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Experimental and Theoretical Studies of the Environmental Sensitivity of the Absorption Spectra and Photochemistry of Nitenpyram and Analogs
Neonicotinoid (NN) pesticides have widespread use, largely replacing other pesticides such as the carbamates. Hence, there is a need to understand their environmental fates at a molecular level in various media, especially water. We report here the studies of a nitroenamine NN, nitenpyram (NPM), in aqueous solution where the absorption cross sections in the actinic region above 290 nm are observed to dramatically decrease compared to those in nonaqueous solvents. Quantum chemical calculations show that addition of a proton to the tertiary amine nitrogen in NPM breaks the conjugation in the chromophore, shifting the absorption to shorter wavelengths, consistent with experiment. However, surprisingly, adding a proton to the secondary amine nitrogen leads to its immediate transfer to the NO2 group, preserving the conjugation. This explains why the UV absorption of ranitidine (RAN), which has a similar chromophore but only secondary amine nitrogens, does not show a similar large blue shift in water. Photolysis quantum yields in aqueous NPM solutions were measured to be Ļ = 0.18 Ā± 0.07 at 254 nm, (9.4 Ā± 1.6) Ć 10-2 with broadband radiation centered at 313 nm and (5.2 Ā± 1.1) Ć 10-2 for broadband radiation centered at 350 nm (errors are 2Ļ). The major products in aqueous solutions are an imine that was also formed in the photolysis of the solid and a carboxylic acid derivative that is unique to the photolysis in water. Combining the larger quantum yields in water with the reduced absorption cross sections results in a calculated lifetime of NPM of only 5 min at a solar zenith angle of 35Ā°, typical of 40Ā°N latitude on April 1. The products do not absorb in the actinic region and hence will be long-lived with respect to photolysis
Visible and near infrared spectroscopy in soil science
This chapter provides a review on the state of soil visibleānear infrared (visāNIR) spectroscopy. Our intention is for the review to serve as a source of up-to date information on the past and current role of visāNIR spectroscopy in soil science. It should also provide critical discussion on issues surrounding the use of visāNIR for soil analysis and on future directions. To this end, we describe the fundamentals of visible and infrared diffuse reflectance spectroscopy and spectroscopic multivariate calibrations. A review of the past and current role of visāNIR spectroscopy in soil analysis is provided, focusing on important soil attributes such as soil organic matter (SOM), minerals, texture, nutrients, water, pH, and heavy metals. We then discuss the performance and generalization capacity of visāNIR calibrations, with particular attention on sample pre-tratments, co-variations in data sets, and mathematical data preprocessing. Field analyses and strategies for the practical use of visāNIR are considered. We conclude that the technique is useful to measure soil water and mineral composition and to derive robust calibrations for SOM and clay content. Many studies show that we also can predict properties such as pH and nutrients, although their robustness may be questioned. For future work we recommend that research should focus on: (i) moving forward with more theoretical calibrations, (ii) better understanding of the complexity of soil and the physical basis for soil reflection, and (iii) applications and the use of spectra for soil mapping and monitoring, and for making inferences about soils quality, fertility and function. To do this, research in soil spectroscopy needs to be more collaborative and strategic. The development of the Global Soil Spectral Library might be a step in the right direction
Genome-inspired molecular identification in organic matter via Raman spectroscopy
Rapid, non-destructive characterization of molecular level chemistry for
organic matter (OM) is experimentally challenging. Raman spectroscopy is one of
the most widely used techniques for non-destructive chemical characterization,
although it currently does not provide detailed identification of molecular
components in OM, due to the combination of diffraction-limited spatial
resolution and poor applicability of peak-fitting algorithms. Here, we develop
a genome-inspired collective molecular structure fingerprinting approach, which
utilizes ab initio calculations and data mining techniques to extract molecular
level chemistry from the Raman spectra of OM. We illustrate the power of such
an approach by identifying representative molecular fingerprints in OM, for
which the molecular chemistry is to date inaccessible using non-destructive
characterization techniques. Chemical properties such as aromatic cluster size
distribution and H/C ratio can now be quantified directly using the identified
molecular fingerprints. Our approach will enable non-destructive identification
of chemical signatures with their correlation to the preservation of
biosignatures in OM, accurate detection and quantification of environmental
contamination, as well as objective assessment of OM with respect to their
chemical contents
van der Waals interaction in nanotube bundles : consequences on vibrational modes
We have developed a pair-potential approach for the evaluation of van der
Waals interaction between carbon nanotubes in bundles.
