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The torsion angle (or angle of twist) about the bond B--C in a series of bonded atoms A--B--C--D is defined as the angle of rotation needed to make the projection of the line B--A coincide with the projection of the line C--D, when viewed along the B--C direction. The positive sense is clockwise.
In general a chain of --CH2-- groups will have a staggered conformation so that torsion angles are 180 for C--C--C--C and 60 for C--C--C--H or H--C--C--H.
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