COMMUNICATIONS TO THE EDITOR
3025
Table I : Values of Surface Gibbs Energy, (in ergs/cm2), and of Entropy,
Y
-b-//bT
HzO
(C4Fa)aN
72.0 0.155
0.080
801
16.35
I
I
y, at
-( b y / b T ) ,
25”
Interface
n-CaHi4
Interface
40.7 -0.086
17.9 0.105
50.4 0.089
I
I
t
I
i
H20
i
H20 + n -
”c
1 “-‘gH14
*
O
OIO
I
I
(C4Fd3N
20
30
k
40
t0c
(C4F9)& is much larger: 0.155 - (-0.086) = 0.241 erg/deg em2. This acccords qualitatively with the larger negative entropy of solution of fluorochemical gases in water than that of alkane gases. The “work of adhesion” is also larger in forming the water-FC43 interface: 72.0 16.35 - 40.7 = 47.7 17.9 - 50.4 than the water-hexane interface: 72.0 = 39.5 erg/cm2. The larger attraction and larger loss of entropy in the former system seem consistent with our view that a fluorochemical, either liquid or gas, is especially effective in destroying hydrogen bonds and setting up stronger dispersion forces in their place. That the results here reported can be regarded as typical for fluorochemical liquids can be inferred from the almost identical solubility of iodine4in four of them; values in mole per cent at 25” are; (C4F9)3N,0.0232; c-C6F11CF3, 0.0210; CBFl6O, 0.0208; C7F18, 0.0180. The amine has no basic character; the color of the solution in each liquid is pure violet. We had a small amount of C7F16 with which we determined the interfacial energy against water at 25, 30, and 35”. The line was parallel to and 1.0 erg/cm2 below the line for the FC43-water interface.
+
+
Figure 1. Surface tensions of HzO, n-C6H14,and (C4F9)$N and interfacial tensions with HsO.
Acknowledgment. We express our thanks t o the 3M Co. for FC43. This work was supported by the National Science Foundation.
changes from 0.155 to 0.089 when covered by n-C6H14, a loss of 0.050 erg/deg em2;but the loss when covered by
(4) J. H. Hildebrand and R. L. Scott, “Regular Solutions,” PrenticeHall, New York, N. Y., 1962, p 164.
C O M M U N I C A T I O N S T O THE E D I T O R Electron Spin Resonance of @-Chloroalkyl Nitroxides. Angular Dependence of @-ChlorineHyperfine Coupling’
Sir: Recent work on the angular dependence of @A uorine hyperfine coupling2-6 has prompted an investigation into the factors influencing @-chlorine hyperfine coupling. The esr spectrum of a-chlorobenzyl t-butyl nitroxide in benzene solution a t room C1 0 - CH3
I l l I I
CeH6-C-N-C-CH3
H
CH3
temperature is shown in Figure 1. The smallest doublets are due to the single @-hydrogen. The over-
lapping quartets are due to coupling from chlorine-35 (75.4%, p = 0.82089)’ and chlorine-37 (24.6%, ~l = 0.68329).’ I t is clear from the spectrum that the isotope of lower abundance has the smaller coupling constant . (1) This work is being supported by AFOSR(SRC)-OAR USAF Grant No. 1069-66. (2) E. T. Strom and A. L. Bluhm, J . Phys. Chem., 74, 2036 (1970) ; E. G. Janzen, B. R . Knauer, J. L. Gerlock, and K. J. Klabunde, ibid., 74,2037 (1970). (3) J. L. Gerlock, E. G. Janzen, and J. K. Ruff, J . Amer. Chem. Soc., 92,2558 (1970). (4) D. Kosman and L. M. Stock, ibid., 92,409 (1970). (5) K. J. Klabunde, ibid., 92, 2427 (1970); G. R. Underwood, V. L. Vogel, and I. Krefting, unpublished results; we acknowledge with appreciation the receipt of a preprint of this work. (6) J. L. Gerlock and E. G. Janzen, J . Phys. Chem., 72, 1832 (1968). (7) J. A. Pople, W. G. Schneider, and H. J. Bernstein, “High-Resolution Nuclear Magnetic Resonance,” McGraw-Hill Book Co., Inc., New York, N. Y., 1959, p 481. The Journal of Physical Chemistry, Vol. 74, No. 16,1970
3026
COMMUNICATIONS TO THE EDITOR group with Ocl = 45" should permit selection of the correct choice of Ocl for the monochloronitroxide radical. The spectrum of trichloromethyl t-butyl nitroxide was obtained by photolysis of BrCC4 a t room temperature in benzene containing 2-methyl-2-nitrosopropane, a method previously used for the preparation of perfluoroalkyl t-butyl n i t r o x i d e ~ . ~The ' ~ ~ nitrogen
Figure 1. Electron spin resonance spectrum of a-chlorobenzyl t-butyl nitroxide in benzene solution.
