Structure and energetics correlations in some chlorohydroxypyridines

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Highlights

Abstract

We have performed a study of the structure and energetics of some chlorohydroxypyridines based on experimental calorimetry techniques and high level ab initio computational calculations. The standard ( = 0.1 MPa) molar enthalpies of formation of 2-chloro-3-hydroxypyridine (2-Cl-3-OHPy), 2-chloro-6-hydroxypyridine (2-Cl-6-OHPy) and 3-chloro-5-hydroxypyridine (3-Cl-5-OHPy) in the crystalline phase, at T = 298.15 K, were derived from the respective standard massic energies of combustion measured by rotating-bomb combustion calorimetry, in oxygen, at T = 298.15 K. The standard molar enthalpies of sublimation, at T = 298.15 K, were measured by Calvet microcalorimetry. From these experimentally determined enthalpic parameters we have derived the standard molar enthalpies of formation of the three compounds in the gaseous phase, at T = 298.15 K: 2-Cl–3-OHPy, −(76.8 ± 2.0) kJ · mol−1; 2-Cl-6-OHPy, −(105.0 ± 1.7) kJ · mol−1, 3-Cl-5-OHPy −(61.2 ± 2.4) kJ · mol−1. These values were compared with estimates obtained from very accurate computational calculations using the G3(MP2)//B3LYP composite method and appropriately chosen reactions. These calculations have also been extended to the remaining chlorohydroxypyridine isomers that were not studied experimentally. Based on B3LYP/6-31G optimized geometries and calculated G3(MP2)//B3LYP absolute enthalpies some structure–energy correlations were discussed.

Introduction

Substituted pyridines are important substructures in a multitude of natural products as well as products of the chemical and pharmaceutical industry. Thermochemical parameters are important in the understanding of chemical issues such as the structure–energy–reactivity relationship in various species and the knowledge gained from these studies can then be used in more applied fields like the chemical and pharmaceutical industry. The present work is part of a systematic thermochemical study of substituted pyridines performed in the Thermochemistry group of Porto for nearly two decades. Over the years different substituted pyridines have been studied experimentally and/or computationally in order to better understand the relation between structure and energetics in this family of compounds: mono- and dihydroxypyridines [1], [2], [3], mono-, di- and trichloropyridines [4], [5], mono- and dibromopyridines [6], trimethylpyridines [7], bipyridines [8], [9], mono- and dicarboxylic acid pyridines [10], [11], [12], mono- and dicarboxylic acid methyl ester pyridines [12], [13], monophenylpyridines [14], monocarboxamide pyridines [15], monoethylpyridines [16], monoacetylpyridines [17], mono-, di- and tri-tert-butylpyridines [18] and mono- and dimethoxypydines [19]. With the present work we intend to enlarge the thermochemical database for disubstituted pyridines by the experimental determination of the standard molar enthalpies of formation of some chlorohydroxypyridines and to use computational chemistry in the understanding of the relation between structure and energetics and to obtain reliable estimates of the enthalpies of formation for those isomers that were not experimentally studied.

In this work we report the standard molar enthalpies of formation of three chlorohydroxypyridine: 2-chloro-3-hydroxypyridine (2-Cl-3-OHPy), 2-chloro-6-hydroxypyridine (2-Cl-6-OHPy) and 3-chloro-5-hydroxypyridine (3-Cl-5-OHPy) in the gaseous phase, at T = 298.15 K, obtained from the standard molar energies of combustion determined by rotation bomb combustion calorimetry and from the values of the standard molar enthalpies of sublimation determined by Calvet microcalorimetry. The structure of these compounds has been discussed from the B3LYP/6-31G calculations and the enthalpies of formation of all possible chlorohydroxypyridine isomers have been derived from G3(MP2)//B3LYP calculations and various appropriately chosen reactions.

Section snippets

Compounds and purity control

The three chlorohydroxypyridines were obtained commercially from Sigma–Aldrich Chemical Co. with the following assigned mass fraction purities: 0.97 for 2-Cl-3-OHPy [CAS Registry No. 6636-78-8]; 0.98 for 2-Cl-6-OHPy [CAS Registry No. 89-64-5]; and 0.98 for 3-Cl-5-OHPy [CAS Registry No. 610-78-6]. These compounds were further purified by repeated sublimation under reduced pressure before the calorimetric experiments, and their purities were assessed by C, H, N and Cl microanalyses and

Computational details

In parallel with the experimental work we have performed computational calculations using the composite G3(MP2)//B3LYP method [31]. All calculations were performed with the Gaussian 03 program [32]. The G3(MP2)//B3LYP method uses the B3LYP/6-31G(d) method in both geometry optimization and calculation of frequencies. The final absolute G3(MP2)//B3LYP enthalpy is obtained from single-point energy calculations at the QCISD(T)/6-31G and MP2/GTMP2Large levels, including the thermal corrections

Experimental results

Determination of purities was assessed by C, H, N and Cl microanalyses and DSC experiments. The results of the microanalyses are presented in table 1. The recorded DSC thermograms did not show any phase transitions between T = 298 K and the fusion temperature of the samples and the mass fraction of purity was higher than 0.9990 for the three compounds. The enthalpies and temperatures of fusion of the crystalline 2-Cl-3-OHPy, 2-Cl-6-OHPy and 3-Cl-5-OHPy were derived from the DSC thermograms as:

Computational results and discussion

The most stable conformations of all chlorohydroxypyridine isomers were obtained at the B3LYP/6-31G level of theory, which is the one used for geometry optimization within the G3(MP2)//B3LYP theory. These are shown in figure 1. The obtained absolute G3(MP2)//B3LYP enthalpies, at T = 298.15 K, are presented in table 7 for the most stable conformations found for the chlorohydroxypyridines. Identical calculations were also performed for the monosubstituted pyridines: chloropyridines and

Conclusions

A combined experimental and computational study has been carried out to obtain the standard molar enthalpies of formation in the gaseous phase, at T = 298.15 K, of 2-chloro-3-hydroxypyridine, 2-chloro-6-hydroxypyridine and 3-chloro-5-hydroxypyridine isomers. In the experimental part of this study the standard molar enthalpies of formation in the crystalline phase were obtained from rotating bomb combustion calorimetry and the standard molar enthalpies of sublimation by Calvet microcalorimetry. In

Acknowledgements

M.S. Miranda thanks Fundação para a Ciência e a Tecnologia, FCT, Lisbon, Portugal for the award of the postdoctoral scholarship (BPD/5594/2001) and for the financial support under the frame of the Ciência 2008 program.

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