From roads to biobanks: Roadkill animals as a valuable source of genetic data.

To protect biodiversity we must understand its structure and composition including the bacteria and microparasites associated with wildlife, which may pose risks to human health. However, acquiring this knowledge often presents challenges, particularly in areas of high biodiversity where there are many undescribed and poorly studied species and funding resources can be limited. A solution to fill this knowledge gap is sampling roadkill (animals that die on roads as a result of collisions with circulating vehicles). These specimens can help characterize local wildlife and their associated parasites with fewer ethical and logistical challenges compared to traditional specimen collection. Here we test this approach by analyzing 817 tissue samples obtained from 590 roadkill vertebrate specimens (Amphibia, Reptilia, Aves and Mammalia) collected in roads within the Tropical Andes of Ecuador. First, we tested if the quantity and quality of recovered DNA varied across roadkill specimens collected at different times since death, exploring if decomposition affected the potential to identify vertebrate species and associated microorganisms. Second, we compared DNA stability across taxa and tissues to identify potential limitations and offer recommendations for future work. Finally, we illustrate how these samples can aid in taxonomic identification and parasite detection. Our study shows that sampling roadkill can help study biodiversity. DNA was recovered and amplified (allowing species identification and parasite detection) from roadkill even 120 hours after death, although risk of degradation increased overtime. DNA was extracted from all vertebrate classes but in smaller quantities and with lower quality from amphibians. We recommend sampling liver if possible as it produced the highest amounts of DNA (muscle produced the lowest). Additional testing of this approach in areas with different environmental and traffic conditions is needed, but our results show that sampling roadkill specimens can help detect and potentially monitor biodiversity and could be a valuable approach to create biobanks and preserve genetic data.

Enhancing CO2 reduction through the catalytic effect of a novel silicon haeckelite-inspired 2D material.

We propose a novel 2D material based on silicon haeckelite (Hck), whose structure contains a silicon atom arranged in a periodic pattern of pentagons and heptagons. Stacking the two layers gives rise to a planar geometry of the layers that compose it. This new structure presents a semiconductor character with a band gap of 0.17 eV. Furthermore, we studied CO2 reduction using molecular hydrogen to form formic acid, carbon monoxide, formaldehyde, methanol, and methane. All these have been studied theoretically at the Grimme D3BJ corrected TPSS/def2-SVP level. A massive biflake containing 132 Si atoms was used to model the Hck surface. According to the results, CO2 capture with Hck is a spontaneous step; in contrast, the same process for silicene mono- and bi-flakes studied previously was endergonic. After the capture of CO2, the addition of H2 to the substrate passes through an intermediate containing a Si-H bond. The formation of Si-H intermediates is the origin of the catalytic effect, facilitating H2 dissociation and acting as the hydrogen atom donor for the substrate. These intermediates are transformed by adding hydrogen atoms and losing water molecules, producing formic acid and formaldehyde as the most probable products, with rate-controlling steps of 29.2 and 27 kcal mol-1, whose values were less than those exhibited by the silicene biflake. This means that the silicon haeckelite biflake presents better catalytic activity than the silicene biflake. The results show that the novel 2D silicon hackelite material has remarkable potential for CO2 capture and reduction. The theoretical analysis of this innovative 2D structure provides valuable insights into the potential applications of silicene-based materials.

Catalytic activity of silicene biflakes for CO2 reduction: a theoretical approach.

