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High resolution Fourier transform infrared spectra and analysis of the ν14, ν15 and ν16 bands of azetidine
Journal of Molecular Spectroscopy 264 (2010) 105–110
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Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms
High resolution Fourier transform infrared spectra and analysis of the m14, m15 and m16 bands of azetidine Taras Zaporozan, Ziqiu Chen, Jennifer van Wijngaarden ⇑ Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
a r t i c l e
i n f o
Article history: Received 9 August 2010 In revised form 18 September 2010 Available online 25 September 2010 Keywords: Azetidine Trimethylene imine FTIR Infrared Synchrotron Ring puckering Rovibrational
a b s t r a c t Rotationally resolved vibrational spectra of the three lowest frequency bands of the four-membered heterocycle azetidine (c-C3H6NH) have been collected with a resolution of 0.00096 cm1 using the far infrared beamline at the Canadian Light Source synchrotron. The modes observed correspond principally to motions best described as: b-CH2 rock (m14) at 736.701310(7) cm1, ring deformation (m15) at 648.116041(8) cm1, and the ring puckering mode (m16) at 207.727053(9) cm1. A global fit of 14 276 rovibrational transitions from the three bands provided an accurate set of ground state spectroscopic constants as well as excited state parameters for each of the three vibrational modes. The ground state structure was determined to be that of the puckered conformer having the NH bond in an equatorial arrangement. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Azetidine or trimethylene imine (c-C3H6NH) is the amine analog of the oxetane (c-C3H6O) and b-lactam (azetidinone, c-C2H4NHCO) heterocycles. Oxetane and azetidinone are common structural features of natural products known for their anti-cancer and anti-bacterial properties. The bioactivity of such molecules, for example Taxol (a chemotherapy drug) and penicillin, is linked to the stability of the four-membered ring subunit [1,2]. Reports of azetidine as a structural feature in natural products are much less common by comparison although derivatives have been found in some systems such as marine sponges [3]. In order to understand such vast differences in the stability and reactivity of large, complex molecules, it is instructive to start with simpler models such as the four-membered heterocycles themselves. The spectra and underlying potential energy surfaces of heterocycles are particularly interesting due to the presence of low frequency vibrational modes and the possibility of finding multiple stable conformers [4]. The low frequency ring puckering and pseudorotation motions of numerous small rings have been investigated and the subject has been extensively reviewed [4–9]. In the case of azetidine, there has been some uncertainty with respect to the potential form of the lowest frequency mode. The far infrared spectrum of the m16 ring puckering mode (and several hotbands) collected by Carreira and Lord [10] was originally fit to an ⇑ Corresponding author. Fax: +1 204 474 7608. E-mail address: [email protected] (J. van Wijngaarden). 0022-2852/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2010.09.010
asymmetric double well potential in which one minimum corresponded to an axial arrangement of the NH bond and the other to an equatorial position. The energy difference between the two conformers was found to be 95 cm1 with the barrier to interconversion lying 441 cm1 above the lower minimum. The far infrared data was later re-analyzed by Robiette et al. [11] and found to be consistent with a single well asymmetric potential with the equatorial conformer being the only stable structure. This is in agreement with ab initio calculations [12,13], Raman intensities [14] and a subsequent far infrared study [15]. The microwave and electron diffraction experiments of Günther et al. [16] confirmed the presence of the equatorial conformer and determined its structure to have a ring puckering angle of 29.7° although their microwave spectrum also contained a weak unassigned feature which was thought to be due to the axial conformer. The more recent microwave and ab initio investigation of López et al. [17] analyzed the rotational spectrum in the ground and several excited ring puckering vibrational states. Their results point to a single well asymmetric potential along the ring puckering coordinate. Interestingly, they also observed tunneling doublets which were attributed to a concerted motion involving NH inversion and ring puckering through a C2v barrier estimated to be 1900–2600 cm1. In addition to the extensive investigation of the ring puckering mode of azetidine, the higher frequency modes have been calculated via ab initio methods and the infrared spectrum has been measured in the gas phase [16,18], in solution [18,19] and in a solid argon matrix [20]. Dutler et al. [18] recorded the gas phase infrared spectrum from 400 to 3400 cm1 at a resolution of 0.12 cm1 and
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assigned the observed transitions to normal modes using scaled ab initio force field calculations. We have adopted their convention for labeling the vibrational modes of azetidine and their descriptions of the normal mode character. In this work, we report the rovibrational assignment and analysis of the m14, m15 and m16 bands of azetidine using data collected at the far infrared endstation of the Canadian Light Source (CLS). The former two modes together account for approximately 70% of the NH out-of-plane bending motion of azetidine and the latter is the ring puckering mode. A global fit of the three bands was performed to obtain a single set of ground state spectroscopic constants for comparison with previous microwave studies. The fit also provided accurate determination of the m14, m15 and m16 band origins and excited state parameters. To our knowledge, this work represents the first high resolution infrared study of azetidine.
