FAR-IR ACTION SPECTROSCOPY OF AMINOPHENOL AND ETHYLVANILLIN: EXPERIMENT AND THEORY Vasyl Yatsyna, Daniël Bakker*, Raimund Feifel, Vitali Zhaunerchyk, Anouk Rijs* Department of Physics, University of Gothenburg, Sweden, *FELIX Laboratory, Radboud University Nijmegen, The Netherlands, ISMS 2015, June 24

IR spectroscopy for structural analysis Capping motif for peptide helix formation J. Phys. Chem. Lett. 2015, 6, 1504−1508 Experimental IR spectra (usually in range cm -1 ) Theoretical spectra Structure!!! Not perfect Can be congested

Far-IR range for structure determination Delocalized backbone vibrations Unique structural fingerprint Direct probe of hydrogen bonding modes Challenging because: ―Weak vibrational bands (high power source needed for gas-phase) ―Influence of anharmonicity and mode coupling ―Poor performance of conventional theoretical approaches ν =298 cm -1 ν =3626 cm -1 Localized Delocalized See talk RB02

Explore far-IR range with semi-rigid molecules Ethyl Vanillin, 22 atoms (3-ethoxy-4-hydroxybenzaldehyde) Aminophenol, 15 atoms 2-AP3-AP4-AP Methyl Acetanilide, 22 atoms 4-MA 2-MA Peptide bond Theory?

Ways to treat anharmonicity  Variational (brute force, very expensive)  Vibrational self-consistent field (VSCF, implemented in GAMESS, suitable for large systems, but dependent on choice of coordinates etc.)  Molecular dynamics (e.g. DFT based Born Oppenheimer MD )  Vibrational second-order perturbation theory (VPT2, implemented in Gaussian, reasonable accuracy): See V. Barone et al. Phys. Chem. Chem. Phys., 2014, 16, Evaluation of Hessians in 6N-11 points See talk RB02 by Daniël Bakker

rotational and vibrational cooling mass selectivity conformer selectivity good spectral resolution Experiment: IR-UV ion dip technique Free Electron Laser for Infrared eXperiments (FELIX) FEL-1: µm ( cm -1 ) FEL-2: 3-45 µm ( cm -1 ) 0.2% spectral bandwidth Molecular conformers M M+M+ M* UV REMPI IR

Ethyl Vanillin: conformers trans-trans cis-trans cis-gauche trans-gauche Phys. Chem. Chem. Phys., 2010, 12, 12486–12493 planar non-planar

Ethyl Vanillin: Harmonic DFT spectra B3LYP/6-311+G** FWHM 1%

Experimental spectra of ethyl vanillin Conformer 1 50 µm 10 µm 20 µm

Experimental spectra of ethyl vanillin Conformer 2 50 µm 10 µm 20 µm Ion gain features For details see E. Buchanan et al., Faraday Discuss., 2011, 150, 209 Conformer 1

Ethyl Vanillin: Harmonic vs anharmonic VPT2 correction cis-trans harmVPT2 RMS, cm MAX, cm B3LYP/6-311+G** x5 Conformer 1

Trans-trans harmVPT2 RMS, cm MAX, cm B3LYP/6-311+G** Ethyl Vanillin: Harmonic vs anharmonic VPT2 correction x5 Conformer 2

Aminophenol experiment vs theory * - NH 2 wagging (inversion) fundamental with DFT B3LYP/aug-pc2, scaling factor= * * * *

Aminophenol experiment vs theory Problematic vibrations: -NH 2 wagging (inversion) -OH, NH 2 wagging Same behavior as for halosubstituted anilines: J. Chem. Phys., Vol. 69, No.2, 1978 * *

Aminophenol: Basis set dependence for VPT2 Moderate basis sets perform well!

Aminophenol: performance of different functionals Basis set: G(d,p) GGA Hybrid +Dispersion/long range Double hybrid

Hybrid scheme for better accuracy  Harmonic frequency calculations are performed with more accurate method (e.g. CCSD(T) or B2PLYP)  Anharmonic PT2 corrections are calculated with less expensive method such as B3LYP/N07D  Computed fundamental frequencies give improved accuracy with lower computational cost MethodRMS, cm -1 B3LYP8.19 PBE09.96 B B2PLYP(cc-pVTZ)/B3LYP(N07D)9.98

Conclusions  Far-IR range is efficient for structure determination of species showing “identical” or congested mid-IR spectra  Anharmonic VPT2 approach aids the assignment, with best performance of B3LYP, PBE0, B3PW91 and B3LYP-D3 functionals with moderate basis set  Normal modes with strongly anharmonic potential need evaluation with other methods, such as variational approach or molecular modeling simulations.

Acknowledgments  For support and discussions:  Anouk Rijs  Vitali Zhaunerchyk  Daniël Bakker  Raimund Feifel  FELIX Laboratory team - for support in experiments, particularly to Lex van der Meer and Britta Redlich  Surfsara for providing time at Lisa computing facility THANK YOU!

3-aminophenol spectra

2-Methyl Acetanilide spectra Mid-IR trans 2MA