Infrared Photodissociation Research Articles

Microhydration Structures of Protonated Oxazole

The initial microhydration structures of the protonated pharmaceutical building block oxazole (Ox), H+Ox-Wn≤4, are determined by infrared photodissociation (IRPD) spectroscopy combined with quantum chemical dispersion-corrected density functional theory calculations (B3LYP-D3/aug-cc-pVTZ). Protonation of Ox, achieved by chemical ionization in a H2-containing plasma, occurs at the most basic N atom. The analysis of systematic shifts of the NH and OH stretch vibrations as a function of the cluster size provides a clear picture for the preferred cluster growth in H+Ox-Wn. For n = 1-3, the IRPD spectra are dominated by a single isomer, and microhydration of H+Ox with hydrophilic protic W ligands occurs by attachment of a hydrogen-bonded (H-bonded) Wn solvent cluster to the acidic NH group via an NH···O H-bond. Such H-bonded networks are stabilized by strong cooperativity effects. This is in contrast to previously studied hydrophobic ligands, which prefer interior ion solvation. The strength of the NH···O ionic H-bond increases with the degree of hydration because of the increasing proton affinity (PA) of the Wn cluster. At n = 4, proton-transferred structures of the type Ox-H+Wn become energetically competitive with H+Ox-Wn structures, because differences in solvation energies can compensate for the differences in the PAs, and barrierless proton transfer from H+Ox to the Wn solvent subcluster becomes feasible. Indeed, the IRPD spectrum of the n = 4 cluster is more complex suggesting the presence of more than one isomer, although it lacks unequivocal evidence for the predicted intracluster proton transfer.

Infrared photodissociation spectroscopic and theoretical study of [Co(CO2)n]+ clusters

The mass-selected infrared photodissociation (IRPD) spectroscopy was utilized to investigate the interactions of cationic cobalt with carbon dioxide molecules. Quantum chemical calculations were performed on the [Co(CO2)n]+ clusters to identify the structures of the low-lying isomers and to assign the observed spectral features. All the [Co(CO2)n]+ (n=2−6) clusters studied here show resonances near the CO2 asymmetric stretch of free CO2 molecule. Experimental and calculated results indicate that the CO2 molecules are weakly bound to the Co+ cations in an end-on configuration via a charge-quadrupole electrostatic interaction. The present IRPD spectra of [Co(CO2)n]+ clusters have been compared to those of Ar-tagged species ([Co(CO2)n]+-Ar), which would provide insights into the tagging effect of rare gas on the weakly-bounded clusters.

Adsorption Forms of Water Molecules on Gas-Phase Platinum Clusters Pt3 + Studied by Vibrational Photodissociation Spectroscopy

Abstract The vibrational spectra of Pt3(H2O) m + (m = 1–4) cluster were measured in the 3000–3800 cm−1 range via infrared photodissociation (IRPD) spectroscopy. The IRPD spectra were recorded through the photodissociation of Pt3(H2O) m +-Ar (m = 1–3) complexes and Pt3(H2O)4 + cations upon vibrational excitation. The spectra were compared to the vibrational spectra of several stable isomers obtained by density functional theory (DFT) calculations and the adsorption forms of the water molecules were subsequently discussed. The IRPD spectra of all the studied Pt3(H2O) m + cations exhibited intense peaks at ∼3600 and 3700 cm−1. This suggested that the water molecules mainly adsorb onto the Pt clusters in molecular form and that each molecule binds directly to a Pt atom via its O atom side. For the water-rich Pt3(H2O)4 + cations, all four water molecules were directly bound to the Pt atoms; however, according to the DFT calculations, the fourth H2O molecule could bind to a first-layer water molecule through hydrogen bonding.

Spectroscopic and Computational Evidence of Intramolecular AuI ⋅⋅⋅H+ -N Hydrogen Bonding.

