JOURNAL O F T H E AMERICAN CHEMICAL SOCIETY Registered in U. S,Patent Ofice.
@ Cogyrighl. 1963,
VOLUME85, NUMBER 4
by the A w k a n Chcmicd Society
FEBRUARY 27, 1963
PHYSICAL AND INORGANIC CHEMISTRY [CONTRIBUTION FROM
FRICK
CHEMICAL LABORATORY, PRINCETON UNIVERSITY, PRINCETON, N. J.]
Solvent Effects in Infrared Spectroscopic Studies of Hydrogen Bonding It2 BY ADAMALLERHAND’
AND P A U L VON
R. SCHLEYER‘
RECEIVED SEPTEMBER 5, 1902 The d e c t of solvents on the stretching vibrations of hydrogen bonded systems has been studied. Contrary to previously held ideas, the frequencies of hydrogen bonded bands, YOH...~, were found to be very solvent sensitive for many intramolecular or intermolecular hydrogen bonds. The solvent shifts of OH.. .O bands are proportional to C=O and other X=O solvent shifts, indicating a possible similarity of mode of interaction of solvents with these chromophoric groups. Existing theories of solvent shifts do not explain this behavior. An empirical linear free energy equation, ( Y O V ’ ) / v o = aG, is proposed for the accurate prediction of the positions of a large number of solvent sensitive infrared bands. The limitations of this equation are discussed. For intermolecular hydrogen bonds involving the interaction of a proton donor ROH with a proton acceptor B in a n “inert” solvent, Y O H . .B is found to depend on the concentration of B. while YOH (the “free” band) is practically independent. As a consequence Au, the difference between the two bands, is concentration dependent, a result mntrary to existing theories. Dielectric constants and refractive indexes were determined for several B S solvent systems to test various “dielectric equations”; these do not explain the observed results. An empirical method is proposed for the prediction of the occurrence of such concentration dependence.
-
+
Infrared spectroscopy is a powerful method for the study of hydrogen bonding.6 One of the most characteristic effects observed is the frequency shift due to hydrogen bonding, Av (in cm.-l), defined as the difference in position between a “free” stretching vibrational band, A-H, in a given environment and its bonded counterpart, A-H . B, displaced to lower frequencies in the same environment.6 Most infrared spectroscopic studies of hydrogen bonding are conducted in the so-called (‘inert’’ solvents, CClr, CS2, etc., since vapor phase measurements pose experimenta! difficulties. Frequently comparisons of Av, which is related to the strength of the hydrogen bond,&have to be made for several systems in which the solvent employed was not the same, either because of the unavailability of data or because of solubility problems. Despite the well known solvent sensitivity of many infrared absorption bands, such as carbonyl and sulfonyl7.*which vary as much as 30 cm.-’ from solvent to solvent, the systematic study of the effect of liquid environment on “free” and hydrogen bonded stretching frequencies has been We generally n e g l e ~ t e d . ~ ~ ’ ~ report here the results of such a study.
..
(1) Preliminary report: A. Allerhand and P. von R. Schleyer, Abstracts, Fourth Delaware Valley Regional Meeting, Am. Chem. SOC.,Philadelphia, Pa., January, 1962, p. 99. This work was taken from the Ph.D. Thesis of A. A., Princeton University, 1962. (2) Paper VI1 of a series on hydrogen bonding; IV, P. von R. Schleyer, J . A m . Chem. SOL., 83, 1368 (1961); V, P.von R. Schleyer and A. Allerhand, ibid., 84,1322 (1962); VI, R. West, D. L. Powell, L.S. Wheatley, M. K. T. Lee and P. von R. Schleyer, ibid., 84,3221 (1962). (3) Merck Foundation Fellow, 1960-1961; Esso Foundation Fellow, 1861-1962. (4) Alfred P. Sloan Research Fellow. (5) G. C. Pimentel and A. L. McClellan, “The Hydrogen Bond,” W. H. Freeman and Co., San Francisco, Cal., 1960, Ch. 3, pp. 67-141. (6) This is the definition t o be used in this paper. T h e symbol “Au” is frequently used t o designate the shift of a n infrared frequency when passing from the gaseous state t o a solvent, but we shall w e V Q u’ for this latter purpose, t o avoid confusion. (7) L. J. Bellamy a n d R. L. Williams, Trans. Faraday Soc., 66, 14 (1959). (8) L. J. Bellamy, C. P. Conduit, R. J. Pace and R. L. Williams, ibid., 66, 1677 (1959).
-
371
An early theoretical attempt to calculate infrared solvent shifts is the Kirkwood-Bauer-Magat equation.11912Suchattemptstocorrelatetheeffectsof solvents on infrared spectra with bulk properties of the solventdielectric constant and refractive index-are sometimes referred to as dielectric +heory.L3 On the other hand there have been attempts in recent years to explain spectral shifts qualitatively on the basis of specific solvent-solute interactions, dielectric effects being considered unimportant.lo,15.16 Also, a combination of non-specific (dielectric) and specific effects has been c o n ~ i d e r e d . ~A~ more . ~ ~ rigorous but mathematically involved approach has been proposed b y Drickamer, Wiederkehr and Benson in which no use is made of the vague notions of ((specific” and “nonspecific’] solvent interactions] rather a bond interaction niodel is used and consideration is given to both attractive and repulsive intermolecular interactions. 14, l8v19 An illustration will show differences in the predictions of some of these approaches. Consider a molecule A-€1, capable of acting as proton donor toward (9) For studies of H-bonded systems in various solvents, see M.-L.Josien and N. Fuson, J . Chem. Phys., 22, 1169 (1854)and ref. 10. (10) L. J. Bellamy and H. E. Hallam, Trans. Faraday Soc., 66, 220 (1959). (11) W. West and R. T. Edwards, J . Chem. Phys., 6, 14(1937). (12) E. Bauer and M. Magat, J . Phys. Radium, 9, 319 (1938). (13) It is assumed, in this approach, t h a t the solute molecule behaves like a polarizable point dipole at the center of a spherical cavity. The effect of the surrounding solvent molecules is taken 8 s that of a continuow dielectric medium surrounding the cavity. An interesting discussion is given in ref. 14. pp. 30-34. (14) R. R. Wiederkehr, Ph.D. Thesis, University of Illinois, 1857; Dissalt. Abslr., 17,2542 (1957). (15) L.J. Bellamy, H. E. Hallam and R. L. Williams, Trans. Faraday Soc., 64, 1120 (1958). (16) L. J. Bellamy and R. L. Williams, Proc. Roy. Soc. (London), AS66, 22 (1960). (17) G.L.Caldow and H. W. Thompson, ibid., 8464, 1 (1860). (18) R. R. Wiederkehr and H. G. Drickamer, J . Chem. Phys., 28,-311 (1958). (19) A. M.Benson a n d H. G. Drickamer, ibid., 27, 1164 (1957).
