Ring Critical Points Research Articles

Electron Density and Molecular Orbital Analyses of the Nature of Bonding in the η3-CCH Agostic Rhodium Complexes Preceding the C-C and C-H Bond Cleavages.

In our recent work, we revisited C-H and C-C bond activation in rhodium (I) complexes of pincer ligands PCP, PCN, PCO, POCOP, and SCS. Our findings indicated that an η3-Csp2Csp3H agostic intermediate acts as a common precursor to both C-C and C-H bond activation in these systems. We explore the electronic structure and bonding nature of these precleavage complexes using electron density and molecular orbital analyses. Using NBO, IBO, and ESI-3D methods, the bonding in the η3-CCH agostic moiety is depicted by two three-center agostic bonds: Rh-Csp2-Csp3 and Rh-Csp3-H, with all three atoms datively bound to Rh(I). IBO analysis specifically highlights the involvement of three orbitals (CC→Rh and CH→Rh σ donation, plus Rh→CCH π backdonation) in both C-C and C-H bond cleavages. NCIPLOT and QTAIM analyses highlight anagostic (Rh-H) or β-agostic (Rh-Csp2-H) interactions and the absence of Rh-Csp3 interactions. QTAIM molecular graphs suggest bond path instability under dynamic conditions due to the nearness of line and ring critical points. Several low-frequency and low-force vibrational modes interconvert various bonding patterns, reinforcing the dynamic η3-CCH agostic nature. The kinetic preference for C-H bond breaking is attributed to the smaller reduced mass of C-H vibrations compared to C-C vibrations.

Metronidazole-Picric acid salt formation: Synthesis, characterization, Hirshfeld surface analysis, single crystal X-ray diffraction, computational studies, and molecular docking studies against Clostridioides difficile

The salt formation of metronidazole (MET) and picric acid (PA) is obtained via slow evaporation of their mixture in methanol which afforded a novel organic salt, C6H10N3O3+⋅C6H2N3O7- (METPA). The structure of METPA salt was analyzed using single crystal X-ray diffraction. METPA crystallizes in the triclinic space group P-1 with cell dimensions: a = 8.2446(1), b = 9.1843(1), c = 11.5832(2) Å, α = 106.190(1)°, β = 99.483(1)°, γ = 106.635(1)°, Volume (Å3) = 778.02(2) and Z = 2. The formation of METPA salt was analyzed by comparing the IR spectrum of MET, PA and METPA salt. In order to visualize the intermolecular interactions in the crystal of METPA salt, Hirshfeld surface analysis was employed. QTAIM calculations were performed using the AIMALL program package to estimate the strength of hydrogen bonding interactions in terms of bond critical points (BCP) and ring critical points (RCP). Molecular docking of the MET and METPA was conducted against three toxins of Clostridioides difficile such as C. difficile toxin A (TcdA), C. difficile toxin B (TcdB), C. difficile transferase (CDT). The results showed that the docking score calculated based on the knowledge-based iterative scoring function ITScore-PP indicates that METPA with higher negative value of docking score in case of all three toxins when compared to MET. This shows that the binding model of METPA salt is more favorable than MET.

Synergetic effects of inter- and intramolecular hydrogen bonding interactions in XC5H3HC = Y···HO···H2O2 complexes (X = N, P, As and Sb; Y = O, S and NH): the role of aromaticity and exchange interactions

