Page O. Stoutland

The thermodynamic driving force for CH activation at iridium

Abstract This paper discusses the relationship between the intermolecular oxidative addition reaction of carbon-hydrogen bonds in organic molecules to transition metal centres, and the dissociation energies of the CH, MH and MR bonds that undergo changes during this process. Earlier studies of transition metal MH and MR bond energies are reviewed, followed by a summary of relative and absolute bond energies measured more recently for the (η 5 -C 5 Me 5 )(PMe 3 )Ir(X)(Y) system. The MH and MR energies are unusually large in this system compared with most others that are presently known; an important exception are those in the thorium series, where intermolecular CH activation is also observed. The IrC and IrH bond energy values are utilized in discussing the propensity of iridium for intermolecular CH insertion, and in predicting thermochemistries for its RH insertion and M(H)(R) reductive elimination reactions. Finally, the physical basis for the strong metal-carbon and -hydrogen bonds in the iridium system is discussed.

Quantitative chemical identification of four gases in remote infrared (9–11 µm) differential absorption lidar experiments

A combined experimental and computational approach utilizing tunable CO(2) lasers and chemometric analysis was employed to detect chemicals and their concentrations in the field under controlled release conditions. We collected absorption spectra for four organic gases in the laboratory by lasing 40 lines of the laser in the 9.3-10.8-mum range. The ability to predict properly the chemicals and their respective concentrations depends on the nature of the target, the atmospheric conditions, and the round-trip distance. In 39 of the 45 field experiments, the identities of the released chemicals were identified correctly without predictions of false positives or false negatives.

Spectroscopic analysis of infrared DIAL measurements

A combined experimental and computational approach utilizing CO2 infrared gas lasers and chemometric multivariate analysis was employed to detect chemicals and their concentrations in the open atmosphere under controlled release conditions. Absorption spectra of four organic gases were collected in the laboratory by lasing 40 lines of a Synrad 15 W CO2 laser in the 9.3 to 10.8 micron range. Several chemometric calibration models were constructed based on this IR data using the Partial Least Squares computational technique. The chemometric models were used to analyze in near real time the field DIAL data acquired over this exact wavelength range at round trip distances of 7 and 13 km. It will be shown that the ability to predict the chemicals and their respective concentrations depends on a variety of factors. In 39 of the 45 experiments, the identities of the released chemicals were correctly identified without predictions of false positives or false negatives. Under the best field conditions, we achieved predictions of absolute concentrations within 30% of the actual values.

Picosecond Infrared Study of Carbonmonoxy Cytochrome c Oxidase: Ligand Transfer Dynamics and Binding Orientations

Cytochrome c oxidase (CcO), an enzyme which catalyzes the reduction of dioxygen to water in the terminal step of the respiratory chain, combines several fundamental chemical processes in performing its function; electron, proton and ligand transfers.[1] The coordination chemistry and ligation dynamics of the cytochrome a 3-CuB site, where O2 and other small molecules such as CO, NO and isocyanates can bind, are essential to the function of the enzyme.[2] The sensitivity of the vibrational frequencies and bandwidths of small molecules to changes in coordination and environment makes infrared spectroscopy uniquely useful as a probe for those processes, particularly at CUB +, which generally is not observable by other spectroscopies. [1,2] Recent time-resolved infrared (TRIR) and visible absorption measurements have shown that coordination to CuB + is an obligatory mechanistic stop for CO entering the cytochrome a 3 heme site and departing the protein after photodissociation. [2] The timescale (> 10−7 s) of the TRIR measurements, however, precluded observation of the ligation dynamics immediately following photodissociation. Here we report a picosecond timescale TRIR study of these events. The results reveal that the photoinitiated ligand transfer of CO from Fea3 2+ to CuB +, which are believed to lie 4–5 A apart [1], occurs within 1 ps.

Time-resolved infrared studies of the dynamics of ligand binding to cytochrome c oxidase

Time-resolved infrared spectroscopy (TRIRS) has been employed to study the reactions of small molecules with the cytochrome a3-CuB site of cytochrome c oxidase (CcO). All phases of these reactions have been investigated, from ultrafast phenomena (hundreds of femtoseconds) to relatively slow processes (milliseconds). The ligation dynamics immediately following photodissociation have been studied using a TRIRS technique with time resolution of less than 1 ps. The rate of photoinitiated transfer of CO from Fea32+ to CuB+ was measured directly by monitoring the development of the transient CuB+-CO absorption. The development of a stationary CuB+ spectrum which is constant until the CO dissociates from CuB+ occurs in less than 1 ps, indicating that the photoinitiated transfer of CO is remarkably fast. This unprecedented ligand transfer rate has profound implications with regard to the structure and dynamics of the cytochrome a3-CuB site, the functional architecture of the protein and coordination dynamics in general. The photodissociation and recombination of CN- has also been studied using a real-time TRIR technique. The CN- recombination rate of 430 s-1 is consistent with a recombination pathway similar to the one we have previously proposed for CO, in which a long-lived barrier to recombination is formed by the binding of an endogenous ligand L to Fea32+. The authors suggest the rate determining step for CN- recombination is the thermal dissociation of the Fea32+-L bond.