Celeste McMichael Rohlfing

Bonding and stabilities of small silicon clusters: A theoretical study of Si7–Si10

Ab initio calculations have been performed to study the structures and energies of intermediate‐sized silicon clusters (Sin, n=7–10). All geometries have been optimized at the Hartree–Fock (HF) level of theory with the polarized 6‐31G* basis set. The harmonic vibrational frequencies have been evaluated at the HF/6‐31G* level of theory. Electron correlation effects have been included by means of fourth order Mo/ller–Plesset perturbation theory. The most stable structure for Si7 is a pentagonal bipyramid and the lowest energy calculated structures for Si8–Si10 correspond to capped octahedral or prismatic geometrical arrangements. The evolution of the cluster geometries with increasing size is discussed. Clusters containing four, six, seven, and ten atoms have been identified as ‘‘magic numbers’’ for small silicon clusters, both theoretically and experimentally. The hybridization and bonding in small silicon clusters is discussed. Our results are used to interpret the recent photoelectron spectra of negative...

Electronic structures of the negative ions Si−2 –Si−10: Electron affinities of small silicon clusters

Accurate ab initio calculations have been performed to investigate the structures and energies of the negative ions of Si2–Si10. The effects of polarization functions, diffuse functions, and electron correlation have been included in these calculations. In most cases, there is a good correspondence between the ground state structures of the negative ions and those of the corresponding neutral species. Adiabatic electron affinities are computed and compared with recent experimental measurements. Si3, Si5, Si8, and Si9 are found to have electron affinities which are larger than their neighbors. This result is interpreted using our previous calculations on the low‐lying states of the corresponding neutral species.

Electronic structures and photoelectron spectra of Si−3 and Si−4

Vibrationally resolved photoelectron spectra of Si−3 and Si−4, recently reported by Kitsopoulos, Chick, Weaver, and Neumark, are interpreted using ab initio quantum chemical calculations of the ground and excited electronic states of the corresponding neutral clusters. The calculated electron affinities as well as the low‐lying excitation energies agree within 0.1–0.2 eV of the experimental values, thus confirming the theoretically predicted structures of neutral and anionic Si3 and Si4 reported previously.

A study of the structure and bonding of small aluminum oxide clusters by photoelectron spectroscopy: AlxOy−(x=1–2, y=1–5)

The structure and bonding of aluminum oxide clusters, AlxOy (x=1–2, y=1–5), are studied with anion photoelectron spectroscopy (PES) and are compared with preliminary ab initio calculations. The spectra were obtained at four detachment photon energies: 2.33, 3.49, 4.66, and 6.42 eV. The 6.42 eV spectrum for AlO− reveals the X 2Σ+ ground state and two excited states of AlO. The 6.42 eV spectrum for AlO2− also shows three states for AlO2: X 2Πg ground state and the A 2Πu and B 2Σg+ excited states. The spectra for Al2Oy− clusters show vibrationally resolved ground states which come from Al sp-type orbitals and also high binding energy excited states, which are mainly of oxygen 2p character. Al2O2, which has a D2h rhombus structure, has an electron affinity (EA) of 1.88 eV and its singlet–triplet excitation energy is measured to be 0.49 eV. Much higher EAs are measured for the larger Al2Oy clusters. The PES spectra of Al2O3−, Al2O4−, and Al2O5− show very similar electronic and vibrational structure. Furthermor...

Structures and stabilities of sulfur clusters

The geometric structures and relative stabilities of small sulfur clusters (S2–S12) are explored by means of ab initio quantum chemical calculations. The effects of polarization functions and electron correlation are included in these calculations. Open forms are more stable for the small clusters up to S4 whereas ring structures are favored for the larger clusters. The stereochemical aspects of cyclo‐S5 and cycle‐S6 exhibit features very similar to that of the valence‐isoelectronic hydrocarbons cyclopentane and cyclohexane. The calculated geometries of S6–S8 and S10–S12 are all in excellent agreement with the experimentally determined structures. Vibrational frequencies have been computed and are compared with experimental data. The relative stabilities of the different sulfur rings are compared to each other.

