## 4 The Current Understanding of Dielectric Relaxation Processes
Figure 4: Temperature dependence of (**a**) the dielectric relaxation strength \(\Delta\varepsilon\) of the \(\alpha\)-relaxation, as well as the underlying microscopic effective dipole moment, based on the assumption that the fluctuating moiety is (**b**) the whole molecule \(\langle u_{eff}\rangle_{mol}\), or (**c**) the COH unit \(\langle u_{eff}\rangle_{COH}\), for several polyalcohols as indicated. The dashed line in (**c**) is a guide to the eye. The data are taken from [28]. In all these classical approaches, the detailed chemical structure of the molecule under study, its conformations [48] and the various chemical potentials of the bonds interconnecting the different polar moieties are not taken into consideration. Additionally, intermolecular (e.g., dipole-dipole) interactions are neglected. It is evident that an atomistic description of the molecular dynamics based on quantum mechanics is required. On the experimental side, methods which enable to measure the effect of orientational polarization on the level of the specific (polar) moieties within a molecular system are called for. In fact, it has been argued that the intra-molecular degrees of freedom in these polycalcohols enable conformational changes of the molecules that correspond to structural relaxation rather than the orientational relaxation of the whole molecule [33]. Consequently, the dipole moment of a submolecular structure, in particular the OH-group with 1.69 D, has been used to obtain Kirkwood correlation factors of \(\sim\)2-2.5 [33]. Following this approach, Equation (4) must be modified to refer to the concentration of the respective dipole species (instead of the concentration of molecules), i.e.,  
Figure 1:0  
where \(N_{COH}\) is the number of COH units per molecule. Concerning the molecular origin and composition of dielectric relaxation peaks, it was recently suggested that generally two types of contributions are present: self-correlations (i.e., the fluctuation/libration dynamics of individual dipoles with respect to their earlier orientation), and cross-correlations (i.e., the fluctuation/libration dynamics with respect to the orientation of neighboring dipoles) [49]. This was concluded from comparative measurements of BDS and photon correlation spectroscopy which respond to molecular dynamics via different physical principles [46]. Such investigations of the polyalcohol series discussed here revealed a complex shape of the dielectric peak shape in sorbitol that indicates pronounced intra-molecular dynamics, while for glycerol a rather simple peak shape led to the conclusion of a rather rigid molecule [34; 35]. This contrasts with an attempt to simulate the dielectric response of several types of glass-forming molecules which succeeded for most of them but failed particularly in glycerol [50]; the reason for this was found in the flexibility of the glycerol molecule, which was not taken into account in that model. Further evidence for the impact of minute details of the molecular structure (and thus of intra-molecular fluctuations) on the inter-molecular dynamics is found by comparing the isomers xylitol, donitol, D-arabitol and L-arabitol: although they have an identical molecular formula and nearly the same structure, their glass transition temperatures are 250 K, 252 K, 261 K and 261 K, respectively [51]. Consequently, also the temperature dependence of their structural relaxation as probed by BDS (often quantified in the fragility parameter) varies considerably [52]. We note that the described complexity of dynamics on the intra- and inter-molecular levels is very likely not restricted to H-bonding materials but may be expected to be inherent to any molecular system that contains several interactions of highly varying strengths and pronounced directionality, e.g., combining van der Waals forces with Coulombic interactions or steric dipoles. This demonstrates the theoretical and experimental demand to access dynamics on a submolecular level.