The use of locally dense basis sets in the calculation of indirect nuclear spin–spin coupling constants: The vicinal coupling constants in H3C–CH2X (X=H, F, Cl, Br, I)

We have calculated the vicinal indirect nuclear spin-spin coupling constants 3J1H1H3J1H1H  in the series of molecules H3C–CH2XH3C–CH2X with X=H, F, Cl, Br, and I at the self-consistent field level and using the second order polarization propagator approximation (SOPPA). We have studied the effect of electron correlation and of the substituents (X=F, Cl, Br, and I) on all four contributions to the coupling constants. But in particular we have investigated the possibility of using locally dense basis sets, i.e., we have carried out calculations with basis sets, where the basis functions on the hydrogen atoms were optimized for the calculation of spin–spin coupling constants whereas on the other atoms smaller, contracted sets of basis functions were used. This changes the results for the couplings by ∼0.3 Hz or 3%. However, the change is almost entirely due to the orbital paramagnetic term and is independent of electron correlation, which enables one to estimate the SOPPA results in the full basis sets. Furthermore we find that the Fermi contact term is the dominant contribution to the vicinal coupling constants, because it is about an order of magnitude larger than the other contributions and because the two orbital angular moment terms almost cancel each other completely. Also the changes in the calculated couplings due to electron correlation are solely due to the Fermi contact term. However, the shifts in the coupling constants caused by the different substituents arise in equal amounts from the Fermi contact and the orbital diamagnetic term, whereas the changes in the orbital paramagnetic term are smaller and are in the opposite direction. In comparison with the experimental data we find very good agreement for C2H6C2H6 and C2H5F.C2H5F. However, the agreement becomes less good with increasing nuclear charge of the substituent X.Fil: Provasi, Patricio Federico. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Aucar, Gustavo Adolfo. 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

The role of explicit solvent molecules in the calculation of NMR chemical shifts of glycine in water

We present the results of a computational study of the NMR properties of glycine in water solution at the level of density functional theory employing the B3LYP functional and the 6-31G(d,p) and pcSseg-2 basis sets, describing the solvent either via the PCM continuous solvation model or PCM with additional explicit water molecules hydrogen-bonded to the solute.We observe that the solvent causes considerable changes in the predicted magnetic shieldings and that the results depend significantly on the number of solvent molecules included in the quantum mechanical treatment.Fil: Caputo, Maria Cristina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; ArgentinaFil: Provasi, Patricio Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Sauer, Stephan P. A.. 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 Tecnológica; Argentin