Gaseous and crystalline phase molecular structures of 4,6-dichloropyrimidine, 2,6-dichloropyrazine and 3,6-dichloropyridazine

Abstract

The gas-phase molecular structures of 4,6-dichloropyrimidine, 2,6-dichloropyrazine and 3,6-dichloropyridazine have been determined by electron diffraction (GED) and ab initio calculations, and are compared to their respective parent compounds to demonstrate the effects of chlorination on ring geometry. The crystal structures of the three dichloro compounds are also reported; the intermolecular contacts leading to distortions in the solid phases have been identified. Gas-phase electron diffraction (GED)Sample preparation. The sample of 4,6-dichloropyrimidine was a gift from Dr R. V. H. Jones of Zeneca plc. Both 2,6dichloropyrazine and 3,6-dichloropyridazine were bought from Lancaster Synthesis at 99 and 98% purity and used in the GED analysis without further purification.GED experiments. Electron scattering intensities were recorded on Kodak Electron Image photographic plates using the Edinburgh apparatus. 15 Six plates (three from the long camera distance and three from the short distance) were recorded for each compound and traced digitally using a computer controlled Joyce Loebl MDM6 microdensitometer 16 at the EPSRC Daresbury laboratory. Standard programs were used for the data reduction 17 with the scattering factors of Fink et al. 18 The weighting points used in setting up the offdiagonal weight matrix, s range, scale factors, correlation parameters and electron wavelengths are given in Table 1. Least-squares correlation matrices for the restrained GED refinement for the compounds under study have been supplied † Scale constants were obtained from the successful scaling of the forcefield for 1,3,5-triazine against a set of experimental IR frequencies.

Experimental and Theoretical Analysis of the Vibrational Spectra and Theoretical Study of the Structures of 3,6-Dichloropyridazine and 3,4,5-Trichloropyridazine

“…Accordingly, in Table only the newly calculated theoretical data are presented. The equilibrium geometry of 3,6-dichloropyridazine determined at the MP2/6-311G** level by Morrison et al (showing only marginal differences from our calculated data) are repeated in the table for better comparison. Experimental data originating from the papers of Almenningen et al and Morrison et al are also presented in Table .…”

“…We have determined the structural paramenters of 3,6-dichloropyridazine in its ground state at the HF/3-21G*, HF/6-31G*, HF/6-31G**, MP2/6-311G**, CCSD/6-311G**, BLYP/6-31G*, and B3LYP/6-31G* levels. Recently Morrison et al published the theoretical equilibrium geometries of 3,6-dichloropyridazine at the HF/3-21G*, HF/6-31G*, MP2/6-31G*, and MP2/6-311G** levels. Accordingly, in Table only the newly calculated theoretical data are presented.…”

“…It is therefore not surprising to find extensive information and, in particular, vibrational analysis about chloropyrimidines and chloropyrazines, although such accurate information does not yet exist for chloropyridazines. Specifically, only a few previous studies exist dealing with the structure , and vibrational and electronic spectra of 3,6-dichloropyridazine. Unfortunately, in the previous experimental vibrational studies, we can prove that the reported spectra showed a high percentage of impurities, which led to a completely mistaken assignment of the spectra.…”

The molecular structures of 2-, 3- and 4-chloropyridine and chloropyrazine in the gas phase by electron diffraction and ab initio calculations †

“…Two further parameters were required to reduce the effects of correlation between the six similar ring distances, namely rC-C av Ϫ rC-N av and rC(6)-N(1) Ϫ rC(2)-N(1). All restrained geometric parameters returned values in the least-squares refinement to within one standard deviation of the restrained value, except for p 13 and the dependent parameter restraint rC(6)-N(1) Ϫ rC(2)-N (1), which refined to within two standard deviations. The R G factor recorded for this refinement was 0.084, indicating that the data are of good quality.…”

Transferability and Physicochemical Interpretation of Canonical Force Fields in Redundant Internal Coordinates: Pyridazine and 3,6-Dichloropyridazine

“…Definition of Coordinates. Pyridazine and 3,6-dichloropyridazine − belong to the structural C 2 v symmetry point group . The experimental structures selected in the present study are taken from refs and for pyridazine and 3,6-dichloropyridazine, respectively.…”