The use of renewable energy sources to reduce carbon dioxide (CO2) emissions has gained significant attention in recent years. The catalytic reduction of CO2 into value-added products is a promising approach to achieve this goal, and silicene biflakes (2Si) have been identified as potential candidates for this task. In this study, we explored the catalytic activity of these structures using density functional theory calculations. Our results show that the reaction pathway involves the adsorption of CO2 onto the silicene surface, followed by the addition of hydrogen molecules to form products such as formic acid, methanol, methane, carbon monoxide, and formaldehyde. Our proposed mechanism indicates that silicene biflakes exhibit a higher affinity for CO2 than single-layer silicon. We also found that the hydrogenation with H2 occurs by adding one hydrogen atom to the absorbed CO2 and another to the surface of 2Si. Intermediate species are reduced by systematically adding hydrogen atoms and removing water molecules, forming formic acid as the most probable product. The rate-controlling step for this reaction has an energy of 32.9 kcal mol-1. In contrast, the process without a catalyst shows an energy of 74.6 kcal mol-1, suggesting that the silicon bilayer is a structure with outstanding potential to capture and reduce CO2. Our study provides important insights into the fundamental mechanisms underlying the silicene-mediated CO2 reduction and could facilitate the development of more efficient catalysts for this process.

Trifluoromethyl-stabilized 2D nitrogen clusters: a theoretical study.

The stability of 2D all nitrogen clusters containing from 6 to 96 nitrogen atoms, terminated with CF3 groups, has been explored using two computational models: dispersion corrected B3LYP functional and scaled opposite spin Møller-Plesset perturbation theory (SOS-MP2). Single point domain-based local pair natural orbital coupled-cluster theory calculations (DLPNO-CCSD(T)) was used for further energy refinement. All systems were found to be minima, and their stability increases with CF3/N ratio. Larger clusters and anion radicals were not dynamically stable, while some of the cation radicals were found to be minima on potential energy surface. The mechanism of cluster stabilization by CF3 groups is related with interaction of orbitals holding lone electron pairs and antibonding sigma orbitals.

Complexes of Li, Na, and Mg with 2D allotropies of second and third period: a theoretical study.

Complexes of Li, Na, and Mg with graphene, silicene, phosphorene nanoflakes (NFs), and their 2D allotropies have been studied at dispersion corrected TPSS/def-TZVP level of theory. The energy partition analysis of the complexes revealed that for most of the complexes exchange and correlation energies represent dominant contributions to the binding with strong charge transfer from the metal atom to a NF. The exceptions are Mg complexes of graphene and phosphorene NFs where binding is due to dispersion and correlation terms. This difference is also reflected in large Mg-NF distances suggesting weak intermolecular interactions in these complexes. The calculated activation energies for metal hopping are easily achievable at room temperatures for carbon and silicon allotropies. However, they are significantly higher for phosphorus allotropies reaching almost 18 kcal/mol. Generally, activation energies for hopping increase with binding energies for graphene, silicene, and phosphorene NFs. This trend does not observe however for graphene, silicene, and phosphorene 2D allotropies.

Water clusters as bifunctional catalysts in organic chemistry: the hydrolysis of oxirane and its methyl derivatives.

This contribution explores the bifunctional catalytic activity of water clusters ((H2O)n with n = 1-5) in organic chemistry similar to that observed in the formation of H2SO4 in acid rain (Chem. Commun., 53, 3516, (2017)). We considered for this purpose the Hydrolysis of Epoxides (HE), in particular, that of oxirane and its methyl derivatives. Surrounding water molecules with H-bond cooperative effects decrease the activation energy of the rate-limiting step of HE in condensed phase, especially when they lead to an anti-periplanar attack on the alkoxide leaving group. Furthermore, the water molecules have a bifunctional catalytic role in HE by (i) increasing the nucleophilic and electrophilic character of the attacking oxygen atom and the leaving group of the reaction, respectively, and (ii) placing the reactants in a suitable disposition for the substitution to occur. Overall, this investigation provides relevant insights into the collective action of water molecules on organic reactions in neutral, basic and acid media.

In silico modeling: electronic properties of phosphorene monoflakes and biflakes substituted with Al, Si, and S heteroatoms.

