Dynamic NMR Study of the Mechanisms of Double, Triple, and Quadruple Proton and Deuteron Transfer in Cyclic Hydrogen Bonded Solids of Pyrazole Derivatives

Systematic analysis of electronic barrier heights and widths for concerted proton transfer in cyclic hydrogen bonded clusters: (HF)n, (HCl)n and (H2O)n where n = 3, 4, 5

The MP2 and CCSD(T) methods are paired with correlation consistent basis sets as large as aug-cc-pVQZ to optimize the structures of the cyclic minima for (HF)n, (HCl)n and (H2O)n where n = 3-5, as well as the corresponding transition states (TSs) for concerted proton transfer (CPT). MP2 and CCSD(T) harmonic vibrational frequencies confirm the nature of each minimum and TS. Both conventional and explicitly correlated CCSD(T) computations are employed to assess the electronic dissociation energies and barrier heights for CPT near the complete basis (CBS) limit for all 9 clusters. Results for (HF)n are consistent with prior studies identifying Cnh and Dnh point group symmetry for the minima and TSs, respectively. Our computations also confirm that CPT proceeds through Cs TS structures for the C1 minima of (H2O)3 and (H2O)5, whereas the process goes through a TS with D2d symmetry for the S4 global minimum of (H2O)4. This work corroborates earlier findings that the minima for (HCl)3, (HCl)4 and (HCl)5 have C3h, S4 and C1 point group symmetry, respectively, and that the Cnh structures are not minima for n = 4 and 5. Moreover, our computations show the TSs for CPT in (HCl)3, (HCl)4 and (HCl)5 have D3h, D2d, and C2 point group symmetry, respectively. At the CCSD(T) CBS limit, (HF)4 and (HF)5 have the smallest electronic barrier heights for CPT (≈15 kcal mol-1 for both), followed by the HF trimer (≈21 kcal mol-1). The barriers are appreciably higher for the other clusters (around 27 kcal mol-1 for (H2O)4 and (HCl)3; roughly 30 kcal mol-1 for (H2O)3, (H2O)5 and (HCl)4; up to 38 kcal mol-1 for (HCl)5). At the CBS limit, MP2 significantly underestimates the CCSD(T) barrier heights (e.g., by ca. 2, 4 and 7 kcal mol-1 for the pentamers of HF, H2O and HCl, respectively), whereas CCSD overestimates these barriers by roughly the same magnitude. Scaling the barrier heights and dissociation energies by the number of fragments in the cluster reveals strong linear relationships between the two quantities and with the magnitudes of the imaginary vibrational frequency for the TSs.

A two-electron reducing reaction of CO2 to an oxalate anion: a theoretical study of delocalized (presolvated) electrons in Al(CH3)n(NH3)m, n = 0-2 and m = 1-6, clusters

Presolvated electron possibility in three oxidation states of aluminum - Al(0), Al(I), and Al(II) - has been theoretically investigated for the Al + 6NH3, Al(CH3) + 5NH3, and Al(CH3)2 + 4NH3 reactions. It has been shown that the metal center adopts a tetrahedral shape for its most stable geometric structure, irrespective of the degree of Al oxidation states. Using different analysis techniques (highest occupied molecular orbital shapes, spin density distributions, and electron delocalization ranges), we showed that presolvated (delocalized) electrons are only formed in the Al(CH3)2(NH3)p coordination complexes when 2 ≤ p ≤ 4. It has also been evidenced that these delocalized electrons being powerful reducing agents allowed two CO2 molecules to be captured and form an oxalate ion in close contact with the [Al2(CH3)2(CH2)2(NH3)4]2+ dication core.

Development of an Iron(II) Complex Exhibiting Thermal- and Photoinduced Double Proton-Transfer-Coupled Spin Transition in a Short Hydrogen Bond

Multiple proton transfer (PT) controllable by external stimuli plays a crucial role in fundamental chemistry, biological activity, and material science. However, in crystalline systems, controlling multiple PT, which results in a distinct protonation state, remains challenging. In this study, we developed a novel tridentate ligand and iron(II) complex with a short hydrogen bond (HB) that exhibits a PT-coupled spin transition (PCST). Single-crystal X-ray and neutron diffraction measurements revealed that the positions of the two protons in the complex can be controlled by temperature and photoirradiation based on the thermal- and photoinduced PCST. The obtained results suggest that designing molecules that form short HBs is a promising approach for developing multiple PT systems in crystals.

