J. Phys. Chem. 1995, 99, 2444-2458
2444
Vibrational Predissociation Dynamics of p-Difluorobenzene-N2 Complexes. Comparison with p-Difluorobenzene- Ar Brian D. Gilbert and Charles S. Parmenter* Department of Chemistry, Indiana University, Bloomington, Indiana 47405 Hye-Keun Oh Department of Physics, Hanyang University, Ansan-Si, Kyungki-Do 425-791, S. Korea Received: July 27, 1994; In Final Form: October 4, 1994@
The SIvibrational predissociation (VP) dynamics and physical characteristics of the p-difluorobenzene-N2 (~DFB-Nz) van der Waals complex are reported. The geometry of the complex is roposed to be similar to the geometries of pDFB-Ar, benzene-Ar, and benzene-NZ, with the NZabout 3.5 above the center of the pDFB ring, and, by analogy to the benzene-NZ complex, with the NZ nearly freely rotating parallel to the aromatic molecular plane. Upper limits to the SI and So van der Waals binding energies of Do' I240 cm-I and DO" I213 cm-', respectively, were obtained. Only two of the nine observed SOpDFB ring modes ( ~ 6 " the symmetric ring stretch and v< the out-of-plane ring puckering mode) appear to be perturbed by complexation, and these only slightly. In S I , none of the observed ring levels appears significantly perturbed, but surprisingly, spectroscopic evidence concerning (Y could not be obtained. In general, YS(or Y16a in Wilson notation) is the most perturbed vibration in aromatic-rare gas van der Waals complexes. Vibrational predissociation from four initial SIring levels lying within the first 800 cm-I of the pDFB-NZ vibrational manifold was characterized using single vibronic level fluorescence spectroscopy. State-to-state dissociation from each level produces the pDFB product molecule in only a few of many accessible SIvibrational levels. Evidence of intramolecular vibration'al redistribution (IVR) within the pDFB-Nz complex is observed for one level. The dissociation is treated by preliminary modeling based on a serial mechanism (involving IVR within the complex followed by VP) that is related to that developed by Kelley and Bernstein [J. Phys. Chem. 1986, 90, 51641 for s-tetrazine-Ar VP. The modeling accounts for the final state selectivity and most but not all of the observed VP channels. The experimental and modeling results are compared with those of the pDFB-Ar complex whose vibrational level structure differs only modestly and in a known way from that of pDFB-N*.
x
Introduction Vibrational predissociation (VP) of aromatic molecule-rare gas van der Waals (vdW) complexes has provided much insight regarding the dissociation dynamics. Among the many aromatic cases that have been so studied, the unusually accessible S I SOspectroscopy of p-difluorobenzene (pDFB)I,*has allowed the VP of the pDFB-Ar complex to be one of the most thoroughly d ~ c u m e n t e d . ~In- ~this report, we describe the characterization of the pDFB-Nz complex and its VP. Specifically, we are interested in how a different solvating species that changes the vdW modes in a known way but leaves the ring modes unchanged influences the VP dynamics. It is fitting that a report on this system appears in the joumal issue dedicated to Professor S . A. Rice. Van der Waals complexes involving large molecules has been one of the many experimental and theoretical areas to which he and his coworkers have made seminal contrib~tions.~-l~ One of these complexes is, in fact, P D F B - A ~ , ~the benchmark for our comparisions with pDFB-N2. s-Tetrazine-Ar was the first aromatic-rare gas vdW complex to be studied, and it still remains one of the best characterized in terms of the complex's properties (structure, dissociation energy, vdW modes) and VP dynamics.9s12.13.15-23 Dissociation from specific S115-'7.20 and So9vibrational levels of s-tetrazine-
* To whom correspondence should be addressed. @
Abstract published in Advance ACS Absfrucrs, February 1, 1995.
Ar exhibits the final product (s-tetrazine) vibrational level selectivity that is now seen to be a general rule for VP of aromatic-rare gas complexes. A crucial insight into the mechanism of the VP process was provided by the time-resolved fluorescence emission spectra.I7,l9 These experiments clearly demonstrated that intramolecular vibrational redistribution (IVR) from an initially excited SIring level in s-tetrazine-Ar could compete with direct VP from the initially pumped level and could set up a serial dissociation path in which dissociation occurred from complex levels with the highly excited vdW modes produced by the IVR. Kelley and BemsteinZ4 (KB) developed a model of the VP that incorporated this serial mechanism. The IVR rate was given by Fermi's golden rule, and the ensuing VP rate from complexes with the highly excited vdW levels was calculated using a restricted RRKM unimolecular rate theory. This general model has provided a good description for the dissociation in several other aromatic-rare gas vdW c ~ m p l e x e s . ~ ~ - ~ ' Both the characteristics of the pDFB-Ar complex and the SI VP dynamics from seven initial ring levels of pDFB-Ar have been fully e~tablished.~-~ The Ar atom was found to lie roughly 3.5 A above the center of the pDFB ring in the SOstate, and the distance decreases only slightly upon S1 excitation. The SOand SI vdW binding energies were determined to be 190 cm-I 5 D( 5 240 cm-' and 160 cm-I 5 DO" 5 210 cm-l. None of the seven observed SO ring levels of pDFB were perturbed upon complexation, and only Yg (the out-of-plane ring
0022-3654/95/2099-2444$09.00/0 0 1995 American Chemical Society
Dynamics of p-Difluorobenzene-N2 Complexes puckering, mode 16a in Wilson notation) was significantly perturbed in SI. VP dynamics from seven initial SI ring levels of pDFB-Ar lying within the first 900 cm-I of the SI vibrational manifold were characterized. For each initial level, the SI levels of the free pDFB molecule dissociation product were formed with extreme selectivity. We have used a modified version of KB’s model to account for the observed selectivity and VP branching ratios.30 In brief reports, we have described how limited and known changes in the vibrational structure of the pDFB-Ar benchmark may be introduced by chemical modification^.^^-^^ It is then instructive to observe the response of the IVR and VP dynamics to these changes. The first complex to be compared with pDFB-Ar was chosen to specifically test the validity of the serial IVRNP model. Ewing suggested that accidental Fermi resonances between the initially excited SI ring level in the complex and a ring-vdW mode combination level could also explain the specific selectivity of VP in aromatic-rare gas vdW complexes.35 Deuterating the pDFB ring in order to compare the VP of pDFB-&-Ar with that of pDFB-Ar tests specifically the result of destroying (or at least changing) any Fermi resonances. It was found that when the same initial SI vibrational levels in the pDFB -Ar and pDFB-&-Ar complexes were excited, the same SI pDFB (pDFB-d4) product levels were formed by VP.32 These results showed that accidental Fermi resonances are not controlling the level selectivity and provided additional evidence for the validity of the serial IVRNP model. Changing the complexing species from Ar to N2 has quite a different effect on the complex level structure. Rather than changing the detailed positions of ring levels as with the pDFBQ-Ar modification, using N2 alters the vdW modes with almost no change of ring levels. In our earlier report concerning a single S I ring level in the pDFB-N2 complex,34 we showed that this specific perturbation of vdW modes increased the IVR rate but had only a modest effect on the observed VP channels themselves. We also briefly described the consequences of replacing a fluorine with a CH3 group to form the p-fluorotoluene complex pFT-Ar. This change introduces an internal rotation that interacts strongly with the ring vibration^.