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Off-resonant electric field induced second harmonic generation (EFISH) and resonant third harmonic generation (THG) measurements of two squarylium dyes are reported. The magnitude of is large for both processes. The EFISH measurement clearly shows that the off-resonant third-order polarizability is negative, a consequence of the dominance of a non-two-photon mechanism. This unusual result is in agreement with our earlier reports on two other squarylium dyes. Resonant THG measurements strongly suggest that the two-photon contribution, while smaller than the one-photon contribution, is not negligible and cannot be entirely ignored when defining the overall thirdorder optical susceptibility. Consequently, the squaryliums are best qualitatively described by a three-level model that truncates to two dominant perturbation terms. Also reported is the isomeric distribution of squaryliums. Proton NMR results are interpreted by way of molecular mechanics to show that the squaryliums can readily exist as a mixture of isomers. The possible effects on nonlinear optical (NLO) measurements via an orientational contribution to the EFISH are discussed. An X-ray structure for the squarylium dye TSQ is presented and interpreted. The X-ray structure clearly suggests that the squarylium dyes are extensively bond delocalized with relatively little bond length alternation.

Structure-resonance theory for r-molecular systems based solely on covalent KekulC structures is justified phenomenologically and by reference to recent theoretical work. The idea of antiaromaticity is shown to be a logical extension of the theory, and a concept of a local ring aromaticity or antiaromaticity is quantitatively defined. Estimates of resonance stabilization energies are substantially lower than predictions based on Hiickel MO theory. The resonance theory results agree with those from SCF-LCAO-MO calculations. he recognition of extremely simple algorisms for T counting KekulC structures' and their permutations induced us to test a semiempirical quantum theory with a basis of KekulC structure functions. The mathematical equivalencies3-j between Hiickel molecular orbital (HMO) and valence bond (VB) theories for the benzenoid class of hydrocarbons led us to expect a correspondence of our results to previously known HMO quantities. The results2 were surprising in that it was found that calculated resonance energies, RE, for an extensive series did not correlate well with HMO delocalization energies (correlation coefficient, 0.493 for resonance energy per electron, REPE). Instead there was a congruity with resonance energy values obtained from SCF-LCAO-MO calculations6 (correlation coefficient, REPE, 0.991).The approach that we use is essentially a quantification of the structural resonance theory traditionally applied to structure-reactivity problems in organic chemistry.' In this paper we try to provide some justification for our procedures which will be described in more detail than in the previous communication.Calculations of resonance energies of several classes of aromatic compounds will then be presented, and comparisons with previous MO results and experimental properties will be delineated. We emphasize throughout that our computational procedure is so easily carried out and leads to such sensible results that it should be the method of choice for calculating resonance energies. Applications to the estimation of heats of formation8 and carcinogenic activitiesg of benzenoid hydrocarbons have already given good results, and a description of bond-order relationships will appear in a following article. Resonance TheoryThe formulism of the method is that of VB theory. l 1 For a n-electronic system each energy eigenfunction is written as given in eq 1. The $(u) are structure func-tions, the cf are coefficients, the index u indicates the type of electron distribution associated with the function, and the subscript i numbers the function. After selecting functions, the coefficients and eigenvalues can be evaluated by solving eq 2, where H is the Hamiltonianmatrix and S is the overlap matrix. A conventional approximation is to assume zero overlap of the wave functions so that off-diagonal elements in the matrix involve Hamiltonian integrals only. A significant simplification in obtaining a ground-state eigenvalue is also provided if one assumes a wave function consisting of equal contributions from the c...

1685 cm-"1; nmr (CDC13) H6 4.41 (s), H3 5.56 (s), OCH3 6.60 (s), gem-dimethyl 8.70 and 8.50 (s). Hydrogenation of 4c in methanol-THF using 5% Pd/C gave 4d.Having demonstrated the utility of this procedure in the synthesis of 6-methoxypenicillins, we focused our attention on the methoxylation of the cephalosporin molecule. The procedure, by analogy, presented two formidable problems: (1) chlorination at the C2 position and (2) 3 double bond isomerization.6 However, treatment of 6a with lithium methoxide (3.5It was now apparent that we could confirm the structure of 7-a-methoxycephalosporin C (7e) obtained OAc C02R2

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