1306
LEOA. WALL,MARYR. HARVEY AND MAXTRYON
melting point and pH change is fortuitous. If the effects on the melting point are due to interactions with the gelatin, such interactions would contribUte to the pH change, the major part of which is due to binding a t the charged groups. It is reasonable to assume that those ions that are most strongly bound a t charged groups would also inter-
Vol. 60
act most strongly a t the polar peptide groups. A precise description of possible complexes between ions and peptide bonds is not possible a t present. Acknowledgment.-This work was supported under Contract No. DA 49-007-MD-298 with the Office of the Surgeon General, Department of the Army, to whom we wish i o express our thanks.
OXIDATIVE DEGRADATION OF STYRENE AND a-DEUTEROSTYRENE POLYMERS BY LEOA. WALL,MARYR. HARVEYAND MAXTRYON National Bureau of Standards, Washington, D. C Received March 2S71968
The oxidation of polystyrene and a-deuterostyrene polymers in the presence of ultraviolet radiation and air a t 60' has been that of polyinvestigated. Initially, the deuterated polymer shows an increase in absorption a t 340 mp of only about styrene. A post-irradiation effect was observed which disappears upon further irradiation. The reaction occurring during post-irradiation of polystyrene consists of two first-order components with activation energies of 16 and 20-24 kcal. per mole. This may indicate the decomposition of two different hydroperoxide structures during the post-irradiation periods. However, it is suggested that one of these components may result from a cis-trans isomerization. I t is concluded tentatively that polystyrene oxidizes mainly a t the a-position of the monomer unit but that the resulting hydroperoxide, although formed, is extremely labile.
Introduction I n earlier investigations a t this Laboratory on the ultraviolet-induced oxidation of polystyrene, the carbonyl and hydroxyl group build-up was followed by infrared spectra2a and the small amountls of volatile components produced were analyzed by mass spectrometry.2b Recently the increase in ultraviolet absorption has been found to be a sensitive indication of d e g r a d a t i ~ n . ~Thus ultraviolet spectrometry, which is usually a more convenient and simpler technique t o employ than infrared or mass spectrometry, has been used in this work. Often the easily accessible regions in the infrared merely show the presence of certain groups without giving definite further information concerning the specific nature of the structure containing these groups. Mass spectrometry, on the other hand, ordinarily gives analyses of volatile fragments which are, a t least a t the low extents of reaction so far studied,2 invariably contaminated with trace impurities such as solvent and solvent oxidation products. The rate of oxygen consumption would be a more direct quantity to study from a mechanistic viewpoint. We have, however, confined ourselves in this study to the changes in ultraviolet transmission in the region of 280 to 400 mp, because previous work has shown that decreased transmission occurred in these regions during the early stages of oxidation.3 In the present work the oxidation behavior of adeuterostyrene polymer was compared with that of polystyrene, the purpose being to establish the site of radical attack, It is well-known that deuterium atoms are abstracted a t much lower rates (1) Presented a t tlie 126th Meeting of the American Chemical Society in New York, N. y . , September 12-17, 1954. (2) (a) B. G. Achhammer, M. J. Reiney and F. W. Reinhart, J . Research Nall. Bur. Slandards, 4 1 , 116 (1951); (b) B. G. Achliammer. M. J. Reiney, L. A. Wall and F. W. Reinhart, J . Polymer S c i . , 8, 555 (1952) : also Natl. Bur. Standards Circular 525, Polymer Degradation Mechanisms, p. 205 (1953). (3) M. J. Reiney, M . Tryon and B. G . Achhammer, J . Research Nall. Bur. Standards, 51, 155 (1953).
