The relatively small diffuse function-augmented basis set, 3-21+G, is shown to describe anion geometries and proton affinities adequately. The diffuse sp orbital exponents are recommended for general use to augment larger basis sets.

Efficient diffuse function‐augmented basis sets for anion calculations. III. The 3‐21+G basis set for first‐row elements, Li–F

A general all-atom force field for atomistic simulation of common organic molecules, inorganic small molecules, and polymers was developed using state-of-the-art ab initio and empirical parametrization techniques. The valence parameters and atomic partial charges were derived by fitting to ab initio data, and the van der Waals (vdW) parameters were derived by conducting MD simulations of molecular liquids and fitting the simulated cohesive energies and equilibrium densities to experimental data. The combined parametrization procedure significantly improves the quality of a general force field. Validation studies based on large number of isolated molecules, molecular liquids and molecular crystals, representing 28 molecular classes, show that the present force field enables accurate and simultaneous prediction of structural, conformational, vibrational, and thermophysical properties for a broad range of molecules in isolation and in condensed phases. Detailed results of the parametrization and validation f...

COMPASS: An ab Initio Force-Field Optimized for Condensed-Phase ApplicationsOverview with Details on Alkane and Benzene Compounds

6. Rehybridization of the Acceptor (RICT) 3908 7. Planarization of the Molecule (PICT) 3909 III. Fluorescence Spectroscopy 3909 A. Solvent Effects and the Model Compounds 3909 1. Solvent Effects on the Spectra 3909 2. Steric Effects and Model Compounds 3911 3. Bandwidths 3913 4. Isoemissive Points 3914 B. Dipole Moments 3915 C. Radiative Rates and Transition Moments 3916 1. Quantum Yields and Radiationless Deactivation Processes 3916

Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures

Azobenzene undergoes trans → cisisomerization when irradiated with light tuned to an appropriate wavelength. The reverse cis →transisomerization can be driven by light or occurs thermally in the dark. Azobenzene's photochromatic properties make it an ideal component of numerous molecular devices and functional materials. Despite the abundance of application-driven research, azobenzene photochemistry and the isomerization mechanism remain topics of investigation. Additional substituents on the azobenzene ring system change the spectroscopic properties and isomerization mechanism. This critical review details the studies completed to date on the 3 main classes of azobenzene derivatives. Understanding the differences in photochemistry, which originate from substitution, is imperative in exploiting azobenzene in the desired applications.

Photoisomerization in different classes of azobenzene

Switches, and Actuators Masahiro Irie,*,† Tuyoshi Fukaminato,‡ Kenji Matsuda, and Seiya Kobatake †Research Center for Smart Molecules, Rikkyo University, Nishi-Ikebukuro 3-34-1, Toshima-ku, Tokyo 171-8501, Japan ‡Research Institute for Electronic Science, Hokkaido University, N20, W10, Kita-ku, Sapporo 001-0020, Japan Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan Department of Applied Chemistry, Graduate School of Engineering, Osaka City University, Sugimoto 3-3-138, Sumiyoshi-ku, Osaka 558-8585, Japan

Photochromism of Diarylethene Molecules and Crystals: Memories, Switches, and Actuators

Potential energy surface crossings in organic photochemistry

In this paper, we identify the most efficient decay and isomerization route of the S1, T1, and S0 states of azobenzene. By use of quantum chemical methods, we have searched for the transition states (TS) on the S1 potential energy surface and for the S0/S1 conical intersections (CIs) that are closer to the minimum energy path on the S1. We found only one TS, at 60° of CNNC torsion from the E isomer, which requires an activation energy of only 2 kcal/mol. The lowest energy CIs, lying also 2 kcal/mol above the S1 minimum, were found on the torsion pathway for CNNC angles in the range 95−90°. The lowest CI along the inversion path was found ca. 25 kcal/mol higher than the S1 minimum and was characterized by a highly asymmetric molecular structure with one NNC angle of 174°. These results indicate that the S1 state decay involves mainly the torsion route and that the inversion mechanism may play a role only if the molecule is excited with an excess energy of at least 25 kcal/mol with respect to the S1 minimum...

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On the Mechanism of the cis−trans Isomerization in the Lowest Electronic States of Azobenzene: S0, S1, and T1

In this paper we use ab initio multiconfigurational second-order perturbation theory to establish the intrinsic photoisomerization path model of retinal chromophores. This is accomplished by computing the ground state (S0) and the first two singlet excited-state (S1, S2) energies along the rigorously determined photoisomerization coordinate of the rhodopsin chromophore model 4-cis-γ-methylnona-2,4,6,8-tetraeniminium cation and the bacteriorhodopsin chromophore model all-trans-hepta-2,4,6-trieniminium cation in isolated conditions. The computed S2 and S1 energy profiles do not show any avoided crossing feature along the S1 reaction path and maintain an energy gap >20 kcal⋅mol−1. In addition, the analysis of the charge distribution shows that there is no qualitative change in the S2 and S1 electronic structure along the path. Thus, the S1 state maintains a prevalent ionic (hole–pair) character whereas the S2 state maintains a covalent (dot–dot) character. These results, together with the analysis of the S1 reaction coordinate, support a two-state, two-mode model of the photoisomerization that constitutes a substantial revision of the previously proposed models.

https://www.pnas.org/content/pnas/97/17/9379.full.pdf

Computational evidence in favor of a two-state, two-mode model of the retinal chromophore photoisomerization

for correcting for the change in charge distribution, and the present approach represents the most accurate method of obtaining an estimate of Eo(S). The resulting values of Eo(s) were -1.19, -1.73, -1.05, and -0.94 V for methylamine, ethylamine, dimethylamine, and trimethylamine, respectively. Likewise, utilizing the values of AGO(,) derived above, one can estimate that Eo(I) has values of about -1.5, -1.9, -1.5, and -1.4 V for the same amines, respectively. The large negative values of Eo(I) confirm the strong reductive power of these R1R2NCR3H' radicals. Previous estimates of Eo(]) in aqueous solution have been in the region of -1.0 V.@ Equilibrium measurements of these quantities in water will be very difficult because of the transitory nature of both the radicals and the iminium products. Thus, this is a case where theoretical investigations of the solution free energies of the iminium species would be of considerable interest, since they have the potential to reduce the uncertainties in the calculation of EO(I). 5. summary In the praent work, thermochemical functions for aminoalkyl and alkylaminium free radicals and related species were derived by combining results from high-level ab initio calculations with ~ ~ ~~

An MC-SCF Study of the S1 and S2 Photochemical Reactions of Benzene

The excited state (2 1 Ag) reaction paths involved in the photochemical transformations of butadiene have been studied via ab initio MC-SCF methods. It is demonstrated that the reaction funnel assumes the form of a conical intersection region where the ground (1 1 Ag) and first excited (2 1 Ag) potential energy surfaces are degenerate. This mechanism is consistent with experimental results for the photochemical isomerization and is also consistent with the observed absence of fluorescence from the 2 1 Ag state. Thus the currently accepted mechanisms for butadiene photochemistry which involve radiationless decay at avoided crossing minima need to be replaced with a model that involves fully efficient return to the ground state via a conical intersection

Fernando Bernardi

Papers

Potential energy surface crossings in organic photochemistry

On the Mechanism of the cis−trans Isomerization in the Lowest Electronic States of Azobenzene: S0, S1, and T1

Computational evidence in favor of a two-state, two-mode model of the retinal chromophore photoisomerization

An MC-SCF Study of the S1 and S2 Photochemical Reactions of Benzene

A conical intersection mechanism for the photochemistry of butadiene. A MC-SCF study