Near-IR Study of Tetraalkylammonium Salt Solutions (1962); (c) E. Fluck, W. Kerler, and W. Neuwirth, Angew. Chem., Int. Ed. End.. 2. 277 (1963): (d) E. Fluck. Adv. Inorg. Chem. Radiochem: 6, 433 (1964); (e)E . Fluck and P. Kuhn, Z. Anorg. Allg. Chem., 350, 263 (1967). (a)G. M. Bancroft, M. J. Mays, and B. E. Prater, Discuss. Faraday Soc., 47, 136 (1969); (b) G. M. Bancroft, M. J. Mays, and B. E. Prater, J . Chem. SOC.A , 956 (1970); (c)G. M. Bancroft, R. E. B. Garrod, and A. G. Maddock, bid., 3165 (1971); (d) G. M. Bancroft, "Mossbauer Spectroscopy", Wiley, New York, N.Y., 1973, Chapters 5 and 6. J. M. Malin, C. F. Schmidt, and H. E. Toma, Inorg. Chem., 14, 2924 (1975). H. E. Toma and J. M. Malin, Inorg. Chem., 12, 1039 (1973). For example A. VBrtes and F. Parak, J. Chem. SOC.,Dalton Trans., 2062 (1972); A. VBrtes, S. Nagy, I. Czako-Nagy, and E. Csakvary, J. Phys. Chem., 79, 149 (1975). W. A. Mundt and T. Sonnino, J . Chem. fhys., 50, 3127 (1969).
"Handbook of Preparative Inorganic Chemistry",Vol. Academic Press, New York, N.Y., 1965, p 1509. J. Matas and T. Zemcik, fhys. Lett., 19, 111 (1965). S. Asperger, 1. Murati, and D. Pavlovic, J . Chem. SOC.A , 2044 G. Brauer, Ed., 11,
(1969).
D. X. West, J. Inorg. Nuci. Chem., 29, 1163 (1967). K. Hofmann, Z. Anorg. Aiig. Chem., 11, 31 (1896). E. Biesalski and 0. Hauser, Z. Anorg. Aiig. Chem., 74, 384 (1912). Reference 7, p 1511.
The Journal of Physical Chemistry, Vol. 82, No. 9, 1978 1051 (14) W. Manchot and P. Woringer, Ber. Dtsch. Chem. Ges., 46, 3514 (19 13). (15) D. J. Kenney, T. P. Fiynn, and J. 8. Gallini, J . Inorg. Nuci. Chem., 20, 75 (1961). (16) H. E. Toma, J. M. Malin, and E. Giesbrecht, Inorg. Chem., 12, 2084 (1973). Dalton Trans., (17) D. Paviovic, I. Murati, and S. Asperger, J. Chem. SOC., 601 (1973). (18) W. Beck, Z. Anorg. Aiig. Chem., 333, 115 (1964). (19) J. R. DeVoe, Natl. Bur. Stand. (U.S.), Tech. Note, No. 404, 208 (1966). (20) C. P. Monaghan, Ph.D. Dissertation, Clemson University, 1976. (21) S. Margulis and J. R. Ehrman, Nuci. Instrum. Methods, 12, 131 (1961). (22) N. N. Greenwood and T. C. Gibb, "Mossbauer Spectroscopy", Chapman and Hall, London, 1971, pp 172 and 183. (23) A. N. Garg and P. S. Goel, J. Inorg. Nuci. Chem., 31, 697 (1969). (24) D. B. Brown and D. R. Shriver, Inorg. Chem., 8, 37 (1969). (25) L. H. Ahrens, Geochim. Cosmochim. Acta, 2, 155 (1952). (26) K. J. Duff, fhys. Rev. B , 9, 66 (1974). (27) S. Isotani and K. Watari, J . Inorg. Nuci. Chem., 38, 501 (1976). (28) N. B. HannayandC. P. Smyth, J . Am. Chem. SOC.,68, 171 (1946). (29) A. J. Gordon and R. A. Ford, "The Chemist's Companion", Wiley, New York, N.Y., 1972, pp 2 f f . (30) K. J. Laidler, "Chemical Kinetics", 2nd ed, McGraw-Hill, New York, N.Y., 1965, p 90.
