The infrared (IR) spectrum of the isolated protonated neurotransmitter dopamine was recorded in the fingerprint range (570-1880 cm À1) by means of IR multiple photon dissociation (IRMPD) spectroscopy. The spectrum was obtained in a Fourier transform ion cyclotron resonance mass spectrometer equipped with an electrospray ionization source, which was coupled to a free electron laser (FEL). The spectroscopic studies are complemented by quantum chemical calculations at the B3LYP and MP2 levels of theory using the cc-pVDZ basis set. Several low-energy isomers with protonation occurring at the amino group are predicted in the energy range 0-50 kJ mol À1. Good agreement between the measured IRMPD spectrum and the calculated linear absorption spectra is observed for the two gauche conformers lowest in energy (DE) and free energy (DG) at both levels of theory, denoted gÀ1 and g+1. Minor contributions of higher lying gauche isomers cannot be ruled out spectroscopically but their calculated energies suggest only minor population in the sampled ion cloud. In all these gauche structures, one of the three protons of the ammonium group is pointing toward the catechol subunit, thereby maximizing the intramolecular NH-p interaction of the positive charge with the aromatic ring. In total, 16 distinct vibrational bands are observed in the IRMPD spectrum and assigned to individual normal modes of the energetically most stable gÀ1 conformer, with deviations of less than 24 cm À1 (average 11 cm À1) between measured and calculated frequencies. Comparison with neutral dopamine reveals the effects of protonation on the geometric and electronic structure.

The existence of a transitional size regime where preferential stabilization alternates between "all-surface" (all atoms on the surface of a cluster) and "internally solvated" (one water molecule at the center of the cluster, fully solvated) configurations with the addition or the removal of a single water molecule, predicted earlier with the flexible, polarizable (many-body) Thole-type model interaction potential (TTM2-F), has been confirmed from electronic structure calculations for (H2O)n, n = 17-21. The onset of the appearance of the first "interior" configuration in water clusters occurs for n = 17. The observed structural alternation between interior (n = 17, 19, 21) and all-surface (n = 18, 20) global minima in the n = 17-21 cluster regime is accompanied by a corresponding spectroscopic signature, namely, the undulation in the position of the most redshifted OH stretching vibrations according to the trend: interior configurations exhibit more redshifted OH stretching vibrations than all-surface ones. These most redshifted OH stretching vibrations form distinct groups in the intramolecular region of the spectra and correspond to localized vibrations of donor OH stretches that are connected to neighbors via "strong" (water dimer-like) hydrogen bonds and belong to a water molecule with a "free" OH stretch.

The gas phase infrared spectrum (3250-3810 cm-1) of the singly hydrated ammonium ion, NH4+(H2O), has been recorded by action spectroscopy of mass selected and isolated ions. The four bands obtained are assigned to N-H stretching modes and to O-H stretching modes. The N-H stretching modes observed are blueshifted with respect to the corresponding modes of the free NH4+ ion, whereas a redshift is observed with respect to the modes of the free NH3 molecule. The O-H stretching modes observed are redshifted when compared to the free H2O molecule. The asymmetric stretching modes give rise to rotationally resolved perpendicular transitions. The K-type equidistant rotational spacings of 11.1(2) cm-1 (NH4+) and 29(3) cm-1 (H2O) deviate systematically from the corresponding values of the free molecules, a fact which is rationalized in terms of a symmetric top analysis. The relative band intensities recorded compare favorably with predictions of high level ab initio calculations, except on the nu3(H2O) band for which the observed value is about 20 times weaker than the calculated one. The nu3(H2O)/nu1(H2O) intensity ratios from other published action spectra in other cationic complexes vary such that the nu3(H2O) intensities become smaller the stronger the complexes are bound. The recorded ratios vary, in particular, among the data collected from action spectra that were recorded with and without rare gas tagging. The calculated anharmonic coupling constants in NH4+(H2O) further suggest that the coupling of the nu3(H2O) and nu1(H2O) modes to other cluster modes indeed varies by orders of magnitude. These findings together render a picture of a mode specific fragmentation dynamic that modulates band intensities in action spectra with respect to absorption spectra. Additional high level electronic structure calculations at the coupled-cluster singles and doubles with a perturbative treatment of triple excitations [CCSD(T)] level of theory with large basis sets allow for the determination of an accurate binding energy and enthalpy of the NH4+(H2O) cluster. The authors' extrapolated values at the CCSD(T) complete basis set limit are De [NH4+-(H2O)]=-85.40(+/-0.24) kJ/mol and DeltaH(298 K) [NH4+-(H2O)]=-78.3(+/-0.3) kJ/mol (CC2), in which double standard deviations are indicated in parentheses.

