Photoelectron spectroscopy of BH−3

Unusual and Conventional Dative Bond Formation by s2 Lone Pair Donation from Alkaline Earth Metal Atoms to BH3, AlH3, and GaH3

Using ab initio electronic structure methods with flexible atomic orbital basis sets, we examined the nature of the bonding arising from donation of an ns² electron pair on an alkaline earth atom (Mg or Ca) into a vacant n'p orbital on the group 13 atom of BH₃, AlH₃, or GaH₃. We also examined what happens when an excess electron is attached to form corresponding molecular anions. Although the geometries of MgBH₃, MgAlH₃, MgGaH₃, and CaBH₃ are found to be much as one would expect for datively bound molecules, CaAlH₃ and CaGaH₃ were found to have very unusual geometries in that their Al-H or Ga-H bonds are directed toward the Ca atom rather than away, as in the other compounds. Internal electrostatic Coulomb attractions between the partially positively charged Ca center and the partially negatively charged H centers were suggested as a source of these unusual geometries. The other novel finding is that the electron affinities (EAs) of all six M'-MH₃ species lie in the 0.7-1.0 eV range, which is suggestive of ionic electronic structures for the neutrals even though the partial charges on the alkaline earth centers are as low as 0.3 atomic units. Partial positive charge on the alkaline earth atoms combined with substantial electron affinities of the BH₃, AlH₃, and GaH₃ groups, but only when distorted from planar geometries, were suggested to be the primary contributors to the large EAs.

Realization of Lewis Basic Sodium Anion in the NaBH3 - Cluster

We report a Na:^- →B dative bond in the NaBH₃ ^- cluster, which was designed on the principle of minimum-energy rupture, prepared by laser vaporization, and characterized by a synergy of anion photoelectron spectroscopy and electronic structure calculations. The global minimum of NaBH₃ ^- features a Na-B bond. Its preferred heterolytic dissociation conforms with the IUPAC definition of dative bond. The lone electron pair revealed on Na and the negative Laplacian of electron density at the bond critical point further confirm the dative nature of the Na-B bond. This study represents the first example of a Lewis adduct with an alkalide as the Lewis base.

One-electron bonds are not "half-bonds"

Despite the success of the molecular orbital (MO) and valence-bond (VB) models to describe the electronic structure and properties of molecules, neither MO nor VB provides an explanation for the nature of the chemical bond. The first to address this problem was Ruedenberg, who showed that chemical bonds result from quantum interference. He developed a method to calculate the interference contribution to the total electronic energy and density and applied it to molecules containing typical two-centre two-electron (2c-2e) covalent bonds. To test the generality of Ruedenberg's hypothesis, we developed a powerful Interference Energy Analysis (IEA) method to calculate the interference contributions of individual chemical bonds to the total energy of diatomic and polyatomic molecules, and showed that any two-electron bond, despite its polarity, results from quantum interference. Nevertheless, many stable molecules are experimentally known whose chemical structures clearly indicate the existence of two-centre one-electron bonds (2c-1e). Therefore, the question remains if quantum interference will be the dominant effect for these systems. This work describes the extension of the IEA for treating two-centre one-electron bonds, making use of a Generalised Product Function (GPF) built from spin coupled wave functions of N electrons in M orbitals, SC(N,M). Several diatomic and polyatomic molecules were analysed and whenever possible the results were compared with the analogous case of a two-electron bond. The results indicate that interference is the dominant effect for the one-electron bonds, which reinforces the role of quantum interference as the central element in chemical bonding theory.

GW100: Comparison of Methods and Accuracy of Results Obtained with the WEST Code

The reproducibility of calculations carried out within many-body perturbation theory at the G₀ W₀ level is assessed for 100 closed shell molecules and compared to that of density functional theory. We consider vertical ionization potentials (VIP) and electron affinities (VEA) obtained with five different codes: BerkeleyGW, FHI-aims, TURBOMOLE, VASP, and WEST. We review the approximations and parameters that control the accuracy of G₀ W₀ results in each code, and we discuss in detail the effect of extrapolation techniques for the parameters entering the WEST code. Differences between the VIP and VEA computed with the various codes are within ∼60 and ∼120 meV, respectively, which is up to four times larger than in the case of the best results obtained with DFT codes. Vertical ionization potentials are validated against experiment and CCSD(T) quantum chemistry results showing a mean absolute relative error of ∼4% for data obtained with WEST. Our analysis of the differences between localized orbitals and plane-wave implementations points out molecules containing Cu, I, Ga, and Xe as major sources of discrepancies, which call for a re-evaluation of the pseudopotentials used for these systems in G₀ W₀ calculations.

