Ring-opening Barrier Research Articles

Hydroxyl and amino dual-functionalized core-shell molecular sieves featuring hydrogen bond donor groups for efficient CO2 cycloaddition

In CO2 cycloaddition reactions, hydrogen bond donor (HBD) groups are considered environmentally friendly substitutes for metals to promote epoxide ring-opening through interactions with nucleophilic anions. A core-shell structured ILs-based catalyst (mSiO2@MCM-NH2-OH) with dual hydrogen bond donors (–OH and –NH2) was synthesized by copolymerization strategy. Through in-depth characterization, it has been demonstrated that the catalyst (mSiO2@MCM-NH2-OH) possesses multiple catalytic active sites including –OH, –NH2, Br- groups, and the synergistic effect of double HBD groups (–OH and –NH2) and Lewis base (Br-) significantly improved the catalytic activity. Meanwhile, the core-shell structure of the catalyst effectively prevents the loss of active components, which makes the yield remain at about 94 % after 10 cycles. Based on Density Functional Theory (DFT) calculations, a synergistic catalytic mechanism, which involves dual hydrogen-bond donors (–OH and –NH2) and Lewis bases (Br-) was proposed. The cooperative interaction between –OH/–NH2 and Br- reduced the ring-opening barrier of epoxide from 58.6 to 32.0 kcal mol−1 significantly, and thereby facilitated the CO2 cycloaddition reaction.

Linear Scaling Relationships for Furan Hydrodeoxygenation over Transition Metal and Bimetallic Surfaces.

Brønsted-Evans-Polanyi (BEP) and transition-state-scaling (TSS) relationships have become valuable tools for the rational design of catalysts for complex reactions like hydrodeoxygenation (HDO) of bio-oil (containing heterocyclic and homocyclic molecules). In this work, BEP and TSS relationships are developed for all the elementary steps of furan activation (C and O hydrogenation and CHx -OHy scission, for both ring and open-ring intermediates) to oxygenates, ring-saturated compounds and deoxygenated products on the most stable facets of Ni, Co, Rh, Ru, Pt, Pd, Fe and Ir surfaces using Density Functional Theory (DFT) calculations. Furan ring opening barriers were found to be facile and strongly dependent on carbon and oxygen binding strength on the investigated surfaces. Our calculations suggest linear chain oxygenates form on Ir, Pt, Pd and Rh surfaces due to their low hydrogenation and high CHx -OHy scission barriers, while deoxygenated linear products are favoured on Fe and Ni surfaces due to their low CHx -OHy scission and moderate hydrogenation barriers. Bimetallic alloy catalysts were also screened for their potential HDO activity and PtFe catalysts were found to significantly lower the ring opening and deoxygenation barriers relative to the corresponding pure metals. The developed BEPs for monometallic surfaces can be extended to estimate the barriers on bimetallic surfaces for ring opening and ring hydrogenation reactions but fails to predict the barriers for open-ring activation reactions due to the change in transition state binding sites on the bimetallic surface. The obtained BEP and TSS relationships can be used to develop microkinetic models for facilitating accelerated catalyst discovery for HDO.

Unraveling reactive oxygen species-dependent toluene mineralization routes via construction of TixSn1−xO2 infinite solid solution photocatalysts

Intermediates seriously affect the reaction process and rate in terms of kinetics. However, there are few studies on the effect of reactive oxygen species on intermediates of toluene mineralization. Herein, SnO2 and TixSn1−xO2 infinite solid solution photocatalysts are prepared and the mineralization ability of Ti0.1Sn0.9O2 is 2.7 times that of SnO2. Introduction of Ti shifts the conduction band position upward, providing thermodynamic possibility for superoxide radicals formation. The sole hydroxyl radicals over SnO2 lead to the generation of benzyl alcohol; while synergetic hydroxyl radicals and superoxide radicals result in benzoic acid existed in solid solution catalysts. Theoretical calculation confirms that benzoic acid has lower ring opening barrier, thereby improving toluene mineralization ability over TixSn1−xO2. The synergetic roles of Ti and Sn in adsorption and activation of water and toluene moleculars are also proposed. This work provides new insights into reactive oxygen radicals regulation to alter intermediates and ease toluene removal.

