5432
J . Phys. Chem. 1989, 93, 5432-5431
Time-Resolved Resonance Raman Studles of Terephthalic Acid Anion Radicals' Ling Qin? G . N. R. Tripathi,* and Robert H. Schuler Radiation Laboratory and Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received: January 20, 1989)
Time-resolved optical absorption and resonance Raman spectroscopies have been used to examine the radicals produced on the microsecond time scale by electron addition to terephthalic acid. In acidic solutions the radical exists predominantly in its monoanionic form with both carboxyl groups fully protonated. In basic solutions the carboxyl groups deprotonate with pK,s of 7.2 and 9.3, Le., 4-5 units higher than for the parent acid. The electronic spectra of the three forms of the radical are similar, all having intense absorptions at -360 and -530 nm. The raonance Raman spectra of these radicals are, however, sufficiently different to allow structural identification and examination of their interconversion. Prominent bands in the resonance Raman spectra in the 161Ck1640-cm-' region are attributed to the v8, CC ring stretching mode. The v1 ring breathing vibrations, which are observed in the 795-830-cm-' region, are strongly enhanced with excitation at 360 nm but not at 530 nm. While the ring vibrations are affected very little on electron addition to terephthalic acid or its dianion, there is drastic change in the vibrational frequencies of the carboxylic groups, showing that the extra charge is largely confined to the antibonding orbital of the carboxyl group. This localization of the charge on the substituent groups explains, qualitatively, the relatively high reactivity of hydrated electrons toward aromatic carboxylic acids as compared to benzene and requires the unpaired spin population to be largely distributed over the aromatic ring, in agreement with ESR data. The bonds linking the carboxylic groups with the phenyl ring are similar in strength (qa 1400 cm-I) to the CO bonds in p-benzosemiquinone radical anion and have a bond order of -1.5.
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Introduction The transient electron adducts of terephthalic acid can be conveniently generated by pulse radiolytic reduction of terephthalate ion in aqueous solution. Previous ESR studies3 have definitively shown that mono- and trianion radicals (TH2'- and T*3-) are formed in mildly acidic (pH -4.5) and strongly basic (pH > 10) solutions, respectively. In these ESR studies the radical dianion (THO2-), which this study shows is the dominant form at pH -8, was not detected, presumably because of line broadening by proton exchange on the time scale of the hyperfine splittings (-lo-' s). Optical absorption studies (vide infra) show that the different forms of the radical have similar optical spectra. Since the vibrational structure of the transient intermediates is not affected by the relatively slow proton-exchange processes, Raman spectroscopy offers the possibility of comparing the populations of the different forms of the radical directly. In this paper we report time-resolved resonance Raman studies of the transient electron adducts of terephthalic acid over the pH range 4-1 1. Fingerprint spectroscopic evidence for the existence of the radical in its dianionic form at pH -8 is given for the first time. The Raman data on the monoanionic and trianionic forms of the radical, which dominate at low and high pH, respectively, show the ring and C-C02/C02H bond structures to be very similar. These Raman data also show that the added charge resides on the carboxylic groups and not on the ring which explains the high reactivity of electrons toward terephthalic acid but not benzene. The systems studied here serve as references for understanding the structures of other more complex aromatic acid radical systems. Experimental Section Solutions of the disodium salt of terephthalic acid (Aldrich), [ ~ i , a ' - ' ~ terephthalic C~] acid (MSD), or terephthalic-d4 acid (Sigma) were prepared in water purified in a Millipore Milli Q system. D 2 0 (>99%) was used when the radicals with deuterium-substituted carboxyl groups were to be examined. The pH was adjusted with HCIO4 or KOH. Phosphate buffer was used in solutions near to neutral. pH measurements, made with an (1) From the Ph.D. Dissertation of L. Qin, University of Notre Dame, 1988. The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-3055 from the Notre Dame Radiation Laboratory. (2) Present address: Department of Biochemistry, University of Illinois, Urbana-Champaign, IL. (3) Neta, P.; Fessenden, R. W. J . Phys. Chem. 1974, 78, 523.
