Temperature Drift of Conformational Equilibria of Butyl Alcohols

Article pubs.acs.org/JPCA

Temperature Drift of Conformational Equilibria of Butyl Alcohols Studied by Near-Infrared Spectroscopy and Fully Anharmonic DFT Justyna Grabska,†,‡,§ Krzysztof B. Beć,*,†,§ Yukihiro Ozaki,§ and Christian W. Huck*,† †

Institute of Analytical Chemistry and Radiochemistry, Leopold-Franzens University, Innrain 80-82, 6020 Innsbruck, Austria Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland § Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan ‡

S Supporting Information *

ABSTRACT: 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.

1. INTRODUCTION Near infrared (NIR) spectroscopy has experienced significant advancement in the recent years, since both the instrumentation and data analysis methods have been evolving constantly.1−5 NIR spectroscopy offers unique capabilities in elucidating structural information on wide range of samples, from simple molecules3,6 to complex biosystems.7 Compared to infrared (IR) spectroscopy, it benefits from simpler and more reliable construction of spectrometers and more convenient optical materials, factors which can frequently be deciding in applied studies. However, NIR spectroscopy also has numerous advantages from the point of view of basic physicochemical investigation. Compared to IR spectroscopy, it gains from different and complementary spectra-structure dependencies (i.e., stronger articulation of bands due to vibrations of functional groups and particularly X−H vibration), allows in particular for clear observation of spectral signals originating from terminal X−H groups in the polymeric species associated through X−H···B hydrogen bonding, and due to a typically low extinction coefficient of organic molecules in the NIR region, © 2017 American Chemical Society

allows a relative ease of studying association mechanisms by investigations of solutions over a wide range of concentrations without the need of using thin layers.1,3 For the reasons above, NIR spectroscopy has played an important role in physicochemical studies on aliphatic alcohols, particularly on the association mechanisms through hydrogen bondings, and the temperature-dependent shift observed for the first overtone band of OH stretching mode. The prominent 2νOH band of aliphatic alcohols has been typically used as a rich source of structural information about molecules in liquid phase, including the formation of hydrogen bonding structures with the change of concentration. For low concentration levels, where a well-defined “free” 2νOH band originating from a molecule exists, a shift of the peak maximum toward a higher wavenumber accompanied by a decline in its intensity as temperature increases can be observed experimentally. This Received: January 20, 2017 Revised: February 19, 2017 Published: February 20, 2017 1950

DOI: 10.1021/acs.jpca.7b00646 J. Phys. Chem. A 2017, 121, 1950−1961

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The Journal of Physical Chemistry A

molecules only - which severely limits their value for applied NIR studies. The methods based on vibrational second-order perturbation (VPT2) theory generally offer a good accuracy for common semirigid molecules, and their computational costs are at least an order of magnitude lower than that of VSCF method.36 On the other hand, VPT2 based approaches typically have faced a major challenge in a form of vibrational resonances, a singularity which frequently demands nonroutine solutions, consequently making them difficult to use in spectroscopic research. However, recent introduction of generalized VPT2 (GVPT2) approach,37,38 in which the problem of resonances has been successfully addressed, has opened new possibilities for NIR studies, as it became possible to calculate NIR modes with a good accuracy and moderate computational cost even for fairly complex molecules.39−41 Recently, usefulness of GVPT2/DVPT2 calculations for NIR examination of simple aliphatic alcohols6 and carboxylic acids42 have been evidenced. In the present work we aim at quantum mechanical reproduction of NIR spectra of four kinds of butyl alcohols, 1-butanol (n-butanol, 1-hydroxybutane), 2-butanol (sec-butanol, 2-hydroxybutane), isobutanol (2-methyl-1-propanol) and tert-butyl alcohol (2-methyl-2-propanol), with special attention to the influence of conformational isomerism of studied molecules to the NIR spectral patterns. The chosen objects of study differ in the number of stable conformers, from a number of 14 (1-butanol), through nine (2-butanol) and five (isobutanol) to one in the case of tert-butyl alcohol. The conformational isomerism of three of the butyl alcohols gives a good opportunity to compare spectral and structural properties of aliphatic alcohols of different kinds. Moreover, temperature dependence of the conformational equilibria of studied objects are investigated with an aid of 2D-COS analysis, which offers considerable advantages in such studies, i.e., by enhancing the spectral resolution, and has been used frequently in NIR studies of alcohols. NIR spectroscopy and 2D-COS approach have proven to deliver major advantages for investigations of the structure and dynamics of alcohols, as evidenced by numerous reports in the literature, i.e., on their molecular structure,16,43−48 the structure of hydrogen-bonding,43,49−52 the interaction with water,44,45,51,53−56 chiral discrimination,50 the association mechanisms,57−60 and finally, the influences of temperature on the above effects.44,45,49,53,57 In these extensive literature reports, NIR spectroscopy offered the advantages that we mentioned above: the notable distinction of OH stretching bands among other bands, the ability to study both associated and free species, due to coexistence of the OH stretching band arising from both bonded and nonbonded moieties, the capability of studying wide range of concentrations and also neat liquid-phase, etc. In these reports, very often the 2D-COS approach has brought notable gains, by being very sensitive to subtle differences in the spectra and increasing the spectral resolution and expanding the complex spectral changes onto a two-dimensional scale, thus providing richer information and unambiguously highlighting the direction and sequence of these changes. In this work the NIR spectra of butyl alcohols in diluted CCl4 solution will be accurately simulated by anharmonic DFT calculations. All major factors influencing the NIR spectra of the respectful molecules will be explained, and contributions stemming from conformational isomers will be elucidated. The subtle temperature-dependent spectral changes, stemming from the drift of the conformational populations, will be reproduced

