Journal of Molecular Spectroscopy 279 (2012) 31–36

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Journal of Molecular Spectroscopy

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The m21 ring puckering mode of 3-oxetanone: A far infrared spectroscopicinvestigation using synchrotron radiation

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 13 August 2012In revised form 23 August 2012Available online 3 September 2012

Keywords:3-OxetanoneFT infraredSynchrotronRing puckering potential

0022-2852/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.jms.2012.08.006

⇑ Corresponding author. Fax: +1 204 474 7608.E-mail address: vanwijng@cc.umanitoba.ca (J. van

a b s t r a c t

The far infrared spectrum of the four-membered heterocycle 3-oxetanone (c-C3H4O2) has been recordedin the region 100–200 cm�1 with a resolution of 0.000959 cm�1 using synchrotron radiation from theCanadian Light Source. The ring puckering mode (m21) and first three hotbands were observed and9165 separate rovibrational transitions were assigned and analyzed. The band centers of the fundamentalband (139.549148(5) cm�1) and first three hotbands (v = 1 ? v = 2: 141.093457(14) cm�1, v = 2 ? v = 3:142.644034(19) cm�1 and v = 3 ? v = 4: 143.989670(26) cm�1) were accurately determined for the firsttime along with the rotational and centrifugal distortion constants that characterize the four excitedvibrational states. Based on our spectra, the potential governing this vibrational mode of 3-oxetanonehas the form of V = ax4 + bx2 (where a = 2.04388 � 105 cm�1/Å4 and b = 4.26392 � 104 cm�1/Å2) alongthe ring puckering coordinate x.

� 2012 Elsevier Inc. All rights reserved.

1. Introduction

Small heterocycles have been the subject of decades of experi-mental and theoretical investigation due to the challenges posedby modeling low frequency vibrations such as ring puckering [1–6]. These modes tend to be anharmonic and the underlying poten-tial function is highly dependent on the ring substituents. Four-membered rings, in particular, are relatively simple in structureand serve as valuable prototypes for probing the connection be-tween potential energy surfaces and observed spectroscopic signa-tures of molecules.

Classically, in four-membered heterocycles, the planarity of thering backbone is dependent on the competition between two fac-tors: the torsional forces exerted along the bonds to minimizethe steric hindrance of neighboring CH2 groups (which favor apuckered ring) and the ring strain (which favors planarity). Thering geometry itself is thus intrinsically linked to the vibrationalpotential energy function governing the puckering motion. Infour-membered rings, this low frequency mode is described by aone dimensional potential function which may be written asV = ax4 + bx2 where x is the ring puckering coordinate, a is a posi-tive constant and b is a constant that can be either positive or neg-ative depending on whether the potential has a single or doublewell, respectively. The ring puckering spectrum therefore providesa distinct fingerprint by which to characterize a four-memberedring via its underlying ring puckering potential function.

ll rights reserved.

Wijngaarden).

Previous experimental efforts to probe the ring puckering modeof small rings have used infrared and microwave spectroscopy.These tools give direct and indirect information, respectively,about the vibrational energy spacing and have shown that mole-cules with similar backbones can have completely different ringpuckering potentials associated with them. For example, the ana-logs of cyclobutane with silicon, oxygen and nitrogen substitutionwithin the ring (silacyclobutane (c-C3H8Si) [7–9], oxetane (c-C3H6O) [10–14] and azetidine (c-C3H6NH) [15–18]) have very dis-tinct potential forms. Silacyclobutane is a puckered ring and thedouble well potential has a large barrier (�440 cm�1) that givesrise to inversion tunneling doubling of the lowest energy states.Oxetane’s potential is also double well but the barrier to planarity(�35 cm�1) lies below the ground vibrational energy level and thering is thus quasiplanar. Finally, azetidine has a nonplanar back-bone like silacyclobutane but is characterized by an asymmetricsingle well potential. In this case, the lone hydrogen atom on nitro-gen moves between an equatorial and axial position during thering puckering motion and the latter structure is not a minimumon the potential energy surface.

