Microsolvation of the Formanilide Cation (FA+) in a Nonpolar Solvent

Feb 19, 2014 - Infrared photodissociation (IRPD) spectra of cationic formanilide (N-phenylformamide) clusters, FA+–Ln, with L = Ar (n = 1–8) and N...
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Microsolvation of the Formanilide Cation (FA+) in a Nonpolar Solvent: Infrared Spectra of FA+−Ln Clusters (L = Ar, N2; n ≤ 8) Johanna Klyne, Matthias Schmies, and Otto Dopfer* Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstrasse 36, 10623 Berlin, Germany S Supporting Information *

ABSTRACT: Infrared photodissociation (IRPD) spectra of cationic formanilide (Nphenylformamide) 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.

1. INTRODUCTION Understanding chemical recognition of biologically active molecules and their interactions with solvent at the molecular level requires the knowledge of the underlying intermolecular interaction potentials. The peptide linkage is an essential component in the structure of proteins, and its geometry depends on its local environment, thereby affecting the polypeptide structure and its biochemical function. Although protein solvation has been studied extensively in solution, limited information is available about the microsolvation, in particular the sequential process of solvation of peptides and DNA. Fragmentation and charge delocalization of radical cations of peptides, DNA, and their building blocks are relevant for questions involving sequencing and charge transport of these biological macromolecules.1 To this end, in this work the stepwise microsolvation of the formanilide (FA, N-phenylformamide, C7H7NO, Figure 1) cation is characterized by vibrational spectroscopy of size-selected clusters generated in a molecular beam and density functional calculations. Formanilide is one of the simplest aromatic amides, with a -HN−COpeptide linkage between a phenyl group and a terminal hydrogen atom. Such aromatic amides exhibit at least two competing binding sites for nucleophilic ligands, namely, Hbonding to the acidic N−H group of the amide and π-stacking to the aromatic π-electron system of the phenyl ring (π-bond). The geometric, vibrational, and electronic structures of isolated FA have been characterized in its ground electronic state (S0) by MW,2−5 IR,6,7 and UV spectroscopy,6,8−11 as well as electron diffraction.12 Along with quantum chemical © 2014 American Chemical Society

calculations, these studies reveal two coexisting isomers of FA (Figure 1), namely, the more stable planar trans-FA rotamer (tFA, anti) and the less stable nonplanar cis-FA rotamer (c-FA, syn), for which the phenyl and amide planes are twisted by ∼35° and the two equivalent minima are separated by a barrier to planarity of 152 ± 2 cm−1.2,3,5,12 A rather high barrier between the cis and trans isomers of FA, calculated as Vb = 5700 cm−1,3 and measured as 7000 cm−1 in solution,13 arises from the partial double bond character of the amide HN−CO bond14 and prevents their facile interconversion under isolated conditions. Depending on the experimental conditions and the temperature, the range of the observed c-FA/t-FA population ratio changes drastically. While most studies in cold molecular beams do not detect the c-FA isomer at all,2,7 or only at the level of below 10%,6,8,9 studies at room or higher temperature or in solution reach the 50% level.3,12,15 At low temperature, tFA is favored thermodynamically, while entropy favors c-FA at higher temperature because of its higher flexibility and the two degenerate minima.3 These abundance ratios have been rationalized by experimental estimates of the energy difference of ΔD0trans−cis = 350 ± 150, 3 276 ± 84,5 and ∼875 cm−1,8 which are in the range of theoretical predictions at various levels of theory.3,8,9,12,16 The reported isomer-selective IR spectra of t-/c-FA cover both the hydride stretch (amide A, νNH, νCH) and fingerprint (amide I−III) ranges and clearly Received: February 3, 2014 Revised: February 19, 2014 Published: February 19, 2014 3005

