IR spectra of para-substituted phenol+–Ar cations: effect of halogenation on the intermolecular potential and O–H bond strength

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.

Stepwise microhydration of aromatic amide cations: water solvation networks revealed by the infrared spectra of acetanilide+-(H2O)n clusters (n ≤ 3)

The structure and activity of peptides and proteins strongly rely on their charge state and the interaction with their hydration environment. Here, infrared photodissociation (IRPD) spectra of size-selected microhydrated clusters of cationic acetanilide (AA⁺, N-phenylacetamide), AA⁺-(H₂O)_n with n ≤ 3, are analysed by dispersion-corrected density functional theory calculations at the ωB97X-D/aug-cc-pVTZ level to determine the stepwise microhydration process of this aromatic peptide model. The IRPD spectra are recorded in the informative X-H stretch (ν_OH, ν_NH, ν_CH, amide A, 2800-3800 cm^-1) and fingerprint (amide I-II, 1000-1900 cm^-1) ranges to probe the preferred hydration motifs and the cluster growth. In the most stable AA⁺-(H₂O)_n structures, the H₂O ligands solvate the acidic NH proton of the amide by forming a hydrogen-bonded solvent network, which strongly benefits from cooperative effects arising from the excess positive charge. Comparison with neutral AA-H₂O reveals the strong impact of ionization on the acidity of the NH proton and the topology of the interaction potential. Comparison with related hydrated formanilide clusters demonstrates the influence of methylation of the amide group (H → CH₃) on the shape of the intermolecular potential and the structure of the hydration shell.

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 (FT-IR/FT-Raman) and computational (HF/DFT) investigation and HOMO/LUMO/MEP analysis on 2-amino-4-chlorophenol

The spectra (FT-IR and FT-Raman) of the present compound; 2-amino-4-chlorophenol (2A4CP) were recorded in the range of 4000-100 cm(-1). All the computational calculations were made in the ground state using the HF and DFT (B3LYP and B3PW91) methods with 6-31++G(d,p) and 6-311++G(d,p) basis sets. From potential energy surface calculation, there are two conformers, Rot-1 and Rot-2 for this molecule. The computational results detected that Rot-1 form is the most stable conformer. Making use of the recorded data, the complete vibrational assignments were made and analysis of the observed fundamental bands of molecule is carried out. The complete assignments were performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQMs) method and PQS program. The shifting of the frequencies in the vibrational pattern of the title molecule due to the substitutions; NH(2) and Cl were deeply investigated by the vibrational analysis. Moreover, (13)C NMR and (1)H NMR chemical shifts were calculated by using the gauge independent atomic orbital (GIAO) method with HF/B3LYP/B3PW91 methods with 6-311++G(d,p). A study on the electronic properties, such as HOMO and LUMO energies, were performed by time-dependent DFT (TD-DFT) approach. Besides frontier molecular orbitals (FMOs), molecular electrostatic potential (MEP) was performed. NLO properties and Mulliken charges of the 2A4CP were also calculated and interpreted. The thermodynamic properties (heat capacity, entropy, and enthalpy) of the title compound at different temperatures were calculated in gas phase.

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.

Cation spectroscopy and binding energy determination for 1,4-benzodioxan-Ar1 and -Ar2 complexes

Cation vibronic spectra are measured for 1,4-benzodioxan (BZD) and van der Waals complexes of BZD with one and two Ar atoms using zero electron kinetic energy and mass analyzed threshold ionization spectroscopy. The spectra of the monomer cation were used to measure the frequencies of the two key low-frequency modes which had previously been extensively studied in the neutral S0 and S1 states. The aliphatic ring twisting mode, nu25, has an energy of 146 cm(-1) in the cation, intermediate between the values found in the S0 and S1 states. The bending, butterfly-like mode nu48 has an energy of 125 cm(-1), which is of higher frequency than either of the neutral states. The S1 spectra of the BZD-Ar1 and BZD-Ar2 complexes are recorded and observed to have modest red shifts from the monomer. The cation spectra of the complexes are also measured using mass analyzed threshold ionization spectroscopy including scans at higher energy which are used to determine the Ar binding energies. The energies for the loss of one Ar atom were determined to be 630 +/- 10 and 650 +/- 10 cm(-1) for BZD-Ar and BZD-Ar2, respectively. The similar cation spectra and similar binding energies indicate that each Ar atom in BZD-Ar2 has a similar binding geometry. Quantum chemical calculations were performed which had fair agreement with the measured binding energies and give some insight into the specific binding 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.

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.

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.

The World of Non-Covalent Interactions: 2006

The review focusses on the fundamental importance of non-covalent interactions in nature by illustrating specific examples from chemistry, physics and the biosciences. Laser spectroscopic methods and both ab initio and molecular modelling procedures used for the study of non-covalent interactions in molecular clusters are briefly outlined. The role of structure and geometry, stabilization energy, potential and free energy surfaces for molecular clusters is extensively discussed in the light of the most advanced ab initio computational results for the CCSD(T) method, extrapolated to the CBS limit. The most important types of non-covalent complexes are classified and several small and medium size non-covalent systems, including H-bonded and improper H-bonded complexes, nucleic acid base pairs, and peptides and proteins are discussed with some detail. Finally, we evaluate the interpretation of experimental results in comparison with state of the art theoretical models: this is illustrated for phenol...Ar, the benzene dimer and nucleic acid base pairs. A review with 270 references.

