Tunneling motion and splitting in the CH2OH radical: (Sub-)millimeter wave spectrum analysis.

The (sub-)millimeter wave spectrum of the non-rigid CH2OH radical is investigated both experimentally and theoretically. Ab initio calculations are carried out to quantitatively characterize its potential energy surface as a function of the two large amplitude ∠H1COH and ∠H2COH dihedral angles. It is shown that the radical displays a large amplitude torsional-like motion of its CH2 group with respect to the OH group. The rotation-torsion levels computed with the help of a 4D Hamiltonian accounting for this torsional-like motion and for the overall rotation exhibit a tunneling splitting, in agreement with recent experimental investigations, and a strong rotational dependence of this tunneling splitting on the rotational quantum number Ka due to the rotation-torsion Coriolis coupling. Based on an internal axis method approach, a fitting Hamiltonian accounting for tunneling effects and for the fine and hyperfine structure is built and applied to the fitting of the new (sub)-millimeter wave transitions measured in this work along with previously available high-resolution data. 778 frequencies and wavenumbers are reproduced with a unitless standard deviation of 0.79 using 27 parameters. The N = 0 tunneling splitting, which could not be determined unambiguously in the previous high-resolution investigations, is determined based on its rotational dependence.

Experimental and theoretical threshold photoelectron spectra of methylene.

The threshold photoelectron spectrum of methylene (CH2), produced by consecutive H atom abstractions on methane, has been recorded using synchrotron radiation. The experimental spectrum spans the region of the X + 2Π u ← X 3 B 1 ionizing transition. It is modeled starting from ab initio bending potentials and using the bending approach introduced by Coudert et al. [J. Chem. Phys. 148, 054302 (2018)] accounting for the quasilinearity of CH2 and the strong Renner-Teller interaction in CH 2 + . This first calculation yields a theoretical threshold photoelectron spectrum which is in moderate agreement with the experimental one. A more accurate approach treating the three vibrational modes is developed for computing the threshold photoelectron spectrum of triatomic C 2 v molecules. This new treatment is tested modeling the already measured threshold photoelectron spectrum of the X + 2Π u ← X 1 A 1 ionizing transition of the water molecule. The threshold photoelectron spectrum of CH2 computed with the new approach compares more favorably with the experimental spectrum and yields an adiabatic ionization potential of 10.386(6) eV.

Optimal orientation of an asymmetric top molecule with terahertz pulses.

Terahertz pulses effects are investigated in an asymmetric top C2v molecule using numerical simulations. The average value of the direction cosine ΦZx is computed solving the time dependent Schrödinger equation for several types of pulses. The H2S molecule taken as a test case is first subject to two short terahertz pulses with a duration smaller than 5 ps, an identical maximum value of the electric field of 2 MV/cm, but a different shape. The thermal average ⟨⟨ΦZx⟩⟩ is calculated for several temperatures, and non-periodic time variations are found even for the lowest temperature. For a given temperature, the maximum orientation achieved is shown to be dependent on the overlap between the absorption spectrum of the molecule and the Fourier transform of the pulse. The maximum orientation is also shown to be closely related to the molecular energy increase. In a second step, the optimal control theory is used to build a 14 ps long few-cycle pulse with the same maximum value of the electric field allowing us to reach a large maximum value of ⟨ΦZx⟩ equal to 0.93. A fairly good understanding of the wavefunction describing the molecule after the pulse was achieved.

Spin-torsion effects in the hyperfine structure of methanol.

The magnetic hyperfine structure of the non-rigid methanol molecule is investigated experimentally and theoretically. 12 hyperfine patterns are recorded using molecular beam microwave spectrometers. These patterns, along with previously recorded ones, are analyzed in an attempt to evidence the effects of the magnetic spin-torsion coupling due to the large amplitude internal rotation of the methyl group [J. E. M. Heuvel and A. Dymanus, J. Mol. Spectrosc. 47, 363 (1973)]. The theoretical approach setup to analyze the observed data accounts for this spin-torsion in addition to the familiar magnetic spin-rotation and spin-spin interactions. The theoretical approach relies on symmetry considerations to build a hyperfine coupling Hamiltonian and spin-rotation-torsion wavefunctions compatible with the Pauli exclusion principle. Although all experimental hyperfine patterns are not fully resolved, the line position analysis yields values for several parameters including one describing the spin-torsion coupling.

