June, 1960
INTI~AMOLISCULAIL HYD~~OG BONDING EN IN MONO-AXOSS
mined partial pressure mlues and those calculated by the total pressure method outlined above is quite sat-sfactory. Calculated results generally agree with experimental results more closely for systems deviating only slightly from ideality than for those systems which are extremely non-ideal. It should be mentioned, however, that experiniental difficulties often are greater for systems h)wiiig large positive deviations from Raoult's Law, aiid it is possible that part of the discrepancy is a result of experimental error. I t is well known that total pressure measurements can be made with grcater armracy than partial pressure measurements. The syFtem ethanol-carbon tetrachloride (shown in Fig. 6) is of special interest. Jf the experinicntal results are correct, it would appear that the carboii tetrachloride does not approach Raoult's Law as a limiting law. This could be interpreted :is iiidicat ng the formation of aggregates of ethanol molecules even a t low ethanol concentrations. Experinie i t s are being undertaken to check both partial am1 total vapor pressure results for the ethanol-carbon tetrachloride system aiid other alcohollion-polar solvent systems. The above aiethod, in addition to yielding a c ~ ~ rate parti,il pressure values, is extremely rapid and easy to uce. It is believed that wide-spread use of :I conr-cnic.nt total pressure method, such as the one preseiitcd hcre, will ultimately lead to the accuinu-
767
latioii of a much greater volume of accurate thermodynamic data for solutions than would be possible using the more difficult conventional total pressure-partial pressure techniques. The program developed here is ayailable to any worker in the field through the Unirersit'y of Oklahoma Computer Laboratory. IBM 650 computing time for the systems reported ranged from less thaii five minutes to a maximum of about 12 minutes. .Ilthough no results are given here for systems iu which one of the compoiieiits dimerizes in the vapor phase (acet.ic acid-carbon tetrachloride, for example), the program has been applied successfully to such systems. These results will be report,ed in a forthcoming paper. I n addition, t,he program is being modified to allow culculation of partial prcss u m for systems in n-hich both rompoiients dimerize in the vapor phase aiid in ~ ~ - h i(c~ho s , 5climerizatioii occurs also. Acknowledgments-The author ir indebted l o I'rofessor William I-iaraiit tilid I \ h . Margaret Crawley of the University of Oklahoma Computer Laboratory for their advice and assistance in de\-eloping the method of computation. The many hours of IBL\I 650 computer time doiiated by the i 1,:iboratory niade this htudy posrihlc. In addition, the author Jyishes to expres~his :ippreciatioii for the financial aid given him liy the Sat,iorial Science Foundation during tlic early phases of this \\-ark.
PItO'l'OS MAGSETIC HESOSASCE STUDIES O S ISTlL1RlOLECULhlt HYDROGES BOSDISG I S 31OSO--4SIOSS OF STE1iIC;lLLT HISDERED SUCCISIC ,ICIDS
The proton magnetic resonance spectra of the monopotassium salt's of certain alkyl-substituted succinic acids have beer] investigated. The results obtained indicat'e that there is a st'rong intramolecular hydrogen bond in the mono-anions of those dicarboxylic acids n-hich exhibit anomalously high ratios between t,heir first and second dissociation constants.
Introduction It is gwerally accepted' that the high ratio of nialeic acid is due to the formation of a strong intramolecular hydrogen bond in its nionoX-ray4 and neutron diff ractioii; anion. studies hs,vc shon~nunequivocally that there is a short (0 . . . H . . . 0'= 2.42-2.44 A.)aiid, a t least statisticall.y, symmetrical intramolecular hydrogen bond in solid moiiopotassium maleate. Proton magnetic resonance (p.m.r.) studies on monosodium and monopotassium maleate in dimethyl sulfoxide (1) Atomic E h e r g y Department, M E A , T:ister%s, Sweden. (2) H. C. Brown, D. H. McDaniel and 0. Hifliger in E. A . Braiide and F.C. Na:hod, "Determination of Organic Structures by Physical c Press, Inc., h-ew York, N. Y., 19.55. l m * l l J . I). Dunite and I,. 1,;. Org(,I, J Ciicni. h'oc., 3 i 4 0 (1953). (-I)S. Darl jIv in 1). H a d z i and H. \V. Ttiornlmon. "Hydrogen Bonding,'' l'erqaiiijju Pres&,Luiidun, 1%9. ( 5 ) 3. I V . i'etcr3on i ~ i i d11. A . L e v y ,
J . Cliem.
l'/ty,s., 2 9 , !I i h ( 1 ' , ~ 5 6 ) .