0022-3654/89/2093-5432$0l.50/0
Orion 8 11 pH meter calibrated with Fisher reference buffers, were accurate to f0.02. The solution was purged of oxygen with N2 before and during the experiments. In most cases the radicals were produced by reaction of terephthalic acids with aqueous electrons generated pulse radiolytically. tert-Butyl alcohol (Baker) or methanol (Aldrich) was added to scavenge OH radicals. The radical trianion was also produced in strongly basic solutions by electron transfer from acetone ketyl radical prepared radiolytically from isopropyl alcohol. In this case the radicals were at higher concentrations and exhibited considerably longer lifetimes because of the absence of counter radical^.^ The optical absorption spectra of the terephthalate radicals were recorded by the spectrophotometric pulse radiolysis methods described previou~ly.~A 5 4 s electron pulse from a 8-MeV linear accelerator was used to produce a radical concentration of -3 pM. Extinction coefficients were determined by reference to the thiocyanate dosimeter.6 Kinetic traces were recorded on the nanosecond time scale with a Biomation 6500 (2 nsjchannel) and on the microsecond time scale with a Biomation 8100 (10 ns/ channel). The pulse radiolysis time-resolved resonance Raman experiment has been described in detail elsewhere.' A 100-ns pulse of 2-MeV electrons from a Van de Graaff accelerator was used to produce a radical concentration of lo4 M. The radicals were probed with a pulsed excimer pumped dye laser operated at a wavelength within an absorption band of the transient of interest (usually 360 or 530 nm). The scattered light was dispersed by a 0.85-m spectrograph (1 10 mm, 1800 groovesjmm grating) and detected with a Princeton Applied Research optical multichannel analyzer (OMA 111) using an intensified photodiode array gated in synchrony with the Raman signals. Time resolution was obtained by a delay between the accelerator and laser pulses. Signal averaging was employed extensively with experiments being carried out at a repetition rate of 7.5 Hz. The Raman data accumulated in the OMA were then corrected for background, smoothed, and plotted off line. The Raman frequencies of the transients were measured by reference to the Raman spectra of ethanol, cyclohexane, or acetonitrile measured under similar experimental
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(4) Qin, L.; Madden, K. P.; Schuler, R. H. J . Phys. Chem. 1988,92,3790. (5) Patterson, L. K.; Lilie, J. Int. J . Radiat. Phys. Chem. 1974, 6, 129. (6) Schuler, R. H.; Patterson, L. K.; Janata, E. J . Phys. Chem. 1980,84,
2088. (7) Tripathi, G. N. R. In Multichannel Image Detectors. II; Talmi, Y., Ed.; American Chemical Society: Washington, DC, 1983; Vol. 236, p 171. Tripathi, G. N. R.; Schuler, R. H. J. Chem. Phys. 1984, 81, 113.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 14, 1989 5433
Terephthalic Acid Anion Radicals
--
30000
'E
w
-z
'z
I pH = 8.3
0
n
Z
a
I-
w
m
LL
0
Y
U
.
v,
Ez
m
a
E? I-
W
2 la
V
z t X W
0 300
500
400
600
-I W U
WAVELENGTH (nm)
I
Figure 1. Absorption spectra of the transient radical anions observed at microsecond time scale on electron addition to terephthalate at pH 13.5 (0),8.3 (A),and 4.6 ( 0 )(see text).
co;
I
-
+H'
pK,
-
4.34
to;
1
+
CO2H
CO2H
I
eiq
1
1
I
20
I
I
40
TIME
I
I
I
80
60
- microseconds
I
*H'
pK,
- 3.5
W
D
0
z a m a
0 v)
m
e,
a 72 *H' 4
pK,
-
D
7.2
$1;
W
I I-
U J W
a
L
TT3
I
Figure 3. Kinetics of the first protonation at pH 8.3.
to; +
I 0
L
I
I
I
0
I
2
3
TIME ( p s )
TH?
Figure 2. Reaction scheme.
Figure 4. Kinetics of protonation at pH 7.2.
conditions. For well-resolved bands these frequencies are known to -1 cm-I.
diffusion controlled in spite of the negative charge on the terephthalate. The Raman data reported below show that this radical sequentially protonates to form its dianion in mildly basic solutions and its monoanion in acidic solutions. The reaction scheme indicated by these various observations is illustrated in Figure 2. In this case, because one starts with the trianionic form of the radical, the acid-base equilibria have to be examined by monitoring the protonation rather than the deprotonation processes. One sees in Figure 1 that the dianionic and trianionic forms of the radical have similar extinction coefficients at 362 nm. At pH 8.3, where both of these forms are expected, the buildup and decay of the radical system can be followed by monitoring the time response of the absorption signal at this isosbestic point, as is illustrated in Figure 3. The dianionic form absorbs somewhat less strongly at 368 nm so that the trace at this wavelength exhibits an initial cusp, reflecting the first protonation step with a period of 1.5 KS. The pK, for deprotonation of the dianion was determined to be 9.3 from the pH dependence of the amplitude of this cusp. At pH -8 the pseudo-first-order rate constapt for protonation is -4.6 X lo's-I. Taking this value, together with the equilibrium constant, we estimate the rate constant for deprotonation of the radical dianion by base to be 2.3 X 1 O l o M-' SI. This value is very reasonable, indicating that a t pH 8 protonation of the trianion is mostly by reaction with water and that reaction with the buffer is not important. At pH 4.6 protonation of the trianion is more rapid and catalyzed by phosphate (500 ns at 1 mM phosphate and 80 ns at 38 mM phosphate). In acidic solutions protonation of the trianion occurs directly by reaction with H+ ( k 5 X 1Olo M-l s-l) or with H2P04- ( k 3 X lo9 M-I s-l). The pK, for deprotonation of the radical monoanion was determined similarly to be. 7.2 by following the cusp at 350 nm where the dianion absorbs somewhat more strongly than does the monoanion. In this case high concentrations of phosphate (-20 mM)
