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W. R. CARPER AND R. M. HEDGES
Fluoranil-Pyridine Charge-Transfer Complexes
by W. R. Carper' Department of Chemistry, California State College at Lo8 Angeles, Los Angeles, Cdifornia
and R. M. Hedges
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Department of Chemistry, Texas A d M Univereity, College Station, Texas (Received January 84, 1966)
Pyridine and nine of its methyl derivatives are complexed with fluoranil in carbon tetrachloride and other solvents a t 25". The spectra may be interpreted as charge-transfer complexes and ion pairs ("inner complexes in the Mulliken sense").
Introduction Charge-transfer complexes between pyridine and various acceptors have been studied extensively by various investigator^*-^ by means of both ultraviolet and infrared spectroscopy. Kosower and co-workers10 have investigated various pyridinium iodide complexes and looked at solvent effects while the solid-state structures of the pyridine and 4-picoline-iodine complexes have been reported by Hassel." In a previous study12 aniline-chloranil complexes were investigated and a successful test of the molecular orbital13 approach was made. As an extension of this type of investigation, pyridine and nine methyl derivatives thereof were complexed with fluoranil in carbon tetrachloride, chloroform, and water a t 25".
Experimental Section Fluoranil (mp 171.5-172.5') was obtained from the Pierce Chemical Co. and used without further purification. Spectral grade solvents were used throughout the investigation. The pyridines were generously donated by the Reilly Tar and Chemical Corp. and were quadruply distilled shortly before their use. Boiling points corresponded to those found in the literature and chemical handbooks. Solutions were made up in the manner described previously, l 2 and measurements were made at 25 f 0.01" with a Beckman DK-1 spectrophotometer.
Results Two new absorption bands appear when fluoranil is added to various methyl derivatives of pyridine in carbon tetrachloride at 25". These new absorption The Journal of Physical Chemistry
bands are listed in Table I. These same absorption bands (both visible and ultraviolet) are seen to increase in intensity with decreasing temperature. This phenomena is reversible and is a consequence of the exothermic charge-transfer complexation. As an electron donor, pyridine offers both the nitrogen lone-pair and the A electrons in the ring. With this in mind, the following questions arise: (1) Are the new transitions n + P * , A + x * , or both? (2) Are the new bands representative of separate complex species, or are they associated with the same complex? In an attempt to answer the first question, molecular (1) To whom correspondence may be addressed. (2) (a) D. L. Glusker and H. W. ThomDson. J. Chem. SOC..471 (1955); (b) V. G. Krishna and M.Chowdhum', J . Phys. Chem., 67, 1067 (1963). (3) R. E. Merrifeld and W. D. Phillips, J . Am. Chem. Soc., 80, 2778 (1958). (4) J. Nag-Chaudhuri and S. Basu, Trans. Faraday SOC.,5 5 , 898 (1959). (5) E. K. Plyler and R. S. Mulliken, J . Am. Chem. SOC.,81, 823 (1959). (6) A. I. Popov and R. T. Pflaum, ibial., 79,570 (1957). (7) A. I. Popov and R. H. Rygg, ibid., 79, 4622 (1957). (8) C. Reid and R. S. Mulliken, ibid., 76, 3869 (1954). (9) R . A. Zingaro and W. B. Witmer, J . Phys. Chem., 64, 1705 (1960). (10) E. M. Kosower, J . Am. Chem. SOC.,77, 3883 (1955); 78, 5838 (1956); 80, 3253 (1958); 82, 2195 (1960); 83, 2013, 3142, 3147 (1961). (11) 0. Hassel and H. Hope, Acta Chem. Scand., 15, 407 (1961); 967 (1961). (12) W. R. Carper, R. M. Hedges, and H. N. Simpson, J . Phys. Chem., 69, 1707 (1965). (13) R . E. Miller and W. F. K. Wynne-Jones, J . Chem. SOC.,2375 (1959).
FLUORANIL-PYRIDINE CHARGE-TRANSFER COMPLEXES
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Table I : Fluoranil-Pyridine Complexes in Carbon Tetrachloride a t 25' Donor'
-New
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Pyridine %Picoline 3-Picoline 4Picoline 2,CLutidine 2,SLutidine 2,BLutidine 3,4Lutidine 3,bLutidino 2,4,6Collidine
transitions,b mr-
396 (1) 385 (1) 393 (1) 382 (2) 375 (1) 355 (1) 355 (1) 377 (1) 390 (2) 369 (1)
(1)
(2) (3) (4) (5) (6) (7) (8) (9) (10)
455 (3) 455 (2) 455 (2) 455 (2) 455 (2) 455 (1) 455 (1) 455 (2) 455 (3) No additional band
' Numbers in parentheses correspond to data points in Figure 2.