The ring puckering mode of 3-oxetanone, the subject of thepresent work, was first studied by Carreira and Lord [19] via infra-red spectroscopy. A more comprehensive infrared and Ramanstudy of 3-oxetanone in the gas and liquid phases was later pre-sented by Durig et al. [20] in which all 21 fundamental bands(including the ring puckering mode) were observed. For the ringpuckering mode in particular, the fundamental and several hot-bands were observed between 140 and 150 cm�1 and both studiesfit the estimated band centers to a Hamiltonian which was

32 Z. Chen, J. van Wijngaarden / Journal of Molecular Spectroscopy 279 (2012) 31–36

consistent with a single well potential with a small degree ofanharmonicity. These experiments are in agreement with the re-cent ab initio calculations of Vansteenkiste et al. [21] which pre-dicted that the ring puckering potential of 3-oxetanone is onlyslightly anharmonic in comparison to other four-membered rings.Gibson and Harris [22] measured pure rotational transitions in thefirst five excited ring puckering states of 3-oxetanone and studiedthe vibrational dependence of the rotational constants. This micro-wave study also confirmed that the ground state of 3-oxetanone isplanar which was later rationalized by Meinzer and Pringle [23]who investigated the balance between torsional strain and ringstrain and their connection to ring planarity in 3-oxetanone and re-lated compounds.

We recently reported the first rotationally-resolved infraredspectrum of 3-oxetanone in the region 360–720 cm�1 using syn-chrotron light from the Canadian Light Source (CLS) as the excita-tion radiation [24]. The observed band centers compared favorablywith our ab initio frequency predictions in this particular spectralregion. The bands correspond to the carbonyl in-plane (m16) andout-of-plane (m20) bending modes as well as a ring deformation(m7). In order to assign rovibrational transitions and treat local per-turbations in the spectra, it was first necessary to establish anaccurate set of ground state spectroscopic constants using groundstate combination differences derived from over 2500 pairs ofinfrared transitions. This first effort thus laid important groundwork for extending our study to include other modes of 3-oxeta-none including the ring puckering mode in the far infrared region.

In this paper, we report the far infrared rovibrational spectrumof the m21 ring puckering mode of 3-oxetanone using data collectedat the CLS in the region 100–200 cm�1. The highly congested spec-trum includes transitions due to the fundamental and first threehotbands of this vibration. A global analysis of 9165 separate infra-red transitions was performed by fixing the ground state parame-ters to those previously determined [24]. The resultingspectroscopic parameters provide accurate and precise character-ization of the v = 1 through v = 4 ring puckering states via the rota-tional and centrifugal distortion constants. The band origins for thefundamental and hotbands were well-determined and used to elu-cidate the potential function of form V = ax4 + bx2 using a leastsquares fitting routine.

2. Experimental details

Rotationally resolved vibrational spectra of the m21 ring pucker-ing mode of 3-oxetanone were recorded at the Canadian LightSource (CLS) using the synchrotron as the infrared light sourceand a Bruker IFS125 HR spectrometer capable of recording FTIRspectra with an unapodized spectral resolution of 0.000959 cm�1.High resolution spectra collected using synchrotron light typicallyshow improvement over conventional thermal sources in terms ofsignal to noise ratio due to its highly collimated nature [25]. Thisallows the synchrotron beam to effectively pass through the smallentrance aperture which is necessary for high resolution experi-ments with minimal loss.

A vapor pressure of 78 mTorr from a liquid sample of 3-oxeta-none (97% purity, Synthonix) was introduced into a 2 m multipasscell with a total absorption pathlength of 72 m. Interferogramswere recorded in the 100–200 cm�1 region utilizing the instru-ment’s entire 9.4 m optical path difference to realize the full spec-tral resolution of 0.000959 cm�1. The linewidths (FWHM) of 3-oxetanone in this region were instrument-limited. The data wascollected with the spectrometer outfitted with a 6 lm Mylarbeamsplitter and a helium-cooled Si bolometer detector. The finalspectrum of the m21 ring puckering mode and its hotbands was ob-tained by averaging 310 interferograms and applying a Fourier

transform. Background interferograms were collected at lower res-olution (0.01536 cm�1), averaged and the result was transformedwith appropriate zero filling. The absorption spectrum was thencalculated.

