Correlations between Structure and Near-Infrared Spectra of

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Correlations Between Structure and Near-Infrared Spectra of Saturated and Unsaturated Carboxylic Acids. An Insight From Anharmonic DFT Calculations Justyna Grabska, Mika Ishigaki, Krzysztof Bernard Be#, Marek Janusz Wojcik, and Yukihiro Ozaki J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b02053 • Publication Date (Web): 17 Apr 2017 Downloaded from http://pubs.acs.org on April 19, 2017

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The Journal of Physical Chemistry

Correlations Between Structure and Near-Infrared Spectra of Saturated and Unsaturated Carboxylic Acids. An Insight From Anharmonic DFT Calculations

Justyna Grabska*1,2, Mika Ishigaki1, Krzysztof B. Beć1, Marek J. Wójcik3 and Yukihiro Ozaki*1

1

Department of Chemistry, School of Science and Technology, Kwansei Gakuin University Sanda, Hyogo 669-1337, Japan

2

Faculty of Chemistry, University of Wroclaw, F. Joliot-Curie 14, 50-383 Wrocław, Poland 3

Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland

Corresponding Authors. Email: [email protected] Email: [email protected] 1 ACS Paragon Plus Environment


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Abstract By near-infrared (NIR) spectroscopy and anharmonic 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.

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1. Introduction Over the last decade near-infrared (NIR) spectroscopy grew in importance throughout a wide field of chemical sciences.1,2,3,4,5,6,7,8,9,10,11,12 A variety of factors contributed here, i.e. developments of instruments, particularly the keystone introduction of NIR hyperspectral imaging,13,14 and simultaneous progress in the area of spectral data analysis.1-15 Compared to infrared (IR) spectroscopy, NIR spectroscopy offers

unique

advantages, increasing its potential for physicochemical as well as applications. 20 ,21,22,23, 24, 25 ,26,27, 28, 29 NIR bands investigations originate from forbidden transitions and the absorptivity of organic samples is usually low enough for feasible measurements in the bulk phase.1-3 Convenient optical path lengths are adequate unlike for IR region, which usually requires the use of thin films, 3,5, 16, 17,18,19

dissolvement, or ATR technique. A NIR spectrum strongly articulates bands due to functional groups containing a light atom, i.e. X-H vibrations that have high anharmonicity 30 , 31 . It provides rich information about intra- and intermolecular interactions, since bands arising from monomers often remain well separated from those of dimers and higher associates.31 Thus, NIR bands of various forms can often be detected separately.1-3 All above aspects contribute to the advantages and uniqueness of NIR spectroscopy in a wide field of basic research and applications. On the other hand, NIR spectroscopy suffers from a major drawback in the form of intrinsic complexity of the spectral data, and the correlations with structural features are not so well-understood as in the region of fundamental vibrations. This has led to the extensive use of statistical based analysis in NIR spectroscopy. The same reasons increase the value of quantum mechanical methods for interpretation of NIR spectra. However, the calculations of overtones and combinations are more challenging than those of fundamental modes. Going beyond quantum harmonic oscillator approximation involves a substantial computational effort. When aiming at medium-sized biomolecules or hydrogen-bonded complexes, this factor can often be decisive. There are few theoretical approaches allowing for the description of molecular vibrations beyond the double-harmonic approximation; vibrational self-consistent field (VSCF), 32 , 33 , 34 , 35 and its correlation-corrected derivative (CC-VSCF), 36 vibrational configuration interaction (VCI) 37 , 38 , 39 , 40 , vibrational coupled-cluster (VCC). 41 , 42 Deperturbed/generalized vibrational second-order perturbation theory (DVPT2/GVPT2)43 has been evidenced to offer a good balance between the general accuracy and affordability, making it suitable for studies in the NIR region,44,45 including medium-sized molecules27 and hydrogen-bonded systems.46 Physical chemistry of carboxylic acids has been deeply explored for multiple reasons.18,23,44,46,47,48,49 As one of the major biological substances, fatty acids and their properties have been a keen interest of scientific investigations, where vibrational spectroscopy is frequently involved. Even basic carboxylic acids such as propionic acid 3 ACS Paragon Plus Environment


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or butyric acid are essential in physiological processes;50,51 all kinds of fatty acids from short-chain to long-chain ones appear in metabolic pathways, and are of relevance to physiology, microbiology, biochemistry and medicine.52,53,54 Knowledge about simple fatty acids increases our understanding of complex biomolecules containing a carboxyl group.55 One of the main focuses of state-of-the-art NIR spectroscopy is spectral imaging of biological samples. 56 , 57 , 58 Recent reports on in-situ and non-invasive monitoring of fish embryo growth by NIR imaging give new insights into the factors determining the fertility rate and allow to follow the metabolism of an ovum13,14. One of important aspects of such studies is the distribution of saturated and unsaturated fatty acids in an egg. A good understanding of spectroscopy of carboxylic acids is of importance also in these kinds of studies. Moreover, investigations have been aimed at the nature of the double hydrogen-bond emerging between carboxyl groups of cyclic dimers of carboxylic acids.46,59,60,61,62,63,64,65 IR and Raman spectroscopies have often been engaged for elucidating the properties of hydrogen-bonded species.66,67,68,69,70,71,72,73,74,75 The IR and Raman studies have frequently been supported by quantum chemical calculations.72,76,77,78 NIR spectroscopy proved to be able to deliver unique insights as well, i.e. by being able to monitor the coexistence of associated and non-associated species, providing information complementary to the one extracted from IR spectra.64, 79, 80, 81, 82, 83, 84 Noble gas-matrix isolation studies and quantum chemical calculations have successfully been used, i.e. by Akai et al. uncovering details on anharmonicity and dimerization of acetic acid.85 Carboxylic acids were somewhat challenging systems for such studies, due to the complexity of NIR spectra. Unlike IR and Raman studies, NIR investigations have never received any substantial support from quantum chemical calculations for the reasons explained above. With the advances in the field of anharmonic computational approaches, many open questions can be attempted to be answered; i.e. pyruvic acid has been studied by Reva et al. in 201444; however they have focused on the low-temperature matrix isolation data. Only recently more detailed studies on the influences of hydrogen-bonding on the corresponding NIR data in solution have been reported.64 In the present work we aim to study NIR spectra of carboxylic acids with saturated or unsaturated aliphatic chain, containing three and four carbons. Propionic acid (propanoic acid), butyric acid (butanoic acid) and acrylic acid (prop-2-enoic acid), crotonic acid (but-2-enoic acid) and vinylacetic acid (but-3-enoic acid) form representative groups of saturated and unsaturated carboxylic acids, respectively (Figure 1a-e). Anharmonic calculations by means of GVPT2/DVPT2 scheme on DFT level will be employed for the interpretation of experimental data. The correlations between the structure and the spectra will be focused on, with particular emphasis on the differences between saturated and unsaturated alkyl chains. Moreover, the 4 ACS Paragon Plus Environment

