15,241 research outputs found
Infrared intensities of amide modes in N‐methylacetamide and poly(glycine I) from ab initio calculations of dipole moment derivatives of N‐methylacetamide
The infrared intensities of the amide modes in N‐methylacetamide (NMA) and poly(glycine I) (PGI) have been studied using ab initio dipole moment derivatives obtained for the peptide group in NMA and an empirical force field refined for PGI. Good agreement is found between the calculated transition moment magnitudes and directions of the amide I and II modes and experimental intensity and dichroism data. By analyzing the separate contributions of each internal coordinate to the total intensity, we are able to understand in detail the origins of the IR intensities of the amide modes. Besides demonstrating one approach by which IR intensities can be studied in complex molecules and polymers, our results also provide a basis for using IR intensities in structural studies of peptides and polypeptides.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70763/2/JCPSA6-82-4-1631-1.pd
Three-geometry and reformulation of the Wheeler-DeWitt equation
A reformulation of the Wheeler-DeWitt equation which highlights the role of
gauge-invariant three-geometry elements is presented. It is noted that the
classical super-Hamiltonian of four-dimensional gravity as simplified by
Ashtekar through the use of gauge potential and densitized triad variables can
furthermore be succinctly expressed as a vanishing Poisson bracket involving
three-geometry elements. This is discussed in the general setting of the
Barbero extension of the theory with arbitrary non-vanishing value of the
Immirzi parameter, and when a cosmological constant is also present. A proposed
quantum constraint of density weight two which is polynomial in the basic
conjugate variables is also demonstrated to correspond to a precise simple
ordering of the operators, and may thus help to resolve the factor ordering
ambiguity in the extrapolation from classical to quantum gravity. Alternative
expression of a density weight one quantum constraint which may be more useful
in the spin network context is also discussed, but this constraint is
non-polynomial and is not motivated by factor ordering. The article also
highlights the fact that while the volume operator has become a preeminient
object in the current manifestation of loop quantum gravity, the volume element
and the Chern-Simons functional can be of equal significance, and need not be
mutually exclusive. Both these fundamental objects appear explicitly in the
reformulation of the Wheeler-DeWitt constraint.Comment: 10 pages, LaTeX fil
Is Barbero's Hamiltonian formulation a Gauge Theory of Lorentzian Gravity?
This letter is a critique of Barbero's constrained Hamiltonian formulation of
General Relativity on which current work in Loop Quantum Gravity is based.
While we do not dispute the correctness of Barbero's formulation of general
relativity, we offer some criticisms of an aesthetic nature. We point out that
unlike Ashtekar's complex SU(2) connection, Barbero's real SO(3) connection
does not admit an interpretation as a space-time gauge field. We show that if
one tries to interpret Barbero's real SO(3) connection as a space-time gauge
field, the theory is not diffeomorphism invariant. We conclude that Barbero's
formulation is not a gauge theory of gravity in the sense that Ashtekar's
Hamiltonian formulation is. The advantages of Barbero's real connection
formulation have been bought at the price of giving up the description of
gravity as a gauge field.Comment: 12 pages, no figures, revised in the light of referee's comments,
accepted for publication in Classical and Quantum Gravit
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Architecture of the Short External Rotator Muscles of the Hip.
