Isomer-Selective Ultraviolet and Infrared Spectra of Jet-Cooled 2

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Building Up Water-Wire Clusters: Isomer-Selective Ultraviolet and Infrared Spectra of Jet-Cooled 2‑Aminopurine (H2O)n, n = 2 and 3 Simon Lobsiger,†,‡ Rajeev K. Sinha,†,§ and Samuel Leutwyler*,† †

Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland Department of Chemistry, University of Virginia, McCormick Road, Charlottesville, Virginia 22904-4319, United States § Department of Atomic and Molecular Physics, Manipal University, Manipal-576104, Karnataka, India ‡

ABSTRACT: 2-Aminopurine (2AP) is an adenine analogue with a high fluorescence quantum yield in water solution, which renders it a useful real-time probe of DNA structure. We report the ultraviolet (UV) and infrared (IR) spectra of size-selected and jet-cooled 9H2AP·(H2O)n clusters with n = 2 and 3. Mass- and species-specific UV/UV holeburning spectroscopy allows to separate the UV spectra of four cluster isomers in the 31200−33000 cm−1 spectral region with electronic band origins at 31339, 31450, 31891, and 32163 cm−1. Using IR/UV depletion spectroscopy in combination with B3LYP calculated harmonic vibrational frequencies, the H-bonding topologies of two isomers of the n = 2 and of two isomers of the n = 3 cluster are identified. One n = 2 isomer (denoted 2A) forms a water dimer chain between the N9H and N3 atoms at the sugar-edge site, the other isomer (denoted 2D) binds one H2O at the sugar-edge site and the other at the trans-amino site between the N1 atom and the NH2 group. For 2-aminopurine·(H2O)3, one isomer (denoted 3A) forms an H-bonded water wire at the sugar-edge site, while isomer 3B accommodates two H2O molecules at the sugar-edge and one at the trans-amino site. The approximate second-order coupled cluster (CC2) method predicts the adiabatic S1 ← S0 transitions of 9H-2-aminopurine and six water cluster isomers with n = 1−3 in very good agreement with the experimental 000 frequencies, with differences of 25 kJ/mol above isomer 2A, and are not considered further. Figure 2 shows the seven lowest energy calculated isomers of 2AP·(H2O)3. The most stable isomer 3A binds all three water molecules at the sugar-edge site, forming a water-wire between N9H and N3 atoms similar to the isomers 1A and 2A of the n = 1 and 2 clusters. The average hydrogen bond length is comparable to isomer 2A of 2AP·(H2O)2. Isomer 3B lies 6.2 kJ/mol above 3A and accommodates a water dimer at its sugaredge and one H2O at the trans-amino site, in a 2 + 1 fashion. In isomer 3C, the water molecules are H-bonded to the same sites, but in a 1 + 2 fashion. The energy of this isomer is 15.3 kJ/mol higher than 3A. Isomers 3D and 3E accommodate all three water molecules in a water-wire motif that is localized at the cisor trans-amino sites. Their relative energies are slightly higher than that of isomer 3C. Isomers 3F and 3G share the water molecules in a 2 + 1 fashion at the two donor−acceptor cis-/ trans-amino sites and have relative energies >24 kJ/mol. We

