1037
NMRSTUDIES ON INTERMOLECULAR ASSOCIATION IN BINARY MIXTURES bacisity of 9-C0Og by using averaged absorption and fluorescence bands probably is not valid. However, excited-state rotation of the carboxyl group does not occur for the carboxylate anion, even in aprotic solvents. Consequently, the absorption and fluorescence of 9-COO- are primarily the structured anthracene 'La transition with only minor CT perturbation. This implies little transfer of electron density toward the -COO- group and a small change in basicity. If ApK1 is calculated by the Forster cycle using the lowest energy vibronic components of the 9-COOH and 9-COO- absorption spectra, a value of ApKl = 0.5 is obtained. Such a value seems more consistent with the spectral behavior of 9-COO- which indicates only a minimal
interaction between the carboxylate group and the ring. Since the excited-state rotation occurs for the un-ionized acid, the Forster assumption of equal entropy for the ground and excited-state ionization is probably not met when fluorescence frequencies are included in calculations. Thus caution must be exercised when calculating excited-state pK,'s of molecular undergoing excitedstate geometry changes. Acknowledgments. This work was supported in part through funds provided by the Atomic Energy Commission under Contract No. AT(30-1)-905. T. C. W. thanks the National Institutes of Health and the Proctor and Gamble Co. for fellowship support.
Nuclear Magnetic Resonance Studies on the Intermolecular Association in Some Binary Mixtures.
I. Chloroform and Proton-Acceptor Solvents1
by Wei-chuwan Lin and Shyr-jin Tsay Department of Chemistry, National Taiwan University, Taipei, Taiwan, Republic of China (Received February 19, 1969)
A graphic method based on nmr measurementshas been developed which enables one to determine the association constant and the chemical shift of the 1:1 molecular complex in binary mixtures. Results obtained for chloroform in various proton-acceptor solvents are presented and discussed in detail.
Introduction Nuclear magnetic resonance studies have revealed that the chemical shift of the chloroform proton in the medium of a proton acceptor is largely dependent on the chloroform concentration and also on the nature of solvent.2 This dependence is attributable to intermolecular association between chloroform and solvent molecules through hydrogen bonding. Chloroform also associates with benzene to form a complex8 whose bonding is similar to other hydrogen bonds, i.e., essentially electrostatic in character.8b Two types of hydrogen-bonded complex with 1: and 2: l8"v5ratios, respectively, have been known between chloroform and a proton-acceptor solvent. Many different methods of nmr approach, such as curve fitting,216limiting slope,6 and temperature variation? as well as a graphic meth~d,~J'have been used to determine the association constant for 1 :1 hydrogenbonded complexes. The present paper is concerned with the proton magnetic resonance of chloroform in a variety of protonlaat'
acceptor solvents. A graphic method will be developed to determine the association constant and the chemical shift due to its 1:1hydrogen-bonded complexes.
Theory I n a solution of a 1 : l solute-solvent complex, an equilibrium is considered to exist between the unasso(1) Based in part on material submitted for the Master's thesis of 8. T. (2) C. M. Huggins, G. C. Pimental, and J. N. Shoolery, J. Chem. Phys., 23,1244 (1955). (3) (a) G. W.Chantry, H. A. Gebbie, and H. N. Mirsa, Spedrochim. Acta, 23A, 2749 (1967); (b) R. J. Abraham, Mol. Phys., 4, 369 (1961). (4) L.J. Andrews, Chem. Rev., 54,713 (1954). (5) A. L. McChellan, S. W. Nicksic, and J. C. Guffy, J. Mol. Spectrosc., 11,340 (1963). (6) P.J. Berkeley, Jr., and M. W. Hanna, J. Phys. Chem., 67, 846 (1963). (7) R. E.Klinck and J. B. Stothers, Can. J . Chem., 44, 37 (1966). (8) R. Marthur, E. D. Becker, P. B. Bradley, and N. C. Li, J. Phys. Chem., 67,2190 (1963). (9) S. Nishimura, C.H. Ke, and N. C. Li, ibid., 72, 1297 (1968).
