On the molecular mechanism of non-radiative decay of nitrobenzene and the unforeseen challenges this simple molecule holds for electronic structure theory

Abstract

In this work, we present a complete mechanistic picture of the non-radiative decay of the mono-substituted aromatic compound nitrobenzene from the bright singlet state to the electronic ground state. This mechanism involves internal conversion (IC) and inter-system crossing (ISC) along three dominating internal coordinates of the nitro group and consistently explains the experimental findings. Relaxation from the lowest triplet state via ISC occurs along the out-of-plane bending coordinate of the nitro group, while initial IC as well as ultrafast ISC into the triplet manifold take place along symmetric NO stretching and ONO bending modes that have not been considered yet. The proposed mechanism is based on high-level single- and multi-reference electronic structure calculations employing ADC3, MOM-CCSD(T), EOM-CCSD, DFT/MRCI and CAS-SCF/NEVPT2 levels of theory, which is, as we will demonstrate, absolutely necessary to assure a reliable and sufficiently accurate theoretical description of nitrobenzene. The need for third-order methods will be traced back to the large double-excitation character of about 50% of the second excited singlet state of nitrobenzene. As a result, second-order methods like approximate coupled-cluster of second order (CC2) and partially even (EOM-)CCSD yield a qualitatively wrong picture of the excited states. Surprisingly, already the description of the ground state geometries is problematic at the CC2 and partially also CCSD level of theory.

Quenching of Charge Transfer in Nitrobenzene Induced by Vibrational Motion

“…12 Electro-optical absorption measurements of nitrobenzene in cyclohexane 14 and low-energy electron-impact excitation of solid films of nitrobenzene, 18 both supported by theoretical calculations, seconded the characterization as an intramolecular CT transition. A recent extensive study employing a wide range of quantum-chemical methodologies backed this conclusion 21 despite showing large 3 discrepancies in the energy of the CT state depending on the method employed.…”

“…[8][9][10] In order to unravel the underlying mechanisms behind this rich photochemistry, it is of fundamental importance to properly characterize its molecular electronic excited states. Therefore, the electronic structure of nitrobenzene was widely investigated in the past by both experimental [11][12][13][14][15][16][17][18] and theoretical [18][19][20][21] techniques. Furthermore, nitrobenzene can be considered the smallest prototype of nitrated polycyclic aromatic hydrocarbons, a class of compounds that constitute an important type of widespread environmental pollutants with phototoxic effects, e.g., stimulating the formation of skin cancer.…”

Insights into the Complex Photophysics and Photochemistry of the Simplest Nitroaromatic Compound: A CASPT2//CASSCF Study on Nitrobenzene

“…8 Accordingly to their results, the 100 fs is due to decay back to the original ground state through a conical intersection (CI) between S 1 and S 0 mainly characterized by the shortening of the ONO angle, which can be reached by surmounting an energy barrier from the S 1 minimum. The high triplet quantum yield associated with the 6 ps transfer from S 1 to the triplet manifold is explained by the degeneracy in energy together with a high value of the spin-orbit coupling (SOC) that characterized the S 1 and T 2 states at the S 1 minimum.…”

Experimental Benchmark Data and Systematic Evaluation of Two a Posteriori, Polarizable-Continuum Corrections for Vertical Excitation Energies in Solution

“…60 This level of theory has previously been found to provide good agreement with experiment for a prototypical nitrobenzene, whereas MP2-, CC2-, and CCSD-optimized geometries deviated significantly. 61 In the case of the amino-methoxy disubstituted nitroaromatics, we find that the dispersion correction systematically improves the predicted solvent shifts, for all QM/PCM combinations considered here. Optimized geometries are available in the Supporting Information.…”