2. EXPERIMENTAL AND THEORETICAL METHODS The measurement setup was described previously.18,19 Briefly, the 2-aminopurine·(H2O)n clusters were formed in a pulsed supersonic expansion of neon carrier gas (Linde, ≥99.995%) at 1.5 bar, which was passed over ice held at −15 °C (∼1.8 mbar H2O pressure), then over 2-aminopurine (Sigma-Aldrich) that is heated to 190 °C and expanded through a heated pulsed nozzle. Mass selected two-color R2PI spectra were recorded by crossing the skimmed molecular beam with the UV excitation beam from a frequency doubled dye laser (350 μJ/pulse, 0.3 cm−1 bandwidth) and ionization beam (266 nm, 250 μJ/pulse) from the Nd:YAG fourth harmonic. The resulting photoions were analyzed with a linear time-of-flight mass spectrometer. For UV/UV holeburning experiments, the holeburning UV pulses were generated by a second frequency-doubled dye laser 300 ns prior to the UV excitation/ionization laser pulses. For UV/UV depletion spectroscopy, the holeburning laser was scanned in frequency instead of the excitation laser. The UV wavelengths of the two dye lasers were calibrated with a WS-6 high-precision wavelength meter and were vacuum corrected. The IR/UV depletion spectra were recorded in the 3000− 3900 cm−1 region by overlapping the UV and mildly focused IR laser beams. The IR laser pulses (∼5 mJ/pulse, 2 cm−1 bandwidth) were produced by a 10 Hz injection-seeded Nd:YAG (Innolas, Spitlight 600) pumped IR OPO/OPA laser system (LaserVision). For the depletion of the groundstate population, the IR laser was fired 100 ns before the UV detection laser; the resulting population depletion was monitored at the respective electronic origins of the respective isomers (see below). The UV excitation and detection lasers were operated at 20 Hz, and the ion signals with and without IR laser recorded as a function of IR frequency. The S0 and 1ππ* state structures of the 2AP·(H2O)n clusters were optimized with the B3LYP density functional and the TZVP basis set and also with the correlated RI-CC2 method and the aug-cc-pVDZ basis set using Turbomole 6.0.29,30 For each cluster isomer, the minimum-energy structure was obtained by checking the different possible rotamer structures; only the lowest energy rotamer was considered for the analysis of the IR and UV spectra. Harmonic frequency calculations were performed at the optimized geometries. The harmonic frequencies have been scaled by a factor of 0.9613, as is B

dx.doi.org/10.1021/jp407127c | J. Phys. Chem. B XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry B

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Figure 1. B3LYP/TZVP calculated geometries of the eight lowestenergy isomers of 9H-2-aminopurine·(H2O)2. The energies (in kJ/ mol) relative to the most stable structure 2A are given in parentheses. The atom numbering of 2-aminopurine is indicated for isomer 2A.

Figure 2. B3LYP/TZVP calculated geometries of the seven lowestenergy isomers of 9H-2-aminopurine·(H2O)3. Energies (in kJ/mol) relative to the most stable structure 3A are given in parentheses.

omitted the structures involving H-bonding at the N7 atom from the study of the 2AP·(H2O)3 isomers because of their high relative energies compared to 3A. 3.2. Resonant Two-Photon Ionization and UV/UV Holeburning Spectroscopy. Figure 3 shows the two-color R2PI overview spectra of the 2AP·(H2O)2 and 2AP·(H2O)3 clusters, which exhibit many sharp vibronic bands in the 31200−33100 cm−1 spectral region. Fragmentation with loss of one H2O molecule can be observed from both 2AP·(H2O)2 and 2AP·(H2O)3. The high density of vibronic features observed in the 2AP·(H2O)+2 mass channel must therefore be attributed to spectral contributions from several n = 2 and 3 isomers, as indicated by dashed vertical lines in Figure 3. Using UV/UV holeburning spectroscopy, contributions from four isomers could be identified in the n = 2 mass channel, as

shown in Figure 4a−d. The lowest-frequency contribution, attributed to an isomer that we denote I has a 000 band at 31339 cm−1. Careful comparison of the n = 2 and 3 mass channels shows the presence of all vibronic bands in the n = 3 mass channel, hence we assign this as an n = 3 cluster. No further bands with comparable intensity are observed to the red of this band in the n = 2 mass channel. The low-intensity band at 31450 cm−1 was identified as the origin of an isomer denoted II, which is not observed in the n = 3 mass channel. The electronic origin of isomer III was detected at 31891 cm−1. The corresponding band is clearly observed in both the 2AP·(H2O)+n (n = 2,3) mass channels, as indicated by a dashed line in Figure 3. The most intense band observed in the n = 2 mass channel is the electronic origin of C



