99
J. Phys. Chem. 1992,96, 99-104
The meta-disubstituted benzenes are included with less depths compared with the mono- and para-substituted benzenes. Moreover, they are not so symmetrically included as expected from the molecular symmetry (CzOfor both of m-hydroxyphenol and m-aminoaniline). Actually one of the two identical groups at the meta positions is more axially included than expected from the symmetrical inclusion model. This trend is clearer as m-aminoaniline than in m-hydroxyphenol. This may be explained in terms of the difference in the abilities of the hydroxyl and amino groups (26) Bergeron, R.; Channing, M. A.; Gilbeily, G. J.; Pillor, D. M. J. Am. Chem. Soc. 1977, 99, 5146. (27) Bergeron, R.; Channing, M. A. Bioorg. Chem. 1976, 5,437. (28) Buvari, A.; Barcza, L. J . Chem. Soc., Perkin Trans. 2 1988, 543.
to form the hydrogen bonds with the peripheral hydroxyl groups of the host cavity. In conclusion we may say that the rotational strength analysis method is sufficiently useful for studying the inclusion geometry of substituted benzene guests. That is, this method enables us to obtain basic information on the substitution effect on the inclusion orientations and depths. Registry No. j3-Cyclodextrin-phenol complex, 7362 1-01-9; j3-cyclodextrinaniline complex, 73621-02-0; j3-cyclodextrin-benzoic acid complex, 68419-51-2; j3-cyclodextrin-p-nitrophenol complex, 61955-24-6; j3-cyclodextrin-hydroquinone complex, 78 153-75-0; j3-cyclodextrin-psalicylic acid complex, 80065-26-5; 8-cyclodextrin-o-nitroanilinecomplex, 78153-70-5; j3-cyclodextrin-m-benzenediolcomplex, 78153-74-9; j3-cyclodextrin-m-benzenediaminecomplex, 78 153-77-2.
Electronic Spectra of 0 - , m-, and p-Dlfluorobenzene Cations: Striking Similarity in Vibronic Coupling between the Neutral Molecule and Its Cation Yuko Tsuchiya, Ken Takazawa, Masaaki Fujii, and Mitsuo Ito* Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan (Received: July 16, 1991)
The D(r,r).- Do transitions of m- and edifluorobenzene (DFB) cations prepared by two-color REMPI have been observed by dissociation spectroscopy. All the spectra due to the transitions from different vibrational levels in Do showed well-resolved vibrational structures. The spectral analysis indicates the existence of a strong vibronic coupling between the D(*,r) state and a nearby D(u,r) state for both m- and eDFB cations. The D(r,r) state was found to be the lowest excited state, contrary to the generally accepted criterion that the lowest excited state of the nonemissive fluorobenzene cation is D(u,r). It was found that the out-of-plane vibration responsible for the vibronic coupling is exactly the same as that of the corresponding neutral molecule in S,for all the difluorobenzene (ortho, meta, and para) cations, indicating similarity in their electronic states.
Introduction Fluorobenzene cations have attracted a great interest with respect to their emissive properties. Fluorobenzene cations having more than two fluorine atoms are highly fluorescent, while monoand difluorobenzene cations are nonemi~sive.'-~ In the cation, there exist two kinds of low-lying electronic excited states. One is the D(u,?r) state due to the transition of an electron from the u-bonding orbital localized mainly in the C-F bond to the highest half-occupied ?r-bonding orbital. The other is the D(?r,r) state arising from the transition of an electron in the second highest *-bonding orbital to the highest half-filled r-bonding orbital. The relative energies of the D(u,?r) and D ( r , r ) states change greatly by the number of the fluorine atoms in the cation. It is well established that in monofluorobenzene cation the D(U,T) state is the lowest excited state and for the cations having more than two fluorine atoms (tri-, tetra-, penta-, and hexafluorobenzene cations) the D ( r , r ) state becomes l ~ w e s t . Phenomenologically, ~ the nature of this lowest excited state is closely related with the emissive property mentioned above. That is, the lowest excited state of the emissive cation is D(*,r) and the nonemissive cation has D(U,T) as the lowest excited state. Difluorobenzene (e, m-, and p-difluorobenzene (DFB)) cations are at a critical position with respect to the energy order of the two electronic states. Difluorobenzene cations are nonfluorescent in and also in the low-temperature matrix.5 Therefore, it has been anticipated that the lowest excited state is D(u,?r). (1) Allan, M.; Maier, J. P. Chem. Phys. Lett. 1975, 34, 442. (2) Allan, M.; Maier, J. P.; Marthaler, 0. Chem. Phys. 1977, 26, 131. (3) Cossart-Magas, C.; Cossart, D.; Leach, S . Mol. Phys. 1979,37,793. (4) Bieri, G.; Asbrink, L.; Von Niessen, W. J . Electron Spectrosc. 1981, 23, 28 1. (5) Bondybey, V. E.; English J. H.; Miller, T. A. J . Mol. Specrrosc. 1980, 81. 455.