phenomenon can be studied in detail, among others, by twodimensional correlation spectroscopy (2D-COS), which offers numerous advantages in this case, i.e. high spectral resolution due to expansion of perturbation driven spectral changes onto two dimensions.8 Butyl alcohols have been investigated many times,9−15 i.e., by NIR/IR spectroscopies and 2D-COS aided by harmonic DFT calculations, in which data on fundamental OH stretching transition were derived.16 An influence of two conformational isomers of 1-butanol was proposed there; internal rotations with respect to internal rotation of C−O bond have been considered,16 but at that moment no solid evidence could be provided for the proposed mechanism. Also, no quantum mechanically calculated data for NIR modes were available before. However, more recent fully anharmonic DFT studies on 2νOH band of aliphatic alcohols suggested that full conformational flexibility (including internal rotation about the C−C bonds) is manifested in the experimental NIR spectra,6 and thus needs to be taken into account as well. Good understanding of molecular structure of aliphatic alcohols in diluted phase is crucial for comprehension of the formation of a complex hydrogen-bonded structure in single phase. The conformational equilibrium of aliphatic alcohols, and in particular the internal rotation about C−O(H) bond, has attracted constant attention9,17−19 with very recent report involving pulsed jet FT microwave spectroscopy and quantum mechanical study as a good example.20 Moreover, subtle effects of hydrogen bonding leadings to the formation of various heteroclusters in a binary alcohol/water and alcohol/hydrocarbon mixtures have recently been reported; these microscale effects are believed to be expressed directly as heterogeneity of such binary systems observed experimentally by NIR spectroscopy.21,22 The coexistence of conformational isomers and the impact of temperature and concentration on conformational equilibria of contributing alcohol may play a role in the formation of such heteroclusters. NIR spectroscopy also has been well established in the field of applied studies in which alcohols were focused on,23 i.e., with very recent reports in this field.24 So far, IR spectroscopy has been a great beneficent of quantum mechanical calculations, since theoretical prediction of fundamental vibrations within harmonic approximation has become a fairly routine approach.25,26 However, this is entirely different for NIR spectroscopy, as it is concerned with nonfundamental transitions, overtones and combination modes, which are unavailable in the harmonic approximation.3 Various theoretical approaches to anharmonic approximation were introduced over the time, however, these often suffered from shortcomings impairing their usefulness in basic and applied spectroscopy. A detailed data on selected overtones can be obtained by resolving vibrational Schrödinger equation; studies utilizing this approach have been reported even recently.27−29 However, for the reproduction of entire NIR region more general approaches are preferable. Here, for example, vibrational self-consistent field (VSCF) has been available for some time now.30,31 Yet, despite substantial computational cost, VSCF often struggle to deliver accurate results on NIR modes, even when more advanced derivatives of the method, such as perturbation corrected (PT2-VSCF) approach, are used.32 More sophisticated approaches, i.e. vibrational configuration interaction (VCI)33 or vibrational coupled-cluster (VCC),34,35 scale exponentially with the number of modes, effectively making them suitable for smallest 1951


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Figure 1. B3LYP/6-31G potential energy surface of isobutanol molecule with respect to rotations about C−O(H) (O1−C2) and C−C(CH2)2 (C3− C2) bonds.

