Excited-state proton transfer in N-methyl-6-hydroxyquinolinium salts: solvent and temperature effects

The excited-state proton transfer (ESPT) reaction of the "super"photoacid N-methyl-6-hydroxyquinolinium (MHQ) was studied using both fluorescence upconversion and time-correlated single photon counting (TCSPC) techniques. The ultrafast ESPT kinetics were investigated in various alcohols and water and determined to be solvent-controlled. The ESPT temperature dependence of MHQ was also studied in various alcohols and compared to that observed for another "super"photoacid, 5,8-dicyano-2-naphthol (DCN2). A full set of kinetic and thermodynamic parameters describing the ESPT was obtained. The protolytic photodissociation rate constant for MHQ was higher than that for DCN2, while the ESPT activation energies of MHQ were smaller. These findings are attributed to the approximately 3 orders of magnitude differences in excited-state acidities of MHQ and DCN2.     

Ultrafast proton transfer of three novel quinone cyanine photoacids

Steady-state and time-resolved emission techniques were used to study the photoprotolytic properties of three recently synthesized strong quinone cyanine photoacids (QCy7 and its sulfonated derivatives). The rate coefficient of the excited-state proton transfer (ESPT), k(PT), of the three dyes is roughly 1.5 × 10(12) s(-1), a high value that is comparable to the solvation dynamics rate of large polar organic molecules in H(2)O and D(2)O. It is twice as fast as the proton transfer rate between two adjacent water molecules in liquid water. We found that, as expected, two of the sulfonated photoacids geminately recombines with the proton at an elevated rate. The accelerated geminate recombination process of the sulfonated derivatives is different from a simple diffusion process of protons. The ESPT rate coefficient of these molecules is the highest recorded thus far.     

Charge redistribution and photoacidity: neutral versus cationic photoacids

A series of pyrene photoacids is used to investigate excited-state proton transfer with time-dependent pump-probe spectroscopy. The deprotonation dynamics of a cationic photoacid, 8-aminopyrene-1,3,6-trisulfonic acid trisodium salt (APTS), shows single exponential dynamics( approximately 30 ps) in water. This is in contrast to what is observed for the neutral photoacids 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) and 8-hydroxy-N,N,N',N',N",N"-hexamethylpyrene-1,3,6-trisulfonamide, which display biexponential dynamics. For the cationic photoacid, the vast majority of the intramolecular charge redistribution does not occur in the protonated state. Instead, the charge redistribution, which is responsible for the photoacidity and the observed spectroscopic changes, occurs primarily following the excited-state proton transfer. The lack of charge redistribution prior to proton transfer causes APTS to display single exponential kinetics. In contrast, the dynamics for the neutral photoacids are multiexponential because major charge redistribution precedes proton transfer followed by additional charge redistribution that accompanies proton transfer. Previous studies of HPTS in water are discussed in terms of the results presented here.     

Temperature dependence of excited state proton transfer in ice

We have studied the excited-state proton-transfer rate of four photoacids in ice as a function of temperature. For all four photoacids, we have found a non Arrhenius behavior of the proton-transfer rate constant, k(PT). d(ln k(PT))/d(1/T) decreases as the temperature decreases. The average slope of ln(k(PT))versus 1/T depends on the photoacid strength (pK*). The stronger the photoacid is, the smaller the slope. For the strongest photoacid 2-naphthol-6,8-disulfonate (2N68DS) the largest slope is 35 kJ/mol at about 270 K, and the smallest measured slope is about 8 kJ/mol at about 215 K. We propose that the temperature dependence of k(PT) in ice at the temperature range 270 > T > 200 K can be explained as arising from contributions of two proton-transfer mechanisms over the barrier and tunneling under the barrier. At very low temperatures T < 200 K, the slope of ln(k(PT)) versus 1/T increases again. At about 170 K, the proton-transfer rate is much slower than the radiative rate, and the deprotonated form of the photoacid cannot be detected in the steady-state emission spectrum. At lower temperatures, T < 200 K, the rate further decreases because of a limitation on the reaction caused by the restrictions on the H2O hydrogen reorientations.     

