Intramolecular vibrational energy redistribution processes

Reaction dynamics as opposed to reaction kinetics strives to unravel the fundamentals of reactions—just how they transpire, how intramolecular vibrational energy redistributions provide energy to the modes most involved along the reaction coordinate, how specihc reaction states progress to specihc product states, why product energy distributions and ratios of alternative products are as they are, and, of course, how fast the basic processes on an atomic scale and relevant timeframe occur. 

Another important effect on the Norrish type I/II ratio is the occurrence of intramolecular vibrational energy redistribution (IVR). For short timescale processes shorter than 10 ps (such as the Norrish type I reaction), IVR is yet far from completed as assumed by statistical theories such as RRKM. The opposite is true for Norrish type II reaction. The reaction only starts after 20 ps, pointing out that IVR seems to be necessary for the reaction. The longer the cai bon chain (the larger... 

Reaction dynamics on the femtosecond time scale are now studied in all phases of matter, including physical, chemical, and biological systems (see Fig. 1). Perhaps the most important concepts to have emerged from studies over the past 20 years are the five we summarize in Fig. 2. These concepts are fundamental to the elementary processes of chemistry—bond breaking and bond making—and are central to the nature of the dynamics of the chemical bond, specifically intramolecular vibrational-energy redistribution, reaction rates, and transition states. 

Recently, however, experimental studies have cast a doubt on this assumption (see Ref. 1 for a review). For example, spectroscopic studies reveal hierarchical structures in the spectra of vibrationally highly excited molecules [2]. Such structures in the spectra imply the existence of bottlenecks to intramolecular vibrational energy redistribution (IVR). Reactions involving radicals also exhibit bottlenecks to IVR [3]. Moreover, time-resolved measurements of highly excited molecules in the liquid phase show that some reactions take place before the molecules relax to equilibrium [4]. Therefore, the assumption that local equilibrium exists prior to reaction should be questioned. We seek understanding of reaction processes where the assumption does not hold. 

Laser pulses of 3 jis duration and a typical fluence of 1.0 J/cm and Infrared absorption cross sections of 10 -10 cm yield photon absorption rates of 10 -10 photons/s, which are much slower than the rate of Intramolecular vibrational energy redistribution In systems such as benzyl anion. Hence, the anions are excited In a sequential process In which the vibrational energy Is redistributed before the next photon Is absorbed. This means that such experiments cannot determine In which vibrational mode(s) the energy resides. 

The first step in a unimolecular reaction involves energizing the reactant molecule above its decomposition threshold. An accurate description of the ensuing unimolecular reaction requires an understanding of the state prepared by this energization process. In the first part of this chapter experimental procedures for energizing a reactant molecule are reviewed. This is followed by a description of the vibrational/rotational states prepared for both small and large molecules. For many experimental situations a superposition state is prepared, so that intramolecular vibrational energy redistribution (IVR) may occur (Parmenter, 1982). IVR is first discussed quantum mechanically from both time-dependent and time-independent perspectives. The chapter ends with a discussion of classical trajectory studies of IVR. 

If the time interval between the formation of the activated intermediate complex and the collision for stabilization is long compared to the lifetime (r, Hk of the intermediate product, (A+M)+, the activated complex will decompose back to the initial reactants. Since the formation of the association products depends on the collision with the third-body gas molecules (N), the entire process is a competition between intermolecular collisional energy transfer and intramolecular vibrational energy redistribution of the excess energy of the activated intermediate complex among its degrees of freedom. 

The ECD process, by its nature, is a very rapid process and bond dissociation occurs faster than the redistribution of intramolecular vibrational energy that occurs with CID. This explains the dissociation of the strong N-C, amine bonds in the presence of the weaker C—N amide bonds in peptides and proteins.93,94 Consequently, any labile PTMs (e.g., phosphorylation, sulfation, 7-carboxylation, N- and O-glycosylation) are preserved and may be unequivocally located in the peptide/protein sequence. See also discussion in Section 9.10.3.2.6 on the use of ECD/ETD and CID/IRMPD for protein/peptide sequencing, and Table 4. 

---