Energy and spectroscopic parameters of neutral and cations isomers of the CnH2 (n = 2-6) families using high-level ab-initio approaches.

Cationic species, previously detected from ion-induced desorption of solid methane by plasma desorption mass spectrometry (PDMS), and neutral species, are investigated using high-level ab-initio approaches. From a set of 25 cationic and 26 neutral structures belonging to CnH2 (n = 2-6) families, it was obtained the energy, rotational constants, harmonic vibrational frequency, charge distribution and excitation energies. The ZPVE-corrected energies, at CCSD(T)-F12; CCSD(T)-F12/RI/(cc-pVTZ-F12, cc-pVTZ-F12-CABS, cc-pVQZ/C) (n = 2-5) and CCSD(T)/cc-pVTZ (n = 6) levels, reveal that the topology of the most stable isomer vary with n and the charge. Out of 674 harmonic frequencies, those with maximum intensity are generally in the 3000-3500 cm-1 range. Analysis of 169 vertical transition energies calculated with the EOM-CCSD approach, suggest three C6H2 species as potential carriers of the diffuse interstellar bands (DIB). Systematic comparison of properties between neutral and cationic species can assist in the structural description of complex matrices.

Deciphering the irradiation induced fragmentation-rearrangement mechanisms in valence ionized CF3CH2F.

The increasing presence of 1,1,1,2-tetrafluoroethane (CF3CH2F) in the atmosphere has prompted detailed studies into its complex photodissociation behavior. Experiments focusing on CF3CH2F irradiation have unveiled an array of ions, with the persistent observation of the rearrangement product CHF2+ not yet fully understood. In this work, we combine density functional theory, coupled-cluster calculations with a complete basis set formalism, and atom-centered density matrix propagation molecular dynamics to investigate the energetics and dynamics of different potential pathways leading to CHF2+. We found that the two-body dissociation pathway involving an HF rearrangement, which was previously considered complex for CHF2+ formation, is actually straightforward but not likely due to the facile loss of HF. In contrast, our calculations reveal that the H elimination pathway, once thought of as a potential route to CHF2+, is not only comparably disadvantageous from both thermodynamic and kinetic points of view but also does not align with experimental data, particularly the lack of a rebound peak at m/z 101-102. We establish that the formation of CHF2+ is predominantly via the HF elimination channel, a conclusion experimentally corroborated by studies involving the trifluoroethylene cation CF2CHF+, a key intermediate in this process.

Phosphine Reactivity and Its Implications for Pyrolysis Experiments and Astrochemistry.

Despite the importance of phosphorus-bearing molecules for life and their abundance outside Earth, the chemistry of those compounds still is poorly described. The present study investigates phosphine (PH3) decomposition and formation pathways. The reactions studied include phosphine thermal dissociation, conversion into PO(2Π), PN(1Σ+), and reactions in the presence of H2O+. The thermodynamic and rate coefficients of all reactions are calculated in the range of 50-2000 K considering the CCSD(T)/6-311G(3df,3pd)//ωB97xD/6-311G(3df,3pd) electronic structure data. The rate coefficients were calculated by RRKM and semiclassical transition-state theory (SCTST). According to our results, PH3 is stable to thermal decomposition at T < 100 K and can be formed promptly by a reaction network involving PH(3Σ-), PO(2Π), and PN(1Σ+). In the presence of radiation or ions, PH3 is readily decomposed. For this reason, it should be mainly associated with dust grains or icy mantles to be observed under the physical conditions prevailing in the interstellar medium (ISM). The intersystem crossing associated with the dissociation of the isomers PON, NPO, and PNO is accessed by multireference methods, and its importance for the gas-phase PH3 formation/destruction is discussed. Also, the implications of the present outcomes on phosphorus astrochemistry are highlighted.

Prediction of Rate Coefficients for the H2CO + OH → HCO + H2O Reaction at Combustion, Atmospheric and Interstellar Medium Conditions.

