Simultaneous Interaction of Hydrophilic and Hydrophobic Solvents

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Simultaneous interaction of hydrophilic and hydrophobic solvents with ethylamino neurotransmitter radical cations: Infrared spectra of tryptamine+-(H2O)m-(N2)n clusters (m,n#3) Markus Schütz, Kenji Sakota, Moritz Raphael, Matthias Schmies, Takamasa Ikeda, Hiroshi Sekiya, and Otto Dopfer J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b07408 • Publication Date (Web): 09 Sep 2015 Downloaded from http://pubs.acs.org on September 16, 2015

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The Journal of Physical Chemistry

Simultaneous Interaction of Hydrophilic and Hydrophobic Solvents with Ethylamino Neurotransmitter +

Radical Cations: Infrared Spectra of Tryptamine -(H2O)m-(N2)n Clusters (m,n≤3) a

b

a

a

b

b

Markus Schütz, Kenji Sakota, * Raphael Moritz, Matthias Schmies, Takamasa Ikeda, Hiroshi Sekiya, Otto a

Dopfer * a

Institut für Optik und Atomare Physik, Technische Universität Berlin, D-10623 Berlin, Germany

b

Department of Chemistry, Faculty of Sciences, and Department of Molecular Chemistry, Graduate School of

Science, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan

* Corresponding authors: [email protected], [email protected]

Abstract Solvation of biomolecules by a hydrophilic and hydrophobic environment strongly affects their structure and function. Here, the structural, vibrational, and energetic properties of size-selected clusters of the +

microhydrated tryptamine cation with N2 ligands, TRA -(H2O)m-(N2)n (m,n≤3), are characterized by infrared photodissociation (IRPD) spectroscopy in the 2800-3800 cm

-1

range and dispersion-corrected density

functional theory calculations at the ωB97X-D/cc-pVTZ level to investigate the simultaneous solvation of this prototypical neurotransmitter by dipolar water and quadrupolar N2 ligands. In the global minimum structure of +

TRA -H2O generated by electron ionization, H2O is strongly hydrogen-bonded (H-bonded) as proton acceptor +

to the acidic indolic NH group. In the TRA -H2O-(N2)n clusters, the weakly-bonded N2 ligands do not affect the +

H-bonding motif of TRA -H2O and are preferentially H-bonded to the OH groups of the H2O ligand, whereas +

stacking to the aromatic π electron system of the pyrrole ring of TRA is less favorable. The natural bond orbital analysis reveals that the H-bond between the N2 ligand and the OH group of H2O cooperatively +

strengthens the adjacent H-bond between the indolic NH group of TRA and H2O, while π stacking is slightly +

noncooperative. In the larger TRA -(H2O)m clusters, the H2O ligands form a H-bonded solvent network attached to the indolic NH proton, again stabilized by strong cooperative effects arising from the nearby positive charge. Comparison with the corresponding neutral TRA-(H2O)m clusters illustrate the strong impact of the excess positive charge on the structure of the microhydration network.

Revised version submitted to JPC A on 3 September 2015

1

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1. Introduction

Biomolecules in living cells are surrounded by water molecules, and the subtle balance of intermolecular interactions determines the hydration structures around these biomolecules. In particular, hydrogen-bonding (H-bonding) interactions play a key role in shaping the hydration network. Although such H-bonds form extended water networks around the biomolecule, the first hydration shell might have the largest impact on its structure and function. In addition, hydrophobic interactions induced by the less polar hydrophobic parts of the biomolecule also play a crucial role to determine biomolecular structures. For instance, some of the globular proteins form a hydrophobic core at the beginning of a folding process, and subsequently the remaining parts of the proteins are folded into the native structure.

1,2

Since the efficiency of biochemical processes such as

enzymatic reactions depends on the biomolecular configuration, their fundamental understanding requires the detailed knowledge of the intermolecular interactions between biomolecule, water, and nonpolar moieties. Laser spectroscopy combined with mass spectrometry applied to cold neutral and ionic biomolecular and aromatic clusters generated in a molecular beam or a cryogenic ion trap is a powerful tool to experimentally probe the structure

3-19

and dynamics

20-26

of such competing intermolecular interactions at the molecular level

with high accuracy and sensitivity. Here, we study clusters of the tryptamine cation with water and molecular +

nitrogen, TRA -(H2O)m-(N2)n, by infrared photodissociation (IRPD) in a tandem mass spectrometer and dispersion-corrected density functional theory calculations.

Tryptamine, with a flexible ethylamino side chain attached to the aromatic indole chromophor, is considered as a simple analogue of the neurotransmitter serotonin and the amino acid tryptophan, and has spectroscopically been characterized in detail in molecular beams. In the neutral ground electronic state (S0), up to seven stable conformers are observed by UV,

27,28

IR,

29

Raman,

30

and microwave

31,32

spectroscopy,

which differ just in the conformation of the flexible side chain and which are connected by relatively low isomerization barriers.

33-35

In contrast, only a single conformer having Gpy(out) conformation is observed in 36-39

the S0 state of TRA-H2O (see Ref. 29 for the notation of the isomers of TRA),

implying that a single water

molecule greatly reduces the flexibility of TRA via conformational locking induced by intermolecular H-bonding. In this structure, H2O is H-bonded as proton donor to the lone pair of the amino nitrogen of the side chain, similar to other protic solvent molecules such as methanol.

27,36

IR spectra of larger TRA-(H2O)m

clusters with m=2 and 3 suggest that a H-bonded (H2O)m bridge is formed between the amino nitrogen and the indolic NH proton.

40

Ionization of TRA has a large impact on the side-chain conformation and the

interaction with solvent molecules. Ionization into the ionic ground electronic state (D0) occurs via removal of 41

an aromatic π electron from the indole ring. Gu and Knee reported zero-kinetic-energy photoelectron (ZEKE) and photoionization efficiency (PIE) spectra of the D0 state of various TRA isomers, substantial impact of the excess positive charge localized mainly on the indole ring

42

41

which reveal the

on the side chain

conformation and the relative energetic order of the isomers. Similarly, the PIE and fragmentation spectra of the TRA-H2O dimer reveal a large intermolecular geometry change upon ionization, and its analysis using density functional theory calculations suggests that photoionization of TRA-H2O from the S1 state induces a substantial rearrangement of the H-bond between TRA

(+)

and water.

42

2


The subsequently measured IRPD

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+

spectrum of TRA -H2O generated by resonant two-photon ionization (R2PI) clearly confirms that the water molecule transfers from the amino group of the ethylamino side chain to the indolic NH group after photoionization with 100% yield, and changes its role from a proton donor in S0/1 to a proton acceptor in D0.

43

+

Although density functional theory calculations suggest the indolic NH …OH2 H-bonding motif to be the final reaction product of the ionization-induced rearrangement reaction, it has experimentally been unclear +

whether it is the global or only a local minimum on the TRA -H2O potential in the D0 state. This remaining +

open question is obviously an important aspect for the pathway of the solvent migration in TRA -H2O, because different potential energy profiles along the H-bond rearrangement may lead to different binding +

motifs of the final TRA -H2O reaction product. Such shuttling processes of H2O (or CH3OH) ligands triggered by photoionization have been inferred for a variety of aromatic clusters, including benzene, aminophenol,

46

formanilide,

47,48

acetanilide,

21,22,49

and aminobenzonitrile,

50,51

44,45

and recent time-resolved

experiments have shown that such solvent rearrangement reactions occur on the picosecond time scale.

20-22

+

Recently, we also reported the IR photodissociation (IRPD) spectra of TRA -(N2)n with n=1-6 to characterize + 52

the microsolvation effect of hydrophobic quadrupolar solvents on TRA .

We found that the indolic NH group

+

+

of TRA is the most favorable H-bonding site for the N2 ligands, whereas the π-stacking sites of TRA are less favorable.

In all previous studies, resonant photoionization (R2PI) of the neutral cluster has been utilized to generate +

TRA -H2O cations in the Franck-Condon region of the D0 state, which produces vibrationally hot clusters far away from the global minimum geometry.

42,43

+

Here, we report for the first time IRPD spectra of TRA -(H2O)m

+

(m=1-3) and cold TRA -(H2O)m-(N2)n (n≤3) clusters generated in a supersonic electron impact plasma +

expansion to explore the microhydration of TRA . This cluster ion source is known to predominantly populate the global minimum of a given cluster size in the D0 state, independent of the most stable structure of the neutral complex.

14,53-62

+

Moreover, the N2 ligands attached to TRA -(H2O)m act as cooling messengers,

63

which

reduces the effective binding energy and temperature of the cluster, leading to IRPD spectra with higher resolution.

14

+

The analysis of the IRPD spectra of TRA -(H2O)m-(N2)n using dispersion-corrected density

functional theory calculations provide insight into the simultaneous intermolecular interactions of hydrophilic +

(H2O) and hydrophobic (N2) solvent molecules with TRA , a topic of recent interest.

45,48,51,64

Comparison with

neutral TRA-(H2O)m clusters reveals the drastic impact of the positive charge on the microhydration network around this prototypical neurotransmitter.

2. Experimental and computational methods +

IRPD spectra of TRA -(H2O)m-(N2)n cluster ions are recorded in a tandem quadrupole mass spectrometer coupled to an electron impact ionization source and an octopole ion trap. extensively to microhydrated aromatic molecular ions.

