Photolysis Data Research Articles

The effect of pH on carbon monoxide binding to Menhaden hemoglobin. Allosteric transitions in a root effect hemoglobin.

FM-’ s-‘; these were assigned as the T state binding rates of the two chains. CO dissociation is also biphasic. At alkaline pH CO binding and dissociation are homogeneous, with rate constants of 1.3 PM-’ s-’ and 0.028 s-‘, respectively; these were assigned as the R state rates for both chains. Full and partial flash photolysis data were collected over the pH range 6.0 to 8.5, and CO equilibrium data from 6.0 to 7.5. These were fit to the twostate model modified to include chain differences. The T and R binding rates, and the R dissociation rate, were assigned; L and the T dissociation rates were calculated. L varies from 4 x lo7 at pH 6.0 to 8 at pH 8.5; the T rates vary from 0.15 and 0.25 at pH 6.0 to 0.02 at pH 8.5. Both Hb and carbon monoxide hemoglobin (HbCO) have pH difference spectra in the Soret and visible regions. The Hb spectrum changes above pH 7.5 and the HbCO spectrum changes below pH 7.0. The transition from R to T state was observed at an Hb-HbCO isosbestic during flash photolysis of HbCO. The total absorbance change at this w-avelength rises, then falls with pH, with a maximum at pH 7.0. The time course of conformation change at pH 7.0 was followed after full and partial flash photolysis and fit to the extended two-state model with an L value of lo” and a 3-fold decrease in the rate of R to T transition for each additional bound CO. The magnitude of static and kinetic absorption changes was compared to the conformation state of Hb and HbCO calculated from the functional measurements. There is poor correlation; both spectral measurements indicate a too great effect of pH on the conformation of both derivatives. It is suggested that optical absorption measurements do not give a quantitative measure of quaternary conformation.

Reactions of the iso‐butyl radical during 1,1′‐azoisobutane pyrolysis

AbstractQuantitative analysis of the products formed in 1,1′‐azoisobutane pyrolyses in the temperature range of 553°–602°K has shown that the major reactions of the iso‐butyl radical are equation image Analysis of initial rate data gave log10k4/(kc)1/2(cm−3/2.mol 1/2.sec−1/2) = 7.54±0.44 − (136.5 + 4.8) kJ/mol/2.303RT, the Arrhenius parameters obtained being in good agreement with thermodynamic data for reaction (4). Measured values of ka/(kc)1/2 where ka is the rate constant of the reaction iC4H9 + AIB → iC4H10 +. AIB were consistent with published parameters determined by photolysis of 1,1′‐azoisobutane. Combination of photolysis and pyrolysis data gave log10 ka/(kc)1/2(cm3/2.mol−1/22.sec−1/2) = 3.68 ± 0.15 − (27.2 ± 1.2) kJ/mol/2.303RT. The crosscombination ratio for methyl and iso‐butyl radicals has been found to be 0.25, indicating that the geometric mean rule does not hold for methyl and iso‐butyl radicals.

ChemInform Abstract: PHOTODISSOCIATION OF THIOPHOSGENE

The photolysis of thiophosgene produces CS(A 1π) below the incident wavelength (1270 A) from which ΔHf °0(SCCl2) ?7.92±1 kcal mol−1 (33.1±4 kJ mol−1) is derived. The quantum yield for the production of Cl atoms has been determined from the amounts of HCl produced from the photolysis of thiophosgene–heptane mixtures. At 2537 A the yield is unity over the pressure region 0.4–80 torr (0.05–10 kPa) of thiophosgene, indicating that the primary photochemical process is almost entirely SCCl+Cl. Both at 3660 and 4358 A the quantum yield of Cl production increases with a decrease in thiophosgene pressure and is 0.90±0.17 and 0.38±0.19, respectively, as the pressure approaches zero. From the pressure dependence of the quantum yield the lifetime of the excited state 1A2 is estimated to be about 55 nsec at 4358 A and about 6 nsec at 3660 A. The photolysis data of the 1A2 state of thiophosgene are correlated with the fluorescence lifetime measured previously. D°0(SCCl–Cl) =63.4±0.5 kcal mol−1 (265.3±2 kJ mol−1) and ΔH...