A Near-Infrared Study of Changes in the Solvent upon Solute-Solute Interactions in Aqueous Solutions of Tetraalkylammonium Salts Carmel Jolicoeur, Jean Paquette, Depatfment of Chemistry, Universit6 de Sherbrooke, Sherbrooke, Quebec J 1K 2R 1, Canada
and Mlchel Lucas DGR, BP no 6, 92260 Fontenay-aux-Roses, France (Received December 5, 1977)
The near-infrared spectra of aqueous solutions of several tetraalkylammonium salts have been recorded in the 1000-nm region under various conditions of temperature and concentration. Differential hydration spectra were obtained at 25 "C and 0.5 m for Bu4NX salts where X = OH-, C1-, NO;, SO:-, HC02-, MeCOf, EtCO;, n-PrCO;, n-BuC02-, n-PenC02-,and n-HepCOz-. The concentration dependence of the near-IR spectra has been investigated at 25 "C over the range 0-1 m for the following electrolytes: n-PrC02Bu4N,n-PenCOzBu4N, and n-HepC02Na. In solutions of R4NBr (R = Me n-Bu), the spectral changes have been followed in the concentration range 0-4 m, at 25 and 60 "C. Additional spectra of water, diluted in weakly interacting solvents and solvent mixtures, have also been obtained to assist in the interpretation of the high concentration data. In the concentration dependence studies, substantial changes of the differential spectra have been found and related to perturbation of the hydration cosphere of the various species. These illustrate the important role of the water solvent in solute-solute interactions, particularly those occurring between two hydrophobic ions. -+
1. Introduction In previous inve~tigations,l-~ the influence of various types of solutes on the near-infrared spectrum of water has been examined systematically to evaluate the potential of near-IR absorption methods in the study of hydration phenomena. During this work, it was often found that modifications of the near-IR absorption bands upon addition of solutes were analogous to changes induced by temperature variations. This observation provided simple grounds for a quantitative interpretation of the spectral changes in terms of perturbations of a postulated hydrogen-bonding equilibrium in liquid ~ a t e r . I - ~ Within the categories of solutes investigated, both hydrogen-bond disrupting (e.g., with NaI, propylene carbonate) and hydrogen-bond enhancing influences (e.g., with Bu4N+Br, tetrahydropyran) could be observed, in addition to the effect of specific interactions of water 0022-3654/78/2082-1051$01 .OO/O
molecules with ions or polar functional groups. Generally, the magnitude of the spectral modifications recorded seemed well related with other physical and thermodynamic parameters used previously to categorize various types of hydration effects. The relatively high sensitivity of the differential near-IR methods used in these studies suggested that, possibly, the influence of hydration effects on solute-solute interactions could also be investigated. The consequence of solute-solvent interactions on the potential energy functions effective among solute particles has been acknowledged for s ~ m e t i m e s . It ~ ?is~now readily accepted that solvation effects play a part in most solute-solute interactions, particularly important in aqueous solutions of large hydrophobic solutes. For example, the anomalous heqts and entropies of dilution of tetraalkylammonium salts have been explained on the basis of mutual interference between the hydration cospheres of
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each solute species.6-8 Quantitative treatment of these effects has been incorporated in statistical models for the calculation of thermodynamic excess f u n c t i o n ~ , ~but J~ these have not provided a molecular description of the role of the solvent. On the other hand, the direct observation, Le., using physical methods, of changes in the hydration cospheres with solute concentration has eluded all attempts. In dilute solutions, only indirect evidences can be found, derived mostly from magnetic resonance ~tudies.'l-'~ In the present report, we describe the results of a near-infrared investigation (1000-nm region) of a few systems, carried out as function of concentration and temperature. Tetraalkylammonium salts with widely different anions have been chosen for this study, in order to identify changes in hydration cospheres occurring with different types of solute species. The new data provide rather straightforward evidence on the role and behavior of solvent water in solute-solute interactions. 