Crystallographic studies of dehydrohalogenation in solid meso-dibromobutane derivatives. II. The crystal and molecular structures of meso-β,β'-dibromoadiponitrile (I), meso-(RSRS)-α,α'-dimethyl-β,β'-dibromoadiponitrile (II) and meso-(RRSS)-α,α'-dimethyl-β,β'-dibromoadiponitrile (III)