Computational study of the influence of solvent on (16)O/(18)O equilibrium isotope effects in phosphate deprotonation reactions

Results from theoretical calculations of (16)O/(18)O equilibrium isotope effects (EIEs) on deprotonation of phosphate and methyl phosphate monoanions as well as their deuterated counterparts are reported. The EIEs are calculated from the Bigeleisen equation using harmonic vibrational frequencies from several quantum mechanical methods (HF, DFT, MP2, and AM1). All methods correctly predict the qualitative trends in the EIEs related to the different isotope substitutions. However, the calculated gas-phase values are found to be systematically higher than those experimentally observed in aqueous solution. On the other hand, the addition of explicit solvent molecules (up to 24 waters) in the first solvation shells of the phosphate ion substantially improves the calculated EIE, which approaches the experimental value with increasing size of the water cluster. The large effects of surrounding water molecules on the phosphate deprotonation EIE can be explained by the strong solute-solvent interactions, which result in solvent coupled vibrational modes of the phosphate ions.     

Laser excited atomic fluorescence for studies of enhancement effects in the direct current plasma

This paper reports a preliminary use of laser excited atomic fluorescence spectrometry (LEAFS) to study analyte population enhancement caused by easily ionized elements (EIEs) in the direct current plasma (DCP). Spatial atom density profiles in the DCP were obtained using resonance fluorescence at the calcium atom line at 422.7 nm, with and without the addition of an EIE. Variations in atom density caused by an EIE were found to be far too small to account for the marked enhancements of atomic emission signals which are caused by EIEs.Direct line fluorescence of the barium ion, excited at 614 nm and detected at 455 nm, was used to probe the effect of an EIE on excited state populations. Measurements in the analytical region of the plasma close to the core revealed that enhancements of fluorescence signals at low laser powers disappeared at laser powers which were sufficient to saturate the atomic transitions. While this result does not clarify any of the mechanisms of excitation in the DCP, it does lend support to two of the fundamental postulates of a recent model of the spectrochemical excitation processes in the DCP. These are first, that the analytical region of the DCP is not in local thermodynamic equilibrium (LTE) and second, that EIE enhancement proceeds by modulating the rates of power distribution among various plasma zones.In the outer zone of the analytical region of the DCP, depressive interferences occurred. These did not disappear upon saturation which indicates that they were not rate effects but effects that resulted from atom density changes.     

Temperature-dependent transitions between normal and inverse equilibrium isotope effects for coordination and oxidative addition of C-H and H-H bonds to a transition metal center

The temperature dependence of the equilibrium isotope effects (EIEs) for coordination and oxidative addition of C-H and H-H bonds to the tungstenocene species {[H2Si(C5H4)2]W} has been determined with the aid of DFT (B3LYP) calculations. The EIE for coordination of CH4 and CD4 does not exhibit typical van't Hoff type behavior in which there is a monotonic variation of EIE with temperature; rather, the temperature dependence of the EIE exhibits a maximum, with inverse values (<1) at low temperature and normal values (>1) at high temperatures. The temperature dependence of the EIE for oxidative addition of CH4 and CD4 differs significantly from that for coordination, with the EIE being normal at all temperatures and approaching infinity at 0 K. In contrast to oxidative addition of methane which is normal at all temperatures, the EIE for oxidative addition of H2 and D2 exhibits a transition from inverse to normal upon raising the temperature. The existence of inverse EIEs in these systems at low temperatures is a result of the zero point energy changes for the products upon isotopic substitution being greater than those for the reactants (H2 or CH4).     

