Geometric Isotope Effect Research Articles

The influence of deuterium on sodium mobility and viscosity of colloidal precursor suspensions yielding template-free nanosized zeolites

The isotopic substitution in chemistry is characterized by two main effects, i.e., the kinetic isotope effect (KIE), associated with differences in the reaction rates, and the geometric isotope effect (GIE) related to crystallographic features. In this work, considering the difference in the weight of deuterium (D) compared to the weight of hydrogen (H), we use the isotopic substitution to investigate the crystallisation mechanism of template-free nanosized faujasite (FAU) type zeolite in colloidal precursors. Тhe basic hydrothermal synthesis solvent - water (H2O), is substituted with heavy water (D2O), aiming to understand the isotopic influence and the effect of H-bond networks on the zeolite crystallisation. The crystallinity, viscosity, conductivity, chemical composition, morphology and local order of the FAU zeolites synthesized in both hydrated (H) and deuterated (D) medium are evaluated with X-ray diffraction, TEM microscopy, EDX and ICP analysis, FT-IR and solid-state NMR spectroscopy. The structural directing agent - Na+, exhibits a lower mobility in the deuterated medium, because of a characteristic higher viscosity compared to hydrated medium. This leads to a slower nucleation and crystallisation in the initial stages of hydrothermal treatment in the colloidal suspensions but also to a significant difference in the Al distribution.

Thermal and Nuclear Quantum Effects at the Antiferroelectric to Paraelectric Phase Transition in KOH and KOD Crystals.

Crystalline KOH undergoes an antiferroelectric (AFE) proton ordering phase transition at low temperatures, which results in a monoclinic bilayer structure held together by a network of weak hydrogen bonds (HBs). The Curie temperature shifts up when the compound is deuterated, an effect that classical MD is not able to catch. For deeper insights into the transition mechanism, we carry out ab initio MD simulations of KOH and KOD crystals by including quantum effects on the nuclei through Feynman path integrals. The geometric isotope effect and the evolution of the lattice parameters with temperature agree with the experimental data, while the purely classical description is not appropriate. Our results show that deuteration strengthens the HBs in the low-T AFE ordered phase. The transition is characterized by the flipping of OH/OD groups along a bending mode. Above the transition, the system is driven into a dynamical disordered paraelectric phase.

Deuteration triggered downward shift of dielectric phase transition temperature in a hydrogen-bonded molecular crystal

Deuteration of hydrogen-bonded phase transition crystals can increase the transition temperatures due to the isotope effect. But rare examples show the opposite trend that originates from the structural changes of the hydrogen bond, known as the geometric H/D isotope effect. Herein, we report an organic crystal, diethylammonium hydrogen 1,4-terephthalate, exhibits a reversible structural phase transition and dielectric switching. Structural study shows the cations reside in channels formed by one-dimensional hydrogen-bonded anionic chains and undergo an order-disorder transition at around 206 K. The deuterated counterpart shows an elongation of the O···O hydrogen bond by about 0.005 Å. This geometric isotope effect releases the internal pressure of the anionic host on the cation guests and results in a downward shift of the phase transition temperature by 10 K.

High-pressure sapphire capillary cell for synchrotron single-crystal X-ray diffraction measurements to 1500 bar

A new sapphire capillary pressure cell for single-crystal X-ray diffraction measurements at moderate pressures (200−1500 bar; 1 bar = 100 kPa) has been developed and optimized for use on beamline I19 at Diamond Light Source. The three-component cell permits optical centring of the crystal andin situpressure modification to a precision of 1 bar. Compression of hexamethylenetetramine and its deuterated analogue to 1000 bar was performed, showcasing the accuracy and precision of the measurements, and highlighting evidence of a geometric isotope effect.

Full-quantum descriptions of molecular systems from constrained nuclear-electronic orbital density functional theory.

