Non-covalent intermolecular carbon-carbon interactions in polyynes

Impact of Halogen Termination and Chain Length on π-Electron Conjugation and Vibrational Properties of Halogen-Terminated Polyynes

We explored the optoelectronic and vibrational properties of a new class of halogen-terminated carbon atomic wires in the form of polyynes using UV-vis, infrared absorption, Raman spectroscopy, X-ray single-crystal diffraction, and DFT calculations. These polyynes terminate on one side with a cyanophenyl group and on the other side, with a halogen atom X (X = Cl, Br, I). We focus on the effect of different halogen terminations and increasing lengths (i.e., 4, 6, and 8 sp-carbon atoms) on the π-electron conjugation and the electronic structure of these systems. The variation in the sp-carbon chain length is more effective in tuning these features than changing the halogen end group, which instead leads to a variety of solid-state architectures. Shifts between the vibrational frequencies of samples in crystalline powders and in solution reflect intermolecular interactions. In particular, the presence of head-to-tail dimers in the crystals is responsible for the modulation of the charge density associated with the π-electron system, and this phenomenon is particularly important when strong I··· N halogen bonds occur.

Topology of electrostatic potential and electron density reveals a covalent to non-covalent carbon-carbon bond continuum

The covalent and non-covalent nature of carbon-carbon (CC) interactions in a wide range of molecular systems can be characterized using various methods, including the analysis of molecular electrostatic potential (MESP), represented as V(r), and the molecular electron density (MED), represented as ρ(r). These techniques provide valuable insights into the bonding between carbon atoms in different molecular environments. By uncovering a fundamental exponential relationship between the distance of the CC bond and the highest eigenvalue (λv1) of V(r) at the bond critical point (BCP), this study establishes the continuum model for all types of CC interactions, including transition states. The continuum model is further delineated into three distinct regions, namely covalent, borderline cases, and non-covalent, based on the gradient, , with the bond distance of the CC interaction. For covalent interactions, this parameter exhibits a more negative value than -5.0 a.u. Å-1, while for non-covalent interactions, it is less negative than -1.0 a.u. Å-1. Borderline cases, which encompass transition state structures, fall within the range of -1.0 to -5.0 a.u. Å-1. Furthermore, this study expands upon Popelier's analysis of the Laplacian of the MED, denoted as ∇2ρ, to encompass the entire spectrum of covalent, non-covalent, and borderline cases of CC interactions. Therefore, the present study presents compelling evidence supporting the concept of a continuum model for CC bonds in chemistry. Additionally, this continuum model is further explored within the context of C-N, C-O, C-S, N-N, O-O, and S-S interactions, albeit with a limited dataset.

Dispersion Interactions between Molecules in and out of Equilibrium Geometry: Visualization and Analysis

The London dispersion interactions between systems undergoing bond breaking, twisting, or compression are not well studied due to the scarcity and the high computational cost of methods being able to describe both the dynamic correlation and the multireference character of the system. Recently developed methods based on the Generalized Valence Bond wave function, such as EERPA-GVB and SAPT(GVB) (SAPT = symmetry-adapted perturbation theory), allow one to accurately compute and analyze noncovalent interactions between multireference systems. Here, we augment this analysis by introducing a local indicator for dispersion interactions inspired by Mata and Wuttke's Dispersion Interaction Density [ J. Comput. Chem. 2017, 38, 15-23] applied on top of an EERPA-GVB computation. Using a few model systems, we show what insights into the nature and evolution of the dispersion interaction during bond breaking and twisting such an approach is able to offer. The new indicator can be used at a minimal cost additional to an EERPA-GVB computation and can be complemented by an energy decomposition employing the SAPT(GVB) method. We explain the physics behind the initial increase, followed by a decrease in the interaction of linear molecules upon bond stretching. Namely, the elongation of covalent bonds leads to the enhancement of attractive dispersion interactions. For even larger bond lengths, this effect is canceled by the increase of the repulsive exchange forces resulting in a suppression of the interaction and finally leading to repulsion between monomers.

Arylene-hexaynylene and -octaynylene macrocycles: extending the polyyne chains drives self-association by enhanced dispersion force

Tetraalkoxyphenanthrylene-hexaynylene and -octaynylene macrocycles, which represent the first examples of isolable arylene-alkynylene macrocycles (AAMs) that contain polyyne chains longer than tetrayne, were synthesized and their self-association behavior was examined. Extending the polyyne chain from diyne to tetrayne, hexayne, and octayne exponentially increased the self-association constant of the macrocycles.

Design and DFT study of nitrogen-rich donor systems for improved photovoltaic performance in dye-sensitized solar cells

Donor modifications, especially through N-annulation, for enhancing the structure–performance relationship of D–π–A systems for DSSC applications.

