Formaldehyde absorption spectrum

However, as discussed in Chapter 4, the absorption spectrum of higher aldehydes cuts off at shorter wavelengths than formaldehyde. This, combined with higher quantum yields for radical production in the 290- to 340-nm range and the fact that HCHO produces 2H02 essentially immediately upon dissociation, makes the photolysis of aldehydes larger than formaldehyde less important at equal concentrations of the aldehydes. 

The ultraviolet absorption spectrum of formaldehyde consists of many sharp discrete bands of Doppler width. The isotopic shifts due to C and O atoms are sometimes 5 to 10 cm-1 in the 3000 to 3100 A region [see Moore (715)]. Hence, it is possible to selectively excite a specific carbon or oxygen isotopic species in mixtures of other isotopic species. 

Spectroscopy (39) and photochemistry (40) of formaldehyde have been extensively studied. The rotational analysis of the absorption spectrum of H2CO in the 250 to 360 nm region indicates that the transition is A(l-A2) X( Ap), which is... 

Since the suggestion of the sequential QM/MM hybrid method, Canuto, Coutinho and co-authors have applied this method with success in the study of several systems and properties shift of the electronic absorption spectrum of benzene [42], pyrimidine [51] and (3-carotene [47] in several solvents shift of the ortho-betaine in water [52] shift of the electronic absorption and emission spectrum of formaldehyde in water [53] and acetone in water [54] hydrogen interaction energy of pyridine [46] and guanine-cytosine in water [55] differential solvation of phenol and phenoxy radical in different solvents [56,57] hydrated electron [58] dipole polarizability of F in water [59] tautomeric equilibrium of 2-mercaptopyridine in water [60] NMR chemical shifts in liquid water [61] electron affinity and ionization potential of liquid water [62] and liquid ammonia [35] dipole polarizability of atomic liquids [63] etc. 

About half of formaldehyde reacts through each of these processes under atmospheric conditions. The absorption spectrum of formaldehyde shows extensive vibrational structure as shown in Figure 7.14, which also shows the photolysis quantum yields into the radical and molecular channels [136]. 

As previously noted, formaldehyde does not present a significant absorption spectrum (without derivatives, see Section 4.1). On the contrary, acetaldehyde, butyraldehyde and benzaldehyde show absorption maxima of different intensities according to absorptivities (Fig. 30). For benzaldehyde, the peak position is close to the one of aromatic rings. 

Quantum yields of formation have been measured for formaldehyde (2 x 10-2), methanol (1.9 x 10-3) and methyl formate (8 x 10-3) [82]. A modification of the absorption spectrum of polymethylacrylate during irradiation has also been observed (Fig. 14). [82]. The change is similar to that observed with polymethylmethacrylate in the same experimental conditions (Fig. 13) and is probably to be ascribed to the same origin. The photodegradation of polymethylacrylate can be visualized as... 

Fig. 2-17. Absorption spectrum and quantum yields for the two photodecomposition channels of formaldehyde in air at 1000 mbar pressure assembled from data of Clark et al. (1978), Horowitz and Calvert (1978), Tang et al. (1979), and Moortgat et al. (1979, 1983).

An example of a single-absorption spectrum illustrating many of the effects discussed in this section is the spectrum of formaldehyde, H2CO, shown in figure B 1.1.2 [20]. This shows the region of the lowest singlet-... 

The Si <- So band system of formaldehyde (CH2O) has been thoroughly studied, and it has been established that the vibrationless Si and So states have A2 and Ai symmetry, respectively, in C2V The Si So absorption spectrum and descriptions of normal vibrations in the Si and So states are shown in Figure P7.1 and in the list below. 

Degeneracies or near-degeneracies of more than two electronic states may play a role in two distinctly different ways. They can (1) affect the dynamics sequentially, i.e. as series of two-state (quasi-)degeneracies, which the system encounters separately during its time evolution and which both play a comparable role. An example of this type has been reported to occur in highly excited states of formaldehyde, severely affecting its VUV absorption spectrum [44]. (2) More than two electronic... 

On-the-Fly Calculation of So Sj Spectrum of Formaldehyde Very recently Tatchen and Poliak computed the absorption spectrum of formal-... 

Figure 10.4 Absorption spectrum of formaldehyde computed according to a senticlassical approach by on-the-fly TD-DFT computations and its comparison with experiments. The spectrum shows remarkable Herzherg-Teller effects. (From J. Tatchen, E. Pollack, J. Chem. Phys. 2009 130, 041103. Copyright 2009. Reprinted with permission of the American Institute of Physics.)...

The identification of the n - ji system in the short wave length absorption spectrum is one of the unanswered questions in the spectroscopy of formaldehyde. The most detailed calculations place the transition at 87270 cm (10.82 eV). Because the Aj state is calculated to lie above the first ionization potential, it would be autoionized by its ionization continuum. Theory also predicts that the molecule is unboimd along the CO coordinate in the upper state. 

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