IR Spectra of Hydrogen-Bonded Complexes of Trifluoroacetic Acid

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

IR Spectra of Hydrogen Bonded Complexes of Trifluoroacetic Acid with Acetone and Diethyl Ether in the Gas Phase. Interaction Between CH and OH Stretching Vibrations Ruslan E. Asfin J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b10215 • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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The Journal of Physical Chemistry

IR Spectra of Hydrogen Bonded Complexes of Trifluoroacetic Acid with Acetone and Diethyl Ether in the Gas Phase. Interaction between CH and OH Stretching Vibrations Ruslan E. Asfin* Department of Physics, Saint Petersburg State University, 7/9 Universitetskaya Nab., 199034 Saint Petersburg, Russian Federation *Corresponding

author.

Tel: +7 812 428 74 19 Fax: +7 812 428 72 40. Email address: [email protected]

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Abstract The spectra of complexes of trifluoroacetic acid with acetone and diethyl ether and their perdeuterated isotopologues were extracted from the spectra of the mixture of the compounds recorded at room temperature. The ν(OH) bands of the complexes with protiated and deuterated acetone notable differ from each other, whereas these ν(OH) bands are practically not affected by the deuteration of the diethyl ether. An assumption about the interaction of CH and OH groups in the (CH3)-C=O···HO fragment is made. According DFT calculations, complexes of trifluoroacetic acid with both acetone and diethyl ether have a cyclic structure with one strong =O···HO hydrogen bond and one weak CH···O= bond. The structural, spectroscopic, and electronic properties indicate an essential role of weak bonds in the total complexation energy of the systems studied.


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Introduction The nature of the ν(XH) band structure in the spectra of complexes with moderate and strong XH···B hydrogen bonds is still an actively discussed topic in spectroscopy.1–7 The complexes involving carboxylic acids play a significant role in the study of systems with hydrogen bonds, because of their physical properties and as models of biologically important interactions. There is a number attempts to describe the profile of a ν(OH) band in the spectra of these complexes using different physical models.8,9,18,10–17 Most of them describe the bands profile as a result of interaction of νOH stretching vibration with low-frequency intermolecular modes and Fermi resonances with combination levels of fingerprint vibrations. Consideration of isotope effects comes down to the COOH/COOD substitution in most cases. The influence of deuterium substitution in groups connected to COOH on the profile of the ν(OH) band in the spectra of formic and acetic acids cyclic dimers was studied by Flakus recently19 and in some earlier studies.20,21 For these effects Flakus has proposed the term 'longrange H/D isotopic effects and explained it by the "relatively strong vibronic coupling of the Herzberg–Teller type".19 In this study we looked for similar effects in the spectra of heterogeneous complexes. Here the spectra of trifluoroacetic acid (TFA, CF3COOH) in the mixture with acetone (dimethyl ketone, DMK, (CH3)2CO) and diethyl ether (DEE, (C2H5)2O) and their perdeuterated analogues DMKD6 ((CD3)2CO) and DEED10 ((C2D5)2O) are presented. From these spectra the spectra of individual TFA···B complexes (B = acetones or diethyl ethers) are extracted and discussed. Our attempt to record the spectra of analogous complexes with acetic acid has failed. Namely, in the spectra of mixtures of acetic acid with acetone or diethyl ether the bands of heterogeneous complexes were absent. The TFA···DMK complexation energy in the gas phase was measured by analyzing the temperature dependence of the vapor density22 and the intensity of the ν(OH) bands of monomers and complexes.23,24 The energy of TFA···DEE complexation was also reported 3 ACS Paragon Plus Environment


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previously.23,24 The spectra of TFA···DMK and TFA···DEE and a number of their H/D isotopologues were recorded in the gas phase and in low-temperature inert matrices,25 however the spectra of complexes were not separated from the spectra of monomers and TFA dimer. Below we discuss the experimental data supported by DFT calculations. The main focus is on the spectral features in ν(C=O) and ν(OH) band regions, especially on the influence of H/D substitution in proton acceptors on the profiles of ν(OH) bands.

