Formaldehyde excited states

TABLE 12.4 Physical Properties of Formaldehyde Excited States ... 

In a molecule with electrons in n orbitals, such as formaldehyde, ethylene, buta-1,3-diene and benzene, if we are concerned only with the ground state, or excited states obtained by electron promotion within 7i-type MOs, an approximate MO method due to Hiickel may be useM. 

This difference is due to the two lone pairs on the oxygen. Of the six valence electrons on the oxygen atom, two are involved in the double bond with the carbon, and the other four exist as two lone pairs. In Chapter 4, we ll examine the IR spectra for these two molecules. The orbitals suggest that we ll find very different frequencies for the two systems. In Chapter 9, we ll look at the transition to the first excited state in formaldehyde. ... 

Here are the predicted frequencies for the first excited state of formaldehyde, along with the corresponding experimental values (the scale factor is the same as for Hartree-Fock frequencies 0.8929) ... 

Unlike formaldehyde, acrolein retains its planar structure in the first excited state. Moving to the first excited state principally affects the C-C-H bond angle, decreasing it almost 5°, and the C-C-C bond angle, increasing it about the same amount. The C-O bond also stretches slightly. 

Ethylene Excited States Methyienecyclopropene Spectra Formaldehyde Experimental Data... 

Casida, M. E., Casida, K. C., Salahub, D. R., 1998, Excited-State Potential Energy Curves from Time-Dependent Density-Functional Theory A Cross Section of Formaldehyde s A Manifold , hit. J. Quant. Chem., 70, 933. 

Lischka H, Dallos M, Shepard R (2002) Analytic MRCI gradient for excited states formalism and application to the mr valence- and n — (3s, ip) rydberg states of formaldehyde. Mol Phys 100 1647... 

More subtle change can occur to a molecule s structure following photo-excitation. For example, the bond angle in methanal (formaldehyde) increases, so the molecule is flat in the ground state but bent by 30° in the first excited state see Figure 9.14. 

To illustrate these energy levels, Figure 2.1 shows formaldehyde as an example, with all the possible transitions. The n —> n transition deserves further attention upon excitation, an electron is removed from the oxygen atom and goes into the n orbital localized half on the carbon atom and half on the oxygen atom. The n-n excited state thus has a charge transfer character, as shown by an increase in the dipole moment of about 2 D with respect to the ground state dipole moment of C=0 (3 D). 

Problem 10-7. Formaldehyde, CH2O, has C2v symmetry. The ground state has symmetry species Ai, and low-lying excited states have symmetry species A2, B2, and Ai. To which excited states are transihons allowed What are the direchons of polarizahon of the allowed transihons ... 

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