Despite the relevance of the H2CO + OH → HCO + H2O reaction for combustion, atmospheric, and interstellar medium conditions, a large discrepancy on energetic and kinetic data for this reaction is still observed in the previous literature. In this work, this hydrogen abstraction reaction has been investigated at the CCSD(T)/CBS level of theory, suggesting that both the prebarrier complex and saddle point are stabilized in relation to the reactants by 3.31 and 1.35 kcal mol-1, respectively. Moreover, from the Gibbs free energy profile of the reaction coordinate, it has been verified that the formation of the prebarrier complex is endergonic, for temperatures above 550 K. Hence, for temperatures lower than 550 K, the reaction is described by a mechanism consisting of three elementary steps, while for higher temperatures it can be assumed to be an elementary reaction. Finally, the prediction of rate coefficients suggests that unified statistical rate theory best applies to the low temperature regime, while canonical variational rate coefficients better fit experimental data at the high temperature regime.

The first microsolvation step for furans: New experiments and benchmarking strategies.

The site-specific first microsolvation step of furan and some of its derivatives with methanol is explored to benchmark the ability of quantum-chemical methods to describe the structure, energetics, and vibrational spectrum at low temperature. Infrared and microwave spectra in supersonic jet expansions are used to quantify the docking preference and some relevant quantum states of the model complexes. Microwave spectroscopy strictly rules out in-plane docking of methanol as opposed to the top coordination of the aromatic ring. Contrasting comparison strategies, which emphasize either the experimental or the theoretical input, are explored. Within the harmonic approximation, only a few composite computational approaches are able to achieve a satisfactory performance. Deuteration experiments suggest that the harmonic treatment itself is largely justified for the zero-point energy, likely and by design due to the systematic cancellation of important anharmonic contributions between the docking variants. Therefore, discrepancies between experiment and theory for the isomer abundance are tentatively assigned to electronic structure deficiencies, but uncertainties remain on the nuclear dynamics side. Attempts to include anharmonic contributions indicate that for systems of this size, a uniform treatment of anharmonicity with systematically improved performance is not yet in sight.

Solvation of barium atoms and singly charged cations in acetonitrile clusters.

The size distributions of neutral and cationic Ba x (CH3CN) n (x = 0, +1; n ≤ 7) clusters, as produced by a standard laser vaporization-supersonic expansion pick-up source, were determined from molecular beam experiments. The size distribution for cations is in the range of n = 1-7, whereas only the n = 1 complex is observed for neutral clusters, and these two features are unaffected by the variables controlling the performance of the cluster source. The distinct behavior is compatible with the expected charge-dipole interactions in the ionic species, which are stronger than the dipole induced-dipole interactions at play in neutral clusters, and it is corroborated by the relative magnitude of the theoretical successive binding energies (SBEs) for the lowest-lying isomers of cationic and neutral clusters with n = 1-5, as computed at the density functional theory level. The theoretical results also allow for the rationalization of the bimodal Ba+(CH3CN)1-7 size distribution, featuring an apparent minimum at n = 3, in terms of chiefly 6s-5d σ hybridization of the Ba+ ions, which ultimately leads to a relatively small third SBE for the Ba+(CH3CN)3 complex, as compared to those for n = 1, 2, and 4. Additional Born-Oppenheimer molecular dynamics simulations on the Ba+(CH3CN)2-4 clusters suggest that all of the ligands are coordinated to the Ba+ ion and prevent considering completion of the first solvent shell as responsible for the bimodal size distribution.

The furan microsolvation blind challenge for quantum chemical methods: First steps.

