COMMUNICATIONS 1772  WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999 1433-7851/99/3812-1772 $ 17.50+.50/0 Angew. Chem. Int. Ed. 1999, 38, No. 12 Axial and Equatorial Hydrogen Bonds in the Tetrahydropyran ´´´ HCl Dimer** Sonia Antolínez, Juan C. Lo  pez, and Jose  L. Alonso* Hydrogen-bond complexes are formed in the gas phase by collisional association of the components upon adiabatic expansion of a high-pressure gas mixture through a nozzle into a vacuum. When the frozen dimers are in collisionless expansion, their rotational spectrum can be observed by conducting pulsed-microwave Fourier transform spectroscopy on the jet within a Fabry±Perot cavity. We report here the first observation of the axial and equatorial hydrogen-bond complexes formed in the gas phase between tetrahydropyran (THP) and hydrogen chloride. The properties of the dimers, especially their geometry, can be readily determined from the rotational spectrum. Gas-phase molecular structures for binary hydrogen-bond- ed complexes have traditionally been obtained with molecular beam Fourier transform microwave spectroscopy. [1, 2] Virtual- ly all of the dimers B ´´´ HX (where the acceptor atom in B is O, S; and X  F, Cl, Br, I) characterized so far contain monomers B with two equivalent nonbonding electron pairs. As a result of the investigation of a large number of complexes, it has become possible to enunciate some simple rules [3] for predicting geometries based on the nonbonding electron pair model of the acceptor molecule B. The hydrogen atom in HX is electrophilic and is assumed to seek the most nucleophilic site of B, which will be along the direction of the nonbonding electron pairs. We have recently studied hydro- gen-bonded complexes of five-membered heteroatomic rings such as tetrahydrothiophene ´´´ HCl, [4] tetrahydrothiophene ´ ´ ´ HF, [5] and tetrahydrofuran ´´´ HCl. [6] In accordance with these electrostatic rules, two equivalent equilibrium confor- mations having pyramidal configurations at O or S have been found. The six-membered ring THP has two nonequivalent lone electron pairs at the O atom in the axial and equatorial positions. We therefore expect two different hydrogen bonds for the THP ´´´ HCl complex (Figure 1). To confirm these expectations a detailed experimental investigation of THP ´´´ HCl has been carried out by molecular beam Fourier trans- form microwave spectroscopy. [7] The rotational spectrum is very sensitive to molecular conformation, so the axial and equatorial forms can be independently studied, just as in the case of a mixture of stable, nonreactive species. Trial rotational constants of the complexes were estimated from the structures of the monomers THP [8, 9] and HCl, [10] according to the above models. The geometrical parameters Figure 1. Representation of the two different hydrogen bonds expected in the THP ´´´ HCl complex: axial (left) and equatorial (right). of the hydrogen bond found for the related tetrahydrofuran ´´´HCl [6] dimer were used to describe the separation and relative orientation of the subunits. The vector sum of the electric dipole moments of the component molecules predicts a strong m a -type spectrum and a weak m c -type spectrum with the pattern of a nearly prolate asymmetric rotor. Initially argon was used as the carrier gas and the observed spectrum consisted of groups of m a -type, R-branch lines separated by about 1200 MHz, which accounts for the B  C value of the axial form. We scanned wide ranges of frequency, and no line presumably belonging to the equatorial conformer was detected. In the hope of observing the spectrum of the equatorial form, a further search was performed with helium as carrier gas. Thus, new bands of weaker intensity appeared along with those attributed to the axial form, and they were readily assigned to the equatorial dimer. In addition to the ground-state rotational spectra of the most natural abundant isotopomers THP ´´´ H 35 Cl and THP´´´H 37 Cl, spectra were also measured for both axial and equatorial conformers of the THP´´´D 35 Cl species. All the transitions exhibit a character- istic quadrupole hyperfine structure. It arises from the interaction of the electric quadrupole moment of 35 Cl or 37 Cl with the electric field gradient at the Cl nucleus that couples the Cl nuclear spin angular momentum I with angular momentum resulting from the overall rotational J to give the total angular momentum F  I  J . The transition frequencies of each isotopomer were fitted in an iterative least-squares analysis [11] based on the Hamiltonian in Equation (a), where H R is the operator associated with the energy of the semirigid rotor given in the I r representation of the A-reduced Watson Hamiltonian by Equation (b). [12] The coefficients A, B, and C are the rotational constants, and D J , D JK , D K , d J , and d K are the quartic centrifugal distortion constants. Hamiltonian H Q describes the interaction of the Cl nuclear electric quadrupole moment with the electric field gradient at the Cl nucleus through the tensors Q(Cl) and V(Cl) according to Equation (c). [13] The extent of the hyper- fine splittings is described in terms of the elements of the quadrupole coupling tensor, which are related to the electric field gradient at the Cl nucleus by Equation (d) (a, b to be permuted over the principal inertial axes a, b, and c). H  H R  H Q (a) H R  A P 2 z  B P 2 x  C P 2 y  D J P 4  D JK P 2 P 2 z  D K P 4 z  2 d J P 2 (P 2 x  P 2 y )  2 d K [P 2 z (P 2 x  P 2 y )  (P 2 x  P 2 y )P 2 z ] (b) [*] Prof. J. L. Alonso, S. Antolínez, Prof. J. C. Lo  pez Departamento de Química Física, Facultad de Ciencias Universidad de Valladolid E-47005 Valladolid (Spain) Fax: ( 349) 983-423264 E-mail: jlalonso@qf.uva.es [**] This work was supported by grants from the Direccio  n General de Investigacio Ân Científica y Te Âcnica (DGICYT, PB96-0366) and the Junta de Castilla y Leo Ân (VA51/96). S.A. gratefully acknowledges a FPI grant from the Ministerio de Educacio  n y Cultura.