-
In a previous work: the electronic spectra of p D F B cation due to the Z B s U ( r , ~ ) Do(2B2,) transition were observed by massselected ion dip spectroscopy. The spectral analysis showed that there exists the strong vibronic coupling between the 2B3U(a,r) state and a nearby ZBzg(u,r)state. It was concluded from the effect of the vibronic coupling on the vibrational frequency that the ZB3u(r,r)state is the lowest excited state in spite of the nonemissive property of this cation. The result obviously contradicts the generally accepted criterion that the lowest excited state of the nonemissive cation is D(u,r). Therefore, the criterion needs to be reexamined. The result obtained for p-DFB cation suggests that the emissive property is not determined simply by the nature of the lowest excited state. It might be possible that the strong vibronic coupling also plays an important role in the emissive property. In this work, we extended the study to e and m-DFB cations. The spectra due to the D(r,*) Do transition were observed by multiphoton dissociation spectroscopy utilizing the photodissociation of the cation in a highly excited state. The cations in the ground state were generated by two-color 1 1' REMPI of the jet-cooled neutral molecule via a particular vibronic level in S1. By suitable selection of the vibronic level in S1 and choice of the laser frequencies, we can populate the ground-state cations to a specific vibrational level. The cations in the selected level were subjected to the measurement of the electronic spectra by dissociation spectroscopy. The electronic spectra of 0-and m-DFB cations exhibit well-resolved vibrational structures. The analysis of the structure shows the existence of strong vibronic coupling between the D(r,*) and D ( u , r ) states similar to the case of the p-DFB cation. It was also found that the lowest excited state is D(r,*) for both 0-and m-DFB cations. +
+
(6) Tsuchiya, Y.; Fujii, M.; Ito, M. Chem. Phys. Lett. 1990, 168, 173.
0022-3654/92/2096-99%03.00/0 0 1992 American Chemical Society
100 The Journal of Physical Chemistry, Vol. 96, No. 1, 1992
Tsuchiya et al. IP. 15333 cm-'
c 1 5 , l O b'
I
v1
Figure 1. Schematic diagram showing the principle of dissociation spectroscopy for the detection of electronic transitions of cation prepared by two-color 1 + 1' REMPI of a neutral molecule in a supersonic jet.
In the course of the study, we realized a striking similarity in the vibronic coupling scheme between the lowest excited states of the neutral molecule and its cation. The out-of-plane vibrations responsible for the vibronic coupling in 0-, m-, and p-difluorobenzene cations are exactly the same as those for the corresponding neutral molecules.' The similarity in the vibronic coupling seems to be general for all the fluorobenzenes, suggesting a very close relation between the electronic states of the molecule and its cation.
Experimental Section The experimental apparatus for supersonic expansion and the setup for the measurement of dissociation (or dip) spectrum were described Figure 1 shows the principle of dissociation spectroscopy. The sample vapor was seeded in 3 atm of He gas and expanded into a vacuum chamber through a 0.4 mm diameter orifice. The output of a XeCl excimer laser (Lambda Physik EMG 103) was split to pump three dye lasers (Molectron DL-14 for v I and v3 and Lambda Physik FL2002 for v2). vi and u2 were the second harmonic of the first and the second dye lasers and were used to ionize the jet-cooled molecule via a specific vibronic level in SIby the v1 + v2 ionization. The powers of v l and v2 were suppressed as low as possible to avoid ionization by u1 or v2 alone. The parent cations generated by the two-color ionization are populated in a specific vibronic level in the ground-state cation (Do) by choosing a suitable vibronic level in SIas a resonant state in 1 1' REMPI. vi and v2 were focused coaxially into a vacuum chamber and crossed the supersonic jet at 15 mm downstream. The third laser light v3 (visible) was used for the electronic transition of the ground-state cation. The excited parent cation produced by the v3 absorption dissociates into fragmented species via various dissociation channels. We selected a fragmented cation of a specific mass number by a Q-mass filter (Extranuclear 4270-9) and the ions were detected by a channel multiplier (Murata Ceratron). The ion signal was amplified by a current amplifier (keithley 427) and integrated by a digital boxcar integrator (Par Model 4402/4420). By monitoring the mass-selected fragmented ion current while scanning the v3 frequency, we obtain the dissociation spectrum corresponding to the electronic absorption spectrum of the parent cation. This spectroscopy (resonant-enh a n d multiphoton dissociation spectroscopy) has also been used by several groups.I0 The laser power of v3 was monitored by a
+
(7) Tsuchiya, Y . ;Takazawa, K.; Fujii, M.; Ito, M. Chem. Phys. Lett. 1991, 183, 107. (8) Tsuchiya, Y . ; Fujii, M.; Ito, M. J . Chem. Phys. 1989, 90, 6965. (9) Mikami, N.; Hiraya, A.; Fujiwara, I.; Ito, M. Chem. Phys. Lett. 1980, 74, 531. (10) (a) Kakoschke, R.; Bosel, U.; Hermann, J.; Schlag, E. W. Chem. Phys. Lett. 1985, 119,467. (b) Weinkauf, R.; Walter, K.; b e l , U.; Schlag, E. W. Chem. Phys. Lett. 1987,141,267. (c) Walter, K.; Weinkauf, R.; Bosel, U.; Schlag, E. W. J . Chem. Phys. 1988,89, 1914. (d) Walter, K.; Bosel, U.; Schlag, E. W. Chem. Phys. Lett. 1989,155, 8. (e) Danis, P. 0.; Wyttenbach, T.; Maier, J. P. J . Chem. Phys. 1988, 88, 3451. (f) Ripoche, X.; Dimicoli, 1.; LecalvE, J.; Piuzzi, F.; Botter, R. Chem. Phys. 1988, 124, 305. (g) Walter, K.; Bosel, U.; Schlag, E. W. Chem. Phys. Lett. 1989, 162, 261. (h) Bieske, E. J.; Mckay, R. 1.; Bennett, F. R.; Knight, A. E. W. J . Chem. Phys. 1990, 92, 4620.