This contribution explores the systematic substitution of phosphorene monoflakes (Mfs) and biflakes (Bfs) with aluminum, silicon, and sulfur. These systems were investigated using density functional theory employing the TPSS exchange-correlation functional and complete active space self-consistent field (CASSCF) calculations. Al and Si substitution produces significant structural changes in both Mfs and Bfs compared to S-substituted and pristine systems. However, in Mfs, all heteroatoms generate a decrease in band gap and the ionization potentials (IP), and an increase in electron affinity (EA) in comparison with pristine phosphorene. Al doping improves the hole mobility in the phosphorene monoflake, while Si and S substitutions exhibit a similar behavior on EAs and reorganization energies. For Bfs, the presence of Si-Si and Al-P interlaminar interactions causes structural changes and higher binding energies for Si-Bfs and Al-Bfs. Regarding the electronic properties of Bfs, substitution with Si does not produce significant variations in the band gap. Nevertheless, it conduces the formation of hole transport materials, which does not occur in Si-Mfs. The same is observed for Al systems, whereas no correlation was identified between the doping level and reorganization energies for S complexes. The substitution with Al and S leads to an opposite behavior of the band gap and IP values, while the EA variation is similar. In summary, the nature of heteroatom and the doping degree can modify the semiconductor character and electronic properties of phosphorene mono- and biflakes, whose trends are closely related to the atomic properties considered. Overall, these computational calculations provide significant insights into the study of doped phosphorene materials.

Novel 2D allotropic forms and nanoflakes of silicon, phosphorus, and germanium: a computational study.

The structural variability offered by 2D materials is an essential feature in materials design. Despite its significance, obtaining assemblies with suitable stability remains a challenge. In this work, we theoretically explore novel silicon, phosphorus, and germanium, analogues of haeckelites at hybrid DFT level. Both 2D systems and nanoflakes (NF) have been studied. All materials have been found dynamically stable; Si-, P-, and Ge- analogues of haeckelites were found to be more stable in comparison to the corresponding honeycomb structure than haeckelites in comparison with graphene. All 2D materials showed metallic behavior; however, the difference between inorganic haeckelites and the corresponding honeycomb allotropes is less than that between haeckelites and graphene. Si-, P-, and Ge-, allotropes have much higher electron affinities (EAs) compared to carbon allotropes, while haeckelites have higher EAs than honeycomb structures. Furthermore, Si-, P-, and Ge-structures also exhibit low hopping activation energies for lithium atoms. It makes these materials potentially promising as a component in Li-ion batteries.

On the strength of hydrogen bonding within water clusters on the coordination limit.

Hydrogen bonds (HB) are arguably the most important noncovalent interactions in chemistry. We study herein how differences in connectivity alter the strength of HBs within water clusters of different sizes. We used for this purpose the interacting quantum atoms energy partition, which allows for the quantification of HB formation energies within a molecular cluster. We could expand our previously reported hierarchy of HB strength in these systems (Phys. Chem. Chem. Phys., 2016, 18, 19557) to include tetracoordinated monomers. Surprisingly, the HBs between tetracoordinated water molecules are not the strongest HBs despite the widespread occurrence of these motifs (e.g., in ice Ih ). The strongest HBs within H2 O clusters involve tricoordinated monomers. Nonetheless, HB tetracoordination is preferred in large water clusters because (a) it reduces HB anticooperativity associated with double HB donors and acceptors and (b) it results in a larger number of favorable interactions in the system. Finally, we also discuss (a) the importance of exchange-correlation to discriminate among the different examined types of HBs within H2 O clusters, (b) the use of the above-mentioned scale to quickly assess the relative stability of different isomers of a given water cluster, and (c) how the findings of this research can be exploited to indagate about the formation of polymorphs in crystallography. Overall, we expect that this investigation will provide valuable insights into the subtle interplay of tri- and tetracoordination in HB donors and acceptors as well as the ensuing interaction energies within H2 O clusters.

The electronic structure of van der Waals heterostructures formed by the nanoflakes of black phosphorene with those of graphene and haeckelites: their complexes with Li.

