The catalytic conversion of 1,2-cyclohexanediol to adipic anhydride by Ru(IV)O(tpa) (tpa ═ tris(2-pyridylmethyl)amine) is discussed using density functional theory calculations. The whole reaction is divided into three steps: (1) formation of α-hydroxy cyclohexanone by dehydrogenation of cyclohexanediol, (2) formation of 1,2-cyclohexanedione by dehydrogenation of α-hydroxy cyclohexanone, and (3) formation of adipic anhydride by oxygenation of cyclohexanedione. In each step the two-electron oxidation is performed by Ru(IV)O(tpa) active species, which is reduced to bis-aqua Ru(II)(tpa) complex. The Ru(II) complex is reactivated using Ce(IV) and water as an oxygen source. There are two different pathways of the first two steps of the conversion depending on whether the direct H-atom abstraction occurs on a C-H bond or on its adjacent oxygen O-H. In the first step, the C-H (O-H) bond dissociation occurs in TS1 (TS2-1) with an activation barrier of 21.4 (21.6) kcal/mol, which is followed by abstraction of another hydrogen with the spin transition in both pathways. The second process also bifurcates into two reaction pathways. TS3 (TS4-1) is leading to dissociation of the C-H (O-H) bond, and the activation barrier of TS3 (TS4-1) is 20.2 (20.7) kcal/mol. In the third step, oxo ligand attack on the carbonyl carbon and hydrogen migration from the water ligand occur via TS5 with an activation barrier of 17.4 kcal/mol leading to a stable tetrahedral intermediate in a triplet state. However, the slightly higher energy singlet state of this tetrahedral intermediate is unstable; therefore, a spin crossover spontaneously transforms the tetrahedral intermediate into a dione complex by a hydrogen rebound and a C-C bond cleavage. Kinetic isotope effects (k(H)/k(D)) for the electronic processes of the C-H bond dissociations calculated to be 4.9-7.4 at 300 K are in good agreement with experiment values of 2.8-9.0.
We report a collaborative maritime archaeological project in Uruguay, one of several Latin American countries where the subject is undergoing review in terms of the ways it is practised and managed. Uruguay is typical of many states where there has been a tension between a heritage-based approach in which the results of investigations are viewed as publicly owned, as opposed to the profit motive in which commercial and personal gain is the underlying ethic. This project was conceived both as a way of assisting the Uruguayan Heritage Commission in promoting the former approach as well as advancing a programme of research into the age of global exploration. This paper sets out the rationale of the initial field season and reflects on subsequent developments.
Differences in aviation safety records around the world should be minimal or nonexistent considering the global nature of aviation. However, significant differences exist and the reasons for those require the attention of governments, regulatory agencies, aircraft manufacturers, airlines and the flying public. One concern is that aircraft are increasingly used beyond their designed economic life. This study focuses on the relationship between aging aircraft and the country where the aircraft was registered. Findings support the idea that in general, the frequency of accidents increases with the age of an aircraft. The study also revealed that there is a correlation between the number of accidents, their severity and aircraft manufacturers. The aircraft’s phase of flight was also related to the number of accidents, with the Approach/Landing phase having the greatest number of accidents. The FAA and JAA have been influential in the development and dissemination of safety programs around the world. However, their efforts have not yet resulted in a world-wide standardization of safety and maintenance programs. This may be an indication that while an aircraft may be a global product, the aviation industry lags behind in becoming a global industry.
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