Experimental and Computational Thermochemical Study of the Three Monofluorophenol Isomers

Abstract

The present work reports the values of the standard (p o ) 0.1 MPa) molar enthalpies of formation in the condensed phase of the three isomers of monofluorophenol derived from the standard molar energies of combustion, in oxygen, to yield CO 2 (g) and HF • 10H 2 O(l), at T ) 298.15 K, measured by rotating bomb combustion calorimetry, as well as the values of the standard molar enthalpies of sublimation or vaporization, at T ) 298.15 K, determined using high temperature Calvet microcalorimetry. Combining the former two experimental quantities, the standard molar enthalpies of formation in the gaseous phase were derived, at T ) 298.15 K:The experimental values of the gas-phase enthalpies of formation of each compound were compared with estimates using the empirical scheme developed by Cox and with the calculated values based on high-level density functional theory calculations using the B3LYP hybrid exchange-correlation energy functional at the 6-311++G(d,p) basis set.

Calorimetric and computational thermochemical study of difluorophenol isomers

“…The computed results are in agreement with a planar substituent arrangement in the molecules of difluorophenols as observed previously for the monofluorophenol isomers [47], as well as for the parent compound [55], with the hydroxyl group lying coplanar to the plane of the phenyl ring, occurring the conjugation of the oxygen's lone pairs with the p-electron of the benzene ring.…”

“…This feature, as previously mentioned, has also been observed earlier in case of 2-fluorophenol [7,47,56], 2,4-difluorophenol [14], and 2,6-difluorophenol [11].…”

“…The poor agreement between experimental and estimated values of D f H m ðgÞ for the 2,4-and 3,4-difluorophenol compounds seems possible due to the fact that the Cox method considers a higher energetic stabilization effect caused by the introduction of a fluorine atom in the para position relatively to the hydroxyl group in the benzene ring than experimentally observed. A similar fact was already observed in the case of 4-fluorophenol, for which the D f H m ðgÞ estimated with this empirical scheme was Àð294:9 AE 1:8Þ kJ Á mol À1 and the experimental value was Àð287:6 AE 2:1Þ kJ Á mol À1 , with D ¼ 7:3 AE 2:8 kJÁ mol À1 [47].…”

“…Zero-point vibrational energies and fundamental vibrational frequencies were scaled by a factor of 0.9887 and 0.9688, respectively [53]. The above-mentioned exchangecorrelation functional was chosen since in a previous work devoted to the thermochemistry of fluorine substituted phenols [47] and anisoles [54] it yielded accurate estimates of gas-phase enthalpies of formation.…”

Experimental standard molar enthalpies of formation of some methylbenzenediol isomers

“…Among these compounds, special interest has been dedicated to phenolic type compounds. Our Research Group has been carrying out an extensive work on the energetics of substituted phenols for which we already reported our studies on alkylsubstituted phenols [1,2], aminophenols [3,4], 3-nitrophenol [5], fluorophenols [6], chlorophenols [7,8], bromophenols [9], cyanophenol [10], thiophenols [10,11], and alkylsubstituted catechols (1,2-benzenediol) [12].…”