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torsion.16 However, no information is available for the important amide I−III and N−H stretch modes (amide A), which are sensitive to secondary structures in proteins. The ZEKE spectra of t-FA−Ar(π) have been interpreted with a πbonded t-FA+−Ar(π) structure in the cation ground state.18 As a result of the additional charge interactions, the binding energy of the π-bond increases by 169 cm−1 upon ionization. However, all three intermolecular frequencies are rather low, consistent with charge delocalization into the amide group. In contrast to Ar, ZEKE and IR spectra of t-FA+−H2O show that H2O binds exclusively to the NH group of t-FA+.17,20,21 Ionization of tFA−H2O complexes with H2O binding to the CO group in the neutral S0 state triggers a CO→NH isomerization reaction, in which the H2O ligand migrates to the NH binding site on the picosecond time scale.20,21,25,26 Such laser-induced shuttling reactions of a H2O ligand have also been observed in the S0 state of t-FA−H2O.22 Here, we present IR spectra of size-selected FA+−Ln clusters with L = Ar (n = 1−8) and N2 (n = 1−2) in the hydride stretch and fingerprint ranges obtained by IR photodissociation (IRPD). The experimental approach is complemented by quantum chemical calculations at the ωB97X-D/aug-cc-pVTZ level. These cluster systems have been selected for a variety of reasons. Very limited spectroscopic data are available for FA+ and its clusters with nonpolar ligands. The reported ZEKE spectra of t-FA−Ar(π) were interpreted by a π-bound structure in the cation state, t-FA+−Ar(π). However, these clusters were generated by resonance-enhanced two-photon ionization (REMPI) of the neutral π-bonded precursor, which suffers from the severe restriction of the Franck−Condon principle in populating minima on the cation ground-state potential by vertical transitions. Similar restrictions also apply to the IR spectrum reported for t-FA+−Ar clusters, which were also prepared by REMPI.20 Here, FA+−Ln clusters are produced in an electron ionization (EI) source, which generates predominantly the most stable isomer of a given cluster ion.27−29 Significantly, the IRPD spectra of FA+−Arn generated by EI unambiguously show that the H-bonded FA+−Ar dimer, denoted FA+−Ar(H), with Ar binding to the amide NH group is the global minimum on the potential energy surface in the D0 state. This isomer has completely escaped detection in previous REMPI-based experiments, which observed only the less stable t-FA+−Ar(π) local minimum.18,20 No information is available for FA+−Arn clusters with n ≥ 2 and FA+−(N2)n. To this end, the present IRPD spectra of FA+−Ln provide the first reliable impression of the stepwise microsolvation of FA+ in a nonpolar hydrophobic solvent, in particular with respect to the competition between H-bonding and π-stacking. Comparison with FA+−H2O studied previously by photoionization, ZEKE, and IR spectroscopy will unravel the differences in solvation of FA+ in nonpolar and polar solvents. The spectral range investigated here covers the important hydride stretch (amide A, νNH, νCH, 2800−3600 cm−1) and (amide I−III, 1100−1900 cm−1) fingerprint ranges, which are known to be sensitive to the structural environment of amides and thus secondary protein structure. In addition, since the Ar and N2 ligands have little impact on the properties of FA+, which can readily be quantified by comparison with the calculations, valuable structural and spectroscopic information of the bare cation can be derived from extrapolation of the cluster spectra. Comparison with neutral FA will then illustrate the effects of ionization on the properties of the aromatic amide. As no information of clusters with c-FA+ is available, we investigate

Figure 1. Structures of t/c-FA+ and t/c-FA+−L clusters with L = Ar and N2 obtained at the ωB97X-D/aug-cc-pVTZ level. Relevant structural, vibrational, and energetic parameters are listed in Table 1 (see Figure S3 in the Supporting Information).

show that the structural properties of the amide group are significantly different in both isomers.6,7 Clusters of FA with nonpolar (L = Ar) and polar ligands (L = H2O) have only been observed for the more abundant t-FA isomer.7,9,10,17−22 t-FA− Ar is found to have a π-bonded equilibrium structure, denoted t-FA−Ar(π), in which the Ar ligand binds by dispersive stacking interactions to the aromatic ring.18 In contrast, in t-FA−H2O clusters, the polar H2O ligand forms an H-bond to either the N−H or the CO group of the amide moiety.7,9,10,19,22 The 2A″ ground electronic state (D0) of FA+ is generated by removal of a bonding π-electron from the highest occupied molecular orbital (HOMO), which is delocalized over the phenyl and amide groups.16,23 Such a delocalization is consistent with the similar ionization energies of benzene and formamide (9.244 and 10.16 eV).24 As a result, both t-FA+ and c-FA+ are planar ions, whereby the energy difference between both rotamers is further increased by 302 ± 7 cm−1 on ionization.16 Zero kinetic energy (ZEKE) photoelectron spectra exhibit a few low-frequency amide modes and the amide 3006