ABSTRACT Intramolecular hydrogen bonding (IHB) interactions were considered in XC5H3HC = Y···HO molecules which include OH···Y unit (Y = O, S and NH) and heteroatom X (X = N, P, As and Sb). Alteration of heteroatoms X in para position causes a change of properties of electron charge densities on these molecules and influences on strength of the IHB interactions. The aromaticity of the ring A (involving IHB interaction) and ring B (involving heteroatoms X) of XC5H3HC = Y···HO molecules was calculated using para-delocalisation index (PDI). Results reveal that the change of X from N to Sb is followed by the decrease in aromaticity of the mentioned rings. However, the strength of the IHB interactions increases during the change of X. Moreover, IHB energies were estimated using some computational methods and a viewpoint based on the properties of ring critical points (RCPs). The IHB energies obtained using a viewpoint based on the properties of RCPs have a better relationship with hydrogen bonding descriptors. Also, intermolecular hydrogen bond (HB) interactions of hydrogen peroxide with the close and open forms of the XC5H3HC = Y···HO molecules were considered. Cooperative effects of the HB and IHB interactions are revealed. Moreover, energy decomposition analysis (EDA) highlights the role of exchange interactions on the stability of the XC5H3HC = Y···HO···H2O2 complexes.

Theoretical Insights into Bifurcated Intramolecular Dihydrogen Bonds.

Two-ring intramolecular π-electron delocalization assisted dihydrogen bonds existing in (1Z,4Z)-1,4-dipentene-3-bora-1,5-diol and its symmetrically substituted derivatives have been analysed here since the MP2/6-311++G(d,p) calculations on these systems were performed. The influence of the coexistence of two intramolecular dihydrogen bonded rings in these molecular structures on properties of intramolecular dihydrogen bonds as well as on the π-electron delocalization within these rings was investigated. The comparison with corresponding structures of typical two-ring, so-called resonance-assisted, RAHB, systems was performed. The results of calculations show that such rings' coexistence leads to the weakening of dihydrogen bonds, similarly as for the typical two-ring RAHB systems. The Quantum Theory of ''Atoms in Molecules'' (QTAIM) was also applied here to get more details about the nature of dihydrogen bonds. Correlations between dihydrogen bond strength measures and other energetic, geometrical and topological parameters were also analysed. It was found that characteristics of bond critical points as well as of ring critical points are useful to estimate the strength of intramolecular dihydrogen bonds in two-ring dihydrogen bonded systems discussed here. The Natural Bond Orbital, NBO, approach parameters are also discussed as useful ones to describe properties of dihydrogen bonded systems.

Quantification of hydrogen bond energy based on equations using spectroscopic, structural, QTAIM-based, and NBO-based descriptors which calibrated by the molecular tailoring approach

ContextHydrogen bonds critically influence the structure and properties of both organic molecules and biomolecules, as well as supramolecular assemblies. For this reason, the development and elaboration of methods for quantitative assessment of hydrogen bond energy is an urgent challenge. In this study, using a large series of hydroxycarbonyl aliphatic compounds with the O‒H···O = C intramolecular hydrogen bond, a bank of hydrogen bond descriptors was created, including spectroscopic, structural, QTAIM-based, and NBO-based parameters. It was shown that the O‒H vibration frequency, OH chemical shift as the spectroscopic descriptors, the O···H hydrogen bond length, O···O distance, and O‒H covalent bond length as the structural descriptors, the electron density and its Laplacian, electron potential energy density in the hydrogen bond critical point, the electron density at the ring critical point as the QTAIM-based descriptors change in a correlated manner. The same correlation is found in change of the charge transfer energy through a hydrogen bond, the occupancy of the O‒H bond antibonding orbital, the Wiberg indices of the O···H hydrogen bond, and the O‒H covalent bond, as well as the polarization of the O‒H bond, which are the NBO-based descriptors. It was also recognized that the specified descriptors from the spectroscopic, structural, QTAIM-based, and NBO-based categories are functionally related to the values of intramolecular hydrogen bond energy, quantified via the molecular tailoring approach. This allowed one to obtain a system of equations for quantitative estimation of intramolecular hydrogen bond energy based on the spectroscopic, structural, QTAIM, and NBO descriptors, which makes such quantification more dependable and reliable.MethodsTo obtain the spectroscopic descriptors, the vibrational spectra and shielding constants were calculated using the GIAO method. Structural descriptors were obtained for the equilibrium geometry of molecules, calculated at the MP2(FC)/6–311 + + (2d,2p) level using the Gaussian 09 program. The QTAIM-based descriptors were calculated using the AIMAll program within the framework of the quantum theory “Atoms in Molecules.” The NBO-based descriptors were calculated using the NBO 3.1 program implemented into Gaussian 09. To quantify the energy of intramolecular hydrogen bonds, molecular fragmentation was used within the molecular tailoring approach.