Despite substantial evidence of short Au⋅⋅⋅H−X contacts derived from a number of X‐ray structures of AuI compounds, the nature of AuI⋅⋅⋅H bonding in these systems has not been clearly understood. Herein, we present the first spectroscopic evidence for an intramolecular AuI⋅⋅⋅H+−N hydrogen bond in a [Cl−Au−L]+ complex, where L is a protonated N‐heterocyclic carbene. The complex was isolated in the gas phase and characterized with helium‐tagging infrared photodissociation (IRPD) spectra, in which H+−N‐mode‐derived bands evidence the intramolecular AuI⋅⋅⋅H+−N bond. Quantum chemical calculations reproduce the experimental IRPD spectra and allow to characterize the intramolecular Au⋅⋅⋅H+−N bonding with a short r Au⋅⋅⋅H distance of 2.17 Å and an interaction energy of approximately −10 kcal mol−1. Various theoretical descriptors of chemical bonding calculated for the Au⋅⋅⋅H+−N interaction provide strong evidence for a hydrogen bond of moderate strength.

Spectroscopic and Computational Evidence of Intramolecular AuI⋅⋅⋅H+−N Hydrogen Bonding

AbstractDespite substantial evidence of short Au⋅⋅⋅H−X contacts derived from a number of X‐ray structures of AuI compounds, the nature of AuI⋅⋅⋅H bonding in these systems has not been clearly understood. Herein, we present the first spectroscopic evidence for an intramolecular AuI⋅⋅⋅H+−N hydrogen bond in a [Cl−Au−L]+ complex, where L is a protonated N‐heterocyclic carbene. The complex was isolated in the gas phase and characterized with helium‐tagging infrared photodissociation (IRPD) spectra, in which H+−N‐mode‐derived bands evidence the intramolecular AuI⋅⋅⋅H+−N bond. Quantum chemical calculations reproduce the experimental IRPD spectra and allow to characterize the intramolecular Au⋅⋅⋅H+−N bonding with a short rAu⋅⋅⋅H distance of 2.17 Å and an interaction energy of approximately −10 kcal mol−1. Various theoretical descriptors of chemical bonding calculated for the Au⋅⋅⋅H+−N interaction provide strong evidence for a hydrogen bond of moderate strength.

Joining Microfluidics with Infrared Photodissociation: Online Monitoring of Isomeric Flow-Reaction Intermediates.

A cryogenic ion trap vibrational spectrometer is combined with a microfluidic chip reactor in a proof-of-principle experiment on the Hantzsch cyclization reaction forming 2-amino-4-phenyl thiazole from phenacyl bromide and thiourea. First, the composition of the reaction solution is characterized using electrospray-ionization mass spectrometry combined with two-color infrared photodissociation (IRPD) spectroscopy. The latter yields isomer-specific vibrational spectra of the reaction intermediates and products. A comparison to results from electronic structure calculations then allows for an unambiguous structural assignment and molecular-level insights into the reaction mechanism. Subsequently, we demonstrate that isomeric and isobaric ions can be selectively monitored online with low process time, i.e., using a single IRPD wavelength per isomer, as the chip reaction parameters are varied.

Vibrational spectroscopy of the hexahydrated sulfate dianion revisited: role of isomers and anharmonicities.

We report on the gas phase vibrational spectroscopy of the hexahydrated sulfate dianion, SO42-(H2O)6, and its fully deuterated isotopologue, SO42-(D2O)6, using infrared photodissociation (IRPD) spectroscopy of the D2-tagged dianions in combination with density-functional-theory calculations on minimum-energy structures as well as finite temperature ab initio molecular dynamics (AIMD) simulations. The IRPD spectra were recorded at an ion trap temperature of 12 K and in the spectral range from 650 to 3800 cm-1, covering the intramolecular modes of the solvent (OH/OD stretches and H2O/D2O bends) at higher energies, those of the solute (sulfate stretches) at intermediate energies and the intermolecular solute librational modes at the lowest energies. Isomer-specific double resonance in combination with messenger-tag dependent IRPD spectra show that only a single isomer is contributing significantly and that this isomer is not the highly symmetric Td but rather the lower symmetry C3 isomer. Temperature-dependent IR multiple photon dissociation spectra of bare SO42-(H2O)6 suggest that the C3 isomer remains the most stable one up to 200 K. The AIMD simulations reveal that the IRPD spectra can only be fully understood when anharmonic effects as well as entropy-driven hydrogen bond network fluctuations are considered.

