J. PhyS. Chem. 1983, 8 7 , 1120-1125
1120
v,)
and torsion (f,) are consistent with the view that nonbonded 0-0 interactions are not significant. The overall success of the quadratic force field used here, as opposed to the Urey-Bradley field which emphasizes nonbonded interactions, further supports this picture. The Raman spectra and force constant calculations on s-NZ03,which can be stabilized after formation by photolysis in the NO matrix, have also provided an improved perspective on the spectra and bonding of this molecule. Seven of the nine fundamental frequencies of s-N203have been assigned with considerable certainty, and those have permitted the calculation of force constants for two different vibrational models. The force constants (from calcn I) show the molecule to have a nitrogen-oxygen double bond (fR = 12.1 mdyn/A) stronger than in free NO2 CfR = 11.0 mdyn/A) or HO-N=O (fR = 11.7 mdyn/A) but weaker than in XO-N=O (fR = 13.0 mdyn/A for X = F or C1) or X-N=O molecules ( f R = 15.2-15.9 mdyn/A). The nitrogen-oxygen single bond constant off, = 3.6 mdyn/A may be compared' to the lower values found for X-O-NOZ molecules (f, = 2.3-3.2 mdyn/A for X = C1, F, H) or for Nz05(f, = 1.6 mdyn/A for OZN-O-NO2). However, the corresponding bond may be considerablystronger as in FO-NO (f, = 6.3 mdyn/A). Although as-Nz03 rather than s-N203is the species formed under normal conditions, it has the weakest bond
(the N-N linkage) in either molecule. Its higher thermodynamic stability thus must result from its three nitrogen-oxygen multiple bonds (see Figure 5a) vs. only two for s - N ~ O ~Lattice . effects may also contribute to its solidstate stability. In the vapor phase as-Nz03is also more stable than s-NZ03,based on the fact that infrared spectra of N2O3 vapors show only the presence of the asymmetric i ~ o m e r .In ~ the nitric oxide matrix as-N203is also preferrentially produced. However, photolysis with red radiation apparently results in rupture of the N-N bond and free NOz and NO are produced. Under continuing photolysis the OZN-NO cannot reform. Instead, one of the oxygen atoms in the NOz attaches to the nitrogen of NO producing O=N-0-N=O. The process is reversed when radiation in the green or blue region is used. Photolysis in this region ruptures a nitrogen-oxygen single bond in s-Nz03and the NOz and NO products rearrange to form as-Nz03. As should be apparent, the nitric oxide matrix plays an important role in these interconversions by providing excess NO molecules which may react with the NOz decomposition products. This is true even though much of the NO is dimerized as ONNO.
Acknowledgment. This research was supported by the National Science Foundation. Registry No. N,Os, 10544-73-7; NO, 10102-43-9.
Thermodynamics of Sodium Chloride Solutions in Steam Kenneth S. Pitzer Department of Chemistry and Lawrence Berkeley Laboratory, University of California. Berkeley, California 94720 (Received: September 7, 1982; I n Final Form: November 3, 1982)
Gas-phase data from mass spectrometry are used to calculate the Gibbs energy of hydration of Na+ and C1ions in steam. A similar hydration model for the ion pair NaCl is fitted to the experimental measurements of the solubility of NaCl in steam. The ionization constant calculated from these sources fits the directly measured values at 1073 K and densities above 0.3 g ~ m - At ~ . lower temperatures reasonable curves interpolate between the calculated values for low density and the direct measurements at higher density. Other thermodynamic properties are calculated for Na+, C1-, and NaCl in steam. The partial molal heat capacity of the ions is very large in the range 700-1000 K; this arises from the enthalpy of dissociation of water from the hydrated ions. The Born equation is compared with these results. A practical application to steam turbine technology is also considered.
