Non-resonant sound transmission through double walls using statistical energy analysis

Although SEA is a suitable framework for predicting sound transmission through double walls it has been found that the standard method of computing the non- resonant coupling loss factor from a room to cavity underestimates the coupling. A revised model for computing this coupling loss factor is presented which gives much better agreement with measured data. This allows better predictions to be made of sound transmission through lightweight double walls.     

Ab initio MO calculations and 17O NMR at natural abundance of para-substituted acetophenones

¹⁷O NMR chemical shifts and calculated (ab initio MO theory) electron densities are reported for a series of para-substituted acetophenones, X? C₄H₆? COCH₃, where X = NH₂, OCH₃, F, Cl, CH₃, H, COCH₃, CN, NO₂. The ¹⁷O shifts are very sensitive to the para substituent and cover a range of some 51 ppm. Donors induce upfield shifts and acceptors downfield shifts. The substituent chemical shifts (SCS) correlate precisely with σ_I and σ_R⁺ using the Dual Substituent Parameter (DSP) method. The derived transmission coefficients ρ_I and ρ_R indicate that polar and resonance mechanisms contribute approximately equally to the observed substituent effects. The shifts also correlate well with calculated π-electron densities (slope = 1500 ppm per electron) confirming their electronic origin. λ values are also reported, and the role of the average excitation energy, ΔE, in determining ¹⁷O SCS values is discussed. It is concluded that variations in ΔE are minor and that the local Δ-electron density is the dominant feature controlling ¹⁷O SCS values.     

Damping of building structures

Design charts are presented which enable the total loss factor of walls and floors to be quickly calculated. A comparison with measured results shows very good agreement. For normal walls and floors it is shown that the damping due to coupling can be approximated by $\text{1/}\text{f}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and that the spread of results from many different walls is small. The approximation can therefore be used for a large variety of walls and floors.     

Temperature dependence of 19F chemical shifts in substituted benzyl fluorides.

The ¹⁹F chemical shifts of substituted benzyl fluorides are shown to be temperature dependent and this is discussed in terms of conformational effects.     

Difficult macrocyclizations: new strategies for synthesizing highly strained cyclic tetrapeptides

[reaction: see text] Cyclic tetrapeptides are an intriguing class of natural products. To synthesize highly strained cyclic tetrapeptides we developed a macrocyclization strategy that involves the inclusion of 2-hydroxy-6-nitrobenzyl (HnB) group at the N-terminus and in the "middle" of the sequence. The N-terminal auxiliary performs a ring closure/ring contraction role, and the backbone auxiliary promotes cis amide bonds to facilitate the otherwise difficult ring contraction. Following this route, the all-L cyclic tetrapeptide cyclo-[Tyr-Arg-Phe-Ala] was successfully prepared.     

Magnetic-domain-wall pinning by regions of weak exchange or anisotropy

The extremely high coercivity of rare-earth cobalt materials calls for a new non-magnetostatic, approach to domain wall pinning. It is shown that the arbitrary weakening of the exchange coupling between two neighbouring layers of spins, or of the magnetocrystalline anisotropy, can give very steep energy gradients and strong pinning even when the perturbations are relatively small.