Localization and Orientation of Dye Molecules at the Surface of a Levitated Microdroplet in Air Revealed by Whispering Gallery Mode Resonances.

Microdroplets offer unique environments that accelerate chemical reactions; however, the mechanisms behind these processes remain debated. The localization and orientation of solute molecules near the droplet surface have been proposed as factors for this acceleration. Since significant reaction acceleration has been observed for electrospray- and sonic-spray-generated aerosol droplets, the analysis of microdroplets in air has become essential. Here, we utilized whispering gallery mode (WGM) resonances to investigate the localization and orientation of dissolved rhodamine B (RhB) in a levitated microdroplet (∼3 µm in diameter) in air. Fluorescence enhancement upon resonance with the WGMs revealed the localization and orientation of RhB near the droplet surface. Numerical modeling using Mie theory quantified the RhB orientation at 68° to the surface normal, with a small fraction randomly oriented inside the droplet. Additionally, low RhB concentrations increased surface localization. These results support the significance of surface reactions in the acceleration of microdroplet reactions.

Alignment of fibrous J-aggregates and the resulting macroscopic optical anisotropy observed in static solution.

J-aggregates, which are supramolecular assemblies that exhibit unique optical properties owing to their excitonic interactions, have potential applications in artificial light-harvesting systems and fluorescence biosensing. Although J-aggregates are formed in solution, in situ observations of their structures and behaviors in solution remain scarce. In this study, we investigated the J-aggregates of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate [DiIC18(3)] in methanol/water (M/W) binary solvents using fluorescence imaging as well as polarized absorption and fluorescence measurements to explore the relationship between their structure and macroscopic optical properties under static conditions. Fluorescence images revealed that the DiIC18(3) J-aggregates have fibrous structures in the M/W = 44/56 (v/v) binary solvent. We measured the polarization-angle dependence of the fluorescence intensity of the fibrous J-aggregates to determine the direction of their transition dipole moment. Furthermore, the J-band absorbance was dependent on the polarization angle of the linearly polarized incident light, even in the absence of an external force such as that generated by a flow or stirring, indicating that the J-aggregates "spontaneously" aligned in solution. We also monitored the time evolution of the degree of alignment of the fibrous J-aggregates, which revealed that the formation and elongation of the fibers induced their alignment, resulting in the observed macroscopic optical anisotropy in solution.

Synthesis of Single-Nanometer-Sized Gold Nanoparticles in Liquid-Liquid Dispersion System by Femtosecond Laser Irradiation.

Gold nanoparticles (AuNPs) show unique optical properties and catalytic activities, and their synthesis from gold ions has been widely studied. One of the additive-reagent-free and noncontact production procedures is the reduction of gold ions in solution by femtosecond laser pulses; however, the aggregation of AuNPs is unavoidable in homogeneous solution. Here, we report the synthesis of single-nanometer-sized AuNPs in a mixture of aqueous HAuCl4 solution and n-hexane (the mixture) and in aqueous HAuCl4 solution (the aqueous solution) by femtosecond laser irradiation in the absence of any additive reagents. Transmission electron microscopy revealed that circlelike colonies consisting of well-separated AuNPs were obtained from the mixture, while highly stacked and agglomerated AuNPs were obtained from the aqueous solution. The mean size of AuNPs in the mixture was nearly independent of the laser irradiation time, whereas that obtained in aqueous solution was gradually shifted to smaller size by laser irradiation period. We propose that the adsorption of primary AuNPs on the surface of hexane microdroplets and the fragmentation of large AuNPs in water by successive laser pulses retain single-nanometer-sized AuNPs in the mixture. The use of liquid-liquid interface on hexane microdroplets in aqueous solution provides a simple and useful environment to synthesize small AuNPs without the aid of surfactants or capping agents.

Synthesis of Bare Iron Nanoparticles from Ferrocene Hexane Solution by Femtosecond Laser Pulses.