Recently the writer' showed that the mass spectrometric measurements on the hydration equilibria for H30f and OH- could be used to calculate the self-ionizationin steam in the region above the critical temperature and at pressures up to maxima increasing from 100 or 200 bar near T,to lo00 bar near lo00 K. Other thermodynamic properties of ions in steam were also calculated. In this paper similar methods are applied to the system NaC1-H20 in the same region of temperature and pressure. Kebarle and have measured the hydration equilibria of gaseous Na+ and C1-. For this system there is the additional problem of the hydration of the NaCl ion (1)K. S.Pitzer, J. Phys. Chem., 86, 4704 (1982). (2)I. Dzldic and P. Kebarle, J. Phys. Chem., 74, 1466 (1970). (3)M. Anshadi, R. Yamdagni, and P. Kebarle, J. Phys. Chem., 74, 1475 (1970). 0022-3654/83/2007-1120$01 S O / O
pair in steam. Reasonable estimates are made for these hydration equilibria for NaCl with guidance from experimental values for the solubility of solid NaCl in steam. Unfortunately there are large differences among the various experimental solubility but it is found that many data are reasonable on this theoretical basis while the others are not. Further experiments are very desirable (4)M.A. Styrikovich, I. Kh. Khaibullin, and D. G. Tskhvirashvili, Akad. Nauk. SSSR, 100, 1123 (1955). (5)M. A. Styrikovich and I. Kh. Khaibullin, Dokl. Akad. Nauk. SSSR, 109, 962 (1956);Engl. transl. 109, 507 (1956). (6)0. L.Martynova, Zh.Fir. Khim., 38, 1065 (1964);Russ. J. Phys. Chem., 38, 587 (1964). (7)M. A. Styrikovich, 0. I. Martynova, and E. I. Mingulina, Dokl. Akad. Nauk. SSSR, 171, 911 (1966);Engl. trans., 171, 783 (1966). (8)S. Sourirajan and G. C. Kennedy, A m . J. Sci., 260, 115 (1962). (9) J. F. Galobardes, D. R. Van Hare, and L. B. Rogers, J. Chem. Eng. Data, 26,363 (1981).
0 1983 American Chemical Society
The Journal of Physical Chemistfy, Vol. 87,No. 7, 1983
Thermodynamics of NaCl in Solutions in Steam
concerning this solubility of NaCl in steam. The ionization of NaCl in steam has been measured by Quist and Marshalllo at densities from 0.3 to about 0.8 g cm-3 over the temperature range 673-1073 K. The calculated ionization constants for lower densities are fully concordant with the measurements of Quist and Marshall.
Gaseous Hydration Equilibria The thermodynamic properties of gaseous NaC1, Na', and Cl- are reasonably well-known. We adopt the values given in the JANAF tabled' as follows: NaCl(s) = NaCl(g)
A.H0298
= 54.9 kcal mol-l
NaCl(g) = Na+(g) + Cl-(g) A H 2 9 8 = 133.22 kcal mol-'
(1)
TABLE I: Enthalpy and Entropy Changes for Successive Hydration of Na', Cl-,and Na'Cl-
- AS" T * / (cal K-'
n
T*/K
mol-')
1 2 3 4+
700 550 4 50 3 50
Na' 21.5 22.2 21.9 25.0
1 2 3 4+
400 400 3 50 330
16.5 20.8 23.2 25.8
1 2 3 4+
500 500 500 500
Na'CI22 22 22 25
-APT*
/
(kcal mol-') 24.0 19.8 15.8 13.8 - 1.5(n - 4 )
c1-
(2)
The JANAF tables also give entropies and heat capacities and, finally, Gibbs energy values at various temperatures from which the equilibrium constants for reactions 1 and 2 can be calculated.
