Intramolecular Raman modes

This is the most puzzling requirement. It is not known why certain crystals, e.g. HMTSF TCNQ or Cu(DMDCNQI)2, conduct very well at low temperatures, but do not form Cooper pairs. One can only speculate that certain intramolecular or intermolecular vibrations or rigid-body librational modes must be right for superconductivity. Hans Kuzmany has suggested [203] that strained molecules, e.g. Cf, have intramolecular Raman modes which may somehow be very important for superconductivity. Recently, fresh attention has been drawn to this issue [34]. 

To provide an example of the two-dimensional response from a system containing well-defined intramolecular vibrations, we will use simulations based on the polarized one-dimensional Raman spectrum of CCI4. Due to the continuous distribution of frequencies in the intermolecular region of the spectrum, there was no obvious advantage to presenting the simulated responses of the previous section in the frequency domain. However, for well-defined intramolecular vibrations the frequency domain tends to provide a clearer presentation of the responses. Therefore, in this section we will present the simulations as Fourier transformations of the time domain responses. Figure 4 shows the Fourier transformed one-dimensional Raman spectrum of CCI4. The spectrum contains three intramolecular vibrational modes — v2 at 218 cm, v4 at 314 cm, and vi at 460 cm 1 — and a broad contribution from intermolecular motions peaked around 40 cm-1. We have simulated these modes with three underdamped and one overdamped Brownian oscillators, and the simulation is shown over the data in Fig. 4. 

Table 3.32 Calculated data relevant to polarizabilities in water dimer for intramolecular vibrational modes, and Raman scattering activities. ... 

The fact that using excitation in the CT absorption a number of Raman bands appears in the intermolecular mode region,while the intramolecular bands practically disappear, can be understood on the basis of the dimeric model. The modulation of the transfer integral t by the intermolecular vibrations, particularly the antiphase translational modes, provides an efficient mechanism for intensity enhancement at resonance with the CT transition. No such mechanism is operative for the intramolecular vibrational modes. 

A vibrational (infrared or Raman) spectrum of a molecule consists of vibrational modes (resulting from the interaction of two or more different vibrations of neighboring bonds). These absorption (IR) or scattering (Raman) modes provide information about features such as the chemical nature (e.g. bond types and functional groups) and molecular conformation (e.g. irons and gauche). They also provide information about the individual molecular bonds (intramolecular interactions) and the interactions between molecules (intermolecular effects). 

At higher frequencies (above 200 cm ) the vibrational spectra for fullerenes and their cry.stalline solids are dominated by the intramolecular modes. Because of the high symmetry of the Cgo molecule (icosahedral point group Ih), there are only 46 distinct molecular mode frequencies corresponding to the 180 6 = 174 degrees of freedom for the isolated Cgo molecule, and of these only 4 are infrared-active (all with Ti symmetry) and 10 are Raman-active (2 with Ag symmetry and 8 with Hg symmetry). The remaining 32 eigcnfrequencies correspond to silent modes, i.e., they are not optically active in first order. 

The Raman spectrum in Fig. 10 for solid Ceo shows 10 strong Raman lines, the number of Raman-allowed modes expected for the intramolecular modes of the free molecule [6, 94, 92, 93, 95, 96, 97]. As first calculated by Stanton and Newton [98], the normal modes in molecular Ceo above about 1000 cm involve carbon atom displacements that are predominantly tangential... 

The thirty-two silent modes of Coo have been studied by various techniques [7], the most fruitful being higher-order Raman and infra-red spectroscopy. Because of the molecular nature of solid Cqq, the higher-order spectra are relatively sharp. Thus overtone and combination modes can be resolved, and with the help of a force constant model for the vibrational modes, various observed molecular frequencies can be identified with specific vibrational modes. Using this strategy, the 32 silent intramolecular modes of Ceo have been determined [101, 102]. 

The Raman and infrared spectra for C70 are much more complicated than for Cfio because of the lower symmetry and the large number of Raman-active modes (53) and infrared active modes (31) out of a total of 122 possible vibrational mode frequencies. Nevertheless, well-resolved infrared spectra [88, 103] and Raman spectra have been observed [95, 103, 104]. Using polarization studies and a force constant model calculation [103, 105], an attempt has been made to assign mode symmetries to all the intramolecular modes. Making use of a force constant model based on Ceo and a small perturbation to account for the weakening of the force constants for the belt atoms around the equator, reasonable consistency between the model calculation and the experimentally determined lattice modes [103, 105] has been achieved. 

Although in the frequency region of the conventionally measured infrared and Raman spectra (400-4000 cm ) only intramolecular modes appear, some particular bands can be sensitive to intermolecular interactions typical of the different modes of packing of chains with identical conformations. 

Iqbal et al (Ref 56) measured the transmission infrared and laser excited Raman spectra of poly crystalline RDX in the range of 40 to 4000/cm. To aid assignments in the spectral region of 400 to 4000/cm, the spectra of two types of N15-labeled samples and the soln spectra in different solvents were also recorded. From these data it was possible to assign many of the observed bands to intramolecular modes of the RDX molecule. The Raman-active lattice modes also were resolved and found to be comparable to the lattice mode frequencies in solid cyclohexane... 

---