Spectroscopic and Theoretical Investigations of Halopyridines, Halobismuthates and a Spiro Ketal
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Infrared and Raman spectra of 2,6-difluoropyridine (26DFPy) and 2,3,5,6-tetrafluoropyridine were recorded and vibrational assignments made for their electronic ground states. Theoretical ab initio and density functional theory (DFT) calculations were used to complement the experimental work. The electronic excited states of these molecules were investigated with ultraviolet absorption spectroscopy and theoretical CASSCF calculations. For 2,6-difluoropyridine the structure is planar in its S1(π,π*) electronic excited state, but a barrier to planarity of 256 cm^-1 was predicted for its S2(n,π*) electronic excited state which has its band origin at 37820.2 cm^-1. For 2,3,5,6-tetrafluoropyridine a barrier of 124 cm^-1 was predicted for the S1(π,π*) state which has its band origin at 35704.6 cm^-1. Lower frequencies for the out-of-plane ring bending vibrations for the electronic excited states result from the weaker π bonding within the pyridine ring in these states. The infrared and Raman spectra of 2-chloro-, 3-chloro-, 2-bromo-, and 3-bromopyridine have been measured and assigned for the electronic ground state. Density functional theory (DFT) calculations at the B3LYP level of theory with the 6-311++G(d,p) basis set for vibrational frequencies produced excellent agreement with the experimental values. MP2 calculation methods with cc-pVTZ basis set were utilized to compute the molecular structures. A shortening of the N-C(2) bond resulting from halogen substitution on the C(2) carbon atom was shown for all 2-halopyridines. Theoretical calculations including ab initio and density functional theory (DFT) methods were carried out to study the bond lengths and vibrational frequencies of halobismuthates and haloantimonates. The results were also compared to experimental crystal structures, and to the infrared and Raman spectra of these species. Although the presence of cations was neglected in the calculations, the observed trends in the bond distances and bond stretching vibrational frequencies were verified. External bonds across from bridging bonds are the shortest and have the highest stretching frequencies for all of the ions investigated. This supports the previously postulated trans effect. The infrared and Raman spectra of the spiro molecule 2-cyclopenten-1-one ethylene ketal (CEK) have been recorded and compared to calculated spectra with good agreement. The structures and conformational energies for the two pairs of conformational minima, which can be defined in terms of ring-bending (x) and ring-twisting (τ) vibrational coordinates, have also been calculated. Utilizing the results from ab initio MP2/cc-PVTZ calculations, a two-dimensional potential energy surface (PES) was generated. The energy levels and wavefunctions for the PES were calculated and the characteristics of these were analyzed.
ultraviolet absorption (UV)
density functional theory (DFT)
Sheu, Hong-Li (2014). Spectroscopic and Theoretical Investigations of Halopyridines, Halobismuthates and a Spiro Ketal. Doctoral dissertation, Texas A & M University. Available electronically from