Depolarization Ratio Measurements Research Articles

ChemInform Abstract: Characteristic Vibrational Frequencies and Normal Modes of the CCO Ligand in Trinuclear Ketenylidene Clusters.

The two stretching vibrations associated with the CCO ligand in (PPN)/sub 2/(Ru/sub 3/(CO)/sub 6/(/mu/-CO)/sub 3/(/mu//sub 3/-CCO)) (1) have been identified by isotopic substitution and fit to an approximate normal-coordinate analysis. The high-frequency CCO stretching component occurs in the carbonyl stretching region (1977 cm/sup /minus/1/), and significant mixing occurs between this CCO stretch and a terminal carbonyl stretch of the same symmetry. The low-frequency CCO stretching vibration has been identified in the Raman spectrum of 1 and in the IR or Raman spectra of the ketenylidene clusters H/sub 2/Ru/sub 3/(CO)/sub 9/(/mu//sub 3/-CCO), (PPN)/sub 2/(Fe/sub 3/(CO)/sub 9/(/mu//sub 3/-CCO)), and (Co/sub 3/(CO)/sub 9/(/mu//sub 3/-CCO))(PF/sub 6/). This mode was found in the 1200-1350-cm/sup /minus/1/ range, with the more positively charged clusters having lower frequencies. The position of the high-frequency CCO stretch of (PPN)/sub 2/(Ru/sub 3/(CO)/sub 6/(/mu/-CO)/sub 3/(/mu//sub 3/-CCO)) is sensitive to isotopic substitution at the central carbon atom of the CCO but relatively unaffected by /sup 13/C substitution at the basal carbon. The converse is true for the low-frequency CCO stretch. This pattern of isotope shifts has a simple explanation in the form of the normal modes, and it should prove useful in the search for CCO on metal surfaces. Several of the framework more » vibrations associated with the M/sub 3/C core of the above ketenylidene clusters are reported. Assignments were made with the aid of Raman depolarization ratio measurements on samples in CH/sub 2/Cl/sub 2/ solution and in some cases on polycrystalline solids. 36 refs., 8 figs., 6 tabs. « less

Infrared, Raman, resonance Raman, and excitation profile studies of tetrakis(acetato)bis(triphenylphosphine)dirhodium and its oxygen-18 and methyl-d3 isotopomers

The resonance Raman spectra of RH/sub 2/(O/sub 2/CCH/sub 3/)/sub 4/(PPh/sub 3/)/sub 2/ and its /sup 18/O and CD/sub 3/ isotopomers have been recorded at ca. 80 K. A band at 289 cm/sup /minus/1/, denoted /nu//sub 1/, is assigned to /nu/(Rh-Rh) and is the dominant progression-forming mode. This band also forms combinations with bands attributable to the tetraacetate cage, such as /nu/(Rh-O) and /delta/(OCO), as well as with those of the axial PPh/sub 3/ ligands. Excitation profiles of /nu/(Rh-Rh) and several other bands are shown to maximize under the 376-nm electronic absorption band, which, by way of Raman band depolarization ratio measurements, is assigned to an axial (z) polarized transition terminating in /sigma/* (RhRh). Band assigned to /nu/(Rh-O) are shown to be dependent on both /sup 18/O and CD/sub 3/ substitution. For Rh/sub 2/(O/sub 2/CCH/sub 3/)/sub 4/(PPh/sub 3/)/sub 2/. Raman spectra obtained with 514.5-nm excitation are also discussed. FTIR spectra (3500-40 cm/sup /minus/1/) of Rh/sub 2/(O/sub 2/CCH/sub 3/)/sub 4/(PPh/sub 3/)/sub 2/ and its /sup 18/O and CD/sub 3/ isotopomers are also presented and assigned. The results provide a firm basis for making vibrational band assignments for other dimetal tetracarboxylates. 18 references, 4 figures, 5 tables.

