Molar Enthalpies Of Formation Research Articles

Thermochemical properties of different 1-(R-phenyl)-1H-imidazoles

Phenyl substituted imidazoles exhibit versatile biological activity. 1-(R-phenyl)-1H-imidazoles with different functional groups R provide a convenient suitcase of molecules with tunable physicochemical properties adjustable for many practical applications. In this work, the absolute vapor pressures of 1-(R-phenyl)-1H-imidazoles (with R = H, 2-methyl, 4-methyl, 2-methoxy, 4-methoxy, 2-fluoro, 4-fluoro, 2-bromo and 4-bromo) at different temperatures have been measured by the transpiration method for the first time. The standard enthalpies of vaporization of these compounds were derived from the temperature dependencies of the vapor pressures. An internal consistency of the standard vaporization enthalpies has been proven by comparison with vaporization enthalpies of parent species, as well as by a group contribution method. A system of group-additivity values is suggested for a quick assessment of vaporization enthalpies of different 1-(R-phenyl)-1H-imidazoles. Gas-phase standard molar enthalpies of formation of 1-(R-phenyl)-1H-imidazoles have been calculated using the high-level quantum-chemical method G3MP2. The combination of experimentally determined standard vaporization enthalpies with the G3MP2 results allows for the prediction of the liquid-phase standard enthalpies of formation for the studied compounds.

Structural Modulation from 1D Chain to 3D Framework: Improved Thermostability, Insensitivity, and Energies of Two Nitrogen-Rich Energetic Coordination Polymers.

Two new energetic coordination polymers (CPs) [Pb(BT)(H2O)3]n (1) and [Pb3(DOBT)3(H2O)2]n·(4H2O)n (2) with 1D and 3D structures were synthesized by employing two rational designed ligands, 1H,1'H-5,5'-bitetrazole (H2BT) and 1H,1'H-[5,5'-bitetrazole]-1,1'-diol ligands (DHBT), respectively. Thermal analyses and sensitivity tests show that the 3D architecture reinforces the network of 2 which has higher thermal stability and lower sensitivity than that of 1. Through oxygen-bomb combustion calorimetry the molar enthalpy of formation of 2 is derived to be much higher than that of 1 as well as the reported CPs. Herein, more importantly, the heats of detonation (ΔHdet) were calculated according to the decomposition products of TG-DSC-MS-FTIR simultaneous analyses for the first time. The calculated results show that ΔHdet of 2 is 23% higher than that of 1. This research demonstrates that 3D energetic CP with outstanding energetic properties can be obtained through efficient and reasonable design.

Calorimetric and computational studies for three nitroimidazole isomers

In the present work, a combined experimental and computational thermochemical study of nitroimidazole isomers was carried out. The standard (p°=0.1MPa) molar enthalpy of combustion, in the crystalline phase, for 4-nitroimidazole was determined, at the temperature of 298.15K, using a static bomb combustion calorimeter. Calvet microcalorimetry experiments were performed to measure its standard molar enthalpy of sublimation. The standard molar enthalpy of formation of 4-nitroimidazole, in the gaseous phase, at T=298.15K, (116.9±2.9)kJ·mol−1, has been derived from the corresponding standard molar enthalpy of formation in the crystalline phase and the standard molar enthalpy of sublimation.Computational studies for 4-nitroimidazole were performed to complement the experimental work. These were also extended to the 2- and 5-nitroimidazole isomers. The gas-phase enthalpies of formation were estimated from high level ab initio molecular orbital calculations, at the G3 level. Also investigated were the tautomeric equilibrium of 4(5)-nitroimidazole in the gaseous phase and it was concluded that the two tautomers are equally stable.

Energies of Combustion and Specific Heat Capacities of Diaminofurazan, Dinitrofurazan and Diaminoazofurazan

Energies of combustion and specific heat capacities of diaminofurazan(DAF), dinitrofurazan(DNF) and diaminoazofurazan(DAAzF) were determined. Energies of combustion for the three compounds are (-13043±119), (-6863±37) J·g-1 and (-12661±54) J·g-1, respectively. The corresponding standard molar enthalpies of formation (ΔfHmΘ) were obtained with the different combustion products. The values of specific heat capacities for the three compounds are (140.8±0.1), (236.8±0.2), (216.9±0.2) J·mol-1·K-1, respectively. Energy of combustion tends to rise with the decrease of oxygen content in molecule (DAF>DAAzF>DNF). Amino group contributes to increase energy of combustion, but nitro group has the opposite effect. As for the specific heat capacity, the change rule is opposite to energy of combustion.

