Vibrational spectroscopy and conformational dynamics of small ring molecules

Abstract

Low-frequency infrared and Raman spectra have been recorded and the one-dimensional asymmetric ring-puckering potential energy functions have been determined for 3-phospholene (CH(,2)CH=CHCH(,2)PH),(' )3-phospholene-1-d(,1), 2-phospholene (CH=CHCH(,2)CH(,2)PH),(' )and 2-phospholene-1-d(,1). A single potential energy function for both isotopic forms of each molecule was determined. The most stable conformation of 3-phospholene was found to be the endo conformer (dihedral angle 18(DEGREES)). The planar and exo conformers are 331 and 785 cm('-1) higher in energy, respectively. One-, two-, and three-dimensional models using a vectorial representation of the ring-puckering, ring deformation, MH(,2) rocking and MX inversion vibrations of four and pseudo-four-membered rings were used to calculate and study kinetic energy terms. Kinetic energy expansions for principle G matrix elements and g(,45) terms, where appropriate, were determined. 1-Silacyclobutane-1-d(,1), CH(,2)CH(,2)CH(,2)SiHD,(' )has been prepared as an isotopic impurity and its mid- and far-infrared spectra have been analyzed. A ring-puckering potential function which simultaneously fits the data for the d(,0), d(,1), and d(,2) isotopic forms has been determined. Calculation of the wave functions and doublets observed for the SiH and SiD stretching bands showed that each of the two lowest very nearly degenerate energy states corresponds to only one distinct molecular conformation. The effect is produced by only a 0.20% difference in the reduced mass of the two conformers. The spectroscopic data associated with the interaction of the ring-puckering and PH inversion vibrations in 3-phospholene has been analyzed. Localized solutions assuming a harmonic PH inversion about the potential energy minima lead to the determination of a potential energy surface with the correct symmetry. Although the problem is complex, the potential energy surface determined satisfactorily reproduces the observed spectroscopic data. Energy levels were calculated using a computer program which was developed involving the complete form of the kinetic energy for two vibrations (i.e., kinetic energy surfaces for g(,44, )g(,55), and g(,45) type terms) and a wide variety of potential energy terms.