Infrared Multiple Photon Dissociation Research Articles

Selective C[sbnd]H bond activation of ethane by free gold clusters

The activation and potential dissociation of ethane mediated by small cationic gold clusters Aux+ (x = 2–4) has been explored by infrared multiple photon dissociation (IR-MPD) spectroscopy and density functional theory (DFT) calculations. The calculations show that the interaction between the gold clusters and ethane is mainly governed by the mixing of the ethane CH3 bond-forming orbitals π−(CH3) with gold d-orbitals. While the CC single bond appears to be unaffected, this mixing leads to the selective activation of up to two ethane C–H bonds and a reduction of the activation barrier for C–H bond dissociation to up to 0.82 eV, making the reaction kinetically feasible at room temperature. In agreement with this, experimental IR-MPD spectra of the complexes Au2(C2H6)2+ and Au2(C2D6)2+ show strong indications for the co-existence of two isomeric structures, one representing the encounter complex and one where a single ethane C–H bond is dissociated resulting in a complex which contains an ethyl group and a bridge-bonded H atom along with a second, adsorbed C2H6 molecule. A similar C–H bond dissociation mechanism is theoretically predicted for the Au3(C2H6)y+ (y = 2,3) and Au4(C2H6)y+ (y = 2,3) complexes, albeit thermodynamically less favorable. IR-MPD spectra of Au3(C2H6)y+ (y = 2,3) and Au4(C2H6)y+ (y = 2,3) confirm the encounter product to be the dominant one, although the co-existence of isomers containing ethyl groups cannot be excluded. Various pathways for C–H bond activation are theoretically explored and the ethane activation mechanism is compared to the gold mediated activation of methane and ethylene.

Inverse Sandwich Cyclopentadienyl Complexes of Sodium in the Gas Phase.

Infrared multiple photon dissociation (IRMPD) experiments and electronic structure computations have been used to explore the structures and energetics of binding in gas phase sodium cyclopentadienyl complexes of the general form Na nCp n-1+. Computational work for the analogous anionic Na nCp n+1- complexes reveals nearly identical energetics for the loss of neutral NaCp units from both cationic and anionic species leading to the conclusion that the binding in the gas phase species is largely electrostatic, arising primarily from ion-ion and ion-dipole interactions. This is supported by an examination of the molecular orbitals of these species, which show no orbital overlap between the Na and Cp moieties. Observation of peaks in the IRMPD spectra, which do not correspond to any of the computed linear absorption frequencies, strongly suggest the contribution of overtone and combination bands.

Sequential activation of methane by Ir+: An IRMPD and theoretical investigation

The sequential activation of up to 4 CH4 molecules by Ir+ is investigated through a gas-phase infrared multiple photon dissociation (IRMPD) experiment and theoretical calculations. A molecular beam apparatus was used to generate Ir+ by laser ablation and expose it to controlled amounts of CH4. Product ions were irradiated with IR light from a free electron laser over the 500–1800 cm−1 spectral range and photodissociation was monitored using a time-of-flight mass spectrometer. Experimental spectra were obtained for five distinct species: [Ir,3C,8 H]+, [Ir,3C,10 H]+, [Ir,4C,10 H]+, [Ir,4C,12 H]+, and [Ir,O,3C,12 H]+. To identify these species, B3LYP/def2-TZVPPD geometry optimizations were performed on a variety of possible structures, with computed IR spectra compared to the experimental IRMPD spectra. This has led to the following assignments: [Ir,3C,8 H]+ = IrCH2(CH3)2+, [Ir,3C,10 H]+ = HIr(CH3)3+, [Ir,4C,10 H]+ = Ir(CH3)2(C2H4)+, [Ir,4C,12 H]+ = a mixture of HIr(CH3)(C2H4)+(CH4), HIr(CH3)2(C2H5)+, Ir(CH3)4+, and (H2)Ir(CH3)2(C2H4)+, and [Ir,O,3C,12 H]+ = (H2O)HIr(CH3)3+. Notably, evidence for CC coupling is observed upon reaction with a fourth methane. Mechanisms for the formation of the observed products were also explored computationally by examining the reaction coordinate pathways for the reactions of methane with HIrCH+, IrCH2+, Ir(CH3)2+, HIrCH2(CH3)+, HIr(CH3)3+, and IrCH2(CH3)2+.

