Localized Molecular Orbital Analysis Research Articles

Exploring Spectral and Electrochemical Behavior of Hydroxy‐N‐Benzylideneanilines by Integrated Theoretical and Experimental Approaches

ABSTRACTThe present work explored the effect of –OH group substitution (o/p) on the spectral and electrochemical behavior of N‐benzylideneaniline. The geometry optimization of unsubstituted and (o/p)‐OH‐substituted analogs revealed the coplanarity of the molecules. The vibrational spectra of the compounds were computed using density functional theory (DFT) and compared with the experimental data. The observed bands were assigned based on total energy distribution (TED). Predicted electronic absorption spectra from time‐dependent density functional theory (TD‐DFT) calculation were compared with the UV–visible spectra of the molecules. The analysis of the lowest spin‐allowed (singlet‐singlet) excited states divulged possible electronic transition. The o‐substituted benzylideneaniline possessed the lowest highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gap among the substituted and unsubstituted analogs. The intramolecular contacts were interpreted using natural bond orbital and localized molecular orbital analysis. The –CH=N– linkage was investigated as a bridge for the electron delocalization from the donor to acceptor moieties. The manifestation of a reduction peak in the cyclic voltammetric studies confirmed the electrochemical behavior in the –OH‐substituted molecule, which was diffusion‐controlled. The discrepancy in the electrochemical property concerning the position of the –OH substituent of the candidate molecules was put forward.

Electron Dynamics in Alkane C-H Activation Mediated by Transition Metal Complexes.

Alkanes, ideal raw materials for industrial chemical production, typically exhibit limited reactivity due to their robust and weakly polarized C-H bonds. The challenge lies in selectively activating these C-H bonds under mild conditions. To address this challenge, various C-H activation mechanisms have been developed. Yet, classifying these mechanisms depends on the overall stoichiometry, which can be ambiguous and sometimes problematic. In this study, we utilized density functional theory calculations combined with intrinsic bond orbital (IBO) analysis to examine electron flow in the four primary alkane C-H activation mechanisms: oxidative addition, σ-bond metathesis, 1,2-addition, and electrophilic activation. Methane was selected as the representative alkane molecule to undergo C-H heterolytic cleavage in these reactions. Across all mechanisms studied, we find that the CH3 moiety in methane consistently uses an electron pair from the cleaved C-H bond to form a σ-bond with the metal. Yet, the electron pair that accepts the proton differs with each mechanism: in oxidative addition, it is derived from the d-orbitals; in σ-bond metathesis, it resulted from the metal-ligand σ-bonds; in 1,2-addition, it arose from the π-orbital of the metal-ligand multiple bonds; and in electrophilic activation, it came from the lone pairs on ligands. This detailed analysis not only provides a clear visual understanding of these reactions but also showcases the ability of the IBO method to differentiate between mechanisms. The electron flow discerned from IBO analysis is further corroborated by results from absolutely localized molecular orbital energy decomposition analysis, which also helps to quantify the two predominant interactions in each process. Our findings offer profound insights into the electron dynamics at play in alkane C-H activation, enhancing our understanding of these critical reactions.

Unveiling the Reduction Mechanism of Pu(IV) by Acetaldoxime.

The separation of plutonium (Pu) from spent nuclear fuel was achieved by effectively adjusting the oxidation state of Pu from +IV to +III in the plutonium uranium reduction extraction (PUREX) process. Acetaldoxime (CH3CHNOH) as a free salt reductant can rapidly reduce Pu(IV), but the reduction mechanism remains indistinct. Herein, we explore the reduction mechanism of two Pu(IV) ions by one CH3CHNOH molecule, where the second Pu(IV) reduction is the rate-determining step with the energy barrier of 19.24 kcal mol-1, which is in line with the experimental activation energy (20.95 ± 2.34 kcal mol-1). Additionally, the results of structure and spin density analyses demonstrate that the first and second Pu(IV) reduction is attributed to hydrogen atom transfer and hydroxyl ligand transfer, respectively. Analysis of localized molecular orbitals unveils that the reduction process is accompanied by the breaking of the Pu-OOH bond and the formation of the OOH-H and C-OOH bonds. The reaction energies confirm that the reduction of Pu(IV) by acetaldoxime is both thermodynamically and kinetically accessible. In this work, we elucidate the reduction mechanism of Pu(IV) with CH3CHNOH, which provides a theoretical understanding of the rapid reduction of Pu(IV).

