Jan Martin and his group at the Weizmann Institute continue to push the envelope in developing a computational rubric that produces computed energies with experimental accuracy. Their latest attempt tries to balance off computational accuracy with performance, and they propose the W3.2lite composite method,1 which includes, among other things, an empirical correction for including triples and quadruples configurations.

Amongst the test molecules they discuss are the benzynes (the ortho, meta, and para diradicals) discussed at great length in Chapter 4.4 of my book. The W3.2lite estimate heats of formations are 112.06 ± 0.5, 125.06 ± 0.5, and 139.03 ± 0.5 kcal mol-1 for the o-, m-, and p-benzyne, respectively. This compares with the experimental2 estimates of 108.8 ± 3, 124.1 ± 3.1, and 139.5 ± 3.3 kcal mol-1, respectively. This demonstrates nice agreement between the computed and experimental values. A similar sized difference is obtained for the singlet-triplet gap of p-benzyne: 5.4 ± 0.6 with W3.2lite and 3.8 ± 0.5 kcal mol-1 estimate from ultraviolet photoelectron spectroscopy.3

References

(1) Karton, A.; Kaminker, I.; Martin, J. M. L., "Economical Post-CCSD(T) Computational Thermochemistry Protocol and Applications to Some Aromatic Compounds," J. Phys. Chem. A 2009, DOI: 10.1021/jp900056w.

(2) Wenthold, P. G.; Squires, R. R., "Biradical Thermochemistry from Collision-Induced Dissociation Threshold Energy Measurements. Absolute Heats of Formation of ortho-, meta-, and para-Benzyne," J. Am. Chem. Soc. 1994, 116, 6401-6412, DOI: 10.1021/ja00093a047.

(3) Wenthold, P. G.; Squires, R. R.; Lineberger, W. C., "Ultraviolet Photoelectron Spectroscopy of the o-, m-, and p-Benzyne Negative Ions. Electron Affinities and Singlet-Triplet Splittings for o-, m-, and p-Benzyne," J. Am. Chem. Soc. 1998, 120, 5279-5290, DOI: 10.1021/ja9803355.

InChIs

o-benzyne: InChI=1/C6H4/c1-2-4-6-5-3-1/h1-4H
InChIKey=KLYCPFXDDDMZNQ-UHFFFAOYAO

m-benzyne: InChI=1/C6H4/c1-2-4-6-5-3-1/h1-3,6H
InChIKey=MDEXXEPHAKMVNO-UHFFFAOYAG

p-benzyne: InChI=1/C6H4/c1-2-4-6-5-3-1/h1-2,5-6H
InChIKey=AIESRBVWAFETPR-UHFFFAOYAI