J. Phys. Chem. 1985,89, 3999-4002
3999
Effect of Nolfaqueous Solvents on the Chemisorptlon and Orientation of Aromatic Compounds at Smooth Polycrystalline Platinum Electrodes: Naphthohydroquinone in Water-Benzene Solutlons Dian Song, Manuel P. Soriaga,* Kenneth L. Vieira, Donald
e. Zapien, and Arthur T. Hubbard*
Department of Chemistry, University of California, Santa Barbara, California 93106 (Received: March 8, 1985)
The chemisorption behavior of naphthohydroquinone (NHQ) at smooth polycrystalline Pt electrodes has been studied in aqueous solutions containing preselected amounts of benzene, benzoic acid, or hydroquinone (HQ), as a means of investigating the influence of homocyclic aromatic (nonaqueous)solvents. Packing density measurementswere made with thin-layer electrodes. The F vs. log C" curves for NHQ were found to be extremely sensitive to the presence of any of the three model aromatic solvents. As little as low5M aromatic solvent was sufficient to alter the adsorption profile of NHQ; at higher aromatic solvent concentrations, the orientational transitions observed with pure NHQ solutions were completely suppressed. Packing density measurements indicated that, at sufficiently high concentrations, the aromatic solvent molecules were chemisorbed vertically and prevented the chemisorption of NHQ in the horizontally oriented state.
Introduction Weakly coordinating solvents (such as water) and electrolytes (such as perchlorate) have been shown to be spontaneously and irreversibly displaced from platinum electrode surfaces by a wide variety of organic compounds." Systematic studies with atomically smooth2 polycrystalline Pt thin-layer electrodes and various aromatic compounds have demonstrated that the resulting chemisorbed organic monolayer consists of close-packed molecules attached in specific, identifiable Identical phenomena have been observed a t well-defined Pt( 111) singlecrystal electrode surfaces;" postulated orientations have been verified by surface infrared spectroscopy."J It has been found that orientational phase transitions within the chemisorbed layer can be induced by changes in adsorbate (solute) concentration,lcVd temperature,If and coadsorption of other surface-active species;lb,qflat-to-vertical orientational transitions are suppressed by surface roughness.1u Adsorbed molecule orientation has been shown to exert a profound influence on the surface chemistry of the adsorbate itself'h-Jg*and on the electrochemical reactivity of (1) (a) Soriaga, M. P.; Hubbard, A. T. J. Am. Chem. SOC.1982, 104, 2735. (b) Soriaga, M. P.; Hubbard, A. T. J. Am. Chem. Soc. 1982,104,2742. (c) Soriaga, M. P.;Hubbard, A. T. J. Am. Chem. Soc. 1982,104,3937. (d) Soriaga, M. P.; Wilson, P. H.; Hubbard, A. T.; Benton, C. S. J. EIeczroanaL Chem. 1982, 142,317. (e) Chia, V. K. F.; Soriaga, M. P.; Hubbard, A. T.; Anderson, S.E. J. Phys. Chem. 1983, 87, 232. (f) Soriaga, M. P.; White, J. H.; Hubbard, A. T. J . Phys. Chem. 1983,87, 3048. (g) Stickney, J. L.; Soriaga, M. P.; Hubbard, A. T.; Anderson, S.E. J. Electroanal. Chem. 1981, 125,73. (h) Soriaga, M. P.; Stickney, J. L.; Hubbard, A. T. J . Electroanal. Chem. 1983, 144, 207. (i) Soriaga, M. P.; Stickney, J. L.;Hubbard, A. T. J . Mol. Catal. 1983, 21, 211. ti) Soriaga, M. P.; Hubbard, A. T. J . Electroanal. Chem. 1983,159, 101. (k) Soriaga, M. P.; Hubbard, A. T. J . Phys. Chem. 1984,88, 1089. (1) Soriaga, M. P.; Hubbard, A. T. J. Phys. Chem. 1984,88, 1758. (m) Soriaga, M. P., Chia, V. K. F.; White, J. H.; Song,D.; Hubbard, A. T. J . Electroanal. Chem. 1984, 162, 143. (n) Soriaga, M. P.; Hubbard, A. T. J. Electroanal. Chem. 1984, 167, 79. ( 0 ) Chia, V. K. F.; Soriaga, M. P.; Hubbard, A. T. J. Electroanal. Chem. 1984, 167, 97. (p) Soriaga, M. P.; White, J. H.; Song, D.; Hubbard, A. T. J. Electroanal. Chem. 1984,171, 359. (9) Soriaga, M. P.;White, J. H.; Song, D.; Hubbard, A. T. J . Phys. Chem. 1984, 88, 2285. (r) Soriaga, M. P.; Binamira-Soriaga, E.; Hubbard, A. T.; Benziger, J. B.; Pang, K.