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I. Fankuchen

Bio: I. Fankuchen is an academic researcher. The author has contributed to research in topics: Diffraction & Single crystal. The author has an hindex of 1, co-authored 1 publications receiving 33 citations.

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TL;DR: A low temperature x‐ray diffraction technique, especially suitable to single crystal work, is described, which prevents moisture condensation and permits rotation and oscillation of the specimen while it is being cooled.
Abstract: A low temperature x‐ray diffraction technique, especially suitable to single crystal work, is described. Crystals are grown in glass capillaries on the x‐ray camera. Position and orientation are determined with the aid of a polarizing microscope on the apparatus. A special specimen holder is used which prevents moisture condensation and permits rotation and oscillation of the specimen while it is being cooled.

33 citations


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TL;DR: In this paper, a new X-ray analysis has confirmed that the space group is Pbca, with four molecules per unit cell, and a = 7$\cdot $460, b = 9$\CDot $666 and c = 7
Abstract: A new X-ray analysis has confirmed that the space group is Pbca, with four molecules per unit cell, and a = 7$\cdot $460, b = 9$\cdot $666 and c = 7$\cdot $034 angstrom. 284 structure factors have been measured from oscillation photographs using CuK$\alpha $ radiation. The structure suggested by Cox (1932) was refined by the Fourier transform method and by two-dimensional Fourier and difference syntheses. The co-ordinates derived from the projections were refined by three-dimensional differential syntheses in which isotropic thermal parameters were used and termination of series errors corrected by the backshift method. After five differential cycles, electron density and difference maps calculated in the mean molecular plane revealed appreciable angular oscillations of the molecules about their senary axes. The refinement was continued by means of three-dimensional differential syntheses with anisotropic thermal parameters for the carbon atoms, and difference syntheses computed in the plane of the molecule and adjacent parallel planes. The criterion finally used to determine the anisotropic parameters was the equality of the observed and calculated atomic peak curvatures, the scale factor being adjusted to give the corrected integrated number of electrons in the molecule. The r.m.s. amplitude of angular oscillation about the senary axis is approximately 7$\cdot $9 degrees. An important consequence of the molecular angular oscillation is that the mean C$\chembond{1,0} $C bond length is increased by 0$\cdot $015 angstrom to 1$\cdot $392 angstrom, the likely limits of error being not less than 0$\cdot $010 angstrom. The result agrees satisfactorily with other determinations of the C$\chembond{1,0} $C distance in benzene, notably Langseth & Stoicheff's value (1956) of 1$\cdot $397$_{4}\pm $ 0$\cdot $001 angstrom from the Raman spectrum of the gas. The carbon ring is accurately planar, individual co-ordinates being only 0$\cdot $0013 angstrom from the mean plane. The final value of the residual R was 9$\cdot $9%. During the refinement, an attempt was made to derive an accurate difference synthesis from which conclusions about bonding electrons could be drawn. The experimental data did not prove sufficiently reliable, and only very tentative conclusions have been obtained. Detailed examination of the crystal structure has shown that the molecules pack together like sheets of six-toothed gear wheels. This allows an easy in-phase oscillation of the molecules about their senary axes, and explains the large r.m.s. amplitudes observed. Occasional rotations of molecules in groups are probably responsible for the narrowing of the nuclear magnetic resonance line above 120 degrees K.

