Simulated IR Research Articles

Two-dimensional heterodyned and stimulated infrared photon echoes of N-methylacetamide-D

The stimulated infrared photon echo of N-methylacetamide-D [NMAD; CH3(CO)ND(CH3)] was measured and used to determine the vibrational frequency correlation function. The correlation function was modeled as a single exponential plus a constant, and it was found that most of the NMAD vibrational frequency distribution is motionally narrowed with a pure dephasing time of 1.12 ps. The two-dimensional infrared (2D IR) spectrum of NMAD was also obtained by heterodyning the echo field with a weak local oscillator pulse. The real and imaginary portions of the 2D IR spectrum exhibit multiple peaks due to υ=0–1 and 1–2 coherences that are excited, which are not resolved in the absolute magnitude of the 2D IR spectrum. Using the correlation function determined from the stimulated photon echo, the 2D IR spectrum was accurately simulated. Resolution enhancement of the 2D IR spectrum was performed by manipulating the photon echo field with window functions. The enhanced experimental and simulated 2D IR spectra are dramatically narrowed.

PM3, AM1, MNDO and MINDO3 semi-empirical IR spectra simulations for compounds of interest for Titan's chemistry: diazomethane, methyl azide, methyl isocyanide, diacetylene and triacetylene

Four semi-empirical methods (PM3, AM1, MNDO and MINDO3) have been tested to find the best auxiliary tool for the gas chromatography/Fourier transform IR spectroscopy/mass spectrometry (GC/FTIR/MS) identification of five compounds of interest for Titan's atmospheric chemistry as test compounds: diacetylene, triacetylene, diazomethane, methyl azide, methyl isocyanide. Of the four methods, MINDO3 can be considered as the most appropriate method to facilitate the identification of such and similar compounds, since (1) the simulated IR spectra best match the experimental spectra for four compounds of five studied; and (2) MINDO3 provides the best linearity between the calculated and experimental frequencies (correlation coefficient of 0.995; a scaling factor of 0.84 can be applied to afford better correspondence between the calculated and experimental wavenumbers). None of the semi-empirical methods tested is able to predict (even approximately) infrared band intensities, and therefore a spectral intensity pattern.

Ab initio molecular dynamics simulation of hydration and ion-exchange processes in low Al-zeolites

First-principles molecular dynamics room-temperature simulations combined with full relaxation of atomic positions of selected structures are performed on gmelinite, a zeolite composed of hexagonal prisms like technologically important faujasites and mordenite. In low-aluminum gmelinite (one Al per cell) the location and the dynamics of the counter-ion are investigated as a function of the degree of hydration. The time evolution of the ion-exchange is simulated for the reaction Na−zeo+NH 4Cl⇌NH 4−zeo+NaCl. The simulated IR bands of both the Na-zeolite and the ion-exchanged zeolite are consistent with the literature data. The calculated energetics shows that protonated structures are disfavored by ∼200 kJ mol −1 compared with Na zeolites and the NH + 4 form is by −8.5 kJ mol −1 more stable than the Na form. The adsorption energies calculated for the products of the ion-exchange reaction are higher than the energy of hydration of the Na zeolite in good agreement with experimental data.

Conformational properties of thiophene oligomers

AbstractThe molecular geometries and the torsional potentials about the inter‐ring C‐C bond in α‐oligothiophenes (α‐nTh, n=2–4) have been calculated by means of conventional ab initio and density functional theory (DFT) calculations employing the hybrid B3LYP and BH&HLYP functionals. The position and the energetics of the critical points in the potential energy curve generated by rotation about the inter‐ring CC bond are shown to be dependent on the computational method. DFT calculations, in comparison with MP2 calculations, favour conjugative interactions, while steric and coulombic interactions are equally treated by both methods. On oligomerization the electron delocalisation increases slightly, the p‐charge being preferentially confined within the rings, although it is sufficient to move the molecular structure towards co‐planarization and to increase the barrier through the perpendicular conformation. The IR and Raman spectra on the relevant rotamers of α‐2Th have been computed at HF/6–31G* and B3LYP/6–31G* levels. The comparison with the experiment is excellent. It has been found that small twisting from the planar conformation has no apparent effects, while 90° twisting and isomerization to the syn‐gauche form produce significant frequency and intensity variations which could be useful probes in conformational studies. The simulated IR and Raman spectra of the α‐2Th rotamers are consistent with a smallπ‐electron delocalisation between the rings.

