Single-conformation spectroscopy of two diastereomers of 1-(4-hydroxy-3-methoxyphenyl)propane-1,2,3-triol (HMPPT) has been carried out under isolated, jet-cooled conditions. HMPPT is a close analog of coniferyl alcohol, one of the three monomers that make up lignin, the aromatic biopolymer that gives structural integrity to plants. In HMPPT, the double bond of coniferyl alcohol has been oxidized to produce an alkyl triol chain with chiral centers at C(α) and C(β), thereby incorporating key aspects of the β-O-4 linkage between monomer subunits that occurs commonly in lignin. Both (R,S)- and (R,R)-HMPPT diastereomers have been synthesized in pure form for study. Resonant two-photon ionization (R2PI), UV hole-burning (UVHB)/IR-UV hole-burning (IR-UV HB), and resonant ion-dip infrared (RIDIR) spectroscopy have been carried out, providing single-conformation UV spectra in the S(0)-S(1) region (35200-35800 cm(-1)) and IR spectra in the hydride stretch region. Five conformers of (R,S)- and four conformers of (R,R)-HMPPT are observed and characterized, leading to assignments for all nine conformers. Spectroscopic signatures for α-β-γ, γ-β-α, and α-γ-β-π chains and two cyclic forms [(αβγ) and (αγβ)] of the glycerol side chain are determined. Infrared ion-gain (IRIG) spectroscopy is used to determine fractional abundances for the (R,S) diastereomer and constrain the populations present in (R,R). The two diastereomers have very different conformational preferences. More than 95% of the population of (R,R) configures the glycerol side chain in a γ-β-α triol chain, while in (R,S)-HMPPT, 51% of the population is in α-β-γ chains that point in the opposite direction, with an additional 21% of the population in H-bonded cycles. The experimental results are compared with calculations to provide a consistent explanation of the diastereomer-specific effects observed.
[Show abstract][Hide abstract] ABSTRACT: The dynamics of structure evolution of nanodiamonds ranging from 22 to 318 atoms of various shapes is studied by density functional tight-binding molecular dynamics. The spherical and cubic nanodiamonds can be transformed into fullerene-like structures upon heating. A number of the transformed fullerenes consist of pentagons and hexagons only. Others contain squares, heptagons, and octagons. One simulated fullerene is an isomer of C(60). The temperature of the transformation depends on the size, shape, and orientation of initial cluster. To be transformed into onion-like fullerenes, the spherical nanodiamonds should have 200 atoms or more, while the cubic ones require 302 atoms or more. The time-resolved energy profiles of all the transformations clearly reveal three-stage transformation character. During the first stage, the energy reduces quickly due to converting sp(3) carbon with dangling bond at the surface into sp(2) one, and the formation of partial sp(2) envelope wrapping the cluster. For the second stage, energy decreases slowly. The remaining interior carbon atoms come to the surface through the hole in the sp(2) envelope, and similar amount of sp(3) and sp(2) atoms coexist. The third stage involves the closure of holes, accompanied by the detachment of C(2) molecules and carbon chains from the edges. The energy decreases relatively fast in this stage. The proposed three-stage transformation pathway holds for all the simulations performed in this work, including those with the instant heating.
The Journal of Physical Chemistry A 06/2011; 115(30):8327-34. DOI:10.1021/jp200449f · 2.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Laser-desorbed quinine and quinidine have been studied in the gas phase by combining supersonic expansion with laser spectroscopy, namely, laser-induced fluorescence (LIF), resonance-enhanced multiphoton ionization (REMPI), and IR-UV double resonance experiments. Density funtional theory (DFT) calculations have been done in conjunction with the experimental work. The first electronic transition of quinine and quinidine is of π-π* nature, and the studied molecules weakly fluoresce in the gas phase, in contrast to what was observed in solution (Qin, W. W.; et al. J. Phys. Chem. C2009, 113, 11790). The two pseudo enantiomers quinine and quinidine show limited differences in the gas phase; their main conformation is of open type as it is in solution. However, vibrational circular dichroism (VCD) experiments in solution show that additional conformers exist in condensed phase for quinidine, which are not observed for quinine. This difference in behavior between the two pseudo enantiomers is discussed.
The Journal of Physical Chemistry A 07/2012; 116(32):8334-44. DOI:10.1021/jp3047888 · 2.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Single-conformation ultraviolet and infrared spectroscopy has been carried out on the neutral peptide series, Z-(Gly)(n)-OH, n = 1,3,5 (ZGn) and Z-(Gly)(5)-NHMe (ZG5-NHMe) in the isolated environment of a supersonic expansion. The N-terminal Z-cap (carboxybenzyl) provides an ultraviolet chromophore for resonant two-photon ionization (R2PI) spectroscopy. Conformation-specific infrared spectra were recorded in double resonance using resonant ion-dip infrared spectroscopy (RIDIRS). By comparing the experimental spectra with the predictions of DFT M05-2X/6-31+G(d) calculations, the structures could be characterized in terms of the sequence of intramolecular H-bonded rings of varying size. Despite the enhanced flexibility of the glycine residues, a total of only six conformers were observed among the four molecules. Two conformers for ZG1 were found with the major conformation taking on an extended, planar β-strand conformation. Two conformers were observed for ZG3, with the majority of the population in a C11/C7/C7/π(g-) structure that forms a full loop of the glycine chain. Both ZG5 molecules had their population primarily in a single conformation, with structures characteristic of the first stages of a "mixed" β-helix. C14/C16 H-bonded rings in opposing directions (N → C and C → N) tie the helix together, with nearest-neighbor C7 rings turning the backbone so that it forms the helix. φ/ψ angles alternate in sign along the backbone, as is characteristic of the mixed, C14/C16 β-helix. The calculated conformational energies of these structures are unusually stable relative to all others, with energies significantly lower than the PGI/PGII conformations characteristic of polyglycine structures in solution and in the crystalline form, where intermolecular H-bonds play a role.
Journal of the American Chemical Society 10/2012; 134(41):17186-201. DOI:10.1021/ja306652c · 12.11 Impact Factor
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