PPT Slide
Anomalous isotope effects
There are several clues that the harmonic approximation
has failed. Usually an isotope shift leads to a frequency
lowering since the frequency is inversely proportional to
the square root of the reduced mass for a particualar
mode. The anomalous shifts seen in the previous frame
are indicative of coupling of the vibrational mode to
other modes. This can be seen definitively in calculations
of adenine. The amino group has a low frequency
puckering vibration that has a bistable potential surface.
It is intrinsically anharmonic. It is also coupled to other
vibrations. We see this by projecting along the low
frequency mode and recalculating the frequencies of the
remaining modes. If they do not shift then they are not
coupled. If they do shift then they are coupled.