We develop a full-quantum formulation of constrained nuclear-electronic orbital density functional theory (cNEO-DFT). This formulation deviates from the conventional Born-Oppenheimer framework, and all nuclei and electrons are treated on an equal footing within the molecular orbital picture. Compared to the conventional DFT, the ground state energy in full-quantum cNEO-DFT inherently includes all vibrational zero-point energies. We also derived and implemented the analytic gradient of the full-quantum cNEO-DFT energy with respect to the quantum nuclear expectation positions. With the analytic gradient, the geometry optimizations are performed, which naturally include the nuclear quantum effects and describe the geometric isotope effects. The full-quantum cNEO-DFT is tested on a series of small molecules and the transition states of two hydrogen transfer reactions. The results are compared with those from conventional DFT, DFT-VPT2, and NEO-DFT with only key protons treated quantum mechanically. It is found that the nuclear quantum effects have notable impacts on molecular equilibrium geometries and transition state geometries. The full-quantum cNEO-DFT can be a promising method for describing the nuclear quantum effects in many chemical processes.

Nuclear Motion in the Intramolecular Dihydrogen-Bound Regime of an Aminoborane Complex.

The 1,3-diaza-2,4-diborobutane (NBNB) molecule serves as the smallest model complex of an intramolecular "dihydrogen bond," which involves a nominally hydrogen-bonding interaction between amine and borane hydrogen atoms. In the present study, the role of this dihydrogen bond in influencing the inherent molecular dynamics of NBNB is investigated computationally with ab initio molecular dynamics and path integral molecular dynamics techniques, as well as vibrational spectra analysis and static quantum chemistry computations. These simulations indicate that the dihydrogen-bonding interaction impacts both the high- and low-frequency motions of the molecule, with the dominant motions involving low-frequency backbone isomerization and terminal amine rotation. Geometric isotope effects were found to be modest. The analysis also addresses the paradoxical fostering of amine rotation via a relatively strong dihydrogen bond interaction. Electrostatic and geometric factors most directly explain this effect, and although some orbital evidence was found for a small covalent component of this interaction, the dynamics and electronic structure suggest that electrostatic contributions are the controlling factors for molecular motion in NBNB.

Geometric isotope effect of deuteration in a hydrogen-bonded host\u2013guest 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.

Effect of hydrogen bonding on the infrared absorption intensity of OH stretch vibrations

We consider how the infrared intensity of a hydrogen-bonded OH stretch varies from weak to strong H-bonds using a theoretical model. We obtain trends for the fundamental and overtone transition intensities as a function of the donor–acceptor distance, a common measure of H-bond strength. Building upon our earlier work using a two-diabatic state model, we introduce a Mecke function-based dipole moment for the H-bond and calculate transition moments using one-dimensional vibrational eigenstates along the H-atom transfer coordinate. The fundamental intensity is found to be over 20-fold enhanced for strong H-bonds, where non-Condon effects are significant. We analyse isotope effects, including the secondary geometric isotope effect. The first overtone intensity varies non-monotonically with H-bond strength; suppression occurs for weak bonds but strong enhancements are possible for strong H-bonds. We also study how these trends are affected by Mecke parameter variations. For a few specific dimers, we compare our results with earlier works.

Inverse Ubbelohde effect in the short hydrogen bond of photosystem II: Relation between H/D isotope effect and symmetry in potential energy profile.

The short hydrogen bond between tyrosine Yz and D1-His190 of photosystem II (PSII) was investigated using multicomponent quantum mechanics, where the quantum fluctuation of a hydrogen nucleus was incorporated into electronic structure calculation. Our computation demonstrated that the deuteration for hydrogen in the short hydrogen bond of PSII led to the reduction of the O…N distance. It indicated an inverse Ubbelohde effect typically recognized in strong and symmetric hydrogen-bonding clusters such as FHF(-) and H3O2-. We confirmed that the relation between the geometric isotope effect and the symmetry of the potential energy profile of FHF(-) was reasonably agreed with that of PSII. According to this agreement, the short hydrogen bond in PSII can be regarded as a short strong hydrogen bond. © 2016 Wiley Periodicals, Inc.

Isotopic fractionation in proteins as a measure of hydrogen bond length.