Volume 74, Number 6 March 6 , 1070
WEI-CHUWAN LIN AND SHYR-JIN TSAY
1038 ciated and associated forms of the proton donor (D) and the acceptor (A): A D AD. The equilibrium constant in terms of mole fraction ( X ) can be expressed as
+ +
+ NOD -
(N’A
(1)
NAD)
where NDOand NAO are the number of moles of proton donor and acceptor initially given, and NADis the number of moles of 1: 1 complex after equilibrium is established. The observed shift of the chloroform proton (6obsd) is therefore the weighted average of the shift due to the unassociated solute (BD) and that due to the complex (BAD), i.e. 6obsd =
NOD - NAD BD NOD
+ NADBAD
(2)
__
NOD
or
NAD __ - dobsd - 6D NOD SAD - 6~
Aobsd
where Aobsd and AAD stand, respectively, for SD) and AD - SD), i.e. &bed
=
dobsd
AAD = SAD
-
(2’)
PAD
-
(dobsd
(3)
SD
- SD
(4)
Substitution of eq 2’ into eq 1 gives
K =
Aobed/AAD
1-
Aobad/AAD
[’
+
XoA
-
(&bed/AAD)XoD
1
where XO’s denote the initial mole fractions. Equation 5 after expansion becomes
=
(
slope X intercept = 1/ 1
3
+-
One has P + 0 as K --+ 0 and P + 1 as K d w , indicating that the behavior of the straight line is sensitive to small change in K , as long as K is small, but rather insensitive when K is large. Now suppose the case in which K for solute dimerization or other non-1:1 complex formation is smalls over a certain range of solute concentration. Then a small value of K for 1:1 solute-solvent complex formation will exhibit a plot with pronounced curvature a t higher concentration range. If the value of K for 1: 1 complex formation is large enough, so that K for dimerization or other non-1 : 1 complex formation becomes negligibly small, the resulting plot will exhibit good linearity. Thus the straight line obtained over a certain range of solute concentration can determine the association constant for the 1:1 solute-solvent complex and rule out unequivocally a weak dimerization or other competitive non-1 : 1complex formation. The chemicals used in the present work were obtained from standard commercial sources and were purified mostly according to conventional procedures, lo but some were purified according to the literature.& The sample solutions were made up by weight under Table I : The Nmr Data for the Intermolecular Association of Chloroform with a Variety of Proton-Acceptor Solvents a t 26”
- 1) A’AD
=
0 (6)
Rearrangement of eq 6 gives
The plot of A o b s d / ( l - X O D ) against X o ~ A ~ , b , d / ( l XOD) gives a straight line whose slope and intercept determines the values of AADand K . I n the present study we are mainly interested in the 1 :1 solute-solvent complex. However, if the formation of any other non-1:1 complex or self-association of solute proceeds competitively in the system, the plot of Aobsd/( 1 - I’D) 218. x o ~ ~ a ~ b PD) ~ ~ / Will ( ~ exhibit a distinct curvature a t higher solute concentration, where these processes are expected to become more proThe JOUTTUZ~ 01Phvaical Chemistry
P
Experimental Section
xoD
(5)
(&
nounced. Even in this case, a linear plot obtained at the lower concentration range may be approximated to the behavior due to the 1 : l complex. Accuracy and reliability of association constants thus determined largely depend on the extension of the linear part of the plot. The product of slope X intercept for the straight line mentioned above can be given by eq 7 as
Range of chloroform mole
Proton aooeptor
fraction studied
(*obsd)XoD+c,, ppm
AAD,
K,
ppm
(mf)-l
Benzene
0.04680.3242
0.853
3.33
0.36
Toluene
0.0216-
0.915
2.31
0.67
1.037
2.26
0.86
Mesitylene Acetonitrile Dimethyl sulfoxide
0.3093 0.03980.3624 0.02900.3364 0.02020.4764
-0.500
-0.65
3.4
-1.250
-1.64
3.2
(10) J. A. Riddick and E. E. Toops, “Organic Solvents,” Interscience Publishers, Inc., New York, N. Y., 1955.
1039
NMRSTUDIESON INTERMOLECULAR ASSOCIATION IN BINARY MIXTURES Table I1 : Literature Values of Association Constants for Chloroform-Proton-Acceptor Complex AAD, ppm
K , (rn0-1
-0.63 -0.973 I O . 0 1 9
3.2 1.14 & 0.04
-0.752
0.91 k 0-05
Proton acceptor
Acetonitrile
Dimethyl sulfoxide a
Reference 6.
b
-1 5 I
zk
0.024
3.8
Results and Discussion The chloroform proton shift a t infinite dilution in the solution of cyclohexane was determined as 5.65 ppm (339.0 Hz) from the plots of chloroform proton shift as a function of chloroform concentration in solution. This value is in good agreement with Jumper's resuW , chemical shift of pure chloroand was taken as 6 ~ the form monomer. Several plots of Aobsd/(l - X%)against X0DAz0b.a/ (1 XOD)for the binary mixtures under study are shown in Figure 1. The results of the determinations are listed in Table I . I n Table 11,some nmr data on association constants in the literature are listed for comparison. Chloroform forms dimers by self-association through weak hydrogen b0nding.l' However, as can be seen from Figure 1,these plots generally exhibit good straight lines with the exception of the chloroform-dimethyl sulfoxide system, which on extension to the right-hand side of Figure 1 gives a distinct curvature above 16 ' chloroform concentration. This indicates that mol % the dimerization effect of chloroform is negligibly small as compared to the effect of chloroform-solvent association, within the range of chloroform concentration studied (below 35 mol %). Breakdown of linearity in a higher concentration range above 16 mol % for the case of a chloroform-dimethyl sulfoxide system mentioned above seems to be due t o the simultaneous formation of the 2 : 1complex in addition to the 1:1 complex.5 Even in this case, the h e a r plot obtained below 16 mol % can be used to determine K and AAD for the 1:l complex. The values of K and AAD thus determined for the 1:1 com-
Ref
Binary mixture Ternary mixture containing CC14 as solvent Ternary mixture containing CCId as solvent Binary mixture