48,51,66-72

14,65

This setup has been applied

+

TRA -(H2O)m-(N2)n clusters are produced in a

pulsed supersonic expansion (20 Hz) by electron and/or chemical ionization (EI) of TRA close to the nozzle orifice and subsequent clustering reactions. The expanding gas mixture is produced by passing N2 carrier gas (5 bar) seeded with trace amounts of H2O through a heated reservoir filled with TRA (410 K). Ions are 3



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extracted from the source through a skimmer into the first quadrupole to mass select the parent cluster ions of interest. These are subsequently irradiated in an adjacent octopole with a tunable IR laser pulse (νIR) generated by an optical parametric oscillator (IR-OPO) pumped by a nanosecond Q-switched Nd:YAG laser. The output of the IR-OPO laser operating at a repetition rate of 10 Hz is characterized by a pulse energy of -1

-1

2-5 mJ in the 2800-3800 cm range and a bandwidth of 1 cm . Calibration accomplished using a wavemeter -1

+

is accurate to better than 1 cm . Resonant excitation of vibrational modes of TRA -(H2O)m-(N2)n leads to the +

evaporation of all N2 ligands, which is the only observed fragmentation channel. In the case of TRA -(H2O)m, +

the IRPD spectrum is monitored in the H2O loss channel. To obtain IRPD spectra of the TRA -(H2O)m-(N2)n parent clusters the respective fragment ions are selected by a second quadrupole and detected by a Daly detector as a function of νIR. Ne and Ar could not be used as tagging atoms (and carrier gas), +

60-62,73-79

+

because

+

of the mass coincidence of highly abundant Ar4 and Ne8 ions which are isobaric with TRA (m/z 160). Although the laser intensity variations are monitored by a pyroelectric detector, the IRPD spectra are not normalized for laser power. Nonetheless, the relative intensities of transitions are believed to be accurate to within a factor of three, mainly due to the variation in the overlap between the laser and ion beams in the octopole.

Dispersion-corrected

density

functional

theory

calculations

are

performed

at

the

unrestricted

+

ωB97X-D/cc-pVTZ level to obtain geometric, vibrational, and energetic properties of TRA -(H2O)m-(N2)n. +

This theoretical level has previously provided results for TRA -(N2)n clusters with satisfactory accuracy.

52

80

Spin

contamination is found to be negligible at this theoretical level. The application of the tight self-consistent field convergence criterion including an ultrafine integration grid as implemented in GAUSSIAN09 is crucial to correctly identify stationary points for the N2-tagged clusters. Harmonic frequencies are scaled by a factor of 0.9415, obtained by matching the experimental O-H stretch frequencies of H2O (ν +

s/a

OH=3657/3756 +

-1

cm ) and f

the indolic N-H stretch fundamental of TRA (approximated by that of the π-bonded TRA -N2 dimer, ν NH=3447 -1 52

cm )

-1

to better than 5 cm . All relative energies and binding energies (Ee, De) are corrected for vibrational

zero-point energies to derive E0 and D0 values. In the natural bond orbital (NBO) analysis,

81

pairs of NBOs

obtained for the α and β electrons have similar shapes. Therefore, although the sum of the dominant second order perturbative energies, E

(2)

i→j*,

for α and β electrons are evaluated, only NBOs for the α electrons are

displayed in the figures.

3. Results and Discussion +

Figure 1 compares the IRPD spectra of TRA -(H2O)m with m=1-3 in the 2800-3800 cm

-1

range to the

+

TRA -N2 spectrum reported previously. This spectral range covers the C-H, N-H, and O-H stretch fundamentals (νCH/NH/OH), which provide detailed information about the evolution of the H-bonded +

microhydration network around the TRA cation in its D0 ground electronic state. In the following, the transitions A-D are assigned to O-H stretch modes (νOH) of the water ligands. The transition E is attributed to +

the indolic N-H stretch vibration (νNH) of TRA , whereas bands F and G are assigned to the asymmetric and symmetric N-H stretch modes of the amino group (ν

a

NH2,

ν

s

NH2),

respectively. The aromatic and aliphatic C-H

4


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stretch (νCH) fundamentals occur also in this frequency range but their calculated IR intensities are much weaker compared to those of νNH/OH. The positions and widths of the transitions observed are listed in Table 1, along with their vibrational and isomer assignments. In the following, we discuss the evolution of the +

TRA -(H2O)m-(N2)n structures as a function of the cluster size m and n by comparing experimental and calculated IR spectra. +

3.1. TRA -H2O +

The IRPD spectrum of TRA -H2O clusters generated in the electron impact (EI) source is compared in +

Figure 2 with the previously reported IRPD spectrum of TRA -H2O produced by R2PI. The positions are listed +

in Table 1, along with the suggested vibrational and isomer assignments. The IRPD spectrum of TRA -H2O -1

generated by EI is dominated by two sharp bands A and B at 3723 and 3635 cm and an intense broad band E in the 3050-3250 cm

-1

-1

range peaking at around 3167 cm . The latter band exhibits partly resolved

substructure, with weaker maxima located at 3076, 3107, and 3146 cm +

-1

on the red side. The overall +

appearance of the TRA -H2O spectrum is quite similar to that of the related indole -H2O complex,

82

in which

H2O is attached as a proton acceptor to the indolic NH group, forming an NH…O H-bond with favorable charge-dipole orientation (Figure S1 in the Supporting Information (SI)). This spectral similarity immediately +

suggests that TRA -H2O observed in the EI-IR spectrum in Figure 2(b) has the same H-bonding motif as that +

-1

of indole -H2O. We readily assign the vibrational bands B and A at 3635 and 3723 cm to the free symmetric and antisymmetric O-H stretch modes ν

s

OH

and ν

a

OH

-1

of the H2O ligand. Their redshifts of -22 and -33 cm from

-1

the frequencies of bare H2O (3657 and 3756 cm ) are typical for cation-H2O clusters with charge-dipole configuration.

14,45,48,51,66-72

The H-bonded indolic N-H stretch mode νNH should be the largest contributor to the -1

-1

f

intense band E at 3167 cm , which corresponds to a redshift of -280 cm from the free ν NH frequency of bare +

-1

+

52

TRA (3447 cm ) estimated from π-bonded TRA -N2 (indicated by arrows in Figures 1 and 2). The additional weaker transitions overlapping with ν

b

NH

arise from Fermi resonance interactions of ν

b

NH

with overtone and

combination bands and/or vibrational mode mixing of νNH with aromatic C-H stretch modes (νCH). These +

+

transitions are quite weak in the TRA -N2 spectrum (Figure 1), indicating that in the TRA -H2O spectrum they gain most of their IR activity from the nearby ν

b

NH

-1

mode. The weak band F observed at 3410 cm is tentatively

assigned to the antisymmetric N-H stretch mode of the amino group, ν

a

NH2.

Consequently, it is absent in the

+

f

corresponding indole -H2O spectrum (Figure S1 in SI). There is no signal in the spectral range of the free ν NH +

-1

+

mode of TRA near 3450 cm , indicating that all observed TRA -H2O complexes have a structure in the D0 state with H2O attached to the indolic NH group as H-bond acceptor. As the EI cluster ion source is known to +

produce predominantly the most stable isomer of a given cluster ion, the observed TRA -H2O structure corresponds indeed to the global minimum in the D0 state. In the S0 state of neutral TRA-H2O, H2O forms a proton donor H-bond to the amino group in the ethylamino side chain of TRA. Thus, the positive charge in +

TRA drastically changes the preferential H-bonding site, which can be understood by analysis of the charge distribution discussed in section 3.5.

5



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+

The previously reported IR-dip spectrum of TRA -H2O generated by one-color R2PI shown in Figure 2(a) is similar in appearance to the IRPD spectrum of the clusters generated by EI shown in Figure 2(b). Hence, R2PI of neutral TRA-H2O triggers an isomerization reaction in the D0 state,

43

which reaches as the final

+

product indeed the global minimum structure of TRA -H2O in the D0 state. The major difference between the +

two production methods (EI and R2PI) is the internal energy of the TRA -H2O ions. The bands in the R2PI-IR spectrum are substantially broader than those of the EI-IR spectrum (in particular band E), indicating that the +

average internal energy of the TRA -H2O clusters is larger using the former production technique. Calculations at the M06-2X/aug-cc-pVDZ level indicate that vertical ionization of TRA-H2O generates cluster -1

cations with at least 2100 cm internal energy with respect to the global minimum in the D0 state.

43

Indeed,

+

the anomalously low dissociation energy of TRA -H2O cations produced by REMPI of TRA-H2O, measured as -1

1750±150 cm , is consistent with such high internal energies resulting from vertical R2PI.

42

The binding

-1

energy of the global minimum structure in D0 is calculated here as 58.5 kJ/mol (5000 cm ). Thus, under the +

single-photon absorption conditions employed in the present work, the TRA -H2O cations detected in the -1

-1

EI-IR spectrum in the 2800-3800 cm range must have an internal energy of the order of 1200-2200 cm , i.e. they are significantly colder than those detected in the R2PI-IR spectrum. As a consequence, the band E mainly assigned as the ν

b

NH

proton donor stretch mode is significantly blueshifted in the warmer R2PI-IR

-1

spectrum (by ~30 cm ) and its substructure is essentially washed out by sequence hot bands involving ν

b

NH.

+

All computed low-energy structures of TRA -H2O shown in Figure 3 have the same H-bonding motif in which H2O is H-bonded as a proton acceptor to the indolic NH group but they differ in the orientation of the ethylamino side chain. The first eight isomer lie within a range of ΔE0=20 kJ/mol. All eight isomers have essentially the same binding energy (D0=58.5-60.8 kJ/mol) and H-bond length (1.741-1.754 Å), indicating that the side-chain conformation has little impact on the strength of the indolic NH…OH2 H-bond. Thus, the +

+

relative energies of the TRA -H2O isomers follow the same energetic order as the TRA isomers,

52

and

monohydration has little impact on the energetic preference of the side chain (< 3 kJ/mol). Other binding sites +

of the H2O ligand to TRA are less stable and not shown here, because they are not detected experimentally. +

The most stable isomer (E0=0) at the employed ωB97X-D/cc-pVTZ level is TRA -H2O with the side chain in Gpy(in) conformation and a dissociation energy of D0=58.5 kJ/mol, followed by the Gph(in) isomer at E0=3.6 kJ/mol and D0=58.5 kJ/mol. Test calculations at other theoretical levels, including B3LYP (cc-pVDZ, aug-cc-pVTZ, 6-311++G**) and M06-2X (cc-pVTZ) give essentially the same results for these two lowest-energy isomers, with energy spacings in the range of 0.2-4.1 kJ/mol (Figure S2 in SI). Only at the B3LYP-D3/aug-cc-pVTZ level, the energetic order of both isomers is reversed with a spacing of 1.1 kJ/mol. +

Bonding of H2O in TRA -H2O is perpendicular to the indole plane with a barrier of 3.5 kJ/mol to internal rotation around the H-bond calculated for the Gpy(in) isomer. +

The IRPD spectrum of TRA -H2O measured with the EI source is compared in Figure 4 to the linear absorption stick spectra computed for the three lowest-energy isomers Gpy(in), Gph(in), and Gph(up), and a complete comparison with the spectra computed for all eight isomers is available in Figure S3 in SI. Clearly, the best agreement is obtained for the most stable isomer, Gpy(in). All theoretical IR spectra predict three 6


Page 7 of 44

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strong vibrational transitions, namely the bound indolic ν ν

s

OH

and ν

a

OH

b

NH

-1

near 3100 cm and the free O-H stretch modes

-1

of the H2O ligand near 3630 and 3720 cm , which agree well with the measured IRPD spectrum.