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Table 1. Selected Geometrical Parameters (r and R, Å; θ, degree), Vibrational Frequencies (ν, cm−1), and Dissociation Energies (De and D0, cm−1) of FA, FA+, and FA+−L Clusters (Figure 1 and Figure S3 in SI) Calculated at the ωB97X-D/aug-cc-pVTZ Levela

a

species

rNH

rCO

νNH

νCO

t-FA c-FA t-FA−Ar(H) t-FA−Ar(π) c-FA−Ar(H) c-FA−Ar(π) t-FA−N2(H) t-FA−N2(π) c-FA−N2(H) c-FA−N2(π) t-FA+ c-FA+ t-FA+−Ar(H) t-FA+−Ar(π) c-FA+−Ar(H) c-FA+−Ar(π) t-FA+−N2(H) t-FA+−N2(π) c-FA+−N2(H) c-FA+−N2(π)

1.0053 1.0076 1.0053 1.0052 1.0077 1.0075 1.0059 1.0052 1.0082 1.0075 1.0120 1.0153 1.0141 1.0119 1.0176 1.0152 1.0162 1.0118 1.0202 1.0150

1.2062 1.2064 1.2064 1.2062 1.2065 1.2063 1.2068 1.2062 1.2070 1.2062 1.1861 1.1856 1.1867 1.1863 1.1861 1.1858 1.1873 1.1856 1.1867 1.1856

3449 (26) 3428 (32) 3450 (50) 3450 (25) 3426 (57) 3428 (33) 3437 (100) 3451 (26) 3417 (108) 3428 (33) 3381 (99) 3348 (123) 3351 (267) 3383 (96) 3310 (318) 3353 (120) 3311 (434) 3384 (96) 3277 (490) 3355 (119)

1759 1757 1758 1760 1769 1759 1758 1759 1755 1759 1812 1802 1811 1812 1802 1803 1807 1812 1799 1805

(361) (702) (375) (353) (677) (691) (378) (352) (684) (690) (132) (243) (141) (129) (226) (242) (142) (130) (232) (249)

De

D0

RFA−L

θNH−L

267 455 260 446 532 654 538 646

157 284 126 393 324 448 332 504

2.88 3.76 2.91 3.68 2.45 3.41 2.44 3.35

165

680 596 661 600 1373 796 1345 815

594 459 551 465 1054 604 1131 580

2.54 3.51 2.52 3.52 2.17 3.26 2.15 3.27

164 171 162

177 180 171 176

IR intensities (km/mol) are listed in parentheses. The scaling factors are 0.969 and 0.944 for frequencies below and above 2500 cm−1.

the properties of c-FA+−L here as well, because these clusters may be produced in the employed EI ion source. Finally, comparison of FA+−Ln with the recently studied clusters of acetanilide (AA+−Ln)30 will reveal the effects of methyl substitution of the amide on its structural, vibrational, energetic, and microsolvation properties.

