Investigation of the use of B3LYP zero-point energies and geometries in the calculation of enthalpies of formation

The use of B3LYP/6–31G^* zero-point energies and geometries in the calculation of enthalpies of formation has been investigated for the enlarged G2 test set of 148 molecules [J. Chem. Phys. 106 (1997) 1063]. A scale factor of 0.96 for the B3LYP zero-point energies gives an average absolute deviation nearly the same as scaled HF/6–31G^* zero-point energies for G2, G2(MP2), and B3LYP/6–311 + G(3df,2p) enthalpies. A scale factor of 0.98, which has been recommended in some studies, increases the average absolute deviation by about 0.2 kcal/mol. Geometries from B3LYP/6–31G^* are found to do as well as MP2/6–31G^* geometries in the calculation of the enthalpies of formation.     

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.     

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.     

Thermochemistry of aromatic ketones. Experimental enthalpies of formation and structural effects

The standard enthalpies of formation Δ_fH^o (liq. or cr.) at the temperature T = 298.15 K were measured using combustion calorimetry for benzophenone (A), 1-indanone (B), -tetralone (C), 9-fluorenone (D), anthrone (E) and dibenzosuberone (F). The standard enthalpies of vaporization Δ_vH^o or sublimation Δ_sH^o of A-F and 5,7-dihydro-6H-dibenzo[a,c]cyclohepten-6-one (G) were obtained from the temperature function of the vapor pressure measured in a flow system. Enthalpies of fusion Δ_mH of solid compounds were measured by DSC. From the enthalpies of formation of the gaseous compounds of A-G the values of their strain enthalpies were derived and structural effects discussed.

     

Computational study of the thermochemistry of organophosphorus(III) compounds

The enthalpies of formation of organophosphorus(III) compounds have been calculated at the G3X, G3X(MP2), and B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) levels of theory using the atomization energy procedure and the method of isodesmic reactions. The Delta f H298 degree values for 50 relatively large molecules with up to 10 non-hydrogen atoms, such as P(CH3)3, P(C2H5)3, P(OCH3)3, n-C4H9OPCl2, [(CH3)2N]2PCl, (C2H5)2NPCl2, and [(CH3)2N]2PCN, have been calculated directly from the G3X atomization energies. A good agreement between the known experimental values and G3X results for 14 compounds provides support to our predictions for remaining species whose experimental enthalpies of formation are unknown or known with relatively large uncertainties. On the basis of our calculations and sometimes conflicting experimental data a set of internally consistent enthalpies of formation has been recommended for organophosphorus(III) compounds. Our computational results call into question the experimental enthalpies of formation of P(C2H5)3 and P(n-C4H9)3. From comparison with most reliable experimental data, the accuracy of the theoretical enthalpies of formation is estimated as ranging from 5 to 10 kJ/mol. The recommended Delta f H298 degree values were used to derive the group additivity values (GAVs) for 45 groups involving the phosphorus(III) atom. These GAVs significantly extend the applicability of Benson's group additivity method and may be used to estimate the enthalpies of formation of larger organophosphorus(III) compounds, where high level quantum chemical calculations are impracticable.     

Gas-phase enthalpies of formation of the fluorobenzenes family and their dewar isomers from ab initio calculations

The standard gas-phase enthalpies of formation, at T = 298.15 K, of the complete series of fluorobenzene and their corresponding dewar isomers have been determined by means of the CBS-QB3 and G3MP2B3 composite approaches. These values have been estimated by using appropriate supporting reactions, such as, reactions of atomization or of atom substitution. The results show that there is a linear dependence between the enthalpy of the most stable n-fluorobenzene and the corresponding n-fluorodewar benzene (n = 0, 1, …, 6). Further, the estimates are always more negative than the experimental results and so, suggested enthalpies of formation for 1,2,3-, 1,2,4- and 1,3,5-trifluorobenzenes and for 1,2,3,4- and 1,2,3,5-tetrafluorobenzenes are those retrieved from G3MP2B3 calculations added by 8 kJ/mol. The interaction of four different M⁺ ions with fluorobenzene and the three difluorobenzenes shows that the σ-interaction with 1,2-difluorobenzene is stronger than π-interaction on these fluorobenzenes.     

C chemical shifts and carbonyl stretching frequencies as structural probes for ferrocenyl ketones

¹³C chemical shifts (δ(CO)) have been measured for a series of aromatic ferrocenyl ketones (FcCOC₆H₄X) and some sterically hindered analogues (FcCOR, R = mesityl, anthracyl, t-Bu, adamantyl). The shifts correlated quite well with those of the corresponding benzophenones. From the carbonyl carbon shifts (δ(CO)), estimates were made of the interplanar angle between the carbonyl sp² and the phenyl ring planes. Substituent effects of the Cp ring carbons are discussed. The solution (CCl₄) infrared spectra of the derivatives were obtained and the carbonyl stretching frequency (ν(CO)) was found to correlate roughly with δ(CO). Estimates were also made of the interplanar angle between the cyclopentadienyl ring plane and the carbonyl plane, for sterically hindered ketones using δ(CO) values.     

