Quantitatively Adequate Calculations of the H-Chelate Ring Distortion upon the S₀ → S₁(ππ*) Excitation in Internally H-Bonded o-Anthranilic Acid: CC2 Coupled-Cluster versus TDDFT

Abstract

The S → S(π → π*) excitation in o-aminobenzoic acid causes strengthening of the N-H···O intramolecular hydrogen bond. The interplay of the hydrogen bond shortening, the hydrogen atom dislocation along the hydrogen bond, and the skeletal relaxation is investigated. These effects often cause the appearance of dual fluorescence from the π-conjugated internally H-bonded molecules, which is traditionally interpreted as the evidence of the excited-state intramolecular proton transfer process: ESPIT. Hence, their quantitative modeling is an important but demanding task for computational photochemistry. Extensive calculations using CC2 method (the perturbative approximation to CCSD coupled-cluster) and TDDFT(B3LYP) were performed with the series of (aug)-cc-pVXZ(X = D,T,Q) basis sets. CC2 predicts remarkable shortening of the O···H distance by 0.273 Å accompanied by the skeleton relaxation that involves considerable distortions of valence angles of the amino group (up to 7.3°) and within the benzene ring (up to 5°). Additionally, moderate changes (<0.046 Å) of the bond alternation in the π-electronic system and the hydrogen atom dislocation along the hydrogen bond (0.043 Å) are predicted. The CC2 method yields 90% of the magnitude of the experimentally based geometry changes, estimated in the earlier studies via Franck-Condon fit to the LIF spectra, while the TDDFT results approach only 65% of the experimental values.

Optical Spectra of Oligofurans: A Theoretical Approach to the Transition Energies, Reorganization Energies, and the Vibronic Activity

“…The energies also decrease more steeply with 1/ n as compared to the experiment (for both absorption and emission). Such discrepancies have not been expected from the method that was used previously in several successful attempts of theoretical analyses of the optical spectra of unsaturated organic systems [ 33 , 34 , 35 , 36 , 37 ]. A closer inspection, however, shows that the respective errors of the CC2-based absorption and emission energies are close to each other, and the behavior is mimicked only by two of the tested DFT functionals: MN15 and tuned-CAM-B3LYP (cf.…”

“…In this study, we aim to calculate the global reorganization energies associated with the investigated electronic transitions, reproduce the available experimental absorption and emission spectra of oligofurans, and identify the normal modes that contribute to the observed vibronic structures. To achieve these goals, we employ the density functional theory (DFT) and its time-dependent variant (TDDFT) using several representative exchange-correlation functionals, as well as the ab initio approach using the approximate coupled clusters singles and doubles model (CC2) [ 30 , 31 , 32 ] that has already proven successful in similar applications [ 33 , 34 , 35 , 36 , 37 , 38 ] and can currently be regarded as the method of choice for theoretical analysis of spectroscopic properties of medium-sized (containing several tens of heavy atoms) organic systems. In order to select the best XC functionals for our purposes, as well as to further test the performance of CC2, we first calculate the electronic transition energies and the related reorganization energies for the oligofurans containing from two to nine rings and compare them with the available experimental data [ 16 , 28 ].…”

Optical Spectra of Oligofurans. A Theoretical Approach to the Transition Energies, Reorganization Energies, and the Vibronic Activity

“…The energies also decrease more steeply with 1/n as compared to the experiment (for both absorption and emission). Such discrepancies have not been expected from the method that was used previously in several successful attempts of theoretical analyses of the optical spectra of unsaturated organic systems [26][27][28][29][30]. A closer inspection, however, shows that the respective errors of the CC2based absorption and emission energies are close to each other, the behavior that is mimicked only by two of the tested DFT functionals: MN15 and tuned-CAM-B3LYP (cf.…”

“…In this study, we aim to calculate the global reorganization energies associated with the investigated electronic transitions, reproduce the available experimental absorption and emission spectra of oligofurans, and identify the normal modes that contribute to the observed vibronic structures. To achieve these goals we employ the DFT/TDDFT methodology with several representative exchange-correlation functionals, as well as the ab initio approach using the approximate coupled clusters singles and doubles model (CC2) [23][24][25] that has already proven successful in similar applications [26][27][28][29][30][31] and currently can be regarded as the method of choice for theoretical analysis of spectroscopic properties of medium size (containing several tens of heavy atoms) organic systems. In order to select the best XC functionals for our purposes, as well as to further test the performance of CC2, we first calculate the electronic transition energies and the related reorganization energies for the oligofurans containing from 2 to 9 rings and compare them with the available experimental data [1,17].…”