: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

We propose an efficient, accurate method to integrate the basins of attraction of a smooth function defined on a general discrete grid and apply it to the Bader charge partitioning for the electron charge density. Starting with the evolution of trajectories in space following the gradient of charge density, we derive an expression for the fraction of space neighboring each grid point that flows to its neighbors. This serves as the basis to compute the fraction of each grid volume that belongs to a basin (Bader volume) and as a weight for the discrete integration of functions over the Bader volume. Compared with other grid-based algorithms, our approach is robust, more computationally efficient with linear computational effort, accurate, and has quadratic convergence. Moreover, it is straightforward to extend to nonuniform grids, such as from a mesh-refinement approach, and can be used to both identify basins of attraction of fixed points and integrate functions over the basins.

Accurate and efficient algorithm for Bader charge integration

By introducing an electron bath that represents the chemical environment in which a chemical species is immersed, and by making use of the second-order Taylor series expansions of the energy as a function of the number of electrons in the intervals between N - 1 and N, and N and N + 1, we show that the electrodonating (omega-) and the electroaccepting (omega+) powers may be defined as omega-/+ = (mu-/+)2/2eta-/+, where mu-/+ are the chemical potentials and eta-/+ are the chemical hardnesses, in their corresponding intervals. Approximate expressions for omega- and omega+ in terms of the ionization potential I and the electron affinity A are established by assuming that eta- = eta+ = eta = mu+ - mu-. The functions omega-/+(r) = omega-/+f -/+(r), where f -/+(r) are the directional Fukui functions, derived from a functional Taylor series for the energy functional truncated at second order, represent the local electrodonating and electroaccepting powers.

Electrodonating and electroaccepting powers.

Critic2: A program for real-space analysis of quantum chemical interactions in solids

Magnetic bistability, as manifested in the magnetization of ferromagnetic materials or spin crossover in transition metal complexes, has essentially been restricted to either bulk materials or to very low temperatures. We now present a molecular spin switch that is bistable at room temperature in homogeneous solution. Irradiation of a carefully designed nickel complex with blue-green light (500 nanometers) induces coordination of a tethered pyridine ligand and concomitant electronic rearrangement from a diamagnetic to a paramagnetic state in up to 75% of the ensemble. The process is fully reversible on irradiation with violet-blue light (435 nanometers). No fatigue or degradation is observed after several thousand cycles at room temperature under air. Preliminary data show promise for applications in magnetic resonance imaging.

https://science.sciencemag.org/content/sci/331/6016/445.full.pdf

Magnetic bistability of molecules in homogeneous solution at room temperature.

A high performance grid-based algorithm for computing QTAIM properties

The ultrafast photodynamics of four‐coordinate nickel(II) porphyrins in noncoordinating solvents has been studied using femtosecond time resolved optical spectroscopy. Unambiguous evidence has been found for the formation of a metastable metal (d,d) excited state in less than 350 fs following excitation of the macrocycle. However, the transient absorption spectrum of this ligand‐field electronic excited state continues to evolve and reaches the steady‐state form only after about 20 ps. This spectral behavior and the attendant complex kinetics can be modeled phenomenologically in terms of a broad distribution of the (d,d) excited states evolving to a narrower distribution. The dynamics are associated with vibrational relaxation. Intramolecular and intermolecular contributions to this process are considered.

Ultrafast vibrational dynamics of a photoexcited metalloporphyrin

The inherent quality of leaving groups in chemical reactions is related to their ionization potential and electron affinity using a quadratic model for the dependence of the energy on the number of electrons. A good leaving group for nucleophilic substitution/elimination reactions is one where the difference in energy between the system with the “optimum” number of electrons and the anion is small. Similarly, a good leaving group for electrophilic substitution/elimination reactions is one where the difference in energy between the system with the optimum number of electrons and the cation is small. This insight allows us to define indices for the quality of leaving groups in nucleophilic and electrophilic reactivity, which we term the nucleofugality and the electrofugality, respectively. These indices are useful not only for predicting the quality of leaving groups in organic reactions, but also for explaining the stability of carbocations, carbanions, and trends in pKa.

Indices for predicting the quality of leaving groups.

The effects of nuclear motion on the ground state and excited state optical spectra of porphyrins are examined in a number of experiments designed to generate excess vibrational energy within the macrocycle. These include time‐resolved spectroscopic measurements following ultrafast radiationless transitions, and static measurements in the gas and in the condensed phase at various temperatures. The excess vibrational energy generated by highly exothermic radiationless transitions is found to induce significant red shifts in both the ground state absorption and excited state emission features. As the excess vibrational energy is dissipated on the time scale of about 10 ps, the optical features blue shift to their steady‐state spectral positions. The red shifts found in the time‐resolved spectra are also observed in the ground state absorption spectra of porphyrins in the gas phase at high temperature. We consider various mechanisms for the spectral shifts, including vibrationally induced reduction of the el...

Time‐resolved and static optical properties of vibrationally excited porphyrins

We introduce a method to compute atomic properties according to the “quantum theory of atoms in molecules.” An integration grid in real space is partitioned into subsets, ωi. The subset, ωi, is composed of all grid points contained in the atomic basin, Ωi, so that integration over Ωi is reduced to simple quadrature over the points in ωi. The partition is constructed from deMon2k's atomic center grids by following the steepest ascent path of the density starting from each point in the grid. We also introduce a technique that exploits the cellular nature of the grid to make the algorithm faster. The performance of the method is tested by computing properties of atoms and nonnuclear attractors (energies, charges, dipole, and quadrupole moments) for a set of representative molecules. © 2008 Wiley Periodicals, Inc. J Comput Chem 2009

Juan I. Rodríguez

Papers

A high performance grid-based algorithm for computing QTAIM properties

Ultrafast vibrational dynamics of a photoexcited metalloporphyrin

Indices for predicting the quality of leaving groups.

Time‐resolved and static optical properties of vibrationally excited porphyrins

An efficient grid‐based scheme to compute QTAIM atomic properties without explicit calculation of zero‐flux surfaces