than protium atoms, and hence such a comparison should enable one to gain basic knowledge of the mechanism of oxidation. If the presence of deuterium atoms alters any of the rate-determining elementary steps of the oxidation process, then the over-all rate will be altered. An analysis of the results in conjunction with other kinetic data will aid in establishing the mechanism in more detail than has hitherto been possible. Deuterium studies have been used previously to ascertain the site of transfer processes in polymerization reactions4r5and in pyrolytic decomposition of polymers.6 Materials and Methods The polystyrene used i n this work was a sample prepared by thermal bulk polymerization a t 120" and had an approximate number average molecular weight, as determined from osmotic pressure measurements, of 237,000. This was the same highly purified polystyrene sample used in earlier work.s The poly-a-deuterostyrene sample wa8 prepared by the bulk polymerization a t 70' of a-deuterostyrene. The monomer was synthesized from acetophenone by reduction with lithium aluminum deuteride to give a-deuteromethylphenylcarbinol, which was dehydrated. Mass spectra gave the following analysis: styrene 1.90%, styrene-dl 97.4270, styrene-dz 0.54%, styrene-& 0.14%. Both polymer samples were purified of monomer, dimer and similar materials by repeated solution in benzene, followed by precipitation in methanol. The final product was dissolved in benzene, the solution frozen, and the solvent, removed by sublimation a t reduced pressure. The extent of puiification was determined by the ultraviolet ahsorption of chloroform solutions of these polymers after each cycle of solution and precipitation as recently descrihed . 3 Films of these purified polymers were cast from benzene solutions by the method described in references 2a and 3 . The film thicknesses used were approximately 0.18 mm. The films were exposed to ultraviolet radiant energy from a sunlamp in air on a rotating turntable 15, cm. from the lamp. The temperature of the table was 60 . This equipment is described in method No. 6021 of Federal Specifica(4) L. A. Wall and D. W.Brown, J . Polymer S c i . , 14, 513 (1954). (5) P. D. Bartlett and F. A. Tate, J . A n . Chem. Soc., 1 5 , 91 (1953). (6) L. A. Wall, D . W . Brown and V. E. Hart, J . Polymer Sci., 16, 157 (1955).
Sept., 195G
OXIDATIVE DEGRADATION OF STYRENE AND a-DEUTEROSTYRENE POLYMERS 1307
tion L-P-406a.' The lamp gives a typical mercury spectrum and peak intensity a t 313 m p . A Beckman Model DU spectrophotometer with ultraviolet accessories was used, with further accessories for temperature control of the sample compartment. The comparison of the rates of ultraviolet absorption after exposure to ultraviolet radiation and after storage in the dark a t room temperature for both polymers was made as described in earlier work.8 The study of the temperature coefficient a t 70, 75 and 80' for the post-radiation reaction in polystyrene was made by removing the sample from the exposure apparatus after a suitable exposure of about 290 hours and immediately placing the sample in the spectrophotometer cell compartment which was controlled a t the indicated temperature. The increased absorbances at 340 mp wave length was measured at suitable time intervals.
Results With purified polystyrene the transmittance progressively decreases on oxidation. I n Fig. 1 the full lines represent the results of exposure for the indicated times in hours to ultraviolet in the presence of air a t a temperature of about 60". The dashed lines show the effect of dark storage3 in air a t room temperature. This post-effect is removed when the film is briefly exposed again. At 340 mp the decrease in transmittance that occurs during the storage period is a maximum.3 The absorption produced in the ultraviolet during these oxidation studies is, as suggested previously, probably due to carbonyl-containing structures. This premise is supported by the fact that plots of the absorbance in the infrared region a t 5.78 p versus the ultraviolet absorbance a t 340 mp, for all samples studied, are essentially linear. I n Fig. 2 there is presented the absorbance of the deuterated polymer and polystyrene as a function of the time of exposure to ultraviolet light in air a t 60". The solid lines represent results obtained by making absorption measurements immediately after the films had been oxidized in the presence of ultraviolet light, while the dashed lines represent the results obtained after an additional storage time of 120 hours. This time was chosen so that a maximum post-effect would be achieved. The difference between the two curves for a given substance is a measure of the total post-effect. Since the curves spread apart, it is seen qualitatively that the post-effect increases with extent of degradation. From this observation it appears that the much smaller post-eff ect in the poly-a-deuterostyrene may be partly due to the lower extent of apparent degradation. The apparent degradation of the deut,erated polymer is markedly less initially, the rate of change in absorbance for this polymer being about a factor of that for the polystyrene. This factor is within the range of possible deuterium isotope effects. It was found that the post-effect developed faster a t higher temperatures. In Fig. 3 the absorbance of polystyrene in the ultraviolet is plotted versus time of storage for several temperatures. Higher temperatures produce a much more rapid increase in absorbance. (7) "Plastics, organic: General specifications, test methods," Federal Specification L-P-406s (Government Printing Office, W a s h ington 25, D. C . , Jan. 24, 1944). ( 8 ) Absorbance is defined as log 1/T, where T i s the transmittance or t h e ratio of the transmitted energy to the incident energy.
