Pairwise Substitution Effects, Inter- and Intramolecular Hydrogen Bonds in Methoxyphenols and Dimethoxybenzenes. Thermochemistry, Calorimetry, and First-Principles Calculations

Abstract

Methoxyphenols are the structural fragments of different antioxidants and biologically active molecules, which are able to form strong intermolecular and intramolecular hydrogen bonds in condensed matter. In the present work, thermochemical, Fourier transform infrared (FTIR)-spectroscopic and quantum-chemical studies of methoxyphenols and its H-bonded complexes in solution and gas phase have been carried out. Thermodynamic properties (standard molar enthalpies of formation, vapor pressure, vaporization enthalpies, sublimation enthalpies, and fusion enthalpies) of 2-methoxyphenol, 3-methoxyphenol, 4-methoxyphenol, 1,2-dimethoxybenzene, 1,3-dimethoxybenzene, and 1,4-dimethoxybenzene have been studied in this work. To verify the experimental data, ab initio calculations of all compounds have been performed using density functional theory (DFT), MP2, and G3 methods. The quantitative analysis of ortho, meta, and para pairwise-substituent effects in methoxyphenols has been performed. Solution enthalpies of methoxyphenols at infinite dilution in proton acceptor solvents have been measured. Calorimetric data shows that intermolecular hydrogen bond strength in complexes of 2-methoxyphenol with organic bases is less than that for 4-methoxyphenol. Two experimental approaches for determination of enthalpy of intramolecular hydrogen bonds in ortho-methoxyphenols were proposed. The new results help to resolve uncertainties in the available thermochemical data on methoxyphenols and dimethoxybenzenes and to realize relations among properties and structures for these compounds.

Secondary organic aerosol formation from biomass burning intermediates: phenol and methoxyphenols

“…Carboxylic acids believed to be ring fragments, as well as muconic acid, a ring-opened C 6 dicarboxylic acid, were also identified. Muconic acid was also identified in the particle phase from benzenediol ozonolysis (Tomas et al, 2003). These results support the current observations that many dicarbonyl species in general trend with the SOA mass, suggesting that they may be semi-volatile in nature.…”

“…The following discussion is based on the work presented in Varfolomeev et al (2010) that examines pairwise substitution effects, and inter-and intramolecular hydrogen bonds in methoxyphenols and dimethoxybenzenes. Varfolomeev et al (2010) show that intramolecular hydrogen bonding plays a significant role on ortho-methoxy substituted phenols. This leads to o-methoxyphenols having a lower phenolic O−H bond dissociation energy (BDE) compared to meta isomers and higher than para isomers.…”

“…This means that the phenolic O−H BDE increases in order the order 4-methoxyphenol < 2-methoxyphenol < 3-methoxyphenol. From the ab initio calculations of Varfolomeev et al (2010), the enthalpy of formation for 2-methoxyphenol (guaiacol) is expressed below:…”

“…The pairwise effect of (ortho-OH-OH) was not calculated by Varfolomeev et al (2010), but assuming that it is less sterically hindered compared to (ortho-OH-OCH 3 ) and is stabilized by intramolecular hydrogen bonding similar to 1,2-dimethoxybenzene, we assume it to be < 4.5 kJ mol −1 . Due to the combined favorability of energetics for the substituent effects and the pairwise effects, this can help explain why some of the syringol rapidly converts to hydroxyguaiacol and even trihydroxybenzene.…”

“…Often these include candidate compounds such as phenol and catechol (Atkinson et al, 1992;Olariu, 2001;Olariu et al, 2002;Berndt and Boge, 2003;Tomas et al, 2003;Coeur-Tourneur et al, 2009;Iinuma et al, 2010;Nakao et al, 2011;Ofner et al, 2011). Many of these compounds can also serve as model systems for understanding aerosol formation from humic substances, the role of water, and associated heterogenous processes (Ammann et al, 2005;Nieto-Gligorovski et al, 2008;Ofner et al, 2010Ofner et al, , 2011Sun et al, 2010).…”