Experimental and computational study of the thermochemistry of the fluoromethylaniline isomers

The standard (po = 0.1 MPa) molar enthalpies of formation in the condensed phase of seven isomers of fluoromethylaniline were derived from the standard molar energies of combustion, in oxygen, to yield CO2(g), N2(g) and HF.10H2O(l), at T = 298.15 K, measured by rotating bomb combustion calorimetry. The standard molar enthalpies of vaporization or sublimation of these compounds, also at T = 298.15 K, were determined using Calvet microcalorimetry, while the enthalpies of fusion of the solid compounds were determined by differential scanning calorimetry. The standard molar enthalpies of formation in the gaseous phase, at T = 298.15 K, were derived from the former two experimental quantities. G3MP2//B3LYP calculations were performed for all possible fluoromethylanilines allowing the estimation of data for the isomers that were not studied experimentally. The Cox scheme was applied with two different approaches for the estimation of the standard molar enthalpies of formation of all the isomers studied, and this led to the conclusion that the literature values for the enthalpies of formation of the meta and para isomers of methylaniline seem to be not reliable. Further G3MP2//B3LYPs calculations on the methylaniline isomers yielded new values for the standard molar enthalpies of formation of the isomers of methylaniline, which have been tested under the Cox scheme, resulting in better estimates.     

计算饱和醇异构体标准生成焓的新方法

基于极性叠加原理,在成功设计烷烃异构体和多氯代烷烃生成焓计算新方法的基础上,设计了一种计算多元醇异构体生成焓的新方法,并合理地假定任一异构体的原子化焓等于三种键(C-C、C-H和C-O-H键)的键能、极性叠加能项以及氢键能项的加和.用这一模型拟合24种原子化焓数据,得到了标准生成焓的估算公式.为了检验预测的精确性,又设计了一种预测方法,使用在排除被预测的化合物条件下回归得到的参数,预测该化合物的生成焓.按这种方法,预测了24种异构体的生成焓.通过该5参数预测的相对于实验值的各种误差(平均绝对误差、均方根误差和最大绝对误差)不仅比7参数的基团法预测的对应误差小得多,而且比相应实验数据的误差还要小.与键加和法比较,该方法的模型包含了极性叠加能和氢键能量,该两项代表了主要的非键相互作用能,表征了不同异构体的结构差异,并大大减少了参数.     

Experimental and computational investigation of the thermochemistry of the six isomers of dichloroaniline

The standard (p(o) = 0.1 MPa) molar enthalpies of formation of 2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-dichloroanilines were derived from the standard molar energies of combustion, in oxygen, to yield CO(2)(g), N(2)(g) and HCl.600H(2)O(l), at T = 298.15 K, measured by rotating bomb combustion calorimetry. The Calvet high-temperature vacuum sublimation technique was used to measure the enthalpies of sublimation of the six isomers. These two thermodynamic parameters yielded the standard molar enthalpies of formation of the six isomers of dichloroaniline, in the gaseous phase, at T = 298.15 K. The gas-phase enthalpies of formation were also estimated by G3MP2B3 calculations, which were further extended to the computation of gas-phase acidities, proton affinities, and ionization enthalpies.     

Energetic study applied to the knowledge of the structural and electronic properties of monofluorobenzonitriles

The present work reports an energetic and structural study of 2-fluoro-, 3-fluoro-, and 4-fluorobenzonitrile. The standard molar enthalpies of formation, in the condensed phase, of the three isomers were derived from the standard molar energies of combustion, in oxygen, at T = 298.15 K. The standard molar enthalpies of vaporization or sublimation (for 4-fluorobenzonitrile), at T = 298.15 K, were measured using high-temperature Calvet microcalorimetry. The combination of these two parameters yields the standard molar enthalpies of formation in the gaseous phase. The vapor-pressure study of the referred compounds was performed by a static method, and the enthalpies of phase transition derived from the application of the Clarke and Glew equation. Theoretically estimated gas-phase enthalpies of formation, basicities, proton and electron affinities, and adiabatic ionization enthalpies were calculated from the G3MP2B3 level of theory. In order to evaluate the electronic properties, the geometries were reoptimized at MP2/cc-pVTZ level, and the QTAIM and NICS were computed. On the basis of the donor-acceptor system, another approach for evaluating the electronic effect for these compounds, using the NBO is suggested. The UV-vis spectroscopy study for the three isomers was performed. The intensities and the band positions were correlated with the thermodynamic properties calculated computationally.     