Analysis of the microwave, terahertz, and far infrared spectra of monodeuterated methanol CH2DOH up to J = 26, K = 11, and o(1).

The first theoretical approach aimed at accounting for the energy levels of a non-rigid molecule displaying asymmetric-top asymmetric-frame internal rotation is developed. It is applied to a line position analysis of the high-resolution spectrum of the non-rigid CH2DOH molecule and allows us to carry out a global analysis of a data set consisting of already available data and of microwave and far infrared transitions measured in this work. The analysis is restricted to the three lowest lying torsional levels (e0, e1, and o1), to K ⩽ 11, and to J ⩽ 26. For the 8211 fitted lines, the unitless standard deviation is 2.4 and 103 parameters are determined including kinetic energy, hindering potential, and distortion effects parameters.

Rotation-induced breakdown of torsional quantum control.

Control of the torsional angles of nonrigid molecules is key for the development of emerging areas like molecular electronics and nanotechnology. Based on a rigorous calculation of the rotation-torsion-Stark energy levels of nonrigid biphenyl-like molecules, we show that, unlike previously believed, instantaneous rotation-torsion-Stark eigenstates of such molecules, interacting with a strong laser field, present a large degree of delocalization in the torsional coordinate even for the lowest energy states. This is due to a strong coupling between overall rotation and torsion leading to a breakdown of the torsional alignment. Thus, adiabatic control of changes on the planarity of this kind of molecule is essentially impossible unless the temperature is on the order of a few Kelvin.

Analysis of the torsional spectrum of monodeuterated methanol CH2DOH.

An analysis of the torsional spectrum of monodeuterated methanol CH(2)DOH is presented. Twenty nine torsional subbands have been assigned in the 20-800 cm(-1) region. The newly assigned subbands and those already available in the literature were analyzed with a theoretical approach accounting for internal rotation of an asymmetrical CH(2)D methyl group. Seventy six subband centers were reproduced with an rms value of 0.09 cm(-1). Spectroscopic parameters corresponding to the generalized inertia tensor and to the hindering potential were determined as well as rotation-torsion distortion constants.

Paired hydrogen bonds in the hydrogen halide homodimer (HI)2.

The HI homodimer was found to have structural and vibrational properties unlike any other previously studied (HX)(2) system, with X = F, Cl, and Br. The infrared spectrum of (HI)(2) is also observed to be distinctly different from the other members of the series. In addition, the interaction energy of the (HI)(2) dimer has been calculated using the coupled-cluster with singles, doubles, and perturbative triples [CCSD(T)] level of theory. A four-dimensional morphed intermolecular potential has been generated and then morphed using available near infrared and submillimeter spectroscopic data recorded in supersonic jet expansions. The morphed potential is found to have a single global minimum with a symmetric structure having C(2h) symmetry. The equilibrium dissociation energy is found to be 359 cm(-1) with the geometry in Jacobi coordinates of R(e) = 4.35 Å, θ(1) = 43°, θ(2) = 137°, and φ = 180°. The infrared spectrum is characterized by pairs of excited vibrational states resulting from the coupling of the two HI stretching modes. A qualitative model using a quadratic approximation has been fitted to obtain an estimate of this coupling. Furthermore, a morphed intermolecular potential for the vibrationally excited system was also obtained that gives a quantitative estimate of the shift in the potential due to the excitation. The submillimeter analysis is consistent with a ground state having its highest probability as a paired hydrogen bond configuration with R(0) = 4.56372(1) Å and an average angle θ=cos(-1)((1/2)) = 46.40(1)° (between the diatom center of mass∕center of mass axis and direction of each component hydrogen iodide molecule). On monodeuteration, however, the ground state is predicted to undergo an anomalous structural isotope change to an L-shaped HI-DI structure with highest probability at R(0) = 4.51 Å, θ(1) = 83°, θ(2) = 177°, and φ = 180°. These results provide a test for large scale ab initio calculations and have implications for the interpretation of photoinduced chemistry and other properties of the dimer.

Magnetic hyperfine coupling of a methyl group undergoing internal rotation: a case study of methyl formate.

