. Can. J. Chem. 53, 986 (1975). Under conditions of slow intermolecular hydroxyl proton exchange 6Jp0H.F < 0, SJtr.nsOH.F > 0, SJcI,OH~F < 0, 4J00H*F < 0 in fluorophenol derivatives. 'J,OHsF is remarkably insensitive to intrinsic substituent effects, yields accurate values of conformational populations of m-fluorophenol derivatives and can be used to demonstrate a buttressing effect on the strength of the relatively weak 0-H-halogen hydrogen bond. 4J,0H.F displays a stereospecific dependence opposite to that of other couplings from a side-chain proton to a ring fluorine nucleus, is apparently very sensitive to the distance between proton and fluorine nucleus and is cited as an example of a negative through-space coupling ,mechanism. Approximate molecular orbital calculations give some support to the last suggestion. [Traduit par le journal]

The long-range spin-spin coupling constants over five bonds between the hydroxyl proton and the ring protons in a series of trihalophenols imply that the intramolecular hydrogen bond strength (negative entlialpy) to fluorine is greater than that to iodine by 75 + 20 cal,mol, whereas the strengths to chlorine and bromine are 460 i 60 caliniol greater than to iodine. If a distinction can be made between chlorine and bromine, then chlorine-hydrogen bonds more strongly by only a few tens of calories per mol. The meas~lrenients were ~u a d e mainly o n dilute solutions in carbon tetrachloride a t 32 ' C . Chern. 52.3037 (1974) Les valeurs des constantes de couplage spin-spin a longue distance sur cinq liens entre le proton de I'hydroxyle et les protons du cycle ont ete mesurees pour une serie de phenols trihalogenes: ces valeurs impliq~le~it que la force du lien hydrogene intramoleculaire (entlialpie negative) vers le fluor est plus grande que celle Lers I'iode par 75 I 20 cal mol alors qne les forces des liens hydrogenes intraniolCculaires vers le chlore et le bronie sont plus grandes que celle de I'iode par 460 i 60 cal/niol. Si l'on peut faire une distinction entre le chlore et le brome on note alors que les atomes de chlore font des liens q~t i sont plus forts: la difference n'est que quelques dizaines de calories par niol. O n a fait les niesures principalenlent sur des solutions diluees dans le tetrachlornre de carbone a 32 C .[Traduit par le journal]

The proton magnetic resonance spectra of some benzylfluoride derivatives yield long-range spin–spin coupling constants between ring protons and protons or fluorine nuclei in the fluoromethyl group. In conjunction with the eigenfunctions for a hindered twofold rotor, the couplings over six bonds are used to show that in 3,5-dichlorobenzylfluoride in solution the C—F bond prefers the benzene plane by 260 ± 50 cal/mol; in close agreement with ab initia and MINDO/3 molecular orbital calculations. The latter method suggests that in a conformation in which the C—F bond lies in a plane perpendicular to the benzene ring, the C – C – F angle reduces to 107.2° and the C – C – H angles become 116.1°, perhaps due to increased conjugation of the C—F bond or fluorine atom with the π electrons of the ring. The observed barrier is presumably a delicate balance between steric interactions, hyperconjugation or p–p conjugation effects, and dipole–dipole interactions between polarized bonds.

A complete analysis (8-spins) is given of the p.m.r. spectrum of aniline-15N, of the spectra of some haloa n i l i n e~-~~N and of 2-aminoacetophenone-'5N. Intermolecular exchange of the amino protons is slow enough for observation of their spin-spin coupling to the ring protons. The magnitudes of the coupling constants between amino protons and 15N or ring protons are a measure of the geometry of the amino group. This is not true of the couplings between 15N and the ring protons. Long-range couplings computed in the CND0/2 and INDO approximations of molecular orbital theory show points of agreement with experiment. For example, their signs and magnitudes are consistent with a nonplanar but not with a planar conformation of aniline. Couplings from 15N to ring protons are also computed for nitrobenzene.L'analyse complete (8 spins) du spectre de r.m.n. est donnee pour I'aniline 15N, quelques haloanilines 15N et pour l'amino-2 acetophenone 15N. L'khange intermoleculaire des protons de l'amino est suffisamment lente pour pouvoir observer leur couplage spin-spin avec les protons du cycle. Les amplitudes des constantes de couplage entre les protons de l'amino et "N ou les protons du cycle donnent une mesure. de la geomttrie du groupe amino. Cela n'est plus vrai pour les couplages entre 15N et les protons du cycle. Les couplages a longue distance calcultrs avec les approximations du CND0/2 et INDO de la theorie des orbitales moleculaires sont en accord avec les valeurs experimentales. Par exemple, les signes et les amplitudes des couplages ne sont compatibles qu'avec la conformation non plane de I'aniline. Les couplages entre 15N et les protons du cycle ont Cte egalement calcults pour le nitrobenztne.

Spin–spin coupling constants over five, six, and seven bonds between protons in different methyl groups are reported for the xylenes, derivatives of benzene, pyridine, pyrimidine, pyridinium salts, p-benzoquinone, and borazine. The coupling magnitudes are characteristic of the substitution pattern in the delocalized systems. Calculations at the INDO-MO-FPT level of methyl proton couplings in N- and B-methylborazines are in agreement with the available experimental evidence in indicating a relatively weak transmission of spin density information via the presumed π electron system of borazine.