As expected, these transitions are not sensitive to the side chain conformation. In contrast, the weakly IR active symmetric and antisymmetric N-H stretch modes of the amino group, ν

s

NH2

-1

and ν

a

NH2,

predicted near

-1

3300-3400 cm as well as the aliphatic C-H stretch modes near 2800-2900 cm are quite sensitive to the structure of the side chain. While the C-H stretch modes in the experimental spectrum are strongly affected by the interaction with the intense ν

b

NH

transition, the relatively isolated νNH2 bands may be used to determine the

side chain conformation, and the by far best agreement is obtained for the most stable Gpy(in) isomer, with respect to both position and relative IR intensity. Hence, we assign the observed spectrum mainly to this +

TRA -H2O isomer, and detailed structural, vibrational, and energetic parameters are listed in Table 1. Specifically, the predicted ν

a

OH

and ν

s

OH

-1

bands at 3738 and 3648 cm exhibit redshifts of -19 and -10 cm

-1

-1

from bare H2O, consistent with the measured shifts of -33 and -22 cm . These shifts arise from the O-H bond elongation of 1.4 mÅ upon formation of the NH…OH2 H-bond. The intense ν -1

b

NH

fundamental is predicted at

-1

3103 cm with huge IR activity (1396 km/mol) and a redshift of -339 cm close to the measured value (-280 -1

cm ), which arises from the elongation of the indolic N-H bond by 21 mÅ upon H-bonding. The N-H stretch fundamentals of the amino group are predicted at ν

s

NH2=3334

and ν

a

NH2=3409

-1

-1

cm , and the latter frequency

matches well the position of band F (3410 cm ) in the measured spectrum. The ν

s

NH2

transition is only visible

-1

as band G in the colder N2 tagged spectra near 3346 cm (Figure 2) discussed below in section 3.2. Further +

+

confirmation for the observation of the Gpy(in) isomer of TRA arises from the analysis of the TRA -N2 spectrum shown in Figure 1 in the C-H stretch range. Comparison of the experimental spectrum with that +

calculated for the H-bonded TRA -N2 isomer in Figure S4 in SI reveals that the observed aliphatic symmetric -1

and antisymmetric C-H stretch bands at 2886 and 2930 cm are well reproduced by those calculated for the +

-1

Gpy(in) isomer in TRA -N2 (2854/2863 and 2926/2941 cm ). +

3.2. TRA -H2O-(N2)n +

+

The IRPD spectra of TRA -H2O-(N2)n with n=1-3 measured in the TRA -H2O channel are compared in +

Figure 2(c)-(e) to that of bare TRA -H2O in 2(b). As can be seen, N2 tagging has a significant impact on the +

appearance of the IRPD spectrum of the TRA -H2O core ion. The transitions are much narrower (Table 1) because the temperature of the N2 tagged clusters is much lower. The binding energy of the N2 ligands is of -1

the order of 8 kJ/mol (~700 cm ), which provides an upper limit of the internal energy of the cluster. The +

observation that single-photon IRPD of TRA -H2O-(N2)3 causes the evaporation of all three N2 ligands implies -1

that the total binding energy of these three N2 ligands is less than the photon energy of 3000 cm , consistent with binding energies below 1000 cm

-1

+

per N2 ligand. Clearly, single-photon IRPD of cold TRA -H2O-(N2)n

clusters in the considered spectral range does not require any internal energy prior to photodissociation. As a consequence of the relatively small binding energies of the N2 ligands, they affect neither the binding site of the H2O ligand nor the conformation of the side chain. Hence, N2 ligands are supposed to attach to the most +

stable Gpy(in) isomer of TRA -H2O, and the IRPD spectra in Figure 2 fully support this scenario. The +

transitions in the O-H stretch range are not only sharper for TRA -H2O-(N2)n but also show systematic shifts 7



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Page 8 of 44

and splittings as a function of n due to different competing N2 binding sites, namely H-bonding to H2O and π stacking to the aromatic indole ring. In addition, in the spectral range between 2850 and 3200 cm covers the H-bonded indolic ν

b

NH

-1

,

which

+

band (E) and the C-H stretch bands, the TRA -H2O-(N2)n spectra are +

relatively independent of the number of N2 ligands, while they strongly differ from that of the bare TRA -H2O ion. The latter result may be attributed to (i) the effect of cooling and (ii) the effects of N2 complexation on the +

force field of TRA -H2O. The observation that sequential N2 complexation shows little effects in this spectral +

range for n=1-3 suggests that cooling may be the more important factor. Similar to TRA -H2O, the indolic ν

b

NH

band should dominate the contribution to the complicated band pattern. Following this scenario, the most intense band E near 3060 cm

-1

is attributed to the ν

b

NH

+

fundamental of TRA -H2O-(N2)n, which implies a

-1

+

redshift of the order of -100 cm compared to the corresponding band in TRA -H2O. Calculations discussed -1

below indeed suggest that part of this shift (-40 cm ) arises from changes in the N-H stretch force field upon H-bonding of the first N2 ligand to H2O (Figure S5 in SI). The effect of the temperature on the frequency of proton donor stretch vibrations is substantial.

14,83

The higher the temperature, the weaker the intermolecular +

bond and the higher the proton donor stretch frequency. The TRA -H2O-(N2)n clusters must be cold and therefore we detect the ν

b

NH

fundamental at lower frequency. In contrast, we can only photodissociate warm

+

TRA -H2O clusters by single-photon absorption. Therefore, the ν

b

NH

band appears at much higher frequency

because the fundamental transition is not observed and the detected sequence hot bands involving this proton donor stretch mode occur at higher frequency than the ν

b

fundamental. Such a behavior was

NH

previously observed for a series of related tagged microhydrated (or solvated) aromatic cluster ions. +

As a result of the colder TRA -H2O-(N2)n clusters, their spectra in the ν

b

NH

21,48,51,73

range are much better resolved,

leading to a complex band pattern (bands Y1-Y7). Although we cannot definitely assign the peaks in the -1

range from 2850 to 3160 cm , the further redshift of the H-bonded ν

b

NH

+

band in the cold TRA -H2O-(N2)n

clusters enhances the Fermi resonance interactions with overtone and combination bands as well as mode mixing with C-H stretch fundamentals. +

Figure 5 shows expanded views of the IRPD spectra of TRA -H2O-(N2)n in the free O-H and N-H stretch -1

range. Obviously, more than two O-H stretch bands are observed above 3600 cm for clusters with n=1-3 N2 ligands and the observed spectral pattern is illustrated in Figure 6. This observation strongly suggests that the +

N2 ligands are attached to different binding sites of TRA -H2O, leading to the detection of structural isomers. The ν

a

OH

and ν

s

OH

vibrations observed near ~3700 and ~3620 cm

-1

can be classified into three groups,

corresponding to the numbers of N2 ligands which are H-bonded to the H2O ligand. Transitions A0/B0, A1/B1, and A2/B2 are attributed to ν

a

s OH/ν OH fundamentals

of isomers with zero, one, and two H-bonded N2 ligands,

respectively. +

-1

-1

The A0 and B0 bands of TRA -H2O-N2 at 3726 and 3638 cm are blueshifted by 3 cm compared to the A +

+

and B bands of TRA -H2O, and are thus attributed to a TRA -H2O-N2 isomer in which the N2 ligand is π-bonded to the indole ring. The much more intense A1 and B1 transitions observed at 3713 and 3622 cm

-1

-1

are redshifted by -10 and -13 cm from A and B, implying that the N2 ligand is H-bonded to the H2O ligand. From the relative band intensities (taking the effects of the calculated IR oscillator strengths into account), one 8


Page 9 of 44

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can clearly infer that the H isomer is significantly more abundant than the π isomer, suggesting that the H-bond is more stable than the π-bond. +

The A1 and B1 bands of TRA -H2O-(N2)2 at 3715 and 3624 cm

-1

are slightly blueshifted by 2 cm

-1

+

compared to the A1 and B1 bands of TRA -H2O-N2, indicating that they arise from an H/π isomer with one -1

H-bonded and one π-bonded N2 ligand. The A2 and B2 transitions at 3697 and 3617 cm are substantially -1

+

redshifted by -26 and -18 cm from A and B of bare TRA -H2O, suggesting that they have to be attributed to a +

2H isomer with two H-bonded N2 ligands. The A0 and B0 bands in the TRA -H2O-(N2)2 spectrum at 3726 and -1

3637 cm are rather weak, indicating that 2π isomers with two π-bonded N2 ligands have very low abundance. +

-1

The TRA -H2O-(N2)3 spectrum is dominated by the A2 and B2 bands at 3697 and 3618 cm , which are +

almost unshifted from those of the TRA -H2O-(N2)2 spectrum and can thus be assigned to a 2H/π isomer with two H-bonded and one π-bonded N2 ligands. Similarly, the weaker B1 and A1 bands at 3716 and 3623 cm

-1

+

are attributed to H/2π isomers. The A0 and B0 bands are completely absent in the TRA -H2O-(N2)3 spectrum, suggesting that the abundance of 3π isomers with three π-bonded N2 ligands are below the detection limit. In +

summary, the preferred cluster growth of the most stable TRA -H2O-(N2)n clusters begins with H-bonding of +

the first N2 ligands to H2O of the Gpy(in) isomer of TRA -H2O, while subsequently less stable π-bound sites are occupied. Obviously, the exact binding sites of the non H-bonded N2 ligands cannot be derived from the O-H stretch frequencies alone. +

Selected calculated structures and relative stabilization energies of TRA -H2O-(N2)n clusters with n=1-3 are displayed in Figures 7-9 and their calculated IR spectra are compared in Figures S6-S8 in SI. Salient +

structural, vibrational, and energetic parameters for the TRA -H2O-N2 isomers are compiled in Table 2. For n=1, there are two distinguishable H and two different π binding sites depending on whether the ligand and the side chain are on the same or opposite sides of the aromatic plane (denoted top and bottom). Similar possibilities and corresponding combinations apply also to the larger clusters (n=2 and 3). However, the simulated IR spectra are not sensitive to the exact position (top or bottom) of the π- or H-bonded ligands. The +

spectral shifts only indicate whether N2 is H-bonded or π-bonded. As observed for TRA -(N2)n clusters, the π-bonded N2 ligands bind to the pyrrole ring of indole.

