Search

Isodesmic reactions, in which chemical bonds are redistributed between substrates and products, provide a general and powerful strategy for both biological and chemical synthesis. However, most isodesmic reactions involve either metathesis or functional‐group transfer. Here, we serendipitously discovered a novel isodesmic reaction of indoles and anilines that proceeds intramolecularly under weakly acidic conditions. In this process, the five‐membered ring of the indole motif is broken and a new indole motif is constructed on the aniline side, accompanied by the formation of a new aniline motif. Mechanistic studies revealed the pivotal role of σ→π* hyperconjugation on the nitrogen atom of the indole motif in driving this unusual isodesmic reaction. Furthermore, we successfully synthesized a diverse series of polycyclic indole derivatives; among quinolines, potential antitumor agents were identified using cellular and in vivo experiments, thereby demonstrating the synthetic utility of the developed methodology. [ABSTRACT FROM AUTHOR]

The possible aromatic conjugation between 3D and 2D aromatic units is in the focus since the synthesis of benzocarborane. It has been showed that in the 3D aromatic icosahedral 1,2‐dicarba‐closo‐dodecaborane systems fused with 2D aromatic rings a global 3D/2D aromaticity does not exist. Despite this fact during the last years several studies proposed interactions between 2D and 3D moieties. Herein, while tuning the size and the effective charge of the (car)borane systems, we demonstrate that global aromatic character can be excluded in any investigated cases, and the detectable conjugative properties can be explained the effect of the well‐known negative hyperconjugation. [ABSTRACT FROM AUTHOR]

Aiming at intramolecular frustrated Lewis pairs (FLPs) based on soft Lewis acidic bismuth centers, a phosphine function was combined with a dichloridobismuthane unit on a phenylene backbone utilizing a scrambling approach. The reaction between two equivalents of BiCl3 and (o‐(Ph2P)C6H4)3Bi yielded (o‐(Ph2P)C6H4)BiCl2(THF), the structure of which indicated Bi...P interactions and thus a pronounced Lewis acidity at the bismuth center that was confirmed by the Gutmann‐Beckett method. However, the system turned out to be insufficient to be utilized for FLP reactivity. Hence, the chloride ligands were exchanged by iodide and C2F5 substituents, respectively. Despite a lower electronegativity the iodide compound exhibits a shorter Bi...P contact, while the C2F5 substituents led to a further decrease of the Lewis acidity, despite their high group electronegativity. DFT calculations rationalized this by a quenching of the Lewis acidity inherent to the σ*(Bi−C) orbital by negative hyperconjugation from occupied p‐orbitals at the F atoms. Furthermore, it turned out that the strength of the covalent Bi−X σ‐bond is a more important factor than the charge at Bi in determining the energetic accessibility and thus Lewis acidity of the antibonding σ*(Bi−C) orbital. [ABSTRACT FROM AUTHOR]

The benzoanellated tricyclic boratirene 2⋅Li(Et2O) was synthesized by reductive dehalogenation of C─borylborirene 1 with Lithium powder, and its distorted structure was determined by X‐ray analysis. The formation of compound 2⋅Li(Et2O) can be described as a result of the insertion of the carbene‐like boron atom into the C─H bond of a methyl group at a duryl ring (duryl ring = 2,3,5,6‐tetramethylphenyl) in the ortho position. This new tricyclic compound contains a highly shielded, formally three‐coordinated boron atom with δ11B = 27 ppm. However, the 11B chemical shift in the range of 65 to 85 ppm is expected for carbon‐bonded three‐coordinated boron atoms. The reason for the high shielding of this boron atom in 2⋅Li(Et2O) is a very strong C─B hyperconjugation. The strained, and negatively charged three‐membered ring in 2⋅Li(Et2O) is an optimal component for a strong hyperconjugative interaction, leading to the formation of a three‐center two‐electron (3c‐2e) σ bond with the empty p orbital of the boron atom, resulting in a strong distortion of the six‐membered ring. [ABSTRACT FROM AUTHOR]

The excited‐state double proton transfer (ESDPT) reaction in the intermolecular hydrogen‐bonded complex 7‐azaindole:piperidin‐2‐one is investigated by quantum chemical calculations with particular emphasis on the H‐bonding interactions within the system. The natural bond orbital (NBO) calculations show that the X─H···Y H‐bonds in the studied complexes, namely, 7‐azaindole:piperidin‐2‐one (N‐form) and 7H‐pyrrolo[2,3‐b]pyridine:3,4,5,6‐tetrahydropyridin‐2‐ol (T‐form, proton transferred tautomer) are aptly described by nY→σ∗(X−H)${n}_{\mathrm{Y}} \to {\sigma }^*({{\mathrm{X}} - {\mathrm{H}}})$ hyperconjugative charge transfer effect (nY${n}_{\mathrm{Y}}$ is non‐bonded lone‐pair on Y‐atom). That the hyperconjugation effect outplays the rehybridization effect is invoked to account for the observed red‐shifting H‐bonds in harmony with the Bent's rule. The differential nature of interactions underlying the origin of X─H···Y H‐bonds in the complexes (N‐form and T‐form) is explored from atoms‐in‐molecules (AIM) calculations. The H‐bonds in the N‐form are found to have a primarily electrostatic origin (closed‐shell interaction) whereas the relatively stronger H‐bonds in the T‐form are assisted by contribution from shared‐shell (covalent) interaction. The nonoccurrence of the double‐proton transfer reaction in the S0 state is argued from the large barrier for the transformation N‐form → T‐form (N‐form constitutes the global minimum on the S0‐PEC), whereas the stability order is reversed together with a marked reduction of the energy barrier in the S1‐PEC accounting for the ESDPT process. [ABSTRACT FROM AUTHOR]

The atomic and electronic structures of two organosilicon polymers [–Ph2Si–(C≡C)2–]n (P1) and [–Ph2Si–(C≡C–C≡C)2–]m (P2) (where Ph is a phenyl group) of acetylene and diacetylene types are studied using the methods of density functional theory and X-ray emission spectroscopy. The SiKβ1 X-ray emission spectra of these polymers are interpreted on the basis of analysis of the distribution of partial electronic states obtained from quantum chemical calculations. The quantitative characteristics of the parameters of the chemical interaction of atoms, such as population, natural charges, and electronic configurations in the studied polymers, are obtained on the basis of analysis of hybrid natural bond orbitals. The values of the polarization coefficients of natural bond orbitals indicate the localization of electron density predominantly on carbon atoms. The electronic configurations for carbon atoms in different fragments differ significantly. For the C atoms of ethynyl (diethynyl) fragments, they are close to the linear σ bond of the sp1.03 (P1) and sp0.95 (P2) type, while for the C atoms of phenyl fragments, they are sp2.42, which are intermediate between sp2 and sp3, in accordance with molecular geometry. The natural charges on Si in both polymers are almost the same: +1.58e, +1.59e (e is the elementary charge), while the natural charges for carbon atoms of the diethynyl group decrease in comparison with the charge for the carbon atom of the ethynyl group from –0.42e to –0.36e. [ABSTRACT FROM AUTHOR]

