Ming-Der Su

A stable species with a formal GeC triple bond – a theoretical study

Abstract The isomerization of triplet and singlet HCGeF to CGe(H)(F) and to (H)(F)CGe have been studied by various computational methods. Our theoretical investigations suggest that electronegative halogen groups can considerably stabilize a carbon–germanium triple bond. In particular, triplet HCGeF, rather than singlet HCGeF, is predicted to be a promising candidate for isolation as a long-lived molecule.

Substituent effects on the stabilization of a germanium-sulfur double bond

Substituent effects on the potential energy surface of X2GeS (X=H, F, Cl, Br, CH3, and SiH3) were studied by using the B3LYP and CCSD(T) methods. Our theoretical investigations indicate that the kinetic and thermodynamic stabilities of substituted germanethiones are strongly dependent on the substituent. That is to say, dihalogen substitution can dramatically stabilize X2GeS with respect to XGe–SX. Thus, F2GeS, Cl2GeS and Br2GeS should be viable candidates for experimental observation.

Density functional studies on the mechanisms of unimolecular reactions of HXCSe (X=H, F, Cl, and Br)

Abstract Reaction pathways for the decomposition of HXCSe (X=H, F, Cl, and Br) on the singlet state potential energy surface have been studied using the B3LYP/6-311G* level of theory. Predicted molecular parameters (equilibrium geometries, dipole moments, atomic charges, and rotational constants) and vibrational IR spectra agree very well with the available experimental data. Five different reaction mechanisms are proposed: (A) 1,1-HX elimination, (B) 1,2-H shift, (C) 1,2-X shift, (D) H and XCSe radical formation, and (E) X and HCSe radical formation. From a consideration of the effect of halogen substitution, the following conclusions emerge: our theoretical findings suggest that selenocarbonyl molecules should be both kinetically and thermodynamically stable with respect to the unimolecular decomposition reactions given above. We also report theoretical predictions of molecular parameters and vibrational IR spectra of some monohalogen substituted selenoformaldehydes, which should be useful for further experimental observations.

A computational study of photochemical isomerization reactions of thiophenes

The mechanisms of the photochemical isomerization reactions were investigated theoretically using three model systems; 2‐methylthiophene, 2‐cyanothiophene, and 2‐phenylthiophene. The CASSCF (10‐electron/eight‐orbital active space) and MP2‐CAS methods were employed with the 6‐311(d) basis set. Three mechanisms, i.e., the internal cyclization‐isomerization route (path A), the zwitterion‐tricyclic route (path B), and the direct route (path C), have been used to explore the real photochemical reaction mechanism of these three model molecules. The structures of the conical intersections, which play a key role in such phototranspositions, were obtained. The intermediates and transition structures of the ground states were also calculated to assist in providing a qualitative explanation of the reaction pathways. Our model investigations suggest that the preferred reaction route is as follows: reactant → Franck‐Condon region → conical intersection → photoproduct. In particular, the conical intersection mechanism described in this work gives a better explanation than either the previously proposed internal cyclization‐isomerization (path A) or the zwitterion‐tricyclic pathway (path B) mechanisms, and is supported by the experimental observations. The results obtained allow a number of predictions to be made.

Substituent effect on relative stabilities of the phosphorus and tin multiple bonds

Abstract The unimolecular rearrangement of XSnP (X=H, Li, BeH, BH 2 , CH 3 , NH 2 , OH, and F) to SnPX is considered using B3LYP and QCISD calculations. The theoretical findings suggest that the doubly bonded SnPX molecule is intrinsically more stable than the triply bonded XSnP species, regardless of the electronegativity of the substituent X attached. On the other hand, the model calculations predict that only bulky aryl substituents can greatly stabilize XSnP over SnPX due to the steric effect.

Relative stability of multiple bonds between germanium and arsenic. A theoretical study

Abstract Substituent effects on the potential energy surface of XGeAs (X=H, Li, Na, BeH, MgH, BH 2 , AlH 2 , CH 3 , SiH 3 , NH 2 , PH 2 , OH, SH, F, and Cl) were investigated by using B3LYP and CCSD(T) methods. The isomers include structures with formal double (GeAsX) and triple (XGeAs) bonds to germanium–arsenic, so a direct comparison of these types of species is possible. Our model calculations indicate that electropositively substituted GeAsX species are thermodynamically and kinetically more stable than their isomeric XGeAs molecules. Moreover, the theoretical findings suggest that F, OH, NH 2 , and CH 3 substitution prefer to shift the double bond (GeAsX) by forming a triple bond (XGeAs).

