Classification of So-Called Non-Covalent Interactions Based on VSEPR Model

The variety of interactions have been analyzed in numerous studies. They are often compared with the hydrogen bond that is crucial in numerous chemical and biological processes. One can mention such interactions as the halogen bond, pnicogen bond, and others that may be classified as σ-hole bonds. However, not only σ-holes may act as Lewis acid centers. Numerous species are characterized by the occurrence of π-holes, which also may play a role of the electron acceptor. The situation is complicated since numerous interactions, such as the pnicogen bond or the chalcogen bond, for example, may be classified as a σ-hole bond or π-hole bond; it ultimately depends on the configuration at the Lewis acid centre. The disadvantage of classifications of interactions is also connected with their names, derived from the names of groups such as halogen and tetrel bonds or from single elements such as hydrogen and carbon bonds. The chaos is aggravated by the properties of elements. For example, a hydrogen atom can act as the Lewis acid or as the Lewis base site if it is positively or negatively charged, respectively. Hence names of the corresponding interactions occur in literature, namely hydrogen bonds and hydride bonds. There are other numerous disadvantages connected with classifications and names of interactions; these are discussed in this study. Several studies show that the majority of interactions are ruled by the same mechanisms related to the electron charge shifts, and that the occurrence of numerous interactions leads to specific changes in geometries of interacting species. These changes follow the rules of the valence-shell electron-pair repulsion model (VSEPR). That is why the simple classification of interactions based on VSEPR is proposed here. This classification is still open since numerous processes and interactions not discussed in this study may be included within it.     

Potent strategy towards strongly emissive nitroaromatics through a weakly electron-deficient core

Nitroaromatics seldom fluoresce. The importance of electron-deficient (n-type) conjugates, however, has inspired a number of strategies for suppressing the emission-quenching effects of the strongly electron-withdrawing nitro group. Here, we demonstrate how such strategies yield fluorescent nitroaryl derivatives of dipyrrolonaphthyridinedione (DPND). Nitro groups near the DPND core quench its fluorescence. Conversely, nitro groups placed farther from the core allow some of the highest fluorescence quantum yields ever recorded for nitroaromatics. This strategy of preventing the known processes that compete with photoemission, however, leads to the emergence of unprecedented alternative mechanisms for fluorescence quenching, involving transitions to dark nπ* singlet states and aborted photochemistry. Forming nπ* triplet states from ππ* singlets is a classical pathway for fluorescence quenching. In nitro-DPNDs, however, these ππ* and nπ* excited states are both singlets, and they are common for nitroaryl conjugates. Understanding the excited-state dynamics of such nitroaromatics is crucial for designing strongly fluorescent electron-deficient conjugates.

Dipyrrolonaphthyridinedione appended with para- or meta-nitrophenyl substituents exhibits strong fluorescence from a ¹ππ* S₁ state.     

A Quantum-Chemical Study on the Photochemical Properties of Azides of a Series of Heteroarenes from Pyridine to Dibenzoacridine in Neutral and Protonated Forms

Molecules of a series of heteroaromatic azides in the ground (S ₀) and the lowest excited singlet (S ₁) states were calculated by the PM3 semiempirical method. It was shown that in the S ₀ state, the azide group in all the azides has quasi-linear geometry and a significant positive charge on the two terminal nitrogen atoms. The azide photoactivity is determined by the population of the σ _NN ^* orbital in the excited state, which is unoccupied in the ground state. The population of this orbital was found to depend on the size and charge of the aromatic π system. For the initial members of this azide series, the σ _NN ^* orbital is populated in both neutral and protonated forms. This is consistent with the experimental data and means that these azides are photoactive. With an increase in the size of the aromatic system, the energetic gap between the σ _NN ^* orbital and LUMO increases. As a result, the σ _NN ^* orbital is not populated in the S ₁ state when a particular threshold size of the π system is achieved, and the azide becomes photo-inactive.__________Translated from Khimiya Vysokikh Energii, Vol. 39, No. 4, 2005, pp. 259–266.Original Russian Text Copyright © 2005 by Budyka, Oshkin.     

A theoretical study of the C–HN hydrogen bond in the methane–ammonia complex

The C–HN hydrogen bond in the methane–ammonia complex is studied by determining its bond dissociation energy (BDE) and the n(N)→σ^*(C–H) interaction. At the MP2(Full)/6-311++G(3df,2p) level of theory with basis set superposition error (BSSE) correction, the BDE was determined to be 2.5 kJ mol⁻¹. The n(N)→σ^*(C–H) interaction at this level of theory was found to be 3.7 kJ mol⁻¹ by natural bond orbital (NBO) analysis. It was also found that the NBO values are in general higher than the BDE values with BSSE correction when they are compared at the same level of theory.     

