J. Phys. Chem. 1991, 95, 10311-10317
Vibrational Mode-Specific Tunneling Splittings in the
10311
States of Deuterated Tropolones
Hiroshi Sekiya,* Yusuke Nagashima, Takeshi Tsuji, Yukio Nishimura,* Akira Mori, and Hitoshi Takeshita Institute of Advanced Material Study and Department of Molecular Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasuga-shi. Fukuoka 816, Japan (Received: March 13, 1991; In Final Form: July 5, 1991)
The electronic spectra have been measured for jet-cooled tropolone-3-d (3DTRN), tropolone-4-d (4DTRN), and tropolone-5-d (5DTRN) to investigate the effect of a very weak perturbation due to the substitution of a deuterium atom in the skeletal seven-membered ring on the average geometries and the tunneling splittings for the vibrationally excited levels in the A states. The vibrational fundamental for ~ ’ 1 3has changed significantly in the order 3DTRN > 4 DTRN > SDTRN, whereas that of dI4has been found to be almost constant. This suggests that ~ ’ 1 3involves the motions of the hydrogen atoms in positions 3, 4, and 5 , while those of the hydrogen atoms at the same positions are insignificant in dI4.The tunneling splittings for d I 3have decreased remarkably in 4DTRN and 3DTRN, indicating that the motions of the hydrogen atoms in positions 3 and 4 are strongly coupled with the motion of the hydroxylic proton in ~ ’ 1 3 . On the other hand, very slight changes have been found in the tunneling splitting of JI4.Similarly, changes in the tunneling splittings for several modes due to the substitution of a deuterium atom had been examined. It has been suggested that the motions of the light atoms as well as the heavy atoms are coupled with the proton-transfer coordinates.
I. Introduction Intramolecular hydrogen bonds in medium-sized molecules such as tropolone (TRN) and malonaldehyde (MA) have been studied extensively both experimentally and theoretically. T R N and MA have a 0 - H - 0 chelate moiety.
A TRN
MA
Spectroscopic studies of these molecules clearly indicate that the intramolecular hydrogen bonds of these molecules are described by symmetric double-minimum potential wells.’-” The tunneling splitting for the zero-point level in the A1B2state of T R N has been measured to be 20 cm-’.’-’ This value is almost identical with the tunneling splitting of 21 cm-’ for the zero-point level in the ground elec_tronicstate of MA.“ Thus the proton tunneling process in the A’B2 state of T R N may be similar to that in the ground state of MA. (1) (a) Alves, A. C. P.; Hollas, J. M. Mol. Phys. 1972,23,927. (b) Alves, A. C. P.; Hollas, J. M. Mol. Phys. 1973, 25, 1305. (2) Redington, R. L.; Redington, T. E. J. Mol. Spectrosc. 1979, 78,229. (3) Rossetti, R.; Brus, L. E. J . Chem. Phys. 1980, 73, 1546. (4) Tomioka, Y.; Ito, M.; Mikami, N. J . Phys. Chem. 1983, 87, 4401. (5) Alves, A. C. P.; Hollas, J. M.; Musa, H.; Ridley, T. J. Mol. Spectrosc.
1985, 109, 99. (6) Redington, R. L.; Chen, Y.; Scherer, G. J.; Field, R. W. J . Chem. Phys. 1988, 88, 627. (7) (a) Sekiya, H.; Nagashima, Y.; Nishimura, Y. Bull. Chem. SOC.Jpn. 1989,3229. (b) Sekiya, H.; Nagashima, Y.; Nishimura, Y. Chem. Phys. Lett. 1981,160,581. (c) Sekiya, H.; Nagashima, Y.; Nishimura, Y . J . Chem. Phys. 1990, 92, 5761.
(8) Sekiya, H.; Sasaki, K.; Nishimura, Y.; Li, Z.-H.; Mori, A,; Takeshita, H. Chem. Phys. Lett. 1990, 173, 285. (9) Redington, R. L.; Redington, T. E.; Hunter, M. A,; Field, R. W. J . Chem. Phys. 1990, 92, 6456. (10) Redington, R. L. J . Chem. Phys. 1990, 92, 6447. (11) (a) Rowe, W. F.; Duerst, R. W.; Wilson, E. B. J . Am. Chem. SOC. 1976, 98, 4021. (b) Baughcum, S. L.; Duerst, R. W.; Rowe, W. F.; Smith, Z.; Wilson, E. B. Ibid. 1981,103,6296. (c) Smith, Z.; Wilson, E. B.; Duerst, R. W. Spectrochim. Acta 1983, 39A, 1 1 17. (d) Baughcum, S. L.; Smith, 2.; Wilson, E. B.; Duerst, R. W. J. Am. Chem. SOC.1984,106, 2260. (e) Turner, P.; Baughcum, S. L.; Coy, S. L.; Smith, Z. Ibid. 1984, 106, 2265.
