Solid State15N NMR and Theoretical Studies of Primary and Secondary Geometric H/D Isotope Effects on Low-Barrier NHN−Hydrogen Bonds

Distinguishing between COOH, COO- , and hydrogen disordered COOH sites in solids with 13 C chemical shift anisotropy and T1 measurements

Since 1993, it has been known that 13 C chemical shift tensor (i.e., δ11 , δ22 , and δ33 ) provides information sufficient to distinguish between COOH and COO- sites. Herein, four previously unreported metrics are proposed for differentiating COOH/COO- moieties. A new relationship is also introduced that correlates the asymmetry (i.e., δ11 -δ22 ) of COOH sites to the proximity of hydrogen bond donating partners within 2.6 Å with high accuracy (±0.05 Å). Conversely, a limitation to all proposed metrics is that they fail to distinguish between COO- and hydrogen disordered COOH sites. To reconcile this omission, a new approach is proposed based on T1 measurements of both 1 H and 13 C. The 13 C T1 values are particularly sensitive with the T1 for hydrogen disordered COOH moieties found to be nearly six times smaller than T1 's from COO- sites.

Detecting the Hydrogen Bond Cooperativity in a Protein β-Sheet by H/D Exchange

The hydrogen bond (H-bond) cooperativity in the β-sheet of GB3 is investigated by a NMR hydrogen/deuterium (H/D) exchange method. It is shown that the weakening of one backbone N-H…O=C H-bond between two β-strands, β1 and β2, due to the exchange of NH to ND of the H-bond donor in β1, perturbs the chemical shift of ¹³C_α, ¹³C_β, ¹H_α, ¹H_N, and ¹⁵N of the H-bond acceptor and its following residue in β2. Quantum mechanical calculations suggest that the -H-bond chemical shift isotope effect is caused by the structural reorganization in response to the H-bond weakening. This structural reorganization perturbs four neighboring H-bonds, with three being weaker and one being stronger, indicating that three H-bonds are cooperative and one is anticooperative with the perturbed H-bond. The sign of the cooperativity depends on the relative position of the H-bonds. This H-bond cooperativity, which contributes to β-sheet stability overall, can be important for conformational coupling across the β-sheet.

An N⋯H⋯N low-barrier hydrogen bond preorganizes the catalytic site of aspartate aminotransferase to facilitate the second half-reaction

Pyridoxal 5'-phosphate (PLP)-dependent enzymes have been extensively studied for their ability to fine-tune PLP cofactor electronics to promote a wide array of chemistries. Neutron crystallography offers a straightforward approach to studying the electronic states of PLP and the electrostatics of enzyme active sites, responsible for the reaction specificities, by enabling direct visualization of hydrogen atom positions. Here we report a room-temperature joint X-ray/neutron structure of aspartate aminotransferase (AAT) with pyridoxamine 5'-phosphate (PMP), the cofactor product of the first half reaction catalyzed by the enzyme. Between PMP NSB and catalytic Lys258 Nζ amino groups an equally shared deuterium is observed in an apparent low-barrier hydrogen bond (LBHB). Density functional theory calculations were performed to provide further evidence of this LBHB interaction. The structural arrangement and the juxtaposition of PMP and Lys258, facilitated by the LBHB, suggests active site preorganization for the incoming ketoacid substrate that initiates the second half-reaction.

Simultaneous H/D and 13C/12C Anomalous Kinetic Isotope Effects during the Sonolysis of Water in the Presence of Carbon Monoxide

Splitting of water molecules driven by ultrasound plays a central role in sonochemistry. While studies of sonoluminescence revealed the formation of a plasma inside the cavitation bubble, much less is known about the contribution of plasma chemical processes to the sonochemical mechanisms. Herein, we report for the first time sonochemical processes in water saturated with pure CO. The presence of CO causes a large increase in the H/D kinetic isotope effect (KIE) to α_H = 14.6 ± 1.8 in a 10% H₂O/D₂O mixture under 20 kHz ultrasound. The anomalous H/D KIE is attributed to electron quantum tunneling in the plasma produced by cavitation. In addition, CO₂ formed simultaneously with hydrogen during the sonochemical process is enriched with the ¹³C isotope, which indicates a V-V pumping mechanism typical for non-equilibrium plasma. Both observed KIEs unambiguously point to the contribution of quantum effects in sonochemical mechanisms.

