The spin–spin coupling constant over five formal bonds between 19F and methyl protons, [Formula: see text], in 2,6-dimethylbenzoyl fluoride is −3.1 Hz. Observation of a nonzero [Formula: see text] indicates an out-of-plane conformation for the carbonyl fluoride group and implies substantial nonbonded repulsions between the methyl and carbonyl fluoride groups. It is argued that [Formula: see text] is as small as −7 Hz when the C—F bond lies cis to a methyl group and that its magnitude is a consequence of the so-called 'through-space' coupling mechanisms. On the basis of INDO–MO–FPT computations, it is suggested that such observed couplings are a composite of large contributions of either sign and, therefore, that observed through-space 1H,19F couplings may be of either sign if conformational averaging occurs.

CHUM. Can. J. Chem. 54, 3666 (1976). The proton magnetic resonance spectra in the absence of inter~nolecular proton exchange for 0.27, 0.59, and 0.79 1l4 solutions of phenol in CC14 are analyzed. The long-range coupling over five bonds between the hydroxyl and ring protons is estimated as 0.33 Hz in the monomer and as 0.20 Hz in the trimer of a monorner-trimer model. Support for the decrease in the coupling 011 trilnerization is found in CNDO/2 and INDO MO FPT calculations. The long-range coupling over six bonds is unobservable, consistent with conformations of the trimer in which the phenol molecules remain planar. Water catalyzes the intermolecular proton exchange and, in its absence, the transfer of hydroxyl protons from ~nolecule to molecule within the associated species is inefficient. TED SCHAEFER, J. B. ROWBOTHAM et KALVIN CHUM. Can. J. Chem. 54, 3666 (1976). The ring proton resonance spectrum of phenol dissolved in CC14 was first analyzed precisely by Castellano et crl. (1). Long-range spin-spin couplings between the hydroxyl proton and the ring protons were not observed, no doubt because of relatively rapid intermo!ecular proton exchange. Such coupling constants are easily observed when strong intramolecular hydrogen bonding to carbonyl or nitro groups exists (2-4). More recently, stereospecific long-range couplings involving the hydroxyl proton have been used to deduce the relative strengths of intramolecular hydrogen bonds to halogen substituents (4-6).In this paper experiments are described which succeed in reducing the rate of intermolecular hydroxyl proton exchange in CCI4 solutions of phenol to the extent that long-range couplings to the ring protons become observable. Self association of phenol in CC14 has been extensively investigated (7-111, so that the present [Research assoclate. summer 1975. coupling data throw some light on the conformation of the phenol molecule in the associated state and on the perturbation of the couplings caused by self association. Some inferences about the mechanism of proton exchange in nonaqueous solutions are also made. ExperimentalThe assumption that water catalyzes the intermolecular proton exchange in CC14 solution dictated the method of sample preparation. Zone refined phenol from Aldrich Chemical Co., rated at 99.9(+)',; purity, with an mp of 40.68 C, and CC14 from Fisher, containing 0.02', of water (certified A.C.S.), were used in the following procedure.Sample tubes, stopcock attachments for purposes of degassing by the freeze-pump-thaw technique, pipettes containing small wads of white Kleenex tissue, and 5 ml vials containing molecular sieve, were kept in beakers on a hot plate in a dry box. The temperature at the bottom of the beaker ranged from 180 to 210-C. After 3 days ~~n d e r these conditions, solutions of phenol in CC14, containing a littlc tetramethylsilane and prepared on a balance outside the dry box. were added to the vials containing the rnolec~rlar sieve. These vials were stoppered and left for a day in the dry box, well away from the ho...

The doubly hydrogen bonded conformation of 2,2′-dihydroxy-4-methoxybenzophenone ensures substantial nonbonded interactions between C—H bonds on neighbouring aromatic rings and gives rise to spin–spin coupling between the protons in those bonds. Because of the relative orientation of the C—H bonds containing the coupled protons, the coupling represents a direct mechanism which probably does not depend on the orbitals of the carbon atom. The observed coupling between the hydroxyl protons, formally over eight bonds, is small and indicative of an inappreciable covalent character of the hydrogen bond, in agreement with recent conclusions that a weak hydrogen bond is best represented by electrostatic and non-bonded forces only.

