Thermochemical properties of 4-N,N-dialkylamino-7-nitrobenzofurazan derivatives (alkyl = methyl, ethyl)

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Highlights

Abstract

The standard (p° = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, in the gaseous phase, for two nitrobenzofurazan derivatives, 4-N,N-dimethylamino-7-nitrobenzofurazan (DMANBF) and 4-N,N-diethylamino-7-nitrobenzofurazan (DEANBF), were derived from their enthalpies of combustion and sublimation, obtained by static bomb calorimetry and by the Knudsen effusion technique, respectively. The results are compared with the corresponding data calculated by the G3(MP2)//B3LYP approach. Computationally, the molecular structures of both compounds were established and the geometrical parameters were determined at the B3LYP/6-31G(d) level of theory.

Introduction

During the last two decades, our Research Group has been involved in a systematic study of the dissociation enthalpy of terminal N–O bonds in heterocyclic compounds [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12].These studies are a contribution to the understanding of the chemical behavior of those organic compounds, particularly as oxidizing agents, enabling the ordering of them on a reactivity scale [1]. Several classes of compounds with N–O terminal bonds have been studied, with particular attention devoted to quinoxaline N,N-dioxides [2], [3], [4], [5], [6], [7], pyrazine N,N-dioxides, [1], [8], [9], phenazine N,N-dioxides [10], benzofurazan N-oxides [1], pyridine N-oxides [11] and dipyridil N-oxides [12]. We have extended recently these studies to 4-nitro-2,1,3-benzothiadiazoles [13] and Miranda et al. also have published studies on the structure, energetics and aromaticity of 2,1,3-benzothiadiazole [14], [15]. In this context, we report herein a new contribution for the energetic characterization of two nitrobenzofurazan derivatives, namely 4-N,N-dimethylamino-7-nitrobenzofurazan (DMANBF) and 4-N,N-diethylamino-7-nitrobenzofurazan (DEANBF), whose structural formula are represented in figure 1.

The enthalpies of formation of these two crystalline compounds and their enthalpies of sublimation, at T = 298.15 K, have been derived from static bomb calorimetry and from Knudsen effusion method, respectively. From these results, the standard (p° = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, were derived, and were compared with estimated data, calculated by the G3(MP2)//B3LYP approach. Computationally, the molecular structures of both compounds were established and the geometrical parameters were determined at the B3LYP/6-31G(d) level of theory.

Section snippets

Synthesis and characterization of compounds

The 4-N,N-dimethylamino-7-nitrobenzofurazan (DMANBF; CAS 1455-87-4) was prepared by a published literature method [16], [17]. The synthetic procedure involved treating an ethanolic solution of 4-chloro-7-nitrobenzofurazan with aqueous dimethylamine (40% by mass). The crude product was collected by filtration under reduced pressure, and was further purified by two crystallizations from propanone. The 4-N,N-diethylamino-7-nitrobenzofurazan (DEANBF; CAS 74392-01-1) was prepared in similar fashion

Computational thermochemistry

The standard ab initio molecular orbital calculations for the 4-N,N-dimethylamino-7-nitrobenzofurazan and 4-N,N-diethylamino-7-nitrobenzofurazan were performed with Gaussian 03 computer code [28], and the composite G3(MP2)//B3LYP approach was the methodology employed [29]. In this method, the geometry full-optimization and calculation of the frequencies of the molecule are done through the hybrid B3LYP method together with the split-valence polarized 6-31G(d) basis set. The zero-point energies

Experimental condensed phase and phase transition

Results of a typical experiment for each compound are presented in table 2; Δm(H2O) is the deviation of the mass of water added to the calorimeter from 2900.0 g; ΔU is the energy correction to the standard state; ΔU(IBP) is the internal energy for the isothermal bomb process calculated according to equation (8). The remaining terms have been previously described [23]ΔU(IBP)=-{εcal+Cp(H2O,l)·Δm(H2O)+εf}ΔTad+ΔU(ign).

The standard massic energies of combustion, Δcu°, for 4-N,N

Final remarks

In the present work, the standard molar enthalpies of formation, in the gaseous phase, for 4-N,N-dimethylamino-7-nitrobenzofurazan and 4-N,N-diethylamino-7-nitrobenzofurazan were determined by experimental (static bomb combustion calorimetry and Knudsen effusion technique) and computational methods (G3(MP2)//B3LYP approach). The experimental values obtained were, respectively, (303.2 ± 4.7) and (272.0 ± 5.2) kJ · mol−1, and using different working reactions, a good agreement with the calculated values

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    Present address: Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India.

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