J. Phys. Chem. 1986, 90. 5615-5619 TEMPERATURE
T, K
I(
+
,I
0
/
I
5
-
lower values of Xo 4.21 Thus, with the assumption of a single distribution of TLS, it can be argued that the diagonal modulation mechanism is more consistent with the existing optical line width or dephasing data. However, the prevailing tendency to fit temperature-dependent data with a single F-type power law may be misleading3’ since it implies that a single mechanism is operative. For optical dephasing it may be necessary to take into account both the intrinsic and extrinsic TLS with the latter determined by the Two two types of TLS would have different distributions, and the extrinsic TLS would favor off-diagonal modulation. A F-type dependence can also be described by a a7m b P type function with m < n and p > n. With this in mind, optical line-width measurements of different transitions of the same impurity in an amorphous host, as well as specific heat and thermal conductivity measurements on one and the same sample, may prove useful for an improved understanding of low-temperature behavior.
IO(
0
5615
IO-^
10.~
Acknowledgment. We thank John M. Hayes for useful discussions. This research was supported by the Division of Materials Research of NSF through Grant DMR-8421363.
NORMALIZED ENERGY
Figure 3. G, for s = 3-5 as a function of
B ( r ) for
A, = 4 and u2 = 5 ,
cf. text. modulation mechanisms. In this regard, specific heat data are attractive. Taken as a whole such data on a wide variety of systems favor a density of states ( p ( E ) ) power law of Eveffwith 0.3 5 veff 5 0.5. Such values due to the intrinsic TLS are consistent with
(31) Varma, C. M.; Dynes, R. C.; Banavar, J. R. J . Phys. C 1982, 15, L1221-LI225. (32) Black, J. L.;Halperin, B. I. Phys. Reo. 8: Solid State 1977,816, 2879. (33) Black, J. L.Phys. Reo. 8: Solid State 1978,817, 2740. (34) Fearey, B. L.;Carter, T. P.; Small, G. J. Chem. Phys. 1986,101,279.
n , r * State of Jet-Cooled Benzophenone As Studied by Sensitized Phosphorescence Excitation Spectroscopy Shin-ichi Kamei, Tatsuya Sato, Naohiko Mikami, and Mitsuo [to* Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan (Received: March 18, 1986)
-
The sensitized phosphorescence excitation spectrum of jet-cooled benzophenone due to the Sl(n,r*) So transition has been measured. It was found that the spectrum consists exclusively of several long progressions of 60 cm-’ which is the in-phase torsional mode of the phenyl rings. The vibrational analysis and the potential calculation show that in the Sl(n,r*)state great geometry changes occur in the dihedral angle between the phenyl rings, the C=O bond distance, and the C-C bonds adjacent to the C=O bond.
The lowest electronic excited state Sl(n,x*)of benzophenone has attracted much attention because of its importance in photochemical reactions. Although the electronic and geometrical structures of the S,(n,x*) have been studied by many investigators, experimental information available is surprisingly incomplete. This is because the electronic absorption spectrum due to the transition from the ground state to the Sl(n,a*)state is very broad even in the vapor phase and exhibits no vibrational structure.’ The broad feature of the vapor absorption spectrum probably arises from the spectral congestion of many hot bands due to the transitions involving low-frequency vibrations. If it is the case, the spectral congestion will be removed by cooling the molecule by a supersonic expansion. Actually, many molecules having broad absorption were found to exhibit well-resolved vibrational structure in the fluorescence excitation spectra taken under the supersonic jet condition.24 However, in the case of benzophenone, this powerful (1) Itoh, T.; Baba, H.; Takemura, T. Bull. Chem. Soc. Jpn. 1978,51,2841. (2) Murakami, J.; Ita, M.; Kaya, K. J . Chem. Phys. 1981, 7 4 , 6505.
