TWO-DIMENSIONAL ANALYSIS OF THE RING-PUCKERING AND PH INVERSION VIBRATIONS OF 3-PHOSPHOLENE
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The ring-puckering energy levels for 3-phospholene, CH2CH = CHCH2PH, in both the ground and excited states of the PH inversion vibration have been established from the vibrational spectra of this molecule. The availablity of such extensive data made it possible to determine a reliable two-dimensional (with the ring-puckering and the P-H inversion as the two coordinates) potential energy surface. For the calculations, the kinetic energy coefficients g44, g55, and the cross term g45 were accurately represented as functions of both coordinates. An initial approximation to the asymmetric two-dimensional potential energy surface was obtained from the one-dimensional ring-puckering function as well as from structural data from microwave spectroscopy. Symmetry constraints established which potential terms were to be utilized. Three separate computer programs were written in order to calculate the vibrational energy levels and to adjust the potential parameters. The most efficient approach utilized a product of two prediagonalized basis sets for the basis functions. Initial attempts to fit the data with a potential surface using terms up through fourth power proved unsatisfactory in fitting data for the inversion excited state. A local solution based on Van Vleck perturbation theory was thus obtained and then expanded, using the symmetry constraints, to the entire solution. The resulting potential surface, which required the use of sixth power terms, gave excellent agreement with the observed spectra. Only five variable parameters were adjusted (for the local solution) in order to fit the 20 observed frequencies. The final potential surface has a central barrier of about 5500 cm-1for the planar configuration. The more stable endo conformation lies 785 cm-1in energy below that of the exo form with similar puckering angle (18). 1983 American Institute of Physics.
