Effects of Ultrafast Solvation on the Rate of Adiabatic Outer-Sphere Electron Transfer Reactions

Recent studies have demonstrated that solvation dynamics in many common dipolar liquids contain an initial, ultrafast Gaussian component which may contribute even more than 60% to the total solvation energy. It is also known that adiabatic electron transfer reactions often probe the high-frequency components of the relevant solvent friction (Hynes, J. T. J. Phys. Chem. 1986, 90, 3701). In this paper, we present a theoretical study of the effects of the ultrafast solvent polar modes on the adiabatic electron transfer reactions by using the formalism of Hynes. Calculations have been carried out for a model system and also for water and acetonitrile. It is found that, in general, the ultrafast modes can greatly enhance the rate of electron transfer, even by more than an order of magnitude, over the rate obtained by using only the slow overdamped modes usually considered. For water, this acceleration of the rate can be attributed to the high-frequency intermolecular vibrational and librational modes. For a weakly adiabatic reaction, the rate is virtually indistinguishable from the rate predicted by the Marcus transition state theory. Another important result is that even in this case of ultrafast underdamped solvation, energy diffusion appears to be efficient so that electron transfer reaction in water is controlled essentially by the barrier crossing dynamics. This is because the reactant well frequency is-directly proportional to the rate of the initial Gaussian decay of the solvation time correlation function. As a result, the value of the friction at the reactant well frequency rarely falls below the value required for the Kramers turnover except when the polarizability of the water molecules may be neglected. On the other hand, in acetonitrile, the rate of electron transfer reaction is found to be controlled by the energy diffusion dynamics, although a significant contribution to the rate comes also from the barrier crossing rate. Therefore, the present study calls for a need to understand the relaxation of the high-frequency modes in dipolar liquids.

High-temperature reactions of alkali and plagioclase feldspars with alkali and alkaline-earth chlorides-formation of celsian

High-temperature reactions (Ca 900-degrees-C) involving albite, K-feldspar or plagioclase and K, Ba-or K, Sr chlorides were experimentally studied. These experiments reveal that the reaction between K-exchanged albite, potash feldspar, or plagioclase and Ba-chloride/Ba-K chloride results in the formation of celsian by the breakdown of the starting feldspar structure above 800-degrees-C. Sr-feldspar does not form under similar conditions. A size-effect of the large M-site cation appears to be responsible for the formation of celsian. The reaction between K-feldspar and barium chloride may be used as a method for synthesizing celsian.

Oxygen-Exchange Reactions Accompanying Oxidation of Vanadyl Sulfate by Diperoxovanadate

The absorption spectrum in the visible range and the, ESR spectrum of vanadyl sulfate were lost on addition of diperoxovanadate. The V-51-NMR spectra revealed that diperoxovanadate was reduced to vanadate and its oligomers. With excess vanadyl, tetrameric vanadate was found to be the major product, During this reaction oxygen was released into the medium. The oxygen-release reaction was inhibited by a variety of organic ligands-imidazole, benzoate, formate, mannitol, ethanol, Tris, DMPO, malate, and asparagine. An oxygen-consuming reaction emerged at high concentrations of some of these compounds, e.g. benzoate and ethanol. Using DMPO as the spin-trap, an oxygen-radical species with a 1:2:2:1 type of ESR spectrum was detected in the reaction mixtures resulting from vanadyl oxidation by diperoxovanadate which was unaffected by addition of catalase or ethanol. The results showed that secondary oxygen-exchange reactions occur which depend on and utilize the intermediates in the primary reaction during diperoxovanadate-dependent oxidation of vanadyl sulfate.

