DFT studies and AIM analysis of intramolecular N–H···O hydrogen bonds in 3-aminomethylene-2 methoxy-5,6-dimethyl-2-oxo-2,3-dihydro-2λ5-[1,2]oxaphosphinin-4-one and its derivatives

The intramolecular N–H···O hydrogen bonds in 3-aminomethylene-2 methoxy-5,6-dimethyl-2-oxo-2,3-dihydro-2λ⁵-[1,2]oxaphosphinin-4-one and its derivatives (F, H, Li, -BeH) were studied by DFT (density functional theory) methods. The results of calculations were obtained at B3LYP/6-311++G(d,p) level on model species, with the resonance-assisted hydrogen bonds (RAHB). Topological parameters such an electron density, its Laplacian, kinetic electron energy density, potential electron energy density, and total electron energy density at the bond critical points (BCP) of H···O/N–H contact bonds from Bader’s ‘Atoms in molecules’ (AIM) theory were analyzed in details. The energy of the N–H···O interactions studied here was found rather weak (E _HB = 2.53–12.08 kcal/mol). The results of AIM ellipticity indicated π-delocalization over all six atoms within ring.     

The influence of steric hindrance on conformation properties and molecular structure of 2,4,6-trinitrobenzenesulfonic acid: gas electron diffraction and quantum chemical calculations

A combined gas-phase electron diffraction and quantum chemical (B3LYP/cc-pVTZ, MP2/cc-pVDZ) study of molecular structure of 2,4,6-trinitrobenzenesulfonic acid (2,4,6-tri-NBSA) was carried out. Quantum chemical calculations showed that 2,4,6-tri-NBSA possesses six conformers, which form three pairs of enantiomers with the relative energy of 0, 4.4/3.9, and 2.5/2.5 kcal/mol. It was experimentally established that at T = 444(5) K a saturated vapor over 2,4,6-tri-NBSA is, predominantly (up to 93 mol.%), represented by a low-energy enantiomers II and II′ characterized by intramolecular hydrogen bond between an H atom of the hydroxyl group and one of the O atoms of the NO₂ group. Experimental internuclear distances for the low-energy enantiomers are (?): r _h1(C–C)_av. = 1.387(4), r _h1(C–S) = 1.811(6), r _h1(S=O)_av. = 1.424(4), r _h1(S–O) = 1.579(4), r _h1(N–O)_av. = 1.214(3), r _h1(C–N)_av. = 1.491(5). Geometry of the conformer II points on existance of strong steric interactions between SO₂OH group and two ortho-nitro groups. Analysis of the orbital interactions between the substituents and benzene ring was carried out. Geometric parameters and energies of transition states between conformers were calculated (B3LYP).     

Synthesis,spectroscopic, thermal and biological aspect of mixed ligand copper(II) complexes

Derivative of 8-hydroxyquinoline i.e. Clioquinol is well known for its antibiotic properties, drug design and coordinating ability towards metal ion such as Copper(II). The structure of mixed ligand complexes has been investigated using spectral, elemental and thermal analysis. In vitro anti microbial activity against four bacterial species were performed i.e. Escherichia coli, Pseudomonas aeruginosa, Serratia marcescens, Bacillus substilis and found that synthesized complexes (15–37 mm) were found to be significant potent compared to standard drugs (clioquinol i.e. 10–26 mm), parental ligands and metal salts employed for complexation. The kinetic parameters such as order of reaction (n = 0.96–1.49), and the energy of activation (E _a = 3.065–142.9 kJ mol⁻¹), have been calculated using Freeman–Carroll method. The range found for the pre-exponential factor (A), the activation entropy (S* = −91.03 to−102.6 JK⁻¹ mol⁻¹), the activation enthalpy (H* = 0.380–135.15 kJ mol⁻¹), and the free energy (G* = 33.52–222.4 kJ mol⁻¹) of activation reveals that the complexes are more stable. Order of stability of complexes were found to be [Cu(A⁴)(CQ)OH] · 4H₂O > [Cu(A³)(CQ)OH] · 5H₂O > [Cu(A¹)(CQ)OH] · H₂O > [Cu(A²)(CQ)OH] · 3H₂O     

Isotherm,thermodynamic and kinetic studies of Sr<Superscript>2+</Superscript> adsorption on spherical TiO<Subscript>2</Subscript>/PAN composites

A spherical polyacrylonitrile–TiO₂ composite adsorbent was prepared and its strontium removal potential was investigated. The Langmuir equation fixed well the equilibrium data. The value of ∆H° = 8.943 kJ/mol and ∆G° = 6.291 kJ/mol at 298 K indicate that the adsorption of strontium onto TiO₂/PAN composite adsorbent is an endothermic and non-spontaneous reaction. The kinetic process was described by a pseudo-second-order rate model very well.     