All transitions assigned in this region were calibrated by com-parison of the observed transition frequencies of residual waterin the 100–200 cm�1 range with those listed in the HITRAN data-base [26]. In the end, we found the measured frequencies to beaccurate to within 0.0001 cm�1 indicating that the spectrometerwas nearly perfectly calibrated. A complete listing of the assignedtransitions is provided as Supplementary material.

3. Spectral assignment and analysis

From the microwave spectrum, it is known that 3-oxetanone isa planar asymmetric top of C2v symmetry [22]. The m21 ring puck-ering mode was first observed by Carreira and Lord in the range of140–150 cm�1 [19] and has B2 symmetry which gives rise to a c-type band structure with selection rules oe M ee and eo M oo. Ourpreliminary survey spectrum revealed a series of sharp Q branchesin the region 139–145 cm�1 as shown in Fig. 1. Based on the ob-served intensities in this work and the assignments reported fromprevious low resolution studies, these observed Q branches wereattributed to the fundamental and first three hotbands of 3-oxeta-none which increase in frequency with the vibrational quantumnumber v. Due to the close spacing of the band centers, whichare separated by only �1.5 cm�1, the P and R branches of thesebands are largely overlapped.

To aid in the assignment of this highly congested spectrum, weconstructed a Loomis–Wood plot to identify patterns of regularlyspaced transitions. Fig. 2 shows a sub-section of the Loomis–Woodplot for 3-oxetanone in the range of 152–163 cm�1 which corre-sponds to the R branches of these bands. The strongest 12 progres-sions in the R branches (sharing common Kc) are denoted here andare assigned to transitions in the fundamental band and first twohotbands. Transitions in the third hotband (v = 3 ? v = 4) are tooweak to observe on the scale of this plot.

For the fundamental band, we used the estimated band originfrom the survey spectrum and the ground state constants previ-ously determined from our recent far infrared study of three higherfrequency vibrations [24] and earlier microwave work [22] to sim-ulate the ring puckering band structure. Once the patterns ob-served in the Loomis–Wood plot were identified based oncomparison with the simulation, ground state combination differ-ences were used to confirm the quantum number assignment oftransitions within the fundamental band. After several progres-sions were assigned and fit using Watson’s A-reduced Hamiltonian,Ir-representation in Pickett’s SPFIT program [27], the simulatedspectrum was refined and the process continued in an iterativefashion until the strongest transitions were accounted for. Overall,3237 transitions between Ka = 0 and 43 with J(max) = 53 were as-signed for the fundamental band, including 400 transitions fromthe strong, congested Q branch.

The assignment of transitions within the first hotband was doneusing lower state combination differences as the lower state (v = 1)of this band was already well-determined from the analysis of thefundamental band. Following the procedure described above, anadditional 2668 P, Q and R rovbirational transitions were assignedfrom the first hotband, spanning quantum number Ka from 0 to 42with J(max) = 44. The subsequent assignment of the second andthird hotbands was carried out in a similar manner. For thev = 2 ? v = 3 band, 2178 rovibrational transitions were assignedbetween Ka = 0 and 37 with J(max) = 50. Due to the low intensityof the v = 3 ? v = 4 band, the higher J transitions of this band aretoo weak to be observed. As a result, only 1082 P, Q and R rovbira-

Fig. 1. Zoomed-in section of the Q branches of the fundamental and first three hotbands of the ring puckering mode of 3-oxetanone.