The thermodynamic properties and stability of tert-butyl peroxyesters of o-and m-carborane-1-carboxylic acids

The paper presents the results of experimental determination of the enthalpies of combustion and formation and the temperature dependence of the heat capacity of the o-and m-derivatives of tert-butyl peroxyesters of carborane-1-carboxylic acid. The entropies and Gibbs energies of formation of these compounds were calculated. The enthalpies, entropies, and Gibbs energies of the o-m transitions of the isomers in the condensed and gaseous states were determined.     

Experimental and computational investigation of the energetics of the three isomers of monochloroaniline

The standard (p degrees = 0.1 MPa) molar enthalpies of formation of 2-, 3-, and 4-chloroaniline were derived from the standard molar energies of combustion, in oxygen, at T = 298.15 K, measured by rotating bomb combustion calorimetry. The Calvet high-temperature vacuum sublimation technique was used to measure the enthalpies of vaporization or sublimation of the three isomers. These two thermodynamic parameters yielded the standard molar enthalpies of formation of the three isomers of chloroaniline, in the gaseous phase, at T = 298.15 K, as 53.4 +/- 3.1 kJ.mol(-1) for 2-chloroaniline, 53.0 +/- 2.8 kJ.mol(-1) for 3-chloroaniline, and 59.7 +/- 2.3 kJ.mol(-1) for 4-chloroaniline. These values, which correct previously published data, were used to test the computational methodologies used. Therewith, gas-phase acidities, proton affinities, electron donor capacities, and N-H bond dissociation enthalpies were calculated and found to compare well with available experimental data for these parameters.     

Thermochemical study of the ethylpyridine and ethylpyrazine isomers

The standard (p(o) = 0.1 MPa) molar energies of combustion in oxygen, at T = 298.15 K, of four liquids: 2-ethylpyridine, 4-ethylpyridine, ethylpyrazine and 2,3-diethylpyrazine were measured by static bomb calorimetry in an oxygen atmosphere. The values of the standard molar enthalpies of vaporization, at T = 298.15 K, were obtained by Calvet microcalorimetry, allowing the calculation of the standard molar enthalpies of formation of the compounds, in the gas phase, at T= 298.15 K: 2-ethylpyridine (79.4 +/- 2.6) kJ mol(-1); 4-ethylpyridine (81.0 +/- 3.4) kJ mol(-1); ethylpyrazine (146.9 +/- 2.8) kJ mol(-1); and 2,3-diethylpyrazine (80.2 +/- 2.9) kJ mol(-1). The most stable geometries of all ethylpyridine and ethylpyrazine isomers were obtained using the density functional theory with the B3LYP functional and two basis sets: 6-31G* and 6-311G**. These calculations were then used to obtain estimates of the enthalpies of formation of all isomers, including those not experimentally studied, through the use of isodesmic reactions. A discussion of the relationship between structure and energetics of the isomers is also presented.     

