Dynamic,computer-assisted interpretation of infrared spectra of condensed-phase mixtures

A knowledge-based system, MIXIR, designed to assist researchers in interpreting the infrared spectra of condensed-phase mixtures is described. This system overcomes many of the inherent limitations of static-rule-based systems. An iterative problem-solving approach is permitted, making use of information gained during the interpretation process. The dynamic rule selection and significance evaluation alos allow the user to provide information on the sample matrix, or to select a subset of the compounds for rule derivation. The MIXIR system was evaluated with a knowledge base compiled using IRBASE, a complementary program, and a test set of 20 mixtures.     

Expert system for interpretation of the infrared spectra of environmental mixtures

A program for the identification of the principal components of mixtures through interpretation of the infrared mixture spectrum (IntIRpret) was developed. This program, which was developed as a preliminary screening tool for unknown organic mixtures, has five main subroutines: the interferogram processing and peak-selection subroutine (PUSHSUB), the automated knowledge-acquisition subroutine (AUTOGEN), the system optimization subroutine (STO), the interpretation subroutine (PAIRS), and final processing subroutine to subtract spectral similarity (PAIRSPLUS). Principal advantages of this system compared to earlier systems are speed, flexibility and accuracy.     

Principal component analysis of the infrared spectra of mixtures

Principal component analysis of the infrared spectra of a series of related mixtures is used to determine the number of compounds present. The use of empirical error estimates makes it possible to determine correctly the number of components even when the spectra of the individual compounds are very similar.     

Two-dimensional infrared (2D IR) spectroscopy. A new tool for interpreting infrared spectra

Two-dimensional infrared (2D IR) spectroscopy is used to study atactic polystyrene. 2D IR is a technique based on time-resolved detection of IR signals in response to an external perturbation, such as mechanical strain. Since different chemical functional groups respond to the applied perturbation at unique and often different rates, characteristic time-dependent variations of the IR-band intensities are observed. Correlation analysis of the dynamic variation of the IR signals yields a new spectrum defined by two independent wave numbers. Peaks located on a 2D IR spectral plane imply interactions and connectivities among chemical functional groups. By spreading convoluted IR bands over two dimensions, the spectral resolution is also greatly enhanced.     

Target factor analysis of infrared spectra of multicomponent mixtures

The target factor analysis (t.f.a.) error criteria discussed earlier are applied to Fourier-transform infrared spectra in order to determine, without any prior knowledge of the experimental error, the number and identities of components in a series of related multicomponent mixtures. The t.f.a. method is also used for quantitative analysis and is compared to regression analysis, which force-fits the data and does not compensate for an impurity of small but measurable quantity. In addition, it is shown how an impurity in a single mixture can be detected, identified, and quantified. An actual experimental example is presented, using solutions of o-xylene, m-xylene, p-xylene, and ethylbenzene in cyclohexane, with chloroform as an impurity.     

Validation of reducing-difference procedure for the interpretation of non-polarized infrared spectra of n-component solid mixtures

Validation of reducing-difference procedure based on subtracting of non-polarized infrared (IR) spectra of n-component solid mixtures is presented. The accuracy and precision are established. The limits of detection are 3.0, 2.5, 1.5 and 1.0 wt.% for 5-, 4-, 3-and 2-component mixtures, respectively. Smoothing procedures for IR spectral analysis, based on Savitzky-Golay or Fourier methods are applied as well. The mean values and relative standard deviations for peak position (ν_i) and integral absorbance (A_i) obtained by this data processing approach have been examined using Student's t-test.     

SpecTool: a knowledge-based hypermedia system for interpreting molecular spectra

Summary A hypermedia environment was designed to set up a collection of reference data, reference spectra and computational tools. The system may serve as an aid for the combined interpretation of molecular spectra. This paper describes the context dependent presentation of the pieces of information.     

NMR methods for unravelling the spectra of complex mixtures

The main methods for the simplification of the NMR of complex mixtures by selective attenuation/suppression of the signals of certain components are presented. The application of relaxation, diffusion and PSR filters and other techniques to biological samples, pharmaceuticals, foods, living organisms and natural products are illustrated with examples.     

