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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

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Page 1: Organic Reaction Dynamics - EPFL · Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created Date: 4/22/2012 10:10:24 AM

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  

Page 2: Organic Reaction Dynamics - EPFL · Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created Date: 4/22/2012 10:10:24 AM

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!  

Page 3: Organic Reaction Dynamics - EPFL · Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created Date: 4/22/2012 10:10:24 AM

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  

Page 4: Organic Reaction Dynamics - EPFL · Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created Date: 4/22/2012 10:10:24 AM

Chapter  6  

Organic  Reac0on  Dynamics  

Discrimina)ng  between  pathways  

Page 5: Organic Reaction Dynamics - EPFL · Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created Date: 4/22/2012 10:10:24 AM

Chapter  6  

Organic  Reac0on  Dynamics  

Discrimina)ng  between  pathways  

Carpenter  et  al.  J.  Am.  Chem.  Soc.  2000,  122,  41.  

Page 6: Organic Reaction Dynamics - EPFL · Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created Date: 4/22/2012 10:10:24 AM

Chapter  6  

Movement  of  the  “Real”  Poten0al  Energy  Surface  

Organic  Reac0on  Dynamics  

R  =  reactant  P  =  product  TS  =  transi0on  state  I  =  intermediate  

Page 7: Organic Reaction Dynamics - EPFL · Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created Date: 4/22/2012 10:10:24 AM

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  

Page 8: Organic Reaction Dynamics - EPFL · Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created Date: 4/22/2012 10:10:24 AM

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  

Page 9: Organic Reaction Dynamics - EPFL · Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created Date: 4/22/2012 10:10:24 AM

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  

Page 10: Organic Reaction Dynamics - EPFL · Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created Date: 4/22/2012 10:10:24 AM

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  

Page 11: Organic Reaction Dynamics - EPFL · Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created Date: 4/22/2012 10:10:24 AM

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  

Page 12: Organic Reaction Dynamics - EPFL · Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created Date: 4/22/2012 10:10:24 AM

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  

Page 13: Organic Reaction Dynamics - EPFL · Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created Date: 4/22/2012 10:10:24 AM

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  

Page 14: Organic Reaction Dynamics - EPFL · Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created Date: 4/22/2012 10:10:24 AM

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  

Page 15: Organic Reaction Dynamics - EPFL · Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created Date: 4/22/2012 10:10:24 AM

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  

Page 16: Organic Reaction Dynamics - EPFL · Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created Date: 4/22/2012 10:10:24 AM

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.  

Page 17: Organic Reaction Dynamics - EPFL · Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created Date: 4/22/2012 10:10:24 AM

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  

Page 18: Organic Reaction Dynamics - EPFL · Organic_Reaction_Dynamics Author: Clemence Corminboeuf Created Date: 4/22/2012 10:10:24 AM

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.