14,534 research outputs found
Synthesis, structure and dynamics of NHC-based palladium macrocycles
A series of macrocyclic CNC pincer pro-ligands based on bis(imidazolium)lutidine salts with octa-, deca- and dodecamethylene spacers have been prepared and their coordination chemistry investigated. Using a Ag2O based transmetallation strategy, cationic palladium(II) chloride complexes [PdCl{CNC–(CH2)n}][BArF4] (n = 8, 10, 12; ArF = 3,5-C6H3(CF3)2) were prepared and fully characterised in solution, by NMR spectroscopy and ESI-MS, and in the solid-state, by X-ray crystallography. The smaller macrocyclic complexes (n = 8 and 10) exhibit dynamic behaviour in solution, involving ring flipping of the alkyl spacer across the Pd–Cl bond, which was interrogated by variable temperature NMR spectroscopy. In the solid-state, distorted coordination geometries are observed with the spacer skewed to one side of the Pd–Cl bond. In contrast, a static C2 symmetric structure is observed for the dodecamethylene based macrocycle. For comparison, palladium(II) fluoride analogues [PdF{CNC–(CH2)n}][BArF4] (n = 8, 10, 12) were also prepared and their solution and solid-state structures contrasted with those of the chlorides. Notably, these complexes exhibit very low frequency 19F chemical shifts (ca. −400 ppm) and the presence of C–HF interactions (2hJFC coupling observed by 13C NMR spectroscopy). The dynamic behaviour of the fluoride complexes is largely consistent with the smaller ancillary ligand; [PdF{CNC–(CH2)8}][BArF4] exceptionally shows C2v time averaged symmetry in solution at room temperature (CD2Cl2, 500 MHz) as a consequence of dual fluxional processes of the pincer backbone and alkyl spacer
On the construction of probabilistic Newton-type algorithms
It has recently been shown that many of the existing quasi-Newton algorithms
can be formulated as learning algorithms, capable of learning local models of
the cost functions. Importantly, this understanding allows us to safely start
assembling probabilistic Newton-type algorithms, applicable in situations where
we only have access to noisy observations of the cost function and its
derivatives. This is where our interest lies.
We make contributions to the use of the non-parametric and probabilistic
Gaussian process models in solving these stochastic optimisation problems.
Specifically, we present a new algorithm that unites these approximations
together with recent probabilistic line search routines to deliver a
probabilistic quasi-Newton approach.
We also show that the probabilistic optimisation algorithms deliver promising
results on challenging nonlinear system identification problems where the very
nature of the problem is such that we can only access the cost function and its
derivative via noisy observations, since there are no closed-form expressions
available
Complex-valued Time Series Modeling for Improved Activation Detection in fMRI Studies
A complex-valued data-based model with th order autoregressive errors and general real/imaginary error covariance structure is proposed as an alternative to the commonly used magnitude-only data-based autoregressive model for fMRI time series. Likelihood-ratio-test-based activation statistics are derived for both models and compared for experimental and simulated data. For a dataset from a right-hand finger-tapping experiment, the activation map obtained using complex-valued modeling more clearly identifies the primary activation region (left functional central sulcus) than the magnitude-only model. Such improved accuracy in mapping the left functional central sulcus has important implications in neurosurgical planning for tumor and epilepsy patients. Additionally, we develop magnitude and phase detrending procedures for complex-valued time series and examine the effect of spatial smoothing. These methods improve the power of complex-valued data-based activation statistics. Our results advocate for the use of the complex-valued data and the modeling of its dependence structures as a more efficient and reliable tool in fMRI experiments over the current practice of using only magnitude-valued datasets
How physics instruction impacts students' beliefs about learning physics: A meta-analysis of 24 studies
In this meta-analysis, we synthesize the results of 24 studies using the
Colorado Learning Attitudes about Science Survey (CLASS) and the Maryland
Physics Expectations Survey (MPEX) to answer several questions: (1) How does
physics instruction impact students' beliefs? (2) When do physics majors
develop expert-like beliefs? and (3) How do students' beliefs impact their
learning of physics? We report that in typical physics classes, students'
beliefs deteriorate or at best stay the same. There are a few types of
interventions, including an explicit focus on model-building and/or developing
expert- like beliefs that lead to significant improvements in beliefs. Further,
small courses and those for elementary education and non-science majors also
result in improved beliefs. However, because the available data oversamples
certain types of classes, it is unclear whether these improvements are actually
due to the interventions, or due to the small class size, or student population
typical of the kinds of classes in which these interventions are most often
used. Physics majors tend to enter their undergraduate education with more
expert-like beliefs than non-majors and these beliefs remain relatively stable
throughout their undergraduate careers. Thus, typical physics courses appear to
be selecting students who already have strong beliefs, rather than supporting
students in developing strong beliefs. There is a small correlation between
students' incoming beliefs about physics and their gains on conceptual
mechanics surveys. This suggests that students with more expert-like incoming
beliefs may learn more in their physics courses, but this finding should be
further explored and replicated. Some unanswered questions remain. To answer
these questions, we advocate several specific types of future studies.Comment: 30 pages. Accepted to Phys Rev ST-PE
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