Article pubs.acs.org/JPCA
Synchrotron-Based Far-Infrared Spectroscopic Investigation and ab Initio Calculations of 3‑Oxetanone: Observation and Analysis of the ν7 Band and the Coriolis Coupled ν16 and ν20 Bands Ziqiu Chen and Jennifer van Wijngaarden* Department of Chemistry, University of Manitoba, Winnipeg Manitoba, R3T 2N2 Canada S Supporting Information *
ABSTRACT: Rotationally resolved vibrational spectra of the fourmembered heterocycle 3-oxetanone (c-C3H4O2) have been investigated in the 360−720 cm−1 region with a resolution of 0.000 959 cm−1 using synchrotron radiation from the Canadian Light Source. The observed bands correspond to motions best described as CO deformation out-of-plane (ν20) at 399.6 cm−1, CO deformation inplane (ν16) at 448.2 cm−1, and the ring deformation (ν7) at 685.0 cm−1. Infrared ground state combination differences along with previously reported pure rotational transitions were used to obtain the ground state spectroscopic parameters. Band centers, rotational and centrifugal distortion constants for the ν7, ν16, and ν20 vibrational excited states were accurately determined by fitting a total of 10 319 assigned rovibrational transitions in a global analysis. The two adjacent carbonyl deformation bands, ν16 and ν20, were found to be mutually perturbed through a first-order atype Coriolis interaction which was accounted for in the multiband analysis. The band centers agree within 3% of the ab initio estimates using DFT theory.
1. INTRODUCTION 3-Oxetanone (c-C3H4O2), along with other four-membered heterocycles, is a fundamentally important species in synthetic and pharmaceutical chemistry. It undergoes numerous ringopening and ring expansion reactions and these, together with the ability to transform its carbonyl group via reduction and Grignard reactions, make it a versatile starting material in multistep organic synthesis.1 Such activity is inherently linked to the instability of highly strained four-membered ring moieties and the structural properties of the ring can clearly influence the rate of nucleophilic substitutions at different sites that open the ring. In drug discovery, 3-oxetanone derivatives are preferred precursors for adding oxetane (c-C3H6O) subunits into pharmaceuticals that serve to block metabolically exposed sites in the bioactive molecule without increasing its lipophilicity.2,3 In this regard, the ring structure of the precursor obviously plays a role in the effectiveness of the blocking agent. To better understand and exploit the chemical and metabolic behavior of 3-oxetanone and related heterocycles, a connection of these activities to the fundamental properties of the ring is needed. In this regard, modest effort has been put into the study of the structure and dynamics of 3-oxetanone to date. Semiempirical4 and ab initio5,6 calculations were used to investigate the structure and ring puckering potential.7 Experimentally, from analysis of the microwave spectrum by Gibson and Harris,8 it has been shown that 3-oxetanone is a planar asymmetric top of C2v symmetry with a permanent dipole moment of 0.887 D. The planar structure was rationalized by Meinzer and Pringle9 who investigated the balance between © 2012 American Chemical Society
torsional strain and ring strain and their connection to ring planarity in 3-oxetanone and related compounds. Carreira and Lord10 reported the first vibrational study of the ring puckering mode of this molecule at 140 cm−1, and a more comprehensive infrared and Raman study in the gas and liquid phases was subsequently presented by Durig et al.11 in which all 21 fundamental bands were assigned and analyzed. To date, the vibrational spectrum of 3-oxetanone has not been studied by high resolution techniques. Our recent far-infrared spectra of the structural isomer 2oxetanone (β-propiolactone)12,13 demonstrate the value of high resolution investigations of small ring molecules. For example, our analysis of the ν13 and ν20 bands served to correct the assignment of the CO deformation in-plane and out-of-plane modes (separated by only ∼20 cm−1), which were misassigned in the original low resolution study of this molecule. Our high resolution spectrum also permitted the analysis of the Coriolis interaction between these two modes leading to a more complete, accurate set of spectroscopic constants for these states. In the current work, we report the far-infrared rovibrational assignment and analysis of the ν7, ν16, and ν20 bands of 3oxetanone using data collected at Canada’s national synchrotron facility. To the best of our knowledge, this is the first high resolution vibrational study of this molecule. On the basis of ab initio calculations of the harmonic and anharmonic vibrational Received: August 2, 2012 Revised: August 28, 2012 Published: August 28, 2012 9490
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Table 1. Comparison of the Experimentally Observed Vibrational Frequencies (cm−1)11 with Computational Predictions (This Work) (B3LYP/6-311++G(2d,3p)) fundamental ν 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 a
experiment a
