H. Bernhard Schlegel

Coherent superposition of ground and excited states of cations can produce ultrafast charge migration that can be monitored by attosecond transient absorption, attosecond pump-IR probe, high harmonic spectroscopy, attosecond streaking, and strong field ionization. Vibrational motion on the ground and excited states of the cations leads to decoherence and decay of charge migration. Computational modeling of charge migration requires suitable methods for calculating energy differences between the electronic states of the cations and potential energy surfaces for the ground and excited states of the cations. The performance of CCSD, EOM-CCSD, EOMCC-IP, CASSCF, and MC-PDFT/L-PDFT has been examined for computing ionization energies, cation excitation energies, and harmonic vibrational frequencies for ground- and excited-state cations. Three sets of molecules that are relevant to charge migration were selected to illustrate different aspects of the calculations: HCCI+ and HCCBr+ have degenerate ground and first excited state of the same symmetry; CH3I+ and CH3Br+ are subject to Jahn–Teller distortion in their ground state; charge migration in vinyl bromide cation and propiolic acid cation involves the ground state and second excited state of the cations. CCSD is limited to the lowest energy state of a given symmetry. EOM-CCSD can be used for excited states. Both are easy to apply, “black box” methods. EOMCC-IP is an alternative that is less expensive than EOM-CCSD and is suitable for both ground and excited states of cations with degenerate vibrational modes. CASSCF with state averaging is needed for Jahn–Teller distortion cases. MC-PDFT is suitable for adding correlation energy to CASSCF and generally improves the CASSCF results. L-PDFT is required for the proper treatment of cations with degenerate or closely spaced electronic states.

Z-trisubstituted alkenes serve as key pharmaceutical building blocks and essential feedstocks for the selective synthesis and modification of drug molecules. Among these compounds, Z-trisubstituted alkenes that contain allylic tertiary amines are significant due to the increased solubility, lipophilicity, and metabolic stability provided by the tertiary amines. However, controlled synthesis of Z-trisubstituted alkenes with allylic tertiary amines has proven difficult, primarily due to thermodynamic challenges associated with the Z-isomer and the synthetic difficulties of accessing allylic tertiary amines. Here, we report a diene ligand-controlled rhodium-catalyzed amination of 1,1-disubstituted fluoromethyl allylic trichloroacetimidates with secondary aliphatic and aryl amines to afford the Z-trisubstituted alkenes bearing allylic tertiary amines in good yields (45–98% yields) and excellent selectivity (E/Z > 1:20). A mechanistic investigation was conducted using both experimental and computational Density Functional Theory (DFT). DFT calculations showed a preference for bidentate coordination of the diene ligand to the rhodium, which is needed for the ionization of the 1,1-disubstituted fluoromethyl allylic trichloroacetimidate to generate anti- and syn-π-allyl-rhodium intermediates. The E- and Z-differentiation occurs prior to nucleophilic attack of the secondary amines onto the more reactive π-allyl intermediates. The syn π-allyl intermediates lead to the minor E-trisubstituted alkene product, whereas the more reactive anti intermediates give the major Z-product. DFT calculations showed that the rate of conversion between reactive anti and syn π-allyl intermediates via π–σ–π interconversion was faster than the rate of nucleophilic attack, establishing a Curtin–Hammett condition. The fluoromethyl substrate was essential for the linear regioselectivity, as it induced a crucial hydrogen bonding interaction between the 1,1-disubstituted fluoromethyl allylic trichloroacetimidate and the secondary amine nucleophile. Overall, we anticipate that this methodology will be broadly applicable to the development of medically relevant Z-alkenes bearing allylic tertiary amines. The mechanistic insights gained will also serve as a foundation for addressing the challenges involved in other classes of Z-configured alkenes.

Recoil-frame momentum imaging resolves orbital-specific strong-field ionization in vinyl bromide, revealing orientation-dependent asymmetry with dominant contributions from HOMO−2.

Nucleobases play diverse structural and functional roles in biological systems. Understanding the fundamental properties of nucleobases is important for their applications as chemical probes of nucleic acid function. As the nucleobases are modified to tune their fluorescence or binding properties, their physical properties such as pK_a may also change. Unlike the canonical nucleobases, modified nucleobases are less well understood in terms of their acid-base properties. Previously, theoretical pK_a values of canonical, naturally modified, and aza-/deaza-modified nucleobases were determined. In this study, the theoretical pK_a values for 25 different fluorescent modified nucleobases (55 total pK_a values) were calculated by using an ab initio quantum mechanical method employing the B3LYP density functional with 6-31+G(d,p) basis set along with an implicit–explicit solvation model. The results of these computations are compared to known experimental pK_a values. The ability to estimate theoretical pK_a values will be beneficial for further development and applications of fluorescent nucleobases.

We report a novel water-soluble cobalt( ii )-based quinoline complex capable of bifunctional water splitting. The electrocatalytic behaviour towards water reduction and oxidation were studied, and corresponding mechanisms were proposed.

