Analysis of the proton nuclear magnetic resonance spectrum of formamide by the double resonance technique

168

T h e high-resolution proton resonance spectrum of formamide has been measured and a~ialyzed. Q u a d r~~p o l e broadening of the resonance signals due t o the N" ru~cleus was eliminated b y s~~b s t i t u t i o n of the isotopic n~~c l e u s NL5. Twenty-four lines were resolved in the spectrum, which is consistent with restricted rotation about the C-N bond. T h e non-equivalence of the two protons in the N1W2 group is characterized b y a s~naller screening constant and a greater spill-spin iriteraction of the proton which is in the trans position relative t o the carbo~iyl group. I t is suggested this difference in the spin coupling of tlie two protons may be associated with different S-H b o~i d lengths. Pronounced ch:i~iges in the chemical shifts and spin-coupling constants result when formamide is dissolved in dilute solution in water and acetone. In water solution the relative magnitudes of the spin coupling of the two protor~s in the N H ? group is revcrsed frorn t h a t observed in pure forrna~nide and the coupling of the aldehyde proton t o N15 varies by nearly 50%. Heating of formamide above room t e~i i p e r a t~~r e causes rotation of the N H I group as well as proton exchange between neighboring molecules. T h e two effects could be separated. From t h e observed changcs in tlie proton spectra as a function of temperature the barrier hindering rotation (~ileasured i l l a 10 mole yo solution of formamide in acetone) was found t o be 18f 3 lical/lnole. T h e activatio~i energy for proton exchange in pure liquid formaniide was found to be 1 0 f 3 l~cal/mo!e. I S T R O D U C T I O NFormamide is generally regarded as an approximately planar molecule. In the absence of free rotation about the C-N bond, the proton resonance spectrum may be expected to be of the type ABX (1) containing a minimum of 12 lines. Assuming the two protons on the nitrogen atom are only slightly non-equivalent the three protons may be labelled as follows:The observed spectrum of ordinary formamide contains only a broadish doublet partially overlapped by a very broad signal arising from the NH:! group. The incomplete resolution and the broad nature of the lines derive froin the quadrupole coupling of the N14 nucleus. This difficulty has been partially overcome by Piette, Ray, and Ogg (2) by use of the double-irradiation technique (3) to decouple the N u nucleus. The resulting spectrum showed six lines of which four could be assigned to the "aldehyde" hydrogen and yielded the coupling constants corresponding to the spin interaction with the two protons in the amide group. The spectrum of the latter could not be completely resolved presumably because of incomplete saturation of the N1.' nucleus. In the present work an alternative approach has been followed. Quadrupole broadening by N14 was eliminated by substitution of the nitrogen isotope N15. Since this has spin I = +, sharp-line proton signals result, permittiilg a complete resolution and analysis of the spectruln as well as a study of proton exchange behavior and intern...

Section: Resultssupporting

confidence: 88%

Proton Magnetic Resonance Measurements of Formamide

Sunners¹,

Piette²,

Schneider³

1960

168

“…However, the q~~a d r u p o l e coupling has no effect upon the coupling of the NH hydrogen to the ring protons. This is a similar situation to that found in formamide (9).…”

Section: ( B ) Pyrrolessupporting

confidence: 88%

The Proton Resonance Spectra of Furan and Pyrrole

Abraham¹,

Bernstein²

1959

Interpretation of the fine structure of the NMR spectra of some five-membered "aron~atic" heterocyclic ring con~pounds indicates that spin coupling constants between adjacent protons on the ring and those across the ring are approximately equal. This is in contrast to the situation in six-membered rings where coupling constants between protons ortho to one another arc much larger than those in the mcta position.Ql~adrupole effects of the N" nucleus in pyrrole broaden the N I i signal but do not interfere with the spin coupling of the N I i proton with other protons. The surprising result is obtained that the spin couplings of the NH proton with cy and 6 protons in the ring arc very nearly equal in magnitude.T h e analysis and interpretation of the high resolution proton magnetic resonance spectra of non-benzenoid aromatics have been largely neglected. Corey et al. (I) have shown that the magnetic resonance spectra of natural compounds containing furan rings are of considerable help in determining the structure of these compounds, a s the position of substitution in the furan ring can be found by inspection of the chemical shift data. ,4nderson (2) has measured the coupling constants and chemical shifts for 2-bromo-5-chlorothiophen, b u t no systematic high resolution studies have so far been reported for 5-membered ring compounds.I t is the purpose of this work t o present the complete analysis of the spectra of the heterocyclic molecules furan and pyrrole and also to investigate the effect of substitution 011 the magnitude of the spin-spin coupling constants in these molecules. W e shall show t h a t the coupling constants obtainecl by the analysis of these spectra are very different from those found for benzene systems.For the analysis of these complex spectra we have employed two simplifying techniques. Firstly, t h a t of substitiitio~~ to reduce the numbel-of interacting protons: deuterium substitution is preferable as the ~nagnetic constants remain unchanged, but methyl substitutioil can also be used in many cases since it only changes the coupling constants by a small amount. Secondly, by observing the specti-uin of the molecule dissolved in a suitable solvent: very often one group of protons ]nay be more affected by metlium effects than the others and this nlay iricrease the chemical shift difference between the groups and siinplify the fine structure considerably. In particular, the intensity clistribution becomes inore anienab!e to simple analytical treatment. This process clces not appear to affect the size of the spin couplilig constants appreciably, only the chemical shifts. Also, these solution spectra help in an ~~nclerstanding ol the n a t~: r e of the molecular colllplexes formed in solution and some preliminary investigations of these effects have been made here.T h e spectra were lneasured with a Vnrian V-4300 high resolution spectrometer, incorporating a field stabilizer and operating a t 40 l l c / s . A spinning sample tube of 5 inin outside diameter was used. T h e standard side-b...

“…and their cis orientation in dF(trans) (major) (16,20,22). No crossover of the major and minor H , bands occurs in D,O, so the isomer assignment must be the same as in DMSO-d,.…”

Section: Resultsmentioning

confidence: 99%

A ¹H and ¹³C nmr study of the radiation-induced degradation products of 2′-deoxythymidine derivatives: N-(2′-deoxy-β-D-erythropentofuranosyl) formamide

Cadet¹,

Nardin²,

Voituriez³

et al. 1981

N-(2′-Deoxy-β-D-erythropentofuranosyl) formamide is an ionizing radiation-induced degradation product of 2′-deoxythymidine. This work describes a facile chemical synthesis of this molecule, and discusses the results of an 1H and 13C nmr study. The rotational barrier to cis–trans isomerization about the amide linkage is calculated, and details of the molecular conformation of the cis and trans isomers in aqueous solution are presented.