HO Hydrogen Bond Research Articles

Boryl Ancillary Ligands: Influencing Stability and Reactivity of Amidinato‐Silanone and Germanone Systems in Ammonia Activation

AbstractWhile the nucleophilic addition of ammonia to ketones is an archetypal reaction in classical organic chemistry, the reactivity of heavier group 14 carbonyl analogues (R2E=O; E=Si, Ge, Sn, or Pb) with NH3 remains sparsely investigated, primarily due to the synthetic difficulties in accessing heavier ketone congeners. Herein, we present a room‐temperature stable boryl‐substituted amidinato‐silanone {(HCDippN)2B}{PhC(tBuN)2}Si=O (Dipp=2,6‐iPr2C6H3) (together with its germanone analogue), formed from the corresponding silylene under a N2O atmosphere. This system reacts cleanly with ammonia in 1,2‐fashion to give an isolable sila‐hemiaminal complex {(HCDippN)2B}{PhC(tBuN)2}Si(OH)(NH2). Quantum chemical calculations reveal that the formation of this sila‐hemiaminal is crucially dependent on the nature of the ancillary ligand scaffold. It is facilitated thermodynamically by the hemi‐lability of the amidinate ligand (which allows for the formation of an energetically critical intramolecular N⋅⋅⋅HO hydrogen bond within the product) and is enabled mech‐anistically by a process in which the silanone initially acts in umpolung fashion as a base (rather than an acid), due to the strongly electron‐releasing and sterically bulky nature of the ancillary boryl ligand.

Boryl Ancillary Ligands: Influencing Stability and Reactivity of Amidinato-Silanone and Germanone Systems in Ammonia Activation.

While the nucleophilic addition of ammonia to ketones is an archetypal reaction in classical organic chemistry, the reactivity of heavier group 14 carbonyl analogues (R2E=O; E=Si, Ge, Sn, or Pb) with NH3 remains sparsely investigated, primarily due to the synthetic difficulties in accessing heavier ketone congeners. Herein, we present a room-temperature stable boryl-substituted amidinato-silanone {(HCDippN)2B}{PhC(tBuN)2}Si=O (Dipp=2,6-iPr2C6H3) (together with its germanone analogue), formed from the corresponding silylene under a N2O atmosphere. This system reacts cleanly with ammonia in 1,2-fashion to give an isolable sila-hemiaminal complex {(HCDippN)2B}{PhC(tBuN)2}Si(OH)(NH2). Quantum chemical calculations reveal that the formation of this sila-hemiaminal is crucially dependent on the nature of the ancillary ligand scaffold. It is facilitated thermodynamically by the hemi-lability of the amidinate ligand (which allows for the formation of an energetically critical intramolecular N⋅⋅⋅HO hydrogen bond within the product) and is enabled mech-anistically by a process in which the silanone initially acts in umpolung fashion as a base (rather than an acid), due to the strongly electron-releasing and sterically bulky nature of the ancillary boryl ligand.

Organoplatinum Chemistry Related to Alkane Oxidation: The Effect of a Nitro Substituent in Ligands Having an Appended Phenol Group

The organoplatinum chemistry of the ligands 2-C5H4N-CH2-NH-C6H3-2-OH-5-X (L1, X = H; L3, X = NO2) and 2-C5H4N-CH=N-C6H3-2-OH-5-X (L2, X = H; L4, X = NO2), which contain an appended phenol substituent, is described. Comparisons are made between the ligands with amine or imine groups (L1, L3 vs. L2, L4) and ligands with X = H or NO2 (L1, L2 vs. L3, L4), and major differences are observed. Thus, on reaction with the cycloneophylplatinum(II) complex [{Pt(CH2CMe2C6H4)(μ-SMe2)}2], ligands L1, L2 and L4 give the corresponding platinum(II) complexes [Pt(CH2CMe2C6H4)(κ2-N,N′-L)], containing a Pt··HO hydrogen bond, whereas L3 gives a mixture of isomeric platinum(IV) hydride complexes [PtH(CH2CMe2C6H4)(κ3-N,N′,O-L3-H)], which are formed by oxidative addition of the phenol O-H bond and which react further with oxygen to give [Pt(OH)(CH2CMe2C6H4)(κ3-N,N′,O-L3-H)]. The differences in reactivity are proposed to be due to the greater acidity of the nitro-substituted phenol groups in L3 and L4 and to the greater ability of the deprotonated amine ligand L3 over L4 to stabilize platinum(IV) by adopting the fac-κ3-N,N′,O-L3-H coordination mode.

