The singlet-triplet energy gap in divalent three, five and seven-membered cyclic C2H2M, C4H4M and C6H6M (M = C, Si, Ge, Sn AND Pb)

Total energy gaps, ΔEt–s, enthalpy gaps, ΔHt–s, and Gibbs free energy gaps, ΔGt–s, between singlet (s) and triplet (t) states were calculated for three, five and seven-membered cyclic C2H2M, C4H4M and C6H6M (M = C, Si, Ge, Sn and Pb) at B3LYP/6-311++G**. The singlet-triplet free energy gaps, ΔGt–s, for C2H2M (M = C, Si, Ge, Sn and Pb) are found to be increased in the order: C2H2Si > C2H2C > C2H2Ge > C2H2Sn > C2H2Pb. The ΔGt–s of C4H4M are found to be increased in the order: C4H4Pb > C4H4Sn > C4H4Ge > C4H4Si > C4H4C. Also, the ΔGt-s of C6H6M are determined in the order: C6H6Pb > C6H6Ge ≥ C6H6Sn > C6H6Si > C6H6C. The most stable conformers of C2H2M, C4H4M and C6H6M are proposed for both the singlet and triplet states. Nuclear independent chemical shifts (NICS) calculations were carried out for determination of aromatic character. The geometrical parameters are calculated and discussed

ELECTRONIC EFFECTS AT α-POSITION OF DIVALENT FIVE-MEMBERED RING CARBENES: SINGLET-TRIPLET ENERGIES THROUGH DFT CALCULATIONS

Thermal energy gaps, ΔEs–t; enthalpy gaps, ΔHs–t; Gibbs free energy gaps, ΔGs–t, between singlet (s) and triplet (t) states of G–C4H3C (G = –NH2, –OH, –CH3, –F, –Cl, –Br, –H, –CF3, –NO2) were calculated at B3LYP/6-311++G** level of theory. DFT calculations indicated that electron donating substituents (G = –NH2, –OH and –CH3) at α position cause to decrease ΔGs–t and electron withdrawing substituents (G = –F, –Cl, –Br, –CF3 and –NO2) lead to increase the ΔGs–t of G-C4H3C. Nuclear independent chemical shifts (NICS) calculations were carried out to determine the aromatic character. KEY WORDS: DFT calculations, Electronic effects, Singlet-triplet energies, Carbene, Five-membered, NICS Bull. Chem. Soc. Ethiop. 2009, 23(1), 151-155

Singlet-triplet gap studies on aryl-cyclopentadienylidenes: indirect electronic effects

Energy gaps, ΔXS–t (X = E, H and G) (∆Xs-t = X(singlet)-X(triplet)) between singlet (s) and triplet (t) states of aryl substituted cyclopentadienylidenes, Ar-C4H3C, were calculated at B3LYP/6-311++G**. Electron donating substituents (G = -NH2, -OH, -CH3, -F, -Cl and -Br) at phenyl group cause to increase and electron withdrawing substituents (G = -CF3 and -NO2) lead to decrease the singlet-triplet energy gaps of Ar-C4H3C. The results of the singlet-triplet gap calculations were fully supported by HOMO-LUMO gaps. KEY WORDS: Carbene, Cyclopentadienylidenes, Singlet-triplet gap, Electronic effects  Bull. Chem. Soc. Ethiop. 2010, 24(2), 311-314

Steric effect studies on solar energy storage of norbornadiene-quadracyclane system: DFT calculations

The aim of this research is to determine the possible solar energy storage in the norbornadiene (1) / quadricyclane (2) system, through involving steric effects on various position of carbon C1, C2 or C7 for 1 and 2; calculating the corresponding energies at B3LYP/6-311G** level of theory. The extent of the solar energy storage is the least for 11-i-Pr (-21.018), 12-t-Bu (-22.525) and 17-i-Pr (-17.753) when the bulk substituents (X) were occured at C1, C2 and C7, respectively

<b>DFT calculations on 1,4-dithiine and S-oxygenated derivatives</b>

The molecular structures of 1,4-dithiine and S-oxygenated derivatives are studied using B3LYP/6-311++G** level of theory. These compounds have 8π-electrons in the ring. This led to stabilization of non-planar conformation. DFT calculations show that 1,4-dithiine, C₄H₄SS, 1,4-dithiine-1-oxide, C₄H₄SOS, 1,4-dithiine-1,4-dioxide, C₄H₄SOSO and 1,4-dithiine-1,1,4-trioxide, C₄H₄SO₂SO; have boat conformation. 1,4-dithiine-1,1-dioxide, C₄H₄SO₂S, have a shadow boat conformation. 1,4-dithiine-1,1,4,4-tetraoxide, C₄H₄SO₂SO₂, have a planar conformation

