Dynamics of Formamide-Water Mixtures Investigated by Linear and Nonlinear Infrared Spectroscopy.

Formamide (FA) exhibits complete miscibility with water, offering a simplified model for exploring the solvation dynamics of peptide linkages in biophysical processes. Its liquid state demonstrates a three-dimensional hydrogen bonding network akin to water, reflecting solvent-like behavior. Analyzing the microscopic structure and dynamics of FA-water mixtures is expected to provide crucial insights into hydrogen bonding dynamics─a key aspect of various biophysical phenomena. This study is focused on the dynamics of FA-water mixtures using linear and femtosecond infrared spectroscopies. By using the intrinsic OD stretch and extrinsic probe SCN-, the local vibrational behaviors across various FA-water compositions were systematically investigated. The vibrational relaxation of OD stretch revealed a negligible impact of FA addition on the vibrational lifetime of water molecules, underscoring the mixture's water-like behavior. However, the reorientational dynamics of OD stretch slowed with increasing FA mole fraction (XFA), plateauing beyond XFA > 0.5. This suggests a correlation between OD's reorientational time and the strength of the hydrogen bond network, likely tied to the solution's changing dielectric constant. Conversely, the vibrational relaxation dynamics of SCN- was strongly correlated with XFA, highlighting a competition between water and FA molecules in solvating SCN-. Moreover, a linear relationship between rising viscosity and the prolonged correlation time of SCN-'s slow dynamics indicates that the solution's macroscopic viscosity is dictated by the extended structures formed between FA and water molecules. The relation between the reorientation dynamics of the SCN- and the macroscopic viscosity in aqueous FA-water mixture solutions was analyzed by using the Stokes-Einstein-Debye equations. The direct viscosity-diffusion coupling is observed, which can be attributed to the homogeneous dynamics feature in FA-water mixture solutions. The inclusion of these intrinsic and extrinsic probes not only enhances the comprehensiveness of our analysis but also provides valuable insights into various aspects of the dynamics within the FA-water system. This investigation sheds light on the fundamental dynamics of FA-water mixtures, emphasizing their molecular-level homogeneity in this binary mixture solution.

Anion Recognition in Solution: Insights from Thermodynamics and Ultrafast Structural Dynamics.

Anion recognition through noncovalent interactions stands as an emerging field in supramolecular chemistry, exerting a profound influence on the regulation of biological functions. Herein, the thermodynamics of complexation between sodium cyanate (NaOCN) and calix[4]pyrrole was systematically investigated by linear and nonlinear IR spectroscopy, highlighting enthalpy changes as the dominant driving force. The overall orientational relaxation of bound anion can be described by an Arrhenius-type activated process, yielding an activation energy of 15.0 ± 1.0 kJ mol-1. The structural dynamics of contact ion pairs (CIPs) formed between Na+ and OCN- in solution showed a negligible temperature effect, suggesting entropy changes as the principal governing factor. Further analysis revealed that anion recognition in solution is mediated by conformational changes of the receptor and collective rearrangement of hydrogen bond dynamics. This study, framed within the paradigms of thermodynamics and ultrafast structural dynamics, substantially advances our comprehension of the microscopic mechanisms underlying anion recognition in the realm of supramolecular chemistry.

Non-negligible Axial Ligand Effect on Electrocatalytic CO2 Reduction with Iron Porphyrin Complexes.

Iron(III) porphyrin complexes have been demonstrated as one of the efficient molecular catalysts for the electrochemical reduction of CO2. However, the role of axial ligands coordinated with a metal center in the complex on the electrochemical CO2 reduction activity has not been fully explored yet. Herein, iron(III) tetraphenylporphyrin thiocyanate (FeTPP-SCN) is synthesized from a commercially available catalyst of FeTPP-Cl by a counteranion exchanging reaction. Cyclic voltammetry measurements showed that the catalytic activity of FeTPP-SCN is noticeably suppressed in the DMF solutions. The structural dynamics of the axial ligand in FeTPP-SCN are further examined by the FTIR and ultrafast IR spectroscopies, where the SCN ligand is employed as the local vibrational probe. Vibrational relaxation measurements showed that the reorientational dynamics of SCN ligands was strongly restricted in DMF solution, suggesting that the subtle electrostatic interaction between the ligands and metal center in the complex can have a non-negligible effect on its catalytic activity.

Distinct Hydrogen Bonding Dynamics Underlies the Microheterogeneity in DMF-Water Mixtures.