Herein we present the results of a blind challenge to quantum chemical methods in the calculation of dimerization preferences in the low temperature gas phase. The target of study was the first step of the microsolvation of furan, 2-methylfuran and 2,5-dimethylfuran with methanol. The dimers were investigated through IR spectroscopy of a supersonic jet expansion. From the measured bands, it was possible to identify a persistent hydrogen bonding OH-O motif in the predominant species. From the presence of another band, which can be attributed to an OH-π interaction, we were able to assert that the energy gap between the two types of dimers should be less than or close to 1 kJ/mol across the series. These values served as a first evaluation ruler for the 12 entries featured in the challenge. A tentative stricter evaluation of the challenge results is also carried out, combining theoretical and experimental results in order to define a smaller error bar. The process was carried out in a double-blind fashion, with both theory and experimental groups unaware of the results on the other side, with the exception of the 2,5-dimethylfuran system which was featured in an earlier publication.

SO3 formation from the X-ray photolysis of SO2 astrophysical ice analogues: FTIR spectroscopy and thermodynamic investigations.

In this combined experimental-theoretical work we focus on the physical and chemical changes induced by soft X-rays on sulfur dioxide (SO2) ice at a very low temperature, in an attempt to clarify and quantify its survival and chemical changes in some astrophysical environments. SO2 is an important constituent of some Jupiter moons and has also been observed in ices around protostars. The measurements were performed at the Brazilian Synchrotron Light Source (LNLS/CNPEM), in Campinas, Brazil. The SO2 ice sample (12 K) was exposed to a broadband beam of mainly soft X-rays (6-2000 eV) and in situ analyses were performed by IR spectroscopy. The X-ray photodesorption yield (upper limit) was around 0.25 molecules per photon. The values determined for the effective destruction (SO2) and formation (SO3) cross sections were 2.5 × 10-18 cm2 and 2.1 × 10-18 cm2, respectively. The chemical equilibrium (88% of SO2 and 12% of SO3) was reached after the fluence of 1.6 × 1018 photons cm-2. The SO3 formation channels were studied at the second-order Møller-Plesset perturbation theory (MP2) level, which showed the three most favorable reaction routes (ΔH < -79 kcal mol-1) in simulated SO2 ice: (i) SO + O2 → SO3, (ii) SO2 + O → SO3, and (iii) SO2 + O+ → SO3+ + e- → SO3. The amorphous solid environment effect decreases the reactivity of intermediate species towards SO3 formation, and ionic species are even more affected. The experimentally determined effective cross sections and theoretical reaction channels identified in this work allow us to better understand the chemical evolution of certain sulfur-rich astrophysical environments.

A theoretical study of three gas-phase reactions involving the production or loss of methane cations.

Hydrocarbon ions are important species in flames, spectroscopy and the interstellar medium. Their importance is reflected in the extensive body of literature on the structure and reactivity of carbocations. However, the geometry, electronic structure and reactivity of carbocations are difficult to assess. This study aims to contribute to the current knowledge of this subject by presenting a quantum mechanics description of methane cation dissociation using multiconfigurational methods. The geometric and electronic parameters of the minimum structure were determined for three main reaction paths: the dissociation CH4(+)→ CH2(+) + H2 and the dissociation-recombination processes CH4(+)↔ CH3(+) + H. The electronic and energetic effects of these reactions were analyzed, and it was found that each reaction path has a strong dependence on the methodology used as well as a strong multiconfigurational character during dissociation. The first doublet excited states are inner-shell excited states and may correspond to the ions that are expected to be formed after electron detachment. The rate coefficient for each reaction path was determined using variational transition state theory and RRKM/master equation calculations. The major dissociation paths, with their rate coefficients at the high-pressure limit, are CH4(+)(X(~)(2)B1) → CH3(+)(A(2)A1') + H((2)S) (k∞(T) = 1.42 × 10(+14) s(-1) exp(-37.12/RT)) and CH4(+)(X(~)(2)B1) → CH2(+)(A(2)A1) + H2((2)Σg(+)) (k∞(T) = 9.18 × 10(+14) s(-1) exp(-55.77/RT)). Our findings help to explain the abundance of ions formed from CH4 in the interstellar medium and to build models of chemical evolution.

