Infrared Laser Spectroscopy of Mass-Selected Carbocations

IR Spectrum and Structure of Protonated Monosilanol: Dative Bonding between Water and the Silylium Ion

We report the spectroscopic characterization of protonated monosilanol (SiH₃ OH₂⁺ ) isolated in the gas phase, thus providing the first experimental determination of the structure and bonding of a member of the elusive silanol family. The SiH₃ OH₂⁺ ion is generated in a silane/water plasma expansion, and its structure is derived from the IR photodissociation (IRPD) spectrum of its Ar cluster measured in a tandem mass spectrometer. The chemical bonding in SiH₃ OH₂⁺ is analyzed by density functional theory (DFT) calculations, providing detailed insight into the nature of the dative H₃ Si⁺ -OH₂ bond. Comparison with protonated methanol illustrates the differences in bonding between carbon and silicon, which are mainly related to their different electronegativity and the different energy of the vacant valence p_z orbital of SiH₃⁺ and CH₃⁺ .

Infrared Multiphoton Dissociation Spectroscopy with Free-Electron Lasers: On the Road from Small Molecules to Biomolecules

Infrared multiphoton dissociation (IRMPD) spectroscopy is commonly used to determine the structure of isolated, mass-selected ions in the gas phase. This method has been widely used since it became available at free-electron laser (FEL) user facilities. Thus, in this Minireview, we examine the use of IRMPD/FEL spectroscopy for investigating ions derived from small molecules, metal complexes, organometallic compounds and biorelevant ions. Furthermore, we outline new applications of IRMPD spectroscopy to study biomolecules.

Spectroscopy of prospective interstellar ions and radicals isolated in para-hydrogen matrices

The p-H₂ matrix-isolation technique coupled with photolysis in situ or electron bombardment produces protonated or hydrogenated species important in astrochemistry.

O2-·[Polar VOC] Complexes: H-Bonding versus Charge-Dipole Interactions, and the Noninnocence of Formaldehyde

Anion photoelectron imaging was used to measure the photodetachment spectra of molecular complexes formed between O₂^- and a range of atmospherically relevant polar molecules, including species with a carbonyl group (acetone, formaldehyde) and alcohols (ethanol, propenol, butenol). Experimental spectra are analyzed using a combination of Franck-Condon simulations and electronic structure calculations. Strong charge-dipole interactions and H-bonding stabilize the complex anions relative to the neutrals, resulting in a ca. 1 eV increase in electron binding energy relative to bare O₂^-, an effect more pronounced in complexes with H-bonding. In addition, broken degeneracy of the O₂-local π_g orbitals in the complexes results in the stabilization of the low-lying excited O₂ (a ¹Δ_g)·[polar VOC] state relative to the ground O₂ (X ³Σ_g^-)·[polar VOC] state when compared to bare O₂. The spectra of the O₂^-·[polar VOC] complexes exhibit less pronounced laser photoelectron angular distribution (PADs). The spectrum of O₂^-·formaldehyde is unique in terms of both spectral profile and PAD. On the basis of these experimental results in addition to computational results, the complex anion cannot be described as a distinct O₂^- anion partnered with an innocent solvent molecule; the molecules are more strongly coupled through charge delocalization. Overall, the results underscore how the symmetry of the O₂ π_g orbitals is broken by different polar partners, which may have implications for atmospheric photochemistry and models of solar radiation absorption that include collision-induced absorption.

Infrared spectroscopic and theoretical study of the HC2n+1O+ (n = 2-5) cations

The carbon chain cations, HC_2n+1O⁺ (n = 2-5), are produced via pulsed laser vaporization of a graphite target in supersonic expansions containing carbon monoxide and hydrogen. The infrared spectra are measured via mass-selected infrared photodissociation spectroscopy of the CO "tagged" [HC_2n+1O·CO]⁺ cation complexes in the 1600-3500 cm^-1 region. The geometries and electronic ground states of these cation complexes are determined by their infrared spectra compared to the predications of theoretical calculations. All of the HC_2n+1O⁺ (n = 2-5) core cations are characterized to be linear carbon chain derivatives terminated by hydrogen and oxygen, which have the closed-shell singlet ground states with polyyne-like carbon chain structures.

