This document is © Elsevier Science, 1994
Presented in part at the 12th Colloquium on High Resolution Molecular Spectroscopy, September 9th to 13th 1991 at Dijon.
Published in part in J. Mol. Struct., 291 (1993) 151-158
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Although the term "cross-conjugation" is found in many publications the concept itself seems to be rather unclear. This is evidenced by the fact that even a clear definition of cross-conjugation is lacking. One requirement that is basic to this concept and is agreed upon by most authors is that there have to be two double bonds that are in conjugation with a third one, but not in a linear arrangement. The p electronic system thus forms a bifurcation. So the simplest cross-conjugated hydrocarbon would be 3-methylene-1,4-pentadiene (I) that can be viewed as an ethene disubstituted geminally with two vinyl groups:
The structure implied by this illustration is not quite correct since the distance between the "inside" hydrogens on carbon atoms 1 and 5 would be smaller than the added van-der-Waals radii. The structure proposed by  is anti-skew with a dihedral angle of about 40 degrees between the planar butadiene fragment and the vinyl group.
Another requirement that some authors introduce is that the two "outer" double bonds are not in conjugation to each other. While this is the case for the aforementioned methylene-pentadiene, it is not true for its cyclic analogue fulvene (II). Although fulvene is considered to be cross-conjugated by many authors, it would not fit to this definition of cross-conjugation.
Some authors expand the definition of cross-conjugation even further: the "outer" double bonds can then be exchanged for "unsaturated centers". These include hetero-atoms with lone electron pairs as nitrogen or oxygen. By this definition even urea is said to be cross-conjugated .
In this paper we will use the first (and most common) definition of cross-conjugation, i.e. there have to be (at least) two double bonds that are conjugated to a "central" double bond in such a way that the p electronic system forms a bifurcation.
Cross-conjugated molecules according to this definition include quinones, fulvenes, organic dyes, fused aromatic hydrocarbons and as mentioned above many bioorganic compounds like steroids, terpenes and alkaloids.
One characteristic feature of many cross-conjugated compounds is an enhanced reactivity. Some compounds are even highly unstable. This is easily seen by looking at the most simple representatives fulvene, cyclopentadienone, cyclohexadienone or benzoquinone.
The first cross-conjugated compound examined by microwave spectroscopy was fulvene (II) [9,10]. Its oxygen analogue cyclopentadienone (III) has also been studied [11,12]. All of these works clearly showed that the five-ring system has a planar geometry. The question about the planarity of the six-membered rings is more difficult to answer and indeed the answer that have been given so far are contradictory. Six-membered heterocyclic systems like g-pyranone and its sulfur analogues were shown to have planar structures  by microwave spectroscopy whereas studies on 4,4-Dimethyl-1-methylene-2,5-cyclohexadiene (IV) by electron diffraction seem to indicate a non-planar structure . The unmethylated parent compound has also been prepared . But as it reacts rather quickly to form a mixture of toluene and dimeric products  only kinetic and thermodynamic studies have been carried out so far.
Note added in proof: In the meantime a microwave study on this compound has also been performed.
The situation is even more difficult for the corresponding dienone (V). It tautomerizes very rapidly to phenol. The detection of the molecule has been claimed only twice: with infrared spectroscopy at -196°C  and with the flowing afterglow technique , but no structural data are available. We tried to produce (V) by flash pyrolysis and to detect the unstable molecule by microwave spectroscopy, but were not able to find any lines originating from the desired product. Instead we measured only the spectrum of phenol  which indicates that even in the gas phase at very low pressures the molecule rearanges very rapidly.
Since the instability of the cyclohexadienone is caused by tautomerism, i.e. migration of a hydrogen atom from C-4 to the oxygen atom, it is evident that double methylation at the 4-position stabilizes this molecule, just as for the methylene compound (IV). So we decided to investigate the dimethyl-substituted compound (IX) that was characterized first by Yanagita, Tahara and Ohki .
The compound thus prepared is stable over a period of at least several weeks when stored under nitrogen or in an evacuated flask, but oxidizes slowly when exposed to air.
The spectra were recorded using a conventional Stark-spectrometer with a thermostatized 3m-cell that allows measurements at constant temperatures between +40°C and -70°C.
The spectrum is typical of an a-type near-prolate rotor. There are large areas with no or few transitions and areas spaced by B+C where many lines lie very close together.
The assignment of some transitions proved to be rather difficult due to many lines originating from vibrationally excited states. We were not too interested in the vibrational behavior of this molecule and so only one state (n36=1) has been investigated in some detail.