The structure and infrared (IR) spectrum of the Ag(+)-phenol cationic complex are characterized in the gas phase by photodissociation spectroscopy and quantum chemical calculations in order to determine the preferred metal ion binding site. The IR multiple photon dissociation (IRMPD) spectrum has been obtained in the 1100-1700 cm(-1) fingerprint range by coupling the IR free electron laser at the Centre Laser Infrarouge d'Orsay (CLIO) with a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer equipped with an electrospray ionization source. The spectroscopic efforts are complemented by quantum chemical calculations at the MP2 and B3LYP levels using the aug-cc-pVTZ basis set. Analysis of the IRMPD spectrum is consistent with a π complex, in which the Ag(+) ion binds to the aromatic ring in an η(1) (B3LYP) or η(2) (MP2) fashion to carbon atoms in the para position of the OH group. Ag(+) bonding to the hydroxyl group in the form of a σ complex is calculated to be less favorable. Comparison of the structural and vibrational properties of phenol, Ag(+)-phenol, and phenol(+) suggests partial charge transfer upon formation of the π complex.

The gas-phase IR spectrum of the protonated neurotransmitter serotonin (5-hydroxytryptamine) was measured in the fingerprint range by means of IR multiple photon dissociation (IRMPD) spectroscopy. The IRMPD spectrum was recorded in a Fourier transform ion cyclotron resonance mass spectrometer coupled to an electrospray ionization source and an IR free electron laser. Quantum chemical calculations at the B3LYP and MP2 levels of theory using the cc-pVDZ basis set yield six low-energy isomers in the energy range up to 40 kJ/mol, all of which are protonated at the amino group. Protonation at the indole N atom or the hydroxyl group is substantially less favorable. The IRMPD spectrum is rich in structure and exhibits 22 distinguishable features in the spectral range investigated (530-1885 cm(-1)). The best agreement between the measured IRMPD spectrum and the calculated linear IR absorption spectra is observed for the conformer lowest in energy at both levels of theory, denoted g-1. In this structure, one of the three protons of the ammonium group points toward the indole subunit, thereby maximizing the intramolecular NH(+)-π interaction between the positive charge of the ammonium ion and the aromatic indole ring. This mainly electrostatic cation-π interaction is further stabilized by significant dispersion forces, as suggested by the substantial differences between the DFT and MP2 energies. The IRMPD bands are assigned to individual normal modes of the g-1 conformer, with frequency deviations of less than 29 cm(-1) (average <13 cm(-1)). The effects of protonation on the geometric and electronic structure are revealed by comparison with the corresponding structural, energetic, electronic, and spectroscopic properties of neutral serotonin.

Experimental IR multiphoton dissociation spectra of cationic niobium−acetonitrile complexes with the formal stoichiometry [Nb,nCH3CN]+, n = 4, 5 (the notation [Nb,nCH3CN]+ was chosen in order to emphasize the formal stoichiometry of a cluster without implying any structural information), as provided by the Free Electron Laser at CLIO, Orsay, France, are compared to ab initio IR absorption spectra throughout the spectral “fingerprinting” range 780−2500 cm-1. For n = 4 the vibrational spectrum in combination with the performed ab initio calculations provides strong evidence for a square-planar high-spin quintet [NbI(NCCH3)4]+ complex. For n = 5, additional vibrational bands between 800 and 1550 cm-1 are interpreted in terms of covalent C−C coupling in [Nb,5CH3CN]+. Screening on the basis of ab initio calculations leads to the assignment of the recorded spectrum to the metallacyclic species [NbIII(NCCH3)3(NC(CH3)C(CH3)N)]+ with an electronic triplet state. The deduced processes upon 4-fold and 5-fold coordination of NbI with CH3CN in the gas-phase are complexation only and reductive nitrile coupling, respectively. The minimum energy pathways of the reductive nitrile coupling reaction in [NbI(NCCH3) n ]+, n = 4, 5, investigated for singlet, triplet, and quintet states (S = 0, 1, 2) by density functional theory, account well for the observed (non)reactivity. In ground state (triplet, S = 1) [NbI(NCCH3)5]+ the reaction is found to be exothermic and the activation barrier amounts to approximately 49 kJ mol-1, whereas for ground state (quintet, S = 2) [NbI(NCCH3)4]+ the corresponding reaction is endothermic and would require an activation of more than 116 kJ mol-1.

