Choose the bond in each pair that you expect to vibrate at the higher wavenumber.
(a) C―N vs. C = N

Verified step by step guidance

1. Understand that the wavenumber in infrared spectroscopy is related to the frequency of vibration of a bond, which depends on the bond strength and the masses of the atoms involved.
2. Recall that stronger bonds typically vibrate at higher frequencies, and thus have higher wavenumbers.
3. Compare the bond strengths: a C=N (carbon-nitrogen double bond) is generally stronger than a C-N (carbon-nitrogen single bond).
4. Consider the bond order: a double bond (C=N) has a higher bond order than a single bond (C-N), indicating a stronger bond.
5. Conclude that the C=N bond is expected to vibrate at a higher wavenumber than the C-N bond due to its greater bond strength and higher bond order.

Key Concepts

Bond Vibrational Frequency

Bond vibrational frequency refers to the energy required for a bond to stretch or compress during vibration. It is influenced by the bond strength and the masses of the atoms involved. Stronger bonds and lighter atoms typically result in higher vibrational frequencies, which are measured in wavenumbers (cm⁻¹).

Single vs. Double Bonds

Single bonds, like C―N, are generally weaker than double bonds, such as C=N, because double bonds involve more shared electrons, increasing bond strength. This increased strength in double bonds leads to higher vibrational frequencies compared to single bonds, as more energy is required to alter the bond length.

Wavenumber

Wavenumber is a unit of frequency used in spectroscopy, defined as the number of wave cycles per unit distance (cm⁻¹). It is directly related to the energy of molecular vibrations; higher wavenumbers indicate higher energy vibrations. In infrared spectroscopy, wavenumbers are used to identify functional groups based on their characteristic vibrational frequencies.