Senthilnathan, V. P.

Singh, Surjit

English

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Iildian Journal of ChemistryVol. 1,"- October 1976, pp. '''-H8Proton Chemical Shifts of Solute Molecules in Binary Solvent Mixtures:Reversal of the Roles of Solute & Perturbing Solvent"V. P_ SENTHILNATHAN & SURJIT SINGHtDepartment of Chemistry, Indian Institute of Technology, Madras 600036Received 5 November 1975; accepted 13 February 1976Values of equilibrium constants for various solute-solvent equilibria have been determinedon the basis of NMR chemical shifts of solutes in binary solvent mixtures. The equilibriumconstants for the system S+P.=SP have been determined by studying the variation of cbemicalshifts of the solute S in rntxtures of polar solvent (P) and nonpolar solvent (N) and also by studyin~the variation of chemtcal shift of P in binary mixtures of solvents Sand N. The K values deter-lIlined by the reversal of the roles of solute and the perturbing solvent have been found to be dif-ferent. An attempt has been rnade to explain this anomaly on the basis of solvation of ordersbi~ber than that involved in 1: 1 interactions.EARLIER. attempts'rs have been made torelate the observed solvent chemical shiftsto 1:1 or higher order interactions betweenthe solute and solvent species. Tnerefore, as in thecase of hydrogen-bonded and charge-transfer com-plexes, existence of variors dipole-dipole, dipole-induced dipole ard other c illision complexes havebeen p.oposed-. F.);, solutes in binary solventsystems, Eq. (1) (similar to relations used for thecalculation of variation in infrared a-id electronicband shifts and ba-rd intensities=t] has been employ-edS for the evaluation of the thermodynamic dataof complex formation:fl.= KptJ..OCp1+KCp ••• (1)where tJ.. = 8-30 and tJ..o= 8c-30. 0 and 00 arerespectively the chemical shifts of solute protonsat infinite dilution in binary solvent mixture con-taining Cp moles of perturbing solvent and in pureinert solvent. oc is the chemical shift for complexin inert solvent. Kp is the equilibrium constantfor the solute (5)-solvent (P) interaction of thetype 5+P~5P. The following two rearrangementsof Eq. (1) have been employed for the evaluationof equilibrium constant:!= 1 +! (2)Il fl.0KpCp tJ..o ••.tJ..Cp= -Kefl.+Kpfl.° ... (3)Kuntz and Johnston' have used Eq. (2) to determineequilibrium constant for various weak interactionsincluding dipole-dipole and dipole-induced dipoletypes.In the various studies carried out for theevaluation of equilibrium constant by spectralmethods, the variation of spectral parameter of thesolute sp~cie;;was 0;11yconsidered. Because of the-FonDS part of Ph.D. thesis of V.P.S. submitted to lIT,Madras.tTo whom all correspondence should be addressed.limitations in the spectral measurements, the va~ia-tion of spectral parameters of the complexingsolvent was seldom followed. In the case ofchemical shift studies such limitations do not exist.To determine whether expressions similar to (1)yield true values of equilibrium constant for thesystem 5+P~5P, in a previous publication", weevaluated K values for two systems by studying thevariation of chemical shift of the solute (5) in amixture of polar solvent (P) and the inert solvent(N) and also by studying the variation of chemicalshift of (P) in a binary solvent mixture of (5)and (N). The K values were not found 10 beequal when roles of solute and solvent were changed.It was thought of interest to extend these studiesto some more systems and give some reasonableexplanation for the anomaly. Studies on varioussystems exhibiting weak interactions are beingreported in this paper.Materials and j;MethodsAU the chemicals used were of AR or LR gradeand were purified by standard methods'". All thesolvent mixtures were prepared by volume per cent.The NMR spectra were recorded employing aVarian A-60-Dspectrometer. As observed by Kuntzand Johnston', nearly 1% solute concentration wasfound to approximately represent the chemical shiftof solute at infinite dilution. All the shifts reportedare with respect to TMS as the internal referenceat 210. The chemical shifts were reproducible withan accuracy of ± 0·2 Hz.Results and DiscussionChemical shift data. of various solutes in binarysolvent mixtures are given in Tables 1-3. Eq.