Molecular Crystal Structure of Acetylacetone at 210 and 110 K: Is the Crystal Disorder Static or Dynamic?

4-Methoxypicolinic Acid N-Oxide: One of the Shortest Hydrogen Bonds Known Characterized by Neutron Diffraction, Inelastic Neutron Scattering, Infrared Spectroscopy, and Periodic DFT Calculations

The present work focuses on the case of an extremely short intramolecular O-H···O hydrogen bond (H-bond) found in 4-methoxypicolinic acid N-oxide (MPANO). The donor···acceptor separation of 2.403 Å makes the H-bond in MPANO one of the shortest H-bonds known. We elucidated the structure and dynamics of the H-bond by two neutron-based techniques, namely, single-crystal diffraction and inelastic scattering (INS) vibrational spectroscopy. We also utilized conventional infrared (IR) spectroscopy as well as quantum chemical computations on isolated and periodic models. Both the protiated and deuterated variants of MPANO were investigated by INS and IR. All the methods used unequivocally confirm the existence of an extremely short, asymmetric H-bond, with the proton located near yet off the midpoint. The main relevant feature of the IR spectrum is an extremely broad, complex, and red-shifted OH (OD) stretching band spanning between 1800 and 500 cm-1 and centered at about 1360 cm-1, which indicates the presence of extensive anharmonicity and coupling with other H-bond modes. Of the modes characteristic of H-bond dynamics, only the out-of-plane OH (OD) bending can clearly be detected in the INS spectra; it has a relatively high frequency indicative of the strength of the H-bond. The computed structure is in excellent agreement with the diffraction measurement when periodicity is taken into account. The calculated harmonic frequencies show a reasonable match with the observed spectral features, whereby the assignment of the IR and INS spectra is facilitated. The hydrogen stretching frequency, however, appears to be significantly overestimated, on account of the limitations of the harmonic approximation and the complex nature of the short H-bond.

Exploring the Potential of Multinuclear Solid-State 1 H, 13 C, and 35 Cl Magnetic Resonance To Characterize Static and Dynamic Disorder in Pharmaceutical Hydrochlorides

Crystallographic disorder, whether static or dynamic, can be detrimental to the physical and chemical stability, ease of crystallization and dissolution rate of an active pharmaceutical ingredient. Disorder can result in a loss of manufacturing control leading to batch-to-batch variability and can lengthen the process of structural characterization. The range of NMR active nuclei makes solid-state NMR a unique technique for gaining nucleus-specific information about crystallographic disorder. Here, we explore the use of high-field ³⁵ Cl solid-state NMR at 23.5 T to characterize both static and dynamic crystallographic disorder: specifically, dynamic disorder occurring in duloxetine hydrochloride (1), static disorder in promethazine hydrochloride (2), and trifluoperazine dihydrochloride (3). In all structures, the presence of crystallographic disorder was confirmed by ¹³ C cross-polarization magic-angle spinning (CPMAS) NMR and supported by GIPAW-DFT calculations, and in the case of 3, ¹ H solid-state NMR provided additional confirmation. Applying ³⁵ Cl solid-state NMR to these compounds, we show that higher magnetic fields are beneficial for resolving the crystallographic disorder in 1 and 3, while broad spectral features were observed in 2 even at higher fields. Combining the data obtained from ¹ H, ¹³ C, and ³⁵ Cl NMR, we show that 3 exhibits a unique case of disorder involving the ⁺ N-H hydrogen positions of the piperazinium ring, driving the chloride anions to occupy three distinct sites.

Vibrational Dynamics of the Intramolecular H-Bond in Acetylacetone Investigated with Transient and 2D IR Spectroscopy