0.8
1
g 0.6 $
3
'i
0.4
E
0.2
0.0
320 340 360 380 Wave length, mp. Fig. 1.-Ultraviolet spectra of polystyrene film as progressive oxidation occurs. ?ours of exposure indicated : , after exposure a t 60 ; - - - -, after storage for 120 hours a t room temperature. 280
.o
1
300
I
I
I
I
An /
/
0
0.8
4 0.6 M
e
8
9
D
$ 4
Yn
d //
0.4
/
Q
0.2
0
0
100 200 300 400 Exposure to sunlamp a t 60" in air, hours. Fig. 2.-Absorbance of oxidized polystyrene films : , immediately after exposure; - - - -, after an additional 120 hours of storage in dark a t 2 5 " ; 0, polystyrene; A, poly-a-deuterostyrene.
Assuming that the absorbances measured in this work are related to the concentration of products of the degradation reaction, a semi-log plot was made of one minus the fractional increase of absorbance, R, versus time of storage of polystyrene at 70" after ultraviolet irradiation for about 290 hours a t 60". This is shown in Fig. 4 (the curve with open circles) where
A , is the limiting absorbance measured during storage, A is the observed absorbance a t time t, and A0 is the initial absorbance before the storage reac-
LEOA. WALL,MARYR. HARVEY AND MAXTRYON
1308
Vol. 60
rates of the two reactions by subtracting the slower from the over-all reaction leaving the faster one. This treatment is common in the case of some radio.58 active decay problems. I n Fig. 4 the over-all reaction is indicated by the curve with open circles and $" the fast and slow reactions by the straight lines. .54 The closed circles indicate the points calculated by + d subtracting the slow reaction from the over-all reaction and were used t o obtain the straight line -50 for the fast reaction. Thus the experimental re2p .46 sults are interpretable as the algebraic sum of two first-order variations in absorbance and thus the $ .42 lbd J data are suggestive of two simultaneous first-order processes. Values of the rate constants obtained from the slopes of these two straight lines a t four .38 i I I different temperatures are shown in Table I. Ac0 1 2 3 4 5 tivation energies are obtained from the slopes of Time, hours. the lines in Fig. 5 . The fast reaction results in a very Fig. 3.--Ultraviolet absorbance at 340 rnb of polystyrene good straight line leading to an activation energy of film as a function of time of storage at three temperatures 16 kcal. per mole, while the slow reaction data yield after ultraviolet irradiation for 290 hours at 60". a less well defined straight line indicating an activa1 .o tion energy of the order of 20-25 kcal. The relative error of the estimate of the fast reaction slopes is much smaller than that of the slow reaction slopes since the over-all magnitudes of the fast reaction slopes are so much larger than the slower. It is assumed, of course, that the absolute errors of the two measurements are of the same order of magnitude. Even considering the decreased accuracy inherent in the method for the slow reaction values, it is concluded that its activation energy is scjmewhat higher than that of the fast reaction. The pre-exponential factors found were 10" set.-' 0.1 G for the slow reaction and lo7 set.-' for the fast reaction. The rate data are given in Table I. I
6.2
I
I
I
W.L.
