Intermolecular hydrogen bonding, as an important site-specific interaction between hydrogen donor and acceptor molecules both in the gas phase and in solution, plays a significant role and has a remarkable influence on the photophysics and photochemistry of chromophores in the hydrogen-bonding surroundings. In recent years, the excited-state structures and dynamics of the intermolecular hydrogen bonding have been widely investigated by using both the experimental and theoretical methods. This review article focuses on the recent research progress of the important intermolecular hydrogen-bonding effects on the non-adiabatic photophysical processes and photochemical reactions. Firstly, the corresponding relationship between electronic spectral red-shift or blue-shift and excited-state intermolecular hydrogen bond strengthening or weakening has been clarified. A dynamic equilibrium induced by the intermolecular hydrogen bond strengthening in the electronically excited state of fluorenone chromophore was used...

Intermolecular hydrogen-bonding effects on photophysics and photochemistry

Size selected water clusters are generated by photoionizing sodium doped clusters close to the ionization threshold. This procedure is free of fragmentation. Upon infrared excitation, size- and isomer-specific OH-stretch spectra are obtained over a large range of cluster sizes. In one application of this method the infrared spectra of single water cluster sizes are investigated. A comparison with calculations, based on structures optimized by genetic algorithms, has been made to tentatively derive cluster structures which reproduce the experimental spectra. We identified a single all-surface structure for n = 25 and mixtures with one or two interior molecules for n = 24 and 32. In another application the sizes are determined at which the crystallization sets in. Surprisingly, this process strongly depends on the cluster temperature. The crystallization starts at sizes below n = 200 at higher temperatures and the onset is shifted to sizes above n = 400 at lower temperatures.

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A size resolved investigation of large water clusters.

The 24 normal and 24 local vibrational modes of the formic acid dimer formed by two trans formic acid monomers to a ring (TT1) are analysed utilising preferentially experimental frequencies, but also CCSD(T)/CBS and ωB97X-D harmonic vibrational frequencies. The local hydrogen bond (HB) stretching frequencies are at 676 cm−1 and by this 482 and 412 cm−1 higher compared to the measured symmetric and asymmetric HB stretching frequencies at 264 and 194 cm−1. The adiabatic connection scheme between local and normal vibrational modes reveals that the lowering is due to the topology of dimer TT1, mass coupling, and avoided crossings involving the H⋅⋅⋅OC bending modes. The HB local mode stretching force constant is related to the strength of the HB whereas the normal mode stretching force constant and frequency lead to an erroneous underestimation of the HB strength. The HB in TT1 is stabilised by electron delocalisation in the O=C–O units fostered by forming a ring via double HBs. This implies that the CO apart ...

Local vibrational modes of the formic acid dimer – the strength of the double hydrogen bond

In this paper, the intermolecular structural study asserted by the vibrational analysis in the stretch frequencies of hydrogen bonds (π⋯H) and dihydrogen bonds (H−δ⋯H+δ) have definitively been revisited by means of calculations carried out by Density Functional Theory (DFT) and topological parameters derived from the classic treatise of the Quantum Theory of Atoms in Molecules (QTAIM). As a matter of fact the π⋯H hydrogen bond is formed between the hydrofluoric acid and the CC bond of the acetylene, but the QTAIM calculations revealed a distortion in this interaction due to the formation of the ternary complex C2H2⋯2(HF). Although the π bonds of ethylene (C2H4), propylene (C2H3(CH3)), and t-butylene (C2H2(CH3)2) are considered proton acceptors, two hydrogen-bond types—π⋯H and C⋯H—can be observed. Over and above the analysis of the π hydrogen bonds, theoretical arguments also were used to discuss the red-shifts in the stretch frequencies of the binary dihydrogen complexes formed by BeH2⋯HX with X = F, Cl, CN, and CCH. Although a vibrational blue-shift in the stretch frequency of the H–C bond of HCF3 due to the formation of the BeH2⋯HCF3 dihydrogen complex was obtained, unmistakable red-shifts were detected in LiH⋯HCF3, MgH2⋯HCF3, and NaH⋯HCF3. Moreover, the alkali–halogen bonds were identified in relation to the formation of the trimolecular systems NaH⋯2(HCF3) and NaH⋯2(HCCl3). At last, theoretical calculations and QTAIM molecular integrations were used to study a novel class of dihydrogen-bonded complexes (mC2H5+ ⋯nMgH2 with m = 1 or 2 and n = 1 or 2) based in the insight that MgH2 can bind with the non-localized hydrogen H+δ of the ethyl cation (C2H5+). In an overview, QTAIM calculations were applied to evaluate the molecular topography, charge density, as well as to interpret the shifted frequencies either to red or blue caused by the formation of the hydrogen bonds and dihydrogen bonds.

