Errol G. Lewars

Polymers and oligomers of carbon dioxide: ab initio and semiempirical calculations

Abstract Ab initio calculations have been performed on the D 2 h cyclic dimer (CO)O(CO)O (1,3-dioxacyclobutane-2,4-dione, 1,3-dioxetanedione), the D 3 h cyclic trimer (CO)O(CO)O(CO)O (1,3,5-trioxacyclohexane-2,4,6-trione, 1,3,5-trioxanetrione), the acyclic oligomers HO(CO 2 ) n H, n = 2–6 (dicarbonic to hexacarbonic acid) and the “infinite” polymer(CO 2 ) n -(poly(carbon dioxide)) of CO 2 . The cyclic molecules were studied at the AM1, HF/3-21G, HF/6-31G ∗ and MP2(FC)/6-31G ∗ levels, and the acyclic molecules at the AM1 and HF/3-21G levels. At the levels used, all these species are minima on the potential energy surface. The cyclic dimer and trimer are thermodynamically unstable with respect to CO 2 by 216 and 283 kJ mol −1 , respectively, at the MP2(FC)/6-31G ∗ level, i.e. 108 and 94 kJ mol −1 per CO 2 unit, while the limiting ( n → ∞) value for the acyclic polymer is 57 kJ mol −1 per CO 2 unit at the AM1 and 82 kJ mol −1 at the 3-21G level. The barrier for the concerted decomposition of the cyclic dimer and trimer to CO 2 is 41 and 80 kJ mol −1 , respectively, at the MP2(FC)/6-31G ∗ level, which corresponds to half-lives at room temperature of 1.1 × 10 −6 and 7.4 s and at 77 K of 4.6 × 10 15 and 1.3 × 10 41 s, respectively.

Density Functional Calculations

Density functional theory is based on the two Hohenberg-Kohn theorems, which state that the ground-state properties of an atom or molecule are determined by its electron density function, and that a trial electron density must give an energy greater than or equal to the true energy (the latter theorem is true only if the exact functional could be used). In the Kohn-Sham approach the energy of a system in formulated as a deviation from the energy of an idealized system with noninteracting electrons. From the energy equation, by minimizing the energy with respect to the Kohn-Sham orbitals the Kohn-Sham equations can be derived, analogously to the Hartree-Fock equations. Finding good functionals is the main problem in DFT. Various levels of DFT and kinds of functionals are discussed. The mutually related concepts of electronic chemical potential, electronegativity, hardness, softness, and the Fukui function are discussed.

Pyramidane: an ab initio study of the C5H4 potential energy surface

Abstract The novel hydrocarbon pyramidane (tetracyclo[2.1.0.01,302,5]pentane, [3.3.3.3]fenestrane), C5H4, with a pyramidal apical carbon, is a minimum not only at Hartree–Fock levels, as was shown in earlier work, but also at the MP2(FC)/6-31G* and MP2(FC)/6-31++G** levels. To investigate the stability and the chemistry of pyramidane, the C5H4 potential energy surface was explored with ab initio HF/6-31G* and MP2(FC)/6-31G* optimizations and single-point QCISD(T)/6-31G* energies. At the QCISD(T)/6-31G*//MP2(FC)/6-31G* level, pyramidane sits in a well with a barrier of 96.1 kJ mol−1 separating it from its most accessible isomer, the carbene tricyclo[2.1.0.02,5]pent-3-ylidene; the next lowest barrier found, 118 kJ mol−1, led to bicyclo[2.1.0]pent-2-en-5-ylidene (relative energies 87.1 and 55.3 kJ mol−1, respectively). Only two C5H4 minima were found to be lower in energy than pyramidane: 1,2,4-cyclopentatriene (“C2 cyclopentadienylidene”) and bicyclo[2.1.0]penta-1(5),2-diene (“C1 cyclopentadienylidene”).