Unraveling the Unique Role of Methyl Position on the Ring-Opening Barrier in Photocatalytic Decomposition of Xylene Isomers

Unraveling the Unique Role of Methyl Position on the Ring-Opening Barrier in Photocatalytic Decomposition of Xylene Isomers

Coupling-oxidation process promoted ring-opening degradation of 2-mecapto-5-methyl-1,3,4-thiadizaole in wastewater

As an important raw material and intermediate of widely used antibiotics cefazolin and cefazedone, 2-mecapto-5-methyl-1,3,4-thiadizaole (MMTD) in antibiotic wastewater is hardly decyclized during wastewater treatment, posing great risk to the environment. This work proposed a green “coupling-oxidation” process to enhance ring-opening of MMTD during antibiotic wastewater treatment. In particular, the significant role of humic substances (HS) as pre-coupling reagent was emphasized. Real HS and different model HS, especially quinones, not only efficiently pre-coupled MMTD (>95%) but also promoted the MMTD removal by MnO2 (from 72.4% to 92.4%). Mass spectrometric analysis indicated that MMTD pre-coupled to HS would be oxidized with ring opening to environmental-friendly sulfonated HS, while direct oxidation of MMTD produced MMTD dimers or sulfonated MMTD that would not undergo ring opening. Theoretical calculations indicated that pre-coupling to HS enabled the ring-opening oxidation by increasing the nucleophilicity and decreasing ring-opening barrier of thiadiazole. Based on the density function theory (DFT), the global nucleophilicity index (Nu) followed the trend of HS-MMTD > MMTD dimer > sulfonated MMTD, while the ring-opening barrier followed the trend of HS-MMTD (274 kJ/mol) < first ring of MMTD dimers (286 kJ/mol) < MMTD (338 kJ/mol). Theoretical calculations further confirmed that the cross-coupled HS-MMTD intermediate was more likely to be decyclized than intermediates from direct oxidation. Moreover, nitrogen, acetaldehyde group, sulfate and CO2 were the products of thiadiazole ring degradation. Pre-coupling of MMTD with HS provides a new idea and strategy in developing a green and sustainable scheme for wastewater treatment.

Unveiling the unconventional roles of methyl number on the ring-opening barrier in photocatalytic decomposition of benzene, toluene and o-xylene

The ring-opening reaction is the rate-determining step for photocatalytic decomposition of aromatic VOCs with different methyl number. However, as a key challenge, the intrinsic relationship between the methyl number and ring-opening reaction efficiency has not been revealed. In order to clarify the ring-opening barriers and enhance the photocatalytic efficiency, the photocatalytic degradation pathway was systematically investigated aiming to elucidate the pivotal roles of methyl number on ring-opening mechanisms. The SnO2 is chosen as a photocatalyst to achieve highly efficient and stable photocatalytic degradation of benzene, toluene, and o-xylene respectively. It is unexpected to discover that the photocatalytic degradation performance and quantum efficiency are significantly elevated along with the increase of methyl number, reaching the highest efficiency for o-xylene with two methyl groups. The photocatalytic degradation efficiency of o-xylene on SnO2 is as high as 78.2 %, much higher than that of toluene (63.4 %) and benzene (55.9 %). The introduction of methyl groups could exert great influence on the distribution of electrons on the phenyl ring, enhance the adsorption and activation of benzene rings, and finally reduce the energy barrier of ring-opening reaction. The methyl number in the benzene ring is dominantly responsible for the elevated decomposition efficiency. This work proposes a new concept in mechanistic understanding of VOCs decomposition and propels the application of photocatalytic technology for efficient degradation of aromatic VOCs.

Theoretical Prediction of the Strong Solvent Effect on Reduced Ethylene Carbonate Ring-Opening and Its Impact on Solid Electrolyte Interphase Evolution

With continuous growth in demand for lithium-ion batteries (LIBs) with higher capacity retention and longer lifetime, solid electrolyte interphase (SEI) formation allowing for tunable structure is critical to improve efficiency in current LIBs and implement next-generation technologies. To enable engineering of the SEI layer, a deeper understanding of the mechanisms and kinetics underlying its formation processes is required. We first demonstrate the importance of the rate of ring opening of reduced ethylene carbonate (c-EC–) to its chain-like conformation (o-EC–). According to our classical molecular dynamics simulations, c-EC– is far more likely to diffuse away from the anode than o-EC–, implying that longer c-EC– lifetimes would correlate with thicker SEI layers and reduced capacity retention. Our work then illustrates that the activation barrier for the c-EC– → o-EC– reaction can be strongly influenced by the entropic effects associated with the reorganization of the surrounding molecules. Ab initio molecular dynamics simulations combined with metadynamics sampling clearly demonstrate a considerable decrease (by a factor of 2.5) in the ring-opening barrier when considering a 50/50 mixture of cyclic EC and linear dimethyl carbonate as the solvent rather than pure liquid EC. This is not predicted by implicit solvent methods owing to their failure to capture the effect of solvent geometry. Our findings also suggest that other perturbations of the local environment not considered, such as those due to interfaces, anions, or other reduction products, may also significantly affect the diffusion/reaction kinetics of EC– radical anion, and, thus, SEI formation and evolution.