Experimental and Calculation Details Experimental. The samples were placed into a stainless cell with BaF2 windows and the gasphase spectra in the 4700 – 800 cm−1 range were recorded with a Bruker IFS125 HR spectrometer at a resolution of 0.5 cm−1. The optical path length was 8.5 cm. A liquid nitrogencooled MCT detector, a KBr/Ge beam splitter, and a globar as a light source were used. All spectra were obtained at a temperature of 20 ± 1 °C. The studied compounds used were obtained from commercial sources. The ampoules with compounds and the cell were attached to a glass vacuum system. After evacuating the cell, the required pressures of compounds were admitted. The acid was admitted firstly, and then acetone or ether was added. A number of spectra of trifluoroacetic acid in the range 2.4 – 10.6 Torr as well as spectra of both isotopologues of acetone (14 – 18 Torr) and diethyl ether (2.4 – 14.5 Torr) were recorded. These data were used to extract the spectra of TFA···DMK(D6) and TFA···DEE(D10) complexes from the spectra of mixtures. The spectra of mixtures of the acid with acetone or ether at total pressure of 4 – 23 Torr were recorded with the ratio of TFA:B in the range 1:1 – 1:4. The all pressures were measured during admission with an U-shaped oil gauge, and after that they could slightly change due to adsorption in the wall of the cell. This did not affect the extraction of spectra of monomers, dimers, and complexes, however did not allow us to estimate the absolute intensity of the bands (see Supporting Information for details).


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The spectra of complexes were obtained by sequential subtraction of the spectra of monomer and dimer of trifluoroacetic acid, and the spectrum of acceptor from the spectra of mixtures. The procedure is discussed in detail below. Calculations. The quantum-mechanical calculations were performed with a Gaussian 16, Revision A.03 package.26 The density functional theory (DFT) method with the B3LYP functional was used to optimize the geometry. Several starting configurations were chosen for the search of local minima. Additionally, some calculations were made with the wB97XD and M062X functionals. The final geometry optimization, the energy and frequency calculations were performed using the 6-311++G(2df,2pd) basis set. The basis set superposition error (BSSE) correction was done by the counterpoise method during the calculation of the geometry and frequencies of the complexes. The anharmonic calculations were performed with a 'freq=anharmonic' keyword. The analysis of the electron densities in the vicinity of bond critical points was performed with the AIM2000 program.27,28

Results and Discussions Results of Calculations. In Figure 1 the optimized structures of studied complexes are presented. The selected geometrical and energetic parameters are collected in Table 1. The structures of monomers and Cartesian coordinates of atoms in optimized structures of monomers and complexes are presented in Supplementary Information in Table S1. Table 1. Selected Calculated Geometric Parameters of TFA, DMK and DEE Monomers, as well as Geometric and Energetic Parameters of TFA···TFA Dimer, TFA···DMK and TFA···DEE Complexes and the Changes of Parameters Upon Complexation (c – m). Parameter Monomer Complex c−m TFA···TFA r(C=O), Å 1.193 1.212 +0.019 r(O−H), Å 0.969 0.999 +0.030 r(O···O), Å 2.671 ΔE, BSSE, kcal/mol 16.79 ΔE, BSSE, ZPE, kcal/mol 15.66 5 ACS Paragon Plus Environment


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TFA···DMK r(C=O), TFA, Å 1.193 1.201 +0.008 r(O−H), TFA, Å 0.969 0.997 +0.028 r(C=O), DMK, Å 1.210 1.220 +0.010 r(C−H), DMK, Å 1.086 1.086 −0.00 r(O···O), Å 2.669 r(O···C), Å 3.480 ΔE, BSSE, kcal/mol 12.02 ΔE, BSSE, ZPE, kcal/mol 13.00 TFA···DEE r(C=O), TFA, Å 1.193 1.200 +0.007 r(O−H), TFA, Å 0.969 1.000 +0.031 r(C−O), DEE, Å 1.416 1.433 +0.017 r(O···O), Å 2.653 r(O···C), Å 3.658 ΔE, BSSE, kcal/mol 10.97 ΔE, BSSE, ZPE, kcal/mol 11.98 The dimer of trifluoroacetic acid has the structure of a planar cycle with two strong hydrogen bridges. This structure corresponds to that obtained previously in crystal29 and in the gas phase.30 The energy of dimerization, taking into account the zero-point vibrational energy (ZPE), is ΔE = 15.7 kcal/mol which is comparable with the experimental value of enthalpy of dimerization of ca. 14 kcal/mole obtained independently by various methods.31–36 The complex of trifluoroacetic acid with acetone has the structure similar to a planar ring. Beside the strong C=O···H-O bridge there is the additional weak interaction between the C=O group of the acid and the C-H group of the ketone’s methyl. The calculated energy of complexation ΔE = 13.0 kcal/mol is in a satisfactory agreement with the value of enthalpy of complexation of 14.4 (1.0) kcal/mol obtained by a vapor density technique22 and essentially higher than the value of 7 (1) kcal/mol derived from spectroscopic measurements at a higher temperature.23 In the complex of trifluoroacetic acid with diethyl ether there are two interactions between the molecules: the hydrogen bond between the oxygen atom of the ether and the hydroxyl group of the acid and another weak hydrogen bond between the C=O group of the acid and one of the hydrogen atoms of the methyl group of the ether. The calculations with functionals taking into account dispersion predict the weak bond with hydrogen atom of CH2 group. The energy of 6 ACS Paragon Plus Environment

