J. Phys. Chem. 1985,89, 2053-2058
2053
recently, Trifunac, Sauer, and Jonah3s have specifically addressed the question of the identity of the high mobility cation. They propose that the high mobility species are R+ and/or RH*+, the means of rapid charge movement being H- and H+ transfer, respectively. The production of R+ by decomposition of RH+. into R+ and H is very endothermic and must occur from excited ions. I w a ~ a k has i ~ ~also presented evidence in favor of R+ and RH2+ being mobile species. Trifunac, Sauer, and Jonah35have suggested a mechanism for cyclohexane radiolysis in which the initial ionization leads to G(R+) = G(RH+.) = 2.5. Insofar as
our low-temperature results can say anything about the room temperature situation, the subnanosecond decomposition of excited cations seems to be only a small effect judging from the almost equal yields of 3MO+ and e; which we measure at midpulse in the glasses (Figure 4). However, our results would be consistent with the significant production of R+ if most of the R+ were to recombine with e; before 20 ns. In the same vein, our finding that G(e;) > G(3MO+) at midpulse at 153 K is most simply interpreted as indicating the rapid formation of a less mobile cation, rather than a more mobile cation, but could be reconciled to the production of R+ by a more complicated mechanism.
(34) J. P. Smith, S. Lefkowitz, and A. D. Trifunac, J. Phys. Chem., 86, 4347 (1982). (35) A. D. Trifunac, M. C. Sauer, and C. D. Jonah, Chem. Phys. Lett., in press. (36) M. Iwasaki, Faraday Discuss., Chem. SOC.,in press.
Acknowledgment. We thank the authors of ref 35 and 36 for preprints of their papers. R e t r y NO. 3MO,2216-33-3; 3MO', 64156-02-1; SQ', 79054-29-8; TMPD', 95693-58-6.
Complexation Equilibria in Systems Composed of Phenols and Oxygen Bases in CCI, R. Wolny, A. KoU, and L. Sobczyk* Institute of Chemistry, University of Wroclaw. 50-383 Wroclaw, Poland (Received: April 26, 1984; In Final Form: October 30, 1984)
The complexation equilibria for a number of systems composed of oxygen bases and 2,6-dichlorophenol derivatives in CCl., have been studied. For some of them and particularly for those containing tri-n-butylamine N-oxide, the intracomplex proton transfer equilibrium has been evidenced. The linear correlation between log Km and ApK, is satisfactorily obeyed. In the case of 2,6-dichlorophenols, because of steric repulsion, a contribution of homoconjugated anions is excluded, while generally for systems containing stronger proton donors and acceptors homoconjugated OHO+ cations play an essential role. The structure of (AH)*B adducts is also discussed, and we showed that the second phenol molecule is attached to the OH group of the first one bound directly to the oxygen base.
Introduction The oxygen bases are characterized by an ability to form hydrogen-bonded complexes of various composition. The concentration ratios of particular forms depend not only on the component's concentrations but also on the electron and steric structure of the oxygen base and proton donor. Owing to two lone electron pairs on the oxygen atom, the number of possible species of (AH),B composition increases in relation to nitrogen bases. In most cases, particularly when the excess of A H is not great and carboxylic acids as proton donors are excluded, one can safely assume that n = 1 or 2. For the complexes with n = 1 the greatest probability is ascribed to configuration, where the proton-donor group is arranged coaxially with a lone electron pair, though one cannot exclude the existence of hydrogen bridges coaxial with the X=O bond (11). As far as the (AH)2B complexes are concerned, quite controversial opinions on their structure can be found in the literature. There is evidence supporting the existence of both structures I11 and IV or V.1-3 It appears that their stability depends mainly on the proton affinity of the oxygen base and steric conditions. Simple proton donors like hydrohalides or HNCS form with strong bases mostly complexes of structure 111, while phenols with substituents at the ortho positions form primarily adducts of structure IV or V. Calculations show that both structures I1 and V are energetically less stabile as compared with I and IV, respe~tively.~~ (1) S. Detoni, D. Had& R. Smerkolj, J. P. Hawranek, and L. Sobczyk, J . Chem. SOC.A , 2851 (1970). (2) A. Koll, M. Rospenk, and L. Sobczyk, Bull. Acad. Pol. Sci., Ser. Sci. Chim.,7, 735 (1972). (3) R. Wolny, A. Koll, and L. Sobczyk, Bull. SOC.Chim. Belg.. 93, 99 (1984). (4) K. Mazur, Ph.D. Thesis, University of Wroclaw, 1973.
