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Understanding and Measurement for the Binding Energy of Hydrogen Bonds of Biomass-Derived Hydroxyl Compounds Yang Luo, Hong Ma, Yuxia Sun, Penghua Che, Xin Nie, Tianlong Wang, and Jie Xu J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b10637 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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The Journal of Physical Chemistry

Understanding and Measurement for the Binding Energy of Hydrogen bonds of Biomass-derived Hydroxyl Compounds Yang Luo,ab Hong Ma,*a Yuxia Sun,ab Penghua Che,a Xin Nie,a Tianlong Wang,a and Jie Xu*a a

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese

Academy of Sciences, Dalian National Laboratory for Clean Energy, Dalian 116023, P. R. China; b

University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

E-mail: [email protected], [email protected]; Phone& Fax: +86-411-84379245.

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ABSTRACT Experimental measurement for the binding energy of hydrogen-bonds (HBs) has long been an attractive and challenging topic in chemistry and biochemistry. In the present study, the binding energy of OH···O HBs can be determined by 1H NMR technique using a set of model biomass-derived hydroxyl compounds, including furfuryl

alcohol,

isosorbide,

(S)-3-hydroxytetrahydrofuran.

By

tetrahydrofurfuryl performing

alcohol,

and

concentration-

and

temperature-variation experiments, we put forward a modified Arrhenius-type equation, in which the compensated natural logarithm of the chemical shift (lnδ + Δδ) is linearly correlated with 1/T. HBs energies can be directly determined by the slope of the plot, and are substantiated by the Density Functional Theory (DFT) theoretical calculations. This study provides a reliable method to measure the binding energy of OH···O HBs in hydroxyl-containing biomass-derived feedstocks. INTRODUCTION Hydrogen bonding is identified as a crucial interaction in chemistry and biological chemistry, and experimental measurement of the binding energy of hydrogen-bonds (HBs) has long been a challenging topic.1,2 The most common approach to predict the HBs energetics (∆E, ∆G, ∆H, and ∆S) depends on the physical properties sensitive to the change of H-bonding, such as chemical resonance (NMR) and absorption frequency (IR and UV).3,4 Particularly, the proton chemical shift (δ) directly reflects the change of electron density (ED) and chemical environments of the H nuclei, making 1H NMR a useful tool for


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HBs investigation. Schaefer performed an initial NMR study to quantify a very nearly linear relationship between theoretical calculated HBs energy (E) and the change of chemical shifts (∆δ) in some ortho-substituted derivatives. They revealed E satisfied the empirical formula (∆δ=−0.4±0.2+E),5 and Afonin et al. further verified it by the Density Functional Theory (DFT).5,6 Reuben also proposed a linear relationship between the DFT HBs energy and ∆δ for aniline derivatives.7 Del Bene et al. found a linear correlation between the computed binding energies and chemical shifts by using the gauge-invariant atomic orbital (GIAO) method.8 Despite these researches revealed HBs energies were related to the change of proton chemical shift, their underlying mechanism has yet to be fully clarified, as well as substantiated by general concepts. Up to date, experimental measurements of HBs energy are currently lacking. Therefore, it is desirable to develop a new approach for determination of the HBs binding energy by NMR spectroscopy. OH···O HBs are ubiquitous in the biomass, and extremely vary the properties in terms of structure, solubility and chemical activity.9-11 As the utility of renewable biomass resource has grown rapidly in recent years, so has the need to deepen the understanding of OH···O HBs for better conversion.12,13 In the previous researches conducted by our laboratory, series of studies have been performed on the catalytic conversion of biomass-derived feedstocks and the critical role of HBs therein.14-18 Our recent work disclosed for the first time an excellent linear correlation between the natural logarithm of OH proton



