Michael H. Palmer

The molecular energy levels of the azoles: A study by photoelectron spectroscopy and ab initio molecular orbital calculations

Abstract HeI photoelectron spectroscopy and ab initio calculations have been applied to the azoles, providing sets of energy levels that correlate well with each other in the upper valence shell region. Observed IPs are assigned to the three π- and to the five o -levels that involve (principally) valence shell p orbitals. The observed vibration structure is not particularly informative as an aid to assignment since both π-and σ-levels give some bands with vibration structure. The calculations provide in addition to eigenvalues (energy levels) a set of eigenvectors, permitting analysis of the bonding characteristics of the levels, and trends apparent within the series.

The electronic states of furan studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio multi-reference configuration interaction calculations

Abstract The results of large-scale ab initio CI calculations on the electronic states of furan, using a multi-reference multi-root CI method (MRD-CI), are related to new VUV absorption and low-energy electron energy-loss (EEL) measurements, leading to detailed spectral assignments. All of the expected 1,3 ππ ∗ states have been assigned, there being improved agreement between theoretical and experimental energies in all cases. A number of new Rydberg series has been identified by experiment and, for these also, the calculations perform well. In addition, cationic states and ground state molecular electronic properties have been computed.

The electronic states of the azines. II, Pyridine, studied by VUV absorption, near-threshold electron energy loss spectroscopy and ab initio multi-reference configuration interaction calculations

Abstract The results of large-scale ab initio CI calculations on the electronic states of 1,3,5-triazine, using a multi-reference multi-root CI (MRD-CI) method are related to new VUV absorption and low-energy electron energy-loss (EEL) measurements, leading to detailed spectral assignments. Vertical transition energies are computed to within 0.6 eV for all and 0.3 eV for most of the low-lying electronic states. The computations are extended to cationic states and to the evaluation of ground state molecular electronic properties of the molecule.

Ab Initio Molecular Orbital Calculations

A series of non-empirical calculations on furan, pyrrole and 1,2,5-oxadiazole are reported in which the effect of polarisation functions added to the minimal 7s 3p basis on each atom is studied. The effect on these planar molecules is largely through theσ rather than theπ-system. A comparison with the results of work with scaled functions is reported. Both series are shown to lead to much improved agreement with the electron spectroscopy energy levels. The effect on the dipole moments of these changes in basis is more variable but, with the exception of furan, the agreement with experiment is improved in the present method.ZusammenfassungFür die Moleküle Furan, Pyrrol und 1,2,5-Oxadiazol wurde eine Reihe von nichtempirischen Rechnungen durchgeführt, in denen der Einfluß von zusätzlichen Polarisationsfunktionen zur minimalen 7s 3p-Basis an jedem Atom untersucht wird. Die Ergebnisse werden mehr durch die Art der Beschreibung des Systems derσ-Elektronen als durch diejenige derπ-Elektronen beeinflußt. Ein Vergleich mit den Ergebnissen bei Verwendung skalierter Funktionen wird durchgeführt. Beide Reihen von Ergebnissen zeigen eine verbesserte Übereinstimmung zu den Energiemeßwerten der Elektronenspektroskopie. Die Änderungen des berechneten Dipolmoments bei derartigen Basisvariationen sind größer als bei früheren Methoden. Die Übereinstimmung mit dem Experiment wird, mit Ausnahme von Furan, jedoch verbessert.

The electronic structure of substituted benzenes; AB initio calculations and photoelectron spectra for the methyl- and fluoro-benzenes and fluorotoluenes

Abstract The valence shell orbital energies for the various title compounds are assigned by a combination of He(I) and He(II) photoelectron spectra and ab initio MO calculations of double and single zeta quality.

The electronic structure of aromatic molecules: ab inito molecular orbital studies and photoelectron spectra for the aza-derivatives of indole, benzofuran and benzothiophen

Abstract The He(I) photoelectron spectra for the aza-derivatives of benzofuran (4 and 7), benzothiophen (8) and indole (3a, 3b, 6a, 9b), and combined He(I) and He(II) spectra of the 1,2-benzisothiazole (5), 1,2,3-benzothiadiazole (11) and benzotriazole (9a) have been obtained and assigned by a combination of heteroatom substituent effects and ab initio molecular orbital calculations. The variations in lone pair levels (LP N LP O , LP S ) and π-levels, between these and the monocyclic compounds are discussed.