Concentration-dependence of specific rotation of optically active glycerol analogues and structurally related compounds: The significance of intermolecular hydrogen bonding

Specific rotation of optically active glycerol analogues, (S)-3-(benzyloxy)propane-1,2-diol [(S)-1], (S)-3-methoxypropane-1,2-diol [(S)-6] and (S)-3-phenoxypropane-1,2-diol [(S)-13], changed its sign from (+) to (–) with increasing concentration in CHCl3, whereas no significant concentration-dependent change was observed in MeOH. The 1H NMR spectrum of (S)-1 showed concentration-dependent changes of the chemical shift in CDCl3. Intermolecular interaction by OH⋯OH hydrogen bonds was suggested by single crystal X-ray crystallography of (S)-13. The concentration-dependent changes of specific rotation of (S)-1, (S)-6, and (S)-13 in CHCl3 can therefore be presumed to be due to self-assembly of these compounds via intermolecular hydrogen bonding at high concentration. The remarkable concentration-dependent changes in specific rotation were also observed to occur in the compounds with related chemical structures, such as (S)-3-hydroxy-4-methoxy-4-oxobutanoic acid [(S)-10], (S)-1-[(2-methylpropan-2-yl)oxycarbonyl]pyrrolidine-2-carboxylic acid [Boc-L-Pro-OH, (S)-14], (R)-3-hydroxy-4,4-dimethylhydrofran-2(3H)-one [(R)-15], (R)-tetrahydrofuran-2-carboxylic acid [(R)-16], (S)-3-methyl-2-[(2-methylpropan-2-yl)oxycarbonylamino]butanoic acid [Boc-L-Val-OH, (S)-17], (R)-oxiran-2-ylmethanol [(R)-18], and (4S)-4-isopropyloxazolidin-2-one [(S)-19]. The existence of intermolecular interaction by C=O⋯HO hydrogen bonds was suggested by single crystal X-ray crystallography of (S)-10, (S)-13, and (S)-14. Thus, this series of chiral compounds have multiple functional groups in appropriate positions that serve as hydrogen-bond donors and/or acceptors, resulting in significant concentration-dependent changes of specific rotation.

Properties of 3-fluoroprop-2-enoic acids, chemical shifts and indirect spin-spin coupling constants: A DFT study

Nuclear magnetic resonance (NMR) parameters, shielding constants, and indirect spin-spin coupling constants (SSCC) in a large set of model compounds were calculated using the combination of the B3PW91, B3LYP, and BHandH functionals with the 6-311+(d,p), 6-311++G(d,p), 6-311++G(2d,p), and 6-311++G(3df, 3pd) basis sets, in order to verify the most appropriate methods for calculating these parameters. Selected methods were applied to calculate the NMR parameters in (2E)- and (2Z)-3-fluoroprop-2-enoic acids. Optimization of structures revealed four stable rotamers of each isomer, E and Z, of 3-fluoroprop-2-enoic acid. The internal molecular energy of s-cis and s-trans syn rotamers indicated that they are favorable in solution. All rotamers of 3-fluoroprop-2-enoic acid are coplanar except E s-trans anti, which has a "twisted" structure due to repulsion between hydrogen atoms CFH and HO. Topological analysis of the electronic structure of molecules including properties of critical points revealed noncovalent F...OH and F...HO bonds in Z s-trans syn and anti rotamers, respectively. The molecules of acid tend to form dimers and trimers in solution via hydrogen bonds O...H, CH...O, and CH...F, with binding energies ranging from a few to over a dozen kcal per mol. Theoretical chemical shifts and SSCCs were analyzed in detail to find a dependence on the geometry of individual rotameters. Calculations revealed a 5J(F,H) of -31 Hz in Z s-trans-anti rotamer containing the F...HO hydrogen bond, while this SSCC ranges from -1.9 to 1.4 Hz in the remaining rotamers. Intermolecular SSCCs across hydrogen bonds range from -3 to 7 Hz in dimers and trimers. Theoretical NMR parameters were compared with available experimental data.