Z. Chen, J. van Wijngaarden / Journal of Molecular Spectroscopy 279 (2012) 31–36 33

tional transitions, about half the number of the assigned transi-tions for the second hotband, were assigned for the third hotbandwith Ka values from 0 to 29 and J values up to 30. Fig. 3 shows aportion of the congested spectrum that arises from these four over-lapping bands. In a narrow spectral window of 0.15 cm�1, there aremore than 20 transitions assigned that span all four vibrationalstates. The assignment of transitions within each band is unambig-uous as both lower and upper state combination differences wereemployed to check assignments.

A global fit consisting of all 9165 assigned rovibrational transi-tions from the m21 fundamental and the first three hotbands wasperformed in Pickett’s SPFIT program using Watson’s A-reducedHamiltonian, Ir-representation [27] with the ground state spectro-scopic constants held fixed to the values previously reported [24].The rms error of the simultaneous four state fit was 0.000098 cm�1

and the complete set of fitted spectroscopic constants is providedin Table 1.

Fig. 2. Loomis–Wood plot for the ring puckering mode of 3-oxetanone in the range of(sharing common Kc) of the fundamental and first two hotbands.

4. Discussion

The rotationally-resolved far infrared spectrum of the ring puck-ering mode of 3-oxetanone allowed unambiguous assignment ofthe fundamental and first three hotbands using lower and upperstate combination differences. The simultaneous fit of over ninethousand infrared rovibrational transitions with such a remarkablylow rms error (0.000098 cm–1) suggests that the model Hamilto-nian employed provides a good description of these four excitedring puckering states (v = 1–4). The four band origins have beenaccurately determined to be: 139.549148(5) cm�1 (v = 0 ? v = 1),141.093457(14) cm�1 (v = 1 ? v = 2), 142.644034(19) cm–1

(v = 2 ? v = 3) and 143.989670(26) cm�1 (v = 3 ? v = 4). These val-ues fall within 0.5 cm�1 of those first reported by Carreira and Lord(140 cm�1, 141.5 cm�1, 143.0 cm�1 and 144.3 cm�1) [19]. The ob-served ring puckering fundamental differs by �10 cm�1 fromthe harmonic frequency calculated using DFT theory (B3LYP/6-

152–163 cm�1 showing the strongest 12 progressions identified in the R branches

Fig. 3. A 0.15 cm�1 spectral window of the R branches of the ring puckering mode of 3-oxetanone showing the congested spectrum. Asymmetric rotor quantum number areused to identify transitions for the fundamental, v = 1 ? v = 2, v = 2 ? v = 3 and v = 3 ? v = 4 states, respectively.

Table 1Spectroscopic constants for the ground state, v = 0 ? v = 1, v = 1 ? v = 2, v = 2 ? v = 3 and v = 3 ? v = 4 states of 3-oxetanone.

m (cm�1) Ground statea v = 0 ? v = 1 v = 1 ? v = 2 v = 2 ? v = 3 v = 3 ? v = 40 139.549148(5) 141.093457(14) 142.644034(19) 143.989670(26)

Rotational constants (cm�1)A 0.4045571(3) 0.403009784(18) 0.40144310(3) 0.39987326(4) 0.39828864(9)B 0.16532500(2) 0.16545977(3) 0.16560914(5) 0.16577090(10) 0.16594099(19)C 0.12297756(2) 0.12330109(4) 0.12362574(7) 0.12393918(14) 0.1242563(3)

Centrifugal distortion constants/�109 cm�1

DJ 21.73(6) 22.642(16) 23.85(3) 23.62(5) 25.87(13)DJK 150.6(4) 144.86(8) 138.04(15) 139.4(3) 129.8(6)DK 103.4(4) 110.79(7) 117.99(13) 110.0(3) 105.8(5)dj 5.28(3) 4.963(14) 4.39(3) 5.34(5) 4.38(15)dk 90.8(8) 87.8(3) 89.6(6) 68.3(13) 90(2)

rms error (cm�1) 0.000159 0.000098

a The ground state parameters were derived from the MW spectrum (Ref. [22]) and infrared ground state combination differences from this work and Ref. [24] and weresubsequently held fixed in the fit of the excited states.

