RING-PUCKERING ENERGY-LEVELS FOR SPIRO COMPOUNDS

The ring-puckering potential energy surface for the two ring-puckering vibrations of a spiro compound is given by V = a(x1(4) + x2(4)) + b(x1(2) + x2(2)) + cx1(2)x2(2), where x1 and x2 are the ring-puckering coordinates for the two rings. Calculations for the energy levels have been carried out on three model systems for which the coefficients a and b were selected from known potential functions for silacyclobutane (barrier of 440 cm-1), cyclopentene (barrier of 232 cm-1), and 2,5-dihydrofuran (no barrier). The interaction term was varied over a range from -a to 3a. The case c = 0 represents no interaction between the two rings, while c = 2a represents the case of free pseudorotation. The energy levels for these two values of c were examined most carefully. The effect of a finite central barrier on the free pseudorotation was also examined in some detail.