2 research outputs found
NH<sub>3</sub> as a Strong HāBond Donor in Singly- and Doubly-Bridged Ammonia Solvent Clusters: 2āPyridoneĀ·(NH<sub>3</sub>)<sub><i>n</i></sub>, <i>n</i> = 1ā3
Mass-
and isomer-selected infrared spectra of 2-pyridoneĀ·(NH<sub>3</sub>)<sub><i>n</i></sub> clusters with <i>n</i> =
1ā3 were measured in the NH and CH stretch fundamental region
(2400ā3700 cm<sup>ā1</sup>) using infrared (IR) laser
depletion spectroscopy combined with resonant two-photon ionization
UV laser detection. The IR depletion spectra reveal three different
H-bonding topologies of these clusters: The <i>n</i> = 1
and 2 clusters form ammonia bridges stretching from the NāH
to the Cī»O group of the <i>cis</i>-amide function
of 2-pyridone (2PY), giving rise to intense and strongly red-shifted
(2PY)ĀNH and ammonia NH stretch bands. For <i>n</i> = 3,
two isomers (3X and 3Y) are observed in the IR spectra: The spectrum
of 3X is compatible with an ammonia-bridge structure like <i>n</i> = 2, with the third NH<sub>3</sub> accepting an H-bond
from C<sup>6</sup>āH of 2PY. The IR spectrum of 3Y exhibits
a broad IR band in the 2500ā3000 cm<sup>ā1</sup> range
and is characteristic of a bifurcated double-bridged structure in
which the first NH3 accepts an H-bond from the (2PY)ĀNH and donates
two H-bonds to the other two ammonias, both of which donate to the
Cī»O group of 2PY. This double-donor/double-bridge H-bonding
pattern increases the acceptor strength of the first ammonia and dramatically
lowers the (2PY)ĀNH stretching frequency to ā¼2700 cm<sup>ā1</sup>. For all clusters the ammonia 2Ī½<sub>4</sub> HNH bend overtones
in the 3180ā3320 cm<sup>ā1</sup> region gain intensity
by anharmonic coupling (Fermi resonance) to the hydrogen-bonded ammonia
NH stretches, which are red-shifted into the 3250ā3350 cm<sup>ā1</sup> region. The experimental results are supported by
optimized structures, vibrational frequencies, and IR intensities
calculated using density-functional theory with the B3LYP and PW91
functionals, as well as with the more recent functionals B97-D and
M06-2X, which are designed to include long-range dispersive interactions
Gas-Phase Cytosine and CytosineāN<sub>1</sub>āDerivatives Have 0.1ā1 ns Lifetimes Near the S<sub>1</sub> State Minimum
Ultraviolet
radiative damage to DNA is inefficient because of the
ultrafast S<sub>1</sub> ā S<sub>0</sub> internal conversion
of its nucleobases. Using picosecond pumpāionization delay
measurements, we find that the S<sub>1</sub>(<sup>1</sup><i>ĻĻ</i>*) state vibrationless lifetime of gas-phase keto-amino cytosine
(Cyt) is Ļ = 730 ps or ā¼700 times longer than that measured
by femtosecond pumpāprobe ionization at higher vibrational
excess energy, <i>E</i><sub>exc</sub>. N<sub>1</sub>-Alkylation
increases the S<sub>1</sub> lifetime up to Ļ = 1030 ps for N<sub>1</sub>-ethyl-Cyt but decreases it to 100 ps for N<sub>1</sub>-isopropyl-Cyt.
Increasing the vibrational energy to <i>E</i><sub>exc</sub> = 300ā550 cm<sup>ā1</sup> decreases the lifetimes
to 20ā30 ps. The nonradiative dynamics of S<sub>1</sub> cytosine
is not solely a property of the amino-pyrimidinone chromophore but
is strongly influenced by the N<sub>1</sub>-substituent. Correlated
excited-state calculations predict that the gap between the S<sub>2</sub>(<sup>1</sup><i>n</i><sub>O</sub>Ļ*) and S<sub>1</sub>(<sup>1</sup><i>ĻĻ</i>*) states decreases
along the series of N<sub>1</sub>-derivatives, thereby influencing
the S<sub>1</sub> state lifetime