Yu-Jong Wu

Direct spectral evidence of single-axis rotation and ortho-hydrogen-assisted nuclear spin conversion of CH3F in solid para-hydrogen

Observation of two weak absorption lines from the E (K = 1) level and one intense feature from A (K = 0) for degenerate modes nu(4) and nu(6) of CH(3)F provides direct spectral evidence that CH(3)F isolated in p-H(2) rotates about only its symmetry axis, and not about the other two axes. An interaction between A and E vibrational levels caused by the partially hindered spinning rotation is proposed. Conversion of nuclear spin between A and E components of CH(3)F is rapid when p-H(2) contains some o-H(2), but becomes slow when the proportion of o-H(2) is much decreased.

INFRARED SPECTRA OF ACETYLENE DILUTED IN SOLID NITROGEN UPON IRRADIATION WITH VACUUM ULTRAVIOLET LIGHT AND ELECTRONS

Infrared spectra and chemical reactions of acetylene diluted in solid nitrogen at 10 K upon irradiation with vacuum ultraviolet (VUV) light and energetic electrons were investigated in separate experiments. Irradiation of the matrix sample with VUV light peaking at 160 and 121.6 nm yielded simple products, including C2H, CN, and isomers of C2N2. In contrast, electron irradiation of a similar sample generated N3, C2H, and various nitriles. The reaction mechanisms for photolysis and radiolysis of the matrix samples are discussed. Our results may help explain the distribution of trace species detected in the atmosphere of Titan. In addition, the UV absorption spectrum of the electron-bombarded icy sample was obtained and might be useful for future spectral investigations of Pluto by New Horizons.

INFRARED AND ULTRAVIOLET SPECTRA OF METHANE DILUTED IN SOLID NITROGEN AND IRRADIATED WITH ELECTRONS DURING DEPOSITION AT VARIOUS TEMPERATURES

We recorded the infrared and ultraviolet absorption spectra of CH4:N2 matrix samples that underwent electron bombardment during deposition in the temperature range of 10–44 K. In contrast to a previous experiment on the IR spectroscopy of electron-bombarded icy samples, methyl and azide radicals became the main products upon electron bombardment during deposition; furthermore, reduced production of nitrile species was observed for deposition at 10 and 20 K. On the other hand, for deposition above 33 K, the observed bands of the radical species (such as methyl and azide) decreased, and bands of large nitriles appeared. This observation may suggest that radical species easily diffuse and recombine to form more complex molecules in solid nitrogen at higher temperatures. Further measurements of similar samples at 10–33 K in the UV region revealed the intense band of azide radicals at 272.5 nm and weak, broad, overlapping features of methyl and azide radicals in the 225–197 nm region. For deposition at 44 K, only a broad feature centered at 219.4 nm was observed, and the possible carriers of nitrile species were proposed based on the corresponding IR spectrum and theoretical predictions of excitation energy. This band is similar to the observed absorption feature of Pluto’s surface recorded by the Hubble telescope in terms of both band position and bandwidth. Our findings therefore further support the suggestion that complex nitrile species may exist on the surface of Pluto.

Identification of a Simplest Hypervalent Hydrogen Fluoride Anion in Solid Argon

Hypervalent molecules are one of the exceptions to the octet rule. Bonding in most hypervalent molecules is well rationalized by the Rundle–Pimentel model (three-center four-electron bond), and high ionic bonding between the ligands and the central atom is essential for stabilizing hypervalent molecules. Here, we produced one of the simplest hypervalent anions, HF−, which is known to deviate from the Rundle–Pimentel model, and identified its ro-vibrational features. High-level ab inito calculations reveal that its bond dissociation energy is comparable to that of dihalides, as supported by secondary photolysis experiments with irradiation at various wavelengths. The charge distribution analysis suggested that the F atom of HF− is negative and hypervalent and the bonding is more covalent than ionic.

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.

Direct infrared observation of hydrogen chloride anions in solid argon

To facilitate direct spectroscopic observation of hydrogen chloride anions (HCl-), electron bombardment of CH3Cl diluted in excess Ar during matrix deposition was used to generate this anion. Subsequent characterization were performed by IR spectroscopy and quantum chemical calculations. Moreover the band intensity of HCl- decays slowly when the matrix sample is maintained in the dark for a prolonged time. High-level ab inito calculation suggested that HCl- is only weakly bound. Atom-in-molecule charge analysis indicated that both atoms of HCl- are negatively charged and the Cl atom is hypervalent.

UV absorption spectrum of allene radical cations in solid argon

Electron bombardment during deposition of an Ar matrix containing a small proportion of allene generated allene cations. Further irradiation of the matrix sample at 385u202fnm destroyed the allene cations and formed propyne cations in solid Ar. Both cations were identified according to previously reported IR absorption bands. Using a similar technique, we recorded the ultraviolet absorption spectrum of allene cations in solid Ar. The vibrationally resolved progression recorded in the range of 266-237u202fnm with intervals of about 800u202fcm-1 was assigned to the A2Eu202f←u202fX2E transition of allene cations, and the broad continuum absorption recorded in the region of 229-214u202fnm was assigned to their B2A1u202f←u202fX2E transition. These assignments were made based on the observed photolytic behavior of the progressions and the vertical excitation energies and oscillator strengths calculated using time-dependent density functional theory.

Formation and identification of borane radical anions isolated in solid argon

The infrared (IR) spectrum of borane(3) anions (BH3-) isolated in solid Ar was recorded; two vibrational modes were observed at 2259.4 and 606.6 cm-1, which were assigned to the BH2 stretching (ν3) and out-of-plane large-amplitude (ν2) modes, respectively. These anions were produced by the electron bombardment of an Ar matrix sample containing a small proportion of B2H6 and H2 during matrix deposition or by the photolysis of single-bridged-B2H5- in an Ar matrix with the selected ultraviolet light. The band positions, relative intensity ratios, isotopic splitting pattern, and isotopic shift ratios of the observed IR features of BH3- are generally in good agreement with those predicted by the B2PLYP/aug-cc-pVTZ method.

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.