A simple variational-numerical approach to the ro-vibrational spectrum of diatomic molecules. An application to CH+

We describe a simple way of obtaining numerically the manifold of energies for ro-vibrational transitions for a centrifugally distorted oscillator, starting from the potential energy of the non-rotating oscillator calculated by an accurate ab initio method. It is shown that the energies so obtained compare well with those obtained variationally. The species of astrophysical interest methylidyne ion, CH(+), has been selected as an example that allow us to show the computational efficiency of the method with respect to the variational one. It is applied for the determination of ro-vibrational levels up J=6, and the spectroscopic parameters corresponding to the ground electronic state X(1)Sigma(+). From the potential energy surface computed at the MRCI/cc-pV5Z level, the fundamental frequency, B(0) and D(0) are determined to be 2724.8, 13.85688 and -1.53322x10(-3)cm(-1), respectively. We provide also an estimation of anharmonic constants.     

Activation of CH4 by gas-phase Mo+, and the thermochemistry of Mo-ligand complexes

The kinetic-energy dependence of the reactions of Mo(+) ((6)S) with methane has been studied using guided ion beam mass spectrometry. No exothermic reactions are observed in this system, as also found previously, but efficient dehydrogenation occurs at slightly elevated energies. At higher energies, MoH(+) dominates the product spectrum and MoC(+), MoCH(+), and MoCH(3)(+) are also observed. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energies (in eV) of D(0)(Mo(+)-C) = 4.55 +/- 0.19, D(0)(Mo(+)-CH) = 5.32 +/- 0.14, D(0)(Mo(+)-CH(2)) = 3.57 +/- 0.10, and D(0)(Mo(+)-CH(3)) = 1.57 +/- 0.09. The results for Mo(+) are compared with those for the first- and third-row transition-metal congeners, Cr(+) and W(+), and the differences in behavior and mechanism are discussed. Theoretical results are used to elucidate the geometric and electronic structures of all product ions as well as the complete potential-energy surface for reaction. The efficiency of the coupling between the sextet and quartet spin surfaces is also quantified.     

Protolytic behavior of hydroxylated Pyrex glass surfaces in NaCl media

Adsorption of hydrogen ions from aqueous NaCl solutions at the Pyrex glass-water interface was investigated by acid-base titration (glass electrode) at 25 degrees C and at the ionic strengths 0.010, 0.030, 0.10, 1.0, and 3.0 mol dm(-3). The pH values ranged from 2 to 7. The Pyrex samples had a specific surface area of 19.2x10(3) m(2)kg(-1) and a porous structure (pores 2.4 nm thick, 280 nm long). The reactions were found to be extremely slow but showed good reversibility. The potentiometric data, due to the small effect of ionic strength on the equilibria, were fitted with a simple nonelectrostatic model based on strong specific interactions of medium ions with deprotonated silanol, >SiO(-), and boranol, >BO(-), as well as with protonated sites. The acid-base properties are described by the reactions and equilibrium constants at the infinite dilution reference state: >SiONa + H(+) <==> >SiOHNa(+), logbeta110Si=3.1+/-0.2; >SiONa + 2H(+) + Cl(-) <==> >SiOH(2)Cl + Na(+), logbeta201Si=6.75+/-0.15; >SiONa + H(+) <==> >SiOH + Na(+), logbeta100Si=1.8+/-0.2, >BONa + H(+) <==>BOH + Na(+), logbeta100B=6.4+/-0.2; >BONa + H(+) <==> >BOHNa(+), logbeta110B=6.6+/-0.2; >BONa + 2H(+) <==> >BOH(+)(2) + Na(+), logbeta200B=11.56+/-0.15.     

Experimental investigation of electronic states of LiCs dissociating to Li(2(2)S) and Cs(5(2)D) atoms

Polarization labeling spectroscopy technique was used to measure excitation spectra of LiCs molecule in the spectral range of 16,000-18,500 cm(-1). Four band systems were observed and assigned to transitions from the ground X(1)Σ(+) state to excited states (4)Ω = 0(+), (5)Ω = 0(+), (5)Ω = 1, and (6)Ω = 1 (in Hund's case (c) notation proper here), the latter three states being fine structure components of the states d(3)Π and e(3)Σ(+), nominally of triplet symmetry. The observed states are characterized spectroscopically and the experimental results are compared with predictions of theoretical calculations, showing accuracy of the theoretical electronic term values better than 100 cm(-1) and of the ω(e) and R(e) constants within 5%.     

