In the investigation of minor incidents involving the spread of radioactivity at atomic energy establishments, identification and measurement is required of the radioactive nuclides involved. These measurements assist in estimating the magnitude of the release and in discovering its source. Examples are given of the role of alpha, beta and gamma spectrometry in Health Physics investigations of such incidents at A.E.R.E. Harwell.Among the items discussed are the contamination of reactor areas by fission products and the activation of reactor construction materials, the spread of fission product activity in disposal operations and the identification of activity in external body contamination. In these and in other cases spectrometry has been found to be a powerful tool for the rapid analysis of radioactive samples.
Pyridyl is a prototypical nitrogen-containing aromatic radical that may be a key intermediate in the formation of nitrogen-containing aromatic molecules under astrophysical conditions. On meteorites, a variety of complex molecules with nitrogen-containing rings have been detected with nonterrestrial isotopic abundances, and larger nitrogen-containing polycyclic aromatic hydrocarbons (PANHs) have been proposed to be responsible for certain unidentified infrared emission bands in the interstellar medium. In this work, the three isomers of pyridyl (2-, 3-, and 4-pyridyl) have been investigated with coupled cluster methods. For each species, structures were optimized at the CCSD(T)/cc-pwCVTZ level of theory and force fields were calculated at the CCSD(T)/ANO0 level of theory. Second-order vibrational perturbation theory (VPT2) was used to derive anharmonic vibrational frequencies and vibrationally corrected rotational constants, and resonances among vibrational states below 3500 cm–1 were treated variationally with the VPT2+K method. The results yield a complete set of spectroscopic parameters needed to simulate the pure rotational spectrum of each isomer, including electron–spin, spin–spin, and nuclear hyperfine interactions, and the calculated hyperfine parameters agree well with the limited available data from electron paramagnetic resonance spectroscopy. For the handful of experimentally measured vibrational frequencies determined from photoelectron spectroscopy and matrix isolation spectroscopy, the typical agreement is comparable to experimental uncertainty. The predicted parameters for rotational spectroscopy reported here can guide new experimental investigations into the yet-unobserved rotational spectra of these radicals.
Nitrogen-substituted polycyclic aromatic hydrocarbons (PANHs) are fundamental species important in biological molecules, astrochemistry, and combustion. They have been detected on meteorites with non-terrestrial isotopic abundances, potentially play a role in prebiotic chemistry, and have been observed in the atmosphere of Titan. They are suggested to be responsible for some of the unidentified infrared (UIR) emission bands in the interstellar medium and have been identified as intermediates in the combustion of biomass. Pyridine is a prototypical nitrogen-containing aromatic molecule, and recent studies suggest that the pyridyl radicals are key intermediates to the formation of PANHs. However, no high-resolution spectra or coupled-cluster calculations of the pyridyl radicals are available. To complement our search for the pyridyl radicals with rotational spectroscopy, we calculated the equilibrium geometries, relative energies, and spectroscopic parameters for the three pyridyl radicals at the CCSD(T) level of theory. In addition, anharmonic vibrational frequencies were evaluated using second-order vibrational perturbation theory. We will discuss the results of our calculations and our progress to measure the rotational spectra of the pyridyl radicals using Fourier transform microwave spectroscopy.
Radical chemistry is thought to play a role in the formation of complex molecules in space, including nitrogen heterocycles and prebiotic species. Radical derivatives of pyridine, one of the simplest nitrogen heterocycles, have been suggested as key intermediates in barrierless pathways to the formation of nitrogen-containing polycyclic molecules. We have calculated the equilibrium geometries of the ortho-, meta-, and para-pyridyl radicals at the CCSD(T)/cc-pwCVTZ level of theory, along with nitrogen quadrupole and hyperfine interaction terms. Spin-rotation terms were calculated at CCSD(T)/cc-pwCVDZ. Vibrational corrections to the rotational constants have also been evaluated at the CCSD(T)/cc-pwCVDZ level using second-order vibrational perturbation theory. We will discuss the results of these calculations as well as efforts to obtain the rotational spectra of these radicals using Fourier transform microwave spectroscopy.
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