ion. The oxidative addition mechanism was originally proposed22i because of the lack of a strong rate dependence on polar factors and on the acidity of the medium. Later, however, the electrophilic substitution mechanism also was proposed. Recently, the oxidative addition mechanism was confirmed by investigations into the decomposition and protonolysis of alkylplatinum complexes, which are the reverse of alkane activation. There are two routes which operate in the decomposition of the dimethylplatinum(IV) complex Cs2Pt(CH3)2Cl4. The first route leads to chloride-induced reductive elimination and produces methyl chloride and methane. The second route leads to the formation of ethane. There is strong kinetic evidence that the ethane is produced by the decomposition of an ethylhydridoplatinum(IV) complex formed from the initial dimethylplatinum(IV) complex. In D2O-DCl, the ethane which is formed contains several D atoms and has practically the same multiple exchange parameter and distribution as does an ethane which has undergone platinum(II)-catalyzed H-D exchange with D2O. Moreover, ethyl chloride is formed competitively with H-D exchange in the presence of platinum(IV). From the principle of microscopic reversibility it follows that the same ethylhydridoplatinum(IV) complex is the intermediate in the reaction of ethane with platinum(II). Important results were obtained by Labinger and Bercaw62c in the investigation of the protonolysis mechanism of several alkylplatinum(II) complexes at low temperatures. These reactions are important because they could model the microscopic reverse of C-H activation by platinum(II) complexes. Alkylhydridoplatinum(IV) complexes were observed as intermediates in certain cases, such as when the complex (tmeda)Pt(CH2Ph)Cl or (tmeda)PtMe2 (tmeda ) N,N,N′,N′-tetramethylenediamine) was treated with HCl in CD2Cl2 or CD3OD, respectively. In some cases H-D exchange took place between the methyl groups on platinum and the, CD3OD prior to methane loss. On the basis of the kinetic results, a common mechanism was proposed to operate in all the reactions: (1) protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, fivecoordinate platinum(IV) species, (3) reductive C-H bond formation, producing a platinum(II) alkane σ-complex, and (4) loss of the alkane either through an associative or dissociative substitution pathway. These results implicate the presence of both alkane σ-complexes and alkylhydridoplatinum(IV) complexes as intermediates in the Pt(II)-induced C-H activation reactions. Thus, the first step in the alkane activation reaction is formation of a σ-complex with the alkane, which then undergoes oxidative addition to produce an alkylhydrido complex. Reversible interconversion of these intermediates, together with reversible deprotonation of the alkylhydridoplatinum(IV) complexes, leads to multiple H-D exchange

Activation of c-h bonds by metal complexes

This critical review presents a comprehensive study of transition-metal carboxylate clusters which may serve as secondary building units (SBUs) towards construction and synthesis of metal–organic frameworks (MOFs). We describe the geometries of 131 SBUs, their connectivity and composition. This contribution presents a comprehensive list of the wide variety of transition-metal carboxylate clusters which may serve as secondary building units (SBUs) in the construction and synthesis of metal–organic frameworks. The SBUs discussed here were obtained from a search of molecules and extended structures archived in the Cambridge Structure Database (CSD, version 5.28, January 2007) which included only crystals containing metal carboxylate linkages (241 references).

/pdf/secondary-building-units-nets-and-bonding-in-the-chemistry-o0m4ttzsgz.pdf

Secondary building units, nets and bonding in the chemistry of metal–organic frameworks

https://anthropology.ucsd.edu/_files/Faculty%20Files/schoeninger-publications/DeNiro-Schoeninger1984.pdf

Nitrogen and carbon isotopic composition of bone collagen from marine and terrestrial animals

Publisher Summary The chapter focuses on a general description of the force fields that are most commonly used at present, and it gives an indication of the directions of current research that may yield better functions in the near future. After a brief survey of current models, mostly generated during the 1990s, the focus of the chapter is on the general directions the field is taking in developing new models. The most commonly used protein force fields incorporate a relatively simple potential energy function: The emphasis is on the use of continuum methods to model the electrostatic effects of hydration and the introduction of polarizability to model the electronic response to changes in the environment. Some of the history and performance of widely used protein force fields based on an equation on simplest potential energy function or closely related equations are reviewed. The chapter outlines some promising developments that go beyond this, primarily by altering the way electrostatic interactions are treated. The use of atomic multipoles and off-center charge distributions, as well as attempts to incorporate electronic polarizability, are also discussed in the chapter.

