J. Phys. Chem. 1083, 87, 2581-2584
these molecules are, in fact, singlet in nature. This measurement does not, however, rule out the possibility of the existence of low-lying triplet mr* (or nr*) levels that could sph-orbit couple to SI.However, the triphenodioxazines are quite photochemically stable: indicating that triplet-state photochemical reactions do not occur with any significant quantum yield. In addition, it is known that the oxazines, which are closely related to the triphenodioxazines, have low triplet quantum yields." Further experiments are planned to determine the nature of the nonradiative decay process. The measured excited-state lifetimes of the triphenodioxazines are typical of dye molecules with strongly allowed So S1transitions. The observed radiative lifetimes, calculated from the measured lifetimes and quantum yields for C12TPDand C12Ph2TPD, agree relatively well with the radiative lifetimes calculated from integration of the absorption and emission spectral2 (seeTable I). The observed radiative lifetime for C12Et2TPDis somewhat smaller than the calculated value, but, as indicated in Table I, the calculated value has a rather large experimental error associated with it. The fluorescence properties of the triphenodioxazine dyes are changed dramatically following incorporation of the negatively charged sulfonate group into the ring structure. The addition of the sulfonate group causes a red shift of the absorption and emission bands, and the spectra of these derivatives are broader than those of the parent compounds. The fluorescence quantum yields of the sulfonated derivatives are much smaller, and the extinction coefficients considerably less, than those of the parent compounds. I t then follows that the fluorescence lifetimes of the sulfonated derivatives are much less than those of the corresponding parent compounds. These properties could be due to an accelerated internal conversion via a reversible transfer of charge from the a
-
(11)K. H.Drexhage in "Dye Lasers", F. P. Schafer, Ed., SpringerVerlag, Berlin, 1973,Chapter 4. (12)S. J. Strickler and R. A. Berg, J. Chem. Phys., 37,814 (1962).
2581
system of the chromophore to the sulfonate group following excitation. Additionally, the solvent used, water, may influence the photophysical properties of these dyes due to ionization of the sulfonate group.
Conclusions The triphenodioxazine dyes are fluorescent dyes which, additionally, possess good photochemical stability and exhibit excellent polymer photostabilizing characteristic~.'-~The observed fluorescence quantum yields range from to -0.6, the lower values occurring for the sulfonated dyes in water. It appears likely that competing nonradiative internal conversion processes are responsible for these lower fluorescence efficiencies. Additional studies of the excited-state properties of these molecules are planned to elucidate the mechanisms of these processes. Acknowledgment. We thank Mr. H. Davis, Dr. F. Purcell, and Mr. R. Kaminski, SPEX, Ind., and Professor J. N. Pitts, Jr., and Dr. D. Lokensgard of the UCR Chemistry Department for assistance in obtaining the quantum yields with their respective spectrofluorimeters. We also thank Mr. A. Johnston of this department for advice regarding the synthesis and purification of the dyes, and Dr. G. C. Newland of Tennessee Eastman Co. for the gift of some C12Ph2TPD. The HeCd laser used in this research was provided by Biomedical Sciences Grant 5507RR-07010-12 from the National Institutes of Health and National Science Foundation Grant CHE77-09163. We also thank Ms. Linda DeLucci for typing the manuscript. This work was supported by a grant from the Universitywide Energy Research Group of the University of California and, in part, by the Energy Sciences Program of the University of California, Riverside. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Registry No. Cl,TPD, 4794-44-9; C12Ph2TPD,13437-05-3; PhzEhTPD,85614-24-0; C12EbTPD,6358-30-1; Direct Blue 108, 1324-58-9.
Pressure Dependence of Sound Absorption in an Aqueous Solution of CaSO,+ C. C. Hsut and F. H. Fisher' Marine physical Laboretciy of the Scrlpps Institution of Oceanography, University of Caiifornk, Sen Diego, Califomis 92152 (Received November 29, 1982)
Sound absorption has been measured in a 0.011 M aqueous solution of C&04 at 25 "C. The values of maximum absorption per wavelength, l@(ax),, are 10.3 f 0.1 at 1atm and 7.7 f 0.1 at 307 atm. The relaxation frequency is 177 f 9 kHz at 1 atm and, within experimental error, is independent of pressure. This corresponds to an inner coordination sphere substitution rate of about lo6, 2 orders below that of 2 X lo8 quoted by Eigen and Ta". At 1 atm, the CaS04 absorption is about 30% that of the same concentration of MgSO,.