The reaction mechanism for the formation of the pseudo‐silaazatriene 1 (LN(SiMe3)2−Si−N−Si(L)NSiMe3; L=PhC−(NtBu)2) by the reaction of 1 equivalent of bis‐silylene (LSi−SiL; L=PhC(NtBu)2) with 3 equivalents of azide Me3SiN3 has been explored at M06/def2‐TZVPP//BP86‐D3(BJ)/def2‐TZVPP level of theory. The reaction mechanism is guided by the high Lewis basicity of the silicon lone pair and the high Lewis acidic character of Si−N bonds, as well as by the high tendency to eliminate N2 from Me3SiN3. The first step of the reaction is the formation of [3+1] cycloaddition product (I1) by the synergetic interactions of the lone pair on the silylene silicon with the π* molecular orbital of Me3SiN3 which is majorly localized on the terminal nitrogen atom, and the donation of the σ‐type lone pair on the nitrogen atom connected to the SiMe3 group with the Si−N σ* orbital on silylene. The elimination of N2 from I1 results in the formation of pseudo‐silaimine intermediate I2 having a dicoordinated, monovalent nitrogen atom. The most favourable pathway involves the second addition of Me3SiN3 followed by N2 elimination resulting bis‐silaimine intermediate I4‐P2. The highly reactive lone pair on the dicoordinated, monovalent nitrogen atom can be coordinated to the Si−N σ* MO of the second silylene moiety resulting in Si−N bond formation and Si−Si bond cleavage. This step is similar to the SN2′ reaction where the incoming nucleophile and the leaving group are on the same side resulting in the formation of silaimine I5. Further, the [3+1] cycloaddition of one more molecule of Me3SiN3 followed by N2 elimination results in the formation of silaimine intermediate I7. I7 undergoes 1,3‐silyl migration leading to the final product 1. The lone pairs on di‐ and tricoordinated nitrogen centres of all the intermediates are stabilized by hyperconjugative interactions. The delocalized hyperconjugative interaction stabilizes the lone pairs in 1. [ABSTRACT FROM AUTHOR]

For designing single-molecule devices that have both conjugation systems and structural flexibility, a hyperconjugated molecule with a σ–π bond interaction is considered an ideal candidate. In the investigation of conductance at the single-molecule level, since few hyperconjugation systems have been involved, the strategy of building hyperconjugation systems and the mechanism of electron transport within this system remain unexplored. Based on the skipped-conjugated structure, we present a rational approach to construct a hyperconjugation molecule using a hydroxyl group, which serves as a bridge to interact with the conjugated fragments. The measurement of single-molecule conductance reveals a two-fold conductance enhancement of the hyperconjugation system having the 'bridging' hydroxyl group compared to hydroxyl-free derivatives. Theoretical studies demonstrate that the hydroxyl group in the hyperconjugation system connects the LUMO of the two conjugated fragments and opens a through-space channel for electron transport to enhance the conductance. [ABSTRACT FROM AUTHOR]

The compound 4-cholorophenoxy acetonitrile (CPAN) was examined using Nonlinear optical properties in the current study. With the aid of normal coordinate analysis and the SQMFF methodology, vibrational analysis of FT-IR and FT-Raman has been conducted. Natural bond orbital analysis is used to analyze the electronic stability of the compound created by hyper conjugative contacts and charge delocalization, and the Frontier molecular orbital energy gap confirms the molecule's optical activity. ESP recognizes the nucleophilic and electrophilic areas of molecules, and the chemical implication of molecules was explained using ELF, LOL. Third-order nonlinear optical (NLO) investigations are carried out on a CPAN crystal using the Z-scan technique, and optical properties are established. [ABSTRACT FROM AUTHOR]

The molecular and high-energy density properties of isolated (3,4-dinitropyrazole, 34DNP) and fused pyrazole ring (1H,4H-3,6-dinitropyrazolo[4,3-c]pyrazole, DNPP) systems were studied by density functional theory (DFT) at the B3LYP/G D3BJ level of theory with def2-TZVP basis set. The theoretically optimized geometrical parameters of both molecules are similar to the corresponding experimental geometrical values. With this, atoms in molecules (AIM), electrostatic, intrinsic bond strength index (IBSI), and natural bond orbital (NBO) properties of the 34DNP and DNPP molecules were studied. These studies show that one of the nitro groups in 34DNP is less stable than other nitro groups present in 34DNP and DNPP molecules. The optimized geometry, AIM analysis, and NBO results confirmed that one of the C-NO2 groups in isolated dinitropyrazole 34DNP makes fewer orbital interactions with the pyrazole ring, and deviation in planarity leads to relatively less stability and acetic than the fused dinitropyrazole DNPP molecule. These studies reveal that the effect of ring fusion in dinitropyrazole derivatives may be used to design high-quality high-energy density materials. [ABSTRACT FROM AUTHOR]

In elimination reactions, the arrangement of leaving groups plays a crucial role in facilitating the reaction. On the other hand, equatorial and axial stereoisomers of a compound have a different share of stability. The presence of a solvent plays a different role in the stability of each stereoisomer. LC-ωPBE and B3LYP methods and the Natural Bond Orbital (NBO) interpretation are used to investigate 3-chloro-8-methyl-8-azabicyclo [3.2.1] octane (1), 3-bromo-8-methyl-8-azabicyclo [3.2.1] octane (2), and 8-methyl-8-azabicyclo [3.2.1] octan-3-yl 4-methylbenzenesulfonate (3). The investigation of thermodynamic parameters shows that equatorial stereoisomers are more stable than axial ones, and by changing the gas phase to n-hexane and water, the stability of equatorial stereoisomers is increased. The stability of the equatorial stereoisomers is the result of the electrostatic model involving dipole-dipole interactions and the combined effects of hyperconjugation interactions and the total steric exchange energy (TSEE). Therefore, it can be expressed that the equatorial stereoisomers of compounds 1-3 show the superior performance in the elimination reaction of 1, 4 in the presence of a solvent, and this performance becomes more evident by changing the polarity of the solvent. [ABSTRACT FROM AUTHOR]