chemical shift (lnδ) and the inverse of temperature (1/T) for a wide range of hydroxyl compounds by 1H NMR spectroscopy.13 On the basis of this study, we put forward a modified Arrhenius-type equation to give the quantitative basis of the relationship between the binding energy of hydrogen-bonds (Ebinding) and OH proton chemical shifts using four typical sugar-derived hydroxyl compounds, which are among the most widely investigated biomass feedstocks. From the concentration variation experiments, the equilibrium constants and parameters in the equation were acquired; Consequently, E binding can be evaluated from the variation of δ as a function of T by performing temperature variation experiments at a certain concentration. Moreover, Ebinding and association constants obtained by 1H NMR experiments match well with the DFT calculations. This work has provided a reliable method to quantitatively measure the binding energy of hydrogen-bonds for biomass sugar-derived hydroxyl compounds. EXPERIMENTAL AND COMPUTATIONAL METHODS Materials. All measured samples, including furfuryl alcohol (FA), isosorbide (ISB), (S)-3-hydroxytetrahydrofuran (3-HTHF), and tetrahydrofurfuryl alcohol (THFA), were purified by vacuum distillation or dried over activated 3 Å molecular sieves followed by vacuum freeze-pump-thaw technique with liquid nitrogen overnight. 5 mm NMR tubes were dried in a vacuum line at room temperature overnight. NMR. The concentration variation experiments were carried out with a Bruker AVANCE III 400 MHz spectrometer. 1H NMR spectra of the typical hydroxyl


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compounds in CDCl3 were recorded at different concentration at 296K (FA: 0.053 mol/L, 0.101 mol/L, 0.143 mol/L, 0.201 mol/L, and 0.420 mol/L). The temperature variation experiments were carried on a Bruker AVANCE III 700 MHz spectrometer in the range of 283-318 K. 1H NMR spectra of the hydroxyl compounds were recorded at a certain concentration (FA: 0.302 mol/L, ISB: 0.127 mol/L, 3-HTHF: 0.504 mol/L, and THFA: 0.309 mol/L). All the 1H NMR spectra were measured using tetramethylsilane (TMS) as an internal standard and chemical shifts were given in parts per million (ppm). DFT calculation. To obtain a better understanding of solvent binding events and related weak interactions, calculations of H-bonds binding energies by DFT were performed. Geometry optimizations for FA, ISB, 3-HTHF, and THFA were carried out at the level of B3LYP/6-311+g(d,p).19-21 The H-bond distance in related molecules were read from the optimized geometries. The binding energies of these biomass-derived hydroxyl compounds were calculated in gas-phase or in CDCl3 solvent with IEFPCM model.22 Ebinding was computed as the energy of the dimer minus the sum of the energies of the two monomers. All computations were performed by using Gaussian 09 and Multiwfn program.23 The intermolecular HBs model and HBs distance for FA, ISB, 3-HTHF, and THFA are shown in Figure S1.

Figure 1. The equilibrium of intermolecular OH···O HBs of furfuryl alcohol.



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Theory. Our study was based on a theory named “continuous association of hydrogen-bonds”, proposed by Redlich and Kister.24 The forming of hydrogen-bonds involved the continuous association of monomer to n-mer, in equilibrium with the backward dissociation reaction in Figure 1. It is assumed that the equilibrium constant is independent of the value of n, which means the first order association constant K1 is far more larger than all the other equal ones, i.e., K1≫K2=K3…=Kn (Kn is the association constant of the nth order), as has been proved by comparison of potential energies in some academic studies.25 Accordingly, the δ and C satisfied the linear equation (1): δ = δ∞ −

δ∞ −δ0 C0

·C

(1)

where δ is an experimental OH proton chemical shift, and δ∞ is the ideal 1H chemical shift of completed H-bonded OH proton, δ0 is the ideal 1H chemical shift of “free” OH proton that does not form the H-bonds, C0 is the analytical concentration of the hydroxyl compound, and C represents concentration of the hydroxyl compound monomer, and is a function of K1, K2, and C0 . According to the equation Cn = K1 · K n−2 · Cn (Cn represents the concentration of 2 nth order complex) and the mass balance equation C0 = C + 2C2 + 3C3 …+ nCn , equation (2) could be deduced: (K1 · K 2 − K 22 ) · C3 − (2K1 −2K 2 − K 22 · C0 ) · C2 − (2K 2 · C0 + 1) · C + C0 = 0

