Inverse Secondary Isotope Effect Research Articles

Isotope Effect and Heavy-Light-Heavy Reactivity Oscillation in the Cl + CHD3/CHT3 Reaction.

Previous experiments and theories have shown the existence of heavy-light-heavy (HLH) reactivity oscillation in the Cl + CH4 reaction and anticipated that similar oscillations should exist in many HLH reactions involving polyatomic reagents. However, the total reaction probabilities for the Cl + CHD3 → HCl + CD3 reaction exhibit only a step-like feature, and the total reaction probabilities for Cl + CHT3 → HCl + CT3 do not show any structure at all. Here, we report seven-dimensional state-to-state quantum dynamics studies for this reaction on the FI-NN PES, and we demonstrate that HLH reactivity oscillations also exist in these two reactions, manifesting as peaks in the reaction probabilities for low product rotational states. These oscillations, however, are obscured in the total reaction probability because of the higher excitation of j ≥ 2 product rotational states. Furthermore, the isotope replacement of nonreactive hydrogen with deuterium and tritium significantly enhances reactivity at collision energies above 0.112 eV, indicating an inverse secondary isotope effect on the probabilities, which is proved to be also caused by HLH mass combination. We also demonstrate that the highly rotational excitation of CHD3 substantially enhances reactivity and the HLH oscillations, similar to HLH triatomic reactions. These observations are completely different from those in the H + CHD3 reaction, which is also a late-barrier reaction. Therefore, the HLH mass combination is very important, which affects not only the reactivity oscillation but also the amplitude and product rotational state distribution and makes the initial rotation excitation play a pivotal role in the reaction.

Isotopic evidence (δ13C, δ37Cl, δ2H) for distinct transformation mechanisms of chloroform: Catalyzed H2-water system vs. zero-valent iron (ZVI)

Catalytic hydrodechlorination is an efficient technology for degrading organochlorinated compounds, such as chloroform (CF), into harmless products. Compound-specific stable isotope analysis (CSIA) of multiple elements is widely used for the investigation of degradation mechanisms. Yet, its application in the study of catalytic hydrodechlorination is still limited. We have applied CSIA to derive chlorine, carbon, and hydrogen isotope enrichment factors (ε) during the degradation of CF over Pd/Al2O3 and over Rh/Al2O3. In addition, the enrichment factors for the same isotopes were derived for the reaction of CF with zero-valent iron (ZVI) for comparison. For the reactions of CF over Pd/Al2O3 and Rh/Al2O3, εC values (−21.9 ± 0.25 ‰ and −23.4 ± 2.3 ‰) and εCl values (−12.1 ± 1.7 ‰ and −10.3 ± 0.6 ‰) were determined, respectively. The corresponding εC and εCl values, for the dechlorination of CF with ZVI were −22.2 ± 2.8 ‰ and −4.7 ± 0.45 ‰, respectively. The apparent kinetic isotope effects (AKIE) of Cl suggest that the transformation mechanism in the catalyzed hydrogen-water system is a non-concerted reaction, unlike the known reductive dechlorination of CF with ZVI. Moreover, dual-isotope slopes (ΛC/Cl) for both catalyzed reactions (ΛPd = 1.8 ± 0.13 and ΛRh = 2.1 ± 0.14) are markedly different than for the ZVI (ΛZVI = 5.8 ± 0.41), reflecting that the reactions proceed in different mechanisms. For hydrogen isotopes, while there was no clear trend for the catalyzed reactions, an inverse secondary hydrogen isotope effect was observed for the reaction of CF with ZVI.

Olefin Cyclopropanation Catalyzed by Iridium(III) Porphyrin Complexes

Tetratolylporphyrinato (TTP) iridium complexes were shown to be extremely active and robust catalysts for the cyclopropanation of olefins using diazo compounds as carbene sources. Ir(TTP)CH3 (1) catalyzed the cyclopropanation of styrene with ethyl diazoacetate (EDA) at −78 °C and achieved 4.8 × 105 turnovers in three successive reagent additions with no sign of deactivation. High yields and moderate trans selectivities were attained for electron-rich and sterically unhindered substrates. A Hammett ρ+ value of −0.23 was determined by competition experiments with para-substituted styrenes. Furthermore, competitive cyclopropanation of styrene and styrene-d8 with EDA and 1 demonstrated a moderate inverse secondary isotope effect of 0.86 ± 0.03. These data are consistent with a catalytic cycle that proceeds through a metalloporphyrin carbene intermediate. Carbene transfer to olefin substrates appears to be rate limiting, as indicated by kinetic studies. Hexavalent iridium halogenato tetratolylporphyrinato comp...

