Theoretical studies of the interaction between enf

Authors: Wiktor Zierkiewicz Danuta Michalska Thérèse ZeegersHuyskens

Publish Date: 2012/12/05

Volume: 19, Issue: 3, Pages: 1399-1405

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Abstract

Increase of the atmospheric concentration of halogenated organic compounds is partially responsible for a change of the global climate In this work we have investigated the interaction between halogenated ether and water which is one of the most important constituent of the atmosphere The structures of the complexes formed by the two most stable conformers of enflurane a volatile anaesthetic with one and two water molecules were calculated by means of the counterpoise CPcorrected gradient optimization at the MP2/6–311++Gdp level In these complexes the CH…Ow hydrogen bonds are formed with the H…Ow distances varying between 223 and 232 Å A small contraction of the CH bonds and the blue shifts of the νCH stretching vibrations are predicted There is also a weak interaction between one of the F atoms and the H atom of water with the Hw…F distances between 241 and 287 Å The CCSDT/CBS calculated stabilization energies in these complexes are between −589 and −466 kcal mol−1 while the enthalpies of formation are between −435 and −322 kcal mol−1 The Cl halogen bonding between enflurane and water has been found in two complexes The intermolecular Cl···O distance is smaller than the sum of the corresponding van der Waals radii The CCSDT/CBS stabilization energies for these complexes are about −2 kcal mol−1Halogenated ethers bearing several F or Cl atoms have been known for many years as narcotic gases In these derivatives the presence of one or several halogen atoms tends to make the CH bonds more acidic which gives rise to specific interactions with surrounding enzymes and neuroreceptors 1 2 3 4 5 6 Enflurane CHClFCF2OCHF2 a volatile anaesthetic is characterized by two CH bonds which can interact with neighboring molecules The structures of the stable conformers of this molecule have been reported in earlier works 7 8 9 The basicity of enflurane and its interaction with guest molecules have been investigated as well 10 11 12 13 Recently the atmospheric chemistry of halogenated ethers such as isoflurane CF3CHClOCHF2 desflurane CF3CHFOCHF2 and sevoflurane CF32CHOCH2F have been studied in the reaction with chlorine atoms and OH radicals with respect to the global warming potentials of these compounds 14 Lane and coworkers 15 studied the reaction of enflurane with chlorine atom and the problems of ozone depletion These authors estimated the global atmospheric lifetime of enflurane as 37 years It is therefore important to investigate the interaction between halogenated ethers and water which is one of the major constituents of the atmosphereAs far as we know no theoretical or experimental data have been reported for the enfluranewater complexes Our work is arranged as follows In the first part we will discuss the structures binding energies and enthalpies of formation of the hydrogen bonded enflurane complexes with water For this purpose we have chosen the two most stable conformers of enflurane The stabilization energies of the complexes have been determined at the MP2/6–311++Gdp and CCSDT/complete basis set CBS levels of theory To estimate the role of the cooperativity or anticooperativity effects the threebody contributions to the total binding energies have been calculated In the second part the Cl halogen bonded complexes between enflurane and water have been investigated at the same levels of theoryFull geometry optimizations followed by the calculations of vibrational frequencies and infrared intensities were performed for the two most stable conformers of enflurane and their complexes with water using an ab initio second order MøllerPlesset perturbation method combined with the 6–311++Gdp basis set 16 17 The counterpoise CPcorrected gradient optimization which eliminates the basis set superposition error BSSE 18 has been used in all calculations of the minimum energy structures of the complexes investigatedThe total stabilization energies of the enfluranewater complexes were determined at the MP2/6–311++Gdp and CCSDT/complete basis set CBS levels of theory The CCSDT/CBS stabilization energy was calculated as the sum of the MP2/CBS stabilization energy and the CCSDT correction term 19 The MP2/CBS energy was extrapolated from the MP2 energies evaluated with the augccpVDZ and augccpVTZ basis sets The extrapolation method of Helgaker et al has been used 20 The CCSDT correction term the difference between the CCSDT and MP2 interaction energies was determined with the augccpVDZ basis set 21 22Enthalpies of formation of the enfluranewater complexes under standard conditions in the gas phase were calculated at the MP2/6–311++Gdp and CCSDT/CBS levels The CCSDT/CBS enthalpy was determined as the sum of the CCSDT/CBS electronic energy and the zeropoint vibrational energy and the thermal correction to enthalpy obtained by the MP2/6–311++Gdp methodThe evaluation of the threebody contribution E3B to the total interaction energy ΔEint of the enflurane complex with two water molecules was performed at the MP2/6–311++Gdp and CCSDT/CBS levels of theory The value of E3B was obtained as the difference between ΔEint of the complex and the sum of three pairwise twobody interaction energies ΔE2B The negative value of E3B means a cooperative effect while the positive one corresponds to an anticooperative interaction in the threebody unit 23

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