Experimental and computational thermochemistry of 3- and 4-nitrophthalic acids

Highlights

• Experimental and theoretical thermochemistry of the 3- and 4-nitrophthalic acids.

• Enthalpies of formation in gaseous and crystalline phases of both acids.

• Isomerization enthalpy of this acids and comparison with their similar anhydrides.

• Boltzmann averaging procedure was used to obtain conformers of both acids.

Abstract

In this work, we present experimental and theoretical thermochemistry of the 3- and 4-nitrophtalic acids. We report their standard molar enthalpies of formation in crystalline phase, at T = 298.15 K, which were obtained from their energies of combustion. The latter were determined through an isoperibolic bomb calorimeter. We measured the change-phase enthalpies, as well as the fusion and sublimation enthalpies, by differential scanning calorimetry (DSC) and thermogravimetry (TG). Also via DSC, we found the heat capacity equations (in crystalline phase) for both isomers as functions of temperature. We determined the enthalpies of formation in both crystal- and gas-phase from the previous measurements. In addition, we computed the enthalpies of formation in gas-phase of both isomers using the composite Gaussian-G4 method and atomization reactions. The final theoretical enthalpies were computed using a weighted Boltzmann averaging procedure, and the weights were estimated using the Gibbs free energy at T = 298.15 K. The absolute differences between theoretical and experimental enthalpies are below 3.3 $\text{kJ}·\text{mo}{\text{l}}^{\text{−1}}$. Finally, we apply our results for obtaining the isomerization enthalpy of 3- to 4-nitrophthalic acid, and difference enthalpies between the nitrophthalic acids and their anhydrides.

Introduction

3- and 4- Nitrophthalic acids are commonly used in the production chains of a wide variety of commercial products such as pigments, dyes, plasticizers, etc. [1]. They are also used as precursors of 4- and 5-nitro-2-benzofuran-1,3-dione, which in turn are used as intermediate reactants in polypeptide synthesis [2]. The nitrophthalic acids constitute some of the basic synthetic blocks for producing polyamides and coordination polymers [3], [4]. Furthermore, they present catalytic properties [5], and act as ligands in metallic coordination complexes [6]. 4-nitrophthalic acid forms proton-transfer compounds [7]. More generally, the acids show potential applications in the synthesis of compounds with interesting magnetic [8], optoelectronic [9], and photoluminescence properties [10]. Coordination and nano-chemistry are other fields wherein these nitrophthalic acid isomers present promising applications [11], [12]. This stems from the acids’ tendency to form strong and flexible hydrogen bonds, both of intra- and inter-molecular type, between different combinations of the oxygens and hydrogens of the two carboxylic groups, and in conjunction with the electronic effects induced by the nitrobenzene group (which may also participate in hydrogen bonding through the –NO₂ functional group) [11], [12]. This provides building blocks for designing supramolecular structures and molecular architectures with special properties [13], [14], [15]. The relative position of the nitro group allows an even bigger number of structural variants [13], [14], [15].

Closer to our group interests, the 3- and 4-nitrophthalic acids can be obtained from 3- and 4-nitrophthalic anhydrides [16], respectively, whereof we have reported their thermochemical properties [17]. In this work, we report our experimental and theoretical procedures to determine the enthalpies of formation of the 3- and 4-nitrophthalic acids (denoted hereafter as 3NFAc and 4NFAc, respectively, and as NFAcs to refer to both isomers taken as a set, Fig. 1), in order to obtain the enthalpies for the anhydride-acid interconversion reactions.

The NFAcs standard molar energies of combustion and standard molar enthalpies of sublimation, at T = 298.15 K, were measured with a static bomb combustion calorimeter, and a thermogravimetry analyser, respectively. Subsequently, from these quantities we calculated the NFAcs standard molar enthalpies of formation in gas-phase, at T = 298.15 K.

Complementary to the experimental determinations, we also report the NFAcs standard molar enthalpies of formation in gas-phase, calculated with the Gaussian G4 composite method combined with atomization reactions and a Boltzmann averaging procedure. We found nine and ten stable conformers, which exclude specular images, for 3NFAc and 4NFAc, respectively.

Both our experimental determinations and theoretical analysis improve the understanding of the NFAcs thermochemical properties.