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Jake A. Tan and Jer-Lai KuoInstitute of Atomic and Molecular Sciences

Academia SinicaTaipei, Taiwan

International Symposium on Molecular Spectroscopy69th Meeting – University of Illinois Urbana-Champaign

Unfolding the Quantum Nature of Proton Bound Symmetric Dimer of (MeOH)2H+:

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Outline

IntroductionMotivationStratagemResults

(MeOH)2H+

Isotope substitution Ar Messenger Effect

Take Home Messages

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Intermolecular Proton Bond (IPB)

• Refers to the interaction between the electronegative atoms of two closed-shell molecules sandwiching an excess proton (H+).

Intermolecular Proton Bond

(IPB)

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What is the relationship between

ΔPA and νIPB?

How about for ΔPA =0?

νIPB

ΔPA

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Coupling between IPB and flanking group motions?

ν IPB

ν ip, sym O-C-O

ν op, sym O-C-O

ν IPB

ν op, free O-H bend

ν op, C-O str

(Me2O)2H+ Exp.

(Me2O)2H+ Calc. (Harm.)

(Me2O)2D+ Exp.

(Me2O)2D+ Calc. (Harm.)

Same case for (MeOH)2H+!

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Outline

IntroductionMotivationStratagemResults

(MeOH)2H+

Isotope substitution Ar Messenger Effect

Take Home Messages

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Motivation

• Most of the experimental literatures do speculate the coupling between IPB and flanking group motions.

– A theoretical study can shed light on the extent of these interactions.

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Stratagem

Quantum Chemistry

Basis Set

Normal Mode Dimensionality

B3LYP/6-311+G(d,p)

Up to 4D

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StratagemDimensionality Aspect

• Insights from One-Dimensional Treatment– The effective scan range

• Insights from Two-Dimensional Treatment – Modes that couple

• Insights from Higher-Dimensional Treatment– Cumulative effect of other modes on IPB stretch

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StrategyMethod of Finite Difference:• For a one-dimensional problem we have:

Where, 𝜇𝑚𝑜𝑑𝑒 1 is the reduced mass for normal mode 1 𝑞𝑚𝑜𝑑𝑒 1 is the coordinate associated with normal mode 1

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Outline

IntroductionMotivationStratagemResults

(MeOH)2H+

Isotope substitution Ar Messenger Effect

Take Home Messages

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(MeOH)2H+

O-O Stretch IPB Stretch

Free O-H BendingOut phase C-O stretch

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(MeOH)2H+

Assignment may be easy with Normal Mode Coordinates

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(MeOH)2H+

F ig. 1 Mixing of the fi rst exci ted states of IPB st re tching mode w i th out phase C-O st re tching mode (upper row ) and f ree O-H bending ( low er row ). Due to a near ly 45° t i l t ing of nodal planes, i t is possible to make an assignment f rom the w avef unc t ions topology. T he co lored contour refe rs to the corresponding PES and the color map values are in te rms of atomic uni t s.

Assignment may NOT easy with Normal Mode Coordinates

IPB Stretch will be based on the larger oscillator strength

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(MeOH)2H+

ν O-O, str ν op, C-O, strν op, O-H bend, str

1D

2D

3D

4D

ν IPB, str

Harmonic

(MeOD)2D+

ν O-O, str

ν op, C-O, str

ν op, O-D bend ν IPB, str

1D

2D

3D

4D

Harmonic

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Outline

IntroductionMotivationStratagemResults

(MeOH)2H+

Isotope substitution Ar Messenger Effect

Take Home Messages

Motivation

• It is well known that Ar tagging do affect the stretching frequency of free O-H.

• How about the IPB bond? • Is Ar really innocent?

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Ar(MeOH)2H+

ν op, C-O, str

ν op, O-H bend ν O-O, strν IPB, str

1D

2D

3D

4D

Harmonic

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Outline

IntroductionMotivationStratagemResults

(MeOH)2H+

Isotope substitution Ar Messenger Effect

Take Home Messages

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Take Home Messages• Intermolecular Proton Bond (IPB) indeed could strongly couple with

some of the flanking group motions.

• Our isotope studies w/ (MeOD)2D+ predicts that coupling will be lessened and a clean spectrum is obtained. Opens for expt. validation

• Argon tagging perturbs the modes of (MeOH)2H+

– IPB is sensitive with the environment– Perhaps studies tagged by He, Ne and other messengers can help us to

understand IPB’s sensitivity with the environment.• Isotope substitution, messenger tagging are useful weapons in resolving the

nature of IPB’s coupling.

In agreement w/ MAJ In addition

ν op, C-O, str ν O-O, str

ν op, O-H bend

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Jer-Lai Kuo Kaito Takahashi