Observation of the Ring-Puckering Vibrational Mode in Thietane

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Observation of the Ring Puckering Vibrational Mode in Thietane Cation Yu Ran Lee, Chung Bin Park, Jiye Hwang, Bongjune Sung, Hong Lae Kim, and Chan Ho Kwon J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b12724 • Publication Date (Web): 13 Jan 2017 Downloaded from http://pubs.acs.org on January 16, 2017

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The Journal of Physical Chemistry

Observation of the Ring Puckering Vibrational Mode in Thietane Cation Yu Ran Lee1†, Chung Bin Park1‡, Jiye Hwang‡, Bong June Sung‡, Hong Lae Kim†, and Chan Ho Kwon† †

Department of Chemistry and Institute for Molecular Science and Fusion Technology, Kangwon National University, Chuncheon 24341, Korea ‡

Department of Chemistry, Sogang University, Seoul 04107, Korea

Abstract We have measured the high-resolution vibrational spectra of a thietane (trimethylene sulfide) cation in the gas phase by employing the vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopic technique. Peaks in the low frequency region of the observed MATI spectrum of thietane originate from a progression of the ring puckering vibrational mode (typical in small heterocyclic molecules), which is successfully reproduced by quantum chemical calculations with one-dimensional (1D) symmetric double-well potentials along the ring puckering coordinates on both the S0 and D0 states, the ground electronic states of neutral and cation of thietane, respectively. The values of the interconversion barrier and the ring puckering angle on the S0 state, the parameters used for the quantum chemical calculations, were assumed to be 274 cm-1 and 26. The barrier and the angle on the D0 state, however, are found to be 48.0 cm-1 and 18.2, respectively, where such small barrier height and puckering angle for the cation suggest that the conformation of thietane cation on the D0 state should be more planar than that of the thietane neutral.

1 

These authors contributed equally to this work. Author to whom correspondence should be addressed. E-mail: [email protected] (C. H. Kwon) and [email protected] (H. L. Kim).

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Introduction Conformation of small heterocyclic ring compounds such as containing nitrogen and sulfur atoms is of fundamental importance because these molecules often constitute a backbone of larger molecules of biological interest.1-3 Since biological activities in many cases depend strongly on their molecular conformations, determination and identification of the molecular conformations on a designated electronic state are one of the essential goals of chemical and biological studies.4,5 The conformation change arises from vibrational motion associated with ring deformation such as the ring puckering and hence spectroscopic studies of this floppy, ring puckering low frequency vibrational mode should thoroughly be carried out.6-8 The spectra of the heterocyclic molecules including four-, five-, and six-membered rings were measured by far infrared and Raman spectroscopy and interpreted taking advantage of quantum chemical calculations.9,10 Vibrational energies and wavefunctions of the molecule on the ground electronic state were calculated with quartic and quadratic double-well potentials along the puckering coordinates, from which the peaks observed in the spectra were successfully identified. The potential parameters in vibrational potential surfaces of the ring puckering motion obtained from spectral analyses provide crucial information on the molecular conformations, interconversion barrier between different structures, and forces responsible for the conformational change. The above mentioned spectroscopic techniques are inapplicable in most cases to molecular ions in the gas phase. One of the classical techniques to measure the vibrational spectra of the molecular ions is the photoelectron spectroscopy. Since the photoelectron spectra in general, however, have low resolution due to electron analyzers, the high resolution spectroscopic techniques such as the zero kinetic energy (ZEKE) spectroscopy have been developed.11 In ZEKE, pulsed field ionization of the molecules in their high Rydberg states followed by delayed extraction of the ZEKE electrons