0022-3654/86/2090-5615$01.50/0
tool of the fluorescence excitation spectroscopy applied to a jetcooled molecule is of no use because of its nonfluorescent nature. Recently developed sensitized phosphorescence excitation spectroscopy combined with the supersonic jet was proved to be very powerful in obtaining the absorption spectrum of a molecule such as benzophenone which has a high phosphorescence quantum yield. We already reported briefly that the sensitized phosphorescence excitation spectrum of jet-cooled benzophenone due to the Sl(n,x*) So transition exhibits a well-resolved vibrational feature. In the present paper, we report the details of the spectrum and also the spectrum of p-fluorobenzophenone in a supersonic jet. The results obtained provide
-
(3) Okuyama, K.; Hasegawa, T.; Ita, M.; Mikami, N. J . Phys. Chem. 1984,88,1711. (4)Okuyama, K.; Mikami, N.; Ita, M. J . Phys. Chem. 1985,89, 5617. (5) Abe, H.; Kamei, S.; Mikami, N.; Ita, M. Chem. Phys. Lett. 1984,109, 217. (6)Kamei, S.; Abe, H.; Mikami, N.; Ita, M . J . Phys. Chem. 1985,89, 3636.
0 1986 American Chemical Society
Kamei et al.
5616 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 I
99.5 158.8
' ,
25604 I
.
27600 27500 WAVE NUMBER I c m-1
2&00
162.2
I 30600 WAVENUMBER I cm-1
I
61.3
I
kl.0 I ,
I
50.8
58.2
I l l I 60.2 '59.4 59.0 I
I
55.5 54.1
I
57.2
I
56.6
I
I
58.0
57.4
I
55.9 55.3
I
I
56.1 55.1
54.8 5 4 . 0
I
I
I
I
60.7 53.8 59.2 50.3 57.3 56.6 55.3 54.1 5 3 . 5 , ! 1 I 1 I I I 1 I
28000
Figure 1. Absorption spectrum of benzophenone vapor (a) and sensitized phosphorescence excitation spectrum of jet-cooled benzophenone (b). The phosphorescence excitation spectrum corresponds to shaded region of the vapor absorption.
us with fairly detailed information on the potential and geometrical structure of benzophenone in the S,(n,a*) state. It was found that the dihedral angle between the two phenyl rings changes greatly by the electronic excitation from the ground state to the SI (n,a*) state together with a great change in the C=O bond length. The changes are largely localized in the C=O and its vicinity and seem to play an important role in photochemical behavior of this molecule. Experimental Section The experimental apparatus for the measurement of the sensitized phosphorescence excitation spectrum in a pulsed supersonic jet has been described e l s e ~ h e r e . Benzophenone ~ or p-fluorobenzophenone was heated to 390 K to increase the vapor pressure to about 3 Torr. The sample vapor seeded in 5 atm of He was expanded into a vacuum chamber at Torr through a 800-pm pulsed nozzle. The laser beam from a dye laser (Lambda Physik FL2002) pumped by a XeCl excimer laser (Lambda Physik EMG 103) crossed the jet IO-" downstream, and the molecule in the jet was excited to its Sl(n,a*) state. The triplet-state molecule produced from the S,(n,s*) state by intersystem crossing travels for 40 ps from the excitation position and collides with a liquid-nitrogen-cooled copper surface that was installed 80-mm downstream from the nozzle. The cold surface is covered with the solid sample or with solid biacetyl. The former was supplied from the jet, and the latter was supplied from its v a p r through another nozzle. The freshly deposited solid serves as a phosphor. When the triplet-state molecules in the jet collide with the solid phosphor, energy transfer from the triplet-state molecule to the phosphor occurs and sensitized phosphorescence is emitted. We detected this emission by a photomultiplier (HTV R585) which was cooled at -20 OC, and the signal was processed by a gated photon-counting system (Ortec 9302, 9315, and 9325 and a homemade gate generator with a 30-ms gate width after a SO-ps delay from the laser firing). With the above procedure, we obtained the sensitized phosphorescence excitation spectrum. Benzophenone and pfluorobenzophenone, which were obtained from Tokyo Kasei, were purified by vacuum sublimation. Results and Discussion Figure l a shows the absorption spectrum of benzophenone vapor due to the Sl(n,r*) So transition.' The spectrum is very broad although there is a faint structure. Figure 1b shows the sensitized phosphorescence excitation spectrum of jet-cooled benzophenone corresponding to the longer wavelength region of the vapor absorption spectrum as indicated in the figure. As seen from the figure, the phosphorescence excitation spectrum exhibits a well-resolved vibrational structure which is in marked contrast to the broad feature of the vapor absorption spectrum. The appearance of the vibrational structure in the jet spectrum is ascribed to the removal of many hot bands in the vapor spectrum by a low temperature attained by the supersonic expansion. The +-
,
60.9
61.1
62.1
I
59:O
159.5
56.8
51.0
58.3
I
I
I
I 26500
I
WAVENUMBER I cm-1
I
I
27000
Figure 2. Sensitized phosphorescence excitation spectrum of jet-cooled benzophenone in the region 26 200-27 000 c d . The band at 26 244. I cm-' is the 0," band. The 60-cm-' progressions and lOO-cm-' progressions are indicated.
