Infrared spectroscopy of the phenol-N2 cluster in S0 and D0: Direct evidence of the in-plane structure of the cluster

Hydrogen Tunneling Exhibiting Unexpectedly Small Primary Kinetic Isotope Effects

Probing quantum mechanical tunneling (QMT) in chemical reactions is crucial to understanding and developing new transformations. Primary H/D kinetic isotopic effects (KIEs) beyond the semiclassical maximum values of 7-10 (room temperature) are commonly used to assess substantial QMT contributions in one-step hydrogen transfer reactions, because of the much greater QMT probability of protium vs. deuterium. Nevertheless, we report here the discovery of a reaction model occurring exclusively by H-atom QMT with residual primary H/D KIEs. 2-Hydroxyphenylnitrene, generated in N2 matrix, was found to isomerize to an imino-ketone via sequential (domino) QMT involving anti to syn OH-rotamerization (rate determining step) and [1,4]-H shift reactions. These sequential QMT transformations were also observed in the OD-deuterated sample, and unexpected primary H/D KIEs between 3 and 4 were measured at 3 to 20 K. Analogous residual primary H/D KIEs were found in the anti to syn OH-rotamerization QMT of 2-cyanophenol in a N2 matrix. Evidence strongly indicates that these intriguing isotope-insensitive QMT reactivities arise due to the solvation effects of the N2 matrix medium, putatively through coupling with the moving H/D tunneling particle. Should a similar scenario be extrapolated to conventional solution conditions, then QMT may have been overlooked in many chemical reactions.

Laboratory IR Spectra of the Ionic Oxidized Fullerenes C60O+ and C60OH. J Phys Chem A 2022;126:2928-2935. [PMID: 35533303 PMCID: PMC9125688 DOI: 10.1021/acs.jpca.2c01329]

We present the first experimental vibrational spectra of gaseous oxidized derivatives of C₆₀ in protonated and radical cation forms, obtained through infrared multiple-photon dissociation spectroscopy using the FELIX free-electron laser. Neutral C₆₀O has two nearly iso-energetic isomers: the epoxide isomer in which the O atom bridges a CC bond that connects two six-membered rings and the annulene isomer in which the O atom inserts into a CC bond connecting a five- and a six-membered ring. To determine the isomer formed for C₆₀O⁺ in our experiment—a question that cannot be confidently answered on the basis of the DFT-computed stabilities alone—we compare our experimental IR spectra to vibrational spectra predicted by DFT calculations. We conclude that the annulene-like isomer is formed in our experiment. For C₆₀OH⁺, a strong OH stretch vibration observed in the 3 μm range of the spectrum immediately reveals its structure as C₆₀ with a hydroxyl group attached, which is further confirmed by the spectrum in the 400–1600 cm^–1 range. We compare the experimental spectra of C₆₀O⁺ and C₆₀OH⁺ to the astronomical IR emission spectrum of a fullerene-rich planetary nebula and discuss their astrophysical relevance.

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.

Development and Verification of Conformer-Specific Vibrational Spectroscopy

Conformers have similar vibrational structures both in neutral (S₀) and cationic (D₀) states owing to the comparable force fields between their nuclei. Nevertheless, there is a continuous development of vibrational spectroscopic techniques to rigorously identify individual conformers in the designated molecule but only in the S₀ state. We developed an inventive conformer-specific vibrational spectroscopic technique to measure identifiable vibrational spectra of individual conformers in both S₀ and D₀ states. We measured isomer-specific vibrational spectra in both states for gas-phase acetone and oxetane isomers from a solution with azeotropic composition to verify the proposed techniques that are based on infrared (IR) resonant vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy. The measured IR dip VUV-MATI and IR hole-burn VUV-MATI spectra for each isomer, which correspond to isomer-specific vibrational spectra in both states, can be represented by IR-resonant VUV photoionization and one-photon VUV-MATI spectra of the binary mixture, respectively, under supersonic expansion conditions. The partial pressures of the individual isomers in the binary mixture with different mole fractions estimated according to the relative peak intensities in the measured spectra provide insights on solute-solvent interactions. We suggest that the verified IR-resonant VUV-MATI spectroscopy can form the basis of effective schemes toward conformational chemistry.

Isolated 2-hydroxypyrene and its dimer: a frequency- and time-resolved spectroscopic study

We investigated isolated 2-hydroxypyrene and its dimer in the gas phase by time- and frequency-resolved photoionisation with picosecond time-resolution.