Evolution of Solute-Water Interactions in the Benzaldehyde-(H2O)1-6 Clusters by Rotational Spectroscopy.

The investigation on the preferred arrangement and intermolecular interactions of gas phase solute-water clusters gives insights into the intermolecular potentials that govern the structure and dynamics of the aqueous solutions. Here, we report the investigation of hydrated coordination networks of benzaldehyde-(water)n (n = 1-6) clusters in a pulsed supersonic expansion using broadband rotational spectroscopy. Benzaldehyde (PhCHO) is the simplest aromatic aldehyde that involves both hydrophilic (CHO) and hydrophobic (phenyl ring) functional groups, which can mimic molecules of biological significance. For the n = 1-3 clusters, the water molecules are connected around the hydrophilic CHO moiety of benzaldehyde through a strong CO···HO hydrogen bond and weak CH···OH hydrogen bond(s). For the larger clusters, the spectra are consistent with the structures in which the water clusters are coordinated on the surface of PhCHO with both the hydrophilic CHO and hydrophobic phenyl ring groups being involved in the bonding interactions. The presence of benzaldehyde does not strongly interfere with the cyclic water tetramer and pentamer, which retain the same structure as in the pure water cluster. The book isomer instead of cage or prism isomers of the water hexamer is incorporated into the microsolvated cluster. The PhCHO molecule deviates from the planar structure upon sequential addition of water molecules. The PhCHO-(H2O)1-6 clusters may serve as a simple model system in understanding the solute-water interactions of biologically relevant molecules in an aqueous environment.

Dimerization dynamics of carboxylic acids in helium nanodroplets.

The dimerization of molecules in helium nanodroplets is known to preferentially yield structures of higher energy than the global energy minimum structure for a number of quite different monomers. Here, we explore dimerization in this environment using an atomistic model within statistically converged molecular dynamics (MD) trajectories, treating the solvent implicitly through the use of a thermostat, or more explicitly by embedding one monomer in a He100 cluster. The focus is on the two simplest carboxylic acids, formic and acetic, both of which have been studied experimentally. While the global minimum structure, which comprises two CO⋯HO hydrogen bonds, is predicted to be the most abundant dimer in the absence of the helium solvent, this is no longer the case once helium atoms are included. The simulations confirm the importance of kinetic trapping effects and also shed light on the occurrence of specific dynamical effects, leading to the occasional formation of high-energy structures away from minima, such as saddle configurations. Theoretically predicted infrared spectra, based on the MD statistics, are in good agreement with the experimental spectra.

A Study of C=O…HO and OH…OH (Dimer, Trimer, and Oligomer) Hydrogen Bonding in a Poly(4-vinylphenol) 30%/Poly(methyl methacrylate) 70% Blend and its Thermal Behavior Using Near-Infrared Spectroscopy and Infrared Spectroscopy.

Inter- and intramolecular hydrogen bonding and their temperature-dependent changes in a poly(4-vinylphenol)/poly(methyl methacrylate)(PVPh 30%/PMMA 70%) blend were investigated using near-infrared (NIR) and infrared (IR) spectroscopy. Band assignments of the fundamentals and first overtones of the OH stretching mode of a free OH group and OH groups in C=O···HO and OH···OH (dimer, trimer, and oligomer) hydrogen bonding of PVPh 30%/PMMA 70% were carried out by comparison between its NIR and IR spectra and comparison with NIR and IR spectra of phenol. The comparison of the NIR spectra of the PVPh 30%/PMMA 70% blend (hereafter, we denote it as PVPh30%) with the corresponding IR spectra reveals that to observe bands arising from the free OH and OH···OH dimer, which is a weaker hydrogen bonding, NIR is better while to investigate bands originating from OH groups in the OH···O=C and OH···OH (oligomer) hydrogen bonds, which are stronger hydrogen bonding, IR is better. Thus, a combination of IR and NIR spectroscopy has provided convincing results for the hydrogen bonding of PVPh30%. The relative intensity of the two bands at 7058 and 6921 cm-1 (I7058/I6921) due to the first overtones of the OH stretching modes of the free OH group and the OH group in the dimer, respectively, increases significantly above 90 °C, which is close to Tg of PVPh. In concomitance with the intensity increase in the relative intensity of the free OH band, the intensity of a band at 1706 cm-1 due to the C=O stretching mode of the C=O···HO hydrogen bond of PVPh30% decreases above 90°C. These results suggest that above the Tg of PVPh the C=O···HO hydrogen bond is broken gradually and that the free OH increases. Of note is that below Tg the intensities of NIR bands due to the OH first overtones of free OH group and OH groups in the OH···OH dimer gain intensity in parallel with temperature increase, and above Tg the intensity of the band derived from the OH···OH group increases linearly much slower than that of the band due to the free OH. Moreover, a band due to an OH···OH oligomer decreases linearly. Hence, it is very likely that the OH···OH oligomers dissociate into free OH groups. Anharmonicity of O-H bonds, which is sensitive to a hydrogen bond, was estimated for the free OH and OH bonds in the C=O···HO and OH···OH (dimer, trimer, and oligomer) hydrogen bonding by comparison between the NIR and IR spectra in the OH stretching band regions.