The use of DOSY experiments to determine the solution structures of coinage metal pyrazolates: The case of {[3,5-(CF3 )2 Pz]Ag}3

A series of DOSY experiments have been carried out to determine the solution stoichiometry of silver(I) 3,5-bis (trifluoromethyl)pyrazolate species. This compound exists as a trimer in the solid state (n = 3) but in solutions of chlorinated solvents, the DOSY data suggest the presence of a mixture of solvent stabilized monomer (n = 1) and dimer (n = 2) in equilibrium. Different approximations have been used including the Stokes-Einstein and the Stokes-Einstein-Gierer-Wirtz equations. Some methodological problems are discussed.

Determination of concerted or stepwise mechanism of hydrogen tunneling from isotope effects: Departure between experiment and theory

Isotope substitution is an important experimental technique that offers deep insight into reaction mechanisms, as the measured kinetic isotope effects (KIEs) can be directly compared with theory. For multiple proton transfer processes, there are two types of mechanisms: stepwise transfer and concerted transfer. The Bell-Limbach model provides a simple theory to determine whether the proton transfer mechanism is stepwise or concerted from KIEs. Recent STM experiments have studied the proton switching process in water tetramers on NaCl(001). Theoretical studies predict that this process occurs via a concerted mechanism, however, the experimental KIEs resemble the Bell-Limbach model for stepwise tunneling, raising question on the underlying mechanism or the validity of the model. We study this system using ab initio instanton theory, and in addition to thermal rates, we also considered microcanonical rates, as well as tunneling splittings. Instanton theory predicts a concerted mechanism, and the KIEs for tunneling rates (both thermal and microcanonical) upon deuteration are consistent with the Bell-Limbach model for concerted tunneling, but could not explain the experiments. For tunneling splittings, partial and full deuteration changes the size of it in a similar fashion to how it changes the rates. We further examined the Bell-Limbach model in another system, porphycene, which has both stepwise and concerted tunneling pathways. The KIEs predicted by instanton theory are again consistent with the Bell-Limbach model. This study highlights differences between KIEs in stepwise and concerted tunneling, and the discrepancy between theory and recent STM experiments. New theory/experiments are desired to settle this problem.

Self-Assembly of Hydrogen-Bonded Cage Tetramers of Phosphonic Acid

The self-association of phosphonic acids with general formula RP(O)(OH)2 in solution state remains largely unexplored. The general understanding is that such molecules form multiple intermolecular hydrogen bonds, but the stoichiometry of self-associates and the bonding motifs are unclear. In this work, we report the results of the study of self-association of tert-butylphosphonic acid using low temperature liquid-state 1H and 31P NMR spectroscopy (100 K; CDF3/CDF2Cl) and density functional theory (DFT) calculations. For the first time, we demonstrate conclusively that polar aprotic medium tert-butylphosphonic acid forms highly symmetric cage-like tetramers held by eight OHO hydrogen bonds, which makes the complex quite stable. In these associates. each phosphonic acid molecule is bonded to three other molecules by forming two hydrogen bonds as proton donor and two hydrogen bonds as proton acceptor. Though the structure of such cage-like tetramers is close to tetrahedral, the formal symmetry of the self-associate is C2.

Multidimensional Hydrogen Tunneling in Supported Molecular Switches: The Role of Surface Interactions

The nuclear tunneling crossover temperature (T_{c}) of hydrogen transfer reactions in supported molecular-switch architectures can lie close to room temperature. This calls for the inclusion of nuclear quantum effects (NQEs) in the calculation of reaction rates even at high temperatures. However, computations of NQEs relying on standard parametrized dimensionality-reduced models quickly become inadequate in these environments. In this Letter, we study the paradigmatic molecular switch based on porphycene molecules adsorbed on metallic surfaces with full-dimensional calculations that combine density-functional theory for the electrons with the semiclassical ring-polymer instanton approximation for the nuclei. We show that the double intramolecular hydrogen transfer (DHT) rate can be enhanced by orders of magnitude due to surface fluctuations in the deep-tunneling regime. We also explain the origin of an Arrhenius temperature dependence of the rate below T_{c} and why this dependence differs at different surfaces. We propose a simple model to rationalize the temperature dependence of DHT rates spanning diverse fcc [110] surfaces.