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dimerization is again essentially higher than the experimental one23 (12 vs 8.4 (1) kcal/mol). Despite the fact that the hydrogen bond in this complex is slightly stronger than that in the TFA···DMK complex (as evidenced by the shorter r(O···O) distance) the calculated total complexation energy is less by 1 kcal/mol. At the same time the r(O···HC) distance in the weak hydrogen bond is essentially longer that in the complex with acetone. It means that the weak interactions play an important role in the complexation energy at least for the TFA···DMK complex. The close values of complexation energies of TFA···DMK and TFA···DEE indicate similar proton acceptor abilities of acetone and ether. The same conclusion was derived previously from the gas phase spectral properties and calculations of complexes of methanol with acetone and dimethyl ether.37 Spectra Extraction. In the spectra of mixtures of TFA with a proton acceptor B one expects to record a superposition of four types of spectra: the spectra of monomers of TFA and B, the spectrum of TFA dimers and most interesting for us spectrum of the TFA···B complex. To identify and isolate the latter the consequent subtraction of spectra should be performed. The spectra of monomeric and dimeric forms of TFA were obtained from the analysis of the acid’s spectra recorded at different pressures, presented in Figure 2(A). It is well known that TFA in the gas phase at temperatures near 300 K exists predominantly in monomeric and dimeric forms. The separation of monomer’s and dimer’s spectra in this case is a rather simple task because some bands are located in non-overlapping regions. The examples of such monomer bands are marked with the symbol "M" in Figure 2(A): the ν(OH) band near 3580 cm−1, the ν(C=O) band near 1830 cm−1, and the δ(OH) band near 1125 cm−1. The spectrum of individual species can be obtained by subtracting the spectrum recorded at a certain pressure from the spectrum obtained at a different pressure with a coefficient chosen to eliminate the characteristic bands of other species. Here, the series of four spectra recorded at different pressures of TFA between 2.5 and 10 Torr was treated in this way. The spectra of monomers and 7 ACS Paragon Plus Environment


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dimers obtained from different pairs of experimental spectra were averaged and the results are presented in Figure 2(B). After the spectra of monomers and TFA dimer are established, the spectra of TFA···B can be extracted from the spectra of mixtures. The spectra of monomers could be easily subtracted because the monomers have separately located bands. However, the bands of TFA dimer and the bands of the complexes strongly overlap with each other and no isolated bands of TFA dimer were found. To overcome this problem, the information on the dimerization constant obtained by processing the spectra of pure TFA and the intensity of the TFA monomer bands were used to find out the appropriate coefficient with which to subtract the spectrum of TFA dimer. As an example of the result, the spectra of mixture of trifluoroacetic acid and acetone-D6, as well as the isolated spectrum of their complex are presented in Figure 3. Some additional aspects of spectra extraction are discussed in Supplementary Information. The Region of C=O Vibrations. In Figure 4 the spectra of the studied monomers and complexes are presented in the spectral region of the ν(C=O) band. The used resolution and probably the pressure and the temperature in the cell do not allow the rotational-vibrational structure of these bands even in the spectra of monomers. The bands of complexes have more or less symmetric shape and the wavenumbers of maxima were used as wavenumbers of vibrational transitions in the complexes. The bands in the spectra of monomers of trifluoroacetic acid and acetone both in protiated and deuterated forms are asymmetric. The central peak of the TFA monomer band (maximum of a Q-branch) was used to determine the wavenumber of the corresponding transition. The value of 1830.0(5) cm−1 is in satisfactory agreement with the previously reported ones: 1826 cm−1,38 1830 cm−1,35 1829 cm−1,39 1827 cm−1,23,24 and slightly differs from 1843 cm−1.40 It should be noted, that all these values were obtained with prism spectrometers. The recently reported value41,42 of 1830 cm−1 is also in a good agreement with the presented wavenumber. The spectrum of protiated acetone is in excellent agreement in the range of intersection with the 8 ACS Paragon Plus Environment

