Unusual Isotope Effects Research Articles

Rotational spectra and internal dynamics of Ne—H2S

Rotational spectra of 15 isotopomers of the Ne-H2S van der Waals complex were measured in the frequency range 4–22 GHz using a pulsed molecular beam Fourier transform spectrometer. Two K = 0 progressions were observed for each of the symmetric isotopomers (with H2S or D2S). This doubling is attributed to an internal rotation motion of the H2S subunit within the complex. These two states can be correlated with the 000 and 101 rotational states of free H2S and D2S. By contrast, symmetry constraints no longer apply to isotopomers with DHS. The excited internal rotor state is no longer metastable, and only one K = 0 progression could be observed. The rotational constants obtained were compared with those of Ar-H2S and Ar—H2O. The ground state rotational constant remained almost constant upon substitution of H with D, showing an unusual isotope effect, similarly to a previous observation in Ar-H2S (GUTOWSKY, H. S., EMILSSON, T., and ARUNAN, E., 1997, J. chem. Phys., 106, 5309). This behaviour is in agreement with the ab initio study by OLIVEIRA, G. D., and DYKSTRA, C. E., 1999, J. chem. Phys., 110, 289. An approximate substitution analysis was carried out to deduce structural information from the ground state rotational constants. Nuclear quadrupole hyperfine structures were observed and resolved or partially resolved for isotopomers containing 33S and D, respectively, and the corresponding nuclear quadrupole coupling constants were determined. These were used to derive information about the internal dynamics of the dimer. Different sensitivities of the quadrupole coupling constants of D and 33S to the extent of out-of-plane motion were revealed.

Analytical three-body interaction potentials and hydrogen bond dynamics of hydrogen fluoride aggregates, (HF)n, n≥3

Oligomeric aggregates of hydrogen fluoride are important prototype molecules for a detailed understanding of the structure, energetics, spectroscopy and dynamics of hydrogen bonding. The pairwise additive description of these oligomers is known to be inadequate. We have sampled the three-body potential for HF at 3000 (HF)3 configurations selected by various classical and quantum sampling techniques, including dynamic sampling based on Voronoi step representation. The counterpoise-corrected Møller-Plesset second-order three-body energies using a double zeta Gaussian basis set with polarization functions (DZP+MP2) at these configurations are fitted by analytical 12-dimensional potentials. Cooperative effects are found to be sizeable and predominantly stabilizing in hydrogen fluoride ring aggregates. Test calculations with larger basis sets and for larger HF aggregates show that in combination with available high quality pair potentials, the analytical three-body terms give an excellent description of the (HF)3 surface in the hydrogen bonding region and a good approximation for clusters up to at least the hexamer. Multidimensional vibrational quantum Monte Carlo calculations indicate that degenerate HF stretch excitation in (HF)3 (3712cm−1) is in close coincidence with (HF)3→3HF dissociation channels at low HF angular momentum, whereas degenerate DF stretch excitation in (DF)3 (2725cm−1) falls slightly below any (DF)3→(DF)2+DF dissociation channels. The (HF)3 potential surface, its stationary points, possible interconversion tunneling pathways, zero point energies, adiabatic channels, unusual isotope effects, fully centrifugal rotational states and the harmonic infrared spectrum are discussed in detail and compared to ab initio calculations and experiment. The applicability of the (1+2+3)-body approach for larger oligomers (3<n<8) is investigated with special emphasis on structure, energetics, infrared and microwave spectra, and predissociation. Neglect of four- and higher-body contributions and hydrogen exchange symmetry is found to affect some properties significantly, but the preference for simple ring structures remains pronounced.

Stripes, vibrations, and superconductivity

We propose a model of a spatially modulated collective charge state of superconducting cuprates. The regions of higher carrier density (stripes) are described in terms of Luttinger liquids and the regions of lower density as a two-dimensional interacting bosonic gas of d_{x^2-y^2} hole pairs. The interactions among the elementary excitations are repulsive and the transition to the superconducting state is driven by decay processes. Vibrations of the CCS and the lattice, although not participating directly in the binding mechanism, are fundamental for superconductivity. The superfluid density and the lattice have a strong tendency to modulation implying a still unobserved dimerized stripe phase in cuprates. The phase diagram of the model has a crossover from 1D to 2D behavior and a pseudogap region where the amplitude of the order parameters are finite but phase coherence is not established. We discuss the nature of the spin fluctuations and the unusual isotope effect within the model.

