Gábor Keresztury

Vibrational spectra of monothiocarbamates-II. IR and Raman spectra, vibrational assignment, conformational analysis and ab initio calculations of S-methyl-N, N-dimethylthiocarbamate

Abstract The IR (3200–30 cm−1) spectra have been recorded for S-methyl-N,N-dimefhylthiocarbamate, (CH3)2NC(O)SCH3, and its isotopomers, S-d3, N-d6 and N-d9, for the gas and liquid. Additionally, the Raman spectra (3200–10 cm−1) for the solid and liquid, with qualitative depolarization ratios, have been obtained for all the isotopes. These data are interpreted on the basis that the s-cis conformer (the S-methyl group oriented eis to the carbonyl group) with Cs symmetry is the only form existing in all three phases for this molecule. A complete vibrational assignment proposed for the -d0 molecule is facilitated by the availability of spectral data for five different isotopomers. A normal coordinate analysis has been carried out utilizing ab initio calculations with the 3–21G* basis set. The potential energy distributions and ab initio calculated frequencies have allowed a clarification of some of the corresponding results obtained from experiment. Structural optimizations and potential surface scan have also been carried out by ab initio calculations with the 3–21G* basis set. These results are compared with some previous studies on this molecule as well as on similar molecules.

Vibrational spectroscopy of triazoles and tetrazole

Abstract This work deals with the analysis of vibrational spectra of three five membered nitrogen heterocycles, the parent compounds: 1,2,3-triazole, 1,2,4-triazole and tetrazole, and their N-deuterated derivatives. The infrared spectra of these compounds were recorded in condensed states, while the Raman spectra were measured without polarization and using both parallel and perpendicular polarizations of scattered light. The fundamental vibrational frequencies were calculated applying the density functional theory with the Becke3P86 functional and the 6-311G(d,p) basis set. The results of the calculations were utilized in the assignment of the vibrational fundamentals, and the measured fundamental frequencies were used to refine the vibrational force constants. The considerable association of these compounds in condensed phase caused shifts of some fundamental frequencies of the isolated molecules. Nevertheless, the relative mean deviations between the measured and calculated frequencies were about 1 % or less for every investigated molecule.

Vibrational analysis of 2-nitrophenol. A joint FT-IR, FT-Raman and scaled quantum mechanical study

Abstract FT-IR (gas, solution, solid) and FT-Raman (solution, solid) spectra of 2-nitrophenol have been recorded in the range of 4000–30 cm−1. The spectra were interpreted with the aid of normal coordinate analysis based on a scaled Becke3–Lee–Yang–Parr/6-31G* density functional force field utilising a set of scale factors introduced recently by Rauhut and Pulay (G. Rauhut, P. Pulay, J. Phys. Chem. 99 (1995) 3093). These scale factors, developed on a small training set of organic molecules containing no hydrogen bonding moieties, were found to be well transferable to 2-nitrophenol including the strong intramolecular (O)H⋯O(N) hydrogen bonding moiety as well. The scaled force field reproduced the experimental frequencies of the molecule by a weighted mean deviation of 10.5 cm−1. Based on the calculated results, 38 fundamentals from a total of 39 were identified and assigned, revising the assignments of earlier experimental studies for several fundamentals.

Vibrational spectra and normal coordinate analysis of phenol and hydroquinone

Abstract The infrared and Raman spectra of hydroquinone have been recorded in solution and in crystalline state (β-modification). The spectra have been assigned with the help of normal coordinate analysis. The calculations have been performed for phenol and the two planar conformers of hydroquinone. The force constants have been computed with the CNDO force method. The scaling factors obtained by fitting the calculated frequencies of phenol to the observed ones have been transferred to hydroquinone.

FTIR Spectroscopy of the Atmosphere. I. Principles and Methods

Abstract In recent years Fourier transform infrared (FTIR) spectroscopy has been the dominant technique used for measuring the infrared (IR) absorption and emission spectra of most materials, with substantial advantages in signal‐to‐noise ratio, resolution, speed, and detection limits. The major advantage of the FTIR technique over other spectroscopic methods is that practically all compounds show characteristic absorption/emission in the IR spectral region and based on this property they can thus be analyzed both quantitatively and qualitatively. The quest for highly sensitive detection methods for atmospheric trace gas samples, either in laboratory setup or in outdoor remote sensing, has been on agenda for several decades. The fast and intensive development of FTIR spectroscopic techniques has propelled the progress of trace gas analysis of the atmosphere. Since the early 1970s the number and scope of FTIR atmospheric measurements has increased steadily. In this review article we are making an attempt to summarize the results of the most significant contributions to the field of FTIR spectroscopy to trace gas analysis of the atmosphere. Beside the basic description of the extractive and open‐path measurement methods, the difficulties connected with collection of appropriate background reference spectra, the ways of qualitative analysis and quantitative evaluation of measured spectra, the problems of calibration, and the effects of spectral resolution on detection sensitivity are discussed. The techniques reviewed include in situ IR absorption measurements over open paths in the field, such as remote sensing using the sun, the sky, or natural hot objects as IR sources of radiation, and also IR emission measurements of hot trace gas sources e.g., stack emissions, exhaust gases of combustion sources, and other industrial effluents.

