Observation of the Low‐Frequency Spectrum of the Water Dimer as a Sensitive Test of the Water Dimer Potential and Dipole Moment Surfaces

Using the helium nanodroplet isolation setup at the ultrabright free‐electron laser source FELIX in Nijmegen (BoHeNDI@FELIX), the intermolecular modes of water dimer in the frequency region from 70 to 550 cm^?1 were recorded. Observed bands were assigned to donor torsion, acceptor wag, acceptor twist, intermolecular stretch, donor torsion overtone, and in‐plane and out‐of‐plane librational modes. This experimental data set provides a sensitive test for state‐of‐the‐art water potentials and dipole moment surfaces. Theoretical calculations of the IR spectrum are presented using high‐level quantum and approximate quasiclassical molecular dynamics approaches. These calculations use the full‐dimensional ab initio WHHB potential and dipole moment surfaces. Based on the experimental data, a considerable increase of the acceptor switch and a bifurcation tunneling splitting in the librational mode is deduced, which is a consequence of the effective decrease in the tunneling barrier.     

Ground‐State Structure of the Proton‐Bound Formate Dimer by Cold‐Ion Infrared Action Spectroscopy

The proton‐bound dicarboxylate motif, RCOO^??H⁺?^?OOCR, is a prevalent chemical configuration found in many condensed‐phase systems. The proton‐bound formate dimer HCOO^??H⁺?^?OOCH was studied utilizing cold‐ion IR action spectroscopy in the range 400–1800 cm^?1. The spectrum obtained at ca. 0.4 K of ions captured in He nanodroplets was compared to that measured at ca. 10 K by photodissociation of Ar‐ion complexes. Similar band patterns are obtained by the two techniques that are consistent with calculations for a C₂ symmetry structure with a proton shared equally between the two formate moieties. Isotopic substitution experiments point to the nominal parallel stretch of the bridging proton appearing as a sharp, dominant feature near 600 cm^?1. Multidimensional anharmonic calculations reveal that the bridging proton motion is strongly coupled to the flanking ?COO^? framework, an effect that is in line with the expected change in ?C=O bond rehybridization upon protonation.     

Vibrational Spectroscopy of the Dehydrogenated Uracil Radical by Autodetachment of Dipole‐Bound Excited States of Cold Anions

Molecules with large enough dipole moments can bind an electron by the dipole field, which has little effect on the molecular core. A molecular anion can be excited to a dipole‐bound state, which can autodetach by vibronic coupling. Autodetachment spectroscopy of a complex anion cooled in a cryogenic ion trap is reported. Vibrational spectroscopy of the dehydrogenated uracil radical is obtained by a dipole‐bound state with partial rotational resolution. Fundamental frequencies for 21 vibrational modes of the uracil radical are reported. The electron affinity of the uracil radical is measured accurately to be 3.4810±0.0006 eV and the binding energy of the dipole‐bound state is measured to be 146±5 cm^?1. The rotational temperature of the trapped uracil anion is evaluated to be 35 K.     

Hydrogen Bond Cooperativity and the Three‐Dimensional Structures of Water Nonamers and Decamers

Broadband rotational spectroscopy of water clusters produced in a pulsed molecular jet expansion has been used to determine the oxygen atom geometry in three isomers of the nonamer and two isomers of the decamer. The isomers for each cluster size have the same nominal geometry but differ in the arrangement of their hydrogen bond networks. The nearest neighbor O? O distances show a characteristic pattern for each hydrogen bond network isomer that is caused by three‐body effects that produce cooperative hydrogen bonding. The observed structures are the lowest energy cluster geometries identified by quantum chemistry and the experimental and theoretical O? O distances are in good agreement. The cooperativity effects revealed by the hydrogen bond O? O distance variations are shown to be consistent with a simple model for hydrogen bonding in water that takes into account the cooperative and anticooperative bonding effects of nearby water molecules.     

Synthesis,Isolation, and Spectroscopic Characterization of Holmium‐Based Mixed‐Metal Nitride Clusterfullerenes: HoxSc3−xN@C80 (x=1, 2)

The synthesis, isolation and spectroscopic characterization of holmium‐based mixed metal nitride clusterfullerenes Ho_xSc_3?xN@C₈₀ (x=1, 2) are reported. Two isomers of Ho_xSc_3?xN@C₈₀ (x=1, 2) were synthesized by the reactive gas atmosphere method and isolated by multistep recycling HPLC. The isomeric structures of Ho_xSc_3?xN@C₈₀ (x=1, 2) were characterized by laser‐desorption time‐of‐flight (LD‐TOF) mass spectrometry and UV/Vis/NIR, FTIR and Raman spectroscopy. A comparative study of M_xSc_3?xN@C₈₀ (M=Gd, Dy, Lu, Ho) demonstrates the dependence of their electronic and vibrational properties on the encaged metal. Despite the distinct perturbation induced by 4f¹⁰ electrons, we report the first paramagnetic ¹³C NMR study on Ho_xSc_3?xN@C₈₀ (I; x=1, 2) and confirm I_h‐symmetric cage structure. A ⁴⁵Sc NMR study on HoSc₂N@C₈₀ (I, II) revealed a temperature‐dependent chemical shift in the temperature range of 268–308 K.     

