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

The kinetic energy dependence of the collision‐induced dissociation (CID) of Ni+n (n=2–18) with xenon is studied by using a guided ion beam mass spectrometer. Bond energies of nickel cluster ions, D0(Ni+n−1–Ni), are determined from measurements of the CID thresholds. Bond energies for neutral nickel clusters, D0(Nin−1–Ni), are derived by combining these ionic bond energies with literature values of ionization energies for Nin. Both D0(Ni+n−1–Ni) and D0(Nin−1–Ni) are found to increase nonmonotonically as a function of cluster size, with local maxima at n=3, 7, and 13 for ionic clusters and at n=6 and 13 for neutral clusters. Examination of the cluster size dependence of nickel cluster bond energies leads to speculations on the likely cluster geometric structures. Examination of the general dissociation behavior over a broad collision energy range shows that nickel cluster ions dissociate primarily by sequential atom loss, although exceptions are noted.

Collision‐induced dissociation of Ni+n (n=2–18) with Xe: Bond energies, geometrical structures, and dissociation pathways

Translational energy dependence of O+(4S) + H2(D2, HD) → OH+(OD+) + H(D) from thermal energies to 30 eV c.m.

The exothermic dihydrogen and methane loss channels for Co + reacting with propane, propane-2-d 1 , propane-2,2-d 2 , propane-1,1,1-d 3 , propane-1,1,1,3,3,3-d 6 , and propane-d 8 are examined. Measurements of the Co + /C 3 H 8 system with a guided ion beam apparatus yield a total cross section that is 13% of the Langevin collision cross section, with H 2 loss favored over CH 4 loss by a factor of 2.4. For Co + reacting with C 3 D 8 , the total cross section decreases by a factor of 2.8 relative to C 3 H 8 , but the branching ratio is unchanged

Transition Metal Ion Mediated C-H and C-C Bond Activation of Alkanes: Dynamical Coupling between Entrance and Exit Channel Transition States

The potential energy surface for activation of methane by the third-row transition metal cation, Au+, is studied experimentally by examining the kinetic energy dependence of this reaction using guided ion beam tandem mass spectrometry. A flow tube ion source produces Au+ primarily in its 1S0 (5d10) electronic ground state level but with some 3D (and perhaps higher lying) excited states that can be completely removed by a suitable quenching gas (N2O). Au+ (1S0) reacts with methane by endothermic dehydrogenation to form AuCH2+ as well as C-H bond cleavage to yield AuH+ and AuCH3+. The kinetic energy dependences of the cross sections for these endothermic reactions are analyzed to give 0 K bond dissociation energies (in eV) of D0(Au+ - CH2) = 3.70 +/- 0.07 and D0(Au+ -CH3) = 2.17 +/- 0.24. Ab initio calculations at the B3LYPHW + /6-311++G(3df,3p) level performed here show good agreement with the experimental bond energies and previous theoretical values available. Theory also provides the electronic structures of the product species as well as intermediates and transition states along the reactive potential energy surface. Surprisingly, the dehydrogenation reaction does not appear to involve an oxidative addition mechanism. We also compare this third-row transition metal system with the first-row and second-row congeners, Cu+ and Ag+. Differences in thermochemistry can be explained by the lanthanide contraction and relativistic effects that alter the relative size of the valence s and d orbitals.

https://collections.lib.utah.edu/dl_files/7d/72/7d726b828a56fd708dcba5dc966fba8c8436589b.pdf

Activation of methane by gold cations: guided ion beam and theoretical studies.

Guided ion beam mass spectrometry has been used to examine the kinetic energy dependence of the reactions of vanadium ions with H/sub 2/, D/sub 2/ and HD. Results for a number of electronic states of V/sup +/ are obtained and show remarkably different behavior. While kinetic energy and electronic energy display no striking differences in their ability to promote reaction, different electronic states (corresponding to different electron configurations) are found to react by different mechanisms. An analysis of this behavior is used to infer a detailed picture of the dynamics of the reactions and a general view of how transition metals interact with molecular hydrogen. Analysis of the threshold behavior of these reactions is used to deduce the VH/sup +/ bond energy of 2.05 +/- 0.06 eV (47.3 +/- 1.4 kcal/mol). This assumes reaction occurs with no activation barrier. 54 references, 8 figures, 3 tables.

P. B. Armentrout

Papers

Collision‐induced dissociation of Ni+n (n=2–18) with Xe: Bond energies, geometrical structures, and dissociation pathways

Translational energy dependence of O+(4S) + H2(D2, HD) → OH+(OD+) + H(D) from thermal energies to 30 eV c.m.

Transition Metal Ion Mediated C-H and C-C Bond Activation of Alkanes: Dynamical Coupling between Entrance and Exit Channel Transition States

Activation of methane by gold cations: guided ion beam and theoretical studies.

Effect of kinetic and electronic energy on the reaction of V+ with H2, HD, and D2