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, H2CC13C:  (2b) and H2C13CC:  (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, C 2 v ) 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, C 2 v ) lies ca. 7 kcal/mol higher in energy than the planar form.

Five isomers of the carbon-rich molecule C5H2 are investigated computationally, using methods based on the coupled-cluster approximation. All of these structures are related to isomers of C3H2 via substitution of hydrogen by ethynyl or attachment of a C2 fragment to a carbene center. The two most stable forms of C5H2 are linear triplet pentadiynylidene (4) and singlet ethynylcyclopropenylidene (6). Both of these isomers have been observed in the laboratory, as has a thirdthe cumulene carbene pentatetraenylidene (5)which is predicted to lie about 15 kcal/mol above the linear triplet. Two other isomers are also studied:  ethynylpropadienylidene (7) and 3-(didehydrovinylidene)cyclopropene (8). Both are found to lie less than 25 kcal/mol above the most stable form of C5H2 and to possess rather large dipole moments. Predictions for the harmonic vibrational frequencies of 12C and mono-13C isotopomers, infrared intensities, and rotational constants are also presented. These should assist efforts to identify these molecules in the laboratory and in the interstellar medium.

1-Diazo-2,4-pentadiyne (6a), along with both monodeuterio isotopomers 6b and 6c, has been synthesized via a route that proceeds through diacetylene, 2,4-pentadiynal, and 2,4-pentadiynal tosylhydrazone. Photolysis of diazo compounds 6a-c (lambda > 444 nm; Ar or N2, 10 K) generates triplet carbenes HC5H (1) and HC5D (1-d), which have been characterized by IR, EPR, and UV/vis spectroscopy. Although many resonance structures contribute to the resonance hybrid for this highly unsaturated carbon-chain molecule, experiment and theory reveal that the structure is best depicted in terms of the dominant resonance contributor of penta-1,4-diyn-3-ylidene (diethynylcarbene, H-C[triple bond]C-:C-C[triple bond]C-H). Theory predicts an axially symmetric (D(infinity h)) structure and a triplet electronic ground state for 1 (CCSD(T)/ANO). Experimental IR frequencies and isotope shifts are in good agreement with computed values. The triplet EPR spectrum of 1 (absolute value(D/hc) = 0.6157 cm(-1), absolute value(E/hc) = 0.0006 cm(-1)) is consistent with an axially symmetric structure, and the Curie law behavior confirms that the triplet state is the ground state. The electronic absorption spectrum of 1 exhibits a weak transition near 400 nm with extensive vibronic coupling. Chemical trapping of triplet HC5H (1) in an O2-doped matrix affords the carbonyl oxide 16 derived exclusively from attack at the central carbon.

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