The mandelamide keto-enol system in aqueous solution. Generation of the enol by hydration of phenylcarbamoylcarbene

Flash photolysis of diazophenylacetamide in aqueous solution produced phenylcarbamoylcarbene, whose hydration generated a transient species that was identified as the enol isomer of mandelamide. This assignment is based on product identification and the shape of the rate profile for decay of the enol transient, through ketonization to its carbonyl isomer, as well as by the form of acid-base catalysis of and solvent isotope effects on the decay reaction. Rates of enolization of mandelamide were also determined, by monitoring hydrogen exchange at its benzylic position, and these, in combination with the ketonization rate measurements, gave the keto-enol equilibrium constant pK(E) = 15.88, the acidity constant of the enol ionizing as an oxygen acid, pQ(E)(a)= 8.40, and the acidity constant of the amide ionizing as a carbon acid pQ(K)(a)= 24.29. (These acidity constants are concentration quotients applicable at ionic strength = 0.10 M.) These results show the enol content and carbon acid strength of mandelamide, like those of mandelic acid and methyl mandelate, to be orders of magnitude less than those of simple aldehydes and ketones; this difference can be attributed to resonance stabilization of the keto isomers of mandelic acid and its ester and amide derivatives, through electron delocalization into their carbonyl groups from the oxygen and nitrogen substituents adjacent to these groups. The enol of mandelamide, on the other hand, again like the enols of mandelic acid and methyl mandelate, is a substantially stronger acid than the enols of simple aldehydes and ketones. This difference can be attributed to the electronegative nature of the oxygen and nitrogen substituents geminal to the enol hydroxyl group in the enols of mandelic acid and its derivatives; in support of this, the acidity constants of these enols correlate well with field substituent constants of these geminal groups.     

Quality of assurance considerations for use of the Fluorlmager SI and fragmeNT analysis software

The Fluorlmager SI (FSI) from Molecular Dynamics is one of several scanning instruments available for the detection of fluorescent emissions associated with DNA samples in a variety of matrices (agarose and polyacrylamide gels, membranes and microplates). In our laboratory, we measured the electrophoretic mobility of randomly amplified polymorphic DNA (RAPD) fragments stained with ethidium bromide in agarose using the FSI to scan gels and the associated Molecular Dynamics software (ImageQuaNT, and FragmeNT Analysis) for analysis. Initial scans and analyses resulted in inconsistent band detection across the same gel and across several scans of the same gel. To determine the best types of calibration for the instrument, several factors were considered and then evaluated. Tests of calibration acceptability were also evaluated. Band detection by FragmeNT Analysis was improved following optimization of matrices and parameters used in calibration and experimental scans. In addition, use of software templates for analysis and modifications in the staining procedure, which have resulted in decreased instrument associated variance, are discussed.     

Chaotic advection,diffusion, and reactions in open flows

We review and generalize recent results on advection of particles in open time-periodic hydrodynamical flows. First, the problem of passive advection is considered, and its fractal and chaotic nature is pointed out. Next, we study the effect of weak molecular diffusion or randomness of the flow. Finally, we investigate the influence of passive advection on chemical or biological activity superimposed on open flows. The nondiffusive approach is shown to carry some features of a weak diffusion, due to the finiteness of the reaction range or reaction velocity. (c) 2000 American Institute of Physics.     

Measurement and analysis of the infrared absorption spectrum of the 2ν2 band of 12CH4

Approximately 500 infrared absorption lines with room-temperature strengths between 3 × 10^?5 and 1 × 10^?2 atm^?1 were assigned to the 2ν₂ band of ¹²CH₄ in the region from 2930 to 3250 cm^?1. These determine 207 of the 212 upper-state energy levels through J′ = 12 as well as a number of levels with J′ = 13 and 14. All but 17 of the levels with J′ ≤ 12 are calculated to 0.03 cm^?1 or better on the basis of a Hamiltonion that contains Coriolis and Fermi interaction terms coupling the upper states of the five bands, 2ν₄, ν₂ + ν₄, ν₁, ν₃, and 2ν₂.     

