set up is the same as that for two-color ionization dip spectroscopy. The molecule in a jet is excited by v l to a specific vibronic state. Then, tunable laser light of v2 is applied. When Y, + v2 reaches the ionization continuum, the cationic ion is generated and an ionization threshold is observed. If the potentials of the excited-state molecule and the ion are similar, transitions with Au = 0 preferentially occur giving the ionization threshold for the vibrational level of the ion corresponding to the vibronic level excited. Therefore, by selecting the vibronic level excited with vl, one can obtain the vibrational level structure of the ion. When the potential and geometrical structures are different between the excited-state molecule and the ion, several thresholds appear determined by the FranckCondon factors between the vibrational levels in the excited state and the ion. Information on large-amplitude motions of an ion can be obtained in this way.
(4) Kittel, C.; Abramson, E.; Kinsey, J. K.; McDonald, S.A.; Reiver, D. E.; Field, R. W.; Katayama, D. H. J . Chem. Phys. 1981, 75, 2056. ( 5 ) Murakami, J.; Kaya, K.; Ito, M. Chem. Phys. Lett. 1982, 91, 401. (6) Suzuki, T.; Mikami, N.; Ito, M. Chem. Phys. Lett. 1985, 120, 333. (7) Suzuki, T.; Mikami, N.; Ito, M. J . Phys. Chem., 1986, 90, 6431.
(8) Fujii, M.; Ebata, T.; Mikami, N.; Ito, M. Chem. Phys. Lett. 1983, 101,' 578. Mikami, N.; Sugahara, Y.;Ito, M. J. Phys. Chem. 1986, 90, 2080. Fujii, M.; Mikami, N.; Ito, M. Chem. Phys. 1985, 99, 193. (9) Okuyama, K.; Mikami, N.; Ito. M. J . Phys. Chem. 1985, 89, 5617. (10) Longuet-Higgins, H. Mol. Phys. 1963, 6, 445.
-
Internal Rotation of a CH, Group The internal rotation of a CH3 group is one of the large-amplitude motions of chemical importance and is extensively studied by microwave, Raman, and infrared spectroscopies. It is expected that internal rotation will change greatly upon the electronic excitation of a molecule. However, our knowledge about this change is limited. It will be shown first how the internal rotation of a CH3 group changes upon electronic e x ~ i t a t i o n . ~ The potential for the internal rotation of a C H 3 group in a planar molecule is expressed by
V(p) = f/2V3(l - COS 3p)
+ ?2V6(1 -COS
6 4
(2)
where cp is the angle of rotation of the CH3 group about the top axis measured from a reference configuration of the CH3 group. The potential for the internal rotation is therefore represented by V3and V,, and by the barrier height to the internal rotation. When the bamer height is 0, the energy levels associated with the internal rotation coincide with those of a one-dimensional free rotor and are given by Bm2. B is the rotational constant of the C H 3 group and m is the rotational quantum number. In the case of nonzero barrier height, the energy levels become irregular and split by tunneling effects. The levels are usually denoted by a combination of the rotational quantum number m of a one-dimensional free rotor and the symmetry species of the permutation inversion grouplo isomorphous to the C3, point group. Figure l a shows the fluorescence excitation spectrum of jetcooled o-fl~orotoluene~ in the spectral region of the first excited singlet state (Sl(a,a*)).Many bands appear on the higher frequency side of the 0,Oband (37 561.5 cm-') at positions displaced by 4-200 cm-' from the 0,O band. These low-frequency bands are assigned to the transitions from two internal rotational levels in the ground state to various internal rotational levels in the excited state as shown in Figure 1b. The selection rules for the transition are a, a,, a2 a2, and e e. The assignments can be confirmed by the observation of the dispersed fluorescence spectra obtained by exciting the individual low-frequency bands in the excitation spectrum. The low-frequency regions of the dispersed fluorescence spectra are shown in Figure 2. The spectra obtained by exciting the bands associated with a l levels in the excited state give a set of bands of identical frequencies, while the spectra from e levels in the excited state give another set of identical bands. The ground-state internal rotational levels of a, species are obtained from the former set and the levels of e species
- -
-
520
The Journal of Physical Chemistry, Vol. 91, No. 3, 1987
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(b) Figure 1. (a) Fluorescence excitation spectrum of jet-cooled o-fluorotoluenein the region of the 0,O band. Frequencies measured from the 0,O band are shown. (b) Schematic diagram showing transitions between internal rotational levels in the ground and excited states. Adapted from ref 9.
