The Energetics of Halogenated Ethylenes (Ethynes) and 1,3-Butadienes (Butadiynes): A Computational and Conceptual Study of Substituent Effects and “Dimerization” (original) (raw)
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Journal of Organic Chemistry, 1987
T h e thermolyses of ethoxyethyne (2a), isopropoxyethyne (2b), tert-butoxyethyne (2c), 1-ethoxy-1-propyne (3a), 1-isopropoxy-1-propyne (3b), 1-tert-butoxy-1-propyne (3c), and 1-ethoxy-1-butyne (4) leading t o ketenes and olefins have been studied by means of the semiempirical SCF MO methods AM1, MNDO, and MIND013 a t the RHF level. The reactions have been found in all cases to be concerted and to exhibit a highly synchronous character. The predicted order of reactivity,-C=CO-t-Bu >-C=CO-i-Pr >-C=COEt, fully coincides with the experimental one, the AM1 calculated activation enthalpies being the closest to the experimentally determined Arrhenius activation energies. Observed deuterium primary and secondary kinetic isotope effects are well reproduced by the calculation. A reaction analysis by correlation of localized molecular orbitals identifies the direction of flow of electron density along the reaction coordinate and suggests that the lack of adaptation of the u-component of the initial triple bond t o the geometrical changes taking place along the reaction path makes an important contribution to the activation energy of the reaction.
Journal of Computational Chemistry, 2007
We present quantum mechanical partition functions, free energies, entropies, and heat capacities of 1,3butadiene derived from ab initio calculations. Our technique makes use of a reaction path-like Hamiltonian to couple all 23 vibrational modes to the large-amplitude torsion, which involves heavy asymmetric functional groups. Ab initio calculations were performed at the B3LYP, MP2, and CCSD(T) levels of theory and compared with experimental values as a reference case. By using the ab initio potentials and projected frequencies, simple perturbative expressions are presented for computing the couplings of all the vibrational modes to the large-amplitude torsion. The expressions are particularly suited for programming in the new STAR-P software platform which automatically parallelizes our codes with distributed memory via a familiar MATLAB interface. Using the efficient parallelization scheme of STAR-P, we obtain thermodynamic properties of 1,3-butadiene for temperatures ranging from 50 to 500 K. The free energies, entropies, and heat capacities obtained from our perturbative formulas are compared with conventional approximations and experimental values found in thermodynamic tables.
To understand the relative isomeric stabilities of 1,3-and 1,4-diheterocyclohexanes and the ultradiagonal strain energy of thiirane, the enthalpies of formation and sublimation of 1,4-dithiane have been measured. The enthalpy of formation for this compound in the solid state is -69.6 ( 2.3 kJ mol -1 , while for the gaseous state, the value is -6.9 ( 2.4 kJ mol. The value for the enthalpy of sublimation is 63.0 ( 0.6 kJ mol -1 . Ab initio molecular orbital calculations at the G2(MP2) and at G3 levels were performed, and the calculated enthalpies of formation are compared with the experimental data. † Structural Effects on the Thermochemical Properties of Sulfur Compounds. Part 2.
Thermodynamic properties of the c5, c6, and c8 n-alkanes from ab initio electronic structure theory
2005
The heats of formation for the n-alkanes C n H n+2 for n) 5, 6, and 8 have been calculated using ab initio molecular orbital theory. Coupled-cluster calculations with perturbative triples (CCSD(T)) were employed for the total valence electronic energies. Correlation-consistent basis sets were used, up through the augmented quadruple zeta, to extrapolate to the complete basis set limit. Geometries were optimized at the B3LYP/ TZVP and MP2/aug-cc-pVTZ levels. The MP2 geometries were used in the CCSD(T) calculations. Frequencies were determined at the density functional level (B3LYP/TZVP), and scaled zero point energies were calculated from the B3LYP frequencies. Core/valence, scalar relativistic, and spin-orbit corrections were included in an additive fashion to predict the atomization energies. The core/valence corrections are not small, (∼1.1 kcal/mol per carbon unit) and cannot be neglected for chemical accuracy. The calculated ∆H f 298 values are-35.0,-40.2, and-50.2 kcal/mol for C 5 H 12 , C 6 H 14 , and C 8 H 18 , respectively, in excellent agreement with the respective experimental values of-35.11 (0.19,-39.89 (0.19, and-49.90 (0.31 kcal/mol. Isodesmic reaction energies are presented for some simple reactions involving C 8 H 18 and are shown not to be strongly method dependent.
Journal of Computational Chemistry, 29, 481 (2008)
The thermodynamic properties of three halocarbon molecules relevant in atmospheric and public health applications are presented from ab initio calculations. Our technique makes use of a reaction path-like Hamiltonian to couple all the vibrational modes to a large-amplitude torsion for 1,2-difluoroethane, 1,2-dichloroethane, and 1,2dibromoethane, each of which possesses a heavy asymmetric rotor. Optimized ab initio energies and Hessians were calculated at the CCSD(T) and MP2 levels of theory, respectively. In addition, to investigate the contribution of electronically excited states to thermodynamic properties, several excited singlet and triplet states for each of the halocarbons were computed at the CASSCF/MRCI level. Using the resulting potentials and projected frequencies, the couplings of all the vibrational modes to the large-amplitude torsion are calculated using the new STAR-P 2.4.0 software platform that automatically parallelizes our codes with distributed memory via a familiar MATLAB interface. Utilizing the efficient parallelization scheme of STAR-P, we obtain thermodynamic properties for each of the halocarbons, with temperatures ranging from 298.15 to 1000 K. We propose that the free energies, entropies, and heat capacities obtained from our methods be used to supplement theoretical and experimental values found in current thermodynamic tables. q 2007 Wiley Periodicals, Inc. J Comput Chem 29: 481-487, 2008
Relative energy of organic compounds I. Hydrocarbons and their oxygen derivatives
Structural Chemistry, 2009
The energies of the following types of compounds are characterized by their calculated relative enthalpies: alkanes and cycloalkanes, alkenes and cycloalkenes, polyolefins and cyclic polyolefins, aromatic hydrocarbons, alcohols and phenols, ethers, peroxides, aldehides and ketones, acetals, carboxylic acids esters, and anhydrides. Stabilization energies of conjugated olefins, benzene, and furan have been estimated.
