Maximum Reaction Temperature Research Articles

DNA quantification via traceable phosphorus measurement through microwave-assisted UV digestion-ion chromatography

Accurate quantification of deoxyribonucleic acid (DNA) is critical for many analyses in molecular biology and genetic tests. We present a method in which the stoichiometrically existing phosphorus content in purified genomic DNA is quantitatively converted into orthophosphate ions by microwave assisted-UV digestion in the presence of microlitre quantities of dilute reagents (HCl, HNO(3), H(2)O(2)). The tandem use of microwave energy and ultraviolet photons for DNA digestion in pressurized quartz vessels enables a maximum reaction temperature of 240 °C resulting in efficient and fast mineralization of high molecular weight DNA within 30 minutes. Compared to hotplate digestion, the digestion time is reduced by a factor of 32. The MW-UV sample preparation approach coupled with the ion chromatographic measurement of phosphate using a high performance (HP) methodology provides an accurate quantitation of phosphorus mass fractions as low as 0.3 μg g(-1), corresponding to a DNA mass of 25 μg. The relative expanded uncertainties (% U) expressed at 95% confidence for these analyses range from 0.2 to 0.6%. Critically, the matrix of the calibrant solution is also matched with respect to the digested matrix anions (chloride, nitrate), without which significant bias in IC performance is observed. The phosphorus content of the calf thymus DNA was also measured using high-performance inductively coupled plasma optical emission spectroscopy (HP-ICP-OES), which provided independent data for comparison with the MW-UV digestion-IC based approach. Ion chromatography requires smaller volume of materials to perform the analysis and could be useful for characterizing primary calibration standards and certified reference materials with low uncertainties.

Microwave-induced torrefaction of rice husk and sugarcane residues

This study utilized microwave irradiation to induce torrefaction (mild pyrolysis) of rice husk and sugarcane residues by varying different parameters, including microwave power level, processing time, water content, and particle size of biomass. Proper microwave power levels are suggested to be set between 250 and 300 W for the torrefaction of these two agricultural residues. With proper processing time, the caloric value can increase 26% for rice husk and 57% for sugarcane residue. Compared to dry rice husk, both maximum reaction temperature and mass reduction ratio increased with higher water content (not over 10%). Moreover, the particle size of biomass needs not to be very small. The mass reduction ratios were 65 wt.%, 69 wt.%, and 72 wt.%, when the sizes were 50/100 mesh, 100/200 mesh, and >200 mesh, respectively. Microwave-induced torrefaction reduces more oxygen/carbon ratio of biomass in comparison with traditional torrefaction. Microwave-induced torrefaction is considered as an efficient and promising technology with great potential.

Investigations for the environmentally friendly production of Na 2CO 3 and HCl from exhaust CO 2, NaCl and H 2O

The conventional Solvay ammonia soda process is a net producer of CO 2 and produces large quantities of ecologically doubtful side products. Therefore a possible solution for this problem was investigated. Theoretical and experimental data are given which show the feasibility of a modified ammonia soda process which delivers Na 2CO 3 and HCl by using exhaust CO 2, NaCl and H 2O. This modified ammonia soda process would not produce the byproduct CaCl 2 as in the conventional Solvay ammonia soda process, would be completely recyclable and could be driven by solar thermal energy. Low maximum reaction temperatures of T ≤ 800 K and an estimated achievable solar efficiency of 10% show that this cycle is not only environmentally friendly but also energetically interesting. Kinetic constants of the main reactions are given which are similar to the ones in the conventional process. The principle of a simple solar thermo-chemical reactor is described. Preliminary economical considerations show that this new process might even be competitive when driven by solar thermal energy instead of using fossil fuels. If this novel process would be implemented worldwide approximately up to 3 × 10 7 tonne of CO 2 could be omitted annually compared with the conventional Solvay ammonia soda process. This would correspond to 0.15% of the annual release of all anthropogenically produced CO 2.

