XOTES
Feb., 1963
513
TABLE I LIXITINOEQUIVALENT CONDUCTAWES AKD Electrolyte
AQ Shedlovsky
Ao
Onsager
THE
Theor. slope
SLOPEOF Exptl. slope
THE
CURVESFROM OKSAGERPLOT % differenoe
a
b
C
16 0.05 0,244 25.54 25.42 17.30 10.84 -37.3 14 .04 .357 27.43 17.59 15.00 -14.75 27.55 13 .03 .376 30 11 30.05 17.96 16.10 -10.34 KNOa 14 .06 .160 14.80 -19.73 33.60 18 44 33.72 SH4XO3 .129 8.78 19 .07 20.00 18.38 33.18 33.25 TIv03 13 .04 .lo6 22.00 21 89 16.75 12.01 -28.30 NaOOCCH3 12 .04 .154 21.75 21.57 16.74 12.22 -27.02 NaOOCCH3.3HZO 76.07 19 .07 ,1053 27.70 17.61 31.02 27.75 TlOOCCH3 -39.40 26 .08 .1852 18.16 11 .oo STLRr 31.75 31.63 Number of concentrations a t which each salt was measured. * The average deviation of each value from the line through these Highest concentration a t which measurements were made. points. LixOa NaNOa
The data for these salts have been analyzed on Shedlovsky’sg empirical extension of the Onsager equation. The plots obtained therefrom have been found to be curved with much varying slopes. A0 values obtained by extrapolating the linear portion of these curves are summarized in Table I. However no immediate explanation is apparant for such deviations. The limiting equivalent conductivities for the above salts are reasonably consistent with Kohlrausch’s principle as shown by the nearly constant differences of A. for various salts having a common ion, Le., Xo(EC+) Xo(Na+) = 2.57 for iodides1 and 2.56 for nitrates, Xo(T1+) - Xo(Sa+) = 5.61 for the acetates and 5.63 for the nitrates, whereas h~(N03)- Xo(Ac-) == 5.43 for the thallium salts and 5.55 for sodium salts. The above comparison of RO values is strong evidence of the reliability of our data. The catabatic nature of the phoreograms for T1NO3 and CH3COOTl is attributed mainly to ion association occurring in the two cases. This type of behavior in such solvents has been attributed to an increase in the relative viscosity of the solution.’O I n this case the relative viscosity of a 0.1 N solution of TlN03 has been found to be 1.088, which may not account for such a deviation. However, the dissociation constant of TIS03 in formamide calculated from Shedlovsky’s method is more than unity, in comparison to the value of 0.52 in water a t 18’ obtained by Davies.ll Owing to large solvent corrections applied in the measurements the results are accurate within 1%. However, good agreement with the previous values confirms the general reliability of our data. Summary Studies have been made concerning the conductometric behavior a t 25’ of 9 uni-univalent electrolytes in formamide a t concentrations in the range 2.5 >c 10-3N . The limiting equivalent conductance 200 X for each electrolyte has been evaluated using Onsager and Shedlovsky extrapolation methods. The applicability of the Kohlrausch lam has been tested for these electrolytes. Acknowledgment.-The authors are grateful to the Head of the Chemistry Department, Gorakhpur 1Jniversity, for providing all facilities, and to the Council of Scientific and Industrial Research for granting a Junior Research Fellowship to G. P. J. (9) T. Shedlovsky, J . Am. Chem. Soe., 64, 1405 (1932). (10) L. R. Dawson, P. G. Sears, and R. H. Graves, ibid., 77, 1986 (1955). (11) C. W. Daviea, Trans. Faladay Soc., 23, 351 (1927).
