Ab initio

About: Ab initio is a research topic. Over the lifetime, 57307 publications have been published within this topic receiving 1624473 citations. The topic is also known as: ab init..

A theoretical investigation of the solution N(7) H ⟷ N(9) H tautomerism of adenine

Abstract: The geometries of the two most important tautomeric forms of adenine and the corresponding methyladenines are investigated by means of quantum chemical calculations at different level of sophistication, ranging from semiempirical methods to correlation corrected ab initio methods at second-order Moller-Plesset perturbation theory level (MP2). The relative stability of the N(7)H ⟷ N(9)H tautomeric forms of adenine are investigated with highly correlation corrected methods, MP3 and MP4. The relative stability is also corrected for solvent interactions and compared with experimental information. N(9)H-adenine is predicted to be the most stable tautomer in both vacuum and in solution. The relative stability is predicted to be between 24.5 and 35.0 kJ/mol in vacuum depending on computation method. In water solution N(7)H-adenine is stabilized more by the solvent, and the corresponding relative energies were found to be between 4.9 and 10.2 kJ/mol. We also found that correlation effects are essential to describe the ground state geometry with a high accuracy. The geometries predicted by semiempirical methods and ab initio calculations without correlation correction show large deviations in some parts of the molecule compared to the MP2 results as well as compared with experimental geometries. © 1995 John Wiley & Sons, Inc.

Competition between n → πAr* and conventional hydrogen bonding (N–H⋯N) interactions: an ab initio study of the complexes of 7-azaindole and fluorosubstituted pyridines

Abstract: In this work, we have investigated a subtle competition between a very weak n → πAr* interaction and a very strong hydrogen bond (N–H⋯N) interaction present in the complexes of 7-azaindole with a series of 2,6-substituted fluoropyridines and observed how the weak interaction modulates the overall structural motif of these complexes in the presence of the strong interaction. We have studied the structures and binding energies of these complexes using MP2 as well as dispersion-corrected DFT calculations. It has been found that the strength of the N–H⋯N interaction in these complexes decreases with increasing fluorination in the fluoropyridine ring while the proximity between the nitrogen atom in 7-azaindole and the aromatic ring of fluoropyridine increases through n → πAr* interaction. Comparison of the binding energy values as well as structural parameters of these complexes calculated at the B3LYP level with those obtained at the MP2, M05-2X, and B97-D levels of theory clearly indicates that the dispersion effect is mostly responsible for this attractive n → πAr* interaction. This conclusion is also supported by localized molecular orbital-energy decomposition analysis (LMO-EDA). The current investigation is the first theoretical study on the n → πAr* interaction in the presence of a conventional strong hydrogen bonding interaction in the molecular system. Thus the present study has great significance for understanding the structures of the biomolecules as well as materials, as these interactions are very often present there simultaneously.

Ab initio prediction of GaN (101-bar0) and (110) anomalous surface relaxation.

Abstract: The results of a study of the surface relaxation of GaN in the framework of the ab initio (all-electron) Hartree-Fock method are presented. We perform total-energy calculations using a two-dimensionally periodic slab model for the most stable nonpolar cleavage faces, namely, the ($10\overline{1}0$) and (110) surfaces of the wurtzite and zinc-blende phases, respectively. For both surfaces, when the energy is minimized the Ga-N surface bonds show a very small rotation angle of about 6\ifmmode^\circ\else\textdegree\fi{} accompanied by a reduction in surface bond length of about 7%. This result differs from the well-accepted model of the GaP (110) and GaAs (110) surfaces, where there is a large rotational angle in the range of 27\ifmmode^\circ\else\textdegree\fi{}-31\ifmmode^\circ\else\textdegree\fi{} and little change in surface bond length. The structure dependence of the calculated density of states suggests that this difference is at least partly due to interaction of the Ga $3d$ states with N $2s$-derived states in GaN. Partial double-bond character in the surface bond may also be important.

Hydrogen bonding and cooperative effects in mixed dimers and trimers of methanol and trifluoromethanol: An ab initio study

Abstract: Ab initio calculations are used to provide information on the mixed dimers and cyclic trimers of the methanol-trifluoromethanol system. In order to better understand the systems, the monomers and their corresponding dimers and trimers are also investigated. Molecular structures and harmonic frequencies are obtained at the B3LYP/6-311++G(d,p) level. Interaction energies are calculated with the MP2 and B3LYP methods using the 6-311++G(d,p), 6-311++G(2d,2p), and 6-311++G(3d,2p) basis sets for the dimers and heterodimers. The 6-311++G(d,p) basis set was used to calculate the interaction energies for the trimers and heterotrimers. Because the primary goal of this study is to examine cooperative effects, particular attention is given to parameters such as O…O distances, electronic charge densities at the bond critical points, enhanced dipole moments, shifts in the stretching frequencies of the donor O–H bond, and the length of the donor O–H bond.