Dissociation of Br2 in shock waves

The rate of dissocation of Br2 in the presence of Ar and Br2 has been investigated using three independent experimental techniques in the same shock tube: molecular absorption spectroscopy (AS), two-body emission spectroscopy (ES), and laser schlieren technique (LS). Present results yield recombination rate constants in good agreement with each other and with earlier high temperature flash photolysis data. The temperature range over which dissociation was studied was extended from 1200 to 3000 °K. Recombination rate constants can be summarized in terms of the following equations: log10krAr(LS) =8.251(±0.002)−1.36(±0.29)log10(T/2300) or log10krAr(LS) =8.378(±0.001)−1.05(±0.30) log10(T/2300). The difference between these two equations arose because in the first equation RBr=RBr2 was assumed, while in the second RBr=RKr was used. Here Rx is the Gladstone–Dale constant of X. These equations are valid between 1600 and 3000°K. The analogous equations for krBr2 are: log10kBr2=8.718(±0.001)−2.35(±0.41)log10(T/1900) and log10krBr2=8.767(±0.001)−2.18(±0.42) log20(T/1900), valid between 1600 and 2000 °K. Furthermore, it is found that log10krAr(AS+ES) =8.182(±0.003)−1.42(±0.18)log10(T/1963), valid between 1180 and 2890 °K. The laser schlieren technique, as applied to chemical reactions in shock waves, was studied. In particular, the effect of finite width laser beam, optical absorption of reacting gas, temperature dependence of its refractive index, and composition dependence of its Gladstone–Dale constant, were investigated. It was found that calculated values of rate constants are sensitive functions of refraction, and hence polarizabilities, of reaction products. Although generally these properties are not known, they can be estimated empirically.

Photodissociation of thiophosgene

The photolysis of thiophosgene produces CS(A 1π) below the incident wavelength (1270 Å) from which ΔHf °0(SCCl2) ?7.92±1 kcal mol−1 (33.1±4 kJ mol−1) is derived. The quantum yield for the production of Cl atoms has been determined from the amounts of HCl produced from the photolysis of thiophosgene–heptane mixtures. At 2537 Å the yield is unity over the pressure region 0.4–80 torr (0.05–10 kPa) of thiophosgene, indicating that the primary photochemical process is almost entirely SCCl+Cl. Both at 3660 and 4358 Å the quantum yield of Cl production increases with a decrease in thiophosgene pressure and is 0.90±0.17 and 0.38±0.19, respectively, as the pressure approaches zero. From the pressure dependence of the quantum yield the lifetime of the excited state 1A2 is estimated to be about 55 nsec at 4358 Å and about 6 nsec at 3660 Å. The photolysis data of the 1A2 state of thiophosgene are correlated with the fluorescence lifetime measured previously. D°0(SCCl–Cl) =63.4±0.5 kcal mol−1 (265.3±2 kJ mol−1) and ΔHf °0(SCCl) =43±1 kcal mol−1 (180±4 kJ mol−1) are derived. The photochemistry of thiophosgene is compared with that of phosgene and formaldehyde. A new method of Cl isotopic enrichment is suggested.

Chemical effects of low energy electron impact on hydrocarbons in the gas phase: III. Cyclopropane

The simulated radiolysis of cyclopropane with low energy electrons (3.5 to 15.0 eV) was investigated. The setup used for the irradiations has been described previously. Appearance curves of the various products formed under electron impact were determined. The features observed on these curves yield various indications. (1) Some products arise from the dissociation of excited molecules. Contributing states are the following ones: a triplet state at 7.4 eV, singlet states at 6.7 and/or 7.7 eV, at 8.55 eV, at 9.4 and/or 9.95 eV and superexcited states lying around 10.2 eV. As in other hydrocarbons studied, the electron impact excitation cross section shows a steep increase at the ionization potential. (2) Other products result from ion fragmentation and ion—molecule reactions. A reaction scheme was proposed to account for the chemical effects associated with excited states and the yields of excited molecules in dissociating states were derived from experimental data. The observations relative to excited molecule fragmentation are in conformity with photolysis data. Additional information on the decomposition processes of molecules excited in the triplet state at 7.4 eV, in the singlet states at 6.7 and/or 7.7 eV and in the superexcited states were obtained. Owing to the complexity of ionic mechanisms it was not possible to distinguish between the contributions of ionization and excitation. Only the radiation chemical yield of products, G(products), was evaluated. The values found for G(products) just above the ionization potential are close to the data obtained in conventional radiolysis which could indicate that secondary electrons having such energies play an important role in radiation chemistry.

