Infrared spectra of one- and two-dimensional fullerene polymer structures:RbC60and rhombohedralC60

Abstract

We compare the infrared spectra of two types of fullerene polymers: the linear-chain RbC 60 and the twodimensional pressure-polymerized rhombohedral C 60 . Both the splitting of the F 1u modes and the structure of newly activated lines are in agreement with fully ordered structures of molecular symmetry D 2h and D 3d , respectively. ͓S0163-1829͑97͒04318-X͔ Polymeric forms of C 60 have been rapidly growing in number and diversity in the last few years. 1 Three different routes were used for their preparation: photopolymerization, 2 pressure-induced polymerization, 3 and reaction with alkali metals. 4 The first two methods yield neutral polymers, while the last one produces a 1:1 fulleride salt whose structure consists of C 60 Ϫ monoanions. The detailed structure of the photopolymer is not yet known, but there exist highprecision structural refinements of the alkali fulleride polymers 5 which are arranged in linear chains, with hexagonhexagon double bonds connecting each ball to two neighboring molecules. For the pressure-polymerized phases, analogous structures were suggested based on model calculations. 6,7 According to these models, in rhombohedral C 60 ͑rh-C 60 ) which forms at 50 kbar and 500-800°C, the bonds are identical but each ball is connected to six neighbors instead of two. The result is a two-dimensional polymer network.From the point of view of molecular structure, these substances represent an amazing variety of several distorted forms of the icosahedral C 60 ball. Vibrational spectroscopy is particularly suited for the investigation of symmetry reduction in fullerene derivatives and a large number of results has appeared, 8,9 Raman spectroscopy clearly dominating. Of particular interest is a recent comparison of the Raman spectra of RbC 60 and the low-temperature polymerized orthorombic C 60 phase 12 by Winter et al., who found one-toone correspondence between the Raman-active vibrations, indicating identical structure. Infrared measurements were reported on several pressure-polymerized phases, 3,10,11 but no systematic analysis has appeared so far.In this paper we compare the infrared spectra of RbC 60 and rh-C 60 . The main spectral features can be perfectly described by the molecular symmetry determined by the bonding pattern, in accordance with the crystal structure. We also discuss variations caused by the addition of charge.RbC 60 and rh-C 60 were synthesized as described previously 3,5 and characterized by x-ray diffraction as single phases. Infrared spectra were taken by a Bruker IFS-28 Fourier transform interferometer in KBr pellets with 1 cm Ϫ1 resolution at room temperature. Figure 1 shows the spectra in the whole vibrational range, and a C 60 spectrum for comparison. The purity of the samples is evidenced by the absence of C 60 lines. Besides

Static and dynamic Jahn-Teller effect in the alkali metal fulleride saltsA4C60(A=K,<

“…Since C 60 is an icosahedral molecule, it has only four infrared active vibrations ͑at 528, 577, 1183, and 1429 cm −1 ͒, 37 all of which belong to the T 1u representation. The shift and the splitting of the highest frequency T 1u ͑4͒ mode has been used as the most sensitive indicator for charge transfer, 38 symmetry change, 39 and bonding 40 in fullerene compounds. The most prominent feature of our spectra is the splitting of this mode ͑shifted to 1350 cm −1 because of charge effects͒ indicating a lowering of symmetry from icosahedral.…”

Low Band Gap and Ionic Bonding with Charge Transfer Threshold in the Polymeric Lithium Fulleride Li4C60

“…Densely spaced modes were also observed in the 50-100 meV energy range in the orthorhombic phase of pressure-polymerized C 60 (both in Raman 46 and IR 47 ) and in IR studies on orthorhombic RbC 60 and CsC 60 . 48,49 The point symmetry in these phases is D 2h , which contains more symmetry elements than the C 2h group of Li 4 C 60 .…”

Computational Termochemistry Study of the C80 Isomers and Their Endo Lanthanum Complexes by Applying Homodesmotic and Isodesmic Reactions

“…It is likely that I h is the more symmetric species, almost all the lower symmetries are subgroups of the I h point group (with the exception of D 5h ) [21,22,23,24]. Identifying an isomer from the same symmetry point group as the classical C 60 fullerene suggests that this will represent the most stable conformation; however this is not the case, actually the C 80 experiments a Jahn-Teller distortion and as a result arise species with lower energy and less symmetry [1,24].…”