observed spectrum is characterized by the long progression of about 60 cm-' in the region 26200-27000 cm-' and also by similar progressions appearing in the region 27 500-28 100 cm-'. Almost all the intensity of the spectrum is exclusively carried by the progressions of 60 cm-', and other spectral features are very weak. Figure 2 shows the spectrum in the region 26 200-27 000 cm-' on an expanded scale. The analysis shows that the spectrum consists of several progressions of 60 cm-' starting from different origins. The most prominent progression is one starting from the longest wavelength weak band at 26 244.1 cm-I and developing to a shorter wavelength until about 13 quanta with the intensity maximum at quantum 7. The frequency interval between the adjacent members of the progression decreases smoothly from 62.2 to 53.5 cm-' with an increase in the vibrational quantum, indicating the anharmonicity of this vibration. A similar but weaker progression also begins from weak bands at 26 343.6, 26 442.5, and 26 538.7 cm-' which are displaced by 99.5, 198.4, and 294.6 cm-' from the longest wavelength member of the first progression at 26 244.1 cm-'. The three bands including the band at 26 244.1 cm-' are members of another progression of about 100 cm-l, showing a slight anharmonicity. It is noted that there exists an intensity alternation in the 60-cm-' progression starting from the member of the IOO-cm-' progression, that is, weak in the 60-cm-l progression starting from the second member, a little stronger in the 60-cm-' progression associated with the third member, and again weak in that associated with the fourth member. It is also of interest to note that the 60-cm-I progressions starting from the second, third, and fourth members of the 100-cm-' progression have the first frequency intervals (61.7, 59.5, and 58.8 cm-I) progressively reduced from that (62.2 cm-') of the 60-cm-I progression starting from 26 244.1 cm-'. The fact that the two different progressions of 60 and 100 cm-' start from a common band at 26 244.1 cm-I clearly shows that So the band at 26 244.1 cm-' is the 0; band of the S,(n,a*) transition. The vibration of 60 cm-' forming the long progressions can be assigned to an excited-state totally symmetric torsional mode of the phenyl rings about the C-C bonds connecting the carbonyl group and the phenyl rings. On the other hand, the excited-state vibration of 100 cm-' which also forms the progression may be assigned to the corresponding asymmetric torsional mode of the phenyl rings because of the observed intensity alternation mentioned above. Assuming C, symmetry for benzophenone molecule, the 60- and lOO-cm-' vibrations are in-phase (A) and out-of-phase (B) torsional modes, respectively. The corresponding ground-state vibrational frequencies are not known, but they are evaluated to be 45 (in-phase) and 50 cm-' (out-ofphase) by Blazevic and Colombo' from their normal coordinate calculation. Assuming that these ground-state frequencies are
-
(7) Blazevic, J.; Colombo, L. J . Raman Spectrosc. 1981, 11, 143.
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5617
n,?r* State of Jet-Cooled Benzophenone 62.2
61.4
61.0
61.1
l
l
l
l
62.2
61.1
61.7
61.2
I
I
I
I
I
59.8
59.1
l
60.2
l
l
l
l
61.1
59.7
59.6
59.3
59.8
I
I
58.5
58.1
I
56.2 54.2
l
l
l
57.8 56.5
I
I
I
~
26500
27000
0
I
I
I 27500
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
r
28000
WAV ENUMBER I c rn-l
Figure 3. Sensitized phosphorescence excitation spectrum of jet-cooled benzophenone in the region 27 500-28 100 cm-I. Two prominent progressions of 60 cm-' are indicated.