Equilibrium isotope effects on noncovalent interactions in a supramolecular host-guest system

The self-assembled supramolecular complex [Ga(4)L(6)](12-) (1; L = 1,5-bis[2,3-dihydroxybenzamido]naphthalene) can act as a molecular host in aqueous solution and bind cationic guest molecules to its highly charged exterior surface or within its hydrophobic interior cavity. The distinct internal cavity of host 1 modifies the physical properties and reactivity of bound guest molecules and can be used to catalyze a variety of chemical transformations. Noncovalent host-guest interactions in large part control guest binding, molecular recognition and the chemical reactivity of bound guests. Herein we examine equilibrium isotope effects (EIEs) on both exterior and interior guest binding to host 1 and use these effects to probe the details of noncovalent host-guest interactions. For both interior and exterior binding of a benzylphosphonium guest in aqueous solution, protiated guests are found to bind more strongly to host 1 (K(H)/K(D) > 1) and the preferred association of protiated guests is driven by enthalpy and opposed by entropy. Deuteration of guest methyl and benzyl C-H bonds results in a larger EIE than deuteration of guest aromatic C-H bonds. The observed EIEs can be well explained by considering changes in guest vibrational force constants and zero-point energies. DFT calculations further confirm the origins of these EIEs and suggest that changes in low-frequency guest C-H/D vibrational motions (bends, wags, etc.) are primarily responsible for the observed EIEs.     

Interferences due to easily ionised elements in a microwave-induced plasma system with graphite-furnace sample introduction

Several aspects of both enhancement and suppression of the analyte emission intensity caused by an easily ionised element (EIE) have been studied in an atmospheric pressure He microwave-induced plasma (MIP). A sequence of experiments, designed to elucidate possible mechanisms of this EIE effect, examines the following aspects: the concentration dependence of the effect for various EIEs; spatially separated vaporisation of EIE and analyte into the plasma; the effect of operating parameters upon the EIE-induced enhancement; the influence of the EIE on the excitation temperature and on the efficiency of coupling of microwave energy to the cavity. The EIE-induced suppression of emission intensity is consistent with reduced power dissipation in the plasma, due to decoupling of the plasma from the microwave power source, whereas the EIE-induced enhancement of emission intensity is best explained by a radiative energy transfer mechanism.     

Experimental evidence for a temperature dependent transition between normal and inverse equilibrium isotope effects for oxidative addition of H2 to Ir(PMe2Ph)2(CO)Cl

The equilibrium isotope effect (EIE) for oxidative addition of H(2) and D(2) to Ir(PMe(2)Ph)(2)(CO)Cl has been measured over a large temperature range, thereby demonstrating that the inverse (<1) EIE previously observed at ambient temperature becomes normal (>1) at high temperature (>90 degrees C). The temperature dependence of the EIE for oxidative addition of H(2) and D(2) to Ir(PH(3))(2)(CO)Cl has been calculated using the geometry and vibrational frequencies obtained from DFT (B3LYP) calculations on Ir(PH(3))(2)(CO)ClH(2) and Ir(PH(3))(2)(CO)ClD(2), and is in accord with the experimentally observed transition from an inverse to normal EIE for oxidative addition of H(2) and D(2) to Ir(PMe(2)Ph)(2)(CO)Cl: the EIE is calculated to be inverse between 0 and 510 K, reach a maximum value of 1.15 at 867 K and then slowly decrease to unity as the temperature approaches infinity. This deviation from simple van't Hoff behavior, and the occurrence of a maximum in the EIE, is the result of the entropy term being temperature dependent. At low temperature, the enthalpy term dominates and the EIE is inverse, whereas at high temperatures the entropy term dominates and the EIE is normal. The observation of both normal and inverse EIEs for the same system indicate that inferences pertaining to the magnitude of an isotope effect at a single temperature may require more detailed consideration than previously realized.     

Equilibrium isotope effects as a probe of nonbonding attractions

The equilibrium isotope effects (EIEs) in the capsule complex (12 + p-xylene + CCl4) were computed with two simple models that mimic two types of contacts, -CH3.Ar and -CH3.CCl4. By using the MP2/6-311++G(d,p) method, the calculated equilibrium constants are consistent with the reported experimental data. Harmonic frequency analyses indicate that vibrational frequency changes caused by nonbonding attractions lead to the observed EIE.     