Thermochemistry of 4-HOC6H4COR (R = H, CH3, C2H5, n-C3H7, n-C4H9, n-C5H11, and n-C6H13) compounds

The thermochemistry of 4-HOC6H4COR (R=H, CH3, C2H5, n-C3H7, n-C4H9, n-C5H11, and n-C6H13) compounds was investigated using enthalpy of vaporization/sublimation measurements by Calvet-drop microcalorimetry and W1-F12 and CCSD(T)-F12 ab initio calculations. Based on the obtained results, standard (p°=1bar) molar enthalpies of formation in the solid, ΔfHmo(cr), and gaseous, ΔfHmo(g), states, at T=298.15K, could be derived for the complete series of compounds. An excellent linear correlation was found when the ΔfHmo(g) values were plotted as a function of the number of carbon atoms in the alkyl side chain (nc), with a CH2 increment of 21.8±0.2kJ·mol−1. Despite the differences in molecular packing between the crystalline 4-HOC6H4COR compounds, approximate additivity of their cohesive energies (as measured by the standard molar enthalpy of sublimation) with nc was also noted, the CH2 increment being 6.6±0.6kJ·mol−1.

Large-scale calculations of gas phase thermochemistry: Enthalpy of formation, standard entropy, and heat capacity

Large scale quantum calculations for molar enthalpy of formation (ΔfH0), standard entropy (S0), and heat capacity (CV) are presented. A large data set may help to evaluate quantum thermochemistry tools in order to uncover possible hidden shortcomings and also to find experimental data that might need to be reinvestigated, indeed we list and annotate approximately 200 problematic thermochemistry measurements. Quantum methods systematically underestimate S0 for flexible molecules in the gas phase if only a single (minimum energy) conformation is taken into account. This problem can be tackled in principle by performing thermochemistry calculations for all stable conformations [Zheng et al., Phys. Chem. Chem. Phys. 13, 10885–10907 (2011)], but this is not practical for large molecules. We observe that the deviation of composite quantum thermochemistry recipes from experimental S0 corresponds roughly to the Boltzmann equation (S = RlnΩ), where R is the gas constant and Ω the number of possible conformations. This allows an empirical correction of the calculated entropy for molecules with multiple conformations. With the correction we find an RMSD from experiment of ≈13 J/mol K for 1273 compounds. This paper also provides predictions of ΔfH0, S0, and CV for well over 700 compounds for which no experimental data could be found in the literature. Finally, in order to facilitate the analysis of thermodynamics properties by others we have implemented a new tool obthermo in the OpenBabel program suite [O’Boyle et al., J. Cheminf. 3, 33 (2011)] including a table of reference atomization energy values for popular thermochemistry methods.

Three new compounds based on 4,4′-azo-1,2,4-triazol-5-one: Synthesis, crystal structure and thermal properties

A series of guanidine salts of 4,4′-azo-1,2,4-triazol-5-one with guanidine(1), aminoguanidine(2), diaminoguanidine(3) and triaminoguanidine(4) was prepared. Compounds 2―4 were characterized by infrared(IR) spectroscopy, elemental analysis and single-crystal X-ray diffraction. Thermal decomposition processes of compounds 1―4 were investigated by differential scanning calorimetry(DSC), and all the compounds showed good thermal stability up to 190 °C. Moreover, these four guanidine salts are more unstable with the increasing number of amino groups. Thermal stability parameters(Te, 0 and Tb) and thermodynamic functions(ΔS≠, ΔH≠ and ΔG≠) for compounds 1―4 were calculated. The constant-volume combustion heats(ΔcU) for compounds 2―4 were determined and tended to increase with the increase of the number of amino groups. The calculated standard molar enthalpy of formation(ΔfHm0) of compounds 2―4 are–541.04,–178.67 and–83.08 kJ/mol, respectively. The impact sensitivities results indicate these four energetic salts are less sensitive than 1,3,5-trinitrotriazacyccohexane(RDX) and 1,3,5,7-tetranitrotetraqza-cyclo-octane(HMX).

