Chemical shifts [D] are assigned by additivity rules. J coupling allows precise assignment of 1H D. Since 13C spectra are obtained with 1H decoupling, additivity rules remain dominant here. 1H{13C} spectra contain J]CH which allow precise assignment of 13C shifts. Unlike D, J is not frequency dependent and may be obtained on low field NMR spectrometers. 1JCH was originally obtained from 13C satellites in 1H NMR spectra. While difficult this stimulated computation of 1J using quantum mechanics. Gaussian calculations are more reliable today and agree better with experiment. Such programs are available in many undergraduate labs. 1JCH for CH4 [125 Hz], C2H4 [156 Hz], C2H2 [259 Hz] increases linearly with the % s character of a C-H bond. Thus 1JCH measures hybridization of a C-H bond. Theory is that s type orbitals have a finite chance of being located in the nucleus while p orbitals have a node at the nucleus and zero chance of location there. 1JCH for CH4 [125 Hz], CH3Cl [150 Hz], CH2Cl2 [178 Hz] and CHCl3 [209 Hz] show electronegativity has a big effect of hybridization. VSEPR theory correctly predicts these molecules are all tetrahedral. Hence Organic Chemistry textbooks which state that once you know the bond angle you know the hybridization and vice versa are wrong. 1JCH shows C-H bonds in CH3Cl are sp2.3, in CH2Cl2 is sp1.8 and in CHCl3 sp1.4. Even ethane is not exactly sp3 hybridized. There are contradictory statements as to the hybridization of C in cyclopropane. C-H hybridization is not the same as for C-C bonds. Bonds between nuclei are hybridized, NOT the atomic nuclei. 1JCH in cyclopropanes [~160 Hz] indicate the C-H hybridization is sp2. C-C hybridization is then sp4. For CHCl3 its Cl-C-Cl bond angle of 110.50 shows that it is tetrahedral as predicted by VSEPR theory. The C-Cl bond length [177 pm] is much greater than that of the C-H [114 pm] hence carbon is no longer at the center of a regular tetrahedron. The use of 1JCH to estimate hybridization makes much more sense; iis indirectly related to bond angles in strained hydrocarbons but not in alkyl halides. This makes for a difficult distinction in teaching hybridization in organic compounds. 1JCH is useful for deciding the assignment of close D in heterocyclics as shown previously for Imidazo[1,4-a]pyridine. Current work focuses on substituted methylenedioxybenzenes which are of interest both as carcinogens and anticancer agents.

1H{13C} NMR allows teaching of hybridization of C-H bonds on an experimental basis

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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