Direct ab initio MD study on the hydrogen abstraction reaction of triplet state acetone from methanol molecule

Solvent re-orientation process of triplet acetone/methanol complex and intermolecular hydrogen atom abstraction reaction on the triplet state energy surface, (CH₃)₂C=O (T₁) + CH₃OH → (CH₃)₂C–OH + CH₂OH in gas phase, have been investigated by means of density functional theory (DFT) and direct ab initio molecular dynamics (MD) methods. The static DFT calculation of hydrogen abstraction reaction at the T₁ state showed that the transition state is 16.4 and 30.9 kcal/mol lower than the energy levels of S₁ and S₂ states, respectively, and 9.2 kcal/mol higher than the bottom of T₁ state. The product state, (CH₃)₂C–OH⋯CH₂OH, is 8.4 kcal/mol lower in energy than the level of T₁ state. The direct ab initio MD calculation showed that the product is rapidly formed within 150 fs and the separated products (CH₃)₂C–OH + CH₂OH were formed. The mechanism of reaction dynamics of the triplet acetone/methanol complex was discussed on the basis of theoretical results.     

Potassium hydrogen trans‐glutaconate monohydrate at 295, 245 and 40 K,and its rubidium analogue at 298 K

A centrosymmetric and short O—H?O hydrogen bond was found in isomorphic crystals of potassium hydrogen trans‐glutaconate monohydrate (potassium hydrogen trans‐pent‐2‐ene‐1,5‐dioate, K⁺·C₅H₅O₄^?·H₂O), (I), and rubidium hydrogen trans‐glutaconate monohydrate (rubidium hydrogen trans‐pent‐2‐ene‐1,5‐dioate, Rb⁺·C₅H₅O₄^?·H₂O), (II). The O?O distance at room temperature is 2.444 (3) Å in (I), and 2.417 (4) Å in (II). The O?O distance for (I) showed no significant decrease at low temperatures.     

Hydrothermal syntheses and characterization of cobalt(II) and nickel(II) complexes of 1,3-adamantanedicarboxylic acid and 1,10-phenanthroline

Two coordination complexes, namely [Co(phen)(H₂O)L]·H₂O and [Ni₂(phen)₂(H₂O)₂L₂]·4H₂O (phen = 1,10-phenanthroline, H₂L = 1,3-adamantanedicarboxylic acid) have been hydrothermally synthesized and structurally characterized by single crystal X-ray diffraction. [Co(phen)(H₂O)L]·H₂O consists of 1D chains of the complex plus lattice H₂O molecules. Interchain hydrogen bonds and π–π stacking interactions assemble the 1D chains into 2D layers. [Ni₂(phen)₂(H₂O)₂L₂]·4H₂O is a binuclear complex which is assembled into a 3D supramolecular structure by strong hydrogen bonds and π–π stacking interactions. Both complexes were characterized by physico-chemical and spectroscopic methods.     

Syntheses and crystal structures of two novel Cu(II) and Co(II) complexes with 3-methyl-4-(p-bromophenyl)-5-(2-pyridyl)-1,2,4-triazole

A new ligand, 3-methyl-4-(p-bromophenyl)-5-(2-pyridyl)-1,2,4-triazole (L) and its complexes, trans-[CuL₂(ClO₄)₂] (1) and cis-[CoL₂(H₂O)₂](ClO₄)₂·H₂O·CH₃OH (2), have been synthesized and characterized by UV, IR, electrospray ionization mass spectrum, elemental analyses, and single-crystal X-ray diffraction methods. In the structure, two L ligands are stabilized by intermolecular π···π interactions between the triazole rings. In the complexes, each L ligand adopts a chelating bidentate mode through N atom of pyridyl group and one N atom of the triazole. Both complexes have a similar distorted octahedral [MN₄O₂] core (M = Cu²⁺ and Co²⁺) with two ClO₄ ⁻ ions in the trans position in 1 but two H₂O molecules in the cis arrangement in 2.     