H/D Isotope Effects on 1H-NMR Chemical Shifts in Cyclic Heterodimers and Heterotrimers of Phosphinic and Phosphoric Acids

Hydrogen-bonded heterocomplexes formed by POOH-containing acids (diphenylphosphoric 1, dimethylphosphoric 2, diphenylphosphinic 3, and dimethylphosphinic 4) are studied by the low-temperature (100 K) ¹H-NMR and ³¹P-NMR using liquefied gases CDF₃/CDF₂Cl as a solvent. Formation of cyclic dimers and cyclic trimers consisting of molecules of two different acids is confirmed by the analysis of vicinal H/D isotope effects (changes in the bridging proton chemical shift, δH, after the deuteration of a neighboring H-bond). Acids 1 and 4 (or 1 and 3) form heterotrimers with very strong (short) H-bonds (δH ca. 17 ppm). While in the case of all heterotrimers the H-bonds are cyclically arranged head-to-tail, ···O=P-O-H···O=P-O-H···, and thus their cooperative coupling is expected, the signs of vicinal H/D isotope effects indicate an effective anticooperativity, presumably due to steric factors: when one of the H-bonds is elongated upon deuteration, the structure of the heterotrimer adjusts by shortening the neighboring hydrogen bonds. We also demonstrate the formation of cyclic tetramers: in the case of acids 1 and 4 the structure has alternating molecules of 1 and 4 in the cycle, while in case of acids 1 and 3 the cycle has two molecules of 1 followed by two molecules of 3.

Revisiting the Structure and Chemistry of 3(5)-Substituted Pyrazoles

Pyrazoles are known as versatile scaffolds in organic synthesis and medicinal chemistry, often used as starting materials for the preparation of more complex heterocyclic systems with relevance in the pharmaceutical field. Pyrazoles are also interesting compounds from a structural viewpoint, mainly because they exhibit tautomerism. This phenomenon may influence their reactivity, with possible impact on the synthetic strategies where pyrazoles take part, as well as on the biological activities of targets bearing a pyrazole moiety, since a change in structure translates into changes in properties. Investigations of the structure of pyrazoles that unravel the tautomeric and conformational preferences are therefore of upmost relevance. 3(5)-Aminopyrazoles are largely explored as precursors in the synthesis of condensed heterocyclic systems, namely pyrazolo[1,5-a]pyrimidines. However, the information available in the literature concerning the structure and chemistry of 3(5)-aminopyrazoles is scarce and disperse. We provide a revision of data on the present subject, based on investigations using theoretical and experimental methods, together with the applications of the compounds in synthesis. It is expected that the combined information will contribute to a deeper understanding of structure/reactivity relationships in this class of heterocycles, with a positive impact in the design of synthetic methods, where they take part.

Diverse Architectures and Luminescence Properties of Group 11 Complexes Containing Pyrimidine-Based Phosphine, N-((Diphenylphosphine)methyl)pyrimidin-2-amine