^^-^^ The interactions are so severe, in fact, that the term “perturbation” of ring level structure is probably inappropriate. The VP response to this change was dramatic. The final level selectivity that is so characteristic of aromatic molecule-rare gas complexes was largely destroyed.34 In this report, we return to the pDFB-N2 complex. We describe its characterization and extend the study of VP to four initial S I levels. The physical properties (geometry, binding energy) of pDFB-Nz were determined using SI-SO fluorescence spectroscopy and will be seen to be not greatly different from those of pDFB-Ar. The VP channels from four S I ring levels of pDFB-N2 were assigned using single vibronic level fluorescence (SVLF) spectroscopy. VP from the four levels produces pDFB with nearly the same selectivity as observed in pDFB-Ar. Evidence of IVR within the S I complex is observed in the SVLF spectrum from one of the initial levels. The VP rates (kvp) for the individual channels have been determined from the SVLF spectra and can be compared with those of pDFB-Ar. Finally, preliminary modeling of pDFB-N2 VP has been performed using our IVRNP model. The predictions are in qualitative agreement with experiment. The report is set forth as follows. First, we present results of the SI-& fluorescence excitation (FE) and SVLF spectra that cover the first 800 cm-l of the S I pDFB-N2 vibrational manifold. The VP channels from four S I pDFB-N2 ring levels
J. Phys. Chem., Vol. 99, No. 9, 1995 2445 are assigned. In the discussion, we describe the geometry of pDFB-N2, the vdW modes, and the upper limits of the SOand SI binding energies, as well as the VP branching ratios, rates, and lifetimes. A short description of the IVRNP model is presented with the results of the preliminary modeling. Throughout the discussion, the pDFB-N2 results are compared with those of pDFB-Ar. We use the conventional notation that distinguishes between the free aromatic molecule and the complex by placing a bar over symbols that refer to the complex. As examples, an S I complex level is 5’ reached by the 5; absorption where as the free pDFB molecule level Oo produced by VP might be detected by the 0; fluorescence band. Band positions are given in cm-I (vacuum). Experimental Procedures Many aspects of the experimental details have been described elsewhere in discussion of our pDFB-Ar s t ~ d i e s . ~We .~ mention here only those changes specific to pDFB-N2. pDFB-N2 complexes were formed by passing 760-2300 Torr of N2 (Air Products, zero grade) through a sample tube containing liquid pDFB (Aldrich, 99%) at 0 “C. This gas mixture was expanded through a 0.8 mm diameter pulsed gas nozzle (General Valve, Series 9). The operating pressure of the vacuum chamber with the nozzle running at 10 Hz and a nozzle pulse width of about 1 ms was 1 x Torr. These expansion conditions led to complexation of about 5% of the pDFB in the beam, with a rotational temperature of Trot 15 K. (See the section on the complex geometry for a rotational contour simulation.) All spectra were obtained at 25-30 nozzle diameters from the nozzle. Decreasing the stagnation pressure caused a reduction in the observed intensities of the pDFB-N2 St SOfluorescence bands. Increasing the stagnation pressure led to a slight increase in the intensity of the complex bands. The signal, however, was soon overcome by the increased noise in the base line. The pDFB-(N2)2 complex was not observed. pDFB-N2 SVLF intensities decreased linearly with the N2 stagnation pressure. A tunable dye laser system (Lambda Physik FL2002E or FL3002, each with 0.4 cm-I bandwidth, frequency doubled by a KDP crystal) pumped by an excimer laser (Lambda Physik EMG 201E or 201E MSC) was used for excitation. The UV excitation energy was 100-250 pJlpulse. The detection systems for total fluorescence and dispersed fluorescence were the same as those described in the pDFB-Ar studies3x5
-
Results The SI-& spectroscopy of free pDFB is well documented in both 300 K bulbs2 and supersonic expansions.’ These studies provide the S I and SO vibrational frequencies. The SO and SI geometry of pDFB was determined from 300 K rotational contours by Cvitas and H01las.~~ Fluorescence Excitation Spectra. Portions of the SI SO fluorescence excitation spectra of pDFB and pDFB-N2 are displayed in Figures 1 and 2. The spectra cover the first 900 cm-I of the SI vibrational ladder and contain both pDFB and pDFB-N2 vibronic transitions. Since the band positions and vibrational frequencies of pDFB are known,’.2 the monomer bands are readily assigned by comparison with previous work. In general, the complex bands are red-shifted 27 cm-I from the corresponding monomer bands. Complex bands are recognizable also by their distinctive rotational contours and by their growth with increasing stagnation pressure. Table 1 lists the displacements and intensities of the observed monomer and
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2446 J. Phys. Chem., Vol. 99, No. 9, 1995
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TABLE 1: Positions and Relative Intensities of the SI SO Fluorescence Excitation Bands for pDFB, pDFB-Nz, and pDFB-Ar" complex band relative monomer band position (cm-I) intensity (%y Dosition (cm-')b band DDFB-N? DDFB-A~ DDFB-N? DDFB-Ar
I'
36820
36840
36860
Frequency (cm-')
h
.-vl
-d
c)
C
3
v
x
0; 30; 8; 27; 6; 27; 5; 6;
-27 -23
-30 -27 -13 -30 -30 -30 -30 -30
-27 -27 -27
9
5
5
11
4 4
5 6
The pDFB-Ar data are from ref 5 . Displacement from the 0; band at 36 858 cm-' (vacuum). Complex/monomer. The relative intensities for a given set of bands, Le., the values of $:O:, are from the same expansion. The intensities for different bands come from different expansion conditions. No significance can be attached to the relative values of 0; vs 30; and so forth.
23 cm-'
36980
0 240 358 403 410 802 818 821
37000
37020
Frequency (cm-') Figure 1. The 0; region (top panel) and 30; region (bottom panel) in the St SO fluorescence excitation spectrum of pDFB seeded in Nz.
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--F-+GP--F-.I I
c' 37160
37180
37200
37220
37240
37260
Frequency (cm-')
h
.-C
vl
5;
Y
s
r3 ._ 5 vl
c)
C
i
37620
37640
37660
Frequency (cm-I)
-
Figure 2. The 8;, 6; region (top panel) and 5;. 6; region (bottom panel) in the Si SOfluorescence excitation spectrum of pDFB seeded in Nz.
