Zero‐kinetic‐energy photoelectron spectroscopy of the hydrogen‐bonded phenol‐water complex

Electronic and vibrational spectroscopies of aromatic clusters with He in a supersonic jet: The case of neutral and cationic phenol-Hen (n = 1 and 2)

Van der Waals clusters composed of He and aromatic molecules provide fundamental information about intermolecular interactions in weakly bound systems. In this study, phenol-helium clusters (PhOH-Hen with n ≤ 2) are characterized for the first time by UV and IR spectroscopies. The S1 ← S0 origin and ionization energy both show small but additive shifts, suggesting π-bound structures of these clusters, a conclusion supported by rotational contour analyses of the S1 origin bands. The OH stretching vibrations of the PhOH moiety in the clusters match with those of bare PhOH in both the S0 and D0 states, illustrating the negligible perturbation of the He atoms on the molecular vibration. Matrix shifts induced by He attachment are discussed based on the observed band positions with the help of complementary quantum chemical calculations. For comparison, the UV and ionization spectra of PhOH-Ne are reported as well.

Isomer-Selective Spectroscopy and Dynamics of Phenol-Arn (n ≤ 5) Clusters

Structures and ionization-induced solvation dynamics of phenol-(argon)_n clusters, PhOH-Ar_n (n ≤ 5), were studied by using a variety of isomer-selective photoionization and vibrational spectroscopic techniques. Several higher-energy isomers were found and assigned for the first time by systematically controlling the experimental conditions of the supersonic expansion. This behavior is also confirmed for the PhOH-Kr₂ cluster. Solvation structures are elucidated by evaluating systematic shifts in the S₁ ← S₀ origin and ionization energies obtained by resonance-enhanced photoionization, in addition to the OH stretching frequency obtained by IR photodissociation. Isomer-selective picosecond time-resolved IR spectroscopy for the n = 2 clusters revealed that the dynamics for the ionization-induced intermolecular π → H site-switching reaction strongly depends on the initial isomeric structure. In particular, the reaction time for the (1|1) isomer is 7 ps, while that for (2|0) is <3 ps. This difference shows that the switching time is determined by the distance of the reaction coordinate between the initial π-site and the final OH-site.

Ultrafast Excited-State Dynamics of Hydrogen-Bonded Cytosine Microsolvated Clusters with Protic and Aprotic Polar Solvents

Microsolvation effects on the ultrafast excited-state deactivation dynamics of cytosine (Cy) were studied in hydrogen-bonded Cy clusters with protic and aprotic solvents using mass-resolved femtosecond pump-probe ionization spectroscopy. Two protic solvents, water (H₂O) and methanol (MeOH), and one aprotic solvent, tetrahydrofuran (THF), were investigated, and transients of Cy·(H₂O)_1-6, Cy·(MeOH)_1-3, and Cy·THF microsolvated clusters produced in supersonic expansions were measured. With the aid of electronic structure calculations, we assigned the observed dynamics to the low-energy isomers of various Cy clusters and discussed the microsolvation effect on the excited-state deactivation dynamics. With the protic solvents only the microsolvated clusters of Cy keto tautomer were observed. The observed decay time constants of Cy·(H₂O) _n are 0.5 ps for n = 1 and ∼0.2-0.25 ps for n = 2-6. For Cy·(MeOH) _n clusters, the decay time constant for n = 1 cluster is similar to that of the Cy monohydrate, but for n = 2 and 3 the decays are about a factor of 2 slower than the corresponding microhydrates. With the aprotic solvent, THF, hydrogen-bonded complexes of both keto and enol tautomers are present in the beam. The keto-Cy·THF shows a decay similar to that of the keto-Cy monomer, whereas the enol-Cy·THF exhibits a 2-fold slower decay than the enol-Cy monomer, suggesting an increase in the barrier to excited-state deactivation upon binding of one THF molecule to the enol form of Cy.

