Experimental and Computational Thermodynamic Study of Three Monofluoronitrobenzene Isomers

Abstract

The present work reports the thermodynamic study performed on three monofluorinated nitrobenzene derivatives by a combination of experimental techniques and computational approaches. The standard (p degrees = 0.1 MPa) molar enthalpies of formation in the liquid phase of the three isomers of fluoronitrobenzene were derived from the standard molar energies of combustion, in oxygen, at T = 298.15 K, measured by rotating bomb combustion calorimetry. The vapor pressure study of the referred compounds was done by a static method and, from the obtained results, the phase diagrams were elaborated, and the respective triple point coordinates, as well as the standard molar enthalpies of vaporization, sublimation and fusion, at T = 298.15 K, were determined. The combination of some of the referred thermodynamic parameters yielded the standard (p degrees = 0.1 MPa) molar enthalpies of formation in the gaseous phase, at T = 298.15 K, of the studied compounds: Delta(f)H(m)(o) (2-fluoronitrobenzene, g) = -(102.4 +/- 1.5) kJ x mol(-1), Delta(f)H(m)(o) (3-fluoronitrobenzene, g) = -(128.0 +/- 1.7) kJ x mol(-1), and Delta(f)H(m)(o) (4-fluoronitrobenzene, g) = -(133.9 +/- 1.4) kJ x mol(-1). Using the empirical scheme developed by Cox, values of standard molar enthalpies of formation in the gaseous phase were estimated and afterwards compared with the ones obtained experimentally, and both were interpreted in terms of the molecular structure of the compounds. The theoretically estimated gas-phase enthalpies of formation were calculated from high-level ab initio molecular orbital calculations at the G3(MP2)//B3LYP level of theory. The computed values compare very well with the experimental results obtained in this work and show that 4-fluoronitrobenzene is the most stable isomer from the thermodynamic point of view. Furthermore, this composite approach was also used to obtain information about the gas-phase basicities, proton and electron affinities and, finally, adiabatic ionization enthalpies.

A Combined Experimental and Computational Thermodynamic Study of Difluoronitrobenzene Isomers

“…32 In previous studies made in our research group, it was established that the correction of +4 kJ · mol -1 suggested by Cox is not sufficient to account for the destabilization effects observed as a result of the steric interaction between a halogen atom and the nitro group as ortho substituents in a benzene ring and that a larger enthalpic correction is needed. Ribeiro da Silva et al 25 suggested an additional correction term of ∼+22 kJ · mol -1 in this case, in addition to the +4 kJ · mol -1 correction recommended by Cox.…”

“…Using the methodology developed by Cox, it was possible to estimate ∆ f H m°( g) for the difluoronitrobenzene isomers according to the four different approaches, presented in Figure 6, taking into account the following literature values of the gasphase standard molar enthalpies of formation: ∆ f H m°( benzene,g) )(82.6 ( 0.7)kJ · mol -1 , 66 ∆ f H m°( fluorobenzene,g))-(115.9 ( 1.4) kJ · mol -1 , 66 ∆ f H m°( nitrobenzene,g) ) (67.5 ( 0.5) kJ · mol -1 , 66 ∆ f H m°( o-difluorobenzene,g) ) -(293.7 ( 0.9) kJ · mol -1 , 66 32 and ∆ f H m°( p-fluoronitrobenzene,g) ) -(133.9 ( 1.4) kJ · mol -1 . 32 In previous studies made in our research group, it was established that the correction of +4 kJ · mol -1 suggested by Cox is not sufficient to account for the destabilization effects observed as a result of the steric interaction between a halogen atom and the nitro group as ortho substituents in a benzene ring and that a larger enthalpic correction is needed.…”

“…This correction has been proven adequate, when applied to other compounds exhibiting the same substitution pattern. [26][27][28][29]32 Taking into consideration all the necessary corrections, the estimated values were calculated for each of the proposed approaches, as presented in Figure 6. The results are compiled in Table 10 along with the experimental results and the deviation ∆ between them.…”

Standard molar enthalpies of formation of monochloroacetophenone isomers

“…The correction term of 4 kJ Á mol À1 suggested by Cox [37] due to steric destabilization, whenever two substituents are attached in adjacent carbon atoms of the ring, need to be further evaluated, since a few cases are already known in the literature in which it is necessary to include a higher value of enthalpic correction of destabilization. In particular, Ribeiro da Silva et al [4,[40][41][42][43][44] found that a large additional correction is necessary to account for the interaction between a halogen atom and the nitro group as a ortho pair of substituents in a benzenic ring.…”

Standard molar enthalpies of formation of 3′- and 4′-nitroacetophenones

“…A large number of nitrobenzene derivatives have been studied in our Research Group [17][18][19][20][21][22][23][24][25][26][27][28][29]. Recently we turned our attention on acetophenone derivatives having already studied some monoand dichloroacetophenone isomers [30,31].…”