Knowledge Bank

The relative intensities of the vibrational components of the $V\leftarrow N$ transition of ethylene have been measured and compared with the squares of the vibrational overlap integrals for torsional oscillation assuming both a harmonic and a sinusoidal barrier. The two barriers give rise to identical overlap integrals for low values of the upper state vibrational quantum number n, but diverge at higher n. The experimental intensities do not agree especially well with either set of calculated intensities for low values of n, but correspond exceedingly well at higher n with those calculated with harmonic oscillator wave-functions. From the n dependence of the intensities, the value of the upper state A rotational constant was determined. It is $5.68cm-1$ for $C2H4(ground=4.82cm-1)$ and $2.61cm-1$ for $C2D4$. It is, therefore, very improbable that the bonding around the carbons is distorted from trigonal towards tetrahedral.