Effect of high pressure on the crystal structures of polymorphs of glycine

The effect of high pressure on the crystal structures of alpha-, beta-, and gamma-glycine has been investigated. A new polymorph, delta-glycine, is obtained from beta glycine. delta-Glycine is monoclinic, P2(1)/a, a = 11.156(4), b = 5.8644(11), c = 5.3417(17) angstrom, beta = 125.83(4)degrees at 1.9 GPa. The transition, which occurs between 0 and 0.8 GPa, proceeds from a single crystal of beta-glycine to a single crystal of delta-glycine, resulting in an equal number of NH center dot center dot center dot O hydrogen bonds but an increase in the number and strength of CH center dot center dot center dot O hydrogen bonds, which act to close-up "holes" that are formed within the layers of beta-glycine in the centers of R-type hydrogen-bonded motifs. Trigonal gamma-glycine begins to undergo a transition to another high-pressure phase, e-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) angstrom, beta = 116.682(10)degrees at 4.3 GPa. The structure consists of layers similar to those observed in alpha-glycine with interlayer separations of 2.38 and 3.38 angstrom and CH center dot center dot center dot O interactions formed between the layers. Monoclinic alpha-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 center dot center dot center dot O hydrogen bonds are not formed up to 6.2 GPa, and they only shorten significantly if they are formed parallel to CH center dot center dot center dot O hydrogen bonds, which strengthen, or vectors across holes which close-up, under pressure.