Uncovering individual hydrogen bonds in rotaxanes by frequency shifts.

We present a theoretical investigation of amide pseudorotaxane IR spectra in the harmonic approximation. In particular, we focus on the effect of axle substitution on the hydrogen bonds that are formed between axle and wheel. Two types of pseudorotaxanes are studied: one with the substituent affecting mostly the axle's carbonyl group and one with the effect influencing primarily the amide NH group. Sizeable red shifts are predicted for the carbonyl stretching frequencies, and large red shifts for the NH stretching frequencies. For the wheel amide groups involved in hydrogen bonding merely with their NH hydrogens, a small shift is observed for the carbonyl stretch mode. A clear relation is observed between the NH stretch shifts and individual hydrogen bond energies. This is confirmed by correlations of the shared electron number with the NH stretch shift showing that this quantity can be taken as an indicator for individual hydrogen bond energies. Axle substitution influences the strengths of the individual hydrogen bonds which is again reflected in the NH stretch frequency shifts. A linear relationship of Hammett's substituent parameters with the NH frequency shifts can be established.

Introducing phase transitions to quantum chemistry: from Trouton's rule to first principles vaporization entropies.

In the present study, we employ quantum cluster equilibrium calculations on a small water cluster set in order to derive thermochemical equilibrium properties of the liquid phase as well as the liquid-vapor phase transition. The focus is set on the calculation of liquid phase entropies, from which entropies of vaporization at the normal boiling point of water are derived. Different electronic structure methods are compared and the influences of basis set size and of cooperative effects are discussed. In line with a previous study on the subject [B. Kirchner, J. Chem. Phys. 123, 204116 (2005)], we find that the neglect of cooperativity leads to large errors in the equilibrium cluster populations as well as in the obtained entropy values. In contrast, a correct treatment of the intermolecular many-body interaction yields liquid phase entropies and phase transition entropies being in very good agreement with the experimental reference, thus demonstrating that the quantum cluster equilibrium partition function intrinsically accounts for the shortcomings of the ideal gas partition function often employed in first principles entropy calculations. Comparing the calculated vaporization entropies to the value predicted by Trouton's rule, it is observed that for entropy calculations the consideration of intracluster cooperative effects is more important than the explicit treatment of the intercluster association even in a highly associated liquid such as water. The decomposition of entropy into contributions due to different degrees of freedom implies the need for the accurate treatment of particle indistinguishability and free volume of translation, whereas minor influences should be expected from the vibrational and rotational degrees of freedom and none from the electronic degrees of freedom.

Coupled Cluster in Condensed Phase. Part I: Static Quantum Chemical Calculations of Hydrogen Fluoride Clusters.

A multiscale approach with roots in electronic structure calculations relies on the good description of intermolecular forces. In this study we lay the foundations for a condensed phase treatment based on the electronic structure of hydrogen fluoride on a very high level of theory. This investigation comprises cluster calculations in a static quantum chemical approach employing density functional theory, second order Møller-Plesset perturbation theory (MP2) and the coupled cluster singles, doubles with perturbative triples method in combination with several basis sets as well as at the complete basis set limit. The clusters we considered are up to 12 monomer units large and consist of ring and chain structures. We find a good agreement of the intramolecular distance obtained from the MP2 approach and the largest basis set. The binding energy of the hydrogen fluoride dimer calculated from coupled cluster at the basis set limit agrees excellently with experiment, whereas the calculated frequencies at all levels agree reasonably well with different experimental values. Large cooperative effects are observed for the ring structures as compared to the chain clusters. The energy per monomer unit indicates the most stable structures to be the ring clusters.

Coupled Cluster in Condensed Phase. Part II: Liquid Hydrogen Fluoride from Quantum Cluster Equilibrium Theory.

Treating the bulk phase with high-level ab initio methods, such as coupled cluster, is a nontrivial task because of the computational costs of these electronic structure methods. In this part of our hydrogen fluoride study we make use of the quantum cluster equilibrium method, which employs electronic structure input of small clusters and combines it with simple statistical mechanics in order to describe condensed phase phenomena. If no parameter adjustment is applied, then the lower quantum chemical methods, such as density functional theory in conjunction with the generalized gradient approximation, provide wrong results in accordance with the description of the strength of the interaction in the clusters. While density functional theory describes the liquid phase too dense due to overbinding of the clusters, the coupled cluster method and the perturbation theory at the complete basis set limit agree well with experimental observations. If we allow the two parameters in the quantum cluster equilibrium method to vary, then these are able to compensate the overbinding, thereby leading to very good agreement with experiment. Correlated methods in combination with small basis sets giving rise to too weakly bound clusters cannot reach this accuracy even if the parameters are flexible. Only at the complete basis set limit, the performance of the correlated methods is again excellent.

How can rotaxanes be modified by varying functional groups at the axle?--a combined theoretical and experimental analysis of thermochemistry and electronic effects.

We present theoretically as well as experimentally determined thermochemical data of the non-covalent interactions in different axle-substituted pseudorotaxanes. The overall interaction energy lies in the region of 35 kJ mol(-1), independent of the substitution pattern at the axle. Because rearrangement energies of 7 and 3 kJ mol(-1) are required for wheel and axle, respectively, the sum of the net interactions of individual non-covalent bonds must exceed 10 kJ mol(-1) to achieve a successful host-guest interaction. The geometrical analysis shows three hydrogen bonds, and the close inspection of the individual dipole moments as well as the individual hydrogen bonds reveals trends according to the different functional groups at the axle. The individual trends for the different hydrogen bonds almost lead to a cancellation of the substitution effects. From solvent-effect considerations it can be predicted that the pseudorotaxane is stable in CHCl(3) and CH(2)Cl(2), whereas it would dethread in water. Comparing experimentally and theoretically calculated Gibbs free enthalpies, we find reasonable agreement if an exchange reaction of one solvent molecule instead of the direct formation reaction is considered.