Electronic and vibrational spectra of protonated benzaldehyde-water clusters, [BZ-(H2O)n≤5]H+: evidence for ground-state proton transfer to solvent for n ≥ 3.

Vibrational and electronic photodissociation spectra of mass-selected protonated benzaldehyde-(water)n clusters, [BZ-(H2O)n]H(+) with n ≤ 5, are analyzed by quantum chemical calculations to determine the protonation site in the ground electronic state (S0) and ππ(*) excited state (S1) as a function of microhydration. IR spectra of [BZ-(H2O)n]H(+) with n ≤ 2 are consistent with BZH(+)-(H2O)n type structures, in which the excess proton is localized on benzaldehyde. IR spectra of clusters with n ≥ 3 are assigned to structures, in which the excess proton is located on the (H2O)n solvent moiety, BZ-(H2O)nH(+). Quantum chemical calculations at the B3LYP, MP2, and ri-CC2 levels support the conclusion of proton transfer from BZH(+) to the solvent moiety in the S0 state for hydration sizes larger than the critical value nc = 3. The vibronic spectrum of the S1 ← S0 transition (ππ(*)) of the n = 1 cluster is consistent with a cis-BZH(+)-H2O structure in both electronic states. The large blueshift of the S1 origin by 2106 cm(-1) upon hydration with a single H2O ligand indicates that the proton affinity of BZ is substantially increased upon S1 excitation, thus strongly destabilizing the hydrogen bond to the solvent. The adiabatic S1 excitation energy and vibronic structure calculated at the ri-CC2/aug-cc-pVDZ level agrees well with the measured spectrum, supporting the notion of a cis-BZH(+)-H2O geometry. The doubly hydrated species, cis-BZH(+)-(H2O)2, does not absorb in the spectral range of 23 000-27 400 cm(-1), because of the additional large blueshift of the ππ(*) transition upon attachment of the second H2O molecule. Calculations predict roughly linear and large incremental blueshifts for the ππ(*) transition in [BZ-(H2O)n]H(+) as a function of n. In the size range n ≥ 3, the calculations predict a proton transfer from the (H2O)nH(+) solvent back to the BZ solute upon electronic ππ(*) excitation.

Microsolvation of the acetanilide cation (AA(+)) in a nonpolar solvent: IR spectra of AA(+)-L(n) clusters (L = He, Ar, N2; n ≤ 10).

Infrared photodissociation (IRPD) spectra of mass-selected cluster ions of acetanilide (N-phenylacetamide), AA(+)-Ln, with the ligands L = He (n = 1-2), Ar (n = 1-7), and N2 (n = 1-10) are recorded in the hydride stretch (amide A, νNH, νCH) and fingerprint (amide I-III) ranges of AA(+) in its (2)A'' ground electronic state. Cold AA(+)-Ln clusters are generated in an electron impact ion source, which predominantly produces the most stable isomer of a given cluster ion. Systematic vibrational frequency shifts of the N-H stretch fundamentals (νNH) provide detailed information about the sequential microsolvation process of AA(+) in a nonpolar (L = He and Ar) and quadrupolar (L = N2) solvent. In the most stable AA(+)-Ln clusters, the first ligand forms a hydrogen bond (H-bond) with the N-H proton of trans-AA(+) (t-AA(+)), whereas further ligands bind weakly to the aromatic ring (π-stacking). There is no experimental evidence for complexes with the less stable cis-AA(+) isomer. Quantum chemical calculations at the M06-2X/aug-cc-pVTZ level confirm the cluster growth sequence derived from the IR spectra. The calculated binding energies of De(H) = 720 and 1227 cm(-1) for H-bonded and De(π) = 585 and 715 cm(-1) for π-bonded Ar and N2 ligands in t-AA(+)-L are consistent with the observed photofragmentation branching ratios of AA(+)-Ln. Comparison between charged and neutral AA((+))-L dimers indicates that ionization switches the preferred ion-ligand binding motif from π-stacking to H-bonding. Electron removal from the HOMO of AA(+) delocalized over both the aromatic ring and the amide group significantly strengthens the C[double bond, length as m-dash]O bond and weakens the N-H bond of the amide group.

Solvent migration in microhydrated aromatic aggregates: ionization-induced site switching in the 4-aminobenzonitrile-water cluster.