https://dasher.wustl.edu/ponder/papers/advprotchem-66-27-03.pdf

Force fields for protein simulations

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Lithium cation complexes with cysteine (Cys) are collisionally activated with xenon in a guided ion beam tandem mass spectrometer and observed to deaminate in addition to loss of the intact amino acid. Source conditions are found to influence the cross sections for these processes considerably, a result interpreted in terms of two isomers, Li+(Cys) and (NH3)Li+(C3H4O2S). Quantum chemical calculations at the B3LYP/6-311G(d,p) level are used to explore the reaction mechanism for this fragmentation process in detail. A complete reaction coordinate surface for the process is elucidated, including all intermediates and transition states. Theoretical molecular parameters for the two isomers and for the rate-limiting transition state for deamination of Li+(Cys) are then used to analyze the threshold energies in the experimental data, providing experimental measurements of the energies of the transition state and various products. These experimental energies are compared with single point energies calculated at t...

Energetics and mechanism for the deamination of lithiated cysteine

Collision-induced dissociation of N3+ (X3Σ−) with Ne, Ar, Kr, and Xe

Guided ion beam tandem mass spectrometry is used to measure the kinetic energy dependent cross sections for reactions of the lanthanide metal gadolinium cation (Gd+) and GdO+ with O2 and for collision-induced dissociation (CID) of GdO2+ with Xe. Gd+ reacts with O2 in an exothermic and barrierless reaction to form GdO+ and O. GdO2+ is also formed in this reaction, but this product ion is formed in a sequential reaction, as verified by pressure dependent measurements and comparison with the results for the reaction of GdO+ with O2. The CID experiments of GdO2+ indicate the presence of two GdO2+ precursor ion populations, assigned to a weakly bound oxygen molecule adduct (Gd+-O2) and an inserted cyclic Gd+ dioxide species (O-Gd+-O). Analysis of the resulting product ion cross sections yields bond dissociation energies (BDEs, D0) for Gd+-O2 and OGd+-O, where the latter BDE is also independently measured in an exchange reaction between GdO+ and O2. The CID experiments also provide the energy of the barrier for the rearrangement of the Gd+-O2 adduct to the inserted O-Gd+-O structure (as identified by loss of a single oxygen atom). The thermochemistry measured here yields D0(OGd+-O) = 2.86 ± 0.08 eV, D0(Gd+-O2) = 0.75 ± 0.11 eV, and a barrier height relative to Gd+-O2 of 0.31 ± 0.07 eV. These data are sufficient to characterize in some detail the potential energy surface of the Gd+ reaction with O2 entirely from experiment. Theoretical calculations are performed for comparison with the experimental energetics and for further insight into the reaction mechanisms.

Gadolinium cation (Gd+) reaction with O2: Potential energy surface mapped experimentally and with theory.

The gas-phase reactivity of the atomic palladium cation with CS(2) is investigated using guided-ion beam mass spectrometry. Endothermic reactions forming PdS(+) and PdCS(+) are observed. Analysis of the kinetic energy dependence of the cross sections for formation of the two products yields the 0 K bond energies of D(0)(Pd(+)-S) = 2.36 +/- 0.11 eV and D(0)(Pd(+)-CS) = 2.67 +/- 0.21 eV. Quantum chemical calculations are used to investigate the electronic structure of the two product ions. In addition, detailed calculations of the potential energy surfaces reveal that the mechanism involves oxidative addition of a CS bond to the metal cation followed by simple bond cleavages. Formation of the ground-state PdS(+) ((4)Sigma(-)) product is spin-forbidden from ground-state Pd(+) ((2)D) whereas PdCS(+) ((2)Sigma(+)) formation is spin-allowed. Crossing points between the doublet and quartet surfaces are located in the region of the SPd(+)(CS) intermediate, which suggests that coupling between the surfaces is efficient, consistent with experimental indications.

P. B. Armentrout

Papers

Energetics and mechanism for the deamination of lithiated cysteine

Collision-induced dissociation of N3+ (X3Σ−) with Ne, Ar, Kr, and Xe

Gadolinium cation (Gd+) reaction with O2: Potential energy surface mapped experimentally and with theory.

Guided ion beam and theoretical studies of the reactions of Pd+ with CS2: thermochemistry of PdS+ and PdCS+.

Thermochemistry of uranium halide ions: reactions of uranium(+) with methyl fluoride, silicon tetrafluoride, methyl chloride and carbon tetrachloride