Photochemistry of Furyl- and Thienyldiazomethanes: Spectroscopic Characterization of Triplet 3-Thienylcarbene

Photolysis (λ \u3e 543 nm) of 3-thienyldiazomethane (1), matrix isolated in Ar or N2 at 10 K, yields triplet 3-thienylcarbene (13) and α-thial-methylenecyclopropene (9). Carbene 13 was characterized by IR, UV/vis, and EPR spectroscopy. The conformational isomers of 3-thienylcarbene (s-E and s-Z) exhibit an unusually large difference in zero-field splitting parameters in the triplet EPR spectrum (|D/hc| = 0.508 cm–1, |E/hc| = 0.0554 cm–1; |D/hc| = 0.579 cm–1, |E/hc| = 0.0315 cm–1). Natural Bond Orbital (NBO) calculations reveal substantially differing spin densities in the 3-thienyl ring at the positions adjacent to the carbene center, which is one factor contributing to the large difference in D values. NBO calculations also reveal a stabilizing interaction between the sp orbital of the carbene carbon in the s-Z rotamer of 13 and the antibonding σ orbital between sulfur and the neighboring carbon—an interaction that is not observed in the s-E rotamer of 13. In contrast to the EPR spectra, the electronic absorption spectra of the rotamers of triplet 3-thienylcarbene (13) are indistinguishable under our experimental conditions. The carbene exhibits a weak electronic absorption in the visible spectrum (λmax = 467 nm) that is characteristic of triplet arylcarbenes. Although studies of 2-thienyldiazomethane (2), 3-furyldiazomethane (3), or 2-furyldiazomethane (4) provided further insight into the photochemical interconversions among C5H4S or C5H4O isomers, these studies did not lead to the spectroscopic detection of the corresponding triplet carbenes (2-thienylcarbene (11), 3-furylcarbene (23), or 2-furylcarbene (22), respectively)

A computational study of the addition reaction of cyclopropenylidene with methyleneimine

The reaction mechanism between cyclopropenylidene and methyleneimine has been systematically investigated at the MP2/6-31+G*level of theory, including geometry optimization, vibrational analysis, and energy property for the involved stationary points on the potential energy surface. The energies of the different species are calculated by the single point energy calculations of CCSD(T)/6-31+G*//MP2/6-31+G*level. It was found that an important initial intermediate (INTA) characterized by spiro-compound structure has been located along the three pathways (1), (2R), and (2L) firstly. After that, another common intermediate (INTB) has been formed via TSB. At last, three different products possessing three- and four-membered ring characters have been obtained through corresponding reaction pathways. In the first reaction pathway (1), a three-membered ring alkyne compound has been obtained. As for the other two reaction pathways (2R) and (2L), the four-membered ring conjugated diene compound has been produced. As a result, the energy barrier of the rate-determining step of the pathway (1) is lower than that of the pathway (2R) and (2L), and the ultima product of pathway (2R) and (2L) is more stable than that of the pathway (1)