Insights into Unconventional Intramolecular CH⋅⋅⋅CO Interaction in Os3(CO)9(μ‐H)2(μ3‐η1: η1: η2‐C16H8) Crystal: A QTAIM, NCI, and NBO Exploration

AbstractThis work presents a study investigating the inter‐ and intra‐molecular interactions within the Os3(CO)9(μ‐H)2(μ3‐η1: η1: η2‐C16H8) crystal. The crystal‘s behavior is analyzed by comparing experimental distances, revealing intriguing interactions. In the isolated molecule, an unconventional pyrene‐C−H⋅⋅⋅CO interaction is observed, an electron transfer from σ(C−H) to π*(CO). Strikingly, the Quantum Theory of Atoms in Molecules identifies similarities to an intramolecular charge‐inverted hydrogen bond, despite its relatively low stability due to proximity to critical points. Energy surface scans demonstrate that the interaction arises from van der Waals strain induced by the crystal‘s packing structure. The proximity between carbonyl and pyrene facilitates electron transfer between σ(C−H) and π*(CO) at distances similar to the crystal structure. A significant correlation is established between total energy and the ratio (|V|/G) of potential energy density (V) to Lagrangian kinetic energy (G) at bond (BCP) and ring (RCP) critical points, underscoring the role of electron delocalization on the pseudo‐ring in determining the existence and characteristics of these interactions. In conclusion, this study provides valuable insights into molecular interactions within the Os3(CO)9(μ‐H)2(μ3‐η1: η1: η2‐C16H8) crystal, enriching our understanding of crystal interactions and offering perspectives for further exploration in this field.

Open shell versus closed shell bonding interaction in cyclopropane derivatives: EDA-NOCV analyses.

Cyclopropane ring is a very common motif in organic/bio-organic compounds. The chemical bonding of this strained ring is taught to all chemistry students. This three-membered cyclic, C3 ring is quite reactive which has attracted both, synthetic and theoretical chemists to rationalize/correlate its stability and bonding with its reactivity and physical properties over a century. There are a few bonding models (mainly the Bent-Bond model and Walsh model) of this C3 ring that are debated to date. Herein, we have carried out energy decomposition analysis coupled with natural orbital for chemical valence (EDA-NOCV) to study the two most reactive bonds of cyclopropane rings of 49 different organic compounds containing different functional groups to obtain a much deeper bonding insight toward a more general bonding model of this class of compounds. The EDA-NOCV analyses of fragment orbitals and susequent bond formation revealed that the nature of the CC bond of the cyclopropane (splitting two bonds at a time out of three CC bonds) ring is preferred to form two dative covalent CC bonds (between a singlet olefin-fragment and an excited singlet carbene-fragment with a vacant sp2 orbital and a filled p-orbital) for the majority (37/49) of compounds over two covalent electron sharing bonds in some (7/49) compounds (between an excited triplet olefin and triplet carbene), while a few (5/49) compounds show flexibility to adopt either the electron sharing or dative covalent bond as both are equally possible. The effects of functional groups on the nature of chemical bond in cyclopropane rings have been studied in detail. Our bonding analyses are in line with the QTAIM analyses which produce small negative values of the Laplacian, significantly positive values of bond ellipticity, and accumulation of electron densities around the ring critical point of C3 -rings. These corresponding QTAIM parameters of C3 -rings are quite different for CC single bonds of normal hydrocarbons as expected. The chemical bonding in the majority of cyclopropane rings can be very similar to those of metal-olefin systems.

