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)
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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
5620
Sugahara et al.
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
TABLE I: Rotational Constants of Pyrimidine excited-state S,(ns*) ground state So” lo = 31072.34 f 0.05 cm-l A’ = 0.2128 f 0.001 cm-’ A” = 0.2089 cm-’ 5’= 0.1948 f 0.001 cm-I 5”= 0.2029 cm-l C ’ = 0.1017 & 0.001 cm? C” = 0.1029 cm-’ B’ = 0.2038 cm-I B” = 0.2059 cm“ K‘ = +0.676 K“ = + 0.887 ‘Calculated from the structure given in ref 7. 6.0
0 -5.0 FREOUENCY / cm-’ Figure 1. Rotational structure of the 0,O band in the fluorescence excitation spectrum of jet-cooled pyrimidine. The band origin is at 31 072.34 cm-I.
I
16bb BAND PYRIM1olNE
+
5.0
0
FREOUENCY / cm-’
-5.0
Figure 3. Calculated rotational structure of C-type transition. Rotational constants used for the calculation are given in Table I. A rotational temperature of 5 K is assumed. 0 - 3.0 FREOUENCY /cm-’ Figure 2. Rotational Structure of the 16b01 vibronic band in the fluorescence excitation spectrum of jet-cooled pyrimidine. The band origin is at 31 435.60 cm-I. +
3.0
gle-tuned etalon controlled by a microprocessor was used for pyrimidine. For the complex, a nitrogen-laser-pumped dye laser (Molectron UV-24 DL-l4P, Rhodamine 640) equipped with a homemade pressure-controlled etalon was used. The calibration of frequencies of the dye laser was done by the fluorescence excitation spectrum of iodine vapor.6 The line widths of the former and latter dye lasers were 0.05 and 0.03 cm-I, respectively, at 645 nm. The second harmonic of the dye lasers was generated by a KDP crystal and used as the light source of the fluorescence excitation spectra. Total fluorescence from the laser-excited free jet was collected by a lens and detected by a photomultiplier (HTV, R562). The detection system is essentially the same as that described elsewhere.]
CALCULATED A -TYPE
+
+
3.0
b
FREOUENCY / c d
-3.0
Figure 4. Calculated rotational structure of A-type transition. Rotational constants are the same as those for Figure 3, and a rotational temperature of 2 K is assumed.
Results and Discussion A . Rotational Analyses of the 0,O and 16b01Bands of Pyrimidine. Figure 1 shows the rotational fine structure of the 0,O band of jet-cooled pyrimidine. The P branch appears on the low-energy side of the intense Q branch, and the R branch is on its high-energy side. The spectrum shows a typical feature for the parallel band of a nearly symmetric top molecule. Since pyrimidine is a nearly symmetric top molecule of oblate type, the parallel band means that the transition moment of the 0,O band is parallel to the axis of the largest moment of inertia, the c axis, which is perpendicular to the molecular plane. Figure 2 shows the rotational structure of the l6bO1band which is located 363 cm-’ higher in energy than the 0,O band. This is the band induced by vibronic coupling with the nontotally symmetric vibration Y~~~ in the excited state. The rotational structure of the band is entirely different from that of the 0,O band. The main peaks may be assigned to RQKand pQKbranches of the K structure in the perpendicular band of the oblate symmetric top. The rotational constants of the ground-state pyrimidine are already available from the molecular structure given by Fernholt et a].’ When the ground-state rotational constants are referred to, the corresponding constants in the excited state can be obtained
in the following way. Since the rotational energy levels of a nearly symmetric top molecule are given by E(JK) = BJ(J + 1) + (C - B)f? and the selection rule for the parallel band is well-known, the excited-state rotational constant B’ in the symmetric top approximation can be obtained from the 0,O band by the combination-difference methoda using the lines in the R and P branches. For the perpendicular band, the spacing between the K’ = 1 K” = 0 (AK = +1) and the K’ = 0 K” = 1 ( A K = -1) lines should be nearly equal to 2(C’- 8’) under the symmetric top approximation, if Coriolis interaction is neglected. As seen in Figure 2, the 16b01band consists of nearly equi-distant rotational lines. The energy difference between the adjacent rotational lines corresponds to 2(C’- 8’). Therefore, by using B’obtained from the 0,O band, we obtain the approximate value of C’. By reference to the approximate values of B’and C’ thus obtained, we carried out a computer simulation9 of the rotational structures on the basis of the asymmetric rigid rotor approximation. In adjusting the rotational constants A’, B’, and C’, the inertia defect was neglected, and A’and B’were assumed to be subject to the constraint that 1/2(A’+ B’) is equal to the observed B’value. The final values of the rotational constants which best reproduce the observed rotational structures of the 0,O and l6bO1 bands are given in Table I, together with the ground-state rotational constants. The calculated spectra are shown in Figures 3 and 4 for the 0,O and 16b01bands, respectively. It is seen that the rotational structure of the 0,Oband is reproduced very well
( 6 ) Gerstenkorn, S.; Luc, P. Atlas du Spectre #Absorption de la Molecule d’lode; Centre National de la Recherche Scientifique: Paris, 1978. (7) Fernholt, L.; Rmming, C. Acta Chim. Scand., Ser. A 1978, A32, 271.