Iron-based nanoparticles (FeNPs) have unique and attractive properties such as superparamagnetism, biocompatibility, and catalytic activity. Although the synthesis of precious metal NPs from a metal in liquid and/or metal salt solution by a pulsed laser has been investigated, comparably little effort has been devoted to examine the production of FeNPs. Here we report the synthesis of carbon-shell free spherical NPs of iron oxide (magnetite) from ferrocene hexane solution by femtosecond near infrared laser pulses. Nanosecond UV laser pulses are used to compare the evolution of the particle size distribution as a function of laser irradiation time. The size of NPs remains constant even for extended exposure to femtosecond laser pulses, whereas it grows with exposure to nanosecond laser pulses. The primary particles are generated by photochemical reactions regardless of pulse duration; however, the fragmentation of NPs by successive femtosecond laser pulses regulates the particle size.

Real-time observation of the photoionization-induced water rearrangement dynamics in the 5-hydroxyindole-water cluster by time-resolved IR spectroscopy.

Solvation plays an essential role in controlling the mechanism and dynamics of chemical reactions in solution. The present study reveals that changes in the local solute-solvent interaction have a great impact on the timescale of solvent rearrangement dynamics. Time-resolved IR spectroscopy has been applied to a hydration rearrangement reaction in the monohydrated 5-hydroxyindole-water cluster induced by photoionization of the solute molecule. The water molecule changes the stable hydration site from the indolic NH site to the substituent OH site, both of which provide a strongly attractive potential for hydration. The rearrangement time constant amounts to 8 ± 2 ns, and is further slowed down by a factor of more than five at lower excess energy. These rearrangement times are slower by about three orders of magnitude than those reported for related systems where the water molecule is repelled from a repulsive part of the interaction potential toward an attractive well. The excess energy dependence of the time constant is well reproduced by RRKM theory. Differences in the reaction mechanism are discussed on the basis of energy relaxation dynamics.

Elevation of the Energy Threshold for Isomerization of 5-Hydroxyindole-(tert-butyl alcohol)1 Cluster Cations.

Isomerization between two hydrogen-bonded (H-bonded) isomers of 5-hydroxyindole-(tert-butyl alcohol)1 cluster cations ([5HI-(t-BuOH)1]+) was investigated in the gas phase. In the S0 state, jet-cooled 5HI-(t-BuOH)1 has two structural isomers, 5HI(OH)-(t-BuOH)1 and 5HI(NH)-(t-BuOH)1, in which the t-BuOH molecule is bound to the OH or the NH group of 5HI. The IR photodissociation spectrum of [5HI-(t-BuOH)1]+ generated by two-color resonant two-photon ionization (2C-R2PI) via the S1-S0 origin of 5HI(NH)-(t-BuOH)1 provided evidence of both [5HI(OH)-(t-BuOH)1]+ and [5HI(NH)-(t-BuOH)1]+ coexisting in the D0 state, indicating that [5HI(NH)-(t-BuOH)1]+ isomerizes to [5HI(OH)-(t-BuOH)1]+ after 2C-R2PI of 5HI(NH)-(t-BuOH)1. The lower limit of the energy threshold for the isomerization of [5HI(NH)-(t-BuOH)1]+ to [5HI(OH)-(t-BuOH)1]+ was experimentally determined to be 3362 ± 30 cm-1, and the corresponding energy threshold for the isomerization of [5HI(NH)-(H2O)1]+ to [5HI(OH)-(H2O)1]+ has been reported to be 2127 ± 30 cm-1. Thus, the energy threshold for the isomerization is elevated by at least 1200 cm-1 when the solvent molecule changes from H2O to t-BuOH. The elevation of the energy threshold is explained by the difference in the stabilization energies of [5HI-(t-BuOH)1]+ and [5HI-(H2O)1]+ in the initial and transition states owing to the larger proton affinity of t-BuOH than H2O.