The electronic structure of heteroaromatic molecules; ab initio calculations and photoelectron spectra for the isomeric-oxazoles and some -oxadiazoles

Abstract The UV photoelectron spectra are reported for isoxazole and 1,3,4-oxadiazole (He(I) region), oxazole and 1,2,5-oxadiazole (He(I) and He(II) regions). Marked fine structure is apparent on some of the spectra. The spectra have been assigned on the basis of ab initio calculations, and by comparison with the azoles and thiazoles. Variations in the lone pairs levels at O-, S- and N- across the azoles, oxazoles, thiazoles and azines are discussed. It is suggested that “through-bond” interactions are more widespread in the azines than previously noted.

The electronic structure of substituted benzenes; ab initio calculations and photoelectron spectra for phenol, the methyl- and fluoro-derivatives, and the dihydroxybenzenes

Abstract The valence shell orbital energies for the ten title compounds are investigated by a combination of He(I), and He(II) photoelectron spectra and ab initio MO calculations of single and double zeta quality.

The electronic structure of substituted benzenes; a study of aniline, the toluidines, phenylenediamines and fluoroanilines by photoelectron spectroscopy and ab initio calculations

The ten title compounds have been investigated by a combination of He(I) and He(II) photoelectron spectroscopy and ab initio molecular orbital studies, of single and double zeta quality. Although the compounds are of low symmetry, the calculations suggest that many of the molecular orbitals have a pseudo-benzene (D6h) character; this is borne out by the systematic trends in the observed spectra. The latter are assigned in some detail, and variations between compounds are discussed, in particular in relation to the corresponding isoelectronic series of substituents in PhX where X = CH3, NH2, OH and F.

The singlet electronic states of pyrrole: a theoretical study by both ab initio multi-reference configuration interaction methods and time-dependent density functional theory and a reconsideration of the experimental VUV spectral data

The singlet electronic excitation spectrum of pyrrole has been reinvestigated by both multi-reference multi-root configuration interaction (CI) calculations and time-dependent density functional theory (DFT) with asymptotically corrected exchange-correlation potentials. The methods used a triple zeta valence + polarization + Rydberg (TZVPR) basis set and a much larger active space than in our previous CI study [Palmer, M. H., Walker, I. C., and Guest, M. F., 1998, Chem. Phys., 238, 179]. Computed vertical excitation energies, oscillator strengths and electronic charge distributions were used to characterize and assign the valence and Rydberg excited states over an energy range of 5–12 eV. A comparison of the present methods with other high-level ab initio studies has been made, including the effects of basis sets and size of CI, and some statistical relationships determined. The present CI vertical excitation energies are generally in closer agreement with the cluster-type methods, especially CC3, than to the various second-order perturbation-type methods (CASPT2, CASPT2-MS, ADC(2) and MRMP). The influence on the excitation energies from exact orbital exchange and multiplicative potentials in hybrid functional development has been investigated. Differences between the CI and the DFT methods vary in the order B97-2 < B97-1 < HCTH < LDA. The differences between hybrid DFT and CI excitations are minimized when the fraction of orbital exchange (ξ) lies in the approximate range 0.2–0.3. The Rydberg and valence-type excitations are seen to be less sensitive than the static polarizability to the inclusion of orbital exchange or multiplicative potentials in hybrid functional development. In order to allow a realistic assessment of the performance of the theoretical studies, the assignment of the experimental electronic spectrum of pyrrole is discussed in detail. Previous conclusions have led to incorrect numbers of Rydberg s- and d-type states, while f-type states have previously been ignored. Some excitations from the second IP, which must occur in the 5–10 eV range, have been reassigned in light of the known small differences between other spectroscopic states and quantum defects. There is an urgent need for higher-resolution studies of pyrrole and related molecules.