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is applied, from which the vibrational spectra of molecular ions can thus be measured from the 00 band at the origin under high resolution. As the same technique but instead of detecting electrons, the mass-analyzed threshold ionization (MATI) spectroscopy detects the molecular ions of designated mass to measure the vibrational spectra of the specific molecular ions.12 The low frequency, large amplitude vibrational motion is in most cases separable from the rest of the other 3N-5 vibrational motions in the molecule and can be treated as one dimensional oscillator. The energies and eigenfunctions can then be calculated by solving one dimensional Schrödinger equation with suitable model potential functions along the ring puckering coordinate, for example.13-15 The variation of potential energies along the ring puckering coordinate is assumed to arise from deformation of the ring angles and torsional motion. Then, interconversion barriers between the two puckered conformations depend upon the methylene torsional forces and ring strain forces. The ring strain is likely to reduce the interconversion barrier and stabilize planar conformations. On the other hand, the torsional forces increase the barrier, thus favoring puckered conformations.16 In addition to the precise structure and detailed conformation of the molecule, forces restoring this ring deformation can be understood by analyzing this low frequency mode in the vibrational spectrum. Four tetrahedral carbon atoms of cyclobutane are responsible for the torsional forces overwhelming the ring strain and provide the interconversion barrier of about 500 cm-1, which should favor the puckered conformation. Substitution of one methylene group by an isoelectronic oxygen, however, reduces the torsional force with little angle strain change and hence the interconversion barrier significantly to about 15 cm-1, thus resulting in almost the planar equilibrium conformation in oxetane.17 Since the open chain CSC angle is smaller than the COC angle, the substitution of O by S in oxetane would reduce the ring strain and hence increase the


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barrier (~270 cm-1), which favors the puckered equilibrium conformation in thietane. The Far IR and microwave spectra of thietane were measured and the peaks in the spectra were identified by quantum chemical calculations with one-dimensional quartic and quadratic potentials, which provided the unambiguous conformational structure on the ground state with the puckering angle of 26 ± 2 and the barrier to the planar conformation of 274  2 cm-1.8,16,18 Removing one electron from a nonbonding orbital in sulfur would make the CSC angle larger in thietane, which would lower the barrier to the planar conformation in the thietane cation. In this paper, we report for the first time, the MATI spectrum of thietane in the gas phase to investigate the ring puckering vibrational mode and to identify the precise conformational structure of the thietane cation. The peaks in the low frequency region of the spectrum are assigned as the progression of the ring puckering vibrational mode in the S0 – D0 transition between the ground electronic states of neutral and cation of thietane. The eigenstates and eigenfunctions of thietane in the S0 and D0 states are obtained by solving the Schrödinger equation with the one dimensional quartic potential function, from which the spectral simulations are successfully carried out with calculated Franck-Condon factors. The spectral simulation with parameters in the potential function provides the precise interconversion barriers through the planar conformation and the puckering angles of thietane on the S0 and D0 state, respectively.


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Experimental methods The MATI spectrum of thietane in the gas phase was taken by the home-built VUV-MATI spectrometer which was described in detail elsewhere except for new electrode assembly and pulsing acheme.19,20 The spectrometer is composed of a source chamber, an ionization chamber with a time-of-flight mass spectrometer, and a Kr gas cell for VUV light generation. The coherent VUV light is generated by four wave difference frequency mixing in Kr, 2l – 2 from two separate dye lasers, where 1 is the resonance frequency for the Kr 5p[l/2]0 – 4p6 transition at 212.556 nm and 2 is the frequency of the tunable, visible light. The UV light at 212.556 nm with ~0.5 mJ/pulses was generated by mixing after doubling of 637.668 nm output of a dye laser (Continuum, ND 6000). The VIS light at 409–420 nm with ~5 mJ/pulses of another dye laser (Lambda Physik, Scanmate 2E) was spatially and temporally overlapped with the 212.556 nm light and loosely focused in the Kr cell with a convex lens (f = 50 cm) to generate the VUV light tunable in the 142.281–143.590 nm range. Then, the VUV light is separated from the residual UV and visible lights and introduced to the modified ion source in a counter propagated manner with the molecular beam. The ion source could apply the very low pulsed field ionization (PFI) voltage and achieve first-order focusing of the MATI ions initially generated with some spatial distribution. The background pressure in the ionization chamber was normally ~10-7 Torr when the nozzle was operated at 10 Hz. A weak spoil field of 0–0.2 V cm-1 was applied to the PFI stage to remove the produced ions. An electric field of 10 V cm-1 was usually applied to ionize thietane in high n, l Rydberg states at ~15 s delay after the VUV laser pulse. Then, the generated MATI ions were extracted applying the appropriate pulsing scheme of electric fields with four electrodes and focused onto the MCP detector at the end of the flight tube. The MATI spectrum is recorded by integration of the mass


