GENERAL DISCUSSIONDr. A. K. Covington (University of Newcdstle upon Tyne) said: I suggest thatthe third explanation (c) of line broadening in trifluoroacetic acid discussed brieflyby Kreevoy and Mead 1 at the end of their paper is the correct one. In anhydroustrifluoroacetic acid there is a band 2 at 1455 cm-1 which has been assigned3 to aC-0 deformation mode and may in part be due to dimers. In the deuterated acidit appears 23 3 at 1426 cm-1. Fig. 1 summarizes the line broadening (half-width)observed for the three acids.194 Some new measurements by Mr. Freeman, Mr.Lilley and myself on trifluoroacetic acid are included for comparison. The closeagreement between these and Kreevoy and Mead’s is striking. The two sets ofmeasurements were made at very different slit widths but the half-width broadening(obtained by subtracting the extrapolated half-width at zero ion concentration) isindependent of the slit width.However, from fig. 1, although the line broadening,,HN03 thwetz1955 d CF$OOH0 Kretvoy4Mcod 1962,19658 Covinrjten FmrnanLLilley 1065Q ,‘#‘O‘965 +0 15 10molarityFIG. 1.in trifluoroacetic acid solutions is linear with concentration up to about 5 M, abovethis it increases sharply. The peak maximum on the other hand shows a steadyfrequency shift from the 1435 cm-1 band found in dilute solutions of the acid andthe sodium salt towards 1455 cm-1 as shown in fig. 2. This seems to be clearevidence that as the solution concentration is increased the 1435 cm-1 band due tothe trifluoroacetate ion, while appearing to broaden as it diminishes in intensity,is being replaced by a band at 1455 cm-1 due to the undissociated acid as the con-centration of this increases.This frequency shift is shown for mixed perchloricand trifluoroacetic acids by fig. 2 of Kreevoy and Mead’s paper.41 Kreevoy and Mead, this Discussion.2 private communication from Dr. Ralph E. Weston, Jr., of Brookhaven National Laboratory.3 Fuson, Josien, Jones and Lawson, J. Chem. Physics, 1952, 20, 1627.4 Covington, Tait and Wynne-Jones, this Discussion.17GENERAL DISCUSSION 177-anhydrous acid 400.00e-sodium salt1 I I I 1 I2 4 6 8 10molarityFIG. 2.I 1.5 10molarityFIG. 3.Raman: x Kreevoy and Mead, 1965 ; +, Covington, Freeman and Lilley, 1965n.m.r. : .. . 0, Hood, Redlich and Reilly, 1955 ; a, Akitt, Covington and Lilley, 196178 GENERAL DISCUSSIONA further disturbing feature of the Raman spectrum of trifluoroacetic acid isthat the 1435 cm-1 band is situated on a sloping base line, which is part of a broadband stretching over some 2000 wave numbers with a maximum at 2000cm-1.This is shown well for sodium trifluoroacetate by Robinson and Taylor 1 in a figuretaken from a microdensitometer trace but a similar broad band is also found in thespectrum of the acid. Until the origin of this band is elucidated the interpretationof interactions in trifluoroacetic acid solutions must remain speculative.Fig. 1 compares the observed line broadening for all three acids for whichmeasurements are so far available23 3 with that observed in sodium nitrate solutionby Vollmar.4 For perchloric acid the line broadening is of similar magnitude asin sodium nitrate while for nitric and trifluoroacetic acids it is greater by a factorof two and three respectively.We think that further evidence must be collectedon many more acids and salts before the phenomenon of line broadening can beunequivocably interpreted.Finally, I should like to comment on the 10 % discrepancy between the valuesof the degree a of dissociation of trifluoroacetic acid found from Raman intensities 2and those found by Hood, Redlich and Reilly 5 from their p.m.r. measurements(see table 2 of Kreevoy and Mead’s paper 2). We have redetermined6 a by thep.m.r. method and find the a values given by Hood, Redlich and Reilly to be incorrect.A critical quantity in the evaluation of a from p.m.r.measurements of the chemicalshift s with respect to water, is s1, the contribution from the hydronium ion.s = s,ap++s,(l -a)p, where’ p = 3x1(2-x),and x is the stoichiometric mole fraction of the acid. Hood, Redlich and Reilly 5assumed for s1 the value found for hydrochloric acid. We have been able to getdown to concentrations as low as 0.1 N and determine s1 experimentally from thelimiting slope of a plot of s1 against p . The value obtained is lower than that as-sumed by Hood, Redlich and Reilly and hence the new a values are higher. Fig. 3compares our results with the earlier results 7 recalculated with the new s1 and withthe values obtained from Kreevoy and Mead’s2 and our own integrated Ramanintensities.All are in good agreement and, in particular, the Raman measurementsare in closer agreement than the modest 10 % accuracy claimed by Kreevoy andMead.2 However, the Raman results lie slightly above the p.m.r. results at con-centrations