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spectrum reported in the GEISA spectroscopic database,43 recorded with a resolution of 0.015 cm−1 at 297.5 K. The absorption in the ν(C=O) band range has a complicated structure and there are two local maxima at 1739.8 and 1728.5 cm−1 in the spectra of DMK and 1743.7 and 1732.0 cm−1 in the spectra of DMKD6. The local minima are observed near 1731.6 and 1739.7 cm−1 for protiated and deuterated forms respectively. The centers of gravity of the bands are located near 1741 and 1736 cm−1. The previously reported wavenumbers of the ν(C=O) band for the protiated form of DMK are 1731,44 1738,45 and 1740 cm−1 (the latter is an implicit value, calculated from tabulated shifts).25 For DMKD6 the reported values are 1732,44 1741,45 and 1740 cm−1.25 It is also reported the value of the center of gravity of the DMK band at 1742 cm−1.3 The most probable origin of such a structure of the ν(C=O) band in the spectra of DMK is the Fermi resonance between the ν(C=O) stretching and combination levels. According to anharmonic calculations the most probably combination levels in protiated acetone are the overtones of γ(C-H)sym and δ(C-C)as as well as the combination γ(C=O)+ν(C-C)as. Fermi resonance with these bands leads to the appearance of four bands: the most intense one is the high-frequency component at 1771 cm−1; the lowest frequency component is located at 1722 cm−1. The center of gravity of these four components is near 1753.5 cm−1. In the calculated anharmonic spectra of deuterated acetone there are no resonances in the region of the ν(C=O) band. However in addition to the carbonyl band near 1747 cm−1, the combinational band of the ν(C-C)sym and the symmetric umbrella vibration of CH3 with a noticeable intensity is located at 1764 cm−1. The center of gravity of these two bands is located near 1758 cm−1. In calculated spectra of complexes of TFA with DMK and DMKD6 there are no the strong Fermi resonances in the region of the ν(C=O) band. Upon formations of the complexes the ν(C=O) bands of both TFA and acetone are red-shifted. The experimental and calculated wavenumbers of these bands and the shifts are collected in Table 2. 9 ACS Paragon Plus Environment


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Table 2. The Experimental (ν(C=O) exp) And Calculated (ν(C=O) calc, Harmonic and Anharmonic) Wavenumbers of the ν(C=O) Bands in the Complexes and the Shifts (Δc-m) Relative To the Wavenumbers of These Bands in Monomers of Trifluoroacetic Acid and Acetone. ν(C=O) exp, Δc-m exp, ν(C=O) calc, Δc-m calc, cm−1 cm−1 harm/anharm, harm/anharm, cm−1 cm−1 ν(C=O) TFA TFA···TFA 1788.6 −41 1808 / 1772 −51 / −54 a b b −72 1779 / 1731 −80 / −95 TFA···DMK 1802.4 −28 1823 / 1788 −36 / −38 TFA···DMKD6 1802.2 −28 1823 / 1787 −36 / −39 TFA···DEE 1795.9 −34 1825/1791 −34 / −35 TFA···DEED10 1796.1 −34 1824/1789 −35/ −37 ν(C=O) DMK TFA··· DMK 1715.2 −26c/ −23d 1747 / 1699 −39 / −54 c d TFA···DMKD6 1706.2 −30 / −38 1737 / 1688 −40 / −70 a Including half of splitting between antisymmetric and symmetric vibrations. See text for details b The mean of symmetric and asymmetric wavenumbers of ν(C=O) c From the center of gravity of the monomer ν(C=O) band d From the maximum of the monomer ν(C=O) band

It is remarkable that the carbonyl band in the spectrum of TFA···DMK complex is located at higher frequency than that in the spectrum of TFA···DMKD6 complex, whereas the maxima of absorption (but not the centers of gravity!) of acetone monomers are located in the reverse order. The harmonic calculations overestimate the wavenumbers of the bands, while anharmonic calculations underestimate them. The shifts of the bands upon complexation are also slightly overestimated both in harmonic and anharmonic case, although for the spectra of TFA···DEE(D10) complexes the calculations perfectly reproduce the experimental data. The slightly larger shift of carboxyl ν(C=O) band upon complex formation in the case of the ethers comparing with the acetones indirectly indicates a stronger hydrogen bond in the TFA···DEE complex. The comparison of the shifts of carboxyl ν(C=O) band in the spectra of TFA···TFA and TFA···acceptor is pointless because both C=O bands in the acid dimer participate in strong 10 ACS Paragon Plus Environment

