States of Formaldehyde

Excellent examples of two methods for computing the energies of excited states are provided by the calculations of Bernu et al. and of Lewerenz and 

CH4 C2H2 C2H4 2 6 methane acetylene ethylene ethane 

CjjHft C3H4 C3H4 C0H0 allene propyne benzene 

A comparison of results shows generally good agreement between the two sets of QMC calculations and between the QMC calculations and experimental measurements. A list of the calculated vibrational energies is given in Table 7, along with the corresponding experimental values. 

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Orbital promotions of this type give rise to states, such as the a and A states of formaldehyde, which are commonly referred to as nn states. In addition, transitions to such states, for example the a — X and A — X transitions of formaldehyde, are referred to colloquially as n — n or n-to-7i, transitions. 

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) ... 

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... 

In Scheme 7, the peroxidic 0-0 bond of the hydroperoxyl group is broken together with /1-scission of the formed alkoxyl radical, and, further, ring closure of alkyl peroxyl diradical may occur. The process generates a hydroxyl radical, methylcarbonyl terminal groups (-CH2-CO-CH3) and dioxetane. The latter is unstable and decomposes into an excited triplet state of formaldehyde and/or excited triplet state of methylcarbonyls (Scheme 8). 

Table 16.10. The leading Rumer tableaux in the wave function for the saddle point state of formaldehyde dissociation.

Results for CIS/6-31G andCIS/6-311+G calculations on the first n 7T ) excited state of formaldehyde are provided below. 

Figure 1.9 Geometry of tai the lowest tit.tt ) excited singlet state ol ethylene, ib) the lowest tJi.n ) excited singlet state of acetylene, and (c) the lowest in. n l state of formaldehyde...

The electronic configuration of the ground and excited states of formaldehyde has been discussed in Section II, E. The transitions... 

Another example of the idea of congruency of transition state and excited state geometry can be found in the chemiluminescent reaction of 1,2-dioxetanes (6). The excited state of formaldehyde is known to be bent, with the... 

Figure 2 4. The calculated excitation energies to some low-lying states of formaldehyde. The basis set, geometry, and other details are found in Refs. [60, 61, 67, 69]...

On this basis, a summary of the primary processes of the first excited singlet state of formaldehyde can be put forth for purposes of comparison. 

A few studies on the photophysics of rotationally resolved states of formaldehyde have been recently published. A. C. Luntz [J. Chem. Phys., j>9, 3436 (1978)] reported the lifetimes of the K -sublevels of the 4 SVL of I CO in a molecular beam. K. Shibuya and E. K. C. Lee [J. Chem. Phys., 69, 5558 (1978)] reported the fluorescence quantum yields from various rotational levels (J1, K ) of the 41 SVL of H2CO. J. C. Weisshaar and C. 3. Moore [J. Chem. Phys., 7J), 5135 (1979)] reported the collisionless decay rates of single rotational levels of the 4 ... 

Such oscillations can be observed by a faint blue luminescence which originates from an electronically excited state of formaldehyde formed in reactions (2) and (3), [118,119],... 

A problem related to obtaining MM potentials for reactions is to devise functions for the excited states of molecules. Blair et al. [74] and DeBolt et al. [75] have parameterized MM potential energy functions for the first excited states of formaldehyde and acetone respectively. The force fields in both cases were of the same form as for the ground states, except that the parameters, especially the partial atomic charges, were altered to reflect the changes that occur in the excited state. 

Blair, J.T., Levy, R.M. and Krogh-Jespersen, K. (1990) Molecular Mechanics Parameters for Electronically Excited States The (n,tr ) Singlet State of Formaldehyde, Chem. Phys. Letts. 166, 429—436. 

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