The crystal structure of perdeuterated methanol hemiammoniate (CD3OD·0.5ND3) determined from neutron powder diffraction data at 4.2 and 180 K

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Abstract
The crystal structure of perdeuterated methanol hemiammoniate, CD3OD·0.5ND3, has been solved from neutron powder diffraction data collected at 4.2 and 180 K. The structure is orthorhombic, space groupPn21a(Z= 4), with unit-cell dimensionsa= 12.70615 (16),b= 8.84589 (9),c= 4.73876 (4) Å,V= 532.623 (8) Å3[ρcalc= 1149.57 (2) kg m−3] at 4.2 K, anda= 12.90413 (16),b= 8.96975 (8),c= 4.79198 (4) Å,V= 554.656 (7) Å3[ρcalc= 1103.90 (1) kg m−3] at 180 K. The crystal structure was determined byab initiomethods from the powder data; atomic coordinates and isotropic displacement parameters were subsequently refined by the Rietveld method toRp≃ 2% at both temperatures. The crystal structure comprises a three-dimensionally hydrogen-bonded network in which the ND3molecules are tetrahedrally coordinated by the hydroxy moieties of the methanol molecule. This connectivity leads to the formation of zigzag chains of ammonia–hydroxy groups extending along thecaxis, formedviaN—D···O hydrogen bonds; these chains are cross-linked along theaaxis through the hydroxy moiety of the second methanol moleculeviaN—D···O and O—D···O hydrogen bonds. This `bridging' hydroxy group in turn donates an O—D···N hydrogen bond to ammonia in adjacent chains stacked along thebaxis. The methyl deuterons in methanol hemiammoniate, unlike those in methanol monoammoniate, do not participate in hydrogen bonding and reveal evidence of orientational disorder at 180 K. The relative volume change on warming from 4.2 to 180 K, ΔV/V, is + 4.14%, which is comparable to, but more nearly isotropic (as determined from the relative change in axial lengths,e.g.Δa/a) than, that observed in deuterated methanol monohydrate, and very similar to what is observed in methanol monoammoniate.

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