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1  

Chapter  7  

Aroma,city  

The  goal  of  this  chapter:    •  Illustra,ng  the  mul,faceted  defini,on  of  aroma,city    •  Quan,fying  (an,)aroma,city.    •  Iden,fying  (an,)aroma,c  molecules  

Il   me   faut   cependant   avouer   que   la   chimie   proprement  dite   ne  m’a   jamais   beaucoup   intéressé.   Pourquoi   ?   Peut-­‐être  parce  que  des  no>ons  telles  que  celles  de  valence,  de  liaisons   chimique   etc.,  m’ont   toujours   semblé   peu   claires  du  point  de  vue  conceptuel.    René  Thom,  1983.    I  have  to  confess  that  I  have  never  been  really  interested  in  chemistry.  Why?  Maybe  because  such  no>ons  as  valence,  bond  lengths  etc.  have  always  seemed  unclear  to  me  from  the  conceptual  point  of  view.  

A  fuzzy  chemical  concept  

Chapter  7  Gonthier,  J.  Steinmann,  S.  N.;  Wodrich  M.  D.;  Corminboeuf,  C.  Chem.  Soc.  Rev.  in  press.  2012  

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2  

What  is  Aroma,city?  

Aroma,city    

Schleyer  and  Jiao,  Pure  Appl.  Chem.  1996,  68,  209-­‐218  

aroma1c  smell  (before  1825)  

Hight  carbon  ra1o  (before  1865)  

Discovery  of  benzene  Faraday  (1825)  

Subs1tu1on  >  addi1on  Erlenmeyer  (1866)  

Benzene  structure  Kekulé  1865  

Exalted  diamagne1c  suscep1bility  Pascal  (1910)  

Electron  sextet  Armit-­‐Robinson  (1925)  

4n+2  electron  Hückel  (1931)  

Ring  current  theory  Pauling  (1936)  

pi  electron  to  contribu1on  to  magne1c  suscep1bilty  London  (1937)  

Ring  current  effect  on  NMR  chemical  shiZ,  Pople  (1956)  

Magne1c  sucsep1bility  exalta1on  Dauben  (1969)  Flygare  (1970)  

Nucleus-­‐Independent  chemical  shiZ  (NICS)    Schleyer  (1996)  

historically  Structure  criteria  Reac,vity  criteria  Energy  criteria  Magne,c  criteria  

Quantum  chemical  evalua1on  of  delocaliza1on  energy  (2000)  

Structural  aspects  of  aroma1city    (not  discussed  here)  

Energy  evala1on  through  theromochemiscal  data    Kis1akowsky  (1936)  

Chapter  7  

Aroma,city  

1865  (Kekulé  structure)  

Chapter  7  

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Aroma,city  

Defini1on:   They   are   unreac1ve   to   common   double   bond  transforma1on.  

Benzene  undergoes  subs1tu1ons  rather  than  addi1ons  reac1ons  (Erlenmeyer,  1866).    

Chapter  7  

Ek =α + 2β cos kπ(n +1)

Ek =α + 2β cos 2kπ(n)

acyclic  chains  

cyclic  systems    (with  even  number  of  atoms)  

n=  number  of  atoms  

Chapter  7  

Hückel  theory  and  Aroma,city  

Defini1on:  Aroma1c  compounds  are  unusually  stable!  

k  =  0,  1,  2…  

k  =  1,  2,  3…  

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4  

Aroma,city  

First  Defini1on:  Aroma1c  compounds  are  unusually  stable!    Quantum  chemical  demonstra,on  using  MO  Theory:      

Num

ber o

f nod

es =

Ene

rgy

Chapter  7  

4n+2  π  electrons  

 E1=α+2β    

E2α+β  

The  stabiliza,on  compared  to  three  isolated    bonds:  8β-­‐  6β  =  2β  The  stabiliza,on  of  hexatriene  

bonds:  7β-­‐6β    =  β  

 E1=α+1.8β    

 E2=α+1.24β    

 E3=α+0.44β    

Aroma,city  

First  Defini1on:  Aroma1c  compounds  are  unusually  stable!    Quantum  chemical  demonstra,on  using  MO  Theory:      

