Adducts of Ammonia DOI: 10.1002/ange.200805104 Adducts of NH 3 with the Conformers of Glycidol: A Rotational Spectroscopy Study** BarbaraM. Giuliano, Sonia Melandri, Assimo Maris, Laura B. Favero, and Walther Caminati* Various chemical problems, difficult to unravel with other techniques, were solved recently by rotational spectroscopy experiments in supersonic jets. The investigations of molec- ular complexes with this technique gave indications on the nature of the forces which bind the constituent molecules, on the preferred sites of interaction, and on the widely unex- plored interactions in the intermediate regime between bonding and nonbonding. Molecular recognition, molecular aggregation, rare-gas atoms forming covalent bonds, and many more phenomena are investigated with this techniques without having to rely heavily on ab initio calculations— although their support is of great help in guiding spectro- scopic searches and interpreting the spectra. Many of the complexes investigated involve one or more molecules of water linked to organic or biomolecules. 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 OÀH w ···O, OÀH···O w , O À H w ···N, N À H···O w , [1–4] where the subscript indicates an atom belonging to the water molecule. These hydrogen bonds are moderately strong, in the range 15–25 kJ mol À1 . Less information is available on molecular complexes of ammonia with organic molecules. The only systems inves- tigated in which ammonia could play the dual proton-donor/ proton-acceptor role are F 3 CH···NH 3 , [5] CH 3 OH···NH 3 , [6] pirrole···NH 3 , [7] and tert-butanol···NH 3 . [8] All the rotational spectra of its complexes with organic molecules revealed only conformers where ammonia, unlike water, acts exclusively as a proton acceptor. A few rotationally resolved electronic-spectroscopy stud- ies on adducts of ammonia with larger molecules confirm this behavior, such as in the case of 1-naphtol···NH 3 , [9] hydro- quinone···NH 3 , [10] and aniline···NH 3 . [11] This trend is in agreement also with the different pK HB values, 0.64 and 1.86, respectively, for water and ammonia. K HB is the equilibrium constant for the association reaction with a proton reference donor, namely 4-fluorophe- nol. [12] The higher value for ammonia shows that ammonia is a stronger hydrogen bond acceptor than water. The formation of molecular complexes can result in various relative conformations. Interesting equatorial/axial equilibria have been encountered while investigating the microwave (MW) spectra of adducts of hydrohalogenic acids with organic molecules. For example, in saturated cyclic ethers, such as tetrahydropyran, two different lone pairs (axial and equatorial) are available at the ether oxygen atom to accept a proton. Alonso and collaborators could assign the supersonic-jet Fourier-transform microwave (FTMW) spectra of both axial and equatorial complexes in the case of tetrahydropyran···HCl, [13] tetrahydropyran···HF, [14] and pen- tamethylene sulfide···HF, [15] thus characterizing relative ener- gies and structural differences. Three conformers have been observed for formamide···water, [4] and six conformers in the molecular recognition study of the self-aggregation of pro- pylene oxide. [16] However, no rotational spectra are available on different adducts originating from the different conforma- tions of a given molecule with a ligand. Ammonia, thanks to its propensity to behave only as proton acceptor, can generate a small number of species in the conformational mixture of its adducts with organic molecules, and these species can there- fore be detected by FTMW spectroscopy. Glycidol (oxiranemethanol) is an interesting chiral mol- ecule, which, in a sense, resembles the palm of a hand, and for this reason it has been used as a probe for molecular recognition in IR [17] as well as MW [18] studies. Glycidol (GLY) provides two possible sites for hydrogen-bond contact: the ring oxygen atom (acceptor) and the OH group (donor/ acceptor). Depending on the orientation of the hydroxy hydrogen with respect to the ring oxygen atom, it can generate two different internally hydrogen-bonded conform- ers, G1 and G2, whose conformations and energy differences have been determined experimentally by MW spectrosco- py. [19] Owing to the small energy difference between the two conformers [DE G2ÀG1 = 3.6(4) kJ mol À1 ], GLY appears to be a suitable molecule with which to investigate the adducts generated with ammonia by the two species. Theoretical calculations were performed before the spectroscopic search. Using the program ORIENT 4.6, [20] we optimized the predicted geometries of the molecular adducts at the highest available rank of the multipole expansion, that is, up to the hexadecapole. Within these preliminary calcu- lations, the structures of ammonia and glycidol were frozen and any structural relaxation upon complexation was neglected, while the dissociation energy was corrected for the zero-point energy contribution. The results obtained with ORIENT are summarized in the Supporting Information. Among twelve energy minima, two [*] Dr. B.M. Giuliano, Dr. S. Melandri, Dr. A. Maris, Prof. Dr. W. Caminati Dipartimento di Chimica “G. Ciamician” dell’Università Via Selmi 2, 40126 Bologna (Italy) Fax: (+ 39) 051-209-9456 E-mail: walther.caminati@unibo.it Dr. L. B. Favero Istituto per lo Studio dei Materiali Nanostrutturati (ISMN, Sezione di Bologna), CNR, via Gobetti 101, 40129 Bologna (Italy) [**] This research was funded by MIUR and the University of Bologna. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200805104. Zuschriften 1122  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2009, 121, 1122 –1125