The diagonal elements in the molecular g-value and magnetic-susceptibility tensors in the principal inertial axis in formaldehyde have been determined by observing the molecular rotational Zeeman effect. A small J dependence was observed in the molecular g values where the absolute values increased with rotational excitation. The g values averaged over the three rotational states studied for each isotopic species are gaa = − 2.899 ± 0.002, gbb = − 0.2256 ± 0.0008, and gcc = − 0.1004 ± 0.0007 for H2CO, and gaa = − 1.445 ± 0.002, gbb = − 0.1917 ± 0.0005, and gcc = − 0.0788 ± 0.0004 for D2CO. The diagonal elements in the molecular magnetic-susceptibility tensor are χaa = − 6.2 ± 0.5, χbb = − 15.8 ± 0.4, and χcc = − 23.0 ± 0.4, all in units of 10−6erg/G2·mole. The sign of the electric dipole moment along the C-O bond in formaldehyde was determined to have +C-O− polarity from the isotope dependence in the above molecular g values. From the g values and the known structure of formaldehyde, the paramagnetic-susceptibility tensor elements can be calculated to be χaap = 29.5 ± 0.1, χbbp = 46.1 ± 0.5, and χccp = 47.7 ± 0.5, all in units of 10−6erg/G2·mole. These data, combined with our above experimental magnetic-susceptibility diagonal tensor elements give the experimental ground-state average values of the sums of squared Cartesian center-of-mass electronic coordinates. The results are 〈0 | Σixi2 | 0〉 = (3.2 ± 0.3) Å2, 〈0 | Σiyi2 | 0 = (5.2 ± 0.3) Å2, and 〈0 | Σizi2 | 0〉 = (11.4 ± 0.3) Å2. The z axis is along the O-C bond and the y axis is in the molecular plane. These new data are used to interpret the ground- and excited-state electronic structure of the formaldehyde molecule. The experimental information is also sufficient to determine the diagonal elements in the molecular quadrupole moment tensor.
The microwave spectrum of OCF2 is examined under high resolution using an L-band waveguide and 1 and 5-kc/sec modulation, where half-widths at half-height of 5–7 kc/sec are obtained. The 19F nuclear-spin-nuclear-spin and the 19F spin—rotation interactions are observed. The experimental 19F spin—rotation constants along the principal inertial axes are Maa = | 19±3 | kc/sec, Mbb = | 13±3 | kc/sec, and Mcc = | 5±3 | kc/sec. The calculated magnitude of the nuclear contribution to the 19F spin—rotation constants show that the signs of the above values are all negative, which then leads to the calculated 19F paramagnetic shielding in OCF2 of σp = −324×10−6. The 19F chemical shift in OCF2 is measured relative to Cl3CF giving 23×10−6. Relating the 19F shielding back to F2 as a reliable standard gives the total shielding at 19F in OCF2 as 236×10−6. Combining this with the paramagnetic shielding obtained from the 19F spin—rotation constants gives the diamagnetic shielding as σd = 560×10−6, which is larger than the corresponding 19F diamagnetic shielding in F2 of 530×10−6. The first-order molecular Zeeman effect was also observed in OCF2 using a zigzag cell in a magnetic field up to 11 kG giving the diagonal elements of the g-value tensor along the principal inertial axes of gaa = | 0.074±0.002 |, gbb = | 0.038±0.003 |, and gcc = | 0.028±0.003 |. Arguments are given to suggest that the signs of the g values are all negative, which yields the paramagnetic susceptibility of χp = 127×10−6. The g values are combined with an estimate of the molar susceptibility to yield the average diamagnetic susceptibility in OCF2 giving a most probable value of 〈ψ0 | ∑ iri2 |ψ0〉=47.4×10−16cm2. A theory is presented which allows an estimation of either the molecular g values or the 19F spin—rotation interaction if one of the two quantities is known. The 19F parameters obtained in this study may be transferable to other systems. Several low-J transitions in OF2 are examined in a 4-ft liquid-N2 absorption cell, and the magnetic properties of 19F in this molecule are re-examined.
The first-order molecular Zeeman effect is observed in vinylidene fluoride to yield the molecular g values along the principal inertial axes. The results are | gaa | = 0.0373 ± 0.002, | gbb | = 0.0480 ± 0.003, and | gcc | = 0.0064 ± 0.003 where the a axis passes through both carbon nuclei and the b axis is in the molecular plane. The negative (positive) molecular g values and the molecular structure are used to give the diagonal elements in the paramagnetic-susceptibility tensor. The results are χaap = 103 (89) × 10−6, χbbp = 108 (92) × 10−6, and χccp = 217(212) ± 10−6 all in ergsG−2·mole−1. These values are compared with the recent corresponding numbers in H2CO, F2CO, H2C2O, and H2CF2. A molecular-orbital theory of paramagnetic susceptibility is employed to interpret the paramagnetic susceptibility in the H2CO, H2C2F2, F2CO, H2C2O, and H2CF2 molecules. A detailed discussion is given concerning the molecular orbitals participating in the n→π* transition in H2CO.
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