The π-π interaction is acknowledged as the predominant factor to determine the molecular packing in organic photovoltaic materials, while other non-covalent intermolecular interactions especially the σ-π hyperconjugation are often ignored. Herein, a perylene diimide (PDI) derivative named FIDT-PDI is designed and synthesized to shed light into the effect of hyperconjugation on the molecular packing and further the photovoltaic performance. Dynamic NMR and 2D NOE NMR demonstrate the formation of intermolecular σ-π hyperconjugation between the C--H bond of the PDI moiety in one molecule and the phenyl sidechain in another molecule of FIDT-PDI. Benefiting from the σ-π hyperconjugation, FIDT-PDI with twisted backbone reversely exhibits more ordered packing and stronger crystallinity compared with another PDI derivative FIDTT-PDI which has better planarity, consequently achieving superior PCE and higher carrier mobility. This contribution is the first paradigm to unravel the structure-property relationship between σ-π hyperconjugation of conjugated materials and corresponding photovoltaic performance. [ABSTRACT FROM AUTHOR]

Electronic structure and proton affinity of Schwesinger bases (Pn bases where n=1–4 indicates the number of phosphorous atoms) were studied using quantum mechanical calculations carried out at M06/def2‐TZVPP//BP86‐D3(BJ)/def2‐TZVPP level of theory. Each phosphorous atom is in the ligating environment of four tricoordinated/dicoordinated nitrogen atoms having one/two lone pairs. While the lone pairs on the tricoordinated/dicoordinated nitrogen atoms act as hyperconjugative donors, the P−N σ* orbitals act as hyperconjugative acceptors. These phosphazene bases are highly stabilized by synergetic hyperconjugative interactions which can accordingly be termed as counterpoise hyperconjugation. The number and the extent of pseudo‐π‐delocalization resulting from hyperconjugative interactions get enhanced as P1 changes to P4. Moreover, additional pseudo‐π‐delocalization starts to appear as P1 changes to P4. The phosphazene P2 has one Ndi(σ)−P−Ndi(σ)−P pseudo‐π‐delocalization which is similar to that of classical π‐delocalization in 1,3‐butadiene. On the other hand, phosphazene P3 has two pseudo‐π‐delocalization similar to that of 1,3‐butadiene and one pseudo‐π‐delocalization similar to that of 1,3‐pentadienyl cation. There are three pseudo‐π‐delocalization similar to that of 1,3‐butadiene and another three pseudo‐π‐delocalization similar to that in 1,3‐pentadienyl cation in P4. Consequently, pseudo‐π‐delocalization is present in all the tetrahedral symmetry planes of P4. Hence, P4 can be considered as a three‐dimensional pseudo‐π delocalized system. Even though the lone pair orbitals are stabilized by hyperconjugative interactions, they are susceptible to protonation. Among the dicoordinated and tricoordinated nitrogen atoms, the proton affinity of the dicoordinated nitrogen atom connected to the tBu group is much higher (257.4 kcal/mol–291.6 kcal/mol), and the proton affinity values increase on going from P1 to P4. The very high proton affinity of this dicoordinated nitrogen atom indicates its superbasic character. [ABSTRACT FROM AUTHOR]

A possibility of manifestation of the anomeric effect of the sp-hybridized carbon atom, which is a component of triple C≡C and C≡N bonds, was studied. The effect of carbon hybridization on the acceptor capacity of the carbon—carbon σ-bonds was evaluated. A high effective electronegativity of the sp-hybridized carbon atom was found to exert a noticeable effect on the magnitude of the anomeric interactions n N → σ ∗ CC . The triple bond in such systems indeed can participate in anomeric interactions, which was proved for N-propargyl substituted bispidines as an example using experimental (XRD) and computational methods. This participation is expressed in the stabilization of the bisectral conformation of the N—CH2—C≡CH and N—CH2—C≡N fragments in the case of N-monosubstituted and N,N′-disubstituted bispidines. The values of charge transfer from an unshared pair of nitrogen atoms to the σ-antibonding orbital of the C≡C—CH2 bond calculated for the model Me2N—CH2—X systems by the NBO method are up to 11.0 kcal mol−1, which is relatively low for the classical anomeric effect: the corresponding value for the N≡C—CH2 fragment is 11.3 kcal mol−1. The anomeric effect manifested by the acetylene group makes it possible to stabilize certain conformations in the bispidine systems containing propargyl substituents, which can affect the reactivity of acetylenes. For the N-benzyl systems (9.4 kcal mol−1) and N-allyl (9.4 kcal mol−1) and N-propyl (10.0 kcal mol−1) derivatives, the hyperconjugation stabilization is weaker and does not play a decisive role in conformational equilibrium. The obtained data are of obvious interest for specialists working in the field of the chemistry of the triple bond of the propargyl type. [ABSTRACT FROM AUTHOR]

The conformations of alkanes and their stability are important topics in basic organic chemistry. Among them, the staggered and eclipsed conformations of ethane are the most fundamental. Current undergraduate organic chemistry textbooks both domestically and internationally provide different explanations for the stability of the staggered conformation of ethane compared to the eclipsed conformation, leading to confusion and making it difficult for undergraduate teaching. In this study, we conducted a survey of literatures, organized different arguments and evidence for the stability of the staggered conformation of ethane, and pointed out the shortcomings of analysis and calculation methods in previous literatures. Through literature research, we believe that both hyperconjugation and steric hindrance should be included in the textbooks. [ABSTRACT FROM AUTHOR]

The structures and energies of various common carbocations, allylic carbocations, and particularly stable α-amino and α-hydroxyl carbocations were calculated using DFT (density functional theory). The results showed that common carbocations are stabilized by the formation of supercovalent structures, rearrangement, and superconjugation effects, whereas allylic carbocations are stabilized by conjugate effects. The stabilities of α-hydroxyl and α-amino carbocations are particularly high and are equivalent to or higher than that of cyclopropylene carbocation. Finally, different methods of carbocation stabilization using amino and hydroxyl groups are suggested. [ABSTRACT FROM AUTHOR]

Multifunctional organoboron compounds increasingly enable the simple generation of complex, Csp3‐rich small molecules. The ability of boron‐containing functional groups to modify the reactivity of α‐radicals has also enabled a myriad of chemical reactions. Boronic esters with vacant p‐orbitals have a significant stabilizing effect on α‐radicals due to delocalization of spin density into the empty orbital. The effect of coordinatively saturated derivatives, such as N‐methyliminodiacetic acid (MIDA) boronates and counterparts, remains less clear. Herein, we demonstrate that coordinatively saturated MIDA and TIDA boronates stabilize secondary alkyl α‐radicals via σB‐N hyperconjugation in a manner that allows site‐selective C−H bromination. DFT calculated radical stabilization energies and spin density maps as well as LED NMR kinetic analysis of photochemical bromination rates of different boronic esters further these findings. This work clarifies that the α‐radical stabilizing effect of boronic esters does not only proceed via delocalization of radical character into vacant boron p‐orbitals, but that hyperconjugation of tetrahedral boron‐containing functional groups and their ligand electron delocalizing ability also play a critical role. These findings establish boron ligands as a useful dial for tuning reactivity at the α‐carbon. [ABSTRACT FROM AUTHOR]