2. Experimental Section The tetrabutylammonium salts used in this study (except the bromide) were prepared directly in solution, through acid-base titrations using freshly prepared tetrabutylammonium hydroxide. The carboxylic acid were distilled before use and their purity was determined as >99% from GLC. The R4NBr salts were recrystallized in methanol or ethanol-ether mixtures and dried under vacuum. The solvents tetrahydrofuran, dichloroethane, and nitromethane were reagent grade quality which were dried on molecular sieves and distilled before use. All aqueous solutions were prepared by weight in distilled deionized water (Continental Deionizer) and were filtered before recording the spectra. Solutions of water in various solvents or solvent mixtures were prepared by volume. The spectra were recorded on a Cary-14 spectrophotometer using a 10-cm reference cell and a variable pathlength working cell. The cell temperature was regulated within 0.01 " C with a circulating water jacket. The procedure followed in recording the differential near-IR spectra has been described in great details el~ewhere.l-~ The cell pathlength adjustments required to account for solute volumes were performed knowing the solution densities, which were measured with a digital flow densimeter.15 The differential spectra were also corrected for the infrared absorption of molecular ions or cosolvent using the spectra recorded in DzO.
3. Analysis of Spectra After allowing for the volume of the solutes and their intrinsic absorption, the differential spectra illustrate the net effect of these solutes on the water spectrum. In previous studies with the tetraalkylammonium salt^,^^^ the spectra were further analyzed to single out the effect of the large R4Nf ions by subtracting the differential spectrum obtained with the corresponding sodium salt. This procedure is, of course, exact only if the small monovalent cations produce negligible spectral modifications compared to the R4N+cations. The differential spectrum assigned to the hydration effects of the R4N+ions consisted of an S-shaped curve, closely similar in shape and position to that obtained when lowering the temperature of pure water or adding nonelectrolytes such as tetrahydropyran.' All of these spectra could be resolved into two components having approximate Gaussian shapes. The integrated intensity of the short wavelength portion of these spectra then served as a quantitative basis for interpretation. With this approach,
C. Jolicoeur, J. Paquette, and M. Lucas
TABLE I: Molal A T * Assigned t o the Bu,N+ Ion (Relative to Na') in 0.50 m Bu,NX Solutions at 25 "C
X OHC1BrNO,-
so,,-
AT*(Bu,N+) I - 1 "C
X
AT *(Bu,N+) i 1 "C
- 10.2 HC0,- 10.8 CH,CO,-11.2 (ll)dC,H,CO,-
-11.8 -9.4a
-9.6 - 10.8 -10.2 T Z - C , H , C O , - ( - ~ . ~ )-~7 . 8 (- 3.0)'
n-C,H,CO,-4.2 n-C,H,,COz-(-4.2)b - 3 . 0 (-2.0)'
a Data obtained at a concentration of 0.5 m in Bu,N+. From spectra recorded at 10 "C. 'From spectra recorded at 40 "C. From ref 2.
a molal AT* could be calculated which gives, by reference to the effect of temperature, the magnitude of the spectral modifications (and the underlying changes in molecular interactions) induced by the various R4N+,relative to Na+. For the data presented below the same procedure were followed wherever possible. The spectra of sodium salts solutions required for the assignment of the R4N+ contribution were taken from a preceding investigati0n.l As discussed earlier2Band is further evidenced in some of the data reported below, the two-Gaussian deconvolution is not accurate in the long wavelength end of the spectra. The origin of such discrepancy may be that three (or more) distinguishable states of the water molecules are involved (e.g., ref 24) or/and that the neighboring combination band (1200 nm) contributes significant intensity in the 1000-1050-nm region. For lack of sufficient evidence to ignore the latter possibility, we did not attempt a three-Gaussian analysis. The neglect of intensity in the long wavelength portion will affect absolute values of equilibrium constant, enthalpy, etc... for the two-state model considered. Indeed, some variance was observed in thermodynamic parameters derived from analysis of three different combination bands (1000, 1200, and 1450 nm) reflecting the limitations of two-Gaussian ana1ysis.l However, in obtaining relative intensity changes due to temperature and solute, we rely exclusively on the highfrequency Gaussian assigned to the weakly perturbed OH groups. The validity of this approach, of course, requires that the shape and position of this component remain invariant with temperature and solute concentration. For most nonelectrolytes and organic ions studied thus far,Iv3 this condition is fulfilled with a short wavelength component centered at 968 nm. With some inorganic ions and concentrated solutions studied here, this component may be shifted significantly. In these cases, greater caution in the interpretation has been emphasizedl~~ and should be recalled.
