Exploring the Structural Properties of Simple Aldehydes: A Monte Carlo and Small-Angle X-ray Scattering Study

Abstract

The structure of simple linear alkanals from propanal to nonanal was studied utilizing configurational bias Monte Carlo (MC) simulations of the aldehydes modeled according to the transferable potential for phase equilibria-united atom force field (TraPPE-UA) and was compared to experimental small-angle X-ray scattering (SAXS) results. This was done by exploiting a recently developed approach for calculating the scattering intensities from theoretically obtained MC data by utilizing the Debye equation (Tomsic et al. J. Phys. Chem. B 2007, 111, 1738). Similar calculations were also performed utilizing a well-established approach based on the reciprocal lattice. Comparison of the calculated scattering data with the experimental SAXS results in the first instance revealed information on the molecular organization in simple aldehydes and in addition also served as a good structural test of the TraPPE-UA force field used to model the aldehydes studied. However, it turned out that such a structural test is a rather strict test for the model which otherwise showed good agreement with the experimental data from the thermodynamic point of view.

The complemented system approach: A novel method for calculating the x-ray scattering from computer simulations

“…Equation (10) reveals very clearly the origin of the unwanted truncation effects that arise when calculating the scattering from simulation data via the Debye equation; failing to take into account the terms with R jk > r c (where r c is the cut-off distance, usually half of the simulation cell's side length) is equivalent to setting all pair distribution functions g AB (r) to zero for r > r c . From the viewpoint of a particle in the centre of the cell, the density at large distances does not tend towards its average value within the system, but instead drops off sharply to zero when crossing the boundary of the cell.…”

X‐Ray Diffraction and Computer Simulation Studies of the Structure of Liquid Aliphatic Aldehydes: From Propanal to Nonanal

“…The simulations were carried out using the OPLS‐AA (“Optimized Potentials for Liquid Simulations – All Atom”) force field . In this model all atoms, including hydrogens, are treated explicitly; this is another difference from the previous structure study on alkanals . Interactions between atoms are described as the sum of two contributions: the non‐bonded and bonded interactions.…”

“…[16] In this model all atoms, including hydrogens, are treated explicitly; this is another difference from the previous structure study on alkanals. [2] Interactions between atoms are described as the sum of two contributions: the non-bonded and bonded interactions. The former consists of the pairwise additive Coulomb potential, accounting for electrostatics, and the 12-6 Lennard-Jones (LJ) potential for the van der Waals interactions (Equation [1]):…”

“…The intramolecular (bonded) forces considered here are the bond stretching (2-body), angle bending (3-body), and the dihedral angle torsion (4-body) interactions. Bond stretching between two covalently bonded i and j atoms is represented by a harmonic potential (Equation [2]):…”

“…They are rather basic and important organic compounds in the organic chemistry educational curriculum and are exploited in numerous practical and scientific applications (see ref. and references therein). Some of the simplest aldehydes, as formaldehyde (metanal, HCHO, one carbon atom), acetaldehyde (etanal, CH 3 CHO, two carbon atoms), and butyraldehyde (butanal, CH 3 CH 2 CH 2 CHO, four carbon atoms), are produced by the chemical industry on the scale of million tons/year .…”