J . Phys. Chem. 1987,91, 5589-5593
5589
Rotational Isomers of m-Cresol and Internal Rotation of the CH, Group in So, S,, and the Ion Hiromi Mizuno, Katsuhiko Okuyama, Takayuki Ebata, and Mitsuo Ito* Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan (Received: April 1 , 1987)
The fluorescence excitation, dispersed fluorescence, and two-color ionization threshold spectra of jet-cooled m-cresol have been observed. The cis and trans isomers of the neutral molecule and its ion arising from orientation of the OH group were identified, and the potential of the internal rotation of the CH3 group of each isomer was determined for the So, SI,and ionic states. It was found that a nearly free rotation of the CH3 group in So becomes a greatly hindered internal rotation by the electronic excitation and ionization, indicating a great enhancement of hyperconjugation in the latter states. The most stable conformation of the CH, group changes from the eclipsed form with respect to the OH group at the meta position for the neutral molecule to the staggered form for the ion.
Introduction
In a series of our papers,'" it was demonstrated that electronic spectroscopy is a powerful means for the study of rotational isomerism of large polyatomic molecules like benzene derivatives. For example, in m-cresol, there are two rotational isomers, cis and trans, arising from orientation of the OH group with respect to the CH3group at the meta position. Vibrational spectroscopy, which has been traditionally used for the study of rotational isomerism, is difficult to apply to such a large molecule because the difference in the vibrational frequency between the two isomers is too small to resolve under usual conditions. However, the two isomers of m-cresol can be easily resolved by electronic absorption spectroscopy. The fluorescence excitation spectrum of jet-cooled m-cresol due to the S l ( r , r * ) So transition gives two different band origins at 35 988.5 and 36 098.3 cm-' which are ascribed to the two rotational isomers of this molecule. The large difference in the excitation energy, 110 cm-I, makes the identification of the two isomers much easier than that by vibrational spectroscopy. In our previous paper,3 we tentatively assigned the origin bands at 35 988.5 and 36 098.3 cm-' to the cis and trans isomers, respectively,on the basis of the frequency shift of each origin induced by the formation of a hydrogen-bonded complex. One of the aims of the present study is to establish the assignments from a different aspect. m-Cresol has a CH3 group which is undergoing internal rotation. Although the CH3 group is located far from the OH group at the meta position, the difference in the orientation of the OH group will cause an appreciable difference in the potential for the methyl internal rotation. The difference in the potential will provide a definite assignment of each isomer. The second aim is to see how the internal rotatio? of the CH3 group changes with electronic excitation and ionizatioh In our prewous papers,68 it was shown that in several meta-substituted toluenes the barrier to the internal rotation of the CH3group greatly increases in going from the electronic ground state to the excited state and to the ionic state, indicating a great enhancement of hyperconjugation. A similar enhancement is also expeCted for the excited and ionic states of m-cresol. It is also interesting to see the difference of the potential in the excited and ionic states between the cis and trans isomers.
-
(1) Oikawa, A.; Abe, H.; Mikami, N.; Ito, M.J. Phys. Chem. 1983, 87, 5083. (2) Oikawa, A.; Abe, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1984, 88, 5180. (3) Oikawa, A,; Ito, M. J . Mol. Spectrosc. 1985, 126, 133. (4) Oikawa, A.; Abe, H.; Mikami, N.; Ito, M. Chem. Phys. Lett. 1985, 116, 50. (5) Yamamoto, S.; Okuyama, K.; Mikami, N.; Ito, M. Chem. Phys. Lett. 1986, 125, 1 . ( 6 ) Okuyama, K.; Mikami, N.; Ito, M. J. Phys. Chem. 1985, 89, 5617. (7) Ito, M. J. Phys. Chem. 1987, 91, 517. (8) Okuyama, K.;Mikami, N.; Ito, M. Laser Chem. 1987, 7, 197.