Theoretical study of Δ-3-(+)-carene oxidation.

In this work, the rate-limiting steps of Δ(3)-carene oxidation by ozone and OH radicals were studied. The thermochemical and kinetic parameters were evaluated using the B3LYP, PBE1PBE and BHandHLYP functionals, coupled cluster methods and the 6-311G(d,p) and 6-311++G(d,p) basis sets. The attack on the double bond may occur in different orientations, leading to different oxidation products. The rate coefficients of each step of the reactions were evaluated using conventional canonical transition-state theory and variational canonical transition-state theory whenever necessary. The theoretical rate coefficient for the ozonolysis mechanism, evaluated at the CCSD(T)/6-31G(d,p)//PBE1PBE/6-311++G(d,p) level, was 2.08 × 10(-17) cm(3) molecule(-1) s(-1). The coefficient for the oxidation initialised by the OH radical, calculated at the BHandHLYP/6-311++G(d,p) level, was 5.06 × 10(-12) cm(3) molecule(-1) s(-1). These values are in reasonable agreement with the experimental results. The importance of these reactions in atmospheric chemistry is discussed.

Molecular Insights into the Binding and Conformational Changes of Hepcidin25 Blood Peptide with 4-Aminoantipyrine and Their Sorption Mechanism by Carboxylic-Functionalized Multiwalled Carbon Nanotubes: A Comprehensive Spectral Analysis and Molecular Dynamics Simulation Study.

In this work, the main purpose is to analyze and understand the mechanism and thermodynamic interactions of carboxylic acid-functionalized multiwalled carbon nanotubes (cf-MWCNTs) and 4-aminoantipyrine (AAP) with human hepcidine25 (Hep25) using multispectroscopic and molecular docking modeling methods, binding free energy calculations, and molecular dynamics (MD) simulations under physiological conditions. AAP belongs to a class of persistent environmental contaminants, and its residue is a potential hazard to human health, exhibiting a high binding affinity with blood peptides. Hepcidin is a 25-residue peptide hormone with four disulfide bonds that regulates the iron balance in vertebrates and contributes to host immunity as a cysteine-rich antimicrobial peptide. Due to their diverse properties and pollutant absorption capabilities, CNTs demonstrate important biological effects in biological applications, particularly in the noncovalent interactions with blood peptides. A comprehensive molecular dynamics simulation integrated with molecular docking methodologies was employed to explore the binding free energy between AAP and Hep25, identify binding sites, elucidate thermodynamic characteristics, and evaluate the binding forces governing their interaction. The investigation delved into elucidating the precise binding site of AAP within the Hep25 protein and thoroughly analyzed the impact of AAP on the microenvironment and conformational dynamics of Hep25. The circular dichroism (CD) experimental results highlight a reduction in ß-sheet composition following the introduction of AAP and cf-MWCNT. In addition, outcomes from fluorescence spectroscopy demonstrate that both cf-MWCNT and AAP significantly attenuated Hep-25 fluorescence via a static quenching mechanism. According to the MD simulations, the presence of AAP induces changes in the secondary structure of Hep25 and enhances its hydrophobicity. Additionally, our findings demonstrated that alongside the alteration in protein structure and functionality induced by contaminants, cf-MWCNTs possess the capability to mitigate the contaminant-induced effects on Hep25 activity while preserving the overarching structural integrity of Hep25. Based on the distance and RDF data, we found that during the simulation the presence of the cf-MWCNT causes the AAP to move away from the Hep25, and as a result fewer and weaker interactions of the AAP with the Hep25 will be observed. Likewise, free energy calculations indicate that the binding of Hep25 to AAP and cf-MWCNT involves electrostatic, π-cationic, and π-π stacking interactions. The research findings offer invaluable insights into the intricate influence of pollutants and carbon nanotubes on protein functionality within the circulatory system and their toxicity in vivo for prospective investigations.