The hydrogen bonding interaction between the amide functional group and water is fundamental to understanding the liquid-liquid heterogeneity in biological systems. Herein, the structure and dynamics of the N,N-dimethylformamide (DMF)-water mixtures have been investigated by linear and nonlinear IR spectroscopies, using the hydroxyl stretch and extrinsic probe of thiocyanate as local vibrational reporters. According to vibrational relaxation dynamics measurements, the orientational dynamics of water is not directly tied to those of DMF molecules. Wobbling-in-a-cone analysis demonstrates that the water molecules have varying degrees of angular restriction depending on their composition due to the formation of specific water-DMF networks. Because of the preferential solvation by DMF molecules, the rotational dynamics of the extrinsic probe is slowed significantly, and its rotational time constants are correlated to the change of solution viscosity. The unique structural dynamics observed in the DMF-water mixtures is expected to provide important insights into the underlying mechanism of microscopic heterogeneity in binary mixtures.

Diagnostic value of serum miR-25-3p in hypertensive disorders in pregnancy.

Hypertensive disorders in pregnancy (HDIP) represent one of the leading causes of maternal and perinatal mortality. microRNA (miR)-25-3p plays roles in HDIP diagnosis. We explored miR-25-3p clinical roles in HDIP. HDIP patients [gestation hypertension (GH), mild preeclampsia (mPE), and severe preeclampsia (sPEz)], and normal pregnant women serving as the control were enrolled. Serum miR-25-3p expression patterns were detected by RT-qPCR. The diagnostic efficacy of miR-25-3p on HDIP was analyzed with a ROC curve. Patients were assigned to the high/low miR-25-3p expression groups according to the median value of miR-25-3p expression. All patients were followed up until delivery, and gestational weeks and pregnancy outcomes were recorded at delivery. The effects of miR-25-3p expression on pregnancy outcomes of GH, mPE, and sPEz patients were analyzed by Kaplan-Meier. miR-25-3p expression in GH, mPE, and sPEz patients was up-regulated. In sPEz patients, systolic and diastolic blood pressure, 24-h urine protein, AST, ALT, GGT, and SCr were increased, and PLT was decreased in the high expression group. High miR-25-3p expression was associated with an increased risk of adverse pregnancy outcomes in PE patients. Collectively, high miR-25-3p expression could aid HDIP diagnosis, and associated with an increased risk of adverse pregnancy outcomes in PE patients.

Structural Dynamics of Short Ligands on the Surface of ZnSe Semiconductor Nanocrystals.

ZnSe semiconductor nanocrystals (NCs) with a size comparable to their Bohr radius are synthesized, and the native capping agents with long hydrocarbon tails are replaced with short thiocyanate (SCN) ligands through a ligand exchange method. The structural dynamics of SCN ligands on the surface of ZnSe NCs in solution is investigated by ultrafast infrared spectroscopy. Vibrational population relaxation of SCN ligands is accelerated due to the specific interaction with the positively charged sites on the surface of NCs. The orientational anisotropy of the bound SCN ligands decayed at a rate much faster than that in the control solution containing Zn2+ cations. From the wobbling-in-the-cone model analysis, we found that the SCN ligand undergoes wobbling orientational diffusion with a relatively large cone semiangle on the surface of ZnSe NCs, and the overall orientational diffusion of bound SCN is found to be strongly dependent on the size of ZnSe NCs.

Exploring the Structure and Complexation Dynamics of Azide Anion Recognition by Calix[4]pyrroles in Solution.

The structure and anion recognition dynamics between calix[4]pyrroles and azide (N3-) anions in the form of its TBA+ and Na+ salts were investigated in dimethyl sulfoxide solutions by Fourier transform infrared (FTIR) spectroscopy and ultrafast IR spectroscopy. Vibrational energy redistribution of the N3- anion in the complex is accelerated through hydrogen bonding interactions with the N-H proton of the receptor. Rotational dynamics of the bound N3- is greatly restricted, demonstrating a distinct countercation effect. The detailed binding modes of N3- with the receptor were further evaluated by the density functional theoretical (DFT) calculations and nuclear magnetic resonance (NMR) spectroscopy. All of these measurements support the notion that the calix[4]pyrroles are capable of capturing the azide anion in solution. However, the calix[4]pyrroles may not necessarily undergo a conformational change to a cone-like geometry when they bind to the azide anion in the solution.

Introducing Water-Network-Assisted Proton Transfer for Boosted Electrocatalytic Hydrogen Evolution with Cobalt Corrole.

