Sentence:  In the branched 21b, the central six-membered ring becomes essentially atropic (NICS=-1.1), as do the adjacent cyclobutadienes (NICS=-0.4), allowing for maximum <WSD>diatropicsm</WSD> of the three terminal cycles (CICS=-10.7; δ=7.24, 7.31ppm). 
Meaning of the word marked with <WSD> tags:  An orthogonal arrangement of groups of atoms within a molecule, often associated with aromaticity.