Effect of intramolecular hydrogen bonding on the empty level structure of gas-phase salicylaldehyde:electron affinities and anion yield upon electron impact

The gas-phase empty level structures of γ-butyrolactone, α-hydroxy-γ-butyrolactone, benzaldehyde, 3-hydroxybenzaldehyde and 2-hydroxybenzaldehyde (salicylaldehyde) are studied by means of electron transmission spectroscopy (ETS) and ab initio 6-31G^** calculations. The most stable conformer of salicylaldehyde is characterised by intramolecular hydrogen bonding which causes relevant geometrical variations and, in particular, a lengthening of the C=O double bond. The empty π^* molecular orbitals of salicylaldehyde with large contribution from the carbonyl group are significantly stabilised with respect to their counterparts in benzaldehyde and in the 3-hydroxy derivative. Dissociative electron attachment spectroscopy (DAS) did not reveal formation of negative fragments at low energy, that is, the presence of dissociative decay channels for the π^* resonances observed in the ET spectra. The total anion current measured at the walls of the collision chamber (close to the electron beam), as a function of the incident electron energy, displays signals peaking at zero energy and at about 0.8 eV in benzaldehyde and in the two hydroxy derivatives. However, the only signal detected using a quadrupole mass filter (which requires a lifetime ≥10⁻⁶ s) is a zero energy peak in salicylaldehyde, due to the molecular anion.     

H2-induced CO adsorption and dissociation over Co/Al2O3 catalyst

The activation of adsorbed CO is an important step in CO hydrogenation. The results from TPSR of pre-adsorbed CO with H2 and syngas suggested that the presence of H2 increased the amount of CO adsorption and accelerated CO dissociation. The H2 was adsorbed first, and activated to form H＊ over metal sites, then reacted with carbonaceous species. The oxygen species for CO2 formation in the presence of hydrogen was mostly OH^＊, which reacted with adsorbed CO subsequently via CO^＊＋OH^＊ → CO2^＊＋H^＊; however, the direct CO dissociation was not excluded in CO hydrogenation. The dissociation of C-O bond in the presence of H2 proceeded by a concerted mechanism, which assisted the Boudourd reaction of adsorbed CO on the surface via CO^＊＋2H^＊ → CH^＊＋OH^＊. The formation of the surface species （CH） from adsorbed CO proceeded as indicated with the participation of surface hydrogen, was favored in the initial step of the Fischer-Tropsch synthesis.     

Assessing solvent effects on the singlet excited state lifetime of uracil derivatives: A femtosecond fluorescence upconversion study in alcohols and D2O

The excited state lifetimes of uracil, thymine and 5-fluorouracil have been measured using femtosecond UV fluorescence upconversion in various protic and aprotic polar solvents. The fastest decays are observed in acetonitrile and the slowest in aqueous solution while those observed in alcohols are intermediate. No direct correlation with macroscopic solvent parameters such as polarity or viscosity is found, but hydrogen bonding is one key factor affecting the fluorescence decay. It is proposed that the solvent modulates the relative energy of two close-lying electronically excited states, the bright ππ^* and the dark nπ^* states. This relative energy gap controls the non-radiative relaxation of the ππ^* state through a conical intersection close to the Franck–Condon region competing with the ultrafast internal conversion to the ground state. In addition, an inverse isotope effect is observed in D₂O where the decays are faster than in H₂O.     

The Role of Intramolecular Hydrogen Bonds vs. Other Weak Interactions on the Conformation of Hyponitrous Acid and Its Mono- and Dithio-Derivatives

High level ab initio and density functional theory calculations have been carried out to investigate the relative stability of the different conformers of hyponitrous acid and its mono- and dithio-derivatives. Geometries and vibrational frequencies were obtained at the B3LYP/6-311+G(d,p) level and final energies through B3LYP/6-311++G(3df,2pd) single point calculations. The reliability of this theoretical scheme has been assessed by comparing these DFT results with those obtained at the G3 level of theory, for some suitable cases. The cis conformers of hyponitrous acid and its mono- and dithio-derivatives are systematically more stable than the trans ones because in the cis conformation a dative interaction between the nitrogen-lone pairs and the σ_NX^* antibonding orbital is significantly favored. Quite interestingly, in general, the conformers presenting an intramolecular hydrogen bond (IHB) are not the global minima of the corresponding potential energy surfaces and only for hyponitrous acid the conformer with a OH ⋅s O IHB is slightly more stable than the cis conformer without IHB. The low stability of the tautomers with IHB is closely related with another weak intramolecular interaction which involves the lone-pairs of the chalcogen atoms and the π_NN^* antibondig orbital, and which is significantly perturbed when the IHB is formed.     

