Temperature-dependent Isotope Effects Research Articles

Position-specific 13C distributions within propane from experiments and natural gas samples

Site-specific carbon isotope measurements of organic compounds potentially recover information that is lost in a conventional, ‘bulk’ isotopic analysis. Such measurements are useful because isotopically fractionating processes may have distinct effects at different molecular sites, and thermodynamically equilibrated populations of molecules tend to concentrate heavy isotopes in one molecular site versus another. Most recent studies of site-specific 13C in organics use specialized Nuclear Magnetic Resonance (NMR) techniques or complex chemical degradations prior to mass spectrometric measurements. Herein we present the first application of a new mass spectrometric technique that reconstructs the site-specific carbon isotope composition of propane based on measurements of the 13C/12C ratios of two or more fragment ions that sample different proportions of the terminal and central carbon sites. We apply this method to propane from laboratory experiments and natural gas samples to explore the relationships between site-specific carbon isotope composition, full-molecular δ13C, thermal maturity, and variation in organic matter precursors. Our goal is to advance the understanding of the sources and histories of short-chain alkanes within geologic systems. Our findings suggest that propane varies in its site-specific carbon isotope structure, which is correlated with increasing thermal maturity, first increasing in terminal position δ13C and then increasing in both center and terminal position δ13C. This pattern is observed in both experimental and natural samples, and is plausibly explained by a combination of site-specific, temperature-dependent isotope effects associated with conversion of different precursor molecules (kerogen, bitumen, and/or oil) to propane, differences in site-specific isotopic contents of those precursors, and possibly distillation of reactive components of those precursors with increasing maturity. We hypothesize that the largest changes in site-specific isotopic content of propane occur when bitumen and/or oil replace kerogen as the dominant precursors. If correct, this phenomenon could have significant utility for understanding gas generation in thermogenic petroleum systems.

Theory of thermal transport in multilayer hexagonal boron nitride and nanotubes

We present a theory for the lattice thermal conductivity ${\ensuremath{\kappa}}_{L}$ of single-walled boron nitride nanotubes (BNNTs) and multilayer hexagonal boron nitride (MLBN), which is based on an exact numerical solution of the phonon Boltzmann equation. Coupling between layers in MLBN and nanotube curvature in BNNTs each break a phonon scattering selection rule found in single-layer hexagonal boron nitride (SLBN), which reduces ${\ensuremath{\kappa}}_{L}$ in these systems. We show that out-of-plane flexural phonons in MLBN and out-of-tube phonons in BNNTs provide large contributions to ${\ensuremath{\kappa}}_{L}$, qualitatively similar to multilayer graphene (MLG) and single-walled carbon nanotubes (SWCNTs). However, we find that the ${\ensuremath{\kappa}}_{L}$'s in BNNTs and MLBN are considerably smaller compared to similar SWCNTs and MLG structures because of stronger anharmonic phonon scattering in the former. A large and strongly temperature-dependent isotope effect is found reflecting the interplay between anharmonic and isotope scattering phonons. Finally, we also demonstrate convergence of BNNTs into SLBN for large-diameter nanotubes and MLBN to bulk hexagonal boron nitride within a few layers.

Temperature- and Solvent-Dependent Binding of Dihydrogen in Iridium Pincer Complexes

Mixtures of deuterium labeled complexes (p-XPOCOP)IrH2-xDx (1-6-d0-2) {POCOP = [C6H2-1,3-[OP(tBu)2]2] X = MeO (1), Me (2), H (3), F (4), C6F5 (5), and ArF = 3,5-(CF3)2-C6H3 (6)} have been generated by reaction of (p-XPOCOP)IrH2 complexes with HD gas in benzene followed by removal of the solvent under high vacuum. Spectroscopic analysis employing 1H and 2D NMR reveals significant temperature and solvent dependent isotopic shifts and HD coupling constants. Complexes 1-6-d1 in toluene and pentane between 296 and 213 K exhibit coupling constants JHD of 3.8-9.0 Hz, suggesting the presence of an elongated H2 ligand, which is confirmed by T1(min) measurements of complexes 1, 3, and 6 in toluene-d8. In contrast, complex 6-d1 exhibits JHD = 0 Hz in CH2Cl2 or CDCl2F whereas isotopic shifts up to -4.05 ppm have been observed by lowering the temperature from 233 to 133 K in CDCl2F. The large and temperature-dependent isotope effects are attributed to nonstatistical occupation of two different hydride environments. The experimental observations are interpreted in terms of a two component model involving rapid equilibration of solvated Ir(III) dihydride and Ir(I) dihydrogen structures.

