NORMAL VIBRATIONS OF FORMIC ACID AND METHYL FORMATE

Normal coordinate calculations have been carried out for methyl formate and formic acid on the basis of three different simplified harmonic force fields. The infrared spectra of DCOOCD3 were obtained and the fundamental modes were assigned. The isotopic species HCOOH, DCOOH, HCOOD, DCOOD, HCOOCH3, DCOOCH3, HCOOCD3 and DCOOCD3 were included in least squares refinements of force constants. Force constants associated with the H(CO)O grouping were assumed to be transferable from the acid to the ester. Calculations were carried out with a Urey-Bradley force field, a valence-type force field derived solely from observed frequencies, and a valence-type force field with carbon-oxygen stretching constants evaluated from bond length data. The average error for 90 observed frequencies was 8 cm-1 (~0.7%) for the Urey-Bradley force field and below 6 cm-1 (~0.6%) for the valence-type force fields. The calculations revealed that the use of deuterated species does not provide sufficient data to obtain values for important skeletal interaction constants. Utilization of bond length data led to a reasonably complete force field, to acceptable values of interaction constants and to a good agreement between observed and calculated frequencies. Application of the Urey-Bradley approximation resulted in CO and C-O stretching constants with apparently too low values. The calculated potential energy distribution as well as the Cartesian displacements for several modes are sensitive to the nature of the approximate force field, despite the good frequency agreement obtained with all three employed approximations. The results suggest that consideration must be given to the nature of an approximate force field before the calculated form of a group vibration is accepted as being physically meaningful. © 1969.