Conformational analysis of 2-halocyclopentanones by NMR and IR spectroscopies and theoretical calculations

The conformational isomerism of 2-chlorocyclopentanone and 2-bromocyclopentanone has been determined through the solvent dependence of the ¹H NMR ³J_HH coupling constants, theoretical calculations and infrared data, using the solvation theory for the treatment of NMR data. In 2-chlorocyclopentanone, the energy difference (E_Ψ-e − E_Ψ-a), in the isolated molecule at B3LYP level of theory, between the pseudo-equatorial (Ψ-e) and pseudo-axial (Ψ-a) conformers is 0.42 kcal mol⁻¹, which decreases in CCl₄ and in acetonitrile solutions, in good agreement with infrared data (ν_CO), despite the uncertainties of the latter method. The conformational equilibrium for 2-bromocyclopentanone is also between the Ψ-e and Ψ-a conformations, with an energy difference (E_Ψ-e − E_Ψ-a), in the isolated molecule at B3LYP level of theory, is 0.85 kcal mol⁻¹ which decreases in CCl₄ and in acetonitrile solutions, also in good agreement with infrared data.     

Polymorphism and dynamics of MBBA as studied by NMR

We report proton NMR experiments on the liquid crystal material N-(p-methoxybenzylidene) p-n-butylaniline (MBBA) at 100MHz in the temperature range 110-350 K. The phase diagram was investigated by means of second moment and spin-lattice relaxation measurements in order to establish connections between dynamics and phase transitions. The results show that in a slow cooling experiment, two processes contribute to the relaxation, a slow ethyl group motion together with reorientation of the methyl groups. For the glassy nematic state, as well as for the phases observed after reheating a quenched sample, only methyl rotation is observed. The correlation times of these various mechanisms were determined and the results compared with those obtained by previous NMR and dielectric analysis.     

Structure of neutral molecules and monoanions of selected oxopurines in aqueous solutions as studied by NMR spectroscopy and theoretical calculations

A methodology enabling investigation of a multicomponent tautomeric and acid-base equilibria by (13)C NMR spectroscopy supported by theoretical calculations has been proposed. The effectiveness of this method has been illustrated in a study of 2-oxopurine, 6-oxopurine (hypoxanthine), 8-oxopurine, and 2,6-dioxopurine (xanthine) in neutral and alkaline aqueous solutions. For each compound a series of (13)C NMR spectra were recorded at pH ranges in which neutral molecules, monoanions and/or dianions occurred in dynamic equilibrium. The carbon chemical shifts for these three forms of the investigated compounds were retrieved from the analysis of pH-dependence of the measured, dynamically averaged values of these parameters. The structures of several stable tautomers of the neutral and monoanionic oxopurine forms were predicted from theoretical calculations and nuclear magnetic shielding constants for (13)C nuclei in these tautomers were calculated. At both calculation steps (molecular geometry optimization and calculation of NMR parameters) the PBE1PBE/6-311++G(2d,p) level of theory was used. The populations of the most stable tautomers were determined from the experimental data analysis exploiting the fact that they were population-weighted averages of the chemical shifts of particular tautomers. It has been shown that only the oxo forms of the investigated oxopurines are present in aqueous solutions and that the determined populations in most cases remain in a qualitative agreement with the calculated free energies of the appropriate tautomers. The obtained results are in general agreement with other literature reports on oxopurine tautomerism and confirm importance of the hydration phenomena for the investigated systems. The data analysis has shown that the best compliance between theory and experiment is obtained when the hydration phenomenon is modeled by discrete hydration augmented by PCM (polarizable continuum solvation model).     