Reduced nucleophilicity: an intrinsic property of the Lewis base atom interacting with H in hydrogen-bonds with Lewis acids HX (X = F, Cl, Br, I, CN, CCH, CP)

Equilibrium hydrogen-bond dissociation energies De for the process B⋯HX = B + HX are calculated at the CCSD(T)(F12c)/cc-pVDZ-F12 level for ∼190 complexes B⋯HX. Reduced nucleophilicities of Lewis bases B are determined.

Magnetization Dynamics on Isotope-Isomorphic Holmium Single-Molecule Magnets

Here we reported the deuteration of the metal-binding equatorial water molecules in a reported Ho^III single-molecule magnet (SMM) with pentagonal-bipyramidal geometry, from [Ho(CyPh₂ PO)₂ (H₂ O)₅ ]³⁺ to [Ho(CyPh₂ PO)₂ (D₂ O)₅ ]³⁺ . The hyperfine structures originating from the nuclear spin of ¹⁶⁵ Ho^III can be clearly observed. Moreover, the resulting magnetization dynamics revealed the switch of the relative relaxation rates for the two isotope-isomorphic complexes-respectively faster/slower at low/high temperature. The noticeable isotope effect arises from not only the paramagnetic metal center but also the diamagnetic ligands, which can be explained by the ab initio calculated tunnel splitting and the involvement of the super-hyperfine interaction related to the difference in the nuclear spin number of protium (¹ H, I=¹ /₂ ) and deuterium (² H, I=1).

NMR Properties of the Cyanide Anion, a Quasisymmetric Two-Faced Hydrogen Bonding Acceptor

The isotopically enriched cyanide anion, (13C≡15N)−, has a great potential as the NMR probe of non-covalent interactions. However, hydrogen cyanide is highly toxic and can decompose explosively. It is therefore desirable to be able to theoretically estimate any valuable results of certain experiments in advance in order to carry out experimental studies only for the most suitable molecular systems. We report the effect of hydrogen bonding on NMR properties of 15N≡13CH···X and 13C≡15NH···X hydrogen bonding complexes in solution, where X = 19F, 15N, and O=31P, calculated at the ωB97XD/def2tzvp and the polarizable continuum model (PCM) approximations. In many cases, the isotropic 13C and 15N chemical shieldings of the cyanide anion are not the most informative NMR properties of such complexes. Instead, the anisotropy of these chemical shieldings and the values of scalar coupling constants, including those across hydrogen bonds, can be used to characterize the geometry of such complexes in solids and solutions. 1J(15N13C) strongly correlates with the length of the N≡C bond.

Molecular dynamics study of water confined in MIL-101 metal-organic frameworks

Molecular dynamics simulations of water adsorbed in Material Institute Lavoisier MIL-101(Cr) metal-organic frameworks are performed to analyze the kinetic properties of water molecules confined in the framework at 298.15 K and under different vapor pressures and clarify the water adsorption mechanism in MIL-101(Cr). The terahertz frequency-domain spectra (THz-FDS) of water are calculated by applying fast Fourier transform to the configurational data of water molecules. According to the characteristic frequencies in the THz-FDS, the dominant motions of water molecules in MIL-101(Cr) can be categorized into three types: (1) low-frequency translational motion (0-0.5 THz), (2) medium-frequency vibrational motion (2-2.5 THz), and (3) high-frequency vibrational motion (>6 THz). Each type of water motion is confirmed by visualizing the water configuration in MIL-101(Cr). The ratio of the number of water molecules with low-frequency translational motion to the total number of water molecules increases with the increase in vapor pressure. In contrast, that with medium-frequency vibrational motion is found to decrease with vapor pressure, exhibiting a pronounced decrease after water condensation has started in the cavities. That with the high-frequency vibrational motion is almost independent of the vapor pressure. The interactions between different types of water molecules affect the THz-FDS. Furthermore, the self-diffusion coefficient and the velocity auto-correlation function are calculated to clarify the adsorption state of the water confined in MIL-101(Cr). To confirm that the general trend of the THz-FDS does not depend on the water model, the simulations are performed using three water models, namely, rigid SPC/E, flexible SPC/E, and rigid TIP5PEw.