correct, the torsional frequency increases greatly upon the electronic excitation for both the in-phase and out-of-phase modes. The existence of the long progression of the in-phase mode and the characteristic intensity distribution for the members of the progression strongly suggest a great change in the geometrical structure of the molecule along the in-phase torsional coordinate, which will be discussed later. The observed spectral pattern in the region 26 200-27 000 cm-' can be given by the excited-state energy levels (in cm-I) expressed by eq 1 where u1 and u2 are the vibrational quantum numbers of
the in-phase and out-of-phase torsional modes, respectively. The agreement between the observed levels and the levels calculated by eq 1 is better than 0.5 cm-' for u1 = 0-11 and u2 = 0-3. It is seen from eq 1 that the anharmonicity constant (-1.50 cm-') due to the coupling of the two modes is fairly large. This anharmonicity constant is responsible for the observed difference in the vI progression coupled with each member of the u2 progression. Now, we turn our attention to the region 27 500-28 100 cm-' which is located about 1100 cm-I higher in energy than the first region. Figure 3 shows the spectrum in the region 27 500-28 100 cm-l on an expanded scale. The spectrum in this region consists again of many progressions of 60 cm-'. The most prominent progression is one having the member of maximum intensity at 27 790 cm-I, and the second prominent progression is one having the highest intensity member at 27 760 cm-'. The origins of these two progressions cannot be identified because of their weak intensities. However, the excited-state vibrational frequencies involved can be obtained from the frequency difference between the band of the maximum intensity (26670 cm-l) in the 60-cm-I progression starting from the 0; band and the maximum intensity member of the progression in the second region mentioned above. Then, we obtain the excited-state vibrational frequencies of 1120 and 1090 cm-I. It is well-known that in the n,a* absorption spectrum of a carbonyl compound C=O stretching vibration in the excited state appears strongly and often forms a long progression. Therefore, 1120 and 1090 cm-' may be assigned to the modes involving the C=O stretching in the Sl(n,a*)state, whose ground-state frequency is 1667 ~ m - ' .As ~ usually seen in carbonyl compounds, the frequency of the C=O stretching vibration greatly decreases upon the excitation to the n,a* state. However, the excited-state C=O stretching frequency of 1100 cm-I in the case of benzophenone is much smaller than those of other carbonyl compounds whose frequencies are in the range between 1200 and 1300 cm-'. The great frequency decrease of the C=O stretching mode causes a great mixing of the mode with the R-C-R symmetric stretching mode (R represents the phenyl ring) whose ground-state frequency is 1130 ~ m - ' .Therefore, ~ we assign the excited-state frequencies of 1090 and 1120 cm-' to two totally symmetric modes arising from substantial mixing of the C=O stretching and R-C-R symmetric stretching. Such a mixing also occurs in p-fluorobenzophenone described later. Besides the
-
I
I
I
27400
I
I
I
I
WAVENUMBERI cm-1
I
I
I
28dOO
Figure 4. Sensitized phosphorescence excitation spectrum of jet-cooled p-fluorobenzophenone. The progressions of 59 cm-I are indicated.
60-cm-' progressions coupled with the above-mentioned modes, there exist many other weak progressions of 60 cm-'. Although the vibrational frequencies involved are not clearly identified, they are vibrations of 1OOC-1200 cm-I. These vibrations are probably the modes essentially localized in the phenyl rings such as 18b (1177 cm-' in ground state), 3 (1162 cm-I), 9b (1090 cm-I), 18a (1021 cm-I), and 5 (981 ~ m - ' ) . Weak ~ appearance of these modes may be explained as a result of weak coupling of the ring mode with the C=O stretching mode. It is concluded from the above that the appearance of many progressions in the second region is due to the mixing of the C 4 stretching and the modes localized in other parts of the molecule. The mixing is large for the stretching of R-C-R which is adjacent to the C = O bond and small for the modes of the rings which are far from the C=O bond. It is also suggested that a large geometry change takes place upon the electronic excitation in the central part of the molecule, that is, in the C = O and C-R bond lengths and the torsional angle about the C-R bond. Absence of the ring modes other than those mixing with the C=O stretching mode indicates that the geometrical change of the phenyl ring is small or absent. Figure 4 shows the sensitized phosphorescence excitation spectrum of p-fluorobenzophenone in a supersonic jet. As seen from the figure, the observed spectral features are very similar to those of benzophenone. The longest wavelength weak band at 26 375.1 cm-' is tentatively assigned to the 0; band, from which the long progression of the in-phase torsional mode of about 59 cm-' develops toward higher frequency with the intensity maximum at ul = 7. The frequency of the in-phase torsional mode changes little in going from benzophenone to p-fluorobenzophenone. This implies that the torsional motion of the ring is essentially about the axis passing through the carbonyl carbon atom, its adjacent phenyl carbon atom and the phenyl carbon atom at the para position. In that case, the moment of inertia of the phenyl ring about the axis does not change in substitution of the para hydrogen atom by the F atom. This fact will be used later in the estimation of the dihedral angle between the two phenyl rings in the Sl(n,a*) state. As seen from Figure 4, the spectral features of p-fluorobenzophenone in the region 27 700-28 200 cm-l are again very similar to those of the corresponding region of benzophenone. There exist two prominent progressions of 59 cm-'. The excited-state vibrational frequencies coupled with these progressions are estimated from the frequency intervals between the highest intensity member of the 59-cm-' progression starting from the 0; band and the corresponding member of the progression in the second region. They are 1140 and 1160 cm-I, which are a little larger than those (1 090 and 1 120 cm-') of benzophenone. These vibrations can be assigned to modes in which the C=O stretching and R-C-R' stretching are strongly coupling similar to the case of benzophenone. Now, we shall consider the geometrical structure and the potential of benzophenone in the Sl(n,a*) state. The ground-state
5618 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
Kamei et al.
