Our Research – University of Copenhagen

Our Research

In The Kjaergaard group we are interested in the study of molecules, reactions and chemistry of atmospheric importance. Our current research is divided into two categories:

  • Spectroscopy
    Determining properties of molecules and hydrogen-bonded complexes, some of which are: absolute intensities of XH-stretching vibrations and equilibrium constants, enthalpies, Gibbs free energies and entropies of complex formation. We are able to obtain these properties by using various different experimental and theoretical spectroscopic methods.
  • Kinetics
    Calculating reaction rates of unimolecular reactions such as atmospheric hydrogen shift reactions and ring breaking reactions. In order to do this, we use our recently developed cost-effective approach to the implementation of multi conformer transition state theory (MC-TST).

Spectroscopy

Absolute intensities of molecules

Spectra of the fundamental and first overtone of the NH-stretching vibration in dimethylamine.

Using our one dimensional local mode model (1D-LM), we are able to determine frequencies and intensities of XH-stretching transitions, which are in very good agreement with experimental results. In the 1D-LM model the XH-stretching transition is considered as isolated due to its high frequency relative to the additional molecular vibrations and coupling to these vibrations is therefore neglected. The LM model is applied to ensure that the peaks observed in our FTIR spectra are correctly assigned. The absolute intensity of the XH-stretching transition is proportional to the integrated absorbance of the XH-stretching peak and inversely proportional to the pressure of the molecule. Absolute intensities of XH-stretching vibrations in molecules can therefore be determined experimentally by recording gas phase FTIR spectra.

Hydrogen-bonded complexes

Hydrogen bonded complex with ethanol as the acceptor and pyridine as the donor molecule.

Hydrogen bonded complex with ethanol as the acceptor molecule and pyridine as the donor molecule.

Networks of hydrogen-bonded molecules are found in molecular clusters in the atmosphere and the thermodynamic stability of the clusters is determined by the strength of the formed hydrogen bonds. The molecular clusters are precursors to aerosols, which have a huge impact on our climate due to their ability to absorb and scatter solar radiation. Aerosol particles remain large sources of uncertainty in climate models today and determining the hydrogen bonding abilities of a range of different molecules will help limit these uncertainties and thereby improve current climate models. 

The complex of t-butanol (tBuOH) and dimethylsulfide (DMS) is detected by subtracting the monomer spectra (green and red) from a mixture spectrum (blue), resulting in a spectrum of the complex (orange).

In order to achieve a better understanding of the formation of clusters in the atmosphere, the QSD group have investigated a number of different hydrogen bonded complexes in the gas phase and determined the equilibrium constant of their formation to help predict if they play a significant role in cluster formation and growth in the atmosphere. In the group we use conventional absorption spectroscopy as well as matrix isolation spectroscopy to examine the complexes as well as theoretical investigations using our local mode model to predict wavenumbers and oscillator strengths of molecular vibrations.

We use conventional FTIR spectroscopy to examine hydrogen bonded complexes as well as matrix isolation spectroscopy, which is helpful when detecting very unstable complexes.

Kinetics

Autoxidation in the atmosphere (Multi-conformer transition state theory)

Reduced carbon species are emitted to the atmosphere from biogenic and anthropogenic sources in huge quantities. These are oxidized in the atmosphere by various different processes such as autoxidation reactions (repeated addition of O2 and H-shifts to a molecule), and the oxidized products have a much lower volatility than the reduced species. Low-volatility compounds play a significant role in the growth of secondary organic aerosols (SOA). SOA affect the radiative balance of the earth, yet quantitative assessment of the effect of SOA on the climate remains a challenge.

In the QSD group we calculate rate constants of various different radical hydrogen shift reactions in order to determine the most likely products, which are formed from the autoxidation of different reduced carbon species. This contributes to understanding of the processes which happen in the atmosphere and the way that the formed species affect the radiative balance of the earth. 

The autoxidation of 3-pentanone.

One of the processes we have studied, is the autoxidation of 3-pentanone (shown in the picture above) with the final reaction product being 4- hydroperoxypentane-2,3-dione. Addition of O2 to radical species happens very fast, making the radical hydrogen shift reaction the ones that determine the overall rate of the autoxidation. In order to determine the rate constants of the hydrogen shift reactions we have developed a cost-effective approach to the implementation of multi conformer transition state theory (MC-TST), which includes tunneling.