Theoretical Modeling of Deuteration-Induced Shifts of the 0–0 Bands in Absorption Spectra of Selected Aromatic Amines: The Role of the Double-Well Potential

Abstract

The harmonic approximation fails for inversion of the NH2 group in the ground state of aromatic amines as this vibration is characterized by a symmetric double-well potential with relatively small energy barrier. In such cases, the standard harmonic vibrational analysis is inapplicable: the inversion frequency calculated for the bottom of the potential well is strongly overestimated, while it attains imaginary values for the planar conformation of the molecule. The model calculations are discussed taking explicitly into account the presence of the double-well potential. The study is initially focused on reproduction of the deuteration-induced shifts of the 0-0 absorption band for anthranilic acid. The (incorrect) harmonic frequency of the NH2 inversion is replaced by a better one, obtained from numerical calculations employing a simple, quartic-quadratic model for the double-well potential, which is parametrized using just the harmonic frequency of the inversion and the height of the energy barrier. This operation brings theoretical results to qualitative agreement with experiment. A still better match is achieved with a modified version of the model that accounts for mixing of the NH2 inversion mode with other normal modes while retaining the initial simplicity of one-dimensional approach. The corrected results show surprisingly good accuracy, with deviations of the calculated shifts from the experimental values reduced to less than 5 cm(-1). In order to test the performance of the model for systems with higher energy barrier for the NH2 inversion, we have measured the LIF excitation spectra of three different amminobenzonitriles. Partial assignment of the 0-0 bands has been achieved based on their relative intensities for samples with different isotopic exchange ratios. Calculated shifts are in excellent agreement with experimental values for the identified bands. Theoretical predictions are used to complete the assignment of the 0-0 bands in the spectra of the studied amminobenzonitriles.

The lowest triplet states of bridged cis-2,2′-bithiophenes – theory vs. experiment

“…For example, it was recently demonstrated for anthranilic acid (AA) that, as a result of the molecular vibrations, it undergoes a dynamic planarization. 51 In order to find out whether such behavior may be observed also for DTP, we have calculated a relaxed potential energy scan along the normal coordinate connecting the two symmetry-related, distorted conformations of DTP. The quartic-quadratic function, which was fitted to the calculated energies, then served as the potential energy in the 1-dimensional Schro ¨dinger equation, which was numerically solved to obtain the ground state wavefunction for the out-ofplane vibration.…”

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].…”