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dimerization of carboxylic acids through hydrogen-bondings and its impact on the respectful NIR spectra will be studied as well. NIR baseline elevation due to the self-association of carboxylic acids will be compared with our conclusions reported for acetic acid.46 The conclusions drawn here will give physicochemical insights into the properties of carboxylic acids, forming valuable support to applied NIR studies of biological samples.13,14

Figure 1. Molecular structures of the major conformational isomers of the studied carboxylic acids: (a) propionic acid; (b) butyric acid; (c) acrylic acid; (d) crotonic acid; (e) vinylacetic acid.

2. Materials and methods 2.1. Experimental The studied chemicals of high purity were purchased from Wako Pure Chemical Industries Japan (carbon tetrachloride, Infinity Pure, min. 99.9%; propionic acid, acrylic acid, butyric acid, crotonic acid, min. 98%) and from Sigma-Aldrich (vinylacetic acid, 97%). The liquid samples were dried by freshly activated molecular sieves (Wako Pure Chemical Industries Japan, 4Å pore size) and stored under nitrogen. Solid crotonic acid was stored in a refrigerator. NIR spectra of the carboxylic acids were measured in CCl4 solutions in a wide range of concentrations (5·10-4 M - 0.05 M) in rectangular quartz cells of 10 mm and 100 mm of optical paths. The spectrometer used in this study was a Perkin Elmer Spectrum One NTS FT-NIR device operating in a transmittance mode. The spectra were collected in the 10,000 – 4000 cm-1 region, with an 4 cm-1 spectral resolution and an interpolated data spacing of 1 cm-1; 256 scans were accumulated every time and each measurement was repeated 3 times, preceded with a background collection; each time the relevant cell filled with solvent was included in the background data. All spectra were acquired at controlled temperature of 298 K. As concluded from the repeated measurements, high stability of experimental conditions was maintained during the data collection. Subsequently, the measured spectra were converted to absorbance scale and minor background fluctuations were normalized by a linear offset of the spectra at 9,000 cm-1, in a region where no meaningful absorption was evidenced. The spectra measured for very low concentrations were additionally smoothed based on the algorithm by Savitzky and Golay.86 2.2. Computational details A systematic conformational analysis of investigated carboxylic acids was carried out as a preliminary step in the computational procedures. An initial scan of potential 5 ACS Paragon Plus Environment


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energy surface was performed about each relevant C-C bond on B3LYP/6-31G87 level in aim to elucidate the local minima. The detailed data derived from the conformational analysis can be found in the Supporting Information. The approximated structures obtained this way were subsequently fully optimized on B2PLYP/def2-TZVP88,89 level, with very tight convergence criteria, ultrafine integration grid, integral accuracy of 10-12, and ultrafine grid for solving coupled perturbed Hartree-Fock (CPHF) equations. With the exceptions of propionic acid and butyric acid, all the resolved conformational isomers were used in the subsequent anharmonic calculations. For propionic acid the cis conformer has less than 1% of calculated abundance. In case of butyric acid, the conformational analysis revealed that six stable, but high energy conformers, have total calculated abundance of 4.5% (with 1.6% of the leading one in this group) and are not likely to influence NIR spectra; therefore these structures were excluded from further studies. Unless stated otherwise, throughout this work B2PLYP double-hybrid functional was used with unfrozen core approximation, meaning that all electrons have been included in correlation calculations. Additionally, the empirical correction for dispersion by means of Grimme’s D3 scheme 90 with Becke-Johnson damping (GD3BJ)91,92,93 and self-consistent reaction field (SCRF)94,95 with polarizable conductor calculation (CPCM)96 solvent model of CCl4 were applied. The resulting optimized structures were subsequently used for anharmonic vibrational analysis by means of GVPT2 calculation scheme; vibrational transitions up to two-quanta have been derived. Standard two-point differentiation for calculation of higher order derivatives was employed within GVPT2 calculations. Boltzmann coefficients97 corresponding to each conformational isomer were obtained from Gibbs free energies calculated at 298 K with additional correction by anharmonic zero-point energy (ZPE) value. The cyclic dimeric structures were obtained by combining all the previously resolved conformational isomers. In the case of cyclic dimers, the geometry optimization and subsequent anharmonic vibrational analysis were carried out on a B3LYP/SNST level of electronic theory. All quantum chemical calculations were carried out in Gaussian 09 Rev. E.01 software.98 The obtained vibrational energies and intensities corresponding to the near-infrared modes, first overtones and binary combination modes, were subsequently used for the modelling of an NIR spectrum for each conformational isomer; for convolution procedure Lorentz-Gauss (Cauchy-Gauss) product function (eq. 1) was employed as a bandshape model. A(ν ) =

a1 1 + a (ν − a3 ) 2 2

2

(

2

× exp − a42 (ν − a3 )

)

(1)

In eq. 1, the parameters a1 and a3 are, respectively, the intensity and wavenumber, obtained in the quantum chemical calculations. The remaining shape parameters, a2 and a4, were set as 0.075 and 0.015, following the best agreement with the 6 ACS Paragon Plus Environment

