Vibrational absorption and circular dichroism studies of (-)-camphanic acid

Vibrational absorption and circular dichroism (VCD) spectra of (-)-(1S,3R)-camphanic acid have been measured in deuterated chloroform solutions at different concentrations (0.005, 0.045, and 0.200 M) in the mid-infrared spectral range. Experimental spectra have been compared with the density functional theory (DFT) absorption and VCD spectra, calculated using the B3PW91 functional and cc-pVTZ basis set for three conformers of both the monomer and the dimer forms of (-)-(1S,3R)-camphanic acid. These calculations indicate that, in the dilute solution, the conformer with intramolecular hydrogen-bonding between the hydroxyl and lactone groups is of lowest energy and represents 70% of the different monomer conformers at room temperature, whereas, in concentrated solution, the dimer formed by intermolecular hydrogen-bonding of carboxyl groups of the two distinct monomer conformations is stabilized. The vibrational absorption and circular dichroism spectra calculated from the Boltzmann population of the individual monomer and dimer conformers are in very good overall agreement with the corresponding experimental spectra, allowing the absolute conformation and configuration of (-)-(1S,3R)-camphanic acid in dilute and concentrated solution, respectively. Experiments were also performed on (-)-(1S,3R)-camphanic chloride for which the populations predicted by DFT calculations are found to be in disagreement with those deduced from experimental spectra.     

Kinetic isotope effects in the thermal gas phase reactions of 1, 2-epoxycyclohexane-3, 3, 6, 6-d4

First-order rate constants for formation of cyclohexanone and 2-cyclohexen-1-ol from 1,2-epoxycyclohexane and 1,2-epoxycyclohexane-3,3,6,6-d₄ have been determined over the temperature range of 677–746°K. The observed kinetic isotope effects are used in an attempt to determine the mechanism for formation of products. A distinction between a biradical and a concerted mechanism for the alcohol formation could not be made. However, if a common biradical is the precursor of both cyclohexanone and 2-cyclohexenl-ol then the rate of ring closure of this biradical must be much faster than the rates of hydrogen transfer to give the ketone and the alcohol.     

Vibrational circular dichroism studies of molecular conformation and association of dipeptides

Vibrational circular dichroism (VCD) and infrared (IR) absorption spectra in the NH stretching region have been measured for the dipeptides, R′COAANHR′'(R′ = Me and tertBu; AA = Ala, Leu, Val and Phe; R′' = Me, isoBu and neoPe). Analyses of the VCD and absorption spectra indicated that the VCD bands for the NH stretching are quite sensitive to the state of hydrogen bonding as well as the local conformation of oligopeptides. VCD spectra exhibit a negative VCD band at 3420-3405 cm⁻¹ due to the C₅ conformer with an intramolecularly hydrogen-bonded five-membered ring. The intermolecularly hydrogen-bonded NH stretching vibration exhibits a characteristic negative—positive couplet from the high wavenumber side due to the antiparallel C₅C₅ dimer formation. Hydrogen-bonded oligomers beyond the dimer formed in highly concentrated solutions give rise to an additional negative VCD band on the lower wavenumber side of the hydrogen-bonded absorption band.     

Vibrational circular dichroism in hydrogen bond systems: Part II. Vibrational circular dichroism of the OH stretching vibration in 2,2-dimethyl-1,3-dioxolane-4-methanol

Vibrational circular dichroism (VCD) of the OH stretching band in 2,2-dimethyl-1,3- dioxolane-4-methanol has been studied. The OH stretching vibration for the intramolecularly hydrogen-bonded species in the (R)-enantiomer gives rise to a positive VCD band but no VCD band for the hydrogen-bond free species. It is shown that the G⁻G⁺ conformation in the intramolecular hydrogen bond system

gives a positive VCD band for the OH stretching vibration. The thermodynamic parameters in equilibrium between the free and intramolecularly hydrogen-bonded species have been determined as ΔH =−7.8 kJ mol⁻¹ and ΔS = −18 kJ K⁻¹ mol⁻¹.     

(3R,4S)-4-(4-Fluorophenyl)-3-hydroxymethyl-1-methylpiperidine: conformation and structure monitoring by vibrational circular dichroism.

Absorption and vibrational circular dichroism (VCD) spectra of the title compound, a common intermediate in synthesis of many pharmaceuticals, were measured and analyzed in order to determine its absolute configuration and prevailing conformations. The analysis was combined with a systematic conformer search based on relative energies as well as with comparison of experimental and computed NMR shifts. The spectra were interpreted on the basis of ab initio simulations. The results indicate that the compound adopts exclusively a chair conformation of the piperidine ring with all the fluorophenyl, hydroxymethyl, and methyl substituents attached in equatorial positions. A limited rotation of the hydroxymethyl group is most consistent with the observed VCD pattern. VCD parameters were found significantly more sensitive to conformational changes than absorption or NMR. Concentration dependence of the absorption spectra indicated aggregation in concentrated solutions, but involved hydrogen bonds probably do not influence molecular conformation.     

