Llew Rintoul

Lattice vibrations of montmorillonite: an FT Raman and X-ray diffraction study

Abstract The lattice vibrations of a series of dioctahedral montmorillonitic clay minerals were studied using FT Raman spectroscopy. The lattice vibrations are assigned in terms of the molecular vibrations of (a) the distorted octahedron MO6 with symmetry S6 in the region 50–200 cm−1, (b) the OHO isosceles triangle of C2v symmetry in the 200 to 270 cm−1 and (c) distorted SiO4 tetrahedra of symmetry C3v in the region 300 to 1200 cm−1. The application of FT Raman spectroscopy to the analysis of montmorillonitic clay structure is elucidated. A series of montmorillonites with different weathering profiles from the Cressfield deposit in Australia were analysed by both FT Raman and X-ray diffraction. The FT Raman spectra of the Cressfield bentonites are shown to be a function of clay crystallinity. X-ray diffraction identified the changes in the montmorillonitic lattice as weathering occurs. The variation in the 001 d spacing changed from 17.53 to 17.36 A and the CDS value increased from 126 to 148 A, indicating an increase in both the order and crystallinity of the montmorillonite on ageing. FT Raman spectroscopy identified changes in the lattice vibrational modes as this weathering is taking place.

Single crystal Raman microscopic study of the asbestos mineral chrysotile

Polarisation Raman microscopy is used to study tubular chrysotile. The OH-stretching region is characterised by the inner surface OH bands at 3695, 3686 and 3678 cm-1 and the inner OH band at 3643 cm-1. The outer OH pointing away from the Mg-layer gives rise to two overlapping bands at 3695 and 3686 cm-1 due to a positional disorder caused by the folding of the layers. These bands are the in-phase vibrations whereas the bands at 3678 and 3643 cm-1 represent the out-of-phase vibrations. The 1102 cm-1 band is an antisymmetric stretching mode of Si–O perpendicular to the sheet. From the 692 and 705 cm-1 bands the first one is assigned to the symmetric Si–O–Si stretch and the second to an outer symmetric translation mode of the Mg–OH oriented sub-parallel to the a-axis. The 709 cm-1 band is assigned to the second outer symmetric translation mode of the Mg–OH oriented at a small angle to the b-axis and c-axis. The 629 and 622 cm-1 bands represent antisymmetric OH–Mg–OH translation modes. The band around 607 cm-1 is described as the symmetric libration mode of the inner Mg–OH group. The 458 cm-1 band is assigned as the v3(a1) mode of SiO4. The 466 cm-1 band is probably an OH translational vibration. A strong band at 388 cm-1 is ascribed to the antisymmetric v5(e) mode of the SiO4 tetrahedron. The band at 432 cm-1 is assigned as an antisymmetric Mg–OH translation mode. In the region between 450 and 200 cm-1 five bands can be observed at 374, 345, 318, 304 and 231 cm-1. The 374, 318 and 304 cm-1 bands are antisymmetric modes, whereas the 345 and 231 cm-1 bands are symmetric modes. The band at 374 cm-1 is associated with a symmetric Mg–OH vibration. At present a more detailed assignment of the other bands is not possible. The band at 199 cm-1 is assigned to the A1g mode of a Mg(O,OH)6 octahedron distorted in the direction normal to the octahedral sheet.

Halogen bonding between an isoindoline nitroxide and 1,4-diiodotetrafluorobenzene: new tools and tectons for self-assembling organic spin systems.

Radical assembly: Halogen bonding has been observed for the first time between an isoindoline nitroxide and an iodoperfluorocarbon (see figure), which cocrystallize to form a discrete 2:1 supramolecular compound in which N--O(.)I halogen bonding is the dominant intermolecular interaction. This illustrates the potential use of halogen bonding and isoindoline nitroxide tectons for the assembly of organic spin systems.The isoindoline nitroxide 1,1,3,3-tetramethylisoindolin-2-yloxyl (TMIO) and 1,4-diiodotetrafluorobenzene readily form a discrete 2:1 complex that shows evidence of relatively strong N--O(.)I halogen bonding. This interaction was characterized in the solid state by single-crystal X-ray analysis, thermal analysis, and vibrational spectroscopy (IR and Raman), backed by density functional theory calculations. EPR spectroscopy performed on a solution of TMIO in pentafluoroiodobenzene, a halogen-bonding donor, indicates that halogen bonding induces an increase in electron density at the nitroxide nitrogen nucleus and an increase in the nitroxide rotational correlation time. Our findings demonstrate the potential of utilizing halogen-bonding interactions to promote the self-assembly of new isoindoline nitroxide tectons for the preparation of organic spin systems.

