Harold Basch

Electronic Energies and Electronic Structures of the Fluoromethanes

The high‐resolution He i and He ii photoelectron spectra of all fluoromethanes in the series CH4 to CF4 and their deuterated analogs have been recorded and are compared with the Koopmans theorem results of near‐Hartree–Fock calculations performed in a Gaussian basis. The agreement is very good in general and offers an unambiguous assignment of almost all of the bands observed. In particular, repeated correlations are demonstrated between the compositions of the orbitals from which the electrons are ejected and the characters of the resulting photoelectron bands. Identifiable trends throughout the series are stressed and an anomalous feature in the CF4 spectrum is noted. Jahn–Teller effects in CH4 and CH3F are clearly evident, but as expected, they are not observed in CHF3 and CF4. Comparison of the photoelectron spectra excited with He i and He ii radiation shows wide variations in the relative intensities of various bands in certain of the more symmetric molecules, suggesting that relative intensities c...

Photoelectron spectroscopy of simple amides and carboxylic acids

Abstract The accidental near-degeneracies of the no (oxygen nonbonding) and π2 (antisymmetric pi) MOs of formamide and formic acid have been investigated using photoelectron spectroscopy and SCF calculations of the successive ionization potentials. It is concluded that ionization from no preceeds that from π2 in both of these compounds, but that the reverse order obtains in M-methylformamide and N,N-dimethylformamide. The close correspondence between the profiles of the no and π2 photoelectron bands and the Rydberg band profiles observed optically is used in making a more detailed assignment of the optical bands. All ionization potentials below 20 eV are reported for each of the four compounds.

On the interpretation of k-shell electron binding energy chemical shifts in molecules

Abstract The relationship between the potential-at-a-point and orbital energy difference methods of computing K-shell electron binding energy chemical shifts, and their affinity to the nuclear shielding effect in NMR spectroscopy is demonstrated.

Interpretation of Open‐Shell SCF Calculations on the T and V States of Ethylene

Self-consistent field calculations are reported on (planar and perpendicular geometric conformers of) the lowest energy triplet and singlet (pi,pi*) orbital configurations of ethylene, which are usually identified with the spectroscopic T and V states, respectively. For the planar conformation the calculation predicts a V state with the characteristics of a Rydberg state, but a T state of expected valence-shell character. The pi* orbital is much too large and the internuclear distance too small for the calculated V state, but are as expected for the T state. It is concluded that the calculated result for the supposed V state in the planar conformation is spurious due to the inadequacy of the Hartree–Fock single-configuration theory. The supposed V state here calculated may be identified with a Rydberg state which has been observed spectroscopically at ~ 1.4 eV higher energy than the V state in the optical spectrum. For the perpendicular conformation, the calculations yield resonable results for both the T and V states. Calculations also have been carried out on a singlet (sigma,pi*) state for planar ethylene; here the size of the pi* orbital is reasonable.

ESCA: chemical shifts of K-shell electron binding energies for first-row atoms in molecules

Abstract Chemical shifts of k-shell electron binding energies for first-row atoms in molecules due their different chemical environments have been obtained from double-zeta basis SCF-MO calculated ground state 1s orbital energies using Koopmans theorem. This computational procedure is valid because it is known that the energy quantities neglected thereby are either approximately invariant for a given atom, or small. The calculated chemical shifts are characteristics of an atom in a functional group, show a linear relationship with net charge, and agree with experimental numbers in the few cases where comparison is possible.

Self‐Consistent‐Field Study of the Series XeFn, n = 2, 4, 6

An ab initio self‐consistent‐field computation has been performed on the series XeF2, XeF4, and XeF6 in a basis of contracted Gaussian functions. The charge distribution, photoelectron and optical spectrum, and one‐electron properties are discussed and compared with available experimental results.

Gaussian-Orbital Basis Sets for the First-Row Transition-Metal Atoms

Contracted Gaussian-type function basis sets have been produced for the first-row transition-metal atoms Sc through Cu for use in molecular SCF calculations. The basis functions consist of 15 s-type, eight p-type, and five d-type Gaussian primitives contracted to a four s-type, two p-type, and one d-type basis set and are exponent and coefficient optimized after contraction. It is shown that this basis set is of single-zeta quality for the core functions and of double-zeta quality for the valence-electron functions. Relaxation of the degree of contraction results in a significant lowering of the total energy.

Self‐Consistent‐Field Study of the Cluster Model in Ionic Salts. I. NiF64−

Self‐consistent‐field calculations have been carried out on the NiF64− cluster in a near‐Hartree–Fock basis (for the valence electrons) of Gaussian functions to test the validity of the model for describing the localized properties of ionic salts. Both the spin‐restricted and spin‐polarized treatments are compared for ground and excited states of the complex ion. From comparison with experimentally obtained results it is concluded that a Hartree–Fock description of the cluster gives reasonably good wavefunctions and energies. Surprisingly, the crystal‐field splitting parameter obtained using just the ground‐state wavefunction is too large by a factor of 3.

Dimerization of Methylenes by Their Least Motion, Coplanar Approach: A Multiconfiguration Self‐Consistent Field Study

The one‐dimensional path of reaction in the least motion, coplanar approach of two methylenes to form ethylene has been calculated using the nonempirical multiconfiguration self‐consistent field (SCF) method in a large Gaussian basis. Orthogonality between open‐ and closed‐shell molecular orbitals of the same symmetry is constrained by a “partitioned basis set” technique which is found to give rapid and uniform convergence to the proper SCF solution. The results obtained for the reaction path suggest that the ground state of bent (∼u2009120°) methylene is the triplet. Therefore, in this study, it is the triplet states (and not closed‐shell singlet states) of two appropriately oriented bent methylenes that correlate with the normal (ground) state of ethylene. For two closed‐shell singlet‐state methylenes the reaction path is found to be purely repulsive.

Hei and Heii Photoelectron Spectra and the Electronic Structures of XeF2, XeF4, and XeF6

The high‐resolution photoelectron spectra of XeF2, XeF4, and XeF6 have been obtained essentially free from any impurity features, and the ionization potentials up to 28 eV have been measured. The results are used, in conjunction with recent ab initio calculations, to assign the molecular orbital electronic structure of these molecules. Agreement between experiment and calculation is fairly good.