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signal and corrected by the visible laser power measured separately. The spectral resolution (~8 cm-1) and the signal size in the MATI spectrum thus measured were simultaneously enhanced by adopting the new electrode assembly and pulsing scheme in the VUV-MATI spectrometer. The gaseous sample from a reservoir where the liquid and the vapor are at equilibrium at ice temperature is introduced through a pulsed nozzle (dia. 500 m, Parker Valve) at the Ar stagnation pressure of 3 atm. The molecular beam is introduced to the ionization chamber through a skimmer (dia. 1 mm, Beam Dynamics) equipped at 3 cm downstream from the nozzle. Thietane was purchase from Aldrich (99 %) and used without further purification.


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Results and Discussion The measured MATI spectrum of thietane is presented in Figure 1, especially in the low frequency region. In the spectrum, the observed peaks would be a progression of a low frequency vibrational mode, the ring puckering mode in the case of thietane cation, which should be assigned by taking advantage of quantum chemical calculations. In MATI spectroscopy, since the pulsed field ionizes the molecule in the Rydberg states that converge to the individual vibrational states of the cation, the MATI spectrum becomes the vibrational spectrum of the corresponding cations measured from the origin at the 0-0 band position. The first peak as the origin, associated with photoionization efficiency curve, thus provide the first adiabatic ionization energy of the molecule. The correct ionization energy of thietane measured in the spectrum at the zero field limit is 8.6493  0.0004 eV because the pulsed field should lower the ionization energy of the molecule.11,12

Figure 1. VUV-MATI spectrum of thietane, observed (top) and simulated (bottom), from the origin. The grey curve shows the photoionization efficiency curve.


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In order to identify the observed peaks in the MATI spectrum, we solve 1D Schrödinger equation along the ring puckering vibrational coordinate and obtain eigenfunctions and their corresponding energies for both the ground (S0) and the ionic ground (D0) states. We also estimate the transition probabilities of vibrational transitions in the S0 – D0 transition. The Hamiltonian operator is set up by employing a ring puckering angle (), where  is the dihedral angle between the two intersecting planes: three carbons (Cl, C2, and C3) construct one plane and two carbons, C2, C3, and a sulfur atom construct another plane (Figure 2).

Figure 2. Dihedral angle between the two planes. The potential energy function should be symmetrical because the potential energy should be the same at  as the thietane puckers from - to +. The potential energy function along the ring puckering mode may be, therefore, described by a double-well shape potential with two identical potential minima. The potential energy function in this study is modeled by quartic polynomials of , i.e., ,

/ ,

where h is the potential energy barrier between the two puckered conformations and z is the


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puckering angle at potential minima. Assuming the same puckering coordinate for thietane and its cation, the Schrödinger equation can be solved for thietane and its cation with a different set of (h, z), respectively. With the use of 300 trigonometric basis functions, i.e.,

and

with an integer m from 1 to 150, we employ the variational principle and diagonalize the 300  300 Hamiltonian matrix, from which we obtain eigenvalues and eigenfunctions corresponding to the vibrational states on the S0 and D0 states, respectively. The potential energy functions for both the ground and the ionic ground states are presented in Figure 3 along with obtained vibrational energy levels.

Figure 3. Potential energy curves for thietane on the S0 and D0 states with calculated vibrational energy levels, respectively. Arrows show vibronic transitions identifying the peaks in the MATI spectrum in Fig 1.


