Band 215, Heft 3 - 3

The bimolecular reaction: C 2 H 5 + Cl 2 4 → C 2 H 5 Cl + Cl is studied in the very low pressure reactor (VLPR) system at 298K starting with Cl/C 2 H 6 /Cl 2 mixtures. Mass conservation is found to be 97 ± 3% for the overall chemical change in the system. From the C 2 H 5 Cl product formation kinetics, k 4 =(1.05±0.05)·10 -12 cm 3 /(molecule-s) is derived. Two independent estimates of the A factor gives A 4 =1.3 ·10 -12 cm 3 /(molecule-s). These parameters yield a positive activation energy of 0.13 kcal/mol in contrast with recently reported negative activation energy values. Some side reactions of small extent are initiated by H atom contamination in the system. The measurement of CH 3 Cl product formed from CH 3 +Cl 2 11 →CH 3 Cl+Cl gives k 11 =(3.4±0.3) · 10 -12 cm 3 /(molecule-s) with an estimated A factor of A 11 =5.0×10 -12 cm 3 /(molecules). The source of CH 3 radicals is the side reaction H+C 2 H 5 →2CH 3 .

The rate constant k 1 for the recombination reaction between CF 3 radicals and O 2 has been measured at 295K over the pressure range of 2-110 Torr, predominately in N 2 . CF 3 radicals were formed by the uv photolysis of CF 3 I, and their kinetic decay monitored in the presence of NO 2 and O 2 by observing the infrared emission from the products of the CF 3 + NO 2 reaction. The data were combined with higher pressure relative rate constant measurements in the literature to yield a fall off curve with the high pressure limiting value of k 1(∞) =(2.55±0.11)·10 -12 cm 3 molecule -1 s -1 and the low pressure third body recombination rate constant k 1(0) =(2.5±0.3)·10 -29 cm 6 molecule -2 s -1 with 2σ quoted errors, and the Troe broadening parameter F c =0.60. The high pressure limiting value is some 35% lower than the currently recommended value, but is believed to be more reliable because of the far shorter extrapolation to the high pressure limit in the present study compared with previous work. Our data also suggest a reevaluation of the bimolecular rate constant of the CF 3 +Cl 2 reaction to (2.3±0.6)·10 -14 cm 3 molecule -1 s -1 .

Trans-glyoxal molecules in a seeded pulsed jet were laser-excited into selected vibrational levels of the T 1 and S 1 states. In the latter case, those specific rovibrational levels were chosen which exhibit S 1 /T 1 gateway coupling. In each case, the long-lived emission into the S 0 state was observed employing a spectrograph/CCD combination, covering 5000 cm -1 at 60 cm -1 FWHM resolution. A model Franck–Condon calculation of the relative ν 5 band intensities demonstrates the structural similarities of the S 1 and T 1 states. However, there are some distinct differences, despite the almost identical S 1 and T 1 vibrational frequencies. The S 1 ← S 0 excitation distorts the molecule significantly more in the ν 5 dimension than does T 1 ← S 0 .

Solutions of fluoranthene exhibit a delayed fluorescence (DF) due to triplet-triplet annihilation, 3 M* + 3 M* → 1 M* + 1 M (P-type DF). In the fluid solvent 3-methylpentane, at low temperatures, the DF consists of two components: delayed monomer fluorescence (DMF) and a slightly red-shifted component, which is assigned to delayed excimer fluorescence (DEF). The temperature dependence of the intensity ratio Q EM of DEF and DMF was measured between 133 K and 293 K. The assignment of the second component of the DF to the DEF of fluoranthene is consistent with the rise and the decay of the DF at 153 K. A kinetic model was developed that describes the reversible geminate association of a 1 M*··· 1 M pair to an excimer, 1 M*··· 1 M⇔ 1 (MM)*. The main features of the kinetic model are: a Smoluchowski equation, a distance-dependent relative diffusion coefficient, an exponential distance dependence of the first-order rate coefficient for the annihilation of a triplet pair, and a short-range potential representing the excimer. The Smoluchowski equation was numerically solved with a new algorithm, which is based on numerical fundamental solutions and allows the successive doubling of the total diffusion time. The kinetic model accurately describes Q EM (T) from ≈150 K (maximum of Q EM (T)) to room temperature. The effective first-order rate constant for excimer dissociation, k diss (T), the equilibrium constant K EM (T) for excimer formation, and the excimer binding enthalpy are estimated. The observability of prompt excimer fluorescence is discussed. A formation enthalpy of the ground-state dimer of ≈-hc×500 cm -1 is estimated. The major part of the DEF spectrum is approximately equal to a spectrum, which is produced by inhomogeneous spectral broadening (i.e. convolution with a Gaussian) and a red-shift of the spectrum of the monomer fluorescence. The DEF spectrum contains no band that corresponds to the 0-0 transition of the monomer fluorescence.

