Microwave spectroscopic and computational study of prototypical isocyanates and isothiocyanates

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Stitsky, Joseph
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This thesis outlines the microwave spectroscopic and computational study of prototypical isocyanates (RNCO) and isothiocyanates (RNCS) to better understand how the geometry around nitrogen is influenced by different side chains (R) and terminal chalcogen substitution. Specifically, this thesis describes the comprehensive research done on allyl isothiocyanate (H2C=CHCH2-NCS) (for a comparison with previous results for allyl isocyanate (H2C=CHCH2-NCO)) along with a study of benzyl isocyanate (C6H5CH2-NCO) which was compared to a set of theoretical predictions for benzyl isothiocyanate (C6H5CH2-NCS). Microwave spectroscopy is a useful technique for these investigations as the data collected using the broadband chirped-pulse Fourier transform microwave (cp-FTMW) and the narrowband Balle-Flygare Fourier transform microwave (FTMW) spectrometers are intrinsically sensitive to the underlying molecular geometries of the conformers in each sample. In the allyl-NCS study, microwave transitions were assigned to two conformers including the first report of the lowest energy form and its heavy atom minor isotopologues which enabled a structural derivation. A comparison of the calculated nitrogen hybridization between allyl-NCS and allyl-NCO revealed a change from a ~ sp1.5 environment in allyl-NCO to a ~ sp environment in allyl-NCS which is consistent with the change in the experimentally derived CNC bond angles. For benzyl-NCO, transitions were assigned to the global minimum conformer and its 13C isotopologues. An analysis of the nitrogen hybridization revealed that the benzyl-NCO nitrogen electronic environment is most similar to that of its allyl counterpart and a similar result was confirmed based on the computational predictions for benzyl-NCS when compared with allyl-NCS. A comparison of the results of this thesis with studies of other RNCO and RNCS species in the literature highlights the subtle balance of effects that the R group and terminal chalcogen substitution have on the properties of isocyanates and isothiocyanates.
Rotational Spectroscopy, Molecular Structure, Computational Chemistry, Conformers