L
' Numbers in parentheses are standard deviations.
\
I
25,000
\6CT orbital calculations were obtained (see Computations section) on pyridine and its derivatives, and the results are given in Table 11. The coefficient of the highest filled donor orbital (in /3 units) was plotted vs. Yct (cm-') of the highest energy band. The result of this plot is contained in Figure 1. The work of NagChaudhuri and Basu4 on iodine complexes is also included.
Table 11: Results of Huckel Molecular Orbital Calculations r-
B coeff Compound
Pyridine 2-Picoline 3-Picoline 4Picoline 2,4Lutidine 2,SLutidine 2,BLutidine 3,4Lutidine 3,SLutidine 2,4,BCollidine
No. of electron centers
6 6 6 6 6 6 6 6 6
B coeff est A 0
B coeff of highest BO
of 2nd highest BO
o.9265 0.9411 0.9365 1.0000 1.0370 0.9415 1.0133 1.0552 0.9799 1.1912
o.7744 0.6762 0.7374 0.6410 0.5754 0.5904 0.6399 0.5849 0.7298 0.5380
0.9176 0.8943 1.0000 0.8977 0.8665 0.7808 0.8916 0.7808 0.7808
of low-
l.oooo
Electron density on nitrogen
o.9043 0.9973 0.8998 0.9720 1.0650 0.9936 1.0962 0.9683 0.8951 1.1637
The linearity of the Figure 1 plot indicates that the two transitions which appear for each of the new complexes are (a) n -t P* (450-m~band) and (b) P + T* (as indicated by Figure 1). If the identification of the transition energies is correct, then there is the question concerning the possibility of isomeric complexes. That is to say, are the transition energies associated with the same species, or
I
27,000 (CM")
Figure 1. p coefficient us. -yot (cm-1): (1) pyridine, (2) 3-picoline, (3) 3,5-lutidine1 (4) 2-picoline, (5) Ppicoline, (6) 2,4lutidine, (7) 3,4lutidine, (8) 2,4,6-collidine.
are there two distinctly different complexes present in the same solution? The next step was to consider the effect of other solvents (chloroform and water) on the same complexes. The spect'ra of a sample pyridine-fluoranil complex in carbon tetrachloride, chloroform, and water is contained in Figures 2 and 3. Each spectrum is broken up into P* and P -F P*) so as to the two separate bands (n indicate the relative intensities of t'he two transit,ions. A summary of the data given in Figures 2 and 3 is contained in Table 111.
Table 111: Analysis of Spectral Data Contained in ~i~~~~ 2 and 3 ca
Solvent
cc1, CHCla H20
n-
-
for the transitionn
A*
A-
86 ( ~ 4 5 mp) 0 3,800 (-450 mp) 10,600 (-370 mp)
A*
600 (-396 mp) 2400 (-396 mp) 6000 (-335 mp)
a ea = apparent molar extinction coefficient = absorbance of pyridine-fluoranil charge-transfer band/ [fluoranil].
Before considering this table, it should be pointed out that the observed spectra in chloroform and water were seen to be temperature independent, thus indicating the presence of a new species which cannot be the same type as that observed earlier in carbon tetrachloride. Examination of Table I11 reveals a blue shift of the two bands as one proceeds to a solVolume 70, Number 10
October 1966
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W. R. CARPERAND R. M. HEDGES
water as compared with a separation c.t^ 4.0 ev) in carbon tetrachloride. All of the above can be represented by the mechanism
D
+ A e C1 (outer complex) +
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Cz (inner complex, D+A-)
Figure 2. Spectra of pyridine-fluoranil solutions in CC14 and CHCls; absorbance us. wavelength (mp): (1) fluoranil, 1.167 X 10-3 M in CCl4; pyridine, 0.505 M; (2) fluoranil, 1.167 X M in CCh; (3) fluoranil, 1.44 x 10-4 M in CHCla; pyridine, 0.505 M ; M in CHCla. (4) fluoranil, 1.44 X
Such a mechanism was first postulated by Mulliken and Reid8 for pyridine-iodine complexes. More recently, Miller and W y n n e - J o n e ~ ’ ~have ? ~ ~ used this model for a variety of trinitrobenzene-amine complexes. The relative intensities are explained by assuming that the geometry of the complex varies from solvent to solvent. That is to say, the inner complex (favored in the more polar solvent of high dielectric constant) exists in a form such that most of the transfer occurs through the nitrogen atom. Consequently, the n + x* transition becomes enhanced as one proceeds to t8hemore polar solvent. As a final point, one would expect that if two separate types of complexes existed simultaneously in solution and if these types were (a) x + T * and (b) n + x*, that the spectrum of (a) would be only mildly perturbed (if at all) by the various changes in solvent. The fact that both transitions are affected simultaneously, coupled with the results in Figure 1, leads the investigators to conclude that the new transition energies are indeed all representative of the same species of complex.