Introduction Fisher1 recently pointed out that aqueous solutions of CaS04should exhibit acoustic absorption with a relaxation frequency around 200 kHz and with a magnitude about that of an equivalent concentration of MgS04. This Contribution of the Scripps Institution of Oceanography, new series. *Departmentof Electrical Engineering, Chung Cheng Institute of Technology, Tahsi, Taoyuan, Taiwan 335,Republic of China.
conjecture was based on experimental acoustic data published by Kurtze and Tamm2 for aqueous solutions of MgS04,MgCrO,, and CaCr04,as seen in Figure 1. Wilson and Leonard3had reported that no sound absorption was (1)Fisher, F. H.Acustica 1981,48, 116-7. Acustica (2)Kurtze, G.;Ta", K. Nature (London) 1951,168,346; 1953,3,33. (3) Wilson, 0 . B., Jr.; Leonard, R. W. J. Acoust. SOC.Am. 1954,26, 223-6.
QQ22-3654/83/2Q87-2581$01.5O/O0 1983 American Chemical Society
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The Journal of Physical Chemistry, Vol. 87, No. 14, 1983
Hsu and Fisher
where the mi and the uiare the concentrations (molal) and partial molal volumes and the reaction rates are indicated by the kij. Sound absorption involving substitution in the inner coordination sphere for reaction 1 is expressed by ax
= ?rPchWK/(&)(w2
+
(2)
K2))
where CUXis the absorption per wavelength, &,the chemical compressibility, Po the isothermal compressibility, w the acoustic frequency (rad/& and K the relaxation frequency. For these pressure-dependent coupled reactions Pchis expressed in X
e
\
Bch
10
(3)
where AVIIIis a normal-coordinate volume change consisting of a linear combination of the AVij for the successive reaction steps and ml' includes consideration of activity coefficients. When m4 is small compared to m2 + m3,we see that the absorption may be written as
,
CaCrO,
(ml' + m2 + m3)m4AVII? = (ml + m2 + m3 + m4)(1000RT)
-' -
(4)
where the maximum absorption per wavelength (ax),, occurs when w = K . We now see that the absorption is a measure of the ion-pair concentration if the u, are independent of concentration. The numerical parameters derived by Eigen and T a " for a four-state dissociation model for MgSO, have been used by Fishe$ to explain both electrical conductanceBand sound absorption5 data as a function of pressure. Conductance data yield the concentration of all ion pairs, m2 + m3 + m4, whereas sound absorption is proportional t o this quantity. The caveat on understanding eq 4 is that, because AVIDis a function of pressure, it is only at a fixed pressure that sound absorption is proportional to ion pairing. This is not as trivial as it may first appear since for a pressure change of 1000 atm there is only a 10% decrease in total ion-pair concentration whereas there is about a 67% decrease in absorption. Only within the context of a multistate model is it possible to explain such seemingly contradictory behavior. The reason for such contradictory behavior is that, because of the volume changes between the various steps, the concentrations of the ion pairs redistribute themselves as pressure increases. Because the results for both conductance and acoustic measurements for CaSO, are similar to those for MgS0, in their dependence on pressure, we believe that the multistate model for CaSO, must be of the same form but with different numerical parameters. Without high-frequency absorption data in CaS04 solutions, it is premature to try to derive these numerical parameters at this time.
MgO&S04 ma
& M&04
(1)
m4
(4) Finher, F. H. J. Acowrt. SOC.Am. 19158, SO, 442-8. (6)Fisher, F. H.J. Acoust. SOC.Am. I96K, 38, 806. (6) Eigen, M.; DeMaeyer, L. 'Techniquee of Organic Chemistry"; Wekberger, A, Ed.;Wiley-Interscience: New York, 1983; Vol. VIII, part 11, pp 895-1051. (7) Eigen, M.; Ta", K. 2.Elektrochem. 1962, 66, 93.
Measurements and Results Measurements were made by using the resonance technique over a frequency range from 35 to 300 kHz. Details of the technique and methods used in this work are the same as described by Hsu in earlier work.1° The CaS04 solutions were prepared from reagent-grade CaSO, and analyzed for calcium by compleximetric titration.ll The ~~~~
~
~
(8)Fisher, F. H. Science 1967, 157, 803. (9) Fieher, F. H.; Fox, A. P. J . Solution Chem. 1979, 8, 309. (IO)Hsu,Cheng-Chih (Paul). -Differential Sound Absorption Technique and Effect of Ion-Pairing and Pressure on Sound Absorption in Seawater and Aqueous Mixtures of Magnesium Sulfate and Sodium Chloride"; Marine Physical Laboratory of the Scripps Institution of Oceanography: San Diego, CA, 1981. S10 Reference 81-34, Nov 1,1981.