Flash photolysis of enzymes.

The photoionization of aromatic residues constitutes a major initial photochemical reaction in the flash photolysis of proteins at gamma greater than 250 nm. The ejected electrons have been observed as eaq- and the disulphide bridge electron adduct, and also must be trapped at unidentified sites. The number of tryptophyl (or tyrosyl) residues photo-ionized at 5 musec delay is approximately equal to the number of exposed residues. The flash photolysis data have been related to inactivation by considering how photolysis of these "photolabile" residues can affect enzymic activity, based on the microstructure and available information about permanent alterations and residue specificities. This analysis indicates that hen lysozyme and papain are inactivated by photolysis of an essential Trp residue, that bovine trypsin is inactivated by photolysis of a Trp residue adjacent to the key catalytic Ser and other pathways initiated by excitation of Tyr and Cys, that the efficient photoionization of Tyr and RNase A is not an important inactivating reaction, and that aromatic residues in subtilisn Carlsberg are photosensitive.

Kinetic analysis of flash photolysis data for sequential first and second order reactions: The photoreduction of triplet fluorenone by dabco

In studying reactions of triplet states by flash photolysis, particularly with laser techniques, one frequently encounters the kinetic problem of an initial pseudo-first order triplet reaction overlaid by a second order decay of products. A simple algorithm is given, adaptable to a desk-top calculator, for treatment of experimental data to obtain rate constants and total product yields (or absorptions) in this situation. The procedure is illustrated in the case of the photoreduction of fluorenone triplet by 1,4-diazabicyclo(2.2.2) octane, a reaction which yields a pair of radical-ions.

A paradox: The thermal rate coefficient for the H+DCl → HCl+D exchange reaction

Previously reported photolysis experiments indicate that the frequency factors associated with the hydrogen-exchange reactions H+DCl μ HCl+D and D+HCl → DCl+H are on the order of 1010 cm3/mol⋅sec. This result indicates that the above processes are associated with very small steric factors, in contrast to what one might be led to expect from recent crossed beam experiments. A series of unadjusted, quasiclassical trajectory calculations have been carried out to compute the thermal rate coefficients and activation parameters for a series of 13 thermal processes of the type A+BC → AB+C, where A=H, D, or Cl and BC=H2, D2, HCl, DCl, or Cl2. In addition, hot-atom yield ratios have been computed from the IRP equation for the reactions D*+DCl → D2+Cl, D*+Cl2 → DCl + Cl as a function of the initial D* laboratory energy. Previously formulated, valence-bond representations were employed for the potential-energy surfaces in all the calculations. The computations yield (1) hot-atom [DCl]/[D2] yield ratios within a factor of 2 of the experimental values; (2) thermal activation energies in satisfactory agreement with experiment for all processes investigated; and (3) frequency factors in reasonable accord with experiment for all the reactions except the hydrogen exchange reactions, where the computed values are a factor of about 103–104 larger than indicated by the photolysis experiments. An investigation of a network of elementary thermal reactions that are apparently occurring in the photolysis experiments indicates that the presently computed thermal rate coefficients for the exchange reactions are too large to be compatible with observed HD and D2 quantum yields. These results make it clear that a serious error exists in either the potential-energy surface, the theoretical scattering calculations, the measured photolysis data, or the interpretation of that data. The agreement of the computed hot-atom yields, rate coefficients, and thermal activation parameters for all reactions except the hydrogen exchange reactions coupled with the results of recent crossed-beam experiments strongly indicate that the error lies in the interpretation of the photolysis data.