( 8 ) Herzberg, G. Molecular Spectra and Molecular Structure; Van Nostrand-Reinhold: New York, 1945; Vol. 11. (9) Nakagawa, T.; Overand, J. J . Mol. Spectrosc. 1974, 50, 333.
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The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5621
Pyrimidine and PyrimidineAr Complex TABLE II: Molecular Structure of Pyrimidinea
parameter*
SIstatee
Sn stated
1.353 1.346 1.394 129.7 114.5 120.4 120.5 0.2132 0.1948 0.1018 +0.670
1.335 1.340 1.395 128.2 115.2 122.6 116.3 0.2092 0.2025 0.1029 +0.874
'C-H bond lengths and angles are not included though they were calculated. *Definitions of the parameters R, and 0, are given in Figure 5b. CEstimated errors of the bond lengths and the angles are f0.002 A and f 0.5 deg, respectively. dThe ground-state bond lengths and angles are slightly different from those given in ref 7 because of the assumption of planar structure in this work. with the C-type transition of the asymmetric top (Figure 3). The result is consistent with the B I symmetry of the Sl(nr*) state. On the other hand, the observed structure of the 16b01band is fairly well reproduced with the A-type transition as seen from Figure 4. In the simulation, the axis switching* in the electronic transition did not need to be taken into account. This means that the axis of the smallest moment of inertia ( a axis) is the same between the ground and excited states. It is concluded therefore that the transition moment of the 16b01band is parallel to the C, symmetry axis ( a axis) in the molecular plane and that the vibronic state belongs to the AI species of the C, molecular symmetry. Since the symmetry of the SIstate is B l , the 16b mode is assigned to the out-of-plane vibration of the bl species in agreement with the previous assignment.' B. Excited-State Molecular Structure of Pyrimidine. As seen from Table I, in spite of a small change in the averaged rotational constant B upon the electronic excitation, the rotational constants in the excited state are considerably different from those in the ground state. A remarkable reduction of the asymmetric parameter K in the excited state is also seen. The smaller K indicates that the excited-state molecule is more asymmetric than the ground-state molecule. The three rotational constants are not sufficient to make unambiguous determination of the excited-state molecular structure. In order to estimate the structure, we have to use some reasonable assumptions for the geometrical change. Qualitative information on the geometrical change upon the electronic excitation may be obtained from the vibrational structures of the absorption and fluorescence spectra of pyrimidine. In the absorption or fluorescence spectrum of pyrimidine vapor,I0 all the observed progressions are exclusively of totally symmetric vibrations, and no progression of nontotally symmetric vibration is found. This means that the molecular symmetry is preserved in the electronic transition and the geometrical change can be represented by the displacement in subspace of totally symmetric normal coordinates. Pyrimidine has nine totally symmetric modes. We assume the absence of rotations of these modes (Duschensky effect) upon the electronic excitation. Then, the displacement is expressed by the space defined by the ground-state normal coordinates. When the absorption or fluorescence spectrum is referred to, major intensity is carried by the progression of the 6a mode (677 cm-' in So). Therefore, we assume for simplicity that the geometrical change is along the 6a normal coordinate. The normal coordinate of 6a in So was obtained from the G-F matrix method by using the Urey-Bradley force field available. The normal coordinate obtained is shown in Figure 5 . Then, the molecular geometry was modified along the 6a coordinate from the original structure of the ground-state molecule to find the displacement of the coordinate which reproduces the excited-state rotational constants. The best structures thus obtained are given (10) Knight, A. E.W.; Lawburgh, C. M.;Parmenter, C . S.J . Chem. P h p . 1975,63,4336.
