Investigation of Intermolecular Hydrogen Bonding in the 2
Investigation of Intermolecular Hydrogen Bonding in the 2 -Pyridone Dimer via Pulsed Terahertz Spectroscopy Tanieka L. Motley and Timothy M. Korter Department of Chemistry Syracuse University Syracuse, NY 13244 -4100
Overview o o o Why 2 -pyridone? Experimental and theoretical methods 2 -pyridone in CCl 4 solution Polycrystalline 2 -pyridone Conclusions
Introduction 2 -pyridone (2 PD) dimer o o o Thymine (T) Uracil (U) Simple mimic for uracil (U) or thymine (T) DNA like: anti-parallel hydrogen bonds Tautomerization is an issue (2 -hydroxypyridine)
Tautomerization o o o Important to know how much of each tautomer is present Allows the determination of peak intensity due to the tautomer 2 PD 2 HPD Log KT = 0. 950 for 2 HPD in CCl 4 KT = [2 PD] / [2 HPD] 2 PD ≈ 90% and 2 HPD ≈ 10% in a CCl 4 solution L. Forlani, G. Cristoni, C. Boga, , P. E. Todesco, E. Del Vecchio, S. Selva, and M. Monari. ARKIVOC (2002), (xi) 198 -215.
Methods Experimental o o 97% 2 PD ground into powder and dissolved into CCl 4, 0. 025 M solution Solutions were sonicated Double well cell, 5 mm pathlength Point-by-point scans Theoretical o o Gaussian 03 (www. gaussian. com) DFT n n o o o B 3 LYP/6 -311++G(2 d, p) Ultrafine grid MP 2/6 -311++G(2 d, p) Full geometry optimizations Harmonic vibrational analyses
Terahertz spectrometer Detector Arm Doubler/ Tripler Beamsplitter Pick 50/50 Splitter THz Arm Pick THz Arm Optical Chopper THz Generator Crystal (Ga. P) Nd: YVO 4 UItrafast. Laser. System Nd: YVO 4 UItrafast Labview PC Dry Air Purge Lock-in Amp. Sample Off-axis Parabolics TH z Delay Stage UV Probe THz 30 fs ~30 fs pulses 800 nm nm ll≈≈800 2. 5 m. J/pulse THz Ti: Sapphire Regenerative Oscillator Amplifier Nd: YLF Balanced Photodetectors 50/50 Polarizer Splitter Beamsplitter ¼ wave Wollaston plate. Prism THz Detection Detector Crystal (Ga. P)
Experimental THz spectrum of 2 PD
Calculated binding energies (BEs) of the dimers Dimer BE* BE w/BSSE & ZPVE correction* 2 PD/2 PD 19. 16 17. 61 2 PD/2 PD (MP 2) 22. 87 19. 06 2 HPD/2 HPD 15. 34 13. 65 2 PD/2 HPD 14. 96 13. 46 * kcal/mole, all calculations done with B 3 LYP/6 -311 G++(2 d, p) unless noted otherwise o o 2 PD dimer has greatest BE 2 PD dimer is more favored Majority of intensity comes from 2 PD vibrations MP 2 Results are similar to DFT
Theoretical and experimental THz spectra of 2 PD dimer
Other molecules or clusters o Other dimers? n o Monomers? n n o Neither tautomer dimer or mixed dimer has absorptions in the unidentified region Have no calculated modes below 165 cm-1 Dimerization occurs at 10 -5 M or greater Tetramers? n n No previous reports on 2 PD or 2 HPD tetramers Our DFT calculations show tetramers with favorable binding energies and modes in our experimental region
Calculated 2 PD tetramer structures o B 3 LYP o Top view Side view MP 2 Top view Side view
Tetramer binding energies (BEs) DFT MP 2 Tetramer BE* BE with BSSE correction* 2 PD 40. 66 38. 89 2 HPD 26. 57 23. 89 2 PD 69. 28 54. 58 2 HPD 57. 82 41. 47 * kcal/mole o o BEs shown are tetramers formed from 4 monomers 2 dimers forming a tetramer is unfavorable MP 2 values are larger then DFT BEs (dispersion) 2 PD tetramer is most favorable
B 3 LYP predicted tetramer spectra
2 PD crystal structure o o o o Known atom positions No solvent interactions considered P 212121 Z =4, = = = 90° a = 13. 585(2) Å, b = 5. 806(3) Å, c = 5. 603(2) Å Crystal 06 (www. crystal. unito. it) DMol 3 (www. accelrys. com) ABINIT (www. abinit. org) H. W. Yang, B. M. Craven. Acta Crystallogr. , Sect. B: Struc. Sci. (1998), 54, 912.
Solid state density functional theory (B) B 3 LYP/6 -31 G(d, p) 0. 85 frequency scalar (A) Experimental far-IR spectrum of polycrystalline 2 PD, in the range of 50 -400 cm-1 (B) Theoretical, calculated using Crystal 06, far-IR spectrum of polycrystalline 2 PD, in the range of 50 -400 cm-1 M. J. Wojcik, W. Tatara, M. Boczar, A. Apola, S. Ikeda. J. Mol. Struc. 596 (2001), 207 -214.
Solid 2 PD THz spectra o o o Three absorptions in experiment In good agreement with theory Rising baseline is due to scattering All modes are intermolecular Optical rotation and translation
Conclusions o o o Spectra were taken of 2 PD in CCl 4 Data suggest most absorptions seen are due to 2 PD dimer Unidentified absorption could be due a tetramer Solid 2 PD spectra were also taken Theory agrees well with experiment More defined system, less uncertainty about assignments Acknowledgements n n Syracuse University CNY-PR AGEP
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