Subpicosecond UV spectroscopy of carbonmonoxy-myoglobin: absorption and circular dichroism studies

A thorough absorption and circular dichroism study is performed in carbonmonoxy-myoglobin with a sub-picosecond visible pump, ultraviolet probe experiment. Differential absorption in the 220-360 nm range shows that the time-resolved response mainly comes from the heme and that aromatic amino acids do not contribute significantly. Time-resolved CD at 260 nm shows no dynamics and confirms this result. On the contrary, a strong CD dynamics is observed at 230 nm. This signal could originate from transient deformation of the alpha-helices in the protein.     

Vibrational circular dichroism (VCD) studies on disaccharides in the CH region: toward discrimination of the glycosidic linkage position

The structural features of carbohydrates are a combination of 1) sequence and types of mono-sugars, 2) stereochemistry of their glycosidic linkages, and 3) their glycosidic linkage sites. We performed the first systematic VCD study on glycoside linking site discrimination. VCD spectra, in the CH stretching region from 2000 to 4000 cm(-1), of eleven glucobioses (trehalose (alpha1-alpha1), neotrehalose (alpha1-beta1), isotrehalose (beta1-beta1), kojibiose (alpha1-2), nigerose (alpha1-3), maltose (alpha1-4), isomaltose (alpha1-6), sophorose (beta1-2), laminaribiose (beta1-3), cellobiose (beta1-4), gentiobiose (beta1-6)) suggested a possible new discrimination method for glyco analysis, while VCD spectra in the mid-IR region distinguished the stereochemistry (alpha or beta) of the glycosidic linkage. Both reducing and nonreducing glucobioses showed different VCD spectral features compared to their constituent D-glucose and the anomer-fixed model compounds. Interresidue interaction such as hydrogen bonding was suggested to cause these spectral differences. Interplay between residues is a common phenomenon and thus VCD analysis could be applicable to other di-, oligo- or poly-saccharides. Several isotropic labeled compounds were also measured to support spectral assignment and interpretation.     

Magnetic circular dichroism studies—XLVI: Structure and the magnetic circular dichroism of trans-2-decalones and bicyclo[2.2.2]octan-2-ones

The magnetic CD spectra of a number of Me substituted trans-2-decalones and bicyelo[2.2.2]octan-2-ones were measured. The results showed a large, and sometimes dominant, contribution to the MCD intensity which could be correlated with the presence of α-substituents lying outside of the plane of the carbonyl chromophore.     

Excited-state absorption and circular dichroism of ruthenium(II) tris(phenanthroline) in the ultraviolet region

Excitation of ruthenium(II) tris(phenanthroline) in the visible region results in the tranfer of an electron from the central atom toward one of the ligands. To probe this excited state, we have performed pump-induced absorption and circular dichroism in the ultraviolet wavelengths, in the intraligand pi-pi* transition region. On top of the bleaching of the ground state transitions, new structures appear in the absorption and CD spectra. Thanks to a classical calculation based on the polarizability theory, we can interpret these features as the result of a strong reduction of the excitonic coupling due to a blue shift of the pi-pi* transition in the reduced ligand accompanied by the onset of new excited-state transitions.     

Optical rotatory dispersion and circular dichroism studies of copoly(propyl-L-aspartate-benzyl-L-aspartate)

Copoly(propyl-L -aspartate-benzyl-L -aspartate)s with various compositions were synthesized by propylation of a parent poly(β-benzyl-L -Aspartate). The variation of helix sense of copolyaspartate obtained against a degree of propylation and temperature was examined by optical rotatory dispersion and circular dichroism method. Copolyaspartate of propylation less than about 70% is in a left-handed α helix conformation and its Moffitt parameter b₀ and molecular ellipticity [θ]₂₂₂ decreased gradually with increasing propylation. On the other hand, a different temperature dependence of b₀ and [θ]₂₂₂ was observed for copolyaspartate of propylation more than about 80%, i.e., the reversible transition of the helix sense from right- to left-handed α helix took place by temperature increasing. These features were basically similar to the variation in helix sense of copoly(ethyl-L -aspartate-benzyl-L -aspartate), although a slight difference was seen for copoly(propyl-L -aspartate-benzyl-L -aspartate). The thermodynamic treatment indicating larger entropy of right-handed α helix suggests much mobile side chain of the right-handed α helix than the left handed one.     

Vibrational spectra and normal coordinate analysis of (CH3)4Sn4S6 with tetrahedral adamantane-type structure

Infrared and Raman spectra were recorded for the (CH₃)₄Sn₄S₆ molecule, which is supposed to have an adamantane-like cage structure. The symmetry group is T_d. A normal coordinate analysis was performed taking advantage of the local trigonal symmetries of the methyl groups. Force constants were deduced with the aid of the experimental assignment of vibrational frequencies.     