Starting from a continuum model, we show that the intertube modes range from
to . Using a non-orthogonal tight-binding approximation
for describing the covalent intra-tube bonding in addition, we confirme a
slight chiral dependance of the breathing mode frequency and we found that this
breathing mode frequency increase by 10 % if the nanotube lie inside a
bundle as compared to the isolated tube.Comment: 5 pages, 2 figure
Electronic excitation of transition metal nitrides by light ions with keV energies
We investigated the specific electronic energy deposition by protons and He
ions with keV energies in different transition metal nitrides of technological
interest. Data were obtained from two different time-of-flight ion scattering
setups and show excellent agreement. For protons interacting with light
nitrides, i.e. TiN, VN and CrN, very similar stopping cross sections per atom
were found, which coincide with literature data of N2 gas for primary energies
<= 25 keV. In case of the chemically rather similar nitrides with metal
constituents from the 5th and 6th period, i.e. ZrN and HfN, the electronic
stopping cross sections were measured to exceed what has been observed for
molecular N2 gas. For He ions, electronic energy loss in all nitrides was found
to be significantly higher compared to the equivalent data of N2 gas.
Additionally, deviations from velocity proportionality of the observed specific
electronic energy loss are observed. A comparison with predictions from density
functional theory for protons and He ions yields a high apparent efficiency of
electronic excitations of the target for the latter projectile. These findings
are considered to indicate the contributions of additional mechanisms besides
electron hole pair excitations, such as electron capture and loss processes of
the projectile or promotion of target electrons in atomic collisions
Photophysics and Inverted Solvatochromism of 7,7,8,8-Tetracyanoquinodimethane (TCNQ)
We report the absorption, fluorescence, and Raman spectroscopy of 7,7,8,8- tetracyanoquinodimethane (TCNQ) in a variety of solvents. The fluorescence quantum yields (QYs) of linear alkane solutions are similar to one another, but QY is shown to acutely decrease in other solvents with increasing polarities. The slope of the solvatochromic plot of absorption maxima is inverted from negative to positive with an increase in solvent polarity. A significant change in the frequency of carbon-carbon double bond stretching modes is not observed in Raman spectra of TCNQ in different solvents. The molar absorption coefficient is determined to calculate the oscillator strength of the absorption band. The radiative decay rate constant calculated from the oscillator strength is approximately ten times larger than that elucidated from the fluorescence lifetime and QY. These spectroscopic parameters reveal that the relaxation occurs from a Franck-Condon excited state to a distinct fluorescence emissive state with a smaller transition dipole moment
Wide-range optical studies on various single-walled carbon nanotubes: the origin of the low-energy gap
We present wide-range (3 meV - 6 eV) optical studies on freestanding
transparent carbon nanotube films, made from nanotubes with different diameter
distributions. In the far-infrared region, we found a low-energy gap in all
samples investigated. By a detailed analysis we determined the average
diameters of both the semiconducting and metallic species from the near
infrared/visible features of the spectra. Having thus established the
dependence of the gap value on the mean diameter, we find that the frequency of
the low energy gap is increasing with increasing curvature. Our results
strongly support the explanation of the low-frequency feature as arising from a
curvature-induced gap instead of effective medium effects. Comparing our
results with other theoretical and experimental low-energy gap values, we find
that optical measurements yield a systematically lower gap than tunneling
spectroscopy and DFT calculations, the difference increasing with decreasing
diameter. This difference can be assigned to electron-hole interactions.Comment: 9 pages, 8 figures, to be published in Physical Review B,
supplemental material attached v2: Figures 1, 7 and 8 replaced, minor changes
to text; v3: Figures 3, 4 and 5 replaced, minor changes to tex
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