Abstract
The present work reports an experimental thermodynamic study of two nitrogen heterocyclic organic compounds, fenclorim and clopyralid, that have been used as herbicides. The sublimation vapor pressures of fenclorim (4,6-dichloro-2-phenylpyrimidine) and of clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) were measured, at different temperatures, using a Knudsen mass-loss effusion technique. The vapor pressures of both crystalline and liquid (including supercooled liquid) phases of fenclorim were also determined using a static method based on capacitance diaphragm manometers. The experimental results enabled accurate determination of the standard molar enthalpies, entropies and Gibbs energies of sublimation for both compounds and of vaporization for fenclorim, allowing a phase diagram representation of the (p,T) results, in the neighborhood of the triple point of this compound. The temperatures and molar enthalpies of fusion of the two compounds studied were determined using differential scanning calorimetry. The standard isobaric molar heat capacities of the two crystalline compounds were determined at 298.15 K, using drop calorimetry. The gas phase thermodynamic properties of the two compounds were estimated through ab initio calculations, at the G3(MP2)//B3LYP level, and their thermodynamic stability was evaluated in the gaseous and crystalline phases, considering the calculated values of the standard Gibbs energies of formation, at 298.15 K. All these data, together with other physical and chemical properties, will be useful to predict the mobility and environmental distribution of these two compounds.
Highlights
Herbicides, known as weed killers, are a broad class of pesticides that are used to control or manipulate undesirable vegetation, such as grasses and weeds, that may compromise the growth and profitability of crops [1]
Gas-liquid chromatography with flame ionization detector (FID). b Determined using Karl Fisher coulometric titration. c Analysis certified by the manufacturer. d Under reduced pressure. e Minimum purity degree announced by the supplier
The vapor pressures of the two compounds studied, measured using the effusion or the static methods, are listed in Table 2, where the effusion pressures are the mean of the results determined through the different effusion orifices at each temperature, presented in detail in Tables S4 and S5 (Supplementary Materials)
Summary
Herbicides, known as weed killers, are a broad class of pesticides that are used to control or manipulate undesirable vegetation, such as grasses and weeds, that may compromise the growth and profitability of crops [1] They are efficient and cost-effective means of controlling nuisance plants when compared to hoeing, mowing, cultivation, or hand pulling and play a key role in farmland management and in reducing labor intensity [2,3,4]. While these chemicals help improve and rise crop yields, they pose risks to the crops themselves which can be sensitive to these agents. The safener fenclorim (Figure 1a) was designed to reduce the damage to rice (Oryza sativa L.)
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