0022-3654/85/2089-2053$01.50/0
X-
Complexation equilibria become considerably complicated when homoconjugated cations and anions [BHB]+ and [AHA]- may be formed. The oxygen bases exhibit a greater tendency than nitrogen bases to form [BHB]+ cations with stronger bases; (5) J. E. Del Bene, J. Chem. Phys., 62, 1314 (1975). (6) L. A. Curtiss and D. J. Frurip, Int. J . Quantum Chem., Quantum Chem. Symp., No. 15, 189 (1981).
0 1985 American Chemical Society
2054
Wolny et al.
The Journal of Physical Chemistry, Vol. 89, No. 10, 1985
therefore, under favorable conditions the complexes of 2:2 ratio can be formed.'** In addition, the problem of intracomplex proton transfer equilibrium AH-B ;=+ A--HB+ is still an open question. One has no doubts that such equilibria occur in several systems consisting of nitrogen bases; however, so far there has not been any evidence confirming the existence of such equilibria in oxygen bases. Moreover, it has been pointed out that in OH-.O hydrogen bonds most probably only one energy minimum for the proton motion can O C C U ~ . ~ Under the experimental conditions maintained in this work, the complexation equilibria can be limited at most to two A H and B molecules and expressed by the following scheme:
/
L' I
DMSEO
/
OTPASO
21
I 0
-2 I
(BHBI'A-
PNO 0/ ~. DPSEOo
I
100
200
/ 300
/
OMS0
I
LOO
I
500 METHANOL [,.,,-l] OH
OTPP0
+B\K2 r
AH
+
-L;
-
B AB AH A HB+A 2
i A H )2B
In the present paper we would like to report the results of studies on the complexation equilibria of various oxygen bases with a series of phenols of varying pK, value with the same steric conditions, that is having chlorine substituents at the 2,6-positions. In our investigations we have paid special attention to a probable proton transfer equilibrium, trying to find some evidence which could support its existence in solutions of 1:1 complexes. We have also attempted to gather some results which would enable us to carry out an analysis of the structure of 2: 1 complexes. The influence of various factors on the stability of homoconjugated ions has also been discussed.
Experimental Section Diphenyl sulfoxide (DPSO), dibenzyl sulfoxide (DBESO), triphenylphosphine oxide (TPPO), trioctylphosphine oxide (TOPO), triphenylarsine oxide (TPASO), 2,6-dichloro-4-nitrophenol (DCNP), pentachlorophenol (PCP), 2,4,6-trichlorophenol (TCP), and 2,6-dichlorophenol (DCP) of commercial origin were purified by repeated crystallization, while diphenyl selenoxide (DPSEO), tri-n-butylamine N-oxide (TBNO), and 2,6-dichloro-4-methylphenol (DCMP) were synthesized and purified according to ref 10, 11, 12, and 13, respectively. Diphenylethylphosphine oxide (DPEPO) and diphenyloctylphosphine oxide (DPOPO) were received from the Institute of Organic and Physical Chemistry of Technical University of Wrociaw. Nonhygroscopic compounds were stored in a desiccator over anhydrous MgS04 while most hygroscopic DPSEO, TPASO, and TBNO which form mono-, di-, and polyhydrates were dehydrated under vacuum at about 100, 190, and 60 OC, respectively. Carbon tetrachloride of AnalaR grade was dried over MgS04, three-fold distilled, and stored over type 4A molecular sieves. The spectra were recorded on a Beckman UV 5240 spectrophotometer by using the cells of 0.1-mm thickness, i.e. for concentrations similar to those used in our studies for other techniques. IR spectra were recorded on a Perkin-Elmer 180 spectrophotometer by using the KBr cells of 2-mm thickness. The equilibrium constants K1 and K2 were determined by measuring the intensity of the first (7) Z. Dega-Szafran, E. Grech, and M. Szafran, J. Chem. Sor., Perkin Trans. 