Charge transfer induced enhancement of near-IR two-photon absorption of 5,15-bis(azulenylethynyl) zinc(II) porphyrins.

Intramolecular charge transfer in 5,15-bis(azulenylethynyl) substituted zinc(ii) porphyrin leads to a significant enhancement of two-photon absorption at near-IR region, which has been investigated by femtosecond Z-scan method.

Experimental and theoretical studies on the nature of weak nonbonded interactions between divalent selenium and halogen atoms.

To investigate the nature of weak nonbonded selenium...halogen interactions (Se...X interactions; X = F, Cl, and Br), three types of model compounds [2-(CH(2)X)C(6)H(4)SeY (1-3), 3-(CH(2)X)-2-C(10)H(6)SeY (4-6), and 2-XC(6)H(4)CH(2)SeY (7-9); Y = CN, Cl, Br, SeAr, and Me] were synthesized, and their (77)Se NMR spectroscopic behaviors were analyzed in CDCl(3). The gradual upfield shifts of (77)Se NMR absorptions observed for series 1-3 and 4-6 suggested that the strength of Se...X interaction decreases in the order of Se...F > Se...Cl > Se...Br. The quantum chemical calculations at the B3LYP/631H level using the polarizable continuum model (PCM) revealed that the most stable conformer for 1-3 is the one with an intramolecular short Se...X atomic contact in CHCl(3) (epsilon = 4.9) and also that the n(X) --> sigma(Se-Y) orbital interaction (E(Se...X)) can reasonably explain the order of strength for the Se...X interactions. On the other hand, the (77)Se NMR absorptions observed for series 7-9 did not shift significantly from the reference compounds (C(6)H(5)CH(2)SeY), indicating the absence of the Se...X interaction for 7-9 presumably due to attenuation of basicity for the halogen atom that is substituted directly to the aromatic ring. These observations suggested that the n(X) --> sigma(Se-Y) orbital interaction is a dominant factor for formation of weak Se...X interactions. Electron correlation was also suggested to be important for the stability.

Nature of nonbonded Se...O interactions characterized by 17O NMR spectroscopy and NBO and AIM analyses.

To investigate the nature of nonbonded Se...O interactions, three series of 2-substituted benzeneselenenyl derivatives [2-(CHO)C6H4SeX (1), 2-(CH2OH)C6H4SeX, (2), 2-(CH2OiPr)C6H4SeX (3); X = Cl, Br, CN, SPh, SeAr, Me] were synthesized. The 17O NMR absorption observed for 17O-enriched aldehydes 1 appeared upfield relative to benzaldehyde (PhCHO), while the opposite downfield shifts relative to benzyl alcohol (PhCH2OH) were observed for 17O-enriched alcohols 2 and ethers 3. The magnitude of both the upfield and the downfield shifts became larger as the electron-withdrawing ability of a substituent X increased. Quantum chemical calculations at the B3LYP level revealed that for all model compounds the most stable conformer has an intramolecular nonbonded Se.O interaction. Thus, the relative 17O NMR chemical shifts (DeltadeltaO) for 1-3 would reflect the strengths of the Se...O interactions. The natural bond orbital (NBO) analysis demonstrated that the stabilization energy due to an nO --> sigma Se-X orbital interaction (ESe...O) correlates with the Se...O atomic distance on a single curve irrespective of the type of the O atom. On the other hand, the atoms in molecules (AIM) analysis showed that the nonbonded Se...O interactions can be characterized by the presence of a bond critical point, the total energy density (HSe...O) of which decreases with strengthening of the interaction. The results suggested that Se...O interactions have a dominant covalent character rather than an electrostatic one.

Quantitative evaluation of weak nonbonded Se...F interactions and their remarkable nature as orbital interactions.

To evaluate weak intramolecular nonbonded Se...F interactions recently characterized for a series of o-selenobenzyl fluoride derivatives (Iwaoka et al., Chem. Lett. 1998, 969-970), the temperature dependence of the nuclear spin coupling between Se and F (J(Se...F)) was investigated for 2-(fluoromethyl)phenylselenenyl cyanate (1a) and bis[2-(fluoromethyl)phenyl] diselenide (1e) in CD2Cl2 and CD3CN. A significant increase in the magnitude of J(Se...F) was observed for both 1a and 1e upon lowering temperature, whereas the values of J(Se...F) for the corresponding trifluoromethyl compounds slightly reduced or remained unchanged at low temperatures. Application of the rapid equilibrium model between two possible conformers revealed that conformer A with an intramolecular Se...F interaction is more stable in enthalpy (DeltaH) by 1.23 kcal/mol for 1a (in CD2Cl2) and by 0.85 and 0.83 kcal/mol for 1e (in CD2Cl2 and CD3CN, respectively) than conformer B, which does not have close Se...F contact. The negligible solvent effects for 1e suggested marginal electrostatic nature of the Se...F interactions. Instead, importance of the n(F) -->sigma*(Se-X) orbital interaction was suggested by quantum chemical (QC) calculations and the natural bond orbital (NBO) analysis.