0022-3654/91/2095-10311$02.50/0
The proton transfer in the A1B2state of T R N has been studied theoretically by using only simple one-dimensional (1D) model^.^,^ On the other hand, two-dimensional (2D)I2 and three-dimensional (3D)I3 models as well as IDl4 models have been presented to describe the potential energy surface (PES)for the proton transfer in MA. Carrington and MillerIz modeled the intramolecular proton transfer with an adiabatic reaction Hamiltonian. The tunneling splitting was calculated by solving a 2D potential energy surface with the two 0-H bond lengths rl and rz as the coordinates. Shida et al.I3 developed the 2D model of Carrington and Miller to a 3D model, where the three-dimensional reaction surface was a function of three coordinates, the two 0-H distances rl and r2, and the 0-0 distance r3. In the 2D and 3D models, the remaining 3 N - 8 or 3 N - 7 degree of freedom in an N-atom system were treated as local harmonic motion separate from the 2D or 3D surface. These calculations demonstrated the limitations of 1D models for polyatomic systems such as MA and suggested that the motions of the atoms in the 0-H-O chelate moiety play important roles in the proton transfer. However, the significance of the motions of the other atoms has not been fully understood. To investigate proton tunneling in medium-sized molecules in more detail, it is crucial to determine the tunneling splittings for various vibrationally excited states and to correlate them with each internal normal coordinate. Although the tunneling splittings for several vibrationally excited states of T R N have been the corresponding internal normal coordinates have not been obtained both experimentally and theoretically. Most of the assignments for the vibrational modes in the IR and Raman spectra of T R N were carried out by analogy with the vibrational modes of tropone.2J6 In Figure 1 is thown the-double minimum potential energy functions for the A1B2and X I A I states of T R N along the proton-transfer coordinate. The value lA’o has been determined for various vibrational state of T R N , tropoloneJs02H (TRNls0,H), and their O D derivatives by measuring electronic spectra in free jets,’,* where A‘,and A”o are the tunneling doublet splittings for the u level in the A’B2 state and the zero-point level in the X I A Istate, respectively. Very recently, a value has been ~~
(12) Carrington, T.; Miller, W. H. J . Chem. Phys. 1986, 84, 4364. (13) Shida, N.; Barbara, P. F.; Almlof, J. E. J. Chem. Phys. 1989, 91, 406 1. (14) Kato, S.; Kato, H.; Fukui, K. J. Am. Chem. SOC.1977, 99, 684. (15) (a) Fluder, B. M.; de la Vega, J. R. J. Am. Chem. SOC.1978, 100, 5265. (b) de la Vega, J. R. Acc. Chem. Res. 1982, 15, 185. (c) Bicerano, J.; Shaefer 111, H. F.; Miller, W. H. J . Am. Chem. SOC.1983, 105, 2550. (16) (a) Ikegami, Y. Bull. Chem. SOC.Jpn. 1961, 34, 94. (b) Ikegami, Y.Ibid. 1963, 36, 11 18.
0 1991 American Chemical Society
Sekiya et al.
10312 The Journal of Physical Chemistry, Vol. 95, No. 25, 1991
0 H 0 COORDINATE Figure 1: Scheeatic diagram of double-minimum potential energy curves for the A and X states of tropolone. The transition between the highwavenumber tunneling doublet components is designated by Hi.
