Organic Reaction Dynamics - EPFL Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created Date:

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Text of Organic Reaction Dynamics - EPFL Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created...

  • 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  à  TS1  à  I  à  TS2  à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  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  t