Chapter  7  

4n  π  electrons  

 E1=α+2β    

E2=E3=α  

The  stabiliza,on  compared  to  three  isolated    bonds:  4β-­‐  4β  =  0  

Num

ber o

f nod

es =

Ene

rgy

 E1=α+1.62β    

 E2=α+0.62β    

The  stabiliza,on  of  butadiene  bonds:  4.48β-­‐4β    =  0.48β  

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Aroma,city  

Defini1on:  Aroma1c  compounds  are  unusually  stable!  Resonance  (old  terminology=mesomerism):  bonding  pagern  described  by  mul,ple  resonance  structures    

Conjuga1on  effects:  Stabiliza,on  when  double  bonds  come  into  conjuga,on    

Hyperconjuga1on/anomeric   effects:   Stabiliza,on   when   orbitals   of   the   π-­‐   and   σ  frameworks  interacts.      

Aroma1c  effects:  Aroma,c  stabiliza,on  energy    

Resonance  or  delocaliza1on  Energy:  The  Pauling-­‐Wheland  defini,on  of  “resonance  energy”    captures  all  these  “delocaliza,on”  effects”.  

)()(RE 1 Ψ−= EE ψ

The   quan,ty   obtained   by   subtrac,ng   the   actual   energy   of   the   molecule   in  ques,on  from  that  of  the  most  stable  contribu,ng  structure.  

Wheland,  “The  Theory  of  Resonance”,  1944,  Page  52.  Chapter  7  

Aroma,city  (RE  and  ASE)  

Conven1onal   Evalua1on   using   a   selec,on   of   reference   systems:   isodesmic   or  homodesmo1c  reac1ons   RE  vs.  ASE  3 C2H6 + 3 C2H4 + 6 CH4 -65.1 kcal/mol

3 C2H4 + + 3 C2H6 -48.5 kcal/mol

3 + 2 -36.0 kcal/mol

3 + + 3 -28.8 kcal/mol

3 + 3 C2H4 -20.5 kcal/mol

3 + 3 -22.5 kcal/mol

RE  

RE  

RE  (historic  but  flawed)  

ASE  value  recommended  

ASE  (from  Dewar)  

ASE  Warning:  these  equa1ons  are  not  balanced  for  hyperconjuga1on  and  branching  effects    (as  discussed  earlier  in  this  course)   Chapter  7  

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Aroma,city  (RE)  

Kis1akowsky’s  seminal  evalua1on  of  the  resonance  energy  of  benzene:    historically  important  but  flawed.    

Chapter  7  

Rappel:    hyperconjuga1on  

Resonance  in  Benzene  (RE)  

Direct  energy-­‐based  computa1onal  approaches:  aroma,c  molecules  are  more  stable  than  their  “localized”  analogues.  One  constructs  a  “localized”  resonance  structure  and  compute  the  delocaliza1on  energy  by  comparing  the  difference  between  the  energy  of  the  standard  molecule  and  that  of  the  lowest-­‐energy  resonance  structure.      

benzenerigid 1,3,5-cyclohexatriene

optimal 1,3,5-cyclohexatriene

VRE

AREΔEc

1.386Å  

1.316-­‐1.314Å  

1.517-­‐1.522Å  

=Adiaba,c  RE=  61.4  kcal  mol-­‐1  

=Ver,cal  RE  91.6  kcal  mol-­‐1  

(BLW)-­‐B3LYP-­‐pVTZ  

Chapter  7  

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7  

Block  Localized  Wavefunc,on  (BLW)  Method  

BLW:  B3LYP/6-­‐31G*  Y.  Mo,  S.  D.  Peyerimhoff,  J.  Chem.  Phys.  1998,  109,  1687–1697.  Y.  Mo,  L.  Song,  Y.  Lin  J.  Phys.  Chem.  A  2007,  111,  8291–8301.  