Quantum mechanics is central to our understanding of chemistry both qualitatively and quantitatively. Modern electronic structure calculations can yield energies and structures of small to medium size molecules to chemical accuracy, thereby providing a computational model for chemistry. A potential energy surface describes the energy of a molecule as a function of its geometric parameters. The features of potential energy surfaces provide the connections between quantum mechanics and the traditional chemical concepts such as structure, bonding and reactivity. This brief perspective presents an overview of tools for exploring potential energy surfaces such as optimizing equilibrium geometries, finding transition states, following reaction paths and simulating molecular dynamics.

For simulations of strong field ionization using time-dependent configuration with a complex absorbing potential (TDCI-CAP), standard molecular basis set must be augmented by several sets of diffuse functions to support the wave function as it is distorted by the strong field and interacts with the absorbing potential. Various sets of diffuse functions used in previous studies have been extended and evaluated for their ability to model the angular dependence of the strong field ionization. These sets include diffuse s, p, d, and f Gaussian functions with selected even-tempered exponents of the form 0.0001 × 2ⁿ placed on each atom. For single-centered test cases, the largest contribution to the ionization rate is from functions with a maximum in the radial distribution close to the onset of the complex absorbing potential, while functions with smaller exponents also contributed to the rate. For molecules, diffuse functions on adjacent centers overlap strongly, leading to linear dependencies. The transformation to remove these linear dependencies mixes functions of different angular momenta making it difficult to assess the importance of individual s, p, d, and f functions in simulating the rate for molecules. As an alternative, a hierarchy of diffuse basis sets was constructed by starting with a small set and adding one or two functions at a time. These basis sets were evaluated for their ability to reproduce the rate and shape of the angular dependence of strong field ionization. When combined with the aug-cc-pVTZ molecular basis set and an absorbing potential starting at 3.5 times the van der Waals radius for each atom, the most diffuse s, p, d, and f functions need to have exponents of 0.0032, 0.0032, 0.0064, and 0.0064, respectively, or smaller. Strong field ionization from electronegative atoms such as oxygen required additional f functions with tight exponents of 0.0512 and 0.1024. Diffuse basis sets that perform well for the angular dependence of the ionization rate with a static field are equally effective for strong field ionization with a linearly polarized 7-cycle 800 nm pulse.

Thiol is less reactive but more selective than alcohol. Alcohol-OH forms a stronger hydrogen bond with displaced bromide anion than thiol-SH, contributing to the difference in their reactivity and selectivity.

This Roadmap article provides a succinct, comprehensive overview of the state of electronic structure (ES) methods and software for molecular and materials simulations. Seventeen distinct sections collect insights by 51 leading scientists in the field. Each contribution addresses the status of a particular area, as well as current challenges and anticipated future advances, with a particular eye towards software related aspects and providing key references for further reading. Foundational sections cover density functional theory and its implementation in real-world simulation frameworks, Green’s function based many-body perturbation theory, wave-function based and stochastic ES approaches, relativistic effects and semiempirical ES theory approaches. Subsequent sections cover nuclear quantum effects, real-time propagation of the ES, challenges for computational spectroscopy simulations, and exploration of complex potential energy surfaces. The final sections summarize practical aspects, including computational workflows for complex simulation tasks, the impact of current and future high-performance computing architectures, software engineering practices, education and training to maintain and broaden the community, as well as the status of and needs for ES based modeling from the vantage point of industry environments. Overall, the field of ES software and method development continues to unlock immense opportunities for future scientific discovery, based on the growing ability of computations to reveal complex phenomena, processes and properties that are determined by the make-up of matter at the atomic scale, with high precision.

Strong field ionization of molecules by intense laser pulses can be simulated by time-dependent configuration interaction (TD-CI) with a complex absorbing potential (CAP). Standard molecular basis sets need to be augmented with several sets of diffuse functions for effective interaction with the CAP. This dramatically increases the number of configurations and the cost of the TD-CI simulations as the size of the molecules increases. The cost can be reduced by making use of spin symmetry and by employing an orbital energy cutoff to limit the number of virtual orbitals used to construct the excited configurations. Greater reductions in the number of virtual orbitals can be obtained by examining their interaction with the absorbing potential during simulations and their contributions to the strong field ionization rate. This can be determined from the matrix elements of the absorbing potential and the TD-CI coefficients from test simulations. Compared to a simple 3 hartree cutoff in the orbital energies, these approaches reduce the number of virtual orbitals by 20-35% for neutral molecules and 5-10% for cations. As a result, the cost of simulations is reduced by 35-60% for neutral molecules. The number of virtual orbitals needed can also be estimated by second-order perturbation theory without the need for test simulations. The number of virtual orbitals can be reduced further by adapting orbitals to the laser field using natural orbitals derived from test simulations. This is particularly effective for cations, yielding reductions of more than 20%.