Experimental Complexation Enthalpy Research Articles

Towards a better understanding of the parameters determining the competition between bromine halogen bonding and hydrogen bonding: An FTIR spectroscopic study of the complexes between bromodifluoromethane and trimethylamine

Previous experimental and theoretical studies of various model compounds dissolved in liquid noble gases have shown that iodine halogen bonding can compete with hydrogen bonding and that experimental information on the thermodynamic equilibria present can be derived. To get a more general grasp of the competition between halogen bonding and hydrogen bonding, and to expand the set of experimental data to other, weaker, halogen donors, solutions in liquid krypton containing mixtures of bromodifluoromethane with trimethylamine or the fully deuterated trimethylamine-d9 are studied using FTIR spectroscopy. Analysis of the experimental data obtained is supported by ab initio calculations, statistical thermodynamics and Monte Carlo-Free Energy Perturbation calculations. Careful comparison of the spectra of the monomers and of the mixtures studied, shows that for all solutions studied, only features due to a hydrogen-bonded complex are observed. The experimental complexation enthalpy for this species is determined to be −14.2(4) kJ mol−1.

Effect of Fluorination on the Competition of Halogen Bonding and Hydrogen Bonding: Complexes of Fluoroiodomethane with Dimethyl Ether and Trimethylamine.

To further rationalize the competition between halogen and hydrogen bonding, a combined experimental and theoretical study on the weakly bound molecular complexes formed between the combined halogen bond/hydrogen bond donor fluoroiodomethane and the Lewis bases dimethyl ether and trimethylamine (in standard and fully deuterated form) is presented. The experimental data are obtained by recording infrared and Raman spectra of mixtures of the compounds in liquid krypton, at temperatures between 120 and 156 K. The experiments are supported by ab initio calculations at the MP2/aug-cc-pVDZ-PP level, statistical thermodynamics and Monte Carlo free energy perturbation calculations. For the mixtures containing fluoroiodomethane and dimethyl ether a hydrogen-bonded complex with an experimental complexation enthalpy of -7.0(2) kJ mol-1 is identified. Only a single weak spectral feature is observed which can be tentatively assigned to the halogen-bonded complex. For the mixtures involving trimethylamine, both halogen- and hydrogen-bonded complexes are observed, the experimental complexation enthalpies being -12.5(1) and -9.6(2) kJ mol-1 respectively. To evaluate the influence of fluorination on the competition between halogen and hydrogen bonding, the results obtained for fluoroiodomethane are compared with those of a previous study involving difluoroiodomethane.

Taking the halogen bonding-hydrogen bonding competition one step further: complexes of difluoroiodomethane with trimethylphosphine, dimethyl sulfide and chloromethane.

To rationalize the driving factors in the competition of halogen bonding and hydrogen bonding, the complexes of the combined halogen-/hydrogen-bond donor difluoroiodomethane with the Lewis bases trimethylphosphine, dimethyl sulfide and chloromethane are studied. For all Lewis bases, ab initio calculations lead to halogen- and hydrogen-bonded complexes. Fourier transform-IR experiments involving solutions of mixtures of difluoroiodomethane with trimethylphosphine(-d9) or dimethyl sulfide(-d6) in liquid krypton confirm the coexistence of a halogen-bonded and hydrogen-bonded complex. Also for solutions containing chloromethane, evidence of the formation of binary associations is found, but no definitive assignment of the multiple complex bands could be made. Using van't Hoff plots, the experimental complexation enthalpies for the halogen- and hydrogen-bonded complex of difluoroiodomethane with trimethylphosphine are determined to be -15.4 (4) and -10.5 (3) kJ mol-1, respectively, while for the halogen- and hydrogen-bonded complexes with dimethyl sulfide, the values are -11.3 (5) and -7.7 (6) kJ mol-1, respectively. The experimental observation that for both trimethylphospine and dimethyl sulfide the halogen-bonded complex is more stable than the hydrogen-bonded complex supports the finding that softer Lewis bases tend to favor iodine halogen bonding over hydrogen bonding.