Dispersion, Rehybridization, and Pentacoordination: Keys to Understand Clustering of Boron and Aluminum Hydrides and Halides.

The structure, stability, and bonding characteristics of dimers and trimers involving BX3 and AlX3 (X = H, F, Cl) in the gas phase, many of them explored for the first time, were investigated using different DFT (B3LYP, B3LYP/D3BJ, and M06-2X) and ab initio (MP2 and G4) methods together with different energy decomposition formalisms, namely, many-body interaction-energy and localized molecular orbital energy decomposition analysis. The electron density of the clusters investigated was analyzed with QTAIM, electron localization function, NCIPLOT, and adaptive natural density partitioning approaches. Our results for triel hydride dimers and Al2X6 (X = F, Cl) clusters are in good agreement with previous studies in the literature, but in contrast with the general accepted idea that B2F6 and B2Cl6 do not exist, we have found that they are predicted to be weakly bound systems if dispersion interactions are conveniently accounted for in the theoretical schemes used. Dispersion interactions are also dominant in both homo- and heterotrimers involving boron halide monomers. Surprisingly, B3F9 and B3Cl9 C3v cyclic trimers, in spite of exhibiting rather strong B-X (X = F, Cl) interactions, were found to be unstable with respect to the isolated monomers due to the high energetic cost of the rehybridization of the B atom, which is larger than the two- and three-body stabilization contributions when the cyclic is formed. Another important feature is the enhanced stability of both homo- and heterotrimers in which Al is the central atom because Al is systematically pentacoordinated, whereas this is not the case when the central atom is B, which is only tri- or tetra-coordinated.

Relativistic Density Functional NMR Tensors Analyzed with Spin-free Localized Molecular Orbitals.

The implementation of fast relativistic methods based on density functional theory, in conjunction with localized molecular orbital (LMO) based analysis, allows straightforward interpretations of NMR parameters in terms of contributions from core shells, lone pairs, and bonds, for compounds containing elements from across the periodic table. We present a conceptual review of a frequently used LMO analysis of NMR parameters calculated in the presence of spin-orbit interactions and other relativistic effects. An accompanying example focuses on the 15 N shielding in a heavy metal complex.

Plutonium Hybrid Materials: A Platform to Explore Assembly and Metal-Ligand Bonding.

We report the synthesis of five new hybrid materials containing the [PuCl6]2- anion and charge-balancing, noncovalent interaction donating 4-X-pyridinium (X = H, Cl, Br, I) cations. Single crystals of the title compounds were grown and harvested from acidic, chloride-rich, aqueous media, and their structures were determined via X-ray diffraction. Compounds 1-4, (4XPyH)2[PuCl6], and 5, (4IPyH)4[PuCl6]·2Cl, exhibit two distinct sheet-like structure types. Structurally relevant noncovalent interactions were tabulated from crystallographic data and verified computationally using electrostatic surface potential maps and the quantum theory of atoms in molecules (QTAIM). The strength of the hydrogen and halogen bonds was quantified using Kohn-Sham density functional theory, and a hierarchy of acceptor-donor pairings was established. The PuIV-Cl bonds were studied using QTAIM and natural localized molecular orbital (NLMO) analyses to delineate the underlying bond mechanism and hybrid atomic orbital contributions therein. The results of the PuIV-Cl bond analyses were compared across compositions via analogous treatments of previously reported [PuO2Cl4]2- and [PuCl3(H2O)5] molecular units. The Pu-Cl bonds are predominately ionic yet exhibit small varying degrees of covalent character that increases from [PuCl3(H2O)5] and [PuO2Cl4]2- to [PuCl6]2-, while the participation of the Pu-based s/d and f orbitals concurrently decreases and increases, respectively.

