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    STUDIA CHEMIA - Ediţia nr.1-2 din 2000  
         
  Articol:   HEATS OF FORMATION OF MEDIUM-RING STRAINED CYCLO- AND POLYCYCLOALKANES: COMPARISON OF AB INITIO GROUP EQUIVALENT SCHEMES WITH THE PM3 AND MMX METHODS.

Autori:  LILIANA CRĂCIUN, JAMES E. JACKSON. 		 
       
         
  Rezumat:  Optimized structures and energies were calculated for 57 small- and medium-ring strained polycyclic aliphatic hydrocarbons using ab initio HF/3-21G, HF/6-31G* and HF/6-31G** as well as P (semiempirical) and MMX (force field) methods. Best fit CH2, CH and C group increments relating ab initio total energies to heats of formation were derived from the experimentally known Hf. The ab initio increments deviate little from those previously reported by Wiberg and by Ibrahim and Schleyer, yielding the expected conclusion that the intrinsically isodesmic group increment approach extends efficiently to small- and medium-ring strained systems. For the present data set, the standard deviation between experimental and calculated heats of formation, from ab initio total energies and group increments, is 1.81 kcal/mol for the RHF/6-31G*//RHF/6-31G* calculation, and 1.74 kcal/mol for RHF/6-31G**//RHF/6-31G**, respectively. Less successful results are obtained from the HF/3-21G, PM3 and MMX data. As expected, systems with fused small rings are especially problematic for the latter methods.  
         
     
         
         
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