(2)

Accordingly, the δ and C satisfied the linear equation (1), and equation (3) could be deduced:


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δ = δ∞ −

δ∞ −δ0 C0

2c

3ab+4c2

· 〈3a + 2√(

9a2

1

) · cos {3 · cos −1

8c3 a2 a·b·c ) − + 27 2C0 3 3 a·b 4c2 2 ( + ) 9 3

(

where a = K1 · K 2 − K 2 2 , b = 2K 2 · C0 + 1, c = K1 − K 2 −

K2 2 ·C0 2

−

2π 3

}〉

(3)

.

Results and Discussion To determine the K1, K2, δ0, and δ∞, the concentration variation experiments were performed by using typical biomass sugar-derived hydroxyl compounds including FA, ISB, 3-HTHF, and THFA. Considering that the majority of solvents such as the deuterium DMSO or D2O may competitively form strong intermolecular H-bonds with OH groups, we decided to perform the 1H NMR experiments in the poor H-bond-accepting solvent CDCl3. The 1H NMR spectra in CDCl3 are recorded and shown in Figures S2-5. As expected, all the observed chemical shifts of the OH protons exhibited downfield when the concentrations were increased. For example, the δ of OH proton of FA shifts from 1.52 ppm at 0.053 mol/L to 2.21 ppm at 0.420 mol/L. It is well accepted that HBs leads to a depletion of electron density surrounding the proton and a deshielded nuclei, thereby the protons involved in the H-bonding show the down field shift in the resonance frequency. Next, we use Origin 8.0 software to appropriate the dependence of proton chemical shift on the concentration by equation (3) (see Figure 2). There is fine coincidence between fitting equation (3) and experimental data for the intermolecular H-bonded OH groups in FA, ISB, 3-HTHF, and THFA, even near their individual saturated concentrations. Hereafter, based on the fitting equation (3), the parameters δ∞ δ0, K1, and K2 were obtained and listed in Table 1. Two of these parameters δ0 and δ∞ are



extremely essential to determine binding energies (Ebinding) of HBs. Previous researches also obtained the equilibrium constant but could not measure the Ebinding in the absence of δ0 and δ∞. For all these hydroxyl compounds, K1 is six orders of magnitude larger than K2, so it can be deduced that the intermolecular H-bonds can hardly form except in monomer and dimer forms. It is noted that δ0 of all these hydroxyl compounds are around 1.0 ppm, indicating the 1H chemical shifts of “free” OH groups are basically the same.

Figure 2. Concentration dependence of the 1H NMR shifts of OH in different biomass sugar-derived hydroxyl compounds in CDCl3.


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Table 1 The parameters determined by concentration variation experiments δ0

δ∞

(ppm)

(ppm)

Substrates

K1 (L mol-1)

K2

∆G

(L mol-1)

(kcal mol-1)

FA

0.94±0.23 2.97±0.22 4.97±0.07

1.00×10-6±7.16×10-8 -0.943±0.008

ISB

1.40±0.06 4.28±0.24 2.47±0.27

2.47×10-6±5.11×10-7 -0.532±0.068

3-HTHF

1.09±0.11 6.38±0.51 0.84±0.06

1.02×10-6±5.49×10-8 -0.067±0.091

THFA

1.73±0.06 3.67±0.23 1.12±0.16

1.23×10-6±2.66×10-7 0.103±0.044

Figure 3. The intermolecular H-bonds dependence on temperature of (a) FA, (b) ISB, (c) 3-HTHF, and (d) THFA.