The electronic structure of the van der Waals heterostructures (HSs) of the phosphorene (P) nanoflakes (NFs) with graphene (G) and its allotropy (H1 and H2) NFs, and their complexes with Li have been studied using dispersion-corrected TPSS functional. According to the calculations, the attractive interactions in HSs come from dispersion. It has a relatively small contribution to the binding energy in Li complexes, especially for these forming complexes with G, H1, or H2 NF side. The binding energies between the individual NFs and Li atoms increase in the order G < H1 = H2 = P. The formation of HSs results in a synergetic effect for Li binding energies. This effect is the most notable for phosphorene binding sites; however, it also holds for G, H1, and H2 NFs. The formation of complexes with Li always leads to the almost complete charge transfer from Li to the NFs or HSs. In the case of HSs, the unpaired electron of Li is always located at the carbon NF side independently on the Li binding location. The activation energies of Li hopping for individual NFs are notably higher for P comparing with G, H1, or H2 NFs. The formation of HSs rises slightly the activation energies of Li hopping due to the increase of binding energies in Li-HS complexes. Graphical abstract.

Electronic structure of hybrid pentaheptite carbon nanoflakes containing boron-nitrogen motifs.

The electronic structure of isomeric graphene nanoflakes (NFs) heavily doped with boron and nitrogen atoms has been explored. Dispersion-corrected B3LYP functional has been used for the geometry optimizations. A complete active space method has been used for the energy evaluations. Combined boron and nitrogen doping promotes polyradicalic antiferromagnetic ground states in the NFs and affects the nanoflake geometry. There is a charge transfer from boron to nitrogen atoms which increases with the doping level. This transfer does not involve carbon atoms. Combined doping reduces both the ionization potentials (IPs) and the electron affinities (EAs) of the NFs similar to nitrogen doping alone. Boron does not affect either IPs or EAs being neither n- nor p-type dopant for the isomeric graphene NFs. All hybrid NFs show a tendency to increase the band gaps with doping level, which is promoted by the increment of the bond length alternation with doping. Finally, the hole reorganization energies for the NFs were found to be lower than the electronic ones, positioning the hybrid NF as hole-transporting systems. Graphical Abstract Color coded natural charge differences between charged and neutral states. The excess of positive charge is green for cation radicals and the excess of negative charge is red in anion radicals.

The effect of chiral N-substituents with methyl or trifluoromethyl groups on the catalytic performance of mono- and bifunctional thioureas.

We evaluated thiourea organocatalysts that incorporate a chiral group which includes a trifluoromethyl moiety and contrasted their performance with non-fluorinated analogs. The comparison between such systems allows the direct study of the NH acidity of a thiourea bonded to an aliphatic substituent. In principle, -CF3 systems feature an enhanced hydrogen bond (HB) donor capacity that is undoubtedly beneficial for HB-catalysis applied to the Baylis-Hillman reaction. We found that the thiourea substituted on both nitrogens with this group accelerates this reaction like Schreiner's thiourea. On the other hand, we observed a different behavior in reactions promoted by bifunctional catalysts (thiourea-primary amine). In the Michael addition of isobutyraldehyde to methyl benzylidenepyruvate, the -CF3 containing catalysts were better than the -CH3 systems, whereas the conjugate addition to N-phenylmaleimide showed the opposite behavior. Theoretical calculations of the transition states indicated that the phenylethyl group in fluorinated and non-fluorinated compounds have different kinds of interactions with the electrophile. These interactions are responsible for a different arrangement of the electrophile and thereby the selectivity of the catalyst. Therefore, it cannot be generalized that in all cases NH acidity correlates with the performance of the catalyst, particularly, with aliphatic substituents that unlike the aromatic ones possess groups that are outside the plane of the thiourea.

Stability of doubly and triply H-bonded complexes governed by acidity-basicity relationships.

