Deuterium Substitution Research Articles

H/D isotope effect on miscibility of polyethylene glycols with benzene homologues

The systematic study of miscibility of polyethylene glycols (PEGs) of four average molecular weights equal to 200 a.m.u., 300 a.m.u., 400 a.m.u., and 600 a.m.u., with regular and deuterated benzene homologues has been presented. All systems show phase diagrams with upper critical solution temperatures (UCSTs). The miscibility of PEGs/benzene derivatives systems increases in the range: PEG/p-xylene > PEG/m-xylene > PEG/ethylbenzene > PEG/o-xylene > PEG/toluene. Deuterium substitution makes the miscibility of the investigated systems better and this finding is consistent with the Rabinovich concept. The miscibility isotope effects grow with the molar mass of polyethylene and the highest value has been obtained for toluene. The scaling equation and NRTL model have been used for describing the phase diagrams.

Deuterium substitution effects on the structural and magnetic phase transitions of a hydrogen-bonded coordination polymer, bis(glycolato)copper(ii)

Bis(glycolato)copper(ii) [Cu(HOCH2CO2)2] shows a structural phase transition around 220 K under cooling and 270 K under heating, and ferromagnetic ordering at 1.1 K. Deuterium substitution of the hydroxyl groups in it induced a large shift in the structural transition temperature while no shift in the ferromagnetic transition temperature took place.

Copper and palladium complexes with substituted pyrimidine-2-thiones and 2-thiouracils: syntheses, spectral characterization, and X-ray crystallographic study

Copper(I) and palladium(II) complexes containing 5-acetyl-6-methyl-1,2,3,4-tetrahydropyrimidine-2-thione (L1), ethyl 4,6-dimethyl-2-thioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate (L2), cis-5-acetyl-6-ethyl-5,6-dihydro-2-thiouracil (L3), and 5,6-dihydro-2-thiouracil (L4) have been synthesized. All complexes were characterized by elemental analysis, IR, 1H, and 13C NMR spectroscopy. To assign bands in the IR spectra of L1 and L2 and complexes with Cu(I) and Pd(II), deuterium substitution of movable protons at N-atoms was used. The crystal structures of two compounds, [Cu(L2)2Cl] and [Pd(L4)2Cl2], were determined by X-ray single-crystal and powder diffraction, respectively. In [Cu(L2)2Cl], copper has a rare coordination number of three and triangular surrounding of two neutral L2 molecules, coordinated through sulfurs, and chloride. In [Pd(L4)2Cl2], palladium has a standard square-planar geometry, formed by two uracil molecules and two chlorides. A new method for the synthesis of 5,6-dihydro-2-thiouracil, starting from β-aminopropionic acid, was suggested.

Spectroscopic study on deuterated benzenes. I. Microwave spectra and molecular structure in the ground state.

We observed microwave absorption spectra of some deuterated benzenes and accurately determined the rotational constants of all H/D isotopomers in the ground vibrational state. Using synthetic analysis assuming that all bond angles are 120°, the mean bond lengths were obtained to be r0(C-C) = 1.3971 Å and r0(C-H) = r0(C-D) = 1.0805 Å. It has been concluded that the effect of deuterium substitution on the molecular structure is negligibly small and that the mean bond lengths of C-H and C-D are identical unlike small aliphatic hydrocarbons, in which r0(C-D) is about 5 mÅ shorter than r0(C-H). It is considered that anharmonicity is very small in the C-H stretching vibration of aromatic hydrocarbons.

Keto-Enol Tautomeric Equilibrium of Acetylacetone Solution Confined in Extended Nanospaces.

We aim to clarify the effects of size confinement, solvent, and deuterium substitution on keto-enol tautomerization of acetylacetone (AcAc) in solutions confined in 10-100 nm spaces (i.e., extended nanospaces) using (1)H NMR spectroscopy. The keto-enol equilibrium constants of AcAc (K(EQ) = [keto]/[enol]) in various solvents confined in extended nanospaces of 200-3000 nm were examined using the area ratios of -CH3 peaks in keto to enol forms. The results showed that the keto form of AcAc in hydrogen-bonded solvents such as water and ethanol increased drastically with decreasing space sizes below about 500 nm, but the size confinement did not induce equilibrium shifts in aprotic solvents such as DMSO. The magnitudes of K(EQ) enhancement were well correlated with solvent proton donicity. It followed from the determination of thermodynamic parameters that the stabilization of intermolecular interactions between protons in water and carbonyl oxygen (C═O) in the keto form of AcAc were promoted by size-confinement, and that the keto form could be energetically and structurally favored in extended nanospaces vis-à-vis the bulk space. Furthermore, the measurements of deuterium dependence of the K(EQ) values verified that the nanoconfinement-induced shifts of keto-enol tautomerization of AcAc are attributable to high proton mobility via a proton hopping mechanism of the confined water.