Demonstrating the synergy of synthetic, mechanistic, and computational studies in a regioselective aniline synthesis.

Tri- and tetrasubstituted anilines are formed in good to excellent yields by the addition of ketones to vinamidinium salts (up to 98%). The reaction proceeds via the formation of dienone intermediates, which react to form an enamine with the liberated amine. In the case of a nitro, or dimethylaminomethylene substituent, the enamines undergo a facile electrocyclic ring closure to form a cyclohexadiene, which goes on to form anilines with a high degree of selectivity (up to 50:1) with a minor competing pathway proceeding via the enol providing phenols. Competition experiments using isotopic substitution reveal that the rate determining step en route to dienone is enol/enolate addition to the vinamidinium salt, which is characterized by an inverse secondary isotope effect (k(H/D) 0.7-0.9). Computational studies have been used to provide a framework for understanding the reaction pathway. The original proposal for a [1,5]-H shift was ruled out on the basis of the calculations, which did not locate a thermally accessible transition state. The minimum energy conformation of the enamine is such that a facile electrocyclic ring closure is ensured, which is corroborated by the experimental studies. A framework for understanding the reaction pathway is presented.

Potential Energy Surface for the Acid- and BF3-Catalyzed Rearrangement of Methylpropene Oxide

The acid- and BF3-catalyzed rearrangement of methylpropene oxide to methylpropanal has been investigated by density functional methods. Isotope effects are calculated and are consistent with the reaction occurring via a carbocation intermediate followed by a 1,2-hydride shift. Inverse secondary isotope effects for hydride migration reflect a combination of changes in C1−H(D) stretching and out-of-plane bending frequencies with a stronger force constant of the nonmigrating C1−H bond at the transition structure than in the carbocation. Gas-phase DFT(B3LYP)/6-31G* level calculations favor a geometry of the carbocation with the boron of BF3 below the plane of the carbon framework while B3LYP/6-31G*(SCI−PCM) single point solvation calculations favor a geometry with the boron in the plane. From this latter conformation hydride migration to either face of the carbocation is equally favored. The transition structure for hydride (deuteride) migration is early. A calculated correction applied to the experimentally observed result (MH/MD = 1.92) gives a primary isotope effect for the reaction (kH/kD = 1.56) close to the theoretically calculated value (kH/kD = 1.68).

A statistical study of the methyl nitrite isomerization reaction: inverse secondary isotope effects

Abstract Monte Carlo transition-state theory (MCTST) calculations were carried out for the methyl nitrite isomerization reaction. The results obtained in this study were compared with previous classical trajectory calculations in which three different potential energy surfaces were employed. Recrossings of the transition state are a source of disagreement between the trajectory and MCTST rates. However, `direct trajectories' are the major reason for the deviations. Analysis of the MCTST rates for several deuterium substitutions shows a slight inverse isotope effect, a result in line with the previous trajectory calculations. However, additional MCTST calculations including corrections for zero-point energy effects predict a normal isotope effect.

Electron Transfer Catalyzed [2 + 2] Cycloreversion of Benzene Dimers.

The catalysis of the [2 + 2] cycloreversion of the anti-o,o'-benzene dimer 1 and the syn-o,o'-naphthalene-benzene dimer 2 through thermal and photoinduced electron transfer is studied using experimental and computational methods. The reaction of the radical cations formed by electron transfer is at least 10(5) times faster than the thermal background reaction. It is demonstrated that the photoinduced electron transfer catalyzed reaction proceeds via an electron transfer sensitized pathway and that the observed inverse secondary deuterium isotope effect of 0.91 +/- 0.02 on the reaction is due to the equilibrium isotope effect on the electron transfer step. The relevance of these findings on the mechanism of the electron transfer catalyzed [2 + 2] cycloreversion of the biologically important cis,syn-cyclobutane-thymine dimer is also discussed.

