Calculating angle strain in actual compounds

•	So far we only have a predicted value of angle strain in planar rings. We need a measure of angle strain in actual compounds, so that we can compare this to the predicted values.

•	The preferred angle in actual compounds can be determined, for an n-membered carbocyclic ring, by taking the average internal angle across a large number crystal structures. Angle strain can then be calculated as the difference between this average internal angle and the ideal sp3 angle of 109.5.

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Calculate, from crystal structure data, the actual angle strain in cyclohexane. For this task you are provided with 5 structures each containing a cyclohexane ring (CSD refcodes: ALIPIU, AZANUK, CYCHEX, CHXDCA and BCYHAC). To view a structure click on its refcode in the Structure Navigator on the right hand side of the main Mercury window. For each structure measure all six internal C-C-C angles, then calculate the average internal angle of each structure and the overall average across the whole set.

 

To measure angles in Mercury

 

1. Set Picking Mode in the tool bar (near the top of the main Mercury window) to the required parameter type, viz. Measure Distance, Measure Angle or Measure Torsion

 

2. Geometrical measurements (intramolecular or intermolecular) can now be made by clicking on e.g., two atoms for a distance, three atoms for an angle or four atoms for a torsion angle.

 

3. To remove all geometrical measurements from the display click on the Clear Measurements button in the tool bar near the top of the main Mercury window.

 

 

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Note: For the purpose of this task only a small number of representative structures are provided. However, if you have full access to the Cambridge Structural Database (CSD) the average internal angle can be calculated across a large number of cyclohexane fragments (see SUPPLEMENTARY MATERIAL: DETERMINING THE PREFERRED INTERNAL BOND ANGLE IN CYCLOHEXANE USING THE FULL CSD).