The first <vibrational overtone> is $v=0\to2$, with $\Delta v=2$. It lies near twice the fundamental wavenumber, generally slightly below it because of <anharmonicity>. The linear-dipole harmonic model forbids it; anharmonic wavefunctions or higher terms in the dipole expansion give a weak intensity. Thus it is generally weaker than the corresponding fundamental.

Solution

Past exam of the chemistry A course of the University of Cambridge / 2025 / Paper 1 / A3 / a / ii / Solution

 Solution (source code)