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Brightening and Locking a Weak and Floppy N-H Chromophore: The Case of Pyrrolidine Susanne Hesse, Tobias N. Wassermann,† and Martin A. Suhm* Institut fu¨r Physikalische Chemie, UniVersita¨t Go¨ttingen, Tammannstrasse 6, 37077 Go¨ttingen, Germany ReceiVed: June 15, 2010; ReVised Manuscript ReceiVed: August 18, 2010
The N-H stretching signature of the puckering equilibrium between equatorial and axial pyrrolidine is analyzed via FTIR and Raman spectroscopy in supersonic jets as a function of aggregation. Vibrational temperatures along the expansion axis can be extracted from the Raman spectra and allow for a localization of the compression shock waves. While the equatorial conformation is more stable in the ground state monomer, this preference is probably switched in the excited state with one N-H stretching quantum. Furthermore, the dominant dimer involves an axial donor and the trimer and tetramer structures seem to prefer uniform axial conformations. The IR intensity is boosted by up to 3 orders of magnitude upon aggregation, whereas the Raman scattering intensity shows only moderate hydrogen bond effects. B3LYP and MP2 calculations provide a reasonable description of the N-H vibrational dynamics under the influence of self-aggregation. In mixed dimers with pyrrole, pyrrolidine assumes the role of a hydrogen bond acceptor. 1. Introduction If the molecular dipole moment changes little with vibrational displacement, hydride stretching vibrations have a low visibility in the infrared spectrum. This is often the case for aliphatic N-H bonds. If the molecule is in addition highly fluxional, its low infrared intensity is distributed over several hot transitions. Such is the case for pyrrolidine (C4NH9), an important building block for alkaloids and pharmaceuticals,1-4 and for proline with a soft ring puckering potential. Recently, the large amplitude puckering dynamics of the pyrrolidine ring has been studied in detail using rotational coherence spectroscopy.5 The equatorial form was found to be 0.2-0.5 kJ mol-1 more stable than the axial one (see also Figure 1 for structures), and the barrier separating them was found to be close to 2.6 kJ mol-1. This has resolved and quantified a long-standing issue that had previously been addressed by electron diffraction6 and microwave/millimeter-wave spectroscopy7,8 as well as electronic structure calculations.5,6,8-12 The very soft and anharmonic pseudorotation potential and the weak transition moments of the equatorial ground state in both microwave and infrared spectroscopy pose challenges to experiment and theory alike. Low temperatures are helpful by concentrating the population on a few, relatively localized states, thus bringing experimental and theoretical standpoints closer together. Aggregation via hydrogen bonds can change the IR transition moment by orders of magnitude. We thus present a supersonic jet study of pyrrolidine and its aggregates, using infrared and Raman excitation of the N-H chromophore. While the coupling of FTIR spectroscopy to jet expansions has now been established for more than two decades,13,14 spontaneous Raman scattering techniques15 have only become feasible for the study of supersonic expansions within the last years.16,17 Our FTIR18,19 and Raman setups20-22 were developed for the study of molecular conformations and aggregation phenomena. The NH stretching fundamental of pyrrolidine has been assigned before in the gas phase via IR spectroscopy near 3356 * Corresponding author. E-mail:
[email protected]. † Present address: Laboratoire de Chimie Physique Mole´culaire, E´cole Polytechnique Fe´de´rale de Lausanne, CH-1015 Lausanne, Switzerland.
Figure 1. N-H stretching region of the gas phase FTIR and Raman spectra of pyrrolidine. (a) FTIR spectrum measured at 0.09 bar and a concentration of ≈1% pyrrolidine in helium. (b) Raman spectrum measured at 0.2 bar and a concentration of e1% pyrrolidine in helium.
