Article pubs.acs.org/JPCA
NH3 as a Strong H‑Bond Donor in Singly- and Doubly-Bridged Ammonia Solvent Clusters: 2‑Pyridone·(NH3)n, n = 1−3 Susan Blaser, Philipp Ottiger, Hans-Martin Frey, and Samuel Leutwyler* Departement für Chemie und Biochemie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland S Supporting Information *
ABSTRACT: Mass- and isomer-selected infrared spectra of 2-pyridone·(NH3)n clusters with n = 1−3 were measured in the NH and CH stretch fundamental region (2400−3700 cm−1) using infrared (IR) laser depletion spectroscopy combined with resonant two-photon ionization UV laser detection. The IR depletion spectra reveal three different H-bonding topologies of these clusters: The n = 1 and 2 clusters form ammonia bridges stretching from the N−H to the CO group of the cis-amide function of 2-pyridone (2PY), giving rise to intense and strongly red-shifted (2PY)NH and ammonia NH stretch bands. For n = 3, two isomers (3X and 3Y) are observed in the IR spectra: The spectrum of 3X is compatible with an ammonia-bridge structure like n = 2, with the third NH3 accepting an H-bond from C6−H of 2PY. The IR spectrum of 3Y exhibits a broad IR band in the 2500−3000 cm−1 range and is characteristic of a bifurcated double-bridged structure in which the first NH3 accepts an H-bond from the (2PY)NH and donates two H-bonds to the other two ammonias, both of which donate to the CO group of 2PY. This double-donor/double-bridge H-bonding pattern increases the acceptor strength of the first ammonia and dramatically lowers the (2PY)NH stretching frequency to ∼2700 cm−1. For all clusters the ammonia 2ν4 HNH bend overtones in the 3180−3320 cm−1 region gain intensity by anharmonic coupling (Fermi resonance) to the hydrogen-bonded ammonia NH stretches, which are red-shifted into the 3250−3350 cm−1 region. The experimental results are supported by optimized structures, vibrational frequencies, and IR intensities calculated using density-functional theory with the B3LYP and PW91 functionals, as well as with the more recent functionals B97-D and M06-2X, which are designed to include long-range dispersive interactions.
1. INTRODUCTION Hydrogen bonds are among the principal intermolecular interactions that control three-dimensional structures, the chemical reactivities, and thereby the biological functions of peptides, proteins, and nucleic acids.1 Hydrogen bonds also play an important role in proton-transfer reactions and in the transport of protons both in bulk media and along water-wires and ammonia-wires.2−12 To study proton-transfer reactions and tautomerization reactions, we have chosen 2-pyridone (2PY), which is a planar cis-amide with an NH proton donor neighboring a CO proton acceptor group. 2PY is an Hbonding analogue of the pyrimidine nucleobases uracil and thymine. The cis-amide function can be bridged by solvent molecules such as H2O and NH3 via two or more hydrogen bonds.13−23 2PY also dimerizes in the gas phase with itself,24−29 with its tautomer 2-hydroxypyridine,30−33 and with the canonical nucleobases.34−37 The S1 ← S0 vibronic spectra of 2PY·(NH3)n (n = 1, 2) have first been measured by Nimlos et al. together with those of the respective 2PY·(H2O)n clusters.13 Held and Pratt investigated the electronic origin bands of the n = 1 and 2 clusters by highresolution laser-induced fluorescence (LIF) spectroscopy.15 By combining the experimental ground- and excited-state rotational constants with the results of model calculations, they determined the cluster structures in detail and established that © XXXX American Chemical Society
they have unidirectionally H-bonded (homodromic) ammoniabridges connecting the NH to the OC group of 2pyridone.15 Zwier and co-workers have studied one- and twomembered hydrogen-bonded ammonia and mixed ammonia/ water bridges stretched across the cis-amide groups of oxindole and 3,4-dihydro-2(1H)-quinolinone using ion-dip IR spectroscopy, accompanied by B3LYP density functional calculations.19,38 The infrared spectra of pure (NH3)n clusters, either jetcooled or doped into Hen clusters, have been spectroscopically studied by several groups.39−41 The pioneering IR laser depletion study of Slipchenko et al. established the shifts of the ν1 NH symmetric stretch, ν3 NH asymmetric stretch and 2ν4 bending overtone frequencies and IR intensities for the n = 2−4 ammonia clusters doped into Hen nanodroplets at T = 0.37 K.39 Esboui and co-workers have performed a density functional theory (DFT) study of 2PY·(NH3)n clusters at the B3LYP/631++G** level and predicted that ground-state proton transfer might be possible for the n = 5 cluster.42,43 Because no Special Issue: Joel M. Bowman Festschrift Received: February 20, 2013 Revised: April 24, 2013
