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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. [ABSTRACT FROM AUTHOR]

A 13 C-enriched spectrum of methane ( 13 CH 4 ) was recorded at Doppler-limited resolution (0.0044 cm −1 ) at 80 K covering the entire Tetradecad region in the 4970–6200 cm −1 . In this paper we present the spectral analysis of the lower part of the 13 CH 4 Tetradecad in the 4970–5470 cm −1 region. Starting with a non-empirical effective Hamiltonian derived by high-order Contact Transformations (CT) from the ab initio potential energy surface (PES), the effective Hamiltonian parameters have been determined for this region by extending the previous DAS (Differential Absorption Spectroscopy) spectral analysis for the upper part of the Tetradecad in the 5853–6200 cm −1 region. In total, 1387 rovibrational transitions were assigned belonging to five cold bands of the Tetradecad up to J max  = 11. Their positions were fitted with an rms deviation of 1.6 × 10 −3 cm −1 . Measured line intensities for 737 transitions were modeled using the effective dipole transition moments approach with an rms deviation of about 10%. Finally, the observed transitions were incorporated to fit simultaneously the 13 CH 4 Hamiltonian parameters for the {Ground state / Dyad / Pentad / Octad / Tetradecad} system and the dipole moment parameters for the {Ground state - Tetradecad} system. [ABSTRACT FROM AUTHOR]

We report on a new approach to ease the computational overhead of ab initio "on-the-fly" semiclassical dynamics simulations for vibrational spectroscopy. The well known bottleneck of such computations lies in the necessity to estimate the Hessian matrix for propagating the semiclassical pre-exponential factor at each step along the dynamics. The procedure proposed here is based on the creation of a dynamical database of Hessians and associated molecular geometries able to speed up calculations while preserving the accuracy of results at a satisfactory level. This new approach can be interfaced to both analytical potential energy surfaces and on-the-fly dynamics, allowing one to study even large systems previously not achievable. We present results obtained for semiclassical vibrational power spectra of methane, glycine, and N-acetyl-L-phenylalaninyl-L-methionine-amide, a molecule of biological interest made of 46 atoms. [ABSTRACT FROM AUTHOR]

Variationally computed infrared spectra in the range [0-5000] cm−1 are reported for the deuterated PH2D and PHD2 molecules from accurate potential energy and dipole moment surfaces initially derived for the major isotopologue PH3( C 3 v ). Energy level and line intensity calculations were performed by using a normal-mode model combined with isotopic and symmetry transformations for the H → D substitutions. Theoretical spectra were computed at 296 K up to Jmax = 30 and will be made available through the TheoReTS information system (http://theorets.univ-reims.fr, http://theorets.tsu.ru). For the very first time, ab initio intensity predictions of PH2D/PHD2 are in good qualitative agreement with the literature. This work will be useful for spectral intensity analysis for which accurate spectral intensity data are still missing. [ABSTRACT FROM AUTHOR]

Vibrational levels of the recently observed high-spin transition (1 5Πu - 1 5&3928;g) of dicarbon [P. Bornhauser et al., J. Chem. Phys. 142, 094313 (2015)] are explored by applying non-linear doubleresonant four-wave mixing and laser-induced fluorescence methods. The deperturbation of the d 3Πg, 3 = 8 and 1 5Πg, 3 = 3 states results in accurate molecular constants for the 3 = 3 "dark" quintet state. In addition, the spin-orbit interaction constant is determined and parameters for the upper Swan level d 3Πg, 3 = 8 are improved. The first excited vibrational state of 1 5Πu is observed by performing perturbation-assisted intersystem crossing via "gateway" states in the d 3Πg, 3 = 6 ~ 1 5Πg, 3 = 0 system. The rotationally resolved spectra yield 11 transitions to 1 5Πu, 3 = 1 that include four spinsubstates. Data reduction results in accurate molecular constants of this vibrational level in the shallow potential energy surface of this state. Finally, 3 = 1 and 2 of the lower quintet state (1 5Πg) are measured by performing perturbation-assisted double-resonant excitation to the 1 5Πu, 3 = 0 state and observing dispersed fluorescence. The obtained molecular constants are compared with high level ab initio computations at the multi-reference configuration interaction (MRCI) level of theory by using a large correlation consistent basis set or, alternatively, by applying the computationally less demanding method of explicitly correlated multi-reference configuration interaction (MRCI-F12). The spectroscopic accuracy of both methods is evaluated by comparison with the experimental findings. [ABSTRACT FROM AUTHOR]