Influence of natural exposure on castor oil based polyurethane reinforced with waste tire rubber.

Waste tire rubber (WTR) has been extensively generated worldwide due to mobility needs growth. About 1.5 billion units are generated annually, constantly discharged in the environment with a few reusability alternatives. Therefore, rubber recovery methods and these residues' transformation into a cost-effective product have gained attention. Aiming to minimize the usage of fossil resources and contributes to a circular economy, it was analyzed the usage of WTR particles (5-20% by weight) in castor oil-based polyurethane foams under natural aging to promote a holistic view of all factors involved in the performance of the foams. Morphological, thermal, chemical, and mechanical properties were determined before and after exposure to open air to observe the impact of photo-oxidation and hydrolysis. The increase in viscosity of pre-polymer during the rubber loading produced greater density foams with smaller cell sizes than neat PU, in which the average cell size increased after the weathering. The rubber contributes to enhancing the compressive behavior in the non-exposed samples. After exposure, the results suggest that degradation may act to increase the crosslinking density even with the presented structural changes such as yellowing and voids. Regarding thermal stability, the rubber promotes a slight decay in the ability to resist a heat flow before and after weathering. Still, the char yield increased, showing a possibility of better fire retardancy for composites. FTIR and UV-vis showed chemical structure changes as Photo-Fries network rearrangement, Norrish I random chain scission, and Norrish II ß-scission. Besides, UV-vis revealed the maximum absorbance in the UVB region, showing that the PU reinforced by WTR can be a promising material for civil coatings.

Theoretical study about the hydrogen abstraction reactions on methyl acetate on combustion conditions.

Methyl acetate is considered a prototype molecule to study biodiesel ignition and combustion and is seen as a possible fuel or fuel additive. For this reason, methyl acetate decomposition paths have been investigated in recent years. In the present study, hydrogen abstraction reactions on methyl acetate by OH(2Π), HO2(2A'), H(2Σg), O(3P), and O2(3Σg) were conducted, and the effect of methodology and anharmonic corrections on the rate coefficients were evaluated. The M06-2X and B3LYP-D3 functionals with the cc-pVDZ, cc-pVTZ, aug-cc-pVDZ, and aug-cc-pVTZ basis set were used for methodology evaluation and the CCSD(T)/cc-pVTZ//M06-2X/aug-cc-pVTZ for the rate coefficients calculation and literature comparisons. The rate coefficients were calculated in the range of 250-3500 K, including tunneling corrections, methyl-hindered rotations, and anharmonic effects calculated by the VPT2 method. The methodology analysis showed that the B3LYP-D3 functional leads to lower activation energies for all elementary reactions studied, and the double-zeta basis is insufficient to calculate precise rate coefficients. The inclusion of anharmonic corrections consistently lowered the rate coefficients of all elementary reactions studied and changed the Arrhenius plot profile with the temperature. Significant anharmonic effects were observed at higher temperatures, being the reaction with O2(3Σg) the most affected by this correction. Differences superior to 105 cm3 molec-1 s-1 in the rate coefficients were observed in some cases.

Kinetic and thermodynamic investigations on the HF elimination reactions from neutral and ionized CF3CH2F.

