J . Phys. Chem. 1985, 89, 5617-5625
Internal Rotation of the Methyl Group in the Electronically Excited State: p -Fluorotoluene
5617 0-,
m-, and
Katsuhiko Okuyama, Naohiko Mikami, and Mitsuo Ito* Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan (Received: July 8, 1985)
The fluorescence excitation and dispersed fluorescence spectra of jet-cooled 0-,m-, and p-fluorotoluene were observed. It was found that the spectra exhibit a feature characteristic of internal rotation of the methyl group in the ground and excited states. Detailed analyses of the spectra provided us with the potentials for the internal rotation in both the ground and excited states. In o-fluorotoluene, a large potential barrier for internal rotation in the ground state was found to be practically removed in the excited state (SI). It was also found that the most stable conformation of the methyl group with respect to the benzene plane in the ground state is the most unstable conformation in the excited state. The situation is entirely different in m-fluorotoluene, where the almost free rotation of the methyl group in the ground state is greatly hindered in the excited state. These interesting results are discussed in terms of intramolecular hydrogen bonding and hyperconjugation in the excited state.
Introduction The internal rotation of the methyl group is a classical subject in molecular spectroscopy. The subject has been extensively studied by infrared and Raman spectroscopy and with microwave spectroscopy.' Information on the internal rotation of a molecule in its electronic ground state has been accumulated to such a level that a detailed account of the origin of the internal rotation is now possible. On the contrary, our knowledge of the internal rotation of an electronically excited-state molecule is very p o ~ r . ~It, is~ very probable that the potential for internal rotation is greatly altered by electronic excitation. Such a change will play an important role in various relaxation processes of the excited-state molecule. Our interest in the internal rotation of the methyl group in the excited state was initiated by our study on the multiphoton ionization spectrum of jet-cooled t o l ~ e n e . ~It was found there that the bands due to internal rotation of the methyl group in the excited state appeared with considerable intensities in the S1 So absorption spectrum. Another initiation came in the course of our recent study on the rotational isomers of substituted benzenes by means of electronic spectra in a supersonic jet.5 We found that, in every substituted toluene, the main vibronic bands in the fluorescence excitation and dispersed fluorescence spectra are accompanied by a low-frequency feature which is probably ascribed to the internal rotational levels in the ground and excited states. Based on these findings, we decided to carry out a more systematic study on internal rotation in the excited state. The molecules chosen for this study are 0-,m-, and p-fluorotoluene. For these molecules, information on the internal rotation of the methyl group in the ground state is available from microwave and will greatly aid us in the determination of the excited-state potential. In the present study, we observed the fluorescence excitation and dispersed fluorescence spectra of 0-,m-, and p-fluorotoluene in supersonic jets. It was found that the spectra exhibit a feature characteristic of internal rotation of the methyl group in the ground and excited states. Detailed analyses of the spectra provided us with the potentials for the internal rotation in both the ground and excited states. The results obtained are quite interesting. In o-fluorotoluene, a large potential barrier for internal rotation in the ground state is practically removed in the excited state (SI). It was also found that the most
-
( 1 ) Miller, F. In "Molecular Spectroscopy";Hepple, P., Ed.; Institute of Petroleum: London, 1968; p 5. (2) Baba, M.; Hanazaki, I.; Nagashima, U. J. Chem. Phys. 1985,82,3938. ( 3 ) Leugers, A. M.; Seliskar, J. C. J . Mol. Spectrosc. 1982, 91, 209. (4) Murakami, J.; Ito, M.; Kaya, K. Chem. Phys. Lett. 1981, 80, 203. (5) Oikawa, A.; Abe, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1984, 88, 5180. (6) Susskind, J. J . Chem. Phys. 1970, 53, 2492. (7) Rudolph, H.; Trinkaus, A. Z . Naturforsch. A 1968, 23A, 68. (8) Rudolph, H.; Seiler, H. Z . Naturforsch. A 1965, ZOA, 1682.
0022-3654/85/2089-5617$01.50/0
stable conformation of the methyl group in the ground state9 is the most unstable conformation in the excited state. The situation is entirely different in m-fluorotoluene, where the nearly free rotation of the methyl group in the ground state is greatly hindered in the excited state. These interesting facts will be discussed in terms of intramolecular hydrogen bonding and hyperconjugation in the excited state.
Experimental Section The pulsed supersonic free-jet apparatus was already described elsewhere.'O The sample was heated to 350 K in a nozzle chamber to obtain sufficient vapor pressure and seeded in H e carrier gas. The gaseous mixture (-3 atm) was expanded into a vacuum chamber ( torr) through an orifice of 0.4-mm diameter. The fluorescence excitation spectra were obtained by monitoring the total fluorescence with a photomultiplier (HTV R-562). The photocurrent was averaged by a boxcar integrator (Brookdeal 9415/9425) and recorded. The exciting light used was the second harmonic of a dye laser (Molectron DL-14P) pumped by a nitrogen laser (Molectron UV-24). The laser resolution was about 1.0 cm-I (fwhm). A higher resolution light source was obtained by inserting an air-spaced etalon into the dye laser cavity and the wavelength was continuously scanned by a hand-made pressure control unit. The spectral resolution was about 0.05 cm-I (fwhm) and the scanning range was 3 cm-l. Wavelength calibration was carried out by the fluorescence excitation spectrum of low-pressure iodine vapor. The dispersed fluorescence spectra were measured by a Nalumi 0.75-m monochromator with a 0.04-mm slit width and the spectral resolution was about 4 cm-' (fwhm). The signal was detected by a photomultiplier (HTV R-928) and recorded by the same integrator system as used for the fluorescence excitation spectra. 0-,m-, and p-fluorotoluene were purchased from Tokyo kasei and purified by distillation.
