Walter C. Hamilton

Neutron‐Diffraction Study of the Structures of Ferroelectric and Paraelectric Ammonium Sulfate

Three‐dimensional neutron‐diffraction data have been collected for both the room‐temperature (paraelectric) and the low‐temperature (ferroelectric) phases of ammonium sulfate. Both phases are ordered. The transition, occurring at 223°K and involving a change from Space Group Pnam at room temperature to Space Group Pna21 in the ferroelectric phase, results in somewhat stronger hydrogen bonds in the ferroelectric phase, although there is no dramatic change in the barrier to rotation of the ammonium ions as a result of the transition. In both phases the two crystallographically independent ammonium ions have quite similar rotational barriers and hydrogen‐bond strengths. The transition also results in less distorted ammonium ions in the ferroelectric phase. At room temperature, there are only two hydrogen bonds with O···H distances less than 2.0 A. In the ferroelectric phase there are six. The mean distance is decreased by 0.1 A in the ferroelectric phase, although the range of distances remains the same. The...

On a nearly proton‐ordered structure for ice IX

Single‐crystal neutron diffraction shows that ice IX, the low‐temperature modification of ice III, has an almost completely proton‐ordered structure in which the ordered component contains two types of water molecules, type 1 in a site of no point symmetry, and type 2 on a twofold axis, each forming four hydrogen bonds in a three‐dimensional framework. The configuration of the water molecules is slightly but significantly altered from that of water vapor. Independent O–D distances, which do not differ significantly from one another, average 0.982± 0.003 A (corrected for thermal motion), and are increased by 0.012 A over the vapor value. The shift in O–D stretching frequency ν1 from vapor to ice IX corresponds to a slope dν1/dr≅ −20 000 cm−1. A−1, which agrees with the slope predicted from spectroscopic anharmonicity constants. D–O–D bond angles (104.7± 0.4°, 106.0± 0.2°) are not decreased from the vapor value (104.5°) despite the smaller O··· O··· O angles into which the water molecules donate protons (98...

Precision neutron diffraction structure determination of protein and nucleic acid components. X. A comparison between the crystal and molecular structures of L‐tyrosine and L‐tyrosine hydrochloride

The amino acid L‐tyrosine (C9H11NO3) and its salt L‐tyrosine hydrochloride (C9H11NO3·HCl) crystallize respectively in the space groups P212121, a=6.913(3) A, b=21.118(10) A, c=5.832(3) A, and P21, a=11.083(5) A, b=9.041(4) A, c=5.099(3) A, β=91.82(3) A. Both structures have been refined by neutron diffraction techniques, and all the hydrogen atoms have been located precisely. The tyrosine molecule occurs in the zwitterion form in pure L‐tyrosine while both the amino group and the carboxyl group are protonated in the hydrochloride. In both crystalline compounds there is a three‐dimensional network of hydrogen bonds. Statistical tests show no significant differences between the bond lengths and valence angles in the compounds except for those atoms involved in hydrogen bonds. The conformational angles of the main chain (C, Cα, N, O1, O2) and those of the side chain differ only slightly in the two compounds while the mutual orientations of the main chain and of the side chain differ completely; in L‐tyrosine...

Ordered Proton Configuration in Ice II, from Single‐Crystal Neutron Diffraction

D2O ice II has a fully deuteron‐ordered structure in which two crystallographically independent types of water molecule form a tetrahedrally linked network of H bonds, with the deuterons lying near but not on the O···O centerlines. Orientation of the molecules is nearly, but not exactly, symmetrical in the donor O···O···O angles. The D–O–D angles (average, 105.4°) do not differ significantly from the angle in D2O vapor (104.5°) although the corresponding O···O···O angles are smaller (average, 93.8°). Agreement between D–O–D and O···O···O angles is improved about 6° (on average) by a distortion of the bond network, from symmetry R3c to R3. The approach to a match between D–O–D and O···O···O angles is only partly responsible for the existence of proton ordering in ice II. Twinning of R3 individuals is established by variations in certain diffraction intensities from crystal to crystal. Mean O–D bond length (0.98 A) is slightly longer than in the vapor (0.957 A). Variations among the individual O–D distan...

Structure of Cubic Ammonium Fluosilicate: Neutron‐Diffraction and Neutron‐Inelastic‐Scattering Studies

The motion of the ammonium ion in the cubic phase of (NH4)2SiF6 has been investigated by the inelastic scattering of slow neutrons. The prominent feature of the neutron energy‐gain spectrum is a moderately broad band peaked at 168±8 cm−1 with a shoulder at 305±25 cm−1. These are assigned to the 1–0 and 2–0 transitions of a rotational motion of the ammonium ion.A precision refinement of single‐crystal neuron‐diffraction data has also been completed. There is a well‐defined disorder of the ammonium groups. Although a model with a static threefold disorder, with each ⅓ hydrogen atom undergoing harmonic vibration gives a very satisfactory fit to the data, such a model is probably unrealistic in view of the fact that the disordered positions are only 0.75 A apart. A more realistic model is one involving an ordered hydrogen atom undergoing thermal motion in a very anharmonic potential well. We propose a model in which the hydrogen atom is relatively free to move over a region of about 1 A2 by small rotations of...

