Coupling constant benzene derivatives

Nuclear Magnetic Resonance Spectroscopy. Nmr is a most valuable technique for stmeture determination in thiophene chemistry, especially because spectral interpretation is much easier in the thiophene series compared to benzene derivatives. Chemical shifts in proton nmr are well documented for thiophene (CDCl ), 6 = 7.12, 7.34, 7.34, and 7.12 ppm. Coupling constants occur in well-defined ranges J2-3 = 4.9-5.8 ... 

The factor ijhb is the bond order between the interacting hydrogen atoms and is unity for H2. Proton-proton bond orders in substituted benzenes as derived from coupling constants and Eq. (21) are given in Table III. The same quantitative significance should not be attached to the proton-proton bond orders of Table III that one gives to the carbon-carbon... 

Photon-Proton Bond Orders in Benzenes as Derived from Coupling Constants... 

Table 4.59. One-Bond and Longer-Range 13C — 13C Coupling Constants (Hz) of 13C-7-Labeled Monosubstituted Benzene Derivatives [133],...

Proton Coupling Constants for Benzene Anions and Some Derivatives (G) (Bedford et al., in press Bolton and Carrington, 1961a Bolton el al., 1962a)... 

Nuclear Magnetic Resonance Spectroscopy. Nmr is a most valuable technique for structure determination in thiophene chemistry, especially because spectral interpretation is much easier in the thiophene series compared to benzene derivatives. Chemical shifts in proton nmr are well documented for thiophene (CDC13), 6 = H2 7.12, H3 7.34, H4 7.34, and H5 7.12 ppm. Coupling constants occur in well-defined ranges J2 3 = 4.9-5.8 J3 4 = 3.45-4.35 J2 4 = 1.25-1.7 and J2 5 = 3.2-3.65 Hz. The technique can be used quantitatively by comparison with standard spectra of materials of known purity. 13C-nmr spectroscopy of thiophene and thiophene derivatives is also a valuable technique that shows well-defined patterns of spectra. 13C chemical shifts for thiophene, from tetramethylsilane (TMS), are C2 127.6, C3 125.9, C4 125.9, and C5 127.6 ppm. 

Various chlorine-substituted benzenes have been studied extensively by ED and MW and the most accurate results were obtained by joint analyses of ED intensities, rotational constants (from MW or high-resolution IR spectra) and dipolar coupling constants from liquid-crystal NMR spectra. Ab initio calculations have also been performed for some of these derivatives246 (Table 27). The primary interest in these studies is the degree of distortion of the benzene ring caused by chlorine substitution but in this context we will discuss only the variation of the C—Cl bond distances. In chlorobenzene this distance [173.90(23) pm] is very slightly longer than that in chloroethylene [173.0(4) pm], similar to the observation for C—F bonds. In the three disubstituted derivatives the C—Cl bond distances shorten only by a few tenths of a pm and are almost independent of the relative position of the two chlorines. These experimental results are reproduced correctly by the ab... 

In the 13C NMR spectra of benzene derivatives, apart from the .Jar, only the meta coupling (3Jcu, but not 2Jqh) is usually resolved. A benzenoid CH, from whose perspective the meta positions are substituted, usually appears as a Jqh doublet without additional splitting, e.g. in the case of 3,4-dime thoxy-p-methyl-p-nitrostyrene (9, Fig. 2.9) the carbon atom C-5 generates a doublet at 8C = 111.5 in contrast to C-2 at 8C = 113.5 which additionally splits into a triplet. The use of CH coup-ling constants as criteria for assigning a resonance to a specific position is illustrated by this ex-ample. 

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