Photochemistry of Butatriene − Spectroscopic Evidence for the Existence of Allenylcarbene

Abstract

The photochemistry of butatriene (2), matrix-isolated in argon at 10 K, was investigated. UV irradiation (248 nm) produces vinylacetylene (1) and the dimer of acetylene and in addition methylenecyclopropene (3) as a major product. The photochemical 2 → 3 rearrangement is reversible, and on visible light irradiation (λ > 420 nm) 3 is transformed back to 2. The formation of 3 from 2 requires a [1,2]-H migration to allenylcarbene (8). According to MP2 and DFT calculations, 8 is a minimum on both the triplet (T) and the singlet (S) potential energy surface (PES). The comparison of measured and calculated infrared spectra suggests that two forms of 8 (either syn and anti form of S-8 or a mixture of S-8 and T-8) can be trapped at low temperatures in the matrix. The rearrangement 2 → 3 proceeds in a stepwise manner with S-8 as intermediate on the S−PES (calculated heat of formation at 298 K, ΔH°f(298) = 131.7 kcal/mol) and T-8 on the T-PES (ΔH°f(298) = 129.0 kcal/mol).

Tetrahedrane—Dossier of an Unknown

“…Three of the four observed products (vinylacetylene, methylencyclopropene, and acetylene) are expected from Equation (9)-(14), while butatriene forms from 2 under influence of visible light. [52] Thus, the formation of all observed species can be explained on kinetic as well as thermodynamic grounds without the intermediacy of 1. Moreover, as depicted in Figure 1, the activation energies for the reactions of 6 (40.7 {45.7} kcal mol À1 ) as well as of 8 (35.8 {46.2} kcal mol À1 ) to 1 are too high to compete with the rearrangements of Equations (9)- (14).…”

“…Combination of both factors, incorporation of the carbene carbon atom into a small ring and its conjugation with an exocyclic p bond, yields a ground-state singlet carbene [a] As B3LYP/6-311 + + G** is reported to predict 1 13 to be a closed shell C 1 structure, with the 0.2 kcal mol À1 higher lying C s symmetric interconversion TS for the anti isomer and DE ST = 3.9 kcal mol À1 ; [52] we computed the C 1 structure at CCSD(T)/cc-pVDZ: it converges to the C s -symmetric energy minimum.…”

“…The same stationary point is found by following the imaginary frequency of another C 2v -symmetric Lewis structure, which can be referred to as cyclobutyne and is predicted to be an energy minimum with MP2/6-31G*, MP4SDTQ/6-31G*//MP2/6-31G*, MP4SDTQ/6-311G**//MP2/6-31G*, MCSCFA C H T U N G T R E N N U N G (4,4)/3-21G, MCSCFA C H T U N G T R E N N U N G (4,4)/6-31G*, TCSCF/3-21G [25,56,57] in contrast to HF/6-31G*, B3 LYP/6-311G**, CASSCFA C H T U N G T R E N N U N G (12,12)/6-311G**, CCSD(T)/6-311G** [21] as well as its triplet pendant. Combination of both factors, incorporation of the carbene carbon atom into a small ring and its conjugation with an exocyclic p bond, yields a ground-state singlet carbene [a] As B3LYP/6-311 + + G** is reported to predict 1 13 to be a closed shell C 1 structure, with the 0.2 kcal mol À1 higher lying C s symmetric interconversion TS for the anti isomer and DE ST = 3.9 kcal mol À1 ; [52] we computed the C 1 structure at CCSD(T)/cc-pVDZ: it converges to the C s -symmetric energy minimum.…”

Finding the Transition State of Quasi-Barrierless Reactions by a Growing String Method for Newton Trajectories: Application to the Dissociation of Methylenecyclopropene and Cyclopropane

“…[19,25] In line with this, the energy of TS2 lies only 2.9 kcal/mol above that of the 1 A excited state of anti allenylcarbene (the ground state is a triplet [19,25]). The reaction leading from MCP to allenylcarbene is endothermic by 41.9 kcal/mol Table 1).…”

Dissociation of the Pyridazine Molecular Ion

“…Therefore, AC +• is mainly formed by the loss of N 2 , but it rapidly isomerizes to MCP , respectively. Experimental evidence for the existence of AC has been reported, 19,20 but not for AC +• to our knowledge. It is likely that AC +• would hardly be observed as a stable species due to its shallow potential well as described above.…”