•  Evaluates  resonance  energies  (RE’s)  without  reference  molecules  

•  “Disables”  selected  interac,ons  

 

–

E(BLW)localized π

system

E(DFT)delocalized π

system

RE

= 65.4 kcal/mol

�

ΨIBLW = ˆ A {Φ1Φ2Φk}

)()3()2()1(2

211 iniiiii niβϕαϕβϕαϕ =Φ

∑=

=im

iijij C1µ

µµχϕ

⎩⎨⎧

≠=

=kiski

ikjl

jlklij

δϕϕ

=  +61.4  kcal/mol  

Chapter  7  

Aroma,city  in  Benzene  (ASE)  

The  ASE   evaluates   the   “extra   benzene   resonance   energy”   rela1ve   to   conjugated  but  non-­‐aroma1c  reference  molecules:  

The  recommended  value  based  on  chemical  equa,ons  is  28.8  kcal  mol-­‐1.  

 

 

 

A  related  aroma,city  probe  is  the  “extra  cyclic  resonance  energy”  (ECRE)  defined  as  the   difference   between   the   resonance   energies   of   a   cyclic   conjugated   compound  and  an  acyclic  polyene  either  with  the  same  number  of  bonds  (ECRE1)  or  with  the  same  number  of  diene  conjuga1ons  (ECRE2).  

 

A   third   alterna,ve   is   the   “isomeriza1on   stabiliza1on   energy”   (ISE)   based   on   the  energy  difference  between  cyclically  delocalized  and  merely  conjugated  isomers.  

Chapter  7  

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8  

Extra  Cyclic  Resonance  Energy  (ECRE)  

ECRE:    Energy   difference   between   the  resonance   energies   of   a   cyclic  conjugated   compound   and   its  corresponding  acyclic  polyene    

ECRE1:  same  number  of  double  bonds    

=  +35.6  kcal/mol  

ECRE  2:  same  number  of  conjuga1ons  

RE – RE

ECRE  >  0  Aroma,c  ECRE  ≈  0  Non-­‐aroma,c  ECRE  <  0  An,aroma,c  Y.  Mo,  P.  v.  R.  Schleyer  Chem.  Eur.  J.  

2006,  12,  2009–2020.  

RE – RE = +21.2 kcal/mol3 x

RE – 3 x RE = +25.5 kcal/mol

BLW:  B3LYP/6-­‐31G*  

+28.8  kcal/mol  (expt.)  

Balanced   for   hyperconjuga,on,   hybridiza,on   and   protobranchings   (as   seen  later  in  the  course).  

Best  experimental  aroma1c  stabiliza1on  energy  (ASE)  es1mate  for  benzene  

3+ 3+

61.4  kcal/mol   25.8  kcal/mol  

12.2  kcal/mol  

10.5  kcal/mol  

=  +24.8  kcal/mol  

=  +29.9  kcal/mol  

Chapter  7  

An,aroma,city  of  D2hCyclobutadiene  

Es,ma,ng  the  destabiliza,on   imparted  by  the  an,aroma,city  of  cyclobutadiene  is  more  of  a  challenge.    According   to   Hückel   Theory,   the   π-­‐electronic   energy   of   CBD   is   twice   that   of  ethene  (i.e.  RE=0).  

ΔHf   37.5   6.7  

2

114  

ΔH°  =  45.7  kcal/mol    

How  to  incorporate  the  strain  effects?  

1.588-­‐1.595Å  

1.317Å

 

Is  the  an,aroma,city  of  CBD  really  more  substan,al  than  the  aroma,city  of  benzene  (~30  kcal/mol)?  

