Prof. Timothy G. Wright
Office: Room A45, Chemistry Building; Ext. 67076
Research
A brief description of our areas of research is given; specific recent references can be found via the Publications link.
Vibrational Coupling in Substituted Benzenes
Vibrational Labelling
We are interested in how the vibrational energy levels in substituted benzenes evolve with changing substituents. If we consider a monosubstituted benzene molecule, such as fluorobenzene, then we will have 30 normal modes. In the harmonic picture, these will all be independent and their motions and energies can be simply calculated using standard quantum chemistry packages. If we change the substituent from fluorine to chlorine etc. then the mass change will lead to changes in the wavenumber of the mode, but we have found that the motion remains largely the same; in particular, this implies that electronic effects from the change of substituent are small. These modes are, however, significantly different to those of benzene, and so labelling such modes with "Wilson" labels is misleading and often ambiguous. These points, together with the small mixing of substituent-localized modes, means that the phenyl-ring-localized vibrations are also similar for molecules such as toluene, phenol etc. This has led to our proposing vibrational labels for monosubstituted benzenes, based on those of fluorobenzene, that allow comparison of modes and vibrational activity during electronic excitation and ionization. Recently, we have also published a paper on the extension of these ideas to para-disubstituted benzenes, based on the vibrational modes of para-difluorobenzene. We have also completed, and are writing up, work on the ortho- and -disubstituted benzenes.
Spectroscopy (REMPI, ZEKE, LIF, DF and 2D-LIF
We are interested in electronic spectroscopy and photoelectron spectroscopy, which allow us to study electronically-excited states and cationic states, respectively. We carry out experiments in supersonic jet expansions (molecular beams), which means that our molecules are both vibratonally and rotationally cold. This simplifies the spectra tremendously, giving rise to discrete spectral lines that can be assigned with confidence. These conditions also allow the formation of molecular complexes (which can be both advantageous and a nuisance!), and in principle also allows the study of reactive species, such as free radicals. In addition, we use lasers as our excitation and ionization light sources.
REMPI and ZEKE Spectroscopy
One variant of electronic spectroscopy we use is resonance-enhanced multiphoton ionization (REMPI), which excites the molecules from the ground electronic state to an electronically excited state (usually the S1 <- S0 excitation in substituted benzenes. A second photon, either from the same laser (a one-colour experiment) or from a second laser (a two-colour experiment) then ionizes the molecules. The resultant cations are sent down a time-of-flight tube and mass (strictly m/q) separated. By electronically gating the mass signal of interest, we can record an absorption spectrum of a specific molecule or molecular complex, even if many such species are present; this also allows us to investigate fragmentation processes. Such REMPI spectra show a wealth of vibrational structure and, for molecules with methyl groups, also torsional structure. We can assign the spectra in their own right, by making use of quantum chemical results and other published results, where they exist; however, for cases where there are interactions between the vibrations (and torsions), then further information is often useful, such as ZEKE spectroscopy or dispersed fluorescence (DF) - see below.
To gain information on the cations, we make use of a variant of photoelectron spectroscopy that detects electrons at each ionization threshold, i.e. the electrons that are produced with zero kinetic energy. For this reason, this technique is often referred to as zero-kinetic energy (ZEKE) photoelectron spectroscopy (ZEKE-PES or simply ZEKE spectroscopy). (Strictly we are detecting electrons that result from the pulsed-field ionization (PFI) of high-lying Rydberg states, and so the technique is sometimes called ZEKE-PFI, but that is not important here.) By monitoring the ZEKE signal as a function of the ionizing laser energy, then we can obtain a vibrationally-resolved spectrum of the cation. These experiments are done using a two-colour technique, whereby we excite the molecule into a selected vibrational level of the S1 state, using a fixed wavenumber from our excitation laser. We then ionize from this level into the cation, using our tuneable ionization laser, detecting the ZEKE electrons as we scan through the cation's vibrational levels. There are a number of pieces of information we can glean from these experiments:
(i) The vibrational energies of the cation, and these can be compared to those in the ground state to deduce bonding changes upon ionization.
(ii) The lowest-energy band in the ZEKE spectrum will, in most cases, correspond to the adiabatic ionization energy, and so give the lowest energy difference between the neutral molecule and the cation; this helps to derive heats of formation.
(iii) We can change the vibrational level that we excite through, which means that the ionization will have different Franck-Condon factors (sometimes even the symmetry has changed); this gives us additional information on the cation.
(iv) In cases where vibrational levels are mixed in the S1 state, then the appearance of the ZEKE spectra can help unpick the interactions that are occurring. This information is complementary to that obtained by dispersed fluorescence, such as carried out by Warren Lawrance's group at Flinders University, Adelaide, Australia, and also time-resolved studies using picosecond lasers, such as those carried out by Katharine Reid (Nottingham, United Kingdom). These studies have given great insight into various Fermi resonances that occur in substituted benzenes, plus also the role torsions play in such systems. These results can give insight into how different bonds are involved in the vibrations and can lead to the idea of controlling the outcome of a chemical reaction.
(v) Simply knowing the energies of all of the levels within a molecule can provide a test of quantum chemical methods, as well as allowing statistical thermodynamics methods to be used to obtain bulk quantities from microscopic ones.