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experimental bandshapes. The above choices of parameters resulted in full-width at half-height parameter of modelled bands of 26 cm-1. The chosen bandshape model has been evidenced in the literature to simulate very well the broadening mechanisms of vibrational bands of molecules in liquid phase, i.e. is supreme to pure Gauss spectral profile. 99, 100 On the other hand, as it will be demonstrated in the Results and Discussion Section, the present work needed to take into account a substantial number of overlapping theoretical bands with largely varying intensities. In such case, Lorentz-Gauss product spectral profile does not tend to influence the neighbouring bands as much as either Lorentz or Voigt profiles, which both decay less rapidly at the band shoulders (Figure 2). However, the choice of the spectral profile itself should not be of major importance for the agreement between theoretical and experimental spectra (Figure 2). The final respective theoretical NIR spectra were obtained as a Boltzmann-weighted sum of the spectra of conformational isomers. The computational approaches applied in the present work have been previously evaluated in our earlier reports.45,46 The choice of density functional, basis set, solvent model and their impact on the calculated NIR spectra were therefore based on our more generally oriented studies involving small molecules.45,46 Additionally, the computational approach chosen by us for the purpose of this study yields data on the first overtones and binary combination modes. However, this approximation is largely sufficient to accurately reproduce NIR spectra of organic molecules in solution phase. This has been evidenced in our recent reports, with the most notable examples of butyl alcohols,101 but also other systems.45,46

Figure 2. The details of the spectral profile (Lorentz-Gauss product) chosen for modelling of NIR bands in comparison with other profiles frequently used in the literature. For the Lorentz-Gauss product, a2 parameter is 0.075; a4 is 0.015 (details in the text). For all provided examples: the position of peak maximum is 0, peak height 1, full-width at half-maximum 26 (arbitrary units).

The effect of the spectral baseline increase due to broadening of the specific combination bands involving stretching and bending modes of the double hydrogen-bonded bridge of the respective cyclic dimers has been accounted in the modelled NIR spectra through band fitting procedure. The band fitting of the two groups of bands has been performed as a least-square minimization by employing Powell gradientless optimization procedure. The details and results of the fitting procedure are presented in the Supporting Information.



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3. Results and discussion 3.1 Dimerization of carboxylic acids and NIR spectral features corresponding to hydrogen-bonding formation As is well-known, carboxylic acids tend to form a double hydrogen-bonded cyclic dimers. In a non-polar solvent this tendency is further strengthened, and monomeric species can only be dominant in high dilution.46 Similar conclusion should be stated for the compounds investigated in this work (Figure S1 in Supporting Information). In a highly diluted CCl4 solution, NIR bands specific to monomers can be identified throughout all five carboxylic acids (Figure S1 and Figure S2a-e in Supporting Information). By comparing the experimental spectra with the theoretical ones calculated for the respective monomers (Figure 1), the bands specific to monomers can be identified (Figure S1 and Figure S2 in Supporting Information). Two NIR bands of monomers are notably specific ones. One of them appears in the vicinity of 6905-6918 cm-1 as a well-defined and intense first overtone band due to the free OH stretching mode (Figure S1 and Figure S2 in Supporting Information). The second one, located at around 5290 cm-1, arises from the combination of OH and C=O stretching modes (Figure S1 and Figure S2 in Supporting Information). With an increase of concentration beyond the lowest level (5·10-4 M) the formation of hydrogen-bonded cyclic dimers can be well evidenced in NIR spectra (Supporting Information). The calculated complexation energy of the dimers studied in this work exceeds 20 kcal/mol, and is relatively uniform, within around 2 kcal/mol differences between structures (Table S8 in Supporting Information). This coincides well with what can be concluded from NIR concentration-dependent spectra; the five studied carboxylic acids demonstrate a similar level of aggregation. Closer examination of the spectra of highly diluted samples (Figure S1-S2 in the Supporting Information) reveals, that the dimerization of these molecules already happens even at the concentration level of 10-4 M. This remains in full agreement with earlier experimental reports on carboxylic acids, i.e. cis-9-octadecenoic acid in CCl4, with a degree of dissociation of 22 to 35% (10-2 to 10-3 M) at 323 K indicated by Iwahashi et al.102 Dimerization of carboxylic acids has been studied by NIR spectroscopy before.18,23,24,46,49,61,63-66,68,71,74-76,78,80,81,83,84 The corresponding spectral features observed in the NIR region are different from those in the IR region.103 For example, the baseline elevation and the appearance of a very broad feature below 6000 cm-1 has been monitored by NIR spectroscopy.84 This phenomenon can consistently be observed in the NIR spectra of the carboxylic acids studied in this work at concentration levels higher than around 5·10-4 M (Figure 3). The origin of this prominent feature of carboxylic acids has not been explained fully. In the previous study of the dimerization 8 ACS Paragon Plus Environment

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of acetic acid, we reported that the baseline elevation may be influenced by the broadening of the two kinds of a combination band of the hydrogen bonded bridge in a cyclic dimer46. The results presented here remain in agreement with those earlier ones; the calculated NIR spectra of the respective cyclic dimers (Boltzmann averaged over all conformational isomers) compared with the experimental ones lead to a similar conclusion. The same two types of combination bands as those in the acetic acid dimer have very high calculated intensities, orders of magnitude higher than other bands in NIR region. No clear trace of these bands, manifested as separated spectral features can be observed in the experimental spectra, regardless of the concentration. These combination bands (a+b, and a+c) involve the following modes of the hydrogen-bonded bridge: (a) out-of-phase (or opposite phase) stretching and (b) in-plane bending modes, each time combined with (c) in-phase stretching mode. The relevant internal coordinates corresponding to these vibrational modes are presented in Figure 4a-c. Further studies need to be aimed at explaining the nature of the observed redshift and broadening of the combination bands involving these particular modes of the hydrogen-bonded bridge (Figure 4). The formation of A-H···B hydrogen-bond brings very well studied effects on IR spectra.104,105 Particularly, the anharmonic coupling of the high frequency B···H stretching mode with low-frequency AB stretching mode, yields well-known redshift and significant broadening of the B···H stretching bands.105 Also other manifestations of hydrogen-bonding in IR spectra, including complex behaviour of molecules in crystalline solid phase, were studied extensively.106 Yet, very little is known about the possible analogous effects on the non-fundamental vibrations, and their expression in NIR spectra. Primarily, the selection rules deciding which bands undergo the shift and broadening remain unclear. It is also important to note, that the baseline elevation below around 6000 cm-1 is very common for a variety of samples. Whether these effects share the vibrational mechanism responsible for their existence, that is the broadening of specific combination bands, yet remains to be explained. At the moment we are unable to provide further insights into these concerns, other than those discussed above. Figure 3. Experimental NIR spectra of carboxylic acids studied in CCl4 solutions at a moderate concentration level (0.05 M).

