133985-08-7Relevant academic research and scientific papers
Heavy-atom tunneling in the ring opening of a strained cyclopropene at very low temperatures
Ertelt, Melanie,Hrovat, David A.,Borden, Weston Thatcher,Sander, Wolfram
, p. 4713 - 4720 (2014)
The highly strained 1H-bicyclo[3.1.0]-hexa-3,5-dien-2-one 1 is metastable, and rearranges to 4-oxacyclohexa-2,5-dienylidene 2 in inert gas matrices (neon, argon, krypton, xenon, and nitrogen) at temperatures as low as 3 K. The kinetics for this rearrangement show pronounced matrix effects, but in a given matrix, the reaction rate is independent of temperature between 3 and 20 K. This temperature independence means that the activation energy is zero in this temperature range, indicating that the reaction proceeds through quantum mechanical tunneling from the lowest vibrational level of the reactant. At temperatures above 20 K, the rate increases, resulting in curved Arrhenius plots that are also indicative of thermally activated tunneling. These experimental findings are supported by calculations performed at the CASSCF and CASPT2 levels by using the small-curvature tunneling (SCT) approximation. Quantum mechanical tunneling: Despite an estimated activation barrier of more than 6 kcal mol -1, the strained cyclopropene 1 rearranges at temperatures as low as 3 K to the carbene 2 in its triplet ground state (see figure). Experiments and theory provide clear evidence that the rearrangement proceeds through heavy-atom tunneling.
1H-bicyclo[3.1.0]hexa-3,5-dien-2-one. A strained 1,3-bridged cyclopropene
Sander, Wolfram,Bucher, G?tz,Reicbel, Felix,Cremer, Dieter
, p. 5311 - 5322 (2007/10/02)
Triplet 4-oxocyclohexa-2,5-dienylidene (5) gives 1H-bicyclo[3.1.0]hexa-3,5-dien-2-one (4) on irradiation into its long-wavelength triplet-triplet absorption band (λ = 508-566 nm). Bicyclus 4 was characterized by IR spectroscopy in partially oriented matrices, by deuterium and oxygen-18 isotopic labeling and by comparison of experimental data with ab initio calculations. 4 is highly labile and readily rearranges back to carbene 5 thermally or on visible light (λ = 470 nm) or infrared irradiation. The rates of the thermal 4→5 rearrangement have been measured in argon, krypton, xenon, and nitrogen matrices, and deuterium kinetic isotope effects have been determined. The data show that 4 is directly transformed into 5, with intersystem crossing being rate determining. At low temperatures (20 K), the rates are independent of temperature, which indicates that the rearrangement occurs via quantum mechanical tunneling. MP2/6-31G(d) calculations show that the cyclopropene ring in 4 is tilted by 129.6° with regard to the cyclopentene ring. The torsional angle between the two carbon 2pπ-orbitals in the cyclopropene ring is 9°, and the pyramidalization angles at C5 are 19.2°. The extra strain energy caused by distortion of the cyclopropene double bond is compensated by the π-stabilization energy of the dienone system. Thus, the total strain energy is only 54 ±1 kcal/mol - comparable to the strain energy of cyclopropene.
