Molecular Zeeman Effect of Cyclobutene, Methylene Cyclopropane, and Methylene Cyclobutane

The rotational Zeeman effect has been observed in cyclobutene, methylene cyclopropane, and methylene cyclobutane. The molecular g values, magnetic susceptibility anisotropies, and molecular quadrupole moments were obtained and they are listed below (the a and b axes are in the molecular plane; the a axis is perpendicular to the carbon–carbon double bond in cyclobutene, and it is along the carbon–carbon double bond in methylene cyclopropane and methylene cyclobutane). The units of χ are 10−6 erg/G2·mol and the units of Qxx are 10−26 esu·cm2: CyclobuteneMethylene cyclopropaneMethylene cyclobutanegaa− 0.0516 ± 0.0007− 0.0672 ± 0.0007− 0.0320 ± 0.0008gbb− 0.0663 ± 0.0007− 0.0231 ± 0.0004− 0.0218 ± 0.0010gcc− 0.0219 ± 0.0006+ 0.0244 ± 0.0004− 0.0184 ± 0.00112χaa − χbb − χcc− 0.9 ± 0.5+ 18.3 ± 0.5− 6.4 ± 0.5− χaa + 2χbb − χcc+ 5.0 ± 0.7+ 14.0 ± 0.6+ 4.3 ± 1.7Qaa− 0.3 ± 0.6− 0.7 ± 0.5− 1.2 ± 1.1Qbb+ 1.6 ± 0.7+ 0.9 ± 0.6− 1.1 ± 2.0Qcc− 1.3 ± 1.0− 0.2 ± 0.9+ 2.3 ± 2.4 The above results, in particular the magnetic susceptibility anisotropies, are compared with similar ring molecules. Local and nonlocal (ring current) contributions to the magnetic susceptibility anisotropies are discussed and an additivity scheme is proposed for local group anisotropies in hydrocarbons.