A comparative ab initio study was performed to determine the properties of the complexes of HO 2 with three nucleophile additives. At the QCISD(T)/6-311++G(2d,2p)//MP2/6-311++G(2d,2p) level including CP correction binding energies D 0 of 7.94 kcal mol -1 , 5.12 kcal mol -1 , and 3.58 kcal mol -1 was obtained for HO 2 ·NH 3 , HO 2 ·H 2 O, and HO 2 ·HF, respectively. The vibrational frequencies of the HO 2 fragment are shifted by several hundred wavenumbers due to the influence of NH 3 , together with a two order of magnitude increase of the IR intensity, making the complex a good candidate for experimental detection. Lower shift is caused by H 2 O and negligible shift by HF. The additive and the radical in the HO 2 ·NH 3 as well as in the HO 2 ·H 2 O complexes are bound together by a hydrogen bond between the nucleophile atom of NH 3 and H 2 O and the H atom of HO 2 . In HO 2 ·HF the H atom of HF is oriented towards the radical center of HO 2 and an additional hydrogen bond is formed between F and the H of HO 2 . The self-reaction of the HO 2 radicals is significantly modified by the presence of the additives. An important factor may be the formation of the complexes studied in this work. While the stability of the complex itself does not explain the complicated influence of the additive on the kinetics of the H 2 O+HO 2 reaction, the study of the properties of the complexes allows one to derive some conclusions on the mechanism of the reaction.

Partially restricted coupled cluster calculations involving connected triple substitutions have been carried out for linear SiC 3 and SiC 5 in their 3 Σ - electronic states. Both species are of substantial interest to interstellar cloud chemistry. Accurate equilibrium structures are established and the equilibrium dipole moments of linear SiC 3 and SiC 5 are predicted to be -4.49 D and -6.20 D, with the positive ends of the dipoles at the silicon sites. Various spectroscopic constants are predicted for different isotopomers of SiC 3 .

The high resolution vibronic spectra of the S 1 ←S 0 transition (ν≈34900 - 35300 cm -1 ) of indole and its derivatives 3-methyl indole, tryptamine and N-acetyl tryptophan amide (NATA) embedded in large (N∼10 3 -10 4 ) helium droplets exhibit an unexpected splitting of the zero phonon lines. Up to four components are observed, also for the different conformers in tryptamine, with spacings of about Δν≈1-2 cm -1 not seen in previous studies of the embedded amino acids tryptophan and tyrosine and other organic molecules. The split levels are attributed to different arrangements of two He atoms from the bath which are localized at the two different sites on the two sides of the indole chromophore. The relative intensities of the different components depend on the particular “side chain” attached to a common indole chromophore.

C 2 concentrations and populations of the ground and the first excited vibrational levels of electronically excited C 2 (d 3 Π) radicals during the thermal decomposition of C 3 O 2 behind shock waves were monitored by emission and absorption measurements in the (0-0) and the (1-0) Swan-bands. Experiments were carried out in mixtures of 1% C 3 O 2 +Ar and 1% C 3 O 2 +He at T=2100–3200 K and p=1.5–7.5 bar. In the whole range of conditions studied an essential nonequlilibrium of electronic and vibrational states of nascent C 2 radicals was observed. For analyzing possible formation and consumption reactions of the excited C 2 radicals, a convenient kinetic scheme of the C 3 O 2 decomposition was developed, which considers both the recombinative pumping process and exchange and quenching reactions of the C 2 (d 3 Π, v=0, 1) states.