Substituent effects on the energetics and aromaticity of aminomethylbenzoic acids

The standard (p0 = 0.1 MPa) molar enthalpies of combustion of six aminomethylbenzoic acids were measured at T = 298.15 K by static bomb calorimetry. With these values, the standard molar enthalpies of formation in the crystalline state were obtained. Combining these results with the standard molar enthalpies of sublimation, the standard molar enthalpies of formation in the gaseous phase were derived. For the 10 possible isomers, the obtained experimental results were compared to and correlated with the relative stability obtained by ab initio calculations at the B3LYP/6-311++G(d,p) level of theory. Seeking a better understanding of the aromatic behavior and energetics of aminomethylbenzoic acids in the gas phase, calculations of NICS values, HOMA indices, and dihedral angles between the aromatic carbon and the amino group, Phi(Ar-NHH), were also performed computationally. The significant differences observed in the energetics, as well as in the NICS values, HOMA indices, and Phi(Ar-NHH) dihedral angles for these 10 isomers suggest a strong dependency on the identity and relative position of the three substituents on the benzene ring. This study points out a marked tendency for a decrease of the ring aromaticity, accompanied by an increase in the respective system stability, as the conjugation between the substituents becomes more extensive.     

Thermochemical and theoretical study of some methyldiazines

The standard (p ⁰ = 0.1 MPa) molar enthalpies of formation for the liquid 2,3-dimethylpyrazine and trimethylpyrazine and the crystalline 2,3-dimethylquinoxaline and tetramethylpyrazine were derived from the standard molar enthalpies of combustion, in oxygen, atT=298.15 K, measured by static-bomb combustion calorimetry. The standard molar enthalpies of vaporization or of sublimation for the same compounds were determined by Calvet microcalorimetry. Ab initio full geometry optimization at the 3-21G and 6-31G^* levels were also performed for all the methylpyrazine isomers. MP2/RHF/3-21G//3-21G and DFT energies were also calculated for all the methylpyrazine isomers, thus allowing us to estimate their isodesmic resonance energies.     

异构现象与最大硬度原理

利用原子-键电负性均衡方法计算了700多个异构体的硬度, 通过与标准生成焓所确定的相对稳定性比较后发现, 多数异构体并不遵守最大硬度原理.     

Theoretical study of the nitroalkane thermolysis. 1. Computation of the formation enthalpy of the nitroalkanes, their isomers and radical products

The gas phase enthalpies of formation of mono-, di-, tri-, tetranitromethane and nitroethane, as well as of their nitrite and aci-form isomers were calculated using different multilevel (G2, G3, G2M(CC5)) and density functional theory (DFT)-based (B3LYP, MPW1B95 and MPWB1K) techniques. The enthalpies of the C-N bond dissociation and isomerization of these nitroalkanes were also calculated. The calculated values of the formation and reaction enthalpies were compared with the experimental data when these data were available. It was found that only the G3 procedure gave accurate (within 1 kcal/mol) results for the formation enthalpy of nitroalkanes, their isomers, and radical products. The G3 procedure and two new hybrid meta DFT methods proposed by Truhlar's group (Zhao, Y.; Truhlar, D. J. Phys. Chem. A 2004, 108, 6908) showed good results for the reaction enthalpies of the nitromethane isomerization and the C-N bond dissociation. Our calculation results were used to analyze thermodynamics of the dissociation and isomerization reactions of the poly nitro-substituted methanes.     

Standard molar enthalpies of formation of the methoxynitrophenol isomers: a combined experimental and theoretical investigation

In this paper we present the calorimetric determination of the standard molar enthalpies of combustion, sublimation, and formation of three methoxynitrophenol isomers: 2-methoxy-4-nitrophenol, 2-methoxy-5-nitrophenol and 4-methoxy-2-nitrophenol.In addition, density functional theory calculations with the B3LYP functional and two different atomic basis sets: 6-31G* and 6-311G** allowed the estimation of the standard molar enthalpies of formation in the gaseous phase, for all possible methoxynitrophenol isomers.The theoretical estimations are in good agreement with the experimental determined standard molar enthalpies of formation.     