<b>The singlet-triplet energy gap in divalent three, five and seven-membered cyclic C₂H₂M, C₄H₄M and C₆H₆M (M = C, Si, Ge, Sn AND Pb)</b>

Total energy gaps, ∆E_t–s, enthalpy gaps, ∆H_t–s, and Gibbs free energy gaps, ∆G_t–s, between singlet (s) and triplet (t) states were calculated for three, five and seven-membered cyclic C₂H₂M, C₄H₄M and C₆H₆M (M = C, Si, Ge, Sn and Pb) at B3LYP/6-311++G**. The singlet-triplet free energy gaps, ∆G_t–s, for C₂H₂M (M = C, Si, Ge, Sn and Pb) are found to be increased in the order: C₂H₂Si > C₂H₂C > C₂H₂Ge > C₂H₂Sn > C₂H₂Pb. The ∆G_t–s of C₄H₄M are found to be increased in the order: C₄H₄Pb > C₄H₄Sn > C₄H₄Ge > C₄H₄Si > C₄H₄C. Also, the ∆G_t–s of C₆H₆M are determined in the order: C₆H₆Pb > C₆H₆Ge ≥ C₆H₆Sn > C₆H₆Si > C₆H₆C. The most stable conformers of C₂H₂M, C₄H₄M and C₆H₆M are proposed for both the singlet and triplet states. Nuclear independent chemical shifts (NICS) calculations were carried out for determination of aromatic character. The geometrical parameters are calculated and discussed

<b>Electronic effects at α-position of divalent five-membered ring carbenes: singlet-triplet

Thermal energy gaps, ΔEs–t; enthalpy gaps, ΔHs–t; Gibbs free energy gaps, ΔGs–t, between singlet (s) and triplet (t) states of G–C4H3C (G = –NH2, –OH, –CH3, –F, –Cl, –Br, –H, –CF3, –NO2) were calculated at B3LYP/6-311++G** level of theory. DFT calculations indicated that electron donating substituents (G = –NH2, –OH and –CH3) at α position cause to decrease ΔGs–t and electron withdrawing substituents (G = –F, –Cl, –Br, –CF3 and –NO2) lead to increase the ΔGs–t of G-C4H3C. Nuclear independent chemical shifts (NICS) calculations were carried out to determine the aromatic character

<b>DFT calculations on 1-hetero-2,5-cyclohexadiene-1-oxide: molecular structure and Stability</b>

Molecular structures of 1-hetero-2,5-cyclohexadiene-1-oxide, XO, are studied and compared with their corresponding deoxygenated compound, 1-hetero-2,5-cyclohexadiene, X, using DFT at B3LYP/6-311++G** level. The most stable boat conformers are found for OO, SO, SeO, PO, AsO, S, Se, N, P, As, and the most stable planar conformers are also found for NO, CO, SiO, GeO, O, C, Si, Ge. Isodesmic reactions to determine the stabilities of XO and X are considered. Nuclear independent chemical shifts, NICS, are calculated for the investigation of the homo-aromatic character of XO and X. The optimised geometries show the bonding in the molecules is explicable in terms of basic chemical concepts. The atomic charges calculated are also reasonable based on the concepts of electronegativity and conjugation

<b>Singlet-triplet gap studies on aryl-cyclopentadienylidenes: indirect electronic effects</b>

Energy gaps, ΔXS–t (X = E, H and G) (∆XS–t = X(singlet)-X(triplet)) between singlet (s) and triplet (t) states of aryl substituted cyclopentadienylidenes, Ar-C4H3C, were calculated at B3LYP/6-311++G**. Electron donating substituents (G = -NH2, -OH, -CH3, -F, -Cl and -Br) at phenyl group cause to increase and electron withdrawing substituents (G = -CF3 and -NO2) lead to decrease the singlet-triplet energy gaps of Ar-C4H3C. The results of the singlet-triplet gap calculations were fully supported by HOMO-LUMO gaps