The behavior of nanothermite reaction based on Bi2O3/Al

We studied the impact of aluminum particle size and the thickness of surrounding alumina layer on the dynamic pressure discharge of nanothermite reactions in the Bi2O3/Al system. A pressure discharge from 9 to 13 MPa was generated using as-synthesized Bi2O3 nano-particles produced by combustion synthesis and Al nanoparticles with size from 3 μm to 100 nm. The maximum reaction temperature was measured to be ∼2700 °C. The estimated activation energy of the reaction was 45 kJ/mol. A very large (several orders of magnitude) difference existed between the rate of the pressure pulse release by nanothermite reactions and by thermite reactions with large aluminum particles. The maximum observed pressurization rate was 3200 GPa/s. The time needed to reach the peak pressure was 0.01 ms and 100 ms for aluminum particles with diameter of 100 nm and 70 microns, respectively. The discharge pressure was a monotonic decreasing function of the thickness of the surrounding alumina layer.

Thermodynamic and experimental study of combined dry and steam reforming of methane on Ru/ ZrO 2-La 2O 3 catalyst at low temperature

Analysis of the effect of adding small amounts of steam to the methane dry reforming feed on activity and products distribution was performed from thermodynamic equilibrium calculations of the system based on the Gibbs free energy minimization method. This analysis is supported by new insights from the direct experimental investigation of the influence of co-feeding with H 2O over a Ru/ZrO 2-La 2O 3 catalyst. Activity measurements were carried out in a fixed-bed reactor but using the operating conditions applicable in a Pd membrane reactor, that is, at maximum reaction temperature below 550 °C. Experimental results were in good agreement with thermodynamics predictions. It was observed that the addition of H 2O into the dry reforming feed strongly affects activity and products distribution. The co-feeding of steam resulted in increasing methane conversion and hydrogen yield but decreasing carbon dioxide conversion and carbon monoxide yield. At a given temperature, syngas composition (H 2/CO ratio) can be tuned by changing the amount of H 2O co-fed. Interestingly the stability of the Ru/ZrO 2-La 2O 3 catalyst was improved by adding steam to the dry reforming reactant mixtures.

Design and Control of a Modified Vinyl Acetate Monomer Process

The vinyl acetate monomer (VAM) process presents several challenging design problems because of the many design optimization variables and several important constraints. The process features a cooled tubular reactor, both gas and liquid recycle streams, two absorbers, and two distillation columns, one involving heterogeneous azeotropic distillation. The major design variables are reactor temperature, reactor size, reactor pressure, ethylene gas recycle and acetic acid liquid recycle. The major constraints are a maximum oxygen flammability limit, a maximum internal reactor temperature limit, and the need to stay above dewpoint temperatures in the reactor. This paper develops an economic optimum flowsheet based on the conditions and parameters provided by M. L. Luyben and B. D. Tyreus over a decade ago as a challenge problem for the academic community for design and control studies (Comput. Chem. Eng. 1998, 22, 867–877). The conceptual design developed produces 18% more VAM product for the same oxygen fresh feed flow rate used in the original design. Compared to the original nonoptimized design, the modified design has a reactor that is twice as large with reactor inlet conditions showing lower oxygen concentrations and higher acetic acid concentrations. Operating pressure is lower, liquid acetic acid recycle is larger, and gas ethylene recycle is smaller. A plantwide control scheme that provides effective disturbance rejection and is significantly different than the structure used in the original design is developed.