4. Results and Discussion 4.1. Influence of Various Counterions on the Differential Spectra of Bu4Nt. The molal AT% obtained for Bu,N+ in 0.5 m solutions of various tetrabutylammonium salts are reported in Table I. It is quite apparent from these data that AT* is always negative and roughly independent of the anion, except in the case of alkylcarboxylates having alkyl residues larger than ethyl. The sign and magnitude of the molal AT* data for the tetraalkylammonium ions, as well as for other solutes containing alkyl groups, have been interpreted earlier as due to the water structure-stabilizing influence of the alkyl residues. This has been substantiated in previous papers from correlations of the AT* with other measures of hydration and solvent structural effects.'i2 From data on homologous series, the averaged methylene group con-
The Journal of Physical Chemistry, Vol. 82, No. 9, 1978
Near-IR Study of Tetraalkylammonium Salt Solutions
1053
Bu4N+Hex10 ,
,,;; Bu4N* But,
,'I
,', ,',
I
920
960
1000
1040
X ( nrnl Figure 1. Differential spectra obtained with various alkylcarboxylates of tetrabutylammonium (Ace-, acetate; But-, butyrate; Oct-, octanoate) at 0.5 m . The spectra have been corrected for solute volume and intrinsic absorption in this region. These spectra represent the combined effect of both organic ions.
tribution to AT* of organic ions at 25 "C has been obtained as -0.4 "C. Furthermore, the temperature dependence of AT* for such species as Bu4N+could be used to predict a large positive contribution (solvent relaxational or structural effect) to the apparent molal heat capacity of the ~ o l u t e . ~ J ~ From the invariant AT* (Bu4N+) observed in the presence of different inorganic anions (0.5 rn) it can be concluded that there occurs little disruption of the hydration cosphere of the ions upon solute-solute interaction. The magnitude of such effects remains within the experimental uncertainty of the near-IR method, i.e., within 10%. This result was expected since it was shown earlier that AT* (Bu4N+)in Bu4NBr solutions was independent of concentration in the range 0-1 M.I These observations provide, however, additional evidence that the differential spectra assigned to the R4N+ions do not arise from anion hydration, or from anion-water-cation triplet interactions. Contrastingly, the data reported for tetrabutylammonium alkylcarboxylates exhibit a sharp drop of AT*(Bu4N+) for the propionates and larger salts; in the presence of the octanoate counterion, AT*(Bu4N+) is reduced to only 25% of its expected value. This can be visualized directly from the differential hydration spectra of various alkylcarboxylates as plotted in Figure 1. There, the spectra reflect the combined influences of the Bu4N+ and RCOO- ions, and their magnitudes decrease with increasing chain length of the carboxylate anions. Such an effect is readily attributed to partial loss of the hydration cospheres of the Bu4N+and RCOO- ions due to clustering of the solute molecules, i.e., micellar type aggregation, which has been evidenced in these sol~tions.'~J~ This process will decrease the water-hydrocarbon contact area and consequently reduce solvent structural effects associated with the hydration of the alkyl groups. Such effect may be described quantitatively in the light of more detailed results on the concentration dependence of AT*. 4.2. Importance of Hydration Effects in Bu4N+ -RCOO- Interactions. We illustrate, in Figure 2, AT data obtained as function of concentration for several carboxylates. The data are given as the sum of anion and cation effects and will be labeled AT(+, -) to distinguish from previous molal ionic data. The concentration dependence of AT(+, -) will thus directly reflect the overall changes in the solvation of the hydrophobic ions with salt concentration. By comparison with values expected from addition of ionic molal AT* (dotted lines in Figure 2) it is apparent that AT(+, -) exhibit, in all cases studied, a destructive effect of the hydration cospheres, increasing with concentration. Several important points may be noted from these results. First, the extent of destructive overlap of the hydration cospheres increases with the size of the carboxylate anion.
