Dissociative Transition State Research Articles

Ab initio study of the inversion barrier in NF3+

Ab initio Hartree–Fock, Moller–Plesset perturbation theory (MP2), and quadratic configuration interaction, using single and double substitutions (QCISD), calculations were carried out for the NF3+ ion. Optimized structures were examined at the various levels of theory. Calculation of the inversion barrier height shows the importance of optimizing the geometry at the post-Hartree–Fock level and the inclusion of polarization functions. The best calculated inversion barrier was 13.3 kcal/mol, compared to an experimental value of 17.3 kcal/mol. The dissociation transition state was computed to determine the well depth of the NF3+ ion and its stability toward dissociation. The computed well depth was 28 and 48 kcal/mol at the SCF and MP2 levels, respectively. © 1994 John Wiley & Sons, Inc.

Variable-Temperature and Variable-Pressure Sodium-23 NMR Study on Dissociation Reaction of Sodium(I) Cryptates in Ethylenediamine

Abstract The dissociation rates of the sodium(I) cryptates of 4,7,13,16,21-pentaoxa-1,10-diazabicyclo[8.8.5]tricosane (C221) and 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (C222) in ethylenediamine have been measured at various temperatures and pressures by the 23Na NMR line-broadening method. The dissociation rate constant at 298 K (kd298), activation enthalpy (ΔH‡), activation entropy (ΔS‡), and activation volume (ΔV‡) are as follows: kd298 = 7.2 s−1, ΔH‡ = 45.9 ± 1.8 kJ mol−1, and ΔS‡ = −74.5 ± 5.4 J mol−1 K−1 for Na+C221, and kd298 = 272 s−1, ΔH‡ = 43.2 ± 1.1 kJ mol−1, ΔS‡ = −53.6 ± 3.7 J mol−1 K−1, and ΔV‡ = 8.9 ± 0.4 cm3 mol−1 for Na+C222. The large positive value of ΔV‡ for Na+C222 indicates the dissociative transition state accompanied by the expansion of the cavity of C222. The electrostatic interaction of the sodium cation with ethylenediamine in the transition state has been discussed in connection with the other solvents by comparison of the values of ΔH‡.

Internal chemical bonding in solutions of simple phosphates and vanadates.

The chemical bonding within structurally related phosphates and vanadates in aqueous solution is compared on the basis of vibrational frequencies obtained by classical Raman spectroscopy. To do this, an empirical relationship between the stretching frequency of P-O and P-OH or P-OR groups and bond strength is developed such that the sum of the PO bond strengths, expressed in terms of average number of electron pairs per bond, is as close as possible to 5.0 for phosphoric acid and various anions and esters thereof. The same approach is used for the corresponding vanadates. The internal bonding in phosphates involves a greater bond strength for P-OH and a smaller strength for P-O than might be expected from a simple consideration of canonical resonance forms. In vanadates, V-OH and V-O are closer to single and double bonds, respectively, than in phosphates, and the force constant for V = O is considerably smaller than for P = O, although that for V-OH and P-OH is similar. Since treating the P-O and V-O groups of simple tetrahedral phosphates and vanadates as independent diatomic oscillators provides good correlations between the respective frequencies and bond strengths, the same correlations are used to approximate the expected stretching frequencies for distorted phosphates and vanadates. The distortions considered are those that presumably characterize associative and dissociative transition states for a concerted transfer of the (PO3-) fragment of a dianionic phosphate group between donor and acceptor oxygens with similar character.(ABSTRACT TRUNCATED AT 250 WORDS)

Hydrolysis of pharmaceutically relevant phosphate monoester monoanions: Correlation to an established structure—reactivity relationship