Effect of Extended Benzannelation Orientation on Bergman and Related Cyclizations of Isomeric Quinoxalenediynes.

A combined computational and experimental study was conducted to examine the effect of extended benzannelation orientation on C1-C5 and C1-C6 cyclization of acyclic quinoxalenediynes. Calculations (mPW1PW91/cc-pVTZ//mPW1PW91/6-31G(d,p)) on terminal and phenylethynyl-substituted 5,6-diethynylquinoxaline and 6,7-diethynylquinoxaline showed C1-C6 Bergman cyclization as the favored thermodynamic reaction pathway, with larger C1-C6 preference for the angular quinoxalenediynes due to gain of a new aromatic sextet. Kinetic studies, as a function of 1,4-cyclohexadiene concentration, revealed retro-Bergman ring opening predominates over hydrogen atom abstraction (k-1 > k2) for 6,7-diethynylquinoxaline while 5,6-diethynylquinoxaline undergoes irreversible Bergman cyclization indicative of a large retro-Bergman ring opening barrier (k2 > k-1). The effect of extended linear versus angular benzannelation on reaction pathway shows in the contrasting photocyclizations of phenylethynyl derivatives. While angular 5,6-diethynylquinoxalines gave exclusive C1-C6 photocyclization, linear 6,7-diethynylquinoxaline afforded C1-C5 fulvene products. Computed singlet-triplet gaps and biradical stabilization energies indicated weak interaction between the nitrogen lone pair and proximal radical center in angular 5,6-diethynylquinoxalines. The overall data indicates extended angular benzannelation effectively renders Bergman cyclization irreversible due to favorable aromatic stabilization energy, while extended linear benzannelation results in increased retro-Bergman ring opening, allowing C1-C5 cyclization to become a competitive reaction channel.

Anomerization reaction of bare and microhydrated d-erythrose via explicitly correlated coupled cluster approach. Two water molecules are optimal.

We present a comprehensive benchmark computational study which has explored a complete path of the anomerization reaction of bare d-erythrose involving a pair of the low-energy α- and β-furanose anomers, the former of which was observed spectroscopically (Cabezas et al., Chem. Commun. 2013, 49, 10826). We find that the ring opening of the α-anomer yields the most stable open-chain tautomer which step is followed by the rotational interconversion of the open-chain rotamers and final ring closing to form the β-anomer. Our results indicate the flatness of the reaction's potential energy surface (PES) corresponding to the rotational interconversion path and its sensitivity to the computational level. By using the explicitly correlated coupled cluster CCSD(T)-F12/cc-pVTZ-F12 energies, we determine the free energy barrier for the α-furanose ring-opening (rate-determining) step as 170.3 kJ/mol. The question of the number of water molecules (n) needed for optimal stabilization of the erythrose anomerization reaction rate-determining transition state is addressed by a systematic exploration of the PES of the ring opening in the α-anomer-(H2 O)n and various β-anomer-(H2 O)n (n = 1-3) clusters using density functional and CCSD(T)-F12 computations. These computations suggest the lowest free energy barrier of the ring opening for doubly hydrated α-anomer, achieved by a mechanism that involves water-mediated multiple proton transfer coupled with the furanose CO bond breakage. Among the methods used, the G4 performed best against the CCSD(T)-F12 reference at estimating the ring-opening barrier heights for both the hydrated and bare erythrose conformers. Our results for the hydrated species are most relevant to an experimental study of the anomerization reaction of d-erythrose to be carried out in microsolvation environment. © 2016 Wiley Periodicals, Inc.

Thermochemistry and Kinetic Analysis of the Unimolecular Oxiranyl Radical Dissociation Reaction: A Theoretical Study.

