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

Methane emissions from natural wetlands constitutes the largest methane source at present and depends highly on the climate. In order to investigate the response of methane emissions from natural wetlands to climate variations, a 1-dimensional process-based climate-sensitive model to derive methane emissions from natural wetlands is developed. In the model the processes leading to methane emission are simulated within a 1-dimensional soil column and the three different transport mechanisms diffusion, plant-mediated transport and ebullition are modeled explicitly. The model forcing consists of daily values of soil temperature, water table and Net Primary Productivity, and at permafrost sites the thaw depth is included. The methane model is tested using observational data obtained at 5 wetland sites located in North America, Europe and Central America, representing a large variety of environmental conditions. It can be shown that in most cases seasonal variations in methane emissions can be explained by the combined effect of changes in soil temperature and the position of the water table. Our results also show that a process-based approach is needed, because there is no simple relationship between these controlling factors and methane emissions that applies to a variety of wetland sites. The sensitivity of the model to the choice of key model parameters is tested and further sensitivity tests are performed to demonstrate how methane emissions from wetlands respond to climate variations.

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A process-based, climate-sensitive model to derive methane emissions from natural wetlands : Application to five wetland sites, sensitivity to model parameters, and climate

The recently introduced full multiple spawning (FMS) method for molecular dynamics beyond the Born–Oppenheimer approximation is tested against exact numerical solution of the coupled nuclear Schrodinger equation for a two-dimensional model problem with two electronic states. The method uses a multiconfigurational frozen Gaussian ansatz for the wave function and the key idea is to expand the size of the basis set only during nonadiabatic events, using available information to predict the regions of phase space where population will be created. This is accomplished via the spawning procedure which keeps the basis size manageable while ensuring that it provides a reasonable approximation to the exact wave function. The parameters that govern the numerical accuracy of the method are discussed in detail. Expectation values and branching ratios are predicted quantitatively over a broad range of energies.

Nonadiabatic molecular dynamics: Validation of the multiple spawning method for a multidimensional problem

The observation of coherent dynamics ensuing from the excitation of molecular systems by femtosecond laser pulses is at the heart of femtochemistry. The time-dependent evolution of coherent superpositions of quantum states, the physical basis for the observation of coherent dynamics and their manipulation are of central importance to coherent control of physicochemical processes. In the early days of femtochemistry, there was significant skepticism regarding the type of information that could be learned from spectroscopic experiments using very short pulses. There were arguments that one could infer the dynamics from frequency-resolved experiments and that the femtosecond experiments did not offer new information. It was also assumed that experiments with very short pulses would smear the available spectroscopic information because of their broad bandwidths. Approximately two decades after the initial experiments, it has become clear that femtosecond experiments have opened an extremely valuable window into the dynamic behavior of atomic and molecular systems that is influencing how we think about physics, chemistry, and biology. In particular, we highlight the fact that some of the highest resolution spectroscopic measurements being carried out employ femtosecond laser pulses. These experiments, specifically those taking advantage of rotational coherence, provide resolution that rivals microwave spectroscopy. The reason spectroscopic information is not lost in femtochemistry experiments is coherence, a property that can be manipulated to control physicochemical processes by a number of different approaches, which will be reviewed here. This review presents a summary of some of the most salient contributions to the field of coherent laser control from an experimentalist’s perspective. While a number of theoretical papers have made key contributions to the field, it is from the experimental successes, as well as failures, that we can best learn how to implement new strategies and develop future applications. Coherent laser control, in the context of this review, encompasses experiments in which the coherent properties of the laser and/or the molecule are required for controlling a particular physicochemical process. We distinguish for each of the experiments between coherence in the laser field(s), * To whom correspondence should be addressed. Phone (517) 3559715 (ext-315). Fax (517) 353-1793. E-mail dantus@msu.edu. 1813 Chem. Rev. 2004, 104, 1813−1859

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Experimental coherent laser control of physicochemical processes.