(DMSO) solution6 have shown protoii signals at very low fields (6 = -15.03 and -15.40, respectively, referred to external water standard) which were ascribed to the internal hydrogen bond. Recently, Eberson7 has described a number of rac-a,a'-dialkyl- arid tetraalkylsuccinic acids with extremely high IC1,'& ratios (in 50% aqueous etha,riol by weight), which were explained in the same 11-ay as the maleic acid ratio. However, as infrared data8 were not quite decisive on this point, it was hoped that p.m.r. studies would give further evidence. Experimental The p.ni.r. measurements were made n-ith a Varian I-4300 spectrometer operating a t a fixed frequency of 40 M c . / . arid equippcd with a TT:uian T7-K 3506 Super Stabilizcr . ~~
( 6 ) 6 . F o r s h , ibid., 31, 852 (1959). ti) 1,. Ebi.r.-on, .[cia Chern. S c a d , 13, 2 1 1 (195iI) Is) 1,. I;bl'r,SUIl, i h l d . , 13, '24 (1989).
L. EBERSON AND S. F O ~ ~ S E N
768
Vol. G4
w
P
Q m
Fig. 1.-€'.in r. spectrum of a solution of the monopotasbium salt of mc-a,a'-di-(/-hutyl)-succinic acid (11)in DMSO (mole fracxtion 0.13). The shifts are given relative t o external water. The assignments are: 6 = -14.66, proton in a strong intramolecdar hydrogen bond; 6 = 2.42, the solvent signal, 6 = 4.07, the protons of the t-butyl groups. The small peak close to the solvent signal is one of the C13H3-satellites surrounding the strong methyl bignal.
-
-
__
-
-
TABLE I
-
--_
- _ ~ - ^I
L H E M I C h l ~bHIFTS ( 6 ) I N J l O N O N ~ T A S S l U h l bALTS OF bUBSTITVTED bUCCINIC ACIDS IS ULCIWJSOLUTIOX (EXTERNAL WATER
STAKDARD )
-._^..
Ahsignmcnt of signals8 for ixoton not involved
6 for proton
111 Llll
Salt (z = mole fraction solute)
KV.
Monopotassium maleate (5 = 0.0G)6 Monopotasbium fumarateb ,\lonopotassium mc-a,a'-diisopropylsucdnate (5 = 0.1) Monopotassium rac-a,a'-di-(t-butyl)-succinate (z = 0.1) Monopotassium rac-a,a'-dicyclohexylsuccinate (z = 0.1) RIonopotassium tetraethylsucciriate (z = 0.1) Monopotassium meso-a,a'-diisopropylsuccinate (z = 0.04) hIonopotassium meso-a,a'-di-(t-butyl)-succinate (z = 0.04) Monopotassium meso-a,&-dicyclohexylsuccinate (5 = 0.03) a In 50% aqueous methanol by weight.' Insoluble in DMSO. The sample colls used were thin-walled glass tubes with a good filling factor. Measurements against an external water standard were made in a cell made of two coaxial glass tubes containing water in the narrow annular space and the solution to be measured in the central tube.9 The resonance shifts were determined in c./sec. by the audio-
I I1 I11 Iv V
VI VI1
ApKa
6.60 1.82 7.78 9.54 7.76 6.64 2.12 1.86 2.29
:..+^.."^I lllurllla,
hydrogen bond
.- " - . - & . . - - " I 111 LL"
IIILCI'III,
hydrogen bond
f I....
SiGO
-15.40
2.42
-14.42 -14.66 -14.42 -15.02
2.42 2.42 2.42 2.42 2.42 2.42 2.42
0. 75 (broad) 0.75 (broad) 0.75 (broad)
the corresponding racemac acids which readily could be obtained in solutions with mole fractions of 0.1-0.2.