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hydrogen bonds. Another problem in estimating the hydrogen bond strength from the experimental shift of the ν(C=O) band is its splitting for TFA dimers into symmetric (Raman active) and antisymmetric (IR active) vibrations, because of the symmetry of the complex. Sometimes it is called Davydov splitting.10,13 To the best of our knowledge there are no data on the Raman shift of the symmetric ν(C=O) band of trifluoroacetic acid in the gas phase. The difference between the wavenumbers of antisymmetric (IR active) and Raman shift of symmetric (Raman active) vibrations of C=O groups in jet-cooled spectra is νas – νs = 56 cm−1;46 in solution in CCl4 this value is νas – νs ≈ 62 cm−1.35,38,40,47 Our calculation gave the value of 59 cm−1 in harmonic calculation and 82 cm−1 in anharmonic one. Thus, approximately 30 cm−1 should be added to the shift of ν(C=O) band upon dimerization of trifluoroacetic acid to connect these shift with the energy of hydrogen bond. This value is also listed in Table 2. The Region of OH Vibration. Interaction of ν(OH) with ν(CH) Modes. The maximum of the ν(OH) band of monomers of trifluoroacetic acid is located near 3580.0 cm−1. The previously reported value of 3587 cm−1,35,38 is seems to be the maximum of the band in low resolution spectra. In the spectra recorded with relatively high resolution this band has the maximum matching with the wavenumber reported here.48 In the spectra of TFA dimers the ν(OH) band is broad with several submaxima (Figure 2). The features of the band coincide generally with those described earlier.35,38 In the Figure 5 the extracted spectra of TFA···DMK(D6) and TFA···DEE(D10) complexes as well as spectra of DMK and DEE monomers are presented in the region of ν(OH) bands of complexes. The ν(OH) band of all complexes is broad and intense, which is characteristic for strong H-bonded complexes. The spectral features of the TFA···DMKD6 and TFA···DEED10 bands (the wavenumbers of submaxima) coincide with the previously reported values25 as a whole. These values are listed in Tables S13 and S14 in the Supporting Information. The most probable reason of some disagreements is that in the literature the unextracted spectra are described25, which included the absorption of monomers of TFA, deuterated acceptor monomers 11 ACS Paragon Plus Environment


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and TFA dimers, whereas here the separated bands of TFA···acceptor complexes are considered. The comparison of the spectra in Figure 5 shows that the ν(CH) bands of protiated diethyl ether in the complex TFA···DEE are sufficiently narrow and well discernible against the background of the ν(OH) band. The ν(CH) bands are located close to the same bands of DEE monomer. On the other hand, in the spectra of the TFA···DMK complex there is no evidence of clearly visible narrow bands in the region of ν(CH) of the DMK monomer. The similar picture is observed in the ν(CD) region in the spectra of the TFA···B complexes: there are well distinguishable bands slightly shifted from the bands of DEED10 monomer in the spectra of TFA···DEED10 complex and there are no sharp bands in this region in the spectra of TFA···DMKD6 complex. In the harmonic approximation the calculations predict a decrease in the total intensity of the ν(CH) bands by 20% and ν(CD) bands by 5% upon formation of complexes of trifluoroacetic acid with corresponding acetones. This of course cannot explain the disappearance of these bands in the spectra. Again, one can see that the ν(OH) bands of TFA···DMK and TFA···DMKD6 noticeably differ from each other, especially near the hole at 3095 cm−1, where the redistribution of intensity is notable. Another distinction is in the low-frequency wing of the band near 2275 cm−1. In contrast, the ν(OH) bands in the spectra of TFA···DEE and TFA···DEED10 complexes are virtually the same except for the contributions of ν(CH) and ν(CD) bands of diethyl ether in the spectra of complexes. To eliminate the ν(CH) bands the part of the spectrum of the TFA···DEE complex in the region 3040 – 2800 cm−1 was replaced by the ν(OH) band of the TFA···DEED10 complex in this region. The same was done for spectra of TFA···DEED10 in the range of ν(CD) band region. The result is shown in Figure 5B by dashed lines. To describe the broad bands it is convenient to use a set of spectral moments of a spectral function S ( )  D( ) / , where D( )  lg( I 0 / I ) . The zero spectral moment M 0 



S ( )d is

band

the integrated intensity of the band. The first normalized spectral moment


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M 1  M 01



S ( )  d is the center of gravity of the band. The second normalized central

band

spectral moment M 2  M 01



S ( )  (  M 1 ) 2 d characterizes the effective half-width of the

band

band 1/ 2  2 M 2 that is close to the full width of the band at half maximum. In Table 3 these moments of the ν(OH) bands are collected for all studied complexes.