IP 1 0 2 .506
75000
75200
75400
75600
Yl* U2 WAVENUMBER ( c m - ' )
Figure 2. Two-color 1 + 1' ionization threshold spectra of m-difluorobenzene (DFB) (a-d) and o-DFB (e and f) in supersonic jets. The ui frequency was fixed to the SIC)" (a), Si9ai (b), Sl10b2 (c), and Si6b' (d) vibronic levels of m-DFB as resonant levels. In o-DFB, uI was set to SIOo (e) and S110a2(f). The onsets of the thresholds are shown by arrows.
photomultiplier (Hamamatsu Photonics 1P28). The spectral resolution of the exciting light (v3) was about 1 cm-I (fwhm). The samples were purchased from Tokyo Kasei and used without further purification.
Results and Discussion We shall begin with m-DFB cation. Figure 2a shows the two-color ionization threshold spectrum, in which the jet-cooled m-DFB molecule was excited to Si@by the fmt laser light v I and the second tunable laser light of u2 was used to ionize the excited molecule. The ion signal was detected by scanning the v2 frequency. In the spectrum, there exists a sharp ionization threshold at 75339 cm-I in total energy ( u l vz), which represents the adiabatic ionization potential (IP,). It is expected from the spectrum that we can produce the ground-state cations in its zero-point level when the vl u2 frequency is set at the frequency slightly higher (- 10 an-I) than IPo. Similar two-color ionization threshold spectra were also obtained by using the vibronic levels of S19a1,SIlob2, and S16bias resonant states. Their spectra are shown in Figure 2 W . In each spectrum a sharp ionization threshold is found. This threshold is due to the vertical ionization from the SIstate in a selected vibrational level to the ground-state cation in the level of the same vibrational mode (v-v transition). The strong appearance of the threshold indicates a large Franck-Condon factor for the v-v transition. From the observed thresholds, the vibrational frequencies of 9a, lob2 (overtone of lob) and 6 b of the ground-state cation were found to be 335,368, and 41 1 cm-' as shown in the figure. The observed threshold spectrum shows that the ground-state cations in the particular vibrational level are generated with a high selectivity by setting vi + v2 at the frequency slightly higher than the respective threshold. We used such vibrationally selected ground-state cations as a sample for the measurement of the dissociation spectrum. Figure 3 shows the dissociation spectrum of the m-DFB cation prepared by the two-color 1 + 1' REMPI of the jet-cooled neutral molecule via the SIOolevel. By setting vi + v 2 at a frequency slightly higher than the threshold of IP,, the parent cations generated are populated only in the DoOovibronic level. The parent cations are excited by u3 and dissociate into fragmented cations of different mass numbers by predissociation or multiphoton
+
+
The Journal of Physical Chemistry, Vol. 96, No. 1, 1992 101
Electronic Spectra of Difluorobenzene Cations
~ 2 0 - 19 9 8 7 9 a 7 (n.0-2) -320-3211
I
l
l
I
'
23500
l
l
I
1
I
24000
I
I
'
9a: 10bF!n:O-2;
I
24500
v,
WAVENUMBER (cm-1) Figure 3. Dissociation spectrum of the m-DFB cation due to the electronic transition from the DoOovibronic level obtained by monitoring the fragmented cations of 86 amu. The parent cations were generated by two-color 1 + 1' REMPI via the SIOo vibronic level and were populated only in DoOo. The progression observed in the transition is shown by a line. dissociation. The fragmented cation of 86 amu was selectively detected by a Q-mass filter for the observation of the electronic transition. The fragmented cation of 86 amu is major product in the dissociation. Figure 3 shows the well-resolved vibronic structure in the energy region from 23 100 to 24 500 cm-'. It is expected that m-DFB cation has two excited states in this energy region, 2Bl(?r,?r)and 2 A l ( a , ~ )The . former is dipole-allowed in the transition from the ground-state Do(2A2),while the latter is dipole-forbidden. The strong appearance indicates that the observed spectrum is due to the ZBI(~,?r) Do transition. This assignment is also supported from the absorption and emission spectra of m-DFB cation in the low-temperature matrix reported by Bondybey et al.,I1J2 who assigned the transition in the same region to 2 B I ( ~ , ~ )Do. Since the parent cations are populated only in the origin of Do, all the observed bands are cold bands starting from the DoOolevel. Thus, the lowest frequency band at 23 169 cm-I can be readily assigned to the 0,O band of the transition. It is seen from the figure that the progression of 200 cm-I develops from the 0,Oband. Other characteristic