AbstractThe kinetics of dephosphorylation of dilute aqueous solutions of 3-phosphoryloxymethyl-5,5-diphenylhydantoin (1) and estrone phosphate (2) were studied as a function of pH, buffer concentration, and temperature at an ionic strength of 0.5M. The resulting pH-rate profiles displayed bell-shaped regions with maxima between pH 3 and 5, where the monoanionic phosphate species predominate; mechanistically, these regions involved spontaneous or water-catalyzed dephosphorylation of the monoanionic phosphate species. The profile of 1 was also described with pathways involving a hydronium ion-catalyzed and a spontaneous or water-catalyzed dephosphorylation of the neutral species. The hydrolytic reactivities of the monoanionic species of the phosphomonoester prodrugs of phenytoin (i.e., 1), estrone (i.e., 2), and 2,2,2-trichloroethanol (from the literature) were well predicted by an established structure-reactivity, free-energy relationship. The correlation of these compounds to this relationship supported a common hydrolytic reaction mechanism. The small Brønsted value of −0.27 was consistent with a dephosphorylation mechanism involving a rapid transfer of the lone phosphoryl proton to the bridge-oxygen atom of the ester linkage, followed by the rate-limiting phosphorus-oxygen bond fission proceeding through a largely dissociative transition state.

Is there a transition state for the unimolecular dissociation of cyclotetraoxygen (O4)?

The cyclo-O4 molecule is isoelectronic with cyclobutane and has been mentioned as a potential new high energy density material (HEDM). The important unresolved question has been whether or not cyclo-O4 is a genuine minimum on the O4 potential energy hypersurface. Here the transition state for cyclo-O4 dissociation to two O2 molecules has been located at a number of levels of theory using a double zeta plus polarization (DZP) basis set. The theoretical methods with which the transition state geometry was optimized include two-configuration self-consistent-field (TCSCF), configuration interaction including all single and double excitations (CISD), coupled cluster including all single and double excitations (CCSD), and the latter with a noniterative correction for connected triple excitations, CCSD(T). The equilibrium geometry of O4 has D2d symmetry, while a stationary point of D2 symmetry is of Hessian index two (i.e., two imaginary vibrational frequencies) at the highest level of theory, DZP CCSD(T). The true transition state lies only slightly lower energetically (classical barrier 7.9 kcal/mole) and is of C2 symmetry with the DZP CCSD(T) method. The activation energy was predicted in a completely consistent manner to be 6.2 kcal/mole. The possibility that this barrier could disappear entirely at higher levels of theory is discussed. It is concluded that the O4 barrier is too low for O4 to be useful as a HEDM.

The substrate reactivity of .mu.-monothiopyrophosphate with pyrophosphate-dependent phosphofructokinase: evidence for a dissociative transition state in enzymic phosphoryl group transfer

mu-Monothiopyrophosphate (MTP), an analogue of pyrophosphate (PPi) with sulfur in place of oxygen in the bridge position, is a substrate for the enzyme pyrophosphate-dependent phosphofructokinase. At pH 9.4 and 6 degrees C, the maximal velocity for the phosphorylation of fructose 6-phosphate (F6P) by MgMTP is about 2.8% of that with MgPPi as the phosphoryl donor. The kinetic mechanism is equilibrium random with rate-limiting transformation of the substrate ternary complex to the product when either MgMTP or MgPPi is the phosphoryl donor. This is known from independent studies to be kinetic mechanism at pH 8.0 and 25 degrees C [Bertagnolli, B. L., & Cook, P. F. (1984) Biochemistry 23, 4101-4108]. The dissociation constant of MgPPi is 14 microM, that of MgMTP is 64 microM, and that of F6P from the enzyme is about 5 mM. The Km values for MgPPi and MgMTP are 14.5 and 173 microM, respectively. MgMTP competes with MgPPi for binding to the enzyme. The values of kcat are 3.4 s-1 and 140 s-1 for MgMTP and MgPPi, respectively, at pH 9.4 and 6 degrees C. The estimated rate enhancement factors are 3.6 x 10(5) and 1.4 x 10(14) for the reactions of MgMTP and MgPPi, respectively. Therefore, MgMTP is a reasonably good substrate for PPi-dependent PKF, on the basis of comparisons of kcat. However, the rate enhancement factors show that the enzyme is a poor catalyst for the reaction of MgMTP. Lesser enzymatic catalysis in the reaction of MgMTP compared with MgPPi is largely compensated for by the greater intrinsic reactivity of MgMTP. Thus, the larger substrate MgMTP is well accommodated in the active site, and the dissociative reaction of MgMTP is well accommodated in the transition state. The results are interpreted to indicate a dissociative transition state for phosphoryl group transfer by PPi-dependent PFK. A modified synthesis and purification of MTP are described, in which (trimethylsilyl)trifluoromethanesulfonate and tetra-N-butylammonium iodide are used in place of iodotrimethylsilane to dealkylate tetramethyl-MTP.