328 citations

Journal ArticleDOI
03 Jun 1977-Science
TL;DR: The early work on the molecular structures of boranes by X-ray diffraction led to a reasonable basis for a theory of chemical bonding different from that which is typical in carbon chemistry, and yielded an understanding of the pleasing polyhedral-like nature of these compounds.
Abstract: This year, 1976, the Nobel Prize in Chemistry has been awarded for research in pure inorganic chemistry, in particular the boranes. May I say that I am most pleased and profoundly grateful. My own orientation to this field has been, as it has in all of my studies, the relationships of the chemical behavior of molecules to their three-dimensional geometrical and electronic structures. The early work on the molecular structures of boranes by X-ray diffraction led to a reasonable basis for a theory of chemical bonding different from that which is typical in carbon chemistry, and yielded an understanding of the pleasing polyhedral-like nature of these compounds. Assimilated by the preparative chemists, the principles helped to establish a large body of a hitherto unknown chemistry, which made a reality of the expectation that boron, next to carbon in the periodic table, should indeed have a complex chemistry. In these nearly thirty years both the theoretical and experimental methods have been applied by us and others to areas of inorganic, physical, organic and biochemistry. For examples, these areas include low temperature X-ray diffraction techniques, and the theoretical studies of multicentered chemical bonds including both delocalized and localized molecular orbitals. An early example is extended Huckel theory, originally developed for studies of the boranes, and even now one of the most widely applicable approximate methods for theoretical studies of bonding in complex molecules. More soundly based theories are presently in use by my research students for studying how enzymes catalyze reactions, details of which are based on the three-dimensional structures by X-ray diffraction methods. Besides illuminating particular problems, these developments may contribute toward the redefinition of areas of chemisttry, and thereby broaden the chemist’s view. Our research in the boranes and their related molecular species crosses areas of inorganic, experimental physical, theoretical and organic chemistry, and includes applications in biochemistry. More simply stated, the area is the study of the relationships of molecular structure to function. BORANES, AND EARLY STRUCTURE STUDIES By now, large numbers of chemical compounds related to polyborane chemistry exist: boron hydrides, carboranes, metalloboranes, metallocarboranes, mixed compounds with organic moieties, and others. These discoveries of preparative chemists are relatively recent. Long ago, Alfred Stock established borane chemistry. He developed the experimental techniques which were required

188 citations

Journal ArticleDOI
TL;DR: In this article, it was shown that a simple model provides the right order of magnitude for the heat of dimerization and indicates the relative importance of Al−C and Al−Al bonds.
Abstract: The trimethylaluminum dimer has a bridge structure with a skeletal symmetry of D2h within the accuracy of a structure determination by x‐ray diffraction of single crystals. Bridge bonds, Al–C, equal 2.24A, vs Al–C bonds of 1.99A for the exterior, ``normal'' methyls. The bridge angle Al–C–Al=70°, the exterior angle C–Al–C=124°.It is shown that a simple model provides the right order of magnitude for the heat of dimerization and indicates the relative importance of Al–C and Al–Al bonds. It is suggested that the sharp bridge angle required for good bridge bonding leads to metal‐metal repulsions which should increase as the size of the metal increases. This probably plays a role in the instability and nature of polymers of the heavier trialkyl‐metal compounds.

125 citations

Journal ArticleDOI
01 Jan 1954-Nature
TL;DR: In this article, the crystal structure of benzene at −3°C was determined and it was shown that the normals to the planes of the molecules are roughly perpendicular to the b-axis and inclined at about 50° to the c-axis.
Abstract: A PRELIMINARY determination1 of the crystal structure of benzene at −22° C. showed that the crystals are orthorhombic. space-group Pbca, with four centro-symmetrical molecules in the unit cell, and that the normals to the planes of the molecules are roughly perpendicular to the b-axis and inclined at about 50° to the c-axis. We have now completed the first stage of refinement of the structure at −3° C. Crystals were grown in Lindemann glass tubes of wall thickness 0.05 mm. and bore 0.36 mm. by the method of Keesom and Taconis2 as modified by later workers3. 288 independent reflexion intensities representing 56 per cent of the total possible with copper radiation were estimated visually from oscillation photographs about all three principal axes. Chromium-K radiation was used to obtain accurate values of the unit cell dimensions by the Straumanis method; the revised dimensions are a = 7.460, b = 9.666 and c = 7.034 A.

66 citations

Journal ArticleDOI
TL;DR: In this article, the crystal structure of the interhalogen group of chlorine trifluoride has been determined at −120°C and the molecular configuration is identical in the two cases.
Abstract: A technique has been developed for handling the extremely reactive interhalogen compounds which makes it possible to investigate their structures by x‐ray diffraction at low temperatures. As the first step in a program of study of the interhalogen group the crystal structure of chlorine trifluoride has been determined at −120°C. The chlorine trifluoride molecule is planar with the point group symmetry mm. The Cl atom is bonded to one F atom at 1.621A and to two F atoms at 1.716A. The F–Cl–F bond angle is 86°59′. In the succeeding pages of this journal, a parallel investigation of the vapor phase by microwave spectroscopy is presented by Dr. D. F. Smith. The molecular configuration is identical in the two cases.

52 citations