Manuel Yáñez

Cooperative (nonpairwise) effects in water trimers: An ab initio molecular orbital study

High levels of ab initio molecular orbital theory were used to study the structures and binding energies of water trimers. These calculations included HF/6‐311++G(2df,2p) geometry optimizations for the 17 structures considered. Harmonic vibrational energies were obtained at the HF/6–311++G(2d,2p) level. The HF potential energy surface present three minima whose geometries were refined at the MP2/6–311+G(d,p) level. The global minimum corresponds to an asymmetric cyclic structure which presents significant cooperative effects with respect to the Cs dimer. To properly describe these nonpairwise effects, ZPE (zero point energy) and correlation corrections must be taken into account. They are reflected in a stiffer intermolecular potential, shorter O–O distances, longer donor O–H bond lengths, larger energies per hydrogen bond (HB), and greater shifts of the donor O–H bond stretching frequencies than the Cs dimer. Contrarily, the other two local minima present HBs which are weaker than those of the dimer. The...

Study of the methanol trimer potential energy surface

The potential energy surface of methanol trimer has been studied through the use of high-level ab initio calculations and density functional methods. The geometries have been optimized at the MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory. The harmonic vibrational frequencies were obtained at the latter level. The final energies for the most stable (CH3OH)n (n=1,3) clusters were calculated in the framework of the G2(MP2,SVP) theory. For these and all the other structures the final energies were also obtained using the B3LYP/6-311++G(3df,2p) approach. Three local minima have been located. The global minimum corresponds to a cyclic structure with two methyl groups on one side of the O–O–O plane and the third one on the other side. The bowl conformer, where the three methyl groups are on the same side of the O–O–O plane, is predicted to be only 0.8 kcal/mol less stable than the global minimum. The third local minimum, where one of the monomers behaves as a biacceptor is predicted to lie much higher...

HIGH LEVEL AB INITIO AND DENSITY FUNCTIONAL THEORY STUDIES ON METHANOL-WATER DIMERS AND CYCLIC METHANOL(WATER)2 TRIMER

The methanol-water dimers and the potential energy surface of the cyclic methanol(water)2 trimer have been studied through the use of high-level ab initio calculations and density functional methods. The geometries have been optimized at the MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory. The harmonic vibrational frequencies were obtained at the latter level. The final energies of the different local minima were calculated in the framework of the G2 and G2(MP2) theories. These values were compared with those obtained using the B3LYP/6-311+G(3df,2p) approach. At all the levels of theory considered the most stable conformer of methanol-water heterodimers corresponds to that in which water behaves as a hydrogen bond donor, in agreement with the most recent experimental evidences [P. A. Stockman et al., J. Chem. Phys. 107, 3782 (1997)]. The energy differences between the different conformers of the cyclic methanol(water)2 trimer are rather small, as well as the energy barriers connecting them. The g...

Cooperative effects in water trimers. The performance of density functional approaches

Abstract The performance of B-LYP, B3-LYP, B3-P86 and B3-PW91 density functional to describe cooperative effects in water trimer was investigated. The geometries and vibrational frequencies of the monomer, the dimer and the trimer were obtained using a 6-31+G(d,p) basis set, while the final energies were calculated at the 6-311++G(3df,2p) level. Our results show that B3-LYP and B-LYP functionals seem to be a good alternative to ab initio calculations to account for cooperative or non-pairwise effects, either if these effects are measured in terms of additive interaction energies or in terms of the so-called cooperativity factors.

Do coupling constants and chemical shifts provide evidence for the existence of resonance-assisted hydrogen bonds?

Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculations have been performed to determine two-bond 17O–17O and 15N–15N spin–spin coupling constants (2h J O–O and 2h J N–N) for ten neutral structures which may exhibit intramolecular O–H–O or N–H–N hydrogen bonds. MP2 chemical shifts of hydrogen-bonded protons have also been evaluated. The molecules include malonaldehyde and its diaza counterpart, and the corresponding saturated analogues. The aim of this study is to investigate whether the magnetic properties of the two-bond spin–spin coupling constants and the chemical shifts of the hydrogen-bonded protons provide evidence for the existence of resonance-assisted hydrogen bonds (RAHBs). The two-bond coupling constant for the equilibrium structures is greater for the hydrogen-bonded unsaturated molecule than for the saturated molecule, a result of the shorter O–O distance and stronger hydrogen bond in the former. However, when the O–O or N–N distances are forced to be the same in corresponding saturated and unsaturated structures, the coupling constants are similar. There is a significant increase in coupling constants when the intramolecular hydrogen bond (IMHB) changes from asymmetric to symmetric, due to much shorter O–O or N–N distances. Symmetrization effects also significantly affect the value of the proton chemical shift. For hydrogen-bonded conformers the trends in chemical shifts and coupling constants are similar. A detailed analysis of the NMR properties of oxygen-containing systems leads to the conclusion that neither the coupling constants nor the proton chemical shifts provide any evidence for the existence of RAHBs. Furthermore, the strength of the O–H···O IMHBs, investigated through the use of appropriate homodesmotic reactions, indicates that the enhanced stability of the IMHB in the unsaturated compound is associated with the σ skeleton of the molecule that allows the oxygen atoms to be in closer proximity than in the saturated analogue.