-
Results and Discussion Fluorescence Excitation Spectra. Figures 1-3 show the fluorescence excitation spectra of jet-cooled 0-,m-, and pfluorotoluene, respectively, in the spectral region of the first excited singlet state (Sl). The origin of the spectrum was found to be at 37561.5, 37385.5, and 36859.9 cm-' for 0-,m-, and pfluorotoluene, respectively. Corresponding vapor absorption spectra at room temperature have been studied by Cave and Thompson" and J ~ s h i . ~ * , When '~ compared with the vapor spectra, the jet spectra exhibit sharp and well-resolved structures. (9) Schwock, D.; Rudolph, H. J . Mol. Spectrosc. 1975, 57, 47. (10) Mikami, N.; Hiraya, A.; Fujiwara, I.; Ito, M. Chem. Phys. Lett. 1980, 74, 531. ( 1 1) Cave, W.; Thompson, H. Discuss. Faraday SOC.1950, 9, 35. ( I 2) Joshi, G. Curr. Sci. 1966, 35, 5 12. (13) Joshi, G. Indian J . Pure Appl. Phys. 1966, 4, 40.
0 1985 American Chemical Society
5618
The Journal of Physical Chemistry, Vol. 89,No. 26, 1985
Okuyama et al.
I
I
I
37500
38000
38500
/cm-'
WAVE NUM BE R Figure 1. Fluorescence excitation spectrum of jet-cooled o-fluorotoluene. Bands associated with internal rotation of the methyl group are indicated.
F
/
I
38000
37500
38500
/cm-i
WAVENUMBER
Figure 2. Fluorescence excitation spectrum of jet-cooled m-fluorotoluene. Bands associated with internal rotation of the methyl group are indicated. F
c H3
I _ ______I ._ 37003
V!A\/EblUM
L 38000
37530
-
.
L 38500/cm-1
R ER
Figure 3. Fluorescence excitation spectrum of jet-cooled p-fluorotoluene.
Especially in o-fluorotoluene, the individual vibronic bands in the vapor spectrum are reported to be very broad. However, as seen in Figure 1, the broad band is resolved into several sharp bands appearing in the higher frequency region displaced by 0-200 cm-I from the lowest-frequency strong band (they are shown by broken
lines in the figure). The low-frequency band structure repeatedly appears to accompany the main vibronic bands. For o-fluorotoluene, some of these low-frequency bands are unusually intense. A similar low-frequency structure is also found in the jet spectrum of rn-fluorotoluene as seen in Figure 2, although the intensities
Methyl Group Rotation in
0,m,
0;
The Journal of Physical Chemistry, Vol. 89, No. 26, 1985 5619
and p-Fluorotoluene
37561.5cm'
i.
m -51.7 83 S 129.3
0 14 22.7
196.5
15 5
1
I I
4e n
5e
I
01 - 0 2
37600
37800 / c m '
Ground State
WAVENUMBER
(a> Figure 4. (a) Fluorescence excitation spectrum of jet-cooled o-fluorotoluene in 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.
of the bands are rather weak. In p-fluorotoluene, there seems to be no such low-frequency bands having a regular structure (Figure 3). However, it was found that a low-frequency band structure does appear on the higher-frequency side of the 0,O band when the spectrum is measured with a high gain (see Figure 8). Therefore, the low-frequency bands also exist in p-fluorotoluene, although their intensities are very weak compared with those of 0-and m-fluorotoluene. The low-frequency band structure commonly observed in 0-,m-, and p-fluorotoluene cannot be ascribed to the vibrational modes in the excited state because such lowfrequency modes are not expected in these molecules. As will be mentioned in a later section, it was concluded that the structure represents the levels of internal rotation of the methyl group in the excited state. In Figures 1-3, the assignments of the main vibronic bands in the fluorescence excitation spectra are given. The assignments were made on the basis of the observation of dispersed fluorescence spectra obtained by exciting the individual main bands which are not presented here. Vibrational analyses of the dispersed fluorescence spectra with Raman and infrared f r e q u e n ~ i e s ' ~ - ' ~ and the observed intensity patterns of the dispersed fluorescence spectra lead us to definite assignments of the main vibronic bands in the excitation spectrum of each molecule, which are indicated in the figures. Table I gives the correspondence between the ground-state and excited-state vibrational frequencies obtained from the above analyses. The infrared and Raman frequencies are also included in the table. The vibrational mode which forms a long progression in the fluorescence excitation or the disperSed fluorescence spectrum is indicated by an asterisk. All the modes appearing in the excitation and dispersed fluorescence spectra are of skeletal vibrations of the benzene ring, whose mode numbering was taken from ref 20. It is seen from the table that the fre(14) Thompson, H.; Temple, R.J . Chern. SOC.1948, 1432. (15) Deb, K. Indian J. Phys. 1962, 36, 59. (16) Green, J. Spectrochim. Acta, Part A 1970, 26A, 1503. (17) Ferquson, E.; Hudson, R.; Nielsen, J. J . Chem. Phys. 1953.21, 10. (18) Joshi, G.; Singh, N. Specfrochirn Acta, Part A 1967, 23A, 1341. (19) Green, J. Spectrochim. Acta, Part A 1970, 26A, 1913. (20) Varsanze, G. 'Assignment for Vibrational Spectra of Seven Hundred Benzene Derivatives"; Adam Hilger: London, 1974.