restriction of the instrument. As concluded from the repeated measurements, high stability of experimental conditions was maintained by the spectrometer and temperature control unit. The collected data were converted to absorbance scale, and minor background fluctuations were normalized by a linear offset of the spectra at 9000 cm−1, in a region where no absorption was evidenced. The assumed experimental procedures provided high quality data and no further spectral pretreatment was necessary. Generalized two-dimensional correlation analysis was performed based on the algorithm developed by Noda et al.61 Whole wavenumber region and the entire temperature range reported in the experimental details have been taken into account. As the reference spectrum both the lowest temperature and the average spectrum were used; as no significant changes between these approaches were noticed, for the further analysis the average spectrum was chosen as the reference. 2.2. Computational Details. 2.2.1. Conformational Analysis. Full conformational analysis was performed for the studied molecules. For 1-butanol and 2−butanol the case of conformational flexibility is straightforward, as there are three stable configurations (gauche+, gauche− and trans) with respect to internal rotations of each of C−C and C−O bonds. This gives 14 conformers of 1-butanol and 9 conformers of 2-butanol, after symmetry operations being taken into account additionally. For isobutanol the conformational flexibility corresponds to the internal rotations of C−O(H) and C−C(CH2)2 bonds. Therefore, for isobutanol a systematic approach with a twodimensional scan over these two rotating bonds (360 deg in 3 deg steps) has been performed at the B3LYP/6-31G level, indicating that nine structures should be considered for further optimizations (Figure 1). After higher level optimizations (as described beneath), five unique conformational isomers of isobutanol were yielded. In case of tert-butyl alcohol, all the internal rotations about C−C bonds and C−O bond lead to degenerated states, thus only one unique conformational isomer should be taken into account. The above findings correspond well to the literature data which were obtained

entirely from the DFT calculated data in theoretically predicted NIR 2D-COS patterns. This will further highlight the role of conformational flexibility of aliphatic alcohols on their spectral features. The conformational selectivity of the hydrogen bonding formations involving alcohol molecules has been recognized in the literature,44,45,51 and the present work may provide further insights into this phenomenon, including recent investigated hydrogen-bonded heteroclusters involving alcohol molecules and the resulting microheterogeneity.21,22 Therefore, we believe that our study very well fits to the current trends in the exploration of the structural and spectroscopic properties of aliphatic alcohols.

2. MATERIALS AND METHODS 2.1. Experimental Section. Butyl alcohols of very high purity were purchased from Alfa Aesar (1-butanol, anhydrous, min. 99.9%; 2-butanol, anhydrous, min. 99%; isobutanol, anhydrous, min. 99%; tert-butyl alcohol, anhydrous, min. 99.5%; all packed under argon in resealable bottles) and used without further purification. Carbon tetrachloride was obtained from Sigma-Aldrich (min. 99.9%, HPLC grade) and dried by freshly activated molecular sieves (Sigma-Aldrich, 4 Å pore size). NIR spectra of the above alcohols were measured in diluted CCl4 solutions (0.1 M) in a Hellma QS quartz cell of 10 mm optical path. The spectrometer used in this study was Büchi NIRFlex N-500 spectrometer equipped with a liquid cell accessory and working in a transmittance mode. The spectra were recorded in the 10000−4000 cm−1 region, with an 8 cm−1 spectral resolution and a resulting 4 cm−1 data spacing; 256 scans were accumulated every time, and each measurement was repeated 3 times, preceded with a background collection. This approach allowed for a full control over the repeatability of the spectra, which was concluded to be very high. The measurements were performed in controlled temperature, provided by the internal sample temperature control of the spectrometer. The measurements for temperature dependent spectra were performed for 14 different temperatures, from 300 to 336 K in 3 K increments, and for 338 K; the upper limit was due to 1952


