Hydrogen-bonded Adducts Research Articles

A Triplet/Singlet Ground-State Switch via the Steric Inhibition of Conjugation in 4,6-Bis(trifluoromethyl)-1,3-phenylene Bisnitroxide.

Ground triplet 4,6-bis(trifluoromethyl)-1,3-phenylene bis(tert-butyl nitroxide) (TF2PBN) reacted with [Y(hfac)3(H2O)2] (hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dionate), affording a doubly hydrogen-bonded adduct [Y(hfac)3(H2O)2(TF2PBN)]. The biradical was recovered from the adduct through recrystallization. Crystallographic analysis indicates that the torsion angles (|θ| ≤ 90°) between the benzene ring and nitroxide groups were 74.9 and 84.8° in the adduct, which are larger than those of the starting material TF2PBN. Steric congestion due to o-trifluoromethyl groups gives rise to the reduction of π-conjugation. Two hydrogen bonds enhance this deformation. Susceptometry of the adduct indicates a ground singlet with 2J/kB = -128(2) K, where 2J corresponds to the singlet-triplet gap. The observed magneto-structure relation is qualitatively consistent with Rajca's pioneering work. A density functional theory calculation at the UB3LYP/6-311+G(2d,p) level using the atomic coordinates determined provided a result of 2J/kB = -162.3 K for the adduct, whilst the corresponding calculation on intact TF2PBN provided +87.2 K. After a comparison among a few known compounds, the 2J vs. |θ| plot shows a negative slope with a critical torsion of 65(3)°. The ferro- and antiferromagnetic coupling contributions are balanced in TF2PBN, being responsible for ground-state interconversion by means of small structural perturbation like hydrogen bonds.

Mechanistic Studies on the Bismuth-Catalyzed Transfer Hydrogenation of Azoarenes.

Organobismuth-catalyzed transfer hydrogenation has recently been disclosed as an example of low-valent Bi redox catalysis. However, its mechanistic details have remained speculative. Herein, we report experimental and computational studies that provide mechanistic insights into a Bi-catalyzed transfer hydrogenation of azoarenes using p-trifluoromethylphenol (4) and pinacolborane (5) as hydrogen sources. A kinetic analysis elucidated the rate orders in all components in the catalytic reaction and determined that 1 a (2,6-bis[N-(tert-butyl)iminomethyl]phenylbismuth) is the resting state. In the transfer hydrogenation of azobenzene using 1 a and 4, an equilibrium between 1 a and 1 a ⋅ [OAr]2 (Ar=p-CF3 -C6 H4 ) is observed, and its thermodynamic parameters are established through variable-temperature NMR studies. Additionally, pKa -gated reactivity is observed, validating the proton-coupled nature of the transformation. The ensuing 1 a ⋅ [OAr]2 is crystallographically characterized, and shown to be rapidly reduced to 1 a in the presence of 5. DFT calculations indicate a rate-limiting transition state in which the initial N-H bond is formed via concerted proton transfer upon nucleophilic addition of 1 a to a hydrogen-bonded adduct of azobenzene and 4. These studies guided the discovery of a second-generation Bi catalyst, the rate-limiting transition state of which is lower in energy, leading to catalytic transfer hydrogenation at lower catalyst loadings and at cryogenic temperature.

Oxo-Stabilised Phosphonium Ylides as Hydrogen Bond Acceptors

Oxo-stabilised phosphonium ylides are found to form crystalline hydrogen-bonded adducts with aromatic carboxylic acids, as confirmed by X-ray diffraction. There is also strong hydrogen bonding in solution as indicated by 13C NMR spectroscopy and this confirmed adduct formation with acetic acid, benzamide, thiobenzamide, benzyl alcohol, benzenesulfinic acid and diphenylphosphinic acid. The X-ray structure of the benzamide adduct was also determined, showing a hydrogen-bonded dimeric structure. A bis(stabilised ylide) was also prepared and is found to form a complex hydrogen-bonded adduct with benzoic acid, ethanol and water.

Real-Time In Situ Volatile Organic Compound Sensing by a Dual-Emissive Polynuclear Ln-MOF with Pronounced LnIII Luminescence Response.