Photoacid-base reaction in ice via a mobile L-defect

A time-resolved emission technique was employed to study the photoprotolytic cycle of two photoacids 2-naphtol-6-sulfonate (2N6S) and 2-naphtol-6,8-disulfonate (2N68DS) in ice in the presence of a low concentration of a weak base fluoride ion. We found that an additional proton-transfer process occurs in ice doped with F- ions. This reaction takes place between a mobile L-defect (created by static F- ions) and the photoacid. We used a diffusion assisted reaction model, based on the Debye-Smoluchowski equation, to account for the direct reaction of the L-defect with the excited photoacid.     

Ultrafast excited-state intermolecular proton transfer of cyanine fluorochrome dyes

Steady-state and time-resolved emission spectroscopy techniques were employed to study the excited-state proton transfer (ESPT) to water and D(2)O from QCy7, a recently synthesized near-infrared (NIR)-emissive dye with a fluorescence band maximum at 700 nm. We found that the ESPT rate constant, k(PT), of QCy7 excited from its protonated form, ROH, is ~1.5 × 10(12) s(-1). This is the highest ever reported value in the literature thus far, and it is comparable to the reciprocal of the longest solvation dynamics time component in water, τ(S) = 0.8 ps. We found a kinetic isotope effect (KIE) on the ESPT rate of ~1.7. This value is lower than that of weaker photoacids, which usually have KIE value of ~3, but comparable to the KIE on proton diffusion in water of ~1.45, for which the average time of proton transfer between adjacent water molecules is similar to that of QCy7.     

trans-[(en)~2(NO~2)Co(O~2CC~5H~5N)]^2^+/Fe(Ⅱ)间电子转移反应动力学及机理研究

合成了新型Co(Ⅲ)配合物trans-[(en)~2(NO~2)Co(O~2CC~5H~5N)](ClO~4)~2, 并通过紫外可见光谱、红外光谱、元素分析和X射线单晶衍射分析进行了表征。同时分别以[Fe(CN)~6]^4^-和[Fe(CN)~5(H~2O)]^3^-作为还原剂, 考察了该配合物被还原的反应动力学行为。结果表明两反应体系分别按外配位界机理和内配位界机理进行电子传递。在25℃, Ⅰ=0.5mol·L^-^1,trans-[(en)~2(NO~2)Co(O~2CC~5H~5N)]^2^+/[Fe(CN)~6]^4^-反应体系的前驱配合物离子对形成常数Q~i~p=29mol^-^1·L, 电子转移速率常数k~e~t=2.4×10^-^4s^-^1,电子转移过程的活化焓△H^≠~e~t和活化熵△S^≠~e~t分别为1.2×10^2kJ·mol^-^1和5.0×10^2J·mol^-^1·K^-^1。在40℃, pH=8.0, Ⅰ=0.1mol·L^-^1,trans-[(en)~2(NO~2)Co(O~2CC~5H~4N)]^2^+/[Fe(CN)~5(H~2O)]^3^-反应体系前驱双核配合物分子内电子转移速率常数为7.0×10^-^5s^-^1。最后讨论了分子轨道对称性, 两金属中心氧化还原电势差等因素对电子转移速率的影响。     

Intra- and intermolecular vibrational energy transfer in tungsten carbonyl complexes W(CO)5(X) (X=CO, CS, CH3CN, and CD3CN)