In this article, the synthesis, structural studies, and luminescence properties of Cu^I, Ag^I, and Au^I complexes of pyrimidine-based phosphine [C₄H₃N₂-2-NH(CH₂PPh₂)] (1) are described. The reactions of 1 with CuX led to the isolation of one-dimensional (1D) chain, tetranuclear ladder, or cyclic derivatives. The structural features of these complexes are greatly influenced by the metal-to-ligand ratio, reaction conditions, and CuX (X = Cl, Br or I) employed. In the case of CuCl and CuBr, one-dimensional coordination polymers [{CuCl}{C₄H₃N₂-2-NH(CH₂PPh₂)}]_∞ (2) and [{CuBr}{C₄H₃N₂-2-NH(CH₂PPh₂)}]_∞ (3) were obtained, whereas CuI afforded tetracopper complex [{CuI}₄{C₄H₃N₂-2-NH(CH₂PPh₂)}₂(NCCH₃)₂] (4) having Cu₄ ladder structure supported by P∩N-bridging coordination of 1. The reaction of 1 with AgOTf yielded unprecedented one-dimensional chain structure [{AgOTf}{C₄H₃N₂-2-NH(CH₂PPh₂)}]_∞ (5), whereas the reaction with AgBF₄ produced a 12-membered dinuclear complex, [{Ag}{C₄H₃N₂-2-NH(CH₂PPh₂)}]₂[BF₄]₂ (6), with each silver atom having a linear geometry. Gold complex [{AuCl}{C₄H₃N₂-2-NH(CH₂PPh₂)}]₂ (7) was synthesized by reacting 1 with [AuCl(SMe₂)]. Compounds 2-4 were also prepared using a pestle and mortar by grinding method in almost quantitative yield. Complex 4 with a Cu···Cu distance of 2.828(5) Å shows high luminescence due to the nonbonded metal···metal interactions.

Cyclic trimers of phosphinic acids in polar aprotic solvent: symmetry, chirality and H/D isotope effects on NMR chemical shifts

The hydrogen-bonded self-associates of dimethylphosphinic (1), diphenylphosphoric (2), phenylphosphinic (3), and bis(2,4,4-trimethylpentyl)phosphinic (4) acids have been studied by using liquid-state NMR down to 100 K in a low-freezing polar solvent, CDF₃/CDClF₂. The H/D isotope effects on ¹H NMR chemical shifts caused by partial deuteration of hydroxyl groups unambiguously reveal the stoichiometry of the self-associates and the cooperativity of their hydrogen bonds. In all cases, cyclic trimers are the dominant form, while cyclic dimers are present as a minor form for 1 and 2. Due to the asymmetry of substituents, cyclic trimers of 3 exist in two isomeric forms, depending on the orientation of the phenyl groups with respect to the plane of the hydrogen bonds. The racemic mixture of 4 leads to the coexistence of up to 64 isomers of cyclic trimers, many of which are chemically equivalent or effectively isochronous. The mole fractions of such isomers deviate from the statistically expected values. This feature could provide information about the relative stabilization energies of hydrogen-bonded chiral self-associates. The complexation of 4 with SbCl₅ (complex 5) suppresses the self-association and 5 exists exclusively in the monomeric form with chemically non-equivalent ³¹P nuclei in RS, SR and RR/SS forms.

Diazole-based powdered cocrystal featuring a helical hydrogen-bonded network: structure determination from PXRD, solid-state NMR and computer modeling

We present the structure of a new equimolar 1:1 cocrystal formed by 3,5-dimethyl-1H-pyrazole (dmpz) and 4,5-dimethyl-1H-imidazole (dmim), determined by means of powder X-ray diffraction data combined with solid-state NMR that provided insight into topological details of hydrogen bonding connectivities and weak interactions such as CH···π contacts. The use of various 1D/2D (13)C, (15)N and (1)H high-resolution solid-state NMR techniques provided structural insight on local length scales revealing internuclear proximities and relative orientations between the dmim and dmpz molecular building blocks of the studied cocrystal. Molecular modeling and DFT calculations were also employed to generate meaningful structures. DFT refinement was able to decrease the figure of merit R(F(2)) from ~11% (PXRD only) to 5.4%. An attempt was made to rationalize the role of NH···N and CH···π contacts in stabilizing the reported cocrystal. For this purpose four imidazole derivatives with distinct placement of methyl substituents were reacted with dmpz to understand the effect of methylation in blocking or enabling certain intermolecular contacts. Only one imidazole derivative (dmim) was able to incorporate into the dmpz trimeric motif thus resulting in a cocrystal, which contains both hydrophobic (methyl groups) and hydrophilic components that self-assemble to form an atypical 1D network of helicoidal hydrogen bonded pattern, featuring structural similarities with alpha-helix arrangements in proteins. The 1:1 dmpz···dmim compound I is the first example of a cocrystal formed by two different azoles.

