Walther Caminati, Sonia Melandri, Adele Dell'Erba and Paolo G. Favero Dipartimento di Chimica "G. Ciamician", Università di Bologna, Via Selmi 2, I-40126 Bologna, Italy. E-mail: caminati@ciam.unibo.it; Fax: +39-051-2099456; Tel: +39-051-2099480 Received 7th December 1999, Accepted 7th January 2000, Published 14th January 2000 The rotational spectrum of the pyridazine···Ne complex, a species observable in supersonic expansions, has been measured by free jet absorption millimeter wave spectroscopy. The equilibrium configuration, dynamics and dissociation energy have been deduced from the spectroscopic constants. The equilibrium distance of Ne with respect to the center of mass of the molecule is 3.26 Å, with the Ne atom tilted by 5.8° from the perpendicular to the center of mass of the ring towards the mid-point of the two nitrogen atoms. The dissociation energy is estimated, from the centrifugal distortion constant DJ, to be ca.0.8 kJ mol–1. information on the dynamics of this series, obtained from the rotational spectra. Fig. 1 shows PRD···Ne, PRD, the switching of the principal axes upon formation of the adduct, and the van der Waals structural parameters. Bonding energies of rare gases with aromatic molecules: rotational spectrum and dynamics of pyridazine···neon 0 1. Introduction "Normal" molecules have bond energies of the order of hundreds of kJ mol–1. These values correspond to a good stability at room temperature. On the contrary, species such as adducts of organic molecules with rare gases have stabilisation energies of 1–4 kJ mol–1, so that they cannot be observed at room temperature.It has been shown, however, that concentrations up to 1% of these species can easily be obtained in the fragile environment of supersonic expansions and can be studied with a variety of spectroscopic techniques.1 Jet cooled rotationally resolved spectra supply the most detailed information on the structure and dynamics of these complexes, being the measured distortion from a rigid rotor behaviour mainly due to the van der Waals vibrations (see for example ref. 2–5). Several of the van der Waals complexes investigated in this way are between a rare gas (RG) atom and an aromatic ring molecule,6 probably because of the high number of interaction centers at an almost equivalent distance from the RG atom.Most of the observed complexes (about ten) involve an Ar atom, while only three of them involve a Ne atom: benzene···Ne,7 pyridine···Ne8 and pyrimidine···Ne.9 This is in accord with the lower interaction energy of Ne with respect to that of Ar: the bonding energy decreases roughly from 3 to 1 kJ mol–1 in going from the Ar to the Ne adducts. Between 1983 and 1996 pulsed molecular beam microwave Fourier transform (MBMWFT) spectrometers were mainly used for the observation of van der Waals complexes, but in 1996 we introduced the use of free jet millimeter-wave absorption spectroscopy (FJMMWA),10 to such a kind of investigation, so giving further impulse to the study of these exotic chemical species.FJMMWA allows for a fast scanning of the spectra (10 GHz day–1 against typically ca. 0.2 GHz day–1), although the sensitivity and the resolution power are worse. With this technique we observed the rotational spectra of pyrimidine···Ar5 and pyridazine···Ar.11 We could then measure, although with some more difficulties, the adducts pyridine···Ne8 and pyrimidine···Ne.9 Here we report, to complete the series, the rotational spectrum of pyridazine···Ne (PRD···Ne), obtained with the FJMMWA technique. We compare the PhysChemComm, 2000, 1 Fig. 1 Principal axes systems and their switching in going from PRD to PRD···Ne. 2. Experimental section The Stark and pulse modulated free jet absorption millimeter-wave spectrometer used in this study has already been described elsewhere.10,12 The complex was formed by flowing neon at a pressure of ca.1.5 bar over the sample heated at about 60 ºC, and expanding the mixture through a pulsed nozzle (repetition rate 5 Hz) with a diameter of 0.35 mm, reaching an estimated "rotational" temperature of about 7 K. Neon (99.995%) was supplied by Linde, and pyridazine by Aldrich. The accuracy of the frequency