Trace element analysis by two-colour laser enhanced ionization spectroscopy in a graphite furnace

We demonstrate improved performance of laser enhanced ionization (LEI) with two-colour excitation in graphite furnace. With a modified electrode arrangement we have been able to measure LEI signals at atomization temperature of 2600°C without interference from the electric heating current. The sensitivity and selectivity are greatly improved when step-wise excitation is used instead of single photon LEI. Detection limits in the pg range are reported for five different elements. Futhermore we demonstrate and discuss how a simple and efficient correction for spectral interference can be performed in two-colour LEI.     

Nonperfect synchronization of reaction center rehybridization in the transition state of the hydride transfer catalyzed by dihydrofolate reductase

It has been suggested that the magnitudes of secondary kinetic isotope effects (2 degrees KIEs) of enzyme-catalyzed reactions are an indicator of the extent of reaction-center rehybridization at the transition state. A 2 degrees KIE value close to the corresponding secondary equilibrium isotope effects (2 degrees EIE) is conventionally interpreted as indicating a late transition state that resembles the final product. The reliability of using this criterion to infer the structure of the transition state is examined by carrying out a theoretical investigation of the hybridization states of the hydride donor and acceptor in the Escherichia coli dihydrofolate reductase (ecDHFR)-catalyzed reaction for which a 2 degrees KIE close to the 2 degrees EIE was reported. Our results show that the donor carbon at the hydride transfer transition state resembles the reactant state more than the product state, whereas the acceptor carbon is more productlike, which is a symptom of transition state imbalance. The conclusion that the isotopically substituted carbon is reactant-like disagrees with the conclusion that would have been derived from the criterion of 2 degrees KIEs and 2 degrees EIEs, but the breakdown of the correlation with the equilibrium isotope effect can be explained by considering the effect of tunneling.     

端视ICP-AES中用铟内标校正钠基体干扰的研究

端视电感耦合等离子体原子发射光谱中易电离元素引起的非光谱干扰,常使分析结果产生偏差。本文就不同浓度钠基体对分析谱线产生的干扰进行了试验和研究,并用铟作为内标元素来补偿钠基体的干扰。得出选择用几个内标元素和多条内标谱线时可以校正钠基体的干扰。     

端视等离子体原子发射光谱法中内标法校正钠基体干扰

端视电感耦合等离子体原子发射光谱在分析过程中易电离元素引起的非光谱干扰 ,常常使分析结果产生偏差。就不同浓度Na基体对分析谱线产生的干扰进行了实验和研究 ,并用Y作为内标元素来补偿钠基体的干扰。得出在Robust条件 ,即高功率和低载气流速条件下 ,选择合适的离子线 ,并且离子线的总能量大于 10eV下 ,用内标Y 4 37.4 94nm可以很好的补偿不同Na含量的干扰。     

A steric deuterium isotope effect in 1,1,3,3-tetramethylcyclohexane

The equilibrium isotope effect (EIE) for the interconversion of the two chair isotopomers of 1-trideutero-1,3,3-trimethylcyclohexane was predicted using geometry and vibrational force constants derived from electronic structure theory at HF, B3LYP, and MP2 levels as input for the program THERMISTP. Agreement between theory and previously reported NMR results is very good (experimental K(eq) = 1.042 +/- 0.001 vs K(eq) = 1.0409 at MP2/6-311G* level, K(eq) = 1.0503 at HF/6-311G*, and K(eq) = 1.0417 at B3LYP/6-311G* level, all at 17 degrees C). In order to investigate the origin of this isotope effect, the calculated EIEs for the monodeuterated isotopomers were analyzed. It has been shown that the hydrogen atom on an axial methyl group which is unusually close to its counterpart on the other axial methyl is responsible for the large (steric) isotope effect in the compound studied.     