Lanthanide metal-organic frameworks (Ln-MOFs) are promising for luminescence detection of volatile organic compound (VOC) vapors, but usually suffer from the silent or quenched Ln3+ emission. Herein, we report a new dual-emissive Eu-MOF composed of the coordinatively unsaturated Eu9 clusters that afford abundant open metal sites to form a confined "binding pocket" to facilitate the preconcentration and recognition of VOCs. Single-crystal structural analyses reveal that specific analytes can replace the OH oscillators in the first coordination sphere of Eu3+ and form a unique hydrogen-bonding second-sphere adduct tying adjacent Eu9 clusters together to minimize their nonradiative vibrational decay. With the promoted Eu3+ luminescence, the MOF realizes real-time in situ visual sensing of THF vapor (<1 s) and shows a quantitative ratiometric response to the vapor pressure with a limit of detection down to 17.33 Pa. Also, it represents a top-performing ratiometric luminescent thermometer.

Electrocatalytic hydrogen evolution reaction by a Ni(N2O2) complex based on 2,2′-bipyridine

A Ni complex containing a N2O2 ligand is active for the hydrogen evolution reaction. Mechanistic studies suggest the formation of hydrogen-bonded adducts facilitates hydride formation and near barrierless hydride protonation.

Crystal and molecular structure of Michler’s ketone as a pure phase

The pure phase crystals of Michler’s ketone, [4-(CH3)2NC6H4]2CO, are reported herein as monoclinic in the space group P21/c with Z’ = 2 by single-crystal X-ray diffraction. The data collection of the title compound proved quite difficult, which could explain why the structure has not been reported previously. The crystallographically independent molecules in the asymmetric unit differ by dihedral angles of 24.60/30.34° and 25.25/27.20° between the essentially planar 4-dimethylaminophenyl (DMAP) rings and the central C2C=O plane of the ketone and dihedral angles of 52.14 and 47.41°, respectively, between the DMAP planes. A DMAP ring of each molecule overlaps in the lattice with a methyl ring that overlays the ring centroid of the opposite number (CMe-Cg distances of 3.452 and 3.505 Å), describing sets of trifurcated CAr-acceptor hydrogen bond interactions (HBIs). The trifurcated HBIs possess H∙∙∙CAr distances of 2.90, 2.75, and 2.86 Å; CMe∙∙∙Car distances of 3.554(5), 3.798(5) and 3.881(5) Å; and ÐCMe-H∙∙∙CAr of 115.0, 121.3 and 167.7°, respectively. Stabilization of the asymmetric unit in the title compound is further achieved by supramolecular synthons such as CMe-H∙∙∙CAr (2.831 Å), CAr-H∙∙∙O (2.561 Å) and CAr-H∙∙∙H-CAr (2.290 Å) and coulombic CMe-Ocontact (3.209 Å). The structures of the two independent molecules in the asymmetric unit of the title compound were compared with the same molecule as found in the following co-crystals, adducts or salts: a halogen-bonded adduct with 1,4-diiodotetrafluorobenzene; a hydrogen-bonded adduct with a fluorinated phenol; a coordination complex between the ketone and a trimeric mercury acceptor; and two salts from protonation or methylation at one of the amino nitrogen atoms.

Transformation of Thioacids into Carboxylic Acids via a Visible-Light-Promoted Atomic Substitution Process.

A visible-light-promoted atomic substitution reaction for transforming thiocacids into carboxylic acids with dimethyl sulfoxide (DMSO) as the oxygen source has been developed, affording various alkyl and aryl carboxylic acids in over 90% yields. The atomic substitution process proceeds smoothly through the photochemical reactivity of the formed hydrogen-bonding adduct between thioacids and DMSO. A DMSO-involved proton-coupled electron transfer (PCET) and the simultaneous generation of thiyl and hydroxyl radicals are proposed to be key steps for realizing the transformation.

Low-temperature oxidation of isopropylbenzene mediated by the system of NHPI, Fe(acac)3 and 1,10-phenanthroline

Highly efficient oxidation of isopropylbenzene mediated by the system of NHPI/Fe(acac)3/Phen has been carried out at temperature as low as 60 °C. Significant improvement of catalysis by NHPI was associated with an enhanced oxidizing ability of Fe(III) tandem with Phen, which caused the intense generation of PINO. Furthermore, NMR observations revealed formation of a hydrogen-bonded NHPI-Phen adduct soluble in acetonitrile and isopropylbenzene. Based on this phenomenon, the system was applicable for the oxidation of solvent-free isopropylbenzene. The promise of the system of NHPI/Fe(acac)3/Phen for the selective synthesis of isopropylbenzene hydroperoxide was demonstrated by oxidation at a low content of Fe(acac)3.

