Krzysztof B. Beć

Spectroscopic and Computational Study of Acetic Acid and Its Cyclic Dimer in the Near-Infrared Region

Anharmonic vibrational analysis of near-infrared (NIR) spectra of acetic acid was carried out by anharmonic quantum chemical calculation in a wide concentration range of its CCl4 solution. By predicting vibrational spectra of acetic acid for the first time over a wide NIR region, it was possible to elucidate the influence of the formation of acetic acid cyclic dimer on its NIR spectrum. Quantum chemical simulations were based on coupled cluster and density functional theory quantum methods. Additionally, Møller-Plesset perturbation theory was employed for the additional calculation of hydrogen bonding stabilization energies. An anharmonic vibrational analysis was performed with the use of generalized second-order vibrational perturbation theory (GVPT2). A hybrid approach was assumed, in which monomeric species was treated by CCSD(T)/aug-cc-pVDZ (harmonic approximation) and B3LYP/SNSD (anharmonic approximation) methods. For the cyclic dimer, B3LYP and B2PLYP single and double hybrid functionals, paired with an SNSD basis set, were employed. DFT calculations were augmented with additional empirical dispersion correction. It was found that quantum chemically calculated vibrational modes in the NIR region are in a good agreement with experimental data. The results of anharmonic vibrational analysis were supported by a harmonic shift analysis, for elucidating the very strong anharmonic coupling observed between stretching modes of hydrogen bonded bridge in the cyclic dimer. However, the calculated wavenumbers for combination modes of double hydrogen bonded bridge in the cyclic dimer, which are very sensitive to the formation of hydrogen bonding, were found to be underestimated by quantum chemical methods. Therefore, by band fitting, the wavenumbers and shape parameters for these bands were found, and the modeled spectra were adjusted accordingly. A high accuracy of simulated spectra was achieved, and a detailed analysis of the experimental NIR spectra of acetic acid was possible, with successful identification of numerous experimental bands, including those which originate from concentration effects. It was also found that the main spectral features observed in the NIR spectra of carboxylic acid upon the formation of hydrogen bond should be accounted for combination modes of the stretching and bending vibrations of double hydrogen-bonded bridge in the cyclic dimers of acetic acid.

Temperature Drift of Conformational Equilibria of Butyl Alcohols Studied by Near-Infrared Spectroscopy and Fully Anharmonic DFT

Conformational isomerism of aliphatic alcohols with respect to the internal rotation of C-O(H) group and its impact on near-infrared (NIR) spectra has been known in the literature. However, no attempt has ever been made to investigate systematically whether and how the conformational flexibility of the aliphatic chain determines the observed NIR data of aliphatic alcohols. In the present study NIR spectra of four kinds of butyl alcohols, 1-butanol, 2-butanol, isobutanol, and tert-butyl alcohol, were investigated in diluted (0.1 M) CCl4 solutions. The experimental NIR spectra of butyl alcohols were accurately reproduced and explained in a fully anharmonic DFT study by means of generalized second-order vibrational perturbation theory (GVPT2). Entire conformational populations were taken into account in each case. On the basis of the theoretical study, influences of conformational flexibility with respect to internal rotations not only about the C-O bond, but also about the C-C bonds have been well evidenced in the experimental spectra. The conformational isomerism affects significantly the shape of NIR spectra. The temperature-dependent NIR spectra of butyl alcohols show changes in the band shape and a blue-shift of the overtone band due to the stretching mode of free OH group, and its intensity decreases with increasing temperature. These effects can be closely monitored by two-dimensional correlation spectroscopy (2D-COS). In this work, the experimental 2D-COS patterns have been successfully reproduced, based on DFT calculated NIR spectra of conformational isomers of the studied molecules and their Boltzmann coefficients over the corresponding temperature range. Thus, the experimentally observed effects are fully reflected in the DFT study, which leads to the conclusion that the main factor in the temperature-dependent spectral changes of 2νOH band of aliphatic alcohols in the diluted phase, where no self-association occurs, is played by the changes in the relative population of their conformational isomers.

Correlations between Structure and Near-Infrared Spectra of Saturated and Unsaturated Carboxylic Acids. Insight from Anharmonic Density Functional Theory Calculations

By near-infrared (NIR) spectroscopy and anharmonic density functional theory (DFT) calculations, we investigate five kinds of saturated and unsaturated carboxylic acids belonging to the group of short-chain fatty acids: propionic acid, butyric acid, acrylic acid, crotonic acid, and vinylacetic acid. The experimental NIR spectra of these five kinds of carboxylic acids are reproduced by quantum chemical calculations in a broad spectral region of 7500-4000 cm-1 and for a wide range of concentrations. By employing anharmonic GVPT2 calculations on DFT level, a detailed interpretation of experimental spectra is achieved, elucidating structure-spectra correlations of these molecules in the NIR region. We emphasize the spectral features due to saturated and unsaturated alkyl chains, the location of a C═C bond within the alkyl chain, and the dimerization of carboxylic acids. In particular, the existence of a terminal C═C bond leads to the appearance of highly specific NIR bands. These pronounced bands are located at wavenumbers where no overlapping with other structure-specific bands occurs, thus making them good structural markers. Most of the spectral differences between these two groups of molecules remain subtle, and would be difficult to reliably ascribe without quantum chemically calculated NIR spectra. Moreover, anharmonic DFT calculations provide insights on the manifestation of hydrogen bonding through distinctive spectral features corresponding to cyclic dimers. The resulting spectral baseline elevation is common for all five investigated carboxylic acids, and remains consistent with previous results on acetic acid.

