Three-center versus four-center elimination in photolysis of vinyl fluoride and vinyl bromide at 193 nm: Bimodal rotational distribution of HF and HBr (v5) detected with time-resolved Fourier transform spectroscopy Following photodissociation of vinyl chloride at 193 nm, fully resolved vibration-rotational emission spectra of HCl in the spectral region 2000-3310 cm Ϫ1 are temporally resolved with a step-scan Fourier-transform spectrometer. Under improved resolution and sensitivity, emission from HCl up to vϭ7 is observed, with JϾ32 ͑limited by overlap at the band head͒ for vϭ1 -3. All vibrational levels show bimodal rotational distribution with one component corresponding to ϳ500 K and another corresponding to ϳ9500 K for vр4. Vibrational distributions of HCl for both components are determined; the low-J component exhibits inverted vibrational population of HCl. Statistical models are suitable for three-center ͑␣, ␣͒ elimination of HCl because of the loose transition state and a small exit barrier for this channel; predicted internal energy distributions of HCl are consistent but slightly less than those observed for the high-J component. Impulse models considering geometries and displacement vectors of transition states during bond breaking predict substantial rotational excitation for three-center elimination of HCl but little rotational excitation for four-center ͑␣, ͒ elimination; observed internal energy of the low-J component is consistent with that predicted for the four-center elimination channel. Rate coefficients 33.8 and 4.9ϫ10 11 s Ϫ1 for unimolecular decomposition predicted for three-center and four-center elimination channels, respectively, based on Rice-Ramsberger-Kassel-Marcus theory are consistent with the branching ratio of 0.81:0.19 determined by counting vibrational distribution of HCl to vр6 for high-J and low-J components. Hence we conclude that observed high-J and low-J components correspond to HCl (v, J) produced from three-center and four-center elimination channels, respectively.
Linear carbon disulfide (denoted as SCS) isolated in solid N 2 or Ar at 13 K was irradiated with light at 193 nm from an ArF excimer laser. In addition to an absorption line of CS at 1277.4 cm -1 , new lines at 881.3 and 520.9 cm -1 were observed after photolysis of SCS in solid N 2 . These lines are assigned to cyclic CS 2 (denoted cyc-CS 2 ) based on results from 34 S-and 13 C-isotopic experiments. Doublet lines of cyc-CS 2 at 876.5 (881.1) and 517.7 (522.7) cm -1 were observed after irradiation of SCS in solid Ar at 193 nm; lines in parentheses are associated with a minor matrix site. Secondary photolysis at 248, 308, 532, 560, or 580 nm diminishes signals of cyc-CS 2 and produces SCS. Theoretical calculations using MP2-full and density-functional methods (BLYP and B3LYP) predict three isomers of CS 2 : SCS, cyc-CS 2 , and linear CSS; relative energies, structures, vibrational wavenumbers, and IR intensities were predicted for each isomer. Cyc-CS 2 has C-S bonds (∼1.74 Å) elongated relative to those of SCS (∼1.56 Å), a S-S bond ∼2.14 Å, and ∠SCS ≈ 76°; it lies ∼73 kcal mol -1 above SCS. Calculated vibrational wavenumbers, IR intensities, and isotopic shifts for cyc-CS 2 fit satisfactorily with experimental results. An asymmetric transition state connecting SCS and cyc-CS 2 is characterized, yielding a ring-opening barrier of ∼24.4 kcal mol -1 (zero-point energy corrected). Photoconversion between linear and cyclic CS 2 in a matrix cage is discussed.
Carbon monoxide (CO) intoxication can result in cognitive deficits and demyelinating changes of the white matter (WM), for which hyperbaric-oxygen (HBO) treatment is considered effective in reducing neuropsychiatric symptoms. This study aimed to analyze cognitive functions and WM diffusion properties in CO intoxication after standard HBO treatment. Seventeen CO intoxicated patients were evaluated 4-6 months after HBO treatment. They also underwent diffusion tensor imaging (DTI) and cognitive assessment, and the results were compared with those from 34 age-matched controls. DTI was transformed into fractional anisotropy (FA) and mean diffusivity (MD) and assessed at every voxel level with tract-based spatial statistics across the brain. Correlation between reduced FA and increased MD with neuropsychological deficits were performed. Cognitive results showed that impairment in executive function, as well as verbal and visual memories, were most prominent. There were extensive areas of increased MD and decreased FA. Correlation analyses showed that memory retrieval, judgment, and verbal generation tasks were related to FA of the frontotemporal WM. MD showed weaker correlation with cognitive deficits. These data suggest that neurologic deficits and WM changes are detectable 4-6 months after HBO therapy. The correlation of WM diffusion with cognitive deficits also suggests that reduced connectivity between different cortical regions is a pathophysiologic mechanism.
Following photodissociation of 2-chloro-1,1-difluoroethene CF2CHCl) at 193 nm, vibration–rotationally resolved emission spectra of HCl(v⩽3) and HF(v⩽4) in spectral regions 2000–2900 and 3050–4410 cm−1, respectively, are detected with a step-scan time-resolved Fourier-transform spectrometer. All vibrational levels of HCl and HF show Boltzmann-type rotational distributions. HCl has an average rotational energy of 23±4 kJ mol−1 and a vibrational energy of 25±5 kJ mol−1, whereas HF has an average rotational energy of 20±4 kJ mol−1 and a vibrational energy of 48±6 kJ mol−1. The observed internal energy distribution indicates that HCl is produced via the three-center (α,α), but HF via the four-center (α,β) elimination. A modified separate statistical ensemble model predicts an internal energy distribution of HCl slightly greater than experimental observation. A modified impulse model taking into account geometries and displacement vectors of transition states during bond breaking predicts satisfactorily the rotational excitation of HF produced from four-center elimination. Ratios of rate coefficients (0.87:0.13) predicted for three-center or four-center elimination channels based on Rice–Ramsperger–Kassel–Marcus theory are consistent with a branching ratio of 0.88:0.12 determined based on observed populations of HCl and HF, respectively. We also compare these experimental and theoretical results with those of photolysis of vinyl halides (CH2CHX, X=F, Cl, or Br) at 193 nm.
CH free radicals in an oxyacetylene flame at atmospheric pressure were detected by means of the two-color resonant four-wave mixing technique. Ground-state grating schemes with ω1=ω2 (pump) and ω3 (probe)=ω4 (signal) were used; ω1(=ω2) and ω3 are in resonance with distinct rovibronic transitions of B 2Σ−−X 2Π and A 2Δ−X 2Π of CH, respectively. Varying the relative polarization of the pump and probe beams significantly affects the ratio of signal to noise. Two schemes with cross polarization pairs (ω1⊥ω2) and (ω3⊥ω4) produced results best suited to detect trace species in the luminous, dense medium; the relative J dependence observed for these schemes agrees with theoretical predictions. The double-resonance feature of this technique, advantageous to identify congested lines, is also demonstrated. When an A–X transition line is employed as a probe, previously unobserved predissociative levels of B 2Σ− are detected; the R(12) line of B←X (1,0) transion has a halfwidth greater than 10 cm−1, corresponding to a lifetime of 0.5 ps.
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