Thermochemical Measurements Research Articles

Stepwise Association of Hydrogen Cyanide and Acetonitrile with the Benzene Radical Cation: Structures and Binding Energies of (C6H6•+)(HCN)n, n = 1–6, and (C6H6•+)(CH3CN)n, n = 1–4, Clusters

Equilibrium thermochemical measurements using the ion mobility drift cell technique have been utilized to investigate the binding energies and entropy changes associated with the stepwise association of HCN and CH(3)CN molecules with the benzene radical cation in the C(6)H(6)(•+)(HCN)(n) and C(6)H(6)(•+)(CH(3)CN)(n) clusters with n = 1-6 and 1-4, respectively. The binding energy of CH(3)CN to the benzene cation (14 kcal/mol) is stronger than that of HCN (9 kcal/mol) mostly due to a stronger ion-dipole interaction because of the large dipole moment of acetonitrile (3.9 D). However, HCN can form hydrogen bonds with the hydrogen atoms of the benzene cation (CH(δ+)···NCH) and linear hydrogen bonding chains involving HCN···HCN interaction. HCN molecules tend to form externally solvated structures with the benzene cation where the ion is hydrogen bonded to the exterior of HCN chains. For the C(6)H(6)(•+)(CH(3)CN)(n) clusters, internally solvated structures are formed where the acetonitrile molecules are directly interacting with the benzene cation through ion-dipole and hydrogen bonding interactions. The lack of formation of higher clusters with n > 4, in contrast to HCN, suggests the formation of a solvent shell at n = 4, which is attributed to steric interactions among the acetonitrile molecules attached to the benzene cation and to the presence of the blocking CH(3) groups, both effects make the addition of more than four acetonitrile molecules less favorable.

Assembly of HCN hydrogen bonding chains in the gas phase. Binding energies and structures of phenylacetylene[rad]+(HCN)n clusters, n = 1–4

The stepwise binding energies (ΔHon−1,n) of 1–4 HCN molecules to the phenylacetylene radical cation (C8H6+) are determined by equilibrium thermochemical measurements. The binding energy (10.5±1kcal/mol) of the C8H6+ (HCN) complex is dominated by the ionic hydrogen bonding interaction (ph-CCHδ+⋯NCH). For higher clusters, the attachment of HCN molecules occurs through two comparable interactions involving the association of HCN molecules (ph-CCHδ+⋯NCH⋯NCH⋯) to form a linear hydrogen bonding chain or the interaction with the ring hydrogen atoms through hydrogen bonding (>CHδ+⋯NCH) and bifurcated charge-dipole structures. An interesting pattern of the assembly of linear hydrogen bonded chains is observed and could result in the formation of molecular wires of nanometer length scale.

Near-absolute equations of state of diamond, Ag, Al, Au, Cu, Mo, Nb, Pt, Ta, and W for quasi-hydrostatic conditions

