On the Relationship between Hydrogen Bond Strength and the Formation Energy in Resonance-Assisted Hydrogen Bonds

Resonance-assisted hydrogen bonds (RAHB) are intramolecular contacts that are characterised by being particularly energetic. This fact is often attributed to the delocalisation of π electrons in the system. In the present article, we assess this thesis via the examination of the effect of electron-withdrawing and electron-donating groups, namely −F, −Cl, −Br, −CF3, −N(CH3)2, −OCH3, −NHCOCH3 on the strength of the RAHB in malondialdehyde by using the Quantum Theory of Atoms in Molecules (QTAIM) and the Interacting Quantum Atoms (IQA) analyses. We show that the influence of the investigated substituents on the strength of the investigated RAHBs depends largely on its position within the π skeleton. We also examine the relationship between the formation energy of the RAHB and the hydrogen bond interaction energy as defined by the IQA method of wave function analysis. We demonstrate that these substituents can have different effects on the formation and interaction energies, casting doubts regarding the use of different parameters as indicators of the RAHB formation energies. Finally, we also demonstrate how the energy density can offer an estimation of the IQA interaction energy, and therefore of the HB strength, at a reduced computational cost for these important interactions. We expected that the results reported herein will provide a valuable understanding in the assessment of the energetics of RAHB and other intramolecular interactions.

Tuning Electronic Structure and Optical Properties of Li@cyclo[18]carbon Complex via Switching Doping Position of Lithium Atom

Doping alkali metal atoms, especially lithium (Li), in nanocarbon materials has always been considered as one of the most effective methods to improve the optical properties of the system. In this theoretical work, we doped a Li atom into the recently observed all-carboatomic molecule, cyclo[18]carbon (C₁₈), and finally obtained two stable configurations with Li inside and outside the ring. The calculation results show that the energy barrier of transition between the two Li@C₁₈ complexes is quite low, and thus the conversion is easy to occur at ambient temperature. Importantly, the electronic structure, absorption spectrum, and optical nonlinearity of the two configurations are found to be significantly different, which indicates that the electronic structure and optical properties of the Li@C₁₈ complex can be effectively regulated by switching the location of the doped Li atom between inside and outside the carbon ring. With the help of a variety of wave function analysis techniques, the nature of the discrepancies in the properties of the Li@C₁₈ complex with different configurations has been revealed in depth. The relevant results of this work are expected to provide theoretical guidance for the future development of cyclocarbon-based optical molecular switches.

Tuning Electronic Structure and Optical Properties of Li@cyclo[18]carbon Complex via Switching Doping Position of Lithium Atom

Doping alkali metal atoms, especially lithium (Li), in nanocarbon materials has always been considered as one of the most effective methods to improve the optical properties of the system. In this theoretical work, we doped a Li atom into the recently observed all-carboatomic molecule, cyclo[18]carbon (C₁₈), and finally obtained two stable configurations with Li inside and outside the ring. The calculation results show that the energy barrier of transition between the two Li@C₁₈ complexes is quite low, and thus the conversion is easy to occur at ambient temperature. Importantly, the electronic structure, absorption spectrum, and optical nonlinearity of the two configurations are found to be significantly different, which indicates that the electronic structure and optical properties of the Li@C₁₈ complex can be effectively regulated by switching the location of the doped Li atom between inside and outside the carbon ring. With the help of a variety of wave function analysis techniques, the nature of the discrepancies in the properties of the Li@C₁₈ complex with different configurations has been revealed in depth. The relevant results of this work are expected to provide theoretical guidance for the future development of cyclocarbon-based optical molecular switches.

The Alcohol Catalytic Mechanism for Schiff Base 1,3-Proton Transfer

In order to explore the catalytic effect of alcohols on the 1,3-proton transfer of 1,1-diphenyl-N-(1-phenylethylidene) methylamine, the reaction potential energy surface was systematically studied at the theoretical level of ωB97-MV / def2-QZVPP // PBE0(D3BJ) / 6-31G**. The results show that the catalytic mechanism of benzyl alcohol can be divided into the acid channel and basic channel, in which the acid channel is the dominant one. In the first step, benzyl alcohol protonated the nitrogen atom of imine to form imine cation and benzyl alcohol anion, and the newly formed benzyl alcohol anion preferentially combined with the proton on C1; in the second step, benzyl alcohol continued to protonize the C3 atom, and the newly formed benzyl alcohol anion combined with the hydrogen on nitrogen, thus completing the whole proton migration process. By means of wave function analysis, it is proved that the stronger the hydrogen bond (O–H···N) is, the lower the free energy barrier is. When alcohols with lower pKa values are used as catalysts, the reaction barrier will be lower.

Effects of External Field Wavelength and Solvation on the Photophysical Property and Optical Nonlinearity of 1,3-Thiazolium-5-Thiolates Mesoionic Compound

<p>The photophysical property and optical nonlinearity of an electronic push-pull mesoionic compond, 2-(4-trifluoromethophenyl)-3-methyl-4-(4-methoxyphenyl)-1,3-thiazole-5-thiolate were theoretically investigated with a reliable computing strategy. The essence of the optical properties were then explored through a variety of wave function analysis methods, such as the natural transition orbital analysis, hole-electron analysis, (hyper)polarizability density analysis, decomposition of the (hyper)polarizability contribution by numerical integration, and (hyper)polarizability tensor analysis, at the level of electronic structures. The influence of the electric field and solvation on the electron absorption spectra and (hyper)polarizabilities of the molecule are highlighted and clarified. This work will help people to understand the influence of external field wavelength and solvent on the optical properties of mesoionic-based molecules, and provide a theoretical reference for the rational design of chromophores with adjustable properties in the future.<br></p><br>

Effects of External Field Wavelength and Solvation on the Photophysical Property and Optical Nonlinearity of 1,3-Thiazolium-5-Thiolates Mesoionic Compound

<p>The photophysical property and optical nonlinearity of an electronic push-pull mesoionic compond, 2-(4-trifluoromethophenyl)-3-methyl-4-(4-methoxyphenyl)-1,3-thiazole-5-thiolate were theoretically investigated with a reliable computing strategy. The essence of the optical properties were then explored through a variety of wave function analysis methods, such as the natural transition orbital analysis, hole-electron analysis, (hyper)polarizability density analysis, decomposition of the (hyper)polarizability contribution by numerical integration, and (hyper)polarizability tensor analysis, at the level of electronic structures. The influence of the electric field and solvation on the electron absorption spectra and (hyper)polarizabilities of the molecule are highlighted and clarified. This work will help people to understand the influence of external field wavelength and solvent on the optical properties of mesoionic-based molecules, and provide a theoretical reference for the rational design of chromophores with adjustable properties in the future.<br></p><br>