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Otto Dopfer

Publications and source records attributed to Otto Dopfer.

17 recordsLinked to original sources
PubMed2006

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.

Aniline Compounds↗
PubMed2006

IR spectroscopic features of gaseous C7H7O+ ions: benzylium versus tropylium ion structures.

Gaseous [C7H7O]+ ions have been formed by protonation of benzaldehyde or tropone (2,4,6-cycloheptatrienone) in the cell of an FT-ICR mass spectrometer using C2H5(+) as a Brønsted acid. The so-formed species have been assayed by infrared multiphoton dissociation (IRMPD) using the free electron laser (FEL) at the CLIO (Centre Laser Infrarouge Orsay) facility. The IRMPD features are quite distinct for ions from the two different precursors, pointing to two different isomers. A number of potential structures for [C7H7O]+ ions have been optimized at the B3LYP/6-31+G(d,p) level of theory, and their relative energies and IR spectra are reported. On this basis, the IRMPD spectra of [C7H7O]+ ions are found to display features characteristic of O-protonated species, with no evidence of any further skeletal rearrangements. The so-formed ions are thus hydroxy-substituted benzylium and tropylium ions, respectively, representative members of the benzylium/tropylium ion family. The IRMPD assay using the FEL laser light has allowed their unambiguous discrimination where other mass spectrometric techniques have yielded a less conclusive answer.

Journal Article↗
PubMed2005

Protonation sites of isolated fluorobenzene revealed by IR spectroscopy in the fingerprint range.

Protonated fluorobenzene ions (C6H6F+) are produced by chemical ionization of C6H5F in the cell of a FT-ICR mass spectrometer using either CH5+ or C2H5+. The resulting protonation sites are probed by IR multiphoton dissociation (IRMPD) spectroscopy in the 600-1700 cm-1 fingerprint range employing the free electron laser at CLIO (Centre Laser Infrarouge Orsay). Comparison with quantum chemical calculations reveals that the IRMPD spectra are consistent with protonation in para and/or ortho position, which are the thermodynamically favored protonation sites. The lack of observation of protonation at the F substituent, when CH5+ is used as protonating agent, is attributed to the low-pressure conditions in the ICR cell where the ions are produced. Comparison of the C6H6F+ spectrum with IR spectra of C6H5F and C6H7+ reveals the effects of both protonation and H F substitution on the structural properties of these fundamental aromatic molecules.

Fluorobenzenes↗
PubMed2005

Hydrogen-bonded networks in ethanol proton wires: IR spectra of (EtOH)qH+-Ln clusters (L = Ar/N2, q < or = 4, n < or = 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.

Argon↗
PubMed2005

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 < or = 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. .

Amino Acid Motifs↗
PubMed2004

IR spectrum and structure of protonated ethanol dimer: implications for the mobility of excess protons in solution.

This Communication reports IR spectra and density functional calculations for the isolated protonated ethanol dimer and its N2-microsolvated complexes, (EtOH)2H+-(N2)n (n = 0-2) to investigate the degree of delocalization of the excess proton in this fundamental building block of an alcohol proton wire. The first spectroscopic characterization of isolated and microsolvated (EtOH)2H+ suggests that the excess proton is (nearly) equally shared between both EtOH units under symmetric solvation conditions (Zundel-type ion, n = 0 and 2), whereas it is largely localized on a single EtOH molecule for asymmetric solvation (Eigen-type ion, n = 1).

Journal Article↗
PubMed2004

Intermolecular interaction in an open-shell pi-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.

Journal Article↗
PubMed2004

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.

Journal Article↗
PubMed2004

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.

Journal Article↗
PubMed2004

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.

Journal Article↗
PubMed2003

Protonation of gas-phase aromatic molecules: IR spectrum of the fluoronium isomer of protonated fluorobenzene.

The IR spectrum of the fluoronium isomer of protonated fluorobenzene (F-C(6)H(6)F(+), phenylfluoronium) is recorded in the vicinity of the C-H and F-H stretch fundamentals to obtain the first structured spectrum of an isolated protonated aromatic molecule in the gas phase. Stable F-C(6)H(6)F(+) ions are produced via proton transfer from CH(5)(+) to fluorobenzene (C(6)H(5)F) in a supersonic plasma expansion. The F-C(6)H(6)F(+) spectrum recorded between 2,540 and 4,050 cm(-1) is consistent with a weakly bound ion-dipole complex composed of HF and the phenyl cation, HF-C(6)H(5)(+). The strongest transition occurs at 3,645 cm(-1) and is assigned to the F-H stretch (sigma(FH)). The antisymmetric C-H stretch of the two ortho hydrogen atoms, sigma(CH) = 3,125 cm(-1), is nearly unshifted from bare C(6)H(5)(+), indicating that HF complexation has little influence on the C-H bond strength of C(6)H(5)(+). Despite the simultaneous production of the more stable ring protonated carbenium isomers of C(6)H(6)F(+) (fluorobenzenium) in the electron ionization source, F-C(6)H(6)F(+) can selectively be photodissociated into C(6)H(5)(+) and HF under the present experimental conditions, because it has a much lower dissociation energy than all carbenium isomers. Quantum chemical calculations at the B3LYP and MP2 levels of theory using the 6-311G(2df,2pd) basis support the interpretation of the experimental data and provide further details on structural, energetic, and vibrational properties of F-C(6)H(6)F(+), the carbenium isomers of C(6)H(6)F(+), and other weakly bound HF-C(6)H(5)(+) ion-dipole complexes. The dissociation energy of F-C(6)H(6)F(+) with respect to dehydrofluorination is calculated as D(0) = 4521 cm(-1) (approximately 54 kJ/mol). Analysis of the charge distribution in F-C(6)H(6)F(+) supports the notation of a HF-C(6)H(5)(+) ion-dipole complex, with nearly the whole positive charge of the added proton distributed over the C(6)H(5)(+) ring. As a result, protonation at the F atom strongly destabilizes the C-F bond in C(6)H(5)F.

Fluorobenzenes↗
PubMed2002

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.

Journal Article↗