bands are ones at 320 and 467 cm-I from the 0,Oband. We attempted to obtain definite assignments for these three excited-state vibrational frequencies of 200, 320, and 467 cm-l. For this purpose, we observed the dissociation spectra of the parent ions in different vibrational states in the ground state. As mentioned before, the generation of the parent cations in a specific vibrational level in Do can be achieved by suitable selection of the SIvibronic level as a resonant state in the two-color 1 1' REMPI and also by suitable selection of the laser frequencies. We produced the parent cations by the two-color 1 1' REMPI of the jet-cooled neutral molecule using the S19a1vibronic level as a resonant state (the vibrational frequencies of 9a in So and SIare 329 and 317 cm-l, respectively). It is expected from Figure 2b that the ground-state cations are mainly populated in Do9al by choosing the v1 + u2 frequency coinciding with the vertical ionization threshold (at 75 339 + 335 cm-I). Then, we observed the dissociation spectrum, which is shown in Figure 4. As seen from the figure, there exist two kinds of progressions: one has the interval of 320 cm-I and the other 200 cm-'. The latter progression develops from the strong band at 23 150 cm-l and is similar to the progression seen in Figure 3. On the other hand, the former progression of 320 cm-I starts from the lowest frequency band at 22 830 cm-'. The energy difference between the band a t 22 830 cm-I and the 0,O band at 23 169 cm-' observed in Figure 3 is 339 cm-l, which coincides with the vibrational frequency of mode 9a(al) in Do obtained by the two-color ionization threshold spectrum. This frequency is comparable with ~ the those in the So and SIstates (329 and 317 ~ m - l ) .Therefore,
-
j
+
(11) Bondytey, V. E.; Miller, T. A,; English, J. H. J . Chem. Phys. 1980, 72, 2193. (1 2) Bondybey, V. E.; English, J. H.; Miller, T. A. Chem. Phys. Lerr. 1979, 66, 165.
23500 24000 V 3 WAVENUMBER(cm-1)
Figure 4. Dissociation spectrum of the m-DFB cation after generation of the parent cation by two-color 1 + 1' REMPI via S19al. The generated cations were populated mainly in the Do9alvibronic level. The progressions in the spectrum are shown by lines.
(22756) t I
-
+
I
I
23000
23000
23500
U, W A V E N U M B E R
24000 (cm-1)
Figure 5. Dissociation spectrum of the m-DFB cation after generation of the parent cation by two-color 1 + 1' REMPI via S16b1. The generated cations were populated mainly in the Do6bl vibronic level. The progressions in the spectrum shown by lines.
band at 22 830 cm-I is assigned to say. Then, the band at 23 150 cm-I can be assigned to 9ai and the frequency of 320 cm-' is ascribed to the mode 9a in the D(T,T) state. As shown in Figure 4, all the main bands are those associated with 9al. This clearly confirms the selective population in the Do9al level by the 1 1' REMPI via S19a1. Next, we observed the dissociation spectrum after generating the ground state m-DFB cation by the two-color 1 + 1' REMPI using S16b1as a resonant state (vibrational frequencies of 6b in So and SI are 51 1 and 444 ~ m - ' ) . ~ It is expected from Figure 2d that the ground-state cation is mainly populated in Do6bl by setting the vI + v2 frequency at the threshold energy of 75 339 411 cm-I. The dissociation spectrum observed under this condition is shown in Figure 5. The lowest frequency band at 22756 cm-I is assigned to the 6b7 band in the D(T,T) Do transition by a similar argument made for the band assignment in Figure 4. The energy difference between this band and the 0,O band a t 23 169 cm-I is 413 cm-l, which coincides with the fundamental frequency of mode 6b in Do (41 1 cm-l) obtained from the two-color ionization threshold spectrum. Then, the strong band at 23 223 cm-I is assigned to 6b, because of its expected large Franck-Condon factor. This leads to the 6b mode of 467 cm-I in the (T,T)state. In the figure, several bands appear as doublet. The doublet probably comes from Fermi resonance between the mode 6b(b2) and the mode 15(b2);such a resonance was actually observed in the SI state of the neutral m~lecule.~ We tentatively assign the stronger component of the doublet to one involving 6b. From the above, we could assign two of the three vibrational frequencies appearing in the spectrum of Figure 3. The remaining frequency is 200 cm-l. We generated the parent cations by the two-color 1 + 1' REMPI via SIlob2 (vibrational frequencies of 10b in So and SI
+
+
-
102 The Journal of Physical Chemistry, Vol. 96, No. 1 , 1992 2n
10 b2
(nZl-4)
r - - ( 200 ) - - . y l 9 6-
,
22800
Tsuchiya et al.