Spectral interferences and background overcompensation in inverse zeeman-corrected atomic absorption spectrometry : Part 2. The effects of cobalt,manganese and nickel on 30 elements and 53 elements lines

The background compensation performance of a transversal alternating-current Zeeman corrector system with the magnet acting on the graphite atomization cell was assessed for 30 elements and 53 element lines in the presence of relatively large amounts of cobalt, manganese or nicke. The study reveaaled three cases of background overcompensation, all being caused by a cobalt line adjacent to the analytical line. When the magnetic field is on (and the background is measured), a σ-component of the cobalt lines overlaps the emission lines of boron (249.7 nm), mercury (253.7nm) and gold (267.6 nm). The interfering effect on boron is small, but mercury and gold are more seriously affected; for both elements a serious negative systematic error is introduced. Manganese and nickel did not give any overcompensation effects on the elements and lines studied. When gold and mercury were measured with the use of the same experimental parameters and a conventional deuterium-arc background corrector, only mercury suffered from spectral interference. The spectral interference of cobalt on mercury, with either type of background correction, can be avoided by selecting a proper furnace program. When gold is tobe measured in the presence of cobalt and with the present Zeeman background-correction system, the 267.6.-nm line should not be used; the more sensitive 242.8 nm line is recommended.     

Applications of Laser-Enhanced Ionization In Analytical Chemistry

Various applications of laser-enhanced ionization (LEI) in analytical chemistry are reviewed. This technique was applied to determine some physical quantities associated with a flame through development of an appropriate model. Determinations of flame temperature and atomization efficiency of an element in the flame are examples. As trace analysis is an important application of this technique, we compare the ion yield induced in a two-step LEI with that in a one-step LEI. The factors governing the ion enhancement via two-step excitation are examined in order to make efficient use of the two-step LEI apparatus. A novel technique was designed to couple flow-injection analysis to a conventional LEI device; in this manner, electrical interference, a severe problem inherent in LEI, was successfully removed.     

Spectral and non-spectral interferences in the determination of thallium in environmental materials using electrothermal atomization and vaporization techniques—a case study

The literature on the determination of Tl in environmental samples using electrothermal atomization (ETA) and vaporization (ETV) techniques has been reviewed with special attention devoted to potential interferences and their control. Chloride interference, which is due to the formation of the volatile monochloride in the condensed phase, is the most frequently observed problem. Due to its high dissociation energy (88 kcal/mol), TlCl is difficult to dissociate in the gas phase and is easily lost. The best means of controlling this interference in ETA is atomization under isothermal conditions according to the stabilized temperature platform furnace concept, and the use of reduced palladium as a modifier. An alternative approach appears to be the ‘fast furnace’ concept, wherein both the use of a modifier and the pyrolysis stage are omitted. This concept requires an efficient background correction system, and high-resolution continuum-source atomic absorption spectrometry (HR-CS AAS) appears to offer the best results. This chloride interference can also cause significant problems when ETV techniques are used. Among the spectral interferences found in the determination of thallium are those due to Pd, the most efficient modifier, and Fe, which is frequently found at high concentrations in environmental samples. Both interferences are due to nearby atomic lines, and are observed only when deuterium background correction and relatively high atomization temperatures are used. A more serious spectral interference is that due to the molecular absorption spectrum of SO₂, which has a maximum around the Tl line and exhibits a pronounced rotational fine structure. HR-CS AAS again showed the best performance in coping with this interference.     

Q-band EPR and ENDOR of low temperature X-irradiated beta-D-fructose single crystals

Beta-D-fructose single crystals were in situ X-irradiated at 80 K and measured using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques at Q-band (34 GHz) microwave frequencies. The measurements revealed the presence of at least four carbon-centered radicals stable at 80 K. By means of ENDOR angular variations in the three principal crystallographic planes, six proton hyperfine coupling tensors could be determined and were assigned to four different radicals by the aid of EIE. Two of the radicals exhibit only beta-proton hyperfine couplings and reveal almost identical EIE spectra. For the other two radicals, the major hyperfine splitting originates from a single alpha-proton hyperfine coupling and their EIE spectra were also quite similar. The similarity of the EIE spectra and hyperfine tensors led to the assumption that there are only two essentially different radical structures. The radical exhibiting only beta-proton hyperfine couplings was assigned to a C3 centered radical arising from H3 abstraction and the other radical suggested to be an open-ring species with a disrupted C2-C3 bond and a double C2-O2 bond. A possible formation mechanism for the latter open-ring radical is presented. By means of cluster density functional theory (DFT) calculations, the structures of the two radicals were determined and a fairly good agreement between the calculated and experimental hyperfine tensors was found.     

