Question

A peak observed at 1690 cm-1 in the mid-IR spectrum of guanine is assigned to the carbonyl stretching vibration

What would be the peak position be if the 488 nm line from the argon-ion laser was used instead of the 514.5 nm line? Give you answer in both nm and in relative cm–1.

What would the relative intensity of the peak be if the 488 nm line from the argon-ion laser was used instead of the 514.5 nm line? (Assume that the power output of the laser was the same at both wavelengths.)

Answer 1

The vibrational frequency is given by the equation

where k is the spring constant of the bond, is the reduced mass given by

The frequency only depends on these factors. It changes only if the symmetry of the molecule changes, property of the neighbouring group, resonance, different isotopes, any facor that effects the electron cloud. It doesn't depend on the power or property of the laser, if so each instrument will give different value and will be difficult to study.

So the answer is 1690 cm^-1 itself.

The intensity of the IR band depends only on the transition dipolemoment. It is not effected by the power of the laser used. The intensity will be the same. Transition dipolemoment is the electric dipolemoment associated with a transition, say from an initial state to a final state. It just depends on the transition.