Physical Review B

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Over the past 5 years, there has been a significant interest in employing terahertz (THz) technology, spectroscopy and imaging for security applications. There are three prime motivations for this interest: (a) THz radiation can detect concealed weapons since many non-metallic, non-polar materials are transparent to THz radiation; (b) target compounds such as explosives and illicit drugs have characteristic THz spectra that can be used to identify these compounds and (c) THz radiation poses no health risk for scanning of people. In this paper, stand-off interferometric imaging and sensing for the detection of explosives, weapons and drugs is emphasized. Future prospects of THz technology are discussed.

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THz imaging and sensing for security applications—explosives, weapons and drugs

Water is of fundamental importance for human life and plays an important role in many biological and chemical systems. Although water is the most abundant compound on earth, it is definitely not a simple liquid. It possesses strongly polar hydrogen bonds which are responsible for a striking set of anomalous physical and chemical properties. For more than a century the combined importance and peculiarity of water inspired scientists to construct conceptual models, which in themselves reproduce the observed behavior of the liquid. The exploration of structural and binding properties of small water complexes provides a key for understanding bulk water in its liquid and solid phase and for understanding solvation phenomena. Modern ab initio quantum chemistry methods and high-resolution spectroscopy methods have been extremely successful in describing such structures. Cluster models for liquid water try to mimic the transition from these clusters to bulk water. The important question is: What cluster properties are required to describe liquid-phase behavior?

Water: From Clusters to the Bulk

Atmospheric aerosol particles of biological origin are a very diverse group of biological materials and structures, including microorganisms, dispersal units, fragments and excretions of biological organisms. In recent years, the impact of biological aerosol particles on atmospheric processes has been studied with increasing intensity, and a wealth of new information and insights has been gained. This review outlines the current knowledge on major categories of primary biological aerosol particles (PBAP): bacteria and archaea, fungal spores and fragments, pollen, viruses, algae and cyanobacteria, biological crusts and lichens and others like plant or animal fragments and detritus. We give an overview of sampling methods and physical, chemical and biological techniques for PBAP analysis (cultivation, microscopy, DNA/RNA analysis, chemical tracers, optical and mass spectrometry, etc.). Moreover, we address and summarise the current understanding and open questions concerning the influence of PBAP on the atmosphere and climate, i.e. their optical properties and their ability to act as ice nuclei (IN) or cloud condensation nuclei (CCN). We suggest that the following research activities should be pursued in future studies of atmospheric biological aerosol particles: (1) develop efficient and reliable analytical techniques for the identification and quantification of PBAP; (2) apply advanced and standardised techniques to determine the abundance and diversity of PBAP and their seasonal variation at regional and global scales (atmospheric biogeography); (3) determine the emission rates, optical properties, IN and CCN activity of PBAP in field measurements and laboratory experiments; (4) use field and laboratory data to constrain numerical models of atmospheric transport, transformation and climate effects of PBAP. Keywords: primary biological atmospheric aerosol; climate; cloud condensation nuclei; biology; atmospheric ice nuclei (Published: 22 February 2012) Citation: Tellus B 2012, 64 , 15598, DOI: 10.3402/tellusb.v64i0.15598

