Molecular symmetry can influence the photochemical fate of molecules by controlling excited-state lifetimes and, consequently, the time window available for secondary reactions. We have investigated the primary photodynamics of aqueous fumarate and maleate, the trans- and cis-isomers of 2-butenedioate, using femtosecond transient infrared absorption spectroscopy. Following π* ← π excitation at 200 nm, both isomers undergo rapid decarboxylation to form CO2 and acrylate with identical quantum yields of maximum Φ = 30 ± 10%, independent of symmetry. Strikingly, excited-state lifetimes differ by more than an order of magnitude: fumarate remains excited for 5.9 ps, while maleate returns to its ground state in less than 0.5 ps. We attribute this disparity to the higher symmetry of fumarate, which may restrict nonradiative decay pathways. These results demonstrate that even when primary photoproducts are unaffected by symmetry, excited-state lifetimes - and thus the potential for subsequent bimolecular reactions - can be strongly impacted.

Near-UV photolysis of pyruvic acid results in decarboxylation within 0.8 ps. In contrast, pyruvate anions excited by near-UV light relax back to the ground state within 50 ps with no detectable photolysis. The sharply different behavior of the acid and its conjugate base provides a mechanistic explanation for the puzzling pH dependence of the pyruvic acid photolysis.

The methodology for calculating distributed atom-atom polarizabilities within the minimal basis iterative Stockholder partitioning framework is extended to coupled cluster (CC2, CCSD, CC3) and multiconfigurational self-consistent field (MCSCF) wave functions, as well as to frequency-dependent perturbations. The bond capacity, defined as the rank-0 atom-atom polarizability, is benchmarked on small molecules at equilibrium geometries, where various DFT functionals show good agreement with the CC3 reference. The same conclusion holds when probing geometry dependence through a change in the bond angle or dihedral angle, but a significant deviation is observed between the methods for reaction pathways such as SN2. The MCSCF implementation is used to study the values for bond breaking, where the rank-0 contribution to the molecular dipole polarizability is found to dominate for various systems. Non-nuclear sites for describing out-of-plane polarizability at the charge-only level are also explored but found to be ineffective with ab initio-derived parameters. Dynamic bond capacities at real and imaginary frequencies are presented and display method dependence consistent with that of the static values.

The methodology for calculating distributed atom-atom polarizabilities within the minimal basis iterative Stockholder partitioning framework is extended to coupled cluster (CC2, CCSD, CC3) and multiconfigurational self-consistent field (MCSCF) wave functions, as well as to frequency-dependent perturbations. The bond capacity, defined as the rank-0 atom-atom polarizability, is benchmarked on small molecules at equilibrium geometries, where various DFT functionals show good agreement with the CC3 reference. The same conclusion holds when probing geometry dependence through a change in the bond angle or dihedral angle, but a significant deviation is observed between the methods for reaction pathways such as SN2. The MCSCF implementation is used to study the values for bond breaking, where the rank-0 contribution to the molecular dipole polarizability is found to dominate for various systems. Non-nuclear sites for describing out-of-plane polarizability at the charge-only level are also explored but found to be ineffective with ab initio-derived parameters. Dynamic bond capacities at real and imaginary frequencies are presented and display method dependence consistent with that of the static values.

A gas-phase sample consisting of lithium iodide, LiI, molecules and their dimer (LiI)2 are Coulomb exploded by an intense 25-fs laser pulse. In the case of LiI, we focus on the double ionization that creates a pair of Li+ and I+ recoil ions. From the kinetic energy distribution of the Li+ ions, extracted using coincidence filtering, we determine the distribution of internuclear distances P(R) via the ground-state potential curve of LiI2+ obtained from an ab initio calculation that accounts for non-Coulombic effects. We find that the center of P(R) is close to the expected internuclear separation based on the three vibrational states of LiI populated, whereas the width of P(R) exceeds the theoretical value by similar to 52%. We discuss whether fragmentation via excited LiI2+ potential curves affects the determination of P(R). In the case of the dimer, (LiI)2, we observe kinetic energies and relative emission directions of Li+, I+, and I2+ recoil ions consistent with Coulomb explosion of the parallelogramshaped dimer after removing up to six electrons by the laser pulse.

We propose a Hessian-based interpolation scheme for generating estimates of first-order saddle points on potential energy surfaces. Given reactant and product structures and associated Hessians, two molecular-specific force-field-inspired energy functions are constructed, and an estimate of the connecting first-order saddle point is calculated as a minimum on the intersection of the two diabatic energy surfaces. The minimum-energy crossing point can be located starting directly from the reactant or product geometry. At the intersection minimum, an approximate Hessian can be constructed that is suitable for initiating a search for the actual saddle point. The construction of the diabatic energy functions and interpolation procedure is computationally efficient and allows for an automated screening of thousands of potential transition structures connecting conformationally flexible reactants and products, as well as creating global reaction networks. For a benchmark set of 262 reactions, the HINT procedure can locate 90% of the transition structures in an automated workflow.