Oximes of 3,7-dimethylocta-2,6-dienal: Green synthesis, preparative separation of all diastereomers and complete assignment of 1H and 13C NMR spectra

A mixture of four diastereomeric oximes was prepared starting from citral, an inexpensive mixture of neral and geranial ((Z)- and (E)-3,7-dimethylocta-2,6-dienal, respectively), in a solvent-free mechano-chemical reaction with NH2OH•HCl/NaOH, at room temperature. All stereoisomeric oximes ((1E,2E), (1Z,2E), (1E,2Z), and (1Z,2Z)) were isolated in pure state by isocratic silica-gel column chromatography. For the first time, the oximes were separated and individually characterized by 1D- and 2D-NMR spectroscopy in two deuterated solvents. Their spectra were fully assigned and mutually compared. Coupling constant values of higher-order multiplets belonging to the –CH2CH2- moiety were determined from (iterative) spin simulations. A careful analysis of the NOESY spectra of all diastereomers revealed that the oximes formed hydrogen-bonded adducts in CDCl3. Under the oximation conditions, the configuration of the C2-C3 double bond in the conjugated enals did not change. Prolonged standing of pure oximes of citral in either CDCl3 or DMSO‑d6 led to isomerization of the CN bond while keeping the conjugated CC bond configuration unaltered. The equilibrium ratio of the anti- to syn-oximes in solution corresponded to their ratio in the original reaction mixture. The conditions of GC were sufficient to cause dehydration of the oximes to geranyl or neryl nitriles, and/or isomerization to occur in the heated areas of the instrument, giving more complex chromatograms than expected and being dependent on the identity of the analyzed oxime.

Bifurcated Hydrogen-Bond-Stabilized Boron Analogues of Carboxylic Acids.

The reactivity of a bulky m-terphenylboronic acid, DmpB(OH)2 [1; Dmp = 2,6-bis(2,4,6-trimethylphenyl)phenyl], toward three different N-heterocyclic carbenes has been examined. The reaction of 1 with 1 equiv of bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) leads to the formation of a hydrogen-bonded carbene boronic acid adduct, 2, featuring strong O-H···C contacts. In contrast, more basic 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (IPr2Me2) and 1,3-di-tert-butylimidazol-2-ylidene (ItBu) deprotonate 1 smoothly to afford the rare anionic boranuidacarboxylic acids 3 and 4, respectively. Structural determination reveals that 3 and 4 bear unprecedented bifurcated hydrogen bonds with a BO- unit as a double hydrogen-bond acceptor, which contribute significantly to stabilization of the highly reactive B═O double bond. Quantum-mechanical calculations were conducted to disclose the unique electronic properties of the multiple bonds, as well as the important hydrogen bonds in these compounds.

Temperature Dependence Study of the Kinetics and Product Yields of the HO2 + CH3C(O)O2 Reaction by Direct Detection of OH and HO2 Radicals Using 2f-IR Wavelength Modulation Spectroscopy.

The HO2 + CH3C(O)O2 reaction consists of three product channels: CH3C(O)OOH + O2 (R1a), CH3C(O)OH + O3 (R1b), and OH + CH3C(O)O + O2 (R1c). The overall rate constant ( k1) and product yields (α1a, α1b, and α1c) were determined over the atmospherically relevant temperature range of 230-294 K at 100 Torr in N2. Time-resolved kinetics measurements were performed in a pulsed laser photolysis experiment in a slow flow cell by employing simultaneous infrared (IR) and ultraviolet (UV) absorption spectroscopy. HO2 and CH3C(O)O2 were formed by Cl-atom reactions with CH3OH and CH3CHO, respectively. Heterodyne near- and mid-infrared (NIR and MIR) wavelength modulation spectroscopy (WMS) was employed to selectively detect HO2 and OH radicals. Ultraviolet absorption at 225 and 250 nm was used to detect various peroxy radicals as well as ozone (O3). These experimental techniques enabled direct measurements of α1c and α1b via time-resolved spectroscopic detection in the MIR and the UV, respectively. At each temperature, experiments were performed at various ratios of initial HO2 and CH3C(O)O2 concentrations to quantify the secondary chemistry. The Arrhenius expression was found to be k1( T) = 1.38-0.63+1.17 × 10-12 exp[(730 ± 170)/ T] cm3 molecule-1 s-1. α1a was temperature-independent while α1b and α1c decreased and increased, respectively, with increasing temperatures. These trends are consistent with the current recommendation by the IUPAC data evaluation. Hydrogen-bonded adducts of HO2 with the precursors, HO2·CH3OH and HO2·CH3CHO, played a role at lower temperatures; as part of this work, rate enhancements of the HO2 self-reaction due to reactions of the adducts with HO2 were also measured.