34 Z. Chen, J. van Wijngaarden / Journal of Molecular Spectroscopy 279 (2012) 31–36

311G++2d3p)) (149 cm�1) [24] and by an even greater amountwhen compared with the value that contains anharmonic correc-tions derived from second order perturbation theory (158 cm�1)[24]. This discrepancy suggests that existing computational proce-dures are still limited in their ability to accurately model such lowfrequency vibrations. The closest predicted value comes from thescaled harmonic frequency (135 cm�1) determined using the scal-ing factor (0.9679) of Andersson and Uvdal (135 cm�1) [28] whichwas derived for a series of smaller molecules. This result is likelyserendipitous as the same scaling factor did not improve the fre-quency predictions of other fundamental bands of 3-oxetanone ex-cept for the CH stretching modes above 2900 cm�1 [24].

The four band origins determined here were used in a leastsquares fitting routine [29] using a Hamiltonian of form:bH ¼ p̂2=2lþ ax4 þ bx2 where the reduced mass l of ring puckeringin 3-oxetanone was assumed to be 151 amu as determined previ-ously [19]. We determined the ring puckering potential to have theform V = ax4 + bx2 where a = 2.04388 � 105 cm�1/Å4 andb = 4.26392 � 104 cm�1/Å2. The positive sign of the b coefficientprovides confirmation that the potential is that of a single well.Comparing the values with those reported by Carreira and Lord(a = 1.86 � 105 cm�1/Å4 and b = 4.31 � 104 cm�1/Å2) [19], the quar-tic coefficient shows the largest discrepancy (�10%). This is likely aresult of the fact that their reported band centers, which were

chosen at the point of maximum absorption from the low resolu-tion scan, are systematically blue-shifted from those determinedin this work. A closer examination of the rotationally-resolved Qbranch of the ring puckering fundamental in Fig. 4, for example,highlights the band center determined from our assignment ofrovibrational transitions in comparison with that chosen in theearlier low resolution study [19]. Mathematically, the larger quar-tic term determined here suggests that the ring puckering poten-tial is flatter about its minimum and less harmonic thanpreviously thought although it is still fairly harmonic when com-pared with those of other four-membered heterocycles [21] suchas silacyclobutane (a = 3.82 � 105 cm�1/Å4, b = �26.06 � 103 cm�1/Å2) [7]. Chemically, as the quartic term in the potential is indicativeof the magnitude of the ring strain while the quadratic coefficientreflects the torsional forces [15], the larger a constant suggests thatring strain (which favors planarity) in 3-oxetanone is greater thanexpected based on earlier studies.

The rotational constants determined via our infrared spectra forthe v = 0 through v = 4 ring puckering states of 3-oxetanone arecompared to those derived from the microwave experiment of Gib-son and Harris [22] in Table 2. Although microwave measurementstypically provide higher precision, the data set for 3-oxetanoneonly contained nine pure a-type rotational transitions samplingrotational energy levels up to J = 3. The result is that the A

Fig. 4. Zoomed-in section of the Q branch of the ring puckering fundamental of 3-oxetanone. The arrow on the left indicates the band origin determined from fitting 3237rovibrational transitions for this band in this work, while the arrow on the right shows the estimated band center from the low resolution infrared study in Ref. [19].

Table 2Comparison between the rotational constants determined from infrared (this work) and microwave (Ref. [22]) experiments for the ground state and the first three excited ringpuckering state of 3-oxetanone.

v = 0 v = 1 v = 2 v = 3 v = 4

Rotational constants (this work) (MHz)A 12128.316(9) 12081.9294(6) 12034.9614(9) 11987.8988(12) 11940.393(3)B 4956.3188(6) 4960.3591(9) 4964.8371(15) 4969.686(3) 4974.785(6)C 3686.7745(6) 3696.4737(12) 3706.206(2) 3715.603(4) 3725.110(9)

Rotational constants (MW, Ref. [22])A 12129.2 (9) 12082.9 (9) 12035.9(9) 11988.8(9) 11940.8(9)B 4956.29(2) 4960.34(2) 4964.85(2) 4969.64(2) 4974.77(2)C 3686.77(2) 3696.45(2) 3706.14(2) 3715.61(2) 3725.05(2)

Fig. 5. Observed rotational constants (A, B and C) of 3-oxetanone as a function of vibrational state.

