2362
J . Phys. Chem. 1985,89, 2362-2366
Structural Effects on the Proton-Transfer Kinetics of 3-Hydroxyflavones Andrew J. G. Strandjord, David E. Smith, and Paul F. Barbara*+ Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 (Received: October 8, 1984)
The excited-state intramolecular proton-transfer dynamics and other photophysical aspects of five methylated derivatives of 3-hydroxyflavone (3HF) have been studied. The compounds are 2’-methyl-, 3’-methyl-, 4’-methyl-, 2’,4’-dimethyl-, and 2’,5’-dimethyl-3-hydroxyflavone. The primed numbers designate position on the phenyl ring of the flavone structure. Measurements of a variety of properties, including pK,, NMR chemical shift of the alcoholic flavone proton, fluorescence Stokes shift, excited-state proton-transfer Arrhenius activation energy (Ea), and other quantities, reveal that the compounds fall into two groups depending on whether the ortho position of the phenyl ring is substituted with a methyl group. The ortho-substituted compounds are less basic, and less hydrogen-bond donating. They also exhibit a weaker intramolecular hydrogen bond and greater E, value. These general trends support the recent hypothesis that the excited-state proton-transfer mechanism of 3-hydroxyflavonesis dominated by energetic factors of a solute/solvent complex rather than relaxation effects involving the dynamics of the solvent configuration.
Introduction The excited-state intramolecular proton-transfer reaction of 3-hydroxyflavone (3HF) is a promising model system for small barrier proton-transfer reactions in solution. In the previous article in this journal,’ we proposed a model for the proton-transfer dynamics of 3HF in alcoholic solvents that ascribes the Arrhenius activation energy E, of the proton-transfer rate constant to energetic factors associated with the 3HF/solvent complex. The mechanism is outlined in Figure 1, where hypothetical structures for the important intermediates are also presented. The definition of the symbols in Figure 1 and elsewhere in the present paper may be found in ref 1. A critical issue in the validity of this model is whether the proton-transfer kinetics are a strong function of the relaxation dynamics of the solvent. The physical basis of the model is that the proton-transfer process is not a function of the relaxation dynamics, but rather is more sensitive to energetic factors of the solute/solvent interactions. The mechanism essentially ascribes the proton transfer to a thermally activated rearrangement, in the sense of transition-state theory, of a 3HF/solvent hydrogen-bonded complex. The major evidence that the proton transfer is thermally activated comes from experiments on solvent effects on the proton-transfer kinetics.’ It is desirable to have other forms of evidence on the thermal activation vs. relaxation dynamics issue. Toward this goal, we have made a study of the proton-transfer kinetics of several derivatives of 3HF. The underlying concept in this work is a simple one-if the proton-transfer kinetics are controlled by the relaxation dynamics of the solvents, then E, should be very similar for a broad range of derivatives in the same solvent, since E, in this case is essentially a reflection of the solvent dynamic properties. In contrast, if the thermal activation mechanism is operating, the E, depend on the structure of the derivative. Certain features of the proton-transfer mechanism of 3HF limit the choice of substituents for derivative studies. Halogen atoms are undesirable because the internal heavy-atom effect could complicate the photophysics of derivatives with this type of substituent. Other common substituents, such as methoxy, nitro, and carboxy, are not practical because the hydrogen-bonding propensity of these functionalities might easily lead to an exceedingly complex distribution of solvated forms in the proton-transfer mechanism. We have avoided the limitations just outlined by using a methyl group as a substituent in these studies. The compounds that have been studied are 2’-Me-3HF, 3’-Me-3HF, 4’-Me-3HF, 2’,4‘Me2-3HF, and 2’,5’-Me2-3HF, where the primed numbers designate the position of the methyl (Me) substituent. The numbering system is portrayed as Alfred P. Sloan Fellow and Presidential Young Investigator.