Lone pair⋯π interactions involving carbonyl π-systems: Experimental and theoretical study of the complexes of COF2 and COFCl with dimethyl ether

In this theoretical and experimental study, the ability of carbonyl fluoride (COF2) and carbonyl chloride fluoride (COFCl) to form noncovalent interactions with the Lewis base dimethyl ether (DME) is assessed. From ab initio calculations, two stable complexes are found for COF2·DME, both formed through a lone pair⋯π interaction. FTIR measurements on liquefied noble gas solutions, supported by ab initio calculations, statistical thermodynamical calculations and Monte Carle Free Energy Perturbation calculations, show that a 1:1 lone pair⋯π bonded complex is found in solution, with an experimental complexation enthalpy of −14.5(3)kJmol−1. For COFCl·DME three lone pair⋯π complexes, as well as a Cl⋯O halogen bonded complex, are found from ab initio calculations. Experimentally, clear complex bands for 1:1 lone pair⋯π complexes are observed, with an experimental complexation enthalpy of −11.4(2)kJmol−1. Furthermore, indications of the presence of a small amount of the halogen bonded complex are also observed.

Lone pair···π interactions involving an aromatic π-system: Complexes of hexafluorobenzene with dimethyl ether and trimethylamine

The formation of complexes between hexafluorobenzene and the Lewis bases dimethyl ether, dimethyl ether-d6, trimethylamine and trimethylamine-d9 was investigated experimentally using FTIR spectroscopy on solutions of liquid krypton. Additional bands found in the spectra of mixtures were assigned to lone pair···π complexes using ab initio calculations at the MP2/aug-cc-pVDZ level. By constructing Van ‘t Hoff plots, experimental complexation enthalpies were determined of −6.0(6)kJmol−1 for the complex with dimethyl ether(-d6) and −6.7(9)kJmol−1 for the complex with trimethylamine(-d9). These values are in good agreement with calculated enthalpy values.

Competition of C(sp²)-X···O halogen bonding and lone pair···π interactions: cryospectroscopic study of the complexes of C₂F₃X (X = F, Cl, Br, and I) and dimethyl ether.

Inspection of the electrostatic potential of C2F3X (X = F, Cl, Br, and I) revealed a second electropositive region in the immediate vicinity of the C═C double bond apart from the σ hole of chlorine, bromine, and iodine, leading to C(sp(2))-X···Y halogen bonding, through which complexes stabilized by so-called lone pair···π interactions can be formed. Consequently, the experimental studies for the complexes of dimethyl ether with C2F3X (X = F, Cl, Br, and I) not only allowed one to experimentally characterize and rationalize the effects of hybridization on halogen bonding but, for the first time, also allowed the competition of C-X···Y halogen bonding and lone pair···π interactions to be studied at thermodynamic equilibrium. Analysis of the infrared and Raman spectra reveals that in the cryosolutions of dimethyl ether and C2F3I, solely the halogen-bonded complex is present, whereas C2F3Br and C2F3Cl give rise to a lone pair···π bonded complex as well as a halogen-bonded complex. Mixtures of dimethyl ether with C2F4 solely yield a lone pair···π bonded complex. The experimentally derived complexation enthalpies for the halogen bonded complexes are found to be -14.2(5) kJ mol(-1) for C2F3I·DME and -9.3(5) kJ mol(-1) for C2F3Br·DME. For the complexes of C2F3Cl with dimethyl ether, no experimental complexation enthalpy could be obtained, whereas the C2F4·DME complex has a complexation enthalpy of -5.5(3) kJ mol(-1). The observed trends have been rationalized with the aid of an interaction energy decomposition analysis (EDA) coupled to a Natural Orbital for Chemical Valence (NOCV) analysis and also using the noncovalent interaction index method.

A cryospectroscopic infrared and Raman study of the C[sbnd]X⋯π halogen bonding motif: Complexes of the CF3Cl, CF3Br, and CF3I with ethyne, propyne and 2-butyne

