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 reactions of Sc' and Ti' with ammonia are studied as a function of translational energy in a guided ion beam tandem mass spectrometer. The effect of electronic energy for the Ti+ reactions is also probed by varying the conditions for forming Ti+. Excited doublet states of Ti' are found to be much more reactive than the a4F ground and b4F first excited states. Both metals form the MH+, MNH', and MNH2+ products, while only Ti+ forms MN+. The results are consistent with reaction that occurs primarily through a covalently bound insertion intermediate, H-M+-NHz, having a singlet and doublet spin state for M = Sc and Ti, respectively. The reactivities of the different electronic states of the reactant ions can be explained by using spin conservation concepts. The thresholds for the cross sections of the endothermic reactions are interpreted to give the 298 K bond energies of Do(Sc+-NH2) = 3.69 i 0.07 eV, Do(Sc+-NH) = 5.16 f 0.10 eV, Do(Ti+-NH2) = 3.69 f 0.13 eV, D"(Ti+-NH) = 4.83 f 0.12 eV, and Do(Ti+-N) = 5.19 f 0.13 eV. The large bond strengths of the M+-NH2 and M+-NH species indicate that the lone pair electrons on the nitrogen atom are involved in the metal-ligand bond.

Ammonia Activation by Sc' and Ti': Electronic and Translational Energy Dependence

The reactions of V+ (5D) with CS2 and COS and the reactions of VS+ with Xe, CO, COS, CO2, and D2 are studied as a function of translational energy in a guided-ion-beam (GIB) mass spectrometer. From these experiments, D0(V+−S) = 3.78 ± 0.10 eV, D0(V+−CS) = 1.70 ± 0.08 eV, and D0(V+−SD) = 2.57 ± 0.15 eV are derived. Verification of D0(V+−S) is achieved by probing reactions of V+ and VS+ in a Fourier transform ion cyclotron resonance mass spectrometer. The good agreement between the thermochemistry obtained in the V+/CS2 system and that from the other systems studied shows that the formally spin-forbidden formation of ground-state VS+ (3Σ-) from V+ (5D) and CS2 has no activation barrier in excess of the reaction endothermicity. At higher energies, the spin-allowed formation of VS+ (5Π) competes efficiently, giving rise to a composite shape of the VS+ cross section. The adiabatic and vertical splittings between the 3Σ- and 5Π states of VS+ are calculated as 1.37 and 1.87 eV at the MR-ACPF level of theory. The...

Experimental and Theoretical Studies of Vanadium Sulfide Cation

Reactions of Pt+ with O2, CO, and CO2 have been studied as a function of kinetic energy using guided ion beam tandem mass spectrometry in order to elucidate the bond energy of the PtO+ product. To further characterize the reaction of Pt+ with CO2, the reverse reaction of PtO+ with CO, and the collision-induced dissociation of PtCO2+ with Xe and Ar are also studied. In all cases, the kinetic energy dependences for these reactions show endothermic behavior except for PtO+ + CO → Pt+ + CO2 and PtCO2+ + Xe → PtXe+ + CO2. Analyses of the endothermic reaction cross sections yield the 0 K bond dissociation energies (BDEs) in eV (kJ/mol) of D0(Pt+−O) = 3.26 ± 0.07 (315 ± 7), D0(Pt+−CO) = 2.28 ± 0.05 (220 ± 5), D0(Pt+−CO2) = 0.62 ± 0.05 (60 ± 5), and D0(Pt+−C) = 5.46 ± 0.05 (527 ± 5), reasonably consistent with available theoretical values. Combining the PtO+ BDE measured here with literature data also yields the ionization energy of PtO as 9.52 ± 0.25 eV. These data, along with ab initio calculations at the B3LYP...

Activation of O2, CO, and CO2 by Pt+: The Thermochemistry of PtO+

The dependence of ion–molecule reactions on kinetic and internal energy is explored from both experimental and theoretical points of view. The conversion of raw data (ion intensities versus laboratory energies) to instrument independent information (reaction cross sections versus center-of-mass energies) is explained in detail. The characteristic experimental behavior of ion–molecule reactions as a function of kinetic energy is illustrated and then theoretically characterized. The conversion and relationship between reaction cross sections and rate constants is provided along with examples. The changes in reaction efficiencies with changes in internal excitation are examined, with appropriate examples. Various explanations for why exothermic ion–molecule reactions are not always efficient are then provided along with several illustrative experimental cases. The factors that influence the efficiency of dissociative processes are also elucidated. The relevance of these various considerations for application to atomic mass spectrometry is explored throughout.

P. B. Armentrout

Papers

Ammonia Activation by Sc' and Ti': Electronic and Translational Energy Dependence

Experimental and Theoretical Studies of Vanadium Sulfide Cation

Activation of O2, CO, and CO2 by Pt+: The Thermochemistry of PtO+

Fundamentals of ion–molecule chemistry

Collision-Induced Dissociation Studies of Co(CO)x+, x = 1-5: Sequential Bond Energies and the Heat of Formation of Co(CO)4