On the magnetic circular dichroism of benzene. A density-functional study

Abstract

Spectroscopy of magnetic circular dichroism (MCD) provides enhanced information on molecular structure and a more reliable assignment of spectral bands than absorption alone. Theoretical modeling can significantly enhance the information obtained from experimental spectra. In the present study, the time dependent density functional theory is employed to model the lowest-energy benzene transitions, in particular to investigate the role of the Rydberg states and vibrational interference in spectral intensities. The effect of solvent is explored on model benzene-methane clusters. For the lowest-energy excitation, the vibrational sub-structure of absorption and MCD spectra is modeled within the harmonic approximation, providing a very good agreement with the experiment. The simulations demonstrate that the Rydberg states have a much stronger effect on the MCD intensities than on the absorption, and a very diffuse basis set must be used to obtain reliable results. The modeling also indicates that the Rydberg-like states and associated transitions may persist in solutions. Continuum-like solvent models are thus not suitable for their modeling; solvent-solute clusters appear to be more appropriate, providing they are large enough.

Vibrational Structure in Magnetic Circular Dichroism Spectra of Polycyclic Aromatic Hydrocarbons

“…The C–C bond lengths become both shorter and longer, but the relative changes do not exceed 4%. Behavior of the other molecules is similar (Table S3) and corresponds to changes previously seen in excited states of benzene or PAH6 ([6] helicene) …”

“…The C-C bond lengths become both shorter and longer, but the relative changes do not exceed 4 %. Behavior of the other molecules is similar (Table S3) and corresponds to changes previously seen in excited states of benzene 45 or PAH6 ([6] helicene). 39 In naphthalene, for example, we can identify vibrational structure of the first and second lowest-energy electronic bands (Figure 4).…”

“…With other functionals (CAM-B3LYP and ωB97X), standard and simplified Tamm–Dancoff approximations were also tested, using the sTD program (). B3LYP and other hybrid functionals were applied because of their previous good performance for similar problems; on LC-BLYP we wanted to document the effect of the GGA approximation, and the sTD approach was explored as a way to allow treating of larger molecules …”

Double‐buffered, heterogeneous CPU + GPU integral digestion algorithm for single‐excitation calculations involving a large number of excited states

“…The electronic structure of the nine‐chromophore model in Figure is nearly semiconductor‐like, and its lowest optically bright transition (at the LRC‐ ω PBE/3‐21G* level of theory) is actually the 27th excited state . Numerous excited states are also required to compute circular dichroism spectra within a sum‐over‐states formalism, as this approach often requires hundreds of excited states to converge, even for small organic molecules . Circular dichroism spectra for pyrrole (C 4 H 5 N) reported in Ref.…”

“…This step also incurs significant cost in single‐excitation theories of excited states, namely, TDDFT and CIS, if the number of excited states is large. This might be the case in a semiconductor, where even truncated models lead to a very large density of states, or in calculations of optical rotation parameters and circular dichroism spectra within a sum‐over‐states formalism, where several hundred excited states may be required to converge the spectrum . AIFDEM calculations use single‐excitation calculations on individual monomers as basis states to describe collective excitations, and as such a large number of monomer excited states is often required.…”