1 Nm3/h 규모 합성천연가스(SNG) 합성 시스템의 운전 특성

본 연구에서는 CO, <TEX>$H_2$</TEX>가 주성분인 모사합성가스를 이용하여 합성천연가스(SNG, Synthetic Natural Gas) 제조공정을 평가하기 위하여, 3종류의 SNG 합성반응시스템을 제안하였다. 제시된 공정은 다단 단열반응시스템, 재순환이 있는 다단 단열반응시스템 그리고 강제냉각방식의 수냉각반응시스템이다. 3개의 연속된 반응기로 구성된 다단 단열반응시스템에서의 1차반응기에서는 온도가 최대 <TEX>$800^{\circ}C$</TEX>까지 상승하였으며, 이로 인한 수성가스전환반응으로 인해 <TEX>$CO_2$</TEX>가 다른 시스템에 비해 많이 생성되었으며, SNG 내의 <TEX>$CH_4$</TEX> 농도는 90.1% 정도를 얻었다. 다단 단열반응시스템의 문제점을 해결하기 위해 재순환이 있는 다단 단열반응시스템에서는 반응기의 온도제어를 위해 일부 전환가스를 재순환한 것으로, <TEX>$CH_4$</TEX>는 최대 96.3%를 얻었다. 이러한 다수개의 반응기로 구성된 단열반응기의 단점을 해결하여 반응기 개수를 줄일 수 있는 쉘과 튜브 형태의 반응기로 구성된 강제냉각방식의 수냉각시스템에서는 쉘 측으로 냉각수를 공급하여 반응열을 흡수하는 형태로, 공급되는 냉각수의 유량과 압력에 의해 온도를 제어할 수 있다. 이 시스템에서는 최대 <TEX>$CH_4$</TEX>는 최대 99.2%를 얻었으며, 1차 반응기인 강제냉각방식의 수냉각반응기 출구에서의 97% 이상의 <TEX>$CH_4$</TEX> 농도를 얻을 수 있음을 확인하였다. In this work, we proposed the three different reactor systems for evaluating of synthetic natural gas(SNG) processes using the synthesis gas consisting of CO and <TEX>$H_2$</TEX> and reactor systems to be considered are series adiabatic reaction system, series adiabatic reaction system with the recirculation and cooling wall type reaction system. The maximum temperature of the first adiabatic reactor in series adiabatic reaction system raised to 800. From the these results, carbon dioxide in product gas as compared to other systems was increased more than that expected due to water gas shift reaction(WGSR) and the maximum <TEX>$CH_4$</TEX> concentration in SNG was 90.1%. In series adiabatic reaction system with the recirculation as a way to decrease the temperature in catalyst bed, the maximum <TEX>$CH_4$</TEX> concentration in SNG was 96.3%. In cooling wall type reaction system, the reaction heat is absorbed by boiling water in the shell and the reaction temperature is controlled by controlling the amount of flow rate and pressure of feed water. The maximum <TEX>$CH_4$</TEX> concentration in SNG for cooling wall type reaction system was 97.9%. The main advantage of the cooling wall type reaction system over adiabatic systems is that potentially it can be achieve almost complete methanation in one reactor.

Induction Curing of Thiol–Acrylate and Thiol–Ene Composite Systems

Induction curing is demonstrated as a novel type of in situ radiation curing that maintains most of the advantages of photocuring while eliminating the restriction of light accessibility. Induction curing is utilized to polymerize opaque composites comprised of thiol-acrylate and thiol-ene resins, nanoscale magnetic particles, and carbon nanotubes. Nanoscale magnetic particles are dispersed in the resin and upon exposure to the magnetic field, these particles lead to induction heating that rapidly initiates the polymerization. Heat transfer profiles and reaction kinetics of the samples are modeled during the reactions with varying induction heater power, species concentration, species type and sample thickness, and the model is compared with the experimental results. Thiol-ene polymerizations achieved full conversion between 1.5 minutes and 1 hour, depending on the field intensity and the composition, with the maximum reaction temperature decreasing from 146 - 87 °C when the induction heater power was decreased from 8 - 3 kW. The polymerization reactions of the thiol-acrylate system were demonstrated to achieve full conversion between 0.6 and 30 minutes with maximum temperatures from 139 to 86 °C. The experimental behavior was characterized and the temperature profile modeled for the thiol-acrylate composite comprised of sub100nm nickel particles and induction heater power in the range of 32 to 20 kW. A 9°C average deviation was observed between the modeling and experimental results for the maximum temperature rise. The model also was utilized to predict reaction temperatures and kinetics for systems with varying thermal initiator concentration, initiator half-life, monomer molecular weight and temperature gradients in samples with varying thickness, thereby demonstrating that induction curing represents a designable and tunable polymerization method. Finally, induction curing was utilized to cure thiol-acrylate systems containing carbon nanotubes where 1 wt% carbon nanotubes resulted in systems where the storage modulus increased from 17.6 ± 0.2 to 21.6 ± 0.1 MPa and an electrical conductivity that increased from <10(-7) to 0.33 ± 0.5 S/m.

Ferrocene Containing Hyperbranched Polyester: Structure and Catalytic Performance for Thermal Decomposition of Ammonium Perchlorate

Structures of ferrocene containing hyperbranched polyester (HBPE-Fc) was characterized by FTIR and H1 NMR. The spectroscopic analysis provided evidence of the successful modification of ferrocene toHBPE. The catalytic performances for thermal decomposition of ammonium perchlorate (AP) were investigated by DSC techniques. The kinetic parameters were computed by the Kissinger method. The results indicated that the presence of HBPE-Fc reduces the maximum decomposition reaction temperature in AP samples. HBPE-Fc presented an efficient catalytic effect on the thermal degradation of AP and the catalytic reactions were characterized by decreased activation energy and increased rate constant of thermal decomposition. The possible catalytic mechanism was proposed.