Chapter  7  

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9  

BLW:  B3LYP/6-­‐31G*  

CBD  Resonance  energy  (RE)  

E(BLW)  Localized    

E(DFT)  Delocalized    

 =  +5.1  kcal/mol  

Extra  cyclic  resonance  energy  

ECRE=  –15.9  kcal/mol  

+26.6 kcal/mol+10.1 kcal/mol

RE – RE

+13.3 kcal/mol

RE – RE 2

Antiaromatic species do NOT have negative RE’s, but are net π stabilized!

ECRE2 based on models with two conjugations

CBD  is  only  modestly    destabilized  by  an1aroma1city!  

+10.5 kcal/mol"

+5.1 kcal/mol" +21.0 kcal/mol"

Flawed  literature  es,mates  –35  to  –45  kcal/mol  

ECRE2  for  D2h  Cyclobutadiene  (CBD)    

Chapter  7  

+ 4 H2C=CH2 4

+ 4 H2C=CH2 4imposed 135 deg

CCC angle

–14.18 kcal/mol

–3.79 kcal/mol

Flawed ASE evaluation

Better ASE evaluation

ECRE2  for  D4h  Cyclooctatetraene  (COT)    

(too large due to the 10-11 kcal/mol D4h COT ring strain)

Homodesmo1c  Equa1on    (nega,ve  aroma,c  stabiliza,on  energy,  ASE,  based  on  four  conjuga,ons)  

Extra  cyclic  resonance  energy  (ECRE)  

ECRE  =  –3.2  kcal/mol  

ECRE2 based on models with four conjugations

COT  is  very  weakly  destabilized  by  an1aroma1city!  

RE RE 4–

+10.5 kcal/mol+38.8 kcal/mol

BLW:  B3LYP/6-­‐31G*  

(B3LYP/6-­‐311+G**with  ZPE)  

Chapter  7  

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10  

C6H6  +  3H2    C6H12    ΔH1  =  -­‐49.8  kcal/mol  C6H8  +  3H2    C6H14    ΔH2  =  -­‐81.0  kcal/mol  

R1  

R2  

R3  

BLW-­‐HF  op,miza,on  on  C6H8:  

             R1                R2                  R3  

HF/6-­‐311+G(d,p)    1.324        1.463        1.329  BLW/6-­‐311+G(d,p)        1.316        1.517        1.312  

Basis  Set   VRE        ARE  

6-­‐311+G(d,p)    23.5          20.8  

ECRE  of  benzene:  

Basis  Set   ECRE1   ECRE2  6-­‐311+G(d,p)      36.7      25.7  

Isomeriza,on  Stabiliza,on  Energy  (ISE):    CH3   CH2  

H  H   ΔH  =  33.2  kcal/mol  

Schleyer,  P.  v.  R.,  Puhlhofer,  F.    Org.  Chem.  2002,  4,  2873.  

Aroma,city  of  Benzene  (ECRE1)  

aR

cR EE1ECRE −=

CC

CX

C

CC

CX

C

cRE

C

C

C

C X

C

C

C

C X

aRE

aR

cR EE2ECRE −=

CC

CX

C

CC

CX

C

cRE

aRE C

C

C

C X

c c

C

C

C

C X

c c

5-­‐Membered  Rings  

Chapter  7  

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11  

X

6-311+G(d,p)

ECRE1 ECRE2 NICS AlH BH CH+ CH- CH2 NH O PH S SiH+ SiH- SiH2

-1.1 -2.7 -28.2 42.8 7.7 34.2 25.8 30.8 21.7 -8.9 34.3 1.4

-3.5 -8.2 -35.4 19.1 3.4 17.9 12.8 17.9 12.7 -15.5 17.2 -0.8

5.4 15.8 46.9 -13.7 -4.1 -14.4 -14.8 -17.2 -15.9 11.2 -14.1 0.3

Table  Extra  cyclic  resonance  energies  (kcal/mol)  and  NICS  values  (ppm)  of  5-­‐membered  rings  C4XH4    