372
ADAM-4LLERHAND
AND PAUL \'ON
proton acceptor €3 to give a hydrogen bonded complex A-H . . . . B. Y O A H is the gaseous A-H stretching frequency of the unassociated molecule, while YOAH ...B is the frequency for the associated gaseous complex. The symbols V ~ A Hand V S A H . , B are the corresponding frequencies in an "inert" solven.t, S. Then in the ga.s phase Avo = V'AH - Y O A H . .B (shift produced by H-bonding in the absence of environmental interactions) and in the solvent, s, Avs = V'AH - V ~ A H . . B (shift produced by €3-bonding in the presence of environmental interactions with s), It has been suggested on the basis of the dielectric theoryzothat, nonspecific effects should be the same for tinassociated and associated species, so that Avs should be identical with Avo and be independent of solvent. Experimentally we will show that this is not generally so. Adopting another point of view,l0 that all solvent shifts are mainly due to specific interactions, V'AH should differ from V'AH because of weak hydrogen bonding between A-I3 and the solvent S; consequently V ~ A Hshould vary with the "proton accepting power" of the solvent. A.ccording1y,lo V ~ A H . . B should he essentially independent of solvent, since A-H is a!ready engaged in a hydrogen bond with R and V'AH..B would be expected to be close to V'AH..B. Since V'AH would not, according to be solvent dependent and PAH..B this viewpoint,'O Avs would depend on the solvent in which the measurement was carried out.. Although we will show that Avs does indeed depend upon the solvent., it will be demonstrated that this is due to variations in both Y'AH..B and PAH. In studying interniolecular hydrogen bonding i t is possible to change the nature of the environment not only by changing the solvent but also by changing the concentration of proton acceptor, B. Again, if it is assumed that dielectric effects are the same for the free and associated absorption frequcncies,'O then VAH a i d VAH..B bzit wot Av should vary with the concentration of B. On the other hand, if we assume the presence of specific interactions only,1° neither VAH nor v ~ . . Bshould vary with the concentrat.ion of R. Both assumptions will be shown to be incorrect. A few examples of the variation of VAH..B wth concentration of proton acceptor B already have been reyorted.'6e21-23 We will show that while VAH is insensitive to the concentration of R , V A H . . B and consequently A 0 can change in complex ways, either increasing, decreasing or remaining constant. Rationalization for this behavior will be presented. Experimental Infrared spectroscopic measurements were carried out on a Perkin-Elmer Model 21 infrared spectrophotometer equipped with a lithium fluoride prism and a Reeder thermocouple detector. The following types of matched cells were used: 0.1, 0.2, 1 and 2-mm. sodium chloride cells; 1, 5, 10 and 20-mm. quartz cells, of a type transparent in the near infrared. Solvent absorption was excessive when cells thicker than 2 mm. were used with solvents containing C-H groups; hence, most of the measurements were carried out in 1-mm. cells. We believe our results to be accurate to within 5 2 cm.-' for the unassociated OH frequencies and to within f 4 cm.-' for the broader absorption bands of the hydrogen bonded OH groups. All measurements were carried out at a n average room temperature of 23'. The small variations of room temperature had no observable effect on the positions of the absorption freq~encies.2~All measurements were made a t least twice and some were repeated later (20) Referee's comments. (21) H. Tsubomura, J . Chem. Phys., as, 2130 (1955). (22) E. V. Shuvalova, Optics and Spectroscopy, 6, 452 (1959). (23) S. S. Rlitra, J . Chem. Phys., 36, 3286 (1962). This paper, which contains some observations similar to ours, appeared during final revisions of our manuscript. (2.1) Small variations of temperature may have a considerable effect on
hydrosen bonded eq>.iilibria but the effect on infrared frequencies is very small. See, e.g., E . D. Becker, Speitj.oihim. Acta, 17, 436 (1961).
R.
SCHLEYER
s5
VOI.
TABLE I SOLVESTEFFECTSo s ISTERMOLECULAR HYDROGEN BOSDISG" CHa
I
CHs-CHOH-CHr CHOH-CHs
CHsO-CH-CHzCHzOH Solvent
YOB
YOH
vOH
VOH
free
bonded
Av
free
bonded
Av
3554 3552 3541 3536 3533 3528 3523 3508 3504 3496 3489
91 92 98 104 92 72 81 100 115 128 120
3631 3626 3623 3625 3611
3556 3554 3542 3537 3536
75 72 81 88 75
Hexane 3645 Cyclohexane 3644 Tetrachloroethylene 3639 Carbontetrachloride 3640 Carbon disulfide 3624 Toluene 3600 Benzene 3604 1,2-Dichloroethane 3608 Dichloromethane 3619 Chloroform 3624 Bromoform 3609 a All values are in cm. -I.
..
..
..
3590
3519
71
..
..
3607
3510
97
..
..
I
.
..
.. ..
,
I
TABLE I1 SOLVENT EFFECTSos ISTERYOLECULAR HYDROGEN BOSDISG" -Methanol-etherSolvent
YOH
yOH
free
bonded
Gaseous phase 368Zb Perfluorooctane 3675 Hexane 3654 Cyclohexane 3649 Tetrachloroethylene 3643 Carbontetrachloride 3643 Carbon disulfide 3628 Benzene 3612 1,2-Dichloroethane 3618 Dichloromethane 3627 Chloroform 3634 Bromoform 3616 Diethyl ether .. a 411 values are in cm.-'.
-Phenol-ethetAv
v0H
YOH
free
bonded
3558' 124 3654' 3530 145 3644 3521 133 3621 3516 133 3617 3502 141 3611 3503 140 3611 3496 132 3591 3490 121 3557 3475 143 3564 3469 158 3584 3451 183 3599 3449 167 3579 3506 , . , , Ref. 26, Ref. 27.
A"
,
3274
3303 3357 3343 3338 3330 3315 3295 3291 3285 3283 3340
270 258 200 268 273 201 242 269 293 314 296
..
TABLE I11 SOLVENT EFFECTSO X L4LCOHOL DIMER HYDROGEN BOSDING" -Methanol--
-Diphenylcarbinol---
YOH
Solvent
dimerb
Av
vOH
VOH
free
bonded
Av
90 Gaseous phase 3592' .. 110 Perfluorooctane 3565 InsolulAe 3625 3494d 131 103 Hexane 3551 102 3487 136 3623 Cyclohexane 3547 115 3487 129 3616 Tetrachloroethylene 3528 3616 121 3484d 132 Carbon tetrachloride 3522 110 121 3484 3605 Carbon disulfide 3518 3471 102 3573 .. Toluene .. 3470 106 3576 107 Benzene 3505 .. .. .. .. 1,2-Dichloroethane .. 3456 Dichloromethane 3594 138 3494 133 3464 140 3604 3482 152 Chloroform 3446 142 3588 Bromoform .. All values are in crn.-l. Monomer band positions are given in Table IT. Ref. 28. d These values differ considerably from those reported in ref. 7. See text. with freshly prepared samples. Excellent reproducibility mas observed. The measurements of static dielectric constants were carried out on a heterodyne beat apparatus2: using a wave length of 575 mp. The refractive indexes were measured on a Bausch and Lomb precision refractometer. (25) C. P. Smyth, "Dielectric Behavior and Structure," hlcGraw-Hill Book Co., New York, N , Y., 1955, p. 209. (26) R. G. Inskeep, F. E. Dickson and J. M. Kelliher, J . Mol. Spectry., 4, 477 (1960). (27) L. J. Bellamy and R . L. Williams, Proc. Roy. Soc. (London), 8264, 119 (1960). (28) R. G. Inskeep, J. 11 Kelliher, P. E. McMahon and B. G. Somers, J . Chem. Phys., 28, 1033 (1938).