The Journal of Physical Chemistry, Vol. 87, No. 14, 1983 2583
Pressure Dependence of Sound Absorption A
.01M CAS04 AT 1 ATM
1 I
1 .o
I
I
I l l l l
1
I
1
I 002
1 10.0
L
I
.01M CAS04 AT 307 ATM
I
'g
005
01
02
05
1
C Mol/l
Flguro 3. Wilson and Leonard data for MgSO, solutions presented as sound absorption per wavelength per unit concentration, a h / c , where c is in moi/L. The curve is theoretical showing the fraction of MgSO, as associated, [MgSO?] /[MgS04] calculated by using Debye-Huckel theory from the conductance data of Fisher and Fox.
X 4
1
P, atm 1 307
I
I
I 1 1 1 l
106(cihm,
10.25 t 0.09 7.71 t 0.14
I
I
1
f,, kHz
1 016p&, cm2/dyn
177 t 9 1 7 2 t 22
2.92 2.02
analysis yielded the following concentration for the solution: 10.8 mM from three samples taken before the acoustic data were taken and 11.0 mM for three samples taken from the resonator after the measurements were made. Therefore, we take the acoustic results reported here to be for a concentration of 0.011 mol/L. The absorption results are shown in Figure 2. Fitting the data to eq 2 yielded the results summarized in Table I. We used values of isothermal compressibility for pure water to calculate oh, Po = 44.7 x IO4 and 41.2 x IO4 bar-l for 1and 307 atm, respectively. The chemical compressibility must be in units of cm2/dyn in eq 3.
Discussion of Results In order to compare the absorption results for CaS04 obtained in this paper with those for an equivalent concentration of MgS04,we have plotted the results of Wilson and Leonard in a format similar to that used by Kurtze and Tamm to yield an absorption per wavelength normalized by concentration, that is, ( a h ) - / c , where c is in mol/L. As seen in Figure 3, (aX),,/c increases linearly with concentration. For MgS04 at 0.011 M we find (ax), = 33 X lo4. This is slightly lower than the value of 35.5 x lo4 by Wilson and Leonard. The line going through (11) Tsunogai, S.; Nishimura, M.; Nakaya, S. Talanta 1968, 15, 385-90.
the data represents the fraction of ion pairs of MgS04 calculated according to the results obtained by Fisher and Foxg from conductance data; this tells us that the absorption is directly proportional to the ion-pair concentration of MgS04. Therefore, we conclude that the strength of CaS04 relaxation is only 31% that of an equivalent concentration of MgSO4. The relaxation frequency for CaS04 of 177 kHz is substantially higher than the value of 130 kHz reported by Wilson and Leonard for 0.01 M MgS04. While this increase seems to be in accord with the progressive increase in relaxation frequency with crystallographic radius as discussed by Eigen and Tamm, the magnitude of the increase differs by more than 2 orders of magnitude. Our results indicate that the rate constant k, for substitution in the inner coordination shell of the calcium ion is -1.1 X lo6 s-l rather than 2 X loa s-l quoted by Eigen and Tamm. Within experimental error, no shift in relaxation frequency is observed at elevated pressure. This is consistent with the results obtained by Hsu'O for 0.02 M MgSO, solutions and by F i ~ h e r . ~ The most significant result is the large pressure dependence of the chemical compressibility, a decrease of 24.8% or 25% for a pressure change of 306 atm. This is even greater than the pressure dependence of &, for 0.02 M MgS04 aqueous solutions for which a 20% decrease was observed for the same pressure change. The reason that we use the four-state reaction scheme in eq 1is that only a four-state model has been able to predict the pressure dependence of both sound absorption and electrical conductance for MgSO, solution^.^ Similarly, for MnS04 solutions, the large pressure dependence of sound absorption12 indicates that a four-state model is required. More accurately, we should say no three-state model such as Jackopin and Yeager13 advanced for MnS04 has been able to account for the large pressure dependence of sound absorption in these solutions. Burgess14recently referred to the Jackopin and Yeager13 work in which a three-state model was used to explain (12) Fisher, F. H.; Wright, W. M. J. Acoust. SOC.Am. 1969,46,574-9. (13) Jackopin, L. G.; Yeager, E. J . Phys. Chem. 1970, 74, 3766-72.
(14) Burgess, J. "Metal Ions in Solution";Horwood: Sussex, England, 1978; Chapter 12.
