Cryogenic Rearrangements of Spiroheptadiyl: Light- or Heavy-Atom Quantum Tunneling?

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Chemical Dynamics

Background:

• Triplet 1,4-spiro[2.4]heptadiyl (SHD) exhibits rapid ring-opening and hydrogen shifts upon photolysis.
• Previous hypotheses involved hot molecule effects and hydrogen tunneling in the reaction mechanism.

Purpose of the Study:

• To elucidate the potential energy surface and kinetics of the SHD cascade reaction.
• To investigate and clarify the fundamental principles of tunneling in chemical reactions involving hydrogen and carbon.

Main Methods:

• Utilized modern computational tools to model the reaction pathway.
• Analyzed the potential energy surface and reaction kinetics.
• Simulated the photolysis of a diazene precursor generating SHD at cryogenic temperatures.

Main Results:

• Disproved mechanisms involving hot molecule effects in ring-opening and hydrogen tunneling in the transfer step.
• Identified heavy-atom tunneling as the primary driver for SHD instability.
• Determined a subsequent photochemical hydrogen shift follows the initial tunneling event.

Conclusions:

• The mechanism of SHD instability is primarily driven by heavy-atom tunneling followed by a photochemical hydrogen shift.
• This finding reverses the originally proposed mechanistic sequence.
• The study corrects common misconceptions regarding the role of tunneling in chemical reactions.