Assessing the Interplay between Functional-Driven and Density-Driven Errors in DFT Models of Water.

We investigate the interplay between functional-driven and density-driven errors in different density functional approximations within density functional theory (DFT) and the implications of these errors for simulations of water with DFT-based data-driven potentials. Specifically, we quantify density-driven errors in two widely used dispersion-corrected functionals derived within the generalized gradient approximation (GGA), namely BLYP-D3 and revPBE-D3, and two modern meta-GGA functionals, namely strongly constrained and appropriately normed (SCAN) and B97M-rV. The effects of functional-driven and density-driven errors on the interaction energies are first assessed for the water clusters of the BEGDB dataset. Further insights into the nature of functional-driven errors are gained from applying the absolutely localized molecular orbital energy decomposition analysis (ALMO-EDA) to the interaction energies, which demonstrates that functional-driven errors are strongly correlated with the nature of the interactions. We discuss cases where density-corrected DFT (DC-DFT) models display higher accuracy than the original DFT models and cases where reducing the density-driven errors leads to larger deviations from the reference energies due to the presence of large functional-driven errors. Finally, molecular dynamics simulations are performed with data-driven many-body potentials derived from DFT and DC-DFT data to determine the effect that minimizing density-driven errors has on the description of liquid water. Besides rationalizing the performance of widely used DFT models of water, we believe that our findings unveil fundamental relations between the shortcomings of some common DFT approximations and the requirements for accurate descriptions of molecular interactions, which will aid the development of a consistent, DFT-based framework for the development of data-driven and machine-learned potentials for simulations of condensed-phase systems.

Computational study of ground-state properties of μ2 -bridged group 14 porphyrinic sandwich complexes.

The structural properties of μ2 -bridged porphyrinic double-decker complexes are investigated and the influence of various ligands, metals, substituents, and bridging atoms on the dominant structural motif is elucidated. A variety of quantum chemical methods including semiempirical (SQM) methods and density functional theory (DFT) is assessed for the calculation of ecliptic and staggered conformational energies. Local coupled cluster (DLPNO-CCSD(T1)) data are generated for reference. The r2 SCAN-3c composite scheme as well as the B2PLYP-D4/def2-QZVPP approach are identified as reliable methods. Energy decomposition analyses (EDA) and localized molecular orbital analyses (LMO) are used to investigate the bonding situation and the nature of the inter-ligand interaction energy underlining the crucial role of attractive London dispersion interactions. Targeted modification of the bridging atom, e.g., by replacing O2- by S2- is shown to drastically change the major structural features of the investigated complexes. Further, the influence of different substituents of varying size at the phthalocyanine ligand regarding the dominant conformation is described.

Reversible C–H Activation in Zirconaaziridine Species: Characterization and Bonding of a Bridging (Amino)alkylidene Complex Active in Alkyne Hydroaminoalkylation

Efforts to synthesize a low-coordinate zirconaaziridine complex supported by a bis(ureate) ligand resulted in the formation of bridging aminoalkylidene 2. Complex 2 is a rare example of an aminoalkylidene complex featuring a μ(η2:η2)-N,C binding mode. This complex was fully characterized by single-crystal X-ray diffraction, NMR spectroscopy, and LIFDI-MS. Analysis of the natural localized molecular orbitals (NLMOs) showed a σ-bonding interaction between the alkylidene carbon and each zirconium center with significant delocalization of the Zr1–C1 σ bond into Zr2, thereby explaining the significant differences in the Zr–C bond lengths observed in the solid state. A solution-phase equilibrium between complex 2 and the putative zirconaaziridine species was supported by evaluating the reactivity of 2 in alkyne hydroaminoalkylation. The reaction of diphenylacetylene with complex 2 produced a five-membered metallacycle product, a key intermediate in alkyne hydroaminoalkylation. This reaction was greatly accelerated in the presence of pyridine, suggesting that the presence of neutral donors is essential to favor the formation of reactive zirconaaziridine complexes. In addition, the catalytic activity of complex 2 in the benchmark hydroaminoalkylation reaction of diphenylacetylene with N-benzylaniline was established.