Determination of aluminum in highly concentrated iron samples: Study of interferences using high-resolution continuum source atomic absorption spectrometry

High-resolution continuum source atomic absorption spectrometry (HR-CS AAS) has been used to investigate spectral and non-spectral interferences found with a conventional line source atomic absorption spectrometer in the determination of aluminum in pharmaceutical products containing elevated iron and sugar concentrations. A transversely heated graphite furnace was used as the atomizer in both spectrometers. The two most sensitive aluminum lines at 309.3 nm and 396.2 nm were investigated and it was found that an iron absorption line at 309.278 nm, in the vicinity of the aluminum line at 309.271 nm, could be responsible for some spectral interference. The simultaneous presence of iron and the organic components of the matrix were responsible for radiation scattering, causing high continuous and also structured background absorption at both wavelengths. The aluminum and iron absorption could not be separated in time, i.e., the iron interference could not be eliminated by optimizing the graphite furnace temperature program. However, an interference-free determination of aluminum was possible carrying out the measurements with HR-CS AAS at 396.152 nm after applying least squares background correction for the elimination of the structured background. Analytical working range and other figures of merit were determined and are presented for both wavelengths using peak volume registration (center pixel ± 1) and the center pixel only. Limits of detection and characteristic masses ranged from 1.1 to 2.5 pg and 13 to 43 pg, respectively. The method was used for the determination of the aluminum contamination in pharmaceutical formulations for iron deficiency treatment, which present iron concentrations from 10 to 50 g l^− 1. Spike recoveries from 89% to 105% show that the proposed method can be satisfactorily used for the quality control of these formulations.     

Analytical applications of laser-enhanced lonization spectrometry in flames and furnaces

This paper is concerned with a critical analysis of the analytical performance of laser-enhanced ionization (LEI) spectrometry in flames and furnaces as an ultra-sensitive trace-element technique. Updated tables of LEI detection limits, both in flames and furnaces, are given. Special attention is paid to interference phenomena which are unique to the LEI technique. Ways of reducing their influence on the analytical signals are proposed. Finally, advantages and disadvantages of LEI spectrometry as a tool for ultra-sensitive trace element analysis are discussed.Presented in part at the 1989 European Winter Conference on Plasma Spectrochemistry, Reutte, Austria     

The problem of arsenic interference in the analysis of Cd to evaluate its extractability in soils contaminated by arsenic

The arsenic (As) spectral interference observed in the determination of cadmium (Cd) by inductively coupled plasma atomic emission spectrometry (ICP-AES) was studied in atomic absorption spectrometry (AAS) using flame (FAAS) and graphite furnace (GFAAS) as atomizers. The soils of 15 kitchen gardens located near two smelters in the North of France were selected according the ratio As/Cd. Four different extracting solutions usually used to evaluate the mobility of Cd were chosen to extract Cd from these soil samples: citric acid 0.11 M, acetic acid 0.11 M, calcium chloride 0.01 M and water. The quantitative determinations of Cd in the 15 soils for each solvent were investigated by ICP-AES at two lines (228.802 and 214.438 nm) and by FAAS or GFAAS with two-way background compensation. Compared to the Cd concentrations measured in the acid solutions and in the CaCl₂ solution after the addition of a chemical modifier, it was clearly demonstrated that the high-speed self-reversal background compensation (HSSR-method) was the method of choice to eliminate the spectral interference of As during Cd determination by FAAS and by GFAAS. In water, it was shown that the deuterium lamp used for the background compensation (D₂-method) was able to eliminate the most of the As interference. In comparison with Cd concentrations in water after adding a chemical modifier, those obtained with the HSSR-method were similar and a very good correlation was obtained between these two methods (R² = 0.995). It was therefore established that the HSSR-method would be able to replace the chemical modifiers to eliminate As interference in the determination of Cd-extractable from As contaminated soils.