Intermolecular dissociation energies of hydrogen-bonded 1-naphthol complexes

Dispersion Control over Molecule Cohesion: Exploiting and Dissecting the Tipping Power of Aromatic Rings

ConspectusWe have learned over the past years how London dispersion forces can be effectively used to influence or even qualitatively tip the structure of aggregates and the conformation of single molecules. This happens despite the fact that single dispersion contacts are much weaker than competing polar forces. It is a classical case of strength by numbers, with the importance of London dispersion forces scaling with the system size. Knowledge about the tipping points, however difficult to attain, is necessary for a rational design of intermolecular forces. One requires a careful assessment of the competing interactions, either by sensitive spectroscopic techniques for the study of the isolated molecules and aggregates or by theoretical approaches. Of particular interest are the systems close to the tipping point, when dispersion interactions barely outweigh or approach the strength of the other interactions. Such subtle cases are important milestones for a scale-up to realistic multi-interaction situations encountered in the fields of life and materials science. In searching for examples that provide ideal competing interactions in complexes and small clusters, aromatic systems can offer a diverse set of molecules with a variation of dispersion and electrostatic forces that control the dominant and peripheral interactions. Our combined spectroscopic and theoretical investigations provide valuable insights into the balance of intermolecular forces because they typically allow us to switch the aromatic substituent on and off. High-resolution rotational spectroscopy serves as a benchmark for molecular structures, as correct calculations should be based on correct geometries. When discussing the competition with other noncovalent interactions, obvious competitors are directional hydrogen bonds. As a second counterweight to aryl interactions, we will discuss aurophilic/metallophilic interactions, which also have a strong stabilization with a small number of atoms involved. Vibrational spectroscopy is most sensitive to interactions of light atoms, and the competition of OH hydrogen bonds with dispersion forces in a molecular aggregate can be judged well by the OH stretching frequency. Experiments in the gas phase are ideal for gauging the accuracy of quantum chemical predictions free of solvent forces. A tight collaboration utilizing these three methods allows experiment vs experiment vs theory benchmarking of the overall influence of dispersion in molecular structures and energetics.

Structural study of 1- and 2-naphthol: new insights into the non-covalent H-H interaction in cis-1-naphthol

Previous microwave studies of naphthol monomers were supplemented by measuring spectra of all ¹³C mono-substituted isotopologues of the cis- and trans-conformers of 1-naphthol and 2-naphthol in their natural abundances. The resulting data were utilized to determine substitution structures and so-called semi-experimental effective structures. Results from electronic structure calculations show that the OH group of cis-1-naphthol points ≈6° out of plane, which is consistent with the inertial defect data of cis- and trans-1-naphthol. The non-planarity of cis-1-naphthol is a result of a close-contact H-atom-H-atom interaction. This type of H-H interaction has been the subject of much controversy in the past and we provide here an in-depth theoretical analysis of it. The naphthol system is particularly well-suited for such analysis as it provides internal standards with its four different isomers. The methods used include quantum theory of atoms in molecules, non-covalent interactions, independent gradient model, local vibrational mode, charge model 5, and natural bond orbital analyses. We demonstrate that the close-contact H-H interaction is neither a purely attractive nor repulsive interaction, but rather a mixture of the two.

Rotational analysis of naphthol-aromatic ring complexes stabilized by electrostatic and dispersion interactions

For complexes involving aromatic species, substitution effects can influence the preferred geometry. Using broadband rotational spectroscopy, we report the structures of three naphthol-aromatic ring complexes with different heteroatoms (furan and thiophene) and alkyl groups (2,5-dimethylfuran). The aim was to analyze the influence of the presence of heteroatoms or alkyl groups on the structure of the complex and the kind of intermolecular forces that control it. Face or edge arrangements can take place in these complexes via π-π or O-H⋯O/O-H⋯π interactions, respectively. All the experimentally observed complexes present O-H⋯O/O-H⋯π interactions with the hydroxyl group, with different structures and intermolecular interactions depending on the heteroatom present in the five-membered aromatic rings, yielding different symmetries in the experimental structure. Structures are experimentally identified through the use of planar moments of inertia. Further results from SAPT calculations show that dispersion and electrostatic interactions contribute similarly to the stabilization of all the studied complexes. These new spectroscopic results shed light on the influence of dispersion and hydrogen bonding in molecular aggregation of systems with substituted aromatic residues.

