Local Modes in H-Bonded Complexes
H. G. Kjaergaard
Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen Ø, Danemark.
Mardi 27 Janvier 2015, 11h00
bibliothèque LCT, tour 12 - 13, 4e étage
The interest in complex formation is multifold in atmospheric research, for example, in radiative transfer, reaction mechanisms and nucleation. Central to all these is the formation of a molecular complex in the first place. We have recently detected a series of binary molecular complexes that all contain an X-H---Y hydrogen bond, where X is O, N, or Cl and Y is O, S, or N. Using infrared spectroscopy, we have measured the fundamental XH-stretching transition for these complexes. By combining the measured intensity of this transition with calculated intensities, we determine the abundance of the complex and thereby the equilibrium constant (Kp) or ΔG for the complex formation.[1-3] For one complex, methanol-dimethylamine (MeOH-DMA), we have in addition to recording the OH-stretching fundamental transition also been able to observe overtone transitions.[3] To calculate intensities of overtones it is imperative that anharmonicity is included. Clearly the accuracy of this combined experimental and theoretical approach to obtain ΔG, depends on the accuracy of the calculated transitions intensity.
We determine the intensity of the XH-stretching transition using a 1D local mode model and high level ab initio calculations to determine the local mode parameters and the dipole moment curve. For water dimer (H2O-H2O), comparison of a 1D local mode model, with a 3D local mode model (for each water unit), VPT2 calculations and experimental measurements, revealed that the reduced dimensionality local mode models overestimates the intensity of the fundamental OH-stretching transitions by a factor of ca 2. In comparison, an harmonic oscillator normal mode calculation (as typically used in standard ab initio programs) also overestimates the fundamental intensity by about a factor of two.[4] VPT2 calculations are difficult for complexes, due to the low frequency modes, and become very time consuming for larger systems.
As an alternative and to obtain better agreement with experiment, we have developed a model based on the local mode model (LMPT) but with the effect of coupling to the other vibrational modes included via a second order perturbation treatment (LMPT).[5] For water dimer, we have included coupling to the six intermolecular low frequency modes and find that this significantly improve agreement with experiment.
[1] L. Du, H. G. Kjaergaard, J. Phys. Chem. A 115, 12097 (2011).
[2] L. Du, J. R. Lane, H. G. Kjaergaard, J. Chem. Phys. 136, 184305 (2012).
[3] L. Du, K. Mackeprang, H. G. Kjaergaard, Phys. Chem. Chem. Phys. 15, 10194 (2013).
[4] H. G. Kjaergaard, A. L. Garden, G. M. Chaban, R. B. Gerber, D. A. Matthews, J. F. Stanton, J. Phys. Chem. A 112, 4324 (2008).
[5] K. Mackeprang, H. G. Kjaergaard, T. Salmi, V. Hänninen, L. Halonen, J. Chem. Phys. 140, 184309 (2014).