Amplitude truncation of Gaussian 1/f(alpha) noises: Results and problems

An interesting property of Gaussian 1/f noise was found experimentally a few years ago: The amplitude truncation does not change the power spectral density of the noise under rather general conditions. Here we present a brief theoretical derivation of this invariant property of band-limited Gaussian 1/f noise and include 1/f(alpha) noises also with 0相似文献   

Chaotic mixing induced transitions in reaction-diffusion systems

We study the evolution of a localized perturbation in a chemical system with multiple homogeneous steady states, in the presence of stirring by a fluid flow. Two distinct regimes are found as the rate of stirring is varied relative to the rate of the chemical reaction. When the stirring is fast localized perturbations decay towards a spatially homogeneous state. When the stirring is slow (or fast reaction) localized perturbations propagate by advection in form of a filament with a roughly constant width and exponentially increasing length. The width of the filament depends on the stirring rate and reaction rate but is independent of the initial perturbation. We investigate this problem numerically in both closed and open flow systems and explain the results using a one-dimensional "mean-strain" model for the transverse profile of the filament that captures the interplay between the propagation of the reaction-diffusion front and the stretching due to chaotic advection. (c) 2002 American Institute of Physics.     

Empirical Line Parameters of H2O near 0.94 μm: Positions, Intensities and Air-Broadening Coefficients

To support the interpretation of remote sensing instruments that use water features near 0.94 μm, the water vapor line parameters have been measured using 16 laboratory spectra of pure water and 9 spectra of water+air mixtures recorded at resolution 0.012 and 0.02 cm⁻¹ with the McMath Fourier transform spectrometer at the National Solar Observatory located at Kitt Peak. Over 3300 line positions and intensities between 9 650 and 11 400 cm⁻¹ and 507 air-broadened widths and 462 pressure-induced shifts in positions between 10 150 and 11 190 cm⁻¹ have been obtained at room temperature for the main isotopomer, H₂¹⁶O. The numbers of width measurements involving the parallel bands are 108 for (121), 192 for (201), and 86 for the (003) upper states; for the perpendicular bands, the numbers of widths are 4 for (022), 78 for (300), and 38 for the (102) upper states. These empirical values have been combined with calculated positions and intensities of H₂¹⁸O and H₂¹⁷O and with available broadening coefficients from other spectral regions to form a new composite database. The sum of 5002 transition intensities for the region is 0.6417 cm⁻²·atm⁻¹ (2.588×10⁻²⁰ cm⁻¹/(molecule·cm⁻²)) at 296 K. This new compilation appears in the 2000 edition of HITRAN.     

Spectral assessment of mesh adaptations for the analysis of the dynamical longitudinal behavior of railway bridges

Summary  Extensive studies, [1], concerning the longitudinal behavior of long railway bridges due to braking forces have been done by measurements in situ, [2], and by statical, [3, 4], as well as dynamical simulations. Thereby, the only consistent numerical realization with respect to the measured data was the dynamical one. However, the consecutive discretizations in space and time with time-dependent system matrices are extremely time consuming due to the moving loads and varying stiffness of the ballast under, and in front of, the moving train. Therefore, every effort should be made to optimize the discretization in the space domain. This paper presents a strategy for assessing the quality of finite elements in space and for applying an adaptive mesh-refinement for this special engineering problem. The method is characterized by a spectral assessment, comparing a certain set of eigenvalues of the actual discretization with those of a very fine and rather exact numerical model. The error estimator introduced in this paper controls a whole set of global eigenvalues with corresponding natural vibration modes in order to assess certain types of shape functions. Thus, the procedure estimates local modifications on the one hand and p-properties on the other by means of global indication. Received 30 August 1999; accepted for publication 26 September 2000