Ot4Aexc. (le)
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Figure 2. Dispersed fluorescence spectra of jet-cooled o-fluorotoluene obtained by exciting the bands in Figure la. The spectra showing ground-state frequencies are obtained by subtracting the fluorescence frequency from the exciting frequency. Adapted from ref 9.
from the latter set. The results also confirm the assignments given for the bands in the fluorescence excitation spectrum. Thus, the internal rotational levels are obtained for both the ground and excited states. From the observed energies, the potential for the internal rotation is obtained by assuming eq 2 and is shown in Figure 3. In the ground state, the height of the barrier to the internal rotation is as large as 228 cm-I, which agrees well with
the value obtained from the microwave study." In the excited state, however, the barrier height dramatically decreases to only 22 cm-I. This means that greatly hindered internal rotation of the CH,group in the ground state becomes a nearly free internal rotation in the excited state. (1 1)
Schwock, D.; Rudolph, H. J . Mol. Specrrosc. 1975, 57, 47.
The Journal of Physical Chemistry, Vol. 91, No. 3, 1987 521
Feature Article
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-
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Figure 3. Potentials and energy levels for the internal rotation of the CH3 group of efluorotoluenein the ground and excited states. Conformations of the CH3 group at several torsional angles are shown. Adapted from ref 9.
The intensity distributions of the internal rotation bands in the fluorescence excitation spectrum and the dispersed fluorescence spectra provide us with the relative positisns between the ground-state and excited-state potential^.^ It was found from a calculation of the Franck-Condon factors that the observed intensity distributions can be reproduced very well by taking the relative shift of cp to be 60°. Therefore, as seen in Figure 3, the most stable conformation of the CH3group in the ground state is the most *unstablen conformation in the excited state. It is known from the microwave study" that the potential minimum in the ground state corresponds to a staggered conformation of CH3 group with respect to the C-F bond in the molecule. This staggered conformation as well as the great barrier height in the ground state are easily understood in terms of repulsive interactions operating between the F atom and the hydrogen atoms of the CH3 group in the ortho position. On the other hand, the most stable conformation in the excited state is nearly an eclipsed form in which one of the H atoms of the CH3 group is directed toward the F atom. This conformation and the very small barrier height in the excited state suggest the existence of intramolecular hydrogen bonding between a hydrogen atom of the CH3 group and the F atom in the ortho position. The small barrier height may be explained as a result of cancellation of the repulsive interaction by the attractive interaction due to the hydrogen bonding. Very recently, Nishimoto et a1.I2 obtained the potential for the internal rotation of the CH3 group of o-fluorotoluene by ab initio calculation. The calculated barrier heights (220 and 30 cm-' for the ground and excited states, respectively) agree very well with the observed ones. They concluded from their results that the electrostatic interaction between the F atom and the hydrogen atoms of the C H 3 group is responsible for the small barrier height in the excited state, supporting the interpretation given above. A dramatic decrease of the barrier height upon electronic excitation is also found in o - t o l ~ i d i n e 'in~ which the F atom in (12) Nishimoto, K., private
communication.
0' +60' +120' F w e 4. Potentials and energy levels for the internal rotation of the CH, -60'
group of m-fluorotoluene in the ground and excited states. Adapted from ref 9. TABLE I: Barrier Heights of Several Meta-Substituted Toluenes in the So and SI States and Calculated r-Electron Densities of Meta Carbon Atoms of Parent Molecules SO
molecule to1uen e m-fluorotoluene m-cresol m-toludine
-H -F -OH -NHI
SI
barrier barrier height, r-electronb height, r-electronb cm-' density cm-' density 4c 1.oo 4 1.02 17' 1.oo 124' 1.04 10' 1.01 200' 1.07 9 1.01 317 1.09
Adapted from ref 13. *-Electron densities of metacarbon atoms of the parent molecules, benzene, fluorobenzene, phenol, and aniline taken from ref 14. CRudolph,H. D. et al. Z.Naturforsch. 1967, 22a, 940. 'From ref 9. 'Okuyama, K.; Mikami, N.; Ito, M., unpublished data.
o-fluorotoluene is replaced by a N H 2 group. Analyses of the fluorescence excitation and dispersed fluorescence spectra (SI-&,) of jet-cooled o-toluidine showed that the barrier height for the internal rotation of the CH3group is 750 cm-' in the ground state, but only 40 cm-I in the excited state. The change in the barrier height upon the electronic excitation is more remarkable than that found in o-fluorotoluene. It seems quite general that the barrier to the internal rotation almost disappears on electronic excitation in ortho-substituted toluenes, although more examples are needed to support the generalization. The disappearance of the barrier on electronic excitation is quite surprising in the usual chemical sense, but this fact illustrates how large is the electronic effect on the internal rotation. The importance of the electronic effect on the internal rotation of a CH3 group is also seen for m-fluor~toluene.~ The potentials determined from the fluorescence excitation and dispersed fluorescence spectra of jet-cooled m-fluorotoluene in a way similar to that for o-fluorotoluene are shown in Figure 4. In the ground state, the barrier height is 17 cm-I, but it increases to as large as 124 cm-' in the excited state. Therefore, the change in the barrier height upon the electronic excitation is just opposite to that in o-fluorotoluene. The small barrier height in the ground state can be explained by the absence of steric hindrance. (13) Okuyama, K.; Mikami, N.; Ito, M. Laser Chem., in
press.
