Photoinduced electron transfer in zinc porphyrin-crown ether supramolecule with Eu3+ electron acceptor in crown void

Photoinduced electron transfer (ET) reactions in the zinc porphyrin-crown ether (ZnPCE) supramolecule, in which one crown ether moiety containing Eu3+ as electron acceptor is covalently linked to zinc porphyrin (ZnP), were studied by flash photolysis. In methanol solutions, highly efficient charge separation occurs via intramolecular ET from (ZnP)-Zn-3 to Eu3+ encapsulated in the crown ether void (k(1) = (3 +/- 1) X 10(3) s(-1)) and intramolecular ET from 3ZnP to uncomplexed Eu2+ (k(2) = (2.5 +/- 0.5) X 10(3) s(-1)). Intermolecular ET from Eu2+ escaped from the crown ether void to ZnP.+ (k(tau) = (4.3 +/- 0.7) X 10(8) M(-1) s(-1)) seems to be the main pathway of charge recombination.

Pressure-induced band gap reduction, orientational ordering and reversible amorphization in single crystals of C70: Photoluminescence and Raman studies

Photoluminescence and Raman scattering experiments have been carried out on single crystals of C70 up to 31 GPa to investigate the effect of pressure on the optical band gap, vibrational modes and stability of the molecule. The photoluminescence band shifts to lower energies and the pressure dependence of the band maxima yields the hydrostatic deformation potential to be 2.15 eV. The slope changes in the pressure dependence of peak positions and linewidths of the Raman modes associated with the intramolecular vibrations at 1 GPa mark the known face-centred cubic-->rhombohedral orientational ordering transition. The reversible amorphization in C70 at P > 20 GPa has been compared with the irreversible amorphization in C60 at P > 22 GPa in terms of carbon-carbon distance between the neighbouring molecules at the threshold transition pressures, in conjunction with the interplay between the intermolecular and intramolecular interactions.

Structure and stability of human telomeric sequence

Telomeric DNA of a variety of vertebrates including humans contains the tandem repeat d(TTAGGG)(n). We have investigated the structural properties of the human telomeric repeat oligonucleotide models d(T(2)AG(3))(4), d(G(3)T(2)A)(3)G(3), and d(G(3)T(2)AG(3)) using CD, gel electrophoresis, and chemical probing techniques. The sequences d(G(3)T(2)A)(3)G(3) and d(T(2)AG(3))(4) assume an antiparallel G quartet structure by intramolecular folding, while the sequence d(G(3)T(2)AG(3)) also adopts an antiparallel G quartet structure but by dimerization of hairpins. In all the above cases, adenines are in the loop. The TTA loops are oriented at the same end of the G tetrad stem in the case of hairpin dimer. Further, the oligonucleotide D(G(3)T(2)AG(3)) forms a higher order structure by the association of two hairpin dimers via stacking of G tetrad planes. Here we show that N-7 of adenine in the hairpin dimer is Hoogsteen hydrogen-bonded. The partial reactivity of loop adenines with DEPC in d(T(2)AG(3))(4) suggests that the intramolecular G quartet structure is highly polymorphic and structures with different loop orientations and topologies are formed in solution. Intra- and interloop hydrogen bonding schemes for the TTA loops are proposed to account for the observed diethyl pyrocarbonate reactivities of adenines. Sodium-induced G quartet structures differ from their potassium-induced counterparts not only in stability but also in loop conformation and interactions. Thus, the overall structure and stability of telomeric sequences are modulated by the cation present, loop sequence, and the number of G tracts, which might be important for the telomere function.