Secondary Deuterium Kinetic Isotope Effects in Irreversible Additions of Hydride and Carbon Nucleophiles to Aldehydes: A Spectrum of Transition States from Complete Bond Formation to Single Electron Transfer

The competitive kinetics of hydride and organometallic additions to benzaldehyde-H and -D were determined at −78 °C using LiAlH4, LiBEt3H, NaBH4, LiBH4, LiAl(O-tert-butoxy)3H, NaB(OMe)3H, NaB(OAc)3H (at 20° C). methyl, phenyl, and allyl Grignard, and methyl-, phenyl-, n-butyl-, tert-butyl-, and allyllithium. The additions of hydride were found to have an inverse secondary deuterium kinetic isotope effects in all cases, but the magnitude of the effect varied inversely with the apparent reactivity of the hydride. In the additions of methyl Grignard reagent and of methyllithium and phenyllithium, inverse secondary deuterium isotope effects were observed; little if any isotope effect was observed with phenyl Grignard or n-butyl- and tert-butyllithium. With allyl Grignard and allyllithium, a normal secondary deuterium kinetic isotope effect was observed. The results indicate that rate-determining single-electron transfer occurs with allyl reagents, but direct nucleophilic reaction occurs with all of the other reagents, with the extent of bond formation dependent on the reactivity of the reagent. In the addition of methyllithium to cyclohexanecarboxaldehyde, a less inverse secondary deuterium kinetic isotope effect was observed than that observed in the addition of methyllithium to benzaldehyde, and allyllithium addition to cyclohexanecarboxaldehyde had a kinetic isotope effect near unity. The data with organometallic additions, which are not incompatible with observations of carbonyl carbon isotope effects, suggest that electrochemically determined redox potentials which indicate endoergonic electron transfer with energies less than ca. 13 kcal/mol allow electron-transfer mechanisms to compete well with direct polar additions to aldehydes, provided that the reagent is highly stabilized, like allyl species. Methyl- and phenyllithium and methyl and phenyl Grignard reagents are estimated to undergo electron transfer with endoergonicities greater than 30 kcal/mol with benzaldehyde, so these react by direct polar additions. A working hypothesis is that butyllithium reagents undergo polar additions, despite redox potentials which indicate less than 13 kcal/mol endoergonic electron transfer, because of the great exoergonicity associated with the two-electron addition, which is responsible for a low barrier for polar reactions.

Dynamical study of the dissociation and elimination channels in the decomposition of methyl nitrite

The dynamics of the two unimolecular reactions that initiate the thermal decomposition of methyl nitrite were investigated by classical trajectories and statistical variational efficient microcanonical sampling-transition state theory. These two channels are (I) O–N bond dissociation to produce CH3O and NO and (II) concerted elimination through a four-center transition state to form CH2O and HNO. In order to perform both types of calculations, a potential energy function was developed, which reproduces reasonably well the energies, geometries, and frequencies selected from the literature. Microcanonical rate coefficients and branching ratios were obtained by each method at total energies ranging from 100 to 240 kcal/mol. The computed branching ratios indicated that reaction I is markedly faster than reaction II, which agrees with the experimental observations. It was found that for energies up to 160 kcal/mol the dynamics of reaction I is intrinsically Rice–Ramsperger–Kassel–Marcus (RRKM), but for the highest energies the behavior becomes intrinsically non-RRKM. The classical trajectories showed that the elimination process takes place via a regular dynamics during the last moments before reaction, which is clear evidence for nonstatistical behavior. Analysis of the trajectory rates computed for the deuterated species revealed that the dissociation process exhibits an inverse secondary isotope effect.

Unusual inverse secondary isotope effects: A statistical study of CD2HNC and CH3NC isomerization

Two formulations of microcanonical transition-state theory are used to compute isomerization rate constants k(E,J=0) for CD2HNC and CH3NC. The first of these uses a Monte Carlo method to approximate the classical flux integral expression for k(E,J=0). This method allows a direct comparison to be made with classical trajectory results. The second approach uses a quantum, harmonic state counting method to evaluate k(E,J=0) within the familiar RRKM formalism. The results of the transition-state theory calculations differ at low energies, but at high energies an unusual inverse secondary isotope effect is observed in both. This is analyzed in terms of the RRKM formalism and it is concluded that the presence of a methyl free-rotor mode at the transition state is responsible for the onset of the inverse isotope effect. A similar, but somewhat larger inverse isotope effect has been observed in high-energy classical trajectory calculations.