Hydrogen Bond and Other Lewis Acid-Lewis Base Interactions as Preliminary Stages of Chemical Reactions

Various Lewis acid-Lewis base interactions are discussed as initiating chemical reactions and processes. For example, the hydrogen bond is often a preliminary stage of the proton transfer process or the tetrel and pnicogen bonds lead sometimes to the SN2 reactions. There are numerous characteristics of interactions being first stages of reactions; one can observe a meaningful electron charge transfer from the Lewis base unit to the Lewis acid; such interactions possess at least partly covalent character, one can mention other features. The results of different methods and approaches that are applied in numerous studies to describe the character of interactions are presented here. These are, for example, the results of the Quantum Theory of Atoms in Molecules, of the decomposition of the energy of interaction or of the structure-correlation method.

Temperature dependence of the vibrational spectrum of porphycene: a qualitative failure of classical-nuclei molecular dynamics

Approximate quantum dynamics succeed in predicting a temperature-dependent blue-shift of the high-frequency stretch bands that arise from vibrational coupling between low-frequency thermally activated modes and high-frequency quantized ones. Classical nuclei molecular dynamics fail and instead predict a red-shift.

Magnetic descriptors of hydrogen bonds in malonaldehyde and its derivatives

The nature of the hydrogen bond, HB, as such is still unknown, though a few of its most fundamental features has been uncovered during the last few decades. At the moment, it is possible to obtain reliable results for only a few of its broadest properties, like magnetic properties. They could give new insights into the physics underlying the strength and features of HBs. In this article we analyze the electronic origin of the NMR spectroscopic parameters of malonaldehyde, MA, and some substituted MAs. These substituted MAs are such that the H-bonds are assisted by one of two phenomena: resonance, RAHB, or charge, CAHB. We have studied the dependences of these parameters on two of the main factors which contribute the most to both phenomena, the geometrical and electronic factors, and found out how they can be used to characterize RAHB or CAHB by means of reliable theoretical calculations. We show that in the set of compounds analyzed here (i) the shielding of the proton of the H-bond can be used as a measure of the strength of the HB and (ii) the relation between the contact and non-contact mechanisms of J-couplings between donor and acceptor atoms is a reliable descriptor of whether the H-bond is resonance assisted or charge assisted.

Influence of Hydrogen Bonds in 1:1 Complexes of Phosphinic Acids with Substituted Pyridines on 1H and 31P NMR Chemical Shifts

Two series of 1:1 complexes with strong OHN hydrogen bonds formed by dimethylphosphinic and phenylphosphinic acids with 10 substituted pyridines were studied experimentally by liquid state NMR spectroscopy at 100 K in solution in a low-freezing polar aprotic solvent mixture CDF₃/CDClF₂. The hydrogen bond geometries were estimated using previously established correlations linking ¹H NMR chemical shifts of bridging protons with the O···H and H···N interatomic distances. A new correlation is proposed allowing one to estimate the interatomic distance within the OHN bridge from the displacement of ³¹P NMR signal upon complexation. We show that the values of ³¹P NMR chemical shifts are affected by an additional CH···O hydrogen bond formed between the P═O group of the acid and ortho-CH proton of the substituted pyridines. Breaking of this bond in the case of 2,6-disubstituted bases shifts the ³¹P NMR signal by ca. 1.5 ppm to the high field.

Geometric H/D isotope effect in a series of organic salts involving short O–H⋯O hydrogen bonds between carboxyl and carboxylate groups

Noticeable elongations of donor–acceptor distances upon deuteration are confirmed in short O–H⋯O hydrogen bonds between carboxyl and carboxylate groups.