0
0.4
0.2
0.8
0.6
mlmol. KQ,']
1.0
Figure' 2. Concentration dependence of the differential spectra (integration expressed at AT(+, -)) for various alkylcarboxylates. Dashed lines indicated expected values based on A T(Bu,N+) and A T(RCO0-) as obtained from solutions of the bromides and sodium salts, respectiveiy.
I
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I
02
I
I
OA
1
06
I
08
l
i
-
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l
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The Journal of Physical Chemistry, Vol. 82, No. 9, 7978
C. Jolicoeur, J. Paquette, and M. Lucas
L
Me,N Br
-
-.2
-
- 2-
0W
Y -
a
m
0:
k
&r,N Br
1
920
I
960
I
I
1000
L
1
920
1040
A
I
I
960
I
1
1000
I
I
104 0
Inm)
Flgure 4. Differential spectra obtained with homologous R,NBr relative to NaBr at various concentrations and temperatures. Each spectrum was corrected for solute volume and intrinsic absorption: dashed lines, data at 1 m from previous work.* B, 1.0 m ; 0 , 2.0 m; B, 3.0 m ; A, 4.0
m.
Near-IR Study of Tetraalkylammonium Salt Solutions
AH(+, -) as needed to calculate the equivalent of h: AH*(+, -)ex = AH*(+, -)m - AH*(+, -)o where AH* are molal values, i.e., AH(+, - ) / m and the subscripts m and 0 refer to finite concentration and limiting values, respectively. The close coincidence in the variation of AH(+, -) and & , w i t hconcentration points to the important role of the solvent in the large exothermic heat of dilution of these electrolytes. From the near-IR data, a major part of this heat of dilution is seen to originate in perturbations of the hydration cospheres of the organic ions. Judging from these results, overlap effects dominate the excess enthalpy, over and above other contributions from the ion-ion Coulombic interactions, or dispersion energies between the solute species. The latter two effects should contribute only to 4L. It is intriguing that cosphere perturbations appear dominant when both ions contain large hydrophobic groups, whereas it is not detectable with Bu4N+ salts having a small hydrophylic anion. It must be expected, surely, that if Bu4N+is surrounded by a "solvent structured shell", the latter will be greatly modified at high concentrations or temperatures. In the following section, we present results obtained to verify this expectation. 4.3. Differential Spectra of R4NBr Salts as a Function of Concentration and Temperature. In Figure 4, we illustrate the differential spectra obtained with the homologous R4NBr series (R = Me, Et, Pr, Bu) in the concentration range 1.0-4.0 m, a t 25 and 60 "C. These spectra have been corrected for the infrared absorption of the organic ions and for the effect of Br- based on the differential spectra of NaBr at the same concentrations. Therefore, they reflect the influence of the R4N+ ions relative to Na' and, possibly, cosphere overlap effects from ion-ion interactions. As is obvious from the results, some of the spectra did not lend themselves to the integration procedure used earlier to evaluate AT*. Thus, only qualitative comparisons can be pursued. The general trends in these data indicate the following: with all R4N+ ions, the magnitude of the differential spectrum decreases with increasing temperature; with Me4N+and Et4N', the magnitude is roughly proportional to salt concentration up to 4.0 m (the same was found with NaBr for which AT* varies lineary with concentration); with Pr4N+and Bu4Nt, a new component appears at -950 nm which increases with concentration and temperature. For the first part, a reduction in the magnitude of the low concentration differential spectra at 60 "C compared to 25 " C is consistent with earlier observations2-21and supports the idea that the solvent structural"modifications by the R4Nt ions become weaker at higher temperature. On the other hand, the new absorption found with solutions of Pr4NBr occurs at wavelengths shorter than those assigned to the "weakly interacting" OH group in pure water, ca. 970 nm. The frequency and the temperature dependence of this component suggests that it may be due to OH groups undergoing still weaker interactions, perhaps comparable to those of water in a nonpolar solvent. To assist this interpretation, we have examined the spectrum of water in several solvents and solvent mixtures. 