The jetcooled highresolution infrared spectrum of formic acid

The jet-cooled high-resolution infrared spectrum of formic acid cyclic dimer Manuel GOUBET 1, Sabath BTEICH 1, Thérèse R. HUET 1, Olivier PIRALI 2, Pierre ASSELIN 3, Pascale SOULARD 3, Atef JABRI 3, Pascale ROY 4, Robert GEORGES 5 1 Université Lille, CNRS, UMR 8523 - Ph. LAM - Physique des Lasers Atomes et Molécules, France des Sciences Moléculaires d’Orsay, UMR 8214, CNRS Université Paris-Sud, France 3 Sorbonne Universités, UPMC Univ Paris 06, UMR 8233, MONARIS, France 4 Ligne AILES - Synchrotron SOLEIL, France 5 Institut de Physique de Rennes, UMR 6251, CNRS Université Rennes 1, France 2 Institut June 21 st 2017 – ISMS – Urbana Champaign

Motivations Formic acid (FA) very abundant organic compound & the simplest carboxylic acid Widely used in chemistry. Abundant in the atmosphere and the ISM. Model system for the properties of carboxylic acids. Dimeric form dominant in the gas phase in standard conditions (STP): ~ 92%. Peculiar thermodynamical properties. Cyclic dimer (FAD) is an elementary system for the Concerted Proton transfer process. IR bands of the dimer are unresolved at room temperature: spectral congestion + Doppler broadening. 2 June 21 st 2017 – ISMS – Urbana Champaign

Spectroscopic issues Prolate asymmetric top (κ ~ -0. 7) belonging to the C 2 h point group. No pure rotation spectrum; 12 IR and 12 Raman active modes. Statistical weights (Ka, Kc) are ee: eo: oe: oo = 1: 3: 9: 3 Series hard to follow. Concerted proton transfer with TS in D 2 h (~ G 8) Rovib. levels split into 2 tunneling states: lower (l) and upper (u). Selection rules different according to band type. Birer&Havenith, Annu. Rev. Phys. Chem. 2009, 60: 263– 75 3 June 21 st 2017 – ISMS – Urbana Champaign

Spectroscopic issues Low frequency intermolecular modes Rapid high density of states. 4 June 21 st 2017 – ISMS – Urbana Champaign

Spectroscopic issues Jet-cooled conditions mandatory to rotationally resolve IR bands. 5 June 21 st 2017 – ISMS – Urbana Champaign

Spectroscopic issues Parts of C-O stretch and 2 combi recorded. No LSCD possible: strong correlation between GS and ES constants. Tunneling states treated separately. Broadband Jet-cooled conditions mandatory for multi-band LSCD analysis. 6 June 21 st 2017 – ISMS – Urbana Champaign

The jet-AILES apparatus 7 June 21 st 2017 – ISMS – Urbana Champaign

The jet-AILES apparatus ces our s l a rn Inte n atio radi n o rotr h c n Sy r litte p s am Be Toroidal mirrors MCT Back exit l e l l para eter m o Bol Magnification factor 1. 14 8 June 21 st 2017 – ISMS – Urbana Champaign

The jet-AILES apparatus Conditions: He @ 5 l/min FA @ 200 g/hr Tinj = 313 K 60 mm x 50 µm slit Pres = 90 Torr Pch = 0. 15 Torr 9 June 21 st 2017 – ISMS – Urbana Champaign

Synchrotron-based jet-cooled far-IR spectrum of FAD @ max res jet, res. 0. 001 cm-1 jet, res. 0. 005 cm-1 cell, res. 0. 1 cm-1 10 June 21 st 2017 – ISMS – Urbana Champaign

Intermolecular in-plane bending 340 co-added interferograms (~12 h of scans) recorded at max. resolution (10 -3 cm-1) using synchrotron radiation + 6 µm mylar beamsplitter + He-cooled Si bolometer. - exp. - sim. l l - sim. u u zoom in the Q branch zoom in the P branch. Qualitative agreement but fit far from experimental accuracy. 11 June 21 st 2017 – ISMS – Urbana Champaign

Intermolecular in-plane bending Loomis-Wood diagrams (Lodyga’s LWWa) Tunneling states observed. Statistical weight issue. R Branch P Branch Ka = 0 l l u u Ka = 3 Ka = 1 12 June 21 st 2017 – ISMS – Urbana Champaign

1200 cm-1 Fermi triad: C-O stretching + combinations 80 co-added interferograms (~4 h of scans) recorded at max. resolution (10 -3 cm-1) using globar source + KBr beamsplitter + MCT photovoltaic detector. - exp. - sim. l u - sim. u l Combi. bands C-O str. 13 June 21 st 2017 – ISMS – Urbana Champaign