Vibrational energy relaxation of degenerate CO stretches of four tungsten carbonyl complexes, W(CO)6, W(CO)5(CS), W(CO)5(CH3CN), and W(CO)5(CD3CN), is observed in nine alkane solutions by subpicosecond time-resolved infrared (IR) pump-probe spectroscopy. Between 0 and 10 ps after the vibrational excitation, the bleaching signal of the ground-state IR absorption band shows anisotropy. Decay of the anisotropic component corresponds either to the rotational diffusion of the molecule or to the intramolecular vibrational energy transfer among the degenerate CO stretch modes. The time constant of the anisotropy decay, tauaniso, shows distinct solvent dependence. By comparing the results for the T1u CO stretch of W(CO)6 and the A1 CO stretch of W(CO)5(CS), the time constant of the rotational diffusion, taur, and the time constant of the intramolecular energy transfer among the three degenerate vibrational modes, taue, are determined as 12 and 8 ps, respectively. The tauaniso value increases as the number of carbon atoms in the alkane solvent increases. After 10 ps, the recovery of the bleaching becomes isotropic. The isotropic decay represents the vibrational population relaxation, from v=1 to v=0. In heptane, the time constant for the isotropic decay, tau1, for W(CO)5(CS) and W(CO)6 was 140 ps. The tau1 for the two acetonitrile-substituted complexes, however, shows a smaller value of 80 ps. The vibrational energy relaxation of W(CO)5(CH3CN) and W(CO)5(CD3CN) is accelerated by the intramolecular energy redistribution from the CO ligand to the acetonitrile ligand. In the nine alkane solutions, the tau1 value of W(CO)6 ranges between 124 and 158 ps, showing the apparent V-shaped solvent dependence with its minimum in decane, while the tau1 value shows little solvent dependence for W(CO)5(CH3CN) and W(CO)5(CD3CN).     

Spectroscopy and femtosecond dynamics of excited-state proton transfer induced charge transfer reaction

Fluorescence spectroscopy and femtosecond relaxation dynamics of 2-{[2-(2-hydroxyphenyl)benzo[d]oxazol-6-yl]methylene}malononitrile (diCN-HBO) and 2-{[2-(2-hydroxyphenyl)benzo[d]thiazol-6-yl]methylene}malononitrile (diCN-HBT) are studied to probe the excited-state proton transfer (ESPT) coupled charge transfer (ESCT) reaction. Unlike most of the ESPT/ESCT systems previously designed, in which ESCT takes place prior to ESPT, both diCN-HBO and diCN-HBT undergo ESPT, concomitantly accompanied with the charge transfer process, such that the ESPT reaction dynamics are directly coupled with solvent polarization effects. The long-range solvent polarization interactions result in a solvent-induced barrier that affects the overall proton transfer reaction rate. In cyclohexane, the rate constant of ESPT of diCN-HBO is measured to be 1.1 ps (9.1 x 10(11) s(-1)), which is apparently slower than that of 150 fs for the parent molecule 2-(2'-hydroxyphenyl)benzoxazole (HBO). Upon increasing solvent polarity to, for example, CH 3CN, the rate of ESPT is increased to 300 fs (3.3 x 10(12) s(-1)). The results are rationalized by the stabilization of proton transfer tautomer, which possesses a large degree of charge transfer character via an increase of the solvent polarity, such that the corresponding solvent-induced barrier is reduced. We thus demonstrate a prototypical system in which the photon-induced nuclear motion (proton transfer) is directly coupled with solvent polarization and the corresponding mechanism is reminiscent of that applied in an electron transfer process.     

Electrochemical response of dopamine at a penicillamine self-assembled gold electrode.

The penicillamine (Pen) self-assembled monolayer (SAM) modified gold electrode (Pen/Au) is demonstrated to catalyze the electrochemical response of dopamine (DA) by cyclic voltammetry. A pair of well-defined redox waves was obtained and the calculated standard rate constant (k(s)) is 3.88 x 10(-3) cm/s at the self-assembled electrode. The electrode reaction is a quasi-reversible process. The oxidation peak of DA can be used to determine the concentration of DA. The peak current and the concentration of DA are a linear relationship in the range of 2.0 x 10(-5) M to 8.0 x 10(-4) M. The detection limit is 4.0 x 10(-6) M. By ac impedance spectroscopy the apparent electron transfer rate constant (k(app)) of Fe(CN)(3-)/Fe(CN)(4-) at the Pen/Au electrode was obtained as 2.08 x 10(-5) cm/s. The Pen SAM was characterized with X-ray photoelectron spectroscopy (XPS), grazing angle FT-IR spectroscopy and contact angle goniometer.     