Vibrational spectra of mixed silicon carbide clusters Si(m)C(n) with m + n = 6 in the gas phase are obtained by resonant infrared-vacuum-ultraviolet two-color ionization (IR-UV2CI for n ≤ 2) and density functional theory (DFT) calculations. Si(m)C(n) clusters are produced in a laser vaporization source, in which the silicon plasma reacts with methane. Subsequently, they are irradiated with tunable IR light from an IR free electron laser before they are ionized with UV photons from an F(2) laser. Resonant absorption of one or more IR photons leads to an enhanced ionization efficiency for Si(m)C(n) and provides the size-specific IR spectra. IR spectra measured for Si(6), Si(5)C, and Si(4)C(2) are assigned to their most stable isomers by comparison with calculated linear absorption spectra. The preferred Si(m)C(n) structures with m + n = 6 illustrate the systematic transition from chain-like geometries for bare C(6) to three-dimensional structures for bare Si(6). In contrast to bulk SiC, carbon atom segregation is observed already for the smallest n (n = 2).

A reaction of the bulky alkylcyclopentadienyliron(II) high-spin complex [Cp 000 Fe(μ-Br)] 2 (1a) (Cp 000 = C 5 H 2 (CMe 3 ) 3 -1,2,4) with phenylmagnesium bromide produced the deep blue dinuclear complex [{Cp 000 Fe} 2 (μ,η 5 :η 5 -H 5 C 6 dC 6 H 5 )] (2) with a bridging bis(cyclohexadienylidene) ligand. Its structural analysis shows a centrosymmetric dimer. Each tri(tert-butyl)cyclopentadienyliron fragment is η 5coordinated to a cyclohexadienylidene moiety in which one carbon atom is bent out of the plane by 0.39 A ˚, exhibiting a bond length of 1.370 A ˚to its symmetry equivalent. Electrospray ionization mass spectra (ESI-MS) from acetonitrile solution confirm nicely the elemental composition of 2 by way of their isotope patterns. Reaction of 1a or its tetraisopropylcyclopentadienyl analogue [ 4 CpFe(μ-Br)] 2 (1b) ( 4 CpdC 5 H(CHMe 2 ) 4 ) with 2,6-diisopropylphenylmagnesium bromide affords the extremely air-sensitive, paramagnetic σ-aryl complexes [Cp 000 Fe(C 6 H 3 i Pr 2 )] (3a) or [ 4 CpFe(C 6 H 3 i Pr 2 )] (3b), whose 4 Cp-Fe distance of 1.92 A ˚is typical for cyclopentadienyliron high-spin complexes. In reactions with copper(I) halides 3a is rearranged to a diamagnetic π complex and coordinated via the ipso carbon atom of the six-membered ring to copper(I) halide fragments to form heterodinuclear complexes [Cp 000 Fe(μ,η 5 :η 1 -C 6 H 3 i Pr 2 )CuCl] (4-Cl) and [Cp 000 Fe( μ,η 5 :η 1 -C 6 H 3 i Pr 2 )CuBr] (4-Br). ESI mass spectra of complexes 4 do not show the molecular cations, but fragmentation to cyclopentadienyliron arene cations and formation of the hexa(tert-butyl)ferrocenium cation on one hand and fusion of complex fragments to oligonuclear complexes with or without inclusion of oxygen or fragments of solvent molecules on the other hand. Three of these oligonuclear complexes formed under the conditions of the ESI-MS experiment, whose elemental composition could be derived from isotope patterns, have been interpreted as [Cp 000 Fe(μ,η 5 :η 1 -C 6 H 3 i Pr 2 )Cu(μ,η 1 :η 5 -OC 6 H 3 i Pr 2 )FeCp 000 ] þ and [{Cp 000 Fe(μ,η 5 :η 1 -C 6 H 3 i Pr 2 )Cu} 2 X] þ (X=Cl, Br). DFT calculations support the structural analysis of 2 and predict the structure of the dication 2 2þ . The crystal structures obtained by X-ray diffraction for 2 and 3b are reported.