2, 1989 (1972). (8) Z. Dega-Szafran and M. Szafran, J. Mol. Sfrucf., 45, 3 3 (1978). (9) M. M. Kreevoy and Kwang-chou Chang, paper presented in part at the Proceedings of the 3rd Euchem Conference on Proton Transfer Equilibria and Reactions in Non-ideal Systems, Padova, 1975. (10) H. Leicester, "Organic Synthcses", Collect. Vol. 2, A. H. Blatt, Ed., Wiley, New York, 1957, p 240. (1 1) M. Cinquini, S . Colonna, and R. Giovini, Chem. Id.(London), 1738 (1969). (12) A. C. Cope and H. Lee, J. Am. Chem. Soc., 79, 964 (1957). (13) G. Mazzara and M. Lambreti-Zanardi, Gazz. Chim. Ifal., 26, 401 (1896).
Figure 1. pK,(BH+) plotted against AvoH(methanol) for oxygen bases: TMNO, trimethylamine N-oxide, PNO, pyridine N-oxide; remaining notations as in text.
overtone of vOH in the broad range of CAHo/CBoconcentration ratios on a Unicam-Cary 14 apparatus with quartz cells of 5- and 10-cm thickness. All operations connected with both preparation of the samples and filling of the cells were carried out exclusively in a drybox.
Results and Discussion Donor-Acceptor Properties of Interacting Components. As a measure of donor-acceptor properties in hydrogen-bonded adducts, one uses widely the scale of ApK, = pK,(BH+) - pKa(AH). The pKa values for all phenols studied in this work are k n 0 ~ n . l ~ Less confident seem to be the incomplete data regarding pK,'s of oxygen bases which have been determined by using different methods.15 Therefore, it seemed desirable to perform a unification of the data based on the easily available AYOH values of the shift of the stretching vibration band for methanol (molar fraction of about 4.5 X in the presence of corresponding bases in CC14. We correlated the AvOH(methanol) values determined in this work with accessible pKa(BH+) values and obtained a fairly well fulfilled linear dependence expressed by pK,'(BH+) = 0.0330AvoH(methanol) - 8.876
(1)
This relation plotted in Figure 1 has been used for searching corrected pK,' values and further calculations of ApK,'. The pK,' scale comprising all the complexes investigated ranges from -10.76 for DPSO-DCMP to +1.61 for TBNO-DCNP systems. Obviously there remains a question as to what extent the ApK, values express sufficiently well the donor-acceptor properties of interacting components in CC14. A justification for the use of this comparative acid-base scale can be a well-fulfilled linear relationship between AvoH(methanol) in CCl, for the oxygen bases and pKa, confirmed in this work. On the other hand, we also found a very good linear correlation between pKa and the voHfrequency of phenols in CCl, in the presence of the respective proton acceptor, e.g. dioxane. The linear equation has the form pK,(phenol) = 0.0291voH(dioxane) - 88.909 The application of vOH instead of AvOH was justified by the fact that for each phenol we have a somewhat different energy of the intramolecular hydrogen bond to the chlorine atom. A small contribution of dioxane (molar fraction of about 2.5 X does not markedly change the properties of the solvent. We also confirmed the linear correlation between vOH and pKa for acetone as a proton acceptor. It seems worthwhile to emphasize here that all literature data collected so far on the pK, values determined in dimethyl sulf(14)
E.P. Serjeant and B. Dempsy, "Ionization Constant of Organic Acids
in Aqueous Solution", Pergamon Press, Oxford, 1979. (1 5) D. D. Perrin, "Dissociation Constants of Organic Bases in Aqueous Solution", Butterworths, London, 1965; supplement, 1972.
