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University of Helsinki Faculty of Science

Molecular spectroscopy and theoretical chemistry


Contact Information

Laboratory of Physical Chemistry
Department of Chemistry
A.I. Virtasen aukio 1
(P.O. BOX 55)
FI-00014 University of Helsinki

Group leader
Prof. Lauri Halonen, D.Sc. CV
Phone: +358(0)2941 50280
email: lauri.halonen


Computational surface science

Proton configurational disorder in ice and ice-like systems

Fig. 1. Two energetically low-lying morphologies of (H2O)20 generated by our program.

Recently, there has been a lot of interest to computationally study ice and ice-like systems. Creating atomic level models is crucial in understanding such systems. However, a huge number of isomers of for ice formation make this a challenging problem. We have studied the proton configurational disorder in ice and ice-like systems computationally using quantum chemical ab initio methods [1, 2]. In addition, a computer program is developed to aid in the generation and analysis of proton configurations [3].

[1] P. Parkkinen, S. Riikonen, and L. Halonen, (H2O)20 water clusters at finite temperatures, J. Phys. Chem. A 117, 9985-9998 (2013).
[2] P. Parkkinen, S. Riikonen, and L. Halonen, Ice XI: Not that ferroelectric, J. Phys. Chem. C 118, 26264-26275 (2014).
[3] P. Parkkinen, S. Riikonen, and L. Halonen, Configurational entropy in ice nanosystems: Tools for structure generation and screening, J. Chem. Theory Comput. 10, 1256-1264 (2014).

Acid ionization on ice and mineral surfaces and atmospheric implications

Fig. 1. Mechanism of acid ionization of an acid HM (M is the base of the acid) on an aqueous surface, viewed from above. Inserts of panels (a-d) give a schematic view of H-bonding near the contact ion pair, where arrows denote direction of H-bond donation. (+) depicts the hydronium ion site. Panels (e-g) demonstrate hydronium migration on the ice basal plane. Two H-bonds are denoted by (1) and (2). H-bonds with their directions reversed during migration are highlighted in green [1,2,5].

Ice and mineral surfaces are ubiquitous in the natural environment. Ice surfaces are covered by a thin, quasi-liquid layer (QLL), with properties intermediate between ice and water. Mineral surfaces such as dust or more extended surfaces such as urban surfaces are wetted with a water layer since water is always present.

Fig. 2. Snapshots from 250 K trajectory displaying steps in H2SO4 ionization on hydroxylated (0001) alpha-quartz with water monolayer: (a) model system, (b-d) uppermost portion [4,5].

If the exposed mineral surface is crystalline, the adsorbed water may be ordered and form an ice-like layer. Heterogeneous chemistry on ice surfaces has important implications for the atmosphere (e.g., presence of ozone holes in high latitude regions). We have studied with accurate ab initio methods and realistic models acid dissociation and proton migration on ice [1], its QLL surface [2], and on wet quartz [3, 4]. Such processes enhance surface pickup of additional adsorbates and surface reactivity. An understanding of these fundamental chemical processes is critical for the atmospheric and environmental sciences. Our work in this area has recently been reviewed in a high-impact journal [5].

[1] S. Riikonen. P. Parkkinen, L. Halonen, and R. B. Gerber, Ionization of nitric acid on crystalline ice: The role of defects and collective proton movement, J. Phys. Chem. Lett. 4, 1850-1855 (2013).
[2] S. Riikonen. P. Parkkinen, L. Halonen, and R. B. Gerber, Ionization of acids on the quasi-liquid layer of ice, J. Phys. Chem. A 118, 5029-5037 (2014).
[3] G. Murdachaew, M.-P. Gaigeot, L. Halonen, and R. B. Gerber, Dissociation of HCl into ions on wet hydroxylated (0001) alpha-quartz, J. Phys. Chem. Lett. 4, 3500-3507 (2013).
[4] G. Murdachaew, M.-P. Gaigeot, L. Halonen, and R. B. Gerber, First and second deprotonation of H2SO4 on wet hydroxylated (0001) alpha-quartz, Phys. Chem. Chem. Phys., 16, 22287-22298 (2014).
[5] R. B. Gerber, M. E. Varner, A. D. Hammerich, S. Riikonen, G. Murdachaew, D. Shemesh, and B. J. Finlayson-Pitts, Computational studies of atmospherically-relevant chemical reactions in water clusters and on liquid water and ice surfaces, Acc. Chem. Res., Article ASAP, (2015).

Molecules Adsorbed on Surfaces

Fig. 1. The ammonia molecule adsorbed on a Ni(111) surface.

Adsorption of molecules on surfaces induces changes in the vibrational spectra. The systems of water and ammonia adsorbed on metal surfaces have been studied using quantum chemical methods [1, 2]. Density functional theory has been used to compute potential energy surfaces. The anharmonic vibrational Hamiltonians have been obtained by combining the exact gas phase kinetic energy operators with the computed potential energy surfaces. The vibrational energy levels have been calculated variationally.

[1] E. Sälli, V. Hänninen, and L. Halonen, Variationally calculated vibrational energy levels of ammonia adsorbed on a Ni(111) surface, J. Phys. Chem. C 114, 4550-4556 (2010).
[2] E. Sälli, S. Martiskainen, and L. Halonen, Computational study of the vibrational structure of the ammonia molecule adsorbed on the fcc(111) transition metal surfaces, J. Phys. Chem. C 116, 14960-14969 (2012).