Neon Matrices Research Articles

OPTICAL ABSORPTIONS OF OXYGENATED CARBON CHAIN CATIONS IN THE GAS PHASE

ABSTRACT The gas-phase electronic spectra of linear OC4O+ and a planar C6H2O+ isomer were obtained at a rotational temperature of ≈10 K. Absorption measurements in a 6 K neon matrix were followed by gas-phase observations in a cryogenic radiofrequency ion trap. The origin bands of the transition of OC4O+ and the A B1 of HCCC(CO)CCH+ lie at 417.31 ± 0.01 nm and 523.49 ± 0.01 nm, respectively. These constitute the first electronic spectra of oxygenated carbon chain cations studied under conditions that are relevant to the diffuse interstellar bands (DIBs), as both have a visible transition. The recent analysis of the 579.5 nm DIB indicates that small carriers, five to seven heavy atoms, continue to be possible candidates (Huang & Oka 2015). Astronomical implications are discussed regarding this kind of oxygenated molecules.

Electronic absorption spectrum of HC7O+

ABSTRACTVibrationally resolved electronic absorption spectrum of 1-oxohepta-2,4,6-triyn-ylium cation has been recorded in a 6 K neon matrix after mass-selected deposition of m/z = 101 cations produced from the 1,2,3,4,5-benzenepentacarboxylic acid in an ion source. The band system with origin at 278 nm is assigned to the 21Σ+ ← X1Σ+ transition of the linear-HC7O+ isomer on the basis of theoretical calculations. Vibrational analysis of the electronic spectrum is consistent with the structure.

Electronic Spectra of Protonated Fluoranthene in a Neon Matrix and Gas Phase at 10 K.

Four electronic systems with origin bands at 759.5, 559.3, 476.3, and 385.5 nm are detected in a 6 K neon matrix following deposition of mass-selected protonated fluoranthene C16H11(+) produced from a reaction of neutral vapor and ethanol in a hot-cathode ion source. Two cationic isomers are identified as the carriers of these band systems. The 559.3, 476.3, and 385.5 nm absorptions are assigned to 4,3,2 (1)A' ← X (1)A' transitions of isomer E(+) (γ-) and the 2 (1)A' ← X (1)A' system at 759.5 nm is of isomer C(+) (α-) of protonated fluoranthene on the basis of theoretical predictions. The electronic spectrum of E(+) was also recorded in the gas phase using a resonant 1 + 1 two-photon excitation-dissociation technique in an ion trap at vibrational and rotational temperatures of 10 K. The 3,2 (1)A' ← X (1)A' transitions have origin band maxima at 558.28 ± 0.01 and 474.92 ± 0.01 nm. Both the 2 (1)A' and 3 (1)A' excited states have a distinct vibrational pattern with lifetimes on the order of 1 ps.

Electronic Characterization of Reaction Intermediates: The Fluorenylium, Phenalenylium, and Benz[f]indenylium Cations and Their Radicals.

Three vibrationally resolved absorption systems commencing at 538, 518, and 392 nm were detected in a 6 K neon matrix after mass-selected deposition of C13 H9 (+) ions (m/z=165) produced from fluorene in a hot-cathode discharge ion source. The benz[f]indenylium (BfI(+) : 538 nm), fluorenylium (FL9(+) : 518 nm), and phenalenylium (PHL(+) : 392 nm) cations are the absorbing molecules. Two electronic systems corresponding to neutral species are apparent at 490 and 546 nm after irradiation of the matrix with λ<260 nm photons and were assigned to the FL9 and BfI radicals, respectively. The strongest peak at 518 nm is the origin of the 2 (1) B2 ←X̃ (1) A1 absorption of FL9(+) , and the 490 nm band is the 2 (2) A2 ←X̃ (2) B1 origin of FL9. The electronic systems commencing at 538 nm and 546 nm were assigned to the 1 (1) A1 ←X̃ (1) A1 and 1 (2) A2 ←X̃ (2) A2 transitions of BfI(+) and BfI. The 392 nm band is the 1 (1) E'←X̃ (1) A1 ' transition of PHL(+). The electronic spectra of C13 H9 (+) /C13 H9 were assigned on the basis of the vertical excitation energies calculated with SAC-CI and MS-CASPT2 methods.

