He I Photoelectron Spectra and Gas-Phase Electronic Structures of End-Functionalized [3]- and [5]-Ladderanes Tomislav Fris ˇc ˇ ic ´ , ² Leo Klasinc,* ,‡ Branka Kovac ˇ , ‡ and Leonard R. MacGillivray* ,² Department of Chemistry, UniVersity of Iowa, Iowa City, Iowa 52242-1294, and Department of Physical Chemistry, The Rudjer Bos ˇkoVic ´ Institute, HR-10002 Zagreb, Croatia ReceiVed: March 23, 2007; In Final Form: NoVember 28, 2007 [3]- and [5]-ladderanes obtained by way of template-controlled syntheses conducted in the organic solid state have been characterized Via He I photoelectron (PE) spectroscopy. The results provide a first correlation with X-ray crystallographic structure data and establish the reliability of quantum chemical DFT (B3LYP/6-31G*) and ab initio HF calculations in predicting geometrical and electronic structures of molecular ladder frameworks. Introduction Molecules with covalent frameworks based on n edge-fused cyclobutane rings (i.e.,[n]-ladderanes [where n ) 3, 4, 5, ...]) are attracting increasing attention in several research areas. 1 In the context of biology, [3]- and [5]-ladderanes have recently been discovered as lipids in intracellular membranes of anam- mox bacteria, which are largely responsible for the denitrification step of the oceanic nitrogen cycle. 2,3 These ladderane lipids have been suggested to be essential for the metabolic cycle of the bacterium by providing extraordinary rigidity to the cellular membranes. 1 In the context of theoretical chemistry, recent computational studies suggest that such frameworks exhibit novel forms of fluxional behavior. 4,5 The introduction of a structural defect within a ladderane framework is expected to result in the formation of sigmatropic shiftamers, which are fluxional molecules in which the defect can move throughout the ladderane framework. Ladderanes have also been implicated as potential rod-shaped building blocks in molecular electronics applications. 6,7 Whereas the chemistry of ladderanes is gaining increasing attention, such significance is underscored by a lack of experimental data that characterizes the ladder frameworks, an issue expected to become increasingly important as ladderanes continue to be synthesized and discovered. This lack of data is largely related to a general difficulty of synthesizing such edge- fused cyclobutane moieties. 8,9 This difficulty is illustrated by two recent total syntheses of a naturally occurring ladderane, which were each achieved in approximately 1% overall yield. The main difficulty encountered in the synthesis was the construction of the ladder portion of the molecule. 10 In this context, we have recently described a method to synthesize all-trans-ladderanes regiospecifically and in quantita- tive yield. 11 The method employs molecules, in the form of linear templates, that assemble and preorganize conjugated polyenes, Via hydrogen bonds, in the organic solid state for stepwise [2+2] photodimerizations. We have used this method to generate gram quantities of end-functionalized [3]- and [5]-ladderanes, namely, 1,2,7,8-tetrakis(4-pyridyl)-[3]-ladderane 1 and 1,2,11,12-tetrakis(4-pyridyl)-[5]-ladderane 2 (Scheme 1). 12 Having achieved the synthesis of 1 and 2, we now wish to report the He I photoelectron spectra of these ladder frameworks. By combining the PE spectra of 1 and 2 and the knowledge gained from our X-ray structure data, we have been able to assess the ability of computational chemistry to predict the ionization bands and electronic structures of the ladderanes. Our studies have allowed us to discuss the only previously reported electronic structure of a ladderane molecule. Experimental and Computational Details Compounds 1 and 2 were prepared according to the literature report. 11 He I PE spectra of 1 and 2 were recorded on a Vacuum Generators UV-G3 instrument 13 with spectral resolution of 30 meV when measured at the full width at half-maximum (fwhm) of the Ar + 2 P 3/2 calibration line. Sample inlet temperatures of 210 and 180 °C were necessary for 1 and 2, respectively, to achieve sufficient vapor pressure in the ionization region. The energy scale of the spectra was calibrated using small amounts of Xe gas added to the sample flow. Stability of 1 and 2 in the vapor phase was checked by mass spectrometry following PES measurements. Electronic structure calculations were performed using the GAUSSIAN 03 program package, 14 including full geometry optimization of the neutral molecule using the density functional method (DFT) with the B3LYP functional 15-17 and 6-31g* basis set as the first step. The combination of this * Corresponding authors. L.R.M.: tel, (319)-335-3504; fax, (319)-335- 1270; e-mail, len-macgillivray@uiowa.edu. L.K.: e-mail, klasinc@irb.hr. ² University of Iowa. ‡ The Rudjer Bos ˇkovic ´ Institute. TABLE 1: Calculated -E DFT and -E HF (eV) Energies and Assignments for 1 and 2 1 2 orbital -ǫDFT assign -ǫHF assign -ǫDFT assign -ǫHF assign h 6.76 nN 9.62 π 6.69 nN 9.56 π h-1 6.8 nN 9.63 π 6.71 nN 9.56 π h-2 6.82 nN 9.84 π 7.10 nN 9.82 π h-3 6.86 nN 9.98 π 7.10 nN 9.9 π h-4 7.15 π 10.08 π 7.52 π 10.04 π h-5 7.28 π 10.27 π 7.52 π 10.08 π h-6 7.32 π 10.27 π 7.53 π 10.18 π h-7 7.42 π 10.34 π 7.68 π 10.26 π h-8 7.54 π 11.23 n N 7.77 π 10.94 σladd h-9 7.72 π 11.31 nN 7.8 π 10.96 σladd + nN h-10 7.71 π 11.36 nN 7.82 π 11.27 nN h-11 7.8 π 11.39 nN 7.92 π 11.3 nN h-12 8.13 σladd 11.64 σladd 8.0 σladd 11.37 nN h-13 8.84 σladd 12.08 σladd 8.21 σladd 11.47 nN +σladd h-14 9.03 σladd 12.65 σladd 8.55 σladd 11.66 σladd 1493 J. Phys. Chem. A 2008, 112, 1493-1496 10.1021/jp072330c CCC: $40.75 © 2008 American Chemical Society Published on Web 01/29/2008