1121
13.1 12.7 11.7
11.1 - 0.5(n - 4 )
17.5 13.5 9.5 9.5 - 0.8(n - 4 )
source and report AHn and ASn for the successive reactions. Table I gives these results for the first four steps in each case together with the approximate mean temperature of K2 = fNa+fCl-/fNaCI (4) measurement. While at low pressures only these first four hydrates are significant, we need a model for further hySince these fugacities are all very small they can be dration to extend calculations to higher pressures and for approximated by the partial pressures, but we retain the Na+ the expression in Table I represents the experimental symbols fNa+,etc. to distinguish these quantities from the values2 for the next two steps, 5 and 6,quite accurately partial pressures in the aqueous system. The enthalpy and and may be used as a basis for extrapolation. Gibbs energy for reaction 1 is uncertain by about 0.5 kcal For the Cl- ion values are reported3for only the first four mol-'; various sets of data are concordant but still leave stages of hydration and these are given in Table I. The this uncertainty. The electron affinity of C1 had been enthalpies for the early stages are much smaller for C1uncertain by 1 kcal or more but has now been measured than for Na+ as would be expected for the larger radius quite a~curately.'~J~ of the C1- ion. Also the increment in this case is much In the aqueous system there are successive hydration smaller, about 0.5 kcal mol-l, and that value is assumed equilibria14 for further hydration. Since we will be making calculations at temperatures Na(H20),_,+ H 2 0 = Na(H20),+ K,+ (5) much higher than those of mass spectrometric measurement, the AC, for successive hydration reactions should Cl(H20),-1- H2O = Cl(H20); K,,(6) be considered. A hydration step converts three modes of translation and three of rotation into six low-frequency NaC1(H20),-1 + H 2 0 = NaC1(H20), K,," (7) vibrational modes in the larger cluster. The torsional with the successive equilibrium constants K,+, K,-, and modes, especially, will be quite anharmonic. Also one P V K,O. term will be lost. Hence, one expects AC, to be 6(R + ?) - 4R or substantially in excess of 2R. For the earlier For our purpose we need the sum of the partial pressures calculations on the hydration of H30+and OH-, the value of all of the hydrates of a given species. We write 3.5R was adopted after exploratory calculations with 3R ~ 4R. The value 3.5R also seems most reasonable the~P,+/PN =~ (s+) + = 1 K1+PH20 K ~ + K ~ ++P ***H ~ ~ and (8) oretically, and it was used in the present calculations. In addition to the molecules of water of hydration bound cP,,-/Pcl- = (s-)= 1 K1-PH20 K ~ - K ~ - P H , o*.. ~ tightly enough to the ion to be measured in the mass spectrometer, there will be some more distant molecules (9) which will still make some contribution to the total Gibbs c P n 0 / P N a C l = (so) = 1 + Ki0PH2o+ K10K20PH,02 + "' energy of hydration. In low-pressure steam this second or outer-shell effect may be negligible, but at the high (10) pressures of interest here it should be considered. In the where the symbols (S+), etc., are convenient abbreviations. earlier paper' on the self-dissociation to H+ and OH- the Also, the partial pressures of the unhydrated species are hydration model was extended to include an alternate assumed to be equal to their fugacities. expression with less rapidly reduced binding for n greater For the ions Kebarle and associates2s3have measured than 4. A better procedure, suggested by Professor R. H. these hydration equilibrium constants at several temperWood, is to use the Born equation for an ion in a dielectric atures in a mass spectrometer with a high-pressure ion with an appropriate radius yielding only the "outer-shell" effect, For this contribution to the Gibbs energy of solvation, the Born equation is (10)A. S.Quist and W. L. Marshall, J.Phys. Chem., 72,684 (1968). K1 = fNaCl
(3)
+ +
+
+
+
(11)D.R. Stull and H. Prophet, Ed., Nut[. Stand. Ref.Data Ser., Natl. Bur. Stand., No.37 (1971). (12)R. S. Berry and C. W. Reimann, J. Chem. Phys., 38,1540(1963). (13)G. Miick and H. P. Popp, 2.Naturforsch. A, 23, 1213 (1968). (14)W.L.Marshall, J.Phys. Chem., 76,720 (1972).
-AhGB = (e2/8mo)(l - l/c)(l/R*)
(11)
where R* is the effective radius of the spherical cavity in the dielectric which contains the ion, t is the dielectric
1122
The Journal of Physical Chemistty, Vol. 87, No. 7, 1983
Pitzer