1 9 7 1-
9 7 7
I
23000
23200
23400
23600
23800 l
!J3 WAVENUMBER ( c m - l )
23200
Figure 6. Dissociation spectrum of the m-DFB cation due to the transition from the D010b2vibronic level. The parent cations generated by two-color 1 + 1' REMPI via S110b2were populated mainly in Do10b2. The progression in the transition is shown by lines. TABLE I: Main Vibronic Bands of the *BI ( r , r ) the m-DFB Cation freq, cm-' assignment freq, cm-l 23 425 22 756 6b7 23 426 (22801) (lob!) 23 437 22 830 sa7 23471 6by 1Obi 22 960 23 475 23 000 23 484 1Ob: 23001 23 489 23 009 23 490 9a t 23 150 23 499 23 169 0: 23 507 (lob! + Ah) 23 185 23 508 lob! 23 197 23 523 23 219 9ay10b: 23 540 23 223 6bt 23 541 23 237 23 557 (lob! + Bb) 23 283 23 559 9a: + Bh 23 316 23 571 23009 + 9ah 23 321 23 591 10b:9af, 23613 23 343 23 624 9a;lOb; 23 359 23 636 lobi 23 374 23 656 23 317 23 667 1Obi 23 394 23 766 23 401 23 962 23 408
+-
Do Transition of
~~
assignment 6bi lob2 9ay 1Ob: 23237 lobi sa:
+
lob: 9ah lob:
~
+ Bh
6b19ah 9a; A; 9ai10b: Ah lobi lobi 6b; + Af, 6b;lOb: 6bh Bh Ch lob: lob:
+
are 227 and 126 cm-I, respectively7). The dissociation spectrum of the cations thus produced is shown in Figure 6. Since the overtone frequency of mode 10b was obtained to be 368 cm-l in our two-color ionization threshold measurement (seeFigure 2b), we expect the appearance of the lobi hot band at 22801 cm-I (23 169 cm-l (0,O)-368 cm-' (lob2)). However, there is no such band at the expected position. The lowest frequency strong band is found at 23 001 cm-I and the progression of 200 cm-I starts from there. The strong appearance of the 200-cm-' progression shows that it is the progression of the overtone of lob in the excited state. It is expected from Franck-Condon factor that the second member of the progression lob2 will be strongest. So,we assigned 1 This assignment locates the lobi the band at 23 001 cm-'to 1Ob2. band at 22 801 cm-I, which exactly coincides with the expected position of this band from the threshold spectrum. Thus, the apparent absence of this band in the spectrum is due to its weakness. It is concluded from the analysis that the frequency of 200 cm-I is the overtone frequency of the mode 10b in the D(a,a) state. Therefore, the fundamental frequency of the mode 10b in the D(a,a) state is 100 cm-I, which may be compared with 184 cm-' in Do. In Table I the observed frequencies of the main vibronic bands in the dissociation spectra are summarized with
23LOO
~
1
23600 23800 24003
~
1
~
1
2L.200 24400
Y3 WAVENUMBER ( c m - ' ) Figure 7. Electronic spectrum of the o-DFB cation obtained by dissociation spectroscopy by monitoring the fragmented cations of 86 amu. The parent cations were generated by two-color 1 1' REMPI via SIOo and were populated only in DoOo. The progression observed in the transition is shown by a line.
+
TABLE XI: Vibrational Frequencies of m-DFB and Its Cation in the Ground nod Excited States and Tbeir Correspoedeaees (cm-') mode sym Dd2A2) D(r,rM2B1) SO('Al)" S1(w,r*)(IB# 9a al 335 320 329 317 6b b2 411 437 511 444 10b bl 184 102 227 126 a
+ Ch
1
From ref 7.
