The equilibrium structure and potential energy surface of calcium monohydroxide in its ground doublet state, X2Σ+ CaOH, have been determined from large-scale ab initio calculations using the spin-restricted coupled-cluster method, RCCSD(T), with basis sets of quadruple- and quintuple-ζ quality. The vibrational−rotational energy levels of the CaOH and CaOD isotopomers were calculated using the variational method. The spectroscopic constants determined are found to be in remarkably good agreement with experimental data.

In this overview we discuss the vibrational spectrum of phosphaethyne, HCP, in its electronic ground state, as revealed by complementary experimental and theoretical examinations. The main focus is the evolution of specific spectral patterns from the bottom of the potential well up to excitation energies of approximately 25,000 cm(-1), where large-amplitude, isomerization-type motion from H-CP to CP-H is prominent. Distinct structural and dynamical changes, caused by an abrupt transformation from essentially HC bonding to mainly PH bonding, set in around 13,000 cm(-1). They reflect saddle-node bifurcations in the classical phase space--a phenomenon well known in the nonlinear dynamics literature--and result in characteristic patterns in the spectrum and the quantum-number dependence of the vibrational fine-structure constants. Two polar opposites are employed to elucidate the spectral patterns: the exact solution of the Schrödinger equation, using an accurate potential energy surface and an effective or resonance Hamiltonian (expressed in a harmonic oscillator basis set and block diagonalized into polyads), which is defined by parameters adjusted to fit either the measured or the calculated vibrational energies. The combination of both approaches--together with classical mechanics and semiclassical analyses--provides a detailed spectroscopic picture of the breaking of one bond and the formation of a new one.

The six-dimensional potential energy surface of hydrogen peroxide, H2O2, has been determined from large-scale ab initio calculations using the coupled-cluster method, CCSD(T), with the basis set of quadruple-ζ quality, cc-pVQZ. The effects of core-electron correlation on the calculated structural parameters and the torsional potential energy function have been investigated. The anharmonic quartic force field has been determined. The vibrational−rotational energy levels of the molecule have then been calculated using the variational method. The calculated molecular properties are found to be in good agreement with experimental data.

We have made energy-momentum maps for the experimental end-over-end rotational energy and the two-dimensional bending vibrational energy, both of which confirm the dominating effects of nontrivial quantum monodromy in cyanogen isothiocyanate. Accidental resonances in the rotational spectra yield accurate intervals between bending states.

AM1 and PM3 semiempirical calculations are reported for the solvent effects on the tautomeric equilibria of 2-pyridonel2-hydroxypyridine and 4-pyridone/4-hydroxypyridine in the gas phase and solution. The solvent effects on the tautomeric equilibria were investigated by self-consistent reaction field (SCRF) theory implemented in the AMPAC and MOPAC program in two different ways: one in which all the solvent relaxation is included in the quantum mechanics and the total energy must be corrected for the solvent change in energy, method A; and a second in which the quantum mechanics directly includes this term, method B. The calculated (AMl, method A) tautomeric equilibrium constants (log K,) for 2-pyridone in the gas phase, cyclohexane, chloroform, and acetonitrile are -0.3,0.3, 0.8, and 1.3, respectively, in good agreement with the experimental data ( -0.4.0.24.0.78, and 2.17, respectively). For 4-pyridone/4-hydroxypyridine differences between calculated log K, for the gas phase, chloroform and acetonitrile ( -6.0, -2.6, and -1.2, respectively) and experimental data ( < -1, 0.11, and 0.66, respectively) are larger but the experimental values are also less certain. The experimental acetonitrile data are disturbed by specific interactions. An extension of the SCRF for aqueous solutions is reviewed.

Quantum monodromy has a strong impact on the ro-vibrational energy levels of chain molecules whose bending potential energy function has the form of the bottom of a champagne bottle (i.e. with a hump or punt) around the linear configuration. NCNCS, cyanogen iso-thiocyanate, is a particularly good example of such a molecule and clearly exhibits a distinctive monodromy-induced dislocation of the energy level pattern at the bending-rotation energy at the top of the potential energy hump. Indeed, NCNCS [B. P. Winnewisser et al., Phys. Rev. Lett. 2005, 95, 243002] and the water molecule [N. F. Zobov et al., Chem. Phys. Lett. 2005, 414, 193-197] were the first two molecules for which experimental confirmation of quantum monodromy was obtained. We used the fast scan sub-millimetre spectroscopic technique (FASSST) to extend the measurements and spectral analysis to pure rotational transitions (end-over-end) in bending vibrational states lying well above the monodromy point. The analysis of 9204 lines assigned to 7 vibrational states, presented here, shows that the topological properties of the bending potential function are mapped onto every aspect of the ro-vibrational energy levels involving excitation of the quasi-linear bending vibration. In order to model the large amplitude dynamics of such a molecular system, and also to achieve some insight beyond satisfactory parameters for reproducing the spectrum, we used the generalized semi-rigid bender (GSRB) Hamiltonian, which is described in some detail. This Hamiltonian provides a good description of the energy levels over the seven bending states observed, coming close to experimental accuracy. Due to high J values of the measured rotational transitions (J

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