The Journal of Physical Chemistry, Vol. 89, No. 10, 1985 2055
Phenols and Oxygen Bases in CCl.,
IOgK2
0
0
t
0
e
o 0
A0 0
e
0
0 0
c
2.
i 0
e o
a
1
0
-10
0 DCNP OPCP oTCP ADCP ODCMP 1
I
I
I
-8
-6
-4
-2
oDCNP OPCP oTCP ADCP eDCMP
0
I
I
.
0
2
4
I1
1
I
I
I
-10
-8
-6
-4
-2
PKb Figure 2. Log K, and log K2 plotted against ApKl for oxygen base-phenol systems in CCI4.
oxideI6 show a very good linear correlation between these values for phenols in Me2S0 and H20. This enables us to suspect that the pK, values determined in water express sufficiently well the relative measure of acid-base properties in other solvents. Equilibrium Constants KI, K2, and K3. Evidence for the existence of AHB and (AH)2B adducts in systems composed of oxygen bases and various proton donors in nonpolar solvents has been reported.'-' Also in the case of phenols used in this work we ascertained, in addition to 1:l complexes, the existence of (AH)2Bspecies. The most important proof is the IR spectra shown in Figure 6 and the numerical analysis of the formation curves discussed below. The formation curves related to the concentration equilibrium constants Kl and K2 for reactions A H + B ti AHB and AHB A H + (AH)2B, respectively, were determined from the extinction of the band ascribed to the first overtone of the stretching vibration voHfor free phenol. Such a procedure was chosen since the overtone bands of free and complexed OH groups are better separated and all the operations seem much easier. For three complexes the formation curves obtained from the fundamental and overtone vibration bands were compared and very good agreement was found. In the calculations we used a program based on the algorithm which fits the formation curve function
+
by means of damped least-squares method." In all cases only one set of solutions has been obtained in a wide range of initial K Iand K2 values. The constant K3 equal to CAHB2/ [ (CAH.,, + CA-...He+)] CBwas determined from electronic spectra at an excess of base. For the system of DCNP with TBNO even a slight excess of base causes a drastic change in the spectrum and the appearance of a longwavelength band which corresponds to the undisturbed phenolate anion. A spectral picture in the UV-vis range is shown in Figure 4, where one may see how essential is the shift of the phenolate ion band by going from the 1:l (A--HB+) to 1:2 (A--BHB+) complexes. In order to analyze the influence of the equilibrium constant K3 on K, and K2, it seemed convenient t o analyze the following equations C E O CE + K~CBCAH + K~K~CBCAH' + ~ K ~ K ~ C A H C(3)B ~
+
CAH' = CAH+ K~CBCAH KIK~CAHCB' + ~KIK~CBCA,~ (4) ~~
(16) W. C. Piliugin, Y. I. Umansky, C. W. Wasin, L. J. Chrustalieva, and A. A. Girfanova, Zh. Obshch. Khim., 50, 1625 (1980). (17) K. Levenberg, Q. Appl. Math., 2, 164 (1944).
