Electronic spectra of protonated benzaldehyde clusters with Ar and N2: effect of ππ* excitation on the intermolecular potential.

Electronic spectra of the S(1)←S(0) transition of dimers of protonated benzaldehyde (BZH(+)) with Ar and N(2) are recorded by resonance-enhanced photodissociation in a tandem mass spectrometer. The S(1) origins observed are shifted to higher frequency upon complexation with Ar (ΔS(1) = 300 cm(-1)) and N(2) (ΔS(1) = 628 cm(-1)). Ab initio calculations at the CC2/aug-cc-pVDZ level suggest an assignment to H-bonded dimers of L = Ar and N(2) binding to the cis isomer of O-protonated BZH(+), yielding values of ΔS(1) = 242 and 588 cm(-1) for cis-BZH(+)-L(H). Electronic ππ* excitation results in a substantial increase of the proton affinity of BZH(+), which in turn destabilizes the intermolecular H-bonds to the inert ligands by 35%. The drastic effects of electronic ππ* excitation on the geometric and electronic structure as well as the strength and anisotropy of the intermolecular potential (H-bonding and π-bonding) are investigated.