522 The Journal of Physical Chemistry, Vol. 91, No. 3, 1987 0
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Figure 5. Two-color ionization threshold spectra of jet-cooled mfluorotoluene. The upper spectrum was taken after exciting the molecule to the internal rotational level of Oal in SIby uI. The thresholds corresponding to the internal rotational levels of m-fluorotoluene ion belonging to a, species appear. The lower spectrum is that obtained after exciting to the l e level in SIwith v I . The e levels of the ion are seen. Adapted from ref 16.
However, the appearance of a larger barrier in the excited state is a somewhat unexpected result. Table I summarizes the barrier heights of toluene, m-fluorotoluene, m-cresol, and m-toluidine in the So and SIstates.I3 All the values except for toluene were determined from the fluorescence excitation and dispersed fluorescence spectra of the jet-cooled molecules. With the exception of toluene, the barrier height is greatly increased by the electronic excitation. In the ground state, the barrier height is less than 20 cm-I for all the molecules. In the excited state, it markedly increases in the order toluene, m-fluorotoluene, m-cresol, and m-toluidine. In the table are shown the calculated a-electron densities at the meta carbon atoms of the parent molecules,14benzene, fluorobenzene, phenol, and aniline in the So and SIstates. It is seen that there exists a good correlation between the barrier height and the r-electron density. In the So state, the a-electron density at the meta carbon atom is nearly equal for all the parent molecules. This is reflected by the nearly free internal rotation of the CH3 group in all the molecules. In the S I state, however, the a-electron density increases markedly in the order benzene, fluorobenzene, phenol, and aniline. In the same order, the barrier height progressively increases in the meta-substituted molecules. The close correlation between the barrier height and the r-electron density indicates that the origin of the barrier to the internal rotation is almost purely an electronic one and probably due to the hyperconjugation of the C H 3 group. Recent ab initio calculations carried out by Nishimoto et al.12 also support the above view. Following the MO picture, the hyperconjugation is expected to be greatly enhanced in going from a neutral molecule to its cationic ion.I5 It is therefore interesting to compare their barrier heights. Two-color ionization threshold spectroscopy can be used
Figure 6. MPI spectrum of jet-cooled mesitylene. The 0,O band region is shown in detail in inlet. Adapted from ref 17.
to obtain the internal rotational levels of the CH3group in an ion. Figure 5 shows the two-color ionization threshold spectra of jet-cooled m-fluorotoluene obtained after exciting the molecule to various internal rotational levels in the S, state.16 The thresholds in the figure represent the internal rotational levels in the ground state of the m-fluorotoluene cation whose assignments are shown in the figure. From the thresholds, one obtains the barrier height for the internal rotation to be about 300 cm-’. This value may be compared with the ground-state barrier height of neutral m-fluorotoluene of 17 cm-I. The great increase in the barrier height from the neutral molecule to the ion also shows that the hyperconjugation is the origin of the barrier to the internal rotation. The results mentioned above clearly demonstrate that internal rotation is greatly affected by the electronic structure of a molecule and gives a good characterization of the electronic state. The fairly strong appearance of the bands associated with the internal rotation of the CH3 group in the SI-Sotransition is partly ascribed to vibronic coupling. Clear evidence for the vibronic coupling is found in the fluorescence excitation spectrum of jet-cooled 1,3,5-trimethylbenzene (me~itylene),~’ which is shown in Figure 6. In this molecule, the Sl(a,a*)state is of A i species in D3h molecular symmetry and the electronic transition from the So(Al’) state to the SI state is forbidden similar to the corresponding transition in benzene. Therefore, the 0,O band is absent in the spectrum. Despite the absence of the 0,O band, several low-frequency bands appear on the higher frequency side of the forbidden 0,O band. The low-frequency bands are those associated with the internal rotations of the CH3groups, which can be confirmed from the dispersed fluorescence spectra. The observation clearly indicates that the low-frequency bands gain their intensities from a vibronic coupling between the SIstate and a higher excited state through the internal rotation of the CH, group. Therefore, internal rotation plays a role in vibronic mixing of excited electronic states, which may lead to a great deformation of the potential in the excited state. Another interesting aspect on mesitylene is that the dispersed fluorescence spectra obtained after exciting the molecule to some internal rotational levels in S I contain fluorescence originating from the zero-point level in SIindicating relaxation to this level.’* It is not certain at present whether the relaxation is due to intramolecular vibrational redistribution (IVR) or to collision. Very recently, Parmenter and Stone19showed from comparison between the dispersed fluorescence spectra of p-difluorobenzene and p fluorotoluene that the internal rotational modes of the CH3group greatly accelerate IVR. A similar acceleration of IVR might occur also in mesitylene. ~
(14) Mihara, P.; Rai, D. Zni. J . Quantum Chem. 1972, 6, 47. Yadav, J.; Mihara, P.; Rai, D. Mol. Phys. 1973, 26, 193. (15) Mulliken, R. S. Tetrahedron 1959, 5 , 2 5 3 .
