The effect of asymmetry of the atomic charge distribution on the positions of terminal atoms as determined with X-rays

Coppens, P.; Coulson, C. A.

doi:10.1107/s0365110x67003573

Cited by 16 publications

(8 citation statements)

References 4 publications

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“…For the nitrogen atom of the cyano group a much better balance is obtained between the triple bond and lone-pair densities and no shift is observed. This explanation is confirmed by calculations by Matthews & Stucky (1971), using a formalism for the centroid of the atomic charge density according to a simple hybridization scheme • (Coppens & Coulson, 1967). This calculation ignores the effect of series termination and thermal modification and can therefore only provide a qualitative estimate.…”

supporting

confidence: 53%

Some implications of combined X-ray and neutron diffraction studies

Coppens¹

1974

Acta Crystallogr Sect B

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A survey is made of about ten combined X-ray and neutron diffraction studies. The theory of X-N difference densities is reviewed for both centric and acentric structures. It is concluded that the phase problem has been incorrectly treated in past work on acentric crystals, leading to a systematic underestimate of the difference density. Expressions are given for the average standard deviation in the difference density due to random errors in both the X-ray and neutron measurements. A survey of experimental peak heights indicates that (a) the peaks in lone-pair regions are generally lower than the maxima in the bonds, (b) double bonds may have lower difference densities than either triple or single bonds. The effect of thermal smearing on a theoretical difference density is analyzed and it is shown that the relatively low height of the lone-pair density is a result of molecular vibrations which affect the sharper lone-pair features more strongly than the overlap density. The first-row atom asphericity shifts are reviewed and discussed in terms of scattering factors associated with the difference density features. The size of the shifts varies with data cut-off and thermal amplitude of the atoms, as the lone pair scattering is relatively dominant in the high-order region. Introduction 'Do you feel that you are now using the theory to calibrate the experiment but as you go along to larger and' larger molecules the experiment will play the dominant role in getting out the really chemically interesting information?' Walter C. Hamilton ACA Symposium, Albuquerque (1973).It seems appropriate at this time to discuss how well the experiment has been calibrated by theoretical calculations and to examine information available from very few studies on larger molecules. Survey of structures studiedThe number of compounds that have been studied by the combined X-ray-neutron technique is limited, because the studies are time-consuming and require careful attention to details of both the X-ray and neutron experiments. A listing of published studies is given in Table 1. They fall in three groups: room-temperature studies of centric (1) and acentric (3) crystals and liquid nitrogen temperature studies of centric crystals (2). The resulting X-ray neutron difference density QX-N, which is further defined below represents the redistribution of the electron density when atoms bond to form a molecule. An example is given in Fig. 1, which shows 0X-N in the plane of the tetracyanoethylene molecule (Becker, Coppens & Ross, 1974). Typically, density is accumulated in the bonds between the atoms and in the lone pair at the 'free' side of the * Editorial note:-This paper was presented at a memorial symposium in honour of Walter C. Hamilton, a former Coeditor of Acta Crystallographica, held on 15 June 1973 at Brookhaven National Laboratory, and sponsored jointly by the American Crystallographic Association and the Brookhaven National Laboratory. nitrogen atom. The nitrogen atom and the ethylenic carbon atom are in negative regions indicating ...

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supporting

confidence: 53%

Some implications of combined X-ray and neutron diffraction studies

Coppens¹

1974

Acta Crystallogr Sect B

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“…• ii calculations with Slater orbitals have shown that the centroid of an oxygen atom of a nitro group is displaced outwards, away from the nitrogen atom (Coppens & Coulson, 1967). Similar calculations for C-N indicate a much smaller displacement of the nitrogen charge center (Coppens, unpublished), because the C---N is a stronger and shorter bond than N-O.…”

Section: Molecular Dimensions and Packingmentioning

confidence: 69%

X-ray structure analysis of cubic tetracyanoethylene and the length of the C[triple-bond] N bond. Application of a double-atom refinement method

Little¹,

Pautler²,

Coppens³

1971

Acta Crystallogr B Struct Crystallogr Cryst Chem

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The crystal structure of a cubic modification of tetracyanoethylene has been studied. The space group is Im3 and the cell edge is 9.736 (5) .~. The bond lengths agree well with those in the monoclinic modification, but there are significant differences between the bond angles in the two forms. The central C=C bond is 1.344 (2) • which is not significantly different from the corresponding bond length in ethylene and considerably shorter than predicted from INDO calculations. A new double-atom refinement method has been applied which, from the X-ray data alone, corrects for the apparent shortening of the C=N bond length, as previously found by comparison of X-ray and neutron diffraction data. The corrected value for C=N (1.166 (2)/~) is close to the electron diffraction result on tetracyanoethylene. The molecular packing in the crystal is such that large cavities surround the positions (0,0,0) and (½, ½, ½).

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“…In many cases the individual atomic asphericity represents a balance between these two factors (see e.g. Coppens & Coulson, 1967), whilst the effect on bond lengths is often the resultant of two atomic asphericities; the results for the cyano group [Table l(c)] are a clear example of these considerations. Furthermore it is not always possible either to predict or to rationalize these effects using simple concepts.…”

Section: Summary and Implicationsmentioning

confidence: 99%

“…A qualitative picture of O asphericity in the nitro group was presented by Coppens & Coulson (1967).…”

Section: Nitrogen-oxygen Bondsmentioning

confidence: 99%

A systematic pairwise comparison of geometric parameters obtained by X-ray and neutron diffraction

Allen¹

1986

Acta Crystallogr B Struct Sci

188

120

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The effects of X-ray atomic asphericity on derived molecular geometry have been examined for ten common substructures involving C, N, O. The Cambridge Structural Database has been used to locate pairs of X-ray (X) and neutron (N) structure determinations of the same compound. Rigid criteria based on experimental temperature, refinement procedures, crystalline form and precision have been applied in selecting suitable X, N structure pairs. Corresponding X and N values for derived parameters have been paired for 46 chemically unique subsets (39 bond-length sets and 7 valence-angle sets). The statistical significance of the distribution of X-N differences (Di) has been assessed in each case via a Wilcoxon matched-pairs

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The effect of asymmetry of the atomic charge distribution on the positions of terminal atoms as determined with X-rays

Cited by 16 publications

References 4 publications

Some implications of combined X-ray and neutron diffraction studies

Some implications of combined X-ray and neutron diffraction studies

X-ray structure analysis of cubic tetracyanoethylene and the length of the C[triple-bond] N bond. Application of a double-atom refinement method

A systematic pairwise comparison of geometric parameters obtained by X-ray and neutron diffraction

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