Rearrangements of a Water Molecule in Both Directions between Two Hydrogen-Bonding Sites of 5-Hydroxyindole Cation: Experimental Determination of the Energy Threshold for the Rearrangement.

Rearrangements of a water molecule in both directions between two hydrogen-bonding (H-bonding) sites of the 5-hydroxyindole (5HI) cation was investigated in the gas phase. IR-dip spectra of jet-cooled 5HI-(H2O)1 revealed that two structural isomers, 5HI(OH)-(H2O)1 and 5HI(NH)-(H2O)1, in which a water molecule is bound to either the OH group or the NH group of 5HI, were formed in the S0 state. The IR photodissociation spectrum of [5HI-(H2O)1](+) generated by two-color resonant two-photon ionization (2C-R2PI) via the S1-S0 origin of 5HI(NH)-(H2O)1 clearly showed that [5HI(OH)-(H2O)1](+) and [5HI(NH)-(H2O)1](+) coexist in the D0 state. The appearance of [5HI(OH)-(H2O)1](+) after R2PI via the S1-S0 origin of 5HI(NH)-(H2O)1 is explained by isomerization of [5HI(NH)-(H2O)1](+) to [5HI(OH)-(H2O)1](+), which corresponds to the rearrangement of the water. In addition, isomerization in the opposite direction was also observed when [5HI-(H2O)1](+) was generated via the S1-S0 origin of 5HI(OH)-(H2O)1. The upper limit of the energy threshold for the rearrangement of the water in [5HI(NH)-(H2O)1](+) was experimentally determined to be 2127 ± 30 cm(-1) from the adiabatic ionization energy of 5HI(NH)-(H2O)1. Above the energy threshold, the water molecule in [5HI-(H2O)1](+) may fluctuate between the two preferential H-bonding sites of 5HI(+).

Simultaneous Interaction of Hydrophilic and Hydrophobic Solvents with Ethylamino Neurotransmitter Radical Cations: Infrared Spectra of Tryptamine(+)-(H2O)m-(N2)n Clusters (m,n ≤ 3).

Solvation of biomolecules by a hydrophilic and hydrophobic environment strongly affects their structure and function. Here, the structural, vibrational, and energetic properties of size-selected clusters of the microhydrated tryptamine cation with N2 ligands, TRA(+)-(H2O)m-(N2)n (m,n ≤ 3), are characterized by infrared photodissociation spectroscopy in the 2800-3800 cm(-1) range and dispersion-corrected density functional theory calculations at the ωB97X-D/cc-pVTZ level to investigate the simultaneous solvation of this prototypical neurotransmitter by dipolar water and quadrupolar N2 ligands. In the global minimum structure of TRA(+)-H2O generated by electron ionization, H2O is strongly hydrogen-bonded (H-bonded) as proton acceptor to the acidic indolic NH group. In the TRA(+)-H2O-(N2)n clusters, the weakly bonded N2 ligands do not affect the H-bonding motif of TRA(+)-H2O and are preferentially H-bonded to the OH groups of the H2O ligand, whereas stacking to the aromatic π electron system of the pyrrole ring of TRA(+) is less favorable. The natural bond orbital analysis reveals that the H-bond between the N2 ligand and the OH group of H2O cooperatively strengthens the adjacent H-bond between the indolic NH group of TRA(+) and H2O, while π stacking is slightly noncooperative. In the larger TRA(+)-(H2O)m clusters, the H2O ligands form a H-bonded solvent network attached to the indolic NH proton, again stabilized by strong cooperative effects arising from the nearby positive charge. Comparison with the corresponding neutral TRA-(H2O)m clusters illustrates the strong impact of the excess positive charge on the structure of the microhydration network.

Macromolecular Crowding Modifies the Impact of Specific Hofmeister Ions on the Coil-Globule Transition of PNIPAM.

