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Specific Interactions of CH2- and CH3-Groups of n-Paraffins with PFTB

The OH stretching frequency νOH of perfluoro-tert.-butanol (PFTB) dissolved in different liquid n-paraffins decreases with increasing chain length n of the solvent. This observation is explained by an overlapping of two different vOH absorption bands induced by interactions between CH2- resp. CHj-groups and the OH-oscillators of PFTB. The decrease of vOH demonstrates an increase of the intermolecular interaction energies. These mean interaction energies between the OH-groups and the solvent molecules increase with n in a similar way as the densities q of the paraffins. The frequency shifts zh'OH (^vOH = vOH(Gas, -vOH) of PFTB as well as the specific volumes V of the paraffins can be described by a sum of CH2- and CH3-group contributions. In this way the role of the cohesion energy for zlvOH is demonstrated. The OH ··· CH2-interaction has been found as 2.5 times larger than OH ··· CH3-interactions. The I ··· CH2 Lennard-Jones parameter £ has been found too as 2.5 times larger than the value of I ··· CH3 interactions. This agreement demonstrates the efficiency of the intermolecular IR spectroscopy. The sum of group contributions for interactions of linear molecules seems to have some advan- tages and may be more intuitive by its simplicity in comparison to explanations with parameter for “non-central forces” as e.g. the acentric factors ω of Pitzer et al.