The hyperfine structure of methyl formate was recorded in the 2-20 GHz range. A molecular beam coupled to a Fourier transform microwave spectrometer having an instrumental resolution of 0.46 kHz and limited by a Doppler width of a few kHz was used. A-type lines were found split by the magnetic hyperfine coupling while no splittings were observed for E-type lines. Symmetry considerations were used to account for the internal rotation of the methyl top and to derive effective hyperfine coupling Hamiltonians. Neglecting the spin-rotation magnetic coupling, the vanishing splittings of the E-type lines could be understood and analyses of the hyperfine patterns of the A-type lines were performed. The results are consistent with a hyperfine structure dominated by the magnetic spin-spin coupling due to the three hydrogen atoms of the methyl group.

Local mode behavior in H(2)Te.

A high-resolution spectrum of hydrogen telluride (H(2)Te) was recorded in the 4050-7000 cm(-1) region. Two bands could be observed at 4900 and 5980 cm(-1) and were assigned as the (20(+/-),1) <-- (00(+),0) and (30(+/-),0) <-- (00(+),0) bands, respectively. Rotational transitions of the two bands were assigned for the most abundant H(2) (130)Te and H(2) (128)Te isotopic species. Line position analyses were carried out to investigate a possible local mode behavior. For the first time we found in H(2)Te strong experimental evidence for such a behavior for the higher-lying band as its two upper vibrational states are only 0.027 and 0.032 cm(-1) apart for the H(2) (130)Te and H(2) (128)Te isotopic species, respectively.

The potential energy surface of the Ar-CO complex obtained using high-resolution data.

A potential energy surface is retrieved for the Ar-CO complex by carrying out a global analysis of its high-resolution spectroscopic data. The data set consists of already published microwave and infrared data and of new microwave transitions which are presented in the paper. The theoretical approach used to reproduce the spectrum is based on a model Hamiltonian which accounts simultaneously for the two large amplitude van der Waals modes and for the overall rotation of the complex. Only the vCO = 0 state is considered. The root-mean-square deviation of the analysis is 18 MHz for the microwave data and 1.4 x 10(-3) cm(-1) for the infrared energy difference data. Fifteen parameters corresponding to the potential energy function are determined in addition to two kinetic energy parameters and two distortion-type parameters. The potential energy surface derived is in good agreement with the one obtained by Shin, Shin, and Tao [J. Chem. Phys. 104, 183 (1996)].

High-Lying Rotational Levels of Water: An Analysis of the Energy Levels of the Five First Vibrational States.

As a continuation of the work carried out on the ground and (010) vibrational states of water (R. Lanquetin, L. H. Coudert, and C. Camy-Peyret, 1999, J. Mol. Spectrosc. 195, 54-57), rotational energy levels for these two states are revisited here and new accurate rotational energy levels are considered for the three next vibrational states, that is, the (020), (100), and (001) states. Experimental rotational energies, along with their uncertainties, are retrieved through analyses of already published data sets and of discharge and flame emission spectra. The maximum value of J for the obtained levels is 25 for the ground state, 21 for the (010) state, and 20 for the three next states. Based on the bending-rotation Hamiltonian approach (L. H. Coudert, 1997, J. Mol. Spectrosc. 181, 246-273), a new theoretical approach is proposed to calculate rotational energies in the five interacting vibrational states under consideration and is used to carry out an analysis of the experimental energies. Comparisons with other existing energy level data sets are also presented. Copyright 2001 Academic Press.

The Rotational-Torsional Spectrum of the g'Gg Conformer of Ethylene Glycol: Elucidation of an Unusual Tunneling Path.

The microwave spectrum of the energetically unfavored g'Gg conformer of ethylene glycol (CH(2)OH&bond;CH(2)OH) is reported. This spectrum is dominated by an interconversion geared-type large-amplitude motion during which each OH group in turn forms the intramolecular hydrogen bond. The microwave spectrum has been analyzed with the help of a Watson-type Hamiltonian plus a 1.4-GHz tunneling splitting. The rotational dependence of this tunneling splitting has been examined using an IAM approach and this yielded qualitative information on the tunneling path the molecule uses to interconvert between its two most stable conformers. Unexpectedly, but in agreement with ab initio calculations, when tunneling occurs between the energetically equivalent g'Gg and gGg' conformers, the OH groups are rotated stepwise through 240 degrees in the sense of a flip-flop rather than a concerted rotation and the molecule goes through the more stable g'Ga and aGg' forms. The electronic reasons for preferring a long rather than a short rotational path via a gGg form are discussed using calculated adiabatic vibrational modes. Copyright 2001 Academic Press.