Ab inito MRSDCI study on the low-lying electronic states of the lithium chloride molecule (LiCl)

Potential energy curves (PECs) for the low-lying states of the lithium chloride molecule (LiCl) have been calculated using the internally contracted multireference single- and double-excitation configuration interaction (MRSDCI) method with the aug-cc-PVnZ (AVnZ) and aug-cc-PCVnZ (ACVnZ) basis sets, where n = T, Q, and 5. First, we calculate PECs for 7 spin-orbit (SO)-free Λ-S states, X(1)Σ(+), A(1)Σ(+), (3)Σ(+), (1)Π, and (3)Π, and then obtain PECs for 13 SO Ω states, X0(+), A0(+), B0(+), 0(-)(I), 0(-)(II), 1(I), 1(II), 1(III), and 2, by diagonalizing the matrix of the electronic Hamiltonian plus the Breit-Pauli SO Hamiltonian. The MRSDCI calculations not including core orbital correlation through the single and double excitations are also performed with the AV5Z and ACV5Z basis sets. The Davidson corrections (Q0) are added to both the Λ-S and Ω state energies. Vibrational eigenstates for the obtained X(1)Σ(+) and X0(+) PECs are calculated by solving the time-independent Schro?dinger equation with the grid method. Thus, the effects of basis set, core orbital correlation, and the Davidson correction on the X(1)Σ(+) and X0(+) PECs of LiCl are investigated by comparing the spectroscopic constants calculated from the PECs with one another and with experiment. It is confirmed that to accurately predict the spectroscopic constants we need to include core-electron correlation in the CI expansion and use the basis sets designed to describe core-valence correlation, i.e., ACVnZ. The SO PECs presented in this paper will be of help in the future study of diatomic alkali halide dynamics.     

Adduct formation between alkali metal ions and anionic LV(V)O(2)(-) (L(2-) = tridentate ONS ligands) species: syntheses, structural investigation, and photochemical studies

Syntheses of alkali metal adducts [LVO(2)M(H(2)O)(n)] (1-7) (M = Na(+), K(+), Rb(+), and Cs(+); L = L(1)(-)L(3)) of anionic cis-dioxovanadium(V) species (LVO(2)(-)) of tridentate dithiocarbazate-based Schiff base ligands H(2)L (S-methyl-3-((5-(R-2-hydroxyphenyl))methyl)dithiocarbazate, R = H, L = L(1); R = NO(2), L = L(2); R = Br, L = L(3)) have been reported. The LVO(2)(-) moieties here behave like an analogue of carboxylate group and have displayed interesting variations in their binding pattern with the change in size of the alkali metal ions as revealed in the solid state from the X-ray crystallographic analysis of 1, 3, 6, and 7. The compounds have extended chain structures, forming ion channels, and are stabilized by strong Coulombic and hydrogen-bonded interactions. The number of coordinated water molecules in [LVO(2)M(H(2)O)(n)] decreases as the charge density on the alkali metal ion decreases (n = 3.5 for Na(+) and 1 for K(+) and Rb(+), while, for Cs(+), no coordinated water molecule is present). In solution, compounds 1-7 are stable in water and methanol, while in aprotic solvents of higher donor strengths, viz. CH(3)CN, DMF and DMSO, they undergo photoinduced reduction when exposed to visible light, yielding green solutions from their initial yellow color. The putative product is a mixed-oxidation (mu-oxo)divanadium(IV/V) species as revealed from EPR, electronic spectroscopy, dynamic (1)H NMR, and redox studies.     

Cation-specific interactions with carboxylate in amino acid and acetate aqueous solutions: X-ray absorption and ab initio calculations

Relative interaction strengths between cations (X = Li (+), Na (+), K (+), NH 4 (+)) and anionic carboxylate groups of acetate and glycine in aqueous solution are determined. These model systems mimic ion pairing of biologically relevant cations with negatively charged groups at protein surfaces. With oxygen 1s X-ray absorption spectroscopy, we can distinguish between spectral contributions from H 2O and carboxylate, which allows us to probe the electronic structure changes of the atomic site of the carboxylate group being closest to the countercation. From the intensity variations of the COO (-) aq O 1s X-ray absorption peak, which quantitatively correlate with the change in the local partial density of states from the carboxylic site, interactions are found to decrease in the sequence Na (+) > Li (+) > K (+) > NH 4 (+). This ordering, as well as the observed bidental nature of the -COO (-) aq and X (+) aq interaction, is supported by combined ab initio and molecular dynamics calculations.     