Vibrational spectra of the CH stretching vibration of diazine molecules, pyrimidine and pyrazine, were measured by infrared (IR)–vacuum ultraviolet (VUV) spectroscopy under the jet‐cooled gas‐phase condition. The observed IR spectra were analyzed by three anharmonic algorithms to account for the Fermi‐resonance (F‐R). The anharmonic analysis of the F‐R pattern was performed with second‐order vibrational perturbation theory (VPT2) with quartic potentials (QPs) at the DFT level of B2PLYP/6–311++G(d,p), followed by vibrational configuration interaction (VCI) method with the same QP. The VPT2 + QP method reasonably reproduced most of the bands in the observed spectra for all the species, especially for pyrimidine, a decent agreement is obtained with respect to the band positions and relative intensities. The analyses of the spectra show that all the observed spectra can be well interpreted by the F‐R between the CH stretching fundamentals and the first overtone and 1 + 1 combination bands involving the in‐plane CH bending vibrations and the contribution of the higher‐order anharmonic coupling to the observed spectra seems to lead to red‐shift of the F‐R patterns. Discrete variable representation based methods with potential energy surfaces at CCSD/aug‐cc‐pVDZ theory with 10 degrees of freedom were also carried out to assess the quality of QP at DFT level. [ABSTRACT FROM AUTHOR]

We present new potential energy surfaces for the interaction of NO(C ²π) with each of Ne, Kr, and Xe. The potential energy surfaces have been calculated using second order Møller-Plesset perturbation theory, exploiting a procedure to converge the reference Hartree-Fock wavefunction for the excited states: the maximum overlap method. The bound rovibrational states obtained from the surfaces are used to simulate the electronic spectra and their appearance is in good agreement with available (2+1) REMPI spectra. We discuss the assignment and appearance of these spectra, comparing to that of NO-Ar. [ABSTRACT FROM AUTHOR]

A new full-dimensional global potential energy surface (PES) for the acetylene-vinylidene isomerization on the ground (So) electronic state has been constructed by fitting ~37000 high-level ab initio points using the permutation invariant polynomial-neural network method with a root mean square error of 9.54 cm-1. The geometries and harmonic vibrational frequencies of acetylene, vinylidene, and all other stationary points (two distinct transition states and one secondary minimum in between) have been determined on this PES. Furthermore, acetylene vibrational energy levels have been calculated using the Lanczos algorithm with an exact (J = 0) Hamiltonian. The vibrational energies up to 12 700 cm-1 above the zero-point energy are in excellent agreement with the experimentally derived effective Hamiltonians, suggesting that the PES is approaching spectroscopic accuracy. In addition, analyses of the wavefunctions confirm the experimentally observed emergence of the local bending and counter-rotational modes in the highly excited bending vibrational states. The reproduction of the experimentally derived effective Hamiltonians for highly excited bending states signals the coming of age for the ab initio based PES, which can now be trusted for studying the isomerization reaction. [ABSTRACT FROM AUTHOR]

Recent developments in single and multireference electronic structure methods and the approaches suitable to generateab initiodata that may be employed in the construction of global molecular potential energy surfaces are reviewed. The most appropriate, robust, accurate and cost effective strategies are discussed in the context of various applications ranging from cold collisions and weakly interacting clusters, to large amplitude motion in covalently bound molecules, as well as reaction and photodissociation dynamics. The relationships between the types and necessary quantity ofab initiodata, and representations through fitting are important, and issues related to symmetry and electronic state degeneracy are mentioned. The impacts of limitations or error in the electronic structure data are discussed in terms of how they are reflected in calculations of spectroscopy, dynamics and kinetics. This discussion includes examples such as the submerged reef feature found along the path to formation of ozone on several published potentials. For that example, a relatively small absolute error in the form of a spurious barrier has profound effects on the dynamics and rates of exchange reactions. The origin of the spurious barrier in ozone and other systems is discussed from an electronic structure standpoint. The effective use of dynamically-weighted state-averaged multireference calculations to obtain robustly convergent global surfaces is detailed. [ABSTRACT FROM PUBLISHER]