Received F e b~u a r y 2, 1978 T E D SCHAEFER and K A L V I N CHUM. Can. J. Chem. 56, 1788Chem. 56, (1978. The analysis of the proton magnetic resonance spectra of 2-methylphenol and of 4-chloro-2-methylphenol in benzene-& and CCI, solutions yields spin-spin coupling constants over five bonds between the hydroxyl proton and the t11etn ring protons. These coupling constants are related to the intramolecular equilibrium between conformers in which the hydroxyl group is oriented cis or trolls t o the methyl group. The free energy difference between the conformers is 0.4 , 0.1 kcal/niol, with the trmls form being the more stable. The concentration dependence of the hydroxyl proton chemical shift of the chloro compound in CCI, suggests that dimerization is relatively ~~ninlportant and yields rough values for the extent of self-association. It is argued that the derived free energy values refer t o the intramolecular equilibrium f o r the monomers.T E D SCHAEFER et KALVIN CHUM. Can. J. Chern. 56, 1788 (1978). L'analyse des spectres de resonance nlagnetique nucleaire du proton du methyl-2 phenol et du chloro-4 methyl-2 phenol dans des solutions d e benzene-d, et de CCI, conduisent aux constantes de couplage spin-spin a travers cinq liaisons entre le proton de I'hydroxyle et les protons du cycle en position I T I P~~. On a etabli une relation entre ces constantes d e couplage et I'equilibre intranioleculaire entre les conformeres dans lesquels le groupe hydroxyle est oriente soit cis ou trm7s par rapport au groupe mtthyle. La difference d'energie libre entre les conformeres est de 0.4 f 0.1 kcal/nlol, la fornie trntls etant la plus stable. La relation q~~i existe entre la concentration et le deplacenlent chimique du proton hydroxyle du conlpose chlore dans CCI, suggire que I'iniportancc d'une dimerisation est faible et fournit des valeurs approximatives concernant le taux d'autoassociation. On presellte des arguments B I'effet que les valeurs des energies libres qui en sont derivees se referent a l'equilibre intranloleculaire des monomeres.[Traduit par le journal] Introduction A direct measurement of the cis-fiatis equilibrium for 2-methylphenol in solution is not available. The stretching frequency of the hydroxyl group in 2-inethylphenol in CC1, solution at 300 K is not resolvable into two components, so that the free energy difference, AG, between the cis (la) and trans (lb)

. Can. J. Chem. 53, 2734Chem. 53, (1975.The carbon-13 satellite peaks in the proton magnetic resonance spectra of 1,3-dioxole and bis-1,3-dioxolyl are analyzed under single and double resonance conditions to yield the signs and magnitudes of proton-proton coupling constants over three, four, and five bonds, and of protonxarbon-13 coupling constants over one, two, and three bonds. The conformational behavior of bis-1,3-dioxolyl contrasts sharply with that of analogous sym-tetrasubstituted ethane derivatives. It is indicated that the two-bond proton-carbon-13 coupling in the ethanic fragment can be used for conformational analysis in a manner similar to vicinal proton-proton couplings. The vicinal three-bond protonxarbon-13 couplings are given for dihedral angles of 180 and 120" and their relative magnitudes are as expected from a Karplus relationship. The two-bond protonxarbon-13 coupling in the olefinic fragment is, at 20.0 Hz, the largest coupling known for such a bond.TED SCHAEFER, KALVIN CHUM, DAVID MCKINNON et M. S. CHAUHAN. Can. J. Chern. 53,2734Chern. 53, (1975.On a analysk les pics satellites causes par le 13C dans les spectres r.m.p. du dioxole et du bis-dioxolyl-1,3; utilisant des conditions de resonance simple et de resonance double, on a pu determiner le signe et I'amplitude des constantes de couplage proton-proton sur trois, quatre et cinq liens et les constantes de couplage proton-13C sur un, deux et trois liens. Le comportement conformationnel du bis-dioxolyl-1,3 est en opposition flagrante avec celui de dCrives analogues de 1'Cthane tetrasubstituk d'une facon symktrique. I1 ressort de ces travaux que le couplage proton-13C sur deux liens de la fraction Cthane pourrait &re utilise pour des besoins conformationnels de la mCme maniere que les couplages proton-proton vicinaux. On donne les couplages vicinaux proton-13C sur trois liens pour des angles dikdres de 180 et 120' et leurs amplitudes relatives correspondent a ce que I'on peut attendre a partir d'une relation de Karplus. Le couplage proton-13C sur deux liens du fragment olCfinique est de 20.0 Hz qui est la valeur la plus ClevCe connue pour un tel couplage.[Traduit par le journal]