Molecular g Values, Magnetic Susceptibilities, Molecular Quadrupole Moments, and Sign of the Electric Dipole Moment in Cyclopropene

Abstract

The high-field rotational Zeeman effect has been studied in cyclopropene and 1,2-dideuterocyclopropene. The molecular g values for cyclopropene are gaa = −0.0897 ± 0.0009, gbb = −0.14915 ± 0.00016, and gcc = 0.05363 ± 0.00077. For the deuterated species they are gaa = −0.0689 ± 0.0010, gbb = −0.13718 ± 0.00015, and gcc = 0.4538 ± 0.00018. The magnetic susceptibility anisotropies, in units of 10−6 erg/G2·mole, are 2χaa − χbb − χcc = 7.1 ± 0.6 and −χaa + 2χbb − χcc = 26.8 ± 0.4. Although only the relative signs of the g values are determined experimentally, the above signs are conclusively assigned on the basis of the molecular quadrupole moments. The a axis is perpendicular to the double bond, and the a and b axes are in the molecular plane. The molecular quadrupole moments in cyclopropene, in units of 10−26 esu·cm2, are Qaa = −0.4 ± 0.4, Qbb = 2.4 ± 0.3, and Qcc = −2.0 ± 0.6. Using the known molecular structure, the diagonal elements of the paramagnetic susceptibility tensor and the anisotropies in the second moment of the electronic charge distribution may be calculated, and they are χaap = 56.1 ± 0.3, χbbp = 80.7 ± 0.2, χccp = 93.4 ± 0.2, all in units of 10−6 erg/G2·mole; and 〈a2〉 − 〈b2〉 = 4.26 ± 0.2, 〈b2〉 − 〈c2〉 = 7.75 ± 0.2, and 〈c2〉 − 〈a2〉 = −12.01 ± 0.2, all in units of 10−16 cm2. Although the bulk susceptibility is unknown, a theoretical value of 〈c2〉 in conjunction with the measured second-moment anisotropies enables the diagonal elements of the diamagnetic and total magnetic susceptibility tensors to be predicted. These are χaad = −82.3 ± 0.3, χbbd = −100.4 ± 0.3, χccd = −133.3 ± 0.3, χaa = −26.2 ± 0.4, χbb = −19.7 ± 0.4, χcc = −39.9 ± 0.4, and χAv = −28.6 ± 0.5, all in units of 10−6 erg/G2 mole. The sign of the electric dipole moment is obtained from accurately measured g values in the two isotopic species, and gives a polarity of +⊲−.