-W. P. Inorg. Chem., in press. (s) Soriaga, M. P.; White, J. H.; Song, D.; Chia, V. K. F.;Arrhenius, P. 0.; Hubbard, A. T.Inorg. Chem., in press. ( t ) Chia, V. K. F.;Stickney, J. L.; Soriaga, M. P.; Rosasco, S.D.; Salaita, G. N.; Hubbard, A. T.; Benziger, J. B.; Pang, K.-W. P. 1. Electroanal. Chem. 1984, 163, 407. (u) White, J. H.; Soriaga, M. P.; Hubbard, A. T. J. Electroanal. Chem. 1984, 177, 89, (v) Soriaga, M. P.;Song, D.; Hubbard, A. T. J. Phys. Chem. 1985,89,285. (w) Soriaga, M.P.;Song, D.; Zapien, D.; Hubbard, A. T. Langmuir 1985,1, 123. (x) Pang, K.-W. P.; Benziger, J. B.; Soriaga, M. P.; Hubbard, A. T. J. Phys. Chem. l984,88,458j. (y) Song, D.;Soriaga, M. P.; Vieira, K. L.: Hubbard, A. T. J. Elecfroanal. Chem., in press. (z) White, J. H.; Soriaga, M. P.; Hubbard, A. T. J . Efectroanaf. Chem., in press. (2) Hubbard, A. T. Acc. Chem. Res. 1980, 13, 177; J . Vac. Sci. Technol. 1980, 17, 49.
unadsorbed redox couples.1z It is of prime importance to extend the above studies to include nonaqueous solvents since, in many organic electrochemical/synthetic reactions carried out in nonaqueous media: mechanisms have been postulated which include adsorbed molecule orientation effects. Phrases such as "steric shielding by the electrode from attack on ohe side of the sub~trate/interrnediate"~ imply preferred modes of surface attachment (orientations). Deviations from the results ip aqueous solutions summarized above are to be expected a priori when nonaqueous media are utilized because these solvents exhibit greater surface activity than pure water.'g The present investigation is one step in this direction: described here are data concerning the chemisorption and orientation of naphthohydroquinone (NHQ) on smooth polycrystalline Pt electrodes in aqueous solutions containing various concentrations of homocyclic aromatic compounds [benzene, benzoic acid, and hydroquinone (HQ)]. The use of HQ as a model (aromatic) nonaqueous solvent is advantageous since the coadsorption packing density of this compound in the presence of N H Q can be easily measured.
Experimental Section Preparation of atomically smooth polycrystalline Pt electrodes has been described.Iu The electrodes were smoothed with successive grades of wet emery paper and polished with 7 Mm and 0.25-pm diamond compounds. The polished electrodes were then treated with hot concefltrated nitric acid. Thermal annealing consisted of heating the polished electrode to redness for 60 s in a gas-oxygen flame, followed by quenching in pyrolytically triply distilled water;5 this heating-quenching cycle was repeated at least 10 times. The active surface area was determined froni hydrogen atom underpotential deposition (UPD) measureIflents.Ius6 The effect of surface roughness on the adsorption, orientation, and electrochemical oxidation of aromatics on Pt in aqueous electrolyte has been discussed. l U Absolute packing density measurements have been described in detail.'" I' was obtained from eq 1, where Ql,Q2, and Q
(3) Baizer, M. M.; Lund, H. 'Organic Electrochemistry"; Mafcel Dekker: New Ydrk, 1983. (4) Eberson, L.; Sternerup, H. Acta Chem. Scand. 1972, 26, 1431. ( 5 ) Conway, B. E.; Angerstein-Kozlowska,H.: Sharp, W. B. A.; Criddle, E. E. Anal. Chem. 1973,45, 1331. (6) Htlbbard, A. T.; Ishikawa, R. M.; Katekam, J. J. Ektroanal. Chem. 1978,86, 271. Ishikawa, R. M.; Hubbard, A. T. J . Electroonal. Chem. 1976, 69, 317.
