Gibbs energy of complex formation – combining infrared spectroscopy and vibrational theory

ABSTRACT Formation and growth of atmospheric aerosols are governed by the Gibbs energy of complex formation (). A number of hydrogen bound bimolecular complexes in the gas phase at room temperature have been detected. In this review, we illustrate how can be determined by combining gas phase infrared spectroscopy and vibrational theory. The XH-stretching (where X is a heavy atom like O) fundamental transition of the hydrogen bond donor molecule in the complex is redshifted and its intensity enhanced upon complexation. This facilitates detection of weak complexes even though the equilibrium is shifted towards the monomers at room temperature. The ratio of the measured and calculated intensity of the vibrational transition is proportional to the complex abundance, which with known monomer pressures gives the equilibrium constant and thus . This approach relies on calculated vibrational transitions in the complexes. An accurate description of the observed bound XH-stretching fundamental transition is challenging due to effects of the low-frequency intermolecular modes. We have developed reduced dimensionality vibrational models within the local mode picture to calculate accurate vibrational intensities. For complexes with an alcohol donor molecule, we find that P, O or S as the acceptor atom of the hydrogen bond results in very similar hydrogen bond strength, whereas N provides a significantly stronger bond.