Spectrophotometric Studies
In order to optimize the experimental conditions for the Raman studies, the absorption spectra were first examined by pulse radiolysis methods. Spectra recorded at pH 13.5, 8.3, and 4.6 are given in Figure 1. In the following discussion it is shown that these spectra represent respectively the trianionic, dianionic, and monoanionic forms of the radical. A similar spectrum for the radical trianion was reported by Gordon et al.? but there is no information in the literature on the spectra of the protonated forms in aqueous solution. It is noted that all three forms of the radical have strong absorption bands at -360 nm and somewhat weaker ones at -530 nm. An absorption spectrum of the terephthalic acid radical anion in methyltetrahydrofuran (MTHF) glass at also shows bands in the 360- and 77 K, obtained by Shida et 530-nm regions with, because of the low temperature, vibrational structure that is better resolved than in aqueous solution at room temperature. In basic solution terephthalic acid is completely ionized so that the terephthalate trianion radical (TO3-) is expected to be formed initially on reduction of terephthalate by eaq-. This expectation is supported by both the absorption and Raman observations reported in the present study. The rate for reaction of eaq-with terephthalate dianion was determined spectrophotometrically at pH 12 by following the pseudo-first-order decay of the hydrated electron at 650 nm and the complementary growth of the radical trianion at 360 nm. The second-order rate constant, 8.5 X lo9 M-' s-I, which is only slightly higher than the value of 7.3 X lo9 M-I s-I previously reported,8 shows the reaction essentially to be (8) Gordon,S.; Hart, E.J.; Thomas, J. K.J . Phys. Chem. 1964, 68, 1262. (9) Shida, T.; Suehiro, I.; Imamura, M. J . Phys. Chem. 1974, 78, 741.
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The Journal of Physical Chemistry. Vol. 93, No. 14, 1989
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0
I\ \UI
m
n
f
(b)
+
J
m
I700
1200
-
700
RAMAN SHIFT crn-l Figure 6. Resonance Raman spectra a t 530 nm: (a) T'3-, (b) TH2'-.
TABLE I: Comparison of the Raman Frequencies (cm-') of Terephthalate (T2-), Its Electron Adducts (T&, TH2-, TH2'-), and p-Benzosemiquinone Radical Anion (PBsQ'-)" ,
I
1600
I
O
I
,
1200
1400
I
I
I
1000
RAMAN SHIFT
t
800
I
,
600
- cm-1
Figure 5. Resonance Raman spectra at 360 nm: (a) T'3-, (b) THa2-, (c) THIO-.
must be used to catalyze the relatively slow final protonation step. The complexity of these studies is illustrated in Figure 4 where a slight dip in the absorption signal at 368 nm, observed at 0.4 ps, manifests the initial formation of the dianion followed by the somewhat slower formation of the monoanion. In Figure 3 these radicals are seen to decay on the 100-ps time scale as the result of second-order processes having rate constants of the magnitude of lo9 M-' s-' . During the course of these studies it was found that when the radical trianion was prepared a t pH 13 by electron transfer from acetone ketyl radical, it decayed much more slowly, Le., only on the millisecond time scale. The rate constant for electron transfer from the ketyl was determined to be 1.73 X lo7 M-' s-' from the growth of the absorption at 360 nm. In the presence of N,O-saturated solutions containing 0.1 M isopropyl alcohol, essentially all of the primary radicals are converted to the acetone ketyl radicals and subsequently to terephthalate radical trianions. In this case other types of counterradicals are virtually absent, and it is clear that the decay noted in Figure 3, where tert-butyl alcohol was used to remove OH radical, results mainly from the cross-reaction with the radicals from tert-butyl alcohol. The second-order reaction between terephthalate radical trianions is intrinsically slow (Clo8 M-' s-' 1. This slow decay rate means, of course, that relatively higher steady-state radical concentrations can be maintained during irradiation. Advantage has been taken of this fact in ESR experiments where the trianion radicals containing I3Cat the natural abundance level were readily ~ b s e r v a b l e . ~Unfortunately, the isopropyl alcohol radical (the acidic form of the ketyl) does not undergo a similar electron transfer so that this approach cannot be used below pH 12.
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Resonance Raman Studies
The Raman spectra of these radicals excited at 360 and 530 nm and observed on the microsecond time scale are given in Figures 5 and 6. The Raman frequencies are summarized in
T.3-c
TH.2-d
1609 1610vs
T2-b
1622 vs
1 1 28 1422 w (vw) 1362 w-m
1170 1182w 824 828 vs (w)
1186 w 827 vs
1570 sh 1413 1107 m-s (s) 1134 w 1063 w 739 680 w 770 m
TH2'-e 1639 vs
PBSQ'-f 1620 vs
1404 (w)
1435 m
1179 w (s) 798 vs (w) 1292 w, b 1605 (sh) 1079 w (m) 648 (m) 1471 w
1161 m 831 vw
approx mode description v8, (ring CC stretch) U1a
(C-X
stretch)g w9. (CH bend) u1 (ring breath) u3 (CH bend) C=O stretch C 0 2 stretch C-OH stretch 6 C 0 2 (scissor) 6 OH (planar bend)
O v = very, s = strong, m = medium, w = weak, sh = shoulder band (360-nmspectra). (The band intensities at 530 nm, when different, are given in parentheses.) bTerephthalate dianion, see ref 10. CThecombination and overtone bands are observed at 3220 (1610 X 2), 3030 (1610 + 1422), 2784 (1610 + 1182), 2709 (1610 + 1107), 2432 (1610 + 828), 2277 (1610+ 680), 2242 (1422 + 828), 2007 (1182 + 828), 1935 (1107 + 828). 1656 (828 X 2), and 1502 (929 + 680) cm-I. dThe spectrum was obtained at 360 nm only. eThe combination and overtone bands are observed at 3270 (1639 X 2), 2432 (1639 + 798), 2379 (1581 + 798), 2092 (1291 + 798, 1980 (1179 + 798), 1877 (1079 + 798), and 1600 (798 X 2) cm-I. fReference 13. EX = C02, C 0 2 H , or 0.