Spectra-structure correlations of saturated and unsaturated medium-chain fatty acids. Near-infrared and anharmonic DFT study of hexanoic acid and sorbic acid

Quantum chemical reproduction of entire NIR spectra is a new trend, enabled by contemporary advances in the anharmonic approaches. At the same time, recent increase of the importance of NIR spectroscopy of biological samples raises high demand for gaining deeper understanding of NIR spectra of biomolecules, i.e. fatty acids. In this work we investigate saturated and unsaturated medium-chain fatty acids, hexanoic acid and sorbic acid, in the near-infrared region. By employing fully anharmonic density functional theory (DFT) calculations we reproduce the experimental NIR spectra of these systems, including the highly specific spectral features corresponding to the dimerization of fatty acids. Broad range of concentration levels from 5·10-4M in CCl4 to pure samples are investigated. The major role of cyclic dimers can be evidenced for the vast majority of these samples. A highly specific NIR feature of fatty acids, the elevation of spectral baseline around 6500-4000cm-1, is being explained by the contributions of combination bands resulting from the vibrations of hydrogen-bonded OH groups in the cyclic dimers. Based on the high agreement between the calculated and experimental NIR spectra, a detailed NIR band assignments are proposed for hexanoic acid and sorbic acid. Subsequently, the correlations between the structure and NIR spectra are elucidated, emphasizing the regions in which clear and universal traces of specific bands corresponding to saturated and unsaturated alkyl chains can be established, thus demonstrating the wavenumber regions highly valuable for structural identifications.

Influence of Non-fundamental Modes on Mid-infrared Spectra: Anharmonic DFT Study of Aliphatic Ethers

Fundamental and non-fundamental vibrational modes, first overtones, and binary combination modes of selected aliphatic ethers (di-n-propylether, di-iso-propylether, n-butylmethyl ether, n-butylethyl ether, di-n-butyl ether, tert-buytlmethyl ether, and tert-amylmethyl ether) were modeled in a fully anharmonic generalized second-order vibrational perturbation theory (GVPT2) approach on the DFT-B2PLYP/SNST level. The modeling procedure of theoretical line shapes took into account conformational isomers of studied molecules. The calculated spectra of the above ethers were compared to the corresponding experimental spectra in the infrared (IR) region (4000-560 cm-1) of the absorption index k(ν) derived from the neat liquid thin-film transmission data. It was found that IR spectra of aliphatic ethers are heavily influenced by the bands originating from non-fundamental modes, particularly from the combination modes in the C-H stretching region (3200-2800 cm-1). Because of the effects of vibrational resonances, the intensities of overtones and combination bands originating from methyl and methylene deformation modes increase sufficiently to influence the experimental line shape in this region. Less significant contributions from non-fundamental vibrational modes were noticed in the lower IR region (1600-560 cm-1), particularly in the vicinity of the C-O stretching band. The 2700-1600 cm-1 region, which is rich in weak bands due to non-fundamental vibrations, was reproduced accurately as well. It was concluded that a fully anharmonic approach allows significantly more accurate reproduction of the complex IR line shapes, particularly in the C-H stretching region of aliphatic ethers. On the basis of the achieved agreement between the experimental and calculated spectra, it may be concluded that the anharmonic GVPT2 method can adequately reproduce the anharmonic effects and vibrational resonances in particular, influencing the IR spectra of aliphatic ethers. The results obtained in this study show that the non-fundamental modes may play a significant role in shaping the IR spectra of aliphatic ethers and similar molecules in the neat liquid phase.

Quantum chemical calculations of basic molecules: alcohols and carboxylic acids

This second part of the review series on quantum chemical methods in near infrared (NIR) spectroscopy covers the practical aspects involving applications of generalised second-order vibrational perturbation theory (GVPT2). Basic considerations are discussed here, with the aim of introducing experimental spectroscopists to the topic. The specifics of fully anharmonic calculations, the notably increased computational cost (when compared to routine harmonic vibrational calculations), the choice of software suite and hardware considerations, and various other factors with high importance for practical use of discussed theoretical methods are covered. These deliberations are directly compared to recent reports on the applications of anharmonic theoretical studies to the experimental NIR spectra of aliphatic alcohols and carboxylic acids.

