V.J Kowalewski

The signs of the long-range spin-spin coupling constants in methyl pyridines as determined by the INDOR technique

Abstract The long-range spin-spin coupling constants between the methyl and the ring protons in several methyl pyridines have been determined, with their signs. The INDOR technique was used for this purpose and a systematic method was developed applicable to these spin systems, which were all of the ABCX3 type. It was found that 6 J H,CH 3 and 4 J H,CH 3 are negative, while 5JH,CH3 is positive, with |4J|>|6J|>|5J|. These results show the same trend as shown by investigations of previous authors in other cyclic compounds.

Conformational effects on 13C NMR parameters in alkyl formates

The 13C NMR spectra of a series of alkyl formates which show the coexistence of s‐cis and s‐trans rotamers at room temperature were measured at 125 MHz. 13C chemical shifts and 1J(CH) couplings are compared for both types of rotamers. Their differences are rationalized in terms of different intramolecular interactions. The 17O NMR spectra of these compounds could be observed only for the most abundant rotamer, which in all cases was identified as the s‐cis rotamer. In the three members of this series with the shortest alkyl chains, a 2J(17O,1H) coupling constant of ca. 40 Hz was observed. In ethyl formate, 13C magnetic shielding constants were calculated using the LORG approach with ab initio optimized geometries in both types of rotamers for the carbon atom β to the dicoordinated oxygen atom. A comparison between the calculated and experimental values yields support for the rationalizations quoted above.

Intramolecular electric field effect on a 1J(C,H) NMR spin–spin coupling constant. an experimental and theoretical study

An experimental study of the effects of intramolecular electric fields on 1J(C,H) coupling constants involving a formyl proton in a series of 5‐X‐salicylaldehydes is reported. The particularly large electric field component along the CcHf bond, calculated for X=NO2 , leads to a measured substituent effect of 7.36 Hz. For other X substituents a linear correlation was found between the calculated substituent electric field component along the Cc—Hf bond and the substituent effect on 1J(Cc ,Hf). To determine if 5‐X substituents affect the intramolecular CO· · ·HO hydrogen bond, NBO analyses were carried out on 5‐NO2 − and 5‐H‐salicylaldehyde, and they were compared with those corresponding to benzaldehyde and phenol. The effects of interactions other than the electrostatic one on 1J(Cc ,Hf) couplings were ruled out. The experimental results are in agreement with theoretical predictions published previously. Copyright

1H, 13C and 19F NMR studies on fluorinated ethers

Abstract The enflurane and ethoxyflurane 1H, 13C and 19F NMR spectra are examined—including sign determination of FF and FH couplings—and considered in the light of previously reported results for methoxyflurane. Conformational differences between methoxyflurane and the former two molecules are indicated by through space FH coupling constants and by the nonequivalence of geminal fluorine nuclei. Populations of conformers about the CC bond are estimated.

A nuclear magnetic resonance study of 2,5-dihydrofuran partially oriented in a nematic phase

Abstract From the study of the proton magnetic resonance spectra of dihydrofuran dissolved in a nematic phase the ratios of interproton distances have been derived.

A simple model of INDOR spectra

Abstract A model is made which describes INDOR spectra as the result of “falling into the trough,” which is produced when the monitored line is split by tickling, plus the effect of the redistribution of the populations of the different levels due to the pumping action of the rf fields. This last effect is calculated using an extension of Blochs “equivalent circuit” model in which, besides the terms representing the spontaneous transition probabilities, terms are added which represent the transition probabilities induced by the rf fields. Calculations made for the AX and AB cases, both homo- and heteronuclear, show good agreement with the specific case studied, as well as with data available in the literature, in spite of the simplifying assumptions which had to be made in the model.

Conformational analysis in 2,4-dinitrophenol by nuclear Overhauser effect experiments

Abstract The cis conformation of the hydroxyl with respect to the nitro group in 2,4-dinitrophenol under fast exchange conditions and the consequent persistence of the intramolecular hydrogen bond are determined. These facts emerge from the existence of a scalar-coupling-of-the-first-kind relaxation interaction linking the phenolic and the ring protons of the compound which is directly established by a nuclear Overhauser effect experiment. Relaxation information is incidentally obtained. Decoupling experiments in solutions of 2-nitro-4-chlorophenol were made showing similar existence of the cis conformation.

Four-bond interproton spin-spin coupling in pyridines

Abstract The INDOR technique has been applied to determine the sign of the coupling constant between protons in the ring position a in 4-aminopyridine and 4-chloropyridine which, contrary to the usual assumptions, were found to be positive, a result in agreement with the trend in other heterocyclic compounds.

The sign of the four-bond FH coupling in methoxyflurane

Abstract The methoxyflurane NMR spectrum was reexamined looking for a through-space transmitted four-bond FH coupling. This coupling was measured and its sign determined relative to the corresponding vicinal FH coupling. This sign determination required a triple irradiation technique, which is also described. A K arplus -like equation for the vicinal FH couplings yields the relative populations of both preferential conformers, and determines the positive sign of both three- and four-bond FH couplings.

INDOR spectra in AXn systems

Abstract A model previously proposed by the same authors for the INDOR spectra of twos-pin- 1 2 systems is generalized for the AXn systems, which present the complications arising from the degeneracy of some of the lines. The basic idea of the model is the superposition of the effects of tickling and the redistribution of populations. Explicit calculations were made for the AX3 case, resulting a reasonable agreement with experimentally observed spectra in methyl formate.