Probing Lewis Acid Base Interactions with Born-Oppenheimer Molecular Dynamics: The Electronic Absorption Spectrum of p-Nitroaniline in Supercritical CO2

The structure and dynamics of p-nitroaniline (PNA) in supercritical CO2 (scCO(2)) at T = 315 K and rho = 0.81 g cm(-3) are investigated by carrying out Born-Oppenheimer molecular dynamics, and the electronic absorption spectrum in scCO(2) is determined by time dependent density functional theory. The structure of the PNA scCO(2) solution illustrates the role played by Lewis acid base (LA-LB) interactions. In comparison with isolated PNA, the nu(N-O) symmetric and asymmetric stretching modes of PNA in scCO(2) are red-shifted by -17 and -29 cm(-1), respectively. The maximum of the charge transfer (CT) absorption band of PNA in scSCO(2) is at 3.9 eV, and the predicted red-shift of the pi ->pi* electronic transition relative to the isolated gas-phase PNA molecule reproduces the experimental value of -0.35 eV. An analysis of the relationship between geometry distortions and excitation energies of PNA in scCO(2) shows that the pi ->pi* CT transition is very sensitive to changes of the N-O bond distance, strongly indicating a correlation between vibrational and electronic solvatochromism driven by LA-LB interactions. Despite the importance of LA-LB interactions to explain the solvation of PNA in scCO(2), the red-shift of the CT band is mainly determined by electrostatic interactions.