Compounds containing the structure -C(=O)-H. The aldehydes are encountered as intermediates in the oxidation of hydrocarbons and other organic substances. Formaldehyde is the most important member of the group. It is present in the photochemical smog, and it also is of concern as an indoor air pollutant. The aldehyde hydrogen atom next to the carbonyl group has a lower vibration frequency than the usual C-H stretch. The bands that fall in the region 2850 to 2650 cm-1 are a good choice for aldehyde monitoring. Formaldehyde in the atmosphere is most easily measured by the absorption features near 2780 cm-1. See SPECTRA, chapter H

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