Multivariate curve resolution applied to Fourier transform infrared spectra of macromolecules: structural characterisation of the acid form and the salt form of humic acids in interaction with lead

The significance of evolving mixtures structural spectroscopic studies might appear limited when the experimental spectra do not present a sufficient quality for a precise interpretation. It is the case when the chemical behaviour of macromolecules is studied on the basis of infrared spectra. If the effective resolution is low, the spectral profiles appear similar despite the applied chemical conditions change. This makes impossible the interpretation of the raw spectra and mathematical treatments are required to separate the different contributions that overlap.To determine the behaviour of the reactive sites of humic acids in the binding with heavy metals, infrared spectra are recorded under various chemical conditions. The cation to be considered is Pb²⁺ and the two chemical variables to be studied are pH and initial lead concentration. Four series of FTIR spectra are recorded, but no visible difference can be directly assigned to the different chemical states of the macromolecules. Multivariate self-modelling curve resolution is thus proposed as a tool for resolving these complex and strong overlapping datasets. First, initial estimates are obtained from pure variable detection methods: it comes out that two spectra are enough to reconstruct the experimental matrices. In a further step, the application of the multivariate curve resolution-alternating least squares (MCR-ALS) algorithm with additional constraints on each individual dataset, as well as on column-wise augmented matrices, allows to optimise the profiles and spectra that appear to be highly characterising the acid and the salt form of the molecule. Moreover, the concentrations profiles associated to these two limit spectral forms allow interpreting the analytical measurements made during the reactions between humic acids and H⁺ or Pb²⁺. Consequently, depending on the initial state of the humic acid, two distinct reactional mechanisms are proposed.     

Double-modulation and conventional ESR spectra of spin-labeled crosslinked polystyrene

The recently introduced method of detecting homogeneous linewidths from inhomogeneously broadened ESR spectra using the double-modulation technique has been applied in parallel with the conventional method to study molecular motion in crosslinked polystyrene. The rotational correlation time in the slow-motion region determined from the homogeneously broadened line varied from 1.68 × 10^?5 to 8.0 × 10^?6 s between 150 and 340 K. Double-modulation spectra appear to be very sensitive to local motion at low solvent concentration. The influence of crosslink density and solvent concentration on both local and overall motion is analyzed.     

An emergency response intercomparison exercise using a synthetically generated gamma-ray spectrum

Although high resolution gamma ray spectrometry serves as the primary analytical technique in emergency response situations, chances for laboratories to practice analysing the type of spectra that may be expected in the early phase of such a situation are limited. This problem is more acute for laboratories in countries that have no nuclear facilities. The analysis of synthetically generated spectra may serve as a useful surrogate for actual spectra; this paper reports the results of a multilateral intercomparison exercise conducted using such a spectrum. Results indicate that the laboratories involved appear to have no problems identifying radioactive isotopes that regularly appear in national and international intercomparisons or exercises. However, some problems are evident for isotopes that are less often encountered which may, however, occur during the early phases of a nuclear accident. Isotopes prone to true coincidence summation proved also to be difficult with regard to correction of activities in some cases. The synthesized spectrum constituted a useful means of comparative analysis of complex spectra multilaterally without the impracticalities of using a sample drawn from a reactor.     

Effect of spiroconjugation on the UV and CD spectra of 1,1′-spirobiindene

U.v. and c.d. spectra of 1,1′-spirobiindene were theoretically explained based on the composite molecule method. The effect of spiroconjugation was shown to appear markedly in the ¹L_a region of indene, making a large splitting of this band, as was expected from PE spectra.     

1064-nm-excited Fourier transform Raman studies of bacteriochlorophyll-a in solid films and in a blue-green mutant of Rhodobacter sphaeroides

1064-nm-excited Fourier transform Raman spectra of bacteriochlorophyll-a (BChl) in various solid films and in chromatophores from a blue-green mutant of Rhodobacter sphaeroides have been obtained. The observed Raman spectra are free from high fluorescence backgrounds and sample degradation. The observed intensities seem to be enhanced because of a pre-resonant effect between the exciting radiation at 1064 nm and the Q_y absorption at 770–870 nm of BChl. The spectral features are substantially different from the Soret and Q_x resonance Raman spectra extensively investigated so far; several bands in the wavenumber region lower than 1200 cm⁻¹ are particularly enhanced in the Q_y pre-resonance Raman spectra. Bands due to both the C₂O and C₉O stretches appear at 1700–1620 cm⁻¹, providing structural information on these carbonyl groups. In the CC stretching region (1620–1490 cm⁻¹), the correlation between band positions and the co-ordination number of central magnesium, which was previously found in the Soret-excited Raman spectra, is preserved in the Q_y, pre-resonance Raman spectra as well. The relative intensities of strong bands in the 1200–1000 cm⁻¹ region appear to be useful for characterizing the BChl state. By using these advantages of the Q_y, pre-resonance Raman spectra, molecular interactions and arrangements of BChl in hydrated films and in the B870 light-harvesting complex of R. sphaeroides are discussed.     