Triple bonds between iron and heavier group-14 elements in the AFe(CO)3- complexes (A = Ge, Sn, and Pb).

Heteronuclear transition-metal-main-group element carbonyl anion complexes of AFe(CO)3- (A = Ge, Sn, and Pb) are prepared using a laser vaporization supersonic ion source in the gas phase, which were studied by mass-selected infrared (IR) photodissociation spectroscopy. The geometric and electronic structures of the experimentally observed species are identified by a comparison of the measured and calculated IR spectra. These anion complexes have a 2A1 doublet electronic ground state and feature an A[triple bond, length as m-dash]Fe triply bonded C3v structure with all of the carbonyl ligands bonded at the iron center. Bonding analyses of AFe(CO)3- (A = C, Si, Ge, Sn, Pb, and Fl) indicate that the complexes are triply bonded between the valence np atomic orbitals of bare group-14 atoms and the hybridized 3d and 4p atomic orbitals of iron.

CO2 Adsorption on Ti3O6–: A Novel Carbonate Binding Motif

CO2 adsorption on Ti3O6–, which serves as a model for an oxygen vacancy on a titania surface, is studied using infrared photodissociation (IRPD) spectroscopy in combination with density functional ...

Communication: Gas phase vibrational spectroscopy of the azide-water complex.

The vibrational spectra of the azide-water complex, N3 -(H2O), and its fully deuterated isotopologue are studied using infrared photodissociation (IRPD) spectroscopy (800-3800 cm-1) and high-level ab initio computations. The IRPD spectrum of the H2-tagged complex exhibits four fundamental transitions at 3705, 3084, 2003, and 1660 cm-1, which are assigned to the free OH stretching, the hydrogen-bonded O-H stretching, the antisymmetric N3 stretching, and the water bending mode, respectively. The IRPD spectrum is consistent with a planar, singly hydrogen-bonded structure according to an MP2 and CCSD(T) anharmonic analysis via generalized second-order vibrational perturbation theory. The red-shift of the hydrogen-bonded OH stretching fundamental of 623 cm-1 associated with this structure is computed within 6 cm-1 (or 1%) and is used to estimate the proton affinity of azide (1410 kJ mol-1). Born-Oppenheimer molecular dynamics simulations show that large amplitude motions are responsible for the observed band broadening at cryogenic temperature. Temperature-dependent (6-300 K) IR multiphoton dissociation spectra of the untagged complex are also presented and discussed in the context of spectral diffusion observed in the condensed phase.

Protonation and Sequential Microsolvation of 5-Hydroxyindole: Infrared Photodissociation Spectra of 5HIH+–Ln with L = Ar and N2 (n ≤ 3)

Protonation and solvation of functionalized heterocyclic aromatic molecules, which often occur as biomolecular building blocks, are important processes in (bio-)organic biochemistry. Herein, we study the protonation and microsolvation mechanisms of 5-hydroxyindole (5HI, 1 H-indol-5-ol, C8H7NO), the chromophore of serotonin, produced by electron and and chemical ionization using infrared photodissociation (IRPD) spectroscopy of mass-selected cold 5HIH+-L n clusters (L = Ar/N2, n ≤ 3) and dispersion-corrected density functional theory calculations (B3LYP-D3/aug-cc-pVTZ). Isomer-selective OH and NH stretch frequencies in the spectral range 3000-3800 cm-1 reveal the coexistence of at least four protonated species: the most stable syn (cis) isomer protonated at the C3 position of indole, both syn- and anti-rotamers protonated at C4 of the phenol ring, and the drastically less stable O-protonated isomer (Δ E0 = 117.1 kJ/mol) stabilized by kinetic trapping. Manipulation of the IRPD conditions (fragmentation channels) facilitates the spectroscopic isolation of O-protonated species. Upon protonation, the acidity of the OH group increases in the order 5HIH+(C3), 5HIH+(C4), and 5HIH+(O), while the acidity of the NH group decreases along this series, strongly affecting the microsolvation motifs of the individual isomers. Comparison of 5HIH+-L to the corresponding neutral and radical cation clusters reveals the impact of both protonation and ionization on the interaction with nonpolar ligands. Furthermore, our results are compared to protonated phenol, for which similar gas-phase protonation mechanisms have been found. Comparison of 5HIH+-L with the corresponding clusters of protonated phenol illustrates the effects of functional substitution and addition of aromatic rings on intermolecular potential.