quencies of all the modes decrease when going from the ground state to the excited state and a substantial frequency decrease is found for some modes. In the present work, we do not discuss further the intramolecular vibrational modes. Internal Rotational Levels of the Methyl Group in the Ground and Excited States. Figure 4a shows the details of the fluorescence excitation spectrum of jet-cooled o-fluorotoluene in the region near the 0,Oband. As mentioned in a previous section, many bands appear on the higher-frequency side of the 0,O band at positions displaced by 4-200 cm-' from the 0,O band. These low-frequency bands are tentatively 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 4b. The energy levels of the internal rotation of the methyl group are denoted by a combination of the rotational quantum number m of a one-dimensional free rotor and the symmetry species of the permutation inversion group isomorphous to C,, point group.21,22The selection rule for the transition is a , a,, a2 a2, and e e. The transition between a, and a2 species can also be allowed by taking into account the rotation of the whole molecule. If the tentative assignments given in Figure 4b are assumed, the bands due to the transition from the l e level in the ground state are hot bands and they should show a temperature effect. However, we could not find an appreciable temperature effect for these bands in the observations of the fluorescence excitation spectrum under the various jet cooling conditions attained by controlling the stagnation pressure. This indicates that the tentative assignments are wrong or the energy separation between the ground-state Oa, and l e levels is very small. In order to check the above assignments, we observed 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 5. It is seen from the figure that the spectra obtained by exciting the O,O, 0 + 51.7, and 0 + 196.5 cm-' bands in the excitation spectrum give a set of identical bands, while the spectra
- -
-
(21) Longuet-Higgins, H. Mol. Phys. 1963, 6, 445. (22) Bunker, P. "Molecular Symmetry and Spectroscopy"; Academic Press: London, 1979.
5620 The Journal of Physical Chemistry, Vol. 89, No. 26, 1985
TABLE II: Internal Rotational Levels of o-Fluorotoluene in Ground and Excited States ( c d )
TABLE I: Correspondence between Vibrational Frequencies in Ground and Excited States (em-')
Ground State
o-Fluorotoluene mode' excited state ground state IR Ramanb 272 15 154 272 370 2 X 10b 23 1 350 526 6b 317 525 426 9b 392 428 576 6a 498 576 1* 705 750 746 1036 18b 924 1037 986 5 947 989 1072 2 X 16a 959 1074 1243 1233 7a* mode" 9a 15 6b 2 X 16b I*
9b 18b 12* 14 modeo 11 15 9b 6a* 6b 1* 18a 13 7a 12
a' a" X 2 a' a' a' a' a' a' a"X 2
species (C3J
C,
a' X
species (C3J Oa I le 2e 3a, 3a; 4e 5e 6a2 6a1
2
C,,
b2 bl bl a, bl a1 a1 a, a1 a1
lJ='
Figure 5. Dispersed fluorescence spectra of jet-cooled o-fluorotoluene obtained by exciting the bands in Figure 4a. For the spectra on the right and left, the frequency scale is given by taking the transition energies to the ground-state Oa, and l e levels as 0, respectively.
