Vicenç Branchadell

Atomic reference energies for density functional calculations

Abstract Atomic ground states are usually degenerate. It is demonstrated that the density functionals for the exchange-correlation energy that are commonly used are not invariant over the set of ground state densities. This leads to uncertainties of the order of 3 to 5 kcal/mol in the atomic ground state energy of second and third period main group elements and the first transition series. A much larger spread in energies is obtained for transition elements if symmetry and equivalence restrictions for the Kohn-Sham orbitals are abandoned. It is recommended that atomic ground states that are actually used to calculate heats of atomization are made explicit, and tables with one choice of atomic ground state energies for the first rows of the periodic system are provided for the local density approximation and for a few generalized approximations.

Density functional study of the Fe–CO bond dissociation energies of Fe(CO)5

Fe(CO)n (n=1–5) complexes have been studied using density functional theory (DFT) methods. Several functionals have been used in the geometry optimizations, harmonic frequencies computation and calculation of the iron–carbonyl bond dissociation energies. Coupled-cluster single double (triple) bond dissociation energies have also been computed for the smaller systems. The obtained results show that DFT methods yield reasonable geometries and vibrational frequencies. Regarding the bond dissociation energies, it is shown that the validity of the results depends on whether there is a change in the atomic state of the metal during the dissociation. When the atomic state is the same for both complexes, the bond dissociation energy computed using gradient corrected functionals is within the range of the experimental values, while when the atomic state changes, DFT methods overestimate the bond dissociation energy due to a poor description of the atomic multiplets.

Self‐Assembly of Chiral trans‐Cyclobutane‐Containing β‐Dipeptides into Ordered Aggregates

Two chiral synthetic β-dipeptides have been constructed, one with two trans-cyclobutane residues and the other with one trans and one cis fragment, 1 and 2, respectively, and investigated to get insight into the non-covalent interactions responsible for their self-assembly to form ordered aggregates, as well into parameters such as their morphology and size. Experimental evidence of the formation of these assemblies was provided by spectroscopy, microscopy and X-ray diffraction experiments that suggest the formation of nanoscale helical aggregates. This process involves a conformational change in the molecules of each dipeptide with respect to the preferred conformation of the isolated molecules in solution. A high-resolution NMR spectroscopy study allowed the determination of the dynamics of the gelation process in [D(8)]toluene and the sol-gel transition temperature, which was around 270 K in this solvent at a concentration of 15 mM. NMR spectroscopy experiments also provided some information about conformational changes involved in the sol-gel transition and also suggested a different gel packing for each dipeptide. These observations have been nicely explained by computational studies. The self-assembly of the molecules has been modelled and suggested a head-to-head molecular arrangement for 1 and a head-to-tail arrangement for 2 to give helical structures corresponding to hydrogen-bonded single chains. These chains interact with one another in an antiparallel way to afford bundles, the significant geometry parameters of which fit well to the main peaks observed in wide-angle X-ray diffraction spectra of the aggregates in the solid state.

Enantioselective synthetic approaches to cyclopropane and cyclobutane β-amino acids: synthesis and structural study of a conformationally constrained β-dipeptide

Abstract Synthetic approaches to carbocyclic compounds, namely cyclopropane and cyclobutane β-amino acids, are presented. One of them is based on enzymatic desymmetrization of meso diesters, leading to the enantioselective production of cis-hemiesters, which afforded β-amino acids through Curtius rearrangements. The enantiomeric excess for the cyclobutane derivatives was 91% whereas the cyclopropanes were obtained in 63% ee. According to another strategy, an enantiomerically pure cyclopropane trans-β-amino acid, bearing a quaternary center, has been synthesized from a homochiral precursor easily available from d -glyceraldehyde. The preparation and structural investigation of the first synthesized cyclobutane containing dipeptide is also described. A hairpin-like conformation of this molecule in the solid state has been demonstrated by X-ray structural analysis, showing crystal packing induced by the presence of the rigid cyclobutane moiety and the formation of intermolecular hydrogen bonds. NMR experiments confirmed that these molecules also tend to produce aggregates in solution. On the contrary, theoretical calculations suggest that intramolecular interactions are important in the gas phase, as expected.

