NOTES
2026
mentioned that the colored compound formed by illumination of 2- (2’-nitro-4’-cyano benzyl)pyri dine in ether-isopentane-ethanol solutions exhibits siinilar absorption peaks to those of the T H complex shown in Fig. 1 and that Wettermark and have suggested that the color is due to an ionic species produced by dissociation of an aci form of the nitro compound. Bleaching. A solution of 0.02 M T H in purified acetone was subjected a t room temperature to 30 cycles of darkening and quick bleaching with no measurable loss in acid concentration. Since the total amount of complex formed was estimated to be a t least 5 mmoles/l. as calculated froin the sum of the absorbances developed and using E = 1870, this suggests that the bleaching reaction may be predominantly a reversal of the color-forming reaction. At somewhat elevated temperatures and in the presence of continuous illumination, some decomposition occurs. The decomposition reaction consists of decarboxylation since 1,3,5-trinitrobenzene (TNB) was found to be the principal product, as identified by infrared spectra and mixture melting point determinations. The rate of decarboxylation under illumination was studied using various initial concentrations of TH, ranging from 0.01 to 0.05 M , and was found to be first order with rate constants of 5.05 X lo-? sec.-? a t 3 5 O , 14.3 X lo-? set.-' at 4 5 O , and 69.7 X lo-? sec.-l a t 56’ (b.p. of acetone). It is interesting to note that solutions of T H in acetone showed no change in acid concentration upon being refluxed in the dark for more than 1day. The bleaching reactions may be summarized thus
€ITAHi!HA
(I) bleaching to form acid, possibly reversal
1 A\
+
(11) decarboxylation, ie., T S B C02 solvent
+
An attempt was made to determine the proportions contributed by paths I and I1 to the total bleaching. This involved comparing the initial darkening rate a t room temperature with an extrapolated value of the decarboxylation rate. This leads to the very tentative estimate that reaction II represents less than 1% of the bleaching reaction a t 25’. T h e Journal of Physical Chemistry
Proton Magnetic Resonance Spectrum of
1,1,2,2,4,4,5,5-0ctadeuteriocyclohexane
by Xorbert Muller and Peter J. Schultz Department of Chemistry, Purdue University, Lafayetle, I n d i a n a (Received February 17,1.964)
It is known1 that the proton magnetic resonance spectrum of cyclohexane a t low temperatures shows much partially resolved hyperfine structure arising from magnetic nonequivalance of the axial and equatorial hydrogen atoms. Because of inadequate resolution it has not been possible to obtain accurate values for the chemical shifts of the axial and equatorial species or for the various spin-spin coupling constants. The chemical-shift values are needed to test predictions based on theoretical models which have been proposed to explain n.m.r. data21a and also to make it possible to refer shifts found for substituted cyclohexanes4~5to logical “standard” values. With this in mind we prepared 1,1,2,2,4,4,5,5-octadeuteriocyclohexane (ODCH) which was expected to yield a proton CD2-C D2
/ \
CH2
\ CH2
/
CD2-CD2
resonance spectrum much simpler than that of ordinary cyclohexane.
Experimental The preparation of ODCH is described in detail elsewhere.‘j Briefly, the procedure used was to exchange all hydrogens of 1,4-cycl~hexanedione~ by refluxing with three portions of D 2 0 to which a trace of acetic anhydride had been added to provide a catalyst. The extent of exchange was monitored by n.m.r. spectroscopy, and it was estimated that the diketone eventually obtained was 96% deuterated. This material was converted to the corresponding bisdithioketal (1) I?. R. Jensen, D. S.Noyce, C. H. Sederholm, and A. J. Berlin, J . Am. Chem. SOC.,82, 1256 (1960); 84, 386 (1962). (2) J . I. Musher, J . Chem. P h y s . , 35, 1159 (1961). (3) A. G. Moritz and N. Sheppard, Mol. Phys., 5 , 361 (1962). (4) N. Muller and W. C. Tosch, J . Chem. P h y s . , 37, 1167 (1962). (5) E. L. Eliel, M. H. Gianni, and T. H. Williams, Tetrahedron Letters, 741 (1962). (6) P. J. Schultz, M.S. Thesis, Purdue University, in preparation. (7) Obtained from Columbia Organic Chemicals.