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term being treated on the second-order perturbation (MP2) level, offer very good accuracy in yielding vibrational frequencies and intensities. B2PLYP functional paired with def2-TZVP basis set has recently been benchmarked thoroughly and it achieved a very good level of accuracy among numerous other methods in deriving harmonic frequencies.65 Anharmonic calculations have been carried out by means of GVPT2/DVPT237,38 method, yielding wavenumbers and intensities of two-quanta transitions, first overtones and binary combination modes. This approximation brings satisfactory results, as higher level overtones and combination modes as well as difference modes influence the NIR spectra of aliphatic alcohols much lesser, as it will be evidenced further in this work. We assumed a hybrid approach for anharmonic calculations, in which the anharmonic force field has been calculated on a lower level of theory; harmonic frequencies yielded on a higher level have been used as the input data for anharmonic calculations. The anharmonic calculations were carried out by employing B3LYP66 singlehybrid functional with triple-ζ SNST basis set; this method achieves very good results when used for GVPT2 calculations.67 During all calculations with the use of double hybrid B2PLYP DFT functional no frozen core approximation was applied, meaning that full electron correlation was taken into account. The entire calculations, including structure optimization, harmonic and anharmonic approximations, were carried out both in vacuum and independently repeated with application of conductor polarizable calculation model (CPCM)68 of CCl4 solvent. The agreement with experimental data was found to be better for the data obtained with incorporation of CPCM model, and thus, this data set is presented in this work and founded the base for the following discussions. The wavenumbers and vibrational intensities obtained in the DFT study were then used for modeling of the theoretical NIR spectra of all resolved conformational isomers of butyl alcohols. The band shape function used in the process was a fourparameter Cauchy−Gauss (Lorentz−Gauss) product function. The band shape function parameters were defined arbitrary for the best agreement with experimental spectra; however, they were kept uniform throughout all the bands in any given simulated spectrum. The final calculated NIR envelopes were yielded as weighted sum of the modeled spectra of conformational isomers with respect to their Boltzmann abundances (Table 1). The Boltzmann coefficients included symmetry prefactors and were obtained from relative Gibbs free energies calculated for the temperatures corresponding to the experimental conditions;69 this approach allowed obtaining temperature-dependent conformer populations of the studied alcohols. The accuracy of determination of Gibbs free energy strongly depends on the reliability of prediction of vibrational frequencies. Therefore, the zero-point corrected values of Gibbs free energy were calculated on the same level as used for the harmonic calculations (CPCM-B2PLYP/def2-TZVP) and confirmed additionally by their recalculation on CPCM-MP2/ aug-cc-pVTZ and CPCM-B2PLYP/aug-cc-pVTZ levels. Although minor differences were noticed, the temperature trends remained similar throughout the entire calculated data set. All quantum mechanical calculations were performed in Gaussian 09 Rev. D.01 software.70 The application of band shape function and subsequent derivation of Boltzmann weighted sum of the theoretical spectra of conformational isomers was performed in MATLAB (The MathWorks, Natick,

earlier on the lower level of theory and further extends these findings.9,11,12,17,62 We have assumed a straightforward naming scheme for conformational isomers of butyl alcohols. The upper cases correspond to conformations about C−C bonds, while the lower cases correspond to conformations about a C−O bond. The prim sign denotes the structures where moieties are pointed in the same direction versus the rest of the chain. The molecular structure of each conformational isomer of butyl alcohols was subsequently optimized on a higher level of theory. Density functional theory (DFT) B2PLYP/def2TZVP63,64 was applied for this purpose, with tight convergence criteria and superfine integration grid. All resolved structures were true minima with no imaginary frequencies existing. All studied structures belong to the C1 point group, with the exception of the TTt conformer of 1-butanol and the Tt conformer of isobutanol, which belong to the Cs point group. The entirety of the resolved structures (Table 1) were used for the subsequent vibrational analysis. Detailed structural data on the studied forms are provided in Supporting Information. 2.2.2. Quantum Mechanical Calculation of NIR Spectra. Harmonic frequencies were obtained on the same level of theory as the structural optimizations (B2PLYP/def2-TZVP). Double hybrid DFT functionals, i.e., B2PLYP with correlation Table 1. Conformational Isomers of Three Kinds of Butyl Alcohols: Calculated Data Based on the Results of CPCMB2PLYP/def2-TZVP Calculationsa 1-butanol

ΔG298 [kcal/mol]

ΔG298 [kJ/mol]

C [%]

TGg TGt TTg TGg′ TTt GGg GGt GGg′ GTg′ GTg GTt G′Gg GG′t GG′g 2-butanol

0.0000 0.0144 0.0477 0.0565 0.0232 0.5265 0.6162 0.7210 0.8032 0.8258 0.8321 1.3811 1.3849 2.1009 ΔG298 [kcal/mol]

0.0000 0.0604 0.1995 0.2363 0.0971 2.2028 2.5782 3.0167 3.3606 3.4552 3.4814 5.7787 5.7945 8.7902 ΔG298 [kJ/mol]

15.8 15.4 14.6 14.4 7.6 6.5 5.6 4.7 4.1 3.9 3.9 1.5 1.5 0.5 C [%]

G−t G−g+ G−g− Tg+ Tt Tg− G+g+ G+ t G+g− isobutanol

0.0000 0.0345 0.0628 0.4022 0.4612 0.4964 0.8848 0.9582 1.1433 ΔG298 [kcal/mol]

0.0000 0.1444 0.2625 1.6829 1.9297 2.0768 3.7020 4.0091 4.7837 ΔG298 [kJ/mol]

20.8 19.6 18.7 10.5 9.5 9.0 4.7 4.1 3.0 C [%]

