Yutaka Iwata

Experimental charge density and electrostatic potential in nicotinamide

The accurate crystal structure of nicotinamide, 3-pyridinecarboxamide, was determined from X-ray and neutron diffraction experiments: C(6)H(6)N(2)O, M(r) = 122.13, monoclinic, P2(1)/c, Z = 4. The electron distribution at 150 K was determined by the maximum entropy method and the electrostatic potential in the crystal was calculated by Fourier convolution of the electron distribution. The electrostatic properties of the nicotinamide molecule depend on the molecular conformation. The asymmetric electrostatic potential field observed above and below the pyridine-ring plane is related to the rotation of the carboxamide group with respect to the pyridine plane. The positive potential peak at the C4 atom of the pyridine ring extends to the C=O-group side of the plane. The asymmetry of the potential on the C4 atom is consistent with the stereospecificity of hydride transfer in NAD(+)/NADH oxidoreduction.

Electron-density distribution in crystals of p-nitrobenzene derivatives

The electron-density distributions in five nitrobenzene derivatives have been examined by the multipole expansion method based on X-ray diffraction data measured with Mo Kα radiation (λ= 0.70926 A) at 120 K. p-Dinitrobenzene, C 6 H 4 N 2 O 4 , (I), M r =168.1, monoclinic, P2 1 /n, a=10.941 (2), b =5.3813 (5), c=5.6684 (4) A, β=92.116 (8) o , V= 333.51 (7) A 3 , Z=2, D x =1.67 Mg m -3 , μ= 0.13 5 mm -1 , R=0.030 for 2045 unique reflections

A Neutron Diffraction Study of the Ferroelectric Transition of CsH2PO4

Hydrogen atoms in ferroelectric phase of CsH 2 PO 4 have been located from low temperature neutron diffraction experiment. Chains of hydrogen bonds extending to both the crystallographic b - and c -axes are confirmed. The hydrogen atom in a shorter O–H··O bond becomes ordered at low temperature phase with bond length O-H=1.02 A auad H··O=1.46 A. Structural change below T c shows that the hydrogen ordering accompanies a slight rotation of PO 4 tetrahedron about the one P–O bond.

Neutron Diffraction Study of the Structure of Paraelectric Phase of Pb5Ge3O11

The structure of paraelectric phase of Pb 5 Ge 3 O 11 was refined by the least squares calculations of single crystal neutron diffraction data at 200°C. Final R value was 0.08 and the type of phase transition was determined as displacive one. The existence of unstable oxygen atom which is lying on the mirror plane perpendicular to the c -axis was found to have relation with the reorientation of GeO 4 group. Atomic displacements through the phase transition were evaluated and used for the calculation of the spontaneous polarization for the point charge model. This gave P s to be 2.0 µC/cm 2 , a smaller value than the observed one of 4.2 µC/cm 2 at room temperature.

A neutron diffraction study on the deuteration effect in CsH2PO4 crystal

The deuteration rate x of partially deuterated crystals, Cs(H 1- x D x ) 2 PO 4 , was determined through the neutron diffraction experiment. It was found that two non-equivalent positions of hydrogen atoms take slightly different deuteration rates. The deuteration rate x 1 of the shorter hydrogen bond which is closely related to the ferroelectric phase transition is smaller than the rate x 2 of the longer one. The observed dependence of the phase transition temperature T c on x 1 is found to be linear in contrast to the concaved dependence predicted in the one-dimensional proton-pseudospin coupled model. The neutron diffraction experiment also revealed that the distance of the shorter hydrogen bond increased linearly with x 1 , whereas that of the longer one remained constant.

A neutron diffraction study on deuterated rochelle salt in the paraelectric phase

Abstract The crystal structure of Rochelle salt was refined at two temperatures of paraelectric phase by using three-dimensional neutron diffraction data from deuterated single crystal. The high temperature (40°C) structure exhibits enhanced anisotropic thermal motion about particular deuterium atoms while the low temperature (-195°C) structure shows quite normal state with lessened anisotropic thermal behaviors. These anisotropies are further refined by split-atom model and it was shown that split-atom model gives better agreement. Based on these structures, phase transition scheme was examined. Previous model of the phase transition does not contradict to the present result but needs some modifications.

Reinvestigation of Low-Temperature Phase Transitions in Rb2ZnBr4

Low-temperature phase transitions in Rb 2 ZnBr 4 single crystal were reinvestigated by means of neutron and X-ray diffractions. Below 116 K some superlattice reflections appeared at the same position (h/2, k/2, 0) as in other members of Rb 2 ZnCl 4 group crystals, where h and k denote the Miller indices in the normal phase (space group P m c n ). The temperature dependence of the integrated intensity of the superlattice reflection changed below 78 K. However, the anomaly around 50 K reported previously was not observed. Below 116 K an incommensurate phase as reported for K 2 ZnCl 4 was not detected. The space group of phase between 78 and 116 K was directly confirmed to be C 1 c 1 by the verification of extinction rules of X-ray diffraction.

X-Ray diffraction and NMR studies on the crystal structures of MABB and MABA at room temperature

Abstract We have investigated the structures of (CH3NH3)3Bi2Br9 (MABB) and (CH3 NH3)3Sb2Br9 (MABA) crystals by X-ray Fourier method and high resolution 15N-NMR in their prototypic phases. A doublet with intensity ratio 2:1 was observed in the NMR spectrum of MABB. The methyl ammonium cations also showed different electron density distributions at two non-equivalent sites. The distribution was examined in connection with splitting of the Br atoms and the NMR results.

Cold neutron spin interferometry and its application to modified spin echo methods

Abstract We propose three kinds of modified neutron spin echo methods using cold neutron spin interferometry, which is based on the coherent superposition principle of the neutron spin. Two kinds of the modified methods are based on a novel quantum precession of neutron spin by multilayer spin splitters and another method on time dependent spin interferometry using RF flippers. The performances of the last type are discussed and the firmer two methods are described in other proceedings.

Neutron structural investigation of the isotope effect of KHCO3

Abstract The crystal structures of KHCO3 and KDCO3 have been analyzed from the three-dimensional neutron-diffraction data measured in the high-temperature phase. In the space group C2/m (Z = 4), both the structures are refined successfully with a disordered model. The final R-factors are 0.055 for KHCO3, and 0.047 for KDCO3, respectively. Fourier analysis also reveals that the proton of KHCO3 and deuteron of KDCO3 have two-equilibrium positions separated by 0.616(4) and 0.647(3) A, respectively.