EDW.\RD IT7. HUGHES AND ~VILLIAM N. LIPSCOMB
19i0
fluoride but twice as much Freon 11 is required compared with Freon 12 thus making it less attractive as a method of synthesis. A t 400” CFC13 presumably reacts with uranium trioxide according to the equation 4CFC13
+ U03
400
& UF4
+ 3COC12 + CCL + C1,
This reaction appears to begin a t 350-360’.
The authors wish to express their thanks to the Wisconsin Alumni Research Foundation for providing financial assistance during the period that the work was being completed a t the University of \Tisconsin. We are indebted for the supply of [CONTIIIBUTION FROM
THE
I-01. OS
Freon 11 and 12 to the kindness of the Kinetic Mfg. Cn., Inc.
Summary Difluorodichloromethane reacts quantitatively with uranium trioxide to produce pure anhydrous uranium tetrafluoride a t elevated temperatures. ~lonofluorotrichloroi~~etliane rcacts similarly but its use has the disadvantage of needing twice as much as in the case of Freon 12. CLEVELAND, OHIO
J A N L - A R ~ 13, ~~
-
19.13
( 1 2 1 The publication of thii paper n a i delayed for war time ~ e c u r -
reasons -The
it>
Edttot.
GATESAND CRELLIN LABORATORIES O F CHEMISTRS, C.4LIFORNIA KO. 10371
INSTITUTE O F
TECHKOLOGY,
The Crystal Structure of Methylammonium Chloride BY EDWARD W. HUGHES] AND WILLIAM N. LIPSCOMB Values of the carbon-nitrogen single-bond dis- The value of a0 was found to be larger by 4 2 and tance in various compounds are of special interest its direction to be a t 45’ to that given by him; because of the occurrence of this bond in amino hence the unit cell actually contains two mole. acids, proteins and related substances. Nu- cules of CH3NH3C1. merous electron diffraction studies2 of gas inoleThe Unit Cell and Space Group.-The macules have yielded values of about 1.47 A., which terial used for this investigation was Eastman are consistent with the usual table of covalent Kodak Co. Red Label Grade methylamine radii.3 On the other hand, distances ranging hydrochloride. Suitable crystals were grown from 1.39 to 1.49 fi. have recently been reported from aqueous solutions by allowing the water to in X-ray crystal structure investigations of coni- evaporate slowly. A needle-like specimen appounds containing tetracovalent nitrogen.2 Be- proximately 0.2 iniii. in thickness and 1.5 mm. in cause of these many different values it was length was selected for the single-crystal photothought desirable to investigate the carbon- graphs. Early experiments indicated that the nitrogen distance in a simple crystal in which a compound is somewhat deliquescent ; hence the minimum number of parameters needed deter- crystal which was selected was dipped in a mixmination; methylammonium chloride appeared ture of paraffin wax and vaselinc. This mixture not only provitlcd L: filiii which protected the to be such a crystal. Methylammonium chloride crystallizes in thc crystal from thc atmosphere but was also found tetragonal system.4 A determination of its suitable for :Lttaching the crystal to a quartz crystal structure has been reported by H e n d r i ~ k s . ~fiber mounted on the goniometer head. The The smallest unit of structure which he found crystal was oriented by means of preliminary comp2tible with his data had the dimensions a = oscillation photographs or Laue photographs. The dimensions of the unit cell were determined -1.28 4. (obtained from powder data) and c = 5.13 A. (obtained from oscillation photographs) ; approximately from layer-line separations and this unit contains one molecule of CH3NH3C1. more accurately from the equatorial reflections The atomic positions were C1- a t 000, N a t * $ z l , on oscillation photographs taken with filtered and C a t 3&, with the most probable values of CuKa radiation. Oscillations about theo a, c the parameters given as z1 = 0.24 and z2 = 0.50. and [ 1101 a y s gave the values a. = (i.01 A. aad \\,-hen the present investigation was begun i t co = 3.03 A. with probable errors of 0.01 A. was thought necessary only to refine the above These dimensions of the unit cell together with parameter values, but it soon became evident that the observed density value,5 1.23 g icc., require the sttucture proposed by Hendricks is incorrect. two molecules of CH&H3Cl per unit cell. The calculated value of the density, 1.216 g./cc., is (1) h‘oyes Fellow. (2) A partial list of values obtained for this distance in various in satisfactory agreement with the observed compounds is given i n Table V. value. -4 set of powder photographs was taken (3) L. Pauling, “The Nature of the Chemical Bond,” second with filtered CuKa radiation by means of the edition, Cornel1 University Press, Ithaca, N. Y . , 1940, p. 164. The multiple-film technique.6 These photographs value 1.365 A. is given b y the slightly modified values of covalent radii proposed by V. Cchomaker and D. P. Stevenson, THISJOURNAL, were successfully indexed on the new unit cell. 63, 3 i (1,941). The only systematic extinction which occurs on (1) I’ Groth, “Chemische Krystallographie.” Vol. I , Wilhelm Engelmann, Leipzig, 1906, p. 168. 15) S. B. Hrndricks, Z. Krisl., 67, 106 (1928).