Figure 4. The relevant vibrational modes of hydrogen-bonded OHO bridge in a cyclic dimer of a carboxylic acid (on the example of the dimer of vinylacetic acid); A: opposite-phase OH stretching; B: in-phase stretching; C: OH bending. The animated file presenting displacements of the atoms is available in the Supporting Information.



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When these very intense bands are modelled with the shape parameters common to all theoretical bands (refer to Section 2.2 for the details of the procedure of convolution), they appear as outstandingly high bands with no resemblance in the experimental spectra (Figure 5a-e). Therefore, we believe that their intensity gets converted into integral intensity, through broadening. The degree of this effect is apparently high enough, that no tare of these bands as separate peaks can be seen in the experimental spectra; instead they contribute to the baseline elevation (Figure 5a-e, Figure 6a-e). In our simulated data, the number of the respective bands is higher than two in each spectrum, due to contributions arising from conformational isomers (please refer to Supporting Information for details on the structures resolved in a conformational analysis and subsequently used in a spectroscopic study). Therefore, we have performed the bandshape fitting parameters for the discussed two kinds of combination band, optimizing the band profile parameters including the wavenumber and bandwidth. We have limited the number of optimized structures to two, reflecting the two kinds of combination bands. The details and quantitative results of the fitting procedure can be found in the Supporting Information. All other theoretical bands were modelled with the parameters described in Section 2.2. Thus, we aimed at reflecting the shift and the broadening of the experimental bands, leading to the baseline elevation. While the combination bands originating from OH vibrations of hydrogen-bonded groups (cyclic dimers) are of very high intensity (Figure 5), the corresponding overtone bands are almost of negligible intensity. These finding remain in full agreement with our previous anharmonic studies on more basic carboxylic acids46 and with combined experimental NIR and computational investigations of acetic acid by Akai et al.85

Figure 5. Calculated Boltzmann-averaged NIR spectra of cyclic dimers of the five kinds of carboxylic acids investigated. in comparison with the experimental spectra measured for medium concentration (0.05 M in CCl4); (a) propionic acid, (b) butyric acid, (c) acrylic acid (d) crotonic acid, (e) vinylacetic acid.

As evidenced in Figure 3, the bands due to monomers can be still noticed at medium concentration (above 0.05 M). Therefore, we reflected this in our simulated spectra by combining the calculated spectra of monomers and dimers with weight coefficients of 0.1 and 0.9, respectively. The weight coefficient were chosen arbitrary to achieve the best agreement with the experimental spectra. Thus, the final agreement between the simulated and experimental spectra increased significantly after 10 ACS Paragon Plus Environment

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incorporating the coexistence of dimers and monomers and the baseline elevation effects, as explained above (Figure 6a-e). It is noteworthy that further increase in the concentration of the studied compounds does not lead to any sharp changes in the NIR spectra, although the baseline effect expands further. Therefore, we conclude that the broadening and frequency shift of the discussed bands are specific for a wide range of concentrations. The above findings should be applicable for other carboxylic acids, since the NIR baseline elevation is common for these compounds.66,103 Moreover, similar spectral features are known for other types of molecules as well,103 and further studies are needed to explain whether it is an effect of the broadening of combination bands specific to hydrogen-bonded complexes.

Figure 6. Final theoretical NIR spectra of the investigated carboxylic acids corresponding to experimental spectra measured in a solution of medium concentration; (a) propionic acid, (b) butyric acid, (c) acrylic acid (d) crotonic acid, (e) vinylacetic acid. Exp.: experimental (0.05 M, CCl4); I: the contribution of bands due to dimers and monomers (weighted, as explained in the text); II: the approximated contribution of bands due to the specific combination bands of dimers (details in the text); calc.: final theoretical spectra (I + II).

3.2 Contributions of overtones and combination modes into NIR spectra of saturated and unsaturated carboxylic acids The contributions from combination bands throughout the entire NIR region of all five kinds of carboxylic acids, both saturated and unsaturated ones, are much more significant than those from the first overtone bands (Figure 7a-e and Figure 8). Only in the region of 7000-6200 cm-1 the first overtone bands are relevant. The entire region below 5300 cm-1 is mainly due to binary combinations. Between 6200 – 5300 cm-1 both kinds of bands are comparatively significant. Due to populations of vibrational levels and lower probability of corresponding transitions, the contributions stemming from higher order overtones and combinations are not expected to be significant, as has been discussed recently.101 The calculated NIR spectra allow to elucidate a pronounced and consistent feature, which would be otherwise very difficult to conclude on the basis of experimental data alone. Note the spectral contributions due to overtone and binary combination bands between 6200 – 5300 cm-1, where these two kinds of contributions are of similar magnitude, as presented in Figure 7. For the saturated acids (Figure 7a-b) the contribution from overtones is relatively lower, and similar in strength to the combination bands. On the other hand, the structures involving C=C bond (Figure 7c-e) exhibit a significantly higher intensities of first overtone bands in 11 ACS Paragon Plus Environment


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this region. Consistent observations in this regard can also be made for peak positions. NIR spectra of saturated carboxylic acids (Figure 7a-b) do not have any bands of a wavenumber higher than 6000 cm-1 in this region. The above findings remain consistent, as evidenced on the example of vinylacetic acid (Figure 8). These dependencies clearly set apart the saturated carboxylic acids from the unsaturated ones. More general conclusions about the structural manifestations that can be directly observed in NIR spectra will be provided in Section 3.3.

Figure 7. The contributions of the first overtone and binary combination bands into NIR region of -1

6500 – 6000 cm of carboxylic acids studied in this work. Higher contribution of overtones in case of unsaturated acids can clearly be identified; (a) propionic acid, (b) butyric acid, (c) acrylic acid (d) crotonic acid, (e) vinylacetic acid.

Figure 8. Contributions to the calculated NIR spectrum on the example of vinylacetic acid. All band intensities are kept to scale. Note a significant level of band overlapping; magnified major regions with better view of details of calculated bands are presented as well.