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isomer IV at 32163 cm−1; we assign it to an n = 2 cluster, since this spectrum does not appear in the n = 3 mass channel. The intense vibronic bands above 31891−32800 cm−1 in the 2AP·(H2O)+3 mass channel (Figure 3b) can all be assigned to isomer III using UV/UV holeburning spectroscopy. The lowenergy region of this spectrum exhibits several bands with about 10 times lower intensity, which are either from other n = 3 isomers or from fragmentation of 2AP·(H2O)4 clusters. Their low intensity prevented a more detailed investigation. Compared to the 9H-2AP monomer, the electronic origin bands of the n = 2 and 3 clusters are red-shifted by δν = −199, −471, −912, and −1023 cm−1, corresponding to increases in H-bond binding energy upon electronic excitation by 2.38, 5.63, 10.90, and 12.23 kJ/mol. For isomers 1A and 1B of 2AP· H2O, these red shifts were δν = −70 cm−1 (0.84 kJ/mol) and −889 cm−1 (10.63 kJ/mol), respectively.18 In the R2PI spectra of the 2AP monomer and the monohydrated isomers A and B, the in-plane intramolecular ′ (corresponding to ν1′ in benzene) is observed as vibration ν16 one of the most intense bands at 759, 763, and 761 cm−1, respectively.17,18 The spectra of all four 2AP·(H2O)n (n = 2,3) isomers in Figure 4 exhibit the corresponding band between 764 and 768 cm−1. The ν′7 and ν′8 vibrations, which are related to the ν6b ′ and ν6a ′ normal modes of benzene, are observed between 450 and 491 cm−1, slightly blue-shifted to the 2AP monomer (434 and 465 cm−1), and comparable to the n = 1 isomers 1A (451 and 481 cm−1) and 1B (465 and 484 cm−1). To characterize the isomers based on their vibronic spectra, we compare the experimental origin frequencies with TDB3LYP/TZVP and CC2/aug-cc-pVDZ calculated 1ππ* state adiabatic energies, as shown in Table 1. For completeness, the values for 9H-2AP17 and for the n = 1 isomers18,19 are also tabulated. For these, the differences between the calculated adiabatic energies and experiment is 1.9−3.3% with the B3LYP/TD-B3LYP methods, but only 0.1 − 0.5% with the CC2 method. Due to the better agreement with the CC2

Figure 3. Two-color R2PI spectrum of 9H-2-aminopurine·(H2O)n (n = 2,3) clusters. The bands indicated with arrows were investigated by UV/UV and IR/UV holeburning spectroscopy.

Figure 4. UV/UV holeburning spectra of 2-aminopurine·(H2O)n (n = 2,3) isomers (a) I (31339 cm−1), (b) II (31450 cm−1), (c) III (31891 cm−1), and (d) IV (32163 cm−1). D



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Table 1. Experimental Electronic Origins of 9H-2Aminopurine and Its Water Clusters, Compared to TDB3LYP/TZVP and CC2/aug-cc-pVDZ Calculated Adiabatic 1 ππ* Transition Energies B3LPY/TZVP Exp. cm−1 9H-2APa 32362 9H-2AP·H2Ob Isomer 1A 32292 Isomer 1B 31473 9H-2AP·(H2O)2 Isomer 2A 32163 Isomer 2B Isomer 2C Isomer 2D 31450 Isomer 2E 9H-2AP·(H2O)3 Isomer 3A 31891 Isomer 3B 31339 Isomer 3C Isomer 3D Isomer 3E a

(IV)

(II)

(III) (I)

RI-C2C/aug-cc-pVDZ

cm−1

ratio (calc./ exp.)

cm−1

ratio (calc./ exp.)

33395

1.032

32398

1.001

33348 32062

1.033 1.019

32370 31305

1.002 0.995

33187 31398 32060 32118 31123

1.032

32249 30767 30939 31291 30282

1.003

32917 31988 31509 32008 31238

1.032 1.021

31890 31159 30774 30883 30610

1.000 0.994

1.021

0.995

From reference 17. bFrom reference 19.

