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Propellane ⇑ Robynne Kirkpatrick A, Tony Masiello B, Matthew Martin A, Joseph W

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Propellane ⇑ Robynne Kirkpatrick A, Tony Masiello B, Matthew Martin A, Joseph W Journal of Molecular Spectroscopy 281 (2012) 51–62 Contents lists available at SciVerse ScienceDirect Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms High-resolution infrared studies of the m10, m11, m14, and m18 levels of [1.1.1]propellane ⇑ Robynne Kirkpatrick a, Tony Masiello b, Matthew Martin a, Joseph W. Nibler a, , Arthur Maki c, Alfons Weber d, Thomas A. Blake e a Department of Chemistry, Oregon State University, Corvallis, OR 97332-4003, USA b Department of Chemistry and Biochemistry, California State University, East Bay, Hayward, CA 94542, USA c 15012 24th Ave., Mill Creek, WA 98012, USA d Sensor Science Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA e Pacific Northwest National Laboratory, P.O. Box 999, Mail Stop K8-88, Richland, WA 99352, USA article info abstract Article history: This paper is a continuation of earlier work in which the high resolution infrared spectrum of [1.1.1]pro- Received 27 July 2012 pellane was measured and its k and l structure resolved for the first time. Here we present results from an In revised form 3 September 2012 0 0 0 0 analysis of more than 16000 transitions involving three fundamental bands m10 ðE À A1Þ; m11ðE À A1Þ, m14 Available online 15 September 2012 00 0 0 00 0 00 ðA2 À A1Þ and two difference bands (m10–m18)(E ÀE ) and (m11 À m18)(E À E ). Additional information 0 00 about m18 was also obtained from the difference band (m15 + m18) À m18 (E À E ) and the binary combina- Keywords: 0 0 tion band (m15 + m18) ðE À A Þ. Through the use of the ground state constants reported in an earlier paper Propellane 1 [1], rovibrational constants have been determined for all the vibrational states involved in these bands. High-resolution infrared spectrum The rovibrational parameters for the (E00) state were obtained from combination–differences and Rovibrational constants m18 0 Coriolis interactions showed no need to include interactions with other states. The m10 (E ) state analysis was also straight-for- 00 Ground state structure ward, with only a weak Coriolis interaction with the levels of the m14 ðA2Þ state. The latter levels are much 0 DFT calculations more affected by a strong Coriolis interaction with the levels of the nearby m11 (E ) state and also by a 00 Anharmonic frequencies small but significant interaction with another state, presumably the m16 (E ) state, that is not directly observed. Gaussian calculations (B3LYP/cc-pVTZ) computed at the anharmonic level aided the analyses by providing initial values for many of the parameters. These theoretical results generally compare favor- ably with the final parameter values deduced from the spectral analyses. Finally, evidence was obtained for several level crossings between the rotational levels of the m11 and m14 states and, using a weak cou- pling term corresponding to a Dk = ±5, Dl = Ç1 matrix element, it was possible to find transitions from the ground state that, combined with transitions to the same upper state, give a value of C = 0.1936515(4) cmÀ1. This result, combined with the value of B = 0.28755833(14) cmÀ1 reported ear- 0 0 0 lier [1], yields a value of 1.586277(3)A Å for the length of the novel axial CC bond in propellane. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction the axial carbons differs significantly from the more normal sp, sp2 or sp3 types observed for carbon and hence this molecule and [1.1.1]Propellane or, more simply, propellane (C5H6) is the pro- its derivatives have been the subject of a number of investigations totype of a whole class of tricyclic organic molecules having three [2–8]. From the outset, it was assumed that the symmetry of pro- medial rings fused to form an axial-axial carbon single bond. The pellane was D3h and this was confirmed by analysis of gas phase name propellane was introduced into the chemical literature by electron diffraction measurements [9] and by vibrational IR and Ginsburg [2]. This unusual structure (Fig. 1) challenges us to re- Raman studies [10]. Our quantum calculations [1] and the detailed think our models of how individual atoms combine to form mole- higher resolution spectral measurements reported here also indi- cules. Of greatest interest is the novel central bond that joins the cate that this is the symmetry of the equilibrium structure of this axial carbon atoms; as seen in the figure, the angle between the ax- simplest case of the propellanes. ial bond and the carbons on each corner of the trigonal bipyramid Once thought to be too unstable to exist, propellane was first