A
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set. The optimized cluster structures are shown in Figure 1; note that all structures are Cs-symmetric with a mirror plane through the 2PY ring. With all four DFT methods employed we found two stable minima for the n = 3 cluster, which we denote 3X and 3Y. With the B97-D and M06-2X functionals, no minimum was found for a homodromic three-membered ammonia-bridge structure (analogous to n = 2 cluster). Such a three-membered bridge structure was reported as the only global minimum in the B3LYP/6-31++G** study of Esboui et al.; we only find it to be a stable minimum with the B3LYP and PW91 functionals, with a structure that is close to 3Y. With the B3LYP and PW91 functionals, we also obtained a locally stable structure similar to 3X, in which the ammonia accepts an H-bond from C3−H and donates an H-bond to the oxygen of the keto group. However, this local minimum is very shallow and the structure easily isomerizes to the 3Y structure during the optimization procedure. With the M06-2X and the B97-D functionals, this structure is unstable and its properties (structure, IR spectrum) could not be calculated. The S0 state harmonic vibrational mode eigenvectors, frequencies and infrared band intensities were calculated at the respective minimum geometries. Anharmonic vibrational frequencies were calculated with the B3LYP functional and the IR intensities were taken from the respective harmonic calculations. Note that all calculated frequencies reported are unscaled. The M06-2X, PW91, and B3LYP calculations were performed using the GAUSSIAN09 package with the VERYTIGHT option for structure optimizations.58 The B97-D calculations were carried out with the TURBOMOLE program package.59 The thresholds were decreased to the following nonstandard values: 10−9 au for SCF convergence and 10−8 au for one-electron density convergence; for the structure optimizations to 10−8 au for the energy change, 6 × 10−6 au for the maximum displacement element, 10−6 au for the maximum gradient element, 4 × 10−6 au for the RMS displacement, and 10−6 au for the RMS gradient. 2.2. Experimental Methods. The 2PY·(NH3)n clusters were synthesized in pulsed adiabatic supersonic jets. Neon (Linde, ≥99.995%) carrier gas was mixed with 0.5% NH3 at ∼1.5 bar backing pressure and passed through a 20 Hz magnetically pulsed nozzle (0.4 mm diameter) containing 2PY (Aldrich, 97%) heated to 95 °C. Mass-selective two-color resonant two-photon ionization (2C-R2PI) spectra were measured by orthogonally crossing the skimmed supersonic jet with the unfocused UV excitation and ionization laser beams that overlapped within the source of a time-of-flight (TOF) mass spectrometer. Excitation was performed with UV pulses (70−80 μJ) from a Nd:YAG pumped frequency-doubled dye laser (Lambda-Physik, 0.4 cm−1 bandwidth) over the range 29 850−32 250 cm−1. The excited-state species were ionized by the fourth harmonic of a Nd:YAG laser (266 nm, 600 μJ/ pulse). Mass- and isotopomer-specific IR spectra were measured with the IR-UV double resonance method44−47,53,60 by overlapping the IR and the two UV laser beams nearly collinearly within the source of a linear time-of-flight mass spectrometer. We held the UV excitation laser energy at ∼500 μJ/pulse to optically saturate the respective S1 ← S0 origin transitions and thereby minimize the effect of laser intensity fluctuations on the ion signal. Every second UV laser pulse was preceded by 100 ns with a counter-propagating, spatially
experiments have so far tested this interesting prediction, we here investigate the mass-specific infrared (IR) spectra of 2PY·(NH3)n, n = 1−3 clusters. These are based on IR cluster depletion combined with mass-selective two-color resonant two-photon ionization (2C-R2PI), or IR/UV depletion spectroscopy for short. We complement the IR spectra studies by DFT calculations of the cluster structures and spectra. Because the n = 3 cluster may also exhibit nonclassical NH···π interactions, we have employed recently developed density functionals that are designed to include dispersive interactions. These calculations reproduce the accurately known cluster structures for n = 1 and 215 and allow us to assign the observed IR bands in the 2600−3700 cm−1 region. The respective cluster structures are shown in Figure 1.