Synthesis of trans-(3S)-amino-(4R)-alkyl- and -(4S)-aryl-piperidines via ring-closing metathesis reaction

[reaction: see text] trans-(3S)-Amino piperidines bearing various alkyl and aryl substituents at the C-4 position were synthesized via a ring-closing metathesis reaction. The absolute stereochemistry was controlled using a protected D-serine as a starting material. Stereoselective hydrogenation of allylamines provided trans-(3S)-amino-(4R)-alkyl- and -(4S)-aryl-piperidines. This procedure presents the first method for the asymmetric synthesis of 4-substituted 3-amino piperidines.     

Vibrational study of (CH3)4NSbCl6 and [(CH3)4N]2SiF6

(CH3)4NSbCl6 and [(CH3)4N]2SiF6 are face-centred cubic compounds at ambient temperature with a = 11.548 and 11.172 A, respectively. The vibrational spectra of these two compounds are discussed in relation to their crystal structure and other compounds containing (CH3)4N+ ions. The assignment of the observed bands is discussed. The Raman and infrared spectra of [(CH3)4N]2SiF6 show that the cations and anions are not distorted inside the crystals and are weakly hydrogen-bonded to each other. The infrared spectrum of (CH3)4NSbCl6 confirms a cubic structure for this compound at ambient temperature, in which cations are in octahedral environments and can be interpreted as being disordered groups. No evidence could be found for the existence of hydrogen bonding between cations and anions in this compound. The phase transition of (CH3)4NSbCI6 is studied by means of Raman spectroscopy. It is believed to be governed by the reorientational motions of tetramethylammonium cations and may be classified as an 'order-disorder' type.     

(3S,4S)-4-氨基-3-羟基-6-甲基庚酸的合成

价廉易得的L-亮氨酸先以苄基同时保护氨基及羧基得(S)-2-(二苄胺基)-4-甲基戊酸苄酯,进而在碱性条件下与乙腈发生亲核取代反应得(S)-4-(二苄胺基)-6-甲基-3-氧代庚腈,再经硼氢化钠选择性还原羰基得(3S,4S)-4-(二苄胺基)-3-羟基-6-甲基庚腈,用双氧水氧化得(3S,4S)-4-(二苄胺基)-3-羟基-6-甲基庚酸,最后在Pd(OH)2/C-H2作用下脱掉苄基得到(3S,4S)-4-氨基-3-羟基-6-甲基庚酸,即(3S,4S)-statine。整个合成路线总产率为33.6%。     

Vibrational and electronic absorption spectra of 4-chloro 2-methyl, 4-chloro 3-methyl and 6-chloro 3-methyl phenols

The infrared spectra of 4-Cl 2-Me, 4-Cl 3-Me and 6-Cl 3-Me phenols have been recorded. The vibrational spectrum has been analysed assuming that the molecules belong toC _s point group and a tentative assignment of the observed frequencies to various modes of vibration has been proposed. The near ultraviolet absorption spectrum of these compounds has also been recorded. Assuming the transition to be electronically allowed the strongest band on the longer wavelength side has been assigned as the (0, 0) band in each case. The spectrum has been analysed in terms of several excited state frequencies which have been correlated with the ground state frequencies observed in the infrared spectrum.     

Vibrational spectroscopic study of the electron transfer for the Cl- + CH3Cl S(N)2 reaction in the gas phase

The vibrational spectra study of electron transfer of Cl- + CH3Cl --> ClCH3 + Cl- reaction has been examined by density function theory (DFT) calculations at 6-311 + +G** level in this paper. This reaction includes old bond rupture, new bond formation and electronic transfer in the intermolecular. The vibrational frequencies and vibrational modes of reactant, precursor complex, transition state, successor complex and product are analyzed. The relationship and the change among them can confirm the rupture of bond, the formation of bond and the process of electron transfer.     

Vibrational analysis of 4-amino pyrazolo (3,4-d) pyrimidine A joint FTIR, Laser Raman and scaled quantum mechanical studies

The FTIR and Laser Raman spectra of 4-amino pyrazolo (3,4-d) pyrimidine have been measured in the regions 4000–400 cm⁻¹ and 3500–100 cm⁻¹, respectively. Utilizing the observed FTIR and Laser Raman data, a complete vibrational assignment and analysis of the fundamental modes of the title compound were carried out. The vibrational frequency which were determined experimentally are compared with those theoretically from force field calculation based on ab initio HF/6−311+G**(d,p) and standard B3LYP/6−311+G**(d,p) methods and basis set combinations for optimized geometries. The observed FTIR and Laser Raman vibrational frequencies were analysed and compared with the theoretically predicted vibrational frequencies. The assignments of bands to various normal modes of the molecules were also carried out. A detailed interpretation of the infrared and Raman spectra of 4-amino pyrazolo (3,4-d) pyrimidine [4AP(3,4-D)P] is also reported based on total energy distribution (TED). The calculated HOMO and LUMO energies shows that charge transfer occur within the molecule. The theoretical FT-IR and FT-Raman spectra for the title molecule have also been constructed.