52

Clearly, N2 binding to the amino group is predicted to

be significantly less stable (D0=4.7 and 6.0 kJ/mol) compared to H-bonding (D0=7.8 and 7.9 kJ/mol) and π-bonding (D0=7.6 and 8.3 kJ/mol), and can thus be excluded from further considerations (Figure 7). In contrast, H-bonding and π-bonding are calculated to be competitive, with energy differences of ΔD0=0.4 kJ/mol for their most stable structures. Thus, it is not surprising to detect both types of isomers in the molecular beam.

The various H, 2H, and 2H/π isomers are predicted to be most stable for n=1-3 (neglecting zero-point correction, Ee). Indeed, the intensities of the ν

a

OH

+

+

bands for TRA -H2O-N2(H) and TRA -H2O-(N2)3(2H/π) are

by far stronger than those for the other isomers (even when correcting for the IR enhancement predicted for H-bonding), implying that the OH groups of H2O are the most preferential H-binding sites for the N2 ligands. 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

However, the intensity of the ν

a

Page 10 of 44

+

band of the TRA -H2O-(N2)2(2H) isomer is comparable to that for

OH

+

TRA -H2O-(N2)2(H/π). For n=2, six isomers are obtained within ΔE0=1.1 kJ/mol (Figure 8). Among them, four +

+

are classified as TRA -H2O-(N2)2(H/π) while only one isomer is classified as TRA -H2O-(N2)2(2H). Thus, the +

+

number of the isomers classified as TRA -H2O-(N2)2(H/π) is four times larger than that of TRA -H2O-(N2)2(2H). +

In other words, the "configuration entropy" or “statistical weight” or “degeneracy” of TRA -H2O-(N2)2(H/π) +

structures is greater than that of TRA -H2O-(N2)2(2H). Therefore, the ν +

TRA -H2O-(N2)2(H/π) overlap, making the ν

a

OH

a

OH

bands of the isomers classified into

-1

band at 3715 cm to be prominent in the n=2 spectrum.

In Figure 6 the evolution of the measured and calculated O-H stretch frequencies ν

s

OH

(B) and ν

a

OH

(A) are

plotted as a function of the cluster size n by considering all computed structures shown in Figures 7-9. Although the theoretical shifts upon N2 solvation are somewhat overestimated (by roughly a factor of two), the computed pattern shows a clear correspondence with the measured one, thereby confirming the given assignments. Similar pattern recognition procedures have previously been successful in assigning the structures of solvation shells.

79,84-88

The line style indicates the solvation path where zero (dotted), one +

(dashed), and two (solid) N2 ligands are bonded to TRA -H2O. In line with the experimental observations, π-bonded N2 ligands induce minor blueshifts in ν

s

OH

and ν

a

OH,

while H-bonding induces substantial redshifts.

In the N-H stretch range of the amino group, the asymmetric N-H stretch mode (band F) is observed at ν

a

NH2=3409±1

-1

+

cm for TRA -H2O-(N2)n independent of n. This value is in good agreement with an assignment +

-1

to the Gpy(in) isomer of TRA as aromatic core, with predicted frequencies in the range of 3409-3416 cm for the various isomers of the n=0-3 cluster sizes (Tables 1 and 2). The corresponding symmetric N-H stretch -1

mode (band G) is somewhat weaker in intensity, and tentatively observed around 3345 cm only in the n=1 -1

and n=2 spectra, again in good agreement with the corresponding calculated frequencies of 3333-3339 cm . Although weak in IR activity, these modes are sensitive to the side chain conformation and thus can be used +

to exclude all other TRA isomers (Figure 4 and S3 and Table T1 in SI). Although the C-H stretch bands of the side chain are sensitive to its conformation (Figure 4 and S3 in SI), their strong coupling with the intense ν

b

NH

band induces strong deviations from the linear IR absorption spectra. Consequently, the many bands Y1-Y7 cannot be used for further confirmation of the side chain structure suggested from the νNH2 transitions. Finally, -1

+

the interpretation of the very weak band X observed near 3503 cm in the TRA -H2O-(N2)n spectra with n=1-3 +

is less obvious. All fundamental frequencies in the fingerprint range of TRA -H2O-(N2)n are predicted below -1

1700 cm , i.e. an assignment to a first overtone is not possible and its interpretation remains open. +

+

Based on the IRPD spectra and the calculations of TRA -H2O-(N2)n, the indolic NH group of TRA and the OH groups of H2O are the preferential H-bonding sites for H2O and N2 ligands. Thus, the H and 2H isomers form a linear or branched H-bonded network. H-bonded networks generally show a cooperative enhancement of the strength of each H-bond. The NBO analysis is applied to quantify the cooperative effect of the H-bonds in the H and 2H isomers. In the NBO model,

81

the interaction strength of conventional σ-type A-H...B H-bonds

is correlated with the donor-acceptor charge transfer interaction from the lone pair orbital of the B atom of the H-bond acceptor to the antibonding σ* orbital of the A-H donor bond. This charge transfer interaction in 10


Page 11 of 44

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A-H...B is evaluated using the second-order perturbative energy, E B and the σ* orbital of the A-H bond, respectively. The E

(2)

i→j*

(2)

i→j*,

where i and j* denote the lone pair of

energies between the indolic N-H antibonding +

orbital and the lone pair of O in H2O are 82.8, 93.0, 102.8, and 105.2 kJ/mol for TRA -H2O and the Htop, 2H, +

and 2H/πtop isomers of TRA -H2O-(N2)1-3, and the corresponding orbitals are visualized in Figure S9 in SI. Thus, E

(2)

i→j*

of the NH...O H-bond increases by 10.2 kJ/mol through the H-bond of the first N2 ligand to H2O

and by almost the same amount (9.8 kJ/mol) through the H-bond with the second N2. On the other hand, the attachment of the third π-bonded N2 ligand does not increase E +

2π isomers of TRA -H2O-(N2)1,2, E

(2)

i→j*

(2)

i→j*

significantly (by 2.4 kJ/mol). For the π and

amounts to 84.3 and 84.1 kJ/mol, i.e. the effects of π ligands on the

NH...O bond are minor. In fact, the small blueshifts of a few cm

-1

observed for π stacking with N2 ligands

indicate that this type of bonding is overall noncooperative and destabilizing for the NH...O bond. In Figure 10, the E

(2)

i→j*

+

energies of TRA -H2O, Htop, 2H, and 2H/πtop are plotted as a function of the NH...O bond length and

a linear correlation is found. Thus, the H-bonds between N2 and H2O cooperatively enhance the neighboring NH...O bond, although the quadrupolar N2 ligands form only weak H-bonds. This cooperative effect is also visible in the NH...O bond length itself, which contracts from 1.754 to 1.727, 1.705, and 1.698 Å for n=0-3, respectively. At the same time, the calculated ν

b

NH

frequency decreases as 3103, 3062, 3007, and 2999 cm

-1

along the same series. Another view of the cooperativity is connected to the proton affinity (PA). The PA of +

H2O-N2 is substantially larger than that of H2O (by the difference of the binding energies of H3O -N2 and H2O-N2). As a consequence, the H-bonded H2O-N2 solvent interacts stronger with the indolic NH proton than bare H2O leading to a stronger NH…O bond. The structure of the H2O-N2 complex has been characterized by microwave spectroscopy

89

and quantum chemical calculations,

90

yielding a nearly linear H-bonded global

minimum with an OH…N2 bond distance of 2.35 Å and a dissociation energy of De=5.1 kJ/mol. Our ωB97X-D/cc-pVTZ calculations yield 2.42 Å, De=4.9, and D0=2.0 kJ/mol. Hence, the presence of the positive +

charge in TRA -H2O-N2 has a strongly stabilizing effect on the OH…N2 bond (2.14 Å, D0=7.9 kJ/mol), again +

illustrating the cooperativity of H-bonding in the TRA -H2O-N2 solvation network. +

3.3. TRA -(H2O)2-(N2)n +

+

The IRPD spectra recorded for TRA -(H2O)2 and TRA -(H2O)2-(N2)2 are compared in Figure 11. These +

spectra can fully be interpreted by TRA -(H2O)2 structures, in which a H-bonded (H2O)2 dimer is attached to +

the indolic NH group of TRA , i.e. the H2O ligands form a H-bonded solvent network. One of these structures is exemplarily shown in Figure 12 and used for the assignment of the experimental spectrum in Figure 11. There are two distinguishable structures because the OH groups of the first H2O ligand are not equivalent. However, the spectral and energetic properties of both isomers are quite similar and cannot be distinguished +

by experiment. Similar to TRA -H2O, in this m=2 structure the (H2O)2 has little interaction with the ethylamino +

+

side chain of TRA , which keeps the Gpy(in) conformation. Comparison with the TRA -H2O spectrum in Figure 1 reveals substantial changes in both the O-H and indolic N-H stretch ranges, clearly indicating that the second H2O ligand is attached to the first one. Hence, isomers in which two individual H2O ligands are +

separately attached to TRA at different binding sites (interior ion solvation) are below the detection limit and -1

not considered further. Bands A and B at 3737 and 3646 cm are attributed to the ν 11


a

OH

and ν

s

OH

bands of the


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 44

-1

terminal H2O ligand, in good agreement with the predicted values of 3747 and 3655 cm . These frequencies -1