Using the modified formalism of [Dorogokupets, Oganov, 2005, 2007], equations of state are developed for diamond, Ag, Al, Au, Cu, Mo, Nb, Pt, Ta, and W by simultaneous optimization of shock-wave data, ultrasonic, X-ray, dilatometric and thermochemical measurements in the temperature range from ~100 K to the melting temperature and pressures up to several Mbar, depending on the substance. The room-temperature isotherm is given in two forms: (1) the equation from [Holzapfel, 2001, 2010] which is the interpolation between the low pressure (x≥1) and the pressure at infinite compression (x=0); it corresponds to the Thomas-Fermi model, and (2) the equation from [Vinet et al., 1987]. The volume dependence of the Grüneisen parameter is calculated according to equations from [Zharkov, Kalinin, 1971; Burakovsky, Preston, 2004] with adjustable parameters, t and δ. The room-temperature isotherm and the pressure on the Hugoniot adiabat are determined by three parameters, K', t and δ, and K0 is calculated from ultrasonic measurements. In our study, reasonably accurate descriptions of all of the basic thermodynamic functions of metals are derived from a simple equation of state with a minimal set of adjustable parameters. The pressure calculated from room-temperature isotherms can be correlated with a shift of the ruby R1 line. Simultaneous measurements of the shift and unit cell parameters of metals are conducted in mediums containing helium [Dewaele et al., 2004b; 2008; Takemura, Dewaele, 2008; Takemura, Singh, 2006], hydrogen [Chijioke et al., 2005] and argon [Tang et al., 2010]. According to [Takemura, 2001], the helium medium in diamond anvil cells provides for quasi-hydrostatic conditions; therefore, the ruby pressure scale, that is calibrated for the ten substances, can be considered close to equilibrium or almost absolute. The ruby pressure scale is given as P(GPa)=1870⋅Δλ/λ0⋅(1+6⋅Δλ/λ0). The room-temperature isotherms corrected with regard to the ruby scale can also be considered close to equilibrium or almost absolute. Therefore, the equations of state of the nine metals and diamond, which are developed in our study, can be viewed as almost absolute equations of state for the quasi-hydrostatic conditions. In other words, these equations agree with each other, with the ruby pressure scale, and they are close to equilibrium in terms of thermodynamics. The PVT relations derived from these equations can be used as mutually agreed pressure scales for diamond anvil cells in studies of PVT properties of minerals in a wide range of temperatures and pressures. The error of the recommended equations of the state of substances and the ruby pressure scale is about 2 or 3 per cent. Calculated PVT relations and thermodynamics data are available at http://labpet.crust.irk.ru.

Laser-Induced Acoustic Desorption Mass Spectrometry

Large thermally labile molecules were not amenable to mass spectrometric analysis until the development of atmospheric pressure evaporation/ionization methods, such as electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI), since attempts to evaporate these molecules by heating induces degradation of the sample. While ESI and MALDI are relatively soft desorption/ionization techniques, they are both limited to preferential ionization of acidic and basic analytes. This limitation has been the driving force for the development of other soft desorption/ionization techniques. One such method employs laser-induced acoustic desorption (LIAD) to evaporate neutral sample molecules into mass spectrometers. LIAD utilizes acoustic waves generated by a laser pulse in a thin metal foil. The acoustic waves travel through the foil and cause desorption of neutral molecules that have been deposited on the opposite side of the foil. One of the advantages of LIAD is that it desorbs low-energy molecules that can be ionized by a variety of methods, thus allowing the analysis of large molecules that are not amenable to ESI and MALDI. This review covers the generation of acoustic waves in foils via a laser pulse, the parameters affecting the generation of acoustic waves, possible mechanisms for desorption of neutral molecules, as well as the various uses of LIAD by mass spectrometrists. The conditions used to generate acoustic or stress waves in solid materials consist of three regimes: thermal, ablative, and constrained. Each regime is discussed, in addition to the mechanisms that lead to the ablation of the metal from the foil and generation of acoustic waves for two of the regimes. Previously proposed desorption mechanisms for LIAD are presented along with the flaws associated with some of them. Various experimental parameters, such as the exact characteristics of the laser pulse and foil used, are discussed. The internal and kinetic energy of the neutral desorbed molecules are also considered. Our research group has been instrumental in the development and use of LIAD. For example, we have systematically examined the influence of many parameters, such as the type of the foil and its thickness, as well as the analyte layer's thickness, on the efficiency of desorption of neutral molecules. The coupling of LIAD with different instruments and ionization techniques allows for broad use of LIAD in our research laboratories. The most important applications involve analytes that cannot be analyzed by using other mass spectrometric methods, such as large saturated hydrocarbons and heavy hydrocarbon fractions of petroleum. We also use LIAD to characterize lipids, peptides, and oligonucleotides. Fundamental research on the reactions of charged mono-, bi-, and polyradicals with biopolymers, especially oligonucleotides, also requires the use of LIAD, as well as thermochemical measurements for neutral biopolymers. These are but a few of the uses of LIAD in our research group.

