CRYSTAL STRUCTURE OF SODIUM BOROHYDRIDE
May, 1947
987
agree very well with those of Pitzer and Gwinn7 and they conclude that the potential barrier restricting rotation is about but a little less than 800 cal. mole-'. Calculation of the potential barrier from their data leads to a value of about 600 cal. mole-'. The calculation of the potential barrier from the entropy measurements reported here is given below. "I.C.T." constants are used for the statistical calculation.
restricting the internal rotation in nitromethane would appear to be rather definitely established in the range of the above determinations. The possibility that the value might lie in the range 8000-10,000 cal. mole-l is definitely excluded by the entropy determination. We thank Dr. Jerome Martin of the Commercial Solvents Corporation for supplying the sample of nitromethane and Drs. J. E. Gordon and L. Guttman for assistance with the calorimetric measurements. TABLE VI11 Summary CALCULATION O F THE ENTROPY O F NITROMETHANE FROM The beat capacity of solid and liquid nitroMOLECULAR DATA methane has been determined from 13 to 300°K. Ideal Gas State, T = 298.1°K., P = 1 atm. cal. deg.-l mole-' The melting point is 244.73"K. (OOC. = Translation 38.25 273.1"K.). The heat of fusion is 2319 cal. mole-'. The vapor pressure a t 25' was found to be External rotation 23.51 ITbration 1.88 3.666 cm. and the heat of vaporization a t this Internal rotation (assumed free) 2.15 temperature as measured calorimetrically is 9147 cal. mole-'. Entropy ccrresponding to free internal rotation 65 79 The observations were used to calculate the Entropy from calorimetric data 65.73 entropy of nitromethane in the ideal gas state a t 298.1°K. and 1 atm. The value found was 63.73 Reduction in entr3py due to restriction of internal cal. deg.-l mole-'. As customary, this value does rotation 0 06 not include the entropy due to nuclear spin. Combining the above entropy value with calThe entropy tables of Pitzer and GwinnI2 show that the 0.06 ca1. deg.-' mole-' corresponds to a culations based on available molecular data, the six-fold potential barrier of 500 cal. mole-', in six-fold potential barrier restricting internal rogood agreement with the values 800 and 600 cal. tation in the nitromethane molecule is found to be mole-'. The limits of accuracy of all the above 500 cal. mole-', in good agreement with values of determinations make values in the range 0-1000 800 and 600 cal. mole-1 found by Pitzer and cal. mole - l possible, but taking all the results to- Gwinn, and De Vries and Collins, respectively, gether, the magnitude of the potential barrier from measurements of heat capacity of the gas.
-
(12) Pitzer and Gwinn,
CONTRIBUTION FROM
J Chem Phys , 10, 4 2 8
THE
(1942)
BERKELEY,CALIFORNIA RECEIVED DECEMBER 14, 1946
GATESAND CRELLINLABORATORIES OF CHEMISTRY, CALIFORNIA INSTITUTE OF TECHNOLOGY, No. 1086)
Crystal Structure of Sodium Borohydride BY A. M. SOLDATE The unexpected properties of sodium boro- Ka! radiation filtered through nickel in a cylindrihydride, such as its stability in vacuum a t cal camera with a 5-cm. radius. The twenty retemperatures as high as 400°,' give 'interest flections observed were indexed on the basis of a to an investigation of its structure, hitherto cubic cell with a0 = 6.15 A. More precise lattice unreported. constant measurements using sodium chloride as Experimental Methods and Results.-A sam- an internal standard gave 6.151 * 0.009 k X for the ple of powdered sodium borohydride was kindly value of UO. No reflections with mixed indices supplied b y Dr. H. C. Brown of Wayne Univer- were observed, the lattice is, therefore, face-censity, who reported i t as being 98-99y0 pure. tered. The unit cell, being face-centered, must contain Specimens suitable for X-ray photography were obtained by sealing powdered material in thin- a multiple of four sodium borohydride molecules. walled Pyrex capillaries about 0.5 mm. in diam- I t was assumed that the correct number of moleeter. To ensure adequate protection of this cules in the unit cell is four. The density calcuhygroscopic substance, the procedure was car- lated with this assumption is 1.074 g./cc. No exried out over sulfuric acid in a sealed enclosure. perimental value is known to us, but this value Powder photographs were made with copper is a reasonable one. There are two ways in which the sodium and boron atoms can be arranged in (1) Private communication from Dr. H. I. Schlesinger, The Unithe unit cell : versity of Chicago.