The temperature variation experiments were carried out in CDCl3 in the range of 283-318 K in a certain concentration (FA: 0.302 mol/L, ISB: 0.127



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mol/L, 3-HTHF: 0.504 mol/L, and THFA: 0.309 mol/L) on a Bruker AVANCE III 700 MHz spectrometer, in order to obtain binding energies of intermolecular HBs in the hydroxyl compounds. Figure 3 shows that with the increase of temperature, the weak interaction of H-bonds become weaker. For example, the chemical shift of H-bonded OH proton of FA moves slightly highfield from 2.19 ppm at 283 K to 1.90 ppm at 318 K. We found that K2 ≪ K1·C0 (see Table 1), so K2 could be ignored according to the data. Then the equation (1) could be simplified to equation (4). δ = δ∞ −

2(δ∞ −δ0 )

(4)

1+√1+8K1 ∙C0

With the Arrhenius equation lnK=−ΔE/RT+lnAArrhenius (ΔE = Ebinding= EDimer － 2EMonomer),26 we can deduce equation (5). ln δ + ln(1 −

δ0

δ

) − 2 ln(1 − δ ) = − δ

Ebinding

∞

R·T

− ln

(δ∞ −δ0 )

(5)

2AArrhenius ·C0 ·δ∞ 2

The previous study of our group has reported an Arrhenius-type equation (lnδ=A’/T+B’) satisfied by a function lnδ of 1/T. Herein, equation (5) can be rewritten as a modified equation (6): A

ln δ + Δ𝛿 = T + B δ

δ

where Δ𝛿 = ln(1 − δ0 ) − 2 ln(1 − δ ), A = − ∞

Ebinding R

, and B = − ln 2A

(6) δ∞ −δ0 Arrhenius ·C0 ·δ∞

2

.

Equation (6) is in good agreement with the Arrhenius-type equation, and lnδ is linear to 1/T. It is remarkable that the values of the parameters (δ0 and δ∞) for the sugar-derived hydroxyl compounds can be obtained from fitting equations via


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concentration variation experiments, and thus Δδ can be successfully figured out as a certain number at any δ. Figure 4 shows the well correlated linear lines of 1/T versus lnδ, as well as the compensated lnδ + Δδ. All of the correlation coefficient R2 is 0.99 for FA, ISB, 3-HTHF, and THFA in Figures 4 and S6-8. Compared with the Arrhenius-type equation (lnδ=A’/T+B’) previously put forward by our laboratory, the compensated Δδ leads to a higher slope for the plot of intermolecular H-bonds (2193.1 for FA, 2213.8 for ISB, 1883.3 for 3-HTHF, and 2979.3 for THFA), as well as higher intercept (-4.92 for FA, -6.09 for ISB, -4.55 for 3-HTHF, and -8.34 for THFA). Based on equation (5), the values of binding energies of intermolecular H-bonds have been estimated by the slop of the fitting curves, as well as AArrhenius by the intercept. The Ebinding values for FA, ISB, 3-HTHF, and THFA are -4.36, -4.40, -3.74, and -5.92 kcal mol-1, respectively.



Figure 4 Plots of 1/T versus lnδ and lnδ + Δδ of OH protons for FA. HBs binding energies of all these hydroxyl compounds were calculated by theoretical calculation at the B3LYP/6-311+g(d,p) theoretical level. As presented in Table 2, the calculated binding energy in gas phase (Etheory, gas) for FA, ISB, 3-HTHF and THFA were -5.23, -5.07, -4.90, and -4.98 kcal mol-1, respectively. By comparing Etheory, gas with Ebinding in Table 2, errors of all these sugar-derived hydroxyl compounds were less than 1.2 kcal mol-1, which was acceptable in the theoretical calculation. It indicated that the DFT binding energies matched remarkably well with the experimental results. To get more accurate results, we further took a detailed DFT calculation with solvent effect taken into account, and found the calculated binding energies for FA, ISB, 3-HTHF, and THFA changed to -4.91, -4.27, -4.90, and -5.00 kcal mol-1, respectively. In contrast to the Etheory,