above 7 M and this may be again symptomatic of the changed natureof the 1435 cm-1 Raman band at these concentrations.Dr. Ralph E. Weston, Jr. (Brookhaven Nat. Lab., N.Y.) (communicated): AsKreevoy and Mead report in their paper, I have also made measurements on Ramanline broadening in aqueous solutions of trifluoroacetic acid.8 These measurements,carried out subsequent to the publication of their first communication,9 have beenlimited to solutions of CF3COOH in H20, CF3COOD in D20, and their sodiumsalts, but have been extended to somewhat higher acid concentrations than thoseused by Kreevoy and Mead.At concentrations where comparison is possible,my results for CF3COOH are similar to theirs. Up to a stoichiometric acid con-centration of 7 M, Av* is linear with concentration; but at higher concentrations1 Robinson and Taylor, Specfrochim. Acta, 1962, 18, 1093.2 Kreevoy and Mead, this Discussion. 3 Covington, Tait and Wynne-Jones, this Discussion.4 Vollmar, J. Chem. Physics, 1963, 39, 2236.5 Hood, Redlich and Reilly, J. Chem. Physics, 1955, 23, 2229.6 Freeman and Lilley, Chem. Comm., 1965, 349.7 Gutowsky and Saika, J. Chem. Physics, 1953, 21, 1688.8 This research was wried out under the auspices of the U.S.Atomic Energy Commission.9 Kreevoy and Mead, J. Amer. Chem. SOC., 1962,84,4596GENERAL DISCUSSION 179it rises sharply. The width of the 1435 cm-1 band in solutions of sodium trifluoro-acetate was essentially independent of concentration. There are, however, certaincomplications attached to the interpretation of the observed broadening.(a) The spectrum of pure CF3COOH exhibits a broad band at 1456 cm-1, witha molar intensity about one-tenth that of the anion band at 1435 cni-1. This bandis presumably due to the dimer, but persists in 10 M CF3COOH, at which con-centration dissociation and dimerization are thought to be negligible.1 In pureCF3COOD (in DzO) the corresponding band is found at 1422 cm-1, while the tri-fluoroacetate band is unshifted in position or intensity.These bands appear tobe present at lower acid concentrations, since the 1435 cm-1 band in 4 M CF3COOHis slightly asymmetric with broadening towards higher frequencies, while the sameband in 4 M CF3COOD is broadened towards lower frequencies.(b) The agreement between values of a determined from intensities of the 1435cm-1 Raman band and from p.m.r. measurements is excellent up to concentrationsof about 7 M CF3COOH. The positive deviation of the Rainan measurementsincreases with increasing concentration, again indicating that a species other thanCF3@OO- is responsible for Raman scattering in this spectral region. The dis-crepancy is larger for solutions of CF3COOD, where the intensity measurementslead to the unlikely conclusion that the concentration of CF3COO- increasesmonotonically with increasing stoichiometric acid concentration.( c ) As Covington, Tait and Wynne-Jones point out, the factors which determineRaman band widths are not well understood.In measurements of the dissociationof perchloric acid by Raman intensities,s Heinzinger and Weston have also deter-mined band widths of the 931 cm-1 perchlorate band. Our results agree with thosereported by Covington, Tait and Wynne-Jones. In particular, we find that theincrease in band width with concentration is essentially the same for solutions ofsodium perchlorate as it is for perchloric acid, up to a concentration of about 8 M(the solubility limit of sodium perchlorate).At higher acid concentrations, wheresome undissociated acid is present, the rate of increase of band width rises sharply.It seems possible that, in addition to the ion-solvent interactions suggestedfor solutions of perchloric and nitric acids, there are hydrogen-bonding effects insolutions of carboxylic acids which will lead to a broadening of vibrational energylevels, and hence to an increase in Raman line width.There are also certain features of the kinetic arguments which do not appearto fit the facts. It is postulated that the spectroscopic lifetime of the trifluoroacetateion A- is determined by the rate of the process,H+ .A-+HA.In this case, it would appear necessary to observe the broadening of a band attribut-able to the H f .A- species, since the conversion of A- to H+ . . . A- and finallyto Hf . A- is too slow (rate constant N 1011 M-1 sec-1) to produce observablebroadening. Yet the authors evidently believe that the 1435 cm-1 band shouldbe assigned to the A- and H+ . . . A- species.Furthermore, there should not be a dependence of the lifetime (as distinguishedfrom the rate of disappearance) of the Hf . A- species on the concentration of Hfor HA, since Hf . A- disappears by a unimolecular process. Of course, the con-centration of H+ . A- may depend on the acid concentration. It therefore appearsdifficult to explain the