SOLL~ENT EFFECTSIN INFRARED STUDIES OF HYDROGEN Cosn~sc;
Feb. 20, 1963
3i3
TABLE IV THEOH
STRETCHING FREQUESCIES, I N Mole % €__ - 1 dioxane 2E 1 t
+
O F PHENOL AND METHASOL I N SOLUTIONS O F DIETHYL ETHERIX CHLOROFORM“ Phenol-ether system---Methanol-dioxane system---Y O H ‘‘free’’ YOH bonded Au voa “free” YOH bonded A”
CM.-’,
4.78 0.358 3599 .. .. 3634 .. , . 4.92 ,362 3599 3283 316 3634 3451 180 4.9: ,363 3599 328.5 314 3634 3451 183 368 3598 3282 316 3634 3450 184 5.17 21 .o 5.65 ,378 3598 3292 306 3631 3453 181 35.4 5.93 ,384 .. 3308 (291) 3635 3461 174 5.14 ,367 .. 3333 (266) .. 3498 (136) 79.3 100.0 4.30 .344 .. 3340 (259) .. 3506 (128) a Dielectric constants at 20’ obtained by interpolation from I. E. Coop, Trans. Faraday SOC.,33, 583 (193T). The dielectric constant of the ether-chloroform system has a maximum a t about 45 mole 70 ether. For the chloroform-ether system, there is a linear variation of n z o D with mole fraction, from 2.090 for pure chloroform to 1.822 for pure ether.
0.0 2.3 3.3 7.4
I
Commercially available compounds were purified by distillation or recrystallization and the melting points and boiling points were compared with literature values. Several compounds were prepared by known methods. “Spectroquality” and “Spectro Grade” solvents were used without further purification, except chloroform, which was shaken several times with concentrated sulfuric acid to remove ethanol preservative, then shaken with water and dried, first with anhydrous XanSOcand then by distilling from P~06. Chloroform so purified was used promptly to avoid decomposition. The concentrations of R-OH were too small to produce any observable changes in the refractive indexes and dielectric constants of the solutions.
Results Variation of Infrared Frequencies with Solvent.Tables 1-111 summarize results on hydrogen bonded OH stretching frequencies in many solvents. The choice of solvents was limited to proton acceptors much weaker than the base B in the hydrogen bond being B. Diethyl ether was used as examined] ROH solvent only in those systems in which the proton acceptor was diethyl ether itself. To avoid proton donor self association the lowest possible concentration of proton donor was used, except in alcohol dimer studies. Table I lists data for cases of intraniolecular hydrogen bonding of the OH , , .0CH3 and O H . . OH types. Tn,b!e I1 gives examples of intermolecular association to diethyl ether as base. In alcohol dimer studies (Table 111) interference from “polymer” bands a t lower frequencies was serious in hexane, cyclohexane, CCI,, C2C14and CS2 even for the sterically hindered alcohol, diphenylcarbinol. The position of the dimer band was dificult to determine with precision for this reason. Several concentrations and path lengths, within the limitations imposed by solvent absorptions, hsd to be used. At certain concentrations of alcohol the composite dimer-polymer band may happen to be nearly symmetrical and can be mistaken for the dimer band. For exaniple, 0.44 114 diphenylcarbinol in CC14 (0.2 mm. cells) shows a very broad band with an apparent On diluting this maximum a t 3410 ern.-'. solution two broad bands become apparent; finally a t 0.05 A4 concentration (10 nim. cells) there is no polymer band. The dimer band a t 3484 cm.-’ gradually disappears on further dilution. The previously reported “dimer” band of diphenylcarbinol a t 3410 =t20 cni.-l in CC!4 and in hexaneI0 corresponds to the fictitious maximum mentioned above. Systems of the type ROH . . . B presented no such difficulties, if the concent.ration of ROH was kept low. Tables 1-111 furnish aniple proof that stretching frequencies of OH groups already engaged in hydrogen bonding are solvent sensitive. See Fig. 1 for a typical illustration. The OH . . . €3 stretching frequency of the methanol-ether complex is lowered by more than :50 cm.-’ in going from the gaseous state to cc14 SOIL+ tion and by more than 100 cm.-l when the solvent is chloroform. The “free” V O H also shifts, as expected, but the magnitude of the shift is different for the
...
“free” and “bonded” frequencies; as a consequence Av varies with environment. The solvents employed can be divided into four groups. (1) Chloroform, bromoform and dichloromethane gave the largest Ads, in some cases as much as 50yo larger than the gaseous state and 25% larger than CCl4. (2) Tetrachloroethylene, carbon tetrachloride, perfluorooctane and 1,2-dichloroethane gnve Av’s smaller than those of group 1 but larger than the gas phase values. For a given hydrogen bonded system the A d s are about the same for all these solvents of group 2 and we believe they may be used interchangeably for such measurements. (3) Carbon disulfide, hexane, cyclohexane I .5
I
I
-C Y C L O H E X A N E CHLOROFORM
’.Ot
.
I
3550
I
3650
cm:‘ Fig. 1.-Solvent effect on the OH stretching frequencies of C H p CH(OCHP)-CH~CI&OH. Concn. : 7 pl,/ml, ; I-mm. cells.
and probably other saturated hydrocarbons gave Av’s lower than solvents in the first two groups and only slightly higher than the gaseous state values. (4) Benzene, toluene and presumably other aromatic hydrocarbons gave the lowest Av’s of all. Hydrogen bonding involving aromatic rings as proton acceptors is well e s t a b l i ~ h e d . ~This ~ produces a lowering of the “free” (actually H-bonded to the aromatic ring) OH stretching frequency and, consequently, low Av’s. Effects of Variation of Proton Donor Concentration.Tables IV-IX show the effect of proton donor concentration on infrared spectra of hydrogen bonded systems. (29) Ref. 5 , p. 202, and references cited therein.
ADAMALL~ERIIAND AND PAUL VON K.
374
SCHLEYER
Vol. 85
TABLE V THEOH STRETCHING FREQUESCIES, IX CM.-~,OF PHENOL AND METHANOL IN SOLUTIONS OF DIETHYL ETHER IN CARBOY TETRACHLORIDE Mole % ether
nz
€ - 1 __ 6
2E
1 A O
+1
(I . 0
2 ,234” 0.226 ,227 0.4 2.246 2.265 ,229 1.5 ,235 4.6 2.329 20.0 2.635 ,261 40.6 3.061 ,289 63.7 3.528 ,314 100.0 4.29’ ,343 Obtained by extrapolation from ref.