calculated adiabatic energies, we used these for preliminary assignments of the n = 2,3 isomers to cluster structures, as shown in Table 1. With the assignments as given, the discrepancies between the adiabatic energies and the observed 000 bands of the n = 2,3 isomers are 0.9%, or to assignments to higher-energy (less stable) groundstate isomers. As shown in Section 3.3, the tentative assignment of the isomers I to IV given in Table 1 is fully confirmed by the IR depletion spectra of the different isomers. 3.3. Isomer-Specific IR Depletion Spectra. 3.3.1. 2Aminopurine·(H2O)2 Isomers. IR spectroscopy is a very sensitive tool to investigate the H-bonding topologies of molecular clusters and can definitely identify and characterize the observed isomers.18,19,22−27 We begin with isomers IV and II, which we have tentatively assigned to the n = 2 isomers 2A and 2D, based on the calculated UV transition energies. Figure 5a shows the mass-selected IR depletion spectrum in the 3000−3900 cm−1 region with the UV detection at the origin of isomer IV (32163 cm−1). The B3LYP calculated IR spectra of the low-energy structures 2A to 2E are plotted in Figure 5b−f. The frequencies of the observed and calculated bands and their mode descriptions are listed in Table 2. The experimental bands at 3721 and 3706 cm−1 can be clearly assigned to free OH stretching vibrations, attesting to the presence of two H2O molecules in this isomer. The H-bonding topology can be sensitively diagnosed by the narrow bands at 3452 and 3563 cm−1, which lie very close to the symmetric and asymmetric NH2 stretch bands of bare 9H-2AP17 at 3466 and 3583 cm−1 and of isomer 1A at 3464 and 3579 cm−1.19 We therefore assign the 3452/3563 cm−1 bands as the free symmetric/asymmetric NH2 stretches of the 2AP moiety. On the other hand, no band is observed in the region of the free N9H stretch at ∼3510 cm−1.19 The presence of the symmetric and asymmetric NH2 stretches and absence of the free N9H stretch imply that the two H2O molecules are H-bonded at the sugar edge of 2AP. Since the calculated IR spectrum of isomer

Figure 5. (a) IR depletion spectrum of isomer IV (UV detection at 32163 cm−1), compared to (b−f) the B3LYP/TZVP calculated IR spectra for the five n = 2 isomers shown in Figure 1a−e. Calculated frequencies are scaled by 0.9613.

Table 2. Experimental Vibrational Frequencies for Isomers IV and II, compared to Calculated Frequencies and NormalMode Description for Isomer 2A and 2D (in cm−1)

a

modes

experimental

B3LYP/TZVPa

Isomer IV O−H(f) O−H(f) NH2a NHs2 O−H(b) O−H(b)/NH(b) N−H(b)/OH(b) Isomer II O−H(f) O−H(f) NH2a O−H(b)/N−H(b) NHs2(b)/O−H(b) N−H(b)/O−H(b) O−H(b)/NHs2(b)

Figure 5a 3721 3706 3563 3452 3384 3200 3147 Figure 6a 3726 3721 3545 3438 3413 3383 3370

Isomer 2A 3717 3704 3570 3454 3304 3171 3110 Isomer 2D 3720 3713 3558 3419 3374 3360 3328

B3LYP/TZVP, scaled by 0.9613.

2A is in good agreement with the experimental IR depletion spectrum, we assign the species IV to the isomer 2A. Based on the B3LYP calculation, the 3147 cm−1 band is assigned to the H-bonded N9H stretch and the bands at 3200 and 3384 cm−1 to the H-bonded OH···N and OH···O vibrations, respectively. E



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fundamental of the trans-amino H2O at 1627 cm−1 and that of the sugar-edge H2O at 1634 cm−1. In the IR spectrum of different conformers of 3-indole-propionic acid·(H2O)2, wide bands at 3200 cm−1 have also been observed and assigned to the water bend overtones.24,32 We have not definitively assigned the very weak band at 3560 cm−1, but note that an analogous weak feature 15 cm−1 above the asymmetric NH2 stretch has been observed in the related 1B complex.19 In that case, we suggested two possible assignments: (1) Tunneling splitting caused by the double minimum potential along the NH2 inversion coordinate, similar to that proposed for two close-lying bands in the IR spectrum of 7H-adenine,33 or (2) a combination of the NH2 symmetric stretch at 3413 cm−1 with the intermolecular Hbond stretch σ.19 3.3.2. 2-Aminopurine·(H2O)3 Isomers. The remaining isomers I and III have been assigned to 2AP·(H2O)3 clusters in Section 3.2 based on the CC2 calculated excitation energies. Figure 7a shows the IR depletion spectrum with UV detection

Figure 6a shows the IR/UV depletion spectrum with the UV detection at the 000 band of isomer II (31450 cm−1). The

Figure 6. (a) IR depletion spectrum of isomer II (UV detection at 31450 cm−1), compared to (b)-(f) the B3LYP/TZVP calculated IR spectra of the n = 2 isomers shown in Figure 1a−e. Calculated frequencies are scaled by 0.9613.