constant or relative permitivity, c0 is the vacuum permitivity, and e the protonic charge. For the dielectric constant of steam at the high temperatures of present interest the equation of Quist and Marshall15 was adopted. Its form is theoretically satisfactory for extrapolation to higher T from the range of experimental measurement. The more recent equation of Uematsu and Franck16is to be preferred at more moderate temperatures but its extrapolation to higher Tis uncertain. Actually, for most of the range of interest, at least, there is reasonable agreement between the two equations. The choice of an effective radius for the outer-shell contribution is somewhat ambiguous. Calculations based on crystallographic radii for ions and hydrogen bonded water, together with the radius corrections found to be appropriate in other Born-equation applications, indicate a radius near 5 A. This radius considers only a single inner shell on C1- but allows for a partial second shell on Na+ as seems appropriate from the mass-spectrometric data. However, this calculation assumes too tight and regular a hydration structure for the high temperatures of present interest. Also, if a water molecule is close enough to an ion to be drawn with the ion through the mass spectrometer, it will be counted in the “inner shell”. These latter arguments suggest a somewhat larger effective radius for the outer-shell calculation. Exploratory calculations for the ionization constant were made for 1073 K with several radii and the best agreement with the experimental data at high density, see below, was obtained for 6 A. This value seems very reasonable, and its use in the Born equation will give an estimate of the outer-shell effect which involves primarily the angular orientation with respect to an ion of water molecules too distant to be strongly bound. There are no molecular-level data on hydrates of the ion pair Na+Cl-. One expects the first two or three molecules of water to attach to the Na+ since the enthalpies for Na+ are much greater than for C1- for the first few stages. But the presence, at greater distance, of the negative charge of the C1- will lower the energy of attachment to Na+ of these first few molecules. Thus a model was set up for the hydration of Na+C1- in which the decimal fractions in the AS,, and AH,, values for Na+ were rounded off and a single values were reintermediate T* adopted; then all -AH,, duced by a constant amount. The partial pressure of aqueous NaCl in equilibrium with the solid is given by
l a shows the data (except those of Sourirajan) for 450 “C (723 K); all of the measurements agree quite well near 100 bar but there are considerable differences below 50 bar. Our model yields a curve midway between the values of Galobardes et al. and the two Russian measurements. Qualitatively the curve must decrease in slope at sufficiently low steam pressure and eventually approach the pressure of unhydrated NaCl molecules. This feature was recognized by Styrikovich, Martynova, and Mingulina’ in their 1966 paper. Figure 1, b and c, shows the temperature dependence at 50 and 100 bar, respectively, and show the gross discrepancy between the measurements of Sourirajan and Kennedp from all others. At the lower temperatures one can fit the Sourirajan data fairly well with a model in which all -AH,, values are increased by 1.5 kcal mol-’ from those listed in Table I. But at the highest temperature there is still a discrepancy of a factor of 30 from the modified model. Indeed the shape of the Sourirajan curves above 900 K implies an impossibly rapid change in the AH of hydration. There are measurements by other investigators for the region of two liquid phases which occurs at higher pressures. The most recent work is that of Parsiod and Plattnerl’ who compare their results with those of others. These recent measurements agree with those of Khaibullin and Borisov,lafrom the same laboratory as Martynova and Styrikovich, and disagree significantly with those of Sourirajan and Kennedp for this higher pressure region. Thus it seems clear that the evidence favors the model fitting the results of Martynova, of Styrikovich et al., and of Galobardes et al.,and those parameters (given in Table I) were used in all further calculations. The details of this model are much less certain than those for Na+ and C1-, and it will probably need revision when better experimental data are available. But it is physically plausible in relationship to the models for the hydration of Na+ and C1- and it represents those experimental data which are internally consistent as well as is possible. Figure Id shows the resulting vapor pressure of aqueous NaCl over a wide range of conditions.
(15) A. S. Quist and W. L. Marshall, J. Phys. Chem., 69,3165 (1965). (16) M. Uematsu and E. U. Franck, J. Phys. Chem. Ref. Data, 9,1291 (1980).
(17) C. J. Parisod and E. Plattner, J. Chem. Eng. Data, 26, 16 (1981). (18) I. Kh. Khaibullin and N. M. Berisov, Teplofiz. Vys. Temp., 4, 518 (1966); Engl. transl., 4, 489 (1966).