Mechanism and kinetics of the reaction of the 2-propargyl radical with ammonia

Gas-phase mechanism and kinetics of the reactions of the 2-propargyl radical (H₂CCCH), an important intermediate in combustion processes, with ammonia were investigated using ab initio molecular orbital theory at the coupled-cluster CCSD(T)//B3LYP/6-311++G(3df,2p) method in conjunction with transition state theory (TST), variational transition state theory (VTST), and Rice–Ramsperger–Kassel–Macus (RRKM) calculations for rate constants. The potential energy surface (PES) constructed shows that the C₃H₃ + NH₃ reaction has four main entrances, including two H-abstraction and two addition channels in which the former are energetically more favorable. The H-abstraction channels occur via energy barriers of 24 (T0/P2) and 26 kcal/mol (T0/P3) forming loose van de Waals complexes, COM_1 (12 kcal/mol) and COM_2 (14 kcal/mol), respectively. These complexes can easily be decomposed via barrier-less processes resulting HCCCH₃ + NH₂ (P2, 14 kcal/mol) and HCCCH₃ + NH₂ (P3, 15 kcal/mol), respectively. The additional channels occur initially by formation of two intermediate states, H₂CCCHNH₃ (35 kcal/mol) and H₂CC(NH₃)CH (37 kcal/mol) via energy barriers of 37 and 40 kcal/mol at T0/1 and T0/5, respectively, followed by isomerization and decomposition yielding 21 different products. These processes are fully depicted in an as-complete-as-possible PES. The rate constants and product branching ratios for the low-energy channels calculated show that the C₃H₃ + NH₃ reaction is almost pressure-independent. For the temperature range of 300–2000 K, the HCCCH₃ + NH₂ is the major product, whereas the minor one, HCCCH₃ + NH₂, has more contribution when temperature increases. Theoretical results on the mechanism and kinetics of the reaction considered may be helpful for future experiments as well as for understanding the role of the propargyl radical in combustion chemistry.     

Synthesis, structure and spectroscopic characterization of a new organic cation diphosphate: [2,6-(C2H5)2C6H3NH3]2H2P2O7?·?2H2O

The chemical preparation, crystal structure and spectroscopic characterization of [2,6-(C₂H₅)₂C₆H₃NH₃]₂H₂P₂O₇ · 2H₂O have been reported. The compound crystallizes in the monoclinic system in space group P2₁/c and cell parameters a = 14.323(2), b = 11.158(3), c = 16.387(2) ? and β = 96.34(3)°; V = 2602.8(9) ?³ and Z = 4. Crystal structure has been determined and refined to R = 0.044, using 3528 independent reflections. The atomic arrangement of the title compound shows anionic layer of formulae [H₂P₂O₇(H₂O)₂] _n ²ⁿ⁻ stacked along the c-axis. The 2,6-diethylanilinium cations establish on both sides of these inorganic layer hydrogen bonds so as to contribute to the intralayer cohesion in the network. The different building species are held together by means of O–H···O and N–H···O intermolecular hydrogen bonds in addition to electrostatic and van der Waals interactions.     

Kinetic and thermodynamic studies of MgHPO<Subscript>4</Subscript> · 3H<Subscript>2</Subscript>O by non-isothermal decomposition data

The thermal decomposition of magnesium hydrogen phosphate trihydrate MgHPO₄ · 3H₂O was investigated in air atmosphere using TG-DTG-DTA. MgHPO₄ · 3H₂O decomposes in a single step and its final decomposition product (Mg₂P₂O₇) was obtained. The activation energies of the decomposition step of MgHPO₄ · 3H₂O were calculated through the isoconversional methods of the Ozawa, Kissinger–Akahira–Sunose (KAS) and Iterative equation, and the possible conversion function has been estimated through the Coats and Redfern integral equation. The activation energies calculated for the decomposition reaction by different techniques and methods were found to be consistent. The better kinetic model of the decomposition reaction for MgHPO₄ · 3H₂O is the F _1/3 model as a simple n-order reaction of “chemical process or mechanism no-invoking equation”. The thermodynamic functions (ΔH*, ΔG* and ΔS*) of the decomposition reaction are calculated by the activated complex theory and indicate that the process is non-spontaneous without connecting with the introduction of heat.     

