Search results for "Isotope effect"
showing 10 items of 61 documents
Double-resonance measurements of isotope shifts and hyperfine structure in Gd I with hyperfine-state selection in an intermediate level
2000
Isotope shifts and hyperfine structure have been measured in the 4f7 5d6s2 9D6 -- X9 D6 (;38 024. 9 cm-1) transition in atomic gadolinium using high- resolution resonance ionization mass spectroscopy. Excitation was performed as a resonance-enhanced two-photon transition with the 4f7 5d6s6p 9F7 state as an intermediate level. Selective population of hyperfine states in the first excitation step allowed assignment of all transitions in the complex hyperfine spectrum of the odd isotopes 155,157Gd and evaluation of the magnetic dipole and electric quadrupole hyperfine structure constants for the X 9D6 state. Measured values for the isotope shifts of all stable Gd isotopes have been used to der…
Pulse-duration dependence of the isotopic effect in simple molecular ions driven by strong laser fields
2011
In this paper we discuss isotopic effects in simple molecular ions subjected to strong laser fields. We show that the intensity of the emitted spectra strongly depends upon both the nuclear mass of the molecular ions and the laser pulse duration. In particular, for short pulse duration [up to 8 optical cycles (o.c.)], we confirm the trend described in the most studied case in which the high-order harmonic generation is more efficient for heavier isotopes; in contrast, an interesting physical phenomenon is predicted for pulses longer than 16 o.c. characterized by an inverse effect in which lighter molecular species are responsible for higher-order harmonic emission.
Isotope Effects and Collective Excitations
1983
Isotopic substitution in the aqueous solvent is discussed as an effective method for probing the role of solvent dynamics in the stability of biomolecular conformation.
Study of the benzene⋅N2 intermolecular potential-energy surface
2003
The intermolecular potential-energy surface pertaining to the interaction between benzene and N2 is investigated theoretically and experimentally. Accurate intermolecular interaction energies are evaluated for the benzene–N2 van der Waals complex using the coupled cluster singles and doubles including connected triples [CCSD(T)] method and the aug-cc-pVDZ basis set extended with a set of 3s3p2d1f1g midbond functions. After fitting the energies to an analytic function, the intermolecular Schrödinger equation is solved to yield energies, rotational constants, and Raman-scattering coefficients for the lowest intermolecular levels of several benzene–N2 isotopomers. Experimentally, intermolecula…
Computational and experimental investigation of intermolecular states and forces in the benzene-helium van der Waals complex
2003
A study of the intermolecular potential-energy surface (IPS) and the intermolecular states of the perprotonated and perdeuterated benzene–He complex is reported. From a fit to ab initio data computed within the coupled cluster singles and doubles including connected triples model for 280 interaction geometries, an analytic IPS including two- to four-body atom–atom terms is obtained. This IPS, and two other Lennard-Jones atom–atom surfaces from the literature, are each employed in dynamically exact (within the rigid-monomer approximation) calculations of J = 0 intermolecular states of the isotopomers. Rotational constants and Raman-scattering coefficients for intermolecular vibrational trans…
The oxidation of alkanes with dimethyldioxirane; a new mechanistic insight
1997
Abstract Primary kinetic isotope effects were measured for the oxidation of cyclohexane and methylcyclohexane with DMDO in solution and in the gas phase. These experiments suggest an electrophilic oxygen insertion mechanism for the oxidation of alkanes by DMDO.
Theoretical Modeling on the Reaction Mechanism of p-Nitrophenylmethylphosphate Alkaline Hydrolysis and its Kinetic Isotope Effects
2015
We have studied the alkaline hydrolysis of p-nitrophenylmethylphosphate (p-NPmP) in aqueous solution by means of polarizable continuum models and by hybrid quantum-mechanical/molecular-mechanical (QM/MM) methods. The theoretical predictions of kinetic isotope effects (KIEs) are in very good agreement with the experimental data, confirming a concerted asynchronous molecular mechanism. In addition, comparison of high level DFT theory with semiempirical AM1/d Hamiltonian has allowed checking the reliability of the later to be used in modeling very large molecular models containing phosphorus atoms.
Mechanistic Studies on the Metal-Free Activation of Dihydrogen by Antiaromatic Pentarylboroles
2012
The perfluoro- and perprotiopentaphenylboroles 1 and 2 react with dihydrogen to effect H-H bond cleavage and formation of boracyclopentene products. The mechanism of this reaction has been studied experimentally through evaluation of the kinetic properties of the slower reaction between 2 and H(2). The reaction is first-order in both [borole] and [H(2)] with activation parameters of ΔH(‡) = 34(8) kJ/mol and ΔS(‡) = -146(25) J mol(-1) K(-1). A minimal kinetic isotope effect of 1.10(5) was observed, suggesting an asynchronous geometry for H-H cleavage in the rate-limiting transition state. To explain the stereochemistry of the observed products, a ring-opening/ring-closing mechanism is propos…
Huge Quantum Symmetry Effect in the O + O2 Exchange Reaction.
2015
We report extensive, full quantum-mechanical calculations for the (16)O + (16)O(16)O → (16)O(16)O + (16)O collisions, for both inelastic and atom exchange processes, using a time-independent method based on hyperspherical coordinates. The rates obtained in the present study are much larger than the previously reported ones for this system. The discrepancy is attributed to a huge symmetry effect that was missing in the studies so far. This effect differs from the well-known isotope effect. Importance of this quantum effect is further confirmed by comparison with results for the (16)O + (18)O(18)O → (16)O(18)O + (18)O, exchange reaction.
Kinetik der Bromierung von Phenolen und phenolischen Mehrkernverbindungen, 5. Mitt.
1984
The kinetics of the bromination of six differently substituted 2,6-bis(hydroxybenzyl)phenols having only one reactivepara position at the phenolic unit in the middle of the molecule were studied in acetic acid at 22°C. The reaction rate decreases if intramolecular hydrogen bonds between one or two hydroxy groups of the adjacent phenolic units and the hydroxy group of the reacting unit become possible, and it is especially low, if these hydrogen bonds are directed to the middle by bulky substituents inortho position. This must be explained by a smaller +M-effect of the hydroxy group of the reacting unit. A kinetic isotope effect is observed in deutero acetic acid, where the reaction rate is …