14N NQR spectroscopy reveals the proton position in N–H⋯N bonds: a case study with proton sponges

The position of the proton in intramolecular N–H⋯N hydrogen bonds has been determined to a high accuracy with ¹⁴N Nuclear Quadrupole Resonance (NQR) spectroscopy.

Predicting anisotropic thermal displacements for hydrogens from solid-state NMR: a study on hydrogen bonding in polymorphs of palmitic acid

The hydrogen-bonding environments at the COOH moiety in eight polycrystalline polymorphs of palmitic acid are explored using solid-state NMR. Although most phases have no previously reported crystal structure, measured 13C chemical shift tensors for COOH moieties, combined with DFT modeling establish that all phases crystallize with a cyclic dimer (R22(8)) hydrogen bonding arrangement. Phases A2, Bm and Em have localized OH hydrogens while phase C has a dynamically disordered OH hydrogen. The phase designated As is a mix of five forms, including 27.4% of Bm and four novel phases not fully characterized here due to insufficient sample mass. For phases A2, Bm, Em, and C the anisotropic uncertainties in the COOH hydrogen atom positions are established using a Monte Carlo sampling scheme. Sampled points are retained or rejected at the ±1σ level based upon agreement of DFT computed 13COOH tensors with experimental values. The collection of retained hydrogen positions bear a remarkable resemblance to the anisotropic displacement parameters (i.e. thermal ellipsoids) from diffraction studies. We posit that this similarity is no mere coincidence and that the two are fundamentally related. The volumes of NMR-derived anisotropic displacement ellipsoids for phases with localized OH hydrogens are 4.1 times smaller than those derived from single crystal X-ray diffraction and 1.8 times smaller than the volume of benchmark single crystal neutron diffraction values.

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.

Geometric isotope effect of deuteration in a hydrogen-bonded host-guest crystal

Deuteration of a hydrogen bond by replacing protium (H) with deuterium (D) can cause geometric changes in the hydrogen bond, known as the geometric H/D isotope effect (GIE). Understanding the GIEs on global structures and bulk properties is of great importance to study structure–property relationships of hydrogen-bonded systems. Here, we report a hydrogen-bonded host–guest crystal, imidazolium hydrogen terephthalate, that exemplifies striking GIEs on its hydrogen bonds, phases, and bulk dielectric transition property. Upon deuteration, the donor–acceptor distance in the O–H···O hydrogen bonds in the host structure is found to increase, which results in a change in the global hydrogen-bonded supramolecular structure and the emergence of a new phase (i.e., isotopic polymorphism). Consequently, the dynamics of the confined guest, which depend on the internal pressure exerted by the host framework, are substantially altered, showing a downward shift of the dielectric switching temperature.

Deuterating a hydrogen bond can change the bond’s geometry, a phenomenon known as the geometric isotope effect (GIE). Here, the authors find that a hydrogen-bonded host–guest crystal, imidazolium hydrogen terephthalate, exhibits significant GIE on its hydrogen bonds, changing its crystal phases and bulk dielectric properties.

NH Tautomerism of a Quadruply Fused Porphyrin: Rigid Fused Structure Delays the Proton Transfer

We report herein NH tautomerism of a freebase derivative of a quadruply fused porphyrin (H₂QFP-Mes, 3), which has one mesityl group at one of the β-positions of the nonfused pyrroles to lower the structural symmetry, allowing us to observe the NH tautomerism with ¹H NMR spectroscopy. Compound 3 was revealed to have the two inner NH protons on the two nonfused pyrroles, and the NH tautomerism of 3 was evidenced by variable-temperature (VT) ¹H NMR experiments in various deuterated solvents. The VT-NMR studies revealed that the activation barrier for the NH tautomerism of 3 was larger than that of tetraphenylporphyrin. The positive activation entropy (ΔS^‡ = 89 J mol^-1 K^-1), determined for the NH tautomerism, can be explained by dissociation of the π-π stacked dimer structure of 3 in the ground state, as evidenced by the crystal structure and NMR measurements. On the basis of the kinetic studies and density functional theory calculations, the stability of intermediates in the NH tautomerism of 3 and the transition states have been discussed in detail.

