Examined here are the structures of complexes of benzophenone microsolvated with up to three water molecules by using broadband rotational spectroscopya nd the cold conditions of amolecular jet. The analysis shows that the water molecules dock sideways on benzophenone for the water monomer and dimer moieties,and they move aboveone of the aromatic rings when the water cluster grows to the trimer.The rotational spectra shows that the water trimer moiety in the complex adopts an open-loop arrangement. Ab initio calculations face ad ilemma of identifying the global minimum between the open loop and the closed loop,which is only solved when zero-point vibrational energy correction is applied. An OH•••p bond and aB ürgi-Dunitz interaction between benzophenone and the water trimer are present in the cluster.T his work shows the subtle balance between water-water and water-solute interactions when the solute molecule offers several different anchor sites for water molecules.
S2 S32) Completion of Reference [17].
The investigation on the preferred arrangement and intermolecular interactions of gas phase solute−water clusters gives insights into the intermolecular potentials that govern the structure and dynamics of the aqueous solutions. Here, we report the investigation of hydrated coordination networks of benzaldehyde-(water) n (n = 1−6) clusters in a pulsed supersonic expansion using broadband rotational spectroscopy. Benzaldehyde (PhCHO) is the simplest aromatic aldehyde that involves both hydrophilic (CHO) and hydrophobic (phenyl ring) functional groups, which can mimic molecules of biological significance. For the n = 1−3 clusters, the water molecules are connected around the hydrophilic CHO moiety of benzaldehyde through a strong CO•••HO hydrogen bond and weak CH•••OH hydrogen bond(s). For the larger clusters, the spectra are consistent with the structures in which the water clusters are coordinated on the surface of PhCHO with both the hydrophilic CHO and hydrophobic phenyl ring groups being involved in the bonding interactions. The presence of benzaldehyde does not strongly interfere with the cyclic water tetramer and pentamer, which retain the same structure as in the pure water cluster. The book isomer instead of cage or prism isomers of the water hexamer is incorporated into the microsolvated cluster. The PhCHO molecule deviates from the planar structure upon sequential addition of water molecules. The PhCHO−(H 2 O) 1−6 clusters may serve as a simple model system in understanding the solute−water interactions of biologically relevant molecules in an aqueous environment.
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...
A gas‐phase nitrogen–nitrogen noncovalent interaction has been unveiled in the nitroethane–trimethylamine complex in an environment free from solvent and matrix effects using rotational spectroscopy in supersonic expansion. Different quantum chemical models (NOCV/CD and NBO) agree in indicating that this interaction largely prevails over the C−H⋅⋅⋅O and C−H⋅⋅⋅N hydrogen bonds. Furthermore, a SAPT analysis shows that electrostatic and dispersion interactions play a comparable role in stabilizing the complex. The conformational landscape exploration and stationary points characterization have been performed using state‐of‐the‐art quantum‐chemical computations providing significant insights on structure determination.
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