I
I
I
0
2
4
A
PK:,
For the systems of the equations described, by applying a three-dimensional grid method, we looked for a solution for which IcAH'(expt1) - CAHo(CalCd)l (5) reaches a minimum. As the initial values of K1, K2, and K3, we used the numbers both much higher and much lower from those obtained in particular experiments. One can prove1*that the fulfillment of the condition 3a C Ki is numerical evidence for the existence of the respective equilibrium. In our case the standard deviation u calculated according to ref 19 and 20 for K1 values listed in Table I is less than 20% of KI and for K2 values less than 7% of K2. The results obtained were also analyzed by using the procedure of Rossotti and Rossotti. The linearity of the plot y/(l
- V c A H = Kl + KIK2CAH(2 - Y)/(1 - Y)
where
one considers as evidence for the existence of AHB and (AH)2B species in the investigated concentration range.21 The influence of the equilibrium constant K3 on Kl and K2 has been taken into account for the most polar complex TBNO-DCNP. It is equal in this case to 580 mol-I.L. The obtained values of K1 and K2 for the whole group of complexes are gathered in Table I and presented in diagrams of log K = f(ApK,') in Figure 2. As one may see, for weaker complexes a linear correlation for both log KI and log K2 with ApK,' is adequately fulfilled up to ApK,' = -3. Then we observe some deviation which corresponds more or less to the region where proton transfer states can be expected (see the subsequent paragraph). Equilibrium Constants KpT. The phenol band corresponding to a ?r x* transition after having been bonded in a complex is shifted bathochromically, and the stronger the hydrogen bond the greater the shift is. In Figure 3 one may see as an example the evolution of the PCP long-wavelength band in complexes with various bases. Almost a linear dependence of Av,. on ApK,' is observed, but after having reached some interval of ApKl a
-
(18) Ch. F. Baes and R. E. Mesmer, "The Hydrolysis of Cations", Wiley, New York, 1976, p 71. (19) E. R. Cohen, K. M. Crowe, and J. W. M. DuMond, "Fundamental Constants of Physics", Interscience, New York, 1957, Chapter 7. (20) B. L. Chrisman and T. A. Tumolillo, Compuf.Phys. Commun., 2,322 ( 197 1). (21) F. J. C. Rossotti and H. Rossotti, "The Determination of Stability Constants and Other Equilibrium Constants in Solution", McGraw-Hill, New York, 1961, p 110.
Wolny et al.
2056 The Journal of Physical Chemistry, Vol. 89, No. 10, 1985 TABLE I: Equilibrium C O M ~ I IK~,Sand K 2for Complexes of Oxygen Bases with 2,6-Mchloropheaols at 25 OC in CCI, DCNP
PCP
DCP
TCP
DCMP
base
K1
K2
KI
Kl
K1
K2
KI
K2
KI
K2
DPSO DBESO TPPO DPEPO DPOPO DPSEO TOP0 TPASO
67.0 148.7 954.5 67 1.8 726.2 702.6 1157.5 1940.5 6837.2
1.9 3.3 6.1 7.2 7.6 10.3 15.8 27.2 50.1
26.4 63.4 588.3 354.7 335.3 376.2 1400.5 1867.4 4951.1
1.5 2.3 7.6 5.7 5.7 7.2 14.9 22.4 44.7
14.0 36.1 127.3 172.1 180.8 149.8 778.8 1250.5 1872.4
1.o 1.5 3.0 3.7 2.1 4.5 7.7 21.0 32.9
7.2 15.1 58.0 88.3 93.3 66.3 220.7 862.7 1071.6
0.4 0.7 1.3 1.8 1.6 1.9 3.8 8.9 26.6
6.8 21.0 81.9 81.8 78.2 67.8 169.7 611.6 83 1.8
1.1 1.6 3.8 3.5 3.4 3.4 10.4 17.8 42.8
TBNO
~
~~
AS,,# [cm?
PCP -OXYGEN BASES 2000
1500
1000
500
C
.. -a
I
4444 4 -L
4
I
0
ApKb
Figure 3. Dependence of Au-. shift of long-wavelength band on ApK,' for PCP complexes with oxygen bases in CC14.