EXTRARETINAL PHOTORECEFTION IN INSECTS

Abstract— Extraretinal photoreceptors are widespread among insects and function in the photoperiodic control of development and in the entrainment of circadian rhythms. The effects of light on the daily and seasonal regulation of brain neuroendocrine activity are mediated solely through extraretinal photoreceptors. In primitive insects, the eyes participate in the entrainment of nonendocrine circadian rhythms such as the locomotor rhythm. In more advanced forms, however, extraretinal pathways appear to be the only pathway for the entrainment of all rhythms thus far examined. But even in this latter case, the eyes sometimes effect a masking of the expression of the overt rhythm. An exact localization of the extraretinal receptors has not been accomplished, but in all studies to date they appear to be associated with the cerebral lobe region of the brain. Action spectra for photoperiodic responses have been determined for a number of insects. In general the responses are maximally sensitive in the blue with a marked decline in the red although exceptions do exist. Complete action spectra for circadian responses have been determined only for two insects. In both cases a plateau of sensitivity extends through the blue with a steep drop at longer wavelengths. From the action spectra data, the extraretinal receptors appear to have a threshold sensitivity less than 3 times 10^-2 J/m². The pigment nature of the receptor is unknown although it appears not to be a carotenoid derivative.     

Mass-analyzed Threshold Ionization Spectroscopy of Rotamers of p-ethoxyphenol Cations and Configuration Effect

Two-color resonant two-photon mass-analyzed threshold ionization （MATI） spectroscopy was used to record the vibrationally resolved cation spectra of the selected rotamers of p- ethoxyphenol. The adiabatic ionization energies of the trans and cis rotamers are determined to be 61565±5 and 61670±5 cm^-1, which are less than that of p-methoxyphenol by 645 and 643 cm^-1, respectively. Analysis on the MATI spectra of the selected rotamers of p-ethoxyphenol cation shows that the relative orientation of the ethoxy group has little effect on the in-plane ring vibrations. The low-frequency OC2H5 bending vibrations appear to be active for both forms of the cation.     

Raman-Spektroskopie und Molekülstruktur

The substitution pattern of benzene derivatives can be determined by means of a series of questions on characteristic Raman bands; frequency range, relative intensity, and degree of polarisation must be considered. Mono-, ortho-, meta-, and para-di-, and 1,3,5-tri-substituted derivatives can be recognised particularly well. Assignments made from Raman spectra appear to us to be more reliable than those obtained from characteristic infrared bands.     

Ground‐State and Excitation Spectra of a Strongly Correlated Lattice by the Coupled Cluster Method

The coupled cluster method is applied to a strongly correlated lattice Hamiltonian, and the coupled cluster linear response method is extended to the calculation of electronic spectra by finding an approximation to a resolvent operator which describes the spectral response of the coupled cluster solution to excitation operators. In this spectral coupled cluster method, the ground and excited states appear as resonances in the spectra, and the resolvent can be iteratively improved in selected spectral regions. The method is applied to a MnO₂ plane model which corresponds to previous experimental works.     

Effect of gold nanoparticles on an aerosil surface on the fluorescence and Raman spectra of adsorbed tryptophan

We discuss the fluorescence and Raman spectra of the amino acid tryptophan (Trp) in the presence of gold nanoparticles in solution and on the surface of highly dispersed silica (aerosil) containing gold nanoparticles (Au-SiO₂). The fluorescence of Trp is efficiently quenched in the presence of gold nanoparticles both in solution and on the SiO₂ surface. The fluorescence and excitation spectra contain bands for molecular Trp and a charge transfer complex between Trp and the nanoparticles. In the Raman spectra of Trp with gold nanoparticles, considerably enhanced intense vibrations appear for the carboxyl and amino groups and also for the benzene and pyrrole rings. The effect of gold nanoparticles on the Raman spectra of Trp in a heterogeneous system is considerably weakened due to strong light scattering by the dispersed silica.     

Infrared,Raman spectra and vibrational assignment of (PF2)2O

The IR spectra of gaseous and solid (PF₂)₂O has been recorded from 80 to 1200 cm^?1. The Raman spectra of gaseous, liquid and solid (PF₂)₂O have also been obtained (30–1000 cm^?1). The spectra of the fluid phases indicate the presence of at least two conformers. The spectrum of the solid phase can readily be interpreted on the basis of a single conformer possessing a symmetry lower than C_2v. A vibrational assignment is proposed for all normal modes except the PF₂ torsions. The results are compared with similar data of related compounds. There appear to be two molecules per primitive cell based upon the low-frequency Raman data.     

