RESUMO
[reaction: see text] This study is a multinational, multidisciplinary contribution to the thermochemistry of dimethyl1,4-cubanedicarboxylate and the corresponding isomeric, cuneane derivative and provides both structural and thermochemical information regarding the rearrangement of dimethyl 1,4-cubanedicarboxylate to dimethyl 2,6-cuneanedicarboxylate. The enthalpies of formation in the condensed phase at T = 298.15 K of dimethyl 1,4-cubanedicarboxylate (dimethyl pentacyclo[4.2.0.0.(2,5)0.(3,8)0(4,7)]octane-1,4-dicarboxylate) and dimethyl 2,6-cuneanedicarboxylate (dimethyl pentacyclo[3.3.0.0.(2,4)0.(3,7)0(6,8)]octane-2,6-dicarboxylate) have been determined by combustion calorimetry, delta(f) H(o)m (cr)/kJ x mol(-1) = -232.62 +/- 5.84 and -413.02 +/- 5.16, respectively. The enthalpies of sublimation have been evaluated by combining vaporization enthalpies evaluated by correlation-gas chromatography and fusion enthalpies measured by differential scanning calorimetry and adjusted to T = 298.15 K, delta(cr) (g)Hm (298.15 K)/kJ x mol(-1) = 117.2 +/- 3.9 and 106.8 +/- 3.0, respectively. Combination of these two enthalpies resulted in delta(f) H(o)m (g., 298.15 K)/kJ x mol(-1) of -115.4 +/- 7.0 for dimethyl 1,4-cubanedicarboxylate and -306.2 +/- 6.0 for dimethyl 2,6-cuneanedicarboxylate. These measurements, accompanied by quantum chemical calculations, resulted in values of delta(f) Hm (g, 298.15 K) = 613.0 +/- 9.5 kJ x mol(-1) for cubane and 436.4 +/- 8.8 kJ x mol(-1) for cuneane. From these enthalpies of formation, strain enthalpies of 681.0 +/- 9.8 and 504.4 +/- 9.1 kJ x mol(-1) were calculated for cubane and cuneane by means of isodesmic reactions, respectively. Crystals of dimethyl 2,6-cuneanedicarboxylate are disordered; the substitution pattern and structure have been confirmed by determination of the X-ray crystal structure of the corresponding diacid.
RESUMO
Thermochemical data, and in particular the enthalpies of formation of oxygen- and sulfur-containing six-membered heterocycles provide essential information on the factors responsible for the contrasting behavior (structural, conformational and reactivity) between these types of compounds. A proper understanding of the experimental observations requires theoretical modeling in order to confirm the relative importance of the steric, electronic, electrostatic and stereoelectronic interactions that are responsible of the enthalpies of formation for the heterocyclic compounds of interest.
Assuntos
Simulação por Computador , Compostos Heterocíclicos/química , Compostos de Enxofre/química , Enxofre/química , Calorimetria/métodos , Estrutura MolecularRESUMO
The enthalpies of combustion and vaporization of 3-buten-1-ol and 3-butyn-1-ol have been measured by static bomb combustion calorimetry and correlation gas chromatography techniques, respectively, and the gas-phase enthalpies of formation, Delta(f)H degrees (m)(g), have been determined, the values being -147.3 +/- 1.8 and 16.7 +/- 1.6 kJ mol(-1), for 3-buten-1-ol and 3-butyn-1-ol, respectively. High level calculations at the G2 and G3 levels have also been carried out. Relationships between the enthalpies of formation of 1-alkanols, 1-alkenols and 1-alkynols and with the corresponding hydrocarbons have been discussed. From the calculated contributions to Delta(f)H degrees (m)(g) for the substitutions of CH(3) by CH(2)OH, CH(3)CH(2) by CH(2)=CH and CH(3)CH(2) by CH triple bond C, we have estimated the Delta(f)H degrees (m)(g) values for 3-buten-1-ol and 3-butyn-1-ol, in excellent agreement with the experimental ones. Delta(f)H degrees (m)(g) values for 1-alkenols and 1-alkynols up to 10 carbon atoms have also been estimated.