Quasiclassical trajectory study of the molecular beam kinetics of the deuterium atom–hydrogen halide exchange reactions

Unadjusted quasiclassical trajectory computations have been carried out to simulate the molecular beam scattering of thermal D atom beams at 2800 °K crossed with beams of HCl and HI at 250 °K. Total reaction cross sections, energy partitioning distributions, and differential scattering cross sections have been computed for the exchange reactions D+HCl → DCl+H and D+HI → DI+H while total reaction cross sections are reported for the corresponding abstractions, i.e., D+HCl → HD+Cl and D+HI → HD+I. For the exchange reactions, the computed reaction cross sections are within the range estimated from the crossed beam experiments. The calculated average energy partitioned into relative translational motion of products is in near quantitative agreement with the beam results, and the predicted differential scattering cross sections appear to be in qualitative accord with the beam experiments. The over-all agreement between theory and experiment indicates that previously computed values for the thermal rate coefficients for the exchange reactions are of the right order and that a systematic error exists in the interpretation of photolysis data in the hydrogen–hydrogen halide systems.

Contributions to analytical chemistry of vitamin B12.

The possibility of determination of small amounts of cyanide (0.02–1.0μg) in Vitamin B12, without distillation of hydrogen cyanide, is demonstrated. Vitamin B12 samples (10−6−4×10−5M), acidified to pH 3–4 with sulphuric or nitric acid, are irradiated in 25-ml volumetric flasks with an ultraviolet lamp (without filter) at 35–40° for 30 min. After cooling, the samples are adjusted to pH 12–13 with sodium hydroxide, the ionic strength is adjusted to 0.1 with potassium nitrate, and cyanide is determined with an ionselective membrane electrode. The calibration curve is prepared with KCN or Vitamin B12. The total error is ±2 %. The practical advantage of the method is that 5–10 samples can be analysed simultaneously, and the equilibria between cyanocobalamine, analogues of Vitamin B12 and potassium cyanide can be studied. The thermolysis and photolysis data can serve for evaluation of the strength of the Co—CN bond in Vitamin B12 and analogous molecules, by comparison with other cyanide complexes which have accurately known stability constants and structures.

Abstraction versus exchange in the reaction of H with DBr

The photolysis of DBr−H2−Br2 mixtures at 185 nm furnishes data which can be analyzed to provide the abstraction to exchange rate coefficient ratio k8/k9 for reactive collisions of H with DBr where H is a translationally hot atom with an average energy of about 1 eV. The results of such experiments are reported here and are analyzed using (1) an assumed mechanism, (2) available independent experimental data, (3) some quasiclassical trajectory data, and (4) an isotope effect model based on collision frequencies to give 0.2 ? k8/k9 ? 0.8. In addition quasiclassical trajectory calculations have been used to generate model data for direct comparison with the experimental data. These calculations, using potential energy surfaces which have been very successful in accounting for a variety of other experimental data, give k8/k9 = 0.24, as well as values for the other rate coefficients in the mechanism. With a single adjustable parameter used to fit the intercept in one experiment, these trajectory calculations are in quite satisfactory agreement with the experimental data provided D−atom exchange with DBr is incorporated as a significant mechanism for thermalization of hot D atoms. It is also shown that the photolysis data are consistent with trajectory calculations that give an abstraction to exchange ratio for thermal H atoms that lies in the range 0.2−1.0.