0022-3654/85/2089-2362$01.50/0
In planning this study, we had anticipated that the protontransfer kinetics of 3’-Me-3HF and 4’-Me-3HF should be similar to unsubstituted 3HF, since the methyl group is known to have weak para- and meta-substituent effects in structure vs. reactivity correlation^.^ On the other hand, for the derivatives with a methyl group at the 2’ (ortho) position, we expected that the situation could be very different due to the ortho effect, which is well-known in the physical organic l i t e r a t ~ r e . ~The ortho effect explains why the equilibrium and kinetic behavior of molecules with a phenyl group that has a bulky ortho substituent, such as o-benzoic acid, do not follow the same substituent vs. reactivity trend as parasubstituted aromatics. Simply, the aspect of the ortho effect we consider here is due to a steric inhibition of resonance. The resonance is inhibited because nonbonding interactions due to the bulky substituent modify the equilibrium torsional angle between the phenyl group and the reactive part of the molecule, Le., the ortho-substituted molecules are less planar. An additional motivation for studying the kinetics of derivatives of 3 H F is the possibility that phenomenological trends might be established between the proton-transfer kinetics and other physically revealing properties of 3HF, such as the pK, and infrared spectrum. Trends of this sort could easily lead to new insight into the proton-transfer mechanism. In particular, it would be interesting to obtain evidence on whether the proton-transfer kinetics is a function of the torsional angle (0) about the bond which joins the phenyl group to the a-pyrone ring. The in-
volvement of 0 in the proton-transfer mechanism has been a long-standing, but unresolved, issue.2 The investigation of the proton-transfer kinetics of the methyl-substituted derivatives offers (1) Strandjord, A. J. G.; Barbara, P. F., preceding paper in this issue. (2) Sengupta, P. K.; Kasha, M. Chem. Phys. Lett. 1979, 68, 382. (3),Lowry, T. H.; Richardson, K. S . “Mechanism and Theory in Organic Chemistry”; Harper & Row: New York, 1981; Chapter 2. (4) Gould, E. S. “Mechanism and Structure in Organic Chemistry”; Holt, Rinehart and Winston: New York, 1959; p 236.
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 1I , 1985 2363
Proton-Transfer Kinetics of 3-Hydroxyflavones
TABLE I: Melting Points and Elemental Analysis of the Derivatives abbreviation compd 3HF 3-hydroxy-2-phenyl-4H-1-benzopyran-4-one 3-hydroxy-2-(2-methylphenyl)-4H-l -benzopyran-4-one 2’-Me-3HF 3’-Me-3HF 3-hydroxy-2-(3-methylphenyl)-4H-1-benzopyran-4-one 4’-Me-3HF 3-hydroxy-2-(4-methylphenyl)-4H1-benzopyran-4-one 2’,4’-Me2-3HF 3-hydroxy-2-(2,4-dimethylphenyl)-4H-l-benzopyran-4-one 2’,5’-Me2-3HF 3-hydroxy-2-(2,5-dimethylphenyl)-4H-l-benzopyran-4-one
mp, “C
%H,M
%xptl
191-192 146-148
76.17
196.5“ 168-170
76.17 76.67
184-186
76.67
75.92 76.33 76.29
76.17
76.86
4.80 4.80 4.80 5.31
76.78
5.31
%HexPtI 4.84
4.91 4.99 5.29 5.16
‘Literature value, 196-198 “C6
I S O ~v I t i o
nL
1
;t
proton
3’
a
c
c
Tranrfer
. I
m C 0)
n I
a V
. I
I
0.
0
T*
1
320
I
I
340
1
I
I
I
380
360
Wavelength (nm) Figure 2. Absorption of 3HF and its methyl derivatives in methanol at ambient temperature. The primed numbers denote the position of the methyl substituent(s) in each compound. derivatives were verified by NMR, IR, thin layer chromatography, and elemental analysis. The melting points and elemental analysis data are summarized in Table I. The pKa (eq 1) determinations for the ground state of the N
2
T 0
Figure 1. Photodynamic model for the excited-state proton transfer of the 3-hydroxyflavonols. In ref 1 this model (which is denoted by model B) is described in detail. important new information on the torsional angle problem. The results we describe herein are discussed in terms of a kinetic scheme that is outlined in Figure 1 and described in further detail in the preceding article in this journal, in which this model is denoted by the label model B. The preceding article also includes a derivation of the underlying photophysical equations that relate the experimental observables to the microscopic rate constants of Figure 1.