Unusually sharp FTIR ν(OH) bands and very weak O[sbnd]H⋯F hydrogen bonds in M2(H2O)1,2B12F12 hydrates (M[dbnd]Na[sbnd]Cs)

We report attenuated total reflection FTIR spectra of microcrystalline samples of M2(H2O)n(B12F12) (MNa, K, Rb, Cs) and Na2(H2O)6(B12Cl12). The ν(OH) bands are unusually narrow compared to the spectra of other alkali metal salt hydrates, with most full widths at half-max less than 20 cm−1 and some less than 10 cm−1. In addition, these bands are not as redshifted compared to the spectra of other alkali metal salt hydrates. These unusual effects are attributed to extremely weak OH⋯F and OH⋯Cl hydrogen bonding between the coordinated H2O molecules and the B12F122− and B12Cl122− superweak anions. The ν(OH) bands in these compounds are as narrow as the bands in the FTIR spectrum of monomeric H2O absorbed in poly(vinylidene fluoride), in which the only hydrogen bonding is OH⋯FC, or as the bands in the IR photodissociation spectrum of the gas-phase species [(Ar)Na(H2O)2]+. These unusual spectra are compared to the literature spectrum of [NEt4]4[Hg(CB11F11)2]2∙H2O, which also exhibits sharp ν(OH) bands with relatively small redshifts from the ν(OH) values for H2O(g).

IR photodissociation spectra of SixH4x-4+ (x = 4–8): Evidence for Si[sbnd]H[sbnd]Si proton bridges

Although silane-type cations of the form SixHy+ are thought to play a significant role in plasma chemistry and astrochemistry, their structural and energetic properties are largely unexplored in the size range x ≥ 3. Herein, infrared photodissociation (IRPD) spectra of mass-selected SixHy+ ions (specifically SixH4x-4+ with x = 4–8, Si4H12+-Si8H28+) are recorded in the SiH stretch range and analyzed by dispersion-corrected density functional calculations at the B3LYP-D3/aug-cc-pVDZ level. The SixH4x-4+ ions are produced in a SiH4/H2/He plasma molecular beam expansion. The IRPD process leads to the loss of SiH4 ligands, which corresponds to the lowest-energy fragment channel. Spectral analysis of the IRPD spectra reveals that all SixH4x-4+ ions have at least one SiHSi bridge. The characteristic fingerprint of these three-center two-electron (3c-2e) bonds is the highly IR active antisymmetric stretch fundamental of the SiHSi bridge (σ SiHSi) occurring in the 1600–2100 cm−1 range, whose frequency strongly depends on the structural and energetic details of the SiHSi bridge. Although the investigated SixH4x-4+ ions can formally be described by the formula Si2H4+(SiH4)x-2, the cluster growth is more complex. The appearance of the σSiHSi bands confirms that all considered SixH4x-4+ ions with x ≤ 5 are formed by polymerization reactions. Larger clusters (x ≥ 6) show evidence for the additional presence of weakly-bonded SiH4 ligands attached to smaller chemically-bonded core ions. Correlations of the properties of the SiHSi bridges (bond distances, bond angles, binding energies, SiH stretch frequencies), which vary between strong symmetric 3c-2e chemical bonds and weak hydrogen bonds, are discussed.