This paper presents a learning opportunity for the experimental investigation of radical photobromination reactions. The experiments are staged as case comparisons of constitutionally isomeric reactants and, in the form presented, allow the development of the inductive effect or the concept of hyperconjugation, which belong to the core concepts of organic chemistry. With different observation methods (disappearance of the bromine colour in the reaction solutions as well as detection of pH changes, changes in the electrical conductance and bromide detection reactions in test solutions), these reactions are suitable in the interweaving of experiment and theory to investigate fundamental mechanistic aspects of radical substitution reactions. These include, in particular, energetics as well as regioselectivity. In addition, the case comparisons are designed to teach learners to distinguish between relevant and irrelevant external structural features. [ABSTRACT FROM AUTHOR]

The conformational space of diallylamine (DAA) was investigated using rotational spectroscopy from 7 to 19 GHz aided by quantum chemical calculations. Extensive conformational searches using density functional theory B3LYP-D3(BJ) and the ab initio MP2 method with the aug-cc-pVTZ basis set identified a total of 42 minima for DAA within ∼22 kJ mol−1. This reveals a strikingly rich conformational landscape for this secondary amine with two equivalent substituents. Experimentally, transitions belonging to four low energy conformers (I, II, III, and IV) were unequivocally assigned in the rotational spectrum, and their patterns were confirmed by the presence of the hyperfine structure owing to the 14N quadrupolar nucleus. The relative intensities of the observed transitions suggest a conformational energy ordering of I < II < III < IV. Natural bond orbital and non-covalent interaction calculations reveal that the geometric preferences for the observed conformers are governed by an interplay of subtle attractive interactions (including hyperconjugation involving the lone pair at nitrogen) and repulsive effects. [ABSTRACT FROM AUTHOR]

For designing single-molecule devices that have both conjugation systems and structural flexibility, a hyperconjugated molecule with a σ–π bond interaction is considered an ideal candidate. In the investigation of conductance at the single-molecule level, since few hyperconjugation systems have been involved, the strategy of building hyperconjugation systems and the mechanism of electron transport within this system remain unexplored. Based on the skipped-conjugated structure, we present a rational approach to construct a hyperconjugation molecule using a hydroxyl group, which serves as a bridge to interact with the conjugated fragments. The measurement of single-molecule conductance reveals a two-fold conductance enhancement of the hyperconjugation system having the ‘bridging’ hydroxyl group compared to hydroxyl-free derivatives. Theoretical studies demonstrate that the hydroxyl group in the hyperconjugation system connects the LUMO of the two conjugated fragments and opens a through-space channel for electron transport to enhance the conductance.

Four methods of computational quantum chemistry are used in a study of hyperconjugation in protonated aromatic molecules. Benzene, benzenium, toluene, and four isomeric forms of toluenium are examined using the self-consistent field level of theory followed by configuration interaction and coupled cluster calculations, as well as density functional theory. Results for proton affinities, geometric parameters, atomic populations, dipole moments, and polarizabilities are reported. The calculated results are in good agreement with previous computational studies and with experimental data. The presence of hyperconjugation is evident from the shortened carbon–carbon bond lengths in the aromatic ring and concomitant changes in dipole moments and polarizabilities. The proton affinities of benzene and toluene compare well with experimental values. The examination of all of the toluenium isomers reveals that the position of the methyl group has a minor impact on the strength of hyperconjugation, although the most stable isomer is found to be the para form. Mulliken population analyses indicate that the addition of a proton contributes to aromatic hyperconjugation and increases the strength of π-bonds at the expense of σ-bonds.

A systematic study on the distortion of a naphthalene ring was performed using steric repulsion between peri-substituents at the 1- and 8-positions. The introduction of bromo groups into the methyl groups of the 1,8-dimethylnaphthalene enhanced the steric repulsion to distort the naphthalene ring. X-ray crystallography revealed that 1,8-bis(bromomethyl)naphthalene had a vertical distortion with a 11.0° dihedral angle (α) between peri-substituents which disturbed the coplanarity of the naphthalene ring. On the other hand, the dihedral angle of 1,8-bis(dibromomethyl)naphthalene was smaller (α = 8.3°) despite the bulkier substituents. In this case, horizontal distortion of the naphthalene ring increased. These distortions should non-electronically activate the naphthalene framework. In order to evaluate their reactivity, nitration and hydrogenation were carried out; however, the 1,8-bis(dibromomethyl)naphthalene was intact under the employed conditions. A DFT calculation suggested that the inertness of the 1,8-bis(dibromomethyl)naphthalene is presumably due to the negative hyperconjugation of the (dibromo)methyl group. [ABSTRACT FROM AUTHOR]

Allenes (>C=C=C<) are classified as cumulated dienes with a linear structure and an sp‐hybridized central carbon atom. We have synthesized and isolated a stable 2‐germapropadiene with bulky silyl substituents. The 2‐germapropadiene allene moiety adopts a linear structure both in the solid state and in solution. An X‐ray diffraction electron‐density‐distribution (EDD) analysis of this 2‐germapropadiene confirmed the linear C=Ge=C geometry with a formally sp‐hybridized germanium atom that bears two orthogonal C=Ge π‐bonds. Based on detailed structural and computational studies, we concluded that the linear geometry of the isolated 2‐germapropadiene most likely arises from the negative hyperconjugation of the silyl substituents at the terminal carbon atoms. The 2‐germapropadiene reacts rapidly with nucleophiles, indicating that the linearly oriented germanium atom is highly electrophilic. [ABSTRACT FROM AUTHOR]

In this paper, the anionic hyperconjugation occurring in alkyl and fluorinated carbanions in the gaseous phase is examined at density functional theory (DFT) at the level of theory B3LYP/6‐31++G (d,p) with empirical dispersion‐GD3 theory. First, a contribution of this study is to confirm that the C‐H bond in (CH3)3C‐H (isobutane) is more acidic than the C‐H bonds in CH3CH2‐H (ethane) and (CH3)2CH‐H (propane) based on a comparison of conjugate base stability. Also, the natural population analysis (NPA) and CHELPG methods are well‐performed for calculating charge distribution on carbon atoms, and in contrast, the Mulliken population is better for hydrogen atoms to describe the anionic hyperconjugation in the valence bond (VB) model. The molecular orbital model accounts for the stability order of alkyl and perfluorinated carbanions through the number, strength, and overlap geometry between the lone electron pair on carbon atom and the vicinal σ*C‐H/σ*C‐F orbitals. On the other hand, the utility of VB theory becomes invalid because of the strong inductive effect of the fluorine element and the steric repulsion among CF3 groups in perfluorinated carbanions. Also, the hybridization of the carbon atom holding the lone electron pair can become more s‐character or p‐character dependent on the size of surrounding groups. [ABSTRACT FROM AUTHOR]

