The molecular structure of 1-methyl-1-silacyclohexane 3 has been determined by gas electron diffraction (GED). The conformational preference of the methyl group was studied experimentally in the gas phase (GED) and in solution (low-temperature (13)C NMR) and by quantum chemical calculations (HF, MP2, and B3LYP with 6-31G basis sets and mPW1PW91/6-311G(2df,p)). Both experimental methods result in a preference of the equatorial position of the methyl group, 68(7)% in the gas phase at 298 K and 74(1)% in solution at 110 K. The calculations predict 68-73% equatorial conformer at room temperature. From coalescence temperatures, Gibbs free energies of activation for ring inversion DeltaG++ (eq --> ax) = 5.81(18) and DeltaG++ (ax --> eq) = 5.56(18) kcal mol(-1) were derived. The calculated values for DeltaG++ (eq --> ax) are 5.92 (B3LYP) and 5.84 kcal mol(-1) (mPW1PW91).

Mass spectra were recorded for (2+n) resonance enhanced multiphoton ionization (REMPI) of HCl as a function of resonance excitation energy in the 82 600-88 100 cm(-1) region to obtain two-dimensional REMPI data. Analysis of ion-mass signal intensities for excitations via the F (1)Delta(2)(v(')=0-2) and the V (1)Sigma(+)(v(')) states as a function of rotational quantum numbers in the intermediate states either revealed near-resonance interactions or no significant coupling between the F (1)Delta(2) and the V (1)Sigma(+) states, depending on quantum levels. Ion-signal intensities and power dependence measurements allowed us to propose photoionization mechanisms in terms of intermediate state involvement. Based on relative ion-signal intensities and rotational line positions we quantified the contributions of Rydberg and valence intermediate states to the photoionization product formation and evaluated coupling strengths for state mixing. Time-dependent density functional theory (TD-DFT), equation-of-motion coupled cluster (EOM-CC), and completely renormalized EOM-CC calculations with various basis sets were performed to derive singlet state potential energy curves, relevant spectroscopic parameters, and to calculate spectra. Experimentally observed spectra and older calculations are compared with the reported ab initio results.

(2+1) REMPI spectra of HX (X=Cl, Br and I) have been recorded and analyzed by simulation calculations to derive rotational constants, band origins and isotope shift values for a number of vibrational bands of Ω=0 states. Our data for HCl compare nicely with those derived by Green et al. by conventional analysis methods [D. S. Green et al., J. Mol. Spectrosc. 150, 303, 354, 388 (1991); D. S. Green and S. C. Wallace, J. Chem. Phys. 96, 5857 (1992)]. New spectroscopic parameters were derived for eight vibrational bands which are assigned to the V(1Σ+) state, for v′=4 of the E(1Σ+) state, as well as for five new bands in HBr. New spectroscopic parameters were derived for four vibrational bands which are assigned to the V state and for v′=1 of the E state in HI. Anomalies observed in energy level spacings, rotational parameters and isotope shift values are interpreted as being largely due to homogeneous interactions between the V and the E states. It is argued that the interaction causes a compression of rovibrational levels in the E state manifold but an expansion of levels in the V state manifold, something which might be expected for a Rydberg to ion-pair interaction. Variations observed in the intensity ratio of O and S line series to Q line series in vibrational bands of the E and V states for HCl and HBr are discussed and mechanisms of two-photon excitation processes are proposed.

Mass spectra were recorded for (2+n) resonance enhanced multiphoton ionization (REMPI) of HCl as a function of resonance excitation energy in the 81,710-82,870 cm(-1) region to obtain two-dimensional REMPI data. Small but significant fragmentations and H(+), Cl(+), as well as HCl(+) formations are found to occur after resonance excitations to the triplet Rydberg states f (3)Delta(2)(v(') = 0), f (3)Delta(1)(v(') = 0), and g (3)Sigma(+)(1)(v(') = 0). Whereas insignificant rotational line shifts could be observed, alterations in relative ion signal intensities, due to perturbations, clearly could be seen, making such data ideal for detecting and analyzing weak state interactions. Model analysis of relative ion signal intensities, taking account of the major ion formation channels following excitations to Rydberg states, its near-resonance interactions with ion-pair states as well as dissociations and/or photodissociations were performed. These allowed verification of the existence of all these major channels as well as quantifications of the relative weights of the channels and estimates of state interaction strengths. The proposed mechanisms were supported by ion signal power dependence studies.

͑2ϩ1͒ resonance enhanced multiphoton ionization spectra have been recorded, simulated, and used to derive energies of rovibrational levels in the F(1 ⌬), E(1 ⌺ ϩ), and V(1 ⌺ ϩ) states for HCl ͑H 35 Cl and H 37 Cl) and HBr ͑H 79 Br and H 81 Br). Spectroscopic parameters derived for the F states compare nicely with those derived by others using conventional analysis methods. Clear evidence for near resonance interactions between the F and the V states is seen for the first time, both in HCl and HBr. Shape of curves for rotational level energy spacings versus rotational quantum numbers are found to depend characteristically on the nature of off-resonance interactions observed between the E and the V states. Model calculations for state interactions, based on perturbation theory, are performed for HCl. These prove to be useful to interpret observed perturbations, both qualitatively and quantitatively. Interaction strengths are evaluated for F to V and E to V state interactions. Variations observed in the intensity ratios of O and S line series to Q line series in vibrational bands of the V state for HCl are discussed and mechanisms of two-photon excitation processes are proposed.