Oscillator strength and linewidth measurements of dipole-allowed transitions in 14N2 between 93.5 and 99.5 nm

Line oscillator strengths in 16 electric dipole-allowed bands of 14N2 in the 93.5-99.5 nm (106,950-100,500 cm(-1)) region have been measured at an instrumental resolution of 6.5 x 10(-4) nm (0.7 cm(-1)). The transitions terminate on vibrational levels of the 3psigma 1Sigma u (+), 3ppi 1Pi u, and 3ssigma 1Pi u Rydberg states and of the b' 1Sigma u (+) and b 1Pi u valence states. The J dependences of band f values derived from the experimental line f values are reported as polynomials in J'(J'+1) and are extrapolated to J'=0 in order to facilitate comparisons with results of coupled-Schrodinger-equation calculations that do not take into account rotational interactions. Most bands in this study reveal a marked J dependence of the f values and/or display anomalous P-, Q- and R-branch intensity patterns. These patterns should help inform future spectroscopic models that incorporate rotational effects, and these are critical for the construction of realistic atmospheric radiative transfer models. Linewidth measurements are reported for four bands. Information provided by the J dependences of the experimental linewidths should be of use in the development of a more complete understanding of the predissociation mechanisms in N2.     

Theoretical study of the electronic structure of MCH(2)(+)(M=Fe,Co,Ni)

State of the art coupled cluster (CC) methods are applied to accurately characterize the ground state electronic structure and photoelectron spectra of transition metal carbene ions MCH(2) (+) (M=Fe, Co, and Ni). The geometries and energies of the lowest energy quartet, triplet, and doublet electronic states as well as several low-lying vertical excitation energies of FeCH(2) (+), CoCH(2) (+), and NiCH(2) (+) are reported. The excitation energies are computed using the equation-of-motion CC and for states of different symmetries, by the energy differences of single reference ground and excited states (Delta-CC). The latter employ several reference states; the unrestricted Hartree-Fock, restricted open shell Hartree-Fock, and unrestricted Kohn-Sham. We conclude that the (2)A(1) electronic ground state of NiCH(2) (+) is separated by about 30.0 kJ/mol from the next highest state, and the lowest (4)B(1) and (4)B(2) states of FeCH(2) (+) as well as the (3)A(2) and (3)A(1) states of CoCH(2) (+) are nearly degenerate. The presence of metal-pi*(MCH(2)) charge transfer states with significant oscillator strengths in the visible/near-UV energy domain of the theoretical spectra of FeCH(2) (+) and CoCH(2) (+) are at the origin of the photofragmentation of these compounds observed after irradiation between 310 and 360 nm.     

Dependence of positive binding energies on side chains--a theoretical prediction on the origin of regular ordering for the amino acid residues in the selectivity filter

The side-chain effects of metalated and protonated dipeptides, including GGH(+)M(+), GAH(+)M(+), AGH(+)M(+), AAH(+)M(+), GWH(+)M(+), GSH(+)M(+), GTH(+)M(+), GFH(+)M(+), GYH(+)M(+), and GVH(+)M(+) (G = glycine, A = alanine, W = tryptophan, S = serine, T = threonine, F = phenylalanine, and V = valine; M = Li, Na, and K), are theoretically explored in this paper on their positive binding energies (PBEs), which are derived from interactions of M+ with the carboxyl oxygen(s). The B3LYP/6-311++G(**)// B3LYP/6-31G(*) calculations suggest that the PBEs of dipeptides with side chain(s) are much smaller than those with no side chain (GGH(+)M(+)). Generally, larger side chains and smaller M(+) radii would lead to fewer PBEs for the M(+) involved systems. On the basis of the direct dependence of PBE on the electrostatic repulsion between two kinds of cations (H(+) and M(+)) in these dipeptide models, it could be reasonably expected that the side-chain effect on the electrostatic repulsion and consequently on the PBEs could offer one good insight, on a chemical-physical basis, into the origin of regular ordering of the amino acids when they form a filter in the K(+) channel protein (MacKinnon, et al. Science 1998, 280, 106).     