2027
NOTES
CHz--5 I
I
CHz--S
CD2-CD2
\ /' C /'\
EkCH2
'/ C /\
CDz-CDz
!I
S-CHz
with ethanedithiol and anhydrous zinc chloride.8 The product had the same melting range (189-191') as material obtained earlierg from orldinary 1,4-cyclohexanedione and produced a similar n.m.r. spectrum with the obvious exception that only a very faint broad peak was found with a chemical shift (-2.33 p,p.m. from tetramethylsilane internal reference) characteristic of the cyclohexane ring protons of the bi~dithioketal.~The protons on the sulfur-containing ring produce a sharp peak at - 3.4 1 p.p.m. The bisdithioketal was reduced to ODCH with W-2 Raney nickel in methylcyclohexane, and the product was isolated by vapor chromatography.6 The behavior of ODCH during chromatography was identical with that of ordinary cyclohexane used in preliminary runs. For n.m.r. measurements 0.17 ml. of ODCH with 0.02 ml. of tetramethylsilane were dissolved in 0.31 ml. of carbon disulfide in a thin-walled sample t)ube which was then sealed under vacuum. For the isotope shift determinatilon the tube was opened, about 0.01 ml. of ordinary cyclohexane added, and the tube resealed. All n.m.r. spectra were obtained as described in ref. 4 .with oscillator frequency 56.4 Mc.p .s.
Analysis of the Spectrum If the magnetic moment of the deuterium nucleus were zero, ODCH at low temperatures would produce a simple four-line spectrum of type AB.'O Allowing for indirect spin-spin coupling between hydrogen and nearby deuterium nuclei, further splitting of each proton signal is expected. Each hydrogendeuterium coupling constant ( J H D ) is 0.153 times as large1' as the analogous J H H . With reasonable values12 of the latter, one may predict the following approximate coupling constants for hydrogen and deuterium atoms bound to adjacent carbon atoms J H e ~ S e
J
b
Figure 1. (a) Proton resonance spectrum of
1,1,2,2,4,4,5,5-octadeuteriocyclohexane a t -95' and 56.4 Mc.p.s.; ( b ) calculated AB spectrum using parameters derived in the text.
The spectrum recorded at -95' and shown in Fig. l a is in qualitative agreemeiit with this prediction, but all the signals are somewhat broader than expected. The peaks at -98.1 and -85.5 c.p.s. should belong to the equatorial protom1 The expected pair of lines from the axial protons is so severely broadened that only the more intense peak produces a maximum, a t -68.5 c.P.s., with the weaker peak appearing as a shoulder on the high-field side. Reducing the temperature to - 110' resulted in no appreciable sharpening of the signals. Thus, it appears that the observed line widths are not due to incomplete "freezing-out" of the conformational inversion. Since the four-line spectrum of an AB system is symmetric about its midpoint, analysis required locating only three of the peaks. For ODCH, the midpoint of the spectrum should lie halfway between the two strong maxima at -85.5 and -68.5 c.P.s., that is at - 77.0 C.P.S. This is rather surprising, since literature v a l ~ e s ~for ~ ~the ~ ' chemical ~ ~ ' ~ shift of ordinary
H = ~J H ~~ DGS,~0.16 C.P.S.
JH.D% 1.8 C.P.S. The subscripts a and e refer to axial and equatorial sites, respectively. It follows that there should be a small but perceptible broadening for the pair of signals from the equatorial hydrogens and a coiisiderably more pronounced broa,dening for the axial hydrogen signals.
(8) J. F. Tinker, J . Org. Chem., 16, 1417 (1951). (9) N. Muller and R. C. Jorgensen, unpublished results.
T h e bisdithioketol prepared by Mr. Jorgensen had the following properties. Anal. Calcd. for CioHisSd: C , 45.4; H , 6.1; S,48.5; mol. wt., 264.4. Found: C , 45.23; H , 6.12; 8,48.51; mol. wt., 279.5. (10) P. L. Corio, Chem. Rev., 60, 374 (1960). ( 1 1 ) H. S. Gutowsky, M. Karplus, and D. M. G m n t , J . Chem. Phys., 3 1 , 1278 (1959). (12) A. C. Huitric, J. B. Carr, W. F. Trager, and B. J. Nist, Tetrahedron, 19, 2145 (1963).