FA+−Ln parent cluster. In addition to IRPD, collision-induced dissociation is applied by filling the octopole with 10−5 mbar N2 collision gas to determine and confirm the composition of a given parent cluster. In general, IRPD leads to a narrow range of fragment channels, and this information can be used to estimate the ligand binding energies.28 IRPD spectra are not normalized for laser intensity fluctuations monitored by a pyroelectric detector. The relative intensities of widely spaced bands are believed to be accurate to within a factor of 2 as a result of the changing spatial overlap between the IR laser and ion beams. The observed peak width in the IRPD spectra originates mainly from unresolved rotational substructure and possible sequence transitions involving low-frequency modes. Hence, the transition widths correlate with the binding energy of the most weakly bound ligand of the cluster, which determines the upper limit of the internal energy deposited in the cluster before leading to metastable decay. Thus, the widths in the FA+−Ln spectra are smaller for L = Ar than for L = N2 and decrease somewhat with the cluster size n. For example, the νNH bands of FA+−L have widths of 10 (Ar) and 12 cm−1 (N2). 2.2. Computational Techniques. Quantum chemical calculations at the ωB97X-D/aug-cc-pVTZ level are carried out for neutral and cationic t-/c-FA(+) and their t-/c-FA(+)−L dimers to determine their structural, energetic, and spectroscopic properties.32 This dispersion-corrected functional accounts well for the electrostatic, induction, and dispersion forces of the considered systems and reproduces the experimental IR spectra and binding energies with semiquantitative accuracy.33 Other functionals, such as B3LYP and M06-2X, were found to be much less accurate in the prediction of IR spectra and binding energies. On the other hand, MP2 calculations reveal high spin contamination. The latter is negligible at the ωB97X-D/aug-cc-pVTZ level, with values of ⟨S2⟩ − 0.75 = 0.026 and 0.004 for t-FA+ before and after annihilation, respectively. All coordinates are allowed to relax

2. EXPERIMENTAL AND COMPUTATIONAL TECHNIQUES 2.1. Experimental Methods. IRPD spectra of massselected FA+−Ln clusters with L = Ar and N2 are recorded in a quadrupole−octopole−quadrupole tandem mass spectrometer coupled to an electron ionization (EI) cluster source.28,29,31 FA (Sigma-Aldrich, >99.9%) heated to 110−140 °C and seeded in either Ar or N2 carrier gas (10 bar) is expanded through a pulsed nozzle. FA+−Ln clusters are generated by electron or chemical ionization of FA followed by three-body aggregation. A typical mass spectrum of the ion source using N2 as carrier gas is available in Figure S1 in the Supporting Information (SI). The desired FA+−Ln clusters are mass-selected in the first quadrupole and injected in the adjacent octopole where they are exposed to the pulsed radiation of a tunable IR optical parametric oscillator pumped by a Q-switched nanosecond Nd:YAG laser. Relevant parameters of the IR laser are a pulse energy of ∼0.3−1 and 2−5 mJ in the fingerprint and hydride stretch range, respectively, a repetition rate of 10 Hz, and a bandwidth of 1 cm−1. Calibration of the IR laser frequency accurate to better than 1 cm−1 is accomplished by a wavemeter. Resonant vibrational excitation of FA+−Ln causes the loss of one or several ligands, which are the only fragmentation channels observed: FA+−Ln + hνIR → FA+−Lm + (n − m)L

(1)

The generated FA −Lm fragments are selected by the second quadrupole and monitored by a Daly detector as a function of the IR laser frequency (νIR) to yield the IRPD spectrum of the +

3007

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during the search for stationary points carried out without any symmetry restrictions, and their nature as minima or transition states is verified by harmonic frequency analysis. Interaction energies (De) are corrected for harmonic zero-point energy to provide binding energies (D0). Harmonic intramolecular vibrational frequencies are subjected to linear scaling by factors of 0.969 and 0.944 in the fingerprint and hydride stretch range, respectively. The former originates from fitting calculated harmonic frequencies of neutral t-FA to observed transitions (Table T1 in SI).6 The latter is derived by matching the calculated νNH frequency of t-FA+ with the experimental value (3380 cm−1) approximated by νNH of FA+−Ar(π). The application of two scaling factors accounts for the different anharmonicities of the fingerprint and hydride stretch modes.34 Reported intermolecular frequencies are unscaled. The charge distribution is analyzed by a natural bond orbital (NBO) analysis.

