Rare Gas Effects on Hyperfine Coupling Constants of BO, AlO, and GaO

Hyperfine-Coupling Tensors from Projected Hartree-Fock Theory

We assess the calculation of hyperfine coupling (HFC) tensors by different variants of Projected Hartree-Fock (PHF) theory. For a set of small main-group S = 1/2 radicals (BO, CO⁺, CN, AlO, vinyl, methyl, ethynyl), spin-symmetry as well as complex-conjugation and point-group symmetry are first broken in a reference determinant, and then variationally restored, in the frame of the modern formulation of PHF theory. Historically, PHF theory was basically restricted to the restoration of spin symmetry from an unrestricted HF determinant (conserving S_z symmetry). This afforded unsatisfactory HFCs. We obtain far better results for isotropic (and anisotropic) HFCs when the variational energy is further lowered by working with generalized determinants that completely break spin symmetry, and when additional symmetries are used. Specifically, complex-conjugation projection recovers a substantial fraction of the dynamical correlation energy in small molecules, and the detailed equations for combined complex-conjugation, spin- and point-group projection in the density-matrix/diagonalization formulation of PHF theory are here reported for the first time. The compact representation of the PHF wave function allows for a straightforward evaluation of the spin-density matrix and of HFC tensors with little effort. The promising performance of PHF theory may motivate the application of post-PHF methods to the calculation of HFC tensors.

Thermochemical properties and electronic structure of boron oxides BnOm (n = 5-10, m = 1-2) and their anions

The molecular and electronic structures of a series of small boron monoxide and dioxide clusters B(n)O(m) (n = 5-10, m = 1, 2) plus their anions were predicted. The enthalpies of formation (DeltaH(f)'s), electron affinities (EAs), vertical detachment energies, and energies of different fragmentation processes are predicted using the G3B3 method. The G3B3 results were benchmarked with respect to more accurate CCSD(T)/CBS values for n = 1-4 with average deviations of +/-1.5 kcal/mol for DeltaH(f)'s and +/-0.03 eV for EAs. The results extend previous observations on the growth mechanism for boron oxide clusters: (i) The low spin electronic state is consistently favored. (ii) The most stable structure of a neutral boron monoxide B(n)O is obtained either by condensing O on a BB edge of a B(n) cycle, or by binding one BO group to a B(n-1) ring. The balance between both factors is dependent on the inherent stability of the boron cycles. (iii) A boron dioxide is formed by incorporating the second O atom into the corresponding monoxide to form BO bonds. (iv) A B(n)O(m)(-) anion is constructed with BO groups bound to the B(n-1) or B(n-2) rings (yielding the B(n-2)(BO)(2)(-) species). This becomes the preferred geometry for the larger boron dioxides, even in the neutral state. The boronyl group mainly behaves as an electron-withdrawing substituent reducing the binding energy and resonance energy of the oxides. (v) The boron oxides conserve some of the properties of the parent boron clusters such as the planarity and multiple aromaticity.