Formaldehyde Frequencies

Here is the route section from the input file  

The values of the variables were taken from an optimization job on formaldehyde. 

A frequency job begins by computing the energy of the input structure. It then goes on to compute the frequencies at that structure. Gaussian predicts the frequencies, intensities, and Raman depolarization ratios and scattering activities for each spectral line  

This display gives predicted values for the first four spectral lines for formaldehyde. The strongest line (IR) is line 4 at 2028.1 cm  

Raw frequency values computed at the Hartree-Fock level contain known systematic errors due to the neglect of electron correlation, resulting in overestimates of about 10%-12%. Therefore, it is usual to scale frequencies predicted at the Hartree-Fock level by an empirical factor of 0.8929. Use of this factor has been demonstrated to produce very good agreement with experiment for a wide range of systems. Our values must be expected to deviate even a bit more from experiment because of our choice of a medium-sized basis set (by around 15% in all). 

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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. ... 

All frequency calculations include thermochemical analysis of the system. By default, this analysis is carried out at 298.15 K and 1 atmosphere of pressure, using the principal isotope for each element type. Here is the start of the ermochemistry output for formaldehyde ... 

This exercise will investigate various carbonyl compounds. Examine the frequencies for the systems pictured below and determine the frequencies associated with carbonyl stretch in each case. In addition, locate the characteristic peak produced b) the single hydrogen attached to the carbonyl for the applicable systems. (We looked a this mode in formaldehyde in Example 4.1.)... 

The largest shifts in frequency occur for methyl acetate, where the hydrogens in formaldehyde are replaced by a methyl group and a methoxy group, and most strikingly in acetyl chloride, where the hydrogen atom is replaced by a chlorine atom and a methyl group. 

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) ... 

Next, we will consider the vibrational frequencies of formaldehyde in acetonitrile, using the Onsager SCRF model and the SCIPCM model. Acetonitrile is a highly polar solvent, with an e value of 35.9. In order to predict the vibrational frequencies, we ll first need to optimize the structure for formaldehyde in this medium. Thus, we ll be running these jobs ... 

Sclutlcn The geometry optimization reveals that the structure of formaldehyde in cyclohexane is essentially the same as it is in acetonitrile. Here are the predicted frequency shifts with respect to the gas phase for the two media ... 

Solution A geometry optimiiation and frequency calculation (both in solution) are needed for each system (we ran the formaldehyde calculations earlier in this chapter). Here are the resulting scaled frequencies associated with carbonyl stretch for each system, along with the corresponding experimental values ... 

Ionization Potentials and Vibration Frequencies of Formaldehyde and its Radical Cation [after Turner (103)]... 

Beckett and Hua (2000) investigated the sonolytic decomposition of 1,4-dioxane in aqueous solution at 25 °C at discrete ultrasonic frequencies. They found that the highest first-order decomposition rate occurred at 358 kHz followed by 618, 1,071, and 205 kHz. At 358 kHz, 96% of the initial 1,4-dioxane concentration was decomposed after 2 h and the pH of the solution decreased to 3.75 from 7.50. Major decomposition intermediates were ethylene glycol diformate, methoxyacetic acid, formaldehyde, glycolic acid, and formic acid. 

Figure 5. IR absorbance spectra of 12C- and 13C-labeled formaldehyde in the frequency regions of 1600-1850 and 2600-3050cm-1.

In nonpolar solvents such as CH2C12 the bridging carbonyl (X) is formed preferentially. It has a remarkably low carbonyl stretching frequency of 1638 cm-1 and an X-ray structure determination of the corresponding bis(diphenylarsino)methane complex shows that there is no Pt-Pt bond (3,9,10). The complex therefore is considered as a dimetal-lated formaldehyde derivative. 

From the theoretical point of view the work of Leszczynski and collaborators159, should be mentioned, where the calculated harmonic vibrational frequencies of thioformaldehyde were compared with those of formaldehyde and selenoformaldehyde. The largest discrepancy (120 cm-1) between calculated and experimental vibrational frequencies was found for the aforementioned CH2 in-plane deformation. 

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