Ti3C2-MXene/Bismuth Ferrite Nanohybrids for Efficient Degradation of Organic Dyes and Colorless Pollutants

The current environmental and potable water crisis requires technological advancement to tackle the issues caused by different organic pollutants. Herein, we report the degradation of organic pollutants such as Congo Red and acetophenone from aqueous media using visible light irradiation. To harvest the solar energy for photocatalysis, we fabricated a nanohybrid system composed of bismuth ferrite nanoparticles with two-dimensional (2D) MXene sheets, namely, the BiFeO3 (BFO)/Ti3C2 (MXene) nanohybrid, for enhanced photocatalytic activity. The hybrid BFO/MXene is fabricated using a simple and low-cost double-solvent solvothermal method. The SEM and TEM images showed that the BFO nanoparticles are attached onto the surface of 2D MXene sheets. The photocatalytic degradation achieved by the hybrid is found to be 100% in 42 min for the organic dye (Congo Red) and 100% for the colorless aqueous pollutant (acetophenone) in 150 min. The BFO/MXene hybrid system exhibited a large surface area of 147 m2 g-1 measured via the Brunauer-Emmett-Teller sorption-desorption technique, which is found to be the largest among all BFO nanoparticles and derivatives. The photoluminescence spectra indicate a low electron-hole recombination rate. Fast and efficient degradation of organic molecules is caused by two factors: larger surface area and lower electron-hole recombination rate, which makes the BFO/MXene nanohybrid a highly efficient photocatalyst and a promising candidate for many future applications.

Intensified C≡C Stretching Vibrator and Its Potential Role in Monitoring Ultrafast Energy Transfer in 2D Carbon Material by Nonlinear Vibrational Spectroscopy

In this work, an intensity-enhanced C≡C stretching infrared (IR) absorption is observed in hexakis[(trimethylsilyl)ethynyl]benzene (HTEB), whose IR transition dipole magnitude becomes comparable to that of a typical C═O stretch, and the enhancement is believed to be due to a joint effect of π-π conjugation and hyperconjugation associated with a terminal trimethylsilyl group. Using dynamical time-dependent two-dimensional infrared (2D IR) spectroscopy, a picosecond intramolecular energy redistribution process is observed between two nondegenerate C≡C stretching modes, whose symmetry breaking is attributed to a noncovalent halogen-bonding interaction between HTEB and solvent CH₂Cl₂. The rigid structure of HTEB and limited structural dynamics are also inferred from the insignificant initial spectral diffusion value extracted from the 2D IR spectra. This work provides the first nonlinear infrared investigation of the conventionally weak C≡C stretch. The methods outlined are particularly important for detailed understanding of the structure-related processes such as vibrational energy transfer in novel C≡C species containing materials such as graphdiyne.

Anion-encapsulating fullerenes behave as large anions: a DFT study

M06L/6-311++G(d,p)//M06L/6-31G(d,p) level density functional theory studies show that the endohedral reaction of C60 with X- (X = F, Cl, Br, OH, NH2, NO2, CN, and ClO) is exothermic by 37.8-65.2 kcal mol-1. The exothermic character of the reaction is drastically reduced in polar and nonpolar solvents due to the lack of direct solvation influence on the encapsulated anion. In all X-@C60, the occupied frontier molecular orbitals (FMOs) are located on X- while the energy levels of FMOs centered on C60 are very similar to those of the C60- radical anion. Molecular electrostatic potential (MESP) analysis of X-@C60 revealed that the negative character of the MESP minimum (Vmin) on the carbon cage increases by ∼72 fold compared to C60, which is very similar to the enhancement in the negative MESP observed on the C60- radical anion. The MESP data and quantum theory of atoms in molecules (QTAIM) analysis of charge, electron delocalization index, and Laplacian of bond critical point (bcp) support significant electron sharing from the anion to the carbon atoms of the fullerene cage, which makes the cage behave like a very large anion in a closed shell configuration. The data are also supportive of a multicenter charge-shift type of bonding interaction between the anion and the carbon cage. The anionic nature of the fullerene cage has been verified in the cases of larger systems such as Cl-@C70, Cl-@C84, and Cl-@C90. The binding of a counter cation K+ with X-@C60 is found to be highly exothermic (∼72 kcal mol-1) and very similar to the binding of K+ with the C60- radical anion (72.9 kcal mol-1), which suggests that C60 in X-@C60 behaves as a closed shell anion.

Sumanene: an efficient π-bowl for dihydrogen storage

The outstanding ability of sumanene, its anionic and dianionic forms and sumanene–M⁺ion-pair complexes (M = Li, Na, K) to bind dihydrogen has been revealed using density functional theory calculations pointing out that these systems could be employed for developing new H₂storage systems.

