COMMUNICATIONS TO THE EDITOR
2418
organic yields due to laOIrn (I.T.)lYI processes in liquid result from a similarity of environmental activation cyclohexane within i0.8%. Detailed experimental and/or decomposition. These similarities were also procedures and equipment used were described p r e found between laoI,lZ8I,and *OBr formed by (I.T.), viou~ly.~~~ (n, r),and (1.T.) processes, respectively, in n-hexane The organic yield of 1301 following neutron irradiation by Wilkey and Willard.' The Auger electron reaction hypothesis9 may explain these phenomena. The and decay of 1301rn in the solid state was 2.8% in the fact that radiative neutron capture induced yields of 7.8 X mole fraction 12sIs system. This low yield bromine were lower than (1.T.)-produced yields of was attributed to a phase separation between iodine 82Brwas found universal for the liquid C6 alkanes.l0 and the cyclohexane (clumping) upon freezing, similar This may indicate that processes additional to Auger to that found in Brz-containing systems.6J*8 When radiation induced reactions occur in iodine and (1.T.)an identical sample was melted 20 sec after a 30-sec activated 82Br, or possibly that internal conversion solid-state irradiation and 1301rn allowed to decay in of (n, 7)-activated bromine occurs before the recoil the liquid system, the yield was 21.701,. Allowing for energy has been dissipated; thus it would be carried the fraction of 1301 born by (I.T.) reactions (0.61),l away from the center of the pocket of fragments caused and the fraction of lmImwhich decayed while the by radiolytic effects. The freeze-thaw t e ~ h n i q u e , ~ sample was solid, the organic yield of laoIdue to (I.T.) which lends itself to a high degree of precision, is a was 1/(0.956 X 0.61) times the 1301yield observed after very valuable tool for studying isomeric transition the lmIm decayed out in the liquid, minus 1.9%, Le., 35.37& The 1.9% is a correction for the organic- induced reactions. ally combined 1301 as a result of solid-state reactions Acknowledgment. This is A.E.C. Document No. induced by (n, y) processes and the fraction of laOIm COO-1617-1. which decayed while the sample was solid. By resolidifying samples containing laoIrn, the high (6) J. A. Merrigan and E. P. Rack, J. Phys. Chem., 69,2795(1965). rate of increase in organic yield due to (I.T.) could be (7) R.M. A. Hahne and J. E. Willard, ibid., 68,2582(1964). halted by the clumping of 12911301m. Thus, growth in (8) M. Milman, J. Am. Chem. SOC., 80,5592(1958). (9) P.R.Geissler and J. E. Willard, J. Phys. Chem., 67, 1675 (1963). 1301 yield due to (I.T.) in the liquid state could be de(10) J. A. Merrigan, J. B. Nicholas, and E. P. Rack, Radiochim. scribed by the e q ~ a t i o n Rt , ~ - RO = R" - RO(l Acta, in press. e-"), where R" is the yield due to (I.T.) after all DEPARTMENT OF CHEMISTRY J. A. MERRIQAN lmIm had decayed, Rt is the yield due to (I.T.) at any OF NEBRASKA J. B. NICHOLAS UNIVERSITY time t, RO is the yield contribution of (n, y) and the R. M. LAMBRECHT LINCOLN, NEBRASKA 68508 part of the (I.T.) processes occurring in the solid state, N. J. PARKS and X is the decay constant for lmIrn.By analyzing E. P. RACK a plot of 1301 organic yield due to (1.T.) 8s. time after RECEIVED MAY6, 1966 irradiation, RS was done in the bromine system,6 a 1mIm half-life of 8.9 i 0.3 min was found. The values of organic yields of 1301 due to lwIrn (I.T.) laoIprocesses process and those of lzsI produced by the lZ7I(n, y) lZEI Anomalous Effect of Pressure on the Protolytic are compared with those due to 82Brm(I.T.) Be2 and Dissociation of Excited States of Nitrophenols 7'3Br(n, y) @Brmin Table I. Table I: Organic Yields of (1.T.)-Produced 1s0I and 82Br and (n, r)-Produced lz*I and NBrm in Cyclohexane at Room Temperature Mole faction of 1 2
x 5.2 x 7.8
10-3 10-3
lroI
1281
82Br
35.3 39.6
36.1 40.5
35.3 39.2
*Brm 22.5 25.4
The similarity between la01 and lzSIyields may indicate a similarity of reaction mechanisms independent of the init,ial kinetic energy of the activated atom. Their likeness to 82Bryields would suggest that activated halogens may trace chemical processes which The Journal of Physical Chsmistry
Sir: I n general, the ionic dissociation of a weak electrolyte in water involves a substantial contraction, and as a corollary the process is favored by an increase of hydrostatic pressure. The contraction arises because the free ions exert a powerful electrostatic attraction on the surrounding solvent and compress it to a greater density than normal. The phenomenon and its cause have been known for many years and were the subject of a review by the writer' in 1963. At that time, some 30 weak acids and bases had been examined under (1) S. D. Hamann in "High Pressure Physics and Chemistry," R. S. Bradley, Ed., Academic Press Inc., New York, N. Y., 1963,Vol. 2,pp
146-156.
