Manuel Alcamí

Charge-transfer-induced structural rearrangements at both sides of organic/metal interfaces

Organic/metal interfaces control the performance of many optoelectronic organic devices, including organic light-emitting diodes or field-effect transistors. Using scanning tunnelling microscopy, low-energy electron diffraction, X-ray photoemission spectroscopy, near-edge X-ray absorption fine structure spectroscopy and density functional theory calculations, we show that electron transfer at the interface between a metal surface and the organic electron acceptor tetracyano-p-quinodimethane leads to substantial structural rearrangements on both the organic and metallic sides of the interface. These structural modifications mediate new intermolecular interactions through the creation of stress fields that could not have been predicted on the basis of gas-phase neutral tetracyano-p-quinodimethane conformation.

Mechanical Isolation of Highly Stable Antimonene under Ambient Conditions

Antimonene fabricated by mechanical exfoliation is highly stable under atmospheric conditions over periods of months and even when immersed in water. Density functional theory confirms the experiments and predicts an electronic gap of ≈1 eV. These results highlight the use of antimonene for optoelectronics applications.

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.

Theoretical study of ionization potentials and dissociation energies of Cnq+ fullerenes (n=50–60, q=0, 1 and 2)

We have evaluated electronic energies of neutral, singly charged and doubly charged fullerenes with sizes n=50–60 using density functional (DFT) theory. For each value of the cluster charge, we have considered around 40 possible structures. We have found that, except for C522+, the most stable isomer always has the minimum possible number of C2 units between adjacent pentagons. We have evaluated adiabatic dissociation energies corresponding to the various dissociation channels leading to the emission of carbon dimers with different charges. Our findings for dissociation leading to C2 emission are in reasonable agreement with the latest experimental values. As a byproduct of our calculations, we have also evaluated the first and second adiabatic ionization potentials. Both dissociation energies and ionization potential are useful data to interpret fragmentation of fullerenes by impact of energetic photons, electrons and ions.

Bond activation by protonation in the gas phase

Abstract The structures of different first-row bases and their protonated species are compared using ab initio quantum-mechanical techniques at the 6-31G* and 6-31+G(d, p) levels of accuracy. A topological analysis of the Laplacian of the electronic charge density offers quantitative information on bond activation upon protonation. We have shown that protonation can be accompanied by bond breaking in cases where the basic center is highly electronegative as in alcohols or fluoroalkanes. In bidentate bases, protonation produces opposite effects depending on the center which undergoes protonation.

The performance of density-functional theory in challenging cases: Halogen oxides

Halogen dioxides (FOO, ClOO, BrOO, OClO, OBrO), their cationic and anionic derivatives and two isomers of ClO3 have been studied by means of density-functional theory (DFT) and the results compared with those from high level ab initio molecular orbital calculations. Three different density functionals (SVWN, B3LYP, and G96LYP) combined with a 6-311+G(2df ) basis set were used to obtain geometries and vibrational frequencies, which were then compared with MP2 (second-order Moller–Plesset), QCISD, and CCSD(T) (coupled-cluster single double triple) results. The B3LYP/6-311+G(2df ) calculations generally give geometries and frequencies in excellent agreement with those calculated from high level ab initio calculations such as CCSD(T). Exceptions, such as ClOO and BrOO, arise when high spin contamination at B3LYP level produces spurious results. Atomisation enthalpies evaluated at B3LYP/6-311+G(3df ) level of theory are observed to be in good agreement with the experimental values. In some particular cases thi...

G2 ab initio calculations on three‐membered rings: Role of hydrogen atoms

G2 ab initio calculations on all ABX three‐membered rings (TMRs) that can be derived from cyclopropane by systematic substitution of the (SINGLE BOND)CH2 groups by (SINGLE BOND)NH or (SINGLE BOND)O groups have been performed. Our results show that the decrease in the A(SINGLE BOND)B bond length as the electronegativity of X increases is significantly larger than that found for the corresponding acyclic analogs. In general, a systematic substitution of the (SINGLE BOND)CH2 groups of cyclopropane by (SINGLE BOND)NH or (SINGLE BOND)O groups implies significant geometric changes that are not reflected in a parallel change of the corresponding conventional ring strain energy (CRSE). When the electronegativity of the groups forming the TMR increases the effect on the CRSE of the system is small, although the charge delocalization inside the ring decreases. The near constancy of the CRSE along the series can be explained in terms of the charge redistribution of the system where the (SINGLE BOND)CH2 groups play a crucial role. There are, however, significant changes in the hydrogenation energies of the TMR investigated; our results show that, when in an ABX three‐membered ring, the electronegativity of X increases the hydrogenation energy of A(SINGLE BOND)B bond decreases and vice versa. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1072–1086, 1998

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+ .

Stable Non-IPR C60 and C70 Fullerenes Containing a Uniform Distribution of Pyrenes and Adjacent Pentagons

The most abundant fullerenes, C(60) and C(70), and all the pure carbon fullerenes larger than C(70), follow the isolated-pentagon rule (IPR). Non-IPR fullerenes containing adjacent pentagons (APs) have been stabilized experimentally in cases where, according to Eulers theorem, it is topologically impossible to isolate all the pentagons from each other. Surprisingly, recent experiments have shown that a few endohedral fullerenes, for which IPR structures are possible, are stabilized in non-IPR cages. We show that, apart from strain, the physical property that governs the relative stabilities of fullerenes is the charge distribution in the cage. This charge distribution is controlled by the number and location of APs and pyrene motifs. We show that, when these motifs are uniformly distributed in the cage and well-separated from one other, stabilization of non-IPR endohedral and exohedral derivatives, as well as pure carbon fullerene anions and cations, is the rule, rather than the exception. This suggests that non-IPR derivatives might be even more common than IPR ones.

Probing the site-dependent Kondo response of nanostructured graphene with organic molecules.

TCNQ molecules are used as a sensitive probe for the Kondo response of the electron gas of a nanostructured graphene grown on Ru(0001) presenting a moiré pattern. All adsorbed molecules acquired an extra electron by charge transfer from the substrate, but only those adsorbed in the FCC-Top areas of the moiré show magnetic moment and Kondo resonance in the STS spectra. DFT calculations trace back this behavior to the existence of a surface resonance in the low areas of the graphene moiré, whose density distribution strongly depends on the stacking sequence of the moiré area and effectively quenches the magnetic moment for HCP-Top sites.