Watching proton transfer in real time: Ultrafast photoionization-induced proton transfer in phenol-ammonia complex cation

In this paper, we give a full account of our previous work [C. C. Shen et al., J. Chem. Phys. 141, 171103 (2014)] on the study of an ultrafast photoionization-induced proton transfer (PT) reaction in the phenol-ammonia (PhOH-NH₃) complex using ultrafast time-resolved ion photofragmentation spectroscopy implemented by the photoionization-photofragmentation pump-probe detection scheme. Neutral PhOH-NH₃ complexes prepared in a free jet are photoionized by femtosecond 1 + 1 resonance-enhanced multiphoton ionization via the S₁ state. The evolving cations are then probed by delayed pulses that result in ion fragmentation, and the ionic dynamics is followed by measuring the parent-ion depletion as a function of the pump-probe delay time. By comparing with systems in which PT is not feasible and the steady-state ion photofragmentation spectra, we concluded that the observed temporal evolutions of the transient ion photofragmentation spectra are consistent with an intracomplex PT reaction after photoionization from the initial non-PT to the final PT structures. Our experiments revealed that PT in [PhOH-NH₃]⁺ cation proceeds in two distinct steps: an initial impulsive wave-packet motion in ∼70 fs followed by a slower relaxation of about 1 ps that stabilizes the system into the final PT configuration. These results indicate that for a barrierless PT system, even though the initial PT motions are impulsive and ultrafast, the time scale to complete the reaction can be much slower and is determined by the rate of energy dissipation into other modes.

Photoionization-induced π ↔ H site switching dynamics in phenol+–Rg (Rg = Ar, Kr) dimers probed by picosecond time-resolved infrared spectroscopy

Picosecond time-resolved infrared spectroscopy of phenol–rare gas dimer cations reveal delocalization of a wavepacket of the single rare gas atom above and below phenol in around 100 ps.

Mass analyzed threshold ionization detected infrared spectroscopy: isomerization activity of the phenol–Ar cluster near the ionization threshold

A new spectroscopic method reveals the barrier and the crucial role of direct photoionization in the π → H site switching in phenol–Ar.

Spectroscopic and ab initio investigation of 2,6-difluorophenylacetylene–amine complexes: coexistence of C–H⋯N and lone-pair⋯π complexes and intermolecular coulombic decay

Hydrogen bond and lone-pair⋯π interactions can coexist.

Exploring the aqueous vertical ionization of organic molecules by molecular simulation and liquid microjet photoelectron spectroscopy

To study the influence of aqueous solvent on the electronic energy levels of dissolved organic molecules, we conducted liquid microjet photoelectron spectroscopy (PES) measurements of the aqueous vertical ionization energies (VIEaq) of aniline (7.49 eV), veratrole alcohol (7.68 eV), and imidazole (8.51 eV). We also reanalyzed previously reported experimental PES data for phenol, phenolate, thymidine, and protonated imidazolium cation. We then simulated PE spectra by means of QM/MM molecular dynamics and EOM-IP-CCSD calculations with effective fragment potentials, used to describe the aqueous vertical ionization energies for six molecules, including aniline, phenol, veratrole alcohol, imidazole, methoxybenzene, and dimethylsulfide. Experimental and computational data enable us to decompose the VIEaq into elementary processes. For neutral compounds, the shift in VIE upon solvation, ΔVIEaq, was found to range from ≈-0.5 to -0.91 eV. The ΔVIEaq was further explained in terms of the influence of deforming the gas phase solute into its solution phase conformation, the influence of solute hydrogen-bond donor and acceptor interactions with proximate solvent molecules, and the polarization of about 3000 outerlying solvent molecules. Among the neutral compounds, variability in ΔVIEaq appeared largely controlled by differences in solute-solvent hydrogen-bonding interactions. Detailed computational analysis of the flexible molecule veratrole alcohol reveals that the VIE is strongly dependent on molecular conformation in both gas and aqueous phases. Finally, aqueous reorganization energies of the oxidation half-cell ionization reaction were determined from experimental data or estimated from simulation for the six compounds aniline, phenol, phenolate, veratrole alcohol, dimethylsulfide, and methoxybenzene, revealing a surprising constancy of 2.06 to 2.35 eV.

Communication: The ionization spectroscopy of mixed carboxylic acid dimers

We report mass analyzed threshold ionization spectroscopy of supersonically cooled gas phase carboxylic complexes with 9-hydroxy-9-fluorenecarboxylic acid (9HFCA), an analog of glycolic acid. The vibrationally resolved cation spectrum for the 9HFCA complex with formic acid allows accurate determination of its ionization potential (IP), 64,374 ± 8 cm(-1). This is 545 cm(-1) smaller than the IP of 9HFCA monomer. The IPs of 9HFCA complexes with acetic acid and benzoic acid shift by -1133 cm(-1) and -1438 cm(-1), respectively. Density functional calculations confirm that Cs symmetry is maintained upon ionization of the 9HFCA monomer and its acid complexes, in contrast to the drastic geometric rearrangement attending ionization in complexes of 9-fluorene carboxylic acid. We suggest that the marginal geometry changes and small IP shifts are primarily due to the collective interactions among one intramolecular and two intermolecular hydrogen bonds in the dimer.