A classical trajectory study of CD2HNC isomerization

Classical trajectory studies of CD2HNC isomerization have been carried out using a simple potential energy function. Mode-specific effects are found to be small, and the fastest rate of reaction is observed for a microcanonical ensemble. Comparison with the CH3NC isomerization reaction reveals an unexpected inverse secondary isotope effect. Rotational excitation about the molecular axis is found to enhance the rate of reaction more than rotation perpendicular to this axis.

Isotope effects and alternative substrate reactivities for tryptophan 2,3-dioxygenase.

Tryptophan 2,3-dioxygenase (EC 1.13.1.12) is a hemoprotein which catalyzes the first step in the oxidative degradation of tryptophan. The reaction is believed to proceed by addition of O2 across the 2,3-bond of the indole ring, followed by decomposition of the resultant dioxetane to give N-formylkynurenine. A primary D2O isotope effect of 4.4 on Vmax/Km was observed at the pH optimum, pH 7.0. This implies that abstraction of the indole proton is at least partially rate-determining. An inverse secondary isotope effect of 0.96 was observed for L-[2-3H]tryptophan at this pH. The secondary isotope effect signals the formation of the C-O bond at C-2. As the rate of proton abstraction increased with increasing pH, the D2O isotope effect decreased to 1.2 at pH 8.5 and the secondary isotope effect increased to 0.92. The rate-determining steps therefore change with increasing pH, and bond formation at C-2 becomes more rate-limiting. The secondary isotope effect did not change significantly with varying O2 concentration so that substrate binding is primarily ordered with O2 binding first. The specificity of the enzyme towards substituted tryptophans shows that substitution of the phenyl ring of the indole is sterically unfavorable. Steric hindrance is highest at the 4- and 7-positions, while the 5- and 6-positions are less sensitive. 6-Fluoro-L-tryptophan was more reactive than tryptophan, and the increased reactivity can be explained by an electronic effect that enhances of the rate of C-O bond formation at C-2.

Direct measurement of the inverse secondary isotope effects of Rh(I)-C 2H 4 and Rh(I)-C 2H 3D utilizing gas chromatography

The inverse secondary equilibrium isotope effects of Rh(I)-C 2H 4 and Rh(I)-C 2H 3D were directly measured using a gas chromatographic column of dicarbonyl-rhodium(I)-3-trifluoroacetyl-1 R-camphorate in squalane solution at 283-333 K. Statistical isotope effects were also obtained from a reduced partition function using harmonic vibrational frequencies of RHC-C 2H 4 complexes and normal-mode analysis. The observed isotope effects were in good agreement with those deduced from the reduced partition function. Thermodynamic data of the inverse isotope effect were Δ D.H H = −469 ± 12 J mol −1 and Δ D.HΔ S = −0.975 ± 0.017 J mol −1 K −1 where Δ D.H H = Δ H D - Δ H H = the difference of the enthalpy changes of the deuterated and non-deuterated compounds and Δ D.H S = Δ S D - Δ S H = the difference of the entropy changes of the deuterated and non-deuterated compounds. The detailed analysis of the force constant most affected, F c-c = 8.39 mdyn/Å, by metal complexation was closer to that of the carbon-carbon double bond (9.1 mdyn/Å) than to that of the carbon--carbon single bond (4.3 mdyn/Å).

Hydrogen isotope effects in the photodissociation of C 2H 2 and C 2D 2 molecules superexcited at 16.85 and 21.22 eV

In the photodissociative excitations of C 2H 2 and C 2D 2 at 16.85 and 21.22 eV, the electronic transitions observed are A 2Δ→X 2Π r, B 2Σ −→X 2Π r and C 2Σ +→X 2Π r or CH and CD radicals, d 3Π g→a 3Π u, e 3Π g→a 3Π u, C 1Π g→A 1Π u and D 1Σ + u→X 1Σ + g of C 2 radical, and the hydrogen Balmer series. The isotopic ratios of σ f(C * 2(H))/σ f(C * 2(D))=1.34 and 1.4, σ f(CH*)/σ f(CD*)=0.67 and 0.9 are obtained from the fluorescence cross sections (σ f) determined at 16.85 and 21.22 eV, respectively, where C * 2(H) and C * 2(D) represent the electronically excited C * 2 radicals produced from the superexcited C 2H ** 2 and C 2D ** 2. At the photon energy of 21.22 eV the ratio of σ f(H*)/σ f(D*)=1.3 is deduced from the fluorescence cross sections of the H and D Balmer lines. The values of the σ f(C * 2(H))/σ f(C * 2(D)) and σ f(H*)/σ f(D*) ratios show a normal primary hydrogen isotope effect. The obtained σ f(CH*)/σ f(CD*) ratios imply the inverse secondary hydrogen isotope effect, which may be due to the tunneling effect to form molecular hydrogen through a cis-bent transition state. The rates of relaxation of the nonionizing superexcited acetylene through radiation are estimated to be about 40% and 60% at 16.85 and 21.22 eV, respectively.