pDFB-Nz transitions. For comparison, the displacements and relative intensities of the pDFB-Ar bands observed by Su, 0, and Parmentels are included. Monomer Bands. The pDFE3 transitions appearing in our excitation spectra are O,: 30:, 8;, 27;, 6:, 27;, 5;, and 6;. All but one are allowed and have type B contours resulting from the transition dipole moment lying along the b inertial axis, as shown in Figure 3. The 27; transition is forbidden. Two
I
I
Figure 3. Allowed electric dipole transition moment axis pe and the
inertial axes (a, b and c) of pDFB (top) and pDFB-Nz (bottom). mechanisms by which this transition may gain intensity have been discussed by other workers.',2 Complex Bands. Five of the pDFB bands are accompanied by a complex band, all displaced 27 cm-' to the red of the corresponding monomer bands. As is the case with pDFBAr: the pDFF3-Nz bands are small peaks with about 1/20 the intensity of the associated monomer bands. The shifts, combined with the different rotational contour observed for the complex, the changes in band intensity with changing beam conditions, and knowledge of the similar pDFB-AI complex provide secure criteria for making the assignments of the pDFB-N2 bands given in Table 1. The ?@ band was found to have a singular 23 cm-' red shift. The unusual shift is consistent with trends observed in the pDFB -Ar fluorescence excitation spectra, where out-ofplane ring modes, such as ~ 3 (an 0 out-of-plane fluorine bend), are more perturbed by complexation than in-plane ring vibrations. In PDFB-A~,~the band had an unusual red shift of 13 cm-I. The band is entirely missing from the pDFB-Nz excitation spectrum. By checking the intensity ratios of the bands in Figure 2 , it is possible to determine if the i: transition could be seen in our experiments if it occurred with the intensity normal for the complex. The intensity ratios of corresponding
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J. Phys. Chem., Vol. 99, No. 9, 1995 2447
Dynamics of p-Difluorobenzene-N2 Complexes
the monomer spectrum. The band positions, relative intensities, and assignments of the emission features in Figure 5 are listed in Table 2. Fluorescence after Exciting 3 240 cm-I). The fluorescence emission spectrum obtained by pumping the %; band 23 cm-' to the red of the 30; pDFB band is shown in the middle of Figure 6. The 302 SVLF spectrum is displayed at the top, and the Oo SVLF spectrum is shown at the bottom. The SVLF spectrum appears to consist of several broad, evenly spaced bands. These bands encompass several sources of fluorescence whose combined contributions causes the bands to be broad. Two general processes contribute to the fluorescence. One source is emission from undissociated vdW complexes. One is complex fluorescence (CF) from the initially pumped level, while the other is emission from another complex level reached by IVR. Another source is emission from SI levels of the free monomer produced by VP. First we consider emission from undissociated complexes. There are four possible pathways for this to occur. The first is from the initially pumped level G2. It is revealed by the transition occurring 316 cm-I to the red of the pump. This band is the most intense expected from The other three possibilities are emission from the 8', % I , and levels reached byIVR. Each will be in combination with high vdW mode excitation. The 8' level probably lies about 65 cm-' below and the most prominent transition would be the 8; band near 36 567 cm-'. Examination of the spectrum in this region shows no evidence of this transition. Emission from the 8' level may be ruled out. The next two possibilities are emission from the and Go levels. The 30; band is at 36 773 cm-', which corresponds to the peak observed 290 cm-I to the red of the pump position. The observation of the -1-0 3015, transition 1144 cm-I from the pump position confirms that the 30' level is reached by IVR. Finally, the most prominent transition, namely, the $ band itself, is observed at 250 cm-' to the red of the pump. The absolute frequency of this band is 36 831 cm-', which corresponds to the frequency of the i: observed in the SVLF in the bottom of Figure 5. Only one pDFB SI level populated by VP appears, namely Oo, with the 0: transition observed at 36 858 cm-'. This is the same frequency at which it is found in the SI SOfluorescence excitation spectrum. Table 3 lists the displacements of the transitions from the 302 and pump positions, as well as the relative intensities of the bands. The broad width of the transitions in the SVLF spectrum in Figure 6 is probably due to emission from hot vdW modes reached by IVR. Such emission broadens the transitions beyond the spectrometer resolution. This effect was observed by Outhouse et al. in the VP of 1-methylindole-Ar vdW complexes.29 Fluorescence after Exciting 6' i410 cm-I). The dispersed fluorescence spectrum obtained from pumping the 6; band is presented in the bottom of Figure 7. The top portion displays the 6' SVLF spectrum. Dashed lines from the monomer to the complex spectrum indicate transitions that are the same in both spectra. It is clear that there are several bands in the complex spectrum that cannot be assigned as CF. The non-CF fluorescence appears to come entirely from the Oo level of pDFB. The first prominent transition is 382 cm-' to the red of the pump position. This displacement corresponds to frequency of the 0; transition of the monomer product. All of the other non-CF emission bands in the spectrum are
1
(z' +
z2
-4
0
-2 AV
2
z2.
4
(cm'l)
Figure 4. Comparison between the pDFB-N2
6; band rotational
contour observed in fluorescence excitation (top panel) at 0.4 cm-I resolution and a calculated contour (bottom panel). The simulation uses T = 15 K and SOrotational constants (the S I values are in parentheses) in cm-' of A = 0.0462 cm-I (0.0472 cm-I), B = 0.0380 cm.' (0.0376 cm-I), and C = 0.0268 cm-' (0.0275 cm-I). These constants are for a complex with a spherical mas: of 28 amu centered over the ring with a bond distance of r" = 3.50 A and r' = 3.40 A. It is assumed that the pDFB SO and S I ring geometries are not changed by complexation.
comp1ex:monomer bands is fairly constant within a given fluorescence excitation spectrum. For example, the ratio -6::6; = 5% in Figure 2, and this should also be the ratio of 8::8:. The ratio of 8::6; 50%. Therefore, the band intensity should be -2.5% of the 6; transition. (We assume here that the Franck-Condon factors and fluorescence quantum yields are not significantly changed upon complexation.) A peak of this size would be observed in the spectrum displayed in Figure 2. Rotational Contours. The rotational contours of pDFB and SO fluorescence pDFB-Nz bands occurring in the SI excitation spectra of Figures 1 and 2 are different from one another. The allowed monomer transitions have dominant P and R branches, indicative of type B rotational contours. The complex transitions are dominated by Q branches, as shown for the $ band in Figure 4. These are type C contours for which pe lies on the c inertial axis. The two sets of inertial axes are shown schematically in Figure 3. Such inertial axis switching occurs also in the pDFB-Ar ~ o m p l e x . ~ Single Vibronic Level Fluorescence (SVLF) Spectra. Dispersed fluorescence spectra have been obtained at a spectrometer fwhm resolution of 17 cm-' by pumping the pDFBN2 absorption bands displayed in Figures 1 and 2. To aid in analysis, the SVLF spectra of the corresponding monomer bands are also shown.
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+
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Fluorescence after Exciting 0'. Figure 5 contains the SVLF spectra obtained when the Oo and 6' levels are excited. The transitions in the monomer spectrum are assigned by comparison to 300 K2 and cold jet' SI SOdispersed fluorescence studies. The pDFB-N2 spectrum was assigned by direct comparison to
-
5'
z2,
30'
0'
zo
+
z2
z2
(z'
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I
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Frequency (an-') Figure 5. Single vibronic level fluorescence spectra after pumping the 0; pDFB (top panel) and $ pDFB-N2 (bottom panel) bands. The spectra have been aligned by their excitation positions (asterisks) so that the cm-' scale indicates displacement from excitation. Band assignments in the 0' spectrum are analogous to those marked in the Oo spectrum. TABLE 2: Band Positions and Relative Intensities of SI SOFluorescence after Pumping the Oo pDFB and
pDFB-Nz Levels , displacement from pump position (cm-I) pDFB pDFB-N2
relative intensity A (cm-')U pDFB pDFB-N2 0
450 840
0 450 834
0 6
19 9
860 890 1258
858 878 1258
2 12 0 2
0
1308 1702 1724
1310 1702 1720
21 14
2116
a
io
0 4 2
-
TABLE 3: Band Positions and Relative Intensities in SI SOEmission after Pumping the @ Band of pDFB-Nz
displacement (cm-I)
assignt
0 216
31 16
0: 6: 8:
250 290
[loo] 2 34 9 16
[loo] 23 44 18 29
5: 6; 3: 5:6: 3y6:
662
15 27
32 35
5: 3:5:
A = monomer displacement - complex displacement.
consistent with Oo emission based on our knowledge of the pDFB Oo SVLF spectrum. The band positions, relative intensities, and assignments for the 6' SVLF spectrum are listed in Table 4. Fluorescence after Exciting 7' (0' 818 cm-I). Table 5 lists the displacements and relative intensities of the bands in the 5' SVLF spectrum that is displayed in the bottom of Figure
+
8.
