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

Guided ion beam mass spectrometry is used to investigate the kinetic energy dependence of the bimolecular reactions of CO2 and CO with Mo+, MoO+, and MoO2+. To obtain a more complete understanding of these systems, we probe MoO2+ and the intermediates, Mo(CO2)+, OMo(CO)+, OMo(CO2)+, and O2Mo(CO)+, by collisional activation experiments with Xe. Thermochemical analyses of the reaction cross sections obtained in this study yield (in eV) D0(Mo+−CO2) = 0.51 ± 0.07, D0(OMo+−CO2) = 0.77 ± 0.03, D0(OMo+−CO) = 0.80 ± 0.08, D0(O2Mo+−CO) = 0.92 ± 0.17, and D0(OMo+−O) = 5.57 ± 0.14 eV. Additional features in some reaction cross sections are assigned to the formation of excited electronic states of the products, thereby allowing a speculative measurement of excitation energies for states of MoO+ and MoO2+.

Reactions of CO and CO2 with Gas-Phase Mo+, MoO+, and MoO2+

The reactions of Mn + with isobutane, neopentane, acetone, cyclopropane, and ethylene oxide are studied as a function of translational energy in a guided ion beam tandem mass spectrometer. The electronic state of the Mn + ions is varied by altering the ionization technique. The a 5 S and a 5 D excited states are found to be more reactive than the a 7 D ground state, although formation of MnCH 3 shows less of an effect than the other major reactions

Reactions of Mn+ with i-C4H10, neo-c5H12, (CH3)2CO, cyclo-C3H6, and cyclo-C2H4O : bond energies for MnCH2+, MnH, and MnCH3

Guided‐ion beam mass spectrometry is used to probe the reaction of CS+2 with Xe as a function of ion kinetic energy from thermal to 16 eV. S+, CS+, and Xe+ are the only observed reaction products. Formation of S+(4Su) is observed, even though it is spin forbidden, providing evidence for the importance of spin–orbit coupling in the dissociation of CS+2. The threshold for formation of S++CS, 4.74±0.04 eV, leads to a heat of formation of the CS radical ΔfH00(CS) of 65.8±0.9 kcal/mol. From this value, we derive the bond energies D00(CS)=169.8±0.9 kcal/mol, D00(S–CS)=103.8±0.9 kcal/mol, and D00(O–CS)=158.7±0.9 kcal/mol. The onset for production of CS+ by collision‐induced dissociation of CS+2 is observed at 6.16±0.07, 0.40 eV above the thermodynamic threshold, but coincident with the threshold for excitation to the C state of CS+2. The cross section for charge transfer to form Xe+ displays a threshold of ≊2 eV, corresponding to the difference in ionization energies of Xe and CS2. This process is significantly...

Collision‐induced dissociation of CS+2. Heat of formation of the CS radical

While numerous investigations of the gas-phase reactions of iron ions have been conducted in recent years, none of these has suggested that the reactivity of the ground state may be over an order of magnitude less than that of the excited states. Previous work in our labs on the reactions of H/sub 2/ with several states of V/sup +4/ and Mn/sup +5/ led us to believe that the /sup 6/D ((4s)(3d)/sup 6/) ground state and the /sup 4/ F(3d)/sup 7/) first excited state of Fe/sup +/ should exhibit very different reactivity with H/sup 2/ primarily because they differ in their electronic configuration. In the present study, they find that Fe/sup +/(/sup 4/F) reacts efficiently with H/sup 2/ but Fe/sup +/(/sup 6/D) is about 80 times less reactive. This result is obtained despite the fact that these states are separated by only 0.25 eV. This work is the first to assess the individual reactivities of such closely spaced electronic levels of a transition-metal ion.

Does ground state Fe+ (iron(1+)) react with hydrogen?

The interactions of alkali metal cations (M+ = Li+, Na+, K+, Rb+) with the amino acid cysteine (Cys) are examined in detail. Experimentally, bond energies are determined using threshold collision-i...

P. B. Armentrout

Papers

Reactions of CO and CO2 with Gas-Phase Mo+, MoO+, and MoO2+

Reactions of Mn+ with i-C4H10, neo-c5H12, (CH3)2CO, cyclo-C3H6, and cyclo-C2H4O : bond energies for MnCH2+, MnH, and MnCH3

Collision‐induced dissociation of CS+2. Heat of formation of the CS radical

Does ground state Fe+ (iron(1+)) react with hydrogen?

An experimental and theoretical study of alkali metal cation interactions with cysteine.