Figure 1. B97-D calculated ground-state structures of the 2pyridone·(NH3)n clusters (n = 1−3). The successive ammonia moieties are labeled A1, A2, and A3. All optimizations converged to Cs symmetric geometries.
2. METHODS 2.1. Density Functional Theory Calculations. The S0 state minimum energy structures of 2PY·(NH3)n with n = 1−3 were fully optimized using density functional theory (DFT). On one hand, we employed the PW91 and B3LYP functionals and the 6-311++G(d,p) basis set. Previous computational studies on H-bonded complexes and clusters18,23,44−52 have confirmed that the PW91 and B3LYP functionals yield good geometries and vibrational frequencies of H-bonded complexes and clusters. The unscaled PW91 harmonic frequencies have been successfully used to assign IR spectra of H-bonded dimers.36,37,48,53−57 In this work we include two newer functionals that are designed to model weak intermolecular interactions, the M06-2X functional [with the 6-311++G(d,p) basis set] and the B97-D functional with the def2-TZVPP basis B
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overlapped IR pulse (∼5 mJ) generated by a 10 Hz LaserVision OPO/OPA system61 that was mildly focused by a 1000 mm CaF2 lens. When the OPO/OPA frequency coincides with an IR transition out of the S0 state level being monitored by R2PI, the population of this level decreases, thereby depleting the R2PI signal from that level. The difference in the 2C-R2PI ion signal obtained without and with the IR laser present was recorded as a function of IR frequency. The IR depletion spectra of cluster size n were recorded in the respective 2PY·(NH3)n+ mass channels. The signal was recorded on a shot-by-shot basis with a LeCroy Wavepro 7300 digital oscilloscope for 50 laser shots per frequency increment. The IR wavelength was recorded in parallel. The data were periodically transferred to a PC and subsequently processed with an IDL (Interactive Data Language) program.
3. TWO COLOR RESONANT TWO PHOTON IONIZATION SPECTRA Figure 2 shows the measured 2C-R2PI vibronic spectra of 2PY·(NH3)n with n = 1 and 2. The electronic origin of the n = 1
Figure 3. (a) Overview of the 2C-R2PI spectra of 2-pyridone·(NH3)n with n = 3 (upper spectrum) and n = 4 (lower spectrum). (b) 2CR2PI spectrum of 2-pyridone·(NH3)3 corrected for the fragmentation from the n = 4 cluster.
denoted 3X. The second group of bands exhibits an intense origin at 30 624 cm−1; this isomer is denoted 3Y. The spectral shifts relative to the 2PY monomer amount to δν = +569 or +594 cm−1 for 3X and δν = +797 cm−1 for 3Y. The n = 4 spectrum is about 10 times weaker than that of n = 3. It also exhibits two groups of bands that lie at similar (but not identical) transition frequencies, which we denote 4X and 4Y. Close scrutiny of the two spectra in Figure 3a reveals that about 70−80% of the R2PI spectrum of the 4Y cluster also appears in the n = 3 mass channel, reflecting 4 → 3 cluster fragmentation. In contrast, 4X exhibits much less fragmentation to n = 3. Figure 3b shows the n = 3 spectrum from which the n = 4 spectrum scaled by 2.5× has been subtracted. This removes a large part of the broad background from the raw n = 3 spectrum and allows us to see additional features, such as a 80 cm−1 vibrational progression of 3Y.
