Crystal polymorphism in pendimethalin herbicide is driven by electronic delocalization and changes in intramolecular hydrogen bonding. A crystallographic, spectroscopic and computational study

Abstract

Pendimethalin, N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine, is a potent herbicide that exists in two differently coloured polymorphic crystal habits. Triclinic pendimethalin I (P1 ¯) is the orange-coloured thermodynamically stable form, whereas monoclinic pendimethalin II (P2 1 /c) is a bright-yellow metastable form. The latter is normally produced first upon cooling the molten chemical, whereas the orange form is formed by a polymorphic phase transition which occurs slowly upon long term storage of the yellow form at temperatures below its melting point. Such phase transitions are rapidly revealed by calorimetry. 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 further study the nature of the polymorphism in terms of intra-and inter-molecular properties. Solid state CP-MAS 13 C NMR spectroscopy was shown to be the method of choice for quantitative analysis of polymorphic mixtures. The differences in spectral properties and crystal habits were investigated by computational methods which included molecular exciton, molecular orbital and molecular mechanics calculations. The dramatic colour change from yellow to orange-red during the polymorphic transition 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 hydrogen and an oxygen of the 6Ј-nitro group. This results in increased overlap between the amino nitrogen's lone pair and the -electron orbitals 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 polymorphs. The slow kinetics of the polymorphic transition are largely governed by the steric interaction of the 1-ethylpropyl side chain and the two nitro groups. During crystallization, the more compact side chain conformation required to form the energetically more stable orange (I) polymorph appears to be more difficult to achieve than that required for the yellow (II) polymorph.

NMR studies of organic polymorphs & solvates

“…In particular, many systems contain nitrogen and CPMAS NMR of 15 N at natural abundance is generally feasible, though frequently requiring overnight accumulation of spectra because the receptivity of this nuclide is only 0.0225 that of 13 C. so-called dipolar dephasing pulse sequence (which tends to eliminate resonances of protonated nitrogens from the spectrum), indicates that N11 is protonated. Nitrogen-15 NMR has been perhaps under-used for organic polymorphism to date, most studies coming from the group of Harris, 5,18,51,52,[65][66][67] with others from Stockton et al, 68 Glaser, Shiftan and Drouin, 69 Schmidt, 70 Lo ´pez et al, 71 Reutzel-Edens et al, 72 and Garcia et al 4 The recent development 73 of the inverse-detection technique (i.e. observation of a rare-spin spectrum by monitoring proton magnetisation) for 15 N CPMAS NMR should result in a rapid expansion of the use of this nuclide for polymorphism studies.…”

Decreasing the Viscosity in CO2 Capture by Amino-Functionalized Ionic Liquids through the Formation of Intramolecular Hydrogen Bond

“…To avoid the formation of strong hydrogen-bonded networks, an efficient method for improving CO 2 capture was reported by non-amine-functionalized ILs, ,,,− whose viscosity did not increase dramatically upon the uptake of CO 2 because of lacking the formation of hydrogen bond. It is well-known that intramolecular hydrogen bond was another hydrogen bond mode that benefits the stability of organic molecules , and biomolecule, , determines the product ratios, and influences physical and chemical characteristics, ,− which could avoid the formation of hydrogen bonding network among IL molecular . Therefore, can we develop a new strategy for avoiding the dramatic increase or realizing the decrease in viscosity during CO 2 capture by ILs making use of intramolecular hydrogen bond?…”

A combined NMR crystallographic and PXRD investigation of the structure-directing role of water molecules in orotic acid and its lithium and magnesium salts

“…[8][9][10][11] Due to its particular sensitivity towards the local environment of nuclei, especially hydrogen, solid-state NMR is a powerful indicator of conformational changes, (subtle) variations in molecular packing and underlying interactions as well as of solid-state dynamics. 12,13 Additionally, as solid-state NMR provides a way of studying crystalline forms as well as powdered samples, dosage forms and even amorphous materials, 14 it is widely used in the field of pharmaceutical research, e.g., for the identification of structures, polymorphs and solvates, 5,[15][16][17][18][19][20][21][22] to study the interactions between the API and excipients 23 and for the investigation of drug stability.…”