High‐temperature dissociation of ethyl radicals and ethyl iodide

AbstractThe decomposition of ethyl iodide and subsequent dissociation of ethyl radicals have been investigated behind incident shock waves in a diaphragmless shock tube by laser‐schlieren (LS) densitometry (1150–1870 K, 55 ± 2 Torr and 123 ± 3 Torr). The LS density‐gradient profiles were simulated assuming that the initial dissociation of C2H5I proceeded by 87% C–I fission and 13% HI elimination. Excellent agreement was found between the simulations and experimental profiles. Rate coefficients for the C–I scission reaction were obtained and show strong falloff. Gorin model RRKM (Rice, Ramsperger, Kassel, and Marcus) calculations are in excellent agreement with the experimental data with E0 = 55.0 kcal/mol, which is in very good agreement with recent thermochemical measurements and evaluations. However, E0 is approximately 2.7 kcal/mol higher than previous estimates. First‐order rate coefficients for dissociation of C2H5I were determined to be k55Torr = 8.65 × 1068 T−16.65 exp(−37,890/T) s−1, k123Torr = 3.01 × 1069 T−16.68 exp(−38,430/T) s−1, k∞ = 2.52 × 1019 T−1.01 exp(−28,775/T) s−1. Rates of dissociation for ethyl radicals were also obtained, and these are in very good agreement with theoretical predictions (Miller J. A. and Klippenstein S. J. Phys Chem Chem Phys 2004, 6, 1192–1202). The simulations show that at low temperatures ethyl radicals are consumed through recombination reactions as well as dissociation, whereas at high temperatures, dissociation dominates. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 433–443, 2012

Spin-forbidden hydrogen atom transfer reactions in a cobalt biimidazoline system

Described here are hydrogen atom transfer (HAT) reactions from high-spin cobalt(II) tris(2,2′-bi-2-imidazoline) (CoIIIIH22bim) to the hydrogen atom acceptors, 2,2,6,6-tetramethyl-1-piperidinyl-oxyl (TEMPO), 2,4,6-tri-tert-butylphenoxyl radical (tBu3ArO˙), and benzoquinone (BQ). The cobalt product is the oxidized and deprotonated, low-spin cobalt(III) complex (CoIIIIIIHbim), and the organic products are TEMPOH, tBu3ArOH, or hydroquinone, respectively. These reactions are formally spin forbidden because the spin state of the reactants is different from that of the products. For instance, quartet CoIIIIH22bim plus doublet RO˙ can have a triplet or quintet ground state, while the CoIIIIIIHbim + ROH product state is a singlet. Kinetics measured in the forward and reverse directions and thermochemical measurements provide a detailed picture of the reactions. The reactions are quite slow: the reaction of 10 mM CoIIIIH22bim with excess TEMPO requires roughly a day at ambient temperatures to reach equilibrium. This is 3400 times slower than the related reaction of the iron analogue FeIIIIH22bim, which is 2 kcal mol−1 more uphill. Mechanistic analyses show that the TEMPO reaction occurs by hydrogen atom transfer (HAT), and this is likely for the tBu3ArO˙ and BQ reactions as well. This is an unusually well defined spin-forbidden HAT system, which serves as a model for more complex multi-spin state HAT processes such as those suggested to occur in cytochrome P450 and metal-oxo model systems. In principle, HAT could occur before, after, or concerted with spin change. Computational studies indicate a reaction mechanism involving pre-equilibrium spin state interconversion of quartet 44CoIIIIH22bim to its doublet excited state 22CoIIIIH22bim, followed by spin-allowed HAT to the organic acceptor. This mechanism is consistent with the available kinetic, thermochemical and spectroscopic measurements. It indicates that the slow rates are due to the large change in geometry between CoIIIIH22bim and CoIIIIIIHbim, rather than any inherent difficulty in changing spin state. The implications of these results for other spin-forbidden or ‘two-state’ HAT processes are discussed.