0022-3654/85/2089-3999$01 SO10 0 1985 American Chemical Society
4000
The Journal of Physical Chemistry, Vol. 89, No. 19, 1985
Song et al.
W
a
w
I
a
I
'\\
I
8 8 f I--
2 W
a a 2
0
0.2 0.4 0.6 0.8 1.0 1.2 POTENTIAL, VOLT VS. AgCl
J
Figure 1. Thin-layer current-potential curves for clean and benzenepretreated smooth polycrystalline platinum electrode in 1 M HzS04: (-) benzene adsorbed from 2 mM aqueous solution; (-- -1 benzene adsorbed from 0.1 mM solution; clean Pt. Thin-layer cell volume, V = 3.65 &; electrode surface area = 1.16 cm2;rate of potential sweep, r = 2 mV s-'; temperature, T = 25 f 1 O C . (e-)
represent, respectively, the charge for quinone/diphenol electrolysis of the subject diphenols after one, two, and multiple (23) fillings of the thin-layer cavity; Qb is the appropriate background charge, F is the Faraday's constant, and A is the electrode surface area determined by hydrogen underpotential deposition.6 In regions where r was independent of concentration, - QZb) was equal to ( Q - Qb). Equation 1 is applicable regardless of whether or not surface-active contaminants are present in solution.lp*v.w Adsorption of the subject compounds was carried out for 180 s, without external electrode potential control, at room temperature. The influence of electrode potentiallo and the qualitative aspects of the kinetics of adsorption'r~w have been reported. The water-benzene solutions were deaerated with oxygen-free nitrogen previously bubbled through a saturation tube containing the same water-benzene solution used in the coverage measurements. Adsorption experiments were always started from clean electrodes; that is, adsorption was competitive between N H Q and nonaqueous solvent. Electrochemical regeneration of clean surfaces was by oxidation at 1.2 V (Ag/AgCl reference) and reduction at -0.20 V in molar H2S04 or HC10,; surface cleanliness was verified by current-potential curves in pure supporting electrolyte.'"
(e2
Results and Discussion Figure 1 shows thin-layer current-potential curves for benzene chemisorbed from aqueous solutions onto a smooth polycrystalline Pt electrode. The dashed curve was obtained when adsorption was from 0.1 mM benzene, while the solid curve was from 2 mM benzene; the dotted curve was for clean (untreated) Pt. It should be noted that these curves were obtained in the absence of unadsorbed benzene; hence, the presence of the large anodic peak at 0.90 V indicates oxidation of irreversibly adsorbed benzene. The anodic peak was smaller when adsorption was from 0.1 mM than that from 2 mM benzene, and is due to Occurrence of a higher packing density a t the higher concentration. This behavior is reminiscent of that of diphenolic and quinonoid compounds:' adsorption from low concentrations led to flat-oriented (lowcoverage) species, while adsorption from high concentrations formed vertically attached (high-coverage) intermediates. However, the anodic oxidation charge (eox, the area under the anodic peak corrected for background oxidation of the Pt surface) is not linearly related to the packing density on going from flat ($) to vertical ($) orientation because the extent of oxidation is lower in the vertical than in the flat Molecular area ca1culationsla for benzene gave packing densities r16= 0.35 nmol cm-2 and r,z = 0.66 nmol cm-2; if it is assumed that benzene adsorption at C? 5 0.1 mM yielded v6-attached species and adsorption at C' 1 2 m M gave v2-oriented intermediates, the measured charges for oxidation of flat- and edge-adsorbed molecules were eo,,+= 1170 pC, and QOx,,2 = 1500 p C . From Faraday's law, Q,, = n,,FAr (where the surface area, A , is 1.18
02
0.4
a6
1.0
0.8
1.2
POTENTIAL, VOLT VS. AgCl
Figure 2. Thin-layer current-potential curves for clean and benzoic acid-pretreated Pt electrode in 1 M HzS04: (-) benzoic acid adsorbed from 2 mM solution; (- - -) benzoic acid adsorbed from 0.1 mM solution; clean Pt. Other experimental conditions were as in Figure 1. (-e)
0.6
E
NHP odxirption In presence of benzene 8 No benzene 0 28 mM benzene 0 SaturaleZ benzene T =
6
0.5 I
I
V
0.4 -1
0
5 0.3 Y
0
2 0.2 0.I
o.o~"'"''"''"''"~ 5.0
- LOG4.0CD,,
" " " " " " '
3.0
(MI
Figure 3. r vs. naphthohydroquinone(NHQ) concentration curves on smooth polycrystalline Pt electrode in aqueous 1 M HCIOPcontaining preselected concentrations of benzene: (e)0 mM benzene; ( 0 )0.28 mM benzene; (0) saturated benzene. The solid lines interconnect the data points and do not represent any theoretical curve. Other experimental conditions were as in Figure 1.
