## 5 Dielectric Spectroscopy-Tomorrow
\[\varepsilon^{*}=\varepsilon^{\prime}-ie^{\prime\prime} \tag{7}\]2 OPS-spectra for the O-H moiety (measured for the stretching vibration \(\
u\)(O-H) at \(\lambda\) = 2966.8 nm). **(a)**\(\Delta A\) describes the modulation of the transmitted detector signal in response to the frequency of the external electrical field \(E_{ext}(\omega)\) at a fixed temperature of 313 K for decreasing field strengths as indicated. \(\Delta A_{0}\) is the noise level measured at the detector if no external electric field is applied. **(b)** is similar to **(a)** but for varying temperatures at fixed field strength as indicated. The data are taken from [56]. As a rough estimate, we compare (Figure 8) the interaction energy of a dipole with a point charge \(W(r)\) and the thermal energy \(E_{th}\). For each degree of freedom, the latter is \(\frac{1}{2}k_{B}T\approx 1.7\times 10^{-21}\) J per entity or \(\sim\)1 \(\frac{\text{kJ}}{\text{mol}}\) at a temperature of 250 K. The former can be calculated as [57]  
\[\varepsilon^{*}=\varepsilon^{\prime}-ie^{\prime\prime} \tag{7}\]3  
where \(q\) is the point charge, \(\mu\) is the absolute value of the dipole moment, \(\theta\) is the angle of the dipole axis with the connection line between the charge and the center of the dipole, \(\varepsilon\) is the (real part of the) permittivity of the medium between charge and dipole, and \(r\) is the distance along this connection line. For an OH moiety (treated as a dipole) interacting with another OH as bond acceptor (treated as a point charge), we use \(q=\frac{1}{2}e\) (where \(e\) is the elemental charge), \(\mu=4.5\) D, \(\theta=0^{\circ}\), \(\varepsilon=3\), and \(r=230\) pm to obtain an interaction energy of \(6.8\times 10^{-21}\) J per OH unit or \(\sim\)41 \(\frac{\text{kJ}}{\text{mol}}\). It is evident that this approximation includes many uncertainties of the involved parameters and thus can only provide a qualitative picture. Orientational Polarization Spectroscopy (OPS) offers completely novel perspectives to selectively study molecular dynamics: (i) By the use of tunable IR sources it will be possible to determine the response of atomistic transition moments separately in dependence of temperate and pressure. By that, detailed insights into the intra- and inter-molecular interactions of the system under study can be achieved. (ii) It is expected that also non-polar groups linked to a polar moiety will respond to the external electrical field. (iii) It is furthermore anticipated that the OPS-response of different polar moieties is complex and shifted in time for the different moieties of a molecular system depending on the potentials of the chemical bonds. (iv) The Debye theory of orientational polarization describes the response of a non-interacting single dipole connected to a heat bath. This delivers an exponential decay in the time domain and the Debye relaxation formula in the frequency domain. It is not expected that this idealized picture holds for molecular systems at the atomistic scale. \[\varepsilon^{*}=\varepsilon^{\prime}-ie^{\prime\prime} \tag{7}\]4 Scheme of the inter-molecular H-bond at varying temperatures demonstrating the increased constraint of the angular orientation of the H-O unit at lower temperatures due to the growing proximity of the H-bond acceptor.