(16) (17) ( 1 8) (19)
~~
Okuyama, K.; Ito, M., unpublished result. Murakami, J.; Ito, M.; Kaya, K. Chem. Phys. Lett. 1981, 80, 203. Okuyama, K.; Ito, M., unpublished result. Parmenter, C. S.; Stone, B. M. J . Chem. Phys. 1986, 84, 4710.
The Journal of Physical Chemistry, Vol. 91, No. 3, 1987 523
Feature Article METHY LGLYOX A L
(a) FLUORESCENCE EX
22500 WAVENUMBER / cm-l (409)
(1371
23000 (b) PHOSPHORESCENCE EX.
I
f83L)
I 23000
I
22500 WAVENUMBER / cm-'
Figure 7. Fluorescence excitation spectrum (a) and simultaneously measured sensitized phosphorescence excitation spectrum (b) of jet-cooled methylglyoxal. The assignments of the internal rotational modes of the CH3 group in SIare shown in (a) and their excited-state frequencies are given in parentheses in (b). Adapted from ref 20.
A CH3 group also plays a characteristic role in intersystem crossing. Figure 7 shows a comparison between the fluorescence excitation spectrum of jet-cooled methylglyoxal due to the S,(n,?r*) So transition and the simultaneously measured sensitized phosphorescence excitation spectrum.20 Both spectra can be interpreted by the repeated appearance of a structural unit consisting of bands associated with the internal rotational levels of the CH3 group in the SI state. The two spectra are identical as far as the band positions are concerned, but the relative intensities are quite different between the two spectra. In the sensitized phosphorescence excitation spectrum, the triplet-state molecule produced from the excited singlet state by intersystem crossing is monitoredS2Therefore, the intensity ratio of a particular vibronic band in the sensitized phosphorescence excitation spectrum to that in the fluorescence excitation spectrum roughly represents the intersystem crossing efficiency of the excited level associated with this vibronic band. The difference in the relative intensities between the two spectra implies difference in the intersystem crossing efficiency among the individual levels. It is seen from the figure that the bands associated with the internal rotational levels belonging to e species appear strongly in the sensitized phosphorescence excitation spectrum compared with those belonging to a species. This fact indicates the existence of mode selectivity in the intersystem crossing. The same mode selectivity is also found in biacetyL20 It is quite remarkable that the intersystem crossing is controlled by the symmetry species of the internal rotation of the CH? group. Theoretical studies on this interesting phenomenon are highly desired.
0 :
0' LEVEL FLUO. SPECTRUM
-
Torsion of the C-C Bond in Conjugated Molecules Torsional motion of a conjugated molecule about the C-C bond attracts great interest with respect to cis-trans isomerization. Vibrational spectroscopy such as Raman and infrared is the most popular means for the study of the torsional motion in a ground-state molecule. The oscillatory torsional motion, however, does not induce a great change in the dipole moment and polarizability of a molecule. Therefore, the torsional mode usually appears weakly in the vibrational spectrum. The weak intensity as well as the low frequency make the observation of the torsional mode difficult in the vibrational spectrum. Very often, groundstate vibrational levels which are hidden or weak in the vibrational spectrum can be seen by observing the levels in emission from an electronically excited state. This is achieved by dispersed fluorescence and stimulated emission spectroscopies. Since the selection rules for electronic transitions are more relaxed than that for vibrational transitions, one can detect many vibrational levels (20) Kamei, S.; Okuyama, K.; Abe, H.; Mikami, N.; Ito, Chem. 1986, 90, 93.