First-principle protocol for calculating ionization energies and redox potentials of solvated molecules and ions: theory and application to aqueous phenol and phenolate

The effect of hydration on the lowest vertical ionization energy (VIE) of phenol and phenolate solvated in bulk water was characterized using the equation-of-motion ionization potential coupled-cluster (EOM-IP-CCSD) and effective fragment potential (EFP) methods (referred to as EOM/EFP) and determined experimentally by valence photoemission measurements using microjets and synchrotron radiation. The computed solvent-induced shifts in VIEs (ΔVIEs) are -0.66 and +5.72 eV for phenol and phenolate, respectively. Our best estimates of the absolute values of VIEs (7.9 and 7.7 eV for phenol and phenolate) agree reasonably well with the respective experimental values (7.8 ± 0.1 and 7.1 ± 0.1 eV). The EOM/EFP scheme was benchmarked against full EOM-IP-CCSD using microsolvated phenol and phenolate clusters. A protocol for calculating redox potentials with EOM/EFP was developed based on linear response approximation (LRA) of free energy determination. The oxidation potentials of phenol and phenolate calculated using LRA and EOM/EFP are 1.32 and 0.89 V, respectively; they agree well with experimental values.

IR Spectroscopy of Microsolvated Aromatic Cluster Ions: Ionization-Induced Switch in Aromatic Molecule–Solvent Recognition

IR spectroscopy, mass spectrometry, and quantum chemical calculations are employed to characterize the intermolecular interaction of a variety of aromatic cations (A+) with several types of solvents. For this purpose, isolated ionic complexes of the type A+–L n , in which A+ is microsolvated by a controlled number (n) of ligands (L), are prepared in a supersonic plasma expansion, and their spectra are obtained by IR photodissociation (IRPD) spectroscopy in a tandem mass spectrometer. Two prototypes of aromatic ion–solvent recognition are considered: (i) microsolvation of acidic aromatic cations in a nonpolar hydrophobic solvent and (ii) microsolvation of bare aromatic hydrocarbon cations in a polar hydrophilic solvent. The analysis of the IRPD spectra of A+–L dimers provides detailed information about the intermolecular interaction between the aromatic ion and the neutral solvent, such as ion–ligand binding energies, the competition between different intermolecular binding motifs (H-bonds, π-bonds, charge–dipole bonds), and its dependence on chemical properties of both the A+ cation and the solvent type L. IRPD spectra of larger A+–L n clusters yield detailed insight into the cluster growth process, including the formation of structural isomers, the competition between ion–solvent and solvent–solvent interactions, and the degree of (non)cooperativity of the intermolecular interactions as a function of solvent type and degree of solvation. The systematic A+–L n cluster studies are shown to reveal valuable new information about fundamental chemical properties of the bare A+ cation, such as proton affinity, acidity, and reactivity. Because of the additional attraction arising from the excess charge, the interaction in the A+–L n cation clusters differs largely from that in the corresponding neutral A–L n clusters with respect to both the interaction strength and the most stable structure, implying in most cases an ionization-induced switch in the preferred aromatic molecule–solvent recognition motif. This process causes severe limitations for the spectroscopic characterization of ion–ligand complexes using popular photoionization techniques, due to the restrictions imposed by the Franck–Condon principle. The present study circumvents these limitations by employing an electron impact cluster ion source for A+–L n generation, which generates predominantly the most stable isomer of a given cluster ion independent of its geometry.

Zero kinetic energy spectroscopy of hydroquinone-water (1:1) complex: A probe for conformer assignment

Zero kinetic energy (ZEKE) photoelectron spectroscopy of the hydroquinone-water (HQW) complex was carried out to characterize its S(1)-S(0) resonantly enhanced multiphoton ionization (REMPI) spectrum in terms of the cis and trans conformers. The ZEKE spectra of the hydroquinone isomers show differences in the Franck-Condon (FC) activity of a few ring modes, viz., modes 15, 9b, and 6b, due to the different symmetries of the two isomers. These modes were used as a "diagnostic tool" to carry out the categorical assignment of the REMPI spectrum of the HQW complex. It was found that the FC activity of these diagnostic modes in the cationic ground state (D(0)) of the water complex is similar as that of the monomer. The two lowest energy transitions in the REMPI spectrum of the water complex, 33,175 and 33,209 cm(-1), were reassigned as the band origins of the cis and trans hydroquinone-water complexes, which is opposite of the previous assignment. The intermolecular stretching mode (sigma) of the complex shows a long progression, up to v(')=4, in the cationic ground state and is strongly coupled to other observed ring modes. The Franck-Condon factors for different members in the progression were calculated using the potential energy surfaces computed ab initio. These agree well with the observed intensity patterns in the progression. The ionization potential of the trans and cis complexes was determined to be 60,071+/-4 and 60,024+/-4 cm(-1), respectively.