Helium nanodroplets are co-doped with a K atom and a CsI molecule, forming a complex of previously unknown structure and location within the droplet. The complex is identified and characterized by femtosecond laser-induced Coulomb explosion and subsequent recording of the momentum distributions of the resulting K+, Cs+, Cs+Hen, I+, and I+Hen fragment ions. The kinetic energy distribution of the K+ ions contains two distinct peaks at 2.2 and 5.5 eV. These values match the outcome of classical molecular dynamics simulations of the Coulomb explosion of the gas-phase K- CsI complex, starting from its ab-initio-calculated equilibrium structure, into (K+,Cs+,I) or (K+,Cs+,I+) fragments, respectively. Covariance maps of the momentum distributions for K+ ions with other fragment ions corroborate the occurrence of these two Coulomb explosion channels, resulting from double and triple ionization of K- CsI. Overall, we conclude that K- CsI is formed at the surface of the droplets with a structure very close to the theoretically determined triangular shape of the gas-phase complex.

Bond capacities can be considered as atom-atom condensed versions of the density response function. They quantify the ease with which the electron density can be transferred between atoms due to differences in potential and are thus central quantities for modeling charge flow in force fields. We describe an implementation of calculating bond capacities by linear response methods with the minimal basis iterative stockholder definition of atoms in molecules. The calculated bond capacities are moderately sensitive to the level of theory at Hartree-Fock, density functional theory, and multiconfigurational self-consistent field and are insensitive to basis set quality beyond a polarized double-ζ quality. The dependence of bond capacities on chemical structure displays a high degree of transferability and conforms to the concept of functional groups. Bond capacities connect all atom pairs in a molecule; however, the magnitude rapidly diminishes as a function of the number of connecting bonds for the nonconjugated system, while a less rapid decay and oscillating pattern is observed for conjugated systems.

We apply transient absorption spectroscopy supported by 2D-IR spectroscopy and density functional theory calculations to determine the primary photolysis of acrylate excited via the transition at 200 nm. Upon photoexcitation, about half of the excited acrylate anions return to the ground state and relax to equilibrium in 5 ps primarily through intermolecular coupling between the carboxylate group and the surrounding water. The rest of the excited acrylate anions dissociate. Three dissociation channels have been identified. In one reaction, decarboxylation of acrylate forms CO2 and CH2CH-. CH2CH- is protonated by water and forms ethene, C2H4, in <0.8 ps. In the second reaction, the excited acrylate anions dissociate to H2CCHO- and CO. In about 20 ps, H2CCHO- picks up a proton from water to produce vinyl alcohol, H2CCHOH. A third dissociation channel forms H2CCHO˙ and CO-. H2CCHO˙ abstracts a hydrogen atom from water and forms vinyl alcohol. Vinyl alcohol will tautomerize to acetaldehyde, but this occurs on a time scale longer than the experimental observation time of 0.56 ns.

We report herein a novel, efficient, and expeditious approach for enantioselective intramolecular carbonylative Heck-Matsuda reaction, employing highly accessible, stable, and cost-effective nitroarenes as masked electrophiles. This tandem process combines the one-pot reduction of nitroarenes to the respective anilines, diazotization, Heck-Matsuda, carbonylation, and cyclization, enabling the synthesis of enantioenriched spirolactones. The method achieves overall yields of up to 76% with excellent enantiomeric ratios of up to 96 : 4 under mild conditions. Isotopically labeled products are readily obtained with near stoichiometric 13C carbon monoxide. Importantly, nitroarenes are used as masked electrophiles, which serve as an advantageous alternative to anilines and aryldiazonium salts for the Heck-Matsuda reaction. This approach thereby avoids the isolation of sensitive aryldiazonium salt intermediates and, consequently, the dangers associated with them. Density Functional Theory (DFT) calculations provide precise insights into the enantioenrichment mechanism, highlighting the significance of Pd carbonyl complexes for efficient diastereoconvergence. Microkinetic modeling of the computationally obtained reaction network results in an enantioenrichment of sub-kcal-accuracy in comparison to the experiment. This work not only showcases the level of complexity achievable in the field of tandem reactions but also highlights the utility of nitroarenes in complex organic transformations, demonstrating their potential for both academic and industrial applications.