Experimental Section The experimental methods we describe here are similar to those described else where.'^^ The proton N M R measurements were made at 300 MHz with a Nicolet NT-300 spectrometer. The flavonol derivatives were synthesized by the method of Smith6 which is a modification of a procedure developed by Algar, Flynn, and Oyamada.’ The compounds were purified by multiple recrystallizations from hexane. The structure and purity of the (5) Strandjord, A. J. G.; Courtney, S. H.; Friedrich, D. M.; Barbara, P. F. J . Phys. Chem. 1983, 87, 1125. Barbara, P. F.; Strandjord, A. J. G. In “MultichannelImage Detectors”;Talmi, Y.Ed.; American Chemical Society: Washington DC 1983; ACS Symp. Ser. p 183. (6) Smith, M. A,; Neumann, R. M.; Webb, R. A. J . Heterocycl. Chem.
Anion
@(p 0
(1)
N
form of the derivatives were made in a similar fashion to that described in detail by Wolfbeis et ale8 The solvent system was 5% methanol by volume in water, and the 3HF concentration was N M. A sodium borate/boric acid buffer system was employed. The pKa values were determined from inflection points in plots of anion optical density vs. pH. The absorption of the anion was monitored at the following wavelengths: 3 H F (402 nm), 3’Me-3HF (403 nm),4’-Me-3HF(404 nm), 2’-Me-3HF(375 nm), 2’,4’-Me2-3HF (377 nm), and 2’,5’-Me2-3HF (377 nm). The infrared measurements were recorded with a Beckman 4850 spectrometer. The solvent for the infrared spectra was CCll which was freshly distilled from CaH2.
Results Absorption and Emission Spectra. The electronic absorption spectra of 3 H F and the methylated derivatives are portrayed in Figure 2. It is interesting to compare the spectra for those compounds that lack an ortho methyl group (3HF, 3’-Me-3HF, and 4’-Me-3HF) to the compounds with an ortho methyl group (2’-Me-3HF, 2’,4’-Me2-3HF, and 2’,5’-Me2-3HF). Molecules in the latter group exhibit spectra with (i) a broader low-energy
1968, 5, 425.
(7) Seshadri, T. R. “The Chemistry of Flavonoid Compounds”; Geissman, T. A., Ed.; Pergamon Press: Oxford, 1962; p 156.
(8) Wolfbeis, 0. S.; Knierzinger, A,; Schipfer, R. J . Photochem. 1983, 21, 67.
2364 The Journal of Physical Chemistry, Vol. 89, No. 11, 1985
Strandjord et al.
TABLE 11: Static Fluorescence and Absorption Parameters of 3HF Derivatives in Various Solvents
I
2'4'
h
Methylcyclohexane 3HF 4' 3' 2' 2',4' 2',5'
29 498 29 239 29 359 30 675 30 769 30 902
19083 18975 19047 19084 19011 19048
1540 1494 1491 1905 1867 1934
3HF 4' 3' 2' 2'4' 2',5'
29 150 28730 29230 30580 30400 30670
24630 24390 24570 24752 24570 24691
4520 4340 4660 5828 5830 5979
18903 18903 18867 18957 19102 19047
1920 1967 1848 2345 2453 2677
3HF 4' 3' 2' 2',4' 2',5'
29036 28490 28735 30211 30084 30211
2-Chloroethanol 3888 4823 24213 23952 3575 4539 3898 4841 23894 24691 3597 5520 24095 3500 5989 3880 5940 24271
19047 19011 19011 19305 19474 19361
2119 2212 2148 2638 2812 2682
3HF 4' 3' 2' 2',4' 2',5'
29154 29053 28785 30599 30450 30571
Trifluoroethanol 24715 3200 4439 2972 4302 24691 24390 2764 4395 25000 3 120 5599 24420 3 108 6030 3326 5850 24721
20040 20060 20100 20449 20713 20449
2599 2910 2914 3410 3420 3092
Methanol 3306 3038 3167 3414 3797 3290
400
500
600
Wavelength (nm) Figure 3. Fluorescence spectra of 3 H F and its methylated derivatives in methanol at ambient temperature. The excitation wavelength was chosen to correspond to the absorption maximum of each compound (see Table 11). The primed numbers denote the position of the methyl sustitutents in each compound.