Time-resolved fluorescence studies on covalently linked porphyrin�nitroarene complexes. Conformational control of photoinduced electron transfer reactions

Time-resolved fluorescence studies were carried out on a series of free-base and zinc(II) derivatives of meso-tetraphenylporphyrins covalently linked to either 1,3-dinitrobenzene (DNB) or 1,3,5-trinitrobenzene (TNB) acceptor units. These acceptor units were linked at different sites (at the ortho, meta or para positions of one of the phenyl groups of meso-tetraphenylporphyrin) to the donor porphyrins such that the resulting isomeric intramolecular donor-acceptor complexes exhibit different centre-to-centre (ctc) distances and relative orientations. Biexponential fluorescence decay profiles observed for several of these covalently linked complexes were rationalized in terms of the presence of ''closed'' and ''extended'' conformers. Detailed analyses of the fluorescence decay data have provided a comprehensive understanding of the photoinduced electron transfer (PET) reactions occurring in systems containing zinc(II) porphyrin donors. It is observed that although DNB-linked zinc(II) complexes follow the trends predicted for the efficiency of PET with respect to donor-acceptor distance, the TNB-linked zinc(II) porphyrins exhibit a behaviour which is dictated by steric effects. Similarly, although the thermodynamic criteria predict a greater efficiency of charge separation in TNB-linked complexes compared with DNB-linked complexes, the reverse trend observed has been attributed to orientational effects. In the complexes containing free-base porphyrin donors, PET is expected to be less efficient from a thermodynamic viewpoint. In a few of these cases, fluorescence quenching seems to occur by parallel mechanisms other than PET.

Anomalin. A Dihydropyranocoumarin Derivative from the Plant Lingusticum elatum

The title compound, 9,10-dihydro-8,8-dimethyl-2-oxo-2H,8H-benzo[1,2-b:3,4-b']dipyran-9,10-diyl 2-methyl-2-butenoate, C24H26O7, contains a highly planar coumarin nucleus and a substituted dihydropyran ring (C), which has a distorted half-chair conformation, with an 8 alpha,9 beta orientation. The conformation of ring C is further supported by the two angelyloxy (2-methyl-2-butenoyloxy) substituents at positions C9 and C10, which are cis oriented and thus cannot both occupy equatorial positions with respect to the plane of ring C. The conformations of the two angelyloxy substituents are different, as indicated by their endocyclic torsion angles. The most striking of these angles are O1'-C2'-C4'=C6' and O1'-C2'-C4'-C5' [-137.7 (5) and 43.7 (5)degrees, respectively, in the chain at C10 and 155.8 (5) and -24.7 (9)degrees, respectively in the chain at C9]. These variations are due to two intramolecular hydrogen bonds, namely, C16-H161 ... O1' [C16 ... O1' 3.056 (7) Angstrom] and C7''-H7Y ... O3'' [C7'' ... O3'' 2.955 (12) Angstrom]. The methyl substituents, C15 and C16, at position C8 are alpha and beta oriented, respectively. The crystal structure is stabilized by a weak C4-H41 ... O3' hydrogen bond [C4 ... O3' 3.297 (6) Angstrom] between the screw-related molecules.

Use of L matrix to obtain reliable ab initio force constants of polyatomic molecules: ethylene as a test system

Direct use of experimental eigenvalues of the vibrational secular equation on to the ab initio predicted eigenvector space is suggested as a means of obtaining a reliable set of intramolecular force constants. This method which we have termed RECOVES (recovery in the eigenvector space) is computationally simple and free from arbitrariness. The RECOVES force constants, by definition, reproduce the experimental vibrational frequencies of the parent molecule exactly. The ab initio calculations were carried out for ethylene as a test molecule and the force constants obtained by the present procedure also correctly predict the vibrational frequencies of the deuterated species. The RECOVES force constants for ethylene are compared with those obtained by using the SQM procedure.

Bile acids in asymmetric synthesis and molecular recognition

This account summarizes the progress made in our laboratory towards the development of new uses of naturally occurring bile acids. Applications in Asymmetric Synthesis (intramolecular coupling, and intermolecular reactions) and Molecular Recognition are described with suitable examples.