Journal of Physical Chemistry A - J PHYS CHEM A, 2001
The bond dissociation energies and the enthalpies of formation of halogenated molecules were theoretically calculated, and the results were compared with the corresponding experimental values in order to examine the reliability of a large number of levels of theory in thermochemical calculations. Density functional theory using a multitude of exchange and correlation functionals, Møller-Plesset perturbation theory, and QCISD-(T) and CCSD(T) methods were employed, with all-electron and effective-core potential basis sets of varying complexity. A small set of 19 molecules was selected, consisting of X 2 , HX, and CH 3 X (X ) F, Cl, Br, and I), the mixed-halogen molecules ClF, BrF, BrCl, IF, and ICl, and H 2 and CH 4 . The calculated bond dissociation energies were corrected for basis set superposition errors and the first-order spin-orbit coupling in the 2 P state of halogen atoms. In addition, the enthalpies of formation of all molecules in the set as well as those of methyl CH 3 and halomethyl radicals CH 2 X were also calculated by using the corresponding atomization reactions, corrected for the spin-orbit coupling in the 3 P state of carbon atom and the 2 P state of halogen atoms. Levels of theory employing the B3P86 functional with moderately large basis sets, augmented with diffusion and polarization functions, were found to be sufficiently reliable in the calculation of bond dissociation energies of closed-shell halogenated molecules. In particular, the B3P86/6-311++G(2df,p) level of theory was found to be the most accurate, with an RMS deviation of 6 kJ mol -1 for 23 bond dissociation energies, with a negligible dependence of the accuracy on the level of theory chosen for the geometry optimization. In addition, the B3P86 functional in combination with small basis sets was found to be superior to B3LYP and MP2 in the calculation of molecular structures. Regarding the calculated enthalpies of formation, G2 theory was the most accurate, with an RMS deviation of 9 kJ mol -1 , followed by several combinations of the B3PW91 and B3LYP functionals with mostly large basis sets. However, the B3P86 functional tends to overbind openshell species, resulting in an underestimation of the enthalpies of formation for polyatomic molecules. Extension of the bond dissociation energy calculations at levels of theory employing the B3P86 functional to a larger set of 60 bonds in 41 halogen-containing molecules revealed systematic errors dependent on the molecular size. Therefore, the calculated bond dissociation energies at the B3P86/6-311++G(2df,p) level of theory were empirically improved by increasing the absolute energies of the radicals by the quantity 9 × 10 -5 ‚N e Hartrees (N e ) total number of electrons of the radical), with a subsequent lowering of the RMS deviation in the larger set to 8.0 kJ mol -1 .
Journal of the American Chemical Society, 1994
In connection with the observed losses of CH3' and CHq from ionized butane (1) and isobutane (2) in the gas phase, ab initio molecular orbital calculations at the UMP2, QCISD, and QCISD(T) levels of theory with the 6-31G(d) and 6-31G(d,p) basis sets have been used to investigate the relevant parts of the C&~O*+ ground-state potential energy surface. The isomerization of 1 to 2 is found to take place via a transition structure (X) consisting of a nonclassical H-bridged propyl cation coordinated to the methyl radical. X lies 19.9 kcdmol above the lowestenergy trans conformer of 1 and 3.6 kcdmol above the energy of the dissociation fragments sec-propyl cation plus methyl radical. In addition to mediating the 1-2 isomerization, X also mediates the losses of both CH3' and C& from 1 through non-minimum energy reaction paths which are energetically accessible. The CHq elimination from 2 is found to take place via a transition structure (XI) which can be viewed as a sec-propyl cation coordinated to the methyl radical. XI is calculated to lie 12.9 kcdmol above 2 and 3.1 kcaymol below the energy of its loosely bound components. These theoretical results are consistent with mass spectrometry experimental findings reported in the literature.
Journal of Computational Chemistry, 2004
Energies of hydrocarbon monoderivatives CH 3 X, C 2 H 5 X, n-C 4 H 9 X, and n-C 5 H 11 X with 16 different substituents X were calculated at the levels B3LYP/6-311ϩG(d,p) and B3LYP/AUG-cc-pVTZ//B3LYP/6-311ϩG(d,p). The results were used to test the validity of the additive rule that has served commonly for estimating the enthalpies of formation ⌬ f H(T). The exact additivity corresponds to zero reaction energy ⌬E of the isodesmic reaction, in which the substituent X is transferred from one alkyl group R to another. Additivity is approximately fulfilled for butyl and pentyl derivatives with the differences less than 0.3 kJ mol Ϫ1 (except charged groups X). Methyl derivatives deviated from the additive rule up to 22 kJ mol Ϫ1 for dipolar groups X and 45 kJ mol Ϫ1 for charged group, in agreement with the available experiments and with the anticipation of all suggested empirical schemes. In addition, smaller deviations of ethyl derivatives (3 or 20 kJ mol Ϫ1 , respectively) were observed here for the first time. There is no correlation between the deviations of methyl and ethyl derivatives; they are also not related to steric effects, and only partly to polarization. Deviations of methyl derivatives are proportional to the electronegativity of the first atom of the substituent; even when the definition of electronegativity is somewhat questionable, one can say in any case that it is controlled by the first atom.