There are many possible VP channels from this high level of pDFB-N2. Emission from a total of five different levels is observed when the 3' level is pumped. CF fluorescence from the initially pumped level is indicated by the dashed lines that
316
-
re1 re1 displacement intensity assignt (cm-I) intensity assignt
-
54 [loo1 46 23
30; 0;
1072 1110
63 86
0;
1144
46
30;
31 40
30:s:
5;
1172 1468
-
5:
J:
ojs: 3:
6:
1510
59
5:
700
25 51
@
25
8;
1540 1572
41 32
30;;:
1042
o:1:
connect the transitions from the 5' level of the monomer spectrum (top half of Figure 8) to those of the complex. Emission from four monomer levels has been assigned. The band at -788 cm-' from the pump position corresponds to the 0; monomer transition. Several other transitions from the Oo level are marked. The band at -390 cm-' is at the frequency of the 6; transition (37 250 cm-I). The band at -1038 cm-I (36 619 cm-I) is the 8; transition from the 8' pDFB level. Finally, the band at -1078 cm-' (36 579 cm-I) is at the frequency of the 81301 transition originating from the 8'30' level. Fluorescence after Exciting (0' f 820 cm-I). The 6* level lies only 3 cm-' above the 5' level. In pDFB's2 and pDFB-M these levels are in a weak Fermi resonance, and the resonance should also be intact in pDFB-Nz. The 6* SVLF spectrum is displayed in the bottom of Figure 9 with the 62 SVLF spectrum of the monomer at the top. There are three
z2
Dynamics of p-Difluorobenzene-N2 Complexes
34800
/?o:
30: 5:
(3021
I
35200
35600
36000
J. Phys. Chem., Vol. 99, No. 9, 1995 2449
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Frequency (cm-l) Figure 6. Single vibronic level fluorescence spectra after pumping the 30; (top panel), %$ (middle panel), and 0; (bottom panel) bands of pDFB and pDFB-Nz. Emission from the and Oo levels in the 30' spectrum is indicated by dashed lines. Emission from the toand 30' levels is assigned in the GZspectrum. The pump positions of the 302 and 30' spectra are indicated by asterisks.
30'
sources of emission other than CF from the initially pumped level. The sources are the 6l, 8I, and Oo levels. The displacements and band intensities are listed in Table 6. The assignments followed from an analysis similar to that of the 5' SVLF spectrum described above. Discussion Geometry of pDFB-Nz. As established by experiment and predicted by theory, the three complexes benzene-N~,"O-~~ b e n ~ e n e - A r , ~ ~and - ~ ~pDFB have nearly identical geometries. The similarity of this trio suggests that the geometry of pDFB-N2 will be a close replica of benzene-N2. On this basis, the pDFB-N2 geometry would have the center of mass located symmetrically 3.5 8, above the ring plane with the N-N bond parallel to the ring plane. The bond distance is almost identical in the SI and SO states, decreasing in SI by only a few percent. The N2 intemal with the bond parallel to the ring plane would be nearly free with a barrier of only a few cm-'. We have attempted a preliminary experimental consistency check of this proposition based on the S1-So 5: rotational band contour observed at about 15 K with 0.4 cm-' resolution. Our attempts to simulate this contour with an asymmetric rotor program that contains the pDFB-N2 geometry described above have not had a satisfactory result. The experimental contour and a contour simulated with the geometry described above are displayed in Figure 4. Temperature variations in the simulation affect primarily the base width rather than the Q-branch-like peak. None of a wide variety of other assumed geometries produces a more satisfactory simulation. The disparity between the experimental and simulated contour is in marked contrast to the good success of similar efforts with -@s4'
pDFB-Ar from the same lab~ratory.~ While pDFJ3-Ar and pDFB-Nz contours differ markedly at 0.4 cm-I resolution, we suspect the geometries are quite similar, as are the geometries of benzene-& and benzene-Nz complexes. It is possible that sequence transitions between intemal rotation states contribute to the contour of pDFB-N2. Such transitions are not included in the simulation. Transitions between intemal rotational states were observed in sub-Doppler SI-& bands of ben~ene-N*~I as well as in a Fourier-transform microwave spectroscopic The geometries of benzene-Ar and benzene-Nz complexes are tightly constrained by microwave and/or SI-SO sub-Doppler s p e c t r o ~ c o p y .Ab ~ ~initio ~ ~ ~ calculations ~~~ have successfully reproduced the g e o m e t r i e ~ . ~That ~ ~ ~of~pDFB-Ar .~~ is less constrained since it is based only on SI-SO band contours at 0.4 cm-I res~lution.~ The structure derived from that contour simulation is essentially identical to the benzene- Ar structure. The predictions of a recent pDFJ3-Ar ab initio calculation are in agreement with the experimental geometry.47 The pDFB-Nz Binding Energy. An upper limit for the SI binding energy (DO') is set by the observation of the VP onset as the SI complex vibrational ladder is climbed. The g2 level, 240 cm-I above Eo, is the lowest complex energy level from which evidence of VP is observed. The upper limit of the binding energy is therefore DO' 240 cm-I. The separation of the monomer and complex absorption bands ADOshown in Figure 1, and listed in Table 1, is the difference between the SI and SO vdW binding energies (ADO= DO' DC,").For the typical ADO= 27 cm-I, this gives DC,"5 213 cm-'. Lower bounds of DO'and DO"have not been determined. The reported dissociation energies for P D F B - A ~ ,pDFB~~~~ N2, and five other aromatic-Ar and aromatic-Nz vdW ~ o m p l e ~ e s ~ are ~ collected - ~ ~ ~in Table ~ ~ ~7. ~A general - ~ ~ ~ ~ - ~ ~ trend is apparent. DO' increases when an Ar atom is replaced with a N2 in all but one of the complexes. The only exceptions are the radical complexes benzyl-Ar and b e n ~ y l - N 2 . ~The ~ difference between the S I and SO binding energies when Ar is replaced with NZ is less predictable. ADOis larger for the N2 ADO complexes of pyrimidine,52aniline,53and 4-eth~laniline.~~ is smaller for the other three. Finally, it is interesting to note that the dissociation energies of pDFB-N2 and pDFB-Ar are (probably) the lowest of those listed in Table 7. Ring Mode Frequencies in pDFB-Nz. Although spectroscopic activity in only five of the ring vibrations of pDFB-N2 has been observed in this study, they appear to follow the general rule that ring vibrational frequencies are not greatly perturbed by c~mplexation.~ A comparison between the monomer and complex ring SO frequencies is given in Table 2. The frequencies are obtained from the Oo and Eo SVLF spectra. The displacements of the observed transitions are the SO ring frequencies and are accurate to within the f 4 cm-I precision of the experiment. Two pDFB-N2 SO ring vibrations, both second overtones, are observed to be perturbed. This is in marked contrast to pDFB-Ar, for which none of the SO vibrations were perturbed. One of the levels is the overtone of Y 8 , out-of-plane ring puckering motion; the other is the overtone of Yg,an in-plane ring distortion. The pDFB-N2 SI frequencies of two fundamentals and two overtones may be extracted from the data listed in Table 1 by observation of displacements of SI-SOcomplex absorption bands from the monomer bands. All have the same 27 cm-' displacement as the 0; bands, except for the band (23 cm-I). Thus none of the observed SI levels are significantly perturbed, although the i: transition is
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2450 J. Phys. Chem., Vol. 99, No. 9, 1995
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0
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0
Frequency (crn-') 6; pDFB-Nz (bottom panel) bands. Band assignments in the 6' spectrum analogous to those in the 6l spectrum are indicated by dashed lines. Emission from the Oo level of the free pDFB VP product is assigned Figure 7. Single vibronic level fluorescence spectra after pumping the 6; pDFB (top panel) and
TABLE 4: Band Positions and Intensities of Emission after Pumping 6: pDFB-Nz
S1
- SO
displacement re1 displacement re1 (cm-I) intensity assignt (cm-') intensity assignt
0
-1
1308
60 0;
-
TABLE 5: Band Positions and Intensities in S1 SO Emission after Pumping the Band of pDFB-Nz displacement re1 (cm-I) intensity
0 450
11
[IO01 38 36
382 450
[lo01 55
;;
1642 1688
832
30
6:
1706
856
18
665:
1755
788 830 840
898 1240
34 97
-1
208 1
892
38
__
1254 1264
52 33
6;3:
2108 2147
1038 1078 1238 1258
46 17 22 8
1278
20
__ 6, 5;
2169
missing. As Table 1 also shows, the situation is much the same for SIpDFE3-Ar where more levels are seen, but in this case YS' is notably perturbed. Mode 8. The band of pDFB-Nz is conspicuous by its absence. In pDFB-Ar, the 8' level was the only SI vibrational energy level greatly perturbed as evidenced by the 13 cm-I red shift of the ii transition relative to the free pDFB absorption vs the typical 30 cm-' red shift.j The pDFB-N2 bands are consistently shifted 3 cm-I to the blue relative to the corresponding pDFB-Ar bands, as shown in Table 1. The shift implies that the $, band of pDFB-Nz would be red-shifted by 10 cm-I if Yg' in pDFB-Nz behaved as in pDFB-Ar. A specific search for the band in that region and elsewhere was unsuccessful. The absence of the transition remains an enigma.