the tentative assignments. Table I1 summarizes the correspondence of the vibrational frequencies between the Do and D(a,a) states of m-DFB cation, together with those in So and SI of the neutral molecule. The important result obtained here is the fact that the vibrational frequency of mode 10b decreases very much in going from Do(184 cm-')to the 2BI(a,a)excited state (100 cm-I). The great decrease is in contrast to the other modes whose frequencies are similar between Do and D(a,a). The difference in the electronic structure between Do and D(a,r) comes from which of the two highest a-bonding orbitals is half-filled. Since both orbitals are bonding, we expect similar vibrational frequencies between the two states. Therefore, the observed frequency change is too large to ascribe it simply to the difference in the electronic state. Such a great change for a specific mode may be explained in terms of vibronic coupling. In the difluorobenzene cation, the D(a,a) and D(u,a) states exist in the same energy region of 3-4 eV above the ground state4 and these two closely lying states can interact by vibronic coupling through an out-of-plane mode. In the case of m-DFB cation, these two states predicted from the calculation are the ZBl(a,a)and ZAl(u,a)states4 Therefore, the vibration which is responsible for vibronic coupling between the two states should be of the symmetry species of bl. The symmetry species is in accord with that of the 10b mode. We explain therefore the small frequency of the mode 10b in D(a,a) as a result of the vibronic coupling between the 2Bl(n,?r)and nearby zA1(u,a)states through the mode 10b(bl). The small frequency of the mode 10b in D(r,a) suggests that the D(r,a) state is located below the D(u,?r) state.13 This energy order is consistent with that obtained from photoelectron spectroscopy4 and MO c a l ~ u l a t i o n s . ' ~ J ~ Similar vibronic coupling between D(r,r) and nearby D(a,a) has also been found for p D F B cation? We expect a similar vibronic coupling also for o-DFB cation, which will be described below. Figure 7 shows the dissociation spectrum of o-DFB cation in a supersonic jet. The parent cations were generated by the vibronic level with the v 1 two-color 1 + 1' REMPI via the SIOo + v2 frequency coinciding to the adiabatic ionization potential of R. L.;Gouterman, M. J . Chem. Phys. 1961, 35, 1059. (14) Davies, D. W. Chem. Phys. Lett. 1977, 48, 565. ( I 5) Iwaki, H.; Murakami, A,; Ohno K.; Katsumata, S.Proc. Symp. Mol. Strucf. Mol. Specfrosc., Osaka 1986, 240. (13) Fulton,
~
The Journal of Physical Chemistry, Vol. 96, No. 1, 1992 103
Electronic Spectra of Difluorobenzene Cations
TABLE I V Vibrational Frequencies of o-DFB and Its Cation in the Ground and Excited States and Their Correspondences (coil) mode sym 1 a, 6b b2 b2 9b 10a b,
loa:
a
i
I
'
I
~
I
~
I
'
I
22400 22600 22800 23000 23200 23400
Y3 WAVENUMBER
i
I
'
I
23600 23800
(cm-I)
Figure 8. Dissociation spectrum of the o-DFB cation after generating the parent cation by twecolor 1 + 1' REMPI via S,loa2. The generated cations were populated mainly in the Do10a2 vibronic level. The progression in the transition is shown by a line. TABLE 111: Main Vibronic Bands of the *B1(*,r) of the o-DFB Cation freq, cm-' assignt freq, cm-' 22656 loa; 23 427 22 922 loa2 23 442 23 455 22 950 23 446 23 075 23 147 23 501 23 580 23 166 23 157 0: 23651 23 215 1 Oai 23 670 23 291 23710 23 797
+-
Do Transition assignt 1Oaa
9bA 6bh loa:
74 979 cm-I (see Figure 2e). Then the cations are populated to the zero-point level of the ground-state cation (Do). The dissociation spectrum was obtained by monitoring the fragmented cation of 86 amu which is the most abundant fragment produced by the dissociation. The spectral region in Figure 7 agrees with that of the 2 B l ( r , r ) Do(2BI)electronic transition of o-DFB cation in the rare gas matices.I6 The lowest frequency band at 23 157 cm-' is assigned to the 0,Oband, because the initial state of the electronic transition is restricted to the DoOo level. A characteristic feature of this spectrum is appearance of the strong band at 23 427 cm-l, which is displaced by 270 cm-l from the 0,O band. The frequency of 270 cm-'represents the lowest vibrational frequency in the excited state, in which we are most interested. To establish the assignment of the 27O-cm-I vibration, we observed the dissociation spectrum of o-DFB cation populated to Do10a2 level. The parent cations in the Do10a2level were produced by the two-color 1 + 1' REMPI via the S110a2vibronic level by selecting the v, + vz frequency equal to the vertical ionization threshold at 74979 + 506 cm-l (see Figure 20. The observed spectrum is shown in Figure 8. A sharp band is observed at 22656 cm-I, which is shifted by -501 cm-I from the 0,Oband at 23 157 cm-I. The fundamental frequency of mode 10a (b,) was found to be 253 cm-I in Do from the two-color ionization threshold spectrum via the SIloazvibronic level (Figure 2f). Since the value of 501 cm-I is close to 2 X 253 cm-I, the band at 22 606 cm-l is readily assigned to the loa; hot band. It is seen from the figure that the progression of 270 cm-' develops from the loa! band. The vibrational frequency of 270 cm-' is the same as that appearing in Figure 7. The observed progression clearly shows that 270 cm-' is the overtone frequency of the mode 10a in the excited state. Then the fundamental frequency should be 135 cm-I, which may be compared with 253 cm-I in Do. Table 111 summarizes the frequencies of the main vibronic bands of the o-DFB cation, and Table IV summarizes the correspondence
-
(16) Lutiro, J. T.; Andrews, L. Chem. Phys. 1985, 97, 121.
