Proton Probability Distribution in the O···H···O Low-Barrier Hydrogen Bond: A Combined Solid-State NMR and Quantum Chemical Computational Study of Dibenzoylmethane and Curcumin

Abstract

We report a combined solid-state (H, H,C, O) NMR and plane-wave density functional theory (DFT) computational study of the O···H···O low-barrier hydrogen bonds (LBHBs) in two 1,3-diketone compounds: dibenzoylmethane (1) and curcumin (2). In the solid state, both 1 and 2 exist in the cis-keto-enol tautomeric form, each exhibiting an intramolecular LBHB with a short O···O distance (2.435 Å in 1 and 2.455 Å in 2). Whereas numerous experimental (structural and spectroscopic) and computational studies have been reported for the enol isomers of 1,3-diketones, a unified picture about the proton location within an LBHB is still lacking. This work reports for the first time the solid-stateO NMR data for the O···H···O LBHBs in 1,3-diketones. The central conclusion of this work is that detailed information about the probability density distribution of the proton (nuclear zero-point motion) across an LBHB can be obtained from a combination of solid-state NMR and plane-wave DFT computations (both NMR parameter calculations and ab initio molecular dynamics simulations). We propose that the precise proton probability distribution across an LBHB should provide a common basis on which different and sometimes seemingly contradicting experimental results obtained from complementary techniques, such as X-ray diffraction, neutron diffraction, and solid-state NMR, can be reconciled.

Elucidating Solvent Effects on Strong Intramolecular Hydrogen Bond: DFT‐MD Study of Dibenzoylmethane in Methanol Solution

“…[10,[15][16][17][18][19][20][21][22][23][24][25][26][27][28] The perplexity caused by the relative stability of the asymmetric and symmetric DBM's protic tautomers is due to the inability of experimental scattering and spectroscopic techniques to resolve ultrafast proton motion in the hydrogen bond, but also possibly due to various experimental conditions and aggregate states in which DBM was studied. [10,[15][16][17][18][19][20][21][22][23][24][25][26][27][28] The perplexity caused by the relative stability of the asymmetric and symmetric DBM's protic tautomers is due to the inability of experimental scattering and spectroscopic techniques to resolve ultrafast proton motion in the hydrogen bond, but also possibly due to various experimental conditions and aggregate states in which DBM was studied.…”

Solid‐State 17O NMR Reveals Hydrogen‐Bonding Energetics: Not All Low‐Barrier Hydrogen Bonds Are Strong

“…[9,16] Figure 3b shows the results of the planewave DFT MD simulations for NA as a2Dproton probability distribution in the q 1 -q 2 space where q 1 = (r OH Àr HN )/2 and q 2 = (r OH + r HN )a re the natural hydrogen bond coordinates commonly used in the valence bond order analysis. [9,16] Figure 3b shows the results of the planewave DFT MD simulations for NA as a2Dproton probability distribution in the q 1 -q 2 space where q 1 = (r OH Àr HN )/2 and q 2 = (r OH + r HN )a re the natural hydrogen bond coordinates commonly used in the valence bond order analysis.…”

“…[9,16] Figure 3b shows the results of the planewave DFT MD simulations for NA as a2Dproton probability distribution in the q 1 -q 2 space where q 1 = (r OH Àr HN )/2 and q 2 = (r OH + r HN )a re the natural hydrogen bond coordinates commonly used in the valence bond order analysis. [9,16] Fore xample,D urlak and Latajka [16c] reported very similar results for benzoylacetone.T hey further showed that the two "peaks" seen in classic ab initio MD simulations merge into one in path-integral MD simulations which take full account of the nuclear quantum effects.T hus,o ur MD data support the conclusion about the LBHB formation in NA.InFigure 3b,the theoretical curve from the valence bond order model is also shown for comparison. [9,16] Fore xample,D urlak and Latajka [16c] reported very similar results for benzoylacetone.T hey further showed that the two "peaks" seen in classic ab initio MD simulations merge into one in path-integral MD simulations which take full account of the nuclear quantum effects.T hus,o ur MD data support the conclusion about the LBHB formation in NA.InFigure 3b,the theoretical curve from the valence bond order model is also shown for comparison.…”

“…[7] Thef irst step in our study was to establish the nature of the HB in NA crystals.A sp revious crystal structures of NA are of only moderate quality, [5] we decided to collect higher quality diffraction data for asingle crystal of NA at different [1b] The 2 Hq uadrupolar coupling constant was 128 kHz, significantly smaller than that expected for ar egular OÀHc ovalent bond, about 200 kHz. [9] We have also obtained the 15 Ni sotropic chemical shift (Figure 2e)a nd chemical shift tensor components for [ 15 N]NA(see Table 1and Figure S1 in the Supporting Information). [9] We have also obtained the 15 Ni sotropic chemical shift (Figure 2e)a nd chemical shift tensor components for [ 15 N]NA(see Table 1and Figure S1 in the Supporting Information).…”

Visualization of H atoms in the X‐ray crystal structure of photoactive yellow protein: Does it contain low‐barrier hydrogen bonds?

“…Their bond energies can be as great as ~37 kcal/mol, and their donor–acceptor distances can be as short as ~2.2 å . Typical O…O distances of LBHBs are ~2.4 å . LBHBs are found in many organic molecules as well as in some enzymes .…”