Structure of the C8H8•+ Radical Cation Formed by Electron Impact Ionization of Acetylene Clusters. Evidence for a (Benzene•+·Acetylene) Complex

Here, we report ion mobility experiments and theoretical studies aimed at elucidating the identity of the C8H8•+ ion formed by electron impact ionization of neutral acetylene clusters. The ion dissociates by a dominant low-energy channel involving the loss of an acetylene molecule, leaving behind a stable benzene radical cation. Using equilibrium thermochemical measurements, the enthalpy and entropy changes of association of acetylene to the benzene radical cation are measured as −4.0 ± 1 kcal/mol and −11.4 ± 2.5 cal/(mol K), respectively. Ion mobility measurement indicates the presence of mainly one isomer with an average collision cross section in helium of 61.0 ± 3 Å2, significantly larger than the calculated cross sections of all of the covalently bonded C8H8•+ ions and in excellent agreement with that of the benzene•+·acetylene complex. The results provide strong evidence that the C8H8•+ ion is predominantly present as an acetylene molecule associated with the benzene radical cation in ionized acetylene clusters.

Thermodynamic characteristics of protolytic equilibria of hexamethylenediamine-N,N,N′,N′-tetraacetic acid

The heats of dissociation of betaine groups of hexamethylenediamine-N,N,N′,N′-tetraacetic acid (H4L) at 298.15 K and the ionic strengths of 0.1, 0.5, and 1.0 (KNO3) were determined by direct calorimetry. The standard thermodynamic characteristics of the ptotolytic equilidria of H4L were calculated using the results from thermochemical and potentiometric measurements made under identical test conditions.

Imidazolium based ionic liquids. 1-Ethanol-3-methyl-imidazolium dicyanoamide: Thermochemical measurement and first-principles calculations

The standard molar enthalpy of formation Δ f H m ° ( l ) of the ionic liquid 1-ethanol-3-methylimidazolium dicyanamide has been determined at 298.15 K by means of combustion calorimetry. First-principles calculations of the enthalpy of formation in the gaseous phase have been performed for the ionic species using the composite G3(MP2) method. The combination of combustion calorimetry with the high-level quantum-chemical calculations allows to estimate the molar enthalpy of vaporization of the ionic liquid under study. It has been established, that the liquid phase enthalpy of formation of this ionic liquid presumably obeys the group additivity rules.

Destabilization of Conjugated Systems of α-Dicarbonyls and of Cyanogen

The formally conjugated system of α-carbonyls in 2,3-butanedione does not impart thermodynamic stabilization, but significant destabilization of 5.9 kcal mol–1. The conjugated triple bonds of cyanogen cause a large thermodynamic destabilization of 11.4 kcal mol–1, which is the difference in the enthalpies of hydrogenation of the first and second triple bonds. Experimental thermochemical and structural measurements and theoretical ab initio (G3) calculations support the destabilizations being reported. This work focuses only on the observable thermodynamic effects (enthalpies of hydrogenation), which are not necessarily related to the ability of conjugated groups to transmit electronic effects.

Pyridinium based ionic liquids. N-Butyl-3-methyl-pyridinium dicyanoamide: Thermochemical measurement and first-principles calculations

The standard molar enthalpy of formation Δ f H m°(l) of the ionic liquid N-butyl-3-methylpyridinium dicyanamide has been determined at 298.15 K by means of combustion calorimetry. Vaporization of the ionic liquid into the nitrogen stream in order to obtain vaporization enthalpy has been attempted, but no vaporization was achieved. First-principles calculations of the enthalpy of formation in the gaseous phase have been performed for the ionic species using the G3MP2 theory. The combination of traditional combustion calorimetry with modern high-level quantum–chemical calculations allows estimation of the molar enthalpy of vaporization of the ionic liquid under study.

Internally consistent thermodynamic database for iron to the Earth's core conditions