Theoretical characterization of the minimum energy path for the reaction H+O2→HO2*→HO+O

The potential energy surface for the reaction H+O2→HO*2 →HO+O has been characterized in the vicinity of the minimum energy path using CASSCF/contracted CI calculations with a basis set which is triple zeta valence quality plus three sets of polarization functions. CASSCF/CI calculations were carried out along the CCI minimum energy path. The latter calculation shows essentially no barrier for addition of an H atom to O2, in agreement with predictions made in earlier studies. The potential surface for recombination of OH and O is complicated by a crossing, at rOO ≈5.5a0, between the surface for electrostatic (OH dipole–O quadrupole) interaction and that for the formation of an O–O chemical bond. This surface crossing results in a small (≈0.5 kcal/mol) barrier.

A theoretical study of small silicon clusters using an effective core potential

Abstract The structures and energies of small silicon clusters are investigated using an effective core potential (ECP), a double-zeta polarized basis set, and fourth-order Moller-Plesset perturbation theory. Excellent agreement is obtained with the results of recent all-electron calculations of Si2-Si10. The ECP is used to examine Si11, which has not been previously characterized. The lowest energy structure of Si11 is found to be a distorted pentacapped trigonal prism, which offers further growth possibilities.

Fast beam photodissociation spectroscopy and dynamics of the vinoxy radical

The spectroscopy and photodissociation dynamics of the vinoxy (CH2CHO) radical B(2A″) ←X(2A″) transition have been investigated by fast beam photofragment translational spectroscopy. We show conclusively that excitation to the B state is followed by predissociation, even for the origin transition. Two photodissociation channels are observed: (1) CH3+CO, and (2) H+CH2CO, with a branching ratio of ≈1:4. The form of the translational energy distributions imply a significant exit barrier to formation of CH3+CO, and a considerably smaller barrier for H+CH2CO formation. Dissociation ultimately proceeds by internal conversion to the ground electronic state; the internal conversion rate appears to be significantly enhanced by a curve crossing with either the A(2A) or C(2A) states. Ab initio calculations of critical points on the global potential energy surfaces aid in determining the dissociation mechanism. We present a simple model for dissociation over a barrier, the statistical adiabatic impulsive model, w...

Fragmentation of small silicon clusters

Abstract Recently we have performed accurate ab initio molecular orbital calculations to study the structures and energies of small silicon clusters (Si n , n =2–10). In this work, the ground state binding energies of the clusters have been used to interpret the results of recent photofragmentation experiments on the ionic silicon clusters up to Si 20 + . Fragmentation predominantly yields daughter ions which are energetically most stable, consistent with a unimolecular dissociation mechanism involving vibrationally excited clusters on the ground electronic potential surface. Clusters containing 6, 7 and 10 atoms have been identified as “magic fragments” for these photodissociation processes, consistent with the experimental results.

An extensive ab initio study of the structures, vibrational spectra, quadratic force fields, and relative energetics of three isomers of Cl2O2

The three lowest‐lying isomers of Cl2O2 have been investigated using state‐of‐the‐art ab initio quantum‐mechanical methods. Electron correlation methods that have been used include second‐order Mo/ller–Plesset perturbation theory, singles and doubles coupled‐cluster (CCSD) theory, and the CCSD(T) method, which incorporates a perturbational estimate of the effects of connected triple excitations. Accurate relative energies have been obtained using the CCSD(T) method in conjunction with large atomic natural orbital basis sets that include up to g‐type functions. Our best estimate is that the ClClO2 and ClOClO isomers lie 0.9±2.0 and 10.1±4.0 kcal/mol higher in energy (0 K), respectively, than the more stable ClOOCl peroxide form. In order to obtain accurate equilibrium geometries it is necessary to include f‐type functions in the one‐particle basis set. The vibrational spectra (including IR intensities) of all three isomers are computed and compared with experimental data for ClOOCl and ClClO2. The theoreti...