High-resolution 500 MHz solution-state 1H and 13C NMR spectra of various poly(lactides) indicate at least hexad stereosequence sensitivity. The poly(lactides) were prepared in vials by melt polymerization of various combinations of l-lactide, d-lactide, and meso-lactide at 180 °C for 3 h using tin(II) bis(2-ethylhexanoate) (tin(II) octoate) as the catalyst in a 1:10 000 ratio. The intensity distribution of the various stereosequence resonances in the NMR spectra indicates a preference for syndiotactic addition during the polymerization process. Minimal evidence of transesterification was observed for these polymerization conditions.

Polylactide (PLA) is a biodegradable polyester formed by the ring-opening polymerization of lactide, the cyclic dimer of lactic acid. Many of the physical properties of PLA are influenced by the amount and distribution of the R and S stereocenters in the polymer chain. NMR spectroscopy is the most common technique used to determine the stereosequence distribution of the polymer. The correct determination of the stereosequence distribution is contingent upon the assignment of the peaks in the NMR spectrum to specific stereosequences. Recently, alternative assignments to the commonly accepted stereosequence assignments have been proposed. If any of these alternative assignments are found to be correct, it would invalidate the conclusions of much of the previous work in understanding the kinetics of lactide polymerization and determining the stereoselectivity of new catalysts. The most significant problem is reconciling the commonly accepted peak assignments, which were based upon statistical probabilities, with contradictory connectivity data observed in a HETCOR NMR experiment. To describe the directionality in PLA, we have modified the nomenclature used to describe directionality in peptides and proteins. For PLA, the end containing the carboxylic group is referred to as the C terminus, and the end with the hydroxyl group is the O terminus. We had previously proposed that the central pairwise relationship (isotactic or syndiotactic, denoted i or s) in the 1 H NMR spectrum is determined by the stereocenter in the lactic acid unit attached to the O terminus and that in the 13 C NMR spectrum it is determined by the central pairwise relationship of the stereocenter in the lactic acid unit attached to the C terminus. One-and two-dimensional NMR techniques, in combination with selective isotopic labeling, were used to show that this relationship is correct and that the commonly accepted assignments are correct. In addition, all of the nondegenerate resonances in the 1 H and 13 C NMR spectrum of polylactide at the tetrad stereosequence level have been assigned.

The excited-state properties of 1-(acy1amino)anthraquinones have been examined using a variety of physical techniques, including static fluorescence spectroscopy, and picosecond and subpicosecond time-resolved fluorescence techniques, as well as picosecond transient absorption spectroscopy. The decay of the singlet excited states is dominated by a combination of efficient internal conversion via excited-state intramolecular proton transfer (ESIPT) and rapid intersystem crossing. For strongly electron withdrawing substituents, the short singlet excited state lifetimes of ca. 100 f 50 ps are ascribed to efficient nonradiative processes in the ESIPT tautomer. For acetylamino and heptanoylamino derivatives in cyclohexane, the anomalously short singlet lifetimes are attributable to fast intersystem crossing and formation of the triplet excited state. Subpicosecond resolution fluorescence studies show that there is an instrument-limited rise of red (ESIPT) emission on the 100-300-fs time scale for 1 -(dichloroacetylamino)anthraquinone, faster that solvent relaxation. For 1 -(chloroacetylamino)anthraquinone, a slower rise in blue (normal tautomer) emission is seen. Calculations and comparisons to related systems show that the tunneling splittings, solvent reorganization energies (for moderately polar solvents), and kT at room temperature are all on the same order of magnitude, giving rise to a subtle interplay among these factors. transfer in plyatomic systems.

This paper describes extensive new ultrafast pump-probe experiments on the photodissociation, geminate recombination, and vibrational relaxation of 12-in various solvents with various counterions. The first measurements on 12-in polar aprotic solvents and in relatively nonpolar solvents are described. Recombination to the 3/2g(2113/2,s) excited state of 12-in solution and the 3/2g(2113/2,g) -3/2u(2113/2,u) transition in solution are assigned for the first time. The new analysis confirms the existence of an extremely rapid (0.3 ps) initial vibrational relaxation component corresponding to a major fraction of the ground-state relaxation ( 2 50% for water), followed by a slower solvent-dependent process on the 3-6 ps time scale. The experimental results herein are compared to the molecular dynamics simulations of 12-that are described in the companion article. The comparison to the simulations reveals that (i) the nonexponential vibrational relaxation dynamics should be attributed to solvent-induced 12-charge flow, and (ii) the Franck-Condon analysis used to analyze the experimental results herein may be inaccurate at high excess energy, owing to charge-transfer contributions to the spectrum.