16969-45-2Relevant academic research and scientific papers
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.
(+)-?-Komplexe in der Gasphase
Cacace, Fulvio,Attina, Marina,Fornarini, Simonetta
, p. 754 - 756 (2007/10/02)
Stichworte: Arenkomplexe * Gasphasenchemie * Massenspektrometrie * Silyl-Kationen
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 basicity measurements of dipeptides that contain valine
Gorman, Greg S.,Amster
, p. 5729 - 5735 (2007/10/02)
Gas-phase basicities of 22 dipeptides that contain valine were measured by a double bracketing method in a Fourier transform ion cyclotron resonance spectrometer. Matrix-assisted laser desorption was used to generate protonated peptide ions which were reacted with reference compounds to bracket the gas-phase basicity. In addition, neutral peptide molecules were formed by substrate-assisted laser desorption and with protonated reference ions to confirm the assignment of the gas-phase basicity. The rate of proton transfer between the protonated molecule of alanylvaline and six reference compounds was measured to examine the behavior of both exoergic and endoergic reactions. Gas-phase basicities of most of the dipeptides were found to be nearly equal to that of their most basic amino acid residue. The results are consistent with an intramolecular hydrogen bond between the N-terminus nitrogen and the amide carbonyl oxygen of a dipeptide. Furthermore, the results suggest that inductive effects cause an increase in the strength of the intramolecular hydrogen bond that the in the basicity of the C-terminus amino acid residue. Dipeptides VF and VY are more basic than their constituent amino acids. These data and molecular mechanics calculations suggest that these two peptides are stabilized by an electrostatic interaction between the N-terminal ammonium ion and the polarizable electrons of the aromatic side chain of the C-terminus.
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.
The Ionic Hydrogen Bond. 2. Multiple NH+...O and CH?+...O Bonds. Complexes of Ammonium Ions with Polyethers and Crown Ethers
Meot-Ner (Mautner), Michael
, p. 4912 - 4915 (2007/10/02)
Complexes of ammonium ions RNH3+ (R = CH3, c-C6H11), (CH3)3NH+, and pyridineH+ with polyethers and crown ethers are observed in the gas phase in the abscence of the solvent effects.The dissociation energies, ΔH0D, of the RNH3+ polyether complexes range from 29.4 kcal mol-1 (for RNH3+*CH3OCH2CH2OCH3) to 46 kcal mol-1 (RNH3+*18-crown-6).The large ΔH0D values for complexes of polydentate ligands indicate multiple -NH+...O-hydrogen bonding.Such mutiple bonding can contribute up to 18 kcal mol-1 to the bonding in RNH3+*CH3(OCH2CH2)3OCH3 and 21 kcal mol-1 in RNH3+*18-crown-6.Multiple interactions are also evident in the (CH3)3NH+*polyether complexes where -CH?+...O-hydrogen bonding seems to occur; and consecutive -CH?+...O-bonds contribute approximately 6, 4, and 2 kcal/mol-1 respectively for up to three such bonds.Total ΔH0D values in the (CH3)3NH+*polyether complexes thus range from 26.7 kcal mol-1 in (CH3)3NH+*CH3O(CH2)2OCH3 to 41 kcal mol-1 in (CH3)3NH+*18-crown-6.Multiple interaction effects, possibly including van der Waals dispersion forces, are observed also in pyridineH+*polyether complexes.Large negative entropies in RNH3+*acyclic polyether complexes vs.RNH3+*cyclic crown ethers make the acyclic polyethers less efficient ligands.
EVIDENCE FOR A SINGLE ELECTRON TRANSFER ACTIVATION IN THE HYDRIDE TRANSFER FROM AN NADH MODEL COMPOUND TO TETRACYANOETHYLENE
Fukuzimi, Shunichi,Kondo, Yuji,Tanaka, Toshio
, p. 751 - 754 (2007/10/02)
New evidence for a stepwise mechanism which requires a single electron transfer activation in the hydride transfer from an NADH model compound, 1-benzyl-1,4-dihydronicotinamide (BNAH), to tetracyanoethylene (TCNE) has been presented based on the effects of pyridine on the stoichiometry of the overall reaction, the rate constant, and the kinetic isotope effect.