3. RESULTS AND DISCUSSION 3.1. Quantum Chemical Calculations. The minimum structures of t-/c-FA+ and their most stable t-/c-FA+−L dimers with L = Ar and N2 obtained at the ωB97X-D/aug-cc-pVTZ level are depicted in Figure 1, and full structural details of t-/cFA(+) are given in Figure S3 in SI. Energetic, structural, and vibrational data relevant for the present work are listed in Table 1. These include intramolecular parameters, namely, the N−H and C−O bond lengths (rNH and rCO) and their stretch frequencies and IR intensities (νNH/CO, INH/CO). Selected intermolecular parameters are also given, namely, bond lengths (R) and bond angles (θ), as well as binding energies (De/0). In the following, the notation t-/c-FA(+)−Ln(xH/yπ) is employed to designate complexes of t-/c-FA(+) with x H-bound and y πbound ligands. IR stick spectra calculated for t-/c-FA+ and t-/cFA+−L in the fingerprint and hydride stretch ranges are compared in Figure 2. A list of all vibrational frequencies and IR intensities of t-FA(+) along with their assignments is available in Table T1 in SI. 3.1.1. FA and FA+. In the ground electronic state (S0), planar t-FA (Cs) is calculated to be ΔE0 = 68 cm−1 (ΔG298 = 59 cm−1) more stable than c-FA, compatible with the experimental estimate of 350 ± 150 cm−1 derived from MW spectroscopy.3 In c-FA, the planar amide group is rotated by φ = 34° with respect to the aromatic plane and the barrier to linearity is calculated as Vb = 141 cm−1, in agreement with the values derived from experiment (φ = 34.7 ± 0.5°; Vb = 152 ± 2 cm−1).3 The rather short N−H bonds, rNH = 1.0053/1.0076 Å for t-/c-FA, correlate with high N−H stretch frequencies, νNH = 3449/3428 cm−1, with a splitting of ΔνNH = 21 cm−1, which agree well with the experimental data (νNH = 3463/3440 cm−1; ΔνNH = 23 cm−1).6 In contrast, the C−O bonds are fairly long for both isomers, rCO = 1.2062/1.2064 Å, with low stretching frequencies of νCO = 1759/1757 cm−1 for t-/c-FA, which are close to the experimental values (νCO = 1742/1741 cm−1).6 tFA is separated by a very large isomerization barrier from c-FA, with a calculated barrier of Vb = 6624 cm−1 in good agreement with the value measured in solution (∼7000 cm−1).13 Hence, both isomers are readily produced and observed in molecular beams.3,6,8,16 Ionization of t-/c-FA into its 2A″ cationic ground electronic state (D0) arises from the removal of a bonding π-electron from the highest occupied molecular π(a″) orbital (HOMO, Figure S4 in SI), which is highly delocalized over both the aromatic ring and the amide group.16 The NBO analysis detailed in

Figure 2. IR stick spectra of t-/c-FA+ and t-/c-FA+-L dimers (L = Ar/ N2) in the fingerprint (amide I−III) and hydride stretch (amide A) ranges obtained at the ωB97X-D/aug-cc-pVTZ level (Table 1, Figure 1).

Figure S5 in SI predicts that 0.32/0.36 e of the positive excess charge is localized on the amide group of t-/c-FA+, as evaluated by the charge differences of t-/c-FA+ and t-/c-FA. In contrast to neutral FA, both t-FA+ and c-FA+ are planar, with t-FA+ being 468 cm−1 more stable (Figure 3). Although the barriers for

Figure 3. Potential energy surface for cis−trans isomerization of FA+ evaluated at the ωB97X-D/aug-cc-pVTZ level. Energies are optimized for internal rotation around the amide OC−NH bond by variation of the dihedral angle θ (C1−N−C−O) between 0° (c-FA+) and 180° (tFA+) in steps of 3.6° in a redundant coordinate scan. c-FA+ is 562 cm−1 less stable than t-FA+, and t-FA+→c-FA+ isomerization involves a significant barrier of Vb = 3222 cm−1 at θ ∼ 90°.

trans−cis isomerization are much lower for the radical cation than for the closed-shell neutral molecule (Vb = 3222 and 2660 cm−1 for trans→cis and cis→trans), they are still substantial so that both isomers could be stabilized in their respective potential wells in the supersonic plasma expansion. Ionization of FA induces remarkable structural changes (Figure S3 in SI). For example, the N−H bond of the amide group elongates for 3008