Figure 2. Overview of the 2C-R2PI spectra of 2-pyridone·(NH3)n, n = 1 and 2.
complex is at 30007 cm−1 and is blue-shifted by δν = +176 cm−1 compared to that for bare 2PY.13,15,23 We have previously given a detailed assignment23 of the inter- and intramolecular bands in the vibronic spectrum.13,15 The electronic origin of 2PY·(NH3)2 lies at 30 189 cm−1 and is shifted by δν = +358 cm−1 compared to that for 2PY.13,15 The 2C-R2PI spectra of the n = 3 and 4 clusters are reported here for the first time and are shown in Figure 3. The n = 3 spectrum in Figure 3a exhibits two distinct groups of bands, one starting at either 30 400 or 30 425 cm−1 with long progressions in low-frequency vibrations; this cluster will be
4. EXPERIMENTAL IR SPECTRA AND COMPARISON TO THEORY 4.1. 2-Pyridone·NH3. The IR/UV depletion spectrum of 2PY·NH3 was detected at the 30 007 cm−1 UV band that is vertically labeled in Figure 2a. The corresponding IR spectrum in the range 2900−3700 cm−1 is plotted in Figure 4 together with the IR spectra calculated with the DFT methods discussed above. The DFT frequencies and band intensities are also given in Table 1. C
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Figure 4. IR/UV-depletion spectrum of 2PY·NH3 (R2PI detection at 30 007 cm−1). The IR stick spectra calculated with the B97-D, M062X, B3LYP, and PW91 functionals are shown for comparison. Note the logarithmic band intensity scale. The respective vibrational bands and intensity modes and the proposed assignments of the experimental IR bands are indicated by dashed lines.
Figure 5. Normal-mode eigenvectors and frequencies of the 2PY and ammonia NH stretches of 2-pyridone·NH3, calculated with the B97-D functional (def2-TZVPP basis set).
The (2PY)NH stretch normal-mode eigenvector is shown in Figure 5a. The multiple band structure between 2950 and 3155 cm−1 arises by intramolecular harmonic mixing of this (2PY)NH stretch with the four CH stretch fundamentals of 2PY. Judging from the IR spectrum, the mixing of the different levels is strong and it is difficult to associate individual bands with specific vibrations. However, because the main contribution to the IR intensity of the 2950−3155 cm−1 group of bands derives from the H-bonded (2PY)NH stretch, we associate it with the most intense bands at 3085/3104 cm−1. Because about ten strong IR bands are observed and there are only one NH and four CH fundamentals, five bands must arise as combination or overtone bands. For the ammonia NH stretches, all methods predict a Hbonded symmetric (s) stretch, a non-H-bonded or “free” symmetric (s′) stretch and a free antisymmetric (as) stretch; the respective normal-mode eigenvectors are shown in Figure 5b−d. We assign the H-bonded and free symmetric NH
