title: Probing Lewis Acid-Base Interactions with Born-Oppenheimer Molecular Dynamics: The Electronic Absorption Spectrum of p-Nitroaniline in Supercritical CO2.
abstract: The structure and dynamics of p-nitroaniline (PNA) in supercritical CO2 (scCO2) at T = 315 K and ρ = 0.81 g cm(-3) are investigated by carrying out Born-Oppenheimer molecular dynamics, and the electronic absorption spectrum in scCO2 is determined by time dependent density functional theory. The structure of the PNA-scCO2 solution illustrates the role played by Lewis acid-base (LA-LB) interactions. In comparison with isolated PNA, the ν(N-O) symmetric and asymmetric stretching modes of PNA in scCO2 are red-shifted by -17 and -29 cm(-1), respectively. The maximum of the charge transfer (CT) absorption band of PNA in scSCO2 is at 3.9 eV, and the predicted red-shift of the π → π* 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 scCO2 shows that the π → π* 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 scCO2, the red-shift of the CT band is mainly determined by electrostatic interactions.
authors.affiliation: †Grupo de Física Matemática da Universidade de Lisboa, Av. Prof. Gama Pinto, 1049-003 Lisboa, Portugal.|‡Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Av. Professor Gama Pinto 2, 1649-003 Lisboa, Portugal.
authors.forename: Benedito J Costa
authors.initials: BJ
authors.lastname: Cabral
authors.affiliation: §Instituto de Física da Universidade Federal da Bahia, Campus Universitário de Ondina, CEP 40210-340 Salvador, Bahia, Brazil.
authors.forename: Roberto
authors.initials: R
authors.lastname: Rivelino
authors.affiliation: ∥Instituto de Física da Universidade de São Paulo, CP 66318, 05314-970 São Paulo, SP, Brazil.
authors.forename: Kaline
authors.initials: K
authors.lastname: Coutinho
authors.affiliation: ∥Instituto de Física da Universidade de São Paulo, CP 66318, 05314-970 São Paulo, SP, Brazil.
authors.forename: Sylvio
authors.initials: S
authors.lastname: Canuto
mesh_terms: D000814:Aniline Compounds / Q000737:chemistry*; D002245:Carbon Dioxide / Q000737:chemistry*; D058116:Lewis Acids / Q000737:chemistry*; D058115:Lewis Bases / Q000737:chemistry*; D056004:Molecular Dynamics Simulation
chemical_list: D000814:Aniline Compounds; D058116:Lewis Acids; D058115:Lewis Bases; D002245:Carbon Dioxide; C019498:4-nitroaniline