Showing papers in "Faraday Discussions of The Chemical Society in 1977"

Semi-classical theory for non-separable systems:. Construction of “good” action-angle variables for reaction rate constants

Abstract: A semiclassical expression for bimolecular rate constants for reactions which have a single activation barrier is obtained in terms of the “good” action variables of the (classical) Hamiltonian that are associated with the saddle point region of the potential energy surface. The formulae apply to non-separable, as well as separable saddle points.

The reaction of F + H2→ HF + H. A case study in reaction dynamics

Abstract: Concepts developed in recent years as a result of experimental and theoretical studies of the dynamics of reactions A + BC → AB + C are examined here in trajectory studies of the reaction F + H2→ HF + H—currently the prototype of exothermic reaction. (i) An “early” barrier, characteristic of substantially exothermic reaction, has the consequence that reagent translation is much more efficient than vibration in promoting reaction, even for energies well in excess of the barrier. (ii) High vibrational excitation in the molecular product stems from the release of H·H repulsion while the new bond, F––H, is still extended (termed “mixed energy-release”); the large zero point vibration in H2 introduces variability in the F––H extension and H·H repulsion, and consequently in the breadth of the observed product vibrational distribution. (iii) Since the product vibrational excitation is governed by the attractive plus mixed energy-release, the slope of the outrun of the energy surface is not, per se, a dominant factor governing product vibrational excitation. (iv) Enhanced reagent vibration, 〈ΔV〉, tends to be channelled into enhanced product vibration, 〈ΔV′〉; the efficiency (〈ΔV′〉= 0.81 ΔV) is dependent on the form of energy surface in the region of “corner-cutting”, corresponding to reaction through extended intermediates F––H–H. (v) Enhanced reagent translation is channelled into product translation plus rotation (〈ΔT′〉+〈ΔR′〉= 1.12 〈ΔT〉); the efficiency provides an indication of the energy required to bend and compress the intermediate FH·H. (vi) The rotational dependence of the reactive cross section, σ(J), provides a sensitive probe of the region of the potential-energy hyper-surface along the entry valley up to the barrier. (vii) Enhanced reagent rotational excitation at first decreases product vibrational excitation and then increases it according to experiment; this effect has not been reproduced theoretically. (viii) Product rotational excitation derives in large part from the release of repulsion in bent configurations: this gives rise predominantly to coplanarity and opposed directions for the product rotational and orbital motions. (ix) Enhanced reagent excitation of all sorts results in significant enhancement in product rotation, due to reaction through more-compressed and bent configurations; the most efficient conversion is from reagent rotation into product rotation (〈ΔR′〉= 1.2 〈ΔR〉). (x) For thermal (300 K) reaction the computed centre-of-mass angular distribution is sharply backward-peaked, and similar for all product vibrational levels. (xi) Enhanced reagent vibration or translation shifts the computed mean scattering angle forward, with ΔT being markedly more effective than ΔV; the “stripping threshold energy” is high, as anticipated for these masses reacting on a strongly repulsive potential-energy surface favouring collinear approach.

A many-body expansion of polyatomic potential energy surfaces: application to Hn systems

Abstract: A method recently proposed to construct explicit functions for the potential energy hypersurfaces of triatomic molecules is extended to deal with larger polyatomic systems. This extension is based upon a many-body expansion of the total potential energy and has the objective of reproducing both the equilibrium properties of any stable molecule on the surface and the asymptotic dissociation limits. The surfaces for Hn clusters have been examined in order to test the convergence of the many-body expansion. Accurate ab initio calculations on H3 and H2 are used to construct a function for the H3 surface, which is shown to be more accurate, when judged by its overall description of the surface, than previous functions. The 2 and 3-body terms obtained from this analysis are combined with ab initio calculations on H4 to deduce a 4-body term and a complete 6-dimensional surface for H4. This surface is shown to have a trapezoidal transition state for the H2– D2 exchange which is 6 kJ mol–1 below the H2 dissociation limit. A reaction path passing through this transition state is determined. The many-body expansion terminated at the 4-body term is used to predict energies for D5h structures of H5 and D6h structures of H6. The optimum bond length for H6 is 1.85a0, in good agreement with calculations. The potential well is, however, of lower energy than for 3H2, showing that a small repulsive 5-body term must be invoked to obtain an accurate energy for this structure.

