Roger L. DeKock

The lithium superoxide radical: symmetry breaking phenomena and potential energy surfaces

Abstract The two lowest electronic states of the lithium superoxide radical, LiO2, have been investigated using ab initio theoretical techniques, including RHF SCF, CISD, Davidson-corrected CISD [CISD + (Q)], UHF SCF, UMP2, UMP3, UMP4(SDTQ), spin-projected UHF and UMP, valence and extravalence CASSCF (CASSCF-v and CASSCF-π), and CISD based on CASSCF natural orbitals (CISD-π). Four basis sets ranging in quality from Li (9s4p/5s2p), O (9s5pld/5s3pld) to Li (10s5pld/6s4pld), O(11s7p2dlf/6s5p2dlf) were employed, these being designated TZP, QZ2P, QZ2P+R, and QZ2P+R+f. The investigation encompassed dissociation energies, relative energies of various conformations, geometrical structures, vibrational frequencies, infrared and Raman intensities, dipole moments, cubic force fields, vibration-rotation interaction constants, and symmetry breaking phenomena. The onset of spatial symmetry breaking in the electronic orbitals of the TZP RHF reference wavefunction for X 2A2 LiO2 leads to an irremovable singularity in the quadratic force constant for antisymmetric LiO stretching at the isosceles-triangle (C2v) geometry d(OO) = 1.3266 A and r(LiO) = 1.7737 A. This anomalous lowering of spatial symmetry from C2v to Cs, makes the two oxygen atoms inequivalent, and thus it becomes necessary to avert the symmetry dilemma in the reference wavefunction to provide unequivocal evidence for a C2v geometrical structure of X 2A2 LiO2 which is consistent with the ionic model. This task is achieved with the CASSCF-π and CISD-π wavefunctions, the latter yielding d(OO) = 1.3405 A, r(LiO) = 1.7937 A, ω1(a1) = 1263 cm−1, ω2(a1) = 740 cm−1, and ω3(b2) = 519 cm−1 at the C2v equilibrium geometry. Final proposals of d(OO) = 1.335 A, r(LiO) = 1.76 A, and D0(LiO2) = 62 kcal/mol are made for X 2A2 LiO2, as indicated by appurtenant studies of X 2ΠgO2−1 and X 2Π LiO. Improved predictions are thereby provided for gas-phase LiO, viz., re = 1.694 A ωe = 814 cm−1, ν0 = 798 cm−1, and D0(LiO) = 86 kcal/mol. The A 2B2 state of LiO2 is also predicted to have a C2v geometrical structure, d(OO) = 1.3497 A and r(LiO) = 1.8612 A being obtained at the TZP CISD level, and a final adiabatic excitation energy of Te = 16.7 kcal/mol is determined. A large differential correlation energy effect is found to be important in obtaining an accurate relative energy for the 2Π, C2∞v, LiOO linear structure, which is found to be a shallow minimum 22.2 kcal/mol above the X 2A2, C2v state with TZP CISD bond lengths of d(OO) = 1.3149 A and r(LiO) = 1.6341 A. Finally, 2Π LiO2 is connected to the X 2A2 and A 2B2 minima via two transition states of 2A″ and 2A′ symmetry, respectively, with interconversion barriers near 1.4 and 1.1 kcal/mol.

On the nature of the first excited states of the uranyl ion

Abstract Results of calculations on the uranylion using the LCAO MO Hartree—Fock—Slater method including relativistic effects are reported. The highest occupied molecular orbital is calculated to be σ u , consisting predominantly of U 5f character. The σ u orbital is the HOMO partly because of “pushing-from-below” by the U 6p orbital, but also as a result of the change in potential of the U 5f electrons with the uranium core elections brought about by relativistic contraction of the core electrons. This effect also determines the character of the first virtual levels (δ u and Φ u , respectively) in equatorial ligand fields.

