Robert J. Buenker

Combined SCF and CI Method for the Calculation of Electronically Excited States of Molecules: Potential Curves for the Low‐Lying States of Formaldehyde

The success of various ab initio methods for the calculation of electronic spectra for polyatomic molecules is discussed with reference to the formaldehyde system. It is found that a limited CI calculation based on the SCF MOs of the ground‐state wavefunction yields inaccurate potential curves for the excited states of H2CO and overestimates their transition energies from the ground state. It is concluded that the major cause for these deficiencies lies in the fact that the ground‐state MOs do not represent a good starting point for the description of the excited states, and thus a second series of calculations is carried out using open‐shell SCF techniques in an attempt to improve upon this situation. The resulting excited‐state potential curves do show the proper angular behavior expected from experimental studies, but the calculated transition energies are too low because the correlation energy error in the SCF treatment is greater for the closed‐shell ground state than for the open‐shell excited spe...

A new interpretation for the structure of the VN bands of ethylene

Abstract Potential surfaces for CC stretch and CH2 twisting of ethylene obtained by the combined SCF CI method lead to the prediction that non-vertical transitions are responsible for the absorption maximum in the VN bands and that the broad diffuse nature of the spectrum in this region could well be caused by the proximity of two states of the same symmetry of the twisted molecule.

Combined SCF and CI Calculations for the Low‐Lying Rydberg and Valence Excited States of Ethylene

A series of nonempirical SCF–MO and CI calculations is carried out for the excited states of ethylene. In the usual manner the SCF treatment itself is seen to underestimate vertical transition energies from the closed‐shell ground state to open‐shell excited states by about 1 eV; an exception is noted, however, in the case of the π → π* singlet–singlet species. A CI(PCMO) treatment, which employs the SCF MOs of a given parent configuration as basis for its own CI expansion, is quite successful in balancing the correlation error, obtaining excellent agreement with experimental transition energies to valence and Rydberg states alike; a possible exception is found in the case of the π → π* singlet–singlet excitation for which the calculated value of 8.32 eV overestimates the location of the V ← N absorption maximum by 0.7 eV. The variational π* MO of the SCF wavefunction for the upper‐state singlet is quite diffuse, but it is argued that this fact is not inconsistent with the known experimental data for the V ← N band system. Since the calculated state is found to correlate with a valence species for antiplanar ethylene, its diffuse character in the planar geometry does not imply that its potential surface should resemble that of a Rydberg state; in addition, its charge density contours emphasize that it should not be associated with a pure Rydberg species even in the planar conformation. The change in character with relative rotation of the methylene groups suggests that the electronic transition moment must be considered explicitly in the theoretical treatment of the intensity distribution in the V ← N bands, and also indicates that the probability of nonvertical transitions to partially rotated structures may well be greater than that of the vertical excitation.

Geometry of Ozone and Azide Ion in Ground and Certain Excited States

A series of ab initio SCF MO calculations for ground and various closed‐shell excited states of O3 and N3− have been carried out with a view toward investigating relationships between the geometries of these states. The calculations give quantitative verification to many of the assumptions made in previous empirical theories dealing with this subject and enlarge the scope of former quantitative schemes to include more quantitative predictions relative to steepness of potential surfaces of molecular states. Thus it is shown that the calculated SCF total energy surfaces of both O3 and N3− can be compared quite concisely in terms of a small number of differentiating orbital‐energy curves.

