Proton transfer in the lowest excited singlet state of 4-methoxyacridine: extraction of kinetic parameters from an incomplete fluorimetric titration curve

The fluorimetric pH titration curve of 4-methoxyacridine cannot be completed because of quenching of the neutral molecule above pH 14. Normalization of the points in the curve with respect to some point, corresponding to $\text{φ}\text{φ}{}_{\mathrm{o}}$ < 1, can be employed to determine the correction factor for conversion to normalization, with respect to the unattainable fluorescence intensity of the neutral molecule, in the absence of excited-state protonexchange or quenching. The excited-state dissociation constant (pK_a^*) for protonated 4-methoxyacridine, calculated from the rate constants for excited-state proton exchange determined by this approach, is in excellent agreement with the value of pK_a^* calculated from a Förster cycle.     

Interactions of singlet oxygen with 2,5-dimethyl-2,4-hexadiene in polar ano non-polar solvents evidence for a vinylog ene-reaction

2,5-Dimethyl-2,4-hexadiene ($\text{1}$)was studied as a singlet oxygen acceptor in various solvents. $\text{1}$undergoes concomitantly the three well-known modes of singlet oxygen reactions: (1) the ene-reaction to give the allylic hydroperoxide $\text{3}$, (2) the (4+2)-cycloaddition to give the endoperoxide $\text{4}$, and (3) the (2+2)-cycloaddition to give the dioxetane $\text{2}$. Beyond that (and in contrast to simple olefins), there are (4) “physical” quenching and (5) a “vinylog ene-reaction” to give the twofold-unsaturated hydroperoxide $\text{5}$. The latter reaction represents a novel mode of singlet oxygen interaction with a substituted 1,3-diene. - Kinetic analysis shows that “physical” quenching, endoperoxide and vinylog ene-product formations proceed with solvent-inde pendent rates; the rates of dioxetane and ene-product formations, however, are solvent-dependent. - A mechanism (Scheme 3) is proposed, according to which endoperoxide formation is due to a concerted singlet oxygen reaction with the s-cis-conformational isomer $\text{1b}$; with the s-trans-isomer $\text{1a}$, “physical” quenching and the vinylog ene-reaction proceed via a non-polar singlet diradical intermediate, whereas the ene-product formation occurs via a per epoxide-like transition state. In aprotic solvents, the dioxetane is mainly formed via a “tight-geometry intermediate”, in methanolic solution via a solvent-stabilized zwitterion; the latter is also responsible for the formation of the methanol-addition product $\text{6}$.     

Fluorescence of ammonia-d3 from its first excited singlet state

Structured emission spectra have been observed from ND₃ excited at 2139 Å and 2144 Å. The emission is short-lived (τ_{$\text{1}\text{2}$} < 10^?10 s) and has been assigned to the ND₃($\text{A}$) → ND₃($\text{X}$) fluorescence transition.     

CEPA calculations of potential energy surfaces for open-shell systems.: IV. Photodissociation of H2O in the A1B1 state

A three-dimensional potential energy surface for the photodissociation of H₂O in its lowest excited singlet state $\text{A}$¹B₁ in C_2v or $\text{A}$¹A″ in C₃ symmetry, respectively, has been calculated with quantum-chemical ab initio methods including electron correlation. The main features of the surface are discussed and qualitative explanations are given for the experimentally observed vibrational and rotational excitations of the product OH(²Π) radicals. The surface will be used in subsequent investigations of the dynamics of the H₂O photodissociation process.     

Theoretical study of the 12A″ (X2II) State of N2 O+: implications for isotopic scrambling

Ab initio calculations suggest that the potential energy surface for the 1²$\text{A}$″ state of N₂O⁺ has a secondary minimum corresponding to a strongly bent structure. This structure is computed to lie below the $\text{A}$²Σ ⁺ state energetically and therefore may be responsible for the isotopic scrambling observed in this energy region.     

The ground-state singlet potential surface for C2H4

Calculations with an extended polarized basis set and Møller-Plesset perturbation theory including triple substitution correlation corrections in the fourth-order treatment indicate that singlet ethylidene (CH₃CH:) is not a local minimum on the C₂H₄ potential energy surface. Rearrangement to ethylene occurs without actuation. Barriers for hydrogen scrambling and for 1,1-hydrogen elimination are estimated.     

Spin non-conservation in low-energy molecular charge-transfer reactions

The reaction H₂O⁺(²B)+NO₂(²A) → H₂O(¹A) + NO₂⁺(¹Σ) occurs at near the collision rate constant 1.2 × 10^?9 cm³ s^?1, in spite of the fact that the reactants produce both a singlet and a triplet state and the products correlate only with the singlet state. This would be expected to yield a statistical weight factor of $\text{1}\text{4}$ to be multiplied by the collision rate constant to obtain the maximum charge-tranfer rate constant. The triplet products of the charge transfer are clearly endothermic. The singlet—triplet intersection has not been identified but the available information about the singlet and triplet states of the intermediate protonated nitric acid molecule is discussed. Four other examples of apparent “spin violation” charge-transfer reactions have been noted H₂O⁺ + NO, N₂O⁺ + NO.CO⁺ + NO and CH₄⁺ + O₂.     

Stable isobenzofuran endoperoxide and its use for oxidation of olefins ans aromatic compounds

1,3-Di-tert-butylisobenzofuran reacts with singlet oxygen with rate constant of 2.8 x 10⁹ M^?1 sec^?1 to give stable endoperoxide $\text{4}$. Thermolysis of the endopepoxide ($\text{4}$) in the ppesence of norbornene and naphthalene gave the corrpesponding epoxide and naphthols, respectively. Addition of Pd(OAc)₂ improved the yield of naphtols considepably.     

