Pre-processing of ultraviolet resonance Raman spectra

The application of UV excitation sources coupled with resonance Raman have the potential to offer information unavailable with the current inventory of commonly used structural techniques including X-ray, NMR and IR analysis. However, for ultraviolet resonance Raman (UVRR) spectroscopy to become a mainstream method for the determination of protein secondary structure content and monitoring protein dynamics, the application of multivariate data analysis methodologies must be made routine. Typically, the application of higher order data analysis methods requires robust pre-processing methods in order to standardize the data arrays. The application of such methods can be problematic in UVRR datasets due to spectral shifts arising from day-to-day fluctuations in the instrument response. Additionally, the non-linear increases in spectral resolution in wavenumbers (increasing spectral data points for the same spectral region) that results from increasing excitation wavelengths can make the alignment of multi-excitation datasets problematic. Last, a uniform and standardized methodology for the subtraction of the water band has also been a systematic issue for multivariate data analysis as the water band overlaps the amide I mode. Here we present a two-pronged preprocessing approach using correlation optimized warping (COW) to alleviate spectra-to-spectra and day-to-day alignment errors coupled with a method whereby the relative intensity of the water band is determined through a least-squares determination of the signal intensity between 1750 and 1900 cm(-1) to make complex multi-excitation datasets more homogeneous and usable with multivariate analysis methods.     

The near ultraviolet absorption spectra of 2,3-, 2,4- and 2,5-dimethylbenzonitriles

A new technique—photoacoustic spectroscopy (PAS—was applied for the study of the electronic transitions in three isomers, 2,3-, 2,4- and 2,5-dimethylbenzonitriles. The PAS spectra were compared with solution and vapour phase spectra. A probable detection of S → T absorption in the molecules is considered to be of significance in the present investigation. It is also shown that two π → π* transitions analogous to the benzene strong 200 nm and weak and forbidden 260 nm transitions could be identified in all three molecules. An interesting feature is regarding the identification of a few combination bands in the excited state of an electronic transition which could be fairly comparable with such combinations observed in the near i.r. PAS spectra of the molecules, in the ground state.     

Ab initio calculations of the ultraviolet resonance Raman spectra of uracil

An equation been derived to calculate, ab initio, the frequencies and intensities of a resonant Raman spectrum from the transform theory of resonance Raman scattering. This equation has been used to calculate the intensities of the ultraviolet resonance Raman spectra from the first π-π* excited state of uracil and 1,3-dideuterouracil. The protocol for this calculation is as follows: (1) The force constant matrix elements in Cartesian coordinate space, the vibrational frequencies, and the minimum energy ground and excited state geometries of the molecule are calculated ab initio using the molecular orbital program Gaussian 92, (2) the force constants in Cartesian coordinates are transformed into force constants in the space of a set of 3N – 6 nonredundant symmetrized internal coordinates, (3) the G matrix is constructed from the energy minimized ground state Cartesian coordinates and the GFL = LΛ eigenvalue equation is solved in internal coordinate space, (4) the elements of the L and L^?1 matrices are calculated, (5) the changes in all of the internal coordinates in going from the ground to the excited state are calculated, and (6) these results are used in combination with the transform theory of resonance Raman scattering to calculate the relative intensities of each of the 3N – 6 vibrations as a function of the exciting laser frequency. There are no adjustable parameters in this calculation, which reproduces the experimental frequencies and intensities with remarkable fidelity. This indicates that the Dushinsky rotation of the modes in the excited state of these molecules is not important and that the simplest form of the transform theory is adequate. © 1995 John Wiley & Sons, Inc.     

The ultraviolet spectra of silocyclobutanes

The UV spectra of Si-substituted silocyclobutanes in vacuum were studied. The bathochromic shift of the absorption band, disclosed by comparison with bands of the corresponding acyclic compounds, can be explained by the highly strained state of the four-membered ring. This same principle in disilocyclobutanes leads to the reduction of the barrier effect of the methylene group, disrupting the interaction of the silicon atoms in the unstrained molecule.     

The Raman spectra of bromoform

In this work the Raman spectrum of bromoform is recorded at various temperatures in the pure compound and in solution in carbon tetrachloride and the features shown by some of the bands are considered in the light of previous spectroscopic data interpretations. In particular, the concentration dependent effects exhibited by the ν₅(e) vibrational mode suggest, by comparison with similar effects previously observed in chloroform and trichloromethyl fluoride, the existence, in the pure liquid at room temperature, of important intermolecular interactions.     

Molecular spectra and molecular structure of heterocyclic compounds. Infrared and near ultraviolet absorption spectra of pyridine N-oxide derivatives

The infrared and ultraviolet spectra of many pyridine N-oxide derivatives have been studied. Ultraviolet spectra show a strong solvent effect in going from aprotic to hydroxylic solvents due to strong hydrogen bonding. Values of δ H° for several systems CH₃OH/XPyNO in tetra-chloroethylene solution were evaluated by infrared techniques and a linear correlation δ H° vs. δ v OH has been found. Features of the infrared and ultraviolet characteristic absorption bands were also investigated.     

