4164 Intramolecular Hydrogen Bond Formation
NOTES %T
in o-Trifluoromethylphenol
' by Frank C. Marler, 111, and Harry P. Hopliins, Jr.*
r
Department of Chemistry, Georgia State University, Atlanta, Georgia 30303 (Received June $4, 1970)
The formation of an intramolecular hydrogen bond between a hydroxyl proton and a fluorine atom on a neighboring group has received considerable attention in recent years.]-7 Infrared studies on this type of system have been hampered by the small shift in the OH stretching frequency which is normally o b s e r ~ e dwhen * ~ ~ a hydrogen bond is formed between a covalently bound fluorine and a hydroxyl hydrogen. All previous studies, to our knowledge, have been performed on molecular systems in which a five-membered ring results from the hydrogen bond formation. The geometry of the five-membered ring and the size of the fluorine atomx0present an unfavorable situation for the formation of a hydrogen bond, if formed at all,1-3,10in these systems. I n an effort to determine if intramolecular hydrogen bond formation between a covalently bound fluorine and hydroxyl hydrogen is influenced by ring size, high resolution infrared studies were performed on o-trifluoromethylphenol, a system in which a six-membered ring will be formed if intramolecular hydrogen bonding occurs.
Experimental Section The spectra were recorded with a Beckman IR-12 spectrometer and a VLT-2 variable temperature unit. The trifluoromethylphenols studied were obtained from the Peninsular ChemResearch, Inc., and purified by vacuum distillation or sublimation. Samples of the o-trifluoromethylphenol were triply vacuum sublimed and sealed in glass ampoules under nitrogen until the solutions were prepared. The final purity of the o-trifluoromethylphenol was determined by a pH titration in aqueous solution to be 99% or better. I n order to prepare solutions containing a minimum quantity of water, spectroscopic grade isooctane which had been dried over molecular sieve was mixed in a dry bag under nitrogen with the samples sealed in the glass ampoules. The resulting solutions were then transferred to the infrared cells in the dry bag under nitrogen. Compensation for the solvent bands in the 3 6 0 0 - ~ m -region ~ was accomplished with a variable path length cell in the reference beam of the spectrometer.
Results The infrared spectra of phenol and the three isomers of trifluoromethylphenol were recorded in the 3600-cm-' region in isooctane solutions dilute enough so that no intermolecular association could be detected. For pheThe Journal of Physical Chemistry, Vol. 74, No. $3,1970
I
0
3680
3660
3640
3620
3600
I 3180
FREPUENCY cm"
Figure 1. Infrared spectrum of o-trifluoromethylphenol in isooctane in the 3600-em-1 region.
no1 and the m- and p-trifluoromethylphenols symmetrical bands with approximately Lorenztian shapes were observed with maxima at 3619.5, 3617.5, and 3622.0, respectively. The band corresponding to the OH stretching mode for o-trifluoromethyphenol was asymmetric and is shown in Figure 1. The observed spectra could be reproduced by two overlapping Lorenztian bands with maxima at 3606 and 3624 cm-'. A series of spectra were determined for an identical solution at several diff went temperatures. Using an iterative procedure, the relative areas of the bands were determined and the ratio of the band centered at 3624 to that at 3606 was evaluated for each temperature studied. The temperatures and the corresponding ratios are given in Table I. The shoulder which appears on the band assigned to
* To whom correspondence should be addressed. (1) H. Bourana-Bataille, P. Sauvageau, and C. Sandorfy, Can. J . Chem., 41, 2240 (1963). (2) A. W. Baker and W. W. Kaeding, J. Amer. Chem. SOC.,81, 5904 (1959).
(3) E. A. Allan and L. W. Reeves, J. Phys. Chem., 66, 613 (1962). (4) J. H. Richards and S . Walker, Trans. Faraday SOC.,55, 220 (1959). (5) P . J. Krueger and H. D. Mettee, Can. J. Chem., 42, 326 (1964). (6) P . J. Krueger and H. D. Mettee, ibid., 42, 340 (1964). (7) D. A. K. Jones and J. G. Watkinson, Proc. Chem. SOC.London, 2371 (1964). (8) D. A. K'. Jones and J. G. Watkinson, J. Chem. SOC.London, 2366 (1964). (9) P. J. Krueger and H. D. Mettee, Can. J. Chem., 42, 288 (1964). (10) L. J. Bellamy and R. J. Pace, Spectrochim. Acta, Part A , 25, 319 (1969).
