3 research outputs found
NH<sub>3</sub> as a Strong H‑Bond Donor in Singly- and Doubly-Bridged Ammonia Solvent Clusters: 2‑Pyridone·(NH<sub>3</sub>)<sub><i>n</i></sub>, <i>n</i> = 1–3
Mass-
and isomer-selected infrared spectra of 2-pyridone·(NH<sub>3</sub>)<sub><i>n</i></sub> clusters with <i>n</i> =
1–3 were measured in the NH and CH stretch fundamental region
(2400–3700 cm<sup>–1</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 N–H
to the CO group of the <i>cis</i>-amide function
of 2-pyridone (2PY), giving rise to intense and strongly red-shifted
(2PY)NH 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>–H of 2PY. The IR spectrum of 3Y exhibits
a broad IR band in the 2500–3000 cm<sup>–1</sup> range
and is characteristic of a bifurcated double-bridged structure in
which the first NH3 accepts an H-bond from the (2PY)NH and donates
two H-bonds to the other two ammonias, both of which donate to the
CO group of 2PY. This double-donor/double-bridge H-bonding
pattern increases the acceptor strength of the first ammonia and dramatically
lowers the (2PY)NH stretching frequency to ∼2700 cm<sup>–1</sup>. For all clusters the ammonia 2ν<sub>4</sub> HNH bend overtones
in the 3180–3320 cm<sup>–1</sup> region gain intensity
by anharmonic coupling (Fermi resonance) to the hydrogen-bonded ammonia
NH stretches, which are red-shifted into the 3250–3350 cm<sup>–1</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
Excited-State Structure and Dynamics of Keto–Amino Cytosine: The <sup>1</sup>ππ* State Is Nonplanar and Its Radiationless Decay Is Not Ultrafast
We
have measured the mass- and tautomer-specific S<sub>0</sub> →
S<sub>1</sub> vibronic spectra and S<sub>1</sub> state lifetimes of
the keto–amino tautomer of cytosine cooled in supersonic jets,
using two-color resonant two-photon ionization (R2PI) spectroscopy
at 0.05 cm<sup>–1</sup> resolution. The rotational contours
of the 0<sub>0</sub><sup>0</sup> band
and nine vibronic bands up to +437 cm<sup>–1</sup> are polarized
in the pyrimidinone plane, proving that the electronic excitation
is to a <sup>1</sup>ππ* state. All vibronic excitations
up to +437 cm<sup>–1</sup> are overtone and combination bands
of the low-frequency out-of-plane ν<sub>1</sub><sup>′</sup> (butterfly), ν<sub>2</sub><sup>′</sup> (boat),
and ν<sub>3</sub><sup>′</sup> (H–N–C<sup>6</sup>–H twist) vibrations. UV
vibronic spectrum simulations based on approximate second-order coupled-cluster
(CC2) calculations of the ground and <sup>1</sup>ππ* states
are in good agreement with the experimental R2PI spectrum, but only
if the calculated ν<sub>1</sub><sup>′</sup> and ν<sub>2</sub><sup>′</sup> frequencies are reduced by a
factor of 4 and anharmonicity is included. Together with the high
intensity of the ν<sub>1</sub><sup>′</sup> and ν<sub>2</sub><sup>′</sup> overtone vibronic excitations, this
implies that the <sup>1</sup>ππ* potential energy surface
is much softer and much more anharmonic in the out-of-plane directions
than predicted by the CC2 method. The <sup>1</sup>ππ*
state lifetime is determined from the Lorentzian line broadening necessary
to reproduce the rotational band contours: at the 0<sub>0</sub><sup>0</sup> band it is τ = 44 ps, remains
at τ = 35–45 ps up to +205 cm<sup>–1</sup>, and
decreases to 25–30 ps up to +437 cm<sup>–1</sup>. These
lifetimes are 20–40 times longer than the 0.5–1.5 ps
lifetimes previously measured with femtosecond pump–probe techniques
at higher vibrational energies (1500–3800 cm<sup>–1</sup>). Thus, the nonradiative relaxation rate of keto–amino cytosine
close to the <sup>1</sup>ππ* state minimum is <i>k</i><sub>nr</sub> ∼ 2.5 × 10<sup>10</sup> s<sup>–1</sup>, much smaller than at higher energies. An additional
nonradiative decay channel opens at +500 cm<sup>–1</sup> excess
energy. Since high overtone bands of ν<sub>1</sub><sup>′</sup> and ν<sub>2</sub><sup>′</sup> are observed in the R2PI
spectrum but only a single weak 2ν<sub>3</sub><sup>′</sup> band, we propose that ν<sub>3</sub><sup>′</sup> is a promoting
mode for nonradiative decay, consistent with the observation that
the ν<sub>3</sub><sup>′</sup> normal-mode eigenvector points toward the “C<sup>6</sup>-puckered”
conical intersection geometry
Gas-Phase Cytosine and Cytosine‑N<sub>1</sub>‑Derivatives Have 0.1–1 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> ⇝ S<sub>0</sub> internal conversion
of its nucleobases. Using picosecond pump–ionization delay
measurements, we find that the S<sub>1</sub>(<sup>1</sup><i>ππ</i>*) state vibrationless lifetime of gas-phase keto-amino cytosine
(Cyt) is τ = 730 ps or ∼700 times longer than that measured
by femtosecond pump–probe 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 τ = 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> = 300–550 cm<sup>–1</sup> decreases the lifetimes
to 20–30 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>π*) and S<sub>1</sub>(<sup>1</sup><i>ππ</i>*) states decreases
along the series of N<sub>1</sub>-derivatives, thereby influencing
the S<sub>1</sub> state lifetime