Structural and functional characterization of second-coordination sphere mutants of soybean lipoxygenase-1.

Lipoxygenases are an important class of non-heme iron enzymes that catalyze the hydroperoxidation of unsaturated fatty acids. The details of the enzymatic mechanism of lipoxygenases are still not well understood. This study utilizes a combination of kinetic and structural probes to relate the lipoxygenase mechanism of action with structural modifications of the iron's second coordination sphere. The second coordination sphere consists of Gln(495) and Gln(697), which form a hydrogen bond network between the substrate cavity and the first coordination sphere (Asn(694)). In this investigation, we compared the kinetic and structural properties of four mutants (Q495E, Q495A, Q697N, and Q697E) with those of wild-type soybean lipoxygenase-1 and determined that changes in the second coordination sphere affected the enzymatic activity by hydrogen bond rearrangement and substrate positioning through interaction with Gln(495). The nature of the C-H bond cleavage event remained unchanged, which demonstrates that the mutations have not affected the mechanism of hydrogen atom tunneling. The unusual and dramatic inverse solvent isotope effect (SIE) observed for the Q697E mutant indicated that an Fe(III)-OH(-) is the active site base. A new transition state model for hydrogen atom abstraction is proposed.

An Unusual Solvent Isotope Effect in the Reaction of W(CO)5(solv) (solv = Cyclohexane or Cyclohexane-d12) with THF

Time- and temperature-resolved infrared absorption spectroscopy is used to probe the reaction of THF with W(CO)5(solv) (solv = cyclohexane, cyclohexane-d12) to form W(CO)5(THF). In cyclohexane-d0, ΔH⧧ for the reaction is determined to be 3.6 ± 0.2 kcal mol-1 (15.2 ± 0.9 kJ mol-1) and ΔS⧧ = −13.7 ± 2.5 eu (−57.3 ± 10.5 J mol-1 K-1). In cyclohexane-d12, ΔH⧧ = 2.4 ± 0.6 kcal mol-1 (10.2 ± 2.5 kJ mol-1) and ΔS⧧ = −18.3 ± 3.5 eu (−76.6 ± 14.6 J mol-1 K-1). The low activation enthalpy in comparison to the (CO)5W−cyclohexane bond dissociation energy, the negative entropy of activation, and comparison with other spectroscopic experiments involving reactions of W(CO)5(cyclohexane) indicate that the reaction proceeds by an associative interchange reaction mechanism. An unusual isotope effect, in which reaction proceeds more slowly in cyclohexane-d12 even though ΔH⧧ is lower, is explained in terms of relative populations of low-frequency vibrations and the relative C−H and C−D bond strengths in the solvent molecule released upon uptake of THF. These experimental results are supported by the results of an ab initio DFT calculation of the structure and vibrational spectrum of W(CO)5(cyclohexane).