3.3 Structure-spectra correlations of carboxylic acid in the NIR region. NIR spectra of carboxylic acids carry rich information about the molecular structure, including the differences between saturated and unsaturated alkyl chains. The respective spectra can be divided into three major spectral subregions, into which different types of bands contribute the most (Figure 3, Figure 9a-b, Figure 10a-c). We will highlight the most useful correlations, allowing to distinguish saturated and unsaturated carboxylic acids, and also those with a terminal C=C bond, with special attention to the role of sp2 CH2 group on the spectral features appearing in NIR region. The studied molecules contain highly specific structural fragments, which allow to draw general conclusions about more complex carboxylic acids, including long-chain fatty acids (LCFA). Propionic acid and butyric acid can be expected to be similar to saturated LCFAs, such as arachidic acid. From the three kinds of unsaturated carboxylic acids, acrylic acid has a terminal CH2 (sp2 CH2) group, and no other methyl or methylene groups. Vinylacetic acid has both kinds of CH2 groups (sp3 and sp2) in its structure, while crotonic acid does not contain CH2 groups at all. The differences between these five structures allow to pick up the spectral differences, which may be common for a wider range of more complex fatty acids.


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Figure 9. Band assignments proposed for NIR spectra of saturated carboxylic acids investigated in medium to high concentration; (a) propionic acid, (b) butyric acid. Band numbers correspond to those in Table 1.

Figure 10. Band assignments proposed for NIR spectra of unsaturated carboxylic acids investigated in medium to high concentration; (a) acrylic acid, (b) crotonic acid, (c) vinylacetic acid. Band numbers correspond to those in Table 2.

3.3.1. The region of 7000-5600 cm-1. In the vicinity of 7100 cm-1 a very weak absorption feature appears in the spectra of the five studied samples (Figure S8-S12 in the Supporting Information). The literature assigns these bands to sum combinations of the second overtone methyl or methylene stretching mode with the relevant bending mode.103 Other than that, a well-separated band originating from monomeric species appears at around 6910 cm-1 (Figure 3, Figure S8-S12 in the Supporting Information), with the position varying between these five acids (6905-6926 cm-1) as reported in Table 1 and Table 2. This band has been useful i.e. for studies on dimerization constant and its temperature dependence107,108,109,110. The remaining wavenumber region between 6300-5600 cm-1 is notably specific, and the spectral features herein can be clearly attributed to the corresponding structures. Saturated carboxylic acids, propionic acid and butyric acid, have very similar lineshape in this region. Despite heavy overlapping of individual bands resulting in a substantial broadening, in each of these two spectra four peaks can be identified, grouped by two (Figure 3, Figure 9, Table 1). The first two peaks are located at 5958, 5922 cm-1 for propionic acid (Figure 9a, Table 1), and 5911, 5883 cm-1 for butyric acid (Figure 9b, Table 1). These two peaks mainly originate from the first overtones as well as various binary combinations of asymmetric stretching modes of CH3 and CH2 groups. The following two peaks, also clearly grouped, can be identified at 5796, 5701 cm-1 (propionic acid, Figure 9a), and 5748, 5678 cm-1 (butyric acid, Figure 9b). These are mainly due to the first overtones and binary combinations of symmetric stretching modes of the respective CH3 and CH2 groups (Table 1); the influence of the overtones is lower in this case (as evidenced for vinylacetic acid in Figure 8; the level of influence of the first overtones throughout NIR region is similarly low among all studied compounds). A contribution from combination mode of symmetric and asymmetric CH2 stretching can be evidenced in this region as well. On the other hand, the NIR spectra of unsaturated carboxylic acids stand out prominently in this region. Acrylic acid (Figure 10a) and vinylacetic acid (Figure 10c) separate also from crotonic acid (Figure 10b) due to the existence of a terminal CH2 13 ACS Paragon Plus Environment


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(CH2(sp2)) group in their structure. It gives rise i.e. to a well-defined and intense band due to the first overtone of νasCH2(sp2) and a combination of νasCH2(sp2) and νCH modes. This band is located at 6172 cm-1 for acrylic acid (Figure 10a) and 6131 cm-1 for vinylacetic acid (Figure 3, Figure 10c). It does not overlap with bands of other kinds of carboxylic acids (saturated ones and crotonic acid), and thus, it can be a very reliable marker for this kind of molecular structure of carboxylic acids. The following two bands of acrylic (6008, 5942 cm-1) and vinylacetic acids (5968, 5908 cm-1), still involving CH2(sp2) vibrations, are additionally influenced by other contributions corresponding to an unsaturated alkyl chain, mainly νCH mode (Figure 10a,c). However, these bands are located in a region where they can overlap with the bands of saturated carboxylic acids. The relevant region of 6100-5900 cm-1 in the case of crotonic acid (Figure 10b) also is populated by prominent bands (νCH + νCH, 2νCH), but remains relatively less specific. It is noteworthy that the spectral features in this region, despite being redshifted by around 30-40 cm-1 in case of a longer chain (vinylacetic acid vs. acrylic acid), in their general appearance remain very similar among these two kinds of unsaturated carboxylic acids. This is a clear trace of the CH2(sp2) group (Figure 10a,c). Toward the lower wavenumbers (5900-5600 cm-1) some spectral differences can be pointed out for the studied unsaturated compounds (Figure 3, Figure 10, Table 2). While for acrylic acid this region remains flat, for the acids containing CH3 or CH2(sp3) groups the first overtones of stretching modes and combination modes of these groups yield bands in this region. The NIR spectrum of vinylacetic acid (Figure 10c) reveals a substantially broadened structure (5850-5550 cm-1); the structure primarily originates from overlapping bands due to CH2(sp3) stretching modes (first overtones of both symmetric and asymmetric stretching modes, and the respective combination modes). For crotonic acid (Figure 10b) the bands origin is similar (the CH3 group instead of the sp3 CH2 group) but broadening sets apart the region compared with the one of vinylacetic acid (Figure 10c, Table 2). This spectral feature of crotonic acid (Figure 10b, Table 2) due to a CH3 group, which is the only unsaturated carboxylic acid involving such structural motif here, remains in agreement with previous experimental observations made for natural fatty acids13,14. In these studies, the NIR spectral region, in which molecular vibrations involving CH3 group contributes the most specifically, has been concluded through analysis of biological membranes identified in NIR hyperspectral imaging data. The present results confirm those earlier findings. The relevant contributions stem from stretching modes of CH3 group; both first overtones and combinations, and both symmetric and asymmetric modes. Moreover, at 5638 cm-1 a band due to in-phase stretching mode of the hydrogen bonded bridge, combined with CH stretching mode appears (Figure 3, Figure 10, Table 2). This band is located at wavenumber notably lower than any of the bands of saturated species. 14 ACS Paragon Plus Environment

