VARIABLE-TEMPERATURE AND VARIABLE-PRESSURE SPECTROSCOPIC STUDIES OF NORBORNADIENE

Differential scanning calorimetry, variable-temperature infrared and Raman spectroscopies, and variable-pressure Raman spectroscopy have been used to study the order-disorder phase transition in norbornadiene (bicyclo[2.2.1]hepta-2,5-diene). In the calorimetric measurements, the phase transition occurred at 201 K on heating and at 175 K on cooling, accompanied by an average entropy change of 41.6 J K-1 mol-1. The large hysteresis of the phase transition was also observed in the variable-temperature vibrational spectroscopic studies. The spectra suggest that the unit cell of the low-temperature phase is tetragonal, containing either two or four molecules. The same phase transition was induced by the application of high pressure, occurring at 6.4 (±1.0) kbar at room temperature. A previously unobserved case of Fermi resonance was discovered to occur between v4 (1563 cm-1) and a combination mode at 1574 cm-1, the point of maximum resonance being about 20 kbar. For the two sets of Fermi resonance doublets in the C-H stretching region, the resonance effects decreased slightly with increasing pressure. © 1990 American Chemical Society.