Bond fission in monocationic frameworks: diverse fragmentation pathways for phosphinophosphonium cations

A series of phosphinophosphonium cations ([R2PPMe3]+; R = Me, Et, i Pr, t Bu, Cy, Ph and N i Pr2) have been prepared and examined by collision-induced dissociation (CID) to determine the fragmentation pathways accessible to these prototypical catena-phosphorus cations in the gas-phase. Experimental evidence for fission of P-P and P-E (E = P, C) bonds, and β-hydride elimination has been obtained. Comparison of appearance potentials for the P-P bond dissociation fragments [R2P]+ (P-P heterolysis) and [PMe3]+˙ (P-P homolysis) shows that heterolytic P-P cleavage is more sensitive than P-P homolysis towards changes in substitution at the trivalent phosphorus center. The facility of β-hydride elimination increases with the steric bulk of R in [R2PPMe3]+. A density functional theory (DFT) study modelling these observed processes in gas-phase, counterion- and solvent-free conditions, to mimic the mass spectrometric environment, was performed for derivatives of [R2PPMe3]+ (R = Me, Et, i Pr, t Bu, Ph and N i Pr2), showing good agreement with experimental trends. The unusual observation of both homolytic and heterolytic cleavage pathways for the P-P and P-C bonds reveals new insight into the fundamental aspects of bonding in monocations and undermines the use of simplistic bonding models.

Lewis acidities and hydride, fluoride, and X- affinities of the BH(3-n)Xn compounds for (X = F, Cl, Br, I, NH2, OH, and SH) from coupled cluster theory

Atomization energies at 0 K and enthalpies of formation at 0 and 298 K are predicted for the BH(4-n)X(n)(-) and the BH(3-n)X(n)F(-) compounds for (X = F, Cl, Br, I, NH(2), OH, and SH) from coupled cluster theory (CCSD(T)) calculations with correlation-consistent basis sets and with an effective core potential on I. To achieve near chemical accuracy (+/-1.0 kcal/mol), additional corrections were added to the complete basis set binding energies. The hydride, fluoride, and X(-) affinities of the BH(3-n)X(n) compounds were predicted. Although the hydride and fluoride affinities differ somewhat in their magnitudes, they show very similar trends and are both suitable for judging the Lewis acidities of compounds. The only significant differences in their acidity strength orders are found for the boranes substituted with the strongly electron withdrawing and back-donating fluorine and hydroxyl ligands. The highest H(-) and F(-) affinities are found for BI(3) and the lowest ones for B(NH(2))(3). Within the boron trihalide series, the Lewis acidity increases monotonically with increasing atomic weight of the halogen, that is, BI(3) is a considerably stronger Lewis acid than BF(3). For the X(-) affinities in the BX(3), HBX(2), and H(2)BX series, the fluorides show the highest values, whereas the amino and mercapto compounds show the lowest ones. Hydride and fluoride affinities of the BH(3-n)X(n) compounds exhibit linear correlations with the proton affinity of X(-) for most X ligands. Reasons for the correlation are discussed. A detailed analysis of the individual contributions to the Lewis acidities of these substituted boranes shows that the dominant effect in the magnitude of the acidity is the strength of the BX(3)(-)-F bond. The main contributor to the relative differences in the Lewis acidities of BX(3) for X, a halogen, is the electron affinity of BX(3) with a secondary contribution from the distortion energy from planar to pyramidal BX(3). The B-F bond dissociation energy of X(3)B-F(-) and the distortion energy from pyramidal to tetrahedral BX(3)(-) are of less importance in determining the relative acidities. Because the electron affinity of BX(3) is strongly influenced by the charge density in the empty p(z) lowest unoccupied molecular orbital of boron, the amount of pi-back-donation from the halogen to boron is crucial and explains why the Lewis acidity of BF(3) is significantly lower than those of BX(3) with X = Cl, Br, and I.