Vibrational spectroscopy of deprotonated peptides containing an acidic side chain

The vibrational properties of four deprotonated dipeptides and three tripeptides containing an acidic residue, either Asp or Glu, have been studied using InfraRed Multiple Photon Dissociation (IRMPD) action spectroscopy in a mass spectrometer. It is found that these spectra fall into two broad categories, including clearly different spectra for sequence-reversed pairs of dipeptides. The observed bands were assigned using Density Functional Theory (DFT) calculations. Aside from the bands pertaining to the CO stretching modes which all lie in the expected 1550–1750 cm−1 range, photon absorption is strongly influenced by the hydrogen bonding patterns, which differ according to whether the acidic residue is located at the C-terminus or not. This leads to two distinct frequency ranges of absorption in between 1250 and 1550 cm−1. In addition, unexpectedly wide absorption around 1600 cm−1 may be attributed to non-classical proton sharing between the two carboxylates when Asp is at the C-terminus.

Structural Characterization of Molybdenum Oxide Nanoclusters Using Ion Mobility Spectrometry–Mass Spectrometry and Infrared Action Spectroscopy

Polyoxometalate clusters possess unique catalytic and electromagnetic properties. The structure and function of polyoxometalates is dictated by complex oligomerization processes, which in turn depend on the solution conditions. In this work, small gas-phase polyoxomolybdate nanoclusters (HMonO3n+11–, n = 1–8, and MonO3n+12–, n = 2–8) were investigated after nanoelectrospray of an acidified solution of ammonium heptamolybdate heptahydrate by ion mobility spectrometry–mass spectrometry (IMS–MS), infrared multiple photon dissociation (IRMPD) spectroscopy, and infrared action spectroscopy in helium nanodroplets. The spectra and collision cross sections obtained were matched to predictions from density-functional theory (DFT) to unravel the structural progression of nanoclusters with increasing size. For doubly charged clusters, transitions among chain (n = 2–3), ring (n = 4–5), and compact (n ≥ 6) structures are observed in IMS–MS and IR spectroscopy experiments, in agreement with low-energy structures from DFT calculations. For singly charged clusters, reduced Coulombic repulsion and hydrogen bonding interactions are found to strongly influence the most stable cluster structure. Notably, a noncovalent ring structure is observed for HMo3O101–, stabilized by a strong intramolecular hydrogen bond, and a compact structure is observed for HMo5O161–, in contrast to the ring structure favored for Mo5O162–.

Online Separation and Identification of Isomers Using Infrared Multiple Photon Dissociation Ion Spectroscopy Coupled to Liquid Chromatography: Application to the Analysis of Disaccharides Regio-Isomers and Monosaccharide Anomers.

The vast array of molecular isomerisms which form the complex molecular structure of carbohydrates is the foundation of their biological versatility but defies the analytical chemist. Hyphenations of mass spectrometry with orthogonal structural characterization, such as ion mobility or ion spectroscopy, have recently shown great promise for distinction between closely related molecular structures. Yet, the lack of analytical strategies for identification of isomers present in mixtures remains a major obstacle to routine carbohydrate sequencing. In this context, an ideal workflow for glycomics would combine isomer separation and individual characterization of the molecular structure with atomistic resolution. Here we report the implementation of such a multidimensional analytical strategy, which consists of the first online coupling of high-performance liquid chromatography (HPLC)-MS and infrared multiple photon dissociation (IRMPD) spectroscopy. The performance of this novel workflow is exemplified in the case of monosaccharides (anomers) and disaccharides (regioisomers) standards. We report that the LC-MS-IRMPD approach offers a robust advanced MS diagnostic of mixtures of isomers, including carbohydrate anomers, which is critical for carbohydrate sequencing. Our results also explain the bimodal character generally observed in LC chromatograms of carbohydrates. More generally, this multidimensional analytical strategy opens the gateway to rapid identification of molecular isoforms with potential application in the "omics" fields.