2931 1847 1457 1134 979 832 683 2962b 1111b 1102b 2924 1430 1250a 1076 955a 448 2956 1284a 1049 401 140
calculated harmonic
calculated harmonic (scaled 0.9679)c
calculated anharmonic
normal mode
symmetry
3037 1888 1488 1311 991 840 688 3081 1120 770 3026 1461 1272 1087 991 449 3080 1158 1070 410 149
2936 1827 1440 1269 959 813 666 2982 1084 745 2929 1414 1231 1052 958 434 2981 1121 1036 397 135
2889 1851 1472 1279 971 822 680 2927 1094 755 2839 1425 1242 1051 958 444 2925 1131 1048 411 158
CH2 sym str CO str CH2 scissoring CH2 wag ring breathing ring def ring def CH2 antisym str CH2 twist CH2 rock CH2 sym str CH2 scissoring CH2 wag ring def ring def CO def in-plane CH2 antisym str CH2 twist CH2 rock CO def out-of-plane ring puckering
A1 A1 A1 A1 A1 A1 A1 A2 A2 A2 B1 B1 B1 B1 B1 B1 B2 B2 B2 B2 B2
From liquid IR spectrum. bFrom liquid Raman spectrum. cReference 21.
a liquid sample (97% purity, Synthonix). For the weak ν7 band, 708 mTorr of vapor was introduced into a 2 m multipass cell, which was set to achieve a total absorption path length of 72 m. Interferograms were recorded in the 550−1130 cm−1 region utilizing the instrument’s entire 9.4 m optical path difference to realize the full spectral resolution of 0.000 959 cm−1. The calculated Doppler line width of 3-oxetanone in this region is such that the observed line widths (fwhm) were instrument limited. For this particular spectral range, the spectrometer was equipped with a KBr beamsplitter and a GeCu detecter. The final spectrum of the ν7 band was obtained by averaging 758 separate interferograms and applying a Fourier transform. Background interferograms were collected at lower resolution (0.015 36 cm−1), averaged and the result was transformed with appropriate zero filling before calculation of the absorption spectrum. The data for the ν16 and ν20 bands were collected in the range 360−550 cm−1 with the spectrometer outfitted with a 6 μm Mylar beamsplitter and a GeCu detector at a reduced pressure of 188 mTorr to prevent saturation of the more intense ν16 band. The spectrum was calculated from 454 interferograms following the procedure described above. The ν7 spectrum was calibrated using a sample of N2O and comparing the observed line positions in the 615−725 cm−1 range with those from the HITRAN database.15 The lower frequency spectrum was corrected using residual water lines in the 360−490 cm−1 range. In the end, we found the measured frequencies in both spectral regions to be accurate to within 0.0002 cm−1. A complete listing of the assigned transitions is provided as Supporting Information.
frequencies, these modes correspond to ring deformation and the CO in-plane and out-of-plane bending modes, respectively. To properly account for the Coriolis interaction between the ν16 and ν20 bands, we first established accurate ground state spectroscopic constants using ground state combination differences from the assigned rovibrational transitions along with the previously reported microwave frequencies.8 Once the ground state parameters were welldetermined, a global analysis of 10 319 separate infrared transitions corresponding to these three vibrations was performed to extract the key spectroscopic parameters that characterize the vibrationally excited states. The experimental results are complemented by ab initio estimates of the band centers using DFT theory. These show close agreement for the ν7, ν16, and ν20 bands reported here but also identify several inconsistencies in the original assignment of the fundamental bands of 3-oxetanone.11
2. EXPERIMENTAL DETAILS The experiment was conducted at the Canadian Light Source (CLS), using the synchrotron as the infrared light source and a Bruker IFS125 HR spectrometer capable of recording infrared spectra with an unapodized spectral resolution of 0.000 959 cm−1. Synchrotron radiation (SR) offers a light source with higher brightness than conventional thermal infrared sources. As SR is highly collimated, it passes through the small aperture needed for high resolution experiments with minimal loss. A recent infrared study of pyrrole compared the high resolution vibrational spectrum recorded using SR from the CLS with that recorded using a conventional thermal (globar) source on the same spectrometer.14 The signal-to-noise ratio of the SR spectrum showed a factor of 8 improvement compared to that of the globar spectrum due to this “synchrotron advantage”. Rotationally resolved vibrational spectra of the ν7, ν16, and ν20 bands of 3-oxetanone were recorded separately in two spectral regions at room temperature using vapor pressure from
3. FREQUENCY CALCULATIONS The harmonic vibrational frequencies of 3-oxetanone were calculated using Gaussian 03W16 at the B3LYP level with a 6311++G(2d,3p) basis set following geometry optimization. Anharmonic corrections were subsequently determined via second-order perturbation analysis in G03W as our previous 9491
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Figure 1. Overview spectrum showing the overlapping c-type ν20 and the b-type ν16 bands of 3-oxetanone between 360 and 500 cm−1.