HIGH-LEVEL AB INITIO VERSUS DFT CALCULATIONS ON (H2O2)2 AND H2O2-H2O COMPLEXES AS PROTOTYPES OF MULTIPLE HYDROGEN BOND SYSTEMS

The performance of B‐LYP, B‐P86, B3‐LYP, B3‐P86, and B3‐PW91 density functionals to describe multiple hydrogen bond systems was studied. For this purpose we have chosen the dimers of hydrogen peroxide and the hydrogen peroxide–water complexes. The geometries and vibrational frequencies obtained with a 6‐311+G(d,p) basis set were compared with those obtained at the MP2 level using the same basis set expansion. The corresponding dimerization energies were obtained using a 6‐311+G(3df,2p) basis set and compared with those obtained using the G2(MP2) theory. Red shiftings of the OH donor stretching frequencies were predicted by all approaches investigated; however, in all cases, the DFT values were sizably larger than the MP2 ones. Similarly, the blue shifting of the torsion of the hydrogen peroxide subunit was larger when evaluated at the DFT level. All functionals reproduced the G2(MP2) relative stabilities of the different local minima quite well. With the exception of the B‐LYP and B3‐PW91 approaches, all functionals yielded binding energies which deviated from the G2(MP2) values by less than 0.5 kcal/mol, provided that G2‐type basis sets were used and that the corresponding BSSE corrections were included.

Density functional theory study on ethanol dimers and cyclic ethanol trimers

The structure and the relative stability of the ethanol dimer and the cyclic ethanol trimer were studied using density functional theory methods. The geometries of the different dimers and trimers were optimized at the B3LYP/6-311+G(d,p) level of theory, while the final energies were obtained at the B3LYP/6-311+G(3df,2p) level. Four different (ethanol)2 complexes were found to be local minima of the potential energy surface, the global minimum being that in which both monomers exhibit a trans conformation. The hydrogen bond (HB) in ethanol dimer is slightly stronger than in methanol dimer, reflecting the enhanced intrinsic basicity of ethanol with regards to methanol. The OH donor stretch appears redshifted by 161 cm−1, while the redshifting undergone by the OH acceptor stretch is negligibly small. The relative stability of the trimers is a function of the number of monomers with a gauche conformation, the global minimum being that in which the three monomers have a trans conformation. As for water and me...

Very strong hydrogen bonds in neutral molecules: The phosphinic acid dimers

Ab initio molecular orbital and density functional theories have been used to study the structures and binding energies of the dimers of phosphinic acid (PA) and its dimethyl derivative (DMPA). For the first compound we have located all possible minima of the potential energy surface, while for the second only the most stable dimer was considered. The geometries were fully optimized at the MP2(full)/6-31+G(d,p) and B3LYP/6-31+G(d,p) levels of theory. The harmonic vibrational frequencies were evaluated at the same levels, while the final energies were obtained using a B3LYP/6-311+G(3df,2p) approach. Both phosphinic acid and its dimethyl derivative form cyclic dimers in the gas phase, where the two monomers are held together by hydrogen bonds (HBs) which are significantly stronger than those found for their carboxylic analogs. The estimated dimerization enthalpies for PA (23.2) and DMPA (23.2 kcal/mol) are the highest reported so far for neutral homodimers in the gas phase and almost twice those measured fo...

The geometry of pyrazole: A test for ab initio calculations

Ab initio calculations on the structure of pyrazole have been carried out at different levels of accuracy. At the Hartree‐Fock (HF) level, the performance of several basis sets, namely 3‐21G, 6‐31G, 6‐31G**, and 6–311G** was investigated. The influence of electron correlation effects also was studied by carrying out geometry optimizations at the MP2, MP4, and QCISD levels. The performance of a density functional method also was evaluated. We have also investigated the possible influence of the frozen core approximation on the final optimized geometry. Three different statistical analyses were considered in determining which geometry is closest to the experimental microwave geometry—namely Paul Curtins diagrams, cluster analysis, and multidimensional scaling. From these analyses, we conclude that there is no asymptotic approach to the experimental geometry by increasing the quality of the theoretical model, although, as expected, the more reliable structures are those obtained at the MP2, MP4, and QCISD levels, as well as those obtained by the B3LYP density functional method. We have also found that the values of the rotational constants are a tight criterion to define the quality of a molecular geometry.

Cu+ binding energies. Dramatic failure of the G2 method vs. good performance of the B3LYP approach

Abstract The Cu + binding energies of a wide set of neutral systems were calculated using G2-based methods and CCSD(T) formalism. The standard G2 formalism is shown to be quite unsuitable to estimate Cu + binding energies, with errors greater than 13 kcal/mol. The relaxing of the inner-valence orbitals or the inclusion of all electrons in the correlation space imply a negligibly small improvement. These failures seem to be related to a poor convergence of the MP series for Cu + . Direct CCSD(T) calculations yield binding energies which are not necessarily closer to the experimental values. Quite importantly, the B3LYP method provides reasonably good Cu + binding energies.