An Unusual Isotope Effect in the Reactions of the Naphthylcarbenes

An Unusual Isotope Effect in the Reactions of the Naphthylcarbenes

An intramolecular theory of the mass-independent isotope effect for ozone. I

An intramolecular theory of the unusual mass-independent isotope effect for ozone formation and dissociation is described. The experiments include the enrichment factor, its dependence on the ambient pressure, the ratio of the formation rates of symmetric and asymmetric ozone isotopomers, the enrichment of ozone formed from heavily enriched oxygen isotopes, the comparison of that enrichment to that when the heavy isotopes are present in trace amounts, the isotopic exchange rate constant, and the large mass-dependent effect when individual rate constants are measured, in contrast with the mass-independent effect observed for scrambled mixtures. To explain the results it is suggested that apart from the usual symmetry number ratio of a factor of 2, the asymmetric ozone isotopomers have a larger density of reactive (coupled) quantum states, compared with that for the symmetric isotopomers (about 10%), due to being more “RRKM-like” (Rice–Ramsperger–Kessel–Marcus): Symmetry restricts the number of intramolecular resonances and coupling terms in the Hamiltonian which are responsible for making the motion increasingly chaotic and, thereby, increasingly statistical. As a result the behavior occurs regardless of whether the nuclei are bosons (16O, 18O) or fermions (17O). Two alternative mechanisms are also considered, one invoking excited electronic states and the other invoking symmetry control in the entrance channel. Arguments against each are given. An expression is given relating the mass-independent rates of the scrambled systems to the mass-dependent rates of the unscrambled ones, and the role played by a partitioning term in the latter is described. Different definitions for the enrichment factor for heavily enriched isotopic systems are also considered. In the present paper attention is focused on setting up theoretical expressions and discussing relationships. They provide a basis for future detailed calculations.

NUCLEAR SIZE AND SHAPE EFFECT IN CHEMICAL ISOTOPE EFFECT OF GADOLINIUM USING DICYCLOHEXANO-18-CROWN-6

ABSTRACT Gadolinium isotopes were fractionated in a liquid-liquid extraction system using dicyclohexano-18-crown-6. The isotope enrichment factors showed breakdowns of the conventional Bigeleisen-Mayer approximation for every condition of the initial phases; the concentrations of gadolinium and hydrochloric acid and the type of organic solvents. The nuclear mass effect and the field shift effect were estimated by use of the isotope pairs 152Gd-160Gd and 154Gd-160Gd. Since both isotopes, 152Gd and 154Gd, have the unique characteristics of the nuclear charge radii, <r2>'s, the unusual isotope effects due to the field shifts took place, while the mass effects of the other isotopes canceled the field shift effects. The maximum isotope effect was observed to be ϵ160,152 =0·0252 ± 0·0024 whose initial aqueous phase was 0·023 M GdCl3 in 12 M HC1, and its organic phase was 0·2 M DC18C6 in chloroform: this was 0·00315 ± 0·00030 in terms of the enrichment factor for unit mass.

Unexpectedly large O37ClO/O35ClO intensity ratios of the fluorescence from the low-energy vibrational levels of OClO (Ã 2A2)

The fluorescence intensity ratios of O37ClO/O35ClO excited to the à 2A2 state have been found to be much larger than that expected from the 37Cl/35Cl natural abundance ratio for several low-energy vibrational levels. For instance, the O37ClO/O35ClO ratios are 1.67 and &amp;gt;10 for the (0,0,0) and (1,1,0) bands, respectively, which are 5 and 30 times larger than those expected from the natural abundance ratio. These unusual isotope effects are observed in a narrow energy region within ∼1000 cm−1 above the electronic origin. It is likely that the isotopomer-selective, near-resonant spin-orbit coupling of the low-energy vibrational levels of the 2A2 state with those of the 2A1 state, which provides a decay channel for the 2A2 state via spin-orbit coupling, is responsible for the unusual isotope effect.

Nuclear magnetic resonance study of the low-temperature localized H(D) motion inα−ScHx(Dx): Isotope effects

Nuclear magnetic resonance measurements of the {sup 45}Sc, {sup 1}H, and {sup 2}D spin-lattice relaxation rates in the solid solutions {alpha}-ScH{sub x} (x=0, 0.11, and 0.27) and {alpha}-ScD{sub x} (x=0.05 and 0.22) have been performed over the temperature range 4.2{endash}294 K. For the hydrided and deuterided samples we have found pronounced deviations of the {sup 45}Sc relaxation rate measured at 86.2 MHz from the Korringa behavior in the range 30{endash}180 K. The excess {sup 45}Sc relaxation rate displays a peak near 100 K in {alpha}-ScH{sub x} and near 110{endash}140 K in {alpha}-ScD{sub x}, resulting from the quadrupole interaction modulated by fast localized hopping of H(D) atoms. The amplitude of the {sup 45}Sc relaxation rate peak in the deuterided samples is found to be much higher than in the hydrided samples with comparable hydrogen content. This unusual isotope effect indicates that the fraction of D atoms participating in the fast localized motion in {alpha}-ScD{sub x} is approximately six times as large as the corresponding fraction of H atoms in {alpha}-ScH{sub x}. The results of our measurements of the {sup 1}H and {sup 2}D relaxation rates are consistent with this conclusion. The strong effect of H{leftrightarrow}D substitution on the fraction of atomsmore » involved in the localized hopping should be accounted for in any microscopic model of hydrogen motion in Sc. {copyright} {ital 1999} {ital The American Physical Society}« less