Table I, and a number of combination and overtone bands, observed in the 1800-3300-~m-~region, are listed as footnotes. Comparison is made in the table with the frequencies of the comparable vibrational modes of the parent moleculesI0 and p benzosemiquinone radical Is is particularly noted that the bands at -800 cm-I, which are very weak in the spectra excited at 530 nm, become very intense with excitation at -360 nm. The Raman spectra of radical anions prepared from several isotopically substituted terephthalic acids have also been obtained to assist in the spectral assignments, and the frequencies observed for the radical trianion are given in Table 11. In general, the (10)Tripathi, G.N. R.; Sheng, S. J. J. Mol. S t r m . 1979, 57, 21. (1 1) Tripathi, G.N.R. J . Chem. Phys. 1981, 74, 6044. Tripathi, G.N. R.; Schuler, R. H. J. Chem. Phys. 1982, 78, 2139. (12)Schuler, R. H.; Tripathi, G. N. R.; Prebenda, M. F.; Chipman, D. M.J . Phys. Chem. 1983,87, 5337. (13) Tripathi, G.N.R.; Schuler, R. H. J. Phys. Chem. 1987, 91, 5881.
Terephthalic Acid Anion Radicals
The Journal of Physical Chemistry, Vol. 93, No. 14, 1989 5435
TABLE 11: Raman Frequencies (cm-') of Isotopically Substituted Terephthalate Trianion Radicals (T'*)" T'3[a,a'J3C2]T-3T"--d, 1610 vs 1609 vs 1586 vs 1422 w 1182 w 1107 m 828 vs 680 w
1388 w 1183 w 1106 m 823 vs 680 w
1417 w 846 s 1115 vw 817 vs 660 vw
360-nmexcitation,
growth and decay of the Raman signals of the individual radicals are completely consistent with the absorption studies. At pH 8 the initial spectrum (at 100 ns) is, for example, that of the radical trianion. This spectrum converts to that of the dianion on the microsecond time scale. Similarly, at pH 5 the spectrum of the radical dianion is observed at short times and that of the monoanion grows in with a period -2 ps. Terephthalate Radical Trianion ( T3-).The Raman spectra recorded at 0.2 ps and later times after the pulse irradiation of solutions at pH 13 (Figures 5a and 6a) are attributed to the terephthalate trianion radical. The bands at 1610, 1422, 1182, 1107, 828, and 680 cm-' are assigned to fundamental vibrations. The shoulder at 1656 cm-' in the 360-nm spectrum is clearly the first overtone of the 828-cm-' vibration in that it is not apparent in the spectrum excited at 530 nm where it is not expected because the 828-cm-I band is weak. This band shifts to 1635 cm-I on ring deuteriation as the 828-cm-' band shifts to 817 cm-' (Table 11). The very weak band at 1522 cm-' is the combination of the fundamentals at 828 and 680 cm-I. The corresponding band in the radical deuteriated on the ring (T'3--d4) is too weak to be observed because of the reduced intensity of the 8 17-cm-' band. Since the 360- and 530-nm absorption bands of this radical are quite intense, only totally symmetric vibrations are likely to be observably enhanced in the resonance Raman spectra. Assuming that the T3-radical has D2h symmetry, only seven such vibrations are expected below 1700 cm-I. The va planar ring distortion mode, which generally appears in the low-frequency region (e600 cm-I),14 is not apparent and appears to be not appreciably resonance enhanced. The six fundamentals observed must therefore be attributed to (using Wilson notations) the vEa (CC stretch), vga (CH bend), and vI (ring breath) phenyl modes, the v~~(in-phase C-C02 stretch) mode, and the v, C 0 2 (in-phase symmetric C O stretch) and 6 COz (in-phase planar COz bend; scissor) modes of the carboxyl groups. Although the vibrational frequencies observed for this radical and its parent]' are similar (see Table I), the extra electron likely occupies an antiboding ( T * ) orbital so that bond properties will change and one cannot base assignments strictly on one-to-one correspondence between the vibrational frequencies of the terephthalate dianion and the trianion radical. The principal questions here are as to the assignments of the bands observed in Figures 5 and 6 at 1422, 1182, and 1107 cm-I. The 1610-cm-' fundamental represents a phenyl mode and can be assigned, with certainty, to V E , as no other phenyl vibration of a, symmetry is expected at this frequency. This band does not shift in the radical containing I3CO2(see Table 11) which indicates that this mode does not involve significant motion of the carboxylic group. The shift observed on ring deuteriation, to 1586 cm-I, is similar to the 23 cm-l found for the vEavibration on deuteriation of p-benzosemiquinone radical anion.I2 Similarly, the 828-cm-l band, which shifts by -5 cm-l in the I3C species, can be safely assigned to vl. Assignment of the 680-cm-' band in the T O 3 - spectrum to the 6 C 0 2 scissors mode is straightforward, as no phenyl ag vibration occurs in this frequency region.I4 The corresponding vibration in the parent molecular anion has been identified at 739 cm-l, indicating an appreciable reduction in frequency (59 cm-I) on the formation of the radical. This comparison indicates that the extra
-
( 14) Varsanyi, G. Vibrational Spectra of Benzene Derivatives: Academic: New York, 1969.
