3898
1.2 log Po units compared to curve 3 for the cyclobutane fall-off at 430' observed by Vreeland and Swinehart.3d As would be expected, the difference (at the lower pressures) between the methylcyclobut ane falloff data at 430' and curve 4 for cyclobutane at the higher temperature (449') is somewhat larger (1.251.4 log Po units). The value of s = 23 (Kassel classical) for niethylcyclobutarie may be compared with Vreeland and Swinehart's cyclobutane value of s = 18.3d I n each case s is approximately 60% of the total number of vibrational degrees of freedom and on this basis the vibrational degrees of freedom of the added methyl group appear to participate almost as effectively in the activation process as the rest of the molecule. This finding would seem t o be in accord with the concept of intramolecular energy transfer during the lifetime of the activated molecule. For methylcyclopropane4 the value of s = 19 which gives a suitable Kassel classical fall-off curve represents an increase of 6 or 7 over the s appropriate for cyclopropane. Moreover, Flowers and Freyg found that the curve for s = 23 reproduces their fall-off data for 1,l-dimethylcyclopropane (which has the same number of vibrational degrees of freedom as methylcyclobutane) , Also of interest is that the fall-off curve obtained by Chesick'o for methylenecyclobutane lies between the curves for methylcyclobutane and cyclobutane. In the low pressure work on cyclopropane'' and cyclob~tane,~c.d it has been observed that the curve for the decrease in rate constant tends to level off at pressures in the region nim. and below. The data in Fig. 1 and 2 for methylcyclobutane do not extend to a pressure of mm. and do not exhibit a noticeable leveling off, Acknowledgment. The authors wish to thank Dr. E. W. Schlag for his computer program and Mr. Carl Whiteman for his assistance. (9) M.C. Flowers and H. M . Frey, J . Chem. Soc., 1157 (1962). (10) J. P.Chesick, J . P h y s . Chern., 65, 2170 (1961). (11) A. D. Kennedy and H. 0. Pritchard, ibid., 67, 161 (1963).
Solvent Dipole Competition for Interamide Hydrogen Bonds'
by James S. Frarizen and Barbara C. Franaen Department of Biochemistry and S u t r i t i o n , Graduate School of Public Health, L'nitersity of Pittsburgh, pittsbtkrgh. Pennsgluania (Received J u n e 29, 29/34)
It has been demonstrated that the hydrogen-bonding interaction between amides is weakened by a dipolar T h e Journal of Physical Chemistry
NOTES
solvent.2 This earlier study has now been extended to another solvent system of varying dipolar character. By using either carbon tetrachloride, F = 0.0 D . , or l,l,l-trichloroethane, p = 1.6 D., or mixtures of the two, a range of solvent dielectric constants from 2.25 to 7.0 can be covered. This system is more appropriate than the cis-trans-dichloroethylene system used previously because no type of solvent-solute hydrogen bonding is possible. The extensive studies of Allerhand and Schleyer3 have revealed no evidence for the participation of methyl groups in hydrogen bonding, and, therefore, any competitive effects by the solvent must be due to dipole-dipole interaction. e-Caprolactam was chosen as the hydrogen-bonding solute since it forms dimers ~ n l y ,thus ~ , ~eliminating the need for making assumptions about relative magnitudes of equilibrium constants as is required for multiple equilibrium systems.5 The extent of association was determined from the variation of the apparent extinction coefficient of the first overtone of the free N-H absorption according to the treatment of Lord and P ~ r r o . ~
Experimental The solvents employed were obtained from Fisher Scientific Co. Spectroanalyzed grade carbon tetrachloride was used without further treatment. The trichloroethane, however, exhibited a strong absorption band at about 273 mF which was probably due t o aromatic contamination. This solvent was, therefore, distilled through a 90-cm. electrically heated column packed with Berl saddles. Using a reflux ratio of 8 : 1, the fraction boiling at 72-73' and having no absorption band at 276 nip was used for the hydrogen-bonding experiments. e-Caprolactam from K and K Laboratories was dried overnight in vacuo at room temperature and used directly. The spectral measurements of the unassociated N-H group in the near-infrared region were carried out as described in the earlier paper2 except for a few minor changes. Since the Gary 14-R spectrophotometer was available, the reverse beam feature of this instrument was exploited. Thus instead of passing the entire wave length range of the source through the sample, only monochromatic radiation traversed the solutions. As a result, heating of the sample by the beam was reduced to (1) This work was supported in part by a Public Health Service Research Grant (GM-10133-02) from the Division of General Medical Sciences. (2) J. S.Franzen and R . E. Stephens, Biochemistry, 2, 1321 (1963). (3) A. Allerhand and P. von R. Schleyer, J . Am. Chem. Soc., 85, 1715 (1963). (4) R. C. Lord and T. J. Porro, 2 . Elektrochem.. 64, 672 (1960). (5) > Davies 'I. and D. K . Thomas, J . Phys. Chem., 6 0 , 763 (1956).
