Standard enthalpy of solution of ammonium nitrate in water at 298 K

We have made calorimetric measurements of the enthalpy of solution of NH₄NO₃(c, IV) in water at 298 K, where (c, IV) indicates the crystal form of amomonium nitrate that is stable from 256 to 305 K. Results of our measurements have been combined with enthalpy of dilution values from Parker to obtain the standard enthalpy of solution of NH₄NO₃ (c, IV) in water at 298.15 K to be ΔH^o = 25.41 kJ mol^?1.     

Onset of double-diffusive convection in a saturated porous layer with time-periodic surface heating

The stability of a fluid saturated, horizontal porous layer in the presence of a solute concentration gradient and time-periodic thermal gradient is examined. The modulated gradient is the result of a sinusoidal upper surface temperature which models the effect of variable solar radiation heating of the layer. Darcy's law and the Boussinesq approximation are employed, and we assume an equation of state linear in temperature and concentration. A linear stability analysis is carried out to obtain predictions for the onset of convection and critical wavenumbers for the system. The critical conditions are obtained via the Galerkin method and Floquet theory. The effects of variable concentration gradient, temperature modulation amplitude and frequency are examined, and compared with the results obtained analytically from the corresponding unmodulated problem. It is shown that instabilities can occur as convective motions which are synchronous or subharmonic with the surface heating, or can be identified via complex conjugate Floquet exponents. The neutral stability curves at the transitions between instabilities are found to be bimodal when the temperature is time-periodic, and are characterized by jumps in the critical wavenumbers. Received February 5, 1998     

Interception of a Rh(I)-Rh(III) dinuclear trihydride complex revealing the dihydrogen activation by [Rh(CO)2{(R,R)-Ph-BPE}

Reaction of [Rh(CO)(2){(R,R)-Ph-BPE}][BF(4)] 1 under 7 bar H(2) provides the dihydride [Rh(H)(2)(CO)(2){(R,R)-Ph-BPE}][BF(4)] 3, which reacts with the neutral hydride [Rh(H)(CO){(R,R)-Ph-BPE}] 2 arising from 3 in THF. The resulting complex is the dimeric monocationic Rh((I))-Rh((III)) complex [Rh(H)(2)(CO)(2){(R,R)-Ph-BPE}][BF(4)] 4.     

Untethered 4,1,2-MC(2)B(10) supraicosahedral metallacarboranes, their C,C'-dimethyl 4,1,6-, 4,1,8- and 4,1,12-MC(2)B(10) analogues, and DFT study of the (4,)1,2- to (4,)1,6-isomerisations of C(2)B(11) carboranes and MC(2)B(10) metallacarboranes

Reduction of the tethered carborane 1,2-μ-(CH(2)SiMe(2)CH(2))-1,2-closo-C(2)B(10)H(10) followed by metallation with {CpCo} or {(p-cymene)Ru} fragments affords both C,C'-dimethyl 4,1,2-MC(2)B(10) and 4,1,6-MC(2)B(10) species. DFT calculations indicate that the barriers to isomerisation of both 4-Cp-4,1,2-closo-CoC(2)B(10)H(12) and 4-(η-C(6)H(6))-4,1,2-closo-RuC(2)B(10)H(12) to their respective 4,1,6-isomers are too high for this to be the origin of the unexpected formation of 4,1,6-MC(2)B(10) products (in marked contrast to the related isomerisation of 1,2-closo-C(2)B(11)H(13) to 1,6-closo-C(2)B(11)H(13)), and, indeed, the 4,1,2-species are recovered unchanged from refluxing toluene. Equally, the DFT-calculated profile for the isomerisation of [7,8-nido-C(2)B(10)H(12)](2-) to [7,9-nido-C(2)B(10)H(12)](2-) suggests that the unexpected formation of 4,1,6-metallacarboranes is unlikely to result from isomerisation of a reduced (nido) carborane following desilylation. Instead, the source of the 4,1,6-MC(2)B(10) compounds is traced to desilylation of 1,2-μ-(CH(2)SiMe(2)CH(2))-1,2-closo-C(2)B(10)H(10) by Li or Na prior to reduction. The supraicosahedral metallacarboranes 1,8-Me(2)-4-Cp-4,1,8-closo-CoC(2)B(10)H(10), 1,12-Me(2)-4-Cp-4,1,12-closo-CoC(2)B(10)H(10) and 1,12-Me(2)-4-(p-cymene)-4,1,12-closo-RuC(2)B(10)H(10) are also reported with all new species characterised both spectroscopically and crystallographically.     

Perceptual differences between low and high rates of stimulation on single electrodes for cochlear implantees

Previous research has shown that increases in the rate of stimulation on a single electrode yield changes in pitch perception until the rate is increased beyond a given critical rate, after which changes in rate are only perceived as changes in loudness. The critical rate beyond which a rate increase no longer elicits a pitch change in most subjects is approximately 300 Hz, although a small number of subjects have been observed to have critical rates up to approximately 1000 Hz. In this article, we sought to determine if increasing the rate of stimulation beyond the critical rate (up to 12.8 kHz) would eventually result in new changes of perception (other than loudness.) Our data replicate the previously observed results that rates between approximately 300 and 1500 Hz are indistinguishable from each other. However, we observed the finding that a rate of stimulation well above the critical rate (starting between 1500 Hz and 12.8 kHz, depending on electrode and subject) can elicit changes in perception. The perceptual differences between these high rates were sometimes but not always labeled as pitch changes. This phenomenon needs further research to assess its potential relevance to speech perception using high rates of stimulation.