+
obtained by exciting 0 + 4.4, 0 + 22.7, 0 83.5, and 0 129.3 cm-' bands give another set of identical bands. This clearly confirms the selection rules a l a l and e e. The internal rotation of the methyl group of o-fluorotoluene in its ground state was studied by Susskind6 from the microwave spectra. H e reported that the potential for internal rotation is expressed by V(p) = V3/2(1 -COS 3p) + V6/2(1 - COS 61p) (1)
-
0 0
92 95 154 168 215
92 95 154 169 214 265 268 333 41 1 500 500
89 92 152 166 213 263 268 33 1 409 499 499
Excited State obsd
calcd'
0
0
4.4 22.7 45.5 51.7 83.5 129.3
4.4 22.7 43.7 51.7 83.6 129.5 185.7 185.8
196.5
V3 = 228.3 cm-I; V, = 6.6 cm-I; E = 5.306 cm-I. *Reference 6. V3 = 227.3 cm-I; V, = 0 cm-I; E = 5.308 cm-I. V3 = 21.8 cm-'; V, =
-13.8 cm-I; E = 5.121 cm-l. with V, = 227.1 cm-l and v6 = 0 cm-I. The reduced rotational constant B was obtained to be 5.308 cm-l. By using this potential, we can readily calculate the energy levels of the internal rotation in the ground state. It was found that the calculated energies reproduce very well the structures of the dispersed fluorescence spectra shown in Figure 5 (see also Table 11). Therefore, we can readily assign the individual bands in the dispersed fluorescence spectra to the ground-state internal rotational levels, which are indicated in the figure. It is seen from the figure that all the bands appearing in the dispersed fluorescence spectra obtained by O,O, 0 51.7, and 0 196.5 cm-l band excitations are assigned to a l species, while all the bands in the spectra obtained by 0 4.4, 0 + 22.7, 0 + 83.5, and 0 + 129.3 cm-' band excitations are assigned to e species. If the selection rule and the internal rotational level structure are taken into account, all the low-frequency bands in the fluorescence excitation spectrum are definitely assigned to the internal rotational levels in the excited state, which is indicated in Figure 4a. In this way, the tentative assignments mentioned above were completely confirmed with the help of the dispersed fluorescence spectra. Now, we can draw the internal rotational level structures in the ground and excited states from the results obtained from the fluorescence excitation and dispersed fluorescence spectra. However, from these spectra alone, we never know the relative energy relation between the e structure and a,, a2 structure because of the severe selection rule. Once the energy separation between any one of the a l species and any one of the e species is known, we can construct the complete level structure in both the ground and excited states. According to the calculation based on the ground-state potential given by Susskind, the energy separation between the ground-state Oal and l e levels is 0.1 cm-I. This small energy difference explains the absence of the temperature effect on the bands due to transitions from the ground-state l e level. By assuming this energy difference, we obtain all the energy levels in the ground and excited states, which are listed in Table 11. Figure 6 shows the details of the fluorescence excitation spectrum of jet-cooled m-fluorotoluene in the region of the 0,O band. The bands indicated by broken lines are considered to be associated with excited-state internal rotational levels. Frequencies
+
(le)
-
0 0
495
a' a' a'' a' a' a' a' a'
0 0
2641273 330 413
'Mode number was taken from ref 20. An asterisk denotes mode forming progression in the fluorescence excitation and/or dispersed fluorescence spectra. References 18 and 19. References 14 and 15. dReferences 16 and 17.
+
obsd
a'
IR Ramanb 157 312 420 455 638 844 1017 1214 1223 728
0 + 4.4 exc.
calcd present work' microwaveb
C.
rn-Fluorotoluene excited state ground state IR Ramanb 293 298 450 446 426 513 508 464 886 516 880 728 728 685 1143 1123 705 1114 746 1112 1004 962 1003 1265 1267 p-Fluorotoluene excited state ground state 137 180 219 287 399 424 410 448 549 639 795, 802 853 845 1001 1194 1213 1230 1237 730
Okuyama et al.
+
+
Methyl Group Rotation in
0
0"
0,m,and
The Journal of Physical Chemistry, Vol. 89, No. 26, 1985 5621
p-Fluorotoluene
37385.5 cm-'
TABLE III: Internal Rotational Levels of m -Fluorotoluene i n Ground and Excited States (cm-I) Ground State calcd suecies ( C d obsd set I' set IIb
.1 I/ 0 -3.8
I 1
1
I 177.1
91.5 105.0
45.3
0
0
18 52 82 132
194
0 0
0 0
18 52
18 49
49 84 133 198 198
84 133 198 198
Excited State obsd
species (C3J
52
calcdC
Oa I
0
0
le 2e
-3.8 45.3
-3.8 45.3 55.6 91.5 105.0 146.4 199.3 202.2
3%
3a1 4e 5e 6az 6a I
91.5 105.0 177.1
v3= 15.9 cm-l; v6 = 8.0 cm-I; B = 5.477 cm-I. bMicrowave, ref 7. V, = 21.8 cm-l; v6 = -13.8 cm-I; E = 5.477 cm-l. ' V 3 = 123.7 cm-'; V6 = -26.4 cm-I; B = 4.763 cm-I. 37600
37400
/cm-~ 0; 36859.9cm'
WAVENUMBER
4
, r n Y I T 253.9 3076
Figure 6. Fluorescence excitation spectrum of jet-cooled m-fluorotoluene in the region of the 0,O band. Frequencies measured from the 0,O band (37 385.5 cm-I) are shown.
0 - 3.8exc. (le)
0
13.7
52.9743
1720
lO0L
0,Oexc. (Oa,)
le
li
0+45,3exc. (2e)
0+91.5exc.
2'I
Ze
1 36800
I 37000
u ,/ern.'
37200
WAVE NUM BE f?
Figure 8. Fluorescence excitation spectrum of jet-cooled p-fluorotoluene in the region of 0,O band, which was taken at a gain higher than that of Figure 3. Frequencies measured from the 0,O band (36 859.9 cm-l) are shown. Bands due to low-frequency vibrational modes are also indicated.
