Thermodynamic and aromaticity studies for the assessment of the halogen⋯cyano interactions on Iodobenzonitrile

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Highlights

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    Vapour pressure study of the iodobenzonitrile isomers by Knudsen effusion technique.

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    Enthalpies of formation of the iodobenzonitrile isomers by combustion calorimetry.

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    NICS, HOMA, SA indices used as aromaticity criteria for iodobenzonitrile isomers.

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    QTAIMs properties, HOMO–LUMO gap, UV–Vis spectra of iodobenzonitriles.

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    Bromo and fluorobenzonitriles are taken as references for the aromaticity analysis.

Abstract

The standard (p° = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, of the 2-, 3- and 4-iodobenzonitrile isomers were derived from the combination of the corresponding standard molar enthalpies of formation, in the condensed phase, at T = 298.15 K, and the standard molar enthalpies of sublimation, at the same temperature, calculated respectively from the standard molar energies of combustion in oxygen, measured by rotating-bomb calorimetry, and from the vapour-pressure study of the referred compounds, measured by mass-loss Knudsen effusion technique. The strength of the halogen-halogen and the halogen-cyano intermolecular interactions, in the crystal, are evaluated by the enthalpies and entropies of phase transition of the iodobenzonitrile derived from mass-loss Knudsen technique and differential scanning calorimetry measurements and compared with those reported to fluorobenzonitrile and bromobenzonitrile isomers. The computational calculations complement the experimental work, using different aromaticity criteria (HOMA, NICS, Shannom Aromaticity, PDI and ATI) for the analysis of the electronic behaviour of each iodobenzonitrile isomer.

Introduction

The benzonitrile derivatives, due to the triple bond of the cyano group, have a fundamental role in the synthesis of specific organic compounds, specifically in polymers with appreciated industrial application for their optical [1], [2], [3], [4], [5], [6], [7], [8] and conductivity properties [9], [10], [11], [12]. In addition, the halogen interactions with the cyano group in the halogenated benzonitriles assign them relevant applications in the materials engineering, in particular by their influence in the mechanical characteristics such as the bending and the hardness.

In this work, calorimetric techniques (rotating-bomb combustion calorimetry and differential scanning calorimetry), UV–Vis spectroscopy and Knudsen effusion technique are used to develop a thermodynamic and electronic study of the iodinated benzonitrile isomers. The standard (p° = 0.1 MPa) molar enthalpy of formation, in the condensed phase, of each isomer was derived from the standard massic energy of combustion, measured by rotating-bomb combustion calorimetry. The measurement of the vapour pressures of the iodobenzonitrile isomers at several temperatures, by the mass-loss Knudsen effusion technique allowed to derive the corresponding standard (p° = 0.1 MPa) molar enthalpies and entropies of sublimation. These data were used to calculate the standard molar enthalpies of formation, in the gaseous phase, at T = 298.15 K.

The literature reports studies of the halogen···halogen [13], [14], [15], [16], [17] and halogen···cyano [18], [19], [20], [21] interactions on halobenzonitrile isomers performed by rotational spectroscopy and X-ray crystallography. The energetic nature of the intermolecular interactions of a set of monofluorobenzonitrile and monobromobenzonitrile isomers has been analyzed in previously works [22], [23], correlating the enthalpies of sublimation with the strength of each predominant interactions. This paper aims to contribute to the knowledge of the intermolecular interactions strengths between the halogen atom and the cyano group and to their influence on the symmetry in the crystal, combining the enthalpies and entropies of phase transition reported in previous works [22], [23] with the results obtained for the three iodobenzonitrile isomers. The thermal analysis study of the iodobenzonitrile isomers has been performed by differential scanning calorimetry (DSC). This DSC study has been extended to the bromobenzonitrile isomers [23] in order to compare the behaviour of the iodo- and bromoderivatives.

The experimental measurements performed in this work in order to derive the enthalpies of formation of the compounds, in the gaseous phase, have been complemented by computational calculations to evaluate the aromaticity of the iodobenzonitrile isomers, using three different criteria, viz. magnetic (Nucleus Independent Chemical Shifts [24], [25], NICS), geometric (Harmonic Oscillator Model of Aromaticity [26], HOMA) and based in molecular electron density (Shannon Aromaticity [27], SA, para delocalization index [28], PDI, and the average two centre index [29], ATI). The UV spectra obtained experimentally were correlated with HOMO–LUMO gap calculated theoretically.

Section snippets

Compounds and purity control

The purity details and the commercial origin of the three iodobenzonitrile isomers are presented in table 1. The purity degree of each isomer was checked by gas chromatography along the purification process, using an Agilent HP-4890 apparatus with an HP-5 column, 5% diphenyl and 95% dimethylpolysiloxane, under nitrogen pressure as carrier gas. To assess a purity degree above 0.999 (mass fraction) needed to the calorimetric and vapour pressure measurements, all the isomers were purified by

Computation details

The computational calculations were performed using the Gaussian 03 software package [51]. Due to the size of the iodine atom, 46 inner electrons were included in SKBJ relativistic effective core potential (ECP) and the SKBJ-VDZ basis set was applied to describe the 5s25p5 valence electrons [52]. The optimised geometries and the fundamental vibrational frequency calculations were performed using the Hartree–Fock theory, the second-order Møller–Plesset perturbation theory (MP2) [53] and the

Experimental enthalpies of formation, in the condensed phase

Detailed results of the combustion experiments performed for each isomer of iodobenzonitrile isomers studied are presented in tables S1–S3 (Supplementary Information).

The energy of the isothermal bomb process, ΔU (IBP), is calculated through equation (2), correcting the energy equivalent, ε(calor), for the deviation of the mass of water used from 5222.5 g, Δm (H2O), where ΔTad is the calorimeter temperature change corrected for the heat exchange, the work of stirring and the frictional work of

The halogen⋯CN energetic contribution, in the gaseous phase

According to the values of ΔintHexp. presented in table 6, the presence of an iodine atom in the benzonitrile has a destabilising effect in the three iodobenzonitrile isomers, reflecting the weak ability of iodine atom to attract electron density through the σ bonds and the weak π – donating effect. The enthalpies of reaction, ΔintHexp., obtained from experimental values reported in literature, were compared with the corresponding computed enthalpies, ΔintHcomp., also presented in table 6,

Final remarks

This work is part of an extensive thermodynamic study on the monohalobenzonitrile isomers, focusing on the effect of the volume and the electronic properties of each halogen atom (F, Br and I) in the thermochemical properties. The combination of experimental techniques (rotating-bomb combustion calorimetry and mass-loss Knudsen effusion technique) allowed the derivation of the standard molar enthalpy of formation, in the gaseous phase, of each iodinated isomer. These data were used to evaluate

Acknowledgements

Thanks are due to Fundação para a Ciência e Tecnologia (FCT), Lisbon, Portugal and to European Social Fund for financial support to Centro de Investigação em Química, University of Porto (strategic project PEst-C/QUI/UI0081/2011). I.M.R. thanks FCT and European Social Fund (ESF) under the Community Support Framework (CSF) for the award of Ph.D fellowship (SFRH/BD/61915/2009).

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