Potential energy surfaces, quasiadiabatic channels, rovibrational spectra, and intramolecular dynamics of (HF)2 and its isotopomers from quantum Monte Carlo calculations

We report analytical representations of the six-dimensional potential energy hypersurface for (HF)2, the parameters of which are closely adjusted to low energy experimental properties such as hydrogen bond dissociation energy (D0=1062 cm−1 ) and vibrational–rotational spectra in the far and mid infrared. We present a detailed analysis of properties of the hypersurface in terms of its stationary points, harmonic normal mode amplitudes, and frequencies for the Cs minimum and C2h saddle point and effective Morse parameters and anharmonic overtone vibrational structure for the hydrogen bond and the HF stretching vibrations. The comparison between experimental data and the potential energy surface is carried out by means of accurate solutions of the rotational–vibrational Schrödinger equation with quantum Monte Carlo techniques, which include anharmonic interactions between all modes for the highly flexible dimer. Two extensions of the quantum Monte Carlo technique are presented, which are based on the clamped coordinate quasiadiabatic channel method and allow for the approximate calculation of excited rotational and vibrational levels. Predictions include dissociation energies D0 for isotopomers (XF)2 with X=μ, D, T (D0=477; 1169; 1217 cm−1 ). Unusual anharmonic isotope effects predicted for the out-of-plane bending fundamental ν6 [378; 276; 295; and 358 cm−1 for (HF)2, (DF)2, (HFDF), (DFHF)] can be understood in simple terms. Centrifugal effects both for the high frequency a-axis rotation and low frequency c-axis rotation are accurately calculated for the vibrational ground state and some excited states, with a best equilibrium center of mass distance Req.ab=5.14a0 between the HF monomers. A very large anharmonic interaction constant x46≊−16 cm−1 is predicted for the hydrogen bond vibration ν4 and for out-of-plane bending ν6. This leads to assignment of our earlier experimental observation of a band at 383 cm−1 as ν6+ν4−ν4(K=1←0) at almost exactly the predicted position. The fundamental ν4 is predicted at 130±10 cm−1. A new, indirect assignment of our experimental data gives ν4≊125 cm−1. Monte Carlo calculations are presented for quasiadiabatic channels and transition states for hydrogen bond dissociation. We present a discussion of symmetry correlations for these channels and symmetry effects in predissociation by rotation, nuclear spin symmetry, and parity violation. Large effects from zero point energy on the three-dimensional quantum centrifugal barriers for rotational predissociation are found. On the basis of the new data, a much improved statistical mechanical estimate for the equilibrium 2HF=(HF)2 is obtained.

Potential energy surface for unimolecular dissociation and rearrangement reactions of the ground electronic state of HFCO

The potential energy surface of the HFCO molecule in its electronic ground state has been investigated with ab initio method, at levels up to MP4(SDTQ)/6-311G**//MP2/6-31G*. At the highest level, the barrier height for molecular dissociation (HFCO→HF+CO) was calculated to be 46.9 kcal/mol with a zero-point energy correction, in good agreement with an experimental estimate and a recent theoretical result. The intrinsic reaction coordinate (IRC) for molecular dissociation was traced and the coupling between the IRC and normal modes as well as that among the normal modes was analyzed along the IRC. The analysis is consistent with the mode specificity of recently observed quasistable vibrational states of HFCO above the dissociation limit. Almost all possible stationary points on the potential surface of the HFCO system have been located, including the rearrangement and atomic dissociation products and transition states, as well as van der Waals complexes. These are compared with the H2CO system. All the species that have an FO bond are unstable or nonexistent, reflecting the weakness of the bond.

