Robert B. Bradley

Tautomerism and Site of Protonation of 1-Methylcytosine: Proof by Nuclear Magnetic Resonance Spin-Spin Coupling

New nuclear magnetic resonance data lead to a completely unambiguous proof that the predominant tautomeric form of 1-methylcytosine is the amino form and that in acid solution the molecule protonates at 3-N. The close similarity of the nuclear magnetic resonance spectra of this compound to the spectra of the cytosine nucleosides indicates that these structures also exist in the nucleosides. These conclusions are reached on the basis of the spectra of analogs labeled with N15 and of the proton spin decoupling measurements made at low temperature.

Double resonance studies of isotope effects on nitrogen chemical shifts

Abstract The effect on nitrogen chemical shifts of 14N 15N isotopic substitution has been examined by 1H{14N} and 1H{15N} heteronuclear double resonance. For the four molecules (CH3)4N+I−, C6H6CH2NC, CH3ONO2 and (CH3)2NNO2 and their 15N anologs the ratio of 15 N 14 N resonance frequencies is R = 1.40275695 ± 8 × 10-8. It is concluded that there is no significant isotope effect on the nitrogen shieldings and that R gives directly the ratio γ ( 15 N ) γ( 14 N ) .

Nuclear Magnetic Resonance of Compounds Related to DDT: Part II. Aromatic Protons

The nuclear magnetic resonance spectra of the aromatic protons of DDT and 31 of its analogs and derivatives have been analyzed. Chemical shifts of these molecules are functions of the electronegativities of the substituents on the ring, as well as those in the aliphatic portion, although substitution of the ring chlorine in DDT by either a nitro or a t-butyl group leads to anomalous values. Molecular orbital calculations show that the chemical shift of the proton ortho to the ring substituent depends upon the π electron density at the corresponding carbon, but the chemical shift of the proton meta to this substituent is independent of the π electron density at the corresponding carbon. The data also indicate that the two aromatic rings in DDT are independent of each other.

The Nuclear Magnetic Resonance Spectra of Compounds Related to DDT. I. Non-Aromatic Protons:

The nuclear magnetic resonance spectra of a series of compounds related to DDT have been studied as they are influenced by ring or aliphatic substituents or by aliphatic structural changes The chemical shift of the benzylic or α proton follows the Hammett equation for ring substituents and the Taft variation of this equation for aliphatic substituents. Alkoxy substituents on the ring show alternation of chemical shift of the —OCH2— group protons with chain length and limiting behavior of the terminal methyl group with chain length. Dehydrochlorination of o,p-DDD produces cis-trans isomers. Coupling constants are shown to be influenced by ring substitution, particularly ortho chlorine.