A Semi-empirical Analysis of the Electronic Spectrum and Molecular Structure of Formaldehyde

The electronic transition energies and ionization potentials of formaldehyde have been calculated by a semi-empirical molecular orbital method, using several different ways of evaluating the diagonal matrix elements, in an attempt to adapt the Pariser–Parr–Pople method to the calculation of σ electron properties. The use of theoretically computed penetration integrals in this semi-empirical calculation lead to incorrect relative separation of the π-, σ-, and nonbonding molecular orbital energies, and hence to failure in predicting n →π* and n → σ* transition energies. A method is developed which reproduces the spectrum of formaldehyde (which includes n–π*, π–π*, and n–σ* electronic transitions) satisfactorily. In paper II of this series, this method is applied to study the spectrum of HCN and CO2.