The rotational spectrum of silacyclohexane

Transcription

1 Journal of Molecular Spectroscopy 229 (2005) The rotational spectrum of silacyclohexane Laura B. Favero a, Walther Caminati b, *, Ingvar Árnason c,ágúst Kvaran c a Istituto per lo Studio dei Materiali Nanostrutturati (ISMN, Sezione di Bologna), CNR, Via Gobetti 101, I Bologna, Italy b Dipartimento di Chimica G.Ciamician dellõuniversità, Via Selmi 2, I Bologna, Italy c Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavík, Iceland Received 18 June 2004; in revised form 30 August 2004 Available online 22 October 2004 Abstract The rotational spectra of the normal and Si-d 2 isotopomers of the chair form of silacyclohexane have been measured by microwave absorption spectroscopy. A partial r 0 structure has been obtained. The rotational spectra of some, the most intense, vibrational satellites have also been measured. They belong to the ring-puckering motions. Their vibrational energies and their shifts of planar moments of inertia with respect to the ground state indicate that the amplitude of these vibrations is larger than in cyclohexane. The dipole moment has also been determined: l a = 0.75(2), l c = 0.280(2), and l tot = 0.80(2) D. Ó 2004 Elsevier Inc. All rights reserved. Keywords: Rotational spectroscopy; Silicon compounds; Six membered ring molecules 1. Introduction Saturated six-membered ring molecules play an important role in organic chemistry because of their flexibility, which allows for interconversion between conformers either in the bare molecule (chair and twisted boat forms), or in substituted derivatives (equatorial and axial species). The prototype molecule, cyclohexane, exists predominantly in the chair form, the twisted boat is estimated to be ca. 20 kj/mol higher in energy [1]. The molecular structure of cyclohexane has been determined by electron diffraction (ED) [2]. The microwave (MW) spectrum is available for the asymmetrically substituted deuterated species [3]. Studies of the structure and conformations of several ring-mono-substituted hetero-derivatives have been performed by ED and/or MW spectroscopy: tetrahydropiran [4,5], tetrahydrothian [6], piperidine [7], silacyclohexane [8], and germacyclohexane [9]. All of them have been observed predominantly in the chair form. An important member of the family, silacyclohexane (from now on SCH, see Fig. 1), has not been investigated by MW spectroscopy. A theoretical study of the conformational equilibrium of SCH is available [10]. Both MM3 and ab initio calculations at various levels show that the upper part of the potential energy surface is characterized by two minima corresponding to two different twisted boat forms, having C 1 and C 2 symmetry, respectively, with energies about kj/mol higher than the chair form. We decided to investigate the rotational spectrum of SCH to gain further information about its structure and ring dynamics. 2. Experimental part 2.1. MW spectroscopy * Corresponding author. Fax: addresses: walther.caminati@unibo.it, Caminati@ciam. unibo.it (W. Caminati). The microwave spectra were recorded in the frequency range GHz with a computer controlled Stark /$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi: /j.jms

