SUBSTITUENT CROWDING IN NITROBENZENES

The rotational motion of adjacent nitro groups in 1,2-dinitrobenzene a nd 1,2,4-trinitrobenzene has been investigated using X-ray diffraction and quantum chemical calculations, Pilot calculations on nitrobenzene indicate that density functional theory provides reliable geometries for this type of compound, Using this method for the substituted compo unds, we find that rotation of the two adjacent nitro groups can take place quite freely within a groove in the potential surface that defin es an interlocking cogwheel mode. The experimental structures of both molecules are found to have nitro group configurations corresponding t o this mode of motion. (C) 1998 Elsevier Science B.V.