ii
ii
assignt
_ -5: -
displacement re1 (cm-]) intensity assignt
__
1352
5
5A6:
5b6:
1624
10
8;
0: 61 _ 5Ag;
1644 1688 1726 1752
69 23 17 26
5: 615: 6: __ 516;
1899 1939
36 20
815: 813015:
2045 2092
40 24
613;
2144
24
6:5:
__ + 5:6: 81 81301
-6: 5;3: 6:
$1
3:
Historically, Y8 ( ~ 1 6in~Wilson notation) has been found to be the mode most perturbed upon complexation of aromatic ring molecules by rare gas atoms or diatomic molecules. In addition to pDFB-Ar,j the dramatic difference in vibronic band shifts for transitions involving mode Y8 (VI&) has been documented for s - t e t r a ~ i n e - A r ? ~aniline-He, ~ ~ ~ ~ ~ ~ -Ne, ~ ~ -Ar,55 pyrimidene- Ar, -N2,52 and p-dichlorobenzene- Ar, -Kr.56 Weber and Ricelo proposed a perturbation theory analysis that, for several of the above complexes, appears to reproduce the experimental observations. The perturbation method proposed by Ewing3j was used to calculate the shift of SI?43 in pDFBAr with an account that the calculation nearly matched the experimental r e ~ u l t .These ~ perturbation treatments also predict that q should be perturbed in SO to the same extent that it is in
Dynamics of p-Difluorobenzene-N2 Complexes
J. Phys. Chem., Vol. 99, No. 9, 1995 2451
-2500
-2000
- 1500
-1000
-500
0
-2500
-2000
-1500
-1000
-500
0
Frequency (cm-' ) Figure 8. Single vibronic level fluorescence spectra after pumping the 5; pDFB (top panel) and
?A
pDFB-Nz (bottom panel) bands. Band assignments in the ?' spectrum analogous to those in the 5' spectrum are indicated by dashed lines. Emission from the Oo, 8l, and 8I3O1levels of the free pDFB VP product is assigned.
S I . This result is in obvious disagreement with experimental frequency to be 44.3 cm-l. These results agree well with the findings in P D F B - A ~ . ~ experimental values of the pDFB-Ar complex of 41 (stretch), The interest in Y E(Yl6a) stems from the primary role it plays 17 (bend), and 23 cm-I (bend).5 The benzene-N2 vdW in s-tetrazine-Ar VP.I6-l9 Rates of VP involving mode 16a frequencies have also been calculated. The stretch is predicted in the s-tetrazine product were found to be dominant relative to be 62 cm-I and the bends are 16 cm-' and 12 cm-l, to those that do not involve this ~ i b r a t i o n . 9 . 'In ~ ~contrast, ~~~~~ r e s p e ~ t i v e l y .The ~ ~ torsional frequency (ty rocking above the mode 8 appears to play little, if any, role in pDFB-Ar v P . 4 ~ ~ ~benzene plane) is predicted to be 53 cm-l, and the internal rotation is predicted to be 14 cm-' with a 20 cm-' barrier in VP from the and levels of pDFB-Nz does form the 8l SI.^ Hobza et aL4' in an ab initio study of benzene-Nz and 8l30' levels of pDFB, but these channels are not special in predicted a stretch frequency of 56.6 cm-I and a torsional terms of their branching ratios (see the following section). frequency of 73.2 cm-I. Fourier transform microwave specTherefore, it appears that as in pDFB-Ar, mode 8 in the N2 t r o ~ c o p ywas ~ ~ used to determine the benzene-Nz stretch complex is not a special mode in the VP dynamics. frequency to be 45.6 cm-' and one of the bends to be 26.5 van der Waals Modes. There is no experimental information cm-'. concerning pDFB-N2 vdW modes. Since they play a central role in the modeling of IVR and VP, we comment here on a Branching Ratios. The ratios for the G2,6', J', and 6' set inferred from other complexes, especially benzene-N2 and levels of pDFB-N2 and pDFB-Ar are listed in Table 8. These pDFB-Ar. ratios define the partitioning of complex decay into its various As depicted in Figure 10 , the switch from an Ar to a N2 channels and are expressed as percentages. The channels are complex partner introduces three additional degrees of freedom. fluorescence from the initially excited complex level, dissociaOne is the N2 high-frequency diatomic vibration (not depicted tion to form the pDFB monomer product in specific SI in Figure 10). The others may be considered true vdW modes. vibrational levels, and IVR to form the complex with ring levels One is a torsion of the N-N bond relative to the ring plane, that are below the dissociation energy. The latter channels occur and the other is rotation of N2 with the bond axis parallel to the only from the complex level 30'. ring plane. When these are added to the stretch and bends that A detailed discussion of how the branching ratios are are analogous to those of pDFB-Ar, five vdW modes are seen extracted from an SVLF spectrum was given previously for to occur for pDFB-N2. P D F B - A ~ . ~The same procedure was used for the pDFB-NZ Insight concerning the frequencies of the pDFB-N2 vdW data, so we give only a brief description here. Diagnostic modes may be gained by analogy to the structurally and transitions in the SVLF spectrum for each emitting level are dynamically similar pDFB-Ar and benzene-Ar, -Nz comchosen, for example, the 0; and bands in the 6' SVLF plexes. For benzene-Ar,4 the predicted stretching frequency is 40 cm-I, the degenerate bends are 11 cm-I each. In an ab spectrum of Figure 7. The branching ratios for the Oo and 6' initio study of benzene-Ar, Hobza et ~ 1 . calculated 4~ the stretch levels are determined from the relative intensities of the
2'
z2
z;
Gilbert et al.
2452 J. Phys. Chem., Vol. 99, No. 9, 1995
-2500
- 1500
-2000
I
I
-2500
I
1
-2000
-1500
0
-500
-1000
I
1
-500
-1000
0
Frequency (cm-l) Figure 9. Single vibronic level fluorescence spectra after pumping the 6; pDFB (top panel) and $ pDFB-Nz (bottom panel) bands. Band assignments in the 6* spectrum analogous to those in the 6* spectrum are indicated by dashed lines. Emission from the O0 and 8l levels of the free pDFB VP product is assigned.
