Rotational spectra parameters, computation

Once quantum chemistry has provided all the information required, that is, rotational and centrifugal distortion constants and, if the case, hyperfine parameters as well as line intensities, a graphical simulation of the rotational spectrum can be performed. The latter requires the knowledge of the experimental technique involved. For example, if the frequency modulation with second harmonic detection is performed, then the second derivative of the natural spectrum is obtained (as seen in Figure 6.2). The graphical representation of the computed spectrum can then be... 

If the g- and hyperfine anisotropies are known from analysis of a solid-state spectrum, the line-width parameters (1, and yt can be used to compute the rotational correlation time, tr, a useful measure of freedom of motion. Line widths in ESR spectra of nitroxide spin labels, for example, have been used to probe the motional freedom of biological macromolecules.14 Since tr is related to the molecular hydrodynamic volume, Ft, and the solution viscosity, r, by a relationship introduced by Debye 15... 

A prominent example in this context is the recent detection of oxadisulfane (HSOH) via rotational spectroscopy [4]. The successful identification of HSOH among the products of the pyrolysis of (t-Bu)2SO was possible due to accurate predictions of the spectroscopic parameters of HSOH. In fact previous searches for HSOH without such predictions were unsuccessful [4]. As outlined by Winnewisser et al. [4], quantum chemical calculations were used to predict the HSOH rotational-torsional spectrum The equilibrium rotational constants were obtained at the CCSD(T)/cc-pCVQZ level of theory and then augmented by vibrational corrections at the CCSD(T)/cc-pVTZ level. Dipole moment components were also computed in order to predict the type of rotational transitions detectable and their intensity. 

Furthermore, a cw-EPR spectrum can be simulated based on quantum mechanics. The most widely used approach in EPR spectral simulation is based on the stochastic Liouville equation (SLE), which treats the electronic and nuclear spins quantum mechanically, while the nitroxide re-orientation motion is treated classically and parameterized in terms of rotational diffusion constants. The SLE approach is extremely efficient and capable of computing a spectrum in a fraction of a second. This enables iterative fitting of experimental spectra, including those that fall within the slow-motion regime. " However, SLE-based spectral simulations depend on the physical model used to describe the nitroxide motion, which usually requires a large number of parameters, and unique determination of nitroxide motion from simulation remains challenging. 

---