This work discusses the possible HF formation routes via recombination reactions from CF3CH2F (R-134a) and its cation. The molecular properties of the main reagents were first evaluated at the M06-2X/cc-pVTZ level. Then, changes in energy (ΔE) for all reactions comprising a possible HF formation from the studied systems were evaluated at the CCSD(T)/CBS//M06-2X/cc-pVTZ level. With the aid of Intrinsic Reaction Coordinate (IRC) calculations for each path, it is found that the HF formation reaction takes place majorly through the "1,2" elimination, resulting in an olefin as the secondary product. In turn, the IRC associated with "2,2" reactions allowed to find a post-barrier complex between the carbene :CHCF3 and HF in its exit channel, with dissociation energy of ∼4 kcal mol-1. Similarly, the cationic system exhibits favoritism towards the "1,2" elimination, and an ion-dipole post-barrier complex is found. The ΔE of such a complex production is negative in both directions, indicating this complex (25.5 kcal/mol more stable than CF3CH2F+) should be a minimum on the R-134a cation surface. However, unlike the neutral "2,2" path, there is no F atom migration transition state for the 2,2-HF elimination from CF3CH2F+. Hence, the F migration is expected to occur simultaneously with the rest of the structural changes towards the ion-dipole complex. The rate coefficients computed at the current level of theory, including corrections for anharmonicity and hindered rotations, showed a reasonable agreement with the available experimental data, inspiring confidence in our predictions for the cationic system.

Carbonyl oxides reactions from geraniol-trans-(3,7-dimethylocta-2,6-dien-1-ol), 6-methyl-5-hepten-2-one, and 6-hydroxy-4-methyl-4-hexenal ozonolysis: kinetics and mechanisms.

A density functional theory (DFT) study of the mechanisms of carbonyl oxide reactions from geraniol-trans, 6-methyl-5-hepten-2-one, and 6-hydroxy-4-methyl-4-hexenal ozonolysis is presented. The geometries, energies, and harmonic vibrational frequencies of each stationary point were determined by B3LYP/6-31(d,p) and BH&HLYP/cc-pVDZ methods. According to the calculations, the ozonolysis reactions are initiated by the formation of van der Waals (VDW) complexes to yield primary ozonides, which rapidly open to carbonyl oxide compounds. These carbonyl oxide compounds react to form dioxanes and hydroperoxides. The hydroperoxides react by isomerization to form stable products. Glyoxal and methyl-glyoxal have been identified as the final product from geraniol-trans, 6-methyl-5-hepten-2-one, and 6-hydroxy-4-methyl-4-hexenal ozonolysis. Our results are in good agreement with the experimental studies.

Kinetics and thermodynamics of limonene ozonolysis.

Using density functional methods, the initial reaction steps of limonene ozonolysis have been investigated with a focus on primary ozonide formation and its decomposition to Criegee intermediates and carbonyl compounds. The ozonide formation is highly exothermic, and the decomposition channels have similar free energies of activation, ΔG(‡), indicating that there is no primary pathway for ozonide decomposition. Assuming that ozonide formation is the rate limiting step, the theoretical rate coefficient, k = 1.6 × 10(-16) molecule(-1) cm(3) s(-1), evaluated at the CCSD(T)/6-31G(d,p)//BHandHLYP/cc-pvdz level and 298.15 K for d-limonene is in good agreement with the experimental value, k(exp) = 3.3 × 10(-16) molecule(-1) cm(3) s(-1). The theoretical Arrhenius expression is also in good agreement with experimental results.

Theoretical study of the addition of OH radicals to trans-geraniol-(3,7-dimethylocta-2,6-dien-1-ol), 6-methyl-5-hepten-2-one, and 6-hydroxy-4-methyl-4-hexenal.

A combined density functional theory and transition state theory study of the gas-phase addition of OH to 3,7-dimethylocta-2,6-dien-1-ol (trans-geraniol), 6-methyl-5-hepten-2-one, and 6-hydroxy-4-methyl-4-hexenal is presented. In this study, all different possibilities for the addition of the OH radical to the C-C double bonds in trans-geraniol, 6-methyl-5-hepten-2-one, and 6-hydroxy-4-methyl-4-hexenal were considered. The geometries, energies, and harmonic vibrational frequencies at each stationary point were determined at the MPW1K/cc-pVDZ and BH&HLYP/cc-pVDZ levels. Global rate coefficients of 0.94 x 10(-10) and 3.1 x 10(-10) cm(3) molecule(-1) s(-1), 2.11 x 10(-11) and 7.53 x 10(-11) cm(3) molecule(-1) s(-1), and 2.70 x 10(-13), and 4.37 x 10(-12) cm(3) molecule(-1) s(-1) were calculated using data obtained at the BH&HLYP/cc-pVDZ and MPW1K/cc-pVDZ levels of theory. These coefficients correspond to the sum of the rate coefficients of the individual paths for trans-geraniol, 6-hydroxy-4-methyl-4-hexenal, and 6-methyl-5-hepten-2-one, when reacting with OH radicals. The calculated rate coefficients are in good agreement with the available experimental data.