Infrared Signatures of Protonated Benzonitrile

Abstract Aromatic hydrocarbons and their protonated ions are important constituents of the interstellar medium (ISM). The recent discovery of benzonitrile (BN; cyanobenzene, C6H5CN) in the ISM suggests that its protonated ion (H+BN) is also present. Herein, we present vibrational signatures of H+BN obtained via infrared photodissociation (IRPD) spectra of its clusters with up to four nonpolar ligands (L = Ar/N2) recorded in the NH (ν NH) and CH (ν CH) stretch range. Protonation of BN occurs at the N atom of the nitrile group. Systematic complexation shifts (Δν NH) observed in the IRPD spectra of H+BN-L n are assigned to cluster structures by comparison to quantum chemical calculations. In the most stable H+BN-L n structures, the first ligand (n = 1) forms a NH+… L ionic hydrogen bond (H-bond), while additional ligands (n = 2–4) are attached to the aromatic ring via π stacking. For L = Ar, a less stable π-bonded H+BN-Ar isomer is also detected, and its IR spectrum provides an accurate experimental estimate of ν NH = 3555 ± 3 cm−1 for bare H+BN, an intense characteristic fingerprint of this ion in the 3 μm range. Comparison of C6H5CNH+ with HCNH+ and CH3CNH+ reveals that the acidity of the NH proton in RCNH+ ions increases in the order R = C6H5 < CH3 < H.

Direct IR Absorption Spectra of Propargyl Cation Isolated in Solid Argon

The direct infrared (IR) absorption spectra of propargyl cations were recorded. These cations were generated via the electron bombardment of a propyne/Ar matrix sample during matrix deposition. Secondary photolysis with selected ultraviolet (UV) light was used for grouping the observed bands of various products. The band assignment of the propargyl cation in solid Ar was performed according by referring to the previous infrared photodissociation (IRPD) and velocity-map imaging photoelectron (VMI-PE) data, and via theoretical predictions of the anharmonic vibrational wavenumbers, band intensities, and deuterium-substituted isotopic ratios. Almost all the IR active bands with an observable intensity were recorded and the ν11 mode was reported for the first time.

Probing the Structures of Solvent-Complexed Ions Formed in Electrospray Ionization Using Cryogenic Infrared Photodissociation Spectroscopy.

The gas-phase infrared photodissociation (IRPD) spectra of solvent-tagged small biomolecules are studied in a cryogenic ion trap at 17 K. In this study para-aminobenzoic acid (PABA) and tyramine molecules are noncovalently tagged with water or acetonitrile in the electrospray ionization (ESI) source. The complexes are then cooled in the cryogenic trap prior to spectroscopic measurements. These molecules provide two putative sites for solvent attachment: the protonated amine (NH3+) and the OH groups. Comparisons of the experimental IR spectra to theoretical spectra obtained with density functional theory show that the NH3+ site is mainly favored. Evidence for the formation of both NH3-bound and OH-bound conformers is found only in tyramine, despite having similar solution- and gas-phase energetics to that of PABA. Since the structures cannot interconvert in the gas phase, this suggests an isomerization during the electrospray process.

Probing the propensity of perchlorate anions for surface solvation by infrared photodissociation spectroscopy

We investigate the perchlorate anion, ClO4-, microhydrated with up to eight water molecules using infrared photodissociation (IRPD) spectroscopy aided by density functional theory computations. The features observed in the IRPD spectra of D2-tagged ClO4-(H2O)n with n = 0-8 in the O-H stretching (2800-3800 cm-1) and fingerprint (800-1800 cm-1) spectral ranges are assigned to vibrational modes of the solvent, i.e., free and hydrogen-bonded O-H stretching and H2O bending modes, and of the solute, i.e., perchlorate stretching modes. The splitting of the triply degenerate antisymmetric stretching mode in the bare ClO4- (1102 cm-1) upon microhydration directly reports on the symmetry of the local solvation network, while the red-shift of the hydrogen-bonded O-H stretching bands in comparison to those of free O-H oscillators (>3700 cm-1) reflects the strength of the anion-water (3550-3650 cm-1) and water-water (3240-3540 cm-1) hydrogen bonds. The stronger water-water interaction leads to an early onset of water-water hydrogen bond formation (n = 2) and eventually asymmetric solvation of ClO4-, evidenced by the population of the second and third hydration shells at n = 5 and n = 8, respectively, before the first hydration shell is completed. The present study provides a bottom-up view of the initial stages of hydrogen-bond network formation around a high surface propensity anion, and these insights are discussed in the context of recent results obtained for the water-vapor interface of perchlorate solutions.