Unusual Isotope Effects of Molybdenum in Chemical Exchange Reaction Using Dicyclohexano-18-Crown-6

Abstract Molybdenum isotopes were fractionated in a liquid–liquid extraction system using dicyclohexano-18-crown-6 (DC18C6). The enrichment factors showed a breakdown of the conventional mass-dependent rule. Some unusual and larger isotope effects were observed in the even atomic mass isotopes, 92Mo and 94Mo. The unusual features in the present study were not responsible for the field shift effect, which was regarded as a primary factor of the anomalous isotope effect in the recent theory, but were due to an anomaly on the vibrational levels. The largest isotope effect was observed in the isotope pair of 94Mo–96Mo, it was ε96,94 = 0.0086 ± 0.0007, its initial aqueous phase was 0.91 M molybdenum chloride, and its organic phase was 0.2 M DC18C6 in chloroform: this was 0.0043 ± 0.0004 in terms of the enrichment factor for unit mass difference.

From Röntgen to Ioffe, from Giessen to Saint Petersburg — Relations between Russian and German physics

Two former professors of physics at Giessen university contributed significantly to the development of the Ioffe Institute: Wilhelm Conrad Rontgen as a teacher of Abram Ioffe, and Wilhelm Hanle, whose effect found various applications, e.g., in the spectroscopy of hot electrons in low-dimensional structures. A few examples will illustrate how topics of their scientific work found a continuation in the research activities at Giessen, but also in the collaboration between Giessen and Saint Petersburg. They range from sodium chloride, the old Rontgen/Ioffe material where we could prove the existence of an unusual isotope effect in nickel-doped crystals, over level-crossing experiments in gases, to GaAs/AlAs superlattices, where level-anticrossing spectroscopy of excitons reveals detailed information about recombination processes and interface quality. A short summary of the efforts to keep the traditionally close and good relations between Russian and German physics vital completes the report.

Unusual Thresholds and Isotope Effects in Al+ + H2/D2/HD Reactions

Ab initio quantum chemistry is used to generate a three-dimensional reactive potential-energy surface for the collision of 1S Al+ ions with ∑g H2 molecules. This surface, in a tessellated and locally interpolated form, is used to generate forces for classical trajectory simulations of the 3.98 eV endothermic Al+ + H2 f AlH+ + H reactions with initial conditions appropriate to a thermal H2 sample and an Al+ beam of specified center of mass collision kinetic energies in the 3-20 eV range. Our findings indicate that the reaction occurs not on (or near) the collinear path, which has no barrier above the reaction endothermicity, but via a near-C2V insertive path which spontaneously breaks C2V symmetry via second-order Jahn-Teller distortion to permit flux to evolve to AlH+ + H products. The strong propensity to “avoid” the collinear path and to follow a higherenergy route is caused, at long range, by the ion-quadrupole interaction between Al+ and H2 and, at shorter range, by favorable overlap between the H2 σu and Al+ 3p obitals. Examination of a large number of trajectories shows clearly that reactive collisions (1) lose much of their initial kinetic energy to the repulsive ionmolecule interfragment potential as the closed-shell Al+ and H2 approach, (2) transfer significant energy to the H-H stretching coordinate, thus weakening the H-H bond, (3) convert initial H2 rotational motion as well as Al+ to H2 collisional angular momentum into rotational angular momentum of the HAlH+ complex, “locking” the H2 moiety into the insertive near-C2V geometry about which twisting motion occurs, and (4) allow the Al+ ion to form a new bond with whichever H atom is nearest it when the system crosses into regions of the energy surface where the H-Al-H asymmetric stretch mode becomes second-order JahnTeller unstable, thus allowing fragmentation into AlH+ + H. These findings, combined with considerations of kinematic factors that distinguish among H2, D2, and HD, allow us to explain certain unusual threshold and isotope effects seen in the experimental reaction cross-section data on these reactions.

