The molecular structure of di-t-butylamine, NH[C(CH3)3]2, as determined by gas-phase electron diffraction: an exceptionally sterically hindered amine

Gaseous di-t-butylamine, NHBu^t₂, has been studied by electron diffraction at a nozzle temperature of ca. 293 K. The diffraction data reveal that this amine is exceedingly hindered. Repulsive steric interactions between the Bu^t groups are mainly relieved by an opening of the CNC valence angle to 135(3)°. Evidence of steric strain may also be found in the non-zero tilts (2–3°) of the Bu^t groups, defining the angle of rotation of these groups about axes through the N-bonded carbon atoms orthogonal to the NC₂ plane. The N---C and C---C bonds, 1.467(13) and 1.561(6) Å, respectively, are on the other hand relatively unstrained. Other principal geometrical parameters are unexceptional: C---H = 1.103(10) Å, NCC = 111.3(1.5)° (mean value) and CCH = 106(2)°. The position of the N-bonded hydrogen atom relative to the NC₂ plane could not be determined. The torsional positions of the Bu^t groups, with respect to the N---C bonds, could not be derived from least-squares refinements. A large number of models with Bu^t groups in various fixed positions were considered; the best fit between the observed and theoretical intensity data was obtained with one Bu^t group being twisted 19° away from the position typified by one C---C bond of this group being anti to the remote N---C bond, and the other Bu^t group similarly twisted by 30°. When viewed along the N---C bonds, the Bu^t groups are twisted in the same direction. Viewed along the C(N)C axis, these groups come close to being staggered with respect to each other. The values of the above bond distances are those of r_a parameters. Parenthesized values denote error limits, being least-squares standard deviations multiplied by a factor of three.     

Theoretical study of the accuracy of the elution by characteristic points method for bi-langmuir isotherms

The bi-Langmuir equation has recently been proven essential to describe chiral chromatographic surfaces and we therefore investigated the accuracy of the elution by characteristic points method (ECP) for estimation of bi-Langmuir isotherm parameters. The ECP calculations was done on elution profiles generated by the equilibrium-dispersive model of chromatography for five different sets of bi-Langmuir parameters. The ECP method generates two different errors; (i) the error of the ECP calculated isotherm and (ii) the model error of the fitting to the ECP isotherm. Both errors decreased with increasing column efficiency. Moreover, the model error was strongly affected by the weight of the bi-Langmuir function fitted. For some bi-Langmuir compositions the error of the ECP calculated isotherm is too large even at high column efficiencies. Guidelines will be given on surface types to be avoided and on column efficiencies and loading factors required for adequate parameter estimations with ECP.     

The molecular structure of gaseous bis(hexamethydisilylamido)mercury(II), Hg{N(Si(CH3)3)2}2, as determined by electron diffraction

Gaseous bis(hexamethydisilylamido)mercury(II), Hg{N(SiMe₃)₂)₂}₂, has been studied by electron diffraction at a nozzle temperature of ca 390 K.

The diffraction data are consistent with a model consisting only of monomers. By assuming the NHgN chain to be linear and the HgHSi₂ fragments to be planar, an equilibrium conformer with a staggered Si₂NHgNSi₂ skeleton of D_ad-symmetry may be brought into a nice agreement with the observed diffraction data. The relatively large value of the vibrational amplitude of the inter-ligand SiSi distance, 0.26(12) A, indicates that the ligands undergo large amplitude vibrations about the NHgN axis. Steric considerations as well as the magnitude of the rotational barrier as estimated from the diffraction data (ca. 2 kcal mol⁻¹) show that this motion is hindered. A model with an eclipsed, co-planar Si₂NHgNSi₂ backbone of D_adsymmetry could not satisfactorily be brought into agreement with the observed diffraction data.

