Predissociation Mechanism Research Articles

Shock-Tube Study of Radiation from S2

Absolute emission intensity measurements of the radiation from sulfur molecules S2 have been obtained in the temperature range from 2300 to 4800°K at six wavelengths spaced between 3132 and 5461 Å. The radiation intensities were proportional to the square of the sulfur atom concentration and were independent of the total pressure of the buffer gas, argon. In addition, the band structure was recorded for the wavelength interval 3600–4100 Å with an image orthicon camera mounted in a spectrograph. The intensities between 3650 and 5461 Å have similar activation energies and are interpreted in terms of the radiative transitions from the B 3Σu− state of molecular sulfur, this state being populated by the mechanism of inverse predissociation. Theoretical calculations of the B 3Σu− bands predicted the band shapes observed with the image orthicon and, together with the measured intensities, provide the electronic f value for the bands. The square of the transition moment [Re(r) / ea0]2 is estimated to have a value of 5.0, which corresponds to an f value of 0.16 on the basis of published Franck–Condon factors. The intensities at 3132 Å have a temperature dependence close to that predicted for transitions from the b 1Δu electronic state.

Fluorescence of nitric oxide. Part 7.—Quenching rates of NO C2Π(v= 0), its rate of radiation to NO A2Σ+, energy transfer efficiencies, and mechanisms of predissociation

The following experimental results are reported : (a) accurate rates of quenching of NO C2Π(v= 0) by N2, CO2, Ar and NO; (b) measurement of the rate of the NO C2Π(v= 0)→NO A2Σ+ cascade radiation (fel= 1.6±0.6); (c) efficiencies of energy transfer in the deactivation of NO C2Π(v= 0) to A2Σ+; (d) evidence for electronic energy transfer in the deactivation of NO D2Σ+(v= 0) to NO A2Σ+ by N2; (e)“anomalous” quenching rates of the δ-bands by N2, CO2 and NO, and (f) partial quenching of the 15N16O δ-emission by Ar. A route via NO a4Π is adopted to account for the predissociation of NO C2Π(v= 0). It is suggested that the levels below F2(7/2) of 14N16O C2Π(v= 0) lie below the dissociation limit, indicating Do(NO)= 52,400±10cm–1. A compilation of the quenching parameters is given.

Metastable Peaks in the Mass Spectra of N2O and NO2. II

Metastable peaks arising from the delayed unimolecular dissociation of NO+ to O++N in the mass spectra of both N2O and NO2 and from the dissociation of N2+ to N++N in the mass spectrum of N2O have been investigated. A new metastable dissociation of N2O2+ to NO++N+ was also studied. Further studies were made on the previously known metastable transitions of N2O+ to NO++N and of NO2+ to NO++O. The appearance potential, the kinetic-energy release in fragmentation, and the half-life were determined for each metastable transition. Comparisons were made of the energetic and half-life characteristics of the metastable NO+ ion as produced from the sources NO, N2O, and NO2, and of the metastable N2+ ion from the sources N2 and N2O. The results are consistent with the unimolecular dissociation of these diatomic ions, N2+ and NO+, proceeding by predissociation mechanisms.

Predissociation effects in the A 2Σ + state of the OH radical

Extensive configuration-interaction calculations are reported for the lowest 4Σ −, 4II and 2Σ− states of the OH radical. The relative ordering of these states and their location with respect to the A 2Σ + state help to define possible predissociation mechanisms.

Photochemistry of Acetanilide. III. The Secondary Processes in the Photochemical Reaction

Abstract The photochemical secondary processes of acetanilide have been studied. The photochemical reaction of p-methylacetanilide at 2537 Å did not produce the p-rearrangement product, but the o-rearrangement one. The quantum yields of the o-rearrangement and of the disappearance of p-methylacetanilide were 0.092 and 0.12 respectively. Comparing the quantum yields of acetanilide with those of p-methylacetanilide, the values of the quantum yields for the predissociation of the N–C bond, the back reaction, and the recombination ratio were estimated to be 0.25, 0.10, and 0.9 respectively. These results indicate that the recombination reaction between the pair radicals formed by the predissociation of the N–C bond occur efficiently in a solvent cage. The rate constants of the back reaction, the o-rearrangement, and the p-rearrangement were proportional to the odd π electron densities of the anilino radical. No intermediate was observed by the flash photolysis. The small dependence of the quantum yields of acetanilide on the temperature supports the predissociation mechanism previously described. On the basis of these studies, the mechanism of the photochemical reaction of acetanilide in the liquid phase has been proposed.

Photoionization of High-Temperature Vapors. IV. TlF, TlCl, and TlBr

The vapors of TlF, TlCl, and TlBr have been studied by photoionization mass spectrometry. From the thresholds for formation of Tl+, the dissociation energies obtained are 4.57 ± 0.02, 3.82 ± 0.02, and 3.42 ± 0.02 eV for TlF, TlCl, and TlBr, respectively. The TlF value provides confirmation for the existence of barriers in the potential energy curves of its π0+3 and π13 states. The ion pairs in TlF and TlCl are formed with considerable kinetic energy, probably by a predissociative mechanism.

