Studies of Phosphazenes. Part 13.' Thermal Rearrangement Reactions of Some Met hoxycyclo phosp hazenes

The methoxycyclophosphazenes [NP(OMe),], (n = 3-6) rearrange on heating to give oxocyclophosphazanes, [N(Me)PO(OMe)],. Isomeric products are formed when n = 4-6. The lH, ,lP, and 13C n.m.r. data for the starting materials and the products are presented. The ethoxy- and n-propoxy-derivatives N,P,( OR)* do not undergo the above rearrangement. The geminal derivatives N,P,R,(OMe), (R = Ph or NHBut) on heating yield both fully and partially rearranged products, namely dioxophosphaz-1 -enes and oxophosphazadienes, as shown by 270- MHz lH n.m.r. spectroscopy. The non-geminal derivative N,P,( NMe,),(OMe), gives only the fully rearranged product N,Me,P,(NMe,),O,(OMe), whose structure has been established from its lH and 31P n.m.r. spectra.

Photofragmentation Reactions Of Thiocarbonyl Compounds

Preferential yield of ring expansion and rearrangement products through α-cleavage of tetramethyl-3-thio-1,3-cyclobutanedione (1) and 3-mercapto-2,2,4-trimethyl-3-pentenoic acid β-(thio lactone) (2) involving diradical and carbene has been observed upon photolysis of 1 and 2.

The Origin of Regioselectivity in a-Cleavage Reactions of Cyclopropenethiones: Potential Role of Pseudo-Jahn-Teller Effect in Substituted Cyclopropenyl Systems

Arylalkylcyclopropenethiones undergo highly regioselective photochemical a-cleavage via thioketene carbene intermediates, giving rise to products derived from the less stabilized carbene. UHF MIND0/3 calculations provide an insight into this unexpected regioselectivity. The nx* triplet of cyclopropenethione is calculated to have a highly unsymmetrical geometry with an elongated C-C bond, a delocalized thiaaUyl fragment, and a pyramidal radicaloid carbon (which eventually becomes the carbene center). From this molecular electronic structure, aryl group stabilization is expected to be more effective at the thiaallyl group rather than at the pyramidal radical center. Thus, the stability of the substituted triplet thione rather than that of the thioketene carbene determines the preferred regiochemistry of cleavage. The unusual structure of the cyclopropenethione triplet is suggested to be related to one of the Jahn-Teller distorted forms of the cyclopropenyl radical. An alternative symmetrical structure is adopted by the corresponding triplet of cyclopropenone, partly accounting for its differing photobehavior. A similar structural dichotomy is demonstrated for the corresponding radical anions as well.

Preparation of Hexafluorodisilane and Reactions of Hexafluorodisilane and Hexachlorodisilane with Sulfur Trioxide

Hexafluorodisilane has been prepared by the fluorination of hexachlorodisilane or hexabromodisilane by potassium fluoride in boiling acetonitrile, in yields approximating 45 and 60% respectively. Hexafluorodisilane has been characterised by infrared spectral data, vapour density measurements and analytical data. Both hexafluorodisilane and hexachlorodisilane are found to react with sulfur trioxide when heated to 400°C for 12 h. The products of reaction are silicon tetrafluoride, silica and sulfur dioxide with hexafluorodisilane while hexachlorodisilane in addition gives rise to hexachlorodisiloxane.

Photochemical reactions of organic crystals

Abstract is not available.

Fractional power dependence of rate on activation energy for reactions with very low internal barriers

The dynamics of reactions with low internal barriers are studied both analytically and numerically for two different models. Exact expressions for the average rate,kI, are obtained by solving the associated first passage time problems. Both the average rate constant, kI, and the numerically calculated long-time rate constant, kL, show a fractional power law dependence on the barrier height for very low barriers. The crossover of the reaction dynamics from low to high barrier is investigated.

Studies of phosphazenes. 28. Reactions of pentachloro- and pentafluoro(triphenylphosphazenyl)cyclotriphosphazenes with sodium methoxide. Mechanistic aspects and their implications for nucleophilic displacement at a tetrahedral phosphorus(V) center

Abstract is not available.