measurements is about 0.05 MHz. 3. Rotational spectrum The first estimate of the rotational constants of PRD···Ne has been based on a model similar to that of Fig. 1, with Ne positioned at 3.4 Å above the PRD plane, along the perpendicular to the center of mass (c.m.), and with the r geometry of PRD as in the isolated molecule.13Table 1 Experimental transition frequencies ( n/ MHz) of PRD···Ne Doubly overlapped transitionsa J'K’a�J''K’’a J'K’a�J''K’’a 13(6)–12(5) 13(7)–12(6) 13(8)–12(7) 13(9)–12(8) 13(10)–12(9) n 61155.00 60053.00 62478.82 67377.71 61414.79 63820.71 68674.59 71126.41 73597.41 60410.06 10(10)–9(9) 11(8)–10(7) 11(9)–10(8) 11(11)–10(10) 12(7)–11(6) 12(8)–11(7) 12(10)–11(9) 12(11)–11(10) 12(12)–11(11) 13(5)–12(4) 14(5)–13(4) 14(6)–13(5) 14(7)–13(6) a n 62790.35 65178.79 67578.29 69991.06 72418.97 13(11)–12(10) 74864.35 13(12)–12(11) 77328.76 64176.59 66551.06 68933.18 a Due to the Ka asymmetry degeneracy of levels, only Ka is given.The experimental spectrum showed several high J high K b-R-type lines, doubly overlapped due to the Ka nearly prolate degeneracy of the involved levels.Later on the m weaker lines with lower Kas, resolved in asymmetry doublets, have also been measured. The measured transition frequencies are reported in Table 1. They have been fitted with Watson’s "S" reduced Hamiltonian (Ir representation).14 Quartic and sextic centrifugal distortion parameters were required to fit the spectrum, in accord with the large amplitude inherent with the Ne motions. Table 2 collects the determined parameters. No hyperfine structure, due either to the quadrupole effects of the two 14N nuclei, or to the Ne tunnelling motion, has been observed. r0a 3.34(1)b 81.2(6) Table 2 Spectroscopic constants of PRD···Ne (Ir representation, S reduction) A / MHz 3121.34(9)a B / MHz 1928.67(10) C / MHz 1912.90(12) DJ / kHz 21.1(4) DJK / kHz 85.5(8) DK / kHz –95.6(4) d1 / kHz s/ MHz –1.89(9) a Standard errors are given in parenthesis in units of last digit. b Number of transitions in the fit.n 71325.71 73731.59 76152.47 67933.41 70301.41 72676.41 75062.13 71679.65 74040.66 76408.71 3.339 81.3 Calc.d 4. Location and dynamics of the Ne atom The effects of the Ne motions (see ahead) make it difficult to give a simple description of the structure of the complex. Table 3 r0 and rs van der Waals geometry for PRD···Ne (a) van der Waals parameters / Å and ° RaCM (b) Ne rs coordinates / Åc Exptl.|x| 0.505(2) 0.511 |y| 0.154(5) 0.0 |z| 3.2968(3) 3.3007 a From the fit of rotational constants. b Error (in parentheses) is expressed in units of the last digit. c Coordinates in the principal axes system of PRD. d Calculated with the r0 parameters above. Non-overlapped transitions J'K’a,K’cJ''K’’a,K’’c J'K’a�J''K’’a 14(4,11)–13(3,10) 14(4,10)–13(3,11) 15(3,13)–14(2,12) 15(3,12)–14(2,13) 16(3,13)–15(2,14) 17(4,14)–16(3,13) 17(4,13)–16(3,14) 14(8)–13(7) 14(9)–13(8) 14(10)–13(9) 15(5)–14(4) 15(6)–14(5) 15(7)–14(6) 15(8)–14(7) 16(5)–15(4) 16(6)–15(5) 16(7)–15(6) d2 / kHz HJ / Hz HJK / Hz HKJ / Hz HK / Hz Nb 0.10(1) –2.4(5) –12.7(9) 44.6(18) –26.9(8) 33 0.11 0 n 61861808.50 63061.10 63306.86 67091.31 73046.97 73068.30 Approximate descriptions of the position of the Ne atom in the complex can be given, however, by a partial r geometry or by the rs substitution coordinates reported in Table 3.The r0 parameters result from a fit of the three rotational constants to the two (RCM and q, see Fig. 1) van der Waals parameters (the geometry of PRD has been fixed to that of the isolated molecule). The rs coordinates have been obtained applying Kraitchmann’s equations15 to the rotational constants of isolated pyridazine and PRD···Ne, supposing a dummy atom of zero mass to be substituted by a Ne atom. The small non-zero values of |y| are likely to be owing to the large amplitude motions underlying this kind of molecular complex (see for example ref.5), and therefore compatible with zero equilibrium values. The obtained values are affected by Coriolis contributions to the moments of inertia, so that they are implicitly approximated values. rsTable 4 Shifts of planar moments of inertia in going from the isolated molecule to the complex with neon