NOTES
3899
Table I : Values of Thermodynamic Functions for the Association of +Caprolactam AGO,
K , 1.
-2,53
-5.4
--9.8
-4 . 9
- 11.4
D.
276 298 313
2.26 2.22 2.20
146 f 6 74.2 f 2 . 8 46.5 f 1 . 9
276 298
3.54 3.38
58.0 f 2 . 3 32.8 f 1 . 3
-2.07
CCld-TCE (1: 1)
298
4.01
22.9 f 0 . 9
-1.86
TCE
276 298 313
7.67 6.89 6.38
23.8 f 0 . 9 13.4 f 0 . 5 8 . 7 i0 . 3
- 1.54
CC14
CClh-TCE (2: 1)
mole -1
A S 0 , cal. mole -1
kcal. mole-'
T ,OK.
Solvent
A H O ,
kcal. mole-'
deg. -1
Dielectric constants of the solvents were determined by the resonance method with the apparatus described by Bender.6 Results and Discussion
0.2
0.4
0.6
1/D.
Figure 1. Dependence of the free energy of caprolactam association on the reciprocal of the solvent dielectric constant. The closed circles represent the d a t a of this paper. The open circles represent d a t a for the association of caprolactam in solvents composed of cis- and/or transdichloroethylene taken from the previous study.2
a negligible amount, and temperature control was captain to within f0.5'. The temperature within the cell was monitored directly by the use of a thermocouple sealed in a glass capillary and press-fitted into the Teflon stopper supplied with the cell. An optical path of 10 cm. was used in all cases, since at the higher concentrations required for shorter cells the monomer-djmer equilibriuirn becomes complicated by the formation of larger aggregate^.^ The estimated maximum error in the reported equilibrium constants is 4%.
As can be seen from Table I, the caprolactam dimer hydrogen bonds are quite sensitive to the composition of the solvent. This is also evident from Fig. 1, which shows that trichloroethane, p = 1.6 D., is less effective in disrupting the dimer complex than cis-dichloroethylene, p = 1.89 D. Such a relation is expected in view of the propensity of the a-electron cloud and of the C-H groups3 of dichloroethylene for hydrogen-bonding to caprolactam monomers. The explanation of the linear relations in Fig. 1, however, is not fully established. Although the free energy of the association reaction varies directly with the reciprocal of the solvent dielectric constant, a theoretical description of the system in which the region between the hydrogen-bonded molecules is treated as a continuum2 is not realistic. The observed linear relations may just be fortuitous sinice from a microscopic point of view there is no evident reason at present for expecting such behavior. I t is more likely that the reduction in the extent of association with increasing dipolar nature of the solvent is due to solvent-solute dipole-dipole interactions. In conclusion, it can be stated that there is a close relationship between hydrogen-bond strength and the molecular environment provided by the solvent. Accordingly, the recent studies of Ritchie arid Pratt' emphasize the significance of solvation effects of this sort. (6) P. Bender, J . Chem. Educ.. 23, 179 (1946). ( i )C. D. Ritchie and A. L. Pratt, J . Am. Chem. Soc., 86, 15'71 (1964).
Volume 68, 'Vumber 12 December, 1964