3
i
Y
0
PI%
- 5
I
I
I
TABLE I RATECONSTANTS AS A FUNCTION OF TEMPERATURE FOR THE REACTION THAT FOLLOWS ~JLTRAVIOLET IRRADIATION OF POLYSTYRENE I N AIR FOR 290 HOURS AT 60" kslow = 10" exp -23,50O/RT sec.-l kfast = lo7 exp - 16,40O/RT sec.-I Temp., 'C.
0.01 100 150 200 250 300 350 Time (min.) Fig. 4 . 4 e m i l o g plot of one minus the fractional increase of absorbance, R, v e r m time; 70" data for posteffect in polystyrene. Straight lines add up to give experimental curve: 0,experimental points; 0 , pohta calculated by difference between experimental curve and extrapolated straight line.
0
50
tion begins. Since this plot does not yield a straight line, the storage reaction as measured by ultraviolet absorption is not a first-order reaction. Plotting the same data to test for a second-order reaction also led to curvature. A calculation of the apparent order of reaction from the slope of a plot of log dR/dt versus log R gave a value of approximately 1.24. On the assumption that two simultaneous independent first-order reactions of widely different rates are producing absorbing products and that the faster reaction is complete before the last measurements were made it is possible t o estimate the
25 70 75 80
Slow reaction k.low, sec. - 1
Fast reaction
7 . 1 x 10-7 0.96 x 10-4 2.26 x 10-4 0.37 X lo-'
9 . 9 x 10-6 4 . 5 4 x 10-4 3.82 x 10-4 7 . 1 9 x 10-4
kr-t, sec. - 1
Previous workg with 7-ray induced degradation of polystyrene in carbon tetrachloride solution with air present gave a post-effect on the viscosity decrease. Temperature had an accelerating action in this process, It is very likely that both posteffects are due to the decomposition of a hydroperoxide intermediate. Discussion As a starting point for a discussion of the oxidation mechanism of polystyrene it is convenient t o choose the generally accepted mechanism of oxidation for olefins'o which is presumably applicable t o polydienes. It is our purpose here to ascertain for polystyrene t o what degree this mechanism is applicable, and to determine the variations and finer (9) L. A. Wall and M. Magat, J . chim. phys., SO, 308 (1953); Modern PIastica, 80, 111 (1953).
(IO) L. Bsteman, Quart. Rm., I,147 (1954).
Sept., 1956
OXIDATIVE
DEGRADATION O F STYRENE AND
a-DEUTEROSTYRENE POLYMERS
1309
details that may be peculiar to this polymer. The mechanism'0 for olefin oxidation, for which a great deal of experimental evidence has been accumulated, is Initiation Thermal Photo Labile substances Radicals Atomic radiation j Propagation R- 0 2 + Rot. ROr RH ROsH RTermination 2R- + R- Rot- --f Stable inactive
1
++
+
2 Rot- +
+
\
I
products
I
ks kr k4
ks
ks
where I is the over-all rate of initiation and the k's are the specific rate constants for the indicated reaction. For the system under study in this investigation, photo initiation is evidently the radical producing process. For a long kinetic chain, i.e., propagation occurring much more often than termination, and using the usual steady-state assumption, the following expressionlo is obtained relating the consumption of oxygen to the rate constants and concentrations of reactants
I / T x 103. Fig. 5.-Plot of -log rate constants versus 1/T: 0, fast reaction; o*slow reaction.