We used our recently proposed acidity-basicity interplay (ABI) model (Chem. Sci., 2018, 9, 4402) and the Jorgensen secondary interactions hypothesis (JSIH) to rationalise the experimentally observed trends in the formation constants of doubly and triply H-bonded systems with -NHO[double bond, length as m-dash]C- and -NHN- interactions. Unlike the JSIH, the ABI interpretation can explain the trends in the complexation of amide/imide homo- and heterodimers as well as ADA-DAD clusters. We found that the strongest H-bonds play a very important role, a condition which offers an alternative to the well established JSIH to modulate the stability of these relevant systems.

Acidity and basicity interplay in amide and imide self-association.

Amides dimerise more strongly than imides despite their lower acidity. Such an unexpected result has been rationalised in terms of the Jorgensen Secondary Interactions Hypothesis (JSIH) that involves the spectator (C[double bond, length as m-dash]OS) and H-bonded (C[double bond, length as m-dash]OHB) carbonyl groups in imides. Notwithstanding the considerable body of experimental and theoretical evidence supporting the JSIH, there are some computational studies which suggest that there might be other relevant intermolecular interactions than those considered in this model. We conjectured that the spectator carbonyl moieties could disrupt the resonance-assisted hydrogen bonds in imide dimers, but our results showed that this was not the case. Intrigued by this phenomenon, we studied the self-association of a set of amides and imides via 1H-NMR, 1H-DOSY experiments, DFT calculations, QTAIM topological analyses of the electron density and IQA partitions of the electronic energy. These analyses revealed that there are indeed repulsions of the type OS···OHB in accordance with the JSIH but our data also indicate that the C[double bond, length as m-dash]OS group has an overall attraction with the interacting molecule. Instead, we found correlations between self-association strength and simple Brønsted-Lowry acid/base properties, namely, N-H acidities and C[double bond, length as m-dash]O basicities. The results in CDCl3 and CCl4 indicate that imides dimerise less strongly than structurally related amides because of the lower basicity of their carbonyl fragments, a frequently overlooked aspect in the study of H-bonding. Overall, the model proposed herein could provide important insights in diverse areas of supramolecular chemistry such as the study of multiple hydrogen-bonded adducts which involve amide or imide functional groups.

The bifunctional catalytic role of water clusters in the formation of acid rain.

State-of-the-art chemical bonding analyses show that water clusters have a bifunctional catalytic role in the formation of H2SO4 in acid rain. The embedded H2O monomers mitigate the change in the chemical bonding scenario of the rate-limiting step, reducing thereby the corresponding activation energy in accordance with Hammond's postulate. We expect that the insights given herein will prove useful in the elucidation of the catalytic mechanisms of water in inorganic and organic aqueous chemistry.

Bifunctional Thioureas with α-Trifluoromethyl or Methyl Groups: Comparison of Catalytic Performance in Michael Additions.

Thioureas are an important scaffold in organocatalysis because of their ability to form hydrogen bonds that activate substrates and fix them in a defined position, which allows a given reaction to occur. Structures that enhance the acidity of the thiourea are usually used to increase the hydrogen-bonding properties, such as 3,5-bis(trifluoromethyl)phenyl and boronate ureas. Herein, we report the synthesis of bifunctional thioureas with a chiral moiety that include either a trifluoromethyl or methyl group. Their catalytic performance in representative Michael addition reactions was used in an effort to compare the electronic effects of the fluorination at the methyl group. The observed differences concerning yields and ee values cannot be attributed solely to the different steric environments; theoretical results indicate distinct interactions within the corresponding transition states. The calculated transition states show that the fluorinated catalysts have stronger N-H···O and C-H···F hydrogen bonds, while the nonfluorinated systems have C-H···π contacts. These results have shown that a variety of hydrogen-bonding interactions are important in determining the yield and selectivity of thiourea organocatalysis. These details can be further exploited in catalyst design.

