Infrared photodissociation spectra of isomeric SiOH+–Ar (n=1–10) complexes

Microhydrated clusters of a pharmaceutical drug: infrared spectra and structures of amantadineH+(H2O)n

Solvation of pharmaceutical drugs has an important effect on their structure and function. Analysis of infrared photodissociation spectra of amantadineH⁺(H₂O)_n=1-4 clusters in the sensitive OH, NH, and CH stretch range by quantum chemical calculations (B3LYP-D3/cc-pVTZ) provides a first impression of the interaction of this pharmaceutically active cation with water at the molecular level. The size-dependent frequency shifts reveal detailed information about the acidity of the protons of the NH₃⁺ group of N-protonated amantadineH⁺ (AmaH⁺) and the strength of the NH⋯O and OH⋯O hydrogen bonds (H-bonds) of the hydration network. The preferred cluster growth begins with sequential hydration of the NH₃⁺ group by NH⋯O ionic H-bonds (n = 1-3), followed by the extension of the solvent network through OH⋯O H-bonds. However, smaller populations of cluster isomers with an H-bonded solvent network and free N-H bonds are already observed for n ≥ 2, indicating the subtle competition between noncooperative ion hydration and cooperative H-bonding. Interestingly, cyclic water ring structures are identified for n ≥ 3, each with two NH⋯O and two OH⋯O H-bonds. Despite the increasing destabilization of the N-H proton donor bonds upon gradual hydration, no proton transfer to the (H₂O)_n solvent cluster is observed up to n = 4. In addition to ammonium cluster ions, a small population of microhydrated iminium isomers is also detected, which is substantially lower for the hydrophilic H₂O than for the hydrophobic Ar environment.

Unraveling the protonation site of oxazole and solvation with hydrophobic ligands by infrared photodissociation spectroscopy

Infrared spectroscopy reveals exclusive N-protonation of the oxazole ring and bifurcated or linear hydrogen bonding with hydrophobic N₂and Ar ligands.

IR Spectrum and Structure of Protonated Monosilanol: Dative Bonding between Water and the Silylium Ion

We report the spectroscopic characterization of protonated monosilanol (SiH₃ OH₂⁺ ) isolated in the gas phase, thus providing the first experimental determination of the structure and bonding of a member of the elusive silanol family. The SiH₃ OH₂⁺ ion is generated in a silane/water plasma expansion, and its structure is derived from the IR photodissociation (IRPD) spectrum of its Ar cluster measured in a tandem mass spectrometer. The chemical bonding in SiH₃ OH₂⁺ is analyzed by density functional theory (DFT) calculations, providing detailed insight into the nature of the dative H₃ Si⁺ -OH₂ bond. Comparison with protonated methanol illustrates the differences in bonding between carbon and silicon, which are mainly related to their different electronegativity and the different energy of the vacant valence p_z orbital of SiH₃⁺ and CH₃⁺ .

Competing Insertion and External Binding Motifs in Hydrated Neurotransmitters: Infrared Spectra of Protonated Phenylethylamine Monohydrate

Hydration has a drastic impact on the structure and function of flexible biomolecules, such as aromatic ethylamino neurotransmitters. The structure of monohydrated protonated phenylethylamine (H(+) PEA-H2 O) is investigated by infrared photodissociation (IRPD) spectroscopy of cold cluster ions by using rare-gas (Rg=Ne and Ar) tagging and dispersion-corrected density functional theory calculations at the B3LYP-D3/aug-cc-pVTZ level. Monohydration of this prototypical neurotransmitter gives an insight into the first step of the formation of its solvation shell, especially regarding the competition between intra- and intermolecular interactions. The spectra of Rg-tagged H(+) PEA-H2 O reveal the presence of a stable insertion structure in which the water molecule is located between the positively charged ammonium group and the phenyl ring of H(+) PEA, acting both as a hydrogen bond acceptor (NH(+) ⋅⋅⋅O) and donor (OH⋅⋅⋅π). Two other nearly equivalent isomers, in which water is externally H bonded to one of the free NH groups, are also identified. The balance between insertion and external hydration strongly depends on temperature.

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.

Infrared photodissociation spectroscopy of Mg(+)(H2O)Ar(n) complexes: isomers in progressive microsolvation

Ion-molecule complexes of the form Mg(H2O)Ar(n)+ (n = 1-8) are produced by laser vaporization in a pulsed-nozzle cluster source. These complexes are mass-selected and studied with infrared photodissociation spectroscopy in the O-H stretch region. The spectra are interpreted with the aid of ab initio calculations on the n = 1-5 complexes, including examination of various isomeric structures. The combined spectroscopic and theoretical studies reveal the presence of multiple isomeric structures at each cluster size, as the argon atoms assemble around the Mg(+)(H2O) unit. Distinct infrared resonances are measured for argon-on-metal, argon-on-OH and argon-on-two-OH isomers.

Ultraviolet and infrared photodissociation of Si(+)(C6H6)n and Si(+)(C6H6)(n)Ar clusters

Ion-molecule complexes of the form Si(+)(C6H6)n and Si(+)(C6H6)(n)Ar are produced by laser vaporization in a pulsed nozzle cluster source. These clusters are mass-selected and studied with ultraviolet (355 nm) photodissociation and resonance-enhanced infrared photodissociation spectroscopy in the C-H stretch region of benzene. In the UV, Si(+)(C6H6)n clusters (n = 1-5) fragment to produce the Si(+)(C6H6)n mono-ligand species, suggesting that this ion has enhanced relative stability. IR photodissociation of Si(+)(C6H6)n complexes occurs by the elimination of benzene, while Si(+)(C6H6)(n)Ar complexes lose Ar. Resonances reveal C-H vibrational bands in the 2900-3300 cm(-1) region characteristic of the benzene ligand with shifts caused by the silicon cation bonding. The IR spectra confirm that the major component of the Si(+)(C6H6)n ions studied have the pi-complex structure rather than the isomeric insertion products suggested previously.

