The two smallest boron clusters (B3 and B4) in their neutral and anionic forms were studied by photoelectron spectroscopy and ab initio calculations. Vibrationally resolved photoelectron spectra were observed for B3 - at three photon energies (355, 266, and 193 nm), and the electron affinity of B3 was measured to be 2.82 ± 0.02 eV. An unusually intense peak due to two-electron transitions was observed in the 193-nm spectrum of B3 - at 4.55 eV and its origin was theoretically characterized. We confirmed that both B3 - and B3 are π and σ aromatic systems with D 3 h symmetry. The photoelectron spectra of B4 - were also obtained at the three photon energies, but much broader spectra were observed. The B4 - anion was found to have the lowest electron detachment energy (∼1.6 eV) among all boron clusters with three or more atoms, consistent with its extremely weak mass signals. The neutral B4 cluster was found to have a D 2 h rhombus structure, which is only slightly distorted from a perfect square. For B4 -, we identified computationally two low-lying isomers (2B1u and 2Ag) both with D 2 h symmetry, with the 2B1u state slightly more stable, which is confirmed through comparison of the calculated spectra with the experimental spectra. The chemical bonding of the two small boron clusters is discussed in terms of aromaticity and antiaromaticity both in the π and σ frameworks. We demonstrated that the aromaticity and antiaromaticity concepts provide us a clear explanation of the chemical structure and bonding in these two boron clusters.

Ionization energies below 20 eV of 10 molecules calculated with electron propagator techniques employing Hartree-Fock orbitals and multiconfigurational self-consistent field orbitals are compared. Diagonal and nondiagonal self-energy approximations are used in the perturbative formalism. Three diagonal methods based on second-and third-order self-energy terms, all known as the outer valence Green's function, are discussed. A procedure for selecting the most reliable of these three versions for a given calculation is tested. Results with a polarized, triple < basis produce root mean square errors with respect to experiment of approximately 0.3 eV. Use of the selection procedure has a slight influence on the quality of the results. A related, nondiagonal method, known as ADC(3), performs infinite-order summations on several types of self-energy contributions, is complete through third-order, and produces similar accuracy. These results are compared to ionization energies calculated with the multiconfigurational spin-tensor electron propagator method. Complete active space wave functions or close approximations constitute the reference states. Simple field operators and transfer operators pertaining to the active space define the operator manifold. With the same basis sets, these methods produce ionization energies with accuracy that is comparable to that of the perturbative techniques.

=An efficient procedure for third-order electron propagator calculations of ionization energies and electron affinities is reported. Diagonal self-energy expressions that are suitable for large molecules are employed. The outer-valence Green's function method also is implemented. An integral transformation program for direct and semidirect algorithms is modified to store only nonzero integrals according to Abelian point group symmetry. Contributions to self-energy matrix elements that depend on electron repulsion integrals with four virtual orbital indices are computed in a direct way. Intermediate batches of integrals are created by sort procedures while avoiding storage of transformed integrals in the main memory. This method permits calculation of electron binding energies for C:-with a 231 atomic orbital basis and for Zn(C5H5)2 with a 220 atomic orbital basis on an IBM RISC/6000 Model 550. During these calculations, the CPU is engaged approximately 90% of the time.

Photoelectron spectra of deoxyribonucleotide anions are interpreted with ab initio, electron propagator calculations. Ground-state structures display hydrogen bonds which are not present in less stable minima that resemble Watson-Crick fragment geometries. For the adenosine and thymidine anions, there are two vertical electron detachment energies (VEDEs) within 0.1 eV of each other that correspond to phosphate- and base-centered Dyson orbitals (DOs). The first VEDE of the cytidine anion belongs to a phosphate-centered DO. The anomalously low VEDE of the guanosine anion is assigned to a base-centered, pi DO. Higher VEDEs of all four anions also are assigned.

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