INFLUENCE OF WATER ON THE SORET BAND OF CHLOROPHYLLS BY P. J. MCCARTIN Division of Physical Chemistry, Instatute of Technology, Unmerszty o f Mannesota, iMinneapolas 14, Mannesota Recezued iWaV 86,1068
When chlorophylls a and b are dissolved in highly purified hydrocarbons, a new absorption band appears on the low energy side of the principal red bend and the fluorescence efficiency has a much lower value than in polar solvents. Traces of polar substances (e.g., moisture) fully “activate” the fluorescence and restore the absorption spectrum to its form in “wet” solutions. These effects are best interpreted in terms of the existence of solvates of these pigments which in the unsolvated state are weakly fluorescent and absorb a t somewhat longer wave lengths in the red.’gZ I n the course of photochemical studies on the chlorophylls we found conspicuous spectral differences between solutions in dry pyridine and solutions in mixtures of pyridine and mater. These differences are most pronounced in the blue-violet region. The effect of water on the spectra is reversible in that removal of water restores the spectra to their forms in dry pyridine. I n view of the basically different photochemical behavior of chlorophylls in dry and in wet pyridine,3 it is important to establjsh the function of the water and to determine whether this function is the same as, or different from, the solvation of the pigments in dry hydrocarbon solutions. Livingston, Watson, and niIcArdlel have described the spectra of chlorophylls in both carefully dried and in wet benzene. The effects of solvation on the Soret bands are as follows: (1) solvation increases the height of the Soret band and especially of its blue component; (2) it increases the separation of the blue and violet bands in chlorophyll1 b; and (3) a t low temperatures the spectrum of a partially solvated solution approaches that of a completely solvated solution. Experimental Chlorophylls a and b were extracted from spinach and purified chromatographically as described e l s e ~ h e r e . ~Pyridine was Mallinckrodt A.R. grade freshly distilled from CaHz. Water was purified by repeated distillation. All spectra were measured in 2-cm. cells on a Cary recording spectrophotometer Model 11. The spectrum observed a t (1) R. Livingston, W. F Watson, and J. McArdle, J . Am. Chem. SOC.,71, 1542 (1949). (2) 6 . Freed and K. Sanoier, d i d . , 76, 198 (1954). (3) T. Bannister, ThesiB, University of Illinois, 1958. (4) R. Livingston and R . Pariser, J . Am. Chem. Soc., 70, 1510 (1948).
514
KOTES
Vol. 67
TABLE 1 PEAKPOSITIONS ( m r ) FOR CHLOROPHYLLS IK PYRIDINE AND
IK 50% AQCEOUS PYRIDIKE Figures in parentheses are molar decadic extinction coefficients a! [ a = log ( l o pX ) l/cd with c in mole 1.-1 and d in em.] based on the values of Zscheile and Comar for ether solutions.6 Peak positions are rtl mp and extinction coefficients are A0.2 x 104. Chlorophyll a (7. 4 X 10 -8 M)-7 ------Chlorophyll b (6.9 X 10-8 M)-----
--
% Water
Red peak
0 50
1.6
670 (8 3 X l o 4 ) 669 ( 7 o x 104)
Blue peak
Red peak
442 (11 5 x 104) 436 ( 8 . 0 X 104)
655 ( 4 0 X IO4) 653 (2 7 x 104)
2
1.4
1.2
1.2
.6
1.0
g T .4 0"
2 m c
x x
104) 104)
x
10-6 M , 2-cm. ).
1.6
1.4
i
j
Blue peak
472 (13 7 467 ( 9 o
1.0
c
0
'g 0.8
.u
0
0.8
0.6
0.6
(b.4
0.4
0.2
0.2
400
350
450 500 550 Wave length (mp).
600
660
700
Fig. 1.-Absorption spectrum of chlorophyll a (7.4 X M, 2-cm. cell) in dry pyridine () and in 507, aqueous pyridine (- - - -). 1.8
-
1.6
-
1.4
-
1.2
-
6
3
I .*
0" 0.8 -
0.4
-
400
450 500 550 600 Wave length ( m d .
650
Fig. 2.-Absorption spectrum of chlorophyll 6 (6.9 X M, 2-cm. cell) in dry pyridine (-) and in 50% aqueous pyridine (- - -).