The values of some relevant key-parameters are: r_a(Hg---N) = 2.01(2) A, r_a(Si---N) = 1.732(9) A, r_a(Si---C) = 1.883(6) A;HgNSi = 116.0(1.0)°, SiNSi = 128.0(2.0)°, NSiC= 111.8(1.2)° and SiCH = 111.0(2.0)°. The trimethylsilyl groups are twisted 25(3)° away from their references positions typified by one Si---C bond of each such group eclipsing the adjacent Hg---N bond, in such a way that the overall symmetry of the model is lowered from D_ad to S₄.     

Speckle interferometry for measurement of continuous deformations

Improvements of a method for measurement of continuous displacements and deformations with digital phase shifting speckle pattern interferometry are presented. The method is based on an algorithm that, with the knowledge of the initial phase, only needs one image at a time to evaluate continuos phase changes due to object deformations. In the improved method, the initial random phase of the speckle pattern is evaluated using a number of phase-shifted images before the deformation under study. This is used for increasing the accuracy of the initial phase estimation and reducing influences from image noise and other measurement disturbances. The phase-shifted speckle patterns are used as references for comparison with the speckle patterns of the deformed object, thereby increasing the reliability and accuracy of the phase estimations of the deformed patterns. The technique can be used for measuring deformations such as transients and other dynamic events, heat expansion as well as other phenomena where it is difficult to accomplish phase shifting during deformation.     

Thermodynamic characterization of the adsorption of selected chiral compounds on immobilized amyloglucosidase in liquid chromatography

Immobilized amyloglucosidase was used as a chiral stationary phase (CSP). First, the retention and enantioselectivity of several model chiral amines and acids were investigated. We found that this CSP was unable to separate the enantiomers of acids, though all selected amines could be resolved. The adsorption of (R)- and (S)-propranolol and its influence on column temperature and 2-propanol content in the eluent were then studied in detail, using a three-step methodology. The adsorption was first evaluated using Scatchard plots; thereafter, the adsorption was characterized in detail by calculating the adsorption energy distribution. With this model-independent information, a better judgment could be made of the possible adsorption models selected in the last step, the model fitting to the data. In the case examined, the bi-Langmuir model (containing nonselective and enantioselective sites) describes the system well. The retention of (R)- and (S)-propranolol at low temperatures increases with the content of 2-propanol in the eluent, due to the increased saturation capacity of the enantioselective sites. The retention is an enthalpy-driven process at both types of sites, whereas the enantioseparation is due to differences between the entropy changes of the two enantiomers at the enantioselective sites. The enthalpy of adsorption at the nonselective sites is almost identical at the two concentrations of 2-propanol in the eluent. Enantioselective adsorption, on the other hand, is more exothermic at higher modifier content (20%). Thus, at high temperatures the retention decreases with increasing modifier content, whereas the opposite (unusual) trend is the case at low temperatures.     

Evaluation of a combined linear–nonlinear approach for column characterization using modern alkaline‐stable columns as model

This study investigates if deeper understanding is achieved when combining nonlinear and linear chromatographic column characterization methods. As test systems, two hybrid columns (Phenomenex Gemini‐NX C₁₈ and Kromasil Eternity C₁₈) and one classic one (Kromasil‐C₁₈) were selected. The nonlinear methods were based on firm adsorption theory and involved determination of adsorption isotherms followed by calculations with a new numerical tool, adsorption energy distribution, on probe components at different pH values. The linear methods involved the hydrophobic subtraction model and selected probe components retention factors as a function of pH. The combined analysis indicated that both complementary and confirmative information can be achieved regarding the actual model systems.     

Combination of schlieren and pulsed TV holography in the study of a high-speed flame jet

This paper presents studies of the propagation of a high-speed turbulent flame jet of an air/hydrogen gas mixture. The experimental results are recorded with the schlieren and the pulsed TV holography method. These methods are compared and combined to benefit from the advantages of each of them. Abel inversion has been used to achieve three-dimensional information i.e. refractive index distributions. Evaluation, calibration and Abel inversion algorithms are described together with experimental results. The results obtained from the different techniques show remarkable similarities concerning both qualitative and quantitative aspects.