5-­‐Membered  Rings  

Chapter  7  

Op,mal  delocalized  and  localized  geometries  for  C5H5+,  C5H5

-­‐  and  C5H6  with  the  6-­‐311+G(d,p)  basis  set  

1.331Å

1.449Å

1.317Å

1.509Å

1.405Å

1.405Å

1.325Å

1.533Å

1.329Å

1.505Å

1.314Å

1.547Å

CC

CX

C

CC

CX

C

1.549Å 1.544Å

X=CH+

CC

CX

C

CC

CX

C

1.405Å 1.547Å

X=CH-

CC

CX

C

CC

CX

C

1.477Å 1.539Å

X=CH2

5-­‐Membered  Rings  

Chapter  7  

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12  

1.410Å

1.410Å

1.347Å 1.347Å

X=CH2+

X=CH2-

X=CH3

C

C

C

C X

1.514Å

1.324Å

1.314Å 1.480Å C

C

C

C X

1.409Å

1.409Å

1.364Å 1.364Å C

C

C

C X

1.520Å

1.322Å

1.321Å 1.524Å C

C

C

C X

1.467Å

1.324Å

1.324Å 1.501Å C

C

C

C X

1.518Å

1.313Å

1.316Å 1.534Å C

C

C

C X

Op,mal  delocalized  and  localized  geometries  for  C5H7+,  C5H7

-­‐  and  C5H8  with  the  6-­‐311+G(d,p)  basis  set  

5-­‐Membered  Rings  

Chapter  7  

Es,mate  of  conjuga,ve  and  hyperconjuga,ve  Stabiliza,ons  

Values are in kcal mol-1 BSE=bond separation energy

Molecule Conventional BSE BLW

2.5 5.7 5.4

5.0 7.8 10.5

3.7 14.8 9.9

0.2 15.1 20.1

NA NA 3.3

36.0 64.4 57.5

H3C CH3

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13  

Aroma,city  

Altnerna1ve  defini1on:  Aroma1c  compounds  exhibits  a  ring  current  !                            Magne1c  Criteria  

δ(1H )/ppm 0  7.3  

TMS  

6.2  

Chapter  7  

•  1H  NMR  chemical  shiZ:  A  criterion  most  o~en  used  experimentally.  

•  Due  to  the  ring  current  induced  by  an  external  magne,c  field,  the  inner  protons  are  shi~ed  upfield,  and  the  outer  protons  are  downfield-­‐shi~ed.  

Magne,c  Aroma,city  

Chapter  7  

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14  

Hydrogens  in  bridging  posi1ons  

δ1H [ppm]

E.  Vogel,  1983    

Magne,c  Aroma,city  

Chapter  7  

But  !!!  

4-­‐5  an,aroma,c    H1:  6.10  

H2:  7.71    nonaroma,c    

H1:  5.78  H2:  6.26    H3:  6.36  4-­‐membered  ring  is  an,aroma,c  

H1:  8.6  H2:  8.1    H3:  8.5  Nonaroma,c  

Magne,c  Aroma,city  

O

H2

H1

H2 H3

H1 H2

H3

H1

Chapter  7  

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15  

7Li+  NMR  Chemical  ShiZ  

•  Lithium  bonding  is  primarily  electrosta,c,  experimental  7Li  chemical  shi~s  generally  shows  ligle  varia,on  among  different  compounds.  

•  Lithium  ca,ons,  typically  coordinate  to  the  π  faces  of  aroma,c  (or  an,-­‐aroma,c)  systems.    

•  This  complexa,on  results  in  a  significant  shielding  (or  deshielding)  of  the  7Li  NMR  signal  due  to  ring  current  effects.  

Magne,c  Aroma,city  

Chapter  7  

7Li+  NMR  as  an  Experimental  probe  

 (aroma,c)  Paquege,  L.  A.  et  al,  JACS,  1990,  112,  8776.  

