Showing papers by "Thomas F. Koetzle published in 1974"

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

Abstract: 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.

Hydrogen bond studies. 89. A neutron diffraction study of hydrogen bonding in 1‐methylthymine

Abstract: A neutron diffraction study of 1‐methylthymine has been carried out. Intensities of 1681 independent reflections were measured at the Brookhaven National Laboratory High Flux Beam Reactor and the structure was refined to an R(F2) of 0.046. The thymine molecules link to form dimers by means of N–H ··· O hydrogen bonds. The dimers appear to be slightly disordered (∼11%) such that dimers using one or the other of the oxygen atoms as hydrogen‐bond acceptors are present in the crystal structure. Evidence is found to suggest the existence of a C–H ··· O hydrogen bond; the H ··· O distance between the methine hydrogen atom in one dimer and the oxygen atom not participating in an N–H ··· O hydrogen bond in a different dimer is 2.045 (3) A. The C ··· O distance is 3.121(2) A and the bond is almost linear [∠C–H ⋯ O=170.9(3)°]. The bond distances between heavy atoms are determined to a precision of 0.002 A or better; the corresponding value for heavy atom‐hydrogen atom bond lengths is 0.006 A.

Hydrogen bond studies. 85. A very short, asymmetrical, intramolecular hydrogen bond: A neutron diffraction study of pyridine‐2,3‐dicarboxylic acid (C7H5NO4)

Abstract: A neutron diffraction study of pyridine‐2,3‐dicarboxylic acid (quinolinic acid) has been carried out. The intensities for 3780 reflexions were measured at the Brookhaven High Flux Beam Reactor. The structure was refined to an R(F2) of 0.054 using starting parameters from a previous x‐ray study. The short intramolecular hydrogen bond [O ··· O: 2.398(3) A] has no symmetry restriction operating on the hydrogen atom position and is asymmetric. The two O ··· H distances are 1.163(5) and 1.238(5) A, respectively, and the O–H–O angle is 174.4(4)°. The asymmetry of the bond can be explained by the intramolecular environment. The strain in the molecule caused by the short hydrogen bond results in long carbon‐carbon bonds to the carboxyl groups [1.516(2) and 1.541(2) A]. The C–O bond lengths [1.219(3), 1.263(2), 1.222(3), and 1.270(2) A] vary depending on the hydrogen‐bond involvement of the oxygen atoms. The mean C–H distance is 1.087 A and the N–H distance is 1.036(4) A. The nitrogen atom is involved in an interm...