Secondary oxygen-18 isotope effects for hexokinase-catalyzed phosphoryl transfer from ATP

Secondary 18O isotope effects in the gamma-position of ATP have been measured on phosphoryl transfer catalyzed by yeast hexokinase in an effort to deduce the structure of the transition state. The isotope effects were measured by the remote-label method with the exocyclic amino group of adenine as the remote label. With glucose as substrate, the secondary 18O isotope effect per 18O was 0.9987 at pH 8.2 and 0.9965 at pH 5.3, which is below the pK of 6.15 seen in the V/K profile for MgATP. With the slow substrate 1,5-anhydro-D-glucitol, the value was 0.9976 at pH 8.2. While part of the inverse nature of the isotope effect may result from an isotope effect on binding, the more inverse values when catalysis is made more rate limiting by decreasing the pH or switching to a slower substrate suggest a dissociative transition state for phosphoryl transfer, in agreement with predictions from model chemistry. The 18O equilibrium isotope effect for deprotonation of HATP3- is 1.0156, while Mg2+ coordination to ATP4- does not appear to be accompanied by an 18O isotope effect larger than 1.001.

Enzymatic and chemical reactivities ofsym-monothiopyrophosphate

The synthesis of sym-monothiopyrophosphate (MTP) is described and the relative hydrolysis rate constants are reported for the dianion, trianion, and tetraanion in aqueous solution. The MTP-dianion undergoes hydrolysis at a rate 5 x 107-fold faster than the dianion of pyrophosphate (PPi) and the trianion undergoes hydrolysis 2.7 x 107-fold faster than PPi-trianion. The entropy of activation for the hydrolysis of MTP-trianion is 0.2 cal/deg/mol, and the enthalpy of activation is 19.7 kcal/mol. The hydrolysis appears to involve a dissociative transition state. The hydrolysis of the tetramethyl ester of MTP, in contrast, appears to involve an associative transition state. The Mg complex of MTP is a reasonable substrate for PPi-dependent phosphofructokinase and inorganic pyrophosphatase, despite its larger size relative to PPi. Therefore, MTP is tolerated within the active sites and in the transition states of enzymes that catalyze phosphoryl transfer from PPi to acceptor molecules. These observations are consistent with the transition states for these enzymes being dissociative in nature, although they do not prove that they are dissociative. MTP is, on the other hand, not a substrate for acetyl-CoA synthase, an enzyme that utilizes the Mg complex of PPi as a specific acceptor for the adenylyl group, an alkylphosphoryl group. Inasmuch as nonenzymatic reactions of this type involve associative transition states, it may be that the larger size of MTP relative to PPi undermines the catalytic process in the active site of acetyl-CoA synthase, which may catalyze adenylyl transfer through an associative transition state.

Sulfuryl transfer catalyzed by phosphokinases

Adenosine 5'-sulfatopyrophosphate is a substrate for nucleoside diphosphate kinase. The reaction appears to proceed through a ping-pong mechanism analogous to the physiological reaction involving ATP, presumably by way of a sulfohistidine intermediate. Unlike the phosphoryl transfer reactions, the corresponding sulfuryl transfers catalyzed by nucleoside diphosphate kinase do not have a strict divalent metal requirement. The estimated rate constants for the metal- and nonmetal-catalyzed sulfuryl transfers differ by less than an order of magnitude and are approximately 1000-fold slower than the corresponding phosphate transfers. These results suggest that the role of the metal ion in nucleoside diphosphate kinase is to coordinate the alpha, beta-phosphates of the substrate. Sulfuryl and phosphoryl transfer probably occur through dissociative transition states.