The enthalpies of formation and reorganization of aromatic radicals

The enthalpies of formation of some biphenyl derivatives were determined. A "double difference" method for calculating the enthalpies of formation of aromatic radicals and the bond dissociation energies was proposed. The enthalpies of formation of the radicals biphenyl, diphenyl oxide, and phenyl oxide were determined. The energies of reorganization of these radicals as well as phenyl and 4-, 3-, and 2-pyridyls were calculated. The sums of the energies of the chemical bonds in the molecular moieties transformed into radicals upon the decomposition of chemical compounds were found to be constant for different compounds. The energies of the chemical bonds in arenes were determined.     

Enthalpies of combustion and formation of 3-formylchromones

The enthalpies of combustion of 3-formylchromone (3F), 3-formyl-6-methylchromone (3F6M) and 3-formyl-6-isopropylchromone (3F6I) were determined by combustion calorimetry. The molar combustion energies () of the 3F, 3F6M and 3F6I are: −(4452.4 ± 1.8), −(5115.6 ± 2.7) and −(6411.4 ± 2.5) kJ mol⁻¹, respectively. The formation enthalpies in the crystalline state () are: −(340.2 ± 2.2), −(355.1 ± 3.1) and −(415.5 ± 3.0) kJ mol⁻¹, respectively.s     

Thermochemistry of adducts of some bivalent transition metal bromides with 4-chloroaniline

The compounds [MBr₂(p-clan)₂] (where M is Mn(II), Fe(II), Co(II), Ni(II), Cu(II) or Zn(II); p-clan = 4-chloroaniline) were synthesized and characterized by melting points, elemental analysis, thermal analysis and electronic and IR spectroscopy. The enthalpies of solution of the adducts, metal(II) bromides and 4-chloroaniline in methanol, 1.2 M aqueous HCl or 25% (v/v) 1.2 M aqueous HCl in methanol were measures and by using thermochemical cycles, the following thermochemical parameters for the adducts have been determined: the standard enthalpies for the Lewis acid/base reactions (Δ_rH°), the standard enthalpies of formation (Δ_fH°), the standard enthalpies of decomposition (Δ_DH°), the lattice standard enthalpies (Δ_MH°) and the standard enthalpies of the Lewis acid/base reactions in the gaseous phase (Δ_rH°(g)). The mean bond dissociation enthalpies of the metal(II)-nitrogen coordinated bonds and the enthalpies of adduct formation in the gaseous phase have been estimated.     

Comparison of calculated DFT/B3LYP and experimental C and O NMR chemical shifts, ab initio HF/6-31G* optimised structures, and single crystal X-ray structures of some substituted methyl 5β-cholan-24-oates

Single crystal X-ray structures (monoclinic space group P2₁) for methyl 3-oxo-5β-cholan-24-oate and methyl 3,12-dioxo-5β-cholan-24-oate have been solved and compared with HF/6-31G* optimised structures. In the crystalline packings the side chains are connected with weak OC(sp³)HO-type of interactions between C25–H and C24–O–C25 and the keto ends with weak C(sp³)HO=C-type of interactions between C4–H and O=C3. The orientations of the side chains, which steric configurations are of great importance to the biological activity of the molecules, are compared with the experimental structure of methyl 3-hydroxy-5β-cholan-24-oate. Probable reasons for the observed differences are discussed. In addition, ¹³C and ¹⁷O NMR chemical shifts of methyl 3-oxo-5β-cholan-24-oate and methyl 3,12-dioxo-5β-cholan-24-oate as well as the epimeric methyl 3-hydroxy-5β-cholan-24-oate and methyl 3β-hydroxy-5β-cholan-24-oate have been calculated (DFT/B3LYP/6-311G*) and compared with the experimental values by linear regression analyses. In general, the correspondence between the theoretical and experimental parameters is good or excellent.     

氯化烷基咪唑系列离子液体标准摩尔燃烧焓和生成焓

用精密氧弹热量计测定了4种离子液体: 氯化1-甲基-3-乙基咪唑(C2MIC), 氯化1-甲基-3-丁基咪唑(C4MIC), 氯化1-甲基-3-戊基咪唑(C5MIC)和氯化1-甲基-3-己基咪唑(C6MIC)的燃烧热, 计算了它们的标准摩尔燃烧焓 和标准摩尔生成焓 , 结合文献中的标准摩尔溶解焓, 估算了烷基咪唑阳离子在水溶液中的标准摩尔生成焓, 以及亚甲基对标准摩尔燃烧焓和标准摩尔生成焓的贡献.     

C NMR spectra of cyclomercurated ferrocenylimines: Substituent effects and conformations in the ferrocenes

Three series of cyclomercurated ferrocenylimines (2-chloromercurio-ferrocenylimines) have been studied using ¹³C NMR spectroscopy. Good to excellent linear relationships have been found to exist between the chemical shifts of the carbon atoms in the ferrocenyl moiety and normal Hammett substituent constants σ_m and σ_p. The δ values of the iminyl carbon atoms show excellent linear correlations with the σ values. In three series of the ferrocenes, the sensitivity of the carbon atoms to the substituent effect is different, which is discussed in terms of the twist angle between the N-phenyl ring and the C---C=N---C plane. The relative sensitivity of the chemical shifts on different positions in ferrocenyl moiety to the substituent effect has also been presented.     

