Feb ., 1960
NOTES
specimens were identified as belonging to the laminarly twinned habit on the basis of morphology, convergent-light interference figures and Laue diffraction photographs. The latter revealed that the twinning had introduced apparently perfect orthorhombic symmetry, indicating that both components of the twin were present in nearly equal amounts. Measurement of the anisotropy in the plane normal to the b-axes of the separate laminae (i.e., in the plane of the optic axes of the twinned crystals), using two different specimens, gave an average value of 1.09 f 0.01, the crystals orienting themselves preferentially with the magnetic field normal to the plane of twinning. From this information and the assumptions that the chlorate ions were truly uniaxial and that both components of the twinned specimens were present in equal amounts, it was possible to calculate a tentative value for the anisotropy of the chlorate ion: this value was K , , - K I = 1.09/cos 2(34”18’) = $2.99, or about half the value, K , , - K l = +5.9, calculated by Mookherji’*from data on monoclinic potassium chlorate obtained by K r i ~ h n a n . ~ Acknowledgment.-The author is indebted to the Allied Chemical and Dye Corporation which granted him a fellowship for the year 1955-1956. (14) 8.hlooliherji, Acta Crust.. 10, 25 (1957).
RESTRICTED INTERNAL ROTATION IN PROTONATED AMIDES BY E. SPINNER Departnkent of Medical Chemistry, The Australian National University Canberra, B.C.T. Australia Received J u l y SI, 1969
The purpose of this note is to resolve the apparent contradiction between infrared spectral results which have shown that protonation in acetamide, la K-ethylacetaniidelb and ureaIcnatakes place preferentially a t the nitrogen atom,:! and nuclear magnetic resonance data for some Nmethyl-amides in strong acid3 that have been regarded as evidence for preferential protonation at the oxygen atom.3 The 1i.m.r. spectra do not show the signal of the prot’oiit’hat has a,dded to the basic &e; thus there is no direct evidence of a hydroxylic or nitrogenhound prot,on. 0-Protonation was postulated solely to explain the splitting of the signal of the Nmethyl hydrogen atonis, observed frequently, but riot invariably, in protonated N-methyl-amides, the argument heiiig that in a cation H0CIZ.KR’.CH3 rotation about the CN bond is restricted by virtue of its partial3” or complete3bdouble bond character (possibility of cisltrans isomerism, etc.) , (1) (a) E. Spinner, Spectrochim. Acta, 95 (1959); C. G. Cannon, hfikrochim. Acta, 562 (1956); hI. Davies and L. Hopkins, Trans. Faraday Soc., 63, 1563 (1957). (2) The cations of acetamide and urea show a prominent band near 2400 ern.-’ which is characteristic of the a+“ group (R. D. Waldron, J . Chem. Phys., 21, 734 (1953); C. Sandorfy, in “The Technique of Organic Chemistry” (Editor A. Weissberger), Vol. IX, Interscience Publishers, Inc., New York, N. Y . . 1956, p. 515. (3) (a) G . Fraenkel and C. Niemann, Proc. N a t l . Acad. S c i . U . 8.A., 44, 688 (1958); (h) A . Berger, A. Loewenstein and S. Meiboom, J . Am. Chem. S o e . , 81, 0 2 (1959).