Stabilization of carbocations CH3+, C2H5+, i-C3H7+, tert-Bu+, and cyclo-pentyl+ in solid phases: experimental data versus calculations

Comparison of experimental infrared (IR) spectra of the simplest carbocations (with the weakest carborane counterions in terms of basicity, CHB₁₁Hal₁₁^-, Hal = F, Cl) with their calculated IR spectra revealed that they are completely inconsistent, as previously reported for the t-Bu⁺ cation [Stoyanov E. S., et al. J. Phys. Chem. A, 2015, 119, 8619]. This means that the generally accepted explanation of hyperconjugative stabilization of the carbocations should be revised. According to the theory, one CH bond (denoted as ) from each CH₃/CH₂ group transfers its σ-electron density to the empty 2p_z orbital of the sp² C atom, whereas the σ-electron density on the other CH bonds of the CH₃/CH₂ group slightly increases. From experimental IR spectra it follows that donation of the σ-electrons from the bond to the 2p_z C-orbital is accompanied by equal withdrawal of the electron density from other CH bonds, that is, the electrons are supplied from each CH bond of the CH₃/CH₂ group. As a result, all CH stretches of the group are red shifted, and IR spectra show typical CH₃/CH₂ group vibrations. Experimental findings provided another clue to the electron distribution in the hydrocarbon cations and showed that the standard computational techniques do not allow researchers to explain a number of recently established features of the molecular state of hydrocarbon cations.

Proton relays in anomalous carbocations dictate spectroscopy, stability, and mechanisms: case studies on C2H5+ and C3H3+

We present a detailed exposition of “Grothuss-like” proton shuttles in C₂H₅⁺ and C₃H₃⁺ that result in anomalous structural and spectral behavior.

Cisplatin Primary Complex with l-Histidine Target Revealed by IR Multiple Photon Dissociation (IRMPD) Spectroscopy

The primary complex obtained from cisplatin and l-histidine in water has been detected and isolated by electrospray ionization. The so-obtained cis-[PtCl(NH₃ )₂ (histidine)]⁺ complex has been characterized in detail by high-resolution mass spectrometry (MS), tandem MS, IR multiple photon dissociation (IRMPD) spectroscopy, and by quantum chemical calculations. The structural features revealed by IRMPD spectroscopy indicate that platinum binds to the imidazole group, which presents tautomeric forms. Thus, depending on the position of the amino acid pendant on the imidazole ring, isomeric complexes are formed that are remarkably different with respect to the ease with which they undergo fragmentation when activated either by energetic collisions or by multiple IR photon absorption. It is shown here how IRMPD kinetics can allow their relative proportions to be estimated.

Amino Acid Oxidation: A Combined Study of Cysteine Oxo Forms by IRMPD Spectroscopy and Simulations

The redox activity of cysteine sulfur allows numerous post-translational protein modifications involved in the oxidative regulation of metabolism, in metal binding, and in signal transduction. A combined approach based on infrared multiple photon dissociation spectroscopy at the Centre Laser Infrarouge d'Orsay (CLIO) free electron laser facility, calculations of IR frequencies, and finite temperature ab initio molecular dynamics simulations has been employed to characterize the gas-phase structures of deprotonated cysteine sulfenic, sulfinic, and sulfonic acids, [cysSO_x ]^- (x=1, 2, 3, representing the number of S-bound oxygen atoms), which are key intermediates in the redox-switching chemistry of proteins. The ions show different structural motifs owing to preferential binding of the proton to either the carboxylate or sulfur-containing group. Due to the decreasing basicity of the sulfenic, sulfinic, and sulfonic terminals, the proton bound to SO^- in [cysSO]^- migrates to the carboxylate in [cysSO₃ ]^- , whereas it turns out to be shared in [cysSO₂ ]^- . Evidence is gathered that a mixture of close-lying low-energy conformers is sampled for each cysteine oxo form in a Paul ion trap at room temperature.