Temperature Dependence of Kinetic Isotope Effects for Enzymatic Carbon−Hydrogen Bond Cleavage

An analysis is reported of the anomalously weak temperature dependence of large kinetic isotope effects (KIEs) observed for enzymes that catalyze carbon−hydrogen bond cleavage. After a critical examination of the experimental data, rate expressions for proton tunneling are used to derive a universal relationship between KIEs and their temperature dependence for a model in which the tunneling coordinate, represented by two crossing Morse potentials or a quartic double-minimum potential, is assisted by a harmonic promoting mode. Since the reactions involve electron transfer to a redox system as well as proton transfer from a CH to an OH bond, both adiabatic and nonadiabatic mechanisms are considered. Model calculations are reported, which show that the derived relationship is valid under a wide range of conditions and depends on a minimum number of model parameters, namely almost exclusively on the proton transfer distance and the tunneling-mode anharmonicity. The results are used to derive proton transfer distances and promoting-mode force constants from the available data on temperature-dependent isotope effects for several enzymatic reactions effecting CH cleavage, including the reactions of linoleic acid catalyzed by soybean lipoxygenase-1 (SLO1), of primary amines catalyzed by methylamine and aromatic amine dehydrogenase (MADH and AADH), and of a dicopper complex that models the active site of monooxygenases and lacks a protein environment. These distances are found to be shorter and the force constants to be larger than those implied by van der Waals radii, but with anharmonicities suitably adjusted, they resemble those encountered in systems with hydrogen bonding. It is proposed that the strong redox system, which is always present in these enzymes, acts so as to withdraw electron density from the C···H···O transfer system. This will tend to shrink the tunneling barrier to a size that allows effective passage of H but not necessarily of D, a condition that may lead to a large KIE. It will also increase the enzyme−substrate force constant, thus reducing the temperature dependence of the KIE. For the reaction catalyzed by SLO1, a transfer distance of 0.9−1.0 Å is found. Similar results are obtained for three artificial mutants from which it is concluded that parts of the protein located outside the reaction site have little influence on the proton transfer. For the reactions catalyzed by MADH and AADH, the corresponding values range from 0.5 to 1.0 Å; however, the values much below 1.0 Å may be unrealistic since they are based on data that show anomalies suggesting that an incomplete kinetic model was used in their derivation. The analysis also yields a transfer distance of about 0.9 Å for the dicopper complex, a value similar to that of SLO1, although no protein is present. Since results for three different systems cluster about the same transfer distance of 0.9−1.0 Å, it is concluded that the large KIEs and their weak temperature dependence are due to a redox-induced C···H···O hydrogen bond.

Temperature-dependent isotope effects in soybean lipoxygenase-1: correlating hydrogen tunneling with protein dynamics.