Kinetics and mechanism of aqua ligand substitution from cis-diaqua(cis-1,2-diaminocyclohexane)platinum(II)perchlorate by diethyldithiocarbamate anion in aqueous medium

The kinetics of the interaction of diethyldithiocarbamate (Et₂DTC) with [Pt(dach)(H₂O)₂]²⁺ (dach = cis-1,2-diaminocyclohexane) have been studied spectrophotometrically as a function of [Pt(dach)(H₂O)₂ ²⁺], [Et₂DTC] and temperature at a particular pH (4.0). The reaction proceeds via rapid outer sphere association complex formation followed by two slow consecutive steps. The first step involves the transformation of the outer sphere complex into an inner sphere complex containing a Pt–S bond and one aqua ligand, while the second step involves chelation when the second aqua ligand is replaced. The association equilibrium constant K _E and two rate constants k ₁ and k ₂ have been evaluated. Activation parameters for both the steps have been calculated (∆H ₁ ^# = 66.8 ± 3.7 kJ mol⁻¹, ∆S ₁^# = −81 ± 12 JK⁻¹ mol⁻¹ and ∆H ₂^# = 95.1 ± 2.8 kJ mol⁻¹, ∆S ₂^# = −34.4 ± 9.1 JK⁻¹ mol⁻¹). The low enthalpy of activation and negative entropy of activation indicate an associative mode of activation for both the steps.     

New trans-[Re6S8(CN)4L2] n− Rhenium Cluster Complexes: Syntheses, Crystal Structures and Properties

Reaction of polymeric compound Cs₄[Re₆S₈(CN)₄S_2/2] with aqueous solution of KOH led to formation of trans-[Re₆S₈(CN)₄(OH)₂]⁴⁻ anion which was crystallized in ordered Cs_1.68K_2.32[Re₆S₈(CN)₄(OH)₂] · 2H₂O (1a) and disordered Cs_1.83K_2.17[Re₆S₈(CN)₄(OH)₂] · 2H₂O (1b) modifications. The presence of two types of apical ligands, inert cyanides and labile hydroxides, opened a way to other trans-[Re₆S₈(CN)₄L₂] ⁿ⁻ rhenium cluster complexes: trans-[Re₆S₈(CN)₄Cl₂]⁴⁻, trans-[Re₆S₈(CN)₄(H₂O)Br]³⁻, and trans-[Re₆S₈(CN)₄Br₂]⁴⁻, crystallized as Cs_1.84K_1.16(H)[Re₆S₈(CN)₄Cl₂] (2), Cs_1.68K_1.32[Re₆S₈(CN)₄(H₂O)Br] (3), (Me₄N)₃(H₅O₂)[Re₆S₈(CN)₄Br₂] (4), and CsK{Cu(H₂O)₂[Re₆S₈(CN)₄Cl₂]} · 4H₂O (5) salts.     

Thermochemical study on ternary complex of dysprosium <Emphasis Type="Italic">m</Emphasis>-nitrobenzoic acid with <Emphasis Type="Italic">o</Emphasis>-phenanthroline

A ternary binuclear complex of dysprosium chloride hexahydrate with m-nitrobenzoic acid and 1,10-phenanthroline, [Dy(m-NBA)₃phen]₂·4H₂O (m-NBA: m-nitrobenzoate; phen: 1,10-phenanthroline) was synthesized. The dissolution enthalpies of [2phen·H₂O(s)], [6m-HNBA(s)], [2DyCl₃·6H₂O(s)], and [Dy(m-NBA)₃phen]₂·4H₂O(s) in the calorimetric solvent (V_DMSO:V_MeOH = 3:2) were determined by the solution–reaction isoperibol calorimeter at 298.15 K to be \Updelta_\texts H_\textm^q \Updelta_{\text{s}} H_{\text{m}}^{\theta } [2phen·H₂O(s), 298.15 K] = 21.7367 ± 0.3150 kJ·mol⁻¹, \Updelta_\texts H_\textm^q \Updelta_{\text{s}} H_{\text{m}}^{\theta } [6m-HNBA(s), 298.15 K] = 15.3635 ± 0.2235 kJ·mol⁻¹, \Updelta_\texts H_\textm^q \Updelta_{\text{s}} H_{\text{m}}^{\theta } [2DyCl₃·6H₂O(s), 298.15 K] = −203.5331 ± 0.2200 kJ·mol⁻¹, and \Updelta_\texts H_\textm^q \Updelta_{\text{s}} H_{\text{m}}^{\theta } [[Dy(m-NBA)₃phen]₂·4H₂O(s), 298.15 K] = 53.5965 ± 0.2367 kJ·mol⁻¹, respectively. The enthalpy change of the reaction was determined to be \Updelta_\textr H_\textm^q = 3 6 9. 4 9 ±0. 5 6   \textkJ·\textmol ^{- 1} . \Updelta_{\text{r}} H_{\text{m}}^{\theta } = 3 6 9. 4 9 \pm 0. 5 6 \;{\text{kJ}}\cdot {\text{mol}}^{ - 1} . According to the above results and the relevant data in the literature, through Hess’ law, the standard molar enthalpy of formation of [Dy(m-NBA)₃phen]₂·4H₂O(s) was estimated to be \Updelta_\textf H_\textm^q \Updelta_{\text{f}} H_{\text{m}}^{\theta } [[Dy(m-NBA)₃phen]₂·4H₂O(s), 298.15 K] = −5525 ± 6 kJ·mol⁻¹.     