Cooperation of σ–π and σ*–π* Conjugation in the UV/Vis and Fluorescence Spectra of 9,10-Disilylanthracene

In 1996, we reported that silyl groups of 9,10-disilylanthracenes significantly affect the UV/Vis and fluorescence spectra. Although the results indicate that the silyl groups have strong electronic effects on anthracene, the details of the mechanisms responsible for this have not yet been clarified. This article describes the analysis of the UV/Vis and fluorescence spectra of 9,10-bis(diisopropylsilyl)anthracene by theoretical calculations. This study reveals that π conjugation of anthracene is extended by cooperation of σ–π and σ*–π* conjugation between the silyl groups and anthracene. This effect increases the transition moment of the π–π* transition of anthracene. As a result, the molecular extinction coefficient of the ¹L_a band and the fluorescence quantum yield are increased.     

Theoretical studies on the electronic structures and absorption spectra of substituted benzenes with one- and two-dimensional charge transfer character

The electronic structures and absorption spectra of one- and two-dimensional charge transfer (CT) molecules based on para-nitroaniline (pNA) and 1,3-diamino-4,6-dinitro- benzene (DADB) have been studied theoretically via semi-empirical and ab initio methods. It is found that the behaviors of optical absorption are strongly influenced by the dimension of CT. Different from the well-known one-dimensional CT molecule of pNA, which shows one intense absorption related to the π → π^* CT transition, two-dimensional CT molecule of DADB exhibits more absorption peaks associated with various low-lying CT transitions in near ultraviolet range. In addition, the relative orientations of transition dipole moment and ground state dipole moment in one- and two-dimensional charge transfer molecules were also discussed.     

Experimental Studies on Isonicotinato Cadmium(II) Complex [Cd(C<Subscript>6</Subscript>H<Subscript>4</Subscript>NO<Subscript>2</Subscript>)<Subscript>2</Subscript>(H<Subscript>2</Subscript>O)<Subscript>4</Subscript>] and Density Functional Calculations

Isonicotinato cadmium(II) complex [Cd(C₆H₄NO₂)₂(H₂O)₄] has been synthesized by hydrothermal method and characterized by elemental analysis, electronic-spectra and thermogravimetric analysis. Density functional theory (DFT) method calculations of the structure, atomic charges distribution, electronic spectra, natural population analysis and the thermodynamic properties at different temperatures have been performed. The calculated results show the electronic transitions are mainly derived from the contribution of bands π → π^* and the decomposition of the title compound should first occur at the bond of Cd—O, then at the bond of Cd—N, which agrees very well with the experimental data.     

Increasing protein stability by engineering the n → π* interaction at the β-turn

Abundant n → π* interactions between adjacent backbone carbonyl groups, identified by statistical analysis of protein structures, are predicted to play an important role in dictating the structure of proteins. However, experimentally testing the prediction in proteins has been challenging due to the weak nature of this interaction. By amplifying the strength of the n → π* interaction via amino acid substitution and thioamide incorporation at a solvent exposed β-turn within the GB1 proteins and Pin 1 WW domain, we demonstrate that an n → π* interaction increases the structural stability of proteins by restricting the ϕ torsion angle. Our results also suggest that amino acid side-chain identity and its rotameric conformation play an important and decisive role in dictating the strength of an n → π* interaction.

Amino acid residues adopt a right-handed α-helical conformation with increasing strength of the n → π* interaction. We also demonstrate a direct consequence of n → π* interactions on enhancing the structural stability of proteins.     

