Ab initio calculations of mode selective tunneling dynamics in CH312OH and CH313OH

Abstract

A simplified formulation of the harmonic reaction path Hamiltonian (RPH) approach is used to calculate mode specific tunneling splittings and stereomutation times in CH312OH and CH313OH. The experimental torsional spectrum is very well reproduced, as well as the few known isotope shifts. The mode specific changes in tunneling splitting are investigated for the excitation of fundamentals and OH stretching overtones. Good agreement between experiment and the RPH model is obtained, except for excitations of modes which are perturbed by anharmonic resonances. The inverted tunneling splittings (E level below A) experimentally observed for the fundamental transitions of the CH-stretching modes ν2 and ν9 and of the CH-rocking mode ν11 are shown to result from a pure symmetry effect and not from a breakdown of vibrational adiabaticity. Introducing a proper geometrical phase factor but retaining the adiabatic separation of the torsional dynamics yields calculated values of Δν̃2=−3.6 cm−1, Δν̃9=−3.2 cm−1, and Δν̃11=−8.2 cm−1 that are in satisfactory agreement with experimental data. Negative tunneling splittings are also predicted for the asymmetric CH-bending modes ν4 and ν10 and the CH3-rocking mode ν7. A smooth decrease of the tunneling splitting is calculated for increasing OH stretching excitation [Δν̃(ν1)=6.2 cm−1,…,Δν̃(6ν1)=1.5 cm−1] in quantitative agreement with experiment [Δν̃(ν1)=6.3 cm−1,…,Δν̃(6ν1)=1.6 cm−1]. The effect is shown to result in about equal parts from the increase of the effective torsional barrier and the effective lengthening of the OH bond.

Ab initio vibrational state calculations with a quartic force field: Applications to H2CO, C2H4, CH3OH, CH3CCH, and C6H6

“…This mode corresponds to the torsional motion of the OH fragment which is expected to be very floppy, and so large amplitude motion should be taken into account to reproduce the fundamental of 12 accurately. 52 Second, the calculated 3 gives a large error of 6.1% in the direct cc-VSCF method, which suggests that perhaps a higher level of electronic structure theory should be used or perhaps high levels of correlation in the vibrational state calculations. Finally, 1 calculated by the cc-VSCF/2MR-QFF method gives a large deviation from the corresponding direct cc-VSCF result of ͒ of methanol obtained from the harmonic approximation, cc-VSCF/2MR-QFF, and the direct cc-VSCF ͑2MR͒ calculations, at the level of MP2/cc-pVTZ.…”

Communication: Conical intersections between vibrationally adiabatic surfaces in methanol

“…Fehrensen et al 7 applied an adiabatic reaction path Hamiltonian to account for both the decrease of the torsional tunneling splittings with OH stretch (ν 1 ) excitation and also the inverted torsional tunneling splittings in the first excited states of the asymmetric CH stretches, ν 2 and ν 9 . They found that a geometric phase of -1 was accumulated for the CH vibrations ν 2 and ν 9 on a torsional rotation of 2π .…”

“…The adiabatic approximation has been applied to methanol, [6][7][8] the high frequency vibrations being the OH and CH stretches (3860, 2999, 2956, and 2844 cm −1 ) and the low frequency coordinate being the torsional angle, γ . Fehrensen et al 7 applied an adiabatic reaction path Hamiltonian to account for both the decrease of the torsional tunneling splittings with OH stretch (ν 1 ) excitation and also the inverted torsional tunneling splittings in the first excited states of the asymmetric CH stretches, ν 2 and ν 9 .…”

Mode-selective stereomutation tunneling and parity violation in HOClH+ and H2Te2 isotopomers

“…The accuracy of the RPH approach itself was discussed in detail previously [25,43]. We have also shown recently that with appropriate empirical corrections the modest intrinsic ab initio error in the electronic structure calculations can be further reduced [63]. Fig.…”