An internally consistent thermodynamic database for pure iron has been established to pressures (P) up to 360 GPa and temperatures (T) up to 7000 K from existing static experimental data and thermochemical measurements. The database includes body‐centered cubic (BCC) phases (αorδphase), the face‐centered cubic (FCC) phase (γphase), the hexagonal close‐packed (HCP) phase (ɛ phase), and the liquid phase. We describe fundamental thermodynamic relations as the Gibbs free energy divided into thermochemical and thermophysical terms. The thermochemical data were evaluated from existing metallurgy databases together with experimentally determined phase relations. The thermophysical term is obtained from the pressure‐volume‐temperature equations of state (EoS) for the phases. We constructed an EoS of the FCC phase from our recent internally‐heated diamond anvil cell (DAC) experimental data and assessed the EoS of the liquid phase from existing laser‐heated DAC experiments together with density data atP= 1 bar, 0.2 GPa, and along the Hugoniot. The HCP‐FCC‐liquid triple point is located atP= 90 GPa andT= 2800 K. The calculated melting temperature of HCP iron at the inner core boundary (P= 330 GPa) is 4900 K and the density change at melting is −1.2%. The core density deficits at the inner core boundary are 8.1 wt.% and 5.3 wt.% for the liquid outer core and solid inner core, respectively. The calculated melting temperature is much lower than that from dynamic shock wave experiments, suggesting that the HCP structure may not be stable in the inner core. We included a hypothetical high‐pressure BCC phase which could be stabilized above 220 GPa by a solid‐solid transition of high‐PBCC‐HCP phases. This hypothetical BCC phase should have a large entropy to give a high melting temperature in order to reconcile the existing discrepancies between the static and shock wave experimental studies.

Thermodynamic characteristics of protolytic equilibria of ethylenediamine-N,N′-diacetic-N,N′-dipropionic acid

Heat effects of protonation and neutralization of ethylenediamine-N,N′-diacetic-N,N′-dipropionic acid at 298.15 K and ion force values of 0.1, 0.5, and 1.0 (KNO3) were measured by the direct calorimetric method. Combined use of the results of thermochemical and potentiometric measurements carried out under the identical experimental conditions permitted to evaluate standard thermodynamic characteristics of the equilibria under study. The results obtained were compared with the corresponding data for related compounds considering the specific features of structure of the diamine complexones.

Thermochemistry of ionic liquid-catalysed reactions. Isomerisation and transalkylation of tert-alkyl-benzenes. Are these systems ideal?

The chemical equilibrium of mutual interconversions of tert-alkyl-benzenes was studied in the temperature range (286 to 423) K using chloroaluminate ionic liquids as a catalyst. The knowledge of the activity coefficients is required in order to obtain the thermodynamic equilibrium constants Ka. A well established procedure, COSMO-RS, has been used to assess activity coefficients of the reaction participants in the liquid phase. Enthalpies of five reactions of isomerisation and transalkylation of tert-alkyl-benzenes were obtained from temperature dependences of the corresponding equilibrium constants in the liquid phase. For the sake of comparison, high-level ab initio calculations of the reaction participants have been performed using the Gaussian-03 program package. Absolute electronic energy values of the molecules have been obtained using B3LYP and G3MP2 level. Using these results enthalpies of reaction of isomerisation and transalkylation of tert-alkyl-benzenes in the liquid phase based on the first principles are found to be in good agreement with the data obtained from the thermochemical measurements.

Thermodynamic characteristics of ionic association in dimethylformamide solutions of barium iodide and zinc and lantane chlorides

Using the results from thermochemical measurements of the dilution enthalpies of BaI2, ZnCl2, and LaCl3 solutions in dimethylformamide, and on the basis of previously proposed methods, the enthalpies and ionic association constants at 298.15 K were found in the abovementioned liquid systems.

Multidimensional GC-Fourier Transform Ion Cyclotron Resonance MS Analyses: Utilizing Gas-Phase Basicities to Characterize Multicomponent Gasoline Samples

Hydrocarbon isomers, present in crude petroleum, may yield similar gas chromatography (GC) retention times and indistinguishable mass spectral patterns. Hence, conventional GC-mass spectrometry (MS) may not provide sufficient data for identification of hydrocarbon isomers. Real-time proton affinity or gas-phase basicity "bracketing" provides an additional dimension to GC-MS analyses. Our GC-fourier transform (FT)-ion cyclotron resonance (ICR)-MS yielded an average mass measurement error of less than 3 ppm for components of a retail gasoline sample. The combined use of concurrent thermo-chemical measurements with GC-FT-ICR-MS data analysis allowed differentiation of various isomers such as C(8)H(10) species.