Macromolecular crowding alters many biological processes ranging from protein folding and enzyme reactions in vivo to the precipitation and crystallization of proteins in vitro. Herein, we have investigated the effect of specific monovalent Hofmeister salts (NaH2PO4, NaF, NaCl, NaClO4, and NaSCN) on the coil-globule transition of poly(N-isopropylacrylamide) (PNIPAM) in a crowded macromolecular environment as a model for understanding the specific-ion effect on the solubility and stability of proteins in a crowded macromolecular environment. It was found that although the salts (NaH2PO4, NaF, and NaCl) and the macromolecular crowder (polyethylene glycol) lowered the transition temperature almost independently, the macromolecular crowder had a great impact on the transition temperature in the case of the chaotropes (NaClO4 and NaSCN). The electrostatic repulsion between the chaotropic anions (ClO4(-) or SCN(-)) adsorbed on PNIPAM may reduce the entropic gain of water associated with the excluded volume effect, leading to an increase in the transition temperature, especially in the crowded environment. Furthermore, the affinity of the chaotropic anions for PNIPAM becomes small in the crowded environment, leading to further modification of the transition temperature. Thus, we have revealed that macromolecular crowding alters the effect of specific Hofmeister ions on the coil-globule transition of PNIPAM.

Weak hydrogen bonding motifs of ethylamino neurotransmitter radical cations in a hydrophobic environment: infrared spectra of tryptamine(+)-(N2)n clusters (n ≤ 6).

Size-selected clusters of the tryptamine cation with N2 ligands, TRA(+)-(N2)n with n = 1-6, are investigated by infrared photodissociation (IRPD) spectroscopy in the hydride stretch range and quantum chemical calculations at the ωB97X-D/cc-pVTZ level to characterize the microsolvation of this prototypical aromatic ethylamino neurotransmitter radical cation in a nonpolar solvent. Two types of structural isomers exhibiting different interaction motifs are identified for the TRA(+)-N2 dimer, namely the TRA(+)-N2(H) global minimum, in which N2 forms a linear hydrogen bond (H-bond) to the indolic NH group, and the less stable TRA(+)-N2(π) local minima, in which N2 binds to the aromatic π electron system of the indolic pyrrole ring. The IRPD spectrum of TRA(+)-(N2)2 is consistent with contributions from two structural H-bound isomers with similar calculated stabilization energies. The first isomer, denoted as TRA(+)-(N2)2(2H), exhibits an asymmetric bifurcated planar H-bonding motif, in which both N2 ligands are attached to the indolic NH group in the aromatic plane via H-bonding and charge-quadrupole interactions. The second isomer, denoted as TRA(+)-(N2)2(H/π), has a single and nearly linear H-bond of the first N2 ligand to the indolic NH group, whereas the second ligand is π-bonded to the pyrrole ring. The natural bond orbital analysis of TRA(+)-(N2)2 reveals that the total stability of these types of clusters is not only controlled by the local H-bond strengths between the indolic NH group and the N2 ligands but also by a subtle balance between various contributing intermolecular interactions, including local H-bonds, charge-quadrupole and induction interactions, dispersion, and exchange repulsion. The systematic spectral shifts as a function of cluster size suggest that the larger TRA(+)-(N2)n clusters with n = 3-6 are composed of the strongly bound TRA(+)-(N2)2(2H) core ion to which further N2 ligands are weakly attached to either the π electron system or the indolic NH proton by stacking and charge-quadrupole forces.

IR spectroscopy of monohydrated tryptamine cation: rearrangement of the intermolecular hydrogen bond induced by photoionization.