Volume 68, Number 7
J u l y , 1964
NOTES
2028
cyclohexane at room temperature, corrected to 56.4 iVlc.p.s., vary from -80.0 to -81.0 C.P.S. depending on solvent. The discrepancy is partly the result of an unexpectedly large deuterium isotope effect and partly of the temperature dependenceL4of the cyclohexane shift, which was not noted in the earlier work.’ The isotope shift appears very clearly in the spectrum of a mixture of ODCH and ordinary cyclohexane a t room temperature. The broadened peak of the deuterated species lies 1.4 i 0.3 C.P.S. or 0.025 p.p.m. upfield from the sharp signal of the normal species. Similar but smaller deuterium effects havc been found in the proton spectra of substituted ethylenes, while shifts of comparable magnitude were reported for compounds of the type HD2CX compared with HaCX.16 The temperature dependence of the chemical shift was studied for ordinary cyclohexane a t about 30 vol. yo in carbon disulfide with tetramethylsilane as internal reference, over the range from 80 to -33”, in which the cyclohexane produces a single, sharp peak. The peak moves upfield linearly as the temperature is reduced, a t the rate of 0.011 * 0.004 c.p.s.jdeg. A similar temperature coefficient has since been reported14 for cyclohexane in a mixed solvent. Since the peak from ODCH is centered at -79.1 i 0.3 C.P.S.a t 3 5 O , the value of -77.0 C.P.S.for the average chemical shift at -95” is not unreasonable. Retmning to the analysis of the low-temperature spectrum, one readily findslO for the geminal H-H coupling constant J H H = 12.6 c.P.s., very near the value reported for methane.16 The difference in chemical shift between the axial and equatorial protons is 26.7 C.P.S. or 0.474 p.p.m. The uncertainty of this value is difficult to estimate but should not exceed 0.01 p.p.m. Although it is a little larger than the 0.455 p.p.m. reported by Jensen, et aL,l it lies quite significantly below the 0.525 p.p.m. estimated by Musher2 using the same data.17 The theoretical AB pattern calculated with these parameters is shown in Fig. lb. Extrapolating back to ordinary cyclohexane a t room temperature one finds that the “standard” shifts for axial and equatorial hydrogen should be -1.19 and - 1.66 p.p.m., respectively, although values differing (13) G. V. D. Tiers, J. P h y s . Chem., 62, 1151 (1958). (14) J. L. Jungnickel, A n a l . Chem., 35, 1985 (1963). (15) E. I. Snyder, J. Phys. Chem., 67, 2873 (1963), and work cited there. (16) M. Karplus, D. H. Anderson, T. C. Farrar, and H. S. Gutowsky. J . Chem. P h y s . , 27, 597 (1957). (17) Since this study was completed, Professor F. 4.L. Anet has told us of unpublished work on cyclohexane-dii which yielded a value of 0.478 p.p.m. for this shift difference, in excellent agreement with our result. We wish to thank Professor Anet for this information.
T h e Journal
of
Physical Chemistru
by perhaps 0.02 p.p.m. from these might be obtained if a solvent other than carbon disulfide were used.
The Thermal Conductivity of Hydrogen-Helium Mixtures’
by Robert S. Hansen, Robert R. Frost, and James A. Murphy Institute for Atomic Research and Department of Chemistry, Iowa State University, A m e s , Iowa (Received February 19,1964)
The anomalous peaks recorded in a conventional gas Chromatograph on passage of a hydrogen sample with helium carrier gas have been discussed by Schmauch and Dinerstein2 and attributed to a minimum in the thermal conductivity of hydrogen-helium mixtures. However, the only published data concerning the variation of thermal conductivity of such mixtures with composition are due to Barua3; the data are insufficiently extensive to document a niinimuni and none is shown in Barua’s graphical presentation. It, therefore, seemed worthwhile to determine this variation in more detail, and also to investigate the possibility of representing it theoretically. Schmauch arid DinersteinZ have also shown that a simple model for heat transfer in the chromatograph thermal conductivity detector suggests that the bridge response should vary linearly with the reciprocal of the gas thermal conductivity. The measurements in the present work are based on this principle. The power supply, bridge circuit, and flow controller of a Research Specialties Co. 600 series gas chromatograph was used together with a four filament flow type detector modified by replacement of the two reference side filaments with 20-ohm precision resistors. Circuit response was monitored by a Moseley x-y recorder. The detector was immersed in an ice bath during all experiments. Gas mixtures were prepared in a previously evacuated vessel and displaced into the system with mercury, Commercially available hydrogen, helium, neon, methane, oxygen, nitrogen, and argon were used without further purification; with the exception of methane (99.0%), H2 (99.5%), and 0 2 (1) Contribution no. 1462. Work was performed in the Ames Laboratory of the U. S. Atomic Energy Commission. (2) L. J. Schmauch and R. A . Dinerstein, A n a l . Chem., 32, 343 (1960). (3) A. K. Barua, T n d i a n J. Phys., 34, 169 (1960).
