In recent years, neonicotinoid insecticides have been the fastest growing class of insecticides in modern crop protection, with widespread use against a broad spectrum of sucking and certain chewing pests. As potent agonists, they act selectively on insect nicotinic acetylcholine receptors (nAChRs), their molecular target site. The discovery of neonicotinoids can be considered as a milestone in insecticide research and greatly facilitates the understanding of functional properties of the insect nAChRs. In this context, the crystal structure of the acetylcholine-binding proteins provides the theoretical foundation for designing homology models of the corresponding receptor ligand binding domains within the nAChRs, a useful basis for virtual screening of chemical libraries and rational design of novel insecticides acting on these practically relevant channels. Because of the relatively low risk for nontarget organisms and the environment, the high target specificity of neonicotinoid insecticides, and their versatility in application methods, this important class has to be maintained globally for integrated pest management strategies and insect resistance management programs. Innovative concepts for life-cycle management, jointly with the introduction of generic products, have made neonicotinoids the most important chemical class for the insecticide market.

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A coupled Hartree–Fock method with individual gauge for localized orbitals (IGLO) proposed previously is reformulated and applied to the calculation of the magnetic susceptibility χ and the chemical NMR-shifts σ of various small molecules. The agreement with experiment is usually very good. Unlike in traditional methods, the results are not very sensitive to the size of the basis, and the application to large molecules does not pose serious problems. The results are analyzed in terms of orbital contributions, and it is shown that local diamagnetic terms are transferable, while local paramagnetic as well as nonlocal contributions are not. Pictorial explanations of the large antishielding effects in F2 and H2CO are given. In CH+3, a large dependence of χ and σ on changes of the geometry (pyramidalization) is found.

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The interaction energies of the dimethylsulfide–methanol (I) and dimethylthiocarbonyl–methanol (II) complexes are calculated as a function of the S⋯H–O distances at various levels of theory and compared to those of their oxygen analogs. At the coupled cluster level the binding energy of (I) is −5.46 kcal/mol, only slightly smaller than the hydrogen bond energy of −5.97 kcal/mol for the corresponding oxygen analog, i.e., the dimethylether–methanol complex. It is also considerably larger than for dimethylether–methylthiol, where S and O of the parent complex are interchanged. Density functional theory is unable to describe these weak interactions properly. Choosing second-order Møller–Plesset perturbation theory, the interaction potential surfaces of both complexes with respect to the three relevant intermolecular coordinates are compared. The interactions in the hydrogen bonds involving sulfur are classified by Morokuma, atoms-in-molecules, and natural bond orbital analyses.

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