Periodic trends in bond dissociation energies. A theoretical study

Bond dissociation energies (BDEs) of all possible A-X single bonds involving the first- and second-row atoms, from Li to Cl, where the free valences are saturated by hydrogens, have been estimated through the use of the G3-theory and at the B3LYP/6-311+G(3df,2pd)//B3LYP/6-31G(2df,p) DFT level of theory. BDEs exhibit a periodical behavior. The A-X (A = Li, Be, B, Na, Mg, Al, and Si) BDEs show a steady increase along the first and the second row of the periodic table as a function of the atomic number Z(X). For A-X bonds involving electronegative atoms (A = C, N, O, F, P, S, and Cl) the bond energies achieve a maximum around Z(X) = 5. The same behavior is observed when BDEs are plotted against the electronegativity χ(X) of the atom X. Thus, for A-X bonds (A = Li, Be, B, Na, Mg, Al, Si), the BDEs for a fixed A increases, grosso modo, as the electronegativity differences between X and A increase, with some exceptions, which reflect the differences in the relaxation energies of the radicals produced upon the bond cleavage. A similar trend, albeit less pronounced, is found for single A-X bonds, where A = C, N, O, F, P, S, and Cl. However, there is an additional feature embodied in the enhancement of the strength of the A-boron bonds due to the ability of boron to act as a strong electron acceptor. The trends in bond lengths and charge densities at the bond critical points are in line with the aforementioned behavior. © 2005 American Chemical Society.Peer Reviewe

Valence and Rydberg states of protonated formaldehyde

MR-CISD and MR-CISD + Q calculations have been performed for the vertical excitations of protonated formaldehyde in comparison to formaldehyde. Singlet and triplet states have been investigated. It is shown that the protonation causes the Rydberg states to be shifted to higher energies by several eV. This finding is discussed by means of the Rydberg formula in terms of quantum defects for the two lowest vertical ionization energies. For protonated formaldehyde the pi-pi(*) valence state is energetically the second lowest state at 9.80 eV, about 1.50 eV below the first Rydberg n-3s state. This finding is in strong contrast to the case of formaldehyde where the pi-pi(*) state is embedded within a series of Rydberg states. (C) 2003 Elsevier Science B.V. All rights reserved

A DFT study of pyramidalized alkenes : 7-oxasesquinorbornenes and 7,7'-dioxasesquinorbornenes

DFT calculations of 7¢–oxasesquinorbornenes and 7,7¢-dioxasesquinorbornenes using the B3LYP/6– 31G* method are reported. All the investigated structures (syn- and anti- derivatives) showed significant non-planarity of the central double bond, with the exception of those anti-derivatives possessing symmetrical structures. The influence of the replacement of the methylene groups at position 7- of the norbornene fragment with oxygen and the introduction of second and third (peripheral) double bonds and benzene rings on the molecular and electronic structures of these molecules have also been investigated

A DFT study of pyramidalized alkenes : 7-oxasesquinorbornenes and 7,7'-dioxasesquinorbornenes

DFT calculations of 7¢–oxasesquinorbornenes and 7,7¢-dioxasesquinorbornenes using the B3LYP/6– 31G* method are reported. All the investigated structures (syn- and anti- derivatives) showed significant non-planarity of the central double bond, with the exception of those anti-derivatives possessing symmetrical structures. The influence of the replacement of the methylene groups at position 7- of the norbornene fragment with oxygen and the introduction of second and third (peripheral) double bonds and benzene rings on the molecular and electronic structures of these molecules have also been investigated