Oxirane structures are important in organic synthesis, and they are important initial products in the oxidation reactions of alkyl radicals. The thermochemical properties (enthalpy of formation, entropy, and heat capacity) for the reaction steps of the unimolecular oxiranyl radical dissociation reaction are determined and compared with the available literature. The overall ring opening and subsequent steps involve four types of reactions: β-scission ring opening, intramolecular hydrogen transfer, β-scission hydrogen elimination, and β-scission methyl radical elimination. The enthalpies of formation of the transition states are determined and evaluated using six popular Density Functional Theory (DFT) calculation methods (B3LYP, B2PLYP, M06, M06-2X, ωB97X, ωB97XD), each combined with three different basis sets. The DFT enthalpy values are compared with five composite calculation methods (G3, G4, CBS-QB3, CBS-APNO, W1U), and by CCSD(T)/aug-cc-pVTZ. Kinetic parameters are determined versus pressure and temperature for the unimolecular dissociation pathways of an oxiranyl radical, which include the chemical activation reactions of the ring-opened oxiranyl radical relative to the ring-opening barrier. Multifrequency quantum Rice Ramsperger Kassel (QRRK) analysis is used to determine k(E) with master equation analysis for falloff. The major overall reaction pathway at lower combustion temperatures is oxiranyl radical dissociation to a methyl radical and carbon monoxide. Oxiranyl radical dissociation to a ketene and hydrogen atom is the key reaction path above 700 K.

Ab initio molecular orbital and density functional studies on the ring-opening reaction of 2H-thiete

Electrocyclic ring opening (ERO) reaction of 2H-thiete has been carried out computationally in the gas phase and the ring opening barrier has been computed. When comparing the ERO reaction of 2H-thiete with the parent hydrocarbon (cyclobutene), the ring opening of cyclobutene is found to exhibit pericyclic behavior while 2H-thiete shows pseudopericyclic nature. Computation of the nucleus-independent chemical shift (NICS) of 2H-thiete adds evidence for pseudopericyclic behavior. Locking of the lone pair of electrons by hydrogen bonding was carried out to understand the extent of involvement of the lone pair of electrons during the course of the reaction. CR-CCSD(T)/6-311+G∗∗ computation was performed to assess the energies of the reactant, transition state and the product more accurately.

Ab initio molecular orbital and density functional studies on the ring-opening reaction of oxetene.

Electrocyclic ring opening (ERO) reaction of 2H-Oxete (oxetene) has been carried out computationally in the gas phase and ring opening barrier has been computed. When comparing the ERO reaction of oxetene with the parent hydrocarbon (cyclobutene), the ring opening of cyclobutene is found to exhibit pericyclic behavior while oxetene shows mild pseudopericyclic nature. Computation of the nucleus-independent chemical shift (NICS) of oxetene adds evidence for pseudopericyclic behavior of oxetene. By locking of lone pair of electrons by hydrogen bonding, it is seen that the pseudopericyclic nature of the ring opening of oxetene is converted into a pericyclic one. CASSCF(5,6)/6-311+G** computation was carried out to understand the extent of involvement of lone pair of electrons during the course of the reaction. CR-CCSD(T)/6-311+G** computation was performed to assess the energies of the reactant, transition state and the product more accurately.

Free Energy Surface for Brønsted Acid-Catalyzed Glucose Ring-Opening in Aqueous Solution

Car-Parrinello-based molecular dynamics coupled with metadynamics simulations were used to determine the mechanism and associated free energy surface for opening the ring structure of cyclic glucopyranose in acidic aqueous solutions. The ring-opening process is initiated by the protonation of the ring oxygen atom and the breakage of the C1-O5 bond. The barrier for this process is about 25 kcal/mol, in good agreement with experimental measurements. Moreover, the glucose cyclic conformation is found to be more stable than the open chain form. The barrier for proton-catalyzed ring-opening in aqueous solution appears to be largely solvent induced due to the high affinity of water molecules for protons.