Structure, energy, vibrational spectrum, and Bader's analysis of π⋯H hydrogen bonds and H −δ ⋯H +δ dihydrogen bonds

The sensitivity limitations of Fourier transform infrared (FTIR) spectroscopy for the detection of molecular clusters formed in rarefied gas expansions can be overcome by synchronizing intense gas pulses at a low duty cycle with rapid interferometer scans. This turns the broadband FTIR approach into a universal cluster spectroscopy tool applicable from the far (200 cm−1) to the near (8000 cm−1) IR. It nicely complements more selective and more restricted laser-based techniques and it provides a gas-phase variant of the matrix-isolation method, the main drawback being substance consumption. A survey over the capabilities, limitations and perspectives of this high-throughput nozzle approach to cluster FTIR spectroscopy is given.

Femtisecond single-mole infrared spectroscopy of molecular clusters.

The highest frequency hydrogen bond fundamental of formic acid dimer, ν24 (Bu), is experimentally located at 264 cm−1. FTIR spectra of this in-plane bending mode of (HCOOH)2 and band centers of its symmetric D isotopologues (isotopomers) recorded in a supersonic slit jet expansion are presented. Comparison to earlier studies at room temperature reveals the large influence of thermal excitation on the band maximum. Together with three Bu combination states involving hydrogen bond fundamentals and with recent progress for the Raman-active modes, this brings into reach an accurate statistical thermodynamics treatment of the dimerization process up to room temperature. We obtain D0 = 59.5(5) kJ/mol as the best experimental estimate for the dimer dissociation energy at 0 K. Further improvements have to wait for a more consistent determination of the room temperature equilibrium constant.

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Communication: The highest frequency hydrogen bond vibration and an experimental value for the dissociation energy of formic acid dimer

The OH bond of methanol, ethanol and t-butyl alcohol becomes more anharmonic upon hydrogen bonding and the infrared intensity ratio between the overtone and the fundamental transition of the bridging OH stretching mode decreases drastically. FTIR spectroscopy of supersonic slit jet expansions allows to quantify these effects for isolated alcohol dimers, enabling a direct comparison to anharmonic vibrational predictions. The diagonal anharmonicity increase amounts to 15–18%, growing with increasing alkyl substitution. The overtone/fundamental IR intensity ratio, which is on the order of 0.1 or more for isolated alcohols, drops to 0.004–0.001 in the hydrogen-bonded OH group, making overtone detection very challenging. Again, alkyl substitution enhances the intensity suppression. Vibrational second order perturbation theory appears to capture these effects in a semiquantitative way. Harmonic quantum chemistry predictions for the hydrogen bond-induced OH stretching frequency shift (the widely used infrared signature of hydrogen bonding) are insufficient, and diagonal anharmonicity corrections from experiment make the agreement between theory and experiment worse. Inclusion of anharmonic cross terms between hydrogen bond modes and the OH stretching mode is thus essential, as is a high level electronic structure theory. The isolated molecule results are compared to matrix isolation data, complementing earlier studies in N2 and Ar by the more weakly interacting Ne and p-H2 matrices. Matrix effects on the hydrogen bond donor vibration are quantified.

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Alcohol dimers--how much diagonal OH anharmonicity?

The effect of strong intermolecular hydrogen bonding on torsional degrees of freedom is investigated by far-infrared absorption spectroscopy for different methanol dimer isotopologues isolated in supersonic jet expansions or embedded in inert neon matrices at low temperatures. For the vacuum-isolated and Ne-embedded methanol dimer, the hydrogen bond OH librational mode of the donor subunit is finally observed at ∼560 cm−1, blue-shifted by more than 300 cm−1 relative to the OH torsional fundamental of the free methanol monomer. The OH torsional mode of the acceptor embedded in neon is observed at ∼286 cm−1. The experimental findings are held against harmonic predictions from local coupled-cluster methods with single and double excitations and a perturbative treatment of triple excitations [LCCSD(T)] and anharmonic. VPT2 corrections at canonical MP2 and density functional theory (DFT) levels in order to quantify the contribution of vibrational anharmonicity for this important class of intermolecular hydrogen bond vibrational motion.

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The effect of hydrogen bonding on torsional dynamics: A combined far-infrared jet and matrix isolation study of methanol dimer

Infrared spectra of isolated methanol–acetone clusters up to tetramers are experimentally characterized for the first time. They show evidence for a nanometer-scale demixing trend of the cold species. In combination with quantum calculations, the mutual repulsion is demonstrated to start beyond three molecular units, whereas individual molecules still prefer to form a mixed complex.

Franz Kollipost

Papers

Communication: The highest frequency hydrogen bond vibration and an experimental value for the dissociation energy of formic acid dimer

Femtisecond single-mole infrared spectroscopy of molecular clusters.

Alcohol dimers--how much diagonal OH anharmonicity?

The effect of hydrogen bonding on torsional dynamics: A combined far-infrared jet and matrix isolation study of methanol dimer

Microscopic roots of alcohol-ketone demixing: infrared spectroscopy of methanol-acetone clusters.