Two types of C[dbnd]O⋯HO hydrogen bonds and OH⋯OH (dimer, trimer, oligomer) hydrogen bonds in PVA with 88% saponification/PMMA and PVA with 99% saponification/PMMA blends and their thermal behavior studied by infrared spectroscopy

In this study, polyvinyl alcohol (PVA) with 88% and 99% degrees of saponification was blended with polymethyl methacrylate (PMMA) to investigate the intramolecular hydrogen bonding in PVA and the intermolecular hydrogen bonding between PVA and PMMA. Differential scanning calorimetry (DSC) results confirmed the miscibility of the PVA/PMMA blends. Temperature-dependent ATR-IR spectra revealed that in PVA with an 88% saponification degree, some CO groups were free while the others were involved in CO···HO hydrogen bonding. In the blends with 88% saponified PVA, two types of CO···HO hydrogen bonds formed: CO(PVA)···HO(PVA) and CO(PMMA)···HO(PVA). Since the glass transition temperature of PVA is lower than that of PMMA, CO(PVA)···HO(PVA) can be broken at a lower temperature than CO(PMMA)···HO(PVA). In the blends with 99% saponified PVA, peaks derived from the CO stretching modes of CO(PMMA)···HO(PVA) were observed for PVA:PMMA = 60:40 and 40:60. In the OH stretching region of PVA and the blends, a broad band was observed in the 3700–3000 cm−1 region, where in OH bands arising from OH···OH (dimer, trimer, oligomer) hydrogen bonds and from the two types of CO···HO hydrogen bonds were expected to appear. In the PVA/PMMA blends with 88% saponified PVA, two types of CO⋯HO hydrogen bonds, CO(PVA)···HO(PVA) and CO(PMMA)···HO(PVA), yielded an OH stretching band at around 3370 cm−1. By contrast, the PVA/PMMA blends with 99% saponified PVA showed only a band corresponding to CO(PMMA)···HO(PVA) hydrogen bonding.

Zirconium and Aluminum MOFs for Low-Pressure SO2 Adsorption and Potential Separation: Elucidating the Effect of Small Pores and NH2 Groups.

