Jake A. Tan

Jake A. Tan 

Post-doc researcher

• B.S. Chemistry, University of the Philippines - Diliman Campus

• Ph.D. Chemistry, National Tsing Hua University

• Molecular Science and Technology Program, Taiwan International Graduate Program (TIGP), Academia Sinica link 

Research Interest

Theoretical and Computational Molecular Spectroscopy

Molecular spectroscopy is the study of the interaction between radiation and matter. Quantum mechanics prescribes that this interaction induces spectroscopic excitations if the photon’s energy is in resonance with the energy gap between two levels. These excitations may be observable, within the provisions of the selection rules, and hence provide rich information on the electronic structure, vibrational structure, and rotational structure of the molecule under study.

Vibrational spectroscopy garnered a vital niche in the chemical and allied fields. Both organic and inorganic chemists routinely used infrared and Raman techniques to probe the vibrational signatures of most chemical species in the chemical abstract. To this day, a full characterization of a compound would not be complete without gleaning on its infrared and or Raman spectrum. In addition to this, astrochemists used infrared measurements to identify organic molecules in the extraterrestrial space. Meanwhile, in the petroleum industry, infrared intensities are used to analyze compositions of petroleum-derived materials. In food quality analysis and control, infrared measurements are conducted routinely for the identification of possible contaminants, which poses health hazards to the consumers.

Vibrational Spectroscopy of the Intermolecular Proton Bond (IPB)

In the cluster community, there has been a continuous debate on the form of protonated water clusters, (H2O)nH+. Some argue that the Eigen form (H3O+) dominates. Others argue that it is the Zundel form (H2O•••H+•••OH2) that dominates. Figure 1 shows a schematic representation of both the Eigen and Zundel form for protonated water. In the Eigen form, the proton is localized to one proton acceptor (water in this case), while in the Zundel form, two proton acceptors (oxygen in this case) share the proton. An intermolecular proton bond (IPB) links the two acceptors. As these species are also ionic, the term ionic hydrogen bond (IHB) is also used in the literature.

Figure 1. Schematic representation for the a) Eigen and b) Zundel form of the proton in aqueous media. The red balls represent oxygen atoms, while the gray balls represent hydrogen atoms.

To understand the concept of Zundel structure, we can begin our discourse at hydrogen bonding. When two chemical species say X-H and Y interact by hydrogen bonding, it is established that the X-H bond distance elongates relative to the non-hydrogen bonded form. Such a change in the bond distance implies that the proton is shared between the electronegative atoms of the two molecules. The stronger the hydrogen bond, the more elongated would the X-H distance be. At strong hydrogen bonded scenarios, the proton is found to be midway between the hydrogen bond donor and hydrogen bond acceptor. This shared proton motif is called a Zundel structure.

An intermolecular proton bond (IPB) refers to an excess proton that is sandwiched between two electronegative atoms of a closed-shell molecule. In a structural perspective, it is considered as an ionic type of hydrogen bonding (A•••H+-B). Chemically, it plays an active role in the transport of the most ubiquitous ion in the aqueous media. Biologically, it is found in enzymatic processes and proton pumps across the mitochondrial membrane.

Direct absorption measurements are not feasible for the spectroscopy of ionic clusters due to their extremely low ion density (< 108 cm-3). The development of infrared multiphoton dissociation (IRMPD) together with the method of messenger tagging infrared predissociation technique (IRPD) paved the way to probe the vibrational regime of ions. Both of these techniques were developed by Yuan-Tseh Lee, a 1986 Nobel prize recipient in chemistry, during the late 80’s. To date with, these two methods are still the standard means of studying ionic clusters.

The IPB stretching signature (νIPB) in the infrared regime is known to be highly dependent on the proton affinity difference (ΔPA) between the constituent closed-shell molecules. It is empirically known that a large (ΔPA) corresponds to a higher wavenumber for νIPB. However, this picture does not hold for symmetric IPBs (A•••H+-A) where ΔPA=0. Based on several experimental measurements, νIPB for symmetric cases occurs around 1000 cm-1. The said frequency is dependent on the identity of A. One reasonable cause for such variation is the coupling of the flanking groups with IPB stretch. I am currently pursuing this major direction.