Photoelectron Imaging Spectra of O2-·VOC and O4-·VOC Complexes

The anion photoelectron imaging spectra of O₂^-·VOC and O₄^-·VOC (VOC = hexane, isoprene, benzene, and benzene-d₆) complexes measured using 3.49 eV photon energy, along with the results of ab initio and density functional theory results are reported and analyzed. Photodetachment of these anionic complexes accesses neutrals that model collision complexes, offering a probe of the effects of symmetry-breaking collision events on the electronic structure of normally transparent neutral molecules. The energies of O₂^-·VOC spectral features compared to the bare O₂^- indicate that photodetachment of the anion accesses a modestly repulsive region of the O₂-VOC potential energy surface, with subtle VOC dependence on the relative energies of the O₂ (X ³Σ_g^-)·VOC ground state and O₂ (a ¹Δ_g)·VOC excited state. In contrast, a significantly higher intensity of the transition to the O₂ (a ¹Δ_g)·VOC excited state relative to the O₂ (X ³Σ_g^-)·VOC ground state is observed for VOC = benzene, with a less pronounced effect observed for VOC = isoprene. Similar spectral effects are observed in the O₄^-·benzene and O₄^-·isoprene PE spectra. Several explanations are considered, with involvement of a temporary anion state emerging as the most plausible.

Photochemistry and infrared spectrum of single-bridged diborane(5) anion isolated in solid argon

Three-center two-electron bonds are important for understanding electron-deficient molecules. To examine such a molecule, we produced a diborane(5) anion with a single-bridged structure upon electron bombardment during matrix deposition of Ar containing a small proportion of diborane(6). The diborane(5) anion was destroyed upon photolysis at 180, 220, 385, and 450 nm, but not at 532 nm. Moreover, the possible formation of neutral diborane(5) was observed upon photolysis at 385 and 450 nm, whereas neutral diborane(3) was observed upon photolysis at 180 and 220 nm. The observed line wavenumbers, relative intensities, and isotopic ratios of the diborane(5) anion agreed satisfactorily with those predicted by density functional theory calculations at the B3LYP/aug-cc-pVTZ level of theory. Thus, this method produced the boron hydride anion of interest with few other fragments, which enabled us to clearly identify the IR spectrum of the diborane(5) anion.

Laboratory confirmation of C60(+) as the carrier of two diffuse interstellar bands

The diffuse interstellar bands are absorption lines seen towards reddened stars. None of the molecules responsible for these bands have been conclusively identified. Two bands at 9,632 ångströms and 9,577 ångströms were reported in 1994, and were suggested to arise from C60(+) molecules (ref. 3), on the basis of the proximity of these wavelengths to the absorption bands of C60(+) measured in a neon matrix. Confirmation of this assignment requires the gas-phase spectrum of C60(+). Here we report laboratory spectroscopy of C60(+) in the gas phase, cooled to 5.8 kelvin. The absorption spectrum has maxima at 9,632.7 ± 0.1 ångströms and 9,577.5 ± 0.1 ångströms, and the full widths at half-maximum of these bands are 2.2 ± 0.2 ångströms and 2.5 ± 0.2 ångströms, respectively. We conclude that we have positively identified the diffuse interstellar bands at 9,632 ångströms and 9,577 ångströms as arising from C60(+) in the interstellar medium.