Gg′ Gg Gt Tg Tt

0.0000 0.0540 0.0565 0.4016 0.5353

0.0000 0.2258 0.2363 1.6803 2.2396

28.3 25.9 25.7 14.4 5.7

a

Detailed data on structural parameters of the conformational isomers are presented in the Supporting Information. 1953


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The Journal of Physical Chemistry A Massachusetts, USA). The band assignments procedure was supported by potential energy distribution (PED) analysis carried out in Gar2Ped software71 with natural coordinate analysis (NCA) performed in accordance with Pulay.72

3. RESULTS AND DISCUSSION 3.1. Experimental and Theoretical NIR Spectra of Butyl Alcohols. All the theoretical spectra presented here are based on the results obtained on B3LYP/SNST level with B2PLYP/def2-TZVP harmonic frequencies used as the input data; both parts were calculated with CPCM solvent model of carbon tetrachloride. No wavenumber scaling was applied and all spectra presented in the entire article are based on the raw results of vibrational frequencies obtained through GVPT2 calculations. As shown in Figure 2−Figure 5, the experimental NIR spectra could be fully reproduced for all four kinds of studied Figure 3. Experimental and calculated NIR spectra of 2-butanol. Boltzmann weighted contributions of conformational isomers are presented as well. For band assignments, only summarized major contributions of vibrational modes are presented here. Refer to Supporting Information for detailed assignments.

Figure 2. Experimental and calculated NIR spectra of 1-butanol. Boltzmann weighted contributions of conformational isomers are presented as well. For band assignments, only summarized major contributions of vibrational modes are presented here. Refer to the Supporting Information for detailed assignments. Figure 4. Experimental and calculated NIR spectra of isobutanol. Boltzmann weighted contributions of conformational isomers are presented as well. For band assignments, only summarized major contributions of vibrational modes are presented here. Refer to Supporting Information for detailed assignments.

alcohols; this includes exact replication of subtle details, i.e., band broadenings and asymmetries. Therefore, NIR spectroscopy combined with anharmonic calculations succeeds in the investigation of conformational population in liquid phase. The experimental spectra of four kinds of butyl alcohols are prominently different from each other, particularly in the region of 5200 to 4600 cm−1 and in between 4500 and 4000 cm−1. These regions in the theoretical lineshapes are almost identical to the experimental ones. Not only has the influence of conformational isomerism on subtle spectral features, i.e., band broadenings, been replicated. Also, evidence of experimental bands that are unequivocally originating from a particular conformer can be pointed out. Particularly in the very sensitive to structural differences region between 5200 and 4600 cm−1, which is populated by bands arising from combination modes of CH, CO stretching and CH deformation modes. A notable example can be pointed out in the spectrum of 2-butanol, where the band at 4862 cm−1 originates almost entirely from the second major G−g+ conformer (Figure 3); similar observation can be made in case of isobutanol (Figure 4). A

slightly less reliable level of prediction of the 6000−5500 cm−1 region can be accounted for the inaccuracies in the calculated intensities of first overtones of CH stretching modes (Figures S2−S5 in the Supporting Information). For all kinds of butyl alcohols it is common that the region of 5300 to 4000 cm−1 is formed by binary combination bands, while first overtones have about equal contributions only to the 6000−5500 cm−1 region. The summed contributions of first overtones and binary combinations for all kinds of butyl alcohols are presented in detail in the Supporting Information (Figures S2−S5). The agreement between the calculated spectra and the experimental data is high enough to be able to unambiguously identify all the experimental bands. Detailed band assignments 1954


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for the studied alcohols, as well as calculated potential energy distributions (PEDs), are presented in the Supporting Information (Figures S6−S17 and Tables S1−S4). 3.2. Details of the OH Stretching Overtone Band. The inhomogeneity of the 2νOH bands of 1-butanol, 2-butanol, and isobutanol has been well evidenced in the experimental NIR studies. A separation of the components of this band has been noticed upon the change of the temperature. Yet, the splitting of the 2νOH band during the temperature shift does not occur for tert-butyl alcohol. Despite severe limitations of experimental investigations in this regards, i.e., due to band overlapping, previous assumptions rightly expected the conformational isomerism to be standing behind the differences observed in the experiment for butyl alcohols. However, these investigations often were forced to rely on assumptions rather than solid evidence when it comes to the manifestation of conformational isomerism in NIR spectra of aliphatic alcohols. In the present study a high level of accuracy in the reproduction of experimental line shape has been also achieved for the first overtone band of OH stretching mode, allowing for a direct explanation of the phenomena discussed above. This is evidenced in Figure 6a−d, in the band shape details of the 2νOH band for 1-butanol, 2-butanol, and isobutanol. The fourth

Figure 5. Experimental and calculated NIR spectra of tert-butyl alcohol. For band assignments, only summarized major contributions of vibrational modes are presented here. Refer to Supporting Information for detailed assignments.