If a deuterated molecule containing strong intramolecular hydrogen bonds is placed in a hydrogenated solvent, it may preferentially exchange deuterium for hydrogen. This preference is due to the difference between the vibrational zero-point energy for hydrogen and deuterium. It is found that the associated fractionation factor Φ is correlated with the strength of the intramolecular hydrogen bonds. This correlation has been used to determine the length of the H-bonds (donor-acceptor separation) in a diverse range of enzymes and has been argued to support the existence of short low-barrier H-bonds. Starting with a potential energy surface based on a simple diabatic state model for H-bonds, we calculate Φ as a function of the proton donor-acceptor distance R. For numerical results, we use a parameterization of the model for symmetric O-H⋯O bonds [R. H. McKenzie, Chem. Phys. Lett. 535, 196 (2012)]. We consider the relative contributions of the O-H stretch vibration, O-H bend vibrations (both in plane and out of plane), tunneling splitting effects at finite temperature, and the secondary geometric isotope effect. We compare our total Φ as a function of R with NMR experimental results for enzymes, and in particular with an earlier model parametrization Φ(R), used previously to determine bond lengths.

H/D isotope effect on charge-inverted hydrogen-bonded systems: Systematic classification of three different types in H3XH···YH3 (X = C, Si, or Ge, and Y = B, Al, or Ga) with multicomponent calculation.

Three different H/D isotope effect in nine H3 XH(D)···YH3 (X = C, Si, or Ge, and Y = B, Al, or Ga) hydrogen-bonded (HB) systems are classified using MP2 level of multicomponent molecular orbital method, which can take account of the nuclear quantum nature of proton and deuteron. First, in the case of H3 CH(D)···YH3 (Y = B, Al, or Ge) HB systems, the deuterium (D) substitution induces the usual H/D geometrical isotope effect such as the contraction of covalent R(C-H(D)) bonds and the elongation of intermolecular R(H(D)···Y) and R(C···Y) distances. Second, in the case of H3 XH(D)···YH3 (X = Si or Ge, and Y = Al or Ge) HB systems, where H atom is negatively charged called as charge-inverted hydrogen-bonded (CIHB) systems, the D substitution leads to the contraction of intermolecular R(H(D)···Y) and R(X···distances. Finally, in the case of H3 XH(D)···BH3 (X = Si or Ge) HB systems, these intermolecular R(H(D)···Y) and R(X···Y) distances also contract with the D substitution, in which the origin of the contraction is not the same as that in CIHB systems. The H/D isotope effect on interaction energies and spatial distribution of nuclear wavefunctions are also analyzed.

Why does deuterium substitution lead to the contraction of X···π distance? Origin of the reverse Ubbelohde effect in XH···π interaction

The H/D/T geometrical isotope effect (GIE) on XH···π interaction in XH···benzene (X = CH3, OH, and NH2) complexes is analyzed by MP2 level of multi-component molecular orbital calculations, which can take account of nuclear quantum nature. The distances between X atom and center of the benzene ring in XD···benzene and XT···benzene complexes are found to be shorter than that in XH···benzene complex, which shows a different trend in the conventional hydrogen-bonded complexes. Such reverse Ubbelohde effect in XH···benzene complexes is explained by the cooperative effect of two kinds of GIEs: (1) primary GIE on the covalent X–H bond and (2) secondary GIE on the H···π distance. We have clearly revealed that the primary GIE is dominant in the GIE of XH···π complexes.

Nuclear quantum effects in a 1-D model of hydrogen bonded ferroelectrics

A one dimensional model of a coupled hydrogen (H) bonding chain is developed and parametrized to density functional theory (DFT) calculations on squaric acid, a prototypical H-bonded antiferroelectric crystal. The energetics of single and collective proton jumps and its dependence on H-bond length, as obtained by DFT, is reproduced quite well in the model despite its simplicity where only hydrogen and oxygen atom positions in (O-H...O) H-bonds and nearest-neighbor coupling between H positions are explicitly included. Classical and path- integral molecular dynamics simulations are performed to shed light on nuclear quantum effects and how they influence the paraelectric phase transition. A large H/D isotope shift in the transition temperature TC as well as a geometric isotope effect is obtained in good agreement with experiment. Fixing the O-O bond length results in shifts of TC to higher temperature but a pronounced isotope shift of TC remains, highlighting the importance of quantum effects beyond the geometrical changes in H-bonds accompanying isotopic substitution. Intermediate between fully atomistic models and simpler Ising-type models, the proposed H-bond chain model is a useful toy model for investigating microscopic mechanisms behind phase transitions in H-bonded ferroelectrics and the detailed role of quantum fluctuations.

