1,721,224 research outputs found
The Anomalous Deuterium Isotope Effect in the NMR Spectrum of Methane: An Analysis in Localized Molecular Orbitals
Full text linkAnomaly explained: The secondary isotope effect on the carbon–hydrogen indirect nuclear spin–spin coupling constant in methane, which is larger than the primary isotope effect, is explained in terms of contributions to the coupling constant from localized molecular orbitals.Fil: Sauer, Stephan P. A.. Universidad de Copenhagen; DinamarcaFil: Provasi, Patricio Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Instituto de Modelado e Innovación Tecnológica. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Instituto de Modelado e Innovación Tecnologica; Argentin
Exact closed-form expressions for unitary spin-adapted fermionic singlet double excitation operators
No full textWe derive exact closed-form expressions for the matrix exponential of the anti-Hermitian spin-adapted singlet double excitation fermionic operators.These expressions enable the efficient implementation of such operators within unitary product state frameworks targeting conventional hardware, and allow for the implementation of ansätze that guarantee convergence to specific spin symmetries.Moreover, these exact closed-form expressions might also lay the groundwork for constructing spin-adapted circuits for quantum devices
A second-order doubles correction to excitation energies in the random-phase approximation
No full textUdgivelsesdato: 20 February 199
On the angular dependence of the vicinal fluorine-fluorine coupling constant in 1,2-difluoroethane: deviation from a karplus-like shape
Full text linkThe angular dependence of the vicinal fluorine−fluorine coupling constant, 3JFF, for 1,2-difluoroethane has been investigated with several polarization propagator methods. 3JFF and its four Ramsey contributions were calculated using the random phase approximation (RPA), its multiconfigurational generalization, and both second-order polarization propagator approximations (SOPPA and SOPPA(CCSD)), using locally dense basis sets. The geometries were optimized for each dihedral angle at the level of density functional theory using the B3LYP functional and fourth-order Møller−Plesset perturbation theory. The resulting coupling constant curves were fitted to a cosine series with 8 coefficients. Our results are compared with those obtained previously and values estimated from experiment. It is found that the inclusion of electron correlation in the calculation of 3JFF reduces the absolute values. This is mainly due to changes in the FC contribution, which for dihedral angles around the trans conformation even changes its sign. This sign change is responsible for the breakdown of the Karplus-like curve.Fil: Provasi, Patricio Federico. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Sauer, Stephan P. A.. Universidad de Copenhagen; Dinamarc
Analysis of isotope effects in NMR one-bond indirect nuclear spin-spin coupling constants in terms of localized molecular orbitals
Full text linkWe recently showed, by analyzing contributions from localized molecular orbitals, that the anomalous deuterium isotope effect in the one-bond indirect nuclear spin–spin coupling constant of methane, also called the unexpected differential sensitivity, can be explained by the transfer of s-orbital character from the stretched bond to the other unchanged bonds [ChemPhysChem, 2008, 9, 1259]. We now extend this analysis of isotope effects to the molecules BH4−, NH4+, SiH4, H2O and NH3 in order to test our conclusions on a wider rage of XH4 compounds and to investigate whether the lone-pair orbitals are really responsible for the absence of a similar effect in water and ammonia as proposed earlier [J. Chem. Phys., 2000, 113, 3121].Fil: Provasi, Patricio Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Instituto de Modelado e Innovación Tecnológica. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Instituto de Modelado e Innovación Tecnologica; ArgentinaFil: Sauer, Stephan P. A.. Universidad de Copenhague. Departamento de Química; Dinamarc
The vibrational and temperature dependence of the indirect nuclear spin-spin coupling constants of the oxonium (H3O+) and hydroxyl (OH-) ions
No full textThe indirect nuclear spin-spin coupling constants of the gas phase oxonium (H3O+) and hydroxyl (OH-) ions, their temperature dependence and isotope shifts are predicted by ab initio calculations. The coupling constants are calculated as a function of the symmetric stretching and the inversional coordinates of H3O+and as a function of the bond length of OH-at the uncorrelated level of the random phase approximation (RPA), at the correlated levels of the second order polarization propagator approximation with coupled cluster singles and doubles amplitudes - SOPPA(CCSD) - and of the multiconfigurational random phase approximation (MCRPA) with a large complete active space wavefunction. Effective ro-vibrational state dependent coupling constants are obtained from these functions and the corresponding ro-vibrational wavefunctions. The effective coupling constants for several states are then used to determine the temperature dependence of the coupling constants. The results are compared with the coupling constants of H2O and the nuclear magnetic shielding constants of H3O+and OH-
Atomic integral driven second order polarization propagator calculations of the excitation spectra of naphthalene and anthracene
No full textAn atomic integral direct implementation of the second order polarization propagator approximation (SOPPA) for the calculation of electronic excitation energies and oscillator strengths is presented. The SOPPA equations are solved iteratively using an integral direct approach and, contrary to previous implementations, the new algorithm does not require two-electron integrals in the molecular orbital basis. The linear transformation of trial vectors are calculated directly from integrals in the atomic orbital basis. In addition, the eigenvalue solver is designed to work efficiently with only three trial vectors per eigenvalue. Both of these modifications dramatically reduce the amount of disk space required, thus, increasing the range of applicability of the SOPPA method. Calculations of the lowest singlet excitation energies and corresponding dipole oscillator strengths for naphthalene and anthracene employing basis sets of 238 and 329 atomic orbitals, respectively, are presented. The overall agreement of our results with experimental spectra is good. The differences between the vertical excitation energies calculated by SOPPA and the position of the maximum intensity peaks in the experimental spectra are within the range of ± 0.35 eV with two exceptions, the 41Agstate of naphthalene and anthracene where a 0.85 eV and 0.41 eV deviation is found, respectively. The relatively large discrepancy for this transition is due to large contributions from two-electron excitations which cannot accurately be described in SOPPA. For naphthalene we find additional excitations to Rydberg states of1Auand1B2usymmetry as compared with previous calculations. © 2000 American Institute of Physics