Figure 6. Details of the first overtone band of OH stretching mode of 1-butanol (A), 2-butanol (B), isobutanol (C), and tert-butyl alcohol (D). Experimental spectrum, fourth derivative of the experimental spectrum, the calculated line shape and contributions to NIR spectra stemming from conformational isomers. 1955


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it is impossible to conclude, whether such trend would prevail in case of other, more complex aliphatic alcohols. 3.3. Temperature Dependence of Conformational Equilibria of Butyl Alcohols. As evidenced in section 3.1 the origins of NIR spectral patterns of four kinds of butyl alcohols can be unambiguously correlated to their conformational isomerism, which is very strongly pronounced in the details of 2νOH band (section 3.2). Therefore, it needs to be assumed that this will have direct implications on the temperature dependence of NIR spectra, a fact that has been extensively studied before.16,44,51,62 However, while sophisticated experimental studies could clearly evidence the existence of complex relations in this matter, they were unable to provide solid evidence for the mechanisms standing behind these observations, and only assumptions could be provided at that time. It is known that conformational equilibria directly depend on temperature, through Gibbs free energy levels, and consequently Boltzmann coefficients for conformational isomers. These values can be calculated from the firstprinciples, and the temperature effects on conformational population can be predicted. For conformationally flexible butyl alcohols the drift of relative Boltzmann abundances, in the 273−368 K temperature range in 3 K increments, are provided in Figure 7a−c. This region includes the temperature region that was studied experimentally. The change of the relative abundances of rotational isomers with increasing temperature is mostly positive for trans conformers (with respect to C−O(H) rotation) and negative for gauche conformers. This does not hold as a general rule, but the trend is clear (Figure 7). These differences in population upon the temperature shift are high enough to induce the change in the intensity and the band shape, resulting in the bandshift of 2νOH peak of these alcohols, which will be further evidenced in Section 3.4. Low rotational barriers of butyl alcohols and similar molecules lead to coexistence of numerous conformational isomers with relatively high abundances in the room temperature. A bandwidth increase in the experimental NIR spectra follows up in such case, due to larger number of significant contributions arising from these conformers. 3.4. Theoretical Reproduction of Synchronous TwoDimensional Correlation Plots of Butyl Alcohols. 2DCOS analysis is a powerful tool that allows for elucidating the spectral changes in this kind of complicated data.8 There has been a substantial evidence reported in the literature on the application of 2D-COS approach to NIR spectra of alcohols and other molecules24,43−60,73,76 but also very complex systems,77−79 showing its potential in elucidating subtle spectral information. In the present study, the changes in the region of the 2νOH band of butyl alcohols occurring with temperature drift can be very well monitored in synchronous 2D plots (Figure 8a−c′). This can be now fully evidenced in our calculated results; furthermore, not only should the C−O(H) rotamers be considered here but also full conformational flexibility of these molecules is likely being expressed. The experimental 2D synchronous plots (Figure 8a−c) reveal changes that are similar for all three kinds of conformationally flexible butanols. These 2D patterns which indicate a change in the band shape and a bandshift, have been successfully reproduced by using calculated temperaturedependent spectra (Sections 3.2 and 3.3). These changes are, in general, comparable between the studied systems, although for 2-butanol the impact of the G-g conformer, which gives a significant contribution in the low wavenumber part of the

