Water–wire clusters: Vibronic spectra of 7-hydroxyquinoline⋅(H2O)3

Switching between Proton Vacancy and Excess Proton Transfer Pathways in the Reaction between 7-Hydroxyquinoline and Formate

Bifunctional or amphoteric photoacids simultaneously present donor (acidic) and acceptor (basic) properties making them useful tools to analyze proton transfer reactions. In protic solvents, the proton exchange between the acid and the base is controlled by the acidity or basicity strength and typically occurs on two different pathways known as protolysis and hydrolysis. We report here how the addition of a formate base will alter the relative importance of the possible reaction pathways of the bifunctional photoacid 7-hydroxyquinoline (7HQ), which has been recently understood to predominantly involve a hydroxide/methoxide transport mechanism between the basic proton-accepting quinoline nitrogen site toward the proton-donating OH group with a time constant of 360 ps in deuterated methanol (CD₃OD). We follow the reaction dynamics by probing the IR-active marker modes of the different charged forms of photoexcited 7HQ, and of formic acid (HCOOD) in CD₃OD solution. A comparison of the transient IR spectra as a function of formate concentration, and classical molecular dynamics simulations enables us to identify distinct contributions of “tight” (meaning “contact”) and “loose” (i.e., “solvent-separated”) 7HQ–formate reaction pairs in our data. Our results suggest that depending on the orientation of the OH group with respect to the quinoline aromatic ring system, the presence of the formate molecule in a proton relay pathway facilitates a net proton transfer from the proton-donating OH group of 7HQ-N* via the methanol/formate bridge toward the quinoline N site.

Water bridges anchored by a C–H⋯O hydrogen bond: the role of weak interactions in molecular solvation

Hydrogen-bonded water bridges are re-directed from a polar NH bond to a weakly activated C(2)–H bond upon N-methylation. Infrared spectra, supported by ab initio calculations, provide direct evidence of the role of the C(2)–H donor in the solvation of the imidazole ring.

Excited-State Proton Transfer in 7-Hydroxy-4-methylcoumarin along a Hydrogen-Bonded Water Wire

TDDFT, RI-CC2, and CIS calculations have been performed for the nondissociative excited-state proton transfer (ESPT) in the S1 state of 7-hydroxy-4-methylcoumarin (7H4MC) along a H-bonded water wire of three water molecules bridging the proton donor (OH) and the proton acceptor (C[double bond]O) groups (7H4MC.(H2O)3). The observed structural reorganization in the water-wire cluster is interpreted as a proton-transfer (PT) reaction along the H2O solvent wire. The shift of electron density within the organic chromophore 7H4MC due to the optical excitation appears to be the driving force for ESPT. All the methods used show that the reaction path occurs in the 1pipi* state, and no crossing with a Rydberg-type 1pisigma* state is found. TDDFT and RI-CC2 calculations predict an exoergic reaction of the excited-state enol-to-keto transformation. The S1 potential energy curve reveals well-defined Cs minima of enol- and keto-clusters, separated by a single barrier with a height of 17-20 kcal/mol. After surmounting this barrier, spontaneous PT along the water wire is observed, leading without any further barrier to the keto structure. The TDDFT and RI-CC2 methods appear to be reliable approaches to describe the energy surfaces of ESPT. The CIS method predicts an endoergic ESPT reaction and an energy barrier, which is too high.

Laser Probes of Conformational Isomerization in Flexible Molecules and Complexes

Molecules with several flexible coordinates have potential energy surfaces with a large number of minima and many transition states separating them. A general experimental protocol is described that is capable of studying conformational isomerization in such circumstances, measuring the product quantum yields following conformation-specific infrared excitation, and measuring energy thresholds for isomerization of specific X --> Y reactant-product isomer pairs following excitation via stimulated emission pumping (SEP). These methods have been applied to a series of molecules of varying size and conformational complexity, including 3-indolepropionic acid (IPA), meta-ethynylstyrene, N-acetyltryptophan methyl amide (NATMA), N-acetyltryptophan amide (NATA), and melatonin. Studies of isomerization in solute-solvent complexes are also described, including a measurement of the barrier to isomerization in the IPA-H2O complex, and a unique isomerization reaction in which a single water molecule is shuttled between H-bonding sites on the trans-formanilide (TFA) molecule.