Proton transfer is vital for many biological and chemical reactions. Hydrogen-bonded water-containing networks are often found in enzymes to assist proton transfer, but similar strategy has been rarely presented by synthetic catalysts. We herein report the Co corrole 1 with an appended crown ether unit and its boosted activity for the hydrogen evolution reaction (HER). Crystallographic and 1 H NMR studies proved that the crown ether of 1 can grab water via hydrogen bonds. By using protic acids as proton sources, the HER activity of 1 was largely boosted with added water, while the activity of crown-ether-free analogues showed very small enhancement. Inhibition studies by adding 1) external 18-crown-6-ether to extract water molecules and 2) potassium ion or N-benzyl-n-butylamine to block the crown ether of 1 further confirmed its critical role in assisting proton transfer via grabbed water molecules. This work presents a synthetic example to boost HER through water-containing networks.

Cationic Effects on the Structural Dynamics of the Metal Ion-Crown Ether Complexes Investigated by Ultrafast Infrared Spectroscopy.

It is usually believed that the binding affinity and selectivity of an alkali metal ion with crown ether are defined by the size matching model. However, the underlying mechanism of the specific host-guest interactions and the structural dynamics of the metal ions confined in the cavity of the crown ethers in the solutions are still not clear. In this report, a series of alkali thiocyanate salts (XSCN; X = Li, Na, K, and Cs) complexed with 18-crown-6 (a typical crown ether) in the chloroform solutions were studied by the polarization-selective infrared pump-probe spectroscopy and the ultrafast two-dimensional infrared (2D IR) spectroscopy. The SCN- counteranions were employed as the local vibrational probe to reveal the specific host-guest interactions in the crown ether complexes. The rotational dynamics and spectral diffusion of SCN- vibration were both measured by ultrafast IR spectroscopy, and it was found that the metal cations hosted by the crown ethers can have a pronounced effect on the rotational dynamics of the counteranions. The reorientational time constants of the SCN- vibration in the complexation follow the order Li+ > Na+ > K+ ≃ Cs+. More importantly, the spectral diffusion dynamics of SCN-, which quantifies the decay of the correlation of the frequency fluctuations in the complexation, was also affected by the metal ions but showed a different order of cationic effect. A detailed analysis of the 2D IR data showed that the spectral diffusion of SCN- counteranion clearly decayed with two different time scales in the complex of 18-crown-6 with K+. The 3-4-fold slowdown in spectral diffusion indicated that the fluctuation of SCN- vibrational transition frequency was strongly affected by the K+ cation due to the geometric constraint imposed by the crown ether. The results should help the researchers to unravel the specific host-guest interactions and further reveal the origination of the binding selectivity of crown ether for metal cations in the condensed phases from the perspective of structural dynamics.

Specific Host-Guest Interactions in the Crown Ether Complexes with K+ and NH4+ Revealed from the Vibrational Relaxation Dynamics of the Counteranion.

The specific host-guest interactions in the corresponding complexes of K+ and NH4+ with typical crown ethers were investigated by using FTIR and ultrafast IR spectroscopies. The counteranions, i.e., SCN-, were employed as a local vibrational probe to report the structural dynamics of the complexation. It was found that the vibrational relaxation dynamics of the SCN- was strongly affected by the cations confined in the cavities of the crown ethers. The time constant of the vibrational population decay of SCN- in the complex of NH4+ with the 18-crown-6 was determined to be 6 ± 2 ps, which is ∼30 times faster than that in the complex of K+ with the crown ethers. Control experiments showed that the vibrational population decay of SCN- depended on the size of the cavities of the crown ethers. A theoretical calculation further indicated that the nitrogen atom of SCN- showed preferential coordination to the K+ ions hosted by the crown ethers, while the NH4+ can form hydrogen bonds with the oxygen atoms in the studied crown ethers. The geometric constraints formed in the complex of crown ethers can cause a specific interaction between the NH4+ and SCN-, which can facilitate the intermolecular vibrational energy redistribution of the SCN-.

Interface Catalysts of Ni/Co2N for Hydrogen Electrochemistry.

The development of active, durable, and nonprecious electrocatalysts for hydrogen electrochemistry is highly desirable but challenging. In this work, we design and fabricate a novel interface catalyst of Ni and Co2N (Ni/Co2N) for hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR). The Ni/Co2N interfacial catalysts not only achieve a current density of -10.0 mA cm-2 with an overpotential of 16.2 mV for HER but also provide a HOR current density of 2.35 mA cm-2 at 0.1 V vs reversible hydrogen electrode (RHE). Furthermore, the electrode couple made of the Ni/Co2N interfacial catalysts requires only a cell voltage of 1.57 V to gain a current density of 10 mA cm-2 for overall water splitting. Hybridizations in the three elements of Ni-3d, N-2p, and Co-3d result in charge transfer in the interfacial junction of the Ni and Co2N materials. Our density functional theory calculations show that both the interfacial N and Co sites of Ni/Co2N prefer to hydrogen adsorption in the hydrogen catalytic activities. This study provides a new approach for the construction of multifunctional catalysts for hydrogen electrochemistry.

