A matrix isolation ESR investigation of the MgCH radical.

The MgCH radical and its magnesium-25, carbon-13, and deuterated isotopologs have been isolated in low temperature neon matrices and examined by the matrix isolation electron spin resonance technique for the first time. The radicals were formed through the reactions of laser ablated natural abundance magnesium metal and magnesium-25 enriched magnesium metal with carbon-13 and deuterated isotopologs of acetone. The MgCH radical was shown to have a X4Σ- ground electronic state, and the magnetic parameters determined for this state were g⊥ = 2.001 81(45), g∥ = 2.0018(10), D = 4970(5) MHz, A⊥(13C) = 115(6) MHz, A∥(13C) = 65(15) MHz, A⊥(H) = 34(6) MHz, A∥(H) = 5(10) MHz, A⊥(D) = 5(3) MHz, A⊥(25Mg) = 82(5) MHz, and A∥(25Mg) = 85(10). Comparisons are made between the electronic structure of this radical and the MgCH3 and MgN radicals. Theoretical hyperfine parameters were also evaluated for the MgCH radical, and a potential energy surface for the low-lying electronic states was constructed using complete active space multiconfigurational self-consistent field theory. The leading configuration (96.6%) for the X4Σ- ground electronic state was shown to be 1σ22σ23σ21π44σ25σ26σ27σ12π12π1 with an Mg-C bond length of 2.041 Å for a fixed C-H bond length of 1.090 Å. The Mg-C bond dissociation energy (De) was 48.26 kcal/mol. The optimized geometry from a density functional theory calculation using the B3LYP functional gave a Mg-C bond length of 2.061 Å and a C-H bond length of 1.090 Å.

The effect of molecular environment on the photoisomerization of urocanic acid.

Urocanic acid, imidazole propenoic acid, is a metabolic product of histidine, which accumulates in skin and is excreted in sweat. It absorbs UV radiation at wavelengths shorter than 340 nm, and its principal photochemical reaction is a trans-cis isomerization about the propenyl double bond. This isomerization to the biologically active cis isomer is implicated in the photoinduced suppression of the immune system of skin. The kinetics of the trans --> cis photoisomerization of urocanic acid has been determined in a number of solvents, spanning a range of polarities. The initial rates of isomerization and the photostationary trans-cis compositions, in all solvents except water, correlate linearly with solvent polarity. This indicates that the isomerization proceeds through a polar intermediate that is stabilized by coulombic interactions with the molecular environment.

The fluorescence and photostabilities of naturally occurring isoflavones.

The 5,7-dihydroxyisoflavones do not emit detectable fluorescence and are significantly more photostable than the two 7-hydroxy-5-deoxyisoflavones, daidzein and formononetin. The latter isoflavones emit fluorescence that depends on the basicity of the solution and the polarity of the solvent. Comparison of the spectral distributions of the emission in methanol in the presence and absence of base indicates that it originates from the conjugate anion excited singlet states. The fluorescence of daidzein and formononetin exhibit large Stokes shifts to longer wavelengths in methanol, ethanol, hexanol and acetonitrile and the magnitudes of these shifts increase with solvent polarity. This indicates that the emission originates from excited singlet states with much larger dipole moments than those of the corresponding ground states. Values of 8.3 D and 6.5 D were determined for daidzein and formononetin respectively. The photostabilities of formononetin and daidzein both increase with increasing solvent polarity and in corresponding solvents, the photostability of daidzein is higher than that of formononetin, behaviour which is attributed to the greater antioxidant activity of daidzein.