Theoretical analysis of intermolecular covalent pi-pi bonding and magnetic properties of phenalenyl and spiro-biphenalenyl radical pi-dimers

Singlet-triplet splittings DeltaEST and intermolecular covalent pi-pi bonding characteristics of the prototypical phenalenyl pi-dimer and eight spiro-biphenalenyl radical pi-dimer structures are analyzed with the aid of restricted and unrestricted density functional theory calculations and paramagnetic susceptibility data fitted using the Bleaney-Bowers dimer model and the Curie-Weiss model. Single determinant approximations for DeltaEST as a function of transfer integrals and on-site Coulomb repulsion energy are presented for the two-electron two-site pi-dimers of phenalenyls and the two-electron four-site pi-dimers of spiro-biphenalenyl radicals. Within the range of intermolecular separation of 3.12相似文献   

Ordering and mobility of ferroelectric liquid crystal dimer as studied by FT-IR spectroscopy with 2D-IR correlation analysis

Both a conservative rapid-scan FT-IR technique and a novel step-scan FT-IR technique with 2D correlation analysis were used to study the orientation and the mobility of a ferroelectric liquid crystal dimer during switching under an electric field. The detailed mutual arrangements of different molecular segments (mesogen, poly(methylene) chain, polysiloxane chain) in a smectic C* phase were derived from the static spectra. It was shown that the long mesogen axis, the average poly(methylene) and the average polysiloxane chain axes do not coincide with each other. The hindered rotation of the carbonyl group is confirmed. Time-resolved FT-IR technique was used to follow the segmental motion with a time resolution of 5 μs. The temperature and electric field strength dependencies of the mobility of these segments are discussed. 2D correlation analysis of time-resolved data reveals small differences in the behavior of the carbonyl and the benzoic rings in the mesogen moieties, that can be explained as differences in the orientation distribution functions of these moieties.     

Alkali-hydrolysis of D-glucono-delta-lactone studied by chiral Raman and circular dichroism spectroscopies

The alkali-hydrolysis of D-glucono-delta-lactone (GDL) was investigated by chiral Raman and circular dichroism (CD) spectroscopies in combination with density functional theory calculation. Based on the characteristic CD bands of GDL and its hydrolysis product, the dynamics of hydrolysis was studied using stopped-flow CD method. Using chiral Raman spectroscopy (CRS), the stereochemical change of GDL owing to the hydrolysis reaction was discussed on the vibrational scale. The CRS results show that the ring-o...     

State of the Cu electrode surface studied by X-ray photoelectron and Auger spectroscopies

Annealing of the copper plate in flame was found (by XPS and X-ray Auger spectroscopy) to enrich the surface copper layers in oxygen in the forms of Cu₂O and adsorbed oxygen. The changes in the surface layers of the copper cathode due to annealing can be the origin of the earlier found enhancement of its activity in the electrocatalytic hydrogenation of citral and trans-2-allyl-6-R-1,2,3,6-tetrahydropyridines (R = Me, All, Ph). This method of the copper electrode pre-treatment provides its long durability, an insignificant decrease in the electrocatalytic activity in time, and the reproducibility of the electrocatalytic hydrogenation of unsaturated organic compounds.     

The magnetic properties of myoglobin as studied by NMR spectroscopy

Deoxymyoglobin has been investigated by NMR spectroscopy to determine the magnetic anisotropy through pseudocontact shifts and the total magnetic susceptibility through Evans measurements. The magnetic anisotropy values were found to be Deltachi(ax)=-2.03+/-0.08 x 10(-32) m(3) and Deltachi(rh)=-1.02+/-0.09 x 10(-32) m(3). The negative value of the axial susceptibility anisotropy originates from the z tensor axis lying in the heme plane, unlike all other heme systems investigated so far. This magnetic axis is almost exactly orthogonal to the axial histidine plane. The other two axes lie essentially in the histidine plane, the closest to the heme normal being tilted by about 36 degrees from it, towards pyrrole A on the side of the proximal histidine. From the comparison with cytochrome c' it clearly appears that the position of the one axis lying in the heme plane is related to the axial histidine orientation. Irrespective of the directions, the magnetic anisotropy is smaller than that of the analogous reduced cytochrome c' and of the order of that of low-spin iron(III). The magnetic anisotropy of the system permits the measurement of residual dipolar couplings, which, together with pseudocontact shifts, prove that the solution structure is very similar to that in the crystalline state. Magnetic measurements, at variance with previous data, demonstrate that there is an orbital contribution to the magnetic moment, micro(eff)=5.5 micro(B). Finally, from the magnetic anisotropy data, the hyperfine shifts of iron ligands could be separated in pseudocontact and contact components, and hints are provided to understand the spin-delocalisation mechanism in S=2 systems by keeping in mind the delocalisation patterns in low-spin S=1/2 and high-spin S= 5/2 iron(III) systems.     