Vibrational Relaxation Dynamics of a Semiconductor Copper(I) Thiocyanate (CuSCN) Film as a Hole-Transporting Layer.

The semiconductor CuSCN film, which is typically used as the hole-transporting layer (HTL) in solar cell studies, has been investigated by Fourier transform infrared (FTIR) spectroscopy and ultrafast transient infrared (IR) spectroscopy. A sharp peak at 2175 cm-1 corresponding to the CN vibrational stretching mode in CuSCN was observed, and the peak frequency remained unchanged by varying the thickness of the CuSCN thin film. Vibrational relaxation measurements showed that the 0-1 and 1-2 transitions of CN stretching can be observed at 2175 and 2140 cm-1, respectively. The heat-induced absorption and bleaching peaks (2167 and 2175 cm-1) can be clearly seen at a waiting time of 40 ps. The vibrational relaxation of the CN stretching mode determined from the 1-2 transition exhibited a biexponential decay with time constants of 7.4 ± 0.5 (90%) and 158 ± 50 ps (10%). Importantly, the abnormal anisotropy decay of the CN stretching mode in the CuSCN thin film was also observed for the first time. A detailed analysis showed that the distinct anisotropy decay curve could be described using a triexponential decay function, which was explained by three different processes: resonance energy transfer (∼8 ps), a thermalization process (∼40 ps), and molecular rotation (∼150 ps). The time scale of the thermalization process caused by the vibrational relaxation in CuSCN is at a time scale of 40 ps, which is important for us to understand the thermally activated charge-transport property of the CuSCN film employed as the HTL. Further UV pump-IR probe measurement revealed that the carrier scattering and relaxation processes in the CuSCN film are strongly associated with the vibrational excitation and relaxation dynamics of the CN stretching mode. It is expected that the fundamental understanding of the vibrational relaxation dynamics of the CuSCN thin film should provide helpful insight to elucidate its role as the HTL in solar cell studies at the molecular level.

Understanding the intramolecular vibrational energy transfer and structural dynamics of anionic ligands in a photo-catalytic CO2 reduction catalyst.

The knowledge of intramolecular vibrational energy redistribution (IVR) and structural dynamics of rhenium photo-catalysts is essential for understanding the mechanism of the photo-catalytic process of CO2 reduction. In this study, the rhenium compound Re(dcbyp)(CO)3NCS (Re-NCS), which served as a model CO2 reduction catalyst, was investigated using two dimensional infrared (2D IR) spectroscopy. The vibrational relaxation dynamics and rotational dynamics of Re-NCS were measured by monitoring both the CO and NCS vibrational modes. The rotational dynamics measurement of the CO vibrational stretch shows a single exponential decay with a time constant of 140 ± 10 ps. In contrast, a bi-exponential decay is needed to describe the rotational dynamics of the NCS stretching mode with time constants of 1.5 ± 0.3 ps and 189 ± 15 ps. The 2D IR experiment indicated that the carbonyl CO vibrational modes in Re-NCS are strongly coupled. Furthermore, the intramolecular vibrational energy transfer between CO and NCS stretching modes was observed and analyzed based on an energy exchange model. The energy down flowing transfer from CN to CO stretching mode was determined using time constants of 50 ps. The relatively slow intramolecular vibrational energy transfer rate suggests that there is a weak coupling between CO and NCS ligands. Further theoretical calculation showed that the coupling strength between CO and CN is relatively weak and is about 5-6 times smaller than the coupling strength between the CO vibrational modes in Re-NCS. The distinct structural dynamics of the NCS ligand in Re-NCS presented in this study should provide a fundamental understanding of the role of an anionic ligand in rhenium photo-catalysts, which is believed to play an important role in the photo-catalytic reduction of CO2.

Ultrafast Hydrogen Bond Exchanging between Water and Anions in Concentrated Ionic Liquid Aqueous Solutions.

The mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) ionic liquids (ILs) and water as a function of IL concentrations have been investigated by Fourier transform infrared (FTIR) spectroscopy and ultrafast two-dimensional IR (2D IR) spectroscopy. FTIR spectra of the mixtures resolve two different types of water species, one interacting with the BF4- anions and the other associated with bulklike water molecules. These two water species are in a dynamic equilibrium through forming different hydrogen bonding configurations which are separated by more than 100 cm-1 in the IR spectra. The structural dynamics of the IL mixtures are further revealed by monitoring the vibrational relaxation dynamics of the OD stretching group of interfacial water molecules hydrogen bonded to BF4- anions. With the increase of the IL bulk concentration, vibrational population and rotational dynamics of the interfacial water molecules can be described by a biexponential decay function and are strongly dependent on the IL concentrations. Furthermore, the ultrafast hydrogen bond exchanging between water and BF4- anions in the ILs are also measured using 2D IR spectroscopy. The average hydrogen bond exchanging rate is determined to be 19 ± 4 ps, which is around 3 times slower than that in the NaBF4 electrolyte aqueous solution. The much slower hydrogen bond exchanging rate indicates that the local structure of ILs and water molecules are strongly mediated by the steric effect of the cationic group in the ILs, which is proposed to be responsible for the formation of the heterogeneous structure in the IL mixtures. By using SCN- as the anionic probe, the structural inhomogeneity in the IL solutions can be confirmed from the distinct rotational dynamics of the SCN-, which is segregated from the rotational dynamics of water molecules in the IL mixtures.

Counterion Effect on Vibrational Relaxation and the Rotational Dynamics of Interfacial Water and an Anionic Vibrational Probe in the Confined Reverse Micelles Environment.

Vibrational relaxation and the rotational dynamics of water molecules encapsulated in reverse micelles (RMs) have been investigated by ultrafast infrared (IR) spectroscopy and two-dimensional IR (2D IR) spectroscopy. By changing the counterion of the hydrophilic headgroup in the RMs formed by Aerosol-OT (AOT) from Na+ to K+, Cs+ and Ca2+, we could determine the specific counterion effects on the rotational dynamics of water molecules. The orientational relaxation time constant of water decreases in the order Ca2+ > Na+ > K+ > Cs+. The SCN- anionic probe and counterion can form ion pairs at the interfacial region of the RMs. The rotational dynamics of SCN- anion significantly decreases because of the synergistic effects of confinement and the surface interactions in the interfacial region of the RMs. The results can provide a new understanding of the cationic Hofmeister effect at the molecular level observed in biological studies.

Structural Dynamics of Dimethyl Sulfoxide Aqueous Solutions Investigated by Ultrafast Infrared Spectroscopy: Using Thiocyanate Anion as a Local Vibrational Probe.

The microscopic structure of dimethyl sulfoxide (DMSO) aqueous solutions was investigated by Fourier transform infrared (FTIR) spectroscopy and ultrafast IR spectroscopy. The structural dynamics of the binary mixtures were reflected by using thiocyanate anion (SCN-) as a local vibrational probe. FTIR spectra of SCN- anion showed that the hydrogen bond networks of water are affected by the presence of DMSO molecules, and the peak position and bandwidth of SCN- anions are red shifted and narrowed accordingly because of the weak hydration in the binary mixture. The vibrational lifetime of the SCN- anion showed almost linear enhancement with the increase of DMSO, which can be explained by the weak interaction between SCN- and the hydrophobic groups in the DMSO molecule. However, the rotational dynamics of SCN- are slowing down significantly and showed a maximum response at XDMSO (mole fraction) of 0.35, which is mainly caused by the confinement of SCN- anions positioned in the vicinity of the complex structure formed between DMSO and water molecules. The concentration-dependent rotational dynamics of water molecules and SCN- anions are having similar behavior, indicating that the complex structure can be formed between water and DMSO molecules because of the strong interaction. The result also demonstrates that the structural inhomogeneity in aqueous solution can be unraveled by monitoring the vibrational relaxation dynamics of SCN- anion serving as the local vibrational probe.

Molecular structure and adsorption of dimethyl sulfoxide at the air/aqueous solution interface probed by non-resonant second harmonic generation.

In this study, non-resonant second harmonic generation (SHG) was used to investigate the molecular structure and adsorption of DMSO at the air/neat DMSO liquid and air/DMSO aqueous solution interfaces. The molecular orientation of interfacial DMSO as a function of the bulk DMSO concentration was investigated by quantitative polarization SHG analysis. For the air/neat DMSO liquid interface, the transition dipole moment of the S[double bond, length as m-dash]O group of DMSO is oriented 140° from the surface normal, where the S[double bond, length as m-dash]O group of DMSO is estimated to be 30° from the surface plane. The orientation of the S[double bond, length as m-dash]O group of interfacial DMSO is not dependent on the bulk DMSO concentration. Furthermore, the concentration-dependent SHG signal confirmed that the antiparallel double layer structure does not form at the air/DMSO water interface. The free energy of adsorption of DMSO at the air/DMSO aqueous solution interface was determined to be ΔGads = -5.6 ± 0.4 kJ mol-1.