2 L.B. Favero et al. / Journal of Molecular Spectroscopy 229 (2005) Conformation and structure Fig. 1. Principal axes system and dipole moment components of SCH. spectrometer based on a Hewlett Packard 8400 C instrument. The radiofrequency-microwave double resonance (rfmwdr) technique [11] was used for the assignment of some transitions. The absorption cell temperature was kept constant at about 20 C. The accuracy of the frequency measurements is about 0.05 MHz Chemical preparation Silacyclohexane was prepared according to a standard literature method by the treatment of (CH 2 ) 5 SiCl 2 with LiAlH 4 in Et 2 O [12]. The Si-d 2 isotopomer was prepared in an analogous way using LiAlD 4 instead of LiAlH Rotational spectrum Trial preliminary calculations of the spectrum were based on the ED data [8]. SCH is an asymmetric top with an oblate character (Ray asymmetry parameter k = 0.247). Therefore the assignment of the spectrum started by applying rfmwdr to asymmetry doublet transitions with K c = 5 or 6. The twin transitions 8 1, 7 7 1, 6 and 8 2, 7 7 2, 6, separated by 2.5 MHz, were assigned first. Later more transitions were measured, either with rfmwdr or Stark modulation technique. They are reported in Table 1. They have been fitted using WatsonÕs quartic reduced hamiltonian (A-reduction and I r -representation [13]). The spectroscopic constants obtained and some statistical parameters of the fits are shown in Table 2. Similarly the spectra of the most intense vibrational satellites for SCH and for the ground state of the Si-d 2 isotopomer have been measured. The corresponding measured transitions and spectroscopic parameters are listed in Tables 1 and 2, respectively. The rotational constants derived matched the chair conformational structure. They were not in good agreement, however, with the rotational constants calculated by using available geometry based on ED (see Table 3). Since six rotational constants are not sufficient to obtain a full geometry of the molecule, we performed ab initio calculations to evaluate theoretical rotational constants to compare with our experimental values. B3LYP and MP2 calculations with the G** basis set [14] gave the results shown in Table 3. Whereas the B3LYP results were not so satisfactory, the MP2 calculations produced rotational constants in good agreement with the experimental values. Starting from the MP2 geometry, we fit the three structural parameters shown at the bottom of Table 3 (the Si C bond length, the C Si C valence angle, and the flap (Si) dihedral angle), to obtain a partial r 0 structure, which supplied differences smaller than 1 MHz between calculated and experimental rotational constants. 5. Dipole moment The dipole moment of normal SCH was determined from the analysis of the second-order Stark effect on two transitions, by adding a dc voltage to the square wave modulation. The cell was calibrated with the 3 2 transition of OCS (l = D [15]). The values Dm/E 2 for the observed Stark lobes are reported in Table 4, along with the results of a least squares fit. The Stark coefficients [16] were calculated using the effective rotational constants in Table Vibrational satellites Relative intensity measurements [17] of the rotational transitions of the vibrational satellites with respect to the ground state lead to an estimate of the vibrational energies for v 1 =1, v 2 = 1, and v 1 = 2, as 200(30), 200(30), and 400(50) cm 1, respectively. They are comparable with the values for ring-puckering motions of other saturated six-membered rings [18]. One can obtain some more information on the nature of the two vibrational modes from the changes of planar moments of inertia of the vibrational satellites with respect to the ground state. The planar moments of inertia (P gg ; g = a,b,c) are defined and related to the rotational constants as P aa ¼ P i m ia 2 i ¼ h=ð16p 2 Þð 1=Aþ1=Bþ1=CÞ, etc., and represent the mass extensions in the directions of the corresponding principal axis. The DP gg are shown in Table 5. The DP aa and DP bb values of the v 1 progression are positive and evenly spaced, corresponding, in a first approximation, to a bending of SiH 2 towards

3 190 L.B. Favero et al. / Journal of Molecular Spectroscopy 229 (2005) Table 1 Experimental transition frequencies (MHZ) J 0 Ka; K 0 c J 00 K 00 a ; K000 c Normal Si-d 2 v =0 v 1 =1 v 1 =2 v 2 =1 v =0 6 4, 2 5 4, , 3 5 4, , 6 6 1, , 6 6 2, , 5 6 2, , 4 6 3, , 5 6 3, , 3 6 4, , 4 6 4, , 2 6 5, , 3 6 5, , 2 6 6, , 1 6 6, , 8 7 1, , 7 7 1, , 7 7 2, , 6 7 2, , 6 7 3, , 5 7 4, , 8 8 1, , 8 8 2, , 9 8 0, , , , 9 9 2, , 9 9 1, , , , , , , a , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , a Doubly overlapped transition, due to the near oblate degeneracy of the involved levels. Only K a is given.