The electronic structure and vibrational frequencies of CNN and SiNN from local density functional methods

The geometry and vibrational frequencies of CNN and SiNN in the 3Σ− state have been calculated by local density functional theory. The geometry of CNN has an NN bond (1.202 A) that is shorter than the CN bond (1.252 A). For SiNN, the NN bond is shorter (1.164 A) than that for CNN and the SiN bond (1.749 A) is like an Si–N single bond. The calculated frequencies for CNN (1597, 1273, and 377 cm−1) can account for the lower two observed transitions but can not account for the observed transition at 2847 cm−1 as a fundamental. The infrared intensities are all predicted to be small for CNN. The calculated frequencies for SiNN (1821, 569, and 321 cm−1) can account for the observed spectrum and we note that ω1 is predicted to have a large infrared intensity. The local density functional results for SiNN and CNN are quite different from previously reported molecular orbital predictions which could not satisfactorily account for the experimental results.

Electronic structure and molecular topology of boron and aluminum suboxides

Abstract Semiempirical and ab initio calculations on the suboxides B 2 O, B 2 O 2 , Al 2 O, and Al 2 O 2 predict linear BOB, linear OBBO, linear AlOAl, and cyclic OAlOAl molecular structures. These structures could not have been predicted on the basis of a set of simple topological rules. The calculations provide additional evidence that the tentative structures adopted on the basis of the available thermochemical and spectroscopic data are correct.

Quantum chemical calculations on the structure of (Cl2F)+ and related molecules

Abstract Molecular orbital calculations using the MNDO method have been completed on several diatomic and triatomic molecules and cations. The predicted most stable triatomic isomers are: (ClClF) + , (FClF) + , HOCl, HOF, ClOCl, FOF, ClOF, and (HFCl) + . For the first six, these calculated isomers are in agreement with the experimentally observed most stable structures. The most stable isomers of the last two are not yet known experimentally. Calculated heats of formation, structures, charge distributions, and Wiberg bond indices are reported. Comparison with ab initio calculations at the 4-31G level is also made.

Synthesis of a new class of β-iodo N-alkenyl 2-pyridones.

A new method for the synthesis of β-iodo N-alkenyl 2-pyridones from substituted 2-propargyloxypyridines has been discovered . These compounds present a unique complement of orthogonal functionality and structural characteristics that are unavailable via other routes. The ready access to these compounds renders them an important entry point for the preparation of more complex N-alkyl pyridone-containing targets.

On the validity of the isolobal principle: Pentaborane(9) and its ferraborane derivatives

Abstract We have completed Fenske-Hall LCAO-MO-SCF calculations on B 5 H 9 , 1-Fe(CO) 3 B 4 H 8 , 2-Fe(CO) 3 B 4 H 8 , and 1,2-[Fe(CO) 3 ] 2 B 3 H 7 . Comparison of orbital contour diagrams of the a 1 and e cluster MOs for B 5 H 9 and 1-Fe(CO) 3 B 4 H 8 demonstrates the validity of the isolobal principle. In addition it is found that the apical and basal BH units of B 5 H 9 have practically identical Mulliken overlap populations for framework cluster-type interactions. Further, in all the ferraboranes the 1-Fe(CO) 3 (apical) units have a larger cluster-type Mulliken overlap population than do the 2-Fe(CO) 3 (basal) units. The Fe(CO) 3 units have less electronic charge than do the BH units. The cluster-type Mulliken overlap population for an Fe(CO) 3 group is much less than that of the isolobal BH unit, but this may result from an artifact of the Mulliken overlap population analysis.

On the structures of H+ (CO)5 and H+ (N2)5

Calculs MNDO des deux structures C 2v et D 3h de H + (CO) 5 et H + (N 2 )5; optimisation par des calculs ab initio au niveau STO-3G et a nouveau sur les bases 4-31 G. A partir de ces trois methodes de calcul, discussion sur la stabilite relative des deux structures

Response to Potential-Energy-Only Models

Commentary on the suitability of the potential-energy-only (POE) model for explaining successive ionization energies.