Nicola Solcà

Infrared spectra of the phenol–Ar and phenol–N2 cations: proton-bound versus π-bound structures

Abstract Infrared spectra of the phenol–Ar/N 2 cations (Ph + –Ar/N 2 ), produced in an electron impact ion source, are analyzed in the vicinity of the O–H stretch fundamental, ν 1 . For Ph + –Ar two isomers are identified by their ν 1 frequency shifts upon complexation: the proton-bound global minimum (Δ ν 1 =−65 cm −1 ) and the π -bound local minimum (Δ ν 1 =+2 cm −1 ). The former isomer represents the first aromatic ion-rare gas (Rg) dimer where the Rg atom does not prefer binding to the aromatic π –electron system. The larger frequency shift of proton-bound Ph + –N 2 (Δ ν 1 =−169 cm −1 ) compared to Ph + –Ar is consistent with a stronger intermolecular bond due to the additional charge–quadrupole interaction. Ab initio calculations support the interpretation of the experimental data for both species.

Protonation of aromatic molecules: competition between ring and oxygen protonation of phenol (Ph) revealed by IR spectra of PhH+–Arn

Abstract The protonation sites of phenol are investigated by IR photodissociation spectroscopy of protonated phenol complexed with one and two Ar ligands, PhH + –Ar n ( n =1,2), in the vicinity of the O–H stretch fundamentals. The complexes are produced in a supersonic expansion of Ph, H 2 , and Ar combined with electron impact ionization. The vibrational analysis of the IR spectra unambiguously reveals the presence of at least two PhH + isomers in the expansion: protonation occurs at oxygen and at the aromatic ring (in para and/or ortho position). This observation represents the first spectroscopic evidence for the existence of several PhH + isomers in the gas phase. The Ar ligands in PhH + –Ar n prefer hydrogen bonding over other binding sites. Quantum chemical calculations support the interpretation of the experimental data and provide further insight into the relative stability of various PhH + isomers and their complexes with Ar.

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.

Microsolvation of the indole cation (In+) in a nonpolar environment: IR spectra of In+–Ln complexes (L=Ar and N2, n≤8)

Infrared photodissociation (IRPD) spectra of complexes composed of the indole cation (In+ = C8H7N+) and several neutral ligands (L = Ar and N2) were recorded in the vicinity of the N–H stretch vibration (ν1) of bare In+ in its 2A″ electronic ground state. The analysis of systematic size-dependent ν1 band shifts and photofragmentation branching ratios in the spectra of In+–Arn (n ≤ 5) and In+–(N2)n (n ≤ 8) provides information about the stepwise microsolvation of In+ in a nonpolar hydrophobic environment, including the existence of structural isomers and the determination of ligand binding energies. The IR spectra of the In+–L dimers reveal two transitions, which are attributed to ν1 fundamentals of the H-bound and π-bound isomers on the basis of their complexation-induced ν1 frequency shifts, Δν1. In both cases, the H-bound isomer is found to be more stable than possible π-bound isomers. The Δν1 shifts are used to derive the first experimental estimate for the proton affinity of the indolyl radical (∼920 ± 30 kJ mol−1). The IR spectra of In+–Arn (n ≤ 5) suggest that the preferred microsolvation path for this cluster system begins with the formation of the H-bound In+–Ar dimer core, which is further solvated by (n − 1) π-bound ligands. In contrast, the spectra of In+–(N2)n with n ≤ 8 suggest that this cluster grows by the formation of an In+–(N2)2 trimer core with two H-bound N2 ligands, to which (n − 2) π-bound N2 molecules are attached. The In+–Ln complexes were generated in an electron impact (EI) ion source, which predominantly produces the most stable isomer of each cluster ion. For several In+–Ln complexes, the geometry of the most stable isomer produced in this ion source differs drastically from the structures previously observed by resonant photoionization of the corresponding neutral precursors, demonstrating the severe restriction of photoionization techniques (given by the Franck–Condon principle) for the spectroscopic characterization of cluster ions. Most of the In+–Ln complexes investigated exhibit a distinct ionization-induced change in the preferred substrate–ligand recognition pattern.

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.

Ionization-induced switch in aromatic molecule–nonpolar ligand recognition: Acidity of 1-naphthol+(1-Np+) rotamers probed by IR spectra of 1-Np+–Ln complexes (L = Ar/N2, n≤ 5)

The interaction of the trans (t) and cis (c) rotamers of the 1-naphthol cation (1-C10H8O+ = 1-Np+ = 1-hydroxynaphthalene+) with nonpolar ligands in the ground electronic state is characterized by IR photodissociation spectra of isolated 1-Np+–Ln complexes (L = Ar/N2) and density functional calculations at the UB3LYP/6-311G(2df,2pd) level. Size-dependent frequency shifts of the O–H stretch vibration (Δν1) and photofragmentation branching ratios provide information about the stepwise microsolvation of both 1-Np+ rotamers in a nonpolar hydrophobic environment, including the formation of structural isomers, the competition between H-bonding and π-bonding, the estimation of ligand binding energies, and the acidity of t/c-1-Np+. t-1-Np+ is predicted to be more stable than c-1-Np+ by 9 kJ mol−1, with an isomerization barrier of 38 kJ mol−1. The OH group in t-1-Np+ is slightly more acidic than in c-1-Np+ leading to stronger intermolecular H-bonds. Both 1-Np+ rotamers are considerably less acidic than the phenol cation because of enhanced charge delocalization. The 1-Np+−Ar spectrum displays ν1 bands of the more stable H-bound and the less stable π-bound t-1-Np+–Ar isomers. Only the more stable H-bound dimers are identified for t/c-1-Np+–L2. Analysis of the Δν1 shifts of the H-bound dimers yields a first experimental estimate for the proton affinity of the t-1-naphthoxy radical (∼908 ± 30 kJ mol−1). The Δν1 shifts of 1-Np+–Ln (n ≤ 2 for Ar, n ≤ 5 for N2) suggest that the preferred microsolvation path begins with the formation of H-bound 1-Np+–L, which is further solvated by (n−1) π-bound ligands. Ionization of 1-Np−Ln drastically changes the topology of the intermolecular interaction potential and thus the preferred aromatic substrate–nonpolar ligand recognition pattern.