2. Experimental details The far infrared endstation at the Canadian Light Source (CLS) is equipped with a Bruker IFS125HR spectrometer which can be used to obtain exceptionally high quality vibrational spectra with a resolution of up to 0.00096 cm1. A recent infrared study of pyrrole (another N containing heterocycle) compared a spectrum recorded using synchrotron radiation from the CLS with that collected using a globar source and showed a factor of eight improvement in the signal to noise ratio when synchrotron light was used [21]. This so-called ‘synchrotron advantage’ arises from the high degree of spatial confinement of the synchrotron beam. When small apertures are employed as for high resolution studies, the synchrotron beam passes through the iris with minimal loss providing greater brightness than conventional sources. The ‘synchrotron advantage’ is typically realized in the 300–800 cm1 spectral range at the CLS at present but efforts are ongoing to increase the level of the advantage and expand its range. Rotationally resolved infrared spectra of the m14, m15 and m16 bands of azetidine were recorded at room temperature using vapor pressure from a liquid azetidine sample (98% purity, Sigma–Aldrich). Although a large 2 m multipass gas cell is available at the CLS providing path lengths of up to 80 m, the cost of the azetidine sample ($450 per g) made filling this cell prohibitively expensive. We thus opted to use the 30 cm White cell set to a total absorption path length of 7 m. For the lowest frequency band corresponding to the m16 ring puckering mode in the 100–350 cm1 range, the spectrometer was outfitted with a 6 lm Ge-mylar beamsplitter and a helium cooled Si bolometer detector. The data were recorded at 0.00096 cm1 resolution with a sample pressure of 700 mTorr. The final spectrum of the m16 band was calculated by averaging 17 separate interferograms collected over 3 h and applying a Fourier transform. The averaged background interferogram, taken at lower resolution (0.01536 cm1) was also transformed and the absorption spectrum calculated using the formula ln(I/Io). Data for the m14 and m15 bands were recorded at 0.00096 cm1 resolution between 500 and 850 cm1 using a KBr beamsplitter, GeCu detector and an optical filter (490–1190 cm1). As the bands were stronger than the ring puckering mode, the pressure in the White cell was reduced to 60 mTorr and 112 interferograms were used to calculate the final spectrum as described for the m16 band above. The m16 spectrum was calibrated using H2O transitions observed in the 130–300 cm1 region and comparing the observed line positions with those in the HITRAN database [22]. The same procedure was followed for the higher frequency spectrum using the observed residual CO2 lines centered near 667 cm1. In the end, the calibration made little difference in the analysis (less than
0.0001 cm1 for most transitions in the m16 spectrum and less than 0.0007 cm1 for most transitions in the higher frequency spectrum) indicating that the Bruker IFS125HR was well-calibrated. A complete listing of the assigned transitions is provided as Supplementary material. 3. Spectral assignment and analysis Azetidine is a near oblate asymmetric top with rotational constants A B and an asymmetry parameter j = 0.95. The ground state structure has Cs point group symmetry with a ring puckering angle of 29.7° relative to the ab-plane of the molecule [16]. The three modes studied in this work correspond to motions that are principally due to ring puckering (m16), ring deformation (m15) and b-CH2 rocking (m14) although the two latter modes also have significant character that corresponds to NH out-of-plane bending and a-CH2 twisting [18]. The microwave spectrum of the ground state and several excited ring puckering states of azetidine was analyzed in terms of the C2v molecular symmetry group to allow for the description of the observed NH inversion tunneling splittings [17]. This led to the determination of two sets of spectroscopic constants for each vibrational state corresponding to symmetric (A1) and antisymmetric (B1) inversion tunneling components. Using the C2v point group, the m16 ring puckering mode and the m15 and m14 modes (having NH out-of-plane bending character) also belong to the B1 irreducible representation. If NH inversion tunneling is coupled to any of these three modes, one would expect a-type rotation–vibration–inversion transitions with A1 symmetry and c-type rotation–vibration transitions with B1 symmetry. Based on the similarity in the rotational constants determined for the NH inversion tunneling states in Ref. [17] (which differ by a few kHz or less), the a- and c-type transitions will not be doubled in the infrared spectrum as the calculated splittings fall below the resolution of the spectrometer. In this work, each of the three vibrational bands of azetidine was first assigned and fit independently. Once the individual assignments were complete, a global analysis of all three bands was performed to obtain a common set of ground state spectroscopic parameters. The analysis is detailed below. 3.1. The m16 ring puckering band An overview spectrum of the m16 ring puckering mode at 207.7 cm1 is shown in Fig. 1. The observed band contour is typical of a-type selection rules (DJ = 0, ±1, DKa = 0 and DKc = ±1) and the line widths were 0.0012 cm1 (FWHM) due to slight pressure broadening at 700 mTorr. Upon closer inspection, a series of regularly spaced progressions was observed in the R branch as shown in Fig. 2. To assign these transitions, the spectrum was first simulated using the available rotational constants and band origin as input to the JB95 program [23]. The R branch progressions were determined to follow a sequence of quantum numbers such that adjacent lines in a given progression follow the trend DJ = 1, DKa = +1 and DKc = 2 toward the band center as labeled in Fig. 2. Once the pattern was recognized, ground state combination differences were used to confirm the quantum number assignment of the first few progressions of this type. In total, 3278 a-type transitions were assigned for the m16 ring puckering mode of azetidine including 928 transitions in the Q branch. The spanned quantum numbers of assigned transitions include those in the P and R branches from J0 = 4 through 55 and in general, all except the highest Ka transitions in any progression were strong enough for assignment. In the Q branch, progressions having K 0c ¼ 5 through 32 were assigned for J values up to 46. The
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Fig. 3. Overview of the a-type band contours of the m14 and m15 bands of azetidine at 736.7 cm1 and 648.1 cm1, respectively. Strong, evenly spaced transitions due to the 667 cm1 bending mode of residual carbon dioxide are also seen in this region. Fig. 1. Overview of the a-type band structure of the m16 ring puckering mode of azetidine at 207 cm1. Strong transitions due to residual water in the gas cell are seen throughout this region.
Fig. 2. Sample spectrum showing the rotational structure of a typical portion of the R branch of the m16 ring puckering mode of azetidine. This region shows three overlapping progressions with representative transitions labeled with asymmetric rotor quantum numbers ðJ0 K 0a K 0c —J00 K 00a K 00c Þ for each vibrational level.
assigned rovibrational transitions of the m16 band were preliminarily fit using Watson’s A-reduced Hamiltonian, Ir-representation in Pickett’s SPFIT program [24]. Although only 17 parameters were used (three rotational constants and five centrifugal distortion constants for each state plus the m16 band origin), the rms error of the fit was only 0.000130 cm1. The spectroscopic constants derived from the analysis of the a-type transitions were used to calculate the c-type transitions (predicted using C2v symmetry considerations) but no such transitions were observed in the spectrum.