Non-empirical calculations on the electronic spectrum of butadiene

Abstract Ab initio SCF and CI calculations of the electronic spectrum of butadiene are reported which employ a gaussian basis of double zeta quality augmented by diffuse 3s and 3p functions. Good agreement is obtained with experimental details of this spectrum, both for π→π* and certain Rydberg transitions, and it is concluded that the important NV 1 and NV 2 absorption systems both involve diffuse upper states of 1 B u symmetry.

ab initio Study on the Stability and Geometry of Cyclobutadiene

Ab initio SCF MO and CI calculations dealing with questions of molecular stability, geometry, and spectra of cyclobutadiene C4H4 are reported. Comparison is made between the ab initio and semiempirical Huckel methods for the treatment of molecules of this type and emphasis is placed upon the role configuration interaction plays in improving the representation of the closed‐shell SCF molecular potential surface. The results of the CI calculations in turn indicate a singlet ground state for cyclobutadiene in a rectangular geometry and a lowest excited triplet of the molecule which possesses a potential minimum for a slightly nonsquare geometry and is everywhere more unstable than the aforementioned singlet; the molecule is not found to be stable with respect to its separation products (two acetylenes), although it appears to exist in metastable equilibrium in the cyclic configuration. An attempt is also made to relate the cyclobutadiene calculations to the study of the geometry of similar molecules; from th...

Theoretical Study of the Geometry and Spectrum of Nitrous Oxide

A series of ab initio SCF MO calculations is carried out for the nitrous oxide molecule as a function of internuclear angle in both NNO and NON arrangements. Investigation of the results of these calculations indicates that this molecule favors the nonsymmetric NNO arrangement mainly as a result of the net stabilization its occupied π MOs derive from having the more electronegative oxygen atom at the terminal position of the system. Generalization of this point leads to a qualitative explanation for the almost universal preference of triatomic molecules for equilibrium structures in which the most electropositive atom of the three lies at the middle position of the system. The calculations are also employed to discuss the applicability of general rules governing questions regarding the geometry of symmetric triatomic molecules to analogous problems concerning the nonsymmetric members of this family. Finally, a large‐scale CI calculation for the linear equilibrium geometry of NNO is carried out in order t...

Ab Initio SCF MO and CI Calculations on the Electronic Spectrum of Benzene

An ab initio SCF MO calculation for the ground state of benzene is reported (total energy equal to −230.3745 hartrees). The set of SCF MOs resulting from this calculation is subsequently employed as the basis for a series of CI treatments investigating the theoretical description of the benzene electronic spectrum. It is found that the inclusion of double and higher excitation configurations in the CI treatment produces significant changes in the calculated results relative to those obtained in the more conventional single excitation CI. Perhaps the most important effects of the higher excitation configurations are observed in the class of excited singlet states; the addition of these configurations causes the 1B2u state to be lower than at least one of the three lowest benzene triplets, in agreement with experimental findings, and also leads to a considerable stabilization of the 1E2g species, making it the second most stable excited singlet in definite contrast with single excitation CI results.

Ab Initio SCF MO and CI Studies of the Electronic States of Butadiene

Ab initio SCF MO calculations carried out for the ground electronic states of cis‐ and trans‐butadiene show the trans configuration to be more stable by approximately 5 kcal, total energy − 154.7103 hartrees. The set of SCF MOs resulting from these calculations is subsequently employed as a basis for a series of configuration–interaction (CI) treatments investigating the electronic spectrum of butadiene. Results of CI treatments which consider only π → π* single excitation configurations bear a definite resemblance to previous semiempirical studies and give rise to the excited state ordering 3Bu, 3Ag, 1Bu, and1Ag, in agreement with the current experimental assignment although the singlets are calculated relatively high in energy. Extension of the CI treatment to include multiple excitation configurations and σ → π* excitation decidely alters the spectrum by lowering the 1Ag to 2.5 eV below the 1Bu, and also by introducing 3Buand3Au states lower than the 1Bu: a σ → π*1Au state is also calculated to lie cl...

Theoretical investigation of the cyclic conformer of ozone

Abstract Ab initio SCF and CI calculations are reported which find the cyclic conformer of ozone to lie 16 kcal/mole above the preferred (open-chain) form of this substance. Polarization functions in the AO basis set are found to be quite important in this determination, strongly favoring the cyclic species. The possible experimental significance of such a relatively stable ring conformer of ozone is assessed.