16969-45-2Relevant articles and documents
Experimental and theoretical study of the secondary equilibrium isotope effect (SEIE) in the proton transfer between the pyridinium-d5 cation and pyridine
Munoz-Caro,Nino,Davalos,Quintanilla,Abboud
, p. 6160 - 6167 (2007/10/03)
In this work we present an experimental and theoretical study of the proton transfer from the pyridinium-d5 cation to pyridine. FT-ICR measurements yield, at 331 K, an equilibrium constant K = 0.809 (Δ rGmo = 0.58 kJ mol-1) for the process, favoring the pyridine form. The structural and bonding changes on protonation of pyridine are analyzed by applying the atoms in molecules theory. As a consequence of electronic density redistribution, we found that on protonation the CN and the CC bonds placed farther from the nitrogen weaken. In addition, the CH and the CC bonds closer to the nitrogen increase their strength. Thermostatistical computation of the equilibrium constant from data obtained at the B3LYP/cc-pVTZ level, within the harmonic approximation, predicts a value of 0.827 (ΔrGmo, value of 0.52 kJ mol-1), in good agreement with the 0.809 ± 0.027 experimental result for a 99.9% confidence level. A simple statistical mechanical model intended to apply under conditions close to the present ones is developed. The model allows for a fine-tuning of the thermodynamic state functions for the equilibrium. This model shows that rather than by translational and rotational variations, the reaction is driven by the changes in zero point energies and in the density of vibrational states. In addition, theoretical analysis of the enthalpic and entropic contributions shows that the ΔrGmo value is determined by the enthalpic part. It is also predicted that the ΔrG mo value decreases with temperature. We found that this effect is due to a higher density of vibrational states in the pyridine-d 5 form. A new model is developed to correct the vibrational partition function for anharmonicity. This model shows that correction for anharmonicity in the low-frequency modes reduces significantly the difference between calculated and experimental K values.
Steric and kinetic isotope effects in the deprotonation of cation radicals of NADH synthetic analogues
Anne, Agnès,Fraoua, Sylvie,Hapiot, Philippe,Moiroux, Jacques,Savéant, Jean-Michel
, p. 7412 - 7421 (2007/10/02)
The deprotonation rate constants and kinetic isotope effects of the cation radicals have been determined by combined use of direct electrochemical techniques at micro- and ultramicroelectrodes, redox catalysis, and laser flash photolysis, over a extended
Gas-Phase Chemistry of Transition Metal-Imido and -Nitrene Ion Complexes. Oxidative Addition of N-H Bonds in NH3 and Transfer of NH from a Metal Center to an Alkene
Buckner, Steven W.,Gord, James R.,Freiser Ben S.
, p. 6606 - 6612 (2007/10/02)
We report here on the gas-phase chemistry of a number of bare transition metal-nitrene and -imido ion complexes, MNH+.Group 3, 4, and 5 atomic metal ions react with NH3 at thermal energies to generate MNH+ via dehydrogenation.A reaction mechanism is proposed involving initial oxidative addition to an N-H bond, in analogy to mechanisms proposed for reactions of gaseous atomic metal ions with hydrocarbons.Cr+ reacts with NH3 via slow condensation to form CrNH3+, as do all group 6-11 atomic metal ions investigated.However, excited-state Cr+ reacts with NH3 via bond-insertion reactions to form CrNH2+ and CrNH+.An unidentified metastable electronic state of Cr+, produced by direct laser desorption of chromium foil, reats with much higher efficiency than does kinetically excited Cr+.FeO+ reacts with NH3 to generate FeNH+ with loss of H2O.Thermochemical studies of VNH+ and FeNH+ involving ion-molecule reactions indicate values of D0(V+-NH) = 101 +/- 7 kcal/mol and D0(Fe+-NH) = 54 +/- 14 kcal/mol, the latter value in accord with D0(Fe+-NH) = 61 +/- 5 kcal/mol obtained from photodissociation.The high bond strength for VNH+ indicates multiple bonding, analogous to that in the isoelectronic VO+, while the weaker bond strength for FeNH+ indicates a single bond, analogous to that in the isoelectronic FeO+.Proton-transfer experiments indicate PA(VN) = 220 +/- 4 kcal/mol from which ΔHf(VN) = 111 +/- 9 kcal/mol and D0(V-N) = 125 +/- 9 kcal/mol are obtained.VNH+ is unreactive with ethene and benzene, but FeNH+ transfers NH to ethene and benzene through metathesis and homologation reactions.A cyclic metalloaminobutane intermediate is consistent with the products of the FeNH+/ethene reaction.