Fig. 4. Sample spectrum showing the rotational structure of a typical portion of the R branch of the m14 mode of azetidine. This region shows three overlapping a-type progressions with representative transitions labeled with asymmetric rotor quantum numbers ðJ0 K 0a K 0c —J00 K 00a K 00c Þ for each vibrational level. The weak clusters of transitions follow c-type selection rules.
with quantum numbers J0 = 5 through 55 and progressions in the Q branch having K 0c ¼ 5 through 30 with J values up to 40. The transitions were fit to the same 17 parameters as for the m16 band and the rms error was 0.000149 cm1. After the a-type band analysis was completed, a number of weak transitions in this region were not yet accounted for as shown in Fig. 4. Based on the spectroscopic constants derived from the a-type spectrum, the unassigned features were determined to be c-type transitions of the m14 mode. This led to the assignment of 3071 c-type transitions (2565 P/R and 506 Q) which were subsequently included in the band analysis. A total of 6404 transitions were fit and the resulting rms error was 0.000159 cm1.
3.2. The m14 band 3.3. The m15 band An overview spectrum of the m14 and m15 bands of azetidine is shown in Fig. 3 and the observed band contours are typical of atype selection rules. The assignment and analysis of the individual bands were carried out as described above for the m16 ring puckering mode. For the m14 band at 736.7 cm1, 3333 a-type transitions were assigned including 902 from the Q branch. The quantum numbers spanned include progressions in the P and R branches
For the m15 band at 648.1 cm1, 2130 a-type transitions were initially assigned of which 250 were from the Q branch. The quantum numbers spanned include progressions in the P and R branches with quantum numbers J0 = 6 through 47 and progressions in the Q branch having K 0c ¼ 4 through 25 with J values up to 40. The Q branch transitions of type Qqp (DJ = 0, DKa = 0 and
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As with the m14 mode, a number of weaker transitions were left unassigned at this stage and the derived spectroscopic constants were used to calculate the c-type spectrum that would arise under C2v symmetry considerations. This model successfully accounted for an additional 2464 transitions (2060 P/R and 404 Q) which were added to the fit. The rovibrational analysis of all 4594 aand c-type transitions of the m15 band yielded an rms error of 0.000133 cm1.
4. Discussion In the present study, we have investigated the three lowest frequency modes of azetidine. Once the assignment and analysis of individual bands was completed, the three line lists were merged in order to perform a simultaneous four state fit and the results are summarized in Table 1. In total, 14 276 transitions were fit in Pickett’s SPFIT program using Watson’s A-reduced Hamiltonian, Ir-representation [24]. The extremely low rms error suggests that the model Hamiltonian provides a good description of these low frequency modes of azetidine. The three bands share a common set of ground state rotational parameters which confirms that the observed bands are those of fundamental transitions rather than hotbands.
4.1. Symmetry considerations and NH inversion tunneling
Fig. 5. The rotational structure of the Q branch of the m15 band of azetidine. (a) The Qqp-type transitions (labeled with quantum numbers J0 K 0a K 0c —J00 K 00a K 00c ) are resolved for a range of J values within a given progression and were readily assigned. (b) By comparison, Qqr-type transitions are not typically resolved and form a continuous background absorption. The dotted lines show the predicted transition frequencies.
DKc = 1) were often left unassigned as there were many unresolved lines in this region forming an almost continuous background absorption in some spectral regions as shown in Fig. 5. This was not an issue in the m14 and m16 bands for which transitions of type Qqp and Qqr were readily assigned. The a-type transitions for the m15 band were preliminarily fit to 17 parameters as described previously for the ring puckering mode and the rms error was 0.000142 cm1.
The lowest frequency mode (m16) corresponds to the ring puckering vibration and the spectral pattern is that of a strong a-type band. As shown in Fig. 2, there are no clusters of lines corresponding to a c-type spectrum of a near oblate top in this region as was seen for the other two bands (Fig. 4). As c-type bands have sharp Q branches, we studied this region of our spectrum closely but did not find such transitions. The infrared study of Egawa and Kuchitsu [15] reported that coupling of the NH inversion and ring puckering motions was only significant in the deuterated isotopologue of azetidine but based on the quality of the more recent microwave [17] investigation which clearly shows the presence of this coupling, we assert that the lack of observed c-type transitions in our infrared spectrum is due to insufficient intensity compared to the dominant a-type transitions. Both a- and c-type transitions were readily observed for the m14 and m15 bands in the P, Q and R branches. This is consistent with the selection rules predicted using the C2v molecular symmetry group and therefore suggests that NH inversion is coupled to these two vibrational modes. This is not surprising as the ab initio force field calculations of Dutler et al. [18] suggest that the m14 and m15 modes are comprised of 40% and 30% NH out-of-plane bending character, respectively.
Table 1 Spectroscopic constants determined in the global analysis of the m16, m15 and m14 bands of azetidine. Ground state
m/cm
1
Rotational constants/cm1 A B C
0 0.38203350(39) 0.37823553(36) 0.2205536(1)
Centrifugal distortion constants/106 cm1 DJ 0.16340(33) DJK 0.1646(16) DK 0.2201(19) dJ 0.04832(16) dK 0.0412(5) 1 Rms error/cm
m16
m15
m14
207.727053(9)
648.116041(8)
736.701310(7)
0.3812410(12) 0.3776991(11) 0.2196705(1) 0.1406(12) 0.081(9) 0.195(9) 0.0376(6) 0.0466(3)
0.38178513(44) 0.37838962(41) 0.2200738(1) 0.15625(38) 0.1455(20) 0.2266(20) 0.04536(19) 0.00417(6)
0.38088655(48) 0.37845869(46) 0.2196753(1) 0.1695(5) 0.1970(21) 0.2391(19) 0.05269(25) 0.0353(6) 0.000145
T. Zaporozan et al. / Journal of Molecular Spectroscopy 264 (2010) 105–110
4.2. Band origins of the m14, m15 and m16 vibrations The band origins of the m14 (736.701310(7) cm1), m15 (648.116041(8) cm1) and m16 (207.727053(9) cm1) vibrations have been precisely determined and are in good agreement with values reported from lower resolution gas phase infrared studies [11,15,16,18]. The scaled ab initio force field calculations of Dutler et al. [18] predicted the band origins to have frequencies of 739.4, 636.6 and 198.7 cm1, respectively, although the authors comment that the agreement for the ring puckering mode was surprising and likely fortuitous as such modes cannot be modeled realistically with harmonic force fields. The DFT (B3LYP (6–31G**)) calculations of Thompson et al. [20] show a similar level of agreement with reported values of 748.7, 657.1 and 212 cm1, respectively. The estimate of the ring puckering frequency (206 cm1) appears to improve with the use of a larger basis set as shown by the B3LYP (6–311++G(2d,p)) calculations of Palafox et al. [19], but the agreement for the m14 mode actually worsens with the expanded basis (and the authors have not reported the m15 frequency). Collectively, these results highlight the importance of complementary experimental and theoretical investigations of the fundamental structural and dynamical properties of small molecules. Spectroscopic data in the far infrared region, in particular, provides a critical test of theoretical model potentials as demonstrated here for the fourmembered heterocycle azetidine. 4.3. The ground state of azetidine The ground state rotational constants determined in this work are compared to those from the microwave investigation of López et al. [17] in Table 2. In the microwave spectrum, the ground state and m16 excited state rotational transitions were doubled. This was attributed to symmetric (+) and antisymmetric () tunneling states corresponding to tunneling between equivalent equatorial conformers. Based on the rotational constants of these two states given in Table 2, the resolution of the far infrared spectrum was not sufficient to observe this splitting. In fact, the ground state values for the two tunneling states are the same to within the reported uncertainties of the microwave experiment. The ground state rotational constants determined in the present far infrared analysis are in good agreement with the microwave data with only the C rotational constant falling slightly outside the bounds of the stated uncertainties. There are small discrepancies for the rotational constants in the m16 state (0.00024 cm1 for A and B) as shown in Table 2 but this is likely a result of a combination of factors including the nature of the data sets involved and the fact that different distortion constants were used in each analysis. In the fit of the microwave transitions, for example, transitions from higher
Table 2 Comparison of the rotational constants determined in the global analysis of the ring puckering mode (m16) of azetidine compared with those from the microwave study.