for pyridine, pyrimidine and PRD. The centrifugal distortion parameters DJ and the dissociation energies of the three complexes are also given Pyridinea DMxx / uÅ2 –1.838 –1.731 –2.762 DMyy / uÅ2 –1.123 –0.971 –1.102 DMzz / uÅ2 186.775 180.830 182.178 DJ / kHz 20.3 19.2 21.1 EB / kJ mol–1 0.81c 0.92c 0.82 a From ref. 8. b From ref. 9. c Values slightly smaller with respect to those reported in ref. 8 and 9 because of a different approximation.Table 5 Parameters describing the van der Waals motions for PRD···Ne ks / N m–1 0.90 nY / cm–1 19.5 kX / N m–1 0.12 Xe / Å a 0.333 kY / N m–1 0.12 re / Å a 3.255 ns / cm–1 31.0 qe / º a 5.8 nX / cm–1 EB / kJ mol–1 20.0 0.82 a The equilibrium position of Ne is shifted in the direction from the center of mass of the ring towards the mid-point between the two nitrogen atoms. Some features provided by the rotational spectrum can be used to give a better description of the location and dynamics of the rare gas atom. They are the centrifugal distortion, mainly reflected in the DJ, DJK and DK parameters of Table 2, and the shifts of planar moments of inertia on going from the monomer to the complex. The planar moments of inertia are defined as: (1) Maa = åimiai2, etc., and are easily obtained from the rotational constants.They represent the mass extension along the principal axes. Their shifts (e.g. DMaa) on going from the isolated molecule to the complex (see Fig. 1) are shown in Table 4 for PRD···Ne and for the related complexes pyridine···Ne and pyrimidine···Ne. Due to the axis inversion upon formation of the complexes the more consistent quantities DMxx, DMyy and DMzz (see Fig. 1) are reported there. The centrifugal distortion parameters DJ, which will be utilised in the model description (see ahead) are reported in the same table. The DMyy value should be zero and the DMxx value smaller within the rigid rotor behaviour.A rather simple interpretation of these deviations has been given for pyrimidine···Ar, in terms of mass dispersion and vibrational Coriolis couplings.5 The treatment of these data is relatively simple if we separate the three Ne vibrational motions. One of them can be considered the stretching between the two centers of mass of the two constituent molecules, while the remaining ones can be thought of as two internal rotations of Ne around PRD. Let us consider first the radial part of the Ne motions (stretching). For asymmetric top complexes in which the stretching coordinate is nearly parallel to the inertial a-axis the stretching force constant (ks) can be estimated by approximating the complex to a molecule made of two rigid parts, by using equations of the type:2,16 ks = 16 p4 ( mA RCM)2 [4BA4 + 4CA4 – (BA – CA)2(BA + CA)2] / (hDJ) (2) PRD Pyrimidineb The subscript A denotes an adduct quantity, mA is the pseudo-diatomic reduced mass, RCM is the distance between the centers of mass of the parts (3.255 Å for PRD···Ne), and DJ is the centrifugal distortion constant.ks, the corresponding harmonic stretching fundamental ( ns ), and the dissociation energy (EB), obtained assuming a Lennard-Jones type potential, are reported in Table 5. Fig. 2 shows the Lennard- Jones potential curve. The EB values are compared in Table 4 to the corresponding values for the related complexes pyridine···Ne and pyrimidine···Ne. It can be seen that the differences among these values are within 10%.Fig. 2 Lennard-Jones potential energy diagrams for the dissociation of PRD···Ne. As to the two Ne internal rotations, their effects are reflected in their contribution to the negative values of DMxx, and DMyy, as mentioned above, and to the anomalously high values of the DJK and DK centrifugal distortion parameters, as outlined in several of the complexes of aromatic molecules with rare gases (see for example ref. 3, 4). The procedure to extract the bond’s van der Waals potential energy parameters from centrifugal distortion is rather complex for low symmetry complexes such as PRD···Ne. For this reason we extracted this information for the Cs symmetry PRD···Ne complex from the DMxx, and Myy, values of Table 4. The two motions are considered local harmonic oscillations on one side of