Vibrational and thermodynamic properties of ferrocene

Abstract

The vibrational and thermodynamic properties of crystalline ferrocene (dicyclopentadienyl iron, Fe(CH)2' Fc), and dilute soluSStions of Fc in the liq-uid crystals 4-n-hexyloxybenzylidene 4'-npropylaniline (HBPA) and terephthal-bis-butylaniline (TBBA) have been studied using Mossbauer spectroscopy. The recoil free fraction (rff) of the S7Fe resonance of crystalline Fc and deuterated Fc has been measured from 4 to 300K. Anharmonic effects are found to be important in crystalline Fe, and a model is presented to account for these effects. Fc undergoes a structural phase transition (T l63.9K for Fc, T = 203.7K for deuterated Fc). The large shift o 0 in T for deuterated Fc can be attributed to a change in the molecular o barrier to rotation of the CSHS rings about the S-fold symmetry axis. Solutions of Fc in the two liquid crystals showed similar behavior of the rff. The rff of Fc in crystalline TBBA shows an anomalous drop starting about SO degrees below the crystal-smetic H transition (ll3C). The resonance becomes unobservable just below (-2 degrees) the transition temperature. The smectic phase of HBPA is known to supercool. Ferrocene in supercooled HBPA shows an anomalous drop in rff similar to that observed in TBBA. In this case, however, the Nossbauer resonance can be observed above the crystalsmectic H transition temperature (29C). The anomalous drop in rff begins near OC and the resonance disappears near SSC. The anomalous drop in rff for ferrocene in HBPA is accompanied by broadening of the Fc Mossbauer resonance. Similar behavior of rff and linewidth has been observed in glasses and supercooled liquids.