Noms
2648 1000
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Vol. 77 others are 2.76 A. The distance of 2.7(i conpares tavorably with the diameter of the H- ion (?.ZA,) as found in LiH. The one short H~-H distance is characteristic of one anion-anion distance of the rutile type structure. Magnesium hydride is optically uniaxial positive with refractive indices 1.95 for the ordinary ray and 1.9G for $he extraordinary ray a t the wave length 5893 ,4.
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I:SI\~ERSITY OF CALIFORSIA LOSALAXOSSCIENTIFIC LABORA~ORY -% 1,os ALAMOS,SEW MEXICO Ji
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The Evaluation of Enzyme-Substrate Dissociation Constants1 141
R A L P H I , U l \ \ A C K 4\D CAR1 A I L M A \ \ ' RhChIVELl S O V h h l B h R
22, 19-54
Iior d n cn~yine-catalyLec1reactloll that cdli be described in terms of the steady-state proccss depicted in equation 1 Ef
10 -5
1.5 1.6 1.7 1.8 103. Fig. l.-a, decoinp. pressure of MgHa; b, reaction rate constant for formation of MgH2; c, Wiberg, Goeltzer and Bauer.* 1.1
1.2
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1.4
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The results here presented indicate that no catalyst is necessary. Magnesium hydride prepared from fine turnings of high purity, doubly distilled magnesium by direct synthesis is a very light gray in color, is sensibly inert t o laboratory air, and reacts slowly with water giving off bubbles of hydrogen. This behavior is in contrast to the high reactivity observed by Wiberg and Bauer for magnesium hydride prepared by the pyrolysis of diethylmagnesium. The difference is probably due to the difference in particle size. X-Ray examination of magnesium hydride shows that i t crystallizes in the tetragonal system with a0 = 4.5168 A.and co = 3.0205 A. The measured density of 1.45 j= 0.03 g . ~ ' c m shows .~ t h a t there are two molecules in the unit cell. The calculated X-ray density is 1.419 g./cm.2. The X-ray powder pattern shows t h a t the two magnesium atoms in the unit cell are in a body-centered configuration, i.e., 2Mg in (000) (3 41. .A consideration of all tetragonal space groups shows t h a t there is one and only one possible choice of positions for the four hydrogen atoms leading to a plausible structure. This is space group P4/mnm (Dit) 2Mg in (000) (4 3 f ) 4H in =k ( X , X , O ) ( X with X = 0.306
+ +, 3 - X,f )
I n this structure (rutile type) each magnesium is coordinated to six hydrogens a t a distance of 1.95 .%., and each hydrogen is coordinated t o $hree magriesiums. One H-H distance is 2.49 A. and the [:I)
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