Computations In order to apply the Dewar approach15it was necessary to obtain molecular orbital calculations on all of the donor molecules. These were calculated by the Huckel method using a modified version of Wiberg’s program16 and an IBI4 7094 digital computer. The results of the calculations are given in Table 11. A six-electron model was used to calculate the energy levels in the usual form ctc
ac
Figure 3. Spectra of pyridine-fluoranil solutions in HzO; absorbance vs. wavelength (mp): (5), fluoranil, 3.48 X 10-6 M ; pyridine, 0.505 M; (6), fluoranil, 3.48 X 10-6 M .
- zpCc= A 0 (antibonding orbital)
+ xPcc = BO (bonding orbital)
The heteroparameter used for nitrogen was -0.19 in accordance with Brown’s work1’ while the methyl inductive parameter used was -0.50.18 (14) R. E.Miller and W. F. K. Wynne-Jones, J. Am. Chem. Soc., 4886 (1961).
vent of higher dielectric constant. Furthermore, the relative intensities (see Table 111) have reversed themselves (the n -+ T* transition is more dominant in the solvent of higher dielectric constant) and also have been compressed (there is a separation of 2.5 ev in The Journal of Physical Chemietry
(15) M. J. S. Dewar and A. R. Lepley, J . Am. Chem. SOC.,83, 4560 (1961). (16) K. B. Wiberg, “Physical Organic Chemistry,” John Wiley and Sons, Inc., New York, N. Y.,1964. (17) R. D.Brown and M. L. Heffernan, dustralian J . Chem., 12, 554 (1959). (18) A. Streitwieser, “Molecular Orbital Theory for Organic Chemists,” John Wiley and Sons, Inc., New Pork, N. Y., 1961.
REFERENCE FRAMES IN RfEMBRANE TRANSPORT
Acknowledgment. This work was supported by Robert A. Welch Foundation Grant A-106. Computer
3049
facilities were provided by the Texas A & M Data Processing Center.
The Choice of Reference Frame in the Treatment of Membrane Transport
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by Nonequilibrium Thermodynamics
by D. C. Mikulecky and S. R. Caplanl W e i z m a n n Institute of Science, Rehovoth, Israel
(Received J a n u a r y 86,1 9 6 6 )
It is shown quite generally for isotropic membranes, and with some restrictions for anisotropic membranes, that in describing transport under stationary-state conditions, the membrane may be chosen legitimately as the frame of reference for a local dissipation function. This is true even when there is no mechanical equilibrium, in other words when viscous flow occurs, provided that the mean density of the membrane over a lateral plane is constant in the direction of flow. The forces conjugate to the membrane-centered flows are the gradients of chemical or electrochemical potential of the permeating species. However, the local dissipation function must be averaged over a cross section a t any depth within the membrane, and hence the flows likewise must represent averages, which measured values usually do. The local phenomenological relations corresponding to the average local dissipation function are then symmetrical. The assumption that this is so is implicit in the Kedem-Katchalsky treatment of membrane processes.* As an example, an important limiting case, that of a set of charged pores having a diameter very large compared to the thickness of the electrical double layer, is examined in detail. In a recent treatment of this model, Kobatake and Fujita obtained unsymmetrical phenomenological relations for the average flow^.^,^ This result is not correct. It turns out that their approach indeed leads to symmetrical relations if the local forces and flows are chosen so as to form a set of conjugate pairs.
Introduction I n the study of transport processes in free solution, using the nonequilibrium thermodynamics of continuous systems, the local center of mass is the usual frame of reference for diffusional Ot’her frames of reference available are the local center of volume or any of the individual components of the system, in particular the solvent. I n membane transport, the most convenient component to choose as a frame of reference, both experimentally and theoretically, is clearly the membrane itsself. Some cases of membrane trans-
port may be approached from the point of view of solution theory. This is possible when the membrane is essentially a system of rather large pores (compared to (1) Biophysical Laboratory, Harvard Medical School, Boston 15, Mass. (2) 0. Kedem and A. Katchalsky, J . Gen. Physiol., 45, 143 (1961). (3) Y. Kobatake and H. Fujita, J . Chem. Phys., 40, 2212, 2219 (1964). (4) Y. Kobatake and H. Fujita, Kolloid-Z.. 196, 58 (1964). (5) S. R. de Groot and P. Maeur, “Non-Equilibrium Thermodynamics,” North-Holland Publishing Co., Amsterdam, 1962.
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