Sílvia Simon

How does basis set superposition error change the potential surfaces for hydrogen-bonded dimers?

We describe a simple method to automate the geometric optimization of molecular orbital calculations of supermolecules on potential surfaces that are corrected for basis set superposition error using the counterpoise (CP) method. This method is applied to the H-bonding complexes HF/HCN, HF/H2O, and HCCH/H2O using the 6-31G(d,p) and D95 + + (d,p) basis sets at both the Hartree-Fock and second-order Moller-Plesset levels. We report the interaction energies, geometries, and vibrational frequencies of these complexes on the CP-optimized surfaces; and compare them with similar values calculated using traditional methods, including the (more traditional) single point CP correction. Upon optimization on the CP-corrected surface, the interaction energies become more negative (before vibrational corrections) and the H-bonding stretching vibrations decrease in all cases. The extent of the effects vary from extremely small to quite large depending on the complex and the calculational method. The relative magnitudes of the vibrational corrections cannot be predicted from the H-bond stretching frequencies alone

On the electron-pair nature of the hydrogen bond in the framework of the atoms in molecules theory

Delocalization indices, as defined in the atoms in molecules theory, have been calculated between hydrogen-bonded atoms in 20 molecular complexes that are formed between several H-donor and acceptor molecules. In general, the delocalization index associated to an intermolecular hydrogen bond depends on the interaction energy of the complex, but also on the nature of the H-donor and acceptor atoms. The intermolecular delocalization index appears to be strongly correlated to the orbital interaction energy term as obtained from an energy decomposition scheme based on conceptual Kohn–Sham theory. Both the atoms in molecules theory and the energy decomposition analysis allow for a characterization of the hydrogen bonds in these complexes. In general, both kinds of analysis reveal that hydrogen bonding in these systems is not an essentially electrostatic interaction. 2003 Elsevier Science B.V. All rights reserved.

C–H⋯O H-bonded complexes: How does basis set superposition error change their potential-energy surfaces?

Geometries, vibrational frequencies, and interaction energies of the CNH⋯O3 and HCCH⋯O3 complexes are calculated in a counterpoise-corrected (CP-corrected) potential-energy surface (PES) that corrects for the basis set superposition error (BSSE). Ab initio calculations are performed at the Hartree–Fock (HF) and second-order Mo/ller–Plesset (MP2) levels, using the 6-31G(d,p) and D95++(d,p) basis sets. Interaction energies are presented including corrections for zero-point vibrational energy (ZPVE) and thermal correction to enthalpy at 298 K. The CP-corrected and conventional PES are compared; the uncorrected PES obtained using the larger basis set including diffuse functions exhibits a double well shape, whereas use of the 6-31G(d,p) basis set leads to a flat single-well profile. The CP-corrected PES has always a multiple-well shape. In particular, it is shown that the CP-corrected PES using the smaller basis set is qualitatively analogous to that obtained with the larger basis sets, so the CP method becom...

Dihydrogen Bonding: Donor―Acceptor Bonding (AH···HX) versus the H2 Molecule (A―H2―X)

Dihydrogen bonds (DHBs) play a role in, among others, crystal packing, organometallic reaction mechanisms, and potential hydrogen-storage materials. In this work we have analyzed the central H-H bond in linear H(4), LiH...HX, BH(4)(-)...HX, and AlH(4)(-)...HX complexes with various X by using the quantitative molecular orbital model contained in Kohn-Sham density functional theory at the BP86/TZ2P level of theory. First, we address the questions of if and how one can distinguish, in principle, between a H...H donor-acceptor DHB and the formation of an H(2) molecule by using the simple H(4) model system. The results of these analyses have been used to gain an understanding of the bonding in more realistic model systems (some of which have been studied experimentally), and how this differs from the bonding in H(4).

Influence of the Side Chain in the Structure and Fragmentation of Amino Acids Radical Cations.

The conformational properties of ionized amino acids (Gly, Ala, Ser, Cys, Asp, Gln, Phe, Tyr, and His) have been theoretically analyzed using the hybrid B3LYP and the hybrid-meta MPWB1K functionals as well as with the post-Hartree Fock CCSD(T) level of theory. As a general trend, ionization is mainly localized at the -NH2 group, which becomes more planar and acidic, the intramolecular hydrogen bond in which -NH2 acts as proton donor being strengthened upon ionization. For this reason, the so-called conformer IV(+) becomes the most stable for nonaromatic amino acid radical cations. Aromatic amino acids do not follow this trend because ionization takes place mainly at the side chain. For these amino acids for which ionization of the side chain prevails over the -NH2 group, structures III(+) and II(+) become competitive. The Cα-X fragmentations of the ionized systems have also been studied. Among the different decompositions considered, the one that leads to the loss of COOH(•) is the most favorable one. Nevertheless, for aromatic amino acids fragmentations leading to R(•) or R(+) start being competitive. In fact, for His and Tyr, results indicate that the fragmentation leading to R(+) is the most favorable process.

