Mirosław A. Czarnecki

Interpretation of Two-Dimensional Correlation Spectra: Science or Art?

It has been shown that the most common perturbations of conventional (one-dimensional) spectra such as random noise, baseline fluctuations, band position, and width changes may complicate two-dimensional (2D) correlation spectra, sometimes making them completely useless. In addition, two different physical causes may generate similar patterns for the synchronous and asynchronous spectra. Some of these effects, such as random noise and baseline fluctuations, can be eliminated from the input data, and one can recover the original appearance of 2D correlation spectra. The other effects, such as the frequency shift and bandwidth variation, cannot be removed from the experimental spectra. In this instance, the number and position of the correlation peaks can be elucidated by simulation studies. This report presents a few examples of typical patterns found in the synchronous and asynchronous spectra affected by those perturbations. Long streaks in 2D correlation spectra reveal extensive baseline fluctuations in the original data set. A simple offset often significantly reduces the extent of this effect. When no reasonable baseline correlations can be performed, the second derivative may solve this problem. In most cases, the perturbation-averaged spectrum is recommended as a reference. However, it has been proved that the calculation of 2D correlation spectra without any reference spectrum may also provide useful information, especially for data heavily influenced by noise or baseline fluctuations. In the majority of real-world systems, the spectral changes are a continuous function of applied perturbation. Thus, 2D correlation spectra yield information about the relative rate of intensity variations rather than the sequence of spectral events.

Two-Dimensional Correlation Spectroscopy: Effect of Band Position, Width, and Intensity Changes on Correlation Intensities

Simulation studies have demonstrated that the effect of a band shift may be completely removed from two-dimensional (2D) synchronous spectra if a shifting band simultaneously changes its intensity. In contrast, the corresponding asynchronous spectrum develops at least two peaks, even for a small shift coupled with an appreciable intensity variation. The separation between these peaks increases upon an increase in the bandwidth. If the spectral data are changing monotonically, the number and positions of the synchronous features can be readily determined from the difference between the first and the last spectrum in the series. The correlation spectrum calculated without the subtraction of reference spectrum, for a single band that shifts with constant intensity, is similar to that calculated without the subtraction of reference, for a band undergoing shift combined with significant intensity variations. The synchronous peaks resulting from the exponentially decaying intensity changes alone are at least 10 times more intense than the corresponding asynchronous peaks, whereas the analogous intensity ratio due to a moderate band shift is discernibly lower. This result proves that the asynchronous spectra are more sensitive to the band shift than the synchronous spectra. Also, the effect of noise is more apparent in the case of the asynchronous spectrum. The bandwidth variation alone generates noticeably weaker correlation intensity than that due to the band position or intensity changes. It has been shown that the asynchronous intensity strongly depends on the overall extent of the intensity changes at particular wavenumbers. As a result, the bands changing their intensities extensively but at similar rates may develop more intense asynchronicity than the bands with distinct difference in the rates of the intensity changes but smaller magnitude of these changes.

Two-Dimensional Correlation Spectroscopy: Effect of Normalization of the Dynamic Spectra

Simulation studies have demonstrated that linear nonselective intensity variations coupled with nonlinear selective intensity changes may develop new features in two-dimensional (2D) correlation spectra. Some types of linear nonselective intensity changes are hardly seen in the normal and synchronous 2D correlation spectrum. In contrast, they may develop quite strong features in the companion asynchronous spectrum, especially if the selective intensity variations for different bands have similar response functions. The simplest way of removing this effect is normalization of the dynamic spectra prior to 2D correlation analysis. This operation can be easily performed if we know the relationship between the perturbation and the nonspecific intensity variations. Otherwise, one has to employ an “internal reference” for normalization of the experimental spectra. The “internal reference” means the band that does not selectively change in its intensity under given perturbation. Fourier transform near-infrared (FT-NIR) measurements of octan-1-ol in CCl4 revealed a strong correlation between the concentration and the integrated intensity of the second overtone of the v(C–H) band. Also a strong correlation was found between the integrated intensity of the same band and the temperature-induced density changes of pure octan-1-ol. Thus, in the NIR region the second overtone of the v(C–H) band can be successfully applied for normalization of both the concentration and temperature-perturbed spectra of numerous organic samples. The most complicated situation appears for a rheo-optical experiment involving pronounced deformation, where any simple normalization of the experimental spectra cannot be applied. In this instance, knowledge of the exact relationship between the strain and the sample thickness is required.