Results There seems no need to present an extensive tabulation of the absolute values of various thermodynamic properties of Na+, C1-, and NaCl in steam since any value can easily be obtained as the sum of the absolute value for the anPNaCl.aq = (fNaCI)(So) (12) hydrous species in the standard state, as given in the JANAF tables, the correction to the desired pressure, and the where fNaCl is the fugacity or vapor pressure of anhydrous change for hydration as calculated from the model defined NaCl and (SO) is the sum over the relative pressures for the hydrated species defined in eq 10. Curves for P N ~ c ~ , above. ~ ~ The change in Gibbs energy for hydration of an ion (Na+ or CY) is as a function of temperature and total pressure were calculated for various enthalpy reductions in the model -AhG/RT = In (Sk)- (AhGB/Rcr? (13) stated above and compared with the sets of experimental where the fiist term, with (S+) or (S-), gives the inner-shell results of Martynova? of Styrikovich et a l . , 4 s 5 of Sourirajan contribution and the second term the outer-shell contriand Kennedy,a and of Galobardes et ale9 Reasonable bution as calculated from the Born equation. For NaCl agreement was obtained with all of the data, except that only the first term of eq 13, with (SO), is applicable. Direct of Sourirajan and Kennedy, for -AH,, values reduced by calculation from the model is easy, but for convenience 6.5 kcal mol-l. The agreement was improved if the reTable I1 gives an array of these AhG values at the same duction for stages 4 and above was only 4.5 kcal mol-’. The temperatures as are used in the JANAF tables. parameters on this basis are given in Table I. Enthalpies and entropies can be obtained from the These measurements of the solubility of sodium chloride temperature derivative of the Gibbs energies, but with in steam are very difficult and the uncertainties in the decreased precision. A few values for 100 bar total pressure results are quite large. Figure 1,parts a, b, and c, compare are given in Table I11 which also includes separate values the experimental data with the calculated curves. Figure
The Journal of Physical Chemistry, Vol. 87,
Thermodynamics of NaCl in Solutions in Steam I
-3c
1
I
I
I
,
,
,
No. 7, 1983 1123
1
1. 723K
-'L,A
A ,
1.2
1.4
- 4 1 ,
1.6 1.8 2.0 109 b h O ) / b o r l
\ j -I2F ,
I
1.0
, 2.2
1.2
1.4
400
I
600
IOOO/T
800
I
1
lo00
T/ K
Figure 1. The saturation vapor pressure of NaCl in steam: (a) as a function of Pat 450 'C (723 K), points from references: 6, solid circles; 4, open circles; 9, triangles; (b) as a function of Tat 50 bar, points from references: 6, circles; 8, triangles; (c) as a function of Tat 100 bar, points from references: 4, circles, 8, triangles. Part (d) shows the vapor pressure ratio, NaCl to H20, over a wide range of T and P . The curves on parts (a) and (d) and the lower curves on (b) and (c) are calculated from the model of Table I; the upper curves on (b) and (c) are discussed in the text.
TABLE 11: Values of - A G / R T of Hydration for Na+, Cl-, and NaCl P/bar 10 10 10 20 20 20 20 20 50 50 50 50 50 100 100 100 100 200 200 200 500 500
T/K 700 800 900 700 800 900 1000 1100 700 800 900 1000 1100 800 900 1000 1100 900 1000 1100 1000
1100
Na+ 18.6 12.9 9.1 21.9 15.6 11.4 8.4 6.1 27.4, 20.1 15.2 11.7 9.0 24.7 19.0 15.0 11.9 24.2 19.3, 15.7 27.7 22.9
e16.4 4.3, 3.1, 8.5, 6.0 4.5 3.5 2.9 12.6 9.1 7.0 5.6, 4.6, 12.6 9.8 7.9, 6.6 13.9 11.3
9.4 18.4, 15.3
NaCl 5.5 3.4 2.0, 6.9 4.5, 3.0 2.0 1.4 9.1 6.3, 4.5, 3.3 2.4 , 8.0 6.0 4.6 3.6 7.7 6.1 5.0 8.6 7.3
TABLE 111: Enthalpies of Hydration and Other Properties for Na+, Cl', and NaCl at 100 bar 800K 900K Hydration, Total -AhH/RT (Na') 58 44 - AhH/RT (el-) 29 20 -AhH/RT(NaCI) 19., 15 01)(Na')
( n )(C1-) (n) (NaCl)
Inner Shell Only 5.2 4.6 3.7 3.1 2.5 2.2
Outer Shell Only, Na' or C1' -AhG/RT 2.4, 1.7, - AhH/RT 8.0 5.3
1OOOK 35 15 12 4.2 2.7 2.0 1.2, 3.7
The ionization constant on a pressure basis in the presence of steam is then
for the outer-shell contribution and values for the mean hydration number (n).
The experimental ionization constants are eiven on a molarity basis; the conversion in the ideal g& region is simple
Ionization Constant for NaCl in Steam The ionization constant for gaseous NaCl in the absence of water, K2,is obtained from data in the JANAF tables.
Ki(M)= Ki(P)/RT (16) The resulting curves for log K i(') as a function of the density of the steam at several temperatures are shown in
1124
The Journal of Fhysical Chemistry, Vol. 87,No. 7, 1983
Pitzer
01
1
400
1
I
I
1000
000
600 T/ K
+
Figure 3. The Gibbs energy of hydration of Na+ CI-at the critical pressure 221 bar. The dotted portion is interpolated between experimental values at low temperature and the present calculated values for high temperature. The dashed curve is for the Born equation. Also shown are mean hydration numbers for Na+ and CI- with a separate scale on the right.