On the unexpected isotope effect on the exchange coupling constant in partially deuterated transition‐metal trihydride complexes

AbstractIn this paper we attempt to explain the unusual isotope effect displayed by the 1H‐NMR scalar H‐H coupling of the hydrides in complexes [CpIrL(H)3]+ (L=various phosphine and phosphite ligands) when one of the equivalent protons is turned into a deuteron (D.M. Heinekey, J.M. Millar, T.F. Koetzle, N.G. Payne, and K.W. Zilm, J. Am. Chem. Soc. 112, 909 (1990)]. It is found that in the case of the perprotio species the system can be described as a three‐coupled well system, but when a deuterium is introduced the system is better described as a double‐coupled well. Finally, it is shown that the coupling in the case of a double‐well system is greater than in an equivalent triple‐well system, in this way giving a plausible explanation to the experimental facts reported.

Vibrational excitation of H2O and HOD molecules produced by reactions of OH and OD with cyclo-C6H12, n-C4H10, neo-C5H12, HCl, DCl and NH3 as studied by infrared chemiluminescence

The room-temperature reactions of OH(OD) radicals with cyclo-C6H12, n-C4H10, and neo-C5H12 have been investigated by observing the infrared chemiluminescence from the H2O(HOD) molecules generated in a fast-flow reactor. These hydrocarbon molecules are representative for abstraction from secondary and primary C–H bonds. The total vibrational energy released to H2O(HOD) was in the range of 〈fv〉=0.55–0.65. The majority (80%–85%) of the vibrational energy is in the stretching modes and the main energy release is to the local mode associated with the new OH bond. The dynamics associated with the energy disposal to H2O(HOD) resemble the H+L−H dynamics for the analogous reactions of F atoms. The data from H2O and HOD are complementary because of the different collisional coupling between the energy levels of the ν1, ν2, and ν3 modes; however, no specific isotope effect was found for the energy disposal to H2O versus HOD for reactions with the hydrocarbon molecules. In contrast, a very unusual isotope effect was found between the OH+HCl and OD+HCl pairs. The latter reaction gave the expected stretching mode excitation of HOD; however, the OH reaction gave H2O molecules with virtually no vibrational energy. This anomalous situation is partly associated with an inverse secondary kinetic-isotope effect, but the main isotope effect is on the dynamics of the energy disposal process itself.

Deuterium kinetic isotope effects and the mechanism of the bacterial luciferase reaction.

A combined experimental and theoretical investigation of the deuterium isotope effects on the bacterial luciferase reaction is described. The experimental studies focus on determining if the unusual aldehydic deuterium isotope effect of approximately 1.5 observed in these reactions is an intrinsic isotope effect resulting from a single rate-limiting step or is a composite of multiple rate-limiting steps. The isotope effect observed is not significantly affected by variation in the aldehyde chain length, changes in the pH over a range of 6-9, use of alphaC106A and alphaC106S site-directed mutants, or chloride substitution at the 8-position of the reduced flavin, though the isotope effect is decreased when the 8-methoxy-substituted flavin is used as a substrate. From these observations it is concluded that the aldehydic isotope effect arises from the change in rate of a single kinetic step. A stopped-flow kinetic analysis of the microscopic rate constants for the reactions of 1-[1H]decanal and 1-[2H]decanal in the bacterial luciferase reaction was carried out, and aldehyde hydration isotope effects were determined. From the results it is estimated that the aldehydic deuterium isotope effect is approximately 1.9 after formation of an intermediate flavin C4a-hydroperoxy hemiacetal. Ab initio calculations were used to examine the transformation of the aldehyde into a carboxylic acid and to predict isotope effects for possible mechanisms. These calculations indicate that the mechanism involving rate-limiting electron transfer from the flavin C4a-hydroxide to an intermediate dioxirane is consistent with the enigmatic aldehydic isotope effect and that the intermediacy of a dioxirane is energetically plausible.