Table 3. The First M1 (cm−1) Normalized Moment and Effective Half-Width ν1/2 (cm−1) of the ν(OH) Band in the Spectra of the Complexes and the Shift of M1 from the Wavenumber of TFA Monomer Band νm. M1, cm−1 ν1/2, cm−1 M1 − νm, cm−1 TFA···TFA 3022 430 −558 TFA···DMK 3032 475 −548 TFA···DMKD6 3042 530 −538 TFA···DEE 2867 435 −713 TFA···DEED10 2868 415 −712 The values of the moments support the previously mentioned observation: the centers of gravity and effective half-widths of ν(OH) bands of TFA···DMK and TFA···DMKD6 are noticeable different from each other, whereas these values are practically the same for TFA···DEE and TFA···DEED10 complexes. Differences in the ν(OH) bands and the absence of pronounced ν(CH)/ν(CD) bands in the spectra TFA···DMK and TFA···DMKD6 evidence indicate a substantial interaction of CH/CD bonds of the methyl group of acetone and the OH bond of the acid, which is absent in case of complexes formed between trifluoroacetic acid and diethyl ether. Similar effects were observed previously for the dimers of acetic acid. In ragout-jet spectra the influence of CD3 ← CH3 substitution on the "fine structure" of the ν(OH) band was noted.10,49 Flakus et al. have studied in particular the influence of deuterium substitution in methyl group on the ν(OH) band.19 A mechanism of this influence was proposed based on the vibronic coupling of the Herzberg–Teller type. Dreyer had to include two γCH3 wagging modes to describe the ν(OH) band by multidimensional DFT calculation and consideration of Fermi-resonances and anharmonic couplings with low frequency modes.11 On the other hand, it seems that the 13 ACS Paragon Plus Environment


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interaction between νOH and νCH modes was not considered in this study. In this study it was shown that vibrations of methyl group of one molecule influence the shape of the ν(OH) band of the other molecule when these molecules form the hydrogen bonded complex. Probably the electronic structure of the (CH3)-C=O···HO fragment plays an important role in this process. In any case one should take into account the CH vibrations when describing the ν(OH) band of complexes with the C=O···HO hydrogen bridge. The first moment M1 of the complicated ν(XH) band can be used to estimate the shift of the band upon complexation.50 This shift is often associated with the strength of the hydrogen bond. There are several different correlations proposed between these two values.51–54 Despite the different types of functions describing these correlations all of them have one common property: a larger shift corresponds to a stronger bond. Thus, one can conclude (see Table 3) that the hydrogen bond in the TFA···DMK complex is slightly weaker than that in TFA··· DEE and the strength of this bond is approximately the same as in the trifluoroacetic dimer (per one bond). It should be reminded that the same conclusion about the relative strength of H-bonds in complexes with acetone and ether was derived above based on the calculated r(O···O) distances and observed shifts of ν(C=O) bands. The results of the AIM analysis28 of the complexes are presented in Supporting Information in Figures S1-S3; the properties of the electron density in the vicinity of intermolecular bond critical points obtained from B3LYP calculations are presented in Table 4. The comparison of these properties calculated with different functionals a collected in Supplementary Information. The correlations between the potential energy density or Lagrangian form of kinetic energy density and the intermolecular hydrogen bond strength in crystals were proposed55,56 and checked for moderately strong H-bonds.57 We have speculatively applied these correlations to our system, to estimate the relative energy of different intermolecular bonds. It should be note, that absolute values of estimated energies are rather meaningless, because the relations predict the energy if the distances between the atoms in the bonds are obtained in the crystals. 14 ACS Paragon Plus Environment