Electronic structure and noncovalent interactions within ion-radical complexes of N-(2-furylmethyl)aniline molecular ions.

We investigate the electronic structure and noncovalent interactions within cation-radical complexes that are relevant in the electron impact mass spectrometry of N-(2-furylmethyl)anilines, 4-R-C6H4-NH-CH2-C4H3O with (R = -H, -OCH3, -CH3, -F, -Cl, -Br). In particular, we consider the reactive intermediates that precede the final products of two previously suggested dissociation pathways for these systems, i.e., (i) a direct cleavage of the NH-CH2 bond and (ii) an isomerization/fragmentation mechanism. The study is performed by means of correlated calculations (UCCSD and UMP2) together with density functionals (UM06 an UM06-2x) along with the triple-ζ quality basis set 6-311++G(2d,2p). In addition, we carried out a topological analysis of the electron density in accordance with the quantum theory of atoms in molecules (QTAIM) together with the examination of the noncovalent interaction (NCI) index. In contrast with previous studies based on the UB3LYP approximation, we could determine the transition states associated with both fragmentation pathways. The Rice-Ramsperger-Kassel-Marcus theory, used to determine the relative importance of these dissociation mechanisms, indicates that whereas the direct cleavage and the isomerization/fragmentation reaction routes have similar constant rates at low energy, the former prevails when the energy of the system is increased. The QTAIM analysis reveals that the charge of the cation-radical complex is mainly located on either a furfuryl (direct cleavage mechanism) or a pyrylium (isomerization/fragmentation pathway) ion and that these units interact with a neutral radical aniline moiety. The localization of the positive charge in either a furfuryl or pyrylium cation is in agreement with the preminecence of the m/z = 81 fragment in the mass spectrometry of N-(2-furylmethyl)anilines. Moreover, the QTAIM properties indicate that the α unpaired electron of the system is principally distributed over the nitrogen and the ortho and para carbon atoms with respect to the -NH group in the R-C6H4-NH unit. The investigation of the NCI index and the intermolecular bond critical points and bond paths gives an account of the NCIs linking the radical cation clusters under consideration. Finally, we found correlations which indicate that the concentration of the m/z = 81 fragment in a mass spectrum is reduced with the interaction energy of the radical complex from which it is originated. Altogether, this work shows how the combination of suitable electronic structure calculations along with post wave function analyses can yield important insights about the formation and properties of cation-radical complexes relevant in mass spectrometry.

Relationships between DFT/RRKM branching ratios of the complementary fragment ions [C5H5O]+ and [M - C5H5O]+ and relative abundances in the EI mass spectrum of N-(2-furylmethyl)aniline.

The energy-dependent branching ratios of the complementary fragment ions [C(5)H(5)O](+) and [HC(6)H(4)NH](+) ([M - C(5)H(5)O](+)), originating from the N-(2-furylmethyl)aniline molecular ion, [HC(6)H(4)NH-C(5)H(5)O](+•), were obtained from Rice-Ramsperger-Kassel-Marcus (RRKM) rate calculations based on density functional theory (DFT) energy profiles. The UB3LYP/6-311G+(3df,2p)//UB3LYP/6-31G(d) level of theory was used to model the competitive reaction mechanisms by which the molecular ion can be fragmented. Initially, eight pairs of products were taken into account, corresponding to the combination of two isomeric structures for each fragment ion and the concomitant radicals, which can be formed by direct dissociations or through some isomerization-fragmentation pathways. A great deal of the obtained pathways was discarded by looking over the kinetic barrier heights and the individual RRKM rate coefficients calculated for all the steps. This way, the potential energy profiles were simplified to only three reaction channels, two pathways to [C(5)H(5)O](+) and one to [M - C(5)H(5)O](+). The pre-equilibrium and steady-state approximations were then applied to different regions of the remaining potential energy profiles, allowing the branching ratios of the complementary fragment ions to be easily calculated and discriminated among the three rival processes. According to these results, the major fragment ion in the ion source is [C(5)H(5)O](+), which is produced as a mixture of two structures, the furfuryl and pyrylium cations, one formed by a direct C-N bond cleavage and the other through an isomerization-fragmentation channel. In turn, the direct fragmentation is the only mechanism to produce [M - C(5)H(5)O](+). To confront these results with the available experimental information, the model was broadened out to the 4-substituted analogues [4-R-C(6)H(4)NH-C(5)H(5)O](+•) in which R = F, Br, Cl, CH(3), and OCH(3), finding excellent correlations of the calculated branching ratios and the relative abundances in the electron ionization mass spectra.

