19F Spin—Rotation and Spin—Spin Interactions, 19F Magnetic Shielding, and Molecular Magnetic Moments for OCF2

Abstract

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.

Fourier Transform Microwave Spectrum, Geometry, and Hyperfine Coupling Constants of Phosphenous Fluoride, OPF

“…Absolute Nuclear Shielding Parameters. The spin−rotation constants of a nucleus A can be written as the sum of a nuclear and an electronic term − where the two terms are given by Here μ N is the nuclear magneton, g A is the g factor of nucleus A, e and m are the proton charge and electron mass, c is the speed of light, r nA is the distance between nucleus n , of atomic number Z n , and nucleus A, L i, g A is the g component of the orbital angular momentum of electron i about nucleus A, r iA is the distance between electron i and nucleus A, and |0〉 and | k 〉 are the ground- and excited-state electronic wave functions, at energies E 0 and E k , respectively. Thus, the nuclear term, (nuc), is seen to depend only on the geometry of the molecule, whereas in order to calculate the electronic term, (el), one requires knowledge of the ground- and excited-state wave functions and their energies.…”

Proton and fluorine nuclear shielding anisotropies

“…For C6HaF values for the in-plane anisotropy of -9 and 289 have been obtained, and for Aa in CHsF -66 and -157. [3] ; mc, [4] ; mw, microwave [5] ; n, nematic phase ; na, [6] ; nb, [7] ; nc, [8] ;nd, [9] ; ca, clathrate [10,11] ; sa, solid [12] ; sb, [13] ; q, calculation (limited basis) [14]. The theoretical considerations given in w 2 should enable the sign at least of a shielding anisotropy to be predicted.…”

“…The assumption AHF(2)= AA (2) has been much used, and the assumption AuF(2)=AB (2) enables shielding anisotropies to be related to diamagnetic anisotropies. It has been used to discuss the relation between nuclear spin-rotation constants and rotational magnetic moments [5]. It would, however, be dangerous to assume even that expectation values of ~HF (2) and /~B (2) have the same sign.…”

The microwave spectrum of cyanophosphine, H2PCN

“…The former term can be expressed in terms of strictly geometric parameters, but the latter term involves the ground state electronic wavefunction and the excited state wavefunctions. There is an intimate relationship between the nuclear spin-rotation tensor, C, and the nuclear magnetic resonance (nmr) shielding tensor, r, as first pointed out by Ramsey [37][38][39] and more explicitly by Flygare [40][41][42][43][44]. The nuclear spin-rotation tensorÕs encoding of the electronic environment of the nucleus has the identical dependence upon sums over matrix elements of the electronic wavefunctions as does the nmr shielding tensor.…”