That nucleophiles preferentially attack at the less sterically hindered carbon of epoxides under neutral and basic conditions has been generally accepted as a fundamental rule for predicting the regioselectivity of this type of reaction. However, this rule does not hold for perfluorinated epoxides, such as hexafluoropropylene oxide (HFPO), in which nucleophiles were found to attack at the more hindered CF3 substituted β-C rather than the fluorine substituted α-C. In this contribution, we aim to shed light on the nature of this intriguing regioselectivity by density functional theory methods. Our calculations well reproduced the observed abnormal regioselectivities and revealed that the unusual regiochemical preference for the sterically hindered β-C of HFPO mainly arises from the lower destabilizing distortion energy needed to reach the corresponding ring-opening transition state. The higher distortion energy required for the attack of the less sterically hindered α-C results from a significant strengthening of the C(α)-O bond by the negative hyperconjugation between the lone pair of epoxide O atom and the antibonding C-F orbital. [ABSTRACT FROM AUTHOR]

Fluorine gauche effects are conformational properties of 2‐fluoroethanes often applied in modern molecular designs. However, the physical origins of fluorine gauche effects are not well understood, with the consensus favoring the established hyperconjugation theory over an emerging electrostatic model. Using a series of model systems, we show that a shift to fluorine gauche effects can be influenced by intramolecular CH⋅⋅⋅π aromatic interactions, a through‐space event. Modulating the π‐ring (forming the aromatic interaction) with substituent groups resulted in a linear Hammett relationship, thus indicating that the CH⋅⋅⋅π interaction has electrostatic features. For instance, attaching a nitro group (an electron‐withdrawing substituent) to the π‐ring weakened the CH⋅⋅⋅π interaction and led to a gauche preference, whereas an anti conformer is preferred with amine as substituent. The experimental results performed by using proton NMR spectroscopy are corroborated by gas‐phase DFT calculations and solid‐state X‐ray crystallography. [ABSTRACT FROM AUTHOR]

Interheteromolecular hyperconjugation is ubiquitous in organic systems, affecting bond length, dipole moments, conformations and so on, while its effect on (de)hydrogenation reactivity in a heterogeneous thermo‐catalytic system has rarely been explored. Herein, the N‐heterocycles containing a benzene ring and aliphatic chain [N‐ethylcarbazole (NEC) and N‐propylcarbazole (NPC)] were utilized to study the correlation between interheteromolecular hyperconjugation and catalytic (de)hydrogenation. Density functional theory calculations, variable‐temperature 1H nuclear magnetic resonance spectroscopy, and catalytic experiments showed that the presented hyperconjugation between NEC and NPC weakened the electron cloud density of aromatic rings and thus facilitated the reactivity with hydrogen featuring unpaired electrons. Therefore, an extremely low temperature of 80 °C was enough for the hydrogenation. Moreover, this interheteromolecular hyperconjugation was general in other N‐heterocycles (e. g. N‐methyindole and NPC) and was also effective to (de)deuterate as revealed by isotope experiments. This work expands the application of interheteromolecular hyperconjugation to heterogeneous thermocatalysis for reversible H2 storage. [ABSTRACT FROM AUTHOR]

Most hydrazone compounds prefer the azine tautomeric states. However, oxygen-/sulfur-substituted compounds prefer hydrazone tautomers. In this study, density functional theory at M062X level with the basis set of 6–311 + + g(2d, 2p), with MP2/cc-pVTZ for reference, was used to investigate the different tautomeric mechanisms between hydrazone and azine forms with oxygen, sulfur, carbon, and nitrogen as negative centers. The energetic stabilities are in the order as oxygen- < sulfur- < imine- < amidino- < carbene-substituted hydrazones with respect to their azine tautomeric structures. Resonance of the molecular structures might be the geometrical basis for their energy stabilities and were estimated based on HOMED indices. Further, the increased proton affinities in the trend as hydroxyl < sulfhydryl < imine terminal groups account for their increasing azine preferences. Proton release was examined for the reversible equilibrium from azine to hydrazone by calculating the transition energy barrier of H transfer. It is favorable to form hydrazone tautomers for oxygen and sulfur containing groups, while it is less favorable for amino-substituted ones. Azine form is the most stable tautomer for methyl substituted. [ABSTRACT FROM AUTHOR]

The organic cation in the title molecular salt, C5H13NCl+·Cl−, exhibits the gauche effect with a C—H bond of the C atom β to the chloro group donating electrons to the antibonding orbital of the C—Cl bond to stabilize the gauche conformation [Cl—C—C—C = −68.6 (6)°], as confirmed by DFT geometry optimizations that show a lengthening of the C—Cl bond relative to that of the anti conformation. Of further interest is the higher point group symmetry of the crystal (\overline{4}), compared that of the that of the molecular cation, which arises from a supramolecular head-to-tail square arrangement of four molecular cations that circulate in a counterclockwise direction when viewed down the tetragonal c axis.

The first regioselective addition reactions to 3-trifluoromethylbenzyne are reported. Triple cycloaddition of ynolates to the benzyne provided 1,8,13-tris(trifluoromethyl)triptycenes with high regioselectivity. 1-Trifluoromethyltriptycenes were regioselectively obtained by the Diels–Alder reaction of anthranoxides with the benzyne. These selectivities are attributed to the electron-acceptor nature of the trifluoromethyl group on the benzyne. [ABSTRACT FROM AUTHOR]

The 1H NMR-based estimation of antiaromaticity in anionic molecules is challenging because of the difficulty in separately evaluating NMR shielding effects due to paratropic ring currents and negative charges. Herein we propose a novel approach for the 7Li NMR-based evaluation of antiaromaticity enabled by inorganic salt-assisted clusterization, which is serendipitously found during our studies on hyperconjugative antiaromaticity. Reduction of a dibenzo[b,f]silepin (1) with lithium afforded the dilithium salt [(Li+)2(thf)5][12−] (2), which is expected to have antiaromatic character with a pseudo-16π-electron system involving hyperconjugation between the anionic π-clouds and the σ*(Si–Me) orbitals, although the evaluation of its antiaromaticity by NMR was difficult. Compound 2 reacted with 0.2 equivalents of O2 gas to form a trimeric cluster [(Li+)(solv.)n][(12−)3(Li9O25+)] (3), which can be understood as a supramolecular complex composed of three molecules of [Li+]2[12−] and two Li2O salts. X-ray diffraction analysis revealed that the [Li9O2]5+ core is surrounded by three dibenzosilepinyl dianions (12−), with multiple Li–π coordinations. The trimeric structure is maintained in a toluene solution according to the 1H and 7Li{1H} NMR spectra, and of particular interest are the significant downfield shifts of 7Li{1H} NMR signals of the Li9O2 core (δ(7Li) = 6.3, 4.4). These explicit downfield shifts are reasonably explained by the paratropic ring currents of the dianionic dibenzosilepin rings, which was supported by theoretical studies. [ABSTRACT FROM AUTHOR]