Charge localization in stacked radical cation DNA base pairs and the benzene dimer studied by self-interaction corrected density-functional theory

The incomplete cancellation of the electron self-interaction can be a serious shortcoming of density-functional theory especially when treating odd-electron systems. In this work, several popular and potentially viable correction schemes are applied in order to characterize the electronic structure of stacked molecular pairs, consisting of a neutral molecule and adjacent radical cation, as a function of separation distance. The unphysical sharing of the positive charge between adjacent molecules separated by 6-7 A is corrected for by applying a new empirical scheme proposed by VandeVondele and Sprik [Phys. Chem. Chem. Phys. 2005, 7, 1363] with a unique choice of parameters. This method is subsequently applied to characterize the electronic structure of two neighboring guanines excised from a canonical Arnott B-DNA structure and will be used in future investigations of certain model DNA fibers.     

Electrostatic influence on rotational mobilities of sol-gel-encapsulated solutes by NMR and EPR spectroscopies

The rotational mobilities of small solute molecules encapsulated in tetramethyl orthosilicate (TMOS) sol-gels have been investigated by EPR spectroscopy of encapsulated nitroxide probes and by high-resolution NMR spectroscopic measurements of transferred NOE's (trNOE's), of T(1)'s, and of T(1)'s in the rotating frame (T(1)rho). The two spectroscopic methods are sensitive to motions on different time scales and hence, are nicely complementary. Suites of neutral, positively, and negatively charged nitroxide probes (EPR) and of simple diamagnetic small molecules (NMR) were selected to disclose influences of electrostatic interactions with the sol-gel walls and to probe the presence of multiple populations of molecules in distinct regions of the sol-gel pores. For neutral and negatively charged solute probes, both techniques disclose a single population with a significantly increased average rotational correlation time, which we interpret at least in part as resulting from exchange between free-volume and transiently immobilized surface populations. The electrostatic attraction between cationic probes and the negatively charged sol-gel walls causes the positively charged probes to be more effectively immobilized and/or causes a greater percentage of probes to undergo this transient immobilization. The EPR spectra directly disclose a population of cationic probes which are immobilized on the X-band EPR time scale: tau(c) greater than or approximately equal 10(-7) s. However, NMR measurements of trNOE's and of T(1)rho demonstrate that this population does exchange with the free-volume probes on the slower time scale of NMR. This approach is equally applicable to the study of solutes within other types of confined spaces, as well.     

Weak molecular complexes as studied by NMR spectroscopy of oriented molecules

Weak molecular interactions such as those in pyridine—iodine, benzene—iodine and benzene—chloroform systems oriented in thermotropic liquid crystals have been studied from the changes of the order parameters as a result of complex formation. The results indicate the formation of at least two types of charge transfer complexes in pyridine—iodine solutions. The pi-complexes in benzene—chloroform and benzene—iodine mixtures have also been detected. No detectable changes in the inter-proton distances in these systems were observed.     

Longitudinal diffusion behavior of hemicyanine dyes across phospholipid vesicle membranes as studied by second-harmonic generation and fluorescence spectroscopies

The adsorption and longitudinal diffusion behaviors of a series of hemicyanine dyes to phospholipid vesicle membranes were studied by second-harmonic generation (SHG) and fluorescence spectroscopies. It was observed that the longitudinal diffusion of cationic hemicyanine dyes takes place immediately after the initial adsorption of these dyes to the outer surface of the vesicle membrane. In contrast, hardly any amount of a zwitterionic hemicyanine dye with a sulfonate group diffused across the vesicle membrane within the measurement time (<2000 s). Based on the difference in the time-course responses of SHG and fluorescence spectroscopies for all of the hemicyanine dyes tested, we propose that hydration of the sulfonate group is mainly responsible for the low diffusivity of the zwitterionic hemicyanine dye.     