400
obtained to be 29 3 10 (33) MHz ( E 1 cm-I) for Aftoof TRNI60H by measuring the microwave spectrum.]' A remarkable finding in our previous studies is that the vfl3(al)and d14(al)modes enhance tunneling, while the H / D and l80/l6Oisotope effects on the tunneling splitting of Jl3(al) is significantly different from that of ~ ' ~ ~ ( a ,To ) . elucidate ~,~ this difference in the H / D and I8O/I6O isotope effects on the tunneling splittings of d13(a1) and /,.,(al), it is important to examine the intramolecular normal coordinates of d13and vfI4. One of the purposes of this work is to investigate proton tunneling in the vibrationally excited states in relation to each normal isotope effects coordinate predicted from the H / D and 180/160 on the vibrational fundamental. For this purpose, we have measured the laser fluorescence excitation spectra of tropolone-3-4 (3DTRN), tropolone-4-d (4DTRN), and tropolone-5-d (5DTRN) H
H
H
D
3DTRN
LDTRN
5DTRN
in addition to the previous measurements of T R N I 6 0 H and It has been suggested that Q13involves TRNI8O2H both the displacements of the atoms in the 0-H-0 chelate ring and the seven-membered ring, whereas QI4predominantly involves the displacements of the atoms in the 0-H-O chelate ring. Such a difference in the normal coordinate has been found to affect the tunneling path significantly. The observed tunneling splittings for several vibrational modes in the deuterated tropolones have been correlated with each average molecular geometry. It has been suggested that the motions of the hydrogen atoms on the seven-membered ring as well as the oxygen atoms are coupled with the proton-transfer coordinates. 11. Experimental Section
The experimental apparatus used was essentially the same as that described previ~usly.~.'The samples 3DTRN, 4DTRN, and
800
600
nC
/
Cni'
Figure 2. Fluorescence excitation spectrum of jet-cooled SDTRN. The broken lines indicate transitions between the high-wavenumber tunneling doublet components. The stagnation pressure of He was 0.2 atm. The bands due to TRN are indicated and by the closed circles, while the bands due to the hydrogen-bonded complex between SDTRN are indicated by the asterisks.
5DTRN were prepared by hydrogenation of 3-bromotropolone, 4-bromotropolone, and 5-brom0tropolone,'~J~ respectively. The isotopic purity of the three deuterated tropolones was estimated to be 285% by measuring the mass spectra. The sample was heated to 50-60 OC by using a coiled heater to increase the vapor pressure. The sample was mixed with He carrier gas and expanded into the vacuum chamber_by, using a pulsed nozzle (0.3-0.4-mm diameter) pressure. The A-X transitions of deuterated tropolones were excited by using a nitrogen laser pumped dye laser (Molectron UV22-DL14). The total fluorescence was collected with a f = 5 cm quartz lens and detected by a Hamamatsu R955 photomultiplier. Fluorescence excitation spectra were obtained by scanning the dye laser (fwhm = 0.7 cm-I). Dispersed fluorescence spectra were measured with a Spex 1702 f = 0.75 m monochromator. 111. Results
A. Electronic Spectra of SDTRN. Figure 2 shows the excitation spectrum of 5DTRN. The origin band has been identified at 27033 cm-', which is blue shifted 15 cm-I from the origin band of TRN.7 The vibronic pattern in the excitation spectrum of 5DTRN is much more complicated than that of T R N shown in Figure 3, for which the vibrational assignments were confirmed previ~usly.~ A number of unidentified bands have been detected in the region A I = 300-850 cm-' in Figure 2. Three prominent bands have been detected at A> = 290, 291, and 296 cm-l. The band at AI = 290 cm-I has been assigned as the origin band for the hydrogen-bonded complex between 5DTRN and water, since the intensity of this band increased when traces of water were intentionally added in the nozzle h ~ u s i n g . ~ . ~ To identify the two bands at AI = 292 and 296 cm-' we have measured the dispersed fluorescence spectra. Figures 4 and 5 show (18) Ito, S.; Tsunatsugu, J.; Kanno, T.; Sugiyama, H.; Takeshita, H. Tetrahedron Lett. 1965, 3659.
(17) Tanaka, T. Private communication
(19) Cho, S. Private communication.
The Journal of Physical Chemistry, Vol. 95, No. 25, 1991 10313
Vibrational Mode-Specific Tunneling Splittings
H
I
I
I
0
200
I
I
I
I
600
400
AP&
Figure 3. Fluorescence excitation spectrum of jet-cooled TRN. The broken lines indicate transitions between the high-wavenumber tunneling doublet components. The bands due to the hydrogen-bonded complex between TRN and water are indicated by the asterisks.
I
I
‘000
*p,&
500
0
Figure 5. Dispersed fluorescence spectrum of 5DTRN obtained by exciting the band at A2 = 296 cm-’ (25626;).
I
I
I
I
1500
1000
500
0
O F / cm”
Figure 4. Dispersed fluorescence spectra of SDTRN obtained by exciting the bands (a) at A? = 292 cm-’ (14;) and (b) A? = 327 cm-’ (14bH;). TABLE I: Wavenumbers (em-’) and Assignments for the Fluorescence Excitation Spectrum of SDTR wavenumber A2 IA’,, - A”,-,I assignment 0 19 21 033 19 19 21 052 77 7 27 110 7 84 27117 27 180 147 5 152 5 27 185 20 1 27 234 a) 27 249 216 4 220 4 21 253 29 1 27 324 36 296