Acetylacetone (AcAc) has proven to be a fruitful but highly challenging model system for the experimental and computational interrogation of strong intramolecular hydrogen bonds. Key questions remain, however, regarding the identity of the minimum-energy structure of AcAc and the dynamics of intramolecular proton transfer. Here, we investigate the OH/OD stretch and bend regions of the enol tautomer of AcAc and its deuterated isotopologue with transient absorption and 2D IR spectroscopy. The OH bend region reveals a single dominant diagonal transition near 1625 cm^-1 with intense cross peaks to lower-frequency modes, demonstrating highly mixed fingerprint transitions that contain OH bend character. The anharmonic coupling of the OH bend results in a highly elongated OH bend excited-state absorption transition that indicates a large manifold of OH bend overtone/combination bands in the OH stretch region that leads to strong bend-stretch Fermi resonance interactions. The OH and OD stretch regions consist of broad ground-state bleach signals, but there is no clear evidence of ω₂₁ excited-state absorptions due to rapid population relaxation arising from strong intramolecular coupling to bending, fingerprint, and low-frequency H-bond modes. Orientational relaxation dynamics persist for timescales longer than the vibrational lifetimes, with polarization anisotropy components decaying within approximately 2 and 10 periods of the O-O oscillation for the OH and OD stretch, respectively. The significant isotopic dependence of the orientational dynamics is discussed in the context of intramolecular mode coupling, diffusional processes, and contributions from proton/deuteron transfer dynamics.

Hydrogen-Bonded and Halogen-Bonded: Orthogonal Interactions for the Chloride Anion of a Pyrazolium Salt

In the hydrochloride of a pyrazolyl-substituted acetylacetone, the chloride anion is hydrogen-bonded to the protonated pyrazolyl moiety. Equimolar co-crystallization with tetrafluorodiiodobenzene (TFDIB) leads to a supramolecular aggregate in which TFDIB is situated on a crystallographic center of inversion. The iodine atom in the asymmetric unit acts as halogen bond donor, and the chloride acceptor approaches the σ-hole of this TFDIB iodine subtending an almost linear halogen bond, with Cl···I = 3.1653(11) Å and Cl···I-C = 179.32(6)°. This contact is roughly orthogonal to the N-H···Cl hydrogen bond. An analysis of the electron density according to Bader's Quantum Theory of Atoms in Molecules confirms bond critical points (bcps) for both short contacts, with ρbcp = 0.129 for the halogen and 0.321eÅ-3 for the hydrogen bond. Our halogen-bonded adduct represents the prototype for a future class of co-crystals with tunable electron density distribution about the σ-hole contact.

Analyzing Dynamical Disorder for Charge Transport in Organic Semiconductors via Machine Learning

Organic semiconductors are indispensable for today's display technologies in the form of organic light-emitting diodes (OLEDs) and further optoelectronic applications. However, organic materials do not reach the same charge carrier mobility as inorganic semiconductors, limiting the efficiency of devices. To find or even design new organic semiconductors with higher charge carrier mobility, computational approaches, in particular multiscale models, are becoming increasingly important. However, such models are computationally very costly, especially when large systems and long timescales are required, which is the case to compute static and dynamic energy disorder, i.e., the dominant factor to determine charge transport. Here, we overcome this drawback by integrating machine learning models into multiscale simulations. This allows us to obtain unprecedented insight into relevant microscopic materials properties, in particular static and dynamic disorder contributions for a series of application-relevant molecules. We find that static disorder and thus the distribution of shallow traps are highly asymmetrical for many materials, impacting widely considered Gaussian disorder models. We furthermore analyze characteristic energy level fluctuation times and compare them to typical hopping rates to evaluate the importance of dynamic disorder for charge transport. We hope that our findings will significantly improve the accuracy of computational methods used to predict application-relevant materials properties of organic semiconductors and thus make these methods applicable for virtual materials design.

Refinement of labile hydrogen positions based on DFT calculations of 1H NMR chemical shifts: comparison with X-ray and neutron diffraction methods

Numerous gas phase electron diffraction, ultra-fast electron diffraction, X-ray and neutron diffraction experiments on β-dicarbonyl compounds exhibiting enol-enol tautomeric equilibrium, with emphasis on acetylacetone and dibenzoylmethane, have so far been reported with conflicting results on the structural details of the O-HO intramolecular hydrogen bond and resulted in alternative hypotheses on the intramolecular hydrogen bond potential function either a double minimum potential corresponding to two tautomeric forms in equilibrium or a single symmetrical one. We demonstrate herein, firstly, that the DFT calculated OH ¹H NMR chemical shifts of acetylacetone and dibenzoylmethane exhibit a strong linear dependence on the computed OO hydrogen bond length of ∼-50 ppm Å^-1 and as a function of the O-HO bond angle of ∼1 ppm per degree, upon the transfer of the hydrogen atom from the ground state toward the transition state. Secondly, the refinement of labile hydrogen atomic positions in intramolecular hydrogen bonds based on the root-mean-square deviation between experimentally determined and DFT calculated ¹H NMR chemical shifts in solution can provide high resolution structures of O-H and O(H)O bond lengths and O-HO bond angles with an accuracy of ∼10^-2 Å and ∼0.5°, respectively. Thirdly, the calculated ¹H NMR chemical shifts in solution of the two ground state tautomers in equilibrium of acetylacetone and dibenzoylmethane are in excellent agreement with the experimental value, even for moderate basis sets for energy minimization. In contrast, the single symmetrical structure in a strongly delocalized system is a transition state with calculated ¹H NMR chemical shifts which strongly deviate from the experimental value. Fourth, the DFT calculated ground state O-H bond lengths of acetylacetone and dibenzoylmethane are in quantitative agreement with the literature data which take into account the effect of quantum nuclear motion. The DFT structural results are critically discussed with respect to the state-of-the-art variable temperature X-ray and neutron diffraction methods.