Synthetic routes to mono- and bi-copper hydrazone complexes uncovered by computer simulation

The chelating properties of two polyfunctional di-2-pyridylketone hydrazone ligands toward the copper ion have been investigated in mono- and bimetallic complex species. The complexation reaction is strongly influenced by the nature of the inorganic salt reagent. Structural characterization of the mono- and bimetallic copper species by powder X-ray diffraction analysis and computer simulation made an important contribution to the discovery of the synthesis conditions. Simulated IR and 1H NMR spectra were also calculated.

Resolution Enhancement of Composite Spectra with Fractal Noise in Derivative Spectrometry

The problem of improving the resolution of composite spectra with statistically self-similar (fractal) noise is considered within the framework of derivative spectrometry. An algorithm of the numerical differentiation of an arbitrary (including fractional) order of spectra is produced by the statistical regularization method taking into account a priori information on statistical properties of the fractal noise. Fractal noise is analyzed in terms of the statistical Hurst method. The efficiency and expedience of this algorithm are exemplified by treating simulated and experimental IR spectra.

PM3, AM1, MINDO3 semi-empirical IR spectra simulations for some nitriles of interest for Titan’s chemistry

A set of the semi-empirical methods (PM3. AM1, MNDO and MINDO3) has been tested to find the best auxiliary tool for the identification of nitriles by gas chromatography/Fourier transform IR spectroscopy/mass spectrometry, considering five nitriles of interest for Titan’s chemistry as test compounds: acetonitrile, acrylonitrile, cyanoacetylene, 2-butynenitrile and dicyanoacetylene. Of the four semi-empirical methods, MNDO can be considered as the most advantageous auxiliary tool for the gas chromatography/Fourier transform IR spectroscopy/mass spectrometry (GC/FTIR/MS) identification of nitriles of interest for Titan’s atmospheric chemistry, since (1) the simulated IR spectra best match the experimental (in some cases AM1 gives comparable results); (2) it provides the best linearity between the calculated and experimental frequencies (correlation coefficient of 0.990); a scaling factor of 0.90 can be applied to afford better correspondence between the calculated and experimental wavenumbers. At the same time, none of the methods is able to predict infrared intensities and a spectral intensity pattern.

Simulations of oligopeptide vibrational CD: effects of isotopic labeling.

Simulated ir absorption and vibrational CD (VCD) spectra of four alanine-based octapeptides, each having its main chain constrained to a different secondary structure conformation, were analyzed and compared with experimental results for several different peptides. The octapeptide simulations were based on transfer of property tensors from a series of ab initio calculations for a short L-alanine based segment containing 3 peptide bonds with relative straight phi, psi angles fixed to those appropriate for alpha-helix, 3(10)-helix, ProII-like helix, and beta-sheet-like strand. The tripeptide force field (FF) and atomic polar tensors were obtained with density functional theory techniques at the BPW91/6-31G** level and the atomic axial tensor at the mixed BPW91/6-31G**/HF/6-31G level. Allowing for frequency correction due to the FF limitations, the octapeptide results obtained are qualitatively consistent with experimental observations for ir and VCD spectra of polypeptides and oligopeptides in established conformations. In all cases, the correct VCD sign patterns for the amide I and II bands were predicted, but the intensities did have some variation from the experimental patterns. Predicted VCD changes upon deuteration of either the peptide or side-chains as well as for (13)C isotopic labeling of the amide C=O at specific sites in the peptide chain were computed for analysis of experimental observations. A combination of theoretical modeling with experimental data for labeled compounds leads both to enhanced resolution of component transitions and added conformational applicability of the VCD spectra.