A new collection of photodissociation and photoionisation cross sections for 102 atoms and molecules of astrochemical interest has been assembled, along with a brief review of the basic physical processes involved. These have been used to calculate dissociation and ionisation rates, with uncertainties, in a standard ultraviolet interstellar radiation field (ISRF) and for other wavelength-dependent radiation fields, including cool stellar and solar radiation, Lyman-α dominated radiation, and a cosmic-ray induced ultraviolet flux. The new ISRF rates generally agree within 30% with our previous compilations, with a few notable exceptions. Comparison with other databases such as PHIDRATES is made. The reduction of rates in shielded regions was calculated as a function of dust, molecular and atomic hydrogen, atomic C, and self-shielding column densities. The relative importance of these shielding types depends on the atom or molecule in question and the assumed dust optical properties. All of the new data are publicly available from the Leiden photodissociation and ionisation database. Sensitivity of the calculated rates to variation of temperature and isotope, and uncertainties in measured or calculated cross sections, are tested and discussed. Tests were conducted on the new rates with an interstellar-cloud chemical model, and find general agreement (within a factor of two) in abundances obtained with the previous iteration of the Leiden database assuming an ISRF, and order-of-magnitude variations assuming various kinds of stellar radiation. The newly parameterised dust-shielding factors makes a factor-of-two difference to many atomic and molecular abundances relative to parameters currently in the UDfA and KIDA astrochemical reaction databases. The newly-calculated cosmic-ray induced photodissociation and ionisation rates differ from current standard values up to a factor of 5. Under high temperature and cosmic-ray-flux conditions the new rates alter the equilibrium abundances of abundant dark cloud abundances by up to a factor of two. The partial cross sections for H2 O and NH3 photodissociation forming OH, O, NH2 and NH are also evaluated and lead to radiation-field-dependent branching ratios.

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Photodissociation and photoionisation of atoms and molecules of astrophysical interest

A fully quantal wavepacket approach to reactive scattering in which the best available H3 potential energy surface was used enabled a comparison with experimentally determined rates for the D + H2(v = 1, j = 1) → HD(v9 = 0, 1, 2; j9) + H reaction at significantly higher total energies (1.4 to 2.25 electron volts) than previously possible. The theoretical results are obtained over a sufficient range of conditions that a detailed simulation of the experiment was possible, thus making this a definitive comparison of experiment and theory. Good to excellent agreement is found for the vibrational branching ratios and for the rotational distributions within each product vibrational level. However, the calculated rotational distributions are slightly hotter than the experimentally measured ones. This small discrepancy is more marked for products for which a larger fraction of the total energy appears in translation. The most likely explanation for this behavior is that refinements are needed in the potential energy surface.

https://science.sciencemag.org/content/sci/257/5069/519.full.pdf

State-to-State Rates for the D + H2(v = 1, j = 1) → HD(v', j') + H Reaction: Predictions and Measurements

State-to-state differential cross sections from photoinitiated bulb reactions

Pour yourself a glass of beer and look closely at the rising bubbles. Careful examination shows that they are seldom distributed uniformly throughout the liquid. Instead, streams of bubbles appear to rise from certain spots on the surface of the glass. Closer inspection reveals that the bubbles rapidly grow in size as they ascend, the volume of each bubble often doubling or more by the time it reaches the top of the glass. In addition, the speed of the bubbles increases as they travel upward.

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Through a Beer Glass Darkly

Experimental values for the absorption coefficient and the branching ratio of HDO at 157.5 nm are reported. The experimental evidence strongly supports the analysis of Zhang et al. (ref.10).(AIP)

Isotope effect in the photodissociation of HDO at 157.5 nm

We have measured state‐to‐state integral rate constants for the reaction D+H2(v,j) →HD(v’=0,1,2;j’)+H, in which the H2 reagent was either in the ground state, H2(v=0,j), or prepared in the first excited vibrational state, H2(v=1, j=1), by stimulated Raman pumping. Translationally hot D atoms were produced via UV photolysis of DI, generating two center‐of‐mass collision energies corresponding to the two I atom spin–orbit states. Resonance‐enhanced multiphoton ionization and time‐of‐flight mass spectrometry were employed to detect the nascent HD product in a quantum‐state‐specific manner. Two experimental geometries were used: (1) a probe‐laser‐induced geometry, in which the same laser both initiated the reaction, by photolysis of DI, and detected the HD and (2) an independent‐photolysis‐source geometry, in which photolysis of DI was carried out by an independent laser. We find that vibrational excitation of the H2 reagent results in substantial HD rotational excitation for each product vibrational state, a...

Neil Shafer

Papers

State-to-State Rates for the D + H2(v = 1, j = 1) → HD(v', j') + H Reaction: Predictions and Measurements

State-to-state differential cross sections from photoinitiated bulb reactions

Through a Beer Glass Darkly

Isotope effect in the photodissociation of HDO at 157.5 nm

Measurement of relative state‐to‐state rate constants for the reaction D+H2(v, j)→HD(v’, j’)+H