Excess Molar Enthalpies for Isomeric Chlorobutanes with Isomeric Butanols

Abstract

Excess molar enthalpies for mixtures of each of the isomers of chlorobutane with each of the isomers of butanol were determined at the temperature 298.15 K and atmospheric pressure. Excess molar enthalpies are positive for all the mixtures. The results are discussed in terms of molecular interactions.     

计算多氯代烷烃异构体生成焓的新方法

在我们提出诱导极性叠加原理并用以解释同分异构体稳定性及设计烷烃异构体标准生成焓计算新方法的基础上,进一步设计了含多个杂原子体系的取代烷烃——氯代烷烃异构体生成焓的五参数的新方法.与目前应用较广的九参数的基团加和法相比,计算精度高、预报性好,其预报的均方根误差、平均误差比实验数据的相应偏差还要小.而且,特别重要的是,由于所用的参数少,它在理论上可以推广到含各种杂原子或基团以及多种杂原子或基团共存的化合物生成焓的计算.     

Aromaticity and stability going in opposite directions: An energetic,structural, magnetic and electronic study of aminopyrimidines

The relation between molecular energetics and aromaticity was investigated for the interaction between the amino functional group and the nitrogen atoms of the pyridine and pyrimidine rings, using experimental thermodynamic techniques and computational geometries, enthalpies, chemical shifts, atomic charges and the Quantum Theory of Atoms in Molecules. 2,4-diaminopyrimidine and 2,4,6-triaminopyrimidine were studied by static bomb combustion calorimetry and Knudsen effusion technique. The derived gaseous-phase enthalpies of formation together with the enthalpies of formation of the three isomers of aminopyridine reported in the literature, were compared with the calculated computationally ones and extended to other diamino- and triaminopyrimidine isomers using the MP2/6-311++G(d,p) level of theory.The results were analyzed in terms of enthalpy of interaction between substituents and, due to the absence of meaningful stereochemical hindrance, strong inductive effects, or intramolecular hydrogen bonds according to QTAIM results, the resonance electron delocalization plays an almost exclusive role in the very exothermic enthalpies obtained. Therefore, this enthalpy of interaction was used as an experimental energetic measure of resonance effects and analyzed in terms of aromaticity. It was found that more conjugation between substituents means less aromaticity according to the magnetic (NICS) and electronic (Shannon) criteria, but more aromaticity according to the geometric (HOMA) criterion.     

Experimental and computational thermochemical study of 2- and 3-thiopheneacetic acid methyl esters

Thiophene-based compounds have widespread use in modern drug design, biodiagnostics, electronic and optoelectronic devices, and conductive polymers. The present study reports an experimental and computational thermochemical study on the relative stabilities of 2- and 3-thiopheneacetic acid methyl esters. The enthalpies of combustion and vaporization were measured by a rotating-bomb combustion calorimeter, Calvet microcalorimetry, and correlation gas chromatography, and the gas-phase enthalpies of formation at T=298.15 K were determined. Standard ab initio molecular orbital calculations at the G3 level were performed, and a theoretical study of the molecular and electronic structure of the compounds studied was carried out. Calculated enthalpies of formation, using atomization and isodesmic reactions are in very good agreement with the experimental results.     

The enthalpies of formation of two dibenzocyclooctadienones

The standard molar enthalpies of formation (Δ_fH_m⁰(s)/kJmol⁻¹) for 2,3:6,7-dibenzocycloocta-2,6-dien-1-one and 2,3:7,8-dibenzocycloocta-2,7-dien-1-one [6H-11,12-dihydro-dibenzo[a,e]cycloocten-5-one (ketone 1) and 10H-11,12-dihydrodibenzo[a,d]-cycloocten-5-one (ketone 2), respectively] were derived from enthalpies of combustion, measured by means of a microbomb calorimeter. The fusion and vaporization enthalpies of these compounds were obtained from DSC and correlation gas chromatography measurements. The standard molar enthalpies of formation in the gas phase were calculated by combining the condensed phase standard molar enthalpies of formation with the fusion and vaporization enthalpies adjusted to 298.15 K. Values for Δ_fH_m⁰(g) of (−39.9±5.5) and (−14.8±5.3) kJ mol⁻¹ were obtained for 2,3:6,7-dibenzocycloocta-2,6-dien-1-one and 2,3:7,8-dibenzocycloocta-2,7-dien-1-one, respectively. Quantum chemical calculations are reported for the compounds investigated experimentally and an additional four isomers. Isomerization enthalpies are derived from computed energies. The enthalpies of formation are also calculated by group additivity, compared with the experimental values and then correlated with the structure of the molecules investigated. The X-ray analysis of ketone 1 is also reported.     

