Dipole moments of linear molecules: A computational molecular spectroscopy study

Jensen, Per

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https://hdl.handle.net/2142/100725 Copy

Description

Title

Dipole moments of linear molecules: A computational molecular spectroscopy study

Author(s)

Jensen, Per

Contributor(s)

• Hirano, Tsuneo
• Li, Hui

Issue Date

2018-06-19

Keyword(s)

Mini-symposium: New Ways of Understanding Molecular Spectra

Abstract

Computation of the dipole moment in a ro-vibrationally averaged molecular state of a triatomic molecule requires the 3D potential energy surface and the associated ro-vibrational wavefunctions. Consequently, in most cases, the experimentally derived value of the dipole moment in the ro-vibronic ground state is compared with the theoretical dipole moment value for the “equilibrium geometry.” We have proposed in recent publicationsa,b,c that any linear molecule, whose potential energy minimum occurs at a linear configuration, is observed as being bent on ro-vibrational average. That gives rise to the question recently asked by a reviewer of our paper:c Why does the averaged dipole moment vanish for a symmetrical triatomic molecule of type ABA, such as CO2? In the present talk we show that there is no contradiction, for linear triatomic molecules, between our proposition of a bent averaged geometry and the experimentally derived, vibrationally averaged dipole moment value. We must consider two facts: 1) for a linear molecule, the rotation about the a axis, which approximately coincides with the molecular axis, cannot be separated from the bending motion described by variation of the bond angle in the instantaneous molecular plane, and 2) the dipole moment function is an odd function of the angle describing this rotation. Therefore, only the a axis component of the dipole moment can be observed (and it, too, vanishes by symmetry for an ABA molecule). Taking CO2 and HCO+ as examples, we show, from the theoretical view point, how the dipole moment is observed in the experimental study, typically in Stark spectroscopy. Our theoretically predicted dipole moment value for the ro-vibronic ground state of HCO+, 3.933 D, is in good agreement with that determined from Stark experiments,d 3.921(31) D (quoted uncertainty in parentheses, in units of the last digit).

Publisher

International Symposium on Molecular Spectroscopy

Type of Resource

text

Language

eng

Permalink

http://hdl.handle.net/2142/100725

DOI

10.15278/isms.2018.TB04

Copyright and License Information

Copyright 2018 Per Jensen

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