Reaction of Laser-Ablated Uranium and Thorium Atoms with H2Se: A Rare Example of Selenium Multiple Bonding.

The compounds H2ThSe and H2USe were synthesized by the reaction of laser-ablated actinide metal atoms with H2Se under cryogenic conditions following the procedures used to synthesize H2AnX (An = Th, U; X = O, S). The molecules were characterized by infrared spectra in an argon matrix with the aid of deuterium substitution and electronic structure calculations at the density functional theory level. The main products, H2ThSe and H2USe, are shown to have a highly polarized actinide-selenium triple bond, as found for H2AnS on the basis of electronic structure calculations. There is an even larger back-bonding of the Se with the An than found for the corresponding sulfur compounds. These molecules are of special interest as rare examples of multiple bonding of selenium to a metal, particularly an actinide metal.

Study of the degree of deuteration on the microhardness of deuterated potassium dihydrogen phosphate crystals

A series of deuterated potassium dihydrogen phosphate (DKDP) crystals with different degrees of deuteration are grown from aqueous solution by the point-seed technique. The microhardness of (100), (001) and so-called ‘tripler’ faces for these DKDP crystals were measured. Initially the hardness number of (001) face for each crystal increases with the increase of the applied load until it reaches 25 g. With further increase in load, the hardness number decreases gradually. The hardness numbers decline with the increase in deuterium content. These composition dependences are expected since the bond strength is weakened by the substitution of deuterium for hydrogen. The hydrogen bond is considered to play the key role in effecting the crystal’s hardness. The visible hardness anisotropy of the different faces is attributed to the inhomogeneous distribution of the oxygen–hydrogen bond on these faces.

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.

Ferroelectric Thin Films of Deuterated Betaine Arsenate

Ferroelectric thin films of partially deuterated betaine arsenate have been grown by evaporation method on NdGaO3 and α-Al2O3 substrates. The films consist of small single-crystal blocks, in which ferroelastic domains have been visualized by polarizing microscope. X-ray diffraction shows that the blocks form a polycrystalline structure of film. Ferroelectric phase transition in DBA films is followed by dielectric anomaly and appearance of dielectric hysteresis loops. The magnitude of dielectric anomaly is small as compared with DBA single crystals. Small value of anomaly is associated with polycrystalline structure of DBA films. The deuteration degree of betaine arsenate films is smaller than that of crystals used for film preparation due to substitution of deuterium by hydrogen ions during the crystallization process.

Reaction Path Bifurcation in an Electrocyclic Reaction: Ring-Opening of the Cyclopropyl Radical.

Following previous work [J. Chem. Phys. 2013, 139, 154108] on a simple model of a reaction with a post-transition state valley ridge inflection point, we study the chemically important example of the electrocyclic cyclopropyl radical ring-opening reaction using direct dynamics and a reduced dimensional potential energy surface. The overall reaction requires con- or disrotation of the methylenes, but the initial stage of the ring-opening involves substantial internal rotation of only one methylene. The reaction path bifurcation is then associated with the relative sense of rotation of the second methylene. Clear deviations of reactive trajectories from the disrotatory intrinsic reaction coordinate (IRC) for the ring-opening are observed and the dynamical mechanism is discussed. Several features observed in the model system are found to be preserved in the more complex and higher dimensional ring-opening reaction. Most notable is the sensitivity of the reaction mechanism to the shape of the potential manifested as a Newtonian kinetic isotope effect upon deuterium substitution of one of the methylene hydrogens. Dependence of the product yield on frictional dissipation representing external environmental effects is also presented. The dynamics of the post-transition state cyclopropyl radical ring-opening are discussed in detail, and the use of low dimensional models as tools to analyze complicated organic reaction mechanisms is assessed in the context of this reaction.

Hydroxy nitrate production in the OH-initiated oxidation of alkenes

Abstract. Alkenes are oxidized rapidly in the atmosphere by addition of OH and subsequently O2 leading to the formation of β-hydroxy peroxy radicals. These peroxy radicals react with NO to form β-hydroxy nitrates with a branching ratio α. We quantify α for C2–C8 alkenes at 295 K ± 3 and 993 hPa. The branching ratio can be expressed as α = (0.045 ± 0.016) × N − (0.11 ± 0.05) where N is the number of heavy atoms (excluding the peroxy moiety), and listed errors are 2σ. These branching ratios are larger than previously reported and are similar to those for peroxy radicals formed from H abstraction from alkanes. We find the isomer distributions of β-hydroxy nitrates formed under NO-dominated peroxy radical chemistry to be different than the isomer distribution of hydroxy hydroperoxides produced under HO2-dominated peroxy radical chemistry. Assuming unity yield for the hydroperoxides implies that the branching ratio to form β-hydroxy nitrates increases with substitution of RO2. Deuterium substitution enhances the branching ratio to form hydroxy nitrates in both propene and isoprene by a factor of ~ 1.5. The role of alkene chemistry in the Houston region is re-evaluated using the RONO2 branching ratios reported here. Small alkenes are found to play a significant role in present-day oxidant formation more than a decade (2013) after the 2000 Texas Air Quality Study identified these compounds as major contributors to photochemical smog in Houston.