Carbon-Hydrogen Bond Dissociation Energies in Alkylbenzenes. Proton Affinities of the Radicals and the Absolute Proton Affinity Scale
Meot-Ner (Mautner), Michael
, p. 5 - 10 (2007/10/02)
Rate constants (k) were measured for proton-transfer reactions from alkylbenzene ions RH+ to a series of reference bases B, i.e., RH+ + B -> BH+ + R*.For exothermic reactions (ΔH -1.For example, the reaction C6H5CH3+ + B -> BH+ + C6H5CH2* is fast (reaction efficiency = k/kcol >/= 0.5) when B = MeO-t-Bu or stronger bases, but k/kcol is significantly smaller when B is n-Pr2O or weaker bases.From the falloff curve of reaction efficiency vs.PA(B), we find PA(n-Pr2O) = PA(C6H5CH2*) + 0.8 kcal mol-1 = 200.0 kcal mol-1.Since PA(C6H5CH2*) is obtained from known thermochemical data, this relation defines the absolute PA of n-Pr2O.Through a ladder of known PA, we then obtain PA(i-C4H8) = 186.8 kcal mol-1; we also obtain the absolute PAs of other oxygen bases.Falloff curves of reaction efficiencies of 3-FC6H4CH3+, C6H5C2H5+, C6H5-n-C3H7+, and C6H5-i-C3H7+ with these reference bases give then the following PAs of R* and R-H bond dissociation energies (Do) (all in kcal mol-1) as R*, PA(R*), Do(R-H): 3-FC6H4CH2*, 197.2, 89.4; , 197.9, 86.2; , 199.1, 86.1; , 199.6, 86.1.In similar manner, rate constants for H+ transfer from C6H5NH2+ to reference pyridines and amines yield PA(C6H5NH*) = 221.5 and Do(C6H5NH-H) = 85.1 kcal mol-1 (1 kcal mol-1 = 4.18 kJ mol-1).
Base Catalysis of Aromatic Nucleophilic Substitution Reactions in Aprotic and Dipolar Aprotic Solvents
Bamkole, Titus O.,Hirst, Jack,Onyido, Ikenna
, p. 889 - 894 (2007/10/02)
For base-catalysed aromatic nucleophilic substitution reactions in benzene, catalysis by added base is observed irrespective of whether the catalyst is a stronger or a weaker base than the nucleophile.In acetonitrile, catalysis is only observed if the catalyst has either approximately the same strength or is a stronger base than the nucleophile.These observations are shown to indicate a difference in the mechanism of catalysis in the two solvents.
Acid-Base Kinetics of Pyridine Studied with a Slow Spectrophotometric Indicator in Methanol
Marshall, David B.,Eyring, Edward M.,Strohbusch, Frank,White, Robert D.
, p. 7065 - 7067 (2007/10/02)
The protonation-deprotonation kinetics of pyridine in methanol has been studied by the electric field jump technique.Kinetic measurements of the electric field insensitive equilibrium B + H+ (k1) BH+ (k-1), where B denotes pyridine, were obtained by coupling the equilibrium with a field-sensitive, visibly colored indicator equilibrium.Use of the "slow" indicator 2,2',4,4',6,6'-hexanitrodiphenylamine allowed observation of the slower of the two relaxation times of the coupled system in a time range where measurements could be made with sufficient precision for a reliable extraction of the rate constants k1 and k-1 from the data.The values obtained are k1 = 1.57 +/- 0.32 * 1010 dm3mol-1s-1 and k-1 = 8.43 +/- 1.69 * 104 s-1.The value for k1 is within the expected limits for a diffusion-controlled reaction in this solvent.An ionic reaction radius of 3.9 Angstroem is calculated from the protonation rate constant value, which indicates that the neutral species involved is probably a hydrogen-bonded pyridine-methanol complex rather than free pyridine.The value for k-1 is 1 order of magnitude greater than the constant for methyl-substituted pyridines, reflecting unsubstituted pyridine's lower basicity in methanol.
