Knowledge Bank

The fluorescence excitation spectrum of p-Cresol dimer displays over 20 vibrational transitions in the first $50cm-1$ of the spectrum. Most of these can be assigned as combinations and overtones of two nearly harmonic modes of 8.5 and $13cm-1$ which are atbtuted to the intermolecular bend and torsion. The spectrum also exhibits a $109cm-1$ mode and its overtone at $217cm-1$ which are due to the the intermolecular stretch, but shows no transitions above $450cm-1$. This is in sharp contrast to the p-crasol H2O spectrum which displays prominant ring modes up to $800cm-1$. Comparison of the dimer and water complex hydrogen bond strengths by use of their stretching frequendes and a crude dialomic model shows the H bonding to be appreciably stronger in the dimer. Thus the lack of intesity above $450cm-1$ in the dimer spectrum cannot be attributed to direct dissociation of the dimer, but is most liky due to IVR which may then be followed by dissociation.