A
r-"
L
Y
\
absorption edge and (ii) a higher energy first-absorption maximum, (Table 11). The molar extinction coefficients at the absorption maximums for the six compounds are all in the range (1.8-2.6) X IO4 L/(mol cm-I) (not shown in Figure 2 or Table 11). The N (A, -408 nm) and T (A, -530 nm) fluorescence bands of these compounds (Figure 3) also exhibit a dependence on whether or not the compound is ortho substituted. In Table I1 we present data on the following emission parameters: the energy maximum of the N fluorescence (v;?), the full width at halfheight of N emission (fwhh,"), the energy maximum of the T band (v?;"), the full width at half-height of the T fluorescence - vEy). (fwhh,,), and the Stokes shift of the N emission The dependence of these parameters is summarized in Table 111, where we give average values for the compounds with ortho substitution (2' = Me) and no ortho substitution (2' = H). The largest variation is observed for the fluorescence Stokes shift of N (vgt- v;?), which increases with ortho substitution. The ortho effect on the Stokes shift has a large contribution from a shift
vz
(vz
d
-
03
0
1
I
1
4.0
3.5
I
I
I
4.5
I
5.0
1 / ~ x 1 0 ~ Figure 4. Arrhenius plots of the right side of eq 3. Each point is the average of five data sets and the sofid lines are linear least-square fits to those points. The compounds studied are 3 H F (O),3'-Me-3HF (m), 2'-Me-3HF ( 0 ) .4'-Me-3HF (A),2',5'-Me2-3HF (a), 2',4'-Me2-3HF
in Y$ and a smaller, but significant, contribution from an increase in YE?. Ortho-substituted molecules also tend to have broader N and T fluorescence bands.
TABLE 111: Reduced Static Fluorescence and Absorption Parameters of 2'-3HF Derivatives in Various Solvents
solvent methylcyclohexane methanol 2-chloroethanol trifluoroethanol
vr;,
WE:,cm" 2'= H 2'=CH3
cm-l 2'= H 2'=CH3
29 365 29036 28811 28997
24530 24015 24598
30 782 30550 30168 30540
24671 24352 24713
fWhhNn, cm-I 2 ' = H 2'=CH3 3170 3787 2978
3500 3693 3 184
vg - v$, 2'=H
cm-I 2'=CH3
4506 4734 4399
5879 5816 5826
VYF,cm-l fwhhTn, cm-l 2 ' = H 2 ' = CH3 2 ' = H 2 ' = CH3 19035 18891 19023 20066
19047 19035 19386 20539
1508 1911 2176 2807
1902 2491 2712 3307
TABLE I V Summarv of the Prowrties of 3HF and the 3HF Derivatives
3HF 3' 4' 2' 2',4' 2',5'
1.45 1.24 1.36 1.79 2.30 2.30
8.35 8.00 8.20 9.80 10.50 10.45
9.59 9.53 9.51 9.10 9.03 9.06
"Calculated by combining the data in Figure 4 with determined by picosecond spectroscopy.
