Organic Reaction Dynamics - LCMD | EPFL .Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created

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  • 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

  • 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 TS1 I TS2 P2 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 P3)

    Organic Reac0on Dynamics

  • Chapter 6

    A + BC 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 transla0onal energy is sufficient for the reac0on to proceed

    Late 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 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 (Edist) 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 -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

  • Mini Quiz 7

    1. Reac)ons are more likely to bypass low energy intermediates in the gas phase than in solu)on. What is the reason?

    2. Can you envision a (2D) poten)al energy curve with P1-I1-I2-P2, that would

    cause the rela)ve energies of the intermediates to dictate the product ra)o.