Finding new adsorbents for the desulfurization of flue gases is a challenging task but is of current interest, as even low SO2 emissions impair the environment and health. Four Zr- and eight Al-MOFs (Zr-Fum, DUT-67(Zr), NU-1000, MOF-808, Al-Fum, MIL-53(Al), NH2-MIL-53(Al), MIL-53(tdc)(Al), CAU-10-H, MIL-96(Al), MIL-100(Al), NH2-MIL-101(Al)) were examined toward their SO2 sorption capability. Pore sizes in the range of about 4-8 Å are optimal for SO2 uptake in the low-pressure range (up to 0.1 bar). Pore widths that are only slightly larger than the kinetic diameter of 4.1 Å of the SO2 molecules allow for multi-side-dispersive interactions, which translate into high affinity at low pressure. Frameworks NH2-MIL-53(Al) and NH2-MIL-101(Al) with an NH2-group at the linker tend to show enhanced SO2 affinity. Moreover, from single-gas adsorption isotherms, ideal adsorbed solution theory (IAST) selectivities toward binary SO2/CO2 gas mixtures were determined with selectivity values between 35 and 53 at a molar fraction of 0.01 SO2 (10.000 ppm) and 1 bar for the frameworks Zr-Fum, MOF-808, NH2-MIL-53(Al), and Al-Fum. Stability tests with exposure to dry SO2 during ≤10 h and humid SO2 during 5 h showed full retention of crystallinity and porosity for Zr-Fum and DUT-67(Zr). However, NU-1000, MOF-808, Al-Fum, MIL-53(tdc), CAU-10-H, and MIL-100(Al) exhibited ≥50-90% retained Brunauer-Emmett-Teller (BET)-surface area and pore volume; while NH2-MIL-100(Al) and MIL-96(Al) demonstrated a major loss of porosity under dry SO2 and MIL-53(Al) and NH2-MIL-53(Al) under humid SO2. SO2 binding sites were revealed by density functional theory (DFT) simulation calculations with adsorption energies of -40 to -50 kJ·mol-1 for Zr-Fum and Al-Fum and even above -50 kJ·mol-1 for NH2-MIL-53(Al), in agreement with the isosteric heat of adsorption near zero coverage (ΔHads0). The predominant, highest binding energy noncovalent binding modes in both Zr-Fum and Al-Fum feature μ-OHδ+···δ-OSO hydrogen bonding interactions. The small pores of Al-Fum allow the interaction of two μ-OH bridges from opposite pore walls with the same SO2 molecule via OHδ+···δ-OSOδ-···δ+HO hydrogen bonds. For NH2-MIL-53(Al), the DFT high-energy binding sites involve NHδ+···δ-OS together with the also present Al-μ-OHδ+···δ-OS hydrogen bonding interactions and C6-πδ-···δ+SO2, Nδ-···δ+SO2 interactions.

Photoelectron Spectroscopy and Theoretical Study on Monosolvated Cyanate Analogue Clusters ECX-·Sol (ECX- = NCSe-, AsCSe-, and AsCS-; Sol = H2O, CH3CN).

Six monosolvated cyanate analogue clusters ECX-·Sol (ECX- = NCSe-, AsCSe-, and AsCS-; Sol = H2O and CH3CN) were investigated using negative ion photoelectron spectroscopy (NIPES). NIPES experiments show that these clusters possess similar spectra overall compared to their respective isolated ECX- anions but shift to higher electron binding energy with CH3CN solvent, stabilizing the excess electrons slightly more than H2O. For the ECX-·H2O series, vertical detachment energies and their increments relative to the bare species are measured to be 3.700/0.370, 3.085/0.415, and 3.085/0.430 eV for NCSe-, AsCSe- and AsCS-, respectively, while the corresponding values in the ECX-·CH3CN series are 3.835/0.505, 3.145/0.475, and 3.135/0.480 eV. Ab initio electronic structure calculations indicate that the excess charges were located at the terminal N and Se atoms in NCSe- and migrated to the central C atom in AsCSe- and AsCS-. For NCSe-, the solvation is driven by the interactions with the two negatively charged terminal ends, while for AsCSe- and AsCS-, the solvation revolves around the interactions with the central C atom, where all the excess negative charge is concentrated. Two nearly degenerate isomers for NCSe-·H2O are identified, one forming a single strong N···H-O hydrogen bond (HB) and the other featuring a bidentate HB with two hydroxyl H atoms pointing to N and Se ends. In contrast, the negative central C atom in AsCSe-/AsCS- allows the formation of a bifurcated HB with H2O. Similar effects are observed for the acetonitrile case, in which the three H atoms of the methyl group interact with the two negatively charged terminal ends in NCSe-, while preferring to bind to the central negative carbon atom in AsCSe-/AsCS-. The different binding motifs derived in this work may suggest different solvation properties in NCSe- versus AsCSe-/AsCS- with the former anion leading to asymmetric solvation at the N end of the solute, while the latter species creates more "isotropic" solvation around the central C equatorial plane.