Excited-State Hydrogen-Atom Transfer along Solvent Wires: Water Molecules Stop the Transfer

Excited-state hydrogen-atom transfer (ESHAT) along a hydrogen-bonded solvent wire occurs for the supersonically cooled n = 3 ammonia-wire cluster attached to the scaffold molecule 7-hydroxyquinoline (7HQ) [Tanner, C.; et al. Science 2003, 302, 1736]. Here, we study the analogous three-membered solvent-wire clusters 7HQ.(NH3)n.(H2O)m, n + m = 3, using resonant two-photon ionization (R2PI) and UV-UV hole-burning spectroscopies. Substitution of H2O for NH3 has a dramatic effect on the excited-state H-atom transfer: The threshold for the ESHAT reaction is approximately 200 cm(-1) for 7HQ.(NH3)3, approximately 350 cm(-1) for both isomers of the 7HQ.(NH3)2.H2O cluster, and approximately 600 cm(-1) for 7HQ.NH3.(H2O)2 but increases to approximately 2000 cm(-1) for the pure 7HQ.(H2O)3 water-wire cluster. To understand the effect of the chemical composition of the solvent wire on the H-atom transfer, the reaction profiles of the low-lying electronic excited states of the n = 3 pure and mixed solvent-wire clusters are calculated with the configuration interaction singles (CIS) method. For those solvent wires with an NH3 molecule at the first position, injection of the H atom into the wire can occur by tunneling. However, further H-atom transfer is blocked by a high barrier at the first (and second) H2O molecule along the solvent wire. H-atom transfer along the entire length of the solvent wire, leading to formation of the 7-ketoquinoline (7KQ) tautomer, cannot occur for any of the H2O-containing clusters, in agreement with experimentally observed absence of 7KQ fluorescence.

Laser-Initiated Shuttling of a Water Molecule Between H-Bonding Sites

The two-step laser excitation scheme of stimulated emission pumping (SEP) induces shifts of a single water molecule between two remote hydrogen bonding sites on trans-formanilide. This reaction can be initiated by selective excitation of either isomer (CO-bound or NH-bound) with different SEP excitation wavelengths. Energy (E) thresholds for isomerization in both directions have been measured [796 wave numbers </= E(CO-->NH) </= 988 wave numbers and 750 wave numbers </= E(NH-->CO) </= 988 wave numbers], and the energy difference DE between the CO-bound and NH-bound isomers was extracted (-238 wave numbers </= DE </= +192 wave numbers).

Elementary Steps in Excited-State Proton Transfer

The absorption of a photon by a hydroxy-aromatic photoacid triggers a cascade of events contributing to the overall phenomenon of intermolecular excited-state proton transfer. The fundamental steps involved were studied over the last 20 years using a combination of theoretical and experimental techniques. They are surveyed in this sequel in sequential order, from fast to slow. The excitation triggers an intramolecular charge transfer to the ring system, which is more prominent for the anionic base than the acid. The charge redistribution, in turn, triggers changes in hydrogen-bond strengths that set the stage for the proton-transfer step itself. This step is strongly influenced by the solvent, resulting in unusual dependence of the dissociation rate coefficient on water content, temperature, and isotopic substitution. The photolyzed proton can diffuse in the aqueous solution in a mechanism that involves collective changes in hydrogen-bonding. On longer times, it may recombine adiabatically with the excited base or quench it. The theory for these diffusion-influenced geminate reactions has been developed, showing nice agreement with experiment. Finally, the effect of inert salts, bases, and acids on these reactions is analyzed.

Grotthus-type and diffusive proton transfer in 7-hydroxyquinoline.(NH(3))(n) clusters

Proton translocation along ammonia wires is investigated in 7-hydroxyquinoline.(NH(3))(n) clusters, both experimentally by laser spectroscopy and theoretically by Hartree-Fock and density functional (DFT) calculations. These clusters serve as realistic finite-size models for proton transfer along a chain of hydrogen-bonded solvent molecules. In the enol tautomer of 7-hydroxyquinoline (7-HQ), the OH group acts as a proton injection site into the (NH(3))(n)cluster. Proton translocation along a chain of three NH(3) molecules within the cluster can take place, followed by reprotonation of 7-HQ at the quinolinic N atom, forming the 7-ketoquinoline tautomer. Exoergic proton transfer from the OH group of 7-HQ to the closest NH(3) molecule within the cluster giving a zwitterion 7-HQ-.(NH(3))(6)H+ (denoted PT-A) occurs at a threshold cluster size of n = 6 in the DFT calculations and at n = 5 or 6 experimentally. Three further locally stable zwitterion clusters denoted PT-B, PT-B', and PT-C, the keto tautomer, and several transition structures along the proton translocation path were characterized theoretically. Grotthus-type proton-hopping mechanisms occur for three of the proton transfer steps, which have low barriers and are exoergic or weakly endoergic. The step with the highest barrier involves a complex proton transfer mechanism, involving structural reorganization and large-scale diffusive motions of the cluster.