M. J . Phys.
RELATNE ENERGY /cm-'
Figure 8. Dispersed fluorescence spectrum of jet-cooled trans-stilbene from the zero-point level in S , . The bands of frequencies underlined are the progression of the even quanta of the torsional mode 37. Adapted from ref 7.
in the ground state, including overtones and combinations. Figure 8 shows a part of the dispersed fluorescence spectrum of jet-cooled trans-stilbene obtained after exciting the molecule to the zero-point level (32234 cm-I) in the S,('B,) ~ t a t e . ~ ~ * l Several low-frequency bands are found at 19,45,74, and 107 cm-I separated from the 0; band. The intensity of the band smoothly decreases with an increase in the frequency. Similar vibrational satellites having similar intensity patterns can be found throughout the spectrum. The low-frequency bands can be assigned to the ground-state levels involving even quanta of the torsional mode 37(a,). The torsional mode 37 is an out-of-phase torsion of the two Ce-Cph bonds, where c, and c p h represent the carbon atoms in the ethylene group and in the phenyl group, respectively. The odd quantum levels of 37 can be detected by the observation of the dispersed fluorescence spectrum obtained by excitation of a hot band at 0: 39 cm-I in the fluorescence excitation spectrum. The hot band is due to the transition from the ground-state 37, level to excited-state 37l level. Therefore, in this dispersed fluorescence spectrum, the ground-state odd quantum levels of 37 can be found. From the observed levels thus obtained, the ground-state potential of 37 is obtained' and is shown in Figure 9. The potential is given vs. the torsional angle defined by the dihedral angle between the two phenyl groups. The potential is very anharmonic and has a flat bottom. It is noticed that the torsional mode has an amplitude as large as 20° even in the zero-point level and the fundamental frequency of 37 is only 8 cm-'. It was also found that the potential reproduces the observed levels of trans-stilbene-d,221 These results show that tram-stilbene is a very flexible molecule in the ground state and that a largeamplitude motion is taking place along the out-of-phase torsional
+
(21) Syage, J. A,; Felker, P. M.; Zewail, A. H. J . Chem. Phys. 1984, 82,
4685.
524
The Journal of Physical Chemistry, Vol. 91, No. 3, 1987
200
4
I
c 7
50 Wdeg Figure 9. Potential and energy levels of the out-of-phasetorsional mode 37 in the ground state. Adapted from ref 7.
It0 64033
63900
V I + 1.'~ ENERGY / c m - l 63800 // 62700 62600 62500 62400
U i - V 2 ENERGY /cm-!
400
400
500
500
600
600
700 1700 , I ,
700
1703
1
1800 1
18W
1900
.
I
1903
2000 I
,
2000
2100 ,
I
2100
RELATIVE ENERGY /cm-1
Figure 11. (a) Two-color ionization dip spectrum and (b) dispersed fluorescence spectrum of jet-cooled trans-stilbene. Both spectra were obtained after exciting the molecule to the Oo level in SI with u l . The bands indicated by open and solid circles are shown on an expanded scale. Adapted from ref 7.
61709 cm-1 - - - - _ _ _- _-Threshold __
VIBRATIONAL ENERGY I cm-'
Figure 12. (a) Two-color ionization dip spectrum and (b) dispersed fluorescence spectrum of jet-cooled trans-stilbene. Both spectra were obtained after exciting the molecule to the SIOolevel with v,. The bands shown by lines are 37tX ( X represents another mode). Adapted from ref 7.
'
61700 ' 61800 Pz ENERGY/cm-I
111'
Figure 10. Two-color ionization threshold spectra of jet-cooled transstilbene obtained after exciting the molecule to the Oo level in SI with Y,. v2 laser power increases from top to bottom. Adapted from ref 7 .
coordinate. The torsional mode in the excited state has a frequency of 48 cm-I and is quite harmonic. The great increase in the vibrational frequency in going from the ground state to the excited state indicates a great increase of a-conjugation upon the electronic excitation. The great anharmonicity of the torsional mode in the ground state is the most characteristic feature of trans-stilbene. The great anharmonicity suggests that the torsional mode couples strongly with other vibrational modes. Actually, the frequencies of the torsional modes show great changes by coupling with other vibrational modes.' In particular, great changes occur upon coupling with mode 25 (a 202 cm-l) (see Figure 8). Thus, the torsional potential is also fiexible and easily deforms by coupling with the other modes. The low frequency and great anharmonicity of the large-amplitude torsional mode of trans-stilbene suggest its important role in relaxation process. The most important relaxation process in a ground-state molecule is intramolecular vibrational redistribution (IVR). However, because of the lack of a convenient means, the study on IVR of a ground-state molecule is severely restricted. In this regard, stimulated emission spectroscopy mentioned in a previous section provides a useful means. Figure 10 shows the two-color ionization threshold spectra of jet-cooled trans-stilbene obtained after exciting the molecule to the level of the Sl state with v,.~.' A sharp ionization threshold is seen at the total energy (vl + v2) of 61 709 cm-', which represents the adiabatic ionization potential slightly red-shifted by the field ionization effect. In the energy region above the threshold, many sharp dips appear and the dip intensity increases with an increase in the v 2 laser power. The appearance of the dips can be seen in