 (An,aroma,c)  Sekiguchi  et  al,  JACS,  1991,  113,  7081.  

Magne,c  Aroma,city  

Chapter  7  

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16  

•  Experimental  7Li  NMR  chemical  shi~s  can  be  well  reproduced  by  modern  computa,ons.  

•  The  clear  advantage  of  using  δ(7Li)  as  a  theore,cal  probe  lies  in  the  possibility  to  provide  a  comparison  with  7Li  NMR  spectrum  of  experimental  Li+  complexes.  

•  However,  the  number  of  Li+  complexes  and  therefore  the  u,lity  of  Li+  as  a  computa,onal  probe  are  rather  limited.    

Magne,c  Aroma,city  

Advantages  and  limita1ons  of  7Li+  NMR  Chemical  ShiZ  

Chapter  7  

Why  not  use  the  absolute  chemical  shielding  of  a  virtual  nucleus  to  probe  

(the  ring  current  effects  of)  aroma>city?                                  -­‐-­‐Schleyer  et  al,  JACS,  1996,  118,  6317.  

Magne,c  Aroma,city  

Chapter  7  

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17  

The twelve outside hydrogens resonate at δ = 9.0 ppm

The six inner hydrogens resonate at δ =-3.0 ppm

Upfield of TMS !!!

Nucleus-­‐Independent  Chemical  Shi~s    (-­‐14.2  ppm)  

1996  Schleyer:  Nucleus-­‐Independent  Chemical  Shi~s  (NICS):    J.  Am.  Chem.  Soc.  1996,  118,  6317.  

NMR  proper1es  of  [18]-­‐annulenes  

HH

HH

HH

HHH

H

H

H

HH H

H

H

H

Magne,c  Aroma,city  

Chapter  7  

-8.6 ppm

+20.9 ppm

-2.0 ppm

NICS  are  the  nega1ve  of  magne1c  shieldings  computed  at  non-­‐nucleus  posi1ons  Nega1ve  (diatropic,  upfield);  posi1ve  (paratropic,  downfield)  

Nucleus-­‐Independent  Chemical  ShiZs  =  NICS  (the  most  used  aroma1city  probe).  

Magne,c  Aroma,city  

Chapter  7  

4/29/12  

18  

benzene cyclohexane cyclobutadiene

Paratropic (B deshielded) Diatropic (B shielded)

Aromatic: long-range magnetic shielding Antiaromatic: long-range magnetic deshielding Non-aromatic:short range magnetic response

PW91/IGLOIII

Magne,c  criteria  :  the  nucleus-­‐independent  chemical  shi~  

Chapter  7  

Grid of NICS points

σ-­‐aroma1city  

“Cyclopropane  should  be  isoconjugate  with  benzene  and  hence  σ-­‐aroma1c.”  

Dewar,  J.  Am.  Chem.  Soc.  1984,  106,  669  

Dewar at Dyson

Chapter  7  

4/29/12  

19  

σ-­‐aroma1city  

Dewar,  J.  Am.  Chem.  Soc.  1984,  106,  669  Chapter  7  

H H

H

H

HH

Walsh  sp2  model  

σ-­‐aroma1city  

Chapter  7  

4/29/12  

20  

Why is the strain in cylcopropane ���and cyclobutane so similar?

angle deviation

60°

109.49° tetrahedral value

angle deviation

109.49°- 60°

49.49°

ring size ring strain kcal/mol

3 4 5 6 7

49.44° 19.44°

1.49° -10.56° -19.13°

27.7 26.8

7.1 0.7 6.8

Baeyer  Stain  (1885)  

Chapter  7  

Dewar’s  original  idea  of  “σ”  aroma1city  

Dewar,  JACS  1984,  106,  669.    Cremer  and  Kraka,  JACS,  1985,  107,  3800  and  3811.  Cremer  and  Gauss,  JACS,  1986,  108,  7467.  