+

are experimentally blueshifted by 15 and 11 cm from those of TRA -H2O, because the positive charge is -1

further away in the m=2 cluster. The transitions C and D observed at 3701 and 3421 cm appear newly in the f

m=2 spectrum and are assigned to the free and bound O-H stretch modes (ν OH and ν -1

ligand predicted at 3716 and 3366 cm . The ν

b

NH

activity (2152 km/mol) is redshifted by -187 cm

+

b

OH)

of the first H2O -1

band predicted for TRA -(H2O)2 at 2916 cm with huge IR

-1

+

from that of TRA -H2O and not readily identified in the

experimental spectrum due to the interaction with many of the C-H fundamentals and the resulting strong intensity loss. As the calculations underestimate the ν

b

NH

frequency (see Figure 4 for m=1), we tentatively -1

assign it to the strongest transition in this spectral range observed at 2964 cm , with an incremental redshift -1

of -203 cm , in good agreement with the prediction. +

The total binding energy of the calculated TRA -(H2O)2 structure amounts to 103.4 kJ/mol. Due to strong cooperative effects arising from the substantially increased proton affinity of (H2O)2 as compared to H2O (808 and 690 kJ/mol)

91

the NH…O bond is much stronger and shorter in the m=2 than in the m=1 cluster (1.664

and 1.754 Å). Thus, the N-H bond elongation and corresponding total N-H stretch shifts are correspondingly larger (ΔRNH=32 and 21 mÅ, Δν

b

NH=-526

-1

and -339 cm ). In parallel, the presence of the positive charge in

+

TRA has a huge stabilizing effect on the H-bond in (H2O)2, which contracts from 1.937 Å in free (H2O)2 to +

1.747 Å in TRA -(H2O)2. This H-bond gives rise to the characteristic intense ν +

b

OH

band, whose frequency in

-1

TRA -(H2O)2 is much lower than in the bare water dimer (3421 and 3601 cm ). +

N2 tagging of TRA -(H2O)2 with two N2 ligands causes narrowing and splittings of the O-H stretch transitions. The N2 ligands may occupy different binding sites, such as the three remaining free OH groups of +

the (H2O)2 and the π-binding sites of TRA . In an attempt to assign the O-H stretch bands, calculated structures, binding energies, and IR spectra of selected isomers are presented in Figure S10 in SI, including those with two π-bonded ligands (2π), one π-bonded ligand and the other one H-bonded either to the inner (H1/π) or terminal H2O molecule (H2/π), and two H-bonded ligands (H1/H2 and 2H2). Only one specific isomer of each isomer class is shown. All these isomers have the same energy to within ΔEe=1.1 kJ/mol (ΔE0=2.8 kJ/mol), and thus may be populated in the supersonic expansion. The spectral assignments suggested by the calculations are given in Table 1. +

3.4. TRA -(H2O)3-(N2)n +

The IRPD spectrum of TRA -(H2O)3 differs largely from that of the m=2 cluster in the O-H and N-H stretch range (Figure 1), indicating that the third H2O ligand is extending the water solvation network rather than +

binding to TRA at a different binding site. Two types of structures are obtained by adding a third H2O ligand +

either to the first or the terminal ligand of TRA -(H2O)2, resulting in a chain-like (I) or a branched solvation network (II), in which a linear or branched water trimer is attached as proton acceptor to the indolic NH group (Figure 12). Both isomers have essentially the same total binding energy (141-142 kJ/mol) and their computed IR spectra are compared in Figure 13. 12


Page 13 of 44

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Attachment of the third H2O ligand in isomer II with symmetric solvation further strengthens and shortens the NH…O H-bond (1.589 Å), leading to a larger N-H bond elongation (ΔRNH=47 mÅ) and frequency redshift (Δν

b

NH=-754

-1

-1

cm ). Its predicted position at 2688 cm is outside the spectral range investigated. Both OH…O

H-bonds in the solvent part of isomer II (1.795 Å) are much shorter than in the free (H2O)2 dimer (1.937 Å) but +

longer than in TRA -(H2O)2 (1.747 Å). The symmetric and asymmetric bound O-H stretch modes are predicted with a relatively small splitting as intense transitions at ν substantially blueshifted from ν

b

OH

bs

OH=3423

and ν

ba

OH=3468

-1

cm , i.e.,

-1

of m=2 (3366 cm ). The corresponding free modes at 3751 and 3657 cm

-1

are much less affected and only slightly blueshifted from those of m=2.

In the chain-like isomer I with asymmetric solvation, attachment of the third H2O ligand has a much smaller impact on the strength of the NH…O bond (1.631 Å), with smaller N-H bond elongation (ΔRNH=38 mÅ) and frequency redshift (Δν

b

NH=-614

-1

cm ). Both H-bonds in the (H2O)3 trimer are not equivalent and shorter (1.673

and 1.780 Å) than in the free dimer. The corresponding bound O-H stretch bands are calculated as ν -1

f

b

OH=3206 -1

and 3414 cm . The two free O-H stretch bands are similar in frequency, ν OH=3722 and 3721 cm , and appear between the symmetric and antisymmetric free O-H stretch fundamentals of the terminal H2O ligand, ν

s/a

OH=3657/3751

-1

cm . While there is only a single isomer II global minimum on the potential (for the same

side-chain conformation), several equivalent and also slightly different isomer I type structures are possible +

(Figure 12), depending on the exact connectivity of the H2O ligands to TRA . As these are all similar in energy and have essentially the same IR spectrum, we consider in the following isomer Ia shown in Figure 12 as a representative isomer I structure in the figures and tables. +

Comparison of the IR spectra of isomer I and II with the IRPD spectra of TRA -(H2O)3 and +

TRA -(H2O)3-(N2)2 suggests the presence of both classes of isomers in the molecular beam. This is expected, because both have similar stabilization energies. The N2-tagged clusters are much colder and show better resolved spectra. Bands C and D3 are characteristic for isomer I, clearly confirming the presence of this type of isomer. The broader features D and D1/D2 are in line with the presence of isomer II. A detailed assignment +

+

of the observed transitions of TRA -(H2O)3 is listed in Table 1. Calculations for TRA -(H2O)3-(N2)2 yield a plethora of different classes of isomers with similar energies and IR spectra, which prevents a reliable assignment of the observed transitions to individual isomers.

3.5. Cluster growth and comparison to neutral TRA-(H2O)m +

The IRPD spectra and quantum chemical calculations of TRA -(H2O)m provide detailed information about the sequential cluster growth and initial steps of microhydration around this prototypical aromatic neurotransmitter cation. The preferential hydration motif is attachment of a H-bonded (H2O)m solvent network to the indolic NH group, leading to linear and branched structures for m≤3. These solvation structures benefit from the large cooperativity of H-bonding of the solvent. In contrast, structures corresponding to interior 13



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Page 14 of 44

aromatic ion solvation, in which individual H2O ligands bind to the various attractive binding sites of the TRA

+

cation core are substantially less stable and not observed. The calculated incremental binding energies in the +

most stable TRA -(H2O)m clusters, ΔD0(m) = D0(m)-D0(m-1), decrease as ΔD0=58.5, 44.9, and 38.2 kJ/mol for +

m=1-3. In all IRPD experiments of the untagged TRA -(H2O)m clusters, only the elimination of a single H2O +

ligand is detected, consistent with these calculated binding energies. In the computed TRA -(H2O)m clusters, the side chain conformation is little affected by the microhydration process, because solvation occurs at the indolic NH group. Unfortunately, for the m=2 and 3 clusters, the weak ν

s/a

NH2

side chain conformation, are blended by the rather intense and broad ν

b

bands, which are sensitive to the

OH

transitions of the water solvent

network. +

The indolic N-H bond properties reveal that the solvent molecules in TRA -(H2O)m increasingly attract the acidic N-H proton as m increases. Hence, the ν of 3442, 3103, 2916, and 2828 cm

-1

b

NH

frequency rapidly decreases with m, with calculated values

for m=0-3 (assuming isomer Ia for m=3). Similarly, the N-H bond

gradually elongates from 1.007 Å (m=0) to 1.045 Å (m=3), while the intermolecular NH…O bond gets stronger (1.754, 1.664, and 1.631 Å for m=1-3). The increasing cooperativity as a function of m is also visible in the E →j*

(2)

i

energies of the NH...O H-bond, which increases almost linearly with m, with values of 82.8, 122.3, and

170.4 kJ/mol for m=1-3, respectively, in essentially linear correlation with the NH…O bond length (Figure 10). This overall trend is compatible with the increasing proton affinity of the water solvent cluster as it increases in size. Nonetheless, at the largest size investigated both spectroscopically and computationally (m=3), the +

indolic NH proton is still largely connected to TRA , as the N-H bond length (1.045 Å) is still much shorter than +

the adjacent NH…O H-bond length (1.631 Å). There is no sign of a proton transfer from TRA to the solvent network for m≤3, and probably many more solvent molecules are required for this process to occur. As the ionization energy of TRA (~7.5 eV)

42

is much lower than that of (H2O)m (>10 eV),

92, 93

the positive charge is

+

largely localized on TRA, justifying the notation of TRA -(H2O)m. Nonetheless, as the ionization energy of (H2O)m clusters decreases as a function of m, there is a monotonic increase in the charge transfer and charge +

delocalization from TRA to the solvent cluster. The NBO charge distribution detailed in Figure S11 in SI with Δq=37, 55, and 63 me for m=1-3. As the charge transfer per solvent molecule decreases with m, the individual solvent molecules become more neutral so that both the free and bound O-H stretch frequencies tend to shift to the blue with increasing cluster size (bands A-D in Figure 1). +

Significantly, while the TRA -Lm clusters with polar protic ligands (L=H2O) prefer the formation of a +

H-bonded solvent network, corresponding IRPD spectra and calculations show that TRA -Lm clusters with aprotic hydrophobic ligands (L=N2) prefer interior ion solvation,

52

because they cannot develop strongly

+

H-bonded solvation networks. Hence, microsolvation in TRA -(N2)n clusters also begins with solvation of the indolic NH proton (n=1 and 2) but is followed by further solvation at the π binding sites. +

52

This situation is

similar for N2 solvation of TRA -H2O, in which the first the two H-bonding OH sites are occupied before less stable π stacking sites are populated.