Change of mechanical and electrical properties of polypyrrole films with dopant concentration and oxidative aging

The process of loss of mechanical and electrical properties of polypyrrole (Ppy) was studied for up to 12 months of aging in air at room temperature. Tensile strength of films highly doped with p-toluene sulphonate was lower than that of lightly doped ones. This difference is attributed to the presence of stress concentrations on the nodular surfaces of the films with high dopant concentrations. Highly doped films were electrically more stable, with a lower rate of loss of conductivity with time, compared with lightly doped ones. The conductivity of the latter initially decreased at a different rate than that observed after a prolonged time, exhibiting two regions in the aging behavior. In contrast, highly doped PPy films followed first-order kinetics with a very small loss of conductivity over the aging period. The rate of decline of conductivity was dependent on aging time, dopant type, and concentration, suggesting a complex mechanism of conductivity decay. Examination of FT-IR spectra of aged Ppy films revealed an increase in intensity of an α, β-unsaturated conjugated carbonyl peak, which may be correlated with the loss in conductivity.

Modelling of post-irradiation events in polymer gel dosimeters

The nuclear magnetic resonance (NMR) spin-spin relaxation time (T2) is related to the radiation-dependent concentration of polymer formed in polymer gel dosimeters manufactured from monomers in an aqueous gelatin matrix. Changes in T2 with time post-irradiation have been reported in the literature but their nature is not fully understood. We investigated those changes with time after irradiation using FT-Raman spectroscopy and the precise determination of T2 at high magnetic field in a polymer gel dosimeter. A model of fast exchange of magnetization taking into account ongoing gelation and strengthening of the gelatin matrix as well as the polymerization of the monomers with time is presented. Published data on the changes of T2 in gelatin gels as a function of post-manufacture time are used and fitted closely by the model presented. The same set of parameters characterizing the variations of T2 in gelatin gels and the increasing concentration of polymer determined from FT-Raman spectroscopy are used successfully in the modelling of irradiated polymer gel dosimeters. Minimal variations in T2 in an irradiated PAG dosimeter are observed after 13 h.

Raman spectroscopic characteristics of phthalocyanine and naphthalocyanine in sandwich-type (na)phthalocyaninato and porphyrinato rare earth complexes

Abstract The Raman spectroscopic data for a series of 50 phthalocyaninato and/or porphyrinato rare earth complexes including monomeric Tb[T(4-Cl)PP]acac, and sandwich-type RE(Pc′)2 [RE=Ce, Eu, Gd, Tb, Y; Pc′=Pc, Pc(OC5H11)8, Pc(OC8H17)8, Pc(C7H15)8], RE(Nc*)2 [RE=La, Ce, Pr, Nd, Eu, Gd, Tb, Er, Y; Nc*=Nc(tBu)4, Nc(SC12H25)8], (Pc)RE(Pc*)RE(Pc*) [RE=Eu, Tb, Er; Pc*=Pc(OC5H11)8], (Pc)Eu(Pc*)Er(Pc*) [Pc*=Pc(OC8H17)8], Eu(Pc)(Pc*) [Pc*=Pc(OC5H11)8, Pc(C7H15)8], Eu(Por)(Pc′) [Por=TPP, TPyP, T(4-Cl)PP, T(4-OMe)PP, T(4-tBu)PP; Pc′=Pc, Pc(C7H15)8], Eu(Por)(Nc) [Por=TPyP, T(4-Cl)PP], HEu(Por)(Pc), Ce(TPyP)(Pc), RE2(Por)2(Pc) [RE=Ce, Eu; Por=TPP, TPyP, T(4-Cl)PP, T(4-tBu)PP], and RE2(Por)(Pc′)2 [RE=Eu, Ce, Y; Por=TPP, TPyP, T(4-Cl)PP, T(4-OMe)PP, T(4-tBu)PP, OEP; Pc′=Pc, Pc(OC8H17)8, Pc(C7H15)8], and (TPyP)RE(Pc*)RE′(Pc*) [RE≠RE′=Eu, Y; Pc*=Pc(C7H15)8] have been collected in the solid state using a Raman microprobe and excitation at 632.8 nm (and in some cases, 780 nm). The Raman characteristics for unsubstituted and substituted (na)phthalocyaninato mono-radical anions Pc′ − (Pc′=Pc, Pc*), Nc* − and dianions Pc′2− (Pc′=Pc, Pc*) are thus comparatively described. The intense band at ca. 1500 cm−1 was confirmed to be the marker band for Pc′ − and Pc′2− anions.

Raman, infrared and XPS study of bamboo phytoliths after chemical digestion

Raman, infrared and X-ray photoelectron spectroscopies have been used to examine the effect of various chemical digestion methods on the composition of bamboo phytoliths. Intact bilobate phytoliths, suitable for interrogation by Raman microprobe analysis, were isolated by a microwave wet ashing technique using hydrogen peroxide with nitric and hydrochloric acids. The occluded phytolith carbon presented evidence of cellulose, lignin and carboxylic acids. Nitrate from the nitric acid used in the digestion was observed in homogenized samples of the isolated phytoliths; in addition to nitrogen of plant origin occluded within the phytolith, which was observed as amine nitrogen and ammonia. Intact bilobate phytoliths were not observed following an exothermic hydrogen peroxide/sulfuric acid digest, suggesting that these structures ruptured during this digestion procedure. The silicate network was significantly altered during isolation using the exothermic hydrogen peroxide/sulfuric digest, with surface hydroxyls undergoing condensation to form a SiO(3) ring structure.