Structure−Energy Relationships in Unsaturated Esters of Carboxylic Acids. Thermochemical Measurements and ab Initio Calculations

Standard molar enthalpies of formation in the gaseous state of a series of alkyl 3-methylbut-2-enoates have been obtained from combustion calorimetry and results from the temperature dependence of the vapor pressure measured by the transpiration method. To verify the experimental data, we have performed ab initio calculations of all compounds. Enthalpies of formation derived from the G3MP2 method are in excellent agreement with the experimental results. Quantitative analysis of strain effects in alkyl 3-methylbut-2-enoates was discussed in terms of deviations of deltafH degrees m(g) from the group additivity rules. Energetics of the cis-trans isomerization of carboxylic acid derivatives was studied using G3MP2 and DFT methods. Values of strain and cis-trans corrections derived in this work provide further improvement on the group-contribution methodology for prediction of the thermodynamic properties of compounds relevant to biodiesel.

The thermodynamic characteristics of step dissociation of trimethylenediamino-N,N,N′,N′-tetraacetic acid

The heat effects of step dissociation of trimethylenediamino-N,N,N′,N′-tetraacetic acid were determined by direct calorimetry at 298.15 K and I = 0.1, 0.5, and 1.0 (KNO3). The thermochemical and potentiometric measurement data obtained under equal experimental conditions were used jointly to determine the standard thermodynamic characteristics of the equilibria studied. The results obtained are compared with the corresponding data on related compounds.

Compositional Control in Electrodeposited NixPt1 − x Films

Electrochemical codeposition of a series of face-centered cubic alloys is demonstrated . The alloy composition is a monotonic function of potential. The Pt-rich alloys are formed at potentials positive to that required to deposit elemental Ni. Codeposition is ascribed to the negative enthalpy of alloy formation that proceeds via a Ni underpotential deposition reaction in concert with Pt deposition. Interestingly, this process occurs at higher Ni underpotentials than anticipated based on extrapolated literature data from thermochemical measurements and ab initio calculations of alloy formation. In contrast, Ni-rich alloys are produced at Ni overpotentials although the films are formed under conditions where pure Ni deposition is otherwise kinetically hindered. The alloy composition corresponding to the transition from underpotential to overpotential deposition is a function of the electrolyte composition. The films were found to be bright and specular over the full range of compositions studied (grain size ). Atomic force microscopy yielded root-mean-square roughness values on the order of for Ni-rich deposits up to thick.

Heterogeneous Proton-Bound Dimers with a High Dipole Moment Monomer: How Could We Experimentally Observe These Anomalous Ionic Hydrogen Bonds?

Electronic structure calculations (CBS-QB3 and G3MP2) have been used to predict a suitable method to experimentally observe the anomalous structure which is predicted to exist in a proton-bound dimer with a high dipole moment monomer. The enthalpy associated with forming the proton-bound dimer from its protonated and neutral monomers is shown to be linearly related to the difference in proton affinities which has been observed experimentally. However, unlike previous experimental studies, the linear correlation is not predicted to depend strongly, if at all, on whether the basic sites are C=O, C=N, or O(H) n-donor bases. Thermochemical measurements, then, are probably not the best method to distinguish between the structures of heterogeneous proton-bound dimers. It has been shown that a suitable method to experimentally observe the anomalous structure of proton-bound dimers containing a high dipole moment monomer (or very polar monomer) is by spectroscopic measurement. The O-H+-O asymmetric stretch is probably not the best infrared band to try to correlate with structure. The best band to observe is one which is in a region of the spectrum not masked by other absorptions and is also sensitive to the proximity of the binding proton. For example, it is shown that the methanol-free O-H stretch is very sensitive to the O-H+ bond distance for a series of heterogeneous proton-bound dimers containing methanol. It is predicted that the free O-H stretch of the methanol/acetonitrile proton-bound dimer is more closely related to the O-H stretch in protonated methanol than the O-H stretch in neutral methanol. Observations of these bands should confirm that the proton is closer to methanol in the methanol/acetonitrile proton-bound dimer despite acetonitrile having a higher proton affinity.