COMMUNICATIONS TO THE EDITOR
2419
Table I: Spectral Shifts and Volume Changes for Some Nitrophenols in Water a t 25” bc RO E/ a p
Compound
bfRO-/bP cm-1
%Nitrophenol 2,5-Dinitrophenol 3-Nitrophenol PNitrophenol 4-Nitro-2,6-dibromophenol CNitro-Zaminophenol a
0.015 (21,790)” -0.035 (20,960) 0.020 (22,680) 0.035 (23,090) 0.030 (23,040) 0.020 (20,280)
-0.050 (25,910)’ -0.060 (25,510) -0.070 (27,170) -0.125 (28,570) -0.125 (28,820) -0.090 (24,040)
bAf/bP
AB* -
atm-’
AT
A 7 om8
AT*
mole--
-0.065
7.7
-15.5
-8
-0.025
3.0
-11.6
-9
-0,090
10.6
-16.1
-5
-0.160
18.9
-11.8
7
-0.155
18.3
-10.9
7
-0,110
13.0
-8.9
4
The numbers in parentheses are the values of t (cm-1) at the half-peak height.
pressure and, without exception, found to become stronger when the pressure was raised. However, it is apparent from the theory of the effect2 that the trend might be reversed if, for some unusual reason, the parent molecules were more strongly solvated than their free ions. The writer has now obtained experimental evidence that this may be the case in the electronically excited states of some p-nitrophenols. The results reported here were obtained as part of a wider investigation of the influence of pressure on the ionization equilibria of substituted phenols and anil i n e ~ .Briefly, ~ the molal dissociation constants (Km = mRO-mH +/mRoH) of nitrophenols in their ground states have been measured by standard spectroscopic methods4 and the behavior of the corresponding constants Km* for the excited states ROH* inferred from the shifts in the long-wavelength absorption spectra of ROH and RO-.6 I n the notation of Wellefl
RT In (Km*/Km)= AH
- AH* = Nhc
X AP
where the approximation sign implies an assumption that the entropy change is the same for ionization of the excited state as for the ground state.’ From Planck’s relationships
b(RT In K m ) / b P = - A V it follows that the changes of partial molar volume in the excited and ground states are related by the formula AV* - A V
=
-Nhc(bAfi/bP)
I n applying this formula to the experimental data, Afi has been taken to be the difference between the
wavenumbers at half the maximum heightg of the longwavelength absorption bands of ROH and RO- (use of the wavenumbers for the peaks gives almost the same
result). The quantities A P and b A f i / b P have been derived from measurements made between 1 and 2000 atm at 25” and at ionic strengths near 0.05 M . Their limiting values a t low pressures are listed in Table I. The estimated experimental errors in A P are A 1 cm3 mole-’, and in A P * - A P are *2 cm3 mole-’. Although AV* may therefore be uncertain to =t3 cm3 mole-’, it is safe to draw the following conclusions: A V is negative for all the phenols listed, as it is for many other^;^ A P * is considerably more positive than A V , and although it is still negative for the o- and mnitrophenols, it is positive for the last three p-nitrophenols. The results imply that the excited state of a p-nitrophenol is abnormal in being more strongly solvated in its un-ionized than in its ionized form. The writer suggests that the reasons are as follows. Molecular orbital calculations1° show that the lowest excited (2) J. Buchanan and 5. D. Hamann, Trans. Faraday Soc., 49, 1425 (1953); S. D. Hamann, “Physicc-Chemical Effects of Pressure,” Academic Press Inc., New York, N. Y., 1957,p 152. (3) S. D. Hamann, iM. Linton, and A. J. Murphy, to be published. (4) R. A. Robinson in “The Structure of Electrolyte Solutions,” W. J. Hamer, Ed., John Wiley and Sons, Inc., New York, N. Y., 1959, p 253. (5) Th. Forster, 2.Elektrochem., 54,42 (1950). (6)A. Weller in “Progress in Reaction Kinetics,” Vol. 1, G. Porter, Ed., Pergamon Press, New York, N. Y., 1961,p 189. (7) In the present context, it is only necessary t o assume that the pressure dependences of TAS and TAS* are the same or are small in comparison with those of AH and AH*. The latter assumption is valid for thermodynamic solvation functions calculated from Born’s formula,3 and it is likely that solvation is the main factor governing the entropy changes of the present reactions. (8) M. Planck, Ann. Phys. Chem., 32,462(1887). (9) See ref 6,pp 197-199. (10) S. Nagakura, J. Chem. Phys., 23, 1441 (1955);J. N. Murrell, “The Theory of the Electronic Spectra of Organic Molecules,” John Wiley and Sons, Inc., New York, N. Y., 1963,Chapter 10.
