Proton spin couphng constants

J.E. Del Bene, I. Alkorta, I.J. Elguero, A systematic comparison of second-order polarization propagator approximation (SOPPA) and equation-of-motion coupled cluster singes and doubles (EOM — CCSD) spin — spin couphng constants for selected singly bonded molecules, and the hydrides NH3, H2O, and HF and their protonated and deprotonated ions and hydrogen-bonded complexes, J. Chem. Theor. Comput. 4 (2008) 967-973. 

There are several experimental tools available for the determination of the H-H distance and the degree of the H-H bonding interaction. Neutron diffraction studies provide an accurate measure of the H-H distance. The measurement of the spin-lattice proton relaxation time, Ti, for an tf -V 2 complex or the proton-deuteron couphng constant, Jhd. for the corresponding isotopically substituted rf -WT) complex via H nuclear magnetic resonance (NMR) spectroscopy provides a quantitative measure of the H-H distance. The frequency of the v(H-H) stretching band, as determined by Raman or infrared (IR) spectroscopy of / -H2 complexes provides semiquantitative information about the strength of the H-H interaction. 

Ruonne NMR data can be collected readily on most spectrometers, requinng only minor adjustments to mstrumentation used to run proton samples The fluonne-19 nucleus is easily detected (relative abundance, 100%, spin, 1/2) and generates a wealth of spectral information to assist in structure elucidation To take full advantage of all the spectral evidence available, H, and chemical shifts and couphng constants should be acquired and correlated... 

This is confirmed by comparing ID double-quantum filtered experiments with increasing recouphng times. Extension of this to 2D SQ/DQ correlation identifies the nature of the dipolar coupled pair, since the couphng partner of the directly detected spin can be identified in the indirect dimension, where the chemical shift is the sum of the shift of the two protons involved. This spectrum (Fig. 4b) exhibits intense and well-resolved cross peaks between CD and PDMS. The dipolar couphng constant between the guest and host must therefore be of the order of, or exceed, a few himdred Hz, proving that whatever fast motion the polymer chain performs, it must be a local one. 

We have now discussed three situations in which the n h- 1 Rule fails (1) when the couphng involves nuclei other than hydrogen that do not have spin = 1/2 (e.g., deuterium. Section 6.13), (2) when there is nonequivalence in a set of protons attached to the same carbon and (3) when the chemical shift difference between two sets of protons is small compared to the coupling constant linking them (see Sections 7.7 and 7.8). 

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