parameters were varied (three rotational constants and five centrifugal distortion constants for the ν7 state plus the band origin), the rms error of the fit was only 0.000 114 cm−1 in the preliminary band analysis. 4.2. Assignment of b-Type ν16 and c-Type ν20 Bands. The ν20 band at 399.6 cm−1 corresponding to CO out-ofplane deformation, has B2 symmetry which gives rise to c-type transitions with selection rules oe↔ee and eo↔oo. The ν16 CO in-plane deformation mode at 448.2 cm−1 has B1 symmetry corresponding to the b-type selection rules eo↔oe and ee↔oo. The assignment of these two bands was difficult not only because the ν20 band is inherently weak but also because its R branch significantly overlaps the P branch of the ν16 band resulting in a very congested spectrum in this region as seen in Figure 1. To aid in the assignment, Loomis-Wood plots were constructed for the ν16 and ν20 bands to identify patterns of the strongest progressions of lines using the Loomis−Wood add-in (LWA)18 program for IgorPro. During the preliminary stages, although there were many transitions that lined up well with the predicted frequencies, a number of transitions were noticeably shifted as shown in Figure 2. The preliminary fits of the individual bands revealed that certain excited states parameters (most notably A and ΔK) were inconsistent with those of the ground state and, more importantly, that this set of constants could not account for the remaining unassigned transitions. Such discrepancies are indicative of a perturbation. Also, as seen in the band profiles in Figure 1, the spectrum of the ν20 band exhibits a 1:3 intensity ratio between the P and R branches which suggests the presence of intensity borrowing as a result of interaction with nearby energy levels. To fully account for these observations, we started by seeking a set of “perturbation free” ground state spectroscopic constants as outlined below. 4.3. Determination of the Ground State Spectroscopic Constants. The ground state spectroscopic constants of 3oxetanone were determined using both infrared and microwave transitions from the current work and ref 8, respectively. Initially, we used rovibrational transitions from the P and R branches of the three bands under study (a data set which includes a-, b- and c-type transitions) to obtain infrared ground
work on the structural isomer 2-oxetanone12 showed that the anharmonic estimates were in better agreement with experiment in this region. This result was achieved at a steep computational price, however, as the anharmonic calculations took 40 times longer to complete. The calculated frequencies for 3-oxetanone are listed in Table 1 along with descriptions of the corresponding motions. The band labels follow the convention established by Durig et al.11
4. SPECTRAL ASSIGNMENT AND ANALYSIS Earlier spectroscopic work has shown that 3-oxetanone is a planar asymmetric rotor of C2v symmetry.8 The three modes studied in this work were first identified by Durig et al.11 and correspond to motions involving CO deformation out-ofplane (ν20), CO deformation in-plane (ν16) and ring deformation (ν7). The portion of the spectrum containing the ν16 and ν20 bands is shown in Figure 1. The rovibrational assignment and analysis of the ν7, ν16, and ν20 bands of 3-oxetanone is detailed below. 4.1. Assignment of the a-Type ν7 Band. The ν7 ring deformation band has A1 symmetry corresponding to a-type selection rules eo↔ee and oe↔oo. Upon initial inspection, there were clear rotational patterns visible in the P and R branches. The estimated band origin from the early low resolution IR study11 together with the ground state rotational constants previously determined from the microwave work8 were used to simulate the rovibrational spectrum of this band. Once the observed patterns were identified on the basis of comparison with those simulated, the assignment of the quantum numbers of the first few transitions was confirmed using ground state combination differences. After several progressions were assigned and fit using Watson’s A-reduced Hamiltonian, Ir-representation in Pickett’s SPFIT program,17 the simulated spectrum was refined and the process continued in an iterative fashion until the strongest transitions were assigned. Overall, 2461 transitions were assigned from the P and R branches up to Ka =29 with J(max) = 58. Another 307 lines from the Q branch were assigned with Ka values between 2 and 35 and J values up to 40. Although only nine excited state 9492