Efectos relativistas y de correlación sobre el parámetro 'J' de la RMN
No full textMany of the chemical properties of the molecules, conformed by elements of the first and second row of the periodic table, can be explained within the non-relativistic Hartree-Fock (HF) theory and when heavier elements are involved a relativistic HF level of approach results in necessary. Nuclear magnetic resonance (NMR) parameters like the indirect nuclear spin-spin coupling constant (J) and the nuclear magnetic shielding ( ) are the most frequently studied to determine their correlation with different inter- and intra-molecular factors
like the effect caused by exchanging substituents. In particular, J offer a powerful tool for the conformational and structural analysis of molecules. However, this parameter is particularly difficult to calculate due to its strongly dependence on the molecular electronic structure and often calculations are affected by
the inconvenience of HF instabilities or quasi-instabilities what leads to unrealistically large values of J. Hence, a higher than HF level of approach is needed to get nearlyconverged results and therefore more precise information of the electronic structure of the molecule under study can be extracted. The inclusion of electron correlation into J calculations rise up considerably the computational cost in a way that it makes impossible those calculations in molecular systems containing more than few atoms, say for instance more than 10. Nevertheless, those computational requirements can be minimized by the application of one of the goals which is the utilization of the so-called locally dense basis sets (LDBS) scheme. On the other hand, some evidence of non-additivity between relativistic and correlation effects suggest the necessity for a correlated relativistic approach. The extraordinary success of the second order Møller-Plesset perturbation theory (MP2) and its simplicity is an attractive choice for the inclusion of electron correlation within the relativistic framework.Muchas de las propiedades químicas de moléculas, conformadas por elementos de la primera y segunda fila de la tabla periódica se pueden explicar mediante la teoría de Hartree-Fock (HF) no relativista. Cuando estan presentes elementos más pesados resulta necesario un cálculo HF relativista. Los parámetros espectroscópicos de la resonancia magnética nuclear (RMN), como la constante de acoplamiento indirecto entre espines nucleares (J) y el apantallamiento magnético nuclear ( ) más frecuentemente estudiados para determinar su relación con diferentes factores inter- e intra-moleculares. En particular, la determinación de J resulta ser una herramienta potente para el análisis conformacional y estructural de moléculas.
Sin embargo, este parámetro es muy difícil de calcular debido a su fuerte dependencia con la estructura electrónica molecular y a menudo los cálculos son afectados por problemas de inestabilidades o cuasiinestabilidades que producen valores erróneamente grandes o pequeños de J. De aquí que será necesario un nivel de aproximación superior al HF para obtener resultados cercanos a la convergencia teórica y por lo tanto información más precisa sobre la estrucura electrónica de la molécula bajo estudio. La inclusión de la correlación electrónica en los cálculos de J elevan de manera considerable el costo computacional de forma que se hacen imposibles esos cálculos en sistemas moleculares conteniendo pocos átomos, digamos por ejemplo más que 10. Sin embargo, dichos requerimientos computacionales pueden ser minimizados por la aplicación de uno de los desarrollos de esta tesis que consiste en la utilización de los así llamados esquemas
de bases localmente densas (LDBS) Por otro lado, alguna evidencia de la no aditividad entre efectos relativistas y de correlación sugieren la necesidad de una aproximación correlacionada relativista. El éxito
extraordinario de la teoría de perturbaciones de segundo orden de Møller-Plesset (MP2) y su simplicidad es una elección atractiva para la inclusión de correlación dentro del marco relativista.Fil: Provasi, Patricio Federico. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas y Naturales y Agrimensura; Argentina
Large long-range F-F indirect spin-spin coupling constants. Prediction of measurable F-F couplings over a few nanometers
Full text linkLarge long-range indirect nuclear spin coupling constants are of great interest for quantum computers. But they are rarely observed and are usually considered very small, unless the coupled nuclear spins are proximate in space. Looking for counterexamples, we have calculated F−F couplings in four different series of acyclic hydrocarbons (alkanes, conjugated polyenes, conjugated polyynes, and cumulenes) where the coupled fluorine nuclei are separated by up to 11 bonds or 1.4 nm. The calculations were carried out at the level of the second-order polarization propagator approximation using locally dense basis sets. This approach has, in recent years, been shown to be particularly successful in reproducing indirect nuclear spin−spin couplings in organic molecules. We find that the F−F couplings in saturated alkanes diminish very quickly with the number of bonds between the coupled fluorine atoms, whereas in the conjugated polyenes and in particular polyynes the F−F couplings can be transmitted over much longer distances. We predict that the F−F coupling over 9 bonds or 1.1 nm is 12 Hz in (1E,3E,5E,7E)-1,8-difluoroocta-1,3,5,7-tetraene and the coupling over 11 bonds or 1.4 nm is 7 Hz in difluorodecapentayne. Analyzing the four Ramsey contributions, we find that the F−F couplings in the polyenes are dominated by the spin−dipolar term, which is known to be favored by π-electronic systems, whereas in the case of the polyynes the orbital paramagnetic terms make the largest contributions, although the spin−dipolar and the Fermi contact contributions are also significant.Fil: Provasi, Patricio Federico. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas y Naturales y Agrimensura. Departamento de Física; ArgentinaFil: Aucar, Gustavo Adolfo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas y Naturales y Agrimensura. Departamento de Física; ArgentinaFil: Sauer, Stephan P. A.. Universidad de Copenhagen; Dinamarc
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