Density functional studies on the effects of hydrogen bonding on the formation of a charge-transfer complex between p-benzoquinone and 2,6-dimethoxyphenol.

The ability to form a ground-state charge-transfer (CT) complex between an electron acceptor, p-benzoquinone (BQ) and an electron donor, 2,6-dimethoxyphenol (DMOPh) was found to be enhanced by H-bonding of BQ to a hydrogen-bond donor, trifluoroacetic acid (TFA) and H-bonding DMOPh to a hydrogen-bond acceptor, 4-(N,N-dimethylamino)pyridine (DMAPy) [Chem. Phys. Lett. 2005, 401, 200]. Here is reported density functional theory (DFT) calculations to study the effect of H-bonding to electron donor and electron acceptor moieties on the ground-state CT complex formation ability between the aforementioned electron donor/acceptor pair. DFT calculations using B3LYP with the 6-311G(d,p) basis set show that the HOMO and LUMO energies of BQ drop on H-bonding to TFA through its C=O groups and the HOMO and LUMO energies of DMOPh increase on H-bonding to DMAPy via its O-H group. BQ molecules hydrogen-bonded as 1:1 and 1:2 complexes to TFA act as stronger acceptors than the bare molecule, while 1:1 complexes of DMOPh and DMAPy act as better donors. Vertical excitation energies for electronic transitions from the ground state to the first few excited states of BQ, DMOPh, DMAPy, and their different complexes have been investigated in the framework of time-dependent density functional theory (TD-DFT) to simulate and interpret experimental ultraviolet absorption spectra. Good agreement between experimental and calculated spectra is established. The enhancement of the CT complex formation ability between the BQ and DMOPh pair is favored by the strong H-bonding interaction of BQ with TFA as well as by the H-bonding interaction of DMOPh with DMAPy.