Raman spectra of the ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]), 1-hexyl-3-methylimidazolium chloride ([C6mim]Cl), and 1-hexyl-3-methylimidazolium hexafluorophosphate ([C6mim][PF6]), and binary mixtures thereof, have been assigned using ab initio MP2 calculations. The previously reported anti and gauche forms of the [C4mim]+ cation have been observed, and this study reveals this to be a general feature of the long-chain 1-alkyl derivatives. Analysis of mixtures of [C6mim]Cl and [C6mim][PF6] has provided information on the nature of the hydrogen bonding between the imidazolium headgroup and the anions, and the invariance of the essentially 50:50 mixture of the predominant conformers informs on the nature of glass formation in these systems.

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In this study we present all electron ab initio Hartree–Fock (HF) and configuration interaction (CI) calculations of 112 electronic states resulting from interaction between two Fe atoms. The basis set used was of double zeta quality, in general, but the 3d orbital was represented by a triple zeta function. The CI calculations of the ground state included sixtuple excitations from a reference configuration. The calculations were performed to the same degree of accuracy for all the 112 low-lying states. The ground state is a 7Δu state with the configuration (3dσg)1.57(3dπu)3.06(3dδg)2.53 (3dδu)2.47(3dπg)2.89(3dσu)1.49 (4sσg)2.00. The equilibrium distance is calculated to be 4.54 a.u. (2.40 Å), and the vibrational frequency to be 204 cm−1. The chemical bond between the iron atoms is a single bond, and it is almost entirely due to the 4sσg molecular orbital. The ’’band’’ of the 112 low-lying states make a large contribution to the partition function, resulting in a lowering of the experimental dissociation energy of the Fe2 molecule. Our mass spectrometric measurements essentially confirm the previous results by Lin and Kant. The experimental dissociation energy D0° of Fe2 has been determined as 18±4 kcal mol−1 or 75±17 kJ mol−1. The corresponding standard heat of formation is 180.5±4 kcal mol−1 or 755±17 kJ mol−1.

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