Figure 5. (a) Normal coordinate of the 6a mode. (b) Definitions of bond lengths and bond angles. Cartesian axes (x, y, z) and principal axes (A, B, C) of pyrimidine are shown at the top.
I
Py-Ar COMPLEX
/I
0.0 BAND
b
+1.0
FREQUENCY /cm-'
-1.0
Figure 6. Rotational structure of the 0,O band in the fluorescence excitation spectrum of the Py-Ar complex. The band origin is at 31 032.24
cm-'.
+l .o
0 -1.0 FREQUENCY/cm-'
Figure 7. Calculated rotational structure of the parallel-type transition of a prolate symmetric top with the rotational constants given in Table 111. A rotational temperature of 2.5 K is assumed. TABLE III: Rotational Constant and van der Waals Bond Length of the Py-Ar Complex
S1(n**)
B,cm-I re, A
ijo = 31032.24 f 0.03 0.0427 f 0.001 3.43 A f 0.05
SO
cm-' 0.0423 f 0.001 3.45 f 0.05
in Table 11, together with the reproduced rotational constants for the SI state. We also tried to find the structure along each of the other normal coordinates. However, the structure which satisfies the set of three rotational constants was not found. This also supports our conclusion that the excited-state structural change is approximately along the 6a normal coordinate. C. Structure of the Py-Ar Complex. It was shown in a previous paper] that the 0,O band of the Py-Ar complex appears at 31 032.2 cm-' which is about 40 cm-' lower in energy than the 0,O band of free pyrimidine. Figure 6 shows the rotational structure of the 0,O band of the complex. An intense Q branch and regular structures in R and P branches are the characteristic feature of a parallel-type band expected for a nearly symmetric top molecule. Since the transition moment of the Sl(nn*) So transition of pyrimidine itself is perpendicular to the molecular plane, the parallel-type band of the complex means that the principal axis of the moment of inertia of the complex is also perpendicular to the pyrimidine molecular plane. This in turn implies that the argon atom lies nearly above the center of the pyrimidine ring plane. Thus, the complex is a nearly symmetric top molecule of a prolate type.
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5622 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
Sugahara et al. vibronic bands depending on the coupling strength between the discrete level and the continuum. In order to obtain the coupling strength, we observed the rotational fine structure of these vibronic bands. Figure 8 shows, for example, the rotationally resolved fluorescence excitation spectrum of the 12,,I band (0 1014 cm-I) of the complex. Half-width of half-maximum (hwhm) of each rotational line was found to be about 0.03 cm-I, which is the average for the lines in the R branch. A similar hwhm was also obtained for the 6a01and 1,' bands of the complex. The observed line width for the predissociative level is about the same as that for the 0,O level of the complex, which is lying below the dissociation limit. It was also found that the width is practically the same as that of the laser. The above facts indicate that the line width resulting from the predissociation broadening is smaller than the laser line width and the observed width 0.03 cm-' can be regarded as a maximum width for the predissociation. If the width is simply determined by the predissociation, the upper limit of the predissociation rate constant can be obtained from the uncertainty relation 1/7 = 2acA3, where the line shape function is assumed to be Lorentzian with the hwhm of A?. Using A? = 0.03 cm-l, we obtained 6 X lo9 as the upper limit of the predissociation rate constant. On the other hand, the lower limit of the rate constant can be estimated from the competition between the fluorescence and dissociation processes from the vibronic levels of the complex. It was shown in a previous paper' that, when the 12, level of the complex is excited, the resonance fluorescence from this level does appear, but the resonance fluorescence is completely absent in the excitation of lower energy levels such as 6a'. The fact can be understood by competition between the resonance fluorescence process and the predissociation process. The fluorescence lifetime of the complex will become shorter with the increase of the vibrational excess energy in the excited state. If the dissociation rate does not depend on the excess energy, the resonance fluorescence from the higher vibronic level may appear before the dissociation of the complex, because of shortening of the fluorescence lifetime. Therefore, if we know the fluorescence lifetime for such a level of the complex, the lower limit of the dissociation rate constant can be estimated. Although the fluorescence lifetime of the complex is not known, it is very likely that the fluorescence decay from the vibronic level of the complex is faster than the fast component of the fluorescence decay from the corresponding vibronic level of pyrimidine molecule whose fluorescence exhibits a double-exponential decay. The fast decay component of the pyrimidine fluorescence was measured by Spears et al." and by Baba et al.12 and the lifetime for the 12' level was reported to be 1.2- 1.4 ns. This value can be regarded as the upper limit of the lifetime of the vibronic state of the complex. Then, the lower limit of the predissociation rate constant is to be 7 X lo8 SKI.It is concluded from the above that the vibrational predissociation rate constant of the Py-Ar complex in the excited state is in the range between 7 X 10' and 6 X lo9 s-l.