Ionization-Induced π → H Site Switching in Resorcinol-Arn (n = 1 and 2) Clusters Probed by Infrared Spectroscopy

Infrared (IR) spectra of resorcinol (Rs)-Ar_n clusters (n = 1 and 2) have been measured in the neutral and cationic ground states (S₀ and D₀) by IR dip and resonance-enhanced multiphoton ionization (REMPI)-IR spectroscopy. The OH stretching vibrations in S₀ keep their frequency regardless of the number of Ar atoms and the conformation of the OH groups in Rs (rotamers RsI and RsII), demonstrating that the Ar atoms are attached to the aromatic π-ring (π-bound structure) in S₀. In the D₀ state, the IR spectra of Rs⁺-Ar_n reflect the difference in the Rs conformations (RsI⁺ and RsII⁺). For n = 1, the IR spectra of both rotamers are almost the same as those of the corresponding monomer cations, indicating that Ar ligands essentially remain π-bonded after ionization. In contrast, the IR spectra of Rs⁺-Ar₂ show hydrogen-bonded and free OH stretching vibrations, demonstrating that for a significant fraction of the clusters, the Ar atoms migrate from the π-bound site to the OH groups. The ionization-induced π → H migration yields are not unity for both rotamers RsI⁺-Ar₂ and RsII⁺-Ar₂. This result is in sharp contrast to phenol⁺-Ar₂, in which one of the Ar atoms migrates to the OH site with 100% yield. The mechanism leading to the nonunity yield in Rs⁺-Ar₂ is discussed in terms of the number of OH binding sites and Franck-Condon factors. The ionization excess energy dependence of the IR spectra of Rs⁺-Ar₂ and its Rs⁺-Ar fragments is discussed in terms of the Ar binding energies estimated from the photoionization and photodissociation efficiency spectra.

Sequential microhydration of cationic 5-hydroxyindole (5HI+): infrared photodissociation spectra of 5HI+–Wn clusters (W = H2O, n ≤ 4)

The hydrogen-bonding properties of the acidic OH and NH groups of the 5-hydroxyindole cation are probed by infrared spectroscopy and DFT calculations of its microhydrated clusters.

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.

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.

Hole-burning spectra of phenol-Arn (n = 1, 2) clusters: resolution of the isomer issue

The hole-burning (HB) spectra of phenol-Arn (PhOH-Arn) clusters with n = 1 and 2 have been measured in a molecular beam to clarify the possible existence of isomers. Two species were identified to give rise to signals in the S1-S0 spectrum recorded for the n = 1 cluster; however, one of the species was found to originate from dissociation of an n = 2 cluster. Similarly, three species were observed in the spectrum of the n = 2 cluster, and two of them were assigned to n = 3 and larger clusters. The spectral contamination from larger size clusters was quantitatively explained by the dissociation after photoexcitation. The analysis of the spectra demonstrates that only a single isomer exists in the molecular beam for both the n = 1 and the n = 2 clusters. In addition to two previously detected intermolecular modes, a third low-frequency mode, assigned to an intermolecular bending vibration, is observed for the first time in the HB spectrum of the n = 2 cluster. The assignments of the intermolecular vibrations were confirmed by ab initio MO calculations. The observation of the third intermolecular vibration suggests that the geometry of the n = 2 cluster has Cs or lower symmetry.

Picosecond IR-UV pump–probe spectroscopic study on the vibrational energy flow in isolated molecules and clusters

Intramolecular vibrational energy redistribution (IVR) and vibrational predissociation (VP) from the XH stretching vibrations, where X refers to O or C atom, of aromatic molecules and their hydrogen(H)-bonded clusters are investigated by picosecond time-resolved IR-UV pump probe spectroscopy in a supersonic beam. For bare molecules, we mainly focus on IVR of the OH stretch of phenol. We describe the IVR of the OH stretch by a two-step tier model and examine the effect of the anharmonic coupling strength and the density of states on IVR rate and mechanism by using isotope substitution. In the H-bonded clusters of phenol, we show that the relaxation of the OH stretching vibration can be described by a stepwise process and then discuss which process is sensitive to the H-bonding strength. We discuss the difference/similarity of IVR/VP between the "donor" and the "acceptor" sites in phenol-ethylene cluster by exciting the CH stretch vibrations. Finally, we study the vibrational energy transfer in the isolated molecules having the alkyl chain, namely phenylalcanol (PA). In this system, we measure the rate constant of the vibrational energy transfer between the OH stretch and the vibrations of benzene ring which are connected at the both ends of the alkyl chain. This energy transfer can be called "through-bond IVR". We investigate the three factors which are thought to control the energy transfer rate; (1) "OH <--> next CH(2)" coupling, (2) chain length and (3) conformation. We discuss the energy transfer mechanism in PAs by examining these factors.