cm2,F is the Faraday, and no, is the number of electrons transferred during electrocatalytic oxidation of chemisorbed benzene), nox,+ = 30 and nox,nz= 20. These nox values are consistent with those measured for chemisorbed benzoquinone (24 and 14 for v6 and 2,3-q2-structures, respective1ylh),and suggest that, under the present conditions, oxidation of q6-oriented benzene is complete (to C02) while oxidation of $-oriented benzene is only partial (to include products of lower oxidation states such as maleic acid, a product previously identified in the oxidation of q2-adsorbed benzoquinone",'). Identical behavior was observed for benzoic acid, Figure 2. The complications afforded by the introduction of benzene-type nonaqueous solvents are clear from Figure 1: (i) benzene interacts spontaneously and irreversibly with the Pt electrode surface, and (ii) depending upon the concentration of benzene in the analyte solution, the chemisorption of benzene may lead to different orientational states. Figure 3 shows packing density vs. concentration curves for naphthohydroquinone (NHQ) in aqueous solutions containing various amounts of benzene; N H Q adsorption profiles in the presence of benzoic acid are shown in Figure 4. Behavior of NHQ in the absence of competing aromatic has been discussed before.IGd Briefly, the r plateau at C < 0.1 mM has been associated with formation of a close-packed layer of qlO-oriented NHQ. The intermediate plateau between 0.1 and 0.5 m M involves tilted molecules which are probably in librational motion since this plateau disappears upon cooling to 5 OC,and is extended to higher concentrations at 45 OC.lf The upper plateau represents formation of rigid vertically oriented species; molecular area calculationsla
Naphthohydroquinone on Pt in Water-Benzene Solutions
The Journal of Physical Chemistry, Vol. 89, No. 19, 1985 4001 0.2
NHQ adsorption in presence of benzolc acid 0 No benzoic ocld OOlmM benzoic ocid 0 0 2 5 m M benzoic acid 0 2 0 mM benzoic acid 8 30 mM benzoic ocid
I
POTENTIAL, VOLT vs. AgCl 0.4 0.6 0.8 1.0 1.2 1
1
I
I
1.4
I
I
.., .. q'a-NHQ
----
k! W
.-.-.
4
8
r)lo-NnQ expoaed to ZmF HQ
clean pt
0
z
-I
i
5 03
a LL
W z
0
2 V
Y
0
I
I
L*02
I
0.2
01
00
Figure 4. r vs. log CONHQ curves on smooth polycrystalline Pt electrodes in aqueous 1 M HC104 containing various amounts of benzoic acid: ( 0 ) 0 mM; ( 6 ) 0.01 mM; ( 0 ) 0.25 mM; (0) 1.0 mM; and ( 0 ) 30 mM benzoic acid. All other experimental conditions were as in Figure 3.
