1 -RH06- PROTON DONOR/ACCEPTOR PROPENSITIES OF AMMONIA ROTATIONAL STUDIES OF ITS MOLECULAR COMPLEXES WITH ORGANIC MOLECULES 64 th OSU International Symposium on Molecular Spectroscopy - - June 22-26, 2009 Dipartimento di Chimica ”G.Ciamician” - Universita’di Bologna B.M. Giuliano, A. Maris, S. Melandri, L.B. Favero, L. Evangelisti, W. Caminati

2 Bologna Molecular Beam Fourier Transform MW Spectrometer: Overview Updated&operating with J.-U. Grabow hardware&software, W.Caminati, A.Millemaggi, J.L.Alonso, A. Lesarri, J.C.Lopez, S. Mata, Chem.Phys.Letters, 392, 1 (2004)

3 Complexes of NH 3 with organic molecules Motivation: Many complexes involving one or more molecules of water linked to organic or biomolecules have been investigated by MW. Water participates in several kinds of hydrogen bonds, acting either as a proton acceptor or a proton donor. The most common hydrogen bonds in these systems are of the type O-H w ···O, O-H···O w, O-H w ···N, N-H···O w with interaction energy in the range kJmol -1. Less information is available on molecular complexes of ammonia with organic molecules. The investigated systems where ammonia could play the double proton donor/acceptor role are just F 3 CH···NH 3, CH 3 OH···NH 3, pirrole···NH 3 and tert-butanol···NH 3. All rotational spectra of its complexes with organic molecules revealed only conformers where ammonia, differently from water, acts exclusively as a proton acceptor. A few rotationally resolved electronic spectroscopy studies on adducts of ammonia with larger molecules confirm this behaviour, such as in the case of 1-naphtol···NH 3, hydroquinone···NH 3, and aniline···NH 3.* Results: We studied the molecular complexes of NH 3 with 5 organic molecules, tert-butanol, glycidol, ethanol, 1,4-difluorobenzene and anisole. We observed quite different shapes and interactions among them. We compare the results to the complexes with water. * J. Chem. Phys. 1986, 84, 5983, Chem. Phys. 1988, 125, 31, J. Mol. Spectrosc. 2008, 251, 224, Chem.Phys.Letters, 2008, 463, 330, J.Chem.Phys. 1996, 104, 8332, J.Chem.Phys. 1997, 106, 908.

4 B.M.Giuliano, M.C.Castrovilli, A.Maris, S.Melandri, W.Caminati and E.A.Cohen, Chem.Phys.Letters, –333 (2008). TBA-NH 3 Structural Parameters R (Å)2.03 Θ (°)171 R θ

5 TBA-NH 3

6 TBA-NH 3 : Internal motions Energia V α 453 cm cm -1 V 3 for the internal rotation of NH 3 is almost 0, according to the Pbb value and to the missing of nearby “E” lines

7 TBA-H 2 O The TBA-hydroxyl hydrogen is tunnelling also in the complex, while water does not practically move TBA-OD species  small tunnelling splittings TBA-OH species  large tunnelling splittings

8 Comparing TBA-H 2 O to TBA-NH 3

9 [1] K.-M. Marstokk, H. Møllendal and Y. Stenstrøm, Acta Chem. Scand., 46, 432 (1992). MW spectra of glycidol revealed hydrogen bonded conformers 1 and 2 [1]. Conformer 1 is more stable by 3.4 (4) kJ/mol. Glycidol-NH 3 : Monomeric Glycidol

10 Glycidol-NH 3 We assigned the MW spectra of both adducts, G1-NH 3 and G2-NH 3. This is one of the few cases in which adducts with two different conformers have been rotationally observed.

11 Glycidol-NH 3

12 Glycidol-NH 3

13 Glycidol-NH 3

14 Ethyl alcohol-NH 3 Also in this case the monomer exists in two different forms. We assigned the MW spectra of both adducts, EAtrans-NH 3 and EAgauche-NH 3. Again, one of the few cases in which adducts with two different conformers have been rotationally observed.

15 Ethyl alcohol-NH 3

16 Anisol-NH 3 From a distributed multipole analysis (DMA) of electron charge distribution. Dissociation energies in kJ/mol.

17 Anisol-NH 3 The data obtained with the model potential calculations, were refined for the six more stable conformers of ANI-NH3 with full geometry optimization on the MP2 counterpoise corrected PES (MP2CP(full))/ G** level of calculation. Only four stationary points were found, since species IV and VI appeared not to be minima, and relaxed to species I and V, respectively.

18 Anisol-NH 3

Isotopomeric Conformational Change in Anisole-Water The deuteration of water produces a conformational change in the Anisole  Water complex, as shown by the scheme. The value of the  angle decreases from 138 to 128 , while the secondary interaction O  H Me is replaced by the O  H Ph one. B.M.Giuliano and W.Caminati, Angew.Chem.Int.Ed.Engl (2005).

20 1,4-diFluorobenzene-NH 3 The σ rather than the  complex was observed. Similar to 1,4-diFluorobenzene-H 2 O. (H. Mäder, personal communication)

21 Conclusions  Ammonia does not show any propensity to behave as a proton donor in complexes with ethers.  Ammonia acts primarily as a proton acceptor in its complexes with alcohols. However, it can play a protic wire role when the partner molecule has a favourable configuration (glycidol). In this case an incipient charge transfer effect is observed. DIFFERENTLY RESPECT TO WATER SIMILARLY TO WATER Ammonia forms σ rather than  complexes with fluoroderivatives of benzene. In all cases, plenty of data on the internal dynamics