4165
NOTES Table I: Ratios of the Integrated Intensity of the trans to the cis Bands Temp,
Ratio
O C
2.1333 1.8975 1* 4959 1.3368 1.2565
25 10 - 15 -25 - 30
the OH stretching mode of o-trifluoromethylphenol is assigned to the cis isomer where intramolecular hydrogen bonding could stabilize this conformation. If the ratio of the extinction coefficients of the cis (3606 cm-l) and trans (3624 cm-I) bands is approximately constant over the temperature range studied, the ratios given in Table I are related to the equilibrium constants for interconversion merely by a constant. A plot of the log of the ratios given in Table I ws. the reciprocal of the absolute temperature is shown in Figure 2. From the slope of the straight line obtained, the enthalpy difference between the cis and trans conformation was determined by standard thermodynamic relationship to be 1.4 kcal/mol.
.4t .3 ’
LOG RATIO
.2
’
I ’
comparisons it appears that an increase in ring size from five t o six members does not influence appreciably the magnitude of the hydrogen bond interaction. These results cannot be adequately compared with the o-fluorophenol system since the OH stretching mode in this system appears to have no detectable asymmetry. They do suggest, however, that the interactions in the cis isomer of o-fluorophenol are too weak to result in an observable frequency shift.
Diaphragm Cell Diffusion Studies with Short Prediffusion Times
by Michael J. Piltal Chemistry Department, University of Tennessee, Knoxville, Tennessee 57916 (Receined May 87, 1970)
The diaphragm cell has proven to be a very useful tool in the study of liquid The basic feature of a diaphragm cell is the porous membrane, which is usually a glass frit separating an upper and a lower compartment containing liquids of different composition. If the concentration of diffusing species is initially zero in the upper compartment, the classical analysise shows that from the duration of the diffusion experiment, t, and the concentration of the diffusing species in the upper and lower compartments at the end of the run, denoted by W and V, respectively, the diffusion coefficients may be calculated from the equation
Here, P is a cell constant determined by calibration, and f is defined by
~
3 00
4.00
5 00
LXIO’
Figure 2. A plot of the log of the ratios tabulated in Table I us. l/T°K.
A separation of 18 cm-I between the cis and trans conformation is considerably less than that found for intermolecular association of phenol with a covalently bound fluorineg (39.7 cm-I). However, the frequency change is almost identical with that reported for the intramolecular association postulated for monofluoroethano15 (15.5 cm-l) and trifluoroethanole (19.0 cm-l). The enthalpy difference derived from these data is significantly smaller than found in the above examples (2.13, 2.07, and 3.32 kcal/mol, respectively). From these
where Vu and VLare the volumes of the upper and lower compartments, respectively, and VD is the volume of the frit. The derivation of eq 1 assumes the concentration distribution in the frit is linear throughout the experiment, although it has been shown’ that as long as the (1) J. N. Northrop and M. L. Anson, J . Gen. Physiol., 12, 543 (1928). (2) A. R. Gordon, Ann. N . Y . Acad. Sci., 46, 285 (1945). (3) R. H. Stokes, J . Amer. Chem. Sac., 72, 763 (1950). (4) J. G. Albright and R. Mills, J . Phys. Chem., 69, 3120 (1965). (5) R. Mills and L. A. Woolf, “The Diaphragm Cell,” Australian National University Press, Canberra, Australia, 1968. (6) See R. A . Robinson and R. H. Stokes, “Electrolyte Solutions,” Academic Press, New York, N. Y., 1959, Chapter 10. (7) C. Barnes, Physics, 5, 4 (1934). The Journal of Physical Chemistry, Vol. 74, No. 83,1970