gas,

the recalculated data in solvent reveals a more excellent fit to the

experimental data in term of FA, ISB, and THFA. In the case of 3-HTHF, the calculated binding energy kept unchanged due to its small steric hindrance and ignorable solvent effect. We noticed the trends are not completely the same, although the Etheory, sol values are extremely close to those extracted from experiments. It may be because the real situation of HBs in CDCl3 solvent is much more complicated than that in gas phase.27-29 There are two binding sites for model H-bonded dimer in the DFT calculations, while multi-binding sites in the actual solvent. Besides, other factors such as steric hindrance also interfered the calculation results. As a consequence, geometry optimizations in theoretical calculation have some


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imperfections. In addition, OH···O HBs in hydroxyl groups belong to weak interactions, and thus leads to relatively close binding energies. All these above factors may result in incompletely identical trends. Nevertheless, the difference in the gained energies and trend variations are within the error range. Base on all of the above, Ebinding values from the temperature-variation experiment are substantiated. Table 2 The experimental and calculated HBs binding energies and parameters using B3LYP/6-311+g(d,p) E Substrates (kcalbinding mol-1)

ED (a.u.)

Laplacian of ED (a.u.)

Etheory, gas

Etheory, sol

(kcal mol-1)

(kcal mol-1)

FA

-4.36±0.18

0.0307

0.1146

-5.23

-4.91

ISB

-4.40±0.11

0.0282

0.1069

-5.07

-4.27

3-HTHF

-3.74±0.03

0.0320

0.1152

-4.90

-4.90

THFA

-5.92±0.09

0.0307

0.1075

-4.98

-5.00

The association constants KC and KT were compared at the same level. Herein, 296 K has been chosen to substitute into the Arrhenius equation to achieve KT. And KC obtained from concentration-variation experiments at 296 K (equals to K1). As illustrated in Figure 5a, it is clear that KC nearly equals to the value of KT at 296 K (see Table S1). This further substantiated that the fitting results obtained from NMR experiments are reliable. According to Tables 1 and 2, the larger association constants (KC and KT) do not represent the higher binding energy. That is to say, the equilibrium constants cannot directly reflect the strength of hydrogen binding on account of different collision frequency, molecular structure, and solvent effect.30



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Figure 5. (a) The comparison of KC and KT; (b) The dependence of ΔG and Etheory, gas. Linear free energy plots of theoretical calculation and association constants are one way of comparing computational and experimental results to bridge the gap between gas phase and solution.31,32 Figure 5b shows graphically a linear correlation between Gibbs free energy changes (ΔG) and the HBs binding energy in gas phase, with a correlation coefficient of 0.99. It indicates that calculation of Etheory,

gas

exactly

corroborates well with the experimental results for ΔG (R2>0.99) in CDCl3 solvent. Therefore, the validity of our modified Arrhenius-type equation has been demonstrated through comparison of KC and KT, along with ΔG and Etheory, gas. Conclusion Measurements for the binding energies of hydrogen bonds in model biomass-derived hydroxyl compounds have been realized by using NMR spectroscopy. We revealed the compensated logarithm of the OH chemical shift (lnδ + Δδ) was linearly correlated with 1/T. Binding energies of HBs are directly determined


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by the slope of the (lnδ + Δδ) versus 1/T plot. The association constants obtained from concentration- and temperature-variation experiments have almost the same values, indicating the concentration- and temperature-variation experiments results are in accordance with each other. The DFT binding energies match remarkably well with the experimental binding energies, and have linear relationship with Gibbs free energy changes. The perspective of this study would be the experimental measurement for HBs binding energies of hydroxyl compounds based on NMR proton chemical shifts. This study not only provides us a new method that may determine the binding energy of HBs for biomass-derived hydroxyl compounds, but also help to deepen the understanding of hydrogen bonding.

Acknowledgements This work was supported by National Natural Science Foundation of China (21233008, 21643013, 21690084), Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020300), and Youth Innovation Promotion Association CAS (2013121)

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