Liquid crystal mixtures for potential infrared applications

Liquid crystals (LCs) with diazo linkages have high dielectric and optical anisotropy. Two newly synthesized liquid crystalline compounds were dissolved in room temperature LC hosts, E7 and PTTP-24/36, to assess their properties. It was found that these mixtures have higher birefringence, larger dielectric anisotropy, and better elastic properties than the hosts. Satisfactory viscoelastic coefficients were also obtained for these mixtures, showing that they are promising LC materials for applications in the near IR region.     

Liquid crystal mixtures for potential infrared applications

Liquid crystals (LCs) with diazo linkages have high dielectric and optical anisotropy. Two newly synthesized liquid crystalline compounds were dissolved in room temperature LC hosts, E7 and PTTP-24/36, to assess their properties. It was found that these mixtures have higher birefringence, larger dielectric anisotropy, and better elastic properties than the hosts. Satisfactory viscoelastic coefficients were also obtained for these mixtures, showing that they are promising LC materials for applications in the near IR region.     

Theoretical interpretation of infrared spectra of N-cyclohexyl-2-pyrrolidone in mixtures of hexane and CDCl3

Summary The infrared spectra of N-cyclohexyl-2-pyrrolidone (NCP) in binary mixtures of hexane and CDCl₃ were interpreted theoretically in the region of C=O stretching vibrations using the AM1 semiempirical method. The results were compared with those obtained for N,N-dimethylacetamide (DMA), cyclohexanone (CX) and propanone (PR). Good correlations were found between the carbonyl stretching frequencies and the theoretical parameters of free and solvated species in equilibria with hexane-CDCl₃ solvent mixtures.

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Theoretische Interpretation von Infrarot-Spektren von N-Cyclohexyl-2-pyrrolidon in Mischungen von Hexan und CDCl₃

Zusammenfassung Es wurden die Infrarot-Spektren von N-Cyclohexyl-2-pyrrolidon (NCP) in binären Mischungen von Hexan und CDCl₃ im Bereich der C=O Streckschwingung unter Benutzung der semiempirischen AM1-Methode interpretiert. Die Ergebnisse wurden mit denen für N,N-Dimethylacetamid (DMA), Cyclohexanon (CX) und Propanon (PR) verglichen. Es wurde eine gute Korrelation zwischen den Frequenzen der Carbonyl-Streckschwingung und den theoretischen Parametern der freien und solvatisierten Spezies im Gleichgewicht mit den Hexan-CDCl₃ Lösungsmittelgemischen gefunden.

     

Validation of a computer-program for the interpretation of infrared spectra

The capability of a modern interpretation system for IR spectra has been tested, in which the process of structural assignment is modelled by methods of fuzzy logic. Figures of merit were generated for different interpretation algorithms. These figures of merit permit the validation as well as a comparison of independent interpretation systems. The influence of the sample preparation practice onto the information content of the resulting spectrum has been evaluated as well. Automatically generated band tables show only restricted reliability. Received: 13 February 1997 / Revised: 1 April 1997 / Accepted: 3 April 1997     

Validation of a computer-program for the interpretation of infrared spectra

The capability of a modern interpretation system for IR spectra has been tested, in which the process of structural assignment is modelled by methods of fuzzy logic. Figures of merit were generated for different interpretation algorithms. These figures of merit permit the validation as well as a comparison of independent interpretation systems. The influence of the sample preparation practice onto the information content of the resulting spectrum has been evaluated as well. Automatically generated band tables show only restricted reliability. Received: 13 February 1997 / Revised: 1 April 1997 / Accepted: 3 April 1997     

Complete sets of structure fragments for interpreting IR spectra using IR databases