Figure 7. Dispersed fluorescence spectra of jet-cooled m-fluorotoluene obtained by exciting the bands in Figure 6. For the spectra on the right and left, the frequency scale is given by taking the transition energies to the ground-state Oa, and le levels as 0, respectively.
excited state Oa,, 3al, and 6al levels, respectively. The tentative assignments of the bands are shown in the figure. In Figure 7 are shown the dispersed fluorescence spectra of jet-cooled m-fluorotoluene obtained by exciting the low-frequency bands in the excitation spectrum. Similar to the case of ofluorotoluene, the spectra can be classified into two groups. One group (O,O,0 + 91.5, and 0 + 177.1 cm-I excitation spectra) gives a set of identical bands, while the other (0 - 3.8, 0 45.3, and 0 105.0 cm-' excitation spectra) gives another set of identical bands, as seen from the figure. The ground-state internal rotation of the methyl group in m-fluorotoluene was studied with microwave spectroscopy by Rudolph and Trinkaus.' The ground-state internal rotational levels calculated by the use of their potential reproduce very well the observed dispersed fluorescence spectra (see Table 111). Therefore, we can easily assign the bands in the dispersed fluorescence spectra as indicated in the figure. The assignments also confirm the tentative assignments made before for the low-frequency bands in the excitation spectrum, which are shown in Figure 6. By using the calculated energy difference between the ground-state Oa, and l e levels, which is 5.4 cm-', we
+
of the bands given in the figure are frequency separations measured from the band at 37 385.5 cm-I. It was found that 0 - 3.8, 0 + 45.3, and 0 + 105.0 cm-' bands change their intensities upon changing the cooling conditions of the jet. Therefore, they must be hot bands. It is strongly suggested from the temperature effect that these bands are due to transitions from ground-state l e level lying just above the ground-state lowest level Oal. If this is the case, the 0 - 3.8, 0 + 45.3, and 0 105.0 cm-I bands will be assigned to the excited state le, 2e, and 4e levels, respectively. On the other hand, the O,O, 0 91.5, and 0 + 177.1 cm-I bands are cold bands. Therefore, they are probably assigned to the
+
+
+
5622 The Journal of Physical Chemistry, Vol. 89, No. 26, 1985
0,Oexc.
0+13.7exc. ( 2e’)
I 2e’
(Oa’l) lil
I
Okuyama et al. TABLE IV: Internal Rotational Levels of p-Fluorotoluene in Ground and Excited States (cm-’)
iri
Ground State species (D3*)
I7 0
0+ 7 4 . 3 exc (4e’)
Oa,‘ 1e” 2e’
0 + 52.9 exc ( 3a’i 1
Le
I
’“ ’”
I
-u 319 w )
0
Figure 9. Dispersed fluorescence spectra of jet-cooled p-fluorotoluene
obtained by exciting the bands in Figure 8. For the spectra on the right and left, the frequency scale is given by taking the transition energies to the ground-stateOa, and le levels as 0, respectively. obtain complete level structures in the ground and excited states, which are given in Table 111. For p-fluorotoluene, the bands due to internal rotation are extremely weak in the fluorescence excitation spectrum. However, they appear clearly in the spectrum measured at a high gain, which is shown in Figure 8. Beside the bands due to internal rotation, several bands from low-frequency benzene ring modes (1 1 and 15) appear and their assignments were established by the observation of the dispersed fluorescence spectra obtained by exciting these bands. Similar to the cases of 0-and m-fluorotoluene, the assignments of the internal rotation bands in the excitation spectrum were carried out by reference to the dispersed fluorescence spectra obtained by exciting these bands, which are shown in Figure 9. The ground-state potential for internal rotation of p-fluorotoluene was determined from a microwave study by Rudolph and Seiler.s They reported V , = -4.8 cm-l and B = 5.460 cm-’. Because of higher symmetry in p-fluorotoluene, the V, term in the potential is equal to zero. The ground-state internal rotational levels calculated with this potential again reproduce the observed dispersed fluorescence spectra, although a slight disagreement is seen for several bands (see Table IV). The disagreement is probably ascribed to Fermi resonance with nearby vibrational levels. From the result, we can establish the assignments of the bands in the excitation spectrum to the excited-state internal rotational levels. The assignments thus obtained are indicated in Figure 9. For p-fluorotoluene, the permutation inversion group is isomorphous to the point group D3,,. The internal rotational levels are denoted therefore by the symmetry species of D3*. The selection rule for D3h is the same as that for C,,, the prim and double prim of D3*species being neglected. The energy levels for internal rotation in the ground and excited states obtained from the results of the fluorescence excitation and dispersed fluorescence spectra are given in Table IV, in which the energy separation between the ground-state Oa,‘ and le” was taken to be equal to the calculated value obtained from the ground-state potential. Potentials for Internal Rotation in the Ground and Excited States. Now, we have obtained the energy level structures for internal rotation of the methyl group in the ground and excited states for all the molecules. From these results, the potentials for internal rotation are determined.23 If we assume that the methyl (23) Lewis, J.; Malloy, T.; Chao, T.; Laane, J. J . Mol. Struct. 1972, 12, 427.
calcd” 0 0
16 48
4e’
51 81
5e“
129
6ai 6al’ 7e”
218 260
3al”
_w
( 7e”)
0 0 17
3ai’
species ( D J
0 + 253.9 exc.
obsd
Excited State obsd
Oal‘ 1e”
0
2e’ 3ap 3al” 4e’ 5 e” 6ai 6al’ 7e” 8e’
13.7
52.9 74.3 108.4 172.0 253.9 307.6
50 82 131
197 197 262
calcdb 0 -0.8 13.4 36.1 52.9 74.4 117.8 176.3 177.1 234.6
307.7
“Microwave, ref 8. V, = -4.8 cm-’; E = 5.460 cm-’. cm-I; B = 4.879 cm-I.