Electronic Characterization of Reaction Intermediates: The Fluorenylium, Phenalenylium, and Benz[f]indenylium Cations and Their Radicals

AbstractThree vibrationally resolved absorption systems commencing at 538, 518, and 392 nm were detected in a 6 K neon matrix after mass‐selected deposition of C13H9+ ions (m/z=165) produced from fluorene in a hot‐cathode discharge ion source. The benz[f]indenylium (BfI+: 538 nm), fluorenylium (FL9+: 518 nm), and phenalenylium (PHL+: 392 nm) cations are the absorbing molecules. Two electronic systems corresponding to neutral species are apparent at 490 and 546 nm after irradiation of the matrix with λ&lt;260 nm photons and were assigned to the FL9 and BfI radicals, respectively. The strongest peak at 518 nm is the origin of the 2 1B2←X̃ 1A1 absorption of FL9+, and the 490 nm band is the 2 2A2←X̃ 2B1 origin of FL9. The electronic systems commencing at 538 nm and 546 nm were assigned to the 1 1A1←X̃ 1A1 and 1 2A2←X̃ 2A2 transitions of BfI+ and BfI. The 392 nm band is the 1 1E′←X̃ 1A1′ transition of PHL+. The electronic spectra of C13H9+/C13H9 were assigned on the basis of the vertical excitation energies calculated with SAC‐CI and MS‐CASPT2 methods.

Double deuterated acetylacetone in neon matrices: infrared spectroscopy, photoreactivity and the tunneling process

The effect of deuteration of acetylacetone (C5O2H8) is explored by means of IR spectroscopy of its single and double deuterated isotopologues trapped in neon matrices. The whole vibrational spectra of chelated enols are very sensitive to the H-D exchange of the hydrogen atom involved in the internal hydrogen bond. UV excitation of double deuterated acetylacetone isolated in neon matrices induces the formation of four open enol isomers which can be divided into two groups of two conformers, depending on their formation kinetics. Within each group, one conformer is more stable than the other: slow conformer interconversion due to a tunneling process is observed in the dark at low temperature. Moreover, IR laser irradiation at the OD stretching overtone frequency is used to induce interconversion either from the most stable to the less stable conformer or the opposite, depending on the excitation wavelength. The interconversion process is of great help to assign conformers which are definitively identified by comparison between experimental and calculated IR spectra. Kinetic constants of the tunneling process at play are theoretically estimated and agree perfectly with experiments, including previous experiments with the totally hydrogenated acetylacetone [Lozada García et al., Phys. Chem. Chem. Phys., 2012, 14, 3450].

The donor OH stretching-libration dynamics of hydrogen-bonded methanol dimers in cryogenic matrices.

FTIR spectra of the methanol dimer trapped in neon matrices are presented. The fundamental, overtone and combination bands involving the donor OH libration and stretching motions were observed in order to extract relevant anharmonicity constants. We find a stretching-libration coupling constant of +43(5) cm(-1) and a diagonal librational anharmonicity constant of -71(5) cm(-1). The spectra are compared to a number of VPT2 calculations and a torsionally localized monomer model in order to enhance previous explanations of the observable OH stretching red-shift upon dimerization.

Theoretical investigations of absorption and fluorescence spectra of protonated pyrene.

