Assignment, fit, and theoretical discussion of the v(10) band of acetaldehyde near 509 cm(-1)

The lowest small-amplitude vibration in acetaldehyde (CH(3)CHO) is the in-plane aldehyde scissors mode via at 509 cm(-1). This mode lies about 175 cm(-1) above the top of the barrier to internal rotation of the methyl group and is relatively well separated from other small-amplitude vibrational states (the next fundamental occurring more than 250 cm(-1) higher). It thus provides an excellent example of an isolated small-amplitude fundamental (bright state) embedded in a bath of dark states. Since the bath states at these energies are not too dense, and since they arise purely from states of the large-amplitude torsional vibration of the methyl rotor, a detailed spectroscopic analysis of interactions between the bright state and the bath states should be possible. This paper represents the first step toward that goal. We have assigned several thousand transitions in the v(10) band (J <= 28, K <= 12), and have carried out a simultaneous fit of 2400 of these transitions (J <= 15, K <= 9) with over 8100 transitions to the torsional bath state levels. Three vibration-torsion interactions, which give rise to rather global level shifts of the order of 1 cm(-1) in the v(10) levels, have been identified and quantitatively fit. A number of vibration-torsion-rotation interactions, which give rise to localized (avoided-crossing) shifts in v(10) have also been determined. The present analysis indicates the need for reliable spectroscopic information on more of the torsional bath states in the immediate vicinity of the v(10) levels. Possible ways of obtaining such information in future studies are considered. (C) 2008 Elsevier Inc. All rights reserved

. Rearrangements of Water Dimer and Hexamer

Abstract. Rearrangement mechanisms of the water dimer and the cage form of the water hexamer are examined theoretically with particular reference to tunneling splittings and spectroscopy. The three lowest barrier rearrangements of the water dimer are characterized by ab initio methods and compared with the results of previous constrained calculations. The acceptor-tunneling pathway does not proceed via a direct rotation around the C2 axis of the acceptor, but rather via relatively asynchronous rotation of the donor about the hydrogen bond and an associated ‘wag’ of the acceptor. Rearrangements between different cage isomers of the water hexamer are studied for two empirical potentials. The experimentally observed triplet splittings may be the result of flip and bifurcation rearrangements of the two single-donor, single-acceptor monomers. Twodimensional quantum calculations of the nuclear dynamics suggest that delocalization over more than one cage isomer may occur, especially in excited states