4.4. Near-IR Spectrum of Water i n Several Solvents. The procedure followed here is essentially the same as that reported by several investigators, for other spectral reg i o n ~ . ~In~ Figure - ~ ~ 5a, the spectrum of pure water is compared to that of water a t various concentrations in tetrahydrofuran (THF). At 50 vol % in THF, the water absorption remains broad and unstructured, appearing slightly shifted to longer wavelengths. As the water concentration is decreased, the band separates into two
The Journal of Physical Chemistry, Vol. 82, No. 9, 1978 1055 -
f\
a
0.11
""": 0
2\
I
I
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d + I
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The Journal of Physical Chemistry, Vol. 82, No. 9, 1978
nitrogen and an interchain space large enough to accommodate the water molecules. Likewise, it can be added that in the micellar solutions studied here, there appears to be no water molecules in a hydrocarbon-like environment. Finally, it was noted that, a t 25 "C, the appearance of the 950-nm absorption in solution of R4NBr seems related to the minimum in the concentration dependence of their apparent molal volumes (4").However, from the 4"data presently available25a t 60 " C , this correlation does not hold.
N. C. Craig, R. A. MacPhail, and D. A. Spiegel
Canada and of the France-Qu6bec scientific exchange program. References and Notes J. Paquette and C. Jolicoeur, J . Solution Chem., 6, 403 (1977). P. Philip and C. Jolicoeur, J. Phys. Chem., 77, 3071 (1973). C. Jolicoeur, N. D. The, and A. Cabana, Can. J. Chem., 49, 2008 (1971). C. Jolicoeur and P. R. Philip, J . Solution Chem., 4, 3 (1975). R. W. Gurney, "Ionic Processes in Solution", McGraw-Hill, New York, N.Y., 1954, Chapter 16. H. S. Frank, J. Phys. Chem., 67, 1554 (1963). W. Y. Wen, J . Solution Chem., 2, 253 (1973). J. E. Desnoyers and C. Jolicoeur, "Modern Aspects of Electrochemistry", J. O'M. Bockris and 6. E. Conway, Ed., Plenum Press, New York, N.Y., No. 5, 1969, Chapter 1. P. S. Ramanathan, C. V. Krishnan, and H. L. Friedman, J . Solution Chem., I,237 (1972). W. H. Streng and W. Y. Wen, J . Solution Chem., 3, 865 (1974). J. Davies, S. Ormondroyd, and M. C. R. Symons, J. Chem. SOC., Faraday Trans. 2, 4, 686 (1972). M. M. Marciacq-Rousselot, A. de Trobriand, and M. Lucas, J . Phys. Chem., 76, 1455 (1972). H. G. Hertz, Wafer, Compr. Treat., 3, Chapter 16 (1973). C. Jolicoeur, P. Bernier, E. Firkins, and J. K. Saunders, J. Phys. Chem., 80, 1908 (1976). P. Picker, E. Tremblay, and C. Jolicoeur, J . Solution Chem., 3, 377 (1974). M. Lucas and A. de Trobriand, C.R. Acad. Sci. (Paris),274, 1361 (1972). J. Lang, C. Tondre, R. Zana, R. Bauer, H. Hoffman, and W. Ulbricht, J . Phys. Chem., 79, 276 (1975). P. A. Leduc and J. E. Desnoyers, Can. J . Chem., 51, 2993 (1973). S. Ablett, M. D. Barratt, F. Franks, M. D. Pedley, and D. S. Reid, "L'eau et les syst6mes biologiques", Colloque International du CNRS, No. 246, ed. CNRS, Paris, 1976, p 105. S. Lindenbaum, J . Phys. Chem., 75, 3733 (1971). M. Lucas, A. de Trobirand, and M. Ceccaldi, J. Phys. Chem., 79, 913 (1975). 0. D. Bonner and Y. S. Choi, J. Phys. Chem., 78, 1723 (1974). A. Bruneau and J. Corset, Can. J . Chem., 52, 915 (1974). G. R. Choppin and N. J. Hornug, Spectrochim. Acta, Part A , 30, 1615 (1974). A. LoSurdo and H. E. Wirth, J . Phys. Chem., 76, 1333 (1972).
