Investigations in X-ray crystallography

Abstract

he crystal structure of glycine silver(I) nitrate has been solved in the non-ferroelectric phase by the heavy atom technique and refined to an R value of about 11% with four sets of intensity data, three sets collected at room temperature and one at –155?°C. The particular crystal with which the low-temperature data were collected was found to have the space group P2?/a, although the reported ferroelectric transition to P2? is at –55?°C.

The crystals are monoclinic with four molecules in a unit cell of dimensions:

a = 5.451(4) Å

b = 19.495(10) Å

c = 5.541(8) Å

? = 100°12?(15?) at room temperature, with space group P2?/a.

The cell dimensions at –155?°C, recorded with crystals that did show the transition to space group P2?, are:

a = 5.562(5) Å

b = 19.591(20) Å

c = 5.556(4) Å

? = 100°12?(15?).

The silver atoms, related centrosymmetrically, are separated by a distance of 2.88 Å, the same distance found in metallic silver. They bridge centrosymmetrical pairs of carboxyl groups, forming dimers of glycines. The glycines in the structure exist as zwitterions, NH??CH?COO?, with the nitrogen atom displaced from the mean plane of the ?-carbon atom and the carboxyl group by 0.55 Å.

The glycine nitrogen atom forms three N–H···O bonds with nonequivalent nitrate oxygens at less than 3 Å distance. The silver atoms are likely to play the most significant role in the ferroelectric transition of this crystal.