Infrared Spectra and Vibrational Assignment of Monomeric Formic Acid

The infrared spectra of monomeric HCOOH, HCOOD, DCOOH, and DCOOD have been studied in the gaseous phase with emphasis on the 800 to 450 cm—1 region. New absorption bands were found for HCOOD at 541 and 512 cm—1, and for DCOOD at 538 and 489 cm—1. These bands are attributed to the a′ OCO bending and to the a″ O–D torsion motions, respectively. In HCOOH and DCOOH the bands arising from the corresponding vibrations apparently coincide at approximately 630 cm—1. An assignment of fundamentals for the four isotopic species has been derived. Excellent agreement is obtained with the Teller-Redlich product rule. Also the calculated entropy agrees with the experimental value from the third law of thermodynamics.