Rearrangement of intermolecular hydrogen bond in a monohydrated tryptamine cation, [TRA(H(2)O)(1)](+), has been investigated in the gas phase by IR spectroscopy and quantum chemical calculations. In the S(0) state of TRA(H(2)O)(1), a water molecule is hydrogen-bonded to the N atom of the amino group of a flexible ethylamine side chain [T. S. Zwier, J. Phys. Chem. A 105, 8827 (2001)]. A remarkable change in the hydrogen-bonding motif of [TRA(H(2)O)](+) occurs upon photoionization. In the D(0) state of [TRA(H(2)O)(1)](+), the water molecule is hydrogen-bonded to the NH group of the indole ring of TRA(+), indicating that the water molecule transfers from the amino group to NH group. Quantum chemical calculations are performed to investigate the pathway of the water transfer. Two potential energy barriers emerge in [TRA(H(2)O)(1)](+) along the intrinsic reaction coordinate of the water transfer. The water transfer event observed in [TRA(H(2)O)(1)](+) is not an elementary but a complex process.

Photoionization-induced water migration in the hydrated trans-formanilide cluster cation revealed by gas-phase spectroscopy and ab initio molecular dynamics simulation.

Photoionization-induced water migration in the trans-formanilide-water 1:1 cluster, FA-(H(2)O)(1), has been investigated by using IR-dip spectroscopy, quantum chemical calculations, and ab initio molecular dynamics simulations. In the S(0) state, FA-(H(2)O)(1) has two structural isomers, FA(NH)-(H(2)O)(1) and FA(CO)-(H(2)O)(1), where a water molecule is hydrogen-bonded (H-bonded) to the NH group and the CO group, respectively. In addition, the S(1)-S(0) origin transition of FA(CO)-(H(2)O)(2), where a water dimer is H-bonded to the CO group, was observed only in the [FA-(H(2)O)(1)](+) mass channel, indicating that one of the water molecules evaporates completely in the D(0) state. These results are consistent with a previous report [Robertson, E. G. Chem. Phys. Lett., 2000, 325, 299]. In the D(0) state, however, [FA-(H(2)O)(1)](+) produced by photoionization via the S(1)-S(0) origin transitions of FA(NH)-(H(2)O)(1) and FA(CO)-(H(2)O)(1) shows essentially the same IR spectra. Compared with the theoretical calculations, [FA-(H(2)O)(1)](+) can be assigned to [FA(NH)-(H(2)O)(1)](+). This means that the water molecule in [FA-(H(2)O)(1)](+) migrates from the CO group to the NH group when [FA-(H(2)O)(1)](+) is produced by photoionization of FA(CO)-(H(2)O)(1). [FA-(H(2)O)(1)](+) produced by photoionization of FA(CO)-(H(2)O)(2) also shows the IR spectrum corresponding to [FA(NH)-(H(2)O)(1)](+). In this case, the water migration from the CO group to the NH group occurs with the evaporation of a water molecule. Ab initio molecular dynamics simulations revealed the water migration pathway in [FA-(H(2)O)(1)](+). The calculations of classical electrostatic interactions show that charge-dipole interaction between FA(+) and H(2)O induces an initial structural change in [FA-(H(2)O)(1)](+). An exchange repulsion between the lone pairs of the CO group and H(2)O in [FA-(H(2)O)(1)](+) also affects the initial direction of the water migration. These two factors play important roles in determining the initial water migration pathway.

Photon induced isomerization in the first excited state of the 7-azaindole-(H2O)3 cluster.

A picosecond pump and probe experiment has been applied to study the excited state dynamics of 7-azaindole-water 1 ∶ 2 and 1 ∶ 3 clusters [7AI(H(2)O)(2,3)] in the gas phase. The vibrational-mode selective Excited-State-Triple-Proton Transfer (ESTPT) in 7AI(H(2)O)(2) proposed from the frequency-resolved study has been confirmed by picosecond decays. The decay times for the vibronic states involving the ESTPT promoting mode σ(1) (850-1000 ps) are much shorter than those for the other vibronic states (2100-4600 ps). In the (1 + 1) REMPI spectrum of 7AI(H(2)O)(3) measured by nanosecond laser pulses, the vibronic bands with an energy higher than 200 cm(-1) above the origin of the S(1) state become very weak. In contrast, the vibronic bands in the same region emerge in the (1 + 1') REMPI spectrum of 7AI(H(2)O)(3) with picosecond pulses. The decay times drastically decrease when increasing the vibrational energy above 200 cm(-1). Ab initio calculations show that a second stable "cyclic-nonplanar isomer" exists in addition to a "bridged-planar isomer", and that an isomerization from a bridged-planar isomer to a cyclic-nonplanar isomer is most probably responsible for the short lifetimes of the vibronic states of 7AI(H(2)O)(3).