B3LYP calculated IR spectra of the n = 2 isomers are plotted in Figure 6b−f. Table 2 lists the experimental and calculated positions of the bands and their normal mode descriptions. The closely spaced bands at 3721 and 3726 cm−1 are again assigned as the two free OH stretches. The diagnostic NH/NH2 stretch region now exhibits only a single band at 3545 cm−1, which is assigned to the asymmetric NH2 stretch. The absence of the free N9H and of the symmetric NH2 stretches indicate that both donors are H-bonded to a water molecule. The B3LYP/TZVP calculation predicts such an Hbonding topology for isomer 2D (see Figure 1), which can be viewed as the combination of isomers 1A and 1B of 2AP· H2O.19 For isomer 1A, the bonded NH and OH stretching vibrations were observed at 3389 and 3434 cm−1, respectively. The positions of these two bands are very close to the 3383 and 3438 cm−1 bands in the spectrum of isomer II. Similarly, for isomer 1B, the bonded symmetric NH2 and O−H stretching vibrations were observed at 3369 and 3418 cm−1,19 in close coincidence with the observed bands at 3370 and 3413 cm−1. The intensities and widths of the spectral features are also similar to those observed for isomer 1A and 1B. On the basis of the agreement with the IR spectra of 1A and 1B, we can assign the species II to the isomer 2D. We tentatively assign the weak and broad band to overtones of the two water bends. The calculation predicts the bend

Figure 7. (a) IR depletion spectrum of isomer III (UV detection at 31891 cm−1), compared to (b−f) the B3LYP/TZVP calculated IR spectra of the five n = 3 isomers shown in Figure 2a−e. Calculated frequencies are scaled by 0.9613.

at the origin of isomer III (31891 cm−1). The B3LYP calculated IR spectra of the five lowest-energy 2AP·(H2O)3 isomers are plotted in Figure 7b−f. Table 3 lists the experimental and calculated positions of the bands and their normal-mode descriptions. The IR spectrum exhibits two bands in the free OH stretch region at 3697 and 3722 cm−1. The calculation for isomer 3A predicts three free O−H stretches at 3697, 3717, and 3719 F



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Table 3. Experimental Vibrational Frequencies for Isomers III and I, Compared to Calculated Frequencies and NormalMode Description for Isomer 3A and 3B (in cm−1)

a

modes

experimental

B3LYP/TZVPa

Isomer III O−H(f) O−H(f) O−H(f) NH2a NHs2 O−H(b) O−H(b)/N−H(b) N−H(b)/O−H(b) O−H(b)/N−H(b) Isomer I O−H(f) O−H(f) O−H(f) NH2a NHs2(b)/O−H(b) O−H(b)NHs2(b) O−H(b)/N−H(b) O−H(b)/N−H(b) N−H(b)/O−H(b)

Figure 7a 3722 3722 3697 3562 3446 3394 3338 3171 3059 Figure 8a 3724 3724 3708 3548 3412 −3373 3188 3162

Isomer 3A 3719 3717 3697 3565 3443 3307 3243 3107 3049 Isomer 3B 3715 3713 3704 3543 3374 3327 3304 3178 3102

B3LYP/TZVP, scaled by 0.9613.

