Molecular g Values, Magnetic Susceptibility Anisotropics, Second Moment of the Charge Distribution, and Molecular Quadrupole Moments in Formic Acid

The high-field rotational Zeeman effect has been observed in formic acid. The molecular g values and anisotropics in the magnetic susceptibilities are g_aa = –0.2797 ± 0.006, g_bb = –0.0903 ± 0.0006, g_cc= -0.0270 ± 0.0006, ²χ_aa -χ_bb - χ_cc = (3.4 ± 0.5) x 10^-6 erg/(G² mole), and 2χ_bb - χ_aa - χ_cc = (9.4 ± 0.3) X 10^-6 erg/(G² mole). Only the relative signs of the g values are determined experimentally. However, by comparing the calculated molecular quadrupole moments (for both all positive and all negative g values) with known values for other similar molecules, the above negative signs are conclusively assigned. The molecular quadrupole moments are Qαα = -(5.3 ± 0.4) x 10^-26 esu cm², Q_bb = +(5.2 ± 0.4) x 10^-26esu cm², and Q_cc = +(0.1 ± 0.4) x 10^-26 esu cm². The a axis is nearly along a line connecting the two oxygen atoms, and this axis makes an angle of 31.6° with the C=0 bond direction. The b axis is also in the OCO plane. The diagonal elements in the total magnetic susceptibility tensor are χ_aa = -(18.8 ± 0.8) x 10^-6 erg/(G² mole), χ_bb = -(16.8 ±0.8) x 10^-6 erg/(G² mole), and χ_cc = –(24.2 ± 0.8) x 10^-6 erg/(G² mole). The diagonal elements in the paramagnetic susceptibility tensor are Xαα^p = (28.8 ± 0.1) x 10^-6, X_pp = (106.5 ± 0.1) x 10^-6, and X_cc^p = (117.2 ± 0.1) x 10^{- 6} all in units of erg/(G² mole). The diagonal elements in the second moment of the electronic charge distribution are 〈²〉 = (26.6 ± 0.3) x 10^-16cm², 〈²〉 = (7.7 ± 0.3) x 10^-16 cm², and〈_c²〉 = (3.5 ± 0.2) x 10^-16 cm².