B. B. Howard, C. F. Jumper, and M. T. Emerson, J . Mol. Spectrosc., 10,117 (1963).
open air, degassed at Dry Ice-methanol temperature, and sealed in 5-mm od tubes in vacuo. The 60-MHz nmr experiments were carried out with a Varian HR-60 high-resolution nmr spectrometer system a t 26 f 1". The signal position of the chloroform proton was measured with reference to the cyclohexane signal by the side-band technique with use of a Hewlett-Packard HP-200CD wide-range oscillator in conjunction with an HP-521C electronic counter with crystal time base. It was thus possible to hold the average deviation within *O.lHz.
-
Method
a
b
b C
Calculatedfromref 5 .
I
I
-1 0
-'*I-
Dimethyl Sulfoxide
-14
Figure 1. Plots Of A,b,d/(l - XDo) against x D o A 2 0 b b d / ( l Xoo) for chloroform in a variety of proton-acceptor solvents,
plex of chloroform-dimethyl sulfoxide are in good agreement with those obtained by McChellan, et at.,6as may be seen on comparing Table I with Table 11. For the CHCla-CH3CN complex, Howard, et uZ.,12 gave two sets of association constants both from measurements in ccl4 solution as listed in Table 11. The discrepancy between their results on the one hand and those obtained by Berkeley, et aL16and ours on the other hand may be attributable specifically to the fact that Howard, et ul., made their measurements in cc& solution. A chloroform-base mixture in the solvent carbon tetrachloride presumably undergoes an appreciable amount of CHCla-solvent association" which may proceed in competition with the predominant association of CHC18-base type even a t very dilute chloroform concentration. A similar discrepancy was noticed by Howard, et aZ.,12for association constants of the EtzO-CHCla compIex; i.e., association constants were determined to' be K = 3.76 f 0.10 and AAD -0.905 f 0.00Sppm from measurements incyclohexane solution, while these turned out to be K = 1.46 f (11) C. F. Jumper, M. T. Emerson, and B. B. Howard, J. Chem Phgs., 35, 1911 (1961). (12) B. B. Howard, C. F. Jumper, and M. T. Emerson, J. Mol. Spectrosc., 10,117 (1963).
Volume 74,Number 6 March 6 , 1070
1040
WEI-CNUWAN LIN AND SHYR-JIN TSAY
*
0.04 and AAD = -1.266 0.018 ppm from measurements in CC4 solution. It is obvious from eq 7 that a t extreme dilution (ie., XOD 0), the first term of eq 7 becomes zero and one obtains
-
This latter formula is the same as that obtained by Huggins, et aL2 It is noteworthy that AAD should not be approximated by (Aobsd)XoD+o, unless K is very large. This may find full support in Table I by comparing the data in the third and fourth columns in conjunction with K values given in the fifth column. The dielectric constant of the solvent may have two effects on the hydrogen bond of solute-solvent complexes, viz. ( 1 ) reduction of the electrostatic force acting on the charge centers of the hydrogen bond and (2) increased bonding due to increased polar character of the proton-acceptor group toward hydrogen atom. These two effects are counterbalanced in the equilibrium and its extent is best described by K . Thus the dependence of the strength of the hydrogen bond on K is straightforward. However, the relationship between hydrogen-bond strength and hydrogen-bond shift is rather complicated. Since a more electronegative group favors stronger hydrogen bonding, the stronger the bonding, the larger the low-field shift would be (shift a). At the same time, the proton shift related to the hydrogen bond is also dependent on the specific geometry of the hydrogen bond in the particular complex structure, which causes a low-field shift of the chloroform proton in the medium of a common base like acetone2 but yields a high-field shift due to ring currents in the case of aromatic solvents like benzeneab (shift b). Thus the net shift due to hydrogen bonding is the algebraic sum of shift a and shift b and is not always a direct function of hydrogen-bond strength. This is why no simple regularity is observed for AAD or (Aobsd)XoD-,O, when one compares the complexes with different structures in Table I. Positive values of (Aobsd)XoD-tO and AAD, i.e., the high-field shift of the chloroform proton in the aromatic solvents, as may be seen in Table I, are of course due to the anisotropic effect of the aromatic ring, indicating that a hydrogen-bonded complex is formed between chloroform and the aromatic molecule so as to locate the chloroform proton perpendicular to the benzene ringl3pI4in such a manner that the dipole axis of chloroform is along the sixfold axis of symmetry of the benzene ring with the proton nearest the b e n ~ e n e . ~ ~I n? 'this ~ position the proton experiences a high-field shift due to the aromatic ring current.ab As has been pointed out by Abraham,3b the complex bonding between chloroform and the benzene ring, similar to other hydrogen bonds, is essentially electrostatic in character but, due The Journal of Phz/sicat Chemistry