Theoretical investigation on the stability of negatively charged formic acid clusters.

Recent experimental results on negatively charged formic acid clusters generated by the impact of (252)Cf fission fragments on icy formic acid target are compared to quantum mechanical calculations. Structures for the clusters series, (HCOOH)nOH(-), where 2 < or = n < or = 4, are proposed based on ab initio electronic structure methods. The results show that cluster growth does not have a regular pattern of nucleation. A stability analysis was performed considering the commonly defined stability function. Temporal behavior of the clusters was evaluated by Born-Oppenheimer molecular dynamics to check the mechanism that provides cluster stability. The evaluated temporal profiles indicate the importance of hydrogen atom migration between the formic acid moieties in maintaining the stability of the structures and the water formation due to hydrogen abstraction by the hydroxyl approach.

The Valsalva maneuver in Chagas disease patients without cardiopathy.

OBJECTIVES: To perform a meta-analysis of studies using the Valsalva ratio (VR) in Chagas disease (ChD) patients without cardiopathy in comparison to control subjects in order to determine if vagal heart modulation is impaired in early forms of ChD. METHODS: The medical literature was systematically searched and reviewed for cross-sectional studies in humans in which the Valsalva maneuver was used to evaluate the autonomic modulation of the heart in ChD patients without cardiopathy. The Hedges g statistic (software Medcalc v.9.4.0.0) was used to combine the results of all studies through the calculation of the summary standardized mean difference (SMD): the mean VR in controls minus the mean VR in ChD patients, divided by the pooled and adjusted standard deviation. The sample size necessary to detect this SMD with 80% of power was calculated using the G*Power software v.3.0. RESULTS: The meta-analysis included 396 patients in 7 studies. Summary mean VRs were 1.87 +/- 0.39 ms in controls versus 1.74 +/- 0.40 ms in ChD without cardiopathy, leading to a statistically significant summary SMD of -0.310 (95% CI -0.513 to -0.106). Considering the summary SMD of -0.31, an alpha error of 0.05 and a 1:1 ratio of ChD and control patients, the estimated total sample size for a study with 80% power was 330 patients. INTERPRETATION: ChD patients without cardiopathy have reduced VR values compared to healthy controls, indicating early vagal dysfunction. Most previous studies could not detect this impairment due to small study samples and insufficient statistical power.

Theoretical investigation on the stability of ionic formic acid clusters.

Recent experimental results on positive charged formic acid clusters generated by the impact of (252)Cf fission fragments (FF) on icy formic acid target are examined in this paper by quantum mechanical calculations. Structures for the clusters series, (HCOOH)(n)H(+) and (HCOOH)(n)H(3)O(+), where 2 < or = n < or = 4, are proposed based on ab initio electronic structure methods. Results show that cluster growth does not present a regular pattern of nucleation. A stability analysis was performed considering the commonly defined stability function, where E is the total electronic energy plus the zero point vibrational energy correction, including the BSSE correction. The stability analysis leads to a picture that is compatible with experimental observations, indicating a decay of the stability with the increase of cluster mass. Temporal behavior of the clusters was evaluated by Born-Oppenheimer molecular dynamics to check the mechanism that provides cluster stability. The evaluated temporal profiles indicate the importance of hydrogen atom migration between the formic acid moieties to maintain the stability of the structures.