Quantum chemical calculation of NIR spectra of practical materials

In the third issue of the series on modern quantum chemical methods in the support role of NIR spectroscopy we continue to introduce the researchers from the field of experimental spectroscopy to practical aspects and applications of modern anharmonic theoretical approaches. The first two issues focused on explaining the necessary theoretical and practical background, allowing readers to get more familiar with the topic. An overview of recent literature reports highlighted the advantages stemming from using quantum chemical calculation in the support role to NIR spectroscopy. These deliberations were based on several cases of small- to medium-sized molecules. This part overviews the topic of applications of quantum theoretical methods to complex molecules with practical significance, which typically prove to be challenging objects for theoretical studies. An exemplary application of presented methodology to the case of Rosmarini folium biological samples is also examined here. The rosemary specific active compound, rosmarinic acid, is a relatively complex polyphenol with growing phytopharmaceutical importance, and therefore provides an excellent object of applied studies. The possibilities of combining the information stemming from quantum chemical calculation with the methods of advanced spectral data analysis, which are commonly used in experimental NIR spectroscopy (chemometrics, two-dimensional (2D) correlation spectra) are also overviewed. Again, these deliberations are based directly on the most recent reports published in the field.

Spectroscopic and Quantum Mechanical Calculation Study of the Effect of Isotopic Substitution on NIR Spectra of Methanol

In this work, we studied methanol and its deuterated derivatives (CH3OH, CH3OD, CD3OH, CD3OD) by NIR spectroscopy and anharmonic quantum chemical calculations. Vibrational bands corresponding to up to three quanta transitions (first and second overtones, binary and ternary combination modes) were predicted by the use of the VPT2 route. The accuracy of prediction of NIR modes was evaluated through density functional theory (DFT) with selected density functionals and basis sets. On the basis of the theoretical NIR spectra, detailed band assignments for all studied molecules were proposed. It was found that the pattern of bands in NIR spectra of deuterated methanols can be used for identification of isotopically equalized forms. Calculations of NIR spectra of all possible forms of CXXXOX (X = H, D) molecules demonstrated that the isotopic contamination can be identified due to a coexistence of bands specific to OH and OD groups. Also, bands from partially deuterated methyl groups can be distinguished in NIR spectra. Since the VPT2 framework is known to be sensitive to inaccuracy in the case of highly anharmonic modes, we obtained an independent insight by numerical solving of the time-independent Schrödinger equation corresponding to the O-X stretching mode scanned within -0.4 to 2.0 Å over a dense grid of 0.005 Å. This way the energies of vibrational levels of the CX1X2X3OX4 (X = H, D) isotopomers and the corresponding transition frequencies were obtained with high accuracy (<0.1 cm-1). The change in normal coordinate influences the reduced mass of the oscillator and thus its frequency. Our results lead to a conclusion that the effect of deuterization of the methyl group introduces a very specific and consistent frequency shift of the first overtone of the O-X stretching mode depending on the substitution of X1, X2, or X3 positions (<2 cm-1). However, the pattern of this shift is not reproduced accurately and is also largely overestimated by VPT2 calculations.

Spectra-structure correlations in NIR region: Spectroscopic and anharmonic DFT study of n-hexanol, cyclohexanol and phenol

We investigated near-infrared (7500-4000 cm-1) spectra of n-hexanol, cyclohexanol and phenol in CCl4 (0.2 M) by using anharmonic quantum calculations. These molecules represent three major kinds of alcohols; linear and cyclic aliphatic, and aromatic ones. Vibrational second-order perturbation theory (VPT2) was employed to calculate the first overtones and binary combination modes and to reproduce the experimental NIR spectra. The level of conformational flexibility of these three alcohols varies from one stable conformer of phenol through four conformers of cyclohexanol to few hundreds conformers in the case of n-hexanol. To take into account the most relevant conformational population of n-hexanol, a systematic conformational search was performed. Accurate reproduction of the experimental NIR spectra was achieved and detailed spectra-structure correlations were obtained for these three alcohols. VPT2 approach provides less reliable description of highly anharmonic modes, i.e. OH stretching. In the present work this limitation was manifested in erroneous results yielded by VPT2 for 2νOH mode of cyclohexanol. To study the anharmonicity of this mode we solved the corresponding time-independent Schrödinger equation based on a dense-grid probing of the relevant vibrational potential. These results allowed for significant improvement of the agreement between the calculated and experimental 2νOH band of cyclohexanol. Various important biomolecules include similar structural units to the systems investigated here. A detailed knowledge on spectral properties of these three types of alcohols is therefore essential for advancing our understanding of NIR spectroscopy of biomolecules.

Advances in Anharmonic Methods and Their Applications to Vibrational Spectroscopies

In this chapter, an overview of anharmonic time-independent approaches and their applications to vibrational spectroscopy will be presented. Attention to their significance for near-infrared (NIR) spectroscopy and studies on complex molecules in condensed phase will be paid. The benefits that are offered by NIR studies and particular difficulties that emerge in case of reproduction of NIR spectra due to the treatment of non-fundamental modes will be highlighted. A short introduction to available anharmonic methodologies, directly overviewed on the basis of the most recent reports in the field, will be presented. An exceptional possibility of elucidation of physicochemical properties, intermolecular interactions, hydrogen bonding, and solvent effects, through investigations of infrared and near-infrared modes, will be examined. Next, recent achievements allowing for accurate and efficient reproduction of experimental spectra in the entire NIR region will be presented. The perspectives for major advances approaching in the field of NIR spectroscopy due to the recent advances in anharmonic theoretical approaches will finally be deliberated.