Quantum Effects for a Proton in a Low-Barrier, Double-Well Potential: Core Level Photoemission Spectroscopy of Acetylacetone

We have performed core level photoemission spectroscopy of gaseous acetylacetone, its fully deuterated form, and two derivatives, benzoylacetone and dibenzoylmethane. These molecules show intramolecular hydrogen bonds, with a proton located in a double-well potential, whose barrier height is different for the three compounds. This has allowed us to examine the effect of the double-well potential on photoemission spectra. Two distinct O 1s core hole peaks are observed, previously assigned to two chemical states of oxygen. We provide an alternative assignment of the double-peak structure of O 1s spectra by taking full account of the extended nature of the wave function associated with the nuclear motion of the proton, the shape of the ground and final state potentials in which the proton is located, and the nonzero temperature of the samples. The peaks are explained in terms of an unusual Franck-Condon factor distribution.

Easy but not straightforward: base and solvent effect on the synthesis of luminescent europium 1,3-di(thien-2-yl)propane-1,3-dionate coordination complexes

Reaction between EuCl3 and the ligand 1,3-di(thien-2-yl)propane-1,3-dione (L) leads to luminescent coordination compounds [EuL3(EtOH)2] (1), [EuL2(i-PrOH)4]Cl (2) and [EuL3(i-PrOH)2] (3), isolated as single crystals in high yield (70%–95%). The Eu/β-diketonate ratio is tuned from 1:2 to 1:3 through the variation of the alcoholic solvent (ethanol and isopropanol) or the base (pyridine and NaOH). In all compounds, Eu3+ ions are eight-coordinated, and in 2, the molecules are arranged in an H-bond supported supramolecular 1D chain. These compositional and structural differences are reflected also on the absorption and emission properties.

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

We report a combined solid-state (¹H, ²H, ¹³C, ¹⁷O) 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-state ¹⁷O 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.

Proton transfer in acetylacetone and its α-halo derivatives

A two-dimensional potential energy function has been applied to study the bent intramolecular H-bonds within acetylacetone and its α-halo derivatives. The theoretically predicted proton transfer barrier heights correlate very well with geometrical parameters and electronic properties related to the H-bond strength.

Mechanisms of reactions of sulfur hydride hydroxide: tautomerism, condensations, and C-sulfenylation and O-sulfenylation of 2,4-pentanedione