Potential energy surfaces for simple chemical reactions:. Application of valence-bond techniques to the Li + HF → LiF + H reaction

Abstract: Ab initio multi-structure valence-bond calculations have been performed to determine the potential energy surface governing the reaction Li + HF → LiF + H. Results for both linear and non-linear nuclear geometries are presented. The system is a prototype for many heavier alkali metal plus hydrogen halide reactions which have been studied using the crossed molecular beams technique. The ab initio valence-bond results are improved by applying corrections, within the framework of the orthogonalized Moffitt (OM) method, for the atomic errors present. The orbital basis set used was of double zeta quality, and was augmented by some extra orbitals. Preliminary calculations on the neutral and ionic diatomic species were performed to ensure the adequancy of the valence-bond structure basis sets used and care was taken to ensure that the basis sets provided an adequate description of F–, HF– and LiF–. The endoergicity of the reaction, ignoring the zero-point vibrational energies, was predicted by the ab initio and OM methods to be 5.8 and 2.5 kcal mol–1 respectively, as compared with the experimental value of 2.6 kcal mol–1. Besides the ground state potential energy surface, several surfaces for excited electronic states have been calculated and are presented. The relationship of the ground state potential energy surface to the reactive cross section, and its variation with energy, is discussed. The ground and excited state potential energy surfaces are compared with previously proposed models and a mechanism for the production of alkali metal ions in hyperthermal alkali metal atom–hydrogen halide collisions is proposed.

Determining anisotropic intermolecular potentials for van der Waals molecules

Abstract: The effects of model-dependence and statistical correlation upon the determination of a potential energy surface from the spectroscopic data for isotopic H2–Ar van der Waals complexes, are critically examined. The new potential thus obtained for this system has the correct theoretical long-range behaviour, and yields predicted differential scattering cross-sections and orbiting resonance energies in good accord with experiment. It is also shown that the secular equation method previously applied to H2+ inert gas complexes may also be successfully used to calculate the properties of much more strongly anisotropic species, such as HCl–Ar.

Electron spin resonance studies of electron attachment to fluorocarbons and related compounds

Abstract: Radical anions of three perfluorocycloalkanes and several halogenofluoromethanes have been detected and identified by e.s.r. studies following γ irradiation at 77 K of solid solutions containing up to 5 mol % of the parent compound in neopentane or tetramethylsilane. The isotropic e.s.r. spectra of c-C3F–6, c-C4F–8 and c-C5F–10 are photobleached by visible light and show the second-order structure characteristic of 6, 8 and 10 equivalent fluorines, respectively, the total 19F coupling being approximately the same value (1170 ± 20 G) in each case. Identical e.s.r. spectra were generated in photoionization experiments using tetramethyl-p-phenylenediamine, confirming the radical anion identifications. The equivalence of the fluorines indicates that the unpaired electron is delocalized over the entire molecular framework in an orbital of high symmetry. The e.s.r. spectra of the CF3X– radical anions (X = Cl, Br, I) were anisotropic and showed clear evidence for axially symmetric hyperfine interactions with three equivalent fluorines and the unique halogen. On this basis, a matrix diagonalization program was used to calculate the line positions and the best-fit e.s.r. parameters obtained. Confirmation of the CF3X– identifications was achieved by parallel photoionization experiments and by studies showing that the decay of the CF3X– spectrum in neopentane above 100 K was accompanied by a growth in the spectrum of the CF3 radical. The spin density distributions calculated from the e.s.r. parameters of these congeneric radical anions suggest that the unpaired electron resides in an a1(σ*) antibonding orbital which is composed largely of the p orbitals from carbon and the unique halogen which lie along the C3v symmetry axis of the radical anion. Consistent with this proposal, the spin densities in the s and p oribtials of the unique halogen increase along the series Cl, Br, I, which is the order expected for the effect of decreasing halogen electronegativity.

Experimental determination of average and equilibrium structures of polyatomic gas molecules by diffraction and spectroscopic methods

Abstract: Various definitions and interrelations of the “geometric structures” of polyatomic molecules are reviewed. Analytical procedures for determining well-defined “structures” are discussed. The variations in average nuclear positions caused by isotopic substitution and their influence on the structure are estimated for OCS, HCN, SO2 and H2O as examples.

