Chapter 6
Organic Reac0on Dynamics
The goal of this chapter: • Understand the role that dynamics play in organic reac0on mechanisms
• Iden0fy important vibra0onal modes for reac0ons by looking at TS geometries
• Dis0nguish different mechanism types (concerted vs. stepwise) using dynamics
Organic Reac0on Dynamics
Chapter 6
Chemical reac0ons occur in the 0me domain, despite the picture of discrete cri0cal points along the poten0al energy surface presented in organic chemistry textbooks.
Classical View – Time Independent (Geometries and Energies of) • Reactants • Stable Intermediates • Products • Transi0on States
Dynamic View – Time Dependent • Both Par0cle Posi0ons and
Momenta • Achieved with either molecular
mechanics (Newtonian) or quantum chemical methods
Dynamics complicates the “clean” picture presented by the 0me-‐independent view, but is a more realis0c descrip0on of how chemical reac0on occur!
Chapter 6
Time-‐independent vs. Time-‐dependent descrip0ons
The solid black line represents a situa0on governed by 0me-‐independent processes:
Intermediate, transi0on State
The doWed line reveals that a reac0on may have excess energy, allowing it to “skip” steps on the minimum energy pathway
Low energy transi0on states and their corresponding intermediates may be bypassed completely if molecules are unable to quickly lose their poten0al energy
Organic Reac0on Dynamics
Chapter 6
Organic Reac0on Dynamics
Discrimina)ng between pathways
Carpenter et al. J. Am. Chem. Soc. 2000, 122, 41.
Chapter 6
Movement of the “Real” Poten0al Energy Surface
Organic Reac0on Dynamics
R = reactant P = product TS = transi0on state I = intermediate
Chapter 6
Movement of the “Real” Poten0al Energy Surface
The direct trajectory from reactants to products is given by: R à TS1 à I à TS2 àP2 Other pathways are possible when the PES is flat (indicated by doWed lines). “Hills” of higher energy can change the path, giving rise to semidirect trajectories leading to different products (e.g., leading from R à P3)
Organic Reac0on Dynamics
Chapter 6
A + BC à AB + C, A Prototypical Reac0on
Transi0on states can occur either early or late in a reac0on, which will require different types of energy to pass through
• Early à transla0onal energy is sufficient for the reac0on to proceed
• Late à vibra0onal energy is necessary for the reac0on to proceed
Molecules must have both the correct transla0onal energy, which moves the reactant molecules towards one another, and vibra0onal energy, which will help the reactants reorient themselves in the correct way to form the products
Organic Reac0on Dynamics
Chapter 6
A + BC à AB + C, A Prototypical Reac0on
Early TS, only transla0onal energy important
Late TS, reactant must have correc0on vibra0onal energy to “turn” on the PES
Organic Reac0on Dynamics
Chapter 6
Organic Reac0on Dynamics – What are they good for? • Can show if reac0on mechanisms proceed in a concerted or
stepwise fashion. • Cycloaddi0on reac0ons represent good examples
Organic Reac0on Dynamics
Chapter 6
The ac0va0on energy of 1,3-‐dipolar cycloaddi0on reac0ons is related to the distor0on energy (ΔEdist) required to distort the dipole and dipolarophile to form the transi0on state geometry This implies that the vibra0onal distor0ons represent an important aspect of the reac0on mechanism
Organic Reac0on Dynamics
Chapter 6
Visualiza0on of the transi0on structures and transi0on vectors (imaginary frequency eigenvectors) Main components of the transi0on vectors • Symmetric stretch of the incipient pair of σ-‐bonds • A dipole bending mode • Symmetric C2Hn bending mode These bending modes make up the transi0on vector leading to the distor0on required for the reac0on to occur
Organic Reac0on Dynamics
Chapter 6
Star0ng for a transi0on state obtained by 0me-‐independent quantum chemical computa0ons, trajectories can be run to es0mate the contribu0ons of various vibra0onal modes, etc. to the reac0ons ac0va0on barrier • Run many trajectories propagated over 0me to get a sta0s0cal sample that
resembles the energy distribu0on of reactants whole collision leads to the TS
These overlayed geometries represent the various conforma0ons when the reac0on passes near the TS Reactants must have the correct vibra0onal modes to obtain these geometries
Organic Reac0on Dynamics
Chapter 6
The overall picture of a reac0on looks like this (N2O + C2H2) • The N2O reactant bends back and forth surrounding the linear 180 geometry • As the reactant approach one another (moving from right to leh), the
energe0cally preferred pathway turns towards the products (boWom leh) • If the N2O bend has insufficient energy, conserva0on of momentum applies
and no reac0on would result ( the reactant would rebound of the leh-‐most energy barrier)
Organic Reac0on Dynamics
Chapter 6
Organic reac0on dynamics show that bending vibra0onal modes of the XYZ reactant must have a large amount of vibra0onal excita0on for the reac0on to occur
• This implies that “X” and “Z” atoms are approaching the C2H2 moiety together, a picture that coincides with a concerted mechanism, and not with a stepwise reac0on
Organic Reac0on Dynamics
Chapter 6
The preference for B over A comes from the trajectory of the atoms involved in the expulsion of N2. The momentum of the CH2 group as the hydrocarbon recoils from the expelled N2 is in the direc0on that directs it down past the plane formed by a planar symmetric radical.
Organic Reac0on Dynamics
Dynamic effects in chemical reac)ons are a topic of current interest.
Carpenter et al. J. Am. Chem. Soc. 2000, 122, 41.
Chapter 6
Overview • Organic reac0on dynamics provide a more realis0c picture of what
happens around a transi0on state • Vibra0onal modes, along with their associated excita0ons, are
important factors in determining if a reac0on will occur • Dynamics can be used to determine the nature of mechanism (e.g.,
stepwise or concerted), as demonstrated by the simple example provided earlier
Organic Reac0on Dynamics
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