Quantum computing (QC) provides a promising avenue for enabling quantum chemistry calculations, which are classically impossible due to computational complexity that increases exponentially with system size. As fully fault-tolerant algorithms and hardware, for which an exponential speedup is predicted, are currently out of reach, recent research efforts have been dedicated to developing and scaling algorithms for Noisy Intermediate-Scale Quantum (NISQ) devices to showcase the practical usefulness of such machines. To demonstrate the usefulness of NISQ devices in the field of chemistry, we apply our recently developed FAST-VQE algorithm and a state-of-the-art quantum gate reduction strategy based on propositional satisfiability together with standard optimization tools for the simulation of the rate-determining proton transfer step for CO2 hydration catalyzed by carbonic anhydrase resulting in the first application of a quantum computing device for the simulation of an enzymatic reaction. To this end, we have combined classical force field simulations with quantum mechanical methods on classical and quantum computers in a hybrid calculation approach. The presented technique significantly enhances the accuracy and capabilities of QC-based molecular modeling and finally pushes it into compelling and realistic applications. The framework is general and can be applied beyond the case of computational enzymology.

Alkali-metal dimers, Ak2, located on the surface of a helium nanodroplet, are set into rotation through the polarizability interaction with a nonresonant 1-ps-long laser pulse. The time-dependent degree of alignment is recorded using femtosecond-probe-pulse-induced Coulomb explosion into a pair of Ak+ fragment ions. The results, obtained for Na2, K2, and Rb2 in both the ground state 1 1Eg+ and the lowest-lying triplet state 13Eu+, exhibit distinct, periodic revivals with a gradually decreasing amplitude. The dynamics differ from that expected for dimers had they behaved as free rotors. Numerically, we solve the time-dependent rotational Schr & ouml;dinger equation, including an effective mean-field potential to describe the interaction between the dimer and the droplet. The experimental and simulated alignment dynamics agree well and their comparison enables us to determine the effective rotational constants of the alkali dimers with the exception of Rb2(13Eu+) that only exhibits a prompt alignment peak but no subsequent revivals. For Na2(13Eu+), K2(11Eg+), K2(1 3Eu+), and Rb2(1 1Eg+ ), the alignment dynamics are well described by a two-dimensional rotor model. We ascribe this to a significant confinement of the internuclear axis of these dimers, induced by the orientation-dependent droplet-dimer interaction, to the tangential plane of their residence point on the droplet.

Nuclear magnetic shielding and spin–spin coupling constants are the fundamental parameters that can be extracted from nuclear magnetic resonance experiments. These parameters contain structural information and can be used to deduce the structure of unknown molecules. The traditional approach based on empirical correlations between structure and spectral parameters may be of insufficient accuracy for a unique assignment of the molecular structure. An alternative approach is to calculate the NMR parameters from first principles for possible structural candidates. The accuracy of the calculated parameters depends on several components, of which the basis set used for expanding the orbitals is one ingredient. The present chapter reviews how basis sets can be constructed to allow a systematic reduction of basis set incompleteness, with specific focus on basis sets for calculating NMR parameters.

The minimal basis iterative Stockholder (MBIS) decomposition of molecular electron densities into atomic contributions is extended from spherical to ellipsoidal atomic basins. Despite the more flexible parametrization, the derived atomic multipole moments do not systematically improve the reproduction of molecular multipole moments and electrostatic potentials relative to a decomposition into spherical atomic densities. The decomposition can be constrained to exactly reproduce molecular multipole moments, in the present work extended up to hexadecapole moments, and this slightly improves the ability to reproduce the electrostatic potential. A byproduct of the ellipsoidal decomposition is a set of atomic parameters that describe the anisotropic decay of the electron density with distance from the nucleus, and this may be useful in developing anisotropic atomic parameters for use in force fields as well as for defining anisotropic atomic densities for use in quantum crystallography.

This study investigates the performance of force field models employing only atomic charges to model the electrostatic interactions and only charge-flow to model the electric polarization. The atomic charges and charge-flow parameters are calculated directly by ab initio methods. The performance for liquid-state properties of models that include 1-bond, 2-bond, and 3-bond charge-flow is probed for chloroform and acetonitrile through measurements of dielectric constants, dipole moments, and infrared spectra. The results indicate that models allowing charge-flow only between directly bonded atom pairs (1-bond) lead to significant deviations compared to models incorporating 2- and 3-bond contributions, suggesting that restricting charge-flow solely between directly bonded atom pairs omits important physical contributions. In addition, modeling polarizability via charge-flow inherently leads to an anisotropy description of the molecular polarizability tensor, with the level of anisotropy constituting a major component in the electrostatic response. The results also suggest that modeling the molecular polarizability by only charge-flow tends to overestimate the electric polarization and, thus, indicate that combinations of rank-0 and rank-1 polarizability are required for an accurate modeling of the electric response.

