The nature of halogen...halogen synthons: crystallographic and theoretical studies

A study of the halogen...halogen contacts in organic compounds using ab initio calculations and the results of previously reported crystallographic studies show that these interactions are controlled by electrostatics. These contacts can be represented by the geometric parameters of the C--X1...X2--C moieties (where theta1=C--X1...X2 and theta2=X1...X2--C; ri=X1...X2 distance). The distributions of the contacts within the sum of van der Waals radii (rvdW) versus thetai (theta1=theta2) show a maximum at theta approximately 150 degrees for X=Cl, Br, and I. This maximum is not seen in the distribution of F...F contacts. These results are in good agreement with our ab initio calculations. The theoretical results show that the position of the maximum depends on three factors: 1) The type of halogen atom, 2) the hybridization of the ipso carbon atom, and 3) the nature of the other atoms that are bonded to the ipso carbon atom apart from the halogen atom. Calculations show that the strength of these contacts decreases in the following order: I...I>Br...Br>Cl...Cl. Their relative strengths decrease as a function of the hybridization of the ipso carbon atom in the following order: sp2>sp>sp3. Attaching an electronegative atom to the carbon atom strengthens the halogen...halogen contacts. An electrostatic model is proposed based on two assumptions: 1) The presence of a positive electrostatic end cap on the halogen atom (except for fluorine) and 2) the electronic charge is anisotropically distributed around the halogen atom.     

Exchange coupling mediated by N-H···Cl hydrogen bonds: experimental and theoretical study of the frustrated magnetic system in bis(o-phenylenediamine)nickel(II) chloride

The title compound crystallizes in the monoclinic P2(1)/c space group with a = 11.2470(3) ?, b = 5.9034(2) ?, c = 12.0886(3) ?, β = 115.143(1)°, and V = 726.58(4) ?(3) and consists of discrete monomeric NiCl(2)(o-phenylendiamine)(2) molecules. Each o-phen ligand coordinates in a bidentate mode with the chloride ions occupying trans positions in the resulting tetragonally distorted octahedral coordination sphere. Two discrete sets of N-H···Cl hydrogen bonds link the octahedral molecules into a two-dimensional network, with type 1 interactions linking adjacent monomers along the c axis and type 2 interactions linking monomers along the diagonals in the bc plane. Analysis of the magnetic data reveals the existence of weak antiferromagnetic coupling within the layers via these hydrogen bonds, in addition to the presence of zero field splitting, with the best fit obtained for a 1d antiferromagnetic model with g = 2.0917(7), J/k = -2.11(4) K [J = -1.47(3) cm(-1)], and D = 1.05(3) cm(-1) [β = D/|J| = 0.72(6)] for the model with D > 0 and g = 2.0911(6), J/k = -2.26(1) K [J = -1.57(1) cm(-1)], and D = -0.86(1) cm(-1) [β = D/|J| = 0.55(6)] for the model with D < 0. Theoretical calculations of the exchange coupling confirm the experimental results, yielding values of J(1) = -1.39 cm(-1) for the type 1 hydrogen bonds and J(2)/k = -0.56 cm(-1) for the type 2 hydrogen bonds.     

Bis(2,3‐di­methyl­quinoxalinium) tribromo­cuprate(I)

The title compound, (C₁₀H₁₁N₂)₂[CuBr₃], contains layers of planar monoprotonated cations. Isolated trigonal–planar [CuBr₃]^2? anions are hydrogen bonded to cations in adjacent layers, providing three‐dimensional stability to the crystal structure.     

Automatic classification of chemical structure databases using a highly parallel array processor

This article describes the use of the ICL Distributed Array Processor (DAP) for the automatic classification of chemical structure databases using the Jarvis-Patrick clustering method. This method is based upon the calculation of a table containing the nearest neighbors for each of the molecules in the database which is to be clustered. These nearest neighbors can be identified very efficiently using the DAP since it allows up to 4096 molecules to be compared with a specified molecule in parallel. Experiments with files of 4096 and 8192 structures from the Fine Chemicals Database show that clustering with the DAP is up to 6.7 times as fast as using a highly efficient, inverted file algorithm on an IBM 3083 mainframe.     

The structure and EPR properties of (CH2OHCH2NH3)2CuCl4

The title compound is a member of the (RNH₃)₂CuCl₄ layer perovskite family, space group C2/c, witha=21.624(5),b=7.511(1),c=7.431(2)Å, =89.56(2)° andV=1206.9(4)ų withZ=4. In the antiferrodistortive structure, the Cu(II) ion assumes an elongated octahedral coordination (unique Cu–Cl distances=2.274(2), 2.288(2), and 3.010(2)Å). The octahedra share corners with four neighbors to form a two-dimensional network. The –NH₃ moieties hydrogen bond to the layers, so that the –C₂H₅OH moieties form sheathes about each metal halide layer. A twofold disorder of the –C₂H₅OH groups is observed. The EPR spectrum is investigated to help understand the magnetic properties of the systems. Theg-values are consistent with strong intralayer exchange coupling. The EPR linewidths show evidence of spin diffusion effects at liquid nitrogen temperature. However, at room temperature, the spin anisotropies lead to broadening of the EPR lines through a phonon modulation mechanism.