Advances in Molecular Structure and Interaction Studies Using Near-Infrared Spectroscopy.

Infrared Spectroscopy Mirosław Antoni Czarnecki,*,† Yusuke Morisawa,‡ Yoshisuke Futami, and Yukihiro Ozaki* †Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland ‡Department of Chemistry, School of Science and Engineering, Kinki University, Higashi-Osaka, Osaka 577-8502, Japan Department of Biological and Chemical Systems Engineering, National Institute of Technology, Kumamoto College, Yatsushiro, Kumamoto 866-8501, Japan Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan

Potential of Fourier Transform Near-Infrared Spectroscopy in Studies of the Dissociation of Fatty Acids in the Liquid Phase

Fourier transform near-infrared (FT-NIR) technique has been employed to determine the degree of dissociation and the thermodynamic parameters of octanoic acid in the liquid phase. FT-NIR absorption spectra of the acid over a temperature range of 15 to 92°C were recorded. From the spectra, intensities of the first overtone of the OH stretching mode due to the monomer form of the acid were extracted and were used to calculate the degree of dissociation and the thermodynamic parameters for the process of dissociation of dimeric octanoic acid into monomer species. In the 7400–6700 cm−1 region there appear many bands that are heavily overlapped; the attempts to apply curve-fitting and deconvolution algorithms to separate individual bands have failed. Here, a simple and reliable method of calculation of the intensities of the first overtone of the OH stretching mode due to the monomer is proposed. Also, an excellent procedure of determining the molar absorption coefficients (integrated and at peak position) through a series of measurements in CCl4 solution at very low concentrations is presented. The bands due to CH vibrations were eliminated in two ways; one way involved taking the concentration as a reference, and the other used the band area of the second overtone of the CH stretching modes as a reference. In the operation the spectrum of neat octanoic acid at 15.1°C was employed. The coefficients obtained by both methods are compared and discussed. The present studies indicate that both band areas and peak heights give results which are found to be in very good agreement.

Resolution Enhancement and Band Assignments for the First Overtone of OH Stretching Mode of Butanols by Two-Dimensional Near-Infrared Correlation Spectroscopy. Part I: sec-Butanol

The first paper in a series devoted to self-association in neat butanols presents the results of two-dimensional (2D) near-infrared (NIR) correlation analysis of temperature-induced spectral variations of sec-butanol. By taking advantage of resolution enhancement in the 2D correlation spectra, it was possible to identify spectral features due to vibrations of the free and associated OH groups in the first-overtone region. On the basis of a few assumptions, band assignments of the various types of OH bonds have been proposed. The monomer band (near 7100 cm−1) can be resolved into three components; two of them are due to a rotational isomerism (7089 and 7116 cm−1), and the third one is attributed to the free terminal OH groups in linear polymers (7055 cm−1). The presence of the 7055 cm−1 band implies that the intensity of the monomer peak cannot be used as a measure of the concentration of the monomer species (except in very diluted solutions). Thus, previous estimations of equilibrium constants and thermodynamic parameters associated with hydrogen-bond dissociation have been subject to unacceptable error. At higher temperatures, a new band near 6550 cm−1 becomes visible. This band originates from bended OHO bond, mostly in the cyclic polymers. In order to obtain more detailed information on the complex mechanism of the thermal dissociation of hydrogen-bonded sec-butanol in the pure liquid phase, the entire experimental temperature range was divided into narrower ranges, and then 2D correlation analysis was performed for smaller data sets. It has been shown that the variations of population of the polymeric species and the cyclic dimers are faster than the corresponding changes for the monomers. At elevated temperatures an appreciable dissociation of the cyclic species takes place.

Two-Dimensional Correlation Analysis of Hydrogen-Bonded Systems: Basic Molecules

Abstract This review provides information on application of two-dimensional (2D) correlation spectroscopy to studies of the molecular structure and hydrogen bonding in basic molecules and their binary mixtures with water. The first part of this review is an introduction to 2D correlation analysis and includes detailed description how to calculate the synchronous and asynchronous spectra from the raw experimental data. An appropriate pretreatment is widely discussed and the MATLAB codes to perform these operations are given. In the second part, the applications of 2D correlation spectroscopy for explorations of basic molecules like aliphatic alcohols, N-methylacetamide (NMA), diols, amino alcohols, carboxylic acids, and water in the pure liquid phases are reviewed. Finally, the 2D correlation studies of binary mixtures of basic molecules with water are discussed. It has been shown that an application of 2D correlation analysis to the complex spectra makes it possible to obtain the information not readily seen in the original data.

