Accurate Calculations of OH-Stretching Intensities with a Reduced-Dimensional Local Mode Model Including Eckart Axis Embedding

Abstract

Absolute OH-and OD-stretching transition intensities have been calculated for a series of alcohols (methanol, ethanol, 2-propanol, 1-propanol, and tert-butanol) with onedimensional (1D) and three-dimensional (3D) local mode models. We compare the calculated intensities for the Δv OH = 1−5 and Δv OD = 1−3 transitions with experimental values. Potential energy and dipole moment surfaces are calculated at the CCSD(T)-F12a/ VDZ-F12 level of theory. The 1D local mode model includes only the OH(D)-stretching mode, whereas the 3D local mode model also includes the CO-stretching and COH(D)-bending modes. We analyze the effect on vibrational intensities of using either a molecule-fixed Eckart frame or a space-fixed Cartesian frame. We find that both Eckart embedding and inclusion of the COstretching and COH(D)-bending modes in the local mode model are important for the OH/OD-stretching fundamental transition intensities, but have a minor effect on overtone intensities. The 3D reduced-dimensional local model, when combined with coupled cluster surfaces, accurately predicts OH/OD-stretching transition intensities and wavenumbers, for all alcohols included in this work.

Predicting OH stretching fundamental wavenumbers of alcohols for conformational assignment: different correction patterns for density functional and wave-function-based methods

“…This puts the ball in theory's court. The very uniform deviations from experiment for the results of the CCSD(T) 1D model, 26 which covers diagonal anharmonicity, indirectly support similar off-diagonal contributions for alcohols. VPT2 calculations at B3LYP-D3, B2PLYP-D3 and MP2 level (all cc-pVTZ basis) suggest a total off-diagonal contribution to the fundamental in the range of À(10 to 12) cm À1 for methanol.…”

“…A recent 1D local mode approach 26 on the CCSD(T)-F12a/VDZ-F12 level for the same eleven conformers yielded MAE = 11 cm À1 and MAX = 13 cm À1 with very uniform overestimations between 9 and 13 cm À1 . This might indicate the possibility of an empirical correction to close the remaining gap from neglected normal mode off-diagonal anharmonicity.…”

“…To compare with results of a second high-level method, we also extracted harmonic wavenumbers from the anharmonic CCSD(T)-F12a/VDT-F12 1D local mode model 26 discussed in the introduction. For this the reported fundamental and first overtone transition wavenumbers are inserted in eqn (5), which can be derived for x i, j = 0 from eqn (1).…”

“…This might indicate the possibility of an empirical correction to close the remaining gap from neglected normal mode off-diagonal anharmonicity. Recovering parts of it by adding the couplings with the CO stretching and the COH bending mode (3D local mode model) 26 removes the general overestimation, but the spread of the deviations becomes larger with values between À6 and +4 cm À1 , resulting in MAE = 2.6 cm À1 and MAX = 6 cm À1 . The CCSD(T) 1D and 3D models are currently computationally affordable for alcohols with up to about six carbon atoms.…”

Estimations of FH···X hydrogen bond energies from IR intensities: Iogansen's rule revisited

“…Theoretical predictions of vibrational intensities required an accurate dipole moment functions and vibrational wavefunctions, which became available only in recent decades due to significant improvement of computational methods and techniques 48,49 . At the moment there are a few theories describing the mechanism of intensification of the stretching mode upon formation of hydrogen bond based on QTAIM theory and charge transfer model 50,52,61 …”