the ring, by a model that describes the A' type mode in the xz plane by the displacement X and the A" type displacement in the y direction by Y:(3) V(X,Y) = (1/2) [kx (X – Xe)2 + ky Y 2], where Xe is the displacement, within the symmetry plane, of the neon atom from the z axis at equilibrium.The calculations were made by using the two-dimensional flexible model approach,17 resolving the range (–2.0 Å, +2.0 Å) into 21 mesh points for each of the X and Y displacements. Since the two pieces of data, DMxx, and DMyy, do not allow one to estimate more than two of the three parameters Xe, kx, and ky, we assumed kx = ky, a condition nearly fulfilled for this kind of complex.3 The results are shown in Table 5.Although the values Xe = ± 0.333 Å were obtained, a comparison with other complexes3 suggests the minus rather than the plus sign and yields Xe = –0.333 Å, which means that the line connecting Ne and the center of mass of PRD is tilted by about 5.8º towards the midpoint between the two nitrogen atoms. In spite of the fact that Ne is much lighter than Ar, the bend vibrational fundamental frequencies, 19.5 and 20.0 cm–1, are considerably lower than the values 30.6 and 32.2 cm–1 for PRD···Ar,10 indicating that PRD···Ne is much floppier than PRD···Ar. 5. Conclusions The rotational spectrum of PRD···Ne has been here reported. The effects of the Ne atom's large amplitude motions on the rotational spectrum have been used to obtain the equilibrium location of the Ne atom and the potential energy surfaces describing its dynamics. The 3D representation of the adduct is given in Fig.3. Fig. 3 3D representation of PRD···Ne. The data used to obtain information on the dynamics of Ne, that is the DMgg (g = x, y, z) and the DJ values, are very similar to those of complexes with the related aromatic azines: pyridine and pyrimidine (see Table 4). Only DMxx is quite larger, in the case of PRD···Ne, with respect to the other two complexes, so reflecting a larger q angle (see Fig. 3). Correspondingly the three EB values are also very similar to each other, within 10%. In all cases the Lennard- Jones potential energy well allows for only three bonded vibrational states.It can be questionable if these adducts are real chemical compounds, but in the light of Fig. 2 it seems that the difference with respect to "normal" chemical compounds is quantitative rather than qualitative. Acknowledgements We thank Mr. A. Millemaggi for technical help, Mrssrs. F. Lugli and A. Prezzi for running some measurements, and the Ministero dell'Università e della Ricerca Scientifica e Tecnologica, and the C.N.R. for financial support. Paper a909649c References 1 Atomic and Molecular Beam Methods, ed. G. Scoles, Oxford University Press, Oxford–New York, 1992, vol. I, II. 2 D. J. Millen, Can. J. Chem., 1985, 63, 1477. 3 R. P. A. Bettens, R. M. Spycher and A. Bauder, Mol. Phys., 1995, 86, 487. 4 J. Makarewicz and A. Bauder, Mol. Phys., 1995, 84, 853. 5 W. Caminati, S. Melandri, P. G. Favero and R. Meyer, Chem. Phys. Lett., 1997, 268, 393. 6 S. E. Novick, Bibliography of Rotational Spectra of Weakly Bound Complexes, 1999, available at http://www.wesleyan.edu/chem/bios/vdw.html. 7 Th. Brupbacher, J. Makarewicz and A. Bauder, J. Chem. Phys., 1994, 101, 9736. 8 A. Maris, W. Caminati and P. G. Favero, Chem. Commun., 1998, 2625. 9 W. Caminati and P. G. Favero, Chem .Eur. J., 1999, 5, 811. 10 S. Melandri, G. Maccaferri, A. Maris, A. Millemaggi, W. Caminati and P. G. Favero, Chem. Phys. Lett., 1996, 261, 267. 11 W. Caminati, A. Millemaggi, P. G. Favero and J. Makarewicz, J. Phys. Chem., 1997, 101, 9272. 12 S. Melandri, W. Caminati, L. B. Favero, A. Millemaggi and P. G. Favero, J. Mol. Struct., 1995, 352–353, 253. 13 W. Werner, H. Dreizler and H. D. Rudolph, Z. Naturforsch. A: Astrophys. Phys. Phys. Chem., 1967, 22, 532. 14 J. K. G. Watson, in Vibrational Spectra and Structure, Vol. 6, ed. J. R. Durig, Elsevier, New York– Amsterdam, 1977, pp. 1–89. 15 J. Kraitchman, Am. J. Phys., 1953, 21, 17. 16 W. G. Read, E. J. Campbell and G. Henderson, J. Chem. Phys., 1983, 78, 3501. 17 R. Meyer, J. Mol. Spectrosc., 1979, 76, 266. PhysChemComm © The Royal Society of Chemistry 2000