Experimental information on the CX⋯π halogen bonding motif was obtained by studying the formation of molecular complexes of CF3Cl, CF3Br and CF3I with ethyne, propyne and 2-butyne in liquid krypton, using FTIR and Raman spectroscopy. For CF3Br, experimental evidence was found for the formation of 1:1 complexes with propyne and 2-butyne only, while for CF3I spectroscopic features confirming the existence of the halogen bonded complexes were observed for ethyne, propyne and 2-butyne. In addition, at higher concentrations of CF3I and 2-butyne, weak absorptions due to a 2:1 complex were also observed. The experimental complexation enthalpies, obtained by using spectra recorded at temperatures between 120K and 140K, are −5.9(3) kJmol−1 for CF3I.ethyne, −5.6(3)kJmol−1 for CF3Br.propyne, −8.1(2)kJmol−1 for CF3I.propyne, −7.3(2)kJmol−1 for CF3Br.2-butyne, −10.9(2)kJmol−1 for CF3I.2-butyne and −20.9(7)kJmol−1 for (CF3I)2.2-butyne. The experimental study is supported by theoretical data obtained from ab initio calculations at the MP2/aug-cc-pVDZ(-PP) and MP2/aug-cc-pVTZ(-PP) levels, and Monte Carlo Free Energy Perturbation (MC-FEP) simulations. The experimental and theoretical values on the CX⋯π halogen-bonding motifs studied are compared with previously reported data for the complexes with ethene and propene and with preliminary results obtained for benzene and toluene.

Exploring the C-X…π Halogen Bonding Motif: An Infrared and Raman Study of the Complexes of CF3X (X = Cl, Br and I) with the Aromatic Model Compounds Benzene and Toluene

The formation of halogen bonded complexes formed between the trifluorohalomethanes CF3Cl, CF3Br and CF3I and the Lewis bases benzene and toluene at temperatures below 150K was investigated using FTIR and Raman spectroscopy. Experiments using liquid krypton as solvent show that for both CF3Br and CF3I substantial fractions of the monomers can be involved in 1:1 complexes. In addition, weak absorptions illustrating the formation of 2:1 complexes between CF3I and benzene are observed. Using spectra recorded at temperatures between 120 and 140 K, observed information on the relative stability was obtained for all complexes by determining the complexation enthalpies in solution. The resulting values for CF3Br·benzene, CF3I·benzene and (CF3I)2·benzene are −6.5(3), −7.6(2) and −14.5(9) kJ mol−1. The values for CF3Br·toluene and CF3I·toluene are −6.2(5) and −7.4(5) kJ mol−1. The experimental complexation enthalpies are compared with theoretical data obtained by combining results from MP2/aug-cc-pVDZ(-PP) and MP2/aug-cc-pVTZ(-PP) ab initio calculations, from statistical thermodynamical calculations and from Monte Carlo Free Energy Perturbation simulations. The data are also compared with results derived for other C-X···π halogen bonded complexes involving unsaturated Lewis bases such as ethene and ethyne.

Comparison of Microhydration Methods: Protonated Glycine as a Working Example

The determination of the structures and relative energies of microsolvated complexes remains an important challenge for theoretical chemistry because the number of possible aggregates rapidly increases when more solvent molecules are considered. Several approaches (chemical intuition, hierarchy of scheme, evolutionary trees, ...) have been proposed previously to identify the most stable geometries. In the present paper, we compare both the structures and interaction energies computed with different computational protocols for the model case of microhydrated protonated glycine, a system for which experimental complexation enthalpies are available. We compare the results obtained, one the one hand, by a hierarchical approach starting with a AMOEBA search followed by DFT and MP2 refinements and, on the other hand, by a Darwinian tree that uses evolutionary logic and counterpoise corrected MP2 calculations. It is shown that the two approaches yield relatively similar conclusions, though some structures can only be identified by one of the two strategies. This paper therefore underlines the interest of applying more than one minima search tactic for pinpointing all possible microsolvated complexes.