Structural role of hydrogen bond networks in amino acid–acid systems. (I) The network with highly polarizable OHO hydrogen bonds in sarcosine–methanesulfonic acid (2:1) crystal

The sarcosine–methanesulfonic acid (2:1) crystal was selected for examination of two problems: relations between different components of the amino acid–acid hydrogen bond network and a role of very strong and highly polarizable OHO hydrogen bond in the main structural units of the crystal: sarcosinium–sarcosine dimers (complexes). Our observations are based on phase transitions of the crystal monitored by DSC, X-ray diffraction and temperature evolutions of selected bands of IR spectra. Our experimental and DFT results provide information on the potential energy profile of the OHO proton and its evolution with temperature. The OO distance of the primary hydrogen bond remains almost unchanged and its proton is strongly delocalized and sensitive on neighbour NHO hydrogen bond. We propose a possible mechanism of the phase transitions and coupling between νCO vibrations of the carboxyl group and moving of the proton in neighbour OHO hydrogen bridge.     

Determination of the Inductive Constant of the Succinimide Group from the Reaction Series of Thermolysis of Nitro Esters

The inductive constant σ* of the succinimide group was determined using the previously obtained equation correlating the rate constant k _l ofliquid-phase homolysis of the O-NO₂ bond in nitro esters with the σ* constants of electronegative substituents. The kinetics of thermolysis of N-nitroxymethyl- and N-(2-nitroxyethyl)succinimide in the melt and solutions was studied manometrically. Adequate data for k _l were obtained. The σ* constant of the succinimide group was determined to be 4.5. The σ* constants for the phthalimide group and some other imide substituents were determined by a similar procedure.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 6, 2005, pp. 954–957.Original Russian Text Copyright © 2005 by Goncharov, Dubikhin, Nazin, Fedorov, Shastin.     

Can BB double bond be as potential proton acceptor: A UB3LYP and UMP2 theoretical study on unusual intermolecular T-shaped XH…π interactions between the triplet state HBBH and HF, HCl, HCN or H2C2

The unusual weak T-shaped XH…π hydrogen bonds are found between the BB double bond of the triplet state HBBH and the acid hydrogen of HF, HCl, HCN and H₂C₂ using UMP2 and UB3LYP methods at 6-311++G(2df,2p) and aug-cc-pVTZ levels. The binding energies follow the order of HBBH…HF > HBBH…HCl > HBBH…HCN > HBBH…H₂C₂, and the hydrogen-bonded interactions in the triplet state complexes HBBH…HX (³B₁) are found to be weaker than those in HCCH…HX and OCBBCO…HX. The analyses of natural bond orbital (NBO) and the electron density shifts reveal that the nature of the T-shaped XH…π hydrogen-bonded interaction is that the lost density from the π-orbital of BB bond is shifted toward the hydrogen atom of HX, leading to the electron density accumulation and the formation of the hydrogen bond. The atoms in molecules (AIM) theory has been also applied to characterize bond critical points and confirm that it is difficult for the ground electronic state of HBBH to be as the hydrogen-bond proton acceptor, perhaps due to the nature of electron-deficient BB double bond.     

Holmium porphyrin compound liquid crystals: Synthesis and properties

5,10,15,20-Tetra-[(p-alkoxy-m-ethyloxy)phenyl]porphyrin and [5-(p-alkoxy)phenyl-10,15,20-tri-phenyl]porphyrin and their holmium(III) complexes are reported. They display a hexagonal columnar discotic columnar (Col_h) liquid crystal phase and were studied by cyclic voltammetry, surface photovoltage spectroscopy (SPS), electric-field-induced surface photovoltage spectroscopy (EFISPS) and luminescence spectroscopy. Within the accessible potential window, all these compounds exhibit two one-electron reversible redox reactions. Quantum yields of Q band are in the region 0.0045–0.21 at room temperature. The SPS and EFISPS reveal that all the compounds are p-type semiconductors and exhibit photovoltaic response due to π–π^* electron transitions.     

Photophysics,Excited‐state Double‐Proton Transfer and Hydrogen‐bonding Properties of 5‐Deazaalloxazines

The photophysical properties of 5‐deazaalloxazine and 1,3‐dimethyl‐5‐deazaalloxazine were studied in different solvents. These compounds have higher values of fluorescence quantum yields and longer fluorescence lifetimes, compared to those obtained for their alloxazine analogs. Electronic structure and S₀–S_i transitions were investigated using the ab initio methods [MP2, CIS(D), EOM‐CCSD] with the correlation‐consistent basis sets. Also the time‐dependent density functional theory (TD‐DFT) has been employed. The lowest singlet excited states of 5‐deazaalloxazine and 1,3‐dimethyl‐5‐deazaalloxazine are predicted to have the π, π* character, whereas similar alloxazines have two close‐lying π, π* and n, π* transitions. Experimental steady‐state and time‐resolved spectral studies indicate formation of an isoalloxazinic excited state via excited‐state double‐proton transfer (ESDPT) catalyzed by an acetic acid molecule that forms a hydrogen bond complex with the 5‐deazaalloxazine molecule. Solvatochromism of both 5‐deazaalloxazine and its 1,3‐dimethyl substituted derivative was analyzed using the Kamlet–Taft scale and four‐parameter Catalán solvent scale. The most significant result of our studies is that the both scales show a strong influence of solvent acidity (hydrogen bond donating ability) on the emission properties of these compounds, indicating the importance of intermolecular solute–solvent hydrogen‐bonding interactions in their excited state.     