* V, = -33.7
top and the benzene framework are rigid rotors, the wave function for internal rotation should satisfy the following equation:
$,,(cp)
where p i s the torsional angle between two rotors measured from the most stable conformation and B is the reduced rotational constant24 of the rotors about the methyl top axis. m is the rotational quantum number of a one-dimensional free-rotor and u denotes the symmetry species in C,, or D3h. The potential V(p) is assumed to be expressed by eq 1. For p-fluorotoluene, the V, term in the equation is equal to zero because of high symmetry. Equation 2 can be solved by expanding $,(p) by a basis set of one-dimensional free-rotor wave functions and by diagonalizing the Hamiltonian matrix. In the calculation, we diagonalized the 32 X 32 Hamiltonian matrix, which was confirmed to be large enough to avoid a truncation problem of the free-rotor expansion. Three unknown parameters B, V,, and V, were determined by adjusting them to give the calculated energies which give a best fit to the observed energies. The relative intensity distributions of the bands due to the internal rotation in the fluorescence excitation spectrum and in the dispersed fluorescence spectra provide us with a useful check of the potentials and also with the relative relation between the ground-state and excited-state potentials with respect to the torsional angle. The eigenvector of the m”al species in the ground state, for example, is expressed by
where t is the eigenvector coefficient determined by diagonalization of the ground-state Hamiltonian matrix. The corresponding excited state eigenvector pm,,,(cisp)also obtained from the coefficientsdetermined by the diagonalization. However, the most stable conformation of the methyl group in the excited state is in general different from that in the ground state. If we assume that the difference in the torsional angle between the most stable conformations in the ground and excited states is 8, the excited + 6’). The Franck-Condon state eigenvector is given by $&,,(cp (24) Lin, C.; Swalen, J. Reu. Mod. Phys. 1959, 31, 841
Methyl Group Rotation in
0,m,
and p-Fluorotoluene
The Journal of Physical Chemistry, Vol. 89, No. 26, 1985 5623 c
TABLE V Potential Parameters and Rotational Constants for Internal Rotation in Ground and Excited States (cm-') and Displacement Angle between Ground- and Excited-State Conformations
ground state
excited state 150
C
7
m'
-
&H3
5e
100 i e
2e
37562
3al 302
le
0-
4&
- - 0
ea1
300
6a2
100 -
4e'
-
3 a;
120
5e
80 60
-
j CH3
3ai
20 n
- 120'
- 60'
0'
+
60'
t
120'
8o
Figure 10. Potentials and energy levels for internal rotation of ofluorotoluene in the ground and excited states. Conformations of the methyl group corresponding to several torsional angles are shown.
factor between the m"a, level in the ground state and the m'a, level in the excited state is therefore expressed by ($B,Tta,(q)I$t%a,(q
+ 6 ) ) = j Zt&nJ3y,rnl COS (3i@ =O
-
(4)
The Franck-Condon factors for other symmetry species can be a2 transition. In the obtained in a similar way, except for a, a2 transition, the transition becomes allowed only case of al when rotation of the whole molecule is taken into account. Therefore, its intensity cannot be evaluated simply by the Franck-Condon factor (which is zero). The displacement angle 6 is determined by searching for the best fit between the observed and calculated intensity distributions. When the most stable conformation in the ground state is known, the excited-state stable conformation can be determined. In Table V are summarized the potential parameters V, and V6,the reduced rotational constant B, and the displacement angle 6 obtained from the analyses mentioned above. Figures 10-12 show the potentials and energy levels in the ground and excited states. Comparison with the calculated and observed level energies
-
t
40
2o 0
-120' -60' 0' +60' +120' Figure 12. Potentials and energy levels for internal rotation of pfluorotoluene in the ground and excited states.
is given in Tables 11-IV. It is seen that the agreement between the observed and calculated energies is satisfactory. In Figures 13 and 14 are shown the comparisons between the observed and calculated intensity distributions of the fluorescence excitation and dispersed fluorescence spectra for 0- and m-fluorotoluene. The agreement is generally very good except for a few bands. The intensity distributions of the p-fluorotoluene were not calculated because of inaccuracy in the excited-state potential obtained from our data, which will be discussed below. The ground-state potentials obtained here may be compared with those obtained from microwave studies. The potential parameters, the rotational constants obtained by microwave studies, and the ground-state energy levels calculated with these values
5624 The Journal of Physical Chemistry, Vol. 89, No. 26, 1985
ABSORPTION Obs.
Oa, l e 2 e
bC3.