Ground state m/cm1 A B C
This work
Ref. [12]a
Ref. [12]
0 0.38203350(39) 0.37823553(36) 0.2205536(1)
+ 0.382035530(13) 0.378234860(13) 0.220553845(15)
0.382035527(13) 0.378234859(13) 0.220553846(15)
+ 0.381217050(22) 0.377723368(21) 0.219670818(10)
0.381216973(23) 0.377722793(21) 0.219670687(25)
m16 ring puckering m/cm1 207.727053(9) A B C
0.3812410(12) 0.3776991(11) 0.2196705(1)
a The +/ labels refer to the symmetric and antisymmetric inversion tunneling states identified in Ref. [18].
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excited states were included (up to m16 = 4) and three sextic distortion constants were used in the fit. The strong agreement of the ground state rotational constants determined in this work with those reported by López et al. leads us to conclude that we have observed the same equatorial conformer of azetidine. After complete analysis of the m14, m15 and m16 bands in the 100–350 cm1 and 500–850 cm1 spectral regions, a second look at the spectra did not reveal many unassigned transitions that could potentially arise from a second conformer. This result provides further confirmation of the single well asymmetric potential of Robiette et al. [11]. 5. Conclusions Using infrared radiation from the Canadian Light Source synchrotron coupled to a Bruker IFS125HR spectrometer, we have successfully measured and analyzed high quality, rotationally resolved spectra of the three lowest frequency bands of azetidine including the long debated ring puckering mode at 207.727053(9) cm1. The ground state structure is that of the equatorial conformer of azetidine and the spectra revealed that NH inversion is coupled to the m14 and m15 modes but we did not observe evidence for this coupling to the large amplitude m16 ring puckering vibration within the resolution and sensitivity of our experiment. High resolution vibrational spectra provide detailed information that is sensitive to the underlying potential energy surfaces of molecules. As a result, this technique offers great potential for elucidating important structural and chemical differences within classes of molecules such as the four-membered heterocycles. Acknowledgments This research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) through the Discovery Grant and University Faculty Award programs. We are grateful to Tim May (Canadian Light Source) for stepping in as beamline scientist to provide technical support during the course of these experiments. We also thank Dr. A.R.W. McKellar for useful discussions during preparation of this manuscript. Appendix A. Supplementary material A complete listing of the assigned rovibrational transitions for the m14, m15 and m16 bands are provided as supplementary data. Supplementary data for this article are available on ScienceDirect (www.sciencedirect.com) and as part of the Ohio State University Molecular Spectroscopy Archives (http://library.osu.edu/sites/ msa/jmsa_hp.htm). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.jms.2010.09.010. References [1] M. Wang, B. Cornett, J. Nettles, D.C. Liotta, J.P. Snyder, J. Org. Chem. 65 (2000) 1059–1068. [2] H.C. Neu, Am. J. Med. 79 (1985) 2–12. [3] A.R. Pinder, Nat. Prod. Prep. 9 (1992) 491–504. [4] A.C. Legon, Chem. Rev. 80 (1980) 231–262. [5] J. Laane, J. Phys. Chem. A 104 (2000) 7715–7733. [6] J. Laane, Int. Rev. Phys. Chem. 18 (1999) 301–341. [7] J. Laane, Annu. Rev. Phys. Chem. 45 (1994) 179–211. [8] J. Laane, Pure Appl. Chem. 59 (1987) 1307–1326. [9] T.B. Malloy, J. Mol. Spectrosc. 44 (1972) 504–535. [10] L.A. Carreira, R.C. Lord, J. Chem. Phys. 51 (1969) 2735–2744. [11] A.G. Robiette, T.R. Borgers, H.L. Strauss, Mol. Phys. 42 (1981) 1519–1524. [12] D. Cremer, O.V. Dorofeeva, V.S. Mastryukov, J. Mol. Struct. 75 (1981) 225–240. [13] R.N. Skancke, G. Fogarasi, J.E. Boggs, J. Mol. Struct. 62 (1980) 259–273.
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D.F. Bocian, G.A. Schick, R.R. Birge, J. Chem. Phys. 75 (1981) 2626–2634. T. Egawa, K. Kuchitsu, J. Mol. Spectrosc. 128 (1988) 469–477. H. Günther, G. Schrem, H. Oberhammer, J. Mol. Spectrosc. 104 (1984) 152–164. J.C. López, S. Blanco, A. Lessari, J.L. Alonso, J. Chem. Phys. 114 (2001) 2237– 2250. [18] R. Dutler, A. Rauk, R.A. Shaw, J. Phys. Chem. 94 (1990) 118–124. [19] M.A. Palafox, N. Iza, M. Gil, J.L. Núñez, Int. J. Quantum Chem. 89 (2002) 25–47.