Theoretical study on Xe⋯N non-covalent interactions: Three hybridization N with XeO3 and XeOF2

The interactions of complexes of XeOF2 and XeO3 with a series of different hybridization N-containing donors are studied by means of DFT and MP2 calculations. The aerogen bonding interaction energies range from 6.5 kcal/mol to 19.9 kcal/mol between XeO3 or XeOF2 and typical N-containing donors. The sequence of interaction for N-containing hybridization is sp3>sp2>sp, and XeO3 is higher than XeOF2. For some donors of sp2 and sp3 hybridization, the steric effect plays a minor role in the interaction with the evidence of reduced density gradient plots. The dominant stable part is the electrostatic interaction. In complex of XeO3, the weight of polarization is larger than dispersion, while the situation is opposite for XeOF2 complexes. Except for the sum of the maximum value of molecular electrostatic potential on Xe atom and minimum value of molecular electrostatic potential on N atom, the otherfive interaction parameters including the potential energy density at bond critical point, the equilibrium distances, interaction energies with the basis set superposition error correction, localized molecular orbital energy decomposition analysis interaction energies, and the electron charge density, show great linear correlation coefficients with each other.

Development of a Many-Body Force Field for Aqueous Alkali Metal and Halogen Ions: An Energy Decomposition Analysis Guided Approach.

Aqueous solutions of alkyl/alkaline metal and halide ions play a crucial functional role in biological systems such as proteins, membranes, and nucleic acids and for interfacial chemistry in geomedia and in the atmosphere. We present the MB-UCB many-body force field for monovalent and divalent ions that includes polarization, charge penetration to describe the short-range permanent electrostatics accurately, as well as a model for charge transfer to better describe the quantum mechanical potential energy surface and its components obtained from the absolutely localized molecular orbital energy decomposition analysis (ALMO-EDA). We find that the MB-UCB force field is in very good agreement with a validation suite of ion-ion and ion-water cluster data, exhibiting overall better cancellation of errors among energy components, unlike the case for other many-body potentials that do not utilize an EDA scheme. However, limitations in the functional form for the classical many-body energy components do limit the best achievable accuracy through complete cancellation of error and warrant further study.

Insights into the Metal-CO Bond in O2M(η1-CO) (M = Cr, Mo, W, Nd, and U) Complexes.

Investigations on the structures and bonding properties of metal carbonyl compounds provide fundamental understandings on the origin of small-molecule activations. Herein, the geometry and bonding trends of a series of isovalent metal oxocarbonyl complexes O2M(η1-CO) (M = Cr, Mo, W, Nd, and U) were studied by combined matrix-isolation infrared spectroscopy and advanced quantum chemical calculations. The title complexes present red shift of C-O stretching bands in the range from 122 to 244 cm-1, indicating the difference of CO activation ability for the series of isovalent metal dioxides. Density functional theory calculations predict T-shaped structures with a C2v symmetry for all the title molecules. O2Nd(η1-CO) bears little resemblance to the other complexes in bonding characters because of the weak interactions between the NdO2 and CO moiety. For the other complexes, natural localized molecular orbital analysis reveals a gradual increase of covalent character in M-CO bonds along the metal series Cr → Mo → W→ U. Energy decomposition analysis with natural orbitals for chemical valence calculations demonstrates that the M-CO bonding patterns conform to the conventional Dewar-Chatt-Duncanson motif. The contributions from orbital interactions in total attractions increase from Cr (41.7%) to U (52.7%). The breakdown of the orbital term into pairwise interactions shows that contributions of the M ← CO σ donation decrease from Cr (59.2%) to U (28.4%), while the M → CO π* backdonation increases significantly from Cr (23.8%) to U (67.3%). The more effective overlap and the better energy matching of U 5f and U 6d valence orbitals with CO π* orbitals result in much stronger U → CO π backdonation than the other metal elements.