Thermodynamic properties of molecular borane amines and the [BH4-][NH4+] salt for chemical hydrogen storage systems from ab initio electronic structure theory

The heats of formation for the borane amines BH3NH3, BH2NH2, and HBNH, tetrahedral BH4-, and the BN molecule have been calculated by using ab initio molecular orbital theory. Coupled cluster calculations with single and double excitations and perturbative triples (CCSD(T)) were employed for the total valence electronic energies. Correlation consistent basis sets were used, up through the augmented quadruple-zeta, to extrapolate to the complete basis set limit. Core/valence, scalar relativistic, and spin-orbit corrections were included in an additive fashion to predict the atomization energies. Geometries were calculated at the CCSD(T) level up through at least aug-cc-pVTZ and frequencies were calculated at the CCSD(T)/aug-cc-pVDZ level. The heats of formation (in kcal/mol) at 0 K in the gas phase are Delta Hf(BH3NH3) = -9.1, Delta Hf(BH2NH2) = -15.9, Delta Hf(BHNH) = 13.6, Delta Hf(BN) = 146.4, and Delta Hf(BH4-) = -11.6. The reported experimental value for Delta Hf(BN) is clearly in error. The heat of formation of the salt [BH4-][NH4+](s) has been estimated by using an empirical expression for the lattice energy and the calculated heats of formation of the two component ions. The calculations show that both BH3NH3(g) and [BH4-][NH4+](s) can serve as good hydrogen storage systems which release H2 in a slightly exothermic process. The hydride affinity of BH3 is calculated to be 72.2 kcal/mol, in excellent agreement with the experimental value at 298 K of 74.2 +/- 2.8 kcal/mol.

Thermodynamic Properties of Molecular Borane Phosphines, Alane Amines, and Phosphine Alanes and the [BH4-][PH4+], [AlH4-][NH4+], and [AlH4-][PH4+] Salts for Chemical Hydrogen Storage Systems from ab Initio Electronic Structure Theory

The heats of formation for the molecules BH(3)PH(3), BH(2)PH(2), HBPH, AlH(3)NH(3), AlH(2)NH(2), HAlNH, AlH(3)PH(3), AlH(2)PH(2), HAlPH, AlH(4)(-), PH(3), PH(4), and PH(4)(+), as well as the diatomics BP, AlN, and AlP, have been calculated by using ab initio molecular orbital theory. The coupled cluster with single and double excitations and perturbative triples method (CCSD(T)) was employed for the total valence electronic energies. Correlation consistent basis sets were used, up through the augmented quadruple-zeta, to extrapolate to the complete basis set limit. Additional d core functions were used for Al and P. Core/valence, scalar relativistic, and spin-orbit corrections were included in an additive fashion to predict the atomization energies. Geometries were calculated at the CCSD(T) level up through at least aug-cc-pVTZ and frequencies were calculated at the CCSD(T)/aug-cc-pVDZ level. The heats of formation of the salts [BH(4)(-)][PH(4)(+)](s), [AlH(4)(-)][NH(4)(+)](s), and [AlH(4)(-)][PH(4)(+)](s) have been estimated by using an empirical expression for the lattice energy and the calculated heats of formation of the two component ions. The calculations show that both AlH(3)NH(3)(g) and [AlH(4)(-)][NH(4)(+)](s) can serve as good hydrogen storage systems that release H(2) in a slightly exothermic process. In addition, AlH(3)PH(3) and the salts [AlH(4)(-)][PH(4)(+)] and [BH(4)(-)][PH(4)(+)] have the potential to serve as H(2) storage systems. The hydride affinity of AlH(3) is calculated to be -70.4 kcal/mol at 298 K. The proton affinity of PH(3) is calculated to be 187.8 kcal/mol at 298 K in excellent agreement with the experimental value of 188 kcal/mol. PH(4) is calculated to be barely stable with respect to loss of a hydrogen to form PH(3).