Proton transfer from 2-naphthol to aliphatic amines in supercritical CO2

The proton transfer from 2-naphthol to aliphatic amines was studied in supercritical CO(2) (scCO(2)) and in cyclohexane as reference solvent, by absorption and fluorescence spectroscopy and by time-resolved emission. Irradiation of 2-naphthol in scCO(2) in the presence of ethyldiisopropylamine shows dynamic fluorescence quenching of the acidic form of 2-naphthol and emission from the basic form. Fluorescence excitation spectra show that the emission of the basic form is originated upon excitation of the acidic form. The interaction between 2-naphthol and the amines is described by the formation of a complex with proton donor-acceptor character in the ground and excited states of 2-naphthol. The acidity increase of 2-naphthol upon electronic excitation to the first excited singlet in scCO(2) is as high as in water. Proton transfer quantum yields of 0.6 can be easily achieved in scCO(2). The results have implications for carrying out acid-base catalyzed reactions in scCO(2).     

An anion-dependent switch in selectivity results from a change of C-H activation mechanism in the reaction of an imidazolium salt with IrH5(PPh3)2

Changing the counteranion along the series Br, BF4, PF6, SbF6 in their ion-paired 2-pyridylmethyl imidazolium salts causes the kinetic reaction products with IrH5(PPh3)2 to switch from chelating N-heterocyclic carbenes (NHCs) having normal C2 (N path) to abnormal C5 binding (AN path). Computational work (DFT) suggests that the AN path involves C-H oxidative addition to Ir(III) to give Ir(V) with little anion dependence. The N path, in contrast, goes by heterolytic C-H activation with proton transfer to the adjacent hydride. The proton that is transferred is accompanied by the counteranion in an anion-coupled proton transfer, leading to an anion dependence of the N path, and therefore of the N/AN selectivity. The N path goes via Ir(III), not Ir(V), because the normal NHC is a much less strong donor ligand than the abnormal NHC. PGSE NMR experiments support the formation of ion-pair in both the reactants and the products. 19F,1H-HOESY NMR experiments indicate an ion-pair structure for the products that is consistent with the computational prediction (ONIOM(B3PW91/UFF)).     

Scandium ion-promoted reduction of heterocyclic N=N double bond. Hydride transfer vs electron transfer

Hydride transfer from 10-methyl-9,10-dihydroacridine (AcrH(2)) to 3,6-diphenyl-1,2,4,5-tetrazine (Ph(2)Tz), which contains a N=N double bond, occurs efficiently in the presence of Sc(OTf)(3) (OTf = OSO(2)CF(3)) in deaerated acetonitrile (MeCN) at 298 K, whereas no reaction occurs in the absence of Sc(3+). The observed second-order rate constant (k(obs)) increases with increasing Sc(3+) concentration to approach a limited value. When AcrH(2) is replaced by the dideuterated compound (AcrD(2)), the rate of Sc(3+)-promoted hydride transfer exhibits the same primary kinetic isotope effect (k(H)/k(D) = 5.2+/-0.2), irrespective of Sc(3+) concentration. Scandium ion also promotes an electron transfer from CoTPP (TPP(2)(-) = tetraphenylporphyrin dianion) and 10,10'-dimethyl-9,9'-biacridine [(AcrH)(2)] to Ph(2)Tz, whereas no electron transfer from CoTPP or (AcrH)(2) to Ph(2)Tz occurs in the absence of Sc(3+). In each case, the observed second-order rate constant of electron transfer (k(et)) shows a first-order dependence on [Sc(3+)] at low concentrations and a second-order dependence at higher concentrations. Such dependence of k(et) on [Sc(3+)] is ascribed to formation of 1:1 and 1:2 complexes between Ph(2)Tz(*)(-) and Sc(3+) at the low and high concentrations of Sc(3+), respectively, which results in acceleration of the rate of electron transfer. The formation of 1:2 complex has been confirmed by the ESR spectrum in which the hyperfine structure is different from that of free Ph(2)Tz(*)(-). The 1:2 complex formation results in the saturated kinetic dependence of k(obs) on [Sc(3+)] for the Sc(3+)-promoted hydride transfer, which proceeds via Sc(3+)-promoted electron transfer from AcrH(2) to Ph(2)Tz, followed by proton transfer from AcrH(2)(*)(+) to the 1:1 Ph(2)Tz(*)(-)-Sc(3+) complex and the subsequent facile electron transfer from AcrH(*) to Ph(2)TzH(*). The effects of counteranions on the Sc(3+)-promoted electron transfer and hydride transfer reactions are also reported.     