275
while in ions I and I1 there is free rotation about the CCO-N bond. However, this latter assumption is fallacious. Even in acetaldehyde, acetyl fluoride, chloride and cyanide, there are barriers restricting rotation14 of 1.08-1.35 kcal./mole, because a CH bond tends to eclipse the C=O bond6; for the restricting barrier in trifluoroacetaldehydea the (somewhat doubtful) very high value of 9.8 kcal./mole has been proposed. Furthermore, rotational isomerism in substances of the type RCOCH2X, due to restricted rotation about the Cco-CCX bond, often has been
+
observed.’ Rotation about the Cco-N bond in ions of the type of I and I1 must be similarly re-
+
+
stricted: an N-CMe or N-H bond will tend to eclipse the C=O hond. In addition, rotation
+
about the K-ChIe bond is restricted: only conforma-
+
tions with staggered C-H and N-H bonds are stable. The possibilities of rotational isomerism thus arising explain all the splittings of proton magnetic resonance bands observed in protonated amides. An N-protonated mono-N-methyl-amide (I) can exist in a “trans” or in a ‘Lgauche’lconformation
+
(trans arrangement of the R-C-N-C
or of the
+
R-C-N-H chain). The K-Me proton signals should be different in the two forms. In the “trans” form of I, which should be energetically
lo
R-C
R--C
)-c
//O
HI
HH ( I ) trans
(I) gauche
R \
0
/I
bH
7 1
\ i /
e1
,,‘ I
H
(11) trans/gauche
7-0
Hb
H
\ ;/H
% \ \
‘H
Ha (I) trans End-on view
H/’
,e, I
R-C
//
‘\.H
H
( I ) gauche
End-on view
//o
C“3
CH3
(11) gauchelgauche
preferred, by analogy with neutral N-methylacetamide,s the proton marked “a” is different from those marked “b,” and two separate 11.m.r. signals are to be expected from them, the splitting being due mostly to interaction with the protons (4) E. B. Wilson, Proc. Natl. Acad. Sei. U . S. A., 43, 816 (1957); C . C . Lin and R. W. Kilb, J . Chem. Phys., 24, 1531 (1956); P. H. Verdier and E. B. Wilson, k b d , 29, 340 (1958). ( 5 ) The author attributea this t o attraction between the C=O and the CH bonds by intramolecular van der Waals-London and induction forces. (6) R. E. Dodd, H. L. Roberts and L. Woodward, J. Chem. Soc.. 2783 11957). (7) S. Mizushima and collaborators, J . Chem. Phys., 20, 1720 (1952); 21, 815 (1953); M. L. Josien and R. Calas, Compt. rend., 240, 1641 (1955); L. J. Bellamy, L. C. Thomas and R. L. %~illiams.J . Chem. SOC.,3704 (1956); 4294 (1957); R. N. Jones and E. Spinner, Can. J . Chem., 36, 1020 (1958). (8) S. hlizushima, et al., J . A m . Chem. Soc., 72, 3490 (1950).
KOTES
276
Tol. 64
f chloride and cesium iodide have been measured attached to the K atom. When the latter are re- a t their respective melting points. placed by deuterons, which appear to interact Data on these compounds, in the range studied, weakly with (possibly on account of the have fast nuclear quadrupole relaxation of d e u t e r o n ~ ~ ~ ) , not been published previously. Materials most of the splitting should disappear, in agreeThe cesium chloride and the cesium iodide used in this ment with observation (cf. the n.m.r. spectra of Ninvestigation were supplied by the Oak Ridge National Labmethyla~etarnide~“ in H2S04and DzS04). oratory. The cesium chloride was resublimed in a Vycor An N-protonated N,N-dimethylamide (11) can sublimation tube. The cesium iodide was of the same purity exist in a “trans/gauche” or in a “gauche/gauche” as the resublimed cesium chloride, and no additional puriconformation; the former should be more stable fication was attempted. Impurities, determined spectrowere Li, 0.1%; K, less than O . l % , Na, less and more abundant. The proton signal from the graphically,$ than 0.1%; and Ca, less than 0.01%. Total impurity was “trans1’ N-methyl group should differ from that estimated to be less than 0.2%. from the “gauche” methyl group, just as the proThe samples were enclosed in platinum crucibles, the heat ton signals are different for the trans and the cis N- contents of which were determined by separate measurements. After filling with sample, the crucibles were evaclCIe group in n neutral N,N-dimethylamide. How- uated, filled with helium, evacuated to approximately 10 ever, the difference between a “trans” and a mm. helium pressure and sealed by platinum welding. ‘‘gauche” position (rotation of 120’) is smaller Measurements