0022-3654/87/2091-5589$01.50/0
In the present paper, we report the fluorescence excitation and dispersed fluorescence spectra of jet-cooled m-cresol. From the observed spectra, the potentials for internal rotation of the C H 3 group in So and SI were accurately determined for the cis and trans isomers. For both isomers, a large increase in the potential barrier was found in going from SOto SI. We also measured the two-color photoionization threshold spectra of the jet-cooled molecule by using the vibronic state in SIas an intermediate state in the ionization. The observed spectra provided us with the internal rotational levels of each isomer of m-cresol ion in its ground state, which gave the potential of the ion. It was found that height of the barrier to the internal rotation dramatically increases upon ionization. The change in the potential with electronic excitation and ionization and the effect of the O H orientation on the potential will be discussed. The most stable conformations of the CH3group with respect to the benzene plane in the neutral and ionic molecules are also discussed. Experimental Section
The experimental apparatus for the measurement of the fluorescence excitation, dispersed fluorescence, and two-color photoionization threshold spectra of a jet-cooled molecule has been described e l s e ~ h e r e . ~m-Cresol J~ was heated to 330 K in a nozzle chamber to obtain sufficient vapor pressure and seeded in 4 atm of He carrier gas. The seeded gas mixture was expanded into a vacuum chamber through a 400-pm-diameter pulsed nozzle. The S, So fluorescence excitation spectrum of the jet-cooled molecule was obtained by monitoring the total fluorescence from S1with a photomultiplier (HTV R-562). The photocurrent signal was averaged by a boxcar integrator (Brookdeal 9415/9425) and recorded on a chart recorder. Frequency-doubled output of a tunable dye laser (Lambda Physik FL2002) pumped by a XeCl excimer laser (Lambda Physik EMG103) was used as an excitation source. The SVL fluorescence spectra from various vibronic levels in S,were dispersed by a Nalumi 0.75-m monochromator and detected with a photomultiplier (HTV R-928). The signal was amplified by a preamplifier (PAR 113) and recorded by the same integrator system as that used for the excitation spectrum. In the measurement of the two-color photoionization threshold spectra,1° the output of the XeCl excimer laser was split by a beam splitter to pump two dye lasers (Molectron DL-14 for u1 and Lambda Physik FL2002 for vz). The first laser light (vl) of the second harmonic of the dye laser (C-540 dye) was used to pump the molecule to a particular vibronic level in SI,and then the second scanning laser light (u2; SR-640) was introduced to induce the transtion from the selected vibronic level in S1to the vibrational
-
(9) See,for example, Mikami, N.; Hiraya, A.; Fujiwara, I.; Ito, M. Chem. Phys. Lett. 1980, 74, 531; Fujii, M.; Ebata, T.; Mikami, N.; Ito, M. Chem. Phys. 1983, 77, 191. (10) Fujii, M.; Ebata, T.; Mikami, N.; Ito, M. Chem. Phys. Lett. 1983, 101, 578.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
5590
Mizuno et al. 0,Oexc.
TWNS
-5.2 0 ' e Cal
73.0
0-3.8 exc.
le
141.2 148.4
I
2e
3a14c
CIS -
3.8 0
66.6
136.3 144.9
-
u 0
130
52
83
200
2e
0 66.0 exc. (2e) 1
-
I
I
36ooo
36200 WAVEWMBER /
36W
cm-'
Figure 1. Fluorescence excitation spectrum of jet-cooled m-cresol due to the S l So transition. The internal rotation bands belonging to cis and trans isomers are shown. The assignments of the internal rotational levels in SI associated with the individual bands of each isomer are indicated. The 0.0 bands of cis and trans isomers are at 35 988.5 and 36098.3 cm-I, respectively.
-
193
-
0
53
130 83
levels of the ion. The ion signal was amplified by a current amplifier and integrated by a boxcar integrator. m-Cresol was purchased from Tokyo Kasei (99%) and used without further purification.