Density functional theory and RRKM calculations of decompositions of the metastable E-2,4-pentadienal molecular ions.

The potential energy profiles for the fragmentations that lead to [C(5)H(5)O](+) and [C(4)H(6)](+*) ions from the molecular ions [C(5)H(6)O](+*) of E-2,4-pentadienal were obtained from calculations at the UB3LYP/6-311G + + (3df,3pd)//UB3LYP/6-31G(d,p) level of theory. Kinetic barriers and harmonic frequencies obtained by the density functional method were then employed in Rice-Ramsperger-Kassel-Marcus calculations of individual rate coefficients for a large number of reaction steps. The pre-equilibrium and rate-controlling step approximations were applied to different regions of the complex potential energy surface, allowing the overall rate of decomposition to be calculated and discriminated between three rival pathways: C-H bond cleavage, decarbonylation and cyclization. These processes should have to compete for an equilibrated mixture of four conformers of the E-2,4-pentadienal ions. The direct dissociation, however, can only become important in the high-energy regime. In contrast, loss of CO and cyclization are observable processes in the metastable kinetic window. The former involves a slow 1,2-hydrogen shift from the carbonyl group that is immediately followed by the formation of an ion-neutral complex which, in turn, decomposes rapidly to the s-trans-1,3-butadiene ion [C(4)H(6)](+*). The predominating metastable channel is the second one, that is, a multi-step ring closure which starts with a rate-limiting cis-trans isomerization. This process yields a mixture of interconverting pyran ions that dissociates to the pyrylium ions [C(5)H(5)O](+). These results can be used to rationalize the CID mass spectrum of E-2,4-pentadienal in a low-energy regime.

Density functional theory and RRKM calculations of the gas-phase unimolecular rearrangements of methylfuran and pyran ions before fragmentations.

The potential energy profiles for the mutual conversion of the isomeric molecular ions [C5H6O]+* of 2-methylfuran, 3-methylfuran and 4H-pyran and the fragmentations that lead to [C(5)H(5)O](+) ions were obtained from calculations at the B3LYP/6-311G++(3df,3pd)//B3LYP/6-31G(d,p) level of theory. The various competing unimolecular processes were characterized by their RRKM microcanonical rate coefficients, k(E), using the sets of reactant and transition state frequencies and the kinetic barriers obtained from the density functional method. In either a high- or a low-energy regime, the pyrylium ion [C5H5O]+ is generated directly from the 4H-pyran molecular ion by a simple cleavage. In contrast, in the metastable kinetic window, the molecular ions of methylfurans irreversibly isomerize to a mixture of interconverting structures before dissociation, which includes the 2H- and 3H-pyran ions. The hydrogen atoms attached to saturated carbons of the pyran rings are very stabilizing at the position 2, but they are very labile at position 3 and can be shifted to adjacent positions. Once 4H-pyran ion has been formed, the C-H bond cleavage begins before any hydrogen shift occurs. According to our calculation, there would not be complete H scrambling preceding the dissociation of the molecular ions [C5H6O]+*. On the other hand, as the internal energy of the 2-methylfuran molecular ion increases, H* loss can become more important. These results agree with the available experimental data.