Acetal substitution reactions of α‐halogenated five‐ and six‐membered rings can be highly stereoselective. Erosion of stereoselectivity occurs as nucleophilicity increases, which is consistent with additions to a halogen‐stabilized oxocarbenium ion, not a three‐membered‐ring halonium ion. Computational investigations confirmed that the open‐form oxocarbenium ions are the reactive intermediates involved. Kinetic studies suggest that hyperconjugative effects and through‐space electrostatic interactions can both contribute to the stabilization of halogen‐substituted oxocarbenium ions. [ABSTRACT FROM AUTHOR]

First‐order rate constants for solvolyses of variously substituted benzhydryl diphenylphosphinates in four aqueous solvents have been measured conductometrically in order to investigate leaving group ability of diphenylphosphinate (DPP), that is, to determine corresponding nucleofuge‐specific parameters (Nf and sf) according to the correlation equation log k (25°C) = sf(Ef + Nf). DPP is the first of phosphorus leaving groups included on the Nf nucleofugality scale which presently covers a reactivity range of 15 orders of magnitude. Application of log k (25°C) versus Ef plots has unequivocally shown that benzhydryl diphenylphosphinates solvolyze in hydroxylic solvents by the SN1 mechanism. As the abovementioned equation and corresponding reactivity parameters can also be employed for predicting heterolytic reactivity of substrates in various solvents, the range of kinetic stability of secondary and tertiary benzylic diphenylphosphinates toward heterolysis has been examined in terms of reaction half‐lives. Further, in order to investigate the origin of stability of the DPP anion and, consequently, the ability of the DPP leaving group, quantum mechanical calculations and the Natural Bond Orbital (NBO) analysis were performed at the M06‐2X/6‐311+G(3df,3pd) level of theory using the SMD solvation model. The NBO analysis has revealed that negative hyperconjugation plays a dominant role in delocalizing the anionic charge in disubstituted phosphinate leaving groups. [ABSTRACT FROM AUTHOR]

Recently, fully π-functional two-dimensional (2D) materials have been reported for electronic device applications. Graphene is one of these 2D materials that is attributed to 2D electron confinement effects and exhibits an aromatic character; however, it is characterized by vanishing the bandgap energy. Hence, research was focused on the discovery of graphene-based 2D materials to reduce the bandgap energy. Herein, we investigate the silagraphene structures (SixCy) using DFT calculations to undertake and improve structural, physico-chemical, and electronic properties. Various types of 2D networks have been investigated by considering C–C and C–Si bonds in relative positions. Both conjugation and hyperconjugation phenomenon have been deeply examined and it seemed that they take advantage of each other depending on the C–C and C–Si bond positions. Localized orbital locator (LOL) and electron localization function (ELF) were also performed to examine the electronic densities in the investigated 2D networks and unveil the electronic properties of the studied materials. [ABSTRACT FROM AUTHOR]

The conformational landscapes of four 1-O-methylated monosaccharides-methyl a-glucose, methyl b-glucose, methyl a-galactose, and methyl b-galactose-were characterized using jet-cooled broadband rotational spectroscopy, supported by density functional theory calculations. A newly designed, simple pulsed nozzle assembly was used to introduced the sugar samples into a jet expansion without thermal degradation, eliminating the need for a complex and expensive laser ablation system. Ten conformers were experimentally identified by assigning their rotational spectra, and the intricate methyl internal rotation splittings were analysed. Notably, methylation alters the directionality of intramolecular hydrogen bonding of a-galactose highlighting its impact on structural preference. Natural bond orbital, intrinsic bond strength, and non-covalent interaction analyses were conducted to explore the interplay between hydrogen bonding and hyperconjugation. A set of σ to σ* neutral hyperconjugative interactions were found to override a strong hydrogen bond, driving a preference for the gauche conformers., (© 2024 Wiley‐VCH GmbH.)

A systematic study on the distortion of a naphthalene ring was performed using steric repulsion between peri-substituents at the 1- and 8-positions. The introduction of bromo groups into the methyl groups of the 1,8-dimethylnaphthalene enhanced the steric repulsion to distort the naphthalene ring. X-ray crystallography revealed that 1,8-bis(bromomethyl)naphthalene had a vertical distortion with a 11.0° dihedral angle (α) between peri-substituents which disturbed the coplanarity of the naphthalene ring. On the other hand, the dihedral angle of 1,8-bis(dibromomethyl)naphthalene was smaller (α = 8.3°) despite the bulkier substituents. In this case, horizontal distortion of the naphthalene ring increased. These distortions should non-electronically activate the naphthalene framework. In order to evaluate their reactivity, nitration and hydrogenation were carried out; however, the 1,8-bis(dibromomethyl)naphthalene was intact under the employed conditions. A DFT calculation suggested that the inertness of the 1,8-bis(dibromomethyl)naphthalene is presumably due to the negative hyperconjugation of the (dibromo)methyl group.

The antiviral active molecule N–(4–chlorophenyl)–2–[(4,6–di–aminopyrimidin–2–yl)sulfanyl] acetamide has been characterized to obtain vibrational signatures via Raman and Fourier transform infrared spectroscopy, comparing the results generated by ab initio calculations. The density functional theory model, performed by GAUSSIAN 09 packages, based on the Becke, 3-parameter, Lee–Yang–Parr exchange correlation functions augmented with 6–311++(d,p) basis set. The geometric equilibrium, inter and intra-molecular hydrogen bond, and harmonic vibrational wavenumbers of N–(4–chlorophenyl)–2–[(4,6–di–aminopyrimidin–2–yl)sulfanyl]acetamide were explored with density functional theory. The stereo-electronic interactions, leading to stability were confirmed using natural bond orbital analysis, which has been further substantiated by vibrational spectral analysis. Hirshfeld surface gives an insight into intermolecular contacts within the Crystal structure. The optimized geometry shows a non-planar structure between the phenyl ring and the pyrimidine ring. Differences in the geometries due to the substitution of the electronegative chlorine atom, intramolecuar and intermolecular contacts due to the amino pyrimidine were analyzed. NBO analysis reveals the formation of two strong stable hydrogen bonded N–H···N intermolecular interactions and weak intramolecular interactions C–H···O and N–H···O. The Hirshfeld surfaces and consequently the 2D-fingerprint confirm the nature of intermolecular interactions and their quantitative contributions toward the crystal packing. The red shift in N–H stretching frequency exposed from IR substantiate the formation of N–H···N intermolecular hydrogen bond. The pharmacokinetic properties were investigated from adsorption, distribution, metabolism, excretion and toxicity results. In-silico docking shows the inhibition activity against virus. [ABSTRACT FROM AUTHOR]