Characterization of self-propagating formation reactions in Ni/Zr multilayered foils using reaction heats, velocities, and temperature-time profiles

We report on intermetallic formation reactions in vapor-deposited multilayered foils of Ni/Zr with 70 nm bilayers and overall atomic ratios of Ni:Zr, 2 Ni:Zr, and 7 Ni:2 Zr. The sequence of alloy phase formation and the stored energy is evaluated at slow heating rates (∼1 K/s) using differential scanning calorimetry traces to 725 °C. All three chemistries initially form a Ni–Zr amorphous phase which crystallizes first to the intermetallic NiZr. The heat of reaction to the final phase is 34–36 kJ/mol atom for all chemistries. Intermetallic formation reactions are also studied at rapid heating rates (greater than 105 K/s) in high temperature, self-propagating reactions which can be ignited in these foils by an electric spark. We find that reaction velocities and maximum reaction temperatures (Tmax) are largely independent of foil chemistry at 0.6±0.1 m/s and 1220±50 K, respectively, and that the measured Tmax is more than 200 K lower than predicted adiabatic temperatures (Tad). The difference between Tmax and Tad is explained by the prediction that transformation to the final intermetallic phases occurs after Tmax and results in the release of 20%–30% of the total heat of reaction and a delay in rapid cooling.

10.2478/s11814-009-0167-x

The effects of the sodium-water reaction by small water/steam leaks on 2.25Cr-1Mo and Mod.9Cr-1Mo steels have been studied. Test specimens were exposed to small leaks of water/steam in 300°C stagnant sodium. The phenomenon where the size of the nozzle hole became larger with time was observed for the two types of material. Enlargement rate was slightly faster in the 2.25Cr-1Mo steel than in the Mod.9Cr-1Mo steel. Test results of the enlargement rate of the nozzle hole itself with an increasing duration of the steam injection were analyzed by using SEM and CAMSCOPE images. The maximum reaction temperature appeared at a 17 mm point from the measuring tool, which agrees with the maximum distance that was affected by a jet flame.

The Effect of Mechanical Activation of Elemental Powders on Combustion Synthesis of NbAl&lt;sub&gt;3&lt;/sub&gt;

The effect of mechanical activation procedures on the combustion synthesis of NbAl3 was investigated. The activation was carried out by a two-step high energy ball milling procedure. In the first milling, aluminum and niobium were milled separately (pre-activation). The mixture of pre-activated powders was then activated in the second milling. Reaction synthesis, by simultaneous combustion mode, was conducted on compacted pellets made of powder mixtures with and without pre-activation. The thermal behavior of the compacted pellets upon heating was recorded and the main thermal combustion reaction characteristics were evaluated. The two-step procedure produced aggregates with a globular dispersion of niobium due to increased particle hardness and decreasing mean particle size during pre-activation milling. Analysis of pellet thermal behavior showed the two-step milling procedure could enhance reaction performance by increasing maximum reaction heating rate and temperature gain during reaction.

Thermal behaviour of ‘metamorphic vermiculite’ in Ca-rich ancient ceramic sherds and experimental ceramics

AbstractA ‘metamorphic vermiculite’-like phase was identified in Ca-rich ancient ceramic sherds from excavations in NW Peloponnese, Greece. Archaeometric investigations have shown that the raw materials used in the production of ancient ceramics were derived from local Plio-Pleistocene sediments. Analysis by X-ray diffraction and scanning electron microscopy-energy dispersive spectroscopy showed that ‘metamorphic vermiculite’ is a common constituent phase of these sediments occurring in three texturally different types: as interstratified white mica-chlorite and chlorite-vermiculite, and white mica-(white mica-chlorite) stacks. In an attempt to elucidate the thermal transformation of ‘metamorphic vermiculite’ present in the calcareous raw materials, we produced experimental ceramics made from these local raw materials and fired them at 700, 750, 850, 950 and 1050°C in a static air furnace. The mixed-layered chlorite-vermiculite was transformed completely at ∼800°C contributing to the formation of new high-T minerals. The two other types of ‘metamorphic vermiculite’ retain their original lath-like shape up to 1050°C and only a few crystals show that they have undergone complete transformation at this temperature. In the latter crystals, numerous nanocrystals were formed sub-parallel to the former cleavages of ‘metamorphic vermiculite’ pseudomorphs, suggesting their contribution to the nucleation of high-T minerals (i.e. ferrian aluminian diopside, spinel, Fe oxides) by reactions with the available CaO. It is suggested that the firing conditions (i.e. maximum reaction temperature of 1050°C, reaction time of 1 h, oxygen atmosphere) which promote disequilibrium reactions, and the greater contribution of the white mica constituent against chlorite in some of the initial structures of ‘metamorphic vermiculite’ may be responsible for the delay of its thermal decomposition at 1050°C.