stretches to the strong and medium-strong bands at 3303 and 3400 cm−1, respectively (Table 1). The much weaker IR band at 3436 cm−1 is assigned as the antisymmetric (a″) stretch fundamental. The remaining intense band at 3248 cm−1 is assigned to the first overtone of the NH3 bend vibration, whose fundamental is calculated to lie at 1650 cm−1 (Table 1). Although the large intensity of this band might seem unusual, the only alternative is to exchange the assignment with that of the H-bonded ammonia NH stretch at 3303 cm−1. We prefer the first alternative because it is consistent with the IR spectra of the n = 2 and 3 clusters, see below. The IR fundamental frequencies calculated with the different functionals are listed in Table 1. Although M06-2X predicts harmonic frequencies that are too far blue-shifted, a much better fit to the experiment is observed for the B97-D harmonic
Table 1. Calculated Infrared Frequencies (cm−1) and Intensities (km/mol) of 2-Pyridone·NH3 B97-Da
a
M06-2Xb
B3LYPb,c
PW91b
label
irrep
frequency
intensity
frequency
intensity
frequency
intensity
frequency
intensity
(2PY)C4−H stretch (2PY)C6−H stretch (2PY)C3−H stretch (2PY)C5−H stretch (2PY)N−H stretch NH3 bend NH3 bend NH3 symm stretch (s) NH3 symm stretch (s′) NH3 antisymm stretch
a′ a′ a′ a′ a′ a′ a′ a′ a′ a″
3100.3 3128.0 3145.5 3159.1 3123.7 1606.3 1629.6 3354.0 3491.4 3534.9
18.3 288.4 7.24 6.4 704.5 50.6 68.8 87.6 33.6 3.7
3213.1 3236.4 3257.9 3271.8 3242.3 1628.1 1656.7 3465.0 3611.9 3658.8
13.9 97.3 4.3 3.3 870.4 56.6 43.5 91.4 51.1 12.3
3014.1 3095.3 3076.1 3086.1 3021.2 1645.9 1664.7 3322.2 3390.3 3427.2
12.7 6.8 9.6 2.7 880.7 93.3 13.0 109.5 34.5 7.1
3102.6 3124.9 3145.8 3159.7 3009.8 1596.3 1617.3 3306.1 3473.4 3530.3
13.4 5.5 3.4 4.6 1159.1 91.1 15.5 179.3 26.1 6.9
def2-TZVPP basis set. b6-311++G(d,p) basis set. cAnharmonic frequencies (intensities from harmonic calculation). D
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calculations. Note that the (2PY)NH stretch is predicted to fall into the center of the CH stretch frequencies. The B3LYP anharmonic calculation is in good overall agreement, note especially that the ammonia NH stretches are in very good correspondence with experiment. However, the (2PY)NH stretch band is shifted about 100 cm−1 too far to the red. The PW91 harmonic IR spectrum is also in good agreement with experiment, but as for the anharmonic B3LYP calculation, the (2PY)NH stretch is too strongly separated from the CH stretches compared to experiment. 4.2. 2-Pyridone·(NH3)2. The IR/UV depletion spectrum of 2PY·(NH3)2 was detected at the 30 189 cm−1 UV band that is vertically labeled in Figure 2b. The IR/UV-depletion spectrum of the n = 2 cluster was measured from 2600 to 3650 cm−1 and is shown in Figure 6 along with the calculated IR spectra.
Figure 7. Normal-mode eigenvectors and frequencies of hydrogenbonded NH stretches of 2-pyridone·(NH3)2, calculated at the B97-D/ def2-TZVPP level.
are assigned to the bands at 3175 and 3225 cm−1. The respective normal-mode eigenvectors and frequencies are shown in Figure 7b,c. The free (s) ammonia NH stretches, shown in Figure 8a,b, are assigned to the IR transitions at 3315 and 3380 cm−1. These show a smaller red shift of −75 to −130 cm−1, relative to n = 1. The two HNH bend overtones are assigned to the narrow and intense bands at 3225 cm−1 for A1
Figure 6. IR/UV-depletion spectrum of 2PY·(NH3)2 (R2PI detection at the n = 2 origin at 30 189 cm−1) compared to the IR stick spectra calculated with the four DFT methods as in Figure 4.
