**Infrared multiphoton dissociation (IRMPD)** spectroscopy is employed to characterize of gas-phase molecular ion structure trapped ion in **Fourier Transform Ion Cyclotron Resonance (FT-ICR)** mass spectrometry and irradiated with infrared photon. Then, the molecular ion will further undergo various low energy reaction dynamics pathway due to photo absorption. In general, the recorded **IRMPD** spectrum with respect to dissociation channels contain key information on the molecular ion structure for the particular reaction channel. We investigate the reaction dynamics of chemical interest by using a combination of experimental and theoretical method to answer sophisticated reaction dynamics.
Often, we conduct density functional theory based molecular dynamics calculations to equilibrate our molecular ions at certain thermal energy based on integration of Newtonian equation.

where *F* and *m* is the force and mass of atom.

The theoretical **IR** spectrum is then calculated by taking the Fourier transform dipole moment autocorrelation function.

Eventually, theoretical spectrum shall provide information about the major population of molecular ion in the reaction chamber as shown above. Sometimes, transient intermediates can be characterized via formation of intra-molecular interaction which induces red shift or blue shift in certain vibrational modes.