Probing the transition state in dissociative adsorption

We have performed calculations for H2 molecules being scattered from a metal surface. The emphasis of the work is aimed at understanding the role of rotational energy on the dissociative adsorption reaction. The time-dependent wavepacket formalism has been employed on a vibrationally adiabatic PES which explicitly contains two rotational degrees of freedom and one translational. The PES is activated (ca. 0.25–1.5 eV) and the value of the barrier depends strongly on the angle that the molecular bond axis makes with the surface. This PES enables a scattering event to be modelled which corresponds to a normally incident beam of hydrogen in the ground vibrational state and the J, mJ rotational state. We have found that the translational energy dependence of the dissociation probability of para- and normal hydrogen is approximately the same. This reflects the fact that the quantum of rotational energy is rather small in comparison to the activation barrier height. More interestingly it has been found that the form chosen for the PES gives rise to effective angular discrimination whereby molecules having only particular orientations will dissociate. This, in turn, gives rise to a characteristic rotational distribution which may be used to explore the symmetry of the dissociative transition state.

Solvent effects on the solvolysis of ADPβS

Solvolysis rates for trialkylammonium and tetraalkylammonium salts of ADPβS at 25°C and ADP at 80°C in water, N,N-dimethylformamide (DMF) and water-DMF mixtures are reported. ADPβS undergoes solvolysis much faster than ADP under all conditions. The solvolysis of tris-triethylammonium ADPβS is 40-fold faster in DMF than in water at 25°C, but the solvolysis of tris-tetramethylammonium ADPβS is only slightly faster in DMF than in water. The solvolysis rate for tris-tetramethylammonium ADPβS increases whereas that for tris-triethylammonium ADPβS decreases upon addition of small percentage of H 2O to DMF. The corresponding salts of ADP do not undergo detectable solvolysis at 25°C, but react at 80°C, where the trialkylammonium and tetraalkylammonium salts exhibit analogous but much smaller effects on solvolysis rates. The results indicate the operation of two effects of DMF in enhancing the solvolysis of ADPβS: (a) DMF decreases the acid dissociation constant of ADPβS, which leads to an increased fraction being in the solvolytically labile dianionic form. (b) The decreased polarity of DMF relative to water enhances the rate by a medium effect of destabilizing the ground state relative to the transition state in a dissociative mechanism. Comparisons of the solvolytic rates for ADPβS relative to ADP suggest that the two react by different mechanism, ADP by a S N2 mechanism via a dissociative transition state and ADPβS by a purely dissociative mechanism via a discrete thiometaphosphate monoanion as an intermediate.

Use of the molecular orbital Hessian for self-consistent-field (SCF) wavefunctions

The molecular orbital (MO) Hessian for the self-consistent-field (SCF) wavefunction is defined as the second derivative of the electronic energy of the system with respect to changes in the MO coefficients. The lowest eigenvalue of the MO Hessian is widely used to determine the stability of SCF wavefunctions. The present study extends the use of the MO Hessian in several directions: prediction of low-lying excited states, qualification of basis sets, identification of MOs involved in chemical reactions, warning of a variational collapse, relative stability of wavefunctions, as well as absolute stability of wavefunctions. An SCF instability index is defined as the number of negative eigenvalues of the MO Hessian, since it is possible that an SCF wavefunction has more than one negative eigenvalue of the corresponding MO Hessian. The MO Hessian is analyzed for the ground and excited states of H 2O, H 2CO and NO 2 at equilibrium geometries and/or at transition states for dissociation and for conformational change. Examples are given of stationary values of the electronic energy with respect to orbital rotations representing local minima, maxima and points of inflection. The relevance of the eigenvalues of the MO Hessian to the physical and chemical properties of the system under consideration is discussed.