A. M. SOLDATE
988
VOI. 69
surrounded by a tetrahedral configuration of four hydrogen atoms and that the structure consists of B H 4 - and N a + ions, intensities were calculated for two arrangements of BH4- tetrahedra: (1) "4 a completely random distribution of tetrahedra "4 */4. in positive and negative configurations (mirror By use of PaulingSherman f values2 and b y images) with hydrogen atoms on cube diagonals; neglect of the scattering power of the hydrogen (2) tetrahedra all positive or all negative throughatoms, intensities were calculated for both ar- out the structure with hydrogen atoms on cube rangements of the sodium and boron atoms. Ta- diagonals. I n both instances the calculated inble I shows good agreement between intensities tensities of { 111) and (400) were the only calculated on the basis of arrangement A and ob- values which differed from corresponding values served intensities. The net effect of absorption of Table I by more than 20%. Agreement beand temperature corrections, omitted here, was tween observed and calculated intensities was not estimated to be small. improved over that shown in Table I with either arrangement of tetrahedra. It was decided that TABLE I i t was impossible to reach any conclusion as to A COMPARISON OF OBSERVED INTENSITIES AND INTENSITIES the positions of the hydrogen atoms from the inCALCULATED WITH SODIUM AND BORONATOMS IN ARtensity data obtained here. RANGEMENTA With the assumption of a covalent boron-hy(hk0 Calcd. intensity Ohs. intensityo drogen bond distance of 1.16 A., a van der Waals 111 23 1 m+ radius of 1.0p A. for hydrogen, and a crystal ra200 562 VS dius of 0.95 A. for sodium, i t cap be seen that the 220 386 S boron-sodium distancq of 3.07 A. and the boron311 138 m boron distance of 4.35 A. are large enough to cause 222 130 mvery little steric hindrance t o rotational displace400 72 f ments of BH4- tetrahedra. Thus, rotating or os33 1 51 f cillating BH4- tetrahedra can be reasonably pre420 145 msumed to exist in the structure and statistical 422 105 f+ distributions of tetrahedra, such as the one al33:;-511 31 fready considered, are of relevance. 44(1 33 fI thank Dr. Linus Pauling for the suggestion of 53 1 24 Vf the problem and for helpful advice during the 442-600 65 f preparation of this report. 620 45 f(A) 4Na a t 0 0 0 , 0 '/z'/z, '/2 0 '/2, '/z '/z0 4B a t '/z '/z'/z,'/2 0 0,O '/z 0,O 0 '/2 (B) 4Na a t 0 0 0,O '/z '/2, '/z0 '/z, '/z '/z 0 4B a t '/4 '/4 '/4, '/4 "4 3/4, "4 '/4 3 / 4 ,
53:: 622
7 38
wf
444
15 14
vvf wf
713-551
640 47 642 108 0 s, Strong; m, medium; f, faint.
f-
+
ff
Intensities calculated for the second arrangement were in marked disagreement with those observed. For example, the calculated intensity of the line {220) is much greater than that of {ZOO) and the calculated intensity of (331) is greater than that of (420). With the assumptions that each boron atom is (2) L. Pauling and J. Sherman, Z . Kvirl., 81, 1 (1932).
Summary Powder photographic X-ray data have been used t o show that the structure of sodium borohydride is based on a face-centered cubic lattice. The unit cell contains four sodium and four boron atoms arranged in the following manner : 4 Na a t 0 0 0, 0 '/z '/z,'/z 0 '/z,'/z '/zO 4B ' / z ' / z '/z, ' / z 0 0, 0 '/z 0, 0 0 '/z. The value 6.151 * 0.009 k X was found for UO. It is probable that the structure consists of tetrahedral BH4- ions and Naf ions. The dimensions of the ions would permit oscillation or rotation of the BH4- tetrahedra. PASADENA, CALIFORNIA RECEIVED NOVEMBER 26, 1946