the entire spectral region from the threshold to the total energy of 2vl, Le., to the point where v 2 becomes equal to v l . A part of the two-color ionization dip spectrum is shown in Figure 11. In the figure is also shown the dispersed fluorescence spectrum from the Oo level in S,. When the dip position is measured by uI - v2, the dip position exactly coincides with that of the band in the fluorescence spectrum measured by ground-state frequency. It is concluded, therefore, that the depopulation of the SI state by vz stimulated emission is responsible for the ion dip. In the figure, the two-color ionization dip spectrum looks noisy. However, all the dips including weak ones represent transitions to the ground-state levels. As a result, more than 500 vibrational levels in the ground state were found in the energy region below 3 150 cm-'. The spectral resolution of the dip spectrum is much better than that of the corresponding dispersed fluorescence spectrum because the former is determined by the resolution of the laser used, but the latter by the resolution of the monochromator. When monitoring the fluorescence from SI instead of the ion signal, the two-color fluorescence dip spectrum is obtained, which is exactly the same as the two-color ion dip spectrum. Comparison between the dip spectrum and the fluorescence spectrum shows that the dip is rather small for strong bands in the fluorescence spectrum and rather large for weak bands. The strong bands in the fluorescence spectrum are mainly associated with fundamental vibrational levels in the ground state and the weak bands with overtone or combination levels. The dip intensity is determined not only by the downward transition probability but also by the IVR rate of the ground-state vibrational level reached by the stimulated emission. Since the hand intensity of the dispersed fluorescence spectrum is determined only by the downward transition probability, the difference between the dip intensity and the fluorescence intensity reflects the IVR rate of the vibrational level. The small dip for the fundamental level indicates that the level has a small IVR rate. When the IVR rate (life time) is small (long) for the vibrational level reached by the u2 stimulated emission, the molecule in such a level has a great chance to come back to the SI state by v 2 absorption. Thereby, the depopulation of the Sl state becomes small, leading to a weak dip. The reverse
The Journal of Physical Chemistry, Vol. 91, No. 3, 1987 525
Feature Article
..
I I
kk't16
I
I
I
I
I
37000
36600
WAVENUMBER/cm-l
- 90'
Io Figure 13. Potentials and energy levels for torsion of the C- bond of tolan in the ground state. Adapted from ref 22. 0'
t
90'
is true for the overtone or combination levels. In Figure 12, the bands shown by lines are those associated with the even quantum torsional modes (370, 372, 374, ...) combined with other vibrational modes.' In the dispersed fluorescence spectrum, the 37, bands (n = 0, 2, 4, ...) belonging to any main vibronic band decrease with the increase of n. However, the dip intensities of the 372 and 374 bands are comparable with or even larger than that of the 370 band. The result clearly shows that the IVR rate is enhanced by the torsional mode and the enhancement becomes larger with an increase in the quantum number n. Many of the congested dips in the dip spectrum are those involving the torsional mode because of its small frequency. Their strong appearance in the dip spectrum compared with the weak intensities of the corresponding bands in the fluorescence spectrum also supports an acceleration of the IVR rate by the torsional mode. The low frequency and great anharmonicity of the torsional mode as well as the strong coupling of this mode with other vibrational modes all contribute to the acceleration of the IVR in trans-stilbene. From an analysis of the laser power dependence of the dip intensity, the IVR rate was determined for several vibrational levels7 The IVR rate generally increases with the vibrational energy. The torsion of the C-C bond in a conjugated molecule is a good measure of *-conjugation. In a S,P* electronic transition, the r-conjugation in general changes greatly, and thereby a great change in the torsional motion occurs. Since there exists a great difference in the potential for the torsion between the ground state and the T,T* excited state, the torsional modes appear strongly in the electronic transition because of large Franck-Condon factors. This is the case for the torsion of trans-stilbene. The torsional motion in tolan (diphenylacetylene) supplies another example.22 The fluorescence excitation spectrum of jet-cooled tolan due to the Blu(a,n*) A,, transition under D2hmolecular symmetry exhibits a sharp vibrational structure having many low-frequency bands with the 0,O band at 35248 cm-I. The low-frequency bands are all due to the transitions involving a torsional mode (a,,) about the C=C bond in the molecule. By selecting an appropriate low-frequency band with laser light, one can selectively prepare the excited-state molecule in a specific quantum state of the torsional mode. The dispersed fluorescence spectra from the various quantum states supply us with groundstate torsional levels with quantum numbers up to 15. From the observed levels, the detailed potential of the torsional vibration is obtained and is shown in Figure 13. The highest quantum state observed (v = 15) lies near the top of barrier. The fact that such a high quantum state near the barrier top is observed demonstrates the potential of dispersed fluorescence spectroscopy for accurate determination of the potential for a large-amplitude motion. The ground-state barrier height of 200 cm-' for the internal rotation about the C=C bond is much higher than that of dimethyl-
-
(22) Okuyama, K.; Ito, M.; Mikami, N. J . Phys. Chem. 1984,88, 1711.