-
TABLE 6: Band Positions and Intensities in SI SO Emission after Pumping the pDFB-Nz Band displacement re1 displacement re1 (cm-I) intensity assignt (cm-I) intensity assignt
3
0
-2 60
1514
12
816;
444 798 838
18
6;
1652
11
5;
25 24
0; 6;
[loo] 49 12
6;
23 20 60
6157 6;
888 1064 1250
1694 1715 1741
6;s:
8;
1918
41
$5;
1288
6;
2068 2142
6 50
1300
13 11
-2-0
2190
12
1336
24
6:
6;: 605I
+
6A5;
613;
6;:: -6i5;
diagnostic transitions. We assume in this step that neither the fluorescence quantum yields (ddv”))nor the fractional fluorescence intensities mv’-v”), the percentage of the total S1 SO fluorescence in an SVLF spectrum that occurs in a specific transition) are altered upon complexation. For pDFB-Ar and pDFB-Nz, both of these assumptions appear valid since the relative intensities of SI-SO transitions are unchanged upon complexation. The branching ratios are obtained using the relationship B(v‘)= Z(d-v’’)/(c$dv’) f(v’-v”)) where B(v’),& (v‘), and flu'-v") are the branching ratio, quantum yield of fluorescence, and fractional fluorescence intensity of the level v’ and transition v’-”’, corresponding to the diagnostic transition with relative intensity Z(d-v”) in the -SVLF spectrum. VP Channels following 2 Excitation. 30’ is the lowest level from which VP is observed in both the pDFJ3-Ar and
-
pDFB-NZ complexes. The VP (IVR)channels are displayed in Figure 11. In both complexes, the only VP channel is the pDFB Oo level. IVR populates the level of each complex. In pDFB-N2, the 6’ level is also reached by IVR. The observations of “hot” complex levels formed by IVR are a direct experimental indication of the validity of the IVRNP model that is discussed in a following section. Although the VP channels are only slightly different for pDFB-N2 and pDFB-Ar, the VP branching ratios are markedly different. Table 8 lists the branching ratios for both complexes. When the 30’ level of pDFB-Ar is excited, approximately 70% of the complexes dissociate to form Oo pDFB. In contrast, this channel accounts for only about 27% of the pDFB-N2 30’ level depletion. The majority channel is IVR to io, a process that is not active for pDFB-Ar. Finally, it is notable that, for pDFB-N2, only 3% of the level population remains in the initially pumped level, compared to nearly 20% for pDFB-Ar. VP Channels following 6’ Excitation. Branching ratios for excitation of the level of pDFB-N2 and pDFB-Ar are listed in Table 8. The branching ratios are essentially the same for both complexes. Emission from the 6’ initial level accounts for nearly 15% of the intensity in the dispersed fluorescence spectra, while emission from the Oo level of the monomer product produces nearly 85% of the intensity. The VP channels are displayed in Figure 12. Here is the first evidence of the final product level selectivity of the dissociation. Three monomer product levels are energetically accessible, but emission from only one is observed. VP Channels following 5’ and Excitation. The 5’ and
30’
30’
z’
z2
J. Phys. Chem., Vol. 99, No. 9, 1995 2453
Dynamics of p-Difluorobenzene-N2 Complexes
TABLE 7: Band Shifts, Dissociation Energies (Do'), and So vdW Bond Distances (r") of Various Aromatic Ring-Ar and Complexes comulex band shift (cm-'P Dn'(cm-lV r" t.4) reference pDFB-Ar -30 190-242 (294) 3.5 3,547 -27 pDFB-N2 200
-
v1
-
0
Y
0
200
400
600
\
a
800
S , Vibrational Energy (cm-') Figure 15. VP lifetimes of complex levels in pDFB-Ar and pDFBN2.
0
Figure 13. VP channels for the 2'level of pDFB-Ar and pDFB-N2. The layout is similar to that of Figure 11. Determination of t v p for pDFB -Ar has been presented in detail el~ewhere.~ Since the pDFB fluorescence lifetimes (tf x 10 ns)6,57,58 are fairly constant for our 0-900 cm-' SI energy range and not altered upon c~mplexation,~ it is possible to estimate values of ~ V using P the branching ratios in Table 8. The VP (or IVR) rates (kprd)from the initially pumped complex level into the different individual product levels are calculated using kprod m (Bfinal/Binitial)kf, where Bfinalis the product channel branching ratio, Binitlalis the branching ratio for remaining in the initially pumped complex level, and kf is the pDFB fluorescence rate (-lo8 s-'). ZVP is then the sum of the individual rates, ZVP = (l/Ckprod). It is evident from Figure 15 that the z v p values of pDFB-N2 are -all shorter than for pDFB-Ar. The only exception is for the 6' level where t v p 2 ns for both complexes. Recall that the branching ratios from this initial level were also essentially the same for both pDFB-Ar and pDFB-N2. The results are consistent with our proposed model, presented below. The pDFB-Ar z v p have also been measured by Jacobsen et d 6 using multiphoton ionization (MPI) with time-of-flight
detection to probe several of the pDFB-Ax initial levels that are of interest in the present study. The MPI measurements are listed in Table 8, and are larger than ours by a (nearly) uniform factor of 2. Their lifetimes do, however, reproduce on a relative basis the pDFB-Ar results from the indirect determinations. Modeling. We present elsewhere30a detailed description of the VP model used to predict the final state selectivity and branching ratios for pDFB-Ar dissociation. Here we present the predictions of our first efforts to apply the model to pDFBN2. In this approach, the modeling differs from that used for pDFB-Ar only by adjustment of certain parameters. We describe the modeling approach only briefly with emphasis on the differences for the two complexes pDFB-Ar and pDFBN2. Our treatment is adapted from KB's modeling of s-tetrazineAr dissociation as a serial IVRNP process.24 For pDFB-Ar and pDFB-N2 it is assumed that no dissociation occurs from the initially pumped complex level. The decay of that level is by IVR (in competition with SI SO fluorescence) to complex levels containing highly excited vdW modes. Dissociation then occurs from the states reached by IVR. By this model, the measured VP lifetimes that describe the decay of the initially pumped complex level are actually those
-
J. Phys. Chem., Vol. 99, No. 9, 1995 2455
Dynamics of p-Difluorobenzene-N2 Complexes
IVR
800
bJ3 &
d 0
600
400
0 Figure 16. Energy level representation of the serial IVRNP model. ?(O,O,O) indicates the ?' level of pDFB-Ar without excitation of the vdW modes. 6'(x,y,z)indicates the 6' level with (x,y,z) quanta of excitation in the vdW bending and stretching modes. Every ring level provides a base for a stack of combination levels comprised of that ring level plus excited vdW states. Two such stacks are shown.