A Short, Slightly Asymmetrical, Intramolecular Hydrogen Bond: A Neutron Diffraction Study of Bis(2‐amino‐2‐methyl‐3‐butanone Oximato)Nickel(II) Chloride Monohydrate {Ni(C5H11N2O)2H}+Cl−·H2O

A neutron diffraction study of bis(2‐amino‐2‐methyl‐3‐butanone oximato)nickel(II) chloride monohydrate {Ni(C5H11N2O)2H}+Cl−·H2O has provided a wealth of precise information concerning the short intramolecular hydrogen bond, the dynamics of rotating methyl groups, and the effects of intermolecular environment on potential functions. The intensities of 3500 single‐crystal reflections were measured at the Brookhaven High Flux Beam Reactor. These data were used in conjunction with the earlier x‐ray determination to locate all atoms including the 25 hydrogen atoms. After least squares refinement with anisotropic temperature factors, the agreement factor R = ΣΔ(F2) / ΣF02 was 0.055. The short intramolecular hydrogen bond O···O [2.420(3) A] is unique in that no bond symmetry is imposed by the space group, and it is slightly asymmetrical. The O–H bond lengths for the hydrogen bond are 1.242(5) and 1.187(5) A, and the O–H–O angle is 169.9(3)°. The potential apparently has a broad, flat, single minimum, shifted tow...

Precision neutron diffraction structure determination of protein and nucleic acid components. VIII: the crystal and molecular structure of the β-form of the amino acidl-glutamic acid

A neutron diffraction study of the β-form of L-glutamic acid, C5H9NO4, has been carried out. The structure is orthorhombic: space groupP212121,a= 5·159(5),b= 17·30(2),c= 6·948(7) Å andz = 4. Least-squares refinements based on 803 reflexions led to a final conventionalR value of 0·026, and bond lengths involving hydrogen atoms have been determined with an average precision of 0·004 Å. The molecule is in the zwitterion form, and no intramolecular hydrogen bonds have been found. The hydrogen atom involved in a strong hydrogen bond between two carboxyl groups in adjacent molecules (0 ... 0 distance 2·519(2) Å) is covalently bonded to the carboxyl group belonging to the side chain of the amino acid. This side chain is buckled with Cδgauche to Cα with respect to the Cβ—Cγ bond. The bond angles involving carbon atoms in the side chain are accordingly strained.

Precision neutron diffraction structure determination of protein and nucleic acid components. XII. A study of hydrogen bonding in the purine‐pyrimidine base pair 9‐methyladenine · 1‐methylthymine

A neutron diffraction study of the 1 : 1 complex between 9‐methyladenine and 1‐methylthymine, C6H7N5· C6H8N2O2, has been carried out. The structure is monoclinic, space group P21/m, with two base pairs per unit cell; cell parameters a =8.304(2), b =6.552(2), c =12.837(3)A, and β=106.83(5)°. The structure has been refined by full‐matrix least squares techniques starting from the x‐ray results of Hoogsteen [K. Hoogsteen, Acta Crystallogr. 12, 822 (1959); Acta Crystallogr. 16, 907 (1963)]. All hydrogen atoms have been located with a precision better than 0.01 A, with the exception of methyl group hydrogens. The thymine molecules appear to be slightly disordered by means of a 180° rotation about N3 ··· C6, which has the effect of interchanging N1 and C5 while leaving the positions of all other atoms approximately unchanged. Between 10% and 13% of the thymine molecules in the structure are disordered in this way. Average refined neutron scattering lengths for nitrogen and carbon are bN=0.910(3) and bC=0.657(...

Precision neutron diffraction structure determination of protein and nucleic acid components. XV. Crystal and molecular structure of the amino acid L‐valine hydrochloride

A neutron diffraction study of L‐valine · HCl has been carried out: space group P21, a = 10.382(2), b = 7.066(1), c = 5.4407(9) A, β = 91.40(2)°, Z = 2. The structure has been refined by full‐matrix least‐squares techniques with anisotropic temperature factors for all atoms and with a Type II anisotropic extinction correction, leading to a conventional R value of 0.031. All hydrogen atoms have been located with a precision of 0.005 A. The structure is stabilized by a three‐dimensional network of one O–H … Cl and three N–H … Cl hydrogen bonds, one for each hydrogen atom that is expected to participate in hydrogen bonding. The potential energy barrier for torsional motion of the ammonium group is estimated to be 6.4 kcal mole−1, or about 3 times larger than those found for the methyl groups. This difference reflects the effects of hydrogen bonding.

Methyl group rotation and the low temperature transition in hexamethylbenzene. A neutron diffraction study

Neutron diffraction studies of single crystals of hexamethylbenzene at 298 K and at 130 K indicate that the molecule in phase II has approximate D3d symmetry. The amplitudes of libration of the methyl group and of rigid body motions of the molecule are consistent with earlier data, except that the barrier to methyl group rotation appears to be lower by about 0.5 kcal/mol (2100 J/mol).Consideration of intra- and inter-molecular hydrogen atom contact distances and calculated potential energy curves using a 6-exp potential function suggest that intermolecular forces are important in determining the barrier to rotation of the methyl groups and that substantial changes in the intermolecular packing must be responsible for the lambda-point transition at 116 K and the consequent profound change in the potential barrier to internal rotation which has been previously observed.