observed broadening and its concentration dependence onthe basis of the mechanism proposed.The described method of studying very rapid reactions is certainly correct in1 Heinzinger and Weston, J.Chem. Physics 1965, 42, 272180 GENERAL DISCUSSIONprinciple. However, it is my opinion that the systems so far examined are notsufficiently free of complication unrelated to exchange broadening to constituteconvincing experimental evidence that the latter phenomenon is indeed responsible.Prof. M. M. Kreevoy (Univ. of Minnesota) (communicated) : In reply to Weston,I think these comments are answered as best I can in my reply to Dr. Covington.The kinetic scheme is correct if A- and H+ . . . A- have identical bands, whileH+ . . . A- and H+ . A- are rapidly interconverted. I cannot, yet, be certain about theorigin of the 1450 cm-1 band in aqueous HA+HC104 mixtures, but it is not detectablein 1 M HA, while broadening of the expected general magnitude still occurs, sothat I do not think it can be the principal cause of the observed broadening in fairlydilute solutions.Dr.A. K. Covington and Mr. T. H. Lilley (Newcastle upon Tyne) (communicated) :At the present stage of the quantitative study of electrolytes by Raman spectroscopyit would seem unsafe to assume as Dr. Kreevoy does that at zero concentration thehalf-width in HN03 and NaN03 solutions should be the same. Vollmar 1 founda difference of 0-3 cm-1 (corrected for slit width) at zero molarity between variousnitrates. He also found frequency shifts of more than 1 cm-1.The selection of a value for our /31 (Kreevoy and Mead’s Avg) is arbitrary butit would seem preferable to assume a value for a dilute solution of the acid of interestinstead of that for a salt since the ion band in the acid is attributed by Kreevoyand Mead to the three species A-, Hf .. . A-, and H+ . A-; only the first of thesewill be present in the salt solution. Dr. Kreevoy’s recalculated values of k2 using/31 = 15.3 cm-1 differ from ours by less than the one figure significance that Dr.Kreevoy ascribed to them. There is little doubt that the quantum mechanicaltheory of Kreevoy and Mead is essentially correct but there remain some doubtsabout (a) the application of it to any of the acids so far studied and (b) the con-sistency of the theory with the postulated kinetic scheme. We agree that the quali-tative interpretation of line broadening in dilute solutions of trifluoroacetic acid(1-3 M) is correct.Prof.Maurice M. Kreevoy (Univ. of Minnesota) said: We believe that the1450 cm-1 band in strong acid is associated with the oriented, hydrogen-bonded,pair, W+. A-. A modification will account for the 1455 cm-1 band in the liquidacid and the 1426 cm-1 band in the deutero acid. Regardless of the origin of thesebands we have shown that the observed broadening cannot be obtained by simplyadding them to an unbroadened A- band. In 1 M HA, 1.9 M HC104 the bandwidth increases by 5 cm-1 in spite of the fact that no 1450 cm-1 band can be detectedin 1 M HA, 7 M HC104. The increase in line width with concentration in neutralsolutions of NafA- or NH,+A- is less by about an order of magnitude than that inacid solutions of comparable electrolyte concentration.No viable alternativehas been offered to the proposition that the broadening observed in acidic solutionsof trifluoroacetate in the 1-3 M concentration range is due to the shortness of thelifetime of the ion.In most respects the HNO3 system described by Covington, Tait and Wynne-Jones seems comparable. The impression, which might be gained from the text,that the broadening is the same in comparable neutral and acidic solutions doesnot seem to be borne out by the data cited. It seems safe to assume that infinitelydilute nitric acid solutions would give the same NO, width at half-height, as dilute,neutral, NaNO3 solutions ; - 15.3 cm-1. It is mentioned in the text that 4.39 MNaN03 gives a width of 16.5 cm-1, an increase of 1.2 cm-1. In order to achieve asimilar concentration of NO 3 in HNO3 solutions a stoichiometric concentration1 Vollmar, J.Chem. Physics, 1963, 39, 2236GENERAL DISCUSSION 181of -6.5 M would be required and a width of - 19 cm-1 would be observed (asinterpolated from the data in table 2). This indicates a broadening -4 cm-1,more than 3 times the broadening in the comparable neutral solution.If the rate constants cited in table 2 are recalculated using 15.3 cm-1 as the widthof the unbroadened line the following values are obtained.Cmole I.