Final answer:
Conjugation in organic molecules leads to an extended pi system, reducing the energy required for π - π* transitions. This results in a shift to a lower wavenumber in IR spectroscopy, as lower energy translates to longer wavelengths of absorbed light.
Step-by-step explanation:
In the context of molecular vibrations and infrared (IR) spectroscopy, the question of whether conjugation causes a shift to a lower or higher wavenumber is addressed. Conjugation refers to a system in organic chemistry where pi bonds are alternating with sigma bonds along a backbone of contiguous p-orbitals. This affects the energy levels of molecular orbitals, particularly the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).
Conjugation leads to an extended pi system, which lowers the energy gap between the HOMO and LUMO. As a result, the energy required for π - π* transitions (from the HOMO to the LUMO) is reduced, corresponding to a longer wavelength of light absorbed. In IR spectroscopy, a reduced energy gap translates to a shift to a lower wavenumber, as wavenumber is inversely proportional to the wavelength of light absorbed. Consequently, absorption bands associated with conjugated systems appear at lower wavenumbers compared to similar non-conjugated systems.