Abstract
The empirically derived assignment of the strongly interacting 51 and 92 vibrational states of trans-HCOOH has recently been reassigned on the basis of anharmonic frequency calculations, and this, in turn, affects the assignment of many higher energy states. Here, we investigate the high-resolution synchrotron-based torsional spectrum of trans-HCOOH, and find experimental confirmation that the proposed reassignment is indeed correct, i.e., that 92 is in fact lower in energy than 51. This is largely based on examining the intensity ratio of transitions with the same rotational quantum numbers between the 92-91 and 51-91 hot bands, which indicates that the 51 [92] state has ∼31% 92 [51] character. We also examined the torsional spectrum of trans-HCOOD, and find that the intensity ratios are consistent with 92 instead being higher in energy than 61 (which is analogous to 51 in trans-HCOOD), as previously determined from higher energy spectra.
Highlights
While both rotamers of gas phase formic acid have been investigated by infrared spectroscopy, it is the trans rotamer that has been the most thoroughly studied
Bonner and Hofstadter were the first to report the infrared spectrum of gas phase formic acid, where they noted an intense band around 658 cm-1 that they assigned to monomeric formic acid
This was assigned to the OCO bending vibration (ν7) several years later,15 and was reobserved with higher resolution at 636 cm-1 where it was reassigned to both OCO bending and OH torsion (ν9)16
Summary
The simplest carboxylic acid, is an important atmospheric species that is known to be responsible for a large fraction of the acidity of typical precipitation. Significant biogenic sources are from plants and soil, bacterial and human metabolism, and the venom of ants and bees. Its biological importance and simplicity makes it a sought after astrochemical species, and it has been detected in several interstellar molecular clouds (Sagittarius B23,4 and others5), and very recently in a protoplanetary disk. In those studies it was the trans rotamer (eclipsed OH and CH bonds7) that was detected, which lies 3.90±0.09 kcal/mol (1365±30 cm-1) lower in energy than the cis rotamer. Recently, the cis rotamer has been detected for the first time in interstellar space, in the far-UV illuminated edge of the Orion Bar.. A large number of additional vibrational bands were reassigned that involve the Fermi coupled 51 and 92 states of transHCOOH.26,27 These calculations, which use high quality ab initio data and very different vibrational methods, suggest that most assignments involving these bands in previous spectroscopic investigations should be switched (note that we use these switched labels throughout, unless stated otherwise). Motivated by this major reassignment, we decided to investigate the synchrotron-based infrared spectra of transHCOOH and HCOOD in an effort to see if their computational predictions hold true
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