Results and Discussion The monopotassium salts of four acids Tt,ith crnnllnnn., mr,thnA uP..,l,tt-PonLopA 3nn very high KI/K~ratios, ViX., rac-c-u,a'-diisopropvl-, rac-a,a'-dicyclohexyl- -and CD Audio Oscillator. The shift measurements were usually rac-a,a';di-(t-butyl)-, reproducible to within A 1 c./sec. The resonance shifts tetraethylsuccinic acid (1-117) wcre chosen for were transferred to thc field independent unit 6 defined by measurement and the monopotassium salts of the the expression corresponding meso acids (T'-T'TI) T\ ere incluc!~.d for comparison (the mmo acids have normal K I I K 2 ratios, which are in approximate agreement with The sample temperature was 25 =k 1' KO attempt was those calculated by the Ilirk~ood-T~~estheimer made to make corrections for the macroscopic susceptibilitv method7). The results of the p.m.r. nieasurements effect, as no reliable value for the susceptibility of DMSO on DMSO solutions of the salts are summarized could be obtained. Materials.-The DRISO was purified by freezing out a in Table I and a topical spectrum is shown in 99.5yo pure atoc.1; three times The monopotassium salts Fig. 1. were prepared as devribed earlier.* The soluhilitiee of the P.m.r. spectra also uere ruii 011 water and potassium salts of the /)Lese acids are much less t1i:tn those of incthanol solutions of compounds I-Il*, but no ________ sign:ilb on the low-field side of thct sol1 ciit signal ( W R ililiad Glas= Co., Laidis>iilc, N. J., U. S. ,i. ,,G'\IUGLlLJ
n l ~ L . ~ a111n L~ l l~V DIUL IILYLIU
U
,.a;nm 'L""'6
I,
'U
IIU.11b1"
A
WL'>'"IU
I""
June, 1966
CATALYTIC ISOMERIZATION OF CYCLOPBOPAKE
could be detected. This does not, however, rule out the possibility that an intramolecular hydrogen bond persists to some extent in these media as a rapid proton exchange between solvent and solute mould be expected to broaden and flatten the proton signal from the intramolecular hydrogen bond with a simultaneous merging with the solvent signal. The corresponding free acids of I-IV in DMSO gave rise t,o a very broad band a t about 6 = -5.0, and accordingly any internal hydrogen bond that might ha\.e been present must be disrupted in this solvent (cf. ref.6) It is widely accepted that the position of a proton resonance signal is shifted toward lower applied fields when the hydrogen atom takes part in a hydrogen bond.l0 The magnitude of the shift can be taken as a rough measure of the hydrogen bond strength.’” The difference in the behavior of the racemic and meso acid salts and an observed constancy of the alkyl proton shifts on passing from the free acid to the nionopotassium salt indicates that in the present case 110 anomalous effect arises from the presence of the cation. One would therefore expect that the proton resonance shift of 6 = -15.40 in potassium hydrogen maleate should be very close to the “lower limit” for 0....H ....0’ hydrogen bonds. Accordingly, the in(10) J. A . I’ople, R‘ G. Schneider a n d H. I. Bernstein, “High-iesolution Nuclear Magnetic Resonance,” McGraw-Hill Book Co., New York, N. Y., 1959.
769
tramolecular hydrogen bond in the salts I-IV should be comparable in strength to that of the maleate mono-anion. The slightly smaller shifts in the case of 1-117 as compared with potassium hydrogen maleate may be an indication that these hydrogen bonds are not symmetrical, but further information is needed to obtain full evidence on this point, e.g., by neutron diffraction data. It is interesting to note that short, strong hydrogen bonds can be formed in suitably substituted saturated dicarboxylic acids without there being any other path connecting 0 and 0’ through which migration of electrons can take place as in the maleate and phthalate mono-anion.“ However, some sort of O=C-C=C-C=O conjugation may be necessary for the formation of a symmetrical hydrogen bond. Acknowledgments.-The authors ~vouldlike to thank Dr. E. Forslind for his kind eiicouragement and generosity in including this work in the general research program of the XMR-group. Thanks are also due to Dr. B. R. Thomas for valuable linguisitic criticism and to Stateiis R5d for Atomforskning, Statens Naturvetenskapliga E’orskningsrid and Statens Tekniska ForskningsrHd which have provided financial support for the NMR-group. The cost of the NMR apparatus has been defrayed by grants from Knut and Alice Wallenbergs Stiftelse. (11) See ref. 4, p . 344.
A GAS ( I H R O ~ ~ , ~ T O G K B P HSTUDk’ IC OF THE CATALYTIC ISOJIEHIZBTIOS OF CYCLOPROPANE’ BY D. W.BASSETT~ A N D H. W. HABGOOD Research Council of Alberta, Edmonton, Alberta, Canada Received December t.9,1950
The general case of a first-order catalytic reaction occurring during elution of a reactant through a chromatographic column is discussed Under conditions of low reactant partial pressure and rapid adsorption relative to the rate of the surface reaction, the fractional conversion of a pulse of reactant passed through a chromatographic column is given by an equation analogous to that for the conversion under similar conditions in a steady-state flow reactor. A major advantage of the chromatogr:rphic technique is that it permits a determination of the extent of adsorption under reaction conditions, and thus of the rate constant for the reaction on the catalyst surface. The method is illustrated by a gas chromatographic study of the catalytic isomerization of cyclopropane on Linde Molecular Sieve 13X. The rate constant of the surface reaction was found to be k = 1.3 X 10’0 exp( -30,000/RT) sec.-l. The heat of adsorption of cyclopropane under reaction conditions was 11.0 kcal. rr,ole-l.
Introduction In recent years, t8hetechniques of gas chromatography have been increasingly applied to the study of heterogeneous cat’alytic reactions. These applications are of two distinct types: those in which gas chromatography is used only as an analytical tool, and those in which gas chromatographic techniques are an integral part of the reaction study. Apart from observations of catalytic reactions occurring during analyses by gas chromatography, reported (1) C o n t r i h t i o n No. 110 from t h e Research Council of .4lberta. Presented at the Division oi Analytical Chemistry Symposium on Gas Chromatography, 137th National hfeeting of the American Chemical Society. Cleveland, Ohio, April, 1960. (‘2) Rr~sear