On Integral INICS Aromaticity of Pyridodiazepine Constitutional Isomers and Tautomers.

The structure, energetics, and aromaticity of c.a. 100 constitutional isomers and tautomers of pyrido[m,n]diazepines (m = 1, 2; n = 2, 3, 4, 5; m ≠ n) were studied at the B3LYP/cc-pVTZ level. The pyrido[1,3]diazepines appear the most, while pyrido[2,4]diazepines are the least stable (ca. 26 kcal/mol). In the pyrido[1,n]diazepine group (n = 2-5), the [1,5] isomers are higher in energy by ca. 4.5 kcal/mol and the [1,4] ones by ca. 7 kcal/mol, and the pyrido[1,2]diazepines are the least stable (ca. 20 kcal/mol). All the most stable pyrido[1,n]diazepines have N-atoms near the ring's junction bond but on opposite sites. The most stable [2,n]-forms are also those with the pyridine ring N6-atom near the junction bond. Surprisingly, for the [1,2]-, [1,3]-, and [1,4]-isomer condensation types of pyridine and diazepine rings, the same N9 > N7 > N6 > N8 stability pattern obeys. The stability remains similar in a water medium simulated with the Polarizable Continuum Model of the solvent and is conserved when calculated using the CAM-B3LYP or BHandHlyp functionals. The ring's aromaticity in the pyridine[m,n]diazepines was established based on the integral INICS index resulting from the NICSzz-scan curves' integration. The integral INICS index is physically justified through its relation to the ringcurrent as demonstrated by Berger, R.J.F., et al. Phys. Chem. Chem. Phys. 2022, 24, 624. The six-membered pyrido rings have negative INICSZZ indices and can be aromatic only if they are not protonated at the N-atom. All protonated pyrido and seven-membered rings exhibit meaningful positive INICSZZ values and can be assigned as antiaromatic. However, some non-protonated pyrido rings also have substantial positive INICSZZ indices and are antiaromatic. A weak linear correlation (R2 = 0.72) between the INICSZZ values of the pyridine I(6) and diazepine I(7) rings exists and is a consequence of the communication between the π-electron systems of the two rings. The juxtaposition of the INICS descriptor of the six- and seven-membered rings and diverse electron density parameters at the Ring Critical Points (RCP) revealed good correlations only with the Electrostatic Potentials from the electrons and nuclei (ESPe and ESPn). The relationships with other RCP parameters like electron density and its Laplacian, total energy, and the Hamiltonian form of kinetic energy density were split into two parts: one nearly constant for the six-membered rings and one linearly correlating for the seven-membered rings. Thus, most of the electron density parameters at the RCP of the six-membered rings of pyridodiazepines practically do not change with the diazepine type and the labile proton position. In contrast, those of the seven-membered rings display aromaticity changes in the antiaromatic diazepine with its ring structural modifications.

Insights on adsorption properties of a DNA base, guanine on nano metal cages (Ag24/Au24/Cu24): DFT, SERS, NCI and solvent effects

A detailed theoretical study was implemented on 2-amino-1,7-dihydropurin-6-one's (ADO). Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) simulations were accomplished to explain the adsorption performance of the ADO drug molecule on metal nanocages. Adsorption energy og ADO was calculated to be -53.90, -50.67 and -63.56 kcal/mol for Ag, Au and Cu cages, respectively. ADO interacts with Ag/Cu through the N5, N4 and N6 atoms with a perpendicular orientation and Au-ADO, the interaction is through O, N3H13 and NH2 groups with a perpendicular orientation. Reactivity computations were used to find the electron density in NMC-containing ADO complexes. The intermolecular interaction forces were found by the establishment of numerous bond critical point, monitored by ring critical points at the midpoint of interaction state. The ADO molecule will also be examined as part of the computational study to ascertain its bioactivity and docking studies. We can get accurate estimates of the atomic contact energies for drug delivery using the docking process.

