Ab initio ‘CLOPPA’ decomposition of the static molecular polarizability tensor

IPPP-CLOPPA Analysis of the Influence of the Methylation on the Potential Energy and the Molecular Polarizability of the Hydrogen Bonds in the Cytosine-Guanine Base Pair

The IPPP-CLOPPA method is applied to investigate the influence of a methyl group on the energy of the hydrogen bonds and the potential energy curve of the bridge protons in model compounds, which mimic the methylated and unmethylated cytosine-guanine base pairs. On the same grounds, this influence on the polarizability of the intermolecular hydrogen bonds of these compounds is also addressed, in order to determine whether this linear response property provides a significant proof of the electronic mechanisms that affect the stabilization of the hydrogen bonds. Results obtained show that the methyl electronic system delocalizes on the hydrogen bond region, and changes of these intermolecular hydrogen bonds are due to this effect of delocalization.

On the truncation of the number of excited states in density functional theory sum-over-states calculations of indirect spin spin coupling constants

It is investigated, whether the number of excited (pseudo)states can be truncated in the sum-over-states expression for indirect spin-spin coupling constants (SSCCs), which is used in the Contributions from Localized Orbitals within the Polarization Propagator Approach and Inner Projections of the Polarization Propagator (IPPP-CLOPPA) approach to analyzing SSCCs in terms of localized orbitals. As a test set we have studied the nine simple compounds, CH4, NH3, H2O, SiH4, PH3, SH2, C2H2, C2H4, and C2H6. The excited (pseudo)states were obtained from time-dependent density functional theory (TD-DFT) calculations with the B3LYP exchange-correlation functional and the specialized core-property basis set, aug-cc-pVTZ-J. We investigated both how the calculated coupling constants depend on the number of (pseudo)states included in the summation and whether the summation can be truncated in a systematic way at a smaller number of states and extrapolated to the total number of (pseudo)states for the given one-electron basis set. We find that this is possible and that for some of the couplings it is sufficient to include only about 30% of the excited (pseudo)states.

Distributed polarizability models for imidazolium-based ionic liquids

Quantum chemical calculations are used to derive distributed polarizability models sufficiently accurate and compact to be used in classical molecular dynamics simulations of imidazolium-based room temperature ionic liquids. Two distributed polarizability models are fitted to reproduce the induction energy of three imidazolium cations (1,3-dimethyl-, 1-ethyl-3-methyl-, and 1-butyl-3-methylimidazolium) and four anions (tetrafluoroborate, hexafluorophosphate, nitrate, and thiocyanate) polarized by a point charge located successively on a grid of surrounding points. The first model includes charge-flow polarizabilities between first-neighbor atoms and isotropic dipolar polarizability on all atoms (except H), while the second model includes anisotropic dipolar polarizabilities on all atoms (except H). For the imidazolium cations, particular attention is given to the transferability of the distributed polarizability sets. The molecular polarizability and its anisotropy rebuilt by the distributed models are found to be in good agreement with the exact ab initio values for the three cations and 23 additional conformers of 1-ethyl-3-methyl-, 1-butyl-3-methyl-, 1-pentyl-3-methyl-, and 1-hexyl-3-methylimidazolium cations.

Evaluation of the molecular polarizability using the IPPP-CLOPPA-INDO/S method. Application to molecules of biological interest

The IPPP-CLOPPA-INDO/S method is introduced to investigate the static molecular polarizability in macromolecules. As an example of application, the polarizability of phospholipidic compounds, with and without the presence of water molecules has been estimated. The IPPP technique was employed to calculate the polarizability of the polar head and the hydrocarbon chains separately to analyze the feasibility of evaluating the total polarizability of the molecule by addition of these two projected results. INDO/S dipole moments of different fragments of the complex molecule were obtained by means of localized molecular orbitals in order to evaluate the charge transfer in the system.

CLOPPA-IPPP analysis of cooperative effects in hydrogen-bonded molecular complexes. Application to intermolecular 2hJ(N,C) spin-spin coupling constants in linear (CNH)n complexes

The cooperative effects on NMR indirect nuclear coupling constants are analyzed by means of the IPPP-CLOPPA approach (where CLOPPA is the Contributions from Localized Orbitals within the Polarization Propagator Approach and IPPP is the Inner Projections of the Polarization Propagator). The decomposition of the J coupling allows one to classify these effects as those due to changes in the geometric structure and those that directly involve the transmission mechanisms. This latter contribution admits a further classification, taking into account its electronic origin. As an example, the cooperative effects on intermolecular 2hJ(N,C) couplings of the linear complexes (CNH)n (n = 2, 3, 4) are discussed.

The Sign and Magnitude of 2hJ(F,F) and 1hJ(F,H) in FH···FH. A CLOPPA Analysis of Their Distance Dependence

The sign change of the intermolecular (2h)J(F,F) coupling in the (HF)2 dimer as a function of the F-F distance is discussed by means of the CLOPPA method. It is found that it is due to the competition of positive and negative contributions involving the interaction of the sigma lone pair of the acceptor nucleus with vacant molecular orbitals localized in the F-H...F moiety and with other molecular orbitals localized in the donor molecule. The origin of the sign of each contribution is fully determined by analyzing the response of the electronic system to the magnetic perturbation at the acceptor F nucleus. (2h)J(F,F) coupling in the FH...F-, which is positive for all F-F distances, is also analyzed in order to look for the differences with the former case.

CLOPPA-IPPP Analysis of Electronic Mechanisms of Intermolecular 1hJ(A,H) and 2hJ(A,D) Spin−Spin Coupling Constants in Systems with D−H···A Hydrogen Bonds

The electronic origin of intermolecular (2h)J(A,D) and (1h)J(A,H) couplings is discussed by means of the CLOPPA-IPPP approach in several model complexes with D-H...A hydrogen bonds. It is found that the origin of these couplings is mainly due to the interaction between the acceptor sigma lone pair and vacant molecular orbitals localized in the D-H...A moiety, regardless of the donor and acceptor nuclei. The problem of the larger absolute value of (2h)J(A,D) compared to (1h)J(A,H) is also addressed.