Octacarbonyl Anion Complexes of Group Three Transition Metals [TM(CO)8 ]- (TM=Sc, Y, La) and the 18-Electron Rule.

We report the gas-phase synthesis of stable 20-electron carbonyl anion complexes of group 3 transition metals, TM(CO)8- (TM=Sc, Y, La), which are studied by mass-selected infrared (IR) photodissociation spectroscopy. The experimentally observed species, which are the first octacarbonyl anionic complexes of a TM, are identified by comparison of the measured and calculated IR spectra. Quantum chemical calculations show that the molecules have a cubic (Oh ) equilibrium geometry and a singlet (1 A1g ) electronic ground state. The 20-electron systems TM(CO)8- are energetically stable toward loss of one CO ligand, yielding the 18-electron complexes TM(CO)7- in the 1 A1 electronic ground state; these exhibit a capped octahedral structure with C3v symmetry. Analysis of the electronic structure of TM(CO)8- reveals that there is one occupied valence molecular orbital with a2u symmetry, which is formed only by ligand orbitals without a contribution from the metal atomic orbitals. The adducts of TM(CO)8- fulfill the 18-electron rule when only those valence electrons that occupy metal-ligand bonding orbitals are considered.

Photodissociation Spectroscopy of Cold Protonated Synephrine: Surprising Differences between IR-UV Hole-Burning and IR Photodissociation Spectroscopy of the O-H and N-H Modes.

We report the UV and IR photofragmentation spectroscopies of protonated synephrine in a cryogenically cooled Paul trap. Single (UV or IR) and double (UV-UV and IR-UV) resonance spectroscopies have been performed and compared to quantum chemistry calculations, allowing the assignment of the lowest-energy conformer with two rotamers depending on the orientation of the phenol hydroxyl (OH) group. The IR-UV hole burning spectrum exhibits the four expected vibrational modes in the 3 μm region, i.e., the phenol OH, Cβ-OH, and two NH2+ stretches. The striking difference is that, among these modes, only the free phenol OH mode is active through IRPD. The protonated amino group acts as a proton donor in the internal hydrogen bond and displays large frequency shifts upon isomerization expected during the multiphoton absorption process, leading to the so-called IRMPD transparency. More interestingly, while the Cβ-OH is a proton acceptor group with moderate frequency shift for the different conformations, this mode is still inactive through IRPD.

Vibrational Characterization of Radical Ion Adducts between Imidazole and CO2.

We address the molecular level origins of the dramatic difference in the catalytic mechanisms of CO2 activation by the seemingly similar molecules pyridine (Py) and imidazole (Im). This is accomplished by comparing the fundamental interactions of CO2 radical anions with Py and Im in the isolated, gas phase PyCO2- and ImCO2- complexes. These species are prepared by condensation of the neutral compounds onto a (CO2) n- cluster ion beam by entrainment in a supersonic jet ion source. The structures of the anionic complexes are determined by theoretical analysis of their vibrational spectra, obtained by IR photodissociation of weakly bound CO2 molecules in a photofragmentation mass spectrometer. Although the radical PyCO2- system adopts a carbamate-like configuration corresponding to formation of an N-C covalent bond, the ImCO2- species is revealed to be best described as an ion-molecule complex in which an oxygen atom in the CO2- radical anion is H-bonded to the NH group. Species that feature a covalent N-C interaction in ImCO2- are calculated to be locally stable structures, but are much higher in energy than the largely electrostatically bound ion-molecule complex. These results support the suggestion from solution phase electrochemical studies (Bocarsly et al. ACS Catal. 2012, 2, 1684-1692) that the N atoms are not directly involved in the catalytic activation of CO2 by Im.