Modified Quadrupole Ion Trap Mass Spectrometer for Infrared Ion Spectroscopy: Application to Protonated Thiated Uridines.

Modifications to a Paul-type quadrupole ion trap mass spectrometer providing optical access to the trapped ion cloud as well as hardware and software for coupling to a table-top IR optical parametric oscillator laser (OPO) are detailed. Critical experimental parameters for infrared multiple photon dissociation (IRMPD) on this instrument are characterized. IRMPD action spectra, collected in the hydrogen-stretching region with this instrument, complemented by spectra in the IR fingerprint region acquired at the FELIX facility, are employed to characterize the structures of the protonated forms of 2-thiouridine, [s2Urd+H]+, and 4-thiouridine, [s4Urd+H]+. The measured spectra are compared with predicted linear IR spectra calculated at the B3LYP/6-311+G(d,p) level of theory to determine the conformers populated in the experiments. This comparison indicates that thiation at the 2- or 4-positions shifts the protonation preference between the 2,4-H tautomer and 4-protonation in opposite directions versus canonical uridine, which displays a roughly equal preference for the 2,4-H tautomer and O4 protonation. As found for canonical uridine, protonation leads to a mixture of conformers exhibiting C2'-endo and C3'-endo sugar puckering with an anti nucleobase orientation being populated for both 2- and 4-thiated uridine. Graphical Abstract ᅟ.

Infrared-Driven Charge-Transfer in Transition Metal-Containing B12X122– (X = H, F) Clusters

Density functional theory (DFT) calculations and infrared multiple photon dissociation (IRMPD) spectroscopy are employed to probe [TM·(B12H12)]- and [TM·(B12H12)2]2- clusters [TM = Ag(I), Cu(I), Co(II), Ni(II), Zn(II), Cd(II)]. A comparison is made between the charge-transfer properties of the clusters containing the hydrogenated dodecaborate dianions, B12H122-, and the fluorinated analogues, B12F122-, for clusters containing Cd(II), Co(II), Ni(II), and Zn(II). IRMPD of the [TM·(B12H12)]- and [TM·(B12H12)2]2- species yields B12H11- via hydride abstraction and B12H12- in all cases. To further explore the IR-induced charge-transfer properties of the B12X122- (X = H, F) cages, mixed-cage [TM(B12H12)(B12F12)]2- [TM = Co(II), Ni(II), Zn(II), Cd(II)] clusters were investigated. IRMPD of the mixed-cage species yielded appreciable amounts of B12F12- and B12H12- in most cases, indicating that charge-transfer to the central TM cation is a favorable process; formation of B12F12- is the dominant process for the Co(II) and Ni(II) mixed-cage complexes. In contrast, the Zn(II) and Cd(II) mixed-cage complexes preferentially produced fragments of the form B xH yF z-/2-, suggesting that H/F scrambling and/or fusion of the boron cages occurs along the IRMPD pathway.

Size Dependent H2 Adsorption on AlnRh+ (n = 1–12) Clusters

The interaction of hydrogen with singly rhodium doped aluminum clusters AlnRh+ (n = 1–12) is investigated experimentally by a combination of time-of-flight mass spectrometry and infrared multiple photon dissociation (IRMPD) spectroscopy. Density functional theory (DFT) is employed to optimize the geometric and electronic structures of bare and hydrogenated AlnRh+ clusters and the obtained infrared spectra of hydrogenated clusters are compared with the corresponding IRMPD spectra. The reactivity of the AlnRh+ clusters toward H2 is found to be strongly size-dependent, with n = 1–3, and 7 being the most reactive. Furthermore, it is favorable for H2 to adsorb molecularly on Al2Rh+ and Al3Rh+, while it prefers dissociative adsorption on other sizes. The initial molecular adsorption of H2 is identified as the determining step for hydrogen interaction with the AlnRh+ clusters, because the calculated molecular adsorption energies of H2 correlate well with the experimental abundances of the hydrogenated clusters. ...