The carbon-13 nuclear magnetic resonance spectra of some α-spirocyclopropyl ketones and related cyclohex-2-en-1-ones and 2-ethylidenecyclohex-3-en-1-ones

The ¹³C NMR spectra of a series of β,γ-unsaturated α-spirocyclopropylcyclohexanones and saturated α-spirocyclopropylcycloalkanones have been analyzed and compared with the spectra of diethyl cyclopropanedicarboxylate and a corresponding spiro acylal. The chemical shifts of the cyclopropane methylene carbons are correlated with spiroactivation of the cyclopropane ring to nucleophilic attack. In the case of the saturated spiro ketones these chemical shifts can also be correlated with their photochemistry. In the SFORD spectra of the spiro ketones the signals of the cyclopropane methylene carbons appear as complex multiplets: this is attributed to second-order coupling resulting from strong coupling between the vicinal cyclopropane protons. The ¹³C NMR spectra of a series of related cyclohex-2-en-1-ones and 2-ethylidenecyclohex-3-en-1-ones have also been analyzed; the chemical shift assignments for the latter corroborate the configurational assignments made on the basis of ¹H NMR spectroscopy.     

Proton and carbon-13 nuclear magnetic resonance spectroscopy and conformational aspects of 5,6,7,8-tetrahydro-4-chromanone derivatives

The 25 MHz ¹³C and 350 MHz ¹H-nmr spectra of the title compounds are reported. Conformational equilibria in variously substituted 5,6,7,8-tetrahydrochromanones are discussed. Compounds bearing a t-butyl group at the 6 position appear to be conformationally homogenous.     

Mass spectrometry of 7,7′,8,8′-tetracyanoquinodimethane and of some of its radical anion salts

We present mass spectrometry experiments on tetracyanoquinodimethane (TCNQ), deuteriated TCNQ and on some TCNQ salts. Apart from the usual nitrile fragmentation, at least two original features appear in the spectra. The first is the formation of TCNQ dimers, as evidenced by their molecular peaks and their fragmentation. The second aspect concerns the spectra of the ammonium salts, which show that hydrogen or alkyl groups add to TCNQ before its decomposition, leading to new fragments (as m/z 141).     

Optical plasmon losses in stabilized Au55 clusters

Au₅₅ cluster compounds are investigated by optical spectroscopy and TEM. The optical spectra appear to be rather structureless, neither showing a collective excitation resonance nor exhibiting distinct absorption bands known from lower nuclearity clusters. We discuss changes of the electronic properties compared to larger Au clusters affecting both, 6sp electrons and5d-6sp interband transitions, the cluster-ligand-interaction being considered as a charge transfer process. We additionally report on a low temperature instability of the cluster compound, which results in changed optical extinction spectra. A characteristic absorption feature at λ=400 nm is attributed to small, ligand-free Au cluster fragments.     

Synthesis and spectroscopic investigations of iron(III) complexes with chlorides and dianionic,symmetrically halogen substituted phthalocyanines as ligands

The synthesis of iron(III) complexes of general formula FeCl(R-pc), where R-pc are dianionic, symmetrically halogen substituted phthalocyanines at the positions 2,9,16,23 or 1,8,15,22, from the corresponding amino substituted derivatives is described (R=Cl, Br, I). The complexes are characterized by UV-visible and infrared spectra, powder X-ray diffraction and magnetic susceptibility measurements. The effect of substituents at the periphery and the basicity of the solvents used on the electronic spectra are discussed. The Q band of the electronic spectra for symmetrically halogen substituted derivatives are redshifted and the substituents at 2,9,16,23- positions are more effective in redshifting the Q bands than those at 1,8,15,22-positions. Depending upon the basicity of the solvents, the ligand-to-metal charge transfer (LMCT) transitions on Q band envelop shift to the higher energy region in the order of pyridine>DMF>DMSO. The infrared absorption signals for C-H and metal-ligand vibrations appear to be sharper for 1,8,15,22 substituted derivatives than for 2,9,16,23 substituted ones.     

n-π Interactions in homoallylic methyl ethers a photoelectron spectroscopic study

The He I photoelectron spectra of several monocyclic and bicyclic methoxy compounds have been interpreted in terms of inductive effects and n-π interactions between the π_cc molecular orbital and the oxygen lone pair orbital n_o. The n-π interactions appear to be small, and are mainly through bond.     

On the relation between absorption and emission spectra of molecules in solution

The universal relation between absorption and fluorescence spectra derived by Stepanov in 1957 is reexamined to assess the relative merits of two of the inferences usually drawn from its use in the analysis of data: elevated excited state temperatures (“warm fluorescence”) and inhomogeneous broadening of the molecular spectra. Of these, only the latter appears to give a satisfactory explanation of the anomalous temperatures derived using the Stepanov method in condensed media, and in particular, a purely “warm fluorescence” interpretation implies that a distinct curvature should appear in plots which experimentally appear to be straight lines.     

Pyrolysis—high-resolution mass spectrometry of biological materials

In a diseased state in man or animals a change at eh molecular level might occur. A pyrolysis-high-resolution mass spectrometric method has been developed to measure these changes. The mass spectra are used as fingerprints. A similar approach is reported using low-resolution mass spectrometry. The difference between low resolution and high resolution is that the number of mass spectral lines is strongly increased and therefore also the information content. As a result, a change at the molecular level will be more pronounced in high-resolution mass spectra. As an application, the results of toxicity studies on Daphnia magna are given. The experiments were performed with paraoxon, parathion and malathion. The changes at the molecular level in Daphnia magna appear to be substance-dependent.