-1
2 ~f 1 1
n22.6D
1.463* 1.458 1.457 1.454 1.437 1.415
0.216 ,214 .214 ,213 ,208 .200
150 10
Fig. 2.-Effect
20
30
50 60 mole % nitrile.
40
system-
AU
VOH bonded
..
3611 3610 3611 3611 3610
--Methanol-ether YOH “free”
..
YOH
system-bonded
..
3643 3642 3644 3643 3643 3642
3339 271 3503 3537 3498 274 3337 3497 274 3337 273 3498 .. 3337 (273) 3500 ... ... .. 3338 (272) .. 3504 1.35OC ,177 .. 3340 (270) .. 3506 Refractive index a t 15’ (ref. 38). 37. Refractive index a t 24.8’ (ref. 38).
At high concentrations of proton acceptor the “free” bands gradually &sappear so that the values for Av have to be estimated. This is justifiable since the concentration dependence of the “free” bands is negligible up to the highest concentrations of proton donor a t which they can be observed. The bonded peaks, YOH..B, behave in peculiar ways; in some cases there is concentration dependence, in others none. Both the proton acceptor and the solvent are involved in this phenomenon. VOH. .ether varied with concentration of ether if chloroform was the solvent (Table IV) but did not change appreciably if the soivent was CCL (Table v ) and yet YOH..dioxane varied with the concentration of dioxane in the latter solvent, CCL (Table IV). An empirical method for predicting such concentration dependencies will be presented below.
0
--Phenol-ether YOH “free”
70
80
90
100
of nitrile concentration on Av of phenol in CCl, solution.
Because of the essential constancy of the positions of the “free” peaks and the frequent variation of the “bonded” peaks, the magnitudes of Av will often depend upon the concentrations of proton acceptor chosen, even if the same solvent is used. Values of Av from intermolecular hydrogen bonding studies are often of great interest for correlation with structure of proton donor or acceptor,30with “basicity”31 or with the enthalpy of hydrogen bonding.32 For these purposes Av’s frequently have been measured in the literature33 by the undesirable technique of using the proton acceptor as solvent. We should like to emphasize that Av measurements should be made in an inert solvent with as low a concentration of proton acceptor as possible; extrapolation t o infinite dilution is the preferable procedure. *a Figure 2 and Tables VI11 and IX provide a graphic illustration of these difficulties. Nitriles, as proton acceptors in CCla solution, show large concentration dependencies of VOH..N and of AV.~’ Binary solutions (30) P. von R. Schleyer and A. Allerhand, J. Am. Chem. Soc., 84, 1322, (1962), and unpublished results. (31) C j . a review, J. Gordon, J. Org. Chcm., 26, 738 (1961); also ref. 5. (32) See ref. 5 and 23 and paper VI, ref, 2. (33) For literature citations, see ref. 23. (34) Despite such evidence t o the contrary,’:-l@-*there is still a residual
Av
.. 139 146 146 145 142 (138) (136)
TABLE VI THEOH STRETCHING FREQUENCIES, IN C M . - ~ ,OF PHENOL IN SOLUTIONS OF DIOXAXE IN CARBON TETRACHLORIDE^ phenoldioxane
As
..
..
YOH
Mole % dioxane
0.0
YOH
“free”
3611 3612 3612 3611 3610 3610
3378 234 3377 235 3373 238 3370 240 3365 245 .. 3357 (253) .. 3347 (263) 100.0 .. 3338 (272) a ( e - 1)/(2e f 1) varies from 0.226 for CClr to 0.224 for pure dioxane. There should be little variation of this function in the mixtures. Refractive indexes of CClAioxane mixtures are given by B. Pesce and V. Lago, Gaze. chim. Ifal., 74, 131 (1944). 1.1 2.3 6.7 11.2 22.1 43.1 72.5
of phenol in acetonitrile, cyclohexyl cyanide and t-butyl cyanide give the anomalous result that the latter compound appears to be the weakest base, contrary to the expected inductive effects of the substituent groups.30 At lower nitrile concentrations in a CC14 solvent, the basicity orders change (Fig. 2); finally, a t infinite dilution, the estimated Av values of the three nitriles correspond to the anticipated order.30 For some other classes of compounds, e.g., ethers, there is much less dependence of Av on proton acceptor concentration in CC4 solution. We will show how each type of behavior can be predicted. Dielectric constants and refractive indexes were measured for a number of the solvent mixtures employed (Tables IV, V, VII, VIII) for purposes of testing dielectric theories. Discussion Frequency Shifts in Different Solvents.-Modifications of positions, shapes and intensities of spectral lines of emitting or absorbing species due to the presence of an environment have prompted much theoretical and experimental work in recent years.39 The spectrum of a gaseous molecule is altered even by the presence of a noble gas such as argon in several ways; the theoretical treatment of such a system is far from belief20 t h a t nitriles are very weak proton acceptors. I t is true that the intramolecular hydrogen bond in o-cyanophenol is quite weak’? the interaction there is of the O H . . T type since the nitrogen atom, because of adverse geometry, cannot bond with the OH group. Nitriles form moderately strong intermolecular hydrogen bonds with alcohols a n d phenolr.’*,’0f’r (35) Ref. 5, p. 471 and references therein cited, also see ref. 15 and 21. (36) M. St. C. Flett, S W l r o c h i m . A d o , 10, 21 (1957); V. Prey and H. Berbalk, Monalsh. Chem., 8% 990 (1951). (37) NBS Circular 514, “Table of Dielectric Constants of PureLiquids,” U. S. Government Printing Office, Washington 25, D. C. (1951). (38) “-andbook of Chemistry and Physics,” 41st edition, Chemical Rubber Publishing Co., Cleveland, Ohio, 1959-1960. (39) An excellent comprehensive review is available: (a) R . L. Williams, Ann. Reports (Chem. SOC.London), 88, 34 (1961). Also see (b) M. K. Wilson in E. A. Braude and W. D. Phillips, Ed., “Determination of Organic Structures b y Physical Methods,” Vol. 11, 1962, pp. 203-206, 224-226; and (c) M. L. Josicn, Prrc and A p p l . Chcm., 4,33 (1962).
.
Feb. 20, 1963
SOLVENT EFFECTSKN INFRARED STUDIES O F HYDROGEK BONDING
375
TABLE VI1 THEOH STRETCHING FREQUENCIES, IN C M . - ~ OF , PHENOL AND METHANOL IN SOLUTIONS OF DIOXANEIN CHLOROFORM’ Mole yo dioxane
-Phenol-dioxane system-“free” VOH bonded
€-1 2e 1
+
ۦOO
AV
VOH
-hlethanol-dioxane system“free” VOE bonded AV
VOH
0.0 4 . 806b 0.359 3599 .. .. 3634 .. .. 1.8 4.68 .355 3341 259 3634 3484 150 3600 2 59 3634 3485 149 9.7 4.338 ,345 3599 3340 19.6 4.019 ,332 3597 3346 251 3634 3488 146 30.0 3.765 .324 (3593) 3342 (255)d 3633 3489 I44 40.9 3.511 ,313 .. 3344 (253)d (3631) 3491 (139)’ e 3498 50.5 3.310 ,303 .. 3347 ( 250)d (135)’ 60.2 3.111 .292 .. 3343 (254)d .. 3501 (132)’ .280 .. 3342 (255)d .. 3503 (130)’ 70.0 2.904 80.2 2.683 ,264 .. 3340 (257)* .. 3504 (129)’ 90.1 2.457 ,246 .. 3341 (256)d .. 3503 (128)f 100.0 2.217’ ,224 .. 3338 (259)d .. 3501 (129)’ a 7 t m varies ~ linearly with mole fraction for this system. The total change from pure chloroform to pure dioxane is very small. The Assuming Y O R “free”at 3597cm.-l. function (n2 1 ) / ( 2 ~ 2 1) is practically constant. b Ref. 37. Ref. 37, by extrapolation. Assuming Y O H “free” a t 3633 crn.-l. e Shoulder.