T ~ - 'values
4 520 4 660 4 340 5 828 5 830 5 979
1621 1621 1620 1625 1623 1624
3 360 3 350 3 350 3 410 3 400 3 410
at ambient temperature; see ref 1 for further details. The
6.16 3.55 2.34 7.76 6.92 13.8 7B-' values
were
The Journal of Physical Chemistry, Vol. 89, No. 1I , 1985 2365
Proton-Transfer Kinetics of 3-Hydroxyflavones The solvent dependence of the spectra is less systematic. There does not seem to be a clear trend in solvent effects or a correlation between solvent effects and substitution for the N absorption bands and the N * and T* emission. Infrared, pK,, and N M R Measurements. A variety of experimental measurements show that the chemical properties of the six compounds fall into two groups, depending on whether there is an ortho methyl group substitutent. The pK, values in Table IV show that the ortho-substituted compounds are significantly less acidic. Proton N M R measurements reveal that the hydroxyl of the ortho-substituted compounds are shifted chemical shift (pH) downfield with respect to the other compounds in the hydrogenbond-accepting solvent, dimethyl sulfoxide (Me2SO). These data imply that the ortho-substituted compounds are less hydrogen-bond donating than the non-ortho-substituted flavonol^.^^^^ The infrared measurements on the carbonyl absorption energy (vc4) and the energy of hydroxyl stretch ( ~ 0are ~ particularly ) interesting. It has previously been shown that the infrared data for 3HF reveal that this molecule possesses a strong to moderate intramolecular hydrogen bond in CC14 as indicated by the following structure. The data we present here (Table IV) for
3'-Me-3HF and 4'-Me-3HF suggest that these molecules also have a strong to moderate intramolecular hydrogen bond in non-hydrogen-bonding solvents such as CC14. The data (vc4 and vOH) for the ortho-substituted compounds are also in the range for intramolecular hydrogen bonding. But, the frequency of the bands for the ortho compounds are significantly shifted toward the regions where non-hydrogen-bonded carbonyl and hydroxyl groups absorb.I0*" The conclusion we draw is that the ortho compounds possess a weaker intramolecular hydrogen bond than the other molecules. Proton-Transfer Dynamics. In the preceding paper in this journa1,l it was shown that a good measure of the slow excited-state proton-transfer (N* T*) rate constant, kslow,can be found in the right side of eq 2. f, is the fraction of slow yield of the T*
-
fluorescence, and T~ is the lifetime of T* emission. FB and FG are the integrated intensities of the N * (blue) and T* (green) emission, respectively. It was also shown in the preceding paper that the temperature dependence of kslowcan be determined from a simpler expression (eq 3), if fs is not significantly temperature dependent. In a
(3) separate article, we have shown that f, is negligibly temperature dependent for 3HF in MeOH and MeOD.I2 In the present study we assume that the same behavior holds for 3'-Me-3HF and 4'-Me-3HF. This assumption is not necessary, however, for the ortho-substituted compound which exhibits very little fast proton-transfer yield, Le., f, > 80%. Figure 4 portrays Arrhenius plots of the right side of eq 3 for the various derivatives. Activation energies E, from this plot are shown in Table IV. The ortho compounds exhibit a significantly greater activation energy than the three compounds that lack an (9) Dierckx, A,; Huyskens, P.; Zeergers-Huyskens,T. J. Chim. Phys. 1%5, 62, 336. (IO) Vinogradov, S. N.; Linnel, R. H. 'Hydrogen Bonding"; Van Nostrand Reinhold: New York, 1971. (1 1) Jose, C. I.; Phadke, P. S . ; Ramakao, A. V. Spectrochim. Acta, Part A 1974,30A,1199. Looker, J. H.; Kagal, S. A,; Edman, J. R. J. Heferocycl. Chem. 1966,3, 61. Chang, C. J. Lloydia 1978,41, 17. (12) Strandjord, A. J. G.; Barbara, P. F. Chem. Phys. Lett. 1983,98.21.
ortho substituent. The Arrhenius A factors depend on ortho substitution as well.
