Koop Lammertsma

Bond properties of Be3-7 clusters

Geometries, binding energies, and bonding properties have been investigated for the Be3–7 clusters with ab initio molecular orbital theory using double (6‐31G*) and triple [6‐311(+)G*] ζ‐basis sets with inclusion of the effects of electron correlation estimated with the Mo/ller–Plesset perturbation (up to full MP4) and coupled cluster (QCI) theories. The geometries and binding energies suggest that the single‐reference approach gives already reasonable results for Be5. For the larger clusters Bader’s electron density analyses reveals small concentrations of charge within groups of three Be atoms.

Protonated (protosolvated) onium ions (onlum dications)

ConclusionsIn this short review we have shown the importance of protosolvation of onium ions (containing non-bonded pairs of electrons) in superacid catalyzed reactions. Such activation can result in unusual reactions such as aromatic alkylation with Meerwein’s salts, aliphatic nitration with nitronium ion, alkylation of saturated hydrocarbons, greatly enhanced activity of acyl cations, etc. Possibly such phenomena may be operative in hydroxylation reactions using protonated hydrogen peroxide in strong acid solutions. Even the reactivity of halonium ions could be enhanced by protosolvation. Consequently, electrophilic protosolvation may play a significant role in strogg acid catalyzed reactions.

Simple mixed hydrides of boron, aluminum, and gallium : AlBH6, AlGaH6, and BGaH6

Equilibrium structures, harmonic vibrational frequencies, and binding energies of the mixed diboranes AlBH6, BGaH6, and AlGaH6 were obtained from ab initio quantum mechanical methods using various basis sets and levels of theory to estimate the effects of electron correlation. Good agreement is found with the experimental IR spectroscopic data on BGaH6. The calculated order of the binding energies with respect to the monomers is AlBH6>B2H6>Al2H6>BGaH6>AlGaH6>Ga2H6. The nature of the bonding in these systems is examined using Bader’s theory of atoms in molecules.

Protonation enthalpies in fluorosulfonic acid using ab initio self-consistent reaction field theory

Electrostatic solvation free energies were computed for several small neutral bases and their conjugate acids using a continuum solvation model called the self‐consistent isodensity polarizable continuum model (SCIPCM). The solvation energies were computed at the restricted Hartree–Fock (RHF) and second‐order Møller–Plesset (MP2) levels of theory, as well as with the Becke3–Lee–Yang–Parr (B3LYP) density functional theory, using the standard 6–31G** Gaussian basis set. The RHF solvation energies are similar to those computed at the correlated MP2 and B3LYP theoretical levels. A model for computing protonation enthalpies for neutral bases in fluorosulfonic acid solvent leads to the equation ΔH  prot, HSO 3F (B)=−PA(B)+ΔEt(BH+)−ΔEt(B)+β, where PA(B) is the gas phase proton affinity for base B, ΔEt(BH+) is the SCIPCM solvation energy for the conjugate acid, and ΔEt(B) is the solvation energy for the base. A fit to experimental values of ΔH  prot, HSO 3F (B) for 10 neutral bases (H2O, MeOH, Me2O, H2S, MeSH, Me2S, NH3, MeNH2, Me2NH, and PH3) gives β=238.4±2.9 kcal/mol when ΔΔEt is computed using the 0.0004 e⋅bohr−3 isodensity surface for defining the solute cavity at the RHF/6–31G** level. The model predicts that for carbon monoxide ΔH  prot, HSO 3F (CO)=10 kcal/mol. Thus, protonation of CO is endothermic, and the conjugate acid HCO+ (formyl cation) behaves as a strong acid in fluorosulfonic acid. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 250–257, 1998

Conjugative effects in W(CO)5 complexed phosphirane rings illustrated by substituent effects on 31P NMR chemical shifts

Abstract Correlations between 31 P NMR chemical shifts and Hammett σ substituent constants are reported for a total of 22 anti - and syn -phosphiranes, obtained from the reactions of the carbene-like phosphinidenes PhPW(CO) 5 and H 3 CPW(CO) 5 with various para -substituted styrenes. The origins of these four sets of correlations with respect to electronic and electrostatic interactions are discussed. Arguments are presented that strongly suggest significant conjugation between the remote p -X-phenyl and trans -P-phenyl groups in the syn - 1 substituted phosphiranes. Electrostatic forces do not play an important role in these PW(CO) 5 complexed phosphiranes and neither do steric interactions.

The structure and bonding of Li3H ion-pair states

Abstract Ab initio calculations on Li 3 H ion-pair states and a topological analysis of the charge density of the resulting optimized structures are reported. The global Li 3 H minimum is a planar C 2v structure with H − −Li + 3 ion-pair character. The pyramidal C 3v structure, which is 21.5 kcal/mol higher in energy (MP2/6-311 + + G ** ), Although vibrational analysis indicates this structure to be a true minimum on the potential energy surface, inclusion of the vibrational zero-point energy makes the thermodynamic stability of this structure questionable.

SYNTHESIS OF 1-PHOSPHINOPHOSPHOLES AND THEIR TUNGSTENPENTACARBONYL COMPLEXES

Abstract A series of 1-phosphinophospholes 4a–d (R = tert -butyl ( a ), iso -propyl ( b ), ethyl ( c ), phenyl ( d )) and their pentacarbonyltungsten complexes 5a–d are synthesized and characterized. The 31 P NMR patterns of both the complexed and free 1-phosphinophospholes follow similar trends. The X-ray structure of 5a reveals an elongated P P bond due to steric encumbrance, while the geometry of the phosphole moiety is not affected.

Rhombic C4. Does it contain the shortest nonbonding C-C distance ?

In this ab initio molecular orbital study the bonding properties between the inverted carbons in rhombic C4 and [1.1.1]propellane are discussed. Rhombic C4, its protonated forms C4H+ and C4H2+2, and its linear isomer have been studied with the 6‐311G(2d,p) basis set and all electron correlation using Mo/ller–Plesset perturbation theory at second order. The rhombic form of C4 is estimated to be essentially isoenergetic with the linear form; the estimated harmonic vibrational frequencies for both are compared with experimental data. The molecular orbital and topological one‐electron density analyses show that the cross‐ring bond of rhombic C4 is topologically unstable (at various levels of theory) which suggests a frozen transition for C–C bond making/breaking. At MP4(SDTQ) this cross‐ring distance is only 1.546 A. The stabilizing properties of rhombic C4 (and C4H+, and C4H2+ ) are compared with those of the well known [1.1.1]propellane, which at MP2/6‐31G* has a longer cross‐ring bond.

Can the H2+4 dication exist?

Abstract No H2+4 structures have been found by ab initio calculations (at many levels through RCISD/6-311 + + G(, 2pd)) to correspond to minima on the singlet potential energy hypersurface.

Degenerate Li-H exchange in first-row hydrides: a charge density analysis

Abstract The degenerate Li-H exchange reaction in first-row hydrides was examined using ab initio molecular orbital theory including optimization of geometries at correlated second-order Moller-Plesset (MP2) perturbation theory and characterization of stationary points. Baders electron density analyses on MP2 wavefunctions was used to delineate the nature of bonding in the lithium complexes. New bonding patterns emerge from this analysis. Ionic bonding dominates the lithium complexation except in the electron-deficient complexes of BeH 2 and BH 3 . Subtle differences exist between the Hartree-Fock and MP2 structures.