0 144.9 exc. ( 4 e ) be
Results and Discussion Fluorescence Excitation Spectrum, Dispersed Fluorescence Spectra, and Two-Color Photoionization Threshold Spectra. Figure 1 shows the fluorescence excitation spectrum of the jetcooled m-cresol due to the S l ( r , r * ) Sotransition. The spectnrm is essentially the same as that reported in a previous paper.* As was already shown in this paper, bands at 35988.5 (cis) and 36098.3 (trans) cm-' are the band origins of the two rotational isomers (cis and trans) of m-cresol arising from orientation of the OH group with respect to the CH3 group at the meta position." Many sharp bands appear in the low-frequency region of 0-300 cm-' to the blue of the strong lowest frequency band. These low-frequency bands can be classified into two groups belonging to the cis and trans isomers as shown in the figure. By referring to the dispersed fluorescence spectra, which will be described later, the bands belonging to each isomer can be assigned to the transitions from the two lowest internal rotational levels in the So state to various internal rotational levels in the SIstate. The assignments of the internal rotational levels in SI associated with the individual bands are given in the figure. The internal rotational levels 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 the C,, point group. The assignments are supported from the dispersed fluorescence spectra, which will be described below. Figure 2 shows the dispersed fluorescence spectra obtained by exciting the individual low-frequency bands of the cis isomer in the excitation spectrum. It is seen from the figure that the spectra obtained by exciting the l e (0 - 3.8 cm-I), 2e (0 + 66.0 cm-'), and 4e (0 + 144.9 em-') bands in the excitation spectrum give a set of identical bands, while the spectra obtained by exciting the Oal (0 cm-I) and 3al (0 136.3 cm-I) bands give another set e and a l of identical bands. Because of the selection rules e a], the former set gives the internal rotational levels of e species in So and the latter the a l levels. The observed results also support the assignments of the excited-state internal rotational levels given in Figure 1. Figure 3 shows the dispersed fluorescence spectra of the trans isomer. As seen in the figure, the spectra are similar to those of the cis isomer. The spectra obtained by exciting the l e (0 - 5.2 cm-I), 2e (0 73.0 cm-') and 4e (0 + 148.4 cm-')
-
+
-
-
+
(11) In ref 2 Oikawa et al. reported the 0,O band of the cis isomer'to be 35 980 cm-I. However, reexamination showed that the value should be revised to 35988.5 cm-'.
200
-
130 83
200
Figure 2. Dispersed fluorescence spectra of cis isomer obtained by exciting the internal rotation bands of cis isomer in the fluorescence excitation spectrum. Frequencies on the abscissa are in reciprocal centime-
ters. TABLE I: Internal Rotatio~lLevels of the CH3 Group of the Cis and Trans Isomers in the Sn State (cm-') _______~ ~
~
cis species Oal
calcd'
0.0
0.0 4.0
le
2e 3a1 4e
5e 6al
trans
obsd
20
20 53 83 130
53 83
193
7e
obsd 0.0
calcdb 0.0 5.0 17
16 50 80 131
130 193
192
258
253
V , = 26 cm-I; V, = -9 cm-l; B = 5.31 cm-I. -8 cm-I: B = 5.30 cm-l.
50 80 128 191 255
* V3 = 11 cm-'; v6 =
TABLE II: Internal Rotational Levels of the CH, Group of the Cis and Trans Isomers in the SIState (cm-')
cis species Oa 1 le 2e 3al 4e
trans
obsd 0.0
calcd"
70 136 149
71 136 148
0.0
obsd 0.0
calcdb 0.0
78 141 154
78 141 155
0.18
0.16
V, = 21 1 cm-I; V6 = -39 cm-I; B = 5.21 cm-I. = -22 cm-I: B = 5.31 cm-'.
V3 = 213 cm-I;
v6
bands in the excitation spectrum give a set of identical bands, while the spectra obtained by exciting the Oa, (0 cm-') and 3a1 (0 + 141.2 cm-') bands give another set of identical bands. The results also support tlie excited-state assignments given in Figure 1 for the trans isomer. Thus, we obtained the internal rotational levels in So and S1, which are listed in Tables I and 11.
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5591
Rotational Isomers of m-Cresol 0 - 5.2 exc. (le)
0,O exc. (Oal)
le
67000
67200
67400
67600
67000
67200
67200
67400
67600
67000
67400
67600
u 5 0 0
0
+
141.2 exc.
( 3al
1
'i'
J
Oal
I
67000
67200 67400 67600 TOTAL ENERGY / cm-1
Figure 5. Two-color ionization threshold spectra of trans isomer obtained after exciting to Oa,, 3al, le, and 4e levels in SIwith ul. Thresholds represent internal rotational levels of the trans isomer ion, whose assignments are indicated. 0 148.4 exc.
