Conformational energies of perfluoroalkanes. Part 1.—Semi-empirical calculations

Abstract

Conformational energies have been calculated as functions of the internal rotation angles about the C-C bonds for n-perfluorobutane in the neighbourhood of its trans and gauche conformations, and for polytetrafluoroethylene (FTFE) in the vicinity of its all-trans form. The energy equation consists of a semi-empirical " 6-exp " potential for the interactions between non-bonded atoms, a dipole-dipole interaction term, and a three-fold " intrinsic " torsion potential having a barrier height Eo for each bond undergoing rotation. The various parameters were adjusted to reproduce CONFORMATIONAL ENERGIES OF (CF,), Jernigan and Flory,lo and Scott and Scl~eraga,~~ for example, provide detailed information about the intramolecular energies of polymethylene and the lower n-alkanes. Iwasaki,ll De Santis, Giglio, Liquori and Ripamonti,12 and McCullough and Mchlahon l 3 calculated intramolecular energies of PTFE as a function of rotation angle about the CF,-CF2 bonds. The effect of varying the parameters in the semiempirical expressions was not investigated, however, nor were energies calculated in the vicinity of the gauche minima of n-perfluorobutane. McCullough and McMahon l 3 and De Santis and co-workers l2 considered. contributions to the intramolecular energy from attractive and repulsive interactions between non-bonded atom pairs, while Iwasaki l1 included an additional contribution from dipole-dipole interactions between the C-F bonds. The inclusion of this latter term would seem to be desirable and has been taken into account in the present calculations. In the earlier calculations,ll-l the ecergy equations did not include a contribution from an intrinsic torsion-potential term. The effect of including such a term in the semiempirical energy expression for the perfluoroalkanes should be investigated, in view of its importance in the expressions for the n-alkane~.~. 4* 6-10 INTRAMOLECULAR ENERGY

Structural studies onA-cation-deficient perovskite-related phases. II. Microdomain formation in ThNb4O12

“…That our equation (1) describes the disorder in Teflon fairly well is further supported by modelpotential-energy calculations (DeSantis, Giglio, Liquori & Ripamonti, 1963;McMahon & McCullough, 1965;Bates, 1967;Bates & Stockmayer, 1968) for free filaments which yield a relatively shallow potentialenergy well for rotations about the C-C bond. For T = 300 K, a shift of kT from the most stable bond angle (about 165 ° ) appears to be equal to about +8 ° (McMahon & McCullough, 1965, Fig.…”

Monte Carlo simulation of a solvated ionic polymer with cluster morphology

“…Although the conformational characteristics of PTFE or Teflon (the predominant structure in the Nafion backbone) are well represented by a three-state model with rotation angles of one trans (φ = 0 o ) and two gauche (φ = ±115 o ) conformations, it has been shown that a four-state model is more physically accurate [3]. Therefore, this simulation adopts the four-state model with rotation angles of two trans (φ = ±15 o ) and two gauche (φ = ±120 o ) conformations.…”

“…The values of σ = 0.2, σ = 2.0, ω = 0.2, and β = 0.5 were identified by comparing the mean of the range for each of the values provided in [3] to place the second bond of the backbone, U 3 to place the third bond of the backbone, and U N to place the last bond of the entire backbone chain. Hence U k is used to place bonds 4…”

“…The first-neighbor dependence was incorporated into this simulation by invoking a statistical weight parameter u(α, η, j) = u α,η,j where α is the state of the preceding bond, η is the state of the current bond, and j is the location of the bond within the molecular chain. Therefore, adapting from the values for the statistical weight parameters arrived at by Bates and Stockmayer in [3], we employ the matrix U j = [u α,η,j ] for bond j, where…”

“…Each iteration of the simulation produces a single Nafion molecule backbone within a cubic grid of dimension (5000Å) 3 . The starting point of each chain is randomly specified and the backbone is dynamically constructed according to a commonly assumed geometry and under the restrictions that it cannot exceed the bounds of the initial grid or collide with the embedded clusters.…”