The effect of high pressure on the crystal structures of α-, β-, and γ-glycine has been investigated. A new polymorph, δ-glycine, is obtained from β glycine. δ-Glycine is monoclinic, P21/a, a = 11.156(4), b = 5.8644(11), c = 5.3417(17) Å, β = 125.83(4)° at 1.9 GPa. The transition, which occurs between 0 and 0.8 GPa, proceeds from a single crystal of β-glycine to a single crystal of δ-glycine, resulting in an equal number of NH⋯O hydrogen bonds but an increase in the number and strength of CH⋯O hydrogen bonds, which act to close-up "holes" that are formed within the layers of β-glycine in the centers of R-type hydrogen-bonded motifs. Trigonal γ-glycine begins to undergo a transition to another high-pressure phase, ε-glycine, at 1.9 GPa, but the transformation is destructive; it is essentially complete at 4.3 GPa. The structure is monoclinic Pn, a = 4.8887(10), b = 5.7541(11), c = 5.4419(11) Å, β = 116.682(10)° at 4.3 GPa. The structure consists of layers similar to those observed in α-glycine with interlayer separations of 2.38 and 3.38 Å and CH⋯O interactions formed between the layers. Monoclinic α-glycine is known to be stable to 23 GPa, and we have obtained a single-crystal structure of this polymorph at 6.2 GPa. Super-short NH⋯O hydrogen bonds are not formed up to 6.2 GPa, and they only shorten significantly if they are formed parallel to CH⋯O hydrogen bonds, which strengthen, or vectors across holes which close-up, under pressure. © 2005 American Chemical Society.
