Microsolvation of the Formanilide Cation (FA⁺) in a Nonpolar Solvent: Infrared Spectra of FA⁺–L_n Clusters (L = Ar, N₂; n ≤ 8)

Abstract

Infrared photodissociation (IRPD) spectra of cationic formanilide (N-phenylformamide) clusters, FA(+)-Ln, with L = Ar (n = 1-8) and N2 (n = 1-6), are recorded in the hydride stretch (amide A, νNH, νCH) and fingerprint (amide I-III) ranges to probe the preferred interaction motifs and the cluster growth. Cold FA(+)-Ln clusters are generated by electron ionization in a supersonic expansion, which generates predominantly the most stable cluster isomers. Size- and isomer-specific νNH frequencies unravel the microsolvation process of FA(+) in a nonpolar (L = Ar) and a quadrupolar (L = N2) solvent. The H-bound FA(+)-L dimer with L binding to the NH proton of the amide group is the most stable isomer, and further ligands are attached to the aromatic ring (π-stacking). Ionization changes the preferred binding motif from π-stacking to H-bonding in FA((+))-L. Quantum chemical calculations at the ωB97X-D/aug-cc-pVTZ level confirm the experimentally derived sequential cluster growth and the vibrational and isomer assignments. The calculated FA(+)-L binding energies of D0(H) = 594/1054 cm(-1) for H-bound and D0(π) = 459/604 cm(-1) for π-bound Ar/N2 ligands are consistent with the observed photofragmentation branching ratios. Ionization of FA results from removal of a bonding π-electron delocalized over the phenyl and amide moieties and thus weakens the N-H bond and strengthens the C-O bond.

Stepwise Microhydration of Aromatic Amide Cations: Formation of Water Solvation Network Revealed by Infrared Spectra of Formanilide+–(H2O)n Clusters (n ≤ 5)

“…t-FA + is calculated to be 468 cm −1 more stable than c-FA + , with high barriers of V b = 3222 and 2696 cm −1 for trans→cis and cis→trans isomerization, respectively. 72 As a result, both isomers might be trapped in their respective potential wells in the supersonic plasma expansion. However, the recent IRPD study of FA + −L n with L = Ar and N 2 conducted in the same setup does not reveal any clear evidence for measurable populations of FA + −L n clusters with the cis isomer.…”

“…Thus, the NH group in c-FA + is a better proton donor and forms stronger H-bonds to solvent molecules. 72 The IRPD spectrum of FA + −Ar in Figure 2 is dominated by the ν NH fundamentals (E) of the H-bound and π-bound isomers of t-FA + −Ar at 3365 and 3380 cm −1 , respectively. 72 The latter value is a very good approximation to the ν NH frequency of bare t-FA + .…”

“…72 The IRPD spectrum of FA + −Ar in Figure 2 is dominated by the ν NH fundamentals (E) of the H-bound and π-bound isomers of t-FA + −Ar at 3365 and 3380 cm −1 , respectively. 72 The latter value is a very good approximation to the ν NH frequency of bare t-FA + . Finally, the calculated O−H bond parameters of H 2 O in its 1 A 1 ground state are r OH = 0.9574 Å, ν s OH = 3656 cm −1 , and ν a OH = 3757 cm −1 , which compare favorably with the experimental data (0.9578 Å, 3657 and 3756 cm −1 ), 90,91 indicating that the chosen ωB97X-D/aug-cc-pVTZ level accurately describes the geometric, vibrational, and electronic structure of H 2 O and both neutral and cationic FA (+) .…”

“…The Ar ligand is assumed to occupy a π-bound position in t-FA + −H 2 O(NH)−Ar(π), as π-bonding of Ar to the aromatic ring is by far the most stable binding site (after the NH site, which is however already occupied by the H 2 O ligand; 72 Ar binding to the neutral H 2 O ligand is much less favorable 83 ). The Ar binds to the C1 atom at a distance of 3.54 Å with a dissociation energy of D 0 = 418 cm −1 .…”

Diastereo-specific conformational properties of neutral, protonated and radical cation forms of (1R,2S)-cis- and (1R,2R)-trans-amino-indanol by gas phase spectroscopy

“…Recent applications of this IRPD approach of tagged ions in the Berlin laboratory include hydrocarbon ions, [48][49][50][51][52][53][54][55][56][57] silicon-containing ions, [58][59][60] and biological molecules and their hydrates. [61][62][63][64][65][66] 2.3.2. Infrared multiple photon dissociation (IRMPD).…”