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As shown in the figure, the energy levels are degenerate and diverging above the barrier. The degeneracy is, however, lifted in the well where the energy levels are split into two components. In a double-well potential, wavefunctions in each well can penetrate through the potential barrier via a tunneling effect and become delocalized, which is more effective close to the top of the barrier.6 The energy difference between the 0+ and 0- levels on the S0 state should be small (2.3 cm-1) because the ground vibrational levels are far below from the top of the barrier. We also estimate the Franck-Condon (FC) factors based on the eigenfunctions and eigenvalues on the S0 and D0 states.21 The transition intensities are estimated by the FC factors that are the squares of the overlap integrals between the two vibrational states multiplied by the Boltzmann factors. From the signal out of noise for the vibrational hot bands in the observed MATI spectrum, the vibrational temperature was estimated to be about 10 K under our skimmed molecular beam condition. The Boltzmann population distribution between the 0+ and 0- level on the S0 state at 10 K is, therefore, employed. The calculated FC factors for each transition (with the parities of the eigenfunctions taken into account) provide the simulated MATI spectra for given parameters such as the barrier height (h). Figure 1 (bottom) depicts the simulated spectrum that overlaps with our experiment best. The calculated transition frequencies and relative intensities are listed in Table 1. As mentioned above, the observed peaks in the low frequency region of the MATI spectrum are indeed the progression of the low frequency, ring puckering vibrational mode of the thietane cation. In the spectral simulation, the potential parameters on both the S0 and the D0 states should simultaneously be varied, which would make the simulation non-trivial. In this respect, the potential parameters on the S0 state such as the barrier height (h = 274 ± 2 cm-1) and the puckering angle at a potential minimum (z = 26 ± 2) were adopted from the literature.8,18 By fitting our experiments with simulated spectra, we found that for the D0 state, the barrier height and the


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puckering angle should be 48.0 cm-1 and 18.2, respectively. Such a small barrier height for the cation suggests that the conformation of thietane cation on the D0 state should be more planar. This difference in the equilibrium conformation on the S0 and the D0 states would lead to the large transition intensity for the 0- – 1 transition compared to the 0+ – 0 transition although the population on the 0- level is rather small at the estimated temperature.

Table 1. Frequencies (in cm-1) and relative intensities for vibrational transitions of the ring puckering mode in the S0 – D0 transition of thietane. VUV-MATI S0→D0a

Frequencyb Intensityc Frequencyb Intensityc

0+ → 0

0

1.00

0

1.00

0- → 1

17

0.85

18

0.91

0+ → 2

82

0.57

82

0.59

0 →3

147

0.30

146

0.27

0+ → 4

227

0.13

228

0.18

0- → 5

314

0.04

313

0.07

0+ → 6

409

0.01

411

0.04

-

a

Calculation

One-photon transition from the neutral electronic ground state (0+ or 0- in S0) to the cationic

ground state (′ in D0). b

Vibrational frequencies of the cation from the origin at the 0+–0 band.

c

Normalized to the intensity of the 0+–0 band.


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Conclusions The vibrational spectrum of thietane cation has been obtained for the first time by VUV-MATI spectroscopy and the peaks observed in the low frequency region are successfully assigned as the progression of the ring puckering mode. By solving the Schrödinger equation with the one dimensional double-well shape potential energy function for the ring puckering vibrational modes in thietane and its cation, we obtain the eigenvalues and eigenfunctions accurate enough to calculate frequencies and intensities of individual vibronic transitions and to reproduce the observed MATI spectrum. The interconversion barrier from the puckered conformation to the planar and the puckering angle for the thietane cation were found to be 48.0 cm-1 and 18.2, respectively.

Acknowledgment This work has been financially supported by the National Science Foundation in Korea (2014R1A1A4A01007152 and 2016R1D1A3B03935921).


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TOC GRAPHICS

KEYWORDS VUV-MATI, Molecular cation, Conformation, Low-frequency vibration, Doublewell potential, Interconversion barrier


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Observation of the Ring-Puckering Vibrational Mode in Thietane

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