P. Vansteenkiste

Silicalite-1 zeolite exhibits a characteristic pentasil framework vibration around 540−550 cm−1. In the initial stages of zeolite synthesis, however, this band is observed at much higher wavenumbers: literature shows this vibration to depend on particle size and to shift over 100 cm−1 with increasing condensation. In this work, the pentasil vibration frequency was derived from theoretical molecular dynamics simulations to obtain the correct IR band assignments for important nanoparticles. The IR spectroscopic fingerprint of oligomeric five-ring containing precursors proposed in the literature was computed and compared with experimental data. Our theoretical results show that, while isolated five-membered rings show characteristic vibrational bands around 650 cm−1, the combination of five-membered rings in the full MFI-type structure readily generates the bathochromic shift to the typical pentasil vibration around 550 cm−1. As opposed to what was previously believed, the IR band does not shift gradually as nanoparticle size increases, but it is highly dependent on the specific way structural units are added. The most important feature is the appearance of an additional band when double five-membered rings are included, which allows for a clear distinction between the key stages of early zeolite nucleation. Furthermore, the combination of the simulated spectra with the experimental observation of this spectral feature in nanoparticles extracted from silicalite-1 clear solutions supports their structured nature. The theoretical insights on the dependency of pentasil vibrations with the degree of condensation offer valuable support toward future investigations on the genesis of a zeolite crystal.

In this paper, the performance of the one-dimensional hindered rotor approach (1D-HR) is evaluated for n-alkanes with hetero-elements O or S. The internal rotations in these molecules show a behavior distinct from those in n-alkanes, for which 1D-HR is a cost-efficient method to describe the thermochemical features (entropy and heat capacity). It turns out that also for ethers, alcohols, sulfides and thiols this approach gives a satisfactory experimental agreement. This work confirms earlier results, and consolidates the assumption that the 1D-HR model is highly suitable for reproducing thermodynamic properties of single chain molecules, and that multi-dimensional coupled hindered rotor approaches (nD-HR) are not necessarily required for attaining high accuracy. Moreover, it seems that the 1D-HR results are almost independent of the details of the level of theory.

Large-amplitude motions, particularly internal rotations, are known to affect substantially thermodynamic functions and rate constants of reactions in which flexible molecules are involved. Up to now all methods for computing the partition functions of these motions rely on the Pitzer approximation of more than 50 years ago, in which the large-amplitude motion is treated in complete independence of the other (vibrational) degrees of freedom. In this paper an extended hindered-rotor model (EHR) is developed in which the vibrational modes, treated harmonically, are correctly separated from the large-amplitude motion and in which relaxation effects (the changes in the kinetic-energy matrix and potential curvature) are taken into account as one moves along the large-amplitude path. The model also relies on a specific coordinate system in which the Coriolis terms vanish at all times in the Hamiltonian. In this way an increased level of consistency between the various internal modes is achieved, as compared with the more usual hindered-rotor (HR) description. The method is illustrated by calculating the entropies and heat capacities on 1,3-butadiene and 1-butene (with, respectively, one and two internal rotors) and the rate constant for the addition reaction of a vinyl radical to ethene. We also discuss various variants of the one-dimensional hindered-rotor scheme existing in the literature and its relation with the EHR model. It is argued why in most cases the HR approach is already quite successful.

With the aid of density functional theory calculations, all conformers of several single-chain alcohols, thiols, ethers, and sulfides are investigated. Starting from earlier computational works on n-alkanes, we construct an extended set of general rules for predicting the number and occurrence of conformers in these oxygen- or sulfur-containing compounds. In alcohols and thiols, it is found that only the conformers generated by internal rotations in the HXCH2CH2CH2 (X = O or S) top are distinctive from those in n-alkanes. In ethers and sulfides, the primary influence of the heteroelement also extends up to three internal rotations, but many more conformers are possible. However, a number of double gauche sequences are forbidden, and therefore, several conformers can be eliminated. These exclusions in particular make up a set of rules for eventually deducing all possible conformers. Furthermore, on the basis of only an exact calculation of these gg conformations in addition to single gauche conformers, it is possible to make an accurate estimate of the relative energy. This two-dimensional approximation scheme constitutes an effective tool for adequately describing the relative energies of all possible conformers at a minimal computational cost.

In this Letter, we unravel the origin of the good-behavior of the one-dimensional hindered rotor model to describe the partition function and derived thermodynamic properties of n-alkanes. The simplified uncoupled model predicts entropies of n-alkanes up to decane with a standard deviation less than 1% (P. Vansteenkiste, V. Van Speybroeck, G.B. Marin, M. Waroquier, J. Phys. Chem. A 107 (2003) 3139). Application of a fully coupled scheme for the internal rotations present in pentane and hexane gives a justification of the success of the uncoupled hindered rotor model based on microscopic grounds. The success of the separable rotor model is due to fortuitous cancellation of errors and cannot be generalized.