Production of deuterated switchgrass by hydroponic cultivation

The bioenergy crop switchgrass was grown hydroponically from tiller cuttings in 50 % D 2 O to obtain biomass with 34 % deuterium substitution and physicochemical properties similar to those of H 2 O-grown switchgrass controls. Deuterium enrichment of biological materials can potentially enable expanded experimental use of small angle neutron scattering (SANS) to investigate molecular structural transitions of complex systems such as plant cell walls. Two key advances have been made that facilitate cultivation of switchgrass, an important forage and biofuel crop, for controlled isotopic enrichment: (1) perfusion system with individual chambers and (2) hydroponic growth from tiller cuttings. Plants were grown and maintained for several months with periodic harvest. Photosynthetic activity was monitored by measurement of CO2 in outflow from the growth chambers. Plant morphology and composition appeared normal compared to matched controls grown with H2O. Using this improved method, gram quantities of switchgrass leaves and stems were produced by continuous hydroponic cultivation using growth medium consisting of basal mineral salts in 50 % D2O. Deuterium incorporation was confirmed by detection of the O-D and C-D stretching peaks with FTIR and quantified by (1)H- and (2)H-NMR. This capability to produce deuterated lignocellulosic biomass under controlled conditions will enhance investigation of cell wall structure and its deconstruction by neutron scattering and NMR techniques.

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.

Isotopic Probe Illuminates the Role of the Electrode Surface in Proton Coupled Hydride Transfer Electrochemical Reduction of Pyridinium on Pt(111)

A recently proposed mechanism for electrochemical CO2 reduction on Pt (111) catalyzed by aqueous acidic pyridine solutions suggests that the observed redox potential of ca. −600 mV vs. SCE is due to the one-electron reduction of pyridinium through proton coupled electron transfer (PCET) to form H atoms adsorbed on the Pt surface (Hads). The initial pyridinium reduction was probed isotopically via deuterium substitution. A combined experimental and theoretical analysis found equilibrium isotope effects (EIE) due to deuterium substitution at the acidic pyridinium site. A shift in the cathodic cyclic voltammetric half wave potential of −25 mV was observed, consistent with the theoretical prediction of −40 mV based on the recently proposed reaction mechanism where pyridinium is essential to establish a high concentration of Brønsted acid in contact with the substrate CO2 and with the Pt surface. A prefeature in the cyclic voltammogram was examined under isotopic substitution and indicated an Hads intermediate in pyridinium reduction. Theoretical prediction and observation of an EIE supported the assignment of the cathodic wave to the proposed reduction of pyridinium through PCET forming Hads and eventually H2 on the Pt surface.

On the tunneling instability of a hypercoordinated carbocation.

C(CH3)5(+), a pentacoordinate carbocation, was recently described as a "fleeting" molecule, that is, stable only at low temperatures. Herein it is shown using theoretical methods that, owing to carbon tunneling, the molecule is not persistent even at 0 K. One possible way to lengthen its lifespan is by deuterium substitution, in spite of the fact that no bond to hydrogen is broken.

Production of bacterial cellulose with controlled deuterium-hydrogen substitution for neutron scattering studies.

Isotopic enrichment of biomacromolecules is a widely used technique that enables the investigation of the structural and dynamic properties to provide information not accessible with natural abundance isotopic composition. This study reports an approach for deuterium incorporation into bacterial cellulose. A media formulation for growth of Acetobacter xylinus subsp. sucrofermentans and Gluconacetobacter hansenii was formulated that supports cellulose production in deuterium (D) oxide. The level of D incorporation can be varied by altering the ratio of deuterated and protiated glycerol used during cell growth in the D2O-based growth medium. Spectroscopic analysis and mass spectrometry show that the level of deuterium incorporation is high (>90%) for the perdeuterated form of bacterial cellulose. The small-angle neutron scattering profiles of the cellulose with different amounts of D incorporation are all similar indicating that there are no structural changes in the cellulose due to substitution of deuterium for hydrogen. In addition, by varying the amount of deuterated glycerol in the media it was possible to vary the scattering length density of the deuterated cellulose. The ability to control deuterium content of cellulose extends the range of experiments using techniques such as neutron scattering to reveal information about the structure and dynamics of cellulose, and its interactions with other biomacromolecules as well as synthetic polymers used for development of composite materials.

