Formaldehyde: Comprehensive Spectral Investigation as a Function of Solvent and Temperature

Absorption, emission, and NMR studies indicate that: (1) Formaldehyde exists as a monomer (form A) in nonpolar solvents to a temperature of approximately −95°C, (2) at −95°C in nonpolar solvents, formation of a formaldehyde polymer (form C) begins which is thermally reversible at temperatures > −90°C. The form C also exists at room temperature in water, dimethyl sulfoxide, dimethylformamide, and tetrahydrofuran. The formaldehyde-formaldehyde interaction is principally of a dipole-dipole type. (3) Formaldehyde can act as a hydrogen-bonding donor or acceptor in a complex with solvent molecules or silica gel (form B). In aromatic hydrocarbons, formaldehyde acts as a donor whereas in 2-methyltetrahydrofuran, chloroform, and silica gel it acts as an acceptor. Also, in 2-methyltetrahydrofuran, forms A and B exist in addition to C. Monomer formaldehyde exhibits a fluorescence from the lowest Sn,π* state in nonpolar solvents to −95°C. Both the forms B and C exhibit emission. In addition to the classification of the forms of formaldehyde, it is possible to classify the solvents into four main groups based on the rate of formation of paraformaldehyde or the fact that formaldehyde essentially totally reacts (strong hydrogen-bond donors or molecules with active α-hydrogen atoms).