Dioxygen/Hydrogen Peroxide Interconversion Using Redox Couples of Saddle-Distorted Porphyrins and Isophlorins.

Interconversion between dioxygen (O2) and hydrogen peroxide (H2O2) has attracted much interest because of the growing importance of H2O2 as an energy source. There are many reports on O2 conversions to H2O2; however, no example has been reported on O2/H2O2 interconversion. Herein, we describe successful achievement of a reversible O2/H2O2 conversion based on an N21, N23-dimethylated saddle-distorted porphyrin and the corresponding two-electron-reduced porphyrin (isophlorin) for the first time. The isophlorin could react with O2 to afford the corresponding porphyrin and H2O2; conversely, the porphyrin also reacted with excess H2O2 to reproduce the corresponding isophlorin and O2. The isophlorin-O2/porphyrin-H2O2 interconversion was repeatedly proceeded by alternate bubbling of Ar or O2, although no reversible conversion was observed in the case of an N21, N22-dimethylated porphyrin as a structural isomer. Such a drastic change of the reversibility was derived from the directions of inner N H protons in hydrogen-bond formation of the isophlorin core with O2 as well as those of the lone pairs of the inner nitrogen atoms of the porphyrin core to form hydrogen bonds with H2O2. The intriguing isophlorin-O2/porphyrin-H2O2 interconversion was accomplished by introducing methyl groups at the inner nitrogen atoms to minimize the difference of the Gibbs free energy between isophlorin-O2/porphyrin-H2O2 states and the Gibbs activation energy of the interconversion. On the basis of the kinetic and thermodynamic analysis on the isophlorin-O2/porphyrin-H2O2 interconversion using 1H NMR and UV-vis spectroscopies and DFT calculations, we propose the formation of a two-point hydrogen-bonding adduct between the N21, N23-dimethylated porphyrin and H2O2 as an intermediate.

Experimental and Computational Studies of Unimolecular 1,1-HX (X = F, Cl) Elimination Reactions of C2D5CHFCl: Role of Carbene:HF and HCl Adducts in the Exit Channel of RCHFCl and RCHCl2 Reactions.

The gas-phase unimolecular reactions of C2D5CHFCl molecules with 94 kcal mol-1 of vibrational energy have been studied by the chemical-activation experimental technique and by electronic-structure computations. Products from the reaction of C2D5CHFCl molecules, formed by the recombination of C2D5 and CHFCl radicals in a room temperature bath gas, were measured by gas chromatography-mass spectrometry. The 2,1-DCl (81%) and 1,1-HCl (17%) elimination reactions are the principal processes, but 2,1-DF and 1,1-HF elimination reactions also are observed. Comparison of experimental rate constants to calculated statistical rate constants provides threshold energies. The potential surfaces associated with C2D5(F)C: + HCl and C2D5(Cl)C: + HF reactions are of special interest because hydrogen-bonded adducts with HCl and HF with dissociation energies of 6.4 and 9.3 kcal mol-1, respectively, are predicted by calculations. The relationship between the geometries and threshold energies of transition states for 1,1-HCl elimination and carbene:HCl adducts is complex, and previous studies of related molecules, such as CD3CHFCl, CD2ClCHFCl, C2D5CHCl2, and halogenated methanes are included in the computational analysis. Extensive calculations for CH3CHFCl as a model for 1,1-HCl reactions illustrate properties of the exit-channel potential energy surface. Since the 1,1-HCl transition state is submerged relative to dissociation of the adduct, inner and outer transition states should be considered for analysis of rate constants describing 1,1-HCl elimination and addition reactions of carbenes to HCl.

Protonation Studies of Molybdenum(VI) Nitride Complexes That Contain the [2,6-(ArNCH2)2NC5H3]2- Ligand (Ar = 2,6-Diisopropylphenyl).