Recombination of bromine atoms between 300 and 6000°K, theory and experiment

The dissociation of Br 2 in Ar was studied in the same shock tube using three different time dependent observables which were needed to measure the rate of the reaction: (1) Br 2 molecular absorption, (2) Br atom two-body emission and (3) density gradient change, detected by laser schlieren technique. It was found that the first observable was most useful in determination of dissociation rate constants between about 1500 and 1800°K and the second observable between about 1200 and 1700°K. Use of these two observables yielded rate constants which were in agreement with those earlier experimental data, which are likely to be most reliable. The laser schlieren technique yielded new dissociation rate constants between 2100 and 3000°K. These data were found to be consistent with the “reliable” “emission” and “absorption” data, as well as with the earlier flash photolysis data. In order to interpret the experimental results and to study the dynamics of the reaction, the dissociation of Br 2 in Ar was studied by 3-D classical trajectory calculations at 1500, 2500, 3500, and 6000°K. In agreement with earlier trajectory studies, it was found that Br 2 molecules react only if their total energy is within a few kT of dissociation limit and that metastable molecules, with total energy above the dissociation limit, are particularly reactive. The average energy, E >, and the average angular momentum, l >, transferred in dissociative and non-dissociative collisions were calculated as a function of total energy of Br 2 molecule. Generally, | E >| (non-dissoc.) and | l >| (non-dissoc.) were found to be considerably smaller than the same quantities for dissociative collisions. For energetic metastable molecules, dissociation could be accomplished by a decrease of internal energy and angular momentum of the molecule through collision. Thus, collisionally induced rotational de-excitation provides an additional mechanism for dissociation of diatomic molecules. The non-equilibrium effects in dissociation have been evaluated at 3500 and 6000°K by the method of multiple collisions. The calculated steady state dissociation rate constants between 1500 and 6000°K are in good agreement with the available experimental data. Finally, the validity of the rate quotient law, i.e. k forward / k reverse = K eg , was demonstrated by trajectory calculation technique.

On the recombination mechanism of halogen atoms in rare gas diluents

A classical method is developed which explicitly considers the ET and RMC components of the total recombination coefficient and interprets them in terms of contributions from classical similar trajectories (ST). In the case of halogen atoms in rare gases, flash photolysis data can be satisfactorily understood if the depth of the rare-gas—halogen interaction is taken to be of the order of 1 kcal/mole. The results of the ST treatment are found to be in essential agreement with those of Monte Carlo trajectory calculations.

Far ultra-violet photolysis of ammonia quantum yield determination for the primary process: NH 3 (ND 3) + hv → NH (ND) + H 2 (D 2)

The gas phase photolysis of NH 3-C 2D 4 and ND 3-C 2H 4 mixtures has been investigated at 147 nm (8.4 eV), 123.6 nm (10 eV) and 104.8-106.7 nm (11.6-11.8 eV). The quantum yield of D 2 in the irradiation of ND 3-C 2H 4 mixtures is independent of the concentration of C 2H 4 and of the pressure of ND 3 (10 to 180 Torr). It is concluded that in these mixtures D 2 is entirely formed by molecular elemination from excited ND 3. The quantum yields of such a process are as follows at these energies: 147 nm, 0.032 ± 0.005; 123.6 nm, 0.244 ± 0.01; and 104.8-106.7 nm, 0.306 ± 0.007 (M/N ex = 0.52 ± 0.02). Although the NH 3-C 2D 4 photolysis data exhibit less reliability, it can be concluded that the quantum yield of molecular H 2 at 147 nm is twice that of melecular D 2. Isotope effects are much less pronounced at higher photon energies. At 147 and 123.6 nm, D 2 is eliminated via primary process: ▪ and/or: ▪ On the basis of the spectroscopic observation of Okabe and Lenzi it can be concluded that at 104.8-106.7 nm the primary process: ▪ must also contribute significantly to the formation of molecular hydrogen. It is suggested that the ND 3 + ions which are formed at these energies do not contribute to the formation of molecular D 2, but that the occurence of an overall process: ▪ cannot be ruled out. The extinction coefficients of NH 3 and ND 3, at the rare gas lines have been redetermined and were found to differ significantly from those derived from published absorption curves.