Computer‐controlled system for the accurate measurement of depolarization ratios

AbstractA computer‐controlled switching device for a Pockel cell is described. Depolarization ratios of Raman bands are measured accurately as the intensities of alternate polarizations are recorded at a time separation equal to that for a single point measurement. Measurements on liquid chloroform agree to within experimental reproducibility with literature values.

A Technique to Measure Entrainment in Cloud by Dual-Polarization Radar and Chaff

A new method for studying the entrainment of air into clouds has been developed and tested, and initial results demonstrating the technique are presented here. This new method is superior to other chaff techniques that allow air parcels to be followed only until the chaff reflectivity becomes indistinguishable from cloud reflectivity. Frequently this is won after the chaff enters the cloud. The new technique uses circular depolarization ratio (CDR) measurements to avoid this limitation. Since the CDR of chaff is typically 20 dB greater than that of most hydrormeteors, the signature provided by CDR allows chaff to be followed into moderately reflective regions of the cloud long after the reflectivity signature of the chaff has disappeared. Circular polarization is superior to linear polarization in measurements such as these because the chaff signatures in circular depolarization do not require assumptions about chaff filament orientation, whereas interpretation of ZDR signatures (from linear polarization) does require such assumptions. In this paper, we discuss the technique, and present initial 8.6 mm wavelength radar measurements of chaff released near the base of a cumulus cloud. These measurements demonstrate that the chaff can be distinguished from its CDR signature well after its reflectivity has been lost in the reflectivity of the cloud. Possible applications as well as limitations of the technique are discussed. An extension of the technique that offers the potential for qualitatively observing electrical fields in cloud is also discussed.

ChemInform Abstract: Charge‐Transfer Organic Crystals: Molecular Vibrations and Spectroscopic Effects of Electron‐Molecular Vibration Coupling of the Strong Electron Acceptor TCNQF4.

AbstractAvibrational analysis of the title compound 7,7,8,8‐tetracyano‐ 2,3,5,6‐tetrafluoroquinodimethane and its monomer radical anion is reported, based on Raman depolarization ratio measurements and IR data.

Low Frequency Raman Spectra of Some Lactams

Abstract Low frequency Raman spectra from 20 to 200 cm−1 of δ-valerolactam and ε-caprolactam in CCl4 solution have been measured at different temperatures (25–80°C). Experimental data were transformed to the R(v) representation and the assignments of some bands are discussed based on measurements of depolarization ratio. The spectral features indicate that C2 symmetry dimers are predominant. Previous assignments of some bands are also revised.

Charge–transfer organic crystals: Molecular vibrations and spectroscopic effects of electron–molecular vibration coupling of the strong electron acceptor TCNQF4

A fundamental aspect in the study of the charge–transfer (CT) organic crystals with ionic or partially ionic ground states is the investigation of the spectroscopic effect of the electron–molecular vibration coupling. 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (TCNQF4), an electron acceptor much stronger than TCNQ, is an outstanding component of many interesting CT systems. A thorough vibrational analysis of the title compound and of its monomeric radical anion is reported. The analysis is based on the Raman depolarization ratio measurements and on infrared data of solutions of both neutral and ionic species as well as on polarized infrared spectra of oriented crystals of the neutral molecule. The vibrational assignment, completed by a normal coordinate analysis (NCA), brings to the identification of the ionization frequency shifts and to the choice of the fundamentals (b1uν19 and b2uν33) whose frequencies are diagnostic of the degree of charge transfer for a TCNQF4 moiety partner of a CT system. Through a combined use of a RHF-CNDO/S(CT) electronic calculation and of the eigenvectors of the radical anion given by the NCA, the numerical evaluation of the linear e–mv coupling constants is carried out. The relative values of these constants are extracted, by applying the dimer model, from the vibronic features of the infrared spectrum of the low temperature phase of Rb–TCNQF4. This compound is recognized, by comparison with the alkali salts of TCNQ, to display a phase transition at T≂130 K from a regular to a dimerized stack structure. From the analysis of all these data it is possible to characterize the spectroscopic vibronic effects related to e–mv coupling in CT systems of TCNQF4. This is successfully verified by the segregated dimerized stack system of DBTTF–TCNQF4 and applied to the not yet characterized system TSF–TCNQF4 which is recognized here as a fully ionic complex with segregated dimerized stack motif. As a consequence, through the vibrational spectra, TCNQF4 can be fruitfully used as a sensitive probe for structural and electronic characterization of its CT complexes.