IR signature of the photoionization-induced hydrophobic→hydrophilic site switching in phenol-Arn clusters

IR spectra of phenol-Arn (PhOH-Arn) clusters with n=1 and 2 were measured in the neutral and cationic electronic ground states in order to determine the preferential intermolecular ligand binding motifs, hydrogen bonding (hydrophilic interaction) versus pi bonding (hydrophobic interaction). Analysis of the vibrational frequencies of the OH stretching motion, nuOH, observed in nanosecond IR spectra demonstrates that neutral PhOH-Ar and PhOH-Ar2 as well as cationic PhOH+-Ar have a pi-bound structure, in which the Ar atoms bind to the aromatic ring. In contrast, the PhOH+-Ar2 cluster cation is concluded to have a H-bound structure, in which one Ar atom is hydrogen-bonded to the OH group. This pi-->H binding site switching induced by ionization was directly monitored in real time by picosecond time-resolved IR spectroscopy. The pi-bound nuOH band is observed just after the ionization and disappears simultaneously with the appearance of the H-bound nuOH band. The analysis of the picosecond IR spectra demonstrates that (i) the pi-->H site switching is an elementary reaction with a time constant of approximately 7 ps, which is roughly independent of the available internal vibrational energy, (ii) the barrier for the isomerization reaction is rather low(<100 cm(-1)), (iii) both the position and the width of the H-bound nuOH band change with the delay time, and the time evolution of these spectral changes can be rationalized by intracluster vibrational energy redistribution occurring after the site switching. The observation of the ionization-induced switch from pi bonding to H bonding in the PhOH+-Ar2 cation corresponds to the first manifestation of an intermolecular isomerization reaction in a charged aggregate.

Determination of the Geometry Change of the Phenol Dimer upon Electronic Excitation

The change of the phenol dimer (PH2) structure upon electronic excitation is determined by a Franck-Condon analysis of the intensities in the fluorescence emission spectra obtained via excitation of seven different vibronic bands. A total of 547 emission band intensities are fitted, together with the changes of rotational constants upon electronic excitation of fi ve isotopomers. These rotational constants are taken from previously published [Schmitt et al. ChemPhysChem 2006, 7, 1241-1249] high-resolution LIF measurements. The geometry change upon electronic excitation of the pipi* state of the donor moiety can be described by a strong shortening of the hydrogen bond, a shortening of the CO bond in the donor moiety, an overall symmetric expansion of the donor phenol ring, and a nearly unchanged acceptor moiety. The resulting geometry changes are interpreted on the basis of ab initio calculations.

Hydrogen-bond assisted enormous broadening of infrared spectra of phenol-water cationic cluster: an ab initio mixed quantum-classical study

The infrared spectrum of phenol-water cationic cluster, [PhOH.H2O]+, taken by Sawamura et al. [J. Phys. Chem. 100, 8131 (1996)] is puzzling in that the peak due to the stretching mode of the phenolic OH (3657 cm-1 for a neutral monomer and 3524 cm-1 for PhOH.H2O) seemingly disappears and instead an extremely broad tail extending down to 2900 cm-1 is observed. The present authors theoretically ascribe this anomalous spectrum to an inhomogeneous broadening of the OH stretching peak caused by the hydrogen bond, the strength of which has been greatly enhanced by ionization of the phenyl ring. Indeed they estimate that the peak position is at 2300 cm-1 and the spectral width can become as wide as 1000 cm-1 at the cluster energy of 32 kcal/mol. This surprisingly wide broadening can be generic in hydrogen-bond systems, which in turn is useful to study the nature of the hydrogen-bond assisted dynamics in various systems such as those in DNA and proteins. To study the present system quantitatively, the authors have developed an ab initio mixed quantum-classical method, in which the nuclear motions on an adiabatic ab initio potential surface are treated such that only the OH stretching motion is described quantum mechanically, while all the other remaining modes are treated classically with on-the-fly scheme. This method includes the implementation of many numerical methodologies, which enables it to deal with a relatively large molecular system. With this theoretical method, the authors analyze the present anomalous broadening in a great detail. In particular, they suggest that one can extract direct information about the hydrogen-bond dynamics with respect to the clear correlation between the vibrational excitation energy of the OH stretching and intermolecular distance by means of a time-resolved infrared spectroscopy: Reflecting the slow and wide-range variation of the intermolecular distance of the relevant hydrogen bond, the time-resolved spectrum is predicted to vary (shift) largely covering the wide range of frequency domain. Thus, it is found that the short-time average along a selected trajectory sensitively reflects the change of the intermolecular distance. The authors also study the effect of internal energy on the hydrogen bonding and the OH spectrum.