Devia1on    due  to    σ-­‐aroma1city  

H2C CH2

H2C

6 σ electronsσ-aromatic

Chapter  7  

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21  

Evidence  for  the  σ-­‐aroma1city  of  cyclopropane  

Anisotropy  of  Current  Induced  Density  (ACID)  Herges  et  al.  Chem.  Rev.  2005,  105,  3758  

Diatropic current Negative NICS

The  proton  chemical  shiZ  appears  upfield  as  compared  to  a  similar  acyclic  molecule    

1H δ(ppm)

cyclopropane Propane (CH2)

0.22 1.1

-42.8 ppm

NICS Moran,  Heine,  Manoharan  and  Schleyer    Org.  LeZ.  2003,  5,  23.  

 

Chapter  7  

Evidence  for  the  σ-­‐an1aroma1city  of  cyclobutane  

ACID Herges  et  al.  Chem.  Rev.  2005,  105,  3758.  

Paratropic current Positive NICSzz

• Dewar   first   argued   that   cyclobutane   should   be   considered   as   σ-­‐an1aroma1city  but  predicted  a  small  destabiliza1on  energy.  

• The  σ-­‐an1aroma1city  hypothesis  is  confirmed  by  magne1c  criteria.  

NICS Moran,  Heine,  Manoharan  and  Schleyer      

Org.  LeZ.  2003,  5,  23.  

+55.1 ppm

Chapter  7  

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22  

Aroma1city  

Chapter  7  

Beyond  σ  and  π,  beyond  organic  chemistry  

P

P

2.204Å

P P

Super  σ-­‐aroma,c  

NICS ~ -50 ppm

Spherical  or  3D  aroma1cicy  

P4

Chapter  7  

closoboranes  

4/29/12  

23  

Science, 2001, Vol. 291, 859.

Prepara,on:  Laser  vaporiza,on    Spectra:  Photoelectron  spectra    Characteriza,on:  measured  and  computed  ver,cal  electron  detachment  energies  (VDE)  agree  well  

Chapter  7  

The  Concept  of  Double  Aroma1city  

•  What  is  double  aroma,city?  –  The  existence  of  two  mutually  orthogonal  Hückel  frameworks  

within  a  single  molecule  •  Stabiliza,on  of  the  structure  by  a  4n+2  π  electron  system  in  the  plane  

of  a  molecule  in  addi,on  to  the  normal  4n+2    π  electron  system  perpendicular  to  the  plane  

zx

y

Chandrasekhar,  Jemmis,  Schleyer  Tetrahedron  LeZ.1979,  3707.  Schleyer,  Jiao,  Glukhovtsev,  Chandrasekhar,  Kraka  J.  Am.  Chem.  Soc.  1994,  116,  10129.  

Radial in-plane

Chapter  7  

4/29/12  

24  

Prototype  Double  Aroma1c  

C6H3+  is  stabilized  by  6π  electrons  and  2  in-­‐plane  radial  

electrons  

H

H H

+15

3

π MO Radial MO

Chapter  7  

Tangen1al  σ  aroma1city

tangential p-p bonded σ- aromaticity

R. Hoffmann: “A chemist view on bonding in extended structures” 1988.

(CH3)82+  

D4h    

But pronounced diatropic ring current: NICSzz=-43

Long  C-­‐C  distance=2.361  Å  

Chapter  7  

4/29/12  

25  

Mini  Quiz  8  

1.   What  is  aroma1city?      2.   Which  isomer  is  the  most  stable,  which  is  the  stronger  acid  and  why?              3.   Iden1fy  each  of  the  following  molecules  as  being  aroma1c,  an1aroma1c  or  

non-­‐aroma1c.  How  would  you  verify  your  predic1on?      

HH

orH

H

2-

N

NH N

HN

Chapter  7