1 L. Spada, Q. Gou, B.M. Giuliano, W. Caminati. 70 th Symposium, Urbana-Champaign, June 22-26, RH06- THE ROTATIONAL SPECTRUM OF PYRIDINE – FORMIC ACID L. Spada, Q. Gou, B.M. Giuliano, W. Caminati. Dipartimento di Chimica ”G.Ciamician” - Universita’di Bologna

2 1)Dimers of carboxylic acids. Strong hydrogen bonds, proton tunnelling, Ubbelohde effect, internal dynamics, Conformational equilibria. The rotational spectra of several complexes of carboxylic acids have been investigated in the last decade, which supplied plenty of information on the nature of the involved hydrogen bonds, on the internal dynamics and, occasionally, on the conformational equilibria. They can be classified as: 2) Adducts of carboxylc acids with water. Hydrogen bonds, water motions. 2) Adducts of carboxylc acids with other organic molecules. Hydrogen bonds

Proton tunneling and potential energy function to the proton transfer in carboxylic acid bi-molecules. v = 0 rotational manifold v = 1 rotational manifold Interactions, States mixing  E 01 B2B2 DIMERS OF CARBOXYLIC ACIDS

Homo dimers, no dipole moment Annu. Rev. Phys. Chem :263–75

ChemPhysChem 2008  (S 0 ) = (7) MHz  (S 1 ) = 271.2(7) MHz  (A) = (12) MHz  (E) = (30) MHz J.Chem.Phys DIMERS OF CARBOXYLIC ACIDS: Proton transfer tunnelling

J.Chem.Phys Microwave measurements of proton tunneling and structural parameters for the propiolic acid–formic acid dimer. A. M. Daly, K.O. Douglass, L.C. Sarkozy, J.L. Neill, M.T. Muckle, D.P. Zaleski, B.H. Pate, S. G. Kukolich  E/MHz = (5) (HH) and 3.35(2) (DD) Proton Tunneling in Heterodimers of Carboxylic Acids: A Rotational Study of the Benzoic Acid−Formic Acid Bimolecule L.Evangelisti,P. Écija,E.J. Cocinero, F. Castaño, A.Lesarri, W. Caminati, R. Meyer J.Phys.Chem.Lett Tunnelling Splittings ObsCalc ΔE 01 (HH)/MHz (6) a ΔE 01 (DD)/MHz8.3(3)8.1 Parameters B 2 / cm -1 = 2442(150) w b = b f 1 / cm -1 = 270 c f 2 / cm -1 = 60 c DIMERS OF CARBOXYLIC ACIDS: Proton transfer tunnelling

JACS 2012 Proton Transfer in Homodimers of Carboxylic Acids: The Rotational Spectrum of the Dimer of Acrylic Acid G. Feng, L.B. Favero, A. Maris, A. Vigorito, W. Caminati, R. Meyer. Gas phase measurements of mono-fluoro-benzoic acids and the dimer of 3-fluoro-benzoic acid A.M. Daly, S.J. Carey, A.M. Pejlovas, K. Li, L. Kang, S.G. Kukolich JCP 2015 HOMO DIMERS OF CARBOXYLIC ACIDS : Proton transfer tunnelling  E/MHz ≈ 563(10)

DIMERS OF CARBOXYLIC ACIDS Conformational equilibria and Ubbelohde effect Conformational equilibria and tunneling dynamics in difluoroacetic acid – acrylic acid bi- molecule G. Feng, Q. Gou, L. Evangelisti, W. Caminati. Angewandte 2014 Difluoro acetic acid dimer

Adducts of carboxylic acids with water MW reports available to my knowledge: -Formic acid – water ( D. Priem, T.-K. Ha, A. Bauder, JCP 2000 ) -Water with propanoic acid, trifluoroacetic acid, difluoroacetic acid, acetic acid (B.Ouyang, T.G. Starkey, B.J. Howard, JPC-A/PCCP ) -Benzoic acid – water (E.G. Schnitzler, W. Jäger, PCCP 2014) In addition, we studied (not yet published), a couple of years ago, the MW spectra of: -Acrilyc acid– water, FCH 2 -COOH – water The main problem with these molecular systems is that many relatively small splitting of the rotational lines have been observed, but not yet interpreted in terms of a tunneling internal motion. Schnitzler and Jäger suggest these splittings as attributable to one of the following motions: (1)wagging of the unbound hydrogen atom of water, through the mirror plane of the BA moiety; (2) rotation of water about the lone pair hydrogen-bonded to BA; (3) rotation of water around its C2 axis. (4) exchange of the two hydrogen-bonded protons, a motion similar to that described for the benzoic acid–formic acid Dimer And conclude: “In summary, tunnelling motion (2) is likely the only candidate for the observed splitting. However, how this tunnelling motion can cause a relatively complicated splitting pattern with four components remains unclear”.

Adducts of carboxylic acids with other organic molecules MW reports available, to my knowledge, for: -Formic acid – formamide ( A.M. Daly, B.A. Sargus, S.G. Kukolich, JCP 2010 ) -Formic acid – pyridine ( THIS STUDY ) -Formic acid – formaldehyde (Q. Gou, L.B. Favero, S. S. Bahamyirou, Z. Xia, W. Caminati, JPC-A 2014) -Formic acid – CO 2 ( Maris et al. Manuscript ready )

HCOOH-Pyridine: MP2/ G(d,p) calculations

HCOOH-Pyridine: Experimental results Planar moment of inertia, P cc : in the range from to uÅ 2 Dissociation energy: ca. 40 kJ/mol r(N---O) hydrox = Å; r(N---O) carbonyl = Å Ubbelohde effect: the substitution of HCOOH with HCOOD causes a shortening of a few mÅ of the r(N---O) distances

Barrier to proton transfer in CA dimers 3D potential energy function V(x, y 1, y 2 ) = s(x) {B 0 (1 – x 2 ) 2 + [f 1 / s(1)] (y 1 - y 1e ) 2 + [f 2 / s(1)]  y 2a 2 } x represents the double hydrogen transfer coordinate, while y 1 and y 2 are the coordinates describing the antisymmetric and the symmetric structural changes when going from the C 2v -symmetric saddle point configuration to either equilibrium point and beyond. s(x) is a shape function depending on x and on the ratio between the barrier B 0 and the dissociation energy D 0, s(x) = 1/ [1 + (B 0 /D 0 )1/2 x 2 ] 2 Proton Transfer