The equilibrium geometry and 75 vibrational normal-mode frequencies of the ground and first excited states of protonated pyrene isomers were calculated and characterized in the adiabatic representation by using the complete active space self-consistent field (CASSCF) method. Electronic absorption spectra of solid neon matrixes in the wavelength range 495-415 nm were determined by Maier et al. and they were analyzed using time-dependent density functional theory calculations (TDDFT). CASSCF calculations and absorption and emission spectra simulations by one-photon excitation equations were used to optimize the excited and ground state structures of protonated pyrene isomers. The absorption band was attributed to the S0 → S1 electronic transition in 1H-Py(+), and a band origin was used at 20580.96 cm(-1). The displaced harmonic oscillator approximation and Franck-Condon approximation were used to simulate the absorption spectrum of the (1) (1)A' ← X[combining tilde](1)A' transition of 1H-Py(+), and the main vibronic transitions were assigned for the first ππ* state. It shows that the vibronic structures were dominated by one of the eight active totally symmetric modes, with ν15 being the most crucial. This indicates that the electronic transition of the S1((1)A') state calculated in the adiabatic representation effectively includes a contribution from the adiabatic vibronic coupling through Franck-Condon factors perturbed by harmonic oscillators. The present method can adequately reproduce experimental absorption and fluorescence spectra of a gas phase.

Spectroscopic identification of ethanol-water conformers by large-amplitude hydrogen bond librational modes

The far-infrared absorption spectra have been recorded for hydrogen-bonded complexes of water with ethanol embedded in cryogenic neon matrices at 2.8 K. The partial isotopic H/D-substitution of the ethanol subunit enabled by a dual inlet deposition procedure enables the observation and unambiguous assignment of the intermolecular high-frequency out-of-plane and the low-frequency in-plane donor OH librational modes for two different conformations of the mixed binary ethanol/water complex. The resolved donor OH librational bands confirm directly previous experimental evidence that ethanol acts as the O⋯HO hydrogen bond acceptor in the two most stable conformations. In the most stable conformation, the water subunit forces the ethanol molecule into its less stable gauche configuration upon dimerization owing to a cooperative secondary weak O⋯HC hydrogen bond interaction evidenced by a significantly blue-shift of the low-frequency in-plane donor OH librational band origin. The strong correlation between the low-frequency in-plane donor OH librational motion and the secondary intermolecular O⋯HC hydrogen bond is demonstrated by electronic structure calculations. The experimental findings are further supported by CCSD(T)-F12/aug-cc-pVQZ calculations of the conformational energy differences together with second-order vibrational perturbation theory calculations of the large-amplitude donor OH librational band origins.

The Electronic Spectrum of the Fulvenallenyl Radical

AbstractThe fulvenallenyl radical was produced in 6 K neon matrices after mass‐selective deposition of C7H5− and C7H5+ generated from organic precursors in a hot cathode ion source. Absorption bands commencing at λ=401.3 nm were detected as a result of photodetachment of electrons from the deposited C7H5− and also by neutralization of C7H5+ in the matrix. The absorption system is assigned to the 1 2B1←X 2B1 transition of the fulvenallenyl radical on the basis of electronic excitation energies calculated with the MS‐CASPT2 method. The vibrational excitation bands detected in the spectrum concur with the structure of the fulvenallenyl radical. Employing DFT calculations, it is found that the fulvenallenyl anion and its radical are the global minima on the potential energy surface among plausible structures of C7H5.

The Electronic Spectrum of the Fulvenallenyl Radical.

The fulvenallenyl radical was produced in 6 K neon matrices after mass-selective deposition of C7H5(-) and C7H5(+) generated from organic precursors in a hot cathode ion source. Absorption bands commencing at λ=401.3 nm were detected as a result of photodetachment of electrons from the deposited C7H5(-) and also by neutralization of C7H5(+) in the matrix. The absorption system is assigned to the 1 (2)B1 ←X (2)B1 transition of the fulvenallenyl radical on the basis of electronic excitation energies calculated with the MS-CASPT2 method. The vibrational excitation bands detected in the spectrum concur with the structure of the fulvenallenyl radical. Employing DFT calculations, it is found that the fulvenallenyl anion and its radical are the global minima on the potential energy surface among plausible structures of C7H5.

