341 research outputs found
Tracking a Paterno-Buchi Reaction in Real Time Using Transient Electronic and Vibrational Spectroscopies
On the Participation of Photoinduced N-H Bond Fission in Aqueous Adenine at 266 and 220 nm:A Combined Ultrafast Transient Electronic and Vibrational Absorption Spectroscopy Study
Unravelling the mechanisms of vibrational relaxation in solution
We present a systematic study of the mode-specific vibrational relaxation of NO(2) in six weakly-interacting solvents (perfluorohexane, perfluoromethylcyclohexane, perfluorodecalin, carbon tetrachloride, chloroform, and d-chloroform), chosen to elucidate the dominant energy transfer mechanisms in the solution phase. Broadband transient vibrational absorption spectroscopy has allowed us to extract quantum state-resolved relaxation dynamics of the two distinct NO(2) fragments produced from the 340 nm photolysis of N(2)O(4) → NO(2)(X) + NO(2)(A) and their separate paths to thermal equilibrium. Distinct relaxation pathways are observed for the NO(2) bending and stretching modes, even at energies as high as 7000 cm(–1) above the potential minimum. Vibrational energy transfer is governed by different interaction mechanisms in the various solvent environments, and proceeds with timescales ranging from 20–1100 ps. NO(2) relaxation rates in the perfluorocarbon solvents are identical despite differences in acceptor mode state densities, infrared absorption cross sections, and local solvent structure. Vibrational energy is shown to be transferred to non-vibrational solvent degrees of freedom (V-T) through impulsive collisions with the perfluorocarbon molecules. Conversely, NO(2) relaxation in chlorinated solvents is reliant on vibrational resonances (V-V) while V-T energy transfer is inefficient and thermal excitation of the surrounding solvent molecules inhibits faster vibrational relaxation through direct complexation. Intramolecular vibrational redistribution allows the symmetric stretch of NO(2) to act as a gateway for antisymmetric stretch energy to exit the molecule. This study establishes an unprecedented level of detail for the cooling dynamics of a solvated small molecule, and provides a benchmark system for future theoretical studies of vibrational relaxation processes in solution
Reaction Dynamics of CN Radicals in Acetonitrile Solutions
The bimolecular reactions that follow
267 nm ultraviolet photolysis
of ICN in acetonitrile solution have been studied using transient
absorption spectroscopy on the picosecond time scale. Time-resolved
electronic absorption spectroscopy (TEAS) in the ultraviolet and visible
spectral regions observes rapid production and loss (with a decay
time constant of 0.6 ± 0.1 ps) of the photolytically generated
free CN radicals. Some of these radicals convert to a solvated form
which decays with a lifetime of 8.5 ± 2.1 ps. Time-resolved vibrational
absorption spectroscopy (TVAS) reveals that the free and solvated
CN-radicals undergo geminate recombination with I atoms to make ICN
and INC, H atom abstraction reactions, and addition reactions to solvent
molecules to make C<sub>3</sub>H<sub>3</sub>N<sub>2</sub> radical
species. These radical products have a characteristic absorption band
at 2036 cm<sup>–1</sup> that shifts to 2010 cm<sup>–1</sup> when ICN is photolyzed in CD<sub>3</sub>CN. The HCN yield is low,
suggesting the addition pathway competes effectively with H atom abstraction
from CH<sub>3</sub>CN, but the delayed growth of the C<sub>3</sub>H<sub>3</sub>N<sub>2</sub> radical band is best described by reaction
of solvated CN radicals through an unobserved intermediate species.
Addition of methanol or tetrahydrofuran as a cosolute promotes H atom
abstraction reactions that produce vibrationally hot HCN. The combination
of TEAS and TVAS measurements shows that the rate-limiting process
for production of ground-state HCN is vibrational cooling, the rate
of which is accelerated by the presence of methanol or tetrahydrofuran
Recombination, Solvation and Reaction of CN Radicals Following Ultraviolet Photolysis of ICN in Organic Solvents
The fates of CN radicals produced
by ultraviolet (UV) photolysis
of ICN in various organic solvents have been examined by transient
electronic and vibrational absorption spectroscopy (TEAS and TVAS).
Near-UV and visible bands in the TEAS measurement enable direct observation
of the CN radicals and their complexes with the solvent molecules.
Complementary TVAS measurements probe the products of CN–radical
reactions. Geminate recombination to form ICN and INC is a minor pathway
on the 150 fs −1300 ps time scales of our experiments in the
chosen organic solvents; nonetheless, large infrared transition dipole
moments permit direct observation of INC that is vibrationally excited
in the Cî—¼N stretching mode. The time constants for INC vibrational
cooling range from 30 ps in tetrahydrofuran (THF) to 1400 ps in more
weakly interacting solvents such as chloroform. The major channel
for CN removal in the organic solvents is reaction with solvent molecules,
as revealed by depletion of solvent absorption bands and growth of
product bands in the TVA spectra. HCN is a reaction product of hydrogen
atom abstraction in most of the photoexcited solutions, and forms
with vibrational excitation in both the C–H and CN
stretching modes. The vibrational cooling rate of the Cî—¼N stretch
in HCN depends on the solvent, and follows the same trend as the cooling
rate of the Cî—¼N stretch in INC. However, in acetonitrile solution
an additional reaction pathway produces C<sub>3</sub>H<sub>3</sub>N<sub>2</sub><sup>•</sup> radicals, which release HCN on a
much longer time scale
Evaluation of a Tennessee statewide initiative to reduce early elective deliveries using quasi-experimental methods
Abstract
Background
Concerted quality improvement (QI) efforts have been taken to discourage the practice of early elective deliveries (EEDs), but few studies have robustly examined the impact of directed QI interventions in reducing EED practices. Using quasi-experimental methods, we sought to evaluate the impact of a statewide QI intervention to reduce the practice of EEDs.
Methods
Retrospective cohort study of vital records data (2007 to 2013) for all singleton births occurring ≥36 weeks in 66 Tennessee hospitals grouped into three QI cohorts. We used interrupted-time series to estimate the effect of the QI intervention on the likelihood of an EED birth statewide, and by hospital cohort. We compared the distribution of hospital EED percentages pre- and post-intervention. Lastly, we used multivariable logistic regression to estimate the effect of QI interventions on maternal and infant outcomes.
Results
Implementation of the QI intervention was associated with significant declines in likelihood of EEDs immediately following the intervention (odds ratio, OR = 0.72; p < 0.001), but these results varied by hospital cohort. Hospital risk-adjusted EED percentages ranged from 1.6–13.6% in the pre-intervention period, which significantly declined to 2.2–9.6% in the post-intervention period (p < 0.001). The QI intervention was also associated with significant reductions in operative vaginal delivery and perineal laceration, and immediate infant ventilation, but increased NICU admissions.
Conclusions
A statewide QI intervention to reduce EEDs was associated with modest but significant declines in EEDs beyond concurrent and national trends, and showed mixed results in related infant and maternal outcomes.https://deepblue.lib.umich.edu/bitstream/2027.42/148522/1/12913_2019_Article_4033.pd
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