Contribution of the empirical dispersion correction on the conformation of short alanine peptides obtained by gas-phase QM calculations

In this work we analyze the effect of the inclusion of an empirical dispersion term to standard DFT (DFT-D) in the prediction of the conformational energy of the alanine dipeptide (Ala2) and in assessing the relative stabilities of short polyalanine peptides in helical conformations, i.e., α and 310 helices, from Ala4 to Ala16. The Ala2 conformational energies obtained with the dispersion-corrected GGA functional B97-D are compared to previously published high level MP2 data. Meanwhile, the B97-D performance on larger polyalanine peptides is compared to MP2, B3LYP and RHF calculations obtained at a lower level of theory. Our results show that electron correlation affects the conformational energies of short peptides with a weight that increases with the peptide length. Indeed, while the contribution of vdW forces is significant for larger peptides, in the case of Ala2 it is negligible when compared to solvent effects. Even for short peptides, the inclusion of an empirical dispersion term greatly improves accuracy of DFT methods, providing results that correlate very well with the MP2 reference at no additional computational cost.

Estimation Of The 79Br Nqr Frequencies Of Bromo-Containing Molecules Using Ab Initio Calculations At Different Levels

Ab initio calculations of bromo-containing molecules on the RHF, B3LYP and MP2 levels and 6−31G(d), 6−31+G(d), 6−311G(d) and 6311+G(d) basis sets were executed. They were used to estimate the 79Br NQR frequencies of these molecules. A satisfactory agreement between experimental and estimated NQR frequencies is obtained for the sum of populations of 13p- and 14p-components of the Br atom valence p-orbitals obtained from the RHF, B3LYP and MP2 calculations (particularly from RHF calculations) with the split valence basis sets 6−311G(d) and 6−311+G(d). The agreement between the experimental and estimated NQR frequencies is worse for the populations of the 9p-components of the Br atom valence p-orbitals obtained from these calculations with the basis sets 6−31G(d) and 6−31+G(d). An analogous conformity was not obtained using the populations of other components of the Br atom valence p-orbitals or their total populations obtained from all above-mentioned calculations.

Relation between Magnetic, Spectroscopic and Structural Properties of Binuclear Copper(II) Complexes of Pentadentate Schiff-base Ligand, Semi-empirical and ab-initio Calculations

The synthesis and characterization of [Cu2(L1)(3,5 prz)] (L1=1,3-Bis(2-hydroxy-3,5-chlorosalicylideneamino) propan-2-ol) 1 and of [Cu2(L2)(3,5 prz)] (L2=1,3-Bis(2-hydroxy-bromosalicylideneamino) propan-2-ol) 2 are reported. The compounds were studied by elemental analysis, infrared and electronic spectra. The structure of the Cu2(L1)(3,5 prz)] complex was determined by x-ray diffraction. The magnetochemical characteristics of these compounds were determined by temperaturedependent magnetic susceptibility measurements, revealing their antiferromagnetic coupling. The superexchange coupling constants are 210 cm−1 for 1 and 440 cm−1 for 2. The difference in the magnitude of the coupling constants was explained by the metal-ligand orbital overlaps and confirmed by ab-initio restricted Hartree-Fock (RHF) calculations. In order to determine the nature of the frontier orbitals, Extended Hückel Molecular Orbital (EHMO) calculations are also reported.

Coriolis coupling constants and inertial defect of B2F4 by ab initio calculations and the iterative method

Structural parameters, harmonic frequencies, and force constants of B2F4 were obtained in internal coordinates from ab initio RHF calculations with the 6-311G* basis set. The results obtained are used to calculate the Coriolis coupling constants and the inertial defect for 10B2F4 and 11B2F4 and to compare these values with those obtained by using the autoconsistency method. Both the ab initio and the iterative force fields, despite some significant differences, reproduce the vibrational wave numbers equally well and yield nearly the same Coriolis coupling constants and inertial defects. The calculated inertial defects for 10B2F4 and 11B2F4 are -0.942 and -0.919 amu Å2, respectively.Key words: Coriolis coupling constants, inertial defect, B2F4, harmonic frequencies, force constants.