+
-
THEOH STRETCHING FREQUENCIES, Mole % acetonitrile
IN
TABLE VI11 CM.-~,OF PHENOL IN SOLUTIONS nl
en.5
n22.5D
-1
2n*
+1
OF
ACETOXITRILEIN CARBOSTETRACHLORIDE
e - 1 2.
+1
U O H of phenold
“Free”
Bonded
0.0 2.234“ 1.46303 0.216 0,226 3611 1.0 2.386 1.45781 .2144 ,240 3610 2.8 2.629 1.45667 ,2143 .260 3610 4 9 2 916 1.45529 .2135 .280 3610 12.3 3.980 1.45024 ,2120 .332 3607 .408 .. 33.8 7.74 1.43338 .2064 65.9 19 1.39920 .195 .46 .. 100 37” 1.34596’ .175 .480 *. b Obtained by interpolation from F. Accascina, S. Petrucci and R. 0 From ref. 37, by extrapolation. Refractive index at 16.5’, from ref. 38. 81, 1301 (1959). c Refractive index a t 15’. from ref. 38.
TABLE IX THEOH
STRETCHING FREQUENCIES, I?: CM.-’, O F PHENOL IN SOLUTIOPiS O F tert-BUTYL CYANIDE AND CYCLOHEXYL CPASIDE IN
CARBON TETRACHLORIDE Mole ?& I-butyl
Y O H of phenol “Free” Bonded
Mole % cyclohexyl --”OH of phenol-cyanide “Free” Bonded Av
-
+
A U
..
..
3458 3451 3446 3434 3420 3411 3405 M. Fuoss, J .
152 159 164 173 (187) (196) (202) A m . Chew. SOL,
A V / Y = C(e 1)/(2e 1) KBM relation A V / V = C’(ns 1)/(2na 1) Modified KBM relation
-
+
+
+
(1) (2)
f A [ ( n 2 l)/(2n2 1)RI B [ ( c - 1)/(2e 1)RI) Pullin relation ( 3 ) AV/U CI 4- CZ(C- 1 ) / ( 2 ~ 1) C3(n2 - 1)/(2na 1) Buckingham relation (4) =
cO
+
+ +
+
simple.*O Ilerice, the effect of a liquid environment on spectra should be extremely difficult to treat quantitatively. The well-known Kirkwood-Bauer-Magat relationship11,12(eq. 1 and 2) has been found unsatisfactory to explain the observed medium shifts of X=O stretching f r e q u e n c i e ~ the , ~ ~experimentally ~~~~~~ most extensively studied vibrations. Improved relationships, notably those of P ~ l l i n (eq. * ~ 3) and of BuckinghamJ3(eq. 4), have been proposed but these also are unsatisfactory.
Despite the insufficiencies of these theoretical treatments for practical predictive purposes, many regularities have been observed in solvent shifts of infrared bands. Empirical linear correlations have been established for the frequencies of different solutes in the same set of solvents; for example, X-H stretching vibrations of a number of compounds are said to shift proportionately in different solvents (odeinfra).15,44,45 Similar proportionalities of a more precise nature have been found for the CEO stretching frequencies of a wide variety of carbonyl compounds in different solvents7; S=O and P=O stretching frequency shifts are also proportional to C=O shifts,8 but the solvent behavior of X=O absorptions is very different from that of X-H band^.^,^^ Strong proton acceptor solvents produce large X-H frequency shifts due to hydrogen bonding but affect X=O vibrations to a much smaller relative extent, e.g., 0-H shifts are much larger than C=O shifts in ether, but the reverse is true in CH2C12. There is also little correlation between the effect of change of solvents upon the frequencies of “free” and hydrogen bonded OH stretching frequencies (Fig. 3),& contrary to expectations based on the simplified dielectric approach mentioned in the introduction.20
(40) See, ex., R.J. Breene, Jr., “The Shift and Shape of Spectral Lines,” Pergamon Press, New York, N. Y.,1961; S. C. Ch‘en and M. Takeo, Rev. M o d . Phys., 29,20 (1957); A. D.Buckingham, Trans. Faraday SOC.,68, 449 (1962). (41) R . B. Archibald and A. D. E. Pullin, Spcctrochim. Acfa, 18, 34 (1958). (42) A. D.E. Pullin, ibid., 16, 12 (1960). (43) A. D.Buckingham, Proc. Roy. SOC.(London), M 4 8 , 169 (1958).
(44) W.Gordy, J. Chcm. Phys., 9, 215 (1941); W.Gordyand S. C. Stanford, ibid., 9, 204 (1941). (45) M.L. Josien and G. Sourisseau in D. Hadti. ed., “Hydrogen Bonding.” Pergamon Press, London, 1959, p. 129. (46) Generally in the literature comparisons are made of the quantities (v’ - uO)/vo, where Y O is the vapor phase frequency and Y O is the frequency in solvent S. We prefer to use the frequencies directly, since Y O is unknown
cyanide
0.0 0.5 1.4 3.8
7.6 12.0
3611 3611 3611 3610 3610 3611
..
3448 3446 3442 3439 24:3*5
24.2 3610 3430 47.8 . . 3423 69.6 . . ““O 100 , , 3410 0 Shoulder.
Av
.. 165
176 (Is7) (”O) (191)
0 . 0 3611 .. 0 . 4 3611 3447 .3 3611 3441 5 . 2 3611 3437 9 . 4 3610 3432 1 5 . 5 3608 3425 35.1 (3605)” 3417 45.7 . . 3-113 ’ . 3409 100 , , 3406
..
183 (lg5)
(202)
376
Vol. 85
ADAMALLERHAND AND PAULVON R . SCHLEYER
t
1
0
VAPOR
12 0
R
0.
11
0
PERFLUOROOCTANE
w
w 3
0 3
9 4 .
I
0
0-
0
0
I o
0
3610
CHLOROFORM
0 4 I
I
3650
7
3690
0
V O HMETHANOL (crnr’).
3560
3520
I-
t
i CYCLOHEXANE
DICHLOROMETH4NE
1
3440 1 1650
0 2
I
‘ OI t 0 ‘3440
‘
I
1
I
3480
352 0
UDH...o METHANOL-ETHER
I
e3560
(cmr0.