cm-1 with very low intensity.10 A long path length cell investigation23 revealed a sequence of further bands below 3356 cm-1, which were attributed to puckering motion. No evidence for a second isomer was observed and the transitions were usually assigned to the equatorial conformation.12 N-H overtone transitions are more pronounced24 but also more difficult to interpret. Raman spectra in the liquid state did provide evidence for two conformations,10 but a correlation of this hydrogenbonded situation to the isolated gas phase is not reliable. Evidence for dimers is even more scarce, concentrating on two studies in CCl4 solution25,26 and on a matrix isolation investigation.23 The approximate vibrational red shifts of 90-100 cm-1 relative to the monomer are poorly determined due to the uncertainty about the monomer conformation, as we will see. In any case, the free and rather weakly hydrogen-bonded N-H modes are sufficiently well decoupled from the lower frequency
10.1021/jp105517b 2010 American Chemical Society Published on Web 09/10/2010
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aliphatic C-H vibrations or framework combination states. This is different for strongly hydrogen bonded systems such as pyrazoles.27 2. Methods FTIR spectra at 2 cm-1 resolution were measured in a pulsed slit jet expansion.19 The jet chamber was crossed by a mildly focused and recollimated Bruker Equinox 55 probe beam on a total length of 0.78 m. Either the equilibrated gas phase or a supersonic jet expansion emerging from a 0.6 m slit nozzle with a slit width of 0.2 mm was probed close to the nozzle over a varying cross section with an average diameter of about 15 mm. The compounds (pyrrolidine 99+ %, pyrrole 99%, Acros) were highly diluted in a carrier gas (99.996% He). For details, see ref 28. Variations in expansion pressure and concentration allowed for an assignment of absorptions to either isolated monomers or dimers or larger clusters. The reported approximate gas phase concentrations are based on Antoine parameters and reference measurements. Spontaneous Raman scattering measurements22 were carried out using gas mixtures of pyrrolidine in He, Ne, or Ar/He. They were expanded via a 8 × 0.05 mm2 slit nozzle, pumped by a 250 m3 h-1 Roots pump. The beam of a frequency doubled cw NdYVO4-laser (Coherent Verdi V18, 18 W, λ ) 532 nm) was focused onto the expansion at distances from the nozzle exit of 1-4 mm. For details, see refs 20 and 21. Variation of the nozzle distance21 or the carrier gas29 can lead to relaxation effects by collisions of the molecules with the carrier gas atoms. At 90° angle, the scattered light was collected and collimated using a fast camera lens (Nikon, L ) 50 mm, f/1.2). It was then focused onto the entrance slit of the monochromator (McPherson Model 2051 f/8.6, f ) 1000 mm) using an achromatic planoconvex lens (Edmund Optics, L ) 50 mm, f/7). A Raman edge filter (L.O.T., L ) 25 mm, OD6.0, T > 90%, 535.4-1200 nm) was used to filter Rayleigh scattered photons before the dispersion by the monochromator. A liquid N2 cooled back-illuminated CCD camera (PI Acton, Spec-10: 400 B/LN, 1340 × 400 pixel) served as the detection device. The primary wavelength calibration of the spectra was carried out using the lines of a Ne I emission light source. A secondary calibration shift by -1 cm-1 was indicated by the experimental IR data of pyrrolidine. Cosmic ray signals were removed by the comparison of block-averaged spectra. Exploratory quantum chemical calculations (optimizations and harmonic frequency calculations) were used to assist the assignment of the spectra and to characterize possible dimer and cluster structures underlying the spectral shifts. They were obtained at B3LYP, B97D, and MP2 level using the Gaussian0330 and Gaussian0931 packages and a range of standard basis sets. 3. Results and Discussion Figure 1 shows conventional gas phase spectra of the N-H stretching fundamental using FTIR absorption and spontaneous Raman scattering techniques. There are regular sequences of bands in the Raman spectrum, a widely spaced one starting at 3325 cm-1 with some additional weaker features (A1) and a more narrow one starting at 3371 cm-1 (A2). As we will show, these sequences are due to N-H stretching transitions from excited states in the ring puckering potential, as illustrated in the schematic drawing in Figure 2. The blue shading of the sequences is due to a deepening of the conformational wells with N-H stretching excitation, which raises the hot band transitions above their ground state counterparts. The gradual
Figure 2. Schematic representation of the puckering process from the more stable equatorial form of pyrrolidine to the axial form (υNH ) 0) and the reversal of the energy sequence after N-H stretching excitation (υNH ) 1). Shown are the sequences of increasing puckering excitation together with the corresponding Raman spectral features from Figure 1. The global minima in the adiabatic curves are marked with a black dot.
spreading of the low frequency sequence with increasing excitation indicates anharmonic contributions in the puckering potential. The transition at 3357 cm-1, which was identified earlier as the band center in the IR23 is now recognized to be a late member of the more intense low frequency sequence. The IR spectrum shows consistent structured absorption in the low frequency sequence range, whereas the high-frequency sequence only appears as a broad, unstructured hump. Note that even the strongest IR lines are very weak, corresponding to an experimental band strength of less than 1 km mol-1. An approximate Boltzmann analysis of the dominant low-frequency Raman sequences in terms of hot band transitions indicates a more or less regular energy level spacing of 70 ( 20 cm-1. This is in line with the hot-band estimate of a puckering mode from microwave spectroscopy32 of 50 cm-1. The weaker signals may correspond to other low frequency modes or to cross-conformation transitions or less likely to inversion-puckering splittings.32 The higher frequency sequence is more pronounced in the Raman spectrum than in the IR spectrum, but its hot band sequence is less straightforward to analyze because of band overlap. From these classical vibrational spectra, a range of conclusions may thus be drawn in the light of the small energy gap between the two puckering conformations:5 (a) The N-H stretching fundamentals of the two conformers are separated by about 46 cm-1. This is more than the estimated ground state energy separation of the conformers. (b) The relative Raman and IR intensities behave oppositely for the two conformations and IR intensities do not exceed 1 km mol-1. (c) N-H stretching excitation deepens the well of the conformer contributing to the lower frequency bands, somewhat increasing the ring puckering frequency which was estimated near 70 cm-1. The anharmonic two-conformer picture is necessarily oversimplified in such a soft puckering case,5 but the regularity of the Raman spectrum suggests that it is still reasonably valid at
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Figure 3. N-H stretching region of the Raman spectra of e1% pyrrolidine in He, measured at a nozzle distance of 2 mm. (a) 0.2 bar gas phase spectrum, scaled by 1/30. (b), (c) Jet spectra for increasing concentrations at a stagnation pressure of =0.7 bar.
