[rycharles]

When hydrogens overlap does this cause a downfield shift in the proton NMR spectra?

I am looking at the chemical shift for maleic anhydride.

http://www.chemicalbook.com/SpectrumEN_108-31-6_1HNMR.htm

The literature value of the two protons associated to the ester group would be 2.2 ppm.  But on this compound it is extremely shifted downfield to 7.04 ppm.

Is this downfield shift due to the overlap of the protons (causing one peak) and spin-spin coupling?  Anyone please elaborate?

[gritch]

When hydrogens overlap does this cause a downfield shift in the proton NMR spectra?

I am looking at the chemical shift for maleic anhydride.

http://www.chemicalbook.com/SpectrumEN_108-31-6_1HNMR.htm

The literature value of the two protons associated to the ester group would be 2.2 ppm.  But on this compound it is extremely shifted downfield to 7.04 ppm.

Is this downfield shift due to the overlap of the protons (causing one peak) and spin-spin coupling?  Anyone please elaborate?

The spectrum is perfectly normal. That compound is an α,β-unsaturated ester. They're quite different chemically from simple esters. A shift of around 7ppm is normal for such compounds.

http://www.chem.wisc.edu/areas/reich/nmr/05-hmr-02-delta.htm
Search "unsaturated" and it should go right to a relevant section.

As for the fact there is only one peak, we need to simply consider the symmetry of the molecule. The molecule has a plane of symmetry right down the middle and along the anhydride oxygen. Both protons would be in an identical chemical environment and so we would expect them to have the same chemical shift as well, leading to a single peak that should integrate to 2.