Ketene: Ion chemistry and proton affinity

Abstract The gas phase ion-chemistry of ketene has been examined using a Quadrupole Ion Store (QUISTOR) as an ion-molecule reaction chamber. The ion chemistry of ketene is remarkably similar to that of the primary aliphatic alcohols and mercaptans and is characterized by associative and eliminative reactions of ions with a labile proton attached to the electronegative atom. The rate coefficients for the disappearance of CH 2 −+ and CO −+ in ketene were found to be 3.8 ± 1 × 10 −10 and 3.2 ± 0.5 × 10 −11 cm 3 molecule −1 s −1 , respectively. The proton affinity of ketene as determined by the bracketing technique was estimated to lie between the proton affinities of dimethyl ether and 2-propanol at a value of 807 ± 8 kJ mol −1 .

Dimers, trimers and oligomers of sulfur oxides: an ab initio and density functional study

Abstract Ab initio (HF/3-21G ∗ ), DFT (B3LYP with basis sets 6-31G ∗ , 6-311+G ∗ and 6-311+G(2d)) and, in some cases, MP2/6-31G ∗ calculations, were done on cyclic dimers, trimers, etc. and on acyclic oligomers (with OH and H on the ends) of sulfur monoxide and sulfur dioxide. The four cyclic (SO) n molecules were (S–O) 2 (1,3,2,4-dioxadithietane, 1a ), (S–O) 3 (1,3,5,2,4,6-trioxatrithiane, 2a ), (S(O)) 4 (tetrathietane 1,2,3,4-tetraoxide, 1b ), and (S(O)) 6 (hexathiane 1,2,3,4,5,6-hexaoxide, 2b ). The four cyclic (SO 2 ) n molecules were the dioxide of 1a (1,3,2,4-dioxadithietane 2,4-dioxide, 1c ), the trioxide of 2a (1,3,5,2,4,6-trioxatrithiane 2,4,6-trioxide, 2c ), the tetraoxide of 1b (tetrathietane 1,1,2,2,3,3,4,4-octaoxide, 1d ) and the hexaoxide of 2b (hexathiane 1,1,2,2,3,3,4,4,5,5,6,6-dodecaoxide, 2d ). The 16 acyclic molecules (oxides of disulfane, trisulfane, etc. and oxides of oxadisulfane, dioxatrisulfane, etc.) were (–S–O–) n , (–S(O)–) n , (–S(O)O–) n , and (–S(O) 2 –) n , with n from 2 to 5 and HO, H at the ends. Most of these species are relative minima on the B3LYP/6-31G ∗ potential energy surface. In energy content, the SO dimer, etc. lie below, and the SO 2 dimer, etc. above, their SO x components, at all the electron-correlated levels.

The quinones of benzocyclobutadiene: a computational study.

The conventional (excluding non-Kekulé, singlet diradical structures) quinones of benzocyclobutadiene were studied computationally. Eight structures were examined, namely (based on the CA names for benzocyclobutenedione), benzocyclobutenedione or bicyclo[4.2.0]octa-1,3,5-triene-7,8-dione, bicyclo[4.2.0]octa-3,5,8-triene-2,7-dione, bicyclo[4.2.0]octa-1,4,6-triene-3,8-dione, bicyclo[4.2.0]octa-1(6),4,7-triene-2,3-dione, bicyclo[4.2.0]octa-1(8), 4,6-triene-2,3-dione, bicyclo[4.2.0]octa-1(6),3,7-triene-2,5-dione, bicyclo[4.2.0]octa-1(8),3,6-triene-2,5-dione, and bicyclo[4.2.0]octa-1,5,7-triene-3,4-dione (the question of resonance or tautomerism for the 2,3-dione pair and the 2,5-dione pair is considered). Using DFT (B3LYP/6-31G*) and ab initio (MP2/6-31G*) methods the geometries of the eight species were optimized, giving similar results for the two methods. The heats of formation of the quinones were calculated, placing them in low-energy (-17 kJ mol(-1), 7,8-dione), medium-energy (79-137 kJ mol(-1), 2,7-, 3,8-, and 3,4-diones), and high-energy (260-275 kJ mol(-1), 2,3- and 2,5-diones) groups. Diels-Alder reactivity as dienophiles with butadiene indicated the 2,7-, 3,8-, and particularly the 3,4-quinone may be relatively unreactive toward dimerization or polymerization and are attractive synthesis goals. Isodesmic ring-opening reactions and NICS calculations showed aromatic/nonaromatic properties to be essentially as expected from the presence of a benzene or cyclobutadiene ring. UV spectra, ionization energy electron affinity, and HOMO/LUMO energies were also calculated.

