CRYSTAL POLYMORPHISM IN PENDIMETHALIN HERBICIDE IS DRIVEN BY ELECTRONIC DELOCALIZATION AND CHANGES IN INTRAMOLECULAR HYDROGEN-BONDING - A CRYSTALLOGRAPHIC, SPECTROSCOPIC AND COMPUTATIONAL STUDY

Pendimethalin, ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzeneamine, is a potent herbicide that exists in two differently coloured polymorphic crystal habits. Triclinic pendimethalin I (P (1) over bar) is the oran ge-coloured thermodynamically stable form, whereas monoclinic pendimet halin II (P2(1)/c) is a bright-yellow metastable form. The latter is n ormally produced first upon cooling the molten chemical, whereas the o range form is formed by a polymorphic phase transition which occurs sl owly upon long term storage of the yellow form at temperatures below i ts melting point. Such phase transitions are rapidly revealed by calor imetry. The crystal structures of the polymorphs have been determined using single crystal X-ray diffraction, Solid state NMR spectroscopy, vibrational spectroscopy and UV-VIS spectroscopy were applied to furth er study the nature of the polymorphism in terms of intra- and inter-m olecular properties. Solid state CP-MAS C-13 NMR spectroscopy was show n to be the method of choice for quantitative analysis of polymorphic mixtures. The differences in spectral properties and crystal habits we re investigated by computational methods which included molecular exci ton, molecular orbital and molecular mechanics calculations. The drama tic colour change from yellow to orange-red during the polymorphic tra nsition is discussed in terms of competing inter- and intra-molecular electronic effects. The driving force for the yellow (II) to orange(I) polymorphic transition is attributed to the change in the electronic delocalization achieved from shortening, strengthening, and partially straightening the 'bent' hydrogen bond between the secondary amino hyd rogen and an oxygen of the 6'-nitro group. This results in increased o verlap between the amino nitrogen's lone pair and the pi-electron orbi tals of the aromatic ring. The calculated lattice stabilization energy due to this process is 4 to 5 kcal mol(-1), and the relative lattice energies are consistent with the observed stabilities of the polymorph s, The slow kinetics of the polymorphic transition are largely governe d by the steric interaction of the 1-etlhylpropyl side chain and the t wo nitro groups. During crystallization, the more compact side chain c onformation required to form the energetically more stable orange (I) polymorph appears to-be more difficult to achieve than that required f or the yellow (II) polymorph.