5. Conclusion The results of the present investigation first provide confirming evidence to previous assignment of near-IR hydration spectra of the R4N+ ions. The differential spectra obtained with various salts of Bu4Nt show that, in the presence of small hydrophylic anions, mutual disturbance of the hydration cospheres of the various ions is within 10% of the total hydration effect a t 0.5 m. However, when both ions contain large hydrophobic groups (e.g., tetrabutylammonium hexanoate), the spectra exhibit an important concentration dependence which has been assigned to partial disruption of the structured hydration cospheres. Based on this interpretation, the spectral data are capable of predicting a major part of the heat of dilution of these salts. Finally, it is shown that concentrated solutions of the larger R4NBr salts exhibit a new absorption band ca. 950 nm. From the spectra of water diluted in various solvents, this component can be assigned to water molecules retained between alkyl chains of pairs, or higher clusters, of the hydrophobic ions. Acknowledgment. The authors gratefully acknowledge the financial support of the National Research Council of
Vibrational Assignments and a Potential Function for 3,3-Difluorocyclopropene-do, - d l , and - d z Norman C. Craig," Richard A. MacPhail, and David A. Splegel Department of Chemistry, Oberiin College, Oberlin, Ohio 44074 (Received November 2, 1977) Publication costs assisted by the Petroleum Research Fund
Complete assignments of the vibrational fundamentals of 3,3-difluorocyclopropeneand its dl and dz isotopic modifications are derived from infrared and Raman spectra. The 15 fundamentals for the undeuterated molecule are as follows, in cm-l: (al) 3150, 1598, 1343, 946, 769, 500; (az)883, 393; (b,) 3128, 1131, 968, 522; (b,) 1094, 680,416. An 18-parameter potential function is fit to the full set of frequencies for the three isotopic species. Contributions of internal coordinates to normal modes are analyzed in terms of potential energy distributions, and frequencies are correlated with those of the closely related molecules, CFz-N=N, CH2-CH=CH, CF2CH=CF, and O=C-CH=CH. Changes in potential constants of ring bonds in going from unsubstituted molecules to fluorine-bearingones correlate with changes in bond lengths. Interpretable CF bond characteristics are also found in these molecules. I
--
-
Introduction Fluorine-bearing C3rings are of current interest because of the marked electronic effect of fluorine substituents on these systems. This effect is evident in the changes in carbon-carbon bond lengths in the C3 rings. Laurie and co-workers have obtained detailed structures for 1,l-difluorocyclopropane and for 3,3-difluorocyclopropene.1~z Compared to the corresponding unsubstituted hydro0022-3654/78/2082-1056$01 .OO/O
I
carbons, the CC bonds adjacent to the fluorine-bearing carbons are shorter and the CC bonds opposite are longer. Ab initio molecular orbital calculations correlate these effects of fluorine substitution with changes in strength of the ring bonds.3 One might also expect to find a correlation with changes in potential constants for the ring bonds. Thus, the CC stretching constants for the adjacent CC bonds would be strengthened and the CC stretching
0 1978 American
Chemical Society
