Atoms-in-molecules analysis of extended hypervalent five-center, six-electron (5c-6e) C(2)Z(2)O interactions at the 1,8,9-positions of anthraquinone and 9-methoxyanthracene systems

To clarify the nature of five-center, six-electron (5c-6e) C(2)Z(2)O interactions, atoms-in-molecules (AIM) analysis has been applied to an anthraquinone, 1,8-(MeZ)(2)ATQ (1 (Z=Se), 2 (Z=S), and 3 (Z=O)), and a 9-methoxyanthracene system, 9-MeO-1,8-(MeZ)(2)ATC (4 (Z=Se), 5 (Z=S), and 6 (Z=O)), as well as 1-(MeZ)ATQ (7 (Z = Se), 8 (Z = S), and 9 (Z = O)) and 9-MeO-1-(MeZ)ATC (10 (Z=Se), 11 (Z=S), and 12 (Z=O)). The total electronic energy density (H-b) at the bond critical points (BCPs), an appropriate index for weak interactions, has been examined for 5c-6e C(2)Z(2)O and 3c-4e CZO interactions of the n(p)(O)...sigma*(Z-C) type in 1-12. Some hydrogen-bonded adducts were also reexamined for convenience of comparison. The total electronic energy densities varied in the following order: (OO)-O-... (3: H-b(r(c)) = 0.0028 au) = (OO)-O-... (6: 0.0028 au) > (OO)-O-... (9: 0.0025 au) >= (NNHF)-H-... (0.0024 au) >= (OO)-O-... (12: 0.0023 an) >> (H2OHOH)-H-... (0.0015 an) > (SO)-O-... (8: 0.0013 au) = (SO)-O-... (2: 0.0013 au) > (SO)-O-... (11: 0.0012 au) = (SO)-O-... (5: 0.0012 au) > (HFHF)-H-... (0.0008 au) = (SeO)-O-... (10: 0.0008 au) = (SeO)-O-... (4: 0.0008 au) > (SeO)-O-... (1: 0.0007 au) >= (SeO)-O-... (7: 0.0006 au) >> (HCNHF)-H-... (-0.0013 au). H-b(r(c)) values for (SO)-O-... were predicted to be smaller than the hydrogen bond of (H2OHOH)-H-... and H-b(r(c)) values for (SeO)-O-... are very close to or slightly smaller than that for (HFHF)-H-... in both the ATQ and 9-MeOATC systems. In the case of Z = Se and S, H-b(r(c)) values for 5c-6e C(2)Z(2)O interactions are essentially equal to those for 3c-4e CZO if Z is the same. The results demonstrate that two n(p)(O)...sigma*(Z-C) 3c-4e interactions effectively connect through the central np(O) orbital to form the extended hypervalent 5c-6e system of the sigma*(C-Z)...n(p)(O)...sigma*(Z-C) type for Z=Se and S in both systems. Natural bond orbital (NBO) analysis revealed that n(s)(O) also contributes to some extent. The electron charge densities at the BCPs, NBO analysis, and the total energies calculated for 1-12, together with the structural changes in the PhSe derivatives, support the above discussion.