Helium Tagging Infrared Photodissociation Spectroscopy of Reactive Ions

The interrogation of reaction intermediates is key for understanding chemical reactions; however their direct observation and study remains a considerable challenge. Mass spectrometry is one of the most sensitive analytical techniques, and its use to study reaction mixtures is now an established practice. However, the information that can be obtained is limited to elemental analysis and possibly to fragmentation behavior, which is often challenging to analyze. In order to extend the available experimental information, different types of spectroscopy in the infrared and visible region have been combined with mass spectrometry. Spectroscopy of mass selected ions usually utilizes the powerful sensitivity of mass spectrometers, and the absorption of photons is not detected as such but rather translated to mass changes. One approach to accomplish such spectroscopy involves loosely binding a tag to an ion that will be removed by absorption of one photon. We have constructed an ion trapping instrument capable of reaching temperatures that are sufficiently low to enable tagging by helium atoms in situ, thus permitting infrared photodissociation spectroscopy (IRPD) to be carried out. While tagging by larger rare gas atoms, such as neon or argon is also possible, these may cause significant structural changes to small and reactive species, making the use of helium highly beneficial. We discuss the "innocence" of helium as a tag in ion spectroscopy using several case studies. It is shown that helium tagging is effectively innocent when used with benzene dications, not interfering with their structure or IRPD spectrum. We have also provided a case study where we can see that despite its minimal size there are systems where He has a huge effect. A strong influence of the He tagging was shown in the IRPD spectra of HCCl(2+) where large spectral shifts were observed. While the presented systems are rather small, they involve the formation of mixtures of isomers. We have therefore implemented two-color experiments where one laser is employed to selectively deplete a mixture by one (or more) isomer allowing helium tagging IRPD spectra of the remaining isomer(s) to be recorded via the second laser. Our experimental setup, based on a linear wire quadrupole ion trap, allows us to deplete almost 100% of all helium tagged ions in the trap. Using this special feature, we have developed attenuation experiments for determination of absolute photofragmentation cross sections. At the same time, this approach can be used to estimate the representation of isomers in a mixture. The ultimate aim is the routine use of this instrument and technique to study a wide range of reaction intermediates in catalysis. To this end, we present a study of hypervalent iron(IV)-oxo complexes ([(L)Fe(O)(NO3)](+)). We show that we can spectroscopically differentiate iron complexes with S = 1 and S = 2 according to the stretching vibrations of a nitrate counterion.

A Grotthuss-like proton shuttle in the anomalous C2H3+ carbocation: energetic and vibrational properties for isotopologues

The proton shuttle in C₂H₃⁺ is reminiscent of the Grotthuss proton transfer mechanism.

Infrared spectrum of the cold ortho-fluorinated protonated neurotransmitter 2-phenylethylamine: competition between NH+⋯π and NH+⋯F interactions

IR spectra of cold rare-gas tagged ions reveal the switch of the preferred conformation of the highly flexible side chain of a prototypical protonated neurotransmitter induced by site-specific aromatic fluorination.

London dispersion in molecular chemistry--reconsidering steric effects

London dispersion, which constitutes the attractive part of the famous van der Waals potential, has long been underappreciated in molecular chemistry as an important element of structural stability, and thus affects chemical reactivity and catalysis. This negligence is due to the common notion that dispersion is weak, which is only true for one pair of interacting atoms. For increasingly larger structures, the overall dispersion contribution grows rapidly and can amount to tens of kcal mol(-1) . This Review collects and emphasizes the importance of inter- and intramolecular dispersion for molecules consisting mostly of first row atoms. The synergy of experiment and theory has now reached a stage where dispersion effects can be examined in fine detail. This forces us to reconsider our perception of steric hindrance and stereoelectronic effects. The quantitation of dispersion energy donors will improve our ability to design sophisticated molecular structures and much better catalysts.

Photoelectron imaging and photodissociation of ozonide in O3(-)⋅(O2)n (n = 1-4) clusters

The photoelectron images of O3 (-) and O3 (-) ⋅ (O2)n (n = 1-4) have been measured using 3.49 eV photon energy. The spectra exhibit several processes, including direct photodetachment and photodissociation with photodetachment of O(-) photofragments. Several spectra also exhibit autodetachment of vibrationally excited O2 (-) photofragments. Comparison of the bare O3 (-) photoelectron spectra to that of the complexes shows that the O3 (-) core is preserved upon clustering with several O2 molecules, though subtle changes in the Franck-Condon profile of the ground state photodetachment transition suggest some charge transfer from O3 (-) to the O2 molecules. The electron affinities of the complexes increase by less than 0.1 eV with each additional O2 molecule, which is comparable to the corresponding binding energy [K. Hiraoka, Chem. Phys. 125, 439-444 (1988)]. The relative intensity of the photofragment O(-) detachment signal to the O3 (-) ⋅ (O2)n direct detachment signal increases with cluster size. O2 (-) autodetachment signal is only observed in the O3 (-), O3 (-) ⋅ (O2)3, and O3 (-) ⋅ (O2)4 spectra, suggesting that the energy of the dissociative state also varies with the number of O2 molecules present in the cluster.