Effect of Immobilization on Gold on the Temperature Dependence of Photochromic Switching of Dithienylethenes

We report the properties and switching characteristics of a series of dithienylethene photochromic switches immobilized on gold. Self-assembled monolayers (SAMs) of three structurally related dithienylethenes were formed on roughened gold bead substrates and studied by surface-enhanced Raman spectroscopy (SERS). These data were compared to SERS spectra obtained by aggregation of colloidal gold, solid state Raman spectra, and Raman spectra calculated using density functional theory (DFT). Two of the dithienylethenes studied have an “asymmetric” design, which was demonstrated earlier to lower the thermal barrier for photochemical ring opening in solution. Herein, we show that, when immobilized on a gold surface, the asymmetric dithienylethenes in fact display a higher thermal barrier than that of their symmetric counterparts. In addition, we show that photochemical ring closing of asymmetric dithienylethenes is inhibited when immobilized on gold.

Potential Energy Surfaces for Rearrangements of Berson Trimethylenemethanes

In this research, thermal rearrangements of the Berson trimethylenemethanes (Berson-TMMs) have been investigated by employing density functional theory (DFT) and high-level ab initio methods, such as the complete active space self-consistent field (CASSCF), multireference second-order Møller-Plesset perturbation theory (MRMP2), multireference configuration interaction singles and doubles (MRCISD), and coupled-cluster singles and doubles with perturbative triples [CCSD(T)]. In all computations Pople's polarized triple-ζ split valence basis set, 6-311G(d,p), is utilized. The relevant portions of the lowest-energy, singlet-spin potential energy surface of the C(4)H(6) (parent TMM), C(6)H(8) (Berson-TMMa), and C(8)H(12) (Berson-TMMc) chemical systems have been explored in order to determine the reaction energies and activation parameters accurately, with the ultimate objective of providing a theoretical account of experiments by Berson on TMMc. The nature of the orthogonal and the planar structures of the parent TMM have been clarified in this study. We have concluded that the orthogonal TMM (1)B(1) minimum has a C(2v) symmetry structure, and there is no pyramidalization in the unique methylene group. It lies at 13.9 kcal mol(-1) above the triplet minimum (3)B(2) at MRCISD level. The closed-shell (1)A(1) state of the planar TMM is not a true minimum but a transition structure (TS) for 180° rotation of the unique methylene group in the orthogonal TMM minimum. It lies at 3.0 kcal mol(-1) above (1)B(1). The planar structures are also involved in the interchange of equivalent orthogonal TMMs (o(1), o(2), o(3)). Many features of the parent TMM are retained in TMMa and TMMc, despite the constraints imposed by the five-membered ring in the latter species. Thus, ring closure to the bicyclic molecules 3a (3c) and 5a (5c) takes place similarly to that in the parent TMM. Likewise, planar TMMa (TMMc) structures are TSs, while orthogonal ones are true minima. The adiabatic singlet-triplet gaps are also similar, being 14.7 (13.0) and 16.5 (16.2) kcal mol(-1) in the orthogonal (o(1)) and planar TMMa (TMMc), respectively. It has been shown here that the substantial reductions in the ring-opening barriers of MCP derivatives 3a (3c) and 5a (5c) can be largely attributed to ring strain in the former and π-bond strain in the latter species.

ChemInform Abstract: The Ring Opening of Cyclopropylidene to Allene. Key Features of the Accurate Reaction Surface.

The global potential energy surface, determined in the first paper [1] for the groundstate ring opening of cyclopropylidene to allene, is complemented by accurate calculations of its key regions. The basis set is extended and polarization functions are included. The full configuration space of four electrons in four reactive orbitals is enlarged to the full configuration space of eight electrons in eight active orbitals by including correlations in the unbroken, but stretched CC sigma bonds. The effect of further single and double excitations is examined. The geometries and relative energies of the critical regions are found to change only little except for the ring-opening energy barrier which is lowered to about 7 kcal/mol, in good agreement with the experimental estimate of about 6 kcal/mol. Furthermore, the bifurcation is shown to occurafter the transition state, in the neighborhood of a conical intersection on the steepest descent path from the ring-opening transition state to the allene isomerization transition state. The steepest descent paths and the conical intersection are documented in detail. The cogwheel-like free internal rotation of the two methyl groups is confirmed by the accurate calculations. A similar richness of features is believed to exist on many potential energy surfaces governing chemical reactions.

Understanding the Planar Tetracoordinate Carbon Atom: Spiropentadiene Dication

The atoms in molecule theory shows that the spiropentadiene dication has a planar tetracoordinate carbon (ptC) atom stabilized mainly through the sigma bonds and this atom has a negative charge. The bonds to the ptC atom have less covalent character than the central carbon from neutral spiropentadiene. The total positive charge is spread along the structure skeleton. The analysis of the potential energy surface shows that the dication spiropentadiene has a 2.3 kcal/mol activation barrier for ring opening.

