16,689 research outputs found
Interview with William A. Fowler
No full textInterview conducted in eight sessions between May 1983 and May 1984 with Willy Fowler, Nobel laureate and Institute Professor of Physics, emeritus. In a career in nuclear physics and nuclear astrophysics that spanned more that sixty years, Fowler was primarily concerned with nucleosynthesis--that is, the creation of the heavy elements by the fusion of the nuclei of lighter elements. In 1957, with Fred Hoyle and Geoffrey and Margaret Burbidge, Fowler coauthored the seminal paper "Synthesis of the Elements in the Stars," now known as B2FH. In it, they showed that all the elements from carbon to uranium could be produced by nuclear processes in stars starting only with the light elements produced in the Big Bang. In the interview, Fowler discusses his early education as a physicist at Ohio State; his work with Charles C. and Tommy Lauritsen at Caltech's Kellogg Radiation Laboratory; the history of nuclear physics and nuclear astrophysics at Caltech; and the evolution of nucleosynthesis. There are recollections of many of his mentors and colleagues, including Robert A. Millikan, Hans Bethe, J. Robert Oppenheimer, the Lauritsens, Fred Hoyle, the Burbidges, Jesse Greenstein, A. G. W. Cameron, Richard P. Feynman, and H. P. Robertson. A 1986 Supplement contains an interview on Fowler's work for the Naval Bureau of Ordnance and the Manhattan Project during the Second World War
July 5, 1842 letter from Wiley P. Fowler to Littleton Fowler
No full text20.3 x 31.8 cm[Address panel:]
Revd Littleton Fowler
Bowlingreen [sic]
Kentucky
Circular postmark stamped in black ink with “PRINCETON” stamped inside top of circle; “KY.” at bottom; “JUL” in middle and “5” handwritten below. Handwritten postage in upper right corner of address panel: 37 1/2 [cents]
Princeton [Ky], July the 5th 1842
My dear Brother
Enclosed you will find two letters of introduction. Mr Allen is a gentleman of great moral worth, of high talents and standing and has ever been my warm and invaluable friend. There is no man in Ky. I esteem more highly. He is sincere, warm-hearted and devoted. He may perhaps not be in Frankfort. I want you to become acquainted with him.
You will find Gov. Letcher a plain, free, open generous man, full of life, wit and fond of his friends, joke and fun, yet of great firmness of character, well versed in human nature and decidedly one of the first[?] practical men in the state.
The family are well. Cousin Jas.[?Jos] is still confined; his fever is stubborn. I fear he is going to have a severe spell. I have not heard one word from Mrs.Smedly.
I hope you and family are all well.
Your brother truly,
W. P. Fowler
[For one of the enclosures mentioned in this letter, see W. P. Fowler’s letter to C. Allen, of July 5, 1842.
Ring currents in tangentially p-p bonded σ-aromatic systems
No full textWe report a theoretical study of ring systems that delocalize electrons in a cyclic array of p orbitals arranged tangentially in σ-bonding fashion. σ-Bonded arrays are compared to conventional π-bonded analogues with respect to orbital symmetry and aromatic/antiaromatic behavior. In a one-to-one correspondence between π and tangential molecular orbitals of a cycle, local rotation turns each π to a tangential basis function, changing bonding interactions to antibonding and inverting the order of filling of molecular orbitals. The ipsocentric ring-current mapping approach is used to evaluate aromaticity on the magnetic criterion. As for conventional π-ring currents, the σ-ring current in tangential p-p bonded systems is dominated by the HOMO-LUMO transition, corresponding to circulation of four electrons in diatropic (4n + 2)-electron cycles but two in paratropic (4n)-electron cycles. The systems examined here utilize either C 2p or Si 3p orbitals for delocalization. Although interchangeable with C with respect to the fundamental orbital symmetry and ring-current rules, Si bonds at greater internuclear distances, a feature that allows easier design of potentially stable σ-aromatic structures. Calculations show the wheel-like Si 10C50H70 structure 6 as a stable, neutral aromatic molecule with a diatropic ring current following the σ-bond path formed by Si 3p orbitals. © 2006 American Chemical Society
Counter-rotating spin-polarised ring currents in odd-electron carbocycles
No full textWe propose a molecular-orbital model to explain how majority and minority spins in odd-π-electron carbocycles sustain counter-rotating magnetic-field-induced ring currents. The model is based on the ipsocentric approach to magnetic response, in which ring currents are dominated by frontier-orbital contributions obeying angular-momentum selection rules. Coupled unrestricted Hartree-Fock ab initio calculations of the ring-current responses for singly charged benzene and planarised cyclo-octatetraene ions confirm the predictions of the qualitative model, and are consistent with correlated MP2 spin-polarised current calculations. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Elements of Abstraction: Space, Line and Interval in Modern British Art