Photoelectron-photofragment coincidence studies of the tert-butoxide anion (CH3)3CO((-)), the carbanion isomer (CH3)2CH2COH((-)), and corresponding radicals

A study of the photodetachment and dissociative photodetachment (DPD) of the C(4)H(9)O(-) isomers tert-butoxide, (CH(3))(3)CO(-), and the α-hydroxy carbanion (CH(3))(2)C(CH(2))OH(-) is reported. Photoelectron-photofragment coincidence spectroscopy was used to study these anions at 387, 537, and 600 nm. Supported by CBS-QB3 ab initio calculations, the product mass and translational energy distributions were found to be consistent with dissociation of either highly excited (CH3)(3)CO radicals or (CH(3))(2)C(CH2)OH alkylhydroxy radicals. Vibrationally resolved photoelectron spectra of stable radicals at 537 and 600 nm in conjunction with Franck-Condon simulations were used to assign the dominant channel to tert-butoxide ((CH(3)3)CO(-)) anions thermalized to a vibrational temperature of 550 K. DPD is assigned to highly vibrationally excited radicals produced by photodetachment of unrelaxed tert-butoxide products formed at an effective source temperature of 1400 K. The higher energy carbanion was found to be a minor channel and was not observed to dissociate. Calculated energetics for photodetachment and DPD of (CH(3))(3)CO(-) and (CH(3))(2)C(CH(2))OH(-) are discussed and compared with the experimental results.

Photo-fragmentation spectroscopy of benzylium and 1-phenylethyl cations

The electronic spectra of cold benzylium (C6H5-CH2 (+)) and 1-phenylethyl (C6H5-CH-CH3 (+)) cations have been recorded via photofragment spectroscopy. Benzylium and 1-phenylethyl cations produced from electrosprayed benzylamine and phenylethylamine solutions, respectively, were stored in a cryogenically cooled quadrupole ion trap and photodissociated by an OPO laser, scanned in parts of the UV and visible regions (600-225 nm). The electronic states and active vibrational modes of the benzylium and 1-phenylethyl cations as well as those of their tropylium or methyl tropylium isomers have been calculated with ab initio methods for comparison with the spectra observed. Sharp vibrational progressions are observed in the visible region while the absorption features are much broader in the UV. The visible spectrum of the benzylium cation is similar to that obtained in an argon tagging experiment [V. Dryza, N. Chalyavi, J. A. Sanelli, and E. J. Bieske, J. Chem. Phys. 137, 204304 (2012)], with an additional splitting assigned to Fermi resonances. The visible spectrum of the 1-phenylethyl cation also shows vibrational progressions. For both cations, the second electronic transition is observed in the UV, around 33,000 cm(-1) (4.1 eV) and shows a broadened vibrational progression. In both cases the S2 optimized geometry is non-planar. The third electronic transition observed around 40,000 cm(-1) (5.0 eV) is even broader with no apparent vibrational structures, which is indicative of either a fast non-radiative process or a very large change in geometry between the excited and the ground states. The oscillator strengths calculated for tropylium and methyl tropylium are weak. Therefore, these isomeric structures are most likely not responsible for these absorption features. Finally, the fragmentation pattern changes in the second and third electronic states: C2H2 loss becomes predominant at higher excitation energies, for both cations.

Excited states of proton-bound DNA/RNA base homodimers: pyrimidines

We are presenting the electronic photofragment spectra of the protonated pyrimidine DNA base homodimers. Only the thymine dimer exhibits a well structured vibrational progression, while the protonated monomer shows broad vibrational bands. This shows that proton bonding can block some nonradiative processes present in the monomer.

Cyanide-arene Meisenheimer complex generated in electrospray ionization mass spectrometry using acetonitrile as a solvent

The C - C bond formation activated under negative electrospray ionization of an acetonitrile solution of 1,3,5-trinitrobenzene is reported. The solvent function is to provide a source of cyanide ion, a highly problematic reagent, which is found to attack the electron-deficient aromatic ring to form a covalently bound anionic complex (Meisenheimer complex). The structure of the complex is elucidated by means of collision induced dissociation mass spectrometry and IR multiple photon dissociation spectroscopy in the 'fingerprint' region.