Potential energy surface for bond exchange among three hydrogen molecules

Abstract: Ab initio electronic structure calculations show that a termolecular, six-centre reaction path for bond exchange among hydrogen molecules is energetically accessible, whereas there appears to be no bimolecular, four-centre path. Our best SCF calculation for hexagonal H6 finds the minimum potential energy lies about 39 kcal mol–1 below the 2H2+ 2H asymptote, or about 69 kcal mol–1 above the 3H2 asymptote. The optimum distance between neighbouring hydrogen atoms is only 0.99 A. This calculation employs ten orbitals (two s, three p, five d functions) on each hydrogen atom, with configuration interaction including all single and double excitations and an approximate correction for quadruple excitations. Calculations using less extensive SCF basis sets are also reported which give the zero point energy for the eleven nondissociative vibrational modes of hexagonal H6 and the potential energy profile along the reaction coordinate for the H6→ 3H2 dissociation. Other geometrical configurations of H6 are treated using the simple “diatomics-in-molecules” approximation. This gives good results for the hexagonal isomer (∼16 kcal mol–1 above our best ab initio energy) and indicates that only hexagonal H6 lies low enough to serve as a transition state for the six-centre reaction.Comparisons with approximate ab initio results for larger Hn polygons give an improved estimate for the cohesive energy of metallic hydrogen. It is also shown that H6 is the only H4m+ 2 system for which a concerted bond exchange reaction can occur. This illustrates the need to supplement orbital symmetry correlations with energetic criteria.

Ionic interactions in transport processes as described by irreversible thermodynamics

Abstract: The application of irreversible thermodynamics to transport in electrolyte solutions is reviewed. The linear “Onsager” coefficients lij should provide a general and more fundamental insight into ion–ion and ion-solvent interactions than special case coefficients like the conductance. The implications of non-zero lij, i≠j, are discussed. Selected lij values for binary and ternary systems are presented, including new results for CuSO4. Estimation methods for lij of ternary and higher order systems are reviewed.

Semi-classical methods for vibrational energy levels of triatomic molecules

Abstract: The method recently proposed by Chapman, Garrett and Miller for semi-classical eigenvalues of non-separable systems has been applied to the triatomics SO2 and H2O. The Hamiltonian is expressed in normal coordinates, using potentials V=V(Δr1, Δr2, Δθ). The energy levels are compared with corresponding quantum mechanical energy levels. For SO2, the fundamental frequencies differ by at most 0.1 cm–1, and for H2O they differ by at most 1.6 cm–1.

Crossed beam studies of endoergic bimolecular reactions: production of stable trihalogen radicals

Abstract: The trihalogens IIF and ClIF and the pseudo-trihalogen HIF have been directly observed as the products of the endoergic bimolecular reactions of F2 with I2, ICl and HI in a crossed molecular beam experiment. At high collision energies a second reactive channel producing IF becomes important. Product angular and velocity distributions show that this IF does not result from a four-centre exchange reaction. Observed threshold energies for the formation of IIF, ClIF and HIF yield lower bounds on the stability of these molecules (with respect to the separated atoms of 69, 81 and 96 kcal mol–1, respectively). Analysis of the product centre-of-mass angular distributions indicate that a slightly non-linear approach is most effective in bringing about reaction to form the stable triatomic radical. These studies reveal a potentially important mechanism for the F2+ I2→ 2IF bulk gas phase reaction.

Free radical oxidation of organic sulphur compounds in aqueous solution

Abstract: Complexed radical cations are formed as transient products in the one-electron oxidation of organic sulphides by hydroxyl radicals. Intermolecular complexes, (R2S ∴ SR2)+, are characterized by a 3-electron bond between sulphur atoms of two different molecules, absorption maxima around 500 nm and an equilibrium (R2S)2+.⇌ R2S+.+ R2S. In addition, intramolecular complex formation yielding [graphic omitted] is observed in the oxidation of 1,4-di, 1,3-di, and 1,3,5-tri-thiane. In these complexes a 3-electron-through-space-bond between two sulphur atoms of the same molecule is established. Absorption maxima are red shifted to ≥600 nm. Stabilization of an oxidized sulphur atom can also be effected by hetero-atoms other than sulphur. R2S ∴ Br(Cl) are formed in the reaction of either R2S with Br2[graphic omitted](Cl2[graphic omitted]) or the oxidized sulphide with Br–(Cl–). Equilibrium constants for the complexes, spectral and kinetic data on the neutral radicals and radical cations are reported and discussed.