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spectrum cover the high Ka levels. Normally, when transitions of these types are combined, there is such broad coverage of energy levels that the ground state parameters are wellcharacterized. In this case, however, the centrifugal distortion constants ΔK and δk remained poorly determined at this stage due to incomplete assignment of the weak, perturbed c-type ν20 band. To remedy this, we began assignment of the c-type ring puckering band (ν21) of 3-oxetanone centered at ∼139.5 cm−1 and produced a final data set containing 2532 infrared ground state combination differences spanning J quantum numbers from 0 to 61, covering Ka values from zero up to 34.19 This was supplemented with the nine previously reported pure rotational transitions8 and used to determine a complete, accurate set of ground state spectroscopic constants using Pickett’s SPFIT program (A-reduction, Ir representation).17 The uncertainty assigned to the pure rotational transitions was 1/1500 that of the infrared transitions to reflect the greater precision of the microwave data. The overall rms error of the ground state fit was 0.000 159 cm−1. The ground state spectroscopic parameters were held fixed in the subsequent infrared analysis of the ν7, ν16, and ν20 bands. 4.4. Perturbation Analysis of the ν16 and ν20 Bands. Under C2v symmetry, the ν16 and ν20 bands of 3-oxetanone have B1 and B2 vibrational symmetries, respectively, which can give rise to a first-order a-type Coriolis interaction. Once the ground state energy levels were characterized in isolation (to remove any effect of the untreated perturbation), we reanalyzed the ν16 and ν20 bands simultaneously by incorporating a symmetryallowed first-order a-type Coriolis interaction parameter Ga into the Hamiltonian. This allowed the assignment of the shifted infrared transitions (as shown in Figure 2). In total, 4143 transitions were assigned for the b-type ν16 band including 949 transitions from the Q branch. The spanned quantum numbers of assigned transitions include those in the P and R branches from J = 1 through 74 with a maximum Ka value of 37. Transitions in the Q branch were assigned with Ka between zero and 32 and J values up to 46. For the weaker c-type ν20 band, 2814 P and R branch transitions between Ka = 4 and 45 with J(max) = 45 were assigned. An additional 594 Q branch transitions with Ka values up to 19 were included to a maximum J of 52. The rovibrational analysis of the 7551 transitions assigned for these two vibrational states yielded a rms error of only 0.000 108 cm−1.
Figure 2. Zoomed-in section of the P branch of the ν16 band showing the effect of Coriolis interaction with the neighboring ν20 band. The stick spectrum shows the predicted transition frequencies in the absence of perturbation. The transition marked with a star is noticeably blue-shifted.
state combination differences for direct fitting of the ground state parameters. Ground state combination differences calculated from a-/b-type lines provide information about the high J and Kc energy levels whereas those from a c-type
Table 2. Spectroscopic Constants (cm−1) for the Ground State, ν20, ν16, and ν7 Bands of 3-Oxetanone ground statea ν 0 Rotational Constants A 0.4045571(3) B 0.16532500(2) C 0.12297756(2) Centrifugal Distortion Constants (×109) ΔJ 21.73(6) ΔJK 150.6(4) ΔK 103.4(4) δj 5.28(3) δk 90.8(8) Ga(16,20) rms error 0.000159
ν20
ν16
ν7
399.588606(6)
448.194670(5)
684.994728(7)
0.40533(2) 0.16542474(3) 0.12309730(3)
0.40484(2) 0.16536694(3) 0.122781360(17)
0.40406372(4) 0.16519845(3) 0.122850816(17)
21.984(11) 140.66(16) 102.56(14) 5.213(6) 101.5(3)
21.882(4) 164.62(15) 112.94(12) 5.367(2) 86.5(3)
21.895(8) 152.03(5) 104.10(5) 5.299(5) 91.90(8)
0.3402(17) 0.000110
a
The ground state parameters were derived from the MW spectrum (ref 8) and infrared ground state combination differences and subsequently held fixed in the fit of the excited states. 9493
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Table 3. Subset of Spectroscopic Constants (cm−1) That Illustrate the Effect of Including and Excluding the First-Order a-Type Coriolis Interaction Term between the ν16 and ν20 Bands without perturbation treatment A 109ΔK Ga rms error
perturbation treatment
ground state
ν20
ν16
ν20
ν16
0.4045571(3) 103.4(4)
0.40295009(7) 11.52(13)
0.40721404(7) 205.16(7)
0.40533(2) 102.56(14)
0.40484(2) 112.94(12)
0.000295
4.5. Global Analysis of the ν7, ν16, and ν20 Bands. A global fit incorporating all 10 319 assigned rovibrational transitions from the ν7, ν16, and ν20 bands was performed in Pickett’s SPFIT program using Watson’s A-reduced Hamiltonian, Ir-representation17 with the ground state spectroscopic constants held fixed. The rms error of the simultaneous three band fit was 0.000 110 cm−1 and the resulting spectroscopic constants are listed in Table 2.