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charge is most likely on the carboxylic groups. In that case the C O bond order will change from 1.5 in the parent molecule to 1.25 in the radical as the result of equal contributions from resonance structures A and B. The v, C 0 2 vibration, observed
-
B
A
at 1413 cm-' in T2-, should therefore appear at a considerably lower frequency in T'3-. The C-C02 bonds, on the other hand, acquire a bond order of 1.5 so that the v7a frequency is expected to be. similar to that observed in semiquinone anion type radicals, i.e., appear in the 1400-cm-' region. The 1422-cm-' band in T3-, therefore, must be assigned to v7a which is observed at 1128 cm-' in terephthalate. The frequency of this vibration drops to 1388 cm-' in the 13C species but is very little affected by ring deuteriation. One of the two prominent bands at 1182 and 1107 cm-I in the T*3-spectrum must be assigned to the v, C 0 2 mode and the other to the vga C H bending mode. In the deuteriated species we observe a weak band at 1115 cm-' and a strong band a t 848 cm-' which clearly represent the v, C 0 2 and vga (CD bend), respectively. This 1115-cm-' band may be correlated with either of the two bands at 1182 or 1107 cm-' in the protonated radical. In the latter case these bands are much more intense, which suggests that the C 0 2 stretching and C H bending motionsI5 are strongly coupled. This mixing may be the reason why there is very little effect of 13Csubstitution on these frequencies. Because TH'" and TH2'- also have bands at 1 180 cm-I, we assign the higher frequency (1 182 cm-I) to the vga (CH bend) and the lower one (1 107 cm-I) to the vs C 0 2 mode. Terephthalic Acid Radical Anion (TH2'-). Below pH 5 Raman studies show that the radical exists in its diprotonated form at times longer than -10 ps. Because of the lower point group symmetry (C2hfor the trans isomer and C, for the cis isomer), the number of possible totally symmetric vibrations in this radical is large compared to the trianion radical. All are, however, not necessarily resonance enhanced. The reduced symmetry also leads to a mixing between the normal vibrations of the D2h symmetry. The prominent band at 1639 cm-I is readily assigned to vga and the band at 798 cm-', which is intense in the 360-nm spectrum but only weak in the 530-nm spectrum, to the v I mode. The band at 1179 cm-I, which shifts to 854 cm-' on ring deuteriation, can be unambiguously assigned to vga. The band at 1079 cm-l, which appears at almost the same frequency in TH2*--d4but shifts to a slightly lower frequency (1072 cm-') in the I3C species, can be assigned to the in-phase C-OH stretching mode. This frequency is 28 cm-I lower than the v, C 0 2 frequency in TO3-, which is as expected. The carbonyl C=O stretching vibration in the radical is likely to appear in the 1500-1650-cm-' region. In the 360-nm spectrum, the shoulder band at 1600 cm-' is clearly an overtone of the 798-cm-' vibration. However, an underlying band at 1605 cm-', which becomes apparent in the 530-nm spectrum where the overtone band of the v 1 vibration should not contribute, can be assigned to the carboxyl C O stretch. This vibration appears at 1570 cm-l in the radical with 13C substitution, at 1574 cm-' in the radical deuteriated on the ring, and at 1580 cm-I in the radical deuteriated on the carboxyl groups. It should be noted that the us, ring vibration is somewhat sensitive to 13Csubstitution (1628 cm-') but not to deuteriation of the carboxyl groups (1631 cm-I). This indicates that the phenyl CC and carbonyl C = O stretching motions are strongly coupled in the THz'- radical. The very weak band at 1471 cm-I in the 360-nm spectrum, which shifts to 1025 cm-l on deuteriation of the carboxylic groups, can be attributed to the 6 O H (planar bend) motion. A weak band at 1404 cm-I, which is clearly discernible in the 530-nm spectrum of TH2'-, is tentatively correlated with the v7a band at 1422 cm-' in T'3-. In the 530-nm spectrum of the radical, we observe two broad bands at 1527 and 1292 cm-', while only the 1292-cm-' band appears in the 360-nm spectrum. These bands shift to 1510 and 1310 cm-I,
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~~
~
~~
(15) Tripathi, G.N. R.: Schuler, R. H. J . Phys. Chem. 1988,92, 5129.