[Ar2N3]Mo(N)(O- t-Bu) (1), which contains the conformationally rigid pyridine-based diamido ligand [2,6-(ArNCH2)2NC5H3]2- (Ar = 2,6-diisopropylphenyl), is a catalyst for the reduction of dinitrogen with protons and electrons. Various acids have been added in order to explore where and how the first proton adds to the complex. The addition of adamantol to 1 produces a five-coordinate bis(adamantoxide), [HAr2N3]Mo(N)(OAd)2 (2a), in which one of the amido nitrogens in the ligand has been protonated and the resulting aniline nitrogen in the [HAr2N3]- ligand is not bound to the metal. The addition of [Ph2NH2][OTf] to 1 produces {[HAr2N3]Mo(N)(O- t-Bu)}(OTf) (3), in which an amido nitrogen has been protonated, but the aniline in the [HAr2N3]- ligand remains bound to the metal. Last, the addition of (2,6-lutidinium)BArF4 (BArF4 = {B(3,5-(CF3)2C6H3)4}-) to 1 yields {[Ar2N3]Mo(N)(LutH)(O- t-Bu)}BArF4, in which LutH+ is hydrogen-bonded to the nitride in the solid state and in dichloromethane with Keq = 412 ± 94 and Δ G = -3.6 ± 0.8 kcal at 22 °C. A similar hydrogen-bonded adduct was formed through the addition of (2-methylpyridinium)BArF4 to 1, but the addition of (pyridinium)BArF4 to 1 leads to the formation of (inter alia) {[HAr2N3]Mo(N)(O- t-Bu)}(BArF4), in which the amide nitrogen has been protonated. The addition of cobaltocene to 3 or {[Ar2N3]Mo(N)(LutH)(O- t-Bu)}(BArF4) leads only to the re-formation of 1. X-ray structural studies were carried out on 2a, 3, and {[Ar2N3]Mo(N)(LutH)(O- t-Bu)}(BArF4).

Selective synthesis and stabilization of peroxidesviaphosphine oxides

MEKPO (methyl ethyl ketone peroxide) and other peroxides can be synthesized selectively and stabilized as hydrogen-bonded phosphine oxide adducts.

Enantioselective Synthesis of Pyrroloindolines via Noncovalent Stabilization of Indole Radical Cations and Applications to the Synthesis of Alkaloid Natural Products.

While interest in the synthetic chemistry of radical cations continues to grow, controlling enantioselectivity in the reactions of these intermediates remains a challenge. Based on recent insights into the oxidation of tryptophan in enzymatic systems, we report a photocatalytic method for the generation of indole radical cations as hydrogen-bonded adducts with chiral phosphate anions. These noncovalent open-shell complexes can be intercepted by the stable nitroxyl radical TEMPO· to form alkoxyamine-substituted pyrroloindolines with high levels of enantioselectivity. Further elaboration of these optically enriched adducts can be achieved via a catalytic single-electron oxidation/mesolytic cleavage sequence to furnish transient carbocation intermediates that may be intercepted by a wide range of nucleophiles. Taken together, this two-step sequence provides a simple catalytic method to access a wide range of substituted pyrroloindolines in enantioenriched form via a standard experimental protocol from a common synthetic intermediate. The design, development, mechanistic study, and scope of this process are presented, as are applications of this method to the synthesis of several dimeric pyrroloindoline natural products.

Reactivity of the Sterically Demanding Siloxanediol Mes2 Si(OH)(μ-O)Si(OH)Mes2 Towards Water and Ether Molecules.

A series of isotopologues of the siloxanediol Mes2 Si(OH)(μ-O)Si(OH)Mes2 (3 a) (Mes=2,4,6-trimethylphenyl) were synthesized by reactions of the corresponding disiloxane precursors Mes2 Si(μ-O)2 SiMes2 (2 a), Mes2 Si(μ-17 O)2 SiMes2 (2 b) or Mes2 Si(μ-18 O)2 SiMes2 (2 c) with an excess of H2 O, H217 O or H218 O. NMR and IR signal assignments for the siloxanediols in benzene are supported by quantum-chemical calculations, which indicate small energy differences between trans and cis conformers, the latter of which exhibits an intramolecular hydrogen bond. 1 H NMR as well as IR data suggest the presence of a mixture of both conformers in C6 D6 . Hydrogen-bonded adducts of Mes2 Si(OH)(μ-O)Si(OH)Mes2 with ethers such as diethylether, dimethoxyethane or dioxane were observed in the solid state, where they form polymeric chain-like structures. The latter appear to be stable only in the crystal. 17 O{1 H} NMR and IR data in THF solution suggest an interaction of 3 a with at least one THF molecule, whereas diethylether appears not to interact. Water adducts form neither in solution nor in the solid state as indicated by NMR and ATR IR data. 17 O{1 H} NMR and ESI-MS experiments illustrate the remarkably high stability of the siloxanediols towards water and show no evidence for intra- or intermolecular oxygen-exchange reactions. In marked contrast, a stepwise exchange of all three oxygen atoms-including the one in the Si-O-Si bridge-occurred in the gas phase, when [Mes2 Si(18 OH)(μ-18 O)Si(18 O)Mes2 ]- was treated with H2 O in the hexapole of an ESI FT-ICR mass spectrometer. The scrambling between the bridging and the other oxygen atoms likely proceeds through cyclic Si2 O2 intermediates.