Critical Evaluation of the Standard Molar Entropies, Enthalpies of Formation, Gibbs Energies of Formation and Heat Capacities of the Aqueous Trivalent Rare Earth Ions, and the Corresponding Standard Molar Entropies, Enthalpies of Formation and Gibbs Energies of Formation of the Thermodynamically Stable RECl3·7H2O(cr) and RECl3·6H2O(cr)

In an earlier evaluation (F.H. Spedding, J.A. Rard, A. Habenschuss, J. Phys. Chem. 81: 1069–1074, 1977) the standard molar entropies of the aqueous trivalent rare earth ions RE3+(aq) were calculated based on the standard molar entropies of the hydrated rare earth chlorides RECl3·7H2O(cr) (RE = La, Pr) and RECl3·6H2O(cr) (RE = Nd, Gd, Tb, Dy, Ho, Er, Lu) at T = 298.15 K and p = 0.1 MPa, along with their corresponding standard molar enthalpies and Gibbs energies of solution, which are known for nearly all of the RECl3·7H2O(cr) and RECl3·6H2O(cr) including YCl3·6H2O(cr). However, the entropies of these RECl3·7H2O(cr) and of several of the RECl3·6H2O(cr) have large uncertainties because the source heat capacities only extend up to T = 223–262 K and not to T = 298.15 K and the crystals used for these measurements contained some occluded solution (water excess). Simple methods are described to correct these entropies of the RECl3·7H2O(cr) and RECl3·6H2O(cr) and to estimate the standard molar entropies of the RECl3·6H2O(cr) from those of the corresponding anhydrous RECl3(cr). By using these entropies, standard molar enthalpies of solution and revised results for the standard molar Gibbs energy changes to form the saturated solutions, revised and CODATA-compatible values of the standard molar entropies of the RE3+(aq), $$ S_{\text{m}}^{\text{o}} ({\text{RE}}^{3 + } ,\,\,{\text{aq}},\,\,298.15\,\,{\text{K}}) $$ where RE = (La, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y), were evaluated or estimated (RE = Pm). By combining these results with critically-assessed standard molar enthalpies of formation of the RE3+(aq), the standard molar Gibbs energies of formation of these trivalent rare earth aquo ions were calculated, as were the corresponding standard molar Gibbs energies and enthalpies of formation of the thermodynamically stable RECl3·7H2O(cr) and RECl3·6H2O(cr) phases at T = 298.15 K and p = 0.1 MPa. In addition, standard molar heat capacities of most of the RE3+(aq) were evaluated from published results for dilute solutions from flow microcalorimetry.

Thermodynamic properties of methylprednisolone aceponate

In the present work, temperature dependence of heat capacity of methylprednisolone aceponate has been measured for the first time over the range from 7K to 346K by precision adiabatic vacuum calorimetry. Based on the experimental results, the thermodynamic functions of the methylprednisolone aceponate, namely, the heat capacity, enthalpy H°(T)−H°(0), entropy S°(T)−S°(0) and Gibbs function G°(T)−H°(0) have been determined for the range from T/K→0–333. The enthalpy of combustion (−14304.4±9.1)kJ·mol−1 of the methylprednisolone aceponate was determined for the first time using high-precision combustion calorimeter. The standard molar enthalpy of formation in the crystalline state (−1465.3±9.8)kJ·mol−1 of compound at 298.15K was derived from the combustion experiments. Using a combination of the adiabatic and combustion calorimetry results the thermodynamic functions of formation of the folic acid dihydrate at T=298.15K and p=0.1MPa have been calculated.

An isoperibolic combustion calorimeter developed to measure the enthalpy of combustion of organic compounds

An isoperibolic combustion calorimeter developed to measure the enthalpy of combustion of organic compounds containing carbon, hydrogen, oxygen, and nitrogen atoms has been assembled, calibrated, and tested. The calorimeter was calibrated with benzoic acid NIST 39j, and the resulting energy equivalent was e(calor) = (10116.8 ± 0.5) J K−1. Salicylic acid, 1,2,4-triazole, and acetanilide were used as test compounds. The main result is that all obtained values of the standard molar enthalpy of formation for the test compound are in agreement with the results available in the literature.

Thermochemical and theoretical study of 2-oxazolidinone and 3-acetyl-2-oxazolidinone

