Effect of High Pressure on the Crystal Structures of Polymorphs of Glycine

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.