Comment on “CCSD study of anharmonic Raman cross sections of fundamental, overtone, and combination transitions”

Equations (36) and (37) in L. N. Vidal, P. A. M. Vazquez, Int. J. Quantum Chem. 2012, 112, 3205 are wrong. The agreement between theoretical and experimental Raman cross sections is greatly improved with use of the corrected expressions

Changing gears to neutral in a polymorph of one-dimensional arrays of cogwheel-like pairs of molecular rotors

We report on a polymorph (2) of an amphidynamic crystal of molecular rods with two helical 1,4-bis(ethynyl)bicyclo[2.2.2]octane rotators where half of the rod-like molecules appear to be shifted with respect to their closest neighbours. This translation takes cogwheel-like pairs of rotators apart in the lattice in such a way that their motion becomes uncorrelated. This property is to be contrasted with the highly correlated motion found to govern the rotators in a recently-published polymorph 1 of the same material. As with polymorph 1, this motion is shown to take place independently of mutations in the handedness of the rotators and of the ‘mutamer’-induced second harmonic generation

Gyrations: The Missing Link Between Classical Mechanics with its Underlying Euclidean Geometry and Relativistic Mechanics with its Underlying Hyperbolic Geometry

Being neither commutative nor associative, Einstein velocity addition of relativistically admissible velocities gives rise to gyrations. Gyrations, in turn, measure the extent to which Einstein addition deviates from commutativity and from associativity. Gyrations are geometric automorphisms abstracted from the relativistic mechanical effect known as Thomas precession

More light on the 2ν5 Raman overtone of SF6: Can a weak anisotropic spectrum be due to a strong transition anisotropy?

Long known as a fully polarized band with a near vanishing depolarization ratio [η s = 0.05, W. Holzer and R. Ouillon, Chem. Phys. Lett.24, 589 (1974)], the 2ν5 Raman overtone of SF6 has so far been considered as of having a prohibitively weak anisotropic spectrum [D. P. Shelton and L. Ulivi, J. Chem. Phys.89, 149 (1988)]. Here, we report the first anisotropic spectrum of this overtone, at room temperature and for 13 gas densities ranging between 2 and 27 amagat. This spectrum is 10 times broader and 50 times weaker than the isotropic counterpart of the overtone [D. Kremer, F. Rachet, and M. Chrysos, J. Chem. Phys.138, 174308 (2013)] and its profile much more sensitive to pressure effects than the profile of the isotropic spectrum. From our measurements an accurate value for the anisotropy matrix-element |⟨000020|Δα|000000⟩| was derived and this value was found to be comparable to that of the mean-polarizability ((000020),α¯¯,(000000)) . Among other conclusions our study offers compelling evidence that, in Raman spectroscopy, highly polarized bands or tiny depolarization ratios are not necessarily incompatible with large polarizability anisotropy transition matrix-elements. Our findings and the way to analyze them suggest that new strategies should be developed on the basis of the complementarity inherent in independent incoherent Raman experiments that run with two different incident-beam polarizations, and on concerted efforts to ab initiocalculate accurate data for first and second polarizability derivatives. Values for these derivatives are still rarities in the literature of SF6

From light-scattering measurements to polarizability derivatives in vibrational Raman spectroscopy: The 2 nu(5) overtone of SF6

The room-temperature isotropic spectrum of SF6 was recorded at the frequency of the 2 nu(5) overtone by running high-sensitivity incoherent Raman experiments for two independent polarizations of the incident beam and for gas densities varying from 2 to 27 amagat. Weak yet observable pressure effects were found. A transparent analysis of the Raman cross-section problem along with the first-ever prediction of the value of the mean polarizability second derivative partial derivative(2)(alpha) over bar/partial derivative q(5)(2) are made and the hitherto underestimated role of the hot bands of SF6 is brought to the wider public. The emergence of an analytic hotband factor is shown whose magnitude is dramatically increased with the order of the overtone and the gas temperature and all the more so upon considering low-frequency molecular vibrations. Our formulas, which in the harmonic approximation are exact, are still applicable to real situations provided certain conditions are fulfilled. For nondegenerated modes, generalization to higher order overtones is made, an issue addressing the much challenging problem of the IR-allowed second overtone bands. The content of this paper is also an invitation towards ab initio derivative-calculations for sulfur hexafluoride, especially given the today\u27s needs in interpreting spectra of significance for greenhouse atmospheric issues