Calculation of Error in Depolarization Ratio Measurement Due to Finite Collection Angle in Laser Raman Spectroscopy

We calculate the error introduced in the measurement of the depolarization ratio (ρ) of strongly polarized lines due to the finite angle of collection employed in a laser Raman spectrometer. The mathematical formulation takes explicit account of both the collection lens and the polarization analyzer, placed to select the polarization of the scattered beam. It is shown that for the commonly employed geometry where the analyzer is rotated by 90° to select the polarized and depolarized intensities, keeping the polarization of the incident beam normal to the mean scattering plane, the relative error (Δ ρ/ρ) does not exceed 8%, even for a collection cone of half angle as large as 25° for ρ as small as 0.01. The present work also shows that the results in the literature which suggest that the relative error in the depolarization ratio is independent of ρ are incorrect.

Circular depolarization ratio and Doppler velocity measurements with a 35‐GHz radar during the Cooperative Convective Precipitation Experiment

All aspects of atmospheric convection on the North American High Plains were studied during the Cooperative Convective Precipitation Experiment. This 3‐month experiment conducted near Miles City, Montana, during the summer of 1981 made use of 14 instrumented aircraft and a network of seven Doppler radars. One of these radars, the National Oceanic and Atmospheric Administration/Wave Propagation Laboratory K band (35 GHz) radar had dual‐polarization capability. The circular depolarization ratio (CDR) from the log receiver of this radar was measured in many different meteorological situations, including nonprecipitating clouds, convective storms with light precipitation, and severe hail storms. The analysis discussed here focuses on measurements near the melting layer where a sharp peak in the CDR profile was observed. A method for classifying particles on the basis of their fall speeds is related to measurements of CDR near this level for more than a 1‐hour period. Preliminary aircraft data are used as an aid in the interpretation.

Measurements of depolarization ratios of gaseous linear chain molecules: A test of the principle of the additivity of bond polarizabilities

The depolarization ratio for a number of n-alkanes and n-perfluoro-alkanes are reported, and a discrepancy for C2H??? with earlier results is explained. A simple model based upon the principle of the additivity of bond polarizabilities is shown to be in disagreement with our results. A model based on the classification of chemical bonds into types and subtypes, according to the chemical environment of these bonds is shown to be in considerably better agreement with our data, although the agreement is not completely adequate.

The infrared and Raman spectra of matrix-isolated MVO systems. The characterization of MVO 3 and MVO 2 molecules

Gaseous alkali vanadates MVO 3 and MVO 2 have been identified by means of IR matrix isolation spectroscopy and their spectra have been interpreted on the basis of C 2v ring structures. Raman spectra have been obtained for matrix-isolated CsVO 3 and CsVO 2. The depolarization ratio measurements of the Raman bands of cesium vanadates confirm the bidentate coordination of the VO 3 − and VO 2 − oxyanions.

Hydrogen bonding in liquid formamide. A low frequency Raman study

Low frequency Raman spectra from 20 to 400 cm−1 of liquid HCONH2, HCOND2, and HCO 15NH2 were studied. Experimental data were directly transformed to the R(v̄) representation, previously described. Assignments of low frequency bands are discussed based on a comparison between frequencies for the isotopic species and measurements of depolarization ratios. Chain structures are the dominant species. An out-of-plane intermolecular mode involving atoms in the hydrogen bond was assigned. The frequency for this mode decreased with increasing temperature.

Measurement of weak depolarization ratios of the Rayleigh scattering of gases using Raman spectroscopy

AbstractUsing a spectroscopic method suggested by Murphy, the depolarization ratios for the Rayleigh scattering of several gases have been measured, lying in the range of 10−2 to 5*10−5. The values obtained are generally lower than those found by previous authors who used the classical method, especially in the case of extremely small depolarization ratios. For hydrogen and deuterium sulphides, which present the smallest depolarization ratios ever observed, a difference is found between the isotopomers although this at the limit of the experimental error.