0
Figure 5. Definition of twisting angles (Y and 0measured from the planar
structure.
I 0'
20'
33'
a
(=a)
Figure 7. One-dimensional potentials of So and SI states with respect to twisting angle. The calculated intensity distribution of the 60-cm-' progression is also shown. g -30
-60
the carbon atom at the para position. Assuming such a motion, the kinetic energy for the torsional motions of the two phenyl rings is expressed by eq 3 where I is the moment of inertia of the phenyl
-7.0
I T = -(A&)2 2
-8.0
O0
Figure 6. Potential due to hydrogen atom-hydrogen atom interactions against twisting angle.
equilibrium geometry of benzophenone is determined by a balance of steric and conjugative effects. Conjugation of the carbonyl group with the phenyl rings favors a planar conformation, while steric repulsion between the H2 and Ht2 hydrogen atoms (see Figure 5) prevents the attainment of coplanarity. Because of this balance, each phenyl ring is rotated by angle a or /3 about the axis shown in Figure 5 . It is known from the molecular structure determined by X-ray crystal analysis8 that a and /3 as defined in the figure are both positive and a = /3 = 33'. Therefore, the molecular structure of benzophenone is C2. In order to see the steric effect, we calculated the potential due to the interactions among the H2, H6 and H'2, Ht6hydrogen atoms of the two phenyl rings. We assumed the atom-atom interaction potential in the form V(r) = A exp(-Br) - Cr-6 (2) where r is the interatomic distance. A , B, and C are constants, whose values are proposed by several authors. In the calculations, we used three sets of A, B, and C proposed by different authors. erg, B = 4.86 A-1, and C = 3.64 X 10-l2 Set I is A = 28.19 X Set I1 is A = 4.58 X erg, B = 4.08 A-I, and C = erg .&6.9 3.42 X erg A6.l0 Set I11 is A = 6.31 X erg, B = 4.64 and C = 1.49 X erg A6." The potentials calculated by using the three sets are shown in Figure 6 against the conrotatory angle a = /3. As seen from the figure, all the sets give a local minimum at a = = 36-40', In order to get the torsional potential, we have to add the potential due to the conjugation. The latter has a minimum at a = /3 = 0' and a maximum at a = /3 = 90'. Therefore, the addition of the conjugation effect shifts the local minimum determined by the hydrogen interactions to a smaller angle. Assuming that the ground-state geometry ( a = /3 = 33') determined from the X-ray analysisB is correct, it is concluded that the decrease of the angle due to the conjugation is the order of 3-7'. Now, we shall consider the excited-state potential for the torsional motion of the phenyl rings. As mentioned before, the in-phase torsional mode of 60 cm-' can be well described by the torsional motion of each phenyl ring about the axis passing through the carbonyl carbon atom, the adjacent phenyl carbon atom, and (8) Fleischer, E. b.; Sung, N.; Hawkinson, S. J . Phys. Chem. 1968, 72, 431 1 . (9) Kitaigorodsky, A. J . Chem. Phys. 1966, 63, 9. (10) Oliver, D. A.; Walmsley, S. H. Mol. Phys. 1969, 17, 617. (11) Dashevsky, V. G.; Struckykov, V. T.; Akoppayan, Z. A. Z h . Srruk. Khim. 1966, 7, 594.