The minimal basis iterative Stockholder (MBIS) decomposition of molecular electron densities into atomic quantities is an attractive approach for deriving electrostatic parameters in force fields. The MBIS-derived atomic charges, however, in general tend to overestimate the molecular dipole and quadrupole moments by ∼10%. We show that it is possible to derive a constrained MBIS model where the atomic charges or a combination of atomic charges and dipoles exactly reproduce the molecular dipole and quadrupole moments for molecules. The atomic multipole moments derived by the constrained procedure are better at reproducing the molecular electrostatic potential (ESP) than the unconstrained atomic multipole moments. They are, furthermore, significantly less conformationally dependent than atomic charges obtained by fitting to the molecular electrostatic potential.

When calculating fragment interaction energies by electronic structure methods employing medium-sized atom-centered basis sets, it is often observed that the effect is systematically overestimated. The common interpretation is that the systematic error arises because the basis set for the complex is more complete than for the isolated fragments, and this is denoted basis set superposition errors. It has been observed, however, that the interaction energy in some cases is underestimated, which defies the interpretation in terms of basis set completeness, and instead suggests that the effect partly is due to basis set imbalance. The imbalance can be removed by explicit optimization of the basis sets for each structure, and it is shown that this to a significant extent reduces the systematic overestimation attributed to basis set superposition error.

High-harmonic spectroscopy is an all-optical technique with inherent attosecond temporal resolution that has been successfully employed to reconstruct charge migration, electron-tunneling dynamics, and conical-intersection dynamics. Here, we demonstrate the extension of two key components of high-harmonic spectroscopy, i.e., impulsive alignment and measurements with multiple driving wavelengths to 1,3-cyclohexadiene and benzene. In the case of 1,3-cyclohexadiene, we find that the temporal sequence of maximal and minimal emitted high-harmonic intensities as a function of the delay between the alignment and probe pulses inverts between 25 and 30 eV and again between 35 and 40 eV when an 800-nm driver is used, but no inversions are observed with a 1420-nm driver. This observation is explained by the wavelength-dependent interference of emission from multiple molecular orbitals (HOMO to HOMO-3), as demonstrated by calculations based on the weak-field asymptotic theory and accurate photorecombination matrix elements. These results indicate that attosecond charge migration takes place in the 1,3-cyclohexadiene cation and can potentially be reconstructed with the help of additional measurements. Our experiments also demonstrate a pathway toward studying photochemical reactions in the molecular frame of 1,3-cyclohexadiene.

In this paper, we develop and analyze a number of perturbation series that target the coupled cluster singles and doubles (CCSD) ground state energy. We show how classical Møller–Plesset perturbation theory series can be restructured to target the CCSD energy based on a reference CCS calculation and how the corresponding cluster perturbation series differs from the classical Møller–Plesset perturbation series. Subsequently, we reformulate these series using the coupled cluster Lagrangian framework to obtain series, where fourth and fifth order energies are determined only using parameters through second order. To test the methods, we perform a series of test calculations on molecular photoswitches of both total energies and reaction energies. We find that the fifth order reaction energies are of CCSD quality and that they are of comparable accuracy to state-of-the-art approximations to the CCSD energy based on local pair natural orbitals. The advantage of the present approach over local correlation methods is the absence of user defined threshold parameters for neglecting or approximating contributions to the correlation energy. Fixed threshold parameters lead to discontinuous energy surfaces, although this effect is often small enough to be ignored, but the present approach has a differentiable energy that will facilitate derivation and implementation of gradients and higher derivatives. A further advantage is that the calculation of the perturbation correction is non-iterative and can, therefore, be calculated in parallel, leading to a short time-to-solution.

The deep ultraviolet photochemistry of aqueous pyruvate is believed to have been essential to the origin of life, and near ultraviolet excitation of pyruvate in aqueous aerosols is assumed to contribute significantly to the photochemistry of the Earth’s atmosphere. However, the primary photochemistry of aqueous pyruvate is unknown. Here we study the susceptibility of aqueous pyruvate to photodissociation by deep ultraviolet and near ultraviolet irradiation with femtosecond spectroscopy supported by density functional theory calculations. The primary photo-dynamics of the aqueous pyruvate show that upon deep-UV excitation at 200 nm, about one in five excited pyruvate anions have dissociated by decarboxylation 100 ps after the excitation, while the rest of the pyruvate anions return to the ground state. Upon near-UV photoexcitation at a wavelength of 340 nm, the dissociation yield of aqueous pyruvate 200 ps after the excitation is insignificant and no products are observed. The experimental results are explained by our calculations, which show that aqueous pyruvate anions excited at 200 nm have sufficient excess energy for decarboxylation, whereas excitation at 340 nm provides the aqueous pyruvate anions with insufficient energy to overcome the decarboxylation barrier.