Subject: Carbenium ion heats of formation.

From: "George P. Ford" <[email-masked]>
Date: Tue, 6 Aug 96 11:26 CDT

"Park, Tae-Yun" writes:
> 
> Is it really true that MOPAC93 is not accurate enough fo delta_Hf
> calculation?  If so, what else do I have to try ?  Is there any other 
> way in the field of computational chemistry which can produce the 
> value of delta_Hf data as correct as possible for various gas-phase 
> carbenium ions??
>
Dear Tae-Yun:
 
How accurate do you need? The AM1 heats of formation for the isomeric C5 
carbenium ions you cite differ by ca 2 kcal/mole. The experimental difference is
about the same amount in the opposite direction, i.e. a relative error of about 
4 kcal/mole in the computed heats of formation. One should probably anticipate 
errors of at least this magnitude with any of the methods in MOPAC93.

Part of the problem has to do with the fact that the relative stabilities of the
isomeric carbenium ions depend on a delicate balance of intramolecular 
interactions. Getting these internal balances right is often the achilles' heel 
of the semiempirical methods. In the present context the failure of AM1 is 
probably related to its failure to fully account for the greater 
hyperconjugative ability of the more highly branched substituents. If so, it 
would fail to fully reproduce the tendency of this effect to offset the higher 
non-bonded repulsions present. As is well known properly accounting for 
non-bonded interactions has been one of the major stumbling blocks in the 
development of semiempirical theories. Perhaps less well known is the fact that 
these methods generally underestimate conjugative, and especially 
hyperconjugative, interactions.

For the levels of accuracy you appear to be seeking you will have to go to more 
elaborate, and considerably more expensive, DFT, or ab initio methods. 

============================================================================
George P. Ford                      |     email:  [email-masked]
Department of Chemistry             |       web:  http://www.smu.edu/~gford/
Southern Methodist University       | telephone:  (214)768-2479
Dallas, Texas 75275                 |       fax:  (214)768-4089
============================================================================