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.

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. .

Intermolecular interaction in an open-shell π-bound cationic complex: IR spectrum and coupled cluster calculations for C2H2+-Ar

The intermolecular potential energy surface (PES) of Ar interacting with the acetylene cation in its (2)Pi(u) ground electronic state is characterized by infrared photodissociation (IRPD) spectroscopy and quantum chemical calculations. In agreement with the theoretical predictions, the rovibrational analysis of the IRPD spectrum of C(2)H(2) (+)-Ar recorded in the vicinity of the antisymmetric CH stretching fundamental (nu(3)) is consistent with a vibrationally averaged T-shaped structure and a ground-state center-of-mass separation of R(c.m.) = 2.86 +/- 0.09 A. The nu(3) band experiences a blueshift of 16.7 cm(-1) upon complexation, indicating that vibrational excitation slightly reduces the interaction strength. The two-dimensional intermolecular PES of C(2)H(2) (+)-Ar, obtained from coupled cluster calculations with a large basis set, features strong angular-radial coupling and supports in addition to a global pi-bound minimum also two shallow side wells with linear H-bound geometries. Bound state rovibrational energy level calculations are carried out for rotational angular momentum J = 0-10 (both parities) employing a discrete variable representation-distributed Gaussian basis method. Effective spectroscopic constants are determined for the vibrational ground state by fitting the calculated rotational energies to the standard Watson A-type Hamiltonian for a slightly asymmetric prolate top.

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.

Infrared photodissociation spectroscopy of Si+(CO2)n and Si+(CO2)nAr complexes — Evidence for unanticipated intracluster reactions

Si+(CO2)n and Si+(CO2)nAr ion–molecule complexes were produced by laser vaporization in a pulsed supersonic expansion. The ions were mass-selected in a reflectron time-of-flight spectrometer and studied with infrared photodissociation spectroscopy near the asymmetric stretch vibration of CO2. Si+(CO2)n clusters fragment by the loss of CO2 whereas Si+(CO2)nAr complexes fragment by the loss of argon. All clusters have resonances near the CO2 asymmetric stretch, but with shifts in frequency that are size dependent. The patterns seen in the small clusters are consistent with electrostatic bonding, while the larger systems provide evidence for an intracluster reaction forming oxide-carbonyl species. Density functional theory was employed to examine the structures of these clusters, and their calculated vibrational frequencies were compared to the measured values. Ligand assembly in Si+(CO2)n complexes is dominated by the presence of the occupied 3p valence orbital of the silicon cation. Key words: ion–molecule complexes, infrared spectroscopy, photodissociation, density functional theory.

Infrared spectra of C(3)H(3)(+)-N(2) dimers: identification of proton-bound c-C(3)H(3)(+)-N(2) and H(2)CCCH(+)-N(2) isomers

Mid-infrared photodissociation spectra of mass selected C(3)H(3)(+)-N(2) ionic complexes are obtained in the vicinity of the C-H stretch fundamentals (2970-3370 cm(-1)). The C(3)H(3)(+)-N(2) dimers are produced in an electron impact cluster ion source by supersonically expanding a gas mixture of allene, N(2), and Ar. Rovibrational analysis of the spectra demonstrates that (at least) two C(3)H(3)(+) isomers are produced in the employed ion source, namely the cyclopropenyl (c-C(3)H(3)(+)) and the propargyl (H(2)CCCH(+)) cations. This observation is the first spectroscopic detection of the important c-C(3)H(3)(+) ion in the gas phase. Both C(3)H(3)(+) cations form intermolecular proton bonds to the N(2) ligand with a linear -C-H...N-N configuration, leading to planar C(3)H(3)(+)-N(2) structures with C(2v) symmetry. The strongest absorption of the H(2)CCCH(+)-N(2) dimer in the spectral range investigated corresponds to the acetylenic C-H stretch fundamental (v(1) = 3139 cm(-1)), which experiences a large red shift upon N(2) complexation (Delta(v1) approximately -180 cm(-1)). For c-C(3)H(3)(+)-N(2), the strongly IR active degenerate antisymmetric stretch vibration (v4)) of c-C(3)H(3)(+) is split into two components upon complexation with N(2): v4)(a(1)) = 3094 cm(-1) and v4)(b(2)) = 3129 cm(-1). These values bracket the yet unknown v4) frequency of free c-C(3)H(3)(+) in the gas phase, which is estimated as 3125 +/- 4 cm(-1) by comparison with theoretical data. Analysis of the nuclear spin statistical weights and A rotational constants of H(2)CCCH(+)-N(2) and c-C(3)H(3)(+)-N(2) provide for the first time high-resolution spectroscopic evidence that H(2)CCCH(+) and c-C(3)H(3)(+) are planar ions with C(2v) and D(3h) symmetry, respectively. Ab initio calculations at the MP2(full)/6-311G(2df,2pd) level confirm the given assignments and predict intermolecular separations of R(e) = 2.1772 and 2.0916 A and binding energies of D(e) = 1227 and 1373 cm(-1) for the H-bound c-C(3)H(3)(+)-N(2) and H(2)CCCH(+)-N(2) dimers, respectively.