Photoluminescence Spectroscopy of Mass-Selected Electrosprayed Ions Embedded in Cryogenic Rare-Gas Matrixes.

An apparatus is presented which combines nanoelectrospray ionization for isolation of large molecular ions from solution, mass-to-charge ratio selection in gas-phase, low-energy-ion-beam deposition into a (co-condensed) inert gas matrix and UV laser-induced visible-region photoluminescence (PL) of the matrix isolated ions. Performance is tested by depositing three different types of lanthanoid diketonate cations including also a dissociation product species not directly accessible by chemical synthesis. For these strongly photoluminescent ions, accumulation of some femto- to picomoles in a neon matrix (over a time scale of tens of minutes to several hours) is sufficient to obtain well-resolved dispersed emission spectra. We have ruled out contributions to these spectra due to charge neutralization or fragmentation during deposition by also acquiring photoluminescence spectra of the same ionic species in the gas phase.

On observing C60+and C602+in laboratory and space

Recently, we have measured the IR absorptions of C60+, C60−, and C602+ in neon matrixes. Many previously unknown absorptions were established. Here we compare our laboratory IR absorption spectra for C60+ and C602+ to the IR emission spectra of several astronomical objects that show C60 emission. We find that IR bands of C60+ are possibly present in the objects Tc1, SMP SMC 16, NGC 7023, NGC 2244, and SMP LMC 02. Infrared emission features possibly due to C602+ were identified in NGC 7023. To help with future observations of fullerene-related DIBs, we also revisited the oscillator strengths of the NIR absorptions of C60+, and report significantly revised values. Additionally, we report the UV oscillator strengths of C60+.

Infrared spectrum of CH3CN–HCl in solid neon, and modeling matrix effects in CH3CN–HCl and H3N–HCl

Several IR bands for two isotopic forms of the CH3CN–HCl complex have been measured in neon matrices at 6 K. Assigned bands for the primary isotopomer include the νCC band at 926 cm−1, νHCl the band at 2677 cm−1, and the νCN band, which occurs as a doublet at 2308/2279 cm−1. The observed frequencies are reasonably consistent with previous observations in solid argon and/or nitrogen, with the exception of νHCl bands, which differed, and were ultimately found to shift systematically across various condensed-phase media. A plot of medium-induced shifts for the νHCl band vs. polarizability of the host substances is essentially continuous; except for the N2-matrix frequency, for which the shift is considerably larger than that expected on this basis. In an effort to clarify the physical underpinnings of these shifts, and their structural implications, a computational study was undertaken, which examined CH3CN–HCl as well as H3N–HCl; a system for which analogous frequency shifts have been observed and characterized previously. Beyond equilibrium structure and frequency calculations, which predict that the ammonia complex is stronger and undergoes a greater degree of structural change in bulk, condensed-phase media, the N–Cl potentials of both complexes were mapped in the gas-phase and in bulk, dielectric media. The predicted medium effects on these potentials are consistent with observations. The CH3CN–HCl curves shift slightly in response to the medium, consistent with a subtle enhancement of the hydrogen bond, and relatively small frequency shifts. The effects of the dielectric media on H3N–HCl are more substantial, and indicate a significant degree of proton transfer, even for low-dielectric environments, which is consistent with the extreme spectral shifts noted previously for this system.

Electronic spectra of oxygen containing polycyclic hydrocarbon cations and the protonated analogues.

