Although density functional theory is widely used in the computational chemistry community, the most popular density functional, B3LYP, has some serious shortcomings: (i) it is better for main-group chemistry than for transition metals; (ii) it systematically underestimates reaction barrier heights; (iii) it is inaccurate for interactions dominated by medium-range correlation energy, such as van der Waals attraction, aromatic−aromatic stacking, and alkane isomerization energies. We have developed a variety of databases for testing and designing new density functionals. We used these data to design new density functionals, called M06-class (and, earlier, M05-class) functionals, for which we enforced some fundamental exact constraints such as the uniform-electron-gas limit and the absence of self-correlation energy. Our M06-class functionals depend on spin-up and spin-down electron densities (i.e., spin densities), spin density gradients, spin kinetic energy densities, and, for nonlocal (also called hybrid)...

Density functionals with broad applicability in chemistry.

Ju Mei,†,‡,∥ Nelson L. C. Leung,†,‡,∥ Ryan T. K. Kwok,†,‡ Jacky W. Y. Lam,†,‡ and Ben Zhong Tang*,†,‡,§ †HKUST-Shenzhen Research Institute, Hi-Tech Park, Nanshan, Shenzhen 518057, China ‡Department of Chemistry, HKUST Jockey Club Institute for Advanced Study, Institute of Molecular Functional Materials, Division of Biomedical Engineering, State Key Laboratory of Molecular Neuroscience, Division of Life Science, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China

Aggregation-Induced Emission: Together We Shine, United We Soar!

The successful isolation and characterization of an N-heterocyclic carbene in 1991 opened up a new class of organic compounds for investigation. From these beginnings as academic curiosities, N-heterocyclic carbenes today rank among the most powerful tools in organic chemistry, with numerous applications in commercially important processes. Here we provide a concise overview of N-heterocyclic carbenes in modern chemistry, summarizing their general properties and uses and highlighting how these features are being exploited in a selection of pioneering recent studies.

An overview of N-heterocyclic carbenes

A comprehensive review is presented on nucleus-independent chem. shift as a criterion for aromaticity. [on SciFinder (R)]

Nucleus-independent chemical shifts (NICS) as an aromaticity criterion.

"United we stand, divided we fall."--Aesop. Aggregation-induced emission (AIE) refers to a photophysical phenomenon shown by a group of luminogenic materials that are non-emissive when they are dissolved in good solvents as molecules but become highly luminescent when they are clustered in poor solvents or solid state as aggregates. In this Review we summarize the recent progresses made in the area of AIE research. We conduct mechanistic analyses of the AIE processes, unify the restriction of intramolecular motions (RIM) as the main cause for the AIE effects, and derive RIM-based molecular engineering strategies for the design of new AIE luminogens (AIEgens). Typical examples of the newly developed AIEgens and their high-tech applications as optoelectronic materials, chemical sensors and biomedical probes are presented and discussed.