Novel alkoxysilane pentacoordinate O [dbnd]V(IV) complexes as supported catalysts for cyclohexane oxidation with dioxygen

A variety of newly synthesized and well characterized alkoxysilane pentacoordinate Oxovanadium(IV) complexes, VO[Sal(PMeOSi)DPTA] 3[a], VO[Cl-Sal(PMeOSi)DPTA] 3[b], VO[Sal(PMeOSi)DETA] 6[a] and VO[Cl-Sal(PMeOSi)DETA] 6[b], (Sal = salicylaldehyde, DPTA = bis(aminopropyl)amine, DETA = diaminoethylamine), have been anchored by covalent bond into the surface of SiO 2 and/or Al 2O 3 via silicon-alkoxide route by a condensation process as supported catalysts. These solid supported catalysts (abbreviated as catalysts A to H) showed high catalytic efficiency in the selective oxidation reaction of cyclohexane using molecular oxygen under relatively mild condition in a micro-batch reactor. The Catalyst C (SiO 2/ 3[a] complex) system exhibits best activity, overall yield 38.5% (TONs, ca. 5.0 × 10 3) as well as high selectivity 98% (cyclohexanol 74%, cyclohexanone 24%). Notably, cyclohexane shows significantly improved yield 44.0%, by the addition of pyrazinecarboxylic acid as a co-catalyst. The TGA indicates these catalysts are stable up to maximum reaction temperature, ca. 473 K and ICP analysis shows there is negligible vanadium loss from the supported catalyst after the reaction, allowing further use of the V-catalyst. The various factors influences ( i.e. temperature, O 2 pressure, reaction time, catalyst amount) were also investigated in the systematic way, to optimize the reaction processes. The impact of radical traps and detection of intermediate peroxy radical were also investigated to establish a radical mechanism.

Computational fluid dynamics simulation of a transpiring wall reactor for supercritical water oxidation

A computational fluid dynamic (CFD) model of a transpiring wall reactor for supercritical water oxidation has been implemented and solved using the commercial software Fluent 6.3. Model results have been validated by comparison with experimental temperature profiles, and the influence of model simplifications on the accuracy of the results has been discussed. It has been found that the model presents some inaccuracies in the calculation of the maximum reaction temperature, which have been attributed to the simplifications in the calculation of thermal properties of the fluids imposed by limitations of the software. The effect of different process parameters has been studied, including transpiring water flow ratio, transpiring water temperature, flow rate and composition of the oxidant (pure oxygen or air) on different indicators of reactor performance such as temperature and composition contours, flow path lines and effluent compositions. The behaviour of three different transpiring wall designs comprising different sections of porous and non-porous materials has also been investigated. It has been found that making the upper cup of the reactor of a porous material has a very little influence on reactor performance, and therefore it is preferable to build this part of the reaction chamber using a non-porous, more durable material. On the other hand, the maximum reaction temperature in the reaction chamber can be increased if the upper part of the reaction chamber is made with a non-porous material, which is favourable for treating diluted feeds. Model results complement previous information obtained by experiments and by simplified models.