However, we can conclude tentatively that the over-all oxidation mechanism in polystyrene is similar to that presented above and that at atmospheric oxygen pressure the reaction is independent of oxygen pressure. The post-effect as well as the decreased solubility observed in oxidized polystyrene require further explanation. Attempts to detect chemically the hydroperoxFortunately, a limiting case exists for high pressures of oxygen when reaction 2 is much faster than ide groups assumed to be formed during oxidation of reaction 3, which leads to a much simpler relation- polystyrene have indicated none or very few such structures. Extensive work directed toward the ship preparation of polystyrene hydroperoxide in appre-d(02)/dt High Press Oa = (I/k,)'/z ks(RH) ( 2 ) ciable quantity has been unsuccessful, although This equation is only applicable to oxidation at good yields of hydroperoxide are obtained when the oxygen pressures where the process is independent rings in the polystyrene are alkylated with isoproof oxygen pressure. It is interesting to note the pyl groups. These latter resultsL2are interpreted as wide variation in behavior of olefins in this re- due to either a steric hindrance for the hydrogen abspect.'O In general, rapidly oxidized materials straction process, k3 in the above mechanism, or a show oxygen dependence up to 800 mm. of oxygen steric inhibition of resonance in the tertiary radical pressure. On the other hand, mere difficultly that is produced by step 3. This would mean a oxidized oletins, for instance hexadecene-1, show no greater activation energy for the abstraction of the dependence above 1 mm. oxygen pressure at a tem- tertiary hydrogen atoms along the polystyrene perature of 45". Intuitively, then, one has some chain compared to that in isopropylbenzene. basis for assuming that oxidation of polystyrene Our investigation indicates that there must be would be independent of oxygen pressure in the small amounts of hydroperoxide structures in the range of conditions used in this study. oxidized polymer even though ordinary chemical If this assumption is valid, then according t o methods fail to detect them. For instance, a polyequation 2 one should obtain a retarded rate due t o styrene powder that was oxidized as a solid for the deuterium isotope effect on the rate constant about 48 hours under an ultraviolet lamp catalyzed k3, provided oxidation normally proceeds through the polymerization of methyl methacrylate a t 47". the abstraction of the tertiary hydrogen atom. The resulting solid was insoluble in benzene indiSince the oxidation is a photo-induced reaction, cating a cross-linked material. Also, the post-efit is assumed that no isotope effects would occur in fect in this study as well as the post-effect on the the initiation process. It is more diflScult to rule viscosity of y-irradiated (in air) carbon tetrachloout effects in the termination process except to say ride solutions of polystyreneg seem most readily exthat any effects would be expected to be small. Dis- plained by a hydroperoxide decomposition. The sociative processes in cmpletely deuterated small activation energy for the post-eff ect herein reported molecules, such as ethane, have been observed t o is in the range for peroxide decomposition, although be more rapid" than in completely undeuterated considerably lower than that for peroxides stable a t species. room temperature. It seems therefore reasonable (11)
L. A. Walland W.J. Moore. J . Am. Chsm. Soc., 74,2840 (1951).
(12) D. J. Meta and R.B. Mesrobian, J . Polymer Sci., 16,345 (1955).
LEOA. WALL, MARYR. HARVEY AND MAXTRYON
1310
to deduce that such hydroperoxides are produced but that they are quite unstable, presumably due t o steric factors, and hence cannot be readily isolated. The resolution of the over-all post-irradiation phenomenon into two first-order components indicates two independent processes that produce structures highly absorbing in the ultraviolet spectral region. I n such a complex system two different hydroperoxide or peroxide decompositions would afford a ready explanation. However, other possibilities exist. For instance, although the fast reaction is likely to be due t o a decomposition of a peroxide of the type OOH
An
n v
w
the slow reaction, which is actually very slight, may be due to diffusion controlled reactions of relatively long lived free radicals or even thermal oxidation of some intermediate such as an aldehyde structure. There remains also the question of the insolubility of partially oxidized polystyrene. The decomposition of such a hydroperoxide discussed above should lead to chain scission and hence lower molecular weight and greater solubility. A cis-trans isomerization13 is another possibility. Some interesting s t ~ d i e s ' ~of~the ' ~ effect of sunlight on benzalacetophenone H H