'
Dd2BI) D(*,*N2BI) Sn('Al)4 S , ( T . ~ * W A , Y 831 765 122 540 513 55 1 512 396 344 442 399 277 135 289 121
From ref 7.
of the fundamental frequencies of several modes between Do and the D(r,r) state of the o-DFB cation. The vibrational frequency of the mode loa decreases very much in going from Do to D(r,r), similar to the case of the m-DFB cation. It is known that in the o-DFB cation the 2Al(u,r) state lies near the *B,(?r,r) statea4 ITherefore, the vibronic coupling between the two states through the out-of-plane mode 10a (b,) is highly plausible. The great decrease of the vibrational frequency in the D(r,r) state strongly suggests the energy order of ZAl(u,r)> 2Bl(r,r). It is concluded that the vibronic coupling between the D(r,r) and D(u,r) states also exists in the o-DFB cation. Since a similar coupling is found also for the cations of m- and pDFB, the vibronic coupling is quite general for all the difluorobenzene cations. Now we shall briefly discuss the emissive property of difluorobenzene cation. The emissive property of the fluorobenzene cation was explained by the criterion that the cation having the lowest D ( r , r ) state is emissive but the cation having the lowest D(u,r) state is nonemissive. In the cases of difluorobenzene cations, they all are nonemissive in spite of their lowest D ( r , r ) states. Thus, the criterion based on the energy order of the electronic states should be reconsidered. One of the possible interpretations is the potential distortion due to the vibronic coupling (pseudo Jab-Teller interaction).17 The potential curve of D(?r,r) is distorted by the pseudo Jahn-Teller interaction with the nearly D(u,r) state. The distortion will accelerate the internal conversion to the ground state as a result of increase of the Franck-Condon overlap between the ground and excited states. In a strong coupling case, D ( r , r ) will greatly mix with D(u,r) due to the nonplanar structures of these states resulting from the strong pseudo-Jab-Teller interaction. Such an electronic mixing will accelerate the internal conversion because of the mixed ( u , r ) character. In the difluorobenzene cation, there exists a low-lying electronic state between ground state and the D(r,r) state. This low-lying electronic state is the split component of the degenerate ground state of the benzene cation resulting from the symmetry reduction. It is highly possible that this state plays an important role in the acceleration of the internal conversion. This possibility was already discussed in a previous paper.I7 The most characteristic feature of the D(r,r) states of m- and o-DFB cations is the existence of the vibronic coupling and the unusually small vibrational frequency of a particular out-of-plane mode in the D ( r , r ) state. In the m-DFB cation, the mode 10b (b,) exhibits a great decrease of its frequency in going from Do(184 cm-l) to D(r,r)(102 cm-I). For the o-DFB cation, the great decrease is seen for 10a (b,), its frequency beiig 262 and 135 cm-l, respectively, in Do and D(r,r). The great frequency decrease is also found for corresponding neutral m ~ l e c u l e .In ~ m-DFB, the same mode 10b (b,) exhibits the great frequency change in going from s0(227 cm-I) to S1(r,r*)(l26 cm-I). In o-DFB, again the frequency of the same mode 10a (b,) decreases very much in going from S0(289 cm-I) to Sl(r,r*)(121 cm-I). Thus, the mode responsible for the vibronic coupling is exactly the same between the neutral molecule and its ion. Tables I1 and IV summarize the correspondences of the vibrational frequencies between the ions and the neutral molecules of m- and o-DFB, respectively. Such a correspondence might be accidental. However, the accident is denied when we see the results for p-DFB and its cation. We reported in a previous papel.6 that the out-of-plane mode 16a (a,,) of the p D F B cation exhibits a great frequency decrease in going from D0(355 cm-I) to D(r,r)(199 cm-I). The frequency decrease (17) Fujii, M.; Tsuchiya, Y . ;Ito, M. J . Mol. Strucr. 1991, 249, 55.
J . Phys. Chem. 1992, 96, 104-107
104
TABLE V Vibrational Frequencies of p D F B and Its Cation in tbe Ground and Excited States and Their Correspondences (cm-')
6a 16a 17b
ai a, bj,
436 355 125
413 199 145
450 422 158
410 175 120
"From ref 6. bFrom ref 18.
in the ion exactly corresponds to that for the neutral molecule, for which the same mode of 16a decreases its frequency in going from S0(422cm-l) to Sl(a,a*)(175cm-l).l8 Table V summarizes the correspondence of the vibrational frequencies between the ion and the neutral molecule of p-DFB.I8 Therefore, the striking correspondence is quite general for all the difluorobenzenes and their cations. Similar to DFB cation, the great frequency decrease of the particular out-of-plane mode in the S1(*,a*) state of the neutral molecule is due to the vibronic coupling between the SI(*,**)state and the S(a,u*) state lying above SI.The fact that exactly the same out-of-plane mode is responsible for both neutral molecule and its ion indicates that the vibronic coupling scheme is very (18) Knight, A. E. W.; Kable, S. H. J . Chem. Phys. 1988, 89, 7139.
