Description of chemical reactions by the method of the full optimized reaction space. The ring-opening of unsubstituted and substituted cyclopropylidene to allene
The ring-opening reaction of singlet cyclopropylidene to singlet D/sub 2d/ allene is studied by means of ab-initio FORS-MCSCF calculations. The energy surface is determined for many intermediate geometries defined by the C-C-C opening angle and the two CH/sub 2/ rotation angles. For each choice of these three variables, the remaining twelve internal coordinates are relaxed by energy minimization. The results obtained differ markedly from those of earlier, less sophisticated calculations. As the C-C-C angle opens, the reaction is found to proceed first on a disrotatory pathway uphill to the transition region around a C-C-C angle of 84/sup 0/, where the reaction path bifurcates. It then follows one of two more or less conrotatory downhill pathways. From an opening angle of about 100/sup 0/ on there exists a state of free synchronized disrotatory motion in the molecule. It is shown that the reaction of the unsubstituted species is non-stereospecific. The characteristics of the bifurcating transition region, the first one ever found for a chemical reaction, are examined in detail and the theory governing similar surfaces is developed and generalized. It is shown that for a surface to exhibit such a bifurcation, a Valley Ridge Inflection (VRI) point must be present. The geometries are first calculated using a minimal basis set. Based on these geometries, extended basis set calculations (including polarization functions) are performed for key points along the reaction path. The main effect of this basis set improvement is a substantive lowering of the ring-opening barrier.
- Research Organization:
- Iowa State Univ. of Science and Technology, Ames (USA)
- DOE Contract Number:
- W-7405-ENG-82
- OSTI ID:
- 7138132
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
400201* -- Chemical & Physicochemical Properties
ALKENES
ALLENE
CHEMICAL REACTIONS
CYCLOALKENES
DATA
DIENES
HYDROCARBONS
INFORMATION
IRREDUCIBLE REPRESENTATIONS
MATHEMATICAL MODELS
MOLECULAR MODELS
MOLECULAR STRUCTURE
NUMERICAL DATA
ORGANIC COMPOUNDS
POLYENES
POTENTIALS
SELF-CONSISTENT FIELD
THEORETICAL DATA