Experimental and theoretical aspects of magnetic circular dichroism and magnetic circularly polarized luminescence in the UV, visible and IR ranges: A review

A historical sketch of the MCD (magnetic circular dichroism) spectroscopy is reported in its experimental and theoretical aspects. MCPL (magnetic circularly polarized luminescence) is also considered. The main studies are presented encompassing porphyrinoid systems, aggregates and materials, as well as simple organic molecules useful for the advancement of the interpretation. The MCD of chiral systems is discussed with special attention to new studies of natural products with potential pharmaceutical valence, including Amaryllidaceae alkaloids and related isocarbostyrils. Finally, the vibrational form of MCD, called MVCD, which is recorded in the IR part of the spectrum is also discussed. A final brief note on perspectives is given.

TURBOMOLE: Today and Tomorrow

TURBOMOLE is a highly optimized software suite for large-scale quantum-chemical and materials science simulations of molecules, clusters, extended systems, and periodic solids. TURBOMOLE uses Gaussian basis sets and has been designed with robust and fast quantum-chemical applications in mind, ranging from homogeneous and heterogeneous catalysis to inorganic and organic chemistry and various types of spectroscopy, light-matter interactions, and biochemistry. This Perspective briefly surveys TURBOMOLE's functionality and highlights recent developments that have taken place between 2020 and 2023, comprising new electronic structure methods for molecules and solids, previously unavailable molecular properties, embedding, and molecular dynamics approaches. Select features under development are reviewed to illustrate the continuous growth of the program suite, including nuclear electronic orbital methods, Hartree-Fock-based adiabatic connection models, simplified time-dependent density functional theory, relativistic effects and magnetic properties, and multiscale modeling of optical properties.

Efficient Protocol for Computing MCD Spectra in a Broad Frequency Range Combining Resonant and Damped CC2 Quadratic Response Theory

Coupled cluster response theory offers a path to high-accuracy calculations of spectroscopic properties, such as magnetic circular dichroism (MCD). However, divergence or slow convergence issues are often encountered for electronic transitions in high-energy regions with a high density of states. This is here addressed for MCD by an implementation of damped quadratic response theory for resolution-of-identity coupled cluster singles-and-approximate-doubles (RI-CC2), along with an implementation of the MCD A term from resonant response theory. Combined, damped and resonant response theory calculations provide an efficient strategy to obtain MCD spectra over a broad frequency range and for systems that include highly symmetric molecules with degenerate excited states. The protocol is illustrated by application to zinc tetrabenzoporphyrin in the energy region of 2-8 eV and comparison to experimental data. Timings are reported for the resonant and damped approaches, showing that a greater part of the calculation time is consumed by the construction of the building blocks for the final MCD ellipticity. A recommendation on how to use the procedure is outlined.

Magnet-Free Time-Resolved Magnetic Circular Dichroism with Pulsed Vector Beams

Magnetic circular dichroism (MCD) is a widely used spectroscopic technique which reveals valuable information about molecular geometry and electronic structure. However, the weak signal and the necessary strong magnets impose major limitations on its application. We propose a novel protocol to overcome these limitations by using pulsed vector beams (VBs), which consist of nanosecond gigahertz pump and femtosecond UV-vis probe pulses. By virtue of the strong longitudinal electromagnetic fields, the MCD signal detected by using the pulsed VBs is greatly enhanced compared to conventional MCD performed with plane waves. Furthermore, varying the pump-probe time delay allows monitoring the ultrafast variation of molecular properties.

Efficient Calculation of Magnetic Circular Dichroism Spectra Using Spin-Noncollinear Linear-Response Time-Dependent Density Functional Theory in Finite Magnetic Fields

Excited-state calculations in finite magnetic fields are presented in the framework of spin-noncollinear linear-response time-dependent density functional theory. To ensure gauge-origin invariance, London atomic orbitals are employed throughout. An efficient implementation into the Turbomole package, which also includes the resolution of the identity approximation, allows for the investigation of excited states of large molecular systems. The implementation is used to investigate the magnetic circular dichroism spectra of sizable organometallic molecules such as a zinc tetraazaporphyrin with two fused naphthalene units, which is a molecule with 57 atoms.

Magnetic Circular Dichroism of Naphthalene Derivatives: A Coupled Cluster Singles and Approximate Doubles and Time-Dependent Density Functional Theory Study

The UV-vis absorption and magnetic circular dichroism spectra of naphthalene and some of its derivatives have been simulated at the Coupled Cluster Singles and Approximate Doubles (CC2) level of theory, and at the Time-Dependent Density Functional Theory (TD-DFT) level using the B3LYP and CAM-B3LYP functionals. DFT and CC2 predict in general opposite energetic ordering of the L_b and L_a transitions (in gas phase), as previously observed in adenine. The CC2 simulations of UV and MCD spectra show the best agreement with the experimental data. Analysis of the Cartesian components of the electric dipole transition strengths and the magnetic dipole transition moment between the excited states have been considered in the interpretation of the electronic transitions and the Faraday B term inversion among the naphthalene derivatives.