Nanometer‐Scale Water‐ and Proton‐Diffusion Heterogeneities across Water Channels in Polymer Electrolyte Membranes

Nafion, the most widely used polymer for electrolyte membranes (PEMs) in fuel cells, consists of a fluorocarbon backbone and acidic groups that, upon hydration, swell to form percolated channels through which water and ions diffuse. Although the effects of the channel structures and the acidic groups on water/ion transport have been studied before, the surface chemistry or the spatially heterogeneous diffusivity across water channels has never been shown to directly influence water/ion transport. By the use of molecular spin probes that are selectively partitioned into heterogeneous regions of the PEM and Overhauser dynamic nuclear polarization relaxometry, this study reveals that both water and proton diffusivity are significantly faster near the fluorocarbon and the acidic groups lining the water channels than within the water channels. The concept that surface chemistry at the (sub)nanometer scale dictates water and proton diffusivity invokes a new design principle for PEMs.     

Theoretical Studies on the Potential Energy Surface and Vibrational Energy Levels of HXeBr

The potential energy surface for the electronic ground state of the HXeBr molecule is constructed from more than 4200 ab initio points calculated using the internally contracted multi-reference configuration interaction method with the Davidson correction （icMRCI ＋ Q）. The stabilities and dissociation barriers are identified from the potential energy surface. The three-body dissociation channel is found to be the dominant dissociation channel for HXeBr. Low-lying vibrational energy levels of HXeBr calculated using the Lanczos algorithm are found to be in good agreement with the available experimental band origins.     

Aqueous Heterogeneity at the Air/Water Interface Revealed by 2D‐HD‐SFG Spectroscopy

Water molecules interact strongly with each other through hydrogen bonds. This efficient intermolecular coupling causes strong delocalization of molecular vibrations in bulk water. We study intermolecular coupling at the air/water interface and find intermolecular coupling 1) to be significantly reduced and 2) to vary strongly for different water molecules at the interface—whereas in bulk water the coupling is homogeneous. For strongly hydrogen‐bonded OH groups, coupling is roughly half of that of bulk water, due to the lower density in the near‐surface region. For weakly hydrogen‐bonded OH groups that absorb around 3500 cm^?1, which are assigned to the outermost, yet hydrogen‐bonded OH groups pointing towards the liquid, coupling is further reduced by an additional factor of 2. Remarkably, despite the reduced structural constraints imposed by the interfacial hydrogen‐bond environment, the structural relaxation is slow and the intermolecular coupling of these water molecules is weak.     

Electronic and Vibrational Spectroscopy of 1‐Methylthymine and its Water Clusters: The Dark State Survives Hydration

Electronic and vibrational gas phase spectra of 1‐methylthymine (1MT) and 1‐methyluracil (1MU) and their clusters with water are presented. Mass selective IR/UV double resonance spectra confirm the formation of pyrimidine‐water clusters and are compared to calculated vibrational spectra obtained from ab initio calculations. In contrast to Y. He, C. Wu, W. Kong; J. Phys. Chem. A, 2004 , 108, 94 we are able to detect 1MT/1MU and their water clusters via resonant two‐photon delayed ionization under careful control of the applied water‐vapor pressure. The long‐living dark electronic state of 1MT and 1MU detected by delayed ionization, survives hydration and the photostability of 1MT/1MU cannot be attributed solely to hydration. Oxygen coexpansions and crossed‐beam experiments indicate that the triplet state population is probably small compared to the ¹nπ* and/or hot electronic ground state population. Ab initio theory shows that solvation of 1MT by water does not lead to a substantial modification of the electronic relaxation and quenching of the ¹nπ* state. Relaxation pathways via ¹ππ*¹–nπ*¹ and ¹ππ *–S₀ conical intersections and barriers have been identified, but are not significantly altered by hydration.     

Concentration‐Dependent Hydrogen‐Bonding Effects on the Dimethyl Sulfoxide Vibrational Structure in the Presence of Water,Methanol, and Ethanol

The effects of hydrogen bonding between dimethyl sulfoxide (DMSO) and the co‐solvents water, methanol, and ethanol on the symmetric and antisymmetric CSC stretching vibrations of DMSO are investigated by means of Raman spectroscopy. The Raman spectra are recorded as a function of co‐solvent concentration and reflect changes in structure and polarizability as well as hydrogen‐bond donor and acceptor ability. In all cases studied a nonideal mixing behavior is observed. The spectra of the DMSO/water system show blue‐shifted CSC stretching modes. The antisymmetric frequencies are always further blue‐shifted than the symmetric stretching ones. The DMSO/methanol system also features blue‐shifted CSC stretching frequencies but at high mole fractions a pronounced red shifting is observed. In the binary DMSO/ethanol system, the co‐solvent also gives rise to blue shifts of the CSC stretching frequencies but restricted to mole fractions between x=0.38 and 0.45. The different magnitudes and occurrences of both blue‐ and red‐shifted spectral lines are comprehensively and critically discussed with respect to the existing literature concerning wavenumbers and Raman intensities in both absolute and normalized values. In particular, the normalized Raman intensities show a higher sensitivity for the nonideal mixing behavior because they are independent of the mole fraction.     