Near-IR molar absorption coefficient for the OH-stretching mode of cis-9-octadecenoic acid and dissociation of the acid dimers in the pure liquid state

The molar absorption coefficient, IµOH, at 1445 nm for the stretching mode of the unbonded OH in the carboxy group of cis-9-octadecenoic acid has been successfully determined by examination and correlation of both the absorbance at 1445 nm for a dilute CCl4 solution of the acid and the degree of dissociation, αo, of the dimeric acid into free molecules in solution. αo was obtained from the ratio of the original concentration of the acid to its apparent concentration measured on a molecular weight apparatus. Even in dilute solutions ranging in concentration from 5 × 10–3 to 2.6 × 10–2 mol dm–3, αo fell within the range 0.35–0.22 at 50 °C, implying that the dissociation of the acid dimers into the monomeric species is not complete. Combination of the IµOH and the apparent molar absorption coefficient of the OH-vibration mode for the monomeric cis-9-octadecenoic acid in the pure liquid state gave the degree of dissociation, α, of the acid dimer into its monomers in the pure liquid phase as a function of temperature, T. Dissociation of the acid dimer occurs even at room temperature and increases with temperature. The α–T relationship has two break points at 30 and 55 °C. The break-point temperatures correspond to the transition temperatures in the liquid structures of cis-9-octadecenoic acid. At 30 °C the quasi-smectic liquid crystal changes to a more disordered liquid crystal, while at 55 °C the disordered liquid crystal is converted into an isotropic liquid. In addition, thermodynamic properties such as the standard enthalpy and entropy for the dissociation of the cis-9-octadecenoic acid dimer into its monomeric species also suggest that the clusters with the quasi-smectic liquid-crystal structure exist in the pure liquid state below 30 °C and the isotropic liquid structure exists above 55 °C.

Fourier Transform Near-Infrared Spectra of N-Methylacetamide: Dissociation and Thermodynamic Properties in Pure Liquid Form and in CCl4 Solutions:

FT-NIR spectra have been measured for N-methylacetamide (NMA) in pure liquid form and in carbon tetrachloride (CCl4) solutions over temperature ranges of 303–368 K and 283–343 K, respectively. For the pure liquid, the FT-NIR spectra give one band due to a free NH group, at least five bands due to free end NH groups, and at least six bands due to hydrogen-bonded NH groups in the region of the first overtone of the NH stretching mode, demonstrating the potential of FT-NIR spectroscopy for investigating the hydrogen-bonding of NMA. The temperature-dependent spectral changes of NMA in the pure liquid indicate that, with increasing temperature, longer-chain oligomers of NMA dissociate into monomeric and dimeric species. For the CCl4 solutions, the intensity of a band assigned to the first overtone of the NH stretching mode of the monomeric NMA has been employed to study the degree of dissociation and thermodynamic properties for the dissociation process. Below 4 × 10−3 M, NMA exists completely in the monomer form, and from the absorbance-concentration plot, the molar absorption coefficient of the first overtone of the NH stretching mode has been obtained. The degree of dissociation and dissociation equilibrium constant K have been calculated for the solutions at various concentrations, and then they have been employed to evaluate the thermodynamic parameters (ΔH, ΔS, and ΔG) for the dissociation of hydrogen-bonded NMA into its monomeric species. The results show that the contribution of the entropy (ΔS) to the standard free energy (ΔG) becomes larger with a reduction in the concentration. It seems very likely that in the dilute solutions shorter-chain oligomers exist predominantly, and they easily dissociate into the monomeric species.

Molecular Self-Assembling of Butan-1-ol, Butan-2-ol, and 2-Methylpropan-2-ol in Carbon Tetrachloride Solutions as Observed by Near-Infrared Spectroscopic Measurements:

The self-associations of butan-1-ol, butan-2-ol, and 2-methylpropan-2-ol (tert-butanol) in the pure liquid state and in carbon tetrachloride solutions have been studied mainly through near-infrared spectroscopic observation at various temperatures. A new analysis assuming a successive association process for the alcohol molecules was applied to the sharp band around 1410 nm (the first-overtone band of the OH stretching vibration mode attributed to free OH monomer and partly to OH polymer); it became clear that the mean association number for each alcohol increases with increasing concentration and decreases with increasing temperature. Comparisons of the association numbers at various constant temperatures for the three kinds of alcohols show that the association abilities are on the order butan-1-ol > butan-2-ol > 2-methylpropan-2-ol.