Alkene loss from metastable methyleneimmonium ions: Unusual inverse secondary isotope effect in ion‐neutral complex intermediate fragmentations

AbstractThe mechanism of propene elimination from metastable methyleneimmonium ions is discussed. The first field‐free region fragmentations of complete sets of isotopically labelled methyleneimmonium ions (H2C = \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm N}\limits^{\rm +} $\end{document}+R1R2: R1 = R2 = n‐C3H7; R1 = R2 = i‐C3H7; R1 = n ‐C3H7; R2 = C2H5; R1 = n‐C3H7; R2 = CH3; R1 = n‐C3H7; R2 = H) were used to support the mechanism presented. The relative amounts of H/D transferred are quantitatively correlated to two distinct mathematical concepts which allow information to be deduced about influences on reaction pathways that cannot be measured directly. Propene loss from the ions examined proceeds via ion‐neutral complex intermediates. For the di‐n‐propyl species rate‐determining and H/D distribution‐determining steps are clearly distinct Whereas the former corresponds to a 1,2‐hydride shift in a 1‐propyl cation coordinated to an imine moiety, the latter is equivalent to a proton transfer to the imine occurring from the 2‐propyl cation generated by the previous step. For the diisopropyl‐substituted ions which directly form the 2‐propyl cation‐containing complex, the rate‐determining hydride shift vanishes. The 2‐propyl cation‐containing complex can decompose directly or via an intermediate proton‐bridged complex. Competition of these routes is not excluded by the experimental results. Assuming a 2:1:3 distribution, a preference for the α‐ and β‐methylene of the initial n‐propyl chain as the source of the hydrogen transferred is detected for n‐propylimmonium ions containing a second alkyl chain R2. This preference shows a clear dependence on the steric influence of R2. During the transfer step isotopic substitution is found to affect the H/D distribution strongly. For the alternative route of McLafferty rearrangement leading to C2H4 loss, specific γ‐H transfer is observed.

Energy‐dependent reversal of secondary isotope effects on simple cleavage reactions: Tertiary amine radical cations with deuterium at remote positions

AbstractDeuterium substitution at remote sites gives rise to inverse secondary kinetic isotope effects on the α‐cleavage of a number of tertiary amines in the ion source, but to normal isotope effects on reactions occurring in the field‐free regions. The change from normal to inverse secondary isotope effects when the internal energy of the reacting ions increases is consistent with transition‐state models that involve slight lowering of vibrational frequencies also for bonds well removed from the site of cleavage. The isotope effect on the reactions of ions of high internal energy is caused by the influence that even small changes of isotope‐dependent frequencies have on the state sums (a statistical weight effect favouring loss of the labelled radical), whereas the behaviour of low‐energy ions is determined by the zero‐point energy changes which favour loss of the unlabelled fragment.

Deuterium isotope effects in the photochemistry of 2,5-dimethyl- and 2,5-di-tert-butyl-1,3,5-hexatrienes

Abstract Deuterium isotope effects on the efficiencies of light-induced rearrangements of 2,5-dimethyl- and 2,5-di- tert -butylhexatrienes are reported. The 4π electrocyclizations show an inverse secondary isotope effect, in agreement with the existence of the theoretically predicted barrier on the potential energy surface of the excited state. In the formation of allylcyclopropene derivatives isotope effects provide evidence for a mechanism involving a biradical intermediate. The surprisingly large decrease in the efficiency of E — Z isomerizations in E - and Z -2,5-di- tert -butyl-1,3,5-hexatrienes after deuteration of the central double bond is ascribed to a reduced frequency of the hydrogen out-of-plane deformation which initiates the reaction.