Kalman-filtering-based angular velocity estimation using infrared attitude information of spacecraft

We study the problem of estimating the spacecraft attitude angular velocity using the signal of the spacecraft attitude angular sen- sors such as the IR horizon sensor, the star sensor, and the full attitude global positioning systems (GPS) sensor for the first time. A signal state model is established. Based on the linear least mean squares (LMS) error criterion, recursive Kalman filtering is developed for the direct esti- mation of angular velocity. Our algorithm requires relatively few compu- tations and relatively small storage quantities. Finally, the new algorithm is applied to a set of physical simulated IR data, and the results have the advantages of smaller time lag, higher accuracy, and perfect smooth- ness. © 2000 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(00)00702-9)

Hydrogen bonding effects in quantum mechanical fore fields of pyrimidine bases: uracil

Using dihydrate uracil as a model of H-bonded compound formed by a uracil and two water molecules in the vicinity of HN1C2O and HN3C4O groups, DFT molecular orbital calculations have been performed in order to evaluate the changes in its simulated IR spectrum. To estimate the effect of intermolecular interactions on the vibrational properties of pyrimidine bases we have used the B3LYP functional and polarised 6–31G basis sets. The normal mode analysis enables us to assign the 30 modes of uracil, the 6 modes of the two water molecules along with the 12 additional modes resulting from their mutual interactions.

Application of a passive polarimetric infrared sensor in improved detection and classification of targets

Automatic detection and recognition of targets by means of passive IR sensors suffer from limitations due to lack of sufficient contrast between the targets and their background, and among the facets of a target. In this paper the results of a suite of polarization-sensitive automatic target detection and recognition algorithms on sets of simulated and real polarimetric IR imagery are presented. A custom designed Polarimetric IR (PIR) imaging sensor is used for collecting real polarimetric target data-three of the four Stokes parameters under a variety of conditions. Then a set of novel algorithms are designed and tested that uses the target and background Stokes parameters for detection, segmentation and classification of targets. The empirical performance results are obtained in terms of the probabilities of detection, false alarm rate, segmentation accuracy, and recognition probabilities as functions of number of pixels on target, aspect and depression angles and under several background conditions (clutter densities) on the polarimetric and non-polarimetrirc data. These results show that a noticeable improvement over the non-polarimetric ATR can be achieved.

Quantum Chemical Investigation of Structures, Rotational Barriers, and Vibrational Spectra of the Rotamers of Ethyl Nitrite (CH3CH2ONO)

Ab initio and density functional (DFT) methods were used to investigate the rotamers of ethyl nitrite. The potential energy surface for rotations around the C-O and the N-O bonds was calculated at different levels of theory. From the four geometrically possible conformers, only three were found to be stable when correlation effects are taken into account in the calculations. This result is consistent with a microwave study of Turner (J. Chem. Soc., Faraday Trans. 2 1979, 317.) as well as with the presented IR spectrum of matrix-isolated ethyl nitrite which clearly distinguishes three different N dO stretching modes. A simulated IR spectrum generated from the results of the DFT calculations is in good agreement with this matrix spectrum and allows us to tentatively assign groups of frequencies.

Regularized Method of Spectral Curve Deconvolution

A regularized method of spectral curve deconvolution is proposed. This method is based on three fundamental principles: the regularized method of solving the convolution equation; the use, instead of the apodization function, of the digital low-pass filter, which permits exact knowledge of its characteristics; and the use of the Fourier transform modulus of the spectrum being treated for obtaining a priori information about the frequency characteristics of the solution and noise, required for determination of the optimum parameters of the regularizing operator. The regularized method of de-convolution permits the acquisition of an approximately stable solution for the deconvolution problem of spectral curves, which moves toward an exact solution with the decrease of the experimental spectrum error. Examples are given of the application of the regularized method of deconvolution to simulated and experimental IR spectra. A conclusion about the expediency of using the given method for resolution enhancement in complex spectra is made.