derivative of the experimental spectra reveals the components contributing into the spectral envelope, which are directly reproduced in the calculated data. Obviously, the experimental 2νOH band shape of tert-butyl alcohol is entirely homogeneous, as only one unique conformational isomer of this molecule exists; again this remains in full agreement with the results of the theoretical conformational analysis. The homogeneity and different behavior of tert-butyl alcohol band has been noticed in earlier studies.73,74 The band shape details of 2νOH of four kinds of butyl alcohols have been extensively studied experimentally in the past; NIR spectroscopy has proved to be particularly helpful in this regard with numerous investigations reported to date.43−60 Czarnecki et al.16 and Maeda et al.73 both rightly expected that the conformational flexibility is standing beneath these subtle effects. However, even with aid of chemometrics and 2D spectroscopy, the experimental data alone could not provide a full picture on this phenomenon, and many questions were not fully answered before. However, the theoretical study offers unambiguous explanation of these phenomena. Maeda et al. observed that the wavenumbers of the contributing peaks shift when solvent is changed; this effect is particularly evident for 2-butanol.73 The reasons behind this were not fully understood before. However, the peak positions resolved experimentally are resembled very well by the conformational contributions calculated in our study. This indirectly suggests that the conformational isomerism of butyl alcohols may impact the interactions with solvent molecules. For 2-butanol this effect has been observed more clearly in the experiment, and we now know that this is due to a higher separation of the conformational contributions, with a notable detachment of the 2νOH frequency of G−g− isomer (Figure 6). Previously, it was difficult to unambiguously trace the discussed effect for other butyl alcohols experimentally, for which the grouping of conformational frequencies is higher, and thus the contribution overlapping and the resulting homogeneity of 2νOH band are larger (Figure 6). Presently, with an aid of anharmonic calculations it becomes possible to follow closely these subtle effects for alcohols, and presumably also for other organic molecules. The influence of conformational isomerism of aliphatic alcohols on the spectral profile of the overtone band of OH stretching vibration has been frequently discussed without solid evidence in the past.16,73,75,76 The former assumption that can be found in the literature, that the major role is played here by only two (gauche and trans) conformers with respect to C− O(H) rotation needs to be revised, however. In the present work it is evidenced that the entire conformational population contributes into complex band shape of 2νOH band. One can notice a trend of 2νOH wavenumbers among gauche conformers (with respect to C−O(H) bond) being usually grouped toward lower wavenumbers, while those of trans conformers toward higher wavenumbers. This observation holds well for isobutanol (Figure S20 in Supporting Information) and also for 1propanol, for which it was recently reported.6 However, after considering our results obtained for 1-butanol (Figure S18 in Supporting Information) and 2-butanol (Figure S19 in Supporting Information) the entire picture becomes more complex, and for example in case of 2-butanol the contributions to 2νOH band originating from gauche conformers (with respect to C−O(H) bond) are dispersed about equally over the corresponding wavenumber region. Therefore, at the moment 1956


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resulting peak maximum. By analyzing Figure 8 the change in the band shape details during the temperature change was exactly reproduced for 1-butanol, in which the theoretical clover pattern is the same as the experimental one. In this case the temperature drift of conformational equilibria lead to a shift of the peak maximum and change of the band shape exactly as predicted by calculation. For the two other butyl alcohols, the reproduction was slightly less accurate, although the calculated 2D patterns also predict the change in the band shape in these two cases. These differences in the experimental and reproduced synchronous 2D contours may be primarily attributed to the inaccuracies of calculated data and the chosen band shape function for convolution of theoretical wavenumbers and intensities. Note that in the theoretical approach assumed by us it is impossible to reliably predict the temperature driven frequency drift itself. Therefore, the probable additional effect of a pure vibrational frequency shift of 2νOH band for each of conformational isomers could not be taken into account. However, considering the high agreement between calculated and experimental spectra, it may be reliably assumed that the temperature-induced population change plays a major for in the experimentally observed change of the 2νOH band anyway. 3.5. Anharmonicity Constants of Conformational Isomers. The anharmonicity of OH stretching vibration of butyl alcohols has frequently been investigated before.16,53,75,81−83 However, these studies were based mostly on experimental methods and did not take the advantage of anharmonic quantum mechanical calculations. As such, a deeper insight into conformationally dependent anharmonicity constants was not possible, even when data on higher overtones, where the separation of band components increases, have been used.75,84 In the present research, another look could be taken at the anharmonicity of the OH stretching vibration of butyl alcohols with respect to their conformational flexibility. The investigated conformational isomers exhibit differences in the νOH anharmonicity constant χ, defined here as x=

2ν01 − ν02 2

(1)

As presented in Table 2, the investigated structures exhibit differences in anharmonicity constant. It can be expected from previous reports on ethanol-solvent interaction studies that the intermolecular interactions of conformational isomers differ in strength; this effect can be observed by vibrational spectroscopy. The values obtained here remain in perfect agreement with the data gathered for butyl alcohols experimentally.16 The differences between the values of different rotational conformers may stand behind the experimentally observed differences in between the band shape of νOH and 2νOH bands;16 this fact may also have additionally extended the difficulties in explaining the observed spectral changes upon temperature increase. It was also evidenced before that the anharmonicity constant depends itself on temperature. However, these conclusions were out of necessity based on the convoluted 2ν OH band, where combined spectral information stemming from conformers was expressed. Again, purely spectroscopic studies, even sophisticated ones, are unable to follow closely these effects, unlike combined spectroscopic and theoretical approach. Further studies are needed to clarify more on this phenomenon.