Spectroscopic Identification of Carbenium and Ammonium Isomers of Protonated Aniline (AnH+): IR Spectra of Weakly Bound AnH+−Ln Clusters (L = Ar, N2)

Infrared photodissociation (IRPD) spectra of mass-selected clusters composed of protonated aniline (C6H8N+ = AnH+) and a variable number of neutral ligands (L = Ar, N2) are obtained in the N-H stretch range. The AnH+ -Ln complexes (n < or = 3) are produced by chemical ionization in a supersonic expansion of An, H2, and L. The IRPD spectra of AnH+-Ln feature the unambiguous fingerprints of at least two different AnH+ nucleation centers, namely, the ammonium isomer (5) and the carbenium ions (1 and/or 3) corresponding to protonation at the N atom and at the C atoms in the para and/or ortho positions, respectively. Protonation at the meta and ipso positions is not observed. Both classes of observed AnH+-Ln isomers exhibit very different photofragmentation behavior upon vibrational excitation arising from the different interaction strengths of the AnH+ cores with the surrounding neutral ligands. Analysis of the incremental N-H stretch frequency shifts as a function of cluster size shows that microsolvation of both 5 and 1/3 in Ar and N2 starts with the formation of intermolecular H bonds of the ligands to the acidic NH protons and proceeds by intermolecular pi bonding to the aromatic ring. The analysis of both the photofragmentation branching ratios and the N-H stretch frequencies demonstrates that the N-H bonds in 5 are weaker and more acidic than those in 1/3, leading to stronger intermolecular H bonds with L. The interpretation of the spectroscopic data is supported by density functional calculations conducted at the B3LYP level using the 6-31G* and 6-311G(2df,2pd) basis sets. Comparison with clusters of neutral aniline and the aniline radical cation demonstrates the drastic effect of protonation and ionization on the acidity of the N-H bonds and the topology of the intermolecular potential, in particular on the preferred aromatic substrate-nonpolar ligand recognition motif.

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.

First observation of a dihydrogen bond involving the Si–H group in phenol-diethylmethylsilane clusters by infrared-ultraviolet double-resonance spectroscopy

We have experimentally identified a dihydrogen bond involving the Si-H group in phenol-diethylmethylsilane (DEMS) clusters for the first time by IR-UV double-resonance spectroscopy. Vibrational shifts to lower frequency of 21-29 cm(-1) were found for the OH stretching vibration of three isomers of the phenol-DEMS clusters. Spectral simulations based on the MP2 calculations also support our observation. In addition to these clusters, dihydrogen bonds were also observed in the phenol-H(2)O-DEMS and (phenol)(2)-DEMS clusters, which exhibited much stronger interactions than the phenol-DEMS clusters.

Competition between van der Waals and hydrogen bonding interactions: structure of the trans-1-naphthol/N(2) cluster

The excitation energy in the multiphoton ionization spectrum of the trans-1-naphthol/N(2) cluster shows only a small red shift with respect to isolated naphthol, indicating a van der Waals pi-bound structure rather than a hydrogen-bonded one. To confirm this interpretation, high-level electronic structure calculations were performed for several pi- and hydrogen-bonded isomers of this cluster. The calculations were carried out at the second order Møller-Plesset (MP2) level of perturbation theory with the family of correlation consistent basis sets up to quintuple-zeta quality including corrections for the basis set superposition error and extrapolation to the MP2 complete basis set (CBS) limit. We report the optimal geometries, vibrational frequencies, and binding energies (D(e)), also corrected for harmonic zero-point energies (D(0)), for three energetically low-lying isomers. In all calculations the lowest energy structure was found to be an isomer with the N(2) molecule bound to the pi-system of the naphthol ring carrying the OH group. In the CBS limit its dissociation energy was computed to be D(0) = 2.67 kcal/mol (934 cm(-1)) as compared to D(0) = 1.28 kcal/mol (448 cm(-1)) for the H-bound structure. The electronic structure calculations therefore confirm the assignment of the experimental electronic spectrum corresponding to a van der Waals pi-bound structure. The energetic stabilization of the pi-bound isomer with respect to the hydrogen-bonded one is rather unexpected when compared with previous findings in related systems, in particular phenol/N(2).

