_chemical_formula[]
Peter Murray-Rust pm286 at cam.ac.ukSun Sep 19 10:40:57 BST 2004
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I would be grateful for clarification on the intended and actual use of the _chemical_formula[] Core dictionary items. The dictionary entry includes ---------------------- Definition _chemical_formula_ items specify the composition and chemical properties of the compound. The formula data items must agree with those that specify the density, unit-cell and Z values. _chemical_formula_iupac '[Mo (C O)4 (C18 H33 P)2]' _chemical_formula_moiety 'C40 H66 Mo O4 P2' _chemical_formula_structural '((C O)4 (P (C6 H11)3)2)Mo' _chemical_formula_sum 'C40 H66 Mo O4 P2' _chemical_formula_weight 768.81 ---------------------- There are at least two different ways of creating such data items: - transcription from a chemical logbook or paper where the formula represents the known chemical constitution. This can be derived from knowledge of preparation, analytical measurements, spectra etc. - summation of the scattering matter determined by the diffraction experiment. In many cases these will not agree. This can be due to disorder, unobserved atoms (e.g. H), etc. It is important to be able to state the composition of the sample independently of the diffraction experiment. (Indeed if the structure is not solved this is probably all that can be stated!) Such compositions are extremely valuable in publishing crystallography to the chemical community and in internal checks of the CIF. However I gather that some software generates _chemical_formula fields directly from the _atom_site and _symmetry information. I think it would be useful to have fields such as: _chemical_formula_iupac_meas _chemical_formula_moiety_meas _chemical_formula_structural_meas _chemical_formula_sum_meas _chemical_formula_weight _meas _chemical_formula_iupac_calc _chemical_formula_moiety_calc _chemical_formula_structural_calc _chemical_formula_sum_calc _chemical_formula_weight _calc This would indicate that the first had been experimentally measured by some means other than diffraction and the second would show that they had been calculated fro the _atom_site + _symmetry information. The current _chemical_formula_iupac _chemical_formula_moiety _chemical_formula_structural _chemical_formula_sum _chemical_formula_weight would give no information about the provenance of this information. P. Peter Murray-Rust Unilever Centre for Molecular Informatics Chemistry Department, Cambridge University Lensfield Road, CAMBRIDGE, CB2 1EW, UK Tel: +44-1223-763069
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