https://www.tandfonline.com/doi/pdf/10.3402/tellusb.v64i0.15598

Primary biological aerosol particles in the atmosphere: a review

The spectral angle mapper (SAM) has been widely used in multispectral and hyperspectral image analysis to measure spectral simi- larity between substance signatures for material identification. It has been shown that the SAM is essentially the Euclidean distance when the spectral angle is small. Most recently, a stochastic measure, called the spectral information divergence (SID), has been suggested to model the spectrum of a hyperspectral image pixel as a probability distribution, so that spectral variations among spectral bands can be captured more effectively in a stochastic manner. This paper develops a new hyper- spectral spectral discrimination measure, which combines the SID and the SAM into a mixed measure. More specifically, letr and r8 denote two hyperspectral image pixel vectors with their corresponding spectra speci- fied bys and s8. Then SAM(s,s8) measures the spectral angle between s and s8. Similarly, SID(s,s8) measures the information divergence be- tween the probability distributions generated by s and s8. The proposed new measure, referred to as the SID-SAM mixed measure, can be imple- mented in two versions, given by SID(s,s8)3tan(SAM(s,s8)) and SID(s,s8)3sin(SAM(s,s8)), where tan and sin are the usual trigonomet- ric functions. The spectral discriminability of such a mixed measure is greatly enhanced by multiplying the spectral abilities of the two mea- sures. In order to demonstrate its utility, a comparative study is con- ducted among the SID-SAM mixed measure, the SID, and the SAM. Our experimental results have shown that the discriminatory ability of the (SID,SAM) mixed measure can be a significant improvement over the SID and SAM. © 2004 Society of Photo-Optical Instrumentation Engineers.

/pdf/new-hyperspectral-discrimination-measure-for-spectral-22cmu66z6t.pdf

New Hyperspectral Discrimination Measure for Spectral Characterization

The pure rotational spectrum of ethyl sulfide has been measured from 12 to 21 GHz in a 1 K jet-cooled expansion using a Fourier-transform microwave (FTMW) spectrometer. Prominent features in the spectrum are assigned to transitions from three conformational isomers. Additional assignments of the 13C and 34S isotopomer spectra of these conformers effectively account for all of the remaining transitions in the spectrum. Accurate “heavy-atom” substitution structures are obtained via a Kraitchman analysis of 14 rotational parameter sets, permitting definitive identification of the molecular structures of the three conformers. Two of the structures designated as the gauche–gauche (GG) and trans–trans (TT) conformers have symmetric forms with C2 and C2v symmetries, respectively, and the third trans–gauche (TG) configuration is asymmetric. The components of the electric dipole moment along the principal inertial axes have been determined from Stark measurements and are consistent with these structural assignment...

The conformational structures and dipole moments of ethyl sulfide in the gas phase

We report computations of the rotational barrier of the methyl group in toluene. The computations are based on the use of the gaussian 92 Program Package with the 6-311G∗∗ basis set. The results are 2.69 wavenumbers at the HF level of approximation and 5.02 wavenumbers at the MP2 level. These results compare favorably with the experimental value of 4.9 wavenumbers. It is shown that inclusion of the vibrational zero-point energy in the calculations is necessary for obtaining satisfactory results.

Theoretical studies of the methyl rotational barrier in toluene

A detailed investigation of phonon modes in DNA macromolecules is presented This work presents experimental evidence to confirm the presence of multiple dielectric resonances in the submillimeter-wave spectra (ie, \ensuremath{\sim}001\char21{}10 THz) obtained from DNA samples These long-wave (ie, \ensuremath{\sim}1\char21{}30 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) absorption features are shown to be intrinsic properties of the particular DNA sequence under study Most importantly, a direct comparison of spectra between different DNA samples reveals a large number of modes and a reasonable level of sequence-specific uniqueness This work establishes the initial foundation for the future use of submillimeter-wave spectroscopy in the identification and characterization of DNA macromolecules

Submillimeter-wave phonon modes in DNA macromolecules.

We present computations of the structure and vibrational frequencies of perylene at the Hartree–Fock and B3LYP (DFT) levels of theory using the standard 6-31G* basis set. A detailed analysis of the normal vibrational modes of perylene along with assignments of both symmetry and molecular motion are also presented. Empirical correction factors for all computational data are calculated and presented. The large size and high symmetry of the perylene make it an interesting candidate for a detailed study of vibrational normal modes of the molecule.

James O. Jensen

Papers

New Hyperspectral Discrimination Measure for Spectral Characterization

The conformational structures and dipole moments of ethyl sulfide in the gas phase

Theoretical studies of the methyl rotational barrier in toluene

Submillimeter-wave phonon modes in DNA macromolecules.

Theoretical studies of the infrared and Raman spectra of perylene