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In this work we show how the VSCF method may be successfully used to describe all fundamental vibrational transitions of several isotopologs of water dimer. By expressing the normal mode displacements in terms of appropriate delocalized internal coordinates we are able to minimize the mode-mode coupling in the PES and thus yield PT2-VSCF frequencies in good agreement with the experiment. The use of curvilinear normal modes is of paramount importance to describe vibrational transitions of the very soft intermolecular modes. Within our approach the maximum calculated error for the (H 2 O) 2 intermolecular frequencies are reduced from 311 cm −1 (Cartesian normal modes) to just 56 cm −1 (curvilinear normal modes). Plots of the diagonal intermolecular potential and of the vibrational wave function illustrate the remarkable effect of different coordinate systems. In conclusion, our PT2-VSCF calculations provide a fair anharmonic description of the fundamental transitions of water dimers.

The intrinsically elusive concepts of electronic “delocalization” and “chemical resonance” are briefly reviewed emphasizing their connection with Spin-Coupled (SC) descriptions of electronic structure. Multiconfiguration Spin-Coupled (MC-SC) calculations are performed to describe the three-center two-electron (3c-2e) bonding in some representative carbenium and nonclassical carbonium ions. Within the MC-SC approach, it is found that these cations present significant electronic energy stabilization when described by more than one valence SC spatial orbital configuration. It is shown that it is necessary to have a superposition of two chemical structures to completely span the orbital valence space of these cations. Two characteristic bonding themes are clearly distinguished. One specific to allyl-type carbenium ions and another specific to the nonclassical carbonium ions. In both situations, the 3c-2e bond is described by two chemical structures. The 3c-2e bond present in these carbocations is described clearly within this conceptual framework. The results point out for the robustness of the Spin-Coupled description in yielding a general picture of bonding, even when considering valence-bond type multiconfiguration effects.

N-Heptane and 2,2,4-trimethylpentane (isooctane) are the key species in the modeling of ignition of hydrocarbon-based fuel formulations. Isooctane is knock-resistant whereas n-heptane is a very knock-prone hydrocarbon. It has been suggested that interconversion of their associated alkylperoxy and hydroperoxyalkyl species via hydrogen-transfer isomerization reaction is the key step to understand their different knocking behavior. In this work, the kinetics of unimolecular hydrogen-transfer reactions of n-heptylperoxy and isooctylperoxy are determined using canonical variational transition-state theory and multidimensional small curvature tunneling. Internal rotation of involved molecules is taken explicitly into account in the molecular partition function. The rate coefficients are calculated in the temperature range 300-900 K, relevant to low-temperature autoignition. The concerted HO2 elimination is an important reaction that competes with some H-transfer and is associated with chain termination. Thus, the branching ratio between these reaction channels is analyzed. We show that variational and multidimensional tunneling effects cannot be neglected for the H-transfer reaction. In particular, the pre-exponential Arrhenius fitting parameter derived from our rate constants shows a strong dependence on the temperature, because tunneling increases quickly at temperatures below 500 K. On the basis of our results, the existing qualitative model for the reasons for different knock behavior observed for n-heptane and isooctane is quantitatively validated at the molecular level.