-
400 420 440 Wave length (mp).
460
Fig. 3.--Soret band of chlorophyll a (approx. 8 cell) a t +24' (- - - -) and -55' (-
was measured in a special Dewar cell. "Five per cent aqueous pyridine" for the purposes of this investigation means a bolution obtained by mixing 5 ml. of water with enough pyridine to produce 100 nil. of solution. All solutions were air-saturated. Figure 1 shows the spectra of chlorophyll a in dry pyridine and in pyridine containing water. The height of the violet shoulder is seen to be almost independent of the solvent composition. B minor effect of the introduction of water is the suppression of the Patellite a t 638 mpb and enhancement of the small peak a t 619 mp. The height of the principal red band is reduced in the aqueous solution. The most prominent effect of water is to reduce the height of the blue component of the Soret band (this change is about half complete in 5% aqueous solutions). Furthermore, the introduction of water produces an unmistakable blue shift in the position of the blue peak. Figure 2 shows the analogous effects in chlorophyll b and in Table I are summarized the chief absorption band positions and molar extinction coefficients for both pigments in dry pyridine and in 5oyOaqueous pyridine. Cooling a solution of either of the chlorophylls in 15% aqueous pyridine restores the general characteristics of the spectra in dry pyridine (Fig. 3).
.g 1.0 -
0.6
380
Discussion The suppression by water of the minor peak at 638 mp (chlorophyll a in dry pyridine) has been explained in terms of the formation of a pyridine hydrate which is less basic than pyridine itself.6 This explanation is not applicable to the changes in the Soret band since the shape of this band in dry pyridine is not characteristic of basic solutions only. An obvious possibility is that in dry pyridine the pig(5) A. A. Kraenovski and G . P. Brin, Dokl. Akad. Nauk S S S R , 80, 527 (1953). (6) F. P.Zacheile and C. L. Comar, Bot. Gas., 102, 463 (1941).
li’eb., 1963 ment is present as one or more pyridinates and addition of water converts the pyridinates to hydrates (in each case the solvate referred to is that which presumably activates the fluorescence). However, the spectral changes produced by adding relatively large quantities of water to pyridine solutions of chlorophylls are in marked contrast to those accompanying hydration of the pigment dissolved in benzene. Moreover, it is known that the absorption spectra of activated solutions of chlorophylls in benzene are independent of the nature of the activat0r.l The present data therefore demonstrate an interaction between chlorophylls and water which is distinct from the process of fluorescence ac tivation. Acknowledgment.-This investigation was made possible by a Research Grant (A-2733 C3) from the Ilivision of Arthritis and Metabolic Diseases of the United States Public Health Service. APPLICATION OF GESERALIZED QLASILATTICE THEORY TO HEAT OF PtlIXIKG D A T h FOR HYDROCARBON-HALOGES SUBSTITUTED HYDROCARBON SYST.ERIS BY J. B. OTT. J. R. GOATES.AND R. L. SNOW Department of Chemzstry, Brigham Young C’n,iaerszty. Provo, Ctah
Reeemed J u l y 2, 1962
It was the purpose of this investigation to determine how well a generalized quasi-lattice theory could describe the heats of mixing in hydrocarbon-halogen substituted hydrocarbon systems and to ascertain the magnitude of the energies of interaction that are required for a quasi-lattice interpretation of such systems. The very high heats of mixing and other related anomalies observed in hydrocarbon-perfluorohydrocarbon systemsl make them of special interest. Barker’s2#ageneralized quasi-lattice model that recognizes different types of sites on the same molecule was used in this study. To apply the theory, it is necessary to fix values for two types of parameters: (a) lattice constants that determine the number and types of contact points on each molecule, and (b) energies of interaction for all possible combinations of contact points. In previous ~ ~ we were r successful k ~ in ~describing ~ hydrocarbon-alcohol systems by assuming that each hydrocarbon tetrahedron occupied one site in a fourfold coordinated lattice. The same assignment was made in this study, and because of the similarity in size of the hydrocarbon and perfluorohydrocarbon tetrahedra, the latter was also assigned one site in the lattice. Each atom of chlorine and bromine was considered to occupy one site. The number of eontact points for each molecule was deduced from the structural formulas of the compounds. Type (I) is uscd to designate a contact point on a saturated hydrocarbon; (S), an aromatic hydrocarbon; (F), a perfluorohydrocarbon; and (Cl) and (Br) for contact points on chlorine and bromine atoms, respecR.L. S c o t t , f . Phya. Chem,, 62, 136 (1958). ( 2 ) J. A. Barker, J . Chdm. Phya., 20, 1526 (3) J. A. Barker, zlizd., 21, 1391 (1963). (1)
315
KCYTB
(1952).