Energy transfer in thermal and hyperthermal collisions between CN(X2Sigma+, v = 2) in selected rotational levels (Ni = 0, 1, 6, 10, 15 and 20) and N2

We report rate coefficients (k(tot,N(i))) for total removal of CN(X(2)Sigma(+), v = 2, N(i)) radicals from selected rotational levels (N(i) = 0, 1, 6, 10, 15 and 20) and for state-to-state rotational energy transfer (k(i-->f)) between levels N(i) and other rotational levels N(f) in single collisions with N(2). CN radicals have been generated using two sources: (a) the pulsed laser photolysis of ICN at 266 nm, which generates translationally 'hot' CN radicals; and (b) the pulsed laser photolysis of NCNO at 570 nm, which generates CN radicals with translational energies close to the average value at 298 K. Comparison of the values of k(tot,N(i)) obtained using these two sources of CN demonstrates: firstly, that the same results are obtained as long as time is allowed for the translationally hot CN radicals generated from ICN to be thermalised before radicals are promoted to a specific rotational level in v = 2 using a tuneable infrared 'pump' laser operating at ca. 2.45 micro m; and secondly, that the rate coefficients decrease, but the averaged cross-sections remain approximately constant, as the excess translational energy in CN radicals is moderated by collisions. With NCNO as the source of CN radicals, the observed values of k(tot,N(i)) do not depend on the delay between the pulses from the photolysis and pump lasers. Finally, we demonstrate that, for the non-reactive collision partner N(2) and with allowances made for the rate coefficients that are too small to measure directly, the sum of the state-to-state rate coefficients, Sigma(f)k(i-->f), for rotational energy transfer from a selected initial level N(i) agrees quite well with the value of k(tot,N(i)) for total transfer from the same initial level. The values of k(tot,N(i)) and of the state-to-state rate coefficients are compared with similar, earlier, results in which helium and argon were the collision partners. The relevance of these results to the study of collisions of CN with reactive collision partners is briefly discussed.     

Femtosecond dynamics on excited-state proton/charge-transfer reaction in 4'-N,N-diethylamino-3-hydroxyflavone. The role of dipolar vectors in constructing a rational mechanism

The excitation behaviors for 4'-N,N-diethylamino-3-hydroxyflavone (Ia) have been investigated via femtosecond fluorescence upconversion approaches to gain detailed insights into the mechanism of the proton/charge-transfer coupling reaction. In polar solvents such as CH2Cl2 and CH3CN, in addition to a slow, solvent-polarity-dependent rate (a few tens of picoseconds(-1)) of excited-state intramolecular proton transfer (ESIPT) reported previously, early femtosecond relaxation dynamics clearly reveal that the proton-transfer tautomer emission consists of a rise component of a few hundred femtoseconds. The temporal spectral evolution at the time domain of zero to a few hundred femtoseconds further resolves two distinct emission bands consisting of a proton-transfer tautomer emission and a time-dependent Stokes shifted emission. The results, in combination with ab initio calculations on the dipolar vectors for normal and tautomer species, lead us to unveil the importance of the relationship of the dipolar vectors among various states, and hence the corresponding solvation energetics in the overall ESIPT reaction. We conclude a similar dipolar character between ground-state normal (N) and excited proton-transfer tautomer (T*) species, whereas due to the excited-state intramolecular charge transfer (ESICT), the normal excited state (N*) possesses a large dipolar change with respect to N and T*. ESIPT is thus energetically favorable at the Franck-Condon excited N*, and its rate is competitive with respect to the solvation relaxation process. After reaching the solvent equilibration, there exists an equilibrium between N* and T* states in, for example, CH3CN. Due to the greatly different equilibrium polarization between N* and T*, both forward and reversed ESIPT dynamics are associated with a solvent-induced barrier. The latter viewpoint of the equilibrium type of ESIPT in Ia is in agreement with the previous reports based on steady-state, picosecond, and femtosecond dynamic approaches.     

