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

Activation of C–H bonds in the sequential reactions of Pt+ + x(CH4/CD4), where x = 1–4, have been investigated using infrared multiple photon dissociation (IRMPD) spectroscopy and theoretical calculations. Pt+ cations are formed by laser ablation and exposed to controlled amounts of CH4/CD4 leading to [Pt,xC,(4x-2)H/D]+ dehydrogenation products. Irradiation of these products in the 400–2100 cm–1 range leads to CH4/CD4 loss from the x = 3 and 4 products, whereas PtCH2+/PtCD2+ products do not decompose at all, and x = 2 products dissociate only when formed from a higher order product. The structures of these complexes were explored theoretically at several levels of theory with three different basis sets. Comparison of the experimental and theoretical results indicate that the species formed have a Pt(CH3)2+(CH4)x-2/Pt(CD3)2+(CD4)x-2 binding motif for x = 2–4. Thus, reaction of Pt+ with methane occurs by C–H bond activation to form PtCH2+, which reacts with an additional methane molecule by C–H bond activat...

/pdf/activation-of-c-h-bonds-in-pt-x-ch4-reactions-where-x-1-4-435wwg6t7z.pdf

Activation of C–H Bonds in Pt+ + x CH4 Reactions, where x = 1–4: Identification of the Platinum Dimethyl Cation

The potential energy surface for the CoC2H6+ system is probed by using a guided-ion beam tandem mass spectrometer in conjunction with a dc discharge/flow tube ion source. Bimolecular reactions of Co+ + C2H6, Co+(C2H4) + H2 (D2), and CoCH2+ + CH4 (CD4) are examined, as well as threshold collisional activation of Co+(C2H6) and Co+(C2H4) with Xe. The results allow details of the [CoC2H6]+ potential energy surface (PES) to be elucidated. Key features of the PES include barrier heights of 1.04 ± 0.11 eV (0.29 ± 0.09 eV in excess of the endothermicity) for the reaction of Co+ with ethane to form CoCH2+ + CH4 and of 0.32 ± 0.12 eV for the exothermic reaction to form Co+(C2H4) + H2. From the collisional activation studies, we determine 0 K bond dissociation energies for Co+−C2H4 and Co+−C2H6 complexes of 1.86 ± 0.07 and 1.04 ± 0.05 eV, respectively, in good agreement with literature values. D0(DCo+−C2H4) = 1.13 ± 0.16 eV is also determined although it is somewhat more speculative.

Probing the [CoC2H6]+ potential energy surface: A detailed guided-ion beam study

Sequential bond energies of Pt(CO)x + (x = 1 -4) determined by collision-induced dissociation

An ion-beam apparatus is employed to study the reaction of singly charged cobalt positive ions with cyclopropane, cyclobutane, cyclopentane, and cyclohexane. In all cases, ring cleavage reactions are observed. These processes find analogy with the decompositions of solution-phase metallacycles by C-C bond cleavage. In all systems but cyclopropane, dehydrogenation of the cycloalkane is also observed. Multiple dehydrogenation occurs with cyclopentane (yielding Co(C/sub 5/H/sub 6/)/sup +/) and cyclohexae (yielding Co(Ct/sub 6/H/sub 8/)/sup +/ and Co(C/sub 6/H/sub 6/)/sup +/). The present results are contrasted with the reactions of Co/sup +/ with the isomeric alkenes.

Ion-beam studies of the reactions of atomic cobalt ions with cycloalkanes in the gas phase. Formation and decomposition of chemically activated metallacycles

Evaluation de la section efficace totale La production de MnO + semble endothermique, la section efficace presentant une barriere d'energie

P. B. Armentrout

Papers

Activation of C–H Bonds in Pt+ + x CH4 Reactions, where x = 1–4: Identification of the Platinum Dimethyl Cation

Probing the [CoC2H6]+ potential energy surface: A detailed guided-ion beam study

Sequential bond energies of Pt(CO)x + (x = 1 -4) determined by collision-induced dissociation

Ion-beam studies of the reactions of atomic cobalt ions with cycloalkanes in the gas phase. Formation and decomposition of chemically activated metallacycles

Transition-metal cluster chemistry: reactions of Mn2+ with O2