Effectiveness factor for zeroth-order catalytic reactions with volume change and nonuniform catalyst activity profiles

Analytical solutions are presented for the effectiveness factor of a zeroth-order reaction with volume change and nonuniform catalyst activity profile in slab, cylinder and spherical pellets. The possibility of shape normalization is considered for a variety of activity profiles and pellet shapes. When the catalyst activity at the external surface of the pellet is non-zero, shape normalization is obtained, which makes the asymptotic behavior of the effectiveness factor identical for small and large values of Thiele modulus, however, the normalization can lead to significant errors, particularly for the case of activity profiles decreasing towards the outer surface of the catalyst.

Michael Reactions of the Anions generated by the Metal-Ammonia Reduction of Benzoic Acids.

A novel method for the construction of carboncarbon bonds is described in which anions obtained by the metal-ammonia reduction of benzoic acid and its derivatives undergo ready Michael reaction with methyl crotonate to give the addition products.

Chemical modification of Methionines of Ribonuclease a with O-Benzoquinone

The accessibility of methionines in RNAase A to reaction with OBQ has been studied at highly acidic pH. The differences between the rate constants of reactions of the methionine and methionines of RNAase A with OBQ is a reflection on the limited accessibility of methionines in the protein conformation. Nevertheless, at sufficiently high OBQ concentration, all the four methionines of the enzyme can be modified. At lower concentration of OBQ, a derivative may be prepared in which a specific methionine is modified. The introduced chromophore ionizes at around pH 3 in this derivative. The derivative has partial activity towards RNA which is enhanced on addition of S-protein.

Thermal decomposition of doped calcium hydroxide for chemical energy storage

Thermal decomposition of Ca(OH)2 with and without additives has been experimentally investigated for its application as a thermochemical energy storage system. The homogeneous reaction model gives a satisfactory fit for the kinetic data on pure and Ni(OH)2---, Zn(OH)2--- and Al(OH)3---doped Ca(OH)2 and the order of reaction is 0.76 in all cases except for the Al(OH)3-doped sample for which the decomposition is zero order. These additives are shown not only to enhance the reaction rate but also to reduce the decomposition temperature significantly. Some models for solid decomposition reactions, and possible mechanisms in the decomposition of solids containing additives, are also discussed.

Role of orbital symmetry in transition metal promoted ring opening reactions of methylenecyclopropanes and cyclobutenes

Transition metals catalyse a variety of organic reactions, of which the ring opening of strained ring organic molecules generated a lot of interest. Theoreticians predicted a metal orbital catalysed pathway, which involved concerted bond breaking and bond forming. On the other hand experimentalists were able to show that the reaction was not proceeding through a concerted pathway by intercepting the intermediates involved. There remained, however, two ring systems methylenecyclopropanes and cyclobutenes—whose reactions with metal complexes seemed to be of a concerted nature. An analysis of the reactions of different metal complexes with these ring systems and the theoretical predictions provide a rationale for understanding these reactions.

Gas-Solid Reactions: Photochemical Oxidation of Thioketones in the Crystalline State

Photochemical oxidation of 11 diaryl thioketones (1-11) was conducted in the solid state. Quite interestingly, of these only six were oxidized to the corresponding carbonyl compound whereas the rest were photostable. However, in solution all were readily oxidized. The difference in behavior between the thioketones in the solid state has been rationalized on the basis of molecular arrangement in the crystal. X-ray crystal structure analyses of four thioketones were carried out in this connection.

A Comparative Study of Electrochemical Kinetic Parameters by the Potential Step Method. Electrode Reactions of CyDTA and EDTA Complexes of Cu(II) at DME

By using the same current-time (I-t) curves, electrochemical kinetic parameters are determined by two methods, (a) using the ratio of current at a given potential to the diffusion-controlled limiting current and (b) curve fitting method, for the reduction of Cu(II)–CyDTA complex. The analysis by the method (a) shows that the rate determining step involves only one electron although the overall reduction of the complex involves two electrons suggesting thereby the stepwise reduction of the complex. The nature of I-t curves suggests the adsorption of intermediate species at the electrode surface. Under these circumstances more reliable kinetic parameters can be obtained by the method (a) compared to that of (b). Similar observations are found in the case of reduction of Cu(II)–EDTA complex.