Figure 4. N-H stretching region of the Raman spectra of e1% pyrrolidine in helium. (a)-(d) Jet spectra expanded at a stagnation pressure of =0.7 bar at different nozzle distances: (a) 1 mm; (b) 2 mm; (c) 3 mm; (d) 4 mm. (e) Trace (a) from Figure 3, gas phase.
room temperature. It would be interesting to extend the very detailed puckering analysis in ref 5 to the adiabatic potential energy hypersurface including N-H stretching excitation. To assign the bands to specific conformers, a supersonic jet expansion is needed. The IR bands are so weak that they will be barely visible above the noise level in a state-of-the-art jet FTIR measurement.33 The polarizability change is more pronounced and the Raman jet spectra shown in Figure 3 indeed reveal the hot band character of the sequences. From the steeper intensity drop of the monomer sequence in the jet, an effective vibrational temperature of 60 ( 20 K may be deduced at a nozzle distance of 2 mm. As expected, it is higher than the estimated rotational temperature of 10-30 K. A more detailed analysis is invited by Figure 4, in which spectra at increasing nozzle distance (from bottom to top) are displayed. At 1 mm, the signal is maximum and the vibrational cooling has already progressed to 70 ( 30 K, as judged from
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Figure 5. Influence of different carrier gases, measured at a nozzle distance of 2 mm: (a) 100% helium; (b) 100% neon; (c) 5% argon in helium. All spectra are measured at a concentration of e0.3% pyrrolidine in the carrier gas. The stagnation pressure is =0.7 bar for (a) and (c) and =0.8 bar for (b).
the hot band structure. It saturates at 60 ( 20 K for 2 mm, where the divergence of the supersonic jet diminishes the overall scattering signal. This comes to a halt at 3 mm and at the same time there is a sharp rise of the vibrational temperature to 160 ( 40 K. Both observations are consistent with the onset of the normal shock wave due to the background pressure of about 3 mbar, but compression wave patterns in a slit nozzle can be complex. At a distance of 4 mm, the vibrational temperature has already reached 200 ( 40 K and the spectrum becomes rather similar to the stationary gas phase spectrum (top trace). The accompanying reduction and disappearance of the peaks marked D, Tr, and Te identifies those as hydrogen-bonded cluster bands, which can be differentiated on the basis of their stagnation pressure or monomer concentration scaling. Before addressing these cluster signals, Figure 5 can be used to identify the most stable monomer conformation. Exchange of He by Ne significantly reduces the relative intensity of the lower-frequency transition A1 and also reduces the puckering temperature in general, as evidenced by the largely eliminated hot-band structure (effective temperature of 30 ( 20 K). Addition of 5% Ar to the He is even more effective in reducing the monomer A1 but leaves more hot band structure due to puckering (effective temperature of 70 ( 20 K). Both experiments show unambiguously that the higher frequency N-H stretching component corresponds to the more stable structure, which is, according to earlier work,5,7 the equatorial conformation. The transformation from the local axial minimum to the global equatorial minimum may occur classically via the puckering coordinate or by quantum-mechanical tunneling of the N-H, in analogy to ammonia.32 The fact that argon is more efficient in the barrier crossing process provides indirect support for a mechanism via temporary complex formation.34 We note that the N-H stretching frequency of the equatorial conformation exceeds that of the axial conformation by 46 cm-1, which is more than the estimated ground state energy difference.5 As a consequence, the axial conformation is slightly lower in energy than the equatorial one, when N-H stretching excitation is added. This switch in conformational sequence is also indicated schematically in Figure 2. The different scaling of the D, Tr, and Te peaks with expansion conditions observed in the Raman spectra is even
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J. Phys. Chem. A, Vol. 114, No. 39, 2010 10495 TABLE 1: Supersonic Jet Peak Positions ν˜ N-H (cm-1) of Pyrrolidine and Pyrrolidine/Pyrrole Mixtures pyrrolidine
Figure 6. N-H stretching region of the IR jet spectra of pyrrolidine in helium at different concentrations and stagnation pressures. (a)-(e) Concentration increases monotonically by a factor of 7 up to ≈1% (stagnation pressure 0.4 bar except for 0.7 in (a)).