A new application of terahertz time-domain absorption spectra in luminescent complexes: characterization of the C-Hπ weak interactions in Cu(I) complexes

Six Cu(i) complexes, [Cu(2,3-f)(bdppmapy)]BF4 (1), [Cu(2,3-f)(bdppmapy)]ClO4 (2), [Cu(2,3-f)(bdppmapy)]CF3SO3 (3), [Cu(imidazo[4,5-f])(bdppmapy)]BF4 (4), [Cu(imidazo[4,5-f])(bdppmapy)]ClO4 (5), and [Cu(imidazo[4,5-f])(bdppmapy)]CF3SO3·MeOH (6·MeOH) (bdppmapy = N,N-bis[(diphenylphosphino)methyl]-2-pyridinamine, 2,3-f = pyrazine[2,3-f][1,10]-phenanthroline, and imidazo[4,5-f] = 1H-imidazo[4,5-f][1,10]-phenanthroline), have been synthesized to explore the effects of counteranions on their crystal structures, photophysical properties, and terahertz (THz) spectra. Time-dependent density functional theory (TD-DFT) shows that the luminescence performance of these complexes is attributed to the metal-to-ligand charge transfer (MLCT) in combination with ligand-to-ligand charge transfer (LLCT). In complexes 1-3, the characteristic peak at 1.4 THz is mainly related to the C-Hπ interaction formed by the H atom on the 4#/5# position of 2,3-f and the benzene ring from the bdppmapy on the adjacent asymmetric unit. The common C-Hπ interaction enhances the rigidity of the structure and has non-negligible influence on the photoluminescence quantum yields (PLQYs): the stronger the C-Hπ interaction is, the higher the quantum yield (QY) is. In complexes 4-6, similar absorption peaks (1.10-1.30 THz) are mainly related to the C-Hπ interactions, and strong absorption peaks (1.50-1.90 THz) are affected by the typical hydrogen bonds N-HF/O and O-HO. These results show that some weak interactions can be characterized by THz time-domain spectroscopy (THz-TDS). So, the THz spectroscopy method would make it possible to tune some of the weak interactions in complex structures to regulate the luminescence of materials.

Benchmark Experimental Gas-Phase Intermolecular Dissociation Energies by the SEP-R2PI Method

The gas-phase ground-state dissociation energy D0( S0) of an isolated and cold bimolecular complex is a fundamental measure of the intermolecular interaction strength between its constituents. Accurate D0 values are important for the understanding of intermolecular bonding, for benchmarking high-level theoretical calculations, and for the parameterization of dispersion-corrected density functionals or force-field models that are used in fields ranging from crystallography to biochemistry. We review experimental measurements of the gas-phase D0( S0) and D0( S1) values of 55 different M⋅S complexes, where M is a (hetero)aromatic molecule and S is a closed-shell solvent atom or molecule. The experiments employ the triply resonant SEP-R2PI laser method, which involves M-centered ( S0 → S1) electronic excitation, followed by S1 → S0 stimulated emission spanning a range of S0 state vibrational levels. At sufficiently high vibrational energy, vibrational predissociation of the M⋅S complex occurs. A total of 49 dissociation energies were bracketed to within ≤1.0 kJ/mol, providing a large experimental database of accurate noncovalent interactions.

Intermolecular dissociation energies of 1-naphthol complexes with large dispersion-energy donors: Decalins and adamantane

The ground-state intermolecular dissociation energies D₀(S₀) of supersonic-jet cooled intermolecular complexes of 1-naphthol (1NpOH) with the bi- and tricycloalkanes trans-decalin, cis-decalin, and adamantane were measured using the stimulated-emission-pumping/resonant two-photon ionization (SEP-R2PI) method. Using UV/UV holeburning, we identified two isomers (A and B) of the adamantane and trans-decalin complexes and four isomers (A-D) of the cis-decalin complex. For 1NpOH·adamantane A and B, the D₀(S₀) values are 21.6 ± 0.15 kJ/mol and 21.2 ± 0.32 kJ/mol, those of 1NpOH·trans-decalin A and B are 28.7 ± 0.3 kJ/mol and 28.1 ± 0.9 kJ/mol, and those of 1NpOH·cis-decalin A and B are 28.9 ± 0.15 kJ/mol and 28.7 ± 0.3 kJ/mol. Upon S₀ → S₁ electronic excitation of the 1NpOH moiety, the dissociation energies of adamantane, trans-decalin, and the cis-decalin isomer C change by <1% and those of cis-decalin isomers A, B, and D increase only slightly (1%-3%). This implies that the hydrocarbons are dispersively adsorbed to a naphthalene "face." Calculations using the dispersion-corrected density functional theory methods B97-D3 and B3LYP-D3 indeed predict that the stable structures have face geometries. The B97-D3 calculated D₀(S₀) values are within 1 kJ/mol of the experiment, while B3LYP-D3 predicts D₀ values that are 1.4-3.3 kJ/mol larger. Although adamantane has been recommended as a "dispersion-energy donor," the binding energies of the trans- and cis-decalin adducts to 1NpOH are 30% larger than that of adamantane. In fact, the D₀ value of 1NpOH·adamantane is close to that of 1NpOH·cyclohexane, reflecting the nearly identical contact layer between the two molecules.