In this paper, standard molar energies of combustion of 2-oxazolidinone and 3-acetyl-2-oxazolidinone are reported. The experimental values, obtained from static-bomb combustion calorimetry, are −(1465.42±0.52)kJ·mol−1 and −(2350.86±0.84)kJ·mol−1, respectively.Using these combustion energy values, the standard molar enthalpies of formation, in crystalline phase, for the aforesaid compounds were calculated. These values were also determined, and the corresponding values are −(430.30±0.96)kJ·mol−1 and −(617.72±1.58)kJ·mol−1, respectively.The standard molar enthalpies of sublimation at T=298.15K were experimentally obtained by using the well-known Knudsen effusion method. The obtained values are (84.20±3.22)kJ·mol−1 and (82.36±1.84)kJ·mol−1, respectively.From the above experimental results, the standard molar enthalpies of formation in gas-phase of the compounds were derived. The values are: −(346.10±3.36)kJ·mol−1 and −(535.36±2.43)kJ·mol−1, respectively.To support the consistency of the experimental results presented here, enthalpies of formation in gas-phase, using the G4 composite method, were carried out as well.Experimental results suggest that the formation of intermolecular hydrogen bonds between the nitrogen (at ring’s position 3, see Fig. 1) with the oxygen (at ring’s position 2, see Fig. 1) contributes to the values of temperature and enthalpy of fusion, enthalpy of sublimation, and enthalpy of formation in solid phase.

Energy characteristics of nitrooxazolidines and their radicals

The standard molar enthalpies of combustion for three methyl and dimethyl nitrooxazolidine derivatives in the liquid and crystalline states were determined using a high-precision static bomb combustion calorimeter. The resulting values were used to derive the standard molar enthalpies of formation of the azidomethyl-N-nitrooxazolidines. The standard molar enthalpies of vaporization for these compounds were measured using a Calvet microcalorimeter. The enthalpies of formation of some nitrooxazolidine-derived radicals and biradicals were calculated.

Thermodynamic Assessment of the Hydrogen Sorption Reaction in the System Li-Si-H

Recently, extensive studies using silicon anodes in lithium ion batteries (LiB) have shown that the Li-Si-system is an attractive alternative to the currently dominating graphite anode in view of costs and specific capacity. Currently the main obstacle of silicon is the poor charge/discharge cycling stability and capacity fading due to the large volume variation during charging/discharging. However, remarkable progress has recently been achieved by using thin films of silicon or silicon nanowires to relieve the mechanical stress [1]. It is indisputable that future progress in lithium silicide applications will require fundamental knowledge of the physicochemical properties of the various lithium silicide phases. Especially accurate thermodynamic quantities are crucial for the understanding as well as modelling of phase, reaction and electrochemical equilibria. In contrast to the numerous phase diagram studies reported in the literature, only few reliable thermodynamic quantities were available. Motivated by this situation, we recently reported experimental heat capacities and entropies of the lithium silicides Li17Si4, Li16.42Si4, Li13Si4, Li7Si3 and Li12Si7 [2,3]. Presently, the standard molar enthalpies of formation of the lithium silicides seem to be one of the most unreliable fundamental data because available values scatter by about 10 % which is unacceptable in view of reliable thermodynamic equilibrium calculations. This contribution is focused on the accurate experimental determination of the standard enthalpies of formation DFH°(298) of the lithium silicides Li17Si4, Li13Si4, Li7Si3 and Li12Si7. For this purpose the hydrogenation/dehydrogenation equilibria in the system Li-Si-H were investigated at three different temperatures (450 K, 475 K, 500 K) using a Sievert’s type apparatus. The measurements based on two different sample types: (i) pure silicide and (ii) a stoichiometric mixture of lithium hydride and silicon. The calculation of the standard enthalpies of formation by using the slopes of the classical van’t Hoff plots results in uncertainties of at least 10 %. Due to this unsatisfying large error, an alternative evaluation method is demonstrated linking the hydrogen equilibrium pressures peq(H2) from the sorption measurements with our precise heat capacity and entropy data of the lithium silicides. The obtained values for the standard enthalpies of formation show strongly reduced uncertainties of less than 2%. Finally, it will be shown that the phase diagram of the Li-Si-system can now be calculated (CALPHAD) with excellent quality using a complete set of experimental thermodynamic data. [1] M. T. McDowell, S. W. Lee, W. D. Nix, Y. Cui, Adv. Mater. 2013, 25, 4966–4985. [2] D. Thomas, M. Abdel-Hafiez, T. Gruber, R. Hüttl, J. Seidel, A. U. B. Wolter, B. Büchner, J. Kortus, F. Mertens, J. Chem. Thermodyn. 2013, 64, 205–225. [3] D. Thomas, M. Zeilinger, D. Gruner, R. Hüttl, J. Seidel, A. U. Wolter, T. F. Fässler, F. Mertens, The Journal of Chemical Thermodynamics 2015, 178–190.