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The Journal of Physical Chemistry, Vol. 93, No. 14, 1989
Qin et al.
respectively, in TD2'-. We tentatively assign them to the V8b ( c c stretch) and the u3 (CH bend) vibrations, which are totally symmetric under c 2 h symmetry but not in DZh.lo The 648-cm-' band in the TH2'- radical, which is observed only on 530-nm excitation and shifts to 624 cm-I in TD2'-, probably represents the u6b (CCC bend) mode although it may involve some 6 C 0 2 component. Because of the low molecular symmetry, the phenyl and carboxylic motions are very much mixed in this radical. It is difficult, therefore, to ascertain the extent to which the changes in the vibrational frequencies from the parent molecule result from localization of the electronic charge on the carboxylic groups. However, the large shift in the carboxylic group frequencies in the TH2'- radical, as compared to the TH2 molecule,10 clearly suggests that the extra electron is largely on the carboxylic groups. Terephthalate Radical Dianion (TIT"). The Raman spectrum of the TH'2- radical was recorded at pH 8.3 and at 40 p s . At this pH there is 10% contamination of the spectrum by contributions from the monoanion and trianion radicals. Because of the low molecular symmetry (Q, the normal modes in this radical are more complex than in the other cases. The intense bands at 1622 and 827 cm-l can, however, be assigned readily to the and u l vibrations. Similarly, the assignment of the 1186-cm-l band to uga is certain, based on its shift to 849 cm-I in the radical deuteriated on the ring. We observe two bands in the 1050-1 1 5 0 - ~ m -region ~ at 1134 and 1063 cm-I, as compared to only one band at 1107 cm-I in the T*3-and at 1079 cm-' in the TH2' spectrum. The 1 134-cm-' band is tentatively attributed to the us C 0 2 (symmetric C O stretch) and the 1063-cm-' band to the C-OH stretching vibration. The C 4 stretching vibration, characteristic of the C 0 2 H group, can, in all probability, be assigned to the shoulder band at 1570 cm-l. The band at 1365 cm-I shifts to a higher frequency (1384 cm-') on ring deuteriation and to a lower frequency (1357 cm-I) on deuteriation of the carboxylic proton which indicates that this vibration is very much mixed with the 6 O H and ring C H bending motions. The frequency drops to 1337 cm-I in the I3C species. In this respect, the 1365-cm-' vibration resembles the 1422-cm-' us C-C02 stretching vibration of the T3-radical. We tentatively assign the 1365-cm-I band in TH'2- to the u7a mode which involves stretching motion of the C-C02 and C-C02H bonds. It appears that these bonds are of similar strength. In the radicals of the type p-Y-C6H4-X', where the C-X and C-Y bonds are nonequivalent, the (CH bend) vibration (- 1000 cm-I), which is symmetry forbidden under the c 2 h and D2h point groups, is generally resonance e n h a n ~ e d . ' ~ Absence of this band in the THO2- spectrum is consistent with the assumption that the C-C02 and C-C02H bonds are of comparable strength. Since the vibrational frequencies of the COz and C 0 2 H groups in this radical are almost identical with those oband TH2'- radicals, it appears that the additional served in the T3electron is almost equally distributed between the C 0 2 and C 0 2 H groups.
structure of their radical anions. While eaq- reacts only slowly with benzene or hydroquinone, which provide less energetically favorable vacant phenyl a* orbitals for its occupancy, it reacts with aromatic carboxylic acids at diffusion-controlled rates. The resonance enhancement of the totally symmetric vibrations in the resonance Raman spectra depends on the Franck-Condon overlaps between the ground and the excited electron state. For this reason, the vibrational modes that appear prominently in the electronic spectra are strongly resonance enhanced in Raman. In the absorption spectrum of the terephthalic acid anion radical in the 360-nm region, the vibronic band corresponding to the ex~ cited-state frequency -800 cm-' is very p r ~ m i n e n t .Another vibronic transition involves an excited-state frequency of 1500 cm-I. In the 530-nm absorption spectrum, the vibronic transition corresponding to the excited-state vibration 1400 cm-' is relatively stronger than the transitions associated with the frequencies -600 and 1000 cm-l. However, there is no vibronic transition of intensity comparable to that of the --800-cm-' transition in the -360-nm spectrum. These differences in the vibronic structures are reflected in the resonance Raman spectra of the radical excited at 360 and 530 nm. While the vibrational frequencies of the carboxylic groups are most profoundly affected on forniation of the radical anions, the phenyl vibrations are relatively more strongly resonance enhanced in the Raman spectra at 360 and 530 nm. This comparison shows that, on electronic excitation, it is the ring structure, and not the carboxylic bonds, which is significantly altered. Unfortunately, the theoretical data currently available in literature are not sufficient to provide more than a qualitative discussion of the nature of the 530- and 360-nm transitions based on the observed resonance enhancement patterns. However, from the strong resonance enhancement of the V8a and u I vibrations in the 360-nm Raman spectra, it is clear that transition involved results in overall expansion of the ring, while the strong resonance enhancement of the uga vibration in the 530-nm Raman spectra suggests that, in the case of this transition, there is a significant elongation of the ring structure along the long molecular axis.
