title: Effects of H/D substitution on thermal vibrations in piperazinium hexanoate-h11, d11.
abstract: The crystal structures of piperazinium hexanoate-h11, 1/2C4H12N2(2+).C6H11O2-, and piperazinium hexanoate-d11, 1/2C4H12N2(2+).C6D11O2-, have been determined from neutron diffraction data collected at 15 K. Nuclear anisotropic displacement parameters have been analyzed to obtain the internal molecular displacements of the H and D nuclei, given by (u(obs)2)-(u(ext)2) where (u(ext)2) is the contribution assuming all H/D to be carried rigidly on the vibrating molecular framework consisting of the heavier nuclei. In both crystal structures the cation ring is well fitted by the rigid-body model and the anion chain by a model with two rigid segments. In the piperazinium cations the corresponding protons in the two structures have about the same internal vibrational directions and magnitudes except for the two N--H protons, perhaps owing to differences in N--H...O hydrogen bonding. The internal vibrations of corresponding H/D in the h11 and d11 anions have approximately the same vibrational directions. The internal mean-square displacements of the H nuclei are systematically greater than the values of the corresponding D nuclei by an average factor 1.7(3). For both anions, normal-mode analyses have been carried out using the force fields derived from ab initio quantum-mechanical calculations with HF/3-21G and HF/6-31G** basis sets. The values of the resultant H/D internal displacements for C--H(D) bond stretching and methylene out-of-plane vibrations are in good agreement with experiment. However, with either basis set, theory predicts methylene in-plane mean-square displacements significantly greater than the experimental values.
authors.affiliation: Department of Crystallography, University of Pittsburgh, PA 15260, USA.
authors.forename: J Q
authors.initials: JQ
authors.lastname: Luo
authors.forename: J R
authors.initials: JR
authors.lastname: Ruble
authors.forename: B M
authors.initials: BM
authors.lastname: Craven
authors.forename: R K
authors.initials: RK
authors.lastname: McMullan
mesh_terms: D002208:Caproates / Q000737:chemistry*; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003903:Deuterium; D006860:Hydrogen Bonding; D008956:Models, Chemical; D015394:Molecular Structure; D000077489:Piperazine; D010879:Piperazines / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D002208:Caproates; D010879:Piperazines; D000077489:Piperazine; D003903:Deuterium