We have obtained uniform frequency scaling factors λharm (for harmonic frequencies), λfund (for fundamentals), and λZPVE (for zero-point vibrational energies (ZPVEs)) for the Weigend–Ahlrichs and other selected basis sets for MP2, SCS-MP2, and a variety of DFT functionals including double hybrids. For selected levels of theory, we have also obtained scaling factors for true anharmonic fundamentals and ZPVEs obtained from quartic force fields. For harmonic frequencies, the double hybrids B2PLYP, B2GP-PLYP, and DSD-PBEP86 clearly yield the best performance at RMSD = 10–12 cm–1 for def2-TZVP and larger basis sets, compared to 5 cm–1 at the CCSD(T) basis set limit. For ZPVEs, again, the double hybrids are the best performers, reaching root-mean-square deviations (RMSDs) as low as 0.05 kcal/mol, but even mainstream functionals like B3LYP can get down to 0.10 kcal/mol. Explicitly anharmonic ZPVEs only are marginally more accurate. For fundamentals, however, simple uniform scaling is clearly inadequate.

/pdf/frequency-and-zero-point-vibrational-energy-scale-factors-2bx2s9rwsf.pdf

Frequency and zero-point vibrational energy scale factors for double-hybrid density functionals (and other selected methods): can anharmonic force fields be avoided?

Mid-infrared high-resolution spectroscopy has proven an invaluable tool for the study of the structure and dynamics of molecules in the gas phase. The advent of frequency combs advances the frontiers of precise molecular spectroscopy. Here we demonstrate, in the important 3-µm spectral region of the fundamental CH stretch in molecules, dual-comb spectroscopy with experimental coherence times between the combs that exceed half an hour. Mid-infrared Fourier transform spectroscopy using two frequency combs with self-calibration of the frequency scale, negligible contribution of the instrumental line shape to the spectral profiles, high signal-to-noise ratio, and broad spectral bandwidth opens up opportunities for precision spectroscopy of small molecules. Highly multiplexed metrology of line shapes may be envisioned.

/pdf/mid-infrared-feed-forward-dual-comb-spectroscopy-34rf1pqkue.pdf

Mid-infrared feed-forward dual-comb spectroscopy.

High accurate, ~ 1 × 10 − 4 cm − 1 , ro-vibrational spectra of the C 2 13 H 4 molecule in the region of 600–1600 cm −1 were recorded with Bruker IFS 120/125 HR Fourier transform interferometers and analyzed in the Hamiltonian model which takes into account Coriolis resonance interactions between all four bands. More than 660, 3870, 2420, and 2550 transitions belonging to the ν 4 , ν 7 , ν 10 , and ν 12 bands were assigned in the experimental spectrum with the maximum values of quantum numbers J max . / K a max . , equal to 38/10, 43/21, 33/16 and 52/18, respectively. To make the ro-vibrational analysis physically more suitable, the initial values of the rotational and centrifugal distortion parameters of the studied bands were theoretically estimated by the use of isotopic relations. On that basis, a set of 55 vibrational, rotational, centrifugal distortion, and resonance interaction parameters was obtained from the fit. They reproduce values of 2934 initial “experimental” ro-vibrational energy levels obtained from nonsaturated unblended lines (more than 9500 assigned transitions of the ν 4 , ν 7 , ν 10 , and ν 12 bands) with the rms error d rms = 0.00014 cm − 1 . Ground state parameters of the C 2 13 H 4 molecule were improved as well.

High resolution ro-vibrational analysis of interacting bands ν4, ν7, ν10, and ν12 of 13C2H4

A methodology for the measurement of mid-infrared absorption cross sections of gaseous molecules at high temperatures and pressures is presented. A rapid-tuning, broad-scan external cavity quantum cascade laser with a tuning rate in excess of 30000 cm − 1 s − 1 has been employed in the measurement of full vibrational bands ( > 100 cm − 1 ) in shock-heated test gases. The approach is demonstrated with measurements of the absorption cross section of ethylene (C2H4) in the 8.5 µm to 11.7 µm region for temperatures from 800 K to 1600 K and pressures from 1 to 5 atm. Decreasing peak strength with temperature is observed as well as pressure-insensitivity. The measurements are compared with existing experimental, empirical, and ab initio databases. Additionally, illustrative absorption cross section measurements of propene (C3H6), 1-butene (1-C4H8), 2-butene (2-C4H8), 1,3-butadiene (1,3-C4H6), and methanol (CH3OH) are presented near 1000 K and 2.3 atm.

Measurement of the mid-infrared absorption spectra of ethylene (C2H4) and other molecules at high temperatures and pressures

In this paper we report a new ground state potential energy surface for ethylene (ethene) C2H4 obtained from extended ab initio calculations. The coupled-cluster approach with the perturbative inclusion of the connected triple excitations CCSD(T) and correlation consistent polarized valence basis set cc-pVQZ was employed for computations of electronic ground state energies. The fit of the surface included 82,542 nuclear configurations using sixth order expansion in curvilinear symmetry-adapted coordinates involving 2236 parameters. A good convergence for variationally computed vibrational levels of the C2H4 molecule was obtained with a RMS(Obs.-Calc.) deviation of 2.7 cm(-1) for fundamental bands centers and 5.9 cm(-1) for vibrational bands up to 7800 cm(-1). Large scale vibrational and rotational calculations for (12)C2H4, (13)C2H4, and (12)C2D4 isotopologues were performed using this new surface. Energy levels for J = 20 up to 6000 cm(-1) are in a good agreement with observations. This represents a considerable improvement with respect to available global predictions of vibrational levels of (13)C2H4 and (12)C2D4 and rovibrational levels of (12)C2H4.

https://hal.archives-ouvertes.fr/hal-01214173/document

A new accurate ground-state potential energy surface of ethylene and predictions for rotational and vibrational energy levels

High-resolution infrared analysis of the ν7 band of cis-ethylene-d2 (cis-C2H2D2)

High-resolution Fourier transform infrared spectrum of the ν11 band of ethylene (12C2H4)

The integrated band intensities of ethylene (12C2H4) in the 640&#x2013;3260&#x2009;cm&#x2212;1 region were determined by Fourier transform infrared (FTIR) spectroscopy. The infrared absorbance spectra of the 𝜈7 and 𝜈10, 𝜈12, 𝜈7

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G.B. Lebron

Papers

High-resolution infrared analysis of the ν7 band of cis-ethylene-d2 (cis-C2H2D2)

High-resolution Fourier transform infrared spectrum of the ν11 band of ethylene (12C2H4)

Integrated Band Intensities of Ethylene (1224) by Fourier Transform Infrared Spectroscopy

The high-resolution FTIR spectrum of the ν4 + ν8 band of trans-d2-ethylene (trans-C2H2D2)

Improved rovibrational constants for the ν6 + ν10 band of ethylene (12C2H4) by high-resolution Fourier transform infrared spectroscopy