NMR studies of active-site properties of human carbonic anhydrase II by using (15) N-labeled 4-methylimidazole as a local probe and histidine hydrogen-bond correlations

By using a combination of liquid and solid-state NMR spectroscopy, (15) N-labeled 4-methylimidazole (4-MI) as a local probe of the environment has been studied: 1) in the polar, wet Freon CDF3 /CDF2 Cl down to 130 K, 2) in water at pH 12, and 3) in solid samples of the mutant H64A of human carbonic anhydrase II (HCA II). In the latter, the active-site His64 residue is replaced by alanine; the catalytic activity is, however, rescued by the presence of 4-MI. For the Freon solution, it is demonstrated that addition of water molecules not only catalyzes proton tautomerism but also lifts its quasidegeneracy. The possible hydrogen-bond clusters formed and the mechanism of the tautomerism are discussed. Information about the imidazole hydrogen-bond geometries is obtained by establishing a correlation between published (1) H and (15) N chemical shifts of the imidazole rings of histidines in proteins. This correlation is useful to distinguish histidines embedded in the interior of proteins and those at the surface, embedded in water. Moreover, evidence is obtained that the hydrogen-bond geometries of His64 in the active site of HCA II and of 4-MI in H64A HCA II are similar. Finally, the degeneracy of the rapid tautomerism of the neutral imidazole ring His64 reported by Shimahara et al. (J. Biol. Chem.- 2007, 282, 9646) can be explained with a wet, polar, nonaqueous active-site conformation in the inward conformation, similar to the properties of 4-MI in the Freon solution. The biological implications for the enzyme mechanism are discussed.

Exceptional isotopic-substitution effect: breakdown of collective proton tunneling in hexagonal ice due to partial deuteration

Multiple proton transfer controls many chemical reactions in hydrogen-bonded networks. However, in contrast to well-understood single proton transfer, the mechanisms of correlated proton transfer and of correlated proton tunneling in particular have remained largely elusive. Herein, fully quantized ab initio simulations are used to investigate H/D isotopic-substitution effects on the mechanism of the collective tunneling of six protons within proton-ordered cyclic water hexamers that are contained in proton-disordered ice, a prototypical hydrogen-bonded network. At the transition state, isotopic substitution leads to a Zundel-like complex, [HO⋅⋅⋅D⋅⋅⋅OH], which localizes ionic defects and thus inhibits perfectly correlated proton tunneling. These insights into fundamental aspects of collective proton tunneling not only rationalize recent neutron-scattering experiments, but also stimulate investigations into multiple proton transfer in hydrogen-bonded networks much beyond ice.

The Structure and Dynamic Properties of 1H-Pyrazole-4-Carboxylic Acids in the Solid State

The structure of three simple pyrazole-4-carboxylic acids unsubstituted in position 1 (NH derivatives) 1H-pyrazole-4-carboxylic acid (1), 3(5)-methyl-1H-pyrazole-4-carboxylic acid (2) and 3,5-dimethyl-1H-pyrazole-4-carboxylic acid (3) are discussed based on crystallographic results (two new structures) and solid-state NMR (CPMAS). Compounds 1 and 3 present polymorphism and one of the polymorphs of each compound shows solid-state proton transfer (SSPT). Compound 2 presents tautomerism that has been studied by NMR, both in the solid-state and in solution at low temperature.

Kinetics of Coupled Double Proton and Deuteron Transfer in Hydrogen-Bonded Ribbons of Crystalline Pyrazole-4-carboxylic Acid

The proton tautomerism of pyrazole-4-carboxylic acid (PCA) has been studied by a combination of 15N CPMAS and 2H NMR spectroscopy and relaxometry. Down to 250 K, PCA forms a hydrogen bonded ribbon where adjacent carboxylic and pyrazole groups are linked by an OH···N and an O···HN hydrogen bond, forming either the tautomeric state A or B. Down to about 250 K, the tautomerism is fast on the NMR timescale and degenerate, corresponding to a phase exhibiting dynamic proton disorder. At lower temperatures, a transition to an ordered phase is observed with localized protons, assigned to an all-syn conformation adopting the sequence of tautomeric states ..ABABA.. The longitudinal 15N relaxation times T 1 of PCA-15N2 have been measured at 9.12 MHz (2.1 T). Because of the low field, a chemical shift anisotropy mechanism could be neglected, and the data were analyzed in terms of a dipolar 1H-15N relaxation mechanism, yielding the rate constants k HH. The rate constants k HD and k DD were obtained from the measurement and analysis of the 2H T 1 values of PCA-15N1-d0.9 and PCA-15N1-d0.1 measured at 46.03 MHz. Within the margin of error, no kinetic isotope effects could be detected, in contrast to previous results reported for the very fast tautomerism of solid benzoic acid dimers and the much slower tautomerism of solid 3,5-diphenyl-4-brompyrazole (DPBrP) dimers. The Arrhenius curves of all three systems were simulated using the Bell–Limbach tunneling model. Evidence for a major heavy atom motion for the tautomerism of PCA is obtained, associated with small angle reorientation of PCA molecules around the molecular axis. The observed proton order-disorder transition and the mechanism of the observed rate process are discussed in terms of a coupling of adjacent tautomeric states.