Theoretical studies upon the electronic structures and spectroscopic properties for a series of luminescent terpyridyl platinum(II) phenylacetylide complexes

A comprehensive calculations were carried out to get a deep insight into the ground- and excited-state electronic structures and the spectroscopic properties for a series of [Pt(4-X–trpy)CCC₆H₄R]⁺ complexes (trpy = 2,2′,6′,2″-terpyridine; X = H, R = NO₂ (1), Cl (2), C₆H₅ (3) and CH₃ (4); R = Cl, X = CH₃ (5) and C₆H₅ (6)). MP2 (second-order Møller–Plesset perturbation) and CIS (single-excitation configuration interaction) methods were employed to optimize the structures of 1–6 in the ground and excited states, respectively. The investigation showed that substituted phenylacetylide and trpy ligands only give rise to a small variation in geometrical structures but lead to a sizable difference in the electronic structures for 1–6 in the ground and excited states. The introduction of electron-rich groups into the phenylacetylide and/or terpyridyl ligands produces two different low-lying absorptions for 1 and 2–6, i.e., Pt(5d) → π^*(trpy) metal-to-ligand charge transfer (MLCT) mixed with π → π^*(CCPh) intraligand charge transfer (ILCT) for 1 and Pt(5d)/π(CCPh) → π^*(trpy) charge transfer (MLCT and LLCT) for 2–6. Remarkable electronic resonance on the whole Pt–CCPh–NO₂ moiety for 1 may be responsible for the difference. Solvatochromism calculation revealed that only LLCT/MLCT transitions showed the solvent dependence, consistent with the experimental observations.     

The Solvent-Like Nature of Silica Particles in Organic Solvents

 Kamlet-Taft’s α (hydrogen bond donor acidity) and π^* (dipolarity/polarizability) values of various silica batches measured in various solvents are presented. The α and π^* parameters for the various solid acids are analyzed by means of Fe(phen)₂(CN)₂ (cis-dicyano-bis-(1,10)-phenanthroline-iron(II), 1), Michler’s ketone (4,4′-bis-(dimethylamino)-benzophenone, 2), and two hydrophilic derivatives of 2, (4-(dimethylamino)-4′-(di-2-hydroxyethyl)-amino-benzophenone (3a) and 4,4′-bis-(di-(2-hydroxyethyl)-amino)-benzophenone (3b) as well as coumarin 153 (4) as solvatochromic surface polarity indicators. Apparent β (hydrogen bond acceptor basicity) parameters for bare silica have been evaluated by means of an aminobenzodifuranone dye (5) as solvatochromic probe. The chemical interpretation of the α and π^* parameters and the nature of the solvent/surface interaction which they reflect are discussed. It can be shown that an increase of the HBA (hydrogen bond accepting) capacity of the solvent significantly decreases the HBD (hydrogen bond donating) capacity of the surface environment, whereas the dipolarity/polarizability value of the silica/solvent interface is a composite of many effects. The classification of the polarity of silica particles in organic solvents compared to pure liquids is outlined.     

Extending bond orbital-connection matrix method to the QSPR study of alkylbenzenes: Some thermochemical properties

The properties of alkylbenzenes were estimated, using the contributions of the σ bonds, the conjugated π bond and the steric effect in alkylbenzene molecule. And a novel bond orbital-connection matrix, conjugated π bond orbital-connection matrix in aromatic molecules, was proposed. The eigenvalues of the conjugated π bond orbital-connection matrix can well express the contribution of the conjugated π bond to the properties of aromatic molecules. Using this eigenvalue together with the parameters proposed in our early works, the bond orbital-connection matrix method was extended successfully to the QSPR studies of alkylbenzenes and a general model was obtained to evaluate the thermochemical properties of alkylbenzenes, that is,

p_{(alkylbenzene)}=aN_C–C+b∑X_1CC+cN_C–H+d∑X_1CH+eS_Z/E+k∑X_1π,