Hydrogen bonds, and σ-hole and π-hole bonds – mechanisms protecting doublet and octet electron structures

For various interactions electron charge shifts try to protect the former doublet or octet electronic structure of the Lewis acid centre.

Inverse Temperature Dependence of Nuclear Quantum Effects in DNA Base Pairs

Despite the inherently quantum mechanical nature of hydrogen bonding, it is unclear how nuclear quantum effects (NQEs) alter the strengths of hydrogen bonds. With this in mind, we use ab initio path integral molecular dynamics to determine the absolute contribution of NQEs to the binding in DNA base pair complexes, arguably the most important hydrogen-bonded systems of all. We find that depending on the temperature, NQEs can either strengthen or weaken the binding within the hydrogen-bonded complexes. As a somewhat counterintuitive consequence, NQEs can have a smaller impact on hydrogen bond strengths at cryogenic temperatures than at room temperature. We rationalize this in terms of a competition of NQEs between low-frequency and high-frequency vibrational modes. Extending this idea, we also propose a simple model to predict the temperature dependence of NQEs on hydrogen bond strengths in general.

Theoretical analysis of geometry and NMR isotope shift in hydrogen-bonding center of photoactive yellow protein by combination of multicomponent quantum mechanics and ONIOM scheme

Multicomponent quantum mechanical (MC_QM) calculation has been extended with ONIOM (our own N-layered integrated molecular orbital + molecular mechanics) scheme [ONIOM(MC_QM:MM)] to take account of both the nuclear quantum effect and the surrounding environment effect. The authors have demonstrated the first implementation and application of ONIOM(MC_QM:MM) method for the analysis of the geometry and the isotope shift in hydrogen-bonding center of photoactive yellow protein. ONIOM(MC_QM:MM) calculation for a model with deprotonated Arg52 reproduced the elongation of O-H bond of Glu46 observed by neutron diffraction crystallography. Among the unique isotope shifts in different conditions, the model with protonated Arg52 with solvent effect reasonably provided the best agreement with the corresponding experimental values from liquid NMR measurement. Our results implied the availability of ONIOM(MC_QM:MM) to distinguish the local environment around hydrogen bonds in a biomolecule.

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.

Strong ionic hydrogen bonding causes a spectral isotope effect in photoactive yellow protein

Standard hydrogen bonds are of great importance for protein structure and function. Ionic hydrogen bonds often are significantly stronger than standard hydrogen bonds and exhibit unique properties, but their role in proteins is not well understood. We report that hydrogen/deuterium exchange causes a redshift in the visible absorbance spectrum of photoactive yellow protein (PYP). We expand the range of interpretable isotope effects by assigning this spectral isotope effect (SIE) to a functionally important hydrogen bond at the active site of PYP. The inverted sign and extent of this SIE is explained by the ionic nature and strength of this hydrogen bond. These results show the relevance of ionic hydrogen bonding for protein active sites, and reveal that the inverted SIE is a novel, to our knowledge, tool to probe ionic hydrogen bonds. Our results support a classification of hydrogen bonds that distinguishes the properties of ionic hydrogen bonds from those of both standard and low barrier hydrogen bonds, and show how this classification helps resolve a recent debate regarding active site hydrogen bonding in PYP.

Deuterium isotope effects on 13C chemical shifts of negatively charged NH…N systems

Deuterium isotope effects on (13)C chemical shifts are investigated in anions of 1,8-bis(4-toluenesulphonamido)naphthalenes together with N,N-(naphthalene-1,8-diyl)bis(2,2,2-trifluoracetamide) all with bis(1,8-dimethylamino)napthaleneH(+) as counter ion. These compounds represent both "static" and equilibrium cases. NMR assignments of the former have been revised. The NH proton is deuteriated. The isotope effects on (13)C chemical shifts are rather unusual in these strongly hydrogen bonded systems between a NH and a negatively charged nitrogen atom. The formal four-bond effects are found to be negative indicating transmission via the hydrogen bond. In addition, unusual long range effects are seen. Structures, (1)H and (13)C NMR chemical shifts and changes in nuclear shieldings upon deuteriation are calculated using density functional theory methods.