Magnetic Circular Dichroism of meso-Phenyl-Substituted Pd-Octaethylporphyrins

Absorption and magnetic circular dichroism (MCD) spectra have been measured and theoretically simulated for a series of palladium octaethylporphyrins substituted at the meso positions with phenyl groups (n = 0-4). Analysis of the spectra included the perimeter model and time-dependent density functional theory (TDDFT) calculations. With the increasing number of phenyl substituents, the molecule is transformed from a positive hard (ΔHOMO > ΔLUMO) to a soft (ΔHOMO ≈ ΔLUMO) chromophore. This is manifested by a drastic decrease of the absorption intensity in the 0-0 region of the Q-band and by the strongly altered ratio of MCD intensities in the Q and Soret regions. Such behavior can be readily predicted using perimeter model, by analyzing frontier orbital shifts caused by various perturbations: alkyl and aryl substitution, insertion of a metal, and deviations from planarity. TDDFT calculations confirm the trends predicted by the perimeter model, but they fail in cases of less symmetrical derivatives to properly reproduce the MCD spectra in the Soret region. Our results confirm the power of the perimeter model in predicting absorption and MCD spectra of large organic molecules, porphyrins in particular. We also postulate, contrary to previous works, that the isolated porphyrin dianion is not a soft chromophore, but rather a strongly positive-hard one.

Magnetic circular dichroism spectra from resonant and damped coupled cluster response theory

A computational expression for the Faraday A term of magnetic circular dichroism (MCD) is derived within coupled cluster response theory and alternative computational expressions for the B term are discussed. Moreover, an approach to compute the (temperature-independent) MCD ellipticity in the context of coupled cluster damped response is presented, and its equivalence with the stick-spectrum approach in the limit of infinite lifetimes is demonstrated. The damped response approach has advantages for molecular systems or spectral ranges with a high density of states. Illustrative results are reported at the coupled cluster singles and doubles level and compared to time-dependent density functional theory results.

Theory of Molecular Vibrational Zeeman Effects as Measured with Circular Dichroism

We present a general theory that enables the first nonempirical computation of molecular vibrational Zeeman effects as are detectable with magnetic vibrational circular dichroism spectroscopy (MVCD). In this method, the second derivatives of the molecular magnetic moment appear to be essential to determine the observable MVCD intensities. Using a quasiharmonic approximation, computations based on our method allowed a band-to-band comparison of simulated to measured spectra. Given this new possibility of its reliable interpretation, MVCD spectroscopy may develop as a useful tool to yield detailed information on molecular vibrational states and structure, including achiral systems.

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

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.

Resolving Electronic Transitions in Synthetic Fluorescent Protein Chromophores by Magnetic Circular Dichroism

The detailed electronic structures of fluorescent chromophores are important for their use in imaging of living cells. A series of green fluorescent protein chromophore derivatives is examined by magnetic circular dichroism (MCD) spectroscopy, which allows the resolution of more bands than plain absorption and fluorescence. Observed spectral patterns are rationalized with the aid of time-dependent density functional theory (TDDFT) computations and the sum-over-state (SOS) formalism, which also reveals a significant dependence of MCD intensities on chromophore conformation. The combination of organic and theoretical chemistry with spectroscopic techniques also appears useful in the rational design of fluorescence labels and understanding of the chromophore's properties. For example, the absorption threshold can be heavily affected by substitution on the phenyl ring but not much on the five-member ring, and methoxy groups can be used to further tune the electronic levels.

A complex-polarization-propagator protocol for magneto-chiral axial dichroism and birefringence dispersion

A schematic representation of magneto-chiral effects.

Relative Stability of the La and Lb Excited States in Adenine and Guanine: Direct Evidence from TD-DFT Calculations of MCD Spectra

The relative position of La and Lb ππ* electronic states in purine nucleobases is a much debated topic, since it can strongly affect our understanding of their photoexcited dynamics. To assess this point, we calculated the absorption and magnetic circular dichroism (MCD) spectra of adenine, guanine, and their nucleosides in gas-phase and aqueous solution, exploiting recent developments in MCD computational technology within time-dependent density functional theory. MCD spectroscopy allows us to resolve the intense S0→ La transition from the weak S0→ Lb transition. The spectra obtained in water solution, by using B3LYP and CAM-B3LYP functionals and describing solvent effect by cluster models and by the polarizable continuum model (PCM), are in very good agreement with the experimental counterparts, thus providing direct and unambiguous evidence that the energy ordering predicted by TD-DFT, La < Lb, is the correct one.

Cob(I)alamin: insight into the nature of electronically excited states elucidated via quantum chemical computations and analysis of absorption, CD and MCD data

The nature of electronically excited states of the super-reduced form of vitamin B(12) (i.e., cob(I)alamin or B(12s)), a ubiquitous B(12) intermediate, was investigated by performing quantum-chemical calculations within the time-dependent density functional theory (TD-DFT) framework and by establishing their correspondence to experimental data. Using response theory, the electronic absorption (Abs), circular dichroism (CD) and magnetic CD (MCD) spectra of cob(I)alamin were simulated and directly compared with experiment. Several issues have been taken into considerations while performing the TD-DFT calculations, such as strong dependence on the applied exchange-correlation (XC) functional or structural simplification imposed on the cob(I)alamin. In addition, the low-lying transitions were also validated by performing CASSCF/MC-XQDPT2 calculations. By comparing computational results with existing experimental data a new level of understanding of electronic excitations has been established at the molecular level. The present study extends and confirms conclusions reached for other cobalamins. In particular, the better performance of the BP86 functional, rather than hybrid-type, was observed in terms of the excitations associated with both Co d and corrin π localized transitions. In addition, the lowest energy band was associated with multiple metal-to-ligand charge transfer excitations as opposed to the commonly assumed view of a single π → π* transition followed by vibrational progression. Finally, the use of the full cob(I)alamin structure, instead of simplified molecular models, shed new light on the spectral analyses of cobalamin systems and revealed new challenges of this approach related to long-range charge transfer excitations involving side chains.