Dispersed Fluorescence Spectra of 1H- and 1D-Indazole

Dispersed fluorescence (DF) spectra of jet-cooled indazole (Ia-h) and its singly-deuterated isotopologue (Ia-d) were recorded following excitation of the origin band of the S 1(1 A´) ← S 0(1 A´) electronic transition. The spectra were analyzed and assigned with the help of vibrational frequency calculations by density functional theory (DFT) at the TPSS/aug-cc-pVTZ level. The calculated and measured S 0 state vibrations are in agreement within ≤ 3%. The obtained results for the two Ia monomers form the basis for an analysis of the electronic spectra of the hydrogen-bonded dimers Ia2-hh, Ia2-dd and Ia2-hd and a determination of the exciton splitting in the dimer in a forthcoming paper.

A comparative study of the hydrogen-bonding patterns and prototropism in solid 2-thiocytosine (potential antileukemic agent) and cytosine, as studied by 1H-14N NQDR and QTAIM/ DFT

A potential antileukemic and anticancer agent, 2-thiocytosine (2-TC), has been studied experimentally in the solid state by ¹H-¹⁴N NMR-NQR double resonance (NQDR) and theoretically by the quantum theory of atoms in molecules (QTAIM)/density functional theory (DFT). Eighteen resonance frequencies on ¹⁴N were detected at 180 K and assigned to particular nitrogen sites (−NH₂, –N=, and –NH–) in 2-thiocytosine. Factors such as the nonequivalence of molecules (connected to the duplication of sites) and possible prototropic tautomerism (capable of modifying the type of site due to proton transfer) were taken into account during frequency assignment. The result of replacing oxygen with sulfur, which leads to changes in the intermolecular interaction pattern and molecular aggregation, is discussed. This study demonstrates the advantages of combining NQDR and DFT to extract detailed information on the H-bonding properties of crystals with complex H-bonding networks. Solid-state properties were found to have a profound impact on the stabilities and reactivities of both compounds.

Figure

Superbasicity of a bis-guanidino compound with a flexible linker: a theoretical and experimental study

The bis-guanidino compound H(2)C{hpp}(2) (I; hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) has been converted to the monocation [I-H](+) and isolated as the chloride and tetraphenylborate salts. Solution-state spectroscopic data do not differentiate the protonated guanidinium from the neutral guanidino group but suggest intramolecular "-N-H...N=" hydrogen bonding to form an eight-membered C(3)N(4)H heterocycle. Solid-state CPMAS (15)N NMR spectroscopy confirms protonation at one of the imine nitrogens, although line broadening is consistent with solid-state proton transfer between guanidine functionalities. X-ray diffraction data have been recorded over the temperature range 50-273 K. Examination of the carbon-nitrogen bond lengths suggests a degree of "partial protonation" of the neutral guanidino group at higher temperatures, with greater localization of the proton at one nitrogen position as the temperature is lowered. Difference electron density maps generated from high-resolution X-ray diffraction studies at 110 K give the first direct experimental evidence for proton transfer in a poly(guanidino) system. Computational analysis of I and its conjugate acid [I-H](+) indicate strong cationic resonance stabilization of the guanidinium group, with the nonprotonated group also stabilized, albeit to a lesser extent. The maximum barrier to proton transfer calculated using the Boese-Martin for kinetics method was 2.8 kcal mol(-1), with hydrogen-bond compression evident in the transition state; addition of zero-point vibrational energy values leads to the conclusion that the proton transfer is barrierless, implying that the proton shuttles freely between the two nitrogen atoms. Calculations determining the gas-phase proton affinity and the pK(a) in acetonitrile both indicate that compound I should behave as a superbase. This has been confirmed by spectrophotometric titrations in MeCN using polyphosphazene references, which give an average pK(a) of 28.98 +/- 0.05. Triadic analysis indicates that the dominant term causing the high basicity is the relaxation energy.