Figure 2. The ionization constant of NaCl in steam at several temperatures. The open symbols are calculated values: the solM symbols are measued values of Qulstand Marshall. The vertical scale is offset for the various curves as indicated.
Figure 2 along with the experimental values of Quist and Marshall.'O At 1073 K the calculated results fit very smoothly with the experimental values. At lower temperatures the curves take an S shape with a gradually steeper slope connecting the gaslike region of lower density with the liquidlike region of higher density. Below the critical temperature of water this would become a discontinuity with a gap in both log p and log Ki between the values for the saturated liquid and the saturated vapor. There is, of course, considerable uncertainty in the interpolated portions of the curves on Figure 2 for the lower temperatures.
Discussion These calculations are made on an ideal gas basis which is a reasonable approximation for steam at surprisingly high pressures at temperatures near or above 1000 K. Nevertheless, the effect of gas imperfection should be considered. The first adjustment is to substitute fugacity for pressure; indeed the densities at which calculated points are placed on Figure 2 correspond to the density of steam at the fugacity of the calculation. The outer-shell term, however, was calculated for the pressure as listed in Table I. Many other small corrections would be required for a complete calculation on a nonideal gas basis, including DebyeHiickel effects. Such a complex calculation is not justified at this time in view of the approximate nature of some input data. It was noted above that, in the Born-equation calculation of the outer shell contribution, the choice of the effective radius was influenced by the fit of the calculated ionization constants to the experimental values at 1073 K. Thus this effective radius is, in this sense, a semiempirical parameter,
but the result was very reasonable from structural considerations. The model for the hydrated Na+Cl- ion pair is, of course, estimated in detail, and empirical in the final adjustment, since there are no molecular-level data for this species. It is interesting to note the various contributions to the large heat capacity of hydration which, for Na+ at 100 bar, is 68R for the interval 800-900 K. The largest contribution is the loss of 0.6 molecule of inner-shell hydration; at 11 kcal mol-' this contributes 34R. Also the H20 in a cluster has a higher heat capacity than the free molecule. With an average of 4.9 water molecules in the inner shell, the AC, per molecule of 3.5R yields 17R. The outer-shell contribution is 16R. Below the critical temperature of water, the partial molal heat capacities of ions are also large but negative. In very approximate terms one may view these large thermal effects in the following manner. Below T,it is the hydrogen bonding between water molecules which is breaking up and this yields a large C, for water. Since the water in the hydration shell around an ion is still tightly bound, the difference is a negative heat capacity ascribed to the ion. But above T,in the range we are considering, the water molecules in steam are separate gaseous molecules. It is now the hydration-shell molecules that are gradually being dissociated from the ion and their dissociation enthalpy contributes the large positive heat capacity ascribed to the ion in this range. Figure 3 shows the Gibbs energy of hydration for the critical pressure, 221 bar, over a wide range of temperature. Also shown is the c w e calculated from the Born equation. This equation was given above, eq 11, but it is now applied to the entire hydration effect-not just the outer-shell contribution. For this purpose the mean effective radius R* = 2(1/R+ + l/R-)-I is taken as 1.86 8, to fit the Gibbs energy of hydration of Na+ + C1- at room temperature. The dielectric constant is taken from Uematsu and Franck.lG The Born equation yields a qualitatively correct
1125
J. Phys. Chem. 1083, 87, 1125-1133
Application to Steam Turbine Technology One practical problem to which these data are relevant is that of solid deposition in the low-pressure stages of steam turbines. This problem is discussed by Galobardes et a1.: who state that the important criterion is the solubility of NaCl in steam in the range 1-5 bar pressure and 400-450 K. Figure Id shows the prediction of our model o). In the range of interest for the solubility (as PNacl/PH this ratio is below which is smaller than the value attainable by current technology for purification of boiler feed water. In this conclusion the present model agrees with the estimate of Galobardes et al.
curve, but there is a large quantitative difference above the critical temperature. Since the Born model assumes a continuous dielectric, one would not expect it to be successful in the high temperature range where there are a few water molecules strongly attached to each ion in the locations for strongest bonding together with a very diffuse outer shell. Also we note that even in the liquid range at lower temperatures, where the Born equation fits a particular property fairly well, different effective radii are needed to fit different properties. Thus in comparison to the 1.86 A needed to fit the Gibbs energy of hydration, one may mention the value of 3.27 A chosen by Smith-Magowan and Woodl9 to fit the heat capacity of aqueous NaCl in the range 500-600 K. To fit the partial molal volume of NaC1, Helgesen and Kirkha" chose 1.81 A which agrees rather closely with our 1.86 A.
