Exploring the Potential Energy Surface of the Association of Cu+ to Oxaziridine, Nitrosomethane, and Formaldoxime

Modelling peptide-metal dication interactions: formamide-Ca2+ reactions in the gas phase

The collision induced dissociation of formamide-Ca(2+) complexes produced in the gas phase through nanoelectrospray ionization yields as main products ions [CaOH](+), [HCNH](+), [Ca(NH(2))](+), HCO(+) and [Ca(NH(3))](2+) and possibly [Ca(H(2)O)](2+) and [C,O,Ca](2+), the latter being rather minor. The mechanisms behind these fragmentation processes have been established by analyzing the topology of the potential energy surface by means of B3LYP calculations carried out with a core-correlated cc-pWCVTZ basis set. The Ca(2+) complexes formed by formamide itself and formimidic acid play a fundamental role. The former undergoes a charge separation reaction yielding [Ca(NH(2))](+) + HCO(+), and the latter undergoes the most favorable Coulomb explosion yielding [Ca-OH](+) + [HCNH](+) and is the origin of a multistep mechanism which accounts for the observed loss of water and HCN. Conversely, the other isomer of formamide, amino(hydroxyl)carbene, does not play any significant role in the unimolecular reactivity of the doubly charged molecular cation.

Isomerization of nitrosomethane to formaldoxime: energies, geometries, and frequencies from the parametric variational two-electron reduced-density-matrix method

The isomerization of nitrosomethane to trans-formaldoxime is treated with the parametric variational two-electron reduced-density-matrix (2-RDM) method. In the parametric 2-RDM method, the ground-state energy is minimized with respect to a 2-RDM that is parameterized to be both size extensive and nearly N-representable. The calculations were performed with an efficient version of the 2-RDM method that we developed as an extension of the PSI3 ab initio package. Details of the implementation, which scales like configuration interaction with single and double excitations, are provided as well as a comparison of two optimization algorithms for minimizing the energy functional. The conversion of nitrosomethane to trans-formaldoxime can occur by one of two pathways: (i) a 1,3-sigmatropic hydrogen shift or (ii) two successive 1,2-sigmatropic hydrogen shifts. The parametric 2-RDM method predicts that the reaction channel involving two sequential 1,2-shifts is about 10 kcal/mol more favorable than the channel with a single 1,3-shift, which is consistent with calculations from other ab initio methods. We computed geometric parameters and harmonic frequencies for each stationary point on the reaction surfaces. Transition-state energies, geometries, and frequencies from the 2-RDM method are often more accurate than those from traditional wave function methods of a similar computational cost. Although electronic-structure methods generally agree that the 1,2-shift is more efficient, the energy ordering of the reactant nitrosomethane and the 1,2-shift intermediate formaldonitrone is unresolved in the literature. With an extrapolation to the complete-basis-set limit the parametric 2-RDM method predicts formaldonitrone to be very slightly more stable than nitrosomethane.

Copper-biomolecule complexes in the gas phase. The ternary way

This review deals with copper complexes of a variety of organic and bioorganic molecules that have been produced as gas-phase ions by electrospray and other ionization methods and studied experimentally by mass spectrometry and theoretically by ab initio and density functional theory computations. Ternary complexes of Cu((II)) allow one to modify the oxidation state and coordination sphere of the copper ion and thus induce novel fragmentations that involve redox and radical-based reactions. Structure elucidation, distinction, and quantitation of leucine and isoleucine isomers in peptides, distinction of enantiomers in chiral compounds, and sensitive detection of antibiotics are some of the highlights of mass spectrometry of ternary copper complexes. Binary copper complexes are mainly represented by Cu((I)) species in which the copper ion displays the properties of a weak Lewis acid.

An Experimental and Theoretical Investigation of Gas-Phase Reactions of Ca2+ with Glycine

The gas-phase reactions between Ca(2+) and glycine ([Ca(gly)](2+)) have been investigated through the use of mass spectrometry techniques and B3-LYP/cc-pWCVTZ density functional theory computations. The major peaks observed in the electrospray MS/MS spectrum of [Ca(gly)](2+) correspond to the formation of the [Ca,C,O(2),H](+), NH(2)CH(2) (+), CaOH(+), and NH(2)CH(2)CO(+) fragment ions, which are produced in Coulomb explosion processes. The computed potential energy surface (PES) shows that not only are these species the most stable product ions from a thermodynamic point of view, but they may be produced with barriers lower than for competing processes. Carbon monoxide is a secondary product, derived from the unimolecular decomposition of some of the primary ions formed in the Coulomb explosions. In contrast to what is found for the reactions of Ca(2+) with urea ([Ca(urea)](2+)), minimal unimolecular losses of neutral fragments are observed for the gas-phase fragmentation processes of [Ca(gly)](2+), which is readily explained in terms of the topological differences between their respective PESs.

CASPT2 Study of the Decomposition of Nitrosomethane and Its Tautomerization Reactions in the Ground and Low-Lying Excited States

The dissociation reaction of nitrosomethane into methyl and nitric oxide and the tautomerization reactions to formaldehyde oxime, nitrone, and methoxy nitrene have been studied with the second-order multiconfigurational perturbation theory (CASPT2) by the computation of numerical energy gradients. The prevailing reactions in both the ground and the excited states are dissociations. The structures of the ground and excited states are compared with the corresponding complete active space SCF (CAS-SCF) geometries. It is found that changes in the individual bond lengths are rather large (0.01-0.02 A), while the character and energetics of the CASPT2 optimizations remain similar to the CAS-SCF values.