Laser desorption/ionization and laser ablation synthesis of new selenium oxide compounds from selenium(IV) dioxide

Laser desorption/ionization (LDI) and/or laser ablation (LA) of selenium dioxide crystals or its mixtures with sodium peroxide were studied using a commercial matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometer. It was found that LDI and LA of selenium (IV) dioxide not only ionizes SeO(2), but also leads to the formation of several positively and negatively singly charged species: SeO(n) (+) (n = 0-2), Se(2) (+), SeO(n) (-) (n = 0-4), Se(2)O(n) (-) (n = 3-7), Se(3)O(n) (-) (n = 4-9), Se(4)O(n) (-) (n = 8-10). A rather high yield of selenium species in the positive ion mode, Se(m) (+) (m = 1-8) and Se(m)OH(+) (m = 3-7), was obtained by using the MALDI approach while the species detected in the negative ion mode, SeO(n) (-) (n = 0-4), Se(2)O(n) (-) (n = 3-7), Se(3)O(n) (-) (n = 4-9), and Se(4)O(n) (-) (n = 9, 10), were the same as those observed during LDI/LA of selenium dioxide. The addition of sodium peroxide to selenium dioxide with the aim of enhancing its oxidation and thus increasing the production of SeO(4) product resulted in extensive cationization of the species with sodium or potassium. The following positively and negatively charged species were identified: Se(+), Se(2) (+), Se(2)OH(+), Se(2)ONa(+), SeO(n) (-) (n = 0-3), and Se(2)O(n) (-) (n = 0, 1, 4). Also observed in mass spectra of such mixtures, various mixed sodium and/or potassium adducts with selenium oxide species, e.g. Se(2)O(4)K(2)Na(-), were identified. In all, 26 totally new species, Se(2)O(n) (-) (n = 3-6), Se(3)O(n) (-) (n = 4-9), Se(4)O(n) (-) (n = 8-10), Se(4)O(11)H(5) (-), Se(4)O(12)H(3) (-), Se(2)O(4)Na(-), Se(2)O(5)HNa(-), Se(2)O(5)HNa(2) (-), Se(3)O(6)K(2)Na(-), Se(3)O(6)K(2)Na(2) (-), Se(2)ONa(+), and Se(m)OH(+) (m = 3-7), were described for the first time. Also, for the first time, the formation of selenium(IV) diperoxide, O-O-Se-O-O or O(2)SeO(2), is described. The stoichiometries of the compounds generated were confirmed using isotopic pattern modeling.     

Influence of d orbital occupation on the binding of metal ions to adenine

Threshold collision-induced dissociation of M(+)(adenine) with xenon is studied using guided ion beam mass spectrometry. M(+) includes all 10 first-row transition metal ions: Sc(+), Ti(+), V(+), Cr(+), Mn(+), Fe(+), Co(+), Ni(+), Cu(+), and Zn(+). For the systems involving the late metal ions, Cr(+) through Cu(+), the primary product corresponds to endothermic loss of the intact adenine molecule, whereas for Zn(+), this process occurs but to form Zn + adenine(+). For the complexes to the early metal ions, Sc(+), Ti(+), and V(+), intact ligand loss competes with endothermic elimination of purine and of HCN to form MNH(+) and M(+)(C(4)H(4)N(4)), respectively, as the primary ionic products. For Sc(+), loss of ammonia is also a prominent process at low energies. Several minor channels corresponding to formation of M(+)(C(x)H(x)N(x)), x = 1-3, are also observed for these three systems at elevated energies. The energy-dependent collision-induced dissociation cross sections for M(+)(adenine), where M(+) = V(+) through Zn(+), are modeled to yield thresholds that are directly related to 0 and 298 K bond dissociation energies for M(+)-adenine after accounting for the effects of multiple ion-molecule collisions, kinetic and internal energy distributions of the reactants, and dissociation lifetimes. The measured bond energies are compared to those previously studied for simple nitrogen donor ligands, NH(3) and pyrimidine, and to results for alkali metal cations bound to adenine. Trends in these results and theoretical calculations on Cu(+)(adenine) suggest distinct differences in the binding site propensities of adenine to the alkali vs transition metal ions, a consequence of s-dsigma hybridization on the latter.     

Multiphoton processes of CO at 230 nm

High resolution kinetic energy release spectra were obtained for C(+) and O(+) from CO multiphoton ionization followed by dissociation of CO(+). The excitation was through the CO (B (1)Sigma(+)) state via resonant two-photon excitation around 230 nm. A total of 5 and 6 photons are found to contribute to the production of carbon and oxygen cations. DC slice and Megapixel ion imaging techniques were used to acquire high quality images. Major features in both O(+) and C(+) spectra are assigned to the dissociation of some specific vibrational levels of CO(+)(X (2)Sigma(+)). The angular distributions of C(+) and O(+) are very distinct and those of various features of C(+) are also different. A dramatic change of the angular distribution of C(+) from dissociation of CO(+)(X (2)Sigma(+), nu(+) = 1) is attributed to an accidental one-photon resonance between CO(+)(X (2)Sigma(+), nu(+) = 1) and CO(+)(B (2)Sigma(+), nu(+) = 0) and explained well by a theoretical model. Both kinetic energy release and angular distributions were used to reveal the underlying dynamics.     