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example, Vb(π→H) = 62 cm−1 and Vb(H→π) = 145 cm−1 for tFA+−Ar (Figure S6 in SI). 3.1.3. FA+−N2. The interaction of N2 with t-/c-FA+ is substantially stronger than for Ar due to its additional negative quadrupole moment and increased parallel polarizability, giving rise to additional electrostatic, induction, and dispersion attraction.27,36−38 Furthermore, the anisotropy of the longrange charge-quadrupole and charge-induced dipole interaction favors a linear over a T-shaped approach of N2 toward the positive charge. Similar to t-/c-FA+−Ar, the energetic and structural differences between t-/c-FA+−N2 are modest (Table 1). The planar H-bound t-FA+−N2(H) dimer is the global minimum on the potential, with R = 2.17 Å, θ = 171°, and D0 = 1054 cm−1. The intermolecular frequencies are predicted as β′ = 17, β″ = 11, γ′ = 98, γ″ = 98, and σ = 88 cm−1. Similar to FA+−Ar, the H-bond in c-FA+−N2 is slightly stronger than in tFA+−N2, with R = 2.15 Å, θ = 176°, and D0 = 1131 cm−1, and slightly higher frequencies of β′ = 28, β″ = 33, γ′ = 109, γ″ = 103, and σ = 95 cm−1. Complexation with N2 has an enhanced impact on the intramolecular structural and vibrational properties as compared to Ar, in particular for the H-bonded isomer. H-bonding of N2 in t-/c-FA+−N2(H) leads to ΔrNH = 4.2/4.9 mÅ, ΔνNH = −70/−71 cm−1, and ΔINH = 338/298%. There is only a minor impact on the C−O bond, with ΔrCO < 1.2 mÅ and ΔνCO < 10 cm−1. In contrast to FA+−Ar, the difference in stabilization between the H-bound global and π-bound local FA+−N2(π) minima is much larger (roughly a factor 2). The π-bonds in t-/c-FA+− N2(π) have R = 3.26/3.27 and D0 = 604/580 cm−1, and intermolecular frequencies are βx = 5/12, βy = 28/32, γx = 62/ 69, γy = 47/51, and σz = 53/49 cm−1. Compared to Ar, the πbound N2 ligand is shifted further toward the amide group. While the H-bonds of N2 are much stronger than those of Ar (factor 2), the π-stacking interactions are more similar for both ligands (difference of ∼30%). Isomerization barriers between the π-bonded local minimum and the H-bonded global minimum are Vb(π→H) = 59 cm−1 and Vb(H→π) = 608 cm−1 for t-FA-N2 (Figure S6 in SI). The low Vb(π→H) barriers are consistent with the small βx frequencies and suggest that it might be quite difficult to experimentally trap the N2 ligand at the π-bound site. π-stacking with N2 has only modest influence on the N−H and C−O bonds, with ΔrNH/CO < 0.5 mÅ and ΔνNH/CO < 7 cm−1. 3.1.4. Cluster Growth. In addition to the H-bound global and less stable π-bound local minima, other local minima exist on the FA+−L potential. However, most of them are much less stable and thus not considered in detail further. For example, D0 = 188 and 235 cm−1 for planar t-FA+−Ar(OCH) and tFA+−Ar(R), respectively, in which the Ar ligand binds to either the amide CH group or the p-CH group of the ring via CH−Ar H-bonds. Similar results are obtained for the related N2 complexes, with the notable exception of the relatively high stability of c-FA+−N2(OCH), D0 = 603 cm−1, in which N2 binds to both the amide CH and the phenyl ortho CH groups via bifurcated H-bonding (this structure is not relevant here, because of the low abundance of c-FA+, vide infra). Hence, on the basis of the dimer potential, the predicted cluster growth in FA+−Ln begins with H-bonding of the first ligand to the NH group and continues by attachment of further ligands to the aromatic ring and other less favorable binding sites. H-bonding and π-stacking compete for L = Ar, so that both isomers may be detected in the molecular beam. In contrast, for L = N2 Hbonding is much stronger than π-stacking, so that the

both isomers by ΔrNH ≈ +7 mÅ, whereas the C−O bond contracts significantly; ΔrCO ≈ −20 mÅ. As a consequence, νNH is red-shifted by ΔνNH = −68/−80 cm−1, whereas νCO is blueshifted by ΔνCO = 53/45 cm−1 for t-/c-FA+. Due to longer intramolecular bonds, both νNH and νCO are significantly smaller for c-FA+ as compared to t-FA+ (by 43 and 10 cm−1) and thus can easily be resolved at the current spectral resolution (