Normal Coordinate Analysis and Intensity Calculations for 1A′ and 1A″ Electronic States of Propynal

A normal coordinate analysis using a quadratic force field is carried out for the ground and first‐excited singlet electronic states of the molecules $\mathrm{HC}\equiv \mathrm{C}·\mathrm{CHO}\mathrm{,\hspace{0.17em}}\mathrm{DC}\equiv \mathrm{C}·\mathrm{CHO}\mathrm{,\hspace{0.17em}}\mathrm{HC}\equiv \mathrm{C}·\mathrm{DCO}$ . Three force constant matrix elements are transferred from ground to excited states. The normal coordinates enable the intensities of a number of transitions to be calculated and favorably compared to those observed. The agreement between calculated and observed frequencies is excellent for all isotopes. Semiempirical SCF MO theory (CNDO/2) is used to correlate the changes of geometry and electronic structure on excitation as well as to account for the remaining observed electronic bands of propynal.