Structural investigations of halogen substituted 1,4-dihydropyridine derivatives: Crystallographic and computational studies

Single crystal structure solution and refinement have been carried out on 1,4-dihydropyridine derivative molecules, namely, 4-(4‑chloro-phenyl)-2,6-dimethyl-1, 4-dihydropyridine-3,5-dicarboxylic acid bis-[(2-chlorophenyl)amide] and 4-(4-fluoro-phenyl)-2,6-dimethyl-1, 4-dihydropyridine-3,5-dicarboxylic acid bis-[(2-chlorophenyl)amide]. Final refinement values, of [I > 2σ(I)] (all data) were refined as 0.0608 (0.0768) and 0.0595 (0.0696) for M-Cl and M-F. Hirshfeld surfaces analysis mapped dnorm surfaces, shape index, curvedness and hydrogen bonding in both the molecules and major relative contributions to the percentage of Hirshfeld surface area for the various intermolecular contacts were calculated. DFT studies using Quantum ESPRESSO were carried out by transformation of the conventional monoclinic unit cell to the primitive unit cell converted back to the original monoclinic cell to compare with the experimental data. The NBO analysis implemented in Gaussian was used to know intermolecular force between two molecular units and to obtain the stabilization energies from donors to acceptors in M-Cl and M-F. The natural localized molecular orbital (NLMO) analysis was also performed to capture the intermolecular delocalization interactions of NBOs.

Antiaromaticity-aromaticity transition of cyclo[16]carbon upon metal encapsulation.

In contrast to aromatic compounds with particular stability, antiaromatic compounds are usually less stable due to their high reactivity and unfavorable formation energies. Cyclo[16]carbon (C16) is a carbon ring molecule with a dual antiaromatic character. In this study, we demonstrate that C16 can be transformed into highly aromatic molecules upon metal encapsulation. The geometrical characteristics, electronic properties and thermodynamic stability of MC16 compounds (M = Ca, Sc, Ti, V, Ce, U) are fully investigated from a theoretical perspective. Based on natural population analysis, atom-in-molecules theory and localized molecular orbital analysis, the nature of the metal-carbon interaction in the MC16 compounds is investigated. It has been proved that the bonding between Ca and C16 corresponds to a typical ionic interaction, while other metal atoms form polar covalent bonds with C16. By analyzing the frontier molecular orbitals and magnetic response of MC16, we have found that all the encapsulated metal atoms donate two electrons to the in-plane π orbitals via either electron transfer or orbital hybridization, which makes the in-plane π orbitals completely satisfy the 4n + 2 (n = 4) Hückel aromaticity rule. The U atom formally transfers four electrons to the carbon ring in total, two to the in-plane π orbitals and two to the out-of-plane π orbitals, which results in the remarkable dual aromaticity feature of UC16. The transformation of aromaticity can be utilized to develop new strategies for the synthesis of novel carbon ring molecules.