Discussion The results of this study show that ortho substitution of the phenyl ring of 3-hydroxyflavonols makes the N form less acidic and less hydrogen-bond donating. Ortho substituion also weakens the intramolelcular hydrogen bond. It seems reasonable, furthermore, to assume that the ortho derivatives are less planar than the unsubstituted molecules about the bond that joins the phenyl group to the rest of the molecule. A simple interpretation of the various effects we have observed is that resonance between the phenyl group and the ring of 3 H F favors a partial negative charge on the oxygen of the hydroxyl group and perhaps also favors a partial positive charge on the carbonyl group, as shown in the following structure. The ortho
compounds are presumably less planar due to nonbonding periplanar interactions and, as a result, are less stabilized and "polarized" by resonance. It seems likely that analogous effects should be operating for the N*, T*, and T forms of the flavonols (see below). Excited-State Displacements. The significant increase in fluorescence Stokes shift we observe for the ortho-substituted derivatives is strong evidence that the phenyl torsional angle is coupled to the modes that are most active in the electronic spectra of 3HF. It is not likely, however, that the Stokes shift (4500-5879 cm-l) is actually a direct manifestation of the torsional vibration which probably has a very low f r e q ~ e n c y . ' ~A more reasonable interpretation is that the phenyl angle affects the spectra indirectly by modulating the excited displacement of one or more unknown large frequency, totally symmetric vibrations of the N and N* forms of 3HF. By analogy to other aromatic molecules with T-T* transitions, the likely candidate for the active modes are ring breathing vibrations of the heteronuclear ring. The broader absorption edge and broader emission bandwidth of the more strongly Stokes-shifted ortho derivatives supports the hypothesis that the Stokes shift is due to a long Franck-Condon progression in the absorption and fluorescence of one or more modes with large excited-state displacements. Furthermore, the hypothetical coupling we propose here, between the phenyl torsion angle and the equilibrium geometry of the ring, is highly consistent with the pK,, NMR, and infrared trends we have already mentioned. In contrast to the large ortho effect found for the spectroscopic properties of the flavonols, the solvent dependence is generally smaller and unsystematic. This is observed in spite of the fact that N forms are probably hydrogen bonded with the solvent in TFE, 2CE, and ethanol.' The small solvent effect may be an indication that the intermolecular hydrogen-bond strengths for the N and N * forms are similar in magnitude. Proton-Transfer Mechanism. The simple fact that we observe a distribution of E, values for flavonol derivatives in the same solvent (MeOD) strongly supports one of the conclusions of the previous paper; that is, the proton-transfer dynamics are not due to the relaxation dynamics of the sovlent coordinates (see Introduction and ref 1). The discussion of the physical basis of the ortho effect on E, is not as straightforward. The ortho effect seems to suggest that the proton-transfer barrier increases as the phenyl group is twisted out of the plane of the large ring in the flavonols. ( 1 3) The torsional vibrational frequencies of phenyl-substituted aromatics are in the range 60-10 cm-I; see for example: Werst, D. W.; Gentry, W. R.; Barbara, P. F. J . Phys. Chem. 1985,89,129.
J . Phys. Chem. 1985,89, 2366-2372
2366
However, the source of this trend is subtle. In the model we have proposed1J4 for the proton-transfer dynamics of 3HF, E, is associated with thermal excitation of coordinates involving the ROH- -3HF intermolecular hydrogen bond as represented in Figure 1. The ortho effect described in this paper can be rationalized by Figure 1 if it is assumed that the orthosubstituted derivative possesses a stronger ROH- -3HF hydrogen bond. Unfortunately, we cannot address this issue directly, because we lack data on the hydrogen-bond-accepting ability of N*.It is reasonable, however, that the hydrogen-bond-accepting ability of N* would exhibit an ortho effect, since we have already shown that the hydrogen-bond-donating ability of the ground-state for N shows a clear ortho effect. It is interesting in this regard to note that the E, vs. the A values are roughly correlated for the different derivatives in this paper and roughly correlated for the different solvents in the preceding paper.’ This suggests that the ortho effect and the hydrogenbond-donating solvent effect1 modulates the effective potential for the proton transfer in a similar fashion, namely, by increasing the ROH- -3HF hydrogen-bond well depth. We recognize, however, that the ortho effect on E, may be partly or wholly due to a modulation of energetic factors associated primarily with the vibrations of the flavone portion of the ROH-3HF complex. Further experiments will be necessary to sort out the complex effects that may be involved in the overall protontransfer process. Our results reveal that the proton-transfer activation energy is a function of the torsional angle of the phenyl group. It is not clear, however, whether the proton-transfer reaction coordinate actually involves a component of phenyl torsional motion (eq 4). The torsional coordinate might participate if the torsional potential was shallow enough, Le., with a torsional barrier that is comparable (14) The model in Figure 1 has some of the elements of a general model thai was previously pro& in the paper: McMorrow, D.; Kas& M. J. Phys. Chem. 1984,88, 2235.