Ion-pair formation between Cd(II), Na(I), and Ag(I) complex ions with 18-crown-6 ether derivatives and various pairing anions in water: an understanding of the ion-pair formation based on the HSAB principle

The ion-pair formation constants (K(MLX)(0)/mol(-1) dm(3)) of CdL(2+) with Br(-) or NaL(+) with N,N-diethyldithiocarbamate ion (DDTC(-)) in water were determined potentiometrically at 25°C; ionic strength (I)→0: L denotes 18-crown-6 ether (18C6) and its mono-benzo derivative for the CdBr(2)-L system and 15-crown-5 ether and 18C6 for the NaDDTC-L one. The formation constant corresponding to the simple salt, NaDDTC, in water was also determined at I→0. Using the log K(CdLX)(0) values of CdLCl(+), CdLBr(+), CdLPic(+), and CdLSO(4), then CdL(2+) and picrate ion (Pic(-)) in water have been classified with the hard and soft acids and bases principle, where the values were available in the literature, except for CdLBr(+). The same classification was examined in NaX-L systems with X(-) = DDTC(-), trifluoroacetate ion, MnO(4)(-), ReO(4)(-), Pic(-), and BPh(4)(-) and the AgPic-L one. Consequently, CdL(2+), NaL(+), and AgL(+) were classified as the hard acids, while Pic(-) and BPh(4)(-) as the hard bases. These results reflected the reactivities of the complex ions in ion-pair formation with X(-) and SO(4)(2-) in water.     

Scavenging effects of metal complexes of water-soluble thiacalix[4]arenetetrasulfonate on superoxide anion radicals

The scavenging effects of metal complexes of thiacalix[4]arenetetrasulfonate (Me-TCAS[4], Me=H?, Fe3(+), Mn3(+), Mn2(+), Cu2(+), and Zn2(+)) on superoxide anion radicals (O??) generated from the xanthine-xanthine oxidase system were investigated by the nitroblue tetrazolium (NBT) method and electron spin resonance (ESR) spin-trapping method using 5,5-dimethyl-1-pyrroline-N-oxide as a trapping reagent. As a reference, calix[4]arenetetrasulfonate (H?)-CAS[4]), calix[6]arenehexasulfonate (H?-CAS[6]) and calix[8]areneoctasulfonate (H?-CAS[8]) were also examined. The results by the NBT method indicated that Fe3(+)- and Mn3(+)-TCAS[4] exhibited the highest O?? scavenging activity among Me-TCAS[4] and H?-CAS[n] (n = 4, 6, 8) in this study. The IC?? values of Fe3(+)- and Mn3(+)-TCAS[4] for O?? scavenging activity were estimated to be 5.3 and 7.8 μM, respectively, and were almost the same as those of tannin acid, catechin and their derivatives, which are known as very effective scavengers of O??. Scavenging activities were in the order of Fe3(+)- and Mn3(+)-TCAS[4]>Mn2(+)-, Cu2(+)-, and Zn(2+)-TCAS[4]>H(2)-TCAS[4] and H?-CAS[n] (n=4, 6, 8). Each activity of Me-TCAS[4] (Me=Fe3(+), Mn3(+), Mn2(+), Cu2(+), and Zn2(+)) was higher than that of the corresponding metal ion, indicating that H?-TCAS[4] has the ability to raise the activity of the metal ion itself by forming a complex. Also, the ESR spin-trapping method revealed that Fe3(+)- and Mn3(+)-TCAS[4] showed high O?? scavenging activities, similarly to the results by the NBT method.     