Diffusion path during internal displacement reactions in multi-component oxides: Reaction between Fe and (Co,Mg)TiO3 solid solution at 1273 K

The reaction between Fe foil and a disc of ilmenite solid solution (Co-0.48 Ni-0.52) TiO3 was studied at 1273 K. At the metal/oxide interface, the displacement reaction, Fe + (Co,Mg)TiO3 = Co + (Fe,Mg)TiO3 occurs, resulting in an ilmenite solid solution containing three divalent cations. Ferrous ions diffuse into the oxide solid solution and cause the precipitation of Co-Fe alloy as discrete particles inside the oxide matrix. The morphology of the product layer was characterized by SEM. Only two phases, alloy and ilmenite, were detected in the reaction zone. This suggests that the local flux condition imposed by ilmenite stoichiometry (Co + Fe + Mg):Ti = 1:1] was satisfied during the reactive diffusion: (J(Co) + J(Fe) + J(Mg)) = J(Ti). The composition of the alloy and the oxide was determined using EPMA as a function of distance in the direction of diffusion. Although Mg does not participate in the displacement reaction, its composition in the ilmenite phase was found to be position dependent inside the reaction zone. The up-hill diffusion of inert Mg is caused by the development of chemical potential gradients as a result of displacement reaction. The evolution of composition gradients inside the reaction zone and the diffusion path in a ternary composition diagram of the system CoTiO3-FeTiO3-MgTiO3 are discussed. (C) 2010 Elsevier B.V. All rights reserved.

Reactions of the hexachlorocyclodiphosphazane [MeNPCl3]2 with primary aromatic amines: formation of highly basic bisphosphinimines

Reactions of hexachlorocyclodiphosphazane [MeNPCl3]2 with primary aromatic amines afforded the bisphosphinimine hydrochlorides [(RNH)2(RN)PN(Me)P(NHMe)(NHR)2]+Cl- (R = Ph 1, C6H4Me-4 2 or C6H4OMe-4 3). Dehydrochlorination of 2 and 3 by methanolic KOH yielded highly basic bisphosphinimines [(RNH)2(RN)PN(Me)P(NMe)(NHR)2] (R = C6H4Me-4 4 or C6H4OMe-4 5). Compounds 1-5 have been characterised by elemental analysis and IR and NMR (H-1, C-13, P-31) spectroscopy. The structure of 2 has been confirmed by single-crystal X-ray diffraction. The short P-N bond lengths and the conformations of the PN, units can be explained on the basis of cumulative negative hyperconjugative interactions between nitrogen lone pairs and adjacent P-N sigma* orbitals. Ab initio calculations on the model phosphinimine (H2N)3P=NH and its protonated form suggest that (amino)phosphinimines would be stronger bases compared to many organic bases such as guanidine.

Reactions of palladium chloride with amino spirocyclic cyclotriphosphazenes: Isolation and Crystal Structures of Novel Mono- and Bimetallic Complexes Formed by the Hydrolysis of a .lambda.5-Diazaphospholane Ring

The reaction of the amino spirocyclic cyclotriphosphazene N3P3(NMe2)4(NHCH2CH2CH2NH) (2) with palladium chloride gives the stable chelate complex [PdCl2.2] (4). An X-ray crystallographic study reveals that one of the nitrogen atoms of the diaminoalkane moiety and an adjacent phosphazene ring nitrogen atom are bonded to the metal. An analogous reaction with the phosphazene N3P3(NMe2)4(NHCH2CH2NH) (1) gives initially a similar complex which undergoes facile hydrolysis to give the novel monometallic and bimetallic complexes [PdCl2.HN3P3(O)(NMe2)4(NHCH2CH2NH2)] (5) and [PdCl{N3P3(NMe2)4(NCH2CH2NH2)}]2(O) (6), which have been structurally characterized; in the former, an (oxophosphazadienyl)ethylenediamine is chelated to the metal whereas, in the latter, an oxobridged bis(cyclotriphosphazene) acts as a hexadentate nitrogen donor ligand in its dianionic form. Crystal data for 4 : a = 14.137(1) angstrom, b = 8.3332(5) angstrom, c = 19.205(2) angstrom, beta = 96.108(7)degrees, P2(1)/c, Z = 4, R = 0.027 with 3090 reflections (F > 5sigma(F)). Crystal data for 5 : a = 8.368(2) angstrom, b = 16.841(4) A, c = 16.092(5) angstrom, beta = 98.31(2)degrees, P2(1)/n, Z = 4, R = 0.049 with 3519 reflections (F > 5sigma(F)). Crystal data for 6 : a = 22.455(6) angstrom, b = 14.882(3) angstrom, c = 13.026(5) angstrom, 6 = 98.55(2)degrees, C2/c, Z = 4, R = 0.038 with 3023 reflections (F > 5sigma(F)).