The key to assigning the spectrum was to examine low-J-transitions with fewer K-components at low temperatures. The cooling however was limited by the vapor-pressure of about 0.008 mbar at -45°C. Most measurements were therefore carried out at about -30°C with a pressure of about 0.01 mbar. In the course of our investigation 108 lines of the vibrational ground state could be assigned (see Table 1) to give a least squares fit with a standard deviation of 30 kHz which corresponds well to the estimated accuracy of our measurements. The mesurements on the n36=1 state were made with low resolution. Therefore the fit for the 28 lines that have been assigned (see Table 2) shows a higher standard deviation of 534 kHz corresponding to the lower accuracy of these measurements. The spectroscopic constants are given in Table 3.
Spectrum of the transition 707-606 registered at a static field of ca. 590 V/cm with a modulation field of 825 V/cm (above) and calculated using Lorentzian line-shapes with full half-widths of 500 kHz (below). The width of the recorded range represents a compromise between small ranges that would be desirable for high accuracy and wide ranges permitting to measure several lines at one time. The broken line shows the zero-field frequency of the transition. The component with M=6 lies outside the recorded range. The signals are labeled with their respective values of M. The signal labeled 3* represents the component M=3 at 590 + 825 = 1415 V/cm. All other M*-components are shifted outside the depicted range.
The molecule under study possesses CS symmetry if the ring is non-planar, whereas the symmetry is C2v if it is planar. So we started off with only CS symmetry imposed on the calculations.
We first investigated cyclohexadienone (V). For initial parameters we optimized its structure with the aid of a molecular modeling package called SYBYL . The package used a TRIPOS-forcefield and the optimized structure was planar no matter if the starting-conformation was planar, boat, chair or half chair. So we twisted the molecule arbitrarily to a conformation where the C6-C1-C2-plane and the C3-C4-C5-plane were at plane angles of 2 and 10 degrees respectively to the C2-C3-C5-C6-plane. This conformation was then used as the initial conformation for the ab-initio optimizations with only CS symmetry required. These were carried out with the GAUSSIAN 86  program suite on a DEC VAX-6440. We performed a full optimization at the HF-SCF level using the Berny algorithm and standard convergence criteria on forces and displacements.
The geometry we thus obtained with a RHF/6-31G* basis set had an essentially planar ring with dihedral angles of about 0.1 degrees. The resulting parameters were taken as starting values for the optimization of the 4,4-dimethylated-compound. The initial parameters for the methyl groups were: 1,10 Å for C-H distances, 109,5 degrees for H-C-H angles and 1,54 Å for C-C distances. As the previous calculation had indicated that the methyl torsion has a very high barrier, the methyl groups were "frozen" at their potential minima. Apart from this constraint all other parameters were allowed to relax simultaneously. As before, only CS symmetry was fixed. The resulting ring structure, again, was essentially planar with dihedral angles of about 0.004 degrees and so we restrained the molecule to the indicated C2v symmetry to obtain a conformation with a perfectly planar ring without having to increase computing time overly.
A planar ring conformation was also obtained using a STO-3G basis set and in semi-empirical calculations with MNDO, MINDO/3 and AM1 hamiltonians using the MOPAC  package. The resulting rotational constants and dipole moments of the RHF/6-31G* and AM1 calculations are compared to the experimental values in Table 4.
There is no possibility to decide whether the ring in dimethyl-cyclohexadienone is planar or not just from the rotational constants or the planar moments. This is due to the fact that the geometry of the methyl groups influences the planar moment Pcc more than the (non-)planarity of the ring. Whereas this planar moment varies by ca. 1.5% for different ring-conformations (dihedral angles below 10 degrees), the differences between two planar structures with different but reasonable geometries of the methyl-groups can be well over 3%. But since the ab-initio and semi-empirical calculations, which resulted in planar structures, predicted all three measured rotational constants with remarkable accuracy (the deviations with RHF/6-31G* are about 0.2% for A and 1% for B and C, with AM1 they are 1.1% for A and 0.3% for B and C) there are good reasons for assuming planarity for the ring.
Note added in proof: Additional information on this work is contained in the dissertation thesis of W.H. (in German).
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|A||3.332158(13) GHz||3.332158(13) GHz|
|B||1.19307386(52) GHz||1.19307386(52) GHz|
|C||1.08221280(45) GHz||1.08221280(45) GHz|
|DJ||0.02149(73) kHz||0.02149 kHz*|
|DJK||1.1008(16) kHz||1.1008 kHz*|
|DK||-68.42(32) kHz||-68.42 kHz*|
|dJ||0.00332(16) kHz>||0.00332 kHz*|
|dK||-2.7513(99) kHz||-2.7513 kHz*|
|GAUSSIAN 86 (RHF/6-31G*)||MOPAC (AM1)||experimental|