Jake A. Tan and Jer-Lai Kuo Institute of Atomic and Molecular Sciences Academia Sinica Taipei, Taiwan International Symposium on Molecular Spectroscopy 69 th Meeting – University of Illinois Urbana-Champaign 1

Outline Introduction Motivation Stratagem Results  (MeOH) 2 H +  Isotope substitution  Ar Messenger Effect Take Home Messages 2

Intermolecular Proton Bond (IPB) Refers to the interaction between the electronegative atoms of two closed-shell molecules sandwiching an excess proton (H + ). 3 Intermolecular Proton Bond (IPB)

What is the relationship between ΔPA and ν IPB ? How about for ΔPA =0? 4 ν IPB ΔPA

Coupling between IPB and flanking group motions? 5 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

(Me 2 O) 2 H + Exp. (Me 2 O) 2 H + Calc. (Harm.) (Me 2 O) 2 D + Exp. (Me 2 O) 2 D + Calc. (Harm.) Same case for (MeOH) 2 H + !

Outline Introduction Motivation Stratagem Results  (MeOH) 2 H +  Isotope substitution  Ar Messenger Effect Take Home Messages 7

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. 8

Stratagem Quantum Chemistry Basis Set Normal Mode Dimensionality B3LYP/6-311+G(d,p) Up to 4D 9

Stratagem Dimensionality 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 10

Strategy Method of Finite Difference: For a one-dimensional problem we have: 11

Outline Introduction Motivation Stratagem Results  (MeOH) 2 H +  Isotope substitution  Ar Messenger Effect Take Home Messages 12

(MeOH) 2 H + O-O StretchIPB Stretch Free O-H BendingOut phase C-O stretch 13

(MeOH) 2 H + 14 Assignment may be easy with Normal Mode Coordinates

(MeOH) 2 H + 15 Assignment may NOT easy with Normal Mode Coordinates IPB Stretch will be based on the larger oscillator strength

(MeOH) 2 H + 16 ν O-O, str ν op, C-O, str ν op, O-H bend, str 1D 2D 3D 4D ν IPB, str Harmonic

(MeOD) 2 D + ν O-O, str ν op, C-O, str ν op, O-D bend ν IPB, str 1D 2D 3D 4D Harmonic

Outline Introduction Motivation Stratagem Results  (MeOH) 2 H +  Isotope substitution  Ar Messenger Effect Take Home Messages 18

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? 19

Ar(MeOH) 2 H + ν op, C-O, str ν op, O-H bend ν O-O, str ν IPB, str 1D 2D 3D 4D Harmonic

Outline Introduction Motivation Stratagem Results  (MeOH) 2 H +  Isotope substitution  Ar Messenger Effect Take Home Messages 21

Take Home Messages Intermolecular Proton Bond (IPB) indeed could strongly couple with some of the flanking group motions. Our isotope studies w/ (MeOD) 2 D + predicts that coupling will be lessened and a clean spectrum is obtained. Opens for expt. validation Argon tagging perturbs the modes of (MeOH) 2 H+ –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. 22 In agreement w/ MAJIn addition ν op, C-O, str ν O-O, str ν op, O-H bend

23 Jer-Lai Kuo Kaito Takahashi