Z. Chen, J. van Wijngaarden / Journal of Molecular Spectroscopy 279 (2012) 31–36 35

rotational constants were not well-determined and the centrifugaldistortion constants were not fit at all. The spectroscopic parame-ters determined in this infrared study are based on analysis of en-ergy levels covering a much broader range of J and Ka values,typically to values greater than 40. Even the lowest intensity hot-band, v = 3 ? v = 4, included transitions to J values of 30 withKa = 0–29. The observed infrared spectra are sensitive to terms inthe Hamiltonian that include the three rotational constants andfive centrifugal distortion constants and as seen in Table 1, thesigns and magnitudes of these parameters are consistent acrossvibrational states as expected. The rotational constants are plottedas a function of vibrational state in Fig. 5 and the plots reveal a

linear relationship which confirms that the ring puckering poten-tial is relatively harmonic [2,11]. The largest slope is observed forthe plot of the A rotational constants and it is seen that the magni-tude of A decreases with increasing vibrational excitation. This canbe rationalized by noting that the a-axis bisects the ring and passesthrough the two oxygen atoms in the equilibrium structure. Onewould expect that with increasing excitation of the ring puckeringmotion, these oxygen atoms spend a greater amount of time off thea-axis which would increase Ia and decrease the value of A. Fur-thermore, both B and C increase with vibrational quantum numberwhich is consistent with the heavy atoms of the ring being, onaverage, closer to the b- and c-axes during ring puckering.

36 Z. Chen, J. van Wijngaarden / Journal of Molecular Spectroscopy 279 (2012) 31–36

5. Conclusions

In this study, we have successfully measured and analyzed ahigh quality, rotationally-resolved far infrared spectrum of thelow-lying ring puckering fundamental (139.549148(5) cm�1) andfirst three hotbands (141.093457(14) cm�1, 142.644034(19)cm�1, 143.989670(26) cm�1) of 3-oxetanone. For the fundamental,the observed band center differs by more than 10% from ab initioestimates suggesting that challenges remain in modeling low fre-quency modes. Accurate rotational constants and centrifugal dis-tortion constants were obtained for all four excited states andthe band centers were fit to determine the ring puckering potentialfunction V = ax4 + bx2 where a = 2.04388 � 105 cm�1/Å4 andb = 4.26392 � 104 cm�1/Å2. The potential is flatter and slightly lessharmonic than previously thought but the linear trend in rotationalconstants as a function of v show that the potential is still fairlyharmonic at least in the region of the lowest five vibrational states.

On a final note, the high quality of the data obtained using syn-chrotron radiation as the infrared source was critical in assigningthis congested spectrum with confidence in that it allowed us toconfirm all quantum number assignments using combination dif-ferences. The accurate description of these low-lying states willbe useful when analyzing rotationally-resolved spectra of 3-oxeta-none in higher frequency regions as these excited ring puckeringstates are appreciably populated at room temperature. For exam-ple, based on the Boltzmann equation at 298 K, the relative popu-lation of the first few excited states: N1/N0 = 0.51 and N2/N0 = 0.26,suggests that hotbands and combination bands in other spectralregions will likely involve some contribution from ring puckering.

Acknowledgments

This research was funded by the Natural Sciences and Engineer-ing Research Council of Canada (NSERC) through the DiscoveryGrants program. The authors are grateful to Brant Billinghurst(Canadian Light Source) for technical support for the experimentsin Saskatoon as well as to W.C.P. Pringle at Wesleyan Universityfor the many interesting and useful discussions.

Appendix A. Supplementary material

Supplementary data for this article are available on ScienceDi-rect (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 withthis article can be found, in the online version, at http://dx.doi.org/10.1016/j.jms.2012.08.006.

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