Aggregation-Induced Emission: The Whole Is More Brilliant than the Parts

The molecular structures, relative energies, vibrational frequencies, and electron affinities for the Cl 2 O n //Cl 2 O - n (n = 1-4) systems have been investigated using hybrid Hartree-Fock/density functional theories (BHLYPand B3LYP) and pure density functional theories (BP86 and BLYP). The three different types of neutral/anion energy differences reported in this research are the adiabatic electron affinity (EA a d ), the vertical electron affinity (EA v r e t ), and the vertical detachment energy (VDE). The basis set used in this work is of double-ζ plus polarization quality with additional s- and p-type diffuse functions, and it is denoted DZP++. The geometries are fully optimized by all four DFT methods. We have predicted a number of possible low-lying local minima, including some that are unprecedented. Most strikingly, in several cases structures that have been observed in the laboratory turn out not to be lowest in energy for a particular chemical composition. Two structures are predicted with a formally seven-coordinate chlorine atom. The global minima for the Cl 2 O n /Cl 2 O - m (n = 1-4) systems are ClOCl (C 2 v ), ClClO - (C s ), ClOOCl (C 2 ), OOCl - 2 (C 2 v ), ClOOOCl (C 2 /C 1 ), ClO(O 2 )Cl - (C s ), trans-ClO(O 2 )OCl (C i ), and trans-ClO(O 2 )OCl - (C i ), respectively. The relative energies of the different minima are reported. Five of the 42 structures predicted here have been determined experimentally. Our theoretical geometries and vibrational frequencies are carefully compared with the limited available experimental results, and the BHLYP functional in general provides the best agreement. The ClClO - and ClClO - 2 ground states might be regarded as Cl - ClO and Cl - ClO 2 complexes, respectively. The adiabatic electron affinities, obtained at the favored DZP++ BLYP level of theory, are 3.12 eV for Cl 2 O, 3.96 eV for Cl 2 O 2 , 3.66 eV for Cl 2 O 3 , and 4.15 eV for Cl 2 O 4 . The adiabatic EAs for the first three systems are similar to those of Br 2 O n (3.14, 3.80, and 3.46 eV with BLYP for Br 2 O n , n = 1-3), and this may reflect the geometric similarity between these bromine and chlorine species. But, EA a d for Br 2 O 4 (1.97 eV) is different from Cl 2 O 4 because the neutral and anionic bromine species have different global minimum geometries from those for Cl 2 O 4 .

Molecular structures, thermochemistry, and electron affinities for the dichlorine oxides: Cl2On/Cl2O-n (n = 1-4)

The stability of a set of 105 five-membered π-electron systems (involving aromatic, non-aromatic and anti-aromatic species) was evaluated using six isodesmic reactions of which two belong to the subclass of homodesmotic reactions, which are based on cyclic and acyclic reference systems. We demonstrate that the ‘Resonance Energies’ derived from isodesmotic schemes have obvious flaws and do not correct or cancel other contributions to the energy, such as the changes of hybridization, homoconjugation of heterosubstituted cyclopentadienes, conjugative interactions of CC or CX (X=N or P) with a π or pseudo π orbital at Y (Y=O, S, NH, PH), strain, etc. as effectively as possible. Likewise, ‘aromatic stabilization energies (ASE)’ derived from homodesmotic schemes based on the acyclic reference compounds do not give satisfactory results. We strongly recommend that only cyclic reference compounds should be used for ASE and other aromaticity evaluations. The analysis is based on ab initio optimized geometries at B3LYP/6-311+G∗∗.

Facts and Artifacts about Aromatic Stability Estimation

Protonated pyrrole cations are produced in a pulsed discharge/supersonic expansion source, mass-selected in a time-of-flight spectrometer, and studied with infrared photodissociation spectroscopy. Vibrational spectra in both the fingerprint and C–H/N–H stretching regions are obtained using the method of tagging with argon. Sharp vibrational structure is compared to IR spectra predicted by theory for the possible α-, β-, and N-protonated structures. The spectral differences among these isomers are much larger than the frequency shifts due to argon attachment at alternative sites. Though α-protonation predominates thermodynamically, the kinetically favored β-protonated species is also observed for the first time (in 3–4 times lower abundance under the conditions employed here). Theoretical investigations attribute the greater stability of α-protonated pyrrole to topological charge stabilization, rather than merely to the greater number of resonance contributors. The far-IR pattern of protonated pyrrole does...

Paul von Ragué Schleyer

Papers

Molecular structures, thermochemistry, and electron affinities for the dichlorine oxides: Cl2On/Cl2O-n (n = 1-4)

Facts and Artifacts about Aromatic Stability Estimation

IR spectroscopy of α- and β-protonated pyrrole via argon complex photodissociation.

Trishomoaromatische Dianionen von 1,3,5‐Triboracyclohexanen

Demonstration of Chiral Enantiomerization in a Four‐Atom Molecule