to the weak ionic character of the C-H bond, is considerably less stable than the more common hydrogen bond. This is consistent with lower values of K obtained by us for chloroform in the medium of benzene, toluene, and mesitylene. Creswell and Allredls studied the chloroform-benzene association in the chloroform-benzene-cyclohexane system with cyclohexane used as an inert solvent to dissolve the equimolar mixture of chloroform and benzene. The K value was then determined as a parameter to be adjusted to make the observed shift os. mole fraction of the 1 : l complex a linear function. They obtained K = 1.06 f 0.30 (mf)-l a t 25". However, using this method it seems rather difficult to rule out other competitive non-1 :1 complex formation, for instance, the formation of the 2: 1 CHCkCeHa complex. In acetonitrile and dimethyl sulfoxide, chloroform exhibits negative values of (Aobsd)xOD-,o and PAD (ie., low-field shifts). The low-field shift of chloroform in dimethyl sulfoxide solvent is greater than in acetonitrile solvent. This is inconsistent with the prediction simply from the values of K , which according to Table I are nearly equal, indicating that the chloroform proton shifts should be quite close to each other. This inconsistency may well be resolved by assuming the structure of these 1:1complexes, respectively, as
Ha
c CL3
As far as the effect of magnetic anisotropy is concerned, the double bond of the sulfoxide group must resemble that of the carbonyl group, which results in negative shielding of the proton along the double-bond axis,13 in agreement with the low-field shift. The triple bond of the nitrile group must resemble a - C s C bond, which gives positive shielding of the proton along the triple-bond axis,la in accordance with the high-field shift. The diametrical opposition between the magnetic anisotropy effects of =S=O and -C=N is expected to give rise to an additional contribution to the relative magnitude of the observed chloroform proton shifts in the two association complexes, Le., CHCla(13) L. M. Jackman, "Application of Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry," Pergamon Press, New York , N. Y., 1959. (14) W.T.Huntress, Jr., J . Phy8. Chem., 73,103 (1969) (15) C. J. Creswell and A. L. Allred, ibid., 66, 1469 (1962). I
NMRSTUDIESON INTERMOLECULAR ASSOCIATION IN BINARY MIXTURES DMSO and CHCh-CH3CN, and thus makes the relative chloroform shift between CHCls-DMSO and CHCIs-CHaCN much larger than that predicted merely from the relative hydrogen-bonding strength or from the relative magnitude of association constants. Strictly speaking, K and AAD determined by eq 7 might still include the influence both from solvent shift and from self-association of chloroform because of Aobsd being defined as
If the solvent effect is taken into consideration, Aobsd a t any concentration may be given by the empiri-
cal formulas*16
I n eq 9, A* is the gas-phase difference in shift between pure donor and reference. y A x is the correction for the anomalous shift exhibited by a gaseous solute when infinitely diluted with the acceptor solvent, where y is the characteristic number6Vlsof the acceptor solvent and A x is the difference in the x value between donor solute and reference solute, x being the characteristic numberazl6of the compound containing the proton. Substitution of eq 9 into eq 7 gives
1041
This equation again enables one to determine K and AAD by plotting [aodsd - A* -yl A z ] / ( l - XOD)against XoD[80bsd - A* -I- yAzI2/(1 - XD') provided that y A z is known for the specific system. The values of K and AAD thus determined by this refined method, unlike the previous values obtained from eq 7 and 3, eliminate automatically the unfavorable contribution from solvent shift due to donor and reference solutes and also eliminate the effect arising from self-association of chloroform. Among the series of binary mixtures under study, it is only the chloroform-acetonitrile system whose characteristic number y A x is known.B Application of eq 10 gave AAD = -0.63 ppm and K = 3.1 for the chloroform-acetonitrile system, whose plot is also shown in Figure l for comparison. Only a small discrepancy is noticed between these two methods of determination. A further improvement in the values of K and AAD with application of the refined method would be expected for the rest of the binary mixtures, especially for ones with smaller K values, if their characteristic number y A x were known. (16) A. A. Bothner-By, J . Mol. Spectrosc., 5,52 (1960).
Volume 74,Number 6 March 6,1970