A DFT study on 1,4-dihydro-1,4-azaborinine annulated linear polyacenes: Absorption spectra, singlet-triplet energy gap, aromaticity, and HOMO-LUMO energy modulation

Linear polyacene (LPA) mimics containing multiple heterocycles have been computationally designed by annulating 1,4-dihydro-1,4-azaborinine moieties to benzene (aB₁ -aB₅ ), naphthalene (aN₁ -aN₅ ), anthracene (aA₁ -aA₅ ), and tetracene (aT₁ -aT₅ ) cores. DFT studies conducted on them using M06L/6-311++G(d,p) method reveal a perfect planar structure for all and suggest the utilization of nitrogen lone pairs for aromatic π-electron delocalization. The computed values of aromaticity indices such as HOMA, NICS, and dehydrogenation energy (E_dh ) of heterocycles support strong aromatic character for each six-membered ring in the LPA mimics. On the basis of the minimum value of the molecular electrostatic potential (V_min ) observed on each LPA unit in the LPA mimics, the extended delocalization of π-electrons is verified. The energetic parameter E_dh showed strong linear correlation with HOMA, NICS and V_min parameters, which strongly supports the multidimensional character of aromaticity in LPA mimics. The electronic property modification is shown by the theoretical absorption spectra data and singlet-triplet energy gap (ΔE_ST ). The bandgap and ΔE_ST tunings are achieved for LPA mimics by selecting appropriate number of azaborinine type units and the size of LPA core used for annulation. © 2017 Wiley Periodicals, Inc.

Dihydrogen Binding Affinity of Polyatomic Anions: A DFT Study

The dihydrogen-binding ability of polyatomic oxohalo anions ClO^-, ClO₂ ^-, ClO₃ ^-, ClO₄ ^-, BrO^-, BrO₂ ^-, BrO₃ ^-, and BrO₄ ^- has been studied at the M06L/6-311++G(d,p) density functional theory and the CCSD(T)/aug-cc-pVTZ//CCSD/aug-cc-pVDZ ab initio theory. The maximum number of dihydrogen adsorbed by the anions (n _max) varies from 17 to 24 in the first coordination shell. As the number of H₂ adsorbed varies from 1 to n _max, the oxochloro and oxobromo anions show a wide range for interaction energy (E _int), namely, 1.5-45.4 kcal/mol for the former and 1.4-46.0 kcal/mol for the latter. These results indicate that both series of anions show very similar and high affinity to bind with several dihydrogen molecules. Further, an increase in the coordination ability and a decrease in the strength of the dihydrogen interaction are observed with an increase in the number of oxygen atoms in the polyatomic anion. In contrast, the neutral oxohaloacids show negligible interaction with dihydrogen. The anion···H₂ noncovalent interactions along with H···H dihydrogen interactions within the complex are ascertained by locating the bond critical points (bcps) in the quantum theory of atoms in molecules analysis. The electron density at the bcp summed up for all of the anion···H₂ interactions (∑ρ_bcp) showed a strong linear relationship with E _int, indicating that the stability of the complex is due to the formation of a large network of noncovalent bonds in the complex. The amount of electron density donated by the anion to the dihydrogen during complex formation is also gauged from the molecular electrostatic potential values at the nuclei (V _n) of all of the atoms in the anion. The hydrogen uptake leads to a significant reduction in the negative character of V _n, and the total change in V _n from all of the anion atoms (∑ΔV _n) is found to be directly proportional to E _int. The polyatomic anions have a very high affinity toward dihydrogen binding, which can be utilized for the development of new hydrogen storage systems.

Massive dihydrogen uptake by anionic carbon chains

The remarkable capacity of anionic and dianionic carbon chains to bind dihydrogen compared to their neutral moieties has been established theoretically and these one dimensional anions could be utilized in developing novel H₂ storage materials.

Carbon rings: a DFT study on geometry, aromaticity, intermolecular carbon–carbon interactions and stability

Strong complimentary electrostatic interactions lead to large number of C⋯C interactions in doubly antiaromatic C_4N dimers while the delocalized system of π electrons in doubly aromatic C_4N+2 dimers suggest very few intermolecular C⋯C interactions.

A DFT study on dihydropyrazine annulated linear polyacenes: aromaticity, stability and HOMO–LUMO energy modulation

Linear polyacene (LPA) mimics containing up to 34 rings have been designed by annulating dihydropyrazine units to LPA cores and they showed enhanced aromatic character than the LPA due to significant mixing of the N-lone pairs with the carbon π-orbitals.