Direct Investigation of the Validity of Vertical Approximation in the Calculation of Transition Moment Matrix Elements:  n → π Transition in Methyl Formate

The absolute optical oscillator strength for the n → π* transition in methyl formate is calculated, within a procedure that has recently been used by our group to determine the optical oscillator strength of symmetry-forbidden transitions and which consists of expanding the (squared) transition moment along the normal coordinates of vibration. In the present work, we extend its application to a symmetry-allowed transition and discuss the applicability of the vertical approximation. To validate the present results, the UV photoabsorption cross-section was measured, and the result is compared with the one previously reported.