High‐Frequency Fe–H Vibrations in a Bridging Hydride Complex Characterized by NRVS and DFT

High‐spin iron species with bridging hydrides have been detected in species trapped during nitrogenase catalysis, but there are few general methods of evaluating Fe?H bonds in high‐spin multinuclear iron systems. An ⁵⁷Fe nuclear resonance vibrational spectroscopy (NRVS) study on an Fe(μ‐H)₂Fe model complex reveals Fe?H stretching vibrations for bridging hydrides at frequencies greater than 1200 cm^?1. These isotope‐sensitive vibrational bands are not evident in infrared (IR) spectra, showing the power of NRVS for identifying hydrides in this high‐spin iron system. Complementary density functional theory (DFT) calculations elucidate the normal modes of the rhomboidal iron hydride core.     

Effect of the Cationic Surfactant Moiety on the Structure of Water Entrapped in Two Catanionic Reverse Micelles Created from Ionic Liquid‐Like Surfactants

The behavior of water entrapped in reverse micelles (RMs) formed by two catanionic ionic liquid‐like surfactants, benzyl‐n‐hexadecyldimethylammonium 1,4‐bis‐2‐ethylhexylsulfosuccinate (AOT‐BHD) and cetyltrimethylammonium 1,4‐bis‐2‐ethylhexylsulfosuccinate (AOT‐CTA), was investigated by using dynamic (DLS) and static (SLS) light scattering, FTIR, and ¹H NMR spectroscopy techniques. To the best of our knowledge, this is the first report in which AOT‐CTA has been used to create RMs and encapsulate water. DLS and SLS results revealed the formation of RMs in benzene and the interaction of water with the RM interface. From FTIR and ¹H NMR spectroscopy data, a difference in the magnitude of the water–catanionic surfactant interaction at the interface is observed. For the AOT‐BHD RMs, a strong water–surfactant interaction can be invoked whereas for AOT‐CTA this interaction seems to be weaker. Consequently, more water molecules interact with the interface in AOT‐BHD RMs with a completely disrupted hydrogen‐bond network, than in AOT‐CTA RMs in which the water structure is partially preserved. We suggest that the benzyl group present in the BHD⁺ moiety in AOT‐BHD is responsible for the behavior of the catanionic interface in comparison with the interface created in AOT‐CTA. These results show that a simple change in the cationic component in the catanionic surfactant promotes remarkable changes in the RMs interface with interesting consequences, in particular when using the systems as nanoreactors.     

Interplay of Exciton Coupling and Large‐Amplitude Motions in the Vibrational Circular Dichroism Spectrum of Dehydroquinidine

A detailed analysis of the computed structure, energies, vibrational absorption (VA) and circular dichroism (VCD) spectra of 30 low‐energy conformers of dehydroquinidine reveals the existence of families of pseudo‐conformers, the structures of which differ mostly in the orientation of a single O?H bond. The pseudo‐conformers in a family are separated by very small energy barriers (i.e., 1.0 kcal mol^?1 or smaller) and have very different VCD spectra. First, we demonstrate the unreliable character of the Boltzmann factors predicted with DFT. Then, we show that the large differences observed between the VCD spectra of the pseudo‐conformers in a family are caused by large‐amplitude motions involving the O?H bond, which trigger the appearance/disappearance of strong VCD exciton‐coupling bands in the fingerprint region. This interplay between exciton coupling and large‐amplitude‐motion phenomena demonstrates that when dealing with flexible molecules with polar bonds, vibrational averaging of VCD spectra should not be neglected. In this regard, the dehydroquinidine molecule considered here is expected to be a typical example and not the exception to the rule.     

An Interpretation of the “Anomalous” Changes in the Low‐Wavenumber Range of the Raman Spectra of L‐Alanine Crystals

“Anomalous changes” in the temperature‐ and pressure‐ dependences of the intensities and wavenumbers of the two low‐wavenumber modes in Raman spectra of single‐crystals of L ‐alanine have been interpreted in terms of a change in relative contributions of stretching and deformational components into the intermolecular vibrational bands. The relative contributions of the two components into a lattice vibration result from a change of relative orientations of molecules linked by hydrogen bonds in a three‐dimensional network on variations of temperature or pressure.