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Despite differences in wavenumbers, the overall appearance of the NIR spectra within these two groups of carboxylic acids (saturated and unsaturated ones) is very consistent in the region of 6300-5600 cm-1; along with the low sensitivity to the aggregation as discussed above, this makes the region strongly specific to the molecular structure of carboxylic acids (Figure 3, Figure 9, Figure 10). Clear trace of saturated or unsaturated alkyl chain, and particularly the appearance of a CH2(sp2) group can be reliably attributed to NIR bands in this region. The appearance of bands due to CH3 and CH2(sp3) stretching modes is also distinctively different from the corresponding bands of saturated acids. Therefore, this region should be useful for NIR studies of biological samples involving long-chain fatty acids. 3.3.2. The region of 4800-4450 cm-1. The second NIR region specific to the molecular structure of carboxylic acids is 4800-4450 cm-1 (Figure 3). Above 4550 cm-1 propionic acid and butyric acid show a similar broadened structure of medium intensity. The major contribution here stems from combinations of stretching modes within the carboxyl group (OH, C=O, C-O) as presented in Table 1. The NIR spectrum of crotonic acid shares to a degree the appearance in this region, with broadened structure of overlapping weak bands. However, the level of overlapping is relatively lower and the origins of the bands are clearly different (combinations of C=C stretching with methyl deformations, and CH stretching). Moreover, toward the higher wavenumber around 4764 cm-1 where no band of the saturated acids exists, a combination band due to C=C and CH stretching modes stands out (Figure 3). However, due to low absorptivity in this region, the difference between saturated acids and crotonic acid remains subtle. On the other hand, the structures with a CH2(sp2) group again give rise to pronounced and specific spectral features in this region. Of note is that both acrylic acid and vinylacetic acid have an intense and well-separated band, at 4746 cm-1 and 4734 cm-1 respectively, arising from a combination of C=C, C=O stretching and asymmetric stretching of CH2(sp2) modes (Figure 3). Another specific NIR band, even more intense and similarly separated one, can be observed at 4483/4489 cm-1 for acrylic and vinylacetic acid respectively (Figure 3, Table 1). This band arises from a combination mode involving scissoring and asymmetric stretching vibration of CH2(sp2) group. Again, these two bands can be treated as clear indicators of the existence of CH2(sp2) moiety in the molecular structure of carboxylic acid, with a high specificity rivalling the one of 6172/6131 cm-1 band (Figure 3, Table 1). 3.3.3. The region of 4450-4000 cm-1. The immerse number of contributing combination bands, and the resulting significant overlapping (Figure 8; intensities of the contributing individual bands and 15 ACS Paragon Plus Environment


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the resulting envelope are presented in the common scale), make this region highly compound-specific, resembling the “fingerprint region” in IR region (Figure 3, Figure 9, Figure 10). Nevertheless, saturated acids seem to rather follow a similar way of grouping of bands, leading to an appearance of three groups of bands with similar relation between intensities among propionic acid and butyric acid. As the differences in the structural features of the examined unsaturated acids are more principal, we conclude that this region carries a substantial amount of structural information, and thus is very useful for sample identification.

Table 1. Band assignments proposed for NIR spectra of saturated carboxylic acids in medium to high concentration (Figure 9).

Table 2. Band assignments proposed for NIR spectra of unsaturated carboxylic acids in medium to high concentration (Figure 10).

4. Conclusions NIR spectroscopy is a powerful tool for non-invasive monitoring of biological processes, even in living organisms. However, due to the complexity of anharmonic effects, our general understanding of NIR spectra is still far from satisfactory. With recent advances in the field of anharmonic quantum chemical methods it has become possible to simulate NIR spectra of complex molecules with an adequate reliability. This makes it a highly capable tool to study correlations between spectral features and structural properties of molecules also in the NIR region. In this work we studied five carboxylic acids, two with a saturated alkyl chain (propionic acid and butyric acid), and three with an unsaturated one (acrylic acid, crotonic acid and vinylacetic acid). These molecules possess structural differences which are distinctly reflected in their NIR spectra. We have reproduced NIR spectra of these carboxylic acids by anharmonic DFT calculations, for very low concentration – where monomers can be observed – and for moderate to high concentration – where cyclic dimers dominate. In case of the latter, which is more relevant to biological studies, the calculated spectra took into account the prominent baseline elevation due to the redshift and broadening of the combination bands involving specific modes of the hydrogen-bonded bridge. The final simulated NIR spectra resembled the experimental ones well and the calculated data reflected the main spectral features of carboxylic acids, including the dimer-specific bands and the baseline elevation observed in the experimental NIR spectra. Based on the results of the theoretical study, it was possible to clearly differentiate 16 ACS Paragon Plus Environment

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the saturated carboxylic acids from the unsaturated ones, throughout several spectral subregions in NIR. The characteristic discrepancy in contributions stemming from overtones and combinations in the saturated/unsaturated compounds could be pointed out. The spectral contribution of CH3 group could be elucidated and remained with full agreement with previous assumptions made on the basis of experimental data. Moreover, among the unsaturated compounds, the location of C=C bond has a clear and extremely prominent impact on the corresponding NIR spectrum. The existence of a sp2 CH2 group leads to the rise of very specific, well-defined and intense NIR bands, at 6172/6131 cm-1, 4746/4734 cm-1 and 4483/4489 cm-1 for acrylic/vinylacetic acids respectively. These bands are of high intensity and are located in wavenumber regions, where no overlapping with the bands of other structures is probable; this fact makes them excellent structural markers. The results and conclusions reported in this work should help to understand NIR spectra of complex long-chain fatty acids with biochemical importance.

Supporting Information This article contains Supporting Information. Additional figures, tables, structural data and details of the band-fitting procedure are included in the Supporting Information. Acknowledgement Calculations have been carried out in Wroclaw Centre for Networking and Supercomputing (http:/www.wcss.pl), under grant no. 375 This work was supported by the National Science Center Poland, grant: 2016/21/B/ST4/02102.