Benzooxirene. Ab initio calculations

Abstract Ab initio calculations have been performed on benzooxirene, the corresponding oxo carbene (“ketocarbene”), and the transition state linking the two. At the highest level used, QCISD(T)/6-31G ∗ //MP2(FULL)/6-1G ∗ with MP2(FULL)/ 6-31G ∗ zero point energy corrections, the relative energies of the oxirene, the transition state and the carbene are 0, 24.6, and −17.8 kJ mol −1 . Correlation energy effects are very important in this system: at the QCISD(T) level the oxirene lies above the carbene, as at the MP4 and HF levels, but at the MP2 level the ordering is reversed. Benzooxirene is probably slightly nonplanar: the HF/6-31G ∗ geometry is C 2 v but the MP2(Fermi contact)/6-31G ∗ geometry is C s with a 6-/3-ring coplanarity deviation of about 6.9 °, although in the MP2(FULL)/6-31G ∗ geometry this is reduced to about 3.1 °.

The effect of substituents on the thermodynamic and kinetic stabilities of alkynols: a semiempirical and ab initio survey of the effect of H, Li, BeH, BH2, CH3, NH2, OH and F

Abstract Semiempirical (MNDO) and ab initio (HF/3-21G, HF/6-31G∗∗, and MP2/6-31G∗∗//HF/6-31G∗∗) calculations have been performed on the alkynols RCCOH, the corresponding tautomeric ketenes RHCCO, and the transition states linking the two, for R is H, Li, BeH, BH2, CH3, NH2, OH and F. The reaction energies and activation energies for the unimolecular ynol-to-ketene isomerization are similar to those for the parent (H) system. Isodesmic reaction calculations show that this is the result of similar effects on both ynol and ketene (and presumably the transition states) by the substituents, resulting in small and erratic changes in relative (with respect to the ketenes) stabilities.

PUSH-PULL SUBSTITUTION EVIDENTLY DOES NOT STABILIZE THE OXIRENE SYSTEM : AN AM1 AND AB INITIO STUDY

Abstract AM1 and ab initio calculations were performed on 1-amino-2-methanoyloxirene (1-amino-2-formyloxirene), the two corresponding oxo carbenes (“ketocarbenes”) resulting from alternative CO cleavage, and the transition states linking these species. The purpose was to see if delocalization of electron density from the double bond by the push-pull effect of the substituents would stabilize the oxirene system by diminishing its antiaromaticity. This stabilization was not seen: although at the AM1 level the oxirene is a relative minimum, it is merely an inflection point at the HF/3-21G level, and at the HF/6-31G∗ and MP2(FC)/6-31G∗ levels the oxirene is merely a transition state.

The Concept of the Potential Energy Surface

The potential energy surface (PES) is a central concept in computational chemistry. A PES is the relationship – mathematical or graphical – between the energy of a molecule (or a collection of molecules) and its geometry. The Born-Oppenheimer approximation says that in a molecule the nuclei are essentially stationary compared to the electrons. This is one of the cornerstones of computational chemistry because it makes the concept of molecular shape (geometry) meaningful, makes possible the concept of a PES, and simplifies the application of the Schrodinger equation to molecules by allowing us to focus on the electronic energy and add in the nuclear repulsion energy later; this third point, very important in practical molecular computations, is elaborated on in Chap. 5. Geometry optimization and the nature of transition states are explained.