The results of our research had advanced our understanding of the IPB motion. The key findings are elaborated in the ensuing discussions. The rest of the article contains a report of the projects I have completed as well as those that are ongoing. The significance of our findings and their materialization in the form of scholarly publications is also mentioned.

A counter-intuitive picture on proton quantum confinement

• A series of proton-bound amines namely: (NH3)2H+, (MeNH2)2H+, (Me2NH)2H+ and (Me3N)2H+ were studied.

• Intuitively, as we enhanced the degree of methylation, the proton affinity of the amines would increase. 

• We then expect that the excess proton will be tightly held in one of the amines causing the force constant to increase.

• This cascade of effects will lead to increasing νIPB from (NH3)2H+ to (Me3N)2H+.

• Such picture from chemical intuition was captured by frequency calculations under static nuclei –calculations conducted at the harmonic approximation.

• However, we have found that upon incorporation of the nuclear motion, the trend is completely reversed. Instead of a blue shift in νIPB, a red shift was observed upon enhancement of methylation. 

• To know more about this project, click here.

                                Figure 2: Counter-intuitive picture arising from nuclear quantum effects

for νIPB on symmetric dimers of ammonia and lower amine homologs.

Why is the “donor-acceptor” coordinate a sine qua non in understanding the vibrational signatures of ionic hydrogen bond?

• The vibrational coupling between the ionic hydrogen bond (IHB)  stretch and donor-acceptor  stretch for H5O2+, (MeOH)2H+, and (Me2O)2H+ were investigated.

• The vibrational states of the donor-acceptor stretch are dark, while the fundamental of the IHB stretch is bright.

• We found that the nνDonor-Acceptor + νIPB combination tone are bright. This suggests that such combination tones borrow intensity from the IHB stretch νIPB.

•  We found that this spectral feature is guaranteed by symmetry for all proton-bound dimers.

• To know more about this project, click here.

Figure 3: A vibrational state interaction diagram which accounts for the observed red shift of IHB stretch a) oversimplified two-state system and b) a more realistic model which accounts the interaction between |1,1> and |2,1>. The interaction between |0,1> and |1,1> leads to two coupled states |+> and    |->, which both contains substantial |0,1> character. This accounts for the observed intensity of the combination band |1,1>. (Taken from publication 2)

Strong and Sensitive Coupling of the IPB with Flanking Group Motions

• Compare with H5O2+, a "triplet" instead of a "doublet" is found in the  1000 cm-1 region.

• The triplet feature is caused by the intermode coupling between the H+ stretch (ν2), out-of-phase C-O stretch (ν3) and out-of-phase CH3 twist coupled to out-of-phase COH bend (ν3).  

• To know more about this project, click here.

Figure 4: The four-dimensional calculation using a) MP2/aug-cc-pVDZ and b) CCSD(T)/aug-cc-pVDZ. In both figures, the blue dots refer to the transition of the first few vibrational states relative to the ground vibrational state. (Taken from publication 6)

Investigating the Isotope effects on the vibrational signature of IPB

• The vibrational signatures of the isotopologues of  (MeOH)2H+ are examined in this study.

• The previously reported strong quantum coupling in (MeOH)2H+ was revisited by representing the vibrational Hamiltonian in “pure state” basis.

• the IHB isotopologues {(CH3OH)2H+ and (CD3OH)2H+} are heavily mixed and has a rich vibrational signature at 850-1100 cm-1.

•  the coupling strengths between IHB stretch and out-of-phase in-plane CH3 rock are stronger than that with an out-of-phase out-plane CH3 rock in (CH3OH)2H+.

• (CH3OD)2D+, and (CD3OD)2D+ are predicted to exhibit a "cleaner" spectrum.

• To know more about this project, click here.

Figure 5: The five-dimensional calculation using a) MP2/aug-cc-pVDZ potential surface and dipole moment functions for the isotopologues of (CH3OH)2H+. (Reproduced from  publication 5 with permission of the PCCP Owner Societies)

Tuning the Vibrational Coupling of H3O+ by Changing its Solvation Environment

• This project demonstrates the sensitivity of the Eigen ion's intermode coupling with respect to the identity of the first solvation shell.

• A series of rare gas solvated hydronium ions (H3O+Rg3, where Rg=Ne, Ar, Kr, and Xe) is examined via reduced-dimensional anharmonic vibrational (RDAV) ab initio calculations.