Studies of the reactions of hydrogen atoms by time-resolved e.s.r. spectroscopy

Abstract: Time-resolved e.s.r. spectroscopy has been used to follow directly the reactions of H atoms produced by pulse radiolysis of acid solutions. Detailed analysis of the time profile of the e.s.r. signal was carried out by means of modified Bloch equations. The increased signal found when a scavenger for OH such as t-butyl alcohol is present is shown to be mainly the result of slower H atom decay by radical–radical reaction. The reaction H + OH does not appear to produce any signal polarization. The decay curves observed in the presence of solute are readily accounted for by the treatment, and good plots of pseudo first-order rate constant against solute concentration are obtained. The absolute rate constants for reaction with H atoms are for methanol 2.5 × 106, for ethanol 2.1 × 107, for isopropanol 6.8 × 107, and for succinic acid 3.0 × 106 dm3 mol–1 s–1. These values are in good agreement with earlier chemical measurements.

Studies of the mechanisms of some organic reactions and photoreactions by semiempirical SCF MO methods

Abstract: Applications of MINDO/3 and MNDDO/2 (an analogous version of NDDO) over the last two years are summarized. (a) Calculations for 185 reactions have led to certain conclusions concerning the requirements that must be satisfied if theoretical calculations of reaction mechanisms are to be significant. Examples of the unfortunate effects of failure in these respects are cited. (b) MINDO/3 calculations of molecular vibration frequencies and isotopic frequency shifts, entropies, specific heats, entropies of activation, and kinetic isotopic effects have given results in good agreement with experiment. (c) Calculations for π cycloaddition reactions confirm our earlier conclusion that such reactions are two-step processes. (d) A number of ene reactions have been found to be synchronous. (e) The mechanisms of several reactions involving 1,2 elimination of hydrogen from positive ions have been studied. (f) Detailed calculations of the C7H+7 and C7H+8 potential surfaces gave results in good agreement with mass spectral evidence. (g) SN2 reactions, carbonyl addition reactions, and reactions of carboxylic acid derivatives, with anionic nucleophiles, are predicted to require no activation in the gas phase. (h) A study of the triplet di-π-methane rearrangement of 1,5-hexadiene has led to an interpretation of the reaction that agrees well with experiment.

Potential energy surfaces for ion-molecule reactions. Intersection of the 3 A 2 and 2 B 1 surfaces of NH + 2

Abstract: The N++ H2 system is one of the few ion-molecule reactions for which detailed molecular beam studies have been carried out. To complement this experimental research, we have performed a theoretical study of two of the low-lying NH+2 potential energy surfaces. The intersection and avoided intersection (for Cs geometries) of the lowest 3A2 and 3B1 surfaces allows a pathway by which the ground state of NH+2 may be accessed without a potential barrier. The electronic structure calculations employed a double zeta plus polarization basis set, and correlation effects were taken into account using the newly developed Vector Method (VM). To test the validity of this basis, additional self-consistent-field studies were performed using a very large contracted gaussian basis N(13s 8p 3d/9s 6p 3d), H(6s 2p/4s 2p). The 3A2 surface, on which N+ and H2 may approach, has a surprisingly deep potential minimum, ∼60 kcal mol–1, occurring at re(NH)∼ 1.26 A and θe(HNH)∼ 43°. Electron correlation is responsible for about 15 kcal of this well depth, which appears fairly insensitive to extension of the basis set beyond the double zeta plus polarization level. The line of intersection (or seam) of the 3A2 and 3B1 surfaces is presented both numerically and pictorially. The minimum energy along this seam occurs at ∼51 kcal below separated N++ H2. Thus for sufficiently low energies one expects N+— H2 collisions to provide considerable “complex formation”. Further molecular beam experiments at such low energies (< 0.5 eV) would be of particular interest.