Electronic and Infrared Spectroscopy of [Benzene−(Methanol)n]+ (n = 1−6)

The microsolvation structure of the [benzene-(methanol)(n)](+) (n = 1-6) clusters was analyzed by electronic and infrared spectroscopy. For the n = 1 and 2 clusters, further spectroscopic investigation was carried out by Ar atom attachment, which has been know as a useful technique for discriminating isomers of the clusters. The coexistence of multiple isomers was confirmed for the n = 1 and 2 clusters, and remarkably, preferential production of the specific isomers occurred in the Ar attachment. The most stable isomer of the n = 1 cluster was suggested to be of the "on-ring" structure where the nonbonding electrons of the methanol moiety directly interact with the pi orbital of the benzene cation moiety. This is a sharp contrast to [benzene-(H(2)O)(1)](+), exhibiting the "side" structure, where the water moiety is bound to the C-H sites of the benzene cation moiety. The structure of the n = 2 cluster was discussed with the help of density functional theory calculations. Spectral signatures of the intracluster proton-transfer reaction were found for n > or = 5. The intracluster electron-transfer reaction leading to the (methanol)(m)()(+) fragment was also seen upon vibrational and electronic excitation of n > or = 4.

Hydrogen-Bonded Networks in Ethanol Proton Wires: IR Spectra of (EtOH)qH+−Ln Clusters (L = Ar/N2, q ≤ 4, n ≤ 5)

Isolated and microsolvated protonated ethanol clusters, (EtOH)qH+-Ln with L = Ar and N2, are characterized by infrared photodissociation (IRPD) spectroscopy in the 3 microm range and quantum chemical calculations. For comparison, also the spectrum of the protonated methanol dimer, (MeOH)2H+, is presented. The IRPD spectra carry the signature of H-bonded (EtOH)qH+ chain structures, in which the excess proton is either strongly localized on one or (nearly) equally shared between two EtOH molecules, corresponding to Eigen-type ion cores (EtOH2+ for q = 1, 3) or Zundel-type ion cores (EtOH-H+-HOEt for q = 2, 4), respectively. In contrast to neutral (EtOH)q clusters, no cyclic (EtOH)qH+ isomers are detected in the size range investigated (q < or = 4), indicative of the substantial impact of the excess proton on the properties of the H-bonded ethanol network. The acidity of the two terminal OH groups in the (EtOH)qH+ chains decreases with the length of the chain (q). Comparison between (ROH)qH+ with R = CH3 and C2H5 shows that the acidity of the terminal O-H groups increases with the length of the aliphatic rest (R). The most stable (EtOH)qH+-Ln clusters with n < or = 2 feature intermolecular H-bonds between the inert ligands and the two available terminal OH groups of the (EtOH)qH+ chain. Asymmetric microsolvation of (EtOH)qH+ with q = 2 and 4 promotes a switch from Zundel-type to Eigen-type cores, demonstrating that the fundamental structural motif of the (EtOH)qH+ proton wire sensitively depends on the environment. The strength of the H-bonds between L and (EtOH)qH+ is shown to provide a rather sensitive probe of the acidity of the terminal OH groups.

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. .

Selective infrared photodissociation of protonated para-fluorophenol isomers: Substitution effects in oxonium and fluoronium ions

Isomer-selective infrared photodissociation (IRPD) spectra are obtained for the first time for protonated polyfunctional aromatic molecules isolated in the gas phase. IRPD spectra of the oxonium and fluoronium isomers of protonated para-fluorophenol (C6H6FO+) were separately obtained by monitoring resonant photo-induced H2O and HF loss, respectively. Analysis of the F-H, O-H, and C-H stretch wave numbers provides valuable spectroscopic information on the chemical properties of these reactive intermediates, in particular on the substitution effects of functional groups.

Spectroscopic Identification of Oxonium and Carbenium Ions of Protonated Phenol in the Gas Phase: IR Spectra of Weakly Bound C6H7O+−L Dimers (L = Ne, Ar, N2)

Structural isomers of isolated protonated phenol (C(6)H(7)O(+)) are characterized by infrared (IR) photodissociation spectroscopy of their weakly bound complexes with neutral ligands L (L = Ne, Ar, N(2)). IR spectra of C(6)H(7)O(+)-L recorded in the vicinity of the O-H and C-H stretch fundamentals carry unambiguous signatures of at least two C(6)H(7)O(+) isomers: the identified protonation sites of phenol include the O atom (oxonium ion, O-C(6)H(7)O(+)) and the C atoms of the aromatic ring in the ortho and/or para position (carbenium ions, o/p-C(6)H(7)O(+)). In contrast, protonation at the meta and ipso positions is not observed. The most stable C(6)H(7)O(+)-L dimer structures feature intermolecular H-bonds between L and the OH groups of O-C(6)H(7)O(+) and o/p-C(6)H(7)O(+). Extrapolation to zero solvation interaction yields reliable experimental vibrational frequencies of bare O-C(6)H(7)O(+) and o/p-C(6)H(7)O(+). The interpretation of the C(6)H(7)O(+)-L spectra, as well as the extrapolated monomer frequencies, is supported by B3LYP and MP2 calculations using the 6-311G(2df,2pd) basis. The spectroscopic and theoretical results elucidate the effect of protonation on the structural properties of phenol and provide a sensitive probe of the activating and ortho/para directing nature of the OH group observed in electrophilic aromatic substitution reactions.