Structure of N-tert-butyloxycarbonyl-[alpha]-aminoisobutyryl-DL-pipecolyl-[alpha]-aminoisobutyric acid methyl ester

C20H35N3O6 (Boc-Aib-DL-Pip-Aib-OMe, Boc = tert-butyloxycarbonyl, Aib = alpha-aminoisobutyric acid, Pip = pipecolic acid, OMe = methoxy), M(r) = 413.5, monoclinic, P2(1)/c, a = 18.055 (3), b = 15.048 (3), c = 17.173 (3) angstrom, beta = 91.7 (1)-degrees, V = 4663.8 (9) angstrom3, Z = 8, D(m) = 1.16, D(x) = 1.178 Mg m-3, lambda(Mo Kalpha) = 0.71069 angstrom, mu = 0.081 mm-1, F(000) = 1792, T = 297 K. The final R value for 4925 [I greater-than-or-equal-to 3sigma(I)] reflections is 0.065 (wR = 0.067). The peptide backbone of the two independent molecules in the asymmetric unit is folded at the -Aib-Pip- sequence to form a type-I (I') beta-bend stabilized by a 1 <-- 4 intramolecular N-H...O=C hydrogen bond between the Aib(3) peptide N-H and Boc urethane C=O groups.

Conformational preferences of α-functionalised keten-S,N-acetals: Potential role of SO and SS interactions in solution

PMR spectra of carbonyl compounds 2a-k reveal significant variations in the population of E and Z isomers on changing the solvent from CDCl3 to DMSO-d6. In non-polar media, the intramolecular N-H…. O hydrogen bonded form is exclusively observed. In DMSO-d6, the alternative Z form is also populated. A similar conformational switch is also noted in the corresponding thiones. Different interpretations are critically analysed. The most consistent explanation is suggested to involvean interplay of N-H….X hydrogen bonding and S…X attractive interaction (X = O,S) in these systems. Ab initio calculations support this interpretation.

Quantum and non-Markovian effects in the electron transfer reaction dynamics in the Marcus inverted region

Electron transfer reactions in large molecules may often be coupled to both the polar solvent modes and the intramolecular vibrational modes of the molecule. This can give rise to a complex dynamics which may in some systems, like betaine, be controlled more by vibrational rather than by solvent effects. Additionally, a significant contribution from an ultrafast relaxation component in the solvation dynamics may enhance the complexity. To explain the wide range of behavior that has been observed experimentally, Barbara et al. recently proposed that a model of an electron transfer reaction should minimally consist of a low-frequency classical solvent mode (X), a low-frequency vibrational mode (Q), and a high-frequency quantum mode (q) (J. Phys. Chem. 1991, 96, 3728). In the present work, a theoretical study of this model is described. This study generalizes earlier work by including the biphasic solvent response and the dynamics of the low-frequency vibrational mode in the presence of a delocalized, extended reaction zone. A novel Green's function technique has been developed which allowed us to study the non-Markovian dynamics on a multidimensional surface. The contributions from the high-frequency vibrational mode and the ultrafast component in the non-Markovian solvent dynamics are found to be primarily responsible for the dramatic increase in charge transfer rate over the prediction of the classical theories that neglect both these factors. These, along with a large coupling between the reactant and the product states, may combine to render the electron transfer rate both very large and constant over a wide range of solvent relaxation rates. A study on the free energy gap dependence of the electron transfer rate reveals that the rates are sensitive to changes in the quantum frequency particularly when the free energy gap is very large.

Stereospecific construction of stereogenic vicinal quaternary carbon atoms. Enantiospecific synthesis of (+)-valerane

Stereo- and enantiospecific synthesis of (+)-valerane starting from R-carvone utilising orthoester Claisen rearrangement and intramolecular diazo ketone cyclopropanation reactions for the construction of the two vicinal quaternary carbon atoms is described. Copyright (C) 1996 Elsevier Science Ltd.