The quantum nature of C-H···metal interaction: vibrational spectra and kinetic and geometric isotope effects of adsorbed cyclohexane.

The nature of C-H···M (M = metal surface) interactions is reviewed based mainly on our recent investigations of cyclohexane on Rh(111). Infrared reflection-absorption spectroscopy measurements at low temperature (∼20 K) have shown that the softened CH stretching band consists of several sharp peaks. At temperatures above 80 K, each peak is broadened, most probably by anharmonic coupling with thermally excited low-energy frustrated translational modes. The origin of fine structure in this band and its similarity to that in hydrogen bond systems are discussed. In addition, novel isotope effects were observed in desorption kinetics and adsorption geometry of cyclohexane on Rh(111) using temperature programmed desorption, ultraviolet photoelectron spectroscopy, and spot profile analysis low-energy electron diffraction. The desorption energy of deuterated cyclohexane (C6 D12 ) is lower than that of C6 H12 (inverse kinetic isotope effect). In addition, the work function change by adsorbed C6 D12 is smaller than that by adsorbed C6 H12 . These results indicate that C6 D12 molecules are slightly more distant from the surface than C6 H12 molecules (vertical geometric isotope effect). A lateral geometric isotope effect was also observed for the two-dimensional cyclohexane superstructures as a result of the repulsive interaction between interfacial dipoles (= work function change). These isotope effects are ascribed to the quantum nature of hydrogen involved in the C-H···M interaction.

H/D isotope effect on structures, binding energies, and basis set superposition errors in F−(H2O)n (n = 1–3) clusters

The H/D isotope effects on structures, binding energies, and basis set superposition errors (BSSEs) of hydrated fluoride anion clusters, F−(H2O)n (n=1–3), are theoretically analyzed by using the MP2 level of multi-component molecular orbital (MC_MO-MP2) method, in which quantum nature of proton/deuteron and electron–electron correlation are directly taken account. Our results clearly show that the additional water molecule to F−(H2O)n−1 cluster forms stronger water–water hydrogen bond than that in simple water cluster, whereas the additional F−–water hydrogen bond formation in F−(H2O)n cluster weakens the original F−–water hydrogen bonds in F−(H2O)n−1 cluster. The BSSEs estimated in the MC_MO-MP2 calculations are slightly larger than those in the conventional MP2 calculations, due to the H/D geometrical isotope effect on the intermolecular distances. Consequently, the order of stability in several F−(H2O)3 cluster isomers cannot be adequately evaluated without BSSE corrections in our MC_MO-MP2 calculations, rather than the conventional MP2 ones.

Effect of quantum nuclear motion on hydrogen bonding.

This work considers how the properties of hydrogen bonded complexes, X-H⋯Y, are modified by the quantum motion of the shared proton. Using a simple two-diabatic state model Hamiltonian, the analysis of the symmetric case, where the donor (X) and acceptor (Y) have the same proton affinity, is carried out. For quantitative comparisons, a parametrization specific to the O-H⋯O complexes is used. The vibrational energy levels of the one-dimensional ground state adiabatic potential of the model are used to make quantitative comparisons with a vast body of condensed phase data, spanning a donor-acceptor separation (R) range of about 2.4-3.0 Å, i.e., from strong to weak hydrogen bonds. The position of the proton (which determines the X-H bond length) and its longitudinal vibrational frequency, along with the isotope effects in both are described quantitatively. An analysis of the secondary geometric isotope effect, using a simple extension of the two-state model, yields an improved agreement of the predicted variation with R of frequency isotope effects. The role of bending modes is also considered: their quantum effects compete with those of the stretching mode for weak to moderate H-bond strengths. In spite of the economy in the parametrization of the model used, it offers key insights into the defining features of H-bonds, and semi-quantitatively captures several trends.

Communication: ab initio simulations of hydrogen-bonded ferroelectrics: collective tunneling and the origin of geometrical isotope effects.