The conformations, equilibrium structures, hydrogen bonds, and non-covalent interactions involved in the mechanisms of tautomerization, condensations, and C-sulfenylation and O-sulfenylation of 2,4-pentanedione by sulfur hydride hydroxide (hydrogen thioperoxide, oxadisulfane, H-SOH) have been studied using BD(T), CCSD(T), and QCISD(T) with the cc-pVTZ basis set and using B3LYP, B3PW91, CAM-B3LYP, PBE1PBE, PBEh1PBE, LC-ωPBE, M06-2X, and ωB97XD with the 6-311+G(d,p) basis set. All levels of theory predict the sulfenyl (H-SOH) tautomer of hydrogen thioperoxide to be lower in energy than the sulfinyl (H2S═O) tautomer. Four reasonable mechanisms were considered for the tautomerization of the sulfenyl tautomer of hydrogen thioperoxide to the sulfinyl tautomer: a cyclic three-membered water-free transition state (TS, CCSD(T) activation energy barrier E(⧧) = 65.1 kcal/mol), a cyclic five-membered transition state with one water molecule (TSH2O, E(⧧) = 31.1 kcal/mol), a cyclic seven-membered transition state with two water molecules (TS2H2O, E(⧧) = 14.5 kcal/mol), and a cyclic nine-membered transition state with three water molecules (TS3H2O, E(⧧) = 5.6 kcal/mol). The mechanisms involve hydrogen-bonded reactant complexes and hydrogen-bonded product complexes. The CCSD(T)-predicted energy barriers for the condensation of hydrogen thioperoxide to form thiosulfinic acid through transition states with zero, one, and two waters are E(⧧) = 42.0, 18.3, and 0 kcal/mol, respectively. Mixed condensation reactions are predicted to afford organosulfur products and compounds containing sulfur-selenium bonds. Hydrogen thioperoxide is predicted to add to 2,4-pentanedione to form C-sulfenylated (sulfide, thioether) and O-sulfenylated (sulfenate ester) products. Similar mechanistic trends and reaction pathways are observed in the tautomerism, condensations, and C-sulfenylation and O-sulfenylation reactions of hydrogen thioperoxide. The water molecules set up proton relay networks (bridges) that reduce ring strain, generate favorable conformations for reactivity, lower energy barriers, and increase the numbers of stabilizing hydrogen bonds and nonbonding interactions.

Hydrogen bond strength and vibrational assignment of the enol form of 3-(ortho-methoxyphenylthio) and 3-(para-methoxyphenylthio)pentane-2,4-dione

The molecular structure of 3-(ortho-methoxyphenylthio) pentane-2,4-dione (o-MPTPD) and 3-(para-methoxyphenylthio) pentane-2,4-dione (p-MPTPD) has been investigated by means of Density Functional Theory (DFT) calculations. The results were compared with 3-(phenylthio) pentane-2,4-dione (PTPD), 3-(methylthio) pentane-2,4-dione (MTPD), and their parent, pentane-2,4-dione (known as acetylacetone, AA). The full optimized geometry, the IR and Raman frequencies and their intensities has been calculated at the B3LYP/6-311++G(∗∗) level of theory. The calculated frequencies were compared with the experimental results. The IR and Raman spectra of o-MPTPD and p-MPTPD and their deuterated analogs are recorded in the 3200-200 cm(-1) range. The quantum theory of atoms in molecules (QTAIM) was applied to calculate the topological parameters of electron density distributions and charge transfer energy associated with the intramolecular hydrogen bond (IHB). Natural bond orbital analysis (NBO) was performed for investigation of electron delocalization in these compounds. According to the theoretical and experimental data, the hydrogen bond strength in the 3-thio-pentane-2,4-dione derivatives is much stronger than that in AA. The results of theoretical calculations are in excellent agreement with the vibrational and NMR spectroscopy data.

Partial rotation of the isopropyl group in the solid state: single-crystal-to-single-crystal phase transformation in a carvacrol derivative

Isopropyl group rotation observed in a single crystal of TACH appears to be a result of the counterbalance of molecular energetics and supramolecular packing in response to the thermal stimulus.

Tuning of the double-well potential of short strong hydrogen bonds by ionic interactions in alkali metal hydrodicarboxylates

The influence of the cation nature on the peculiarities of short strong H-bonds within hydrocarboxylate crystals is revealed.

CONFORMATIONAL STABILITY, MOLECULAR STRUCTURE, AND INTRAMOLECULAR HYDROGEN BONDING OF THENOYLTRIFLUOROACETONE

Complete conformational analysis of all possible keto and enol forms of thenoyltrifluoroacetone (TTFA) was carried out using density functional theory with the B3LYP functional and the 6-31G**, 6-311G**, and 6-311++G** basis sets. In addition, the geometries and energies of the four most stable chelated conformers and their corresponding open structures were obtained at the MP2/6-31G** level of theory. The energy differences between the four stable chelated enol conformers, in the gas phase, calculated at the B3LYP levels are negligible. However, calculations at the MP2 level indicate that the B2 conformer (the hydroxyl group in the - CF 3 side) is significantly more stable than others, in agreement with the X-ray diffraction results. The calculated intramolecular hydrogen bond (IHB) energy E HB and the strength of the bond have been compared, and an imperfection in the prevalent method of evaluating E HB has been perceived. The IHB of TTFA was compared with those in several β-dicarbonyls.