[20] C.A. Thompson, R.C. Mowrey, T.P. Russell, J. Mol. Struct. (THEOCHEM) 491 (1999) 67–80. [21] D.W. Tokaryk, J. van Wijngaarden, Can. J. Phys. 87 (2009) 443–448. [22] L.S. Rothman et al., J. Quant. Spectrosc. Radiat. Transfer 96 (2005) 139–204. [23] D.F. Plusquellic, JB95 software. Available from:
Contents lists available at ScienceDirect
Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms
High resolution Fourier transform infrared spectra and analysis of the m14, m15 and m16 bands of azetidine Taras Zaporozan, Ziqiu Chen, Jennifer van Wijngaarden ⇑ Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
a r t i c l e
i n f o
Article history: Received 9 August 2010 In revised form 18 September 2010 Available online 25 September 2010 Keywords: Azetidine Trimethylene imine FTIR Infrared Synchrotron Ring puckering Rovibrational
a b s t r a c t Rotationally resolved vibrational spectra of the three lowest frequency bands of the four-membered heterocycle azetidine (c-C3H6NH) have been collected with a resolution of 0.00096 cm1 using the far infrared beamline at the Canadian Light Source synchrotron. The modes observed correspond principally to motions best described as: b-CH2 rock (m14) at 736.701310(7) cm1, ring deformation (m15) at 648.116041(8) cm1, and the ring puckering mode (m16) at 207.727053(9) cm1. A global fit of 14 276 rovibrational transitions from the three bands provided an accurate set of ground state spectroscopic constants as well as excited state parameters for each of the three vibrational modes. The ground state structure was determined to be that of the puckered conformer having the NH bond in an equatorial arrangement. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Azetidine or trimethylene imine (c-C3H6NH) is the amine analog of the oxetane (c-C3H6O) and b-lactam (azetidinone, c-C2H4NHCO) heterocycles. Oxetane and azetidinone are common structural features of natural products known for their anti-cancer and anti-bacterial properties. The bioactivity of such molecules, for example Taxol (a chemotherapy drug) and penicillin, is linked to the stability of the four-membered ring subunit [1,2]. Reports of azetidine as a structural feature in natural products are much less common by comparison although derivatives have been found in some systems such as marine sponges [3]. In order to understand such vast differences in the stability and reactivity of large, complex molecules, it is instructive to start with simpler models such as the four-membered heterocycles themselves. The spectra and underlying potential energy surfaces of heterocycles are particularly interesting due to the presence of low frequency vibrational modes and the possibility of finding multiple stable conformers [4]. The low frequency ring puckering and pseudorotation motions of numerous small rings have been investigated and the subject has been extensively reviewed [4–9]. In the case of azetidine, there has been some uncertainty with respect to the potential form of the lowest frequency mode. The far infrared spectrum of the m16 ring puckering mode (and several hotbands) collected by Carreira and Lord [10] was originally fit to an ⇑ Corresponding author. Fax: +1 204 474 7608. E-mail address: [email protected] (J. van Wijngaarden). 0022-2852/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2010.09.010
asymmetric double well potential in which one minimum corresponded to an axial arrangement of the NH bond and the other to an equatorial position. The energy difference between the two conformers was found to be 95 cm1 with the barrier to interconversion lying 441 cm1 above the lower minimum. The far infrared data was later re-analyzed by Robiette et al. [11] and found to be consistent with a single well asymmetric potential with the equatorial conformer being the only stable structure. This is in agreement with ab initio calculations [12,13], Raman intensities [14] and a subsequent far infrared study [15]. The microwave and electron diffraction experiments of Günther et al. [16] confirmed the presence of the equatorial conformer and determined its structure to have a ring puckering angle of 29.7° although their microwave spectrum also contained a weak unassigned feature which was thought to be due to the axial conformer. The more recent microwave and ab initio investigation of López et al. [17] analyzed the rotational spectrum in the ground and several excited ring puckering vibrational states. Their results point to a single well asymmetric potential along the ring puckering coordinate. Interestingly, they also observed tunneling doublets which were attributed to a concerted motion involving NH inversion and ring puckering through a C2v barrier estimated to be 1900–2600 cm1. In addition to the extensive investigation of the ring puckering mode of azetidine, the higher frequency modes have been calculated via ab initio methods and the infrared spectrum has been measured in the gas phase [16,18], in solution [18,19] and in a solid argon matrix [20]. Dutler et al. [18] recorded the gas phase infrared spectrum from 400 to 3400 cm1 at a resolution of 0.12 cm1 and
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assigned the observed transitions to normal modes using scaled ab initio force field calculations. We have adopted their convention for labeling the vibrational modes of azetidine and their descriptions of the normal mode character. In this work, we report the rovibrational assignment and analysis of the m14, m15 and m16 bands of azetidine using data collected at the far infrared endstation of the Canadian Light Source (CLS). The former two modes together account for approximately 70% of the NH out-of-plane bending motion of azetidine and the latter is the ring puckering mode. A global fit of the three bands was performed to obtain a single set of ground state spectroscopic constants for comparison with previous microwave studies. The fit also provided accurate determination of the m14, m15 and m16 band origins and excited state parameters. To our knowledge, this work represents the first high resolution infrared study of azetidine.