Deterioration of hydrogen-bonded superprotonic conductors belonging to CsHSO4–CsH2PO4–H2O salt system: a single-crystal neutron diffraction investigation

Single-crystal neutron diffraction investigation on Cs4(HSO4)3(H2PO4) and Cs6H(HSO4)3(H2PO4)4 superprotonic crystals, belonging to CsHSO4–CsH2PO4–H2O salt system, is under taken to elucidate the precise hydrogen atom positions in these crystals. The investigation revealed that these crystals are very sensitive to the ambient conditions and can undergo deterioration due to fluctuation in air moisture content. Cs6H(HSO4)3(H2PO4)4 crystals are more stable as compared to Cs4(HSO4)3(H2PO4). Crystal structure of Cs6H(HSO4)3(H2PO4)4 is obtained both before and after deterioration, it is found that the asymmetric O–HO hydrogen bond between the PO4 and SO4 ions of this crystal becomes stronger after deterioration. This led to the shrinkage of the unit cell, and most likely prevented further deterioration. Diabatic state model for hydrogen bonds is used to obtain the energy contour for the O–H···O hydrogen bond of Cs6H(HSO4)3(H2PO4)4 crystal. The influence of this change in the hydrogen bonding on the proton conduction ability of the crystal is discussed.

Interaction mechanism of calcite and four representative organic molecules: Experiments and DFT study

Calcite crystal-organic molecule interaction governs the wettability formation and alteration of carbonate reservoirs, and influences the oil recovery. However, its in-depth interaction mechanism in atomic scale have not been classified clearly. Regarding this, this study combines the atomic force microscopy (AFM), fourier transform infrared spectrometer (FTIR), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) to systemically investigate the adsorption regularities and mechanisms of four representative organic molecules of crude oil on calcite surface. AFM results illustrated that the adsorption amount of the four organic molecules on calcite surface is in the order of benzoic acid>pyridine>thiophene>toluene. FTIR and XPS results explicated that the COOR and OH functional groups are formed in the benzoic acid adsorption; the Ca of calcite surface and N of pyridine are involved in the interaction, and OH functional groups are also formed in pyridine adsorption; only the OH bond is formed in thiophene adsorption, and no bond is formed in toluene adsorption. DFT results verified the experiments results and further revealed the interaction mechanism in atomic scale. Firstly, DFT results showed that the adsorption energies of benzoic acid, pyridine, thiophene and toluene on calcite surface are −1.39 eV, −0.97 eV, −0.49 eV and −0.31 eV, respectively. Secondly, DFT results explicated that the Ca-O ionic bond and HO covalent bond are formed in benzoic acid-calcite interaction; the Ca-N ionic bond and HO hydrogen bond are formed in calcite-pyridine interaction; only the HO hydrogen bond is formed in calcite-thiophene interaction; No chemical bond is formed in calcite-toluene interaction.

Structural, spectroscopic and thermal characterizations combined with DFT calculations and Hirshfeld analysis of a novel hydrogen-bonded complex: p-phenylenediammonium dinitrate

The compound p-phenylenediammonium dinitrate (PPDN) has been synthesized and its structure characterized by FT-IR, Raman and single crystal XRD techniques. Thermal behavior of this compound has been studied by TGA/DSC analysis. The three-dimensional framework observed in the PPDN structure is mainly maintained thanks to conventional N–HO hydrogen bonds and weak C–H…O contacts. In the p-phenylenediammonium cation, four carbon atoms of the aromatic ring are disordered over two positions with half occupancy. The geometry, modes of vibrations, atomic charges, thermodynamic parameters and frontier molecular orbitals of the PPDN molecule have been calculated by DFT method at B3LYP/6–311++G(d,p) and B3LYP/cc–pVTZ levels. The intermolecular bonding interactions within this molecule have been investigated using the natural bonding orbital (NBO) analysis at B3LYP/cc–pVTZ level. The 3-D Hirshfeld surface and the associated 2-D fingerprint plot, calculated for the PPDN molecule, have showed the clearly dominance of the H…O/O…H and H…H contacts which together represent more than 80% of the total surface.

IR Spectra of Hydrogen-Bonded Complexes of Trifluoroacetic Acid with Acetone and Diethyl Ether in the Gas Phase. Interaction between CH and OH Stretching Vibrations.