Raman scattering: Investigation of nematic and smectic ordering

Quantitative measurements of vibrational Raman depolarization ratios have been used to study molecular orientational order in nematic and for the first time, in smectic liquid crystals. Temperature dependence of the depolarization ratios of the terminal C≡N and the central biphenyl link vibrational bands of two alkylcyanobiphenyl compounds (CBn) and a cyclohexane derivative (PCH7) has been investigated extensively. Two experimental technics have been used: a conventional triple monochromator and the Raman microprobe to overcome the spurious depolarization due to the long wave director fluctuations. The orientational order parameters <P2≳ and <P4≳ were then calculated. The results have been discussed assuming repulsive forces are dominant for the CBn compounds and using the Maïer–Saupe theory in the case of PCH7.

Raman scattering in lysozyme solutions

Previous measurements on the OH stretching behavior in lysozyme solutions do not agree with each other. A careful measurement is performed using an isotopic substitutional technique in order to improve the precision. It is shown that, taking into account the contribution of the OH groups of lysozyme, elsewhere neglected, the interaction between lysozyme and water does not give any significant contribution to the Raman spectrum of the OH stretching. The conclusion is also supported by depolarization ratio measurements.

General theory of the Raman scattering close to a plane surface. Evanescent Raman spectra

We present a general theory of Raman scattering near the surface of a plane dielectric medium. All possible cases are considered but only the ones in which evanescent waves are present are discussed (evanescent excitation or/and evanescent scattering). In each case the intensity and angular distribution patterns of the scattered light are discussed. The problem of measurement of depolarization ratios of Raman bands is also considered.

Theoretical studies of the collision-induced Raman spectrum of carbon dioxide

Ab initio calculations have been performed on carbon dioxide to obtain the first derivatives of the dipole-quadrupole polarizability A and of the first hyperpolarizability β in the bending and asymmetric stretching modes. These derivatives are used to assess the relative contributions of A and β to the collision-induced Raman spectrum. It is concluded that most of the intensity arises from the mechanism involving the A tensor. Reasonable agreement is obtained with measured depolarization ratios and relative intensities for the v 2 and v 3 modes in compressed gaseous CO2.

Dilute solution properties of thermotropic polymers: aromatic polyazomethines

The dilute solution properties of some polyazomethines have been investigated. Poly(nitrilo-2-methyl-1,4-phenylene nitrilomethylidine-1,4-phenylene methylidine) (MePNPM) was studied. The measurements of intrinsic viscosities, molecular weight and depolarization ratio were made in 96% sulphuric acid and in methane sulphonic acid for these polymers. A side by side aggregation is observed with time in the second solvent. In order to obtain a well defined law of viscosity, fractionation by gel permeation chromatography was made. A small amount of aggregated particles was observed for the samples which have been submitted to a thermal treatment. Nevertheless, a well defined law of viscosity has been obtained. From the viscosity law and the variation of the apparent molecular anisotropy, the rigidity of the molecule has been evaluated; the persistence length q which characterizes this rigidity has been found to equal 150 A ̊ ± 25 A ̊ .

Measurements of depolarization ratios and polarizability anisotropies of gaseous molecules

Values for the depolarization ratio ( ϱ T V) of Rayleigh scattered light are reported for a large number of gas molecules. When possible, the results are combined with refractive index data to yield the anisotropy in the optical polarizability.

On the assignment of the NH2 stretching modes in the vibrational spectrum of formamide

Abstract Infrared and Raman spectra of liquid formamide and its deuterated analog were examined. Depolarization ratio measurements in the NH 2 stretching region indicated that the previous assignment of the NH 2 band was erroneous. The re-interpretation of the liquid formamide vibrational spectra is presented. The i.r. spectra of dilute formamide solutions in various organic bases gave additional support for the proposed assignment.