Molecular-orbital calculations isotope effects

Co2(CO)q system, reveals that the reactions proceed through mononuclear transition states and intermediates, many of which have established precedents. The major pathway requires neither radical intermediates nor free formaldehyde. The observed rate laws, product distributions, kinetic isotope effects, solvent effects, and thermochemical parameters are accounted for by the proposed mechanistic scheme. Significant support of the proposed scheme at every crucial step is provided by a new type of semi-empirical molecular-orbital calculation which is parameterized via known bond-dissociation energies. The results may serve as a starting point for more detailed calculations. Generalization to other transition-metal catalyzed systems is not yet possible. 

The rate of reaction of Mel with [Rh(CO)2l2] was measured and compared with the rate of reaction of Mel and the kinetic isotope effect (KIE) was shown, through ab initio molecular orbital calculations, to be consistent with an 5 2 mechanism proceeding through a linear (Rh-C-I ) transition state [33]. 

Molecular orbital calculations of the w-electron distribution in pyridine predict that more 4- than 2-aminopyridine should be formed in the Tschitschibabin reaction.4 The fact that no 4-aminopyridine can be detected when the two positions are allowed to compete for a deficiency of sodamide (see, e.g., Abramovitch et al 268) has led to the suggestion that the observed orientation in this reaction depends on the relative ease of elimination of a hydride ion from C-2 and C-4 and not upon the initial mode of addition (which, by implication, must take place predominantly at C-4 as predicted by the molecular orbital calculations).4 This hypothesis necessitates that the addition step be rapidly reversible and that the second stage, the elimination of hydride ion, be the rate-determining one (Scheme VII). Although it seems reasonable to assume that the hydride ion eliminations are the slow steps in this reaction, the fact that no deuterium isotope effect was observed in the reaction of 3-picoline-2d and of pyridine-2d with sodamide implies that the first stage must be virtually irreversible,268 as was found also in the case of the addition of phenyllithium to pyridine.229 The addition stage must, therefore, be the product-... 

Tetrahedral Anions. The hydrolysis of the [BHJ anion in aqueous DMSO has the kinetic form for a general-acid-catalysed reaction, but with the catalytic coefficients and k-s.+ very different to the values observed for aqueous media. This difference is ascribed to the inability of DMSO to act as a proton relay system. A series of molecular orbital calculations suggest that BH5 could be a metastable intermediate in the hydrolysis of [BH They indicate the presence of identifiable BH3 and Hg subunits in the structure. The authors conclude, however, that limitations in the theoretical methods used do not yet allow experimental and computational results to be reconciled (at least to within a few kcal). The acid-cat ysed ammonolysis in liquid ammonia is first-order in both [NHJ+ and [BHJ, but is appreciably slower than the corresponding reaction in water the rate is markedly decreased by increases in ionic strength. This may be due to the participation of a water molecule in the latter case. The rates of reduction of a variety of substrates by [BHJ have been reported, and for the reduction of ketones it is suggested on the basis of H/T isotope effects that a four-centre transition state is involved. 

Phosphorus.—Oxoanions and Related Species. Ab initio and semi-empirical molecular orbital calculations on the metaphosphate anion [PO3]- attribute the electrophilic reactivity of this species to a low-lying acceptor orbital of a symmetry, nearly degenerate with a n molecular orbital. This species, metaphosphate, is suggested as intermediate in one of the two pathways involved in phosphorylation by phosphorocreatine. Kinetic isotope effect studies indicate that metaphosphate is also generated in the hydrolysis of 2,4-dinitrophenylphosphate ... 

Very recently, Singleton has suggested a new way of calculating isotope effects illustrated by the bromonium ion [61]. This method could possibly also be used for tautomeric systems. Another new approach is the multicomponent molecular orbital method for direct treatment of nuclear quantum effects [62]. The basic idea is to incorporate the nuclear wave function and in particular the proton wave function directly into the electronic structure calculation. This approach has great potential but has so far been tested only for secondary isotope effects on chemical shifts [63]. The geometric isotope effect has also been looked into based on Pauling valence-bond orders [20]. 

Oxo-transfer reactions involving molybdenum have been reviewed. The structure of trans-[Ru(0)2(py)2(0Ac)2] shows a relatively long Ru=0 bond which may be important in the role of the complex as an oxo-transfer reagent. Kinetics and mechanisms for epoxidations by [Ru(terpy)(tmen) (H20)] have been reported. Molecular orbital calculations on the formation of epoxides from Ru(IV)=0 and ethene suggest that the preferred pathway involves a nucleophilic attack on the oxo oxygen. Similar studies have been carried out on the oxidation of alcohols. The mechanism of oxidation of alcohols by [Ru OL] " where HL=[2-hydroxy-2-(2-pyridyl)ethyl]bis[2-(2-pyridyl)ethyl]amine has been investigated. The reactions are first-order in both reagents, and kinetic isotope effects on the... 

The density functional theory calculations of primary 14C KIE and secondary deuterium kinetic isotope effects (SKIE)220 did not reproduce satisfactorily all the experimentally determined 14C KIE and deuterium (4,4-2H2)- and 6,6-2H2-SKIE, though the non-local DFT methods provide transition state energies on a par with correlated molecular orbital theory221. 

O Neil JR (1986) Theoretical and experimental aspects of isotopic fractionation. Rev Mineral 16 1-40 Oi T (2000) Calculations of reduced partition function ratios of monomeric and dimeric boric acids and borates by the ab initio molecular orbital theory. J Nuclear Sci Tech 37 166-172 Oi T, Nomura M, Musashi M, Ossaka T, Okamoto M, Kakihana H (1989) Boron isotopic composition of some boron minerals. Geochim Cosmochim Acta 53 3189-3195 Oi T, Yanase S (2001) Calculations of reduced partition function ratios of hydrated monoborate anion by the ab initio molecular orbital theory. J Nuclear Sci Tech 38 429-432 Paneth P (2003) Chlorine kinetic isotope effects on enzymatic dehalogenations. Accounts Chem Res 36 120-126... 

MOPAC AMI AMI is used in the electronic part of the calculation to obtain molecular orbitals, the heat of formation and its derivative with respect to molecular geometry. MOPAC calculates the vibrational spectra, thermodynamic quantities, isotopic substitution effects and force constants for molecules, radicals, ions,... 

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