Compared to the n = 1 IR spectrum the respective IR bands all shift to lower frequencies. The largest shift is observed for the hydrogen-bonded (2PY)NH stretch, which is assigned to the intense band at 2890 cm−1. The B97-D calculated normal-mode eigenvector is shown in Figure 7a. The spectral shift of −200 cm−1 from n = 1 to 2 is due to the nonadditive increase of hydrogen-bond strength in the homodromic (2PY)NH···NH3···NH3···OC bridge, which moves the NH fundamental below the CH stretch region. This shift also substantially decreases the intramolecular coupling between the NH and CH vibrations. Note that for n = 2 the multiple-band structure extends from 2850 to 3150 cm−1. The CH fundamentals are located at the upper end (3000−3150 cm−1) of this pedestal; hence the bands at the lower end (2850−3000 cm−1) must be combination or overtone bands of bending and twisting fundamentals near the (2PY)NH group, which gain intensity by Fermi resonance with the NH stretch. Three of these vibrations appear weakly in the IR spectrum of n = 1 at 2900, 2950, and 3000 cm−1 (Figure 4). The two ammonia units (A1 and A2 in Figure 1) give rise to three pairs of ammonia NH stretches. The H-bonded stretches
Figure 8. Normal-mode eigenvectors and frequencies of the free NH stretches of 2-pyridone·(NH3)2, calculated at the B97-D/def2-TZVPP level. E
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Table 2. Calculated Infrared Frequencies (cm−1) and Intensities (km/mol) of 2-Pyridone·(NH3)2 B97-Da
a
M06-2Xb
B3LYPb,c
PW91b
label
irrep
frequency
intensity
frequency
intensity
frequency
intensity
frequency
intensity
(2PY)N−H stretch (2PY)C4−H stretch (2PY)C6−H stretch (2PY)C3−H stretch (2PY)C5−H stretch NH3 (1) bend NH3 (1) bend NH3 (2) bend NH3 (2) bend NH3 (1) symm stretch (s) NH3 (2) symm stretch (s) NH3 (1) symm stretch (s′) NH3 (2) symm stretch (s′) NH3 (1) antisymm stretch NH3 (2) antisymm stretch
a′ a′ a′ a′ a′ a′ a′ a′ a′ a′ a′ a′ a′ a″ a″
2921.6 3099.2 3123.2 3143.7 3197.9 1610.0 1637.1 1622.0 1663.5 3170.1 3305.0 3454.5 3469.5 3528.8 3527.7
1559.76 18.1 7.6 6.5 6.8 50.5 60.3 23.2 12.6 398.3 371.9 26.0 74.9 0.9 2.6
3031.5 3210.7 3231.6 3254.7 3267.4 1634.7 1671.1 1654.6 1703.9 3331.7 3453.3 3578.1 3603.8 3649.5 3655.4
1641.2 11.7 4.5 1.5 3.0 59.3 34.7 29.9 63.8 336.9 310.0 44.0 120.8 9.2 6.9
3097.0 3163.6 3189.7 3205.5 3218.4 1594.5 1651.2 1659.5 1672.5 3286.2 3396.2 3518.5 3540.0 3580.6 3588.3
1333.2 12.2 5.6 3.2 4.6 50.03 49.2 44.0 33.9 341.2 339.4 29.9 80.2 4.4 3.4
2810.3 3101.2 3119.7 3144.2 3157.9 1601.2 1612.6 1657.2 1671.3 3103.1 3273.4 3444.5 3463.0 3515.7 3522.9
1801.1 16.4 13.0 4.3 5.7 53.8 34.0 16.8 12.9 493.9 515.8 20.0 56.4 4.1 3.3
def2-TZVPP basis set. b6-311++G(d,p) basis set. cAnharmonic frequencies (intensities from harmonic calculation).