for the N R step. No attempt is made in the modeling to predict those lifetimes, even on a relative scale as different initial complex levels are pumped or as the lifetime from a given pDFB-N2 level is compared with that for pDFB-Ar. More sophisticated modeling and a much improved experimental view of the vdW mode structure are needed for these comparatively subtle experimental aspects of the vibrational dynamics. Instead, the modeling focuses on selecting the correct dissociation channels from the many possibilities and on the more demanding task of predicting the branching ratios among these channels. The early failures with pDFE3-Ar modeling have shown that predicting even such blunt dissociation characteristics as the specific VP channels is a difficult task. The success of KB's approach in an increasing number of application^^^-^' is a tribute to the fundamental integrity of their approach. The schematic in Figure 16 shows that the final states of the IVR occur as sets of highly excited vdW modes, with each set occurring in combination with some specific ring level. The modeling treats the IVR as an array of competitive processes to these individual sets. In that competition, the IVR rate to one or at most a few of these sets is much greater than to any of the other sets so that selectivity among sets is achieved. That selectivity picks out certain lower ring levels with their stacks
of vdW modes. To a large extent, it is the identity of these ring modes that determines the final dissociation channels. The relative rates of IVR to the various sets of excited vdW modes are predicted by the standard golden rule expression with contributions from the ring modes and vdW modes treated separately. The expression factors into components of squared average matrix elements for ring mode interactions and for vdW mode interactions multiplied by the density of vdW states @vdW. The pDFB-Ar modeling shows that the relative rates among sets of vdW levels built on various ring levels are established primarily by the identity of the ring levels. The vdW mode factors play only a secondary role. The largest N R rates occur into those few vdW mode stacks for which ring level quantum changes in the IVR process are limited to Av = 1 or 2, and it is this distinction of rates that ultimately produces the VP channel selectivity. Since the ring modes of pDFB-N2 and pDFB -Ar complexes are essentially identical, the modeling predicts similar IVR and VP channel selectivity for the two complexes. In the main, this similar VP channel selectivity is observed. Differences between the dissociation dynamics of the two complexes are predicted by the modeling to be modest and principally the consequence of their different vdW mode
Gilbert et al.
2456 J. Phys. Chem., Vol. 99, No. 9, I995
50
Pump
I
90
PDFB-Ar
302
Experiment
I
90
7 pDFB-Ar !!!!?IpDFB-N,
60
60 n
n
5 30
5 30
0
.c 4 )
. Y e
2e a o
2
3
3 90
b n o
.-
90
2
2
m
m
60
60
30
30
0 100 200 300 400 500 600 700 800 900 1000 Ev&l
700 6oo
500
0
( I I I / ( I I I I I
1
1 1
pDFB-N,
t
30'
0
3
Oo
0"
Figure 18. Experimental (top panels) and calculated (bottom panels) VP branching ratios for the g2(left, the branching ratios for the 301 and Go channels of the model have been enlarged by a factor of 20) and (right) levels of pDFB-Ar and pDFB-N2.
z1
Pump 3 90
Pump 6*
Experiment
Experiment
90
0pDFB-Ar pDFB-N,
60
0
100 200 300 400 500 600 700 800 900 1000 E,dW
Figure 17. Plots of (bottom).
@vdW
vs
6vdW
(cm-')
for pDFB-Ar (top) and pDFB-N:!
structures. The greatest distinction in this structure is ultimately the increased density of vdW states (@vdW) in pDFB-N2 caused by the additional vdW modes. Figure 17 shows @vdW plotted against the energy contained in the vdW modes, EvdW. It is seen that the maximum density for the pDFB-N2 is an order of magnitude greater than that for pDFB-Ar. We note that the qualitative change in @vdW for pDFB-N2 vs pDFB-Ar is not at all reflected in the very similar branching ratio predictions for the two complexes, nor in the experimentally observed branching ratios. The point emphasizes the strictly secondary role played by the vdW mode structure of these two complexes in determining the VP outcome. On account of the low sensitivity of the banching ratio modeling (and the actual VP) to details of the vdW mode structure, the paucity of experimental information about vdW modes in aromatic complexes does not present large problems for such relatively crude modeling. Incorporation of refinements concerning vdW modes and @vdW will not greatly change the predictions. For the @vdW calculations, we use only four of the five pDFBN2 vdW modes and all three pDFB-Ar modes. The two bends, v, = 17 cm-', vY = 23 cm-I, and the stretch Y, = 41 cm-' are used with the same sets of frequencies for both complexes. These values are taken from pDFB-Ar discussions given e l ~ e w h e r e . ~The ? ~ torsional pDFB-N2 frequency ty = 17 cm-I is assumed to be essentially the same as one of the bends. Calculated for benzene-N:! suggest that the torsion should perhaps be higher (cu. 53-73 cm-I), but there is no experimental information for any vdW complex. We assume that the nearly free internal rotation of N2 in pDFB-N2 is only poorly coupled to other modes, and its possible contributions to IVR are not included in the modeling. With four vdW modes
-
60 n
n
5
5 30
30
0
0
.e Y
. Y e
2o n 0
:0
c
2E t
Model
90
.M
c
u 90
c!
a 60
30
30
0"
8'
8'30'
0 6'
L Model
L 6'
Figure 19. Experimental (top panels) and calculated (bottom panels) VP branching ratios for the 7' (left) and (right) levels of pDFB-Ar and pDFB-N2.
z2
for pDFB-Nz, the @vdW counting extends up to EvdW = 4D{ = 880 cm-'. For pDFB-Ar, the count extends only to 3D{ = 660 cm-'. Anharmonicity according to the standard formula WeXe = V2/4Do' was incorporated for every mode. (A recent pulsed field ionization study has assigned an SI bend fundamental as 31 cm-l and the stretch as 41 cm-l for benzeneAr.59)
The calculated branching ratios for pDFB-N2 are compared with the experimental ratios in the display of Figures 18 and 19. The calculated and experimental ratios for P D F B - A ~are ~~ included for comparison. The experimental branching ratios in these figures are for the levels reached only by IVR or VP and have been renormalized for comparison with the modeling. For all initial levels in both complexes, the modeling reproduces the existence of level selectivity in the VP product as well as the choice of major channel. Considering the large number of possible channels for some of the initial states (see Figures 1 1 14), these qualitative successes suggest that the basic premises of the modeling are correct. The modeling is most successful for pDFB-Ar, where it not only reproduces the channels but also predicts qualitatively the branching ratios. Its most serious problem centers on the close