and the absence of quinone/diphenol electroactivity in the adsorbed stateId (similar to that in the unadsorbed form) are indications of a 2,3-q2 orientational state. The following features are noteworthy from Figures 3 and 4. (i) The adsorption profile of N H Q is extremely sensitive to low levels of surface-active impurities; concentrations as low as 10 pM aromatic were sufficient to blur the packing density transitions. The presence of surface-active impurities may help to explain the wide variability among adsorption isotherms reported in the earlier literature for aromatic compounds on Pt surfaces.IY (ii) The packing density of N H Q decreased as the concentration of benzene/benzoic acid was increased, the extent of suppression being greater at low N H Q concentrations where qI0-NHQ would have been formed. This decrease in r N H Q is due to increased adsorption of the aromatic solvent as its concentration is increased. (iii) In the presence of 2 mM benzoic acid, r N H Q at 2 mM N H Q was about 75% of that measured in pure water, indicating preferential chemisorption of N H Q relative to benzoic acid. Preferential chemisorption of N H Q relative to H Q was reported earlier.Iw (iv) At benzene/benzoic acid concentrations exceeding 2 mM, where these two compounds are expected to chemisorb vertically, adsorption of q'O-NHQ was completely suppressed, and no packing density transitions were apparent. Evidently, under these conditions, N H Q and the aromatic solvent molecule are adsorbed in edgewise orientations; that is, at high benzene/benzoic acid concentrations, adsorption of N H Q occurs solely in the edge orientation. These observations and conclusions are corroborated by results from studies on the competitive chemisorption between N H Q and HQ; in this instance, the indiuidual and total packing densities (r, = r,,? + FHQ) can be measured simultaneously.'v3wFigure 5 shows thin-layer current-potential curves for irreversible electrochemical oxidation of q'O-NHQ before (dotted curve) and after (dashed curve) exposure to 2 mM HQ; also shown are anodic oxidation curves for clean and v2-HQ pretreated Pt. It is seen that when $O-NHQ was exposed to 2 mM HQ, the peak height and potential of the subsequent oxidation curve shifted to values intermediate between those for pure q2-NHQ and pure q2-HQ.lcqd The same trend was noted when q6-HQ was exposed to 2 mM NHQ.IV Evidently, insertion (coadsorption) of H Q from concentrated solutions into a layer of flat-oriented N H Q yielded a mixture of q2-reoriented N H Q and q2-coadsorbed HQ. The influence of H Q (as model homocyclic aromatic solvent) concentration on N H Q adsorption can be seen in Figure 6 which
I
1
0.8 1.0 1.2 POTENTIAL, VOLT vs. AgCl 0.4
0.6
I
1.4
Figure 5. Thin-layer current-potential curves for clean and pretreated smooth polycrystalline Pt electrode in aqueous 1 M H,S04: (-) q2hydroquinone (HQ) pretreated Pt; $O-NHQ pretreated Pt; (- - -) $"-NHQ pretreated surface exposed to 2 mM HQ solution; (.- .) clean Pt. All other experimental conditions were as in Figure 1. (e-)
NHQ OdsOrptian in presance of HQ
+00 001 mM No HQ
05
g
04
HQ
0 0 8 m M HQ
0 025 mM Ha 0 2.0 mM H Q
T = 25°C
w
d 03
E
Y
p 02 01
Figure 6. r vs. log CONHQ curves on smooth polycrystalline Pt electrodes in aqueous 1 M HCIO4 containing preselected amounts of HQ: ( 8 )0 mM; ( 6 ) 0.01 mM; (0) 0.08 mM; ( 0 )0.25 mM; and ( 0 )2.0 mM HQ. All other experimental conditions were as in Figure 3. NHQ adsorption in presemaI of HQ (fixed). 2mM
,,:C
0
50
0 40
30
20
2
-LOG C",Q(M) Figure 7. Individual ( F H Q and r N H Q ) and total ( r H Q + rNHQ =),'l packing densities on smooth polycrystalline Pt as a function of NHQ concentration in aqueous solutions containing a fixed amount of HQ (2.0 mM): (0)r N H Q ; ( 0 )r H Q ; ( 0 )I'Total.All other experimental conditions were as in Figure 3.
shows N H Q chemisorption profiles at the following H Q concentrations: 0, 0.01,0.08, 0.25, and 2.0 mM. The data presented in Figure 6 are strikingly similar to those obtained when benzene or benzoic acid were present in solution, Figures 3 and 4. Significant changes in the shape of the N H Q adsorption isotherm were brought about by H Q at concentrations as low as 0.01 mM. In the presence of 0.08 mM HQ, the N H Q packing density transitions were barely noticeable; at higher H Q concentrations,
J . Phys. Chem. 1985,89, 4002-4007
4002
evidence for orientational transitions was completely suppressed. The absence of a packing density (orientational) transition when PHQ exceeded 0.1 mM was not unexpected since, at COHQ = 2 mM and in the absence of competing surface-active material, H Q spontaneously forms a closed-packed q2-oriented adlayer.'c,d Hence, any N H Q adsorption when C N H Q < 0.1 mM and C'HQ = 2.0 mM can occur only in the q2-orientation due to molecular area limitations. Direct evidence for this postulate appears in Figure 7 which shows the individual and total packing densities as a function of N H Q concentration when H Q was fixed at 2.0 mM. (i) As expected, r H Q decreased and rmQincreased as was increased; since r H Q = r p H Q at @NHQ 1 mM (lower than @HQ = 2 mM), preferential adsorption of NHQ relative to H Q is indicated.Iw (ii) The total packing density was virtually un-
-
emQ
changed (-0.58 nmol cm-2) at all N H Q concentrations studied; that is, the sum r N H Q r H Q was a constant regardless of the r N H Q / r H Q ratio in the mixed adlayer. This result is understandable only ifNHQ and HQ were each chemisorbed in vertical orientations in which their respective molecular area requirements were very similar, a requirement which is fulfilled only by the 2,3-q2 orientation."Vd
+
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the Air Force Office of Scientific Research for support of this research. Registry No. NHQ, 571-60-8; NQ, 123-31-9; HCI04, 7601-90-3; Pt. 7440-06-4; benzene, 71-43-2; benzoic acid, 65-85-0.