‘Volume 70, Number 7 July 1966
COMMUNICATIONS TO THE EDITOR
2420
state is a highly polar charge-transfer one resembling the valence bond structure
and this conclusion is supported by experimental evidence that the analogous excited state of p-nitroaniline has a very large dipole moment.11 The charges are far enough apart to act independently on the solvent. When protolytic dissociation occurs, the charge on the anion RO- becomes delocalized and some bound solvent is released. It is worth remarking that density datal2 show that the electrically analogous dissociation of zwitterions of amino acids
+ O2C-R-NH3
+02C-R-NH2
+ H+
bX/d
L
1.0
2.0
I
3.0
I
4.0
I
5.0
J
6.0
VIA
(in water)
Figure 1.
also involves a small increase of Volume. Ward and Habgood'). The linear relation confirms the suggestion'J that the adsorption of carbon dioxide (like that of carbon monoxide) is essentially a polariDIVISIONOF PHYSICAL CHEMISTRY S. D. HAMANN zation effect caused by the electrostatic field due to AND INDUSTRIAL COMMONWEALTH SCIENTIFIC the cations in the zeolite. The X and Y series give RESEARCH ORGANIZATION separate lines indicating that the field strength calcuMELBOURNE, AUSTRALIA lations have not fully taken into account the differences RECEIVED MAY9, 1966 between the X- and Y-type zeolites. It is interesting to note that in the case of transition metal cation Y zeolites, the COS frequency is practically unchanged from the gasphase value, although the electric field Carbon Dioxide Adsorbed on was calculated to be very close to that in Mg2+-conLinde X and Y Zeolites taining zeolites. (11) J. Czekalla and G. Wick, Z . Elektrochem., 65,727 (1961). (12) H.H.Weber, Biochem. Z.,218,l (1930).
Sir: In a recent article,l Ward and Habgood reported on carbon dioxide adsorbed on Linde X zeolites. They found that in the alkali earth metal cation substituted zeolites the asymmetric stretching vibration of the adsorbed carbon dioxide was at a higher frequency than in the gas phase and dependent on the cation present. They attributed this shift to an ion-dipole interaction resulting in a linear adsorption of the C02 molecule. This phenomenon is very similar to our observation2 on the cation dependence of the vibration of adsorbed carbon monoxide, which we explained as due to a polarization of the carbon monoxide molecule in the electrostatic field of the cation. We also observed a frequency shift in the case of COZadsorbed on Linde Y zeolites, and we were able to put the field dependence on a semiquantitative basis. The method of calculating the electrostatic field in the neighborhood of the cation has been described p r e v i o u ~ l y . ~ ~ ~ Figure 1 shows the frequency of the asymmetric stretching vibration of adsorbed C02 plotted against the calculated field strength (filled circles represent our values, and open circles represent values from The Journal of Physical Chemistry
(1) 3.W. Ward and H. W. Habgood, J. Phys. C h m . , 70, 1178 (1966). (2) C. L.Angel1 and P. C. Schaffer,ibid., 70,1413 (1966). (3) P. E. Pickert, J. A. Rabo, E. Dempsey, and V. Schomaker, Ades Congr. Intern. Catalyse, Se, Amsterdam, 1064,714(1965). Actual values of the field strength based on an improved model were kindly made available by Dr. E. Dempsey.
UNIONCARBIDE RESEARCH INSTITUTE UNIONCARBIDECORPORATION TARRYTOWN, NEWYORK
C. L. ANGELL
RECEIVED MAY19, 1966
The Reactions of Thermal Hydrogen Atoms with Ethanol and Ethanol Free Radicals at ??OK
Sir: The reactions of hydrogen atoms and trapped free radicals in alcohol glasses at 77°K have been the As a result subject of considerable (1) R. 5.Alger, et d., J. C h m . Phys., 30,695 (1959). ( 2 ) R.H.Johnsen, J . Phys. Chem., 65,2144(1961); 67,831 (1963). (3) P.J. Sullivan and W. 5.Koski, J . Am. Chem. Soc., 86, 159 (1964).