IR Spectra of C2H5+-N2 Isomers: Evidence for Dative Chemical Bonding in the Isolated Ethanediazonium Ion

The potential energy surface (PES) of C(2)H(5)(+)-N(2) is characterized in detail by infrared photodissociation (IRPD) spectroscopy of mass-selected ions in a quadrupole tandem mass spectrometer and ab initio calculations at the MP2/6-311G(2df,2pd) level. The PES features three nonequivalent minima. Two local minima, 1-N(2)(H) and 1-N(2)(C), are adduct complexes with binding energies of D(0) = 18 and 12 kJ/mol, in which the N(2) ligand is weakly bonded by electrostatic forces to either the acidic proton or the electrophilic carbon atom of the nonclassical C(2)H(5)(+) ion (1), respectively. The global minimum 3 is the ethanediazonium ion, featuring a weak dative bond of D(0) = 38 kJ/mol. This interaction strength is sufficient to switch the C(2)H(5)(+) structure from nonclassical to classical. The 1-N(2)(C) isomer corresponds to the entrance channel complex for addition of N(2) to 1 yielding the product 3. This reaction involves a small barrier of 7 kJ/mol as a result of the rearrangement of the C(2)H(5)(+) ion. The partly rotationally resolved IRPD spectrum of C(2)H(5)(+)-N(2) recorded in the C-H stretch range is dominated by four bands assigned to 3 and one weak transition attributed to 1-N(2)(H). The abundance ratio of 1-N(2)(H) and 3 estimated from the IRPD spectrum as ∼1% is consistent with the calculated free energy difference of 12 kJ/mol. As the ethanediazonium ion escaped previous mass spectrometric detection, the currently accepted value for the ethyl cation affinity of N(2) is revised from -ΔH(0) = 15.5 ± 1.5 to ∼42 kJ/mol. The first experimental identification and characterization of 3 provides a sensitive probe of the electrophilic character and fluxionality of the ethyl cation. Comparison of 3 with related alkanediazonium ions reveals the drastic effect of the size of the alkyl chain on their chemical reactivity, which is relevant in the context of hydrocarbon plasma chemistry of planetary atmospheres and the interstellar medium, as well as alkylation reactions of (bio)organic molecules (e.g., carcinogenesis and mutagenesis of DNA material).

Microsolvation of the ammonia cation in argon: II. IR photodissociation spectra of NH3+–Arn (n=1–6)

Abstract Mid-infrared photodissociation spectra of NH3+–Arn (n=1–6) complexes in the electronic ground state have been recorded in the vicinity of the N–H stretch vibrations of the ammonia cation. The rovibrational analysis of the transitions in the spectrum of the NH3+–Ar dimer (n=1) is consistent with a planar, proton-bound equilibrium structure with C2v symmetry. The three N–H stretching fundamentals occur at ν 1 ( a 1 )=3177.4±1 cm −1 , ν 3 ( a 1 )=3336.0±1 cm −1 , and ν 3 ( b 2 )=3396.26±0.13 cm −1 , and the combination band of ν1 with the intermolecular stretching vibration is observed at ν 1 +ν s ( a 1 )=3305.5±2 cm −1 . The relatively long lifetime with respect to predissociation (τ>250 ps) and modest complexation-induced frequency shifts (|Δν 1,3 | cm −1 ) of the N–H stretch fundamentals imply weak coupling between the intramolecular and intermolecular degrees of freedom. The linear intermolecular proton bond in the ground vibrational state is characterized by an interatomic H–Ar separation of 2.27 A and a harmonic stretching force constant of ≈12 N/m. Observed tunneling splittings in the ν3(b2) band are attributed to hindered internal rotation through potential barriers separating the three equivalent H-bound global minima. By comparison with theoretical data, the frequency of the infrared forbidden ν1 fundamental of free NH3+ is estimated from the NH3+–Ar spectrum as 3234±15 cm −1 , the currently most accurate value based upon experimental measurements. The vibrational spectra of the larger NH3+–Arn complexes (n=2–6) display distinct frequency shifts and splittings of the N–H stretching vibrations as a function of cluster size. The spectra are consistent with cluster geometries in which the first three Ar ligands fill a primary solvation subshell by forming equivalent intermolecular proton bonds (n=1–3) leading to planar structures with either C2v or D3h symmetry. The next two Ar ligands fill a second subshell by forming equivalent p-bonds to the two lobes of the 2pz orbital of the central N atom leading to cluster structures with C3v (n=4) and D3h symmetry (n=5). The first Ar solvation shell around the interior NH3+ ion is closed at n=5 and the 6th Ar ligand occupies a position in the second solvation shell. The dissociation energies of the H-bonds and p-bonds are estimated from photofragmentation branching ratios as D 0 ( H )≈950±150 cm −1 and D 0 ( p )≈800±300 cm −1 , respectively. In general, the intermolecular H-bonds significantly weaken the intramolecular N–H bonds, whereas the p-bonds slightly strengthen them. Properties of the intermolecular bonds and the cluster growth in NH3+–Arn are compared to related AHk+–Arn cluster systems.