Question

The chemical shifts ( \(2.4 \mathrm{ppm}\) ) of acetylenic hydrogens \((\mathrm{RC} \equiv \mathrm{C}-\mathrm{H})\) are considerably more toward higher magnetic fields than those of alkene hydrogens (4.6 to \(6.9 \mathrm{ppm}\) ). Show how this shielding effect might be explained in terms of the atomic orbital representation of acetylenes.

Short answer

In acetylene molecules, the carbon-hydrogen bond is formed by the overlap of the hydrogen's s orbital with an sp hybrid orbital of the carbon atom, resulting in a strong bond and higher electron density around the hydrogen nucleus. This leads to a stronger shielding effect on the acetylenic hydrogen, causing a lower chemical shift. On the other hand, in alkene molecules, the carbon-hydrogen bond is formed by the overlap of the hydrogen's s orbital with an sp2 hybrid orbital, resulting in lower electron density around the hydrogen nucleus and a weaker shielding effect, leading to a higher chemical shift.

Step-by-step solution

1. Recall the atomic orbital representations of acetylenes and alkenes

In an acetylene molecule, the carbon-carbon triple bond comprises of one σ bond and two π bonds. The σ bond is formed by the hybridization of one s orbital with one p orbital from each carbon atom, forming two sp hybrid orbitals. The remaining two p orbitals of each carbon atom overlap with the p orbitals of the adjacent hydrogen and carbon atoms, forming two π bonds. The hydrogen atoms are bonded to the sp hybrid orbital of the carbon atom. In an alkene, the carbon-carbon double bond comprises one σ bond and one π bond. The σ bond is formed by the hybridization of one s orbital with two p orbitals of each carbon atom, forming three sp2 hybrid orbitals. The remaining p orbitals of each carbon atom overlap with the p orbitals of the adjacent sp2 hybridized carbon atoms, forming one π bond. The hydrogen atoms are bonded to the sp2 hybridized orbital of the carbon atom.

2. Understand the concept of chemical shifts and shielding

Chemical shifts are a measure of the resonance frequency of a nucleus in a magnetic field, relative to a reference frequency. This resonance frequency depends on the electronic environment of the nucleus, and the chemical shift is affected by the nearby electrons that shield the nucleus from the external magnetic field. Shielding is the phenomenon where electrons in atomic and molecular orbitals cause a decrease in the magnetic field experienced by the nucleus, resulting in a higher resonance frequency and a smaller chemical shift.

3. Explain the shielding effect of acetylenic hydrogens compared to alkene hydrogens

In acetylenes, the carbon-hydrogen bond is formed by the overlap of the hydrogen's s orbital with an sp hybrid orbital of the carbon atom, resulting in a σ bond. In this configuration, the carbon-hydrogen bond is relatively stronger and more tightly bound. The electron density around the acetylenic hydrogen is higher due to the hydrogen being closer to the sp hybrid orbital of the carbon atom. This results in a stronger shielding effect on the hydrogen nucleus, leading to a lower chemical shift. In alkenes, the carbon-hydrogen bond is formed by the overlap of the hydrogen's s orbital with an sp2 hybrid orbital of the carbon atom. The electron cloud around the alkene hydrogen is less dense than the electron cloud around the acetylenic hydrogen because of the less s character in the sp2 hybridized orbital compared to the sp hybridized orbital. This results in a weaker shielding effect on the alkene hydrogen nucleus, leading to a higher chemical shift. So the more significant shielding effect of acetylenic hydrogens compared to alkene hydrogens can be explained by the atomic orbital representation of acetylenes, where the electron density around the hydrogen nuclei is higher due to the overlap of the hydrogen's s orbital with the sp hybrid orbitals of the carbon atom.

Key concepts

Shielding Effect

The shielding effect is a foundational concept in Nuclear Magnetic Resonance (NMR) spectroscopy.

The fundamentals of the shielding effect rely on the presence of electrons around a nucleus, which serve as a buffer against the external magnetic field. During an NMR experiment, nuclei in a molecule are exposed to a strong magnetic field, and the behavior of these nuclei is observed. The electrons in the molecule generate their own local magnetic fields, which oppose the external field, thus 'shielding' the nucleus.

Impact of Shielding on Chemical Shifts

More electrons or denser electron clouds between the magnetic field and a nucleus increase the shielding effect. Consequently, the nucleus feels a reduced magnetic field and requires less energy to flip its spin state, resulting in an upfield (lower frequency) chemical shift on the NMR spectrum. Understanding this concept is vital in interpreting the NMR spectral data, particularly when comparing different types of hydrogen atoms in organic molecules.

Atomic Orbital Hybridization

Atomic orbital hybridization is a concept in chemistry that describes the mixing of atomic orbitals to form new hybrid orbitals.

These hybrid orbitals have different shapes and energies compared to the original atomic orbitals, and they play a crucial role in determining molecular geometry and bond properties. In NMR spectroscopy, hybridization affects the electronic environment of nuclei and influences the shielding effect.

Hybrid Orbitals in Different Bonds

For example, sp hybridized orbitals in acetylenes are involved in forming stronger, shorter bonds compared to sp2 hybridized orbitals in alkenes. The different hybridizations thus affect the electron cloud density around the attached hydrogen nuclei, changing their shielding and resulting in observable differences in NMR chemical shifts.

Acetylenic Hydrogen

In the context of NMR spectroscopy, acetylenic hydrogens refer to the hydrogen atoms bonded to sp hybridized carbon atoms in alkyne molecules.

The bonding orbital for acetylenic hydrogen is comprised of 50% s-character, which makes it more electron-dense and closer to the nucleus compared to bonds with less s-character. This increased electron density means greater shielding of the acetylenic hydrogen nuclei, leading to smaller chemical shifts (more upfield signals) on the NMR spectrum.

It's this relationship between hybridization and shielding that helps interpret why acetylenic hydrogens appear at around 2.4 ppm in their chemical shifts, displaying the impact of strong electron shielding in the vicinity of the hydrogen nuclei.

Alkene Hydrogen

Alkene hydrogens are the hydrogens attached to sp2 hybridized carbon atoms in alkene molecules.

These carbon atoms mix one s orbital with two p orbitals to form three sp2 hybridized orbitals — with roughly 33% s-character — which bond with hydrogen. Due to the reduced s-character in these orbitals, alkene hydrogens have less electron density around them compared to acetylenic hydrogens, resulting in decreased shielding hence a higher chemical shift in NMR, typically ranging from 4.6 to 6.9 ppm.

This lower level of shielding of alkene hydrogen nuclei makes them appear further downfield on an NMR spectrum, illustrating the nuanced differences in chemical environments and the precision of NMR in distinguishing molecular structures.