異なる初期組成粉末を用いた Bi2212 超伝導丸線材の組織観察

Bi2212 round wires are one of candidates for application in high magnetic fields and low temperatures because of its high performance of Jc. Furthermore, round wires are suitable to make complicated coils and Rutherford cables. Bi2212 round wires are fabricated by simple heat treatment process of partial melting and slow cooling. However, the Jc values are sensitive to reaction temperatures, especially maximum reaction temperature (Tmax). In this research, we fabricated Bi2212 round wires with different compositions. We prepared four powders with the nominal compositions of between Bi=2.00 and Bi=2.25. Highest Ic value of 260 A (Jc=200 A/cm2) at 4.2 K and 10 T were obtained in Bi2.25 sample with Bi rich composition. From the observations of microstructures, we observed small amount of bridging of filaments in high Ic sample. To understand the growth mechanism of bridging, we report the results of quench experiments during heat processing.

Autothermal hydrogen generation from methanol in a ceramic microchannel network

In this paper, the authors present the first demonstration of a new class of integrated ceramic microchannel reactors for all-in-one reforming of hydrocarbon fuels. The reactor concept employs precision-machined metal distributors capable of realizing complex flow distribution patterns with extruded ceramic microchannel networks for cost-effective thermal integration of multiple chemical processes. The presently reported reactor is comprised of five methanol steam reforming channels packed with CuO/γ-Al 2O 3, interspersed with four methanol combustion channels washcoated with Pt/γ-Al 2O 3, for autothermal hydrogen production (i.e., without external heating). Results demonstrate the capability of this new device for integrating combustion and steam reforming of methanol for autothermal production of hydrogen, owing to the axially self-insulating nature of distributor-packaged ceramic microchannels. In the absence of any external insulation, stable reforming of methanol to hydrogen at conversions >90% and hydrogen yields >70% was achieved at a maximum reactor temperature of 400 °C, while simultaneously maintaining a packaging temperature <50 °C.

Acrylic Bone Cements Modified With Bioactive Filler

AbstractBioactive cuttlebone Sepia officinalis particles that contain collagen were used to fill poly (methyl methacrylate‐co‐styrene) bone cements, varying size and concentration of filler particles. Cuttlebone was characterized by X‐ray diffraction and plasma atomic emission spectrophotometer. Maximum reaction temperature and cement setting time were determined for composites and reference (copolymer without filler), along with NMR determination of residual monomer concentration. Mechanical properties characterization included stress‐strain, bending, compression, fracture toughness and storage modulus tests. Mechanical results for composites filled with 10 and 30% weight of cuttlebone, complied with norm requirements which opens the possibility for using cuttlebone particles as bioactive filler.

Analysis of micro-leak sodium-water reaction phenomena in a sodium-cooled fast reactor steam generator

The effects of the sodium-water reaction by small water/steam leaks on 2.25Cr-1Mo and Mod.9Cr-1Mo steels have been studied. Test specimens were exposed to small leaks of water/steam in 300°C stagnant sodium. The phenomenon where the size of the nozzle hole became larger with time was observed for the two types of material. Enlargement rate was slightly faster in the 2.25Cr-1Mo steel than in the Mod.9Cr-1Mo steel. Test results of the enlargement rate of the nozzle hole itself with an increasing duration of the steam injection were analyzed by using SEM and CAMSCOPE images. The maximum reaction temperature appeared at a 17 mm point from the measuring tool, which agrees with the maximum distance that was affected by a jet flame.

Reactivity study of combustion for coals and their chars in relation to volatile content

To determine the effect of volatile matter on combustion reactivity, the pyrolysis and combustion behavior of a set of four (R, C, M and K coals) coals and their chars has been investigated in a TGA (SDT Q600). The maximum reaction temperatures and maximum reaction rates of the coals and their chars with different heating rates (5-20 o C/ min) were analyzed and compared as well as their weight loss rates. The volatile matter had influence on decreasing the maximum reactivity temperature of low and medium rank coals (R, C and M coals), which have relatively high volatiles (9.5-43.0%), but for high rank coal (K coal) the maximum reactivity temperature was affected by reaction surface area rather than by its volatiles (3.9%). When the maximum reaction rates of a set of four coals were compared with those of their chars, the slopes of the maximum reaction rates for the medium rank coals (C and M coals) changed largely rather than those for the high and low rank coals (R and K coals) with increasing heating rates. This means that the fluidity of C and M coals was larger than that of their chars during combustion reaction. Consequently, for C and M coals, the activation energies are lower (24.5-28.1 kcal/mol) than their chars (29.3-35.9 kcal/mol), while the activation energies of R and K coals are higher (25.0-29.4 kcal/mol) than those of their chars (24.1-28.9 kcal/mol).