0.3402(17) 0.000108
between Ga and A. Overall, the rms error of the fit was reduced by a factor of 3 when Ga was included, suggesting that this set of constants provides a better description of the 7551 transitions assigned in the carbonyl deformation region of the spectrum. The Coriolis interaction term was found to have a value of 0.3402(17) cm−1, and the positive sign indicates that this is a positive perturbation in which the R branch of the lower frequency band (ν20) and the P branch of the higher frequency band (ν16) gain intensity while the other branches are depleted. This leads to the asymmetry in the band profiles shown in Figure 1. Classically, this occurs when two perpendicular vibrational transitions (in this case the ν16 in-plane and ν20 outof-plane CO deformations) are enhanced by the rotation of the molecule producing Coriolis forces in the same sense as the vibrational angular momentum of the adjacent mode.20 In this study, the band origins of the ν20 (399.588 606(6) cm−1), ν16 (448.194 670(5) cm−1), and ν7 (684.994 728(7) cm−1) modes were accurately determined for the first time and compare well with the gas phase infrared spectrum reported by Durig et al. (401 cm−1, 448 and 683 cm−1).11 These values can be compared with the ab initio predictions of the harmonic and anharmonic frequencies in Table 1. An additional column listing scaled harmonic frequencies is also included which were determined using an empirical scaling factor (0.9679) derived from comparing experimental and ab initio frequencies for a set of 125 small molecules.21 When the band centers determined in this work are compared with the harmonic frequencies derived using DFT theory (410, 449, and 688 cm−1), the calculated values differ by less than 10 cm−1 from their experimental counterparts. The scaled harmonic (397, 434, and 666 cm−1) and anharmonic frequencies (411, 444, and 680 cm−1) provide no systematic improvement in this spectral region. This may be serendipitous or may suggest that these particular modes of 3oxetanone are fairly harmonic. The latter assertion is consistent with the ab initio derived potential for the ring puckering mode of 3-oxetanone, which has a small anharmonic correction compared with those of related heterocycles.7 Upon comparison of the band assignments of all 21 vibrational fundamentals of 3-oxetanone provided by Durig et al.11 with the ab initio estimates in Table 1, several notable features were identified. At this level of theory, the harmonic values, in general, match the experimental frequencies to within 3% across the entire vibrational spectrum. The scaled harmonic values provide the best estimates (well within 1%) of the highest energy vibrations in the CH stretching region above 2900 cm−1 but provide no systematic benefit over the other methods in predicting the frequencies of bending and deformation motions. This suggests that empirical scaling factors are of limited use for predicting far-infrared spectra of such molecules. Our ab initio study also identified several inconsistencies with the original assignments given in ref 11. The most obvious discrepancy is with the ν10 fundamental,
5. DISCUSSION The low resolution survey of 3-oxetanone by Durig et al.11 provided a general overview of the approximate frequencies, selection rules and intensities of the fundamental vibrations of this molecule. This served as a preliminary guide for our high resolution work in the far-infrared region and allowed accurate characterization of the ground state as well as the ν7, ν16, and ν20 excited states. The simultaneous fit of over ten thousand infrared transitions with such a remarkably low rms error (0.000 110 cm−1) suggests that the model Hamiltonian employed provides a good description of these three rovibrational bands. At a more fundamental level, the high resolution spectra have also confirmed the nature of the vibrational motions that are involved in these transitions as the observed patterns are consistent with selection rules related to primarily in-plane (ν7, ν16) and out-of-plane (ν20) modes. The unambiguous assignment of the rotational structure was critical in this regard as the low frequency modes of the structural isomer 2-oxetanone were originally misassigned when only the unresolved band profiles were observed.13 Although the microwave spectrum of 3-oxetanone offers greater precision, the data set included only nine a-type transitions that sampled energy levels to J = 3. In the present work, energy levels up to J = 61 and covering a broad range of Ka and Kc values were used to derive the ground state spectroscopic parameters via ground state combination differences. This method removes the upper state dependence to isolate any perturbation and thus ensures that the resulting spectroscopic constants are physically meaningful rather than effective fit parameters. We believe that our treatment of the ground state in