Thermodynamic properties of two microporous materials for Na2[M2B12O21] (M = Co2+, Cu2+)

Two transition metal borates with microporous structures for Na2Co2B12O21 and Na2Cu2B12O21 have been synthesized and characterized by XRD, FT-IR, and elemental analysis. The molar enthalpies of solution of Na2Co2B12O21(s) and Na2Cu2B12O21(s) in 1moldm−3 HCl(aq) were measured by microcalorimeter at T=298.15K, respectively. The molar enthalpies of solution of CoCl2·6H2O(s) and CuCl2·2H2O(s) in the mixture solvent of 2.00cm3 of 1moldm−3 HCl(aq) in which 5.30mg of H3BO3 and 0.84mg of NaCl being added were also measured, respectively. With the incorporation of the previously determined enthalpy of solution of H3BO3(s) in 1moldm−3 HCl(aq), together with the standard molar enthalpies of formation for CoCl2·6H2O(s)/CuCl2·2H2O(s), NaCl(s), H3BO3(s), HCl(aq) and H2O(l), the standard molar enthalpies of formation of – (9072.4±9.7)kJmol−1 for Na2Co2B12O21(s) and – (8887.5±9.7)kJmol−1 for Na2Cu2B12O21(s) at T=298.15K were obtained on the basis of the appropriate thermochemical cycles. It is obvious that Na2Co2B12O21(s) is more thermal stable than that of Na2Cu2B12O21(s).

Thermodynamic properties of 2,7-di-tert-butylfluorene – An experimental and computational study

This work presents a comprehensive experimental and computational study of the thermodynamic properties of 2,7-di-tert-butylfluorene. The standard (po=0.1MPa) molar enthalpy of formation in the crystalline phase was derived from the standard molar energy of combustion, measured by static bomb combustion calorimetry. The enthalpies and temperatures of transition between condensed phases were determined from DSC experiments. The vapour pressures of the crystalline and liquid phases were measured between (349.14 and 404.04)K, using two different experimental methods. From these results the standard molar enthalpies, entropies and Gibbs energies of sublimation and of vaporization were derived. The enthalpy of sublimation was also determined using Calvet microcalorimetry.The thermodynamic stability of 2,7-di-tert-butylfluorene in the crystalline and gaseous phases was evaluated by the determination of the standard Gibbs energies of formation, at the temperature 298.15K, and compared with the ones reported in the literature for fluorene.A computational study at the G3(MP2)//B3LYP and G3 levels has been carried out. A conformational analysis has been performed and the enthalpy of formation of 2,7-di-tert-butylfluorene has been calculated, using atomization and isodesmic reactions. The calculated enthalpies of formation have been compared to the experimental values.

Investigation on molar heat capacity, standard molar enthalpy of combustion for guaiacol and acetyl guaiacol ester

The molar heat capacities (Cp) of guaiacol (CAS 90-50-1) and acetyl guaiacol ester (AGE, CAS 613-70-7) were determinated from 290K to 350K by differential scanning calorimetry (DSC), and expressed as a function of temperature. Two kinds of group contribution models were used to estimate the molar heat capacities of both guaiacol and AGE, the average relative deviation is less than 10%. The standard molar enthalpies of combustion of guaiacol and AGE were −3590.0kJ·mol−1 and −4522.1kJ·mol−1 by a precise thermal isolation Oxygen Bomb Calorimeter. The standard molar enthalpies of formation of guaiacol and AGE in a liquid state at 298.15K were calculated to be −307.95kJ·mol−1 and −448.72kJ·mol−1, respectively, based on the standard molar enthalpies of combustion. The thermodynamic properties are useful for exploiting the new synthesis method, engineering design and industry production of AGE using guaiacol as a raw material.

Thermodynamic properties of vitamin B9

In the present work temperature dependence of heat capacity of vitamin B9 (folic acid dihydrate) has been measured for the first time in the range from (6 to 333)K by precision adiabatic vacuum calorimetry. Based on the experimental values, the thermodynamic functions of the vitamin B9, namely, the heat capacity, enthalpy H°(T)−H°(0), entropy S°(T)−S°(0) and Gibbs function G°(T)−H°(0) have been determined for the range from T→(0 to 333)K. The value of the fractal dimension D in the function of multifractal generalization of Debye’s theory of the heat capacity of solids was estimated and the character of heterodynamics of structure was detected. Enthalpy of combustion (−8942.8±7.5)kJ·mol−1 of the vitamin B9 was measured for the first time using a high-precision combustion calorimeter. The standard molar enthalpy of formation in the crystalline state (−1821.0±7.9)kJ·mol−1 of B9 at 298.15K was derived from the combustion experiments. Using a combination of the adiabatic and combustion calorimetry results, the thermodynamic functions of formation of the folic acid dihydrate at T=298.15K and p=0.1MPa have been calculated. The low-temperature X-ray diffraction was used for the determination of coefficients of thermal expansion.