One-electron oxidation of a hydrogen-bonded phenol occurs by concerted proton-coupled electron transfer

The hydrogen-bonded phenol 2-(aminodiphenylmethyl)-4,6-di-tert-butylphenol (HOAr-NH2) was prepared and oxidized in MeCN by a series of one-electron oxidants. The product is the phenoxyl radical in which the phenolic proton has transferred to the amine, *OAr-NH3+. The reaction of HOAr-NH2 and tris(p-tolyl)aminium ([N(tol)3]*+) to give *OAr-NH3+ + N(tol)3 has Keq = 2.0 +/- 0.5, follows second-order kinetics with k = (1.1 +/- 0.2) x 105 M-1 s-1 (DeltaG = 11 kcal mol-1), and has a primary isotope effect kH/kD = 2.4 +/- 0.4. Oxidation of HOAr-NH2 with [N(C6H4Br)3]*+ is faster, with k congruent with 4 x 107 M-1 s-1. The isotope effect, thermochemical arguments, and the dependence of the rate on driving force (DeltaDeltaG/DeltaDeltaG degrees = 0.53) all indicate that electron transfer from HOAr-NH2 must occur concerted with intramolecular proton transfer from the phenol to the amine (proton-coupled electron transfer, PCET). The data rule out stepwise paths that involve initial electron transfer to form the phenol radical cation *+HOAr-NH2 or that involve initial proton transfer to give the zwitterion -OAr-NH3+. The dependence of the electron-transfer rate constants on driving force can be fit with the adiabatic Marcus equation, yielding a large intrinsic barrier: lambda = 34 kcal mol-1 for reactions of HOAr-NH2 with NAr3*+.     

Proton translocation in monomolecular Langmuir-Blodgett films including 2-naphthol and 1,4-anthraquinone derivatives

The scope of the present work is the investigation of proton transport through monomolecular Langmuir-Blodgett (LB) films. The films were formed from amphiphilic molecules: 2-naphtholo-6-sulfonamide of dodecylamine (N) and 1,4-anthraquinono-2 sulfonamide of dodecylamine (A). The 2-naphthol derivative can act as a proton donor due to excited state proton transfer (ESPT) and the 1,4-anthraquinone group can play the role of proton acceptor because of protonation of the reduced form if it is present. Absorption and emission spectra of LB films containing N and A were registered and separated into component bands. Individual absorption and emission peaks observed were assigned to given forms of chromophores. The behavior of different component bands reflects the state of anthraquinone dependent on proton concentration. A correlation of rate and efficiency of ESPT, with changes of the spectra of A, may be expected to yield information concerning the transport of protons from N to A. The influence of the donor-acceptor distance, sample humidity, film arrangement and the presence of protonophores (Gramicidin A) on proton transfer is studied. Our results indicate that the proton can be transported through the film but its concentration vanishes at the distance greater than 30 A. The efficiency of proton transfer depends strongly on water content, film structure and the presence of ion channels.     

Proton transfer from C-6 of uridine 5'-monophosphate catalyzed by orotidine 5'-monophosphate decarboxylase: formation and stability of a vinyl carbanion intermediate and the effect of a 5-fluoro substituent