Dynamics of p-Difluorobenzene-N2 Complexes similarity of its predictions for the two complexes. The experimental data show that the IVR and the VP of the two complexes have substantial differences, particularly in the extra channels that occur for pDFB-Nz. None of these channels are predicted by the modeling. The worst case concems the initial level G2for which the dominant channel G2 %I, an IVR process, is notpredicted to be competitive by the model. Increased VP Rates in pDFB-Nz. Figure 15 shows that, for all of the initial vdW complex ring levels pumped, the VP lifetimes for pDFB-N2 are shorter than those of pDFB-Ar. In another report,34 we compared the ?' SVLF spectra of both complexes where the relative fluorescence intensity from the undissociated complex served as a qualitative gauge of the relative VP rates. A comparison of the intensity of the transition from the initially pumped level demonstrated that there was significantly faster depletion of J' level of pDFB-N2. In Table 8, it is found that this sort of comparison is valid for all of the initially pumped ring levels of pDFB-N2 vs pDFB-Ar. For each initial level, the branching ratio for remaining in that level decreases when N2 is substituted for Ar. The shorter initial complex state lifetimes for pDFB-N2 are consistent with the concept of an IVR that is accelerated by the increased @vdW accompanying the switch to N2 and Ar. In this respect, the pDFB-N2 results are also consistent with the modeling. Other vdW complexes also exhibit such an increased VP rate as @vdW grows. For both indole-Ar, - C a Z 5 and 1-methylindole-Ar, -CH429 complexes, the VP rate increased as the solvating species was changed from Ar to C a . The increased VP rates were found, using a VP model similar to our own, to depend mostly upon QvdW. In and 4-ethylaniline-M2* (M = Ar, N2, and CH4) complexes, the VP rates increased progressively as the number of vdW modes grew. There appears to be only one instance in which increasing the number of vdW modes did not raise the VP rate. By comparing line widths observed in dispersed fluorescence spectra, Hopkins et ~ 1 showed . ~ evidence that the VP rate of n-(f)-octylbenzeneNZwas slower than that of the corresponding complex with Ar as the solvent. Conclusions. S 1-SO fluorescence spectroscopy has been used to begin the characterization of the pDFB-N2 vdW complex. None of the four observed SIenergy levels in the complex were found to be perturbed upon complexation. Two of the SO levels were found to be perturbed. The complex SI SO absorption bands were all red-shifted by 23-27 cm-' relative to the corresponding pDFB bands. The $ transition was not observed. In pDFB-Ar the SI ring puckering mode Vg' (Vl6a)) is severely perturbed as is the analogous mode in many other aromatic molecule-rare gas complexes. Curiously, absorption involving that mode in pDFB-N2 is missing entirely. The nearly identical geometries of benzene- Ar, -N241-43,45,46,61 and p D l 3 - M ~were ~ ~ used as guides for our proposed pDFBN2 structure. In SO, the N2 center of mass is assumed to lie about 3.5 8, above the pDFB center of mass. Upon excitation the intermolecular distance is expected to contract by a few percent. As in ben~ene-N2,'".~~-~~ the diatomic is likely parallel to the ring plane and a nearly free rotor. Rotational contour simulations using a wide variety of geometries were unable to reproduce the contours. Evidence for VP is first observed in the G2SVLF spectrum as excitation climbs the Slvibrational ladder. This observation sets the upper bound of the vdW binding energy at DO'5 242 cm-I. The SO binding energy is DO" = DO' - 27 cm-'. The VP channels from the G 2 , ;I, ?I, and Z2 levels have been identified. The VP produces pDFB in only a select few
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J. Phys. Chem., Vol. 99, No. 9, 1995 2457 of the possible product states. Preliminary modeling using a serial IVRNP mechanism reproduces the selectivity, but not always all of the specific VP channels. In comparison with P D F B - A ~ , ~ -many ~ , ~ ~aspects of pDFBN2 are similar. The binding energies, geometries, complex band shifts, and the major VP channels are all nearly the same for the two complexes. The major change is the presence of two additional vdW modes. Their presence increases the rate of IVR in the complex and, hence, the rate of VP as expected on the basis of the IVRNP model. Acknowledgment. The financial support of the NSF is appreciated. We thank Drs. Meng-Chih Su, K. W. Butz, and C. J. Purse11 for helpful discussions. We thank Ms. Kah Tan for providing the simulations of the pDFB -N2 band rotational contour. References and Notes (1) Knight, A. E. W.; Kable, S. H. J. Chem. Phys. 1988, 89, 7139. (2) Coveleskie, R. A.; Parmenter, C. S. J. Mol. Spectrosc. 1981, 86, 86. (3) Butz, K. W.; Catlett Jr, D. L.; Ewing, G. E.; Krajnovich, D.; Parmenter, C. S. J. Phys. Chem. 1986, 90, 3533. (4) 0, H.-K.; Parmenter, C. S.; Su,M.-C. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 253. ( 5 ) Su, M.-C.; 0,H.-K.; Parmenter, C. S. Chem. Phys. 1991,156,261. (6) Jacobsen, B. A,; Humphrey, S.; Rice, S. A. J. Chem. Phys. 1988, 89, 5264. (7) Sulkes, M.; Jouvet, C.; Rice, S. A. Chem. Phys. Lett. 1982, 93, 1. (8) Stephenson, T. A.; Rice, S. A. J. Chem. Phys. 1984, 81, 1083. (9) Weber, P. M.; Rice, S. A. J. Chem. Phys. 1988, 88, 6120. (10) Weber, P. M.; Rice, S. A. J. Phys. Chem. 1988, 92, 5470. (11) Rosman, R. L.; Rice, S. A. J. Chem. Phys. 1987, 86, 3292. (12) Weber, P. M.; Rice, S. A. J. Chem. Phys. 1988, 88, 6107. (13) Weber, P. M.; Buontempo, J. T.; Novak, F.; Rice, S. A. J. Chem. Phys. 1988, 88, 6082. (14) Rice, S. A. In Dynamics of Polyatomic van der Waals Complexes; Halberstadt, N.; Janda, K. C., Eds.; Plenum Press: New York, 1990; p 189. (15) Smalley, R. E.; Wharton, L.; Levy, D. H. J. Chem. Phys. 1978, 68, 2487. (16) Brumbaugh, D. V.; Kenny, J. E.; Levy, D. H. J. Chem. Phys. 1982, 78, 3415. (17) Ramaekers, J. J. F.; Krijnen, L. B.; Lips, H. G.; Langelaar, J.; Rettschnick, R. P. H. Laser Chem. 1983, 2, 125. (18) Ramaekers, J. J. F. Ph.D. Dissertation, University of Amsterdam, Amsterdam, 1983. (19) Heppener, M.; Kunst, A. G. M.; Bebelaar, D.; Rettschnick, R. P. H. J. Chem. Phys. 1985, 83, 5341. (20) Brumbaugh, D. V.; Kenny, J. E.; Levy, D. H. J. Chem. Phys. 1983, 78, 3415. (21) Haynam, C. A.; Brumbaugh, D. V.; Levy, D. L. J. Chem. Phys. 1984, 80, 2256. (22) Brocks, G.; Hyugen, T. J. Chem. Phys. 1986, 85, 3411. (23) Haynam, C. A.; Brumbaugh, D. V.; Levy, D. H. J. Chem. Phys. 1984, 80, 2256. (24) Kelley, D. F.; Bemstein, E. R. J. Phys. Chem. 1986, 90, 5164. (25) Outhouse, E. A.; Bickel, G. A.; Demmer, D. R.; Wallace, S. C. J. Chem. Phys. 1991, 95, 6261. (26) Nimlos, M. R.; Young, M. A,; Bemstein, E. R.; Kelley, D. F. J. Chem. Phys. 1989, 91, 5268. (27) Hineman, M. F.; Kim, S. K.; Bemstein, E. R.; Kelley, D. F. J. Chem. Phys. 1992, 96, 4904. (28) Hineman, M. F.; Bemstein, E. R.; Kelley, D. F. J. Chem. Phys. 1993, 98, 2516. (29) Outhouse, E. A.; Demmer, D. R.; Leach, G. W.; Wallace, S. C. J. Chem. Phys. 1993, 99, 80. (30) 0,H.-K. Ph.D. Dissertation, Indiana University, Bloomington, IN, 1989. (31) Semmes, D. H.; Baskin, J. S.; Zewail, A. H. J. Chem. Phys. 1990, 92, 3359. (32) Elston, H. J.; Gilbert, B. D.; Parmenter, C. S.; 0, H.-K.; Stone, T. A,; Su, M.-C.; Zhao, Z.-Q. In Proceedings of the 6th International Conference on Time Resolved Vibrational Svectroscouv: ' ', Lau. A.. Ed.: Spn'nger-Verlag: Heildelberg, 1993; p 14. (33) Parmenter, C. S.; Gilbert, B. D.; Oh, H.-K.; Su, M.-C.; Zhao, Z.Q. Leituvos Fizikos kurnalas 1994, 34, 114. (34) Gilbert, B. D.; Parmenter, C. S.; Su, M.-C.; Oh, H.-K.; Zhao, Z.0. A D D ~Phvs. . B 1994. 59. 397. (j5) Ew;ng, G. E. j . Phys. Chem. 1986, 90, 1790. (36) Okuyama, K.; Mikami, N.; Ito, M. J. Phys. Chem. 1985,89,5617.
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