Cak
3al Le 5 e 6a,
EMISSION
Figure 13. Comparisons between the observed intensity distributions of the fluorescence excitation and dispersed fluorescence spectra of ofluorotoluene and the calculated intensity distributions.
are given in Tables 11-IV. For o-fluorotoluene, the calculated energies are slightly different from the observed energies. In microwave spectroscopy, the interaction between the internal rotation and the rotation of whole molecule, which causes a slight shift of the rotational level associated with an internal rotation level from that without the interaction, is used for the determination of the potential. When the potential barrier is high, the potential obtained by this method is inaccurate because of a very small shift of the rotational level. This is the case for ground-state o-fluorotoluene, in which the barrier height is as large as 220 crn-]. On the other hand, in the dispersed fluorescence spectroscopy, the ground-state internal rotational levels are directly observed up to high quantum states, and they are used for the potential determination. For these reasons, we believe that the potential obtained from the analysis of the dispersed fluorescence spectra (V, = 228.1 cm-I, V, = 6.6 cm-', B = 5.306 crn-') is more accurate than the potential obtained from the microwave spectrum (V, = 227.2 cm-', V, = 0 cm-', B = 5.308 cm-I). For ground-state m-fluorotoluene, two sets of potentials (set I and I1 in Table 111) are proposed from the microwave study. As seen from the table, the two sets give the same calculated energies except for the energy order of 3a1 and 3a2 levels. The observed energy of 52 cm-' for 3al supports set 11, which predicts 52 cm-'. Since the ground-state potential barrier is low in mfluorotoluene, the potential obtained from the microwave study is considered to be accurate enough. Therefore, we did not try to calculate the potential by using the observed energies. Actually, the set I1 potential gives calculated energies which agree very well with the observed energies except for a slight difference for the 6al level (see Table 111). For the same reason, we adopted the ground-state potential obtained from the microwave study in the case of p-fluorotoluene. As seen in Table IV, the calculated energies obtained by using this potential nicely reproduce the observed energies although an appreciable difference is seen for the 6al level. The most interesting results obtained in this study are the potentials for internal rotation in the excited state. Especially in o-fluorotoluene, the barrier for internal rotation shows a dra-
Okuyama et al. matic reduction from 228 to 22 cm-l upon electronic excitation, as seen from Figure 10. That is, the greatly hindered internal rotation of the methyl group in the ground state becomes almost free rotation in the excited state. Another interesting point about o-fluorotoluene is that the most stable conformation of the methyl group is quite different between the ground and excited states. As seen from Figure 10, the potential minimum in the ground state corresponds to a staggered conformation of methyl group with respect to the C-F bond: while the stable conformation in the excited state is nearly an eclipsed form, that is, the most stable conformation in the ground state is the most unstable one in the excited state and vice versa. Such a result was obtained from an analysis of the intensity distributions of the fluorescence excitation and dispersed fluorescence spectra in terms of Franck-Condon factors. As mentioned before and also as seen in Figure 4a, the bands due to the internal rotation in the fluorescence excitation spectrum of o-fluorotoluene are unusually intense. Such an unusual intensity distribution is never explained by similar stable conformations of the methyl group between the ground and excited states and is explained only by assuming a great conformational change. A somewhat unexpected result obtained for m-fluorotoluene is that the barrier height for internal rotation increases to about seven times as large as that in the ground state upon electronic excitation (see Table V and Figure 11). That is, the almost free rotation in the ground state is greatly hindered in the excited state. This is just the reverse of the case for o-fluorotoluene, where the greatly hindered internal rotation in the ground state becomes almost free rotation in the excited state. However, in m-fluorotoluene, there is no change in the stable conformation of methyl group between the ground and excited states, although we do not know the actual stable conformation of the methyl group with respect to the benzene plane. In p-fluorotoluene, the ground state barrier height of 4.8 cm-' is in the same order as that of toluene. In the excited state, the barrier height increases to 33 cm-'. Possible Causes of the Large Change in Potential upon Electronic Excitation. The results obtained in the present study are summarized as follows: (1) The ground-state potential barrier decreases in the order of 0-,m-,and p-fluorotoluene. This may be simply explained by steric hindrance between the methyl group and the F atom. (2) The o-fluorotoluene potential barrier decreases very much upon electronic excitation and the internal rotation of the methyl group becomes almost freely rotating in the excited state. The most stable conformation of the methyl group is the staggered form with respect to the C-F bond in the ground state, but it changes to the eclipsed form in the excited state. (3) In contrast to o-fluorotoluene, the m-fluorotoluene potential barrier increases very much upon electronic excitation. However, there is no change in the most stable conformation of the methyl group between the ground and excited states. (4) For all the molecules, the contribution of the V6 term to the potential increases upon electronic excitation. (5) The rotational constant B decreases upon electronic excitation for all the molecules. The decrease is particularly large in m- and p-fluorotoluene. All the facts listed above except for (1) were somewhat unexpected. Especially, it was a great surprise for us that the large barrier of o-fluorotoluenein the ground state is practically removed in the excited state. The large barrier in the ground state is easily explained by the great steric hindrance between the methyl group and the nearby F atom. The stable conformation of the staggered form in the ground state is also understood in terms of repulsive interactions operating between the F atom and the hydrogen atoms of the methyl group in the ortho position. The practical disappearance of the barrier in the excited state cannot be ascribed to a large geometrical change of methyl group because the rotational constant B does not change greatly upon electronic excitation. Therefore, we may still expect large repulsive interactions in the excited state. For the almost free rotation of the methyl group in the excited state, the repulsive interactions should be cancelled out by some attractive interactions between the methyl
Methyl Group Rotation in
0,m,
and p-Flnorotoluene
The Journal of Physical Chemistry, Vol. 89, No. 26, 1985 5625
AB SORPT I ON 1.150
I
U
L
Oar
le
3al
2e
4e
5e
601
F
bc,,
1.100
EM1SSIO N
I t
7e
5e
Le
2e
le
621 3a1 Oal
n
4e
7e
5e
Le
Figure 15. Relation between the methyl top angle a and the C-H bond length TCH which reproduces the calculated moment of inertia of the methyl group in the excited state (SI).Ground-state values are also 2e
le
901 601 301 Oal
plotted.