Separating Proton and Electron Transfer Effects in Three-Component Concerted Proton-Coupled Electron Transfer Reactions.

Multiple-site concerted proton-electron transfer (MS-CPET) reactions were studied in a three-component system. 1-Hydroxy-2,2,6,6-tetramethylpiperidine (TEMPOH) was oxidized to the stable radical TEMPO by electron transfer to ferrocenium oxidants coupled to proton transfer to various pyridine bases. These MS-CPET reactions contrast with the usual reactivity of TEMPOH by hydrogen atom transfer (HAT) to a single e-/H+ acceptor. The three-component reactions proceed by pre-equilibrium formation of a hydrogen-bonded adduct between TEMPOH and the pyridine base, and the adduct is then oxidized by the ferrocenium in a bimolecular MS-CPET step. The second-order rate constants, measured using stopped-flow kinetic techniques, spanned 4 orders of magnitude. An advantage of this system is that the MS-CPET driving force could be independently varied by changing either the pKa of the base or the reduction potential (E°) of the oxidant. Changes in ΔG°MS-CPET from either source had the same effect on the MS-CPET rate constants, and a combined Brønsted plot of ln(kMS-CPET) vs ln(Keq) was linear with a slope of 0.46. These results imply a synchronous concerted mechanism, in which the proton and electron transfer components of the CPET process make equal contributions to the rate constants. The only outliers to the Brønsted correlation are the reactions with sterically hindered pyridines, which apparently hinder the close approach of proton donor and acceptor that facilitates MS-CPET. These three-component reactions are compared with a related HAT reaction of TEMPOH, with the 2,4,6-tri-tert-butylphenoxyl radical. The MS-CPET and HAT oxidations of TEMPOH at the same driving force occurred with similar rate constants. While this is an imperfect comparison, the data suggest that the separation of the proton and electron to different reagents does not significantly inhibit the proton-coupled electron transfer process.

A new approach on diminutive effects for non-covalent interactions: fused bicyclic hydrogen-bonded complexes of hypohalous acids with fluoromethanol

ABSTRACTThe computational study of dimer and trimer complexes of fluoromethanol (CH2FOH) with one or two molecules of hypohalous acids (HOX; X = F, Cl or Br) was carried out at the MP2/aug-cc-pVDZ computational level. Also to show the reliability of these results, the binding distance and interaction energy have been compared with large basis set aug-cc-pVTZ level for some systems. The 1:1 ratio gives two different hexagonal and heptagonal ring complexes and the 1:2 ratio of CH2FOH/HOX leads to fused bicyclic hydrogen-bonded trimer adducts constructed from hexagonal and heptagonal rings. The results show diminutive effects in the range of 0.36–0.78 kcal/mol for the trimer bicyclic systems. Redshifts with bond elongations were observed for H–O and C–F bonds, while blueshifts along with bond contractions were seen for the C–H and the X–O bonds. The CH2FOH/HOX complexes were analysed using the Quantum Theory of Atoms in Molecules (QTAIM) and natural bond orbital methodologies.

Direct observation of light-driven, concerted electron–proton transfer

The phenols 4-methylphenol, 4-methoxyphenol, and N-acetyl-tyrosine form hydrogen-bonded adducts with N-methyl-4, 4'-bipyridinium cation (MQ+) in aqueous solution as evidenced by the appearance of low-energy, low-absorptivity features in UV-visible spectra. They are assigned to the known examples of optically induced, concerted electron-proton transfer, photoEPT. The results of ultrafast transient absorption measurements on the assembly MeOPhO-H---MQ+ are consistent with concerted EPT by the instantaneous appearance of spectral features for MeOPhO·---H-MQ+ in the transient spectra at the first observation time of 0.1 ps. The transient decays to MeOPhO-H---MQ+ in 2.5 ps, accompanied by the appearance of oscillations in the decay traces with a period of ∼1 ps, consistent with a vibrational coherence and relaxation from a higher υ(N-H) vibrational level or levels on the timescale for back EPT.