Figure 14. Fluorescence excitation spectrum of jet-cooled 1,2,4,5-tetrafluorobenzene in the low-frequency region. Progressions of 1 12" are shown by solid lines. Adapted from ref 28.
acetylenez3which is less than 4 cm-I. Since there is no appreciable steric hindrance between the two phenyl groups, the barrier in tolan is exclusively electronic in origin, that is, n-conjugation throughout the molecule. In the excited-state, the *-conjugation is further enhanced and the barrier height increases to about 1300 cm-I Stronger r-conjugation in the excited state is also found for biphenyl" and p t e r p h e n ~ l . In ~ ~the ground state, these molecules have nonplanar structures in which the neighboring phenyl groups twist about the C-C bond by an appreciable angle. In the a,** excited state, the molecule becomes planar. Reflecting the great change in geometrical structure, a long progression of the torsional mode of the C-C bond in the excited state appears in the onephoton resonant MPI spectrum with the highest quantum number greater than 20.24 A similar long progression of a torsional mode is also found for benzophenone in the sensitized phosphorescence excitation spectrumZ5of the Sl(n,?r*) So transition. For all these molecules, the excited-state potential is quite harmonic. The corresponding ground-state potential should be anharmonic, but, at present, it is not known. I
-
-
Butterfly Tunneling The vibrational structures of the SI So absorption spectra of monosubstituted benzenes such as fluorobenzene, toluene, phenol and aniline are rather simple and their analyses are now almost complete. However, the analysis is far from complete for all the polysubstituted benzenes except for p-difluorobenzene. The spectra of the polysubstituted benzenes exhibit complicated and irregular vibrational features even for the molecules in jets. In some cases, the complicated vibrational structures arise from the existence of rotational isomers as was recently revealed for several substituted phenols.27 As was shown in a previous section, the irregular vibrational structures found in many substituted toluenes are due to the internal rotation of the methyl group and to its drastic change upon electronic e x ~ i t a t i o n . ~The origin of the irregularity, however, is still unclear for most of the polysubstituted benzenes. The irregularity suggests the existence of an anharmonic vibrational mode in the excited state which is absent in the ground state. Such a situation is found in 1,2,4,5-tetrafluorobenzene2* which will be briefly described below. Figure 14 shows the fluorescence excitation spectrum of jetcooled 1,2,4,5-tetrafluorobenzenedue to the &(*,**) So
-
(23) Olson, W. B.; Papusek, D. J . Mol. Spectrosc. 1971, 37, 527. (24) Murakami, J.; Ito, M.; Kaya, K. J . Chem. Phys. 1981, 74, 6505. (25) Murakami, J.; Okuyama, K.; Ito, M. Bull. Chem. SOC.Jpn. 1982,55, 3422. (26) Kamei, S.; Sato, T.; Mikami, N.; Ito, M. J . Phys. Chem. 1986, 90, 5615. (27) Oikawa, A.; Abe, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1984, 88, 5180. Ito, M.; Oikawa, A. J. Mol. Struct. 1985, 126, 133. Oikawa, A.; A h , H.; Mikami, N.; Ito, M. Chem. Phys. Lett. 1985, 116, 50. Yamamoto, S.; Okuyama, K.; Mikami, N.; Ito, M. Chem. Phys. Lett. 1986, 125, 1. (28) Okuyama, K.; Kakinuma, T.; Fujii, M.; Mikami, N.; Ito, M. J. Phys. Chem. 1986, 90, 3948.
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The Journal of Physical Chemistry, Vol. 91, No. 3, 1987
It0
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transition.28 The spectrum is characterized by many low-frequency bands in the frequency region 50 500 cm-’ above the 0,O band, some of the intensities of which are larger than or comparable to that of the 0,O band at 36 555 cm-I. The observed vibrational features are quite unusual, showing strong intensities for the low-frequency bands and a lack of apparent regularity in the spectrum. A careful examination of the spectrum shows, however, that there exists a structural unit which repeatedly appears everywhere. The first structural unit consists of the bands located at 0, 57, 134, 234, 348, and 467 cm-’ from the 0,O band, which are connected by a solid line in the figure. A similar structural unit develops from other vibronic bands as indicated in the figure. When the dispersed fluorescence spectrum is obtained by exciting each of the low-frequency bands, a long progression with intervals of 388 cm-’ appears having a slight anharmonicity and an intensity distribution characteristic of the level excited. The frequency of 388 cm-’ is the overtone frequency of 1l(b3,,) mode in the ground state which is active in the infrared and has been reported29to be 193 cm-I. The vibrational mode 1l(b3J in the ground state is a butterfly motion of the C F bonds qualitatively represented by
a ag
0 9 Figure 15. Double-minimum potential and energy levels for the butterfly motion of 1,2,4,5-tetrafluorobenzenein the excited state. The energy levels are denoted by the symmetry species of the permutation-inversion group isomorphous to D2,,. Adapted from ref 28.