The dependence of the preferred microhydration sites of 4-aminobenzonitrile (4ABN) on electronic excitation and ionization is determined through IR spectroscopy of its clusters with water (W) in a supersonic expansion and through quantum chemical calculations. IR spectra of neutral 4ABN and two isomers of its hydrogen-bonded (H-bonded) 4ABN-W complexes are obtained in the ground and first excited singlet states (S0, S1) through IR depletion spectroscopy associated with resonance-enhanced multiphoton ionization. Spectral analysis reveals that electronic excitation does not change the H-bonding motif of each isomer, that is, H2O binding either to the CN or the NH site of 4ABN, denoted as 4ABN-W(CN) and 4ABN-W(NH), respectively. The IR spectra of 4ABN(+)-W in the doublet cation ground electronic state (D0) are measured by generating them either in an electron ionization source (EI-IR) or through resonant multiphoton ionization (REMPI-IR). The EI-IR spectrum shows only transitions of the most stable isomer of the cation, which is assigned to 4ABN(+)-W(NH). The REMPI-IR spectrum obtained through isomer-selective resonant photoionization of 4ABN-W(NH) is essentially the same as the EI-IR spectrum. The REMPI-IR spectrum obtained by ionizing 4ABN-W(CN) is also similar to that of the 4ABN(+)-W(NH) isomer, but differs from that calculated for 4ABN(+)-W(CN), indicating that the H2O ligand migrates from the CN to the NH site upon ionization with a yield of 100%. The mechanism of this CN→NH site-switching reaction is discussed in the light of the calculated potential energy surface and the role of intracluster vibrational energy redistribution.

Experimental observation and quantum chemical characterization of the S1 ← S0 transition of protonated naphthalene-argon clusters.

We report on the photodissociation spectrum of protonated naphthalene(+)-argon complexes (NpH(+)-Ar) recorded by excitation into the first excited singlet electronic state. Unlike previous electronic spectra of the free molecule (NpH(+)), both the α and the ß isomer could be observed for the Ar adducts. Detailed information on the S0 and S1 state of both isomers is provided by quantum chemical calculations. An assignment of observed vibrational bands is proposed based on Franck-Condon simulations.

Microsolvation of the 4-aminobenzonitrile cation (ABN+) in a nonpolar solvent: IR spectra of ABN(+)-L(n) (L=Ar and N2, n≤4).

IR photodissociation (IRPD) spectra of mass-selected cluster ions of 4-aminobenzonitrile (ABN(+)) with up to four Ar and N2 ligands are recorded over the spectral range of the N-H stretching vibrations (ν(s/a)) of ABN(+) in its (2)B1 ground electronic state. ABN(+)-L(n) clusters are produced in an electron impact cluster ion source, which predominantly generates the most stable isomer of a given cluster ion. Vibrational frequency shifts of ν(s/a) provide information about the sequential microsolvation process of ABN(+) in a nonpolar solvent. In ABN(+)-(N2)n, the first two ligands fill a first subshell by forming hydrogen bonds to the acidic protons of the amino group, whereas further ligands bind more weakly to the aromatic ring (π bonds). Although the preferred cluster growth sequence in ABN(+)-Ar(n) is similar, several isomers are observed because the hydrogen bonds are only slightly stronger than the π bonds. Quantum chemical calculations at the M06-2X/aug-cc-pVTZ level confirm the cluster growth sequence derived from the IR spectra and provide further details of the intermolecular potential. The calculated binding energies of D0(H)=532 and 895 cm(-1) for hydrogen-bonded and D0(π)=512 and 530 cm(-1) for π-bonded Ar and N2 ligands are consistent with the observed photofragmentation branching ratios. Comparison between ABN(+)-L(n) and the corresponding clusters with the aniline cation demonstrates that the NH protons of the amino group become slightly more acidic upon H→CN substitution at the para position. Comparison between charged and neutral ABN((+))-L dimers indicates that ionization switches the preferred ion-ligand binding motif from π to hydrogen bonding.

Infrared spectrum and structure of the adamantane cation: direct evidence for Jahn-Teller distortion.