Model Study of the Photochemical Rearrangement Pathways of 1,2,4‐Oxadiazole

The mechanisms of photochemical isomerization reactions are investigated theoretically by using a model system of 1,2,4- oxadiazole with the CAS(14,9)/6-311G(d) and MP2-CAS-(14,9)/ 6-311++G(3df,3pd)//CAS(14,9)/6-311G(d) methods. Three reaction pathways are examined, including 1) the direct mechanism, 2) the ring contraction-ring expansion mechanism, and 3) the internal cyclization-isomerization mechanism, which lead to two types of photoisomers. The theoretical findings suggest that conical intersections play a crucial role in the photorearrangement of 1,2,4-oxadiazoles. These model investigations also indicate that the preferred reaction route for 1,2,4-oxadiazole, which leads to phototransposition products, is as follows: reactant → Franck-Condon region → conical intersection → photoproduct. In other words, the direct mechanism is a one-step process that has no barrier. These theoretical results agree with the available experimental observations.

Structures, vibrational spectra, and relative energies of HXSiS (X = H, F, and Cl) isomers

The potential energy surface for the decomposition of HXSiS (X = H, F, and Cl) on the singlet state has been explored by B3LYP and CCSD(T) calculations. Five different types of reaction are proposed: (A) 1,1-HX elimination, (B) 1,2-H shift, (C) 1,2-X shift, (D) H· and XSiS· radical formation, and (E) X· and HSiS· radical formation. These results show interesting trends for the HXSiS isomers. Our theoretical investigations suggest that the doubly bonded species HXSiS should be the lowest energy structure among the isomers from both kinetic and thermodynamic viewpoints. We also report theoretical predictions of molecular parameters and vibrational infrared (IR) spectra of the monohalogen-substituted silanethione, which should be useful for future experimental observations.

Mechanistic Investigations on the Photoisomerization Reactions of 1,2-Dihydro-1,2-Azaborine

The mechanisms of the photochemical isomerization reactions were investigated theoretically by using a model system of 1,2-dihydro-1,2-azaborine with the CAS(6,6)/6-311G(d,p) and MP2-CAS-(6,6)/6-311++G(3df,3pd)//CAS(6,6)/6-311G(d,p) methods. Three reaction pathways, which lead to three kinds of photoisomers, have been examined. The structures of the conical intersections, which play a decisive role in such photorearrangements, were obtained. The thermal (or dark) reactions of the reactant species have also been examined by using the same level of theory to assist in providing a qualitative explanation of the reaction pathways. The model investigations suggest that the preferred reaction route for 1,2-dihydro-1,2-azaborine, which leads to the Dewar 1,2-dihydro-1,2-azaborine photoproduct, is as follows: reactant→Franck-Condon region→conical intersection→photoproduct. The results obtained allow a number of predictions to be made.

A computational study of the reactivities of four-membered heavy carbene systems.

The potential energy surfaces for the chemical reactions of four‐membered N‐heterocyclic group 14 heavy carbene species have been studied using density functional theory (B3LYP/LANL2DZ). Five four‐membered group 14 heavy carbene species, (i‐Pr)2NP(NR)2E:, in which E = C, Si, Ge, Sn, and Pb, were chosen as the model reactants in this work. Also, four kinds of chemical reactions, CH bond insertion, water addition, alkene cycloaddition, and dimerization, have been used to study the chemical reactivities of these group 14 four‐membered N‐heterocyclic carbene species. Basically, our present theoretical work predicts that the larger the ∠NEN bond angle of the four‐membered group 14 heavy carbene species, the smaller the singlet‐triplet splitting, the lower the activation barrier, and, in turn, the more rapid, its chemical reactions to various chemical species. Moreover, our theoretical investigations suggest that the relative carbenic reactivity decreases in the order: C > Si > Ge > Sn > Pb. That is, the heavier the group 14 atom (E), the more stable is its four‐membered carbene toward chemical reactions. As a result, our results predict that the four‐membered group 14 heavy carbene species (E = Si, Ge, Sn, and Pb) should be more kinetically stable than the observed carbene species and, thus, can be also readily synthesized and isolated at room temperature. Furthermore, the singlet‐triplet energy splitting of the four‐membered group 14 carbene species, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict their reactivities. The results obtained allow a number of predictions to be made.