Electron hole formation in acidic zeolite catalysts

The formation of an electron hole on an AlO(4)H center of the H-ZSM-5 zeolite has been studied by a hybrid quantum mechanics/shell-model ion-pair potential approach. The Becke-3-Lee-Yang-Parr (B3LYP) and Becke-Half&Half-Lee-Yang-Parr (BHLYP) hybrid density functionals yield electron holes of different nature, a delocalized hole for B3LYP and a hole localized on one oxygen atom for BHLYP. Comparison with coupled cluster calculations including single and double substitutions and with perturbative treatment of triple substitutions CCSD(T) and with experimental data for similar systems indicate that the localized description obtained with BHLYP is more accurate. Generation of the electron hole produces a substantial geometry relaxation, in particular an elongation of the Al-O distance to the oxygen atom with the unpaired electron. The zeolite framework stabilizes the positive charge by long-range effects. Our best estimates for the vertical and adiabatic ionization energies are 9.6-10.1 and 8.4-8.9 eV, respectively. Calculations for silicalite, the all-silica form of ZSM-5, also yield a localized electron hole, but the energy cost of the process is larger by 0.6-0.7 eV. The deprotonation energy of H-ZSM-5 is found to decrease from 12.86 to 11.40 eV upon electron hole formation.

Prevalence of Eight-Membered Hydrogen-Bonded Rings in Some Bis(cyclobutane) β-Dipeptides Including Residues with Trans Stereochemistry†

Three new bis(cyclobutane) beta-dipeptides have been synthesized from appropriate derivatives of cis- and trans-2-aminocyclobutane-1-carboxylic acid, respectively. The predominance of eight-membered hydrogen-bonded rings has been manifested for (trans,trans)- and (trans,cis)-beta-dipeptides while the formation of six-membered rings is preferred for the (cis,trans)- beta-dipeptide similarly to the previously described (cis,cis)-diastereomer.

Diastereofacial selectivity in uncatalyzed Diels-Alder cycloadditions involving α,β-unsaturated esters and lactones with stereogenic centers containing oxygen functiionalities

Abstract Both experimental and theoretical studies on the Diels-Alder cycloadditions of dienophiles 1 , 2 and 3 to different dienes show that the observed diastereofacial selectivity results from a combination of electronic and steric interactions. The traditional Felkin-Anh mode is not appropriate for these cases. Several new chiral synthetic building blocks have been prepared.

Spin-forbidden N2O dissociation in Cu–ZSM-5

Abstract Spin-forbidden N–O dissociation has been studied for N 2 O, [Cu–ON 2 ] + , and ZCu–ON 2 . For N 2 O and [Cu–ON 2 ] + the minimum energy points of singlet–triplet surface crossing have been located. Moreover, the energy necessary to reach the crossing points has been estimated from the energy profiles of singlet and triplet states at different values of the N–O distance. The comparison of the results obtained for N 2 O, [Cu–ON 2 ] + , and ZCu–ON 2 shows that the catalytic effect of the zeolite can be directly related to the strength of the Cu–O bond in ZCuO.

Comparison of density functional and coupled cluster methods in the study of metal–ligand systems: Sc–CO2 and Cu–NO2

The structure, binding energies, and vibrational frequencies have been determined for the 1A1 state of the η2‐O,O coordination mode of Cu–NO2 and the 2A1 state of the η2‐O,O coordination mode of Sc–CO2. Calculations have been done using coupled cluster methods and methods based on the density functional theory. The results obtained show that all the levels of calculation lead to very similar equilibrium geometries and vibrational frequencies, while different results are obtained for the binding energy. For Sc–CO2 density functional methods overestimate the binding energy with respect to coupled cluster, while for Cu–NO2 the density functional binding energies are lower than the coupled cluster value. In both cases the inclusion of the exact Hartree–Fock exchange into the functional leads to an improvement of the density functional result. Our best estimates for the binding energies of Sc–CO2 and Cu–NO2 are 25 and 50 kcal mol−1, respectively.

Activation of CO2 and SO2 by Boryl(phosphino)carbenes

The stable boryl(phosphino)carbene 1 can cleave small organic dioxide molecules. With CO2 and SO2, 1 gives, respectively, the phosphacumulene ylide [Mes(i Pr2N)B‐O‐P(CCO)(Ni Pr2)Mes] (see scheme and structure) and boryl(phosphoryl)sulfine [Mes(i Pr2N)B‐C(SO)‐P(O)(Ni Pr2)Mes] which have been structurally and spectroscopically characterized.