The spectrum in Figure 1 was obtained in connection with spin trapping experimentss wherein t-butyl hypochlorite was added to phenyl t-butylnitrone (PBN)8 in the absence of oxygen. The same spectrum was obtained from benzene solutions of trichlorosilane or trimethylchlorosilane containing PBN to which tbutylpero~alate~ was added. Although it is probable that in the latter system t-butyl hypochlorite is also responsible for the reaction, the mechanism of production of the monochloro nitroxide derivative is not understood a t this time. The addition of chlorine to PBN, for example, only produces benzoyl t-butyl nitroxide.8 The spectrum of a monochlorobenzyl nitroxide is particularly useful for the investigation of the angular dependence of p-chlorine coupling because the magnitude of the p-hydrogen coupling is related to the dihedral angle OH, i.e., the angle between the P-hydrogencarbon-nitrogen plane and the carbon-nitrogen-p-orbital plane, from which the dihedral angle Ocl can be obtained. From the Heller-McConnell equationlo
AbH
= (B1
-
+ B2
BrCCla
+ hv
I
I
4--CMea
-t
.CC13-*CCl&-CMe3
coupling is almost the same as for the monochloro derivative (Table I) but the p-chlorine coupling is 2.29 G. This indicates that I is probably the preferred conformation for the monochloro derivative.
Table I : Nitrogen and Chlorine Hyperfine Coupling in Chloroalkyl &Butyl Nitroxidesa Radical
AN
Aci
AH
T ,O C
Oc1,b
0.75
35
18'
12.2 12.1
6.05, 4.88 6.1 6.2
0.86 0.88
63 90
13.2
3.3
35
30'
CCI3-N-CMea
12.4
2.3
35
45"
CC13-A-CC13~
11.8
1.25
0. CeHsCHC1-I!J-CMet.
0. CaH&C12-
0.
k- CMe3
I
0-
12.12
a I n gauss with an error (average deviation) i w O . 1 G. parent dihedral angle. Reference 14.
45O
* Ap-
COS' O ) p N
where BI S o,l1BO 50," PN = 0.37,12OH is calculated to be 78". The calculated ecl is thus either 18" as in I or 42' as in 11.
I
0.
0
I1
Temperature variation has very little effect on either the p-hydrogen or p-chlorine coupling. Thus the chlorine coupling increases by 0.15 G and the hydrogen coupling increases by 0.13 G with increase in temperature in the range 35-90". At temperatures below 35" the line width increases substantially with no appreciable change in spacing between the lines. These findings strongly suggest that the a-chlorobenzyl t-butyl nitroxide is essentially locked in a conformation where 6 1 E 18 or 42". The spectrum of trichloromethyl t-butyl nitroxide is of interest because the freely rotating trichloromethyl The Journal of Physical Chemistry, Vol. 74, No. 16, 1070
The spectrum of a third nitroxide of interest has been obtained by dissolving PBN previously exposed to tbutyl hypochlorite in the gas phase14 in carbon tetrachloride. The spectrum consists of 15 lines with some additional partially resolved splitting which can be assigned to a)a-dichlorobenzyl t-butyl nitroxide. (8) E. G. Jansen and B. J. Blackburn, J . Amer. Chem. Soc., 91,4481 (1969). (9) P.D.Bartlett, E. P. Benzing, and R. E. Pincock, (bid.,82, 1762 (1960). (10) C. Heller and H. M. McConnell, J . Chem. Phys., 32, 1535 (1960). (11) M. D.Sevilla and G. Vinoow, J . Phys. Chem., 72, 3647 (1968),
and references therein. (12) Assuming A N = 35.61 ,ON 0.93P O ; P.B.Ayscough and F. P. Sargent, J . Chem. SOC.,Sect. B , 907 (1966); E.G. Jansen and J. W. Happ, J . Phys. Chem., 73,2335 (1969). (13) After the completion of this work a previous report on the trichloromethyl t-butyl nitroxide spectrum became available: I. H. Leaver, G. C . Ramsay, and E. Susuki, Aust. J . Chem., 22, 1891 (1969); A N = 12.73, Aci = 2.41 G. The radical was produced by photolysis of indole in carbon tetrachloride containing 2-methyl-2nitroso pro pane. (14) E.G.Janzen and J. L. Gerlock, Nature, 222,867 (1969).