Structural, physico-chemical landscapes, ground state and excited state properties in different solvent atmosphere of Avapritinib and its ultrasensitive detection using SERS/GERS on self-assembly formation with graphene quantum dots

Avapritinib is a complex molecule with many heterocyclic rings developed for the treatment of advanced PDGFRA D842V-mutant gastrointestinal stromal tumours. The manuscripts presents the study of the structure, ground state and excited state reactivity and stability indices in different solvent atmosphere using density functional theory in B3LYP/aug-cc-pVDZ level. Different molecular properties like reactivity surface, average local ionization energy, nuclear and electronic electrostatic potential, reduced density gradient assay, and localized molecular orbital analysis are studied in detail. The various intramolecular electron delocalisations were studied using natural orbital formalism and intermolecular interactions analysed by evaluating the extended of non-covalent interactions. Another interesting study is the enhancement in Raman activity of the compound when it forms a self-assembly with graphene quantum dots leading to SERS/GERS, which is studied by incorporating Grimme's dispersion into the previous functional. The electronic spectrum was studied in detail using time dependent density functional theory using long range corrected CAM-B3LYP functionals. The compound showed good light harvesting efficiency also. The local information entropy study provides uncertainty of electrons in spatial distribution.

Evaluation of an Efficient 3D-RISM-SCF Implementation as a Tool for Computational Spectroscopy in Solution.

The 3D-RISM-SCF solvent-model implementation of Gusarov et al. [ J. Phys. Chem. A 2006, 110, 6083-6090] in the Amsterdam density functional program has been improved and extended. In particular, an accurate yet efficient representation of the solute electrostatic potential is provided. The Coulomb-potential fitting of many DFT codes can be used advantageously in this context. The extra effort compared to a point-charge representation is small for a given SCF cycle and compensated by faster SCF convergence. This allows applications to large solutes, as demonstrated by evaluation of the solvatochromism of Reichardt's dye. In general, TDDFT applications to excitation energies in solution stand out and are highlighted. Applications to the 17O NMR chemical shifts of N-methylformamide in different solvents also demonstrate the distinct advantages of 3D-RISM over continuum solvents. Limitations are observed in this case for water solvent, where the solvent shielding is overestimated. This shortcoming applies also to the 17O gas-to-liquid shift of water, where we used localized molecular orbital analyses for a deeper understanding. For such cases of extremely strong solute-solvent interactions, couplings between solute and solvent orbitals induced by the magnetic perturbation are relevant. These clearly require a quantum-mechanical treatment of the most closely bound solvent molecules. Except for such extreme cases, 3D-RISM-SCF is very well suited to treat solvent effects on NMR parameters. More serious limitations pertain to the treatment of vibrational spectra, where the absence of the coupling between solute and solvent vibrational modes limits the accuracy of applications of 3D-RISM-SCF. The reported extended, efficient, and numerically accurate 3D-RISM-SCF implementation should provide a useful tool to study chemical and spectroscopic properties of molecules of appreciable size in a realistic solvent environment.

Probing the Electronic Structure of a Thorium Nitride Complex by Solid-State 15N NMR Spectroscopy.

The solid-state 15N NMR powder spectra of the thorium nitride complex, [K(18-crown-6)(THF)2][(R2N)3Th(μ-15N)Th(NR2)3] ([K][1-15N], R = SiMe3), and the thorium amide complex, [Th(NR2)3(15NH2)] (2-15N), were recorded. The spectrum for [K][1-15N] represents the first reported solid-state 15N NMR data for an actinide complex. The experimentally measured tensor spans are Ω = 847 ppm for [K][1-15N] and Ω = 237 ppm for 2-15N. Both shielding tensors exhibit axial symmetry, which for [K][1-15N] is consistent with a local rotational symmetry of its 15N-labeled nitride ligand. For 2-15N, the axial asymmetry can be rationalized by a quasi-free Th-NH2 bond rotation in the solid-state. Density functional theory calculations overestimate the tensor span somewhat for [K][1-15N], but provide isotropic shifts in good agreement with both the solid-state and solution values for both complexes. Natural localized molecular orbital analyses of the nuclear shielding reveal that the larger tensor span in [K][1-15N] vs 2-15N is primarily a consequence of more pronounced covalency of the σ(N-Th) bonds and large spin-orbit coupling due to significant Th 5f orbital contribution to those bonds, impacting the principal components of the shielding tensor perpendicular to the Th-N-Th axis. Overall, our analysis confirms the involvement of the 5f orbitals in Th-N multiple bonds and further demonstrates the value of solid-state NMR spectroscopy for interrogating actinide-ligand bonding.