and Results than that betmeen a trans and a cis position Heat content measurements were made in a (rotation of 180”); the splitting of the N-Rfe proton signal might therefore be expected to be Bunsen ice calorimeter of the type described in smaller in the (N-protonated) cation than in the detail by Ginnings and Corruccini.4 The calineutral N,N-dimethylamide; this is, in fact, always bration factor for the unit, established electrically, was found to be 270.44 0.66 joule per gram of the case.3a The three hydrogen atoms in a “trans” or mercury. Additional calibration tests made on a “gauche” N-methyl group in I1 (and in a “gauche” synthetic sapphire sample, a t four different temperN-Me group in I) are all different from one another atures, yielded enthalpy values that checked acand should all give different proton signals. Such cepted values5 with less than 0.15% deviation. The experimental procedure used in the caloriband splitting has, however, not been observed. In protonated N-methylacetamide the splitting of metric studies is described. A sample of approximately 18 g., sealed in the the signal is only observed in special favorable conditions (e.g., in 72y0 perchloric acid only after platinum container, was heated to an approximate addition of dioxane) ; in protonated N,N-dimethyl- predetermined temperature in a Marshal furnace acetamide it is not observed in 72Oj, perchloric designed so that it could be shunted to obtain a acid.3a This shows that sometimes the life time region of uniform temperature. The temperature of the isomer may be quite short compared to the of the furnace was measured with a platinumtime required for the completion of a nuclear mag- platinum 10% rhodium thermocouple previously netic transition (the exact environmental condi- calibrated against a National Bureau of Standards tions being critical). Such short life times are thermocouple. Once it had been assured that the much more readily explained if the isomerism is entire sample and container had reached the test attributed to (moderately) restricted rotation temperature, the container was dropped into the calorimeter, the system was closed and the heat + about a single bond (like a C-N bond in I or II), evolved in cooling to 273.15”K. was measured. rather than to (strongly) restricted rotation about This procedure was repeated a t temperature ina CN bond with a large amount of double bond tervals of approximately 30°K. over the 273.15 character, in an amide cation protonated at the to 1172.0”K. range. In each run the previously oxygen atom. determined heat content of the container for that temperature was deduced from the total heat evolved. HIGH TEMPERATURE HEAT CONTENT The experimentally determined heat content AND ENTROPIES OF CESIUM CIlLORIDE values of the samples are listed in Table I. They are expressed in defined calories per mole and the AKD CESIUM IODIDE’ molecular weights are in accord with the 1956BY C. E. KAYLOR,~ G. E. R A L D AND EN~ 1957 Atomic Weight Report. DONALD F. SMITH^ Cesium chloride exhibited two phase changes Cffntrihtzonfrom the Southern Experiment Station, Region IT, Burrau within the temperature range studied. One change of Manes, U. S. Department o f the Interaor. a n d the School of Chemistry Unzversity of Alabama, Unaveratu, Alabama was a transition from the alpha to the beta form Received August 3, 1969 and the other from solid beta to the liquid at the This paper presents the results of heat content melting point. The heat of transition was found measurements throughout the temperat ure range to be 581 cal. per mole a t 742.5”K. and the heat of 273.15” to 1172”K., with calculated entropies for fusion at the melting point 918°K. was determined cesium chloride and cesium iodide. The heat of as 4964 cal. per mole. (3) E. E. Creitr, Bureau of Xlinca, Southorn Experiinont Station, transition for cesium chloride a t 742.5”K. has been Ala. measured, and the heats of fusion of cesium Tuscaloosa, (4) D. C. Ginnings and R. J. Corruccini, J . Research, Nail. Bur.
*
( I ) The work upon which this report is based operation with t h e University of Alabama. (2) Buieau of Mines. Tuscaloosa, Ala.
W ~ carried B
out in co-
Standards, 88, 583 (1047). ( 5 ) I). C. Ginnings and 522 (1953).
G. T.
Furukawa, J. Am. Chem. SOC. 75.
