Gyrgy Tarczay, Gbor Magyarfalvi
ON THE ADVANTAGES OF MATRIX-ISOLATION VIBRATIONAL CIRCULAR DICHROISM SPECTROSCOPY Gyrgy Tarczay, Gbor Magyarfalvi Laboratory of Molecular Spectroscopy Elemr Vass Laboratory for Chiroptical Structure Analysis Institute of Chemistry, Etvs University, Budapest, Hungary 61st International Symposium on Molecular Spectroscopy, Columbus, 2006 Photoelastic modulator (PEM)
Vibrational circular dichroism (VCD) spectroscopy FT-IR Polarization filter Photoelastic modulator (PEM) MCT detector Sample Multiplexer Lock-in amplifier Optical filter Advantages of VCD spectroscopy
Unlike in electronic CD spectroscopy (ECD) the investigated chiral molecule does not need to have any chromophoric groups VCD spectra have richer structural content than ECD spectra the absolute configuration determination can be based on more assigned bands ab initio calculation of VCD spectra can be carried out much more accurately the probability of an incorrect assignment is much smaller than in the case of ECD spectroscopy Commercially available spectrometers from the late 90s (BioTools, Bruker, Jasco) Matrix-isolation VCD spectroscopy Previous Studies I.
a-pinene Matrix VCD VCD in CCl4 Matrix IR IR in CCl4 D. W. Schlosser, F. Devlin, K. Jalkamen, and P. J. Stephens, Chem. Phys. Lett. 88, 286 (1982) Matrix-isolation VCD spectroscopy Previous Studies II.
a-pinene Matrix VCD Matrix IR Neat liquid VCD Neat liquid IR D. O. Henderson and P. L. Polavarapu, J. Am. Chem. Soc. 108, 7110 (1986) Conclusions of previous matrix-isolation VCD studies
Well resolved features Intensity gain compared to spectra of neat liquids or solutions Better correspondence between experiment and computations BUT No work has been published after these two studies It seems that the more labor-intensive matrix VCD measurements are not worth the candle in the case of single-conformer and/or weakly interacting molecules WHAT IS THE CASE WITH MORE COMPLEX SYSTEMS? molecules with multiple conformers and/or H-bonding capability chiralchiral and chiralachiral molecular complexes chiral radicals Conformers of 2-amino-1-propanol
3 torsional angles 25 conformers 10-15 conformers are predicted to be present over 1% at room temperature 5 conformers have been identified in the matrix isolation IR spectra R. Fausto, C. Cacela, M. L. Duarte, J. Mol. Struct , 365 (2000) IR and VCD spectra of R-2-amino-1-propanol
Matrix-isolation VCD 10 Matrix-isolation IR Neat liquid VCD Neat liquid IR Wavenumber/ cm-1 G. Tarczay, E. Vass, G. Magyarfalvi, Angew. Chem. Int. Ed. 45, 1775 (2006) Calculated and matrix-isolation IR and VCD spectra of R-2-amino-1-propanol
(BPW91/TZ2P) IR (BPW91/TZ2P) G. Tarczay, E. Vass, G. Magyarfalvi, Angew. Chem. Int. Ed. 45, 1775 (2006) Conformers of Ac-Ala-NHMe
j B3LYP/6-31++G** gD dD aL eD bL(D) eL aD dL gL bL ~40% gL ~54% 0 f dD ~5% 0 j eD ~0.1% IR spectrum of Ac-Ala-NHMe
bL gL dD Amide A vibrational mode Amide I and II vibrational modes MI-IR Calculated Wavenumber / cm-1 Wavenumber / cm-1 G. Pohl, G. Magyarfalvi, E. Vass, A. Perczel, G. Tarczay, to be submitted. Experimental and calculated VCD spectra of Ac-Ala-NHMe
Wavenumber / cm-1 Ar matrix Solution (CH2Cl2) Calculated bL gL dD DA - + G. Pohl, G. Magyarfalvi, E. Vass, A. Perczel, G. Tarczay to be submitted. Rotatory strength surfaces of amide I vibrational modes of the bL conformer of Ac-Ala-NHMe
j f j j-je j-je f-fe f-fe G. Pohl, G. Magyarfalvi, E. Vass, A. Perczel, G. Tarczay, to be submitted. Potential energy surface (PES) and rotatory strength surfaces (RSSs) of bL Ac-Ala-NHMe
(Amide I modes) PES DE /cm-1 r0 r0 j-je j-je re re f-fe f-fe G. Pohl, G. Magyarfalvi, E. Vass, A. Perczel, G. Tarczay, to be submitted. Calculated VCD and IR spectra of theg'Gg' R-2-amino-1-propanolwater complex
4000 3500 3000 2500 2000 1500 1000 500 Wavenumber/cm-1 IR VCD G. Tarczay, E. Vass, G. Magyarfalvi, Angew. Chem. Int. Ed. 45, 1775 (2006) Complexation of Ac-Ala-NHMe and water
8 K 15 K 8 K bL gL A 8 K H2O Wavenumber / cm-1 Matrix-isolation VCD spectroscopy
ADVANTAGES Investigation of complex systems (multiple conformers, complexes, radicals) Simultaneous determination of configuration and conformation It can help the assignment of matrix isolation IR spectra Neither aggregates nor complexes with solvents have to be considered for theoretical calculations LIMITATIONS Maximum size of the molecule that can be evaporated (Knudsen cell) Maximum size of the molecule that can be calculated (parallel codes) DIFFICULTIES thick and homogenous matrices are required (long deposition times) light scattering on thick layers (long measurements) possible artifacts due to non-parallel alignment, strength in window material (CsI BaF2), interference, background (dual modulation) Matrix 2006 Etvs University
Gyrgy Tarczay Gbor Magyarfalvi Elemr Vass Matrix isolation remains an activity involving anti-social hours of work. Cradock and Hinchliffe: Matrix Isolation, Cambridge University Press Gbor Bazs Edit Mtyus Gbor Pohl