Electronic Structure, Enhancement Patterns, and Electronic Transitions
The resonance energy between the structures C and D is fairly high (24 kcal/mol).I7 The C-OH stretching frequency in terephthalic acid appears at 1295 cm-I,l0 which shows that the C-OH bond has significant double-bond character, Le., that resonance structure D contributes significantly to the molecular description. In the radical anion, however, the C-OH stretching frequency drops to 1079 cm-I, indicating that a resonance structure of type D, with positive charge on the hydroxyl oxygen, contributes less importantly to the molecular structure, Le., that the OH bond is more like that in the alcohol radicals where the pKas are 11-1 2.
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-
It is clear from the preceding discussion that the vibrational data on these radical anions are consistent with the ground electronic state structure resulting from the resonance between the valence-bond structures of types A and B, which implies that the additional electron is largely localized on the oxygen atoms. The p electron on the carboxyl carbon in the radical anions forms a P bond (bond order -0.5) with the p electron on the adjacent carbon atom, and, as a result, the spin density is largely transferred to the ring, as is evident from the ESR data.4 The conclusion that the extra electronic charge is mostly on the carboxylic groups and not on the ring also finds support from a theoretical treatmentI6 which shows the vacant P* orbital of the carboxylic group to be lower in energy than the lowest vacant orbital of the phenyl group, thus favoring electron addition to the carboxylic group. There is an obvious relationship between the reactivity of hydrated electrons toward aromatic carboxylic acids and the electronic (16) Nagakura, S.; Tanaka, J. J . Chem. Phys. 1954, 22, 236.
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-
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The Acidity Constants
In an earlier section, we noted that the proton dissociation constants (pKas) of the terephthalic acid radicals are 4-5 units higher than in the parent acid. These changes in the acidity constants can be explained, qualitatively, in terms of the information the vibrational spectra provide on the structure of the radical anions. The aromatic carboxylic acids are considerably more acidic than the alcohols because of the resonance between the following structures:I7 R-C'
U '
RC -:;
'OH
C
'OH
D
-
Comparison with Semiquinone Radicals
The similarity between these radicals and p-benzosemiquinone radical anion in Table I is not surprising since an appreciable fraction of the unpaired spin is on the ring and that of the charge is on the substituent in both type of radicals. As implied by structures A and B, the C-C02 bonds have considerable double-bond character, as do the C-0 bonds in p-benzosemiquinone (17) Pauling, L. The Narure of rhe Chemical Bond; Cornel1 University Press: Ithaca, NY, 1960.
J . Phys. Chem. 1989, 93, 5437-5444 radical anion.l1.l2 The similarity of the v7? frequency of the trianion radical to that of the semiquinone anion indicates that these bonds are of similar strength. While protonation of the carboxylate oxygen has little effect on the ring or C-X (X = COz or COZH) bonds, protonation of p-benzosemiquinone anion, where oxygen is directly conjugated to the ring, has a drastic effect on the C O bond structure, which becomes very much like that of the phenoxy1 C O bond ( v , ~ 1500 The radical anions of terephthalic acid have 11 prr electrons while the p-benzosemiquinone anion has only 9. so it is difficult
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5437
to compare the nature of the electronic transitions in these types of radicals. However, there is some similarity in the enhancement pattern of the phenyl vibrations in the 530-nm Raman spectra of terephthalic acid radical anions and the 430-nm spectrum of p-benzosemiquinone anion. The vga, vga, and v1 vibrations appear at almost similar relative intensities. 11,12 The 360-nm transition in the terephthalic radical anions is probably similar in this respect to the 3 15-nm transition of the p-benzosemiquinone anion, although due to the lack of the resonance Raman data on the latter radical, a direct comparison is not possible.
Temperature and Solvent-Polarity Dependence of the Absorption and Fluorescence Spectra of a Fixed-Distance Symmetric Chlorophyll Dimer S. G . Johnson, G . J. Small,* Ames Laboratory-US. Iowa 50011
Department of Energy and Department of Chemistry, Iowa State University, Ames,
D. G . Johnson, W. A. Svec, and M. R. Wasielewski Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: January 23, 1989)
Temperature-dependentabsorption and fluorescencespectra and line-narrowed fluorescence and excitation spectra are reported for a dimer consisting of two methylpyrochlorophyllide a molecules that share a vinyl group at the 2-position of each macrocycle. The data (obtained for three solvents of widely differing polarity) show that the dimer exists in two conformations (A and B) and that excited-state relaxation from A to B onsets near the glass transition temperature ( T J . Molecular modeling suggests that the two conformations are related by "bicycling" of the two single bonds joined to the vinyl group linkage. At sufficiently low temperature, the solvent dynamics are rate limiting for the conformational relaxation. For a solvent of sufficiently high polarity (DMF), the excited state of B is shown to access a new radiationless decay channel for T R Tg A charge-transfer state is suggested to be important for this decay. The model presented is shown to provide a qualitative explanation for the frequency domain and recently obtained picosecond and fluorescence quantum yield room-temperature data.