Computational study on the conversion of an aziridine and iminium salt to a 1,2-diamine

DFT calculations employing the SCRF-B3LYP/6-311+G(d) level of theory have been employed in order to quantitatively characterize the mechanism of the ring-opening reaction of (s)-1-benzyl-2-methyl-aziridine and an iminium salt to a 1,2-diamine, in the presence of lithium iodide. Computational results indicate that the optimal reaction pathway may proceed via an aziridinium cation intermediate mechanism, rather than by an S N2 mechanism. Self-consistent reaction field (SCRF) calculations, using the PCM model and THF as solvent, showed an energetic barrier for aziridine ring-opening to be on the order of ∼10.9 kcal/mol; indicating the process to be thermodynamically spontaneous. The catalytic role of I − and substituent effects are also elaborated upon herein.

The design of molecules containing planar tetracoordinate carbon

A novel preference for planar tetracoordination was observed over the conventional tetrahedral arrangement in a new series of C 5H 2, C 5H 4, C 5H 4 1+/2+ and related compounds. The stability of these molecules is assessed with the ring-opening barriers, HOMO–LUMO gap, singlet–triplet energy differences and nucleus independent chemical shift values.

Molecular orbital calculations of ring opening of the isoelectronic cyclopropylcarbinyl radical, cyclopropoxy radical, and cyclopropylaminium radical cation series of radical clocks.

Detailed molecular orbital calculations were directed to the cyclopropylcarbinyl radical (1), the cyclopropoxy radical (2), and the cyclopropylaminium radical cation (3) as well as their ring-opened products. Since a considerable amount of data are published about cyclopropylcarbinyl radicals, calculations were made for this species and related ring-opened products as a reference for 2 and 3 and their reactions. Radicals 1-3 have practical utility as "radical clocks" that can be used to time other radical reactions. Radical 3 is of further interest in photoelectron-transfer processes where the back-electron-transfer process may be suppressed by rapid ring opening. Calculations have been carried out at the UHF/6-31G*, MP4//MP2/6-31G*, DFT B3LYP/6-31G*, and CCSD(T)/cc-pVTZ//QCISD/cc-pVDZ levels. Energies are corrected to 298 K, and the barriers between species are reported in terms of Arrhenius E(a) and log A values along with differences in enthalpies, free energies, and entropies. The CCSD(T)-calculated energy barrier for ring opening of 1 is E(a) = 9.70, DeltaG* = 8.49 kcal/mol, which compares favorably to the previously calculated value of E(a) = 9.53 kcal/mol by the G2 method, but is higher than an experimental value of 7.05 kcal/mol. Our CCSD(T)-calculated E(a) value is also higher by 1.8 kcal/mol than a previously reported CBS-RAD//B3LYP/6-31G* calculation. The cyclopropoxy radical has a very small barrier to ring opening (CCSD(T), E(a) = 0.64 kcal/mol) and should be a very sensitive time clock. Of the three series studied, the cyclopropylaminium radical cation is most complex. In agreement with experimental data, bisected cyclopropylaminium radical cation is not found, but instead a ring-opened species is found. A perpendicular cyclopropylaminium radical cation (4) was found as a transition-state structure. Rotation of the 2p orbital in 4 to the bisected array results in ring opening. The minimum onset energy of photoionization of cyclopropylamine was calculated to be 201.5 kcal/mol (CCSD(T)) compared to experimental values of between about 201 and 204 kcal/mol. Calculations were made on the closely related cyclopropylcarbinyl and bicyclobutonium cations. Stabilization of the bisected cyclopropylcarbinyl conformer relative to the perpendicular species is much greater for the cations (29.1 kcal/ mol, QCISD) compared to the radicals (3.10 kcal/mol, QCISD). A search was made for analogues to the bicyclobutonium cation in the radical series 1 and 2 and the radical cation series 3. No comparable species were found. A rationale was made for some conflicting calculations involving the cyclopropylcarbinyl and bicyclobutonium cations. The order of stability of the cyclopropyl-X radicals was calculated to be X = CH2 >> X = O > X = NH2+, where the latter species has no barrier for ring opening. The relative rate of ring opening for cyclopropyl-X radicals X = CH2 to X = O was calculated to be 3.1 x 10(6) s(-1) at 298 K (QCISD).