2D-LIF Spectroscopy
Very recently we have successfully constructed the UK's first (currently only) 2D-LIF spectrometer . This allows us to record a disperseed fluorescence (DF) spectrum at each excitation wavenumber as we scan through the vibrational, torsional and vibration-torsional(vibtor) energy levels of an electronically-excited state. By piecing all of these together, we can investigate coupled levels, similar to (iv) above, but now in much more detail, and by projecting the populatons of the excited state onto the group state, rather than the cation. We have just (December 2017) submitted our first paper, and we hope many will follow.
Studies of NO-containing Complexes
Expanding mixtures of gases into vacuum via a supersonic-jet expansion can cause clusters/complexes to form when the different atoms/molecules stick together. Because the environment is largely collisionless, then complexes can exist for long enough to excite and ionize them using the techniques of REMPI and ZEKE spectroscopy (see above). We have performed many such studies on complexes containing the NO molecule. This is an unusual molecule since it has an unpaired electron (and so is a radical), yet is stable. We have studied complexes of NO with rare gases (RG) (=Ne, Ar, Kr and Xe) as well as with molecules, N2, CO, CH4 and other alkanes. The electronic excitations are localized on the NO molecule, but intermolecular vibrational modes can be observed. By exciting the pi* electron on NO to higher and higher orbitals (Rydberg orbitals), the properties should converge to those of the cation, which indeed we find. In the lower electronic states, however, there is competition for the cationic core between the unpaired electron and the rare gas atom (or molecule).
The interpretation of these spectra, particularly in the lower states, turns out to be far from straightforward, as there are often no obvious progressions. In a collaboration with Nick Besley (Nottingham, UK) and Jacek Klos (Maryland, USA) we calculated potential energy surfaces for NO-RG complexes, showing that one can simulate the spectrum in close agreement with the experiment, and hence obtain assignments for the observed bands. It was clear that the complexity of the spectra arose from the significant anisotropy in the potential energy surfaces.
In time, we wish to continue work on these species, such as recording ZEKE spectra. It is known that NO+ complexes are prevalent in the ionosphere and are the precursors for aerosol and rain drop formation, via the formation of protonated water complexes. The complicated chemistry that occurs in the upper reaches of our atmosphere is difficult to unravel and requires input from laboratory-based studies, as well as quantum chemistry and modelling.
Studies of Metal-containing Complexes (Experiment)
It is possible to get metal atoms into supersonic-jet expansions via laser ablation. For such studies, we take a small rod of metal and mount it inside the vacuum chamber, and connected to a pulsed valve. We focus a pulsed laser onto the rod, which is rotated and translated, to allow a fresh surface to be exposed to the laser, and this leads to a relatively stable metal atom signal. The metal atoms are carried into the vacuum chamber via a pulse of inert gas, and either the inert gas, or molecules entrained in it, can then stick to the metal atoms. We can think of this as forming atomic or molecuar complexes, or as primitive examples of coordination complexes. The latter allow us to examine the interactions that occur between metal atom and ligand in great detail. We have examined Au-RG complexes in depth, obtaining detailed spectra that allowed us to examine the gold-RG interactions, as well as the role of different electronic states in the observed spectra. Particularly of interest were the differing behaviour of the spin-orbit states formed during the 6p <- 6s electronic excitation, with the different rare gas atoms. Future work will include ZEKE studies of these species, together with ligands that are molecular.
This work forms the basis of understanding metal-ligand interactions at the most fundmental level. Combined with ZEKE spectrscopy, which gives information on the cations, we can investigate both model systems in inorganic coordination chemistry, as well as species which appear in the upper reaches of our atmopshere, and lead to the formation of sporadic metal layers, for example.
Studies of Metal-containing Complexes (Theory)
Incipient chemical bonding
By calculating extremely accurate potential energy curves for M+-RG complexes (RG = rare gas), we have investigated the idea of incipient bond formation. For M = alkali metal, we find that the interactions are almost solely physical, in that physical models reproduce the interaction potential extremely closely; while for other species, such as Au+-Xe, there is definite evidence for chemical bonding contributions. In between these, we find a number of cases where hybridization occurs, which can be thought of as a first step towards chemical interaction. For the light group 2 metals (Be and Mg), the hybridization is s-p, while for the heavier ones Ca-Ra, then the hybridization is s-d. we have used various concepts to investigate the bonding, such as population analyses, molecular orbital plots, electron density plots and Birge-Sponer plots. In a few cases, we have also investigated some M+-Rg2 complexes and for the alkali metals find that a simple electrostatic model explains the bent/linear trends in geometry. For M = group 2, again hybridization is an important occurrence.
Ion transport calculations
In collaboration with Larry Viehland (Chatham University, USA), we use our potentials to obtain transport properties of cations in rare gases. Such data is usueful in determining the interactions of metal ions with rare gases over a wide range of the potential, and so provides a astringent test of the calculated potentials, The results are useful in determining loss mechanisms in flow tube experiments for instance, as well as at a more fundamental level. In addition, in some cases we have been able to test experimental results and make deductions about the contributions of different spin-orbit states of the metal cations (and other cations) in the experiment.
Spoaradic metal layers
Observations of the upper atmosphere (mesophere/lower thermosphere) show that sudden appearances of neutral metal atoms occur. At one time it was thought these were directly linked to meteor activity, but in fact are due to clouds of metal ions (originating in the ablation of meteoroids) descending through the Earth's atmosphere and undergoing a range of ion-molecule reactions, followed by dissociative neutralization with electrons. By calculating the energy levels in the metal complexes, we can use statistical thermodynamics to obtain kinetic parameters as a function of altitude and so model the chemistry. Good agreement is found with LIDAR observations, suggesting the chemical models are correct.
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