I + -(AB)2 2
(3)
ring about the axis mentioned above and it was assumed to be equal to that of benzene. Equation 3 is rewritten by eq 4 where
T = I2 [ -!-(A& 2112 = -9'' 1 2
+ AB)]2 + :[$(A&
+ ;Q22 1
- AB)
1 (4)
Q1and Q2are the in-phase and out-of-phase torsional coordinates, respectively. The potential energy is given by eq 5. Using the observed frequencies of ul = 60 cm-I and v2 = 100 cm-I, we obtain 1 V = -k(Aa2 2
+ AB2) + k'AaAp
k = 3.61 X dyn cm/rad and k'= -1.58 X dyn cm/rad. The interaction force constant k'is very large. The corresponding ground-state frequencies are not known. However, Blazevic and Colombo' assigned a broad Raman line at 57 cm-' observed in the CC1, and benzene solutions to both v I and u2 in the ground state. Taking this value, the ground-state force constant is k = 1.70 X dyn cm/rad and k' = 0. Therefore, the torsional force constant k increases very much by the electronic excitation, and the coupling of the two phenyl rings also becomes strong in the excited state. The results obtained strongly suggest that in the Sl(n,r*) state the C-R bond gains more double bond character than that in the ground state. This also implies that the geometrical structure of benzophenone becomes more coplanar in the S,(n,.n*) state than in the ground state. Now, we shall consider the change in the equilibrium angles a and /3 by the electronic excitation. It is apparent from the observed spectrum that there must be a great difference in the angle between the Soand S,(n,?r*) states. The observed intensity distribution of the 60-cm-' progression provides us with information about the angle change. We assumed one-dimensional oscillators for both the in-phase torsional modes of the So and SI states but with displaced equilibrium angle b a (=sa) between the So and SI states. For the ground-state, a one-dimensional oscillator of 57 cm-' was assumed. Then, we calculated Franck-Condon factors for the transitions from the ground-state zero-point level to various excited-state torsional levels by assuming various values of 6a (ESP). It was found that the observed intensity distribution of the 60-cm-' progression starting from the 0;band can be well reproduced by Sa(=Sp) = 13' (see Figure 7). The intensity calculation does not tell us about the direction of the angle change. However, the force constants obtained above indicate that the angle should be smaller in the Sl(n,r*) state than in the Sostate.
J. Phys. Chem. 1986, 90, 5619-5622 It is, therefore, concluded that the benzophenone molecule in the
Sl(n,?r*)state is more coplanar by 13O than in the Sostate. When the ground-state angle of CY = /3 = 33O obtained from the crystal structure analysis is adapted, the excited-state angle is a = /3 = 20°. The results obtained are parallel to the calculated results by the extended Hiickel method which predicted CY = p = 38' and 32' for the So and Sl(n,a*) states, respectively.12 In summary, the n,a* state of benzophenone was fairly well elucidated by the sensitized phosphorescence excitation spectrum of the jet-cooled molecule. It was found that the geometrical (12) Hoffmann, R.; Swenson, J. R. J . Phys. Chem. 1970, 74,415. (13) Holtzclaw, K. W.; Pratt, D. W. J . Chem. Phys. 1986, 84, 4713.
5619
change of the molecule induced by the electronic excitation occurs mainly in the central part of the molecule, that is, in the C=O bond length, in the C-R bond lengths, and in the twisting angle of the phenyl ring about the C-R axis.
Note Added in Proof: Recently, Holtzclaw and PrattI3 reported a similar spectrum of jet-cooled benzophenone using fluorescence excitation detection. Although the spectrum is similar to ours, the interpretation is different between the two groups. Acknowledgment. We are grateful to K. Okuyama and M. F u j i for valuable discussions. Registry No. Benzophenone, 119-61-9;p-fluorobenzophenone,34592-6.
