TABLE V.

Energy gaps along the ring opening reaction of cyclobutene calculated with respect to the cyclobutene ground state energies. In the last column we reported the estimated C₁⋯C₄ bond distance at the top of the TS2 cyclization barrier. All QMC calculations are done with the ECP pseudopo-tential to replace the core electrons of the carbon atoms.

	s-trans (kcal/mol)	gauche (kcal/mol)	TS2 (kcal/mol)	RC1C4 (Å)
VMC1	−11.83(16)	−9.14(11)	36.25(11)	2.1389(2)
VMC2	−11.93(11)	−8.65(11)	36.51(10)
LRDMC1	−11.83(31)	−9.11(30)	35.34(31)
LRDMC2	−12.22(28)	−9.07(29)	35.23(29)
HFa		−13.05	49.26	2.130b
MP2c	−8.1	−5.5	35.3	2.131
MP4c	−9.8	−7.2	35.3
QCISD(T)d		−8.47	34.42
B3LYPe		−12.82	31.59	2.137
B3LYPd		−12.15	31.62	2.138
B3LYPa		−12.55	36.46
BLYPf	− 14.03	−10.25	31.41	2.142
NL-Pc	−13.5	−9.3	30.9	2.148
NL-SCFc	−13.2	−9.3	31.3
Exp34			34.5(7)g
Exp33			34.3(8)g
Exp32			34.1(5)g
Exp35	−10.54h

^aEnergies from Ref. 42 with basis set 6-311G**.

^bStructure obtained with 6-31G* from Ref. 38.

^cThe MP energies are obtained on MP2/6-311G** geometries, while the DFT calculations refer to a NL-SCF/TZ+2P geometrical optimization from Ref. 41.

^dBoth results obtained on B3LYP optimized geometries with 6-311++G** basis sets from Ref. 43.

^eCalculations with 6-311+G(d,p) basis set from Ref. 45.

^fCalculations with 6-31G* basis set from Ref. 38.

^gTo compare these results with the quantum chemical calculations presented we have removed the ZPE contributions estimated with the QCISD(T) method and reported in Ref. 43, which are equivalent to +1.6 kcal/mol. The published experimental values are 32.9(7),³⁴ 32.7(8)³³ and 32.5(5).³²

^hFrom the published experimental value of −11.5³⁵ we have removed the ZPE contributions estimated through the HF calculations of Ref. 38, and equal to +0.96 kcal/mol.