Shock-Tube Study of the Radiative Combination of Oxygen Atoms by Inverse Predissociation

Absolute emission-intensity measurements of the radiative combination of oxygen atoms O(3P) in the temperature range between 2500° and 3800°K are presented. The emission intensity was recorded simultaneously in six spectral intervals in the wavelength range between 2300 and 4511 Å. The absolute emission intensity was found to be proportional to the square of the oxygen atom concentration and to be characterized by an activation energy of 28.9 ± 2.2 kcal mole− 1. These results are interpreted in terms of an inverse predissociation mechanism in which the oxygen atoms combine along a repulsive potential energy surface with a transition to the B 3Σu− state of molecular oxygen and a subsequent radiative transition to the ground electronic state of oxygen. The rate constant for the over-all radiative combination of oxygen atoms was found to be 4.3 × 106exp(− 28 900 ± 2200 / RT) cm3mole− 1·sec− 1 for the wavelength range 2300–5000 Å.

Pseudohalides. Part IV. Infrared spectra and thermochemistry of the complexes PhBX2,L (X = Cl, Br, NCO, or NCS; L = C5H5N or EtOAc), and relative acceptor strengths of halogeno- and pseudohalogenoboranes; with notes on the chemistry of four-co-ordinate isocyanato- and isothiocyanato-boranes

Eight complexes PhBX2,L (X = Cl, Br, NCO, or NCS; L = C5H5N or EtOAc) have been prepared and characterised. Their i.r. spectra have been examined and assignments are proposed. The most significant feature is probably the progressive decrease in ν(CO) in the series PhBX2,EtOAc from 1639 (X = NCO), to 1597 (X = NCS), 1582 (X = Cl), and 1563 cm.–1(X = Br) compared with 1714 cm.–1 in free EtOAC. These changes reflect the inverse order of acceptor strengths (X = Br > Cl > NCS > NCO) in PhBX2. The same result is found calorimetrically, using pyridine as the ligand; but for the EtOAc–PhBX2 system there are complicating side reactions (the 1 : 1 complexes dissolve exothermically in excess ligand). The condensed-phase heats of the reaction (where l = liquid and c = crystal): PhBX2(l)+ py(l)= PhBX2,py(c) are –39·1, –33·2, ∼–28·7, and –26·2 kcal. mole–1 for X = Br, Cl, NCS, and NCO, respectively.The compounds B(NCO)3,py, B(NCS)3,py, o-C6H4O2BNCO,py, Ph2BNCO,py, and PhB(Cl)NCO,py are reported. B(NCX)3,py (X = O or S), was obtained from BY3,py (Y = halogen) and AgNCX (probably by a predissociation mechanism); the other complexes were prepared directly from their components. Reactions with alcohols and amines are described. Four-co-ordinate pseudohalogenoboranes generally behave like their three-co-ordinate progenitors. An exception is found in the B(NCO)3,py–RR1NH system which leads to B(NH·CO·NRR1)3.

Time-resolved studies of emission from I2(B3Πou+)

The emission from I2(B3Πou+)→I2(X1Σ+g)+hv has been monitored following the flash photolysis of CF3I. The study of the emission from this banded system using a rapid scan spectrometer shows that it commences approximately at the onset of the B–X continuum and that continuous emission resulting from the two-body recombination of I(52P½) and I(52P3/2), into which the B state resolves, is not important. Thus the emission in the banded region results from the production of I2(B) by recombination in the presence of a third body according to I(52P½)+ I(52P3/2)+ M →I2(B)+ M. There is no contribution from an inverse predissociation mechanism for which the energy barrier would be large. The rate equation for the concentration of I2(B) can be solved with a number of experimentally justifiable assumptions and the time-dependent concentrations of the ground state and excited atoms derived from previous observations on spin orbit relaxation. Thus the variation of the I2(B—X) emission, monitored at a single wavelength, can be correlated with the kinetics of I(52P½) and I(52P3/2).

Primary Processes in Photodecomposition

When energy is absorbed by a molecule at a particular bond by a process of transition to a repulsive energy level or a sufficiently high point on an attractive level, rupture always results within one vibration period. All other photodecomposition processes are properly included under the title of predissociation. Thus, rupture may also result at the locus of absorption as the result of either spontaneous or induced adiabatic transition from an attractive to a repulsive or ``weakly attractive'' level. These are the only mechanisms of predissociation in diatomic molecules. In polyatomic molecules, spontaneous or induced shifts of energy or rearrangements of groups, resulting in decomposition, may also occur. Illustrations of the various processes suggested are cited from the photochemistry of sulfur, iodine, ozone, and the aldehydes. In the case of formaldehyde it has been possible to distinguish potential hypersurfaces and to indicate two different types of predissociation: one in a single hypersurface and the other a spontaneous transition between two hypersurfaces. The method used is extended to other aldehydes. Suggestions are made as to the interpretation of the photochemistry of several other substances and as to several problems which require solution.