In the previous post, I looked at a class of molecule known as hexaphyrins, inspecting bond length alternation (BLA) at the so-called meso position, the carbon atom joining two pyrrole rings. A search of the difference in bond lengths at this position had shown two significant clusters of crystal structures.
The effect this has is illustrated below. A pyrrole with an NH group contributes two π-electrons to the conjugated periphery whereas a pyrrole with just an N contributes only one. Thus the four NH rings in EGIJEK contribute overall two more π-electrons to the periphery than EGIHUY with just two NH rings. This increases the π-electron count from 26 to 28, driving EGIJEK from a 4n+2 into a 4n π-electron count (n=7) and making it formally anti-aromatic. This in turn induces strong bond length alternation (as for example in cyclobutadiene).
Here is a reprise of the bond length table shown in the previous post, but for EGIJEK.
Meso distances, Å | abs(Δr) | |
---|---|---|
EGIJEK crystal, Ci symmetry, DOI: 10.5517/ccrts2d | ||
1,43657 | 1.37034 | 0.06623 |
1.35661 | 1.44895 | 0.09234 |
1.42356 | 1.37287 | 0.05069 |
B3LYP+GD3BJ/Def2-SVPP (FAIR DOI: 10.14469/hpc/6194) | ||
1.44019 | 1.38259 | 0.0576 |
1.37732 | 1.44481 | 0.06749 |
1.43261 | 1.38568 | 0.04693 |
ωB97XD/Def2-SVPP (FAIR DOI: 10.14469/hpc/6194 ) | ||
1.44575 | 1.37537 | 0.07038 |
1.36214 | 1.45508 | 0.09294 |
1.44221 | 1.36958 | 0.07263 |
Of the two functionals used in the calculations, the ωB97XD form slightly over-estimates the BLA, with the B3LYP slightly under-estimating it. This seems to tally with the earlier observations made for cyclo[n]carbons.
Perhaps these sorts of molecules might form useful reality checks on calculating bond length alternations in large ring cyclic conjugated molecules.
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