148 research outputs found
Implementation of the IPPP–CLOPPA–INDO/S method for the study of indirect nuclear spin coupling constants and its application to molecules containing tin nuclei
Get PDFThe inner projection of the polarization propagator, using contributions from localized orbitals, IPPP–CLOPPA, and using the intermediate neglect of the differential overlap model parameterized for spectroscopy, INDO/S, was implemented and used to calculate indirect nuclear spin coupling constants. The resulting model was tested on a group of small- and medium-size model compounds by comparing its performance with that of other semi-empirical methods and experiments where available. It is shown that in general the INDO/S approximation with the use of S N 2 (0) and r N −3 atomic parameters taken from the INDO and AM1 approaches is the most suitable scheme to describe coupling constants. The introduction of atomic parameters for S N 2 (0) and r N −3 in the case of heavy nuclei like Sn, is a critical step. The correction of the bonding beta parameter for this nucleus was also necessary within the INDO/S approximation to improve the accuracy and to better account for indirect relativistic effects. The application of this parameterization was accomplished in a series of tetrastannacyclohexanes and different pathways for coupling transmission were analyzed.Fil: Botek, Edith L.. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura; ArgentinaFil: Aucar, Gustavo Adolfo. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Cory, Marshall G.. University of Florida; Estados UnidosFil: Zerner, Michael C.. University of Florida; Estados Unido
Electron Heat Fluxes for the Global Ionospheric Networks
Get PDFAbstract This paper proposes an alternative approach for calculating electron heat fluxes by combining the Belgian 3D Plasmaspheric Model (Pierrard, Botek, & Darrouzet, 2021, https://doi.org/10.3389/fspas.2021.681401) and an analytical electron temperature formalism developed by Khazanov (2011), https://doi.org/10.1007/978‐1‐4419‐6797‐8. Such information is essential for 3D ionospheric modeling networks, which use the heat flux parameter as an upper boundary condition to calculate electron temperature, Te, at altitudes below 700–800 km. The theoretical approach presented in this letter does not identify the nature of the magnetospheric cold electron temperature heating sources. Instead, it relies on the empirical thermal structure that is incorporated in the Belgian plasmasphere model and their calculation of total plasmaspheric cold plasma content. We also extended our results to plasmaspheric altitudes and provided cold electron heating rates along the geomagnetic field lines from the southern to the northern hemisphere for different magnetic and solar activities
Análisis CLOPPA-RPA de propiedades de respuesta lineal usando funciones de onda ab initio y semiempíricas
Get PDFEn el presente trabajo de tesis, por un lado, se analizan en forma teórica diversos problemas relacionados con el estudio de parámetros espectroscópicos de RMN y de polarizabilidades moleculares con el método IPPP-CLOPPA (Inner Projection of the Polarization Propagator-Contributions from Localized Orbitals to the Polarization Propagator Approach). Por el otro, se presentan modificaciones y/o nuevas implementaciones que son particularmente apropiadas para tratar algunos de estos problemas. Se analizan diferentes efectos electrónicos y estructurales en fenetoles en base al apantallamiento magnético. Se propone una modificación (AM1*) de los parámetros para Sn en el método RPA AM1 que mejora significativamente el cálculo de acoplamientos indirectos de espin nuclear J(XSn). Se presenta la implementación del método IPPP-CLOPPA en el contexto de cálculos semiempíricos INDO/S para el análisis tanto del tensor de acoplamiento de espin nuclear como del tensor de polarizabilidad estática molecular, lo que resulta muy promisorio dada la flexibilidad de dicho método semiempírico. Se obtienen, así, resultados de acoplamientos J(XSn) donde se produce una notable mejora con respecto a los hallados con el método AMI*. En lo referente a la polarizabilidad molecular, se comparan los valores de esta propiedad en la serie etano, etileno y acetileno con distintas bases a nivel semiempírico, agregándose orbitales de polarización, con un estudio similar a nivel ab initio para analizar la aplicabilidad del método. Se aplica luego al estudio de moléculas de interés biológico. La interrelación entre el efecto de sustituyente en acoplamientos ³J(C1M)α y la polarizabilidad del enlace C3—Mα en l-X-3-M-biciclo[l.l.l]pentanos sustituidos (X=H y M=H, F y CH3) se discute con los métodos mencionados tanto a nivel ab initio como INDO/S. Los métodos presentados demuestran ser una herramienta interesante para el análisis de diversos problemas en el campo de la química cuántica.On the one hand, different problems related to the study of NMR parameters and molecular polarizabilities with IPPP-CLOPPA (Inner Projection of the Polarization Propagator - Contributions from Localized Orbitals to the Polarization Propagator Approach) method are analyzed. On the other hand, modifications and/or implementations particularly appropriated to deal with some of these problems are presented. Different electronic and structural effects in phenetols are analyzed with the magnetic shielding tensor. A modification (AM1*) of Sn parameters in RPA AM1 method is proposed, which significantly improves indirect nuclear coupling constants calculations J(XSn). Implementation of IPPP-CLOPPA method with INDO/S semiempirical approach is presented for the analysis of indirect nuclear coupling and static molecular polarizability tensors that results to be very promising taking account the flexibility of this semiempirical method. Results from J(XSn) couplings are obtained where a notable improvement is obtained over the results with AM1*. Values from semiempirical molecular polarizabilities with different basis adding polarization orbitals in the series ethane, ethylene and acetylene are compared with a similar ab initio study to analyze the applicability of the method. It is applied later to the study of molecules of biological interest. The relation between substituent effects in ³J(C1M)α and the polarizability of C3—Mα bond in 1-X-3-M-biciclo [1.1.1] pentanes (X=H y M=H, F y CH3) is discussed with mentioned ab initio and semiempirical INDO/S methods. The methods presented here are a useful tool for the analysis of different problems related to Quantum Chemistry.Fil:Botek, Edith Lilián. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina
CLOPPA-RPA analysis of molecular lineal responses properties using ab initio and semiempirical wavefunctions
No full textInfluence of dispersed radiation in quantification of images in Nuclear Medicine. An experimental work.