band appears which is characteristic for the phenolate anion associated with the cation via the hydrogen bond. Only in certain complexes are the bands which can be ascribed separately to the HB and PT forms in equilibrium visible. In most cases these bands are overlapped and difficult to ascribe, in particular when the fraction of the PT form is small and only a shoulder in the absorption curve occurs. Apart from TBNO as a base, it is possible to show the appearance of a small absorption only in the case of TPASO with the strongest proton-donor DCNP. For this reason we shall be analyzing the PT equilibrium for the example of the TBNO complexes. In Figure 4 we showed the spectra of the TBNO complexes with phenols for various ratios of the concentration of acid and base. In order not to complicate the picture, we presented only bands of pure phenols, 1:l acid to base ratio (2:l for the system DCNP-TBNO), and at fivefold excess of the base. For DCMP one may only observe a band of the HB form independently of the exof base. However, already in the case of DCP complexes it is seen that an inflection of the band occurs, which can be interpreted as a small contribution of the anionic form. Greater excess of TBNO causes a distortion of the band, suggesting an inducing of the anionic form contribution which results from an increase of environment permittivity on the one hand and quite possibly from self-solvation on the other. Nev-
Ab0
350
300
Inml TBNO*DCNP
",,
......._._ 560 [ nml Figure 4. Electronic spectra of TBNO-phenol complexes in CC14: (1) 300
LOO
phenol; (2) phenol-TBNO, 1:l ratio (for DCNP-TBNO system, 2.1
ratio); (3) phenol-TBNO, 1:5 ratio (d = 0.1 mm, 25 "C). ertheless, one does not see any appearance of the complexes [BHBI'A-, since the position of the anionic band in such an ion pair should be more clearly shifted bathochromically. These effects are more distinct in the case of TCP. The PT form fraction is already visible here, and the effect of TBNO e x e s on the PT equilibrium is even stronger. At the same time one may notice a weak wing appearing on the envelope of the A--HB+ band, which can be ascribed to a free anion in the complex [BHB]+A- (A N 350 nm). In the PCP complex the equilibrium is shifted toward the PT form and the system is very sensitive to an excess of TBNO. In addition to ion pairs A--HB+, a high concentration of [BHB]+A-
The Journal of Physical Chemistry, Vol. 89, No. 10, 1985 2057
Phenols and Oxygen Bases in CCl, TABLE II: Proton Transfer Equilibrium Constants K m for TBNO-Phenol Complexes at 25 'C in CCI,
phenol DCMP DCP
ApK,' -2.09 -1.74
TCP
-1
PCP DCNP
K ~= T C~T/CH,
.oo
0.25 1.61
log K ~ T
0.090 0.318
-1.046 -0.498
0.771
-0.113
1.635 10.523
0.214 1422
log K,
2
1
ApK;
-1
-2
Figure 5. Log Km plotted against ApK;.
ion pairs absorbing at about 350 nm is reflected. The TBNODCNP complexes occur predominantly in the form of ion pairs, and an excess of TBNO practically shifts the equilibrium completely toward the ion pairs of type [BHB]+A-. The phenolate anion band for this complex lies at wavelengths up to 430 nm. The position of this band suggests a weak disturbance of the Aanion by the [BHB]+ counterion. In A--HB+ ion pairs strong hydrogen bonding causes a shift of this band toward shorter wavelengths. In the case of twofold excess of DCNP in relation to TBNO, one may see a weak band at about 300 nm (except the nonbonded phenol band at -290 nm), which suggests the existence of a small amount of the weakly bonded second phenol molecule in the (AH)2B complex (see next section). Analysis of the band position for the weakest complexes in this group, for example DPSO-DCNP, indicates approximately the same band position, i.e. about 300 nm, which confirms our assumption. The quantitative analysis of the band contours in the UV region based on the computer aid separation into components ascribed to HB and PT forms enabled us to determine the mole fractions of these forms which are collected in Table 11. The analysis has been carried out by assuming as a reference the band position and molar extinction of the phenolate anion in CCI, with a small amount of saturated KOH solution in methanol. Although the UV spectrum (both the band position and its intensity) of the hydrogen-bonded ion pair does not depend largely on the solwe admitted the procedure used in this work should provide better results than the application of molar extinction coefficients measured in methanol. It has been proved that the influence of alcohol added even in fivefold excess compared with that applied in our case can be neglected. We also found that the position of the a band for the zwitterionic form of TBNO. DCNP adduct in CCl, appeared to be identical with that recorded for the same phenol in CCl, with some amount of saturated KOH solution in methanol (about 1%). According to the Huyskens and Zeegers-Huyskens concept,24 a linear relation between log KpT and ApK, should be anticipated
+
log KpT = ~ P K , C
(6)
and really, as can be seen in Figure 5, such a relation is satisfactorily fulfilled in our case. The and C values found in this (22) A. Koll, M. Rospenk, and L. Sobcyk, J. Chem. Soc., Furuduy Trum.