14


Page 15 of 44

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+

It is instructive to compare the properties of ionic TRA -(H2O)m clusters with those of neutral TRA-(H2O)m to determine the effects of the positive charge on the microhydration process. Ionization into the cationic D0 state is accomplished by removal of a bonding π electron delocalized over the indole moiety (Figure S11 in SI), f

and thus weakens the N-H bond, as also seen by the substantial drop in the ν NH frequency from 3524 to 3447 -1

cm upon ionization.

29, 52

As a consequence, ionization of TRA drastically enhances the acidity of the indolic (+)

NH proton and changes the preferred microhydration motifs in TRA -(H2O)m. While in TRA-H2O (m=1) the H2O ligand acts as a proton donor in a H-bond to the amino group, HOH…N, this configuration becomes repulsive in the cation and the H2O ligand prefers the much more favorable indolic NH site, where it binds as a proton acceptor in a favorable charge-dipole configuration, NH…OH2. The NBO charge distribution detailed in Figure S11 in SI illustrates that the N atom of the NH2 group in neutral TRA carries high negative charge (-843 me) and thus acts as a proton acceptor for H-bonding with H2O in the S0 state. Ionization of the π orbital of the indole ring implies that the former neutral ring (-24 me) becomes highly positively charged (+894 me), and thus the most positively indolic N atom (+438 me) can act as strong proton donor for H-bonding with H2O in the D0 state. Hence, ionization triggers a NH2→NH isomerization reaction of the H2O ligand, which has been detected by IR and photoionization spectroscopy.

42, 43

For larger TRA-(H2O)m clusters with m=2 and 3, IR

spectra suggest the formation of H-bonded (H2O)m bridges between the indolic NH group and the amino group in the side chain, which strongly benefit from the facile conformational changes of the flexible side chain.

40

+

In the TRA -(H2O)m cation clusters, however, the strongly directional linear ionic H-bonds between

the indolic NH group and (H2O)m lead to linear and branched solvent architectures with little interaction between the solvent network and the distant ethylamino side chain. Thus, similar to the m=1 cluster, ionization of the larger TRA-(H2O)m clusters causes a drastic reorganization of the hydration structure, which are then interesting targets for time-resolved experiments planned in the near future to probe the solvent reorganization in real time by picosecond IR spectroscopy.

3.6. Comparison with indole

20, 21

+

+

It is instructive to compare the properties of solvated TRA clusters with those of the corresponding indole clusters

57,82,94

+

to extract the subtle effects of the ethylamino side chain on the acidity of the indolic NH proton. +

+

The N-H bond in indole -H2O is slightly more acidic than that in TRA -H2O (1.031 and 1.028 Å) and is thus a better proton donor in the NH…O H-bond with H2O, with calculated bond lengths of 1.732 and 1.754 Å and bond dissociation energies of De/D0=70.0/63.2 and 65.0/58.5 kJ/mol. measured dissociation energy of 4790±10 cm measured ν

b

NH

-1

(57.3±0.1 kJ/mol).

+

94

The former value agrees well with the As a result of the stronger bond, the

+

-1

frequency of indole -H2O is lower than that in TRA -H2O (3128 and 3167 cm ), in agreement -1

with the calculated values of 3069 and 3103 cm , respectively. Unfortunately, there are no studies available +

+

for indole -(H2O)m with m≥2 and any indole -(H2O)m-(N2)n complexes for comparison.

15



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Page 16 of 44

4. Concluding remarks

The initial steps of microhydration of a prototypical aromatic ethylamino neurotransmitter cation have been +

determined by IRPD spectra of size-selected TRA -(H2O)m clusters with m≤3 and dispersion-corrected density functional theory calculations at the ωB97X-D/cc-pVTZ level. Tagging of these clusters with N2 ligands produces colder clusters, which exhibit higher spectral resolution and provide insight into the solvation of +

microhydrated TRA -(H2O)m clusters by hydrophobic quadrupolar ligands. All clusters are produced in an electron impact cluster ion source, which produces predominantly the most stable isomers. The preferred cluster growth begins with hydration of the indolic NH proton by the first H2O ligand (m=1), which then acts as a proton donor for further H2O ligands (m=2 and 3), leading to the formation of a H-bonded solvation (H2O)m network. This network is strongly stabilized by cooperative effects, which are quantified by the NBO analysis. Interior ion solvation, in which individual H2O ligands are solvating the aromatic cation at different binding sites are not observed. Although the NH proton gradually shifts toward the (H2O)m cluster as m increases, there is no proton transfer to solvent observed in the size range investigated. Similarly, small partial charge +

transfer from TRA to the solvent network increases monotonically with cluster size m, although the positive +

charge remains largely on the TRA cation due to the disparity in the ionization potential. The cluster growth of +

+

TRA -(H2O)m is qualitatively different from that of TRA -(N2)n, indicating the difference of microsolvation of indolic neurotransmitter cations in a protic dipolar (hydrophilic) and quadrupolar (hydrophobic) solvent. +

+

Comparison between TRA -H2O and indole -H2O reveals that the substitution of the ethylamino side chain slightly reduces the acidity of the indolic NH group leading to weaker H-bonds to the solvent. The preferred N2 +

solvation of TRA -H2O begins with solvation of the OH groups of H2O before less stable π stacking sites are occupied. Such N2 solvation scenarios have previously been observed for related monohydrated aromatic cations,

48,51

and suggest that H-bonding of N2 to H2O is somewhat stronger than π stacking to the positively

charged aromatic ion. Apparently, microhydration and subsequent N2 solvation of the indolic NH proton has +

little impact on the conformation of the ethylamino side chain in TRA .

The additional interactions arising from the excess positive charge imply that the topology of the +

interaction potential in TRA -(H2O)m is quite different from that of the neutral TRA-(H2O)m clusters with respect to both the interaction strength and the geometries of the global minima. As a consequence, photoionization of neutral TRA-H2O triggers an NH2→NH isomerization reaction, in which the H2O ligand initially bound as proton donor to the NH2 group migrates upon ionization to the indolic NH site, where it binds as a proton acceptor.

43

Similarly, the structures of the larger neutral and ionic hydrates are quite different. While in neutral

TRA-(H2O)m clusters with m=2 and 3, the H2O molecules form a H-bonded bridge between the NH2 and NH +

groups of TRA, such configurations are not favorable in the TRA -(H2O)m cations, in which the strongly linear H-bonded solvent network is attached to the NH group with essentially no interaction with the NH2 group. It will be interesting to study the dynamics and energetics of these ionization-induced solvent migration and rearrangement reactions in real time by picosecond time-resolved vibrational and electronic spectroscopy as a function of the ionization excess energy, the degree of hydration, and the isomeric structure. experiments are currently underway. 16


20-26

Such

Page 17 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Acknowledgements This research was supported by the German-Israel Foundation for Scientific Research and Development (no. 1164-158.5/2011), Deutsche Forschungsgemeinschaft (DO 729/4), the JSPS core-to-core program (22003) and the Grants-in-Aid for Scientific Research C (no. 25410022), Scientific Research B (no. 26288010), Scientific Research on Innovative Area (no. 26104527). We thank Ilana Bar and Masaaki Fujii for fruitful discussions. +

+

Supporting Information Available: (1) IR spectrum of indole -H2O, (2) energies of TRA -H2O isomers, (3) +

+

+

IR spectra of TRA -H2O isomers, (4) IR spectra of TRA -N2, (5) IR spectra of TRA -H2O(-N2), (6) IR spectra of +

+

+

TRA -H2O-N2 isomers, (7) IR spectra of TRA -H2O-(N2)2 isomers, (8) IR spectra of TRA -H2O-(N2)3 isomers, +

(9) NBO orbitals and energies, (10) IR spectra of TRA -(H2O)2-(N2)2 isomers, (11) NBO charge distributions. This material is available free of charge via the Internet at http://pubs.acs.org.

17



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Page 18 of 44

Table 1. Positions, widths (FWHM, in parentheses), and suggested vibrational and isomer assignments of the +

vibrational transitions observed in the IRPD spectra of TRA -(H2O)m-(N2)n compared to frequencies of the most stable isomers calculated at the ωB97X-D/cc-pVTZ level. molecule/cluster

ν exp/cm-1

H 2O

3756

a

3657

a

3745

a

3735

a

3654

a

3601

a

3447

a

(H2O)2

TRA

+ +

TRA -H2O

ν

a s

ν calc/cm-1 b

OH

3757 (56)

OH

3658 (5)

a

OH

3745 (72)

ν OH

3726 (68)

s

3649 (12)

ν

f

ν ν

OH

b

OH

f

isomer

3442 (169)

Gpy(in)

a

OH

3738 (117)

Gpy(in)

OH

3648 (51)

Gpy(in)

NH2

3409 (11)

Gpy(in)

NH

3103 (1396)

Gpy(in)

B 3635 (12)

ν

F 3410 (~20)

ν

E 3167 (~60)

ν

s

a b

FR

c

Gpy(in)

FR

c

Gpy(in)

FR

c

Gpy(in)

FR

c

Gpy(in)

Y7 3076

FR

c

Gpy(in)

A0 3726 (6)

ν

Y5 3146 Y6 3107

A1 3713 (5)

ν

B0 3638 (6)

ν

a

s

B1 3622 (5)

ν

X 3503 (4)

?

F 3409 (4)

ν

G 3346 (~6)

E 3065 (19)

Y1 3149 Y2 3137 Y3 3098 Y4 3085 Y5 2997 Y6 2974 Y7 2919

ν

ν

OH

3739 (117)

Gpy(in) πtop

OH

3718 (173)

Gpy(in) Htop

OH

3648 (51)

Gpy(in) πtop

OH

3605 (271)

Gpy(in) Htop

3410 (11)

Gpy(in) Htop

3416 (10)

Gpy(in) πtop

3333 (9)

Gpy(in) Htop

3339 (8)

Gpy(in) πtop

3062 (1509)

Gpy(in) Htop

3104 (1072)

Gpy(in) πtop

a s

a

s

d

3550 (298)

ν NH ν

Y2 3175

TRA -H2O-N2

ν

A 3723 (14)

Y1 3183

+

vibration

NH2

NH2

b

NH

FR

c

Gpy(in)

FR

c

Gpy(in)

FR

c

Gpy(in)

FR

c

Gpy(in)

FR

c

Gpy(in)

FR

c

Gpy(in)

FR

c

Gpy(in) 18


Page 19 of 44

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+

TRA -H2O-(N2)2

A0 3726 (9) A1 3715 (4) A2 3697 (8) B0 3637 B1 3624 (6)

ν

s s

?

