Structure and Molecular Correlation of Liquid Alkanes

Abstract

The paper reports structural investigations of a series of liquid a l k a n e s , f r o m n -C 6 H 1 4 ( n -h e x a n e ) t o n -C 1 6H 3 4 ( n -h e x a d e c a n e ) , d i c h e r i n g b y t w o methyl groups (CH2)2, n-C 20 H 42 (n-eicosane) and n-C30H62 (n-triacontane) performed in temperatures from 20°C to 70°C. The average angular distribution of intensity was determined by dichraction of monochromatic X-ray radiatioii Mo Kα , λ = 0.7107 Α and the functions of radial distribution of electroii density were calculated for the studied compounds. Mean structural parameters including distances between molecules and atks, coordination numbers as well as correlation parameters were obtained. The suitability of the method for structural studies of n-alkanes with the number of carbon atoms from 6 to 30 is discussed.PACS numbers: 61.25.Εm I n t r o d u c t i o nHexane was the first aliphatic liquid studied by X-ray methods [1]. In its X-ray diffraction [2,3] and electron diffraction [4] studies it was established that the mean distance between carbon atoms in the chain was C(1)-C(2) = 1.53 ± 0.01 Α and the distance C(1)-H(11) = 1.12 λ. So far X-ray diffraction studies of liquid n-alkanes have been reported by Hewlett [5] This paper presents results of X-ray structural investigations of eight n-alkanes (Tables I and II) with an even number of carbon atoms. Such n-alkanes as n-tetradecane, n-hexadecane and n-triacontane have not been studied by X-ray diffraction yet [12]. Alkanes have practically zero dipole moment (μ = 0) [13] and their centre of symmetry is between the middle carbon atoms.(1089)

Molecular Correlations in Liquid mono- and Dlichloroalkanes

“…Measurements were carried out for the angles 3° < ^9 < 60° at every 0.2°( where 29 is the scattering angle) by the transmission method [3,4] with the incident and diffracted beams symmetric to each other with respect to the surface of the flat sample. The scattered radiation was detected by a VA-G-120 proportional counter coupled with a pulse detector and a line printer.…”

The Role of Intermolecular Interactions in the Electro-Optical Kerr Effect in Liquid Alkanes

“…Liquids possess local order [48,49] as evidenced by X-ray diffraction in particular [17,18,50,51], revealing liquid local order that closely resembles the crystal structure. For example, n-pentane shows both a parallel local order, where the molecules are arranged in the same direction and are shifted along the molecular axis by about 2.5Å with respect to each other, and a perpendicular local structure resembling letter T [18]. In addition to the local order, and in contrast to crystals, molecules in liquid participate in collective rotations [52].…”

“…If r ii is the radius of the cavity occupied by a single molecule in the liquid, the magnitude of R ii is twice the radius R ii = 2r ii . The X-ray studies of liquid n-alkanes [18] indicate that the average distance between the molecules in the first coordination sphere equals 5.38Å. This value was used in the calculation of E (2), (4), (6), (7).…”

Density, Enthalpy of Vaporization and Local Structure of Neat N-Alkane Liquids

“…These molecules have general molecular formulas of the form CH 3 -(C n-2 H 2n-4 )-CH 3 or more compactly C n H 2n+2 , where n is the number of carbon atoms in the molecule. The structure of n-alkanes was the topic of some of the first X-ray diffraction studies of aliphatic molecules in liquids [14]. These studies helped establish and inform our understanding of molecular structure, like bond lengths, angles, and dihedral angles.…”