2. Experimental details The far infrared endstation at the Canadian Light Source (CLS) is equipped with a Bruker IFS125HR spectrometer which can be used to obtain exceptionally high quality vibrational spectra with a resolution of up to 0.00096 cm1. A recent infrared study of pyrrole (another N containing heterocycle) compared a spectrum recorded using synchrotron radiation from the CLS with that collected using a globar source and showed a factor of eight improvement in the signal to noise ratio when synchrotron light was used [21]. This so-called ‘synchrotron advantage’ arises from the high degree of spatial confinement of the synchrotron beam. When small apertures are employed as for high resolution studies, the synchrotron beam passes through the iris with minimal loss providing greater brightness than conventional sources. The ‘synchrotron advantage’ is typically realized in the 300–800 cm1 spectral range at the CLS at present but efforts are ongoing to increase the level of the advantage and expand its range. Rotationally resolved infrared spectra of the m14, m15 and m16 bands of azetidine were recorded at room temperature using vapor pressure from a liquid azetidine sample (98% purity, Sigma–Aldrich). Although a large 2 m multipass gas cell is available at the CLS providing path lengths of up to 80 m, the cost of the azetidine sample ($450 per g) made filling this cell prohibitively expensive. We thus opted to use the 30 cm White cell set to a total absorption path length of 7 m. For the lowest frequency band corresponding to the m16 ring puckering mode in the 100–350 cm1 range, the spectrometer was outfitted with a 6 lm Ge-mylar beamsplitter and a helium cooled Si bolometer detector. The data were recorded at 0.00096 cm1 resolution with a sample pressure of 700 mTorr. The final spectrum of the m16 band was calculated by averaging 17 separate interferograms collected over 3 h and applying a Fourier transform. The averaged background interferogram, taken at lower resolution (0.01536 cm1) was also transformed and the absorption spectrum calculated using the formula ln(I/Io). Data for the m14 and m15 bands were recorded at 0.00096 cm1 resolution between 500 and 850 cm1 using a KBr beamsplitter, GeCu detector and an optical filter (490–1190 cm1). As the bands were stronger than the ring puckering mode, the pressure in the White cell was reduced to 60 mTorr and 112 interferograms were used to calculate the final spectrum as described for the m16 band above. The m16 spectrum was calibrated using H2O transitions observed in the 130–300 cm1 region and comparing the observed line positions with those in the HITRAN database [22]. The same procedure was followed for the higher frequency spectrum using the observed residual CO2 lines centered near 667 cm1. In the end, the calibration made little difference in the analysis (less than
0.0001 cm1 for most transitions in the m16 spectrum and less than 0.0007 cm1 for most transitions in the higher frequency spectrum) indicating that the Bruker IFS125HR was well-calibrated. A complete listing of the assigned transitions is provided as Supplementary material. 3. Spectral assignment and analysis Azetidine is a near oblate asymmetric top with rotational constants A B and an asymmetry parameter j = 0.95. The ground state structure has Cs point group symmetry with a ring puckering angle of 29.7° relative to the ab-plane of the molecule [16]. The three modes studied in this work correspond to motions that are principally due to ring puckering (m16), ring deformation (m15) and b-CH2 rocking (m14) although the two latter modes also have significant character that corresponds to NH out-of-plane bending and a-CH2 twisting [18]. The microwave spectrum of the ground state and several excited ring puckering states of azetidine was analyzed in terms of the C2v molecular symmetry group to allow for the description of the observed NH inversion tunneling splittings [17]. This led to the determination of two sets of spectroscopic constants for each vibrational state corresponding to symmetric (A1) and antisymmetric (B1) inversion tunneling components. Using the C2v point group, the m16 ring puckering mode and the m15 and m14 modes (having NH out-of-plane bending character) also belong to the B1 irreducible representation. If NH inversion tunneling is coupled to any of these three modes, one would expect a-type rotation–vibration–inversion transitions with A1 symmetry and c-type rotation–vibration transitions with B1 symmetry. Based on the similarity in the rotational constants determined for the NH inversion tunneling states in Ref. [17] (which differ by a few kHz or less), the a- and c-type transitions will not be doubled in the infrared spectrum as the calculated splittings fall below the resolution of the spectrometer. In this work, each of the three vibrational bands of azetidine was first assigned and fit independently. Once the individual assignments were complete, a global analysis of all three bands was performed to obtain a common set of ground state spectroscopic parameters. The analysis is detailed below. 3.1. The m16 ring puckering band An overview spectrum of the m16 ring puckering mode at 207.7 cm1 is shown in Fig. 1. The observed band contour is typical of a-type selection rules (DJ = 0, ±1, DKa = 0 and DKc = ±1) and the line widths were 0.0012 cm1 (FWHM) due to slight pressure broadening at 700 mTorr. Upon closer inspection, a series of regularly spaced progressions was observed in the R branch as shown in Fig. 2. To assign these transitions, the spectrum was first simulated using the available rotational constants and band origin as input to the JB95 program [23]. The R branch progressions were determined to follow a sequence of quantum numbers such that adjacent lines in a given progression follow the trend DJ = 1, DKa = +1 and DKc = 2 toward the band center as labeled in Fig. 2. Once the pattern was recognized, ground state combination differences were used to confirm the quantum number assignment of the first few progressions of this type. In total, 3278 a-type transitions were assigned for the m16 ring puckering mode of azetidine including 928 transitions in the Q branch. The spanned quantum numbers of assigned transitions include those in the P and R branches from J0 = 4 through 55 and in general, all except the highest Ka transitions in any progression were strong enough for assignment. In the Q branch, progressions having K 0c ¼ 5 through 32 were assigned for J values up to 46. The
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Fig. 3. Overview of the a-type band contours of the m14 and m15 bands of azetidine at 736.7 cm1 and 648.1 cm1, respectively. Strong, evenly spaced transitions due to the 667 cm1 bending mode of residual carbon dioxide are also seen in this region. Fig. 1. Overview of the a-type band structure of the m16 ring puckering mode of azetidine at 207 cm1. Strong transitions due to residual water in the gas cell are seen throughout this region.
Fig. 2. Sample spectrum showing the rotational structure of a typical portion of the R branch of the m16 ring puckering mode of azetidine. This region shows three overlapping progressions with representative transitions labeled with asymmetric rotor quantum numbers ðJ0 K 0a K 0c —J00 K 00a K 00c Þ for each vibrational level.
assigned rovibrational transitions of the m16 band were preliminarily fit using Watson’s A-reduced Hamiltonian, Ir-representation in Pickett’s SPFIT program [24]. Although only 17 parameters were used (three rotational constants and five centrifugal distortion constants for each state plus the m16 band origin), the rms error of the fit was only 0.000130 cm1. The spectroscopic constants derived from the analysis of the a-type transitions were used to calculate the c-type transitions (predicted using C2v symmetry considerations) but no such transitions were observed in the spectrum.
