Difluorobenzenes

Question. List the symmetry elements of each of the following molecules (a) 1,2,3-trifluorobenzene, (b) 1,2,4-trifluorobenzene, (c) 1,3,5-trifluorobenzene, (d) 1,2,4,5-tetrafluoro-benzene, (e) hexafluorobenzene, (f) l,4-dibromo-2,5-difluorobenzene. 

XB2u — XAg system of 1,4-difluorobenzene, with a weak overlapping sequence band labelled Bj. (Reproduced, with permission, from Cvitas, T. and Hollas, J. M., Mol. Phys., 18, 793, 1970)... 

The example we consider is the two-photon fluorescence excitation specfrum of 1,4-difluorobenzene, shown in Figure 9.29 and belonging to the >2 point group. The transition between the ground and first singlet excited state is Table A. 3 2 in Appendix A shows that 82 = r(T ) and, therefore, according to Equation (7.122), the electronic transition is allowed as a one-photon process polarized along the y axis which is in-plane and... 

Figure 9.29 Two-photon fluorescence excitation spectrum of 1,4-difluorobenzene. The upper and lower traces are obtained with plane and circularly polarized radiation, respectively, but the differences are not considered here. (Reproduced, with permission, Ifom Robey, M. J. and Schlag, E. W., Chem. Phys., 30, 9, 1978)...

Nevertheless, 1,4-difluorobenzene has a rich two-photon fluorescence excitation spectrum, shown in Figure 9.29. The position of the forbidden Og (labelled 0-0) band is shown. All the vibronic transitions observed in the band system are induced by non-totally symmetric vibrations, rather like the one-photon case of benzene discussed in Section 7.3.4.2(b). The two-photon transition moment may become non-zero when certain vibrations are excited. 

Figure 9.5 f shows a ZEKE-PE specfmm of f, 4-difluorobenzene in which fhe firsf laser, of energy /zcvj, was timed to fhe Og band of fhe 5i-5o system. The second laser, of energy hcv2, was fhen timed across fhe M -fo-M ionization band system. 

Figure 9.51 A zero kinetic energy photoelectron (ZEKE-PE) resonant two-photon spectrum of 1,4-difluorobenzene in which the first photon excites the molecule of the zero-point level of the S-[ excited electronic state of the molecule. (Reproduced, with permission, from Reiser, G., Rieger, D., Wright, T.G., Muller-Dethlefs, K. and Schlag, E.W., J. Phys. Chem., 97, 4335, 1993)...

The resolution of the ZEKE-PE spectmm of 1,4-difluorobenzene can be compared with, for example, that of the ultraviolet photoelectron spectmm of benzene in Figure 8.12. The greatly increased resolution in the ZEKE-PE spectmm is attributable mostly to the fact that only photoelectrons with zero kinetic energy are being detected. It is also partly attributable to the molecules being in a supersonic jet this has the effect of sharpening the bands because of the restricted rotational populations in the ground state of the molecule. 

DIFLUOROBENZENES Interest in the commercialization of difluoroaromatics in crop protection chemicals and dmgs (Table 5) continues to be strong. Numerous liquid crystals containing the 1,2-difluorobenzene moiety have been synthesized. Table 6 lists physical properties of commercially significant intermediates such as o-, m-, and -difluorobenzene, 2,4-difluoroaniline and 2,6-difluorobenzonitrile. The LD q values for the three isomeric difluorobenzenes are identical 55 g/m for 2 h (inhalation, mouse) (127). 

Photochemical brominations offluorobenzene or difluorobenzenes produce hexabromocyclohexane derivatives in low yield. Tri- and tetrafluorobenzenes are unreactive under these conditions [17 (Table 6). Analogous free-radical chlorination of benzotrifluoride produces a 9% yield of the hexachloro adduct [iS]... 

Dihalo-5-fluoroben/oic acids, obtained by oxidation of the corresponding acetophenones, have received considerable attention as intermediates for antibac-tenal fluoroquinolones [3reacts with acetyl chloride to give 2,4,5-trifluoroacetophenone [29] (equation 19) 1,3,5-Tn-fluorobenzene and 1,2-difluorobenzene yield 2,4,6-trifluoroacetophenone and 3,4-difluoroacetophenone, respectively [30] (equations 20 and 21). 

Carboxylic acids and their anhydrides acy late a variety of benzene derivatives, fused ring systems, and heterocyclic compounds. An improved procedure for the preparation of l,4-difluoroanthracene-9,10-dione involves reacting phthalic anhydride and 1,4-difluorobenzene to prepare an intermediate carboxylic acid [35] Intramolecular acylation in polyphosphonc acid completes the synthesis (equahon 24). 

In this paper we present the relative energies of the isomers of the phenylenediamines, dihydroxybenzenes and difluorobenzenes from ab initio calculations using large basis sets and including correlation corrections at the MP-2 level. These calculations were done at the geometry optimized structures. We also include zero-point energy corrections based on our calculated force fields. 

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