-;3-143.994-816.617.488.079.239.7910.301011 kp1. mole-; sec-10.60.60.60.80.91.01.11.21.41012 klsec-11.81.51.00.90.90.90-70.70.6The values appropriate to dilute solutions would seem to be 0.6 x 1011 1.mole-1sec-1 for k:! and -2 x 1012 sec-1 for kl, the dissociation rate constant. The com-parable values for the trifluoroacetic system are N 1.5 x 1011 1. mole-1 sec-1 and- 4 x 1011 sec-1. The dissociation rate for nitric acid is larger, as befits a strongeracid, and the recombination rate is smaller.None of these rates are diffusion-controlled. The interconversion of A- andH+ . . . A- is relatively slow, but the data for salts strongly suggest that the cor-responding bands are displaced by no more than 1 cm-1. The bands for Hf . . . A-and H+ . A- may be separated by - 15 cm-1, but these species can be interconvertedwithout a translational diffusion, by a rotation of the A- unit.This may well bevery fast. In that case the average lifetime of the A- species (that is, A-, H+ . . . A-,and H+ . A-) will be obtained from the overall line width. This is an approximation,and along with the various experimental uncertainties, suggests that no more thanone significant figure be attributed to the derived rate constants.The perchlorate band is not broadened until very high acid concentrations arereached. This observation in itself supports the " chemical " explanation givenfor the broadening in acidic trifluoroacetate and nitrate solutions, since some un-specified " physical " explanation would, presumably, apply to perchlorate, also.The broadening that is observed all occurs in a region of concentrations where neutralsalt solutions also give considerably broadened bands. The derived rate constantsshould be regarded as no more than upper limits.Prof. R.J. Gillespie (McMaster University) said: Dr. Covington has remarkedthat new measurements of the dissociation of perchloric acid in aqueous solutionby the Raman method are in marked disagreement with the results of n.m.r. measure-ments. Dr. White and I have pointed out that the n.m.r. method is unreliableas it is based on the assumption that the chemical shift of the H3O+ ion is independentof the composition of the acid+water mixture.1 Since the degree of hydration ofthe hydronium ion must decrease with increasing acid concentration this assumptioncannot be justified.Mr. T. H. Lilley (Uniuersity of Newcastle upon Tyne) said : Regarding the degreesa of dissociation of acids obtained by the Raman and p.m.r.procedures, in the p.m.r.method it is assumed 2 that when the acid is completely dissociated the chemical shiftis directly proportional to the function p , defined by Gutowsky and Saika.2 Devi-ations from direct proportionality are attributed to association. For the strong1 Gillespie and White, Can. J. Chem., 1960, 38, 1371.2 Gutowsky and Saika, J. Chem. Physics, 1953,21, 1688182 GENERAL DISCUSSIONacids deviations do not occur until high concentrations and some environmentaleffect, rather than association, might be the cause. The ammonium ion in waterhas similar properties to the hydronium ion1 and since for the ammonium ionassociation does not occur, deviations from proportionality between chemical shiftand the analogous p function for the ammonium ion would indicate the naivity ofthe interpretation of the p.m.r.results. In fig. 1 results obtained by Hindman 2p = 2X/l+xFIG. 1.for ammonium salts have been recalculated. Deviations from linearity occur atabout 5 m for the nitrate, chloride and bromide. The insolubility of the perchlorateprevents measurements at concentrations greater than about 2 m. Raman andp.m.r. work in this laboratory 3 has shown that perchloric acid is completely dis-sociated up to approximately 6 M but at higher concentrations the a values obtainedby the two methods diverge. The differences are possibly due to greater environ-mental effects in the p.m.r. measurements than in the Raman measurements.Prof. €3. E. Conway (Ottawa) said: Wynne-Jones remarked on the minimumfound in Zyate ion mobility in H20+H202 mixtures at the high H202 end of thecomposition range. A similar result is found for Iyonium ion mobility in HCl+methanol +water mixtures 4 where the conductance minimum is around 85-90 %methanol. At that composition, the conductance is close to that of KCl in thesame solvent indicating normal bulk transference of the H3Of ion. Water in themethanol+ water mixture acts as a proton trap ; on one side of the conductanceminimum, anomalous transfers can occur favourably between H20 and H30+,while on the other side of the minimum anomalous transfers can occur betweenCH30H; and CH30H. Transfer between H30+ and CH30H will not be so facileif the base strength of CH3OH is less than that of H2O (the apparent base strengthwill itself depend on composition).1 see, e.g., Frank and Evans, J. Chem. Physics, 1945, 13, 507. Kaminsky, Disc. Faraday Soc.,3 Covington, Tait and Wynne-Jones, Proc. Roy. Soc. A , 1965,286,235. Akitt, Covington, Freeman4 Conway, Borcks, Linton, J . Chem.Physics, 1956,24,834.1957,24, 171.and Lilley, Chem. Comm., 1965,349.2 Hindman, J. Chem. Physics, 1962, 36, 1OOO