+
+1.0
0
FREOUENCY/cm-'
-'"
Figure 8. Rotational structure of the 1201 band in the fluorescence excitation spectrum of the Py-Ar complex. The full line width (0.06 cm-I) of the laser used is given.
Assuming the prolate symmetric top for the complex, the observed rotational lines in the R and P branches were readily assigned. The rotational constants of the ground (B'q and excited (8')states can easily be obtained from the combination-difference method from the lines in the R and P branches. The result is given in Table 111, and the calculated rotational structure is shown in Figure 7 . The calculated structure reproduces very well the observed one. It is seen that the rotational constant in the excited state is slightly larger than that in the ground state. The larger rotational constant is evident from the observed spectrum, in which the interval of the rotational lines in the R branch increases with an increase of J and the reverse is seen for the P branch. By use of the derived structure of the complex with the argon atom on the line perpendicular to the pyrimidine ring and passing through the center of the ring, the van der Waals bond length re defined by the distance between the argon atom and the center of the ring can be calculated from the rotational constants. We assumed in the calculation that the structure of the pyrimidine moiety in the complex is the same as that of free pyrimidine molecule given in a previous section. The calculated van der Waals bond lengths in the ground and excited states are given in Table 111. The ground-state bond length of 3.45 A is very close to the distance estimated from the hard sphere model consisting of spheres of known van der Waals radii of argon (1.88 A) and of nitrogen and carbon atoms with a-electrons (1.70 A) in the pyrimidine framework. In this model, the argon atom sits in a shallow valley of the a-electron cloud at the center of the pyrimidine ring. The van der Waals bond length in the excited state is 3.43 A, which is slightly shorter than that in the ground state. As seen from Table 11, a marked increase in the C-N bond length R , and the C-C-C bond angle e4 of the pyrimidine ring occurs by the electronic excitation. This change makes the pyrimidine ring in the excited state more expanded than in the ground state. The expansion might contribute to the decrease in the van der Waals bond length. However, the decrease of the van der Waals bond length in the excited state is more reasonably explained in terms of the increase of polarizability of the pyrimidine in the complex by the electronic excitation. The shorter bond length in the excited state is also supported from the fact that the dissociation energy of the complex is larger in the excited state than in the ground state as indicated from the red shift of the 0,O band of the complex relative to that of pyrimidine. D. Predissociation Rate of the Complex. As described in previous papers,l*2the Py-Ar complex undergoes vibrational predissociation when the vibronic levels of the complex lying above the dissociation limit such as 6a1,6bZ,l', and 12l levels are excited. Since these levels couple with the dissociation continuum, one can expect a broadening of the rotational line belonging to these
Acknowledgment. The authors are grateful to Dr. T. Nakagawa (Fujitsu Co.) for permission to use the program BC3 for the rotational analysis and to Dr. H. Takeuchi for his help in the calculation of normal coordinates. We also thank Dr. K. Yamanouchi (Tokyo University) and M. Suzuki (Hiroshima University) for their assistance in the calculation. Registry No. Ar, 7440-37-1; pyrimidine, 289-95-2. ( 1 1 ) Spears, K. G.; El-Manguch, M. Chem. Phys. 1977,24, 65. (12) Baba, H.; Ohta, N.; Sekiguchi, 0.;Fujita, M.; Uchida, K. J . Phys. Chem. 1983,87,943.