Unlocking the potential of ovalene: A dual-purpose sensor and drug enhancer

Utilizing the Density Functional Theory (DFT) and Time-dependent Density Functional Theory (TD-DFT) with B3LYP-D3 and cam-B3LYP-D3 and 6-311G++ (d, p) basis set, we have investigated the adsorption and non-covalent interactions (NCI) of 5-hydroxymethylfurfural (HMF) on the surface of Ovalene. Gibbs free energy of restricting atoms, their 4-energy sub-atomic orbitals examination for the electron/charge dispersion, impact of charge move, non-direct optical properties (NLO), QTAIM, dipole second, RDG-isosurfaces, hardness, synthetic potential, electrophilicity list, SERS, and UV-Apparent spectra investigation have been sightseen through achievement the DFT recreations. In the gas phase, the HMF–Ovalene complex was simulated to have a free energy of −5.55 kcal/mol, while in the solvent phase; it was simulated to have a free energy of around −19.80 kcal/mol. The binding complex geometry is greatly influenced by the solvent, which also increases its stability. In addition, the formation of various bond critical points (BCPs) and ring critical points (RCPs) at the midpoint of the interaction state were used to monitor the formation of intermolecular interaction (IMI) forces. The solvent effect also appeared to have a significant impact on the calculated changes in enthalpy H, entropy S, and total energy E. Moreover, the docking study shows that the HMF-Ovalene complex has a binding affinity double as compared to the isolated HMF drug molecule. HMF and HMF-Ovalene complex sits deep in the ligand-binding pocket and directly interacts with TYP 108, THR 112, THR 194, PHE 432, and TRP 158. The binding affinity for HMF and 3RZE was found to be −5.00 whereas with 7DFL was −480 kcal/mol. These energies increase to −11.00 and −9.70 kcal/mol respectively when Ovalene serves as a drug delivery surface.

Adsorption of Pyrimidin-2-amine (PA) on Graphene Quantum Dots (GQDs): Non-covalent Interaction Study

A detailed theoretical study was implemented on a novel drug molecule “Pyrimidin-2-amine (PA)”. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) simulations were accomplished to explain the adsorption performance of the PA drug molecule on graphene quantum dots (GQD). Binding and adsorption energy, 4-energy molecular orbitals analysis, charge transfer effect, QTAIM, dipole moment, NLO properties, frontier molecular orbital analysis (FMO), the density of states (DOS), RDG-isosurfaces, UV–Visible, and SERS spectra analysis have been explored while performing the DFT simulations. The total adsorption energy of PA was calculated to be −3.45, –22.48, −20.32, −3.78, and −4.47 Kcal/mol for the corresponding CC-PA, BB-P-PA (where “P” stands for the para position of doped elements), NN-P-PA, BN-O-PA (where “O” stands for the ortho position of doped elements), and BN-P-PA complexes respectively. The intermolecular interaction forces were dogged by the establishment of numerous bond critical points (BCPs), monitored by ring critical points (RCPs) at the midpoint of the interaction state.

Spectroscopic, reactivity analysis and docking studies of 3-(adamantan-1-yl)-4-(4-fluorophenyl)-1-[(4-phenylpiperazin-1-yl)methyl]-4,5-dihdyro-1H-1,2,4-triazole—5-thione: DFT and MD simulations

An adamantane-thione derivative, 3-(adamantan-1-yl)-4-(4-fluorophenyl)-1-[(4-phenylpiperazin-1-yl)methyl]-4,5-dihdyro-1H-1,2,4-triazole—5-thione (ATT) is investigated experimentally and theoretically to predict its various chemical and electronic properties. The solvation energies are predicted in acetone, DMSO, ethanol and water and acetone is a better solvent due to high value of solvation energy. When compared to the gas phase, the chemical descriptors of ATT decreased in the solution phase. The adamantane cage breathing mode is assigned at 785 (theoretical) and 784 cm−1 (IR). Topological study of the atom in molecules (AIM) shows the ring critical points corresponding to the phenyl, pyrazine, triazole and 4-fluorophenyl rings.From molecular dynamics (MD) simulations, solvent accessible surface area (SASA) values are from 656 to 736 and 289 to 294 (Å2) for unbound and bound structures.