Binding of Divalent Metal Ions with Deprotonated Peptides: Do Gas-Phase Anions Parallel the Condensed Phase?

Chelation complexes of the histidine-containing tripeptides HisAlaAla, AlaHisAla, and AlaAlaHis with Ni(II) and Cu(II) having a −1 net charge are characterized in the gas phase by infrared multiple-photon dissociation (IRMPD) spectroscopy and density functional theory calculations. We address the question of whether the gas-phase complexes carry over characteristics from the corresponding condensed-phase species. We focus particularly on three aspects of their structure: (i) square-planar chelation by the deprotonated amide nitrogens around the metal ion (low-spin for the Ni case), (ii) metal-ion coordination of the imidazole side chain nitrogen, and (iii) the exceptional preference for metal-ion chelation by peptides with His in the third position from the N-terminus, as in the amino terminal Cu and Ni (ATCUN) motif. We find that square-planar binding around the metal ion, involving bonds to both deprotonated backbone nitrogens, one of the carboxylate oxygens and the N-terminal nitrogen, is the dominant binding motif for all three isomers. In contrast to the condensed-phase behavior, the dominant mode of binding for all three isomers does not involve the imidazole side chain, which is instead placed outside the coordination zone. Only for the AlaAlaHis isomer, the imidazole-bound structure is also detected as a minority population, as identified from a distinctive short-wavelength IR absorption. The observation that this conformation exists only for AlaAlaHis correlates with condensed-phase behavior at neutral-to-basic pH, in the sense that the isomer with His in the third position is exceptionally disposed to metal ion chelation by four nitrogen atoms (4N) when compared with the other isomers. These results also emphasize the divergence between the conformational stabilities in the gas phase and in solution or crystalline environments: in the gas phase, direct metal binding of the imidazole is overall less favorable than the alternative of a remote imidazole that can act as an intramolecular H-bond donor enhancing the gas-phase stability.

Topology and Electronic Density Driven Generation of Alkali Cation Complexes.

The formation and characterization of K+ and Cs+ complexes originating from the cooperativity of three non-covalent interactions is explored. The tridimensional preorganization of the naphthothiophene platform displays a favorable well-defined bay region combining a π fragment and a carbonyl moiety flanking a central sulfur atom. A joint theoretical and experimental infrared multiple photon dissociation (IRMPD) study allowed deciphering the key contribution of the orthogonal phenyl fragment to the elaboration of alkali metal complexes. In combination with S and CO interactions, the π-cation interaction significantly enhances the binding energies of naphthothiophene derivatives.

Spectroscopic Identification of the Carbyne Hydride Structure of the Dehydrogenation Product of Methane Activation by Osmium Cations.

The present work explores the structures of species formed by dehydrogenation of methane (CH4) and perdeuterated methane (CD4) by the 5d transition metal cation osmium (Os+). Using infrared multiple photon dissociation (IRMPD) action spectroscopy and density functional theory (DFT), the structures of the [Os,C,2H]+ and [Os,C,2D]+ products are explored. This study complements previous work on the related species formed by dehydrogenation of methane by four other 5d transition metal cations (M+ = Ta+, W+, Ir+, and Pt+). Osmium cations are formed in a laser ablation source, react with methane pulsed into a reaction channel downstream, and the resulting products spectroscopically characterized through photofragmentation using the Free-Electron Laser for IntraCavity Experiments (FELICE) in the 300-1800cm-1 range. Photofragmentation was monitored by the loss of H2/D2. Comparison of the experimental spectra and DFT calculated spectra leads to identification of the ground state carbyne hydride, HOsCH+ (2A') as the species formed, as previously postulated theoretically. Further, a full description of the systematic spectroscopic shifts observed for deuterium labeling of these complexes, some of the smallest systems to be studied using IRMPD action spectroscopy, is achieved. A full rotational contour analysis explains the observed linewidths as well as the observation of doublet structures in several bands, consistent with previous observations for HIrCH+ (2A'). Graphical Abstract ᅟ.