Figure 7. Comparison of the IR and Raman jet spectra of pyrrolidine in helium. The spectra included are (d) from Figure 6 (lower spectrum) and (c) from Figure 3 (upper spectrum).
more evident in the corresponding jet-FTIR spectra (Figure 6). While the monomer bands are almost completely absent, redshifted bands grow in with concentration. Their growth rate and bandwidth increases with increasing bathochromic shift, inviting an assignment in terms of the smallest cluster sizes, dimer, trimer, and tetramer. The relative weakness of the monomer bands points at an IR intensity enhancement due to hydrogen bonding by several orders of magnitude. The band profile is qualitatively reminiscent of that of a N2-matrix FTIR spectrum found in ref 23. Due to the lack of assignments in the matrix, a direct comparison of peak positions or shifts is difficult and does not appear useful at this point. Figure 7 compares a selected Raman jet spectrum with an FTIR jet spectrum. While the most red-shifted D bands (D1-D3) agree in position between the two spectra, the least red-shifted ones (D4 and D5) are missing in the IR spectrum. This points to their hydrogen bond acceptor origin, without profit from IR enhancement. One can see in Figure 5 that the dimer
label
ν˜ N-H
Te1 Te2 Te3 Tr1 Tr2 D1 D2 D3 D4 A1 D5 A2
3165 3200 ≈3204 3220 3233 3261 3268 3286 3307 3325 3365 3371
pyrrolidine/pyrrole
label
ν˜ N-H
C1 C2 P
3194 3227 3533
bands do not evolve uniformly upon a change of carrier gas. The peak marked D1 gains in intensity when switching to Ne and certainly corresponds to the most stable dimer. The other D-peaks tend to stay constant with the exception of the band marked D2, which decreases like the A1 satellite. It cannot be ruled out that it is a puckering hot band that still survives in the dimer. The D3 and D4 bands may correspond to less stable dimers, whereas D5 is likely a superposition of several bands. The Tr and Te bands are characteristically shifted between the two spectra in Figure 7, with the totally symmetric Raman transitions being lower in energy. This is a consequence of Davydov coupling between nearly equivalent oscillators35 in the underlying (quasi)symmetric clusters, which leads to different selection rules for IR and Raman probing. The experimental peak positions are summarized in Table 1. The following conclusions may thus be drawn from the jet spectra without reference to any calculations: (d) The more stable monomer conformation has a higher NH stretching frequency. (e) There are at least two dimer conformations present, with the more stable one having the lowest NH stretching vibration. (f) The trimers and tetramers have (nearly) equivalent monomers with sizable oscillator coupling and cooperative hydrogen bond enhancement, indicative of a ring structure. Next, we will explore simple theoretical model chemistries to see how well they explain these experimental findings. For the monomer energy sequence, the published theoretical predictions have been ambivalent. Some predict the axial structure or a related twist form to be lower6,9-11 and others the equatorial structure,12 depending on the electronic structure treatment.8 MP2 and CCSD(T) calculations require large basis sets to converge toward the correct energy sequence.5 The most recent calculations are uniform in predicting the equatorial structure below the axial one and the latter without symmetry breaking.5 The very flat, boxlike axial minimum5 is consistent with the diverging sequence observed in the lower frequency region of the NH spectrum. Inspection of the dipole moment change along the N-H stretching coordinate at various levels of computation reveals a qualitative reason for the better visibility of the axial conformations in the IR spectrum. The dipole change occurs mostly perpendicular to the N-H bond axis, in the direction of the N lone pair, whereas the component along the bond goes through a minimum close to the equilibrium distance. Therefore, the stretching band of the axial conformation is mostly of a-type, with pronounced Q-band transitions. In the equatorial conformation, the dipole change is mostly in the direction of the nearoblate top C axis and the intensity is more spread out among the rovibrational transitions. In addition to the weaker IR
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TABLE 2: Comparison of the Equatorial and Axial Conformations of the Pyrrolidine Monomers at Different Levels of Calculation and by Analysis of the Hot Band Multiplet in the Experimental Gas Phase Raman Spectraa method
basis
ν˜ eq - ν˜ ax
Ieq/Iax
σ′eq/σ′ax
E0eq - E0ax
ax ν˜ lowest
∠CCCCax
B3LYP B3LYP B97D MP2 MP2 MP2 MP2 experiment
6-31+G(d) 6-311++G(d,p) TZVP/fit 6-311+G(d) cc-pVTZ aug-cc-pVDZ aug-cc-pVTZ anharmonic
41 37 36 20 33 34 33 46
0.04 0.3 0.4 0.5 1.4 0.5 1.3