ANALYSIS OF THE CHANGES ON THE POTENTIAL ENERGY SURFACE OF MENSHUTKIN REACTIONS INDUCED BY EXTERNAL PERTURBATIONS

Abstract A quantitative analysis of the changes induced by solvation and static uniform electric fields on the potential energy surface of the SN2 Menshutkin-type reaction between ammonia and methyl chloride has been performed with the help of different indexes. These indexes have been defined to account for the structural and electronic degree of advance of the transition state with respect to the reactant complex and ion pair product through the use of geometrical parameters, dipole moments and electron density distributions. Indexes reveal that external perturbations yield transition states which are both electronically and structurally advanced as compared to the transition state in the gas phase. The overall study is complemented with HOMO-LUMO orbital considerations, quantification of the global charge density redistributions by means of quantum molecular self-similarity measures and an analysis of the topological features of electron density distributions.

Electron transfer from aromatic amino acids to guanine and adenine radical cations in π stacked and T-shaped complexes

Similar redox properties of the natural nucleobases and aromatic amino acids make it possible for electron transfer (ET) to occur between these sites in protein-nucleic acid complexes. Using DFT calculations, we estimate the ET rate from aromatic amino acid X (X = Phe, His, Tyr and Trp) to radical cations of guanine (G) and adenine (A) in dimers G-X and A-X with different arrangement of the subunits. We show that irrespective of the mutual orientation of the aromatic rings, the electronic interaction in the systems is strong enough to ensure effective ET from X to G(+) or A(+). Surprisingly, relatively high ET rates are found in T-shaped dimers. This suggests that pi stacking of nucleobases and aromatic amino acids is not required for feasible ET. In most complexes [G-X](+) and [A-X](+), we find the excess charge to be confined to a single site, either the nucleobase or amino acid X. Then, conformational changes may initiate migration of the radical cation state from the nucleobase to X and back. The ET process from Trp and Tyr to G(+) is found to be faster than deprotonation of G(+). Because the last reaction may lead to the formation of highly mutagenic species, the efficient repair of G(+) may play an important role in the protection of genomic DNA from oxidative damage.

The Role of Aromaticity, Hybridization, Electrostatics, and Covalency in Resonance-Assisted Hydrogen Bonds of Adenine-Thymine (AT) Base Pairs and Their Mimics.

Hydrogen bonds play a crucial role in many biochemical processes and in supramolecular chemistry. In this study, we show quantum chemically that neither aromaticity nor other forms of π assistance are responsible for the enhanced stability of the hydrogen bonds in adenine–thymine (AT) DNA base pairs. This follows from extensive bonding analyses of AT and smaller analogs thereof, based on dispersion-corrected density functional theory (DFT). Removing the aromatic rings of either A or T has no effect on the Watson–Crick bond strength. Only when the smaller mimics become saturated, that is, when the hydrogen-bond acceptor and donor groups go from sp2 to sp3, does the stability of the resulting model complexes suddenly drop. Bonding analyses based on quantitative Kohn–Sham molecular orbital theory and corresponding energy decomposition analyses (EDA) show that the stronger hydrogen bonds in the unsaturated model complexes and in AT stem from stronger electrostatic interactions as well as enhanced donor–acceptor interactions in the σ-electron system, with the covalency being responsible for shortening the hydrogen bonds in these dimers.

A fuzzy-atom analysis of electron delocalization on hydrogen bonds.

The extent of electron delocalization is quantified for set of cyclic complexes exhibiting two or more hydrogen bonds (HBs). In particular, the delocalization index (DI) between the atoms directly involved in the HB, and the ING (a normalized n-center delocalization index) have been evaluated using several fuzzy-atom schemes, namely Becke, Becke-ρ, Hirshfeld, and Hirshfeld-Iterative. The results have been compared with the widely used Quantum Theory of Atoms in Molecules (QTAIM) atomic definition. The DI values are found to correlate very well with geometrical or topological descriptors widely used in the literature to characterize HB systems. Among all fuzzy-atom methods, the ones that can better accommodate the different partial ionic character of the bonds perform particularly well. The best performing fuzzy-atom scheme for both pairwise and n-center electron delocalization is found to be the Becke-ρ method, for which similar results to QTAIM model are obtained with a much reduced computational cost. These results open up a wide range of applications of such electron delocalization descriptors based on fuzzy-atoms for noncovalent interactions in more complex and larger systems.

MS-CASPT2 Study of Hole Transfer in Guanine–Indole Complexes Using the Generalized Mulliken–Hush Method: Effective Two-State Treatment

Because hole transfer from nucleobases to amino acid residues in DNA-protein complexes can prevent oxidative damage of DNA in living cells, computational modeling of the process is of high interest. We performed MS-CASPT2 calculations of several model structures of π-stacked guanine and indole and derived electron-transfer (ET) parameters for these systems using the generalized Mulliken-Hush (GMH) method. We show that the two-state model commonly applied to treat thermal ET between adjacent donor and acceptor is of limited use for the considered systems because of the small gap between the ground and first excited states in the indole radical cation. The ET parameters obtained within the two-state GMH scheme can deviate significantly from the corresponding matrix elements of the two-state effective Hamiltonian based on the GMH treatment of three adiabatic states. The computed values of diabatic energies and electronic couplings provide benchmarks to assess the performance of less sophisticated computational methods.