CNDO calculation of dipole moment derivatives and infrared intensities of formaldehyde

Abstract

The complete neglect of differential overlap (CNDO) approximate wavefunctions for formaldehyde have been applied to the calculation of the derivatives of the dipole moment with respect to the symmetry coordinates (∂p/∂S_j). Agreement between the calculated derivatives and the experimental values given by Hisatsune and Eggers for the A₁ symmetry species is exceptionally good, permitting an interpretation of the experimental results in terms of electronic distributions in the molecule. The agreement for the B₁ and B₂ symmetry species is also very good except for the asymmetric in‐plane CH bend. However, the theory successfully predicts that charge reorientation for this motion is quite different from that which occurs in the out‐of‐plane CH bend, so that these motions are represented by CH bond moments of opposite polarity. An analysis is given of the results in terms of derivatives with respect to internal coordinates (∂p/∂r_k). Comparison of ∂p/∂r_CO values from a series of X₂CO molecules (and also with ∂p/∂r_CS for X₂CS molecules) reveals an interesting trend. An analysis of the various contributions to the calculated ∂p/∂r_k values is presented, with an explanation of the calculated results for the out‐of‐plane bending coordinate ∂p/∂γ.