The use of near infrared Fourier Transform techniques in the study of surface enhanced Raman spectra

Near infrared Fourier Transform Raman spectroscopy has been used to study the SERS of a number of electrode-solution interfaces. These measurements are illustrated by the following examples: the adsorption of pyridine on Ag, Cu and An surfaces; the adsorption of ferri- and ferrocyanide ions on An electrodes in two different support electrolytes; the behaviour of the corrosion inhibitors benzotriazole and 2-aminopyrimidine at Cu surfaces. Measurements of the DSERS spectra of pyridine at Ag electrodes and of normal Raman spectra of pyridine at Pt electrodes are also reported. The results are also compared with data taken by conventional methods in the visible region and the advantages of this newly developed technique are assessed.     

The Raman spectra of the cylophanes

Raman spectra have been obtained for the entire series of [2_n]-cyclophanes and along with the i.r. spectra, several band assignments have been proposed. A correlation of the frequencies of the in-plane ring modes of the cyclophane with those of the identically substituted methylated benzene has been made. The CH₂ stretching bands have been found to move to higher frequency as the degree of steric hindrance increased. Tentative assignments have been proposed for the aromatic CH out-of-plane wag, and a possible origin for the distorted benzene band is discussed.     

The vacuum ultraviolet spectra of methylated silanes

The vacuum ultraviolet spectra of monomethylsilane, monomethylsilane-d₃, dimethylsilane, dimethylsilane-d₂, trimethylsilane, trimethylsilane-d₁, trimethylmonofluorosilane and tetramethylsilane have been recorded in the range 135 nm to 190 nm.     

Excited states of the polyacetylenes. Analysis of the near ultraviolet spectra of diacetylene and triacetylene

The 2000–2800 Å system of diacetylene and the 2300–3000 Å system of triacetylene have been investigated. Analysis of the vibrational structure indicates that we observe transitions to a ¹Δ_u upper electronic state for both molecules and transitions to a ¹Σ^?_u state for diacetylene.     

Raman and surface Raman spectroscopy with ultraviolet excitation

We record the accurate and reliable Raman spectra of benzoic acid (BA), p-nitrobenzoic acid (PNBA) and o-nitrobenzoic (ONBA) in aqueous solution with ultraviolet excitation. And we find that the ultraviolet (UV) Raman spectrum of aqueous BA solution has one-to-one correspondence to that of BA solid whereas the others are less resemble to the solid counterparts. We also report surface Raman spectroscopy of them in silver colloid without any enhancement in UV region and call it surface-unenhanced Raman spectroscopy (SUERS) while the surface-enhanced Raman scattering (SERS) effects are perfect in near infrared or visible regions. It demonstrates the SERS effects are strongly dependent on the excitation wavelength. On the basis of the experiments, we discuss the mechanism of SERS excited in different regions.     

The infrared and Raman spectra of pyromellitic dianhydride

The infrared and Raman spectra of solid state samples of pyromellitic dianhydride have been measured. The infrared—Raman mutual exclusion rule has been observed and the frequencies have been tentatively assigned on the basis of D_2h symmetry. The values of the CO and skeletal ring stretching frequencies have been interpreted in terms of a conjugated π-system.     

The infrared and Raman spectra of guanidinium salts

The i.r. and Raman spectra of guanidinium chloride, guanidinium perchlorate, and the deuterated derivatives of these salts, have been determined as solids and in solutions.     

The infrared and Raman spectra of carbonyl diisocyanate

The infrared and Raman spectra of carbonyl diisocyanate have been recorded. Vibrational assignments have been made on the basis that two planar conformations are present both in the liquid and vapour phases: cis—cis and cis—trans. Simple normal coordinate calculations have been used to confirm the assignments.     

The lineshape of motion-averaged isotropic Raman spectra

The conventional Kubo model can be used to describe the spectrum modulation by slow to fast motion, i.e. even in the range of applicability of perturbation theory. However, the Kubo model assumes the gaussian shape of static spectra. A model free of this assumption has been used to show that experimental data can be interpreted provided the initial static spectrum has non-gaussian wings.     

The infrared and Raman spectra of trans- and cis-tetrachloro-tetramethylbicyclopropylidene

The title compounds trans- and cis-2,2,2',2'-tetrachloro-3,3,3',3'-tetramethyl-bicyclopopylidene were synthesized, and their infrared and Raman spectra were recorded. Non-coincidence between the IR and Raman bands of the trans compound suggested C(2h) symmetry and a planar ring system. In the cis compound most of the IR and Raman bands coincided and a C(2v) symmetry seems likely. The exocyclic CC double bond gave rise to a medium/weak Raman band at 1,847 cm(-1) in the trans compound. In the cis derivative IR and Raman bands both at 1,825 cm(-1) were observed. From similarities with related molecules, the ring breathing, the antisymmetric ring stretch, the CCl(2) out-of-phase and in-phase stretch and the out-of-plane ring bending modes have been tentatively assigned for the trans and cis compounds.     

The infrared and Raman spectra of octachloro- and octabromocyclotetrasilane

The infrared and Raman spectra of cyclo-Si₄Cl₈ and cyclo-Si₄Br₈ have been measured and assigned. The selection rules indicate that the Si₄ ring is not planar. A normalcoordinate analysis (NCA) has been performed. The SiSi stretching-force constants are calculated to be 1.75–1.5·10² Nm^?1 and are lower than in the corresponding perhalogenated five- and six-membered rings. The SiCl and SiBr force constants seem to be somewhat higher (2.7–2.35·10² Nm^?1).     

Raman spectra phthalocyanines

Raman spectra of amorphous phthalocyanine thin films have been studied. Theoretical and experimental correlations in polarization ratios are applied to vibrational assignments of symmetry species and to the problem of molecular orientation in thin solid films.