Fig. 5.-Correlation of hydrogen bonded OH stretching trequencies with dielectric equations : ( a ) Kirkwcod-Bauer-Magat function (eq. 1); ( b ) Pullin function (eq. 6 of ref. 42). Solverits are: 1, gaseous phase: 2 , hexane; 3, cyclohexane; 4, diethyl ether; 5 , tetrachloroethylene; 0, carbon tetrachloride; 7, carbon disulfide; 8, benzene; 9, 1,2-dichlotoethane: 10, dichloromethane; 11, chloroform; 12. bromoform.
$LEX...
/p
5 $6 3.
8
effects on “free” and hydrogen bonded OH stretching frequencies.
t
-
DICHLOROMETHANE
BROMOFORM
Fig. ,?.-Solvetit
I
50 -
I,2-DICHLOROETHANE
0
t
0
$
5 86 0 2
+
0 HEXANE 0 CYCLOHEXANE
I
I-
6
10
1
I
I
I
1660
1670
1680
1690
I 1700
1 1710
V c z 0BENZOPHENONE (cm;’) (ref 7).
Fig. 4.-Correlation between Y O H . . O of methanol-ether ~ c - 0of benzophenone in various solvents.
phase value for the H-bonded complex and not the free X-H molecule. vs is the observed frequency in a solution. a is a function of the particular infrared vibration of a given molecule and is a measure of its solvent sensitivity. G is a function of the solvent only.47 Values of G are compiled in Table X ; they were determined from the best fit of solvent shifts given in the literature for the carbonyl bands of dimethylf~rmamide~ and of benzophenone’ and the sulfonyl band of dimethyl
and
JVe find that solvent shgts of X=O stretching frequencies are proportional to solvent shifts of X-I-I ...B stretching frequencies, i.e., of proton donor groups already engaged in intermolecular or intramolecular hydrogen bonding. An example is given in Fig. 4. ,411 other X-H.. .B frequencies in Tables 1-111 behave similarly. This proportionality implies that solvents interact in a similar manner with X=O and with X-H ...B stretching frequencies. These results cannot be reconciled with the “specific-interactions-only” approach advocated chiefly by Bellamy and his school.10s16p16It is hard to see how the “specific interactions” of solvents with X=O and X-H ...B groups could be similar. itre propose an empirical linear free energy relationship (5) for the correlation of solvent sensitive infrared vibrations ( u ” - p * ) / v ’ = UG (5) Y O is the stretching vibration in thevapor phase; for hydrogen bonded X-H ...B vibrations it refers to the vapor for many OH vibrations. For any given species Y O is a constant, so that only a scale change is involved. The two types of plots are equivalent for showing lnearity relationships for different spectral shifts.
TABLE S SOLVEXT SHIFTPARAMETERS Solvent
G
Solvent
G
\-acuum 00 Benzene so Perfluorooctane 36“ Dioxane 86 Hexane 44 Methyl iodide 89 Cyclohexane 49 hcetonitrile 03 Dibutyl ether 61 Pyridine 94 Triethylamine 62 1,2-Dichloroethane 95 Diethyl ether 64 Nitromethane 09 Tetrachloroethylene 64 Dichloromethane 100 Carbon tetrachloride 69 Chloroform 106 Carbon disulfide 74 Dibromomethane 10s Toluene 74 Bromoform 118 Based on methanol-ether and phenol-ether ROH B solvent shifts (Table I and Fig. 6 and 7).
. ..
(47) Equation 5 is similar in form and intent to that proposed by S. Brownstein (Can.J . Chrm., 98, 1590 (1960)), log (K’ solv/k ethanol) = SR for the correlation of the effects of solvents upon equilibria, spectra and reaction rates. The G constants (Table X) are related to Brownstein’s S values, but there are discrepancies. Hence, eq. 5 is much more accurate for the prediction of infrared spectral shifts. The a constants are related to the slopes of “BHW” plots,lb termed “S” constants by Hallam.*,” (48) E. A. Cutmore and H. E. Hallam, Trans. Faraday Soc., 68,40 (1962). (49) H. E. Hallam and T.C. Ray, dbid., 68, 1299 (1962).
SOLVENT EFFECTSIN INFRARED STVDIES OF HYDROGEN BONDING
Feb. 20, 1983 I
I
3560Q\
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
3600
VAPOR
--
E
0
0
U
3520
0,
I
-I
n
-ii
PERFLUOROOCTANE
\a,
u
E
I
377
0
HEXANE CYCLOHEXANE
ETHER\ CCI,
8,o
E
W
3560
In
CCI,
-I
0
z 4
I I-
W
5
;
I
CHLOROFORM
3440 0
0
'0.
DICHLOROMETHANE
\
s
f
\
I
1
I
I
BROMOFORM I
20
40
60
80
100
I
120
3550
.
3530
-
1
I
I
I
I
I
0
CHLOROFORM
I
l
l
l
l
l
l
l
l
l
\ l
60 80 100 120 G. Fig. 8.-Correlation of the OH stretching frequency of the methanol dimer with G of the solvents.
20
0
Fig. 6.-Correlation of the OH stretching frequency of the methanol-diethyl ether complex with G of the solvents.
\
DICHLOROMETHANE
3480
G
I
3520
40
CYCLOHEXANE
e
g
Y
0
I
0
TOLUENE
36 BENZENE
I,2-DICHLOROETHANE DICHLOROMETHANE
CHLOROFORM
?
100
3480
0
BROMOFORM
0
20
40
60
1
120
80
G.
Fig ;.-Correlation of the OH stretching frequency of the intramolecularly hydrogen bonded complex of CHa-CH( 0CHa)C H ~ C H J O Hnith G of the solvents
sulfoxide.8An arbitrary value of 100 was assigned to dichloromethane to fix the scale.50 T o use eq. 5 it is not necessary to know y o , rather this value can be estimated easily. Equation 5 is used in the form vs = vo - aGvo; vS is plotted against G. The intercept a t G = 0 gives vo and the slope -avo, from which a can be calculated. vo values, estimated in this manner, come within 8 cm.-' or better of the experimental values, where these are available. Equation 5 is just a modified Kirkwood-BauerMagat relationship in which the dielectric constant 1) has been replaced by the emfunction (E - 1)/(2e pirical parameter G, obtained from infrared spectroscopic data on X=O stretching frequencies. A comparison of Figs. 5 and 6 shows the advantage of G over ( E - 1)/(2e 1) for predictive purposes. Equation 5 also gives more accurate results than the relationships
+
+
(50) Much work has been published on solvent effects on X=O stretching frequencies, especially C=O bands.',","," However, there are large discrepancies between the results reported by different workers. Table X will need to be revised when more accurate values are available. (51) (a) N.S.Bayliss, A. R. H. Cole and H. Little, Australian J . Chem., 8 , 26 (1955); (b) M. L. Josien and J . Lascombe, J . Chim. P h y s . , 62, 162 (1955); (c) T.V. Yakovleva, A. G. Maslennikova and A. A. Petrov, Optics end Spect., 7 , 383 (1959); (d) K.Inuzuka, M. I t o and S. Imanishi, Bull. Chem. SOL.Japan, 94, 467 (1961); (e) R. E. Kagarise and K. B. Whetsel, S p e c f r o c h i n . Acta, 17, 869 (1961); 18, 341 (1962); (f) H.W. Thompson and D J. Jewell. ;bid., 19, 254 (1958).