J. Phys. Chem. 1083, 87, 2584-2592
2584
sound absorption in MnS04 solutions a t atmosphere pressure. This model, however, fails to account for the observed pressure dependence of absorption, and, in fact, predicts an increase of absorption with pressure. This is because the isothermal compressibility in eq 2 of the solvent decreases more rapidly with pressure than the chemical compressibility calculated with their model. The fact that the three-state model does not predict the observed pressure dependence of absorption does not mean that the Eigen and Tamm four-state model referred to earlier is the final answer just because it predicts very closely the observed pressure dependence of both conductancegand absorption5J0J5in MgS04 solutions. Fisher and Foxg point out that the value of K,, the dissociation (15)Hsu,C. C.; Fisher, F. H. J . Acoust. SOC.Am., submitted for publication.
constant for MgSO,, obtained from conductance data is 0.0062 compared to the Eigen and Tamm value of 0.019 from the parameters which best predict the observed pressure dependence of both conductance and sound absorption. Our measurements reported here as well as the results obtained earlier for MgS0, and MnS0, solutions indicate that the multistate dissociation concept is the only framework in which we can understand the observed pressure dependence of both sound absorption and conductance data. However, additional work is necessary to obtain quantitative agreement on an absolute basis between both conductance and absorption data. Acknowledgment. This work was supported by the National Science Foundation Grant NSF OCE81-13068. Registry No. CaSO,, 7778-18-9.
Kinetics of Proton-Transfer Reactions of p-Nitrophenoi in N-Methylformamide T. Oncescu, A.-M. Oancea, Polltechnical Instltute of Bucharest, Instltute of Chemisby, Department of Physical Chemistry, Bucharest 7003 1, Romania
and L. De Maeyer' Max-Planck-lnstltut fur blophysikallsche Chemie, P 3 4 0 0 Gijttingen, West Germany (Recelved:April 12, 1982; I n Final Form: December 20, 1982)
Proton-transfer reactions involving p-nitrophenol (PNP) in N-methylformamide (NMF) have been studied by using the 2'-jump technique. The rate constants for p-nitrophenol protolysis and solvolysis as well as the rate constants for the autoprotolysis of the solvent are reported and discussed. The influence of the lifetime of hydrogen bonds on the mechanism of protolytic reactions is analyzed. The energetic constraints on these reactions are compared with other experimental data on some interactions of NMF as a solute with acids and bases in aqueous and nonaqueous media. The strong autoprotolysis of NMF as a solvent is related to the high polarizability of its H-bond acceptor sites, leading to strong H bonds with acids that are also characterized by a high polarizability. This property may slow down protolytic reactions between acids and bases in NMF if their aqueous pK, values are separated by less than 10 units.
Introduction Recent studies on proton-transfer equilibria in Nmethylformamide1i2(NMF) revealed the amphiprotic behavior of this solvent, characterized by an autoprotolysis constant of 1.81 X M2. As in any other amphiprotic solvent, the proton concentration in the presence of a weak acid like p-nitrophenol (PNP) is established by means of the coupled solvolytic and protolytic processes:
The study of these fast processes in aqueous systems has led to a better understanding of acid- and base-catalyzed
reactions in this m e d i ~ m .Although ~ ~ ~ there is a continuing interest in the effect of other solvents on such reactions, much less is known about the kinetics of proton transfer in nonaqueous media. Among the many available solvents, the amides are not commonly studied since their purification is tedious; after the introduction of acidic or basic solutes the solutions are not very stable. On the other hand, NMF and other liquid amides are highly polar solvents with high dielectric constants, able to dissociate many inorganic salts. The temperature-jump technique with Joule heating is therefore well suited for studying fast proton reactions in this medium. Previously reported equilibrium data' on the system p-nitrophenol (PNP)/p-nitrophenoxide (PNP-) indicated that the buffers PNP/PNP-Na+ were much more stable than the acidic and, especially, basic solutions in NMF, and less sensitive to the remaining acid-base impurities. This is readily understandable since the pK of PNP (5.16)
(1) T. Oncescu, A.-M. Oancea, and L.De Maeyer, J. Phye. Chem., 84, 3090 (1980). (2) T. Oncescu, A,-M. Oancea, and L. De Maeyer, J.Phys. Chem., in press.
(3) M. Eigen, Angew. Chem., Int. Ed. Engl., 3, 1 (1964). (4) M.Eigen and L. De Maeyer in "Techniques of Chemistry", A. Weiuberger, Ed., Vol. VI, Part 11, Wiley-Interscience, New York, 1974, p 63-146.
121
sc-v3 LY SI 8
0022-365418312087-2584$01.50/0
0 1983 American Chemical Society
