Search results for "IFT"
showing 10 items of 2193 documents
Conceptualizing ecosystem tipping points within a physiological framework
2017
Connecting the nonlinear and often counterintuitive physiological effects of multiple environmental drivers to the emergent impacts on ecosystems is a fundamental challenge. Unfortunately, the disconnect between the way “stressors” (e.g., warming) is considered in organismal (physiological) and ecological (community) contexts continues to hamper progress. Environmental drivers typically elicit biphasic physiological responses, where performance declines at levels above and below some optimum. It is also well understood that species exhibit highly variable response surfaces to these changes so that the optimum level of any environmental driver can vary among interacting species. Thus, specie…
Chromogenic detection of nerve agent mimics
2008
The current rise in international concern over criminal terrorist attacks via chemical warfare (CW) agents has resulted in an increasing interest in the detection of these lethal chemicals. Among CW species, nerve agents are extremely dangerous and their high toxicity and ease of production underscore the need to detect these deadly chemicals via quick and reliable procedures. A number of detection systems have been developed, most of them based on enzymatic and physical methodologies. However, these usually show limitations such as low selectivity, lack of portability and a certain complexity in their use. An alternative to these classical methods that has been gaining interest in recent y…
Comparison of different theory models and basis sets in the calculation of 13C NMR chemical shifts of natural products.
2004
The influence of the calculation method in mimicking experimental 13C NMR chemical shifts of 15 low-polarity natural products singularly containing 10–20 carbon atoms was investigated by employing different quantum chemistry approaches and basis sets, both in the preliminary geometry optimizations and in the following single-point 13C GIAO calculations of the NMR chemical shifts. The geometries of the involved species were optimized at the PM3, HF, B3LYP and mPW1PW91 levels whereas the 13C NMR parameters were determined at the HF, B3LYP and mPW1PW91 levels. Different combinations of basis sets were also tested. The consistency and efficiency of the considered combinations of geometry optimi…
Four-Component Relativistic DFT Calculations of C-13 Chemical Shifts of Halogenated Natural Substances
2015
We have calculated the (13) C NMR chemical shifts of a large ensemble of halogenated organic molecules (81 molecules for a total of 250 experimental (13) C NMR data at four different levels of theory), ranging from small rigid organic compounds, used to benchmark the performance of various levels of theory, to natural substances of marine origin with conformational degrees of freedom. Carbon atoms bonded to heavy halogen atoms, particularly bromine and iodine, are known to be rather challenging when it comes to the prediction of their chemical shifts by quantum methods, due to relativistic effects. In this paper, we have applied the state-of-the-art four-component relativistic density funct…
Population Properties of Compact Objects from the Second LIGO-Virgo Gravitational-Wave Transient Catalog
2021
Abbott, R., et al. (LIGO and Virgo Collaboration)
Search for Gravitational Waves Associated with Gamma-Ray Bursts Detected by Fermi and Swift during the LIGO-Virgo Run O3a
2022
Abbott, R., et al. (LIGO and VIRGO Collaboration)
Acidity and basicity of primaryN-phenylnitramines: catalytic effect of protons on the nitramine rearrangement
2002
Para-substituted N-phenylnitramines were prepared either by oxidation of diazonium salts or by nitration under alkaline or acidic conditions. Isotopic [15N-NO2] labelling indicated that the bands characteristic of the N-nitro group appear in the 1318–1323 and 1585–1607 cm−1 regions. In the nitrogen NMR spectra, the nitramino group gives two resonances at −193 ± 3 (NH) and −32 ± 3 ppm (NO2). The chemical shifts in proton and carbon NMR spectra are predictable, based on increments and the additivity rule. The spectral data indicate the lack of conjugation between the nitramino group and another substituent bound to the ring. It seems to contradict the well-known fact that substituents strongl…
Noble Gases as Magnetic Probes in Fullerene Chemistry
2016
Since the early 1990s of the previous century, the use of magnetic active noble gas atom to indirectly probe the size and symmetry of fullerene cage has been reviewed. Molecular modeling of nuclear shielding (and chemical shift) using gauge-including atomic orbital (GIAO) approach at several levels of theory, including Hartree–Fock (HF), density functional theory (DFT), and coupled cluster (CC), is discussed. The advantage of using GIAO calculation to predict and closely reproduce experimental 3He and 129Xe NMR spectra is shown.