Thermodynamic factors limiting the preservation of aromaticity of adsorbed organic compounds on Si(100): Example of the pyridine

Abstract

Using pyridine as an example, a thermodynamic analysis of the low temperatures adsorption of aromatic organic molecules with a N atom on the Si(100) surface is presented. This study is restricted to the case of an equilibrium with the gas phase. Dative attachment which is the only way to preserve aromaticity is the more stable form of adsorbed pyridine in dilute solutions at low temperatures. Two factors limit the domain of stability of dative attachment: repulsive interactions between dative bonds prevent them from being present in concentrated solutions while aromaticity contributes to a decrease in the entropy, which explains the vanishing of dative bonds at high temperatures even in dilute solutions.

A Possible Application of the Contribution of Aromaticity to Entropy: Thermal Switch

“…An example of this universal property is the adsorption of pyridine on Si(100): experimental results [5,6] showed that dative attachment which is the only way to preserve aromaticity is the favored form of adsorbed pyridine in dilute solutions at low temperatures. Recent theoretical analysis [7][8][9][10] confirmed that dative configuration is the more stable configuration. Two factors limit the domain of stability of dative attachment: repulsive interactions between dative bonds prevent them from being present in concentrated solutions while aromaticity contributes to a decrease in the entropy.…”

“…When heated, aromatic grafted species (dative bonded) are converted to non-aromatic species which explains the vanishing of dative bonds at high temperatures even in dilute solutions as experimentally observed [5,6]. Since aromatic configuration is favored by lower enthalpy, this conversion appears driven by the entropic gain resulting from the vanishing of aromaticity [7,8] justifying here the deliberate use of possible negative contribution of aromaticity to entropy.…”

“…ΔS was assumed to be close to the contribution of aromaticity to entropy deduced for pyridine from thermodynamical measurements [7,8].…”

“…In order to discuss the relevance of this approach, let us use the results of earlier studies [7,8] for pyridine adsorbed on Si(100) and Ge(100), even if these materials are certainly not the easiest ones to handle. Of particular interest for our purpose is the spectroscopic study associated to DFT calculations of gold-molecule-gold junctions made by Yoshida et al [14].…”

“…We assume the difference in entropy between aromatic (dative) and non-aromatic (tetrapod) configurations to be the one evaluated for pyridine in our earlier study [7,8] i.e., 17 cal·mol −1 ·K −1 .…”