ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
Recently we have shown that the lowest triplet state (T0) of pyridine, incorporated in a single crystal of benzene, may be studied by electron spin-echo (ESE) spectroscopy. From the nitrogen hyperfine structure in the ESE detected electron paramagnetic resonance (EPR) spectra, we were able to conclude that pyridine, a planar molecule in the ground state, becomes nonplanar upon excitation into T0. Here we report the results of a detailed investigation of this distortion and of the electronic nature of the lowest triplet state of pyridine-d5. We have performed electron spin–echo envelope modulation (ESEEM) spectroscopy. From the modulation spectra, the electron-nuclear double resonance (ENDOR) frequencies corresponding to the various deuterium nuclei are obtained. Analysis of the dependence of these frequencies on the orientation of the magnetic field with respect to the triplet fine-structure axes system allows for a determination of the deuterium hyperfine and quadrupole tensors. From these tensors and the known nitrogen hyperfine tensor, the structure and spin-density distribution of pyridine in its lowest triplet state are deduced. Pyridine adopts upon excitation into T0 a boatlike structure, in which the nitrogen atom is tilted by about 40° with respect to the plane through the ortho- and meta-carbon atoms and the para-carbon/para-deuterium fragment by about 10° with respect to this plane. Thereby the hybridization of the ortho-carbon atomic orbitals strongly deviates from that for aromatic hydrocarbons and becomes almost sp3; the hybridization of the atomic orbitals on the other carbon atoms changes much less, while the nitrogen atomic orbitals remain sp2 hybridized. Approximately half of the spin density is found to be localized on the nitrogen atom with a remarkable distribution over the atomic orbitals: the π orbital carries 40%, the n orbital 10%. The rest of the spin density is distributed over the para- (30%) and ortho-carbon atoms (10% each). The lowest triplet state of pyridine is neither an nπ* state nor a ππ* state, but a state of mixed character as the result of a strong vibronic coupling between the 3B1 (nπ*) and 3A1 (ππ*) states.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.457374