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Figures

Figure 1. Molecular structures of the major conformational isomers of the studied carboxylic acids: (a) propionic acid; (b) butyric acid; (c) acrylic acid; (d) crotonic acid; (e) vinylacetic acid.

Figure 2. The details of the spectral profile (Lorentz-Gauss product) chosen for modelling of NIR bands in comparison with other profiles frequently used in the literature. For the Lorentz-Gauss product, a2 parameter is 0.075; a4 is 0.015 (details in the text). For all provided examples: the position of peak maximum is 0, peak height 1, full-width at half-maximum 26 (arbitrary units). 18 ACS Paragon Plus Environment

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Figure 3. Experimental NIR spectra of carboxylic acids studied in CCl4 solutions at a moderate concentration level (0.05 M).



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Figure 4. The relevant vibrational modes of hydrogen-bonded OH···O bridge in a cyclic dimer of a carboxylic acid (on the example of the dimer of vinylacetic acid); A: opposite-phase OH stretching; B: in-phase stretching; C: OH bending. The animated file presenting displacements of the atoms is available in the Supporting Information.


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Figure 5. Calculated Boltzmann-averaged NIR spectra of cyclic dimers of the five kinds of carboxylic acids investigated. in comparison with the experimental spectra measured for medium concentration (0.05 M in CCl4); (a) propionic acid, (b) butyric acid, (c) acrylic acid (d) crotonic acid, (e) vinylacetic acid.



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Figure 6. Final theoretical NIR spectra of the investigated carboxylic acids corresponding to experimental spectra measured in a solution of medium concentration; (a) propionic acid, (b) butyric acid, (c) acrylic acid (d) crotonic acid, (e) vinylacetic acid. Exp.: experimental (0.05 M, CCl4); I: the contribution of bands due to dimers and monomers (weighted, as explained in the text); II: the approximated contribution of bands due to the specific combination bands of dimers (details in the text); calc.: final theoretical spectra (I + II). 22 ACS Paragon Plus Environment

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Figure 7. The contributions of the first overtone and binary combination bands into

NIR region of 6500 – 6000 cm-1 of carboxylic acids studied in this work. Higher contribution of overtones in case of unsaturated acids can clearly be identified; (a) propionic acid, (b) butyric acid, (c) acrylic acid (d) crotonic acid, (e) vinylacetic acid.



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Figure 8. Contributions to the calculated NIR spectrum on the example of vinylacetic acid. All band intensities are kept to scale. Note a significant level of band overlapping; magnified major regions with better view of details of calculated bands are presented as well.


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Figure 9. Band assignments proposed for NIR spectra of saturated carboxylic acids investigated in medium to high concentration; (a) propionic acid, (b) butyric acid. Band numbers correspond to those in Table 1. 25 ACS Paragon Plus Environment


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Figure 10. Band assignments proposed for NIR spectra of unsaturated carboxylic acids investigated in medium to high concentration; (a) acrylic acid, (b) crotonic acid, (c) vinylacetic acid. Band numbers correspond to those in Table 2.



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Tables Table 1. Band assignments proposed for NIR spectra of saturated carboxylic acids in medium to high concentration (Figure 8). Wavenumber / cm-1

Band number

Experimental

Band assignment

Calculated

(a) propionic acid 1

6911

6927

2νOH (m)

2

5958

5922

2[νasCH3, νasCH2]; νasCH3 + [νas’CH3, νasCH2]

5884

νasCH3 + νasCH3; [νas’CH3, νasCH2] + [νas’CH3, νasCH2];

3

5922

νasCH2 + [νas’CH3, νasCH2] 4

5796

5780

νsCH3 + νsCH2; 2νasCH2

5

5701

5710

νsCH2 + νasCH2

6

5504

5476

7

5410

5389

νbridge(in-phase) + νsCH3 2νbridge(opp.-phase)

8

5294

5300

νC=O + νOH (m)

9

4822

4794

δipOH, δwaggCH2+ νOH

10

4692

4670

[νC=O, δipOH] + [νas’CH3, νasCH2]

11

4667

4607

[νC=O, δipOH] + [νas’CH3, νasCH2]

12

4563

4547

[νC=O, δipOH] + νsCH3;

13

4431

4403

δas’CH3 + νasCH3; δscissCH2 + [νas’CH3, νasCH2]; δas’CH3 + [νas’CH3, νasCH2]

14

4359

4238

δipOH + νbridge(opp.-phase);

15

4240

4224

νasCH3 + νbridge(opp.-phase); νas’CH3 + νbridge(opp.-phase); δwaggCH2 + [νas’CH3, νasCH2]

16

4194

4156

[δrockCH2, δrockCH3] + νsCH3

17

4067

4031

δwaggCH2 + νbridge(opp.-phase)

18

4018

4002

1

6908

6919

2

5911

5875

3

5883

5832

4

5748

5795

[δrockCH2, δrockCH3] + νbridge(in-phase); [δrockCH2, δrockCH3] + νasCH2

(b) butyric acid 2νOH (m) 2νasCH3; [νasCH2, νas‘CH3] + νasCH2; 2νas‘CH3;

νasCH2 + νas‘CH3 νsCH2 + νsCH3 28

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5

5678

5749

νsCH2 + νasCH2

6

5288

5292

νC=O + νOH, νasCH2 + νOH (m)

7

4680

4648

νC=O + νasCH2

8

4629

4585

δsCH3 + νOH, νCO+νOH (m)

9

4562

4532

νbridge(opp.-phase) + νsCH2

10

4386

4380

11

4342

4328

[δscissCH2, δasCH3] + [νasCH2, νas‘CH3]; [das‘Ch3] + [νasCH2, νas‘CH3]; [δipOH, δscissCH2] + [νasCH2, νas‘CH3]; dsCH3 + νasCH2; [νC=O, δipOH] + [νasCH2, νas‘CH3];

12

4316

4303

13

4260

4256

δsCH3 + νsCH3; δscissCH2 + νasCH2; δsCH3 + [νasCH2, νas‘CH3]; νC=O + νbridge(in-phase) δrockCH2 + δsCH3;