Enthalpies of mixing of acetonitrile with isomeric butanols: Enthalpies of hydrogen-bonded complexes

The enthalpies of mixing of four isomeric butanols with acetonitrile were determined at 30°C by a Calvet type microcalorimeter. All the four systems showed endothermic behaviour. The enthalpies of hydrogen bonded complex formation were determined by means of a thermochemical cycle. 1-butanol formed a stronger bonding (−13kJ/mole) than the other three isomers (−11.20kJ/mole). The strength of the H-bond of alcohol with C ≡ N is much less than that with C-N. NCL communication no. 4883     

A DFT investigation of dissociation of nitrous acid,transformation of its monomeric isomers,their dimeric isomers,and dimerization

The structure optimization of the monomeric isomers of the nitrous acid HONO, their cyclic dimers (HONO)₂, and transition states of their transformations has been carried out using the density functional theory calculations at the B3LYP/6‐311++G(d,p) and B3LYP/aug‐cc‐pVQZ levels of theory. Three monomeric isomers (c, t, and z isomers) and six dimeric isomers (cc, ct, cz, tt, zz, and tz isomers) were found. Dissociation of the most stable of the HONO isomer (t isomer) was investigated and its rate constant was obtained at the UB3LYP/aug‐cc‐pVQZ level. Energetics, thermodynamic properties, rate, and equilibrium constants of the monomeric and dimeric isomerizations of the nitrous acid were obtained. Dimerization energies, enthalpies, and free energies of the dimeric isomers, (HONO)₂ were also obtained. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Combined experimental and computational study of the thermochemistry of methylpiperidines

To understand the influence of the methyl group in the stability and conformational behavior of the piperidine ring, the standard (p0= 0.1 MPa) molar enthalpies of formation of 1-methylpiperidine, 3-methylpiperidine, 4-methylpiperidine, 2,6-dimethylpiperidine, and 3,5-dimethylpiperidine, both in the liquid and in the gaseous states, were determined at the temperature of 298.15 K. The numerical values of the enthalpies of formation in the liquid and in the gaseous state are, respectively, -(95.9 +/- 1.6) and -(59.1 +/- 1.7) kJ.mol(-1) for 1-methylpiperidine; -(123.6 +/- 1.4) and -(79.2 +/- 1.6) kJ.mol(-1) for 3-methylpiperidine; -(123.5 +/- 1.5) and -(82.9 +/- 1.7) kJ.mol(-1) for 4-methylpiperidine; -(153.6 +/- 2.1) and -(111.2 +/- 2.2) kJ.mol(-1) for 2,6-dimethylpiperidine; and -(155.0 +/- 1.7) and -(105.9 +/- 1.8) kJ.mol(-1) for 3,5-dimethylpiperidine. In addition, and to be compared with the experimental results, theoretical calculations were carried out considering different ab initio and density functional theory based methods. The standard molar enthalpies of formation of the four isomers of methylpiperidine and of the 12 isomers of dimethylpiperidine have been computed. The G3MP2B3-derived numbers are in excellent agreement with experimental data, except in the case of 2,6-dimethylpiperidine for which a deviation of 9 kJ.mol(-1) was found. Surprisingly, the DFT methods fail in the prediction of these properties with the exception of the most approximated SVWN functional.