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 exothermicity of the chemi-ionization reaction Sm + O - SmO+ + e has been re evaluated Guided ion beam tandem mass spectrometry (GIBMS) has been used to determine a value for the bond dissociation energy of SmO+ of 573 +/- 007 eV, as determined by the observed reactivity of Sm+ and SmO+ with several species Combined with the established Sm ionization energy, this value indicates an exothermicity of the chemi-ionization reaction of 008 +/- 007 eV, 02 eV smaller than previous literature determinations This value agrees well with 014 +/- 017eV, independently determined from a new measurement of the ionization energy of SmO using resonantly enhanced two-photon ionization (REMPI) and pulsed-field ionization zero kinetic energy (PFI-ZEKE) photoelectron spectroscopy, 57427 +/- 00006 eV, combined with literature bond energies of SmO The evaluated thermochemistry also suggests that D0(SmO) = 583 +/- 007 eV, consistent with but more precise than literature values Implications of these results for interpretation of chemical release experiments in the thermosphere were evaluated

Thermochemistry and Reactivity of Metals Engaged in Chemiionization

The reaction of Ti/sup +/ with methane is studied as a function of translational energy in a guided ion beam tandem mass spectrometer. The effect of electronic excitation is also studied by varying the conditions for forming Ti/sup +/. The a/sup 2/F excited state is found to form the two main products, TiH/sup +/ and TiCH/sub 2//sup +/, substantially more efficiently than the a/sup 4/F ground and b/sup 4/F first excited states. The results indicate that reaction occurs primarily through a doublet H-Ti/sup +/-CH/sub 3/ intermediate. The reactivities of the different electronic states of Ti/sup +/ can be explained by using simple molecular orbital concepts and spin conversation. The thresholds for these reactions and for related reactions in other systems are interpreted to give D/sup 0/(Ti/sup +/-H) = 54.2 +/- 2.5, D/sup 0/(Ti/sup +/-CH/sub 3/) = 57.5 +/- 2.8, D/sup 0/(Ti/sup +/-CH/sub 2/) = 93.4 +/- 3.5, and D/sup 0/(Ti/sup +/-CH) = 121 +/- 4, all in kcal/mol.

Ammonia Activation by V+: Electronic and Translational Energy Dependence.

Studies of small gas-phase transition metal cluster cations (2–18 atoms) are reviewed with an emphasis on the thermodynamic information acquired. For the metal–metal bond energies, the cluster results deviate from the bulk-phase enthalpy of vaporization in a manner than can be quantitatively described by the spherical drop model that accounts for the surface free energy using bulk-phase parameters (within 10–20%). Binding energies of atomic H and O adsorbates to such clusters are found to vary extensively for the smallest clusters and to reach values lying close to bulk-phase adsorbate energies for all five metal systems that have been investigated (V, Cr, Fe, Co, and Ni). For molecular fragments (C, CH, CH2, NH, and NH2), binding energies to clusters are also found to plateau for larger clusters (greater than about 10 atoms). These asymptotic values may be useful in estimating such quantities on surfaces. In the case of atomic N adsorbates, variations in the adsorbate energies with cluster size remain appreciable through the size range studied, which appears to be a consequence of activating the very strong N2 bond.

Gas-Phase Perspective on the Thermodynamics and Kinetics of Heterogeneous Catalysis

Collision-induced dissociation (CID) of [Th,xC,xO]+, x = 3-6, with Xe is performed using a guided ion beam tandem mass spectrometer (GIBMS). Products are formed exclusively by the loss of CO ligands. Analyses of the kinetic energy-dependent CID product cross sections yield bond dissociation energies (BDEs) of (CO)x-1Th+-CO at 0 K as 1.09 ± 0.05, 0.82 ± 0.07, 0.63 ± 0.05, and 0.70 ± 0.05 eV, respectively. Different structures of [Th,xC,xO]+ were explored using various electronic structure methods, and BDEs for CO ligand loss from precursor [Th,xC,xO]+ complexes were computed. Both experimental and theoretical results corroborate that the structures of [Th,xC,xO]+, x = 3-6, formed experimentally are homoleptic thorium cation carbonyl complexes, Th+(CO)x. The nonmonotonic trend in experimental BDEs is reproduced theoretically, although ambiguities in the spin states of the x = 4-6 complexes (doublet or quartet) remain. BDEs calculated at the coupled cluster with single, double, and perturbative triple excitations (CCSD(T))/cc-pVXZ//B3LYP/cc-PVXZ (X = T and Q) level and a complete basis set (CBS) extrapolation agree reasonably well with the experimental values for all complexes. Thorium oxide ketenylidene carbonyl cations, OTh+CCO(CO)y, y = 1-4, were calculated to be the most stable structures of [Th,xC,xO]+, x = 3-6, respectively; however, these are not observed in our experiment. Potential energy profiles (PEPs) having either quartet or doublet spin calculated at the B3LYP/cc-pVQZ level suggest that the failure to observe OTh+CCO(CO)y, y = 1-4, is the result of a barrier corresponding to the C-C bond formation, making the formation of OTh+CCO(CO)y inaccessible kinetically under the present experimental conditions.

Sequential Bond Dissociation Energies of Th+(CO)x, x = 3-6: Guided Ion Beam Collision-Induced Dissociation and Quantum Computational Studies.

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.

P. B. Armentrout

Papers

Thermochemistry and Reactivity of Metals Engaged in Chemiionization

Ammonia Activation by V+: Electronic and Translational Energy Dependence.

Gas-Phase Perspective on the Thermodynamics and Kinetics of Heterogeneous Catalysis

Sequential Bond Dissociation Energies of Th+(CO)x, x = 3-6: Guided Ion Beam Collision-Induced Dissociation and Quantum Computational Studies.

Ammonia Activation by Sc+ and Ti+: Electronic and Translational Energy Dependence.