Structure of the ammonia dimer studied by density functional theory

Self-consistent Kohn–Sham density functional calculations have been carried out to study the structure of the ammonia dimer. The local-density approximation yields unusually large binding energy and short internitrogen distance compared with the experimental and more accurate theoretical data. The results from the Becke–Perdew gradient-corrected functionals are generally in good agreement with those at the SCF MP 2 level when the geometry is fully optimized with various large basis sets. With our best estimation, the staggered quasi-linear structure (C_s) is 0.6 kcal/mol lower in energy than the symmetric cyclic one (C_2h). The hydrogen-bonded N—H bond in the staggered quasi-linear structure is found to be 0.008 Å longer than the N—H bond in ammonia. In our calculations, we could not find the minima on the energy surface corresponding to the two asymmetric cyclic structures suggested by microwave spectra and coupled pair functional calculations. © 1994 John Wiley & Sons, Inc.     

Helix change of polypeptides in the solid state as studied by NMR spectroscopy

The conformational characterization of solid polypeptides has been carried out by means of high-resolution solid-state NMR. It has been demonstrated that high-resolution solid-state NMR spectroscopy is a very useful means for elucidating the structure of polypeptides and proteins in the solid state.     

Free‐radical copolymerization of styrene and itaconic acid studied by 1H NMR kinetic experiments

The free‐radical copolymerization of itaconic acid (IA) and styrene in solutions of dimethylformamide and d₆‐dimethyl sulfoxide (50 wt %) has been studied by ¹H NMR kinetic experiments. Monomer conversion versus time data were used to estimate the ratio k_p · k_t^−0.5 for various comonomer mixture compositions. The ratio k_p · k_t^−0.5 varies from 5.2 · 10⁻² for pure styrene to 2.0 · 10⁻² mol^0.5 L^−0.5 s^−0.5 for pure IA, indicating a significant decrease in the rate of polymerization. Individual monomer conversion versus time traces were used to map out the comonomer mixture–composition drift up to overall monomer conversions of 60%. Within this conversion range, a slight but significant depletion of styrene in the monomer feed can be observed. This depletion becomes more pronounced at higher levels of IA in the initial comonomer mixture. The kinetic information is supplemented by molecular weight data for IA/styrene copolymers obtained by variation of the comonomer mixture composition. A significant decrease in molecular weight of a factor of 2 can be observed when increasing the mole fraction of IA in the initial reaction mixture from 0 to 0.5. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 656–664, 2001     

Silver nitrate-acetonitrile and iodine-benzene complexes as studied by NMR in thermotropic liquid crystals

Silver nitrate-acetonitrile and π iodine-benzene complexes in thermotropic liquid crystals have been studied by ¹H, ²H, and ¹³C NMR spectroscopy and by optical microscopy. Evidence for at least two silver complexes in each liquid crystal is presented.     

Structure,dynamics and interactions in polymer systems as studied by NMR spectroscopy

The possibilities of NMR spectroscopy in studies of interactions in polymer systems are demonstrated on the example of two types of macromolecular complexes: (i) By measuring ¹H NMR high resolution line intensities, the formation of ordered associated structures of syndiotactic (s) poly(methyl methacrylate)(PMMA) in mixed solvents was quantitatively characterized. The obtained results permit us to assume that the mechanism by which the solvent affects self-association of s-PMMA involves specific interactions of the solvent molecules with PMMA units. Solid state high resolution ¹³C NMR spectra of associated s-PMMA gels were also measured and compared with the spectra of a solid s-PMMA sample. (ii) By using ¹³C solid state NMR spectroscopy, the differences in the structure of the amorphous and crystalline phases in pure poly(ethylene oxide) and its complexes with p-dichlorobenzene or p-nitrophenol were characterized. Prounounced differences also in the dynamic structure of the crystalline phase in these systems are indicated by the relaxation times T₁(C), T_1ρ(C) and T_1ρ(H).     