this analysis has established a complete, accurate set of ground state spectroscopic parameters of 3oxetanone as listed in Table 2 which are useful in this and future rotationally resolved studies of this heterocycle. During the initial assignment of the closely spaced ν16 and ν20 bands, the rotational constant A and the centrifugal distortion constant ΔK seemed to be effective constants and varied more than expected from the analogous ground state parameters as shown in Table 3. Furthermore, these effective parameters did not account for many of the weaker transitions in this spectral region. The addition of the Coriolis term provided better consistency among the spectroscopic constants as seen in Table 3 although the excited state A rotational constants were less well-determined due to a high correlation 9494
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which was assigned on the basis of the Raman spectrum of liquid 3-oxetanone. The experimental value of the band center (1102 cm−1) differs by more than 300 cm−1 from the calculated value (770 cm−1). This misassginment is likely a result of the low Raman intensity of the ν10 band, which is calculated to be less than 2% that of the nearest Raman active mode (ν7) less than 100 cm−1 away. Another discrepancy was discovered in the assignment of the ν4 (CH2 wag) and ν18 (CH2 twist) modes at 1134 and 1284 cm−1, respectively. These are reversed according to the ab initio calculations. The former should be at higher frequency (1311 cm−1 harmonic) than the latter (1158 cm−1 harmonic). This could easily be confirmed from a high resolution spectrum in this region in the future as the wag and twist modes obey a-type and c-type selection rules, respectively. As the band envelopes of these modes are approximately 60 cm−1 wide, the observation of the ν4 and ν18 bands in the original study was likely obstructed by the neighboring ν13 (1272 cm−1) and ν14 (1087 cm−1) vibrations, which are estimated to be approximately 1 and 2 orders of magnitude more intense, respectively. Finally, the band centers reported for the ν5 ring breathing and ν15 ring deformation bands differed by ∼15 cm−1 on the basis of the liquid IR and Raman spectra and the ν15 band was not assigned in the gas phase spectrum at all. On the basis of our calculations, the band centers for these two ring vibrations differ by only one wavenumber and the ν15 band is actually a factor of 3 more intense! As we collected spectra up to 1130 cm−1 during our study of the ν7 vibration, we serendipitously observed the ν5 and ν15 bands which are centered near 982 and 983 cm−1, respectively. The former is weaker with a clear a-type Q branch and the latter is more intense and obeys b-type selection rules as expected on the basis of the symmetry of the ring breathing and deformation motions. In fact, we have partially assigned the b-type band but there is evidence of a strong perturbation which will be difficult to unravel without assignment of the buried a-type spectrum.
Article
ASSOCIATED CONTENT
S Supporting Information *
Complete listing of the assigned rovibrational transitions for the ν7, ν16, and ν20 bands. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: (204)474-8379. Fax: (204)474-7608. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) through the Discovery Grants program. The authors are grateful to Brant Billinghurst (Canadian Light Source) for technical support for the experiments in Saskatoon.
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REFERENCES
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6. CONCLUSIONS In this work, we have assigned 10 319 rovibrational transitions from the ν7, ν16, and ν20 modes of 3-oxetanone and derived accurate band centers and rotational and centrifugal distortion constants for these three vibrational states. The perpendicular modes corresponding to carbonyl deformation (ν16 and ν20) were mutually perturbed and a first-order a-type Coriolis parameter was employed to treat this interaction. The ground state of 3-oxetanone has been extensively characterized through fitting of infrared ground state combination differences that sample quantum states up to J = 61 and Ka = 34 along with the previously reported microwave transitions.8 These parameters will be useful for further rotationally resolved spectra of 3oxetanone. On the basis of our ab initio calculations, we have identified several inconsistencies in the assignment of fundamental bands in the original low resolution vibrational study of 3-oxetanone. The current work has demonstrated the importance of high resolution infrared spectroscopy in characterizing low frequency vibrations of organic building blocks and has laid important groundwork for further studies on this and related molecules. 9495
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The Journal of Physical Chemistry A
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