Fig. 4. Sample spectrum showing the rotational structure of a typical portion of the R branch of the m14 mode of azetidine. This region shows three overlapping a-type progressions with representative transitions labeled with asymmetric rotor quantum numbers ðJ0 K 0a K 0c —J00 K 00a K 00c Þ for each vibrational level. The weak clusters of transitions follow c-type selection rules.
with quantum numbers J0 = 5 through 55 and progressions in the Q branch having K 0c ¼ 5 through 30 with J values up to 40. The transitions were fit to the same 17 parameters as for the m16 band and the rms error was 0.000149 cm1. After the a-type band analysis was completed, a number of weak transitions in this region were not yet accounted for as shown in Fig. 4. Based on the spectroscopic constants derived from the a-type spectrum, the unassigned features were determined to be c-type transitions of the m14 mode. This led to the assignment of 3071 c-type transitions (2565 P/R and 506 Q) which were subsequently included in the band analysis. A total of 6404 transitions were fit and the resulting rms error was 0.000159 cm1.
3.2. The m14 band 3.3. The m15 band An overview spectrum of the m14 and m15 bands of azetidine is shown in Fig. 3 and the observed band contours are typical of atype selection rules. The assignment and analysis of the individual bands were carried out as described above for the m16 ring puckering mode. For the m14 band at 736.7 cm1, 3333 a-type transitions were assigned including 902 from the Q branch. The quantum numbers spanned include progressions in the P and R branches
For the m15 band at 648.1 cm1, 2130 a-type transitions were initially assigned of which 250 were from the Q branch. The quantum numbers spanned include progressions in the P and R branches with quantum numbers J0 = 6 through 47 and progressions in the Q branch having K 0c ¼ 4 through 25 with J values up to 40. The Q branch transitions of type Qqp (DJ = 0, DKa = 0 and
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As with the m14 mode, a number of weaker transitions were left unassigned at this stage and the derived spectroscopic constants were used to calculate the c-type spectrum that would arise under C2v symmetry considerations. This model successfully accounted for an additional 2464 transitions (2060 P/R and 404 Q) which were added to the fit. The rovibrational analysis of all 4594 aand c-type transitions of the m15 band yielded an rms error of 0.000133 cm1.
4. Discussion In the present study, we have investigated the three lowest frequency modes of azetidine. Once the assignment and analysis of individual bands was completed, the three line lists were merged in order to perform a simultaneous four state fit and the results are summarized in Table 1. In total, 14 276 transitions were fit in Pickett’s SPFIT program using Watson’s A-reduced Hamiltonian, Ir-representation [24]. The extremely low rms error suggests that the model Hamiltonian provides a good description of these low frequency modes of azetidine. The three bands share a common set of ground state rotational parameters which confirms that the observed bands are those of fundamental transitions rather than hotbands.
4.1. Symmetry considerations and NH inversion tunneling
Fig. 5. The rotational structure of the Q branch of the m15 band of azetidine. (a) The Qqp-type transitions (labeled with quantum numbers J0 K 0a K 0c —J00 K 00a K 00c ) are resolved for a range of J values within a given progression and were readily assigned. (b) By comparison, Qqr-type transitions are not typically resolved and form a continuous background absorption. The dotted lines show the predicted transition frequencies.
DKc = 1) were often left unassigned as there were many unresolved lines in this region forming an almost continuous background absorption in some spectral regions as shown in Fig. 5. This was not an issue in the m14 and m16 bands for which transitions of type Qqp and Qqr were readily assigned. The a-type transitions for the m15 band were preliminarily fit to 17 parameters as described previously for the ring puckering mode and the rms error was 0.000142 cm1.
The lowest frequency mode (m16) corresponds to the ring puckering vibration and the spectral pattern is that of a strong a-type band. As shown in Fig. 2, there are no clusters of lines corresponding to a c-type spectrum of a near oblate top in this region as was seen for the other two bands (Fig. 4). As c-type bands have sharp Q branches, we studied this region of our spectrum closely but did not find such transitions. The infrared study of Egawa and Kuchitsu [15] reported that coupling of the NH inversion and ring puckering motions was only significant in the deuterated isotopologue of azetidine but based on the quality of the more recent microwave [17] investigation which clearly shows the presence of this coupling, we assert that the lack of observed c-type transitions in our infrared spectrum is due to insufficient intensity compared to the dominant a-type transitions. Both a- and c-type transitions were readily observed for the m14 and m15 bands in the P, Q and R branches. This is consistent with the selection rules predicted using the C2v molecular symmetry group and therefore suggests that NH inversion is coupled to these two vibrational modes. This is not surprising as the ab initio force field calculations of Dutler et al. [18] suggest that the m14 and m15 modes are comprised of 40% and 30% NH out-of-plane bending character, respectively.
