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X-ray structure analyses of orthorhombic betaine fumarate and monoclinic betaine maleate yielded space group  Pnma ,  a  1 =26.510(5) Å,  a  2 =6.684(1)Å,  a  3 =6.376(1)Å,  Z =4,  ρ  calc =1.371g cm -3 ,  ρ  obs =1.370 g cm -3  and, respectively, space group  P 2 1 / n ,  a  1 =6.132(1)Å,  a  2 =10.823(2)Å,  a  3 =17.246(3)Å,  α  2 =92.87(3)°,  Z =4,  ρ  calc =1.355 g cm -3 ,  ρ  obs =1.351 g cm -3 . In both compounds the hydrogen betaine cations are joined to the corresponding anions by strong H-bonds with O···O distances of about 2.48Å. Large single crystals of optical quality of betaine maleate have been grown from aqueous solutions. Further, we determined their coefficients of thermal expansion and their elastic and thermoelastic constants. In comparison to the pronounced anisotropy of the mechanical properties of betaine fumarate, the elastic constants of betaine, maleate appear only slightly anisotropic. Between 80 K and 305 K no phase transision could be detected in betaine maleate. However, softening of certain acoustic modes at temperatures below about 160 K might be a hint at structural instabilities at even lower temperatures.

Crystal structure and elastic properties of betaine fumarate and of betaine maleate, two isomers of ((CH 3 ) 3 NCH 2 COOH . OOC(CH) 2 COOH