No full textThe book is the catalogue of the exhibition Elements of Abstraction: Space, Line and Interval in Modern British Art, which the author curated from the collections of the Tate Gallery, London, the Arts Council, London, Southampton City Art Gallery and private collections. The author provided three essays, 'The Geometry of Modern British Art', 'West Country Constructivism', and 'Abstract Art and the Decline of Modernism' to advance critical histories of three distinct moments of importance in the development of British abstract art. A fourth, edited by him, was by a research student under his supervision (Alan Fowler) and covered Systems Art and Constructionism
Ring-current signatures in shielding-density maps
No full textShielding-density functions interpret nuclear magnetic shieldings point-wise, through the Biot–Savart law. For aromatic, antiaromatic,
and non-aromatic p-systems (benzene, cyclooctatetraene, borazine), the characteristic pattern of current density leads
to a distinctive map for the p-contribution to the perpendicular components of 1H shielding-density. Hence, global and localised
currents have distinguishable signatures, dependent on separation between outward and return circulations in the current–density
map. Aromatic, diatropic p-current produces an intense deshielding ipso spike, weakly cancelled by a remote shielding ridge; paratropic
anti-aromatic p-current inverts these features; localised p-currents produce cancelling shielding/deshielding spikes. Ringcurrents
also contribute characteristically to out-of-plane carbon shieldings
Frontier-orbital ring currents and the annulene analogy
No full textPerimeter and ring currents induced in planar monocycles and poly cycles by perpendicular magnetic fields are commonly taken as diagnostic of aromaticity and antiaromaticity in these systems. Diatropic π currents are associated with aromaticity, paratropic with its opposite. The ipsocentric method is an accurate and economical way of calculating ab initio current-density maps. In π systems it provides a natural interpretation of ring currents in terms of nonredundant orbital contributions, governed by simple symmetry rules for π-π* virtual excitations and dominated by the frontier electrons. Thus, in planar [4n + 2] monocycles, the product of π HOMO and π* LUMO symmetries includes that of the in-plane translations and leads to a 4-electron diatropic current. In planar [4n] monocycles, this product includes the symmetry of the in-plane rotation and leads to a 2-electron paratropic current. Perturbation arguments based on the monocycle explain the opposite senses of the π ring currents in naphthalene (diatropic) and pentalene (paratropic) as consequences of their different frontier-orbital symmetry products. In a generalization to heterocycles, ring current maps for benzotriazole (BtH), its conjugate base (Bt-) and the cation (Bt +) are calculated at an ab initio level. The diatropic current of the two 10π systems and the paratropic current of the 8π system are rationalized in terms of the perturbed-annulene orbital model, giving an explanation of the applicability of simple electron counting in these cases: where the frontier-orbital structure remains close to that of the [4n+2]/[4n] annulene, so does the current-density map
Visualising aromaticity of bowl-shaped molecules
No full textSuperposition of slices (planar maps of induced current density calculated within the ipsocentric pseudo-π model of electronic response to a magnetic field) gives a simple route to visual diagnosis of ring-current aromaticity in bowl-shaped molecules. Results are presented for currents in the recently synthesized indenocorannulene precursors of [60]fullerene. © the Owner Societies 2011
Non-linear ring currents: Effect of strong magnetic fields on π-electron circulation
No full textFinite-field calculations of non-linear induced current density in representative [4n + 2] and [4n] π systems, benzene and flat cyclooctatetraene (COT), show that a strong uniform perpendicular magnetic field enhances benzene π-diatropicity and decreases COT π-paratropicity. The non-linear current is stronger by two orders of magnitude in COT, but still small: at 1a 0 height and in a field of 25 T, third-order effects contribute -5 ppm to the first-order π ring current. Classical arguments based on radial contraction of charge density rationalise the non-linear response of benzene, but that of COT depends on a specific orbital-rotation effect, characteristic of paratropic π systems. © 2004 Elsevier B.V. All rights reserved
Aromaticity, polarisability and ring current
No full textElectric dipole polarisability has been proposed as an alternative to magnetic criteria of aromaticity. The ipsocentric formulation of the molecular response to a magnetic field gives selection rules for diatropic ring current that coincide with those for in-plane polarisability. Thus, induced π current strength might correlate, if at all, with HOMO contributions to the in-plane components of polarisability of diatropic systems. No equivalent correlation is expected a priori for anti-aromatic paratropic systems, or clamped systems in which diatropic current is quenched by interaction with π systems. Ab initio calculations of π current density maps and dipole polarisability contributions confirm the expected correlation for monocycles, but show the limitations of polarisability as a measure of aromaticity. © 2003 Elsevier B.V. All rights reserved
- …