The exchange for deuterium of the C-6 protons of uridine 5'-monophosphate (UMP) and 5-fluorouridine 5'-monophosphate (F-UMP) catalyzed by yeast orotidine 5'-monophosphate decarboxylase (ScOMPDC) at pD 6.5-9.3 and 25 °C was monitored by (1)H NMR spectroscopy. Deuterium exchange proceeds by proton transfer from C-6 of the bound nucleotide to the deprotonated side chain of Lys-93 to give the enzyme-bound vinyl carbanion. The pD-rate profiles for k(cat) give turnover numbers for deuterium exchange into enzyme-bound UMP and F-UMP of 1.2 × 10(-5) and 0.041 s(-1), respectively, so that the 5-fluoro substituent results in a 3400-fold increase in the first-order rate constant for deuterium exchange. The binding of UMP and F-UMP to ScOMPDC results in 0.5 and 1.4 unit decreases, respectively, in the pK(a) of the side chain of the catalytic base Lys-93, showing that these nucleotides bind preferentially to the deprotonated enzyme. We also report the first carbon acid pK(a) values for proton transfer from C-6 of uridine (pK(CH) = 28.8) and 5-fluorouridine (pK(CH) = 25.1) in aqueous solution. The stabilizing effects of the 5-fluoro substituent on C-6 carbanion formation in solution (5 kcal/mol) and at ScOMPDC (6 kcal/mol) are similar. The binding of UMP and F-UMP to ScOMPDC results in a greater than 5 × 10(9)-fold increase in the equilibrium constant for proton transfer from C-6, so that ScOMPDC stabilizes the bound vinyl carbanions, relative to the bound nucleotides, by at least 13 kcal/mol. The pD-rate profile for k(cat)/K(m) for deuterium exchange into F-UMP gives the intrinsic second-order rate constant for exchange catalyzed by the deprotonated enzyme as 2300 M(-1) s(-1). This was used to calculate a total rate acceleration for ScOMPDC-catalyzed deuterium exchange of 3 × 10(10) M(-1), which corresponds to a transition-state stabilization for deuterium exchange of 14 kcal/mol. We conclude that a large portion of the total transition-state stabilization for the decarboxylation of orotidine 5'-monophosphate can be accounted for by stabilization of the enzyme-bound vinyl carbanion intermediate of the stepwise reaction.     

Proton transfer reactions from H3+ ions to N2, O2, and CO molecules

The rate constants for proton transfer from H₃⁺ ions to N₂, O₂, and CO have been measured as function of hydrogen buffer gas partial pressure. The rate constant for proton transfer from H₃⁺ to N₂ shows a very large pressure dependence, increasing from 1.0 × 10^?9 cm³/s at low H₂ partial pressures to 1.7 × 10^?9 cm³/s at high H₂ partial pressures. The rate constants for proton transfer from H₃⁺ to O₂ and CO are constant with partial pressure of H₂; giving values of 6.4 × 10^?10 cm³/s and 1.7 × 10^?9 cm³/s, respectively. The roles of excess vibrational energy in H₃⁺ ions and of equilibrium between forward and back reaction are discussed. Back reaction is observed only for the reaction of H₃⁺ ions with O₂, and an equilibrium constant of K = 2.0 ± 0.4 at 298 K has been determined. From these data the proton affinity of O₂ is deduced to be 0.47 ± 0.11 kcal/mole higher than that of H₂.     

Kinetic study of IO radical with RO2 (R = CH(3), C2H5, and CF3) using cavity ring-down spectroscopy

The reactions of iodine monoxide radical, IO, with alkyl peroxide radicals, RO(2) (R = CH(3), C(2)H(5), and CF(3)), have been studied using cavity ring-down spectroscopy. The rate constant of the reaction of IO with CH(3)O(2) was determined to be (7.0 +/- 3.0) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K and 100 Torr of N(2) diluent. The quoted uncertainty is two standard deviations. No significant pressure dependence of the rate constant was observed at 30-130 Torr total pressure of N(2) diluent. The temperature dependence of the rate constants was also studied at 213-298 K. The upper limit of the branching ratio of OIO radical formation from IO + CH(3)O(2) was estimated to be <0.1. The reaction rate constants of IO + C(2)H(5)O(2) and IO + CF(3)O(2) were determined to be (14 +/- 6) x 10(-11) and (6.3 +/- 2.7) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K, 100 Torr of N(2) diluent, respectively. The upper limit of the reaction rate constant of IO with CH(3)I was <4 x 10(-14) cm(3) molecule(-1) s(-1).