and at 3248 cm−1 for A2. The calculations predict the two antisymmetric NH stretches to be very weak. The respective normal-mode eigenvectors and frequencies are shown in Figure 8c,d. Their frequencies differ by only 1 cm−1; hence we assign both to the weak band at 3429 cm−1 in Figure 6. These bands are shifted by only −7 cm−1, compared to those for n = 1. The frequencies and intensities for the n = 2 cluster calculated with the different functionals are listed in Table 2. Again, the B97-D harmonic frequencies and intensities are in very good agreement with the observed frequencies. The M062X and in this case also the B3LYP calculated spectra are clearly shifted too far to the blue. The PW91 calculation is also in good agreement with the experimental IR spectrum but predicts the H-bonded (2PY)NH and ammonia NH stretches at too low frequencies. 4.3. 2-Pyridone·(NH3)3. Because for n = 3 the 2C-R2PI spectrum indicates the existence of two cluster isomers (see above), the IR spectrum of isomer 3X was measured while detecting at the 30 425 cm−1 R2PI band, whereas that of isomer 3Y was detected at 30 624 cm−1 (Figure 3). The two IR spectra differ very significantly. The experimental and calculated IR spectra of 3X and 3Y, computed with the same four DFT functionals as above, are shown in Figures 9 and 10. The calculated frequencies and IR band intensities are listed in Tables 3 and 4. A striking feature of the IR spectrum of 3X in Figure 9 is the absence of an intense band at low frequency corresponding to the (2PY)NH stretch. For n = 0−2, the (2PY)NH stretch shifts progressively to the red, but not a single IR absorption band is observed below 3000 cm−1 in Figure 9. A possible cause is desolvation of the cis-amide group, which causes the (2PY)NH stretch to shift to the blue. The calculated IR spectra of isomer 3X in Figure 9 support this explanation: The B97-D and M062X calculations predict that the (2PY)NH stretch shifts to higher frequency. Only the anharmonic B3LYP calculation predicts a further red shift, in disagreement with the spectrum. Although the PW91 functional also predicts a blue shift of (2PY)NH stretch, it is small, and the entire IR spectrum remains too far red-shifted. Most of the bands in the 3200−3400 cm−1 region are assigned to NH stretch vibrations of the 2PY and of the A1, A2, and A3 ammonia units, as indicated in Figure 9. The ammonia
Figure 9. IR/UV-depletion spectrum of isomer 3X of 2PY·(NH3)3 (with R2PI detection at 30 425 cm−1), compared to the four DFT calculated IR spectra.
bend overtones are assigned to the two sharp bands at 3225 and 3240 cm−1. The third bend overtone could not be identified and may be overlapped with another band. The antisymmetric stretch vibrations of all three ammonia units have low calculated intensities; they are not experimentally observed. In contrast to the IR spectra of the other clusters, that of isomer 3Y exhibits a weak and broad absorption feature extending from 2500 to 2900 cm−1, which we assign to the Hbonded (2PY)NH stretch (Figure 10). The integrated area of F
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increases its strength as an H-bond acceptor beyond that of the A1 subunits in n = 1 and 2 in which A1 is a single H-donor. This strengthens the (2PY)NH···N hydrogen bond and decreases the (2PY)NH stretch frequency. Indeed, the DFT calculated IR spectra of isomer 3Y predict large red shifts of the (2PY)NH stretch relative to n = 1 and 2. The B97-D and the M06-2X methods predicts shifts of −250 to −300 cm−1, placing the (2PY)NH stretch within the observed broad IR absorption. In contrast, the B3LYP and the PW91 calculations predict shifts of −600 to −650 cm−1, which is lower than the broad feature. The four intense and medium-width IR bands between 3200 and 3514 cm−1 are assigned to the hydrogen-bonded NH stretches of ammonias A1 to A3. Because isomer 3Y is nonsymmetric and A1 is a double H-bond donor, both of its Hbonded NH stretches are red-shifted. Unlike the antisymmetric stretches of A2 and A3, which do not shift much and have very small intensities, the (as) transition of A1 is now also intense; we assign it to the 3314 cm−1 band. The free NH stretches form a group of medium-strong and sharp bands at 3398, 3414, and 3425 cm−1. The HNH bend overtone bands of the three ammonia units are not as clearly visible for isomer 3Y as for 3X. However, the bands in the multiple band structure between 3200 and 3500 cm−1 are broader and could thus comprise several close-lying bands. Figure 10. IR/UV-depletion spectrum of isomer 3Y of 2PY·(NH3)3 (R2PI detection at 30 624 cm−1), compared to the four DFT calculated IR spectra. The broad and weak absorption feature between 2500 and 2900 cm−1 is shaded in gray.
5. DISCUSSION 5.1. (2PY)NH Stretch Frequency. Figure 11 plots the experimental (2PY)NH stretch frequencies of 2PY·(NH3)n for n = 0−3 and compares them to the calculated harmonic frequencies. The B97-D harmonic frequencies are seen to be in overall best agreement with experiment, the differences being typically
