Vibrational Spectra and Density Functional Calculations of Bridged [14]Annulenes with an Anthracene Perimeter Laura Moroni, Cristina Gellini, and Pier Remigio Salvi* Dipartimento di Chimica, UniVersita ´ di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino, Firenze, Italy Chung-Jing Liu and Emanuel Vogel Institut fu ¨ r Organische Chemie, UniVersita ¨ t zu Ko ¨ ln, Greinstrasse 4, 50939 Ko ¨ ln, Germany ReceiVed: January 23, 2002; In Final Form: May 6, 2002 Vibrational spectra of two representative bridged [14]annulenes with an anthracene perimeter, 1,6:8,13-ethane- 1,3-diylidene[14]annulene and 1,6:8,13-propane-1,3-diylidene[14]annulene (4 and 5 in Figure 1, respectively), are presented and discussed on the basis of density functional calculations with the B3LYP functional and 6-31G** and cc-pVDZ basis sets. Infrared and Raman spectra of polycrystalline samples have been measured at room temperature. The Raman spectra have been obtained exciting at 647.1 and 1064 nm, that is, in pre- and off-resonance excitation conditions, respectively. Calculated structures of 4 and 5 are aromatic according to geometric criteria of aromaticity. Observed vibrational frequencies and infrared and Raman intensities of 4 and 5 are well reproduced by the present calculation using scaling factors of linearly condensed aromatic hydrocarbons taken from the literature. A correlation between vibrational modes of anthracene and ring modes of 4 and 5 is attempted in agreement with the aromatic nature of the [14]annulene ring. The effect of reduced symmetry and bridge structure on infrared and Raman intensities is discussed. I. Introduction The Hu ¨ckel molecular orbital theory of benzene, according to which aromatic character is general for cyclically conjugated ring systems containing (4n + 2) π electrons and being planar, provided the impetus for the development of annulenes and their chemistry. 1,2 It was Sondheimer who pioneered the synthesis of an almost complete series of annulenes ranging from [12]- to [30]annulene. 3 The properties of these and other annulenes, especially those of the rigid bridged-type, 4-6 vindicated theory most impressively. Because [10]annulene represents the next higher, potentially aromatic homologue of benzene, it was a prime synthetic goal. Among the various stereoisomers of [10]annulene conceivable, the all-cis and the mono-trans isomers could be made but turned out to be reactive polyolefins. 7 Evidently, because of steric constraints, these molecules pronouncedly deviate from planar- ity. In striking contrast to the parent [10]annulene, 1,6-methano- [10]annulene (1 in Figure 1), as well as its 1,6-epoxy and 1,6- imino analogues, in which the bridge compels the 10-membered ring to adopt a near planar conformation, qualify as aromatic molecules. 8 It is a special feature of the “Hu ¨ckel-aromatic” 1 that it bears a formal relationship to the classical aromatic hydrocarbon naphthalene. This relationship invited the synthesis of syn-1,6: 8,13-bismethano[14]annulene (2) and its anti isomer (3) pos- sessing anthracene perimeters. 9,10 As anticipated, it is the conformation of the perimeters of these molecules that deter- mines their π-electron structure: the syn isomer, though slightly bent because of the steric interference of the inner bridge hydrogen atoms, is aromatic, whereas the anti isomer, existing as a fluxional molecule, suffers loss of aromaticity because of severe distortion of the C 14 perimeter. Two other bridged [14]annulenes, being very rigid and thus particularly relevant to the present work, derive from 2 by proper manipulation of the CH 2 -bridges. Removal of the inner bridge hydrogen atoms with formation of a carbon-carbon bond leads to 1,6:8,13-ethane-1,3-diylidene[14]annulene (4), which exhibits aC 14 perimeter flattened out almost completely. 11 Thus, 4 suggests a comparison with planar anthracene (6). If, on the other hand, the inner hydrogen atoms of 2 are replaced by a * To whom correspondence should be addressed. Figure 1. Molecular structures of 1,6-methano[10]annulene (1), syn- 1,6:8,13-bismethano[14]annulene (2), anti-1,6:8,13-bismethano[14]- annulene (3), 1,6:8,13-ethane-1,3-diylidene[14]annulene (4), 1,6:8,13- propane-1,3-diylidene[14]annulene (5) and anthracene (6). The atomic numbering of the ring C atoms and the molecular reference system are common to the six molecules and shown for convenience only for 2. Additional C atoms of 4 and 5 forming the bridge are explicitly numbered. 6554 J. Phys. Chem. A 2002, 106, 6554-6562 10.1021/jp020221m CCC: $22.00 © 2002 American Chemical Society Published on Web 06/21/2002