BackgroundMuscle architecture, or the arrangement of sarcomeres and fibers within muscles, defines functional capacity. There are limited data that provide an understanding of hip short external rotator muscle architecture. The purpose of this study was thus to characterize the architecture of these small hip muscles.MethodsEight muscles from 10 independent human cadaver hips were used in this study (n = 80 muscles). Architectural measurements were made on pectineus, piriformis, gemelli, obturators, quadratus femoris, and gluteus minimus. Muscle mass, fiber length, sarcomere length, and pennation angle were used to calculate the normalized muscle fiber length, which defines excursion, and physiological cross-sectional area (PCSA), which defines force-producing capacity.ResultsGluteus minimus had the largest PCSA (8.29 cm2) followed by obturator externus (4.54 cm2), whereas superior gemellus had the smallest PCSA (0.68 cm2). Fiber lengths clustered into long (pectineus - 10.38 cm and gluteus minimus - 10.30 cm), moderate (obturator internus - 8.77 cm and externus - 8.04 cm), or short (inferior gemellus - 5.64 and superior gemellus - 4.85). There were no significant differences among muscles in pennation angle which were all nearly zero. When the gemelli and obturators were considered as a single functional unit, their collective PCSA (10.00 cm2) exceeded that of gluteus minimus as a substantial force-producing group.ConclusionsThe key findings are that these muscles have relatively small individual PCSAs, short fiber lengths, and low pennation angles. The large collective PCSA and short fiber lengths of the gemelli and obturators suggest that they primarily play a stabilizing role rather than a joint rotating role
Experimental Demonstration of >230{\deg} Phase Modulation in Gate-Tunable Graphene-Gold Reconfigurable Mid-Infrared Metasurfaces
Metasurfaces offer significant potential to control far-field light
propagation through the engineering of amplitude, polarization, and phase at an
interface. We report here phase modulation of an electronically reconfigurable
metasurface and demonstrate its utility for mid-infrared beam steering. Using a
gate-tunable graphene-gold resonator geometry, we demonstrate highly tunable
reflected phase at multiple wavelengths and show up to 237{\deg} phase
modulation range at an operating wavelength of 8.50 {\mu}m. We observe a smooth
monotonic modulation of phase with applied voltage from 0{\deg} to 206{\deg} at
a wavelength of 8.70 {\mu}m. Based on these experimental data, we demonstrate
with antenna array calculations an average beam steering efficiency of 50% for
reflected light for angles up to 30{\deg}, relative to an ideal metasurface,
confirming the suitability of this geometry for reconfigurable mid-infrared
beam steering devices
Season of the year influences infection rates following total hip arthroplasty
To research the influence of season of the year on periprosthetic joint infections. METHODS We conducted a retrospective review of the entire Medicare files from 2005 to 2014. Seasons were classified as spring, summer, fall or winter. Regional variations were accounted for by dividing patients into four geographic regions as per the United States Census Bureau (Northeast, Midwest, West and South). Acute postoperative infection and deep periprosthetic infections within 90 d after surgery were tracked. RESULTS In all regions, winter had the highest incidence of periprosthetic infections (mean 0.98%, SD 0.1%) and was significantly higher than other seasons in the Midwest, South and West (P \u3c 0.05 for all) but not the Northeast (P = 0.358). Acute postoperative infection rates were more frequent in the summer and were significantly affected by season of the year in the West. CONCLUSION Season of the year is a risk factor for periprosthetic joint infection following total hip arthroplasty (THA). Understanding the influence of season on outcomes following THA is essential when risk-stratifying patients to optimize outcomes and reduce episode of care costs. © The Author(s) 2017
Quantization of static space-times
A 4-dimensional Lorentzian static space-time is equivalent to 3-dimensional
Euclidean gravity coupled to a massless Klein-field. By canonically quantizing
the coupling model in the framework of loop quantum gravity, we obtain a
quantum theory which actually describes quantized static space-times. The
Kinematical Hilbert space is the product of the Hilbert space of gravity with
that of imaginary scalar fields. It turns out that the Hamiltonian constraint
of the 2+1 model corresponds to a densely defined operator in the underlying
Hilbert space, and hence it is finite without renormalization. As a new point
of view, this quantized model might shed some light on a few physical problems
concerning quantum gravity.Comment: 14 pages, made a few modifications, added Journal-re
Feasible Environmental Kuznets and Institutional Quality in North and Southern African Sub-regions
One of the goals of Africa as a developing continent is to grow and also reduce environmental pollution. Most studies investigate the presence of inverted U-shaped environmental Kuznets curve (EKC) using pollutants such as carbon dioxide (CO2), the use of point pollutants such as suspended particulate matter (SPM) is not so popular in literature. Similarly, most studies that assess the role of institutions in the income-pollution nexus do not investigate their capability in achieving feasible turning on the EKC. Focusing on three pollutants, namely: CO2, nitrogen oxide (N2O) and SPM, this study employs system generalised method of moments to assess the role of institutions in two sub-regions (North and Southern Africa) in attaining EKC turning points. Results, among others, indicate that the both sub-regions did not attain a level of average income capable of turning EKC round for CO2 and N2O but do for SPM. It is also revealed that Southern Africa attained EKC faster than North Africa. It is therefore recommended that for the purpose of achieving the goal of green growth, the institutional quality should be strengthened in the two Africa sub-regions, particularly in
Southern Africa
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