An eco-friendly method for the synthesis of 1,2,4-triazole-Schiff base derivatives in aqueous medium and DFT calculations

A detailed optimization study was carried out for the synthesis of 1,2,4-triazole-Schiff base derivatives. In previous studies, such reactions were performed either by conventional methods and using solvents such as ethanol, methanol, or THF in a microwave reactor for a certain period of time. However, in our study, the highest reaction yield was tried to be obtained by changing parameters such as temperature, time, and solvent system by using a monomode microwave reactor in aqueous medium. Thus, five novel 1,2,4-triazole-Schiff base derivatives were synthesized and characterized by FT-IR, NMR, and MS. Further, DFT (density functional theory) and AIM (atoms in molecules) calculations have been carried out to gain a better understanding of the molecular geometry, electronic properties, and chemical reactivity of the synthesized compounds. It was found that the pyrazole and phenyl groups of all compounds are almost in the same plane and that the benzotriazole ring is inclined by a torsion angle of 71.14 to 80.38° These molecular geometries exhibit at least three additional ring critical points (RCPs) resulting from three to five intramolecular contacts.

Study of O[sbnd]H···O bonded-cyclic dimer for 2,5-Dihydroxyterephthalic acid as aided by MD, DFT calculations and IR, Raman, NMR spectroscopy

Molecular Dynamics-simulated OH···O bonded-cyclic dimer structure, called Dc, has been proposed for 2,5-Dihydroxyterephthalic acid a potential organic linker in Hydrogen-bonded Organic Frameworks (HOF). Among many dimer species bound by inter-molecular OH···O bonds, Dc was chosen because of its relatively highest lifetime of 18.3 ps as indicated by the radial distribution functions (RDF). Experimentally observed intense but broad down-shifted IR absorptions peak at ∼ 3023 cm−1 and bands near 2556 cm−1 combined with staggered occurrence of IR and Raman bands in other spectral regions suggested that the proposed cyclic dimer appeared to possess an extended quasi-centrosymmetric configuration. Upon optimization of Dc at B3LYP/6-311++G(d,p) level, it yielded geometry and vibrational spectra in agreement with experimental spectra. Further, we have characterized both intra- and inter-molecular OH···O bonds in Dc using stabilization energy E(2) and other molecular orbital (MO) properties from NBO analysis. Bond critical points (BCPs) and ring critical points (RCPs) deduced from electron density (ρ(r)), its laplacian (∇2ρ(r)) and potential energy density (v(r)) using AIM calculations have provided complementary perspectives. Additionally, a combination of quantitative analysis and graphical visualization of the charge regions where non-covalent interactions occur in real space intra- and inter-molecular OH···O bonds, van der Waals and repulsive steric clash interactions have been obtained from NCI calculations.

Accurate Ring Strain Energies of Unsaturated Three-Membered Heterocycles with One Group 13-16 Element.

High-quality ring strain energy (RSE) data for 1H-unsaturated (CH)2X parent rings, where X is a group 13–16 element, are reported in addition to the 2H-isomers of the pnictogenirene rings. RSE data are obtained from appropriate homosdesmotic reactions and calculated at the DLPNO-CCSD(T)/def2-TZVPP//B3LYP-D3/def2-TZVP(ecp) level. 1H-Tallirene and 1H-plumbirene have unique donor–acceptor structures between an acetylene π(CC) orbital and an empty p orbital of a metallylene subunit (a Dewar–Chatt–Duncanson description) and therefore cannot be described as proper rings but as pseudocyclic structures. Also, 1H-indirene and 1H-oxirene lack ring critical points and constitute borderline cases of pseudorings. 1H-Unsaturated rings exhibit enhanced RSE compared to their saturated homologues. The mechanism of ring strain relaxation by increasing the s character in the lone pair (LP) of group 15–16 elements is remarkable and increases on descending the groups. Furthermore, RSE is affected by the aromatic character of group 13 rings and certain aromatic or antiaromatic character in group 14 or 15–16 rings, respectively, which tend to vanish on descending the group as shown by NICS(1) values. 2H-Unsaturated rings were found only for group 15 elements (although only 2H-azirine shows a proper cyclic structure) and displayed lower RSE (higher stability) than the corresponding 1H-isomers.