On the weakly C–H⋯π hydrogen bonded complexes of sevoflurane and benzene

A vibrational assignment of the anaesthetic sevoflurane, (CF(3))(2)CHOCH(2)F, is proposed and its interaction with the aromatic model compound benzene is studied using vibrational spectroscopy of supersonic jet expansions and of cryosolutions in liquid xenon. Ab initio calculations, at the MP2/cc-pVDZ and MP2/aug-cc-pVDZ levels, predict two isomers for the 1 : 1 complex, one in which the near-cis, gauche conformer of sevoflurane is hydrogen bonded through its isopropyl-hydrogen atom, the other in which the same conformer is bonded through a bifurcated hydrogen bond with the fluoromethyl hydrogen atoms. From the experiments it is shown that the two isomers are formed, however with a strong population dominance of the isopropyl-bonded species, both in the jet and liquid phase spectra. The experimental complexation enthalpy in liquid xenon, ΔH(o)(LXe), of this species equals -10.9(2) kJ mol(-1), as derived from the temperature dependent behaviour of the cryosolution spectra. Theoretical complexation enthalpies in liquid xenon were obtained by combining the complete basis set extrapolated complexation energies at the MP2/aug-cc-pVXZ (X = D,T) level with corrections derived from statistical thermodynamics and Monte Carlo Free Energy Perturbation calculations, resulting in a complexation enthalpy of -11.2(3) kJ mol(-1) for the isopropyl-bonded complex, in very good agreement with the experimental value, and of -11.4(4) kJ mol(-1), for the fluoromethyl-bonded complex. The Monte Carlo calculations show that the solvation entropy of the isopropyl-bonded species is considerably higher than that of the fluoromethyl-bonded complex, which assists in explaining its dominance in the liquid phase spectra.

A cryospectroscopic and ab initio study of the cyclopropane·(HCl)x van der Waals complexes

The mid-infrared (4000–400cm−1) and the far-infrared (300–50cm−1) spectra of cyclopropane/hydrogen chloride mixtures, dissolved in liquefied argon (LAr) and in liquefied nitrogen (LN2) are discussed. In the spectra of the LAr solutions, evidence was found for the formation of 1:1 and 1:2 van der Waals complexes. In the LN2 solutions, only the 1:1 complex could be observed. From spectra recorded at different temperatures, the complexation enthalpy ΔH° for the 1:1 complex was determined to be −7.8(2)kJmol−1 in LAr and −5.6(3)kJmol−1 in LN2, while for the 1:2 complex in LAr, a value of −14.9(7)kJmol−1 was derived. For all investigated species, ab initio geometry optimizations and frequency calculations were made at the MP2/6-31+G(d,p) and at the B3LYP/6-311++G(d,p) level, and the complexation energies were calculated at the MP2/6-311++G(d,p) level. The calculations predict the existence of two isomers of the 1:2 complex, one with the two HCl molecules bonded to different carbon–carbon bonds, while in the other the second HCl molecule is bonded to the first. Using Free Energy Perturbation Monte Carlo simulations to calculate the solvent influences, and using statistical thermodynamics to account for the zero-point vibrational and thermal contributions, from the experimental complexation enthalpy the complexation energy for the 1:1 complex was estimated to be −14.5(6)kJmol−1. Comparison of the calculated with the experimental data suggests that the observed 1:2 complex is a mixture of the two isomers.

Methylenecyclopropane−Boron Trifluoride van der Waals Complexes; an Infrared and DFT Study

The formation of weakly bound molecular complexes between methylenecyclopropane (MeCP) and BF3 dissolved in liquid argon has been investigated using FTIR spectroscopy. Experimental evidence was found for the formation of a 1:1 complex in which the BF3 moiety binds to the exocyclic CC double bond, whereas at higher concentrations of BF3, weak absorption bands due to MeCP·(BF3)2 were also observed. Using spectra recorded at different temperatures between 105 and 129 K, the complexation enthalpy for the 1:1 complex in liquid argon was determined to be −10.7(3) kJ mol-1. Structural and spectral information on the complexes was obtained from density functional calculations at the B3LYP/6-311++G(d,p) level. Applying Free Energy Perturbation Monte Carlo calculations to account for the solvent influences and statistical thermodynamics to calculate the zero-point vibrational and thermal influences, the complexation energy for the 1:1 complex was estimated, from the experimental complexation enthalpy, to be −16.0(1...

Quantitative approach to molecular interactions using the electronegativity equalization method

An internally consistent approach for describing molecular interactions is presented, based on the electronegativity equalization method (EEM). The gas-Phase complexation of alkali-metal cations by water and ammonia is taken as an example. Charge reorganisation in the molecules due to the interaction with the cations can be accurately predicted and the complexation enthalpies can be estimated. Good correlations with experimental complexation enthalpies are found (the absolute values depend on the definition of ‘charge’).