1, 77, 2309 (1981).
(23) M. Rospenk, I. G. Rumynska, and W. M. Schreiber, Zh. Prikl. Spektrosk., 35,156 (1982). (24) P. Huyskens and Th. Zeegen-Huyskens, J. Chim. Phys. Phys.-Chim. Biol., 62, 81 (1964).
I
TBNO + PCP
I.-
I
ldoolboozooo
I
1
zwx)
3ooo
J
3500
cm-l Figure 6. IR spectra in the 1600-3500-cm-' region for the complexes of TBNO. Phenol-TBNO ratio: (1) 0.01-0.01 M, (2) 0.05-0.01 M; (3) test beam, (1) reference beam, (2) KBr (d = 2.0 mm, 25 " C ) .
study are equal to 0.495 and 0.210, respectively. These results are in fairly good agreement with data obtained from dipole moment measurement^.^ Let us recall that the dipole moment enhancement evoked by the hydrogen bond formation can be interpreted in terms of a changeable contribution of the proton transfer polar state. Therefore, we found a confirmation of the concept in which a special role of proton transfer equilibrium is ascribed in the interpretation of hydrogen bond polarity. Slight differences between the results of direct UV spectroscopic measurements of molar fraction of PT form and those estimated from dipole moment measurements are most probably due to the underestimation of additional charge transfer and inductive effects. In conclusion, it seems evidenced that also in the case of oxygen base hydrogen-bonded complexes the proton transfer equilibrium plays an important role. On the other hand, our results seem to indicate that in the systems containing oxygen bases it is always necessary to take into account some contribution of homoconjugated [BHB]' cations. The omission of this circumstance may lead to incorrect interpretation of IR spectra and dipole moments. The [BHB]+ homoconjugated cations composed of oxygen bases are characterized by a very broad intense absorption in the lowfrequency IR region, and also, the contribution of [BHBI'A- ion pairs may cause a drastic increase of the effective dipole polarizability.
J. Phys. Chem. 1985, 89, 2058-2064
2058
Icm-’I
Figure 7. IR spectra of PCP complexes with TBNO and triethylamine (TEA): solid line, phenol to base ratio 1:l; dotted line, phenol to base ratio 5:l; dashed line, differential spectrum.
The Structure of ( A w 2 B Adducts. The analysis of steric conditions might show that the arrangement of two 2,6-dichlorophenol molecules at one oxygen atom seems to be rather impossible. One could expect the structure of (AH)2B adducts to be of type IV or V. For a confirmation of such an anticipation the IR spectra have been studied of 2:l adducts in which two phenol molecules are evidently nonequivalent. In Figure 6 are shown the absorption spectra of TBNO-phenol systems in the 1600-3 500-cm-’ region. The differential spectra obtained when a solution of stoichiometry CAo/CBo= 5 has been put into the test beam and that of CAo/CBo = 1 into the reference beam indicate the contribution of at least two different types of hydrogen bonds. There are two
alternative interpretations of the observed picture. Either the equilibrium of I and 111 complexes is present or a contribution of IV adducts takes place, in which the second molecule of phenol is attached to the oxygen atom of complexed phenol. To solve the question, all the spectra of the PCP-TBNO system for Cmo/CBoequal to 1:l and 5:l were analyzed. The PCP-TBNO system in CC14 belongs to the so-called inversion region where the proton transfer equilibrium HB * PT appeared to be extremely sensitive to the environment effects. The spectrum of 1:l species is typical for nearly pseudosymmetric O.-H.-O bonds with the gravity center of low-frequency protonic background absorption a t about 1000 cm-’. For the systems with CAHo/CBo > 1 one observes a drop in the absorption in the 400-1000-cm-’ region and an increase in the absorption between 1000 and 1600 cm-I. This means that a change in the potential energy curve takes place, favoring the proton transfer ionic form. Thus, an association of the second phenol molecule must take place via the lone electron pair of the hydroxyl group in the AHB complex, causing an increase of its proton donor ability. In the case of structure I11 one should expect the opposite effect, Le. a decrease of the proton transfer contribution, because of lowering the proton acceptor affinity of lone electron pairs. More evidence of such an association could be the comparison of the results obtained for the PCP-TEA (triethylamine) system in which there is only one possible way of bonding additional phenol molecules to the AHB complex, namely via lone electron pairs of hydroxyl groups. In Figure 7 we compared the evolution of IR spectra for PCP-TEA and PCP-TEA and PCP-TBNO systems at an excess of PCP under the same conditions.