Quantum chemistry study on the promoted reactivity of substituted cyclooctynes in bioorthogonal cycloaddition reactions

AbstractIn the present research, we focus on the energetics and electronic aspects of enhanced reactivity in the regioselective bioorthogonal 1,3-dipolar cycloaddition reaction of various substituted cyclooctynes with methyl azide, applying quantum chemistry approaches. In this respect, we assessed the structural and energetic properties of regioisomeric products and their corresponded transition states and calculated the reaction electronic energy changes and energy barriers through the cycloaddition pathways. The obtained results revealed that the trifluoromethyl substitution and fluorination of cyclooctynes lead to improved reactivity, in conjunction with increased exothermicity and decreased activation energy values. On the other hand, quantum theory of atoms in molecules computations were performed on some key bond and ring critical points that demonstrated the stabilizing topological properties of electron density and its derivatives upon trifluoromethyl substitution and fluorination of propargylic carbon of cyclooctynes which can be regarded as the essential origin of enhanced reactivity.

The reaction path of cyclooctatetraene dimerization revisited

AbstractThe electron density and the electron localization‐delocalization matrices (LDMs) are used to revisit the cyclooctetraene (COT) dimerization mechanism. The global minimum of a COT monomer (the tub geometry) exhibits a rare topological feature, giving rise, not to one, but to two ring critical points and a cage critical point necessary to satisfy the Poincaré‐Hopf relation. The energy profiles were used to identify the rate‐limiting step of the reaction: the penultimate transition state of a series of five transition states in total. While the monomers themselves have zero dipolar polarization, the dimer has a nonnegligible total dipole moment reaching almost 1 debye with fluctuations in strength and orientation along the reaction path. The reaction can hence, possibly, be manipulated with intense laser fields. This classic reaction has been used to elucidate whether the electron LDMs reflect structural or energetic similarity. It is found that LDMs are excellent monitors of structural/electronic similarity between different species on the reaction coordinate. A reaction can be characterized by a three‐dimensional hypermatrix whereby the matrix elements change as a function of the reaction coordinate, which can be represented as a parallelepiped of matrix elements. A study of the electron density of the system as the reaction progresses identifies and characterizes the bond paths that drive the reaction. It is hoped that this dynamic picture of the evolution of the electron density complements the usual arrow pushing reaction mechanisms used by organic chemists.

Novel microporous B6N6 covalent organic framework (COF) as an electrochemical sensor for the ultra-selective detection of nitroaniline isomers; a DFT outcome

