Cyclobutyne: Minimum or Transition State?

Area of Science:

• Theoretical Chemistry
• Computational Chemistry
• Organic Chemistry

Background:

• Cyclobutyne has been a subject of extensive theoretical investigation and speculation.
• Previous theoretical studies proposed the existence of singlet cyclobutyne as a stable minimum.

Purpose of the Study:

• To investigate the electronic structure and stability of cyclobutyne using high-level theoretical methods.
• To resolve discrepancies in previous theoretical predictions regarding the nature of singlet cyclobutyne.

Main Methods:

• Coupled cluster computations
• Multireference configuration interaction (MRCI) calculations
• Density Functional Theory (DFT) for vibrational analysis

Main Results:

• Singlet cyclobutyne is confirmed as a transition state, not a minimum, with a ring-puckering imaginary frequency.
• Two equivalent cyclopropylidenemethylene minima were identified, with a barrier height of approximately 23 kcal/mol.
• Triplet cyclobutyne was found to be a genuine minimum, lying about 15 kcal/mol above the lowest singlet state.

Conclusions:

• The ground state of cyclobutyne is a transition state, challenging previous theoretical models.
• Singlet cyclopropylidenemethylene in C2v symmetry is also a transition state, contrary to prior predictions.
• The study provides new insights into the strain energy and bonding characteristics of cyclobutyne and its isomers.