Acknowledgment. We thank Mr. Jerzy Jafiski for his assistance in the calculations. Reniptry NO. DPSO, 945-51-7; DBESO, 621-08-9; TPPO, 791-28-6; TOPO, 78-50-2; TPASO, 1153-05-5; DCNP, 618-80-4;PCP, 87-86-5; TCP, 88-06-2;DCP, 87-65-0;DPSEO, 7304-91-8;DCMP, 2432- 12-4; DPEPO, 1733-57-9; DPOPO, 29701-85-7;TBNO, 7529-21-7.
The Equal 0 Analysis. A Comprehensive Thermodynamics Treatment for the Calculation of Liquid Crystaltine Phase Diagrams Gerald R. Van Hecke Department of Chemistry, Harvey Mudd College, Claremont, California 91 711 (Received: June 19, 1984: In Final Form: February 1 , 1985)
Equilibrium biphasic regions in isobaric binary phase diagrams are usually calculated on the basis of setting equal the chemical potentials of each component in each phase. However, setting equal the total Gibbs energies of the two phases in equilibrium will define a composition that as a function of temperature will also describe the phase coexistence region. This procedure, called the equal G analysis, is applied to isobaric liquid crystalline phase diagrams and shown to offer ease of computation and wide applicability. Further, liquid crystalline phase diagrams are classified as ideal, nonideal, or reentrant depending on the values of the thermodynamic properties of heat capacity and excess Gibbs energy. To illustrate the technique, two phase diagrams are calculated: the binary system of 2-[4-n-heptylphenyl]-5-[4-(n-heptyloxy)phenyl]pyrimidineand 2-[4n-nonylphenyl]-5-[4-(n-nonyloxy)phenyl]pyrimidine,which exhibits miscible isotropic, smectic A, C, F, and G phases, and the binary system of 4-(n-octyloxy)-4’-cyanobiphenyl and 4-n-heptyl-4’-cyanobiphenyl, which is reentrant with a minimum temperature-composition point in the reentrant region.
Introduction Mixtures, especially binary mixtures of liquid crystals, have been, and continue to be, extensively studied. Very often such study involves the determination of the phase diagram exhibited by a particular pair of mesogens. While a few efforts to quantify such diagrms have been reported using thermodynamic’ or sta(1) (a) Domon, M.; Billard, J. Pramana, Suppl. No. I , 131. (b) Cox, R. J.; Johnson, J. F. Zbm J . Res. Develop. 1978, 22, 51. (c) Van Hecke, G. R. 3. Phys. Chem. 1979,83, 2344.
tistical thermodynamic techniques,2 in the main binary phase diagrams have not been calculated or fit to any quantifiable theory. In this paper is presented a simple thermodynamic approach for the quantification of liquid crystal phase diagrams. The approach (2) (a) Humphries, R. L.; Luckhurst, G. R. Proc. R. SOC.London, Ser. A 1976, 352,41. (b) Phaovibul, 0.; Tang, I-M. 3. Sci. SOC.Thailand 1980, 6, 112. (c) Palffy-Muhoray, P.; Dunmar, D. A.; Miller, W. H.; Balzarini, D. A. “Liquid Crystals and Ordered Fluids”, Vol. 4, Griffin, A. C., Johnson, J . F., Eds.; Plenum Press: New York, 1984; p 615.
0022-3654/85/2089-2058$01 .50/0 0 1985 American Chemical Society