No full textCircular dichroism of helical structures using semiempirical methods
No full textA general semiempirical scheme has been elaborated to simulate circular dichroism (CD) spectra of supramolecular systems. This approach adopts the analytical method of Beck and Hohlneicher [Theor. Chem. Acc. 101, 297 (1999).] to evaluate the one- and two-center integrals over Slater atomic orbitals. The performance of the method, employing INDO/S and CNDO/S semiempirical parametrizations, has been assessed by considering (i) the effect of the size of the singly excited states manifold, (ii) the origin invariance, and (iii) comparisons with the experimental and other theoretical spectra of several helicenes as well as pyridine-pyrimidine oligomers, which can adopt helical conformations. The main results are (i) the INDO/S parametrization with rather small excitation manifolds is able to reproduce, at low computational costs, the experimental CD spectra of several helicenes as well as CD simulations performed at ab initio and time-dependent density functional theory level of approximation; (ii) in the series of homohelicenes, the rotatory strength of the lowest-energy band increases almost linearly with the size of the helix; (iii) as evidenced by the study of tetradodecyloxy helicene bisquinone, packing effects can change the sign of remarkable CD bands, which are used to assign the structure configuration.</p
Effective molecular polarizabilities and hyperpolarizabilities in all-trans hexatriene clusters
No full textMolecular aggregation effects on the static and dynamic electronic polarizability and hyperpolarizability are estimated within the time-dependent Hartree-Fock scheme by using a decomposition procedure over the atomic orbitals. For each molecule, the total property value consists in an algebraic sum of two contributions: an intrinsic part coming only from the atomic orbitals belonging to the molecule and a mixed contribution coming from the crossed terms associated with atomic orbitals describing the target molecule and its neighbors. All-trans hexatriene molecular clusters are examined to illustrate this decomposition scheme and to analyze the electronic cooperative behavior of the building units.</p
Semiempirical time-dependent unrestricted Hartree-Fock approach for the determination of molecular linear and nonlinear optical properties
No full textThe time-dependent unrestricted Hartree-Fock (TDUHF) iterative procedure has been implemented at the semiempirical level in the MOPAC 2000 package to determine the (non)linear optical responses of open-shell systems. In addition to providing the dynamic quantities, this procedure enables the decomposition of the total (hyper)polarizabilities into their spin contributions. Its application to reference molecules using AM1 level is reported to illustrate the capabilities of the method compared to ab initio and experimental results. Although for small systems the semi-empirical scheme fails in reproducing the highly-correlated ab initio values, the TDUHF/AM1 approach appears as an efficient tool for studying larger conjugated systems.</p
Análisis CLOPPA-RPA de propiedades de respuesta lineal usando funciones de onda ab initio y semiempíricas
No full textEn el presente trabajo de tesis, por un lado, se analizan en forma teórica diversos problemas relacionados con el estudio de parámetros espectroscópicos de RMN y de polarizabilidades moleculares con el método IPPP-CLOPPA (Inner Projection of the Polarization Propagator-Contributions from Localized Orbitals to the Polarization Propagator Approach). Por el otro, se presentan modificaciones y/o nuevas implementaciones que son particularmente apropiadas para tratar algunos de estos problemas. Se analizan diferentes efectos electrónicos y estructurales en fenetoles en base al apantallamiento magnético. Se propone una modificación (AM1*) de los parámetros para Sn en el método RPA AM1 que mejora significativamente el cálculo de acoplamientos indirectos de espin nuclear J(XSn). Se presenta la implementación del método IPPP-CLOPPA en el contexto de cálculos semiempíricos INDO/S para el análisis tanto del tensor de acoplamiento de espin nuclear como del tensor de polarizabilidad estática molecular, lo que resulta muy promisorio dada la flexibilidad de dicho método semiempírico. Se obtienen, así, resultados de acoplamientos J(XSn) donde se produce una notable mejora con respecto a los hallados con el método AMI*. En lo referente a la polarizabilidad molecular, se comparan los valores de esta propiedad en la serie etano, etileno y acetileno con distintas bases a nivel semiempírico, agregándose orbitales de polarización, con un estudio similar a nivel ab initio para analizar la aplicabilidad del método. Se aplica luego al estudio de moléculas de interés biológico. La interrelación entre el efecto de sustituyente en acoplamientos ³J(C1M)α y la polarizabilidad del enlace C3—Mα en l-X-3-M-biciclo[l.l.l]pentanos sustituidos (X=H y M=H, F y CH3) se discute con los métodos mencionados tanto a nivel ab initio como INDO/S. Los métodos presentados demuestran ser una herramienta interesante para el análisis de diversos problemas en el campo de la química cuántica
Theoretical Design of Substituted Tetrathia-[7]-Helicenes with Large Second-Order Nonlinear Optical Responses
No full textIncreasing response: Semiempirical calculations show that substituting tetrathia-[7]-helicene in positions 7 and 8 (see graphic) as well as in positions 2 and 13 by adequate donor/acceptor pairs can lead to a substantial increase of the second-order nonlinear optical response
- …