Experimental and Theoretical Investigations of Infrared Multiple Photon Dissociation Spectra of Aspartic Acid Complexes with Zn2+ and Cd2.

Complexes of aspartic acid (Asp) cationized with Zn2+: Zn(Asp-H)+, Zn(Asp-H)+(ACN) where ACN = acetonitrile, and Zn(Asp-H)+(Asp); as well as with Cd2+, CdCl+(Asp), were examined by infrared multiple photon dissociation (IRMPD) action spectroscopy using light generated from a free electron laser. A series of low-energy conformers for each complex was found using quantum chemical calculations to identify the structures formed experimentally. The main binding motif observed for the heavy-metal complex, CdCl+(Asp)[N,CO,COs], is a charge-solvated, tridentate structure, where the metal center binds to the backbone amino group and carbonyl oxygens of the backbone and side-chain carboxylic acids. Likewise, the deprotonated Zn(Asp-H)+(ACN) and Zn(Asp-H)+(Asp) complexes show comparable [N,CO-,COs](ACN) and [N,CO-,COs][N,CO,COs] coordinations, respectively. Interestingly, there was only minor spectral evidence for the analogous Zn(Asp-H)+[N,CO-,COs] binding motif, even though this species is predicted to be the lowest-energy conformer. Instead, rearrangement and partial dissociation of the amino acid are observed, as spectral features most consistent with the experimental spectrum are exhibited by a four-coordinate Zn(Asp-NH4)+[CO2-,COs](NH3) complex. Analysis of the mechanistic pathway leading from the predicted lowest-energy conformer to the isobaric deaminated complex is explored theoretically. Further, comparison of the current work to that of Zn2+ and Cd2+ complexes of asparagine (Asn) allows additional conclusions regarding populated conformers and effects of carboxamide versus carboxylic acid binding to be drawn.

Resolution and Assignment of Differential Ion Mobility Spectra of Sarcosine and Isomers.

Due to their central role in biochemical processes, fast separation and identification of amino acids (AA) is of importance in many areas of the biomedical field including the diagnosis and monitoring of inborn errors of metabolism and biomarker discovery. Due to the large number of AA together with their isomers and isobars, common methods of AA analysis are tedious and time-consuming because they include a chromatographic separation step requiring pre- or post-column derivatization. Here, we propose a rapid method of separation and identification of sarcosine, a biomarker candidate of prostate cancer, from isomers using differential ion mobility spectrometry (DIMS) interfaced with a tandem mass spectrometer (MS/MS) instrument. Baseline separation of protonated sarcosine from α- and β-alanine isomers can be easily achieved. Identification of DIMS peak is performed using an isomer-specific activation mode where DIMS- and mass-selected ions are irradiated at selected wavenumbers allowing for the specific fragmentation via an infrared multiple photon dissociation (IRMPD) process. Two orthogonal methods to MS/MS are thus added, where the MS/MS(IRMPD) is nothing but an isomer-specific multiple reaction monitoring (MRM) method. The identification relies on the comparison of DIMS-MS/MS(IRMPD) chromatograms recorded at different wavenumbers. Based on the comparison of IR spectra of the three isomers, it is shown that specific depletion of the two protonated α- and β-alanine can be achieved, thus allowing for clear identification of the sarcosine peak. It is also demonstrated that DIMS-MS/MS(IRMPD) spectra in the carboxylic C=O stretching region allow for the resolution of overlapping DIMS peaks. Graphical Abstract ᅟ.