(4) J. R. Goates, R.L. Snow, and M. R. Jam&, J . Phys. dhem., 65, 385 (1961). ( 5 ) J, Rh Goatds, Rh LASnow, and Jh B. O t t , ibid., 66, 1301 (l@32).
tbely. The number arid type of contact points assigned to each compound used in this study are: n-C6H14,14(1); ?%-C7H16,16(I); i-CsH18, 18(I); Cy-CgH12, 12(I) ; CaH6, 12(s); COF14, 14(F); CiF16, 16(F); cC14, 12(C1); CzH4Cl2,4(I) and G(C1); C2H6Br, 5(I) and 3(Br); C2H4Br2, 4(I) and 6(Br). The interaction energy is defined as the energy change ia the quasichemical reaction 1/2(i-i) l/~(j-j) = i-j, where i and j represent the various types of contact points. The energies of interaction were fixed empirically at values ithat produced the best fit of experimental data taken from the literature. The average of the absolute values of the deviations a t mole fractions 0‘2, 0.4, 0.5, 0.6, and 0.8 was the criterion of best fit. The method of making the calculations has been described p r e v i o ~ s l y . The ~ ~ ~rather lengthy computations involved were made on an IBM-650 computer. Table I records tlhe empirically determined energies of interaction. The first three systems listed involved aromatic-aliphatic hydrocarbon type contacts only. The agreement among the three values, each determined from a different system, is fairly good. The average value of 70 cal./mole for the I-S interaction is in rough agreement with the 82 cal./mole value obtainedfor the same interaction from a study of alcoholhydrocarbon ~ y s t e r n s . ~The fourth system constitutes a test of the assumDtion that interactions between I type contact points on different kinds of molecules can be-assigned approximately zero energy The 5th-8th sysiems involve only a single (non-zero energy) type interaction in each system and allowed values for three different hydrocarbon-halogen interactions to be obtained. The discrepancy in the I-F values determined from the two different systems is rather large. The average value of 152 cal./mole, however, will reproduce the heat of mixing data to within 77” of the experimental values of both systems. The fact that a brainched chain hydrocarbon is involved in the 8th system may have something to do with the discrepancy. The theory is too crude and the data are too limited, however, to attempt to distinguish between normal and branched chain hydrocarbons. Systems 9-11 each involve two (non-zero energy) interactions, which 81-8assumed to be equal, allowing the values for four additional hydrocarbon-halogen interactions to be determined. The superscripts introduced at this point are used to distinguish between halogens that are from different types of molecules. For example, a chlorine in CCl, (Cl) has different properties than a chlorine in C2H4C12 (Cl”). In a similar manner a bromine in CzHsBr (Br’) is distinguished from one in CzH4Brz (Br”). Systems 11-15 are more complex in that three types of interactions had to be considered. Two out of the three constants required in each of these systems already had been determined by the study of the first ten systems. The remaining constant was then fixed a t the value that produced the best fit of the experimental data. Column four of Table I lists the one energy of interaction determined by a study of a given system; column three lists the assumptions made about the energy of the other interactions involved in that system. Of particular in1,erest is the comparison of the 14 cal./rnole 1 4 1 interaction and the approximately 150
+