c=c-c=o
Vol. 60
enough wave length to accqunt for that produced by the post-effect, although it is estimated that the extinction coefficient is of the correct order of magni tude. Hence, although this structure is probably not correct for the product of the dark reaction, this type of structure is indicated. The cross-linking of polystyrene that occurs during oxidation may be the result of simple photolytic dissociation producing radicals, which combine, and hydrogen atoms.2b This process would be expected to increase as more absorbing oxygenated groups were formed in the polymer. Such a process would be similar to the cross-linking produced by atomic radiation. 16-18 The processes occurring during the photo-induced oxidation of polystyrene appear to involve three different stages, which are presumably taking place simultaneously under the usual oxidation conditions. Stage 1.-During exposure to light the chain mechanism discussed above is operating, producing hydroperoxide structures in the polymer chain. H
0 0 Polymer
+ O2+M~~Hz-C-CH~-
--CHz-Cw
The hydroperoxide concentration presumably reaches a small steady value. Stage 2.-During dark storage the peroxide or peroxides decompose thermally to produce a group highly absorbing in the ultraviolet spectrum. H
appear to be pertinent to this point. The ultraviolet spectrum of this compound changes drastically when the material is irradiated with sunlight. l 4 The absorbance in the region 270 to 350 mp decreases while in the region 225 to 260 mp the absorbance increases. The changes are considerably smaller in the latter region. The interpretation of this phenomenon made by the earlier authors14 is that a dimerization was occurring, destroying the conjugation responsible for the longer wave length absorbance. However, more recently it has been shown to be the result of a photo trans-cis isomerization16 and the cis-benzalacetophenone was isolated. This material is thermolabile and bright yellow in color. Several thermal cis to trans conversions have been reportedla having activation energies of 25 kcal. and low frequency factors of about lo4. This effect is highly reminiscent of the removal of the post-eff ect by subsequent irradiation mentioned here and previously.3 It is suggestive that the structure of benzalacetophenone is closely related to that for polystyrene and is a reasonable one to expect from the decomposition of the speculated hydroperoxide of polystyrene. Unfortunately, the absorption of benzalacetophenone is not a t a long
The conjugated structure shown on the right is probably not correct but is representative of a likely type of group. It is felt that numerous substances or structures are actually present and hence there is difficulty in interpreting the observed spectra. I n this respect we have evidence (see Fig. 4) for two processes contributing to the dark reaction. The possibility exists that one of these processes is a thermal cis-trans isomerizationla and the result of a small concentration of a cis-isomer produced during the photo stages of the process. Stage 3.-Finally, on subsequent exposure of the stored polymer, several different reactions
(13) G. M. Wynian, Chem. Reus., 66, G25 (1953). (14) N. H. CroIiiwt.11 and W. R. Watson, J . Org. Chern., 14, 411 (1949). . 1 2 , 4090 (15) R. E. Lutz and R. H. .lordan, J . A m . C h ~ m Roc., (1950).
(16) M . Dole, C. D. Keeling and D. G . Roso, J . A m . Chem. Soc., 7 6 , 4304 (1054). (17) E. J. Lawton, A. I f . BuPclie and H. J. R d w i t , N a l u w , 172, 76 (1 953). (18) A. Charlenby, ibid., 111, 167 (1Y53).
PROTON MAGNETIC RESONANCE OF HYDROGEN BONDING OF PHENOL
Sept., 1956
occur to cross-link the polymer and to reduce the absorption in the ultraviolet. H wC=C-C
/’
00
r+
Photolytic decomposition
\-+
Further oxidation
I
+
trans to cis isomerization
All three processes shown above are quite feasible. Photolytic decomposition could readily remove the absorption and also lead to cross-linking. The structure written would be highly susceptible to
1311
oxidation and by this process the removal of the ultraviolet absorption could occur. Finally, the isomerization process found with benzalacetophenone provides a very facile process that would decrease the ultraviolet absorption. There are many points, such as the over-all rate of oxygen consumption, that need further investigation. Further work on other deuterated styrenes is now in progress. However, the basic features of the oxidation of polystyrene and associated reactions, although quite complex, seem to be understandable in terms of current mechanisms.