Cryptic meteoric diagenesis in freshwater bivalves: Implications for radiocarbon dating

Shells of freshwater bivalves are commonly used for radiocarbon dating late Pleistocene archaeological and vertebrate fossil sites, thus providing important constraints on late Pleistocene human dispersal and megafauna extinction hypotheses. The reliability of bivalve shells for dating rests partly on the ease with which subsequent diagenetic alteration can be recognized; typically, wherein original shell aragonite is replaced by calcite in meteoric environments. Here we document late Pleistocene freshwater bivalve shells wherein meteoric diagenesis involved syntaxial overgrowth of aragonite cement on original aragonite shell biocrystals. Aragonite cement was identified in situ using Raman microspectroscopy and formed rather than calcite as a result of unusually high Mg:Ca ratios in local groundwaters. Thus, altered shells contain diagenetic 14C, rendering their dates unreliable, but they may slip past common vetting techniques because (1) epitaxial cements are not readily apparent petrographically because they are in optical continuity with adjacent biocrystals; (2) X-ray diffraction indicates that no calcite is present; (3) alteration is not apparent in cathodoluminescence studies; and (4) stable isotopes of C and O are difficult to interpret in shells that originate in terrestrial-meteoric environments. Hence, although freshwater with a high Mg:Ca ratio is not common, groundwater chemistry should be considered before accepting bivalve-based radiocarbon dates uncritically. More broadly, meteoric diagenesis in carbonate rocks is generally characterized by the dissolution of aragonite or its conversion to calcite. Our data show that such is not invariably the case, even in fully terrestrial, freshwater systems.

Single crystal raman spectroscopy of cerussite

Abstract Raman and infrared active modes of cerussite were assigned according to their symmetry species and compared to other aragonite group minerals. Small satellite bands at 823 and 1031 cm-1 on the low-wavenumber side of the fundamental vibrations ν2 and ν1, respectively, have been assigned to the isotopic substitutions of 13C and 18O. The Raman active ν1 and ν2 carbonate modes are observed at 1051 and 835 cm-1. The absence of the B2g component of the ν1 and ν2 vibrations has been explained by the small coupling between the Ag and B2g modes. The Raman active ν3 carbonate anti-symmetric stretching mode is observed at 1361(Ag), 1376 (B1g), 1419 (B3g), and 1477 (B2g) cm-1, while the corresponding infrared active bands are observed at 1396, 1432, and 1456 cm-1. The Raman active ν4 carbonate bending mode is observed at 673 (Ag), 668 (B2g), 681 (B1g), and 694 (B2g) cm-1. The corresponding infrared bands are observed at 670, 679, and 698 cm-1. In both ν3 and ν4 the factor group splitting between the B1g and B3g modes is 1 to 3 times smaller than the separation of the Ag and B2g modes. Raman active lattice vibrations are detected at 120 (B3g), 132 (Ag), 148 (B1g), 152(B2g), 174 (B2g), 179 (B1g), 213 (Ag), 226 (B3g), and 243 cm-1 (B2g). Corresponding infrared active bands are detected at 573, 543, 573, 423, 375, 290, 205, 165, 146, and 134 cm-1. Raman bands at 949, 966, 989, 1000, and 1104 cm-1 and at 922, 946, 967, 988, 996, and 1007 cm-1 in the infrared spectra are assigned to combination and overtone bands. Raman bands at 1676 (Ag), 1689 (Ag), 1730 (B3g), and 1740 (B1g) cm-1 are ascribed to combination modes of ν1 + ν4 with bands at 2052 and 2092 cm-1 assigned to 2ν1. Corresponding infrared bands are observed at 1729 and 1740 cm-1 (ν1 + ν3). Bands at 2359, 2409, 2471, and 2521 cm-1 are ascribed to ν1 + ν3, with broad bands at 1246 and 1323 cm-1 assigned to 2ν4 modes.

Radiation Dose Distribution in Polymer Gels by Raman Spectroscopy

The Raman spectroscopy of polymer gel dosimeters has been investigated with a view to developing a novel dosimetry technique that is capable of determining radiation dose within a micrometer of spatial resolution. The polymer gel dosimeter, known as the PAG dosimeter, is typically made up of acrylamide, N,N′-methylene-bis-acrylamide, gelatin, and water. A polyacrylamide network within the gelatin matrix forms in response to an absorbed dose. The loss of monomers may be monitored by corresponding changes to the Raman spectrum. Principal component analysis offers a simple method of quantifying the absorbed radiation dose from the Raman spectrum of the polymer gel. The background luminescence in the spectrum increased significantly with dose and is shown to originate in the glass of the sample vial. The competing effects of elastic scatter, which increases with dose due to the formation of polymer, and sample absorption were quantified and found to introduce errors of up to 5% under certain conditions. Raman spectra as a function of distance from the air–surface interface have been measured for samples that were subjected to doses delivered by a clinical linear accelerator. The depth dose profile thus obtained compared favorably with “gold standard” ion-chamber measurements.