cm−1. Since the latter frequencies are very close, they probably cannot be resolved experimentally and correspond to the 3722 cm−1 band, which is twice as intense as the 3697 cm−1 band. The 3400−3600 cm−1 region that is diagnostic for free NH and −NH2 stretches exhibits two narrow bands at 3446 and 3562 cm−1. These are assigned as the symmetric and asymmetric NH2 stretching vibrations, since they lie very close to the symmetric and asymmetric NH2 stretches of 2AP, isomer 1A and isomer 2A (see above). The presence of these two bands combined with the absence of the free N9H stretch implies that the water molecules are H-bonded to the sugar-edge site of 2AP. Based on these observations, we assign species III to the isomer 3A. Based on the B3LYP calculations, we assign the three broad bands at 3059, 3338, and 3394 cm−1 to the Hbonded O−H stretching vibrations of the three H2O molecules, whereas the band at 3171 cm−1 arises from the H-bonded N9H stretching vibration. Figure 8a shows the IR depletion spectrum with UV detection at the 000 band of isomer I (31339 cm−1); the calculated IR spectra are plotted in Figure 8b−f. Table 3 lists the experimental and calculated frequencies. The narrow bands at 3708 and 3724 cm−1 are assigned to the free OH stretches. Similar to isomer 3A, three IR bands are predicted at 3704, 3713, and 3715 cm−1. The two close-lying higher-frequency bands are probably overlapped, giving rise to the band at 3724 cm−1 in the spectrum. Again, this is supported by its intensity, which is twice that of the 3708 cm−1 band. The 3548 cm−1 band is assigned as the asymmetric NH2 stretch, which was observed at 3560 cm−1 for isomer 1B.18 Two relatively broad bands are observed at 3373 and 3412 cm−1; their band centers are similar to those observed at 3369 and 3418 cm−1 for isomer 1B.18 This indicates that at least one H2O molecule is bonded to the trans-amino site, which is supported by the absence of the free symmetric NH2 stretch from its usual position in the monomer (∼3466 cm−1). Also, there is no evidence for the free N9H stretch (expected at 3510 cm−1), which indicates that the

Figure 8. (a) IR depletion spectrum of isomer I (UV detection at 31339 cm−1), compared to (b−f) the B3LYP/TZVP calculated IR spectra of the five n = 3 isomers shown in Figure 2a−e. Calculated frequencies are scaled by 0.9613.

sugar-edge site is also occupied. Only isomers 3B and 3C are consistent with this H-bond topology. Due to the better agreement of experimental and calculated frequencies and the lower energy of 3B, we prefer to assign species I as isomer 3B.

4. DISCUSSION 4.1. UV Spectra. Figure 9 shows the two-color R2PI spectra in the 30500−32200 cm−1 spectral region, as measured in the 2AP·(H2O)+n (n = 1 − 3) mass channels. The UV spectrum of cluster 1A lies just outside, to the blue of this range.19 All identified cluster isomers 2A, 2D, 3A, and 3B fragment to some extent into the next lower 2AP·(H2O)+n−1 (n = 1, 2) mass channels with 266 nm ionization. The 3B isomer fragments strongly into the n = 2 mass channel, whereas the water-wire isomer 3A fragments much less efficiently. The cluster isomers 2B and 2C are calculated to have binding energies similar to 2D or even larger. Their CC2 calculated adiabatic S1 ← S0 transitions are indicated in Figure 9, including ±0.6% error bars. While there are no bands attributable to 2B or 2C in the n = 2 mass channel, weak features in the 30500− 31300 cm−1 region are observed in the n = 1 mass channel that might arise from the fragmentation of 2B and 2C. However, these bands are ∼10 times lower than those of isomer 2D, precluding IR spectroscopy and detailed identification. In the n = 2 mass channel, there are no features below the origin of the 3B isomer, with the exception of a weak band at 31231 cm−1. This band, together with a number of other weak features in the 30930−31250 cm−1 range is also observed in the n = 3 mass G