Time-resolved fluorescence studies on covalently linked porphyrin�nitroarene complexes. Conformational control of photoinduced electron transfer reactions

Time-resolved fluorescence studies were carried out on a series of free-base and zinc(II) derivatives of meso-tetraphenylporphyrins covalently linked to either 1,3-dinitrobenzene (DNB) or 1,3,5-trinitrobenzene (TNB) acceptor units. These acceptor units were linked at different sites (at the ortho, meta or para positions of one of the phenyl groups of meso-tetraphenylporphyrin) to the donor porphyrins such that the resulting isomeric intramolecular donor-acceptor complexes exhibit different centre-to-centre (ctc) distances and relative orientations. Biexponential fluorescence decay profiles observed for several of these covalently linked complexes were rationalized in terms of the presence of ''closed'' and ''extended'' conformers. Detailed analyses of the fluorescence decay data have provided a comprehensive understanding of the photoinduced electron transfer (PET) reactions occurring in systems containing zinc(II) porphyrin donors. It is observed that although DNB-linked zinc(II) complexes follow the trends predicted for the efficiency of PET with respect to donor-acceptor distance, the TNB-linked zinc(II) porphyrins exhibit a behaviour which is dictated by steric effects. Similarly, although the thermodynamic criteria predict a greater efficiency of charge separation in TNB-linked complexes compared with DNB-linked complexes, the reverse trend observed has been attributed to orientational effects. In the complexes containing free-base porphyrin donors, PET is expected to be less efficient from a thermodynamic viewpoint. In a few of these cases, fluorescence quenching seems to occur by parallel mechanisms other than PET.

Asymmetric Diels-Alder Reactions of Chiral Acrylates of Cholic Acid Derivatives

New steroid-based chiral auxiliaries 6, 9, and 12 have been synthesized from readily available cholic acid. These new chiral auxiliaries place the reactive and the shielding sites in a 1,5 relationship to each other. Diels-Alder reaction of cyclopentadiene with corresponding acrylate esters (7, 10, and 13) have been examined. Acrylates 7 and 10 yielded cycloadducts with 29-88% diastereomeric excess with excellent endo selectivity in the presence of an excess of Lewis acids such as AlCl3, BF3.OEt(2), FeCl3, SnCl4, TiCl4, and ZnCl2. Treatment of acrylate 7 with cyclopentadiene in the presence of BF3.OEt(2) at -80 degrees C gave the endo adduct (>99%) with 88% de. Lewis acid catalyzed and uncatalyzed reactions of acrylates 7 and 10 with cyclopentadiene yielded cycloadducts with opposite stereochemistry. The chiral auxiliary was recovered in a nondestructive manner only via iodolactonization. Acrylate ester of alcohol 12 did not show any selectivity in either catalyzed and uncatalyzed reactions with cyclopentadiene. The presence of a flat aromatic surface at C-7 of the steroid was found to be essential to effect high diastereoselection.

Analysis of geometric and strain effects in homo-Diels-Alder reactions

Molecular mechanics calculations have been carried out to quantify the key geometric and strain effects which are likely to control the homo-Diels-Alder reactivity of 1,4-dienes. The criteria considered include C1..C5 and C2..C4 distances in the diene, twist angle of the two pi units, and the magnitude of strain increase as a result of cycloaddition. By first considering these factors in a number of non-conjugated dienes with known reactivity, the ranges of values within which the reaction is favoured are proposed. Calculations are also reported on several substrates which have not been investigated so far. Promising systems for experimental study are suggested which, in addition to being intrinsically interesting, would place the present proposals on a firm basis.