F 3409 (4)

ν

Y2 3132 Y4 3089 Y5 2989 Y7 2920 A1 3716 (5) A2 3697 (8) B1 3623 (5)

ν ν

s

3648 (51)

Gpy(in) 2H Gpy(in) 2π

OH

3606 ±1 (262 ±30)

OH

3596 (239)

NH2

NH2 NH

Gpy(in) H/π Gpy(in) 2H

e

3412 ±3 (10 ±1) 3336 ±3 (9 ±1)

e

Gpy(in)

e

Gpy(in)

3007 (1881)

Gpy(in) 2H

3064 ±5 (1526 ±74)

Gpy(in) H/π

3105 (905)

Gpy(in) 2π Gpy(in)

FR

c

Gpy(in)

FR

c

Gpy(in)

FR

c

Gpy(in)

FR

c

ν ν ν

a a s s

E 3056 (21)

ν

A 3737 (4)

OH

Gpy(in) H/π

FR

?

Y7 2925

3670 (455)

e

c

X 3504 (4)

Y5 2996

Gpy(in) 2π

OH

b

ν

Y4 3085

3739 (116) 3718 ±3 (176 ±8)

a

B2 3618 (4)

Y2 3133

TRA -(H2O)2

ν

s

OH OH

a

X 3503 (3)

Y1 3141

+

ν

a

ν

E 3062 (28)

TRA -H2O-(N2)3

ν

a

B2 3617 (7)

G 3344 (6)

+

ν

OH OH

OH OH

b

NH

Gpy(in) 3720 ±1 (178 ±4)

e

3671 ±1 (470 ±17)

Gpy(in) H/2π e

Gpy(in) 2H/π

3606 (256 ±23)

e

Gpy(in) H/2π

3595 (225 ±14)

e

Gpy(in) 2H/π

3004 ±5 (1842 ±18)

Gpy(in) 2H/π

3060 ±1 (1565 ±35)

Gpy(in) H/2π

FR

c

Gpy(in)

FR

c

Gpy(in)

FR

c

Gpy(in)

FR

c

Gpy(in)

a

OH

3747 (99)

Gpy(in)

C 3701 (13)

ν OH

3716 (95)

Gpy(in)

B 3646 (20)

ν

s

3655 (29)

Gpy(in)

X 3503 (9)

?

D 3421 (35)

ν

OH

3366 (834)

Gpy(in)

NH2

3333 (8)

Gpy(in)

2916 (2152)

Gpy(in)

G 3353 (7) E 2964

ν

f

ν ν

OH

b s

b

NH

Y1 3295

Gpy(in)

Y2 3253

Gpy(in)

Y3 3132

Gpy(in)

Y4 3054

Gpy(in) 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Y5 3001

Gpy(in)

Y6 2884 +

TRA -(H2O)2(N2)2

A1 3744 (6)

A2 3727 (6)

Gpy(in) ν

ν

a

a

OH

OH

3746 (98)

Gpy(in) 2π

3748 (96)

Gpy(in) H1/π

3727 (160)

Gpy(in) H2/π

3728 (159)

Gpy(in) H1/H2

3718 (88) C1 3704 (5)

TRA -(H2O)3

f

ν OH

B1 3649 (5)

s

ν

ν

?

D1 3414 (24)

ν

A 3733 (11) C1 3714 (14) C2 3705 (8)

ν

ν

OH

b

b

a

OH

OH

3718 (99)

Gpy(in) 2π

3717 (89)

Gpy(in) H2/π

3696 (347)

Gpy(in) 2H2

3670 (309)

Gpy(in) H1/π

3673 (310)

Gpy(in) H1/H2

3654 (29)

Gpy(in) 2π

3656 (37)

Gpy(in) H1/π

3624 (165)

Gpy(in) H2/π

3626 (151)

Gpy(in) H1/H2

3619 (141)

Gpy(in) 2H2

3365 (783)

Gpy(in) 2π

3385 (813)

Gpy(in) H1/π

3325 (973)

Gpy(in) H2/π

3351 (934)

Gpy(in) H1/H2

3275 (1076)

Gpy(in) 2H2

3751 ±1 (93 ±38)

Gpy(in) (I+II)

f

3722 ±1 (81 ±2)

Gpy(in) (I)

f

3722 ±1 (81 ±2)

Gpy(in) (I)

f

OH

ν OH ν OH ν OH

3722 ±1 (81 ±2)

Gpy(in) (I)

B 3645 (~15)

s

3657 ±1 (22 ±5)

Gpy(in) (I+II)

OH

3468 (1011)

Gpy(in) (II)

OH

3423 (496)

Gpy(in) (II)

OH

3414 (569)

Gpy(in) (I)

OH

3206 (1242)

Gpy(in) (I)

ν ν ν ν ν

TRA -(H2O)3(N2)2

OH

Gpy(in) 2H2

C3 3692 (10)

D ~3400 (broad)

+

s

X 3503 (11)

D2 3372 (45)

+

f

ν OH

C2 3679 (6)

B2 3639 (3)

Page 20 of 44

A 3734 (4) C1 3716 (4) C2 3706 (5) C3 3691 (6)

OH

ba bs b b a

OH

Gpy(in) (I+II)

f

ν OH

Gpy(in) (I+II)

f

Gpy(in) (I+II)

f

Gpy(in) (I+II)

f

ν

ν OH ν OH

C4 3676 (10)

ν OH

Gpy(in) (I+II)

B 3623 (~20)

s

Gpy(in) (I+II)

D1 3402 (78)

ν ν

OH

b

Gpy(in) (I+II)

OH

20


Page 21 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


D2 3349 (66)

ν

D3 3160 (30) a

ν

b b

Gpy(in) (I+II)

OH

Gpy(in) (I)

OH

+

+

Frequencies measured for bare TRA (approximated by π-bound TRA -N2) H2O, and (H2O)2 taken from Ref.

52 and 95-97. b

IR intensities in km/mol are listed in parentheses.

c

FR = Fermi resonance and/or mode mixing of ν

b

NH

with overtone/combination bands and/or C-H stretch

modes. d

If not stated otherwise, only the most stable isomer of an isomer family is listed.

e

Average values for all considered isomers.

21



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Page 22 of 44

+

+

Table 2. Selected geometrical parameters, vibrational frequencies, and binding energies of H2O, the Gpy(in) conformer of TRA and its TRA -H2O-N2 clusters (Figure 7) evaluated at the ωB97X-D/cc-pVTZ level. Parameter

b

RNH/Å

TRA

+

a

H2 O

1.0072

ROH/Å

0.9569

+

+

+

+

+

+

+

TRA -H2O

TRA -H2O-N2(Htop)

TRA -H2O-N2(Hbottom)

TRA -H2O-N2(π top)

TRA -H2O-N2(π bottom)

TRA -H2O-N2(NH2bottom)

TRA -H2O-N2(NH2top)

1.0279

1.0308

1.0308

1.0280

1.0278

1.0278

1.0278

0.9583/0.9583

0.9620/0.9578

0.9578/0.9621

0.9583/0.9582

0.9583/0.9583

0.9583/0.9583

0.9583/0.9583

RNH…O/Å

1.754

1.727

1.727

1.748

1.754

1.755

1.754

θNHO/deg

170.1

169.7

169.5

169.7

170.3

170.3

170.4

τNHOH/deg

102.5

104.6

103.4

87.4

103.5

102.7

103.5

θ1/degb

114.8

114.8

114.9

114.8

111.0

114.9

114.9

114.8

τ1/degb

1.0

1.0

1.0

1.0

-3.8

0.5

1.1

1.1

τ2/degb

-145.2

-142.9

-143.3

-141.8

-102.3

-144.8

-144.1

-144.7

νaOH /cm-1

3757(46)

3738 (117)

3718 (173)

3718 (175)

3739 (117)

3738 (116)

3738 (116)

3739 (116)

νsOH/cm-1

3658 (5)

3648 (51)

3605 (271)

3606 (272)

3648 (51)

3648 (51)

3648 (51)

3648 (51)

νf/bNH/cm-1

3442 (169)

3103 (1396)

3062 (1509)

3063 (1563)

3104 (1072)

3103 (1393)

3104 (1411)

3104 (1394)

νaNH2/cm-1

3409 (13)

3409 (11)

3410 (11)

3410 (11)

3416 (10)

3409 (11)

3410 (15)

3409 (27)

νsNH2/cm-1

3334 (13)

3334 (10)

3333 (9)

3334 (9)

3339 (8)

3333(10)

3335 (13)

3333 (19)

58.5 (4894)

66.5 (5558)

66.3 (5545)

66.8 (5584)

66.1 (5529)

64.5 (5396)

63.3 (5287)

D0/kJ mol a

-1 c

IR intensities in km/mol are given in parentheses.

b

θ1 = θC3C8C9, τ1 = τC4C3aC3C8, τ2 = τC3aC3C8C9.

c

Numbers in cm are listed in parentheses.

-1

22


Page 23 of 44

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Figure captions +

+

Figure 1. IRPD spectra of TRA -(H2O)m with m=1-3 recorded in the TRA -(H2O)m-1 fragment channel. The positions, widths, and vibrational and isomer assignments of the transitions observed are listed in Table 1. For +

+

comparison, the IRPD spectrum of TRA -N2 recorded in the TRA channel is also shown. indicate the positions of the ν +

s

OH

and ν

a

OH

-1

52

The arrows f

modes of bare H2O (3657 and 3756 cm ) and the indolic ν NH mode

-1

of bare TRA (3447 cm ). +

+

Figure 2. IRPD spectra of TRA -H2O-(N2)n with n=0-3 recorded by monitored in the TRA fragment channel of +

(b, n=0) or the TRA -H2O channel (c-e, n=1-3, indicated as n→0), respectively. The spectra (b-e) are +

recorded for clusters generated in the electron impact (EI) source. The IRPD spectrum of TRA -H2O by resonant two-photon ionization (R2PI) is shown for comparison (a).