PROTON MAGNETIC RESONANCE STUDIES OF THE HYDROGEN BONDING OF PHENOL, SUBSTITUTED PHENOLS AND ACETIC ACID’ BY CHARLES M. HUGGINS, GEORGE C. PIMENTEL AND Department of Chemistry and Chemical Engineering, University of California, Berkeley, California,
JAMES N. SHOOLERY Varian Associates, Palo Alto, California Received March 88,1966
The H-bond shift of the proton magnetic resonance was measured for phenol, 0-,m- and p-chlorophenol, 0-cresol, and acetic acid over the concentration range accessible in CCll solution and, for acetic acid, in acetone solution. The proton resonance behavior can be correlated with the known B-bond properties of these compounds. The effects of steric hindrance, intramolecular H-bonding and H-bonding with the solvent are observed. The correlations among H-bond shifts, infrared frequency changes and H-bond energies, are examined.
Proton magnetic resonance measurements have shown that a significant chemical shift accompanies the formation of a hydrogen b ~ n d . ~(Hereafter ,~ this shift will be called an H-bond shift.) Hence nuclear magnetic resonance (NMR) measurements potentially offer a means of detecting and studying hydrogen bond (H-bond) formation. To investigate the relationships between the NMR properties and the deductions based on other physical measurements, we have made proton resonance studies of several substituted phenols: phenol, (I); ocresol, (11): p-chlorophenol, (111); m-chlorophenol, (IV); and o-chlorophenol, (V); and of acetic acid, (VI). Experimental The experimental technique and equipment have been described.3h For the phenoh, the magnetic field was 0400 oersteds and the precession frequency 40 megacycles except for a few check measurements a t 7050 oersteds and 30 megacycles; for acetic acid, the magnetic field was 7050 oersteds. Sample temperature was about 28”. The compounds were each studied in CCI, solution from the limit of detectability i n dilute solution to the solubility limit. A reference solute, cyclohexane, was included (5% by volume) in the phenol solutions to avoid the necessity for a correction for change of the bulk diamagnetic suweptibility and to nullify instrument drift. Specific solvent effects might, of course, interfere with th? use of an internal standard. They are not likely to be piesent when both solute and standard are present in the same solvent environment and the solvent has high symmetry. Acetic acid was studied i n ace(1) This material was submitted in partial satisfaction of t h e requirements for t h e Ph.D. degree b y C . M . Huggins, University of California. 1955. ( 2 ) U. Liddel and N. F. Ramaey, J . Ckern. P h ~ s . 19, , 1008 (19.51); -1. 1’. Arnold and hl. E . I’ackard, i b i d . , 19, 1608 (1951). ( 3 ) (a) It. S. Giltowsky and A . Saika, ;bid.,11, IC88 (1953); (b) C. RI. IIuggiiiR, C;. C. I’iiivmtpl and .I. N . Slloolrry, i6id., 13, 124.1 (1955).
‘
tone solution as well as i n CCI,. The methyl group proton resonance of acetic acid was used as reference. All chemicals were of analytical or reagent grade. Their infrared spectra were examined for spurious absorptions.
Results Phenol Derivatives.-Each solution displayed only one proton resonance which could be attributed to the 0-H proton. For each compound the resonance attributed to the 0-H proton has a marked concentration dependence. This behavior is that expected from a system in which protons are present in various molecular species which are in rapid equilibrium. The concentration dependence reflects changes in the relative amounts of these molecular species. The measured resonance shifts, 6, and solution compositions are shown in Figs. 1 and 2. The dimensionless factor 6 is defined in accordance with the usage of Gutowsky and Saikae3* This convention differs in algebraic sign from the “shielding” definition, often designated s or u. H,
-H H,
x
X 106
106
VI.
H , = reference proton resonance field (cyclohexane) H = proton resonance field of 0-H proton 6 is calculated from vr, the known resonance fre-
quency (40 or 30 megacycles), and the value of (v, - Y) obtained by the side-band audio oscillator measurement (see ref. 3b). The accuracy of measurement of 6 was limited at high concentrations by interference of the phenyl proton resonance a t about 6 = 5.4, a t low concentrations by the signalto-noise ratio, arid for compound 111 a t all corI(w1trations by nil unexplained txoadening relative to the other compounds.