Molecular interpretation of the linear relationship between the entropy and the enthalpy of activation of charge transfer reactions in polar liquids

The well-known linear relationship (T?S# =??H# +?, where 1 >? > 0,? > 0) between the entropy (?S#) and the enthalpy (?H#) of activation for reactions in polar liquids is investigated by using a molecular theory. An explicit derivation of this linear relation from first principles is presented for an outersphere charge transfer reaction. The derivation offers microscopic interpretation for the quantities? and?. It has also been possible to make connection with and justify the arguments of Bell put forward many years ago.

Role of free valences and spin densities on the migrating atom in location of saddle points in radical-exchange reactions

The principle of the conservation of bond orders during radical-exchange reactions is examined using Mayer's definition of bond orders. This simple intuitive approximation is not valid in a quantitative sense. Ab initio results reveal that free valences (or spin densities) develop on the migrating atom during reactions. For several examples of hydrogen-transfer reactions, the sum of the reaction coordinate bond orders in the transition state was found to be 0.92 +/- 0.04 instead of the theoretical 1.00 because free valences (or spin densities) develop on the migrating atom during reactions. It is shown that free valence is almost equal to the square of the spin density on the migrating hydrogen atom and the maxima in the free valence (or spin density) profiles coincide (or nearly coincide) with the saddle points in the corresponding energy profiles.

Further studies on Norrish type II reactions including a reaction in the first excited singlet state and cyclization of 1:4 biradicals

The Norrish type II processes of methyl-2,2-dimethyl- cyclopropyl ketone, alpha-alkoxy acetones and alkyl pyruvates have been examined using the AM1 semi-empirical molecular orbital method with complete geometry optimization at the partial configuration interaction level in the restricted Hartree-Fock (RHF) frame. The results reveal that the methyl-substituted cyclopropyl ketone has a constrained geometry favourable for hydrogen abstraction from the gamma-position relative to the carbonyl group in the excited singlet state. The presence of the ether oxygen atom in the beta-position relative to the carbonyl group in alkoxy acetones and alkyl pyruvates leads to increased reactivity relative to alkyl monoketones and diketones respectively. The cyclization of 1:4 biradicals has been studied in the unrestricted Hartree-Fock (UHF) frame, and the results reveal that the 1:4 biradical derived from alkoxy acetones readily cyclizes to form oxetanols. On the other hand, in the 1:4 biradicals derived from methyl-substituted cyclopropyl ketone, the three-membered ring breaks readily to form an enol intermediate. Delocalization of an odd electron in 1:4 biradicals derived from alkyl pyruvates is thought to make cyclization difficult.

Organometallic Chemistry of Diphosphazanes. 11. Reactions of fac-[Mo(CO)3(MeCN){.eta.2-Ph2PN(Pri)PPh(DMP)}] (DMP = 3,5-Dimethyl-1-Pyrazolyl) with Diphosphazanes and Diphosphinoalkanes and Oxidative Addition with Iodine