Yield and decay of the hydrated electron in proton tracks. A pulse radiolysis study

Abstract: Direct observation of the time dependence of the yield of e–aq produced in water by nanosecond pulses of 3 MeV protons is reported for the first time. From the effect of added hydroxide ion, G(e–aq) is estimated to be at least 3.6 at 10–11 s, falling to 2.3 at 10–9 s. The experimental measurements do not agree with predictions of the present diffusion model; modifications to improve the model are discussed.

Rotational spectroscopy of van der Waals molecules

Abstract: Results of molecular beam electric resonance spectroscopy on the systems Ar + HCl, Ar + HF Ar + ClF, Kr + ClF, Ar + OCS, (HF)2, HF + HCl and HF + ClF are reviewed. Some reduction, of the spectral data to intermolecular potentials is presented.

Formation of radical cations and zwitterions versus demethoxylation in the reaction of OH with a series of methoxylated benzenes and benzoic acids. An example of the electrophilic nature of the OH radical

Abstract: Using spectrophotometric and conductometric pulse radiolysis, in situ e.s.r. and product analysis techniques, the reaction of OH radicals with mono-, di- and tri-methoxylated benzenes and benzoic acids has been further studied in aqueous solution. It is found that, in addition to radical cations or zwitterions, phenoxyl radicals and methanol are formed. With the methoxylated benzenes the yield of phenoxyl radical is the same as the yield of methanol, and the sum of the yields of radical cation and of phenoxyl radical (or methanol) is ∼100% of G(OH). The ratio of the yield of radical cation to the yield of phenoxyl radical depends on the number and on the positions of the methoxyl groups relative to each other. The yields of phenoxyl radical are low when the methoxyl groups are meta to one another, whereas the yields are high for substrates with methoxyl groups in an ortho or para relation. A similar dependence on substrate structure of the distribution of products, i.e., radical zwitterions, phenoxyl radicals and methanol, is found for methoxylated benzoic acids.The results are quantitatively interpreted in terms of electrophilic addition of OH to the aromatic ring, the positions of attachment being governed by the ortho-para directing effect of the electron-donating methoxyl groups. H-abstraction from the methyl groups is negligible. The radical cations or zwitterions are derived from OH adducts formed by addition of OH to ring positions not occupied by methoxyl groups, whereas the phenoxyl radicals are produced by elimination of methanol (k∼ 4 × 104 s–1 at pH 7) from OH adducts formed by attachment of OH to ring carbons carrying methoxyl groups.

Contribution of non coulombic forces to ion-pair formation in some non-aqueous polar solvents

Abstract: Ion-pair formation of alkali and tetra-alkylammonium salts in propanol, acetonitrile and propylene carbonate has been investigated with the help of precise conductivity measurements in dilute solutions (10–4 < c < 10–2) in the temperature range from –45 to +25°C.1 The temperature dependence of the association constant, KA, yields the thermodynamic data ΔG0A, ΔH0A and ΔS0A which are divided into a coulombic ion–ion and a residual term. Contributions from ion-solvent interaction depend on the nature of the solvent and the electrolyte compound. The distances of closest approach of the ions in the ion-pairs are compared with crystallographic radii (alkali salts) or radii deduced from the ionic size (tetra-alkylammonium salts).

Quantum chemical calculation of intermolecular interaction potentials, mainly of van-der-Waals type

Abstract: After a discussion of the differences between chemical and intermolecular interaction and a discussion of the asymptotic 1/R expansion from the viewpoint of new theoretical work, the calculation of van der Waals constants based on natural states of the interacting systems is outlined. For finite distances the problems that arise with perturbational and variational approaches are discussed. The appropriate choice of the variational ansatz is derived with particular attention to the coupling of intra- and inter-subsystem correlation effects. Finally some practical calculations are reviewed.