Cation-Size-Dependent Conformational Locking of Glutamic Acid by Alkali Ions: Infrared Photodissociation Spectroscopy of Cryogenic Ions.

Consolidated knowledge of conformation and stability of amino acids and their clusters is required to understand their biochemical recognition. Often, alkali ions interact with amino acids and proteins. Herein, infrared photodissociation (IRPD) spectra of cryogenic metalated glutamic acid ions (GluM+, M = Li-Cs) are systematically analyzed in the isomer-specific fingerprint and XH stretch ranges (1100-1900, 2600-3600 cm-1) to provide a direct measure for cation-size-dependent conformational locking. GluM+ ions are generated by electrospray ionization and cooled down to 15 K in a cryogenic quadrupole ion trap. The assignment of the IRPD spectra is supported by density functional theory calculations at the dispersion-corrected B3LYP-D3/aug-cc-pVTZ level. In the global minimum of GluM+, the flexibility of Glu is strongly reduced by the formation of rigid ionic CO···M+···OC metal bridges, corresponding to charge solvation. The M+ binding energy decreases monotonically with increasing cation size from D0 = 314 to 119 kJ/mol for Li-Cs. Whereas for Li and Na only the global minimum of GluM+ is observed, for K-Cs at least three isomers exist at cryogenic temperature. The IRPD spectra of cold GluM+ ions are compared to IR multiple-photon dissociation spectra measured at room temperature. Furthermore, we elucidate the differences of the impact of protonation and metalation on the structure and conformational locking of Glu.

Structural characterizations of protonated homodimers of amino acids: Revealed by infrared multiple photon dissociation (IRMPD) spectroscopy and theoretical calculations

Infrared multiple photon dissociation (IRMPD) spectroscopic and theoretical studies of protonated homodimers of amino acids generated by electrospray ionization in the gas phase have been reviewed.Results show that proton affinity (PA) may be applied as a probe to predict their structural type: salt-bridged or charge-solvated. Proline can be viewed as a reference. For an amino acid with a PA value higher than that of proline, the most stable conformation of its protonated homodimer tends to prefer salt-bridged conformation; otherwise, charge-solvated conformation is expected to be the most stable. However, side chain effects may cause the inaccuracy in structural determination due to the strong interactions with the charge, which makes the charge-solvated structure more stable even for species with high PA values. Temperature effect on distribution of different isomers is also very important. In lots of cases, the coexistence of multiple isomers is general, which makes the explanation of an overall IRMPD spectrum difficult. So a statistical view on the distribution of optimized isomers is very helpful.

Characterization of Trinuclear Oxo Bridged Cobalt Complexes in Isolation

Abstract This study elucidates molecular structures, fragmentation pathways and relative stabilities of isolated trinuclear oxo bridged cobalt complexes of the structural type [Co3O(OAc)6(Py)n]+ (OAc=acetate, Py=pyridine, n=0, 1, 2, 3). We present infrared multiple photon dissociation (IR-MPD) spectra in combination with quantum chemical calculations. They indicate that the coordination of axial pyridine ligands to the [Co3O(OAc)6]+ subunit disturbs the triangular geometry of the Co3O core. [Co3O(OAc)6]+ exhibits a nearly equilateral triangular Co3O core geometry. The coordination of one or two pyridine ligands disturbs this arrangement resulting in isosceles triangular Co3O core geometries (in the cases of n=1 and 2). Coordination of three pyridine ligands (n=3) results in an equilateral triangular Co3O core geometry as in the case of n=0. Collision induced dissociation (CID) studies reveal that the complexes undergo a consecutive elimination of pyridine and acetate ligands with increasing excitation energy. Relative stabilities of the complexes decrease with the number of coordinated pyridine ligands. The presented results help to gain a fundamental insight into the molecular structure of trinuclear oxo bridged cobalt complexes void of any external effects such as crystal packing or solvation.