Size-selected vibrational spectra of phenol-(H2O)n (n=1–4) clusters observed by IR–UV double resonance and stimulated Raman-UV double resonance spectroscopies

OH and CH stretching vibrations of bare phenol, phenol-(H2O)n clusters (n=1–4), and partially deuterated clusters in the S0 state were observed by using IR–UV double resonance and stimulated Raman-UV double resonance spectroscopies. Characteristic spectral features of the OH stretching vibrations of the phenol as well as of the H2O sites were observed, which are directly related to their structures. The cluster structures were investigated by comparing the observed spectra with the calculated ones obtained by the ab initio molecular orbital calculation with (self-consistent field) SCF 6-31G and SCF 6-31G* basis sets given by Watanabe and Iwata. It was found that for the clusters with n≥2, the isomer of ring form hydrogen-bonded structure is most stable and the simulated IR spectra based on the calculated structure showed good agreements with the observed ones. For a particular cluster, which was assigned as an isomer of the n=4 cluster, an anomalous IR spectrum was observed. Two forms of the isomer are proposed with respect to the structure of water moiety: (1) an ‘‘ice’’ structure and (2) an ‘‘ion-pair’’ structure. The relative IR absorption cross sections of each bands were also investigated for the clusters with n=1 to 4. It was found that the IR absorption cross section of the phenolic OH stretching vibration of the n=1 cluster increases by a factor of 6 compared to that of bare phenol and it further increases with the cluster size.

Enhancing infrared spectra by Fourier deconvolution using the Cauchy—Gauss product function

Chapados, C., Trudel M. and Miletic, J., 1994. Enhancing infrared spectra by Fourier deconvolution using the Cauchy—Gauss product function. Chemometrics and Intelligent Laboratory Systems, 22: 209–219. The method of Fourier deconvolution using Cauchy—Gauss product functions is being developed that allows the enhancing of experimental infrared spectra more adequately than with Cauchy (or Lorentz) functions. The mathematical development to obtain the Fourier transform of the product function is given. Two methods can be used to obtain the inverse Fourier transform of line shape functions: an analytical and a numerical method. The numerical method gives some errors which limit the degree of enhancement while the analytical method overcomes these difficulties. The numerical method is the only way to obtain an inverse Fourier transform of an experimental spectrum, and some aliasing is introduced by the algorithm used. The comparison between analytical and numerical methods of enhancing a simulated IR band is done to determine the limit of the procedure. The IR carbonyl band of methyl acetate is used as an example to show the effectiveness of the procedure described.

Sodalite from Vetralla (Roman potassic province) and Bancroft (Ontario, Canada): observed and simulated IR spectra

Infrared spectra of sodalites from Vetralla and Bancroft are compared to simulated spectra calculated from the available structural data and from empirical force constants applied to a simplified part-central-force potential-energy model. Calculation gives 18 vibrational modes including the 370 cm-1 mode observed by other authors in Raman spectra only. Increasing Si-O stretching constant implies variation of 3 high-frequency modes. The band group in the 750–650 cm-1 region comprises 3 modes strongly influenced by fal-o and subordinately by fsi-al. The 4 modes in the 500–350 cm-1 region are influenced by fo-si-o with weak contribution of O-oscillation, while the 295 cm-1 band is strongly influenced by fo-o, fNa-o and fal-o

Image Processing For Target Detection Using Data From A Staring Mosaic Infrared Sensor In Geosynchronous Orbit

A set of multistage image processing algorithms has been developed to perform change detection in image sequence analysis. These algorithms are tailored to the task of moving target identification (MTI). Conventional differencing algorithms are compared to a new spatial differencing algorithm. Subsequent processing is accomplished with thresholding and a suboptimal implementation of a 2-D matched filter designed for simplicity of calculation. The image processing algorithms are then coupled with a simple tracking/detection algorithm. The resulting combination of processing shows very good performance for detecting targets in simulated mosaic IR images, with a projected probability of false alarm less than one per 1011 frames and a probability of detection for targets within the model approaching unity.