Entropy-driven rearrangement of the water network at the hydrated amide group of the trans-formanilide-water cluster in the gas phase.

Photoionization-induced rearrangement of the water network in the trans-formanilide 1:4 cluster, FA-(H(2)O)(4), has been investigated by using IR-photodissociation spectroscopy and quantum chemical calculations. The IR spectrum of FA-(H(2)O)(4) in the S(0) state shows that the observed cluster has a cyclic hydrogen-bonded structure where the CO group and the NH group of FA are bridged with four water molecules, consistent with the reported structure [E. G. Robertson, Chem. Phys. Lett., 2000, 325, 299]. However, the corresponding cyclic hydrogen-bonded structure in the D(0) state of [FA-(H(2)O)(4)](+) is a minor product arising from photoionization via the S(1)-S(0) origin of FA-(H(2)O)(4). The dominant product has an extended H-bonded structure, where the intermolecular hydrogen bond between the hydrogen of the OH group of a water molecule and the CO group is dissociated. This is the first observation of a photoionization-induced rearrangement of the water network in [FA-(H(2)O)(4)](+). Through DFT calculations, we conclude that the rearrangement occurs due to entropic effects.

Photoionization-induced water migration in the amide group of trans-acetanilide-(H2O)1 in the gas phase.

IR-dip spectra of trans-acetanilide-water 1:1 cluster, AA-(H(2)O)(1), have been measured for the S(0) and D(0) state in the gas phase. Two structural isomers, where a water molecule binds to the NH group or the CO group of AA, AA(NH)-(H(2)O)(1) and AA(CO)-(H(2)O)(1), are identified in the S(0) state. One-color resonance-enhanced two-photon ionization, (1 + 1) RE2PI, of AA(NH)-(H(2)O)(1) via the S(1)-S(0) origin generates [AA(NH)-(H(2)O)(1)](+) in the D(0) state, however, photoionization of [AA(CO)-(H(2)O)(1)] does not produce [AA(CO)-(H(2)O)(1)](+), leading to [AA(NH)-(H(2)O)(1)](+). This observation explicitly indicates that the water molecule in [AA-(H(2)O)(1)](+) migrates from the CO group to the NH group in the D(0) state. The reorganization of the charge distribution from the neutral to the D(0) state of AA induces the repulsive force between the water molecule and the CO group of AA(+), which is the trigger of the water migration in [AA-(H(2)O)(1)](+).

Excited-state triple-proton transfer in 7-azaindole(H(2)O)(2) and reaction path studied by electronic spectroscopy in the gas phase and quantum chemical calculations.