The explanation of the anomeric effect in terms of underlying quantum properties is still controversial almost 70 years after its introduction. Here, we use a method called Relative Energy Gradient (REG), which is able to compute chemical insight with a view to explaining the anomeric effect. REG operates on atomic energy contributions generated by the quantum topological energy decomposition Interacting Quantum Atoms (IQA). Based on the case studies of dimethoxymethane and 2-fluorotetrahydropyran, we show that the anomeric effect is electrostatic in nature rather than governed by hyperconjugation. [ABSTRACT FROM AUTHOR]

The successful use of 1,3,4,2-triazaphospholenes (TAPs) as organo-catalysts stresses the need for efficient synthetic routes to these molecules. In this study, we establish the [1 + 4]-cycloaddition of PBr3 to azo-pyridines as a new approach to preparing pyrido-annellated TAPs in a single step from easily available precursors. The modular assembly of the azo-component via condensation of primary amines and nitroso compounds along with the feasibility of post-functionalization at the P–Br bond under conservation of the heterocyclic structure allows, in principle, to address a wide range of target molecules, which is illustrated by prototypical examples. The successful synthesis of a transition metal complex confirms for the first time the ability of a TAP to act as a P-donor ligand. Crystallographic studies suggest that hyperconjugation effects and intermolecular interactions induce a qualitatively similar ionic polarization of the P–Br bonds in TAPs as in better known isoelectronic diazaphospholenes. [ABSTRACT FROM AUTHOR]

The report on phosphaphthalimide (1), the phosphorus analogue of the phthalimide anion, dates back to forty years ago. However, the presence of π-delocalization between two-coordinated phosphorus centre and neighbouring carbonyl groups in 1 has been underestimated. Herein, sodium salts of 1 were obtained through a convenient procedure on a relatively large scale with a modified procedure. Reactivity studies demonstrated that 1 is indeed a good electrophile and the essential role of π-delocalization in 1 controlling its ambident properties. NBO analysis revealed the p–π conjugation and p–σ* hyperconjugation in 1 affecting its bond lengths in opposite ways. [ABSTRACT FROM AUTHOR]

This study describes the synthesis, theoretical investigations, and photocatalytic degradational properties of a new (pyrazine)(meso-tetrakis(4-tert-methoxyphenyl)-porphyrinato)-cadmium (II) ([Cd(TMPP)-Pyz]) complex (1). The new penta-coordinated CdII porphyrin complex (1) was characterized by various spectroscopic techniques, including FT-IR, NMR, UV-visible absorption, fluorescence emission, and singlet oxygen, while its molecular structure was studied using single crystal X-ray diffraction. The UV–Vis spectroscopic study highlighted the redshift of the absorption bands after the insertion of the Cd(II) metal ion into the TMPP ring. The co-coordination of the pyrazine axial ligand enhanced this effect. A fluorescence emission spectroscopic study showed a significant blueshift in the Q bands, accompanied by a decrease in the fluorescence emission intensity and quantum yields of Φf = 0.084, Φf = 0.06 and Φf = 0.03 for H2-TMPP free-base porphyrin, [Cd(TMPP)] and [Cd(TMPP)(Pyz)] (1) respectively. Singlet oxygen revealed that the H2-TMPP porphyrin produced the most efficient singlet oxygen quantum yield of (ΦΔ = 0.73) compared to [CdTMPP] (ΦΔ = 0.57) and [Cd(TMPP)(Pyz)] (1) (ΦΔ = 0.13). In the crystal lattice, the [Cd(TMPP)Pyz] was stabilized through non-covalent intermolecular interactions (NCI), such as the hydrogen bonds C-H···N and C-H···Cg. Additionally, crystal explorer software was then utilized to measure the quantitative analysis of the intermolecular interactions in the unit cell of the crystal structure and established that the C-H···π interaction dominated. The Natural bond orbital (NBO) analysis revealed that each molecule is stabilized by hyperconjugation and charge delocalization. As a photocatalyst, the coordination complex 1 showed excellent photocatalytic activity toward the degradation of Levafix Blue CA reactive dye (i.e., dye photo-degradation of 80%). [ABSTRACT FROM AUTHOR]

This study describes the synthesis of imidazolidines, hexahydropyrimidines, and 1,5-diazabicyclo[3.2.1]octane. These compounds are used as models to study the hyperconjugation using nuclear magnetic resonance (NMR) spectroscopy through the 13C–1H one-bond coupling constants (1 J C,H) and the 1H chemical shifts. The molecular structures are corroborated by single-crystal X-ray diffraction, and the electron distribution is studied through electron mapping obtained by using Hirshfeld surface analysis to determine intermolecular and intramolecular interactions. The NMR data show that the 1 J C,H ax are lower than 1 J C,H eq as a consequence of the n N →σ* C – H interaction, while the 1 J C,H ax are very similar to 1 J C,H eq in the β-carbon to the nitrogen. This is mainly due to the stabilization energy (E int 18–19.2 kJ/mol) by the hyperconjugative σ Cβ–H eq →σ* C – N interactions. This effect is marked in the chemical shift of 1H, where H eq shifts to a lower frequency than H ax. Finally, the Hirshfeld surface analysis shows that the atoms involved in hyperconjugation play an important role in the intermolecular and intramolecular interactions. The obtained results can support new study models that would afford an accurate determination of interactions in molecular recognition and predict interactions between drugs and proteins, which in turn will help the design and development of new drugs. [Display omitted] [ABSTRACT FROM AUTHOR]

The numbers in Figure 2B in this Research Article are labelled incorrectly.The Δα and Δβ labels in Figure 2B in the original manuscript are swapped relative to the correct labels/values provided in Supporting Information Table S18. In addition, the "+0.5" entry in Figure 2B should be "+0.1" (the value is correct in Table S18). These numbers are not discussed in the main text, so there are no changes to the discussion or the conclusion. A corrected version of Figure 2 is provided.GRAPH [Extracted from the article]

The chalcogen bonding (ChB) ability of Te is studied in symmetrical diaryl ditellurides ArTeTeAr. Among the two Te σ‐holes, the one along the less polarized Te−Te bond was calculated as the more electropositive. This counter‐intuitive situation is due to the hyperconjugation contribution from Te lone pair to the σ* of the adjacent Te which coincides with σ‐hole along the more polarized Te−Ar bond. ArTeTeAr showed notable structural features in the solid state as a result of intermolecular Te⋅⋅⋅Te ChB, such as a Te4 rectangle through dimer aggregation or a triangular Te3 motif, where one Te interacts with both Te atoms of a neighboring molecule through both its σ‐hole and lone pair, in a slightly frustrated geometry. Lewis acidity of ArTeTeAr was also evaluated by NMR with R3PO as σ‐hole acceptors in different solvents. Thus, 125Te NMR allowed monitoring Te⋅⋅⋅O interaction and delivering association constants (Ka) for 1 : 1 adducts. The highest value of Ka=90 M−1 was measured for the adduct between ArTeTeAr bearing CF3 groups and Et3PO in cyclohexane. Notably, by using nBu3PO, Te⋅⋅⋅O interaction was revealed by 19F‐1H HOESY showing spatial proximity between CF3 and CH3 of nBu3PO. [ABSTRACT FROM AUTHOR]