Theoretical prediction of vibrational spectra. The a priori scaled quantum mechanical (SQM) force field and vibrational spectra of pyrimidine

Abstract The complete harmonic force field of pyrimidine has been computed at the ab initio Hartree—Fock level using a 4–21 Gaussian basis set. In order to compensate the systematic overestimations of the force constants at the aforementioned level of quantum mechanical approximation, the theoretical force constants were empirically scaled by using nine scale factors. (The values of all these scale factors were previously determined by fitting the theoretical force field of benzene to the observed vibrational spectra of benzene.) The resulting a priori scaled quantum mechanical (SQM) force field is regarded as the most accurate and physically the most correct harmonic force field for pyrimidine. This force field was then used to predict the vibrational spectra of pyrimidine- h 4 and pyrimidine- d 4 . On the basis of these a priori vibrational spectra uncertain assignments have been confidently resolved. After a few reassignments, the mean deviations between the experimental and calculated frequencies are below 9 and 18 cm −1 for the non-CH stretching in-plane and the out-of-plane vibrations, respectively. Computed IR intensities are generally in agreement with experiments at a qualitative level.

Intramolecular hydrogen-bonding in 2-nitroresorcinol. A combined FT-IR, FT-Raman and computational study

Abstract The vibrational properties of 2-nitroresorcinol have been studied by a combined experimental and theoretical analysis. The FT-IR and FT-Raman spectra of the compound have been recorded in the mid- and far-IR range (4000–150 cm−1). Symmetry related experimental information was obtained from polarisation IR and Raman measurements. The interpretation of the spectra was aided by quantum chemical calculations carried out at the B3-LYP/6-31G* level. Both the spectra and calculations support the C2v structure of the molecule with a symmetric intramolecular –O–H ⋯ O–N–O ⋯ H–O– hydrogen bonding interaction. The deficiencies of the computations for the molecular vibrations were corrected by the scaled quantum mechanical (SQM) method of Pulay. Using the standard scale factors developed for B3-LYP/6-31G* force fields, an rms deviation of 9.7 cm−1 was achieved between the gas-phase experimental and SQM frequencies. As a result of our SQM analysis, all the 42 fundamentals of the molecule were assigned. A comparison with the analogous hydrogen bonding moiety in 2-nitrophenol indicates a somewhat stronger hydrogen bonding interaction in the title molecule. This is accompanied, however, by an increased strain in the six-membered –C–O–H ⋯ O–N–C– rings.

Normal Raman and surface enhanced Raman spectroscopic experiments with thin layer chromatography spots of essential amino acids using different laser excitation sources

A comparative study of the feasibility and efficiency of Raman spectroscopic detection of thin layer chromatography (TLC) spots of some weak Raman scatterers (essential amino acids, namely, glycine and L-forms of alanine, serine, valine, proline, hydroxyproline, and phenylalanine) was carried out using four different visible and near-infrared (NIR) laser radiations with wavelengths of 532, 633, 785, and 1064 nm. Three types of commercial TLC plates were tested and the possibility of inducing surface enhanced Raman scattering (SERS) by means of Ag-sol was also investigated. The spectra obtained from spotted analytes adsorbed on TLC plates were of very different quality strongly depending on the excitation wavelength, the wetness of the samples, and the compounds examined. The best results were obtained with the simple silica TLC plate, and it has been established that the longest wavelength (lowest energy) NIR excitation of a Nd:YAG laser is definitely more suitable for generating normal Raman scattering of analyte spots than any of the visible radiations. Concerning SERS with application of Ag-sol to the TLC spots, 1-3 orders of magnitude enhancement was observed with wet samples, the greatest with the 532 nm radiation and gradually smaller with the longer wavelength excitations. It is shown, however, that due to severe adsorption-induced spectral distortions and increased sensitivity to microscopic inhomogeneity of the sample, none of the SERS spectra obtained with the dispersive Raman microscope operating in the visible region were superior to the best NIR normal FT-Raman spectra, as far as sample identification is concerned.

On the Raman spectroscopic determination of phase distribution in polyethylene

Abstract A methodological improvement is proposed for increasing the accuracy of the standard Raman spectroscopic method for the determination of the phase distribution in semicrystalline polyethylene (PE). A computer-assisted least-squares decomposition of the overlapping Raman bands, using analytical band shapes, is applied instead of the manual decomposition procedure suggested earlier. Comparison of the results obtained by the two methods shows that the manual decomposition systematically overestimates the amorphous content of PE at the expense of the interfacial content. Sums of 60% Gaussian and 40% Lorentzian functions give the best approximatiion to the measured Raman band shapes of PE. Our conclusion that the interfacial content in semicrystalline PE samples is about twice as high as was suspected before, seems to be supported by the measured densities.

FTIR spectroscopy of the atmosphere Part 2. Applications

Abstract The basic principles and methods of FTIR spectroscopy of the atmosphere were summarized in our previous paper 1. Thanks to the continuous technical development of FTIR spectroscopy (increasing throughput, dynamic alignment, more sensitive detectors, brighter sources, increasing scanning speed, development of focal plane array detectors, flexible spectral manipulations and data handling, etc.) in the last decade, this method has offered a great number of unique applications. In this review article, attempt to summarize the results of the most significant and frequent applications of FTIR spectroscopy to the study of the atmosphere. The possibilities of techniques applied in this field, the extractive and open path measurement methods, and the in situ IR absorption measurements such as remote sensing using the sun, the sky, or natural hot objects as IR sources of radiation are discussed. We have made a special focus to FTIR emission spectroscopy, the so‐called passive technique, since there are a number of originally hot gaseous samples such as volcanic plumes, automobile gases, stack gas plumes, or flames. Most of the subjects discussed in this article can be closely related to environmental analysis of the atmosphere. There is a wide range of atmospheric environmental applications of FTIR spectroscopy; therefore, we have focused our attention in the second part of the article on applications of FTIR spectroscopy in the atmosphere (troposphere) and stratosphere. We have summarized the basic literature in the field of special environmental applications of FTIR spectroscopy, such as power plants, petrochemical and natural gas plants, waste disposals, agricultural, and industrial sites, and the detection of gases produced in flames, in biomass burning, and in flares.