Solvation dynamics of rhodamine 800 in water and D 2O

Time-resolved resonance heterodyne optical Kerr-effect spectroscopy is employed to measure the solvation dynamics of rhodamine 800 in water and D 2O. Unlike other large dye molecules, rhodamine 800 solvation dynamics in water exhibits a long-time component which we attribute to hydrogen-bond formation (breaking). We also find a rather unusual large isotope effect which does not exist in other dyes. An important part of the experimental data is a bimodal solvation correlation function with an ultrafast femtosecond component < 100 fs.

A simple light-driven transmembrane proton pump.

Light-induced lipophilic porphyrin/aqueous acceptor charge separation across a single lipid-water interface can pump protons across the lipid bilayer when the hydrophobic weak acids, carbonylcyanide m-chlorophenylhydrazone and its p-trifluoromethoxyphenyl analogue, are present. These compounds act as proton carriers across lipid bilayers. In their symmetric presence across the bilayer, the positive currents and voltages produced by the photogeneration of porphyrin cations are replaced by larger negative currents and voltages. The maximum negative current and voltage occur at the pH of maximum dark conductance. The reversed larger current and voltage show a positive ionic charge transport in the same direction as the electron transfer. This transport can form an ion concentration gradient. The movement of protons is verified by an unusual D2O isotope effect that increases the negative ionic current by 2- to 3-fold. These effects suggest that an interfacial pK shift of the weak acid caused by the local electric field of photoformed porphyrin cations/acceptor anions functions as the driving force. The estimated pumping efficiency is 10-30%. Time-resolved results show that proton pumping across the bilayer occurs on the millisecond time scale, similar to that of biological pumps. This light-driven proteinless pump offers a simple model for a prebiological energy transducer.

Intramolecular hydrogen bonding of the enol forms of β-ketoamides and β-ketothioamides. Deuterium isotope effects on 13C chemical shifts

Deuterium isotope effects of 13C chemical shifts are studied in a series of enol and keto forms of β-ketoamides and the corresponding thioamides. In addition, the 2,6-cyclohexanediketo-1-amides and thioamides are studied. The effects of ring size (five- and six-membered rings) on the isotope effects and the tautomeric nature of the systems are also looked into. Rather unusual isotope effects are found for the amides, indicating a tautomeric system of the CONHRCOHNHR type. This is supported by the 17O chemical shift studies. The isotope effects of the simple amides are compared with those of the tetracyclines and piroxicams. The study of N-phenyl-3-phenyl-3-oxo-propiothioamide at low temperature reveals that this thioamide exists as a mixture of s-cis and s-trans species. The isotope effects and the influence of intramolecular hydrogen bonding in the two species can thus be studied. Thioamides of indan-1,3-diones show tautomeric behaviour, as revealed by very large deuterium isotope effects of both signs. Deuteriation shifts the equilibrium in the direction of the thioamide. Finally, the tendency of a series of β-hydroxy esters, thioesters, anhydrides, amides, thioamides, aldehydes and ketones to become tautomeric is discussed in terms of hydrogen bonding, isotope effects, 2 ΔC(OD), and the nature of the acceptor.

Unusual inverse secondary isotope effects: A statistical study of CD2HNC and CH3NC isomerization

Two formulations of microcanonical transition-state theory are used to compute isomerization rate constants k(E,J=0) for CD2HNC and CH3NC. The first of these uses a Monte Carlo method to approximate the classical flux integral expression for k(E,J=0). This method allows a direct comparison to be made with classical trajectory results. The second approach uses a quantum, harmonic state counting method to evaluate k(E,J=0) within the familiar RRKM formalism. The results of the transition-state theory calculations differ at low energies, but at high energies an unusual inverse secondary isotope effect is observed in both. This is analyzed in terms of the RRKM formalism and it is concluded that the presence of a methyl free-rotor mode at the transition state is responsible for the onset of the inverse isotope effect. A similar, but somewhat larger inverse isotope effect has been observed in high-energy classical trajectory calculations.