We have investigated the excited-state multiple-proton/hydrogen atom transfer reactions in the 7-azaindole water clusters, [7AI](H(2)O)(n) (n = 2,3), in the gas phase by combining electronic spectroscopy and quantum chemical calculations. The fluorescence excitation (FE) spectrum of 7AI(H(2)O)(2) has been observed by monitoring visible emission. In contrast, no vibronic bands are detected in the FE spectrum of 7AI(H(2)O)(3) when the visible emission is monitored. The dispersed fluorescence spectra of 7AI(H(2)O)(n) (n = 2,3) have been measured. The excitation of +180 cm(-1) band from the electronic origin of 7AI(H(2)O)(2) enhances the visible emission as compared with the 0-0 excitation. The +180 cm(-1) band is assgined to an intermolecular mode (σ(1)) of the cyclic hydrogen-bonded ring structure. The calculated S(1)-S(0) absorption spectrum for the cyclic hydrogen-bonded structure is in agreement with the FE spectrum around the 0-0 region. The excitation of σ(1) significantly promotes the reaction and generates the tautomeric form of 7AI(H(2)O)(2). These experimental results on 7AI(H(2)O)(n) (n = 2,3) are very similar to those on 7AI(CH(3)OH)(n) (n = 2,3) and 7AI(C(2)H(5)OH)(n) (n = 2,3). We conclude that the excited-state triple proton/hydrogen atom transfer (ESTPT/HT) occurs in 7AI(H(2)O)(2). Cuts of the potential energy surfaces along the proton/hydrogen atom transfer coordinates of 7AI(H(2)O)(n) (n = 2,3) and 7AI(CH(3)OH)(n) (n = 2,3) are comparatively calculated by quantum chemistry calculations (RI-CC2/cc-pVDZ and TD-DFT(B3LYP)/cc-pVDZ) to explore the mechanism of the ESTPT/HT reaction. The calculated results suggest that concerted proton transfers occur in 7AI(H(2)O)(2) as well as in 7AI(CH(3)OH)(2), whereas the potential barrier for the excited-state quadruple proton transfer in 7AI(H(2)O)(3) and 7AI(CH(3)OH)(3) is higher than those for ESTPT. The theoretical results are consistent with the observation of ESTPT/HT in 7AI(H(2)O)(2).

Electronic spectra of two long-lived photoproducts: double-proton transfer in 7-hydroxyquinoline dimer in a 2-methyltetrahydrofuran glass matrix.

Photoreactions of 7-hydroxyquinoline (7-HQ) in low-temperature (77-100 K) 2-methyltetrahydrofuran glass matrices are investigated using electronic spectroscopy. We have observed fluorescence excitation and fluorescence spectra of two long-lived species generated by irradiation of UV light (230-400 nm). The dominant species responsible for the fluorescence spectrum between 470 and 600 nm was assigned to the S(1)-->S(0) (pipi*) transition of the keto form of cyclic 7-HQ dimer [(7-HQ)(2)] produced by excited-state double-proton transfer, the corresponding S(1)-S(0) fluorescence excitation spectrum of which was detected between 360 and 510 nm. Temperature dependence of the fluorescence excitation spectra showed the occurrence of keto --> enol isomerization in the S(0) state of (7-HQ)(2) due to a back double-proton transfer. A very slow rate for the keto --> enol isomerization implies that the potential barrier height for the back double-proton transfer reacion is substantially high. Theoretical calculations at the MP2/aug-cc-pVDZ level of theory indicate that the enol and keto forms of cyclic (7-HQ)(2) are nonplanar, therefore a large change in the geometry is necessary for the back double-proton transfer. A second long-lived species that emits between 410 and 600 nm has been tentatively assigned to the D(3)(2A'') --> D(0)(1A'') transition of the 7-quinolinoxyl radical on the basis of calculated electronic transition energies for possible candidates obtained by MS-CASPT2/aug-cc-pVDZ level calculations as well as IR study of 7-HQ in argon matrices [Sekine, M.; Nagai, Y.; Sekiya, H.; Nakata, M. J. Phys. Chem. A 2009, 113, 8286]. Photoreaction processes leading to the two long-lived species have been discussed.

Formation of a dual hydrogen bond in the N-H...C=O moiety in the indole-(N-methylacetamide)1 cluster revealed by IR-dip spectroscopy with natural bond orbital analysis.

