The Trimethylamine–Formic Acid Complex: Microwave Characterization of a Prototype for Potential Precursors to Atmospheric Aerosol

Gou

et al. 2018

Angew Chem Int Ed

The rotational spectra of three C-deuterated isotopologues of the dimer of formic acid have been measured, thanks to the small dipole moment induced by asymmetric H! Ds ubstitution(s). Fort he DCOOH-HCOOH species,t he concerted double proton transfer of the two hydroxy hydrogen atoms takes place between two equivalent minima and generates at unneling splitting of 331.2(6) MHz. This splitting can be reproduced by a3Dmodel with abarrier of 2559 cm À1 (30.6 kJ mol À1 )aso btained from theoretical calculations.Carboxylic acids form considerably stable dimers,t hat are sufficiently abundant at experimental conditions in as upersonic jet expansion. An umber of molecular complexes involving carboxylic acids [1][2][3][4][5][6][7][8][9][10] have been investigated by rotational spectroscopy,t ou nderstand the nature of their non-covalent interactions and to have information on their internal dynamics and on their conformational equilibria. Much attention has been paid to the complexes of carboxylic acids,m ainly to their dimers [1] and to their adducts with water. [2] Plenty of data have been obtained on the hydrogenbonded dimers involving proton tunneling,t he Ubbelohde effect and conformational equilibria. [1] HCOOH (formic acid; FA)isthe prototype of the carboxylic acids family and for this reason it is involved in most of the investigations of carboxylic acids bonded to molecules containing other functional groups, such as its adducts with H 2 O, [2a] N(CH 3 ) 3 , [3] CO 2 , [4] thesimplest aldehyde CH 2 O, [5] thesimplest amide H(CO)NH 2 , [6] ethers, [7] ketones, [8] freons, [9] azines [10] and its reactivity with alcohols. [11] An interesting feature of the dimer of FA,( FA) 2 ,i st he concerted transfer of the two hydroxylic protons,amotion in ad ouble minimum potential. It can generate tunneling doubling observable in molecular spectra, and makes them useful to studies of the tunneling process.T he precise tunneling splittings (DE 01 )m easured by microwave (MW) spectroscopy for the acrylic acid dimer [1g] and for the formic acid-benzoic acid bi-molecule [1h] have been reproduced by model calculations with barriers at or close to values obtained ab initio.In Figure 1w es how the proton transfer motion in the formic acid dimer along as ymmetric double minimum potential energy profile.

mentioning

confidence: 99%

The Barrier to Proton Transfer in the Dimer of Formic Acid: A Pure Rotational Study

Gou

et al. 2018

Angew Chem Int Ed

“…Several molecular complexes involving carboxylic acids [3][4][5][6][7][8][9][10][11][12] have been investigated by rotational spectroscopy to understand the nature of their non-covalent interactions and to have information on their internal dynamics and on their conformational equilibria. Most attention has been paid to the complexes of carboxylic acids,mainly to their dimers [3] and to their adducts with water.…”

mentioning

confidence: 99%

“…[4] Plenty of data have been obtained on the dimers,c oncerning proton tunneling, [3a] the Ubbelohde effect, [3b] and conformational equilibria. [3c] HCOOH (FA) is the prototype of the carboxylic acids family and for this reason it is involved in most of the investigations of carboxylic acids with molecules containing other functional groups,s uch as its adducts with H 2 O, [4a] N(CH 3 ) 3 , [5] anhydrides, [6] thes implest aldehyde (CH 2 O), [7] the simplest amide (CH(CO)NH 2 ), [8] ethers (dimethylether), [9] ketones (cyclobutanone), [10] esters (isopropylformate), [11] and azines (pyridine). [12] However,n oM Ws tudies of complexes between carboxylic acids and alcohols are reported.…”

mentioning

confidence: 99%

The Borderline between Reactivity and Pre‐reactivity of Binary Mixtures of Gaseous Carboxylic Acids and Alcohols

Spada

et al. 2017

Angew Chem Int Ed

By mixing primary and secondary alcohols with carboxylic acids just before the supersonic expansion within pulsed Fourier transform microwave experiments, only the rotational spectrum of the ester has been observed. However, when formic acid was mixed with tertiary alcohols, adducts have been formed and their rotational spectra have been easily measured. Quantum mechanical calculations have been performed to interpret the experimental evidence. In the present study esterification takes place without catalyst.To attain a satisfactory conversion a catalyst is generally needed, and yet the employment of one of the reactants in excess is necessary. [2] We discovered a very simple method to obtain esters from a gas phase 1:1 mixture of carboxylic acids and primary and secondary alcohols without need of catalyst. The details are reported below.Several molecular complexes involving carboxylic acids [3][4][5][6][7][8][9][10][11][12] have been investigated by rotational spectroscopy, in order to understand the nature of their non-covalent interactions and to have information on their internal dynamics and on their conformational equilibria. Most attention has been paid to the complexes of carboxylic acids, mainly to their dimers [3] and to their adducts with water. However, no MW studies of complexes between carboxylic acids and alcohols are reported. For this reason, we thought to investigate the prototype of this kind of complex, that is formic acid-methyl alcohol. Unexpectedly, we were not able to assign its rotational spectrum. Such a failure could be due, among to other reasons, to the complications related to the low V 3 barrier underlying the barrier to internal rotation of the methyl group, as well to the inversion of the methyl group from above to below the formic acid plane. In order to understand what was going on, we decided to start the investigation of the adducts carboxylic acids-alcohols from the adduct formic acid with a series of primary, secondary and tertiary alcohols. In details, we made supersonic expansions, with ca. 1% of carboxylic acid and 1% of alcohol in He for the following combinations: HCOOH-CH 3 OH, HCOOH-C 2 H 5 OH, HCOOH-(CH 3 ) 2 CHOH and HCOOH-(CH 3 ) 3 COH, that is formic acid mixed with methyl alcohol and with primary, secondary and tertiary alcohols, respectively.In all cases but the last one, it was not possible to observe the spectra of the adducts, but strong rotational transitions that we discovered to belong to the esters. Then we replaced linear alcohols with cyclic alcohols, like cyclohexanol (secondary) and 1-methylcyclopropanol (tertiary). Again, for the secondary alcohol we could observe only the rotational spectrum of the ester, and for the tertiary alcohol only that of the adduct. Finally, we exploited the replacement of HCOOH with carboxylic acids with stronger (CF 3 COOH) and weaker (pivalic acid) acidity. In the first case we observed only the ester, while in the second case the experiment did not succeed, because pivalic acid was rapidly obstructing our nozzl...

“…Several molecular complexes involving carboxylic acids have been investigated by rotational spectroscopy to understand the nature of their non‐covalent interactions and to have information on their internal dynamics and on their conformational equilibria. Most attention has been paid to the complexes of carboxylic acids, mainly to their dimers and to their adducts with water .…”

Section: Methodsmentioning

confidence: 99%

“…HCOOH (FA) is the prototype of the carboxylic acids family and for this reason it is involved in most of the investigations of carboxylic acids with molecules containing other functional groups, such as its adducts with H 2 O, N(CH 3 ) 3 , anhydrides, the simplest aldehyde (CH 2 O), the simplest amide (CH(CO)NH 2 ), ethers (dimethylether), ketones (cyclobutanone), esters (isopropylformate), and azines (pyridine) …”

Section: Methodsmentioning

confidence: 99%

The Borderline between Reactivity and Pre‐reactivity of Binary Mixtures of Gaseous Carboxylic Acids and Alcohols

Spada

et al. 2017

Angewandte Chemie