Rotation dynamics do not determine the unexpected isotropy of methyl radical EPR spectra

A simple first-principles electronic structure computation, further qc (quantum chemistry) computation, of the methyl radical gives three equal hf (hyperfine) couplings for the three protons with the unpaired electron. The corresponding dipolar tensors were notably rhombic and had different orientations and regular magnitude components, as they should, but what the overall A-tensor was seen by the electron spin is a different story! The final g = (2.002993, 2.002993, 2.002231) tensor and the hf coupling results obtained in vacuum, at the B3LYP/EPRIII level of theory clearly indicate that in particular the above A = (-65.19, -65.19, 62.54) MHz tensor was axial to a first approximation without considering any rotational dynamics for the CH3. This approximation was not applicable, however, for the trifluoromethyl CF3 radical, a heavier and nonplanar rotor with very anisotropic hf coupling, used here for comparison. Finally, a derivation is presented explaining why there is actually no need for the CH3 radicals to consider additional rotational dynamics in order for the electron to obtain an axially symmetric hf (hyperfine) tensor by considering the simultaneous dipolar couplings of the three protons. An additional consequence is an almost isotropic A-tensor for the electron spin of the CH3 radical. To the best of our knowledge, this point has not been discussed in the literature before. The unexpected isotropy of the EPR parameters of CH3 was solely attributed to the rotational dynamics and was not clearly separated from the overall symmetry of the species. The present theoretical results allowed a first explanation of the "forbidden" satellite lines in the CH3 EPR spectrum. The satellites are a fingerprint of the radical rotation, helping thus in distinguishing the CH3 reorientation from quantum rotation at very low temperatures

Conformational Analysis using 2D NMR Spectroscopy Coupled with Computational Analysis as an Aid in the Alignment Procedure of 3DQSAR Studies

The alignment of molecules in 3D-QSAR studies is an important step that affects considerably the outcome and the quality of CoMFA and CoMSIA models and subsequently the drug design and synthesis. Several applications will be reviewed with regard to the flexibility of the structures involved and the alignment procedures applied. Emphasis will be given to the contribution of NMR spectroscopy coupled with theoretical calculations to provide bioactive conformers in environments, which mimic the biological site of action. The aim of this review article is to inform medicinal chemists involved in the design and synthesis of novel drugs about the existing weaponry, which can be of aid in their research work