Application of irreversible thermodynamics to transport in self-complexing electrolytes

Abstract: The study of transport in self-complexed aqueous electrolytes, particularly those of the metal halides, has been pursued since the middle of last century. In 1859 Hittorf observed negative transport numbers for cadmium in concentrated solutions of cadmium chloride and cadmium iodide and for zinc in concentrated solutions of zinc chloride and zinc iodide. These results have been verified by subsequent work. Arrhenius, in 1902, noted the requirement of negative complexes of cadmium to explain such effects and expected a number of such to exist in solution. McBain, surveying the effects of complexes and hydrolysis in 1907, concluded that the concentrations of simple uncomplexed ions would be negligibly small when such anomalous transport phenomena were observed. The effect of complexing is less dramatic for electrical conductance and diffusivity of these salts, but both are lower than would be expected if the salts were dissociated.This paper is concerned with the more quantitative explanation and prediction of such data, using as a basis the theory of irreversible thermodynamics. Any predictive treatment will require a knowledge of the concentrations of free ions and complexes in solution, together with their corresponding mobilities. The theory of irreversible thermodynamics requires in addition that kinetic coupling interactions between these species be recognised. The Onsager mobility coefficients of the binary electrolyte have been determined experimentally for a number of these self-complexed salts. These are shown to be functions of the more fundamental direct mobilities of the free ions and complexes, together with their coupling coefficients, one with another, in all possible combinations.The experimentally derived binary coefficients are shown to have an anomalous concentration dependence, varying in degree according to the degree of complexation of the salt. Choosing aqueous cadmium iodide, which is the most complexed of the group IIB halides, predictive calculations have been made which reproduce the observed concentration dependencies of the binary coefficients, electrical conductance, transport number and salt diffusivity. The method employs Pikal's restatement of Onsager limiting law theory in macroscopic irreversible thermodynamic terms.Additional data on cadmium isotopic diffusion in cadmium iodide are presented. There too, anomalous behaviour is observed, particularly an initial maximum in diffusion coefficient. An extension of the theoretical method to include isotopic experiments is presented and once more, using Pikal's basic treatment these isotopic diffusion features are reproduced. In both cases the predictive capability of theory is limited by the concentration limits for Onsager theory.

X-ray diffraction and structure of NiCl2 aqueous solutions

Abstract: X-ray diffraction data for two aqueous solutions of NiCl2 are analysed. A model including only nearest-neighbour interactions does not yield theoretical structure functions in full agreement with the experimental results, but satisfactory agreement can be reached by simply adding interactions between the hydrated cation and external water molecules, without considering ion–ion interaction.Regarding ionic hydration, the mean coordination geometries of Ni2+ and Cl– ions are found to be octahedral with mean distances cation–water (2.05 – 2.06 A) and anion–water (3.13 – 3.14 A) in accordance with those already found.

Radiative transitions for molecular collisions in an intense laser field

Abstract: Quantum mechanical and semiclassical approaches are discussed for the study of molecular collisions in an intense laser field. Both a coherent state and Fock state representation of the photon field are investigated. The collision dynamics is described in terms of transitions between two electronic-field potential energy surfaces, where each surface depends on the field-free adiabatic surfaces and electric dipole transition matrix elements as functions of nuclear coordinates. The electronic-field surfaces exhibit avoided crossings (on the real axis) due to the radiative coupling at the resonance nuclear configurations, and other parts of these surfaces are similar to the field-free adiabatic surfaces with one of them shifted by ħω for single photon processes. Metastable states, formed at some collision energies, are conjectured to occur in the field, although absent from the field-free case. From a spectroscopic point of view, changes in energy spectra are expected from those of the individual collision-free species. Numerical results are presented for the collinear collision process Br(2P3/2)+ H2(v= 0)+ħω→ Br(2P1/2)+ H2(v= 0).

Spin correlation in the geminate recombination of radical ions in hydrocarbons. Part 2—Time resolved single-photon counting study of the magnetic field effect

Abstract: Pulse shapes of scintillations produced by beta-particles have been measured by time-resolved single-photon counting. Magnetic fields and deuteration of the solute enhance the fluorescence of para-terphenyl and other solutes. This is due to electron-nuclear hyperfine interaction in the radical ions which interconverts the singlet and triplet wave-functions of the ion pairs. The extent of the effect gives a measure of the relative initial yields of singlet and triplet ion pairs. The magnetic field effects very considerably among the solvents studied—n-hexane, iso-octane, cyclohexane, methyl-cyclohexane, trans-decalin, mixed decalins, squalane and benzene.