Determining the Intermolecular Structure in the S0 and S1 States of the Phenol Dimer by Rotationally Resolved Electronic Spectroscopy

The rotationally resolved UV spectra of the electronic origins of five isotopomers of the phenol dimer have been measured. The complex spectra are analyzed using a fitting strategy based on a genetic algorithm. The intermolecular geometry parameters have been determined from the inertial parameters for both electronic states and compared to the results of ab initio calculations. In the electronic ground state, a larger hydrogen-bond length than in the ab initio calculations is found together with a smaller tilt angle of the aromatic rings, which shows a more pronounced dispersion interaction. In the electronically excited state, the hydrogen-bond length decreases, as has been found for other hydrogen-bonded clusters of phenol, and the two aromatic rings are tilted less toward each other.

Franck−Condon Simulations of Clusters: Phenol−Nitrogen

Multidimensional Franck-Condon simulations of the resonance enhanced multiphoton ionization (REMPI) and mass-analyzed threshold ionization (MATI) spectra of phenol-nitrogen are obtained from CASSCF, MRCI, and SACCI optimized geometries. In the REMPI simulations, the results are unsatisfactory, as the transitions associated with intermolecular modes are widely underestimated and much less intense than those associated with intramolecular modes. Conversely, the simulations of the MATI spectra show a good similarity to experiment. The best simulations are obtained in both instances from the SACCI optimized geometries. Furthermore, the simulations suggest that the two most prominent Franck-Condon envelopes present in the MATI spectra are due to the sigma and sigma + ngamma' combination bands in accord with the assignments of the MATI spectra of the analogous phenol-carbon monoxide cluster.

Structures and rearrangement reactions of 4-aminophenol(H2O)1+ and 3-aminophenol(H2O)1+ clusters

In this paper the structures of 4-aminophenol(H2O)1+ and 3-aminophenol(H2O)1+ clusters are investigated in molecular beam experiments by different IR/UV-double resonance techniques as well as the mass analyzed threshold ionization spectroscopy yielding both inter- and intramolecular vibrations of the ionic and neutral species. Possible structures are extensively calculated at the level of density functional theory (DFT) or at the ab initio level of theory. From the experimental and theoretical investigations it can be concluded that in the case of 4-aminophenol(H2O)1 one O-H...O hydrogen-bonded structure exists in the neutral cluster but two structures containing either an O-H...O or a N-H...O hydrogen-bonded arrangement are observed in the spectra of the ionic species. This observation is a result of an intramolecular rearrangement reaction within the ion which can only take place if high excess energies are used. A reaction path via the CH bonds is calculated and explains the experimental observations. In the case of 3-aminophenol(H2O)1+ only one O-H...O bound structure is observed both in the neutral and ionic species. Ab initio and DFT calculations show that due to geometrical and energetical reasons a rearrangement cannot be observed in the 3-aminophenol(H2O)1+ cluster ion.

Classical trajectory calculations of intramolecular vibrational energy redistribution. II. Phenol-water complex

Ab initio classical trajectory calculations have been applied to the intramolecular vibrational energy redistribution process of an O-H stretching vibration for phenol cation, [phenol]+, and its hydrogen-bonded water complex, [phenol-water]+. In phenol cation, a single narrow peak in the power spectrum, obtained by Fourier transformation of the autocorrelation function of its total momentum, indicates that the initial energy given to the O-H stretching oscillator of the phenol moiety is conserved and no energy flow occurs. On the other hand, for phenol-water cation, the calculated broadened power spectrum implies that the initial energy is not conserved and the energy flow causes an energy redistribution among various vibrational modes.