The 2ν3 Raman overtone of sulfur hexafluoride: Absolute spectra, pressure effects, and polarizability properties

Of the six normal vibrations of SF6, ν3 has a key role in the mechanisms of radiative forcing. This vibration, though inactive in Raman, shows up through the transition 2ν3 allowing for a complementary view on the asymmetric stretch of the molecule. Here, we look back into this topic, which has already caught some interest in the past but with some points been left out. We make a systematic incoherent-light-scattering analysis of the overtone with the use of different gas pressures and polarization orientations for the incident beam. Absolute-scale isotropic and anisotropic spectra are reported along with natural and pressure-induced widths and shifts, and other spectral features such as the peaks corresponding to the (experimentally indistinguishable) interfering channels E g and F 2g hitherto seen solely as two-photon IR-absorption features. We make the first-ever prediction of the SF6 polarizability second derivative with respect to the ν3-mode coordinate and we develop a heuristic argument to explain why the superposition of the three degenerate stretching motions that are related to the ν3 mode cannot but generate a polarized Raman band

Does rapid HIV disease progression prior to combination antiretroviral therapy hinder optimal CD4 + T-cell recovery once HIV-1 suppression is achieved?

Objective: This article compares trends in CD4+ T-cell recovery and proportions achieving optimal restoration (>=500 cells/µl) after viral suppression following combination antiretroviral therapy (cART) initiation between rapid and nonrapid progressors. Methods: We included HIV-1 seroconverters achieving viral suppression within 6 months of cART. Rapid progressors were individuals experiencing at least one CD4+ less than 200 cells/µl within 12 months of seroconverters before cART. We used piecewise linear mixed models and logistic regression for optimal restoration. Results: Of 4024 individuals, 294 (7.3%) were classified as rapid progressors. At the same CD4+ T-cell count at cART start (baseline), rapid progressors experienced faster CD4+ T-cell increases than nonrapid progressors in first month [difference (95% confidence interval) in mean increase/month (square root scale): 1.82 (1.61; 2.04)], which reversed to slightly slower increases in months 1–18 [-0.05 (-0.06; -0.03)] and no significant differences in 18–60 months [-0.003 (-0.01; 0.01)]. Percentage achieving optimal restoration was significantly lower for rapid progressors than nonrapid progressors at months 12 (29.2 vs. 62.5%) and 36 (47.1 vs. 72.4%) but not at month 60 (70.4 vs. 71.8%). These differences disappeared after adjusting for baseline CD4+ T-cell count: odds ratio (95% confidence interval) 0.86 (0.61; 1.20), 0.90 (0.38; 2.17) and 1.56 (0.55; 4.46) at months 12, 36 and 60, respectively. Conclusion: Among people on suppressive antiretroviral therapy, rapid progressors experience faster initial increases of CD4+ T-cell counts than nonrapid progressors, but are less likely to achieve optimal restoration during the first 36 months after cART, mainly because of lower CD4+ T-cell counts at cART initiation

Reversible Control of Crystalline Rotors by Squeezing Their Hydrogen Bond Cloud Across a Halogen Bond-Mediated Phase Transition

  We report on a crystalline rotor that undergoes a reversible phase transition at 145 K. Variable-temperature X-ray and 1H spin −lattice relaxation experiments, and calculations of rotational barriers, provide a description (i) of the way in which the rotators’ dynamics changes back and forth at the onset of the phase transition and (ii) of the mechanism responsible for the abrupt switching of the crystalline rotors from a very low-energy 4-fold degenerate equilibrium state, in which the rotation is ultrafast (9.6 GHz at 145 K), to a single higher-energy state associated with a slower motion (2.3 GHz at 145 K). Our results provide evidence that the reversible change observed in the rotational barriers at the transition is due to a cooperative modulation of the C −Hrotator···Istator hydrogen bond cloud across a C −I stator···Istator−C halogen bond-mediated phase transition. In addition, we report evidence for second-harmonic generation from this material, thereby confirming with a second example the benefit of using polarized light to probe the torsional degree of freedom of chiral helix blades, as well as symmetry and dimensionality of large collections of chiral rotors in the solid state.