This paper studies the possibility of revealing various fragments (not given beforehand) in a structure by analyzing the structures selected from the database as a result of a retrieval by the IR spectrum of the compound; the fragments range from those with two nodes (e.g., C=O, C=C) to those with some limited number of connected nodes. It is shown that complete fragment sets reflecting the composition of the selected compound (fragment compositions) contain the information about the set of bonded fragments of the compound. Various factors affecting analysis of the compiled list of fragments are considered using a particular example. Different techniques for revealing structural information are discussed. Scientific and Technical Center of Chemical Informatics, Novosibirsk Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 2, pp. 368–378, March–April, 1996. Translated by L. Smolina     

Computer-assisted interpretation of infrared spectra

Pattern recognition and artificial intelligence programming techniques for the interpretation of infrared spectra are compared in an effort to determine the best technique for assisting the solution of actual structural elucidation problems. For several reasons, artificial intelligence is the method of choice. When information pertaining to a large number of classes is required, an excessively large training set would be needed with pattern recognition procedures. If the program makes a mistake, it must be alterable so that a similar error will not occur again. Artificial intelligence programs are amenable to this correction procedure.     

Far infrared spectra of hexachalcogenohypodiphosphates

The far infrared (fir) reflection spectra of the hexachalcogenohypodiphosphates M₂P₂X₆ (M = Mg, Cd, Mn, Fe, Co, Ni, Pb, Sn; X = S, Se) have been studied. $\text{TO}\text{LO}$ splittings are determined from Kramers-Kronig analyses, the splitting is large for the lattice vibration modes of ferroelectric Sn₂P₂S₆ but very small for Ni₂P₂S₆. Measurements with polarized light permit an assignment of the A_2u and E_u modes of the ethane like P₂S₆ units in Mn₂P₂S₆. Factor group analyses were performed for all hitherto described hexachalcogenohypodiphosphates. The fir absorption spectra of the less symmetric compounds Hg₂P₂S₆, Hg₂P₂Se₆, Ca₂P₂S₆, TiP₂S₆, $\text{In}{}_{\mathrm{<mtext>4</mtext><mtext>3</mtext>}}\text{P}{}_2\text{S}{}_6\text{, In}{}_{\mathrm{<mtext>4</mtext><mtext>3</mtext>}}\text{P}{}_2\text{Se}{}_6$, and CuCrP₂S₆ are given.     

Hierarchical clustering of infrared spectra

The generation of a hierarchical tree of 500 infrared spectra, using the recently proposed fractal or 3-distances-clustering method is described and discussed. The objects of clustering are infrared spectra of polymer compounds which are represented as sets of 80 complex Fourier coefficients, obtained by fast Fourier transformation of digitized absorbance spectra. The generated hierarchical tree, with a maximum height of 20 and an average height of 12 levels, yields a very satisfactory clustering scheme with respect to the structure of the compounds involved. In addition to very good clustering, a 100% retrieval (prediction) ability was obtained. This was achieved by the use of an iterative procedure after the initial tree had been generated. Additionally, the tree was tested with 240 infrared spectra of different compounds which were taken into account during the generation of the tree. The retrieval success of these test runs is discussed with respect to the structural similarity of the compounds to which the “unknown” spectra were linked.     

Impact of finite-temperature and condensed-phase effects on theoretical X-ray absorption spectra of transition metal complexes

The impact of condensed-phase and finite-temperature effects on the theoretical X-ray absorption spectra of transition metal complexes is assessed. The former are included in terms of the all-electron Gaussian and augmented plane-wave approach, whereas the latter are taken into account by extensive ensemble averaging along second-generation Car–Parrinello ab initio molecular dynamics trajectories. We find that employing the periodic boundary conditions and including finite-temperature effects systematically improves the agreement between our simulated X-ray absorption spectra and experimental measurements. © 2018 Wiley Periodicals, Inc.     

Variable-temperature infrared spectra of VO2

The infrared spectra of VO₂ powder in KBr from 1300 to 400 cm⁻¹ during heating and cooling through the metal-insulator transition are reported. In the high-temperature metallic region, the spectra are nearly featureless, while below the transition temperature well-developed vibrational bands are observed. The spectra at room temperature before and after heating are superimposable, a result consistent with the reversibility of the metal-insulator transition.