similar. The similarity implies the existence of a close relationship between the electronic states of the neutral molecule and its ion. In the case of the neutral molecule, the S,(r,r*)state arises from the transition of an electron in the highest occupied *-bonding molecular orbital to the lowest vacant **-antibonding molecular orbital. In the corresponding ion, the molecular orbitals associated with the D(a,a) excited state are the two highest a-bonding orbitals. Therefore, the highest a-bonding orbital is common for the neutral molecule and its ion. However, the other orbital is quite Werent. The one for the neutral molecule is the antibonding a* orbital while the one for the ion is the second highest bonding a orbital. This situation is also similar for the S(a,u*) state of the neutral molecule and the D(u,*) state of its ion. The great difference in the molecular orbitals participating in the electronic states of the neutral molecule and ion suggests great difference in their electronic structures and also in the vibronic coupling scheme. If the observed correspondence is not accidental, there must be a close relationship between the different molecular orbitals which are involved in the neutral molecule and its ion. It will be quite interesting to elucidate the origin of the similarity from theory. In the present paper, we showed a striking similarity in the vibronic coupling between each isomer of difluorobenzene and its ion. However, such a similarity seems to exist also for other molecules and their cations. Registry No. m-DFB, 65308-07-8; o-DFB, 65308-08-9.
A Discharge Flow-Photoionization Mass Spectrometric Study of Hydroxymethyl Radicals (H,COH and H,COD): Photoionization Spectrum and Ionization Energy W. Tao, R. B. Klemm,* Brookhaven National Laboratory, BIdg. 81 5, Upton, New York 1 1 973
F. L. Nesbitt, and L. J. Stief NASAjGoddard Space Flight Center, Laboratory for Extraterrestrial Physics, Greenbelt, Maryland 20771 (Received: July 31, 1991; In Final Form: September 6, 1991)
The photoionization spectrum of H2COH was measured over the wavelength range 140-170 nm by using a discharge flow-photoionization mass spectrometer apparatus with synchrotron radiation. Hydroxymethyl radicals (H2COH and H2COD) were generated in a flow tube by the reaction of F atoms with CH3OH(D). Ionization energies (IE) were determined directly from photoion thresholds. The IE values, 7.56 f 0.02 and 7.55 f 0.02 eV for HzCOH and H2COD,respectively, are consistent with previous measurements. Also, the dissociative ionization process, presumed to be H3CO* HCO' + H2,was observed with a threshold at 8.61 f 0.06 eV.
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Introduction The H~COHradical and its isomer, H~CO,are important intermediates in combustion and atmospheric processes while the radical ion, H~COH+,is important in interstellar molecule formation. In hydrocarbon combustion chemistry,IJ H$OH and H ~ C Oare formed in reactions such as O/OH + C H ~ O H+ H~COH+ O H / H ~ Oand c~~+ o2+ H ~ C O+ 0. ln the oxidation of hydrocarbons in polluted atmosphere^,^ H2COH is generated predominantly via the OH + C H ~ O Hreaction while
H 3 C 0 (+NO2) is the product of C H 3 0 2+ NO. In cold interstellar clouds, ion-molecule reactions predominate; and it has been W3gestd4 that the reactions of c+or CHj+ with C H 3 0 H may yield H2COH' which, on dissociative electron recombination, fOrms H2CO. Although numerous studies have been performed to characterize these radicals, the information concerning ionization of H2COH and H 3 C 0 is limited mainly to indirect studies.s of H2COHf,as derived from The heat of formation appearance potential studies5-10and the proton affinity5*"J2of
(1) Hoyermann, K.; Loftfield, N. S.; Sievent, R.; Wagner, H. Gg. Eighteenth Symposium (International) on Combusrion; The Combustion Institute: Pittsburgh, 1980; p 831 and references therein. ( 2 ) Warnatz, J. In Combustion Chemistry; Gardner, W. C., Jr., Ed.; Springer-Verlag: New York, 1984, and references therein. (3) (a) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Ken, J. A.; Troe, J. J . Phys. Chem. Ref.Data 1989, 18, 881 and references therein. (b) Neshitt, F. L.; Payne, W. A.; Stief, L. J. J . Phys. Chem. 1988, 92, 4030 and references therein.
(4) Huntress, W., personal communication. Cited in: Gottlieh, C. A.; Ball, J. A.; Gottlieb, E. W.; Dickinson, D. F. Astrophys. J . 1979, 227, 422. ( 5 ) Lias, S.G.; Bartmcss, J. E.; Liebman, J. F.; Holmes, J. L.; Lavin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17 (Suppl. No. 1). (6) Refaey, K. M. A,; Chupka, W. A. J . Chem. Phys. 1968,48, 5205 and references therein. (7) Haney, M.A.; Franklin, J. L. Trans. Faraday Soc. 1969.65, 1794 and references therein. (8) Losing, F. P. J . Am. Chem. Soe. 1977, 99, 7526. (9) Berkowitz, J. J . Chem. Phys. 1978, 69, 3044.
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0022-3654/92/2096- 104$03.00/0 0 1992 American Chemical Society
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