The spectra of complexes of trifluoroacetic acid (TFA) with acetone and diethyl ether (DEE) and their perdeuterated isotopologues were extracted from the spectra of the mixture of the compounds recorded at room temperature. The ν(OH) bands of the complexes with protiated and deuterated acetone notably differ from each other, whereas these ν(OH) bands are practically not affected by the deuteration of DEE. An assumption about the interaction of CH and OH groups in the (CH3)-C═O···HO fragment is made. According to density functional theory calculations, complexes of TFA with both acetone and DEE have a cyclic structure with one strong ═O···HO hydrogen bond and one weak CH···O═ bond. The structural, spectroscopic, and electronic properties indicate an essential role of weak bonds in the total complexation energy of the systems studied.

Cooperative hydrogen bond between piperidine-ethanol and 2,6-dichloro-4- nitrophenol

2-(1-Piperidine)ethanol (PEt) and 2,6-dichloro-4-nitrophenol (DCNP) form 1:1 and 1:2 hydrogen-bonded complexes. Their molecular structures and spectroscopic properties have been characterized by X-ray diffraction, the B3LYP/6-311++G(d,p) calculations, Raman, FTIR and UV–vis spectroscopy. In the 1:1 complex, piperidinium-ethanol cation and DCNP− anion form a centrosymmetric dimer through two NH⋯O and OH⋯O hydrogen bonds of 2.735(1) and 2.588(1) Å, respectively. Two independent PEt·(DCNP)2 molecules (A and B) exist in the 1:2 complex. In each of them the PEt+H cation, neutral DCNP and DCNP− anion molecules interact through the cooperative N+H⋯O−⋯HO⋯HO hydrogen bond. The experimental FTIR spectra of the 1:1 and 1:2 complexes show a broad absorption in the 3570-2200 cm−1 region, while the computed IR spectra of these complexes show significant differences in the above range. However, the experimental Raman and FTIR, and computed IR spectra of the 1:1 and 1:2 complexes show a great similarity in the fingerprint region. The UV–vis spectra of both complexes in ethanol show the absorbance band at ca. 395 nm, suggesting the presence of the DCNP− anion. Diagrams of the HOMO and LUMO of molecules are presented. The Mulliken atomic charges for atoms participated in the cooperative hydrogen bonds were given.

Mass density, sound velocity, mixing enthalpy, 1H NMR, Ab initio calculations and intermolecular interactions in binary mixtures of N-methylimidazole + water, +methanol, +ethanol, +1-propanol, +2-propanol

Over a full molar fraction range, for binary mixtures of x N-methylimidazole (hereafter abbreviated to N-mim) + (1 − x) water, +methanol, +ethanol, +1-propanol, +2-propanol, the mass density and sound velocity at (288.15, 298.15, 308.15, 318.15, 328.15) K, the mixing enthalpy at 298.15 K, and 1H NMR measurements, combined with the Ab initio calculations, have been carried out.The excess molar volume, excess average volume expansivity, excess sound velocity, excess isentropic compressibility, and the molar mixing enthalpy were calculated and fitted to a Redlich-Kister equation. The partial molar enthalpy of N-mim, the difference between the partial molar volume and the molar volume of N-mim, and the protons chemical shift changes were calculated and compared. The chemical shifts of N-mim protons, hydroxyl protons, and α-H (proton on the carbon atom next to the –OH group) of alcohols in the mixtures containing alcohols were explained by a model proposed by Gupta et al..The properties indicate that the intermolecular interactions between unlike molecules are stronger than those between like molecules. An increase in temperature has a greater influence on the mixture containing water than on pure components, and has a greater influence on pure components than on mixtures containing alcohols. The predominant interactions between unlike molecules are hydrogen-bonded complexes in the mixture containing water, and are N-mim: alcohols = 1:1 hydrogen-bonded networks in the mixtures containing alcohols, where the alkyl groups of alcohols are electron-withdrawing groups, and the methyl group of N-mim is an electron-donating group. Ab initio calculations proved that the N⋯HO hydrogen bond in the mixtures is more stable and stronger than HO⋯HO in pure water or in pure alcohols and N⋯HC in pure N-mim. The strength sequence of the intermolecular interactions between unlike molecules for the mixtures at 298.15 K is N-mim–water > N-mim–methanol > N-mim–ethanol > N-mim–1-propanol ≥ N-mim–2-propanol.

