a b s t r a c tDrill cores obtained from Lake Peté n Itzá , Peté n, Guatemala, contain a $85-kyr record of terrestrial climate from lowland Central America that was used to reconstruct hydrologic changes in the northern Neotropics during the last glaciation. Sediments are composed of alternating clay and gypsum reflecting relatively wet and dry climate conditions, respectively. From $85 to 48 ka, sediments were dominated by carbonate clay indicating moist conditions during Marine Isotope Stages (MIS) 5a, 4, and early 3. The first gypsum layer was deposited at $48 ka, signifying a shift toward drier hydrologic conditions and the onset of wet-dry oscillations. During the latter part of MIS 3, Peté n climate varied between wetter conditions during interstadials and drier states during stadials. The pattern of clay-gypsum (wet-dry) oscillations during the latter part of MIS 3 ($48-23 ka) closely resembles the temperature records from Greenland ice cores and North Atlantic marine sediment cores and precipitation proxies from the Cariaco Basin. The most arid periods coincided with Heinrich Events when cold sea surface temperatures prevailed in the North Atlantic, meridional overturning circulation was reduced, and the Intertropical Convergence Zone (ITCZ) was displaced southward. A thick clay unit was deposited from 23 to 18 ka suggesting deposition in a deep lake, and pollen accumulated during the same period indicates vegetation consisted of a temperate pine-oak forest. This finding contradicts previous inferences that climate was arid during the Last Glacial Maximum (LGM) chronozone (2172 ka). At $18 ka, Peté n climate switched from moist to arid conditions and remained dry from 18 to 14.7 ka during the early deglaciation. Moister conditions prevailed during the warmer Bolling-Allerod (14.7-12.8 ka) with the exception of a brief return to dry conditions at $13.8 ka that coincides with the Older Dryas and meltwater pulse 1A. The onset of the Younger Dryas at 12.8 ka marked the return of gypsum and hence dry conditions. The lake continued to precipitate gypsum until $10.3 ka when rainfall increased markedly in the early Holocene.

The S1↔S0 and S2↔S0 vibronic spectra of the supersonically cooled 2-pyridone dimer (2PY)2 and its 13C-, d1-, and d2-isotopomers were investigated by two-color resonant two-photon ionization and fluorescence spectroscopies. For the C2h symmetric (2PY)2–h2 and (2PY)2–d2 complexes, the 2PY moieties are equivalent and the S1↔S0 (1Ag↔1Ag) transition is forbidden. A single H/D or 12C/13C isotopic substitution reduces the symmetry to Cs, so that (2PY)2–13C and (2PY)2–d1 both exhibit S1↔S0 and S2↔S0 transitions. The S1/S2 state exciton splittings are 43.6 cm−1 and 52.4 cm−1, respectively. These are analyzed in terms of a Frenkel model and compared to calculated splittings based on ab initio monomer transition dipole moments. For (2PY)2–d1, whose 2PY subunits are different, an excitation transfer time of 318 fs is calculated from the exciton splitting. The S1↔S0 and S2↔S0 spectra are analyzed and assigned. Several bu intermolecular vibrations of S1 appear via vibronic coupling to the S2(Bu) state. Combination of the fluorescence data from excitation of the S1 and S2 origins and vibrational excitations of (2PY)2–h2, (2PY)2–d1, and (2PY)2–d2 allows the determination of the six S0 state intermolecular vibrational frequencies.

The 2-pyridone dimer, (2PY)2, which is linked by two antiparallel N–H⋅⋅⋅O hydrogen bonds, is a model for hydrogen bonded nucleotide base pairs, e.g., the uracil dimer. Mass- and isomer-selected S2←S0 vibronic spectra of supersonically cooled (2PY)2 were measured by laser two-color resonant two-photon ionization and UV/UV-holeburning techniques. The latter allows the identification of the spectrum of the 2-pyridone⋅2-hydroxypyridine mixed dimer, present at ≈5% relative concentration. S2→S0 fluorescence emission spectra show dominantly the hydrogen bond shearing vibration ν4″ at 98.5 cm−1 and the stretching vibration ν6″ at 163.5 cm−1. The hydrogen bond stretching vibration force constant was determined to be 75.4 N/m, or 37.7 N/m per hydrogen bond, a very high value. The ν2″ (au) torsional and the ν3″ (bu) slanting vibrations were also identified. Ground state structures, rotational constants, harmonic intermolecular and intramolecular vibrational frequencies, interaction, and dissociation energies were calculated using Hartree–Fock and density functional (B3LYP) methods. The B3LYP/6-311++(2d,2p) results are in excellent agreement with all experimental observations.

The 2-pyridone dimer, (2PY) 2 , has two antiparallel N-H‚‚‚O H-bonds analogous to nucleobase dimers. The gas-phase rotational constants and all six intermolecular vibrational frequencies of (2PY) 2 have been previously measured, providing benchmarks for theory. The structure, rotational constants, vibrational frequencies, and binding and dissociation energies of (2PY) 2 were calculated at the correlated level using second-order Møller-Plesset perturbation theory (MP2) with medium to very large basis sets. The MP2 binding energy limit was extrapolated to the complete basis set (CBS) as D e,CBS ) -22.62 ( 0.07 kcal/mol. Higher order correlation energy contributions to D e at the CCSD(T) level are destabilizing (+0.77 kcal/mol). This implies that (2PY) 2 is the most strongly bound doubly hydrogen-bonded dimer known so far. The Hartree-Fock contribution to D e,CBS is only ≈65%. Several medium-size basis sets yield MP2 D e 's within (5% of the CBS value, as well as structure, rotational constants, and intermolecular vibrations in good agreement with experiment. The PW91 density functional method also shows very good performance with regard to all properties calculated, comparable to MP2. The results imply that correlated methods combined with carefully chosen medium-size basis sets may give near-quantitative results for the structures, binding energies, and intermolecular vibrational frequencies of nucleic acid base dimers.

The 2-aminopyridine2-pyridone (2AP2PY) dimer is linked by N-H...O=C and N-H...N hydrogen bonds, providing a model for the Watson-Crick hydrogen bond configuration of the adenine.thymine and adenine.uracil nucleobase pairs. Mass-specific infrared spectra of 2AP2PY and its seven N-H deuterated isotopomers have been measured between 2550 and 3650 cm(-1) by IR laser depletion combined with UV two-color resonant two-photon ionization. The 2PY amide N-H stretch is a very intense band spread over the range 2700-3000 cm(-1) due to large anharmonic couplings. It is shifted to lower frequency by 710 cm(-1) or approximately 20% upon H bonding to 2AP. On the 2AP moiety, the "bound" amino N-H stretch gives rise to a sharp band at 3140 cm(-1), which is downshifted by 354 cm(-1) or approximately 10% upon H bonding to 2PY. The amino group "free" N-H stretch and the H-N-H bend overtone are sharp bands at approximately 3530 cm(-1) and 3320 cm(-1). Ab initio structures and harmonic vibrations were calculated at the Hartree-Fock level and with the PW91 and B3LYP density functionals. The PW91/6-311++G(d,p) method provides excellent predictions for the frequencies and IR intensities of all the isotopomers.