Absolute line intensities for formic acid and dissociation constant of the dimer

Abstract

Absolute line intensities in the nu(6) and nu(8) interacting bands of trans-HCOOH, observed near 1105.4 and 1033.5 cm(-1), respectively, and the dissociation constant of the formic acid dimer (HCOOH)(2) have been measured using Fourier transform spectroscopy at a resolution of 0.002 cm(-1). Eleven spectra of formic acid, at 296.0(5) K and pressures ranging from 14.28(25) to 314.0(24) Pa, have been recorded between 600 and 1900 cm(-1) with an absorption path length of 19.7(2) cm. 437 integrated absorption coefficients have been measured for 72 lines in the nu(6) band. Analysis of the pressure dependence yielded the dissociation constant of the formic acid dimer, K(p)=361(45) Pa, and the absolute intensity of the 72 lines of HCOOH. The accuracy of these results was carefully estimated. The absolute intensities of four lines of the weak nu(8) band were also measured. Using an appropriate theory, the integrated intensity of the nu(6) and nu(8) bands was determined to be 3.47 x 10(-17) and 4.68 x 10(-19) cm(-1)(molecule cm(-2)) respectively, at 296 K. Both the dissociation constant and integrated intensities were compared to earlier measurements.

The HITRAN2012 molecular spectroscopic database

“…[267]. Recently, Vander Auwera et al [262] reported absolute line intensities measurements for the ν 6 and ν 8 bands using…”

“…This edition of HITRAN constitutes a major update of the information provided for formic acid: the 9-μm region has been completely replaced [262,263], and there is the first inclusion of the 5.6-μm region [264]. These regions correspond to the strong ν 6 and ν 3 bands, respectively.…”

Communication: The highest frequency hydrogen bond vibration and an experimental value for the dissociation energy of formic acid dimer

“…These equilibrium constants fall in two broad categories -vapor density measurements 27 and spectroscopic monomer determinations. 26 The average value for K p p • from the vapor density results (including photoacoustic resonance, 28 but excluding the data which need major temperature extrapolation) is about 265(20) Pa, whereas spectroscopy yields significantly higher values above 350 Pa. Combining the two types of data set with a double weight for the single reliable spectroscopic source, one arrives at 300(50) Pa. Having narrowed down the partition function uncertainty in the present work to less than 10%, this K p discrepancy is now determining the accuracy of the resulting dissociation energy of formic acid dimer.…”

Vapor pressures of substituted polycarboxylic acids are much lower than previously reported

“…Thus, the mixing ratio of organic vapor (partial pressure of order 1 × 10 −3 Pa or less) in an atmosphere of ∼ 80 × 10 3 Pa continuously flowing organic-free N 2 would be of order 1 × 10 −8 or lower. Assuming that di-and mono-carboxylic acids have similar dimerization properties (the dissociation constants for monocarboxylic acids in the gas phase are large, e.g., formic acid, 3.6 × 10 2 Pa at 296 K, Vander Auwera et al, 2007), that the effect of chain length on enthalpy of dimerization is minimal (cf., Eq. 7 of Tsonopoulos and Prausnitz, 1970) and solving the quadratic equation as described in, e.g., Zuend et al (2011), the fraction of molecules dimerized would be ∼ 1 per 1 × 10 8 for a sample with p •,L (T ) ∼ 1 × 10 −6 Pa.…”