Reactive scattering via a persistent collision complex as a function of impact parameter

A recently proposed microcanonical transition state theory for angular distributions of reactive scattering arising from a persistent collision complex is extended to the full range of impact parameters. At larger impact parameters the angular distribution is governed by the precessional motion of the symmetric top transition state for complex dissociation while at smaller impact parameters the angular distribution is determined by the disposition of the product orbital angular momentum estimated from phase space theory. The extended theory predicts an angular distribution for F+C 2H 5I reactive scattering which is in excellent agreement with recent experimental measurements at an initial translational energy E ≈ 38 kJ mol −1.

Effect of solvent on the reactions of coordination complexes. Part 10. kinetics of solvolysis of cis(chloro)(1‐amino‐propan‐2‐ol)bis‐(ethylenediamine)cobalt(III) in methanol + water, propan‐2‐ol + water, and t‐butanol + water media

AbstractThe solvolysis of cis(chloro)(1‐amino‐propan‐2‐ol)bis(ethylenediamine)cobalt(III) in aqueous alcoholic media using methanol, propan‐2‐ol, t‐butanol as cosolvents, resulted in the formation of the (N,O) chelated productcis[Co(en)2(NH2CH2CHOHCH3)]3+. The pseudo first order rate constant decreased with increasing molfraction (Xorg) of alcohols, the decrease being less marked as the bulkiness of hydrophobic moiety of alcohol increased. The plots of log 𝓀obs vs. reciprocal of the bulk dielectric constant of the solvents, log 𝓀obs vs. Grunwald‐Winstein solvent parameter and log 𝓀obs, vs. Xobs under isodielectric condition (Ds = 50, at 50°C) for CH3OH/H2O, C2H5OH/H2O, (CH3)2CHOH/H2O, (CH3)3COH/H2O, (CH2)2(OH)2/H2O, and (CH3)2 C  O/H2O in water rich media indicated that both solvent structural effects and presumably the hydrophobic interaction appreciably mediate the reaction. The calculated values of the relative transfer free energy at 25°C[ΔGt(C3−)‐ΔGt(i.s)(s←w)] where C3+ and i.s. denote the dissociative transition state {cis[Co(en)2(NH2CH2CHOHCH3)]3+}* and the initial state, respectively, indicated that the tripositive transition state is more effectively solvated by the mixed solvent media, than the dipositive initial state, the effect appeared to be more significant with increasing Xorg. The plots of activation enthalpy and entropy against Xorg exhibited maxima and minima indicating that enthalpy and entropy changes associated with the solvent shell reorganization of the reactant contribute to the overall activation parameters.

Theoretical study of low-lying states of H3O

The first two doublet and quartet states of H3 O have been surveyed by multiconfiguration self-consistent field/first-order configuration interaction (MCSCF/FOCI) calculations in C3v symmetry. Geometries of the minima on the doublet surfaces have been optimized by MCSCF gradient techniques and energies obtained by large-scale multireference single and double excitation CI calculations. The correlation diagram linking the minima to different dissociation limits has been established. A local minimum in C3v symmetry of ion-pair character is shown to be unstable with respect to dissociation to H2 +H+O. The lowest state, 1 2 A1 in C3v , (1 2 A′ in Cs ) may be characterized as an H3 O+ core surrounded by an oxygen 3s Rydberg electron. Transition states for dissociation of the 1 2 A′ state to H2 O+H and to OH+H2 have been investigated. An extremely low barrier height, 3.58 kcal/mol without vibration, 0.4 kcal/mol for H3 O, an estimated 1.3 kcal/mol for D3 O, with zero-point energy, is found for dissociation of the 1 2 A′ state to H2 O+H. Within the uncertainty of the calculation it is not possible to predict whether the lowest state of H3 O should be observable experimentally. The relationship of these results to experimental observations for H3O is discussed.