(6) J J deLange J hl Robertion, and I W‘ooduard. (London), A171. 398 (193Y),
$OF
PYOC. Roy.
Oct., Inlo
CRYSTAL STRUCTURE OF METHYLAMMONIUM CHLORIDE
1971
the oscillation and powder photographs is hkO with h. k odd. The Laue symmetry as found from photographs both parallel and perpendicular to co is - ~ - , ' I u u zThese ~ ~ . data indicate7 that the space group is D& - PJ,/nmm. If for the present we disregard the hydrogen atoms wc have, with two atoms of each kind in the unit cell, thc following possible point positions'
+
(a) 000; a30 (b) 00%; $++ (c)
032; $02
Neither the C nor N atoms can be in (a) or (b) and still forni covalent bonds; consequently we chose (c), with point symmetry CdV,and locate NH3+ a t O i z l ; io%, and CH3 a t 0422; $022. If we assume the normal van der Waals radii for the atoms and a nornial C-N distance, the dimensions of the unit cell do not allow sufficient room for the C1- ions along the four-fold axis; hence the C1ions must be in (a) or (b). The choice between these positions is arbitrary. A rough consideration of the hkO intensities (cf. Table 111) also requires that the positions for C1- be either (a) or (b). I t must be pointed out that the hydrogen atoms of the CH3NH3+groups cannot conform to this space group unless the group is rotating about the carbon-nitrogen axis or unless the hydrogens are randomly oriented with respect to rotation about this axis. A consideration of the rotation of these groups is presented in a subsequent section. A further test of the structure, and one which distinguishes our structure from the one proposed by Hendricks, is the pyroelectric test. His structure, which has no center of symmetry, consists of alternate layers of Cl-. ions, NH3+ groups, and CH, groups and is therefore strongly polar; i t would show a. strong pyroelectric effect. Our proposed structure, shown i n Fig. 1, has a center of symmetry a t $io, etc., and hence could show no pyroelectric s-ffeet. In order to test for pyroelectricity the crystal was attached to a single fiber of silk by means of a microscopic drop of cement. 'The crystal was suspended in liquid air and then removed. Since the crystal then showed no tendency to be attracted to the neck of the Dewar and since no anisotropic growth of ice crystals was observed, i t was concluded that the pyroelectric test was negative. Determination of the Parameters.-Complete sets of oscillation photographs were taken with filtered MoKa radiation about the a axis and the [ 1101 axis, and with filtered CuKa radiation about the c axis. The multiple-film techniquee was used; when M o K a radiation was used, 0.001-inch copper sheets were interleaved between the films in order to reduce the intensities by a suitable factor. The peak densities of re(7) "Internationale Tabellen zur Bestimmung von Kristallstrukturen." Gebriider Borntriiger, Berlin, 1935.
+----aw Fig. 1.-The
unit cell of methylammonium chloride.
flections in the density range 0.3 to 0.8 (including background) were measured with the aid of an Eastman Densitometer, Model B, fitted with an opening 0.012-inch in diameter, and then all of the reflections were estimated visually using the densitometered values as a basis. In the densitometered range the intensity was taken proportional to the peak density of a reflection minus the density of the neighboring background. Structure factors were calculated from these estimated intensities after correction for Lorentz and polarization factors and, on the powder photographs, for the multiplicity factors. In the photographs about the a axis and [110] axis, for which the needle axis was horizontal, the intensities were estimated of only those reflections for which the incident and reflected rays passed through relatively short paths in the crystal. The various oscillation photographs about a given axis were placed on the same relative scale by comparison of corresponding structure factors with those obtained from the powder photographs. The observed structure factors, F h k l , in a given zone were placed on the proper absolute scale by determination of the scale factor CY such that z w h k l ( c u F L k l - F h k l ) 2 is a minimum, where W h k l is the weighting factor referred to below and FLk1 is the structure factor calculated from the final values of the parameters. The resulting structure factors are listed in Tables I, I1 and TABLE I FACTORS OF h01 REFLECTIONS
STRUCTURE THE
TABLE I1
STRUCTURE FACTORS OF THE hhl REFLECTIOSS
h01
Foix.
pca~cd.
hhl
F"Ib
FmId
001 101 200 201 002 102 202 301 003 103 302 400 203
19.7 11.6 39.2 17 1 18 9 3.4 11.6
21..5 10.1 39.0 17.4 19 8 5 2 14.7 5 0 4.4 -1.9 1.3 18.7 4.8
001 110 111 002 112 220 221
19,:
21.5
5 8
3.2 2.4
