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).

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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

Spin‐orbit state‐selected reactions of Kr+(2PJ), J=3/2 and J=1/2, with isotopic molecular hydrogen (H2, D2, and HD) to form KrH+ and KrD+ are investigated using guided ion beam techniques Reaction cross sections for each spin‐orbit state are measured as a function of the relative translational energy of the reactants from near thermal energies up to 20 eV cm At low energies, the cross sections for reaction with H2 and D2 are each about 26 times larger for the 2P3/2 ground spin‐orbit state of krypton ion than for the 2P1/2 excited state For the HD reaction, the 2P3/2 reactivity is enhanced by a factor of about 42 A higher‐energy process, which exhibits an apparent activation energy of about 1 eV, is present only for the 2P3/2 spin‐orbit state The Kr+(2P3/2) spin‐orbit state exhibits a strong intermolecular isotope effect, showing the unusual dependence σ(HD)>σ(H2)>σ(D2) for the total reaction cross sections at low energies Cross sections for the Kr+(2P1/2) state show a much smaller dependence on t

Spin‐orbit state‐selected reactions of Kr+(2P3/2 and 2P1/2) with H2, D2, and HD from thermal energies to 20 eV c.m.

Ion beam studies of organometallic chemistry. High energy "sampling" of reaction intermediates involved in carbon-carbon bond cleavage by transition metals

Intrinsic transition metal-carbon double bond dissociation energies: periodic trends in M+-CH2 bond strengths

The kinetic energy dependences of the reactions of Fen+ (n=2–15) with CD4 are studied in a guided ion beam tandem mass spectrometer over the energy range of 0–10 eV. All reactions exhibit thresholds and two main products, FenD+ and FenCD2+, are formed. These primary products decompose at higher energies to form secondary and higher order products, FenCD+, FenC+, Fen−1D+, Fen−1CD2+, Fen−1CD+, and Fen−1C+. The cross-section magnitudes for the dehydrogenation products, FenCD2+, are observed to vary considerably as a function of cluster size; subsequent dehydrogenation to form FenC+ becomes more facile for larger clusters. Thresholds for the various primary and secondary reactions are analyzed and bond energies for iron cluster cation bonds to C, CD, CD2, and CD3 are determined. As a function of cluster size, these bond energies rapidly reach relatively constant values, which are argued to lie close to bulk phase values. The relative magnitudes in these bond energies are consistent with simple bond order cons...

Activation of methane by size-selected iron cluster cations, Fe n + (n=2-15): Cluster-CH x (x=0-3) bond energies and reaction mechanisms

Statistical phase space theory is applied to the reactions of carbon(1+)ions with isotopic molecular hydrogen (H2, D2, and HD) to form methyliumylidene (CH+ and CD+). Reaction cross sections and isotope effects as a function of kinetic energy are compared to recent experimental results. The important features of the potential energy surfaces for the C+(2P)+H system are reviewed. The treatment of multiple electronic potential energy surfaces and excited state product channels in phase space theory is discussed. Phase space theory accurately reproduces the relative behavior of the experimental cross sections in the threshold region. Also, the cross section magnitude is reproduced if only those potential energy surfaces without energy barriers in the entrance channel are included in the phase space model. Phase space theory fails, however, to predict quantitatively the kinetic isotope effects or the shape of the cross section at higher energies where excited state CH+ production may be important.

P. B. Armentrout

Papers

Spin‐orbit state‐selected reactions of Kr+(2P3/2 and 2P1/2) with H2, D2, and HD from thermal energies to 20 eV c.m.

Ion beam studies of organometallic chemistry. High energy "sampling" of reaction intermediates involved in carbon-carbon bond cleavage by transition metals

Intrinsic transition metal-carbon double bond dissociation energies: periodic trends in M+-CH2 bond strengths

Activation of methane by size-selected iron cluster cations, Fe n + (n=2-15): Cluster-CH x (x=0-3) bond energies and reaction mechanisms

C+(2P)+H2(D2,HD)→CH+(CD+)+H(D). II. Statistical phase space theory