Understanding the role of ethylene glycol in a remarkable catalyst-free Strecker reaction of a-CF3 ketoimine: A theoretical study

Density functional theory (DFT) calculations have been performed to understand the role of Ethylene Glycol (EG) in a remarkable catalyst-free Strecker reaction of α-CF3 ketoimine. The fluorine hydrogen bonds involved in titled reaction have been studied. To find the suitable calculation methods for the weak CF⋯H interaction, we chose seventeen density functionals and MP2 method for calculation of the structures of three typical CF⋯H hydrogen-bonded systems. Their structures were computed and compared with the available experimental data. The results show that the M05-2X and M06-2X functionals are better than the other methods. Based on these facts, the interactions between the α-CF3 ketoimine and EG were investigated at the M06-2X/6-31G(d,p) level. And the results indicate the presence of strong CF⋯HO hydrogen bond in the complexes between α-CF3 ketoimine and solvent EG. Moreover, we also studied the reaction mechanism of the titled reaction with or without the assistance of EG at the M06-2X/6-31G(d,p) level. The results show that the two possible pathways without the aid of EG have very high free energy barriers and the reaction can't proceed, which is consistent with the experiment fact. However, with the assistance of EG, the reaction activation free energy barrier can decrease greatly. For the two-EGs model with two EG molecules, the reaction free energy barrier decreases from 47.5 kcal mol−1 to 24.7 kcal mol−1. The two EGs play an important role to stabilize the transition state and decrease the activation barrier greatly. The present theoretical calculations imply the role of solvent EG in the titled interesting catalyst-free Strecker reaction of α-CF3 ketoimine. Our theoretical calculations provide a satisfactory explanation for the experiment.

The Role of Hydrogen Bond in the Mechanism of Autocatalytic Reaction between Acetic Anhydride and tert-Butyl Hydroperoxide

The role of hydrogen bonding in the autocatalytic reaction of acetic anhydride with tert-butyl hydroperoxide in CC14 has been elucidated. The stage-by-stage mechanisms of formation of hydrogen-bonded complexes. Structure and energy parameters of stable conformers of the compounds and energy of the major and transition states have been determined by means of B3LYP/aug-cc-pVDZ and B3LYP/aug-cc-pVTZ methods. The contribution of the O···HO hydrogen bonds in the stabilization of the complexes has been determined using the noncovalent interactions index. 1-Hydroxy-1-alkylperoxy compound is the most stable intermediate.

Experimental and DFT simulation study of a novel felodipine cocrystal: Characterization, dissolving properties and thermal decomposition kinetics

In this study, a new cocrystal of felodipine (Fel) and glutaric acid (Glu) with a high dissolution rate was developed using the solvent ultrasonic method. The prepared cocrystal was characterized using X-ray powder diffraction, differential scanning calorimetry, thermogravimetric (TG) analysis, and infrared (IR) spectroscopy. To provide basic information about the optimization of pharmaceutical preparations of Fel-based cocrystals, this work investigated the thermal decomposition kinetics of the Fel–Glu cocrystal through non-isothermal thermogravimetry. Density functional theory (DFT) simulations were also performed on the Fel monomer and the trimolecular cocrystal compound for exploring the mechanisms underlying hydrogen bonding formation and thermal decomposition. Combined results of IR spectroscopy and DFT simulation verified that the Fel–Glu cocrystal formed via the NH⋯OC and CO⋯HO hydrogen bonds between Fel and Glu at the ratio of 1:2. The TG/derivative TG curves indicated that the thermal decomposition of the Fel–Glu cocrystal underwent a two-step process. The apparent activation energy (Ea) and pre-exponential factor (A) of the thermal decomposition for the first stage were 84.90 kJ mol−1 and 7.03 × 107 min−1, respectively. The mechanism underlying thermal decomposition possibly involved nucleation and growth, with the integral mechanism function G(α) of α3/2. DFT calculation revealed that the hydrogen bonding between Fel and Glu weakened the terminal methoxyl, methyl, and ethyl groups in the Fel molecule. As a result, these groups were lost along with the Glu molecule in the first thermal decomposition. In conclusion, the formed cocrystal exhibited different thermal decomposition kinetics and showed different Ea, A, and shelf life from the intact active pharmaceutical ingredient.