Acknowledgment. I thank Professor R. H. Wood for his valuable suggestion discussed above. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Division of Engineering, Mathematics, and Geosciences of the US.Department of Energy under Contract No. DE-AC03-76SF00098. Registry No. NaCl, 7647-14-5; steam, 7732-18-5.
(19)D. Smith-Magowan and R. H. Wood, J.Chem. Thermodyn., 13, 1047 (1981). (20)H.C.Helgesen and D. H. Kirkham, Am. J. Sci., 276,97 (1976).
Excited-State Dynamics of 3-Hydroxyflavone A. J. G. Strandlord, S. H. Courtney, D. M. Friedrich,+and P. F. Barbara" Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 (Received: September 7, 1982; I n Final Form: November 9, 1982)
We have investigated the time- and wavelength-resolvedfluorescence spectroscopy of 3-hydroxyflavone(3HF) in several solvents and find that the photodynamics are well described by a kinetic model that includes two pathways for excited-state intramolecular proton transfer (ESIPT). We are unable, at present, to identify a single dominant effect that controls the ESP" kinetics in all solvents. Our data reveal an irreversible excited-state isomerization that occurs at a rate which is temperature and solvent dependent. This rate is exceptionally small (lOs-lO1l s-') as compared to the published unresolvably rapid kinetics (>lo" s-') observed for the other ESIPT molecules studied to date. In this paper we describe in detail the fluorescence kinetics of 3HF and propose some preliminary mechanistic interpretations of these data.
Introduction Excited-state intramolecular proton transfer (ESIPT) has been observed to be an efficient mechanism for adiabatic, irreversible, excited-state rearrangement in a broad range of molecular examples.' The ESIPT process in many compounds has been found to be unresolvably rapid (km > 10" s-'), even at low temperature, placing upper limits on potential barrier height for proton transfer of less than 1 kca1/moL2* An apparent exception to "barrierfree" ESIPT is found in the studies by Woolfe and Thistlethwaite (WT),' Sengupta and Kasha (SK),* and Itoh et al. (IT)9of 3-hydroxyflavone (3HF). Because of the adjacent carbonyl and hydroxyl groups in the ypyrone ring of the flavone, 3HF may exist in one of two principal tautomeric forms, the normal form (N) and the proton-transferred form (T).9 The stable ground-state tautomer of 3HF is apparently the N form (see Discussion). 3HF shows two distinct fluorescence bands, as originally suggested by Frolov et al.,1° and is confirmed by the time-unresolved spectral study of SK.8 The blue band (413 +Permanent address: Holland, MI 49423.
Chemistry Department, Hope College, 0022-365418312067-1125$01.50/0
N -
T -
nm) and the green band (543 nm) are solvent and temperature dependent. The two bands have similar excita(1) Two useful reviews of ESP" can be found aa follows: W. Klopffer, Adu. Photochem., 10, 311 (1977);D. Huppert, M. Gutman, and M. J. Kaufmann, Adu. Chem. Phys., 47,643 (1981). (2)K. K. Smith and K. J. Kaufman, J.Phys. Chem., 82,2286 (1978). (3)P.F.Barbara, P. M. Rentzepis, and L. E.Brus, J.Am. Chem. SOC., 102,2786 (1980). (4)P.F.Barbara, L. E. Brus, and P. M. bntzepis, J.Am. Chem. SOC., 102,5631 (1980). (5)H. Shizuka, S.Mataui, Y. Hirata, and I. Tanaka, J. Phys. Chem., 80,2070 (1976);81,2243 (1977). (6)G. J. Woo& and P. J. Thistlethwaite, J. Am. Chem. SOC.,102,6917 (1980). ( 7 ) G. J. Woolfe and P. J. Thistlewaite, J. Am. Chem. SOC.,103, 6916-23 (1981). (8)P.K.Sengupta and M. Kasha, Chem. Phys. Lett., 68, 382 (1979).
@ 1983 American Chemical Society
