Dr Michael H Palmer
Main methods of study
We use of ab initio Hartree-Fock SCF, CI and/or M??ller-Plesset methods to study the ground and electronically excited states of molecules. Studies include organic and inorganic types of molecule. We use GAMESS-UK, MOLPRO and GAUSSIAN-09 for specific types of investigation. The work is heavily computational including conventional and parallel processors on Unix based systems. We do not have an experimental program, but collaborate with spectroscopists on a routine basis, for the interpretation of specific spectral problems, or long term projects.
1. The electronically excited states of aromatics and alkenes
The oldest of the spectroscopic techniques is in the UV range, and have been studied for over 100 years. Much of the early interest for chemists was in determining specific features of UV spectra for structural information in complex molecules. With the advent of IR and NMR in the 1960's, UV and vacuum UV (VUV) spectra, were largely deserted; this was because of the complexity of the spectra. Although the general principles were well established, most of the interpretation was empirical. Only in the last 30 years, with the advent of super-computers, has it been possible to put this work on a firm basis of theory.
Our UV and VUV spectra for polyatomic molecules, such as aromatics and hetero-aromatics, are studied by synchrotron radiation in the range up to about 12eV; generally; the spectra contain many discrete absorption maxima as well as broad bands with poorly defined structure. These spectra will contain both Rydberg (sharp) and valence states (often broad). The order of the electronic states cannot generally be decided without recourse to major theoretical chemical study.
We use multi-reference multi-root singles and doubles CI, usually termed MRD-CI, to give an interpretation of all low-energy states, for each symmetry, in both the singlet and triplet manifolds. Re-assignment of UV-photoelectron spectra without recourse to Koopmans' type or other ground state approaches follows naturally as part of these studies. When the MRD-CI strategy is applied to these problems, a set of reference configurations (which may total ~200), from which all single and double excitations are to be derived, is set up. The programme sets up a matrix of all the configurations and diagonalises the corresponding energy matrix; the lowest set of energy solutions and the excited state dipole moments, provide the interpretation of the spectra. A development of this is that of using state averaging techniques; it is possible to determine the equilibrium structures for many of these excited states.
2. Interpretation of NQR and microwave spectral Quadrupole Coupling Constants
Almost all elements of the Periodic Table have one or more quadrupolar isotopes. In molecules, such nuclei yield quadrupole coupling constants (QCC) as part of the electronic structure. Microwave spectroscopy (MW) provides the best method of investigating these QCC. In principal, all data concerning sign, direction and magnitude can be evaluated from the spectra. However, in practice, much of this data is not fully determined owing to insufficient combinations of isotopic substitution data, and sheer complexity of the spectra. Similarly, in the solid state, nuclear quadrupole resonance gives magnitudes of coupling constants, but no sign or direction for each element!! The use of high-level ab initio calculations provides such an interpretation. Such calculations may be either molecular, cluster or lattice in type, depending on the system. Our recent studies cover a range of common isotopes, including: l4N, l0,11B, 33S, 17O and halogen (Cl, Br and I).
- M. H. Palmer, M. Coreno, M. de Simone, C. Grazioli, S. Vronning Hoffmann, N. Jones, K. Peterson, R. A. Aitken, and C Rouxel,"The photoelectron spectra of the isomeric 1- and 2-methyltetrazoles; their equilibrium structures and vibrational analysis by ab initio calculations,"J. Chem. Phys., 149, 034305 (2018);
- M. H. Palmer, S. Vronning Hoffmann, N. C. Jones, M. Coreno, M. de Simone and C. Grazioli,'The valence and Rydberg states of difluoromethane: a combined experimental vacuum ultraviolet spectrum absorptionand theoretical study by ab initio configuration interaction and density functional computations,'J. Chem. Phys.,148, 214304 (2018).
- M. H. Palmer, M. Biczysko, K. Peterson, C.Stapleton, S. Wells,"Structural and Vibrational Properties of Iodopentafluorobenzene: A Combined Raman and Infrared Spectral and Theoretical Study,"J. Phys. Chem. A, (2017), 121(41), 7917.
- M. H. Palmer, M. Biczysko, A. Baiardi, M. Coreno, M. de Simone,C. Grazioli, S. Vronning Hoffmann, N. C. Jones, and K. A. Peterson,'The ionic states of difluoromethane: a reappraisal of the low energy photoelectron spectrum including ab initio configuration interaction computations, 'J. Chem.Phys., 147, 074305 (2017).
- M. H. Palmer, S. Hoffmann, N. C. Jones, M. Coreno, M. de Simone, C. Grazioli, K. Peterson, A. Baiardi, Teng Zhang, and M. Biczysko, 'A combined theoretical and experimental study of the valence and Rydberg states of iodopentafluorobenzene.'J. Chem. Phys., 146, 174301-12 (2017).
- M. H. Palmer, T. Ridley, S. V. Hoffmann, N. C. Jones, M. Coreno, M. De Simone, C. Grazioli, Teng Zhang, M. Biczysko, A. Baiardi, and K. Peterson,'Combined theoretical and experimental study of the valence, Rydberg and ionic states of fluorobenzene', J. Chem. Phys., 144, 204305/1-204305/20 (2016).