The crystal structure of benzene at — 3°C

Abstract

A new X -ray analysis has confirmed that the space group is Pbca , with four molecules per unit cell, and a = 7⋅460, b = 9⋅666 and c = 7⋅034 Å. 284 structure factors have been measured from oscillation photographs using Cu Kα 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⋅9°. An important consequence of the molecular angular oscillation is that the mean C—C bond length is increased by 0⋅015 Å to 1⋅392 Å, the likely limits o f error being not less than 0⋅010 Å. The result agrees satisfactorily with other determinations o f the C—C distance in benzene, notably Langseth & Stoicheff’s value (1956) of 1⋅397 4 ± 0⋅001 Å from the Raman spectrum of the gas. The carbon ring is accurately planar, individual co-ordinates being only 0⋅0013 Å from the mean plane. The final value of the residual R was 9⋅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 o f 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°K.

The interaction between phenylalanine rings in proteins

“…Whilst these interactions will undoubtedly contribute to stability, as do the interactions between almost all the atoms in the protein's hydrophobic core, our results do not support the idea of a separate class of interaction. Except for a small a 3 different ring-ring interactions occur in the benzene crystal specified as in [4] b 10 of the 162 Phe-Phe interactions involve the C& atom of one Phe ' Lines represent ring-planes edge-on sub-group, with a special disposition of the phenylalanines, the interaction geometry is essentially random. In proteins the preferred 'edge-toface' aromatic-aromatic interaction [7,8] is found, but so are many other less favourable orientations.…”

“…The geometry and energetics of such interactions will be crucial for protein structure, specificity and activity. The classical herringbone pattern observed for benzene in the crystal structure [4] involves perpendicular, cogwheel and parallel interactions between the benzene rings (see table 1). In contrast in B-DNA the purine and pyrimidine bases are stacked with their planes parallel.…”

Influence of rotational motion of molecules on the thermal conductivity of solid SF6, CHCl3, C6H6, and CCl4

“…Solid benzene under the pressure of its own saturated vapor has only one crystallographic modification: it has the orthorhombic spatial symmetry Pbca ( ) D h 2 15 with four molecules per unit cell [9,10]. Benzene melts at 278.5 Ê and the melting-caused change in the entropy is DS f /R = 4.22 [3], which is much higher that the Timmermans criterion for OD phases.…”

Phonon scattering and heat transfer by "diffusive" modes in solid C_{6}H_{6}

“…Solid benzene under the pressure of its own saturated vapor has only one crystallographic modification: it has the orthorhombic spatial symmetry P bca (D 15 2h ) with four molecules per unit cell [3,4]. Benzene melts at 278.5 K and the melting-caused change in the entropy is ∆S f /R =4.22 [5], which is much higher that Timmermans criterion for orientationally disordered phases.…”