IR-dip spectra in the NH stretch regions have been measured for the S(0) state of the indole/N-methylacetamide 1:1 clusters (Ind-NMA(1)). We identified two structural isomers of Ind-NMA(1) that possess an N-H...O=C hydrogen bond. The redshifts of the NH stretch fundamental of the indole moieties in Ind-NMA(1) are larger than that for Ind-(H(2)O)(1) [Carney, Hagemeister, and Zweir, J. Chem. Phys. 108, 3379 (1998)], indicating that the strength of the N-H...O=C hydrogen bond in Ind-NMA(1) is stronger than that of the N-H...O-H hydrogen bond in Ind-(H(2)O)(1). On the basis of the natural bond orbital analysis we suggest that two lone pair orbitals of the O atoms in the NMA moiety form a dual hydrogen bond with the NH group designated by N-H:::O=C. Owing to the dual nature of the N-H:::O=C hydrogen bond its strength in Ind-NMA(1) is larger than that of the N-H...O-H hydrogen bond in Ind-(H(2)O)(1).

Charge transfer interaction of intermolecular hydrogen bonds in 7-azaindole(MeOH)n (n = 1, 2) with IR-dip spectroscopy and natural bond orbital analysis.

Resonance-enhanced two-photon ionization (RE2PI) spectra of the deuterated 7-azaindole [7AI](MeOH)(n) (n = 1-3) clusters in the 0-0 region of the S(1)-S(0) (pi pi*) transition and IR-UV ion-dip spectra of the deuterated 7AI(MeOH)(n) (n = 1, 2) in the NH and OH stretch regions are observed in the gas phase to investigate the effect of charge transfer delocalization interaction on intermolecular hydrogen bonds. Two and three isotopomers are identified for 7AI(MeOH)(1)-d(1) and 7AI(MeOH)(2)-d(2), respectively, where 7AI(MeOH)(1)-d(1) has a deuterium atom in the NH or OH group and 7AI(MeOH)(2)-d(2) has two deuterium atoms in the NH and OH groups or in the two OH groups. The local modes of the NH and OH groups are successfully observed in the IR-dip spectra upon deuteration. The NH and OH stretch fundamentals of the 7AI(MeOH)(1)-d(1) and 7AI(MeOH)(2)-d(2) clusters are red-shifted from the corresponding ones of the 7AI and MeOH monomers. The observed red-shifts in 7AI(MeOH)(1)-d(1) and 7AI(MeOH)(2)-d(2) are correlated with the second-order perturbative energies obtained by the natural bond orbital analysis, suggesting that the charge transfer delocalization interaction plays an important role in stabilizing the intermolecular hydrogen bonds in 7AI(MeOH)(n) (n = 1, 2).

Spectroscopic study on the structural isomers of 7-azaindole(ethanol)(n) (n=1-3) and multiple-proton transfer reactions in the gas phase.

The resonance-enhanced two-photon ionization (RE2PI) and laser-induced fluorescence excitation spectra were recorded for the S(1)-S(0)(pipi( *)) region of the 7-azaindole(ethanol)(n) (n=1-3) [7AI(EtOH)(n) (n=1-3)] clusters in the gas phase to investigate the geometrical structures and the multiple-proton/hydrogen atom transfer reaction dynamics. Four and two structural isomers were identified for 7AI(EtOH)(2) and 7AI(EtOH)(3), respectively. Density functional theory calculations at the B3LYP/6-31++G( * *)/6-31G( *) level predicted four different conformations of the ethyl group for 7AI(EtOH)(2), in good agreement with the observation of the four structural isomers in the RE2PI spectra. Visible fluorescence from the tautomeric forms was observed in the S(1) states for all isomers of 7AI(EtOH)(2), but no sign of double-proton/hydrogen atom transfer and quadruple-proton/hydrogen atom transfer has been obtained in the electronic spectra of 7AI(EtOH)(1) and 7AI(EtOH)(3), respectively. These results suggest that the multiple-proton transfer reaction is cluster-size selective, and the triple-proton/hydrogen atom transfer potential is dominated by the cyclic hydrogen-bonded network in 7AI(EtOH)(2). The excitation of the in-phase intermolecular stretching vibration prominently enhances the excited-state triple-proton/hydrogen atom transfer reaction.