Energy levels of iodine monochloride near the first dissociation limit

Absorption transitions to vibrational levels close to the A state dissociation limit of ICI have been examined using a two-photon sequential absorption technique. The discrete rotational structures of I³⁷ Cl bands to within 0.7 cm^?1 of the limit have been selectively excited and analysed. A value of 17557.514 ± 0.030 cm^?1 has been obtained for the I(²P^o_{$\text{3}\text{2}$}) + Cl(²P^o_{$\text{3}\text{2}$}) dissociation energy D_e, relative to the minimum of the ICI ground state potential well. The two-photon technique can be used to excite and display separately the high resolution absorption spectra of different isotopic species of a molecule which are contained in a mixture.     

Cycloaddition of singlet oxygen and 4-methyl-1,2,4-triazoline-3,5-dione (MTAD) to methoxycyclooctatetraene (MCOT)

The endoperoxides $\text{6}$ and $\text{7}$, respectively 9-methoxy- and 1-methoxy-7,8-dioxabicyclo[4.2.2]deca-2,4,7-trienes, and the urazoles $\text{8}$ and $\text{9}$, respectively 9-methoxy- and 2-methoxy-7,8-diazantricyclo[4.2.2]deca-2,4,9-trien-$\text{N}$-methyl-7,8-dicarboximide, were obtained in the cycloaddition of singlet oxygen and $\text{4}$-methyl-1,2,4-triazoline-3,5-dione (MTAD) to methoxycyclooctatetraene (MCOT).     

Electronic control of stereoselectivity-21 : Stereochemical parallelism between dienophilic capture and singlet oxygenation of isodicyclopentadiene derivatives

The stereochemistry and product distribution resulting from reaction of 4',5',6',7'- tetrahydrospirol[cyclopropane-1,2']-[4,7]methano[2$\text{H}$]indene(5), endo-2-methyl(6a) and 2,2-dunethyl-4,7-dihydro-4,7-methano-2$\text{H}$-indene (6b), as well as 4',5',6',7'-tetrahydrospiro[cyclopentane-1,2']-[4,7]methano-[2$\text{H}$]indene (7) with singlet oxygen have been determined. Stereochemical assignments to the diepoxide products were readily deduced by ¹³C-NMR comparison with the spectra of the parent isomcrs of established structure (X-ray). To unravel the stereochemistry of the epoxy aldehydes, recourse was made to 2D NOE experiments The observed stereosclectivity and reaction profile of each substrate are analyzed and placed in proper mechanistic and energetic perspective.     

1,1-Diaminosubstituted 2-azaallenium salts: preparation and barriers to topomerization

Dialkylcarbodiimides, $\text{5}$, react with iminium salts, $\text{6}$, to afford the formal 1,1-diaminosubstituted 2-azaallenium salts $\text{3}$. According to dynamic NMR measurements compounds $\text{3}$ must be regarded as alkylidene-guanidinium salts $\text{A}$, undergoing a fast topomerization via a transition state $\text{B}$ with allene geometry (2G⁼₂₆₅ = 45.9 ± 1 kJmol^-1 for $\text{3f}$) and a slower rotation around the peripheral C-N bonds (transition state $\text{C}$; 2G⁼₂₉₀⁼ 58.0 ± 1 kJmol^-1 for $\text{3f}$).     

Solvent and deuterium isotope effects on the lifetime of singlet oxygen determined by direct emission spectroscopy at 1.27 μm

The relative lifetime of singlet molecular oxygen have been evaluated, as a function of solvent and deuterium substitution, $\text{via}$ measurement involving the direct emission of singlet oxygen at 1.27 μm.     

Second excited singlet state emission spectroscopy of thiocarbonyls. I. Thiophosgene

Well resolved emission spectra have been recorded after dye laser excitation of low-lying vibronic levels in the $\text{B}$(¹A₁) state of Cl₂CS. The effects of substrate pressure, added SF₆ and excitation wavelength on the spectra are reported. A Franck-Condon analysis suggests that the excited state CS bond length is 0.5 Å longer than in the ground state.     

Chemistry of singlet Oxygen. XXVII. Directing effect of methoxy group in additions to methoxystyrenes

A methoxy group on a styrene double bond directs O₂ attack $\text{cis}$, giving 1,4 addition if the phenyl is $\text{cis}$, or ene reaction if a CH₃ is $\text{cis}$. If no substituent is $\text{cis}$, 1,4 attack on the $\text{trans}$ phenyl occurs at a slower rate.     

Novel adducts of sulfides and 4-substituted 1,2,4-triazoline-3,5-diones ( TAD ) A similarity of TAD and singlet oxygen( 1O2 )

The 4-substituted 1,2,4-triazoline-3,5-diones( $\text{2}$ ) afford with sulfides( $\text{1}$ ) having acidic α-hydrogens the 1,4-disubstituted urazoles( $\text{3}$ ). Reactivity of the triazolinedione shows similarity in possible reaction modes with that of singlet oxygen.     

The synthesis of the endoperoxide of cyclooctatetraene via photosensitized singlet oxygenation

7,8-Dioxabicyclo[4.2.2.]deca-2,4,9-triene ($\text{3}$), the hitherto unknown endoperoxide of cyclooctatetraene, was prepared via cycloaddition of singlet oxygen and characterized by spectroscopic and chemical means.