Development of approaches for deuterium incorporation in plants.

Soon after the discovery of deuterium, efforts to utilize this stable isotope of hydrogen for labeling of plants began and have proven successful for natural abundance to 20% enrichment. However, isotopic labeling with deuterium ((2)H) in higher plants at the level of 40% and higher is complicated by both physiological responses, particularly water exchange through transpiration, and inhibitory effects of D2O on germination, rooting, and growth. The highest incorporation of 40-50% had been reported for photoheterotrophic cultivation of the duckweed Lemna. Higher substitution is desirable for certain applications using neutron scattering and nuclear magnetic resonance (NMR) techniques. (1)H(2)H NMR and mass spectroscopy are standard methods frequently used for determination of location and amount of deuterium substitution. The changes in infrared (IR) absorption observed for H to D substitution in hydroxyl and alkyl groups provide rapid initial evaluation of incorporation. Short-term experiments with cold-tolerant annual grasses can be carried out in enclosed growth containers to evaluate incorporation. Growth in individual chambers under continuous air perfusion with dried sterile-filtered air enables long-term cultivation of multiple plants at different D2O concentrations. Vegetative propagation from cuttings extends capabilities to species with low germination rates. Cultivation in 50% D2O of annual ryegrass and switchgrass following establishment of roots by growth in H2O produces samples with normal morphology and 30-40% deuterium incorporation in the biomass. Winter grain rye (Secale cereale) was found to efficiently incorporate deuterium by photosynthetic fixation from 50% D2O but did not incorporate deuterated phenylalanine-d8 from the growth medium.

Initial excited-state structural dynamics of 6-substituted uracil derivatives: femtosecond angle and bond lengthening dynamics in pyrimidine nucleobase photochemistry.

Substituents on the pyrimidine ring of nucleobases appear to play a major role in determining their initial excited-state structural dynamics and resulting photochemistry. To better understand the determinants of nucleobase initial excited-state structural dynamics, we have measured the absorption and resonance Raman excitation profiles of 6-deuterouracil (6-d-U) and 6-methyluracil (6-MeU). Simulation of the resonance Raman excitation profiles and absorption spectrum with a self-consistent, time-dependent formalism shows the effect of the deuterium and methyl group on the photochemically active internal coordinates, i.e. C5C6 stretch and C5X and C6X bends. The methyl group on either the C5 or C6 position of uracil equally increases the excited-state reorganization energies along the C5C6 stretch. However, a lower reorganization energy of the C5X + C6X bends in 6-MeU than uracil and 5-MeU shows that C6 methyl substituents reduce the bending reorganization energy. In addition, deuterium substitution at either C5 or C6 has a much smaller effect on the initial excited-state structural dynamics than methyl substitution, consistent with a mass effect. These results will be discussed in light of the resulting photochemistry of pyrimidine nucleobases.

Deuterium Isotope Effect on Solution Behavior of Thermoresponsive Star-Shaped Poly(2-isopropyl-2-oxazoline)

Thermosensitive star-shaped poly(2-isopropyl-2-oxazoline) (molar mass M≈21000 g mol−1) in D2O solution was studied by the static and dynamic light scattering methods. The behavior of the polymer investigated in deuterated water is similar qualitatively to that observed previously in undeuterated water. At the same time, the considerable quantitative changes of polymer behavior in D2O were seen. Deuterium substitution of solvent affects the phase transition temperature by decreasing its value by 1°C. The temperature interval of phase transition in D2O solution expands (by about 1°C) in comparison with that in H2O solution.

Phase behavior of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide with chloroform and/or chloroform/carbon tetrachloride mixed solvent

The phase behavior of liquid mixtures of the ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([C2MIM][NTf2]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([C4MIM][NTf2]), 1-pentyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([C5MIM][NTf2]) with chloroform and chloroform/carbon tetrachloride mixed solvent including the effect of deuterium substitution in chloroform has been studied. Using the cloud point method the phase diagrams were obtained showing very complicated miscibility behavior. Thus phase diagrams with the upper critical solution temperature (UCST) were obtained in most cases however under some conditions the phase diagrams with the lower critical solution temperature (LCST) or even with the both (UCST and LCST) were also observed. The “hourglass” type of the phase diagrams constitutes another type of the phase behavior observed in this study. Deuterium substitution affects the miscibility very seriously, in particular in the systems with the mixed solvent (CHCl3/CCl4). In general the worse miscibility is observed, and deuteration induces large isotope shifts of the UCST and LCST, provoking in some cases the transfer from the UCST/LCST behavior to the “hourglass” phase diagram.