Theoretical study of the neutral hydrolysis of methyl formate via a concerted and stepwise water-assisted mechanism using free-energy curves and molecular dynamics simulation

A procedure previously described by us is used for the theoretical study of chemical reactions in solution by means of molecular dynamics simulation, with solute–solvent interaction potentials LJ (12-6-1) derived from ab initio quantum calculations. We apply the procedure to the case of the neutral hydrolysis of methyl formate, HCOOCH₃ + 3H₂O → HCOOH + CH₃OH + 2H₂O in aqueous solution, via concerted and stepwise water-assisted mechanisms. We use the solvent as reaction coordinate, and the free-energy curves for the calculation of the activation energies. The theoretical calculation for the thermodynamics of this hydrolysis reaction in aqueous solution, assisted by three water molecules, is in agreement with the available experimental information. In particular our study gives values of ΔG ^≠ = 28.88 and 28.17 kcal/mol for the concerted and stepwise mechanisms, close to the experimental activation barrier of 28.8 kcal/mol, and a significant improvement over the values of 48.05 and 45.66 kcal/mol found in another similar study using the PCM model.     

Syntheses, structures and properties of two new neodymium complexes with N-P-acetamidobenzenesulfonyl-glycine acid

Two new neodymium complexes, [Nd₂(abglyH)₆(2,2′-bipy)₂(H₂O)₂] · 4H₂O 1 and {[Nd(abglyH)₃(H₂O)₂] · (4,4′-bipy) · 7H₂O}_n 2 (abglyH₂ = N-P-acetamidobenzenesulfonyl-glycine acid, 2,2′-bipy = 2,2′-bipyridine, 4,4′-bipy = 4,4′-bipyridine), have been synthesized and their structures have been measured by X-ray crystallography. In 1, nine-coordinated Nd(III) ions are bridged by two syn–syn bidentate and two tridentate bridging carboxylate groups from four different abglyH⁻ anions to form dinuclear motifs, which are further connected into a 3-D supramolecular framework via hydrogen bonds between the binuclear motifs and the uncoordinated water molecules. In 2, eight-coordinated Nd(III) ions are linked by six carboxylate groups adopting a syn–syn bidentate bridging fashion to form a 1-D inorganic–organic alternating linear chain. These polymeric chains generate microchannels extending along the a direction, and these cavities are occupied by discrete tetradecameric water clusters, which interact with their surroundings and finally furnish the 3-D supramolecular network via hydrogen bonds. At the same time, π–π stacking interactions between benzene rings from abglyH⁻ anions also play an important role in stabilizing the network.     

A two-dimensional ladder-type network in the 2:1 co-crystal of 1,2,5-thiadiazole-3,4-dicarboxylic acid and 4,4′-bipyridine

Abstract  

The crystal structure of the 2:1 co-crystal of 1,2,5-thiadiazole-3,4-dicarboxylic acid and 4,4′-bipyridine, (C₄H₂N₂O₄S)₂·C₁₀H₈N₂, has been determined by X-ray diffraction at the monoclinic space group C2/c with cell parameters of a = 21.388(7) ?, b = 6.735(2) ?, c = 14.877(5) ?, β = 110.431(3)°, and Z = 4. There are one molecule of thiadiazole and a half molecule of bipyridine in the asymmetric unit. The dihedral angle between the pyridine ring planes is 40.5(3)°. Two intramolecular O–H···N [2.730(7) ?] and O–H···O [2.433(6) ?] hydrogen bonds are observed in the thiadiazole molecule. In the crystal structure, the molecules form a unique two-dimensional ladder-type network linked by intermolecular O–H···N [2.704(4) ?] hydrogen bonds and S···O [3.100(5) ?] heteroatom interactions.     

