Ana I. González

PERFORMANCE OF DENSITY FUNCTIONAL THEORY METHODS FOR THE TREATMENT OF METAL-LIGAND DICATIONS

Abstract The performance of different density functional theory approaches for the treatment of MgX 2+ (X=H 2 O, CH 2 O, CH 3 OH, NH 3 , CH 2 NH, HCN, CH 3 NH 2 , NH 2 OH) and CaX 2+ (X=H 2 O, NH 3 ) metal-ligand dications was investigated. The DFT results were compared with high-level ab initio calculations carried out at the QCISD(T)/6-311+G(3df,2p)//QCISD/6-311G* and CCSD(T)/6-311+G(3df,2p)//QCISD/6-311G* levels of theory. In general, the hybrid DFT methods yield X–Mg 2+ bond distances which are too short compared with the QCISD/6-311G* optimized ones. In contrast, non-hybrid DFT approaches, such as BLYP or G96LYP, yield longer X–Mg 2+ bond distances, which are in better agreement with the QCISD ones. The DFT methods investigated, with the exception of the G96LYP approach, yield Mg 2+ binding energies 2.0 to 6.0 kcal/mol larger than those obtained using high-level ab initio techniques. These differences are smaller when the metal dication is Ca 2+ .

The structure and stability of Sb4H+ clusters: The importance of nonclassical structures

The structure and relative stabilities of the different Sb4H+ clusters were investigated by means of high level ab initio calculations. For this purpose we have developed a split valence and an extended basis set for the treatment of Sb-containing compounds to be used with different effective core potentials available in the literature. The split-valence basis set reported seems to reproduce nicely the geometries and vibrational frequencies of different Sb-containing compounds, provided that electron correlation effects are included at the MP2 level. When the extended basis set is used, within the framework of the G2(ECP) theory, the atomization enthalpies of antimony derivatives are reproduced within ±3 kcal/mol. A systematic study of the Sb4H+ potential energy surface (PES) using these basis sets, showed that the global minimum is the result of the side protonation of the Sb4 tetrahedral molecule. In this species the hydrogen is covalently attached to two Sb atoms through the formation of a three-center...

EXPLORING THE POTENTIAL ENERGY SURFACES OF THE REACTIONS OF O+(4S) AND O+(2D) WITH AMMONIA

Abstract High-level ab initio calculations in the framework of the G2 and CBS-Q theories have been performed for the [H 3 ,N,O] + doublet and quartet state cations. The geometries of the different stationary points of both potential energy surfaces (PESs) were optimized at the QCISD/6-31+G(df,p) level. The bonding characteristics of doublet and quartet state cations are rather different. The latter are weakly bound species involving one-electron linkages, while the former present normal covalent bonds. The global minimum of the doublet PES is the HO-NH 2 + 2D species, which has a N–O linkage with a partial double-bond character. This means that the ionization of hydroxylamine implies an important enhancement of the stability of the N–O bond, which upon ionization becomes much shorter, although its stretching frequency is significantly blue-shifted. For the quartets the global minimum can be viewed roughly as the interaction between O in its 3 P ground state and NH 3 + . From the characteristics of both [H 3 ,N,O] + doublet and quartet state potential energy surfaces we conclude that the products of O + ( r S) and O + ( 2 D) reactions with ammonia are significantly different. For reactions involving O + in its 4 S ground state, only the charge transfer process should be observed, in agreement with the experimental evidence. In reactions involving O + in its 2 D first excited state, the charge transfer process is energetically disfavored with respect to H and H 2 loss. Hence, the major product ions should be H 2 NO + , HNO + , NOH + , HNOH + and in a much smaller proportion NH 3 + , NH 2 + ( 1 A 1 ), and NH + ( 2 Π). The exothermicity of H 3 + reactions with NO to yield NOH + , HNO + , and H 2 NO + (which may be important depletion mechanisms for NO in the interstellar clouds) is also discussed.

Energetics of addition versus insertion mechanisms in the Si+(2P) + HCOOH reaction

Abstract High-level ab initio calculations have been performed to investigate the preference of insertion processes with respect to the formation of adducts in the Si + + formic acid reaction in the gas phase. We have found that the reactivity patterns shown by Si + in reactions with methanol and formaldehyde are significantly different from those exhibited with formic acid, which has both functional groups. The most stable product of the reaction between Si + and HCOOH corresponds to the insertion of the monocation into the COH bond of the neutral. Mostly importantly, the Si + association to the carbonyl oxygen atom is only 4.9 kcal/mol less favourable. All investigated local minima lie below the reactants in energy. In agreement with the experimental evidence, the formation of SiOH + as a possible product of the Si + + HCOOH reaction is predicted to be exothermic by 41.7 kcal/mol. The distonic character of the products is discussed as well as the harmonic vibrational frequencies of the global minimum.

Acetamidine-Mg+(2S) complexes; the performance of different exchange and correlation functionals

Abstract The structures, harmonic vibrational frequencies and bonding characteristics of the different isomers of acetamidine [NH2C(CH3)NH] and of the related complexes formed upon Mg+(2S) association have been investigated using the SVWN, BVWN, BP86, BLYP, B3LYP, B3P86 and B3PW91 density functional approaches at the 6-31G∗ level. The DFT optimized geometries are compared with those obtained at the MP2/6-31G∗ and the QCISD/6-31G∗ levels. All hybrid functionals yield rotational constants which differ by less than 0.3% from the ab initio ones, whereas the deviations for the SVWN, BP86 and BLYP functionals are ten times greater. The corresponding acetamidine-Mg+ binding energies were compared with those obtained at the G2(MP2) level. In general those functionals which include gradient corrections to both the exchange and the correlation parts give binding energies differing from the G2(MP2) values by less than 2.0 kcal/mol, provided that a 6-311 + G(3df, 2p) basis set is used. Our results also show that, as far as binding energies are concerned, all these functionals yield nearly equal results, without privileging the hybrid schemes over the non-hybrid ones. Overall, DFT approaches are a reasonably good alternative to high level ab initio calculations provided a flexible basis set (6-311+G(3df, 2p)) is used for the expansion.