We present dynamical studies of the CNϩH 2 reaction based on an empirical potential energy surface that is derived from high quality ab initio calculations. The ab initio calculations, which use a multireference configuration interaction method with large correlation consistent basis sets, indicate that the linear HHCN barrier is about 4.3 kcal/mol above CNϩH 2 , and that there is no reaction path which connects CNϩH 2 to the stable intermediate H 2 CN, although there is a path for dissociation of H 2 CN to HϩHCN. The empirical surface is written as a sum of two-, three-, and four-body terms, with the two-and three-body terms for HCN based on an accurate global surface that describes both the HCN and HNC force fields. The four-body terms are developed so as to describe the HHCN linear saddle point and the H 2 CN minimum accurately, as well as dissociation of H 2 CN into HCNϩH, and the ridge which separates the abstraction and H 2 CN dissociation pathways. Other features of the potential surface, such as the HCNH cis and trans minima, and the pathways leading to the formation of HNCϩH are also described, though less accurately. Three different choices for the HHCN saddle point properties are considered. We find that the surface which matches the ab initio barrier energy most accurately gives rate constants that are too low. Much better agreement is obtained using a 3.2 kcal/mol barrier. The trajectory results show typical dependence of the CNϩH 2 reactive cross sections on initial translational energy and initial vibration/rotation state, with CN behaving as a spectator and H 2 playing an active role in the reaction dynamics. Analysis of the HϩHCN products indicates that both the C-H stretch and bend modes are significantly excited, with bend excitation showing strong sensitivity to the saddle point properties and to reagent translational energy. At translational energies below 20 kcal/mol, direct H abstraction is strongly favored over addition elimination.

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Despite the fact that lead poisoning is the most common disease of environmental origin in the United States, the spectroscopic properties of aqueous Pb(II) coordination compounds have not been extensively investigated. Spectroscopic techniques that can be used to probe the fundamental coordination chemistry of Pb(II) will aid in both the development of water-soluble ligands that bind lead both tightly and selectively and the characterization of potential biological targets. Here, we report the preparation and characterization of a series of Pb(II) complexes of amido- derivatives of EDTA. The 207Pb chemical shift observed in these complexes (2441, 2189, and 1764 ppm for [Pb(EDTA)]2-, Pb(EDTA-N2), and [Pb(EDTA-N4)]2+, respectively) provides an extremely sensitive measure of the local environment and the charge on each complex. These shifts help to map out the lead chemical shift range that can be expected for biologically relevant sites. In addition, we report the first two-dimensional 207Pb-1H heteronuclear multiple-quantum correlation (HMQC) nuclear magnetic resonance spectra and demonstrate that this experiment can provide useful information about the lead coordination environment in aqueous Pb(II) complexes. Because this technique allows 207Pb-1H couplings through three bonds to be identified readily, 207Pb-1H NMR spectroscopy should prove useful for the investigation of Pb(II) in more complex systems (e.g., biological and environmental samples).

The ab initio potential energy surface of the ArCO2 cluster is calculated using the supermolecular Mo/ller–Plesset perturbation theory (S-MPPT) and dissected into its fundamental components; electrostatic, exchange, induction, and dispersion energies. The surface contains a single minimum for the perpendicular approach of Ar toward the C atom which has a well depth of ∼210 cm−1 at R=6.5 a0. This value is obtained using an extended basis set supplied with the bond functions and the fourth order supermolecular Mo/ller–Plesset calculations, and is expected to be accurate to within ±5%. The areas of the surface corresponding to the collinear approach of Ar to CO2 contain an extended plateau. The saddle point in this region for R=9.0 a0 is stabilized by 117 cm−1. The analytical pair potential for Ar–CO2 obtained by fitting to the individual interaction components is provided. The three-body effects in the related cluster, Ar2CO2, are examined for two configurations of the Ar2CO2 cluster. The overall nonadditivity is dominated by the three-body dispersion effect; however, the exchange nonadditivity is the most anisotropic. The sources of this anisotropy are discussed.

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