Interaction of Ionic Biomolecular Building Blocks with Nonpolar Solvents: Acidity of the Imidazole Cation (Im+) Probed by IR Spectra of Im+−Ln Complexes (L = Ar, N2; n ≤ 3)

The intermolecular interaction between the imidazole cation (Im+ = C3N2H4+) and nonpolar ligands is characterized in the ground electronic state by infrared photodissociation (IRPD) spectroscopy of size-selected Im+-Ln complexes (L = Ar, N2) and quantum chemical calculations performed at the UMP2/6-311G(2df,2pd) and UB3LYP/6-311G(2df,2pd) levels of theory. The complexes are created in an electron impact cluster ion source, which predominantly produces the most stable isomers of a given cluster ion. The analysis of the size-dependent frequency shifts of both the N-H and the C-H stretch vibrations and the photofragmentation branching ratios provides valuable information about the stepwise microsolvation of Im+ in a nonpolar hydrophobic environment, including the formation of structural isomers, the competition between various intermolecular binding motifs (H-bonding and pi-bonding) and their interaction energies, and the acidity of both the CH and NH protons. In line with the calculations, the IRPD spectra show that the most stable Im+-L dimers feature planar H-bound equilibrium structures with nearly linear H-bonds of L to the acidic NH group of Im+. Further solvation occurs at the aromatic ring of Im+ via the formation of intermolecular pi-bonds. Comparison with neutral Im-Ar demonstrates the drastic effect of ionization on the topology of the intermolecular potential, in particular in the preferred aromatic substrate-nonpolar recognition motif, which changes from pi-bonding to H-bonding. .

Isomer-selective detection of microsolvated oxonium and carbenium ions of protonated phenol: Infrared spectra of C6H7O+–Ln clusters (L=Ar/N2, n⩽6)

Infrared photodissociation (IRPD) spectra of clusters composed of protonated phenol (C(6)H(7)O(+)) and several ligands L are recorded in the O-H and C-H stretch ranges using a tandem mass spectrometer coupled to a cluster ion source. The C(6)H(7)O(+)-L(n) complexes (L=Ar/N(2), n=1-6) are generated by chemical ionization of a supersonic expansion. The IRPD spectra of mass selected C(6)H(7)O(+)-L(n) clusters obtained in various C(6)H(7)O(+)-L(m) fragment channels (m<n) display the unambiguous fingerprints of at least two different C(6)H(7)O(+) nucleation centers: the oxonium ion (5) and the carbenium ion(s) corresponding to protonation of phenol in ortho and/or para position (1/3). These two classes of C(6)H(7)O(+)-L(n) isomers show very different fragmentation behavior upon IR excitation, facilitating the assignment of the observed vibrational transitions. The vibrational frequency shifts as a function of cluster size reveal that the microsolvation of 1/3 and 5 in Ar and N(2) begins with the formation of intermolecular hydrogen bond(s) to the acidic OH group(s) and proceeds by the formation of intermolecular pi-bonds to the respective six-membered rings. The analysis of photofragmentation branching ratios yields estimated ligand binding energies of the intermolecular OH- and pi-bonds for solvation of the different C(6)H(7)O(+) isomers. The effects of microsolvation on the properties of 1/3 as reactive intermediates in electrophilic aromatic substitution reactions are discussed. Comparison of clusters of protonated phenol with those of neutral phenol reveals the drastic protonation-induced changes in the topology of the intermolecular potential of aromatic molecules interacting with a nonpolar solvent. Moreover, the results show that the IRPD process can be used to selectively generate a spectroscopically clean ion beam of either 1/3 or 5 with some control over their internal energies.

The structure of the phenol-nitrogen cluster: A joint experimental andab initiostudy

The rotationally resolved LIF spectra of four different isotopomers of the phenol--nitrogen cluster have been measured to elucidate the structural parameters of the cluster in ground and electronically excited (S1) state. The fit of the rotational constants has been performed by a genetic algorithm and by an assigned fit to the line frequencies. The results of both methods are compared. The intermolecular structures are fit to the inertial parameters and are compared to the results of ab initio calculations for both states. This fit was performed under the restriction that the geometry of the monomer moieties do not change upon complexation. Of the remaining five intermolecular parameters two dihedral angles were fixed due to the planarity of the complex, which was inferred from the inertial defects of all isotopomers. The distance of the nearest nitrogen atom to the hydrogen atom of the phenolic hydroxy group is found to decrease upon electronic excitation of the chromophore considerably more than predicted from ab initio calculations. This deviation between theory and experiment can be traced back to the absence of electron-electron correlation in the performed complete active space self-consistent field calculations. The shortening of the OH...NN "hydrogen" bond upon electronic excitation is in agreement with the increased dipole moment of phenol in the S1-state.