A PRECISE ANALYSIS OF THE H-1 NUCLEAR-MAGNETIC-RESONANCE SPECTRUM OF 2-PHENYL-1,3-DITHIANE - RING PUCKER, SIGNS OF LONG-RANGE J(H,H), INTERNAL ROTATIONAL BARRIER, AND VAN-DER-WAALS SHIFTS

The H-1 nuclear magnetic resonance spectrum of 2-phenyl-1,3-dithiane, as a dilute solution in a CS2-C6D12-TMS solvent mixture at 300 K, is a nalyzed to yield 8 chemical shifts and 22 distinct coupling constants, (n)J(H,H), n = 2-6. The coupling constant between H-2 and the para pr oton indicates, first, that the bisected conformer (phenyl plane perpe ndicular to the pseudo plane of the dithiane ring) is most stable and, second, that the apparent twofold barrier to rotation about the Csp(2 )-Csp(3) bond is 9.6 kj/mel. The AM1, STO-3G and STO-3G* computations confirm the twofoldedness of the barrier, the AM1 barrier is 9.4 kJ/m ol. The empirical equation, (3)J(H,H) = 13.36 cos(2) phi + 9.59 cos ph i - 0.43, reproduces the vicinal coupling constants of the CH,CH,CHI f ragments and implies puckering angles phi(ee), phi(ea), and phi(ae) of 54 degrees, 61 degrees, and 64 degrees, respectively. It is implied t hat (3)J at phi = 0 degrees is larger than at phi = 180 degrees. This results is discussed in terms of the latest theoretical approach to (3 )J in the HCCH fragment. The (4)J(H,H) signs and magnitudes for the CH 2CH2CH2 fragment agree reasonably well with theory. For the CH2SCH fra gment, (4)J(H,H) values are positive, in contrast to corresponding num bers in the propanic fragment, perhaps the first experimental values f or certain rigid orientations about a heteroatom. INDO MO FPT computat ions on propane, dimethyl ether, and dimethyl sulfide confirm the expe rimental trend in (4)J(H,H). (2)J(H,H) and (5)J(H,H) values are compar ed to those in related molecules. The striking differential shifts of the axial and equatorial protons are attributed to differential van de r Waals interactions with the 3p lone-pair orbital on sulfur. A compar ison of the ring proton chemical shifts with those in phenylcyclohexan e and isopropylbenzene implies that C-S bonds are weaker net electron donors by hyperconjugation than are C-C bonds. It is also proposed tha t the ortho protons are deshielded by intramolecular van der Waals int eractions with the 3p orbitals on the sulfur atoms.