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

A guided-ion beam mass spectrometer is used to study the reactions FeCH2+ + D2 and Fe+(6D,4F) + CD4, thereby experimentally probing the [FeCH4]+ potential energy surface (PES). The results obtained are compared to recent theoretical results. Experiment and theory agree that dehydrogenation of methane by Fe+ is hindered by a tight, four-center transition state complex. The major discrepancy observed between experiment and theory is in the height of this barrier, which theory predicts to be 75 kJ/mol (92 kJ/mol after correction for zero-point energies) versus our experimental result of 41 ± 6 kJ/mol. Our results can be interpreted by using phase space theory to help understand how various features on the PES control branching ratios among the various channels observed, including extensive hydrogen scrambling in the FeCH2+ + D2 reaction. Lastly, results for the [FeCH4]+ PES are compared to those for the [CoCH4]+ PES obtained in a previous study in order to help assess the effect of potential energy surfaces ...

Reaction of FeCH2+ + D2: Probing the [FeCH4]+ Potential Energy Surface

The kinetic-energy dependence for the reactions of Con+ (n=2–20) with O2 is measured as a function of kinetic energy over a range of 0to10eV in a guided ion-beam tandem mass spectrometer. A variety of Com+, ComO+, and ComO2+ (m⩽n) product ions is observed, with the dioxide cluster ions dominating the products for all larger clusters. Reaction efficiencies of Con+ cations with O2 are near unity for all but the dimer. Bond dissociation energies for both cobalt cluster oxides and dioxides are derived from threshold analysis of the energy dependence of the endothermic reactions using several different methods. These values show little dependence on cluster size for clusters larger than three atoms. The trends in this thermochemistry and the stabilities of oxygenated cobalt clusters are discussed. The bond energies of Con+–O for larger clusters are found to be very close to the value for desorption of atomic oxygen from bulk-phase cobalt. Rate constants for O2 chemisorption on the cationic clusters are compare...

Guided ion beam studies of the reactions of Crn+ (n=2–18) with O2: Chromium cluster oxide and dioxide bond energies

Thresholds for collision-induced dissociation of Fe(N2)x+ (x = 1−5) and Fe(CH2O)x+ (x = 1−4) with xenon are measured by using guided ion beam mass spectrometry. Values for the 0 K (N2)x-1Fe+−N2 bond energies (in eV) are determined to be 0.56 ± 0.06, 0.86 ± 0.09, 0.47 ± 0.06, 0.56 ± 0.04, and 0.64 ± 0.04 for x = 1−5, respectively. Values for the 0 K (CH2O)x-1Fe+−CH2O bond energies (in eV) are determined to be 1.43 ± 0.07, 1.79 ± 0.08, 1.06 ± 0.05, and 0.83 ± 0.06 for x = 1−4, respectively. The bond energy for Fe+−Xe is determined as 0.47 ± 0.08 eV. The results for the singly ligated species are in good agreement with values in the literature. These bond energies are compared with those for several other FeLx+ series including previously published values for Fe(CO)x+ (x = 1−5), which are reevaluated in light of theoretical information. The observation that the relative bond strengths vary nonmonotonically with the number of ligands is discussed in terms of spin conservation and ligand field theory.

Gas-Phase Metal Ion Ligation: Collision-Induced Dissociation of Fe(N2)x+ (x = 1−5) and Fe(CH2O)x+ (x = 1−4)

Reactions of Fe{sup +} with CH{sub 3}X (X = Cl, Br, I) are studied by guided ion beam techniques. State-specific reaction cross sections for production of FeCH{sub 3}{sup +} and FeX{sup +} are presented for the {sup 6}D ground and the {sup 4}F first excited states of Fe{sup +}. The overall behavior seen in these reactions is similar to that seen in the analogous reactions of Co{sup +} and Ni{sup +}, discussed in the preceding paper in this issue. The two states of Fe{sup +} exhibit large differences in reactivity, with the {sup 4}F state generally being more reactive than the {sup 6}D state for production of FeX{sup +} and FeCH{sub 3}{sup +}. The only exception is for the exothermic formation of FeCH{sub 3}{sup +} below 0.7 eV in the CH{sub 3}I system. We postulate that this is due to a potential energy surface crossing that is avoided at low kinetic energies due to spin-orbit interactions and is permitted at higher energies. Analysis of the threshold behavior of the endothermic reactions provides two determinations of D{degree} (Fe{sup +}-CH{sub 3}) = 2.49 {plus minus} 0.13 and 2.47 {plus minus} 0.07 eV, in good agreement with previous values. Lower limits are placedmore » on the bond energies for Fe{sup +}-X.« less

Guided ion beam studies of the state-specific reactions of iron(1+)(6D,4F) with methyl halides (CH3X; X = Cl, Br, I)

The kinetic energy dependencies of the reactions of Fe+n (n=2–15) with D2 are studied in a guided ion beam mass spectrometer. The only products observed are FenD+ (n=2–15) and FenD+2 (n=9–15). All reactions are observed to exhibit thresholds, except for formation of Fe9D+2. Threshold analyses of the endothermic reactions lead to binding energies for the first deuterium atom to the cluster ions as a function of cluster size. The Fe+n–D bond energies are compared to previously determined metal–metal bond energies, D0(Fe+n–Fe). The bond energies of Fe+n–D vary nonmonotonically with n, and parallel those for Fe+n–Fe except for notable differences at n=5, 8, 12, and 14. These trends are rationalized in terms of electronic and geometric structures for the Fe+n clusters. Arguments are presented to suggest that the thresholds measured for FenD+2 production correspond to barriers for chemisorbtion.

P. B. Armentrout

Papers

Reaction of FeCH2+ + D2: Probing the [FeCH4]+ Potential Energy Surface

Guided ion beam studies of the reactions of Crn+ (n=2–18) with O2: Chromium cluster oxide and dioxide bond energies

Gas-Phase Metal Ion Ligation: Collision-Induced Dissociation of Fe(N2)x+ (x = 1−5) and Fe(CH2O)x+ (x = 1−4)

Guided ion beam studies of the state-specific reactions of iron(1+)(6D,4F) with methyl halides (CH3X; X = Cl, Br, I)

Guided ion beam studies of the reactions of Fe+n (n=2–15) with D2: Cluster–deuteride bond energies as a chemical probe of cluster structures