• Our calculations revealed that Fermi resonance between the first overtones of O-H bends and the fundamentals of O-H stretches led to complex spectral features from 3000 to 3500 cm-1. Such interaction is not only sensitive to the type of rare gas messengers surrounding the H3O+ ion; it also exhibits an anomalous H to D isotope effect.

• Although it is accepted that visible combination tones (~1900 cm-1) do arise from the complex coupling between the hindered rotation and the H-O-H bends, the origin of their intensities is not yet clearly understood. We found that the intensity of these combination tones could be much stronger than their fundamental H-O-H bends.

• To know more about this project, click here.

Figure 6: A vibrational state interaction diagram that describes the coupling of the H-O-H bending overtones (|2,0> and |0,2>) and combination tones (|1,1>) with the fundamental stretching modes for H3O+•Rg3. (Reproduced from publication 4 with permission from the PCCP Owner Societies)

Publications

1. “Infrared spectra and anharmonic coupling of proton-bound nitrogen dimers N2-H+-N2, N2-D+- N2, and 15N2-H+-15N2 in solid para-hydrogen” H.-Y. Liao, M. Tsuge, J. A. Tan, J.-L. Kuo, and Y.-P. Lee, Phys. Chem. Chem. Phys. 19, 20484-20492, DOI: 10.1039/C7CP03847J, (2017) (2017 PCCP Hot Articles)

2. “Why is the “donor-acceptor” coordinate a sine qua non in understanding the vibrational signatures of ionic hydrogen bond?” J. A. Tan and J.-L. Kuo, a chapter in Progress in Theoretical Chemistry and Physics: Quantum Systems in Physics, Chemistry, and Biology: Selected Proceedings of QSCP-XX, A. Tadjer, R. Pavlov, J. Maruani, E. J. Brändas, G. Delgado-Barrio, eds., Springer, Cham, Vol. 30, pp. 251-269 (2017)

3. “Communication: Trapping a Proton in Argon: Spectroscopy and Theory of the Proton-Bound Argon Dimer and Its Solvation” D. C. McDonald, D. T. Mauney, D. Leicht, J. H. Marks, J. A. Tan, J.-L. Kuo, and M. A. Duncan. J. Chem. Phys., 145, 231101, DOI: 10.1063/1.4972581, (2016)

4. “Tuning the Vibrational Coupling of H3O+ by Changing Its Solvation Environment” J. A. Tan, J.-W. Li, C.-c. Chiu, H.-Y. Liao, H.T. Huynh, and J.-L. Kuo, Phys. Chem. Chem. Phys. 18, 30721-30732, DOI: 10.1039/C6CP06326H, (2016)

5. “A Closer Examination on the Coupling between Ionic Hydrogen Bond (IHB) Stretch and Flanking Group motions in (CH3OH)2H+: The Strong Isotope Effects”, J. A. Tan and J.-L. Kuo, Phys. Chem. Chem. Phys. 18, 14531-14542, DOI: 10.1039/C6CP00309E, (2016)

6. “Strong Quantum Entanglement in the Vibrational Signatures of an Intermolecular Proton Bond: The Case of (CH3OH)2H+”, J. A. Tan and J.-L. Kuo, J. Phys. Chem. A, 119 (46), 11320- 11328. DOI: 10.1021/acs.jpca.5b10554, (2015).

7. “Proton Quantum Confinement on Symmetric Dimers of Ammonia and Lower Amine Homologs”, J. A. Tan, J.-W. Li and J.-L. Kuo, a chapter in Progress in Theoretical Chemistry and Physics (vol. 29): Frontiers in Quantum Methods and Application in Chemistry and Physics, M. A. Nascimento, J. Maruani, E. J. Brӓndas, G. Delgado-Barrio, eds., Springer, London, 2015, Vol. 29, Chap. 5, pp. 77-89.

8. “Modern Experiments in General Chemistry I,” R. B. Gross, E. C. Abenojar, and J. A. Tan, Department of Chemistry, Ateneo de Manila University, 2011.

9. “Modern Experiments in General Chemistry II,” R. B. Gross, J. A. Tan and E. C. Abenojar, Department of Chemistry, Ateneo de Manila University, 2010.