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We conclude that solvation by water-wire clusters H-bonded between N9H and N3 hardly stabilizes the 1ππ* state. The larger shift of isomer 3A may reflect a stabilizing interaction with the amino group (see Figure 2). By contrast, all cluster isomers with one or two water molecules in the trans-amino position (H-bonded between NH2 and N1) exhibit large spectral red shifts, corresponding to stabilizations of the 1ππ* excited state of 10−12 kJ/mol. Thus, the stabilization of the S1(ππ*) state of 2-aminopurine by water clusters is highly regiospecif ic. 4.2. H-Bond Patterns and Binding Energies of Different Cluster Isomers. The relative binding energies in Figure 1 predict the water-wire clusters 2A, 2B, and 2C as the most stable n = 2 isomers. Similarly, Figure 2 predicts that the water-wire 3A has the largest binding energy of the n = 3 clusters. Analogous water-wire motifs have been diagnosed and discussed in the spectra of other microhydrated biomolecules, such as tryptamine·(H2O)n, in which two and three water molecules form a water bridge that links the indole NH donor with the amino group acceptor on the ethylamine side chain.24 Similarly, the serotonin·(H2O)n n = 1 and 2 clusters both exhibit a single conformer, with a water molecule or water dimer bridge connecting the 5-hydroxy group to the ethylamine side chain.34 In the latter systems, the cooperative strengthening of the H-bonds in the water-wire and between two parts of the flexible molecule results in “conformational locking”,24 condensing a number of conformers of the bare tryptamine or serotonin monomer into a single isomer.24,34 Beyond the water-wire motif, the relative cluster binding energies strongly depend on the H-bonding sites in 2AP: Comparing the 2A, 2B, and 2C water-wires in Figure 1, the sugar-edge site (2A) is seen to bind most strongly, followed by the trans-amino site (2B, 12 kJ/mol higher) and the cis-amino binding site (2C, 15 kJ/mol higher). Since the terminal Hbonds of the 2A and 2C wires are to the same N3 acceptor, the difference between the strong N9H donor and the weak NH2 donor translates to a 15 kJ/mol difference in binding energy. An analogous dependence on the H-bonding site appears for the three-member water-wire clusters 3A, 3D, and 3E in Figure 2. Again, the strongest H-bonds are formed within the sugaredge site (3A), the trans-amino trimer being less strongly bound (3D, 16 kJ/mol higher), followed by the cis-amino site (3E, 18 kJ/mol higher). On the other hand, Figure 2 also brings out the competition between water-wire clusters and aminopurine−water H bonds, showing that water-wires do not always “win”: Because of the relatively weakness of the 2-amino donor, it is energetically advantageous to break the trimer water-wires of isomers 3D and 3E and place one or two H2O molecules in the sugar-edge site, as in 3C or 3B. 4.3. Relative Populations of Water-Cluster Isomers. We have previously observed two isomers of the 2AP monohydrate: sugar-edge 1A and trans-amino 1B.19 The n = 2 and 3 clusters discussed here also both exhibit a sugar-edge isomer (2A, 3A) and a trans-amino (2D, 3B) isomer. The UV spectra in Figure 9b,c provide evidence for further weakly populated cluster isomers such as 2B, 2C, 3C, and 3D. Since the isomers of types B to D are predicted to have substantially lower binding energies than the A isomers, it is clear that the ensemble of hydrate clusters of a given size does not reach thermodynamic equilibrium. The reason for this is the large distance of the trans-amino site to the sugar-edge and the cis-amino sites: H2O molecules

Figure 9. Two-color R2PI spectra in the 30500−32200 cm−1 spectral region recorded in the 2-aminopurine·(H2O)+n (n = 1,2,3) mass channels.

channel, hence it arises from a n = 3 or 4 cluster. As the CC2 calculated adiabatic S1 ← S0 transitions for 3C, 3D, and 3E in Figure 9c indicate, these bands might be assigned to the isomer 3C or 3D. Because of the low intensities, we did not pursue the identification of these species.

Figure 10. Plot of the CC2/aug-cc-pVDZ calculated adiabatic transition energies of 9H-2-aminopurine and the 9H-2-aminopurine· (H2O)n cluster isomers 1A, 1B, 2A, 2D, 3A and 3B versus the experimental 000 bands.

Figure 10 collects and compares the CC2 calculated adiabatic ππ* excitation energies to the experimentally observed electronic origins of 2AP and the 2AP·(H2O)n water clusters 1A, 1B, 2A, 2D, 3A, and 3B, as analyzed above or previously.18,19 As mentioned above, the agreement of calculated and experimental values is very good, with differences

Isomer-Selective Ultraviolet and Infrared Spectra of Jet-Cooled 2

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