4 L.B. Favero et al. / Journal of Molecular Spectroscopy 229 (2005) Table 2 Spectroscopic parameters of SCH Normal Si-d 2 v =0 v 1 =1 v 1 =2 v 2 =1 v =0 A (MHz) (2) a (2) (2) (1) (2) B (MHz) (6) (6) (1) (8) (7) C (MHz) (7) (8) (1) (9) (9) D J (khz) 0.44(5) 0.52(5) [0.44] b 0.59(6) 0.43(5) D JK (khz) 0.19(5) 0.4(1) [ 0.19] [ 0.19] 0.31(5) D K (khz) 0.42(7) 0.6(2) [0.42] [0.42] [0.42] d J (khz) 0.150(5) 0.146(8) 0.154(4) 0.156(6) 0.133(6) d K (khz) 0.28(2) 0.28(5) [0.28] 0.26(2) 0.11(3) N c J max r d (MHz) a Standard error in parenthesis in units of the last digit. b Values in bracket fixed at the ground state values of the normal species. c Number of fitted transitions. d Standard deviation of the fit. Table 3 Calculated and experimental rotational constants and partial r 0 structure of SCH Calculated Experimental B3LYP a MP2 a ED b MP2 + fit Rotational constants Normal A (MHz) B (MHz) C (MHz) Si-d 2 A (MHz) B (MHz) C (MHz) Structural parameters Si C (3) 1.880(2) \C Si C (1.4) 104.6(1) Flap (Si) c (4.1) 137.2(2) a G** basis set. b From [8]. c Flap angle from the plane of C2, C3, C5, and C6. Table 4 Stark coefficients and dipole moment components of SCH J 0 ðk 0 a ; K0 c Þ J 00 ðk 00 a ; K00 c Þ M Dm/E2 (Hz V 2 cm 2 ) Observed Calculated 6(2,4) 5(2,3) (5,2) 6(5,1) Dipole moment (D) l a = 0.75(2) l b = 0.0 a l c = 0.280(2) l tot = 0.80(2) a By symmetry. coplanarity with the adjacent C2 and C6 carbon atoms. The negative value of DP bb for v 2 = 1 suggests that this state belongs to a twisting motion of the C2 and C6 (and possibly C3 and C5) carbon atoms. Table 5 Low energy vibrational states of SCH: vibrational energies and shifts of planar moments of inertia with respect to the ground state v 1 =1 v 1 =2 v 2 =1 E a vib (cm 1 ) 200(30) 400(50) 200(30) DP aa (uå 2 ) DP bb (uå 2 ) DP cc (uå 2 ) a From relative intensity measurements. 7. Conclusions In this paper we report information relevant to conformation, structure, dipole moment, and internal dynamics of SCH from its rotational spectrum. Rotational constants were obtained for SCH and its Si-d 2 isotopomer. MP2 calculations give structural values in

5 192 L.B. Favero et al. / Journal of Molecular Spectroscopy 229 (2005) good agreement with the experimental results. Shifts of second moments of inertia in going from the ground to the excited state indicate that the vibrational satellite belongs to ring-puckering and ring twisting motions. Acknowledgment We thank the University of Bologna, the Ministero dellõistruzione, dellõuniversità e della Ricerca for financial support. References [1] See for example E.L. Eliel, S.H. Wilen, Stereochemistry of Organic Compounds, Wiley-Interscience Publication, John Wiley & Sons, New York, [2] J.D. Ewbank, G. Kirsch, L. Schäfer, J. Mol. Struct. 31 (1976) 39. [3] J. Dommen, T. Brupbacher, G. Grassi, A. Bauder, J. Am. Chem. Soc. 112 (1990) 953. [4] J.C. Lopez, J.L. Alonso, R.N. Villamanan, J. Mol. Spectrosc. 147 (1986) 67. [5] U. Spoerel, W. Stahl, W. Caminati, P.G. Favero, Chem. Eur. J. 4 (1998) [6] R.W. Kitchin, T.B. Malloy, R.L. Cook, J. Mol. Spectrosc. 57 (1975) 179. [7] C.C. Costain, J.E. Parkin, P.J. Buckley, Bull. Am. Phys. Soc. Ser (1968) 833; J.E. Parkin, P.J. Buckley, C.C. Costain, J. Mol. Spectrosc. 89 (1981) 465; H. Ehrlichmann, J.U. Grabow, H. Dreizler, N. Heineking, M. Andolfatto, Z. Naturforsh. A 44 (1989) 841. [8] Q. Shen, R.L. Hilderbrandt, J. Mol. Struct. 54 (1979) 121. [9] Q. Shen, S. Rhodes, Y. Tacheuchi, K. Tanaka, Organometallics 11 (1992) [10] I. Arnason, G.K. Thorarinsson, E. Matern, Z. Anorg. Allg. Chem. 626 (2000) 853. [11] F.J. Wodarczyk, E.B. Wilson, J. Mol. Spectrosc. 37 (1971) 445. [12] R. West, J. Am. Chem. Soc. 76 (1954) [13] J.K.G. Watson, in: J.R. Durig (Ed.), Vibrational Spectra and Structure, vol. 6, Elsevier, New York/Amsterdam, 1977, pp [14] Gaussian03, Revision B.04. Gaussian Inc, Pittsburgh, PA, [15] I.M.L.J. Reinartz, A. Dymanus, Chem. Phys. Lett. 24 (1974) 346. [16] S. Golden, E.B. Wilson Jr., J. Chem. Phys. 24 (1948) 669. [17] A.S. Esbitt, E.B. Wilson Jr., Rev. Sci. Instrum. 34 (1963) 901. [18] See for example F.A. Miller, H.R. Golob, Spectrochim. Acta 20 (1964) 1517.