The electronic transitions of 9-fluorenone FL(+) and 2,3,6,7-dibenzotropone DBT(+) cations were detected in 6 K neon matrices following a mass-selective deposition. The absorptions at 649.2 and 472.2 nm are assigned to the 2 (2)B1←X̃(2)A2 FL(+) and 2(2)A(')←X̃(2)A(') DBT(+) transitions. Absorption spectra of protonated 9-fluorenone H(+)-FL and 2,3,6,7-dibenzotropone H(+)-DBT have also been measured. Protonation of the oxygenated polycyclic aromatic hydrocarbons is carried out in a hot cathode source via in situ produced protonated ethanol. Vibrationally resolved absorptions commencing at 423.3 nm of H-FL(+) and two band systems of H-DBT(+) with origins at 502.4 and 371.5 nm are assigned to the 2(1)A(')←X̃(1)A(') electronic transition of 9-hydroxy-fluorenyl cation and 1 (1)A←X̃(1)A, 2 (1)A←X̃(1)A of 2,3,6,7-dibenzocycloheptenol cation. The assignments are based on vertical excitation energy calculations with time dependent density functional theory, symmetry adapted cluster configuration interaction, and MS-CASPT2 methods.

Slow ground state molecules from matrix isolation sublimation

SynopsisWe describe a cryogenic beam of 7Li2 dimers from sublimation of a neon matrix where Li atoms have been implanted via laser ablation of solid precursors of LiH. Laser absorption spectroscopy measured: T∼7 K, Trot ∼ 6K, drift velocity of 130 ms-1 with molecular density of 109 cm-3. The formation of molecules in a matrix offers new possibilities.

Electronic Spectra of Corannulenic Cations and Neutrals in Neon Matrices and Protonated Corannulene in the Gas Phase at 15 K

Abstract Three absorption systems starting at 624.1, 601.2, and 590.0 nm were detected in a 6 K neon matrix following deposition of mass selected m/z = 250 ions produced from corannulene vapour in a hot cathode ion source. The two latter systems were also observed after deposition of neutral corannulene in solid neon with concomitant bombardment of the matrix with argon ions. The features in the absorption spectrum are assigned to the 42A′′ ← X 2A′′ transition of cylobutadieno-benzo[ghi]fluoranthene cation and to the 32A′ ← X 2A′′ and 32A′′ ← X 2A′ transitions of two Jahn-Teller structures of bowl-shaped corannulene cations, respectively. The assignment is based on excitation energies calculated with the SAC-CI and CASPT2 methods. The electronic absorption spectrum of protonated corannulene has onsets at 515.1 and 398.8 nm in a neon matrix, following deposition of a mass-selected beam produced by reactions of corannulene with EtOH2 +. The absorptions are assigned, on the basis of theoretical predictions, to the 3,41A ← X 1A transitions. The electronic spectrum was also recorded in the gas phase using a resonant multiphoton fragmentation technique in an ion trap at vibrational and rotational temperatures of 15 K. The 3,41A ← X 1A transitions are observed with origin bands at 521 ± 1 nm and 396.4 ± 0.1 nm. The 31A excited electronic state indicates fast internal conversion of ≈ 5 fs, while the 41A state has a lifetime of ≈ 0.2 ps. A distinct vibrational pattern is discernible in the 41A ← X 1A transition.

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.

Electronic spectra of linear HC5H and cumulene carbene H2C5.

The 1(3)Σu (-)←X(3)Σg (-) transition of linear HC5H (A) has been observed in a neon matrix and gas phase. The assignment is based on mass-selective experiments, extrapolation of previous results of the longer HC2n+1H homologues, and density functional and multi-state CASPT2 theoretical methods. Another band system starting at 303 nm in neon is assigned as the 1(1)A1←X˜(1)A1 transition of the cumulene carbene pentatetraenylidene H2C5 (B).

Fluorine-Rich Fluorides: New Insights into the Chemistry of Polyfluoride Anions.

Polyfluoride anions have been investigated by matrix-isolation spectroscopy and quantum-chemical methods. For the first time the higher polyfluoride anion [F5 ](-) has been observed under cryogenic conditions in neon matrices at 850 cm(-1) . In addition, a new band for the Cs(+) [F3 ](-) complex in neon is reported.