Table 3. Relative Energies for Barrier Heights and Total Reaction Energies for the Bergman-like Cyclizations Examined in This Study^a.

elementary step	pathway	ΔE‡ (kcal mol–1)	ΔrE (kcal mol–1)
Penta-1,4-diyne Anion
5a → 5c	OSS (Cs)	+108.6	+106.5
	CSS (C2v)	+65.6	+32.9
	CSS (C1)		+21.8
(Z)-Hexa-3-ene-1,5-diyne
theoryc	CSSb	+34.3	+12.6
	tripletc	+32.9	–35.9
experimentd,e,f		+28.2d	+8.5d or +13e
Hepta-1,6-diyne Cation
7a → 7c	singlet	+30.7	–1.9
	triplet	+0.96	–46.6
7c → 7d	singlet	+6.1	–38.9
7a → 7d (stepwise)	singlet	+30.7	–40.8
Octa-1,7-diyne Dication
8a → 8cTL	triplet	+4.1	–21.9
8a → 8d	singlet	+41.8f	–73.9

^aAll barriers and reaction energies are adiabatic on their surfaces and were computed at the (SF-)UCCSD/cc-pVDZ level of theory for singlet and triplet pathways, respectively. Also included for comparison are theoretical and experimental estimates for the cyclization barrier and reaction energy of the canonical Bergman cyclization of (Z)-hexa-3-ene-1,5-diyne.

^bRelative energies constructed from absolute electronic energies computed at the SF-CCSD/cc-pVDZ level of theory with an unrestricted reference wave function, taken from Table S3 and in the Supporting Information of ref (22).

^cRelative energies constructed from absolute electronic energies computed at the CCSD/cc-pVDZ level of theory with an unrestricted reference wave function, taken from Table S7 and in the Supporting Information of ref (22).

^dValue taken from Figure 4 in ref (5).

^eValue taken from eq 10a in ref (75).

^fApproximate upper bound to this cyclization barrier estimated along a frozen-string reaction pathway⁷¹ constructed along the lowest singlet surface and computed at the CCSD/cc-pDVZ level of theory using a restricted reference wave function.