Double Chalcogen Bonds: Crystal Engineering Stratagems via Diffraction and Multinuclear Solid-State Magnetic Resonance Spectroscopy.

Group 16 chalcogens potentially provide Lewis-acidic σ-holes, which are able to form attractive supramolecular interactions with electron rich partners through chalcogen bonds. Here, a multifaceted experimental and computational study of a large series of novel chalcogen-bonded cocrystals, prepared using the principles of crystal engineering, is presented. Single-crystal X-ray diffraction studies reveal that dicyanoselenadiazole and dicyanotelluradiazole derivatives work as promising supramolecular synthons with the ability to form double chalcogen bonds with a wide range of electron donors including halides and oxygen- and nitrogen-containing heterocycles. Extensive 77 Se and 125 Te solid-state nuclear magnetic resonance spectroscopic investigations of cocrystals establish correlations between the NMR parameters of selenium and tellurium and the local chalcogen bonding geometry. The relationships between the electronic environment of the chalcogen bond and the 77 Se and 125 Te chemical shift tensors were elucidated through a natural localized molecular orbital density functional theory analysis. This systematic study of chalcogen-bond-based crystal engineering lays the foundations for the preparation of the various multicomponent systems and establishes solid-state NMR protocols to detect these interactions in powdered materials.

Multiple bonds of heavier group 14 compounds: H2CYX2 and HXCYX2 (X = F, Cl, Br and I, Y = Si and Ge)

The heavier group 14 compounds, H2CYX2 and HXCYX2 (X = F, Cl, Br and I, Y = Si and Ge), were investigated by ab initio method. Equilibrium geometries, energies and bonding properties of H2CYX2/HXCYX2 were analyzed. Bader’s atoms-in-molecule (AIM), Pipek-Mezey localized molecular orbital and fuzzy bond orders (FBO) analyses were performed to investigate the bonding interactions in H2CYX2/HXCYX2. The CY bond in H2CYX2/HXCSiX2 shows double bond feature, which is the main reason for that all H2CYX2/HXCYX2 are planar structures or close to planar structures. YX and CX bonds have partial double bond feature as well due to the p-π conjugation effect between X atom and CY π bond. The thermodynamic stability of H2CSiX2 decreases gradually as X goes from F to I, while that of HXCGeX2 has an opposite tendency. The trend of the thermodynamic stability in H2CGeX2 and HXCSiX2 is not pronounced. For H2CYX2 and HXCSiX2, the CY bond is weakened gradually as X goes from F to I, and the CSi bond in H2CSiX2 (or HXCSiX2) is stronger than the CGe bond in its Ge counterpart. In addition, the CY bond in most of HXCYX2 is weaker than that of H2CYX2.

Rapid Identification of Halogen Bonds in Co-Crystalline Powders via 127 I Nuclear Quadrupole Resonance Spectroscopy.

127 I nuclear quadrupole resonance (NQR) spectroscopy is established as a rapid and robust method to indicate the formation of iodine-nitrogen halogen bonds in co-crystalline powders. Once the relevant spectral frequency range has been established, diagnostic 127 I NQR spectra can be acquired in seconds. The method is demonstrated for a series of co-crystals of 1,4-diiodobenzene. Changes in the 127 I quadrupolar coupling constant (CQ ) by up to 74.4 MHz correlate with the length of the C-I donor covalent bond and inversely with the I⋅⋅⋅N halogen-bond length. The predictive power of this technique is validated on two previously unknown co-crystalline powders prepared mechanochemically. Single-crystal growth via co-sublimation and structure determination by single-crystal X-ray diffraction cross-validates the findings. Natural localized molecular-orbital analyses provide insight into the origins of the quadrupolar coupling constants.