The fundamentals: the IR spectrum of the adamantane cation, C(10)H(16)(+), has been derived by resonant IR photodissociation of weakly bound C(10)H(16)(+)⋅L(n) clusters. The analysis of the IR spectrum provides the first spectroscopic characterization of this fundamental cycloalkane carbocation in the gas phase and direct evidence for the Jahn-Teller distortion in the (2)A(1) ground electronic state.

Structures and IR/UV spectra of neutral and ionic phenol-Ar(n) cluster isomers (n ≤ 4): competition between hydrogen bonding and stacking.

The structures, binding energies, and vibrational and electronic spectra of various isomers of neutral and ionic phenol-Ar(n) clusters with n ≤ 4, PhOH((+))-Ar(n), are characterized by quantum chemical calculations. The properties in the neutral and ionic ground electronic states (S(0), D(0)) are determined at the M06-2X/aug-cc-pVTZ level, whereas the S(1) excited state of the neutral species is investigated at the CC2/aug-cc-pVDZ level. The Ar complexation shifts calculated for the S(1) origin and the adiabatic ionisation potential, ΔS(1) and ΔIP, sensitively depend on the Ar positions and thus the sequence of filling the first Ar solvation shell. The calculated shifts confirm empirical additivity rules for ΔS(1) established recently from experimental spectra and enable thus a firm assignment of various S(1) origins to their respective isomers. A similar additivity model is newly developed for ΔIP using the M06-2X data. The isomer assignment is further confirmed by Franck-Condon simulations of the intermolecular vibrational structure of the S(1) ← S(0) transitions. In neutral PhOH-Ar(n), dispersion dominates the attraction and π-bonding is more stable than H-bonding. The solvation sequence of the most stable isomers is derived as (10), (11), (30), and (31) for n ≤ 4, where (km) denotes isomers with k and m Ar ligands binding above and below the aromatic plane, respectively. The π interaction is somewhat stronger in the S(1) state due to enhanced dispersion forces. Similarly, the H-bond strength increases in S(1) due to the enhanced acidity of the OH proton. In the PhOH(+)-Ar(n) cations, H-bonds are significantly stronger than π-bonds due to additional induction forces. Consequently, one favourable solvation sequence is derived as (H00), (H10), (H20), and (H30) for n ≤ 4, where (Hkm) denotes isomers with one H-bound ligand and k and m π-bonded Ar ligands above and below the aromatic plane, respectively. Another low-energy solvation motif for n = 2 is denoted (11)(H) and involves nonlinear bifurcated H-bonding to both equivalent Ar atoms in a C(2v) structure in which the OH group points toward the midpoint of an Ar(2) dimer in a T-shaped fashion. This dimer core can also be further solvated by π-bonded ligands leading to the solvation sequence (H00), (11)(H), (21)(H), and (22) for n ≤ 4. The implications of the ionisation-induced π → H switch in the preferred interaction motif on the isomerisation and fragmentation processes of PhOH((+))-Ar(n) are discussed in the light of the new structural and energetic cluster parameters.

Microhydration effects on the electronic spectra of protonated polycyclic aromatic hydrocarbons: [naphthalene-(H2O)(n = 1,2)]H+.

Vibrational and electronic spectra of protonated naphthalene (NaphH(+)) microsolvated by one and two water molecules were obtained by photofragmentation spectroscopy. The IR spectrum of the monohydrated species is consistent with a structure with the proton located on the aromatic molecule, NaphH(+)-H(2)O. Similar to isolated NaphH(+), the first electronic transition of NaphH(+)-H(2)O (S(1)) occurs in the visible range near 500 nm. The doubly hydrated species lacks any absorption in the visible range (420-600 nm) but absorbs in the UV range, similar to neutral Naph. This observation is consistent with a structure, in which the proton is located on the water moiety, Naph-(H(2)O)(2)H(+). Ab initio calculations for [Naph-(H(2)O)(n)]H(+) confirm that the excess proton transfers from Naph to the solvent cluster upon attachment of the second water molecule.

Electronic spectra of protonated benzaldehyde clusters with Ar and N2: effect of ππ* excitation on the intermolecular potential.