Optical and Resonance Raman Studles of the 1:l and 2:l Complexes of Hexamethylbenzene with Tetracyanoethylene Morgan L. Smith and Jeanne L. McHale* Department of Chemistry, University of Idaho, Moscow, Idaho 83843 (Received: March 19, 1985)
The electron donor-acceptor (EDA) complexes of hexamethylbenzene (HMB) and tetracyanoethylene (TCNE) have been studied by optical and resonance Raman spectroscopy in a variety of solvents. The visible absorption spectra of the 1:l and the 2:1 complexes in cyclohexane have been resolved by multiwavelength linear regression of concentration-dependentabsorbance data. The resonance Raman spectra of CCl,, CH2C12,and cyclohexane solutions of HMB and TCNE have been obtained with 5145-Aexcitation. A low-frequencymode at 163-169 cm-' is assigned to the first overtone of the donor-acceptor stretch, vDA. In cyclohexane, the second overtone of uDA is also observed, but is overlapped by an a2" out-of-plane bending vibration of the methyl groups of HMB, which becomes allowed in the 2:l complex due to vibronic coupling. A simple molecular orbital picture of the bonding in the 1:l and 2:l complexes is proposed which explains the resolved visible spectra as well as the low-frequency Raman data. It is concluded that the second donor molecule is not bound by "charge-transfer" forces, and that, although the ground state 2:l complex has D2* symmetry, the two D-A bonds are of unequal strength.
I. Introduction The existence of electron donoracceptor (EDA) complexes of the type D2A in solutions of high donor concentration is wellestablished.',2 The intense "charge-transfer" electronic transition has frequently been exploited to determine the equilibrium constants for the formation of 1:1 and 2:l complexes from the concentration dependence of the optical absorbance at the wavelength of maximum ab~orbance.~Although including ternary complexes in the model has avoided some of the wavelength- and concentration-dependent discrepancies which were found in the early work, the question of the nature of the binding of the second donor molecule has not really been addressed. Is the Structure of the 2:l complex D-D-A or D-A-D? Is the second donor molecule bound by charge-transfer forces, or are other interactions (van der Waals, electrostatic, etc.) at work? How does the excited state of the 2:l complex differ from that of the 1:l complex? In this paper, we use optical and resonance Raman spectroscopy to address these questions for the EDA complexes of hexamethylbenzene (HMB) and tetracyanoethylene (TCNE) in cyclohexane, CH2C12,and CCl, solutions. H M B and T C N E form a fairly strongly bound T-T* EDA complex. AHo for the 1:l complex has been reported to be -8.44 kcal/mol in CH,C124 and -7.75 kcal/mol in CC14.5 Previously
(3) Foster, R. "Molecular Complexes", Vol. 2, Foster, R., Ed.; Crane, Russak and Co., Inc.: New York, 1974; Chapter 3. (4) Rossi, M.; Buser, U.; Haselbach, E. Helu. Chfm.Acta 1976, 59, 1039,
TABLE I: Equilibrium Constants of HMB-TCNE Complexes solvent K,,L mol-' K2,L mol-' ref
cyclohexane CH,CI,
CCI,
339-2808 293 f 20 280 f 20 11.988 f 0.008 16.8 17 f 2 19.6 180 149 173
30
24 f 4 20 f 4
a b (at 533 nm, 33.5 "C) b (at 580 nm,33.5 "C) c
4 6.1
d e a b
f 14
a
'Mouchet, J.-P.; Rousset, Y . J . Chim. Phys. 1980, 7 7 , 529. *Reference 12. 'Herndon, W. C.; Feuer, J.; Mitchell, R. E. J . Chem. Soc., Chem. Commun. 1971, 435. dMerrifield, R. E.; Phillips, W . D. J . A m . Chem. SOC.1958, 80, 2778.
