S H O R T C O M M U I ~ I C A T I O I ~ S 775

Acta Cryst. (1954). 7, 775

T h e c r y s t a l s t r u c t u r e o f p h t h a l i c a c i d . By T. G. I). vA~ SCH~KWYK, Crystallographic Laboratory, Caven- dish Laboratory, Cambridge, England

(Received 17 June 1954)

X-ray measurements on phthal ic acid give:

a = 5.00, b = 14-2, c = 9-60 A, fl = 93-5 ° .

The space group is ei ther C2/c or Cc, and the absence of pyro- and piezo-electric effects is consistent wi th the choice of C2/c. With 4 molecules in the uni t cell, the molecule mus t have an axis of symmet ry which coincides wi th a twofold axis. This is sufficient to exclude the possibility of any intramolecular hydrogen bonding.

A Pa t te rson project ion on (010) gives the ti l t of the benzene ring to the (100) plane as approximate ly 45% By assuming a regular benzene ring of C-C distance 1.4 /~, approximate z coordinates for the a toms could be determined. Wi th the further aid of t t a rke r -Kaspe r inequalit ies an electron-densi ty project ion on (100) was obtained. The final project ion is shown in Fig. 1 (R ----

b/4 ,,

0"17). In the project ion on (001) two benzene rings over- lap (R = 0.18).

The length of the intermolecular hydrogen bond be- tween adjacent carboxyl groups across a centre of sym- me t ry is 2.67 (~=0.05) /~ (see Fig. 2). The possibility of

T

~,~ - . . . . . . . . . . . . . . . . . . . . . :,':: de ~'-" ........................ ~-'- • /crest e

0 1 2 3 l, 5 A , , , , , , , Q - c 0 - 0

Fig. 2. Structure viewed along a axis. Hydrogen bonds are represented by broken lines.

0 1 2 3 4 5h I I I 1 I t

Fig. I. Electron density projected on (100).

de termining the electron distr ibut ion in the hydrogen bond, wi th the help of Geiger-counter measurements , is at present being investigated.

Full details will be published later.

Acta Cryst. (1954). 7, 775

P h t h a l i c a c i d a t l o w t e m p e r a t u r e . By A. L. VEENE~Dm~_L and C~OLnq~ H. MACG~LAVRY, Laboratory for General and Inorganic Chemistry, University of Amsterdam, The Netherlands

(Received 7 July 1954)

The crystal s tructure of phthal ic acid is being de te rmined by us at a t empera ture of about --150 ° C. Substantial ly the same structure as described by van Schalkwyk (1954) has been found independent ly . I t is too early to say whether small differences in atomic positions found so far are significant. We in tend to pursue the invest igation in order to s tudy tempera ture influence, by comparison of

our results wi th those of van Schalkwyk. We wish to t hank Mr van Schalkwyk for sending us a copy of his paper.

R e f e r e n c e SC~_LKWYK, T. G. D. VAN (1954). Acta Cryst. 7, 775.