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