Electronic spectra of the S(1)←S(0) transition of dimers of protonated benzaldehyde (BZH(+)) with Ar and N(2) are recorded by resonance-enhanced photodissociation in a tandem mass spectrometer. The S(1) origins observed are shifted to higher frequency upon complexation with Ar (ΔS(1) = 300 cm(-1)) and N(2) (ΔS(1) = 628 cm(-1)). Ab initio calculations at the CC2/aug-cc-pVDZ level suggest an assignment to H-bonded dimers of L = Ar and N(2) binding to the cis isomer of O-protonated BZH(+), yielding values of ΔS(1) = 242 and 588 cm(-1) for cis-BZH(+)-L(H). Electronic ππ* excitation results in a substantial increase of the proton affinity of BZH(+), which in turn destabilizes the intermolecular H-bonds to the inert ligands by 35%. The drastic effects of electronic ππ* excitation on the geometric and electronic structure as well as the strength and anisotropy of the intermolecular potential (H-bonding and π-bonding) are investigated.

IR spectra of protonated benzaldehyde clusters, C(7)H(7)O(+)-L(n) (L=Ar,N(2);n

Infrared photodissociation (IRPD) spectra of mass-selected protonated benzaldehyde (C(7)H(7)O(+),BZH(+)) and its weakly bound clusters with Ar and N(2) produced in an electron impact source are recorded in the C-H and O-H stretch ranges. The experimental results are supported by ab initio and density functional calculations. Analysis of the IRPD spectrum of the BZH(+) monomer is consistent with the presence of the cis and trans isomers of the oxonium ions, which is confirmed by the cluster spectra. No signature of the less stable carbenium ions is detected. Frequency shifts in the IRPD spectra of dimers and trimers provide information about the preferred intermolecular ligand binding site (pi-bonding versus H-bonding) and the corresponding interaction strength. H-bonding to the OH group of the oxonium ions of BZH(+) is found to be favored over pi-bonding to the aromatic ring for both Ar and N(2). There are significant differences in the microsolvation structure and energetics of the cis and trans oxonium isomers of BZH(+) due to the rather different acidities of their OH groups and isomer-dependent effects arising from steric hindrance. The large positive partial charge of the protonated formyl group implies that the cluster growth of the larger clusters continues by further solvation of the protonated substituent rather than the aromatic ring.

Infrared spectra and quantum chemical characterization of weakly bound clusters of the benzoyl cation with Ar and H(2)O.

Weakly-bound clusters of the closed-shell benzoyl cation (C(6)H(5)CO(+), PhCO(+)) with Ar and H(2)O are investigated by infrared (IR) spectroscopy, mass spectrometry, and quantum chemical calculations in order to characterize the interaction of a closed shell aromatic cation with a nonpolar and a polar ligand. PhCO(+)-L dimers are produced by electron ionization of benzaldehyde in a supersonic plasma expansion. IR photodissociation (IRPD) spectra of PhCO(+)-L with L = Ar and H(2)O are analyzed in the C-O, C-H, and O-H stretch ranges (2000-3900 cm(-1)). The potential energy surface of the PhCO(+)-L dimers is characterized at the MP2/6-311++G(2df,2pd) level to locate the various minima and determine their energetic and vibrational properties. PhCO(+)-Ar prefers intermolecular π-bonding to the aromatic ring with a bond energy of D(0) = 6 kJ mol(-1). The weak interaction implies that the IRPD spectrum of PhCO(+)-Ar is very close to the IR spectrum of bare PhCO(+). The detection of the C-H and C-O stretch fundamentals provides valuable information about the C-O and C-H bond strengths in this prototypical aromatic acylium ion. Moreover, a variety of weak combination and overtone bands are assigned. The global minimum on the PhCO(+)-H(2)O potential has a planar charge-dipole configuration with D(0) = 41 kJ mol(-1) (with only the two H(2)O protons being out-of-plane), in which the lone pairs of H(2)O interact with the positive partial charges on the carbonyl carbon atom and the proton of the CH group in ortho position. The experimental IRPD spectra are in accord with the calculated global minima. The analysis of the charge distribution shows that the PhCO(+) cation is best represented as an oxocarbenium ion (Ph-C(+)[double bond, length as m-dash]O) with smaller contributions of the ketene structure (Ph(+)[double bond, length as m-dash]C[double bond, length as m-dash]O). This view is supported by the geometrical and vibrational properties of PhCO(+) as well as the shape of the intermolecular PhCO(+)-L dimer potentials.