I. Introduction Interest in the primary charge separation process and the excited electronic state structure of photosynthetic reaction centers (RC) has, for many years, stimulated studies of model dimeric and oligomeric chlorophyllic systems. In the case of bacterial RC, the importance of a special bacteriochlorophyll (BChl) pair for the initial phase of charge separation was established prior to the structure determinati~nsl-~ for Rhodopseudomonas viridis and Rhodobacter sphaeroides. The diversity of model systems constructed by covalent linkage, nucleophilic linkage, and self-aggregation is impressive, and their study has provided valuable insight on singlet and triplet excitation and charge delocalization.4-6 In what follows, we mention only a few of the model systems studied. Fong proposed' a dimer of Chl a.H20 as a model for the primary electron donor (PED) of photosystem I (P700) whose assembly is dependent on solvent conditions. Shipman et aL8 presented a similar model for P700 involving dimers formed by nucleophilic linkage of Chl a monomers. Nucleophiles employed (1) Deisenhofer, J.; Epp, 0.;Miki, K.; H u h , R.; Michel, H. J. Mol. Biol. 1984, 180, 385. (2) Allen, J. P.; Feher, G.; Yeates, T. 0.;Rees, D. C.; Deisenhofer, J.; Michel. H.: Huber. R. Proc. Natl. Acad. Sci. U.S.A. 1986. 83. 8589. (3) Chang, C. H.; Tiede, D.; Tang, J.; Smith, U.; Norris; J.; Schiffer, M. FEES Lett. 1986, 205, 82. (4) Katz, J. J.; Hindman, J. C. In Biological Events Probed by Ultrafast Laser Spectroscopy; Alfano, R. R., Ed.; Academic Press: New York, 1982; p 119. (5) Wasielewski, M. R. In Light Reaction Path of Photosynthesis; Fong, F. K., Ed.; Springer-Verlag: West Berlin, 1982; p 234. (6) Boxer, S. G. Biochim. Biophys. Acta 1983, 726, 265. (7) Fong, F. K. Proc. Natl. Acad. Sci. U.S.A. 1974, 71, 3692. (8) Shipman, L. L.; Cotton, T. M.; Norris, J. R.; Katz, J. J. Proc. Natl. Acad. Sci. U.S.A. 1976, 73, 1791.
0022-3654/89/2093-5437$01.50/0
were ROH, RSH, or RNH2, where R is an alkyl chain. A model for the PED state of Rb. sphaeroides, P870, suggested by Wasielewski et aL9 is a pair of bacteriochlorophyllide a molecules covalently bound by an ethylene glycol diester bridge. A similar model for P700 was also constructed by Wasielewski et a1.I0 in which two Chl a were linked by the same bridge. Boxer and Closs" presented a P700 model that is the ethylene glycol diester of methylpyrochlorophyllide a. Pellin et al.l2 have synthesized a P700 electron-acceptor model system consisting of an ethylene glycol diester of pyrochlorophyll a and two molecules of pyropheophorbide e t h y l e n e glycol monoester. The pyropheophorbide a molecules acted as electron acceptors for the dimer. Yuen et al.13 have constructed a model for a P700-antenna system with a tris(pyrochlorophy1lide a)-tris(hydroxymethy1)ethane triester. Two of the molecules folded upon one another when the solvent conditions were sufficient with the third molecule acting as an antenna. Wasielewski5 proposed a model for P700 consisting of a bis(chlorophyl1 a ) cyclophane. Bucks et a1.I4 have investigated several dimers of chlorophyllic species (pyropheophorbide a, pyropheophytin a) and trimers as models for primary electron donors and primary electron donor-antenna systems. Agostiano et a l l 5 (9) Wasielewski, M. R.; Smith, U. H.; Cope, B. T.; Katz, J. J. J . Am. Chem. SOC.1977, 99,4172. (10) Wasielewski, M. R.;Studier, M. H.; Katz, J. J. Proc. Natl. Acad.Sci. U.S.A. 1976, 73, 4282. (11) Boxer, S . G.; Closs, G. L. J . Am. Chem. SOC.1976, 98, 5406. (12) Pellin, M. J.; Wasielewski, M. R. Nature 1979, 278, 54. (13) Yuen, M. J.; Closs, G. L.; Katz, J. J.; Roper, J. A,; Wasielewski, M. R.; Hindman, J. C. Proc. Natl. Acad. Sci. U.S.A. 1980, 77, 5598. (14) Bucks, R. R.; Netzel, T. R.; Fujita, I.; Boxer, S. G. J . Phys. Chem. 1982, 86, 1947. (15) Agostiano, A.; Butcher, K. A.; Showell, M. S.; Gotch, A. J.; Fong, F. K. Chem. Phys. Lett. 1987, 137, 37.
0 1989 American Chemical Society