(4) which is a rearrangement of Eq. (1), was usedfor the determination of equilibrium constant.Cp/fl. =.1/{KptJ..O)+Cp/tJ..o ... (4)The values of correlation coefficient (r), K, and lioobtained by the method of least-squares for varioussystems are summarized in Table 4. It canSENTHILNATHAN & SINGH: PMR SPECTRAL BAND INTENSITIES & SOLUTE-SOLVENT INTERACTIONSTABLE 1 - CHEMICALSHIFT DATA (Hz) IN BINARYSOLVENTMIXTURESWITHCYCLOHEXANEAS THE INERT SOLVENT% of CH2Cla in CHCl. Dioxane in CHCla CHCla in dioxaneperturberCp A Cp A Cp A5 0·617 1·5 0·617 2·0 0·588 6·510 1·233 2'5 1·233 3·5 1-180 14·515 1·850 3·0 1·850 4·0 1·763 17·530 3-700 6·5 3·700 6·5 3·325 25·0SO 6·167 8·0 6·167 8·5 5-875 30·070 8'633 10·0 8·633 10·0 8·225 33·090 IHOO 11·0 11'100 11-0 10·575 34·5TABLE2-CHE:IUCAL SHIFTDATA(Hz) FORSOLUTESIN BINARYSOLVENTMIXTURESWITHCYCLOHEXANEASTHE INERTSOLVENT% of CHCl. in CCl2 = CHGl CCl2 = CHCI in CHCI. Acetone in CH2ClaperturberCp A Cp A Cp A5 0'789 1·510 1-113 2·0 1·579 2·515 1'670 2·5 1·850 3·0 2·369 3·530 3·340 4·0 3-700 4·5 4·738 4·5SO 5'567 5·0 6·167 7·5 7·896 6'570 7·794 7'0 8·633 9'0 11·055 7·590 10'021 8'5 11-100 11·5 14'214 9'0TABLE3- CHEMICALSHIFTDATA(Hz) FORSOLUTESIN BINARYSOLVENTMIXTURESWITHCYCLOHEXANEASTHE INERT SOLVENT% CHCla in CH2Cli CH2Cl2 in acetone Acetone in CCIs=CHCl CCI2=CHCI in acetoneof per-turber Cp A Cp A Cp A Cp A5 0·789 2'0 0·683 5·5 0·557 0·5 0·683 8'010 1-579 3·0 1·366 11·3 1-113 1'0 1·366 11·015 2·369 4·0 2·048 13·5 1-670 1'5 2·048 16·030 4·738 6·5 4·097 19·5 3'340 3·0 4·097 25·050 7·896 10·0 6·828 23·5 5·567 4·5 6·828 33·070 11·055 12·0 9·559 26·5 7·794 6·0 9·559 37·090 12·290 30·5 10'021 7·0 12·290 40·0be noticed that in no case the equilibriumconstant is found to be the same when the rolesof solute and perturbing solvent are reversed.For the system CHCla+acetone, the K p value is5·49 when determined by the variation of chemicalshifts of CHCls and 1·52 when determined by thevariation of chemical shifts of acetone protons.Similar differencesin the Kp values can be observedin the other cases also. However, different valuesof correlation coefficient (r<O·99) show perfect fitof the chemicalshift data to Eq. (1). One concludes,therefore, that the term Kp does not represent thetrue equilibrium constant. Scott-! has recentlyargued that the Kp values found by the methodssimilar to the ones under discussion for weakinteractions are not the real association constants.These are the so-called "sociation constants"which yield the number of complexes formed inexcess of those calculated on the basis of randomprobabilities. Alternatively, on the basis of aplausible quasi-lattice model of the solutions12-16,random collisions between donor and acceptormolecules lead to the equilibrium constant valuesK = z, where z is the number of the nearestneighbours. However, if the derivation of Eq. (1)is considereds on the same line as Langmuir'sadsorption isotherm where the changes in spectralTABLE 4 - RESULTS OF LEAST-SQUAREANALYSISFOR EQ. 4FORTHECHEMICALSHIFT DATAIN BINARYSOLVENTMIXTURESSystems* ,. K AO(litre/mole)CHCl. in acetone+ 0·99 0·485 61·0Acetone in CHCl. t 0·95 0·075 22'SAcetone in C.H. t 0·98 0·307 30·0C.H. in acetone+ 0·94 0·170 16·0CH3NO. in dioxane'[ 0·99 0·776 10·7CHCl. in CH2Cl2 0·96 0·103 21·9CH.CI2 in CHCl3 0·97 0·121 19·3Dioxane in CHCl. 0·99 0·217 15·3CHCl3 in dioxane 1·00 0·416 42'3CHCl3 in CCI2=CHCl 0·94 0·123 14·4CCI2=CHCl. in CHCla 0·91 0·057 28·7Acetone in CH2Cla 0·98 0·152 12-4CH2Cl2 in acetone 0·99 0'258 38·6Acetone in CCl2=CHCI 0·96 0·032 29·9CCI2=CHCl in acetone 0·99 0'203 56·0*Cyclohexane is taken as the inert solvent.