NMR and FT-IR Studies on the Association of Derivatives of Thymidine, Adenosine, and 6-N-Methyl-Adenosine in Aprotic Solvents

Associates of 3',5'-O-TBDMS protected derivatives of the 2'-deoxy forms of the nucleosides adenosine, 6-N-methyl-adenosine and thymidine (henceforward simply addressed by their parents' names) and further model systems in dichloromethane and Freon (CDClF2/CDF3) solutions are studied at low temperatures by 1H NMR and FT-IR spectroscopy. N⋯N distances in hydrogen bonds are estimated from chemical shifts of protons in hydrogen bonds employing geometric and spectroscopic hydrogen bond correlations. These distances are in turn employed to derive N–H stretching frequencies from IR spectroscopic hydrogen bond correlations which may be compared to corresponding experimental results. Three isomeric hydrogen bonded dimers of thymidine are characterized in Freon solution at 120 K. Binary associates of thymidine and a series of pyridines are studied; estimated N⋯N distances in the range of 3.08 to 2.85 Å are qualitatively correlated to shifts of N–H stretching bands where in all cases considerable contributions are found in the spectral region below 3000 cm-1. For adenosine, three isomeric binary associates with 4-nitrophenol are found allowing for an assessment of site-specific acceptor capabilities. In associates of thymidine and adenosine, Watson-Crick and Hoogsteen type 1:1 associates (estimated N⋯N distances of 2.85 and 2.90 Å) as well as 2:1 associates bearing only marginally longer H-bonds could be characterized. Two 1:1 associates between thymidine and 6-N-methyl-adenosine are described that are exclusively bonded via N–H⋯N bridges of about 2.97 and 3.08 Å for Watson-Crick and Hoogsteen sites, respectively, which leads to the conclusion that cooperative effects among coupled N–H⋯O and N–H⋯N hydrogen bonds in A-T base pairs are significant as formation of the N–H⋯O bond induces a contraction of around 0.15 Å in the neighboring N–H⋯N bond.

Solvent and H/D isotope effects on the proton transfer pathways in heteroconjugated hydrogen-bonded phenol-carboxylic acid anions observed by combined UV-vis and NMR spectroscopy

Heteroconjugated hydrogen-bonded anions A···H···X(-) of phenols (AH) and carboxylic/inorganic acids (HX) dissolved in CD2Cl2 and CDF3/CDF2Cl have been studied by combined low-temperature UV-vis and (1)H/(13)C NMR spectroscopy (UVNMR). The systems constitute small molecular models of hydrogen-bonded cofactors in proteins such as the photoactive yellow protein (PYP). Thus, the phenols studied include the PYP cofactor 4-hydroxycinnamic acid methyl thioester, and the more acidic 4-nitrophenol and 2-chloro-4-nitrophenol which mimic electronically excited cofactor states. It is shown that the (13)C chemical shifts of the phenolic residues of A···H···X(-), referenced to the corresponding values of A···H···A(-), constitute excellent probes for the average proton positions. These shifts correlate with those of the H-bonded protons, as well as with the H/D isotope effects on the (13)C chemical shifts. A combined analysis of UV-vis and NMR data was employed to elucidate the proton transfer pathways in a qualitative way. Dual absorption bands of the phenolic moiety indicate a double-well situation for the shortest OHO hydrogen bonds studied. Surprisingly, when the solvent polarity is low the carboxylates are protonated whereas the proton shifts toward the phenolic oxygens when the polarity is increased. This finding indicates that because of stronger ion-dipole interactions small anions are stabilized at high solvent polarity and large anions exhibiting delocalized charges at low solvent polarities. It also explains the large acidity difference of phenols and carboxylic acids in water, and the observation that this difference is strongly reduced in the interior of proteins when both partners form mutual hydrogen bonds.