1H-19F spin-lattice relaxation spectroscopy: proton tunnelling in the hydrogen bond studied by field-cycling NMR

Proton tunnelling in the hydrogen bonds of two fluorine substituted benzoic acid dimers has been investigated using field-cycling NMR relaxometry. The close proximity of the (19)F nuclei to the hydrogen bond protons introduces heteronuclear (19)F-(1)H dipolar interactions into the spin-lattice relaxation processes. This renders the (1)H magnetisation-recovery biexponential and introduces multiple spectral density components into the relaxation matrix characterised by frequencies that are sums and differences of the (19)F and (1)H Larmor frequencies. Using field-cycling NMR pulse sequences that measure the spin-lattice relaxation and cross-relaxation rates we demonstrate how some of these multiple spectral density components can be separately resolved. This leads to an accurate determination of the correlation times that characterise the proton tunnelling motion. A broad spectrum of relaxation behaviour is illustrated and explored in the chosen samples and the investigation is used to explore the theory and practise of field-cycling NMR relaxometry in cases where heteronuclear interactions are significant.

Triple proton transfer of excited 7-hydroxyquinoline along a hydrogen-bonded water chain in ethers: secondary solvent effect on the reaction rate

A large secondary solvent effect on the reaction rate has been experimentally observed in the excited-state tautomerization of a 7-hydroxyquinoline (7HQ) molecule complexed cyclically with two water molecules in ethers. The proton acceptance of a water molecule from the enolic group of 7HQ is the rate-determining step while the proton donation of a water molecule to the imino group of 7HQ is followed rapidly to complete the triple proton transfer of the 7HQ.(H2O)2 complex in both diethyl ether and di-n-propyl ether. The rate constant of the tautomerization is larger in diethyl ether than in di-n-propyl ether due to the more polar environment around the complex in diethyl ether. Although the activation energies of the proton transfer are similar in both ethers, the kinetic isotope effect of the rate constant is larger in di-n-propyl ether than in diethyl ether. We attribute these kinetic differences to dissimilarity in the polarities of the two secondary solvents.

Mechanisms of Double Proton Transfer. Theory and Applications

An analytical two-dimensional (2D) potential-energy surface based on two equal hydrogen bonds coupled by a correlation term, recently introduced [J. Chem. Phys. 127 (2007) 174513] to describe the dynamics of double proton transfer, is reviewed and generalized. It is then applied to the evaluation of proton transfer dynamics in a number of realistic systems, namely several molecules and dimers that exhibit various degrees of correlation between the motions of the two protons. The three parameters required to generate this 2D potential are derived from electronic structure and force field calculations, such that they include implicitly the effect of coupled skeletal modes. It follows that explicit introduction of such coupled modes is not required to obtain the basic relations that define the stationary points of the 2D surface, and thereby the reaction mechanism. Based on these relations, a detailed analysis is reported of a variety of systems exhibiting double proton transfer, including, apart from previously investigated porphine and porphycene, representing weak correlation, and the formic and benzoic acid dimers, representing strong correlation, two newly investigated systems which shed light on the hitherto not represented intermediate correlation category, namely naphthazarin, and the 4-bromopyrazole dimer.

Self-assembly of 3,5-bis(ethoxycarbonyl)pyrazolate anions and ammonium cations of beta-phenylethylamine or homoveratrylamine into hetero-double-stranded helical structures

Hydrogen-bonded double-stranded hetero-helices are formed when reacting sodium 3,5-bis(ethoxycarbonyl)pyrazolate with beta-phenethylammonium or homoveratrylammonium chloride, in which one of the strands is defined by the ammonium cations and the other one by the pyrazolate anions.