why is the chemical shift of RCH3 lower than RCH2R in a carbon-13 NMR, is it because the carbon in RCH3 is attached to more hydrogens (and less carbons) and carbon is more electronegative than hydrogen so the electrons are held closer to carbon, causing a shielding effect from the magnetic field. Whereas in RCH2R there is less of a shielding effect as it's attached to more carbons and less hydrogens. Is this right?