The hydrogen-atom transfer in soybean lipoxygenase-1 (SLO) exhibits a large kinetic isotope effect on k(cat) (KIE = 81) near room temperature and a very weak temperature dependence (E(act) = 2.1 kcal/mol). These properties are consistent with H small middle dot transfer that occurs entirely by a tunneling event. Mutants of SLO were prepared, and the temperature dependence of the KIE was measured, to test for alterations in the tunneling behavior. All mutants studied exhibit KIEs of similar, large magnitude at 30 degrees C, despite an up to 3 orders of magnitude change in k(cat). E(act) for two of the mutants (Leu(754) --> Ala, Leu(546) --> Ala) is larger than for wild-type (WT), and the KIE becomes slightly more temperature dependent. In contrast, Ile(553) --> Ala exhibits k(cat) and E(act) parameters similar to wild-type soybean lipoxygenase-1 (WT-SLO) for protiated substrate; however, the KIE is markedly temperature dependent. The behavior of the former two mutants could reflect increased reorganization energies (lambda), but the behavior of the latter mutant is inconsistent with this description. We have invoked a full H* tunneling model (Kuznetsov, A. M.; Ulstrup, J. Can. J. Chem. 1999, 77, 1085-1096) to explain the temperature dependence of the KIE, which is indicative of the extent to which distance sampling (gating) modulates hydrogen transfer. WT-SLO exhibits a very small E(act) and a nearly temperature-independent KIE, which was modeled as arising from a compressed hydrogen transfer distance with little modulation of the hydrogen transfer distance. The observations on the Leu(754) --> Ala and Leu(546) --> Ala mutants were modeled as arising from a slightly less compressed active site with greater modulation of the hydrogen transfer distance by environmental dynamics. Finally, the observed behavior of the Ile(553) --> Ala mutant indicates a relaxed active site with extensive involvement of gating to facilitate hydrogen transfer. We conclude that WT-SLO has an active site structure that is well organized to support hydrogen tunneling and that mutations perturb structural elements that support hydrogen tunneling. Modest alterations in active site residues increase lambda and/or increase the hydrogen transfer distance, thereby affecting the probability that tunneling can occur. These studies allow the detection and characterization of a protein-gating mode in catalysis.

Evaluating kinetic fractionation factors used for reconstructions from oxgen and hydrogen isotope ratios in plant water and cellulose

Oxygen and hydrogen kinetic isotope enrichments occur in plant leaf water during evapotranspiration. Kinetic fractionation factors for 18O derived empirically from a variety of isotope studies of plant water compare favourably with fractionation factors estimated from theoretical consideration of leaf energy balance and boundary layer dynamics. Kinetic fractionation during evapotranspiration is sensitive to the nature of the boundary layer, which is controlled by leaf size and morphology. Generally, plants with small segmented leaves have a lower component of turbidity in the leaf boundary layer, which results in higher Ck values, than do plants having large simple leaves and more turbulent boundary layers. Equivalent analysis of 2H enrichment in plant leaf water reveals less consistent kinetic relations with leaf size and morphology and an apparent temperature-dependent isotope effect acting in opposition to evaporative enrichment. Accounting for this temperature-dependent isotope effect helps to (1) reconcile the 2Ck inconsistencies for different leaves and (2) explain a temperature effect in white pine which White et al. (1994) attributed to variable biochemical fractionation during cellulose synthesis. Improved characterization of kinetic effects during evapotranspiration has implications for isotopic study of plant water use and for constraining interpretations of paleoclimate based on the isotopic composition of plant tissues.

Dihydride Complexes of the Cobalt and Iron Group Metals: An Investigation of Structure and Dynamic Behavior

The previously reported cationic dihydride complexes (PP3)MH2+ (M = Co, Rh and Ir; PP3 = P(CH2CH2PPh2)3) have been prepared using improved synthetic methods. Variable-temperature 1H and 31P NMR spectra of these complexes reveal complex dynamic behavior. The hydride region 1H NMR spectra have been accurately simulated at all temperatures using a simple site permutation model after taking into consideration the opposite signs of the cis and trans H−P coupling constants. Partial deuteration of the hydride ligands in the rhodium and cobalt complexes is achieved by exposure to D2. In the partially deuterated samples, no evidence is found for a bound dihydrogen ligand, but the involvement of a dihydrogen species in the dynamic process which interchanges the two hydride positions remains a mechanistic possibility, as indicated by a kinetic isotope effect kH/kD = 1.3(1). The partially deuterated samples exhibit large and temperature-dependent isotope effects on the 1H NMR chemical shifts observed for the hydride ...

Unusual temperature-dependent isotope effects in the reactions of phenylcarbene with cyclohexene and cyclohexane

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTUnusual temperature-dependent isotope effects in the reactions of phenylcarbene with cyclohexene and cyclohexaneT. G. Savino, K. Kanakarajan, and M. S. PlatzCite this: J. Org. Chem. 1986, 51, 8, 1305–1309Publication Date (Print):April 1, 1986Publication History Published online1 May 2002Published inissue 1 April 1986https://doi.org/10.1021/jo00358a027RIGHTS & PERMISSIONSArticle Views232Altmetric-Citations28LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (714 KB) Get e-Alerts Get e-Alerts