Rotationally Resolved Fluorescence Excitation Spectra of Jet-Cooled Pyrimidine and Pyrimidine-Argon van der Waals Complex Yoshiyuki Sugahara, Naohiko Mikami,* and Mitsuo Ito Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan (Received: April 15, 1986)
-
Rotational structures of the fluorescence excitation spectra of Sl(nr*) So transitions of jet-cooled pyrimidine and its van der Waals complex with Ar have been observed. The rotational constants of the SI-state pyrimidine were obtained from the rotational analyses of 0,Oand 16b1bands. The molecular structure in the excited state was determined from the excited-state rotational constants by assuming a molecular deformation along the 6a normal coordinate upon the electronic excitation. The van der Waals bond distance between argon and the pyrimidine ring of the complex was also obtained for the ground and excited states from the rotational analysis of the complex spectrum based on the symmetric top approximation. The rotational structure of the vibronic band involving the predissociative excited level of the complex was also observed. The line width of the rotational line shows that the vibrational predissociation rate constant is smaller than 6 X lo9 s-'.
In the last years, we have studied the energetics and dynamics of the pyrimidineargon (Py-Ar) van der Waals complex prepared in a supersonic free The SI state of the complex exhibits many interesting features of vibrational predissociation such as fragmentation to a pyrimidine fragment with a pzrticular vibronic energy and competition among predissociation, intramolecular vibrational redistribution (IVR), and photoionization. Because of these characteristic features, the Py-Ar complex is regarded as a good material for exploring the general mechanism of vibrational predissociation of a large molecular complex. However, before going to the detailed study, we must have accurate knowledge on the geometrical structure of the complex in the SI state. The main purpose of the present study is to establish the geometrical structure of the complex from the rotational analyses of several vibronic bands in the high-resolution fluorescence excitation spectrum of the Sl(na*) So transition. The structure of the pyrimidine molecule in Sl(na*) was first studied by Innes et al.3*4from the rotational analysis of the vapor absorption spectrum. However, their results were not enough to determine the excited-state structure because they obtained only the rotational constant B'. Therefore, we were forced to begin with the rotational analysis of the pyrimidine molecule. Here, we report first the molecular structure of pyrimidine in SI obtained from the rotational analyses of the 0,O and 16b01bands of the jet-cooled molecule. Since the SIstate is the B, species in the C , point group, the 0,O band of the SI So transition should be polarized per-
-
+
(1) Abe., H.; Ohyanagi, Y . ;Ichijo, M.; Mikami, N.; Ito, M. J . Phys. Chem. 1985,89, 3512. (2) Mikami, N.; Sugahara, Y . ; Ito, M. J . Phys. Chem. 1986, 90, 2080. (3) Innes, K. K.;Merritt, J. A,; Tincher, W. C.; Tilford, S. G. Nature (London) 1960, 187, 500. (4) Innes, K. K.; McSwiney, H. D., Jr.; Simmons, J. D.; Tilford, S. G. J . Mol. Spectrosc. 1969, 31, 76.
0022-3654/86/2090-5619$01 S O / O
pendicular to the molecular plane. On the other hand, the rotational structure of the l6bO1band, which is due to a vibronically induced transition, is quite different from that of the 0,O band. The analysis of the former band gives us an approximate value of the rotational constant C', which is difficult to obtain from the latter band. Accurate values of the rotational constants in the excited state were obtained by a computer simulation of the observed rotational structures of the 0,O and 16b01bands. The excited-state molecular structure was determined from the rotational constants based on an assumption of the geometrical change in the electronic transition. Next, we observed the rotational structure of the 0,O band of the jet-cooled Py-Ar complex. By referring to the molecular structure of pyrimidine and by assuming a T-shape structure for the complex, we obtained the van der Waals distance between Ar and Py of the complex in the ground and excited states from the simulation of the observed rotational structure. A reduction of the van der Waals bond length in the excited state was found. Finally, the predissociation rate of the vibronic state of the complex was discussed from the line width of the rotational line of the vibronic band associated with the predissociative level.
Experimental Section Jet-cooled pyrimidine was prepared in a pulsed free expansion of the mixture of helium gas and pyrimidine vapor at room temperature. The Py-Ar complex was generated in the free expansion of pyrimidine vapor seeded in pure argon gas (1 atm). The apparatus of the pulsed supersonic free jet is described el~ewhere.~ In the measurement of the fluorescence excitation spectrum, an excimer (XeC1) laser-pumped dye laser (Lambda Physik EMGlO2MSC FL-2002E. Rhodamine 640) combined with an an-
+
~
~~
(5) Mikami, N.; Hiraya, A.; Fujiwara, I.; Ito, M. Chem. Phys. Lett. 1980, 74, 531.
0 1986 American Chemical Society