Ionic solution theory for non-ideal solvents. Potential of mean force between ions

Abstract: A comparison is made between fully microscopic models of an ionic solution in a dipolar solvent, in which the solvent as well as the solute is treated on a molecular basis, and quasimacroscopic models, in which the solvent is treated as an ideal continuum. The central objects of inquiry are the molecular potentials of mean force, especially the ion-ion potential in the limit of infinite solute dilution.

Relaxation processes in aqueous solution: a Raman spectral study of hydrogen bonded interactions

Abstract: Vibrational relaxation and rotational reorientation times for the nitrate ion in aqueous solutions of group I and group II metal nitrates have been measured at room temperature using polarised Raman spectra. The vibrational relaxation occurs by energy transfer to hydrogen-bound water molecules. In dilute solution all vibrational relaxation times approach a common value of 1.4 ps. At higher concentration there is a marked cation dependence with relaxation being more rapid in the presence of highly polarizing cations. It is shown that hydrogen bonding to only one water molecule is necessary to describe the relaxation process. The rotational reorientation times of the nitrate ion reflect constraints imposed by both hydrogen bonding to water and coulombic interaction with the cationic species. The low concentration limited values of τOR are different for different electrolytes. Also in contrast to vibrational relaxation (which shows linear dependence on cation concentration) the variation of rotational reorientation with concentration is non-linear with the curvature reflecting different contributions to the constraining field.

Mechanisms for radiation damage in DNA constituents and DNA. Reactions of the N1-substituted thymine π-cation radicals

Abstract: Reactions of the N1-substituted thymine π-cation radicals were investigated by e.s.r. in several aqueous glasses. In 8 mol dm–3 NaOD (NaOH) spectra suggestive of OD–(OH–) addition to position 6 in the π-cations of 1-methylthymine, and thymidine were found immediately after u.v. photolysis at 77 K. Production of the same radicals by electron attachment to 1-methyl-5-bromo-6-hydroxythymine and 5-bromo-6-hydroxythymidine in 8 mol dm–3 NaOD confirms the OD– addition mechanism. Results found for these brominated compounds in 12 mol dm–3 LiCl(D2O) after electron attachment show that the a6(H) splitting was sensitive to changes in substituents at position 1 as well as changes in environment. This variation in splitting is shown to be accounted for by small conformational changes in the radicals. In 8 mol dm–3 NaClO4π-cations of the substituted thymines gave evidence for both deprotonation and OD– addition.

Formation of H-addition radicals in some pyrimidine and purine derivatives

Abstract: In pyrimidines with an unsubstituted C5 = C6 bond, H-addition via an anionic stage, the “ionization path”, occurs preferentially at position C6 and direct hydrogen addition, the “excitation path”, at position C5. In adenine derivatives, the “excitation path” yields C8-addition radicals and the “ionization path” C2-addition radicals. However, when the molecule is doubly protonated, C8-addition radicals are preferentially formed by protonation of the electron adduct. In pyrimidine crystals, the C5-addition radicals transform into C6-addition radicals upon irradiation with light of λ 400 nm. In purine crystals, the C8-addition radicals can also be transformed into C2-addition radicals in this way. In “van der Waals crystals”, this transformation is reversible upon storage at room temperature. In “polar crystals”, the transformation is irreversible. This indicates that the C6-addition radicals of pyrimidines and the C2-addition radicals of adenine derivatives need a polar environment to be stabilized. INDO calculations support this conclusion. While the “ionization path” could not be detected in “van der Waals crystals” both paths have been observed in “polar crystals”. These observations bear out the view that, in nonpolar environments, ionization is followed by geminate ion recombination with eventual subsequent homolytic dissociation of atomic hydrogen.

Unusual ion solvation energies in high temperature water

Abstract: Classical hydration theory predicts a decreasing temperature stability for ions in aqueous solutions while the energy of ion-solvent interaction increases. Born theory has been modified to take into account a non-electrostatic entropy term which accounts for loss of degrees of freedom of solvating molecules, and a change in standard state volume. Such a theory predicts very large solvation energies (>105 cal mol–1 at 300°C); this new phenomenon has been verified by measurement of the heats of solution of ionic salts at 300°C in a pressurized calorimeter.