水合六硼酸钡的合成与热化学研究

硼原子的硼酸盐种类繁多,结构复杂多样,其中,有一些硼酸盐具有特殊的性能,如:2ZnO·3B_2O_3·3.5H_2O、2CaO·3B_2O_3·5H_2O可用作阻燃材料~([1]),β-BaB_2O_4(β-BBO)~([2])、Ba_2Be_2B_2O_7~([3])、BaAl_2 B_2O7_~([4])等因其具有良好的光学性质,被广泛用作非线性光学材料.因此,进行硼酸盐的合成及性质研究有着重要的意义.     

Experimental and computational thermodynamic study of ortho-, meta-, and para-methylbenzamide

The Knudsen mass-loss effusion technique was used to measure the vapour pressures of the three crystalline isomers of methylbenzamide. From the temperature dependence of the vapour pressures, the standard molar enthalpies of sublimation and the enthalpies of the intermolecular hydrogen bonds N−H⋯O were calculated. The temperature and molar enthalpy of fusion of the studied isomers were measured using differential scanning calorimetry. The values of the standard (p^° = 0.1 MPa) molar enthalpy of formation in the crystalline phase, at T = 298.15 K, of the compounds studied were derived from their standard massic energies of combustion measured by static-bomb combustion calorimetry. From the experimental values, the standard molar enthalpies of formation in the gaseous phase, at T = 298.15 K, were calculated and compared with the values estimated by employing computational calculations that were conducted using different quantum chemical methods: G3(MP2), G3, and CBS-QB3. Good agreement between experimental and theoretical results is verified. The aromaticity of the compounds has been evaluated through nucleus independent chemical shifts (NICS) calculations.     

Electronic properties of Pt in bimetallic systems: photoemission and molecular-orbital studies for Pt---Al surface alloys

The adsorption of Pt on polycrystalline Al leads to the formation of surface alloys. The electronic properties of these systems have been examined using XPS and ab initio SCF calculations. The Pt---Al surface alloys display a Pt(5d) band that appears at much higher binding energy (≈ 1.8 eV) than in metallic Pt. This is accompanied by positive shifts in the Pt 4f (≈ 1.2 eV) and Al 2s (≈ 0.2 eV) levels. The Pt---Al bond is complex, involving an Al(3s, 3p) → Pt(6s, 6p) charge transfer and a Pt(5d) → Pt(6s, 6p) rehybridization that localize electrons in the region between the two metal centers.     

A Gaussian-3 investigation on the stabilities and bonding of the nine N10 clusters

A Gaussian-3 investigation has been performed to examine the stabilities of the nine N₁₀ isomers. G3 energies at 0K, enthalpies and heats of formation at 298K have been calculated. The most stable structure is VIII, which consists of three five-membered rings with a bowl-shape structure. The thermodynamic stability trend of the nine N₁₀ clusters is VIII > V > IX > VII > IV > VI > III > I > II, which is different from the previous theoretical results. Natural bond orbital (NBO) and atom in molecules (AIM) analysis have been carried out to study the bonding of these isomers.     

Solution enthalpies of hydrates LnCl3·xH2O (Ln=Ce–Lu)

Trichlorides of the lanthanide elements Ln=Ce–Lu form: (a) isotypic hexahydrates LnCl₃·6H₂O with a coordination number (CN) 8 for the Ln³⁺ ions. (b) Two isotypic groups of trihydrates LnCl₃·3H₂O, in the first group Ln=Ce-Dy the CN is 8; the structure of the second group Ln=Er–Lu is unknown. With Ho no trihydrate exists; a dihydrate is formed. (c) Two isotypic groups of monohydrates LnCl₃·H₂O with unknown structure – Ln=Ce–Dy and Ln=Ho–Lu. For all compounds and for anhydrous chlorides LnCl₃ solution enthalpies were measured with an isoperibolic calorimeter. The Δ_solH⁰ values do not depend only on the difference (lattice enthalpies/hydration enthalpies), but also on the state in solution. According to Spedding the CN of the Ln³⁺ ions against water changes from 9 to 8 between Nd and Sm, causing minima in the series of solution enthalpies. Dihydrates LnCl₃·2H₂O are found for Ln=Ce, Pr, Nd, Sm and presumably for Eu and Gd. They are not yet well characterised.     

Standard enthalpies of formation of Sr2CuO3 and Ca2CuO3

The enthalpies of reactions between alkaline-earth cuprates M₂CuO₃ (M = Ca, Sr) and hydrochloric acid were measured in a hermetic swinging calorimeter at 298.15 K. The M₂CuO₃ samples were prepared by solid-phase synthesis from calcium or strontium carbonate and copper oxide and characterized by X-ray powder diffraction, EDX and wet analysis. The standard enthalpies of formation obtained for the cuprates, −1431 ± 4 kJ mol⁻¹ for Ca₂CuO₃ and −1374 ± 3 kJ mol⁻¹ for Sr₂CuO₃, are discussed and compared with previous experimental and assessed values.