The powerful independent gradient model (IGM) method has been increasingly popular in visual analysis of intramolecular and intermolecular interactions in recent years. However, we frequently observed that there is an evident shortcoming of IGM map in graphically studying weak interactions, that is its isosurfaces are usually too bulgy; in these cases, not only the graphical effect is poor, but also the color on some areas on the isosurfaces is inappropriate and may lead to erroneous analysis conclusions. In addition, the IGM method was originally proposed based on promolecular density, which is quite crude and does not take actual electronic structure into account. In this article, we propose an improvement version of IGM, namely IGM based on Hirshfeld partition of molecular density (IGMH), which replaces the free-state atomic densities involved in the IGM method with the atomic densities derived by Hirshfeld partition of actual molecular electron density. This change makes IGM have more rigorous physical background. A large number of application examples in this article, including molecular and periodic systems, weak and chemical bond interactions, fully demonstrate the important value of IGMH in intuitively understanding interactions in chemical systems. Comparisons also showed that the IGMH usually has markedly better graphical effect than IGM and overcomes known problems in IGM. Currently IGMH analysis has been supported in our wavefunction analysis code Multiwfn (http://sobereva.com/multiwfn). We hope that IGMH will become a new useful method among chemists for exploring interactions in wide variety of chemical systems. 

Independent gradient model based on Hirshfeld partition: A new method for visual study of interactions in chemical systems

Recently, Iyakutti and coworkers published an interesting paper studying characteristics of cyclo[12]carbon and cyclo[18]carbon as well as their interactions with graphene (Mater. Sci. and Eng. B 263 (2021) 114895). However, we found some of their calculations are obviously unreasonable and a few conclusions are misleading. Here, we make some remarks on this paper according to common theoretical chemistry knowledge and our recent research experiences on cyclo[n]carbon systems, and meantime we revisit the interaction between the cyclo[18]carbon and graphene using a more reliable calculation model in combination with some novel analysis methods to provide complementary viewpoint to Iyakutti's work.

Comment on “18 and 12 – Member carbon rings (cyclo[n]carbons) – A density functional study”

Potential optical molecular switch: Lithium@cyclo[18]carbon complex transforming between two stable configurations

Planarity is a very important structural character of molecules, which is closely related to many molecular properties. Unfortunately, there is currently no simple, universal, and robust way to measure molecular planarity. In order to fill this evident gap, we propose two metrics of molecular planarity, namely molecular planarity parameter (MPP) and span of deviation from plane (SDP), to quantitatively characterize planarity of molecules. MPP reflects the overall degree of deviation of the structure from a plane, while SDP represents the span of the structural deviation relative to the fitting plane; respectively, they are complementary to each other. The examples in this article demonstrate that these metrics have strong rationality and practicality. They can not only be used to investigate the planarity of the entire molecule, but also measure the planarity of local structures, and they can even be employed to study variation of molecular planarity during a dynamic process. In addition, we also propose a new representation, namely coloring atoms according to their signed deviation distance to the fitting plane. This kind of map allows researchers to intuitively and quickly recognize position of the atoms in the system relative to the fitting plane. It can be seen from the examples that this representation is very useful in graphically exhibiting molecular planarity. The methods proposed in this work have been implemented in our open-source analysis code Multiwfn, which can be freely obtained via http://sobereva.com/multiwfn
 . The use is very simple and rich file formats are supported as input file.

Simple, reliable, and universal metrics of molecular planarity.

Inspired by recent experimental observation of molecular morphology and theoretical predictions of multiple properties of cyclo[18]carbon, we systematically studied the photophysical and nonlinear optical properties of cyclo[2N]carbons (N=3-15) allotropes through density functional theory. This work unveils the unusual optical properties of the sp-hybridized carbon rings with different sizes. The remarkable size dependence of the optical properties of these systems and their underlying nature are profoundly explored, and the relevance between aromaticity and optical properties are highlighted. The extrapolation curves fitted for energy level of frontier molecular orbitals, maximum absorption wavelength, and (hyper)polarizability of considered carbon rings are presented, which can be used to reliably predict corresponding properties for arbitrarily large carbon rings. The findings in this study will facilitate the exploration of potential application of cyclocarbons in the field of optical materials.

Remarkable Size Effect on Photophysical and Nonlinear Optical Properties of All-Carboatomic Rings, Cyclo[18]carbon and Its Analogues.

Although the unique cyclo[18]carbon (C18) realized by recent experiments has been greatly concerned, it has so far remained elusive. In contrast, its precursors C18-(CO)n (n = 6, 4, and 2), which can be separated stably, are of more practical significance. In this paper, the bonding character, electron delocalization, and aromaticity of the C18-(CO)n (n = 6, 4, and 2) with out-of-plane and in-plane dual π systems (πout and πin) perpendicular to each other are studied by combining quantum chemical calculations and wavefunction analyses. These cyclocarbon oxides exhibit alternating long and short C-C bonds and extensive electron delocalization, and a significant diatropic induced ring current under the action of external magnetic field is therefore observed, which reveals the aromatic characteristic in the molecules. The global electron delocalization and significant influence of the number of intramolecular carbonyl (-CO) on the two sets of π conjugated systems have been focused on, and the essential reason for the distinct difference in the overall aromaticity of the molecules was also clarified. It seems that the substituent -CO groups hinders the electron delocalization of the πin system but has relatively small effect on the πout system, resulting in the molecules with less -CO group showing greater aromaticity.

/pdf/bonding-character-electron-delocalization-and-aromaticity-of-13481xv44q.pdf

Bonding character, electron delocalization, and aromaticity of cyclo[18]carbon (C18) precursors, C18-(CO)n (n = 6, 4, and 2): Focusing on the effect of -CO groups

The electronic spectra and (hyper)polarizability of C18-(CO)n (n = 2, 4, and 6) are studied using theoretical calculations to reveal the effect of introducing carbonyl (-CO) groups on the molecular optical properties. Successive introduction of -CO groups is observed to cause a red-shift in the absorption spectrum, but maximum absorption of all molecules is mainly due to the charge redistribution within the C18 moiety. The (hyper)polarizabilities of the cyclocarbon oxides present an ascending trend with the -CO groups in the molecule, and the higher-order response properties are more sensitive. With (hyper)polarizability density analysis and (hyper)polarizability contribution decomposition, the fundamental reasons for the difference of (hyper)polarizability of different molecules are systematically discussed from the perspective of physical and structural origins, respectively. Significant optical resonances under the frequency-dependent fields are found for the (hyper)polarizabilities of the cyclocarbon oxides, which is in contrast to the insignificant influence on their polarizability.

Photophysical properties and optical nonlinearity of cyclo[18]carbon (C18) precursors, C18-(CO)n (n = 2, 4, and 6): focusing on the effect of the carbonyl groups.

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.

/pdf/tuning-electronic-structure-and-optical-properties-of-li-4jym6kdaxa.pdf

Xia Wang

Papers

Potential optical molecular switch: Lithium@cyclo[18]carbon complex transforming between two stable configurations

Remarkable Size Effect on Photophysical and Nonlinear Optical Properties of All-Carboatomic Rings, Cyclo[18]carbon and Its Analogues.

Bonding character, electron delocalization, and aromaticity of cyclo[18]carbon (C18) precursors, C18-(CO)n (n = 6, 4, and 2): Focusing on the effect of -CO groups

Photophysical properties and optical nonlinearity of cyclo[18]carbon (C18) precursors, C18-(CO)n (n = 2, 4, and 6): focusing on the effect of the carbonyl groups.

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