43

The positions of the transitions

observed are listed in Table 1, along with their vibrational and isomer assignments. The arrows indicate the positions of the ν +

s

OH

and ν

a

OH

-1

f

modes of bare H2O (3657 and 3756 cm ) and the indolic ν NH mode of bare

-1

TRA (3447 cm ). +

Figure 3. Most stable structures of TRA -H2O obtained at the ωB97X-D/cc-pVTZ level. Relative stabilization energies (E0) and intermolecular binding energies (D0) are given in kJ/mol. H-bond lengths (RNH...O) are given in Å. +

Figure 4. IRPD spectrum of TRA -H2O (a) compared to linear IR absorption stick spectra of the three most stable structures (b-d) evaluated at the ωB97X-D/cc-pVTZ level (Figure 3). The intensities of the indolic N-H stretch fundamentals (ν

b

NH)

-1

predicted at ~3100 cm in (b-d) are multiplied by 0.1. +

Figure 5. IRPD spectra of TRA -H2O-(N2)n with n=0-3 in the vicinity of the free N-H and O-H stretch fundamentals (recorded with the OPO laser power optimized in this spectral range). The positions of the transitions observed are listed in Table 1, along with their vibrational and isomer assignments. The arrows indicate the positions of the ν +

s

OH

and ν

a

OH

-1

f

modes of bare H2O (3657 and 3756 cm ) and the indolic ν NH mode

-1

of bare TRA (3447 cm ).

Figure 6. Experimental (black, left) and calculated (blue, right) ν

s

OH

(B) and ν

a

OH

(A) frequencies of

+

TRA -H2O-(N2)n as a function of the cluster size n. All structures displayed in Figures 7-9 are included. The line style indicates the solvation path where zero (dotted), one (dashed), and two (solid) N2 ligands are +

bonded to TRA -H2O. The arrows indicate the positions of the ν

s

OH

and ν

a

OH

modes of bare H2O (Table 1).

+

Figure 7. Stable structures of TRA -H2O-N2 obtained at the ωB97X-D/cc-pVTZ level. Relative stabilization energies (E0, Ee) and the total binding energies (D0) and those of the N2 ligands (in parentheses) are given in kJ/mol.

23



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Page 24 of 44

+

Figure 8. Stable structures of TRA -H2O-(N2)2 obtained at the ωB97X-D/cc-pVTZ level. Relative stabilization energies (E0, Ee) and binding energies (D0) and those of the N2 ligands (in parentheses) are given in kJ/mol. +

Figure 9. Stable structures of TRA -H2O-(N2)3 obtained at the ωB97X-D/cc-pVTZ level. Relative stabilization energies (E0, Ee) and binding energies (D0) and those of the N2 ligands (in parentheses) are given in kJ/mol. Figure 10. Plot of the E

(2)

i→j*

energies of each local NH...O H-bond as a function of the bond length between +

the O atom of H2O and the H atom of the indolic NH group for TRA -(H2O)m=1-3 and the Htop, πtop, 2H, 2π, and +

2H/πtop isomers of TRA -H2O-(N2)n=1-3. +

+

Figure 11. IRPD spectra of TRA -(H2O)2 and TRA -(H2O)2-(N2)2 compared to linear IR absorption stick +

spectrum of TRA -(H2O)2 evaluated at the ωB97X-D/cc-pVTZ level (Figure 12). The intensity of the indolic -1

N-H stretch fundamental predicted at ~2900 cm is multiplied by 0.4. The traces of the weak transitions in the +

IRPD spectrum of TRA -(H2O)2 are recorded at higher sensitivity. +

+

Figure 12. Stable structures of (H2O)2, TRA -(H2O)2 and TRA -(H2O)3 obtained at the ωB97X-D/cc-pVTZ level. Relative stabilization energies (E0, Ee) and binding energies (D0) are given in kJ/mol. Bond lengths are given in Å. +

+

Figure 13. IRPD spectra of TRA -(H2O)3 and TRA -(H2O)3-(N2)2 compared to linear IR absorption stick +

spectra of TRA -(H2O)3 isomers evaluated at the ωB97X-D/cc-pVTZ level (Figure 12). The intensities of the indolic N-H stretch fundamentals are multiplied by 0.5.

24


Page 25 of 44

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30


Figure Page 311of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


nf NH

TRA+-N2

ns OH naOH

E E TRA+-H2O

B

A

F D

TRA+-(H2O)2 E

B C A

D

TRA+-(H2O)3

2800

3000

3200

3400

C2 C1 C3 B A

3600

n IR / cm-1


3800

Figure 2


Page 32 of 44

nf NH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

TRA+-H2O

ns OH

naOH

E

(a) R2PI

B

A

E (b) EI B

A

F TRA+-H2O-(N2)n (c) 1  0 Y7

Y1

G

F

G

F

X

(d) 2  0

(e) 3  0

2800

3000

3200

n IR /

3400

3600

cm-1


3800

Figure Page 333of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


Gpy(in)

Gph(in)

Gph(up)

Anti(up)

a

b 3 3a 2 1

4 5 7a 7 6

1.754

1.753

E0 = 0.0 D0 = 58.5

+3.6 58.5

Anti(py)

1.741

1.748

+16.1 60.3

+12.4 59.2

Anti(ph)

1.741

1.744

+15.3 59.6

Gpy(up)

1.741

1.742

+16.9 60.4


plane

+17.9 60.3

+18.4 60.8

Figure 4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


ns OH naOH

nf NH

E (a) EI

B

F nbNH x0.1

(b) Gpy(in)

A naOH

ns OH

nC2H ns NH2 naNH2

(c) Gph(in)

(d) Gph(up)

2800

3000

x0.1

x0.1

3200

3400

3600

3800

n IR / cm-1


Page 34 of 44

Figure Page 355of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


nf NH

naOH

ns OH

A

(a) TRA+-H2O B

F

G

F

A0

B0

X

B1

(c) 2  0 G

A1

B1

TRA+-H2O-(N2)n (b) 1  0

A2 A1

B2 F

X

A0

B0 B2

(d) 3  0

B1 X 3300

3400

3500

n IR /

3600

A2 A1

3700

3800

cm-1


Figure 6

3760 A 3740

3760

n aOH

3740

p

2p

3720

3720 H

H/p

H/2p

2H

2H/p

3700

3700

n exp/ cm-1

3680

3680 n s OH

B

3660

3660

3640

3640

3620

3620

3600

3600

3580

3580

3560

3560 0

1

2

3

0

1

2

n calc/ cm-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


3

n


Page 36 of 44

Figure Page 377of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


p top

Htop

E0 = 0 (Ee = 0) D0 = 66.8 (8.3)

Hbottom

+0.46 (-0.78) 66.3 (7.8)

NH2bottom

+2.26 (+2.95) 64.5 (6.0)

+0.32 (-0.75) 66.5 (7.9)

p bottom

+0.67 (+1.63) 66.1 (7.6)

NH2top


+3.55 (+4.21) 63.3 (4.7)

Figure 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


Hbottom/p top

E0 = 0 (Ee = 0) D0 = 74.9 (16.4)

p top/p bottom

+0.59 (+2.63) 74.3 (15.8)

Htop/p top

+0.01 (-0.36) 74.9 (16.4)

Htop/p bottom

+0.67 (+1.70) 74.2 (15.7)


Page 38 of 44

Htop/Hbottom

+0.19 (-0.14) 74.7 (16.2)

Hbottom/p bottom

+1.06 (+1.65) 73.9 (15.3)

Figure Page 399of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


2H/p top

E0 = 0 (Ee = 0) D0 = 83.3 (24.7)

Hbottom/2p

+0.90 (+2.42) 82.4 (23.8)


Htop/2p

+0.41 (+2.07) 82.9 (24.3)

2H/p bottom

+1.06 (+2.07) 82.2 (23.7)

Figure 10

170

(H2O)3 (II)

Page 40 of 44

TRA+-L

160 150

(2) Ei →j∗ / kJ mol-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


140 130 120 110

(H2O)2 H2O-(N2)3 (2H/p top)

100

H2O-(N2)2 (2H)

90

H2O-N2 (Htop)

80

1.58

1.62

1.66 1.70 RNH…O / Å

H2O-N2 (p top) H2O-(N2)2 (2p) H2O 1.74


1.78

Figure Page 4111 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


D1 TRA+-(H2O)2-(N2)2 IRPD

D2 X E

D

TRA+-(H2O)2 IRPD

TRA+-(H2O)2 Gpy(in)

2800

B1C2 A2 A1 B2 C1

C

X

B

A

ns OH

nf OH naOH

G

nbNH

nbOH

x0.4

3000

3200

n IR /

3400

3600

cm-1


3800

Figure 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


Gpy(in)-(H2O)2

Page 42 of 44

Gpy(in)-(H2O)3 (Ia)

0.958 1.747 0.976

1.039 1.664

0.956 0.973 1.780 1.673

1.045

1.631

0.957

D0 = 103.4

0.956 0.965

0.984 0.956

(H2O)2

E0 = 0 (Ee = 0) D0 = 141.6

1.937 0.958

Gpy(in)-(H2O)3 (II)

Gpy(in)-(H2O)3 (Ib) 0.973

0.958

0.958 0.957 1.796

0.957

1.783 1.675

0.971

1.054

0.985

1.589

1.045 1.632 0.956

1.795

0.957 0.958

0.958 0.957

+0.2 (+1.8) 141.5

+0.4 (-0.1) 141.3


Figure Page 4313 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


D3

D2 D1

C3 C2 C1

TRA+-(H2O)3-(N2)2 IRPD

C4 B A

D

TRA+-(H2O)3 IRPD

nbNH

TRA+-(H2O)3 Gpy(in) (Ia)

C1 C2 C3 B A

nbOH

x0.5

nbOH nf OHna OH ns OH nbNH

TRA+-(H2O)3 Gpy(in) (II)

nbaOH

x0.5

nbs OH a ns OH n OH

2600

2800

3000

3200 3400 -1 n IR / cm

3600


3800


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure TOC

TRA+-H2O TRA+-H2O-N2


Page 44 of 44

Simultaneous Interaction of Hydrophilic and Hydrophobic Solvents

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