(very !) Preliminary “global” analysis Fit (Watson A-red. Hamiltonian on independent tunneling states) of: intermolecular in-plane bending lines C-O stretching lines from this work C-O stretching lines from Goroya et al. C-O stretch (u) GS (l) intermol. bend (l) Param. / MHz this work Goroya this work A 6065. 057 6066. 702 6049. 394 6050. 522 6078. 670 B 2286. 376 2287. 383 2277. 136 2278. 345 2254. 806 C 1661. 609 1663. 875 1653. 001 1654. 950 1643. 331 DJK -0. 0127 0. 0057 -0. 0276 -0. 0132 -0. 0193 DJ 0. 0019 0. 0028 0. 0056 0. 0066 0. 0020 Here again: qualitative agreement but fit far from experimental accuracy. More coherent set of constants (i. e. centrifugal distortion). 14 June 21 st 2017 – ISMS – Urbana Champaign

O-H out-of-plane bending C=O stretching Medium c-type band Strong b-type band Only the Q-branch observed with jet-AILES. Colder QCL spectrum @ MONARIS with observed P and R branches. Full band recorded @ jet-AILES. Parts of recorded by Ortlieb & Havenith. QCL spectrum Jet-AILES spectrum More HR data available for a complete analysis. 15 June 21 st 2017 – ISMS – Urbana Champaign

Now and next Broadband Jet-cooled IR spectra of FAD for multi-band LSCD analysis. Jet-AILES apparatus (~ 40 K) and QCL sp @ MONARIS (~ 10 K). 4 fundamental & 4 combination bands, rotationally resolved, available. Concerted proton transfer tunneling splitting clearly observed. LSCD analysis in progress. Concerted proton transfer process accessible. 2 D-PES calculations in progress. Spectral simulation using appropriate model: call for theoreticians’ help! Thermochemical study of the monomer – dimer equilibrium. Retrieve Gibbs free energy at room temp as for acetic acid. DG 0 16 June 21 st 2017 – ISMS – Urbana Champaign

Acknowledgments € § SOLEIL and the AILES staff for support under proposals § Labex Ca. PPA through the PIA under contract ANR-11 -LABX-0005 -01, by the Regional Council Hauts de France and by the European Funds for Regional Economic Development (FEDER). § Gd. R SPECMO § The french research agency ANR under contracts (ncpmol, ncpchem, topmodel) § The "Action sur projet INSU Physique et Chimie du Milieu Interstellaire" (PCMI) 17 June 21 st 2017 – ISMS – Urbana Champaign

Acknowledgments M. Goubet, Th. Huet Ph. LAM – Université Lille 1 The jet-AILES team: P. Soulard, P. Asselin MONARIS – Université Paris VI O. Pirali, P. Roy AILES, synchrotron SOLEIL R. Georges, J. Courbe IPR – Université de Rennes 1 M. Cirtog A. Moudens J. B. Brubach 18 June 21 st 2017 – ISMS – Urbana Champaign

Slit-jet adiabatic expansion Shock diamonds “Silence zone” (highly supersaturated) Impact pressure (Torr) Stagnation reservoir 100 – 1000 Torr Residual gas < 1 Torr Shear layer Nozzle aperture: 60 mm × 18 m First shock Second shock 20 slm Argon / P 0 = 800 Torr / Pres = 0. 50 Torr 19

Density in the jet Carrier gas: 5× 1016 – 1017 molecule. cm-3 Active molecule: few percent (> 1015 molecule. cm-3) Mechanical pumping (Roots) Liquid N 2 trap Edwards drystar 80 m 3/h 20 Edwards EH 500 + EH 2600 Roots pumps

Rotational temperature in the jet Exp. Resolution : 0. 002 cm-1 Transmittance Trot = 31 1. 5 K Ar 30 slm / H 2 O 100 g/h Trot = 15 1 K Ar 20 slm / H 2 O 12 g/h Shock wave Trot = 10. 3 0. 2 K 111 110 ortho 101 000 para 202 101 ortho Wavenumber / cm-1 21 Ar 20 slm / H 2 O 4 g/h

Kinetics of clustering in a supersonic expansion a IR beam probes jet < 10 mm b d c e (b) Kn max = 0. 125 << 10 Ä Rayleigh formula applies (c) M = 17 @ 10 mm Ä Efficient enthalpy kinetic conversion: T < 6 K @ 5 mm (d) (e) Density drops down rapidly Ä Clustering very close to exit 22