Heterogeneous Coordination Environments in Lithium-Neutralized Ionomers Identified Using 1H and 7Li MAS NMR

The carboxylic acid proton and the lithium coordination environments for precise and random Li-neutralized polyethylene acrylic acid P(E-AA) ionomers were explored using high speed solid-state ¹H and ⁷Li MAS NMR. While the ⁷Li NMR revealed only a single Li coordination environment, the chemical shift temperature variation was dependent on the precise or random nature of the P(E-AA) ionomer. The ¹H MAS NMR revealed two different carboxylic acid proton environments in these materials. By utilizing ¹H-⁷Li rotational echo double resonance (REDOR) MAS NMR experiments, it was demonstrated that the proton environments correspond to different average ¹H-⁷Li distances, with the majority of the protonated carboxylic acids having a close through space contact with the Li. Molecular dynamics simulations suggest that the shortest ¹H-⁷Li distance corresponds to un-neutralized carboxylic acids directly involved in the coordination environment of Li clusters. These solid-state NMR results show that heterogeneous structural motifs need to be included when developing descriptions of these ionomer materials.

Reaction pathways of proton transfer in hydrogen-bonded phenol-carboxylate complexes explored by combined UV-vis and NMR spectroscopy

Combined low-temperature NMR/UV-vis spectroscopy (UVNMR), where optical and NMR spectra are measured in the NMR spectrometer under the same conditions, has been set up and applied to the study of H-bonded anions A··H··X(-) (AH = 1-(13)C-2-chloro-4-nitrophenol, X(-) = 15 carboxylic acid anions, 5 phenolates, Cl(-), Br(-), I(-), and BF(4)(-)). In this series, H is shifted from A to X, modeling the proton-transfer pathway. The (1)H and (13)C chemical shifts and the H/D isotope effects on the latter provide information about averaged H-bond geometries. At the same time, red shifts of the π-π* UV-vis absorption bands are observed which correlate with the averaged H-bond geometries. However, on the UV-vis time scale, different tautomeric states and solvent configurations are in slow exchange. The combined data sets indicate that the proton transfer starts with a H-bond compression and a displacement of the proton toward the H-bond center, involving single-well configurations A-H···X(-). In the strong H-bond regime, coexisting tautomers A··H···X(-) and A(-)···H··X are observed by UV. Their geometries and statistical weights change continuously when the basicity of X(-) is increased. Finally, again a series of single-well structures of the type A(-)···H-X is observed. Interestingly, the UV-vis absorption bands are broadened inhomogeneously because of a distribution of H-bond geometries arising from different solvent configurations.

Ab initio calculations of deuterium isotope effects on chemical shifts of salt-bridged lysines

Deuterium isotope effects measure the change in chemical shift on substitution of a proton by deuterium. They have been calculated by direct treatment of the H/D nuclear quantum effect using a multicomponent ab initio molecular orbital method based on a non-Born-Oppenheimer approximation. This method enables the determination of both the electronic and the protonic (deuteronic) wave functions simultaneously and can directly calculate the geometrical difference induced by H/D isotope effects. The calculations show that the one-bond deuterium isotope effects on (15)N nuclear shielding, (1)Δ(15)N(D), in ammonium and amines decrease as a counterion or water molecule moves closer to the nitrogen. (1)Δ(15)N(D) and (2)Δ(1)H(D) of the NH(3)(+) groups of lysine residues in the B1 domain of protein G have been calculated using truncated side chains and also determined experimentally by NMR. Comparisons show that the structures in solution are different from those in the crystal and that solvation plays an important role in weakening the hydrogen bonds.

The hydrogen-bonding transition and isotope-dependent negative thermal expansion in H3Co(CN)6

We investigate the structures of H(3)Co(CN)(6) and D(3)Co(CN)(6) and their temperature dependence using a combination of advanced neutron and x-ray diffraction techniques. Lattice parameter refinements show marked temperature- and isotope-dependent effects in the thermal expansion behaviour. Reverse Monte Carlo modelling of neutron total scattering data characterizes the distribution of D atom positions in the deuterated compound; analysis of these distributions reveals that the two N-H/D bonds become increasingly different with increasing temperature, suggesting a mechanism for the observed thermal expansion anomalies. We discuss these results in the context of the previous crystallographic, spectroscopic and theoretical results for H(3)Co(CN)(6) and 'super-short' N···H···N systems in general.

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.