TABLE 111: Internal Rotational Levels of the CH, Group of the Cis and Trans Isomers in the Ionic Ground State (an-') cis trans species obsd calcd" obsd calcdb
4e
+
Oal
IC
5e
253
131
0
80
Figure 3. Dispersed fluorescence spectra of trans isomer obtained by exciting the internal rotation bands of trans isomer in the fluorescence excitation spectrum. Frequencies on the abscissa are in reciprocal centimeters.
I
66900
67100
3al 4iOai o
67300 6r1
3a1
66900
4e
4e
le li
67100
67300 7e
J2c 2r
9
le 2e 3a1 4e 5e 6a1 7e
0.0' 0.0
103 192 198 26 1 321 366
0.0 0.0
102 191 197 264 318 367
0.0 0.0
O.Od
0.0
96 171 184 237
292 355
96 173 183 238
292 351
V3= 328 cm-'; V, = -20 cm-'; B = 5.1 1 cm-l. V3 = 276 cm-I; V, = -12 cm-I; B = 5.28 cm-I. 'The energy of the ionization potential is 66910 cm-l. "The energy of the ionization potential is 67069 cm-'.
the internal rotational levels of each isomer in the ionic state. The assignments given in the figure are confirmed later by FranckCondon calculations. It was difficult to resolve the thresholds due to the Oal and l e levels of the ion because of their small frequency difference. The internal rotational levels of the ions and their assignments are given in Table 111. Potential for Internal Rotation in So,S , , and Ion. We obtained the energy level structures for the internal rotation of the CH, group in the So, SI,and ionic states for each isomer of m-cresol. From these results, the potential for the internal rotation can be determined for the three states. If we assume the CH, group and phenol group are rigid rotors, the wave function $(cp) for the internal rotation of the CH3 with respect to the OH group should satisfy6J2
J
66900
67100
67300
66900
67lOO
67300
TOTAL ENERGY/ cm-1
Figure 4. Two-color ionization threshold spectra of cis isomer obtained after exciting to Oa,, 3al, le, and 4e levels in SIwith Y , . Thresholds represent internal rotational levels of cis isomer ion, whose assignments are indicated.
where cp is the torsional angle between the two rotors and B is the reduced rotational constant of the rotors about the CH, top axis. The potential function V(cp)is assumed by
Figures 4 and 5 show the two-color photoionization threshold spectra of the cis and trans isomers, respectively. To measure the spectra, the first laser light of v 1 was used to excite the molecule to a selected internal rotational level in the S1state and the second laser frequency u2 was scanned while the ion current was monitored. The selection rule for the transition from an internal rotation level in SI to the ionization continuum associated with an internal rotational level of the ion is the same as that for the dispersed fluorescence spectrum. The thresholds due to the adiabatic ionization appear at 66910 and 67 069 cm-' for the cis and trans isomers, respectively. The other thresholds represent
Equation 1 can be solved by expanding $(cp) by a basis set of one-dimensional free rotor wave functions and by diagonalizing the Hamiltonian matrix. Three unknown parameters B, V,, and V, were determined by optimizing them to give a best fit to the observed energy levels. The calculated energy levels are listed in Tables 1-111 together with the best fit parameters. It is seen that the agreement between the observed and calculated energies is satisfactory for the So, SI,and ionic states of each isomer.
V(p) = Y2V3(l - cos 3cp)
+ y2V6(l - cos 6p)
(2)
(12) Lewis, J.; Malloy, T.;Chao, T.; Laane, J. J . Mol. Struct. 1972, 12, 421.
5592
Trans
Oal ~ C a l ~ ~ O b s
Oai
3ai
le
lj
Calc.. ,Obs.
Oar 11
le 2e 4e5e
6ai
I[
Trans
Oal
Cis
Oal
, lb
Mizuno et al.
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
le
3al
I
le 2e 4e5e 7e Be
6a1
Cis
L II.
le
le
Oai
3ai
6ai
4
Oar
2e 4e5e 7e
hi
le 2e 4e5e 7e
Figure 6. Comparisons between observed and calculated intensity distributions of internal rotation bands in the dispersed fluorescence spectra for cis and trans isomers.