Figure 14. Comparisons between the observed intensity distributions of
explanation of how the hyperconjugation exerts an influence on the barrier height. The above explanations are tentative and a more full account is required from a theoretical view point. From the experimental group and the F atom. One of the possibilities we may think of view point, the extension of the work to other substituted toluenes is intramolecular hydrogen bonding between a hydrogen atom of is highly desired for understanding these interesting phenomenon. the methyl group and the F atom in the ortho position. Such Such work is now in progress in our laboratory. hydrogen bonding is in general extremely weak in the ground state. Finally, we would like to make a comment on the geometrical However, in the excited state, it is possible that the hydrogen structure of he methyl top in the excited state. As mentioned bonding is greatly enhanced because of a great change in the before, the reduced rotational constant B decreases when going electron densities of the two substituents upon electronic excitation. from the ground state to the excited state for all the molecules. For o-fluorotoluene, the attractive interaction due to hydrogen The decrease is especially large in m- and p-fluorotoluene. Since bonding would slightly overcome the repulsive interactions, rethe methyl top is much lighter than the fluorobenzene framework, sulting in the eclipsed conformation of the methyl group with the change in B can be ascribed to a geometical change of the respect to the C-F bond in the excited state. methyl top upon electronic excitation. An asymmetrical deforThe situation is entirely different in m-fluorotoluene, where mation of the methyl group breaking down C3"local symmetry steric hindrance and hydrogen bonding are practically absent. The e transitions in can be ruled out because of the absence of a, absence of the steric hindrance is illustrated by the low barrier the fluorescence excitation and dispersed fluorescence spectra. for internal rotation in the ground state. However, in the excited Therefore, the deformation must be a symmetric opening of the state, the barrier increases to about 7 times as large as the barrier methyl top and/or simultaneous change of all the C-H bond in the ground state. The large increase of the barrier upon lengths. Assuming that the geometrical structure of 0-, m-,or electronic excitation is just the reverse of the case in o-fluorop-fluorotoluene in the excited state is the same as that of fluotoluene, where the barrier is greatly decreased upon electronic robenzene in its corresponding excited state (C-F = 1.343 A, C-C excitation. This suggests that the cause of the barrier change in = 1.435 A, and C-H = 1.070 A),*' we can evaluate the moment m-fluorotoluene is different from that in o-fluorotoluene. A of inertia of the methyl top alone from the B value. The moment possible origin for the increase in the barrier height in the excited of inertia was found to be 3.347, 3.604, and 3.588 amu A2 for state is hyperconjugation of the methyl group. Since the F atom 0-, m-,and p-fluorotoluene in the excited state. In Figure 15 are is a strong electron-withdrawing substituent, the a electron density shown the curves showing the relations between the top angle a at the meta carbon atom (with respect to the F atom) in the ring and the C-H bond length rCHwhich reproduce the moments of is expected to be larger than that at the ortho or para carbon atom. inertia obtained above. The ground-state values for a and rCH In the excitation of a a electron in the ring, the localization of are also plotted in the figure. The figure suggests that there must the a electron at the meta carbon atom is further enhanced, as be a considerable change in a and rCHupon the electronic exindicated by CNDO/S calculations for f l u o r o b e n ~ e n e . The ~ ~ ~ ~ ~ citation, especially for m- and p-fluorotoluene. great increase of the a electron density probably induces enAcknowledgment. We thank Prof. E. Hirota, Institute for hancement of the hyperconjugation of the methyl group attached Molecular Science, and A. Oikawa for stimulating discussions. to the meta carbon atom, resulting in a great increase in the barrier height in the excited state. However, we have no reasonable Registry No. o-Fluorotoluene, 95-52-3; m-fluorotoluene, 352-70-5; p-fluorotoluene, 352-32-9. the fluorescence excitation and dispersed fluorescence spectra of mfluorotoluene and the calculated intensity distributions.
-
~~
~~
(25) Yadav, J.; Mishra, P.; Rai, D. Mol. Phys. 1973, 26, 193. (26) Mishra, P.; Rai, D. fnr. J . Quanr. Chern. 1972, 6, 47.
(27) Cvitas, T.; Hollas, J.; Kirby, G. Mol. Phys. 1970, 19, 305.