14

4183

4172

δipOH + [νasCH2, νas‘CH3]; δrockCH2 + [νasCH2, νas‘CH3];

15

4155

4141

δrockCH2+ νsCH2 [δrockCH2, δrockCH3] + [νasCH2, νas‘CH3];

16

4079

4038

[δwagg CH2, δipOH] + νbridge(opp.-phase); [δrockCH2 δrock’CH3] + νsCH2

17

4051

[δwaggCH2, δrockCH3] + νsCH2;

4023

[δscissCH2, δipOH] + νbridge(in-phase)



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Table 2. Band assignments proposed for NIR spectra of unsaturated carboxylic acids in medium to high concentration (Figure 10). Wavenumber / cm-1

Band number

Experimental

Band assignment

Calculated

(a) acrylic acid 1

6920

6922

2νOH (m) 2νasCH2(sp2);

2

6172

νCH + νasCH2(sp2); νasCH2 (sp2) + νasCH2 (sp2)

6173,6142

νsCH2 (sp2) + νasCH2(sp2); 3

6008

νsCH2 (sp2) + νsCH2(sp2);

6060

νasCH2(sp2) + νCH 4

5942

6017

2νCH

5

5286

5275

νC=O + νOH (m)

6

4746

4752

7

4726

4738

[νC=C, νC=O] + νasCH2(sp2);

8

4663

4688

[νC=C, νC=O] + νsCH2(sp2);

9

4611

4660

10

4536

4516

δscissCH2(sp2) + νasCH2(sp2);

11

4483

4494

δscissCH2(sp2) + νasCH2(sp2);

12

4399

4406

[δipCH, δipOH] + νasCH2(sp2);

13

4388

4375

δipCH + νasCH2(sp2);

[νC=C, νC=O] + νasCH2(sp2); [νC=C, νC=O, δipOH] + νCH

[νC=C, νC=O, δipOH] + νsCH2(sp2); [νC=C, νC=O, δipOH] + νCH;

[νC=C, νC=O, δipOH] + νbridge(opp.-phase); 14

4347

4308

15

4312

4267

16

4228

4204

17

4171

4165

18

4148

4149

19

4115

4109

20

4055

4078

δipCH + νsCH2(sp2); δipCH + νCH [νC=C, νC=O] + νbridge(opp.-phase);

δipCH + νsCH2(sp2); [δipCH, δrock CH2(sp2)] + νasCH2(sp2); [νC=C, νC=O, δipOH] + νbridge(in-phase); [δipCH, δrockCH2(sp2)] + νasCH2(sp2); [δipCH, δrockCH2(sp2)] + νCH; [δipCH, δrockCH2(sp2)] + νasCH2(sp2); [νC=C, νC=O] + νbridge(in-phase);. [δipCH, δrockCH2(sp2)] + νCH; [δipCH, δrockCH2(sp2)] + νsCH2(sp2);

(b) crotonic acid 30 ACS Paragon Plus Environment

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1

6926

6943

2νOH (m)

2

5974

6066

νCH + νCH

3

5910

5991

2νCH; νCH + νCH

4

5866

5944

2νCH; νCH + νCH 2νasCH3

5

5762

5851

νsCH3 + νasCH3; νasCH3 + νasCH3;

6

5728

5770

2νas‘CH3; νsCH3 + νas‘CH3

7

5638

5708

8

5343

5379

νbridge(opp.-phase) + νCH νbridge(in-phase) + νsCH3

9

5278

5263

νC=O + νOH (m)

10

4894

4905

δipOH + νOH (m)

11

4764

4728

[νC=C, νC=O] + νCH

12

4730

4695

νC=C + νCH

13

4682

4649

[νC=C, νC=O] + νasCH3

14

4606

4585

νC=C + νas‘CH3

15

4575

4561

16

4344

[νC=C, νC=O, δipOH] + νas‘CH3; [νC=C, νC=O, δipOH] + νsCH3

νC=C + νbridge(opp.-phase); δas‘CH3 + νas‘CH3; [δipOH, δipCH] + nCH;

4340

δsCH3 + νasCH3; δsCH3 + νsCH3;

17

4310

4288

18

4245

4246

δipCH + νasCH3; [δipCH, δipOH] + νCH

19

4160

4160

[δrockCH3, δipCH] + νCH

20

4140

4133

[δrockCH3, δipCH] + νCH

21

4073

4071

[δrockCH3, δipCH] + νasCH3

δsCH3 + νas‘CH3;

(c) vinylacetic acid 1

6905

6920

2νOH (m)

2

6131

6090

2νasCH2(sp2); νCH + νasCH2(sp2)

3

5968

6014

νsCH2(sp2) + νCH

4

5908

5960

2νCH; νsCH2(sp2) +

5

5721

6

5648

2νasCH2(sp3);

5880, 5761,5739

νasCH2 (sp3) + νasCH2(sp3); νbridge(opp.-phase) + νasCH2(sp2)

5652

2νsCH2(sp3); νsCH2(sp3) + νsCH2(sp3) 31

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7

5637

5622

2νsCH2(sp3)

8

5292

5295

νC=O+νOH (m)

9

4904

4860

δipOH + νOH (m)

10

4734

4746

νC=C + νasCH2(sp2)

11

4617

4643

νC=C + νsCH2(sp2)

12

4489

4481

δscissCH2(sp2, sp3) + νasCH2(sp2)

4381

δscissCH2(sp2, sp3) + νasCH2(sp3); [δwaggCH2(sp3), δipCH] + νasCH2(sp2) δscissCH2(sp2, sp3) + νasCH2(sp3); [δipCH, δipOH] + νsCH2(sp2); [δwaggCH2(sp3), δipCH] + νsCH2(sp2);

13

4382

14

4280

4315,4291

15

4198

4189

16

4092

4110

[δwaggCH2(sp3), δipCH] + νsCH2(sp3); [δrockCH2(sp2), δtwistCH2(sp3)] + νasCH2(sp2); [δrockCH2(sp2), δtwistCH2(sp3)] + νsCH2(sp2); [δwaggCH2(sp3), δipOH] + νbridge(opp.-phase);

17

4044

4032

[δipCH, δipOH] + νbridge(opp.-phase);

δscissCH2(sp2, sp3) + νbridge(in-phase);


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Correlations between Structure and Near-Infrared Spectra of

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