Conformations of Protonated AlaDap and DapAla Characterized by IRMPD Spectroscopy and Molecular Modeling.

Oligopeptides containing 2,3-diaminopropionic acid (Dap) serve as a unique model to study conformational effects on the ionizability of a side-chain group. In this study, conformations of acetylated isomeric dipeptide ions containing alanine (Ala) and Dap, AlaDapH+ and DapAlaH+, are studied by infrared multiple photon dissociation (IRMPD) spectroscopy and computation. The IRMPD spectra are characterized in detail by comparing them with theoretical IR spectra of a set of low-energy conformations calculated at the ωB97X-D/6-311+G(d) level of theory. The averaged IR spectra according to the Boltzmann distribution of the set of conformations have a good match to the IRMPD spectra. The characteristic amide I band of AlaDapH+ appears to be downshifted compared to that of DapAlaH+. The relative positions of the amide band suggest a stronger hydrogen-bonding interaction between the charged side-chain amino group and the amide carbonyl groups in AlaDapH+ than in DapAlaH+. The stronger hydrogen bonding in the former is likely due to a better alignment of the N-H and O═C bonds, which enables an effective sequestering of the positive charge at the amino group. The effect results in a higher proton affinity of acetylated dipeptides with the Dap residue at the C-terminus.

Structures of the dehydrogenation products of methane activation by 5d transition metal cations revisited: Deuterium labeling and rotational contours.

A previous infrared multiple photon dissociation (IRMPD) action spectroscopy and density functional theory (DFT) study explored the structures of the [M,C,2H]+ products formed by dehydrogenation of methane by four, gas-phase 5d transition metal cations (M+ = Ta+, W+, Ir+, and Pt+). Complicating the analysis of these spectra for Ir and Pt was observation of an extra band in both spectra, not readily identified as a fundamental vibration. In an attempt to validate the assignment of these additional peaks, the present work examines the gas phase [M,C,2D]+ products of the same four metal ions formed by reaction with perdeuterated methane (CD4). As before, metal cations are formed in a laser ablation source and react with methane pulsed into a reaction channel downstream, and the resulting products are spectroscopically characterized through photofragmentation using the free-electron laser for intracavity experiments in the 350-1800 cm-1 range. Photofragmentation was monitored by the loss of D for [Ta,C,2D]+ and [W,C,2D]+ and of D2 in the case of [Pt,C,2D]+ and [Ir,C,2D]+. Comparison of the experimental spectra and DFT calculated spectra leads to structural assignments for all [M,C,2H/2D]+ systems that are consistent with previous identifications and allows a full description of the systematic spectroscopic shifts observed for deuterium labeling of these complexes, some of the smallest systems to be studied using IRMPD action spectroscopy. Further, full rotational contours are simulated for each vibrational band and explain several observations in the present spectra, such as doublet structures in several bands as well as the observed linewidths. The prominent extra bands in the [Pt,C,2D/2H]+ spectra appear to be most consistent with an overtone of the out-of-plane bending vibration of the metal carbene cation structure.

Gas phase vibrations of an anionic, hydrogen-bonded homodimer of a nucleobase analogue: Isocytosino-8-trifluoromethylquinolone

Synthesis and spectra of isocytosino-8-trifluoromethylquinolone (1), as well as the gas phase InfraRed Multiple Photon Dissociation (IRMPD) spectra in the fingerprint region of the corresponding deprotonated anion (3), its d3 analogue, the monodeprotonated homodimer (2), and its d7 analogue are reported here. The anions represent nucleobase analogues having the hydrogen bonding pattern ADAAD (where A stands for acceptor and D stands for donor), in which the site of negative charge is unambiguous (as opposed to guanine, which has more than one acidic nitrogen). The match between experimental vibrational spectra and calculation is good, except for the out-of-plane HNH bends of the undeuterated and deuterated monomer anions between 400 and 600 cm−1. The anionic homodimers form in a parallel orientation.