+e+ +
+
The appearance of the 388-cm-l progression in the dispersed fluorescence spectra leads us to assign the low-frequency bands in the fluorescence excitation spectrum to the even quantum levels of the 11 mode in the excited state. Since the S, So transition of 1,2,4,5-tetrafluoroberizeneis BZu(SI) A,@,) under the D2hmolecular symmetry, the vibronic bands involving the odd quanta of the 11 mode in the excited state are forbidden in one-photon absorption. They can, however, appear in the two-photon absorption spectrum. The observation of the two-photon resonant five-photon ionization spectrum of the jet-cooled molecule was carried outz8and the excited-state levels of 111, 113, and 115 were found to be 4, 85, and 181 cm-I, respectively. All the observed levels of the 11 mode-the even quantum levels from the fluorescence excitation spectrum and the odd quantum levels from the two-photon resonant five-photon ionization spectrum-are used in the determination of the potential in the excited state. The energy levels can be reproduced very well by the Gaussian-type double minimum potential given by
- -
V(q) = ‘/2kq2+ A exp(-a2q2)
(3)
where q is the tunneling coordinate which is close to the normal coordinate of mode 11 in the excited state. The calculated potential and energy levels are shown in Figure 15. It was also found that the potential explains very well the observed intensity distributions of the 11 vibronic bands in the fluorescence excitation spectrum, the dispersed fluorescence spectra, and the two-photon resonant five photon ionization spectrum. It is concluded from the potential obtained that 1,2,4,5-tetrafluorobenzene is undergoing a large-amplitude motion along the coordinate of mode 11. In the excited-state zero-point level, quantum mechanical tunneling is t,.king place between the two equivalent out-of-plane conformations in which the plane involving the four F atoms is displaced from the benzene plane toward both directions perpendicular to the benzene plane. The tunneling splitting of the zero-point level is 4 cm-’ and the height of the barrier separating the two conformations is 80 cm-I. This is a new type of tunneling, and may be called “butterfly tunneling”. The origin of the flexibility in the excited-state molecule is probably a vibronic coupling between the S l ( r , r * )state and a higher r,u* state in which u* is mainly localized in the C-F bond. (29) Green, J. H. S . ; Harrison, D. J. Spectrochim. Acta 1976, 32A, 1185. Eaton, V. J.; Pearce, R. A. R.; Steele, D.; Tindle, J. W. Spectrochim. Acta 1976, 32A, 663.
The example of 1,2,4,5-tetrafluorobenzeneshows that a rigid molecule in the ground state becomes nonrigid in an excited state. Such a change in the rigidity will be important in the relaxation and chemical reactions of excited-state molecules. The complicated and irregular vibrational structures observed in the spectra of many polysubstituted benzenes contain much information on large-amplitude motions in the excited state, as was the case for 1,2,4,5-tetrafluorobenzene.The elucidation of such a large-amplitude motion will contribute a great deal to the dynamics of excited-state molecules.
Concluding Remarks Spectroscopy and dynamics of molecules having large-amplitude motions have been briefly described with examples mainly studied in our laboratory. The potentials of a large-amplitude motion in both the electronic ground and excited states can be accurately determined by the use of various laser electronic spectroscopies applied to the molecule in a supersonic jet. Simultaneous measurements of the electronic spectra of the molecule by different laser spectroscopies are proved to be useful in the study of dynamics of large-amplitude motion. It was demonstrated that large-amplitude motion plays an important role in molecular relaxation processes. The study on large-amplitude motion has a great significance in elucidating the role of anharmonicity in various relaxation processes in a molecule. This role can be studied in the low vibrational energy region where the spectral measurements are easily accessible. The information obtained from the study of low-frequency large-amplitude motion will also be useful in understanding the relaxation processes of a rigid molecule in a highly excited vibrational state where anharmonicity is usually large. Another interesting type of large-amplitude motion is lowfrequency modes in a van der Waals molecule. Since the dissociation energy of a van der Waals molecule is small, the anharmonicity of the van der Waals mode should be large. The great anharmonicity and the small frequencies of van der Waals modes will greatly affect relaxation processes of a van der Waals molecule such as vibrational dissociation. When the constituent molecule in a van der Waals molecule has an intramolecular large-amplitude motion, coupling between the large-amplitude motion and the van der Waals mode will be expected because of their similarity in vibrational frequency. Such a dynamic coupling might be important in understanding the relaxation processes in the condensed phase and in biological systems. Acknowledgment. I express a deep appreciation to my collaborators K. Okuyama, T. Suzuki, M. Fujii, and N . Mikami for their essential contribution to the work.