[Data from ref. (13).parameters of solute molecules in binary solventmixtures represent the extent of solvation, thisanomaly can be well understood.The concept of incomplete solvation can beassumed in the derivation of such an expression747INDIAN J. CHElf., VOL. HA, OCTOBER 1976the role of solute and perturbing solvent is reversed.This is because of the fact that the value of Kis governed by specific values of rates of solvationand desolvation (K = k/k') and the solvation numberwhich determines the value of x and hence a~.If the validity of the above-mentioned assumptionsis justified - though the exact implications of theseassumptions in the derivation of the solvationequations are not too clear - and if proportionalityconstant for the expression a~oc x is worked out,the a~ and a~ovalues can be employed to evaluategiving the solvation numbers. More work in this direction:J!'KCp is in progress and will be communicated shortly.x = ...(5)l+KCp Referenceswhere solvation constant, K = k/k' (k and k' are , 1. LUSSAN, C., J. ckim, Phys., 60 (1963), 1100.respectively the specific rate constants for solvation Z. HANNA, M. W. & ASHBAUGH, A. L., J. phys. Chem., 63and desolvation of 5 by P). On increasing the (1964), 811.b f rt bi 1 I P d th I t 3. FOSTER. R. & FVFE, C. A., Trans. Faraday Soc., 61num er 0 pe ur mg mo ecu es ,aroun e so u e (1965). 1626.molecules, the electric field capable of polarizing the 4. KUNTZ (Jr), 1. D. & JOHNSTON Or), M. D., J. Am. chem,solute molecules will increase. Hence the change Soc .• 89 (1967), 6008.in spectral parameter ; of 5 will increase with 5. SENTHILNATHAN. V. P. & SINGH, S., Spectrocbim. Acta,increasing x. Taking a~as a direct measure of the 29A (1973), 981.6. SENTHILNATHAN, V. P. & SINGH, S., Spectrochim, Acta,amount of P involved in solvation, the above 30A (1974), 285.expression can be written as 7. SENTHILNATHAN. V. P. & SINGH, S., Indian J. Chem.,12 (1974), 1311.ar::= Ka~oCp (6) 8. LASZLO, P., Progress in nuclear magnetic resonance spectro-•• l+KC ~ ... scopy, Vol. 3, edited by J. W. Ems!ey, J. Fenney &p L. H. Sutcliffe (Pergamon Press, London), 1967, 231.The expression (6) is similar to Eq. (1) except 9. SENTHILNATHAN. V. P. & SINGH, S., Proc. Indianthat 1:1 interaction constant Kp is replaced by the Nat. sa. Acad., 40 (1974). 199.1: n solvation constant, K. Thus, Kp values deter- 10. WEISSBERGER, A., Technique oj organic chemistry. Vol. VIImined above are identical with the' K values. (Interscience, New York). 1955. /:11. SCOTT, R. L.. J. phys. Chem .• 75 (1971) 3843.Eq, (1) derived on the basis of 1:1 interaction 12. SCOTT, R. L.. Proceedings oj the third internationalijeon-only cannot explain the variation of Kp with the [erence on coordination compounds, Amsterdam. 1955.change of roles of solute and solvent. But on the U: ~:~~:): ~.:,{:h~~:~/~~~ie~~9~;~p'!f!~ on the physicalbasis of Eq. (6) the K values can be different when chemistry oj weak complexes, Enster, England, 1967.similar to the derivation of adsorption isotherm.Let the amount of polar solvent needed to solvateall the solute molecules to the maximum extentpossible be xO' If the amount of polar solventinvolved in solvation at a concentration Cp is x,then for the equilibrium,I<S+P ~ (S)P1<'the ratio x/:J!' = 6 is given bykCp(I-6) = k'6748

Proton Chemical Shifts of Solute Molecules in Binary Solvent Mixtures: Reversal of the Roles of Solute & Perturbing Solvent

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Proton Chemical Shifts of Solute Molecules in Binary Solvent Mixtures: Reversal of the Roles of Solute & Perturbing Solvent

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