Table 1 Spectroscopic constants determined in the global analysis of the m16, m15 and m14 bands of azetidine. Ground state
m/cm
1
Rotational constants/cm1 A B C
0 0.38203350(39) 0.37823553(36) 0.2205536(1)
Centrifugal distortion constants/106 cm1 DJ 0.16340(33) DJK 0.1646(16) DK 0.2201(19) dJ 0.04832(16) dK 0.0412(5) 1 Rms error/cm
m16
m15
m14
207.727053(9)
648.116041(8)
736.701310(7)
0.3812410(12) 0.3776991(11) 0.2196705(1) 0.1406(12) 0.081(9) 0.195(9) 0.0376(6) 0.0466(3)
0.38178513(44) 0.37838962(41) 0.2200738(1) 0.15625(38) 0.1455(20) 0.2266(20) 0.04536(19) 0.00417(6)
0.38088655(48) 0.37845869(46) 0.2196753(1) 0.1695(5) 0.1970(21) 0.2391(19) 0.05269(25) 0.0353(6) 0.000145
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4.2. Band origins of the m14, m15 and m16 vibrations The band origins of the m14 (736.701310(7) cm1), m15 (648.116041(8) cm1) and m16 (207.727053(9) cm1) vibrations have been precisely determined and are in good agreement with values reported from lower resolution gas phase infrared studies [11,15,16,18]. The scaled ab initio force field calculations of Dutler et al. [18] predicted the band origins to have frequencies of 739.4, 636.6 and 198.7 cm1, respectively, although the authors comment that the agreement for the ring puckering mode was surprising and likely fortuitous as such modes cannot be modeled realistically with harmonic force fields. The DFT (B3LYP (6–31G**)) calculations of Thompson et al. [20] show a similar level of agreement with reported values of 748.7, 657.1 and 212 cm1, respectively. The estimate of the ring puckering frequency (206 cm1) appears to improve with the use of a larger basis set as shown by the B3LYP (6–311++G(2d,p)) calculations of Palafox et al. [19], but the agreement for the m14 mode actually worsens with the expanded basis (and the authors have not reported the m15 frequency). Collectively, these results highlight the importance of complementary experimental and theoretical investigations of the fundamental structural and dynamical properties of small molecules. Spectroscopic data in the far infrared region, in particular, provides a critical test of theoretical model potentials as demonstrated here for the fourmembered heterocycle azetidine. 4.3. The ground state of azetidine The ground state rotational constants determined in this work are compared to those from the microwave investigation of López et al. [17] in Table 2. In the microwave spectrum, the ground state and m16 excited state rotational transitions were doubled. This was attributed to symmetric (+) and antisymmetric () tunneling states corresponding to tunneling between equivalent equatorial conformers. Based on the rotational constants of these two states given in Table 2, the resolution of the far infrared spectrum was not sufficient to observe this splitting. In fact, the ground state values for the two tunneling states are the same to within the reported uncertainties of the microwave experiment. The ground state rotational constants determined in the present far infrared analysis are in good agreement with the microwave data with only the C rotational constant falling slightly outside the bounds of the stated uncertainties. There are small discrepancies for the rotational constants in the m16 state (0.00024 cm1 for A and B) as shown in Table 2 but this is likely a result of a combination of factors including the nature of the data sets involved and the fact that different distortion constants were used in each analysis. In the fit of the microwave transitions, for example, transitions from higher
Table 2 Comparison of the rotational constants determined in the global analysis of the ring puckering mode (m16) of azetidine compared with those from the microwave study.
Ground state m/cm1 A B C
This work
Ref. [12]a
Ref. [12]
0 0.38203350(39) 0.37823553(36) 0.2205536(1)
+ 0.382035530(13) 0.378234860(13) 0.220553845(15)
0.382035527(13) 0.378234859(13) 0.220553846(15)
+ 0.381217050(22) 0.377723368(21) 0.219670818(10)
0.381216973(23) 0.377722793(21) 0.219670687(25)
m16 ring puckering m/cm1 207.727053(9) A B C
0.3812410(12) 0.3776991(11) 0.2196705(1)
a The +/ labels refer to the symmetric and antisymmetric inversion tunneling states identified in Ref. [18].
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excited states were included (up to m16 = 4) and three sextic distortion constants were used in the fit. The strong agreement of the ground state rotational constants determined in this work with those reported by López et al. leads us to conclude that we have observed the same equatorial conformer of azetidine. After complete analysis of the m14, m15 and m16 bands in the 100–350 cm1 and 500–850 cm1 spectral regions, a second look at the spectra did not reveal many unassigned transitions that could potentially arise from a second conformer. This result provides further confirmation of the single well asymmetric potential of Robiette et al. [11]. 5. Conclusions Using infrared radiation from the Canadian Light Source synchrotron coupled to a Bruker IFS125HR spectrometer, we have successfully measured and analyzed high quality, rotationally resolved spectra of the three lowest frequency bands of azetidine including the long debated ring puckering mode at 207.727053(9) cm1. The ground state structure is that of the equatorial conformer of azetidine and the spectra revealed that NH inversion is coupled to the m14 and m15 modes but we did not observe evidence for this coupling to the large amplitude m16 ring puckering vibration within the resolution and sensitivity of our experiment. High resolution vibrational spectra provide detailed information that is sensitive to the underlying potential energy surfaces of molecules. As a result, this technique offers great potential for elucidating important structural and chemical differences within classes of molecules such as the four-membered heterocycles. Acknowledgments This research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) through the Discovery Grant and University Faculty Award programs. We are grateful to Tim May (Canadian Light Source) for stepping in as beamline scientist to provide technical support during the course of these experiments. We also thank Dr. A.R.W. McKellar for useful discussions during preparation of this manuscript. Appendix A. Supplementary material A complete listing of the assigned rovibrational transitions for the m14, m15 and m16 bands are provided as supplementary data. Supplementary data for this article are available on ScienceDirect (www.sciencedirect.com) and as part of the Ohio State University Molecular Spectroscopy Archives (http://library.osu.edu/sites/ msa/jmsa_hp.htm). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.jms.2010.09.010. References [1] M. Wang, B. Cornett, J. Nettles, D.C. Liotta, J.P. Snyder, J. Org. Chem. 65 (2000) 1059–1068. [2] H.C. Neu, Am. J. Med. 79 (1985) 2–12. [3] A.R. Pinder, Nat. Prod. Prep. 9 (1992) 491–504. [4] A.C. Legon, Chem. Rev. 80 (1980) 231–262. [5] J. Laane, J. Phys. Chem. A 104 (2000) 7715–7733. [6] J. Laane, Int. Rev. Phys. Chem. 18 (1999) 301–341. [7] J. Laane, Annu. Rev. Phys. Chem. 45 (1994) 179–211. [8] J. Laane, Pure Appl. Chem. 59 (1987) 1307–1326. [9] T.B. Malloy, J. Mol. Spectrosc. 44 (1972) 504–535. [10] L.A. Carreira, R.C. Lord, J. Chem. Phys. 51 (1969) 2735–2744. [11] A.G. Robiette, T.R. Borgers, H.L. Strauss, Mol. Phys. 42 (1981) 1519–1524. [12] D. Cremer, O.V. Dorofeeva, V.S. Mastryukov, J. Mol. Struct. 75 (1981) 225–240. [13] R.N. Skancke, G. Fogarasi, J.E. Boggs, J. Mol. Struct. 62 (1980) 259–273.
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