Thermodynamic study on six tricyclic nitrogen heterocyclic compounds by thermal analysis and effusion techniques
Introduction
This study is focused on compiling new thermodynamic data on sublimation and vaporization of six tricyclic nitrogen heterocyclic compounds. In this series, two compounds have a linear aromatic ring structure analogous to anthracene with one or two nitrogen atoms (acridine and phenazine), while four compounds have three fused benzene rings, like those of phenanthrene or naphthalene, with one nitrogen atom (phenanthridine) or two, in different positions of the two external rings 1,7-, 1,10- and 4,7-phenanthroline (Fig. 1). Compounds belonging to this class received increasing attention in the last years due to their properties, being probably the most important one its intercalating ability due to the presence of lone pair(s) on nitrogen atom(s) [1].
Acridine and its derivatives (i.e., the acridine-3,6-diamine, or its N,N,N′,N′ tetramethyl derivative, known as proflavin and acridine orange respectively) are disinfectant bacteriostatics against gram-positive bacteria or may be used as chemotherapeutic agents because of their ability to intercalate DNA, disrupt its synthesis and inhibit topoisomerase enzymes [2]. Similarly, several phenazine compounds found in nature and produced by pseudomonas, streptomyces, and pantoea agglomerans bacteria are biologically active [3]. Furthermore, preparation and use of acridine as a chemical intermediate, in the manufacture of dyes and in the synthesis of drugs may result in its release into the environment through various waste streams. Acridine’s emissions are mainly originated by diesel exhaust, coal-burning effluent from residential furnaces, catalyst regeneration flue gas from a gas-oil stock of an oil refinery, and from coal tar and coke-oven. If released into the atmosphere, acridine will exist in the ambient atmosphere in both vapor and particulate phases due to its not negligible extrapolated vapor pressure (0.018 Pa at 298.15 [4]). Phenanthridine is an isomer of acridine, whose importance is due to the fact that it is the basis of DNA-binding fluorescent dyes through intercalation [5]. 1,10-phenanthroline (commonly known simply as phenanthroline) like their isomers 1,7- and 4,7-phenanthroline (all three compounds considered in this study) are extensively used as bidentate chelating ligand in coordination chemistry, due to their ability to form strong complexes with transition metal ions. Ferroin, which is the dication complex [Fe(phen)3]2+ (being phen = 1,10-phenanthroline) is commonly used for the photometric determination of Fe(II).
So, in order to better understand the environmental risks due to their possible presence in the atmosphere the thermodynamic properties of these six tricyclic nitrogen heterocyclic compounds, with particular reference to vapor pressure and enthalpy related to sublimation and vaporization, should be known. On the other hand, a reliable determination of vapor pressure can be done by performing experiments using two or even more different techniques in suitable temperature ranges above the same phase or on two different phases (i.e., above the solid and the liquid), like our group already made in the recent past [6], [7] and in the present study. In this context, the aim of this investigation is to present our contribution for providing recommended standard sublimation enthalpies and entropies of these six compounds adjusted at 298.15 K, once the sublimation and vaporization enthalpies were determined at the averages of their respective experimental temperature ranges from the temperature dependencies of vapor pressure. These trends were determined using both effusion (Knudsen Effusion Mass loss, and Torsion Effusion, KEML and TE, respectively) and Isothermal Thermogravimetry (ITG). Effusion experiments were performed above the solids (in the range T < Tm, where Tm is the melting temperature extrapolated from the corresponding onset DSC peak), while for ITG experiments the temperatures were selected above the liquid phase in the range T > Tm. Comparing the results obtained using the above-mentioned two different approaches (in particular the intersection of both the solid and liquid lines at T = Tm) the internal consistency of the obtained vapor pressure data was checked.
Section snippets
Chemicals
All substances were purchased by Aldrich and needed a further purification by vacuum sublimation. Detailed information concerning molecular formulas, molar masses, CAS numbers and final purities (expressed as mass fraction) are reported in Table 1. Fig. 1 reports their structural formulas.
DSC experiments for melting characterization
DSC experiments were carried out using a Stanton Redcroft STA-625 simultaneous TG/DSC apparatus, consisting of two open aluminium crucibles of cylindrical shape, one empty for the reference and the other
Results and discussion
The DSC curves of all the compounds tested were given in Fig. 2, from which the melting temperatures as well as the standard molar enthalpies ΔcrlH0m (Tm) were determined and the values were compared in Table 2 with those available in literature [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]. A significant number of literature data were found for acridine, several values for phenanthridine and phenazine and only
Conclusions
In this manuscript, isothermal thermogravimetry and effusion techniques (both torsion and Knudsen effusion mass loss) were used to determine vapor pressure values above the liquid and solid phases, respectively. Effusion techniques and comparison with the available literature data enabled to verify the internal consistency of vapor pressure determined by the isothermal thermogravimetry. To achieve this goal the standard molar vaporization enthalpies and the molar enthalpies of fusion determined
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