Structure of 4,6-Diacetylresorcinol

C10H10O4, M(r) = 194.19, monoclinic, P2(1)/c, a = 7.089 (1), b = 11.361 (1), c = 11.656 (1) angstrom, beta = 100.45 (3)-degrees, V = 922.92 (1) angstrom 3, Z = 4, D(m) = 1.410 (5), D(x) = 1.397 Mg m-3, lambda(Cu K-alpha) = 1.5418 angstrom, mu(Cu K-alpha) = 0.89 mm-1, T = 300 K, F(000) = 408, final R = 0.057 for 1701 observed reflections. The molecule is almost planar, with O(9) and O(12) of the acetyl groups deviating by 0.074 (1) and 0.071 (2) angstrom from the mean plane of the benzene ring. The bond lengths and bond angles of the benzene ring are normal. There are intramolecular hydrogen bonds between O(9) and H(14) and between O(12) and H(13); there are no intermolecular hydrogen bonds. The molecules are packed in layers parallel to the ac plane and are held together essentially by van der Waals interactions.

Structure of 7-ethylamino-6-methyl-4-trifluoromethylcoumarin

C13H12F3NO2, M(r) = 271.2, triclinic, P1BAR, a = 5.029 (2), b = 7.479 (2), c = 17.073 (5) angstrom, alpha = 97.98 (2), beta = 95.54 (3), gamma = 103.62 (3)-degrees, V = 612.4 (4) angstrom 3, Z = 2, D(m) = 1.463, D(x) = 1.471 g cm-3, lambda(Mo K-alpha) = 0.71069 angstrom, mu = 1.23 cm-1, F(000) = 280, T = 298 K, final R value is 0.041 for 2047 observed reflections with \F(omicron)\ greater-than-or-equal-to 6-sigma(\F(omicron)\). The N-C(sp2) bond length is 1.356 (2) angstrom. The N and C atoms of the ethylamino group deviate by < 0.15 angstrom from the plane of the aromatic ring. Short intramolecular contacts, C(3)...F(17) 2.668 (3) angstrom [H(3)...F(17) 2.39 (2) angstrom, C(3)-H(C3)...F(17) 98 (1)-degrees], C(5)...F(18) 3.074 (3) and C(5)...F(19) 3.077 (3) angstrom exist in the structure. The crystal structure is stabilized by intermolecular N-H...O hydrogen bonds with N(12)-H(N12) 0.79 (3), H(N12)...O(11)' 2.36 (3), N(12)...O(11)' (x - 1, y + 1, z) 3.105 (3) angstrom and N(12)-H(N12)...O(11)' 155 (2)-degrees.

Synthesis Based on Cyclohexadienes. 5. A New Approach to the Synthesis of a-Ring Aromatic Steroids - a Formal Total Synthesis of (+/-)-Estrone

A new strategy for the construction of A-ring aromatic steroids which resulted in the formal total synthesis of estrone is described. Thus reaction of the adduct (9), obtained from 1-methoxy-4-methylcyclohexa-1,4-diene and acrolein, with 3-(m-methoxyphenyl)propylmagnesium bromide followed by oxidation afforded the bicyclo[2.2.2]octene derivative (14). Acid-catalysed rearrangement of (14) followed by an intramolecular Michael addition resulted in the cis tetraenone (18) which was dehydrogenated with palladium chloride to the pentaenone (22). Li/NH3 reduction of (22) gave 3-methoxy-D-homoestra-1,3,5(10)-trien-16-one (31) which has been converted into the methyl ether (37) of marrianolic acid, and its methyl ester (38).

Synthetic Studies Towards Prismanes - Rearrangements In Some Pentacyclic Precursors And X-Ray Crystal-Structure Of A Novel Heptacyclic Ether

In order to gain access to the heptacyclic tetraone 3, efforts were directed towards the utilisation of the major 'unwanted' [4 + 2]-adduct 11 of tetrachlorodimethoxycyclopentadiene and norbornenobenzoquinone. Epoxides derived from the diol and dimethoxy derivatives of the adduct 11 undergo facile Wagner-Meerwein rearrangement resulting in the required endo, syn, endo stereochemistry as well as methano-bridge functionalisation to deliver 18 and 24, respectively. However, intramolecular ether formation, occurring via the capture of carbocation intermediate with the transannularly poised oxygen functionality, is a more facile process. Attempts to cleave the ether linkage resulted in the formation of a novel transannularly cyclised twisted bowl shape heptacyclic compound 30 and its structure has been established through X-ray crystallography.