the monoclinic space group P&/a with 4 iiiolecules of hTaH2(C6O7Hj)in a cell of dimensions (I = 9.64SAfi.,b = 11.6SOA., c = 7.481,4., /3 = 105”15’. The observed density was 1.747 g. cm.-&while that calculated was 1.749 g. ~ m . - ~ T. h e standard and in error in axial lengths is less than i 0 . 1 3 ~ o angle less than +6’. The observed density was reproducible to 1%. The analysis was based on 1917 independent reflections photographed with C u K a and MoKa radiation. Of these 223 were too weak to be nieasured. T h e trial structure was found from a threedimensional sharpened F2-mapand refined by twoand three-dimensional difference Fourier series and b y three-dimensional least-squares methods with individual isotropic temperature factors.* -\t the present stage of refinement the reliability factor K is 1 3 . 2 7 for all reflections. .U1 seven hydrogen ,ttonis hCl\c been located oil ;I dificrence synthesis.
2 51 1
i 2ir L
1 ,?
E-
t
io
VOLUME, mi
Fig. 2.
favor recombination of the “hot” C r with radicals produced by radiation damage, in spite of their low concentration relative to water molecules. The extent of the radiation-induced decomposition can be seen from the distribution of the total Cr, initially present as Cr(NCS)6-3. Extensive coil--4 occurred, but the L-ersion to Cr(NCS)&(H?O) amount of lower complexes formed is relativelv small. The ratio of tvuns- to cis-Cr(NCS)2(H20)4’is 0.31 for both the radioactivity and the total Cr, as compared to the equilibrium value3 of 0.5 a t 370°K. ,About 11% of the radioactivity fell outside of the main peaks, and two small unidentified peaks can be seen in the cation elution curve. These niay be due to complexes with fragments of the S C S - group, such as C r ( N C ) ( H 3 0 ) 2 ? or Cr ( S H.J (Hp O ) * ” . Acknowledgments.- the author wishes to thank the Research Corporation for a grant for the purchase of counting equipment. He also is grateful to the National Distillers Corporation for the use of their facilities a t the Industrial Reactor Laboratory, Plainsboro, S . J., and especially to D r . D. H . Greenberg, who made all of the arrangements for the irradiation. 11r. F. C. Simons assisted with the chemical analyses. FRICKCHEMICAL LABORATORY PRISCETOS ~TSIVERSITY
S:IEI,I)OS ~ A U P R I A S
PRINCETOS, SEW JERSEY RECEIVBI) . ~ P K I L8, l Y N )
THE
STATE OF IONIZATION OF CRYSTALLINE SODIUM DIHYDROGEN CITRATE‘
Sir: The structure of sodium dihydrogen citrate has been determined by three-dimensional X-ray diffraction methods. This substance crystallizes in 1 1 ) Siil,p n : d Cxiirer I i i i t i
Fig. 1.
The covalerit bond lengths and angles in thcs citrate ion are shown in the diagram. Of the three carboxyl groups, the two end carboxyls have C-0 distances which are non-equivalent (1.20, 1.34 and 1.21, 1.33 A.) whereas the central carboxyl has two equivalent C-o distances ( 1 . x , I .83 k.1. TIIC bond angles confirm these relations. These findings together with the observed attachment of hydro3 demonstrate t h a t i t is the gen atoms t o 01 and 0 central carboxyl group which is ionized in this crystal. It is interesting to note that nuclear magnetic resonance measurements3indicate t h a t in solution the singly ionized citrate is predominately ionized in the end carboxyl groups. Although full discussion of the structure will be reported elsewhere i t is worth while noting that the sodium ion is octahedrally coordinated to oxygens. These octahedra occur in centrosyiiimetrically related pairs with one edge shared ~ 0 ~ 0 ; ’Each ). iriolecule forms twc, bitlentatc chelate rings, o m with each of the sodium ions of the abo\re-irieiitiorictl pairs of octahedra. 01ie of the chelate rings is five-inembered (NaO&&:jO;] and the other is six-membered (Na’O&$,C4C&hj. 1 2 ) \Ye are indebted t o I’rincetoii T-ni\.er?ity a n d t o J n d i i n a l l n i er,ity for t h e iise ( i f t h p i r c , l r n : ) i i titiiin f.icililies. LO -1 l . o v n e n \ t e i n i t i i ~ l 1 1 1 l < t , I ) r r t \ ,’1’111s ] < , I . K \ 1 1 , 8 2 , ? i l l > , I !IliO)
\
COMMUNICATIONS TO THE EDITOR
June 3 , 1'360
2965
These are hydrogen bonding interactions of the type B - H f . . . 0 H 2 . 2 , 5It has been estimated that for equal polar effects of substituent groups on nitrogen, this hydrogen bonding interaction is responsible for a decrease in base strength in aqueous solution of a tertiary relative to a primary amine of about 3.6 powers of ten.ja In certain non-aqueous solvents tertiary alkyl amines, as expected by electronic arguments, are actually more T h e J o h n s Hophins Univerbasic than primary amines.6 In the region 44G4TG HzSOd, a very simple J E K N Y P. GLUSKER D. VAN DER HELM correlation exists between ( H R - Ho) and the 1i:ARNER E. L O V E 4 activity of water, cf. Fig. 1. Seven arylmethanols MARILYX L. DORSBERC were used to establish the H R function in this region A L. PATTERSON and within experimental error no dependecce of
The oxygens O1 and Ob are cuordiiiated to other sodium ions (01and Oa take no part in the coordination). The structure is notable for the large numberaof close intra- and intermolecular contacts (
