Structures, Automerizations, and Isomerizations of C₃H₂ Isomers

Abstract

A combination of experiment and theory provides insight into the structure and rearrangements of various C3H2 isomers. Photolysis of [13C]diazopropynes 6a−c under matrix isolation conditions affords C3H2 isomers containing a single 13C-label. With the aid of computed vibrational frequencies and intensities (CCSD(T)/cc-pVTZ), the seven 13C-isotopomers of triplet propynylidene 1a,b, singlet propadienylidene 2a−c, and singlet cyclopropenylidene 3a,b are readily distinguished by IR spectroscopy. Monitoring the distribution of the 13C-label during photolysis at either λ = 313 ± 10 nm or λ > 444 nm reveals the involvement of two photochemical automerization processes. At λ = 313 ± 10 nm, triplet propynylidene and singlet cyclopropenylidene photoequilibrate. The interconversion does not occur by a simple ring closure/ring opening mechanism, as hydrogen migration accompanies the interconversion. At λ > 444 nm, H2CC13C: (2b) and H2C13CC: (2c) rapidly equilibrate. Various lines of evidence suggest that the equilibration occurs through a cyclopropyne transition state. Computational results confirm that the planar isomer of singlet cyclopropyne (4a, C2v) is the transition state for the interconversion of 2b and 2c. Unexpectedly, the calculations predict that the isomer of this compound containing a tetrahedral carbon atom (4b, C2v) lies ca. 7 kcal/mol higher in energy than the planar form.