The thermal cycloaddition of substituted thiobenzophenones with phenylallene: mechanism and linear free energy relationships of reaction rates

The reaction rates for the thermal cycloadditions of substituted thiobenzophenones with phenylallene in CH3CN at 40 °C were determined and analysed by linear free energy correlations. Using a Hammett-type relationship, the best correlations were obtained with the σ–-parameter (ρ–= 1.09 ± 0.06) indicating that, in the transition state, mesomeric interaction occurs between –M substituents and an electron-releasing reaction centre. The small ρ–-value is in agreement with a 1,4-diradical intermediate in a (π2s+π2s+π2s)-cycloaddition.Separate correlation of electron-donating and electron-withdrawing substituents gives rise to two lines with different slopes, each with a much better correlation coefficient and ψ. This observation indicates that the thione system is relatively more senstive to electron-donating substituents than to electron-withdrawing substituents i.e. a π-repulsive saturation interaction occurs with the π-acceptor thiocarbonyl function. Additional analysis with Charton's DSP-equation (ρR/ρ1= 1.82) shows that the contribution of resonance effect is 69% and the contribution of inductive effect is 31%. Bromilow's DSP–non-linear resonance effect equation yields a value of +0.38 ± 0.01 for the electron demand parameter', indicating the enhanced responance interaction of electron-acceptor substituents with the thione system in the transition state. An inverse secondary isotope effect on the rate constant of disapperance of thione during the reaction with PhCHCCH2, PhCDCCH2, and PhCDCCD2 was observed; k2(Hα)/k2(Dα)= 0.91 ± 0.02 and k2(2Hγ)/k2(2Dγ)= 0.75 ± 0.01. These values are less than unity and consistent with attack at C-β and indicative for a change in hybridization at C-α and C-γ. The observed isotope effect when using D3CO-CHCCH2 instead of H3CO-CHCCH2, k2(H)/k2(D)= 1.20 ± 0.02 represents a rotational isotope effect arising from an increase in mass on substitution of hydrogen by deuterium. Experiments with an optically active allene (+)-PhC(H)CCHBut yielded products which were optically inactive, which points to a non-chiral intermediate i.e. the allylic diradical. We observed also a low solvent effect which is in agreement with the reaction mechanism i.e. a 1,4-diradical-mediated (π2s+π2s+π2s)-cycloaddition.

The resonance Raman spectrum of the aniline radical cation

The time-resolved resonance Raman spectrum of the aniline radical cation has been observed using pulse radiolysis methods. This radical exhibits a large inverse secondary isotope effect on deuteration of the NH 2 group on the CN stretching frequency. Various spectral features indicate that this radical is structurally similar to the phenoxyl radical.

On the mechanism of the acid-catalyzed hydrolysis of uridine to uracil. Evidence for 6-hydroxy-5,6-dihydrouridine intermediates.

In acidic media, the 5,6-double bond of uridine is rapidly hydrated to give a small amount of 6-hydroxy-5,6-dihydrouridine (Urd-H2O), the mechanism of which is known from studies of the acid-catalyzed dehydration of Urd-H2O (Prior, J. J., Maley, J., and Santi, D. V. (1984) J. Biol. Chem. 258, 2422-2428). In addition to dehydration, Urd-H2O also undergoes direct hydrolysis of the N-glycosidic bond in acidic solution. The kinetics of the above reaction demonstrates that Urd-H2O, or an intermediate in the pathway leading from Urd to Urd-H2O, is kinetically competent to account for the hydrolysis of the N-glycosidic bond of Urd. The hydrolysis of (1'-2H)Urd proceeds with an alpha-secondary deuterium isotope effect of kH/kD of 1.11 at 25 degrees C. This isotope effect is sufficiently large to implicate carbonium ion character at the 1'-carbon during hydrolysis but, since it is not the maximal value expected, suggests that N-glycoside cleavage is rate-determining with a transition state intermediate between reactant and products. Importantly, the hydrolysis of [6-3H]Urd proceeds with a substantial inverse secondary isotope effect of kT/kH = 1.15 at 25 degrees C which indicates some degree of sp2 to sp3 rehybridization of C-6 of the pyrimidine moiety during hydrolysis. From the data available, it appears that an important pathway in the hydrolysis of the N-glycoside bond of Urd involves either spontaneous cleavage of Urd which is protonated at the 5-carbon or a protonated species of Urd-H2O. The studies described here, together with the known susceptibility of the 6-position of pyrimidine heterocycles toward nucleophiles, permits the proposal of chemically reasonable mechanisms for enzyme-catalyzed cleavage of N-glycosidic bonds of pyrimidines.