The relative intensity distributions of the internal rotational bands in the fluorescence excitation and dispersed fluorescence spectra provide us with a useful check of the potentials in So and SI and also with the relationship of the potentials with respect to the torsional angle. The Franck-Condon factors between the internal rotational levels in So and SIwere calculated and compared with the relative intensity distributions of the fluorescence excitation and dispersed fluorescence spectra. The comparisons between the dispersed fluorescence spectra and the Franck-Condon factors are shown in Figure 6 . As seen from the figure, the calculated intensity distributions well reproduce the observed ones when the same most stable conformations of the CH3 group with respect to the OH group in both So and S1are assumed. The good agreement between the observed and calculated intensity distributions indicates a high reliability of the potentials obtained. Franck-Condon calculations were also carried out between the internal rotational levels in the S1and ionic states. In the two-color photoionization threshold spectra shown in Figures 4 and 5, the height of the threshold associated with a particular rotational level of the ion is proportional to the transition probability from an internal rotational level in SI pumped by v 1 to the selected level of the ion. Therefore, relative heights of the thresholds in each spectrum can be compared with calculated relative FranckCondon factors between the pumped SIlevel and various internal rotational levels in the ion. It was found that the observed relative heights are greatly different from those calculated by the Franck-Condon factors when we assume the same stable conformations of the CH3 group in both the SI and ionic states. However, when we take the relative shift in the torsional angle between the most stable CH3 conformations in the SI and ionic states to be 60°, a good agreement between the observed and calculated thresholds was obtained as shown in Figure 7 . In Figure 8 are shown the potentials and energy levels in So, SI,and the ion for the cis and trans isomers. For both isomers,
Figure 7. Comparisons between observed and calculated threshold heights in two-color ionization threshold spectra for cis and trans isomers.
5e
300
I\
ION
200-
"
.IE
100-
J
0
>
'q I
w v
I
2 0 0 w f l 100
0
J W 't7' 5c
5e
-L o-'
so
101-
2 c
0, -120'
-60'
0'
60'
126
-120'
-66
0'
60'
120'
Cis Trans Figure 8. Potentials and energy levels for internal rotation of the CH3 group of cis and trans isomers of m-cresol in the So, S,, and ionic states. The abscissa is the torsional angle. The angles of Oo and 60° correspond to eclipsed and staggered configurationsof the CH3group with respect to the OH group at the meta position.
the barrier heights are very small in So but they dramatically increase in S1. However, the most stable conformation of the CH3 group is same in both So and SI. The result is similar to those of m-fluorotoluene and m-toluidine,6 where the barrier heights greatly increase in going from So to S1without change in the most stable conformation of the CH, group. For the m-cresol ion in its ground state, the barrier height becomes larger than that in S1, and a change in the CH3 conformation occurs. That is, the most stable conformation in So or Sl is the most unstable con-
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5593
Rotational Isomers of m-Cresol
Therefore, in the ionic state, each of the hydrogen atoms of the CH, group will carry considerable positive charge. On the other hand, it is known from our previous study', that the OH group of phenol greatly increases its hydrogen-bonding power (with a proton-accepting molecule) in going from the neutral molecule to its cationic ion. This indicates a large positive charge at the
hydrogen atom of the OH group in phenol ion. A similar positive charge is also expected for the OH group of m-cresol ion. It is very probable from the above that both the hydrogen atoms of the CH3 group and of the OH group have large positive charges in m-cresol ion. Then, a considerable coulombic repulsive force, which is a long-range force, is operating between the hydrogen atoms of the CH, group and the hydrogen atom of the O H group, even though the distance between the two groups is fairly large (about 4 A). One of the conclusions derived from the above consideration is about the stable conformation of the CH, group in the ion. Because of the repulsive interaction, the most stable conformation of the CH3 group will be a staggered one with respect to the O H group. The second conclusion is that the cis isomer ion has a larger barrier height than the trans ion because of the shorter distance in the former between the hydrogen atom of the O H group and the CH, group than that in the latter. Therefore, the isomer ion having the barrier height of 328 cm-' is assigned to cis and the one