Attenuated Deuterium Stabilization of Hydrogen-Bound Complexes at Room Temperature

We have observed nine bimolecular hydrogen- or deuterium-bound complexes at room temperature using Fourier transform infrared (FTIR) spectroscopy. The complexes were formed using methanol or ethanol as hydrogen bond donors, as well as deuterated isotopologues of these, in order to study isotopic effects on hydrogen bonds. The complexes were formed using either a dimethylether- (O) or trimethylamine (N) acceptor, to facilitate comparison of two different types of hydrogen or deuterium bonds, OH(D)·O and OH(D)·N. For each complex, the characteristic OH- or OD-stretching fundamental band in the bimolecular complex was observed. The Gibbs energy of complex formation was determined at room temperature for each complex to compare the relative stability of hydrogen- and deuterium-bound bimolecular complexes. It is well known that deuterium-bound complexes are more stable at low temperatures because of the lower frequency of its intermolecular modes and thus a lower zero point vibrational energy. However, at room temperature, entropic contributions to the stability should also be considered. At room temperature, we find the Gibbs energy of complex formation for each pair of corresponding hydrogen- and deuterium-bound complex to be similar. The similar values of the Gibbs energies at room temperature is explained from a difference in the entropy, upon complexation, which favors the formation of the hydrogen-bound complex more than the deuterium-bound complex at higher temperatures.