The density functional theory (DFT) simulations are performed to study the sensitivity and selectivity of a novel (star shape) boron nitride two-dimensional covalent organic framework (B6N6) towards adsorption of toxic nitroaniline isomers. The binding energies of nitroaniline isomers onto the B6N6 material have been evaluated with hybrid B3LYP functional along with third order Grimme dispersion (D3) method in order to accurately account for dispersion effects. Noncovalent interaction index (NCI) and quantum theory of atoms in molecules (QTAIM) illustrate that weak van der Waals forces are the main contributor in stabilizing the reported complexes. The BSSE corrected binding energies of oNA, mNA and pNA on B6N6 are -15.42, -14.73 and -14.34 kcal/mol, respectively. These results reveal that the sensitivity and selectivity of B6N6 decreases in the order of oNA > mNA > pNA. The effect of aromaticity of nitro-aniline isomers in stabilizing the complexes is studied through the ring critical point (RCP) based on Bader atoms in molecule analysis. The sensitivity of B6N6 for nitroaniline isomers is further justified by the electronic properties. For example, the density of states (DOS) reveal that the new occupied energies states appear between original HOMO and LUMO orbitals of pristine B6N6 upon complexation with nitroaniline isomers. The newly generated occupied orbitals are closer to the Fermi energy level which increase the conductivity of B6N6, thus the significant amount of NBO charge transfer is noticed. The amounts of charge transfer in oNA@B6N6, mNA@B6N6 and pNA@B6N6complexes are 0.05, 0.09 and 0.05 e−, respectively. The percentage contribution of orbitals in charge transfer is estimated by the orbital composition analysis. Finally, the effect of increasing concentration of nitroaniline molecules on the sensing properties of B6N6 is also studied, which shows that the binding energies are less negative per molecule by increasing the number of nitroaniline units. The findings of this study reveal that the B6N6 star like COF material shows excellent sensing abilities for the toxic molecules. In summary, B6N6 material is a valuable addition with ultrahigh sensitivity as well as selectivity for future electrochemical sensor devices.

Rotamerism and intramolecular interactions of n-propyl halides and their partially and fully fluorinated derivatives. Electronic structure, topology and vibrational spectroscopy

n-Propyl halides (C3H7X; X = Cl, Br, I) and their partially and fully fluorinated analogues were investigated through quantum mechanical calculations and vibrational spectroscopy. Two energetic minima, corresponding to the rotational isomers (s-trans and s-gauche) were identified for all the species. The Bader quantum theory of Atoms-In-Molecules (AIM) analysis identified some bond and ring critical points for CF3-type gauche compounds involving F atoms on the CF3 group and X and F atoms on the CXF2 group. Such non-covalent intramolecular interactions are found not only responsible for the physical properties of n-propyl halides, but also the features of 19F NMR of n-perfluoropropyl iodide. The comparison between experimental vibrational spectra of some compounds and the calculated ones in both harmonic and anharmonic approximation indicated the frozen phase of these compounds is dominated with s-trans isomers, whereas the liquid phase is a mixture of both isomers. Rovibrational coupling was hypothesized to be responsible for the anharmonicity reflected in the vibrational spectra of C3H7X (X = Br and I). Based on the maps of Molecular Electrostatic Potential (MEP) surfaces, a σ-hole was featured for all 24 compounds in both isomers. The thermodynamics of isomerisation and fluorination was thoroughly analysed and well supported by the Highest Occupied Molecular Orbitals (HOMOs) and MEP.

Topological electron density properties at critical points along aromatic rings as reactivity and regioselectivity descriptors in electrophilic substitutions

AbstractIn order to evaluate the substituent effect on the reactivity and regioselectivity of aromatic rings with respect to electrophilic substitution reactions, we considered several benzene derivatives containing activating (−NH2, −NHCH3, −N(CH3)2, −OH, −OCH3, and −CHCH2) and deactivating (−F, −Cl, −Br, −COH, −COCH3, −COOCH3, −COOH, −COCl, −CN, and −NO2) groups. In this sense, we propose alternative theoretical descriptors of reactivity and regioselectivity, which are based only on electron density properties of initial substrates. The descriptors investigated here were obtained from the quantum theory of atoms in molecules (electron density curvatures at the ring critical point [RCP] and at the critical points of carbon nuclei along with kinetic and potential energy densities at the RCP) and from the interacting quantum atom formalism (intra‐atomic energy values of carbon atoms). The topological parameters at the RCPs and at carbon nuclei within the aromatic moiety were particularly successful in this task, being able to provide quantitative predictions of reactivity as compared with traditional resonance parameters used to that same end.