Studies on mono- and dinuclear bisoxime copper complexes with different coordination geometries

Two new mono- and dinuclear Cu(II) complexes, namely [CuL¹]·0.5H₂O (1) and [(Cu₂(L²)₂)(DMF)]·0.5DMF (2) (H₂L¹ = 1,2-bis{[(Z)-(3-methyl-5-oxo-1-phenyl-1H-pyrazolidin-4(4H)-yl)(phenyl)]methylene-aminooxy}ethane; H₂L² = 1,3-bis{[(Z)-(3-methyl-5-oxo-1-phenyl-1H-pyrazolidin-4(4H)-yl)(phenyl)] methyleneaminooxy}propane), have been synthesized and characterized by X-ray crystallography. The unit cell of complex 1 contains two crystallographically independent but chemically identical [CuL¹] molecules and one crystalline water molecule, showing a slightly distorted square-planar coordination geometry and forming a wave-like pattern running along the a-axis via hydrogen bonding and π···π stacking interactions. Complex 2 has a dinuclear structure, comprising two Cu(II) atoms, two completely deprotonated phenolate bisoxime (L²)²⁻ moieties (in the form of enol), and both coordinated and hemi-crystalline DMF molecules. Complex 2 has square-planar and square-pyramidal geometries around the two copper centers, whose basic coordination planes are almost perpendicular and form an infinite three-dimensional supramolecular network structure involving intermolecular C–H···N, C–H···O, and C–H···π(Ph) hydrogen bonding and π···π stacking interactions of neighboring pyrazole rings.     

Two 3D supramolecules with helices constructed by hydrogen bonds based on p-thioacetatebenzoic acid ligand

Abstract  

p-Thioacetatebenzoic acid (H₂L) and a combination of N-donor ligands such as 4,4′-bipyridine (4,4′-bipy) and 1,3-bi(4-pyridyl)propane (bpp) with metal ions Mn(II) and Ni(II) give rise to two coordination polymers, namely, [Mn(HL)₂(bpp)₂(H₂O)₂] _n (1), [NiL (4,4′-bipy)(H₂O)₃] _n ·nH₂O (2). 1 features an unusual “8” shaped double layer by hydrogen bonds and two different types of helical chains are arrayed alternatively in the 2D double layer framework, which further extends into a 3D supramolecular structure through C–H···O hydrogen bonds. 2 consists of 1D chains which further connect with each other via hydrogen bonds to form the final 3D framework including two different types of helical structure. Photoluminescence study reveals that 1 displays intense structure-related fluorescent emission bands (λ _ex = 369 nm) at 414 nm in the solid state at room temperature. Electrochemical property of 2 reveals that the process of the redox is irreversible.     

Theoretical study of bifurcated bent blue-shifted hydrogen bonds CH<Subscript>2</Subscript>···Y

Ab initio quantum chemistry methods were applied to study the bifurcated bent hydrogen bonds Y··· H₂CZ (Z = O, S, Se) and Y···H₂CZ₂ (Z = F, Cl, Br) (Y = Cl⁻, Br⁻) at the MP2/6-311++G(d,p) and MP2/6-311++G(2df,2p) levels. The results show that in each complex there are two equivalent blue-shifted H-bonds Y···H-C, and that the interaction energies and blue shifts are large, the energy of each Y···H-C H-bond is 15–27 kJ/mol, and Δr(CH) = −0.1 − −0.5 pm and Δv(CH) = 30 − 80 cm⁻¹. The natural bond orbital analysis shows that these blue-shifted H-bonds are caused by three factors: large rehybridization; small direct intermolecular hyperconjugation and larger indirect intermolecular hyperconjugation; large decrease of intramolecular hyperconjugation. The topological analysis of electron density shows that in each complex there are three intermolecular critical points: there is one bond critical point between the acceptor atom Y and each hydrogen, and there is a ring critical point inside the tetragon YHCH, so these interactions are exactly H-bonding.