0
"
IO
20
30
SO
60 mol4 Y. dioxane.
40
70
80
90
I
IOC
Fig. 9.-Effect of dioxane concentration on the OH stretching frequency of the phenol-dioxane complex in CC1, solution.
proposed by Pullin (3)42and by Buckingham (4)43and is simpler to use. It follows from its definition and from the literature that eq. 5 holds for C=O, S=O and P=O bands, and to a lesser degree N=O band~.7,8''~Our results indicate that it also applies to hydrogen bonded X-H...B stretching frequencies; Fig. 6-8 show excellent linear plots of intermolecular and intramolecular hydrogen bonded 0-H stretching frequencies against G for the hydrogen bonded complexes we have studied (Tables 1-111). Equation 5 also holds but less satisfactorily for some reported c a r b o n - h a l ~ g e n C=C53 , ~ ~ ~ ~ ~and other stretching frequency solvent shifts; Table X I lists values of a for a number of infrared bands which are correlated by eq. 5. Possible interpretations of the magnitude of the solvent sensitivity of infrared vibrations, measured here by a constants, have been commented on by Bellamy and his ~ o l l a b o r a t o r s . ~ ~Among 8 ~ ~ ~ other factors, steric hindrance by substituent groups seems to decrease the solvent sensitivity in some cases, possibly by diminishing access of the solvent to the chromophoric group. .B, are particularly Hydrogen bonded vibrations, XH.. sensitive to changes in solvent, but for very strong hydrogen bonds this solvent sensitivity tends to disappear.lo (52) (a) D. H.Whiffen, Trows. Fareday Soc., 49, 878 (1953); (b) G. Chiurdoglu and J. Reisse, Bull. Soc. Chim. Belg., 70, 472 (1961). (53) P.V. Huong, J. Lascombe and M.-L. Josien, J . Chim. Pkys., 68,694 (19G1).
ADAMALLERHAND AND PAULVON R. SCHLEYER
378
Vol. s5
TABLE XI CORRELATION OF SOLVENT SENSITIVITY OF INFRARED BANDS Compound
Acetone Acetophenone Benzophenone Diisopropyl ketone Acetaldehyde Methyl acetate Dimethylformamide Acetone Diisopropyl ketone Acetaldehyde Dimethyl sulfoxide Diphenyl sulfoxide Phosphoryl chloride hlethyl nitrite
Chromophore
c=o c=o c=o
c--o c=o c=o -0 C=O overtone C-0 overtone C=O overtone
c-0 2-0
P=O N=O cis N=O trans
”0,
ern. -1
X=O Bands 1738 1709 1716 1730 1747 1770 1681 3456 3443 3472 1103 1059 1324 1620 1678
a X IO4
Degree of correlation
Ref.
1.3 1.3 1.2 1.3 1.3 1.8 2.3 1.2 1.2 1.3 4.2 1.6 2.1 0.7 1.6
Very good Excellent Excellent Excellent Very good Very good Excellent Very good Good Very good Excellent Good Excellent Fair Fair
5la
0.8
Good Poor Poor Poor Very poor Very poor Very poor Very poor Very poor Very poor
54 15 15 25 48 48 48 48 Here Here, 27
Very good Good Very good Very good Very good Excellent Very good Very good
Here Here 1 Here Here Here Here 1
0.7
4 points only
1 10 10
0.5 1.2
Good Good
53 55
1.8 3.1 1.6 ca. 0
Good Fair Fair
49, 52a 49 49 5la 56
-
3
I
5la 5la
-
5la 5la 5la
s s
8
s s
X-H Bands Propyne Pyrrole Aniline p-Toluidine p-Chloroaniline Methanol Phenol Methanol dimer Benzhydrol dimer Diisopropyl carbinol dimer Methanol-ether Phenol-ether CHsCH( 0CHa)CHzCHzOH 2,4-Dihydroxypentane N-( /3-hydroxyethylpiperidine) Methanol-R,N+Xo-Nitrophenol 8-Hydroxyquinoline 1-Heptyne N-Cyanodimethylamine Chloroform Methyl chloride cis-Dichloroethylene Acetonitrile Cyclohexanol
C-H (free) hT-H N-H asym. N-K sym. N-H asym. N-H sym. N-H asym. N-H sym.
3530 3502 3419 3475 3387 3497 3412 3682 3654
0-H 0-H
X-H. 0 - H . . .O 0 - H . . .O 0-H.. .O 0 - H . . .O 0-H.. .O 0-H . . .O intra. 0-H. .0 intra. 0-H , . . N intra. 0-H . .X 0-H . . .O intra. 0 - H . . .N intra.
. .
c=c C=N C-C1 asym. c-c1 c-c1
c-c
C-0 axial
I
.B Bands 3592 (3524) (3560) 3558 (3412) (3598) (3594) (3552)
..
(3240) (3431)
CGX Bands 2127 2235 c-X Bands 773 732 857 919 (ca. 1065)
All solvent sensitive infrared vibrations would not be expected to vary linearly with the G values of the solvents; a number of chromophores give poor correlations (Table XI). The theoretical treatment of Drickamer and \Viederkehr14*18 resolves the total interaction energy between a molecule and its surroundings into four contributions: electrostatic, inductive, dispersive, and repulsion interaction energies; the frequency shifts depend upon the derivatives of these interaction energies.14 If the inductive energy contributions are much larger than the other factors, the dielectric approach, of which eq. 5 is an empirical modification, may prove useful. For C=O bond vibrations of several carbonyl compounds (which give very good (54) P. V. Huong and J. Lascombe, Compf. rend., 264, 2543 (1962). ( 5 5 ) J. P. Jesson and H. W. Thompson, Spcctrochim. Acto, 18,217 (1968). (56) 0. Chiurdoglu and W. Masschelein, Bull. SOC. Chim. Bclg., 70, 307 (1981).
..
..
, .
.. ..
.. .. ..