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Nevertheless these estimations show that the O···HO bond energy in the TFA···DMK complex is lower than that in the TFA···TFA dimer, and noticeably lower that in the TFA···DEE complex. On the other hand, the energy of the weak C-H···O= bond in the complex between the acid and the acetone is much higher than that in the TFA···DEE complex. Thus, the difference in the strength of the weak interactions makes the TFA···DMK complex more stable that the TFA···DEE complex. This also can be used to explain the difference in the complexation energies of TFA···DMK obtained by a vapor density technique22 in the temperature range 21 – 40 °C and by spectroscopic measurements23 in the range 20 – 200 °C. Indeed, at high temperatures, the weak intermolecular bond could be broken, and only the O-H···O hydrogen bond would contribute to the measured energy. Table 4. The AIM Properties of the Intermolecular Bond Critical Points of Complexes Studied: Electron Densitya (ρ), Laplacian of ρ (2ρ), Potential Energy Density (V), Lagrangian Form of Kinetic Energy Density (G), the Estimated Bond Energies from V(ΔE (V)) and from G (ΔE (G)) CP ρ, a.u. V, a.u. ΔE (G)b, ΔE (V)c, 2ρ, a.u. G, a.u. kcal/mol kcal/mol TFA···TFA BCP (3,-1) 0.04844 0.09443 0.03482 −0.0460 9.0 8.9 O···HO TFA···DMK BCP (3,-1) 0.04663 0.09728 0.03417 −0.0440 8.9 8.6 OH···O BCP (3,-1) 0.00976 0.03181 0.00689 −0.0058 1.2 1.1 O···HC TFA···DEE BCP (3,-1) 0.05019 0.09559 0.03599 −0.0481 9.4 9.4 OH···O BCP (3,-1) 0.00590 0.01890 0.00399 −0.0033 0.4 0.6 O···HC a ρ was calculated by B3LYP/6-311++G(2df,2pd) method b ΔE (G) = (0.448∙G − 0.0012)∙ 627.509 (Ref. 56) c ΔE (V) = −0.31∙V∙ 627.509(Ref. 56)

Conclusions The IR spectra of mixtures of trifluoroacetic acid with acetone and diethyl ether and their perdeuterated analogues have been recorded in the gas phase at the room temperature. The spectra of complexes formed between the acid and the proton acceptors have been extracted. 15 ACS Paragon Plus Environment


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Additionally the equilibrium structure, harmonic and anharmonic vibrational frequencies of these complexes have been obtained by DFT calculations. It was found that the profile of the ν(OH) band in the spectra of complexes of trifluoroacetic acid with diethyl ether virtually does not depend on deuteration of the ether whereas these bands differ noticeably in the spectra of the complex of the acid with acetone and acetone-D6. Besides, there are no pronounced ν(CH) or ν(CD) bands in the spectra of TFA···DMK and TFA···DMKD6 complexes. This is the signature of strong interaction between the CH3 (CD3) and OH groups, which should be taken into account in calculations of the ν(OH) band profile. This effect has been observed for the first time in a heterogeneous complex. The calculations show that the complexes between trifluoroacetic acid and both acetone and diethyl ether have a cyclic structure with one strong hydrogen bond and one weak bond CH···O. This weak interaction seems to play an important role in the energetics of the complex formation. Although the H-bond in the TFA···DMK complex is weaker than that in TFA···DEE complex, based on structural, spectroscopic, and electronic properties, the total complexation energy at a relatively low temperature is larger in the first complex.

Supporting Information The file with supporting information contains the tables with Cartesian coordinates of atoms in the complexes and monomers studied; the wavenumbers of maxima of the bands in experimental spectra, calculated frequencies and intensities in harmonic approximation, selected frequencies and intensities calculated in anharmonic approximation; calculated energies of complexation; description of ν(OH) bands; positions and parameters of bond critical points; some aspects of spectra extraction; discussion of ways to obtain the equilibrium constant.

Acknowledgements R.E.A. is grateful to Dr. S. Melikova and Dr. P. Tolstoy for fruitful discussions. Author acknowledges a financial support of the Russian Science Foundation (Project 18-13-00050). The 16 ACS Paragon Plus Environment

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spectra were obtained with core facilities of RC “Geomodel” of St. Petersburg State University. The calculations have been performed using the computer resources of the Resource Centre of SPbGU (http://cc.spbu.ru).

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Figure Captions Figure 1. The optimized structures of complexes calculated at B3LYP/6-311++G(2df,2pd) level of theory: (A) the dimer of trifluoroacetic acid; (B) the TFA···DMK complex; (C) the TFA···DEE complex.

Figure 2. (A) The experimental spectra of trifluoroacetic acid at 6.5 Torr (a) and 3.2 Torr (b). The symbol M denotes the monomer bands. (B) The spectra of TFA monomer (d) and dimer (c) as the result of extraction from experimental spectra at different pressure.

Figure 3. The spectra of a mixture of TFA (4.1 Torr) with DMKD6 at total pressure of 12.3 Torr (a) and the isolated spectrum of the TFA···DMKD6 complex (b).

Figure 4. The spectra of the monomer of trifluoroacetic acid (TFA); the dimer of trifluoroacetic acid (TFA···TFA); monomers of protiated (DMK) and deuterated (DMKD6) acetones; TFA···DMK and TFA···DMKD6 complexes; monomers of protiated (DEE) and deuterated (DEED10) diethyl ethers; TFA···DEE and TFA···DEED10 complexes in the region of ν(C=O) bands.

