Dimers of the Higher-Energy Conformer of Formic Acid: Experimental Observation

Abstract

We report on the first experimental observation of formic acid dimers composed of two molecules of the higher-energy cis conformer. The cis−cis formic acid dimers are prepared in an argon matrix by selective vibrational excitation of the ground state trans conformer (deuterated form HCOOD) combined with thermal annealing of the matrix at about 30 K. Five cis−cis formic acid dimers are predicted by ab initio calculations (interaction energies from −16.9 to −27.2 kJ mol −1 ), and these structures are used for the assignment of the experimental spectra. Selective vibrational excitation of the obtained cis−cis dimers leads to the formation of several trans−cis dimers, which supports the proposed assignments.

Hydrogen-Tunneling in Biologically Relevant Small Molecules: The Rotamerizations of α-Ketocarboxylic Acids

“…This was necessary to ensure genuine results as it is known that IR radiation in the spectral range of the OH-stretching mode may increase t u n n e l i n g r a t e s d u e t o v i b r a t i o n a l e x c i t ation. 3,4,[6][7][8][9][10]12,14,16,30,32,33,42 Between measurements, the windows of the vacuum shroud were covered to shut out ambient light.…”

“…The quantum mechanical nature of the phenomenon is further underlined by a significant influence of isotopic substitution on the rate of conversion, which is by orders of magnitude slower, but not halted entirely, in the corresponding O-d-carboxylic acids. [7][8][9][10]13,14,33,34 Numerous examples of QMT in carboxylic acids have been found in recent years (Scheme 1), the most prominent being t h e i n t e r c o n v e r s i o n s o f c o n f o r m e r s o f f o r m i c…”

“…The quantum mechanical nature of the phenomenon is further underlined by a significant influence of isotopic substitution on the rate of conversion, which is by orders of magnitude slower, but not halted entirely, in the corresponding O-d-carboxylic acids. [7][8][9][10]13,14,33,34 Numerous examples of QMT in carboxylic acids have been found in recent years (Scheme 1), the most prominent being 4,6,10,[12][13][14][15][16]21,22,24,26,28,30,32 acetic (1b), 5,7,8,10,15,21,28 propionic (1c), 9,21,28,29 S(−)-2-chloropropionic (1d), 25 as well as propiolic acid (1e) 31 and the aromatic meta-chlorobenzoic acid (2) along with variously para-substituted benzoic acid derivatives (3a−f). 20,23 Remarkably, spectral proof for the existence of the monomeric (E)-isomer of 3 and several of its para-substituted derivatives was provided only in 2010.…”

Infrared spectra of formic acid clusters in noble gas matrices

“…In the case of formic acid, the cyclic dimer (denoted as FAD hereafter) has been studied with electron diffraction techniques [2][3][4], and spectroscopic methods [5][6][7] quite extensively. Other metastable dimeric species (open dimer) have also been observed in helium nanodroplets [8], and in noble gas matrices [5,[9][10][11] in combination with infrared photo-excitation techniques.…”

“…In the case of formic acid, the cyclic dimer (denoted as FAD hereafter) has been studied with electron diffraction techniques [2][3][4], and spectroscopic methods [5][6][7] quite extensively. Other metastable dimeric species (open dimer) have also been observed in helium nanodroplets [8], and in noble gas matrices [5,[9][10][11] in combination with infrared photo-excitation techniques. Experimental studies of larger clusters of formic acid (HCOOH) n (n > 2), on the other hand, have been limited in mass spectrometry [12][13][14], and no spectroscopic observation has been reported to our knowledge.…”