Recombination of Iodine Atoms by Flash Photolysis over a Wide Temperature Range. II I2 in He, Ar, Xe, N2, CO

The recombination of iodine atoms in the presence of I2 and He, Ar, Xe, N2, and CO has been studied by flash photolysis between 300–1173°K. The rate constants (in liters2 mole−2 · sec−1) and their temperature dependences may be represented by logkrHe=9.136 − 1.716 log (T/300) + 0.994 log2(T/300),logkrAr=9.439 − 2.418 log (T/300) + 1.911 log2(T/300),logkrXe=9.652 − 2.787 log (T/300) + 1.678 log2(T/300),logkrN2=9.641 − 2.645 log (T/300) + 1.880 log2(T/300),logkrCO=9.729 − 2.606 log (T/300) + 1.457 log2(T/300).The present recombination rate data for He, Ar, and N2 overlap the available shock wave dissociation rate data by 70 to 300°K. Above 1000°K, the recombination rate constants calculated from shock wave data are generally higher and show a steeper negative temperature dependence than the flash photolysis rate constants obtained at the same temperatures. The rate constant krI2 has also been determined from rate experiments on iodine—helium mixtures, from room temperature to 723°K. The present k2I2 data agree well with the flash photolysis data obtained over the same temperature range from iodine—argon experiments [J. A. Blake and G. Burns, J. Chem. Phys. 54, 1480 (1971)]. The two sets of krI2 measurements, when combined, may be represented by logkrI2=12.122 − 5.844 log (T/300) + 2.163 log2(T/300).

Formation of Excited Singlet States in the Nanosecond Pulse Radiolysis and Nanosecond Flash Photolysis of Aromatic Molecules in Liquid and Solid Solutions

The absorption spectra of the excited singlet states of naphthalene and 1:2 benzanthracene have been observed in the nanosecond pulse radiolysis and flash photolysis of these solutes in cyclohexane, benzene, and polystyrene. The effect of xenon, piperylene, and excimer formation on the absorption spectra provides evidence for the assignment of these spectra to excited singlet states. The spectra are very similar both in shape and intensity to the triplet spectra; hence, little growth of the triplet state is observed as the excited singlet state decays. An extinction coefficient at 560 mμ for the 1:2 benzanthracene excited singlet is measured as ε560 = 9700M−1·cm−1. Using the photolysis data it is shown that approximately equal yields of excited singlet and triplet states are initially formed in the radiolysis of these systems. Large yields of ions are also formed in the pulse radiolysis of 1:2 benzanthracene in polystyrene and polymethyl-methacrylate.

Recombination of Br atoms by flash photolysis over a wide temperature range

The recombination rate constant of Br atoms in argon has been measured up to 1273°K using a flash photolysis technique. At room temperature and up to 480°K, results agree with the literature, but at 1150°K the recombination rate constant is (2·7±0·3)×108 l.2/mole2 sec, which is about one third of the rate constant predicted from shock-wave studies of Br2 dissociation. The discrepancy can be explained by considering a model for coupling of vibrational relaxation and dissociation. The main assumptions of the model are that the vibrational relaxation in dissociating gas at reasonably low temperatures can be described by a single relaxation time and that diatomic molecules dissociate from the uppermost vibrational levels. The model reconciles the shock wave and flash photolysis data. In this model, the reduced vibrational relaxation time for Br2 in argon at 1200°K was taken to be equal to 0.95 µsec atm.An alternate model, which considers a coupling of electronic excitation and dissociation processes, is also discussed. This model suggests that the discrepancy observed may be explained if it is assumed that the activation energy of the transition between the ground and excited electronic states of Br2 is between 25 and 35 kcal/mole. There are no known electronic states in this energy region. The temperature dependence of the recombination rate constant between 300 and 2300°K can be expressed by the equation log kr= 13·35–1·6 log T, where kr is expressed in l.2/mole2 sec.

Vacuum Ultraviolet Photochemistry, Part II. Nitrous Oxide at 1849 A

The photolysis of nitrous oxide at 1849 A has been studied and the quantum yield ratios φN2O/φN2, φN2O/φO2, and φN2O/φNO measured. The former two ratios compare well with the same ratios at 1470 A and the latter ratio is pressure dependent. It appears that the photolysis mechanism at 1849 A is similar to that at 1470 A. The absorption coefficient of N2O is calculated from the photolysis data and found to compare well with previous work.