Figure 5. The spectra of TFA···DMK and TFA···DMKD6 complexes, protiated acetone (DMK), deuterated acetone (DMKD6), TFA···DEE and TFA···DEED10 complexes, protiated diethyl ether (DEE), and deuterated diethyl ether (DEED10), in the range of ν(OH) bands of the corresponding complexes. The dashed lines (TFA···DEE, TFA···DEED10) is the most probable shape of the ν(OH) band of the TFA···DEE complex under the ν(CH) bands and the most probable shape of the ν(OH) band of TFA···DEED10 complex under the ν(CD) bands.


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Figure 1. The optimized structures of complexes calculated at B3LYP 6 311++G(2df,2pd) level of theory: (A) the dimer of trifluoroacetic acid; (B) the TFA···DMK complex; (C) the TFA···DEE complex.



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Figure 2. (A) The experimental spectra of trifluoroacetic acid at 6.5 Torr (a) and 3.2 Torr (b). The symbol M denotes the monomer bands. (B) The spectra of TFA monomer (d) and dimer (c) as the result of extraction from experimental spectra at different pressure.


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Figure 3. The spectra of a mixture of TFA (4.1 Torr) with DMKD6 at total pressure of 12.3 Torr (a) and the isolated spectrum of the TFA···DMKD6 complex (b).



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Figure 4. The spectra of the monomer of trifluoroacetic acid (TFA); the dimer of trifluoroacetic acid (TFA···TFA); monomers of protiated (DMK) and deuterated (DMKD6) acetones; TFA···DMK and TFA···DMKD6 complexes; monomers of protiated (DEE) and deuterated (DEED10) diethyl ethers; TFA···DEE and TFA···DEED10 complexes in the region of ν(C=O) bands.


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Figure 5. The spectra of TFA···DMK and TFA···DMKD6 complexes, protiated acetone (DMK), deuterated acetone (DMKD6), TFA···DEE and TFA···DEED10 complexes, protiated diethyl ether (DEE), and deuterated diethyl ether (DEED10), in the range of ν(OH) bands of the corresponding complexes. The dashed lines (TFA···DEE, TFA···DEED10) is the most probable shape of the ν(OH) band of the TFA···DEE complex under the ν(CH) bands and the most probable shape of the ν(OH) band of TFA···DEED10 complex under the ν(CD) bands.



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TOC Graphic


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Figure 1. The optimized structures of complexes calculated at B3LYP 6 311++G(2df,2pd) level of theory: (A) the dimer of trifluoroacetic acid; (B) the TFA···DMK complex; (C) the TFA···DEE complex. 82x123mm (300 x 300 DPI)

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Figure 2. (A) The experimental spectra of trifluoroacetic acid at 6.5 Torr (a) and 3.2 Torr (b). The symbol M denotes the monomer bands. (B) The spectra of TFA monomer (d) and dimer (c) as the result of extraction from experimental spectra at different pressure. 82x80mm (600 x 600 DPI)


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Figure 3. The spectra of a mixture of TFA (4.1 Torr) with DMKD6 at total pressure of 12.3 Torr (a) and the isolated spectrum of the TFA···DMKD6 complex (b). 82x59mm (600 x 600 DPI)



Figure 4. The spectra of the monomer of trifluoroacetic acid (TFA); the dimer of trifluoroacetic acid (TFA···TFA); monomers of protiated (DMK) and deuterated (DMKD6) acetones; TFA···DMK and TFA···DMKD6 complexes; monomers of protiated (DEE) and deuterated (DEED10) diethyl ethers; TFA···DEE and TFA···DEED10 complexes in the region of ν(C=O) bands. 82x99mm (600 x 600 DPI)


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Figure 5. The spectra of TFA···DMK and TFA···DMKD6 complexes, protiated acetone (DMK), deuterated acetone (DMKD6), TFA···DEE and TFA···DEED10 complexes, protiated diethyl ether (DEE), and deuterated diethyl ether (DEED10), in the range of ν(OH) bands of the corresponding complexes. The dashed lines (TFA···DEE, TFA···DEED10) is the most probable shape of the ν(OH) band of the TFA···DEE complex under the ν(CH) bands and the most probable shape of the ν(OH) band of TFA···DEED10 complex under the ν(CD) bands. 169x80mm (600 x 600 DPI)



TOC Graphic 82x44mm (300 x 300 DPI)


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IR Spectra of Hydrogen-Bonded Complexes of Trifluoroacetic Acid

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