Figure 7. Calculated temperature (273−368 K) dependence of Boltzmann coefficients (B2PLYP/def2-TZVP) of conformationally flexible butyl alcohols: 1-butanol (A), 2-butanol (B), and isobutanol (C).

band, can be seen. This is again in agreement with the splitting which has been observed experimentally.73 As it is well-known8,80 a simple shift of a band, without any changes in the band shape details or maximum intensity, is reflected by an absolutely symmetric clover pattern in the 2D synchronous spectrum. Any deviation from this symmetry indicates the change in a band shape, which can occur simultaneously to the shift of the peak maximum position. Therefore, 2D synchronous plots are very sensitive to even very subtle band shape changes, and can be used for monitoring the change in the band details, which in linear NIR spectra would be otherwise difficult to follow. Even more so if the major effect of the changes of the band components is the shift of the 1957


Article


Figure 8. Experimental and calculated synchronous 2D plots of butyl alcohols in the region of 2νOH band resulting from temperature perturbation (300−368 K in 3 K steps); 1-butanol (A, expt; A′, calcd), 2-butanol (B, expt; B′, calcd), isobutanol (C, expt; C′, calcd). Red: positive peaks. Blue: negative peaks.

4. CONCLUSIONS We have investigated the NIR spectra of four kinds of butyl alcohols in diluted CCl4 solution; the temperature-dependent spectra were collected as well. By applying fully anharmonic quantum mechanical approaches, successful reproduction of the experimental data was achieved. Solid evidence based on the linear and 2D NIR spectra simulated entirely by calculations were provided for mechanisms which could previously be only assumed. These studies also provide a new insight on the

conformational isomerism influencing NIR spectra on the molecular level. Conformational isomerism of butyl alcohols, which is well expressed in their respectful NIR spectra, gives distinct temperature-dependent changes, particularly in the region of 2νOH band. These effects were monitored by 2D synchronous plots, and were subsequently reproduced by quantum mechanical calculated data. Moreover, the temperature drift 1958



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Table 2. Anharmonicity Constant Dependence of Conformational Flexibility of Four Kinds of Butyl Alcohols: Calculated Data Based on the Results of CPCM-B2PLYP/ def2-TZVP Calculations νOH [cm−1] GGg′ GGg TGg GTg G′Gg TTg TGg′ GG′t GTg′ GTt TTt TGt GGt GG′g

3627.7 3621.7 3633.3 3622.9 3635.8 3637.1 3643.9 3640.5 3627.9 3645.5 3642.0 3644.4 3645.6 3651.1

G‑g‑ G+g+ G+g‑ Tg+ G+t Tg‑ Tt G‑t G‑g+

3596.2 3628.0 3617.2 3621.6 3630.2 3611.7 3630.1 3628.8 3634.7

Gg Gg′ Tg Gt Tt

3627.1 3627.2 3635.6 3647.0 3655.7

−

3611.1

2νOH [cm−1] 1-butanol 7085.0 7073.5 7097.5 7076.9 7102.9 7105.6 7119.4 7113.0 7087.7 7123.4 7116.7 7121.5 7124.4 7135.8 2-butanol 7021.3 7085.2 7064.6 7073.6 7091.0 7053.9 7091.1 7088.5 7100.4 isobutanol 7083.7 7084.9 7102.1 7126.2 7144.1 tert-butyl alcohol 7052.7

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.B.B.). *E-mail: [email protected] (C.W.H.).

anharm const [cm−1]

ORCID

Krzysztof B. Beć: 0000-0003-0822-5098 85.1 85.0 84.5 84.5 84.4 84.3 84.2 84.0 84.0 83.8 83.7 83.7 83.4 83.2

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Calculations have been carried out in the Wroclaw Centre for Networking and Supercomputing (http://www.wcss.pl), Grant No. 375.

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REFERENCES

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85.5 85.4 84.9 84.8 84.8 84.7 84.5 84.5 84.5 85.2 84.7 84.5 83.9 83.6 84.7

of conformational equilibria can be directly followed by theoretically reproduced data as well. The evidence presented in this work bring new understanding of NIR spectroscopy and physical chemistry of aliphatic alcohols, offering new answers for the old problems, i.e., the nature of the temperature-dependent change of band shape and shift of the OH first overtone band or the relations between temperature and anharmonicity constants. Further investigations should be aimed at the role of conformational flexibility in intermolecular interactions, both in single-phase such as association through hydrogen bonding, and in multiphase, i.e., interactions with solvent molecules, and in particular the formation of microheterogeneous clusters involving alcohol molecules.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b00646. Additional figures, detailed band assignments and PED tables for studied molecules and detailed data on geometries of the investigated structures (PDF) 1959


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Temperature Drift of Conformational Equilibria of Butyl Alcohols

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