Bond dissociation energies and structures of CuNO(+) and Cu(NO)(2)(+)

The bond dissociation energies of CuNO(+), Cu(NO)(2)(+), and CuAr(+) are determined by means of guided ion beam mass spectrometry and quantum chemical calculations. From the experiment, the values D(0)(Cu(+)-NO) = 1.13 +/- 0.05, D(0)(ONCu(+)-NO) = 1.12 +/- 0.06, D(0)(Cu(+)-Ar) = 0.50 +/- 0.07, and D(0)(Cu(+)-Xe) = 1.02 +/- 0.06 eV are obtained. The computational approaches corroborate these results and provide additional structural data. The relative values of D(0)(Cu(+)-NO) and D(0)(Cu(+)-Xe) are consistent with the approximately thermoneutral formation of CuXe(+) upon interacting CuNO(+) with xenon. The sequential bond dissociation energies of Cu(NO)(2)(+) exhibit a trend similar to those of other Cu(I) complexes described in the literature. Although metathesis of nitric oxide to N(2) and O(2) is of considerable interest, no evidence for N-N- or O-O-bond formations in Cu(NO)(n)(+) ions (with n up to 3) is obtained within the energy range studied experimentally.     

Gas-phase reactions of carbon dioxide with atomic transition-metal and main-group cations: room-temperature kinetics and periodicities in reactivity

The chemistry of carbon dioxide has been surveyed systematically with 46 atomic cations at room temperature using an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. The atomic cations were produced at ca. 5500 K in an ICP source and allowed to cool radiatively and to thermalize by collisions with Ar and He atoms prior to reaction downstream in a flow tube in helium buffer gas at 0.35 +/- 0.01 Torr and 295 +/- 2 K. Rate coefficients and products were measured for the reactions of first-row atomic ions from K(+) to Se(+), of second-row atomic ions from Rb(+) to Te(+) (excluding Tc(+)), and of third-row atomic ions from Cs(+) to Bi(+). CO(2) was found to react in a bimolecular fashion by O atom transfer only with 9 early transition-metal cations: the group 3 cations Sc(+), Y(+), and La(+), the group 4 cations Ti(+), Zr(+), and Hf(+), the group 5 cations Nb(+) and Ta(+), and the group 6 cation W(+). Electron spin conservation was observed to control the kinetics of O atom transfer. Addition of CO(2) was observed for the remaining 37 cations. While the rate of addition was not measurable some insight was obtained into the standard free energy change, DeltaG(o), for CO(2) ligation from equilibrium constant measurements. A periodic variation in DeltaG(o) was observed for first row cations that is consistent with previous calculations of bond energies D(0)(M(+)-CO(2)). The observed trends in D(0) and DeltaG(o) are expected from the variation in electrostatic attraction between M(+) and CO(2) which follows the trend in atomic-ion size and the trend in repulsion between the orbitals of the atomic cations and the occupied orbitals of CO(2). Higher-order CO(2) cluster ions with up to four CO(2) ligands also were observed for 24 of the atomic cations while MO(2)(+) dioxide formation by sequential O atom transfer was seen only with Hf(+), Nb(+), Ta(+), and W(+).     

Dramatic effect of the metal cation in dealkylation reactions of phosphinic esters promoted by complexes of polyether ligands with metal iodides

Metal ion electrophilic catalysis has been revealed in dealkylation reactions of phosphinic esters 1-4 promoted by complexes of polyether ligands 5-7 with metal iodides MI(n) (M[n+] = Li(+), Na(+), K(+), Rb(+), Ca(2+), Sr(2+), Ba(2+)) in low polarity solvents (chlorobenzene, 1,2-dichlorobenzene, and toluene) at 60 degrees C. The catalytic effect increases with increasing the Lewis acid character of the cation, in the order Rb(+)< K(+)< Na(+)< Li(+) and Ba(2+)< Sr(2+)< Ca(2+). The results are interpreted in terms of a transition state where the complexed cation (M[n+] subset Lig) assists the departure of the leaving group Ph(2)P(O)O(-) and, at the same time, favors the attack at carbon of the nucleophile I(-) ("push-pull" mechanism). The rate sequence found for 1-4 (Me > Et > i-Pr and t-Bu) shows that this reaction can be utilized for the selective dealkylation of these substrates.     

Hydrated electron extinction coefficient revisited

The extinction coefficient of the hydrated electron (e(-))aq generated by pulse radiolysis is evaluated relative to the methyl viologen radical cation (*)MV(+), whose extinction coefficient at 605 nm has been carefully measured in the past. We find that the room temperature (e(-))aq extinction coefficients reported in the literature are underestimated by 10-20%. We obtain = 22,700 M(-1) cm(-1) for the 20 degrees C hydrated electron at 720 nm, assuming the (*)MV(+) extinction is 13,700 M(-1) cm(-1) at 605 nm. This has implications both for second-order reaction rate measurements of (e(-))aq and for the estimate of its integrated oscillator strength.