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Isotopic Substitution (isotopic + substitution)
Selected AbstractsAb initio quantum-mechanical prediction of the IR and Raman spectra of Ca3Cr2Si3O12 Uvarovite garnetINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 2 2010L. Valenzano Abstract The IR and Raman spectra of uvarovite (Ca3Cr2Si3O12) garnet were simulated with the periodic ab initio CRYSTAL code by adopting an all-electron Gaussian-type basis set and the B3LYP Hamiltonian. The two sets of 17 F1u Transverse-Optical (TO) and Longitudinal-Optical (LO) frequencies are generated, together with their intensities. As regards the IR experimental spectrum, only five peaks are available, that are in excellent agreement with the calculated data (mean absolute difference smaller than 5.2 cm,1). The analysis of the TO-LO eigenvalue overlaps permits to establish a correspondence between LO and TO modes. The set of experimental Raman peaks is much reacher (23 out of 25) and the agreement with our calculations excellent ( smaller than 6 cm,1). Isotopic substitution is used to identify the zones of the spectrum where Cr and Ca contributions are relevant. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source] Asymmetric Cyclopropanation of Optically Active (1-Diethoxyphosphoryl)vinyl p -Tolyl Sulfoxide with Sulfur Ylides: A Rationale for DiastereoselectivityEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 4 2005Wanda H. Midura Abstract The title sulfoxide (S)-(+)- 1a was found to react with sulfur ylides affording the corresponding cyclopropanes in high yields. With fully deuterated dimethyl(oxo)sulfonium methylide, (CD3)2S(O)CD2, the cyclopropanation reaction occurred in a highly diastereoselective manner producing the cyclopropane 4a - d2 as a major diastereomer in which the newly formed quaternary ,-carbon atom is chiral due to isotopic substitution (CH2 vs. CD2). The diastereomer 4b - d2, having the opposite configuration at the ,-carbon atom, was obtained starting form the 2,2-dideuterio substituted vinyl sulfoxide, (S)-(+)- 1a - d2, and the nondeuterated ylide. The diastereomeric ratio in both reactions was found to be ca. 10:1. The reaction of (S)-(+)- 1a with diphenylsulfonium isopropylide yielded the cyclopropane (+)- 7 as a single diastereomer. X-ray structural studies of the crystalline 1-phosphorylvinyl sulfoxide 9 as well as density functional calculations (B3LYP/6-31G*) on (1-phosphoryl)vinyl sulfoxides revealed the origin of the experimentally observed diastereoselectivities and allowed us to propose a transition state model for the cyclopropanation reaction of chiral 1-phosphorylvinyl sulfoxides. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Isotope and disorder effects in the Raman spectra of LiHxD1,x crystalsJOURNAL OF RAMAN SPECTROSCOPY, Issue 8 2001V. G. Plekhanov Most of the physical properties of a solid depend on its isotopic composition in some way or another. Scientific interest, technological promise and increased availability of highly enriched isotopes have led to a sharp rise in the number of experimental and theoretical studies with isotopically controlled crystals. A great number of stable isotopes and well-developed methods for their separation have made it possible to grow crystals of C, LiH, ZnO, ZnSe, CuCl, GaN, GaAs, CdS, Cu2O, Si, Ge and ,-Sn with a controllable isotopic composition. Among these compounds, LiH possesses the largest value of the isotope effect. The great number of theoretical and experimental data suggest that the isotopic composition of a crystal lattice exerts some influence on the vibrational properties of crystals. These effects are fairly large and can be readily measured by modern experimental techniques (ultrasound, Brillouin and Raman scattering and neutron scattering). In addition, crystals of different isotopic compositions possess different Debye temperatures. This difference between an LiH crystal and its deuterated analogue exceeds a 100 K. Very pronounced and general effects of isotopic substitution are observed in phonon spectra. The scattering lines in isotopically mixed crystals are not only shifted (the shift of LO lines exceeds 100 cm,1) but are also broadened. This broadening is related to the isotopic disorder of the crystal lattice. It is shown that the degree of a change in the scattering potential is different for different isotopic mixed crystals. In the case of germanium and diamond crystals, phonon scattering is weak, which allows one to apply successfully the coherent potential approximation (CPA) for describing the shift and broadening of scattering lines. In the case of lithium hydride, the change in the scattering potential is so strong that it results in phonon localization, which is directly observed in experiments. The common nature of the isotopic and disorder effects in a wide range of crystals is emphasized. Copyright © 2001 John Wiley & Sons, Ltd. [source] Growth and characterization of isotopic natGa15N by molecular-beam epitaxyPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S2 2009Yong-zhao Yao Abstract Isotopically enriched gallium nitride films, Ga14N and Ga15N, have been fabricated by molecular-beam epitaxy to study the effects of nitrogen atomic mass variation on structures and properties of GaN. The phonon frequency shift due to the isotopic substitution was clearly observed using Raman spectroscopy. The lattice constants of Ga15N differed from those of Ga14N; the unit cell volume of Ga15N was approximately 0.06% less than that of Ga14N. Temperature-dependent photoluminescence measurements revealed that the recombination mechanism in Ga14N and Ga15N was the same in the temperature range of 4-50 K, and the band gap energy difference was Eg15 -Eg14 = 6.0 ± 0.1 meV. This Eg difference is discussed in terms of volume shrinkage and change in phonon-electron interaction due to isotopic substitution. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] New Fourier transform infrared based computational method for peptide secondary structure determination.BIOPOLYMERS, Issue 2 2001Abstract Fourier transform infrared (FTIR) experiments in dimethylsulfoxide, a solvent incapable of H donation, demonstrate that H , D isotopic replacement on the amide side of peptide bonds involves modifications of both the position and intensity of the amide I band. The effect of the isotopic substitution is particularly significant in the 1710,1670 and 1670,1650 cm,1 regions, which are generally associated with ,-turns and ,-helices. This behavior, attributed to the existence of intramolecular H-bonds in the polypeptide chain, is directly correlated to the presence of different secondary structures. Utilizing the effects induced by isotopic substitution, a method for the quantitative determination of the percentage of intramolecular H-bonds and the correlated secondary structures is proposed. The method consists of three principal steps: resolution of the fine structure of the amide I band with the determination of the number and position of the different components; reconstruction of the experimentally measured amide I band as a combination of Gaussian and Lorentzian functions, centered on the wave numbers set by band-narrowing methods, through a curve-fitting program; and quantitative determination of the population of the H-bonded carbonyls and the correlated secondary structures by comparison of the integrated intensities pertaining to the components with homologous wave numbers before and after isotopic exchange. The method is tested on a synthetic fragment of proocytocin that was previously analyzed by NMR techniques using the same solvent systems. © 2001 John Wiley & Sons, Inc. Biopolymers (Biospectroscopy) 62: 95,108, 2001 [source] Geometric H/D Isotope Effects and Cooperativity of the Hydrogen Bonds in PorphyceneCHEMPHYSCHEM, Issue 2 2007Mohamed F. Shibl Dr. Abstract We investigate the primary, secondary, and vicinal hydrogen/deuterium (H/D) isotope effects on the geometry of the two intramolecular hydrogen bonds in porphycene. Multidimensional potential energy surfaces describing the anharmonic motion in the vicinity of the trans isomer are calculated for the different symmetric (HH/DD) and asymmetric (HD) isotopomers. From the solution of the nuclear Schrödinger equation the ground-state wavefunction is obtained, which is further used to determine the quantum corrections to the classical equilibrium geometries of the hydrogen bonds and thus the geometric isotope effects. In particular, it is found that the hydrogen bonds are cooperative, that is, both expand simultaneously even in the case of an asymmetric isotopic substitution. The theoretical predictions compare favorably with NMR chemical-shift data. [source] Assignments and hydrogen bond sensitivities of UV resonance Raman bands of the C8-deuterated guanine ringJOURNAL OF RAMAN SPECTROSCOPY, Issue 9 2002Akira Toyama Isotope-edited Raman spectroscopy, a combination of site-selective isotopic labeling and Raman difference spectroscopy, is a useful method for studying the structure and interaction of individual nucleic acid residues in oligonucleotides. To obtain basic data for applying isotope-edited Raman spectroscopy to guanine residues, we studied the vibrational modes of UV resonance Raman bands of the C8-deuterated guanine ring by examining the wavenumber shifts upon seven isotopic substitutions (2- 13C, 2- 15N, 6- 18O, 7- 15N, 8- 13C, 9- 15N and 1,- 13C). The hydrogen bond sensitivities of the Raman bands were also investigated by comparing the Raman spectra recorded in several solvents of different hydrogen bonding properties. Some of the Raman bands were found to be markers of hydrogen bonding at specific donor or acceptor sites on the guanine ring. The Raman bands, which shift on C8-deuteration, remain in the difference spectrum between the unlabeled and C8-deuterated guanine rings. Among them, a negative peak around 1525 cm,1 and a strong positive/negative peak pair around 1485/1465 cm,1 serve as markers of hydrogen bonding at N7 and C6O, respectively. Another weak positive/negative peak pair around 1025/1040 cm,1 is sensitive to hydrogen bonding at the proton donor sites (N1,H and N2,H2). The applicability of the hydrogen bond markers has been tested by using a 22-mer oligonucleotide duplex containing eight guanine residues and its analog in which a single guanine residue is C8-deuterated. The difference spectrum shows that the hydrogen bonding state of the guanine residue at the labeled position is consistent with the Watson,Crick base pair structure of DNA. Isotope-edited Raman spectroscopy is a useful tool for studying the hydrogen bonding state of selected guanine residues in oligonucleotides. Copyright © 2002 John Wiley & Sons, Ltd. [source] | ||||||||||