Reaction paths for the dissociation ã 3A″ CH2CO→X̃ 3B1 CH2 + X̃ 1Σ+ CO

An ab initio investigation of the (CIIs) in-plane bent 3A″ CH2CO→X̃ 3B1 CH2+X̃ 1∑+CO and the (CIs) out-of-plane bent 3A′ CH2CO→X̃ 3B1 CH2+X̃ 1∑+CO dissociation paths has been performed. Geometrical structures, vibrational frequencies, and quadratic force constants have been determined at the DZP SCF and DZP CISD levels of theory for the X̃ 1A1, 3A″, and 3A′ states of ketene and for the 3A″ and 3A′ transition states for dissociation. The DZP CISD structure for à 1A″ ketene is also reported. Final energetic predictions for triplet ketene dissociation have been obtained from large-basis (QZ2P and QZ2P+f) UMP4(SDTQ) calculations at the DZP CISD geometries. The CIIs stationary point for 3A″ ketene dissociation is a true transition state with r(C–C)=2.071 Å at the DZP CISD level of theory. The corresponding CIs stationary point for 3A′ ketene is actually a super transition state for the interconversion of two equivalent 3A″CIIs transition states for dissociation. Final theoretical predictions of Te=19 400 cm−1 and T0=19 150 cm−1 are made for the adiabatic excitation energy of the ã 3A″ state of ketene, and a value of 22.3 kcal/mol is proposed for the 3A″ dissociation energy.

Effect of solvent on the reactions of coordination complexes. Part 1.—Kinetics of solvolysis of cis-(bromo)(benzimidazole)bis(ethylenediamine)cobalt(III) in methanol–water media

The kinetics of the solvolytic aquation of cis-(bromo)(benzimidazole)bis(ethylenediamine)cobalt(III) have been investigated at 35, 40, 45, 50.2 and 54.8 °C in aqueous methanol media of methanol content 0–80 wt %. The pseudo-first-order rate constant decreases with increasing methanol content. Plots of log ksaqvs. D–1s(where Ds is the dielectric constant of the solvent mixture) and log ksaqvs. the Grunwald–Winstein Y parameter or revised YCl(1-adamantyl Chloride) parameter are non-linear. The plot of log ksaqvs. the molefraction of methanol (XMeOH), which deviates from linearity at 35, 40 and 45 °C, is an excellent straight line (r= 0.998) at 50 and 54.8 °C over the entire composition range studied. It is evident that the solvation phenomenon plays dominant role and that the rate of aquation is mediated by the dual solvent vectors, the overall basicity and acidity of the solvent mixtures. At 25 °C the calculated values of the transfer free energy of the dissociative transition state, cis-{[Co(en)2(bzmH)]3+}*, relative to that of the initial state, cis-[Co(en)2(bzmH)Br]2+, for the transfer of the ions from water to the mixed solvent were positive at all solvent compositions (except 80 % MeOH), exhibiting a maximum at XMeOH= 0.46. This reflects the propensity of the said tripositive transition state and the dipositive initial state towards solvation interaction with the medium. In contrast to the activation free energy, which varies linearly with XMeOH, the plots of activation enthalpy and entropy against XMeOH exhibit maxima at XMeOH= 0.12 and 0.46 and minima at XMeOH= 0.02 and 0.25; thereafter both ΔH‡ and ΔS‡ decrease steeply to significantly low values at XMeOH= 0.69 (ΔH‡= 80.1 ± 4.3 kJ mol–1 and ΔS‡=–91 ± 13 J K–1 mol–1), manifesting the effect of solvent-shell reorganisation around the complex ion both in the initial state and in the transition state as the composition is varied.

The photofragmentation dynamics of 3-cyclopentenone in the gas phase

Vibrational energy disposal to the CO product of the photofragmentation of 3-cyclopentenone has been studied by time-resolved CO laser absorption spectroscopy. The nascent vibrational distribution of CO is N 0 = 7.88, N 1 = 0.187, N 2 = 0.024. The measured vibrational distribution is substantially colder than that predicted on statistical grounds. This indicates that the product fragments become vibrationally decoupled from one another before the full reaction exoergicity has been released. Our results suggest that such decoupling may occur at the dissociation transition state.