to the observed activation energies. This problem will not be resolved until an accurate determination of the torsional potential is made.
Conclusions and Summary The excited-state intramolecular proton-transfer dynamics and other photodynamical and chemical properties of five methylated derivatives of 3-hydroxyflavone have been studied. Measurements of a variety of properties, including the pK, of the N form, the N M R chemical shifts of the hydroxyl proton of the N form, the fluorescence Stokes shift of the N* emission, and the protontransfer activation energy, E,, reveal that the compounds fall into two groups depending on whether the ortho position of the phenyl ring is substituted with a methyl group. The ortho compounds are less basic, and less hydrogen-bond donating. They also exhibit a weaker intramolecular hydrogen bond and a E,. The trends generally support the hypothesis that the proton-transfer mechanism is dominated by energetic factors of a hydrogen-bonded complex between the 3HF and the solvent rather than relaxation dynamics of the solvent configuration. Acknowledgment. Acknowledgment is made for partial support of this research to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the National Science Foundation (Grant No. CHE-8351158). We thank Prof. T. R. Hoye for assistance with the synthesis and characterization of the compounds described in this paper. Registry No. 2’-Me-3HF, 9591 1-70-9; 3’-Me-3HF, 9591 1-71-0; 4‘Me-3HF, 19275-68-4; 2’,4’-Me2-3HF, 9591 1-72-1; 2’,5’-Me2-3HF, 9591 1-73-2.
Pressure Saturation of the Collisional Quenching of the 3B1State of SO2. 2. S. J. Strickler* and Robert D. Ito Department of Chemistry, University of Colorado, Boulder, Colorado 80309 (Received: November 8, 1984)
Measurements are reported of the temperature dependence of collisional quenching of the ,B1state of SO2and of the emission spectrum at high pressure. These results are compared with predictions of the kinetic model and the radiationless transition model suggested by Strickler and Rudolph to explain the saturation of quenching at high pressures. The models and their possible modifications are discussed further. It is concluded that the collision-induced intersystem crossing model is most likely to be the correct explanation for the collisional quenching.
Introduction The )B1state of sulfur dioxide is of interest because it may play an important part in atmospheric photochemistry and also for its usefulness in testing theories of radiationless transitions in small molecules. In studies of the quenching of the ,B1state, the decay mechanism has been interpreted in terms of the following procewes:
-so2+
,so2
kP
,SO2 + M
-
h ~ ,
SO2 + M (2) Unimolecular nonradiative processes are not expected to be important in such a small molecule, and indeed the zero pressure limit of the quantum yield of phosphorescence following direct k,,
0022-3654/85/2089-2366$01.50/0
excitation of the triplet state has been shown to be unity within experimental error.’ Since the ’B1state has about 25 000 cm-’ of excess electronic energy, it may be expected to be reactive. One known reaction is
+
+
3s02 SO* A SO, SO
(3)
The rate of this chemical quenching must be added to that of the physical quenching of eq 2. On the basis of reactions 1-3, the lifetime of the 3BIstate would be expected to be given by 7-’=
k , + (kqso, + k,)Pso,
(1) Su, F.; Calvert, J. G. Chem. Phys. Lett. 1977, 49, 305.
0 1985 American Chemical Society
(4)