The use of fac-[Mo(CO)(3)(MeCN)(eta(2)-L(1))] (1a) {L(1) = Ph(2)PN(Pr-i)PPh(DMP)}(2) as a precursor to metalloligands and bimetallic, heterotrimetallic, and heptacoordinated complexes is reported. The reaction of 1a with diphosphazane, dppa, or a diphosphinoalkane such as dppm or dppe yields the fac-eta(1)-diphosphine substituted metalloligands, fac-[Mo(CO)(3)(eta(2)-L(1))(eta(1)-PXP)] {PXP = dppa (2), dppm (3), and dppe (4)}. These undergo isomerization to yield the corresponding mer-diphosphine complexes (5-7). Oxidation of the uncoordinated phosphorus atom of the mer-eta(1)-dppm-substituted complex eventually provides mer-[Mo(CO)(3)-(eta(2)-L(1)){eta(1)-Ph(2)PCH(2)P(O)Ph(2)}](8). The structure of the latter complex has been confirmed by single crystal X-ray diffraction {triclinic system, P ($) over bar 1; a = 11.994(3), b = 14.807(2), c = 15.855(3) Angstrom; alpha = 114.24(1), beta = 91.35(2), and gamma = 98.95(1)degrees; Z = 2, 4014 data (F-0 > 5 sigma(F-0)), R = 0.066, R(W) = 0.069}. Treatment of the dppe metalloligand 7 with [PtCl2(COD)] yields the heterotrimetallic complex cis-[PtCl2{mer-[Mo(CO)(3)(eta(2)-L(1))(eta(1)-dppe]}(2)] (9). Attempts to prepare a related trimetallic complex with the dppm-containing metalloligand were unsuccessful; only the tetracarbonyl complex cis-[Mo(CO)(4)(eta(2)-L(1))] (1b) and cis-[PtCl2(eta(2)-dppm)] were obtained. Reaction of la with dppe in the ratio 2:1 yields the mer-mer dinuclear complex [{mer-[Mo(CO)(3)(eta(2)-L(1))]}(2)(mu-dppe)] (10) bridged by dppe. Oxidation of 1a with iodine yields the Mo(II) heptacoordinated complex [MoI2(CO)(2)(eta(3)-L(1))] (11) with tridentate PPN coordination. The same Mo(II) complex 11 is also obtained by the direct oxidation of the tetracarbonyl complex cis-[Mo(CO)(4)(eta(2)-L(1))] (1b) with iodine. The structure of 11 has been confirmed by X-ray diffraction studies {monoclinic system, Cc; a = 10.471(2), b = 19.305(3), c = 17.325(3) Angstrom; beta = 95.47(2)degrees; Z = 4, 3153 data (F-0 > 5 sigma(F-0)), R = 0.049, R(W) = 0.051}. This complex exhibits an unusual capped-trigonal prismatic geometry around the metal. A similar heptacoordinated complex 12 with a chiral diphosphazane ligand {L(3) = (S,R)-P(h)2PN-(*CHMePh)*PPh(DMP)} has also been synthesized.

Reactions of cyanogen radical with alkanes and an explanation for negative temperature dependence of rate constants

Reactions of cyanide radicals with alkanes have been investigated by ab initio methods. It is found that the potential energy surface for reaction of CN with a primary C-H bond in methane has a small positive barrier while reactions of CN with a secondary and a tertiary C-H bond in alkanes are barrierless at the correlated level. A simple explanation for the obtained negative temperature dependence of rate constants for reactions of CN with a secondary and a tertiary C-H bond in alkanes are given in terms of the collision theory of bimolecular reactions. It is shown that for barrierless reactions the negative temperature dependence of the rate constants is attributed to the variation of the pre-exponential factor with temperature.

Study of the reactions of pyridinium poly(hydrogen fluoride) with some ternary oxides

The products of the reaction of pyridinium poly(hydrogen fluoride), PPHF, with KIO3, Na2SnO3, NaBiO3, K2CrO4, Na2MoO4 and Na2WO4 were KIO2F2; Na2SnF6; NaHF2, BiF3; K3CrF6, KHF2, (PyH)(3)CrF6; NaHF2, (PyH)(2)MoO2F4.2NaHF(2); and (PyH)(2)WO2F4.2NaHF(2), respectively, while KClO3, KBrO3 and KlO(4) react with complete decomposition to form KHF2 as the fluorinated product. This differential reactivity and mode of reaction has been discussed in terms of the oxidation state of the central atom, the nature and strength of the bonds and the complex behaviour of the formed intermediate or fluorinated products that undergo complexation or solvation with pyridine and/or hydrogen fluoride.