Mediation of ultrafast light-harvesting by a central dimer in phycoerythrin 545 studied by transient absorption and global analysis

We report ultrafast femtosecond transient absorption measurements of energy-transfer dynamics for the antenna protein phycoerythrin 545, PE545, isolated from a unicellular cryptophyte Rhodomonas CS24. The phycoerythrobilins are excited at both 485 and 530 nm, and the spectral response is probed between 400 and 700 nm. Room-temperature measurements are contrasted with measurements at 77 K. An evolution-associated difference spectra (EADS) analysis is combined with estimations of bilin spectral positions and energy-transfer rates to obtain a detailed kinetic model for PE545. It is found that sub pulse-width dynamics include relaxation between the exciton states of a chromophore dimer (beta 50/60) located in the core of the protein. Energy transfer from the lowest exciton state of the phycoerythrobilin (PEB) dimer to one of the periphery 15,16-dihydrobiliverdin (DBV) bilins is found to occur on a time scale of 250 fs at room temperature and 960 fs at 77 K. A host of energy-transfer dynamics involving the beta 158, beta 82, and alpha 19 bilins occur on a time scale of 2 ps at room temperature and 3 ps at 77 K. A final energy transfer occurs between the red-most DBV bilins with a time scale estimated to be approximately 30 ps. The role of the centrally located phycoerythrobilin dimer is seen as crucial-spectrally as it expands the cross-section of absorption of the protein; structurally as it sits in the middle of the protein acting as an intermediary trap; and kinetically, as the internal conversion and subsequent red-shift of the excitation is extremely fast.     

Early events in the photochemistry of 1,2,3-thiadiazole studied by ultrafast time-resolved UV-vis and IR spectroscopies

Photochemistry of 4-methyl-5-carboethoxy-1,2,3-thiadiazole (MCT) in solution was studied at room temperature using UV-vis and IR transient absorption spectroscopies (λ(ex) = 266 nm). Three deactivation pathways are discussed: thiirene formation, thioketene generation, and the internal conversion by repopulating MCT in the ground state. Ultrafast techniques show a very fast rise (<0.4 ps) of thiirene species in acetonitrile, possibly as a result of a rearrangement in the singlet excited state of MCT in concert with molecular nitrogen extrusion. The remarkable stability of thiirene in solution on a millisecond time scale is limited by the process of dimerization (thiirene-thiirene, thiirene-thioketene). NMR analysis permitted identification of stable photoproducts as 1,3-dithietane, 1,3-dithiole, and thiophene derivatives.     

Solid-state NMR spectroscopy as a tool supporting optimization of MALDI-TOF MS analysis of polylactides

We report systematic structural studies of poly(l-lactide) (PLLA) employing matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) and solid-state NMR spectroscopy. (13)C cross polarization magic angle spinning (CP/MAS) NMR data for 1,8-dihydroxy-9-anthracenone (DT), 2,5-dihydroxybenzoic acid (DHB), 2-(4-hydroxyphenylazo)-benzoic acid (HABA), and trans-3-indoleacrylic acid (IAA), four matrices commonly used in MALDI-TOF analysis of polymers, were analyzed to test the influence of crystallization conditions (solvent, inorganic salt) on sample morphology. (13)C principal elements of chemical shift tensor (CST) and line-shape analyses were employed to study of the nature of hydrogen bonding and to evaluate the crystallinity and amorphicity of the pure polymer. NMR parameters for PLLA were compared with data for polylactide crystallized with the four matrices under different conditions with the addition of two inorganic salts as cationization agents. This study revealed that the semicrystalline structure of the polymer does not change when it is embedded in the matrix.