Ab initio simulations that account for nuclear quantum effects have been used to examine the order-disorder transition in squaric acid, a prototypical H-bonded antiferroelectric crystal. Our simulations reproduce the >100 K difference in transition temperature observed upon deuteration as well as the strong geometrical isotope effect observed on intermolecular separations within the crystal. We find that collective transfer of protons along the H-bonding chains - facilitated by quantum mechanical tunneling - is critical to the order-disorder transition and the geometrical isotope effect. This sheds light on the origin of isotope effects and the importance of tunneling in squaric acid which likely extends to other H-bonded ferroelectrics.

Thermal Properties and Structures of CsHSO4 and CsDSO4 Crystals

Differential scanning calorimetry, thermogravimetric-differential thermal analysis, and Xray diffraction measurements were performed on cesium hydrogen sulfate (CsHSO 4) and deuterated CsDSO 4 crystals. The proton and deuterated compounds were confirmed to exhibit the superionic phase transition at 415.9 and 413.4 K, respectively. The II-III transition for the proton compound was observed in the temperature range of about 330400 K. The thermal decomposition and dehydration reactions of both compounds began at around 460 K. The decomposition continued up to around 1050 K, and the dehydration ended at around 720 K. The weight losses in the temperature ranges of 460-720 K and 720-1050 K were caused by the evaporation of H(D) 2O and SO 3, respectively. The space group symmetries and structural parameters, in phase III (monoclinic, P21/n) for CsHSO 4 and in phase II (monoclinic, P21/c) for CsHSO 4 and CsDSO 4, were determined at room temperature. The expansion of O-H-O hydrogen bond caused by the substitution of deuterium for hydrogen was observed to be 0.015(4) A. The geometric isotope effect on hydrogen-bond structure upon deuteration was realized in the CsHSO 4 crystal. The difference in morphology between as-grown CsHSO 4 and CsDSO 4 crystals was suggested to be caused by the large expansion of the O-H-O hydrogen bond upon deuteration on crystallization in D 2O aqueous solution.

Why is N···Be distance of NH3H+···DBeH shorter than that of NH3D+···HBeH? paradoxical geometrical isotope effects for partially isotope‐substituted dihydrogen‐bonded isotopomers

The partial isotope substitution for the change of geometrical parameters, interaction energies, and nuclear magnetic shielding tensors (σ) of dihydrogen-bonded NH3X(+)···YBeH (X, Y = H, D, and T) systems is analyzed. Based on the theoretical calculation, the distance between heavy atoms RN···Be of NH3H(+) ···DBeH is clearly found to be shorter than that in NH3D(+)···HBeH. Such apparently paradoxical geometrical isotope effect (GIE) on RN···Be is revealed by the cooperative effect of two kinds of (1) primary covalent-bonded GIE and (2) secondary dihydrogen-bonded one. We have demonstrated that (1) the covalent bond lengths become shorter by heavier isotope-substitution and (2) the dihydrogen-bonded distance RX···Y becomes shorter by heavier Y and lighter X isotope-substitution due to the difference of electronic structure reflected by the nuclear distribution. We have also found that interaction energy of NH3H(+)···DBeH is stronger than that of NH3D(+)···HBeH and isotopic deshielding effect of magnetic shielding becomes large in lighter isotope.

Geometric isotope effects on small chloride ion water clusters with path integral molecular dynamics simulations

The geometric isotope effects on the structures of hydrated chloride ionic hydrogen bonded clusters are explored by carrying out path integral molecular dynamics simulations. First, an outer shell coordinate is selected to display the rearrangement of single and multi hydration shell cluster structures. Next, to show the competition of intramolecular and intermolecular nuclear quantum effects, the intramolecular OH∗ stretching and intermolecular ion–water wagging motions are studied for single and multi shell structures, respectively. The results indicate that the intermolecular nuclear quantum effects stabilize the ionic hydrogen bonds in single shell structures, while they are destabilized through the competition with intramolecular nuclear quantum effects in multi shell structures. In addition, the correlations between ion–water stretching motion and other cluster vibrational coordinates are discussed. The results indicate that the intermolecular nuclear quantum effects on the cluster structures are strongly related to the cooperation of the water–water hydrogen bond interactions.