High temperature crystallographic and thermodynamic investigations on synthetic calzirtite (Ca2Zr5Ti2O16)

In the context of formation and stability of synthetic rock (SYNROC), studies on crystal structure and thermodynamic parameters of a mineral analogous material, namely calzirtite has been undertaken. The ambient temperature structural studies revealed a tetragonal (I41/acd) fluorite related super-structure with about 5% intermixing of Ti and Zr sites for calzirtite. High temperature structural studies indicated the retention of tetragonal structure of calzirtite up to 1673 K. The average axial thermal expansion coefficient of calzirtite between 297 and 1473 K is found to be 12.7 × 10−6/K for a-axis and 9.2 × 10−6/K along the c-axis. The thermodynamic parameters of calzirtite have been determined employing high temperature solution calorimetry. The standard molar enthalpy of formation of Ca2Zr5Ti2O16(s) at 298 K is found to be −8964.16 ± 12.59 kJ mol−1. The isobaric heat capacity of the compound has been derived from the enthalpy increment (HT-H298) data measured by drop calorimetric technique. Based on the experimental results thermodynamic functions like Cp,m°, Sm°, H°,G°, −(GT°−H298°T),ΔfH298°andΔfGT° for calzirtite (Ca2Zr5Ti2O16,s) has been generated.

Crystal Structure and Improved Synthesis of 1‐(2H‐Tetrazol‐5‐yl)guanidium Nitrate

AbstractEnergetic derivatives of tetrazoles are one of the key areas of research focus in pursuit of novel high energy materials, useful as propellants and explosives. Herein, the crystal structure and an improved synthetic procedure of 1‐(2H‐tetrazol‐5‐yl)guanidine (1) and its nitrate salt (2) are reported. The compounds were structurally characterized by spectroscopic (FT‐IR, 1H NMR, 13C NMR) and elemental analysis. The molecular structure of tetrazolyl guanidium nitrate (2) was solved using low temperature single‐crystal X‐ray diffraction. 2 crystallized as its hemihydrate in the orthorhombic space group Fdd2, with a crystal density of 1.69 g cm−3. Thermal behavior and decomposition of the molecules were studied with differential scanning calorimetry (DSC). Molar enthalpy of formation (ΔfHm) of compound 2 was back calculated from heat of combustion (ΔcH0) value obtained experimentally using bomb calorimetric measurements. Lattice enthalpy of 1‐(2H‐tetrazol‐5‐yl)guanidium nitrate was directly calculated from measured crystal density using Jenkins equation. Preliminary ballistic parameters of the compound were predicted and compared with reported high nitrogen tetrazole derivatives.

Thermochemical study of dichloromethylpyrimidine isomers

The standard (po=0.1MPa) molar enthalpies of formation in the condensed phase, ΔfHmo, of 2,4-dichloro-5-methylpyrimidine, 2,4-dichloro-6-methylpyrimidine, 4,6-dichloro-2-methylpyrimidine and 4,6-dichloro-5-methylpyrimidine were derived from the standard molar energies of combustion, ΔcUmo, in oxygen, to yield CO2 (g), N2 (g) and HCl·600H2O (l), at T=298.15K, measured by rotating bomb combustion calorimetry. The standard molar enthalpies of vaporization or sublimation, Δcr,lgHmo, for these compounds, at T=298.15K were determined by high temperature Calvet microcalorimetry. Combining these values, the following enthalpies of formation in the gas phase, at T=298.15K, were then derived: 2,4-dichloro-5-methylpyrimidine, (79.6±4.1)kJmol−1, 2,4-dichloro-6-methylpyrimidine, (70.5±3.0)kJmol−1, 4,6-dichloro-2-methylpyrimidine, (68.7±3.3)kJmol−1, and 4,6-dichloro-5-methylpyrimidine, (78.1±2.5)kJmol−1. The gas-phase enthalpies of formation were also estimated by G3 theoretical calculations, which were extended to the computation of gas-phase enthalpies of formation of the other dichloromethylpyrimidine isomers, namely, 2,5-dichloro-4-methylpyrimidine, 4,5-dichloro-2-methylpyrimidine and 4,5-dichloro-6-methylpyrimidine, whose experimental study was not performed.