of 276 cm-' to trans. Then, the neutral isomers having the barrier heights of 26 and 11 cm-' are ascribed to the cis and trans isomers, respectively, in agreement with the assignments made in a previous paper4. As mentioned before, the most stable conformation of the ground-state neutral molecule differs from that of the ion by 60' in the torsional angle. Since the conformation of the ion was concluded to be the staggered form, the conformation of the ground-state molecule should be of the eclipsed form in which one of the hydrogen atoms of the CH3 group is directed toward the O H group. It is not easy to rationalize the eclipsed form by simple consideration, and a detailed account of the electronic structure is needed. Finally, we will discuss briefly the potential in S I . Both the cis and trans isomers of the neutral molecule exhibit a large increase in the potential barrier in going from Soto S1. The height of the barrier in S1 is 211 and 213 cm-I for the cis and trans isomers, respectively, which are compared with 26 and 11 cm-' in So. The large increase in the barrier height with electronic excitation is generally found for meta-substituted toluenes such as m-fluorotoluene and m-toluidine, indicating a large enhancement of the hyperconjugation.8 The barrier height in S1 increases in the order of m-fluorotoluene (124 cm-I), m-cresol (210 cm-'), and m-toluidine (317 cm-'). This order is the same as the increasing order of a-electron density at the meta carbon atom of the parent molecules, fluorobenzene (1.04), phenol (1.07), and aniline (1.09), in the S1 state. The parallel orders indicate that the origin of the barrier height in SI is almost purely electronic and is due to hyperconjugation. The barrier height in S1 is nearly equal between the cis and trans isomers. This implies no appreciable contribution of the orientation of the OH group to the barrier height. The most stable conformation of the C H 3 group in Sl is the same as that in So, and it is the eclipsed form. Although we cannot afford any reasonable explanation for the eclipsed form, this particular conformation seems to provide important information on the nature of hyperconjugation.
(13) Gonohe, N.; Abe, H.; Mikami, N.; Ito, M. J. Phys. Chem. 1985,89, 3642.
Acknowledgment. We thank N. Mikami and M. Fujii for stimulating discussions. Registry No. m-Cresol, 108-39-4; m-cresol radical cation, 607 15-74-4.
formation in the ion. Such a conformational change is similar to that found for o-fluorotoluene,6 where the change by 60' in the torsional angle occurs between So and S I . Identification of Rotational Isomer and Conformation of the CH,Group. We will discuss first the potentials of the So neutral molecule. The height of the barrier to the internal rotation is small for each isomer, and the potential minimum corresponding to the stable conformation of the CH, group is very flat. It is seen from Table I that the absolute value of V, is smaller than that of V, and is nearly equal for the cis and trans isomers. However, the V, value for the cis isomer (26 cm-') is about 2 times larger than that for the trans isomer (1 1 cm-'). The difference in the V, value comes from the difference in orientation of the OH group at the meta position between the two isomers. In a previous paper? we tentatively assigned the bands at 35 988.5 and 36 098.3 cm-' in the fluorescence excitation spectrum to the origins of the cis and trans isomers, respectively, on the basis of the frequency shift of the origin band induced by the formation of hydrogen-bonded complexes. If the assignments are correct, the potentials in Figure 8 correspond to the cis and trans isomers. Assuming an atomatom interaction between one of the hydrogen atoms of the CH, group and the hydrogen atom of the OH group, the larger barrier height in the cis isomer than that in the trans isomer seems reasonable. However, the barrier height is determined not only by nonbonded atom-atom interaction but also by the details of electronic structure. Therefore, the observed difference of the barrier height (1 5 cm-') between the two isomers is too small to derive any conclusion. A clue for the identification might be obtained from the potentials of the ions. The potential barrier is dramatically increased by the ionization as seen in Figure 8. The ground-state barrier heights in the neutral cis and trans isomers of 26 and 11 cm-l can be compared with those in the cis and trans isomer ions of 328 and 276 cm-', respectively. The great increase of the barrier height by the ionization indicates a great enhancement of hyperconjugation in the ionic state. Since the hyperconjugation results in an increase of double-bond character for the C-C bond connecting the CH, group and the phenyl ring, the enhancement of the hyperconjugation means an appreciable contribution of the following structures to the ground state of the ion: I
.L
H'
H-C-H
I
H'
I
H-C-H