Phenotype-guided transposon mutagenesis has emerged as a valuable tool to access cryptic metabolites encoded in bacterial genomes. Recently, the method was demonstrated by inducing silent biosynthetic gene clusters in Burkholderia thailandensis. Amongst the isolated metabolic products, thailandene A exhibited promising antibiotic activity. By assignment, the linear polyenic aldehyde contained a labile motif, where an ostensible chiral secondary alcohol was interlaced in an allylic and a homoallylic constellation. Our attention was drawn to the pseudo-symmetric relationship between the heterofunctionalities, indicating the transformation of a dodecapentaenedial scaffold. Centering on an iterative cross-coupling protocol, the assigned all- E -(12 R)-hydroxydodecapentaenal moiety was assembled by combining Zincke chemistry with the MIDA-attenuated Suzuki reaction developed in the Burke laboratory. Thus, according to the devised strategy, the mixed 1,2-bisborylated vinyl linchpin was consecutively functionalized with 5-bromodienal derivatives in a doubly orthogonal fashion. However, when the synthetic material was matched against the bacterial isolate, inconsistencies were observed. A re-examination of the cryptic natural product was conducted by juxtaposing analytical data from experiment and density functional theory calculations, in which hydroperoxide was evaluated as a candidate metabolite present in the bacterial isolate. [ABSTRACT FROM AUTHOR]

The semipinacol rearrangement is one of the classic yet useful synthetic tools in organic synthesis. However, semipinacol rearrangements involving heteroatom-migration are rare. Reported herein is a boryl-migratory semipinacol rearrangement of α-hydroxyallylboronates and α-hydroxypropargylboronates triggered by diverse halogen-, oxygen-, sulfur- and selenium-containing electrophiles. The protocol leads to a mild and facile access to organoborons bearing valuable functionalities. The σ (C—B) hyperconjugation is believed to be the key factor that leads to the observed exclusive chemoselectivity and enhanced reactivity. Synthetic utilities of the formed products were demonstrated. [ABSTRACT FROM AUTHOR]

High level DLPNO–CCSD(T) electronic structure calculations with extended basis sets over B3LYP–D3 optimized geometries indicate that the three methyl groups in caffeine overcome steric hindrance to adopt uncommon conformations, each one placing a C–H bond on the same plane of the aromatic system, leading to the C–H bonds eclipsing one carbonyl group, one heavily delocalized C–N bond constituent of the fused double ring aromatic system, and one C–H bond from the imidazole ring. Deletion of indiscriminate and selective non-Lewis orbitals unequivocally show that hyperconjugation in the form of a bidirectional –CH 3   ⇆ aromatic system charge transfer is responsible for these puzzling conformations. The structural preferences in caffeine are exclusively determined by orbital interactions, ruling out electrostatics, induction, bond critical points, and density redistribution because the steric effect, the allylic effect, the Quantum Theory of Atoms in Molecules (QTAIM), and the non-covalent interactions (NCI), all predict wrong energetic orderings. Tiny rotational barriers, not exceeding 1.3 kcal/mol suggest that at room conditions, each methyl group either acts as a free rotor or adopts fluxional behavior, thus preventing accurate determination of their conformations. In this context, our results supersede current experimental ambiguity in the assignation of methyl conformation in caffeine and, more generally, in methylated xanthines and their derivatives. [ABSTRACT FROM AUTHOR]

Aim. To study the effect of electronic (α- and β-hyperconjugations) and steric (noncovalent interactions) factors on the structures of olefinic radical cations. Results and discussion. The effect of intramolecular dispersion interactions on the structures of crowded alkenes in the neutral and ionized forms has been studied at the density functional theory (DFT) level with and without dispersion corrections included, as well as at the MP2 theory level with medium size basis sets. The results obtained are compared to the available experimental data. An excellent agreement has been found between the experimental and MP2/DFT-computed geometries of sesquihomoadamantene, adamantylidene adamantane, bis-2,2,5,5-tetramethylcyclopentylidene, bis-D3-homocub-4-ylidene, and bis-CS-homocub-8-ylidene in the neutral and ionized forms. The experimental ionization potentials are better reproduced with the DFT-methods. Experimental part. The structure and composition of compounds were proved by the methods of 1H and 13C NMR-spectroscopy, and GC-MS-analysis. Elemental analysis was performed for the compounds obtained. Conclusions. The twisting of the olefinic moieties in the sesquihomoadamantene and adamantylidene adamantane radical cations is determined by the balance between the σ-π-hyperconjugation and residual one-electron π-bonding and is close to that of the prototypical ethylene radical cation (29°). The twisting reaches 55° for the bis-2,2,5,5-tetramethylcyclopentylidene radical cation due to substantial steric repulsions between methyl groups. At the same time, the ionized states of bis-D3-homocub-4-ylidene and bis-CS-homocub-8-ylidene retain their planarity due to β-CC-hyperconjugation and intramolecular dispersion attractions.

Herein we propose a new strategy for hyperconjugative antiaromatic compounds utilizing negative charges and design the 5,5-diphenyldibenzo[b,f]silepinyl dianion (pseudo 16π-electron system) in which negative hyperconjugation occurs between the anionic π-cloud and the σ*(Si–Ph) orbital. Essentially, reduction of the dibenzo[b,f]silepin with lithium readily generated a dilithium salt of the dibenzosilepinyl dianion, and its hyperconjugative antiaromaticity has been evidenced by the upfield shifts of 1H NMR signals and theoretical calculations, including large NICSzz values and ACID plots. [ABSTRACT FROM AUTHOR]

The decomposition of the molecular total energy in their hyperconjugative, steric and electrostatic effects can lead to interesting interpretations about the stereoelectronic effects that govern their geometry and properties. In this work, we have studied homologous series of perfluoroalkanes, perchloroalkanes, perfluorosilanes and perchlorosilanes, and all molecules have preference for helical geometries. According to Natural Bond Orbitals (NBO) calculations, the silanes have their helical geometry stabilized by hyperconjugative interactions, as well as the perfluoroalkanes. However, it was surprisingly difficult to disclose that steric interactions are ruling the helical geometry preference in perchloroalkanes by comparing the NBO analysis and the Quantum Theory of Atoms ins Molecules (QTAIM). Although perchloroalkanes have extremely intense steric interactions between Cl lone pairs, some of them were underestimated by the NBO analysis, which showed the opposite behavior compared with QTAIM that indicates steric effects as the leading forces to helical geometry preference. [ABSTRACT FROM AUTHOR]