is acute rather than obtuse. Clearly the bonding hybridization at synthesized in 1982 by Wiberg and Walker [11]. Wiberg and coworkers [10] did an extensive study of the spectrum of propel- ⇑ Corresponding author. Fax: +1 541 737 2062. lane in the gas phase using infrared and Raman spectroscopy with À1 À1 E-mail addresses: [email protected], [email protected] (J.W. Nibler). spectral resolutions of 0.06 cm and 2 cm , respectively. They 0022-2852/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jms.2012.09.001 52 R. Kirkpatrick et al. / Journal of Molecular Spectroscopy 281 (2012) 51–62 pellane is difficult to separate from ether using fractional distillation [10], it was converted, using the protocol of Alber and Szeimies [17], to solid 1,3-diiodobicyclopentane, a relatively stable molecule that can be stored and reconverted to propellane. Recon- version to propellane was achieved by addition of sodium cyanide (in DMSO, dry) under nitrogen, with stirring for thirty minutes [17]. Pure propellane was then extracted using fractional distilla- tion at room temperature at a pressure of 13.3 Pa (0.1 Torr). A trap cooled to À78 °C and coated with hydroquinone was used to col- lect the product; the hydroquinone served as a radical scavenger to reduce potential dimerization on the trap surfaces. The internal gold-coated surfaces of the 20 cm IR absorption cells employed to record some spectra were also coated with hydroquinone but this was not done with the long path White cell used and the rate of decomposition was not noticeably higher in the latter case. Hence the need for the hydroquinone coating is not clear. Fig. 1. Views of the m10, m11, m14, and m18 normal modes of the D3h structure of 2.2. Spectral measurements [1.1.1]propellane. Spectra were taken using Bruker 120 or 125 FTIR spectrometers at the Pacific Northwest National Laboratories.1 The spectrometers were able to obtain approximate values for many of the fundamen- employ a Globar light source, various detectors, and either a 20 cm tal vibrational frequencies and some information about its normal absorption cell or a 1 m multipass White cell. The spectrometer opti- coordinates and dipole derivatives. While they observed some J cal path (except that in the absorption cell) was evacuated. Boxcar structure in several bands, they were unable to identify any K apodization was used and interferograms were collected single- structure due to the relatively low instrumental resolution avail- sided with both forward and backward scans. Table 1 summarizes able at that time. The high symmetry and small size of this mole- the instrumental conditions that were used for recording the spectra. cule have led us to use improved high-resolution spectroscopic and Fig. 2 gives a survey view of the infrared spectrum of the region ab initio theoretical methods to reexamine in detail the spectral 1060–1200 cmÀ1 that contains the IR active fundamental bands and structural properties of this intriguing molecule [1,12–14]. m , m , and m . Fig. 3 shows the relatively weak difference band Such spectroscopic studies can provide accurate values of rovib- 10 11 14 m À m (band origin 466 cmÀ1) while Fig. 4 similarly displays ronic parameters that can serve as a direct test of quantum calcu- 10 18 the m À m band (band origin 366 cmÀ1); these were used to help lations, which are often affected by the choice of basis set and 11 18 establish both the m and m parameters. computational model. Conversely, as in this work, the quantum 18 11 prediction of basic rovibrational parameters can also serve as an indispensible guide in the initial analysis of complicated or con- 3. Band analyses gested spectra. The work presented here deals with the determination of 3.1. Energy levels parameters that describe the m10, m11, and m14 fundamental vibra- tions of propellane, obtained from an analysis of the three funda- Propellane is an oblate symmetric top for which the term en- mentals as well as of the m10 À m18, m11 À m18 and (m15 + m18) À m18 ergy of a given vibrational state v is given by difference bands. In this global fit, ground state parameters were Ev ¼ Gðv; lÞþFvðJ; K; lÞ; ð1Þ fixed at values determined previously [1]. In that work the effect of any perturbations on upper states was removed by taking en- where Gðv; lÞ is the vibrational term, and the rotational term for ergy differences between pairs of transitions having the same non-degenerate vibrational states (i.e. l =0)Fv ðJ; KÞ is given by upper levels and different lower state levels so that accurate spac- 2 J 2 2 JK ings between ground state rotational levels were used in the fit. Fv ðJ; KÞ¼Bv JðJ þ 1ÞþðCv À Bv ÞK À Dv J ðJ þ 1Þ À Dv JðJ (This combination-difference method is described in Ref.
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