+
COMMUNICATIONS TO THE EDITOR
3027 Water Structure Promotion by
C1 0 . CH3
I I I
Large Organic Anions
CBH,-C-N-C-CH~
I
Cl
I
CH3
The chlorine coupling is 3.3 G. I n a previous paperj2 an argument was made for a conformational preference of I11 over IV in a,a-difluorobensyl t-butyl nitroxide on the basis of steric and dipole-dipole interactions. Space-filling models show that for the dichloro derivative, I11 is clearly favored
Sir: Large organic cations such as tetraalkylammonium ions have been shown t o promote structure, or increase the hydrogen bonding of water.’+ The special interaction between these ions and water has been referred to as “hydration of the second kind.”4 It has been pointed out,6,B for example, that the B coefficient of the Jones-Dole viscosity equation has large positive values for these “hydrophobic” ions, and the larger the cation the more positive the B coefficient. This is in
Iv
I11 X=C1,
F
over IV for steric reasons. The eclipsing of the phenyl and oxygen in I11 is strongly offset by the greater freedom of motion possible for the two chlorine atoms. I n 111,8cl = 30”. The data for the three chloro-substituted nitroxides described shows that the @-chlorinecoupling increases with a decrease in dihedral angle from 45” to near 0” very much like hydrogen and fluorine as recently pointed out.2r3 Examples of j3-chloroalkyl radicals where the dihedral angle is greater than 45” are not available at this time. I n bis(trichloromethy1) nitroxide16 the j3-chlorine coupling is considerably lower than in trichloromethyl t-butyl nitroxide (1.25 us. 2.3 G). Although, this may be due in part to a smaller spin density on nitrogen in the bis(trichloromethy1) nitroxide,lBthe nitrogen coupling does not decrease enough to account for the decrease in @-chlorine coupling. Small differences in planarity of the nitroxide function induced by different steric interactions in the two radicals may also produce differences in the nitrogen as well as the chlorine couplings. (15) H. Sutcliffe and H. W. Wardale, J . Amer. Chem. SOC.,89, 5487 (1967). (16) Since the trichloromethyl group is more electron withdrawing than the t-butyl group, V mould be favored over V I (see E. G. Janzen, Accounts Chem. Res., 2,279 (1969), for discussion and examples).
DEPARTMENT OF CHEMISTRY
THEUNIVERSITY OF GEORGIA ATHENS,GEORGIA30601 RECEIVED nI.4RCH
Pr,Nt
500-
F-/?I0-
c r I-
I
I
I
I
I
I
Figure 1. Apparent molal heat contents of 1 m aqueous solutions a t 25”: A, chloride salts; 13, sodium salts. Ionic radii for acetate, butyrate, and valerate were estimated from molecular models. Other ionic radii are taken from tables in R. A. Robinson and R. H. Stokes, “Electrolyte Solutions,” 2nd ed, Academic Press, New York, N. Y., 1959. +L values of sodium fluoride, chloride, bromide, iodide and acetate, and lithium, sodium, potassium, and cesium chloride taken from V. B. Parker, “Thermal Properties of Aqueous Uni-univalent Electrolytes,” NSRDS-NBSB, National Bureau of Standards, Washington, D. C., 1965.
(1) H. S. Frank, 2.Phys. Chem., ( L e i p z i g ) , 228, 364 (1965). (2) 8. Lindenbaum, J . Phys. Chern., 70, 814 (1966). (3) A. H. Narten and S. Lindenbaum, J . Chem. Phys., 51, 1108 (1969). EDWARD G. JANZEX (4) H. G. Hertz and M. D. Zeidler, Ber. Bunsenges. Phys. Chem., BRUCER . K N A U E R 68, 82 (1964). LEWIST. WILLIAMS (5) R. L. Kay, T. Vituccio, C. Zawoyski, and D. F. Evans, J . Phys. W. B. HARRISON Chem., 70, 2336 (1966). 8, 1970 (6) R. A. Home and R. P. Young, J . Phys. Chern., 72, 1763 (1968). The Journal of Physical Chemistry, Vol, 74, N o . 16. 1970