.. 3.1 1.8 1.9 2.7 3.4 2.6 2.3 2.8 ca. 0 ca. 0
.. Poor
correlations), the inductive interactions accounted for 65% of the observed shifts.14 For the k N stretching frequency of acetonitrile (which correlates only moderately well), dispersion and repulsion contributions to the and eq. 5 is less satisfactory solvent shifts are large,14-18 in this case. Other poor correlations may be due to similar factors. The behavior of “free” XH bonds with a change in solvent is particularly important. As expected from Fig. 3, there is no general correlation of XH stretching vibrations with eq. 5 because most of the solvents listed in Table X, e.g., the aromatic compounds, the mono and dihalogenated hydrocarbons and especially the solvents containing 0 and N atoms, are capable of forming weak to strong hydrogen bonds with proton donors. Hydrogen bonding of XH groups with these solvents will give rise t o frequency shifts in excess of those expected
SOLVENT EFFECTSIN INFRARED STUDIES OF HYDROGEN BONDING
Feb. 20, 1963
I
35
379
I
I
37
39
1
6- I
2L*I 102. Fig. 11.-Kirkwood-Bauer-Magat E-l
2E+I
plot for POH..O of tne phenoldiethyl ether complex in chloroform solutions.
x102,
to the large difference in dielectric constants between the two solvents, CH&N and CCI,. This i s not general behavior. Both dioxane and CC14 have low dielectric constants yet there are large concentration frequency merely on the basis of bulk property effects, empirically shifts (Table VI and Fig. 9). The failure of eq. 1 is furmeasured by G constants. At best, the “free” OH ther emphasized in a plot of the phenol-ether system in stretching frequencies in Tables 1-111 might correlate chloroform solvent (Fig. 11 and Table IV). with G constants only for the truly “inert” solvents, The solvent shift parameters G are helpful in prei.e., those incapable of acting as proton acceptors. dicting when Y R O H . . B will depend on the concentration These solvents include : perfluorooctane, hexane, cycloof B and when not. Carbon tetrachloride and ether hexane, tetrachloroethylene, carbon tetrachloride and have similar G values; when ether is used as the proton perhaps chloroform (a proton donor). Even with this acceptor and CC1, the solvent there is practically no limited list of solvents poor correlation of XH frevariation of Av with ether concentration (Table V). quency shifts with eq. 5 are obtained (Table XI). Chloroform and ether have very different G values and A critical examination of the correlations of various VOH..B is concentration dependent; in this case Av deX H stretching vibrations in different solvents with each creases with increasing proton donor concentration beother reveals that agreements are generally less satisfactorys7 than often a ~ ~ ~ m e d ~ especially ~ . ~ ~ ~for . ~ . cause ~ ~ -ether ~ 6 has a lower G value than CHC13 (Table IV). The opposite behavior is observed in Tables VI, VI11 the “inert” solvents. Lines often do not pass through and I X where Av increases with increasing proton donor the origin. A convincing rationalization for the solvent concentrations; in these cases G for the proton donors dependence of XH frequencies appears not yet to have exceeds that of the solvents. W h e n the inert solvent S been offered. and the proton acceptor B have similar G values, we conThere is no direct relationship between the G values in clude that VROH .B will be solvent concentration independTable X and the “basicities” or proton accepting ability ent; when the two G values are very dz$erent, V R O H B will of the various solvents. Triethylamine, diethyl ether be solvent concentration dependent. Of the systems we and tetrachloroethylene all have similar G values. Nor have examined, only the phenol-dioxane-chloroform is there any straightforward relationship between these system in Table VI1 fails to conform to this generalizaG values and the functions (nz - l ) / ( 2 n z 1) or tion. Actually, G values could be determined for each 1) or a combination of them. Sol(e - 1 ) / ( 2 ~ mixed solvent as well as for the pure solvents themvents with high refractive indexes or dielectric constants selves. generally have high G constants, but not all solvents A test of these ideas is illustrated by the following with high G values have high refractive indexes or diexample. Y S O for dimethyl sulfoxide in cc& and in the electric constants. A theoretical basis for these empure liquid are reported to be identical.s Consequently pirical G parameters is yet to be found. the G values of CC14 and dimethyl sulfoxide should Changes in Solvent Concentrations.-Both from the be similar. According to the above generalization point of view of the simplified dielectric approach20 VCH,OH OC(CH,)* should be the same in C C t solvent and in and the “specific-interactions-only” hypothesis, the pure dimethyl sulfoxide, despite the vast difference beresults of Tables IV, VI-IX are unexpected, since tween the dielectric constants for these two compounds, both V R O H .. B and Av vary with concentration of pro2.2 vs. 49,58respectively. Experimentally, however, the ton acceptor in an “inert” solvent. The variation of Y C H ~ O HOS(CH~),values were found to be much lower in VROH..Bis greatest a t low concentraions of B (Fig. 2 and pure dimethylsulfoxide (3369 cm.-l) than in 1 molar 9). A comparison of Tables IV and V shows that the ccI4 solution (3406 cm.-l). This behavior was found variation or constancy of Y R O H . , B with concentration of not to be due to a failure in the generalization concerning proton acceptor depends both on the nature of proton G values and solvent shift variations in mixed solvents, acceptor B and the “inert” solvent. It appears imbut rather due to an incorrect reported value for vso of possible to explain the observed results using equations dimethyl sulfoxide (1071 crn.-l)s (pure liquid) upon 1-4, with one excepton. The Y O H . . B frequency of which our G estimates were based. Using the correct phenol-acetonitrile in cc14 solution varies linearly with value (1055 cm.-l), a G value of about 108 can be esti(e - 1)/(2e 1) (Fig. 10). This quantitative agreement is in accord with suggestions in the literat~re16~~smated for dimethyl sulfoxide. Behavior in Mixed “Inert” Solvents.-Many specthat such frequency shifts with concentration are due troscopic measurements have been carried out in mixed Fig. 10.-Kirkwood-Bauer-Magat plot for VOH..N of the phenolacetonitrile complex in CCId solutions.
+
+
+
(57) See Fig. 7 and 8 of ref. 39c; of Table 11.
similar behavior is exhibited by the data
(58) H. L. Schlafer and W. Schaffernicht,Ansew. Ckcm., 72, 618 (1960).
P. F. Cox
380 1.5
7
Vol. 85 TABLE XI1 BEHAVIOR OF PHENOL IN “INERT” SOLVENT MIXTURES
Solvent mixtures Cyclohexane CClr (20: 17 CClr 4- CSz (3:2 v./v.)
+
Cyclohexane
3639 3642
+ hexane (8: 1 + CCla (7: 1
Perfluorooctane w./w.) Perfluorooctane w./w.) FC-75&satd. with F C - 7 P satd. with
0 4U
03-
CCla cyclohexane
Y O H phenol, cm.-’ (max.) 3614 (symmetrical) 3606 (slight asymmetry on low freq. side) 3608 (definite asymmetry on low freq. side) 3614 (definite asymmetry on low freq. side)
3639, 3624 (see tent and Fig. 12a) 3642, 3623 (see Fig. 12b) 3640, 3623 (similar to above) 3640 (slight asymmetry on low freq. side)
Or FC-75 is a mixture of perfluorocyclic ethers (Minnesota Mining and Mfg. Co.). Its behavior is very much like that of perfluorooctane, and it is much more readily available. The free band of phenol in FC-75 is at 3645 em.-’; there is no tendency to hydrogen bond to the oxygen atom in this solvent. Perfluorotributylamine (FC-47) behaves similarly in these respects, but solubilities are lower in this solvent.
02-
01
(1 : 1 v./v.)
Hexane f CSz (10:7 v./vJ
05-
o
+ C&
v./v.)
-
0 cmrl, Fig. 12.-The appearance of the OH band of dilute phenol in (a) perfluorooctane hexane (8: 1 w./w.); (b) perfluorooctane 4- CCI, (7: 1 w./w.); concn. of phenol
